New Fre: ttiers 

lsetepic ~esearch: ........ ~ 

Probes, l .n P11: _,es 

Smali--Saffiple JVnaiY 

New Frontiers in Stable Isotopic 

Research: Laser Probes, Jon Probes, 

and Small-Sample Analysis 
The SHRIMP (Sensitive High mass Resolution lon Micro 

Probe) instrument at the Research School of Earth 

Sciences, The Australian National University. In the 

right foreground are the duoplasmatron for primary-ion 

beam generation and the sample chamber where the 

secondary-ion beam is generated and extracted. The 

electrostatic sector analyzer is in the right rear of the 

photo, and the 1-m radius, 6-t magnetic sector analyzer 

is in the rear center. The ion collector system is in the 

left rear. See Eldridge and others (p. 163) for details of 

SHRIMP construction and operation. 


New Frontiers in Stable 

Isotopic Research: Laser 

Probes, lon Probes, and 

Small-Sample Analysis 

W.C. SHANKS Ill and R.E. CRISS, editors 

Results of a workshop held in Reston, Va., January 12-13, 1988, 

on laser microprobe and ion microprobe analysis techniques 

performed on geological samples for stable and rare gas 

isotopic ratios 

U.S. GEOLOGICAL SURVEY BULLETIN 1890 
DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, Jr., Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

Any use of trade, product, or firm names in this publication is for descriptive purposes 

only and does not imply endorsement by the U.S. Government 

UNITED STATES GOVERNMENT PRINTING OFFICE: 1989 

For sale by the Books and Open-File Reports Section, 

U.S. Geological Survey, Federal Center, 

Box 25425, Denver, CO 80225 

Library of Congress Cataloging in Publication Data 

New frontiers in stable isotopic research : laser probes, ion probes, and 

small-sample analysis. W.C. Shanks Ill and R.E. Criss, editors. 

(U.S. Geological Survey bulletin ; 1890) 

"Results of a workshop held in Reston, Va., January 12-13, 1988, on laser 

microprobe and ion microprobe analysis techniques on geological samples for stable and rare gas isotopic ratios." 

Bibliography: p. 

Supt. of Docs. no. : Supt. of Docs. no. : 119.3:1890 

1. Isotope geology-Congresses. 2. Microprobe analysis-Congresses. 

I. Shanks, Wayne C. II. Criss, R.E. Ill. Series. 

QE75.B9 no. 1890 

[QE501.4.N9] 

557.3 s-dc20 

[551, .028] 89-600096 

CIP 
PREFACE 

Approximately 70 persons attended a workshop on January 12 and 13, 1988, 

entitled "New Frontiers in Stable Isotopic Research: Laser Probes, Ion Probes, and 

Small-Sample Analysis" held at the USGS National Center, Reston, Va. The program 

(see p. XI) included 16 talks on new methods for performing high-precision stable and 

rare-gas isotopic analyses on submicrogram quantities of minerals. The workshop was 

organized by W.C. Shanks III (USGS) and R.E. Criss (USGS) and was sponsored by 

the USGS Development of Assessment Techniques Program. ---

Bruce Doe (USGS Assistant Director for Research) presented an introductory 

address in which he emphasized the USGS commitment to innovative research. 

I. Lynus Barnes (National Bureau of Standards) led off the workshop by summarizing 

the current state of the art in conventional mass spectrometry, secondary ion mass 

spectrometry, and resonance ionization mass spectrometry using thermal vaporization 

and tunable lasers. Alfred Nier (University of Minnesota, Minneapolis, Minn.) 

described terrestrial and extraterrestrial applications of small volume, double-focusing 

mass spectrometers of the type he designed for the Viking lander. 

Two predominant themes emerged in the remainder of the workshop: ( 1) the use 

of a focused laser beam (laser probe) to vaporize 10- to 100-J-1-m cavities followed by 

dynamic or static mass spectrometry to analyze resultant gases and (2) the use of a 

focused ion beam (ion probe) to sputter a 1- to 30-J-1-m spot to produce secondary ions 

that are analyzed in a large-radius double-focusing mass spectrometer. Both technologies have been successfully employed in certain radiogenic and microanalytical applications but are only now being developed for high-precision stable isotopic analysis. 

Ian Wright (The Open University, England) discussed 813C and 8180 analysis by static 

mass spectrometry of very small amounts of gas released by stepped heating of separated minerals. This procedure allows the systematic elimination of contaminants from 

very small samples. Henryk Herman (BP Research Centre, England) reported complete 

conversion of calcite to C02 with no isotopic fractionation under carefully selected 

conditions of laser ablation of calcite. Everett Gibson (NASA Johnson Space Center, 

Houston, Tex.) noted that laser techniques are essentially perfected for 815N analyses. 

He also utilized a laser probe to study fluid inclusions and a number of mineral and 

rock fragments and made chemical (not isotopic) analyses on the released gas with a 

quadrupole mass spectrometer. Problems with laser probes discussed by these workers 

principally relate to inefficient ion production, incomplete conversion to the desired 

gas, and difficulties in handling very small gas samples. Other applications of laser 

techniques to microanalytical isotopic problems will be important in future stable isotopic research. For example, David Cole (Oak Ridge National Laboratory, Oak Ridge, 

Tenn.) discussed possible future stable isotopic analysis by resonance ionization spectroscopy, in which a secondary laser is tuned to selectively ionize specific species in a 

vapor cloud produced by laser ablation. In addition, Robert Burruss (USGS, Denver, 

Colo.) discussed the potential for nondestructive analysis of carbon and oxygen isotopes in fluid inclusions by use of laser Raman spectroscopy. 

Ion probe techniques also offer great promise for high-precision stable isotopic 

microanalysis. Stewart Eldridge (Australian National University, Canberra, Australia) 

reported remarkable success analyzing sulfur isotopic values on 30-j.J-m spots with the 

SHRIMP (Selective High mass Resolution Ion MicroProbe), attaining a precision of 

±2 otoo. Of particular importance are his observations that different generations of 

Preface VII 
VIII Preface 

sulfide minerals having radically different 834S values can occur in single samples. 

Nobu Shimizu (M.I.T., Cambridge, Mass.) also reported good success with ion probe 

determinations of 834S and showed that matrix effects, which invariably enrich the 

light isotopes, depend on the kinetic energy of the secondary ions. The ultimate precision of ion probe analyses on very small volumes (::::; 1 f.Lm3

) of material depends 

directly on the natural abundance of the isotopic species in question. Accordingly, 

there are large differences in the precision of 834S measurements e2

S/ 34S = 20) and 

8180 measurements e6Q/18Q = 500 in normal samples), With precise determinationS 

of 8180 being more difficult. However, Bruno Giletti (Brown University, Providence, 

R.I.) reported preliminary 8180 measurements of homogeneous plagioclase from a volcanic rock with a precision of about ±0.5 otoo, for measurements taken on a single 

day. He also reviewed a variety of geologic and petrologic problems where microanalytical capability for 8180 would result in substantial scientific gains. Ernst Zinner 

(Washington University, St. Louis, Mo.) reported useful 8180 and 8170 measurements 

on small grains from meteorite samples, which commonly have highly variable and 

extreme oxygen isotopic ratios relative to terrestrial samples, and he also illustrated the 

versatility of the ion probe by showing useful measurements forTi, Si, Mg, Ca, and 

H isotopes in meteorites. Richard Hervig (Arizona State University, Tempe, Ariz.) 

discussed the current technology of primary ion beams for ion probe measurements 

and reported on his attempts to eliminate surface charging effects and to measure 8180 

values in thin sections. The authors all discussed difficulties that arise from the insulating nature of most silicate samples, which leads to surface charging, and the additional 

problem of using inherently unstable electron multipliers to count minor ion beams. 

Nonetheless, ion probes provide extremely good spatial resolution, are less destructive 

than laser techniques, and have already permitted many remarkable measurements to 

be made. 

Several talks were scheduled on 40 Ar/ 39 Ar and other rare-gas studies in hope of 

adapting the technology that has been so successfully utilized by noble-gas investigators to stable isotopic microanalysis. Brent Dalrymple (USGS, Menlo Park, Calif.) 

discussed the use of continuous argon-ion lasers for stepped heating of separated mineral grains. He pointed out that this technique offers numerous advantages, such as 

rapid low-blank analysis of very small samples, over conventional 40 Ar/ 39 Ar analysis. 

Ed Spooner (University of Toronto, Ontario, Canada) discussed laser probe 40 Ar/ 39 Ar 

dating of fine-grained illites. The above techniques offer potential for stable isotopic 

work because they are safer (argon laser beam is visible) and contaminants can be 

eliminated by stepped heating. In addition, Paul Hearn (USGS, Reston, Va.) presented 

an application of an air abrasion mill for separation of diagenetic rims on potassium 

feldspar. J.K. Bohlke (Argonne National Laboratory, Argonne, Ill.) presented results 

of an innovative approach of analyzing chemical and isotopic species in fluid inclusions in which a laser microprobe is used to open fluid inclusions in irradiated samples 

for noble gas mass spectrometry. 

At the workshop, we offered to the authors and to others present the possibility 

of publishing a symposium volume as a U.S. Geological Survey Bulletin. The 

response was so positive that we decided to proceed. A target date of April 15, 1988, 

was set for manuscript submittal. Thirteen manuscripts were received, covering most 

of the topics discussed at the workshop, and two additional topics: 815N analysis of 

diamonds by use of the laser probe and static mass spectrometry (by I.A. Franchi and 

others) and laser microprobe microanalysis of coal (by P.C. Lyons and others). All 

manuscripts were subjected to thorough peer review, including scientific reviews by 

Shanks or Criss or both and at least one reviewer from outside the USGS. 

We are highly impressed with the bright future of stable isotopic microanalytical 

techniques, which ultimately will permit precise isotopic determinations on a variety of 

samples that are up to a billion times smaller than those used in conventional analyses. 

Such techniques will open numerous new avenues of research, will clearly revolutionize current understanding of stable isotopic geochemistry, and will make obsolete the 
conventional analysis of certain isotopically heterogeneous geologic materials. As an 

example of the many possible future applications that the conference portends, we 

point out that there is immediate potential for laser application to oD analyses of small 

samples that require only heating to release structural or included water. We feel 

certain that future research using ion probes and laser probes will lead to routine 

microanalysis of the isotopic ratios of carbon, oxygen, and sulfur and many other elements in ordinary samples, with accuracies and precisions similar to conventional macroanalysis techniques. 

We wish to acknowledge the helpful assistance of Irene Harrel, who organized 

registration, Jeffrey Thole, who operated the projection facilities, Susan Socks, who 

translated manuscripts on a wide variety of digital formats into the USGS computer 

system, Linda Shanks, who allowed 70 scientists to invade her house, and John Watson, 

who did an outstanding job of technical editing on the manuscripts. 

W.C. Shanks III 

R.E. Criss 

Preface IX 

PROGRAM 

National Center Auditorium 

TUESDAY, JANUARY 12, 1988 

8:30-8:45 AM INTRODUCTORY ADDRESS. Bruce Doe, Assistant 

Director for Research, U.S. Geological Survey, 

Reston, Va. 

8:45-9:30 AM "RIMS, SIMS, and 23°: Recent advances in mass 

spectrometry." I. Lynus Barnes, National Bureau 

of Standards, Washington, D.C. 

9:35-9:50 AM COFFEE BREAK 

9:50-10:35 AM "Isotope analyses with small-volume mass spectrometers." Alfred 0. Nier, Department of Physics, 

University of Minnesota, Minneapolis, Minn. 

10:40-11:25 AM "Applications of static mass spectrometry to stable 

isotope studies." I. P. Wright, Department of 

Earth Sciences, The Open University, Milton 

Keynes, England 

11:30-1:00 PM LUNCH 

1:00-1:45 PM "Laser ablation in microsampling for geological applications and stable isotope analysis." Henryk Herman, BP Research Centre, Sunbury-on-Thames, 

England 

1:50-2:20 PM "A quadrupole mass spectrometer laser microprobe 

system for analysis of gases released from geological samples." Everett K. Gibson, Jr., NASA 

Johnson Space Center, Houston, Tex. 

2:25-2:40 PM COFFEE BREAK 

2:40-3:00 PM "USGS laser Raman spectroscopy laboratory: Introduction to the technique, hardware, and opportunities for stable isotope studies of individual fluid 

inclusions." Robert C. Burruss, U.S. Geological 

Survey, Denver, Colo. 

3:05-3:20 PM "Geological applications of resonance ionization spectroscopy." David R. Cole (with N. Thonnard), Oak 

Ridge National Laboratory, Oak Ridge, Tenn. 

3:25-4:00 PM GROUP DISCUSSION of laser probe techniques in 

stable isotope research. 

WEDNESDAY, JANUARY 13, 1988 

8:30-9:15 AM "SHRIMP ion microprobe analysis of sulfur isotope 

values in minerals." C.S. Eldridge (with W. Compston, I.S. Williams, and R.A. Both), Geology 

Department, Australian National University, Canberra, Australia 

9:20-9:30 AM "Analysis of sulfur isotopes in sulfides with a Cameca 

IMS 3f." A. MacFarlane and Nobu Shimizu, 

Department of Earth, Atmospheric, and Planetary 

Science, Massachusetts Institute of Technology, 

Cambridge, Mass. 

9:35-9:50 AM COFFEE BREAK 

9:50-10:35 AM "Use of ion microprobes to measure natural oxygen 

isotope abundances in minerals." Bruno J. Giletti, 

Department of Geology, Brown University, Providence, R.I. 

10:40-11:25 AM "Ion probe isotopic analysis of extraterrestrial materials." Ernst Zinner, McDonnell Center for the Space 

Sciences, Washington University, St. Louis, Mo. 

11:30-1:00 PM LUNCH 

1:00-1:30 PM "Charge neutralization and ion microprobe analysis of 

oxygen isotopes in insulating phases." Richard L. 

Hervig, Department of Chemistry, Arizona State 

University, Tempe, Ariz. 

1:35-2:30 AM GROUP DISCUSSION of future developments in ion 

probe analyses. 

2:30-2:45 PM COFFEE BREAK 

2:45-3:05 PM "40 Ar/ 39 Ar dating of submicrogram samples with a 

continuous laser." G. Brent Dalrymple, U.S. Geological Survey, Menlo Park, Calif. 

3:10-3:30 AM "Continuous laser probe 40 Ar/ 39 Ar and conventional 

Kl Ar dating of illites associated with U mineralization, N. Saskatchewan, Canada: Comparison of 

results, and comparison with pitchblende U-Pb 

dates." Ed Spooner and Colin Bray, Department of 

Geology, University of Toronto, Ontario, Canada. 

3:35-3:45 PM "40 Ar/ 39 Ar dating of secondary overgrowths using 

3:50-4:10 PM 

4:15-5:00 PM 

quantitative separation via air abrasion and gravity 

settling techniques." Paul P. Hearn, Jr., U.S. 

Geological Survey, Reston, Va. 

"Simultaneous analyses of noble gas isotopes and 

halogens in irradiated fluid inclusions using a laser 

microprobe noble gas mass spectrometer." J.K. 

Bohlke, C. Kirschbaum, and J. Irwin, Argonne 

National Laboratory, Argonne, Ill. 

SUMMARY DISCUSSION 

Program XI 

CONTENTS 

Preface VII 

Program XI 

Isotope analyses with snJ<:dl-volume mass spectrometers-by Alfred 0. Nier 1 

Carbon isotopic analy~is of small samples by use of stepped-heating extraction and 

static mass spectrome ry -by I. P. Wright and C. T. Fillinger 9 

Laser microprobe-quadntpole mass spectrometer system for the analysis of gases and 

volatiles from geologi.:: materials- by Everett K. Gibson, Jr., and Robert H. 

Carr 35 

Application of lasers in ~mall-!'>ample stable isotopic analysis-by LA. Franchi, 

S.R. Boyd. I.P. \\/right. and C.T. Fillinger 51 

Simultaneous analyses of noble-gas isotopes and halogens in fluid inclusions in 

neutron-irradiated quartz veins by use of a laser-microprobe noble-gas mass 

spectrometer-by J.K. Bohlke, C. Kirschbaum, and J. Irwin 61 

The GLM Continuous La~er System for 40Ar/ 39 Ar dating: Description and performance 

characteristics-- by G Brent Dalrymple 89 

Application of the laser microprobe (LAMMA 1000) to the microanalysis of coal 

constituents-by Paul C. Lyons, John J. Morelli, and David M. Hercules 97 

Quantitative separation or authigenic overgrowths from mineral grains by air-abrasion 

and gravity settling techniques-by Paul P. Hearn, Jr., Jerry D. Gandy, 

John N. Aleinikoff. a:1d Raymond W. Werre 111 

Resonance ionization sp~ctroscopy and its potential application in geosciences- by 

Robert D. Willis. Nrrbert Thonnard. and David R. Cole 117 

Use of the ion micropt"CJ :1e to measure natural abundances of oxygen isotopes in 

minerals- by Bruno J . C i letti and N obu Shimizu 129 

Charge neutralization ar d oxygen isotopic analysis of insulators with the ion 

microprobe-by Rich ird L. Hervig, Robert M. Thomas, and Peter Williams 137 

Isotopic measurements \vith the ion microprobe-by Ernst Zinner 145 

Sulfur isotopic analyses on the SHRIMP ion microprobe-by C.S. Eldridge, 

W. Compston. I.S. \~rilhams. and J.L. Walshe 163 

Contents XIII 

Isotope Analyses with Small-Volume Mass 

Spectrometers 

By Alfred 0. Nier1 

Abstract 

Accurate isotope abundance measurements by mass 

spectroscopy became practical in the 1930's after the 

invention of the electrometer vacuum tube and the transverse electron-beam ion source. The introduction of magnetic sector mass analyzers greatly reduced the weight 

and power requirements of instruments and made practical the wide use of mass spectrometers for isotope and gas 

analyses. The advent of the space age stimulated the 

development of high-performance miniaturized instruments carried on sounding rockets, Earth satellites, and 

space probes. The techniques that were developed have 

been transferred to laboratory instrumentation. Among 

applications that have been made are the isotopic analysis 

of traces of mercury found in meteorites and the isotopic 

analysis of the helium extracted from small samples of 

extraterrestrial dust and from processed and native 

metals. 

INTRODUCTION 

Isotope analysis by mass spectroscopy had its practical beginning in the early 1930's, although measurements 

had been made before that time. Dempster ( 1918), after 

constructing the first magnetic deflection mass spectrometer, analyzed a number of elements, as did a few others who 

used instruments based on his design. Aston (1919), with 

his mass spectrograph, as he called it, and with later 

improved versions of the instrument, measured both masses 

and abundances of a large number of elements. Because 

Aston's abundances were determined from the blackening 

of photographic plates, the precision was not high. A few 

investigators, using thermal ionization for the production of 

ions, studied the isotopic constitution of elements such as 

lithium, potassium, and rubidium. Smythe and Mattauch 

(1932), by using a mass separator based on a radiofrequency filter array, studied the isotopic constitution of 

oxygen. 

The development of techniques for making accurate 

isotopic analyses on a routine basis had to wait for two 

important innovations: (1) the invention of the electrometer 

tube and (2) the introduction of the transverse electronbeam ion source. The former, a vacuum tube having an 

extremely high input impedance, made possible the measurement of very small ion currents-below 10- 16 A. It 

Manuscripts approved for publication March 3, 1989. 1 School of Physics and Astronomy, University of Minnesota, 

Minneapolis, MN 55455. 

replaced the quadrant electrometer, whose operation was an 

art in itself. The transverse electron-beam ion source, 

introduced by Bleakney (1930, 1932), made possible the 

design of mass spectrometers having improved resolution 

and other desirable characteristics. The stage was set for 

measurement of precise isotopic abundances including, of 

course, variations in abundance ratios. 

The mass spectrometers of the 1930's that were 

employed for measurement of isotopic abundances mainly 

used 180° magnetic deflection of the ions to create mass 

spectra. It was realized that by building larger instruments, 

higher resolution and better performance could be obtained. 

The ratio of energy spread of the ions leaving the ion source 

compared to the ion accelerating voltage would be reduced, 

as would the ratio of slit widths to the radius of curvature of 

the ion trajectories in the magnetic analyzers. As instruments grew so did their magnets, with the result that by 

1940 successful isotopic analysis instruments employing 2-t 

magnets were in use. These were powered by 5-kW 

generators whose output voltages had to be stabilized by 

ingenious electronic devices. 

A drastic reduction in the weight and power requirement of the electomagnet resulted when a principle published independently by Barber (1933) and Stephens (1934) 

made possible the construction of magnetic sector mass 

spectrometers (Nier, 1940). During World War II, many 

sector instruments were employed for measuring the enrichment achieved in the uranium isotope separation plants, for 

analyzing the process stream in the large gaseous diffusion 

plant in Oak Ridge, Tenn. (Nier and others, 1948), and for 

hunting vacuum leaks in such plants (Nier and others, 

1947). 

Following World War II, as scientists returned to 

civilian life, there was a surge of interest in mass spectrometry-particularly in the application of isotope studies to 

geological and cosmological problems. The do-it-yourself 

era of construction of mass spectrometers largely came to an 

end as commercial instruments became available, and mass 

spectrometers became tools available to many more investigators. As the drive for higher performance and precision 

grew, so did the size of the instruments. Ninety-degree 

magnetic sector mass spectrometers having radii of 30 em 

became standard in many laboratories. In such instruments, 

the ion path length from ion source to collector exceeded 

1m. 

An interesting modification of the sector geometry 

was proposed by Cross (1951). In this geometric configuration, the ions enter and leave the magnetic sector at a 

Isotope Analyses with Small-Volume Mass Spectrometers 
suitably chosen angle rather than at right angles as in the 

usual application, and one obtains higher dispersion as well 

as a desirable focusing of the individual beams in the z 

direction, perpendicular to the x-y plane of the trajectories. 

Successful commercial instruments have been built on the 

basis of this principle. 

While the determination of isotopic abundances was 

proceeding, a parallel effort was being made in the measurement of precise isotopic masses. Here, much higher 

resolution and stability were required, and, as a result, the 

instruments were more complex. In a typical case, the ions, 

after leaving the source, traveled through an electric sector 

analyzer in tandem with a magnetic sector analyzer. The 

combination produced what is called double focusing and 

insured that ions of a given mass leaving the ion source with 

a spread in energy as well as angle would focus sharply at 

a single position along the focal plane. The single-focusing 

instruments previously described provide only angle focusing. 

Among the double-focusing geometries employed in 

the pre-World War II era were those of Aston ( 1919), 

Dempster (1935), and Bainbridge and Jordan (1936). In 

these instruments, ions were detected by the blackening of 

photographic plates mounted along the focal planes. 

Through precise calibration, masses were related to position 

along the photographic plate. Such instruments were called 

mass spectrographs to distinguish them from mass spectrometers, where ions were detected electrically. 

MINIATURIZED MASS SPECTROMETERS 

When the space age arrived and rockets and space 

probes became available for conducting research in the 

Earth's upper atmosphere, or in the atmospheres of other 

planets, it was natural that mass spectrometry would be 

employed for studying the composition of atmospheric 

gases. In such investigations, in addition to magnetic 

deflection analyzers, a number of other mass analysis 

systems were successfully employed (Bennett, 1950; Paul 

and others, 1958). 

Weight, physical size, and power consumption are of 

minor consequence in laboratory instrumentation, but in 

space applications, this is not the case. Instruments must be 

small, light, and consume a minimum of power. In addition, they must be rugged so they can withstand the severe 

vibrational environment encountered in rocket launches and 

maneuvers. These requirements must be met without too 

much degradation of the performance found in laboratory 

instrumentation. 

For our laboratory investigations involving space 

flights, these factors led to the adoption of the MattauchHerzog geometry (Mattauch and Herzog, 1934), because it 

appeared best able to meet the requirements without seriously compromising performance and versatility. Figure 1 

2 New Frontiers in Stable Isotopic Research 

electric analyzer 

Figure 1. Schematic diagram showing the angular focusing 

properties of the Mattauch-Herzog double-focusing mass 

spectrometer. A diverging beam leaving the ion source 

through slit S0 , located at the focus of the electric sector 

analyzer, emerges from this sector as a parallel beam. In 

the magnetic analyzer, the trajectories are such that the 

parallel beam focuses on a focal plane, the position along 

the plane being proportional to the square root of the 

ratio of mass to charge of the ions. Ions having two 

different mass-to-charge ratios are shown for illustration. 

A beam leaving the ion source and having a spread in 

velocities (not shown here) also emerges from the electric 

analyzer as a parallel beam that, for a given mass-to-charge 

ratio, focuses at the same point as the ions that have an 

angular spread. Hence the expression double focusing. 

Convergence is to first order only. A diaphragm placed 

near the exit of the electric analyzer limits the angular and 

velocity spread sufficiently so that the first-order approximation pertains. At one point along the focal plane (and 

closely aprroximated for nearby points) second-order 

angular focusing exists (Johnson and Nier, 1953) and permits a larger angular spread than would otherwise be the 

case. It is this feature that provides sharper peaks (see figs. 

4 through 8) and hence better effective resolution than 

found in conventional sector instruments. 

is a schematic drawing showing the essential features of the 

Mattauch-Herzog geometry. Ions leave the ion source 

through a grounded defining slit, S0 , and pass between the 

plates of a cylindrical capacitor sector before entering the 

space between the poles of a magnet. Ions of different mass 

focus along a focal plane as shown. The existence of the 

well-defined focal plane lends itself to the use of multiple 

collectors for the simultaneous measurement of several 

masses of ions. 

As in all magnetic deflection instruments, spectra can 

be swept by changing either the ion accelerating potential or 

the magnetic field. In the flight instruments, a permanent 

magnet was employed to conserve weight and power. The 

use of several collectors made it possible to cover an 

extended mass range without employing either excessively 

high or low ion accelerating potentials. In laboratory 

instruments, an electromagnet can be employed to give 

additional flexibility. 

In the flight programs, several sizes of instruments 

were employed. The radii of curvature of the outermost ion 

beams used were 1 in (2.54 em), 1.5 in (3.81 em), and 2.5 
~108.--4~4~------------------------------~ 

... 

~Jo'or--rl~~------2-~e.------------~-----~~ 

<t 

;1012 

Q) 

... 

81o' 4 

c: 

0 

- Decreasing Mass--

Figure 2. Mass spectrum of atmosphere obtained at an 

altitude of 135 km during the descent of the Viking Lander 

to the surface of Mars on july 20, 1976. From laboratory 

calibration of the instrument before the flight, the principal peaks are related to atmospheric species as follows: 

44, C02 ; 40, Ar; 32, 0 2 ; 28, C02 fragment, N2 , and CO; 22 

and 20, doubly charged C02 and Ar, respectively; 18, 

residual instrument H20; 16, C02 , CO and 0 2 fragments, 

and 0; 14, N2 fragments; 12, C02 and CO fragments. 

Other smaller peaks are produced by species containing 

minor abundance isotopes. A careful analysis of the 30, 29, 

and 28 peaks led to the conclusion that NO was present 

and that the isotopic ratio for nitrogen is differerent than 

on the Earth (Nier and McElroy, 1977). 

in (6.35 em), depending on the resolution requirements and 

weight restrictions of the particular program. 

Instruments based on this geometry were used in a 

number of rocket flights to the Earth's upper atmosphere, in 

the Atmosphere Explorer-C. -D, and -E satellites (Nier 

and others, 1973), in the Viking Mars Project (Nier and 

Hayden, 1971), the Pioneer-Venus program (von Zahn and 

others, 1979), and in balloon flights to the Earth's stratosphere to study ozone and other minor constituents (Mauersberger and Finstad, 1979; Murphy and Mauersberger, 

1985). 

Figure 2 shows the mass spectrum of the Martian 

atmosphere obtained as the first Viking Lander spacecraft 

descended to the surface of the planet on July 20, 1976 

(Nier and others, 1976a). The resolution was rather low 

because of the small size of the instrument and the fact that 

the slits had been made relatively wide to insure stability in 

the measurements (the cruise to Mars took 10 months, and 

the instrument had to be released for integration in the 

spacecraft some months before launch). In spite of the 

relative crudeness of the measurements, some significant 

discoveries were made. The composition of the atmosphere 

was studied, and the amounts of Ar, N2 , 0 2 , CO, and NO 

were determined quantitatively in the presence of the much 

more abundant C02 . Carbon and oxygen were shown to 

have isotopic constitutions similar to terrestrial constitutions. A careful analysis of the spectrum gave convincing 

evidence that the 15N/ 14N ratio in the Martian atmosphere is 

approximately 60 percent higher than in the Earth's atmosphere, an observation which may provide important clues 

about the history of the planet's atmosphere (Nier and 

others,l976b; McElroy and others, 1976; Nier and McElroy, 1977). 

LABORATORY APPLICATIONS OF 

MINIATURIZED MASS SPECTROMETERS 

It became obvious that techniques learned in building 

high-performance miniaturized instruments for flight would 

also be applicable to laboratory investigations. Application 

of the techniques led to the development of a versatile 

high-performance laboratory instrument suitable for precise 

isotope and gas analyses. 

A schematic drawing of a somewhat larger laboratory 

instrument having an outermost radius of 3. 75 in (9.53 em) 

(Nier and Schlutter, 1985) and employing an electron 

bombardment ion source is shown in figure 3. It was 

initially designed primarily for the study of 3

He/ 4He ratios 

in the low parts per million range but has been used for 

other isotopic investigations as well. The collector slits are 

spaced for collecting masses 2, 3, and 4 simultaneously. 

For the 3

He/ 4He measurements, the mass 4 collector has a 

relatively wide slit, and an electrometer amplifier is used for 

measuring the 4

He ion current. The mass 3 ion collector slit 

is relatively narrow so that it can resolve 3He from HD and 

H3 • For this collector, an ion-counting electron multiplier is 

employed. 

Figure 4 is a mass spectrum for xenon obtained with 

the instrument shown in figure 3. The intermediate collector 

was employed in this case. It is interesting to note that the 

valleys between the peaks, when compared to the peaks, are 

down by four orders of magnitude. The small peaks in the 

odd mass positions are the result of hydrocarbon impurities 

that were present at the time the spectrum was run. 

For a different research program, an instrument was 

constructed that employed a thermal ion source rather than 

the electron bombardment source shown in figure 3. It was 

used for isotopic studies of neodymium (Nier and others, 

1983). A typical spectrum is shown in figure 5. The 

"tailing" of peaks was so small that, for all practical 

purposes, interference between adjacent peaks was negligible. 

Because the instruments can be thoroughly baked, 

have a relatively small internal surface and volume, and 

employ gold wire gaskets, inleakage is negligible and 

outgassing is low. The instruments can thus be used in the 

static mode and are ideal for the analysis of small gas 

samples. Two examples follow. 

Isotope Analysis of Small Samples of Mercury 

Neutron activation experiments performed on 

Allende and several other meteorites (Jovanovic and Reed, 

Isotope Analyses with Small-Volume Mass Spectrometers 3 
SOURCE SLIT So 

IN GROUND PLATE G 

R 

ION 

l FOCUSING 

LENS 

ELECTRIC 

ANALYZER 

20cm 

Figure 3. Cross-sectional schematic of prototype laboratory instrument (Nier and Schlutter, 1985) similar to that 

employed for most of the present research. In the prototype instrument, RE, R1 , R2 , and R3 were 16.03, 9.53, 8.25, 

and 6.74 em, respectively. Slits 50 , 51 , 52 , and 53 had widths 

of 0.0076, 0.051, 0.013, and 0.051 em, respectively. The 

perfect second-order focusing occurs for the ions reach1976a,b) suggested that the 196Hg/ 202Hg abundance ratio in 

the mercury found in some meteorites may be anomalous. 

An attempt to verify this effect was made by extracting the 

mercury from samples of the Allende meteorite, purifying 

it, and introducing it into an instrument such as shown in 

figure 3, operated in the static mode (Nier and Schlutter, 

1986). Figure 6 shows the mass spectrum obtained when 12 

ng (1.3 x 10-6 cm3 at STP) of normal laboratory mercury 

are introduced. Note that the ion currents for the principal 

isotopes are of the order of 104 ions per second. 

As is to be expected, the peaks drop with time 

because the mercury is absorbed on the gaskets and probably on the very clean surfaces of the walls of the instrument 

housing. Since measurements are made by peak stepping 

and the data are fed into a computer, allowance can be made 

4 New Frontiers in Stable Isotopic Research 

-----y 

ELECTRON 

MULTIPLIER 

MAGNET POLE FACE 

ing the outermost collector. In the present version of the 

instrument, the outer collector is used for most of the 

measurements for which high resolution is required, and 

slits 51 and 52 have widths of 0.015 and 0.051 em, respectively. R, repeller; Sh, shield; P, plate for holding exit slits 

and collector assemblies. 

for the decay of the peaks with time when the isotope ratios 

are calculated. "Memory" (cross-contamination that is due 

to desorption) between samples was a matter of some 

concern. Through the use of mercury having a drastically 

different isotopic composition, it was demonstrated that, 

with the baking techniques used between samples, memory 

was not a problem. 

The mass spectrometric measurements did not confirm anomalous mercury isotopic abundances in the meteorite studied. The reason for this apparent contradiction is 

not clear but could be that an error occurred in the neutron 

activation experiments of Jovanovic and Reed (1976a,b). 

Another possible explanation may be found in the difference in mercury extraction techniques used in the two 

methods of analysis. 
Xenon Isotopes 

104 

0 

z 

0 103 u 

IJJ 

(/) 

129 131 132 

n ! 134 136 

130 n ~ 128 n I 

I 

~ 

........ 

en 

.... 

z 

:::::> 

0 

10 (.) 2 

124 126 

to' I I ~~I 'I " 

I 

Lll ' j ~ ' Jt 

~ ~ ' 

lui ·- MASSFigure 4. Mass spectrum of commercial atmospheric 

xenon obtained with the prototype instrument shown in 

figure 3. Individual ions were counted by an electron 

multiplier, and the count rate is plotted on a logarithmic 

scale. 

Helium in Extraterrestrial Dust Particles 

Helium extracted from particles found in the deep 

Pacific has been observed to have a 3

He/ 4He ratio of 10-4 , 

or more, as contrasted with 1.4X 10-6 for normal atmospheric helium (Merrihue, 1964; Amari and Ozima, 1985; 

Fukumoto and others, 1986). This information, together 

with other evidence, has led to the assumption that the 

particles are of extraterrestrial origin. The precise reason for 

the anomaly is not clear and has been attributed to the solar 

wind or solar flare bombardment of particles in the interplanetary medium, to spallation by cosmic rays, or to the 

possibility that it is of primordial origin. 

Rajan and others (1977) examined single stratospheric particles of mass in the low nanogram range and 

showed they contained 4

He. Their experiments could not 

measure 3

He. Determination of the isotopic ratio of 3He to 4

He for such particles appeared to be an appropriate task for 

the instrument described in the present paper. With the slit 

widths and operating conditions chosen for the work 

described here, the sensitivity in the static mode of operation was such that 3 x 106 atoms of helium were required to 

produce a count rate of 1 count per second at the output of 

the ion counting system. The background counting rate, at 

t 

1-

z 

w 

0:: 

0:: 

:::> 

u 

z 

0 

H 

142 

ISOTOPES of NEODYMIUM 

144 

146 

143 

145 

MAssFigure 5. Typical mass spectrum for normal neodymium 

obtained with a modified instrument having a thermal ion 

source in place of the electron bombardment source 

shown in figure 3. In this case, the slits 50 and 51 were 

0.0076 and 0.023 em, respectively, and a vibrating reed 

electrometer was employed for measuring the ion currents that are plotted on a linear scale with arbitrary units. 

positions in the spectrum where ions are not expected, is 

typically less than 0.005 counts per second. To gain 

experience in working with small samples, experiments 

were carried out with single grains of lunar soil as well as 

deep Pacific magnetic fines. 

A small square (approximately 5 x 5 x 0.025 mm) 

of previously outgassed tantalum foil was folded around the 

material to be tested and mounted in a manifold attached to 

the mass spectrometer. The foil was heated by passing an 

electric current directly through it. Temperatures of up to 

1600 oc were employed. The gas released was passed 

through two liquid-nitrogen-cooled charcoal traps in tandem 

before it was admitted to the mass spectrometer. The mass 

spectrometer had attached to it an additional charcoal trap as 

well as a liquid-nitrogen-cooled cell, on the wall of which a 

fresh film of titanium had been evaporated to reduce the 

hydrogen background in the instrument. While the mass 

spectrometer separates 3

He from HD, it is obviously advantageous to reduce the HD peak as much as possible. 

In the course of the work, it was found that in running 

instrument blanks (that is, heating the foil and the electrical 

attachment leads without a sample), the small amount of 

helium released sometimes had a higher 3He/ 4He ratio than 

found in atmospheric helium. H. Craig (personal commun., 

Isotope Analyses with Small-Volume Mass Spectrometers 5 
0 

z 

0 

(..) 

LiJ 

C/) 

...... 

C/) 

z 

Q 

10 ------------------------------------~ 

202 

200 r-1-. 

201 f \ 

f,.J..\ : \ , I 

1 

199 ,..,. 

198 ,n, I 1 

\ 

~ ; 1 f J I 

i 1 r 

\ ' I 

1 I \ I \ i I 

, I i! 

:i , I 

i 

11 i, !, il 

196 

J, 

: ~ 

J \ 

j ' i 

, 

! 

l 

I t 

~ i . ·j' j, J ~J_rJ ~~~~~------ MASS204 

!'"\ 

) ' 

Figure 6. Mass spectrum of normal laboratory mercury. 

1988) reports that he and colleagues observed high 3He/4He 

in tantalum used for extraction furnaces. They attributed the 

effect to tritium diffusion studies in metals which were 

subsequently reprocessed and sold by distributors. Mamyrin 

and Tolstikhin (1984) reported a similar effect. 

Our observation led to a study of the helium content 

of various metals, including specimens of native metals. It 

was found that many of the common metals used in the 

construction of apparatus exhibited anomalous 3

He/4He 

ratios in the helium released upon heating. The same was 

true of samples of some native metals. Figure 7 shows the 

spectrum for the mass 3 region for a 2.8-mg sample from 

the surface of a piece of native copper from the upper 

peninsula of Michigan. Here the 3

He/ 4He ratio is unusually 

high-4.2X 10-3 . Other small samples from the same piece 

did not show the effect. A random study of milligram-sized 

samples removed from larger specimens of other native 

metals such as gold, FeNi3 , and iron also showed anomalies 

in the helium isotopic ratio in some cases. At present, we do 

not have a good explanation for the phenomenon. It has 

been surmised that in the case of the native metals this 

anomaly may be due to the diffusion of hydrogen into metal 

and the subsequent decay of its tritium into 3He that is 

trapped. Effects of cosmic rays, such as observed by Kurz 

(1986) and Craig and Poreda (1986), might also be responsible. 3

He could be much more abundant in the Earth than 

is generally appreciated. Clearly, it is an interesting subject 

worthy of more systematic investigation. 

6 New Frontiers in Stable Isotopic Research 

HELIUM in 2.8 mg NATIVE COPPER SAMPLE 

4He = 3.85 X 1 03 ions/s 

101 

c 

z 

0 HD u 

w 

(h 

..... 

(h 

z 

Q 

100 

MAssFigure 7. Mass spectrum of mass 3 region showing 3

He 

extracted from a sample of native copper along with the 

residual HD in the instrument. Superimposed is the spectrum remaining when the helium is pumped away. Tensecond integrations were used in obtaining data points. 

For comparison, figure 8 shows the spectrum 

obtained for the mass 3 region for the helium extracted from 

1.5 x 10-3 cm3 at STP of air. The 3

He in this amount of air 

is approximately 10- 14 cm3 at STP. The instrument blank 

accounts for approximately one-third of the 3

He peak. 

When correction is made for this residual, the calculated 3

He/ 4He is (1.6±0.4)X 10-6 , in excellent agreement, considering the small size of the sample, with 1.4X 10-6 , the 

accepted value for the ratio. The amount of air used in the 

test was deliberately chosen to be small so that the 3

He 

would be comparable to the amount that is seen in some of 

the smallest samples used in our investigations. 

SUMMARY 

The availability of rockets and other space vehicles 

stimulated the development of high-performance, miniaturized mass spectrometers for the study of the composition of 

the Earth's upper atmosphere and the atmospheres of other 
101 

0 

z 

0 

u 

LIJ 

<h 

z 

<h 

-

Q 

100 

10-1 

HELIUM in 1.5 x 1 o-3 em 3 STP of AIR 

3~ ~ 

J~ I 

I 

4He = 1.35 X 1 o5 ions/s 

HO 

~ 

Figure 8. Mass spectrum of mass 3 region showing 3He 

together with the residual HD in the mass spectrometer 

after the helium extracted from 10-3 cm3 of air at STP is 

introduced. In practice, a spectrum such as this is used 

only for illustration. In making 3He/ 4He determinations on 

small samples, many readings are obtained at the 3He 

mass "position" to average out statistical fluctuations that 

appear in the counting rates of single determinations. 

Blanks are always run, and peak shapes are carefully 

studied. With the slits employed in the present work, the 

HD contribution at the 3He position is less than 1 percent 

of the height of the HD peak. Over many months of 

running, the peak shapes have been found to be remarkably reproducible. If narrower slits were employed, the 3

He and HD would naturally be better resolved, but at the 

expense of sensitivity and "flatness" of peak tops. In view 

of the high stability and reproducibility of the apparatus in 

its present form, we felt that from the standpoint of 

ultimate accuracy, retaining sensitivity and flat-top peaks 

was more important than having sharp peaks with valleys 

between them. 

planets. The technology that evolved is transferable to 

laboratory instrumentation and has particular application to 

analysis of small samples. High-performance, miniaturized 

mass spectrometers are capable of making precision measurements of isotope abundances. The small volume and 

surface area insure high sensitivity with a minimum of 

residual background. An instrument employing the 

Mattauch-Herzog geometry, with a radius of 9.53 em in the 

magnetic field, had a sensitivity sufficient to detect less than 

10- 14 cm3 at STP of 3He in the presence of a much larger 

HD instrument residual from which it was resolved. The 

mass spectrometer is applicable to isotopic studies as 

diverse as those of the helium in cosmic dust particles or of 

traces of mercury in meteorites. 

ACKNOWLEDGMENTS 

The instrument used in the investigation was made 

possible by grants from the Planetary Atmosphere Program 

of the National Aeronautics and Space Administration 

(NASA) and from the Geochemistry Program of the 

National Science Foundation (NSF). The research itself was 

supported by a grant from the Planetary Materials and 

Geochemistry Program of NASA. I am indebted to my 

associate, Dennis Schlutter, for some of the measurements 

reported here and to Dr. E.J. Olsen of the Field Museum of 

Natural History of Chicago for samples of native metals 

used in the research. Samples of the Allende meteorite used 

for the mercury studies were kindly provided by Dr. George 

Reed of the Argonne National Laboratory. 

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Bleakney, W., 1930, Probability and critical potentials for the 

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Craig, H., and Poreda, R.J., 1986, Cosmogenic 3

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Cross, W. G., 1951, Two-directional focusing of charged particles 

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He/ 4He ratios in deep-sea 

Isotope Analyses with Small-Volume Mass Spectrometers 7 
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2245-2253. 

Johnson, E.G., and Nier, A.O., 1953, Angular aberrations in 

sector shaped electromagnetic lenses for focusing beams of 

charged particles: Physical Review, v. 91, p. 10-17. 

Jovanovic, S., and Reed, G.W., 1976a, 196Hg and 202Hg isotopic 

ratios in chondrites: Revisited: Earth and Planetary Science 

Letters, v. 31, p. 95-100. 

---1976b, Interrelations among isotopically anomalous mercury fractions from meteorites and possible cosmological 

inferences: Science, v. 193, p. 888-891. 

Kurz, M.D.,1986, Cosmogenic helium in a terrestrial igneous 

rock: Nature, v. 320, p. 435-439. 

Mamyrin, A., and Tolstikhin, LN., 1984, Helium isotopes in 

nature: Amsterdam, Elsevier, p. 15. 

Mattauch, J., and Herzog, R., 1934, Uber einen neuen Massenspektrographen: Zeitschrift flir Physik, v. 89, p. 78~795. 

Mauersberger, K., and Finstad, R., 1979, Further laboratory 

studies and stratospheric flight of a mass spectrometer beam 

system: Review of Scientific Instruments, v. 50, p. 

1612-1617. 

McElroy, M.B., Yung, Y.L., and Nier, A.O., 1976, Isotopic 

composition of nitrogen: Implications for the past history of 

Mars' atmosphere: Science, v. 194, p. 70-72. 

Merrihue, C., 1964, Rare gas evidence for cosmic dust in modern 

Pacific red clay: Annals of the New York Academy of 

Sciences, v. 119, p. 351-367. 

Murphy, D.M., and Mauersberger, K., 1985, Operation of a 

microchannel plate counting system in a mass spectrometer: 

Review of Scientific Instruments, v. 56, p. 220-226. 

Nier, A.O., 1940, A mass spectrometer for routine isotope 

abundance measurements: Review of Scientific Instruments, 

V. 11, p. 212-216. 

Nier, A.O., and Hayden, J.L., 1971, A miniature MattauchHerzog mass spectrometer for the investigation of planetary 

atmospheres: International Journal of Mass Spectrometry and 

Ion Processes, v. 6, p. 339-346. 

Nier, A.O., and McElroy, M.B., 1977, Composition and structure 

of Mars' upper atmosphere: Results from the neutral mass 

spectrometers on Viking 1 and 2: Journal of Geophysical 

Research, v. 82, p. 4341-4349. 

8 New Frontiers in Stable Isotopic Research 

Nier, A.O., and Schlutter, D.J., 1985, High-performance doublefocusing mass spectrometer: Review of Scientific Instruments, v. 56, p. 214-219. 

---1986, Mass spectrometric study of the mercury isotopes in 

the Allende meteorite: Proceedings of the 17th Lunar and 

Planetary Science Conference, pt. 1, Journal of Geophysical 

Research, v. 91, p. E124-E128. 

Nier, A.O., Stevens, C.M., Hustrulid, A., and Abbott, T.A., 

1947, Mass spectrometer for leak detection: Journal of 

Applied Physics, v. 18, p. 30-33. 

Nier, A.O., Abbott, T.A., Pickard, J.K., Leland, W.T., Taylor, 

T.I., Stevens, C.M., Drukey, D.L., and Goertzel, G., 1948, 

Recording mass spectrometer for process analysis: Analytical 

Chemistry, v. 20, p. 188-192. 

Nier, A.O., Potter, W.E., Hickman, D.R., and Mauersberger, 

K., 1973, The open-source neutral mass spectrometer on 

Atmosphere Explorer--C, -D, and -E : Radio Science, v. 8, 

p. 271-276. 

Nier, A.O., Hanson, W.B., Seiff, A., McElroy, M.B., Spencer, 

N.W., Duckett, R.J .. , Knight, T.C.D., and Cook, W.S., 

1976a, Composition and structure of the Martian atmosphere: 

Science, v. 193, p. 78~788. 

Nier, A.O., McElroy, M.B., and Yung, Y.L., 1976b, Isotopic 

composition of the Martian atmosphere: Science, v. 194, p. 

68-70. 

Nier, A.O., Murthy, V.R., and Dosso, L., 1983, Minmass: A 

small double focusing mass spectrometer for geochemical 

research: EOS, Transactions of the American Geophysical 

Union, v. 64., p. 334. 

Paul, W., Reinhard, H.P., and von Zahn, U., 1958, Das Elektrische Massenfilter als Massenspektrometer und Isotopentrenner: Zeitschrift flir Physik, v. 152, p. 143-182. 

Rajan, R.S., Brownlee, D.E., Tomandl, D., Hodge, P.W., 

Farrar, H., and Britten, R.A., 1977, Detection of 4He in 

stratospheric particles gives evidence of extraterrestrial origin: Nature, v. 267, p. 133-134. 

Smythe, W.R., and Mattauch, J., 1932, A new mass spectrometer: Physical Review, v. 40, p. 429-433. 

Stephens, W.E., 1934, Magnetic refocussing of electron paths: 

Physical Review, v. 45, p. 513-518. 

von Zahn, U., Krankowsky, D., Mauersberger, K., Nier, A.O., 

and Hunten, D.M., 1979, Venus thermosphere: In situ 

composition measurements, the temperature profile, and the 

homopause altitude: Science, v. 203, p. 768-769. 
Carbon Isotopic Analysis Of Small Samples 

by Use of Stepped-Heating Extraction and 

Static Mass Spectrometry 

By I.P. Wrighe and C.T. Pillinger1 

Abstract 

Highly sensitive static-vacuum mass spectrometry 

has been developed for the determination of the stable 

isotopic compositions of extremely small samples of gaseous carbon-bearing species. The small sample capability 

of instruments of this type complements extraction procedures that are able to produce small amounts of unionized material in an appropriate gaseous form (which in 

the case of carbon is C02). One such technique is stepped 

heating, which can be performed either under vacuum 

conditions (pyrolysis) or in the presence of pure oxygen 

(combustion). Stepped pyrolyses are used to liberate 

trapped C02 gas, while stepped combustion enables 

carbonaceous components of different sorts to be separated according to the temperature at which the material is 

oxidized to form C02 gas. In order to obtain the most 

useful information from stepped-heating experiments, it 

is necessary to appreciate some of the problems that can 

arise either during the experiment or during interpretation of the results. These problems are discussed at some 

length and include (1) the practical difficulties associated 

with the use of pyrolyses for carbonaceous samples, (2) 

dependence of the combustion temperature on the grain 

size of diamond and on the C/H ratio of organic materials, 

(3) the effect of oxygen pressure or sample size (that is, 

the 0/C ratio) on the nature of the release profile, (4) the 

use of repeat combustions at specific temperature steps to 

ensure more complete resolution of two components, (5) 

problems associated with the use of equal and unequal 

temperature increments and the implementation of a 

normalization procedure to correct for unequal step sizes, 

and (6) the effects of poor control of temperature and (or) 

time during the experiment, for which a simple mathematical procedure is presented that can be used to diminish 

the resultant artifacts in the acquired stepped-heating 

profile. The data acquired from the analyses of meteorite 

samples are used to demonstrate some of the techniques 

that can be used during interpretation of stepped-heating 

results. 

INTRODUCTION 

Measurement of carbon stable isotopic compositions 

of samples of geological interest has the potential to provide 

1 Planetary Sciences Unit, Department of Earth Sciences, The Open 

University, Walton Hall, Milton Keynes, MK7 6AA, England. 

information about the source of the constituents (for example, in sedimentary geochemical studies) and has at least 

limited utility in determining temperatures of formation (for 

example, from an appraisal of carbon isotopic fractionation 

between ( 1) C02 and CH4 in well gases and (2) calcite and 

graphite in metamorphic rocks). Carbon isotopic studies 

have been applied to all areas of geology, including 

research on igneous, sedimentary, and metamorphic rocks, 

ore deposits, oil and gas reserves, and oceanography 

(Faure, 1986; Valley and others, 1986; Hoefs, 1987; Kyser, 

1987). In addition, the carbon isotopic compositions of 

various extraterrestrial materials have been studied extensively in order to improve the understanding of pre-, syn-, 

and postsolar system formational processes (Pillinger, 

1984; Kerridge, 1985). 

Conventionally, carbon isotopic measurements are 

made by converting the sample of interest to C02 gas, prior 

to determination by dynamic mass spectrometry. (See 

figure 1. The instrument, which is continuously pumped, is 

referred to as dynamic because sample and reference gases 

alternately enter the mass spectrometer by viscous flow.) 

The isotopic composition of the sample gas is calculated as 

a 813C value (Craig, 1957) relative to an internationally 

recognized standard known as PDB (Peedee belemnite), the 

difference being expressed as parts per thousand (per mil, 

o/oo) Some geological studies might require highly precise 

813C values (±0.1 to ±0.05 o;oo), but, on the other hand, 

many interesting studies can be carried out successfully 

with somewhat larger errors ( <± 1 o/oo). Up until about 

1983, carbon isotopic measurements of the desired precision were made by use of dynamic gas-source mass spectrometers of the type pioneered in the late 1940's (Nier, 

1947; McKinney and others, 1950). However, even with 

the advent of improved commercial equipment, it was still 

necessary to use microgram amounts of carbon for isotopic 

analyses. This requirement proved to be prohibitive for 

some studies, and thus from the early 1970's onward, 

many studies were devoted to exploring ways of decreasing 

sample size requirements. One technique uses a variant of a 

noble-gas-type mass spectrometer. (See figure 2. The 

instrument, referred to as static, differs from the dynamic 

version in that it is not pumped during the analysis.) By 

1983, the first carbon isotopic studies were undertaken by 

use of static mass spectrometry: these initially concentrated 

on isotopically anomalous materials in meteorites (Carr and 

Carbon Isotopic Analysis of Small Samples 9 
Table 1. Some attempts to make small-sample B13C measurements 

Reference Size 

(nanomole) 

Bridger and others, 1974 ............. 300 

Schoeller and Hayes, 1975........... 25 

Matthews and Hayes, 1978........... 20 

Fallick and others, 1980 ............ . 

Wada and others, 1982 . . . . . . . . . . . . . . 45 

Halas and Krouse, 1983 . . . . . . . . . . . . . 50 

Swart and others, 1983b . . . . . . . . . . . . . 8 

Wright and others, 1983 ............ . 

±0.1 

±.5 

±5 

±2 

±.04 

±.2 

±10 

±.25 

Comments 

Conventional dynamic mass spectrometry. 

Computer control, ion-counting, Hedilution. 

Gas chromatography-mass spectrometry. 

Static mass spectrometry (CD4 ). 

Conventional mass spectrometry. 

Variation of flow injection. 

Carr and others, 1986. . . . . . . . . . . . . . . . ± 1 

Zinner and Epstein, 198 7 . . . . . . . . . . . . -. 0000 1 ± 10 

Gas mixing ("isotope dilution"). 

Static mass spectrometry (CD4 ). 

Static mass spectrometry (C02). 

Ion probe. 

Prosser and others, 1988 ............ . . 001 

others, 1983), and so the high levels of precision alluded to 

above were not needed. The technique rapidly improved 

and by 1984 was applied to problems of terrestrial 

geochemistry where precisions of ± 1 o; oo or better were 

necessary (Mattey and others, 1984). The instrument and 

extraction system have subsequently evolved to the point 

where they have become a facility that can be used routinely 

for a wide variety of investigations. 

One method able to provide small quantities of C02 

for analysis by static mass spectrometry is laser-probe 

extraction. The potential of this technique was originally 

demonstrated for isotopic analyses of noble gases by 

Megrue (1967). Subsequently, laser probes have been used 

for extracting light elements in the form of C02 , H20, S02 , 

N2 , and such (Norris and others, 1981; Shankai and others, 

1984; Franchi and others, 1985; Sommer and others, 1985; 

Jones and others, 1986). Franchi and others (1986; this 

volume) have described a laser-probe system capable of 

making isotopic determinations of carbon, oxygen, and 

nitrogen by use of static mass spectrometry. The technique, 

which offers the chance to make isotopic measurements of 

solid target materials at spatial resolutions of about 50 ~J-m, 

is successful because extremely low blanks can be obtained. 

It is highly probable that analytical instrumentation of this 

type will play a significant role in future light-element 

studies. However, there are a number of problems that need 

to be solved (Franchi and others, 1986; this volume) before 

laser-probe extraction becomes a routine procedure for 

light-element isotopic analyses. 

A more generally applicable technique for preparing 

carbon for analysis by static mass spectrometry is one of 

stepped heating, either in vacuo (stepped pyrolysis), or in 

an atmosphere of pure oxygen (stepped combustion). 

Despite their apparent simplicity, these procedures are less 

than straightforward and require a certain amount of skill 

and experience in order to produce high-quality data. For 

this reason, a detailed account of some of the problems 

associated with stepped heating is given herein. 

10 New Frontiers in Stable Isotopic Research 

±2.5 Static mass spectrometry (C02) • 

SMALL-SAMPLE ANALYSIS FOR CARBON 

Over the years, numerous attempts have been made to 

determine the isotopic composition of progressively smaller 

amounts of carbon. Most efforts have generally involved 

modification of existing equipment; table 1 shows the 

sample sizes and precisions in 813C obtained by these 

techniques. Additional studies, which have described either 

modifications to instruments or attempts to fully understand 

instrumental effects, with a view to improving precision or 

decreasing sample size requirements, are given by Shackleton (1965), Deines (1970), Coplen (1973), Mook and 

Grootes (1973), Fallick and Baxter (1977), and Fallick 

(1980; 1983). 

Prior to the advent of commercially available mass 

spectrometers, instruments such as that described by Nier 

(1947), and subsequently modified by McKinney and others 

(1950), were able to measure 813C to precisions of ±0.1 ofoo 

on samples of about 0.4 x 10-3 mol of carbon (McCrea, 

1950). By the early 1970's, instrument companies were 

producing equipment able to perform 813C measurements to 

±0.1 ofoo on 0.3 X 10-6 mol of carbon (Bridger and others, 

1974), but they were very infrequently operated at this level 

of sensitivity. The smallest samples that commercial mass 

spectrometers can accommodate have remained at about 

this level, although precision has improved ( ±0.02 ofoo, 

Brunnee and others, 1977). 

Schoeller and Hayes (1975) and Hayes and others 

(1978) describe a technique that utilized a single collector, 

gas-source, dynamic mass spectrometer fitted with a pulsecounting electron multiplier to increase sensitivity. The 

three ion beams (produced from C02) were focused sequentially onto the detector, by computer control of the accelerating voltage (this in itself was an innovation at the time). 

In order to prevent isotopic fractionation of the small 

amounts of C02 , the gas was diluted with helium. This 

action ensured preservation of high pressures in the inlet 

and thus viscous flow through the capillaries into the mass 

spectrometer. Isotopic values to a precision of ±0.5 ofoo 

were obtained on samples of about 25 x 10-9 mol of C02 , 
although measurement times of an hour were needed. 

Analyses of smaller samples (8 x 10-9 mol) were possible, 

although without correspondingly increased measurement 

times, levels of precision became somewhat poorer. Des 

Marais (1978, 1983) successfully applied the heliumdilution technique to the analysis of small amounts of 

carbon from lunar samples. 

Using a combination of gas chromatography and 

mass spectrometry, Matthews and Hayes (1978; 1979) 

demonstrated the potential of this arrangement to analyze 

samples of C02 as small as 20x 10- 9 mol. By tuning the 

single-collector mass spectrometer to monitor selectively 

the masses of interest, they obtained precisions of ±5 ofoo, 

a level considered adequate for the isotopic-labeling studies 

that were undertaken by use of the equipment. 

Undoubtedly, it is possible to use conventional instrumentation to determine carbon and oxygen isotopic compositions of samples smaller than the manufacturers recommend. Indeed, in this laboratory, by use of the SIRA 24 

mass spectrometer (V.G. Isogas, Winsford, U.K.) it has 

proved possible to make 813C measurements on samples as 

small as 20X 10-9 mol of C02 , with acceptable levels of 

precision (±0.5 ofoo). Wada and others (1982), using 

modified proprietary equipment (MAT-250, Finnigan, Bremen, West Germany), claim to be able to make 813C 

measurements to precisions of ± 0. 04 o; oo on samples as 

small as 45 x 10-9 mol of C02 . This claim has been 

substantiated by a study of microzoning in 8 13C of graphite 

and calcite isolated from marble by a combination of 

microtome cutting and exfoliation procedures (Wada, 

1988). However, in the conventional system, pressures of 

sample and reference gases need accurate balancing, a 

procedure which, while not difficult, is time consuming if 

the instrument has been set up to perform automatic 

analyses of normal-sized samples. In addition, the effects of 

any contaminant gases become increasingly significant with 

smaller samples, so it is extremely important to produce 

samples of the highest purity. Failure to control such factors 

conspires to produce spurious 813C values. From experience of ultrasmall sample analysis in which dynamic mass 

spectrometry is used, reliable results are generally obtained 

only when pure C02 is produced and a limited range in 813C 

values can be anticipated (erroneous values can thereby be 

identified). This situation arises, for instance, in carbonate 

analyses. The use of commercially available equipment for 

ultrasmall-sample analyses of complicated samples (organic 

materials) or complete unknowns (some extraterrestrial 

materials) seems unwise at present. 

To avoid the problems of admitting extremely small 

quantities of C02 into a dynamic mass spectrometer, Swart 

and others ( 1983b) utilized a technique of gas mixing 

(described fully in Wright and others, 1984). The procedure, which was developed for elucidating the nature of 

isotopically unusual carbon-bearing materials in meteorites 

(8 13C up to + 1,100 ofoo), involved mixing small amounts of 

the 13C-rich sample gas with larger quantities of isotopically 

normal reference gas in the inlet of a conventional dynamic 

mass spectrometer. The resultant gas mixture, which may 

have been up to 10 times the size of the original sample, 

was then admitted to the mass spectrometer and analyzed in 

the normal way. It was possible to calculate 813C of the 

sample (albeit with errors of approximately ± 10 ofoo, which 

was adequate for the meteorite investigation) because the 

relative amounts of sample and reference gases, the 813C of 

the reference gas, and the measured 813C value of the mixed 

gases were known. 

Halas and Krouse (1983) and Halas (1985) used a 

system whereby C02 sample gas was transferred into a 

capillary close to the ion source of a normal dynamic mass 

spectrometer and subsequently forced into the instrument by 

a second (''pusher") gas. The technique is similar in some 

respects to flow-injection analysis used in gas chromatography (Stewart, 1981) and proceeds by the sample gas 

becoming compressed to the same pressure as the pusher 

gas. At high pressures in the capillary, mixing is limited to 

the very interface of the two gases, and so a column of 

sample gas (considerably longer than wide) is slowly forced 

into the mass spectrometer. In using this technique, all of 

the sample gas is admitted to the instrument for analysis. 

Halas and Krouse tested a variety of pusher gases, with the 

most suitable arrangement for carbon analyses being the use 

of reference C02 to force sample C02 into the mass 

spectrometer. In theory, 813C measurements can be made 

by a comparison of ion-beam intensity ratios recorded from 

the sample gas, with those from the reference gas flowing 

through the same inlet some time later. Precisions of about 

±0.2 ofoo are possible on samples as small as 50 X 10-9 mol 

of carbon. 

The static mass spectrometers and associated gashandling systems developed by Fallick and others (1980), 

Wright and others (1983), Carr and others (1986), and 

Prosser and others ( 1988) have been used extensively for 

small-sample analyses. The most successful carbon instrument constructed thus far, that described by Carr and others 

(1986), has to date performed around 7,500 individual 

isotopic analyses and has the capability to routinely determine 813C of nanomole samples of carbon with precisions 

of < ± 1 o; oo (samples as small as 40 x 10- 12 mol can be 

determined with somewhat larger errors of ± 10 ofoo). 

The ion probe has great potential in the analysis of 

extremely small amounts of carbon in solid specimens 

(grains, polished sections, and such) at high spatial resolution ( ~ 1 f.Lm). Thus far, this device has been used to 

make 813C measurements only of extremely small grains 

extracted from meteorites (Niederer and others, 1985; 

Zinner and Epstein, 1987; Zinner and others, 1987). Precisions in 813C for isotopically anomalous meteoritic materials range from ± 10 to ± 140 ofoo (Zinner and Epstein, 

1987), but it should be possible under optimum conditions 

to achieve ±3 ofoo. 

Carbon Isotopic Analysis of Small Samples 11 
STATIC MASS SPECTROMETRY 

History of Development for Stable Isotopic 

Analyses of Active Gases 

The concept of static mass spectrometry as a highly 

sensitive method of making isotopic measurements of 

gaseous species was first demonstrated by Reynolds ( 1956) 

for noble-gas analysis, although somewhat earlier, Aldrich 

and Nier (1948) had used a more complicated approach for 3

He/ 4He measurement, whereby the gas removed by pumping from a dynamic mass spectrometer was recycled. 

Further developments in noble-gas static mass spectrometry 

have been documented by, among others, Schaeffer and 

Zahringer (1958), Merrill (1974), Clarke and others (1976), 

Hohenberg (1980), and Stacey and others (1981). Such 

instruments are used routinely in most investigations of 

noble-gas geochemistry (Bohlke and others, this volume) 

and potassium-argon and 40 Ar/ 39 Ar geochronology (Dalrymple, this volume). 

The potential for a static mass spectrometer to carry 

out quantitative measurements of active gases was first 

investigated by Irako and others (1975) and independently 

by Gardiner and others (1978) and Gardiner and Pillinger 

( 1979). The instrument used in the study of lrako and others 

was an all-glass mass spectrometer equipped with a Faraday 

cup detector. Two different ionization sources were 

appraised: a hot filament (conventional electron impact) and 

a cold cathode discharge. lrako and others (1975) demonstrated that N 2 , CO, and CH4 could be analyzed by use of 

a hot filament for ionization but that H2 , 0 2 , and C02 were 

too unstable under such conditions, and the use of a cold 

cathode was required for their analysis. Degradation of 

gases by thermal cracking on a hot filament (sometimes 

referred to as "pumping") was also investigated by Gardiner 

and Pillinger (1979) by use of an all-metal mass spectrometer fitted with an electron multiplier to increase the 

instrument's sensitivity. In order to reduce thermal decomposition as far as possible, a low work function LaB6 

filament was used at low trap current (4 f.LA; operating 

temperature ~700 °C). It was concluded that reliable 

measurements of C02 by static mass spectrometry would be 

extremely difficult, while N2 incurred problems associated 

with mass spectrometer background. However, CH4 , and 

more particularly its deuterated analog, CD4 , were found to 

be sufficiently stable in the mass spectrometer (half-life of 

> 11 hours, under the conditions used) for consideration as 

analysis gases for carbon isotopic measurements, and the 

long-duration measurement times gave rise to high levels of 

precision. 

Gardiner and others ( 1978) attempted to measure the 

o13C of so-called hydrolyzable carbon, in the form of CD4 , 

liberated by the action of DCI on lunar soils. Unfortunately, 

the analyses suffered from the effects of hydrogen partial 

12 New Frontiers in Stable Isotopic Research 

labeling manifested as a large contribution at mass to charge 

(m/z) 19 from 12CD3H+ (

12CD4+/ 12CD3H+ = 2 to 10). 

This uncontrollable effect precluded isotopic measurements 

because of the associated 13CD3H+ interference at m/z 20. 

However, it was demonstrated that carbon isotopic measurements of a pure reference gas were relatively reliable. 

Following the work of Gardiner and others (1978), a 

further investigation by Fallick and others ( 1980) demonstrated that the molecular dissociation of CD4 under electron impact did not cause isotopic fractionation. As such, 

use of a higher trap current was advocated (100 f.LA), with 

concomitant increases in ion beam currents, and improved 

stability and sensitivity, which in tum permitted replacement of the electron multiplier by a more reliable Faraday 

collector. It was concluded that o13C measurements having 

precisions of ±2 otoo should be possible, provided that pure 

CD 4 was available for analysis. 

In order to derive a generally applicable extraction 

technique capable of producing CD 4 from a variety of 

carbonaceous materials, Abell and others ( 1979) and 

McNaughton and others (1983) investigated the possibilities 

of forming CD4 directly from C02 . The philosophy was that 

C02 that formed by combustion of a range of carbon 

components could be subsequently reduced to CD4 for 

analysis by high-sensitivity mass spectrometry. The method 

was based on the so-called Sabatier reaction (Sabatier and 

Senderens, 1902): 

catalyst 

C02 + 4D2 ---> CD4 + 2D20 

400 °C 

Sample C02 was transferred to a small-volume vessel 

containing a suitable catalyst (nickel or ruthenium) and 

subsequently pressurized with pure deuterium gas. The 

catalyst was then heated to about 400 oc for 20 min, after 

which time excess D2 , and any D20 formed during the 

conversion, was selectively removed by use of a Zr/ AI 

getter. Deuteromethane was then admitted to the static mass 

spectrometer (Wright and others, 1983). Although many 

problems were encountered with this technique, some 

successes were obtained with nanomole quantities of carbon 

(McNaughton and others, 1983). However, the method was 

found to be inapplicable because of an intermittent and 

uncontrollable blank/memory problem. Regardless, the 

dual-Faraday-collector static mass spectrometer constructed 

for the analyses (Wright and others, 1983) was found to be 

capable of making o13C measurements of nanomole 

amounts of a pure CD 4 reference gas to precisions of 

<±0.25 O/oo. 

The experience gained in the interval between the 

studies of Gardiner and others (1978) (who state that" ... the 

behaviour of C02 in the static mass spectrometer is found to 

be very unstable and obviously would be unsuitable for 

isotopic studies .... ") and Wright and others (1983) allowed 
the construction of a static mass spectrometer for isotopic 

analysis of C02 • The features of the mass spectrometer and 

associated inlet system (described fully by Carr and others, 

1986), which allow o13C and 8180 measurements of nanomole amounts of C02 to precisions of approximately ± 1 

ofoo, are the use of three Faraday collectors for simultaneous 

ion-beam detection and a strict protocol of equalizing ion 

source pressures of reference and sample gases. The mass 

spectrometer is a 90° magnetic sector instrument that has a 

normal entrance and exit angle for the ion beams. The 

volume of the instrument is estimated to be about 1 L; 

typical source pressures are 1 x 10-5 torr. The sensitivity of 

the mass spectrometer (which is about 1 x 10-5 A torr- 1

) is 

maximized by use of a wide source-exit slit of about 0.5 

mm; the central (m/z 45) collector also has a slit of about 

0.5 mm, so the resulting resolution (m/~m) for this collector is ~ 100. The three collectors are arranged in a straight 

line rather than at the points of focus for the ion beams, but 

wide slits on the m/z 44 and 46 collectors negate the 

necessity for accurate alignment. The dispersion of the ion 

beams at the collectors is about 2 mm. The abundance 

sensitivity of the instrument is poor compared to a conventional instrument because of the relatively higher source 

pressures used, being 370 ppm and 100 ppm for the 

proportions of m/z 44 at the 45 and 46 collectors, respectively. The stability of the instrument is considerably 

improved over those used in previous work (that is, Gardiner and others, 1978; Gardiner and Pillinger, 1979; 

Fallick and others, 1980; Wright and others, 1983) by 

substitution of the LaB6 filament with one of tungsten, 

which has a considerably longer operating lifetime and 

improved ionization efficiency. The elimination of regular 

filament replacement enables the mass spectrometer to 

remain under static conditions for long periods of time. The 

effect of this substitution is to improve the background in 

the region of the mass spectrum where C02 is analyzed. 

The success of the analytical procedure lies in its associated 

extraction system (Carr and others, 1986), which is capable 

of performing incremental heating with acceptably low 

blank levels (see below). A second, more advanced triplecollector static mass spectrometer has subsequently been 

constructed by Prosser and others (1988) and is able to 

determine o13C and 8180 on samples down to 1 X 10- 12 mol 

to a precision of ±2.5 ofoo. 

In addition to carbon isotopic measurements, static 

mass spectrometers have been used for the determination of nitrogen isotopic compositions (Brown and Pillinger, 1981; Frick and Pepin, 1981; Murty and Marti, 

1985). Descriptions of the nitrogen isotope ratio mass 

spectrometers and associated extraction systems currently in 

use in our laboratory are given by Wright and others (1988) 

and Boyd and others (1988), respectively. Some effort has 

also been given to the assessment of using static mass 

spectrometry to make oD determinations (Wright and Pillinger, 1983). 

VARIABLE 

VOLUMES 

SAMPLE 

GAS 

COLO 

Fl NGERS 

M/Z 46 

M/Z 45 

M/Z 44 

Figure 1. Schematic diagram showing the triple-collector 

dynamic mass spectrometer. Instrument is shown collecting the three ion beams pertinent to the isotopic analysis 

of C02 (namely, m/z 44, 45, and 46). Dynamic mass 

spectrometers permit rapid alternation of sample and 

reference gases, but rather large samples are required to 

ensure accurate balancing of the inlet pressures, viscous 

flow in the capillary leaks, and continuous pumping of the 

analyzer tube. m/z, mass to charge ratio. 

Nature of Operation of Static Mass 

Spectrometry 

The static, gas-source mass spectrometer differs from 

the conventional dynamic isotopic ratio instrument (fig. 1) 

in a number of ways (see Wright, 1984). In the latter, the 

sample and reference gases, contained within different parts 

of a dual-inlet system, continuously flow (hence dynamic) 

through capillary tubing to a changeover valve (Murphey, 

1947; Begbie and others, 1972; Halas, 1979); this device 

allows either of the two gases to be directed to the ion 

source of a continuously pumped mass spectrometer while 

the other is removed through a separate pumping system. 

The nature of this operation ensures that differential isotopic 

fractionation effects are avoided and that pressure changes 

in the ion source are minimized when changing between one 

gas and the other. However, three areas of sample wastage 

can be identified. First, large amounts of sample and 

reference gas, located in reservoirs of adjustable volumes to 

allow equivalent flow rates through the changeover valve, 

are required to ensure nonfractionating viscous flow 

through the capillaries (for example, see Halsted and Nier, 

1950). For small-sample analysis, C02 may be transferred 

cryogenically to a low-volume cold finger located in proximity to the end of the capillary, but only a small fraction of 

the gas is used for isotopic measurements since it is 

necessary to retain a high gas pressure at the inlet end of the 

capillary. Second, while the reference gas is being analyzed 

in the mass spectrometer, the sample gas is bled to waste 

through a pump connected to one of the outlets of the 

Carbon Isotopic Analysis of Small Samples 13 
REFERENCE GAS 

SAMPLE 

GAS 

M/Z 46 

M/Z 45 

M/Z 44 

Figure 2. Schematic diagram showing the triple-collector 

static mass spectrometer. Instrument is shown collecting 

the three ion beams pertinent to the isotopic analysis of 

C02 (namely, m/z 44, 45, and 46). Because the mass 

spectrometer is not pumped during an analysis, this type 

of instrument is extremely sensitive. m/z, mass to charge 

ratio. 

changeover valve. Third, the efficiency of a typical ion 

source is about 5 x 10-4 ions per molecule (Beckinsale and 

others, 1973); that is, for every 2,000 gas molecules 

flowing through the source, only 1 is ionized and thus has 

the potential to contribute to the measurement. 

The operation of a static mass spectrometer is somewhat more straightforward (fig. 2). Sample gas is admitted 

through a single, wide-bore inlet into the static volume of 

the mass spectrometer (static implies that the instrument is 

isolated from the pumping system). After a period of gas 

equilibration, the inlet valve is closed and the analysis 

proceeds. Following the measurement period, the valve to 

the pumping system is opened and the residual gas removed 

from the system. When base pressure is achieved 

(<1 X 10-9 torr), an equivalent amount of reference gas, 

metered out by use of a variable volume and a capacitance 

manometer, is introduced and analyzed in an analogous 

fashion. 813C and 8180 values are then determined from a 

comparison of ion-beam intensity ratios recorded from the 

sample and reference gases. 

The improvement in sensitivity over the conventional 

technique is achieved since almost all of the sample gas is 

admitted to the mass spectrometer and thus has the potential 

to contribute to the measurement. For carbon dioxide 

analyses, the mass spectrometer is similar to that employed 

conventionally (identical construction materials, same magnetic sector, and so on). However, there are some important 

differences. For instance, because of the high source 

pressures used in static mass spectrometry, the ion source 

configuration needs to be of a "nude" design in order to 

prevent localized high-pressure regions that would result in 

a number of unwanted ion-molecule reactions and accom14 New Frontiers in Stable Isotopic Research 

Table 2. Mass spectrum in the region pertinent to 

isotopic analysis of C02 

[Interferences can take the form of those species occurring at the 

same masses. In addition, there are problems with gases such as 

S02 and H20, which do not interfere directly] 

m/z 44 

t2C3Hs + 

12C3H6D+ 

and so on 

m/z 45 

13ct60 t6o+ 

tzct60 n 0 + 

Direct interferences 

tzc213CHs + 

12C3H7D+ 

and so on 

m/z 46 

13C160 17o+ 

12c160 1s0 + 

12C170 17o+ 

I2Cl3C2Hs + 

I2C3H6Dz+ 

and so on 

panying isotopic-fractionation effects. Another difference 

between static and dynamic mass spectrometry is that in the 

former technique the thermal degradation half-life of C02 is 

30 to 60 s, so ion-beam intensity ratios must be measured 

rapidly and simultaneously by multiple collection. 

A problem c0'mmon to both dynamic and static mass 

spectrometry is the need to have pure C02 gas for analysis. 

