BLEED BLEED
Argon Thermochronology of Mineral Deposits— 


A Review of Analytical Methods, Formulations, and


Selected Applications


By Lawrence W. Snee 

U.S. Geological Survey Bulletin 2194 

U.S. Department of the Interior 

U.S. Geological Survey 
U.S. Department of the Interior 

Gale A. Norton, Secretary 

U.S. Geological Survey 

Charles G. Groat, Director 

Version 1.0, 2002 

This publication is only available online at: 

http://pubs.usgs.gov/bul/b2194/ 

Any use of trade, product, or firm names in this publication 

is for descriptive purposes only and does not imply 

endorsement by the U.S. Government 

Manuscript approved for publication August 23, 2002


Published in the Central Region, Denver, Colorado


Graphics by L.W. Snee 


Photocomposition by Norma J. Maes


Edited by L.M. Carter

Contents 

Abstract ................................................................................................................................................. 1


Introduction........................................................................................................................................... 1


Acknowledgments .............................................................................................................................. 2


Basis of the 40Ar/39Ar Technique ....................................................................................................... 2


Formulations of the Method........................................................................................................ 2


Standards ....................................................................................................................................... 4


Thermal Release Experiments and Age Spectra .................................................................... 5


Error Analysis ................................................................................................................................ 8


39Ar/37Ar (Apparent K/Ca) Diagrams........................................................................................... 9


Correlation Diagrams ................................................................................................................... 9


Argon Loss ..................................................................................................................................... 10


Extraneous Argon ......................................................................................................................... 11


39Argon Recoil................................................................................................................................ 11


Closure Temperature and Diffusion........................................................................................... 12


Some Minerals Useful for 40Ar/39Ar Geochronology....................................................................... 14


Amphiboles .................................................................................................................................... 14


White Micas .................................................................................................................................. 16


Brown Micas ................................................................................................................................. 18


Illite.................................................................................................................................................. 18


Alkali Feldspar ............................................................................................................................... 19


Low-Temperature Potassium Feldspar.............................................................................. 20


Sanidine and Anorthoclase ................................................................................................. 20


Adularia................................................................................................................................... 20


Plagioclase .................................................................................................................................... 20


Whole Rocks.................................................................................................................................. 21


Alunite and Jarosite ..................................................................................................................... 21


Manganese Oxides....................................................................................................................... 22


Others .............................................................................................................................................. 22


Standard Analytical Techniques ....................................................................................................... 23


Argon Thermochronology Applied to Mineral Deposits................................................................ 24


General Strategy ........................................................................................................................... 24


Some Examples ............................................................................................................................. 24


Panasqueira, Portugal, Tin-Tungsten Deposit.................................................................. 24


Cornubian Batholith and Associated Mineral Deposits, Southwest England ............. 26


Red Mountain Intrusive System and Associated Urad-Henderson ..............................


Molybdenum Deposits, Colorado..................................................................................... 28


Eastern Goldfields Province, Western Australia.............................................................. 31


References Cited ................................................................................................................................. 32


Figures 

1. Diagram showing typical 40Ar/39Ar age spectrum................................................................... 8


2. Diagram showing typical 39ArK/

37ArCa spectrum...................................................................... 9


3. 	 Diagrams of age and K/Ca spectra, and back-scattered electron image of 


Alaskan white mica sample ....................................................................................................... 10


4. Hypothetical isochron diagram and hypothetical inverse correlation diagram................ 12


5. 	 Age and isochron diagrams and chemical plot for Brooks Range, Alaska, 


white micas ................................................................................................................................... 13


III

6. Graphs of examples of Turner’s theoretical argon-loss diagrams.............................. 14


7. Graph of Harrison’s (1981) hornblende argon-loss behavior ....................................... 14


8. Diagram showing 40Ar/39Ar age spectrum for a sample with excess 40Ar ................. 15


9. Diagram showing 40Ar/39Ar age spectrum for a sample exhibiting 39Ar recoil .......... 15


10. Graph showing example of a cooling curve for a mineralized area in Idaho............ 16


11. 	 Diagrams and drawing of complex argon systematics of a Precambrian 


amphibole from northern Pakistan ................................................................................... 16


12.	 Back-scattered electron images of a white mica grain showing complexities of 


metamorphism and deformation that can affect argon systematics.......................... 17


13.	 Diagram of a typical age spectrum for a biotite sample that contains 


some chlorite ........................................................................................................................ 18


14.	 Diagram showing age spectra for size fractions of illite from 


Jerritt Canyon, Nev. ........................................................................................................... 19


15. Back-scattered electron image of adularia formed along cracks in sanidine ......... 21


16. Diagram showing age spectrum for 1,834 Ma alunite from Brazil.............................. 22


17. Sketch map of Portugal showing location of Panasqueira .......................................... 25


18.	 Diagrammatic cross section of the Panasqueira granite cupola that is host to 


the Sn-W deposit ................................................................................................................ 25


19. Drawing of sample 207 exhibiting the two substages of the oxide-silicate stage.... 25


20. Composite age-spectrum diagram for muscovites from OSS I and OSS II ............... 25


21. Summary diagram of ages of mineralization at Panasqueira, Portugal..................... 25


22. Geologic sketch map of southwest England showing Cornubian batholith .............. 26


23.	 Diagram showing summary geochronology for Cornubian batholith and 


associated mineral deposits .............................................................................................. 27


24. Sketch surface geology map of Red Mountain intrusive center................................. 28


25.	 Composite age-spectrum diagram for argon samples from Red Mountain 


intrusive system and Urad-Henderson mineral deposit ............................................... 29


26. Three-dimensional block diagram of Red Mountain intrusive center........................ 29


27.	 Regional-scale geologic sketch map of Yilgarn-block gold deposits, 


Western Australia ............................................................................................................... 30


28. 	 Diagram summarizing currently available geochronologic data for gold 


deposits in Western Australia ........................................................................................... 31


Tables 

1. 	 Averages with standard deviations and ranges for production ratios 


determined for irradiations in the Geological Survey TRIGA reactor 


(GSTR) by the Denver argon geochronology laboratory 


from year 1987 to year 2002............................................................................................... 4


2. 	 Production ratios determined for irradiations in GSTR done by the 


Denver argon geochronology laboratory and used in argon data reductions 


from year 1987 to year 2002 ............................................................................................... 6


3. Stages of mineralization of the Cornubian batholith, southwest England................. 27


IV

Argon Thermochronology of Mineral Deposits—


A Review of Analytical Methods, Formulations, and 


Selected Applications 

By Lawrence W. Snee 

Abstract 

40Ar/39Ar geochronology is an experimentally robust and 

versatile method for constraining time and temperature in geologic processes. The argon method is the most broadly applied 

in mineral-deposit studies. Standard analytical methods and formulations exist, making the fundamentals of the method well 

defined. A variety of graphical representations exist for evaluating argon data. A broad range of minerals found in mineral 

deposits, alteration zones, and host rocks commonly is analyzed 

to provide age, temporal duration, and thermal conditions for 

mineralization events and processes. All are discussed in this 

report. The usefulness of and evolution of the applicability of the 

method are demonstrated in studies of the Panasqueira, Portugal, 

tin-tungsten deposit; the Cornubian batholith and associated 

mineral deposits, southwest England; the Red Mountain intrusive system and associated Urad-Henderson molybdenum deposits; and the Eastern Goldfields Province, Western Australia. 

Introduction 

The 40Ar/39Ar isotopic dating technique has evolved over 

the past 10 years into the most commonly applied geochronologic method used in mineral-deposit research. The method has 

gained this popularity as a result of its natural versatility and 

experimental robustness. Because potassium-bearing minerals 

commonly form during processes associated with mineralization, either by direct precipitation from mineralizing fluids or as 

a result of alteration, an argon geochronometer (meaning, a datable mineral) is often present, and that mineral can be used in 

determining the age of the mineralization event. Perhaps more 

importantly, because argon geochronology is a precise analytical 

method, it is commonly possible to resolve the age of thermal 

events that were closely spaced in time. This capability is especially applicable for defining duration and timing of events in 

complex ore deposits. 

Before 1988, even though high-precision argon geochronology was routinely employed in geologic framework studies, 

it had not been shown to be a valuable tool for unraveling the 

fundamental questions of economic geology. To test the applicability of high-precision 40Ar/39Ar geochronology in economic 

geology, Snee and others (1988) used the detailed paragenetic 

framework defined by Kelly and Rye (1979) for the 

Panasqueira, Portugal, tin-tungsten deposit to provide the geologic backdrop to move the method into applications that were 

more fundamental. From this deposit, carefully characterized 

muscovites were used to address mineral-deposit questions that 

reached beyond simply attempting to determine the apparent age 

of this world-class deposit. In fact, this study not only resolved 

the age of the deposit to less than +0.30 percent absolute, but 

also defined the duration of hydrothermal activity. The study 

also showed that thermal activity was episodic as well as spatially and temporally complex. The detailed geologic understanding of the Panasqueira deposit provided the physical 

framework for the definition of empirical constraints on diffusion of argon from muscovite (of structural state 2M1). Thus 

from the study on the Panasqueira tin-tungsten deposit, Snee 

and others (1988) showed that high-precision 40Ar/39Ar geochronology is truly a thermochronologic method that experimentally 

connects time and temperature and makes its use in mineraldeposit studies valuable. 

Since 1988, many exciting new applications of high-precision 40Ar/39Ar thermochronology in mineral-deposit research 

have been employed. Relatively simplistic studies to determine 

the age of deposits are now routinely producing precision of less 

than 0.2 percent; these precise dates commonly provide critical 

data for constraining genetic models and exploration strategies 

(Losada-Calderon and others, 1994; Ford and Snee, 1996; Marsh 

and others, 1997; Lamb and Cox, 1998; Haeberlin and others, 

1999). The applicability of the method also is expanding as 

argon systematics of other datable minerals, such as alunite, 

jarosite, hollandite, illite, and adularia, are being unraveled. 

