Evaluation of the 

Lead-Alpha (Larsen) 

Method for Determining 

Ages of Igneous Rocks

GEOLOGICAL SURVEY BULLETIN 1097-A

This report concerns work done in part 

on behalf of the U.S. Atomic Energy 

Commission and is published with the 

permission of the Commission

Evaluation of the 

Lead-Alpha (Larsen) 

Method for Determining 

Ages of Igneous Rocks

By DAVID GOTTFRIED, HOWARD W. JAFFE, and FRANK E. SENFTLE

STUDIES IN GEOCHRONOLOGY

GEOLOGICAL SURVEY BULLETIN 109 7-A

This report concerns work done in part 

on behalf of the U.S. Atomic Energy 

Commission and is published with the 

permission of the Commission

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1959
UNITED STATES DEPARTMENT OF THE INTERIOR 

FRED A. SEATON, Secretary

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington 25, D.C. - Price 35 cents (paper cover)
CONTENTS

Page

Abstract__ _ _________-_--_____-_-_____-_-__---____-_-_-__---_-_-_ 1

Introduction._ ____________________________________________________ 2

Acknowledgments _________________________________________________ 3

Method __________________________________________________________ 3

Selection of samples___________-____-_____..-_-_-_____.._-______ 3

Sample preparation.___________________________________________ 5

Thorium to uranium ratios in zircon, monazite, and xenotime.______ 10

Age calculations.______________________________________________ 13

Radioactivity measurements.___________________________________ 19

Lead analysis.________________________________________________ 22

Evaluation of the assumptions._____________________________________ 24

Crystallization of zircon in a magma.____________________________ 24

Nature of lead in accessory minerals.____________________________ 27

Evidence from crystal chemistry___________________________ 27

Relation of optical properties to lead content of zircon.________ 30

Zircon of low lead content._________________________________ 33

Isotopic analysis of lead.___________________________________ 33

Nature of lead in allanite, apatite, and sphene.________________ 36

Acid leaching of zircon_________________________________________ 37

Internal consistency of the method._________________________________ 41

Lead-alpha ages of monazite, xenotime, and thorite._______________ 41

Zircon of different lead content and alpha activity.________________ 41

Comparison with other methods______________________________.______ 46

Lead-alpha ages of some igneous rocks younger than Precambrian. __ 47

Lead-alpha versus isotopic lead ages. __.-_-_-____-_-_____________ 48

Lead-alpha versus Ar40 :K40 and Sr87 :Rb87 ages____._______ 54

Summary and conclusions_________--_-__-_________________________. 56

Literature cited.__________________________________________________ 58

ILLUSTRATIONS

Page 

PLATE 1. Zircon crystals from syenite, Lillian mine, Central City,

Colo________.________________________ Facing 54

FIGURE 1. Variation of factor c with the thorium-uranium ratio.________ 15

2. Variation of the calculated age from the true age.____________ 17

3. Variation of k with the thorium-uranium ratio...__________ 18

4. Comparison of calculated and measured alpha activity for

zircon and monazite__---_-__--______-----_-----________ 20

5. Index of refraction (omega) of Ceylon zircon and the relation

to lead content. _______________________________________ 31

6. Lead-alpha ages of accessory minerals from igneous rocks of

known geologic age compared with Holmes' B time scale. __ 49

in
IV CONTENTS

TABLES

FagTABLE 1. Thorium to uranium ratios in zircon______-_______-_-_______ 11

2. Thorium to uranium ratios in monazite. __________________ 13

3. Percent deviations from true age in the calculated age due to

choice of factor c for different thorium-uranium ratios, _____ 16

4. Constants for zircon, monazite, and xenotime used in age calculation..... _________________________________________ 16

5. Equations used in age calculations. ______________________ 19

6. Comparison of alpha activities and conversion coefficients for

zircon_____________ ________ ____ 21

7. Comparison of alpha activities and conversion coefficients for

raonazite_ ____________________________________________ 21

8. Precision of duplicate spectrographic analyses of lead in zircon

and monazite____-________-_--_-_---_-_--------_----__- 23

9. Comparison of emission spectrographic lead determinations with

those made by other analytical methods ______________ 25

10. Hafnium content, hafnium to zirconium ratio, and radioactivity 

of zircon from igneous rocks of the batholith of southern 

California_________________ ___ ________ 26

11. Lead found in orthoclase, biotite, and in some igneous rocks from

southern California and southwestern Colorado ____________ 29

12. Data for Ceylon zircon______ _ ____ ______ 32

13. Lead-alpha ages of zircon containing less than 10 ppm lead____ 34

14. Original-lead content of iso topically analyzed zircon __________ 35

15. Original lead content of isotopically analyzed monazite_____-_ 37

16. Comparison of lead-alpha ages of allanite, apatite, and sphene

with zircon_-_____________-_--_____-_-----_--__------_- 38

17. Lead-alpha ages of zircon before and after leaching with acids. __ 39

18. Comparison of lead-alpha ages of monazite, xenotime, thorite,

and zircon___________________________________________ 42

19. Comparison of lead-alpha ages of zircon fractions of different

radioactivity and lead content from the same rock._________ 44

20. Comparison of lead-alpha ages of zircon fractions of different

crystal size from the same rock________________________._ 45

"21. Lead-alpha ages of some igneous rocks younger than

Precambrian___________________________________________ 50

22. Comparison of lead-alpha ages with isotopic ages_____________ 51
STUDIES IN GEOCHRONOLOGY

EVALUATION OF THE LEAD-ALPHA (LARSEN) METHOD 

FOR DETERMINING AGES OF IGNEOUS ROCKS

By DAVID GOTTFREED, HOWARD W. JAFFE, and FRANK E. SENFTLE

ABSTRACT

The age of an igneous rock can be determined from the lead-alpha activity 

ratios of certain accessory minerals (zircon, monazite, xenotime, and thorite) 

provided that, (1) these minerals crystallized contemporaneously with the 

enclosing rock-forming minerals, (2) their lead is all of radiogenic origin 

formed by decay of uranium and thorium after formation of these minerals, 

and (3) they have neither lost nor gained parent or daughter products since 

the time of crystallization.

The largest single source of analytical error is in the lead analysis, particularly 

for minerals which contain less than 10 ppm of lead. For zircon and other 

accessory minerals that contain more than 10 ppm of lead the analytical precision is 4 to 10 percent of the mean of duplicate measurements. Comparisons 

between the alpha emission measured by thick source alpha counting and that 

calculated from quantitative determinations of the uranium and thorium content of zircon and monazite indicate that the alpha activity measurements 

have an accuracy of ±5 percent.

Where the method has been applied to a large number of samples of suites 

of igneous rocks younger than Precambrian, the scatter pattern of the age data 

is what would be expected from the experimental errors. In general, the standard deviation from the mean age is about 10 percent or less for minerals from 

suites of rocks older than Cretaceous. For younger rocks, which contain zircon 

with very low lead content, the standard deviation from the mean is considerably greater than 10 percent but is less than 10 million years. Inasmuch 

as zircon, monazite, and xenotime from the same rocks give ages which agree 

within limits of error of the method, it seems that the presence of common lead 

or the loss or gain of parent or daughter products do not contribute significantly to the errors in the age measurements of most of the rocks tested that 

were younger than Precambrian. Age determinations have been made on many 

geologically well dated rocks in order to test the validity of the method by use 

of geologic evidence. With few exceptions, the age data agree with the geologic 

sequence of events and with the Holmes' B geologic time scale.

Lead-alpha ages are in good agreement with many of the ages determined by 

the lead isotope, Ar*°: K40, and Sr87 : Rb87 methods on minerals from unmetamorphosed igneous rocks younger than Precambrian.. However, the agreement
2 STUDIES IN GEOCHRONOLOGY

with other methods is poor for zircon from Precambrian rocks inasmuch as the 

lead-alpha age corresponds to the Pb206 : U238 age which is commonly lower than 

the Pb207 : Pb208 age and concordant Ar40 : K40 and Sr87 : Rb87 ages on mica. When 

concordant lead isotope ages are obtained on Precambrian zircon, the leadalpha age will also agree within the limits of error of the method.

As yet no satisfactory explanation has been found for the wide discrepancies 

in the measured ages of Precambrian zircon, but it seems likely that lead has 

been lost by partial or complete recrystallization in response to metamorphic 

processes. Lead-alpha ages determined on metamorphosed igneous rocks will 

generally not correspond to the true age of igneous activity, but may approach 

the age of metamorphism.

INTRODUCTION

A method for determining the approximate age of accessory minerals of igneous rocks based on the ratio of total lead content to the 

measured alpha activity of the mineral has been described by Larsen, 

Keevil, and Harrison (1952). Their investigations showed that zircon was most suitable for age measurements. During the past few 

years extensive research has been done to test the precision, accuracy, 

and applicability of the method to zircon and other accessory minerals 

from igenous rocks varying greatly in composition, geologic occurrence, and age. Monazite, xenotime, and thorite seem to be as suitable as zircon for age work. Apatite, allanite, and sphene have given 

erratic results and seem to be unsatisfactory.

The lead-alpha age determined for the accessory minerals will be 

the age of the enclosing rock if the following conditions are satisfied: 

(1) The accessory minerals are contemporaneous with the other rockforming minerals, (2) all the lead in the accessory minerals has 

formed from the decay of the radioactive elements present at the 

time of crystallization of the mineral, (3) neither parent nor daughter 

products has been lost or gained since the time of crystallization of 

the accessory minerals. Thus, by measuring the amount of the parent 

radioactive elements (thorium and uranium) in the mineral, and the 

total amount of daughter element formed (total lead), the age of 

the rock can, subject to the above qualifying assumptions, be calculated. As the alpha radioactivity of the specimen is a function of the 

thorium and uranium content, it is convenient to use it as a measure of 

the parent elements. Thus, the rock age can be calculated, accepting 

the above assumptions, by merely measuring the total lead content and 

the alpha activity of a zircon sample separated from a rock. These 

measurements can be made rapidly and at relatively low cost.

The purpose of this paper is to describe in detail the method and 

techniques that have been used to obtain the lead-alpha activity ratios 

on the suitable radioactive accessory minerals; to compare the calculated ages with the geologic sequence of events, and with ages obtained
LEAD-ALPHA METHOD FOR DETERMINING AGES OF IGNEOUS ROCKS 3

by other physical methods; to evaluate the assumptions involved, and 

to point out the applicability and the limitations of the method.

Because of the nature of the assumptions inherent in the method 

and the greater uncertainty in the analytical data it should be stressed 

at the outset that the lead-alpha method is considerably less precise 

than any of the isotopic methods currently employed in age work. A 

single age measurement by the lead-alpha method may provide only 

a crude approximation of the true age of the mineral. Several age 

measurements on suites of the same or different minerals from the 

same geologic setting provide more information and are essential in 

evaluating the age results.

Probably the lead-alpha method will be most useful in providing 

rapidly a large number of age determinations on igneous rocks with 

sufficient accuracy to be useful in many geologic problems. Since the 

solution of many geologic problems is dependent on age determinations of many samples, the methods described and evaluated in this 

report can be of much help to geologists.

ACKNOWLEDGMENTS

The authors express appreciation to Prof. E. S. Larsen, Jr. for 

continuous guidance and encouragement in the work presented here. 

Prof. P. M. Hurley of the Massachusetts Institute of Technology gave 

considerable assistance with problems regarding alpha activity measurements. Dr. H. D. Holland of Princeton University kindly made 

available some zircons measured for their uranium and thorium content. We are grateful to Dr. George Tilton of the Carnegie Institution 

of Washington, Henry Faul of the U.S. Geological Survey, and Dr. 

L. T. Silver of California Institute of Technology for permission to 

use unpublished isotopic data on accessory minerals and for making 

possible interlaboratory checks on the same material. Many colleagues in the U.S. Geological Survey contributed samples for measurement and helpful discussions on geological interpretations. Lorin 

R. Stieff, Thomas W. Stern, E. S. Larsen 3d, and George Phair have 

given the authors many suggestions and assistance in the preparation 

of this report.

This work was done partly on behalf of the Division of Research 

of the U.S. Atomic Energy Commission.

METHOD

SELECTION OF SAMPLES

In developing and testing the lead-alpha method for determining 

the age of accessory minerals, a considerable amount of data has been 

obtained relative to the distribution and the nature of the accessory 

minerals in igneous rocks ranging widely in chemical composition and
4 STUDIES IN GEOCHRONOLOGY

geologic setting. This information can be usefully applied to the 

sampling of rocks to be dated and has a bearing on the amount of 

time and effort required to obtain an age determination.

Zircon is more widespread in igneous rocks than any other accessory 

mineral suitable for lead-alpha age work. In most igneous rocks 

the zircon content ranges from 10 to 200 ppm. There is a general 

correlation between the quantity of zircon, the radioactivity of the 

zircon and the rock type containing the zircon. In a calc-alkalic batholith, the amount of zircon in gabbro is very small (a few parts per 

million) and the alpha activity very low. Tonalites and granodiorites 

contain about 100 to 200 ppm; the alpha activity of the zircon from 

granodiorites may be two to three times that of the zircon from tonalites. Granites commonly contain a small amount of zircon but the 

alpha activity of this zircon may be much greater than that of the 

zircon from the less siliceous rocks. Frequently granites contain 

zircon crystals that vary widely in alpha activity and on such rocks 

two or more age measurements should be made on separate zircon 

fractions that differ in their activity and lead content.

Monazite and xenotime occur principally in the late stage differentiates that usually contain garnet or muscovite or both. On a few 

occasions it has been possible to make age determinations on two or 

more minerals from the same or related rocks. Ages of monazite and 

xenotime are particularly desirable for very young rocks as these 

minerals are relatively high in alpha activity and lead content.

Fine-grained extrusive rocks, which range from quartz latite to 

rhyolite in composition, yield relatively small amounts of zircon and 

this zircon is usually low in alpha activity. Basaltic rocks contain 

insufficient amounts of zircon for such age determinations. The 

highly siliceous coarser grained rocks which are from stocks associated 

with a volcanic series generally have sufficient amounts of zircon. 

Several volcanic tuffs have also yielded sufficient amounts of zircon 

for such age determinations.

The rocks of the border zones of batholiths frequently show structural and textural features that may result from granulation and 

deformation taking place when the marginal facies of the batholiths 

were almost completely crystallized or at a much later time. Age 

determinations on minerals from such rocks should yield the time of 

rock formation if the structure is of protoclastic origin. If the 

structures were formed as a result of a much later period of dynamic 

metamorphism the lead-alpha ratio will record the time of metamorphism if the minerals have been completely recrystallized or if 

recrystallization is not complete, an age somewhere between the original crystallization of the mineral and the time of metamorphism.

The following points have, in general, been found useful in guiding 

the selection of rocks for age determinations, although the number
LEAD-ALPHA METHOD FOR DETERMINING AGES OF IGNEOUS ROCKS 5

and kinds of samples will vary with the nature of the geologic problem 

at hand:

1. The specimens should be as fresh as possible, and free of inclusions of older rock and sulfide minerals. Pyrite and molybdenite 

contain lead and concentrate with the zircon.

2. In an area containing a variety of intrusive rocks that seem to 

be comagmatic, rocks varying widely in composition should be selected 

in order to obtain suitable accessory minerals differing in alpha 

activity, lead content, and chemical composition.

3. In a volcanic series, the more siliceous rocks and associated 

coarser stocks should prove most suitable. Basalts contain insufficient 

zircon to yield enough for analysis.

4. To derive the necessary amount of accessories, at least 50 pounds 

of rock should be collected. About 100 milligrams of the mineral 

is required for an age determination. For gabbros 200 pounds may 

be needed to obtain the required amount of zircon.

5. Samples with textures and minerals of normal igneous rocks 

are most favorable. Minerals from some rocks of Precambrian age 

and from those with metamorphic textures may yield ages much 

younger than the age of their original crystallization.

SAMPLE PBEPABATION

Generally, a sample of 50 pounds of rock is processed for recovery 

of the accessory minerals. Small chips are fed into a roller-type 

grinder for size reduction to a 60 mesh grain size. In order to avoid 

the risk of contamination from lead, specially prepared stainless steel 

sieves are used in place of the usual soldered brass variety. The 

sieves used are 36 inches long, 18 inches wide, and 3 inches deep, and 

are made by tacking stainless steel-wire screening to an unpainted 

wooden frame.

After the sample is sieved, the 60 mesh fraction is further processed; the +60 mesh fraction is reserved for additional material if 

needed. The 60 mesh fraction represents, on an average, about 50 

to 75 percent of the rock and will contain virtually all of the zircon 

and other heavy accessory minerals. Experience has shown that the 

yield of heavy accessory minerals obtained from regrinding the + 60 

mesh fraction is negligible. Although the size of the accessory zircon 

crystals tends to increase with the grain size of the host rock, the 

zircon crystals of plutonic rocks, with the exception of pegmatite, are 

smaller than 60 mesh in about 90 percent of several hundred rocks 

processed in compiling this report. Usually, the zircon crystals are 

not broken in the grinding operation, but are sprung or "popped" out 

of the enclosing minerals as free, whole crystals.

507200 59 2
6 STUDIES IN GEOCHRONOLOGY

The 60 mesh fraction of the rock is then passed over a Wilfley 

wet shaking table for bulk removal of the light minerals, principally 

the feldspars and quartz. As the Wilfley table automatically deslimes 

the sample, the time-consuming process of hand sieving to remove the 

 325 mesh fines is avoided. Further, the Wilfley table has been 

effective in the recovery of 400 mesh zircon, normally discarded by 

dry sieving. It also has the advantages of speed and economy by 

eliminating the need for large amounts of bromoform.

The sample is passed over the Wilfley table twice. The first pass 

deslimes the sample and provides an efficient recovery of heavy minerals. The heavy-mineral concentrate is tabled a second time for 

a refinement of the concentrate. The table is efficient in recovering 

the heavy minerals (density >3.0) with the exception of micas, 

which concentrate with the quartz and feldspar. Representative 

fractions of the light minerals are reserved and can be purified for 

other types of work.

The heavy-mineral concentrate of the Wilfley table is dried and then 

passed through the Frantz separator in the vertical position with 

the vibrator removed, at a setting of 0.05 ampere for the rapid removal of magnetite. Repeated passes at increasing currents will 

remove virtually all the ferromagnetic minerals. The maximum setting of 1.5 amperes in the vertical position is equivalent to about 

0.4 ampere on the Frantz separator set up in the normal horizontal 

position (cross slope 10°, longitudinal slope 10°) with use of the 

vibrator. This concentrates the zircon, with practically no loss, in 

the relatively small nonmagnetic fraction. If xenotime and monuzite 

are present, the vertical Frantz separation should be carried out to 

a maximum of 1.0 ampere to prevent their concentration in the magnetic fraction.

The nonmagnetic concentrate from the vertical Frantz separation, now weighing tens rather than hundreds of grams, is then separated successively in bromof orm (density =2.86) and in methylene 

iodide (density =3.3). Ordinary Clerici solution, prepared from 

salts of thallous malonate and formate, should not be used for the 

separation of minerals on which lead-uranium determininations are to 

to be made. Both T1+ and Pb*2 have large ionic radii (1.49 and 1.32 

A, respectively) and a close geochemical affinity in sulfides such as 

galena, from which thallium is recovered as a commercial byproduct 

of metallurgical processes. Clerici solution may therefore contain 

a large amount of lead, and represents a potential source of lead contamination. Although Clerici solution is inherently water-soluble, 

thallium has been detected in zircon concentrates that have been separated in it, even after treatment with nitric acid. This may result 

from the formation of a small amount of insoluble chlorides of thai-
LEAD-ALPHA METHOD FOR DETERMINING AGES OF IGNEOUS ROCKS 7

Hum. A Clerici solution from which lead has been, extracted by a 

chemical procedure is occasionally used for specialized separations 

that require a density of about 4.0. In general practice, however, 

bromoform and methylene iodide are the only heavy liquids required. 

The less expensive liquid, acetylene tetrabromide, is not used instead 

of bromoform because of its greater viscosity and higher toxicity. 

For a similar reason, as well as for economy, acetone rather than 

carbon tetrachloride is used exclusively for washing the concentrates. 

As a precautionary measure, heavy liquids used for separation of minerals for lead-alpha age work are not used for any other purpose. 

Acetone and methylene iodide contain very little lead as determined 

by analysis:

acetone < 1X 10~8g Pb per ml 

methylene iodide < 1X 10~8g Pb per ml.