Table 2 shows some of the species that occur in the region 

of the mass spectrum of interest for C02 (m/z 44-46) and 

cause direct interference (for example, CS + and hydrocarbons, such as C3H8 +). Other species (not shown) may have 

an indirect effect (for example, the tailing of Ar+ or so+ 

and the presence of H20, which has a tendency to crack and 

form reactive ionic fragments). In a conventional isotopic 

ratio mass spectrometer, following admission of an impure 

gas sample, it is possible to perform a mass scan in order to 

assess the purity of the C02; a contaminated sample may be 

recovered and treated to chemical/cryogenic purification 

procedures in order to remove the interfering species. In the 

case of static mass spectrometry, because of the small 

amounts of gas encountered, it is often necessary to admit 

the entire C02 sample to the instrument for analysis (larger 

samples can be split and conserved in the inlet but if left for 

long periods of time may become contaminated by outgassed species). Because of the short half-life of C02 in a 

static mass spectrometer, the primary objective following 

admission of gas is to make isotopic measurements. The 

presence of impurities is often recognized by changeable 

ion-beam intensity ratios during the analysis because of 

interferences in the mass spectrum at m/z 44 to 46. In severe 

cases of contamination, there will be a disagreement 

between the measured and expected ion-beam intensities 

because, having established the relationship for pure reference gas between quantity of C02 (measured in the inlet via 
a capacitance manometer) and rn/z 44 ion-beam intensity, 

one can infer the presence of contaminants in sample gases 

by an unexpectedly low measured peak height. Unfortunately, a mass scan after isotopic measurement is not 

informative, because active gases in a static vacuum environment often suffer from the effects of thermal cracking 

and (or) reaction of ionic radicals to produce other (sometimes stable) species, from adsorption onto clean surfaces, 

from reactions with the filament, and so on. Ideally, the 

process used to prepare carbon for analysis (see next 

section) should result in the production of pure C02 . The 

instrument described by Prosser and others (1988) incorporates an analytical quadrupole mass spectrometer to assess 

the purity of the sample gas before it is admitted to the 

isotopic ratio mass spectrometer. 

STEPPED HEATING 

Principle of Operation 

In the technique of evolved-gas analysis, geological 

samples are subjected to an increasing temperature ramp; 

gaseous products are continuously removed and analyzed 

by a detector such as a quadrupole mass spectrometer 

(Gibson and Johnson, 1972). Unfortunately, because of the 

nature of isotopic ratio mass spectrometry, it is not possible 

to perform this type of experiment and measure the o13C of 

C02 released from samples undergoing linear heating. 

Rather, it is necessary to allow C02 to accumulate until 

there is a sufficient amount to permit isotopic measurements. It is thus appropriate to incrementally increase the 

temperature of the sample, leave it for a certain length of 

time without further adjustment of the temperature, and 

collect the evolved gases; this process is referred to as 

stepped heating. (In principle, if the temperature increments 

were made infinitely small, the heating/combustion extraction could be considered as having been carried out by use 

of a linear heating profile, but experiments of this nature 

await development.) 

The analysis of small samples of geological materials 

by use of stepped heating offers the advantages of contaminant removal and selective volatilization/combustion of 

different carbonaceous materials. During stepped pyrolysis, 

a sample is heated under vacuum at progressively higher 

temperatures (generally from room temperature to 

-1,300 °C). Gases evolved from a discrete temperature 

increment are collected and purified prior to admission of 

pure C02 to the mass spectrometer. Stepped combustion is 

similar except that the sample is heated in an atmosphere of 

pure oxygen. In the absence of oxygen, organic materials 

(including biological contaminants) break down to lowmolecular-weight volatile species (CH4 , C2H6 , C2H6 , CO, 

C02 , and so on). As such, it is customary to perform a 

combustion extraction up to about 500 °C regardless of the 

investigation. This procedure ensures that all organic material is converted to C02 and subsequently removed. Failure 

to do this can result in spurious isotopic results at higher 

temperatures because of interferences in the mass spectrum 

(m/z 44 to 46) from various contaminant ions. Above 

500 °C, combustion can be continued, or, alternatively, the 

sample can be pyrolyzed in order to release trapped gases 

from fluid inclusions, vesicles, and such. During pyrolysis, 

carbonaceous gases such as CH4 , CO, and C02 may be 

evolved. It is possible to separate these gases prior to 

conversion of each species to C02 for isotopic analysis 

(Sakai and others, 1976). However, the relationship of the 

carbon-bearing species originally present in the sample to 

the gas that is subsequently evolved on heating is not always 

straightforward. For instance, finely divided carbon (from 

graphite or organic materials) may react with oxygen in 

silicates to produce CO and C02 • Furthermore, the presence 

of hydrated minerals may cause carbon to be liberated as 

CH4 . Thus, it cannot always be assumed that an evolved gas 

was originally present in the sample as the trapped species. 

Stepped-heating techniques for carbon have been 

used and (or) described by, amongst others, Briggs and 

Kitto (1962), Des Marais (1978), Swart and others (1983a), 

Carr and others (1986), and Halbout and others (1986). The 

procedures used in our laboratory (Swart and others, 1983a; 

Carr and others, 1986) involve heating schedules of 30-min 

steps in which the temperature is increased as quickly as 

possible at the start of each step. Thus, there may be a time 

period of up to 5 min while the temperature is adjusted, with 

the balance of the 30 min at the nominal step temperature. 

Common to all combustion techniques is the need to 

generate a supply of oxygen. An additional constraint for 

small carbon-sample analyses is that the blank from this 

oxygen must be <0.5 ng C. A convenient way to generate 

oxygen is via the thermal decomposition of copper(II) oxide 

(CuOo-x>• where x = 0 in the case of stoichiometric 

Cu(II)O and 0 to 0. 5 for copper oxide that has been 

pretreated); changing the temperature of the Cu00 _x) 

allows control of the oxygen pressure. Any Cu00 _x) used 

in a stepped-combustion apparatus needs to be able to 

generate a suitable pressure of oxygen repetitively (up to 20 

steps per day). Furthermore, once the copper oxide is 

pretreated and the carbon blank removed, it is desirable to 

keep the same Cu00 _x) for as long as possible (several 

months) before replacement (in fact physical replacement is 

avoided; rather, when the copper oxide becomes oxygen 

depleted (x = 0. 5), it is usual to resorb oxygen from another 

source). 

Two methods of introducing the oxygen to the sample 

for combustion have been evaluated. In the techniques of 

Swart and others (1983a) and Carr and others (1986), 

chosen to achieve the lowest carbon blanks, oxygen evolved 

from Cu00 _x) is cryogenically trapped onto a finger 

containing pellets of a 5-A molecular sieve. Momentarily 

Carbon Isotopic Analysis of Small Samples 15 
warming the molecular sieve to room temperature desorbs 

oxygen (at a pressure of -100-500 torr in the small volume 

of the extraction system) but leaves any C02 from carbon 

impurities in the CuO(l-x) adsorbed onto the molecular 

sieve. The purified oxygen can then be admitted to the 

sample combustion vessel. At the end of the combustion 

step, oxygen that is surplus to the experiment is removed by 

resorbing the gas onto the molecular sieve finger, which is 

immersed in liquid nitrogen ( -196 °C), while sample C02 

is held on a cold finger also at liquid nitrogen temperature. 

While this method supplies oxygen of the highest purity, its 

availability in a fixed amount is sometimes unsatisfactory, 

especially in cases where large amounts of oxidizable 

materials are present, since the oxygen pressure gradually 

decreases with time as various components in the sample 

undergo chemical reactions (for example, hydrogen-, 

carbon-, and sulfur-bearing species are combusted to H20, 

C02 , and S02 , respectively; iron is converted to iron oxide, 

and so on). A danger of decreasing oxygen pressure is that 

the carbon will start to be extracted as CO; eventually, 

combustion may cease altogether. In the case of 

hydrocarbon-containing samples, graphitization (the transferring of one form of carbon into another) may occur. In 

any case, the effect of a continuously changing pressure in 

the oxygen supply is generally undesirable. An additional 

problem occurs when relatively large amounts of C02 are 

involved, because the procedure used to remove excess 

oxygen can result in the loss of small, but variable, amounts 

of C02 onto the molecular sieve (this procedure, although 

resulting in low carbon yields, does not appear to be 

accompanied by any isotopic fractionation). 

As an alternative to using a fixed aliquot of oxygen, 

it is possible to use a copper oxide furnace within the 

volume of the combustion vessel. This usage has the 

advantage that excess oxygen can be resorbed without 

opening the extraction vessel to another part of the vacuum 

line, and, more importantly, it is not necessary to use a 

molecular-sieve trap. Furthermore, by varying the temperature of the copper oxide, oxygen can be made available in 

a continuous supply at an appropriate pressure (because the 

equilibrium reaction CuO(l-x)~Cu + 0(1-x) is temperature 

dependent). The disadvantage of this method is that prior to 

its use for combustion of samples, considerable time is 

required to rid the CuO(l-x) of carbon contaminants that 

would otherwise contribute to the system blank. To achieve 

a low blank ( <0.5 ng C as C02 during the course of a 

30-min combustion), the CuO(l-x) is repetitively cycled 

between 600 and 950 °C to combust any carbon-containing 

species to C02 (with previously untreated CuO(l-x)• this 

process takes several days). In addition, if air is admitted to 

the previously "blanked-out" Cu00 _x)• (because of accidental vacuum failure or when a new sample is loaded into 

the combustion vessel), the cycling procedure has to be 

repeated, albeit in a more limited way. To prevent the 

Cu00 _x) from being exposed to the atmosphere unneces16 New Frontiers in Stable Isotopic Research 

sarily, samples are loaded into an "air-lock" section of the 

vacuum line that is isolated from the extraction vessel; 

following evacuation, the sample can be transferred to the 

combustion vessel through a gate valve (similar to the 

procedure described by Boyd and others, 1988). 

During the combustion interval, a cold trap ensures 

removal of species such as C02, S02, and H20 and thus 

prevents any possibility of these gases becoming adsorbed 

onto the copper oxide. A high-temperature platinum surface 

prevents formation of nitrogen oxide (this problem is dealt 

with extensively by Boyd and others, 1988). At the end of 

a combustion step, excess oxygen is resorbed onto 

CuO(l-x)· Following a pyrolysis extraction, carbon-bearing 

gases may include C02 , CO, CH4 , and higher hydrocarbons. If the C02 , CO, and hydrocarbons have different 

813C values (as result of fractionation during the experiment or because of an inherent property of the sample), it 

will be impossible to unambiguously comprehend the isotopic data unless each species is converted to C02 for 

isotopic analysis. 

Purification of evolved gases is accomplished cryogenically. First, gases that are condensible in liquid nitrogen are separated on a cold finger from noncondensible 

species (02 , N2 , and noble gases, and in the case of 

pyrolyses, CO and CH4 ). The temperature of the cold finger 

is then raised to about - 140 oc in order to distill C02 • S02 , 

H20, and any higher hydrocarbons from a pyrolysis are left 

in the trap (the appropriate apparatus and technique are 

described by Carr and others, 1986). A problem occurs for 

cryogenic separation if large amounts of H20 are present, 

because gases such as C02 may become trapped in the ice 

structure, possibly forming clathrates (which would have an 

idealized formula of C02 ·53/4H20; see Davidson, 1973). In 

water-rich samples, it may be necessary to employ a second 

cold trap at -90 °C to remove H20 before cryogenic 

separation of the remaining gases is attempted. Figure 3 

shows that, for a closed volume, when large amounts of 

gases are handled (that is, high vapor pressures), small 

changes in temperature can result in very large increases in 

pressure. Thus, when trying to separate large samples of 

C02 from S02 , it may become necessary to raise the 

temperature of the cryogenic trap to as high as -100 °C, an 

action that has the effect of desorbing some so2 (in order to 

avoid this problem completely, it is sometimes necessary to 

cryogenically remove the evolved C02 into a second cold 

trap and thus effectively lower the instantaneous vapor 

pressure). As a general rule, the most effective separations 

are obtained by monitoring the pressure of the gases 

liberated from the variable temperature trap; if the onset of 

so2 distillation is detected, the trap can be cooled down 

again and the process repeated or a second cold trap 

implemented. On the whole, cryogenic separations are most 

easily carried out and usually most successful when the 

smallest possible amounts of gases are handled. 
TEMP. 

("[) 

-250 L__.L...l....Ll.J_WL____L_..L.LLLU.Li.____L__l_..LLLWL_~..J.....L.LU..U.J....__.L..-J......L.L...LJ..U 

0·01 0 1 10 100 1000 

VAPOR PRESSURE (torr) 

Figure 3. Vapor pressures versus temperature of the gases 

most commonly encountered during stepped-heating 

experiments (data from Weast, 1984). 

Applications of Stepped Heating 

Figure 4 indicates how various carbon-containing 

species behave during stepped combustion. The kerogen 

sample (Gilmour and Fillinger, 1985) shows that organic 

materials burn below about 500 °C. Biological contaminants such as spores, dust, and fibers burn over temperature 

intervals that are similar to those at which kerogen burns 

(Des Marais, 1983). Thus, at the very least, stepped 

combustion can be used for separating high-temperature 

indigenous carbon from organic contamination. At intermediate temperatures ( 400-700 °C)' amorphous carbon burns 

(Grady, 1982), and carbonates decrepitate to yield the oxide 

of the mineral and C02 gas (Swart and others, 1982). At 

higher temperatures, elemental forms of carbon, such as 

graphite and diamond, combust (Grady and others, 1985), 

while carbides, such as cementite (Fe3C, or the meteoritic 

equivalent cohenite {Fe,NihC), burn at temperatures 

greater than 900 oc (Grady and others, 1986). 

When previously unstudied samples are analyzed by 

stepped combustion, problems may occur when one tries to 

decide, from an appraisal of the release data, which 

carbonaceous components are present. It might appear from 

figure 4 that, when a pure compound is combusted, a single 

release of carbon is liberated over a diagnostic temperature 

interval. However, identifications based purely on combustion temperature may be erroneous. Figure 5 shows the 

combustion temperature of diamonds of different grain sizes 

(Ash and others, 1987). The finest grain size ( ~3 nm) 

combusts at temperatures of 450 to 550 °C, while a single 

chip of diamond ( ~ 100 ~m) burns at temperatures greater 

than 700 °C. Intermediate grain sizes define a trend of 

increasing combustion temperature with increasing grade 

(fig. 5). The rate of combustion is presumably dependent on 

100r-- KEROGEN 

-

0 ~ 

100..-- AMORPHOUS CARBON 

z 0 0 r- ~ Q) 100 t- CALCITE cr: 

~ 

LJ 

~ 

_J ~ 

1-

0 

1- 0 - lJ_ 100 t- GRAPHITE 0 

~ 

.........-

0 

_J 

w _I 0 L...-...-, 

>- 100 t- DIAMOND 

r-'1-

0 ~ - 100 - COHEN ITE ( Fe 3 C) 

r---

o~~l--~'--~'--~~--~~~~~ 0 200 400 600 BOO 1000 1200 

TEMPERATURE (°C) 

Figure 4. Combustion temperatures of kerogen, amorphous carbon, calcite, graphite, diamond, and cohenite. 

In each case the yield of carbon from a particular step is 

expressed as the percentage of the total carbon. Step 

increments in each example are 100 oc. Grain size (compare fig. 5) may also affect combustion temperature. 

surface area; results from the finest fraction demonstrate 

that even in the case of a single diamond chip, combustion 

begins at temperatures as low as 450 oc. However, the 

relatively low surface to volume ratio would result in an 

extremely slow combustion rate at these temperatures. At 

temperatures above approximately 700 °C, combustion rate 

is considered to be fairly insensitive to temperature. 

Carbon Isotopic Analysis of Small Samples 17 
YIELD 

0·5 

0·0 

1-0 

0·5 

0·0 

1-0 

OF 0·5 

CARBON 

(% OF TOTAL oc-1) 

0·0 

1-0 

0·5 

0·0 

1·0 

0·5 

0·0 

SINGLE CHIP f- ( ... 100 ,urn) I -

r--

- - 30-60 ,urn 

- - -

_] 

1- - 8-15}Jrn 

1- -

-

~ 

1- - 0-2,urn 

f- -

J - f- - 0.003 ,urn -

- -

~ I J I I 

400 500 600 700 800 900 

TEMPERATURE (

0

[) 

Figure 5. Variation of combustion temperature of diamond with grain size. The single chip is from a natural 

terrestrial diamond, the three intermediate size fractions 

are commercially available diamond grits, and the finest 

fraction is from a carbonaceous chondrite. Yields of carbon are expressed as percent of total carbon oc- 1 ; step 

sizes are generally 50 oc, except for the diamond chip, 

where they are 100 oc. 

In the case of organic materials, the combustion 

temperature is dependent upon H/C ratio (Gilmour, 1986). 

Some examples are given in figure 6. Immature kerogens, 

such as those found in the Green River Shale, have a lower 

degree of cross-linking and thus have a relatively high 

hydrogen content. Materials of this nature combust at low 

temperatures (less than 400 °C). With increasing maturity, 

18 New Frontiers in Stable Isotopic Research 

2·0 COMBUSTION OF ORGANICS 

lO 

H/C(at.) 

0· 0 L----L---____JL------1.-~t...-..L...~ 

200 400 600 800 

TEMPERATURE (°C) 

Figure 6. Variation of combustion temperature of organic 

materials versus atomic (at.) H/C ratio. The combustion 

temperature is defined as that at which the maximum 

release is obtained during the stepped-heating extraction. 

Step sizes are typically 25 to 35 °C. 

the concomitant loss of hydrogen leads to the formation of 

more highly condensed matter and a consequent increase in 

combustion temperature. In the extreme, when all hydrogen 

has been lost, the organic material is graphitized and thus 

combusts at a temperature similar to that of pure graphite 

( ~800 oc; see fig. 4). A good pictoral representation of 

these maturation processes is given by Hayes and others 

(1983). Yet a further example of variation in release 

temperature is demonstrated in the work of Milodowski and 

Morgan ( 1980), where it is shown that the temperature of 

thermal decrepitation of carbonates, in a flowing stream of 

nitrogen and oxygen, depends on chemical composition. 

The range in decomposition temperature is from 300 °C 

(cerussite, PbC03) to >800 oc (strontianite, SrC03 , and 

witherite, BaC03). 

The success of static mass spectrometry as applied to 

the analysis of C02 liberated by stepped heating rests on the 

extraction procedure having a suitably low carbon blank. 

Figure 7 shows, among other things, a typical blank 

obtained by stepped combustion; all of the experiments 

shown in this figure were performed by use of fixed aliquots 

of oxygen, rather than a continuous supply. Between 200 

and 600 °C, the total blank is about 50 ng C, equivalent to 

about 10 ng C per step when using 100 °C increments. This 

component arises from a small amount of airborne organic 

contamination introduced when the extraction system was 
YIELD 

(ng C) 

((1 

MORB 45° N. \r ___ L __ _ 

I 

I 

I 

:----1 I 

' ' I I I ..... .,J 

: L ___ J 

0 200 400 600 800 1000 1200 

TEMPERATURE (

0

() 

Figure 7. Yield of carbon in nanograms (ng) (log scale) 

versus temperature, obtainable by the extraction system 

described by Carr and others (1986). The shaded histogram is the system blank. The plots labeled Allende CC1, 

MORB 45° N., and Quartz refer to samples of a carbonaceous chondrite, a basaltic glass from the mid-ocean 

ridge, and a fluid-inclusion-rich sample of quartz, respectively. If sample sizes are chosen properly, the amounts of 

C02 available for analysis are always greater than 50 times 

the blank at each step; use of a sufficiently large quartz 

sample is precluded by the extraction vessel's volume. 

opened to the atmosphere. Below 200 °C and above 600 °C, 

the blank, which is < 1 ng Cas C02 per step, is independent 

of temperature, but proportional to the length of time used 

for the extraction (30 min). These blank components are 

from carbon dissolved in the glassware that comprises the 

extraction line, together with the inherent contamination 

present in the oxygen. 

The three samples shown in figure 7 are typical in the 

sense that they represent the specimen sizes normally 

available and that can be manipulated easily and accommodated by the extraction system. Thus, the sample labeled 

Allende CC1, which is 50 percent carbon, represents a 

typical amount of material that remains after extensive · 

processing of a primitive meteorite; its weight, 69 J.Lg, can 

be determined accurately by use of an electronic balance 

(samples an order of magnitude smaller than this can be 

weighed but with about ±5 percent errors). The Quartz and 

MORB 45° N. samples are the largest that can be loaded 

into the system (at 17.15 and 15.25 mg, respectively). 

Allende CC1 (Ash and others, 1987) consists mainly 

of fine-grained elemental forms of carbon. Although most 

of the carbon in the sample combusts over a temperature 

range close to that of the major blank release, yields are 

considerably above blank levels. No blank correction is 

therefore necessary, and isotopic measurements are uncompromised. It can be seen that the sample-to-blank ratios are 

somewhat lower above 600 °C, but again, corrections are 

not routinely applied. 

For the basaltic glass MORB 45° N. (Exley and 

others, 1986), it can be seen that below 600 °C the amounts 

of carbon liberated are similar to, or a little larger than, the 

system blank. The slight enhancement is caused by the 

presence of a small amount of contaminant organic material 

on the surface of the sample. Above 600 °C (which in this 

case represents a stepped pyrolysis), large amounts of 

indigenous C02 are released compared to blank levels. 

Carbon released from the quartz sample (Carr, 1985) 

is liberated in the same temperature range as the blank and 

only a factor of 3 to 5 in excess of it. The sample was 

studied in an attempt to acquire data about the isotopic 

composition of C02 in fluid inclusions. However, with such 

a low sample-to-blank ratio, it is difficult to assess whether 

the apparent bimodal release is caused by an overlap of 

(lower temperature) organic contamination and fluid inclusion gas or represents two generations of fluid inclusions 

superimposed on a release of blank. The true nature of the 

sample might be elucidated by analysis of a larger amount 

of material (not possible with this extraction system), but if 

a large component of the carbon is surficial organic contamination on the sample itself, then this would not necessarily solve the problem. Clearly, stepped-heating extraction is more immediately suitable for some studies than 

others. 

Of importance to isotopic investigations is the question of whether isotopic fractionation accompanies the 

progressive burning of a sample. Swart and others (1982) 

demonstrated that stepped combustion of graphite and 

progressive decrepitation of carbonate did not result in 

carbon isotopic fractionation. A stepped combustion of a 

single diamond chip (D.W.McGarvie, unpub. data, 1988), 

shown in figure 8, demonstrates that no systematic or 

significant carbon isotopic fractionation was induced (the 

temperature interval over which the diamond burned is from 

650 to 1,000 °C). Some small isotopic variations appear to 

be outside analytical errors, but these may be due to internal 

isotopic variations within the diamond, which from other 

studies are known to be of the order of a few per mil (Swart 

and others, 1983c; Boyd and others, 1987). Also shown on 

figure 8 is a line corresponding to a mass-dependent loss of 12C relative to 13C (that is, a Rayleigh distillation with loss 

Carbon Isotopic Analysis of Small Samples 19 
DIAMOND 

13 

II cPDB 

(%o) 

0 20 40 60 80 100 

% COMBUSTED 

Figure 8. 813C versus cumulative release of carbon from a 

stepped combustion of a single chip of diamond (shaded 

area). PDB, Peedee belemnite. The temperature interval of 

the extraction is from about 650 to 1,000 oc (step increments 

are variable, between 20 and 50 °C). The precision of the 

isotopic measurements is ±1 to ±2.5 oJoo, as indicated by 

the height of the shaded areas. The line labeled MASS 

FRACTIONATION represents the result that would be produced by a kinetic isotope effect based on an assumed 

mass-dependent loss of atomic carbon (for such a process 

the fractionation factor ex, = 1.041; that is, y13112). This line 

has been calculated so as to correspond to the same 

percentage yield increments of the sample. Clearly, no 

systematic or significant isotopic fractionation occurs during stepped combustion. 

of atomic carbon assumed). This represents an extreme case 

but is the only form of isotopic fractionation that can be 

anticipated from a sample composed of pure carbon. 

Clearly, stepped combustion of the sample did not produce 

an isotopic fractionation of this magnitude. It is possible 

that, during combustion, at the very surface of the diamond 12C02 may be formed at a faster rate than 13C02 , or faces of 

different crystallographic orientation may bum preferentially. However, after a 30-min combustion period a skin of 

diamond corresponding to between 5 and 10 percent of the 

total mass would be removed and converted to C02 with no 

net isotopic fractionation. 

While combustion of graphite and diamond does not 

induce any observable carbon isotopic fractionation, it 

should be noted that progressive combustion of terrestrial 

kerogen samples does result in a reproducible change in 

o13C value (Gilmour and Pillinger, 1985). This change is 

not due to kinetic isotope fractionation during combustion 

but is a result of either ( 1) 12C enrichment in the more easily 

combustible side-chain and aliphatic materials compared to 

the relatively stable aromatic core of the kerogen (a result 

20 New Frontiers in Stable Isotopic Research 

of kinetic isotopic fractionation during kerogen formation) 

or (2) the resolution of marine and terrigenous components 

of different o 13C values. 

Although isotopic fractionation during combustion 

does not appear to be a problem, fractionation might be 

expected to occur in pyrolysis experiments where diffusive 

release of C02 is encountered. However, no such effect was 

observed during studies of MORB (mid-ocean ridge basalt) 

glasses (Mattey and others, 1984; Exley and others, 1986). 

Presumably, over a specific, narrow temperature interval, 

the samples either melted (thereby liberating gases from 

vesicles or from within the structure of the glass) or the 

confining pressure of fluid inclusions was exceeded, and 

fracturing of the sample and nonfractionating loss of gas 

resulted. In either case, diffusive loss prior to melting was 

of limited significance. 

Inherent Artifacts in Stepped-Heating Profiles 

In order to ascertain what information can be derived 

from a stepped-combustion plot, it is necessary to understand fully some of the problems inherent in the technique. 

These vagaries are manifested as artifacts on steppedcombustion plots that can be recognized and accounted for. 

Figures 9 and 10 illustrate examples of common problems. 

The plots are schematic but based on observations. For 

various reasons, not least of all because of difficulties 

associated with cryogenic purification of C02 (discussed 

above), problems of oxygen access or depletion, localized 

hot spots in the furnaces, and so on, it is desirable to use the 

smallest sample compatible with good precision in o13C and 

representative of the material under investigation. Of 

importance here is the ratio of available oxygen, 0, to the 

amount of combustible and oxidizable materials, CM, 

present in the sample. In systems containing pure carbon, 

the 0/CM is equivalent to the 0/C ratio. Figure 9A shows 

an example of a stepped combustion under a relatively low 

0/CM ratio, that is, a large sample under normal conditions 

or a small sample under low oxygen pressure. The release 

profile is broad and positively skewed (relative to the 

low-temperature part of the release, which rises sharply, the 

profile at high temperature has a more shallow appearance 

and is possibly an exponential decay). The extended nature 

of the release is a result of the inability of oxygen gas to 

gain access to the whole of the sample and complete that 

part of the reaction for which the activation energy has been 

reached; furthermore, C02 produced by the combustion 

may not be removed effectively. Such effects are undesirable in situations where multicomponent samples are being 

analyzed, since the releases of carbon from individual 

components will overlap and poor resolution will result. 

Figure 9B shows the effect of using a much higher 0/CM 
A 

TEMP 

YIELD 

TEMP. TIME 

Figure 9. Schematic diagrams showing the effect of the 

ratio of oxygen pressure to sample size on stepped combustion profiles (A through D) and the use of repeat 

combustions at discrete temperature steps (£, F). A represents combustion of a large sample under normal conditions or a small sample under low oxygen pressure. 

Note the exponential tail on the high-temperature part of 

the release. 8 shows the same sample combusted under 

conditions of high oxygen pressure or a small sample 

under normal conditions. C shows that multiple combustions at individual temperature steps may help in confining the release from a large sample to a narrow temperature interval (the shaded region of a particular step 

demonstrates that additional carbon is released by simply 

keeping the temperature constant). D shows a typical 

stepped-heating profile for a single component; T1 and T2 

represent two arbitrary temperatures, one lower and one 

higher than the maximum release. E and F show the 

effect of repeat combustions at T1 and T2 , respectively. 

ratio, that is, a small amount of the same sample under a 

relatively high pressure of oxygen. The release now occurs 

over a narrow temperature interval, and the sample bums at 

a uniform rate, so the indication is that all the constituent 

grains have access to oxygen, and C02 is able to escape. 

This type of release is more desirable for steppedcombustion analysis, as it may allow components of 

broadly similar combustion temperature to be resolved. 

Because multicomponent samples can consist of several 

carbonaceous materials of widely differing concentrations, 

it may be necessary to use quite a large sample in order for 

there to be enough of a minor component to allow measurement. Thus, to avoid the situation depicted in figure 9A. it 

may be necessary to employ multiple combustions at a 

particular temperature when analyzing large samples. Figure 9C is an attempt to demonstrate this effect, although 

data would probably not be plotted like this in practice 

(instead, the results from multiple combustions would be 

compounded). Swart and others (1983b) and Ash and others 

( 1988) have used this procedure to try to resolve minor 

entities, with slightly higher combustion temperatures, from 

a major component. The latter authors have taken the 

procedure to extreme, with the low-temperature combustions enduring many hours before being advanced to higher 

temperatures. Clearly, the exact temperature program cannot be determined before an unknown sample is analyzed. It 

is necessary to perform a series of trial experiments to 

assess the best course of action. Manual stepped combustions as a preliminary to long-duration automated extractions would seem to be a route for future developments in 

technique. 

The use of multiple combustions at discrete temperatures is complicated by the dependence of oxidation rate on 

temperature. Figure 9D shows a stepped-combustion profile 

for a hypothetical carbon-bearing component. If another 

aliquot of the same sample was subjected to a temperature 

increment of T1 -T0 , followed by multiple combustions at 

this relatively low temperature, then it may take many 

repeat steps in order to bum all of the component (fig. 9E), 

since the combustion rate is slow at low temperatures. A 

temperature increment of T2 - T0 followed by repetitive 

combustions at T2 may result in the sample burning completely in just a few repeat steps because of the faster 

reaction rate (fig. 9F). Differences in combustion rates 

might be used to try to separate components of similar 

combustion temperatures. 

The selection and control of the temperature increments used in stepped-heating experiments can have a 

profound effect on the nature of the release profiles. To 

illustrate this, figure lOA depicts the release of C02 that 

might be expected when a hypothetical component is treated 

to a linear-heating extraction (examples of C02 release from 

various samples can be found in Gibson and others, 1974). 

For the purposes of the following discussion, the x axis of 

figure 1 OA is considered to represent temperature; however, 

it should be appreciated that a linear-heating schedule 

implies a temperature ramp directly proportional to time 

(and thus, the x axis in this example may equally well 

represent time instead of temperature). As stated above, it is 

not possible to perform a linear-heating extraction by use of 

the stepped-combustion apparatus, so in reality the sample 

in figure lOA would have to be heated in discrete temperature increments. When these temperature increments are 

equal (corresponding to the vertical lines in fig. lOA), a 

reasonable representation of the actual release is obtained 

(fig. lOB). However, in figure lOC, the same release is 

depicted if it is assumed that the temperature of one of the 

steps were underestimated. The nature of the profile is now 

Carbon Isotopic Analysis of Small Samples 21 
A D 

YIELD 

8 E 

c F 

TEMP TEMP 

(U =UNDERESTIMATE) 

Figure 10. Comparison of stepped-heating data acquired 

in equal and unequal temperature increments and the 

effect of normalizing yield data to a oc- 1 interval. A shows 

a gas release obtained by use of linear heating. The 

vertical bars, which are equally spaced, represent constant 

temperature increments. B shows the stepped-heating 

profile that would be obtained. C shows the effect of 

underestimating the temperature of the third step. 0 

shows a gas release obtained by use of linear heating. The 

vertical bars, which are unequally spaced, represent temperature increments of different sizes. E shows the 

stepped-heating profile that would be obtained. F shows 

the same data but with yield normalized to a oc- 1 interval. 

less straightforward and could even be interpreted as two 

closely overlapping releases (in practice, additional information would be gained from the isotopic data, and a single 

release may have been inferred). Temperatures are measured erroneously in a number of ways, but four of the most 

commonly encountered problems are ( 1) physical movement of the temperature-measuring device (thermocouple) 

with respect to the sample during the extraction, (2) the 

tendency for an operator to log constant temperature increments in order to produce an apparently regular heating 

schedule, since stepped-heating extractions require adjustment of temperature to successively higher values, (3) the 

temperature of a resistance-wire furnace may often momentarily exceed the step temperature because of the use of an 

increased current in order to change temperature as rapidly 

as possible, and (4) temperatures may drift during the 

22 New Frontiers in Stable Isotopic Research 

course of the experiment (because of fluctuations in voltage, drafts, operator distraction). The combination of these 

effects might result in errors of measured temperatures of 

±5 to ±10 °C. 

In trying to obtain an accurate representation of the 

temperature increments experienced by a sample during a 

stepped-heating experiment, one may find it appropriate to 

record a value considered to represent the mean temperature 

for the step as a whole. This can often be quite a subjective 

exercise, but, in principle, if done accurately, the readings 

correspond to a stepped extraction in unequal temperature 

increments (the vertical lines in fig. lOD). If the yield 

versus temperature relationship is plotted (fig. lOE), a 

rather misleading representation is obtained, one that suggests a bimodal release. For this reason, it is advisable to 

normalize yield data to a constant step interval. The most 

convenient normalization technique is to recalculate the 

data to a yield oc- 1 basis. It can be seen from figure lOF 

that, when normalized data are plotted, a fair representation 

of the real situation is achieved. 

During analyses of unknown samples, it is often 

desirable to perform stepped extractions in temperature 

increments of :::;25 °C; experiments of this nature are 

referred to as high-resolution extractions. It is noteworthy 

that stepped combustions undertaken in small increments 

are often less reproducible than those performed in temperature increments of 100 oc, undoubtedly because of an 

inability to reproduce and control the temperature in small 

increments. The absolute accuracy of an individual measurement is probably ± 25 °C, although under optimum 

conditions the internal error during each experiment may be 

approximately ±5 to ± 10 °C. 

In addition to controlling the temperature accurately 

during a stepped-heating experiment, reproducing the time 

period for each extraction is also necessary. A computercontrolled stepped-heating extraction could be expected to 

give the best reproducibility of time and temperature and 

thereby minimize the incidences of operator error. However, for data already acquired, it is possible to suppress the 

effects of poor experimental control by subjecting raw data 

to an appropriate mathematical correction procedure. The 

results from a stepped-heating experiment may be treated to 

a form of digital smoothing, in the same way that low 

abundance peaks can be resolved from "noisy" massspectral data (Trott and Beynon, 1979). An even simpler 

approach to removing spurious irregularities in the yield 

profile is reforming the data, a process (see fig. 11) that is 

carried out in two stages. First, the raw yield data for each 

pair of adjacent steps are used to calculate the hypothetical 

yield for a step that would have been obtained over a 

temperature interval extending between the midpoints of 

adjacent steps. Second, the revised yield data are recalculated in the same manner to coincide with the original, 

measured temperature increments (this is an iteration of the 

above calculation). An example of the complete operation is 
Cl 

_J 

w 

>-

A 

-----, I 

X 

8 

TEMPERATURE 

Figure 11. Example of how to reform stepped-yield data. A 

shows a release profile containing inconsistencies in two 

adjacent steps, such as might be experienced from an 

erroneous temperature measurement. Band C show the 

intermediate and final results of mathematical processing, 

respectively. X, Y, and Z are the yields of three consecutive temperature steps. R shows step Y, after having been 

reformed. T1 - T2 , arbitrary temperatures. 

shown in figure 11. Here, figure llA represents a release 

profile containing inconsistencies in two adjacent steps (of 

yields X and Y), of the type that might be experienced from 

an erroneous temperature measurement. Figures 11B and 

11 C show the intermediate and final results of mathematical 

processing, respectively. The equation that describes the 

whole operation is given by 

The notation is consistent with that in figure 11. X, Y, and 

Z are the yields (in absolute units, meaning parts per million 

or nanograms carbon rather than oc- 1

-normalized data) 

from three consecutive temperature steps of temperatures 

T2 , T3 , and T4 , respectively. R is the reformed yield 

corresponding to the original step, Y. 

In the special case of the first temperature step where, 

in effect, step X is missing, the reformed yield is calculated 

from Yand Z only (R corresponding to the same temperature 

interval as Y): 

Similarly for the last temperature step: 

=~ (( X+Y) (T3 -T2)) R 2+ T -T x 2 3 1 

Following calculation of the reformed yields, the data 

can then be normalized to a oc- 1 basis.The technique is 

suitable for application where errors in temperature measurement are greater than 50 percent of the step size. 

Because the temperature errors may be as great as ±25 °C, 

the technique is suitable for stepped extractions undertaken 

in steps of less than 50 oc. 

In addition to smoothing the yield profile, it is 

possible to calculate the 813C values for the reformed data 

as well: 

13 813CxX+2813CyY+813CzZ 

CR X+2Y+Z 

Where 813CR is the reformed 813C value from the middle of 

three consecutive steps having yields and 813C values of X, 

813Cx; Y, 813Cy; and Z, 813Cz. 

For the initial step of an experiment (step X missing) 

and for the ultimate step 

Mathematical smoothing of stepped-combustion data 

should be restricted to the process of interpreting the sample 

under investigation. Some examples of this procedure are 

given later on. 

Interpretations of Stepped-Heating Data 

In simple materials (pure compounds) there may be 

few, if any, problems associated with the interpretation of 

stepped-heating data. However, in multi component samples, such as meteorites, the data may be complicated by the 

presence of different materials having similar combustion 

Carbon Isotopic Analysis of Small Samples 23 
temperatures, and so on. Furthermore, during a steppedcombustion experiment, carbon in different locations may 

bum at temperatures that are governed by the nature of the 

site rather than the form of the carbon. For instance, some 

carbonaceous chondrites contain carbon both finely dispersed in the matrix and also enclosed within spinel grains 

(Tang and others, 1988). The carbon in spinel is referred to 

as "protected" and may not bum until the host mineral 

begins to crack, melt, or otherwise allows access of oxygen 

gas; this burning usually occurs at temperatures greater than 

that at which the unprotected component bums. 

There are two ways in which the interpretation of 

stepped-heating data can be expedited. The first involves 

the preparation of a suite of chemical or physical separates 

from the parent sample. For example, whole-rock samples 

could be treated with (1) organic solvents, or water, (2) 

various mineral acids, (3) progressively stronger acids, (4) 

wet-chemical oxidizing agents, or (5) gas-phase oxidation 

at preselected temperatures. The residues produced during 

these procedures can be further subdivided by physical 

separation (magnetic properties, grain-size, density). 

Stepped-combustion analyses of the whole suite may then 

elucidate the carbon inventory of the sample under investigation. A comprehensive study of this type can be found in 

Tang and others (1988). 

Much simpler applications of the process can still 

give useful information; for example, if a whole-rock 

sample is treated with acid, then both the whole rock and 

insoluble residue can be analyzed by stepped combustion 

and the data compared. In a hypothetical case (fig. 12), 

consisting of a major acid-soluble component that has a 

broad release, and a small acid-insoluble component combusting at higher temperatures, the release profile is the sum 

of the insoluble and soluble components (fig. 12C). In the 

ideal case, stepped heating of the acid-insoluble sample 

(fig. 12D) will produce a release profile identical to the 

hypothetical case. Comparison of the data acquired from 

both the whole sample and the acid residue then allows one 

profile to be subtracted from the other, with the result that 

two components can be deconvolved. In practice, such a 

situation rarely exists, the acid treatment producing instead 

a somewhat different effect than might have been anticipated. Figure 12E shows a common problem; treatment 

with acid may result in the removal of a large amount of 

silicate matrix or a mantle of a "protecting" mineral that, in 

tum, may allow the acid-insoluble component to bum at a 

lower temperature. This effect is often accompanied by the 

combustion interval spanning a narrower temperature 

range. Clearly, the profile in figure 12E cannot now simply 

be subtracted from the measured release to allow deconvolution into the two components. Further complications may 

arise (fig.l2F) if not all of the acid-soluble component is 

removed by the acid and (or) some of the acid-insoluble 

component is lost during handling (a serious problem for 

fine-grained material). Again, comparison of the two mea24 New Frontiers in Stable Isotopic Research 

A D 

YIELD 

TEMPERATURE 

Figure 12. The effects of preparing acid-resistant residues 

from complex samples. The three plots on the left-hand 

side show the combustion characteristics of two hypothetical components, one acid soluble (A) and the other acid 

insoluble (8). During a stepped-combustion extraction of a 

sample containing both these components, plot C would 

be obtained. The plots on the right-hand side (D through 

G) illustrate four possible end results following acid dissolution of the sample. D, Stepped heating of acidinsoluble sample in an ideal case. E, Stepped heating of an 

acid-treated sample when the treatment caused removal 

of a silicate matrix or "protecting" mineral, allowing the 

acid-insoluble component to burn at a lower temperature. 

F, Complications that arise if not all of the acid-soluble 

component is removed by the acid and (or) some of the 

acid-insoluble component is lost during handling. G, 

Release temperature of acid-insoluble component occurring at a higher temperature than expected. 

sured profiles produces a less than straightforward insight 

into the nature of the sample. Finally, in figure 12G, the 

release temperature of the acid-insoluble component is 

shown as occurring at a higher temperature than expected. 

This situation might occur if a number of fine particles 

physically aggregate during acid treatment or are otherwise 

chemically altered. In volatile products of the acid treatment 

may coat the acid-insoluble component and raise its apparent combustion temperature. In practice, a combination of 

all these illustrative effects may occur and make interpretation of the sample very difficult without an appreciation of 

how chemical/physical separations may affect the nature of 

stepped-heating profiles. 

In addition to the use of chemical and physical 

separates, the carbon components of a particular specimen 

can be deconvolved by detailed appraisal of an individual 

stepped-heating profile. Halbout and Robert (1986) have 

attempted to undertake this task by using a mathematical 
10~------~------~ 

YIELD 

(arb.) 5 

YIELD 

(arb.} 5 

0 

+80 

~13c +60 

(o/oo} +40 

+20 

0 

200 

c 

400 600 800 1000 

TEMPERATURE (

0

() 

Figure 13. Theoretical effect of mixing two gas releases 

(redrawn from Halbout and Robert, 1986) (yield in arbitrary units). A, Two idealized releases, X and Y, with the 

maximum yield of the minor component, Y, occurring at 

600 oe. B, The yield profile obtained by summing together 

the releases X and Y. Note that the apparent maximum 

release of the minor component is displaced toward a 

lower temperature than 600 oe. C, The isotopic profile 

obtained with X and Y assumed to have one values of 0 

ofoo and +150 Ofoo, respectively. The maximum measured 

one value occurs at a higher temperature than 600 oe and 

is considerably lower than the actual value of component 

Y. 

approach based on temperature-related diffusion 

(expounded in detail for the case of stepped pyrolysis by 

Albarede, 1978). Figure 13A shows the apparent combustion behavior of two hypothetical components, X and Y 

(each assumed to be in a pure state). X is considered to be 

in the higher abundance and is released to the maximum 

extent at 400 °C, whereas the release of Y is centered on 600 

°C. Figure 13B shows the combustion release profile 

obtained when the two components are summed together; 

the apparent release of the minor component is displaced to 

a temperature lower than 600 °C. Figure 13C shows the 

measured 813C values, assuming component X to have a 

"normal" isotopic composition (813C = 0 oJoo) and component Y to be 13C enriched (8 13C = + 150 oJoo). The 

maximum measured 813C value is considerably attenuated 

from the true value because of mixing and occurs at a higher 

temperature than 600 oc. 

A consequence of the Halbout and Robert modeling 

is that relatively high 813C values are obtained over large 

temperature ranges because of the shallow exponential 

decay assumed for a minor component (for example, 

component Y in fig. 13). This observation would seem to be 

corroborated by stepped-combustion analyses of meteorite 

samples, where high 813C values are encountered over the 

temperature range of ~500 to 1,300 °C. However, Carr and 

others (1983) and Swart and others (1983b) have shown 

that, in fact, there are several different 13C-rich components 

that combust at high temperature. Applying the Halbout and 

Robert model to meteorite data could therefore be misleading. It is considered that, in contrast to the Halbout and 

Robert model, the combustion interval of minor components that bum at high temperature are rather narrow 

because of rapid oxidation rates (this is certainly true for the 

system described herein that uses small samples and relatively high oxygen pressures). Thus, the high-temperature 

(exponential) part of a release will have a considerably 

steeper decay than that shown for component Y in figure 13. 

Indeed, the release of a minor high-temperature component 

can probably be approximated by a Gaussian profile (the 

high-temperature profiles of major components are unquestionably described by relatively shallow exponential 

decays-see next section). 

If a Gaussian profile is assumed for a minor hightemperature component, effects similar to those proposed 

by Hal bout and Robert ( 1986) are still observed when 

mixtures are modeled. To illustrate this, figure 14A shows 

the exponential tail of a hypothetical major component with 

813C of 0 oJoo; the effects of a superimposed minor component (8 13C of + 100 ofoo) with a release temperature of 

progressively higher values are shown in figures 14B-E. In 

figure 14B the minor release occurs in a very steep part of 

the exponential decay, and, thus, the measured yield profile 

does not give a clear indication of a second component. As 

the release of the minor component is progressively shifted 

to more shallow parts of the exponential decay (figs. 

14C-E), the yield profile reflects more accurately its 

existence. In all cases the 813C values show the presence of 

more than one component; however, even in figure 14£ the 

measured 813C value of the minor component is compromised because of mixing. Note that in figures 14C-E the 

maximum 813C value occurs in the step following the 

Carbon Isotopic Analysis of Small Samples 25 
TEMPERATURE 

Figure 14. The effect on the measured release (yield in 

arbitrary units, left-hand axis) and isotope profiles (813C, 

right-hand axis) of a minor component, depicted as a 

symmetrical Gaussian profile (shaded region) being liberated on the exponential part of a major release. A shows 

the major release only; B through E show the effects oft he 

minor release occurring in progressively more shallow 

parts of the exponential tail of the major component. Note 

that the minor component becomes better resolved in the 

progression B through E butthatthe measured 813Cvalues 

are always lower than the true value of the minor compor:'Jent (+100 otoo). As in the Halbout and Robert modeling, 

the maximum measured 813C occurs at a higher temperature than that which appears to represent the release of 

the minor component. 

maximum in yield. It is important to appreciate this effect 

when interpreting complicated stepped-combustion profiles; 

otherwise, the release temperature of a minor component 

may be overestimated. 

Some Examples 

If the necessity for accurate and reproducible control 

of temperature and time is fully appreciated, the results of 

high-resolution stepped-heating experiments can be informative. Well-controlled experiments should be the norm 

rather than the exception. However, an inexperienced or 

inattentive operator may produce poor-quality data. The 

problems associated with high-resolution stepped combustions are exemplified by data acquired from individual 

diamond fragments (fig. 15). The experiment shown in 

figure 15A was conducted by use of unequal temperature 

increments (an attempt to document accurately the temperature of each step); the yield versus temperature relationship 

conforms to a single broad release of carbon between 650 

and 950 oc (analogous to the temperature range for diamond 

shown in fig. 4). The nature of the carbon release, which 

gradually decays with each consecutive step from 750 oc 

onwards, suggests that the oxidation rate of diamond at 

elevated temperatures is not highly temperature dependent 

26 New Frontiers in Stable Isotopic Research 

ppm c •c-1 

5000 

8 

700 800 900 

TEMPERATURE ("C) 

700 800 900 

TEMPERATURE ("CI 

ppm C •c-1 c 

700 800 900 

700 800 900 

Figure 15. Stepped combustion of diamond by use of 

small temperature increments. The measured carbonyield data, in weight percent carbon (wt% C), in A were 

acquired by use of unequal steP. sizes. 8 shows the effects 

of normalizing the data to a oc-1 interval, and c shows the 

data after being subjected to a mathematical smoothing 

procedure. D, E, and F show measured, oc- 1

-normalized, 

and smoothed data, respectively, for a diamond combusted in equal temperature increments. 

(if the extent of combustion had been related to the 

temperature, the unequal step sizes would have produced a 

far more irregular profile). Thus, in this case, carbon 

combustion appears to be controlled by the timing of each 

step rather than by the temperature (and so the x axis could 

be replaced by one calibrated in time, rather than temperature). When the data are normalized to account for the 

different step increments (parts per million carbon per °C, 

fig. 15B), the release profile becomes apparently complex 

(correction of the yield data to account for different temperature increments is erroneous if the combustion reaction 

is dependent on time rather than temperature). However, 

the effects of poor temperature control may also produce the 

complicated oc- 1

-normalized plot. Indeed, applying a ±5 

°C error to each temperature step can explain the ostensibly 

complex normalized yield data. It can be seen that subjecting the yield data to the mathematical smoothing procedure 

described earlier (fig. 15C, dealt with in more detail below) 

has helped to portray the simplicity of the release. 

The profile shown in figure 15D represents an attempt 

to analyze the sample in constant (20 °C) increments in 

order to avoid some of the problems encountered in the 
previous experiment. The use of constant temperature 

increments results in a oc- 1

-normalized profile (fig. 15£) 

that is similar in appearance to the yield plot (except for the 

steps below 700 °C and above 940 °C, which are not 20 oc 

increments). However, the experiment has not produced a 

single release of carbon. As in the previous example, it is 

difficult to divorce problems associated with poor control of 

temperature or time. If it is assumed that diamond exhibits 

an oxidation rate that is independent of combustion temperature, then figure 15D is explained if time intervals of 

unequal duration were used (although, it would be necessary to accept that perfectly accurate temperature control 

was achieved, a situation that seems unlikely). On the face 

of it, problems associated with reproducing the duration of 

steps might seem to be trivial compared to problems 

associated with controlling the temperature. However, it 

should be stressed that high-resolution stepped-combustion 

experiments may take typically 10 to 15 hours to conduct; 

as such, it is often a tedious burden on the operator to 

adhere rigorously to the time schedule. Assuming that the 

yield of carbon is directly proportional to the extraction time 

and applying an error of ± 7 min to each step of the 

experiment could produce a yield profile that conforms to 

the expected single broad release. However, as in the case 

of the previous example, the effects of poor temperature 

control must also be considered. 

The application of the smoothing procedure to diamond data is shown in figures 15C and 15F. When the 

irregularities that are due to experimental conditions have 

been suppressed, it is possible to appraise the combustion 

data more critically. It is interesting to note that in figure 

15F there is a small but distinct release of carbon at about 

850 to 900 °C. This effect can also be noted as a distinct 

shoulder on the release shown in the reformed data of figure 

15C (this was not necessarily obvious in the yield versus 

temperature profiles of the raw data). The agreement 

between the two experiments suggests that the small release 

is not an experimental artifact but may instead represent the 

combustion of a relatively more stable part of the diamond 

(a core of different crystal habit, perhaps). Clearly, this 

property should be explored to obtain further information 

about diamond, but thus far the investigation has not been 

pursued. 

An example of the reforming process as applied to the 

yields and isotope measurements of a meteorite sample (a 

carbonaceous chondrite, Y82042, McGarvie and others, 

1987) is shown in figure 16. The raw data describe a 

seemingly complicated release pattern that would have 

several different components burning. When the data are 

reformed, it can be seen that there are three major releases 

of carbon with perhaps a further minor component at about 

700 °C (the carbon burning from 600 to 700 °C results from 

the "tail" of a more major release of organic materials). The 

813C values show that the 3 or 4 releases of carbon are all 

isotopically light, having 813C values of about -20 o!oo. At 

RAw DATA REFORMED 

[ 

ppm "(-1 

100 aoo 900 1000 1100 1200 600 100 aoo 9oo 1000 1100 1200 Boo TEMPERATURE 1"[1 

Figure 16. Raw and reformed data for the hightemperature combustion of an HCI-resistant residue from 

the carbonaceous chondrite Y82042. The yield is plotted as 

the histogram (left vertical scale) and 8 3C data as solid 

dots (right vertical scale). (PDB, Peedee belemnite.) 

temperatures between the releases of light carbon, 813C 

values increase. The reformed data can be interpreted as 

several discrete releases of light carbon superimposed on an 

an overall low abundance of isotopically heavy carbon 

components. An understanding of the carbon components in 

this sample is considerably facilitated by the use of a 

smoothing procedure. 

Before attempting to interpret the components present 

in a complicated meteorite sample, it is necessary to first 

appreciate the extent to which combustion of a major 

component may overlap with the release of higher temperature materials. This consideration is especially important 

in studies of meteorites such as carbonaceous chondrites, 

where high-temperature minor components (present at the 

parts per million level) may become obscured by the tail 

from indigenous organic materials (with a carbon content of 

2 to 5 weight percent). Figure 17 shows those parts of the 

release profiles that are approximately exponential for two 

chemical separates of the Murray meteorite, an HCl residue 

of the Y791824 meteorite, and also a terrestrial diamond. 

The sample named Murray A Y shows an exponential 

decrease in the yield that covers more than 3 orders of 

magnitude. It would seem impossible to derive an effect 

like this during a stepped-heating extraction from anything 

other than the extended combustion of a single component 

(for the purposes of this example, complex meteoritic 

organic materials are considered as a single component). 

Thus, when evaluating stepped-heating data of unknown 

samples, it is important to consider that low-temperature 

components may continue to bum and thus produce exponentially decreasing amounts of carbon with increasing 

temperature. 

Figure 18 shows reformed stepped-combustion data 

(T>800 °C) for an HCl residue of a CM2 carbonaceous 

chondrite·,c Pollen (D.W.McGarvie, unpub. data, 1988). A 

casual inspection shows there to be an exponential tail from 

organic materials with 813C of about -20 o!oo, superimposed 

Carbon Isotopic Analysis of Small Samples 27 
DIAMOND 

102 MURRAY AY 

c 

ppm oc-1 

~ 10 1 

400 500 600 700 800 900 1000 1100 

TEMPERATURE (

0

() 

Figure 17. Yields of carbon (log scale) versus temperature 

for a number of samples. Murray AY and AY4a are chemical and physical separates of the CM2 carbonaceous 

chondrite Murray, Y791824 is an HCI-resistant residue of a 

whole-rock sample of another CM2 carbonaceous chondrite, and the diamond is a type II sample from an 

unknown locality. Only those parts of each release that 

conform to an exponential decay are shown. 

on which is a high-temperature release (~1,100-1,200 °C) 

of carbon having a 813C value of more than +80 ofoo. The 

release profile thus has the appearance of a sample that 

contains just two components; intermediate 813C values 

between 900 and 1,100 oc reflect overlapping combustion. 

Inspection of the data reveals that the maximum 813C value 

occurs at a higher temperature than the carbon release 

discernible at about 1,100 °C. At first sight, this would 

appear to be compatible with the modeling of Halbout and 

Robert (1986), discussed previously. However, when the 

yield data are replotted on a log scale (fig. 19), it is 

immediately apparent that above 950 °C there are at least 

two high-temperature minor carbon components (the dashed 

line, which is a best-fit exponential through the six points 

that correspond to the lowermost temperatures of the 

experiment, represents the release of carbon from organic 

materials). Since the yield data do not return to the 

exponential decay at the highest temperatures, additional 

components may be present. If the nature of the release is 

explained as a mixture of carbon from the tail that is due to 

organic materials (813C = -20 ofoo) and other minor 

components, then calculation of the "true" 813C values for 

28 New Frontiers in Stable Isotopic Research 

c 

ppm oc-1 

POLLEN HCl 

("REFORMED" DATA) +100 

+50 

0 

-50 

900 1000 1100 1200 

TEMPERATURE (°C) 

13c 

~ PDB 

%o 

Figure 18. Stepped-combustion plot (800 to 1,260 °C) of an 

HCI residue prepared from the Pollen CM2 carbonaceous 

chondrite. Yield (histogram, left vertical scale) and 813C 

data (solid dots, right vertical scale) have been reformed 

to remove any inconsistencies in the measured profile. 

(PBD, Peedee belemnite.) 

the minor amounts of high-temperature carbon shows them 

to be not significantly higher than the measured values 

(because of the steep slope of the exponential release). 

However, it is already known from stepped-combustion 

studies of chemical and physical separates of CM2 carbonaceous chondrites such as Murchison and Murray (Swart 

and others, 1983b; Tang and others, 1988) and HClresistant residues of other CM2 meteorites (McGarvie and 

others, 1987) that the 813C values of high-temperature 

carbon components are in the range + 500 to + 1 ,500 of oo. 

Thus, a simple mixing calculation is not effective in 

describing the components in Pollen. 

An alternative approach in explaining the Pollen data 

is to fit a more complicated decay profile to the lower 

temperature major component (such as that labeled A+ Bin 

fig. 19). Curve A+ B has been calculated with an assumed 

mixture of an exponential decay from meteoritic organics 

(813C = -20 otoo) and a constant offset, for example from 

a blank contribution (also with 813C = -20 otoo). Lines A 

and B have been chosen such that their combined effect 

when summed together intersects the final measured point 
YIELD 

OF(, 

ppm"(~ 

POLLEN HCl 

O.OQ1L-----l90L0 ___ 1...L000 ____ 11.L0-0 --~::-:'-----' 

TEMPERATURE (°C) 

Figure 19. Measured yield of carbon (solid dots, log scale) 

versus temperature for the Pollen Hel residue shown in 

figure 18. The dashed line represents a best fit through the 

part of the release that defines an exponential decay. The 

line labeled A represents an offset, as might be expected 

to arise from a constant blank contribution. Line B represents a theoretical exponential decay, selected such that 

the cumulative effect of adding A and B together is to 

intersect the yield point measured at the highest temperature. Actual yields, therefore, indicate the presence of 

two minor components: one between 1,025 and 1,200 oe 

and a second, less obvious than the first, from about 900 

to 1,025 oc. 

(0.017 ppm C oc- 1 at 1,210-1,260 °C). The line describing 

the offset (A, fig. 19) is about five times higher than that 

which would be expected for a normal system blank; thus, 

at very least, this detailed appraisal of the Pollen data 

appears to have demonstrated an abnormally high blank 

during the analysis. Regardless, the 813C data can now be 

corrected under the assumption that mixing took place with 

a component that conforms to the release labeled A+ B. The 

component has a 813C of -20 ofoo. The calculated 813C data 

are shown in figure 20, along with the exponentially 

decreasing release ascribed to organic materials plus constant blank (shaded histogram). The calculated isotopic 

values at low temperatures (T about 900 °C) appear to show 

high degrees of 13C enrichment; this enrichment is largely 

1.0 POLLEN HCL 

+1000 

+500 

0 

900 1000 

TEMPERATURE (oC) 

Figure 20. Measured and calculated results for Pollen Hel 

residue in which mixing of a tail from a major organic 

component is assumed. The shaded histogram corresponds to the tail component (derived from curve A+ B in 

fig. 19; note that the histogram has been calculated so as 

to match the temperature increments of the sample). The 

nonshaded histogram shows the measured yield data. The 

measured one values (solid dots) are shown and compared to those calculated for the minor component (open 

circles), assuming that the major organic component has 

one =-20 ofoo. PDB, Peedee belemnite. 

due to the positioning of the exponential release (the 

location of line B in fig. 19). Of more importance is that, 

between 850 and 1,200 °C, three releases of minor amounts 

of carbon can be discerned, and, perhaps somewhat unexpectedly, the calculated 813C values of these components 

are seen to be relatively 13C poor. 

Thus, the Pollen data are in some ways similar to 

those acquired for Y82042 (fig. 16) because of the observation of both isotopically light and heavy high-temperature 

carbon components. There is every indication that in 

carbonaceous chondrites, between four and seven different 

components bum at high temperatures. The mathematical 

modeling used to derive this conclusion offers a very 

different view from that which may have been attained by 

use of a more simplistic outlook of the type advocated by 

Hal bout and Robert ( 1986). However, it needs to be 

stressed that modeling of this nature should be used only to 

gain an insight into the potential carbon inventory of a 

Carbon Isotopic Analysis of Small Samples 29 
particular sample (calculated yields and o 13C values of each 

putative component are, of course, model dependent). 

Following this process, it should then be possible to devise 

appropriate separation schemes, or stepped-heating schedules, that can further elucidate the sample under investigation. 

CONCLUDING REMARKS 

High-sensitivity static mass spectrometry allows isotopic measurements to be made on subnanomole amounts of 

carbon as C02 ; current instrumentation produces precisions 

of<± 1 otoo for o13C (o180 measurements are also possible 

of course, but with somewhat poorer levels of precision of 

about ± 2. 5 ot oo; however, thus far no attempts have been 

made to exploit, or indeed improve, this capability). An 

instrument being tested appears to be capable of making s::l3c d s::l8 . u an u 0 measurements on ptcomole quantities of 

C02 with precisions of ±2.5 otoo. 

In order for carbon to be extracted in the form of 

C02 , samples can be submitted to incremental heating 

either with oxygen (combustion) or without (pyrolysis). 

Stepped combustion allows separation of carbon-bearing 

components on the basis of oxidation temperature. In many 

favorable instances, different materials can be successfully 

resolved in carefully controlled experiments. In particular, 

indigenous high-temperature carbon components are easily 

d~stinguished from organic contaminants. Small samples 

gtve the best results for stepped-combustion experiments, 

since materials are able to burn, or degrade, at a more 

uniform rate and over more tightly constrained temperature 

intervals, two factors that lead to improved separation of 

components. Furthermore, low-volume extraction systems 

can be employed, resulting in reduced system blanks. 

Because small samples give lower amounts of CO for 

1 . 2 ana ysts, the stepped-combustion approach is more suited to 

static mass spectrometry than the conventional dynamic 

procedures. 

For its full potential to be realized, stepped combustion needs to be performed under the most carefully 

controlled conditions of temperature, time, oxygen, pressure, and so on. Up to now, these factors have not always 

been carefully controlled; therefore, a carbon release profile 

can exhibit features that experience has shown are artifacts 

of poor technique. A number of vagaries associated with 

stepped combustion have been dealt with herein. One of the 

main problems is that in order to resolve components, it is 

often desirable to employ a high-resolution heating schedule 

(that is, temperature steps :5 25 °C); while it is possible to 

make measurements of relative increments in temperature to 

accuracies of :5 ± 5 oc, the absolute temperature of the 

sample may be in error by ±25 °C. Thus, with step sizes of 

:525 oc it becomes increasingly difficult to compare data 

from replicate analyses or from measurements of different 

elements (carbon and nitrogen, for instance, which for 

30 New Frontiers in Stable Isotopic Research 

technical reasons, have to be measured on separate instruments and thus different aliquots of the same sample). Poor 

control of either the temperature or the length of time used 

for an individual step during an experiment may result in 

release profiles that appear (erroneously) complex. However, data already acquired may be mathematically 

reformed to smooth out errors of this nature. 

The combustion process lends itself ideally to mathematical modeling so that small components overlapped by 

a major one may be resolved if an exponential tailing effect 

is assumed. So far, only the very simplest and most easily 

recognized variables have been considered in modeling. 

Parameters such as the grain size of the sample and oxygen 

pressure have yet to be taken into account. The temperature 

regime experienced by a sample could be much more 

thoroughly monitored and time averaged. However, even 

with absolutely perfect temperature control of a furnace 

there are still problems to be considered, such as (1) 

temperature gradients within the extraction vessel, (2) 

positioning of the sample with respect to the temperature 

measurement device, (3) exothermic or endothermic reactions or changes of state involving various materials (not 

necessarily carbonaceous) in a multicomponent sample that 

causes localized temperature variations, and ( 4) the presence within a particular sample of catalysts, surface poisoning agents, or protecting materials that can cause materials 

to combust at unexpected temperatures or affect the gasphase chemistry. 

In carbon isotopic studies, as stepped-combustion 

processes become more fully understood, it should become 

possible to perform reproducible experiments to smaller 

temperature increments. If heating schedules can be tailored 

so as to approximate linear temperature ramps, mathematical modeling of the data will become straightforward and 

indeed more informative. Extremely small-temperatureincrement (ultrahigh resolution) stepped-combustion extractions will undoubtedly require the use of a new generation 

of high-sensitivity static mass spectrometers. True linear 

heating may require completely new developments in mass 

spectrometry. The highest levels of sensitivity for carbon 

isotopic studies may be found eventually in a combination 

of ion- or laser-probe extraction followed by resonance 

ionization mass spectrometry. However, in the meantime 

stepped heating followed by carbon isotopic analysis in 

which static mass spectrometry is used will continue to 

provide valuable information in new and previously studied 

areas of geochemistry and cosmochemistry, particularly 

when all the vagaries associated with the technique are 

taken into account. 

ACKNOWLEDGMENTS 

The development of the stepped-heating techniques 

and static mass spectrometers has been made possible by 
grants from the Nuffield and Wolfson Foundations and the 

Science and Engineering Research Council. We acknowledge all those members of the Planetary Sciences Unit past 

and present whose unpublished data have been included in 

this paper. The manuscript has been improved by a critical 

appraisal from Monica Grady and thoughtful reviews by Pat 

Shanks and Bob Criss. I.P. Wright thanks the USGS and 

the organizers of the "New Frontiers in Stable Isotopic 

Research" workshop for providing funds to allow attendance at the meeting. 

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34 New Frontiers in Stable Isotopic Research 

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730-732. 
Laser Microprobe-Quadrupole Mass Spectrometer 

System for the Analysis of Gases and Volatiles from 

Geologic Materials 

By Everett K. Gibson, jr.,1 and Robert H. Carr2 

Abstract 

A laser microprobe connected to a computercontrolled benchtop quadrupole mass spectrometer is 

capable of extracting gases and volatiles from a wide 

variety of geologic materials. The released gases are identified by mass spectrometry. A capacitance manometer is 

used to quantify the yield of released gases. Reference 

minerals have been analyzed in order to determine the 

nature of volatiles released by laser interaction with the 

surfaces. Minerals analyzed include carbonates (calcite, 

magnesite, dolomite, and azurite), a sulfate (barite), a 

sulfide (pyrite), and diamond. Two separate studies have 

been carried out to show the utility of the newly developed analysis system: (1) analysis of a weathering product 

associated with glass in an SNC meteorite and (2) analysis 

of different types of fluid inclusions in an Archean barite. 

INTRODUCTION 

The abundance, distribution, and chemical forms of 

the light volatile elements present in geologic materials can 

provide valuable information concerning the nature, origin, 

and subsequent evolutionary history of individual samples. 

For example, the ratio of carbon dioxide to water in many 

fluid inclusions can be used to infer the nature of mineralizing solutions and, from phase relations, to indicate the 

pressure/depth regime in which the host mineral formed 

(Roedder, 1972, 1984; Roedder and Bodnar, 1980). Mineral identification also can be enhanced by the determination of gases liberated upon breakdown of the sample by 

pyrolysis, combustion, or acid dissolution. 

Many of the macroscopic methods used in the determination of volatile species essentially have been nonselective, with the possible multiplicity of origin of the extracted 

gases often presenting an insurmountable limitation to 

interpretation (Harris, 1981; Sommer and Schramm, 1983). 

"Probe" methods are becoming increasingly popular in a 

large range of scientific studies, because more detailed 

experimental data often can be obtained by using more 

localized analyses. The ability to selectively extract and 

1 SN2, Planetary Sciences Branch, NASA Johnson Space Center, 

Houston, TX 77058 . 2 SN4, Experimental Planetology Branch, NASA Johnson Space 

Center, Houston, TX 77058. Present address: ICI Europa Ltd., Everslaan 

45 , B-3078 Kortenberg, Belgium. 

analyze areas 10 to 40 J.Lm in diameter offers great potential 

for study of important problems associated with the identification, origin, and elemental and isotopic composition of 

the materials. Laser extraction of gases from geologic 

samples has been undertaken in a variety of circumstances 

to facilitate this finer scale approach (Tsui and Holland, 

1979; York and others, 1981; Kotra and Gibson, 1982). 

Laser-microprobe extraction of carbon, nitrogen, and oxygen from samples for stable isotopic measurements has been 

shown by Franchi and others (1986; this volume) to be a 

viable technique, but further refinements are required. 

In this paper we describe an instrumental system that 

combines the ability of a laser probe to interact selectively 

with a small, predetermined locality in a sample with the 

analytical capability of a benchtop quadrupole mass spectrometer. The quadrupole mass spectrometer permits qualitative information to be obtained on the volatiles released 

from the samples. A capacitance manometer is included in 

the system to allow direct quantitation of the gases extracted 

(Sommer and others, 1985). 

EXPERIMENT A liON 

Apparatus 

The laser microprobe-quadrupole mass spectrometer 

computer system is shown in figure 1 , and a schematic 

diagram of this system is shown in figure 2. The main 

sections are (1) the quadrupole mass spectrometer, (2) a 

vacuum valve and capacitance manometer section, (3) a 

vacuum pumping system, and (4) the sample chamber. Not 

shown in figure 2 are the laser and the sample-viewing 

microscope. A more detailed diagram of the sample chamber is given in figure 3. 

Mass Spectrometer 

The quadrupole mass spectrometer is an HP-5970B 

Mass Selective Detector (Hewlett Packard, Ltd., Palo Alto, 

Calif.) designed primarily for use with gas chromatographic 

systems. The electron-impact ionization source, hyperbolic 

mass filter, electron multiplier, power supply, drive electronics, and air-cooled turbo molecular pump are contained 

laser Microprobe-Quadrupole Mass Spectrometer System 35 
Figure 1. Laser microprobe-quadrupole mass spectrometer with the computer that controls the system (located on 

the right). The laser microprobe, along with its associated 

sample chamber, lighting, and photographic system, is 

shown in the center. The benchtop quadrupole mass 

spectrometer, along with its heated inlet line, is to the left 

of the laser microprobe. In the lower left are the control 

system for the vacuum system and the capacitance 

manometer readout units for quantifying the amounts of 

gases released during analysis. 

in a single, compact (54-kg) unit, and an external twostage direct-drive mechanical pump provides the backing 

vacuum. 

Ionization of the neutral gas species entering the mass 

spectrometer is achieved by impact of electrons, with an 

energy of 70 e V, from either of two filaments that are user 

selectable by means of an external switch. The quadrupole 

mass filter consists of four molybdenum rods, 203 mm 

long, precisely formed to a hyperbolic shape. Detection of 

the positive ions extracted by the mass filter is accomplished by a continuous dynode electron multiplier, which 

has a maximum voltage setting of 3,000 V. The analyzer 

chamber is heated by means of heating tape that is covered 

by insulating material held in place by an aluminum shroud. 

During normal operation, the ion source end is held at about 

280 °C, while the detector end is appreciably cooler, about 

75 °C. The pumping system evacuates the analyzer to a 

background pressure of about 2 x 10-7 torr, as measured 

by an ion gauge located on the rear of the analyzer flange. 

Water is the major contaminant, with minor atmospheric 

gases and traces of pump oil. 

Realtime control of the mass spectrometer is accomplished via the drive electronics located in the main unit. 

Operating parameters are entered via the data system 

(HP-59970 ChemStation), which also runs software for the 

automatic adjustment of voltages in the ion source, calibration, and mass assignment and scanning control of the mass 

spectrometer. An Autotune program is available to optimize 

36 New Frontiers in Stable Isotopic Research 

instrument parameters automatically by comparing a library 

spectrum of perfluorotributylamine (PFTBA) with spectra 

of the PFTBA released from a small cylinder attached to the 

analyzer flange. Data acquisition programs control the 

operating mode of the instrument, while editing software 

allows plotting and tabulating mass spectra, background 

subtractions, and so on. Peripherals attached to the ChemStation are a combined fixed- and floppy-disc unit 

(HP-9133H), a graphics plotter (HP-7475A), and a printer 

(HP-2930). 

Vacuum Valve System and Capacitance Manometer 

The mass selective detector is normally used with an 

interfaced gas chromatograph. For the laser microprobe to 

be interfaced to the mass spectrometer, several modifications must be made to the sample inlet portion of the 

system. When not intended for use with chromatographic 

systems, the inlet of the HP-5970 mass spectrometer is 

supplied with a blank gasketed vacuum fitting (VCR, 

Cajon, Macedonia, Ohio). This fitting has been adapted by 

having a 22-cm length of 6-mm (outside diameter) by 4-mm 

(inside diameter) stainless steel tube welded through it such 

that the tubing terminates close to the mass spectrometer's 

ion source. The other end of the tube is welded to a 

high-vacuum bellows valve (Nupro, Willoughby, Ohio) 

that acts as the inlet valve (V 1 , fig. 2). An aliquoter formed 

from two other valves (V 2 and V 3) allows a small (1.6 cm3

) 

volume of gas to be isolated from the larger (11.2 cm3

) 

volume, which contains a capacitance manometer and cold 

finger and is isolated, in tum, from the pumping system by 

a valve (V 4 ). The valve system can be heated above room 

temperature. Thermocouples are positioned strategically 

within the oven to monitor temperatures via a digital 

thermometer (11KC, Omega Engineering, Inc., Stamford, 

Conn.). 

The sensitivity of the capacitance manometer for 

determining absolute pressures with extreme linearity has 

been described by Sommer and others (1985). The capacitance manometer employed has a bakable unit with a 1 to 

10- 6 torr range (315BHS Baratron, MKS Instruments, 

Burlington, Mass.), housed in an individual oven that is 

operated via a control unit (MKS-273). Output from the 

sensor head is passed, via a nonbakable transducer, to a 

multiplexor unit (MKS-274) for temperature compensation 

and, hence, to a high-accuracy electronic display unit 

(MKS-270B). A 0- to 10-V output from the display unit can 

be connected to a chart recorder so pressure changes may be 

visualized more easily. 

Vacuum Pumping System 

In addition to the mass spectrometer pumps, an 

auxiliary pumping system is provided to evacuate the 

sample chamber after samples are loaded. The auxiliary 

pumping system allows a high vacuum to be maintained 
IONIZATION 

GAUGE 

MASS 

SPECTROHETER 

DIFFUSION 

PUHP 

FLEX ISECTION 

--, 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

: OVENS 

~----, I I 

I 

I 

I 

I 

I 

I 

CAPACITANCE 

HANOHETER 

SAHPLE 

CHAMBER 

Figure 2. Schematic diagram of the laser microprobe-mass spectrometer system. The laser enters the sample chamber 

through an optical window. Valves V1-V7 and components are discussed in the text. 

within portions of the system that are not on line to the mass 

spectrometer. An air-cooled oil-vapor diffusion pump 

(E040/55, Edwards High Vacuum, Crawley, England) 

backed by a rotary pump (E2M2, Edwards) provides the 

vacuum, while a pipeline valve (fig. 2, V5) (PVK40, 

Edwards) and a backing/roughing valve (V 6) (BRVlO, 

Edwards) enable the diffusion pump to be isolated during 

initial pump down. Ionization and thermocouple gauges, 

operating from a controller (Type 843, Varian, Lexington, 

Mass.), monitor the vacuum. Another Nupro valve (V7) can 

be used to admit purified air to the system for venting. 

Laser Microprobe 

The microprobe employed for gas extraction is an 

industrial-grade laser system that is normally used for 

excitation in spectrochemical analysis (Laser Mark III, 

Jarrell-Ash, Waltham, Mass.). A pulse of 1.06-f.J.-m light is 

generated by a Q-switched xenon-pumped neodymium 

glass laser and directed onto the sample by means of a 

trinocular metallurgical microscope (K16-10-801, American Optical Scientific Instruments, Buffalo, N.Y.). The 

energy output of the laser pulse is controllable in the range 

0.1 to 1 J. There are five aperture settings available for the 

mechanical adjustment of the final diameter of the laser 

beam. Beam diameters range from 10 to 200 f.J.-m. A 35-mm 

camera is permanently mounted on the microscope to 

document analyses, while a Polaroid camera can be interchanged with one of the eyepieces to take "instant working 

pictures" of the sample under examination. 

The microscope objectives used must be "air-spaced" 

objectives. Should ordinary microscope objectives that are 

not air spaced be used, catastropic rupture of the lens within 

the objectives can occur. Caution must be observed with the 

objectives during the pulsing of the laser, or serious injury 

might occur to the system's operators if the objectives 

"explode" and spray glass throughout the laboratory. The 

use of air-spaced objectives, however, has several disadvantages for the laser-microprobe studies. Magnification 

laser Microprobe-Quadrupole Mass Spectrometer System 37 
VCR 

FITTING 

SAMPLE CHIP 

cu 

GASKET 

SAMPLE 

STAND 

Figure 3. Diagram of the sample chamber used with the laser microprobe. 

levels above 15 x cannot be obtained when air-spaced 

objectives are used (no known commercial source exists for 

such objectives). If higher magnification objectives were 

available, the working distance would be so small that 

focusing on the sample inside a sample chamber through an 

optical window could not be achieved. Should the laser be 

accidentally focused on the optical window and fired, a 

rupture of the sample chamber's window may occur, and 

glass might be blown around the laboratory. Use of any 

industrial-grade laser for sample studies requires extreme 

caution, and only qualified operators should use such 

equipment. 

Sample Chamber 

The sample chamber (fig. 3) is constructed from three 

stainless steel, copper-gasketed vacuum flanges (MDC, 

Hayward, Calif.) 33 mm in diameter and 6 mm thick that 

are attached to the mechanical stage of the laser microprobe. A 60-mm length of 6-mm-diameter stainless steel 

38 New Frontiers in Stable Isotopic Research 

tubing connects the central flange to the valve systems by 

means of a VCR fitting (Cajon). A length of flexible tubing 

on the valve system allows for movement of the sample 

chamber under the laser-probe microscope. The top flange 

of the chamber has a quartz window (optical grade) 20 mm 

in diameter sealed to it, through which the sample may be 

viewed. The optical window must be constructed of materials that permit essentially all of the laser's energy to pass 

without any absorption. Optical-grade quartz or sapphire is 

the typical window material used for the 1.06-f.,Lm wavelengths obtained with neodymium glass lasers. Other window materials may be required for different wavelengths 

available with other types of lasers. The bottom flange may 

also have a window or may be all metal, depending upon 

whether transmitted or reflected light is used to illuminate, 

respectively, thin sections or optically thick samples. If a 

second windowed flange is used, a spacer is required to 

protect the glass from the microscope stage. The chamber 

and connecting tubing ~ay be wrapped with heating tape if 

required. 
Sample chips are supported in the chamber in a "cup" 

in the end of a piece of brass rod 12 mm in diameter. If 

required, the sample may be embedded in a low-vaporpressure epoxy resin (for example, TorrSeal, Varian, Palo 

Alto, Calif.). Thin sections are supported on a drilledthrough stand. Details of the preparation of thin sections 

have been given previously (Sommer and others, 1985). 