(See, for example, Love and others, 1988; Brooks and Snee, 

1996; Vasconcelos and others, 1994; Lippolt and Hautmann, 

1995; Folger and others, 1996; Hall and others, 1997.) Many 

imaginative studies (Goldfarb and others, 1991, 1993; Kontak 

and others, 1994; Powell and others, 1995; Kent and McCuaig, 

1997; Leach and others, 1998; Reynolds and others, 1998) have 

been published on complex thermochronologic details of mineral deposits; these studies are pressing the method across new 

frontiers. Laser-probe studies (Dong and others, 1997; Onstott 

and others, 1997; Clark and others, 1998) also are pushing the 

technique in new directions by opening the possibility of analyzing alteration assemblages in place without the need for mineral separation. And now the method is being combined not 

only with basic geologic framework details but also with other 

high-precision geochronologic, isotopic, and paleomagnetic 

methods to both certify the argon data and build a more complex and complete geochronologic framework for these deposits. (See, for example, Kontak and others, 1990; Perkins and 

others, 1990; Geissman and others, 1992; Perkins and others, 

1992; Chesley and others, 1993; Zweng and others, 1993; Keppie and others, 1993; Giuliani and others, 1994; Kent and 

McDougall, 1995; Miller and others, 1994, 1995; RibeiroAlthoff and others, 1997; Hofstra and others, 1999.) 

Part of this review was prepared to accompany lectures at a 

short course on the use of geochronology in mineral-deposit 

1

research given at the University of Western Australia in January 

2001. The purpose of this expanded version of the original 

course notes is to publish procedures and assumptions used in 

the U.S. Geological Survey (USGS) Denver, Colo., argon laboratory and to provide a more comprehensive understanding of 

the analytical fundamentals and applications of the 40Ar/39Ar 

geochronologic technique. To that end, I do the following 

herein: 

• Summarize the basic principles underlying the method, 

including the general formulations for calculating 

apparent age and analytical errors 

• Give an assessment of argon standards used in our 

laboratory 

• Discuss closure temperature as a concept, and review 

closure-temperature ranges for typical minerals 

• Describe processes of argon loss and thermal resetting 

of minerals 

• Describe the phenomenon of 39Ar recoil 

• List minerals that are useful in geochronologic studies 

of mineral deposits 

Upon this analytical foundation I build a strategy for dating 

mineral deposits and I review some mineral-deposit studies that 

have been done in the Denver USGS argon geochronology laboratory. 

Acknowledgments 

This report is based on more than 15 years of laboratory 

studies conducted in the U.S. Geological Survey argon geochronology laboratory, Denver, Colo. More than 100 guest investigators have used this laboratory, resulting in more than 300 reports 

and abstracts. Each guest brought his or her unique manner of 

looking at geology and geochronology to our laboratory and to 

each experiment. The results of these studies form the foundation for this report. Equally important are the individuals who 

have built and maintained our laboratory and who have prepared 

rocks and minerals for analysis. These individuals include John 

Chesley, Ross Yeoman, Dan Miggins, Shahid Mirza Baig, Gary 

Davidson, Amy Bern, Libby Prueher, Steve Harlan, Cayce 

Lilleseve, and Brian Penn. Without their efforts, sometimes over 

24-hour long periods, the laboratory would not have functioned. 

I also express gratitude to the U.S. Geological Survey TRIGA 

reactor crew, especially Tim Debey, Paul Helfer, Rick Perryman, 

and Darryl Liles for their expert maintenance and operation of 

the USGS research reactor. They provided virtually all the neutrons that activated our samples. Some of the results described 

in this report have been derived from published results from 

other laboratories. The dedication of these researchers has led to 

the robust nature of the 40Ar/39Ar geochronologic method. 

Lorna Carter has turned my feeble attempt at writing into reasonable English. Special thanks are extended to Professor David 

Groves of the Centre for Global Metallogeny, University of 

Western Australia (UWA), and the University administration for 

granting me a Gledden Visiting Senior Fellowship in early 2001. 

This fellowship provided me the opportunity to refresh my mind 

and to develop a first draft of this report to accompany course 

notes for a series of lectures at UWA. Subsequent studies with 

David Groves, Neal McNaughton, and Noreen Vielreicher, supported by funding from the Australian Research Council and the 

Australian Mineral Industries Research Association, are revealing the power of the argon method in addressing problems of 

Archean gold deposits. 

Basis of the 40Ar/39Ar Technique 

Formulations of the Method 

The 40Ar/39Ar dating technique is a variant of the conventional K-Ar method and is based on the formation of 39Ar during 

irradiation of potassium-bearing samples in a nuclear reactor. To 

obtain a date by this technique, a sample of unknown age and a 

standard of known age are irradiated together to produce 39Ar 

from 39K by fast-neutron bombardment. Wänke and Konig 

(1959), who irradiated meteorite samples weighing approximately 5 g (grams), described the earliest application of the technique. They used a counting technique to measure the activities 

of products formed from three reactions, 39K(n,p)39Ar, 40Ca(n, 

α)37Ar, and 40Ar(n, λ)41Ar. (The reaction expressed as 39K(n,p)39Ar, for example, signifies that a fast neutron is incorporated within the 39K atom with the resultant ejection of a proton causing the formation of 39Ar.) Soon afterwards 

Sigurgeirsson (1962), Merrihue (1965), Merrihue and Turner 

(1966), and Mitchell (1968) established the foundation for mass 

spectrometric determination. (The historical development of 

the 40Ar/39Ar method is fascinating and can be read in these 

papers or in summaries in Faure (1986), Snee (1982b), and 

McDougall and Harrison (1999).) 

In the early experiments, the importance of the 40Ar/39Ar 

technique lay in three aspects. First and most important, no 

direct chemical analysis of potassium is required, unlike the conventional K-Ar method, in which both 40K and 40Ar must be 

measured separately and quantitatively. In a conventional K-Ar 

analysis, argon (a gas) in one aliquot is measured by isotopedilution, gas-source mass spectrometry. Potassium (a solid) in a 

separate aliquot is determined by some other analytical method 

such as flame photometry, X-ray fluorescence, or isotope-dilution, solid-source mass spectrometry. This poses the danger that, 

because of sample inhomogeneity, different potassium and (or) 

argon contents may exist in each aliquot. By contrast, in the 40Ar/39Ar technique, because only the relative isotopic abundances or ratios of argon are measured during a single procedure, 

sample-inhomogeneity problems are circumvented, thus improving the accuracy. Similarly, greater precision results because of 

the use of gas-source mass spectrometry, an extremely precise 

analytical technique. The second advantage of the 40Ar/39Ar 

technique, which was recognized in the early experiments, is the 

ability to analyze precisely very small (milligram-size) samples. 

This was particularly valuable in early applications for dating 

meteorites and later for lunar samples. Perhaps the greatest 

advantage of the technique, later described by Merrihue and 

Turner (1966), is the ability to release argon from samples by 

step-wise (incremental) heating instead of one-step total fusion. 

2 Argon Thermochronology of Mineral Deposits 
The result is a series of dates from a single sample that potentially reveals information about the distribution of argon within 

the sample. The combination of these advantages increases both 

the accuracy and precision of the 40Ar/39Ar method over the conventional K-Ar technique. However, the 40Ar/39Ar technique will 

suffer if proper corrections are not made for interfering radiation-induced isotopes; fortunately, these corrections are well 

known, routinely made, and discussed herein. 

For the purposes of this review, I consider it important to 

outline in some detail the formulations underlying the 40Ar/39Ar 

dating technique. Modern analytical details and formulations of 

the 40Ar/39Ar dating technique are similar to those comprehensively described by Dalrymple and others (1981) in U.S. Geological Survey Professional Paper 1176, “Irradiation of samples for 40Ar/39Ar dating using the Geological Survey TRIGA reactor.” 

In the following discussion some additional experimental considerations that have been revealed since 1981 are included to 

provide the accurate foundation of current analytical procedures. 

The determination of a sample’s age is based on the production of 39Ar from 39K, and the age is directly proportional to the 40Ar/39Ar ratio of the sample. As derived by Grasty and Mitchell 

(1966) and modified by Mitchell (1968), the production of 39Ar 

can be calculated from the relationship, 

39ArK = 39K∆T ∫ φ(ε)σ(ε)dε , (1) 

where: 

subscript K refers to potassium-derived 39Ar; 

∆T is the length of irradiation; 

φ(ε) is the neutron flux density at energy ε; 

σ(ε) is the capture cross section of 39K for neutrons having 

energy ε; and integration is performed over the entire energy 

spectrum of neutrons. 

The formulation of the age equation results from dividing 

this equation into the K-Ar age equation, 

40ArR = 40K (λε/λ)(e (λ

ε

+λ

β−)t

-1), (2) 

where: 

40ArR is radiogenic argon derived from the natural decay of 

potassium; 

λε is the decay constant for the decay of 40K to 40Ar; 

λβ− is the decay constant for decay of 40K to 40Ca; 

λ is the total decay constant for the decay of 40K to 40Ca and 40Ar; and 

t is the time since decay began following the closure of the system to loss of daughter products. 

The resultant rearranged expression is 

(eλt

-1)/(40ArR/

39ArK) = (λ/λε)(39K/40K)∆T ∫ φ(ε)σ(ε)dε. (3) 

The integrated quantity in this equation is very difficult, if 

not impossible, to evaluate directly; but in any particular irradiation, if a “monitor” (standard of known age) is irradiated adjacent to the sample of unknown age, the resulting relationship is 

1 = [(eλt

m-1)/(40ArR/

39ArK)m]/[(eλt

u-1)/(40ArR/

39ArK)u], (4) 

where subscripts m and u refer to “monitor” and “unknown,” 

respectively. Rearranging this equation and using the 

conventions J = (eλt

m-1)/(40ArR/

39ArK)m (Grasty and Mitchell, 

1966) and F = 40ArR/

39ArK (Dalrymple and Lanphere, 1971) 

results in 

tu = (1/λ)ln(JF +1), (5) 

which is the standard 40Ar/39Ar age equation. J is commonly 

referred to as the fluence parameter and F is the ratio of radiogenic 40Ar to potassium-derived nucleogenic 39Ar. The universally used decay constants are those recommended by Steiger 

and Jäger (1977), that is, λε = 0.581×10-10/yr, λβ− = 

4.962×10-10/yr, and λ = λε + λβ− = 5.543×10-10/yr. 