The mineral fraction heavier than methylene iodide is then passed 

through the Frantz separator in the normal position at successive 

settings of 0.1 to 1.5 amperes, stepped-up 0.1 ampere per pass. Allowing for some variation in magnetic response because of mineral 

stain and compositional difference, the three minerals used for leadalpha age determinations will concentrate as follows:

_____Amperes_____ 

NonmagMagnetic at netie at 

Xenotime__________________________________________ 0. 4 ___..

Monazite__________________________________________ 0. 5-0. 6 _____

Fresh zircon___ ___ _____________ ______ 1. 5

Metamict varieties of zircon are extremely variable in magnetic response and may concentrate at settings ranging from 0.5 to 1.5 amperes. In addition to the current control, the longitudinal and cross 

slope may be varied to produce a different response to separation. Use 

of a virtually "flat" Frantz separator at maximum 1.5 amperes has 

given excellent results in separating slightly metamict zircon and rutile 

from fresh zircon.

The mineral fraction that is nonmagnetic at 1.5 amperes on the 

Frantz separator may contain minerals other than zircon. At this 

stage of separation the concentrate is composed of minerals having a 

density greater than 3.1 (allowing for some trapping of minerals 

whose density is near that of methylene iodide), and those that show 

no magnetic response up to 1.5 amperes on the Frantz separator. 

These may include apatite (especially the strontium or rare-earth rich 

varieties), barite, celestite, fluorite, pyrite, molybdenite, topaz, scheelite, zoisite, corundum, rutile, and more rarely, kyanite, sillimanite, 

cassiterite, and galena. On rare occasions, a totally nonmagnetic variety of sphene may concentrate with the zircon. Different procedures
8 STUDIES IN GEOCHRONOLOGY

are required for the removal of the different minerals that concentrate 

with zircon so that each sample becomes a special job in mineral separation after the magnetic separation is completed. The different 

contaminating minerals are removed as follows:

Apatite. This mineral has an average density of 3.18 and will 

normally float in pure methylene iodide (density=3.3). As most 

methylene iodide liquid media fall short of a density of 3.3 and as some 

apatites have a density above 3.18 when strontium, rare earths, and 

manganese are present in significant amounts, apatite may sink with 

zircon in methylene iodide. Even with careful stirring of the liquid, 

some apatite may be mechanically trapped by zircon grains during the 

separation. The authors removed apatite by leaching the concentrate 

in hot concentrated nitric acid for 15 minutes. The effects of acidleaching on zircon, monazite, and xenotime are elaborated under the 

section on "acid-leaching."

Pyrite. This mineral, when fresh, is normally nonmagnetic and 

often concentrates with zircon. Nonmagnetic pyrite is commonly 

removed by leaching in hot concentrated nitric acid as described for 

apatite. A very efficient means of removing pyrite is by flotation in 

a small laboratory flotation cell.

Molybdenite. This sulfide is particularly undesirable in the zircon 

concentrates because it often carries an appreciable amount of lead 

(about 1,000 ppm); it is a fairly common accessory mineral of granitic rocks, Although present in very small amounts in the rock, it may 

become abundant in the zircon fraction after many concentration procedures. A large part of the molybdenite can be removed by shaking 

the concentrate several times from one sheet of glossy paper to another. 

The molybdenite flakes will adhere to the paper as do the mica flakes. 

The concentrate is then leached in hot 1 to 1 aqua regia for about 15 

minutes. Molybdenite is not as soluble as pyrite and will not dissolve 

in nitric acid alone.

Topaz, zoisite, fluorite, and corundum. Although zoisite (density 

>3.2) and fluorite (density=3.18) will normally float in methylene 

iodide, they will occasionally sink for the reasons cited for apatite. 

Topaz (density=3.58) and corundum (density=4.00), however, are 

heavier than methylene iodide and are removed by separation in leadfree Clerici solution (density commonly adjusted to 4.1).

Barite and celestite. Both sulfates are undesirable contaminants 

in the zircon concentrates and are difficult to remove. The similarity 

in the size and charge of the large divalent ions, Ba+8, Sr*2, and Pb+2 

(ionic radii=1.43, 1.27, and 1.32A, respectively) indicates that lead 

may be expected to enter readily the lattice of either barite or celestite. 

Barite has also been reported to be radioactive owing to the presence 

of radium and its daughter products. Eemoval of barite from a
LEAD-ALPHA METHOD FOR DETERMINING AGES OF IGNEOUS ROCKS 9

zircon concentrate to be used for an age determination is therefore 

imperative. The density of barite (density=4.5) is so close to that of 

fresh zircon (density=4.6 to 4.7) that a density separation cannot be 

made. Flotation has not been tried and may prove successful. At 

present, barite is removed from zircon by a method developed by R. P. 

Marquiss (formerly of the U.S. Geological Survey). The concentrate 

is treated in a stoppered flask with a 2 N sodium carbonate solution 

kept under continuous agitation by use of a magnetic stirrer. Temperature is maintained at 90° C. by means of an infrared lamp placed 

about 6 inches from the flask. After treatment with the sodium 

carbonate solution for 4 to 8 hours, the acid insoluble barium sulfate 

is converted to the carbonate which is dissolved readily by washing 

the sample in hydrochloric acid.

Of the heavy accessory minerals listed, only apatite and pyrite are 

commonly present in the zircon concentrate. For this reason the 

nitric acid leach has become a standard step in the purification 

procedure.

Following the acid leach, the zircon concentrate is carefully examined under both the binocular microscope and in oil immersions under 

the petrographic microscope. An immersion in an index liquid of 

1.92 is most practicable, as the liquid will match the <o index of refraction of fresh zircon, and most impurity grains will show up with a 

marked negative relief. The zircon crystals are then spread out thin 

on a glass slide and any impurities are removed by suction with use of 

a glass tube attached to a laboratory vacuum hose (Senftle, 1951). 

The vacuum cleaning is carried out on several splits of zircon until a 

sample of 100 mg of zircon is obtained. This final hand-picking operation is perhaps the most critical step in the mineral separation procedure, inasmuch as each grain is observed under the binocular microscope. No more than two samples per day should be handpicked due 

to the high element of fatigue involved which may result in carelessness. The final 100 mg sample of zircon crystals used for the age 

determination must be closer to 100 percent than to 99 percent in 

purity. In practice, this is readily obtained for fresh zircons, with 

greater difficulty experienced with metamict and stained zircons, and 

those containing abundant inclusions. A systematic study of the inclusions in zircon and their possible relation to the common lead 

reported in some zircons, will require investigation by isotopic analysis. Generally, zircons with inclusions give the same age as zircons 

without inclusions separated from differentiates of the same 

batholith.

The handpicked sample of zircon is hand ground to a fine powder 

with a boron carbide mortar and pestle. The use of ordinary mortars 

may result in the addition of foreign lead in the sample. The mortar
10 STUDIES IN GEOCHRONOLOGY

and pestle are cleaned with acetone between samples. As many zircons are zoned, the fine grinding will homogenize the sample to produce a more representative alpha count.

As many of the zircons examined for age determinations contain 

less than 10 ppm lead, precautions must be taken to avoid laboratory 

contamination. No sample should be left uncovered on a sheet of 

paper for more than the time required for the separation procedure. 

Spectrographic analyses on material scraped from the Frantz separator showed the following lead content :

Parts per 

million

Scraping from the track _______________ __ __ 1, 000 

Paint on the magnet __________________ _ _ _______ - _ - 10 

Paint on the vibrator __________________ ____________ 500

A similar analysis made of laboratory dust collected over a period 

of time showed 1,000 ppm lead, apparently contributed by old paint 

from pipes in the laboratory. Dust blown and circulated by laboratory air conditioners also represents a potential contamination hazard. 

Most of this hazard can be avoided by keeping all samples bottled 

except when they are actually being worked on.

THORIUM TO URANIUM RATIOS IN ZIRCON, MONAZITE, AND

XENOTIME

A knowledge of the range of thorium to uranium ratios in the different accessory minerals is necessary in order to assign more accurate 

constants to the lead-alpha age equation and to the conversion factors 

which are used to correct the thick source alpha counts into units of 

thin source alpha counts. A compilation of the data from the more 

recent literature and some new data for zircon are given in table 1. 

Several independent methods have been applied to the measurement 

of the thorium and uranium contents of the minerals but rarely has 

the same material been analyzed by more than one method. The data 

obtained by isotope dilution analysis are probably accurate to ±1 

percent (Tilton and others, 1955; Tilton, 1956). The expectable errors in the results obtained by the gamma-ray spectrometry method 

are discussed by Hurley (1956) and Hurley and Fairbairn (1957) and 

vary depending on the different proportions of uranium and thorium 

in the sample. For the samples that have been chemically analyzed 

for uranium and measured for alpha activity, the thorium content

was calculated from the equation Th =" nnocn where a is alpha

activity per milligram per hour; U the uranium content in parts per 

million and Th the thorium content in parts per million. Using this 

method the accuracy in the thorium to uranium ratios may be about 

±20 percent.
TABLE 1. Thorium to uranium ratios in 

Locality

Boulder Creek batholith, Colorado.-

Source

.. do..... ..

Accessory mineral.

Accessory mineral.

.....do..... 

_ .do...... 

..... do.......... ...

. do-.... .

.... .do...... 

Accessory mineral. 

.....do...... __

..... do...... .

.... .do........ .....

 do...... 

Method of analyses

.....do,.. .. . . ...... ............ .

Uranium and thorium by X-ray fluorescence.. 

Qamma-ray spectrometry. . ....

. do... 

Alpha counting and uranium analyses __ ....

_ . do..... 

.... .do... ......... . .. . ... ..

.... .do... .. ...... .. .... ...

..... do..... .... ...... -

Nu

sa
12 STUDIES IN GEOCHRONOLOGY

In zircon the thorium to uranium ratios show the greatest variation 

for samples of pegmatitic origin. The thorium-uranium ratios in the 

pegmatite zircon from Ceylon are less than 0.1 as determined by 3 

different methods. Pegmatite zircons from Oklahoma have an average thorium-uranium ratio of about 4. The data on the zircon from 

the rocks of the Boulder batholith of Montana and the Boulder Creek 

batholith of Colorado should prove more typical of zircon from the 

average igneous rocks. These batholiths are composed of the ordinary calc-alkalic series of rocks which are similar to those forming 

the great batholiths of western North America. The thorium-uranium ratios of zircon from the Boulder batholith range from 0.4 to 

2.7 and average 0.9; those from the Boulder Creek batholith range 

from 0.4 to 2.6 and average 1.1. For accessory zircon from rocks that 

are chiefly granite in composition Hurley and Fairbairn (1957) 

report an average thorium-uranium ratio of 0.4, and for zircon of pegmatitic origin, a thorium-uranium ratio of 1.0. When more data 

become available on zircon from rocks of other petrographic provinces it may be possible to make fairly accurate estimates of the 

thorium-uranium ratios of zircon from a given series of rocks of a 

particular region. At present it seems reasonable to assign a thoriumuranium ratio of 1 for the average accessory zircon.

The thorium-uranium ratios for monazite (table 2) given in the 

literature are for the most part on samples of pegmatitic origin. 

From the data given in table 2 it seems that in general the highest 

thorium-uranium ratios are found in monazite derived from pegmatites or from deposits of unusual origin. The monazite from the 

unusual barite carbonate rock;from Mountain Pass, Calif., has an 

average thorium-uranium ratio of nearly 1,300. In pegmatitic monazite from other regions the average thorium-uranium ratio ranges 

from about 50 to 400. For monazite occurring as an accessory mineral of ordinary granitic rocks, the thorium-uranium ratios are in 

general lower. The monazite of rocks from New England have an 

average thorium-uranium ratio of about 25, and a similar ratio is found 

for monazite from the southern Appalachians, the placer concentrate 

from Idaho, and from a granite from the Grand Canyon in Arizona. 

A thorium-uranium ratio of only 2.5 was found in one sample of accessory monazite containing an unusually high content of uranium. As 

with zircon, most of the lead-alpha ages have been determined on 

accessory minerals from granitic rocks rather than from pegmatites. 

From sparse data an average thorium-uranium ratio of 25 has been 

assigned to accessory monazite of igneous rocks.

Xenotime is present in granitic rocks in much smaller amounts than 

either monazite or zircon. As a result no analyses for both uranium 

and thorium have been made on accessory xenotime from igneous 

rocks. Analyses of two samples of xenotime (Palache, Berman, and
LEAD-ALPHA METHOD FOR DETERMINING AGES OF IGNEOUS ROCKS 13

Frondel, 1951) of pegmatitic origin indicate that they contain more 

uranium than thorium. Thus an average thorium-uranium ratio of 

0.5 is tentatively assigned to xenotime.

TABLE 2. Thorium to uranium ratios in monazite

Locality

Southern Appalachians.

New England ____

Portland, Conn. ......

Grand Canyon, Ariz..

Manitoba, Canada __ 

Northwest Territory, 

Canada.

Japan ______ . ....

Ceylon....... .........

South Africa ...........

Source

Pegmatlte(?)..._ 

Accessory 

mineral.

...-_do-_. .......

.... .do.. ....... .

.....do... .......

Accessory 

mineral.

Pegmatite .......

Barite-carbonate rock.

-...do.. ........

 -do... .......

- do..........

Number of 

samples

11

2 

1

10 

1 

1

1

2 

1

2 

1

2 

1

1

1 

5 

1 

6

Thorium-uranium ratio

Range

8-25 

310-486

13-75

71-77

56-106

1, 280-1, 325

12-104

25-90

Average

20

398 

2.5

25 

750 

60

35

74 

25

81 

20

1,300 

24

23 

24

80 

28 

48

Reference

Mertie (1953).

Holmes (1948a). 

W. C. Overstreet, U.S. Oeol. 

Survey (written communication, 1956). 

Hurley and Fairbairn (1957). 

Holmes (1948a). 

George Phair, U.S. Geol. 

Survey (written communication). 

Tilton and Nicolaysen (1957). 

Holmes (1948a). 

L. R. Stieff and T. W. Stern, 

U.S. Geol. Survey (written 

communication, 1957). 

Holmes (1948a). 

L. R. Stieff and T. W. Stem 

(written communication, 

1956). 

Jafle (1955).

K. S. Larsen, Jr., U.S. Geol. 

Survey (written communication, 1956). 

Hurley and Fairbairn (1957). 

Holmes (1948a). 

Do. 

Do. 

Tilton and Nicolaysen (1957).

AGE CALCULATIONS

The basic equations used for the age calculations are similar to 

those outlined by Keevil (1939). His age equations were presented 

in terms of the ratio of the total radiogenic lead to the thorium and 

uranium content. Thus, to calculate the age of an accessory mineral 

where the age is 300 million years or less, Keevil has shown that the 

age is given approximately by

7.18X103 Pb m 

U+0.312Th ( '

where Pb, U, and Th are the lead, uranium, and thorium contents of

the mineral in parts per million and t is the age in millions of years.

The constants in this equation and throughout the paper have been

recalculated from the following half-life data:

T* (U238 )=4.5lXl09 y 

TX (U235 )=7.13X108 y 

TH(ThZ32 )=1.4QX10 y

507200 59 3
14 STUDIES IN GEOCHRONOLOGY

The lead is assumed to be the total radiogenic lead formed from the 

time of crystallization of the mineral without contamination with 

common lead.

The theoretical thin source alpha activity, a (in units of alphas per 

mg per hour) is given by

«=0.366 U+ 0.0869 Th (2) 

Hence, the age, £, in terms of the alpha activity is

^_J2>632+624Th:TJ-| Pb_ Pb , . 

*~L 1 + 0.312 Th:U J T~C~ (6)

where t is in millions of years, Pb is in parts per million, and a is the 

alpha activity given in equation (2).

Measurements of alpha activity may be obtained by thin-sourcecounting techniques (Keevil and Grasham, 1943). True thin sources, 

however, are difficult and time consuming to prepare, and the specific 

alpha activity is very low. For these reasons it is more desirable 

to use thick-source-counting techniques in the laboratory and, where 

necessary, convert the thick-source count (E, in alphas per square 

centimeter per hour) to a thin-source count (a in alphas per milligram 

per hour) by a suitable conversion factor.

Hurley and Fairbairn (1953) have calculated a conversion coefficient, R ^ for zircon and obtain 0.493 assuming a thoriumuranium ratio of about 1. Their calculation is based on a value of 

ju,<#=2.22X10~3 derived from the Bragg-Kleeman law (JSTogami and 

Hurley, 1948; Beharrell, 1949). The conversion coefficient can be 

obtained directly on accessory minerals that have been analyzed for 

thorium and uranium by calculating alpha from equation (2), and 

measuring the thick-source count experimentally. Column 7, tables 

6 and Y, shows the conversion coefficients obtained in this way for 

zircon and monazite. The value 0.496 compares well with that calculated for zircon by Hurley and Fairbairn. Conversion factors for 

other accessory minerals can be calculated in a similar manner.

The variation of the factor c in equation (3) with the thoriumuranium ratio is shown in figure 1. It is obvious from the figure that 

the knowledge of an approximate value of the thorium-uranium ratio 

is necessary. If one chooses an average value for c for a given mineral, 

some error will be reflected in the age due to deviations from the true 

thorium-uranium ratio. Table 3 shows the percent deviation in the 

calculated age from the true age for different thorium-uranium ratios 

due to a given choice of the factor c. For instance, in ordinary zircon 

the thorium-uranium ratio in most samples varies about a value of 

1.0. It is rarely greater than 5 or less than 0.1. Hence, by choosing 

c 2,450 one would not normally expect an error in age greater than
LEAD-ALPHA METHOD FOR DETERMINING AGES OF IGNEOUS KOCKS 15

e Noiivnta NI » aoiovj
16 STUDIES IN GEOCHKONOLOGY

9 percent due to deviations from an assumed thorium-uranium 

ratio of 1.

Thus for an accessory mineral such as zircon, an approximate value 

for c can be selected that will generally produce an error of less than

10 percent in the calculated age due to deviation of the thoriumuranium ratio from an assumed average. The c factors suggested 

for use with some accessory minerals are shown in table 4.

TABLE 3. Percent deviations from true age in the calculated age due to choice 

of factor c for different thorium-uranium ratios

c

2,000.. 

2, 050.... ..- ........

2, 100 

2, 150.. _ ......... _ ..........

2,200.. ____ ..... __ ........

2,250. 

2,300......... ..................

2,350. ..

2,400.... 

2,450 

2,500 

2, 550. __ ___ ........

Thorium-uranium ratios

0.1

-23.4 

-21.5 

-19.6 

-17.7 

-15.8 

-13.9 

-12.0 

-10.1 

-8.2 

-6.2 

-5.0 -2.4

0.5

-21.5 

-19.6 

-17.6 

-15.7 

-13.7

-11.8 

-9.8 

-7.8 

-5.7 

-3.9 

-1.9 

0.0

1.0

-19.5 

-17.5 

-15.5 

 13.5 

-11.4 

-9.4 

-7.4 

-5.4 

-3.4 

-1.4 

+0.6 

+2.6

5

-11.4 

-9.1 

-6.9 

-4.7 

-2.5 

-0.3 

+1.9 

+4.2 

+6.4 

+8.6 

+10.8 

+13.1

10

-7.6 

-5.3 

-2.9 

-0.6 

+1.7 

+3.9 

+6.3 

+8.6 

+10.9 

+13.2 

+15.5 

+17.8

15

-5.7 

-3.4 

-1.0 

+1.3 

+3.7 

+6.0 

+8.4 

+10.7 

+13.1 

+15.4 

+17.8 

+20.2

25

-40 

-1.6

+0.8 

+3.2 

+5.3 

+8.0 

+10.4 

+12.8 

+15.2 

+17.5 

+19.9 

+22.4

TABLE 4. Constants for zircon, monazite, and xenotime used in age calculation

Mineral

Range of measured

thoriumuranium ratios

0. 1-5. 0

2. 5-1, 300

» . 2-1. 0

Selected

thoriumuranium

ratio

1.0

25

i 0. 496

1 . 392

8 .50

c

2,485

2,085

2,550

talO-«

1.56

.65

1.71

' Experimental values. 

' Based on limited data. 

* Theoretical values.

Because of the gradual decay of the parent elements, the above 

equations are not strictly true for old minerals. In order to keep the 

error due to the decay of the parent element about the same as that 

in the constants previously discussed, that is, a few percent, the 

approximate age equation (3) should not be used for minerals older 

than 200 million years. For instance, using equation (3), a 500 million year-old zircon with a thorium-uranium ratio of 1.0 will have a 

calculated age about 4 percent larger than the true age, and a 2,000 

million-year-old zircon with the same thorium-uranium ratio will be 

in error by about 20 percent. (See figure 2.)