In order to stop the inside of the optical window from 

being coated with material sprayed upward from the sample 

during the laser's pulse, a shield can be employed. A 

microscope slide's cover slip can be stationed immediately 

below the optical window of the sample chamber to receive 

the sample spray. After numerous laser pulses, the cover 

slip will be coated with material and should be changed. If 

material is allowed to be deposited on the optical window of 

the sample chamber, the laser's energy will be dissipated 

and difficulty will develop in observing and focusing on the 

sample. Light for illumination of samples is provided by 

incandescent sources via fiber optic cables and lenses, and 

various filters and polarizers may be used as required. 

Procedures for Analysis 

Preparation procedures for the samples depend 

largely on the nature of the sample and the type of analysis 

to be undertaken. Thin sections are prepared by extensive 

polishing followed by decontamination (Sommer and others, 1985), while rock fragments may be analyzed with 

essentially no pretreatment, as is the case with most samples 

described here. For thin-section studies, sections as thick as 

100 to 150 JJ.m are preferred so that the laser beam does not 

drill through the sample and into the embedding medium. 

Our studies have shown that, should the laser beam penetrate into the embedding medium (epoxy), molecular species are released that make the interpretation of the mass 

spectra difficult. It is possible to subtract spectra of the 

embedding medium from the sample spectra (should the 

beam penetrate through the thin section) to give a "corrected" spectrum, although this is not completely satisfactory. 

In order to characterize the interactions of the laser 

probe's 1.06-J..t.m wavelength beam with geologic materials, 

known minerals have been used as standards. Where 

possible, single crystals about 2 mm across have been used. 

In some cases, clusters of smaller crystals have been 

employed. Samples were embedded in vacuum sealant in 

the brass sample holder and left for several hours while the 

epoxy hardened. The sample holder was placed in the 

vacuum chamber, which was then sealed by being bolted to 

the microscope stage. The sample chamber was connected 

to the vacuum system with the VCR fitting, and heating 

tape was wrapped around exposed parts of the system. The 

vacuum chamber was allowed to pump down for about 30 

min, via the auxiliary pumping system, to a pressure of 

about 10- 5 torr before being left pumping through the mass 

spectrometer overnight. 

Samples were viewed through the microscope, and 

the areas chosen for analysis were centered under the laser's 

focal point by movement of the stage. Mass spectrometer 

acquisition parameters were set via software, normally 

scanning the mass to charge (m/z) range of 3 to 100 at 2Hz 

with the electron multiplier voltage set at 2,000 V. Background spectra were obtained for a short time (30 s), and 

then the laser was fired. Gases produced in the chamber by 

the laser's interaction with the sample expanded directly 

into the mass spectrometer to be ionized and detected. Once 

the gases had been pumped away, more background spectra 

were obtained while the sample was repositioned and the 

microscope refocused. The laser was then fired again. The 

laser could be fired every 15 s once its capacitor bank had 

been recharged. However, with repositioning the sample 

and refocusing, the time interval between laser shots was 

typically 1 min. 

Upon completion of data acquisition, the editing 

software was used to examine the analytical results. Average background spectra were calculated from mass scans 

preceding the laser shot and subtracted from the spectra of 

sample gases. Individual ion profiles (ion abundances 

versus time) were examined to establish whether lowabundance species were genuinely released by the laser shot 

or whether they could be accounted for by variations in the 

system's background. 

DISCUSSION AND RESULTS OF 

EXPERIMENTATION 

Types of lasers 

Depending upon the type of lasing system, lasers can 

be operated in either a continuous or a pulsed mode. Pulsed 

lasers having narrow pulse widths offer the best possibility 

for sample decrepitation or gas release from solid samples, 

since they give a very rapid release of the gases, and 

minimal interaction of the volatiles with the laser's beam 

occurs. Some of the different types of lasers available for 

use are listed in table 1. Lasers occur in a wide variety and 

have many different characteristics. The wavelength and 

operating parameters are characteristic to each type. The 

wavelengths of lasers available range from the ultraviolet 

through the visible and near infrared out to many hundreds 

of micrometers for some devices in the far infrared. 

Characteristics of some of the different types of lasers 

available are shown in table 1, which also demonstrates that 

not all the optimum characteristics can be obtained in a 

single laser. For example, to obtain highest peak pulse 

power, one may have to sacrifice some other optimum 

laser Microprobe-Quadrupole Mass Spectrometer System 39 
Table 1. Laser characteristics (after Ready (1971) and Lasers and Applications (1986)) 

[-,unknown; CW, continuous wavelength] 

Peak pulse 

Wavelength CW power power 

Laser (J..Lm) (W) (W) 

He-Ne 0.6328 0.001-0.1 

Ar+ .4880 1-10 20 

.5145 

Arion .457 .002-25 25 

.530 

Kr+ .6471 .5 2 

.5681 

.5208 

.4762 

C02 10.6 10-5,000 105 

Nz .3371 not CW 105 

Ruby .6943 1 to 106 

Nd glass 1.06 to 4 x 109 

Nd YAG 1.06 1-250 to 105 

GaAs .84-.9 1-10 1-100 

Excimer .157 .4-250 

.193 

.222 

.248 

.308 

.351 

properties, such as high beam divergence angle or high 

pulse repetition rate. 

An example of the variability of the effects of laser 

interaction with samples has been seen with a Q-switched 

neodymium glass laser and a neodymium glass power laser 

(Franchi and others, 1986; this volume; unpub. data, 1988). 

The Q-switched laser operating at 1 J energy output produced craters in calcite ranging up to 200 IJ.m in size, 

whereas the power laser operating at the same energy output 

produced only a surface discoloration with the same samples. The Q-switched laser delivered the same amount of 

energy at the sample surface in a shorter time interval than 

the power laser. A summary of specifications that can be 

useful for determining the type of laser to be chosen for a 

particular study is given in table 2. It is obvious that the 

mode of laser operation, the power, pulse length, and 

repetition rates are important parameters that must be 

considered. 

laser Interactions with Geologic Samples 

The interaction of laser beams with geologic materials needs to be researched more in order for a full understanding of what happens to the sample to be gained. The 

absorption or transmission of radiation by the target materials determines the type of process (ablation, mechanical 

disruption, cratering, or melting) that occurs within the 

sample. For example, if the target is pure optical-quality 

40 New Frontiers in Stable Isotopic Research 

Pulse 

Pulse energy rep. rate Beam angle 

(J) (pulses/s) (mrad) 

rarely used pulsed 0.2-1 

mJ 60 .5-1 

Continuous .4-2.68 

mJ 60 .5-1 

0.1-5 ~100 ~1-4 

1-100 2 X 20 

1-500 60 1-10 

1-5000 1-10 .5-10 

.01-1 to 10,000 2-20 

<1 mJ 100-1,000 20 X 400 

.02-1,500 l/6o-750 .05-10 

quartz, the laser beam (from a neodymium glass laser) will 

be transmitted through the material and leave no noticeable 

effects on the sample. However, should the quartz contain 

microscopic dislocations or defects, the portion of the 

radiation absorbed by the target would cause mechanical 

disruption of the surface of the sample because of fracturing 

or spalling in the region where the beam impinges on the 

sample. An example of such an interaction can be seen in 

figure 4A. Alternatively, if radiation from the laser beam is 

strongly absorbed within the sample, melting can occur. 

Olivine is a silicate that has a large mafic component that 

strongly absorbs the 1.06-~J.m wavelength from a neodymium glass laser and thus produces extensive melting (fig. 

4B). 

In order to understand the effects of lasers on geologic samples, a program to characterize laser interactions 

with various materials should be carried out, especially for 

those materials and laser energies in which the proposed 

studies are to be made. Examples of the neodymium glass 

laser (operating at 0.4 J) interactions with a variety of 

minerals can be seen in figure 5. The laser interactions 

produce fracturing and spalling with the quartz samples. 

With carbonates, the laser pits vary, depending upon the 

number of absorbing components present. With calcite (fig. 

5A), very slight melting may occur, whereas with dolomite 

(fig. 5B), greater melting occurs. In the case of barite (fig. 

5C), no noticeable melting occurs in the target area. Mafic 

minerals such as olivine typically absorb the laser energy, 

so melting within the laser pit results. However, should 
Table 2. Lasers useful for high-power effects (after Ready (1971) and Lasers and Applications (1986)) 

Pulse 

Power repetition 

Laser Mode of operation (W) Pulse length rate 

Ruby Normal pulse 2 x 105 peak 0.2-10 ms Low 

Ruby Q-switched 109 5-30 ns Low 

Ruby Picosecond pulse High 10 ps Low 

Nd glass Normal pulse ~106 peak .5-10 ms Low 

Nd glass Q-switched to 4 x 109 peak 6-50 ns Low 

Nd glass Picosecond pulse to 1012 peak 10 ps Low 

Nd YAG Continuous to 250 

Nd YAG Continuously pumped, 104 peak, 10 average 200 ns to 5,000 pulses/s 

repetitively Q-switched 

C02 Continuous 10-5,000 

C02 Repetitively pulsed 105 peak, 100 average 10-100 !J-S 100 pulses/s 

C02 Repetitively Q-switched 105 peak, 1.5 average 200 ns 400 pulses/s 

Arion Continuous 

Excimer Normal pulse 

A 

Figure 4. Scanning electron microscope photographs of 

craters obtained by use of a Q-switched neodymium glass 

laser operating at 0.4-J energy level. Scale bars are 100 IJ-m. 

A, Laser-pitted quartz sample showing evidence of 

minerals contain numerous fractures and dislocations, melting along with fracturing results (figs. 5A,C). Sulfide 

minerals such as pyrite show extensive melting within the 

laser pit (fig. 5D). 

The laser is extremely selective, but care must be 

used to avoid a significant contribution of volatiles from 

omnipresent microinclusions and other contaminants. Even 

in minerals that do not show significant melting (quartz, 

calcite, and so on), the laser can heat the surrounding 

minerals, and decrepitation of other inclusions or release of 

volatiles from the crystal (in sites other than fluid inclusions) would seem likely. To avoid this, exercise caution 

to 25 

to 250 .03-80 to 750 pulses/s 

8 

mechanical spalling and disruption of the target. B, Laserpitted olivine sample showing extensive evidence of melting of the silicates because of absorption of the laser's 

energy. 

and determine "inclusion-free" mineral blanks for every 

sample. 

Carbonates 

To establish the level of reproducibility of the laserextraction technique, a single crystal of azurite 

(2CuC03 ·Cu(OHh) was subjected to repeated laser shots. 

A ratio of the peak area of an individual species to the peak 

area of all species in a scan (total ion current) appears to 

have less scatter than the areas of a single mass observed 

after replicate shots into a material. The reproducibility of 

gas abundances released from the laser ablation of the 

laser Microprobe-Quadrupole Mass Spectrometer System 41 
A 

c 

Figure 5. Scanning electron microscope photographs of 

laser pits in a variety of minerals. Scale bar in photographs 

is 100 f.Lm. The operating power of the Q-switched neodymium glass laser used to produced the laser pits was 0.4 j. 

A, Calcite target showing evidence of melting in the 

bottom of the crater. Disruption and fracturing of the 

calcite appears to be assisted by the crystallographic 

crystal is usually within 10 to 15 percent for replicate 

analysis. This degree of reproducibility might be expected, 

considering possible differences within a naturally formed 

crystal and slight variations in the crystal orientation with 

respect to incident radiation. 

Typical spectra from the laser-microprobe analysis of 

four different carbonates-calcite (CaC03), dolomite 

(CaC03 ·MgC03), magnesite (MgC03), and azurite-are 

given in figure 6. The four carbonates were analyzed by use 

of the same identical laser power settings; thus, the analysis 

conditions were identical. In all cases, the most abundant 

species occur at m/z values of 44 and 28, which correspond 

42 New Frontiers in Stable Isotopic Research 

8 

D 

boundaries of the target. B, Dolomite target showing a 

greater degree of melting because of absorption of the 

laser's energy. C, Barite target showing evidence of fracture of the crystal. D, Pyrite target showing extensive 

melting of the crystal because of absorption of the laser's 

energy. Peaks have been produced in the center of the 

crater. 

to the ions C02 + and CO+ . The ratios of these two ions in 

the different spectra vary, however, with m/z of CO being 

more abundant than that of C02 in the calcite and dolomite 

analyses, while the reverse is true of the azurite data. The 

CO and C02 ion abundances from the magnesite analysis 

are almost equal. With further detailed work, it may be 

possible to distinguish different carbonates by their lasermicroprobe-volatilized mass spectra. The variations of m/z 

28 to m/z 44 ratios are indeed interesting. It is clear that 

more work is needed to determine the relative contributions 

of the following two phenomena to the intensity of m/z 28: 

(1) the laser-produced CO from the carbonate and (2) the 
3.0E6 

ll 

u 2.0E6 c 

Ill 

"0 

c 

:l 

_g 

a: 1.0E6 

0 

A 

5.0E6 

4.0E6 

ll 

u 

c 3.0E6 Ill 

"0 

c 

:l 

_g 2.0E6 a: 

1. 0E6 

121 

8 

6.121E6 

5.121E6 

ll 4.121E6 0 

c 

Ill 

"0 

c 3.0E6 :l 

_g 

a: 2.121E6 

1 .121E6 

121 

c 

6.121E6 

5.121E6 

ll 

0 4.0E6 c 

Ill 

"0 

c 3.0E6 :l 

_g 

a: 2.0E6 

1. 0E6 

0 

D 

Scan 273 C2.411 min) of DATA:CALCITE.D 

,

1

k SUBTRACTED 

44 

""' 12 

( I ,II ,,,[W_ 50 70 79 .I I I I I 

10 20 30 40 50 60 70 80 Mass/Charge 

Scan 172 (1.589 min) of DATA:DOLOM.D 

/1 SUBTRACTED 28 

44 

~ 

16 

""-

IL .II 55 67 78 II .I ,I. I I I I 

1121 2121 3121 4121 5121 6121 7121 8121 Mass/Charge 

Sean 349 C3.12113 min) of DATA:MAGNES.D 

2~ SUBTR ~.CTED 44 

16 

I 12 

32 

93 

I 

90 100 

91 

I 

9121 1121121 

91 I Ill .II II ( 50 58 67 78 84 I I I I I / 1121 20 3121 40 5121 6121 70 8121 90 Mass/Charge 

Scan 31212 C2.577 min) of DATA:AZURITE.D 

SU 4~ TRflCTED 

28 

Figure 6. Background-subtracted I mass 

spectra produced by interactions of laser 

microprobe with four different carbonates: (A) calcite, (8) dolomite, (C) magnesite, and (D) azurite. Note the 

increase in m/z 44 between calcite and 

azurite. The laser's energy is sufficiently 

16 strong to disrupt the bonding in the 

"-.,_ carbonates and produce a major contri- ,L 70 84 97 55 

""' ""' ""' 

bution of CO (m/z 28). Laser operating II d I conditions are the same as those given in 10 20 30 40 50 60 70 80 90 100 Mass/Char e figure 5. 

Laser Microprobe-Quadrupole Mass Spectrometer System 43 
Sc a.n 108 ( 1 . 04 1 m;n) of' DATA:EET.D 

8 SUBTRACTED 

2.5E6 

2.0E6 I) 

u 44 c 

Ill 1. SE6 "-. 'tJ 

c 

:I 

..0 

a: 1. 0E6 

s.ee:s 

99 

67 80 

I I "-. 

0 

10 20 30 40 50 60 70 80 90 100 

Ma.ss/Ch a.r e 

Figure 7. Background-subtracted spectrum of gases released by laser study of alteration 

product associated with EETA79001 meteorite. Note that the relative abundance of the m/z 

28 and 44 peaks is identical to that seen from the calcite data in figure 6A. 

mass-spectrometer-ion-source-produced CO from the carbon dioxide released by the laser irradiation of the carbonate. Therefore, the variations in the data (fig. 6) from 

different carbonates may reflect differences in the laser 

interaction with different minerals, possibly being a function of the amount of energy absorbed by the target and the 

binding energies of the carbonate ions in the host mineral's 

lattices. 

Identification of Carbonate Weathering Product 

One of the meteorites found in Antarctica 

(EET A 79001) is believed, largely as a result of the discovery of trapped gases closely matching Viking spacecraft 

data (Bogard and Johnson, 1983; Becker and Pepin, 1984; 

Carr and others, 1985), to have originated on Mars. Interest 

has recently focused on the secondary minerals to see if 

Martian weathering products can be identified. White crystalline material surrounding glassy inclusions in the interior 

of EETA79001 had been tentatively identified as calcium 

carbonate on the basis of energy dispersive X-ray spectrometry (Wentworth and Gooding, 1986). As part of a detailed 

study of this material, the laser microprobe was used to 

determine the nature of the volatile species released from 

both the putative carbonate and exposed parts of the 

underlying groundmass. The major ions are CO and C02 

(fig. 7), consistent with the identification of the unknown 

material as carbonate. The proportions of these two ions are 

also similar to the calcite data in figure 6A. It was initially 

believed that the unknown carbonate might be a hydrocarbonate mineral; however, the spectra obtained failed to 

show the presence of m/z 18 that would be indicative of the 

presence of a hydrated mineral. No gases could be detected 

44 New Frontiers in Stable Isotopic Research 

at these masses after the laser was fired into the groundmass 

glass, which was the host for the carbonate phase. The 

results showed that the alteration product was a simple 

carbonate and not a hydrocarbonate phase. 

Barite Study 

Samples of fluid-inclusion-free barite crystals were 

subjected to laser microprobe-gas analysis to determine 

what gases were released from barite. Figure SA shows the 

total ion current (TIC) versus time profile from the analysis 

of a single crystal of barite (BaS04). The three peaks in the 

profile are of different sizes (despite being produced with 

the same laser power settings) and show the different 

quantities of gases within three different inclusions within 

the barite crystal. Figure SB is the background-subtracted 

spectrum from the final shot at 1.5 min after the start of data 

acquisition. Sulfur-bearing species are observed at m/z 

values of 64 (S02 +), 48 (SO+), and 32 (S+ or 0 2 +), 

together with abundant m/z values for 28 and 44, presumably from trapped CO and C02 or coexisting carbonate 

within the barite along with m/z value of 18 from H20+. 

Minor peaks at m/z values of 60 and 76 indicate the likely 

presence of COS and CS2 , while peaks possibly from the 

less abundant 34S-isotope species can also be observed at 

m/z values of 34, 50, and 66. 

Volatiles in 3.4-b.y.-old North Pole Barites 

Examination of fluid inclusions within the 3.4-b.y.-

old barites from the North Pole deposit from the Warrawoona Group of Northwestern Australia reveal a complex 

morphology and array of fluid inclusions. This study, along 
A 

B 

5.121E6 

4.121E6 

u 

0 

c: 3.121E6 Ill 

"'0 

c: 

:I 

.ll 2.121E6 a: 

1 .121E6-

121 

1. 2E6 

1. 0E6 

Q) 

0 8.121E5 c: 

Ill 

"'0 

- c: 6.121E5 :I 

..c 

a: 

4.121E5 

2.121E5 

121 

-

TIC of DRTR:BRRITE.D 

121.5 1 • 121 

Time (min. ) 

1. 5 

Scan 186 (1.526 min) of DATA:BARITE.D 

(;~ SUBTRACTED 

64 

I 

32 

I 

48 

~ 

18 

~ 44 

60 

I I I II I I ~ . 2121 30 40 5121 60 

Mass/Charge 

66 

( 76 

I 

I 

70 

Figure 8. Laser microprobe-gas analysis study of barites. A, Total ion current (TIC) versus 

time profile for the analysis of a single barite crystal. The three peaks represent the three 

laser shots. B, Background-subtracted spectrum for the final laser shot. In addition to the 

sulfur ions (m/z 64, 48, 32) evidence of trapped carbonate within the barite can be seen 

from the m/z 44 and 28 ions. Other values are 18, H20+; 60, COS; 76, CS2 ; 34, 50, and 66, 

the less abundant 345-isotope species. 

with previous work (Rankin and Shepherd, 1978; Bourcier 

and others, 1984), has identified at least four generations of 

inclusions. The inclusion types are (I) large (20-40 f..Lm) 

intergrain inclusions having variable-sized vapor bubbles 

with one, two, or three phases, but which may also include 

carbon dioxide fluids or even a single phase; (II) smaller 

(10-30 f..Lm) inclusions concentrated near grain boundaries 

containing three, or less commonly, two, vapor phases, 

aqueous and carbon-dioxide liquid, and a pale yellow 

daughter crystal; (III) small (1-5 f..Lm) highly reflective 

carbon-dioxide-rich secondary and other small daughter 

crystal-bearing fluid inclusions; and (IV) very small (<1-5 

f..Lm), numerous aqueous inclusions evenly distributed 

throughout the grain interiors. Inclusions distributed within 

the interior of large grains are types I, III, and IV, with type 

II inclusions in or near grain boundaries. It has been 

assumed (Bourcier and others, 1984) that the type I and IV 

inclusions primarily relate to the formation of the barite and 

that the type II and III inclusions are secondary or were 

produced during later tectonic events that include a lower 

greenschist regional metamorphism about 2.95 b.y. ago 

(Hickman, 1981) and an event about 2.7 b.y. ago (Marston 

Laser Microprobe-Quadrupole Mass Spectrometer System 45 
and Grover, 1981). While the morphological distinctions of 

the inclusions are readily apparent, any attempt to correlate 

these inclusion types with primary mineral formation or 

secondary mineralization has been hampered by a lack of 

compositional data on volatiles from the individual 

inclusions. 

Laser-microprobe and bulk (vacuum crushing) determinations were made of released volatiles from the fluid 

inclusions. Standard minerals yield reproducibilities of 

about ± 5 percent. Minimum detection limits were found to 

be about 10 -ll mol of gas. 

Individual inclusions of types I, II, and III were 

analyzed by the laser microprobe (type IV inclusions were 

not analyzed because of their small size). Figure 9 is a 

C-H-0 elemental plot of the data with the CO, C02 , CH4 , 

and H20 fields indicated. (This elemental plot is used to 

avoid the uncertainties of expressing individual volatile 

species after extraction and mass-spectrometric analysis.) 

The type I large intergrain inclusions yield C-0 compositions of about 18, 40, and 42 percent, respectively (fig. 9). 

Only a slight trace of S02 was detected in type I inclusions. 

Type II and III (grain boundary and highly reflective 

intergrain inclusions) yield C-H-0 compositions of about 

25, 30, and 45 percent, respectively (fig. 9). These inclusions typically have S02 and H2S present in their spectra. 

For comparison, the C-H-0 of the bulk inclusions 

(obtained by crushing) yield compositions of 3, 63, and 34 

percent, respectively. 

The results indicate that the type I inclusions have 

volatile compositions distinct from the type II and III 

inclusions. In addition, the type I, II, and III inclusions are 

significantly richer in carbon and sulfur gases relative to the 

bulk inclusions (assumed to be predominantly type IV). 

Type I, II, and III inclusions are enriched in carbon phases 

and require that some form of decarbonation occurred 

during the various metamorphic events that have affected 

these rocks. The presence of both H2S and S02 indicates 

that some inclusions have undergone volatile loss or addition since entrapment or during the replacement of gypsum 

by barite. The bulk-inclusion composition indicates a large 

aqueous contribution that cannot at this time be related to 

either primary or secondary mineral genesis. Addition of 

water may have occurred during some later process. 

Sulfides 

Several sulfide minerals have been subjected to the 

laser microprobe-mass spectrometer study in order to determine the gaseous species released during laser interactions. 

Figure 10 shows a spectrum from the analysis of pyrite 

(FeS2). The main species observed is CS2 , the carbon 

possibly originating as the ubiquitous organic contamination found on all kinds of samples (Carr and others, 1985). 

Other abundant peaks can be attributed to H20, CO, S, 

46 New Frontiers in Stable Isotopic Research 

c 

C-H-0 Plot 

GR - Grain Boundary 

I - Interior Primary 

Figure 9. C-H-0 plot of gases released from 3.4-b.y.-old 

barite from North Pole deposit from the Warrawoona 

Group of Northwestern Australia. The CO, C02 , CH4 , and 

H20 fields are indicated. GR, Rl, and I refer to types II, Ill, 

and I fluid inclusions, respectively, discussed in the text. B 

refers to fluid-inclusion determinations made from bulk 

crushing of the barite. Only type I inclusions are believed 

to be primary. Note that the bulk crushing of the barite 

released volatiles having substantially different characteristics than the primary inclusions that could be selectively 

opened with the laser microprobe. 

C02 , SO, and S02 . Several sulfide minerals, including 

galena (PbS), cinnabar (HgS), chalcocite (CuS2), chalcopyrite (CuFeS2), and sphalerite (ZnS), have been studied, and 

all show the formation of species similar to those observed 

for pyrite. 

Diamonds 

The genesis of diamond is receiving increasing attention because an understanding of how and where they are 

formed could provide information on mantle processes and 

the composition of the early Earth. The volatile species 

trapped in inclusions in some diamonds may give vital clues 

in this regard (Bibby, 1982). Experiments where diamonds 

have been crushed have, apparently, shown the presence of 

methane in many samples (Roedder, 1984), but it is known 

that methane can be formed in crushing devices from the 

action of one metallic surface against another (Gibson and 

Andrawes, 1978). Thus, controversy still surrounds the 

validity of some of these results. Laser extraction of the 

gases is potentially a very clean technique; hence, using the 

laser probe to attempt to fracture or cleave a diamond might 

yield important information on the genesis of diamonds. 

A type I diamond from the Koidu kimberlite formation in Sierra Leone, Africa, containing two graphite 
Scan 243 (2.143 min) of DATA:PYRITE.D 

SUBTRACTED 6 

5.0E6 

4.0E6 GJ 

0 

I: 

«< 3.0E6 "'C 

I: 

:J 

..a 32 

a: 2.0E6 I 

28 44 

18 

""' 

I 

1. 0E6 I 

97 

84 \ 0 I 

10 20 30 40 50 60 70 80 90 

Mass/Charge 

Figure 10. Background-subtracted spectrum of gas released from laser interactions with 

pyrite. Carbon disulfide (CS2) is the major volatile released from the sulfide. The carbon 

source is from residual carbon species present within the vacuum system. 

Scan 214 (1.904 min) of DATA:DIAMOND.D 

2.0E6 9 SUBTRACTED 

1. 8E6 

1. 6E6 

GJ 1. 4E6 

0 

I: 1. 2E6 «< 

"'C 1. 0E6 I: 

:J 

..a 8.0E5 a: 

6.0E5 50 

I 4.0E5 

2.0E5 64 78 91 

I I I 

0 

10 20 30 40 50 60 70 80 90 100 

Mass/Charge 

Figure 11. Background-subtracted spectrum of gases released from type I diamond during 

cleaving of diamond in area of graphite inclusion. Carbon monoxide and carbon dioxide 

were major gases released upon cleaving diamond. The m/z 50 ions (CF2 ) result from the 

previous HF etch of the sample to remove adhering silicates. Note the absence of a peak 

at m/z 14 (N+). 

inclusions, was used for the initial experiment. Focusing on 

the surface of the diamond produced only a very shallow 

crater (approximately 30 J.Lm in size), and no gases could be 

detected by the mass spectrometer. When the laser was 

directed at one of the inclusions, the diamond fractured and 

gases were released. Figure 11 shows a backgroundcorrected spectrum. The main peak can be identified as CO, 

since daughter rn/z 12 (C+) and 16 (0+) peaks are present, 

but there is no rnlz 14 peak (N+), which would be present 

if the rn/z 28 peak was caused by N2 . Franchi and others 

(this volume) noted the presence of nitrogen in diamond. 

Our study of gases within diamonds was related only to the 

voids or inclusions in the diamond and did not measure 

nitrogen dissolved within the crystal structure of diamond. 

Franchi and others' study involved oxygen combustion of 

the diamond that resulted in the total destruction of the 

crystal. It is understandable that we did not note the 

presence of nitrogen within the inclusions that were opened. 

Laser Microprobe-Quadrupole Mass Spectrometer System 47 
The peaks at rn/z values 24 to 26 may be due to C2 + 

species, and the small peak at rn/z 44 is most likely from 

traces of C02 . No methane was detected, as revealed by the 

absence of a rn/z 15 peak (rn/z 15 is the major peak 

produced by methane within the mass spectrometer). An 

unusually large peak at rn/z 50 is present in the spectrum. 

However, this diamond had been treated with hydrofluoric 

acid to remove adhering silicates; thus, the rn/z 50 contribution results from CF2 • Laser-microprobe studies of additional diamonds that had not been treated with HF acid 

failed to show the presence of rn/z 50. Further work is 

necessary to determine gas populations of diamonds from a 

variety of occurrences, but the successful laser fracturing of 

this sample indicates that detailed analyses of trapped 

volatiles in diamonds will be possible in the future. 

SUMMARY 

The laser microprobe-quadrupole mass spectrometer 

analyzer has been successfully used to analyze volatiles and 

trapped fluids and vapors in a wide variety of materials. The 

microfocusing capabilities of the laser microprobe, coupled 

with a quadrupole mass spectrometer, result in a powerful 

analytical tool for analysis of volatiles in materials. This 

technique has been applied to a variety of geologically 

interesting materials and also has applications for the study 

of volatiles associated with synthetic materials. The laser 

microprobe-quadrupole mass spectrometer analysis technique is an important weapon in the arsenal of methods 

available for studying the abundances and distributions of 

low-molecular-weight compounds in small samples. 

ACKNOWLEDGMENTS 

Samples were provided by James L. Gooding, 

Stephen Haggerty, and the American Museum of Natural 

History, New York. Special thanks go to William Davidson 

(LEMSCO) for skilled technical assistance. Robert H. Carr 

was an NRC Research Associate at the NASA Johnson 

Space Center. Funding for this work was from NASA's 

Planetary Materials and Planetary Biology Program 

Offices. 

REFERENCES CITED 

Becker, R.H., and Pepin, R.O., 1984, The case for a martian 

origin of the shergottites: Nitrogen and noble gases in 

EET A 7900 1: Earth and Planetary Science Letters, v. 69, p. 

225-242. 

Bibby, D.M., 1982, Impurities in natural diamond: Chemistry and 

Physics of Carbon, v. 18, p. 1-91. 

48 New Frontiers in Stable Isotopic Research 

Bogard, D.O., and Johnson, P., 1983, Martian gases in an 

Antarctic meteorite?: Science, v. 221, p. 651-654. 

Bourcier, W.L., Gibson, E.K., Jr., and Kotra, R.K., 1984, 

Characterization and gas analysis of fluid inclusions in 

Archean barites from the Warrawoona Group, Northwestern 

Australia: Abstracts of Papers Submitted to the Lunar and 

Planetary Science Conference, no. 15, March 12-16, Houston, Texas, p. 82-83. 

Carr, R.H., Grady, M.M., Wright, I.P., and Fillinger, C.T., 

1985, Martian atmospheric carbon dioxide and weathering 

products in SNC meteorites: Nature, v. 314, p. 248-250. 

Franchi, I.A., Wright, I.P., Gibson, E.K., Jr., and Fillinger, 

C. T. , 1986, The laser microprobe: A technique for extracting 

carbon, nitrogen and oxygen from solid samples for isotopic 

measurements: Proceedings of the 16th Lunar and Planetary 

Science Conference, Journal of Geophysical Research, Supplement 91, no. B4, p. 0514--0524. 

Gibson, E.K., Jr., and Andrawes, F.F., 1978, Nature of the gases 

released from lunar rocks and soils upon crushing, in Merrill, 

R.B., ed., Lunar and planetary surfaces: Proceedings of the 

9th Lunar and Planetary Science Conference, v. 2, March 

13-17, Houston, Texas, p. 2433-2450. 

Harris, D.M., 1981, The microdetermination of H20, C02, and 

so2 in glass using a 1280 °C microscope vacuum heating 

stage, cryopumping, and vapor pressure measurements from 

77 to 273 K: Geochimica et Cosmochimica Acta, v. 45, p. 

2023-2036. 

Hickman, A.H., 1981, Crustal evolution of the Pilbara Block, 

Western Australia, in Glover, J.E., and Groves, D.I., eds., 

Archaean geology: Special Publication No. 7, Geological 

Society of Australia, p. 57-69. 

Kotra, R.K., and Gibson, E.K. Jr., 1982, Direct analysis of fluid 

and vapor inclusions using laser microprobe-gas chromatography: EOS, Transactions of the American Geophysical 

Union, v. 63, p. 450. 

Lasers and Applications, 1986, 1987 buying guide technical 

handbook and industry directory: Torrance, California, 

Lasers and Applications, 660 p. 

Marston, R.J., and Grover, D.I., 1981, The metallogenesis of 

Archaean base-metal deposits in Western Australia, in 

Glover, J.E., and Groves, D.I., eds., Archaean geology: 

Special Publication No. 7, Geological Society of Australia, 

p. 409-420. 

Rankin, A.H., and Shepherd, T.J., 1978, H2S-bearing fluid 

inclusions in baryte from the North Pole deposit, Western 

Australia: Mineralogical Magazine, v. 42, p. 408-410. 

Ready, J.F., 1971, Effects of high-power laser radiation: Academic Press, 433 p. 

Roedder, E., 1972, Composition of fluid inclusions, in Fleischer, 

M., ed., Data of geochemistry, 6th ed.: U.S. Geological 

Survey Professional Paper 440-JJ, p. JJ1-JJ164. 

---1984, Fluid inclusions: Reviews in Mineralogy, v. 12, 644 

p. 

Roedder, E., and Bodnar, R.J., 1980, Geologic pressure determinations from fluid inclusion studies: Annual Reviews of Earth 

and Planetary Science, v. 8, p. 263-301. 

Sommer, M.A., and Schramm, L.S., 1983, An analysis of the 

water concentrations in silicate melt inclusions in quartz 

phenocrysts from the Bandelier Tuff, Jemez Mountains, New 

Mexico: Bulletin of Volcanology, v. 46, p. 299-320. 
Sommer, M.A., Yonover, R.N., Bourcier, W.L., and Gibson, 

E.K., Jr., 1985, Determination of H20 and C02 concentrations in fluid inclusions in minerals using laser decrepitation 

and capacitance manometer analysis: Analytical Chemistry, 

V. 57, p. 449-453. 

Tsui, T. F. , and Holland, H. D. , 1979, The analysis of fluid 

inclusions by laser microprobe: Economic Geology, v. 74, p. 

1647-1653. 

Wentworth, S.J., and Gooding, J.L., 1986, Shergottite 

EET A 79001: Petrologic heterogeneity and secondary alteration in lithology C: Meteoritics, v. 21, p. 536-537. 

York, D., Hall, C.M., Yanase, Y., Hanes, J.A., and Kenyon, 

W. J. , 1981 , 40 Ar/39 Ar dating of terrestrial minerals with a 

continuous laser: Geophysical Research Letters, v. 11 , p. 

1136-1138. 

laser Microprobe-Quadrupole Mass Spectrometer System 49 

Application of Lasers in Small-Sample 

Stable Isotopic Analysis 

By I.A. Franchi, 1 S.R. Boyd,1 I.P. Wright,1 and C.T. Pillinger1 

Abstract 

Two techniques, both employing lasers, have been 

developed to sample geological materials for lightelement stable isotopic analysis. Both techniques offer 

advantages in sample characterization and spatial resolution over conventional techniques. The first technique is 

laser sectioning and is primarily limited to diamonds. Thin 

wafers of diamond are precisely sectioned into ~100-

microgram blocks that can then be analyzed on a high 

sensitivity mass spectrometer. This technique has been 

successfully used to investigate isotopic variation of carbon and nitrogen, as well as nitrogen concentration, in 

coated diamonds. 

The second technique involves a laser microprobe 

that is used to melt or vaporize small areas ( <100 micrometers in diameter) of a sample to liberate gases that can 

then be analyzed in the mass spectrometer. This technique has been evaluated for determining the isotopic 

composition of carbon, nitrogen, and oxygen. The results 

have shown that useful o15N measurements can be 

obtained with the laser microprobe and that o13C values 

can be determined to within ±3 per mil. However, 8180 

measurements are currently unreliable. 

A laser microprobe study of the meteoritic polymict 

breccia Bencubbin confirms that isotopically heavy nitrogen (o15N=995 per mil) is distributed throughout metal 

and silicate clast and matrix and thus must have been 

incorporated prior to brecciation. 

INTRODUCTION 

The complex evolution and polymineralic nature of 

many geological materials result in a heterogeneous elemental, and often isotopic, distribution throughout a sample, and it is well known that information on this internal 

variation can reveal much about the origin and history of a 

rock. Although individual components can be separated 

from a sample and analyzed by conventional whole-rock 

techniques, this is a time-consuming procedure requiring 

relatively large amounts of sample. Better characterization 

of the sample can be achieved if components can be 

analyzed in situ. To this end, a number of analytical 

techniques have been developed for geological studies, such 

as the electron microprobe for elemental abundance determinations, the ion probe, which has the added capability of 

1 Planetary Sciences Unit, Department of Earth Sciences, The Open 

University, Milton Keynes, MK7 6AA, England. 

determining isotopic composttlons, and the laser microprobe for the study of noble gases. However, the application of such techniques to the study of the light elements 

(primarily H, C, N, and 0) is currently rather limited 

because of analytical problems. 

The determination of C, N, and 0 elemental abundances by use of the electron microprobe suffers from 

problems associated with the detection of weak secondary 

X rays, while isotopic analyses with the ion probe suffer 

from problems of resolving interfering species; because of 

these problems, both methods result in large uncertainties. 

In the case of nitrogen, the ion probe also suffers from the 

additional problem in that it is necessary to analyze nitrogen 

as CN-, the result being that only samples containing 

appreciable quantities of carbon can be studied (Zinner and 

others, 1987). 

The large gains in sensitivity that have been achieved 

with the use of static-vacuum mass spectrometry in the 

analysis of the light elements (Carr and others, 1986; 

Wright and others, 1988; Wright and Pillinger, this volume) 

have allowed the development of two techniques (both 

employing lasers) that offer good sample characterization 

and spatial resolution for stable isotopic measurements of 

C, N, and 0. In the first technique, a laser is used to dissect 

a sample into small blocks that can then be analyzed by 

conventional techniques. In the second technique, a laser 

microprobe is used, one similar in design to that employed 

in the study of noble gases (Megrue, 1967; Plieninger and 

Schaeffer, 1976, Bohlke and others, this volume). 

LASER SECTIONING 

Natural diamonds can exhibit serial zoning with 

respect to carbon and nitrogen isotopic composition, nitrogen concentration, and nitrogen aggregation state (Boyd 

and others, 1987). The nitrogen in diamond may be present 

within a variety of structures (single nitrogen, nitrogen 

pairs, and higher aggregates) that give rise to characteristic 

absorption spectra in the infrared. Other structures that can 

develop in diamond, termed platelets, may also contain 

nitrogen (see Davies, 1984, for recent review). The causes 

of the zoning include multistage growth from isotopically 

distinct reservoirs and isotope fractionation processes operating within a single growth step. The zoning can be used to 

deduce a simple history for a given diamond sample. 

Synthetically produced diamonds can be sector zoned with 

Application of Lasers in Small-Sample Stable Isotopic Analysis 51 
~Ladder section" ''Chocolate block N 

Figure 1. Sketches showing laser sectioning of diamond 

plates. A plate (about 5X5X0.3 mm) is cut from the center 

of the stone by a diamond saw. This plate is then laser 

sectioned in one of two patterns to produce either a 

"ladder section" or a "chocolate block." 

respect to nitrogen concentration, nitrogen isotopic composition (Boyd and others, 1988a), and possibly carbon isotopic composition. This zoning is due to physical/chemical 

differences between the various growing facets. 

In order to study the growth zoning present within a 

diamond, the sample is first prepared in the following way. 

A plate, usually about 5 mm in diameter, 300 to 500 J.Lm 

thick is cut from the diamond by a diamond saw (fig. 1). 

When removed, the plate has a polished surface and is 

immediately amenable to study by both infrared (IR) and 

cathodoluminescence (CL) techniques, the latter being used 

to investigate zoning of impurities within the diamond. 

Both of these techniques can be used as a guide for the next 

stage of preparation of the plate, although for practical 

purposes the IR and CL techniques are usually applied after 

final preparation. The last stage of preparation involves 

mounting the plate upon a computer-controlled stage and 

dissecting it with a Q-switched 12 mW neodymium YAG 

laser. The two dissection patterns that have been most used 

("ladder sections" and "chocolate blocks") are indicated in 

figure 1 , and in both cases the sections are cut parallel to 

<111>. 

The block sizes from ladder sections are cubes about 

400 to 500 J.Lm on a side that weigh between 300 and 500 

J.Lg. During sectioning, each block is individually labeled by 

the laser to prevent misidentification once the samples are 

removed from the stage. The size of block ensures ease of 

handling. This type of preparation has been used for the 

analysis of natural diamonds where the growth zones are 

both serial and symmetrical. 

52 New Frontiers in Stable Isotopic Research 

Chocolate blocks are smaller than those from ladder 

sections, being typically 250 x 250 X 500 J.Lm and having 

a mass of about 100 J.Lg. Although it is possible to label the 

individual blocks with the laser, this practice has been 

abandoned because there is no way of knowing before the 

IR and CL results are obtained which blocks are going to be 

analyzed. Instead, as the cutting pattern is in the form of a 

grid, each block is mapped and given a unique grid 

reference. If CL and IR indicate that a single block will 

contain insufficient nitrogen for a precise nitrogen isotopic 

measurement, then adjacent blocks can be taken and a 

composite analyzed. This technique is applicable when the 

zoning within the diamond is either sectored or asymmetrical. Although the laser can cut smaller block sizes, cutting 

them this size is impractical, as even with the 250-J.Lm grid 

pattern, in excess of 400 blocks may result from a single 

diamond plate. 

Certain diamonds (termed "coated diamonds") consist of an octahedral "core" surrounded by a fibrous "coat" 

that may have cube faces (Kamiya and Lang, 1965). Figure 

2 shows the results obtained from a coated diamond (see 

Boyd and others, 1987) that was dissected into a ladder 

section of 500-J.Lm cubes. 813C values obtained from the 

coat (exterior) were between -6.7 and -7.1 ofoo, whereas 

the core of the diamond was lighter (813C = - 8. 7 o; oo). 

Nitrogen isotopic measurements were anticorrelated with 

carbon, the lowest 815N values being within the coat ( -3.9 

to -5.5 o; oo) and the highest occurring within the core 

( + 4. 7 to + 5 .1 o; oo). The coat contained between 1 , 180 and 

1,400 ppm nitrogen, present as A-feature nitrogen (pairs of 

atoms), whereas the core contained more nitrogen (1 ,640 to 

1,810 ppm), and, although the A-feature IR-absorption 

spectra dominated, platelet absorptions were also detected. 

THE LASER MICROPROBE 

The laser-microprobe technique employs a neodymium glass, non-Q-switched pulsed laser (maximum output 

5 J) to melt and (or) vaporize small areas of sample to 

liberate the gases of interest. The sample is loaded in a 

demountable stainless steel turret composed of two 70-mm 

flanges (fig. 3) connected to a glass extraction line (fig. 4). 

The uppermost flange contains a quartz window through 

which the sample can be viewed in reflected light and the 

laser beam focused on the sample surface (both with the 

same microscope). An arrangement of three flanges with 

quartz windows in the top and bottom flanges allows thick 

sections to be viewed in transmitted light. One important 

feature of the sample turret is the glass cover slip placed 

above the sample to protect the underside of the quartz 

window from molten material that can be ejected from 

laser-produced pits. The sample turret, complete with 

sample, is evacuated to 10-6 torr (normally 2-3 days) at 

::;300 °C. 
...... 

u 

~ 

c.o 

-6.0 

-7.0 

-8.0 

lSOO 

l(XX) 

soo 

0 

----<r--o---o--o- ~ 5 

--o-

~ -+- ........ -o-

- 0 -o--

........ 

--o-

-o- • ·-- -o- --o-~ -5 

- -- --- - - - -

Platelets 

r 

A-feature ' Nitrogen 

~ ~- -~ 

coat I core r coat 

5mm 

Figure 2. Typical results of &13C, &15N, and nitrogen concentration measurements from a ladder section of a 

coated diamond. The variation in the infrared (IR) absorption spectra characteristic of platelet and A-feature nitrogen (pairs of atoms) is also shown. Data from Boyd and 

others (1987). Open circle, &15N (right axis); filled circle, 

&13C (left axis). 

To extract gases, the sample turret is isolated from the 

pumps and the laser (already focused on the surface) is fired 

at the sample. Manipulation of the sample turret under the 

fixed laser beam is achieved by three micrometers that 

allow movement in the x, y, and z directions. Connection to 

the glass purification section is achieved through two 

stainless steel flexible sections and a pyrex glass to metal 

seal. The gases evolved from the laser-produced pits 

(roughly cylindrical in shape and typically 50 to 100 !J-m 

wide by up to 150 f.Lm deep) are purified by chemical 

processing, and interfering species are cryogenically separated before being analyzed in one of two online highsensitivity static-vacuum mass spectrometers, one dedicated 

to C02 (see Carr and others, 1986) and the other to nitrogen 

(see Wright and others, 1988). The laser-microprobe 

extraction technique has been evaluated for a number of 

different materials by Franchi and others (1986a). The 

following is a summary of the observations of that study and 

their main conclusions. 

Eyepiece c::::::::> 

Prism 

Microscope 

Objective 

Nd Glass 

Laser 

Sample 

Holder 

Figure 3. Schematic diagram of a sample turret, the laser, 

and the viewing optics. 

Carbon and Oxygen 

Carbonates would seemingly offer the least number 

of problems for the laser microprobe, given their high 

concentrations of carbon and oxygen and the fact that the 

carbon is already present in an oxidized form. However, 

experiments to date have not yielded results within the 

precision of the mass spectrometer. Figure 5 is a summary 

of the results from a number of laser-microprobe analyses of 

calcite relative to the true 813C and 8180 values of the 

sample. The carbon data are mostly within ±3 ofoo of the 

expected values, and with the exception of one obviously 

anomalous point, all the measured values are slightly light. 

This enrichment in 12C is probably due to an admixture of 

light carbon from either trapped atmospheric C02 (813C = 

-8 o/oo) and (or) organic material on or within the sample 

(813C = -25 ofoo) or from the system blank (813C = -30 

ofoo). Unfortunately, because of its variability, it was 

impossible to correct for the system blank. 

The 8180 value of the C02 liberated from the calcite 

is 10 to 25 ofoo lighter than the true value (fig. 5). The 

obvious sources of contaminating oxygen are atmospheric 

0 2 and C02 , but as can be seen from figure 5, it is 

impossible to produce the observed results from an admixture of sample and atmospheric gases. Another possibility is 

that the offset in 8180 values may be due to an effect 

normally observed when C02 is generated from carbonates 

by pyrolysis as a result of the variable fractionation 

observed when only two of the three oxygen atoms in the 

carbonate group are converted to C02 . However, McCrea 

Application of Lasers in Small-Sample Stable Isotopic Analysis 53 
Inlet Section Purification Sec:lion 

I I 

1 Combustion Main 1 Backing 1 section vacuum 1 Main Vacuum vacuum 

Main Vacuum. 

.------.~ .. 

: :~ ~ ~Ltil__ ~: ~.... J~ ~ L 

~ • r---ljl ~ ~! ~ I 

Mass Spectrometa' 

Figure 4. Schematic diagram of glass extraction line used 

for laser-microprobe analyses for nitrogen. The extraction 

line is made of pyrex glass, with quartz used when high 

temperatures are required. Most valves are of a special 

construction that allows an independent backing vacuum 

to be maintained between the atmosphere and sections 

(1950) has shown that this effect is small (approximately 4 

o; oo) and produces a fractionation in the opposite direction to 

that required to explain the laser microprobe results. Water 

vapor, which can have a light isotopic composition, is also 

an unlikely source of 160 because of the small amounts 

present and the short time scale for equilibration. The fact 

that the walls of the sample chamber are metal and 

relatively cool also rules out the possibility of any isotopic 

exchange reactions. At present, it is not known why the 

measured 8180 values are lighter than the true values, and 

more work is required in order to understand the source of 

the light oxygen. 

Oxidation of graphitic carbon in an atmosphere of 

oxygen ( =250 torr) by use of the laser microprobe was less 

successful than the experiments with the calcite. Only small 

amounts of oxidized carbon (CO plus C02) were generated, 

and the 813C values were 3 to 8 o; oo lighter than the true 

value. However, because of the small amounts of gas being 

generated, it was impossible to determine whether this was 

due to isotopic fractionation during oxidation or simply was 

a blank contribution (Franchi and others, 1986a). Recently, 

a mass spectrometer has been developed that has better 

precision and smaller sample capability than that employed 

in this study. With a dedicated laser-microprobe extraction 

system, progress is expected in the measurement of high54 New Frontiers in Stable Isotopic Research 

n 

~ 

~ ("I 

~-

li 

.a 

n ~ i ~Twm 

~ "C n 5!. .. 0 

~- R ~ = ij' ~ 0 

>< 

~· ~ ("I 

along which sample gas flows. Details can be found in 

Boyd and others (1988b). The system employed for 

carbon analyses is very similar, the only difference being 

in the arrangement of valves and volumes in the inlet 

section. 

precision 813C values of carbonates and in understanding 

the problems of 8180 measurement, as well as the possibility of developing laser-induced oxidation of materials such 

as graphite, diamonds, carbides, apd kerogen. 

Nitrogen 

The laser microprobe was also evaluated for nitrogen 

isotopic analysis by Franchi and others ( 1986a) by use of a 

number of materials having a range of nitrogen concentrations. In general, the results were very favorable, particularly with the more nitrogen-rich materials. The range in 

815N values and nitrogen concentrations from analyses of 

artificial titanium nitride (table 1) illustrates a number of 

features of this technique. Variation in 815N value is due to 

different target areas being sampled, the results from 

repeated analyses in the same target area (target area 3) 

being reproducible within the precision of the mass spectrometer. Because the mean 815N value of the three target 

areas (-4. 6 o; oo) is close to the value obtained (-3. 8 o; oo) 

by stepped heating of a relatively large sample of this 

material, it is suggested that there is no discernible isotopic 

fractionation of the nitrogen during extraction and that the 

variations observed reflect isotopic heterogeneity within the 

material. 
u 

~ 

~ 

~ 

<l 

'Q' 

~ 

-

0 

QO 

~ 

~ 

<l 

10. 

• 

0-~-------------~--------- 1 • 

-10-

10. 

Atmos. COz 

0- -----------------------· 

-10- • 

Atmos. 0 2 

• • 

-20- • • I I I I I I 

0 1 2 3 4 5 6 7 

Number of Pulses 

fi§ure 5. Results of laser-microprobe analyses for 813C and 

81 0 values of calcite. The results are shown as ~8 values 

(8TRUE - 8MEASURED) against the number of pulses used to 

liberate the gas analyzed. Laser output energy was 5 j. 

Analyses are of C02 only, as no CO above blank levels was 

detected. Apart from one apparently anomalous point, all 

the 813C values are slightly lighter than expected. No 

correction for the blank was performed (813C = -30 O/oo) 

and therefore may account for the small shift. The cause of 

the large shift to light 8180 values has not yet been 

identified. 

Table 1. Results of laser-microprobe analysis of artificial 

titanium nitride 

[Laser output energy was 5 J. The &15N value of the TiN obtained from 

stepped combustion is -3.8 ofoo] 

Target No. of Yield Yield 81sN 

area pulses (ng N) (weight percent N) (per mil) 

1 153.9 15.4 -4.7 

2 156.2 15.6 -2.0 

3 147.4 14.7 -7.5 

3 64.5 6.4 -6.5 

3 22.9 2.3 -7.5 

The yield from the first pulse at each target area is 

also reproducible within the uncertainties of the mass 

spectrometer, and it is only when repeated analyses of the 

same target area are attempted that the yields become 

progressively lower. It appears that a number of factors 

combine to produce this effect. Nitrogen-poor titanium 

metal produced from the first pulse may collect in the 

60- D 

- D 

.-.... l•y Q \ ~ 40- D 

- I -

z - D 

lt') 

~ 20- ~ 

c • Matrix - D Inclusions 

0 . I I . I I 

0 200 400 600 800 

Nitrogen Concentration (ppm) 

Figure 6. Results of laser-microprobe analyses for 8

15N of 

the Murchison meteorite. The data envelope shows the 

range of values obtained from whole-rock samples of 

Murchison in other laboratories employing more conventional techniques (Kerridge, 1985). Laser-microprobe analyses of the matrix lie within this envelope, but most of the 

analyses of small, individual, light-colored inclusions display lower nitrogen concentrations and more variable 

815N values. Laser output energy for matrix was 5 J, but for 

inclusions ranged from 1 to 5 J, depending on the size of 

the inclusion. Between 5 and 10 pulses were necessary for 

each isotopic measurement of the matrix and up to 18 

pulses for each inclusion. 

bottom of the pit; therefore, some energy from subsequent 

pulses could be expended in heating this material. Focusing 

on the bottom of the pit is also more difficult, particularly 

because ejected material coats the underside of the cover 

slip. This coating may also attenuate subsequent pulses of 

the laser beam. These problems can be avoided by always 

focusing the laser beam on a fresh piece of sample surface 

free from ejected material. The concentration of nitrogen 

(=15 weight percent) is low compared to the expected 

value of 23 weight percent. Possibly, the pit volume was 

overestimated or perhaps some of the titanium nitride 

ejected from the pit was ejected without decomposing. 

The results from a study of the carbonaceous chondrite Murchison (fig. 6) also indicate that the laser microprobe is a useful tool for nitrogen analyses. Five analyses of 

the matrix define a narrow range of nitrogen concentration 

(620 to 790 ppm) and o15N values (+38.5 to +44.7 ofoo) 

that are in excellent agreement with whole-rock determinations by other laboratories employing more conventional 

techniques. However, the results from a number of lightcolored inclusions reveal a much wider range of o 15N 

values ( + 12.4 to +60.6 ofoo), as well as lower nitrogen 

concentrations (fig. 6). That the inclusions having the 

highest nitrogen concentrations have o15N values closest to 

those obtained from the matrix suggests that these inclusions were completely penetrated by the laser and that 

underlying matrix was sampled. Care must be taken to 

Application of Lasers in Small-Sample Stable Isotopic Analysis 55 
Table 2. Results of laser-microprobe analysis of the iron 

meteorite Uwet 

[Laser output energy was 5J; nm, not measured] 

Target No. of Yield Yield 81sN 

phase pulses (ng N) (ppm N) (per mil) 

Schreibersite 12 3.4 62 -66 

Schreibersite 21 1.6 17 -67 

Schreibersite 25 1.6 14 -76 

Kamacite 30 .6 3 nm 

Kamacite 25 .7 4 nm 

ensure that the laser is focused on areas of interest that are 

thicker than the depth of a laser-produced pit, typically 100 

to 150 ,_...,m. 

Although it appears to have been possible to obtain 

high nitrogen yields from the Murchison meteorite, problems were encountered when the iron meteorite Uwet was 

analyzed. Stepped heating of a whole-rock sample and an 

acid residue of this meteorite (employing a revised version 

of the technique used by Franchi and others (1986a)) have 

shown that the iron-nickel metal phase (kamacite) contains 

approximately 12 ppm nitrogen and the phosphide crystals 

(schreibersite) 165 ppm (Franchi, 1988), both with a 815N 

value of -96 ot oo. However, the laser microprobe could 

extract only small amounts of nitrogen from this meteorite 

(table 2), with yields of only 3 or 4 ppm from the kamacite 

and up to 62 ppm from the schreibersite. Even these yields 

may be high, as the heavy 815N values of the nitrogen 

extracted from the schreibersite (-66 to -76 ot oo) indicate 

that up to 30 percent of the nitrogen analyzed may represent 

terrestrial contamination. Therefore, as little as 25 percent 

of the indigenous nitrogen in this meteorite may have been 

liberated by the laser microprobe, probably due to incomplete degassing of molten metal or even the formation of 

nitrides as the ejecta cooled. Low recovery rates of gases 

from laser-induced melting or vaporization of geological 

materials is well recorded; for example, Kirschbaum (1988) 

reported yields of only 25 percent for argon and 13 percent 

for xenon from inclusions in the Allende meteorite. The 

results of Franchi and others (1986a) indicate that for some 

materials nitrogen also suffers from low yields. 

A LASER-MICROPROBE STUDY OF BENCUBBIN 

The laser microprobe has been used to investigate the 

distribution of isotopically heavy nitrogen in the Bencubbin 

meteorite. This meteorite is a mixture of iron-nickel metal 

and achondritic silicate clasts (plus a few chondritic clasts) 

welded together in a shock-produced glassy matrix 

(McCall, 1968; Newsom and Drake, 1979). The presence 

of primitive trace-element abundances in both the metal and 

silicate clasts indicates minimal planetary processing 

(Kallemeyn and others, 1978; Newsom and Drake, 1979). 

The silicate clasts also have an unusual oxygen isotopic 

56 New Frontiers in Stable Isotopic Research 

.... 

-I 

~ 

e 

1 

8: 0.1 ._, 

z 

0.01 

·~ 

0 400 800 1200 

Temperature \C) 

Figure 7. Stepped-heating release profiles of samples of a 

metal and silicate clast from the Bencubbin meteorite. 

Solid lines, metal clast; broken lines, silicate clast. The 

histogram displays the yield of nitrogen per degree centigrade (note log scale), the angled lines the 815N value from 

each step. Both samples produce a major release of heavy 

nitrogen over the temperature range 900 to 1,100 oc and a 

small release between 600 and 800 °C. The 815N values of 

gas released from both samples are very similar at all 

temperatures. Diagram after Franchi and others (1986b). 

composition (Clayton and Mayeda, 1978). However, perhaps the most intriguing feature of this meteorite is that 

whole-rock samples contain nitrogen (around 50 ppm) 

highly enriched in 15N, with 815N values up to +973 otoo 

(Prombo and Clayton, 1985). Similar nitrogen-release profiles from high-resolution stepped heating of samples of 

metal and silicate clasts (fig. 7) indicate that there may be a 

single source for the heavy nitrogen, possibly a minor phase 

common to both types of clast (Franchi and others, 1986b). 

Therefore, determining the location of the heavy nitrogen 

within the meteorite is important, as this would help 

constrain the mechanism by which the nitrogen was incorporated into the meteorite and thus constrain possible 

origins of the 15N enrichment. 

The only phase that has been identified in both types 

of clasts is the glassy matrix material, which surrounds all 

the clasts and, in places, forms penetrating veins. It is 

considered that only 10 percent, or possibly 20 percent at 
the very most, of the metal and silicate clast samples could 

have been matrix material that had been inadvertently 

sampled. Therefore, as the metal clast contained 82 ppm 

nitrogen (8 15N = +868 of oo) and the silicate clast 45 ppm 

(815N = +789 o/ oo), the matrix must contain several 

hundred parts per million if it is the carrier phase. Unfortunately, obtaining samples of the matrix is difficult 

because of the very hard, yet ductile nature of the meteorite. 

Stepped-heating extractions of two matrix samples liberated 

nitrogen having concentrations of 14 and 94 ppm and 815N 

values of +878 and +888 of oo, respectively (Franchi, 

1988). These results were rather inconclusive, being the 

lowest and highest nitrogen concentrations found in this 

study and reflecting the very heterogeneous nature of the 

matrix. 

Further information on the location of the heavy 

nitrogen yielded by high-resolution stepped heating of acid 

residues of the meteorite established that the nitrogen is 

concentrated in a minor phase with a uniform 815N value of 

about +995 of oo (Franchi and others, 1986b). Analytical 

transmission electron microscopy has indicated that the only 

possible carriers are submicron chromium-rich sulfide 

grains. As yet, these very small grains have not been 

identified in whole-rock specimens of Bencubbin. Therefore, direct analysis of the nitrogen was considered to be the 

best means of determining its distribution. Rather than by 

acquisition of more, or larger, samples of the matrix for 

further stepped-heating experiments, it was hoped that the 

distribution of nitrogen within the meteorite could be 

determined by laser-microprobe analysis. 

Twenty-one different target areas (metal clasts, silicate clasts, or matrix) have been analyzed by use of the laser 

microprobe. The results are presented in figure 8. To 

minimize errors on the yield calculations, the pit volumes 

were estimated from scanning electron microscope photographs of the sample after analysis. Whereas stepped 

heating revealed a degree of similarity in the nitrogen 

release profile from the various clasts, the laser microprobe 

appears to distinguish considerable 815N variations between 

the metal (8 15N = +420 to +958 of oo) and silicate (8 15N = 

+ 190 to +253 of oo) clasts. However, as the heavy nitrogen 

is known to have a uniform isotopic composition (815N = 

+995 o! oo), the range in 815N values must be due to 

sampling of a second component that has a much lighter 

815N value. The only isotopically light nitrogen observed 

during the stepped-heating extraction was that released 

below 600 °C (fig. 7) and is believed to be terrestrial 

contamination. It is therefore probable that the isotopically 

light nitrogen component sampled during the laser analyses 

is also terrestrial contamination. 

The similar ranges of 815N values for the metal clasts 

and matrix (fig. 8) imply that they suffered similar degrees 

of contamination. However, the much lighter 815N values 

from the silicate clasts indicate more severe contamination. 

As the contamination is most probably concentrated on the 

z 

10 

o Metal Clast 

e Silicate Clast 

• Matrix 

c 

b 

+ 200 +400 +600 +800 + 1000 

S15N (%o) 

Figure 8. Results of laser-microprobe analyses of the Bencubbin meteorite. Laser output energy was 5 J. Between 6 

and 20 laser pulses were employed for each analysis. The 

laser microprobe appears to resolve the different types of 

target areas on the basis of their 815N value and nitrogen 

content. However, to be consistent with the steppedheating extractions, the data can be interpreted in terms 

of mixing an indigenous component (815N = =995 O/oo) 

with a contaminant component (about 50 ppm) with a 815N 

value of 0 O/oo. The metal clasts scatter along the curve 

having an assumed 5 ppm of heavy nitrogen (line a), 

whereas the silicate clasts and matrix require higher 

heavy-nitrogen concentrations of between 10 ppm (line b) 

and 15 ppm (line c). 

sample surface as adsorbed atmospheric gases and organic 

materials, it is expected that the contamination would be 

degassed or volatilized where it interacts with the laser 

beam and also where the hot molten ejecta blanket falls on 

the sample surface. Figure 9 shows that the pit sizes and 

ejecta blankets in the metal clasts and matrix are similar but 

that the ejecta blankets around pits in the silicate clasts are 

approximately twice the width (four times the area) of those 

in the material and matrix. Therefore, if a uniform layer of 

adsorbed gases and organic materials is assumed, the range 

in 815N values obtained from the laser microprobe can be 

explained by adding variable amounts of surface contamination having a low 815N value to the isotopically heavy 

nitrogen indigenous to the sample. 

To obtain an estimate of the concentration of the 

heavy nitrogen in the various materials liberated by the laser 

microprobe, mixing lines have been plotted on figure 8. The 

contamination component was assumed to have a 815N 

value of 0 of oo (not unreasonable) and an abundance 

equivalent to a concentration of 50 ppm. The indigenous 

nitrogen is known to have a 815N value of +995 of oo 

(Franchi and others, 1986b), and the lines for three different 

concentrations (5, 10, and 15 ppm) have been calculated. 

As can be seen in figure 8, the data from the metal clasts fall 

around the mixing line that has an assumed concentration of 

Application of Lasers in Small-Sample Stable Isotopic Analysis 57 
Figure 9. Laser-produced pits in different parts of the 

Bencubbin meteorite. A, Silicate clast. B, Metal clast. C, 

Matrix. Laser output energy was 5 j. The pits produced in 

each material are all similar, approximately 150 1-lm deep 

and ranging from about 80 1-lm across in the silicate clasts 

to 100 f.Lm across in the metal clasts. However, the diameter of the ejecta blankets around the pits in the silicate 

clasts (around 500 f.Lm) are much larger than those around 

pits in the metal clasts and matrix (200 to 300 f.Lm). Scale 

bars all100 f.Lm. 

58 New Frontiers in Stable Isotopic Research 

heavy nitrogen of 5 ppm, whereas the silicate clasts and the 

matrix require higher concentrations of between 10 and 15 

ppm. These results clearly indicate that the matrix is not the 

source, or carrier, of the heavy nitrogen but that the heavy 

nitrogen is distributed throughout the meteorite. 

Curiously, the apparent concentration of isotopically 

heavy nitrogen in the clasts and matrix sampled by the laser 

is considerably lower than the concentrations determined by 

stepped heating. It was noted that the nitrogen concentrations in the iron meteorite Uwet determined by the laser 

microprobe were also considerably lower than the values 

obtained by stepped heating, with, at most, 30 percent of 

the nitrogen being recovered. That the metal clasts in 

Bencubbin have lower heavy-nitrogen concentrations than 

the silicate clasts and matrix, in contrast to the values 

obtained by stepped heating (fig. 2), suggests that the 

recovery rates from the metal were lower than from the 

matrix and silicate clasts. There are two possible effects that 

could influence the recovery rate. The first is the thickness 

of the ejecta blanket; the relatively thin blanket produced 

from the silicate clasts (fig. 9A) would presumably degas 

more readily than the thicker blankets obtained from the 

metal clasts and matrix (figs. 9B and C). The second effect 

is that as the metal cools it reacts with some of the free 

nitrogen to form nitrides, similar to the process that may 

have been responsible for the low yields from the iron 

meteorite Uwet. Such a problem is unlikely to arise during 

laser interaction with the matrix target areas, as large 

amounts of oxygen, produced from the breaking of the 

silicon-oxygen bonds in the silicate portions of the matrix, 

would be more likely to react with any hot metal to form 

metal oxides. Thus, the results from the laser-microprobe 

study corroborate the interpretation of the stepped-heating 

experiments that the heavy nitrogen is distributed throughout the metal and silicate clasts as well as the matrix. The 

implication of such a distribution is that the heavy nitrogen 

was present within the clasts prior to the brecciation and 

was not the result of late-stage in situ nuclear reactions or 

implantation processes occurring in a regolith environment. 

CONCLUSIONS 

The results from these preliminary experiments show 

that lasers are of considerable value in the study of 

light-element stable isotopic compositions. The laser's sectioning technique has produced excellent results in the study 

of diamonds, and the technique is currently being evaluated 

for other types of geological materials. The results from the 

laser microprobe indicate that this may prove to be a very 

powerful tool in stable isotope geochemistry. At the present 

time, useful o15N measurements can be made on various 

materials, but o13C and 8180 determinations are less reliable. However, development of mass spectrometers capable 

of measuring o15N or o13C and 8180 values with even 

greater analytical sensitivity and the development of the 
appropriate gas-handling techniques, currently in progress, 

are required before analysis of a wide range of geological 

materials from a single laser pulse can be realized. 

ACKNOWLEDGMENTS 

Financial support for this work by the Science and 

Engineering Research Council and the Natural Environment 

Research Council is gratefully acknowledged. Laser sectioning of diamonds by Michael Seal of D. Drukker and 

Zn., Amsterdam, is also most gratefully appreciated. Helpful reviews by Pat Shanks, Bob Criss, and Everett Gibson 

have led to a much-improved manuscript. 

REFERENCES CITED 

Boyd, S .R., Mattey, D.P., Pillinger, C. T., Milledge, H.J., 

Mendelssohn, M.J., and Seal, M., 1987, Multiple growth 

events during diamond genesis: An integrated study of carbon 

and nitrogen isotopes and nitrogen aggregation state in coated 

stones: Earth and Planetary Science Letters, v. 86, p. 

341-353. 

Boyd, S.R., Pillinger, C.T., Milledge, H.J., Mendelssohn, M.J., 

and Seal, M., 1988a, Fractionation of nitrogen isotopes in a 

synthetic diamond of mixed crystal habit: Nature, v. 331, p. 

604-607. 

Boy-d, S.R., Wright, I.P., Franchi, I.A., and Pillinger, C.T., 

1988b, Preparation of sub-nanomole quantities of nitrogen 

gas for stable isotopic analysis: Journal of Physics E: Scientific Instruments, v. 21, p. 876-885. 

Carr, R.H., Wright, I.P., Joines, A.W., and Pillinger, C.T., 

1986, Measurement of carbon stable isotopes at the nanomole 

level: A static mass spectrometer and sample preparation 

technique: Journal of Physics E: Scientific Instruments, v. 

19, p. 798-808. 

Clayton, R.N., and Mayeda, T.K., 1978, Multiple parent bodies 

of polymict brecciated meteorites: Geochimica et Cosmochimica Acta, v. 42, p. 325-327. 

Davies, G., 1984, Diamond: Bristol, Adam Hilger, 255 p. 

Franchi; I.A., 1988, Nitrogen isotopic variation in irons and other 

Fe-Ni rich meteorites: Ph.D. thesis, The Open University, 

272 p. 

Franchi, I.A., Wright, I.P., Gibson, E.K., and Pillinger, C.T., 

1986a, The laser microprobe: A technique for extracting 

carbon, nitrogen, and oxygen from solid materials for isotopic measurement: Journal of Geophysical Research Supplement, v. 16, p. 514-524. 

Franchi, I.A., Wright, I.P., and Pillinger, C.T., 1986b, Heavy 

nitrogen in Bencubbin-A light-element isotopic anomaly in 

a stony-iron meteorite: Nature, v. 323, p. 138-140. 

Kallemeyn, G.W., Boynton, W.V., Wills, J., and Wasson, J.T., 

1978, Formation of the Bencubbin polymict meteorite breccia: Geochimica et Cosmochimica Acta, v. 42, p. 507-515. 

Kamiya, Y., and Lang, A.R., 1965, On the structure of coated 

diamonds: Philosophical Magazine, v. 11, p. 347-356. 

Kerridge, J .F., 1985, Carbon, hydrogen and nitrogen in carbonaceous chondrites: Abundances and isotopic compositions in 

bulk samples: Geochimica et Cosmochimica Acta, v. 49, p. 

1707-1714. 

Kirschbaum, C., 1988, Carrier phases for iodine in the Allende 

meteorite and their associated 129Xe/ 127I ratios: A laser 

microprobe study: Geochimica et Cosmochimica Acta, v. 52, 

p. 679-699. 

McCall, G.J.H., 1968, The Bencubbin meteorite: Further details, 

including microscopic character of host material and two 

chondrite enclaves: Mineralogical Magazine, v. 36, p. 

726-739. 

McCrea, J .M., 1950, On the isotopic chemistry of carbonates and 

a paleotemperature scale: Journal of Chemical Physics, v. 18, 

p. 849-857. 

Megrue, G.H., 1967, Isotopic analysis of rare gases with a laser 

microprobe: Science, v. 157, p. 1555-1556. 

Newsom, H.E., and Drake, M.J., 1979, Theoriginofmetalclasts 

in the Bencubbin meteoritic breccia: Geochimica et Cosmochimica Acta, v. 43, p. 689-707. 

Plieninger, T., and Schaeffer, O.A., 1976, Laser probe 39Ar-40Ar 

ages of individual mineral grains in lunar basalt 15607 and 

lunar breccia 15465: Geochimica et Cosmochimica Acta 

Supplement 7, v. 40, p. 2055-2066. 

Prombo, C.A., and Clayton, R.N., 1985, A striking nitrogen 

isotopic anomaly in the Bencubbin and Weatherford meteorites: Science, v. 230, p. 935-937. 

Wright, I.P., Boyd, S.R., Franchi, I.A., and Pillinger, C.T., 

1988, High-precision determination of nitrogen stable isotope 

ratios at the sub-nanomole level: Journal of Physics E: 

Scientific Instruments, v. 21 , p. 865-8 7 5. 

Zinner, E., Tang, M., and Anders, E., 1987, Large isotopic 

anomalies of Si, C, N and noble gases in interstellar silicon 

carbide from the Murray meteorite: Nature, v. 330, p. 

730-732. 

Application of lasers in Small-Sample Stable Isotopic Analysis 59 

Simultaneous Analyses of Noble-Gas Isotopes and 

Halogens in Fluid Inclusions in Neutron-Irradiated 

Quartz Veins by Use of a Laser-Microprobe 

Noble-Gas Mass Spectrometer 

By j.K. Bohlke, 1 C. Kirschbaum/ and j. lrwin3 

Abstract 

Simultaneous analyses of halogens and noble-gas 

isotopes in preselected fluid inclusions in minerals can 

provide unique information useful for (1) defining and 

delineating fluid reservoirs in ancient hydrothermal and 

metamorphic systems, (2) determining the sources of 

fluids and dissolved salts and determining some characteristics of rocks along fluid flow paths, and (3) documenting and quantifying mixing, boiling, and (or) unmixing of 

fluids in relation to mineral precipitation. In this study, a 

high-sensitivity noble-gas mass spectrometer combined 

with a laser sampler was adapted to analyze natural and 

neutron-induced Ar, Kr, and Xe isotopes released from 

fluid inclusions in minerals. Mineral samples were neutron irradiated prior to analysis to produce Ar, Kr, and Xe 

isotopes from Cl, K, Ca, Se, Br, Te, I, Ba, and U. Preliminary results indicate that natural abundances of Ar, Kr, and 

Xe isotopes, as well as products of Cl, K, Br, and I, can be 

measured simultaneously in less than 10-9 liter of dilute 

fluid released from hydrothermal quartz by laser decrepitation. 

Aqueous fluid inclusions in Cretaceous vein quartz 

from Alleghany, in the Sierra Nevada, California, contain 

variable amounts of C02 (<1 to 6 molal) and small 

amounts of chlorine (::s0.2 molal). Fluid inclusions having 

intermediate C02 contents in three vug quartz chips from 

the Oriental mine have several similarities to hightemperature meteoric geothermal ground waters: atmospheric isotope ratios of Kr and Xe, moderately elevated 40Ar/ 36Ar ratios (-345-535), and absolute 36Ar concentrations between approximately 25 and 100 percent of airsaturated water values. These data are consistent with at 

least two interpretations of the origin of gold quartz veins: 

(1) the veins formed from ascending dilute fluids that 

originated at the Earth's surface, equilibrated with air, and 

subsequently acquired C02 at greater depths in the crust 

either by participating in prograde metamorphic reactions 

1 CMT-205, Argonne National Laboratory, Argonne, IL 60439; 

now at the U.S. Geological Survey, National Center, MS 954, Reston, VA 

22092. 2 Charles Evans and Associates, 301 Chesapeake Dr., Redwood 

City, CA 94063. 3 Department of Physics, University of California, Berkeley, CA 

94720. 

or by dissolving deep magmatic gas fluxes or (2) the veins 

formed within a broad zone of mixing between ascending 

magmatic or metamorphic fluids and descending meteoric 

waters, such that neither endmember is represented by 

the analyzed samples. Fluid inclusions in two other quartz 

chips yield linear arrays (mixing lines?) in plots of 132XeP6

Ar, 84KrP6Ar, and CI/36Ar between (1) a chlorinebearing component relatively enriched in the he~vier 

noble gases and (2) a chlorine-free component relatively 

enriched in argon. These data, mostly from milky microfractured quartz, may indicate either that (1) mixtures of 

two fluid types (vapor + liquid?) were trapped as fluid 

inclusions or (2) milky quartz contains large amounts of 

contaminant gases. 

All of the Alleghany fluid inclusions have high 1/CI 

ratios (-103 times seawater), and many appear to be 

relatively enriched in Xe, possibly because the fluids 

acquired I and Xe from organic-rich marine sedimentary 

rocks along flow paths. The chlorine concentrations of the 

fluid inclusions are significantly less than that of seawater, 

and the Br/CI ratios are typical of low-salinity ground 

waters that have not interacted with strongly fractionated 

residual evaporite brines or salt deposits. Absence of 

significant marine and evaporitic salt sources within the 

metamorphic belt may be partly responsible for some of 

the chemical transport properties of the Cretaceous 

hydrothermal system that have led to gold ore deposition. 