Dating material by this method would be a simple matter of 

measuring the 40Ar/39Ar ratios of the sample and standard after 

irradiation if not for the fact that numerous other argon isotopeproducing reactions occur during irradiation, and nonradiogenic 

(atmospheric or extraneous) argon is always present. Several 

investigators (Stoenner and others, 1965; Mitchell, 1968; Brereton, 1970; Turner, 1971a; Dalrymple and others, 1981; Roddick, 

1983) have reported details of these interfering reactions, and 

standard methods are now employed to make quantitative corrections for the interferences. Of these reactions those with the 

potentially most serious deleterious effects are 41K(n,d)40Ar, 40K(n,p)40Ar, 40Ca(n,nα)36Ar, 42Ca(n,α)39Ar, and 35Cl(n,α)36Cl, 

with subsequent β− decay to 36Ar. (The 35Cl interference can 

become serious if irradiated samples are not analyzed within 6 

months after irradiation.) Several additional reactions produce 

minor to trivial quantities of interfering argon isotopes, including 43Ca(n,nα)39Ar, 40K(n,d)39Ar, 40K(n,nd)38Ar, 41K(n,α)38Cl 

with subsequent β− decay to 38Ar, 39K(n,nd)37Ar, 42Ca(n,nα)38Ar, 43Ca(n,α)40Ar, and 44Ca(n,nα)40Ar. Fortunately two nuclear 

reactions, 40Ca(n,α)37Ar and 37Cl(n,γ)38Cl, with subsequent β−

decay to 38Ar, produce isotopes of argon that are used to determine extent of interferences of argon isotopes produced from 

calcium and chlorine. An additional correction must be made 

for the decay of 37Ar because it is radioactive; 37Ar decays by 

electron capture with a half life of 35.1 + 0.1 days to 37Cl 

(Stoenner and others, 1965). 39Ar is also radioactive and decays 

by β− emission to 39K; because its half life is 269 + 3 years, corrections for this decay in most experiments are virtually unnecessary—although we always make this correction as well. In 

addition, pure crystalline salts of CaF2 and K2SO4 are commonly 

irradiated with samples to directly measure the argon-isotope 

production ratios from these interfering reactions. Our laboratory includes CaF2 and K2SO4 with each group of samples irradiated and has found some variation in production ratios from 

irradiation to irradiation. Although the variation is generally 

minor, in some cases, such as dating of young whole-rock basalt 

samples, significant inaccuracies may result in the calculation of 

apparent age if careful corrections are not made. Table 1 shows 

the averages with standard deviations and ranges for the six production ratios for CaF2 and K2SO4 irradiated in the Denver argon 

geochronology laboratory from 1987 until the present (2002). 

Table 2 is a compilation of all the production ratios for the 

majority of 75 irradiations (DD1 through DD75) that were used 

to calculate the averages in table 1. From table 1, the large variation of 1.560×10-2 to 4.970×10-3 in the (40Ar/39Ar)K ratio is 

important. For young potassium-rich samples this variation 

could result in great inaccuracy if the proper ratio is not applied 

to the correction. 

Basis of the 40Ar/39 Basis of the 40Ar/39Ar Technique Ar Technique 3
Table 1. Averages with standard deviations and ranges for production ratios determined for irradiations in the Geological Survey TRIGA reactor (GSTR) by the Denver argon geochronology laboratory 

from year 1987 to year 2002. 

Production ratio Average Std. deviation High Low Number 

(

37Ar/39Ar)K 1.645×10-4 +0.729×10-4 3.38×10-4 5.5×10-5 52 

(

38Ar/39Ar)K 1.312×10-2 +0.009×10-2 1.343×10-2 1.300×10-2 55 

(

40Ar/39Ar)K 9.219×10-3 +2.039×10-3 1.560×10-2 4.970×10-3 55 

(

39Ar/37Ar)Ca 6.984×10-4 +0.811×10-4 9.900×10-4 5.050×10-4 49 

(

36Ar/37Ar)Ca 2.683×10-4 +0.078×10-4 2.90×10-4 2.35×10-4 53 

(

38Ar/37Ar)Ca 4.4×10-5 +1.9×10-5 1.16×10-4 1.8×10-5 50 

Along with necessary corrections for interfering nuclear 

reactions, corrections must be made for naturally occurring 

isotopes of argon (40Ar, 38Ar, and 36Ar) that exist throughout 

nature and are incorporated in varying amounts in standards and 

unknowns. These incorporated argon isotopes are commonly 

referred to as atmospheric or extraneous argon and may have 

their origin from the minor amounts of argon that are present 

before a mineral closes to argon diffusion or from true atmospheric argon incorporated within the mineral structure during 

its life or released from the extraction system while an experiment is conducted. Fortunately, the present-day argon atmospheric ratios of 40Ar/36Ar and 38Ar/36Ar are well known and 

easy to measure in most laboratories; accepted values are 295.5 

for (40Ar/36Ar)At and 1,581 for (40Ar/38Ar)At. 

Taking into account the preceding discussion on interfering 

isotopes of argon, in order to determine the actual F (that is, 40ArR/

39ArK ratio) of a sample or standard, the isotopic abundances of five argon isotopes (40Ar, 39Ar, 38Ar, 37Ar, and 36Ar) are 

measured by mass spectrometry. Then, the corrections for all 

irradiation-produced interfering isotopes of argon are made. 

Next, the correction for atmospheric argon is made using the 

measured atmospheric argon ratio as determined from measuring atmospheric argon on the same mass spectrometer. This 

measured ratio also will be used to correct the argon abundances 

for mass discrimination that takes place in all mass spectrometers. F in its simplest form is calculated from the following 

expression. 

F = 40ArR/

39ArK = 

{(40Ar/39Ar) - 295.5[(36Ar/39Ar) - (36Ar/37Ar)Ca(37Ar/39Ar)] -

(40Ar/39Ar)K}/ 

{1 - (39Ar/37Ar)Ca(37Ar/39Ar)}. (6) 

In this form, no correction is made for Cl-derived argon isotopes 

and other relatively minor interference corrections are ignored. 

In the argon geochronology laboratory at the USGS, Denver, we 

correct for all minor K-, Ca-, and Ar-derived interfering argon 

isotopes as well as Cl-derived argon isotopes. The resultant formulation for F is complex as follows: 

F = {A- [C3C4((E-C5B)/(1- C4 C5))]-C1[G-C10C6(D-C7((E-C5B)/ 

(1-C4C5))-C8(B-(C4(E-C5B)/(1-C4C5)))- C1(G-(C2(E-C5B)/(1-

C4C5)))-C2(E-C5B)/(1-C4C5))/(1-C9 C10C6)]}/{B-[C4(E-C5B)/(1-

C4C5)]}, (7) 

where F=40ArR/

39ArK, 

A=40Ar, 

B=39Ar, 

D=38Ar, 

E=37Ar, 

G=36Ar, 

C1=(40Ar/36Ar)At, 

C2=(36Ar/37Ar)Ca, 

C3=(40Ar/39Ar)K, 

C4=(39Ar/37Ar)Ca, 

C5=(37Ar/39Ar)K, 

C6=(36Ar/38Ar)Cl, 

C7=(38Ar/37Ar)Ca, 

C8=(38Ar/39Ar)K, 

C9=(38Ar/36Ar)At, 

C10=ClD, with subscript D = corrected for decay.

Standards 

Many standards are used for 40Ar/39Ar geochronology. 

Reviews of some of the more commonly used are presented in 

Roddick (1983), McDougall and Harrison (1999), and Dalrymple and others (1993). At the USGS Denver argon geochronology laboratory, we use two standards. Our primary standard is a 

hornblende, MMhb-1, from the McClure Mountain Complex in 

southern Colorado with percent K = 1.555, 40ArR=1.624×10-9 

mol/g, and K-Ar age=520.4 + 1.7 Ma (Alexander and others, 

4 Argon Thermochronology of Mineral Deposits 
1978; Dalrymple and others, 1981; Samson and Alexander, 

1987). Hornblende MMhb-1 is used worldwide as an interlaboratory 40Ar/39Ar standard and is particularly appropriate for use 

as a standard with samples of Mesozoic and older age. 

Our secondary standard is a sanidine from the Fish Canyon 

Tuff, FCT, also from southern Colorado. Steven and others 

(1967) determined a conventional K-Ar age for this sanidine of 

28.5 + 0.8 Ma. We have calibrated sanidine FCT against hornblende MMhb-1 in our laboratory with a resultant 40Ar/39Ar age 

of 27.84 + 0.04 Ma. FCT is particularly useful as a standard for 

samples with Cenozoic and Mesozoic age. 

A great deal of disagreement exists as to the actual age of 

hornblende MMhb-1 (Dalrymple and others, 1993; Renne and 

others, 1994; McDougall and Harrison, 1999); the published 

international mean age is 520.4+1.7 Ma; the Denver argon geochronology laboratory has used this value until recently. The 

disagreement results because the range in apparent ages determined in 18 laboratories is nearly 15 m.y., a difference in age of 

nearly 3 percent, and all ages are used to calculate the mean. 

Thus, the accuracy of this age is in question. Because the precision of the method is now commonly in the 0.2 percent range, 

and because the method is applied to problems requiring greater 

accuracy, such as resolution of the age of time units on the geologic age scale, a more accurate age is necessary. In view of this, 

Renne and others (1994, 1998a, 1998b) have evaluated both the 

accuracy of isotopic dating methods and the intercalibration of 

geochronologic methods. In a comprehensive review of argon 

geochronology standards, Renne and others (1998b) compared 

the commonly used standards, recalibrated two of these commonly used standards, and assessed the quality of each standard. 

From their careful work, among other conclusions, they recommended that the best age for MMhb-1 hornblende, as calibrated 

against the newly determined age of 98.79+0.96 Ma for their primary standard GA-1550 biotite, is 523.1+2.6 Ma, excluding the 

error in decay constant, and 523.1+4.6 Ma including decay constant errors. A slightly higher age of 525.1, with identical errors, 

is calculated if the primary standard used for calibration is GHC305 biotite. Because Renne and others (1998b) considered the 

K-Ar data for GHC-305 to be less accurate than those for GA1550, we have adopted an age of 523.1 Ma for MMhb-1 hornblende as well as their best age of 28.02+0.16 Ma for FCT sanidine. For most experiments on Phanerozoic samples, this 0.4-

percent difference is relatively unimportant. However, for timescale studies or analyses of Archean-age rocks, this difference 

can be important and must be recognized. 