By expanding the exponentials in the fundamental age equation to 

the second term, Keevil (1939) has shown that a good approximate age
LEAD-ALPHA METHOD FOK DETERMINING AGES OF IGNEOUS KOCKS 17

5000

{£,4000

o

3000

LU

 < O

o

LU

5 2000

1000

1000 2000 3000

TRUE AGE, IN MILLIONS OF YEARS

FIOUBB 2. Variation of the calculated age from the true age.

4000

equation can be obtained for minerals up to 1,500 million years. This 

equation is:

(4)

where t is the age given by equation (3). The variation of k with the 

thorium-uranium ratio is given in figure 3 and can be shown to be,

77.2+6.20 Th = U ;

'" 4.06 X1011 (l+0.312 Th:U) ^

Equation (4) is an extremely simplified equation; several approximations were made in its derivation. As Keevil has pointed out,
18 STUDIES IN GEOCHRONOLOGY

_ o .3
LEAD-ALPHA METHOD FOK DETEEMTNTNG AGES OF IGNEOUS EOCKS 19

these approximations are such that the errors compensate to a great 

extent. For ages greater than about 700 million years the approximations used overcompensate and the equation gives ages which are lower 

than the true age. However, the correction is small, being less than 

1 percent to as much as 1,700 million years. At 4,000 million years 

the correction amounts to about 5.5 percent (fig. 2) . It can be shown 

that this error is approximately a function of t0s for ages less than 

4,000 million years, and for the purpose of this work a correction can 

be most simply obtained by adding the term + 3.4X 10"9 t0s to equation 

(4) . Thus, for rocks older than 1,700 million years the age will be,

3 (6)

More rigorous equations can be used to calculate the age of old rocks 

and minerals (Roques, 1956) but generally these are complicated and 

cumbersome to handle for routine work. Equations (3), (4), and 

(6) are preferable from the point of simplicity as well as being sufficiently accurate compared to the experimental data currently available for lead-alpha age calculations. Table 5 summarizes the equations 

that can be used depending on the approximate age of the mineral.

TABLE 5. Equations used in age calculations

Range 

(millions of years)

Equation

0-200 (ppm)

a per mg per hr 

200-1, 700 

1, 700-4, 000 T=(t-l/2 fc*2) + 3.4X10-' (t- 1/2 fc*2) 3

RADIOACTIVITY MEASUREMENTS

The alpha activity is determined by counting with conventional ion 

chamber equipment and thick zircon sources. Small hemispherical 

chambers constructed with a drawer to admit the sample are the 

most suitable. The chamber is flushed with argon gas and the voltage 

is adjusted high enough to operate in the proportional region. 

Under these conditions, the background is generally less than 10 

alpha counts per hour. Variations in the alpha count rates with a 

change in emanation of radon or thoron due to grinding has not 

been detected for zircon or monazite. Counting rates measured 

immediately after grinding showed no measurable difference from 

those measured several days after grinding. If an emanation effect 

is present, it must be small in these minerals.

The zircon is finely ground and placed in a sample dish, care being 

exercised to have the top surface as smooth as possible. The sample 

trays are circular, being about 1 cm in diameter and 0.1 mm deep, to
20 STUDIES IN GEOCHRONOLOGY

insure a depth infinitely thick for the alpha particles. The top outer 

edges are beveled to facilitate cleaning off excess zircon powder when 

loading the trays. For samples of low radioactivity, it is sometimes 

desirable to use larger pans if sufficient sample is available. For such 

sample holders, it is difficult to consistently prepare samples with a 

smooth surface. A technique found useful in preparing smooth surfaces is to cover the roughly filled pan with a piece of glazed paper 

and carry out the final smoothing operation with a spatula on the top 

side of the paper cover. When the paper is removed a smooth surface 

results, which is suitable for alpha counting.

The data given in tables 6 and 7 show the comparison between the 

thin-source alpha activity calculated from the uranium and thorium 

content (column 5) with the value obtained by thick-source counting 

(column 6) and the conversion factor /? (column 7).

Using the average conversion factor, the calculated and measured 

alpha activities are plotted in figure 4. This figure indicates that

ZIRCON ONLY

ALPHAS PER MILLIGRAM PER HOUR (MEASURED)=ALPHAS 

PER SQUARE CENTIMETER PER HOUR x CONVERSION. FACTOR R

100 200 300 400 500 600 700 800

12,000 

11,000 

10.000 

9000 

8000 

7000 

6000 

5000 

4000 

3000 

2000 

1000

/

/

4

£

*

4

EXPLANATION

t

Monazito 

+ 

Zircon

^

A

^

f

-

7

/

$*

/ 

7+ /

/

/

/

/

/

/

/

-}/

1000 

900 

800

700 > 

§ 

600 §

M

500 

400 

300 

200

100 

0

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10.000 11.000 12.000

MONAZITE ONLY

ALPHAS PER MILLIGRAM PER HOUR (MEASURED)=ALPHAS 

PER SQUARE CENTIMETER PER HOUR XCONVERSJON FACTOR »

FIGURE 4. Comparison of calculated and measured alpha activity for zircon (scale at 

right and top) and monazite (scale at left and bott»m)i.
LEAD-ALPHA METHOD FOK DETERMINING AGES OF IGNEOUS ROCKS 21 

TABLE 6. Comparison of alpha activities and conversion coefficients for zircon

Sample

1

BB-23 » 

BB-1- . ....

BB-3 ...... .... .... ...

BB-4a ...... ... ... ...

BB-4b. 

BB-5 ... _

BB-6 ....... .... ._ ... ... 

BB-7 .. 

BB-15 .. .....

BB-17 ...... ......

BB-18 ....... . 

BB-20 ...

BB-21... - . ......

BB-22....... ................ .

BB-24 ...... ..... .....

BB-25.. ----- _ -

BB-27 ... .... ... .....

BB-28.. ... ... ... ... ...

BB-29.. ......... ..........

BB-33. ....... ...

BB-34 .

R-1932 »- ..

R-3185 «- _ _

Z-22. ... ... ... ... .... .

Mean __ . _______ .

Uranium 

(ppm)

2

1,600

755

411

479

493

450

613

575

390

319

387

330

457

1,430

678

617

450

517

890

670

620

460

1,550

2,650

Thorium 

(ppm)

3

1,220

611

464

335

349

328

335

412

150

276

248

890

363

1,215

733

639

366

507

534

904

569

400

183

2,180

Thoriumuranium 

ratio

4

0.76

.81

1.1

.70

.71

.73

.55

.72

.39

.87

.64

2.7

.79

.85

1.1

1.0

.81

.98

.60

1.3

.92

.87

.12

.82

Alpha i 

(a per mg 

perhr)

5

690

329

190

204

210

193

253

246

155

140

163

198

198

627

311

281

196

233

371

323

276

202

582

1,059

E (a per 

cm' per hr)

6

1,349

663

365

408

371

414

473

467

355

316

286

412

438

1,043

692

572

391

503

819

706

554

412

1,152

2,109

'-i

7

0.511

.496

.521

.500

.566

.466

.535

.527

.437

.443

.570

.480

.452

.601

.449

.491

.501

.463

.453

.458

.498

.490

.505

.502

.496

' Calculated from a per mg per hr=0.366 U + 0.0869 Th (uranium and thorium In ppm).

» All samples prefixed BB analyzed by H. Holland, Prlnceton University, by X-ray fluorescence.

' Chemical analyses by F. Cuttitta, U.S. Geological Survey.

*Isotope dilution analyses by Tilton and others (1957).

TABLE 7. Comparison of alpha activities and conversion coefficients for monazite

Sample

P476E-2 - 

SQ-81- .

SQ-12_. 

O-7- .

45-MT-73 

49-MT-18 

45-MT-215-. 

45-MT-128-. 

45-MT-157 

45-MT-234- 

45-MT-160-. 

45-MT-235-. 

45-MT-154-. 

45-MT-101- 

45-MT-172-

53-Be-3 ... 

GS-436-55 

Locality and rock type

1

Front Range, Colo., 

pegmatite.

monazite from carbonate rock.

placer. 

Southeastern Atlantic 

States. 

. .do.-. __ . __ . 

 do.... ..... 

..... do..- ..... 

... -do..- .... . 

.... . do... ... ....... 

.. _ do _ .. _ ........ 

.... . do.- ... 

.-..do.-- _ 

 do.-- - 

 do. .... .... ......

North Carolina, 

Cherryville quartz 

monzonite. 

Yamba Lake, Northwest Territory, 

Canada.

Collector

George Phair..

and D. R.

Shaw.

and J. C. 

Olson.

Jr. 

J. B. Mertie, 

Jr. (1953). 

..... do. 

 ~ do.-- - 

 -do. 

 .do. 

 -do. 

 do. 

 ..do. 

 do. 

... -do. 

. .do.. ......

W. C. Overstreet.

R. E. Folinbee.

Thorium (percent)

2

4.78 

2.65

2

CC

3 91

5.94

3.88 

3.16 

5.68 

6.49 

6.21 

6.90 

4.56 

5.58 

4.44 

4.17

4.9 

4.23

Uranium (percent)

3

0.08 

.002

.002

.136

.15

.22 

.23 

.25 

.26 

.27 

.28 

.31 

.35 

.37 

.50

2.0 

.21

Thoriumuranium 

ratio

4

60 

1,325

1,280

94

39

18 

14 

22 

25 

23 

25 

15 

16 

12 

8

2.5 

20

Alpha* 

(a per 

mg per 

hr)

5

4,447 

2,310

2,232

3,287

5,711

4,177 

3,588 

5,851 

6,592 

6,384 

7,021 

5,098 

6,130 

5,212 

5,454

11, 578 

4,445

E

(a per 

cm2 per 

hr)

6

11,420 

5,597

5,570

8,171

14,155

10,506 

9,607 

15,096 

16, 295 

17,065 

17, 462 

12, 611 

17,000 

14, 355 

14, 436

28,492 

11,042

R =E«

7

0.389 

.413

.401

.402

.403

.398 

.373 

.388 

.405 

.374 

.402 

.404 

.361 

.363 

.378

.406 

.403

.392

1 Calculated from a per mg per hr=0.366 U-f-0.0869 Th (uranium and thorium in ppm). 

507200 59 1
22 STUDIES IN GEOCHRONOLOGY

the activity determined by counting methods agrees on the average 

within 5 percent with the alpha activity calculated from the thorium 

and uranium content. It also shows that the conversion factor is 

relatively independent of the thorium-uranium ratio over a wide 

range of activities. Experience has shown that the larger errors 

can be attributed to incorrect thorium or uranium determinations 

and possibly to a lesser extent to the effects of the thorium-uranium 

ratio on the conversion factor.

The ion chamber method of alpha counting of zircon is currently 

being used by the California Research Corp., Massachusetts Institute 

of Technology, and the U.S. Geological Survey laboratories, and a 

scintillation method is being used by Princeton University and Columbia University laboratories. In general, the interlaboratory 

checks agree within 5 percent. From these checks, and also from the 

alpha-counting experience in this and other laboratories, the alphacounting data probably are accurate to within 5 percent.

LEAD ANALYSIS

With further development of the lead-alpha method of age determination, an emission spectrographic method for the quantitative 

analysis of trace amounts of lead in accessory minerals was developed 

by Waring and Worthing (1953). They originally developed the 

method to cover a concentration range of 0.5 to 1,000 ppm lead and 

have since extended the range to 3,000 ppm lead (C. L. Waring, written communication). Lead is determined by direct current arc excitation of a 50 mg sample consisting of 12.5 mg of the mineral and 37.5 

mg of lead-free sodium carbonate. The method as originally described (Waring and Worthing, 1953) used bismuth as an internal 

standard. Additional testing has shown that equal precision can be 

obtained without internal standards by exposing on the same plate 

as the mineral three or more external standards of known lead content; standards are selected to bracket the lead content of the mineral 

(C. L. Waring, written communication). Thus a curve for each 

plate is drawn showing the relations between blackening of the lead 

line (2833.1 A) and the lead content in the standards; from this 

curve the lead content of the mineral can be derived. The standards 

at present are prepared by diluting the National Bureau of Standards 

standard sample 91, opal glass, of known lead content, with lead-free 

quartz. These standards cover the range 0.5 to 933 ppm lead.

Separate standards for the analysis of lead in monazite were prepared by adding lead to a cerium phosphate base material in 

concentrations that ranged from 1,000 to 3,000 ppm lead. It was 

eventually determined that both the silica and cerium phosphate were 

equally satisfactory base materials for the preparation of standards
LEAD-ALPHA METHOD FOR DETERMINING AGES OF IGNEOUS ROCKS 23

for the analysis of lead in monazite (C. L. Waring, written 

communication, 1956).

The precision of the method was estimated by Waring and Worthing 

(1953) to be 6 to 10 percent of the mean on the basis of replicate 

analyses of a limited number of samples. Since 1953, duplicate lead 

analyses have been made on 279 samples of zircon with a lead concentration that ranged from 0.4 to 350 ppm and 27 samples of monazite 

that contained from 25 to 2,000 ppm lead. The precision for both 

minerals varies with the concentration of lead (table 8). The precision in table 8 is at the 90 percent confidence interval computed 

by using differences between duplicates as shown by Youden (1951). 

For further details of the method, the reader is referred to the paper 

by Waring and Worthing (1953).

The method has since been applied with satisfactory results by 

other investigators (Roques, 1956; Hee and Flesch, 1957; Saillard 

and others, 1957; and A. E. Stoddard, California Research Corp., 

written communication, 1956). Webber and others (1956) used an 

emission spectrographic method standardized by addition of known 

amounts of lead-barium glass to zircon. This calibration consistently 

yielded higher results than those obtained by Waring and Worthing. 

The lead-alpha ages reported by Webber and others (1956) were 

based upon lead analyses calibrated by standards used by Waring 

and Worthing.

TABLE 8.- -Predsfon of duplicate spectregraphic analyses of lead m zircon and 

monazite

Number of duplicate analyses

Zircon

12____ _______________________

30...... __.._. ___.______.__.__._.__

84____ ________________________________

51______--------_-__ _______

74__ ___ _ _____________ _ . ....

17_ _______ __ ________________

11 ___________ ___________________

Monazite

6 ________________________ _

5 __ _ _-_ __ ---.__-_ ___________

9. __ -_-. ____ ________________ _ _.

7 _ __ _ _ _

Lead range (ppm)

0.5-2 2-5

5-20

20-35

35-100

100-200

200-350

25-100

100-500

500-1, 000

1, 000-2, 000

Precision (ppm)

±0.2

±1.0

±1.5

± 2. 5

±2.8

± 4.9

±7.6

±4.2

±7.0

±72

±80

The chemical dithizone mixed-color method (Sandell, 1950) and its 

modification (Powell and Kinser, 1958) have been used to determine 

lead in accessory minerals. Powell and Kinser show a precision of ±1
24 STUDIES IN GEOCHRONOLOGY

percent for lead in monazite. The isotope dilution method of analysis 

of lead in zircon, monazite, and other accessory minerals has been 

described by Tilton and others (1955). The isotope dilution method 

probably has an absolute error of less than 3 percent.

A comparison of lead analyses made by different methods on the 

same minerals is given in table 9. For the most part, the results are in 

good agreement. However, in several cases, notably samples 6-37, 

44-MT-121, SQ-81, and F-22, the results are in disagreement by 

75 to 300 percent; for each of these samples the emission spectrographic value is the lower. Half of the spectrographic lead values 

for the monazite from Mountain Pass were determined using the 

zircon standards and half using prepared cerium phosphate standards 

approximating the stoichiometric base composition of monazite. 

Both sets of data agree within 5 percent. Hence, base composition 

cannot be offered as an explanation for the disagreement with the 

higher values obtained by isotope dilution and modified colorimetric 

methods. The colorimetric method (Sandell, 1950) gave values still 

lower than the spectrographic method and showed good precision on 

any given day. Over a period of time, however, the content of lead 

determined colorimetrically varied about 30 percent of the mean 

(Jaffe, 1955). The results obtained by the colorimetric method of 

Sandell may be low because in the presence of large amounts of 

calcium and phosphorous some of the lead is carried down by insoluble 

phosphates to produce an incomplete dithizone extraction (Sandell, 

1950).

Although most of the data obtained by different methods agree, 

the accuracy claimed for the chemical and spectrographic method does 

not always agree. Where errors occur, they may be large, even though 

high precision may be obtained. The sources of the errors in the 

cases considered have not been established. More research is being 

done on analytical problems connected with the samples that yield 

discrepant results.

EVALUATION OF THE ASSUMPTIONS

CBYSTALLIZATION OF ZIBCON IN A MAGMA

A knowledge of the time of formation and the crystallization history of zircon in plutonic rocks is essential to a proper geologic interpretation of the lead-alpha ages. Zircon or other accessory minerals 

must have crystallized at the same time as the containing rock in 

order to have meaning in geochronologic dating. Although zircon has 

a wide distribution in a series of rocks of a calc-alkalic batholith, 

there is little agreement in the literature on the problem of whether 

zircon crystallizes with the early or later formed rocks during magmatic differentiation.
LEAD-ALPHA METHOD FOR DETERMINING AGES OF IGNEOUS ROCKS 25

TABLE 9. Comparison of emission spectroffraphic lead determinations with those 

made T>y other analytical methods

Sample

7 99

Z-4.. .........

6-37 

S-22__.. 

F oo

GS-516... ....

Standard

No. 1. 

490T-22L.. 

S2-MT-8-- 

49-MT-15-.-.

SQ-81 

Locality

.....do .... ..... .....

Toluca, N.C.. ____

Southern

Appalachians.

Zatalla, Ga__. .........

Mineral

..... do. 

..... do.... 

 do... .....

..... do _

... ..do -

 do... .....

.....do... .....

Emission 

spectrography i

462

437

430

44A

428

352,335

QQA «M A

Oftrt QJQ

65

67

66

68

94fi

5

570

580

155

3,000

1,290

1, 250, 1, 250

1, 200, 1, 230

1, 250, 1, 230

1, 230, 1, 250

1, 240, 1, 220

1, 240, 1, 245

1,042

1,050

1,038

1,030

1,045

1,030

900,880

890,880

880,890

890,890

880,890

920,912

1,100,1,080

1, 110, 1, 120 

1, 160, 1, 140

1, 140, 1, 150

1, 110, 1, 140

1, 130, 1, 140

1,150,1,150

1,200,1,190

1, 185, 1, 150

1, 150, 1, 140

1, 130, 1, 170 

1,200

Total lead 

(ppm) isotope 

dilution

>461

 376

 127

»240

> Q *

*3,720

* 1,070

» 1, 770

< 1, 740

............

Colorimet- ric 2 (Sandell 

method)

7QA

785 

710

703

765

770

930

920

678

675

1,070 

1,040

Colorimet- ric * (modified)

610

400

39&

3,580,3,530

1 240

1,250

1,260

1,090

1,070

QAft

860

830

860

1,780

1,760 

1,740

1,770

 

IC. L, Waring and H. W. Worthing, U.S. Geological Surrey, analysts.

s R. A. Powell, U.S. Geological Survey, analyst.

»G. R. Tllton, analyst (Tilton and others, 1955 ; Waring and Worthing, 1953).

« L. R. Stleff, U.S. Geological Survey, analyst.

If the time required for the emplacement and crystallization of a 

composite batholith is short (a few million years) it will make little 

difference whether zircon crystallizes early, late, or continuously. The 

difference between the time of crystallization of the accessory minerals 

and that of the major rock-forming minerals will be within limits 

of error of the lead-alpha method. Larsen (1945) has estimated 

that the time required for the crystallization of the batholith of 

southern California was on the order of a few million years. How-
26 STUDIES IN GEOCHEONOLOGY

ever, on the basis of potassium-argon dating Evernden and others 

(1957) concluded that the time elapsed between the oldest and youngest intrusions of a series of igneous rocks that they dated from 

Yosemite National Park was 18 million years. In some areas and 

particularly in Precambrian terraines, rocks of igneous appearance 

may have been formed by complex metamorphic processes. Little 

is known regarding the time element involved in the formation of 

these rocks.

Poldervaart (1956) and Larsen and Poldervaart (1957) conclude 

that zircon crystallizes early in calc-alkalic rocks. Because of the 

uniformity of morphological features they believe that the zircon 

crystallized under uniform conditions which would most likely prevail for a short period of time during the early stages of crystallization of the magna. Moorhouse (1956) believes zircon crystallizes late 

on the basis of its textural relations to other minerals and the association of the zircon with late forming minerals. If the zircon forms 

early over a short range of crystallization, it is reasonable to assume 

that it would have uniformity in composition as well as size and shape. 