INTRODUCTION 

Performing simultaneous analyses of noble-gas isotopes (Ar, Kr, and Xe) and halogens (Cl, Br, and I) in fluid 

inclusions could greatly increase our ability to define past 

fluid reservoirs and characterize their interactions in ancient 

hydrothermal systems. Ratios among these components 

may be used to determine likely sources of fluids and 

dissolved salts and to distinguish among processes such as 

fluid mixing or boiling. Furthermore, analyses of fluid 

inclusions in various igneous and metamorphic materials 

are essential for characterizing otherwise inaccessible reservoirs of volatiles in the Earth. In this paper, we report 

preliminary analyses of noble-gas isotopes and halogens in 

Noble-Gas Isotopes and Halogens in Fluid Inclusions in Neutron-Irradiated Quartz Veins 61 
fluid inclusions in gold-bearing quartz veins from the Sierra 

Nevada metamorphic belt, California. The data were 

obtained by use of a high-sensitivity noble-gas mass spectrometer combined with a laser sampling system. Simultaneous analyses of both gases and electrolyte species were 

made possible by neutron irradiation, which produced 

isotopes of Ar, Kr, and Xe from K, Ca, Cl, Br, Se, Ba, Te, 

U, and I (see table 1). By use of the laser sampling system, 

small preselected areas of crystals containing as little as 

10- 10 to 10-9 Lj_ of fluid inclus_i_()nS were analyzed. 

Gold quartz veins occur in deformed metamorphic 

belts of diverse ages worldwide. Many of the chemical and 

isotopic characteristics of the hydrothermal fluids responsible for veins of this type (for example, high C02 , high 

o180) were similar to those expected of "metamorphic" 

fluids (Kerrich and Fyfe, 1981; Phillips and Groves, 1983). 

However, it is not known for certain whether the fluids 

originated predominantly as ( 1) highly exchanged, deeply 

circulating meteoric waters, (2) expelled pore fluids, or (3) 

fluids degassed from hydrous minerals or magmas (Fyfe 

and Kerrich, 1984; Burrows and others, 1986; Nesbitt and 

others, 1986). Many of the chemical and light stableisotopic characteristics of these fluid types converge 

because of fluid-rock interactions at low fluid to rock ratios. 

Oxygen and hydrogen isotopic analyses of fluids and 

minerals may be diagnostic of seawater or meteoric-water 

participation in hydrothermal systems, depending on the 

altitude and latitude of the recharge area (Craig, 1961, 

1963; Taylor, 1974; White, 1974; Criss and Taylor, 1983; 

Wickham and Taylor, 1985); however, these isotope systems can yield ambiguous results, and they have not yet 

satisfactorily determined the ultimate origin of C02-rich 

gold quartz vein fluids (Nesbitt and others, 1986; Pickthorn 

and others, 1987; Bohlke and others, 1988). Ratios of 

noble-gas isotopes and halogens provide numerous alternative or complementary indicators of different fluid types and 

their sources. Also, analyses of noble gases and halogens 

can provide critical data for determining whether vein 

mineralization occurred in response to either (1) unmixing 

of C02-rich and H20-rich fluids during ascent (Robert and 

Kelly, 1987) or (2) mixing of ascending fluids with shallower reservoirs. The laser sampling system is important in 

this context because secondary fluid inclusions unrelated to 

mineralization may be abundant in large samples and 

because mixing and unmixing of fluids at the time of 

trapping are most convincingly documented at the level of 

individual inclusions or small groups of inclusions (Roedder, 1984). 

The analyses reported here were performed both to 

test the technique and to provide preliminary answers to 

fundamental questions about the sources of fluids and 

mechanisms of mineralization in gold quartz veins. Further 

work is currently in progress to refine the technique, to 

extend the Sierran vein study, and to apply the technique in 

studies of other hydrothermal systems. 

62 New Frontiers in Stable Isotopic Research 

BACKGROUND 

Noble Gases and Halogens in Ground Waters 

and Fluid Inclusions 

The published record contains numerous data on 

halogens (especially Cl and Br) in ground waters, relatively 

few on noble gases (mostly He and Ar) in crustal fluids, and 

very few on either halogens or noble gases in fluid inclusions. The following brief selective survey of some of the 

causes and observed effects of halogen and noble-gas 

fractionations in natural fluids is presented to indicate some 

of the potential applications and to provide a basis for 

comparison for data obtained by the new multicomponent 

fluid-inclusion analyses described below. 

The solubilities of the heavy noble gases in water 

increase with mass in the sequence Ar < Kr < Xe; both the 

relative and absolute solubilities are functions of salinity, 

temperature, and pressure (Smith and Kennedy, 1983; 

Shock and others, in press). Adsorption coefficients for 

these gases on solid surfaces also increase with mass 

(Ozima and Podosek, 1983), whereas the opposite appears 

to be true for solution in silicate melts (Lux, 1987). The 

concentrations and isotopic compositions of Ar, Kr, and Xe 

in many natural ground waters indicate that a large component of these noble gases dissolved in approximate equilibrium with the gases in air at ambient temperatures (Mazor 

and Wasserburg, 1965; Mazor, 1972, 1976; Herzberg and 

Mazor, 1979; Kennedy and others, 1985). Systematic 

variations in the absolute and relative abundances of Ar, Kr, 

and Xe, ranging over as much as one order of magnitude, 

have been observed in many surface-derived (meteoric) 

geothermal fluids. These variations have been interpreted in 

part as results of subsurface boiling, incorporation of 

fractionated gases from various aquifer rocks, and subsequent mixing (Mazor, 1976; Kennedy and others, 1985; 

Smith and Kennedy, 1985). The isotopic compositions of 

Ar, Kr, and Xe probably are not fractionated by more than 

a few percent by equilibrium vapor-liquid exchange (Ozima 

and Podosek, 1983). However, many waters that originated 

at the Earth's surface contain significant amounts of 

"excess" radiogenic 40 Ar acquired during water-rock interactions (Mazor, 1976; Kennedy and others, 1985; Smith 

and Kennedy, 1985; Zaikowski and others, 1987); the 40 Ar/ 36 Ar ratios of such fluids are functions of fluid 

residence time and the age and composition of the rocks. 

The noble-gas compositions of fluids exsolved from magmas or metamorphic rocks at depth in the Earth's crust have 

not been completely characterized, but they may deviate 

significantly from those of surface-derived fluids (Ozima 

and Podosek, 1983; Allegre and others, 1987; Fisher, 

1985). The 40 Ar/ 36 Ar ratios of mantle-derived magmatic 

rocks range from approximately atmospheric (296) to 

greater than 20,000, presumably depending in part on the 
mantle source region and degree of crustal contamination. 

Kennedy and others (1985) suggest that a "magmatic" 

component of argon having 40 Ar/ 36 Ar~500 may be present 

in Yellowstone spring waters. Excess 40 Ar amounting to as 

much as 90 to 99 percent of the total argon content has been 

detected in fluid inclusions from crystals in metamorphic 

quartz veins and igneous pegmatites (Rama and others, 

1965; Naydenov and others, 1972). 3

He/4He ratios have 

been interpreted widely as indicators of mantle volatiles, 

water-rock interactions, and crustal gas fluxes in aqueous 

systems (Craig and others, 1978; Kennedy and others, 

1985; Torgersen and Clarke, 1985; Simmons and others, 

1987) but will not be discussed further because the laser 

microprobe has not yet been adapted to measure helium 

isotope ratios. 

The relative and absolute abundances of halogens 

(Cl, Br, and I) differ by orders of magnitude in waters from 

different sources (White, 1957; White and others, 1963: 

Collins and Egleson, 1967; Rittenhouse, 1967; Whittemore, 

1984). For example, Br/Cl ratios of concentrated subsurface 

brines range from < 10- 1 to > 10 1 times that of seawater, 

depending in part on whether they are "neoformed" (from 

salt dissolution) or "residual" (from salt crystallization), 

respectively (Holser, 1979). I!Cl ratios of ground waters are 

almost always greater than that of seawater, and they are 

greatest (~ 103 times that of seawater) in relatively dilute 

waters associated with organic matter. Previous halogen 

analyses of fluid-inclusion-bearing hydrothermal minerals, 

obtained by neutron activation (Wickman and Khattab, 

1972; Luckscheiter and Parekh, 1979; Behr and Gerler, 

1987) and by chemical analysis of leachates (Kozlowski and 

Karwowski, 197 4), have yielded a wide range of Br/Cl 

ratios ( ~0.001 to 0.03), most of which are significantly 

greater than that of seawater ( ~0.0015). Nordstrom and 

others ( 1985) state that dilute ground waters and saline 

fluid-inclusion leachates in granitic rocks at Stripa, Sweden, have indistinguishable halogen ratios (Br/Cl = 0.004; 

I!Cl = 0.0001) and infer that fluid inclusions are the source 

of ground-water salts. Ratios of Br/Cl and I!Cl are not likely 

to change significantly during boiling or fluid unmixing, but 

their utility in detecting mixing of distinct fluid reservoirs 

has been documented (Whittemore, 1984). 

In a study of natural and neutron-induced argon 

isotopes released from irradiated samples of quartz veins 

from granite-hosted tungsten deposits by stepwise heating 

and crushing, Kelley and others (1986) identified (1) a 

potassium-correlated component, presumed to have been 

released from solid mica inclusions, (2) a chlorinecorrelated component, similar to air-saturated water, having 

a small amount of excess 40 Ar, attributed to fluid inclusions, and (3) a calcium-correlated component having a 

large amount of excess 40 Ar. This study established the 

importance of analyzing simultaneously natural and 

neutron-induced components of argon, which could be 

related to other independent information on the electrolyte 

chemistry of fluid inclusions (see also Turner, 1988). Irwin 

and others (unpub. data, 1988) have found evidence for 

several distinct sources of argon isotopes in irradiated 

metamorphic minerals analyzed by use of a lasermicroprobe sampling system coupled with a high-sensitivity 

mass spectrometer (Kirschbaum, 1988). The current study 

represents an initial attempt to use the laser microprobe for 

analyzing Ar, Kr, and Xe isotopes in fluid inclusions in 

irradiated hydrothermal vein quartz (Bohlke and others, 

1987; Kirschbaum and others, 1987). The multicomponent 

noble-gas mass spectrometric approach for simultaneously 

analyzing Ar, Kr, Xe, Cl, K, Ca, Br, Se, Te, U, and I (table 

1) was first used by Turner (1965) for meteorite studies but 

has not previously been used in fluid inclusion studies. 

Gold Quartz Veins in the Sierra Nevada 

Gold quartz veins were deposited in previously 

deformed and metamorphosed rocks in the Cretaceous 

Sierran forearc approximately 110 to 140 m.y. ago (Bohlke 

and Kistler, 1986). The Sierran veins resemble 

"mesothermal-," "greenstone belt-," or "metamorphic-" 

type gold quartz veins that occur in other deformed metamorphic belts worldwide (Lindgren, 1933; Kerrich, 1981; 

Groves and others, 1985). The veins are distributed along 

fault zones at both regional and local scales, they occur in 

a variety of host rocks, and they typically are enveloped by 

carbonatized metasomatic wall rocks (Ferguson and Gannett, 1932; Coveney, 1981; Bohlke, 1986). The veins 

consist mainly of massive white ("milky") quartz in which 

there are scattered open cavities lined with relatively clear 

euhedrally terminated ("vug") quartz crystals. The veins 

probably were formed by repeated episodes of deformation 

and precipitation of quartz in fluid-filled open spaces. 

Wall-rock alteration is dominated by retrograde carbonatization reactions that were probably caused by ascending 

fluids that acquired C02 at greater depths (Bohlke, 1986). 

Stable isotope data and phase equilibria for minerals 

in veins and altered wall rocks in the Alleghany district 

indicate that the metasomatic hydrothermal fluids (at 

325±50 oc and 2± 1 kbar) were isotopically heavy (8 180 = 

+9 to + 14 o/oo; 8D = -50 to -10 ofoo) and C02 rich ( ~ 

10 mole percent C02 ) (Marshall and Taylor, 1981; Bohlke 

and Kistler, 1986; Bohlke, in press). Many of the fluid 

inclusions observed in vein quartz from Alleghany have 

relatively high homogenization temperatures (approximately 180 to 250 °C), and a small proportion of them 

contain as much as 10 mole percent C02 (Coveney, 1981; 

Weir and Kerrick, 1987; Bohlke, unpub. data, 1988), 

consistent with the fluid properties estimated from phase 

equilibria. However, many of the fluid inclusions have 

significantly lower C02 contents and lower homogenization 

temperatures (as low as 110 °C). Also, most analyzed fluids 

obtained from bulk samples of decrepitated or crushed vein 

Noble-Gas Isotopes and Halogens in Fluid Inclusions in Neutron-Irradiated Quartz Veins 63 
Table 1. Neutron reactions, production rates, and approximate detection limits 

[(x, y) = add x, lose yin a nuclear reaction; -13 = lose a beta particle; E.C. = electron capture] 

Reaction Rate of productiona 

atoms product 

atoms reactant 

38Arc/Cf = 21.5 * 10- 37C/ (n,y) 38C/ -P 38Ard 9 

~ ~ 

39Ar~~ = 12.0 * 10- 39K (,/ 9 .p) 39Ar ~ 

40ca (J,o.) 37Ar ~ 

31ArcJCat =6.04 * 10-9 

83Kr5JSe = .860 * 1o- s2Se (n,y) 9 s3se -P sJ8, -P 83K,.J ~ ~ ~ 

82Kr8 ,1Br = 212. * 10- 81Br (n,y) 82Br -P 82K,-d 9 

~ ~ 

130Te (n,y) 131Te -P 131/ ~ ~ 

-P n1Xed 

-4 

131Xer/Te = 16.8 * 10-9 

128Xerf/ = 1283 * 1o-9 

t3080 (n,y) DtBa E.C. Blcs E.C. 13tx d 

~ ~ ~ e 

131Xe8 jBa =2.52 * to-9 

t36Xeu!U = 56.2 * to-9 

36Ar 

84Kr 

132xe 

Detectable 

limit of concentration 

detection b in 10-6 g fluidc 

atoms reactant ppm reactant 

8.3 * 1012 490 

5.4 * 1012 350 

120 * 1012g 8300 g 

8.0 * 1012 1050 

.o4o * 1012 5.3 

.59 * 1012/l,i 130 h,i 

.0089 * 1012 1.9 

4.0 * 1012h,i 900 h,i 

.076 * 1012; 30 i 

470 * 103 31 * 1o-6 

14 * 103 1.9 * to-6 

4.6 * 103 t.o * to-6 

a = thermal neutron fluence = 2.07 * 1017 n/cm2

; "reactant" refers to the sum of all naturally occurring isotopes of the element. 

b = corresponds to 2 times the average propagated uncertainties, after subtraction of blanks and extrapolation to time zero. 

c = equivalent to a cubic aqueous inclusion 100 tJ.m on a side. 

d = some stable natural abundance also present. 

e = monitored reactions. 

f = fast neutrons (all others are thermal neutrons). 

g = detection limit increases from this value with time owing to the 35 day half-life of 37 Ar. 

h = detection limits are higher initially and approach these values as intermediate products decay with half-lives of 8 days (

1311) to 12 days (131Ba). 

i = Te, Ba, U detection limits are calculated assuming they are mutually exclusive (no mutual 131Xe interference). 

quartz from Alleghany have 8D values that are too low 

( -100 to -50 o; oo) to have been in equilibrium with 

hydrothermal sericite (Marshall and Taylor, 1981; Bohlke 

and Kistler, 1986; Bohlke and others, 1988). Massive milky 

vein quartz appears to yield more negative 8D values than 

euhedral vug quartz, possibly because the milky quartz 

contains a larger proportion of late (modem?) secondary 

fluid inclusions (Bohlke and others, 1988). Because the 

64 New Frontiers in Stable Isotopic Research 

Cretaceous paleolatitude and altitude of the Sierra Nevada 

metamorphic belt are not precisely known, it is not clear 

what the 8D values of contemporary meteoric waters were 

at the time the veins formed. 

Occurrences of low-grade disseminated gold mineralization within altered wall rocks at Alleghany have been 

attributed in part to lithologically controlled metasomatic 

reactions between wall rocks and the relatively C02-rich 
fluid (Bohlke, in press), whereas occurrences of high-grade 

gold mineralization within the massive quartz veins commonly appear to be associated with relatively low temperature, C02-poor fluid inclusions (Coveney, 1981). No firm 

petrographic evidence has yet been found for C02-H20 

immiscibility in the Sierran veins, nor has it been demonstrated whether or not fluid mixing contributed to vein 

mineralization. 

Fluid inclusions in vein quartz at Alleghany occur in 

growth bands parallel to euhedral crystal faces (primary 

inclusions?) and in crossing healed fractures (pseudosecondary and secondary inclusions). The inclusions tend to be 

larger (up to several hundred microns across) in vug crystals 

but smaller (a few microns or less) and indistinct in milky 

quartz. In a few large vug crystals, it has been observed that 

inclusions in outer growth bands have lower homogenization temperatures and higher ice melting temperatures than 

those farther inward; however, primary and pseudosecondary inclusions commonly have fairly uniform characteristics 

within centimeter-sized chips. All fluid inclusions from 

Alleghany appear to have chlorinities less than that of 

seawater (Coveney, 1981; Weir and Kerrick, 1987; Bohlke, 

unpub. data, 1988). 

METHOD OF INVESTIGATION 

Samples Studied 

Analyses were performed on four chips of vein quartz 

from the Oriental mine that were selected to represent three 

different elevations and two contrasting quartz textures 

(euhedral vug quartz, massive milky quartz) and on one 

heterogeneous chip of vein quartz from the Gold Crown 

mine. All of the samples are from mineralized veins that 

have carbonatized metasomatic halos. 

Samples 578-9v (65 level, Drain vein split), 553v 

(500 level near Oriental shaft), and 643v (1300 level, east 

of Granite Stope) are euhedral quartz crystals from vugs at 

different levels of the Oriental mine. Previous studies 

indicate that vug quartz crystals from the Oriental mine 

contain fluids that have relatively high 8D values (-54± 14 

otoo; Bohlke and others, 1988). Dense populations of 

irregular to equant fluid inclusions (mostly pseudosecondary) near the bases of these crystals contain two or three 

visible fluid phases and trace amounts of fibrous birefringent daughter minerals at room temperature (see Coveney, 

1981). Liquid C02 was observed in many of the larger (50 

to 200 j.Lm) inclusions in 553v and 643v. C02-H20 

clathrate melting behavior was observed in many of the 

inclusions that did not have visible liquid C02 in all three of 

the vug samples. The majority of the fluid inclusions 

analyzed with the laser microprobe contained little or no 

visible liquid C02 at room temperature, but most of them 

probably contained on the order of 1 to 5 mole percent C02 . 

Fibrous daughter minerals were observed in approximately 

half of the fluid inclusions in 553v and 643v but are 

relatively rare in 578-9v. The daughter minerals are tentatively identified as phyllosilicates and (or) carbonates (Coveney, 1981). Most of the fluid inclusions in splits of the 

analyzed vug crystals homogenize between 135 and 200 °C; 

higher temperatures typically correspond to higher C02 

contents. C02 clathrate melting temperatures in 553v and 

643v range from 8.8 to 9.8 oc (consistent with -0.1-0.4 m 

NaCl equivalent), with a pronounced mode between 9.0 and 

9.4 °C (-0.3 m NaCl equivalent). C02 melting temperatures in the same samples are between -57.5 and -56.5 oc 

and indicate that C02 is much more abundant than CH4 • 

Preliminary analyses of salts from decrepitated C02-bearing 

fluid inclusions in 553v and 643v by energy-dispersive 

X-ray analysis in a scanning electron microscope 

(SEM/EDX) yield alkali to chloride ratios between 1 and 6. 

These data indicate that bicarbonate may be the dominant 

anion in many of the fluids. If dilute NaHC03 and NaCl 

have the same effect on clathrate melting temperatures, then 

the melting temperatures and SEM/EDX data indicate that 

most of these inclusions have approximately 0.05 to 0.20 m 

Cl (that is, 0.1 to 0.4 times that of seawater). Inclusions in 

which clathrate was not observed have ice melting temperatures 2::-3.1 °C, consistent with ~0.9 m Cl ifNaCl is the 

only solute (not likely), or ~0.4 m Cl if NaCl and C02 are 

the only solutes, or ~0.1 m Cl if NaCl, C02 , and NaHC03 

are solutes with Na/Cl equal to 3. All of the available data 

indicate that the majority of fluid inclusions in 553v and 

643v contain approximately 0.05 to 0.2 m chlorine and that 

0.1 m chlorine probably is equal to the average within a 

factor of 2. 

Sample 643m (adjacent to 643v in the Oriental vein) 

is a chip of massive milky white quartz. Previous studies 

indicate that massive milky quartz from the Oriental mine 

commonly contains fluids that have relatively low 8D 

values ( -90±9 otoo; Bohlke and others, 1988). Fluid 

inclusions in 643m are abundant but generally small (a few 

micrometers or less). Most of the inclusions contain one or 

two visible fluid phases at room temperature, and they are 

too small to provide reliable melting or homogenization 

data. The massive quartz is highly fractured, and many of 

its fluid inclusions are clearly secondary or pseudosecondary; nevertheless, liquid C02 appears to be present in some 

of them. 

Sample 671 (adit level near winze) is a euhedrally 

terminated quartz crystal with milky quartz at the base from 

a vug in the Gold Crown mine. The larger fluid inclusions 

in the clearer parts of this sample are generally similar in 

appearance and melting behavior to those in 553v and 643v, 

except that slightly lower clathrate melting temperatures 

(8.6-9.4 °C) of the 671 inclusions indicate slightly higher 

apparent salinities. 

Because the Oriental vein mineralization had not been 

dated previously, the laser microprobe also was used to 

sample argon isotopes from hydrothermal phengitic micas 

Noble-Gas Isotopes and Halogens in Fluid Inclusions in Neutron-Irradiated Quartz Veins 65 
Laser 

(Nd Y AG, Q-switched) 

to Hg 

Diffusion 

Pump 

Mass Spectrometer 

(Glass, 4.5", 60° Sector) 

N2 Trap 

to Hg 

Diffusion 

Pump 

Figure 1. Schematic diagram of the laser-microprobe noble-gas mass spectrometer (from Kirschbaum, 1988). 

for 40 Ar-39 Ar age determinations. The analyzed micas are 

green chromium-rich mariposites in a chip of dolomitized 

serpentinite from the wall of the Oriental vein at the west 

end of the 1300 level (Sample 611 in Bohlke, 1986). 

Analytical Procedure 

The apparatus used for the laser-microprobe extraction and mass-spectrometric analyses of Ar, Kr, and Xe 

isotopes is depicted in figure 1. Descriptions of the instrumentation and general analytical procedures are given in 

Kirschbaum (1986, 1988). The system includes a 4.5-inch 

radius 60° -sector glass mass spectrometer with a BauerSigner GS-98 source and a Johnston multiplier coupled to a 

pulse-counting detector optimized for low count rates (Kirschbaum, 1988). Fluid inclusions were decrepitated with a 

Jarrel-Ash 45-604 laser microprobe focused through a 

petrographic microscope. 

For the present study, small ( ~ 5 mm diameter) 

unmounted quartz chips containing fluid inclusions were 

ground and polished with silicon carbide grit paper in 

distilled water in order to minimize sources of contamination. The polished chips were sealed in evacuated quartz 

tubes along with a standard hornblende flux monitor 

(MMHb-1) (Alexander and others, 1978) and irradiated in 

the University of California, Berkeley, TRIG A Mark III 

reactor for 3 megawatt hours (MWh). The irradiated chips 

were photographed, mapped, and then mounted in the mass 

66 New Frontiers in Stable Isotopic Research 

spectrometer sample chamber with drops of high-vacuum 

sealant (Vac-seal). The chamber was then evacuated and 

baked at ~ 100 °C. 

Fluid inclusions were sampled by 1 to 2-J.LS Qswitched laser pulses at an energy setting of 1 J. Each 

analysis consisted of between 2 and 50 laser pulses focused 

on a small area of the surface of a polished quartz chip 

where one or more fluid inclusions (~50 to 200 J.Lm 

diameter) had been identified within 50 to 100 J.Lm of the 

surface. Multiple laser shots produced excavated pits that 

are approximately 100 to 500 J.Lm in diameter and depth and 

have conchoidal surfaces and no microscopic evidence of 

melting. Examination of chips after several analyses indicates that deeper fluid inclusions also may have been 

sampled by cracks that formed around the excavated pits. In 

some cases, the sampled volume appears to have been as 

deep as 1 to 2 mm. The exact mechanism by which the 

deeper inclusions were degassed is not known, but it is 

possible that multiple laser shots heated the fluid inclusions 

sufficiently to decrepitate them. In order to characterize 

potential contaminants within the quartz or on the surfaces 

of the samples, several analyses were made in areas of chips 

far from fluid inclusions. Laser-excavated pits in inclusionfree areas appear to be similar to those in inclusion-bearing 

areas. 

Released gases were held in the sample chamber for 

10 to 15 min in contact with a clean titanium surface and a 

Zr-Al alloy (SAES) getter and then were equilibrated with 

the mass spectrometer for 10 min before ionization and 

analysis. Sixteen mass numbers were counted in eight 
cycles over a period of approximately 2 h. The counting 

sequence for one cycle was 100 s each for 132, 136, 131, 

129, 128, and 127; 10 s each for 35, 36, 37, 38, 39, and 40; 

and 50s each for 84, 83, 82, and 81. Masses 35, 37, 81, 

and 127 were monitored for Cl, Br, and I contamination in 

the mass spectrometer. Br and I contaminations were 

insignificant; Cl contamination was significant but stable 

and was included in the blank corrections. Blanks (no laser 

shots) were run before each analysis and averaged over a 

period of several days (corresponding to approximately 5 to 

10 determinations). A single average blank value was then 

subtracted from each sample analysis performed within the 

same time period. Typical blanks were in the order of 103 to 

104 atoms for masses 81-136 (Kr and Xe from air), 105 

atoms for mass 38 (mainly HCl), 106 atoms for masses 36 

and 37 (mainly HCl and Cl, respectively), and 108 atoms 

for mass 40 (Ar from air?). 

Blank-corrected Ar, Kr, and Xe isotope data for 

fluid-inclusion-bearing and inclusion-free quartz, along 

with two sets of typical blank values, are listed in table 2. 

Blank-corrected argon isotope data for the hornblende 

standard are given in table 3. Data for all Ar isotopes, 84Kr, 

and 132Xe are given in counts per second (cps); data for all 

other Kr and Xe isotopes are given as ratios with 84Kr or 

132Xe. 40 Ar data are not reported for analyses with large gas 

yields because of excessive detector deadtime corrections 

for count rates greater than ~150,000. The sensitivity ofthe 

instrument is such that 1 cps Xe = 14,000 atoms Xe 

(Kirschbaum, 1988). Sensitivities for Ar (1 cps = 18,000 

atoms) and Kr (1 cps = 16,000 atoms) were assigned to be 

consistent with their relative ionization potentials. The 

sensitivity ratios derived from these values are within ± 10 

percent of those determined independently for the RARGA 

mass spectrometer with a similar Bauer-Signer ion source 

(Smith and Kennedy, 1985; B.M. Kennedy, personal commun., 1988). All ratios reported or discussed in this paper 

refer to atomic (or molar) proportions, whereas ppm refers 

to parts per million by weight. 

Irradiation Parameters 

Irradiation reactions, production rates, and approximate limits of detection for the quartz analyses are listed in 

table 1. Noble-gas products of neutron reactions are designated by subscripts identifying the primary reactants (e.g., 38 Are1 refers to 38 Ar produced from Cl). All neutron 

irradiation reactions used to calculate the abundances of 

dissolved salts in the fluid inclusions were calibrated by use 

of hornblende (MMHb-1) as a standard for neutron flux 

monitoring. The production rates for 38 ArCI, 39 ArK, and 

37 Area were calculated from the hornblende data in table 3 

and the ratios among Cl, K, Ca, and 40 Ar (radiogenic) in 

MMHb-1 given by Roddick (1983). Corrections were made 

for the interfering isotopes 36 Area• 38 Area• 39 Area• 37 ArK, 

38 ArK, and 40 ArK by use of the correction factors listed by 

Dalrymple and others ( 1981) for the U.S. Geological 

Survey TRIG A reactor. These corrections were found to be 

insignificant (::s0.5 percent) for all but 38ArK, which 

amounts to approximately 10 to 15 percent of total 38 Ar 

released from the hornblende. Independently determined 

correction factors for 36 Area, 39 Area, and 40 ArK for the 

Berkeley TRIG A reactor (G. Curtis, personal commun., 

1988) yield practically identical results. 

The mica ages and associated errors were calculated 

from equations given in Dalrymple and others (1981). The 

average 1 value used to calculate 40 Ar-39 Ar ages is 

9. 85 x 10-4 , with a population standard deviation of 

0.33X 10-4 (table 3). 

The thermal neutron fluence (<P) for this irradiation, 

calculated from 

e

8 AretfCl)MMHb-1 <P 

0.2423 X a 37 e1 

(1) 

where a 37 Cl is the thermal neutron cross section for 37 Cl 

and 0.2423 is the ratio 37Cl/Cl, was 2.07 X 1017 n/cm2. 

Production rates of Kr and Xe isotopes from Br, Se, Ba, Te, 

U, and I were calculated from analogs of equation 1 by use 

of thermal neutron reaction cross sections and isotopic 

abundances of the reactants from Walker and others ( 1984). 

Uncertainties 

The uncertainties quoted in the tables and illustrated 

in the following figures are based on ( 1) the population 

standard deviation for the set of approximately 5 to 10 blank 

determinations used to correct a group of analyses and (2) 

the standard deviation of .data about the line used to 

extrapolate count rates (or ratios) to time zero. Several 

potential sources of systematic or random errors are not 

reflected in the uncertainties assigned to individual analyses. These include sample contamination, selective release 

of gases from fluid inclusions, and errors in the assumed 

irradiation parameters and mass spectrometer sensitivities, 

for example. 

The relative sensitivities and transmission efficiencies 

of the mass spectrometer for Ar, Kr, and Xe are not 

precisely known and could be in error by as much as ±20 

percent or more (Kirschbaum, 1986; B.M. Kennedy, personal commun., 1988). Also, because neutron reaction 

cross sections depend on the neutron energy spectrum, the 

calculated noble-gas production rates for unmonitored thermal neutron reactions may be incorrect by varying amounts. 

For example, the ratio e28Xe1/I):e8 Are/Cl) from table 1 is 

approximately 20 to 40 percent smaller than those reported 

by Kirschbaum (1988) for several monitors irradiated in the 

University of Missouri Research Reactor facility, whereas 

the ratios e28Xe/I):e8 Are/Cl), (

131XeBa/Ba):e8 AretfCl), 

Noble-Gas Isotopes and Halogens in Fluid Inclusions in Neutron-Irradiated Quartz Veins 67 
Table 2. Count rates (counts per second) and some calculated quantities for blanks and blank-corrected quartz analyses 

Data for mass numbers 35-40, 84, and 132 are given in counts per 

second (cps). Other data are given as ratios (as indicated in the 

left-hand column). For each mass number or ratio, the first row 

indicates the value, and the second row indicates 1 standard 

deviation, calculated from uncertainties in the blank and in the 

extrapolation of count rates to time zero. F values (lower rows) are 

ANALYSIS 

SHOTS 

SYMBOL 

35 

36 

BLANK1 * BLANK2* 1508 1510* 1551 1553 

0 0 50 10 

T S S 

867.50 864.60 83.20 51.80 46.60 65.00 

54.10 13.10 38.40 37.60 58.48 58.32 

61.52 51.90 327.70 124.20 1392.00 245.50 

12.48 7.26 18.30 18.02 19.57 13.17 

defined in the text. 1 cps Ar = 18,000 atoms Ar, 1 cps Kr = 

16,000 atoms Kr, and 1 cps Xe = approximately 14,000 atoms 

Xe. Blank1 and Blank2 are typical averages of 5 to 10 determinations; all other analyses are blank corrected. Asterisks designate 

analyses that do not appear in the figures and were not used in 

calculations because of small yields and (or) large uncertainties. 

1555 1556 1558 1560 1562* 1565 1567* 1569 

45 20 2 6 3 50 15 20 X y y y Z X T T 

68.86 -14.87 -15.80 88.78 86.20 150.50 -18.29 -19.05 

58.44 65.81 58.65 58.60 58.55 60.21 58.43 58.23 

50.76 664.70 381.80 781.00 292.20 131.90 41.95 1071.00 

16.57 14.12 14.77 13.34 14.56 14.53 14.83 15.81 

37 226.30 

21.80 

241.80 

10.60 

46.16 

12.65 

20.05 

16.86 

27.29 

29.19 

29.17 30.32 4. 63 3.40 23.40 

28.71 

32.33 

28.36 

58.09 

27.87 

0.47 

28.67 

12.81 

28.91 

38 

39 

40 

81/84 

82/84 

83/84 

84 

127/132 

128/132 

129/132 

131/132 

132 

136/132 

K/CL 

40/36 

F84 

Fl32 

FBR 

FI 

CL/36 

14.94 

3.30 

2.22 

0.44 

10767.00 

2651.00 

0.10 

0.03 

0.23 

0.02 

0.16 

0.06 

0.52 

0.12 

0.27 

0.06 

0.13 

0.04 

1.11 

0.05 

0.89 

0.09 

0.48 

0.07 

0.33 

0.05 

13.00 

1.16 

2.90 

0.91 

98.84 

9.74 

7.23 

8.19 

35.38 

9.22 

12.21 

8.25 

9154.00 134900.00 41868.00 

782. 80 9346. 00 6530. 00 

0.05 

0.06 

0.21 

0.06 

0.16 

0.04 

0.39 

0.16 

0.22 

0.12 

0.15 

0.07 

1.05 

0.08 

0.93 

0.12 

0.31 

0.11 

0.33 

0.06 

0.02 

0.01 

0.31 

0.02 

0.20 

0.01 

10.28 

0.42 

0.09 

0.22 

2.30 

0.30 

1.06 

0.21 

1.17 

0.20 

0.88 

0.15 

0.38 

0.10 

-0.03 

0.03 

0.26 

0.01 

0.20 

0.02 

3.96 

0.44 

0.50 

0.27 

1.27 

0.33 

1.15 

0.35 

1.16 

0.30 

0.54 

0.13 

0.66 

0.18 

350.60 

5.78 

15.33 

1. 76 

o.oo 

0.00 

-0.01 

0.00 

0.24 

0.00 

0.20 

0.00 

54.87 

0.89 

0.02 

0.03 

0.90 

0.03 

1.00 

0.01 

0.75 

0.01 

9.67 

0.26 

0.31 

0.01 

28.75 29.01 

65.15 10.91 

3.96 4.27 

3.19 3.93 

1.70 1.84 

90495.00 14401.00 

4808.00 3987 .oo 

-0.03 -0.10 

0.02 0.12 

0.24 

0.01 

0.20 

0.01 

12.19 

0.28 

0.02 

0.05 

0.87 

0.10 

0.94 

0.03 

0.77 

0.04 

2.30 

0.16 

0.33 

0.01 

0.17 

0.03 

0.20 

0.03 

1.56 

0.35 

0.20 

0.22 

0.16 

0.04 

0.92 

0.07 

o. 78 

0.08 

o. 77 0.17 

0.34 

0.04 

30.93 30.14 

201.30 98.17 

3.81 3.92 

7.90 3.01 

1.86 1.73 

0.00 131915.00 

0.00 5766.00 

0.03 o.os 

0.01 0.02 

0.29 

0.02 

0.22 

0.01 

23.81 

0.52 

0.01 

0.04 

1.24 

0.05 

0.93 

0.03 

0.82 

0.03 

3.79 

0.14 

0.31 

0.01 

0.34 

0.01 

0.20 

0.02 

11.99 

0.54 

-0.06 

0.05 

0.77 

0.10 

0.94 

0.03 

0.80 

0.02 

2. 35 

0.14 

0.34 

0.03 

211.20 

4.68 

9. 57 

l. 79 

o.oo 

o.oo 

-0.03 

0.02 

0.25 

0.01 

0.23 

0.00 

28.32 

o. 75 

o.oo 

0.06 

0.81 

0.04 

0.98 

0.02 

o. 79 

0.02 

5.42 

0.21 

0.32 

0.01 

1.17 1.60 0.35 1.82 0.31 0.30 5.13 0.19 0.20 0.27 

175.02 176.38 411.66 337.10 0.00 368.62 283.71 0.00 345.51 o.oo 

0.37 0.32 1.35 1.37 1.69 2.13 1.32 1.54 1.35 1.56 

8.07 6.25 2.79 4.53 7.25 9.79 15.76 5.96 6.42 7.25 

0.23 0.05 1.34 0.85 0.99 1.22 -1.60 1.26 2.82 0.99 

128.20 113.97 813.63 830.20 1406.73 1498.09 778.50 901.55 968.88 961.11 

0.256E+07 0.291E+07 0.529E+07 0.451E+07 0.298E+07 0.360E+07 0.126E+07 0.535E+07 0.322E+07 0.384E+07 

66.55 

s. 71 

4.98 

1.81 

35.88 

3. 69 

16.27 

1.84 

19.14 

4.06 

1. 65 

1. 76 

89751.00 41161.00 24804.00 

5148.00 4812.00 3697.00 

-0.03 

0.02 

0.26 

0.02 

0.19 

0.01 

8.05 

0.35 

0.11 

0.11 

0.88 

0.09 

1.02 

0.05 

0.82 

0.04 

1.15 

0.14 

0.34 

0.03 

-0.04 

0.05 

0.25 

0.02 

0.21 

0.01 

3.33 

0.25 

0.05 

0.06 

0.10 

0.01 

0.98 

0.02 

0.83 

0.04 

2.44 

0.15 

0.31 

0.02 

-0.04 

0.12 

1.04 

0.24 

0.18 

0.03 

1.46 

0.30 

0.32 

0.48 

2.86 

1.42 

1.21 

0.25 

0.97 

0.23 

0.21 

0.12 

0.25 

0.09 

313.50 

6.43 

17.79 

2.03 

o.oo 

0.00 

0.00 

0.02 

0.29 

0.01 

0.20 

0.01 

41.14 

0.96 

0.01 

0.02 

1.31 

0.03 

1.00 

0.02 

0.80 

0.02 

6.22 

0.23 

0.34 

0.01 

0.77 2.63 0.26 0.28 

307.16 312.06 591.28 0.00 

1.18 1.09 1.50 1.65 

4.13 19.31 5.22 6.06 

1.93 0.67 5.02 1.41 

1251.63 103.20 808.30 1066.53 

0.185E+07 0. 391E+07 0.125E+08 0.488E+07 

ct 31XeTe/Te):e8 Arc/Cl), and e28Xe/I):(82Krs/Br) from 

table 1 are within approximately ± 10 percent of the values 

reported by Turner (1965) and Hohenberg and others (1981) 

for monitors irradiated in the Brookhaven and Missouri 

reactors, respectively. In combination, the potential errors 

listed above may be taken to imply that calculated ratios 

between different noble gases (Ar:Kr:Xe) or halogens 

(Cl:Br:I) could be systematically in error by as much as 

50+ percent. However, these particular uncertainties 

should have relatively little effect on ratios such as Cl/ 36Ar, 

K/Cl, 40 Ar/ 36 Ar, 129Xe/ 132Xe, and 83Kr/ 84Kr. It is 

expected that these sources of error will be reduced in future 

analyses by use of synthetic fluid inclusions as neutron 

reaction monitors and gas ratio standards. The analytical 

isotopic fractionation is estimated to be less than 0. 5 

percent/amu for Xe (B.M. Kennedy, personal commun., 

1988). 

68 New Frontiers in Stable Isotopic Research 

It is possible that daughter minerals containing some 

of the analyzed components were not completely degassed 

when fluids were released from the fluid inclusions. For 

example, if significant amounts of K, Ca, and Ba were 

incorporated into carbonate or phyllosilicate daughter minerals after a fluid inclusion was trapped, and if these 

daughter minerals retained neutron-induced noble gases 

during an analysis, then the concentrations of these three 

elements in the original trapped fluid may have been higher 

than indicated by the analysis. Nevertheless, independent 

evidence indicates that potassium-bearing daughter minerals did release a significant fraction of neutron-induced 39 Ar 

(see below). Furthermore, significant quantities of the 
Mass 37 count rates are not corrected for 37 Ar decay, as they are 

indistinguishable from Cl blanks. 0.0 cps is reported for mass 40 

when the count rate was too high (> 150,000 cps) for reliable 

deadtime correction. Symbols listed in the table and plotted in the 

figures (see fig. 2) are as follows: D (diamond), vug quartz 578-9v 

(Drain vein split of Oriental vein, 65 level);Y (inverted triangle), 

ANALYSIS 

SHOTS 

SYMBOL 

35 

36 

37 

1571* 

5 

c 

-39.92 

58.35 

20.32 

14.55 

-17.50 

28.74 

1842 

10 

s 

27.78 

54.88 

750.80 

17.58 

25.15 

22.23 

1845 1847 1849 

10 10 10 

s s s 

-28.92 12.97 -1.10 

54.47 14.03 14.70 

173.30 437.60 220.60 

13.38 11.74 8.83 

-5.44 -1.32 7.73 

22.59 12.55 14.43 

1851 

10 

s 

8.49 

13.60 

727.00 

9.72 

48.96 

11.44 

vug quartz 553v (Oriental vein, 500 level); T (triangle), vug quartz 

643v (Oriental vein, 1300 level); S (square), massive milky quartz 

643m (Oriental vein, 1300 level); C (circle), vug quartz 671 (Gold 

Crown vein, adit level); X (star), inclusion-free quartz (samples 

553v and 671) 

1853 1855 1857 

20 20 20 

c c c 

-9.16 27.15 12.30 

14.33 13.53 14.77 

107 .so 1799.00 334.90 

8.61 14.41 7.79 

-22.99 22.17 0.18 

14.44 12.09 13.79 

1859 1861 1863 

12 15 20 

T T T 

18.64 5.09 42.15 

13.39 13.24 13.67 

138.50 842.90 1406.00 

9.61 10.26 9.41 

6.16 50.22 37.22 

15.22 22.17 13.31 

1865 

10 

T 

-10.27 

13.52 

673.90 

12.00 

0.67 

16.78 

38 3.98 

3.57 

171.00 41.89 103.30 55.39 162.00 27.37 342.60 68.31 38.54 

1.48 2.22 

221.80 354.40 

5.04 3.25 

190.00 

4.26 3.67 6.19 1.63 1. 99 2. 70 6. 55 4.80 

39 3.07 

1. 76 

12.27 4.97 9.52 3.60 12.26 3.10 15.21 

2.36 0.98 1.17 

6.10 5.30 

0.92 1.41 

13.74 30.23 

0.95 1.50 

11.77 

0.51 0.63 1.60 1.49 1.57 

40 9583.00 

3944.00 

0.00 64655.00 o.oo 117159.00 0.00 44337 .oo 0.00 

0.00 1736.00 0.00 

0.00 74221.00 

0.00 2563.00 

o.oo o.oo 

o.oo o.oo 

o.oo 

o.oo 3012.00 o.oo 6515.00 0.00 

81/84 0.20 

0.64 

-0.01 0.00 0.02 0.01 o.oo o.oo 0.00 

0.01 0.01 0.00 

0.08 -0.02 

0.03 0.01 

-0.02 o.oo 

o.oo o.oo 

o.oo 

0.01 

82/84 

83/84 

84 

127 I 132 

128/132 

129/132 

131/132 

132 

136/132 

K/CL 

40/36 

F84 

Fl32 

FBR 

FI 

CL/36 

-0.05 

0.24 

0.18 

0.09 

0.33 

0.29 

0.28 

0.29 

1.11 

0.56 

1.03 

0.18 

1.10 

0.20 

0.28 

0.21 

0.34 

0.09 

0.01 0.01 

0.23 

0.01 

0.22 

0.00 

20.71 

0.35 

0.01 

0.03 

0.88 

0.03 

0.95 

0.02 

0.86 

0.04 

2.99 

0.11 

0.32 

0.03 

0.26 

0.01 

0.32 

0.03 

5.68 

0.23 

0.20 

0.08 

1.04 

0.12 

0.98 

0.08 

1.02 

0.10 

0.88 

0.12 

0.69 

0.15 

0.01 0.02 

0.50 

0.03 

0.19 

0.01 

15.36 

0.67 

0.04 

0.06 

0.79 

0.07 

0.99 

0.02 

0.87 

0.03 

2.38 

0.15 

0.38 

0.02 

0.68 

0.14 

0.24 

0.02 

7.03 

0.35 

0.02 

0.04 

1.05 

0.14 

1.12 

0.05 

0.88 

0.05 

1.29 

0.13 

0.33 

0.02 

0.25 

0.01 

0.19 

0.01 

21.79 

0.27 

0.05 

0.03 

0. 78 

0.04 

1.03 

0.02 

0.85 

0.03 

3. 51 

0.17 

0.29 

0.02 

33.33 0.73 0.95 0.81 0.46 0.86 

471.60 0.00 373.08 o.oo 531.09 0.00 

0.70 1.19 1.41 1.51 1.37 1.29 

14.20 4.16 5.31 5.68 6.09 5.04 

-23.06 0.93 1.76 10.02 11.16 1.74 

26957.07 1253.67 1427.57 1255.15 1408.17 1514.16 

0. 377E+06 0.185E+07 0. 250E+07 0. 224E+07 0. 294E+07 0.162E+07 

halogens (Cl, Br, and I) are not likely to be incorporated 

into daughter minerals except in fluid inclusions that have 

much higher salinities than those described here. 

RESULTS 

Argon Isotopes 

Laser-microprobe analyses of fluid-inclusion-bearing 

quartz from Alleghany yielded approximately 5 to 20 times 

as much 36 Ar, 38 Ar, 39 Ar, and 40 Ar as did blank runs or 

analyses of inclusion-free quartz. Ratios among these isotopes indicate that small populations of fluid inclusions 

contained measurable amounts of Ar, K, and Cl. 

0.28 

0.03 

0.21 

0.01 

3.02 

0.20 

0.14 

0.15 

0.61 

0.19 

0.69 

0.13 

0.89 

0.12 

0.46 

0.12 

0.19 

0.09 

0.24 

0.01 

0.20 

0.01 

22.95 

0.28 

0.16 

0.10 

0.40 

0.05 

1.14 

0.08 

0.95 

0.09 

0.93 

0.13 

0.37 

0.14 

0.26 

0.01 

0.22 

0.02 

5.74 

0.33 

0.29 

0.15 

1.03 

0.21 

1.01 

0.08 

0.85 

0.10 

0.52 

0.11 

0.22 

0.05 

0.25 

0.02 

0.20 

0.02 

5.10 

0.17 

0.18 

0.10 

1.11 

0.16 

0.99 

0.05 

0.81 

0.08 

0.79 

0.14 

0.38 

0.04 

0.30 

0.01 

0.14 

o.oo 

30.42 

0.29 

0.04 

0.01 

0.94 

0.07 

0.98 

0.01 

0.85 

0.02 

5.83 

0.25 

0.32 

0.01 

0.29 

0.01 

0.21 

0.01 

47.44 

0.27 

0.03 

0.02 

0.80 

0.01 

0.91 

0.03 

0.81 

0.02 

8.48 

0.22 

0.31 

0.01 

0.25 

0.01 

0.21 

0.01 

24.71 

0.69 

0.15 

0.03 

1.42 

0.05 

1.01 

0.02 

0.88 

0.02 

3.08 

0.17 

0.35 

0.02 

0.78 5.97 2.03 0.76 0.39 0.60 0.33 

411.29 0.00 0.00 535.89 0.00 o.oo 0.00 

1.20 0.55 0.74 1.58 1.55 1.45 1.58 

4.42 0.54 1.62 5.97 7.23 6.30 4.77 

1.60 8.13 2.83 0.87 2.16 2.02 0.83 

539.60 1033.05 1445.95 1023.14 1243.77 1067.02 1015.06 

0.307E+07 0.118E+06 0.748E+06 0.420E+07 0.350E+07 0.298E+07 0.437E+07 

Corrections for all interfering Ar isotopes were found 

to be insignificant in the calculation of K, Cl, and Ca 

abundances from the quartz analyses ( <0.1 percent of the 

total count rates) when the correction factors listed in 

Dalrymple and others (1981) were used. This result was not 

changed when production data from other sources (Saito 

and Ozima, 1977; G. Curtis, personal commun., 1988) 

were used in the calculations. 

The relatively small amounts of argon released from 

inclusion-free quartz have 40 ArP6 Ar ratios indistinguishable 

from that of air, some of which was probably adsorbed on 

the sample surfaces or in cracks. Seven analyses of fluidinclusion-bearing quartz (all available data) yield 40 Ar/ 36 Ar 

ratios between 345 and 535. These elevated 40Ar/ 36Ar 

ratios are associated with 38 Ar produced from chlorine 

during irradiation, but there is not a clear correlation 

Noble-Gas Isotopes and Halogens in Fluid Inclusions in Neutron-Irradiated Quartz Veins 69 
Table 3. Argon isotope data for hornblende monitor 

(MMHb-1) 

[Data are in counts per second (blank corrected)· 37 Ar count rates are 

d ~ * ' correcte for decay; Ar = radiogenic (nonatmospheric) 40Ar. (]' = 1 

standard deviation, based on uncertainties in the blanks and in the 

extrapolation of count rates to time zero. 

(eb-1) 

J (40 Ar *I 39 ArK)' 

where A.= 5.543X10- 10 and 'T = 518.9 m.a. (Steiger and Jager, 1977; 

Roddick, 1983)] 

Isotope 

or ratio 

36Ar 

(]' 

37Ar 

(]' 

38Ar 

(]' 

(]' 

~Ar 

(]' 

40 Ar*/39 ArK 

~Ar*/37Arca 

~Ar*/ 38ArCl 

Analysis 

1514 1528 1654 

3.69 15.29 13.42 

9.85 9.36 5.18 

142.5 622.7 347.6 

35.9 58.5 109.7 

6.72 19.49 

1.02 2.39 

68.5 207.6 

9.7 4.3 

23,855 72,717 

3,709 2,545 

332.9 329.2 

159.8 109.8 

4,453 4,929 

1,002 1,013 

18.20 

2.74 

175.0 

2.3 

64,421 

3,624 

346.1 

174.2 

4,534 

964 

1745 

5.88 

4.49 

767.4 

237.6 

26.20 

1.75 

293.8 

5.0 

106,878 

9,352 

358.3 

137.2 

4,969 

931 

1747 

2.44 

4.89 

406.4 

249.5 

14.83 

1.74 

193.9 

1.7 

64,365 

3,438 

328.4 

156.7 

5,406 

1,016 

between the apparent relative abundances of "excess" 40 Ar 

and chlorine (fig. 2). 

39Ar/36Ar d 38Ar/36Ar . .j: fl 'd . 1 . an ratios 10r m me uswns 

are shown in figure 3. The relative amounts of neutron-

. d d 39Ar d m uce 

38Ar . d' . K an ci m tcate that most of the flmd 

inclusions have K/Cl ratios between 0.1 and 1.0, with milky 

quartz 643m values generally higher (0.6±0.2) than vug 

quartz 553v and 643v values (0.4±0.2). K/Cl ratios of 

decrepitated salts analyzed by SEM/EDX also are higher for 

milky quartz 643m (0.2±0.1) than for vug quartz 553v and 

643v ( <0.1), but the SEM/EDX values are systematically 

lower than those indicated by the corresponding argon 

isotope data. The differences between apparent K/Cl ratios 

determined by the two techniques may be rationalized if it 

is assumed that (1) significant amounts of potassium are 

present in daughter minerals, (2) neutron-induced 39 ArK 

produced in the daughter minerals was released with laser 

decrepitated gases, and (3) potassium in daughter minerals 

was not released with thermally decrepitated salts. 39 Ar 

could have been released to the fluid during irradiation by 

recoil from fine-grained fibrous daughter minerals, or it 

could have been released by laser heating during the 

analysis. The average K/Cl ratio of the vug quartz fluids 

determined from the argon isotopes (0.4) is approximately 

consistent with an independent estimate of the C02-bearing 

70 New Frontiers in Stable Isotopic Research 

hydrothermal fluid composition in which KINa =0.1 (calculated from mica-feldspar phase equilibria in altered vein 

wall rocks, Bohlke, in press), ifN a/Cl =4. 0 (within the range 

of SEM/EDX analyses of decrepitated salts). 

The fact that analyses of inclusion-free quartz and 

other analyses having low 38 Ar/36 Ar ratios (that is, low Cl) 

yield relatively high K/Cl ratios (fig. 3) indicates that there 

may be a small amount of potassium in the quartz that is not 

in fluid inclusions. It is not known whether this excess 

potassium resides within the quartz matrix, possibly as 

minute solid inclusions of mica, or represents some surface 

contamination. Although the uncertainties are large, it 

appears that the analyses of inclusion-free quartz also may 

contain a small chlorine component, either from surface 

contamination or from small fluid inclusions in adjacent 

areas. 

The yields of 37 Ar were not significantly different 

from the blank values; thus, calcium was not present in 

measurable concentrations. From the data in tables 1 and 2 

it is estimated that the Ca/Cl ratio in the dissolved component of the fluid inclusions is less than approximately 3. 

SEM/EDX analyses of decrepitated salts indicate that the 

Ca/Cl ratio may be as small as 0.01 or less; however, it is 

possible that both techniques failed to detect small amounts 

of calcium that may be present in carbonate daughter 

minerals. 

Krypton and Xenon Isotopes 

Fluid-inclusion-bearing quartz yielded approximately 

5 to 20 times as much krypton and, xenon as inclusion-free 

quartz. With a few anomalous exceptions, all analyses of 

quartz, with or without fluid inclusions, yielded 83Kr/ 84Kr, 

129x 1132x d 136x 1132x . . . 11 . d. . e e, an e e ratios statishca y m tstmguishable from those of air, consistent with the interpretation that U, Se, Te, and Ba were not present in amounts 

sufficient to produce detectable noble gas isotopes during 

the irradiation. From the data in tables 1 and 2, it is 

estimated that U/Cl:50.001, Se/Cl:50.2, Te/Cl:50.01, and 

Ba/Cl:50.07 in the fluid phases of the inclusions. There is 

no clear evidence of natural "excess" xenon or krypton 

isotopes that, if present, might have been acquired by the 

hydrothermal fluids during reactions with uranium-bearing 

rocks before the fluids were trapped or might have come 

from isotopically anomalous (mantle?) source regions. 

In contrast, almost all fluid-inclusion analyses . ld d 82K ;s4K d 12sx ;I32x . . 'fi 1 yte e r r an e e ratios stgm tcant y 

greater than those of air. The data, summarized in figure 4, 

indicate that measurable amounts of 82Kr and 128Xe were 

produced by neutron reactions with bromine and iodine and 

that there are significant variations in the I!Br ratios of the 

sampled fluids. 
0 

0-+~~~._~~_.~~--~--~~_.--------~~~~-

c.o 

+D==578-9v 

+Y==553v 

-th-T==643v 

-$- 8== 643m 

--$-- C== 671 

%X== qtz (no FI) 

0 

0 

AIR. 

~ ~~--~~~~~~~~~~~~~~--~~~~~~-

0.15 0.20 0.25 0.30 0.35 

38Arj36Ar 

Figure 2. 40ArP6Ar versus 38ArP6Ar for fluid-inclusionbearing and fluid-inclusion-free quartz from Alleghany, Calif. Each data point represents one lasermicroprobe analysis. The error bars represent 1 

standard deviation in the blanks and in the extrapolated count rates at time zero. The symbols for this 

and subsequent figures are as follows: D (diamond), 

vug quartz 578-9v (Drain vein split of Oriental vein, 65 

level); Y (inverted triangle), vug quartz 553v (Oriental 

vein, 500 level); T (triangle), vug quartz 643v (Oriental 

Absolute Abundances 

From microthermometric cooling and heating analyses, the majority of fluid inclusions in vug quartz 553v and 

643v are estimated to contain between 0.05 and 0.2 m Cl 

and to average approximately 0.1 m Cl ( ~3,600 ppm Cl by 

weight). This average chlorine concentration may be used 

vein, 1300 level); S (square), massive milky quartz 643 

m (Oriental vein, 1300 level); C (circle), vug quartz 

671 (Gold Crown vein, adit level); X (star), inclusionfree (no Fl) quartz (samples 553v and 671). Fluidinclusion-bearing quartz analyses yield 40Ar/ 36Ar 

ratios greater than that of air, but there is no apparent correlation between 40Ar/ 36Ar and 3 Arc1• 

Inclusion-free quartz analyses yield 40ArP6Ar ratios 

indistinguishable from that of air, possibly from small 

amounts of adsorbed gas. 

to estimate the quantities of fluid that were sampled and the 

absolute concentrations of the other analyzed components 

(all calculations based on the average chlorine value are 

estimated to be uncertain by a factor of 2). Gases extracted 

by six laser shots aimed at a group of fluid inclusions in 

sample 553v (Analysis 1560) indicate that 5.4X 1013 atoms 

(9.0X 10- 11 mol) of Cl were present in the sampled fluid. 

Noble-Gas Isotopes and Halogens in Fluid Inclusions in Neutron-Irradiated Quartz Veins 71 
0 

......-4 

• 0 

lD 

0 . 

0 

0 

0 

• 0 

K/Cl = 0.1 ------

0.15 0.20 0.25 0.30 0.35 

38Arj36Ar 

Figure 3. 39ArP6Ar versus 38Ar/36Ar. See figure 2 for 

key to symbols. The calculated K/CI ratios are based 

on the relative production rates of 39ArK and 38Arc1 

listed in table 1. The apparent K/CI ratios generally 

are smaller in fluid-inclusion-bearing vug quartz 553v 

This chlorine could have been dissolved in 0.9x 10-9 L of 

water, if the water contained 0.1 m Cl. This quantity of 

water is equivalent to the contents of 1 cubic inclusion 

measuring approximately 97 J.Lm on a side, or 10 cubic 

inclusions measuring 45 J.Lm on a side. This -is not unreasonable for the microfractured volume sampled during the 

run, although the amount of fluid actually sampled is not 

precisely known. In contrast, 45 laser shots into an 

inclusion-free area of the same quartz chip (Analysis 1555) 

yielded less than one-tenth as much 38 Ar and could not have 

sampled more than 2 X 10- 11 L of water with the same 

chlorine concentration. 

72 New Frontiers in Stable Isotopic Research 

and 643v and are larger in inclusion-free quartz and 

other samples with relatively low 38ArP6Ar. A small 

amount of potassium not in fluid inclusions may be 

present on the sample surfaces or within the quartz. 

From the data in tables 1 and 2, the average Oriental 

mine vug quartz fluid from 553v and 643v (eight analyses), 

assuming 0.1 m Cl ( ~3,600 ppm Cl), is found to contain in 

the order of 2.3 X 10-4 m Br ( ~ 19 ppm Br) and 0.9x 10-4 

m I ( ~ 11 ppm I). The Cl and Br concentrations are roughly 

one-fifth those of seawater, while the I concentration is 

about 200 times that of seawater. The same fluid apparently 

contains~ 1,400 ppm K, :s9,000 ppm Ca, :s 1,200 ppm Se, 

:s1,000 ppm Ba, :S140 ppm Te, and :S40 ppm U. 

The average ClP6

Ar ratio of this fluid (4.0X 106) 

corresponds to 2.5x 10-8 m 36Ar. Air-saturated water 

contains approximately 4.6x 10- 8 m 36 Ar at 20 °C (calcu-
l!':l 

• 0 

~ 

~ ~ . ~ 0 

00 

-.......-...... ~ Sf ~ C"'J 

* $+~~~& • C'l 0 

00 

C\1 . .AIR 

0 

+ ~ 

• 0 

0 0.5 1 1.5 2 2.5 

128Xej132Xe 

Figure 4. 82Kr/ 84Kr versus 128Xe/132Xe. See figure 2 for key to symbols. Fluid-inclusion-bearing quartz contains 

significant amounts of neutron-induced 82Kr8 r and 128Xe1, whereas inclusion-free quartz does not. 

lated from data in Shock and others, in press). The presence 

of 0. 1 m N aCl in the recharging water would lower the 

argon solubility by less than 5 percent (Smith and Kennedy, 

1983). Because of the uncertainties and possible real 

variability in the chlorine contents of these fluid inclusions, 

it may be concluded that the 36 Ar concentrations are equal 

to or somewhat less than air-saturated water but probably 

not substantially greater (that is, approximately 25 to 100 

percent of air-saturated water concentrations). The same 

conclusion holds approximately for the absolute concentrations of 84Kr and 132Xe, although the apparent 132Xe/ 36 Ar 

ratios are somewhat higher than expected for air-saturated 

water (see below). 

DISCUSSION 

Contamination 

There are numerous potential sources of contamination, including gases adsorbed on the quartz chips, water or 

gases trapped in cracks during sample preparation, and 

fingerprints. Nevertheless, several observations indicate 

that the bulk of both natural and neutron-induced noble 

gases were sampled mainly from fluid inclusions, at least in 

vug quartz samples 578-9v, 553v, and 643v from the 

Oriental mine. 

Noble-Gas Isotopes and Halogens in Fluid Inclusions in Neutron-Irradiated Quartz Veins 73 
1. Yields of both natural and neutron-induced noble-gas 

isotopes from laser shots into fluid inclusions were 

much larger than blanks and much larger than yields 

from laser shots into inclusion-free areas of the same 

quartz chips. This is true even though the inclusion-free 

areas were hit with the maximum numbers of laser shots 

(see table 2). 

2. Inclusion-bearing quartz yielded significantly smaller 132Xe/ 36 Ar, 132Xe/Cl, 132Xe/I, and K/Cl, and larger I/Cl 

and Br/Cl ratios than inclusion-free quartz. These differences indicate that a source of gas compositionally 

distinct from adsorbed gas or quartz matrix gas was 

present where fluid inclusions were present. 

3. Halogen ratios of fluid-inclusion-bearing quartz 

(Br/Cl= 1-3 x 10-3 , IIC1=7-13x 10-4) are distinct from 

those of human sweat (Br/Cl= 1 x 10-4 , I/Cl=2X 10-6

) 

(ICRP, 1975). 

4. Total chlorine yields are reasonable compared to rough 

estimates of sampled inclusion volumes and salinities. 

5. The available 40 Ar/ 36 Ar ratios from the fluid-inclusionbearing quartz are greater than those of air or inclusionfree quartz. Thus, argon from inclusion-bearing quartz 

was not the same as argon in adsorbed air, trapped air, 

or trapped laboratory water. 

6. Calculated argon concentrations are less than or equal to 

experimentally determined 1-bar air saturation values 

and are well within the range encountered in thermal 

ground waters. Also, the majority of Cl/ 36Ar, 84Kr/Cl, 

and 132Xe /Cl ratios are fairly uniform (most are within 

a factor of 2 to 3). These observations indicate that large 

proportions of trapped contaminant air were not sampled 

in most cases. Rather, it is concluded that the bulk of the 

natural noble gases from inclusion-bearing vug quartz, 

including 36 Ar, were present as dissolved components of 

aqueous fluid inclusions of relatively uniform compositions. 

The latter criteria (in 6) are not completely satisfied 

for milky quartz 643m from the Oriental mine or for vug 

quartz 671 from the Gold Crown mine. Analyses of these 

samples yield more variable, and in some cases much 

smaller, Cl/ 36 Ar ratios. Some of these data indicate that an 

anomalous gas-rich component is present in addition to the 

liquid-rich fluid inclusion component. 

Dissolved Atmospheric Gases 

The measured isotopic ratios of krypton and xenon 

and the calculated 36 Ar concentrations for fluid inclusions 

in vug quartz 578-9v, 553v, and 643v from the Oriental 

mine appear to be consistent with the interpretation that the 

sampled fluids originated at the Earth's surface. The 40 Ar/ 36 Ar ratios of the Alleghany fluid inclusions, though 

74 New Frontiers in Stable Isotopic Research 

higher than atmospheric, are similar to those of modem 

meteoric hydrothermal waters. In an earlier study, Kelley 

and others (1986) concluded that meteoric hydrothermal 

fluids trapped in some Paleozoic quartz veins approximately 

retained their initial low-temperature air-saturated-water 

argon concentrations while participating in 400 oc reactions 

at depth and acquiring moderate amounts of excess 40 Ar. 

Similar Ar, Kr, and Xe isotopic ratios might be acquired by 

nonmeteoric fluids from rocks that adsorbed atmospheric 

noble gases previously at low temperature, or possibly from 

other (mantle?) sources, but it would be fortuitous if the 

absolute concentrations of desorbed or deep-source gases 

were near air-saturated water values. 

Most of the sampled fluid inclusions contained significantly less than the maximum observed C02 concentration observed at Alleghany, and it is possible that the 

noble-gas data do not accurately represent the C02-rich 

endmember fluids. Nevertheless, all of the fluid inclusions 

observed in the vug quartz samples clearly were trapped at 

high temperatures and pressures, they appear to have 

roughly uniform salinities and K/Cl ratios, and a large 

proportion of them either contain liquid C02 at room 

temperature or form clathrate on cooling. Thus, although 

they may represent mixtures of C02-rich and C02-poor 

fluids, they probably were trapped at or near the time of 

vein formation and wall-rock metasomatism. If the analyzed fluids were indeed largely meteoric in origin, there 

remain at least two possible explanations for the origin of 

the veins: (1) if the noble gases in the analyzed fluids are 

essentially the same as those in the C02-rich endmember, 

then it is possible that meteoric water circulated to depths 

below which the veins were formed, acquired C02 from 

prograde devolatilization reactions or other (mantle?) 

sources, and then formed the veins while ascending; (2) if 

the noble gases in the analyzed fluids are somewhat 

different from those in the C02-rich endmember, then it is 

possible that meteoric water circulated only as deep as the 

veins were formed. In the latter case, the veins could 

represent vertically extensive zones of mixing between 

descending surface waters and ascending magmatic or 

metamorphic fluids. In either case, the noble gas data 

permit the interpretation that surface waters reached depths 

of at least 5 to 10 km in fault zones within the Sierran 

forearc metamorphic terrane. This interpretation is consistent with 0 and H isotopic evidence for deep meteoric water 

circulation elsewhere in the Cordillera of western North 

America (Criss and Taylor, 1983; Nesbitt and others, 1986; 

Murowchick and others, 1987), whereas 0 and H isotopic 

data from the Sierran veins themselves are not conclusive 

(Bohlke and Kistler, 1986; Bohlke and others, 1988). The 

Alleghany fluids do not resemble seawater or trapped 

marine pore fluids, because most of the fluid inclusions 

appear to be significantly less saline than seawater. 
Table 4. Argon isotopic data for hydrothermal sericite 

[Data are in counts per second (blank corrected). 37 Ar count rates are not 

corrected for decay, as they are indistinguishable from the blank values. 40 Ar *, radiogenic (nonatmospheric) 40 Ar. Ages were calculated from 

equations summarized in Dalrymple and others ( 1981) and decay constants 

from Steiger and Jager (1977). cr = standard deviation based on uncertainties in blanks and in extrapolation of count rate to time zero] 

Isotope Analysis 

1534 1536 1538 

36Ar -6.96 14.45 5.66 

a 9.08 9.26 8.49 

37Ar 14.0 -5.1 7.7 

a 19.4 22.2 18.3 

38Ar 1.06 8.69 5.91 

a 2.39 2.37 2.33 

39Ar 191.1 438.5 494.8 

a 3.7 4.2 6.7 

40Ar 11,054 30,674 33,738 

a 1,747 1,858 1,833 

40 Ar*/39 ArK 68.6 60.2 64.8 

Age (m.y.) 118.0 104.0 111.7 

Natural Radiogenic Gases 

40Ar/ 36Ar ratios between 345 and 535 (fig. 2) indicate that the fluid inclusions contain significant amounts of 

nonatmospheric 40 Ar that was (1) present in the fluids when 

they were trapped and (or) (2) produced by potassium decay 

within the fluid inclusions since they were trapped. Lasermicroprobe argon isotopic analyses of hydrothermal sericites in altered serpentinite adjacent to the Oriental vein 

(table 4) yield an average 40 Ar/ 39 Ar age of 111 million 

years, indistinguishable from Rb/Sr and K/ Ar ages of 

sericites from other veins in the Alleghany district (Bohlke 

and Kistler, 1986). The dated sericites are believed to have 

formed during wall-rock carbonatization reactions in equilibrium with fluids with approximately 10 mole percent 

C02 (Bohlke, in press), similar to the highest concentrations observed in some of the vug quartz fluid inclusions. 

A 40 Ar-39 Ar isochron for the approximate age of the 

veins (11 0 Ma) is shown in figure 5. The fluid inclusion 

data (fig. 5) plot well above the 110 Ma isochron and 

indicate that unsupported "excess" radiogenic 40 Ar is 

present. In the 110 million years since the formation of the 

veins, production of 40 Ar from decay of potassium within 

the inclusions will have increased the 40 Ar/ 36 Ar ratios by 

less than 1 percent. The measured ratios therefore are 

indistinguishable from the initial ratios at the time of 

trapping, which appear to have been somewhat variable. If 

one component of the dissolved gas in each fluid sample 

had an atmospheric 40 Ar/ 36 Ar ratio and contained all of the 36 Ar, then the concentrations of excess 40 Ar were approximately 2-8 X 10- 6 m (assuming 0.1 m Cl), or approximately 15 to 45 percent of the total 40 Ar concentrations. 

The 40 Ar/ 36 Ar ratios of the Alleghany hydrothermal 

fluids at the time of trapping were higher than those of many 

low-temperature ground waters and hot springs, which 

commonly are between 295 and 320 (Mazor, 1976; Mazor 

and Truesdell, 1981; Mazor and others, 1988), but similar 

to those measured in some hot springs and well waters from 

the Yellowstone, Wyoming (~295-650), and Valles, New 

Mexico ( ~320-665), geothermal areas (Kennedy and others, 1985; Smith and Kennedy, 1985). These values also are 

similar to those determined by Kelley and others ( 1986) for 

meteoric fluids trapped in granite-hosted quartz veins associated with tungsten mineralization at Carrock Fell (2::715) 

and Hemerdon Ball (2::385), England. Thus, the Alleghany 40 Ar/ 36 Ar data are consistent with the hypothesis that the 

analyzed quartz trapped high-temperature meteoric fluids 

that circulated at depth. The measured 40 Ar/ 36 Ar ratios are 

not as high as those reported or implied for some Paleozoic 

basin brines (up to 2,100, Zaikowski and others, 1987), for 

fluid inclusions in a "metamorphic" quartz vein ~ 7 ,500?, 

Rama and others, 1965), and for fluid inclusions in igneous 

pegmatites (3,000-30,000?, Naydenov and others, 1972). 

Because several Kr and Xe isotopes are produced by 

natural fission of 238U, it is likely that some "excess" 

quantities of radiogenic Kr and Xe are present in subsurface 

waters that have "excess" radiogenic 40 Ar produced by 

natural decay of 4

°K. In the same way that ratios of excess 40 Ar/ 4

He in basin brines reflect the K/U ratios of aquifer 

rocks, so might the ratios of excess 40 Ar/ 136Xe. To test this 

possibility, and to check the consistency of the argon and 

xenon isotope data, we have plotted 40 Ar/ 36 Ar versus 136Xe/ 129Xe in figure 6. The calculated slopes in figure 6 

are based on the assumption that the elemental and isotopic 

ratios were fixed initially by equilibrium with air between 0 

and 25 oc. These slopes indicate that for K/U2:: 104 (a value 

typical of rocks in the Earth's crust, Zartman and others, 

1961), little change may be expected in 136Xe/ 129Xe 

because of uranium fission for the range of 40 Ar/ 36 Ar ratios 

observed in the Alleghany quartz. The Alleghany quartz 

data do not indicate a correlation between excess 40 Ar and 

excess 136Xe, as almost all analyses yield 136Xe/ 129Xe 

ratios indistinguishable from that of air. Natural excess 

radiogenic 136Xe may be detected in meteoric fluids with 

higher 40 Ar/ 36 Ar ratios or in other fluid types with lower 129Xe/ 36 Ar ratios. The relatively high apparent 136Xe/ 36 Ar 

ratio of one analysis in figure 6 could indicate that some 

neutron-induced gas from uranium was sampled, but the 

significance of this datum in questionable, as it is clearly 

anomalous (see table 2). 

Halogen Ratios 

Halogen data for the Alleghany fluid inclusions are 

summarized in figure 7 and are compared to other data in 

Noble-Gas Isotopes and Halogens in Fluid Inclusions in Neutron-Irradiated Quartz Veins 75 
0 

0-+--~_.--~~--~~--~~~~~~~--~~--~~ 

c:c 

0 

0 

lO 

--------- --- ------

0 

0 

~ 

------ .-- .-- .-- t = 110 Ma 

~------------------

0 

0 

~-+--------~~----~--~~~~--~~--~~--~-+-

0.00 0.05 0.10 0.15 

39Arj36Ar 

Figure 5. 40Ar-39Ar isochron plot with fluid inclusion 

data. See figure 2 for key to symbols. 40Ar-39Ar 

isochrons are based on the production rate of 39ArK 

listed in table 1 (corresponding to j = 9.85X10-4 ). 

The approximate age of the veins (and likely maxifigure 8. The fractionation factor F fw is a convenient 

expression of halogen ratios relative to the composition of 

the largest and most uniform near-surface reservoir (seawater): 

F~w I 

(i/Cl)sample 

(i/Cl)sw 

(2) 

where (Br/Cl)sw = 1.52X 10-3 and (I!Cl)sw = 8.38 X 10-7 

(Sverdrup and others, 1942; Fuge and Johnson, 1986). 

76 New Frontiers in Stable Isotopic Research 

mum (t) time since trapping of the fluid inclusions) is 

110 Ma. Fluid inclusions contain 40Ar in excess of that 

which could have been produced by in situ decay of 

potassium since entrapment, whereas inclusion-free 

quartz does not. 

There is a small but systematic difference between 

F1sw values in massive milky quartz 643m (1 ,400± 100) and 

those in vug quartz 553v and 643v (1 ,030± 100). The 

uncertainties in the calculated Br/Cl ratios are relatively 

large, and there are no distinct differences in F ~~ among the 

different quartz samples. Two analyses of milky quartz 

643m yield anomalously high F ~~ values, but the significance of these data is uncertain. 

Most of the analyzed Alleghany fluid inclusions have 

chlorine concentrations significantly less than that of seawater, and F ~~ values near those of seawater and most other 

low-salinity fluids, including geothermal waters in volcanic 
0 

0 

lO 

0 

0 

~ 

0 

0 

C\1 

0 0.5 

136Xej129Xe 

1 

Fi~ure 6. Relative abundances of radiogenic 40Ar and 13 Xe. See figure 2 for key to symbols. Natural production curves are based on data from Ozima and 

Podosek (1983) and the assumption that all isotopes 

in the fluid initially were in equilibrium with air 

(asw(O) = air-saturated water at 0 °C). Most crustal 

terranes (fig. 8). The hydrothermal fluids trapped in these 

inclusions do not resemble marine pore fluids and apparently did not obtain strongly fractionated salts from either 

residual (high Br/Cl) or neoformed (low Br/Cl) evaporite 

brines (fig. 8; Holser, 1979). These results are consistent 

with the lack of evidence for evaporite deposits within the 

Sierra Nevada metamorphic belt and may be characteristic 

of low-salinity fluids responsible for gold mineralization. 

Absence of marine and evaporite salt sources may be 

critical for determining the chemical transport properties of 

hydrothermal fluids responsible for gold ores deficient in 

rocks have K/U between 103 and 105 (for example, 

Zartman and others, 1961). The general lack of evidence for excess radiogenic 136Xe is consistent with 

the relatively low amounts of excess radiogenic 40Ar 

in the Alleghany quartz fluids. 

base metals (see Hedenquist and Henley, 1985; Henley, 

1985). The bulk of the Alleghany data contrast with the 

results of several previous analyses of hydrothermal fluid 

inclusions, which yielded generally higher Br/Cl ratios 

(Wickman and Khattab, 1972; Kozlowski and Karwowski, 

1974; Luckscheiter and Parekh, 1979; Behr and Gerler, 

1987). At least a few of the earlier analyses pertained to 

saline (residual?) brine inclusions in minerals associated 

with base-metal (Pb, Zn) deposits (Behr and Gerler, 1987). 

The Alleghany fluids have 1/Cl ratios approximately 

1 ,000 times that of seawater, significantly higher than most 

Noble-Gas Isotopes and Halogens in Fluid Inclusions in Neutron-Irradiated Quartz Veins 77 
0 500 

Figure 7. Halogen ratios (F~~ versus Frw) for the 

Alleghany fluid inclusions. See figure 2 for key to 

symbols. F~~ and F rw are defined by equation 2. Data 

for 671 are not shown because they have large uncersaline brines and geothermal waters but similar to some 

low-salinity oil-field brines and to thermal springs associated with mercury deposits in the California Coast Ranges 

(fig. 8). High iodine contents in some natural subsurface 

waters may result from reactions with iodine-rich marine 

organic matter in sedimentary rocks (White, 1957; Collins 

and Egleson, 1967). The mechanism and timing of iodine 

transfer from the organic components of sediments to pore 

waters is not well understood. Nevertheless, the high I/Cl 

ratios of the Alleghany fluid inclusions may indicate that the 

hydrothermal fluids responsible for vein mineralization 

extracted iodine from organic-rich marine sediments or 

metasedimentary rocks. 