The issue of standard-age inaccuracy becomes increasingly 

problematic for comparison of Proterozoic and older argon ages 

to dates determined by other isotopic systems, especially the UPb system (Renne and others, 1998a). Inaccuracies of only 1 

percent at 2,500 Ma translate to 25 m.y., and serious discordance 

between isotopic systems may result simply because of standard-age inaccuracy and not actual geologic age difference. A 

case in point is in the giant Western Australia gold belt where the 

viability of the argon system applied to Archean problems has 

been questioned because of discordances between argon dates 

determined by Kent and McDougall (1996) and reasonable ages 

based on geology and other isotopic dating (Witt and others, 

1996). The discordance and apparent inconsistency likely result 

from inaccuracies in the assumed age of the argon irradiation 

standard (James Dunlap, Australian National University, written 

commun., 2001). 

Because the supply of the original MMhb-1 hornblende 

standard is nearly depleted, recently USGS argon geochronologists recollected at the McClure Mountain Complex locality in 

south-central Colorado from which the original MMhb-1 standard was taken, and have begun to process a new hornblende 

standard, MMhb-2 (Kunk and others, 1994). We are in the process of carefully calibrating the age of this new hornblende standard using (1) a high-precision pipetting method calibrated at the 

National Institute of Standards and Technology to precisely 

(within 0.1 percent) and accurately (within 0.25 percent, excluding the decay constant error) determine the argon content, and 

(2) an isotope-dilution thermal ionization mass spectrometry 

method at the National Institute of Standards and Technology to 

precisely (within 0.1 percent) and accurately (within 0.25 percent) determine the potassium content. The potassium analyses 

have been completed with percent K = 1.5500+0.0011 (1σ, six 

determinations) for hornblende MMhb-2. The potassium content of MMhb-1 was also redetermined by this method and is 

1.55713+0.00072 (1σ, three determinations). Once the argon 

content has been accurately determined, the standard will be 

released for worldwide use and the accuracy of the 40Ar/39Ar 

method will be comparable to its precision. 

This calibration is by so-called “first principles” (Lanphere 

and Dalrymple, 2000) and when completed will provide an analytically accurate measurement. However, others (Roddick, 

1983; Renne and others, 1998b; McDougall and Harrison, 1999) 

have argued that MMhb-1 (and presumably MMhb-2) hornblende is an inhomogeneous material at the single-grain level 

and thus not adequate for modern, high-precision, small samplesize analyses. It is also of an age (≈520 Ma) and potassium content that its usefulness for monitoring very young samples is 

marginal. Other laboratory standards are being proposed for the 

international 40Ar/39Ar dating standard. Clearly, considering the 

high precision and robust nature of the 40Ar/39Ar method in contrast to its lower accuracy, it is time for the argon community to 

evaluate potential standards and to accurately determine the age 

of the accepted standard(s). Renne and others (1994, 1998a, 

1998b) have begun this process, and others, such as Roddick 

(1987) and Baksi and others (1996), are contributing to the better characterization of 40Ar/39Ar standards. 

Thermal Release Experiments and Age Spectra 

The 40Ar/39Ar method was first used in “total-fusion” experiments in which an irradiated sample was completely melted and 

all isotopes of argon were measured in a single analysis to 

calculate a date for the sample (for example, Turner, 1970a,b). 

This total-fusion date is roughly analogous to a conventional KAr date for the sample, except that neither isotopic concentration 

measurement nor direct measurement of potassium content is 

Basis of the 40Ar/39Ar Technique 5 
-- -- --

--

--

-- -- --

-- -- --

Table 2. Production ratios determined for irradiations in GSTR done by the Denver argon geochronology 

laboratory and used in argon data reductions from year 1987 to year 2002. 

[ – indicates not determined ] 

Package (

37Ar/39Ar)K (38Ar/39Ar)K (40Ar/39Ar)K (39Ar/37Ar)K (36Ar/37Ar)K (38Ar/37Ar)K Comments 

Pre-DD8 .002200 .01300 .006270 .000673 .000264 .000032 From Dalrymple and 

others, 1981 

DD8 .00019 .01300 .007520 30 hrs irradiation 

DD9 .000448 .01300 .012600 .000255 .000069 100 

DD10 .000201 .01320 .012900 .000749 .000275 .000049 30 

DD11 .000208 .01310 .012100 .000263 .000051 30 

DD12 .000182 .01300 .009070 .000699 .000266 .000028 30 

DD13 .000139 .01300 .008760 .000708 .000269 .000029 30 

DD14 .000111 .01310 .008780 .000655 .000269 .000035 30 

DD15 .000158 .01320 .009120 96 

DD16 .000234 .01300 .011350 .000666 .000264 .000037 30 

DD18 .000146 .01305 .015600 .000746 .000251 .000030 30 

DD19 .000164 .01301 .009500 .000682 .000271 .000031 30 

DD23 .000174 .01304 .004970 .000675 .000261 .000037 20 

DD24 .000174 .01302 .008840 .000825 .000261 .000059 40 

DD25 .000128 .0130 6 .011800 .000990 .000264 .000032 30 

DD26 .000235 .01310 .010110 .000648 .000270 .000037 25 

DD27 .000110 .01313 .007700 .000641 .000263 .000064 60 

DD28 .000083 .01306 .008780 .000768 .000261 .000030 30 

DD30 .000082 .01306 .009184 .000636 .000270 .000032 35 

DD31 .000082 .01306 .009184 .000636 .000270 .000032 30 

DD32 .000656 .000256 .000037 20 

DD33 .000110 .01307 .009760 .000681 .000270 .000027 30 

DD34 .000080 .01310 .007135 .000675 .000265 .000035 123 

DD35 .000176 .01312 .006660 .000779 .000266 .000067 33 " 

DD36 .000110 .01307 .009760 .000681 .000270 .000027 21 

DD37 .000149 .01313 .008990 .000694 .000280 .000037 30 

DD38 .000149 .01313 .008990 .000681 .000270 .000030 25 

DD41 .000260 .01310 .015200 .000841 .000235 .000030 30 " 

DD42 .000099 .01318 .007500 .000630 .000290 .000021 30 

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required. Very soon after the first uses of the 40Ar/39Ar method, is an interpretive tool, the character of which can be evaluated 

it was realized that a sample could be progressively degassed in within a theoretical framework to interpret the apparent distriincreasing temperature increments (Merrihue and Turner, 1966). bution of potassium and argon within the sample. Turner 

A date can be calculated for each increment of gas evolved and (1968) showed that the dates for the temperature increments of 

the dates can be plotted sequentially and weighted by percent of some meteorites were identical within analytical precision, and 

total released argon to form an age spectrum. The age spectrum that when plotted on an age-spectrum diagram, which shows the 

6 Argon Thermochronology of Mineral Deposits 
--

-- -- --

-- -- --

-- -- --

--

-- --

--

-- -- --

--

--

-- -- -- --

Table 2—Continued. Production ratios determined for irradiations in GSTR done by the Denver argon geochronology laboratory and used in argon data reductions from year 1987 to year 2002. 

[ – indicates not determined ] 

Package (

37Ar/39Ar)K (38Ar/39Ar)K (40Ar/39Ar)K (39Ar/37Ar)K (36Ar/37Ar)K (38Ar/37Ar)K Comments 

DD43 .000055 .01313 .007950 .000680 .000280 .000056 30 hrs irradiation 

DD44 .000092 .01315 .008050 .000268 .000051 16 

DD45 .000150 .01308 .007950 .000635 .000266 .000045 30 

DD46 .000150 .01309 .008000 .000674 .000269 .000044 60 

DD47 .000100 .01308 .008034 .000609 .000271 .000039 20 

DD49 .000110 .01306 .007800 .000595 .000270 .000024 30 

DD50 .000127 .01308 .006200 .000505 .000271 .000018 30 

DD51 .000134 .01316 .008400 .000670 .000272 .000053 52.25 

DD52 .000175 .01317 .009750 8 

DD53 .000175 .01309 .007920 .000685 .000268 .000044 28.5 

DD54 .000685 .000275 .000044 40 

DD55 .000670 .000271 .000080 30 

DD56 .000145 .01310 .010400 .000786 .000271 .000035 20 

DD57 .000300 .01304 .009350 .000720 .000270 .000047 30 

DD59 .000129 .01321 .006850 .000665 .000270 .000058 30 

DD60 .000225 .01311 .007400 .000685 .000270 .000045 25 

DD61 .000175 .01311 .010400 .000634 .000270 .000046 15 

DD62 .000115 .01321 .007100 .000635 .000266 60 

DD63 .000225 .01337 .012000 .000274 25 

DD64 .000175 .01318 .009050 .000692 .000274 20 

DD65 .000338 .01322 .011530 .000915 .000273 .000083 20 

DD66 .000320 .01322 .009315 .000648 .000275 .000086 20 

DD67 .000120 .01326 .009480 .000698 .000272 .000039 30 

DD68 .000100 .01326 .008500 30 

DD70 .000140 .01312 .009150 .000675 .000274 .000045 30 

DD71 .01318 .006000 .000818 .000271 .000050 20 

DD72 .01307 .008950 .000779 .000268 .000027 15 

DD73 .01325 .010500 18 

DD74 .000192 .01343 .010100 .000687 .000267 .000116 129 hrs 

DD75 .000207 .01323 .009050 .000736 .000269 .000035 30 

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date of each temperature increment as a function of percent 39Ar laboratory. In contrast, I use “age” or a specific type of “age,” 

released during the experiment, the spectra formed “plateaus” such as “emplacement age,” “cooling age,” or “age of metamor­

(fig. 1). (Until geologic significance is placed upon the analyti- phism,” to refer to a date that has been constrained by geologic 

cal number, I prefer to use “date,” “apparent age,” or “numerical data and is interpreted to have geologic meaning.) 

age” to refer to the analytical number determined by solving the Ideally a sample dated by the 40Ar/39Ar age-spectrum tech­

“age” equation using isotopic data produced for a sample in the nique will yield concordant dates for all temperature steps. If a 

Basis of the 40Ar/39Ar Technique 7 
120 

110 

100 

90 

800 10 20 30 40 50 60 70 80 90 100 

PERCENT 39ArK RELEASED 

40Ar/39Ar AGE SPECTRUM FOR BIOTITE DVZ-2 

tp = ± 0.18 Ma 

Plateau: apparent age of all steps 

overlapping within 95% confidence level 

93.71 

Figure 1.A typical 40Ar/39Ar age spectrum. Vertical axis plots apparent 

age; horizontal axis is cumulative percent 39Ar released, from 0 to 100 

percent. Lowest extraction temperature step is plotted at left; each progressively higher temperature step is plotted successively to right to 

form spectrum of apparent dates obtained from the sample during the 

heating experiment. Length of corresponding segment represents percentage of 39Ar released during each heating step. Vertical thickness of 

bar shows 2σ error envelope above and below age of that individual 

step. This age spectrum is for a biotite from the Domenigoni pluton in the 

Peninsular Ranges batholith south of Riverside, Calif.; plateau date is 

93.71+0.18 (1σ) Ma. 

sample’s age spectrum is 100 percent concordant, that is, all 

temperature steps yield identical dates within two standard deviations (2σ) of the weighted mean date for all temperature steps, 

then clearly the best date for the sample is a weighted mean of 

the dates of all temperature steps. The geologic significance of 

the resultant date must then be evaluated using geologic or other 

independent constraints. 