On the other hand, if zircon crystallizes continuously throughout 

differentiation the later formed zircon should be gradually enriched 

in uranium, thorium, and hafnium, which are progressively concentrated in the residual liquids. A study of the alpha activity, hafnium 

content, and hafnium to zirconium ratio on zircon separated from 

suites of calc-alkalic rocks has been undertaken to provide some information relative to this problem. Kesults of Gottfried, Waring, and 

Worthington (1956) on zircon separated from a suite of rocks from the 

batholith of southern California are given in table 10. These data 

show a progressive increase of hafnium and radioactivity in zircon 

from later formed rocks. This evidence indicates that zircon crystallizes continuously during the crystallization of the magma.

TABLE 10. Hafnium content, hafnium to zirconium ratio, ana radioactivity of 

zircon from igneous rocks of the batholith of southern California.

Rock type

Tonallte . _ ________

Quartz monzonite and graniteNumber

of

samples

1

8

6

6

Percent hafnium »

Range

0 97-1 12

1.07-1.43

.92-2.02

Average

1.0

1.04

1 01

1.38

Hafnium-zirconium

ratio i

Range

0 021-0 024

.023- .034

.020- .042

Average

0.020

.023

.027

.031

Activity (a per

mg per hr)

Range

143-594

433-1,235

396-4,660

Average

45

256

773

1,586

i The values for the hafnium and hafnium to zirconium ratios are based on improved standardization 

techniques and thus differ from those reported by Gottfried, Waring, and Worthing (1956).

Because of the relatively high degree of stability of zircon, chemically and mechanically, as compared to most other minerals, relict
LEAD-ALPHA METHOD FOR DETERMINING AGES OF IGNEOUS ROCKS 27

zircon may be incorporated into a magma by assimilation or other 

processes. Unless the relict zircon has undergone complete recrystallization its age would be greater than that of the host rock. Twice 

thus far, it seems likely that relict zircon has been measured for age. 

An age of 460 million years was determined on zircon from a syenite 

from the border zone of the Idaho batholith. The syenite may be 

equivalent in age to the rocks from the main mass of the Idaho batholith whose mean age has been determined by Larsen and others (1958) 

as 108 million years by the lead-alpha method. Inclusions of amphibolite were present in the syenite and B. F. Leonard (written communication, 1956) has suggested it may have been contaminated by the 

Precambrian formations which the syenite intrudes.

In the La Sal Mountains of Utah, lead-alpha age determinations on 

zircon separated from three different samples of the diorite porphyry 

gave 380, 410, and 470 millions of years. The age of the diorite porphyry probably is Cenozoic on the basis of geologic field evidence. 

Abundant xenoliths of schist and amphibolite are in the outcrops of the diorite and according to C. B. Hunt and A. C. Waters, 

(Waters, 1955) the diorite magma was derived from partial fusion of 

Precambrian amphibolite. Lead-alpha ages of 50 and 24 million 

years, corresponding to Tertiary on the geologic time scale, were obtained on zircon from the monzonite and syenite porphyries which contain relatively few xenoliths. This age pattern may be explained on 

the basis of relict zircon in the diorite porphyry and predominantly 

magmatic or recrystallized zircon in the monzonite and syenite porphyries. However, until isotopic analyses of the lead in the zircon 

from the diorite porphyry are made the possibility of common lead in 

the zircon cannot be ruled out.

With the exception of the two examples cited, the good agreement 

between lead-alpha ages of zircon and the known geologic age of the 

rocks from different parts of the time scale indicate that in normal intrusive rocks the problem of relict zircon is not serious. Thus, with 

only a few exceptions, the assumption that the zircon is of the same age 

as the host rock seems valid.

NATURE OF LEAD IN ACCESSORY MINERALS 

EVIDENCE FROM CRYSTAL CHEMISTRY

Larsen and others (1952) assumed that bivalent lead would not substitute for quadrivalent zirconium in the zircon lattice at the time of 

crystallization, and therefore they concluded that the lead in zircon 

was primarily all of radiogenic origin. This reasoning was based 

upon information pertaining to the mode of occurrence of lead in
28 STUDIES IN GEOCHRONOLOGY

igneous rocks and on a comparison of the following ionic radii of 

Goldschmidt (1954):

Angstrom units Angstrom units Angstrom units

Zr4*_________ 0. 87 Pb4*_________ 0. 84 Pb41_________ 1.32

U4*_________ 1.05 Ce48_________ 1.18 K4 _________ 1.33

Th44_________ 1.10 La**_________ 1.22 Sr42 _________ 1.27

Hf+«_________ .86 Y4S_________ 1.06 Ca41_________ 1.06

According to Goldschmidt (1954) and Kankama and Sahama 

(1950), the lithophile lead in igneous rocks is present in silicate and 

phosphate minerals as Pb42 ions. Because of the similarity of the 

ionic radii of Pb42, Sr+2, K4, and Ca42, bivalent lead and strontium are 

captured mostly in potassium minerals such as feldspar and biotite and 

to a lesser extent in calcium minerals such as sphene, apatite, and 

augite. Goldschmidt (1954) reports 5 to 100 ppm lead in potassium 

feldspar, 3 to 10 ppm lead in augite, and as much as 50 ppm lead in 

apatite. The data given in table 11 further illustrate the tendency of 

Pb+2 to concentrate in orthoclase and biotite of several igneous rocks. 

Larsen and Keevil (1947) have shown that lead concentrates in the 

most silicic magmatic rocks in the batholith of southern California 

because lead, potassium, and silica are progressively concentrated 

during magmatic differentiation. This is further borne out by the 

recent data of Wedepohl (1956) in a more detailed investigation of the 

geochemistry of lead. Tilton and others (1955) note in their detailed 

study of the lead, uranium, and thorium in a Precambrian granite 

from Ontario, that the nonradiogenic lead in the rock, indicated by 

the concentration of Pb204, resided principally in perthite and resisted 

strongly any tendency to concentrate in the zircon which contained 

none. They showed that uranium and thorium were highly concentrated in zircon, sphene, and apatite.

The data indicate, as would be predicted from principles of crystal 

chemistry, that Th+4, U44, Hf44, and Y43 may substitute for Zr44 in 

zircon on the basis of similarity of ionic radii. On the other hand, 

Pb42 probably would not proxy for Zr44 in zircon because of the large 

dissimilarity in ionic radii. The concentration of Pb42 in potassium 

minerals where it substitutes for K4 again reaffirms the principles of 

substitution based on similarity of ionic radii.

A comparison of sizes of the Pb44 and Zr44 ions shows a close similarity and the possible occurrence of Pb44 ions in a magma must be 

taken into consideration. On the basis of ionization potentials Ahrens 

(1953) points out that Pb+2 is stable in most geologic environments as 

the ionization potential difference between Pb42 and Pb44 is large and 

that only under extreme oxidizing conditions would ionization produce 

the quadrivalent lead found in the mineral plattnerit© (PbO2 ). If
LEAD-ALPHA METHOD FOR DETERMINING AGES OF IGNEOUS ROCKS 29

such oxidizing conditions prevailed in a magma, it is highly probable 

on the basis of their oxidation potentials that nearly all the Fe+2 would 

be converted to Fe+3 before a significant part of the lead would become 

quadrivalent. Evidence for such oxidizing conditions is not found in 

normal igneous rocks.

Although the large Pb+2 ion probably would not proxy for Zr+ * in 

zircon, it might enter more readily the structure of monazite or xenotime substituting for the relatively large trivalent ions of the cerium 

group of rare-earth metals. Similarly, the ionic radii of Th+4 and U+4 

are very close to those of the trivalent rare-earth ions and may more 

readily enter the lattices of monazite and xenotime than zircon. There 

is some evidence that Th+4 enters the lattice of monazite accompanied 

by Si+4 in a coupled substitution of TV4 Si+4 for Ce+3 P+5 , maintaining 

crystal neutrality ( Jaffe, 1955) . The ionic radii of the trivalent rareearth ions are given below :

Cerium earth group

La+a 

Ce+a

Nd+l 

Sm+

(Angstrom units) 

._ 1.22 

._ 1.18 

._ 1.16 

._ 1.15 

._ 1.13 

._ 1.13

Yttrium earth group

(Angstrom units) 

Y+s _________ .- __ 1.06 

Gd+s __________ 1. 11

Tb+s _ _ .

Dy+3_

Ho+s

Er+s

Tm+3

Yb+3_ __

Lu+s

. ___ 1. 09

1.07

1.05

1.04

1.04

__ 1.00

.99

TABLE 11. Lead found in orthoclase, Motite, and in some igneous rocJcs from 

southern California and soutMvestern Colorado

[Determined spectrographieally by C. L. Waring, U.S. Geological Survey]

Sample Rock type

Lead (parts per million)

Orthoclase Biotite Rock

Southern California

SLR-229___--____-__--_

SLR-685__ ____________

EL38-126_-___ _ ____ _-

E38-28-______- -_- ____

SLR-2242____. _-_-___-_

SLR-596 ____ ._____.__

SLR-135_-_-----------_

EL38-265______ ------ __-

EL38-167-. _ __________

Norite

Tonalite

_do .

do

do

__ _ do__-_

Granite

do

50

45

60

43

45

70

12

27

34

30

10

20

10

9

12

10

20

15

Southwestern Colorado

90 ________ '2' ^

\

507200 59-
30 STUDIES IN GEOCHRONOLOGY

The trivlaent ions of the cerium-earth group, characteristic of 

monazite, are larger than those of the yttrium-earth group characteristic of xenotime. Although this difference is not large, only relatively 

small percentages of the cerium-earth metals enter the structure of 

xenotime and similarly only minor amounts of the yttrium-earth 

metals enter the structure of monazite. Although the ionic radii of 

Th+* and U+* are similar, the larger ion, Th+4, follows the cerium-earth 

group in monazite and the smaller ion, U+4, follows the yttrium-earth 

group concentrating in greater amounts in xenotime. Thus, regarding monazite and xenotime, considerations of crystal chemistry 

suggest that greater amounts of original lead can be found in these 

two minerals than in zircon. However, the percentage of original 

lead of the total lead content is small owing to the formation of radiogenic lead from relatively large concentrations of uranium and 

thorium in monazite and xenotime as compared to zircon.

RELATION OF OPTICAL PROPERTIES TO T/B!AT» CONTENT OF ZIRCON

Mineralogic and radioactivity studies of large clear unzoned crystals of zircon from Ceylon have shown that systematic relations exist 

between the changes in physical properties and the breakdown of the 

crystal structure due to alpha-particle bombardment. The nature of 

the processes involved have been treated in detail by Holland and 

Gottfried (1955) and the age of the zircon has been reported by Gottfried, Senftle and Waring (1956). According to the first report the 

indices of refraction of the zircon decreased as a function of the 

radiation dosage, that is, the alpha-particle emission rate times the 

age of the mineral. The same relation should exist between the 

optical properties and lead content if all the lead in the zircon is a 

function of the radioelement content and its age. However, if variable amounts of original lead were present in zircon no correlation 

would be found between lead content and any of the other physical 

properties, such as the indices of refraction and the density. With 

the optical method, the index of refraction, omega, can be measured 

rapidly, accurately, and on every grain, and only a relatively few 

grains are needed in an immersion for each determination. The data 

for the Ceylon zircon are given in table 12, and a plot of the index of 

refraction against lead content is given in figure 5. The curve 

obtained is of considerable interest and permits the following 

observations:

1. Zircon with less than 100 ppm of lead has an index of refraction 

for that of normal zircon (n =1.92) and the curve is virtually 

flat.

2. As the lead increases beyond 100 ppm the zircon shows a rapid 

decrease in the index of refraction, and at a concentration of
fUl

cr

cc

u_ 

o

2 

o

1.92

1.90

1.88

1.86

1.84

1.80

100 200 300 

LEAD CONTENT, IN PARTS PER M

FIGURE 5. Index of refraction (omega) of Ceylon zircon and th
32 STUDIES IN GEOCHRONOLOGY

500 ppm of lead the zircon is practically isotropic with an index 

of refraction of 1.810.

3. When more than 500 ppm of lead are present the curve approaches 

the abscissa asymptotically.

TABLE 12. Data for Ceylon zircon

Sample

1 

4-33 _

2..

4-36. 3-16

5-1

3-11. .

3-42 

3... ... 

2-37 .

2-18 _

2-13 

4 .

2-23 

2-17 

6 ...... ...

7 __ _ .

1-2 

9. ........ .

10 

1-26 .. 

Index of refraction

Omega

1.921 

1.923 

1.922 

1.920 

1.916 

1.918 

1.910 

1.903 

1.905 

1.900 

1.893 

1.875 

1.872 

1.860 

1.852 

1.846 

1.830 

1.825 

1.819 

1.811 

1.810

Epsilon

1.972 

1.978 

1.973 

1.965 

1.972 

1.958 

1.948 

1.932 

1.945 

1.923 

1.913 

1.886 

1.888 

1.870 

1.862 

1.855 

1.834 

1.828 

1.822 

1.813 

1.810

Specific 

gravity '

4.680 

4.669 

4.653 

4.605 

4.593 

4.577 

4.550 

4 482 

4.440 

4.425 

4.421 

4.325 

4.279 

4.202 

4.165 

4.140 

4 048 

4.029 

3.966 

3.965 

3.956

Lead 

(ppm) 2

22

37 

65 

91 

80 

88 

115 

143 

150 

148 

200 

196 

205 

227 

270 

275 

392 

440 

450 

529 

498

a per mg 

per hr

103 

150 

273 

430 

352 

380 

533 

649 

643 

652 

913 

8/SO 

882 

985 

1,245 

1,185 

1,583 

2,040 

1,815 

2.197 

2,210

Lead-alpha 

age (millions of 

years)

532 

609 

590 

527 

565 

575 

536 

548 

578 

563 

545 

573 

576 

572 

538 

576 

613 

536 

613 

594 

560

1 H. Holland, Princeton University, analyst.

2 C. L. Waring, U.S. Geological Survey, analyst.

In zoned zircon, the various zones differ in their optical properties 

and an average index of refraction is difficult to obtain. Usually the 

indices of refraction of the different zones can be directly measured 

along the edges of broken crystals. The area of each zone can be estimated and an approximation of the lead content of the zoned crystals 

can be obtained from the curve.

Although the curve is based entirely on unzoned zircon from Ceylon, 

the same relations were found for practically all zircons from the 

other localities. The high degree of correlation between the lead content and the index of refraction of many zircon samples, strongly 

suggests that nearly all the lead is of radiogenic origin.

Accordingly, an order of magnitude estimate of the lead content 

of zircon can be obtained by a knowledge of the index of refraction 

alone. This information is useful in the following respects:

1. It is possible when analyzing for lead in zircon by the spectrographic method (Waring and Worthing, 1953) to bracket 

zircon samples with suitable standards. In this manner lead 

analyses can be obtained in an initial attempt. Otherwise additional runs would be necessary which may not be possible 

when only a small amount of zircon is available.

2. When wide discrepancies have been found between the lead content determined from spectrographic analysis and the amount
LEAD-ALPHA METHOD FOR DETERMINING AGES OF IGNEOUS ROCKS 33

estimated, further work has revealed either an error in the 

analysis or the presence of previously undetected lead-bearing 

impurity grains other than zircon.

3. It is possible to estimate the amount of zircon required to insure 

extraction of a sufficient quantity of lead for isotopic analysis.

ZIRCON OF LOW I.EAD CONTENT

The presence of original lead in zircon would have a maximum effect 

on the measured age of those zircons that contain little lead and low 

alpha activity To evaluate the age determined on such samples it is 

necessary that the geologic age of the parent rocks be accurately known 

from geologic evidence or from other independent methods. The 

igneous rocks of the San Juan Mountains of Colorado contain zircon 

low in lead and alpha activity, and the geologic ages of the rocks are 

Miocene and Pliocene. The lead content of these zircons range from 

0.4 to 7 ppm and the measured lead-alpha ages from 10 to 23 million 

years. Agreement between the geologic age and lead-alpha age has 

been obtained on zircon with low lead content (less than 10 ppm of 

lead) from the well-dated igneous rocks of the Miocene age of Colorado, Oligocene age of New Mexico, Eocene age of Utah and Montana, 

Late Cretaceous age of Alberta, Canada, very late Cretaceous to very 

early Tertiary age of Montana, early Late Cretaceous age of Baja 

California, and the Middle to Late Devonian age of New Hampshire 

(table 13). It can be readily shown that the addition of as little 

as 2 ppm of original lead of crystallization in zircon containing 

less than 10 ppm lead would yield errors in the lead-alpha age from 

500 percent in Miocene rocks to 25 percent in Devonian rocks. The 

fact that the lead-alpha ages of zircon from these rocks of known 

geologic age fall within limits of error in their proper sequence regardless of their lead content is convincing evidence that the problem 

of original lead contamination is negligible in most accessory zircon. 

If original lead is present in significant amounts it must be postulated that it has been deposited in quantities that were small and 

proportional to the amount of thorium and uranium in the zircon 

lattice at the time of crystallization.

ISOTOPIC ANALYSIS OF UBAD

The development of surface ionization techniques has made possible 

the isotopic analysis of small amounts of lead in zircon and other 

common rock-forming minerals. To date, most of the isotopic analyses of lead in zircon have been obtained by Tilton and others (1957) 

and by Henry Faul of the U.S. Geological Survey.

A tabulation of the data of Tilton and others (1957), Tilton (written communication, 1958), Faul in Davis and others (1957), and O. 

Kuovo (written communication, 1956) on the amount of original lead
34 STUDIES IN GEOCHRONOLOGY

TABLE 13. Lead-alpha ages of zircon containing less than 10 ppm lead

Rock type 

Field No. 

Locality 

Collector

QL-8. 

San Juan Mountains, 

Summitville quadrangle, Colorado. 

George Phair and David 

Gottfried.

GL-6. 

San Juan Mountains, 

Creede quadrangle, Colorado. 

George Phair and David 

Gottfried.

RLS-1. 

Cerrfflos Hills, N. Mex. 

R. L. Smith.

P50-49. 

Bearpaw Mountains, 

Mont. 

W. T. Pecora.

stock. 

HTM-1. 

Tintic district, Utah. 

H. T. Morris.

2415. 

Alberta, Canada. 

F. E. Folinsbee.

52-C-10a. 

Boulder batholith, Mont. 

R. W. Chapman.

52-C-60. 

Boulder batholith, 

Montana. 

R. W. Chapman.

BC-1-5. 

San Jose pluton, Baja 

California. 

L. T. Silver.

SV-1. 

San Vincente, Baja California. 

David Gottfried, L. R. 

Stieff, and T. W. Stern.

NH-Edi-56J-18. 

Exeter pluton, between 

Exeter and Epping, 

N.H. 

H. W. Jaffe and J. B. 

Lyons.

Mineral

Zircon .....

.....do.....

.. do.. 

.....do.....

..... do.....

_ do.....

 ..do.....

..... do.....

__ do_ _

.....do.....

.. do.....

a per

mg 

per hr

188

232

58

56 

770

290

275 

187

223

268

160

203

42

123

75

Lead 

(ppm)'

0. 7, 0. 8

(0. 75)

1. 05. 1. 1 

(1.05)

0. 4, 0. 6

(0.5) 

0.4

9,11

(10) 

5. 2, 5. 6

(5.4)

5. 0T 5. 1 

(5. 05)

3.7

4.5 

5. 5, 6. 0

(5.75) 

4.6

6

1. 8, 2. 0

(1.9) 

5. 1, 5. 0

(5.05) 

9,10

(9.5)

Leadalpha age 

(millions 

of years)

10

11 

21

17 

32

46

46 

49

50 

53

71

73

112

102

307

Probable geologic age

Miocene (Larsen and Cross,

1956). 

Do.

Oligocene (post-Espinaso

voleanics, pre-Abiquiu(?) 

formation) (Stearns, 1953).

Middle Eocene (Green River) (Brown and Pecora, 

1949).

Middle Eocene (Green

River) (H. T. Morris and 

T. S. Lovering, written 

communication, 1956).

Late Cretaceous (Maestrichtian) (Folinsbee and others, 

1957).

Late Cretaceous to early

Tertiary (Laramide) 

(Knopf, 1957).

Do.

Early Late Cretaceous (postAlbian, pre-Maestrichtian) 

(Silver and others, 1956).

Do.

Early Devonian, pre-Mississippian(?) (Billings, 

1956).

i The figure in parentheses is the average used tn deriving the calculated ages.
LEAD-ALPHA METHOD FOR DETERMINING AGES OF IGNEOUS ROCKS 35

found in single zircon crystals and zircon concentrates is given in table 

14. The data for the samples analyzed by Tilton indicate that the original lead content is less than 10 percent of the total lead present in 17 

of the 20 samples. Of these, 14 samples showed only negligible 

amounts of original lead. The zircon concentrates from Capetown, 

South Africa, and Quartz Creek, Colo. contained 21 and 26 percent 

original lead, respectively. Two concentrates of zircon from the 

Bodom granite and Rapakivi granite of Finland contained no 

detectable original lead.