78 New Frontiers in Stable Isotopic Research 

1000 

FI 

1500 2000 

tainties. Fluid-inclusion-bearing quartz data are distinct from inclusion-free quartz data and sweat (for 

example, fingerprints) and indicate that (Br/CI)F1 :;::::: 

(Br/CI) 5w and (I/CI)F1 >> (1/CI)sw· 

Noble-Gas Ratios 

Ratios of natural noble-gas isotopes for the Alleghany 

fluid inclusions are summarized in figure 9 and are compared to other data in figure 10. The noble-gas fractionation 

factor Ffir (Smith and Kennedy, 1983) is analogous to that 

used to describe halogen ratios, but with air as the normalizing reservoir: 

p~ir I 

(i/36 Ar)sample 

(i/36 Ar)air 

(3) 
I 

I Stripa ±a t-ground waters - f [N+85] 

..,.,.__.,.,_j 

trapped brines in 

marine salts 

[WHW63) 

oil-field brines 

(>30000ppm Cl) 

[WHW63) 

! Stripa i ±a Lgranite leachate 

L--1 [N+85] 

NaCaCl brines (no oil?) 

(>30000ppm Cl) 

[WHW63] 

u 

1: 

t

trapped brines in 

nonmarine salts 

[WHW63) 

I ±a 

Hawaiian 

rain 

[DWWB63) 

Sierra Nevada 

"soda springs" 

[BKMP81] 

+s~!dns~!(?)lii:--~=~====s:-------, brines 

[BS69,W81) 

0 

0 500 1000 

Fr 

Figure 8. Halogen ratios (f~~ versus F ~w) for Alleghany quartz compared to other data for natural 

waters. Sources of data: A86, C.N. Alpers (personal 

commun., 1987); BKMP81, Barnes and others (1981); 

BS69, Brewer and Spencer (1969); DWWB63, Duce 

and others (1963); EM77, Ellis and Mahon (1977); 

N +85, Nordstrom and others (1985); W81, White 

(1981); WR62, White and Roberson (1962); WHW63, 

where (

84Kr/36 Ar) . = 2 068 x 10- 2 and (

132Xe/ 36 Ar) . = air · air 

7.446X 10-4 (Ozima and Podosek, 1983). 

The relative abundances of Xe, Kr, and Ar have 

distinctive patterns in the different sample types: (1) analyses of fluid inclusions in Oriental mine vug quartz 553v 

and 643v cluster near average values ofF:~ (1.53± .09) 

and F ~~2 (6.25±.74); (2) analyses of Oriental mine milky 

quartz 643m define a linear array approximately centered on 

the vug quartz cluster; and (3) analyses of Gold Crown mine 

quartz 671 define an apparent extension of the 643m array 

to lower values ofF:~ and F ~~~ (both to less than 1.0). 

Most of the Oriental mine fluid inclusions have 

apparent F ;~ values between those of air and lowtemperature air-saturated waters (fig. 9). The roughly simCoast Range 

Hg mine waters [WHW63,WR62) 

1500 2000 

White and others (1963). ±a represents ±1 standard 

deviation for a group of analyses. The area shaded 

with vertical lines encloses all of the data points 

shown in figure 7, except for two analyses with high 

Br/CI; the area shaded with dots corresponds to vug 

quartz 553v and 643v. The Alleghany quartz data 

resemble those of other low-salinity fluids associated 

with organic-rich marine sedimentary rocks. 

ilar F :~ values (fig. 10) of geothermal-water analyses 

reflect the fact that air is a major source of noble gases in 

many thermal springs and ground waters (Mazor, 1976; 

Kennedy and others, 1985). However, the Oriental mine 

fluid inclusions differ from most geothermal waters or gases 

in their relatively high apparent 132Xe/ 84Kr ratios and F ~~~ 

values (fig. 10). Some excess xenon could be present on the 

sample surfaces, as xenon is the most readily adsorbed from 

air and the most difficult to desorb under vacuum, except 

perhaps with the laser. Some excess xenon also could be 

present in the interiors of the quartz crystals, possibly 

because it was selectively adsorbed and trapped during 

crystal growth (Ozima and Podosek, 1983; Matsubara and 

others, 1988). Analyses of inclusion-free quartz yield the 

Noble-Gas Isotopes and Halogens in Fluid Inclusions in Neutron-Irradiated Quartz Veins 79 
AirSaturated 0 

Water"" 

(oC) ~ 

t 

25 . . . . 

. . . . 

. 

SS3v 

+643v ------------------~~-----------+------

lO . 

0 

I 

0 5 10 15 20 

F132 

Figure 9. Noble gas ratios (1:~ verSl:JS F a~~). See 

figure 2 for key to symbols. F~ and F a~~ are defined 

by equation 3. The reference curve for air-saturated 

water at various temperatures was calculated from 

data in Shock and others (in press) and is approximately consistent with data in Smith and Kennedy 

(1983); reference curves for Rayleigh (open-system) 

distillation were calculated from data in Shock and 

others (in press) with an assumed initial composition 

highest F ~~~values (see fig. 9) and are qualitatively consistent with some form of xenon contamination; however, the 

gas yields from inclusion-free quartz are probably too small 

to account for all of the apparent fluid-inclusion enrichments. For example, the amount of xenon-rich gas released 

80 New Frontiers in Stable Isotopic Research 

corresponding to air-saturated water at 0 oc. The 

points on the distillation curves refer to the fraction 

of the original dissolved argon remaining in the 

liquid (Ar/Ar0 = 0.5 is labeled). Data from 553v and 

643v, falling in a tight cluster, indicate relatively little 

gas fractionation. Data from 643m and 671 define a 

linear trend (dotted line) roughly parallel to that 

defined by air, air-saturated water, and boiling water, 

but shifted to higher f 132 and (or) lower F84 • 

by inclusion-free quartz in sample 553v, analysis 1555 

(9.1 x 105 atoms of 36Ar, F ~~~= 15.8), subtracted from the 

gas released from inclusion-bearing quartz in the same chip, 

analysis 1560 (1.4X 107 atoms of 36Ar, F ~~~= 7.3), would 

yield F ~~~= 6.6, which would not greatly alter its position 
sedimentar)' rocks ~ 

(F84 = 1-100, F132 = 10·10000).,.. 

[PH080] 

oL-------------~------------~~------------~------------~ 0 5 10 15 20 

F132 

Figure 10. Noble-gas ratios (F ~~versus F ~~~ for Alleghany quartz compared with other data for 

natural waters. Sources of data are KLRS85, Kennedy and others (1985); M76, Mazor (1976, 

assuming 40Ar/ 36Ar=295.5-arrow indicates correction for 10 percent excess radiogenic 40Ar); 

MW65, Mazor and Wasserburg (1965, as in M76); PH080, Podosek and others (1980); SK85, 

Smith and Kennedy (1985). "Depleted" waters have ::::25 percent of 20 oc air-saturated argon 

concentrations. The area shaded with vertical lines encloses all of the data points for 

fluid-inclusion-bearing quartz shown in figure 9; the area shaded with dots corresponds to vug 

quartz 553v and 643v. ASW, air-saturated water. 

in figure 9. Selective adsorption of gases released from the 

fluid inclusions during analysis presumably would cause the 

measured Xe/ Ar ratios to be too low. 

If excess 132Xe is present in some of the fluid 

inclusions, then it must have been present in the hydrothermal fluids at the time of trapping, as the other xenon 

isotopes generally do not indicate that significant amounts 

of uranium fission products are present. Many sedimentary 

rocks have high Xe/Ar and Xe/Kr ratios, owing to selective 

adsorption of xenon from ground water or seawater, such 

that F ~~ = 101

-104 and F;~ = 10°-102 in a variety of 

argillaceous and siliceous sedimentary rocks and carbonaceous materials (Podosek and others, 1980; Ozima and 

Podosek, 1983; Matsuda and Nagao, 1986). Although the 

behavior of noble gases in rock-water systems at high 

temperatures and pressures is not well known, it is possible 

that the Alleghany fluids acquired excess xenon with 

approximately atmospheric isotopic ratios by desorption or 

recrystallization of xenon-rich sedimentary rocks along 

their flow paths. This xenon enrichment could take place 

with relatively little change in the F :~ value of the fluid 

because the initial Kr/Xe ratio of the fluid would have been 

much greater than that of the gas desorbed from the rocks. 

An excess xenon component from organic-rich marine 

Noble-Gas Isotopes and Halogens in Fluid Inclusions in Neutron-Irradiated Quartz Veins 81 
argillaceous sedimentary rocks also would be consistent 

with the high observed I/Cl ratios in the fluid inclusions. 

The correlation between F ;~ and F :~~ displayed by 

the 643m and 671 data is roughly parallel to the fractionation produced by vapor phase separation at <300 °C. Two 

reference curves for Rayleigh (open-system) distillation of 

noble gases from boiling fluids at low pressures are shown 

in figure 9. The overall slope of the linear trend defined by 

analyses of 643m and 671 could indicate that the fluid 

inclusions contained mixtures of fluids (liquids ± vapors) 

that boiled or unmixed either before or as they were 

trapped. However, as there is no independent evidence for 

phase separation, it is difficult to explain F :~ and F :~~ 

values less than 1. 0 by such processes involving meteoric 

water, and the data array is remarkably linear, in contrast to 

the Rayleigh curves. These data may therefore represent 

mixtures of gases from two (or more) distinct sources. One 

endmember appears to be slightly enriched in 84Kr and 132Xe relative to the vug quartz fluids; the other endmember 

is relatively enriched in 36 Ar. Low F :~ and F :~~ values 

(::::; 1. 0) are not common in geothermal waters but have been 

reported for well water from Valles, New Mexico, where 

argon-rich gases may have been acquired by fluids from 

glassy volcanic rocks (Smith and Kennedy, 1985). Experiments indicate that silicate liquids exposed to air during 

eruption may contain noble-gas components with low F 

values (Lux, 1987), which presumably could be leached 

later by hydrothermal fluids. The possibility remains that 

the Xe/ Ar sensitivity or transmission ratios assumed for the 

mass spectrometer are significantly in error; however, we 

cannot yet rationalize an error of the direction and magnitude required to produce the apparent shift in the array of 

fluid inclusion data relative to the shallow meteoric water 

data (fig. 10). 

Halogen and Noble-Gas Correlations 

Processes such as phase separation and fluid mixing 

are most likely to be distinguishable on plots relating noble 

gases to halogens, as in figures 11 and 12. For example, 

vapor phase separation at <300 oc would be expected to 

cause significant changes in the noble-gas ratios of the 

residual liquid (see fig. 9) but would not alter its halogen 

ratios. Fluid inclusions that trapped boiling or unmixing 

fluids, or fluids that had boiled or unmixed just prior to 

trapping, would therefore tend to plot in horizontal arrays in 

figure 11. In contrast, mixing of fluids from different 

sources could result in arrays having a wide variety of 

slopes, depending on the past history of each fluid. The 

systematic difference in F :w between analyses of vug 

quartz (553v and 643v) and milky quartz (643m) indicates 

that the fluid inclusions in these two groups of samples from 

the Oriental mine are not simply related by vapor-phase 

82 New Frontiers in Stable Isotopic Research 

separation. However, each quartz sample considered separately could have trapped fluids that had undergone small 

amounts of phase separation. 

Analyses of vug quartz 553v and 643v yield relatively 

high Cll 36 Ar ratios and no positive correlation between 

Cl/ 36 Ar and either F :~ or F :~~ (fig. 12). Because the 

solubilities of Ar, Kr, and Xe converge at high temperatures, it is possible that, at the temperatures estimated for 

carbonate metasomatism along the veins (T>300 °C), 

separation of a vapor phase (C02 ?) could remove a portion 

of the noble gases without fractionating them significantly. 

Fluid inclusions containing such fluids would define nearly 

vertical arrays in figures 12 A and B. The vug quartz data are 

permissive of a small amount of vapor separation, but the 

restricted range of Cl/ 36 Ar ratios is consistent with the 

general lack of petrographic evidence of such a process. 

Analyses of milky quartz 643m and Gold Crown 

quartz 671 yield relatively low Cl/ 36 Ar ratios and exhibit 

linear positive correlations between Cl/ 36 Ar and both F ~I 

and F :~r • The slopes of the data arrays defined by 643m 

alone are approximately consistent with the effects of vapor 

loss at relatively low temperatures (see also fig. 9); however, the colinearities of 643m and 671 data in figures 9 and 

12 may be more plausibly interpreted as mixing lines 

between two endmembers, one gas-rich or essentially chlorine free (Cl/ 36Ar = 0.0; F :~ = 0.50; F :~~= 0.25) and the 

other a chlorine-bearing fluid (Cl/ 36Ar2:4.2X 106

; 

F 84 2:2.15; F :~~;:::: 10.0). Mixtures of these two components 

are indicated by dotted lines in figures 9, 11 , and 12. The 

significance of the low Cl/ 36 Ar endmember is not known, 

as it was detected only in a single sample from the Gold 

Crown mine. If it is assumed that all of the analyzed gases 

came predominantly from fluid inclusions with the measured chlorine concentrations, then some of the aqueous 

671 fluids must have been greatly enriched in noble gases 

(preferentially argon) relative to air-saturated water. Alternatively, the low Cl/ 36 Ar component could represent unobserved (small?) vapor-phase inclusions or some form of 

unidentified contamination, either of which may be present 

mainly in the massive milky forms of quartz. Work is in 

progress to determine if these apparent mixing lines may be 

analytical artifacts or if they reflect multistage hydrothermal 

processes involving, for example, acquisition of various gas 

ratios from rocks, vapor-liquid separation, and subsequent 

mixing (see Smith and Kennedy, 1985). 

CONCLUSIONS 

The abundances of Ar, Kr, and Xe isotopes naturally 

dissolved in hydrothermal waters, and of Ar, Kr, and Xe 

isotopes produced by neutron irradiation of dissolved K, Cl, 

Br, I, and possibly Ca, Ba, U(?), Te(?), and Se(?) are 

sufficient to be detected in individual large fluid inclusions 

or clusters of small inclusions by use of the laser-
0 

0 

0 

C\1 

0 

0 

lCJ 

0 

...... ? ........... ? ........ . 

5 

F132 

10 

Figure 11. F ~w versus F a~;. See figure 2 for key to symbols. F ~~;and F ~ware defined by equations 2 and 

3. Data for 671 are not shown because they have large uncertainties in F ~w. Data from different samples 

are relatively distinct in f 1 but exhibit considerable overlap in f 132 . Horizontal arrays on this plot would 

be consistent with progressive vapor-phase separation before or during fluid trapping or with mixtures 

of fluids with constant 1/CI (for example, the dotted line). 

microprobe noble-gas mass spectrometer. The relative 

abundances of Ar, Kr, and Xe isotopes. and K, Cl, Br, and 

I have been successfully measured simultaneously in 10- 10 

to 10-9 L of dilute C02-bearing aqueous fluid inclusions in 

irradiated vein quartz from Alleghany, California. 

Fluid inclusions in euhedral quartz crystals from vugs 

in the Oriental mine have relatively uniform ratios of 

halogens and noble gases. The absolute and relative concentrations of chlorine and bromine indicate that the fluids 

were not undiluted seawater, nor did they obtain salts from 

strongly fractionated evaporite brines. Absence of salinity 

sources such as these within the metamorphic belt may have 

helped to determine the metal transport properties of the 

Sierran hydrothermal system. The fluid inclusions are 

greatly enriched in iodine, possibly acquired from organicrich sediments. There is some evidence that vug quartz and 

milky quartz from the same sample site contained different 

fluids, distinguished by slightly different I/Cl ratios; however, the data do not indicate whether these fluids mixed at 

the time of vein mineralization. The isotopic compositions 

of krypton and xenon, and the absolute concentrations of 36 Ar, indicate that the C02-bearing dilute fluid inclusions in 

Noble-Gas Isotopes and Halogens in Fluid Inclusions in Neutron-Irradiated Quartz Veins 83 
A 

0 

0 

0 

0 

0 

J-.40 <lO 

\.0 

~ 

~0 

og 

0 

0 

lO 

C\l 

0 

. .. . . . 

0.5 

.·· .... .. . 

... ···· .. 

1 

Figure 12. CI/ 36Ar versus F ~r _(A) and _F a~; (8). See 

figure 2 for key to symbols. F ~rand F a~; are defined 

by equation 3. Reference curves for Rayleigh (opensystem) distillation were calculated from data in 

Shock and others (in press) with an assumed initial 

composition corresponding to air-saturated water at 

0 oc with 0.1 m Cl. The labels on the distillation 

curves refer to the fraction of original dissolved 

vug quartz could contain a significant component of meteoric water, which obtained noble gases from air. Moderately elevated 40 Ar/ 36 Ar ratios in the fluid inclusions cannot 

be due to in situ potassium decay but are consistent with 

exchange between meteoric thermal waters and potassiumbearing rocks before trapping. These data indicate that 

meteoric water was present at least as deep as the veins were 

84 New Frontiers in Stable Isotopic Research 

Rayleigh 

Distillation~ 

1A 

AirSaturated 

Water 

(oC) 

1.5 

Fs4 

2 2.5 

argon remarnrng in the liquid (Ar/Ar0 = 0.5 is 

labeled). Data from vug quartz 553v and 643v could 

be consistent with small amounts of vapor loss at 

high temperatures. Data from 643m and 671 define a 

linear trend (dotted line) consistent with mixing of 

two distinct gas components of uncertain origin (see 

also figs. 9 and 11). 

forming, but they do not rule out the presence of another 

deeper C02-rich fluid source. 

Fluid inclusions in massive milky quartz from the 

Oriental mine and a heterogeneous crystal from the Gold 

Crown mine define common linear arrays in noble-gas 

correlation diagrams. These correlations may be interpreted 

as mixing lines between two noble-gas components, one of 
B 

0 

0 

0 

0 

0 

0 

lC':) 

0 

0 

0 

0 

0 

lO 

C\1 

0 

0 

Air- so Saturated~ 

Water v 

(oC) 

+ ... ····· .. . .. . ·· 

Figure 12.-Continued. 

.·· .. · 

which has unusually low Kr/ Ar and Xe/ Ar ratios and may 

not be associated with chlorine. The significance of these 

apparent mixtures is currently under investigation. 

Summaries of data in the literature indicate that ratios 

among dissolved Ar, Kr, and Xe in ground waters vary by 

factors of at least 3 (Kr/Ar) to 10 (Xe/Ar), whereas ratios 

among Cl, Br, and I vary by factors of more than 10 (Br/Cl) 

to 1,000 (I/Cl). Because these elements and the noble-gas 

isotopic ratios exhibit contrasting behavior with respect to 

fluid-rock interactions, phase separation, and source characteristics, simultaneous halogen and noble-gas analyses of 

fluid inclusions are expected to provide unique resolving 

power for ( 1) determining the sources of fluids and dis5 

F132 

.. .. .. · .. · 

.. .. .. .. ... __ ........,,.._ __ 

10 

solved salts and (2) distinguishing processes such as mixing, boiling, and unmixing in a variety of fossil and 

evolving active hydrothermal systems. 

ACKNOWLEDGMENTS 

The noble-gas analyses were performed in the laboratory of J.H. Reynolds (University of California, Berkeley), who directed the construction of the instrument and 

supported our attempts to use it on terrestrial materials. 

W .E. Glassley helped to broaden the scope of the terrestrial 

investigations. Laboratory and computing facilities at 

Argonne National Laboratory were provided under the 

Noble-Gas Isotopes and Halogens in Fluid Inclusions in Neutron-Irradiated Quartz Veins 85 
supervision of N.C. Sturchio. E. Shock assisted with 

calculations of noble-gas solubilities. We thank D.R. 

Dickey (Oriental mine) for generous assistance in the field. 

This study was funded in part by the U.S. Department of 

Energy, Office of Basic Energy Sciences, under Contract 

W-31-109-Eng-38, by the National Aeronautic and Space 

Administration under Contract NAG 9-34, and by the 

Department of Energy under Contract DE-AC03-

76SF00098. J.H. Reynolds, B.M. Kennedy, S.P. Smith, 

R.E. Criss, W.C. Shanks, J. Abrajano, and N.C. Sturchio 

made numerous suggestions for improving the interpretation and presentation of the data. 

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The GLM Continuous Laser System for 

40ArP9

Ar Dating: Description and 

Performance Characteristics 

By G. Brent Dalrymple 

Abstract 

The GLM is a laser system designed for 40Ar/ 39Ar 

dating of single mineral grains of mass as little as 1 

microgram. The instrument consists of a continuous 

argon-ion laser for sample heating and fusion, laser 

optics, a small extraction/cleanup system, and an ultrasensitive, ultralow-background rare-gas mass spectrometer. 

Radiogenic 40Ar yields commonly exceed 99 percent, and 

the precision of the ages measured with the GLM is 

typically 0.2 to 0.5 percent for homogeneous samples. The 

GLM may be used for total fusion dating of single- and 

multiple-grain samples, for age spectrum analysis of single 

grains, and as a microprobe to determine apparent ages 

from small areas within single grains. 

INTRODUCTION 

In the 40 Ar/ 39 Ar method of K-Ar dating, the sample 

is irradiated with fast neutrons to induce the reaction 39K(n,p) 39Ar. The age of the sample is then calculated from 

the 40 Ar/ 39 Ar ratio after appropriate corrections for interfering Ar isotopes from the atmosphere and from undesirable neutron reactions with Ca and K. 

1 40Ar 

t=-loge (1 +JX~9 ) X. - Ar 

where X. is the decay constant of 4

°K ( = 5. 5 34 x 10 - 10yr- 1). 

The conversion factor, J, for the reaction 39K(n,p) 39Ar is 

determined by irradiating a monitor mineral of known age 

with the samples whose age is to be determined (Merrihue 

and Turner, 1966; Brereton, 1970; Dalrymple and Lanphere, 1969, 1971 ; Lanphere and Dalrymple, 1971 ; Dalrymple and others, 1981). 

The method can be used in two different ways. If all 

of the argon is released by fusing the mineral in a single 

heating, the result is a total fusion age. If the argon is 

released from the sample in steps by incrementally heating 

the sample to progressively higher temperatures, the result 

is a series of ages known as an age spectrum. 

The 40 Ar/ 39 Ar method, which only requires the 

measurement of isotope ratios rather than abundances, has 

several advantages over the conventional method, wherein 

the argon is measured by isotope dilution and the potassium 

is measured on a different sample aliquant by one of several 

techniques, most commonly flame photometry. These 

advantages include (1) higher precision, (2) smaller sample 

size, (3) elimination of problems caused by sample inhomogeneity, and (4) the ability to recover age and thermal 

history information from age spectra. Disadvantages of the 40 Ar/ 39 Ar method include (1) the necessity of access to a 

nuclear reactor, (2) shipping and handling radioactive 

materials, (3) normalization of results to a monitor mineral 

of known age, and (4) interferences from neutron reactions 

that produce undesirable argon isotopes. These disadvantages, however, are easily dealt with and are outweighed by 

the above-mentioned advantages. 

Most commonly, the argon analysis for 40 Ar/ 39 Ar 

dating is done with the same or similar equipment and 

techniques used for the conventional (isotope dilution) 

method of analysis. The sample is fused in ultrahigh 

vacuum by either induction heating or an internal resistance 

furnace, the reactive gases are removed by some combination of molecular sieve and hot metal and metal-oxide 

getters, and the mass analysis is done with a rare-gas mass 

spectrometer (Dalrymple and Lanphere, 1969). These procedures usually require sample weights in the range of 

0.01-1.0 g, depending on the age and potassium content of 

the sample. 

A laser was first used for rare-gas isotopic analysis by 

Megrue (1967), who volatilized small volumes of meteoritic material with a pulsed laser and analyzed the helium and 

neon isotopes released. Later, Megrue (1973) applied this 

method to the 40 Ar/ 39 Ar dating of a lunar breccia. The 

pulsed laser probe can produce 40 Ar/ 39 Ar total fusion ages 

from a small area (typically 20 to 200 J.Lm diameter) within 

a single crystal (for example, Muller and others, 1977; 

Sutter and Hartung, 1984) but lacks the capability of 

producing age spectra or of analyzing whole, single mineral 

grains. Because of these disadvantages, the pulsed laser 

probe has been used only minimally for dating terrestrial 

materials. 

York and others ( 1981) were the first to demonstrate 

that a continuous laser could be used not only to produce 40 Ar/ 39 Ar ages from single crystals but also to determine 

age spectra. Since then, continuous laser probes at the 

University of Toronto, Princeton University, and the U.S. 

Geological Survey at Menlo Park have been used to date 

tektites and illites (Glass and others, 1986; Bray and others, 

1987), to determine the distribution of apparent ages within 

single crystals (York and Hall, 1986; Phillips and Onstott, 

The GLM Continuous Laser System for 40Ar/ 39 Ar Dating 89 
MICROSCOPE 

LASER 

SYSTEM 

LASER BEAM 

\ 

MASS SPECTROMETER 

CLEANUP 

SAMPLE 

CHAMBER 

COMPUTER D 

Figure 1. Block diagram of the GLM, a continuous laser system for 40Ar/ 39Ar dating of single mineral 

grains. 

1986), to produce age spectra from single grains of biotite 

and hornblende (Layer and others, 1987), to circumvent 

xenocrystic contamination in volcanic rocks (LoBello and 

others, 1987), and to determine highly precise ages by the 

use of sanidine crystals from Oligocene rhyolite flows 

(Dalrymple and Duffield, 1988). These laser probes consist 

of a continuous laser for sample fusion, a small-volume 

extraction/cleanup system, and an ultraclean, ultrasensitive 

rare-gas mass spectrometer. Not only can the instruments 

produce both total fusion and incremental heating data, they 

yield 40 Ar/ 39 Ar ages with precisions as good or better than 

any other method of 40 Ar/ 39 Ar dating from samples that 

weigh as little as 1 J.Lg. 

The purpose of this paper is to describe the GLM 

(Great Little Machine), the U.S. Geological Survey continuous laser probe, and its performance characteristics. 

DESCRIPTION OF THE GLM 

The GLM (fig. 1) consists of a laser system, a sample 

chamber monitored by a microscope and closed-circuit 

television, a cleanup system, and an ultrasensitive, 

ultralow-background rare-gas mass spectrometer controlled 

by a computer. The cleanup system, sample chamber, and 

laser system were designed for full computer control, but 

the hardware necessary for implementing this feature is 

presently lacking. 

90 New Frontiers in Stable Isotopic Research 

The Laser System 

The laser system (fig. 2) consists of a continuous 

laser, beam expander, beam steerers, beam switch, power 

meter, and neutral density filters. The laser is a 5-W 

argon-ion laser (Coherent Innova 90-5) with principal 

wavelengths at 488 nm and 514 nm and a maximum 

all-lines output of 7. 5 W. The output is continuously 

variable from about 25 m W to 7. 5 W and is regulated to 

about 0.2 percent (RMS) in light regulation mode. The laser 

is equipped with a wavelength selector for minimizing laser 

spot diameter when necessary, an internal aperture for 

controlling the transverse mode, an internal shutter that 

interrupts lasing for safety, and a RS232 interface for 

computer control. 

The laser beam, which exits the laser with a diameter 

of 1.5 mm at the l/e2 points, is passed through a 10 x 

Galillean beam expander (Oriel model15600) that is used to 

focus the beam onto the sample. The beam expander has an 

aperture of 23 mm, a fused silica diverging input lense with 

a focal length of - 8 mm, and an output lens with a focal 

length of + 80 mm. The output lens is an achromatic 

cemented doublet with negligible spherical aberration and 

very low wavefront distortion. Even though it is a cemented 

doublet, there is no damage to the output lens because the 

power density of the expanded beam is only about 0.04 
BEAM-STEERING MIRRORS 

5~ CONTINUOUS 

Ar-ION LASER 

N .o. FILTERS 

lOx BEAM 

EXPANDER 

BEAM SWITCH 

POWER METER 

SAMPLE 

CHAMBER 

LASER 

BEAM 

Figure 2. Schematic diagram of the laser system of the GLM. The neutral density (N.D.) filters are inserted into the light 

path during focusing and alignment of the laser beam. 

W/mm2

. Both the input and output lenses are coated with a 

multilayer, antireflection coating that reduces the combined 

surface reflection, scatter, and absorption losses to less than 

0. 8 percent for wavelengths from 400 to 700 nm and thus 

minimizes power loss within the expander. 

The laser beam is directed vertically into the sample 

chamber by first-surface beam-steering mirrors held on 

adjustable mounts attached to vibration-damped rods (Newport model 675). Rotational and angular adjustment of the 

mirrors, by micrometers, permits easy and precise alignment of the focused laser beam with the crosshairs of the 

microscope. The mirrors are specifically coated (Newport 

DM.5) for efficient reflection (>99 percent) of wavelengths 

from the argon-ion laser. A beam switch (Advanced Kinetics LSW-25-L, modified with mirror for argon-ion wavelengths) installed in the beam path is used to switch the 

beam away from the sample chamber and into the detector 

of a power meter, which also serves as a beam dump. The 

switch is a safety feature but also permits monitoring of the 

laser beam power independent of the internal laser power 

meter. For visually focusing the beam and aligning the 

beam with the microscope cross hairs, neutral density filters 

that decrease the beam intensity by 1/1000 are temporarily 

inserted into the beam path. 

The Sample Chamber 

The sample chamber (fig. 3) consists of two 2. 75-in 

stainless steel vacuum flanges (Varian Conflat). The bottom 

of the chamber is a bored out blank flange into which a short 

inlet tube is welded. The end of the inlet tube is fitted with 

a 1.33-in mini-Conflat Flange that is attached to a 0.375-in 

flexible stainless steel hose leading to the remainder of the 

extraction/cleanup system. All welds are internal to the 

vacuum system to prevent virtual leaks. The upper part of 

the sample chamber is an unmodified, 1.5-in, zero-length 

viewport with 7056 glass (Varian 954-5127). 

The sample planchette, which sits upon a stainless 

steel platform to minimize the distance to the vacuum 

viewport and improve the viewing angle of the microscope, 

is made of copper. It has 21 machined and numbered 

sample wells 1. 04 mm deep and 9. 5 mm in diameter into 

which loose sample grains are placed. The planchette is 

covered with a 1.6 mm-thick pyrex coverplate to prevent 

fused material from being deposited onto the vacuum 

viewport and to retain mineral grains, particularly biotite 

and hornblende, in their sample wells. 

The sample chamber is attached to an x-y stage, 

which permits motion of the chamber under the laser beam. 

The GLM Continuous laser System for 40Ar/ 39 Ar Dating 91 
LASER BEAM 

PYREX COVER PLATE 

I 

I 

I I 

?056 GLASS VIEWPORT 

I 

I 

I 

I 

I 

I 

1 •• n~~.n •• 1 

Cu SAMPLE 

_____________ PLANCHETTE ~ 

S .s. PLATFORM to cleanup system --1 

-

Figure 3. Schematic diagram of the sample chamber of the GLM. The body of the chamber is made from 

two 2.75-in Varian Conflat flanges., S.S., stainless steel. 

Once the beam is aligned with the microscope cross hairs, 

then any grain can be brought coincident with the 

crosshairs, and hence the laser beam, by manipulation of 

the stage. 

A stereo microscope is used to align the laser beam 

with the individual mineral grains by use of the ocular 

crosshairs and to view the samples when the laser beam is 

not directed into the sample chamber. The microscope is 

fitted with a solid state color closed circuit television 

camera (Panasonic WV-CD120) attached to a 13-in monitor (Panasonic CT-1330M) and a video cassette recorder 

(Panasonic NV-8950) that are used to monitor and record 

the fusion process. The microscope is equipped with a 

long-pass filter between the objective lenses and the sample 

chamber. The filter is of colored glass with 50 percent 

transmission at 550 nm and a transmission of approximately 

10-5 for wavelengths less than 520 nm. 

The Extraction/Cleanup System 

The extraction/cleanup system (fig. 4) is constructed 

primarily of type 304 stainless steel and has a volume of 

approximately 175 cm3

, which includes the sample chamber and the flexible stainless steel hose. The small volume 

permits admission of 90 percent of the sample gas into the 

mass spectrometer by expansion and obviates the need for 

gas transfer by the use of activated charcoal. The system 

92 New Frontiers in Stable Isotopic Research 

consists of a Zr-V-Fe getter (SAES St172/H1116-10/300C) 

operated at room temperature to remove primarily hydrogen, a Zr-Al getter (SAES AP10-GP) operated at 450 octo 

crack hydrocarbons and remove other reactive gases, and a 

pyrex LN2 finger to isolate condensibles. The system is 

pumped by a 20-L/s vacion pump (Varian 911-5030) and a 

50-L/s turbomolecular pumping system (Balzers TSU050), both equipped with bakeable valves (Varian 

951-5014) so that they can be isolated from the system. The 

entire system, including the sample chamber, which can be 

removed from the x-y stage and placed in the oven, can be 

baked at 300 oc. 

The Mass Spectrometer 

The mass spectrometer is a 90° sector directionfocusing instrument with a radius of 15 em (Mass Analyser 

Products model 216) and a Baur-Signer ion source (model 

GS-98). It is equipped with a Zr-Al getter (SAES GP-50W) 

operated at room temperature and a 30 L/s vacion pump 

(Varian 911-5032) and is bakeable to 350 °C. The analyzer 

was manufactured and assembled by means of techniques 

designed to provide the lowest possible background and 

desorption. 

The magnetic field is switched under digital computer 

control by a Hall probe for feedback. Resolution of the field 

control system is 15 ppm in mass per digital step. The 
LASER 

BEAM 

FLEXIBLE s.s. TUBE 

SAMPLE CHAMBER 

Zr-V-Fe 

Zr-Al 

GETTERS 

I 

VALVE 

LIQUID LN2 TRAP 

to mass 

spec. 

VACUUM PUMP 

Figure 4. Schematic diagram of the extraction/cleanup system of the GLM. Only one of the two vacuum 

pumping systems is shown, for simplicity. S.S., stainless steel. 

magnet has curved-entrance pole faces to correct for a ='R 

aberrations. With the source defining slit 0.4 mm wide and 

the collector entrance slit 1.44 mm wide, the flat-topped 

peak to half-width ratio is approximately 0.7 at a resolution 

of 80. 

The instrument has tandem ion collectors: a retractable Faraday cup mounted on a linear-motion feedthrough 

and a fixed 18-stage electron multiplier (Thorn EMI 

9642/3A). Each collector has an independent amplifier, the 

outputs from which are fed to a Solartron (model 7060) 

integrating digital voltmeter (IDVM). In normal operation 

only the electron multiplier is used. Mass spectrometer 

operation is completely controlled by computer (Zenith 

241) via an IEEE-488 buss controller. The total volume of 

the analyzer is less than 1,500 cm3

. 

OPERATION 

Operation of the GLM is relatively simple (fig. 5). 

First, the extraction line is isolated from the vacuum pumps 

by closing the valves, and the line is left static for 5 min. The 

getters and LN2 trap are active, so the accumulating "blank" 

is also cleaned during the isolation. Next, the mass spectrometer is isolated from its vacuum pump, and the blank is 

admitted to the spectrometer and analyzed. For the blank 

analysis, four sets of data are taken. Each set consists of 

Start 

with blank 

Start -----7 

no blank 

min. 

0 

5 

9 

14 

19 

25 

r Isolate extraction line 

L Accumulate blank 

~ Admit blank to m .s. 

J L m.s. analysis <4 sets) 

~ Open system to pump 

J L Pump m.s. & extraction line t Isolate extraction line 

J / Fuse sample & clean gas t Admit sample to m.s. 

L m.s. analysis <6 sets) 

~ Open system to pump 

Figure 5. Schematic diagram of procedure for the GLM. 

Without the background measurement, a 40Ar/ 39Ar age 

analysis takes less than 10 min. m.s., mass spectrometer. 

four 1.28-s integrations for each of the masses 36, 37, 38, 

39, and 40 as well as the baselines between each mass. After 

the blank analysis, the extraction line and mass spectrometer are pumped for 5 min, then the extraction line is again 

isolated for 5 min. During this isolation period, the sample 

The GLM Continuous Laser System for 40 Ar/ 39 Ar Dating 93 
Table 1. Typical results of 40Ar/ 39Ar total fusion age analyses by use of the GLM 

continuous laser system 

[

40 Arr, radiogenic 40 Ar] 

Sample Mineral No. of grains Sample wt. 1 40Ar, Age2 (Ma) no. (grain size) (micrograms) 10-14 mol percent 

85G001 sanidine 10-11 100 2.07 98.4 28.17 ± .32 

(180-250 J.Lm) 12-14 4.27 99.1 28.21 ± .21 

10-14 190 4.53 99.0 28.24 ± .21 

10-14 5.95 98.8 28.11 ± .19 

20-25 10.00 99.1 28.21 ± .17 

10-12 6.34 99.5 28.27 ± .19 

mean = 28.20 ± .06 

MMHb-1 hornblende 1 22 3.59 99.9 520.2 ± 3.7 

( 180-250 J.Lm) 3 72 7.77 99.7 515.5 ± 2.9 

3 4.68 99.8 521.2 ± 3.3 

3 35 5.70 99.9 521.7 ± 3.1 

2 2.54 99.8 516.1 ± 4.5 

4-5 137 14.18 99.3 521.6 ± 2. 7 

mean = 519.4 ± 2.8 

SB-3 biotite 1 15 2.81 98.5 164.3 ± 1.5 

(180-250 J.Lm) 4 15 3.21 99.7 164.1 ± 1.4 

3 1.73 97.7 161.2 ± 2.1 

4 17 3.69 97.5 163.0 ± 1.3 

2 18 4.25 98.3 164.8 ± 1.2 

4 3.03 97.7 161.4 ± 1.4 

mean = 163.1 ± 1.5 

1 Estimated from the diameter of the glass sphere that often forms on fusion. 2 Decay constants: A e=0.581 X 10- 10 yr- 1

' J...[3=4.692X 10- 10yr- 1• Errors are estimates of the standard 

deviation of precision (Dalrymple and Lanphere, 1971). 

is fused or heated (for an incremental heating experiment), 

and the sample gas is cleaned by exposure to the getters and 

LN2 trap. At the end of the 5-min period, the mass 

spectrometer is isolated from its pump and the sample gas is 

admitted to the spectrometer and analyzed, taking six sets of 

data. The entire procedure takes approximately 25 min. 

After the analysis, the extraction line and mass spectrometer 

are pumped for 10 min, during which time the data from the 

analysis are reduced, and the procedure then may be 

repeated. During data reduction, the blank for each isotope 

is subtracted from the sample values after appropriate 

adjustments for baselines. 

Because the blank is relatively constant during the 

day, it is not necessary to measure it before each sample 

analysis; thus, the time for an analysis is reduced to less 

than 10 min (fig. 5), and 20 to 30 age measurements are 

allowed during a single day. Complete automation of the 

system will eliminate operator error and permit unattended 

operation but will not shorten the time for a single analysis. 

PERFORMANCE CHARACTERISTICS 

Mineral grains are fused easily and in a few seconds 

with the GLM laser. The power required to fuse a mineral 

94 New Frontiers in Stable Isotopic Research 

grain of given mass is primarily a function of the power 

density, which varies inversely as the square of the focused 

beam radius and the ability of the mineral grain to absorb 

laser light of 488-nm and 514-nm wavelength. The dark 

minerals, biotite and hornblende, for example, require only 

100 to 200 mW for complete fusion. Muscovite and 

plagioclase typically require 1 to 2 W. In contrast sanidine, 

which is relatively transparent to the argon-ion laser beam, 

commonly requires 5 to 7 W and once fused into a 

transparent glass sphere cannot be refused with the power 

available. 

In addition to single grains, multiple grains can be 

fused simultaneously, provided the grains are in proximity 

to one another in the sample pan. For multiple-grain fusion, 

which is sometimes desirable to increase sample mass and 

hence precision, the laser is focused on a single grain. 

When that grain melts, the surface tension of the silicate 

melt "pulls in" the surrounding grains, which are incorporated into the melt. As many as 30 grains of 300-J.Lm 

diameter have been fused in less than 2 s by this method. 

The mass spectrometer has a fundamental sensitivity 

of 1. 3 rnA/torr with the ion source operated at an emission 

current of 250 J.LA. The electron multiplier, which is 

operated at 2.25 kV, provides a gain of about 4.2 x 104 and 
a realized sensitivity of about 3. 5 x 10- 14 mol/V of signal to 

the IDVM. The data system has a dynamic range of 106 and 

a lower measurement limit of 0.00001 V, so that it is 

possible to determine argon-isotope ratios to within ±0.1 

percent even for isotope concentrations less than 1 X 10- 14 

mol. The sensitivity of the instrument could be increased by 

a factor or two or three by increasing the emission current of 

the ion source, and perhaps another factor of two or so 

(relative to multiplier noise) by increasing the voltage to the 

multipler, but so far this has not proved necessary. 

The background in the mass spectrometer is typically 

1.5 x 10- 18 mol (9X 105 atoms) for m/z=36, 1 X 10- 17 mol 

for m/z=37, 2x 10- 18 mol for m/z=39, and 1.5 x 10- 16 

mol for m/z=40. A typical blank, which includes the mass 

spectrometer background, consists of about 2x 10- 18 mol 

of m/z=36, 1 X 10- 17 mol of m/z=37, 4X 10- 18 mol of 

m/z=39, and 4X 10- 16 mol of m/z=40. The blank varies 

somewhat from day to day but is relatively constant 

throughout the day. For example, the mean blank for 27 

January 1988 consisted of (4.38±0.14)X 10- 16 mol of 

mlz=40, (4.16±0.35)X 10- 18 molofm/z=39, (9.72±0.35) 

x 10- 18 of m/z=37, and (1. 74±0.69) x 10- 18 of m/z=36, 

where the errors are the calculated standard deviations of 

seven blank runs taken during the day. 

The precision of age measurements made with the 

GLM is as good or better than can be obtained by any other 

analytical method, even though the masses of sample 

analyzed are typically three to four orders of magnitude 

smaller than required by other analytical techniques. Table 

1 shows some representative results for three different 

minerals. The precision of the results for the sanidine and 

the hornblende are typical. The biotite, however, is from a 

quartz diorite that has suffered some postcrystallization 

heating, and the poorer precision for that mineral is probably due to inhomogeneity among the individual grains. 

Because the GLM system is so clean, the atmospheric 

contamination is very low, and radiogenic 40 Ar percentages 

commonly exceed 99 percent. The very low atmospheric 

contamination is at least partly responsible for the excellent 

precision obtained by the GLM. 

The Toronto continuous laser probe has been successfully used to date single potassium feldspar grains as young 

as 0.5 Ma (LoBello and others, 1987). The results obtained 

to date with the GLM indicate that the system is capable of 

producing 40 Ar/ 39 Ar ages of good precision for samples 

having high ratios of K/Ca as young as 0. 1 Ma and has the 

potential of resolving differences in apparent age of as little 

as 0. 2 percent for high K/Ca samples as young as a few Ma 

(Dalrymple and Duffield, 1988). 

FUTURE IMPROVEMENTS 

The following improvements are planned for the 

GLM. 

1. Improved beam expander that will allow precise focusing and beam alignment. 

2. Motor-driven valves that will allow computer control of 

the extraction line and mass spectrometer isolation and 

pumping cycles, as well as the introduction of the 

sample gas into the mass spectrometer. 

3. Infrared microscope that will permit temperature measurement of individual grains for incr~mental heating 

and permit complete computer control of the heating/fusion process. 

4. Motor drives for the x-y stage that will allow computer 

control of the sample positioning. 

ACKNOWLEDGMENTS 

I am grateful to Derek York, Chris Hall, and Tullis 

Onstott, who generously shared their thoughts on continuous laser probes and the designs of the Toronto and 

Princeton instruments. I also thank R.E. Criss, M.A. 

Lanphere, W.C. Shanks, and N.D. Sherrill, who reviewed 

the manuscript and offered several suggestions for improvement. 

REFERENCES CITED 

Bray, C.J., Spooner, E.T.C., Hall, C.M., York, D., Bills, T.M., 

and Krueger, H. W., 1987, Laser probe 40 Ar/ 39 Ar and conventional K/ Ar dating of illites associated with the McClean 

unconformity-related uranium deposits, north Saskatchewan, 

Canada: Canadian Journal of Earth Science, v. 24, p. 10-23. 

Brereton, N.R., 1970, Corrections for interfering isotopes in the 40Ar/ 39 Ar dating method: Earth and Planetary Science Letters, v. 8, p. 427-433. 

Dalrymple, G.B., and Duffield, W.A., 1988, High precision 40 Ar/ 39 Ar dating of Oligocene rhyolites from the MogollonDatil volcanic field using a continuous laser system: Geophysical Research Letters, v. 15, p. 463-466. 

Dalrymple, G.B., and Lanphere, M.A., 1969, Potassium-argon 

dating: San Francisco, W. H. Freeman and Co. , 25 8 p. 

--1971, 40 Ar/ 39 Ar technique of K-Ar dating: A comparison 

with the conventional technique: Earth and Planetary Science 

Letters, v. 12, p. 300-308. 

Dalrymple, G.B., Alexander, E.C., Jr., Lanphere, M.A., and 

Kraker, G. P., 1981, Irradiation of samples for 40 Ar/ 39 Ar 

dating using the Geological Survey TRIGA reactor: U.S. 

Geological Survey Professional Paper 1176, 55 p. 

Glass, B.P., Hall, C.M., and York, D., 1986, 40Ar/ 39Ar laserprobe dating of North American tektite fragments from 

Barbados and the age of the Eocene-Oligocene boundary: 

Chemical Geology (Isotope Geosciences Section), v. 59, p. 

181-186. 

Lanphere, M.A., and Dalrymple, G.B., 1971, A test of the 40 Ar/ 39 Ar age spectrum technique on some terrestrial materials: Earth and Planetary Science Letters, v. 12, p. 359-372. 

Layer, P.W., Hall, C.M., and York, D., 1987, The derivation of 40 Ar/ 39 Ar age spectra of single grains of hornblende and 

biotite by laser step-heating: Geophysical Research Letters, 

v. 14, p. 757-760. 

The GLM Continuous Laser System for 40Ar/39 Ar Dating 95 
LoBello, Ph., Feraud, G., Hall, C.M., York, D., Lavina, P., and 

Bernat, M., 1987, 40 Ar/ 39 Ar step-heating and laser fusion 

dating of a Quaternary pumice from Neschers, Massif Central, France: The defeat of xenocrystic contamination: Chemical Geology (Isotope Geoscience Section), v. 66, p. 61-71. 

Megrue, G.H., 1967, Isotopic analysis of rare gases with a laser 

microprobe: Science, v. 157, p. 1555-1556. 

---1973, Spatial distribution of 40 Ar/ 39 Ar ages in lunar 

breccia 14301: Journal of Geophysical Research, v. 78, p. 

3216-3221. 

Merrihue, C., and Turner, G., 1966, Potassium-argon dating by 

activation with fast neutrons: Journal of Geophysical 

Research, v. 71, p. 2852-2857. 

MUller, H.W., Plieninger, T., James, O.B., and Schaeffer, O.A., 

1977, Laser probe 39 Ar-40 Ar dating of material from consortium breccia 73215: Proceedings of the 8th Lunar Science 

Conference, p. 1489-1499. 

96 New Frontiers in Stable Isotopic Research 

Phillips, D., and Onstott, T.C., 1986, Argon diffusion profiles in 

mantle derived phlogopite: A laser probe study [abs.]: EOS, 

Transactions of the American Geophysical Union, v. 67, p. 

1248. 

Sutter, J.F., and Hartung, J.B., 1984, Laser microprobe 40 Ar/ 39 Ar dating of mineral grains in situ: Scanning Electron 

Microscopy, v. 4, p. 1525-1529. 

York, D., and Hall, C.M., 1986, Continuous-laser probe thin 

section chrontouring of sediments [ abs.]: TERRA cognita, v. 

6,p.117. 

York, D., Hall, C.M., Yanase, Y., Hanes, J.A., and Kenyon, 

W.J., 1981, 40Ar/ 39Ar dating of terrestrial minerals with a 

continuous laser: Geophysical Research Letters, v. 8, p. 

1136-1138. 
Application of the Laser Microprobe (LAMMA 1000) 

to the Microanalysis of Coal Constituents 

By Paul C. Lyons, john j. Morelli,1 '

2 and David M. Hercules 1 

Abstract 

The laser microprobe mass analyzer (LAMMA 1000) 

was used to detect trace and minor elements in coal 

components. Laser micro mass spectrometry (LAMMS) 

signatures were obtained easily on the maceral and mineral components in coal. Macerals in the same coal sample 

show major differences with respect to partitioning of 

many elements such as Li, K, Ca, Ti, Fe, Sr, Y, and Ba. The 

elemental partitioning differences are explained as being 

due primarily to varying amounts of illite, kaolinite, phosphate minerals, and other minerals occurring as microscopic and submicroscopic inclusions in the macerals. 

Other elements, such as Cr, V, and Ga, which were 

commonly detected in vitrinites, appear to be mainly of 

organic origin. 

This microprobe can be applied to semiquantitative 

microanalysis of some elements detected in coal, but 

development of this capability will probably require the 

use of maceral standards that are of maximum purity and 

that have uniform concentrations of the elements to be 

analyzed. Early indications from our group's experience 

with LAMMS are that it has potential for elemental mapping of minerals and macerals in coal and for organic 

analysis of coal macerals but needs more development. 

Stable isotopic ratios such as 4°Ca! 88Sr, 4°Ca!138Ba, 23Na/ 4

°Ca, 88Sr/ 24Mg, and 56Fe/4°Ca, obtained by LAMMS 

across maceral and mineral interfaces, could be related to 

marine, terrestrial, and diagenetic geochemistry. 

INTRODUCTION 

Coal is a mixture of minerals and discrete morphological organic components called macerals. Chemically, 

coal is composed of inorganic compounds (minerals) and 

complex macromolecular organic compounds. Microprobe 

techniques have been applied to the microchemical analysis 

of coal. These techniques include the (1) electron microprobe (Dutcher and others, 1964; Minkin and others, 1979; 

Denoyer and others, 1982; and Lyons and others, 1984a), 

(2) scanning transmission electron microscope (STEM; 

Minkin and others, 1983; Palmer and Wandless, 1985), (3) 

scanning electron microscope/energy-dispersive X-ray analysis (SEM/EDX; Augustyn and others, 1976; Boateng and 

Phillips, 1976; Finkelman and others, 1976; Stanton and 

1 University of Pittsburgh, Department of Chemistry, Pittsburgh. 

PA 15260. 2 Present address: McDonnell Douglas Research Laboratories, P.O. 

Box 516, St. Louis, MO 63166. 

Finkelman, 1979; Finkelman, 1981; Lyons and others, 

1982; Makjanic and others, 1983; Thome and others, 

1983), (4) proton-induced X-ray emission (PIXE; Chen and 

others, 1981; Minkin and others, 1982, 1987), and (5) 

secondary ion mass spectrometry (SIMS; Gaines and Paige, 

1983; Wolf and others, 1983; Mcintyre and others, 1985; 

Martin and Mcintyre, 1985; Martin and others, 1986). 

These techniques were applied principally to elemental 

analysis of the inorganic (mineral) components and, to a 

lesser extent, to the elements in the maceral components of 

coal. Traditionally, microchemical techniques have been 

used for elements with atomic number (Z) = 11 to 92. 

The application of laser micro mass spectrometry 

(LAMMS; Morelli and others, 1988) to the microanalysis of 

coal offers the advantage of both inorganic and organic 

analysis. Pioneering work on the application of the laser to 

coal analysis was done in the middle and late 1960's 

(Vastola and Pirone, 1966; Joy and others, 1968; Kam and 

Sharkey, 1968; Vastola and others, 1970). This work is 

summarized in Vastola and McGahan (1983). Early problems with the technique were due to poor beam resolution 

(optimum 50 f.Lm) and the lack of high-speed computer 

techniques for data acquisition, reduction, and analysis. 

Nevertheless, early laser spectral data were obtained for the 

microlithotype vitrite from different rank coals (Vastola and 

Pirone, 1966). However, the early work of Vastola and 

Pirone ( 1966) indicated that a laser surface microprobe that 

had fine spatial resolution was needed for maceral 

microanalysis; this need was fulfilled by the development of 

LAMMA (LAser Microprobe Mass Analyzer) 1000 in the 

early 1980's (Heinen and others, 1983; Hercules, 1984). 

This paper illustrates some of the work accomplished 

in the laser microanalysis of coal, a team effort by the U.S. 

Geological Survey and the University of Pittsburgh, Department of Chemistry. This work has resulted in LAMMS 

spectra of common macerals and minerals found in bituminous coal. The potential of LAMMS for stable isotopic 

gem:hemistry of coal is briefly addressed. 

COAL MACERAL TERMINOLOGY 

The fundamental organic constituents of coal are 

called macerals, which are comparable to minerals in 

inorganic rocks. Macerals are classified on the basis of 

optical reflectance into three groups: the liptinite, vitrinite, 

and inertinite maceral groups, in order of increasing reflecApplication of the laser Microprobe (lAMMA 1 000) to the Microanalysis of Coal Constituents 97 
tance. However, reflectance is rank dependent, so this order 

of reflectance is not applicable in low-volatile bituminous 

and higher rank coals. 

Macerals are distinguished in reflected or fluorescent 

light on the basis of morphology, size, degree of cellularity, 

fluorescent properties, and botanical origin (Stach and 

others, 1982). In Euramerican bituminous and anthracitic 

coals, macerals of the vitrinite group are the major macerals 

and typically occur in bands. Three distinct macerals of this 

group are telinite (cellular), collinite (noncellular), and 

vitrodetrinite (small, angular, transported vitrinite). Generally, both vitrinite and inertinite macerals in Euramerican 

bituminous coals are nonfluorescent, so they can be readily 

distinguished from liptinite macerals. Thus, an oval-shaped 

fluorescent body with a lumen would be called sporinite, a 

maceral of the liptinite maceral group. Fusinite, like telinite, is cellular but has a higher reflectance like other 

inertinite macerals. 

The Advantages of the LAMMA System 

There are several compelling reasons for considering 

the application of the LAMMA 1000 (Leybold-Heraeus 

GMBH) to the microanalysis of coal constituents. It is 

basically a surface analytical system that has a depth of 

penetration of 0.1 to several micrometers (Leybold-Heraeus 

GMBH, undated). LAMMS can be performed on the same 

polished surface that is analyzed by standard optical petrographic procedures (Stach and others, 1982). Also, the 

LAMMS has the capability to analyze most elements, 

including the light elements (Z < 11), such as H, Li, Be, 

and B, and it has parts-per-million sensitivity for many 

elements (Hercules, 1984). The LAMMA 1000 can function as both an organic and inorganic microprobe, unlike 

most microprobes. It is capable of detecting in coal elemental ions and organic and inorganic molecular ions up to a 

mass of 500 amu. Also, after a vacuum is obtained, this 

system can analyze coal constituents rapidly, and many 

spectra can be stored in 1 min. An adequate vacuum 

normally takes about 10 to 20 min to obtain for small coal 

samples. Another advantage of the LAMMA 1000 is that 

relatively large samples, up to 10 em across, can be 

mounted on the sample holder so that large areas of the coal 

surface containing many macerals and minerals can be 

analyzed. It is therefore possible to map the distribution of 

large macerals and minerals (Wilk and Hercules, 1987; 

Morelli and others, 1987) and to determine the spatial 

distribution of elements or compounds. This capability is 

particularly useful near mineral and maceral interfaces. The 

rapid LAMMS ionization ( 15 ns) also reduces the likelihood 

of extensive ion recombination (Balasanmugan and others, 

1981). Another advantage is that that the shallow depth of 

penetration of the laser beam reduces the possibility of 

analyzing material not seen on the surface by optical 

98 New Frontiers in Stable Isotopic Research 

techniques~ SIMS (Blattner and Evans, 1980) is limited by 

sample roughness and surface contamination that are not 

problems with LAMMS; preliminary laser bursts using 

LAMMA can eliminate surface contaminants. 

There are other advantages to microanalysis when the 

LAMMA 1000 is used. First and foremost is its capability 

to analyze areas as small as 5 J.Lm in diameter. Thus, 

macerals, maceral inclusions, cell walls, mineral matter, 

and even intracellular inclusions can be analyzed for both 

their inorganic and organic constituents. LAMMS can 

analyze hydrocarbon components that cannot be analyzed 

by most other microprobes. Because data can. be obtained 

readily on both cations and anions by reversing polarity, 

both positive and negative LAMMS spectra can be 

obtained. Also, LAMMS ionization can be varied over a 

range of ionization conditions by the use of variable laser 

power density (Denoyer and others, 1982; Leybold-Heraeus 

GMBH, 1983). Thus, complex organic components can be 

ionized under conditions that promote parent-ion formation 

without causing extensive fragmentation and possible 

molecular rearrangement. 

THE LAMMA 1000 SYSTEM 

A schematic of the LAMMA 1000 system is shown in 

figure 1. Details of this system can be found in Hercules 

(1984), Lyons and others (1984b, 1987), and Morelli and 

others (1988). 

The LAMMA 1000 can be used in either an observation or analysis mode. The laser, ionization, and products 

can be observed in the analysis mode under 100 x total 

magnification. A 250 x total magnification is used in the 

observation mode to pinpoint the target with x-y-z manipulators having movements of 70, 50, and 50 mm, respectively. 

A Q-switched, high-power Nd Y AG (Yttrium Aluminum Gamet) laser is used for analysis. The fundamental 

mode is A = 1 ,060 nm, but the LAMMA is normally used 

in the frequency-quadrupled mode of A = 265 nm. The 

laser pulse is 20 ns. The intensity of the laser can be 

continuously varied with crossed polarizers so that optimum 

ionization can be obtained by trial-and-error procedures. A 

time-of-flight mass spectrometer having a drift tube of 1. 8 

m is used for separation of the ions that are always singly 

charged, either negatively or positively, in coal microanalysis. Either negative or positive ions can be analyzed by the 

LAMMA system. 

The electronics of the LAMMA 1000 system consist 

of an open 17 -dynode copper-beryllium multiplier that has 

good signal-to-noise characteristics, as shown in LAMMS 

spectra of this paper. The signal is preamplified ( 10 x) and 

fed into a Biomation 8100 transient recorder that has 2 

kilobytes of memory (Morelli and others, 1988). The data 

can be transferred in digital form from the transient 
observation 

mode 

analysis 

mode 

perpendicular obser:vati~ ITI to pump ob~tive ob)E!Ctrve OO carnage 

1on lens system 

!~Ito pump 

mchned sample illumination 00 

telescope 

telescoptl 

CRT-display 

strip chart recorder 

CRT-display 

[g 

[JJJ 

strip chart recorder 

Figure 1. Schematic of the LAMMA 1000. Courtesy of Leybold-Heraeus GMBH. 

recorder to a Hewlett Packard 1000E Series computer. 

Under ideal conditions, the spectral resolution of the 

LAMMA instrument is approximately 800 (m/~m) at mass 

350. Data can be transferred from the computer to a 

graphics terminal where hard copies of individual or averaged spectrum can be plotted in about 5 min. The spectra 

can also be observed or monitored on an oscilloscope during 

analysis. A maximum of 60 LAMMS spectra can be 

obtained on the same target during 1 min of analysis. 

Detection Limits 

The detection limits for selected metals analyzed by 

the LAMMA 1000 system are usually in the range 0.1 to 

1.0 ppm (Hercules, 1984). For the alkali metals (Li, Na, K, 

Rb, Cs), the detection limits are 0.1 to 0.5 ppm, and for the 

alkaline earth elements (Mg, Ca, Sr, Ba), the detection 

limits are 0.4 to 1.0 ppm. The detection limits for Al, Pb, 

Ag, and U are 0.2, 0.3, 1.0, and 2.0 ppm, respectively. 

These limits were determined in a noncoal matrix and 

may represent optimum detection limits. However, from 

quantitative data (Lyons and others, in press a), it is known 

that gallium, in the range 1 to 10 ppm, can be detected by 

LAMMS in a vitrinite (coal) matrix. Thus, it is assumed on 

the basis of relative ionization potential that the detection 

limits for the alkali metals in a vitrinite matrix are this good 

or better. 

The LAMMS detection limits for some elements are 

relatively poor. Hercules (1984) reported a detection limit 

of 20 ppm for copper. The detection limit for silicon in the 

positive spectrum is much less. For example, Lyons and 

others (in press b) show that LAMMS spectra of clays, 

which contain major amounts of Si, show only weak peaks 

for Si but intense peaks for Al. This same study shows that 

Application of the laser Microprobe (LAMMA 1 000) to the Microanalysis of Coal Constituents 99 
Figure 2. SEM micrograph of a microcrater (melt type) 

produced by the LAMMA 1000 in a coal maceral. Note the 

micron and submicron mineral grains in and around the 

microcrater. Scale 10 1-1m. Micrograph courtesy of John 

Evans, U.S. Geological Survey. 

the common appearance of silicon in clays as Sio; and 

Si03 ions in negative LAMMS spectra perhaps explains 

silicon's lack of detection in LAMMS positive spectra. 

Apparently, silicon does not readily form stable Si + ions 

during LAMMS ionization but forms mainly oxygencontaining anions. 

Crater Characteristics 

The optimum LAMMA crater is subcircular and 

about 1 J.Lm in diameter (Hercules, 1984). In a coal matrix, 

the crater is elliptical, about 5 J.Lm in shortest dimension and 

20 J.Lm in longest dimension, and has a maximum depth 

estimated to be 5 J.Lm but may be considerably less when 

lower laser power densities are used. This type of crater, 

here called a melt crater (fig. 2), is different from the crater 

produced in some minerals where the cleavage appears to 

have influenced the crater characteristics. The latter type of 

crater, here called a shock crater (fig. 3), appears to show 

the characteristics of a shock impact, although there was 

probably also melting, ionization, and devolatilization near 

the center of the crater. In some minerals such as spodumene, the depth of the laser crater produced by the 

LAMMA 1000 has been determined optically to be about 

0.1 J.Lm (E. Roedder, U.S. Geological Survey, retired, 

1 00 New Frontiers in Stable Isotopic Research 

Figure 3. Photomicrograph of the microcraters (shock 

type) produced by the LAMMA 1000 in the mineral magnesite. Scale 10 1-1m. Micrograph courtesy of johann Gieb, 

Jnstitut fUr Geologie und Paleontologie, Lahnberge, Federal Republic of Germany. 

personal commun., 1986). After each laser burst on the 

same target, the crater size appears to enlarge slightly in 

diameter but less significantly in depth. Thus, the LAMMA 

1000 is not useful for depth profiling in coal. 

Relationship of LAMMA to Other Microprobes 

Although most microprobes have beam spatial resolution of < 10 J.Lm, the depth of penetration of the various 

microprobe beams varies considerably. For example, PIXE 

has a minimum depth of penetration of 50 J.Lm or more 

(Minkin and others, 1982), whereas SIMS has an penetrative depth of 0.1 J.Lm or less (Mcintyre and others, 1985). 

Thus, SIMS is surface sensitive and LAMMS is not except 

at lower power densities. Also, both SIMS and LAMMS 

can detect the entire range of elements from hydrogen to 

uranium. 

The nature of the beam varies among most microprobes. In the SEM/EDX, PIXE, and SIMS, charged 

particles are used: electrons; protons; and Ar, He, Cs, and 

0 ions, respectively. Alternatively, LAMMS uses a neutral 

beam, normally ultraviolet light (}\ = 265 nm), for 

microanalysis. 

These differences have some analytical consequences. Typically, if charged particles are used for anal-
ysis, there are charging problems encountered during the 

analysis. The charging problem is circumvented by coating 

the analyte with some conductor such as graphite or gold. 

Although LAMMS has some minor charging problems, the 

surface does not require coating because a change in the 

electrostatic potential can usually eliminate any charge that 

may build up. 

Another difference among the microprobes is the 

method of detection of the atoms or ions. The SEMIEDX 

and PIXE use X-ray spectrometers for detection of the 

atoms, whereas LAMMS and SIMS use mass spectrometers, which are based on the mass/charge (m/z) differences 

among the ions. 

Preliminary collaborative work on the same samples 

by use of PIXE, SIMS, and LAMMA, in cooperation with 

J.A. Minkin, (U.S. Geological Survey) and R.R. Martin 

(University of Western Ontario), indicates that SIMS and 

LAMMS yield similar mass spectra. Thus, SIMS and 

LAMMS spectra can be compared easily. 

Procedures Used in LAMMA Coal Microanalysis 

The procedures used in LAMMS analysis of coal 

constitutents are given in Lyons and others (1987). First, 

coal macerals are observed on polished surfaces through a 

petrographic microscope, and circles (250 to 500 J.Lm in 

diameter) are inscribed so that the general micro-area can be 

found easily with the LAMMA 1000 optical subsystem. 

Photomicrographs are used as maps to help align the laser 

optics on the target. 

Both positive and negative LAMMS spectra are 

normally obtained in separate analysis steps. However, 

most of the elemental signals, excluding the halogens (F, 

Cl, and Br), are seen in the positive-ion mode, which is the 

mode most frequently used in the microanalysis of coal 

constituents. The optimum laser energy is determined by 

trial-and-error techniques outside of the target area or on a 

similar maceral or mineral. Normally, a laser energy at the 

source equivalent to about twice the threshold of ionization 

is used. The sample-stage manipulators are used to align the 

laser on the target. One or more single preliminary spectra 

are obtained to optimize laser energy conditions; subsequently, 12 additional spectra are obtained and averaged. 

The data for the individual and averaged spectra are stored 

in digital form in the computer. 

In coal microanalysis, unit mass calibration of the 

LAMMA 1000 is easily accomplished by use of commonly 

occurring coal isotopic signals having known m/z values at 

23 e3Na), 39 e9K), and 56 CS6Fe) in the lower mass range 

and the organic peaks at m/z 61 (C5H), 85 (C7H), and 109 

(C9H) in the higher mass range. Peaks at m/z up to about 

500 have been observed occasionally in LAMMS coal 

analysis, but generally the signals are below m/z 200. 

LAMMA SIGNATURES AND THEIR 

INTERPRET A liONS 

Because LAMMS is fundamentally, at present, a 

qualitative tool with parts-per-million sensitivity for most 

elements (Hercules, 1984), it is ideally suited to the 

detection of elements in minerals and macerals in coal (see 

Lyons and others, 1984b, 1987; Morelli and others, 1988). 

These suites of elements and occasional molecular peaks in 

particular macerals or minerals have been referred to as 

LAMMA "fingerprints" (that is, signatures, Lyons and 

others, 198 7). The reproducibility of major isotopic peaks is 

±8 percent (at best), which gives LAMMS the potential for 

semiquantitative analysis (Morelli and others, 1988). 

It has been shown that two types of vitrinite (telocollinite and corpocollinite) in the same coal can be distinguished by LAMMS (Lyons and others, 1987). On the basis 

of Li, Ca (m/z 40 and 44), Ti, Sr, Ba, F, Cl, and a peak at 

mlz 65, which were lacking or absent in the nonbanded 

vitrinite ( corpocollinite), these authors demonstrated the 

potential of LAMMS as a "fingerprinting" system. In 

addition, they showed that the mineral pyrite (FeS2), 

identified on the basis of chemistry and crystal habit, is 

readily "fingerprinted" with LAMMS peaks at m/z 32, 56, 

88, 112, 120, 144, and 176, which correspond to s+, Fe+, 

FeS+, Fe2 +, FeS2 +, Fe2S+, and Fe2S2 +, respectively. 

Other LAMMS spectra of vitrinites are shown in 

figures 4 and 5. Figure 4 is the positive LAMMS spectrum 

of vitrinite from the Indiana V coal bed (Middle Pennsylvanian, Late Carboniferous), which shows an intense vanadium signal at m/z 51 and a weaker signal at m/z 39 

(potassium). These are contrasted with the vitrinite from the 

Liddell coal bed (Permian) of the Sydney Basin, Australia 

(fig. 5), which shows peaks at m/z 90 (zirconium) and 

161-164 (dysprosium). These two elements are rarely 

observed in the LAMMS spectra of coal macerals. However, the similarity of the spectra in most other respects may 

indicate that similar soil conditions or geochemical processes, rather than different plant precursors (Carboniferous 

tropical plants as opposed to Permian temperate Gondwana 

plants), exercised the major control over the microchemistry of the vitrinites. 

The signal at m/z 65 in positive spectra of vitrinite 

and inertinite macerals has not been unequivocally identified. It is a molecular signal, not an elemental signal, 

because there is no major isotopic peak at m/z 65; typically 

it is relatively intense in high-sulfur coals (fig. 4) and in 

some intertinite macerals (fig. 6). Where m/z 65 is 

extremely intense, as in figure 6, an inertinite maceral from 

the Wilgus coal bed, Ohio, it is associated with peaks at m/z 

64 and 66. That these three peaks also occur when there is 

little or no iron indicates that they are not due to pyritic 

sulfur. S2 and compound ions S02 , S03 , and so on, would 

normally be expected in negative spectra and not in positive 

Application of the laser Microprobe (lAMMA 1 000) to the Microanalysis of Coal Constituents 1 01 
1400 

Na Ca VITRINITE 

1200 

\ AI 

Ti 

v 

1000 I I 

-

>R 

~800 

._j 

c 

600 

Q) 

0: 

400 

200 

0 

0 50 100 150 200 250 300 

M/Z 

Figure 4. LAMMS positive spectrum of vitrinite from the Indiana V coal bed (Middle Pennsylvanian), 

Lynnville mine, Warwick County, Indiana. Peaks at C7 , C11 , and C15 are part of a Codd series; m/z 65 = 

HS2(?). Rei. Int., relative intensity. 

1400 Ti 

I VITRINITE 

1200 

1000 

-

"*-

-800 

._j 

c 

Q) 

600 

0: 

400 

200 

0 

0 50 100 150 200 250 300 

M/Z 

Figure 5. LAMMS positive spectrum of vitrinite from the Liddell coal bed (Permian), Sydney Basin, 

Australia. Peaks at CrC15 are Codd series; m/z 65 = HS2 (?); m/z 112-113 =cadmium(?). Rei. Int., relative 

intensity. 

spectra. We speculate here that the peaks at mlz 64, 65, and 

66 are due to S2 + , HS2 +, and H2S2 + of probable organic 

origin. LAMMS characterization of organosulfur compounds is necessary to support this assertion. Note also in 

figure 6 in the LAMMS spectrum that the peak at m/z 77 is 

interpreted as a benzene derivative (C6H5), which would be 

expected from pyrolysis of aromatic compounds. Also, note 

in the LAMMS signature of this maceral the relatively 

intense peaks for Mg, Ca, Co, Sr, Y, Ba, and La. 

Negative LAMMS spectra usually are not informative 

in the microanalysis of coal macerals. However, the spec102 New Frontiers in Stable Isotopic Research 

trum of vitrinite from the Appalachian basin (fig. 7) shows 

the presence of 0 (rnlz 16), OH (m/z 17), F (rnlz 19), and 

Cl (rnlz 35 and 37). The 0, OH, F, and Cl could be derived 

from clays in the vitrinite (see Lyons and others, 1987). 

However, these elements could also be a part of organic 

molecules composing the vitrinite. 

Some examples of positive-ion LAMMS spectra of 

minerals from coals are shown in figures 8 through 11. 

Sphalerite from the Herrin (No. 6) coal bed of the Illinois 

basin (fig. 8) shows the principal Zn isotopic signals at m/z 

64, 66, and 68, as well as the signals at m/z 56 and 58, 
200 

160 

-120 

~ _. 

c 80 

Q) 

a: 

40 

0 

0 

Na 

\ 

Mg 

AI 

/ K Ca 

\ 

40 

Fe 

64 

Fe \ 66 /© 

80 

M/Z 

Sr 

y 

I SECRETINITE 

(INERTINITE) 

113 

120 160 

Figure 6. LAMMS positive spectrum of the inertinite maceral secretinite (see Lyons and others, 1986) from 

the Wilgus coal bed (Upper Pennsylvanian), Lawrence County, Ohio. Coal sample courtesy of G.K. 

Merrill, University of Houston. Interpretations of numbered mass to charge peaks are 64 = 52 (?); 65 = 

HS2 ; 78 = C6H6 (benzene); 85 = C7H; 98 = C8H2 ; 109 = C9H; 113 =not identified; 121 = C10H. Rei. Int., 

relative intensity. 

100 

80 VITRINITE 

eo 

70 

- eo ~ 

-_. eso 

c: CsH 

40 

CD 

a: 

30 

OCN 

20 

10 

0 

0 10 20 so 40 50 eo 70 eo 80 100 

M/Z 

Figure 7. LAMMS negative spectrum of vitrinite from the Raymond coal bed (Upper Pennsylvanian), 

Kanawha County, West Virginia. Rei. Int., relative intensity. 

corresponding to Fe and Ni, respectively. Peaks at rn/z 69 

and 71 are probably attributable to gallium. A LAMMS 

spectra of cleat calcite (calcite filling a micro fracture) with 

associated impurities is shown on figure 9. The signals at 

rn!z 40, 42, and 44 indicate high concentrations of calcium. 

The peaks at rn/z 27 (due to Al) and at 54 and 56 (due to Fe) 

perhaps indicate some aluminum-bearing mineral impurities 

and, perhaps, Fe in the calcite structure. Figure 10 is a 

LAMMS spectrum interpreted as "illite" associated with the 

inertinite maceral of figure 6. The signals at rn/z 27, 39 and 

Application of the laser Microprobe (LAMMA 1 000) to the Microanalysis of Coal Constituents 103 
50 

SPHALERITE 

40 Zn 

-

* 

- 30 

......,; 

c Zn 

Q) 

0::: 20 

Zn 

Fe 

10 

0 

0 20 40 60 80 100 

M/Z 

Figure 8. LAMMS positive spectrum of sphalerite from the Herrin (No. 6) coal bed (Middle Pennsylvanian), Peoria County, Illinois. Interpretation of peak at m/z 37 = C3H. Rei. Int., relative intensity. 

Specimen courtesy of James C. Cobb, Kentucky Geological Survey. 

100 Na AI Ca 

\ I I CLEAT CALCITE 

80 a Ca 

I -

* 60 

-...,...; 

c: 

Q) 40 Fe 

a: 

20 

0 

0 20 40 60 80 100 

M/Z 

Figure 9. LAMMS positive spectrum of cleat calcite with aluminum-bearing mineral impurities and, 

perhaps, iron in the calcite structure. The sample is from the Wilgus coal bed (Upper Pennsylvanian), 

Lawrence County, Ohio. Rei. Int., relative intensity. 

41, and 54 and 56 correspond to AI, K, and Fe, respectively, which are the major LAMMS signals characteristic 

of illite (Lyons and others, in press b). The titanium signal 

at rnlz 48 may have the same mineralogical origin. The 

alkaline earth peaks at m/z 24, 40 and 44, 88, and 138, 

which correspond to Mg, Ca, Sr, and Ba, respectively, are 

interpreted as being due to one or more associated minerals 

1 04 New Frontiers in Stable Isotopic Research 

(for example, calcite or other carbonate minerals, phosphates, barite, and so on). An arsenic-bearing pyrite in the 

cell walls of the Lower Bakerstown coal bed, Maryland 

(fig. 11) may imply the derivation of arsenic (as well as 

sulfur) from degradation of organic matter in the cell walls. 

Minkin and others (1984) previously noted arsenic in pyrite 

in coal by microprobe techniques. 
200 AI 

160 

-

?fl. 120 --+-i 

c 

Q) 80 Na a: 

40 

0 

0 

K 

\ K 

I 

Ca 

Fe 

40 

Fe 

I 

80 

M/Z 

''ILLITE"+ UNKNOWN 

Ba 

120 160 

Figure 10. LAMMS positive spectrum of "illite" from the Wilgus coal bed (Upper Pennsylvanian), 

Lawrence County, Ohio. Coal sample courtesy of G.K. Merrill, University of Houston. Rei. Int., relative 

intensity. 

100 s \ s 

c I 

Fe 

I CELL -WALL PYRITE 

80 

~ 60 

-

As 

20 

0 

0 50 100 150 200 250 300 

M/Z 

Figure 11. LAMMS positive spectrum of arsenic-bearing pyrite in the cell wall of a pyrite coal ball, Lower 

Bakerstown coal bed (Upper Pennsylvanian), Garrett County, Maryland. Rei. Int., relative intensity. 

Potential of LAMMA for Organic Microanalysis 

of Macerals 

LAMMS has already been applied to the "fingerprinting" of organic polymers. For example, epoxy and paraffin 

(Nitsche and others, 1978), polyethylene, polystyrene, and 

phenoxl resin (Dutta and Talmi, 1982) and other organic 

polymers (Gardella and others, 1980; Hercules and others, 

1982; Hercules, 1983) have been successfully "fingerprinted" by LAMMS techniques. Their work therefore opens 

up the possibility that macerals also can be "fingerprinted" 

by LAMMS, because organic polymers are homogeneous 

Application of the laser Microprobe (LAMMA 1 000) to the Microanalysis of Coal Constituents 1 05 
MAP NUMBER 

:1:1:1 

COAL KACBKAL 

LPl 

PYRITE 

IN VITRINITE 

MAP OF MASS 56 

(Fe) 

POLARITY + 

AREA MAPPED 

350 X 350 J,Jm 

,//.~//~/~~A~~hY~fl~~//~/ASTEP SIZE 

Figure 12. LAMMS map of pyrite (black) in a vitrinite 

matrix (light), unnamed coal bed (Middle Pennsylvanian), 

Narragansett basin, Massachusetts. The map is of m/z 56 

(Fe+). From Wilk and Hercules (1987) and Morelli and 

substances somewhat like macerals. More experimentation 

with different laser energies and microchemical surface 

techniques is necessary to test this possibility. 