For a sample to exhibit 100 percent concordancy in apparent ages across all temperature steps is unusual. Most dated 

samples display some discordancy on the age spectrum because 

of a physical or chemical disturbance, such as the presence of 40Ar that was not derived from the in-place decay of 40K, or the 

loss of some 40Ar after the original closure of the sample to 40Ar 

diffusion. However, even if an age spectrum shows some 

degree of discordancy, the interpreted dates usually hold geologic significance. Some commonly used terms for interpreted 

dates derived from an age spectrum are total-gas date (or age), 

integrated date (or age), plateau date (or age), and preferred 

date (or age). The total-gas date from a thermal release experiment is the average of the dates of all temperature steps for the 

sample weighted according to percentage of released 39Ar in 

each step; the total-gas date is comparable to a conventional KAr date but is more precise and more accurate for the reasons 

previously stated. An integrated date is virtually identical to the 

total-gas date but is preferred by some others. The plateau date 

is the weight-averaged date for that part of the age spectrum 

composed of contiguous gas fractions that together represent 

more than 50 percent of the total 39Ar released from the sample 

and for which no difference in apparent age can be detected 

between any two fractions at the 95 percent confidence level 

(Fleck and others, 1977). The term plateau has been used by 

8 Argon Thermochronology of Mineral Deposits APPARENT AGE (Ma)

others (for example, Lanphere and Dalrymple, 1971; Dalrymple 

and Lanphere, 1974; Harrison, 1983; see also McDougall and 

Harrison, 1999) to refer to a “high-temperature segment” of an 

age spectrum throughout which the dates of the plateau-defining 

steps are concordant at the 95 percent confidence level. In some 

cases, by this usage, a single step may define a plateau. Finally, 

some age spectra show near concordancy, and weight-average 

dates of the near-concordant parts of the spectrum may have 

some geologic meaning. I refer to these dates as preferred dates 

that consist either of an apparent plateau comprising less than 50 

percent of the total released 39Ar or of fractions of gas whose 

dates overlap within three standard deviations (3σ) of the 

weighted mean. 

Error Analysis 

In order to determine the actual 40ArR/

39ArK ratio (F) of a 

sample or standard, we measure the isotopic abundances of five 

argon isotopes (40Ar, 39Ar, 38Ar, 37Ar, and 36Ar) by mass spectrometry. Each of these laboratory measurements has an associated error estimated as the standard deviation of analytical 

precision. These analytical errors, along with those resulting 

from the measurement of reactor-induced interfering argon isotopes and atmospheric argon isotopes, are propagated in our 

error calculation for F. In the Denver argon geochronology laboratory, we also independently estimate an error for each measured F by using a pooled coefficient of variation for numerous 

measurements of multiple splits of an extracted gas fraction 

from a single sample; this pooled coefficient of variation is 0.11 

percent, at 1σ. Both error estimates are calculated, and the 

larger estimate is used for any analysis to ensure a conservative 

error. For an error estimate on any calculated date, the error in 

the J-value is included in the estimate. Errors in J are analytical 

errors calculated at 1σ from the calculation of the J-value associated with monitors (standards) adjacent to each unknown. Our 

error equation is modified from that published by Dalrymple and 

others (1981) in which we derive our error formula by differentiation of our equation for F; this equation includes errors in chlorine-derived argon isotopes and other minor interferences. 

Roddick (1987, 1988) and Scaillet (2000) presented an alternative approach to error analysis that avoids the need to make simplifying assumptions. Roddick developed a more rigorous 

numerical error propagation that takes into account the direct 

effect of the analytical error of each measurement on F. 

To evaluate an age spectrum, we compare the errors associated with the F-values of adjacent fractions of gas at the 95 percent confidence level (2σ) using the critical value test (McIntyre, 

1963; Dalrymple and Lanphere, 1969). The Critical Value, C.V. 

equals 1.96 [(σ1)2

/n1+ (σ2)2/n2]1/2, where σ1 and σ2 are the standard deviations of the analytical measurements (in this case, the 

F-values) and n1 and n2 are the number of measurements. If two 

adjacent fractions are analytically identical in F-value, the difference in their F-values must be less than the calculated critical 

value. If a plateau is defined for a sample, the apparent age of 

the plateau segment is the weighted mean of the apparent ages of 

the steps on the plateau, and the error in the plateau date is the 

standard deviation (1σ) of the apparent ages included in the plateau-date calculation. 
Ca 

Ultimately apparent ages of several samples are compared. 

To decide whether an absolute difference exists between two 

apparent ages, we again employ the critical value test. If the difference between two apparent ages is less than their critical 

value, an age difference has not been established. This does not 

mean that there is no age difference, but only that if one exists, it 

cannot be resolved by our method. In contrast, if the difference 

between two apparent ages is greater than the critical value, an 

apparent age difference has been detected. Apparent ages of 

populations also are compared in this way. After a population is 

defined on some criterion (for example, all belong to the population of a single vein set), all dates determined for that population 

are averaged and a weighted mean apparent age with associated 

standard deviation can be calculated. 

39Ar/37Ar (Apparent K/Ca) Diagrams 

During irradiation 39Ar is produced from 39K and 37Ar is 

produced from 40Ca. After correction for interfering argon isotopes, 39Ar/37Ar ratios provide valuable information on the relative distribution of K with respect to Ca in a sample. An 39Ar/ 37Ar diagram is complementary to the sample’s age spectrum 

diagram and displays the relative 39Ar/37Ar ratio for each temperature step of the sample plotted against percent 39Ar 

released. The 39Ar/37Ar ratio is directly proportional to the K/ 

Ca ratio corresponding to each temperature step of the analysis. The proportionality constant is controlled by the ratio of 

fast to thermal neutrons during irradiation and thus not only is 

reactor dependent but also can change from irradiation to irradiation in a single reactor. For the Geological Survey TRIGA 

reactor (GSTR), Dalrymple and others (1981) determined the 

relationship to be 

K/Ca = (0.49+0.09) 39ArK/

37ArCa (8) 

The constant of proportionality was calculated using data 

from 19 samples for which K/Ca was measured by independent 

chemical means. The Denver argon geochronology laboratory 

has not repeated this experiment, but based on more than 70 irradiations since 1986, using hornblende MMhb-1 as our standard, 

we have observed that its 39ArK/

37ArCa always lies between 0.40 

and 0.41. The reported K/Ca of this hornblende is 0.208 (Samson and Alexander, 1987), which is within error of the K/Ca calculated using the preceding equation with our measured 39ArK/ 37ArCa values of 0.40 to 0.41. So for samples irradiated in 

GSTR, a semiquantitative K/Ca can be calculated for a sample, 

using this equation. This, however, is not a universal equation, 

because the fast-to-thermal neutron ratios of other reactors are 

not the same as that of GSTR. 

An 39ArK/

37ArCa diagram for a sample can prove to be quite 

useful in the interpretation of complex age spectra. From our 

experience, for example, standard amphibole will exhibit a constant 39ArK/

37ArCa ratio throughout the temperature range of 

release of structurally controlled argon (fig. 2). 

In contrast, it is common for argon that is released at lower 

extraction temperatures—and less commonly at higher extraction temperatures—to have strikingly different 39ArK/

37ArCa as 

well as different apparent ages. 

39ArK/37ArCa FOR HORNBLENDE JMZ-2 

1.5 

1.0 

0.5 

0.0 0 10 20 30 40 50 60 70 80 90 100 

0.5 39ArK/37ArCa = K/Ca 

PERCENT 39ArK RELEASED 

Figure 2. A typical 39ArK/

37ArCa relation. Analogous to an age-spectrum diagram, vertical axis plots 39ArK/

37ArCa; horizontal axis is cumulative percent 39Ar released from 0 to 100 percent. Each 

segment of the line corresponds to 39ArK/

37ArCa for a temperature 

step. For the TRIGA reactor, K/Ca is approximately 0.5(39ArK/

37ArCa). 

This plot is for a hornblende from the Peninsular Ranges batholith; 

its age spectrum defines a plateau that is ≈90 percent concordant 

with a plateau date of 101.9+0.2 Ma and a constant K/Ca throughout 

the plateau. 

We have interpreted these different ratios to reflect contributions of argon from a variety of sources, such as fluid inclusions, intergrown minerals such as pyroxene, plagioclase, 

biotite, or actinolite, or the influence of system blank. In studies 

of white micas, Roeske and others (1995), Christiansen and Snee 

(1994), and Till and Snee (1995) used K/Ca ratios with isochron 

diagrams and back-scattered element images to define the phases 

reflected in complex argon release spectra. Thus, an 39ArK/ 37ArCa diagram is an effective graphical tool for elucidating 

argon systematics within a sample (fig. 3). 

Correlation Diagrams 

Two correlation diagrams are used to interpret 40Ar/39Ar 

data, and in most cases either or both are used to confirm agespectrum results or to try to derive useful information from complex age spectra. These diagrams are particularly useful for 

determining and illustrating the age of the sample and the composition of the trapped (the nonradiogenic) component(s) of the 

sample. The isochron diagram (fig. 4A) (Merrihue and Turner, 

1966) is a plot of 40Ar/36Ar versus 39Ar/36Ar after reactorinduced interferences have been deleted. The inverse correlation 

diagram (fig. 4B) is a plot of 36Ar/40Ar versus 39Ar/40Ar with 

reactor-induced interferences removed (Turner, 1971b). 