TABLE 14. Original lead content of isotopically analyzed zircon

Locality

Bedstone, N.H ______ .

Tory Hill, Ontario... 

DO-..... ......-......

Cardiff uranium mine, Ontario. 

Llano, Tex ____ .. ........

Natural Bridge, N.Y _ ..

Wichita Mountains, Okla ... 

Pikes Peak, Colo.. . .... 

Bagdad, Ariz..-_. . ........

Capetown, South Africa __

Quartz Creek, Colo ..........

Marys Rock Tunnel, Va .... 

Cranberry, N.C _______

T&^sen dome, Baltimore, 

Md. 

Phoenix dome, Baltimore, 

Md. 

Canada Hffl, N.Y . ....

Philadelphia, Pa.... .....

Storm King, N. Y.._~ ... 

Espoo, Finland _______

Lemi, Finland _______

Col de la Grosse Pierre, 

Vosges. 

Wembach, Scbwarzwald .....

Martinskapelle, Scbwarzwald. 

Martmskapelle, Schwarzwald, leached with hot 

hydrochloric acid. 

Halbmeil, Scbwarzwald. __

Rock type

Granite ________

Limestone-syenite 

contact rock. 

Pegmatite, sample A~ 

sample B..

 ..do........ 

.. do..... ... ......

. .do... . 

Granodiorite gneiss ....

.- do..... ..

 do... ...... ... 

 .. do..... .... .....

... . .do... ...... ...

 ..do... ..... ..... .

 . .do..... ... ... ... ...

 ..do..... ..... 

 -do.... .... ... 

 - do .... ... ... ...

Total lead 

(ppm)

32.5

127 

461 

123

485

53.8 

127

63.5 

376 

33.3 

338 

66.5 

210 

82.6

59.5 

65

104

64 

167 

337

73 

166

46 

163

86

78 

17

Original lead

Parts per 

million

gO. 4

SI. 7

§5.5 

§1.5 

§6

<0.6 

SI. 6

2.6 

33 

3 

40 

14 

54 

<1.0

<1.0 

<1.0

 0.6

<1.0 

<1.5 

<5 C1)

(') 

23

33

5.1 

%

29

31 

<1

Percent

<1.2

<1.3 

<1.2 

<1.2 

<1.2

<1.1 

<1.3

4.1 

8.8 

9 

11.5 

21 

26 

<1.2

<1.7 

<1.5

<1.4

<1.6 

<1.0 

<1.5

32 

20

11 

59

34

40 

<5

Reference

Tilton and others 

(1957). 

Do. 

Do. 

Do. 

Do.

Do. 

Do.

Do. 

Do. 

Do. 

Do. 

Do. 

Do. 

Q. R. Tilton 

(written communication, 

1958). 

Do. 

Do.

Do.

Do. 

Do. 

Do. 

O. Kuovo (written 

com munlcation) . 

Do. 

Faul, in Davis and 

others (1957). 

Do.

Do. 

Do.

Do.

Do. 

Do.

i Not detectable.

The amount of original lead in the European zircon concentrates 

analyzed by Faul are in sharp contrast to those discussed above. Except for the one sample which contains less than 5 percent original 

lead, 5 samples contained amounts of original lead ranging from 

11 to 40 percent. It is of interest to note that hot hydrochloric 

acid treatment of the zircon concentrate from Martinskapelle re-
36 STUDIES IN GEOCHRONOLOGT

duced the total lead content from 163 ppm of lead to 86 ppm 

of lead. The amount of original lead was reduced from 59 percent to 

34 percent. Thus a larger fraction of original lead is leached from 

the concentrate as compared to radiogenic lead. This would suggest 

that much of the soluble original lead is not present in the zircon 

lattice, but rather is present surficially.

Isotopic analyses of 13 monazite leads show that only 1 to 7 percent 

of the total lead is original (table 15). The lead found in most monazites is almost entirely radiogenic Pb208 produced from the decay of 

Th232. For example, monazite from Mountain Pass, Calif. (Jaffe, 

1955) contained 98.85 percent of radiogenic lead consisting of 98.34 

percent Pb208 and only 0.51 percent of Pb206, the latter produced from 

the decay of U238. Only 1.15 percent of the total lead was found to 

be original lead based upon correction of the isotopic analysis of the 

monazite lead by that of Mountain Pass galena lead. The isotopic 

abundances of the monazite lead in atom percent were as follows:

S04 i06 S07 S08 Sum

Total Pb.._____-____ 0. 017 0. 782 0. 258 98. 943 100.000

Original Pb_______ . 017 . 270 . 258 . 601 1. 146

Radiogenic Ph..___ _____ .512 _____ 98.342 98.854 

NATURE OF UEAD IN ALJLANITE, APATITE, AND_SPHENE

Lead-alpha ages were determined on several samples of allanite, 

sphene, and apatite and compared with the age determined on zircon 

from the same rocks. The ages determined on these minerals do not 

show the consistency obtained for zircon. Allanite, apatite, and 

sphene gave ages that were consistently older than those obtained on 

zircon from the same rock. The old ages determined suggest that 

allanite, sphene, and apatite either contain large amounts of original 

lead or they have all undergone a loss of uranium and thorium. In 

all probability the old ages of allanite result from a high content of 

original lead because of the following reasons:

1. Pb+2 (ionic radius=1.32 A) often replaces Ca+2 (ionic radius= 

1.06 A) in magmatic calcium minerals such as augite.

2. Allanite from Mountain Pass, Calif, (sample SQ-13, table 16) was 

associated with galena in the same rock, and field and petrographic observations indicate that they are cogenetic. The 

allanite was well crystallized with a high birefringence, strong 

pleochroism, and n = 1.818, which indicates it to be a very fresh 

variety of this mineral. The close association with galena indicates that a large amount of lead was available at the time 

of crystallization. The very fresh nature of the allanite suggests the mineral has not lost uranium or thorium by alteration.
LEAD-ALPHA METHOD FOR DETERMINING AGES OF IGNEOUS ROCKS 37 

TABLE 15. Original lead content of isotopicalty analyzed monazite

Locality and reference

Mountain Isa mine, Queensland, Australia __ 

Holmes, 1948a. 

Las Vegas, near Santa Fe, N. Mex_ ________

Holmes, 1948a. 

Huron Claims, Manitoba, Canada _________

Holmes, 1948b. 

Trent Range, Colo_______________________

George Phair (oral communication). 

Mountain Pass, Calif _ ______________ __

Jaffe, 1955. 

Goodhouse, Cape Province, South Africa____ 

Tilton and Nicolaysen, 1957. 

Bush veld complex, South Africa ___ _______

Tilton and Nicolaysen, 1957. 

Brown Derby mine, Gunnison, Colo- ______ _

Tilton and Nicolaysen, 1957. 

Van Rhynsdorp district, Cape Province, 

South Africa __ ____________________ _

Tilton and Nicolaysen, 1957. 

Yamba Lake, Northwest Territory, Canada. _ 

L. R. Stieff and T. W. Stern (oral communication) . 

Bikita district, South Rhodesia_ ___________

Holmes, 1954. 

Salisbury, South Rhodesia _ ______________

Holmes, 1954. 

Irumi Hills, South Rhodesia. __________ _ _

Holmes, 1954.

Total lead 

(ppm)

2,850 

3,720 

15,240 

1,485 

* 1, 130 

3,770 

5,800 

2,635

3,040 

6,810

3,450 

9,500 

5,800

Original lead

Parts per 

million

75 

63 

150 

19 

13 

38 

68 

22

202 

None

None 

200 

250

Percent

2.6 

1.7 

1.0 

1.3 

1.1 

1.0 

1.2 

0.9

6.6

2. 1 

4.3

' A new value of 1,740 ppm was determined by isotope dilution by Q. R. Tilton and L. R. Stiefl (oral 

 communication).

3. The allanites highest in activity (Conway granite and granite of 

Kubidoux Mountain, table 16) give ages closest to the probable 

geologic age and the zircon age. This would be expected because with increasing age, more radiogenic lead is produced 

and the original lead content becomes less significant. 

A high original lead content can also be expected in sphene which 

has a defective lattice, and is capable of being a host for a great variety 

of ions (Sahama, 1946). Magmatic apatite also has a great capacity 

for rare-earth ions and is a common host mineral for lead.

ACID LEACHING OF ZIRCON

Some of the zircon analyzed for lead-alpha age determination shows 

a surface coating or stain which generally can be removed by leaching 

in concentrated nitric acid or 1 to 1 aqua regia. Other samples of 

zircon concentrates contain an abundance of pyrite or molybdenite 

which concentrates with zircon. Prior to the development of an 

effective small laboratory flotation cell, the sulfides had been removed

507200 5S
38 STUDIES IN GEOCHRONOLOGY

TABU: 16. Comparison of lead-alpha ages of allanite, apatite, and sphene with

zircon

Rock type 

Field No. 

Locality 

Collector

Woodson Mountain granodiorite. S-2.

Southern California batholith.

E. S. Larsen, Jr.

Granite of Rubidoux Mountain.

Z-15.

Southern California batholith.

E. S. Larsen, Jr.

Quartz monzonite --_.- ____

G-21.

Southern California batholith.

E. S. Larsen, Jr.

Quartz monzonite.- _______

PB-4.

Sierra Nevada batholith,

near Bishop, Calif.

P. C. Bateman.

Con way granite- _____ ...

White Mountain plutonicvolcanic series, New

Hampshire.

A. Butler.

Barite-carbonate rock ______

SQ-13.

Associated with alkalic

syenite and shonkmite,

Mountain Pass, Calif.

H. W. Jaffe.

Shonkinite ____________

NM-1.

Mountain Pass, Calif.

T. B. Nolan and W. T.

Pecora.

Granodiorite ___________

HCD-62.

Gem stocks, Coeur d'Alene

district, Idaho.

A. B. Griggs.

Granodiorite ___________

HCD-63.

Gem stocks, Coeur d'Alene

district, Idaho.

A. B. Griggs.

Mineral

Allanite ...

Zircon....

Allanite _

Zircon..-.

Allanite _

Zircon....

Allanite _

Zircon.. _ .

Allanite 

Zircon....

 do ...

Allanite _

Apatite. ..

 do... _

Zircon _ .

 do- ..

Zircon

and

thorite.

Sphene ....

 do... 

Zircon....

e

2,085

2,485

2,085

2,485

2,085

2,485

2,085

2,485

2,085

2,485

2,485

2,085

2,485

2,485

2,485

2,485

O ftlQ /, Ulo

2,485

2,485

2,485

o per 

mg 

per hr

582

433

1,143

2,700

698

610

523

792

1,602

298

855

61

120

130

270

270

1 ±t 710 toy

143

80

292

Lead 

(ppm)

197

21

87

106

210

23

180

35

203

22.5

63.5

470

150

115

110

110

100

25

15

11

Leadalpha 

age (millions of 

years)

700

121

160

98

620

94

720

110

260

188

185

16,000

3,100

2,200

932

932

116

460

430

94

Probable geologic age

Early Late Cretaceous

(Larsen, 1948).

Do.

Mesozoic (Miller,

1946).

Late Jurassic (Hinds,

1934).

Mississippian(?)

(Billings, 1956).

Late Permian(?)

(Lyons and others,

1957).

Pre-Tertiary (Olson

and others, 1954).

Do.

Cretaceous or early

Tertiary (A. B.

Griggs, written communication, 1956).

Do.

by acid leaching. Apatite, normally removed in methylene iodide, 

may also concentrate with zircon either by being trapped by zircon in 

the liquid separation or by virtue of containing heavy ions such as Sr42 

and Ce*3 in substitution for Ca*2 in the lattice. In practice, apatite 

that does not float in methylene iodide is removed by acid leaching. 

Acid-leaching experiments were carried out on several samples 

of 80+200 mesh size crystals of both fresh and metamict zircon 

of different ages from several localities (table 17). Only small to
LEAD-ALPHA METHOD FOR DETERMINING AGES OF IGNEOUS ROCKS 39

TABLE 17. Lead-alpha ages of zircon before and after leaching with adds

Rock type 

Field No. 

Locality 

Collector

3469. 

Birch Creek, Ruby quadrangle, central Alaska. 

M. White and J. Stevens.

3881.

Mount Fairplay, Fortymile district, east-central 

Alaska. 

J. J. Matzkoand A. Nelson.

HJ-47-54. 

Gabbro-red rock complex 

near Mellen, Wis. 

H. L. James.

Wichita Mountains, Okla. 

F. L. Hess.

Hawkeye granite gneiss. ....... P-3.

Loon Lake quadrangle, 

Adirondack Mountains, 

N.Y. 

A. W. Postel.

Lyon Mountain granite gneiss. 

P-l. 

Dannemora, Adirondack 

Mountains, N.Y. 

A. W. Postel.

Olivine-quartz syenite .........

P-2. 

Ausable Forks, Adirondack Mountains, N.Y. 

A. W. Postel.

Granite _ - __________

EI-3. 

Bodom, Finland. 

Earl Ingerson.

Mineral

zircon.

... -do.........

.....do.........

a per mg 

per hr

1,342

1,322 

1,620

1,134

1,930 

1,476

1,270

2,600 

1,594

1,550 

103

101 

1,380

1,292

1,190 

1,168

185

187

215 

213

165

157 

150

146 

157

Lead 

(ppm)>

30

29 

68

45

72 

63

50

115 

72

62 

25

26 

352

350

343 

339

51

48,48,51,48 

(48)

60 

60

52, 53, 55, 56

(54) 

52

101

101 

103

Leadalpha age 

(millions 

of years)

56

55 

104

99

93 

106

98

110 

112

99

575

608 

568

600

637 

642

649

607

656 

660

761

770 

1,480

1,510 

1,450

Acid treatment

Boiling concentrated 

nitric acid 20 min.

Boiling 1 to 1 nitric 

acid 20 min. 

Untreated. 

Boiling 1 to 1 nitric 

acid 20 min. 

Boiling concentrated 

nitric acid 20 min. 

Untreated. 

Boiling 1 to 1 nitric 

acid 20 min.

Boiling concentrated 

nitric acid 20 min.

Boiling concentrated 

nitric acid 20 min.

Boiling 1 to 1 aqua 

regia, 20 min. 

Untreated. 

Boiling 1 to 1 aqua 

regia, 20 min.

Untreated.

Boiling concentrated 

nitric acid 20 min.

Untreated. 

Boiling concentrated 

nitric acid 20 min.

Untreated.

Boiling concentrated 

nitric acid 20 min.

Untreated.

Boiling concentrated 

nitric acid 20 min. 

Boiling 1 to 1 aqua 

regia 20 min.

1 The figures in parentheses are the averages used in deriving the calculated ages.

negligible changes could be detected in either the alpha activity or 

lead content of the zircon before and after acid leaching. A change 

was not considered significant unless it exceeded the analytical reproducibility of ±5 percent for the alpha activity or ±10 percent 

for the lead determinations.

A detailed leaching study was made of zircon from the granite from 

Mount Fairplay, Alaska (John Matzko, H. W. Jaffe, and C. L. Waring, 

written communication). Zircon from this rock was separated into 

three fractions having distinctly different magnetic susceptibilities.
40 STUDIES IN GEOCHRONOLOGY

Leaching of each fraction with nitric acid produced a 30 to 40 percent 

loss in alpha activity. The lead, however, showed a proportional 

decrease with the loss of alpha activity and the lead-alpha ages were 

basically unchanged after acid treatment, as shown in table 17. This 

indicated that neither uranium, thorium, nor lead had been selectively 

leached, but rather there had been solution of either metamict zones 

of zircon or thorite from the crystals. Inasmuch as the crystals were 

heavily stained before acid treatment, the identity of the soluble 

material was not established.

One sample of metamict zircon from the Wichita Mountains, Okla., 

showed a 7 percent loss in alpha activity after leaching for 20 minutes 

in 1 to 1 aqua regia. The lead remained virtually unchanged and the 

acid-leached sample therefore yielded an age about 7 percent older 

than the untreated zircon. Another specimen of metamict zircon 

from the same locality showed no significant change after acid leaching.

Metamict zircon from granite from Tory Hill showed a 10 percent 

loss in alpha activity after acid treatment. A lead value lower by 3 percent is within the limit of analytical error and may not be considered 

significant. The acid-leached zircon therefore showed an age about 

7 percent higher than the unleached sample.

Other samples of both fresh and metamict zircon, ranging from 

48 to 343 ppm lead and in activity from 101 to 1,190 alpha per milligram per hour showed no loss of either alpha activity or lead content 

after acid treatment for 15 minutes in boiling concentrated nitric acid 

or aqua regia.

The results, therefore, indicate that acid leaching of zircon may 

produce a 5 to 10 percent loss in alpha activity in some samples of 

metamict zircon and no measurable loss in fresh zircon. Selective 

loss of lead could not be demonstrated in any of the samples. Acid 

treatment of fresh zircon will therefore not affect the lead-alpha age; 

acid treatment of metamict zircon may cause an increase in the age by 

5 to 10 percent in some instances and no change in others.

It should be borne in mind that all varieties of metamict zircon 

do not fall into one category. Kostyleva (1946), has recognized two 

types of metamict zircon as follows :

1. The metamict state without the entry of water and with almost the 

theoretical composition of zircon.

2. The metamict state with the entry of water and with deviations 

from the composition of zircon. This group would include such 

varieties as cyrtolite.

All of the metamict zircon on which acid-leaching studies were made 

(table 17) fall into Kostyleva's first category. Cyrtolite and other 

metamict varieties of Kostyleva's second type might be subject to 

a much larger alpha activity loss and lead loss from acid leaching.
LEAD-ALPHA METHOD FOR DETERMINING AGES OF IGNEOUS ROCKS 41

Very little data on acid-wash experiments on zircon are reported 

in the literature. Tilton (1956) reported a lead loss of 15 percent on 

a zircon from a granite from Tory Hill, Ontario, after boiling the 

powdered mineral for 15 minutes in 1 to 1 aqua regia.

INTERNAL. CONSISTENCY OP THE METHOD

LEAD-ALPHA AGES OF MONAZITE, XENOTIME, AND THORITE .

Although most of the lead-alpha ages have been determined on 

zircon, on many occasions it has been possible to obtain ages on two or 

three different minerals from the same rocks. Inasmuch as the isotopic 

data indicate that the amount of original lead in monazite is negligible 

this mineral should be suitable for the lead-alpha method. No data 

are available on the isotopic composition of lead in xenotime.

Several igneous rocks ranging in composition from diorite to 

granite contained from about 1 to 10 ppm thorite. This thorite is 

generally isotropic or has low birefringence with an average index of 

refraction of about 1.76. Because it is heavy (density >3.2) and 

nonmagnetic it is concentrated with the zircon in the final separation 

product. Thorite is readily soluble in acid and can be removed easily 

from the zircon separate. Natural mixtures of these two minerals 

have shown ages that are in close agreement with the ages obtained on 

zircon after leaching the thorite. A few isotopic analyses of thorite 

indicate the common lead content is small (Nier, 1939).

A comparison of ages obtained on two or more of these minerals 

separated from the same rock, ranging from Tertiary to Precambrian 

in age, is given in table 18. The close agreement of ages on these different suites of cogenetic minerals containing widely different amounts 

of lead and alpha activity and different proportions of uranium and 

thorium demonstrates the internal consistency of the method.

ZIRCON OF DIFFERENT LEAP CONTENT AND ALPHA ACTIVITY

Zircon concentrates separated from the same rock often contain fresh 

and metamict varieties of zircon that contain widely different amounts 

of lead and radioactivity. Although zircon is ordinarily nonmagnetic 

on the Frantz separator, some varieties of zircon can be concentrated 

in different magnetic fractions. When both varieties are present in a 

concentrate or a single crystal, two or more age measurements can be 

made on samples that contain different amounts of lead and alpha 

activity. This technique was used by Larsen, Waring, and Berman 

(1953) on zircon in pegmatite from Oklahoma that contained both 

fresh and metamict zones in the same crystal. Lead-alpha activity 

ratios on the fresh and metamict zircon that contained about 40 and 

350 ppm lead, respectively, showed ages that agreed to within ±10 

percent. Lead-alpha age measurements also agreed to within ±10
42 STUDIES IN GEOCHRONOLOGY

TABLE 18. Comparison of lead-alpha ages of monazite, teenotime, thorite, and

zircon

Rock type 

Field No. 