POTENTIAL OF LAMMA FOR QUANTITATIVE 

WORK IN COAL MICROCHEMISTRY 

Preliminary work indicates that LAMMS has some 

semiquantitative potential for the microchemical analysis of 

coal macerals (Morelli and others, 1988). Reproducibility 

of the laser energy is ± 8 percent at best; therefore, 

averaging of multiple LAMMS shots (as shown for mixtures of benzalkonium chloride) is necessary for quantitive 

analysis (Hercules, 1984). Calibration plots for lithium and 

lead in standard samples indicate a linear relationship 

(Kaufmann and Hillenkamp, 1979; Wechsung and others, 

undated) in which standard deviations are less than 5 

106 New Frontiers in Stable Isotopic Research 

10 J,Jm 

others (1987). The 350-,.,_m by 350-,.,_m area covered in the 

figure represents 1,225 LAMMS analyses at 10-,.,_m intervals. The map essentially shows a pyrite grain ( -250 ,.,_m 

across) surrounded by vitrinite. 

percent at concentrations greater than 10 ppm (Kaufmann 

and Hillenkamp, 1979). However, reproducibilities in practical analyses are generally ± 15 percent (relative standard 

deviation) at best. Hercules (1984) has shown, using 

average LAMMS data, that there is a good correlation with 

high-performance liquid chromatography for the quantitative analysis of some organic compounds. Positive correlation of analytical results were also obtained between 19F 

NMR (Nuclear Magnetic Resonance) and LAMMS in 

analyses of polyvinylidine fluoride (Hercules, 1983). 

LAMMA "MAPPING" 

Illustrations of LAMMS for mapping of the distribution of elements in a vitrinite matrix are given in figures 12 

and 13. Figure 12 shows the distribution of Fe+ (rnlz 56) 

from pyrite in a vitrinite matrix (Wilk and Hercules, 1987; 
Figure 13. LAMMS map of pyrite (light area left of center) 

surrounded by vitrinite (darker areas), unnamed coal bed 

(Middle Pennsylvanian), Narragansett basin, Massachusetts. Map is of mlz 84 (C7 ), negative spectra, same area as 

figure 12. From Wilk and Hercules (1987). The 350-jJ-m by 

Morelli and others, 1987). Figure 13 shows the same area at 

m!z 84 (C7). The 350 ~m x 350 J-Lm area covered in figures 

12 and 13 represents 1 , 225 LAMMS analyses at 10-J-Lm 

intervals. The "maps'' (figs. 12 and 13) essentially show a 

pyrite grain ( ~250 J-Lm across) surrounded by vitrinite. 

Although SIMS is clearly superior to LAMMS in mapping 

(Mcintyre and others, 1985; Martin and others, 1986), both 

have ion-imaging capability for imaging of the same microareas. However, charging problems with SIMS (Wilk and 

Hercules, 1987) may make it less superior in this respect for 

automated mapping of microsurfaces of coal. 

POTENTIAL OF LAMMS FOR STABLE ISOTOPIC 

GEOCHEMISTRY OF COAL 

The application of stable isotopic geochemistry has 

focused on the elements H, C, N, 0, and S (Anderson and 

MAP NUMBER 

114 

COAL IU.CBRAL 

LPl 

PYRITE 

IN VITRINITE 

MAP OF MASS 84 

(Organic) 

POLARITY -

AREA MAPPED 

350 X 350 JJm 

~"'*"r"""""to.A STEP SIZE 

10 .um 

350-fJ-m area covered in the figure represents 1,225 

LAMMS analyses at 10-jJ-m intervals. The map essentially 

shows a pyrite grain (~250 fJ-m across) surrounded by 

vitrinite. 

Arthur, 1983), all abundant in coal. Although LAMMS has 

sufficient mass resolution to detect the more abundant 

isotopes such as 12C, 13C, 160, 180, and 32S and 34S, the 

natural variation in abundance of the isotopes is too small 

for LAMMS, at present, to be a viable technique for 

measuring fractionation of the isotopes because the reproducibility of the major isotopic signal in LAMMS is ±8 

percent. With averaging of many isotopic ratios, reproducibility might be improved to a few percent, at best, which is 

still too imprecise for useful isotopic ratio determination. 

One of the limitations of the LAMMA system is that, 

at present, it has only unit mass resolution. For element 

identification and determination of concentration, the 

masses of interest must be free of interference by other 

elements or molecules, such as hydrocarbons (CxHy), having the same nominal mass to charge ratio. An example is 

m!z 27, the same nominal mass to charge for Aland C2H3 . 

Application of the Laser Microprobe (lAMMA 1 000) to the Microanalysis of Coal Constituents 1 07 
Multiply charged ions used in spark-source mass spectrometry to verify the presence of many elements are not 

available from the laser source. Two techniques can be used 

to produce interference-free peaks: (1) removal of the 

carbonaceous material by pretreatment or (2) use of a mass 

spectrometer of sufficiently high resolving power to "split 

off' interfering ions in a multiplet spectrum. The combination of a laser ion source and a high-resolution massto-charge analyzer would have this capability. 

The greatest potential of LAMMS for stable isotopic 

geochemistry is mapping of common elements found in coal 

across mineral and maceral interfaces. Thus, 4°Ca!88Sr, 4

°Ca!138Ba, 23Na/40Ca, 88SrP4

Mg, 56Fe/4

°Ca, and other 

elemental ratios from carbonates, clays, phosphates, barite, 

and other minerals possibly could be related to saltwater, 

terrestrial, and diagenetic influences on the peat. Such 

mapping could be related to 34SP2

S and 160/180 ratios 

obtained by conventional techniques from samples from the 

same bands in the coal. Lyons and others (in press c) have 

shown that sulfate, probably from marine water, was 

introduced after the peat was laid down; bacterial sulfate 

reduction caused a fractionation of sulfur so that the pyrite 

shows a regular increase of 34S with depth in the peat (coal). 

CONCLUSIONS 

Recent results (Lyons and others, 1984b, 1987; 

Morelli and others, 1987) show that the LAMMA 1000 has 

the capability for qualitative microprobe analysis of trace or 

minor elements in macerals or minerals in coal. LAMMS 

signatures can be obtained easily on minerals and on 

vitrinite and inertinite macerals. The partitioning of elements among closely associated macerals can be studied by 

use of LAMMS by normalizing isotopic signal intensities to 

the intensity of the m/z 85+ organic peak (C7H) (Morelli 

and others, 1988). 

These studies indicate that some elements, such as 

AI, K, Ca, Ti, Fe, Sr, and Ba in coal macerals, can be 

explained as being due to microscopic and submicroscopic 

mineral matter (fig. 2; see also Minkin and others, 1983; 

Palmer and Filby, 1984) dispersed in the macerals. Other 

elements, such as Cr, V, and Ga, which have rather uniform 

intensities relative to m/z 85+, appear to be partly or mainly 

complexed in organic compounds. One challenge for 

LAMMS is to develop a semiquantitative capability for the 

microanalyses of coal components, which will probably 

require the use of maceral elemental standards that have 

rather uniform elemental concentrations. LAMMS has 

some potential for mapping isotopic ratios such as 

40Ca/88Sr, 4oCa/138Ba, 23Na/40Ca, 88Sr/24Mg, and 

56Fe/4

°Ca across mineral and maceral interfaces in coal. 

These ratios could be related to 160/ 180 and 32S/34S ratios 

obtained by conventional techniques and to marine, terrestrial, and diagenetic geochemistry. 

108 New Frontiers in Stable Isotopic Research 

ACKNOWLEDGMENTS 

The authors thank Z.A. Wilk, formerly of the Chemistry Department, University of Pittsburgh, for supplying 

the LAMMA maps (figs. 12 and 13); John Evans, U.S. 

Geological Survey, for allowing us to use his SEM micrograph (fig. 2); Johann Gieb, Institut fiir Geologie und 

Paleontologie, Lahnberge, Federal Republic of Germany, 

for supplying an SEM micrograph (fig. 3); J. Sharkey, 

University of Pittsburgh, for supplying information on the 

potential of LAMMS; and C.A. Palmer and R.B. Finkelman, U.S. Geological Survey, for reviewing the paper. 

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study of a coal surface using a laser microprobe mass 

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Gardella, J.A., Jr., Hercules, D.M., and Heinen, H.J., 1980, 

Mass spectrometry of molecular solids: Laser Microprobe 

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Heinen, J.J., Meier, S., Vogt, H., Wechsung, R., 1983, 

LAMMA 1000, a new laser microprobe mass analyzer for 

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Hercules, D.M., Day, R.J., Balasanmugan, K., Dang, T.A., and 

Li, C.P., 1982, Laser microprobe mass spectrometry 2: 

Applications to structural analysis: Analytical Chemistry, v. 

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Joy, W.K., Ladner, W.R., and Pritchard, E., 1968, Laserheating 

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Lyons, P.C., Finkelman, R.B., Thompson, C.L., Brown, F.W., 

and Hatcher, P.G., 1982, Properties, origin, and nomenclature of rodlets of the inertinite maceral group in coals of the 

central Appalachian Basin, U.S.A.: International Journal of 

Coal Geology, v. 1, p. 313-346. 

Lyons, P.C., Hatcher, P.G., Minkin, J.A., Thompson, C.L., 

Larson, R.R., Hamlin, Z.A., and Pheifer, R.N., 1984a, 

Resin rodlets in shale and coal (Lower Cretaceous), Baltimore Canyon Trough: International Journal of Coal Geology, 

V. 3, p. 257-278. 

Lyons, P.C., Hercules, D.M., Mattern, D., Sellers, G.M., 

Brown, F.W., Simon, F.O., and Hatcher, P.G., 1984b, 

Laser microprobe characterization of coal macerals: 1984 

International Chemical Congress of Pacific Basin Societies, 

Honolulu, Hawaii, December 16-21, 1984, Abstracts of 

Papers, No. 07C14. 

Lyons, P.C., Hatcher, P.G., Brown, F.W., 1986, Secretinite: A 

proposed new maceral of the inertinite maceral group: Fuel, 

V. 65, p. 1094-1098. 

Lyons, P.C., Hercules, D.M., Morelli, J.J., Sellers, G.A., 

Mattern, D., Thompson-Rizer, C.L., Brown, F.W., and 

Millay, M.A., 1987, Application of laser microprobe 

(LAMMA 1000) to "fingerprinting" of coal constituents in 

bituminous coal: International Journal of Coal Geology, v. 7, 

p. 185-194. 

Lyons, P.C., Palmer, C.A., Bostick, N.H., Fletcher, J.D., 

Brown, F.W., Brown, Z.A., Krasnow, M.R., and 

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and trace elements in vitrinite concentrates in a rank series 

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Lyons, P.C., and Alpern, B., eds., Peat and coal, 2, Coal: 

Classification, coalification, mineralogy, trace-element 

chemistry, and oil and gas potential of coal: International 

Journal of Coal Geology, v. 13. 

Lyons, P.C., Hosterman, J.W., Lineman, D., Morelli, J.J., and 

Hercules, D.M., in press b, Laser microprobe (LAMMA 

1000) chemical analysis of clay minerals: Potential application to in situ analysis of clay minerals in coal: XI International Congress of Carboniferous Stratigraphy and Geology, 

Beijing, China, August 31-September 4, 1987, Compte 

Rendu. 

Lyons, P.C., Whelan, J.C., and Dulong, F.T., in press c, Marine 

origin of pyritic sulfur in the Lower Bakerstown coal bed, 

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and Alpern, B., eds., Peat and coal, 1: Origin, facies, and 

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v. 12. 

Makjanic, J., Odic, 1., Jasic, M., Marijanovic, P., Raos, D., 

Rendic, D., and Valkovic, V., 1983, Elemental analysis of 

coal using X-ray spectroscopy: Fuel, v. 62, no. 11, p. 

1247-1249. 

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using Secondary Ion Mass Spectrometry (SIMS): 1985 International Conference on Coal Science, Proceedings, p. 

796-799. 

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MacPhee, J.A., 1986, SIMS imaging in the study of coal 

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Mcintyre, N.S., Martin, R.R., Chauvin, W.J., Winder, C.G., 

Brown, J.R., and MacPhee, J.A., 1985, Studies of elemental 

distributions within discrete coal macerals: Fuel, v. 64, p. 

1705-1712. 

Minkin, J.A., Chao, E.C.T., and Thompson, C.L., 1979, Distribution of elements in coal macerals and minerals: Determination by electron microprobe: American Chemical Society, 

Division of Fuel Chemistry, April 1979, Preprints (papers 

presented at Honolulu), v. 24, no. 1, p. 242-249. 

Minkin, J.A., Chao, E.C.T., Thompson, C.L., Nobiling, R., and 

Blank, H., 1982, Proton microprobe determination of elemental concentrations in coal macerals: Scanning Electron 

Microscopy, 1982/1, p. 175-184. 

Minkin, J.A., Chao, E.C.T., Thompson, C.L., Wandless, M.V., 

Dulong, F.T., Larson, R.R., and Neuzil, S.G., 1983, Submicroscopic ( < 1 J..Lm) mineral contents of vitrinites in 

selected bituminous coal beds, in Gooley, R., ed., Microbeam 

analysis-1983: San Francisco Press, p. 27-30. 

Minkin, J.A., Finkelman, R.B., Thompson, C.L., Chao, E.C.T., 

Ruppert, L.F., Blank, H.,and Cecil,C.B., 1984, Microcharacterization of arsenic- and selenium-bearing pyrite in Upper 

Freeport coal, Indiana County, Pennsylvania: Scanning Electron Microscopy, 1984/IV, p. 1515-1524. 

Application of the laser Microprobe (lAMMA 1 000) to the Microanalysis of Coal Constituents 1 09 
Minkin, J.A., Chao, E.C.T., Blank, H., and Dulong, F.T., 1987, 

Proton microprobe analysis of trace-element variations in 

vitrinites in the same and different coal beds: Scanning 

Electron Microscopy, v. 1, no. 2, p. 503-513. 

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Pittsburgh Coal Conference, September 28-0ctober 2, 1987, 

Pittsburgh, Proceedings, p. 166--177. 

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with the LAMMA instrument: Mikrochimica Acta, 1988, III, 

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V. 63, p. 318-328. 

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elements in coal minerals of selected eastern United States 

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110 New Frontiers in Stable Isotopic Research 

of bituminous coal: Optical and SEM identification of its 

constituents: Scanning Electron Microscopy, 1979/1, p. 

465-471. 

Thome, L., McCormick, G., Downing, B., and Price, B., 1983, 

Some aspects of the analysis of coal by X-ray fluorescence 

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148 p. 

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of coal: American Chemical Society, 151st Meeting, Division of Fuel Chemistry, Symposium on Pyrolysis Reactions 

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2, p. C53-C58. 

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Leybold-Heraeus GMBH, 14 p. 

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International Journal of Mass Spectrometry and Ion Physics, 

V. 46, p. 487-490. 
Quantitative Separation of Authigenic Overgrowths 

from Mineral Grains by Air-Abrasion and Gravity 

Settling Techniques 

By Paul P. Hearn, Jr., jerry D. Gandy, john N. Aleinikoff, and Raymond W. Werre 

Abstract 

Techniques that permit the removal and isolation of 

secondary overgrowths on sand-sized mineral grains are 

described. The surface layers of sized mineral separates 

are removed as micron-sized flakes by tangential collisions with the ceramic walls of an air-driven abrasion mill. 

The abraded surface material is subsequently separated 

and concentrated by suspension in water and collection 

on acetone-soluble filters. These simple techniques facilitate the physical separation of samples of authigenic 

overgrowths or reaction rims on mineral grains for a wide 

variety of analytical determinations. 

INTRODUCTION 

In a variety of current problems in applied and 

experimental geochemistry, it is desirable either to remove 

or to obtain physical samples of surface layers or coatings 

on mineral grains. Some examples include (1) separation of 

secondary overgrowths on feldspar and quartz grains in 

sedimentary rocks for 40 Ar/ 39 Ar age determinations, oxygen isotopic analyses, and structural state determinations, 

(2) separation of inherited components in zircon samples for 

improved uranium-lead age determinations, (3) removal of 

glass from sanidine grains in volcanic rocks for more 

accurate age determinations, ( 4) removal of secondary 

generations of chlorite and actinolite from hornblende 

grains that have undergone retrograde metamorphism, and 

(5) separation of reaction products developed on grain 

surfaces during mineral-water reaction experiments. We 

describe here a simple and relatively inexpensive air-driven 

abrasion device that is useful in a number of applications of 

this nature. 

The air-driven abrasion mill was modified from a 

design developed by one of the authors (Aleinikoff) for use 

in uranium-lead analyses of zircon. The mill differs from 

earlier designs (Bandyopadhyay, 1976; Krogh, 1982) by 

having increased sample capacity and by employing sintered alumina as the abrasive medium. 

METHODS 

Mill Construction and Operating Conditions 

The abrasion mill consists of a ceramic abrasion 

chamber and a stainless steel head assembly that are held 

together with a simple aluminum frame (fig. 1). The 

abrasion chambers were machined from pressed blocks of 

powdered alumina (99. 5 percent Al20 3) and then sintered at 

high temperature by a commercial ceramics company 

(R& W Products, Inc.). 

The air inlet port (see fig. 1) is connected to a 

regulated supply of compressed air, which enters the 

abrasion chamber through four angled slots on the base of 

the head assembly and creates a circular flow pattern. 

Abrasion occurs by tangential collisions between grains and 

the walls of the abrasion chamber (fig. 2A-C). Most of the 

abraded material is trapped on the stainless steel screens 

covering the outlet port. Particles that pass through the 

screens are collected in a simple trap consisting of a 

graduated cylinder filled to about one-third capacity with 

water. Most of the coarser particles (the cores of the original 

grains) remain in the bottom of the mill. 

Two different versions of the abrasion mill were 

constructed-one having an internal diameter of 2 in, and a 

larger version having an internal diameter of 4 in (fig. 3). 

The smaller mill is suitable for samples weighing up to 600 

mg, while the larger version will accept up to 3 g of sample. 

Typical inlet air pressures are 2 to 10 psi and 20 to 50 psi 

(dynamic) for the small and large version, respectively. The 

2-in mill uses tygon tubing with simple pipe-to-tubing 

friction fittings; the higher operating pressures of the 4-in 

mill necessitate the use of high-pressure hose and fittings on 

the inlet side, but tygon tubing can still be used for the 

exhaust line. A trap capable of removing contaminating 

particulate matter, water, and oil should be placed upstream 

of the inlet port for both mills. 

In general, the speed of the abrasion process is 

determined by air-flow velocity (controlled by inlet air 

pressure) and the mass of sample in the mill. Typical run 

times for separation of overgrowths on potassium feldspar 

grains range from 2 to 4 hours for maximum sample loads. 

In cases where less sample is required, as in the removal of 

overgrowths from zircon, considerably less run time is 

necessary. In any case, optimum air pressure and run times 

are best determined by trial and error for specific sample 

types. In cases where separates of overgrowths or grain 

coatings are being produced, it is important not to continue 

the run for too long so as to avoid contamination of the 

separate. In contrast, when surface layers are being 

removed to produce clean "cores," longer run times will be 

necessary. 

Separation of Authigenic Overgrowths from Mineral Grains 111 
Piece 1 

112 New Fro n t' •ers in Stabl e Isoto p•c · Research 

AIR 

OUT 

t 

150 

N 

----____:....-1 i 

J 

AIR 

IN 

t 

K 
w 

v 

Piece 4 

Figure 1. Schematic diagram showing construction of airdriven abrasion mill. Specifications for small (2-in I.D.) 

and large (4-in I.D.) versions are indicated in the listing 

below the figure. All dimensions are given in inches. 

Dimensions in parentheses refer to large mill. 

1 - Head assembly (stainless steel) 

2 - 0-ring (note: 0-ring groove should be cut in large mill) 

3 - Outlet-port screen (stainless steel) 

4 - Machine screws 

5 - Abrasion chamber (sintered alumina) 

r

'---r-, 

I ' 

I /'\ 

I S \ 

v ~ 0 I I R 

L L----_-_L ___ /_/_/~-~----~J 

eN-=r 

Piece 5 

Letter Small mill 

A 

B 

c 

D 

E 

F 

G 

H 

J 

K 

L 

M 

N 

0 

p 

Q 

R 

s 

T 

u 

v 

diameter 2.375 

Drill through 0.250 (0.500) in on 

centerline and machine to fit tubing 

0.875 

1.187 

0.562 ± .005 

0.233 

0.093 

On centerline bore to 1.225 ± .005 in 

(1.630 ± .005 in) to 0.265-in depth, 

then countersink 82° to 0. 750-in 

diameter 

Drill and tap 2 holes for 4-40 (6-32) 

machine screws, 3/s in deep, 180° 

apart on 1.000-in (1. 312 in) bolt 

circle 

Cut 4 slots at 20° to face, .015 in 

wide, 0.250 (0.437) in deep (K), 

90° apart 

0.250 

1.437 diameter 

Drill 0.125-in hole through body but 

not flange, machine to fit tube fitting 

1.812 

2.000 ±.002 

0.562 ±.005 

0.250 

2.375 

radius 1 

0.750 

1.000 

1.246 ±.002 

W Drill #31 (#26) holes, 180° apart on 

X 

y 

1.000- (1.312) in bolt circle, countersink for 4-40 (6-32) machine 

screw 

0.250 

1.995 ± .001 

Large mill 

diameter 4. 7 50 

1.750 

2.125 

1.125 ± .005 

0.280 

0.093 

0.437 

3. 000 diameter 

3.500 

4.000 ±.002 

1.125 ±.005 

0.437 

4.800 

radius 2 

1.000 

1.312 

1.622 ±.002 

0.250 

3.996 ±.001 

Separation of Authigenic Overgrowths from Mineral Grains 113 
A 8 

C D 

114 New Frontiers in Stable Isotopic Research 
A 

8 

Figure 3. A, Photograph of 2-in I. D. abrasion mill showing 

configuration of inlet air supply, regulator, and water trap. 

B, Photograph of 4-in I.D. abrasion mill showing regulator 

(mounted on wall), high-pressure inlet-air hose and quicksnap fittings, and scaled-up water trap. 

.,... Figure 2. A, Scanning electron micrograph of a sized 

separate of detrital potassium feldspar grains that have 

well-developed authigenic overgrowths. Source rock: 

Butterly Dolomite (Lower Ordovician), near Davis, 

Okla. B, Micrograph of the above sample after 4 hours 

in small abrasion mill at inlet pressure of 10 psi. C, 

Micrograph of abraded overgrowth material. D, Micrograph of same field as in C seen with cathodoluminescence detector; note luminescing contaminant grain 

of high-temperature potassium feldspar. 

Separation of Abraded Surface Material 

Surface material that is separated from abraded cores 

is recovered in the following manner. The mill is disassembled, and, with distilled water, the abraded dust is carefully 

rinsed from the outlet port screens and the transfer tube into 

a teflon beaker. While a few larger fragments containing 

core material often will be included with the fine fraction, 

these can be separated if they are allowed to settle and the 

finer fraction is suctioned off with a pipette. Alternatively, 

for small samples this separation can be done by centrifugation. In both cases the appropriate settling times can be 

computed for specific phases by the Stokes equation (see 

appropriate sections in Folk, 1968, and Jackson, 1975). The 

particle size of the abraded surface material will vary 

somewhat according to the physical properties of each 

sample (hardness, cleavage, and so on) and the microtopography of grain surfaces. However, our experience with 

abrasion of potassium feldspar, quartz, and zircon indicates 

an average particle size of 1 to 5 J.Lm. 

The final separate of abraded surface material is most 

easily concentrated by filtration through a cellulose nitrate 

membrane filter, which may then be dissolved by repeated 

acetone rinses. The purity of the final separate should be 

assessed with a petrographic microscope, or better still, 

with a scanning electron microscope. In the case of authigenic overgrowths on detrital potassium feldspar, separates 

should be evaluated by use of a luminoscope or a scanning 

electron microscope that has a cathodoluminescence detector (see fig. 2D). Fragments of detrital potassium feldspar 

luminesce with a bright blue color and are easily distinguishable from authigenic particles, which do not luminesce 

(Nickel, 1978; Kastner and Siever, 1979). 

REFERENCES CITED 

Bandyopadhyay, T., 1976, A grinder-polisher for fabricating 

minispheres: Indian Journal of Earth Sciences 3, p. 109-112. 

Folk, R.L., 1968, Petrology of sedimentary rocks: Austin, Hemphill's, 170 p. 

Jackson, M.L, 1975, Soil chemical analysis-ABvanced course, 

2d ed., lOth printing: Published by the author, Madison, WI 

53705, 895 p. 

Kastner, M., and Siever, R., 1979, Low temperature feldspars in 

sedimentary rocks: American Journal of Science, v. 279, p. 

435-479. 

Krogh, T.E., 1982, Improved accuracy ofU-Pb zircon ages by the 

creation of more concordant systems using an air abrasion 

technique: Geochimica et Cosmochimica Acta, v. 46, p. 

637-649. 

Nickel, E., 1978, The present status of cathode luminescence as a 

tool in sedimentology: Minerals Science Engineering, v. 10, 

p. 73-100. 

Separation of Authigenic Overgrowths from Mineral Grains 115 

Resonance Ionization Spectroscopy and its Potential 

Application in Geosciences 

By Robert D. Willis, 1 Norbert Thonnard,1 and David R. Cole2 

Abstract 

Resonance ionization spectroscopy (RIS) provides 

the basis for several new analytical techniques that have 

outstanding potential in the geosciences. The RIS-based 

methods feature high sensitivity (parts-per-million to 

parts-per-trillion range), elemental selectivity (103 to 109 ), 

and generality (applicable to all elements except helium 

and neon). We discuss the RIS process and describe the 

application of RIS to the analysis of chemically processed 

samples, noble-gas samples, and to in situ analysis of 

solids. Applications of RIS in the geosciences include 

quantitative major, minor, and trace analysis, isotopic 

analysis, microprobe and depth-profiling analysis, and 

ultrasensitive noble-gas analysis. 

INTRODUCTION 

This paper provides a brief overview of resonance 

ionization and examines the potential of RIS techniques in 

the geosciences. Resonance ionization spectroscopy is a 

laser-based multistep photon-absorption process ending 

with the ionization of an atom. As a method of ionization 

that is both highly selective and very efficient, RIS provides 

the basis for several new analytical techniques that have 

demonstrated impressive, and in many cases unique, capabilities in the analysis of solids, liquids, and gases. 

The concept of RIS was introduced by Hurst and 

co-workers (Hurst and others, 1975; Payne and others, 

1975) at the Oak Ridge National Laboratory, who used RIS 

to detect a single atom of cesium in a background of 1019 

argon atoms and 1018 CH4 molecules (Hurst and others, 

1977). The concept was subsequently generalized (Hurst 

and others, 1979) to include all elements of the periodic 

table except for helium and neon, which were excluded for 

practical reasons. 

Applications of RIS are being developed at a number 

of university and government laboratories worldwide as 

well as in the commercial sector. The areas of application 

include basic research into sputtering processes (Winograd 

and others, 1982; Kimock and others, 1984), spectroscopy 

of atoms and molecules (Rempe and Walther, 1987; Agostini and others, 19 8 7; Berry, 198 7), materials analysis and 

1 Atom Sciences, Inc., 114 Ridgeway Center, Oak Ridge, TN 

37830. 2 Geosciences Group, Chemistry Division, Oak Ridge National 

Laboratory, Oak Ridge, TN 37831. 

surface analysis (Pellin and others, 1984; Gruen and others, 

1986; Parks and others, 1983, 1985), medical and environmental applications (Moore and others, 1987; Rimke and 

others, 1987), ultrasensitive noble-gas measurements 

(Thonnard and others, 1987b), and elementary particle 

physics (Fairbank and others, 1987). 

One can point to several demonstrations of RIS 

techniques in the geosciences: the recent measurement of 

rhodium concentrations at the Cretaceous-Tertiary boundary 

by Bekov and others (1988), isotopic measurement of 

subnanogram quantities of rhenium and osmium by Walker 

and Fassett (1986) and Walker and others (1988), and the 

measurement of 81Kr in ground water by Willis and others 

(1989). In general, however, the RIS-based techniques are 

relatively unknown to the geoscience community, primarily 

because of the newness of the methods, some of which are 

still undergoing fairly rapid development, and because the 

techniques are largely confined to research laboratories. 

The situation is changing, however, as the RIS techniques 

mature, and we believe that the RIS methods will contribute 

increasingly to the geosciences. 

THE RIS PROCESS 

In nonresonant photoionization (fig. 1A) atoms can be 

ionized by the simultaneous absorption of several photons 

whose total combined energy exceeds the ionization potential or by the absorption of a single photon of sufficiently 

high energy. Either process is inefficient, as photonabsorption cross sections are low, and only a small fraction 

of the atoms in the laser beam is ionized. If the laser beam 

is focused to increase the photon flux, the reaction rate can 

be raised, but due to the decreased ionization volume, the 

overall efficiency remains low. In addition, the process is 

nonselective and ionizes any element within the laser beam. 

In the RIS process (fig. lB) the photon energy is 

matched to the energy difference between energy levels that 

are unique to a specific element (resonance). As photonabsorption cross sections are many orders of magnitude 

larger for resonant transitions than for nonresonant transitions, the photon flux from available lasers is therefore 

sufficient to saturate the bound-bound transitions without 

focusing. In a three-photon RIS scheme (fig. lB), finding a 

second excited state that is within 1.16 e V of the ionization 

potential is usually possible and thereby allows saturated 

ionization from the second excited state with a large flux of 

1 ,064-nm-wavelength infrared photons. These photons 

have insufficient energy to be a source of interference that 

Resonance Ionization Spectroscopy and its Potential Application in Geosciences 117 
FIRST 

-----EXCITED ---t-- STATE 

FIRST 

-EXCITED--~­

STATE 

A 

B 

Figure 1. Comparison of nonresonant photoionization (A) 

with resonance ionization spectroscopy (B). Because discrete transitions unique to a selected element are used in 

B, the ionization process is element specific and much 

more efficient than A. 

results from nonresonant multiphoton ionization. It is therefore possible to saturate the ionization process for a selected 

element with essentially no ionization of other elements. 

With properly chosen laser schemes, the ionization efficiency of the selected element can be as much as 109 times 

higher than for other elements (Fassett and others, 1983; 

Beekman and Thonnard, 1989). 

The properties of saturated ionization for most elements make single atom counting feasible with the RIS 

process. With RIS, all atoms of the selected element within 

the laser beam (which can be several millimeters in diameter) will be ionized and hence counted with almost unit 

probability, while very few interfering counts will be 

generated. In addition, the RIS process is very general, as 

illustrated in figure 2. By use of lasers that are commercially available and frequency-converting optical elements, 

together with published spectroscopic data, three-color RIS 

schemes have been developed that allow saturated ionization of 68 elements. By use of more elaborate techniques to 

generate tunable vacuum ultraviolet (VUV) photons 

118 New Frontiers in Stable Isotopic Research 

(Tompkins and Mahon, 1981; Kramer and others, 1983; 

Herman and others, 1985), it is possible to ionize the noble 

gases Ar, Kr, and Xe and also some of the other elements 

on the right-hand side of the periodic table with this 

preferred RIS scheme. An additional 13 elements can be 

ionized with this same apparatus if a two-photon transition 

to the first excited state is used, though with some loss in 

sensitivity and selectivity. 

In our discussion of the RIS process, we assume the 

existence of a source of free atoms in their ground state (the 

population that is generally of most interest). Thus, gaseous 

samples can be probed directly with the RIS lasers, solids 

are typically analyzed by a sputtering ion beam to convert 

bound atoms of the solid into the (predominantly) neutral, 

bound state atoms in the gas phase, and liquid samples can 

be deposited on a filament for thermal atomization or can be 

dried on a substrate and sputtered as a solid. 

In table 1, we have classified the periodic table from 

the RIS user's point of view. For purposes of comparison, 

we indicate, for the different groups of elements, "ballpark" 

sensitivities that are presently achieved or can be expected 

reasonably with SIRIS (sputter-initiated resonance ionization spectroscopy), the solids analysis instrument at Atom 

Sciences employing sputter atomization. Elements within 

groups A and B (table 1) can be ionized with the preferred 

three-color scheme with sensitivities at the parts-per-billion 

level and selectivities greater than 109 • All elements of 

group A have resonance wavelengths that are available from 

a single dye combination: DCM in the first dye laser and 

R590 in the second, if required. Analysis of elements in 

group B requires changing dyes in the dye lasers, a 15- to 

30-min procedure. Analysis of elements in group C by a 

two-photon transition to the first excited state yields sensitivities of 0.1 to 1 ppm and selectivities of 103 and 106

. The 

loss in sensitivity is due to the reduced ionization volume 

(as the laser beam is often focused to a 0.1-mm diameter) 

and decreased ionization efficiency because two-photon 

transitions are difficult to saturate, while selectivity is 

reduced because of the increased multiphoton nonresonant 

ionization from the intense (focused) ultraviolet (UV) laser 

beam. However, if tunable VUV radiation is generated-a 

difficult step, but already implemented for krypton and 

xenon-the sensitivity for elements in groups C and D can 

be similar to that for groups A and B. (Atom Sciences is 

presently working on VUV generation for As, C, H, N, 0, 

P, Se, and Ar.) Once spectroscopic data become available, 

elements in group E will probably fall into groups A and B. 

Although RIS can be achieved for many elements by 

use of continuous-wave lasers, the generality illustrated in 

figure 2 presently can be achieved only with pulsed lasers. 

The very high peak power of pulsed lasers enables efficient 

generation of UV and VUV radiation in nonlinear processes, so the entire periodic table is accessible. In addition, 

the high peak power assures saturation of the RIS process, 

possible only for a few elements with use of continuous-
~ Preferred three-color RIS scheme 

~ Tvo-photon transition for first step 

~ Vacuum ultraviolet wavelength for first step 

Figure 2. Periodic table identifying those elements for 

which practical RIS schemes have been identified. Commercially available laser systems can ionize 68 elements by 

use of a three-color scheme to achieve saturated ionizawave lasers. The pulsed neodymium YAG (yttrium aluminum garnet) laser system, because of its high peak power 

and convenient and reliable operation, is used frequently for 

the RIS process. The repetition rate of this system, however, is limited to tens of hertz. Higher repetition rates can 

be achieved with excimer (hundreds of hertz) and copper 

vapor (thousands of hertz) lasers, but at considerable loss 

of peak power and flexibility. 

The use of pulsed lasers in the RIS process has 

important consequences in terms of sample utilization and 

the effective duty cycle of the technique. With pulsed 

atomization sources, it is possible to achieve very efficient 

sample utilization by properly matching the atomization and 

ionization duty cycles (Kimock and others, 1984). However, the sensitivity of the RIS-based techniques as measured in ions detected per second is fundamentally limited 

by the repetition rate of the laser system, as discussed later 

in the paper. 

RIS-BASED METHODS AND APPLICATIONS 

The high ionization efficiency, element specificity, 

and applicability to all elements (except helium and neon) 

~ Not amenable to RIS using available lasers 

~ Insufficient data; RIS scheme not assigned 

~ RIS achieved at Atom Sciences 

tion with very low interference. An additional13 elements 

can be ionized by the use of two-photon transitions to the 

first excited state. To date, RIS has been achieved for most 

elements by researchers around the world. 

make RIS an ideal ultrasensitive analysis tool. The RIS 

method requires free atoms, and as the ionizing laser beams 

are usually only several millimeters in diameter, it is 

desirable for these free atoms to be concentrated in the laser 

beams. For many applications, isotopic information is also 

of interest. For elements with an atomic number greater 

than 5, the RIS technique does not readily provide isotopic 

selectivity; therefore, a mass-analysis step is desirable in the 

detection system. But, a mass-analysis step is not always 

necessary; the first demonstration of single atom counting 

when using RIS was achieved in a proportional counter in 

which single cesium atoms were detected in a background 

of 1019 argon atoms and 1018 CH4 molecules (Hurst and 

others, 1977), while recent work by Niemax ( 1985) has 

demonstrated ultratrace isotopically resolved element detection at the picogram level when using resonance excitation 

and collisional ionization in a thermionic diode detector. 

The following sections describe in a general way the 

implementation of RIS in techniques now being applied to 

the analysis of chemically processed samples, noble-gas 

samples, and solid samples. We shall also discuss specific 

applications of RIS that demonstrate the potential of these 

techniques to problems in the geosciences. 

Resonance Ionization Spectroscopy and its Potential Application in Geosciences 119 
Table 1. Classification of RJS schemes 

Group Element Number of 

elements 

A. DCM dye for first laser; AI, B, Ba, Bi, Ca, Co, Cs, Cu, 39 

R590 for second laser Dy, Er, Fe, Ga, Gd, Ge, Hf, 

(if needed) Ho, In, K, La, Mo, Na, Ni, Pu, 

Rb, Re, Sc, Si, Sn, Ta, Tb, 

Tc, Th, Ti, Tl, Tm, U, V, W, Zr 

B. Other dye combinations Ag, Au, Be, Cd, Ce, Cr, Eu, 29 

Hg, Ir, Li, Lu, Mg, Mn, Nb, 

Nd, Os, Pb, Pd, Pr, Pt, Rh, 

Ru, Sb, Sm, Sr, Te, Y, Yb, Zn 

c. Two-photon transition As, Br, C, Cl, H, I, N, 0, P, 13 

for the first step Rn, S, Se,Xe 

D. Vacuum ultraviolet Ar, F, Kr 3 

(VUV) for the first step 

E. Insufficient data Ac, Am, At, Bk, Cf, Cm, Es, 17 

Fm, Fr, Lr, Md, No, Np, Pa, 

Pm, Po, Ra 

Not currently amenable to RIS: Ne, He 

Total 

Analysis of Chemically Processed Samples 

Figure 3 shows the RIS enhancement of solid-source 

thermal-ionization mass spectrometry, called resonance 

ionization mass spectrometry (RIMS). In this analytical 

technique, the sample, be it biological tissue or fluid, a 

mineral or other solid, or an environmental water sample, is 

chemically processed to separate and concentrate the analyte and deposit it on a filament. Quantitation is typically 

achieved with the isotope dilution technique. Heating of the 

filament vaporizes the sample, while the RIS laser beams 

ionize the vapor cloud. Because of the pulsed ionization, 

time of flight (TOF) can be used for mass analysis. Poor 

sample utilization resulting from the low duty cycle of the 

laser has been improved considerably by pulsing the current 

to the vaporization filament (Fassett and others, 1984). 

The RIMS technique has already made a number of 

valuable contributions to the field of geoscience. For 

example, Walker and Fassett ( 1986) used RIMS to measure 

the isotopic compositions of microgram and picogram 

quantities of rhenium and osmium. The reported precisions 

and accuracies were ~ 1 percent and 1 to 5 percent for 

microgram and picogram quantities, respectively. The 

selectivity of RIMS was valuable in eliminating Re/Os 

isobaric interferences but even more crucial in enabling the 

authors to correct accurately for hydrocarbon interferences. 

In a more recent study, Walker and others (1988) measured 187Re/186Qs and 187 Os/ 860s isotopic ratios in a series of 

120 New Frontiers in Stable Isotopic Research 

2 

103 

komatiite flows and obtained precisiOns of ~2 percent 

(essentially controlled by counting statistics). Bekov and 

others (1988) used a variation of RIMS to provide the first 

data on rhodium concentrations at the Cretaceous-Tertiary 

boundary. A mass analyzer was used to improve the 

selectivity. The quoted detection limits for rhodium in 

chemically pretreated samples were 3 parts per trillion (ppt), 

and the authors claim detection limits as low as 1 ppt for the 

platinum-group elements and, in principle, their isotopes. 

Such sensitivity could be useful in geochemical prospecting 

and in studying the origin of the noble metals (Bekov, 

1989; Chen and others, 1987). 

Most of the lanthanide- and actinide-group elements 

can be ionized by use of the preferred three-color scheme 

(fig. 2), for which the sensitivities are at the parts-perbillion level. This region of the periodic table is also rich in 

isotopic systems that have isobaric overlaps-Pu!U, 

Nd/Sm, Tc/Mo, and Lu/Yb-and provides opportunities to 

demonstrate the elemental selectivity of RIS. Donohue and 

others (1982), for example, used RIMS to measure neodymium and samarium isotopic ratios in binary mixtures in 

which there had been no chemical separation. The elimination of isobaric interferences was demonstrated, and the 

authors concluded that precisions of ±0.5 percent for 

isotopic ratio measurements could be achieved reasonably 

with an improved laser system and proper data-collection 

scheme. 
RIS Ion Extraction 

~and Secondary-Ion 

~Repeller Electrode 

1 Filament Heating 

Current 

Figure 3. Schematic drawing of the resonance ionization 

~ass spectrometer (RIMS) technique. The sample, which 

1s processed chemically, is deposited on a filament for 

vaporization. The RJS laser beams ionize only the selected 

element. 

Ultrasensitive Noble-Gas Measurements 

A technique for the analysis of noble-gas samples, 

first developed at Oak Ridge National Laboratory (Hurst 

and others, 1985), is illustrated in figure 4. Typical diameters of RIS laser beams are only several millimeters, while 

mass spectrometer volumes are several liters; thus, for 

noble-gas samples containing fewer than 1,000 atoms, the 

probability of even a single noble-gas atom being in the RIS 

laser beam would be small. Unacceptably long analysis 

times would be required. It is possible to concentrate the 

noble-gas atoms in the RIS laser beams by employing a cold 

finger (Hurst and others, 1984) that freezes the noble gas 

onto a small spot. A pulsed, heating laser releases the 

noble-gas atoms into the ionization region just before the 

RIS lasers are fired. Combining this "atom bunching" with 

the high ionization efficiency and selectivity of RIS has 

made it possible to detect selected isotopes of krypton with 

fewer than 1,000 atoms in a 6-L mass spectrometer, even in 

the presence of 2x 108 residual gas atoms (10- 9 torr 

vacuum). 

The pioneering work of Hurst's group at Oak Ridge 

National Laboratory is being carried forward at Atom 

dt. 

lfj]Detector 

RIS Ion Extraction 

~Electrode 

20-60 K 

Cold Finger 

Figure 4. RIS analysis of noble-gas samples. The sample is 

introduced into the static mass spectrometer, frozen onto 

the cold finger, and subsequently released by a pulsed, 

heating laser a few microseconds before the RIS lasers are 

fired, thereby concentrating the atoms in the RJS laser 

beams. 

Sciences, where efforts are presently under way to extend 

the m(~thod to any isotope of the heavy noble gases (Ar, Kr, 

Xe, Rn). One application being developed at Atom Sciences is to date ground water by measuring the concentration of the cosmogenic radioisotope 81Kr (half-life = 

210,000 years). An example of this work is shown in figure 

5. The figure is a TOF spectrum of a processed krypton 

sample extracted from approximately 50 L of ground water 

showing the detection of 6,200 atoms of 81Kr in the TOF 

apparatus. Details of the sample processing and analysis are 

given by Thonnard and others (1987a,b). The same 

approach can be used in dating polar ice, a key step toward 

interpreting the atmospheric record preserved in the ice. 

Other radioisotopes of interest that are difficult to 

detect by decay counting are 39 Ar (half-life = 269 years) 

and 85Kr (half-life = 10.7 years), which are valuable 

"clocks" for measuring the rates of deep-ocean circulation 

e9 Ar) and new ocean-water formation (

85Kr). Although an 

RIS scheme for argon has not yet been demonstrated, it 

should be possible. In the area of cosmochemistry, much 

has been learned about the evolution of the solar system 

from isotopic analysis of small noble-gas samples extracted 

from meteorites. These measurements demand high sensitivity and selectivity, especially against hydrocarbon backgrounds in the mass spectrometer. Turner's group at Sheffield (Turner, 1987) is using RIS to study xenon anomalies 

Resonance Ionization Spec:troscopy and its Potential Application in Geosciences 121 
0 0~----------------------------------~ 

> § 

r-1 

10 

c 

01 

.... 

(f) 

lL. 

0 

II 

(f) 

H 

cr: 

C\1 

0 

10 

~ 

0 

0 

~ 

0 

10 

0 

10 

0 

Milk River #2 

50 liters of water 

----

78 80 

\ 

\ 

/ 6200 81Kr 

82 84 86 

Mass (amu) 

Calibration: 

2.88xl06 86Kr 

88 90 

Figure 5. Krypton mass spectrum for sample extracted 

from 50 L of water from the Milk River aquifer in Canada. 

The time-of-flight (TOF) apparatus detected 6,200 atoms of 81Kr. Because the magnitude of the 81 Kr signal was only 

slightly less than that of modern water, an age of less than 

100,000 years is indicated. Decay counting would have 

required processing 100,000 L of water. amu, atomic mass 

units. 

in meteorites. Another application of ultrasensitive noblegas measurements is 81Kr-Kr dating in which radioactive 81 Kr and stable spallation krypton are measured to determine cosmic-ray exposure ages of chondritic meteorites and 

lunar samples (Marti, 1982; Eugster, 1989). 

Direct Analysis of Solids 

For direct analysis of solid surfaces, two techniques 

are showing great promise. Material can be removed from 

the sample surface with either an atomizing laser pulse 

(Beekman and others, 1980) or a sputtering ion beam (Parks 

and others, 1983), with subsequent ionization of the 

expanding cloud of neutral atoms by RIS laser beams (fig. 

6). The former is called LARIS (laser atomization RIS), 

while the latter is called SIRIS (sputter-initiated RIS). 

When the atomizing laser or sputtering ion beam is focused, 

regions in the micrometer or submicrometer size regime can 

be analyzed with high sensitivity. Imaging can be achieved 

122 New Frontiers in Stable Isotopic Research 

Sputter Ion Beam 

~Ion ~ Detec<o' 

RIS Ion Extraction 8 and Secondary-Ion 

Repelle< Eleot<ode 

Figure 6. Direct analysis of solid samples by the use of 

sputter-initiated RIS (SIRIS) or laser-atomization RIS 

(LARIS). Because resonance ionization acts on the preponderance of the neutral atoms evolved from the sample, 

sensitivity is increased and matrix effects are reduced. 

by rastering the beam over the surface, while depth profiling is possible by collecting data as the sample is eroded 

during the analysis. The two techniques will probably 

become complementary. SIRIS, utilizing the wellcontrolled linear sputtering process, is developing into a 

precise analytical technique that has high lateral and depth 

resolution, while LARIS, perhaps less quantitative, promises lower ultimate detection limits by several orders of 

magnitude. 

Both techniques rely on resonance ionization of the 

neutral atoms removed from the sample. This process offers 

two distinct advantages over secondary ion mass spectrometry (SIMS). First, the neutral fraction of ejected particles is 

often 99 percent or greater, so SIRIS and LARIS should be 

capable of achieving lower detection limits for most elements. Secondly, because the neutral flux is analyzed, 

SIRIS and LARIS measurements are potentially much less 

matrix dependent than SIMS and should be more quantitative. Because the RIS methods virtually eliminate isobaric 

interferences, the mass-resolution requirements of the 

detector are in general greatly reduced compared to the 

SIMS technique. A low-resolution TOF detector suffices 

for many applications and offers nearly 100 percent transmission efficiency. 

Figures 7 and 8 illustrate some of the salient features 

of SIRIS and LARIS by use of results obtained at Atom 

Sciences. Figure 7 shows LARIS TOF spectra of a silicon 

sample containing 58 ppm gallium (Beekman and 

Thonnard, 1989). The lower spectrum (lasers tuned to 

gallium resonance) shows the simplicity of the TOF spec-
...... 

rn 

c: 

01 

•.-1 

" 0 

\C) 

0 

(T) 

0 

Ulo 

IJ...q' 

0 ... 

I0 

0 

.... 

0 

w 

0 

N 

I 

0 

RIS Wavelength Off-resonance X200 

J ,, ,l.kJU .. IL, I.J L. 

RIS wavelength On-resonance 

RIS 

Ga• 

5 10 15 20 

TOF (}-IS) 

Figure 7. Laser-atomization resonance ionization time-offlight (TOF) mass spectrum of a silicon sample containing 

58 ppm gallium (lower plot). Total mass range is 0 to 133 

amu. The simplicity of the spectrum, which has only a hint 

of a signal at 4.6 J.LS, results from direct ionization of silicon 

from the atomization laser, which was adjusted to deliver 

4X107 W/cm2

• With the RIS laser detuned by only 0.07 nm, 

some signals are seen in the 200 x expanded scale view 

(upper plot). Comparison of the resonant 69Ga si§nal with 

the nonresonant mass 28 signal indicates 3 x10 greater 

gallium resonance ionization than nonresonant silicon 

ionization and thus demonstrates the remarkable selectivity of RIS. 

trum. The remarkable selectivity of RIS is illustrated by 

comparing the on-resonance gallium signal to the offresonance silicon signal (upper spectrum). Figure 8 shows 

the correlation of the SIRIS signal for vanadium versus the 

certified National Bureau of Standards (NBS) values for a 

suite of standards spanning almost six orders of magnitude 

in vanadium concentration. The fact that these measurements comprised three different matrix materials (pure 

vanadium, stainless steel, and "pure" iron) is of importance 

because it illustrates the matrix-independence of the 

method. 

(/).-1 

H 0 

a: .. -t 

H 

(f) 

0 

Eo 

:J .. -t 

·rl 

'0 

ro ... Cb 

ro > ... -t 

Figure 8. Correlation of vanadium SIRIS signal versus 

accepted National Bureau of Standards concentration values. Excellent agreement is obtained over nearly six 

decades even though three different matrix materials were 

used (pure vanadium at 10°, stainless steel at 10-2 to 10-4 , 

and "pure" iron at 7X 10-6). Because the data for pure 

vanadium were obtained with 1/200th of the sputtering ion 

current used for the remaining samples, the indication is 

that sputtering is a linear process, which is important for 

quantitation. pc, picocoulomb; SRM's, standard reference 

materials. 

A schematic diagram of the SIRIS apparatus developed at Atom Sciences is shown in figure 9 and is described 

in more detail by Parks and others (1983; 1985). Briefly, 

the sample is sputtered by use of a pulsed (30 Hz) argon ion 

beam produced in a duoplasmatron microbeam ion gun. 

Typical parameters for the sputtering beam are 5 ~A of 

current in a 1-~s pulse, ion energy of 10 ke V, and a spot 

size of 100 ~m (reducible to 5 ~m with reduced current). 

The laser system comprises two tunable dye lasers pumped 

by a 30-Hz neodymium YAG laser and can generate any 

wavelength between about 220 and 1,064 nm. Mass analysis is provided by either a TOF analyzer or a magnetic 

sector. SIMS ions constitute background in the SIRIS data 

and are rejected by means of an energy analyzer. SIRIS ions 

are detected by an electron multiplier with a conversion 

dynode operated in the analog or single-ion-counting mode. 

The target chamber maintains a base pressure of 4X 10-9 

torr. 

The sensitivity of SIRIS-type instruments has been 

studi,ed at Atom Sciences and elsewhere. At Atom Sciences, a detection limit of 2 ppb for gallium in silicon has 

been reported (Parks and others, 1984), and at Argonne 

National Laboratory, Pellin and co-workers (Young and 

Resonance Ionization Spectroscopy and its Potential Application in Geosciences 123 

I 
TUNABLE DYE LASER 

Figure 9. Schematic diagram of SIRIS apparatus, which 

consists of a pulsed-ion source, tunable laser system, 

ultrahigh vacuum system, extraction electrodes, and 

either a double-focusing magnetic sector mass spectromothers, 1987; Pellin and others, 1986) reported detection 

limits of 0.5 ppb for 54Fe in silicon and 2 ppb for 56Fe in 

silicon. This last result is a particularly impressive demonstration of the elemental selectivity of RIS; without this 

selectivity, the 56Fe signal would be overwhelmed by the 

signal from the 28Si2 dimer. 

Development of the LARIS technique is not as far 

along as SIRIS, but a detection limit of 10 ppb for gallium 

in silicon was recently demonstrated (Beekman and 

Thonnard, 1989) in a very simple system after only 5'00 

laser shots (50-s analysis time). Simple operational changes 

could lower the detection limit to below 0.1 ppb, while the 

potential of achieving sub-parts-per-trillion sensitivity is 

realistic. 

As an analytical tool, SIRIS has much in common 

with SIMS and the ion microprobe, and a list of potential 

applications of SIRIS in geochemistry would look very 

familiar to users of SIMS techniques. The broad generality 

of RIS as illustrated in figure 2, the sub-parts-per million to 

parts-per-trillion sensitivites for much of the periodic table, 

and the low matrix dependence provide SIRIS with a 

tremendous potential for quantitative in situ trace analysis 

of solids. It is well known that the SIMS spectra of 

geological samples are overwhelmingly complicated by the 

presence of molecular ion species. While these can be 

eliminated by energy filtering of the secondary ions or by 

using spectrometers that have very high mass resolution, the 

loss in signal intensity is substantial (Shimizu and Hart, 

1982), and the elemental isobaric interferences remain. The 

remarkable selectivity of RIS (particularly for those elements in groups A and B of table 1) affords tremendous 

simplification of the mass spectra of complex samples. 

124 New Frontiers in Stable Isotopic Research 

TIME~F-FLIGHT MASS ANAI.tzER 

eter or a time-of-flight mass analyzer. Secondary ions 

generated in the sputtering process constitute a background noise in SIRIS and are eliminated with the use of 

time and energy discrimination. 

The SIRIS system at Atom Sciences has demonstrated a lateral resolution down to 5 to 10 J..Lm and a depth 

profiling resolution of ~30 A and thus potentially enabled 

studies of ( 1) the distribution of major, minor, and trace 

elements in minerals, (2) elemental diffusion gradients, (3) 

the magnitude of isotopic zoning or inhomogeneities in 

individual mineral grains, (4) the extent of isotopic variation that is due to changes in fluid type, fluid source region, 

and temperature as preserved in growth zones of hydrothermal and authigenic minerals, and (5) the degree of isotopic 

equilibration between individual grains of different phases. 

When VUV laser schemes become available, it may be 

possible to study the light stable isotopes (C, H, N, 0, S, P, 

and others in group C of table 1) in different mineralogical 

settings where these elements are present at the trace level. 

Such studies could provide insight, for example, into the 

nature of solid solution of these elements in oxides and 

silicates (see Freund, 1987). Because of interferences from 

the major elements, or poor sensitivity as in the case of 

nitrogen, these studies would be difficult or impossible with 

SIMS. 

Zinner (this volume) has pointed out that the limited 

sample size and relatively large isotopic anomalies make 

extraterrestrial samples ideal subjects for ion-microprobe 

analysis. Both primitive meteorites and interplanetary dust 

particles (IDP's) contain considerable evidence for the 

existence of preserved interstellar material that was formed 

prior to the formation of the solar system. Neutron-rich 

isotopes such as 48Ca, 50Ti, 54Cr, and 64Ni are of interest, 

as they provide evidence of supernova events in the presolar 

system (Andrew Davis, James Frank Inst., University of 

Chicago, personal commun., 1986). The iron group of 
elements are among the group A and B elements (table 1) 

that can be detected by SIRIS with high sensitivity, and the 

expected high selectivity should eliminate isobaric interferences that complicate the SIMS data. The existence of 

hydrocarbons in Comet Halley dust particles has stimulated 

interest in the abundances and distribution of biogenically 

important elements including H, C, N, 0, P, and S. As 

discussed earlier, ionization of these elements by a twophoton transition for the first excitation step results in 

reduced sensitivity (in the few parts-per-million range) and 

selectivity. Even so, the present SIRIS technique may offer 

advantages over SIMS in some applications. For example, 

the direct determination of nitrogen concentrations in extraterrestrial samples is not feasible with SIMS because of the 

low ion yields for nitrogen. Instead, nitrogen concentrations 

are inferred by the measure of the yield of CN ions. Isobaric 

interferences complicate this measurement, and the CN 

yields are influenced by the carbon content of the matrix 

(Zinner, this volume). In this particular application, the 

SIRIS technique probably offers unique capabilities. For 

example, nitrogen contents and isotopic compositions in 

diamonds could be investigated on a much finer scale than 

described by Boyd and others ( 1988), who observed large 

isotopic fractionations that were due to the influence of 

crystal structure. With improved techniques for generating 

VUV light for the first excitation step, the ultimate sensitivity and selectivity of the SIRIS technique can be 

approached for this important set of elements. 

The previous paragraphs have emphasized potential 

advantages of the SIRIS and LARIS techniques and possible applications of these techniques in the geosciences. We 

must balance this discussion by stating that the SIRIS and 

LARIS techniques have not yet, to our knowledge, been 

demonstrated on complete geological samples. The added 

complexity of these samples will undoubtedly pose new 

problems for these techniques as it did in the case of the 

microprobe. Indeed, the present situation is perhaps analogous to that confronting the SIMS technique when its 

capabilities were first being extended to the fields of 

geochemistry and cosmochemistry. SIRIS and LARIS are 

still suffering some of the "growing pains" of a new 

technique. Researchers have begun only recently to address 

key analytical concerns such as matrix effects and fractionation effects in relatively simple matrices. Limits to SIRIS 

and LARIS sensitivity that are due to the duty cycle of these 

techniques is another area of concern. In the remaining 

paragraphs, we would like to briefly discuss the issues of 

matrix effects, fractionation effects, and duty cycle. 

Figure 8, discussed earlier, suggests that matrix 

effects with SIRIS are minimal. Parks and others (1985) 

investigated the SIRIS yield for silicon and boron in various 

matrices, and the results are presented in table 2. The 

maximum matrix effect observed in this limited study was 

approximately a factor of 4 for boron in BPSG. This matrix 

is a type of glass (boron phosphide silica glass), and the 

Table 2. Relative SIRIS response for silicon and boron in 

various matrices 

Matrix Element Relative SIRIS response 

Si Si 1.00 

Al Si .96 

PdSi2 zsSi .96 

PdSi2 3oSi .96 

TiSi2 Si 3.60 

GaAs Si 3.65 

Si B 1.00 

GaAs B .77 

BPSG1 B .23 

1 Boron phosphide silica glass. 

poor relative yield is probably due to charge buildup in the 

target. Our experience is that factors of 3 or 4 in the relative 

yield from different matrices are the exception rather than 

the rule; in SIMS measurements, matrix effects can be as 

high as several orders of magnitude. Matrix effects are 

expected to be small for two reasons: (1) the yield of 

sputtered neutrals does not depend strongly on the properties of the surface and (2) the ionization process takes place 

in the gas phase, decoupled from surface effects. This 

relative matrix independence should greatly improve the 

prospects for quantitative analysis. 

Stable isotopic measurements are basic to geochemistry, and it is important to assess the ability of SIRIS to 

measure isotopic ratios. To date, efforts have focused by 

and large on quantitative trace analysis rather than on 

isotopic measurements, so data are sparse. In the case of 

SIRIS , fractionation effects will occur both in the sputtering 

process and in the ionization step. However, with proper 

choices of the sputtering-beam pulse width, the laser pulse 

width, and the time delay between the sputtering-beam 

pulse and the ionization laser, the ionization probability of 

the ejected neutrals (in the absence of isotopic effects in the 

RIS process itself) will be independent of the particle 

velocity (or mass) (Kimock and others, 1984). Our experience at Atom Sciences indicates that fractionation effects 

are dominated by the RIS process itself. In isolated cases we 

have observed fractionation effects between odd- and evenmass isotopes of about 10 percent, whereas for odd or even 

isotopes taken separately, the maximum error in the ratios is 

typically 2 percent. The odd-even effect is presumably 

caused by a dependence of the ionization probability on the 

polarization of the photons used in the excitation step(s) 

(Fairbank and others, 1989). Figure 10 shows a SIRIS 

TOF spectrum of pure molybdenum. Integration of the 

individual peak areas yielded natural molybdenum abundance ratios within 2 to 3 percent with no corrections 

applied to the raw data. 

We believe that with careful attention to saturation of 

the RIS process, polarization of the lasers, and by running 

Resonance Ionization Spectroscopy and its Potential Application in Geosciences 125 
0 

0 

>~ 

.§ 

r-to 

10(1) 

c 

0) 

•r1 

(/)0 

tO LL 

0 

t-

(/)0 

...... ~ 

a: 

...... 

(/) 

0 OC\J 

::E. 

0 

92 94 96 98 100 

Mass (amu) 

Figure 10. SIRIS time-of-flight (TOF) spectrum of pure 

molybdenum. The resolution m/.t:lm of the TOF detector 

was approximately 500, and the analysis time was 1 min 

(600 shots). The measured isotopic ratios agreed with the 

natural abundance ratios within 2 to 3 percent. amu, 

atomic mass units. 

standards next to samples, isotopic ratios can be measured 

with precisions and accuracies better than 0.5 percent. In 

fact, analysis of calibration spectra used in the krypton work 

described earlier shows precision of 0. 25 percent and 

accuracies ranging between 0.5 and 1.5 percent for the 

major isotopes. 

Factors limiting the sensitivity of SIRIS, as discussed 

by Kimock and others (1984), are the low duty cycle, which 

is due to use of pulsed lasers, and the brightness of the 

primary ion beam. For the typical case of a repetition rate of 

30 Hz and ion-beam pulse width of 1 J.1S, the duty cycle is 

3 x 10-5 • This low duty cycle can be offset partly by the 

higher yield of ejected neutrals compared to secondary ions 

for specific cases. In analysis for elements having very poor 

secondary ion yields (10- 5 or less, for example), the SIRIS 

signal level can be significantly higher for SIRIS than for 

SIMS and thus allow access to analysis for elements not 

previously detectable. Nevertheless, for those elements having good SIMS sensitivity, the signal intensity (ions 

detected per second) may be two orders of magnitude 

smaller than for SIMS. One should note, however, that the 

sample utilization, that is, the amount of sample consumed 

during the analysis, is very low since the sputtering beam is 

pulsed and is frequently much less than in SIMS because of 

the higher overall efficiency. 

The duty cycle problem can be improved by using 

brighter ion beam sources delivering a few milliamps of 

current in a spot size of several millimeters and using the 

higher repetition rate or continuous-wave lasers discussed 

126 New Frontiers in Stable Isotopic Research 

earlier. For microprobe analyses, however, the present 

microbeam ion guns are already space-charge limited and 

cannot be made brighter. Analysis for some elements, 

especially those having high secondary ion yields and few 

potential isobaric or molecular interferences, will probably 

continue being done advantageously with SIMS. But, the 

accessibility to almost all elements of the periodic table with 

high sensitivity, good quantitation, and virtually no interferences will make the SIRIS technique valuable. 

SUMMARY AND CONCLUSIONS 

Resonance ionization spectroscopy (RIS) is a method 

of ionization in which narrow-band lasers are used to ionize 

elements selectively and efficiently. It is the basis for 

several ultrasensitive elemental analysis techniques including, but not limited to, (1) analysis of chemically processed 

samples deposited on a filament for vaporization (RIMS), 

(2) ultrasensitive noble-gas measurements, and (3) direct 

analysis of solid samples by the use of either sputterinitiated RIS (SIRIS) or laser-atomization RIS (LARIS). In 

the latter methods, pulsed-ion sputtering or pulsed-laser 

atomization is used to vaporize atoms from a solid sample, 

and the RIS laser(s) then ionize(s) atoms of a selected 

element that are detected and counted. Because the neutral 

flux is analyzed, SIRIS and LARIS measurements are 

potentially much less matrix dependent than SIMS and 

should be more quantitative. These two surface-analysis 

techniques can analyze impurities in a variety of materials 

(minerals, glasses, semiconductors, metals) with sensitivities in the low parts-per-million to low parts-per-billion 

range. The absence of isobaric interferences that are due to 

the selectivity of the RIS process greatly simplifies the 

analysis of complex samples. In addition to elemental 

analysis, isotopic information is obtained by use of either a 

TOF analyzer or a magnetic-sector energy· analyzer. Both 

the SIRIS and LARIS techniques are capable of microprobe 

analyses if one focuses the sputtering beam (SIRIS) or the 

atomization laser (LARIS). With recent development in 

liquid-metal ion sources, spatial resolutions of 1,000 A are 

possible with SIRIS. 

Applications of RIS in the geosciences have been 

relatively limited to date, owing to the newness of the 

methods. However, we expect a growing interest in these 

techniques as their capabilities continue to improve and 

become better known. 

ACKNOWLEDGMENTS 

The authors gratefully acknowledge Ceci Steele for 

typing this paper. D.R. Cole would like to thank David 

Wesolowski for his careful review of this paper. Support of 

D.R. Cole was provided by the Division of Engineering and 

Geosciences, Office of Basic Energy Sciences, U.S. 
Department of Energy, under contract DE-AC05-

840R21400 with Martin Marietta Energy Systems, Inc. 

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128 New Frontiers in Stable Isotopic Research 

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Bristol, England, p. 163-168. 
Use of the lon Microprobe to Measure Natural 

Abundances of Oxygen Isotopes in Minerals 

By Bruno j. Giletti1 and Nobu Shimizu2 

Abstract 

lon microprobe analysis with the Cameca model 4f 

permits determination of oxygen isotopic compositions in 

crystal surfaces in volumes as small as 1 micrometer on a 

side, although larger volumes are needed to reduce analytical uncertainties to less than 1 per mil. Analyses of a 

plagioclase feldspar yielded blocks of data of 100 mass 

spectrometer cycles that had standard deviations of the 

mean of approximately 0.7 per mil for 1 a. A succession of 

such blocks of data on the same specimen yielded maximum differences on the order of 1.1 per mil. The ability to 

reproduce differences between specimens of different 

8180 values and to obtain such data for different minerals, 

however, remains to be determined through testing. 

DESIRABLE CONDITIONS FOR 180 ANALYSIS 

The principal advantage of using an ion microprobe 

(or secondary ion mass spectrometer, SIMS) (Slodzian, 

1980) to measure oxygen isotopic compositions in minerals 

is that a very small area (and volume) is analyzed, so 

detailed studies of inhomogeneous particles as well as of 

very small particles are permitted. This capability opens up 

the possibility of answering questions that were not previously solvable with the use of the standard extraction 

(employing BrF5 , ClF3 , or F2) and mass spectrometric 

analysis. 

Before examining the performance of an ion microprobe, it is useful to consider some of the problems and 

goals of such analyses in order to assess the specific needs 

of the analyst. Examples of the problems would include (1) 

determination of the 8180 values of coexisting minerals in 

normal, closed-system, rapidly cooled igneous rocks to 

assess the degree of equilibration, (2) a similar determination for slowly cooled igneous or metamorphic rocks, where 

gradients will be found in most minerals, with the objective 

of measuring the cooling rate (see Giletti, 1986), (3) 

measurement of 8180 values of fossil shells to determine 

changes in the environment of the organism during its life 

cycle, and (4) measurement of 8180 values of ore and 

gangue minerals to assess the nature of the ore fluid over the 

time of the deposition. A different category of problem, 

1 Department of Geological Sciences, Brown University, Providence, RI 02912. 2 Department of Earth Atmospheric and Planetary ScienceS, Massachusetts Institute of Technology, Cambridge, MA 02139. 

concerning the relative abundances of all three oxygen 

isotopes (16, 17, and 18) and the interpretation of early 

solar system history, is discussed by Zinner (this volume). 

Examples of more complex geological processes 

include multiple metamorphic episodes or open system 

behavior where fluids enter and (or) leave the rock specimen in question. In these cases, chemical reactions or 

recrystallization may affect some minerals and not others 

(for example, alteration, metasomatism, solutionprecipitation). Figure 1 shows a cartoon example of an 

igneous rock that has had a history that is somewhat more 

complex than simple cooling. The minerals are generally 

normal in appearance, but the fine-grained band labeled 

"Other" is suggestive of a later history, which may be 

fracturing and vein filling, a small shear zone, or another 

event. The mineral grains are not intended to depict the 

results of particular events, so their shape is not intended to 

appear diagnostic. 

It clearly would be desirable to analyze the small 

mineral particles as well as to measure 8180 values in 

traverses across the various larger mineral particles. In 

addition, given our present knowledge of diffusional 

anisotropies in quartz (Giletti and Yund, 1984) and hornblende (Farver and Giletti, 1985), such traverses should 

also be measured in different directions. 

In the example of figure 1, an area for analysis should 

be on the order of <50 J.Lm. If detail is sought in the tiny 

grains, then < 10 tJ..m is preferable. In addition, the depth of 

the analyzed area should be small, preferably < 1 tJ..m. It is 

possible to imagine fine-grained rocks where the volume 

analyzed is a few micrometers across in all dimensions. 

This volume should be contrasted with the normal oxygen 

extraction and mass spectrometer analysis, where sample 

volumes are 2 to 5 mm3

• 

A cube of quartz, 1 tJ..m on a side yields 108 180 

atoms. A reasonable useful yield of ions of 0. 3 percent 

would result in counting 3 x 105 180 atoms, with a 

counting uncertainty of 2 otoo (1 a). By increasing the size 

to 2 tJ..m on a side, the uncertainty would be lowered to 0.7 

otoo. Consequently, the ion probe analysis represents a 

potential improvement of approximately 3 orders of 

magnitude in volume of sample, or approximately 2 orders 

of magnitude if a comparable precision of 0.1 to 0.2 otoo, 

comparable to normal mass spectrometer analysis, can be 

attained. 

Oxygen isotopic compositions and their variations in 

nonhomogeneous-minerals have been measured routinely, 

by using ion microprobes, when very large variations have 

Use of the lon Microprobe to Measure Oxygen Isotopes in Minerals 129 
t 

5 mm 

Figure 1. Hypothetical thin section of a granite illustrating 

scale on which analyses are needed. Minerals are Qtz, 

quartz; PI, plagioclase; Kf, microcline; Hb, hornblende; 

Biot, biotite; Mag, magnetite; and the fine-grained band 

labeled "Other," some fine-grained material that may be the 

result of later deformation, recrystallization, or fracturing 

and deposition. 

resulted from doping the mineral with isotopically enriched 

oxygen (Giletti and others, 1978; Giletti and Yund, 1984). 

The objective of the present discussion is to focus on 

measuring naturally occurring, and thus very small, variations in oxygen isotopic composition in these extremely 

small volumes of material. These have not yet been 

reported. 

Needless to say, such analyses should give a high 

degree of precision. For cases in which there is compositional zoning in the o180 values, this zoning may be several 

per mil, but it might also be only 1 or 2 ofoo, or even less. 

This zoning may reflect late-stage changes in ambient 

fluids, late-stage growth of the crystal, or late-stage alteration. 

If the rock had a simple history, it may still have o180 

zoning resulting from diffusional exchange during cooling 

(Dodson, 1986). The Dodson model, together with oxygen 

diffusion data in hornblende (Farver and Giletti, 1985) and 

plagioclase (Giletti and others, 1978), were used to compute the compositional gradient of 5180, parallel to the c 

axis, in a hornblende crystal that was in contact with 

plagioclase and cooled at 10 or 100 °C per million years (see 

fig. 2). The ordinate is taken as the difference from the 

value in the adjacent plagioclase. Analyses that give uncertainties of 0. 2 o; oo or better are needed if the cooling rate is 

to be determined. 

ION MICROPROBE FEATURES AND 

OPERATION 

Most ion microprobe studies have used the Cameca 

IMS 3f or earlier versions of the ion probe produced by 

130 New Frontiers in Stable Isotopic Research 

18 !::.. 0 

(o/oo) 

-2 

--------------- ---------------------.:::::::::--- ---

l00C/m.y. ', 

0.2 olool 

100 ~m 

t-; 

HORNBLENDE (parallel to c-axis) 

' \ 

\ 

\ 

\ 

\ 

\ 

\ 

I 

I 

I 

\ 

-3,_----------------~----------------~ 

Distance from Center of Crystal (mm) 

Figure 2. Difference in 180 concentration between a hornblende crystal and an adjacent homogeneous reservoir of 

plagioclase as a function of distance from the center of the 

hornblende crystal to its edge. Variation is due to variation 

of isotopic fractionation factor during cooling in the 

vicinity of the closure temperature and is also due to the 

slowing down of the oxygen diffusion rate as the rock 

cooled at a rate of 10 or 100 oc per million years (m.y.). 

Horizontal straight lines show mean ~ 180 value for the 

entire crystal. otoo, per mil. 

various companies. We shall be reporting on the newer 

model of the Cameca, the IMS 4f. It is useful to treat this in 

terms of the earlier model, however, so that the other papers 

(Hervig and others, this volume; Zinner, this volume) can 

refer to the description given here. Some of the new features 

of the 4f will then be introduced. 

Figure 3 shows a schematic drawing of the Cameca 

IMS 3f ion microprobe as published by the Cameca Company. Its operation is straightforward in principle, and a 

very brief description will be given to illustrate ion probe 

characteristics and operation in general. 

An ion microprobe consists of two parts: (1) the 

source and ion optics of a primary beam and (2) the ion 

optics of the mass analyzer of the secondary beam. The 

sample surface is the object that is common to the two parts. 

The purpose of the primary beam is to direct ions of 

moderately high energy at the sample surface in order to 

knock (sputter) sample atoms from the surface. These 

sample atoms will include positively or negatively charged 

ions, and these can be accelerated into the mass analyzer 

part as a secondary beam. The mass analyzer then separates 

the ions according to mass and either measures the abundance of a given mass or displays an image of the sample, 
Electromagnet 

Cs.+ Source 

Electronic Signal 

Collector 

20 

19 

Secondary Beam Flight Tube 

Sample 

Image on 

Screen 

Figure 3. Schematic drawing of the Cameca IMS 3f ion 

microprobe, with primary beam at bottom and secondary 

ion optics at center and top. See text for description. 

Modification of a figure from the Cameca Company, 

Courbevoie, France. 

showing where the given mass ions derive (creating a 

"map" of the abundance distribution of that mass in the 

sample surface). 

The operation of the ion probe can now be considered 

in more detail (see fig. 3). A primary ion beam, created in 

the source regions 1 or 2, can be atoms of Cs +, o-, 0 2 +, 

Ar+, or other gases (see Hervig and others, this volume). 

The cesium ions are produced by heating molten cesium 

metal so as to evaporate cesium atoms and having these 

come into contact with very hot metal so the atoms are 

ionized to Cs +. The gas ions are created by introducing 

oxygen, argon, or another gas into a duoplasmatron, where 

the applied high voltage causes the atoms to ionize into a 

plasma. In either case, the ions are accelerated by several 

thousand volts (usually 10 kV) toward the sample. The 

polarity of the applied potential will select the sign of the 

charge on the ions being accelerated. The beam is cleaned 

up by means of a magnetic sector "mass spectrometer" to 

give ions of a single mass at region 3, then the beam is 

focused and shaped to impinge on the sample at 5. The 

cleanup by the mass spectrometer at 3 results in a primary 

beam that consists of one mass of the gas being used. Where 

the ion is a single atom, only one isotope of the gas is 

accelerated to the sample. The primary beam is focused into 

a small cross-sectional area as it arrives at 'the sample. This 

focusing occurs in the primary beam column (between 3 and 

5) and results in a beam of circular cross section whose 

DETAIL OF KEY FEATURES OF ION PROBE ANALYSIS 

Secondary Beam 

Primary Beam 

o-, o; , Ar+, Cs+ 

Figure 4. Schematic drawing of local geometric relations 

of primary and secondary ion beams with relation to 

sample in Cameca ion microprobe. Scale is such that the 

area of sample struck by the primary beam may be in the 

range of 1 to a few hundred micrometers in diameter. 

diameter can be chosen. Generally, the beam can be given 

a diameter between less than 1 to a few hundred micrometers. In addition, the beam can be swept over a rectangular 

area, much as in a cathode ray oscilloscope or TV tube. 

This action is called rastering. The size of the rastered area 

can be chosen and may be 0 to 400 J.Lm across, where these 

dimensions refer to the area swept by the center of the 

primary beam. 

The atoms in the sample surface are sputtered off by 

the cascades of collisions induced by the primary beam ions 

when they strike the surface (fig. 4). The sputtered material 

consists of several different species: neutral atoms, positively or negatively charged atoms, or clusters of atoms. 

The charged ions are predominantly singly charged, although 

multiply charged ions are observed also. The multi-atom 

fragments consist mostly of two atoms, though larger 

groupings are observed, with the abundances dropping by 

orders of magnitude for each additional atom above two. 

These multi-atom fragments need not follow simple rules of 

chemistry but may be made up of any two atoms in the 

sample. In silicates, the common pair is the metal oxide 

(KO, SiO, NdO, and so on), again, with no relevance to the 

valence properties. The vast majority of the atoms do come 

off as single atoms or ions. 

The multi-atom fragments of single charge act much 

as do the single-atom ions. If such a fragment has the same 

Use of the lon Microprobe to Measure Oxygen Isotopes in Minerals 131 
"molecular" weight as the atomic weight of a single atom 

being analyzed, the fragment will augment the signal 

received for that mass and give a greater apparent abundance to the isotope being studied. 