Although some have argued that one has more utility than the 

other (Roddick and others, 1980; McDougall, 1985; Phillips and 

Onstott, 1986), the “two types of diagrams***yield the same 

information provided the correct mathematics are used for estimating correlation coefficients and for the least squares fit” (Dalrymple and others, 1988, p. 589). Choice of one over the other 

simply comes down to personal preference. Excellent discussions of the constructions, formulations, and error analysis of 

Basis of the 40Ar/39Ar Technique 9 

39ArK/37Ar 
these diagrams are presented in the reports just listed as well as 

York (1969), Roddick (1978), and Wendt and Carl (1991). 

We use correlation diagrams to evaluate the nature of the 

trapped nonradiogenic argon component and to help unravel 

complex argon age spectra. Heizler and Harrison (1988) and Till 

and Snee (1995; fig. 5) showed how this approach can be useful. 

Both studies demonstrated that multiple trapped components of 

nonradiogenic argon can be revealed by careful isochron analyses and some pure luck. In both studies, age spectra that did not 

provide unambiguous age information were interpretable with 

associated isochron analysis. 

Argon Loss 

In contrast to the simple argon release displayed in a plateau spectrum, Turner (1968) showed that some age spectra 

exhibited a distinct increase in dates across an age spectrum 

from low-temperature to high-temperature extraction steps (fig. 

6). Turner showed theoretically that an age spectrum will 

exhibit a step-up in dates if argon is lost from a sample in a geologic environment by thermally activated volume diffusion. 

The age spectrum will exhibit a plateau if the sample had 

never been disturbed after formation or if the sample had been 

completely reset by a younger thermal event (end-member 

curves 1.0 and 0). Depending on the amount of thermal disturbance, that is, the percentage of argon lost, the date of the 

younger, low-temperature fractions will be equal to, or older 

than, the age of the thermal disturbance that affected the sample, 

whereas the date of the older, high-temperature fractions will be 

equal to, or younger than, the original age of closure. This stepup in dates exhibited by an age spectrum and apparently resulting from argon loss due to geologic activity has been experimentally reproduced by Harrison (1981) from hornblende (fig. 7) 

and has been observed by numerous investigators from hornblende, muscovite, and potassium feldspar. 

The reliability of hornblende, muscovite, and potassium 

feldspar to record geologically induced thermally activated 

argon loss as exhibited in an age spectrum produced by experimental degassing in the laboratory has been called into question 

by Lee and others (1991). They contended that hydrous minerals, in particular, but potassium feldspar as well, undergo 

changes, including dehydration, melting, and phase conversions, 

during heating under vacuum. These heating-induced changes 

prevent release of argon by thermally activated volume diffusion 

from the mineral. Thus, according to Lee and others (1991), 

experimental reproduction of the geologic phenomenon of argon 

loss is impossible. 

Although the mechanism responsible for the development 

of apparent argon-loss age spectra from some hornblendes, 

potassium feldspars, and muscovites may not be completely 

understood, there is no question that this pattern is exhibited by 

many, if not most, of these minerals after they undergo partial 

argon loss due to reheating in the geologic environment. 

Besides thermally activated volume diffusion, several other possible mechanisms can explain apparent argon-loss spectra. For 

example, two or more phases of different isotopic ages may 

10 Argon Thermochronology of Mineral Deposits 

1,000 

500 

0 

180 

140 

100 

60 

0 50 100 

130.0±0.4 Ma 125.2±0.3 Ma 

127.1±0.5 Ma 119.3_ 

±0.3 Ma 

A PERCENT 39ArK RELEASED 

EXPLANATION 

CH128, coarse grained 

CH128, fine grained 

Figure 3 (above and following page). Diagrams of age and K/Ca spectra, and back-scattered electron image of Alaskan white mica sample. A, 

Age spectra for fine-grained and coarse-grained fraction of white mica 

from Cosmos Hills, Alaska (Christiansen and Snee, 1994) show a bimodal 

age distribution that is mirrored by the K/Ca diagram; grain sizes show different apparent ages. K/Ca indicates that high-K white mica (phengite in 

this case) controls argon in high-age part of hump; low-K white mica (paragonite) controls argon in low-age part of spectrum. B, Back-scattered 

electron image confirms complex intergrowth; ph, phengite; pg, paragonite; ms, muscovite; cld, chloritoid. 

coexist in one rock. If two different phases of similar character, 

such as the white micas, phengite and muscovite, coexist in a 

mineral separate, and if each phase degasses in vacuum over different temperature ranges, then the resultant age spectrum will 

represent a mixture of the age spectra for both samples and will 

resemble an argon-loss spectrum (Till and Snee, 1995). Similarly, a single-phase mineral separate that has several grain sizes 

or structural types that degas over different temperature ranges 

under vacuum can produce the same result (Cosca and others, 

1992). In both of these examples, the apparent argon-loss spectrum can be interpreted as if it had resulted from thermally activated volume diffusion. That is, the apparent age of the younger, 

lower temperature fractions is approximately the age of, or older 

than, the age of thermal resetting. Therefore, apparent argonloss spectra give us considerable potential for understanding the 

thermal histories of complex geologic environments. However, 

we must use caution when interpreting apparent argon-loss spectra because other, nongeologic factors, such as extraction-system 

blanks and sample impurity, can affect the character of an age 

spectrum. APPARENT AGE (Ma) APPARENT K/Ca 
Basis of the 40Ar/39Ar Technique 11

Extraneous Argon

Dalrymple and Lanphere (1969) defined extraneous 40Ar as 

consisting of two types—excess 40Ar and inherited 40Ar. 40Ar results from other than in-place radioactive decay; inherited 40Ar is incorporated in a system from mineral grains that retain 

an older reservoir of argon. The mechanism for the incorporation of excess 40Ar into a rock or mineral is unknown, but several 

possible explanations exist. 

an environment that contains argon with an 40Ar/36Ar composition greater than 295.5 (the present-day atmospheric argon composition) and if this enriched argon is incorporated into the rock 

or mineral, the initial 40Ar/36Ar ratio of the mineral will have 

“excess 40Ar” relative to present day. , hydrothermal 

fluids are known to carry argon that commonly has an isotopic 

composition greater than 295.5, perhaps due to fluid interaction 

with rocks containing radiogenic argon. or fluids that derive 

argon from very old, potassium-rich rocks, incorporated 40Ar/ 36Ar ratios could conceivably be much greater than 100,000. 

these fluids alter or become incorporated in a rock or mineral, 

excess 40Ar can be added to that rock or mineral. 

Several studies (such as Lanphere and Dalrymple, 1971, 

1976; Kaneoka, 1974) have documented that large quantities of 

excess 40Ar in a sample will produce a saddle-shaped 40Ar/39Ar 

age spectrum that exhibits anomalously old dates for lower temperature and higher temperature extraction steps (fig. 8). 

quantities of excess 40Ar commonly only affect the lower temperature extraction steps, resulting in an L-shaped spectrum. 40Ar-enriched argon derived from the lower temperature parts of 

the release spectrum commonly is associated with low-retention 

sites, such as fluid inclusions, defects, and alteration; higher relative K/Ca is common for these steps. 40Ar-enriched argon 

derived from the higher temperature parts of the spectrum is 

likely associated with low-potassium, high-retention mineral 

phases, such as pyroxene; lower relative K/Ca is common for 

these higher temperature steps.

39Argon Recoil

39Ar recoil occurs as a direct result of the production of 39Ar 

from the 39K(n,p) 39Ar nuclear reaction. When a fast neutron 

collides with a 39K nucleus, a proton is ejected to form an 39Ar 

nucleus. The ejected proton causes “recoil” of the 39Ar atom. 

To interpret an 40Ar/39Ar age spectrum as representing the 

natural behavior of argon released from a sample requires that 

none of the features of the spectrum be produced by the experimental method itself. The recoil of 39Ar from a sample during 

irradiation is a function of the experimental method and violates 

this assumption. The result of the loss of 39Ar from a sample is 

the anomalous increase in the 40Ar/39Ar of the material and an 

increase in the apparent age (fig. 9). 

The mean energy of the recoil in most research reactors 

ranges between 100 and 200 keV (Turner and Cadogan, 1974). 

Using the work of Davies and others (1963), Turner and Cadogan 

(1974) reasoned that if an outer layer of material were affected by 

recoil, its mean depth of depletion would be 0.082 μm. 

pler terms, if the 39Ar atom is in a weakly held location, it could 

B

Basis of the 40Ar/39Ar Technique

Excess 

If a rock or mineral crystallizes in 

Similarly

F

If 

Small 

In sim-
0.004 

0.003 

Atmospheric argon ≈ 295.5 

600 

800 

1,000 

Sample initial 40Ar/

36Ar 

Slope ∝ age 

1/Sample (40Ar/

36Ar)

i 

1/Sample 40Ar/

39Ar; 

Intercept ∝ age 

40Ar/36Ar 

36Ar/40Ar 

0.002 

400 

0.001

200 

0 

0 20 40 60 80 100 0 

0 2 4 A 39Ar/

36Ar B 39Ar/

40Ar 

Figure 4. Hypothetical isochron (A) and hypothetical inverse correlation (B) diagrams. Slope of line in the isochron diagram is directly proportional to age, and intercept is initial 40Ar/36Ar. Generally accepted 40Ar/36Ar atmospheric ratio is 295.5. Intercepts of line in inverse correlation diagram are 1/(40Ar/36Ar)i

 (where i=initial; vertical axis) and 1/F (horizontal axis). 

be completely ejected from the 39K site perhaps ending up in an 

adjacent site or being completely ejected from the material. 

Huneke (1976), Huneke and Smith (1976), and Onstott and others (1995) showed that 39Ar recoil could affect age spectra of 

samples in which potassium is principally located in fine-grained 

phases adjacent to potassium-poor minerals, such as in finegrained basalts. We have shown (Folger and others, 1996) that 39Ar recoil has a significant effect on illite/white mica grains that 

are less than 10 μm. In this study, for grain sizes less than 0.1 

μm, we documented that as much as 28 percent of the 39Ar was 

recoiled completely out of the sample; the amount of recoiled 

39Ar was indirectly proportional to grain size decreasing essentially to zero from samples with grain sizes exceeding 10 μm. 