Locality 

Collector

Lov-1.

T. S. Lovering. 

Diorlte.... ...... _ ... _.-

Z-24.

Clayton Peak stock, near 

Brighton, Utah. 

M. Crittenden.

630-210.

Lost Horse Creek, near 

Idaho-Montana State line. 

R. W. Chapman.

CPR-122.

Lost Horse Creek, near 

Idaho-Montana State line. 

B. F. Leonard and 0. P. 

Ross.

CPR-123.

Idaho-Montana State line. 

B. F. Leonard and C. R. 

Ross.

Alaskite _____________

52C-8.

Boulder batholith, Montana. 

R. W. Chapman.

PB-3.

Sierra Nevada batholith, 

near Bishop, Calif. 

P. C. Bateman.

63 PB-10.

Yosemite National 

Park, Calif. 

P. C. Bateman.

L-113.

Idaho batholith, near 

Stanley, Idaho. 

E. S. Larsen, Jr.

L63-573a.

batholith, Idaho. 

B. F. Leonard.

Q-199. 

Idaho batholith, 15 miles

ley, Idaho. 

David Qottfried.

Woodson Mountain granodiorite. 

Z-16.

Lescanso Junction, southern 

California batholith. 

E. S. Larsen, Jr.

Woodson Mountain granodiorite. S-6.

Northeast of Descanso Junction, southern California 

batholith. 

E. S. Larsen, Jr.

Mineral

Monazite. ..

Zircon .......

Thorite.....

Zircon _ ....

Monazite. - .

Zircon .......

Monazite ... .

Zircon ......

Thorite.....

Thorite- ....

Thorite.. 

Monazite --.

Zircon _ . ...

Xenotime... 

Zircon .......

Xenotime.-.

Monazite.-. 

Zircon _ _ ..

a, per

mg. per 

hr.

7,657

1,920

5,795

3,330

. 3, 213

275

. 3,386

257

2,925

262

6,733

4,990

4,670

400

10, 370

330

1,375

825

2,726

340

6,617

6, 025 

1,970

6,400 

1,235

6,430 

1,180

Lead 

(ppm)i

55,58

(56. 5) 

15, 15, 16, 16

(15.5) 

115

65

79

6.2

90,90

(90)

6.0,7.0 

(6.5)

80,80

(80) 

5. 0, 6. 0

(5.5) 

231

127

205

15

454

15,16

(15.5) 

70

30

144,148

(146) 

13, 15

(14) 

250, 256

(253) 

220 

90

260 

50

360 

46

Leadalpha age 

(millions 

of years)

15

20

40

48

51

56

55

63

57

63

72

63

88

93

88

117

102

90

112

102

93

93 

114

104 

101

117 

97

Probable geologic age

Unknown (T. S.

Lovering, written 

communication,

1954). 

Unknown (M.

Crittenden, written

communication, 

1966).

Cretaceous (Ross,

1952).

Do.

Cretaceous (Ross,

1952).

Late Cretaceous or

early Tertiary

(Knopf, 1957). 

Late Jurassic (Hinds.

1934).

Do.

Cretaceous (Ross and

Forrester, 1947).

Do.

Do.

Early Late Cretaceous 

(Larsen, 1948).

Early Late Cretaceous 

(Larsen, 1948).
LEAD-ALPHA METHOD FOR DETERMINING AGES OF IGNEOUS ROCKS 43

TABLE 18. Comparison of lead-alpJia ages of monazite, xenotime, thorite, and

zircon Continued

Rock type 

Field No. 

Locality 

Collector

Bethlehem gneiss ................ NH-S-4-52-1.

Hampshire plutonic series, 

New Hampshire. 

J. B. Lyons.

Pegmatite intrusive into Toluca 

quartz monzonite. 

490t-16. 

Cleveland County, N.C. 

W. C. Overstreet.

Pegmatite intrusive into Toluca 

quartz monzonite.

530t-14. 

Cleveland County, N.C. 

W. C. Overstreet.

Granodiorite gneiss _______

54J-1.

U.S. Highway 211, 1.6 miles

west of Sperryville, Va. 

H. W. Jaflfe.

Pegmatite ____________

62081.

Tambane, Nyasaland, Africa. 

C. E. Tilley.

Pegmatite. ___________

Co-R-Goo. 

Greenville, Routt County, 

Colo. 

R. S. Cannon.

Mineral

Monazite. -_ 

Xenotime .

Monazite. -. 

Zircon. ______ 

_. do____ 

Monazite... 

Zircon ___

 _-do.__._-

Zircon .......

_. do--__._.

Xenotime _

a, per

mg. per 

br.

2,922 

4,063 

242

5,685 

456 

452

5,464 

652

8,786

686

854

1,634

185

375

1,749

2,390

Lead 

(ppm)J

470 

530 

30

1,050 

81 

82

1,000 

124

2,000

135

160

430

47

85

1, 250, 1, 270

(1,260) 

1, 550, 1, 560 

(1,555)

Leadalpha age 

(millions 

of years)

342 

266 

297

380 

427 

435

377 

456

468

470

449

539

600

538

1,430

1,420

Probable geologic age

Late Devonian (?) 

(Billings, 1956).

Paleozoic (Griffltts 

and Overstreet, 

1952).

Paleozoic (Griffltts and 

Overstreet, 1952).

Precambrian (J. C.

communication,

1955). 

Late Precambrian (C.

E. Tilley, written

communication,

1953). 

Precambrian (R. S.

Cannon, written 

communication, 

1957).

1 The figures in parentheses are the averages used in deriving the calculated ages.

percent on a suite of 21 gem-quality crystals of zircon from Ceylon 

which ranged from 22 to 500 ppm of lead (Gottfried, Senftle, and 

Waring, 1956). Some of the data of these investigations and other 

similar examples are given in table 19.

In addition to obtaining different varieties of zircon on the basis of 

magnetic susceptibility, sieving the concentrate into different size 

fractions has been used as another means for obtaining samples of 

zircon that are different in alpha activity and lead content. Zircon 

crystals range in size from 80 mesh to less than 400 mesh and usually 

are liberated from the rock during crushing as entire crystals. In 

general the smaller zircon crystals contain either the same or greater 

amounts of radioactivity than the larger zircon crystals from the same 

rock. Effects of alteration by solutions, metamorphism, or diffusion 

of lead would be much greater in minute zircon crystals that have a 

high lead content and that have undergone extensive radiation damage. 

Close agreement in age between different size fractions has nearly 

always been obtained on zircon from rocks ranging in age from Tertiary to Precambrian. The data are given in table 20.
44 STUDIES IN GEOCHRONOLOGY

TABLE 19. Comparison of lead-alpha ages of zircon fractions of different radioactivity and lead content from the same rock

Rock type 

Field No. 

Locality 

CoDector

LS-7-55. 

North La Sal Mountains, Utah. 

D. R. Shawe.

3881. 

Mount Fairplay, Fortymjle district, east-central Alaska. 

J. J. Matzko and A. Nelson.

White. Mountain plutonic-volcanic series, Lake Wlnnepesaukee quadrangle. New Hampshire. 

H. W. Jaflfe and J. B. Lyons.

Scituate granite gneiss __ ___

Q-53-37. 

North Scituate and Georgiaville 

quadrangles, Rhode Island. 

A. W. Quinn.

Gem gravels, derived from pegmatite. 

Ceylon.

AND-3. 

Wet Mountains, Coster County, 

Colo. 

Q. D. Singewald.

Nepheline syenite.. _________

62117. 

Nyasaland, Africa. 

C. E. Tilley.

EI-3. 

Bodom, Finland. 

Earl Ingerson.

1203. 

McGrath, Minn. 

R. G. Schmidt.

Pegmatite __ ______ . .......

Wichita Mountains, Okla. 

F. L. Hess.

Mineral

. do......

-..do......

....do......

....do...... 

....do......

.... do......

 .do......

... -do......

.... do......

a per ing 

per hr

3,180

3,720 

1,134

1,270 

1,550

260

650 

251

352

103 

273 

643

882 

1,185 

1 245 

1,815 

2,210

319

452 

770 

1,060 

1,680 

3,520

344

358 

380

489

146

184 

318

353

168

1,180 

1,190 

1,230 

1,380

Lead 

(ppm)i

70,70

(70) 

75,79 

(77)

45

50 

62

18,20

(19) 

46,48 

(47)

35

44

22 

65 

150 

205 

275 

270 

450 

498

78

115 

192 

290 

430 

980

75

85 

86 

105

101

120 

228 232

(230) 

255,260 

(257)

43

340 

343 

360 

352

Leadalpha age 

(millions 

of years)

55

51 

99

98 

99

182

180 

337

303

532 

590 

578 

576 

576 

538 

613 

560

580

601 

590 

644 

605 

655

519

562 

637

512

1,510

1,440 

1,560

1,570 

579

638 

637 

648 

567

Probable geologic ago

Tertiary (Waters,

1955). 

Mesozoic(?) (Mertie,

1937). 

Mississippian(?)

(Billings, 1956). 

Late Permian(T) 

(Lyons and others, 

1957).

Pre-Pennsylvanlan,

(Devonian? or older). 

(Quinn and others, 

1957).

Precambrian (Wadla 

and Fernando, 1944),

Later than Precambrain

regional metamorphism; upper age limit 

is not known (Q. D, 

Singewald, written 

communication, 

1956).

Late Precambrian

(C. E. Tilley, 

written communication, 1954).

Kahma, written 

communication, 1955),

,1949).

(Taylor, 1915).

»The figures in parentheses are the averages used in deriving the calculated ages.
LEAD-ALPHA METHOD FOR DETERMINING AGES OF IGNEOUS ROCKS 45

TABLE 20. Comparison of lead-alpha ages of zircon fractions of different crystal

size from the same rock

Rock type 

Field No. 

Locality 

Collector

Quartz latite porphyry

Plutonic.

GL-6.

San Juan Mountains, Colo.

David Gottfried

and George Phair.

Oranodiorite 

ELS-2.

Valles Mountains,

N.Mex.

E. L. Smith.

Monzonite 

RLS-1. 

Cerrillos Hills, N.

Mex.

E. L. Smith.

Quartz monzonite _ - 

HTM-1.

Silver City stock,

Tintic district,

Utah.

H. T. Morris.

Porphyritic potassic

syenite. P-50-49.

Bearpaw Mountains, Mont.

W. T. Pecora.

Oranodiorite .

JPO-1.

Batholith, Limones, Puerto Rico.

J. P. Owens.

Isabella granodiorite , EMM-1.

Kern River area,

California.

E. M. MacKevett.

Syenite.-. - 

VT-Sy-56 J-12.

White Mountain

plutonic-volcanic

series, Mount

Ascutney, Vt.

H. W. Jafle and

J. B. Lyons.

Mount Osceola granite..

99 BNH-67A.

White Mountain

plutonic-volcanic

series, Franconia

quadrangle, New

Hampshire.

A. Butler.

Cowesett granite ___ - Q-48-3.

Cowesett, R.I.

A. W. Quinn.

Mineral

Zircon __

...do.... 

...do.......

... do... 

... do.......

... do.......

...do.... 

 do.......

...do.... 

 do.... 

Mesh size

-80+200

-200+400

-80+400

-400

-80+400

-80+200

-200+400

-400

-100+200 

-200+400

-80+100

-100+200

-80+200

-200+400

-80+200

-200+400

-400

-80+200

 200+400

-80+200

-200+400

-80+200

-200+400

-400

a per mg 

per hr

56

58

424

547

760

770

780

840

187 

223

290

275

141

212

283

320

351

166

214

446

508

134

217

322

Lead 

(ppm)»

0.4

0.4,0.6

(0.5)

4.0,4.0

(4.0)

3.0,4.0

(3.5)

13, 14. 15

(14) 

9.11

00}

10,12

(11)

14, 15, 17

(15. 5)

3.7

4.5

5. 2, 5. 6

(5.4)

5. 0, 5. 1

(5. 05)

3. 0, 3. 0

(3.0)

4. 0, 5. 0

(4.5)

9,10

(9.5)

11,12

(11. 5)

13,14

(13. 5)

11,12

(11- 5)

20,17

(18. 5)

33,37

(35)

38,39

(38.5)

14.16

(15)

24,25

(24)

33,35

(34)

Lead 

alpha age 

(millions 

of years)

17

21

23

16

46

32

35

46

49 

50

46

46

53

53

83

89

96

172

211

195

188

272

272

257

Probable geologic age

Miocene (Larsen and

Cross, 1956).

Tertiary (C. S. Eoss

and R. L. Smith,

written communication, 1956).

Oligocene (post-Espinaso volcanics, 

pre-Abiquiu(?)

formation) (Stearns,

1953).

Middle Eocene 

(Green River)

(H. T. Morris and

T. S. Levering,

written communication, 1956).

Middle Eocene

(Green River)

(Brown and

Pecora, 1949).

Eocene (Kaye, 1957).

Late Jurassic(?)

(Hinds, 1934).

Mississippian(?)

(Billings, 1956).

Late Permian(?)

(Lyons and others,

1957).

Mississippian(?)

(Billings, 1956).

Late Permian(?)

(Lyons and others,

1957).

Mississippian(?)

(Quinn and others)

1957.

See footnote at end of table.
46 STUDIES IN GEOCHRONOLOGY

TABLE 20. Comparison of lead-alpha ages of zircon fractions of different crystal 

size from the same rock Continued

Rock type 

Field No. 

Locality 

Collector

Winnepesaukee quartz 

diorite. NH-WQd-56-J-16. 

Lake Winnepesaukee quadrangle, 

New Hampshire. 

H. W. Jaffe and 

J. B. Lyons.

AB-2. 

Ripogenus dam, 

Maine. 

A. J. Boucot.

AB-6. 

Pocologan Harbor, 

Charlotte County, 

New Brunswick, 

Canada. 

A. J. Boucot.

SQ-1. 

Giant's Range near 

Mount Iron, 

Minn. 

Q. M. Schwartz.

M-3655. 

Saganaga batholith, Sea Qull 

Lake, Cook 

Co'inty, Minn. 

Q. M. Schwartz.

Mineral

Zircon..... 

...do.......

 do.......

...do.......

_.-do.-..___

Mesh size

-80+200 

-200+400

-80+200

-200+400 

-80+200

-200+400 

-400

-80+200

-200+400

-200+400

a per mg

per hr

353 

391

212

230 

164

229 

232

393

420 

63

62

Lead 

(ppm)i

38,40 

(39) 

45,47 

(46)

28

30 

24,26

(25) 

34,33 

(34) 

34,36 

(35)

233,236

(235) 

229, 233 

(231)

57, 57, 59

(58) 

53,54 

(54)

Lead 

alpha age 

(millions 

of years)

269 

285

320

316 

368

354 

364

1,310

1,220 

1,900

1,800

Probable geologic age

Late Devonian(?) 

(Billings, 1956).

(Boucot, 1954). 

Pre-Middle Silurian(?) (A. J. 

Boucot, written 

communication, 

1956).

goman?) (Q. M. 

Schwartz, written 

communication, 

1956).

rentian) (G. M. 

Schwartz, written 

communication, 

1956).

i The figures in parentheses are the averages used in deriving the calculated ages.

COMPARISON WITH OTHER METHODS

Independent evidence concerning the reliability of the lead-alpha 

method can be obtained in two of the following ways:

1. Comparing the lead-alpha age with the geologic age of igneous 

rocks whose position in the geologic time scale is fairly well 

known on the basis of stratigraphic-paleontologic evidence. In 

this manner it can be determined whether the age measurements 

are correct with regard to their relative value.

2. Comparing the lead-alpha ages with concordant ages obtained by 

isotopic Pb206 : U238, Pb207 : U235, Pb207 : Pb206, Sr87 : Kb87, and 

Ar40 : K40 methods. These comparisons would give information 

as to the absolute value of the ages inasmuch as different minerals containing different parent and daughter isotopes are used 

in two of the other methods.
LEAD-ALPHA METHOD FOB DETERMINING AGES OF IGNEOUS ROCKS 47

LEAD-ALPHA AGES OF SOME IGNEOUS BOCKS YOUNGER THAN

PRECAMBRIAN

In order to compare lead-alpha ages with those obtained by geologic 

methods many age measurements have been made on minerals from 

suites of igneous rocks whose age can be assigned to different parts 

of the geologic time scale after the Precambrian. Within this segment 

of geologic time, relative geochronology has been established primarily 

by stratigraphic-paleontologic methods. Lead-alpha ages on some 

series of rocks ranging in age from late Tertiary to early Paleozoic 

are listed in table 21. In several localities the geologic time relations 

between fossiliferous sediments and the igneous rocks are accurately 

known within fairly narrow limits. For most of the other igneous 

rock series, the assigned geologic ages are probably correct. Generally, the average ages obtained for a related series of igneous rocks by 

the lead-alpha method fall in the same sequence as the geologic age of 

the rocks and are correct as to relative value.

Those localities containing rocks that are suitable for establishing 

physical time points in the geologic column are as follows:

1. San Juan Mountains, Colo. Extrusive and intrusive rocks of the 

Potosi volcanic series and Fisher quartz latite are of Miocene 

age on the basis of fossil plants and insects from the Creede 

formation (Larsen and Cross, 1956).

2. Cerillos Hills, N". Mex. Stocks intrude the Galisteo and Espinaso 

formations of late Eocene age and are overlain by Abiquiu (?) 

formation of early Miocene age (Stearns, 1953).

3. Tintic district, Utah. Plant fossils of Green Eiver age (middle 

Eocene) are found in fresh-water limestone interlayered with 

agglomerates which are correlated with the intrusive rocks 

(T. S. Lovering and H. T. Morris, written communication, 

1956).

4. Bearpaw Mountains, Mont. Volcanic rocks overlie the Wasatch 

formation (early Eocene) and are interlayered with tuffaceous 

sediments containing plant and vertebrate fossils of Green 

River age (middle Eocene) (Brown and Pecora, 1949).

5. Alberta, Canada, Kneehills tuff lies in the upper part of the 

Edmonton formation, which contains a dinosaurian fauna, of 

Maestrichtian age (Late Cretaceous), and is overlain by 

Paskapoo beds of Tertiary (probably Paleocene) age (Folinsbee and others, 1957).

6. Baja California. Granitic rocks intrude the San Telmo formation of Albian age (late Early Cretaceous) and are overlain 

by the Eosario formation of Maestrichtian age (Late Cretaceous) (Silver, and others, 1956, and Woodford and Harris, 

1938).
48 STUDIES IN GEOCHRONOLQGY

7. New Hampshire plutonic series, New Hampshire. Intrusive 

rocks are younger than the Littleton formation (Devonian) 

and older than the Moat volcanics of possible Mississippian age 

(Billings, 1956).

8. St. George, New Brunswick, Canada. Granite intrudes the Eastport formation (Late Silurian and possibly Early Devonian) 

and is overlain unconformably by the Perry formation which 

contains Late Devonian fossils and cobbles of the granite.

9. Oliverian plutonic series. Intrusive rocks are younger than 

Clough quartzite (Early or Middle Silurian) and may be 

younger than the Littleton formation (Middle Devonian) 

(Billings, 1956).

10. Highlandcroft plutonic series, New Hampshire. Intrusives are 

younger than the Ammonoosuc volcanics (Late Ordovician?) 

and older than the Fitch formation (Middle Silurian) (Billings, 1956).

11. Strasburg, Virginia. Tuff 7 feet above base of Martinburg shale 

of Middle and Late Ordovician age.

The average lead-alpha age on accessory minerals of rocks from 

these localities are plotted in figure 6 on the Holmes' B time scale 

(Holmes, 1947). Considering the uncertainty as to the geologic age 

of some of the minerals used by Holmes and the uncertainties inherent in the lead-alpha method, the agreement is highly satisfactory.

The experimental data from which all the lead-alpha ages have 

been calculated are given by Jaffe and others (1959). The geologic 

significance of the -age measurements for the rocks of New England 

are discussed in detail by Lyons and others (1957), Quinn and others 

(1957), and for the Mesozoic batholiths of western North America 

by Larsen and Schmidt (1958) and Larsen and others (1958).

LEAD-ALPHA VERSUS ISOTOPIC LEAD AGES

Application by Tilton and others (1957) of the isotope dilution 

method of analysis for the determination of small amounts of uranium, 

thorium, and lead has made possible the determination of ages on 

zircon from the ratios Pb206 :U238, Pb207 :U235, Pb208 :Th232, and 

Pb207 : Pb206. Comparisons of lead-alpha and isotopic ages obtained 

either on the same material or on samples from related rocks are given 

in table 22. Ages obtained from the isotopic ratios are frequently 

discordant and usually show the pattern Pb208 : Th232<Pb206 : IP38 

<Pb207 : IP^Pb207 : Pb206. When these ages agree to within some 

specified limit, about ±10 percent of each other, they are said to be 

concordant. An example of concordancy in table 22 is shown by the 

excellent agreement of the four isotopic ages obtained on zircon from 

Ceylon and a typical discordant pattern is shown by the isotopic data
LEAD-ALPHA METHOD FOR DETERMINING AGES OF IGNEOUS ROCKS 49

CARBONIFEROUS

Juan Mountains, Colo.

kCerriltos Hills, N. Mex.