These sputtered ions are then accelerated by a 4,500-

V potential, as the secondary beam, in the direction of areas 

7 and 8 in figure 3. Normally, less than 1 percent of the 

sputtered atoms are ions of the appropriate charge to be 

accelerated as the secondary beam and measured. The beam 

is bent by the electrostatic analyzer at 9 and the magnet at 

13, which produces the mass separations commonly 

encountered in a mass spectrometer. The electrostatic 

deflection of the beam at 9 results in a small dispersion 

(region 11) of the ions, depending on their energies. 

The Cameca instrument takes advantage of this effect 

to remove some of the multi-atom fragments from the 

secondary beam. It was observed that the energy distributions of the single-atom ions differ from those of the 

multi-atom fragments. More of the single-atom ions have 

higher energy. If a slit is placed at region 11 , only atoms of 

higher energy can be selected to enter the magnet. The 

result is that atoms of the isotope being analyzed will be 

reduced in number by the slit, but multi-atom fragments 

will be far more reduced, thereby "purifying" the secondary 

beam. This technique is called energy filtering. An alternative way to effect this filtering is to keep everything else the 

same and reduce the accelerating voltage from 4,500 V. 

This technique is called a voltage offset. The voltage offset 

is often in the range 30 to 100 V. Less offset voltage will 

not produce much effect, and more will often reduce the 

signal received so drastically that the sacrifice in counting 

statistics of the signal is considered too great. 

An alternative to energy filtering to remove the 

extraneous multi-atom fragments is to distinguish them 

from the single-atom ions on the basis of mass. The 

interfering fragment will have the same nominal mass as the 

isotope being studied, but there will be a very small 

difference in the masses. By using the mass spectrometer of 

the ion probe under conditions of high mass resolution, it is 

sometimes possible to separate the beams of the fragment 

ions from the single-atom ions and to measure only the 

desired isotope. As in the case of energy filtering, the price 

paid for the higher mass resolution is a decrease in signal 

intensity. Consequently, although mass resolutions in 

excess of 15 ,000 are attainable with the Cameca ion probes, 

the signals become highly attenuated, so that precise abundance measurements are difficult for low-concentration 

species. 

Once the beam leaves the magnet at 13 (fig. 3), it can 

be collected for study in different ways. It is then either 

imaged at 19 or is measured by use of either a Faraday cup 

or an electron multiplier in region 20-23. The imaging at 19 

occurs on a phosphorescent screen viewed with a microscope or on a cathode ray oscilloscope display. The ion 

optics are designed to produce a focused image of the ion 

132 New Frontiers in Stable Isotopic Research 

beam at the detector at 18 so that an image of the sample 

surface is seen. Regions of high concentration of the isotope 

being imaged will appear as bright areas on the image. 

Quantitative measurement of the ion beam is effected 

by deflecting the beam into the region 20-23, where an 

electron multiplier or DC amplifier is used. Oxygen isotopic 

analysis performed with this instrument, with some modifications, is discussed by Hervig and others (this volume) 

and by Zinner (this volume). 

The Cameca 4f model is different from the 3f in 

several respects, but we shall consider only two, which are 

important to the 180 analyses. The principal innovation is 

the normal-incidence electron gun (Slodzian and others, 

1987). As discussed also by Hervig and others and by 

Zinner (this volume), a serious problem arises in the 

analysis of electrical insulators. If a Cs + primary beam is 

used, charging of the sample surface takes place, which 

alters the net accelerating potential of the secondary beam 

and distorts the local field at the sample surface where the 

ions are collected for their trip through the analyzer optics. 

The result is drastic reduction of the secondary beam signal 

(2 or 3 orders of magnitude). 

The solution to this problem in the case of the 4f is 

shown in figure 5. Electrons are generated by an electron 

gun mounted partway along the flight tube of the secondary 

beam (in the vicinity of regions 7 and 8 of figure 3, which 

depicts the Cameca 3f) and are accelerated and then 

deflected "upstream" toward the sample surface. The accelerating potential is identical to that applied to accelerate the 

o- ions of the secondary beam, approximately 4,500 v' 

but in the opposite sense. The result is that, if the sample 

acquires a positive charge owing to the Cs + primary beam, 

the electrons will neutralize it. If there is no charge buildup, 

a weak magnet deflects the electrons from the surface of the 

sample. As a result, charge neutralization will occur as 

though "on demand," and signal intensity is maintained at a 

high level. Additional focusing is required for the secondary 

beam because distortion is introduced in the course of 

controlling the electron beam. This adjustment is made as 

shown on the right-hand side of figure 5. 

The second modification in the 4f enables peaks to be 

switched electrostatically. This capability permits rapid 

alternation between masses 16 and 18, without the delays 

inherent in driving the mass spectrometer electromagnet to 

the appropriate field strengths for the different masses. The 

result is that modest variations in the signal intensity with 

time can be offset by reducing the time spent on one mass 

during such variations. Data collection times can be made 

quite short (we used 100 ms for mass 16 in some of our 

measurements). Such short counting times and rapid 

switching are possible if the electron multiplier is used. 

Owing to the slower reponse time of the amplifier of the 

Faraday cup collector, peak switching is not normally used. 
Sample sur1ace 

Electron11 

Secondary Beam 

Flight Path 

Figure 5. Schematic drawing of the charge neutralization device used on the Cameca 4f ion microprobe. 

Left-hand end of diagram shows same geometry as in 

figure 4 except inverted left-to-right. Remainder of 

diagram is part of flight tube for secondary beam. 

Strong oxygen signals (mass 16 approximately 5 x 

105 counts per second (cps)) were obtained by use of an 

8-pA primary beam of somewhat uncertain size on the order 

of 0. 5 to 1 ~m in diameter. 

RESULTS 

The results presented were obtained on the demonstrator instrument at the Cameca factory in Courbevoie, 

France. The sample was a plagioclase from a Lake County, 

Oregon, basalt, chosen in the hope that a plagioclase 

quenched in a lava would be homogeneous. Specific operating conditions used with the Cameca IMS 4f to measure 180/ 160 ratios are delineated in the following list. 

Lake County plagioclase is from a basalt (therefore 

quenched so as to avoid gradients) 

Primary beam = Cs + ions 

Spot size of primary beam = 1 ~m 

Primary beam current = ~ 10 pA 

Secondary beam = 0- ions 

Normal-incidence electron gun employed (to neutralize 

sample charge) 

Mass resolution = approximately 1 ,500 

Electromagnet 

Electromagnets 

for secondary oon 

trajectory correctoon 

~>Quadrupole· lenses 

~ 

Sccondnry 

Ions 

Filament is source of electron beam that is deflected 

"upstream" in the secondary beam, back to the sample 

surface. Modification of a drawing from the Cameca 

Company, Courbevoie, France. 

Electrostatic peak switching, count 16 for 0.1 s and 18 for 

5 s 

Each data set is 10 blocks of 10 scans each ( 100 ratios) 

When rastered, used 50- by 50-~m area to start off, then 

reduced to 15- by 15-~m area for data collection 

Mass 18 counts per scan = (0.5 to 1) x 104 

The analyses consisted of sets of data obtained by 

finding the ratio of mass 18 abundance to mass 16 abundance. A given ratio was found by taking the ratio of the 

signal at mass 18 relative to that for mass 16, where the 

latter is the weighted mean of the two mass 16 values 

measured just before and just after the mass 18. The 

weighting is based on the time intervals between the 

collection of the mass 18 and the two mass 16's, as these are 

not symmetrically disposed in time about the 18. Background counts as well as monitoring of the 30Si signal were 

also part of the mass spectrometer cycle. Blocks of data 

consisting of 100 such ratios were taken. 

Table 1 shows the results of a single day of analyses 

and includes a single analysis in which a small rastered area 

was used instead of no rastering by the primary beam. The 

data are presented in the order they were taken. Each entry 

is the mean of the 100 cycles described above. Spacing 

occurs to indicate where the sample was moved to begin 

sputtering in a new spot on the crystal. The uncertainties are 

given as standard deviations of the mean, in per mil. 

Use of the lon Microprobe to Measure Oxygen Isotopes in Minerals 133 
Table 1. Reproducibility of 180 analyses by use of the 

Cameca IMS 4f ion microprobe 

[All data were run continuously in 1 day. Sample is a plagioclase from 

Lake County, Oregon. Extra spacing between run numbers indicates a 

change to a new spot on crystal surface] 

180/160 +1- 1a Mean Stand. dev. Stand. dev. 

Run 

(X 103

) (otoo) (X 103) 

from mean from A1 

(otoo) (otoo) 

32 1.979 0.7 0.8 

33 1.977 .35 .2 

34 1.9764 .63 1.97747 .5 1.0 

35 1.9722 .78 2.5 

36 1.9821 .76 1.97715 2.5 1.1 

37 1.9729 .6 3.3 

38 1.9794 .57 .1 

39 1.9835 .52 2.1 

40 1.9817 .64 1.9794 1.2 O=A1 

41 1.9784 .67 .4 

42 1.9801 .60 1.2 

43 1.9747 .59 1.9777 1.5 .8 

44 (miniraster) 1.9972 .8 3.4 

45 (miniraster) 1.9887 .69 1.9938 2.6 7.3 

1 Arbitrary value against which the others will be compared. 

DISCUSSION 

It can be seen from table 1 that these are preliminary 

data given as a progress report. While the Cameca Company was quite generous in allocating 5 days to this study 

(following another briefer visit 6 months earlier), the time 

was too short to indicate the quality of the data that 

ultimately can be obtained from this instrument. 

Before discussing the results further, it is useful to 

consider the key factors that control the determination of 

high-precision 8180 data. One of these is the statistics of the 

measurement. The primary ion beam can be made quite 

intense and thereby produce a strong secondary beam 

signal. Use of a Cs + primary beam and 0- secondary ions 

is no exception. It is relatively easy to obtain a mass 16 

signal that is 1 million cps, which means that the mass 18 

signal in oxygen is 2,000 cps. If counted for 5 s, this rate 

would give a mass 18 standard deviation of 1 percent for 

one such count. 

A problem with such count rates lies in the dead time 

of the electron multiplier used to measure the counts. Dead 

time is the interval from the acceptance of a count by the 

counter until the counter is ready to accept the next count. 

Dead times on the order of 25 ns are common in the 

multipliers used on Cameca ion probes. For high count 

rates, the electron multiplier will not be able to keep up, and 

a correction must be made for those counts that it misses. 

Such a correction is not accurate enough for the oxygen 

work if the rates are on the order of millions of counts per 

second. 

134 New Frontiers in Stable Isotopic Research 

The relative abundances of the oxygen mass 16 to 

mass 18 are 500 to 1. The choice is either to increase the 

count rate so that the mass 18 can be counted with better 

precision, with consequent loss of precision in the huge 

mass 16 signal, or to reduce the count rate for mass 16 to 

approximately 500,000 cps, with a loss in the mass 18 

precision. 

Consider a specific case of dead-time correction. For 

a 25-ns dead time and a count rate of 500,000 cps, the 

multiplier would fail to record 1. 25 percent of the counts, 

and the apparent rate would need to be increased by that 

amount. If the count rate were 600,000 cps, the multiplier 

would miss 1. 5 percent of the counts. Suppose that the dead 

time should really be 30 ns; then, the corrections would 

need to be 1.5 and 1.8 percent, respectively. Clearly, the 

measured ratio of mass 18 to 16, for instance, depends 

directly on the correction for the dead time. If a correction 

is made, however, the two count rates would still be in the 

ratio of 1.2 to 1. 

Our experience is too limited to permit us to make a 

definitive statement, but it appears that the measured ratio 

will vary from day to day by as much as several per mil. 

Further, the absolute ratio is not known well. As a result, it 

appears desirable to measure the ratio and compare it to that 

for a standard that is mounted in the same sample mount and 

is measured on the same day. If this is done, and the same 

dead-time correction is used for all the measurements, the 

relative values of the ratios (~ 18

0's) could still be correct. 

The problem would arise again, however, if the correction 

became large (say > 10 percent), because uncertainties are 

inherent in the way the correction can be made. If the same, 

relatively low, count rate is used for all measurements, then 

there is less chance for discrepancies. 

The logical solution is to count mass 18 for a long 

time, but this solution permits greater variations in the 

signal intensity that are due to a slight drift as well as small 

fluctuation in the signal intensity. Alternatively, an electron 

multiplier that has a shorter dead time might be sought. 

A second factor in obtaining high precision is the 

fractionation of the isotopes during the analysis. By that we 

mean the steady-state measurement of a different isotopic 

ratio in the ion probe from that of the sample. If sputtering 

causes fractionation, the result is that a different composition is left on the sample surface. This process will, in tum, 

affect the composition of the sputtered material until a steady 

state is reached that yields a sputtered sampling that is the 

same as the sample itself. Despite this, however, the 

fraction of sputtered atoms that is ionized needs not have the 

same isotopic composition as the sputtered material (see 

Shimizu and Hart, 1982). These authors also found that 

where the sputtered ions were sampled, relative to the 

aligned optic axes of the primary and secondary beams, was 

critical to the degree of fractionation that occurred. Consequently, careful alignment and maintenance of the same 
1.9970 - REPRODUCIBILITY OF OXYGEN ISOTOPE RATTO 

1.9940 - . 

' Used minirasler 

1.9910 

1.9880 

180/160 

1.9850 

1.9820 

1.9790 

X X 

1.9760 

1.9730 }-

1.97 

I i ~ 4 

ARBITRARY SPOT LOCATION ON CRYSTAL 

Figure 6. 180/160 ratio measured on different places of a 

single crystal of Lake County, Oregon, plagioclase. Spacing between the two lines is 1 otoo and is given to assist in 

assessing reproducibility. Note data point where rastering 

was employed. 

conditions of operation may prove essential to optimum 180 

analysis. 

Another possible cause of fractionation is if not all the 

ions produced and accelerated into the secondary beam are 

allowed to travel to the collector. The technique of energy 

filtering, used to reduce interferences by multi -atom fragments, may also transmit different masses selectively. It 

may be best to operate without energy filtering. 

Figure 6 shows the data from table 1. The results of 

all the analyses fall within 1.1 otoo of each other. This fact 

is encouraging but not definitive. It is necessary to assess 

the performance of the instrument in two other critical 

ways. The first is to be able to compare the results of 

measurements taken on two different specimens, with 

different 8180 values, and obtain the correct value of this 

difference. 

The second is to be able to measure the same two 

specimens at a significantly later date (days or weeks) and 

still obtain the same difference. It would be even more 

desirable if the same 180/ 160 ratios were obtained on the 

same specimen on different days. Preliminary data taken on 

days different from the day when the values of table 1 were 

obtained suggest that the last may not be possible (see also 

Hervig and others, this volume). Note how important the 

modest change in operating conditions, namely the use of 

rastering, is to the data. It remains to be seen if rastering 

will give reliable, though different absolute, values. 

It will be necessary to make numerous additional 

measurements before the uncertainty in the measured 180/ 160 ratios can be known. The measurements show 

considerable promise but are a beginning at best. The 

absolute values of the ratios are quite different (see table 1) 

from the expected values, where the 180 might be expected 

to be 0.203 percent of the total oxygen. This difference is 

probably due to fractionation at the sputtering site. The 

absolute value is not a critical constraint, however, if 

differences in the 180/ 160 ratios can be measured well. In 

that sense, this system would be no different from standard 

mass spectrometry, where the ratio is determined relative to 

a standard. A standard could be mounted alongside the 

unknown and the A 180 between them determined. The 

same standard could then be used to compare with other 

specimens also, much as in the case of standard mass 

spectrometry. 

A great deal remains to be done. First is to reproduce 

the same A 180 between two specimens on different days. 

Second is to determine the optimum operating conditions to 

achieve the smallest uncertainty in this reproducibility test. 

Third, the difference in A 180 should be varied between 

different specimens, and the correct difference should be 

obtained in all cases. Finally, the assessment of the effect of 

the crystal matrix remains to be made. Can a feldspar be 

compared to a quartz, to a mica, or to a nonsilicate such as 

magnetite (see Hervig and others, this volume)? Will 

multiple standards be needed? It should be clear from these 

questions that a more detailed treatment of the data presented is not warranted at this point. The data need to be 

compared with other results of the sort just described before 

a proper assessment of the capabilities of the Cameca 4f can 

be made. It should be noted that closer scrutiny may lead to 

larger or smaller uncertainties than are shown here. 

CONCLUSIONS 

Preliminary measurements have been made of 180/ 160 ratios by use of the Cameca IMS 4f ion microprobe. The normal-incidence electron gun makes for considerable stability and a high secondary o- beam intensity. 

Rapid electrostatic peak switching permits avoidance of 

drift and some variation problems that arise from a fluctuating beam. Replicate measurements on a single crystal of 

plagioclase fall within a range of 1.1 otoo. It is not yet 

known if replication of differences between two specimens 

can be achieved over long time periods of days to months. 

It is also necessary to determine the effects of comparing the 180/ 160 ratios of different minerals. While promising 

results were obtained, the definitive assessments of the 

Cameca 4f for 180/ 160 analysis remain to be performed. 

ACKNOWLEDGMENTS 

The authors wish to thank the Cameca Company for 

making time available on their demonstrator model, IMS 

4f. Particular thanks are due to H.N. Migeon and J.-J. Le 

Goux for their generous cooperation in helping us to run the 

Use of the lon Microprobe to Measure Oxygen Isotopes in Minerals 135 
tests. We also wish to thank Bob Criss and Pat Shanks for 

organizing this workshop and for their efforts, together with 

those of Ernst Zinner, for reviewing this manuscript. 

REFERENCES CITED 

Dodson, M.H., 1986, Closure profiles in cooling systems: Material Sciences Forum, v. 7, p. 145-154. 

Farver, J .R., and Giletti, B .J., 1985, Oxygen diffusion in amphiboles: Geochimica et Cosmochimica Acta, v. 49, p. 

1403-1411. 

Giletti, B.J., 1986, Diffusion effects on oxygen isotope temperatures of slowly cooled igneous and metamorphic rocks: Earth 

and Planetary Science Letters, v. 77, p. 218-228. 

136 New Frontiers in Stable Isotopic Research 

Giletti, B.J., and Yund, R.A., 1984, Oxygen diffusion in quartz: 

Journal of Geophysical Research, v. 89, p. 4039-4046. 

Giletti, B.J., Semet, M.P., and Yund, R.A., 1978, Studies in 

diffusion, pt. III, Oxygen in feldspars, an ion microprobe 

determination: Geochimica et Cosmochimica Acta, v. 42, p. 

45-57. 

Shimizu, N., and Hart, S.R., 1982, Isotope fractionation in 

secondary ion mass spectrometry: Journal of Applied Physics, v. 53, p. 1303-1311. 

Slodzian, G. , 1980, Microanalyzers using secondary ion emission: 

Advances in electronics and electron physics, supplement 

138: Academic Press, p. 1-44. 

Slodzian, G., Chaintreau, M.P., and Dennebouy, R.C., 1987, 

Self-regulated potential at insulating surfaces in presence of a 

strong electrostatic extraction field: Cameca News, May 

1987, p. 1-6. 
Charge Neutralization and Oxygen Isotopic Analysis 

of Insulators with the lon Microprobe 

By Richard L. Hervig,1 Robert M. Thomas/ and Peter Williams2 

Abstract 

The Cameca IMS 3f ion microprobe at Arizona State 

University has been modified to allow the analysis of 180/160 ratios on 10- to 20-micrometer spots in insulating 

minerals in thin section. A primary beam of K+ orcs+ is 

used, and charge is neutralized by an electron flood gun 

operating at -7.5 kilovolts. Negative secondary ions are 

accelerated to -4.5 kiloelectronvolts into the mass spectrometer, where 180- is detected on the electron multiplier by pulse counting, and 160- is detected on the 

electron multiplier or directly measured by an electrometer. In either configuration, internal precisions of 1.2 to 1.7 

per mil (1 standard error of the mean) in 180/160 ratios can 

be achieved. Preliminary analyses, showing quartz to be 

10 to 15 per mil lighter than hydrothermally altered plagioclase, suggest that matrix effects are important. 

INTRODUCTION 

The microanalysis of minerals in thin section for 

stable isotopic ratios is a desirable goal for researchers 

working on the ion microprobe (secondary ion mass spectrometer). A major impediment is charge buildup on 

minerals during analysis. The ion microprobe uses an 

energetic ion beam to strike a selected mineral. Because 

most rock-forming minerals are insulators, different degrees 

of local charge are bound to build up on the sample surface. 

This charging acts like an electrostatic lens at the sample, 

deflecting the secondary-ion beam away from the mass 

spectrometer and commonly reducing the secondary-ion 

current to the level of instrumental noise. Great strides in 

overcoming this problem for isotopic measurements have 

been made by the research group at Washington University 

in St. Louis under the leadership of Ernst Zinner. In 

measuring &D, o13C, &15N, and &180 in extraterrestrial 

samples, they have avoided the problem of charging by 

pressing small ( -10 J.Lm) particles into a conducting matrix 

of gold foil. When the primary-ion beam overlaps the 

particle and the metal, excess charge on the particle "leaks" 

off to the foil (Zinner, this volume). Although this technique has produced results of fundamental importance in the 

study of the solar system, it cannot be applied to the study 

of minerals in thin section, so mineral zoning studies are 

inaccessible. 

1 Center for Solid State Science, Arizona State University, Tempe, 

AZ 85287. 2 Department of Chemistry, Arizona State University, Tempe, AZ 

85287. 

Recent technological developments (new normalincidence electron guns) may provide a means of effective 

charge neutralization that will enable microanalysis of thin 

sections (Slodzian and others, 1986; Giletti and Shimizu, 

this volume), but the technique has not been adequately 

tested. This contribution reviews various techniques that 

may provide some means of charge neutralization and 

shows the results of preliminary oxygen isotopic microanalyses of minerals in thin section. 

Statement of the Problem 

Historically, ion-microprobe analyses of insulating 

phases have used a negatively charged primary-ion beam of 

oxygen directed toward samples coated with a thin conducting layer of carbon or noble metal (Lovering, 1975). Most 

commonly, a positive potential is applied to the sample to 

drive positive secondary ions into the mass spectrometer 

(see fig. 1 of Eldridge and others, this volume). Although 

the sample is at high positive potential, a net negative 

charge builds up in the crater formed as the impacting 

primary beam erodes the conducting film. This charge on 

the sample is approximately compensated when the abundant secondary electrons that are produced hop to the 

conducting coat around the crater. At worst, the crater 

becomes biased to slightly negative values relative to the 

rest of the sample surface. If this charging increases to high 

enough values, the secondary-ion beam will become unstable and the intensity will drop because it will be partially 

deflected from the mass spectrometer slits. 

In the case of analysis for oxygen isotopes, two 

problems that arise from the above approach are immediately apparent: (1) an oxygen primary beam will contaminate the sample and (2) the positive secondary-ion yield of 

oxygen is very small. The latter can be addressed by 

changing the polarities on the extraction optics of the ion 

probe so that negative ions are collected and analyzed; the 

negative secondary-ion spectrum of oxides shows very 

intense signals for oxygen and other electronegative elements. Achieving a change in the primary ion species from 

oxygen to another element is more difficult. If a positive 

primary ion impacts an insulator while negative ions and 

secondary electrons are extracted, the sample charges to 

high, positive values (see fig. 2 of Eldridge and others, this 

volume). 

Charge Neutralization and Oxygen Isotopic Analysis of Insulators with the lon Microprobe 137 
Because of the above difficulties, our initial efforts to 

analyze minerals for oxygen isotopes were aimed toward 

developing non-oxygen negative primary-ion beams 

(Hervig and Williams, 1986). Later, alternative methods of 

negative secondary-ion analysis in insulators by the use of 

positive primary beams and auxiliary high-energy electron 

flood guns were developed. The results of these experiments are described below and are followed by discussion 

of some preliminary isotopic determinations on natural 

samples and laboratory standards made in experiments in 

which primary K+ and Cs+ beams were used. 

EXPERIMENTATION 

This work was carried out in a Cameca IMS 3f ion 

microprobe analyzer equipped with a standard duoplasmatron ion source, an alkali-metal ion source (constructed 

in-house), and a primary beam mass filter. The duoplasmatron was modified by eliminating the plasma expansion cup 

and restricting the intermediate electrode aperture to a 

diameter of 1h6 in (with a channel length of I/s in). All 

primary ion currents were measured with the Cameca 

Faraday cup assembly and corrected for secondary electron 

errors by calibration against a Faraday cup installed at the 

sample position. All samples were sputter coated with a thin 

(100-200 A) conducting layer of Au-Pd alloy prior to 

insertion into the sample chamber. 

Alternative Non-Oxygen Negative Primary Jon 

Beams 

Fluorine 

We first investigated the feasibility of generating a Fprimary-ion beam in the Cameca duoplasmatron. Reuter 

( 1984) reported that high primary currents of CF 3 + could be 

generated in the same model ion probe by attaching a tank 

of an 0 2-CF4 mix to the duoplasmatron. Oxygen was used 

as the support gas instead of argon because with the latter, 

CF4 was observed to polymerize on one of the electrodes 

and thus accelerate the degradation of the duoplasmatron. 

When we fed a similar gas into the duoplasmatron and 

extracted negative ions, we found large signals for 160-

(>200nA) and 19F- (>40 nA). The fluorine beam was 

stable for several hours at a time and generated a similar 

normalized count rate for 180- from a standard quartz as a 160- primary beam. Corrosion of the duoplasmatron was 

accelerated relative to use of pure oxygen, but only by a 

factor of about four. Unfortunately, the signal for 180-

relative to 160- increased with time during a single analysis. The most likely reason for this is contamination of 19Fby 180H-. The mass-resolving capacity of the Cameca 

138 New Frontiers in Stable Isotopic Research 

primary column is not sufficient to resolve these two ion 

species. We concluded that F- is not acceptable for the 

analysis of 180/ 160 ratios. A.O. Nier (University of Minnesota, personal commun., 1988) has recently suggested 

mixing CF4 with a tank of isotopically pure 160 2 . Presumably, 160H3 - would be of negligible intensity. 

Cyanide 

The 12C14N- ion is known to be very readily formed. 

For safety reasons, HCN was ruled out as a feed gas. 

Instead, we used a mixture of 25 percent C02 : 75 percent 

air. This mixture gave a maximum of 300 nA CN-, 

although 50 to 100 nA was more common. At the same 

time, an abundant 160- signal was obtained (maximum of 

800 nA), so conventional analyses could use this species 

without changing the feed gas to the duoplasmatron. Initially, we had problems with primary-beam stability. By 

testing several mixtures, we found that the most stable eNbeams were achieved with relatively high ratios of N2 to 

C02 . Secondary-ion count rates for 180- on quartz normalized to primary-beam intensity were higher than obtained by 

use of 160- by approximately the ratio of the masses of the 

primary beams (26116). At 8 ke V impact energy, count 

rates (counts per second) were > 107 cps/nA for 160- and 

>2 x 104 cps/nA for 180-. 

A serious problem developed when CN- generation 

resulted in severe corrosion of the duoplasmatron. Operation for 24 hours eroded the aperture in the intermediate 

electrode to more than twice its initial diameter and large 

whiskers grew from the electrode to the anode. The corrosion problem was solved by designing a new intermediate 

electrode. A mild steel electrode was machined with a large 

aperture (1/s-in diameter), and a Macor machinable ceramic 

insert with a I/I6-in aperture was pressed inside this. This 

magnetic electrode, having an insulating, nonmagnetic 

insert, thus had a large magnetic aperture but the optimum 

physical aperture. This design has worked well for routine 

operation with Ar+ and 0 2 + primary beams, and we have 

used this design exclusively since mid-1985. Enlargement 

of the intermediate electrode aperture during operation with 

routine gases has been nearly absent, so our intermediate 

electrodes no longer wear out. 

Although this development was favorable, whiskers 

continued to grow rapidly from the cathode during operation with cyanide. At best, a stable CN- beam would last 

for about 8 hours, but eventually, the drift would be too 

large for data to be taken. In the stable part of one of these 

sessions, we analyzed a mantle-derived olivine (Fo90) four 

times, obtaining good internal precision (1 standard error of 

the mean ranged from 1 to 1. 5 o; oo). The arithmetic mean of 

the four analyses on different points had a standard deviation of 3 ofoo. Although somewhat encouraging, the erratic 

behavior of the CN- primary beam forced us eventually to 

abandon this approach. 
E-GUN HOUSING 

. ~LIGHT TUBE 

DEFL. 

PS 

Figure 1. Schematic drawing of the electron flood gun 

assembly. As received, the gun was mounted on a 23/4-in 

Conflat flange. An adapter tube was constructed with a 

Cephalex flange for mating to the Cameca sample chamber. The four boxes shown in the electron gun housing 

represent the four lenses supplied with the gun, but only 

the first and last lenses were energized. A deflector 

Positive Alkali Metal Primary Beams with an 

Electron Flood Gun 

While oxygen primary beams (positive or negative) 

enhance positive secondary-ion count rates (relative to an 

Ar+ primary beam), a primary beam of cs+ greatly 

enhances negative secondary-ion count rates (relative to 

Ar+). Routine use of cesium ion sources in ion microprobe 

analyses obviates the need for exotic (and corrosive) feed 

gases for the duoplasmatron. Our cesium source was made 

at Arizona State University (Lareau and Williams, 1986). 

The problem, as stated earlier, is that the input of positive 

primary ions and the extraction of negative secondary ions 

and secondary electrons lead to severe positive charge 

buildup in the analysis crater on insulating samples. We 

approached this problem by using an electron flood gun to 

supply additional electrons to the sample surface. 

Successfulness of this technique hinges upon three 

requirements: ( 1) a high-energy electron gun is needed to 

permit electrons to reach the sample, which is at a potential 

of -4,500 V, (2) the impact energy of the electrons must be 

selected such that the yield of secondary electrons is less 

than 1, and (3) proof of charge neutralization rests on 

obtaining stable secondary-ion signals from samples for 

indefinite periods and (or) generating sharp images of 

negative secondary ions from insulating materials. 

/J 

~ .... 

E-GUN MOUNTING 

FLAWGE W/HV 

FEEDTHROUGHS 

assembly (DEFL. ASSY.) and flight tube were attached to 

the immersion lens (IMMER. LENS), which defines a plane 

parallel to, and 5 mm away from, the sample surface. 

Typical operating conditions were (1) filament voltage of 

-7.5 kV, (2) filament current of 5 A, (3) first lens voltage of 

-1.2 kV, and (4) last lens voltage of -3.4 kV. HV, high 

voltage; DEFL. PS, deflector power supply. 

Electron Flood Gun 

A 10-keV electron gun (Model No. CE-537) was 

purchased from Cliftronics, Inc. , of New Jersey. The gun is 

a high current, four-lens design with a tungsten filament. It 

was mounted on an auxiliary port on the Cameca sample 

chamber so that the electron beam struck the sample at an 

angle ~30° to the sample normal (fig. 1). Although the 

diameter of the port is only slightly larger than the electron 

gun diameter, operation of the potentials up to 9 kV is 

possible routinely without arcing. A grounded flight tube 

between the gun and the immersion lens housing prevents 

charge buildup on insulators near the electron trajectory; the 

einzel lens nearest the sample must also be well screened 

from stray electrons. The flight tube incorporated deflectors 

for beam steering, but usually these were not needed. 

Electron currents (measured at the sample) up to several 

hundred microamperes have been obtained and used for 

studies in electron-stimulated desorption (Gillen and 

Williams, 1986; Williams and Gillen, 1987). For charge 

neutralization, we reduced the current to 0.5 to 1.5 J.LA. 

The choice of an appropriate electron accelerating 

potential was extremely important because the secondary 

electron yield of many oxides is a function of electron 

impact and angle and can be greater than unity at particular 

electron energies. For example, the secondary electron 

yield of ~ 500 e V electrons on MgO is much greater than 5 

Charge Neutralization and Oxygen Isotopic Analysis of Insulators with the Jon Microprobe 139 
(Dekker, 1958). Because secondary electrons will be accelerated away from the sample, an electron gun providing 

electrons at such energies causes the sample to charge more 

positively, opposite to the desired effect. Secondary electron yields peak at energies of a few hundred electronvolts 

(Kanaya and others, 1978) and fall below unity at very low 

and very high energies. Thus, effective neutralization of 

positive charge can be achieved by delivering to the sample 

either very low-energy electrons, as in the new normalincidence Cameca electron gun (Slodzian and others, 1986; 

Giletti and Shimizu, this volume) or very high-energy 

electrons (Eimpact ~ 3ke V) as in this work. 

We originally felt that the secondary electron problem 

could be avoided by installing permanent magnets near the 

sample to produce a weak magnetic field (100-200 G) 

parallel to the sample surface and return the low-energy 

secondary electrons to the sample. Although this approach 

seemed to work, we later discovered that the magnets were 

not needed if an appropriate electron energy were used. 

Additional concerns arose about possible mass fractionation 

related to the magnets, so this approach was abandoned. 

The best evidence for the absence of sample charging 

is the ability to obtain well-resolved negative secondary-ion 

images of insulating samples when using a defocused 

primary beam (Slodzian and others, 1987). We have been 

able to obtain such images from certain samples but not 

from others, and the success or failure of imaging attempts 

does not depend on the presence of the electron gun. 

Rather, it appears that certain minerals are more conducive 

to imaging than others. Biotites give clear negative 

secondary-ion images, but quartz and plagioclase do not. A 

simple explanation is that the cleavage traces in biotite 

retain residues of the noble-metal coating that originally 

covered the surface. These residues in the traces provide a 

leakage path for excess charge to the conducting coat, while 

a 1-nA, ~ 150-J.Lm diameter, defocused Cs + beam completely removed the coat from polished quartz and plagioclase crystals. 

The focused alkali-metal primary beam has been 

imaged clearly imaged by 160- secondary ions on mineral 

grains at beam diameters :550 J.Lm. In this case, the electron 

gun is absolutely required; switching off the filament 

voltage eliminates the image. In addition, we have developed techniques to generate stable and intense asecondary-ion count rates for long periods of time. 

Detection of Secondary Ions 

The natural abundance ratio of 180 to 160, being 

1 :500, conflicts with the dynamic range of the electron 

multiplier, as count rates for 180- must be kept below ~ 103 

counts per second (cps) in order to keep count losses on the 160- signal below 1 percent. Analysis times are quite long 

at these conditions, but at higher count rates, the error in 

determining the counting system dead time becomes signif140 New Frontiers in Stable Isotopic Research 

icant. The alternative of using an electrometer to measure 

both isotopes is much more attractive, but up to now we 

have not been able to obtain stable signals for 180- > 1 x 

106 cps (

160- = 5 x 108 cps). The Cameca electrometer 

has a variable offset current that corresponds to a count rate 

of ~ 105 cps, so signals of ~ 106 cps cannot be measured 

accurately. A situation arises where we must either give up 

a factor of 102 to 103 in secondary-ion intensity (and 

analysis speed) or measure one isotope on the electrometer 

and the other with the electron multiplier. McKeegan 

(1987) has successfully analyzed oxygen isotopes by using 

only the electron multiplier, but analyses require ~45 min 

and precision is rarely better than ±2 ofoo. We have tested 

the analysis of 160 on both detectors, finding that ratios can 

be precisely measured either way, but errors in the absolute 

ratios are very large when two different detection devices 

are used. The advantage to such a technique is in searching 

for mineral zoning. As each analysis requires only ~20 

min, internal precision of <1 ofoo can be achieved, and 

absolute ratios are not so important. Important parameters 

include the settling time and offset current of the Faraday 

cup electrometer, the dead time on the electron multiplier, 

and the degree of charging on the sample. 

Analytical Conditions 

Primary Beam.-During most analyses reported 

here, a Cs + beam was generated from a General Ionex ion 

source assembled at Arizona State University. As part of a 

separate study, this source was loaded with Na, K, and Rb, 

as well as Cs, to test the relative effectiveness of different 

alkali beams in ion-microprobe analysis (Lareau and 

Williams, 1986). On some occasions, analyses for oxygen 

isotopes were made with K+ as the analyzing beam. At all 

times, the beam was mass analyzed (for 133Cs + or 39K+). 

Primary currents ranged from 0. 1 nA to 5 nA (best results 

at 0.1 nA) with a corresponding range in beam diameter 

from 10 to 30 J.Lm. . 

Secondary Beam.-The sample was held at -4.5 kV 

and negative ions were accepted into the mass spectrometer 

from a circular area 20 J.Lm in diameter defined by a 

combination of electrostatic lenses and physical apertures. 

Analysis.-The analysis was begun by sputtering a 

crater for 10 min to reach stable levels of charging and 

constant secondary-ion counts. A 10- to 15-min analysis 

would follow, during which < 1 ng of sample would be 

consumed at primary currents of ~ 1 nA. The energy 

window was wide open (energy bandpass of 130 e V) and 

translated to discriminate against ions with <0 e V excess 

energy (those formed above the sample in the gas phase). 

Each cycle of measurements began by ramping the sample 

voltage to values ± 100 V from the initial value of 4,500 V 

while monitoring the secondary-ion intensity of 180-. The 

sample voltage was then returned to the centroid of the 

voltage-intensity peak as a means to compensate for sample 
2.10 

-

0 2.09 - 0 

0 

,.... Interior Edge 2.08 

_. >< 

0 2.07 r- t ! CD I ! ! ,.... ! ! - 2.06- ! 0 ~ co 

,.... 2.05 I I I 

0 200 400 600 800 1000 

Distance {J.lm) 

Figure 2. Step scan of oxygen isotopic ratios across a 

plagioclase crystal in a hydrothermally altered granite 

from the Isle of Skye, Scotland. K+ primary beam was at 5 

nA and 30 J.1m in diameter and rastered over a 50 J.1m x 50 

J.1m area. Secondary ions were collected from a circular 

area 20 J.1m in diameter in the center of the crater. 160-

intensity was measured on a Faraday cup, and 180- was 

measured on an electron multiplier. Errors are ~iven as 1 

standard error of the mean. The reported 180/ 6

0 ratios 

are approximately 30 otoo too high, probably as a result of 

using two different detectors for the different isotopes 

and (or) the result of electronic effects. No clear-cut 

spatial trend in 180/160 is apparent in this sample. 

charging. This procedure showed that the sample charged to 

-50 to -80 V. Intensities were measured on masses 18, 

17, 16.5 (when the electrometer offset signal was determined), and 16. 

Other Variables.- At this time, we are experimenting with different operating conditions. Keeping the field 

aperture constant at 750 1-1m, we compared analyses by 

using different transfer optic lens settings. Analyses with 

transfer optics set to give an analyzed area 20 1-1m in 

diameter gave slightly higher ion intensities, but errors in 

isotopic ratios were identical to those when transfer optics 

gave an area 60 1-1m in diameter. There is an advantage to 

using the larger analyzed area in that aligning the secondary 

beam on the ion optical axis is easier. 

RESULTS AND DISCUSSION 

There are two questions we want to answer in 

developing this technique. Can we obtain the same 180/ 160 

ratio several times on the same crystal, and how much does 

chemical composition affect the relative yield of the two 

oxygen isotopes (that is, how serious are matrix effects)? 

The results of nine analyses spaced at ~ 1 00-1-1m 

intervals are shown in figure 2 for a crystal of plagioclase in 

a thin section of granite from the Isle of Skye, Scotland, 

supplied by John Ferry of Johns Hopkins University. This 

2.180 

2.170 

~02.160 

,...0 

-o 

=o,... 2.15o 

,...~ 

2.140 

2.130 

Clear 

Interior 

I 

i 

~ 

Turbid 

Edge 

Plagioclase 

feldspar 

Quartz 

! i 

0 100 200 300 400 500 600 700 

Distance {J..Lm) 

Figure 3. Step scan of oxygen isotopic ratios across same 

plagioclase crystal as in figure 2. Analytical conditions 

were identical except that the primary beam was Cs+ at 1 

nA focused to a 25-J.1m diameter and rastered over a 50 J.1m 

x 50 J.1m square. Note that the two analyses of an adjacent 

quartz crystal appear to have a significantly lower 180/160 

ratio than the plagioclase, an unreasonable situation that 

probably arises from large (>10 otoo) matrix effects. 

sample was selected because large 180 depletions caused by 

interaction of rocks with low- 180 meteoric-hydrothermal 

fluids are known to be widespread in this region (Forester 

and Taylor, 1977). This crystal has homogeneous majorelement chemistry in its clear core (left side of the figure) 

but has a "turbid" rim produced during a hydrothermal 

event. Errors in each analysis were calculated as 1 standard 

error of the mean and range from 0.4 otoo to 1.4 otoo. Even 

though there are some textural differences associated with 

the last two analyses as the turbid zone on the right side of 

the figure is approached, it appears that, to the precision of 

the ion-microprobe analyses ( ~ 1 otoo), the crystal is homogeneous. 

The absolute ratios are higher than expected by 

roughly 30 otoo. These high values may be due to a poorly 

calibrated feedback resistor (1010 0) in the electrometer. 

Such problems are inevitable when two detection modes are 

used, and we have not yet tried to improve this performance 

but have instead concentrated on obtaining reproducible 

relative ratios that have good internal precision. 

Not shown in figure 2 is an analysis of a quartz grain 

adjacent to the plagioclase. The measured ratio was 7.5 otoo 

lighter than the mean value for plagioclase. Because texturally clean quartz in a granite would be expected to be 

isotopically heavier (typically by 2 to 20 otoo) than feldspar 

altered by meteoric hydrothermal fluids (Forester and Taylor, 1977; Criss and Taylor, 1983), our results show that 

matrix effects are important in ion-microprobe analyses of 

minerals for oxygen isotopes. 

One year later, we returned to this same grain and 

performed another traverse from the clear interior to the 

turbid rim. These data are shown on figure 3 with two 

Charge Neutralization and Oxygen Isotopic Analysis of Insulators with the lon Microprobe 141 
analyses of an adjacent quartz grain. The 180 signal is 

relatively high again, this time by ~so ofoo. Subsequent 

failure of the electron multiplier preamplifier confirmed that 

electronics probably contribute to the high ratio. Errors (1 

standard error of the mean) are less than ±2 ofoo. Once 

again, the plagioclase appears to be isotopically homogeneous. Quartz shows low 180/ 160 ratios again, this time by 

~ 15 O/oo. 

At the present time, we are encouraged by several 

recent developments. As we have not been able to increase 180- count rates to levels at which the Cameca Faraday cup 

can be used, we have lowered the primary beam intensity 

from 1-5 nA to 0.1--0.5 nA. Because this change caused 

the primary beam spot size to decrease to ~ 10 ~m, we 

shifted to a stationary beam ( ~ 10 f.Lm), and the observed 

count rates (108 cps for 160- and 5 X 105 cps for 180-) 

were slightly lower than those observed with the rastered 

beam at the higher primary currents. The energy window 

has been closed down to a bandpass of 40 e V. Degree of 

charging, which had been ~so to 80 V, is now 10 to 12 V. 

Stability of the secondary-ion signal is much improved, and 

internal precisions ( 1 standard error of the mean) for 

15-min analyses are routinely between 1 and 2 ofoo. We 

have tested whether it is important to maintain a small 

analyzed area (20-f,Lm diameter) or shift to a lesser magnification (60-f.Lm diameter). No improvement has been seen, 

but the ease of alignment of the secondary-ion beam on the 

optic axis at the latter conditions may be critical for later 

analyses of small samples (for example, Fahey and others, 

1987). We have tested whether high precisions are achievable with both isotopes on the electron multiplier. Count 

rates were reduced by offsetting the sample voltage by 100 

eV so that 160 intensities were <5 X 105 cps. Although 

analyses take 45 min, internal precisions of± 1.4 ofoo have 

been obtained (fig. 4). Matrix effects are often reduced 

when high-energy ions are used. We have not yet tested 

whether this is true in oxygen isotopic analyses. 

FUTURE STUDIES 

If recent improvements in the technique can be 

maintained, we will be working with various collaborators 

on determining the extent of zoning of minerals in hydrothermally altered granites and mylonites. Analysis of authigenic and detrital quartz is an obvious application, and 

available bulk-analyzed samples of coexisting quartz and 

magnetite will enable us to test matrix effects. 

We face several problems in determining absolute 

and precise oxygen-isotopic ratios. We do not understand 

the contribution of the electron multiplier electronics to the 

measured isotopic ratios, and if we continue to analyze 

isotopes on two different detectors, determination of absolute ratios may not be feasible. In addition, absolute 

calibration of the isotopic ratios requires comparison with 

142 New Frontiers in Stable Isotopic Research 

400000 2.100 

0-16 2.050 

300000 2.000 

>- 0 0 - CD t/) 1.950 ,.... 0 

200000 0 c: - ,.... C1) 1.900 0 

- 18/16 co 

c: 1.850 ,.... ~ 100000 

1.800 

0 1.750 

0 20 40 60 80 10u 

Cycle# 

Figure 4. Recent analysis for 180 and 160 in NBS 610 

silicate glass. Both isotopes were measured on the electron multiplier, and 160 is reported as counts per second 

(open squares, left scale). Each cycle represents a measurement of 160 and 180. Also shown is the dimensionless 180/160 ratio (solid squares, right scale). The upward drift 

of the ratio at the end of the analysis represents a decrease 

in 160 (caused by minor instrument instability), while 180 

remained constant. One standard error of the mean was 

1.4 otoo. Analysis time was 45 min. 

appropriate homogeneous, bulk-analyzed standards. 

Although careful bulk analyses show very good estimates of 

error, the degree of isotopic variation in individual grains 

contained in any mass spectrometer run is unknown, so 

microanalysis of standard grains from a bulk-analyzed batch 

will be somewhat uncertain. 

Concerning precision, if we keep count rates low so 

that the electron multiplier can be used for both oxygen 

isotopes, the problem arises that over 8 hours is required to 

collect 107 counts for 180 (0.3 o!oo precision). Although 

many problems can be studied at the currrent precisions of 

1 to 2 ofoo (1 standard error of the mean), analyzing both 

isotopes at higher intensities on a Faraday cup is very 

desirable. Thus, we are currently rebuilding the Cameca 

secondary-ion Faraday cup in the hopes of measuring 180-

at intensities in the range of 105 to 106 cps (1 0- 14 to 10 -u 

A) without significant noise. One other problem that may 

become significant is that because of the high currents we 

use on the electron gun, certain beam-sensitive phases may 

not be amenable to our technique. 

CONCLUSIONS 

Use of an alkali-metal ion source and a high energy 

electron flood gun can control charge buildup to the point of 

allowing 180/ 160 ratios of 10- to 20-~m spots in insulating 

phases to be determined to a precision of 1 ofoo. Matrix 

effects between quartz and plagioclase feldspar probably 

exceed 10 ofoo. The degree of isotopic homogeneity we 

should expect in a mineral showing homogeneous majorelement chemistry remains unknown. 
ACKNOWLEDGMENTS 

Research to support our development of techniques 

for oxygen-isotopic analysis with the ion microprobe has 

been supported by a grant from the Petroleum Research Fund 

(# 18512-ACS to Peter Williams). Helpful comments from 

Ian Hutcheon, Bob Criss, Pat Shanks, and Bruno Giletti 

were appreciated. The Department of Physics machine shop 

at Arizona State University has played an integral role in 

producing these results. 

REFERENCES CITED 

Criss, R.E., and Taylor, H.P., Jr, 1983, An 180/ 160 and D/H 

study of Tertiary hydrothermal systems in the southern half of 

the Idaho batholith: Geological Society of America Bulletin, 

V. 94, p. 640-663. 

Dekker, A.J., 1958, Secondary electron emission: Solid State 

Physics, v. 6, p. 251-311. 

Fahey, A.J., Goswami, J.N., McKeegan, K.D., and Zinner, E., 

1987, 26Al, 244Pu, 50Ti, REE and trace element abundances 

in hibonite grains from CM and CV meteorites: Geochimica 

et Cosmochimica Acta, v. 51, p. 329-350. 

Forester, R.W., and Taylor, H.P., Jr., 1977, 180/ 160, D/H, and 13C/ 12C studies of the Tertiary igneous complex of Skye, 

Scotland: American Journal of Science, v. 277, p. 136-177. 

Gillen, J.G., and Williams, P. 1986, Electron-stimulated desorption microscopy, in Romig, A.D., Jr., and Chambers, W.F., 

eds., Microbeam analysis-1986: San Francisco, San Francisco Press, p. 109-112. 

Hervig, R.L., and Williams, P., 1986, Non-oxygen negative ion 

beams for oxygen isotopic analysis in insulators, in Benninghoven, A., Colton, R.J., Simons, D.S., and Werner, H.W., 

eds., Secondary ion mass spectrometry, SIMS V: Berlin, 

Springer Series in Chemical Physics, v. 44, p. 152-154. 

Kanaya, K., Ono, S., and Ishigaki, F., 1978, Secondary electron 

emission from insulators: Journal of Physics D: Applied 

Physics, v. 11, p. 2425-2437. 

Lareau, R.T., and Williams, P. 1986, Survey of alkali primary ion 

sources for SIMS, in Benninghoven, A., Colton, R. J., 

Simons, D.S., and Werner, H.W., eds., Secondary ion mass 

spectrometry, SIMS V: Berlin, Springer Series in Chemical 

Physics, v. 44, p. 149-151. 

Lovering, J.F., 1975, Application of SIMS microanalysis techniques to trace element and isotopic studies in geochemistry 

and cosmochemistry: National Bureau of Standards Special 

Publication 427, p. 135-178. 

McKeegan, K.D., 1987, Oxygen isotopes in refractory stratospheric dust particles: Proof of extraterrestrial origin: Science, v. 237, p. 1468-1471. 

Reuter, W., 1984, CF3 +:An alternative primary beam source for 

the sensitive detection of electropositive elements, in Benninghoven, A., Okano, J., Shimizu, R., and Werner, H.W., 

eds., Secondary ion mass spectrometry, SIMS IV: Berlin, 

Springer Series in Chemical Physics, v. 36, p. 54-56. 

Slodzian, G., Chaintreau, M., and Dennebouy, R., 1986, Selfregulated potential at insulating surfaces in the presence of a 

strong electrostatic extraction field, in Romig, A.D., Jr., and 

Chambers, W.F., eds., Microbeam analysis-1986, San 

Francisco, San Francisco Press, p. 78. 

---1987, SIMS: Self-regulated potential at insulating surfaces 

in presence of a strong electrostatic extraction field: Cameca 

News, May 1987, p. 1-6. 

Williams, P., and Gillen, J.G., 1987, Direct evidence for coulombic ejection of electron-desorbed ions: Surface Science, 

V. 180, p. L109-L112. 

Charge Neutralization and Oxygen Isotopic Analysis of Insulators with the lon Microprobe 143 

Isotopic Measurements with the lon Microprobe 

By Ernst Zinner1 

Abstract 

The principles and applications of isotopic ratio 

measurements by microprobe secondary ion mass spectrometry (SIMS) are reviewed. After a discussion of general instrumental aspects and measurement techniques 

for isotopic analysis, a more detailed treatment is given for 

the elements H, C, N, 0, Mg, Si, S, Ca, Ti, and Pb. 

Examples include measurements of D/H ratios in interplanetary dust particles; of C, N, and Si isotopes in 

presolar SiC; of Ca, Ti, and 0 isotopes in meteoritic 

hibonites; and of U-Pb dating of terrestrial and extraterrestrial zircons. 

INTRODUCTION 

Isotope geology has been dominated by two techniques: gas mass spectrometry and thermal ionization mass 

spectrometry (see White and Wood, 1986). These two 

traditional branches of mass spectrometry are distinguished 

by the physical mechanism of ion production. In the last 

decade secondary ion mass spectrometry (SIMS) has 

become an increasingly important tool for the geoscientist. 

The main two applications are measurements of trace 

elements and isotopic ratios on a small spatial scale 

(Shimizu and Hart, 1982a). 

In the ion microprobe, secondary ions are produced 

by ion bombardment of the sample to be analyzed. The 

focusing of the bombarding ions into a fine beam allows the 

in situ analysis of individual mineral phases. The main 

advantages of the technique are 

1. High sensitivity. Detection limits for certain elements 

are as low as 1 ppb even in complex geological samples. 

2. Small sample size. Meaningful analyses have been 

reported on subpicogram ( < 10 -l2 g) amounts of total 

sample, involving in extreme cases the measurement of 

less than 1 femtogram ( < 10 -Is g) of a specific element. 

3. Measurement of elements that are difficult to analyze by 

other techniques. These include isotopic measurements 

of iron and osmium. 

These advantages are to be compared with problems that are 

inherent in the technique: 

1. The sputtering process produces a large variety of 

molecular secondary ions along with atomic ions. These 

molecular ions interfere with the atomic ions of interest. 

1 McDonnell Center for the Space Sciences and Physics Department, Washington University, St. Louis, MO 63130. 

In geological samples, the problem of interfering molecular species depends on the composition of the analyzed 

mineral, so the analysis of each element is a special 

problem. 

2. The ionization efficiencies of different elements vary by 

many orders of magnitude. Certain elements are easily 

ionized and can be measured fairly readily, while other 

elements (such as noble gases) practically cannot be 

measured at all. 

3. In addition to the variation of the ionization efficiency 

between different elements, the ionization efficiency of 

a given element depends strongly on the chemical 

composition of the sample from which the element is 

sputtered. This so-called "matrix effect" is one of the 

major problems of quantitative elemental analysis by 

SIMS. 

The ion microprobe has been hailed as the ultimate 

weapon of the geoscientist for more than 20 years (Lovering, 197 5), but its emergence as a practical instrument has 

been tied to the advent of a second generation of ion 

microprobes that include the Cameca IMS 3f (Lepareur, 

1980) and the Sensitive High mass Resolution Ion MicroProbe (SHRIMP) at the Australian National University at 

Canberra (Clement and others, 1977). These instruments 

have technical features such as high mass-resolving power 

and energy filtering that help to overcome the problems of 

the presence of molecular isobaric interferences and the 

matrix dependence of secondary-ion yields. 

While SIMS has been applied to the trace-element 

analysis of geological samples of both terrestrial and extraterrestrial origin (Shimizu and Hart, 1982a; Crozaz and 

Zinner, 1986; Veizer and others, 1987), the majority of 

isotopic measurements have been performed on extraterrestrial material. The reason is that the isotopic variations 

discovered in interplanetary dust particles (IDP's) and 

primitive meteorites are much larger than those found in 

terrestrial geological materials (for example, Clayton and 

others, 1973, 1988; Begemann, 1980; Wasserburg and 

Papanastassiou, 1982; Anders, 1988). However, important 

isotopic measurements have been made on terrestrial rocks 

and include prominently U-Pb dating of individual zircons 

and the study of the distribution of S and Pb isotopes in 

ore minerals (Hart and others, 1981; Froude and others, 

1983; Williams and others, 1984; Eldridge and others, this 

volume). 

Although the field of SIMS isotopic measurements is 

still in a state of vigorous development, it has reached a 

degree of maturity that makes an overview desirable. The 

present paper has this general objective but will emphasize 

Isotopic Measurements with the lon Microprobe 145 
the practical aspects of isotopic analysis with the ion 

microprobe at the expense of a detailed historical account of 

all studies reported in the literature. It will concentrate on 

measurements made with the Cameca IMS 3f ion microprobe and on analysis techniques developed for this instrument at the Washington University ion microprobe laboratory. At the same time, it will review the work of other 

experimenters either with the Cameca IMS 3f or with other 

instruments. The next section provides a general discussion 

of the techniques and problems of isotopic analysis by 

SIMS. This discussion is followed by more detailed treatments of the isotopic measurement of a series of elements 

ranging from hydrogen to lead. 

GENERAL ASPECTS OF ISOTOPIC 

ANALYSIS BY SIMS 

Instrumentation 

This section is restricted to the discussion of practically important questions of isotopic measurements by 

SIMS. For an introduction to SIMS, the reader is referred to 

the more general literature on that subject (Slodzian, 1980~ 

Williams, 1983; Benninghoven and others, 1987) and to 

Giletti and Shimizu (this volume). A wealth of extremely 

useful details on isotopic measurements with the Cameca 

IMS 3f ion microprobe can be found in the Ph.D. theses of 

McKeegan (1987a) and Fahey (1988). 

The ion microprobe is a mass spectrometer in which 

the ions are produced through sputtering by a finely focused 

primary-ion beam. Two ion sources are available: a duoplasmatron capable of delivering beams of oxygen or argon 

ions, and a cesium source. Primary oxygen ions are used for 

the analysis of positive secondary ions because the presence 

of oxygen on the sample surface significantly enhances 

positive ion yields. The presence of cesium enhances the 

yield of negative secondary ions. A duoplasmatron can 

generate o- and 02 + ions, but the negative beam is 

preferred for insulating samples because it minimizes sample charging. Additional discussion of primary beam generation and of the sputtering process is given by Hervig and 

others (this volume). 

Sample Preparation 

Since the sample surface is part of the secondary-ion 

extraction system (Slodzian, 1980), the sample surface has 

to be flat and conducting. For the analysis of positive 

secondary ions, polished sections of geological samples are 

coated with 200 to 1,000 A of carbon or gold. For isotopic 

analysis, the primary beam sputters a small (3-20 1--lm) hole 

through this coating. In all practical cases, charging of the 

analyzed area under the 0- beam is less than 20 V and can 

be compensated for . 

146 New Frontiers in Stable Isotopic Research 

On the other hand, bombardment with Cs+ ions and 

extraction of negative secondaries result in intolerable 

charging conditions on polished surfaces of insulating 

samples, even if they are metal coated. In order to avoid this 

problem, samples to be analyzed in the negative secondary 

ion mode under a Cs + primary beam are pressed into gold 

foil (McKeegan and others, 1985; McKeegan, 1987a). This 

method works well as long as individual grains of samples 

are smaller than 20 to 30 1-Lm and has been employed for 

isotopic measurements of H-, c-, o-, CN-, and Si-. 

Recently, Slodzian and others (1987) designed a charge 

compensation device for the negative ion mode that is 

reported to make negative secondary-ion analysis from 

insulating samples possible (also see Hervig and others, this 

volume). This device is now standard equipment on the 

Cameca IMS 4f instruments. 

Mass Spectrometer 

In both the Cameca IMS 3f and the SHRIMP, the 

mass spectrometer is of the double focusing magnetic type 

(Clement and others, 1977 ~ Lepareur, 1980). Because of the 

wide energy distribution of sputtered secondary ions, only a 

double focusing mass spectrometer gives the high massresolving power (MRP) found in these instruments without 

a significant loss of transmission. With few exceptions, all 

isotopic measurements discussed in the present paper rely 

on the use of high mass resolution for the elimination of 

molecular isobaric interferences. While the suppression of 

molecular ions by energy filtering has been successfully 

applied for the measurement of trace elements (Shimizu and 

Hart, 1982a~ Zinner and Crozaz, 1986), for isotopic ratio 

measurements the use of high mass resolution has several 

advantages: 

1. Certain interferences such as hydrides cannot be suppressed very efficiently by energy filtering. For example, the presence of an 28SiH signal might present a 

substantial interference for Si isotopic measurement, 

whereas the Si concentration can be obtained from the 28Si signal, which is more likely to be free of interferences (signals from N2 + and co+ are usually negligible 

for the analysis of Si). 

2. The loss of signal incurred by the selection of only 

high-energy ions (energy filtering) is usually much more 

severe than that accompanying analysis at high mass 

resolution. 

3. The precision requirements for isotopic analysis are 

normally much higher than those for elemental abundance measurements and make it of prime importance to 

maximize the secondary-ion signal. 

The Cameca IMS 3f/4f is an ion microscope that 

produces a direct image of the sample surface by massselected secondary ions. Although for isotopic measurements the primary beam is focused into a small spot on the 
sample, the imaging capability is extremely helpful for 

inspection of the analyzed area for the presence of inclusions, for the tuning of the secondary beam to highmass-resolution conditions, and for maintaining reproducible tuning conditions, an important function for the measurement of intrinsic mass fractionation effects (Fahey and 

others, 1987a,b). 

Both the Cameca IMS 3f/4f and the SHRIMP employ 

a mass spEctrometric geometry of electrostatic analyzer 

followed b) magnet. This arrangement provides dispersion 

in ion energy and, with the aid of an energy slit, the 

possibility of energy selection (Lepareur, 1980; Slodzian, 

1980). Res:riction to a limited energy window (typically 

25-50 V) of low-energy ions helps in tuning to high MRP 

conditions. 

Detection System 

Mass -analyzed secondary ions either are counted 

individually in an electron multiplier or their current is 

measured in a Faraday cup. The vast majority of isotopic 

measure me llts reported so far have been made by ion 

counting in the electron multiplier. Several problems are 

associated with ion counting: 

1. The detection efficiency is a function of the ion species 

(element, molecule, charge). For a given element the 

detectio l1 efficiency varies with isotopic mass (Zinner 

and others, 1986a). 

2. For precise isotopic measurements, the dead time of the 

detectio l1 system must be known with high accuracy, but 

the dead time depends on the detected ion species 

(Zinner and others, 1986a). 

The lSotopic mass dependence of the detection efficiency mimics a linear mass fractionation that is, however, 

smaller thall the isotopic mass fractionation produced in the 

sputter-ionization process. Both fractionation effects can be 

treated tog,!ther as "instrumental mass fractionation." The 

determination of the counting system dead time is best 

achieved by isotopic measurements of a standard of known 

isotopic composition. Such a standard should have at least 

three isotopes of varying and well-known abundances, and 

the element should lend itself to ion probe analysis. The 

number of elements having these characteristics is limited. 

The most favorable example is titanium (Fahey and others, 

1987a). The dependence of the dead time on the ion species 

might pres~nt a limit to the precision of isotopic measurements by single ion counting. 

Onl) few isotopic measurements employing current 

measuring techniques have been published (Ireland and 

others, 19~:6a; Chaussidon and others, 1987). This is still a 

field to be worked at, since subsequent measurements by 

Ireland (1988) indicated that the Faraday cup results of 

Ireland an'l others ( 1986a) probably were in error. 

5 

4 41Ti 

Ill 

o4J 

~ 

:::s 3 0 

u 

..... 

0 

1:10 2 \ 0 

~ \ '

7

TiH 

1 

Mass (amu) 

Figure 1. High-mass-resolution spectrum of the mass 48 

region that shows the structure of the 48Ti+ and 48Ca+ 

peaks and was obtained from meteoritic hibonite in the 

Cameca IMS 3f ion microprobe. The peaks are resolved 

without interference even though no valley separates 48Ti 

and 48Ca. Separation of 48Ti and 48Ca requires a massresolving power (MRP) in excess of 10,500. Also shown is 42TiH, which is clearly resolved under these high MRP 

conditions. amu, atomic mass units. 

Isotopic Measurement Techniques 

Isotopic analysis of a given element requires a careful 

evaluation of the molecular and atomic isobaric interferences encountered at the masses of the measured isotopes. 

These interferences determine the MRP needed to resolve 

them or, if they cannot be resolved, the procedures to be 

used to correct for their contribution to the measured signal. 

Mass-Resolving Power 

There is no completely satisfactory definition of the 

MRP, because the requirements for mass resolution depend 

not only on the mass difference and relative intensities of 

the peaks to be resolved but also on the detailed shape of the 

peaks in question. 

We define the mass-resolving power as MRP = 

m/..lm where ..lm is the 10-percent width of a given peak at 

mass m. This is an arbitrary operational definition that 

differs from other definitions, which typically define MRP 

by a 1-percent valley separation between peaks (for example, Ireland and others, 1985). However, in many cases the 

signal of a given peak can be measured virtually without 

interference even if it is not separated by a valley from a 

neighboring peak. This fact is demonstrated by figure 1, 

which shows a high-mass-resolution spectrum of mass 48 

with the peaks of 48Ti + and 48Ca +. Although the two peaks 

are not well separated, the contribution of the low-mass tail 

of 48Ca at the center of the 48Ti + peak is less than 10-4 and 

Isotopic Measurements with the lon Microprobe 147 
thus negligible within the precision of typical ion microprobe titanium isotopic measurements (Fahey and others, 

1985, 1987a). 

The MRP required for the elimination of isobaric 

interferences in many cases exceeds the capabilities of 

present-day instruments. The practical maximum of the 

MRP for isotopic measurements is ~ 15,000 counts per 

second (cps) in the Cameca IMS 3f/4f, approximately the 

same as that in the SHRIMP (Ireland, 1988). The mass 

separation between 48Ti and 48Ca corresponds to an MRP of 

10,500. Examples of isobaric interferences of smaller mass 

separations are 46Ca with 46Ti, and soy and socr with soTi, 

with the mass difference between socr and soTi corresponding to an MRP of 40,000. An even more important example 

to geochronologists is the separation between 87Rb (mass = 

86.909188 amu) and 87Sr (mass = 86.908884 amu), which 

would require a currently unattainable MRP of 2.9 x 10s. 

In some of these cases the contributions of interfering 

isobars can be subtracted if other isotopes of the interfering 

species can be measured. An example of this "peak stripping" technique is the correction of 48Ca + + for magnesium 

isotopic measurements (Hutcheon, 1982). The correction 

procedures for 46Ca, soy, and socr interferences in the case 

of titanium isotopic measurements have been discussed in 

detail by Fahey and others (1987a). 

Peak Jumping 

In the absence of multiple collector systems, ion 

probe isotopic measurements are made by peak jumping. 

Under computer control the magnetic field is stepped to 

values corresponding to the centers of the peak tops. These 

values can be located reliably as long as the magnetic field 

is cycled through the same sequence of values and the peaks 

are centered periodically (Huneke and others, 1983; Fahey 

and others, 1987a). Dwelling times at peak tops can be 

adjusted, depending on the signal intensity of different 

isotopes. Counts from many cycles are averaged to minimize the effects of both random and systematic changes in 

the secondary-ion signals. Because of a finite settling time 

of the magnet, dwelling times of much less than 1 s are not 

practical. 

Much faster cycling can be achieved by electrostatic 

peak switching as implemented in the Cameca IMS 4f. This 

method has the potential advantage that signal fluctuations 

can be averaged out more effectively. However, no systematic isotopic studies by electrostatic peak switching have 

been reported to date. Furthermore, a true advantage is 

achieved only under a limited set of circumstances. Isotopic 

measurements in this laboratory indicate that for count rates 

of up to ~20,000 cps a precision commensurate with 

counting statistics can be obtained by magnetic peak switching. More rapid switching would be of advantage only if the 

count rate of the least abundant isotope were substantially 

higher while that of the most abundant isotope were 

148 New Frontiers in Stable Isotopic Research 

still low enough as to not lead to a loss of precision because 

of limited knowledge of the dead time of the pulse-counting 

system. 

Instrumental Mass Fractionation 

For precise isotopic measurements, the signal ratios 

measured in the detection system have to be related to the 

true isotopic abundance ratios in the analyzed sample. 

There are two effects that change the original isotopic ratios 

of the sample: one is the instrumental mass fractionation, 

and the other is the response of the detection system. The 

latter has already been discussed for an electron multiplier 

pulse-counting system. The instrumental mass fractionation 

is a combination of the ionization probability as function of 

isotopic mass during sputtering and the selection effects 

imposed on the secondary ions between emission from the 

sample surface and collection in the detector. This effect 

has been studied by a number of experimenters (Slodzian 

and others, 1980; Jull, 1982; Shimizu and Hart, 1982b; 

Gnaser and Hutcheon, 1987, 1988) who found that generally the light isotopes are ionized preferentially relative to 

the heavier ones. The magnitude of this isotopic mass 

fractionation varies from element to element and is 

inversely related to the ionization efficiency. For a given 

element, mass fractionation is a function of the secondaryion emission angle and velocity as well as the chemical 

composition of the sample (matrix effect). 

Since we are still far from a detailed understanding of 

the basic phenomenon of ion emission by sputtering and 

since transmission conditions of secondary ions in a SIMS 

instrument are too complex to be taken into account 

theoretically, in practice a correction for the instrumental 

mass fractionation is based on either external or internal 

calibration. 

External Calibration.-In this method, the isotopic 

ratios measured in the unknown sample are compared with 

those measured in an external standard that has a known 

isotopic composition. Standard and sample should have the 

same chemical composition, and the measurements must be 

made under identical conditions. Since the instrumental 

mass fractionation strongly depends on the energy of the 

secondary ions (Slodzian and others, 1980; Gnaser and 

Hutcheon, 1987), it is important to select identical energy 

windows and to compensate for shifts in the selected energy 

because of sample charging (Fahey and others, 1987a,b). 

For elements such asH, Li, B, C, and N that have 

only two stable isotopes, the instrumental massfractionation correction necessarily must rely on external 

standards. However, also for elements having more than 

two stable isotopes, such as oxygen and silicon, SIMS 

isotopic measurements have been reported as deviations 

from the ratios measured in standards (Fahey and others, 

1987c; McKeegan, 1987a; Zinner and others, 1987). In 

certain cases, when models for the production of isotopic 
effects in the sample are available, the observed corrected 

deviations in elements having more than two isotopes can 

be split into contributions that can be attributed to different 

mechanisms. A prime example are the magnesium isotopic 

results obtained from refractory inclusions in meteorites that 

show evidence for 26Mg enrichments resulting from the 

decay of primordial 26 Al. In these cases, the intrinsic (that 

is, corrected for instrumental mass fractionation) magnesium isotopic effects are divided into a contribution from 

natural mass-dependent fractionation processes and 26Mg 

excesses attributed to the decay of primordial 26 Al (Macdougall and Phinney, 1979; Hutcheon, 1982; Huneke and 

others, 1983; Hinton and Bischoff, 1984; Fahey and others, 

1987b). The same decomposition of isotopic effects has 

been made for titanium and calcium SIMS isotopic measurements (Fahey and others, 1987a; Hinton and others, 

1987a). 

Traditionally, the deviations of isotopic ratios from 

those of terrestrial standards are expressed as 8 values in per 

mil ( o; oo), where for the isotope i of element E 

[

8Ei= 1 ,000 (E./E.) r J measure ct -1 J . 

(E/ E) standard 

E{ is the reference isotope. For example, 8170sMow and 

8 80sMOW are the per-mil deviations of the 170/ 160 and 180/ 160 ratios from those of SMOW (standard mean ocean 

water). However, for elements having three and more 

isotopes, if the intrinsic isotopic effects are decomposed 

into mass-dependent fractionation (F), due to physicochemical processes, and into non-mass-dependent (also 

called nonlinear) effects, due to other mechanisms (for 

example, radioactive decay, nucleosynthetic processes), 

8Ei denotes only the non-mass-dependent deviation. Thus, 

in the above example of meteoritic inclusions that have 26Mg excesses, 826Mg would stand for only the 26Mg 

excess after a correction for mass-dependent fractionation 

effects (intrinsic and instrumental). 

Internal Calibration. -For elements that have three 

or more stable isotopes, if one is interested in only nonmass-dependent effects and if one has convincing arguments that such effects are present in only some of the 

isotopes, one can eschew the use of external standards and 

make an internal mass fractionation correction by normalizing the measured isotopic ratios to an isotope pair. By 

definition, the two selected isotopes should be free of 

non-mass-dependent effects, but in many cases this situation is far from certain. The ambiguity resulting from the 

choice of the normalizing isotope pair has been discussed in 

the literature (Niederer and others, 1985). 

For ion microprobe analysis, the internal calibration 

approach has the advantage that a higher precision can be 

achieved by it than by external calibration. Furthermore, in 

many cases, no suitable standards that have matching 

chemical compositions are available. Still, there is a basic 

problem associated with internal calibration as well as with 

the decomposition of isotopic anomalies into massdependent and non-mass-dependent effects: the mass 

dependence of the isotopic fractionation has to be known. 

For the internal calibration this involves the instrumental 

fractionation as well as the intrinsic fractionation of the 

sample studied. In principle, the fractionation laws can be 

different for these two effects. 

There is good evidence that the instrumental isotopic 

fractionation (a) in the ion probe follows an "exponential" 

law that was originally found for thermal ionization mass 

spectrometric (TIMS) instruments through measurements of 

doubly spiked calcium samples (Russell and others, 1978), 

(E/ E) measured [mi] a 

(E/ E)sampte mj 

where mi and mj are the atomic masses of isotopes i and j. 

This law has been clearly demonstrated for measurements 

on titanium and calcium (Zinner and others, 1986b; Fahey 

and others, 1987a). Figure 2 shows titanium measurements 

on terrestrial and lunar standards after internal normalization to the 46Ti-48Ti isotope pair under the assumption of a 

linear and an exponential mass-fractionation law. The 

divergence between the two normalizations depends on the 

size of the fractionation. In the case of calcite, which shows 

the largest instrumental fractionation for calcium (19.2 

otoo/amu), the difference between the 848Ca values computed after a 40Ca-44Ca normalization according to the two 

laws is 11.5 ofoo. 

For other elements, either the instrumental mass 

fractionation is not large enough or the absolute isotopic 

ratios of the sample are not known well enough to test the 

form of the fractionation law. For example, the typical 

instrumental mass fractionation for Mg is 8 otoo/amu, so 

there is a difference of only 0.25 otoo for 826Mg between the 

exponential and linear law. On the other hand, the discrepancy between 26Mg/ 24Mg ratios reported for terrestrial 

samples (see Ireland and others, 1986a) is much larger than 

this difference. 

Precision 

It has been repeatedly stated that ion microprobe 

isotopic measurements are intrinsically less precise than 

measurements by other mass spectrometric methods. This 

statement is certainly true for measurements that require 

external calibration for elements such as H, C, and N. For 

magnesium, a precision of < 1 o; oo (2rr) can be achieved 

(Brigham and others, 1986; Ireland and others, 1986a; 

Fahey and others, 1987b; Hinton and others, 1988a), and 

for potassium, Hinton and others (1987b, 1988b) reported a 

precision of better than 0.2 otoo (2rr). 

Isotopic Measurements with the lon Microprobe 149 
8;/48(%o) 

4 

3 

2 

1 

46 

3 

2 

1 

-1 

46 

LINEAR LAW 

47 48 

''EXPONENTIAL' LAW 

47 48 

tTf 

49 so 

e Ti metal 

• Rutile 

A Ilmenite 

49 so 

Mass (amu) 

Figure 2. Plotted are 8(iTi/48Ti) values of titanium ratios 

measured in different standards after normalization to the 

~i-48Ti pair (stars), where the superscript i refers to the 

isotope of interest. An "exponential" law better describes 

the instrumental mass fractionation in the ion probe than 

a linear law. A linear fractionation law was used for the 

upper graph, an "exponential" law for the lower one. 

Note that the apparent excesses of -1 o;oo for 849Ti and -3 

o!oo for 850Ti in the upper graph are clearly shown in the 

lower graph to be artifacts of the inappropriate correction 

procedure (from Fahey and others, 1987a). 

For internal calibration measurements, the answer as 

to the ultimate precision of SIMS isotopic measurements 

has not been obtained yet. Limiting factors are the knowledge of the exact form of the instrumental massfractionation law and, for an ion counting system, the 

accuracy with which its dead time is known. Measurements 

of current charge in a Faraday cup currently suffer from 

inadequate designs both in the Cameca IMS 3f/4f (unpub. 

data from this laboratory) and the SHRIMP (Ireland, 1988). 

The limitation in precision imposed by fluctuations in the 

secondary-ion signal due to instabilities in the primary-ion 

150 New Frontiers in Stable Isotopic Research 

beam and sample heterogeneity will be overcome only by 

multiple collection systems. Such a system is presently 

being tested for the SHRIMP. 

The relationship between statistical precision and 

amount of sample consumed during analysis is straightforward. If V is the volume of the consumed sample, <J' its 

density, and A its average atomic weight, the number of 

atoms of a given isotope collected during isotopic analysis 

is 

Here, N is Avogadro's number, C a the atomic concentration 

of the element measured, I the isotopic abundance, 11 the 

ionization efficiency, T the fraction of emitted ions that are 

detected, and d the duty cycle of the measurement. 

To illustrate this relationship, we consider the carbon 

isotopic analysis of a 1-j.Lm SiC grain (Zinner and others, 

1987; Tang and others, 1988). Such a grain contains ~3 x 

1010 carbon atoms and 3 x 108 13C atoms. With a useful ion 

yieldS (product of 11n of ~3 x 10-4 and a duty cycle of 

50 percent, 45,000 13C atoms are counted, and a statistical 

precision of 4. 7 o; oo (1 <J') results. From a 0.1-j.Lm grain only 

45 13C atoms will be counted. The situation is worse for 

minor and trace elements as well as for elements that have 

low-abundance isotopes. As a consequence, a 1 ofoo precision cannot be achieved for oxygen isotopic measurements 

on a 1-j.Lm spatial scale. 

ISOTOPIC MEASUREMENTS OF INDIVIDUAL 

ELEMENTS 

There are essentially three factors that affect the 

feasibility of isotopic analysis of a given element in the ion 

microprobe: ionization efficiency, isobaric interferences, 

and isotopic abundances. 

As a general rule, elements on the left side of the 

periodic table readily form positive secondary ions, and 

elements on the right side form negative secondary ions. 

There are, however, exceptions to this simple picture. For 

example, in spite of the fact that C and 0 as well asP have 

relatively high yields of negative secondary ions, N does 

not form stable negative ions at all. 