Closure Temperature and Diffusion 

The apparent age calculated for a mineral from its accumulated radioactive decay products is the time when the chemical 

system of that mineral became closed to diffusion of that particular radioactive decay product. Many early studies (Jäger, 1962, 

1965, 1967; Jäger and others, 1967; Hart, 1964; Aldrich and others, 1965; Armstrong and others, 1966; Hanson and Gast, 1967; 

Hanson and others, 1975; Berger, 1975; Berger and York, 1981, 

among others) showed that isotopic dates for minerals in the 

Rb-Sr and K-Ar systems gave discordances that were best 

explained as cooling ages that were younger than age of formation or ages that had been thermally reset. These studies also 

showed that different minerals exhibited characteristic retentivity to diffusion of daughter products, reflecting different isotopic 

closure temperatures for each mineral. Closure to diffusion is 

controlled chiefly by temperature but also by cooling rate 

(Carslaw and Jaeger, 1959; Dodson, 1973), chemical compositional variation (for example, Till and Snee, 1995), and structural 

12 Argon Thermochronology of Mineral Deposits 

state variation or strain history (Cosca and others, 1992). Closure to diffusion takes place over a temperature range; each 

mineral has its characteristic closure-temperature range. The 

apparent age is a measure of when the mineral cooled through 

the closure-temperature range. The closure-temperature range is 

higher and narrower for fast cooling, and conversely, is lower 

and broader if cooling was slow. For the 40Ar/39Ar system, closure temperatures of the most commonly used geochronometers 

are reasonably well known; and 40Ar/39Ar isotopic dates provide 

a thermochronology that connects time and temperature. 

The argon geochronometers that have the best documented 

closure temperatures are hornblende, biotite, muscovite, and 

potassium feldspar. Commonly accepted closure temperatures 

that span a range from rapid (1,000 °C/m.y.) to slow cooling 

(5°C/m.y.) are 580°–480 °C for hornblende (Harrison, 1981), 

325°–270 °C for muscovite (Snee and others, 1988; 2M1 structural state), 340°–280 °C for biotite (McDougall and Harrison, 

1999; Snee, 1982b); and a wide range of >300 °C to <150 °C for 

microcline and orthoclase feldspars (McDougall and Harrison, 

1999). (Each of these is discussed in more detail in the following sections.) When different minerals with different argon-closure temperatures are used in combination for some geologic 

terranes, cooling curves (such as shown in fig. 10) can be 

derived. 

Again, as with the applicability of the concept of volume 

diffusion to 40Ar/39Ar studies, argument exists within the argon 

community on the validity of the closure-temperature concept. 

Villa (1997) argued that field observations going back to the 

original work defining the closure concept (Jäger, 1967), when 

viewed in the context of newer work, can be interpreted entirely 

as a function of isotopic inheritance. Our work and that of many 

others have shown that mixed argon reservoirs arising from 

intergrowth of phases can simulate the behavior predicted for 

6 
13 

88ATi5Y 

White mica 

150,000 

100,000 

50,000 

675 ± 2 Ma 

1 4 2 

5 

3 

6 

7 

8 

9 

10 12 

13 11

14 

15 

900° C and lower 

950° C and higher 

0 1,000 2,000 

C 

0 

EXPLANATION 

39Ar/36Ar 

40Ar/36Ar 

60,000 

80,000 

40,000 

20,000 

0

0 1,000 2,000 3,000 

87ATi54C 

White mica 

D 

40Ar/36Ar 

39Ar/36Ar 

750° C and lower 

1,050° C and higher 

800° C to 1,000° C 

7 6

8 

10 9

1 2 

4 

3 

5 

11 

12 

13 

190 ± 4 Ma

600 ± 2 Ma 

Mixed 

EXPLANATION 

x 

xx x xxx x x

x xx x 

6.0 

6.5 

7.0 

7.5 

0 0.2 0.4 0.6 

Mg/(Mg=Fe) 

Si pfu 

E 

86ATi91A - beards 

86ATi91A - coarse 

88ATi5Y - fine 

88ATi5Y - coarse 

87ATi54C 

volume diffusion and thus could be interpreted to support the 

conclusions of Villa (1997). wever, study after study shows 

an indisputably clear relationship between the ages of various 

phases and geologic cooling after some thermal process. 

Furthermore, this relationship extends beyond the 40Ar/39Ar system into the U-Pb and Rb-Sr systems. The reality probably lies 

in between the extremes of the arguments, and ultimate 

800° C and lower 

850° C and higher 

682 ± 5 Ma 

B 

40Ar/36Ar 

39Ar/36Ar 

60,000 

40,000 

20,000 

86ATi91A 

White mica 

0 500 1,000 

0 

EXPLANATION 

86ATi91A 

88ATi5Y 

87ATi54C 

54C 

5Y 

91A 

APPARENT AGE (Ma) 39Ar/37Ar 

100 

200 

300 

400 

500 

600 

700 

150 

300 

450 

600 

750 

PERCENT 39ArK RELEASED 

10 20 30 40 50 60 70 80 90 100

A 

0 

Figure 5. Age and isochron diagrams and chemical plot for 

Brooks Range, Alaska, white micas (Till and Snee, 1995). A, Age 

spectra and K/Ca composite diagram; B, C, D, isochron diagrams; E, 

chemical plots 

rovian and high-pressure metamorphism of Cretaceous age overprinting Precambrian-age gneisses. 

degrees of apparent argon loss. 

become increasingly disturbed with increasing degree of partial resetting and concomitant argon disturbance (shown from B to C to 

D). 

pfu, Si per formula unit; beards, late-growth white mica developed 

around older biotite grains. 

Basis of the 40Ar/39Ar Technique 

resolution will require other approaches to resolve the conflict. 

In view of this, Dunlap (2000; see also York, 1984) assessed 

argon diffusion from the aspect of the natural environment by 

evaluating cooling rates over geologic time. s 

Ho

These three white micas are from an area of BarThe white micas show varying 

Isochrons for the three samples 

Chemistry shows compositional complexity of the samples; Si 

Dunlap’
APPARENT AGE (Ga) 

0.2 

0.5 

1.0 

2.0 

3.0 

4.0 

4.5 

FRACTION OF 39Ar RELEASED 

0 0.5 1.0 

A 

RELATIVE 40Ar/ 39Ar RATIO 

0.01 

0.02 

0.05 

0.1 

0.2 

0.5 

1.0 

1.0 

0.98 

0.95 

0.9 

0.8 

0.6 

0.4 

0.2 0.1 

0.05 

0 

FRACTION OF 39Ar RELEASED 

RELATIVE 40Ar/39Ar RATIO 

APPARENT AGE (Ga) 

0 0.5 1.0 0.01 

0.02 

0.05 

0.1 

0.2 

0.5 

1.0 

0.2 

0.5 

1.0 

2.0 

3.0 

4.0 

4.5 

B 

0.98 

0.90 

0.95 

0.8 

0.6 

0.4 

0.2 

0 

conclusion is that ancient (Precambrian) orogens as a whole 

exhibit slower postorogenic cooling. When minerals like biotite 

are exposed in the crust to prolonged periods at elevated temperatures approaching or slightly lower than their closure-temperature range, the minerals have a tendency to lose argon. 

observation is correct, it lends support to the closure-temperature 

and diffusion concepts. 

Some Minerals Useful for 40Ar/39Ar 

Geochronology 

Amphiboles 

Amphiboles are widespread in igneous and metamorphic 

rocks and contain as much as 2 percent potassium. 

primarily, but to some extent other amphiboles, has proved to be 

exceptionally retentive of radiogenic argon having a high closure 

temperature at or above 550°C (Harrison, 1981). 

emplacement, cooling, and uplift of plutonic rocks, hornblende 

forms the argon geochronologic proxy for a zircon crystallization date, assuming sufficient estimates for the cooling period 

after emplacement; and if the hornblende date is combined with 

zircon U-Pb dates, important high-temperature cooling constraints can result (for example, Premo and others, 1998). 

blende in metamorphic rocks is also valuable for defining 

cooling history after metamorphism; moreover, in cases where 

hornblende is recovered from metamorphic rocks that formed at 

a temperature below the argon closure temperature for the hornblende in the rock, an 40Ar/39Ar date for that hornblende is one of 

the best estimates for the age of metamorphism. 

also is present in a wide range of volcanic rocks and in skarns 

and provides a means for dating the volcanism and contact metamorphism. Amphibole has been used to date skarns (Berger and 

others, 1983; Chesley and others, 1993) and amphibole-facies 

associated mineralization (Napier and others, 1998), but its 

greatest utility in mineral-deposit geochronologic studies comes 

in the definition of premineralization cooling history of host 

rocks. wing the thermal history of the host is important in 

order to limit the possible interpretations of the ages of phases 

Figure 6. Examples of Turner’s theoretical argon loss diagrams. 

Sample assumed to be 4.55-Ga meteorite affected from 0 percent 

to 100 percent (0 to 1.0) argon loss during a 500-Ma thermal event. 

Family of curves mimics degree of argon loss expected in argon 

age spectra. A, family of curves for sample of uniform-size 

spheres; B, family of curves for sample of lognormal distribution. 

Reproduced with permission of Grenville Turner (from Turner, 

1968). 

Natural thermal 

release 

40Ar/39Ar AGE SPECTRA FOR HARRISON (1981) HORNBLENDES 

400 

300 

200 

100 

APPARENT AGE (Ma) 

0 20 30 40 50 60 70 80 90 100

PERCENT 39ArK RELEASED 

EXPLANATION 

After hydrothermal 

heating at 850 °C 

for 32.9 days 

Theoretical 

Figure 7. Hydrothermal heating of hornblende (modified from 

Harrison, 1981). 

ural hornblende shows a plateau formed by more than 95 percent 

of the released 39Ar. 

that was hydrothermally treated at 850°C for 32.9 days, and subsequently irradiated and argon dated, shows stepping-up pattern 

with increased temperature of release that is typical of apparent 

argon loss. 

according to Turner’s (1968) prediction. 

sion from Contributions to Mineralogy and Petrology, “Diffusion of 

40Ar in hornblende,” by T. Mark Harrison, 

© Springer Verlag. 

Harrison. 