^Bearpaw Mountains, Mont, and Tintic, Utah 

t Central City, Colo.

Kneehills tuff. 

Alberta, Canada 1 Baja, Calif.

EXPLANATION

 

Lead-alpha age

x 

Holmes'age

Spinelli quarry, Conn. New Hampshire plutonic series, N. H. 

. St. George, New Brunswick. Canada 

Oliverian plutonic series, N. H.

Bedford, N.Y.* 

Martinsburg shale, Strasburg, Va.*

Highlandcroft plutonic serk 

N. H.

200 300 

MEASURED AGE. IN MILLIONS OF YEARS

FIGURE 6. Lead-alpha ages of accessory minerals from igneous rocks of known geologic 

age compared with Holmes' B time scale.

on zircon from the Pikes Peak granite of Colorado. The Pb208 : Th232 

ages are often considerably low even when the other three ages are 

in close agreement as is shown for the zircon from the granite from 

Tory Hill, Canada. Discordant ages are most commonly attributed 

to the following causes:

1. Differential loss or gain of uranium, thorium, and lead.

2. Presence of common and old radiogenic lead.

3. Loss of radon.

4. Errors in experimental techniques.

Any of these causes will have effects of different orders of magnitude 

on a given lead ratio or discordant age pattern; they are discussed 

in detail by Kulp and others (1954), Kulp and Eckelmann (1957), 

Stieff and Stern (1956), and Faul (1954). The effect of recent alteration causing a loss of lead, uranium, thorium, or radon is much less 

for the Pb207 :Pb206 ratio than for the Pb206 :U238 and Pb208 :Th232 

ratios. However, the Pb207 : Pb206 ratio is most sensitive to mass 

spectrometric errors and to the presence of original lead. When the 

Pb207 : Pb206 ages are greater than the Pb206 : U238 ages the pattern can
50 STUDIES IN GEOCHRONOLOGY

TABLE 21. Lead-alpha ages of some igneous rocks younger than Precambrian

Locality

formation).

Tintic, Utah.. ._ -_ .. 

Southern California batholith, California. 

Sierra Nevada batholith, California _

Coast Range batboliths, Washington, 

British Columbia, and Alaska. 

White Mountain plutonic volcanic 

series, New Hampshire. 

Westerly granite and related rocks, 

Rhode Island. 

Quincy granite and related rocks, 

Rhode Island and Massachusetts. 

Scituate granite gneiss and related 

rocks, Rhode Island. 

New Hampshire plutonic series, New 

Hampshire.

Canada.

Oliverian plutonic series, New Hampshire. 

Littleton, New Hampshire ______

Highlandcroft plutonic series, New 

Hampshire.

Number 

of rocks

1

8

1

3

1

1

2

1

2

7

6

25 

15 

17

16 

5 

6 

2 

4 

8 

1

1

5

1

1

4

Number 

of determinations

1

10

1

1

3

1

4

4

2

3

1

8

6

27 

20 

21

23 

12 

6 

5 

5 

12 

1

2

5 

1

1

4

Range 

(millions 

of years)

10-24

19-28

32- 46

39- 50

46- 46

53- 58

63- 76

96-112

92-136 

81-123 

90-135

92-134 

171-212 

208-274 

257-275 

289-337 

266-346

316-320

287-337

368-425

Mean 

age and 

standard 

deviation 

(millions 

of years)

15

18± 5

18

23

23± 5

37

40rb 7

45± 5

46± 0

55± 2

53

71± 4

101± 5

110±13 

102±11 

108±12

105±13 

1S6±14 

234±23 

270± 7 

306±18 

296±29 

301

318± 2

311±26 

298

353

385±27

Probable geologic 

age

Pliocene(7).

Miocene.

Do.

Do.

Oligocene or

Miocene. 

Do.

Middle Eocene.

Do.

Eocene (?).

Late Cretaceouspre- Oligocene.

Cretaceous. 

Do.

Late Jurassic or 

Cretaceous.

Late Jurassic or 

Cretaceous. 

Mississippian(?) .

Late or postCarboniferous. 

Mississippian(?).

Devonian (?) or 

older. 

Late Devonian(7).

Post Early

Devonian. 

Middle or Late 

Devonian (?). 

Early and Middle

Devonian.

Ordovician. 

Late Ordovician.

be attributed to either the loss of lead or radon or the gain of uranium, 

provided original lead is not a major factor. Thus, a single discordant age pattern provides no information as to which of the causes 

has played a predominant role.

The amount of uranium in zircon is, in general, nearly equal to or 

else greater than the amount of thorium present. Inasmuch as a given 

amount of uranium produces about four times the alpha activity as the 

same amount of thorium, most of the radiogenic lead in zircon is 

Pb206. Thus, the lead-alpha ages can be considered essentially 

Pb206 : U238 ages. Close agreement between lead-alpha and Pb206 : U238 

ages has been obtained on most zircon samples from Conway, N.H.; 

Ceylon; Wichita Mountains, Okla.; Pikes Peak, Colo.; Llano County, 

Tex.; Tory Hill, Canada; and Bodom, Finland. The wide discrepancy between the lead-alpha and the Pb206 : U238 age on the zircon
TABLE 22. Comparison of lead-alpha ages with 

Locality

Boulder batholith, Montana.

Southern California batholith.

Pala . _

Baja California-----.----..

Sierra Nevada batholith, 

California. 

Yosemite National 

Park.

Coast Range batholiths. 

Washington: 

Colville batholith.. 

Chelan batholith .......

Rock type

Bentonite above tuff. . 

Quartz monzonite __ 

Alasklte -

Tonalite to granite. ...

La Grulla granodiorite.

Gateway granodiorite. 

El Capitan granite.---

Half Dome quartz 

monzonite.

Johnson granite porphyry. 

Cathedral Peak granite.

Quartz diorite and 

granodiorite. 

-....do....... . .--

Minerals

do

 ..do.... 

 do.. 

do

Zircon....... .

Biotite . -

Blotite . -

Thorite. __ ------

Biotite-- - .

 ..do.... ... - -

Age in m

Leadalpha 

(mean 

age)

53

71 

72

109

117 

110

99 

103

94

117 

88

95 

113

Pb*>«:

UM8

115

Pbsor:

UMS

115

Pb

T
TABLE 22. Comparison of lead-alpha ages with isotop

Locality

British Columbia: 

Similkameen batholith.

Bayonne batholith 

Batholith near Vancouver. 

Yukon Territory: 

Itsi batholith.-------

White Mountain plutonicvolcanic series, New 

Hampshire.

Wichita Mountains, Okla...

Central City, Colo., Lillian 

mine.

Natural Bridge, N.Y 

Tory Hill, Ontario _ ......

Sharp Mountain mass..

Enchanted Bock pluton 

Legion Creek mass _ ..

Kock type

Quartz dlorite and 

granodiorite. 

_..-.do... .. _ ...

.... .do... 

- .do.. . ..

Pegmatite(?) ..........

Limestone-syenite 

contact.

 .do . 

_ ..do..... _ .........

 .do... 

Minerals

-.-..do . _ ......

Biotite ... ......

 ..do..... ..

...-.do..... ... ..

.....do. .........

Biotite 

..... do.............

Biotite ......

 ..do...... ....

Zircon (fresh) Zircon (metamict).

Biottte. ___ .....

.....do......... ....

 do. _ .

 ..do.. ... .....

.....do... ..........

 do.. ... .

... ..do ... .....

 do..... .....

..... do ..... ...

... .-do........... ..

Age in mil

Leadalpha 

(mean 

age)

114 

108

188

320

670

600

630 

630

675

1,250

710

065

865 

880

845 

830 

9.15

pbSOS;

u»«

187

540

514

520

624

1,340

1,025

1,030

Pb»7:

U»»

184

544

522

527

707

1,345

1,065

1,050

Pb«

Th

1
LEAD-ALPHA METHOD FOR DETERMINING AGES OF IGNEOUS ROCKS 53

O* ^i O1 "

 R" C"

S "2 °<§ *3 <§

o>. o;-' && a--' a

P S P P !

ia S » » <

^S x® x.Sx.2 x' !» ' Os" d oa^c^-^ S"

? « 8 E E m K> (

2

QfCl G

S feo S? So^SXS° .? fe & B^.fesfc® Bfcrf

° 5-0 S-o gS-g g-o 3| S'ggS^! S §

^Q ofi frtA *rtft S Tlft SSftft SQ °3O H "j-^ flQ mM P

S 

r . 

^3 03 rw ^*0 .<»rH 5^'S X^ C8rK 5*0 X^ r*« ^^ 03rt« ^^§^0^5 CO _ " w rK »t* S W^s'B

8

!

o

1

1

§o=

1 1

""If

MS

c c

T >C

lith. 

Petrick quarry ......... Mel1(vn. Win. -- 

§§

f 'f

i

fH

ll I

WE^ s

c

Minnesota:

c 

5

* 

1

1 f=

8

8

4*

S 

n

!

i i

j j

coot

1

;

i

! i

i

i Si

!-

<

8

r1

c «

Cloud St. 

Babbitt.. . 

»

SIT

p>

n,<of 

» *j *

c c

5 

(i

!

*

n 

T

!

C

r-

'il

Wts

C>c

i 

j

4 F

^ 5

!

1 ll

PQ Nl5

o

\ ll

J ^0

ffi

oo

c

3

fi 

s5

t

Muscovite.. 

6

i

!

!

i

-

si

Copperlifls rhyolite. 

Granite __ 

Canada..... Ontario, Bodom, Finland __ 
54 STUDIES IN GEOCHRONOLOGY

from Natural Bridge, N".Y., is due to a discrepancy in the results of the 

lead analyses of the sample. A content of 127 ppm of lead was obtained by Tilton by isotope dilution as against 66 ppm determined 

repeatedly by emission spectrographic analyses on splits of the same 

material. Further work is in progress to determine the cause of this 

discrepancy. The lead-alpha age on monazite from a granitic rock 

from Baja California is in satisfactory agreement with the preliminary 

isotopic lead-uranium ages reported to the writers by L. T. Silver 

(California Inst. Technology, written communications). In order to 

make a proper comparison between the two methods the Pb208 : Th232 

age is necessary. This determination has not yet been made.

A Precambrian syenite from the Lillian mine near Central City, 

Colo., contained both zircon and uraninite. There can be little doubt 

that these minerals are cogenetic inasmuch as crystals of uraninite are 

present as inclusions in many zircon crystals (plate 1). The leadalpha age obtained on hand-picked zircon crystals free of uraninite, 

agreed to within 10 percent of the concordant isotopic ages on uraninite obtained by George Phair (written communication).

Tilton and others (1957) have shown that isotopic ages on most 

zircon containing significant amounts of original lead are badly discordant and that concordant ages were obtained on nearly all samples 

for which original lead was not detected. However, for most zircon 

samples the lead-alpha ages, which are based on total lead content, are 

in close agreement with the Pb206 : U238 ages for which corrections for 

original lead have been applied. These checks indicate the relatively 

minor effect of the presence of original lead on the lead-alpha ages, and 

that if the presence of original lead were the major cause for discordant 

ages in zircon, then the Pb207 : Pb206 ratios would show ages much 

greater than the probable true age. The difficulties in interpreting 

discordant age patterns have yet to be resolved and the choice of any 

of the individual ages will continue to be largely speculative unless 

independent evidence is brought to bear on the problem.

LEAD-ALPHA VERSUS Ar*°: K40 AND Sr87 :Bb87 AGES

The decay of the radioactive istopes of the alkali metals K40 and 

Rb87 to Ar40 and Sr87, respectively, provides methods for measuring 

the age of potassium and rubidium bearing minerals. The decay 

schemes are as follows: K40 decays both by beta emission to Ca40 and 

by electron capture to Ar40 ; Rb87 decays by beta emission to Sr87. 

Recent measurements of the decay constants used for the Ar40 : K40 

ages have been published by Wetherill and others (1956), and for the 

Sr87 :Rb87 ages by Aldrich and others (1956). They were obtained 

primarily by comparing the Ar40 : K40 and Sr87 : Rb87 ratios in micas 

against concordant lead-uranium ages on uraninite from the same
GEOLOGICAL SURVEY BULLETIN 1097 PLATE 1

ZIRCON CRYSTALS FROM SYENITE, LILLIAN MINE, CENTRAL CITY,

COLORADO

A, Zircon crystals at low magnification ( X 50) showing uraninite (black crystal at center) intergrown with 

zircon. B, Intergrown zircon-uraninite crystal at high magnification (X 150). C, Autoradiograph of 

same crystal showing alpha tracks emanating from uraninite.

LEAD-ALPHA METHOD FOE DETEEMINING AGES OF IGNEOUS EOCKS 55

geologic setting. Using this method, the decay constant for electron 

capture for K40 and the branching ratio were found to be 

0.557±0.026X10-10 yr-1 and 0.117±0.015, respectively, and the halflife of Eb87 to be 5.0±0.2X 1010 years. By virtue of the close agreement of these values with those obtained by laboratory counting 

experiments it seems that the decay constants are not seriously in 

error. Inasmuch as each alkali metal parent isotope, Rb87 and K40, 

decays to chemically different daughter products, one an alkaline 

earth metal, and the other a noble gas, concordant Ar40 : K40 and 

Sr87 : Rb87 mica ages should be close to the absolute ages of these 

minerals and provide an independent means of evaluating the leadalpha and isotopic ages obtained on zircon and other accessory minerals from the same or related rocks.

Comparison of lead-alpha ages with published Ar40 : K40 and Sr87 : 

Rb87 ages are given in table 22. The same constants have now been 

adopted by other investigators so that the Ar40 : K40 and Sr87 : Rb87 

ages from different laboratories are comparable in this respect. Wetherill and others (1955), Folinsbee and others (1956), and Carr and 

Kulp (1957) have found discrepancies between the Ar^rK40 ages on 

mica and feldspar and have shown that, in general, micas retain more 

argon than the feldspars.

Close agreement between the lead-alpha zircon age (53 million 

years) and the feldspar age (52 million years) has been obtained for 

the Kneehills tuff and between the lead-alpha ages (72 million years) 

and feldspar age (71 million years) for the Boulder batholith of 

Montana. However, Knopf (1956) favors a calculated age of 87 

million years (based on a branching ratio of 0.089) on the grounds 

of argon leakage for the feldspar from the Boulder batholith. Age 

measurements on micas from these rocks are clearly needed to determine whether or not there has been a loss of argon from these 

feldspars.

A large number of lead-alpha age determinations have been obtained on accessory minerals from rocks of the batholith of southern 

California (Larsen and others, 1958). Isotopic ages have not been 

determined on minerals from the major rock types, but have been 

made on feldspar and lepidolite from the pegmatites at Pala, Calif. 

The pegmatites are genetically related to the batholith (Larsen, 1948) 

and the average lead-alpha ages obtained on zircon (109 million 

years), monazite (117 million years), and xenotime (110 million 

years) from the igneous rocks are in satisfactory agreement with 

Ar40 : K40 feldspar and lepidolite ages (92 and 100 million years) and 

Rb87 : Sr87 ages ranging from 102 to 121 million years. Lead-alpha 

ages obtained on granitic rocks of the Sierra Nevada batholith and 

Coast Range batholiths are principally the same as those obtained on
56 STUDIES IN GEOCHEONOLOGY

rocks of southern California. Comparisons of lead-alpha with 

Ar40 : K40 biotite ages can be made for three rocks from Yosemite National Park. Zircon from the 3 rocks gave lead-alpha ages of 103, 94, 

and 117 million years as compared to mica ages of 93,92, and 84 million 

years. Thorite from one of these rocks gave a lead-alpha age of 88 

million years. The Ar40 : K40 ages determined on two samples of bulk 

granitic rock and a feldspar phenocryst from the same area gave ages 

of 67,72, and 88 million years; these low values may result from argon 

leakage. The lead-alpha ages on zircon from scattered intrusive 

masses of the Coast Eange batholith check closely with three 

Ar40 : K40 ages on biotite from other intrusive bodies of this batholith.

Age measuremnts by each of the methods discussed have been made 

on granitic rocks from Conway, N.H.; Pikes Peak, Colo., and Llano 

County, Tex. The lead-alpha age (188 million years) and isotopic 

lead-uranium ages on the zircon (187,184 million years) from the Conway granite are in remarkably close agreement with the Ar*°:K40 

(175 and 182 million years) and Sr87 : Bb87 (170 and 185 million years) 

ages obtained on biotite in two different laboratories (Tilton and 

others 1957, Hurley and Fairbairn 1957).

In contrast to the agreement for most of the lead-alpha ages with 

ages determined by other methods on minerals younger than Precambrian, most of the lead-alpha ages on Precambrian rocks are 

not in good agreement with Ar40 : K40 and Sr87 : Bb87 ages. The age 

data on the Pikes Peak granite show the general pattern which is 

repeated in other Precambrian localities. The lead-alpha age (675 

million years) is closest to the lead-uranium isotopic ages (624 and 707 

million years), whereas the Pb207 :Pb206 age (980 million years) 

checks more closely with Ar40 : K40 (1,030 million years) and Sr87 : Eb87 

(1,020 million years) ages for the biotite. In nearly every case where 

the lead-alpha ages failed to agree with the biotite ages the lead-alpha 

ages were low. The differences found were as follows: nearly 10 percent low for the granite from Babbitt and McGrath gneiss, about 20 

percent for the granitic rocks from Texas, and from 30 to 40 percent on 

samples from Pikes Peak, Mellen, Eockville, St. Cloud, and Bellingham. Although Sr87 : Eb87 and Ar40 : K40 ages on mica are usually in 

very close agreement, wide age discrepancies have been found on both 

muscovite and biotite from the Copper Cliff rhyolite of Phemister 

(1957), and the Concord granite. In both of these cases the lead-alpha 

ages agreed most closely with the highest ages obtained on the micas.

SUMMARY AND CONCLUSIONS

The data given in this report permit an evaluation of the limitations 

of the lead-alpha age method and its applicability to geologic problems. Sources of error include: experimental errors of the alpha
LEAD-ALPHA METHOD FOR DETERMINING AGES OF IGNEOUS ROCKS 57

counts and lead determinations; presence of original lead; variations 

from the assumed thorium-uranium ratios; possible susceptibility of 

minute zircon crystals to loss or gain of thorium, uranium, or lead by 

diffusion, leaching, or metamorphism; and application of the simplified age equations to rocks older than Cambrian. With regard to 

analytical techniques, the largest source of error is in the lead analyses, 

especially in zircon that contains only a few parts per million of lead. 

For zircon and other minerals that contain more than 10 ppm of lead, 

the content of lead can be measured with a precision of ±6 to 10 percent. Agreement between the alpha emission measured by thicksource alpha counting and that calculated from the uranium and 

thorium content of zircon and monazite indicates that the alpha activity measurements have an accuracy of about ± 5 percent.

Where the method has been applied to a relatively large suite of 

cogenetic rocks younger than Precambrian, the scatter pattern of the 

age data is what would be expected from the experimental errors. In 

general, the standard deviation from the mean age is about 10 percent 

or less for minerals from rocks older than Cretaceous and younger 

than Precambrian. For younger rocks, which contain zircon with a 

very low lead content, the percent error is much greater but is generally less than 10 million years.

Inasmuch as zircon, monazite, thorite, and xenotime from the same 

or related rocks give ages which generally agree within the limits of 

error of the method, it has been shown that errors in the assumptions 

with regard to original lead and the loss or gain of parent and daughter 

products do not contribute significantly to the errors in the ages for 

most of the rocks, younger than Precambrian, tested.

Age determinations have been made on many geologically well dated 

igneous rocks in order to test the validity of the method by using 

geologic evidence. With few exceptions, the age data agree with the 

geologic sequence of events and hence the method can be used for distinguishing the ages of igneous rocks provided their age difference 

is not within the limits of error of the method. The lead-alpha ages 

on these samples are in close agreement with the lead-uranium age& 

on minerals that are probably of equivalent geologic age and form 

the basis of the Holmes time scale (Holmes, 1947).