The presence and intensity of isobaric interferences 

depend on the chemical composition of the sample. Because 

of the shape of the mass defect curve (the mass defect is the 

difference between the mass of a given nucleus and the sum 

of its constituent nucleons), there is a systematic relationship between the atomic mass of a given element and the 

MRP required to separate the most common isobaric interferences (for example, Liebl, 1980). This situation is 

graphically displayed in figure 3, which shows a plot of the 

deviation of the nuclidic masses of the stable isotopes from 
50 • • u 

Tb • 

+ ~+CH, .:::cu ,.......,., C::::::~" :l 0 E m =20,000 

as 

E ....._,., -50 

tn 

tn 

Cl) 

u 

>< -100 w 

tn 

tn 165 mamu as -150 :E 

0 50 100 150 200 25 

Atomic Mass Number A 

Figure 3. Mass excess (in milli-amu, mamu) of the nuclides 

plotted against atomic mass number A. The mass excess is 

defined as the difference between the mass of a given 

nucleus in atomic mass units (1 amu = 1112 of the mass of 12C) and the atomic mass number A (number of nucleons). 

This plot provides a simple graphical way to compute mass 

differences of isobaric interferences. Singly charged ions 

are represented by vectors from zero to the particular 

nuclide point. Mass excesses of molecular ions are 

obtained by vector additions. Doubly charged ions are 

their respective atomic mass numbers (the unit of atomic 

masses is defined as l/12 of the mass of 12C). For example, 

for 56Fe with atomic mass number A equal to 56 and a mass 

of 55.93494 amu, this deviation is -0.06506 amu. For 

atomic mass numbers A smaller than 100, most molecular 

isobaric interferences have larger atomic masses than those 

of the atoms, but the required m/Llm increases with A. 

Molecular species such as 28Si2 or 40Ca0 have larger 

masses than 56Fe, as can be seen from figure 3. The 

situation is reversed for high atomic numbers. For example, 

molecular species (HfSi, Zr20) interfering with lead have 

smaller atomic masses than the lead isotopes (see fig. 3). As 

a consequence, elements having atomic masses up to 60 and 

lead are most suitable for SIMS isotopic analysis at high 

MRP. 

represented by vectors having the same direction but half 

the length of singly charged ion vectors. The vertical 

distance between two vectors representing two isobaric 

sr.ecies at a given mass number (for example, 56Fe and 2 Si2) gives the mass difference, ~m. The MRP required to 

separate the two species is then N~m since A= m. Also 

shown are lines of constant MRP equal to m/~m, indicating what mass differnce (~m) is separated at any particular 

mass number for a given MRP (adapted from Liebl, 

1980). 

Elements having low-abundance isotopes sometimes 

pose special problems. An example is the measurement of 170, whose abundance is only 0.04 percent of that of 160. 

Although the separation of 17 0 and 160H requires only a 

moderate MRP of 4,700, the 160H- signal from most 

samples is much higher than the 170- signal, so special 

correction procedures are necessary (McKeegan, 1987a). 

What follows is to a large extent based on the 

experience gained at the Washington University ion microprobe laboratory but includes also many results reported by 

other experimenters. Table 1 presents a list of parameters 

characterizing high-mass-resolution measurements with the 

Cameca IMS 3f ion microprobe for a series of selected 

elements. Column 2 gives the preferred mode of analysis. 

Positive secondary ions are produced by an o- beam with 

Isotopic Measurements with the lon Microprobe 151 
Table 1. Parameters for isotopic measurements when using the Cameca IMS 3f/4f 

[MRP, mass-resolving power] 

Sensitivity Preferred MRP Precision (2<T) Element mode (m/dm) ( cou nts/nNwt% (in per mil) Problems 

H 

c 

N 

0 

Mg 

Si 

Ca 

Ti 

Pbt 

of element) 

H-, Cs+ 500 3X 106 

c-. cs+ 3,500 2X 105 

CN-, Cs+ 6,000 2X 105* 

o-, Cs+ 6,000 8X 104 

Mg+. o- 3,000 2-3X 105 

si+. o- 3,000 1-2X 104 

Si-, Cs+ 3,000 105 

ca+, o- 12,000 2.5X 104 

Ti+, o- 10,000 2X 104 

Pb+. o- 9,000 

t Measurements made with the SHRIMP. 

* With 50 weight percent C present. 

an energy of 17 ke V, and negative secondary ions are 

produced with a Cs + beam of 14.5 ke V energy. For the 

MRP (m!Llm) listed in column 3, Llm is the 10 percent peak 

width as defined previously. The sensitivity in column 4 is 

given as the number of ion counts per second per nanoampere of primary beam current per weight percent concentration of the analyzed element. Sensitivity depends on 

many conditions and can vary by as much as a factor of 2 to 

3~ the numbers given are typical values for practical 

applications. Except for Hand Mg, all measurements were 

made at a magnification corresponding to a 150-J.Lm imaged 

field (Lepareur, 1980); H and Mg were measured with 

magnification corresponding to 150-J.Lm and 75-J.Lm imaged 

fields (McKeegan, 1987a). Column 5 lists the precision that 

can be achieved in these measurements, albeit with some 

effort. In many practical cases the actual precision is much 

less than this, particularly in cases where it is sufficient for 

the purpose of the specific measurement. More detailed 

discussions follow. 

Hydrogen 

Hydrogen isotopes have been measured as positive 

secondary ions (Hiriton and others, 1983) and negative 

secondary ions (Zinner and others, 1983~ McKeegan and 

others, 1985). Negative secondaries have the advantage that 

the H2 - interference is less than 0.5 percent of the Dsignal, whereas for positive ions the H2 + signal is greater 

than the D+ signal by up to a factor of 100 (see fig. 1 of 

McKeegan and others, 1985). The precision of 30 o!oo is not 

that of single measurements, which can have much higher 

precision, but the typical variation between measurements 

of individual grains of a given standard. This variation may 

be the result of the lack of control of the ion emission 

152 New Frontiers in Stable Isotopic Research 

~30 Low deuterium abundance. 

~5 

? Low 15N; BO interference; 

need presence of C. 

2 Low 170; OH interference. 

.5 

.5 

.5 

1.0 

48Ti, 88Sr + + interference; 

low 48Ca abundance. 

1.0 48Ca, 5

°Cr interferences. 

geometry in the case of grains pressed into gold and may be 

smaller if measurements can be performed on polished 

surfaces. However, no systematic studies of this aspect 

have been made yet. 

The most important result obtained by D/H measurements in the ion probe has been the discovery of large 

deuterium excesses in interplanetary dust particles (IDP's) 

(Zinner and others, 1983; McKeegan and others, 1985, 

1987) and primitive meteorites (Hinton and others, 1983; 

McKeegan and Zinner, 1984). Figure 4 shows the ranges of 

oD values measured in different individual IDP's. The 

deuterium excesses are heterogeneously distributed on a 

micrometer scale, as the analysis of single fragments of 

certain IDP's gives large variations in oD, ranging in the 

most extreme case from -320 to +2,700 ofoo. One analysis 

of such a fragment consumes only 10- 11 g of sample. The 

carriers of deuterium excesses in IDP's and meteorites seem 

to be carbonaceous materials. Because of their small sizes, 

imaging of the deuterium distribution in the ion probe 

(McKeegan and others, 1987) appears to be a promising 

approach. 

Carbon 

The large interference of 12CH-, whose signal is 

usually much higher than that of 13C-, makes high mass 

resolution a necessity (see fig. 3 of McKeegan and others, 

1985). Since carbon has only two isotopes, an external 

standard must be used. NBS-21 (graphite) has been used in 

this laboratory (McKeegan and others, 1985 ~ Zinner and 

others, 1987). The instrumental mass fractionation for 

carbon is typically 45 to 50 ofoo/amu but studies by 

McKeegan and others ( 1985) indicated that matrix effects 

between very different substances are smaller than 5 ofoo, 
oD (SMOW) 

I, I, I I I I I I I 

-250 ' 500 1000 1500 2000 2500 ..... I 

.....__ ! 

:.=::r7 Olivines 

-1 ' 

I t 

I 

1- : - Pyroxenes : - I 

I 

I r;-

;- : I I 

: ~ I 

~ Layer Lattice Silicates I 

-.-

:-+-- I 

+ I - I 

I I 

I: I 

I I I I I I I I 1 

Figure 4. The hydrogen isotopic composition measured in 

34 interplanetary dust particles (lOP's). Plotted is the range 

of oD values (normalized to standard mean ocean water 

(SMOW)) observed in individual fragments of the lOP's. 

Many of the lOP's show large deuterium excesses that, as 

indicated by the range spanned by individual measurements, vary on a micrometer scale (the lOP's are about 10 

f.Lm in size). "Oiivines," "Pyroxenes," and "Layer Lattice 

Silicates" indicate the infrared classes of the lOP's established from Fourier transform infrared measurements. The 

fourth category comprises particles that do not fall into 

any of these classes. Vertical dashed lines denote the 

range of oO values observed in terrestrial rocks. 

which is the magnitude of the variation between measurements on different grains of a given substance. 

Ion microprobe isotopic measurements have been 

made in IDP's and materials from primitive meteorites. 

While anomalies measured in IDP' s are only of limited size 

(McKeegan and others, 1985), primitive meteorites contain 

SiC whose carbon isotopic composition differs from terrestrial material by up to 7,000 otoo (Zinner and Epstein, 1987; 

Zinner and others, 1987; Tang and others, 1988). Figure 5 

shows a plot of 13C excesses measured in small agglomerates of separates from the Murchison carbonaceous chondrite (Tang and others, 1988). These separates are mixtures 

of isotopically heavy SiC and an isotopically normal pure 

carbon phase. The SiC shows large 13C excesses, mostly 

between 500 and 2,000 otoo, with individual data points for 

813C ranging up to 7,000 ot oo. On the basis of this extremely 

anomalous isotopic composition and the fact that SiC 

cannot be formed from a gas of solar system composition, 

Zinner and others (1987) argued for a presolar origin of the 

SiC in the atmospheres of certain stars. 

Nitrogen 

Because nitrogen does not form negative ions and its 

positive secondary-ion yield is extremely low, nitrogen 

... o 

""'c.o 

2500 

2000 

1500 

1000 

500 

0 

6000 

4000 

2000 

0.6 

Ill 

~ 

I 

0 0.2 

............ 

+ 

+ 

0.8 

~} 

0.4 

+ D Hlllullocl 

0 Hllllighl 

• Hlllduk 

-i ~t-CH 

~ ~~ 

~~~ ~ f-!-1 ~I-t-t 

~~ 

f-+i~ ..-1 

I I 

0.6 0.8 1.0 1.2 1.4 

Si/C 

Figure 5. Plot of o13C versus Si/C measured in separates 

from the Murchison carbonaceous chondrite. The different symbols denote different size fractions of acid residues, some of which had also been oxidized in a r,lasma 

discharge ("ashed"). The correlation between 81 C and 

Si/C provides evidence for multiple carbonaceous components, namely isotopically normal pure carbon and isotopically heavy SiC. The SiC in these separates shows large 

and variable 13C excesses and is probably of presolar 

origin (from Tang and others, 1988). HM, residue of grains 

in size range of 0.03 to 0.2 f.Lm; HN, residue of grains in 

size range of 0.2 to 2.0 f.Lm. 

isotopes can be measured only in very special cases. In the 

presence of C, N yields a very intense CN- signal, so it is 

possible to measure N isotopes in carbonaceous materials in 

the form of CN- (Carey and others, 1987). Figure 6 shows 

high-mass-resolution spectra at the atomic masses 26 and 27 

obtained from a Murchison separate containing mostly SiC 

(Tang and others, 1988). While an MRP of 4,300 is 

sufficient to separate 12C15N from 13C14N at mass 27, a 

higher MRP is required for the separation of 11 B 160 from 12C15N (boron is a contaminant apparently introduced 

during the chemical treatment that led to the separation of 

SiC). 

Silicon carbide grains identified in the carbonaceous 

meteorites Murchison and Murray contain isotopically 

highly anomalous nitrogen, providing additional evidence 

for their extrasolar origin. Ion probe measurements (Zinner 

and others, 1987; Tang and others, 1988) revealed isotopically light as well as heavy nitrogen with 815N values 

ranging from -850 otoo to +4,300 otoo. The variability of 

the nitrogen and the carbon isotopic compositions of meteoritic SiC indicate different stellar sources. 

Oxygen 

Oxygen-rich samples under Cs + bombardment have a 

high yield of 0- secondary ions. The main problems 

Isotopic Measurements with the lon Microprobe 153 
Murchison Residue HN 

100000 

12 14 C N 

10000 

rll 

~ 

= 1000 

0 = 

u 100 

10 

26.00 26.005 26.01 26.995 27.00 27.005 

Figure 6. High-mass-resolution spectra of the mass 26 and 

mass 27 regions measured in a Murchison separate consisting mostly of SiC. The CN- obtained from nitrogenbearing carbonaceous samples is used to measure nitrogen isotopic ratios from the 12C14N and 12C15N peaks. The 

latter is resolved not only from 13C14N but also from the 

interfering 11B160. HN, residue of grains in size range of 

0.2 to 2.0 f-lm. 

associated with isotopic measurements of oxygen are the 

low abundances of 170 and 180, and the presence of 160H-

(separated from 170 by 3.6 x 10- 3 amu, corresponding to 

an MRP of 4,710) that can be up to 100 times larger than the 170- signal. If only the 180/ 160 ratio is measured, the 

problem is less severe, since in the case where 160H- = 

100 X 170-, 170H- would be only 0. 7 percent of the 180-

signal. In addition, the H2

160- signal is usually negligible 

and furthermore is separated from 180 by 11.4 x 10- 3 

amu, corresponding to an MRP of only 1,570. For a more 

detailed discussion of 180/ 160 measurements, see Hervig 

and others (this volume). 

For the measurement of all three oxygen isotopes, the 160H- interference requires a careful evaluation and correction of the contribution of the tail of the 160H- peak to 

the signal in the center of the 170- peak. In the procedure 

developed by McKeegan (1987b) the tail of the 160- peak 

is monitored at a distance from the 160- peak center that 

corresponds to the mass difference between 170 and 160H. 

The 160 tail to 160 peak ratio is then applied to the 160Hsignal to obtain the tail correction to 17 0-. 

Measurements on different terrestrial oxide standards 

(spinel and hibonite) showed that their isotopic differences 

can be reproduced within 2 ofoo (2a) if measurements on 

many individual grains are averaged (McKeegan, 1987b). 

However, individual measurements on different grains vary 

by typically 5 to 8 o/oo from the average. As already 

discussed for hydrogen isotopic measurements, these fluctuations might reflect variations in the local geometry of 

grains pressed into gold foil. Obtaining measurements on 

polished sections made by the Cameca IMS 4f and its 

charge compensation electron gun is highly desirable so it 

154 New Frontiers in Stable Isotopic Research 

can be established whether better control of the local 

geometry can lead to a higher precision for individual 

measurements. 

Ion microprobe isotopic measurements have provided 

important information on the distribution of an 160-rich 

component, first discovered by Clayton and others (1973), 

in early solar system materials. McKeegan ( 1987b) presented evidence for the extraterrestrial origin of refractory 

stratospheric dust particles on the basis of their 160-rich 

isotopic composition. Fahey and others (1987c) showed that 

hibonites exhibiting both excesses and depletions in 48Ca 

and 50Ti have 160 excesses. These excesses range from 40 

ofoo to 70 o/oo, surpassing the value previously observed for 

the spinel fraction from CM and CV meteorites (Clayton 

and others, 1977; Clayton and Mayeda, 1984). Furthermore, the large variations in the oxygen isotopic compositions of individual spinel grains from Murchison and 

Murray (McKeegan, 1987b; Grossman and others, 1988; 

Zinner and Tang, 1988) indicate that several oxygen reservoirs were responsible for the distribution of oxygen isotopes in the early solar system. 

It is worth mentioning that the "best" absolute value 

for the 170/ 160 ratio has been obtained from SIMS measurements. Although oxygen isotopic measurements relative to SMOW can be made with exceedingly high precision, the 170/ 160 ratio itself has been poorly known. Under 

the assumption that the C8

0/ 160) SMOW ratio is given 

by the value of Baertschi (1976), Fahey and others 

(1987c), from measurements on terrestrial spinel, obtained 

(

170/ 160)sMOW = 0.00038288± .00000028 (2amean). 

Magnesium 

By far, most ion microprobe isotopic measurements 

have been made for magnesium. The following provides 

only a limited list of papers reporting such measurements: 

Bradley and others (1978), Lorin and Christophe MichelLevy (1978), Macdougall and Phinney (1979), BarMatthews and others (1982), Hutcheon (1982), Huneke and 

others ( 1983), Hutcheon and others ( 1983, 1987), Clayton 

and others (1984), Hinton and Bischoff (1984), Lorin and 

Havette (1984), McKeegan and others (1985), Brigham and 

others (1986), Ireland and others (1986a), Fahey and others 

(1987a,b), Hinton and others (1988a), Ireland (1988). The 

reason for this abundance of measurements is the fact that 

the ion microprobe is uniquely suited for in situ measurements of 26Mg excesses believed to be due to the decay of 

live 26Al in the early solar system (see Wasserburg and 

Papanastassiou, 1982). 

Evidence for live 26 AI is given by the distribution of 26Mg excesses that are proportional to the Al/Mg ratio. An 

example is shown in figure 7, where the 26Mg/ 24Mg ratios 

(after correction for instrumental and intrinsic mass frac-
tionation) measured in different mineral grains of a coarsegrained Allende refractory inclusion are plotted against the 

Al/ 24Mg ratios. Measurements were made in the ion microprobe that allows the measurement of both the Mg isotopes 

and the Al/Mg ratio in individual phases. All data points lie 

on a line having a slope of 3. 9 x 10-5 . Interpreted as an 

isochron, this line corresponds to an 26 AI/ 27 AI ratio of 3. 9 

X 10- 5 at the time of formation (isotopic closure) of this 

inclusion. 

Magnesium isotopic measurements are among the 

simplest ones to be made by SIMS. At low MRP, the 

interference from 48Ca + + can be corrected for by peak 

stripping (Hutcheon, 1982), but care must be taken that 

possible MgH interferences can be excluded safely. Highmass-resolution analysis is preferred (Huneke and others, 

1983; McKeegan and others, 1985; Ireland and others, 

1986a) where an MRP of 3,000 eliminates all significant 

interferences and still yields a high sensitivity (table 1). 

Measurements of the intrinsic mass fractionation of a 

sample with the use of an external standard can be made 

with a precision of ~0.5 ofoo (2cr) (Ireland and others, 

1986a). Approximately the same precision can be achieved 

for internal calibration measurements of o26Mg after normalization to the 24Mg-25Mg pair (Hutcheon, 1982; Ireland 

and others, 1986a). Normalizations were made with an 

assumed linear instrumental mass-fractionation law and 

with a standard 26Mg/ 24Mg ratio. Although values for this 

ratio reported in the literature vary widely (see above), ion 

probe measurements on terrestrial materials from this laboratory (McKeegan and others, 1985; Fahey and others, 

1987b) are consistent with the ratios of Catanzaro and 

others (1966) within 0.3 ofoo (2cr). 

Evidence for 26Mg excesses associated with 26 AI 

(half -life 7 X 105 years) has been found in 10 different 

meteorites, but it is still not clear how widely 26 AI was 

distributed in the early solar system. The most comprehensive ion probe study on refractory inclusions from the 

Allende carbonaceous chondrite was presented by Hutcheon 

( 1982). He showed that only inclusions of one type had 

well-defined AI-Mg isochrons corresponding to an initial 26 Al/ 27 AI ratio of 4 to 5 x 10-5 • Other inclusions had 

isochrons of much smaller slopes or had 26Mg excesses that 

failed to be correlated with the Al/Mg ratio. While such 

observations can be explained by invoking late formation or 

redistribution of Mg and AI after 26 AI decay, the distribution of 26 AI in hibonites from primitive meteorites is even 

more puzzling (Ireland and others, 1986a, 1988a; Fahey 

and others, 1987a; Ireland, 1988). Hibonite is among the 

first minerals believed to condense from a gas of solar 

composition (Davis and others, 1982; Kornacki and Fegley, 

1984). Meteoritic hibonites exhibit the largest calcium and 

titanium isotopic anomalies found to date (see below). 

However, hibonites that have the largest Ca and Ti effects 

and trace element patterns indicating early formation do not 

show any evidence for 26 A1, while hibonites that have small 

0.16 

Allende 3529-Z 

~0.15 Anorthite #2 .. 

... 

-0/J 

~ 

~ 0.14 

Pyroxene + Melilite 

0.13 

0 100 200 300 400 500 

AI/ 24Mg 

Figure 7. Plot of 26Mg/ 24Mg versus AI/ 24Mg measured in 

different minerals of a coarse-grained Ca-AI-rich inclusion 

(CAl), 3529-Z, from the carbonaceous meteorite Allende. 

The magnesium isotopes in this inclusion show 26Mg 

excesses that are correlated with the AI/ 24Mg ratio and 

thus indicate that the 26Mg excesses originate from the in 

situ decay of live 26AI (half-life 7.2 x 105 years). If this were 

indeed the case, then the slope of the correlation line 

gives the 26AI/ 27 AI ratio at the time of formation of this 

inclusion (from MacPherson and others, 1988). 

or no Ca and Ti anomalies and trace element patterns 

indicating a complex formation history have 26Mg* (radiogenic) that are consistent with an initial 26 Al/ 27 Al ratio of 

5X 10-5 (Ireland, 1988; Ireland and others, 1988a). 

Heterogeneity of 26 Al in the early solar system is a 

possible explanation for the above observations (Hutcheon, 

1982). Also, Fahey and others (1987b) interpreted the 

observation that the CZ6 Al/ 27 Al)0 in the rim of a refractory 

inclusion from Efremovka is larger than that in the interior 

as evidence for an inhomogeneous 26 AI distribution. Still, 

such an explanation for the hibonites is not really satisfying 

(Ireland and others, 1988a), and the truth may lie somewhere else. 

A series of ion probe magnesium isotopic studies was 

concerned with the intrinsic isotopic fractionation in refractory meteoritic materials (Huneke and others, 1983; Clayton 

and others, 1984; Hinton and Bischoff, 1984; Hinton and 

others, 1985; Fahey and others, 1987b). The most intriguing observations were made by Macdougall and Phinney 

(1979) and Hutcheon and others (1983) who found extreme 

magnesium isotopic fractionations (up to 345 ofoo/amu) that 

varied on a submicrometer scale in a hibonite-spinel sample 

from Murchison. No magnesium fractionation of comparable magnitude has ever been found in any other object. 

Silicon 

Silicon isotopes have been measured as positive 

(Huneke and others, 1983; Clayton and others, 1984; 

Isotopic Measurements with the lon Microprobe 155 
McKeegan and others, 1985) and negative secondaries 

(Zinner and others, 1987). The silicon signal per primary 

beam current for a given sample is higher for negative 

secondary ions (see table 1), but since the sputter rate 

(number of sputtered atoms per incident ion) of Cs + is 

roughly 40 times that of 0-, the ionization efficiency 

(number of secondary ions per sputtered atom of a given 

species) is much higher for positive secondaries. The 

instrumental mass fractionation, however, is much larger 

for positive (-32 ofoo/amu) than for negative ions ( -8 

ofoolamu). Comparison measurements in this laboratory for 

both ion polarities on silicon and SiC indicate that also for 

silicon the instrumental mass fractionation is best described 

by the "exponential" law. Ion probe measurements of 

silicon isotopes in terrestrial materials disagree with the 

values reported by Barnes and others (1975): if normalized 

to the 28Si-29Si pair with e9

Si/ 28Si)norma1=0.0506331 

(Barnes and others, 1975), the average for 30Si/ 28Si is 

0.033474 ± .000014 (2amean) rather than 0.0336214. 

Ion probe isotopic measurements of silicon in IDP' s 

did not detect any deviations from the terrestrial composition (McKeegan and others, 1985). Measurements in meteoritic materials revealed intrinsic mass-fractionation effects 

but no discernible non-mass-dependent effects (Huneke and 

others, 1983; Clayton and others, 1984) until Zinner and 

others ( 1987) and Tang and others ( 1988) found large 

anomalies in SiC from Murray and Murchison. Figure 8 

displays their data in a three-isotope plot. Some data points 

are considerably displaced from the terrestrial composition 

along a slope 1/2 fractionation line, but many others lie far 

above or below the line. The distribution of the points in the 

plane indicates that at least three distinct isotopic components are needed to explain the silicon isotopic variability 

found in presolar SiC from primitive meteorites. 

Sulfur 

Sulfur isotopic measurements have been made with 

the Cameca IMS 3f (Chaussidon and others, 1987; N. 

Shimizu, personal commun., 1987) and the SHRIMP (Eldridge and others, this volume) ion microprobes. Both 

positive and negative secondary ions produced by abombardments were used. No analyses with Cs+ bombardment have been reported to date, but preliminary studies in 

this laboratory indicate that sulfides (galena, troilite) conduct well enough that negative ion analysis with a Cs + 

beam is feasible in these minerals. Analyses were restricted 

to measurements of 32S and 34S in terrestrial rocks. A 

detailed discussion of these studies can be found in Eldridge 

and others (this volume). 

Potassium 

Two types of potassium isotopic measurements have 

been reported: a study of the potassium isotopic mass 

156 New Frontiers in Stable Isotopic Research 

150 

• MurHM 

100 

• MurHN 0 

50 

·- 0 00 

g: 60 

tO 

-50 

40 

-100 20 

-150 0 

-200 +--.-----r----.---,-----,,-,--....---,----r--,----.----,-r--,----r--1 

-250 -200 -150 -100 -50 0 50 100 150 

o

30 Si 

Fi~ure 8. Three-isotope plot of ae9Si/28Si) versus 

8( 0Si/28Si) measured in SiC samples from the Murray (My) 

and Murchison (Mur) carbonaceous chondrites. The 

squares and diamonds stand for different size fractions. 

Most data points are far from the terrestrial massfractionation line, which has a slope of 1h and passes 

through the mean terrestrial composition (labeled EARTH) 

that defines the origin (0,0). All ordinary ("massdependent") fractionation processes such as diffusion or 

equilibrium partitioning produce deviations that fall on 

this line; samples that do not conform to the terrestrial 

fractionation line bear witness for distinct primordial isotopic reservoirs. The extremely anomalous silicon isotopic 

compositions are further evidence for the presolar origin 

of the meteoritic SiC. Inset shows expanded plot in the 

vicinity of EARTH and the terrestrial fractionation line 

(from Tang and others, 1988). HM and CF, residue of 

grains in size range of 0.03 to 0.2 J.Lm; HN, residue of 

grains in size range of 0.2 to 2.0 J.Lm. 

fractionation in the Earth, the moon, and meteorites (Hinton 

and others, 1987b, 1988b) and the search for 41K excesses 

that are due to the decay of 41Ca (half-life 1.9X 105 years) 

in refractory inclusions from pnm1t1ve meteorites 

(Hutcheon and others, 1984). The first study involved 

measurements in high potassium phases at low MRP (AEI 

IM-20 ion probe) and, with the use of external standards, 

achieved a precision of 0. 2 o; oo (2a), better than that 

achieved by thermal ionization mass spectrometric analyses. The potassium isotopic composition was found to be 

light by approximately 1 ofoo/amu both in lunar samples and 

meteorites relative to the Earth, in disagreement with 

fractionation models based on U/K ratios that predict heavy 

potassium for the moon. The second type of measurements 

were made with a Cameca IMS 3f in phases with extremely 

high CalK ratios at an MRP of ~5,000, which eliminates 

interferences for 41K+ such as 4

°CaH+, 25Mg160+, and so 

forth, but not 40Ca42Ca + + and 26Mg56Fe + +. These doubly 

charged ions pose an extremely serious problem. From 
measurements in pyroxene from Allende inclusions, Hutcheon and others (1984) could set an upper limit of 8 x 10-9 

on the 41Ca/ 4°Ca ratio in these objects at the time of their 

formation. This low value would require an interval of ~2 

x 106 years between 41Ca production and inclusion formation if the 41Ca to 40Ca production ratio was 10-4 . 

Calcium and Titanium 

Calcium and titanium isotopic ratios have been measured at low MRP with the AEI IM-20 ion probe at the 

University of Chicago (Hinton and others, 1985, 1987a). 

The calcium measurements were restricted to 4

°Ca, 42Ca, 44Ca, and, while Hinton and others (1987a) measured all 

titanium isotopes, they had to make an interference correction for the contribution of 48Ca + to the 48Ti + signal. Since 

in refractory objects such as hibonites from primitive • 

meteorites the 48Ca shows the largest effect among the 

calcium isotopes, it is preferable to separate the 48Ca + and 48Ti + signals. 

The necessary resolution can be achieved in the 

Cameca IMS 3f/4f (see fig. 1) and in the SHRIMP. Both 

instruments have been used to measure titanium isotopes 

under high MRP conditions (Fahey and others, 1985, 

1987a; Ireland and others, 1985; Ireland, 1988). The 

SHRIMP has the advantage that at high MRP its ion 

transmission is higher than that of the Cameca IMS 3f/4f by 

a factor of 20 to 50. This advantage is a direct consequence 

of its larger dimensions and its higher extraction voltage for 

secondary ions. The technique for the measurement of 

titanium at high MRP has been described in detail by Fahey 

and others (1987a). They include a discussion of the 

interference correction for 46Ca, 50V, and 5

°Cr that cannot 

be resolved from 46Ti and 50Ti. 

Measurements of all calcium isotopes except 46Ca 

have been made at an MRP similar to that of the titanium 

measurements (Zinner and others, 1986b; Fahey and others, 

1987c). In most minerals measured so far, the 48Ti+ signal 

is much more intense than the 48Ca + signal. In these cases, 

the 48Ca + signal is measured by first locating the center of 

the 48Ti peak and then changing the magnetic field by an 

amount corresponding to the mass difference between 48Ti 

and 48Ca. Although a contribution from the tail of the 48Ti + 

peak to the 48Ca + signal cannot be completely avoided, a 

tail correction is made similar to that discussed for the 17 0 

peak. 

Ion probe isotopic measurements of calcium and 

titanium in hibonites revealed large anomalies (fig. 9) that 

exceed those observed previously by thermal ionization 

mass spectrometry by more than an order of magnitude 

(Zinner and others, 1986b; Fahey and others, 1987c). The 

largest effects are found for 48Ca and 50Ti and are correlated 

in the sense that excesses of 48Ca accompany excesses of 50Ti and deficits of 48Ca accompany deficits of 50Ti. Since 

200 

lfll 

13-13 100 

i 

€ MY-H3 

0 1;1 .. ., EIIIJI!I 

11!1!1 El 

BB-5 

-100 

-100 0 200 300 

Figure 9. Calcium and titanium isotopes measured in 

meteoritic hibonite (Ca [AI, Ti, Mg]120 19) show correlated 

anomalies in 48Ca and 5D-ri: both isotopes show either 

excesses or deficits. However, these 48Ca and 50Ti anomalies are completely decoupled from 160 and 26Mg anomalies in the same samples. The 48Ca and 5

D-ri correlated 

effects are evidence for the nucleosynthetic origin of the 

Ca-Ti isotopic anomalies. The excesses are thought to be 

produced by neutron-rich nuclear statistical equilibrium 

processes during supernova explosions. The compositions with deficits in the neutron-rich isotopes consist 

probably of several nucleosynthetic components. An 

appropriate mixture of these and the neutron-rich component resulted in the average solar system isotopic 

composition of calcium and titanium. Plotted are ion 

probe measurements of hibonites from the meteorites 

Murray, Murchison, and Lance. Data for Murray MY-H3 

are from Fahey and others (1987a) and Zinner and others 

(1986b), for Murchison BB-5 from Fahey and others 

(1987c), and for Murchison 13-13 from Ireland and others 

(1988b). (Courtesy ofT. Ireland.) 

different nucleosynthetic processes in different astrophysical sites are responsible for the production of different 

calcium and titanium isotopes (see, for example, Niederer 

and others, 1985), the isotopic variability in hibonites 

represents incomplete mixing of materials carrying different 

isotopic components in the early stages of the solar system. 

In case of the correlated 48Ca and 50Ti excesses, the 

nucleosynthetic process responsible can be identified as 

neutron-rich nuclear statistical equilibrium burning (Cameron, 1979; Hartmann and others, 1985). The ion microprobe, with its capability of small spot analysis, makes it 

possible to find large isotopic effects that provide a link to 

the nucleosynthetic sources of solar system materials. 

Lead 

The main application of lead isotopic measurements 

is the determination of U-Pb ages. Although the first efforts 

were made at low MRP with peak stripping (Andersen and 

Hinthorne, 1972; Hinthorne and others, 1979), U-Pb measurements are now exclusively made on the SHRIMP at 

high MRP (Compston and others, 1984). Aided by the high 

Isotopic Measurements with the lon Microprobe 157 
ion transmission of the SHRIMP at high MRP, Compston 

and co-workers have made U-Th-Pb dating of zircons into a 

routine procedure. In addition to the measurement of lead 

isotopes, they have developed techniques for the quantitative measurements of Th/U and Pb/U ratios (Compston and 

others, 1984). An MRP of 9,000 (10 percent width definition) is used to resolve the lead isotopes from interferences 

such as Zr20, Zr20H, HfSi, HfSiH, and combinations of 

SiO and rare-earth elements. The determination of the Th/U 

and Pb/U ratios relies on corrections made from the measured ThO+ and uo+ signals. Ion microprobe dating of 

zircons has resulted in a large number of applications, 

including the identification of 4.2x 109

-year-old terrestrial 

zircons (Froude and others, 1983), evidence for unsupported radiogenic Pb in Antarctic zircons (Williams and 

others, 1984), U-Pb dating of lunar zircons (Compston and 

others, 1984), and the determination of U-Pb ages of 

meteoritic perovskite grains from the Murchison and 

Allende carbonaceous chondrites (Ireland and others, 

1986b). 

Hart and others (1981) used the Cameca IMS 3f 

operated at low MRP to study the zoning of the lead isotopic 

composition within a terrestrial galena crystal and found 

large variations in 208Pb/ 206Pb and 207Pb/ 206Pb ratios. 

Unfortunately, to date no correlated sulfur isotopic measurements were made on this sample. Such studies can 

provide important information on the genesis of ore deposits. 

Other Elements 

Ion probe isotopic measurements have been reported 

for a number of other elements. Although some of the 

applications of these measurements are potentially of high 

interest, none of them has become routine so far. Isotopic 

analyses of the elements listed in the following may require 

very different techniques and thus cannot be summarized in 

a simple way. The reader is referred to the original 

references. 

Lithium isotopes in meteoritic inclusions have been 

measured by Hutcheon and others ( 1977) and Phinney and 

others (1979); the latter investigators also measured boron 

isotopes. No isotopic anomalies were found for these 

elements. A series of chromium isotopic measurements was 

made in a search for short-lived 53Mn (half-life 3.7X 106 

years) in meteoritic material (Scatena-Wachel and others, 

1984, 1985; Hutcheon and others, 1985). Although Hutcheon and others (1985) found some evidence of 53Cr 

excesses in iron meteorites, the lack of a correlation with 

the 55Mn/ 52Cr ratio makes it doubtful that substantial 

amounts of 53Mn were present in these objects. 

Fahey and Zinner ( 1987) failed to find iron isotopic 

anomalies in a Lance inclusion that has large 48Ca and 50Ti 

anomalies (Fahey and others, 1986). The neutron-rich 

158 New Frontiers in Stable Isotopic Research 

nuclear statistical equilibrium process that produces 48Ca, 50Ti, and 54Cr is predicted to produce also 6

°Fe (half-life 

1.5 x 106 years) (Cameron, 1979; Hartmann and others, 

1985), but ion probe searches (Hinton and others, 1984; 

Scatena-Wachel and others, 1985) for evidence of 60Ni 

excesses due to 60Fe decay were negative. Hutcheon and 

others (1987) reported isotopic studies of Mg, Fe, Mo, Ru, 

and W in Fremdlinge from Allende refractory inclusions. 

While they found evidence for mass-dependent fractionation of molybdenum, they did not observe large nonmass-dependent isotopic anomalies. 

Ion microprobe measurements of 87Sr/ 86Sr ratios in 

terrestrial carbonates have been reported by Exley (1983), 

Exley and Jones (1983), and Scatena-Wachel and others 

( 1986). A prerequisite for such measurements are low 

rubidium concentrations. Because the separation of 87Rb 

and 87Sr requires an MRP of 2.9X 105

, in situ Rb-Sr dating 

• by ion microprobe mass spectrometry is out of the question 

for present-day instruments. For osmium isotopic measurements, SIMS proved to be superior to previous techniques 

(Luck and others, 1980). SIMS allowed the determination 

of Re-Os ages of terrestrial samples (Allegre and Luck, 

1980; Luck and Allegre, 1982) based on a new determination of the 187Re half-life from the study of iron and 

carbonaceous meteorites (Luck and others, 1980; Luck and 

Allegre, 1983)-' 

CONCLUSION AND OUTLOOK 

Secondary ion mass spectrometry has been applied 

successfully to isotopic measurements of many different 

elements. Although most of the work reported so far has 

been done on extraterrestrial materials, ion microprobe 

measurements of sulfur and lead isotopes have been important developments for geochemistry and geochronology. 

As for the future, improvements for SIMS isotopic 

analysis can be expected to be associated with increased 

selectivity and sensitivity. It is doubtful that future instruments will rely on substantial increases of mass spectrometer MRP for improvements in selectivity (separation of 

atomic ions of interest from interferences), although new 

concepts such as Fourier transform mass spectrometry 

should not be excluded. It is more likely that selection by 

laser resonance ionization will play a major role. 

An important increase in sensitivity can be achieved 

by multiple collection systems. Furthermore, such systems 

would result in increased precision of isotopic ratio measurements at high count rates because effects from fluctuations in the secondary ion signal would be eliminated (for 

example, instabilities of the primary beam current, beam 

position, inhomogeneities in the analyzed sample). 

Resonance ionization is also promising for improvements in sensitivity. Useful ion yields in present-day 

instruments are on the order of 10-4 to 10-3 . More 
complete, selective ionization could lead to either the 

analysis of smaller samples (femtogram analysis) or to 

improvements in the precision of isotopic measurements 

(for example, complete ionization would eliminate instrumental mass-fractionation effects). However, such capabilities presently exist only in theory; their realization will 

require further developments in laser technology (for example, higher repetition rates and higher laser power densities). 

ACKNOWLEDGMENTS 

I gratefully acknowledge the important role of R.M. 

Walker in establishing the Washington University ion probe 

laboratory. A.J. Fahey and K.D. McKeegan made essential 

contributions to many of the isotopic measurement techniques discussed in this paper. I thank E.G. Koenig for her 

help with the preparation of the manuscript. This work was 

supported by NASA Grant NAG9-55 and NSF Grant 

EAR-8719528. 

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730-732. 
Sulfur Isotopic Analyses on the SHRIMP lon 

Microprobe 

By C.S. Eldridge,1

' 

2 W. Compston/ I.S. Williams,2 and j.L. Walshe1 

Abstract 

The ion microprobe offers the user the advantages of 

high spatial resolution, in situ analytical capabilities, and 

respite from the tedium and hazards of sample preparation for conventional isotopic analyses. Because the only 

sample preparation needed is sample polishing, photographing, and carbon coating, the microprobe allows 

relatively rapid analyses of small samples as compared to 

selective abrasion or other physical means of isolating 

microsamples for conventional mass spectrometry. Like 

other mass spectrometers, however, each ion microprobe 

must be examined to quantify mass fractionation, demonstrate linearity, and discover if there are residual contamination, or memory, effects on measured isotopic ratios. 

Moreover, additional steps must be taken to ensure that 

appropriate standards are used and that standards are 

analyzed in the same analytical conditions as the 

unknown. lon microprobe analyses are limited by instrumental analytical precision, commonly around 1 to 3 per 

mil (2a), with accuracy equaling precision. 

Of the stable isotopes, the sulfur isotopic system is 

the easiest to analyze precisely on the ion microprobe 

because of the abundance of the minor isotope e4S). The 

ion microprobe SHRIMP has been used to study the sulfur 

isotopic systematics of sediment-hosted massive sulfide 

deposits, and important new data have been obtained 

from the McArthur River H.Y.C. and Mount lsa lead-zinc 

deposits, Australia, and the Rammelsberg deposits, West 

Germany, as well as the Salton Sea geothermal system, 

California. These studies have shown in some cases that 

the isotopic variability may actually be several times that 

found in conventional studies because of the averaging 

effects of conventional sample preparation, that mineralization in such deposits need not be syngenetic, and that 

biogenic sulfide may not have contributed sulfur to basemetal deposition at the site of ore precipitation. 

The next step in ion microprobe analyses, the 

improvement of precision, is being pursued by several 

methods. Investigations of multiple collection and 

increase of secondary-ion yields through use of negatively 

charged ions or laser-enhanced positive secondary ions 

are under way or planned. 

INTRODUCTION 

Research performed on the light stable isotopes (sulfur, oxygen, carbon, and hydrogen) has had a major impact 

1 Geology Department, The Australian National University, GPO 

Box 4. Canberra, A.C.T. 2601. Australia. 2 Research School of Earth Sciences, The Australian National 

University, GPO Box 4, Canberra, A.C.T. 2601, Australia. 

on many subdisciplines of the geosciences (Valley and 

others, 1986; Kyser, 1987). Analytical precisions of conventional analyses are such that many fundamental natural 

processes, both equilibrium and kinetic, can be examined 

by use of the light stable isotope ratios. Often, though, 

stable isotopic studies are limited by the amount of sample 

necessary for analyses. Typically, around 10 mg of pure 

mineral separate must be concentrated from the hand 

sample and converted to a purified gas for mass spectrometric analysis. Traditional methods of physical or chemical concentration of the chosen mineral are incapable of 

discriminating between various generations, and it is 

increasingly clear, through conventional studies involving 

careful dissecting of relatively coarse-grained materials or 

those enlisting microanalytical techniques, that rocks and 

individual mineral grains sometimes contain more information concerning their origins and subsequent modifications 

than can be obtained easily by bulk sampling and analytical 

means. While it is possible to lessen the amount of sample 

required for conventional analysis by holding the sample 

inlet system of a conventional mass spectrometer in a static 

mode and not sacrificing gas to the pumping system, this 

type of analysis offers no advantage in relating small 

samples to specific textural features. Therefore, sampling 

techniques have been developed for conventional isotopic 

measurements of single grains or parts of single grains, 

including crushing and abrasion, step heating of handpicked crystals, and laser ablation. However, all of these 

sampling techniques are limited by labor-intensive steps 

involved in liberation, purification, and transfer of gas to 

the mass spectrometer. Furthermore, some of the methods 

mentioned above may produce data that are only average 

isotopic values for the sample and may sacrifice the 

sample's important textural information that is crucial to 

proper data interpretation. Alternatively, the ion microprobe offers sampling on a very fine scale (15 to 30 1-1m), 

eliminates the difficult and potentially hazardous aspects of 

sample preparation for conventional mass spectrometry, 

and provides largely nondestructive in situ analyses that can 

be related to the microtextural features of the sample. 

ANALYTICAL METHOD 

Analysis of the stable isotopic composition of solids 

in the ion microprobe requires little sample preparation. The 

sample must be cut to a size that will fit into the sample 

Sulfur Isotopic Analyses on the SHRIMP lon Microprobe 163 
QUADRUPOLE LENS 

ENERGY ANALYZER 

FIELD CLAMPS 

SHRIMP ANALYZER 

ION OPTICS 

FARADAY CUP 

ION MULTIPLIER 

Figure 1A. Schematic drawing of the SHRIMP overall 

geometry. The major components are the primary column, where the primary beam is generated; the source 

chamber, site of secondary-ion formation and extraction; 

electrostatic sector analyzer (cylindrical, 85°) that allows 

transmission of ions within an energy bandpass of 150 eV 

in 25 keV; 72.5° nonnormal entry sector magnet for mass 

analysis with a dispersion of 773 mm x (~m/m); and the 

collector, which has a detection system for simultaneous 

holder and must also be polished. Typically, a polish 

finishing with 1-J.Lm diamond abrasive is sufficient. The 

SHRIMP (Sensitive High mass Resolution Ion MicroProbe) 

sample holder is circular, has a diameter of 25.4 mm, and 

holds samples no thicker than 7 mm. Diverse materials, 

from appropriately trimmed thin sections to polished blocks 

to assemblages of grains, once oriented to allow space for 

the standard plug and mounted in epoxy, can be used. The 

epoxy prevents samples from acting as virtual leaks inside 

the vacuum chamber, and the mounts must be coated 

(typically with carbon, but copper or gold can be used) to 

dissipate charge during analyses. Once under vacuum in the 

SO\lfCe chamber, any portion of the mount can be analyzed 

164 New Frontiers in Stable Isotopic Research 

PRIMARY 

ION SOURCE 

counting of eight masses. All major segments of the 

instrument are joined by bellows to allow movement and 

the best possible overall alignment. The collector housing 

is on rails and is wired to a computer-driven motor so that 

it can be located for the optimum peak shape position for 

each isotope. The housing is also on two independently 

movable plates and can be tilted left or right as well as 

nose up or down in order to best match the slits with the 

incoming ion beams. 

by controlling drive motors attached to the sample-holding 

arm. Continuous viewing, in reflected light only, of the 

sample is permitted during analyses. Photomicrographs of 

relevant portions of these mounts, for navigational purposes, must be prepared prior to analytical sessions. 

The ion microprobe obtains material for analysis by 

directing a primary-ion beam onto the sample surface. 

Impact of the primary ions dislodges material from the 

sample, a small portion of which is ionized, and these 

secondary ions are then directed into the mass analysis 

portion of the instrument (figs. 1, 2). Descriptions of the 

interaction of the primary beam with the sample and models 

of secondary ion formation can be found in several papers 
SHRIMP SOURCE BEAM STEERING HOLLOW 

CATHODE 

ION SOURCE ION OPTICS 

BEAM STEERING 

LENS 1 LENS 2 

CHANGER 

MICROSCOPE 

Figure 1 B. Schematic drawing of the SHRIMP primary-ion 

source and secondary extraction ion optics. The primaryion beam is generated in an IONEX duoplasmatron fitted 

with a cold, hollow, nickel cathode. The primary-ion beam 

can be operated, like most commercially available instruments, in critical illumination where the extraction aperture is imaged and the beam current is proportional to the 

primary-beam spot size on the target. However, Kohler 

illumination, where an aperture between the two primary 

column einzel lenses is imaged and the spot size is 

essentially fixed while the beam current (ion density) may 

be varied, is more commonly used. Different areas of the 

target are accessed when the sample is moved under the 

primary-ion beam by means of x andy positioners, and the 

target can be focused in the z dimension through a 

micrometer attached to the sample arm. Focusing in the z 

(for example, Schwarz and Helms, 1981; Williams, 1982; 

Shimizu, 1986, and references therein), and more detail of 

the operation of the SHRIMP ion microprobe is contained in 

papers by Clement and others ( 1977) and Ireland and others 

(1986). Primary ions are accelerated at 10 k V, and they can 

be focused onto the target by use of either critical or Kohler 

PRIMARY BEAM 

SECONDARY BEAM 

LENS 3 

BEAM STEERING 

BEAM MONITOR SOURCE SLIT 

dimension is used essentially to compensate for surficial 

imperfections such as polishing relief. The primary beam 

impacts at 45° to the sample surface, and secondary ions 

are extracted normal to the plane of the sample, accelerated to ~11 kV across a 1-mm gap. Secondary ions are 

focused through three lens systems, passed through the 

source slit (commonly at 80 1-1m wide), and finally steered 

into the mass-analysis system. Approximately 10 percent 

(the exact amount is dependent on the target matrix) of 

the secondary-ion beam is trimmed by the secondarybeam monitor. Counts from the monitor are divided into 

the counts obtained for each isotope at the collector, and, 

in a manner similar to multiple collection, ratios are 

corrected for fluctuations in the intensity of the primary 

beam. 

illumination to produce a minimum crater size of between 

15 and 30 J.Lm in diameter. The primary beam yields a 

current of 2 to 7 nA at the target. Because all of the atoms 

contained in that crater are removed and have the potential 

for entering the mass analyser, the mass spectrometer 

portion of the ion microprobe needs to be able to resolve 

Sulfur Isotopic Analyses on the SHRIMP lon Microprobe 165 
t 

Secondary ions to mass 

analyzer 

Galena lattice 

Figure 2. Schematic view of the sputtering event in which 

a primary beam of negatively charged oxygen ions is used 

and positively charged secondary ions of sulfur and lead 

from galena are produced. The high ratio of Pb+ to s+ 

illustrates the low ionization efficiency of sulfur in this 

analytical mode. 

possible isobaric overlaps on the masses of interest (table 

1). The mass spectrometer portion is double focusing, with 

the secondary ions, accelerated to 11 kV, initially being 

focused by the electrostatic analyser (a passive energy 

filter), which allows only ions of a selected energy to pass 

into the magnetic mass analyser, and then being focused 

into the collector by the magnetic sector. 

SHRIMP was designed by S.W.J. Clement, W. 

Compston, and G. Newstead (Clement and others, 1977), 

who used equations for overall geometry from Matsuda 

( 197 4). It was constructed largely at the Research School of 

Earth Sciences of the Australian National University. Its 

primary constraints were that it have a mass resolution of 

10,000 (m/.6-m) and as high a sensitivity as possible. The 

sensitivity, a function of the width of divergence-limiting 

slits, increases with the length of the flight tube and size of 

the magnet. The magnet has a radius of 1 m, maximum field 

of 5 kG, has nonlaminated pole pieces, weighs 6 t, and is 

water cooled. The electrostatic sector, as demanded by the 

equations (Matsuda, 1974), has a radius of 1.27 m. To 

minimize stray magnetic fields acting on the secondary ions 

before they actually enter the magnetic sector, the electrostatic sector is surrounded by Mu metal shielding inside the 

vacuum and mild steel outside. The path traversed by the 

secondary ions through these two sectors to the collector is 

approximately 7.4 m long. High vacuum is maintained by 

three 400-L/s turbomolecular pumps, two pumping differentially on the source and one on the collector housing, and 

one 300-L/s oil diffusion pump on the electrostatic analyzer. 

Typically, a vacuum of approximately 4X 10-8 torr is 

achieved throughout, except in the primary column, where 

it is of lower quality due to the gas needed for the primary 

beam. 

Sulfur Isotopic Analyses 

Sulfur isotopic analyses offer the best chance of 

precise measurements in the ion microprobe because of the 

relatively high ratio of heavy to light isotope (table 1). 

Currently, analyses of sulfur isotopes in SHRIMP utilize a 

negatively charged oxygen ion primary beam and extract 

positively charged 32S and 34S secondary ions. As sulfur 

ionizes more readily to form negatively charged ions, this is 

not the optimum system for sulfur isotopic analyses~ however, the use of the negative oxygen ion beam causes the 

least disruption to the operation of the facility. The mass 

resolution required for stable isotopic analyses is not particularly great (table 1). For sulfur isotopic work, a mass 

resolution of 2,000 is sufficient to prevent 160 2 + (from the 

primary beam or from sulfates) from overlapping with 32S+, but a mass resolution of 4,500 is needed to separate 64Zn2+ from 32S+. Little 64Zn2+ reaches the collector, 

however, even during sulfur isotopic analyses of sphalerite 

(Eldridge and others, 1987b), so it may be safe to work at 

Table 1. Some factors affecting stable isotopic analyses on the ion microprobe 

Possible common ~m Required 

Natural abundance ratios isobaric overlaps (milli-amu) resolution (m/~m ) 

s .......... 32S/ 34S = 22:1 1602+; 64zn2+ on 32s+ 17.75; 7.52 1 ,800; 4,252 

32S/ 33S = 127:1 H32S+ on 33S+ 8.438 3,908 

H33S+ on 34S+ 11.416 2,976 

c .......... 12C/ 13C = 90:1 H12C+ on 13C+ 4.470 2,910 

24Mg2+ on 12c+ 7.478 1,605 

26Mg2+ on Be+ 12.057 1,078 

0 .......... 160/ 180 = 489:1 H/6

0+ on 180+ 11.405 1,578 

H ......... . HID= 6,666:1 H2+ on D+ 1.548 1,301 

4He2+ on D+ 12.80 1,564 

166 New Frontiers in Stable Isotopic Research 
mass resolution of 2,000 for most sulfides and sulfates. 

SHRIMP sensitivity decreases so little with increasing 

resolution that analyses are often carried out at the same 

resolution needed for zircon analyses (mass resolution of 

6,500) without detrimental effects. Because of the high 

sensitivity at this high mass resolution, no efforts are made 

at energy filtering on this instrument, in contrast to analyses 

done on other instruments (Shimizu, 1986). 

Ions of the two sulfur masses are counted sequentially 

in an ETP (Australian manufacture) 1 0-stage ion multiplier, 

and the signals passed through two 10 x cascaded amplifiers 

to the discriminator. Mass 32 is counted for 3 s and then 

mass 34 for 10 s in computer-controlled cyclic magnetic 

field stepping. There is a 1-s delay, before each mass is 

counted, to let the magnet settle at the new field value. It 

has also been found that the optimum peak shape for each 

mass is obtained at slightly different collector positions, so 

the collector is moved toward the magnet for measuring 

mass 32 and away from the collector for mass 34 while the 

magnetic field value is altered and settled. The difference in 

position is usually on the order of 1 mm. The typical 

analysis comprises measurement of 30 34S/ 32S ratios and 

takes approximately 30 min. During this period, a crater of 

nearly 30 J.Lm in diameter and 5 J.Lm deep is removed from 

the target. If pyrite is used, for example, this would 

correspond roughly to 4X 1014 atoms. At typical count 

rates, because of all the measuring time lost to cycling, only 

1 in ~ 10 7 atoms is actually ionized, matches the acceptance 

of the mass analyser, and is counted. 

Despite the inefficiency, SHRIMP performs sulfur 

isotopic measurements reliably in this analytical mode. 

Data from early experiments have demonstrated that the 

precision is approaching the limit imposed by counting 

statistics (approximately ±2 per mil (o/oo), 2a). The analyses, 

corrected for sputtering-induced fractionation by use of 

standards, are accurate within this precision. Instrumental 

response is linear over a wide range in o34S values for many 

sulfur-bearing minerals, there do not appear to be any 

effects that are due to charging of the sample, and there are 

no memory effects (Eldridge and others, 1987b). In 

SHRIMP analyses, it has been found that measured sulfur 

isotopic ratios are fractionated, relative to those measured 

conventionally, with the light isotope always enriched. 

Fractionation appears linear, and therefore correction of ion 

microprobe data to the Canyon Diablo scale is straightforward. However, two significant features of ion microprobe 

analysis must be closely monitored. First, the degree of 

enrichment varies in accordance with the mineral analyzed 

in a manner that parallels light isotopic enrichment in 

equilibrium isotopic partitioning experiments. Sulfates give 

ion microprobe ratios that are highly enriched ( ~6 percent/ 

amu) in 32S, while galena yields ratios less enriched in 32S 

( ~ 1.5 percent/amu), and the iron and copper-iron sulfides 

fall accordingly in between (Eldridge and others, 1987b). 

Second, while the relative order of this fractionation 

appears constant from one analytical session to the next, the 

absolute value may shift by several per mil even with just a 

sample change. Therefore, it is imperative to measure the 

isotopic compositions of unknown minerals relative to a 

standard of like matrix and to measure the compositions of 

the standard and unknowns in the same analytical session 

(Eldridge and others, 1987b). To accomplish this, a special 

method of casting epoxy mounts has been developed. 

Samples are placed on double-sided tape inside a 25 .4-mm 

holder, and a 10-mm metal rod, with its circumference 

lightly coated with silicon grease, is placed beside the 

sample and perpendicular to the glass plate. When cured, 

the disks all have uniform hollows that allow a tight 

press-fit of an epoxy mount 10 mm in diameter, holding 

minerals of known isotopic composition, so that measurements of standards and unknowns can be interspersed in a 

given analytical session. 

APPLICATIONS 

The greatest advantages of ion microprobe determinations are their in situ analytical capabilities and their 

spatial resolution. One area that requires both of these 

features but tolerates low precision ( ± 2 o; oo) because of a 

naturally large range in isotopic composition is the study of 

sediment-hosted massive sulfide deposits. Such ores have 

been the subject of study for many years and have remained 

controversial because of the unresolved role of biogenic 

sulfide and questions as to the timing of formation relative 

to host rocks. Two Phanerozoic deposits (the H.Y.C. at 

McArthur River and the lead-zinc at Mount Isa, Australia), 

one Paleozoic (Rammelsberg, West Germany), and one 

possible modem analog (Salton Sea geothermal system, 

California) have been studied by use of SHRIMP. 

H.Y.C. at McArthur River 

The H. Y. C. deposit in the N orthem Territory of 

Australia is one of the world's best examples of shalehosted lead-zinc mineralization, and it has been studied 

over the past 20 years by several investigators (for example, 

Croxford, 1968; Williams, 1978a,b; Rye and Williams, 

1981; Williams and Logan, 1986). The deposit comprises 

~230 million metric tons of mineralization containing 9 

percent zinc and 4 percent lead. Mineralization occurs over 

a vertical interval of 55 m on average, distributed concordantly (fig. 3A) in eight beds of dolomitic and carbonaceous 

siltstones that are thought to have been deposited in an 

emergent sabkha environment (Williams and Logan, 1986). 

The sulfide minerals in the H. Y. C. are noted for their fine 

grain size ( <200 J.Lm in diameter) and lack of any apparent 

metamorphic overprint. The host rocks for the mineralization are often found to be quite pyritic, and the relationship 

between the pyrite and base-metal sulfides has remained 

Sulfur Isotopic Analyses on the SHRIMP lon Microprobe 167 
B 

F 

SSGS 

G H 

168 New Frontiers in Stable Isotopic Research 

• 
Figure 3. These pairs of photomicrographs are of samples 

used in each of the four applications of SHRIMP sulfur 

isotopic analyses described in the text. Photos on the left 

illustrate how the sulfides are distributed in the host 

rocks, and on the right actual analytical craters are shown 

with their 834S values, in per mil, relative to Canyon Diablo 

troilite. The scale for each of the rock photos is given in 

the caption, while the -30-~J.m in diameter craters serve as 

scales for the detail pictures. 

A, Mineralized dolosiltstone from the No.7 orebody of the 

H.Y.C. deposit at McArthur River. Sulfide minerals comprising pyrite, sphalerite, and galena are generally distributed parallel to the bedding of the host rock; however, 

there are some minor crosscutting veinlets of galena. In 

this sample the crosscutting galena is isotopically indistinguishable from that found along the bedding. The long 

dimension is approximately 12 mm. 

8, The left photo shows typical textural relations between 

pyrite and base-metal sulfides of the H.Y.C. at McArthur 

River. Py2 appears to be overgrown and crosscut by 

sphalerite (sp) and galena (gn). The py2 is isotopically 

zoned (2 to 25 otoo), having a mean composition quite 

distinct from that of the base-metal sulfide. The 1 otoo in 

the left-center of the photo is galena. The right photo was 

taken approximately 0.8 mm away from the left photo, and 

it shows the usual isotopic relationship between the 

fine-grained py1 crystals and the coarse-grained py2 • The 

py1 ( -6 otoo) is considerably lighter than the adjacent, 

isotopically zoned py2 (1-10 otoo) or the base-metal sulfide in the same sample. 

C, This sample of Mount I sa ore from the No. 14 orebody 

contains bands of sulfides displaying the typical bimodal 

size distribution. The field of view is -7 mm wide. 

D, The larger photo shows adjacent coarse-grained and 

fine-grained pyrite from Mount lsa, both distributed parallel to the bedding of the host rock but having quite 

different isotopic compositions. The fine-grained, framboidal pyrite (py) is much heavier (-30 otoo) than the 

coarse-grained, cubic pyrite (-14 otoo) or associated basemetal sulfide(-20 otoo). Sphalerite is indicated by sp. The 

smaller photo (upper left corner) shows the temporal 

relationship between the coarse-grained, isotopically light 

pyrite and the fine-grained, isotopically heavy pyrite. This 

composite crystal, an overgrowth of cubic on framboidal 

pyrite, has an isotopic composition between that (20 otoo) 

of the two endmember pyrites. 

E, A sample of copper-rich ore from Rammelsberg made 

up of graded beds of sulfide clasts having variable proportions of pyrite, chalcopyrite, sphalerite, and galena. The 

field of view is -10 mm wide. 

F, The left photo of Rammelsberg ore is an assemblage of 

cubic pyrite clasts, of drastically differing isotopic compositions, surrounded by sphalerite and minor galena (not 

visible). The right photo shows gross isotopic disequilibrium between pyrite ( -10 otoo) and an overgrowth of 

chalcopyrite (12 otoo). 

G, Fine-grained sediment from 4,029 ft (1 ,228m) down the 

Salton Sea geothermal system State-2 well, with framboidal pyrite (light gray, mottled) occurring parallel to the 

bedding, crosscut in places by a vein of chalcopyrite (lightest gray, massive). The field of view is -7 mm in the long 

dimension. 

H, The photo on the left, from the Salton Sea geothermal 

system, shows some of the fine-grained pyrite (py), now 

engulfed in chalcopyrite (ccp), and the photo on the right 

shows some pristine framboidal pyrite several millimeters 

from the chalcopyrite. The pyrites are isotopically indistinguishable (--33 otoo), regardless of their proximity to 

the chalcopyrite. The base-metal sulfide ( -4 otoo) is quite 

distinct from the pyrite in isotopic composition despite 

the fact that it surrounded some of the pyrite crystals. 

enigmatic because of the inability to obtain pure mineral 

separates for isotopic analysis. The situation is further 

complicated for the analyst in that the pyrite occurring with 

the base-metal sulfides comprises two forms: py 1 , a bright 

yellowish-white, cubic, high-relief mineral typically overgrown by py 2 , a brownish mineral that is often framboidal 

and is of low relief. Understanding which, if either, pyrite 

type was genetically related to the base-metal mineralization has proven impossible until investigation by ion microprobe. 

In the SHRIMP investigation, no textural evidence 

has been found to counter the paragenetic sequence of 

sulfide deposition first described by Williams ( 1978a); 

however, isotopic systematics have been found to be more 

complicated than suggested from earlier conventional isotopic work (Smith and Croxford, 1973). Throughout the 

deposit, isotopic variation spans 60 ofoo, with pyrite being 

the most heterogeneous mineral (py 1, - 15 to 12 0/ oo; py 2, 

-1 to 45 otoo). Though both pyrite types may be heterogeneous within samples, py 1 varies less (that is, < 10 o;oo) than 

py 2 ( <46 o; oo). Within samples, py 1 and py 2 are usually 

quite distinct isotopically with the df1.l-py 1 typically around 

10 ofoo. Base-metal sulfides, with o S values limited to the 

range -5 to 14 otoo, have considerably less variable isotopic 

compositions than either pyrite type. Base-metal sulfides 

can be less than, equal to, or greater than py 1 in o34S value 

but are usually less than py2 (fig. 3B). It is possible to find 

single samples that contain nearly the entire range of 

isotopic variability of the deposit. For example, in a 1-cm2 

area of one sample from the No. 6 ore body, py 1 was -12 

ofoo, py2 was 45 otoo, and sphalerite was 0 otoo. This 

variation alone is more than three times that found throughout the whole deposit by means of conventional isotopic 

study (Smith and Croxford, 1973). 

The isotopic heterogeneity of py 1 and py 2 and the 

consistent enrichment of 34S in py 2 as compared to py 1 

suggest that these two pyrites may both have been formed 

through biogenic reduction of seawater sulfate such that the 

sediment was open to sulfate during the formation of py 1 

and closed before py2 began to precipitate. Isotopic disequilibrium between base-metal sulfides and pyrite is found 

throughout the deposit and implies that no sulfide from 

either pyrite was utilized in precipitating the lead or zinc 

sulfides. The isotopic heterogeneity between sphalerite and 

pyrite or galena and pyrite also indicates that little movement toward isotopic equilibration has occurred in 1. 8 

billion years since deposition (Eldridge and others, unpub. 

data, 1988). 

Sulfur Isotopic Analyses on the SHRIMP lon Microprobe 169 
Mount lsa Lead-Zinc 

The ore deposit at Mount Isa has traditionally been 

divided into the lead-zinc and the copper orebodies, and 

each has been mined and milled separately (Bennet, 1965; 

Perkins, 1984). Lead-zinc ores comprise nearly 100 million 

metric tons at 7 percent lead and 6 percent zinc ( Gustafson 

and Williams, 1981) and are largely conformable with the 

Urquhart shale, while the copper ore occurs in a highly 

recrystallized, often fractured, silica dolomite. The geologic setting for the Mount Isa lead-zinc ore is thought to be 

similar to that of the McArthur River deposit, as the 

Urquhart shale is dolomitic and has many features consistent with deposition in an intermittently emergent basin 

(Neudert and Russel, 1981). To date, SHRIMP analyses 

have been concentrated in the lead-zinc mineralization with 

the aims of identifying the sulfides that were part of the host 

rocks and those that may have been introduced, as well as 

what might have been formed as a part of the metamorphic 

events. 

Like the mineralization at McArthur River, Mount Isa 

ore hosts two pyrite types (fig. 3C); however, they are 

found in a somewhat different style from McArthur River. 

There is fine-grained (often framboidal) pyrite that ranges in 

isotopic composition from 2 to 30 o; oo and a coarse-grained, 

cubic pyrite that varies only from --10 to 27 ofoo (fig. 3D). 

This coarser pyrite generally occurs with the lead and zinc 

mineralization and can be found as overgrowths on the 

framboidal pyrite (fig. 3D). The coarse-grained pyrite to 

fine-grained pyrite isotopic relationships vary little within a 

sample but widely from sample to sample, where the 

coarse-grained pyrite may have &34S values lower than, 

equal to, or higher than the adjacent fine-grained pyrite. As 

at McArthur River, the fine-grained pyrite does not appear 

to have contributed sulfur at the site of base-metal precipitation. Moreover, isotopic disequilibrium between finegrained pyrite and adjacent base-metal sulfides appears to 

have survived greenschist metamorphism of the deposit 

(Eldridge and others, 1985b; 1986b). 

Rammelsberg 

The Rammelsberg ore body, like the McArthur River 

and Mount Isa lead-zinc deposits, is hosted in and is 

generally conformable with sedimentary rock. It is considerably smaller than either of the previously mentioned 

orebodies, with only 25 million metric tons of ore at 20 

percent zinc, 8 percent lead, and 1 percent copper. Though 

the ore is hosted in shale and may have formed in a rift 

environment (Sawkins and Burke, 1980) like the McArthur 

River and Mount Isa deposits, Rammelsberg ore has features that clearly distinguish it from that found at McArthur 

River or Mount Isa. In addition to the familiar sulfides 

interbedded with shale, Rammelsberg ore has graded beds 

of sulfide minerals (fig. 3E), and the orebody is mineral170 New Frontiers in Stable Isotopic Research 

ogically zoned ascending from pyritic to barite ore (Anger 

and others, 1966) as are the Japanese Kuroko deposits 

(Ohmoto and Skinner, 1983). 

Only a few isotopic analyses from one sample of 

clastic Rammelsberg ore have been carried out, but within 

that 4-cm2 area nearly the entire isotopic range from the 

whole ore body, as determined conventionally (Anger and 

others, 1966), was found by use of the ion microprobe. The 

magnitude of variation in &34S values is similar to that at 

Mount Isa lead-zinc (--30 ofoo); however, the ore at Rammelsberg is a physical mixture where neighboring clasts of 

pyrite may differ in &34S value by the full 30 ofoo (fig. 3F), 

in contrast to Mount Isa lead-zinc ore, where heterogeneity 

is most notable from stratum to stratum. As at McArthur 

River and Mount Isa, the base-metal mineralization has a 

more restricted range in &34S values than does the pyrite. 

Chalcopyrite at Rammelsberg has been found to be~ 13 ofoo 

regardless of whether it has overgrown isotopically light 

( -13 o/oo) pyrite or isotopically heavy (13 o/oo) pyrite. Thus 

it would appear that base-metal mineralization required no 

sulfur from preexisting iron sulfides. This isotopic heterogeneity has prevailed through greenschist metamorphism of 

the orebody (Eldridge and others, 1985a; 1986a; 1988). 

Salton Sea 

The Salton Sea geothermal system (SSGS) is located 

in a tectonically active continental rift zone where hot (up to 

365 °C) and highly saline hydrothermal brines interact with 

Pliocene-Pleistocene fluvial and lacustrine sediments 

(McKibben and others, 1987). A modem continental rift 

setting where hydrothermal fluids encounter evaporitic 

sediments has been postulated as a modem analog for 

ancient sediment-hosted sulfide deposits such as McArthur 

River or Mount Isa lead-zinc (see discussion by Williams in 

Williams and Logan, 1986), so it is logical to study sulfide 

formation in the SSGS as a possible corollary to the ancient 

occurrences. SSGS sediments include evaporitic anhydrite 

and often contain diagenetic pyrite. Mineralization, characterized by fine-grained sulfides deposited in sediments and 

subvertical veins (fig. 3G), is apparently formed from fluids 

that are relatively sulfur poor (Skinner and others, 1967; 

McKibben and Elders, 1985), and it has been proposed that 

base-metal-sulfide precipitation involved recycling of biogenic sulfide stored in the sedimentary pyrite (White, 

1968). 

Cursory conventional isotopic studies have revealed 

only a narrow range in &34S values for the system (McKibben, 1986), while ion microprobe investigations have 

demonstrated a range of over 40 o; oo within a sample and as 

many as three generations of isotopically distinct sulfides 

within several samples (McKibben and Eldridge, in press). 

As in the three older deposits, described above, the hydrothermal base-metal sulfides have been found as overgrowths 
on earlier pyrite, yet no evidence of sulfur recycling has 

been discovered (fig. 3H). Ion microprobe studies have also 

shown that disequilibrium between closely intergrown biogenic pyrite and continental evaporitic anhydrite has been 

preserved to depths in excess of 3,000 m and to metamorphic grades of lower amphibolite facies (Eldridge and 

McKibben, 1988; McKibben and Eldridge, in press). The 

ion microprobe data, combined with conventional analyses 

of aqueous H2S and S04 , have demonstrated that the sulfide 

used in precipitation of the base-metal sulfides has come 

largely from hydrothermal partial reduction of anhydrite 

sulfate and could not have involved diagenetic sulfide from 

the site of mineralization. 

SUMMARY 

Thus, by the use of SHRIMP, it has been found that 

all of these deposits share some characteristics. Individual 

samples have been found in each deposit that contain nearly 

the whole range or more isotopic heterogeneity than has 

been discovered throughout the entire orebody when conventional means have been used (fig. 4); isotopic heterogeneity has been preserved over very small distances (tens of 

micrometers to millimeters) for very long periods of time 

and through various degrees of metamorphism (from 

diagenesis to lower amphibolite grade), and no direct 

evidence has been found to suggest incorporation of locally 

derived biogenic sulfide at the site of ore precipitation. 

Moreover, isotopic compositions, together with the preserved textural settings of the minerals, can be used to 

constrain the timing of mineralization. For example, the 

fact that the deposits are hosted in sediments, and may be in 

part generally conformable with the sediments, is no guarantee that the formation of sulfides was contemporaneous 

with deposition of the host sediments. Clearly, the ongoing 

mineralization at the Salton Sea is largely epigenetic, and 

the combination of ore textural and isotopic data suggests 

that mineralization at McArthur River and Mount Isa were 

also epigenetic. Rammelsberg, on the other hand, was 

likely to have formed syngenetically. 

FUTURE DEVELOPMENTS 

Improvement of analytical prec1s10n would allow 

application of this powerful in situ analytical technique to a 

wide range of topics, and several avenues for better sulfur 

isotopic analyses are open. The ion counting system can be 

reconfigured for multiple collection, more efficiently using 

the ions produced in sputtering by eliminating the time 

wasted in magnetic field cycling. Compared to single 

collection, such a system could be used either to produce an 

increased number of analyses of a given precision in a given 

time or the same number of analyses, of increased precision, in a given time. Additionally, the method of produc10 

5 

conventional 

McARTHUR RIVER 

0 

o34S (per mil) 

Figure 4. Comparison of conventional and SHRIMP sulfur 

isotopic data from four areas of sulfide mineralization. 

Black squares are data from sulfide minerals that may 

include, depending on the deposit, pyrite, pyrrhotite, 

sphalerite, galena, and chalcopyrite. The white squares 

are data from sulfate minerals, chiefly anhydrite, but may 

include gypsum. Conventional data sources: McArthur 

River, Smith and Croxford, 1973; Mount lsa lead-zinc, 

Solomon, 1965; Smith and others, 1978; Rammelsberg, 

Anger and others, 1966; Salton Sea, McKibben, 1986. 

SHRIMP data sources: McArthur River, Eldridge and others, unpub. data, 1988; Mount lsa lead-zinc, Eldridge and 

others, 1985b; Rammelsberg, Eldridge and others, 1988; 

Salton Sea, McKibben and Eldridge, in press. 

tion and extraction of secondary ions in the source chamber 

can be altered. A tunable laser can increase the ions 

produced per atom sputtered (Willis and others, this volume) and thus increase the secondary-ion count rate. 

Alternatively, negative secondary ions can be extracted and 

counted (fig. 5). In experiments with SHRIMP, it has been 

found that, with the change to extraction of negative 

secondary ions, count rates increase dramatically for 

galena, somewhat for other sulfides, and not at all for pyrite 

Sulfur Isotopic Analyses on the SHRIMP lon Microprobe 171 
Secondary + ions to mass 

Primary ions from analyzer 

source 0 (j ® (j t 

~ ~ M\ ~~-s=::: .. ~ ~ t v h+\ ·:._._._.·:_::;.:. ~ ~ evh+\~-., vEj ~ 

Galena lattice 

Figure 5. Schematic view of the sputtering event in which 

a primary beam of positively charged oxygen ions is used 

and negatively charged sulfur and lead ions are produced. 

Note the increased production of sulfur ions relative to 

lead as compared to figure 2, as well as the exit of 

electrons from the target surface. 

169~ 0 4-nA primary beam 

Positive secondary ions 

galena 

(/) 168 111 20-nA primary beam 

N Negative secondary ions (") 

19t sphalerite N 

~ 

a: 18 w chalcopyrite I 

<( 3 

(9 

w pyrrhotite 2 ~ pyrite 

TARGET MINERAL 

Figure 6. Comparitive secondary-ion yields for sulfide and 

sulfate minerals from use of positive and negative secondary ions. There is no compensation for charging effects in 

the negative-ion mode, and galena is the only mineral to 

benefit to the full extent possible. See Eldridge and others 

(1987a) for more details. 

or sulfates (fig. 6). It is thought that charging of the sample 

surface, which is due to removal of electrons as well as 

negatively charged secondary ions, impedes this method of 

operation. It is not entirely clear why poor conductors like 

sphalerite give somewhat increased secondary-ion yields of 

negative ions, while better conductors, like pyrite, do not. 

However, charge compensation, by use of an electron flood 

gun (fig. 7), may raise all levels of secondary-ion output to 

similar values for all matrices (Hervig and others, this 

172 New Frontiers in Stable Isotopic Research 

Galena lattice 

Figure 7. Schematic view of the sputtering event showing 

how an electron flood gun, bombarding a wide area, may 

replace the electrons lost to the secondary-ion extraction 

process and eliminate sample charging, which drastically 

reduces secondary-ion yields for most minerals. 

volume; Giletti and Shimizu, this volume). Alternatively, 

pressing a very small sample into a block of gold and 

overlapping the primary beam onto the block may also work 

(Zinner, this volume). While no testing has been done to 

compare laser-enhanced emission of positively charged and 

charge-compensated emission of negatively charged secondary sulfur ions, multiple collection of secondary ions, 

with yields enhanced by either method, may allow the 

precision of ion microprobe analyses to equal that afforded 

by conventional analyses ( ± 0. 1 of oo). 

ACKNOWLEDGMENTS 

We wish to thank Dr. J.J. Foster of the Research 

School of Earth Sciences for his unfailing assistance in 

keeping SHRIMP in tune. Advice on photography from R. 

Wilde-Brown and P. Willis of the Research School of Earth 

Sciences, The Australian National University, was quite 

helpful. The Australian Research Grants Scheme supported 

the efforts of the first author, and he was also granted a 

visiting fellowship by the Research School of Earth Sciences; support from both is gratefully acknowledged. We 

are grateful to Dr. N. Williams of Carpentaria Exploration 

Co. Pty., Ltd., and Dr. D.J. Patterson of Mount Isa Mines, 

Ltd., for their cooperation and assistance in the McArthur 

River and Mount Isa projects, respectively. The work on the 

Salton Sea geothermal system benefited from the support of 

Dr. M.A. McKibben of the University of California, 

Riverside, through National Science Foundation grant EAR 

8617430 and Department of Energy grant DE FG03 

85ER13408. Rammelsberg research was aided by Dr. H. 
Sperling of Preussag Aktiengesellschaft and National Science Foundation grants EAR 8319402 and EAR 8319654 to 

Dr. H. Ohmoto of the Pennsylvania State University. 

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