Comparison of ages by the lead-alpha method has been made on 

zircon and monazite on which isotopic ages have been determined ^ 

zircon closely associated with isotopically dated uraninite; and on 

zircon and monazite from the same or related rocks for which argonpotassium and strontium-rubidium ages are available. In view of the 

paucity of the data the following conclusions regarding the cross 

checks are tentative:

1. The lead-alpha ages determined on zircon are primarily equivalent 

to the Pb208 : Usages.
58 STUDIES IN GEOCHRONOLOGY

2. When concordant lead-uranium and Pb207 : Pb206 ages are obtained, 

the lead-alpha ages are generally in close agreement with the 

isotopic ages regardless of their geologic age.

3. When a discordant age pattern is obtained, the Pb206 :!!238 and 

hence the lead-alpha ages are lower than the Pb207 : Pb206 age; 

the last, in general, is in closest agreement with the strontiumrubidium and argon-potassium ages obtained on micas.

4. In general, the lead-alpha ages agree with the isotopic, lead-uranium, argon-potassium and strontium-rubidium ages determined on minerals younger than Precambrian.

5. Often the lead-alpha ages are much lower than those obtained by 

the other methods on minerals from Precambrian rocks.

As yet, we have no satisfactory explanation for the wide age 

discrepancies obtained from the Precambrian rocks, but it seems 

likely that in some areas the minerals have lost lead by partial or 

complete recrystallization resulting from later metamorphic 

processes. Although considerable doubt exists in the interpretation 

of the results on Precambrian rocks, the lead-alpha method may yield 

useful information with regard to the postcrystallization history of 

a region even if its usefulness for dating the primary crystallization 

of minerals is restricted.

Fundamental studies are needed on a large number of carefully 

selected samples in order to correlate the age results with a complex 

geologic environment. Regardless of the method applied to the 

dating of minerals or rocks, the interpretation of the results from a 

geologic standpoint can be no better than our understanding of the 

history of a rock and its relationship to its geologic environment.

LITERATURE CITED

Ahrens, L. H., 1953, The use of ionization potentials, pt. 2, Anion affinity and

geochemistry: Geochim. et Cosmochim. Acta, v. 3, p. 1-29. 

Aldrich, L. T., Wetherill, G. W., and Davis, G. L., 1956, Determinations of

radiogenic Sr87 and Rb87 of an interlaboratory series of lepidolites: Geochim.

et Cosmochim. Acta, v. 10, p. 238-240. 

Aldrich, L. T., Wetherill, G. W., Tilton, G. R., and Davis, G. L., 1956, Half-life

of Rb87 : Phys. Rev., v. 103, p. 1045-1047. 

Baadsgaard, H., Goldich, S. S., Nier, A. O., and HofEman, J. H., 1957, The reproducibility of A*°/K*° age determinations: Am. Geophys. Union Trans., v. 38,

p.539-542. 

Beharrell, J., 1949, Absorption of alpha rays in thick sources: Am. Geophys.

Union Trans., v. 30, p. 333-336. 

Beveridge, A. J., and Folinsbee, R. E., 1956, Dating Cordilleran orogenies: Royal

Soc. Canada Trans., v. 50, p. 19-43. 

Billings, M. P., 1956, The geology of New Hampshire, pt. 2, Bedrock geology:

New Hampshire State Plan, and Devel. Comm., Concord, 203 p. 

Boucot, A. J., 1954, Age of the Katahdin granite [Maine]: Am. Jour. Sci., v. 252,

p. 144-148.
LEAD-ALPHA METHOD FOR DETERMINING AGES OF IGNEOUS ROCKS 59

Brown, R. W., and Pecora, W. T., 1949, Paleocene and Eocene strata in the

Bearpaw Mountains, Montana: Science, v. 109, p. 487-489. 

 arr, D. R., and Kulp, J. L., 1957, Potassium-argon method of geochronometry:

Geol. Soc. America Bull., v. 68, p. 763-784. 

Chapman, R. W., Gottfried, David, and Waring, C. L., 1955, Age determinations

on some rocks from the Boulder batholith and other batholiths of western

Montana: Geol. Soc. America Bull., v. 66, p. 607-609. 

«Cormier, R. F., 1957, Comparison of Rb/Sr ages measured for biotite and

muscovite from the same rock sample: Mass. Inst. Technology, 4th Ann.

Prog. Rept, NYO-3937, p. 72-74. 

Davis, G. L., and others, 1957, The age of rocks and minerals: Carnegie Inst.

of Washington, Ann. rept. of the Director of the Geophys. Lab., 1956-1957,

p. 169. 

Evernden, J. F., Curtis, G. H., and Lipson, J., 1957, Potassium-argon dating of

igneous rocks: Am. Assoc. Petroleum Geologists Bull., v. 41, no. 9,

p. 2120-2127. 

Paul, Henry, 1954, Nuclear Geology: New York, John Wiley and Sons, Inc.,

414 p. 

Polinsbee, R. E., Lipson, J., and Reynolds, J. H., 1956, Potassium-argon dating:

Geochim. et Cosmochim. Acta, v. 10, p. 60-68. 

Folinsbee, R. E., Ritchie, W. D., and Stansberry, G. F., 1957, The Crowsnest

volcanics and Cretaceous geochronology: Alberta Soc. Petroleum Geologists

Guidebook, 7th Ann. Field Conf., Waterton, 1957, p. 20-26. 

Goldich, S. S., Baadsgaard, H., and Nier, A. O., 1957, Investigations in A^/K*0

dating: Am. Geophys. Union Trans., v. 38, p. 547-551. 

Ooldschmidt, V. M., 1954, Geochemistry: Oxford, Clarendon Press. 

Gottfried, David, Senftle, F. E., and Waring, C. L., 1956, Age determination of

zircon crystals from Ceylon: Am. Mineralogist, v. 41, p. 157-161. 

Gottfried, David, Waring, C. L., Worthing, H. W., 1956, Hafnium content,

hafnium to zirconium ratio, and radioactivity of zircon from igneous rocks

[abs.]: Geol. Soc. America Bull., v. 67, p. 1700. 

Griffitts, W. R., and Overstreet, W. C., 1952, Granitic rocks of the western

Carolina Piedmont: Am. Jour. Sci., v. 250, p. 777-789. 

H6e, Arlette, and Flesch, Louis, 1957, Age absolu apparent des zircon d'Espaly

[Haute-Loire]: Acad. sci. [Paris] Comptes rendus, v. 244, no. 13, p.

1796-1798. 

Herzog, L. F., and others, 1956, Rb-Sr analysis and age determination by mass

spectrometry and related studies [abs.] :'Internat. Geol. Cong., 20th, Mexico

City. Resumenes de los trabajos presentados, p. 217-218. 

Hinds, N. E. A., 1934, The Jurassic age of the last granitoid intrusives in the

Klamath Mountains and Sierra Nevada, California: Am. Jour. Sci., 5th

ser., v. 27, p. 182-192. 

Holland, H. D., and Gottfried, David, 1955, The effect of nuclear radiation on

the structure of zircon: Acta Cryst, v. 8, p. 291-300. 

Holmes, Arthur, 1947, The construction of a geological time scale: Geol. Soc.

Glasgow Trans., v. 21, pt. 1, p. 117-152.

 1948a, Monazite as a geological timekeeper: Royal Soc. Edinburgh Proc., 

v. 63, pt. 2, p. 115-124.

 1948b, The oldest known minerals and rocks: Geol. Soc. Edinburgh 

Trans., v. 14, pt 2, p. 176-194.

 - 1954, The oldest dated minerals of the Rhodesian Shield: Nature, v.

173, p. 612-614.
60 STUDIES IN GEOCHRONOLOGY

Hurley, P. M., 1956, Direct radiometric measurement by gamma-ray scintillation* 

spectrometer; pt. 1: Uranium and thorium series in equilibrium; pt. 2: 

Uranium, thorium, and potassium in common rocks: Geol. Soc. America 

Bull., v. 67, p. 395-404, 405-^11.

Hurley, P. M., and Fairbairn, H. W., 1953, Radiation damage in zircon a 

possible age method: Geol. Soc. America Bull., v. 64, p. 659-673.

 1957, Abundance and distribution of uranium and thorium in zircon,

sphene, apatite, epidote, and monazite in granitic rocks: Am. Geophys^

Union Trans., v. 38, p. 939-944. 

Hurley, P. M., Pinson, W. H., Fairbairn, H. W., and Cormier, R. F., 1957,.

Comparison of A^/KW and Sr^'/Rb87 ages on biotite [aba] : Am. Geophys.-

Union Trans., v. 38, p. 396. 

Hutchinson, R. M., Jaffe, H. W., and Gottfried, David, 1954, Magmatic trends

and absolute age determinations of Precambrian intrusives of centrai

Texas [abs.]: Geol. Soc. America Bull., v. 65, p. 1266. 

Jaffe, H. W., 1955, Precambrian monazite and zircon from the Mountain Pass

rare earth district, San Bernardino County, California: Geol. Soc. America^

Bull., v. 66, p. 1247-1256. 

Jaffe, H. W., Gottfried, David, Waring, C. L., and Worthing, H. W., 1959,

Lead-alpha age determinations of accessory minerals of igneous rocks (1953-

1957) : U.S. Geol. Survey Bull. 1097-B. 

Kaye, C. A., 1957, Notes on the structural geology of Puerto Rico: Geol. Soc..

America BulL, v. 68, p. 103-118. 

Keevil, N. B., 1939, The calculation of geologic age: Am. Jour. ScL, v. 237r

p. 195-214. 

Keevil, N. B., and Grasham, W. B., 1943, Theory of alpha ray counting from

solid sources: Canadian Jour. Research, v. 21, p. 21-36. 

Knopf, Adolph, 1956, Argon-potassium determination of the age of the Boulder

bathylith, Montana: Am. Jour. Sci., v. 254, p. 744-745.

 1957, The Boulder bathylith of Montana: Am. Jour. Sci., v. 255, p, 

81-103.

Kostyleva, B. B., 1946, On metamictic disintegration of the zircon group of minerals, in Problems of mineralogy, geochemistry, and petrography (translated by Taisia Stadnichenko) ; U.S. Geol. Survey TEI-369, 16 p., issued by 

U.S. Atomic Energy Comm. Tech. Inf. Service Ext., Oak Ridge, Tenn.

Kulp, J. L., Bate, G. L., and Broecker, W. S., 1954, Present status of the lead 

method of age determination: Am. Jour. Sci., v. 252, p. 345-365.

Kulp, J. L., and Eckelman, W. R., 1957, Discordant U/Pb ages and mineral 

type: Am. Mineralogist, v. 42, p. 154-164.

Larsen, B. S., Jr., 1945, Time required for the crystallization of the great batholith of southern and Lower California: Am Jour. Sci., v. 243-A, Daly 

Volume, p. 369-416.

' 1948, Batholith and associated rocks of Corona, Elsinore, and San Luis 

Rey quadrangles, southern California: Geol. Soc. America Mem. 29, 182 p,

Larsen, B. S., Jr., and Cross, Whitman, 1956, Geology and petrology of the San 

Juan region, southwestern Colorado: U.S. Geol. Survey Prof. Paper 258, 

303 p., 4 pis.

Larsen, B. S., Jr., Gottfried, David, Jaffe, H. W., and Waring, C. L., 1958, Leadalpha age determinations of the Mesozoic batholiths of western North 

America: U.S. Geol. Survey Bull. 1070-B, p. 35-62, pis. 5-6.

Larsen, E. S., Jr., and Keevil, N. B, 1947, Radioactivity of the rocks of the 

batholith of southern California: Geol. Soc. America Bull., v. 58, p. 483-494.
LEAD-ALPHA METHOD FOR DETERMINING AGES OF IGNEOUS ROCKS 61

Larsen, B. S., Jr., Keevil, N. B., and Harrison, H. O., 1952, Method for determining

the age of igneous rocks using the accessory minerals: Geol. Soc. America

Bull., v. 63, p. 1045-1052. 

Larsen, B. S., Jr., and Schmidt, B. G., 1968, A reconnaissance of the Idaho batholith and comparison with the southern California batholith: U.S. GeoL

Survey Bull. 1070-A, p. 1-33, pi. 1. 

Larsen, E. S., Jr., Waring, C. L., and Berman, Joseph, 1953, Zoned zircon from

Oklahoma: Am. Mineralogist, v. 38, p. 1118-1125. 

Larsen, L. H, and Poldervaart, Arie, 1957, Measurement and distribution of

zircon in some granite rocks of magmatic origin: Mineralog. Mag., v. 31, no.

238, p. 544-564. 

Lyons, J. B., Jafle, H. W., Gottfried, David, and Waring, C. L., 1957, Lead-alpha

ages of some New Hampshire granites: Am. Jour. Sci., v. 255, p. 527-546. 

Mertie, J. B., Jr., 1953, Monazite deposits of the southeastern Atlantic States:

U.S. Geol. Survey Cire. 237,31 p. 

 1937, The Yukon-Tanana Region, Alaska: U.S. GeoL Survey Bull. 872,

276 p. 15 pis. 

Miller, W. J., 1946, Crystalline rocks of southern California: Geol. Soc. America

Bull., v. 57, p. 457-540. 

Moorhouse, W. W., 1956, The paragenesis of accessory minerals: Econ. Geology,

v. 51, p. 248-262. 

Nier, A. O., 1939, The isotopic constitution of radiogenic leads and measurements

of geologic time, pt 2: Phys. Rev., v. 55, p. 153-163. 

Nogami, H. H., and Hurley, P. M., 1948, The absorption factor in counting alpha

rays from thick mineral sources: Am. Geophys. Union Trans., v. 29, p. 335-

340.

Olson, J. C., Shawe, D. R., Pray, L. C., and Sharp, W. N., 1954, Rare-earth mineral deposits of the Mountain Pass district, San Bernardino County, Calif.:

U.S. Geol. Survey Prof. Paper 261, 75 p., 13 pis. 

Palache, O., Berman, H., and Frondel, C., 1951, Dana's system of mineralogy,

7th ed., v. 2: New York, John Wiley and Sons, Inc. 

Phemister, T. C., 1957, The Copper Cliff rhyolite in McKim Township, District

of Sudbury: Ontario Dept. Mines, 65th Ann, Rept., v. 65, pt 3, (1956), p.

91-116. 

Pinson, W. H., Jr., 1957, Comparison of K/A and Rb/Sr ages of three Precambrian rocks: Mass. Inst. Technology, 4th Ann. Prog. Rept. NYO-3937, p.

39-43. 

Poldervaart, Arie, 1956, Zircon in rocks, pt. 2, Igneous rocks: Am. Jour. Sci., v.

254, p. 521-554. 

Powell, R. A., and Kinser, O. A., 1958, Dithizone method for determination of

lead in monazite: Anal. Chemistry, v. 30, no. 6, p. 1139-1141. 

Quinn, A. W., Jaffe, H. W., Smith, W. L., and Waring, C. L., 1957, Lead-alpha

ages of Rhode Island granitic rocks compared to their geologic ages: Am.

Jour. Sci., v. 255, p. 547-560. 

Rankama, Kalervo, and Sahama, Th. G., 1950, Geochemistry: Chicago, Univ.

Chicago Press, 912 p. 

Reynolds, J. H., 1957, Comparative study of argon content and argon diffusion in

mica and feldspar: Geochim. et Cosmochim. Acta, v. 12, p. 117-184. 

Roques, Maurice, 1956, Determination de 1'age absolu du granite carbonifere du

Mayet de montagne [Allier]: Acad. Sci [Paris], Comptes rendus, v. 242,

p. 528-530. 

ROSB, C. P., 1952, The eastern front of the Bitterroot Range, Montana: U.S. Geol.

Survey Bull. 974-B, p. 135-175, pi. 4.
62 STUDIES IN GEOCHRONOLOGY

Ross, C. P. and Forrester, J. D., 1947, Geologic map of the State of Idaho: U.S. 

Geol. Survey and Idaho Bur. Mines and Geology.

Sahama, Th. G., 1946, On .the chemistry of the mineral titanite: Finland, Oomm. 

Geol. Bull. 138, p. 88-120.

Saillard, Nicole, Scheibling, M. Gaston, and Hee, Arlette, 1957, Sur la d£termination de la teneur en plomb des zircons par une method speetrographique: 

Acad. Sci [Paris], Comptes rendus, v. 244, no. 5, p. 609-611.

Sandell, E. B., 1950, Colorimetric determination of traces of metals, in Chemical 

analysis, v. 3,2d ed.: New York, Interscience Publishers, Inc.

Senftle, F. E., 1951, Apparatus for separation of mineral grains: Am. Mineralogist,, v. 36, p. 910-912.

Silver, L. T., Stehli, F. G., and Alien, C. R., 1956, Lower Cretaceous prebatholithic rocks of northern Baja California, Mexico, [abs.] : Intemat. 

Geol. Cong., 20th, Mexico City, Resumenes de los trabajos presentados, p. 30.

Stearns, G. E., 1953, Tertiary geology of the Galisteo-Tonque area, New Mexico: 

Geol. Soc. America Bull., v. 64, p. 459-507.

Stieff, L. R., and Stern, T. W., 1956, Interpretation of the discordant age 

sequence of uranium ores, in Contributions to the geology of uranium and 

thorium by the U.S. Geol. Survey and Atomic Energy Oomm. for the United 

Nations Internat. Oonf. on peaceful uses of atomic energy, Geneva, Switzerland, 1955: U.S. Geol. Survey Prof. Paper 300, p. 549-555.

Taylor, O. H., 1915, Granites of Oklahoma: Oklahoma Geol. Survey Bull. 20, 

108 p.

Tilton, G. R., 1956, The interpretation of lead-age discrepancies by acid-washing 

experiments: Am. Geophys. Union Trans., v. 37, p. 244-230.

Tilton, G. R., Davis, G. L., Wetherill, G. W., and Aldrich, L. T., 1957, Isotopic 

ages of zircon from granites and pegmatites: Am. Geophys. Union Trans., 

v. 38, p. 360-371.

Tilton, G. R., and Nicolaysen, L. O., 1957, The use of monazites for age determination : Geochim. et Oosmochim. Acta, v. 11, p. 28-40.

Tilton, G. R. and others, 1955, Isotopic composition and distribution of lead, 

uranium, and thorium in a Precambrian granite: Geol. Soc. America Bull., 

v. 66, p. 1131-1148.

Wadia, D. N., and Fernando, L. J. D., 1944, Ilmenite, monazite, and zircon (sessional paper 6 of 1926, revised) Gems and semi-precious stones of Ceylon: 

Ceylon, Dept. Mineralogy, Recs., Prof. Paper 2, p. 13-44.

Waring, C. L., and Worthing, Helen W., 1953, A spectrographic method for the 

determination of trace amounts of lead in zircon and other minerals: Am. 

Mineralogist, v. 38, p. 827-833.

Waters, A. C., 1955, Volcanic rocks and the tectonic cycle, m Poldervaart, Arie, 

ed., Crust of the earth: Geol. Soc. America Spec. Paper 62, p. 703-722.

Webber, G. R., Hurley, P. M., and Fairbairn, H. W., 1956, Relative ages of eastern 

Massachusetts granites by total lead ratios in zircon: Am. Jour. Sci., v. 254, 

p. 574-583.

Webster, R. K., Morgan, J. W., and Smales, A. A., 1957, Some recent Harwell 

analytical work on geochemistry: Am. Geophys. Union Trans., v. 38, p. 

543-546.

Wedepohl, Karl Hans, 1956, Untersuchungen zur Geochemie des Bleis: Geochim. 

et Cosmochim. Acta, v. 10, p. 69-148.

Wetherill, G. W., Aldrich, L. T., and Davis, G. L., 1955, AW/KW ratios of feldspars 

and micas from the same rock: Geochim. et Cosmochim. Acta, v. 8, p 

171-172.
LEAD-ALPHA METHOD FOR DETERMINING AGES OF IGNEOUS ROCKS 63

Wetherill, G. W., Davis, G. L., and Aldrich, L. T., 1957, Age measurements on

rocks north of Lake Huron [abs.] : Am. Geophys. Union Trans., v. 38, p. 412. 

Wetherill, G. W., and others, 1956, Decay constants of K40 as determined by the

radiogenic argon content of potassium minerals: Phys. Rev., v. 103, p.

987-989. 

Woodford, A. O., and Harriss, T. F., 1938, Geological reconnaissance across Sierra

San Pedro Martir, Baja, California: Geol. Soc. America Bull., v. 49, p.

1297-1336. 

Woyski, M. S., 1949, Intrusives of central Minnesota: Geol. Soc. America Bull.,

v. 60, p. 999-1016. 

Youden, W. J., 1951, Statistical methods for chemists: New York, John Wiley

and Sons, Inc., 128 p.

O