If this 

Hornblende, 

In studies of 

HornHornblende 

Kno

Age spectrum diagram (blue) for undisturbed, natAge spectrum (red) for the same hornblende 

Dashed green curve represents 15 percent argon loss 

Reproduced with permisv. 78, p. 326, figure 1, 1981, 

And with permission of the author, T. Mark 

14 Argon Thermochronology of Mineral Deposits 
40Ar/39Ar AGE SPECTRUM FOR DEATH VALLEY WHOLE-ROCK BASALT 

3 

2 

0

0 10 20 30 40 50 60 70 80 90 100 

1 

t

max= 300±10 ka 

t

tg=820±50 ka 

PERCENT 39ArK RELEASED 

Figure 8. 40Ar/39Ar age spectrum for a sample with excess 40Ar. Sample of whole-rock basalt from floor of Death Valley, Calif., displays anomalously old apparent ages for lower 

temperature and higher temperature steps. Low step in “saddle” is interpreted to be a 

maximum age estimate for the eruption of the basalt; ttg= total-gas date; tmax= maximum estimated date for sample. 

40Ar/39Ar AGE SPECTRUM FOR WHOLE-ROCK BASALT SRB-3 

20 

15 

10 

5 

0

0 10 20 30 40 50 60 70 80 90 100 

PERCENT 39ArK RELEASED 

Figure 9. 40Ar/39Ar age spectrum for a sample exhibiting 39Ar recoil. Characteristic steppingdownward in age spectrum results from the loss of 39Ar by recoil out of sample during irradiation. 

Loss of 39Ar increases the apparent 40Ar/39Ar ratio and age. APPARENT AGE (Ma) APPARENT AGE (Ma) 

dated from the mineralized rocks. Lund and others (1986; this 

report, fig. 10) used amphibole dates from the host rocks of epithermal gold-bearing quartz veins in central Idaho to uniquely 

interpret the white mica dates as formation ages and not cooling 

ages. Other studies that have followed this approach include 

Snee and others (1995) and Goldfarb and others (1991, 1993, 

1997). 

Because most argon from hornblende apparently is released 

in the vacuum chamber during dehydration and decomposition 

as the hornblende is heated, much debate rages over the meaning 

of an age spectrum generated from step-wise heating experiments. Because the purpose of step-heating experiments in vacuum is to simulate volume diffusion resulting from heating in 

the geologic environment, this dehydration destroys that 

Some Minerals Useful for 40Ar/39Ar Geochronology 15 
? ? ? Metamorphic 

biotite 

Metamorphic 

hornbende 

Granitic 

muscovite Vein 

muscovite 

Granitic 

microcline 

High T 

Low T 

Low-temperature regional cooling 

Granite 

emplacement 

Gold-vein 

formation 

Post-metamorphism 

cooling 

500 

400 

50 60 70 80 

300 

200 

100 

0 

AGE (Ma) 

TEMPERATURE (°C)

40Ar/39Ar AGE SPECTRUM FOR HIMALAYAN 

HORNBLENDE 2,500 

2,000 

1,500 

1,000 

500 0 10 20 30 40 50 60 70 80 90 100

PERCENT 39ArK RELEASED 

APPARENT AGE (Ma) 

tp = ± 8 Ma 

2,031 ± 8 Ma 

573 ± 4 Ma 

A 

1.5 

1.0 

2.0 

0.5 

0 0 0 20 30 40 50 60 70 80 90 100

PERCENT 39ArK RELEASED 

39ArK/37ArCa 

39ArK/37ArCaFOR HIMALAYAN HORNBLENDE 

Hornblende 

Actinolite Chlorite/biotite 

B 

0.5 millimeters 

Hornblende 

Actinolite 

Biotite 

Chlorite 

HIMALAYAN HORNBLENDE 

C 

simulation. xamples of apparent argon 

loss (that is, monotonic rise in apparent age with increasing 

extraction temperature) exist. xtensive experience, 

many of these cases are directly a function of multiple generations of hornblende in a single sample or overgrowths of other 

amphiboles on an earlier formed hornblende (Snee and others, 

1995; Baig, 1990; fig. 11). Whatever the case, valuable information is derived from step-heating experiments, and derived age 

spectra allow recognition of excess argon. Also, the lower temperature steps commonly remove argon that is derived from fluid 

inclusions and other mineral impurities and prevents this argon 

from diluting K-lattice-derived argon released from the true 

amphibole sites. 

Some additional studies that explore the argon systematics 

of amphibole include Harrison (1981), Harrison and Fitz Gerald 

(1986), Onstott and Peacock (1987), Gaber and others (1988), 

Baldwin and others (1990), Kelley and Turner (1991), Wartho 

and others (1991), Lee and others (1991), Lee (1993), Rex and 

others (1993), Cosca and O’Nions (1994), Wartho (1995), and 

Dahl (1996a). 

White Micas 

Muscovite with as much as 10 percent potassium is common in metamorphic rocks, peraluminous plutonic rocks, pegmatites, and many mineral deposits. 

muscovite of structural polytype 2M1 (two-layered monoclinic, type 1) is the predominant species of white mica in 

Barrovian-type assemblages, whereas phengite (structural 

Figure 10. Example of a cooling curve for a mineralized area in Idaho 

(modified from Lund and others, 1986). 

cooling after regional metamorphism; shape and trajectory of the curve 

are controlled by ages and closure-temperature estimates of dated minerals; bars represent approximate uncertainties in age and temperature. 

Dotted line represents emplacement of granitic rocks and subsequent 

gold-vein formation. 

Figure 11. Complex argon systematics of a Precambrian amphibole 

from northern Pakistan. A, Age spectrum; B, K/Ca diagram. C, Drawing 

of a hornblende from thin section of sample that produced the age data, 

illustrating the complexities inherent in some amphiboles (Baig, 1990). 

K/Ca plot reflects influence on lower temperature steps of release pattern by presence of minor amounts of chlorite and biotite in the sample. 

Release of argon from actinolite affects low to intermediate temperature 

steps. 

that retains its Precambrian age. 

1,898 

1

Nonetheless, numerous e

From our e

In metamorphic rocks, 

Dashed line represents regional 

Majority of age spectrum is controlled by argon from hornblende 

16 Argon Thermochronology of Mineral Deposits 
A B 

Figure 12. Back-scattered electron images of a white mica grain showing complexities of metamorphism that can affect argon systematics. A, Deformed white mica has strained and unstrained realms with slightly different chemistry. B, Two generations of 

phengite (ph1, ph2) with potentially different ages (Till and Snee, 1995). Mg/(Mg+Fe) values are indicated in parentheses; str, strained; 

unstr, unstrained. 

polytype 3T; three-layered trigonal) is formed under high-pressure metamorphic conditions. Many geochronology studies 

have been done using white mica because of its high potassium content, geologic diversity, and argon retentivity. White 

micas can show complicated argon retention characteristics, 

but for well-behaved muscovites, numerous field studies (starting with the early work of Hanson and others, 1975, and Jäger, 

1967) indicate that argon closure occurs at a temperature 

higher than that for biotite. An estimate for the closure-temperature range of muscovite, based on geologic information 

including fluid inclusion filling temperatures, is in the range of 

270 °–325°C, covering slow to rapid cooling rates (Snee and 

others, 1988). Recently, Hames and Bowring (1994) and Kirschner and others (1996) have modeled 40Ar/39Ar age spectrum and laser-probe results to derive a closure-temperature 

estimate for muscovite of about 410 °C. 

Many studies have shown that complicated argon age spectra are exhibited by some metamorphic white micas, especially 

in situations in which white micas of differing composition and 

age are intergrown (fig. 12; also figs. 3A, 3B, 5A–E; Scaillet, 

1996; Hodges and others, 1994; Roeske and others, 1995; Till 

and Snee, 1995; Christiansen and Snee, 1994; Chopin and 

Maluski, 1980; Hammerschmidt and Frank, 1991; and Wijbrans 

and McDougall, 1986). Several suggestions have been made to 

account for these complexities. Scaillet and others (1992) 

showed that high-Si, Fe-rich phengite in the western Alps was 

completely reset under particular metamorphic conditions but 

that under these same conditions high-Si, Mg-rich phengite was 

not completely reset. In contrast, similar complexities in white 

mica argon systematics have also been ascribed to argon loss due 

to volume diffusion (Wijbrans and McDougall, 1986), thermal 

reequilibration of argon systematics in previously metamorphosed rocks (Wijbrans and McDougall, 1986; Hammerschmidt 

and Frank, 1991; Hames and Hodges, 1993; Reddy and others, 

1996; Hames and Cheney, 1997), and the influence of 

deformational history on argon reequilibration (Chopin and 

Monie, 1984; Scaillet and others, 1990; Hammerschmidt and 

Frank, 1991). Similarly, complex age spectra in low-grade 

metamorphic rocks have been attributed to effects resulting from 

grain size or mica polytype (Cosca and others, 1992). An added 

complexity exists when paragonite and K-white mica are intergrown at the micrometer scale under blue-schist or greenschist 

facies conditions as demonstrated by Roeske and others (1995). 

The closure temperature of phengite is unknown, although 

it must be higher than that for muscovite. Till and Snee (1995) 

showed that preexisting phengite of structural state 3T present in 

blue-schist facies rocks that underwent remetamorphism was not 

reset at temperatures as high as 525 °C but was completely reset 

at temperatures on the order of 580 °–620 °C. Other studies that 

have explored the complexity of argon systematics in white mica 

include Wijbrans and McDougall (1988), Wijbrans and others 

(1990), Hames and Cheney (1997), Ruffet and others (1995), 

Dahl (1996b), and Dalla Torre and others (1996). Clearly more 

must be done to understand white mica argon systematics. 

Muscovite is a highly versatile mineral in 40Ar/39Ar studies 

of mineral deposits. Snee and others (1988) showed that carefully characterized muscovites could be used to define age and 

duration of mineralization in the long-lived Panasqueira (Portugal) tin-tungsten deposit. Chesley and others (1993) combined 

muscovite 40Ar/39Ar geochronology with other isotopic systems 

to define the age and duration of the Cornubian batholith 

(England) and the many associated mineral deposits. With the 

worldwide economic interest in gold deposits and the ubiquitous 

association of muscovite with orogenic gold mineralization, 

muscovite argon geochronology is a highly reliable means to 

define age constraints of these large systems. Goldfarb, Snee, 

and associates used muscovite geochronology of orogenic gold 

deposits constrained by the cooling history of host rocks to elucidate the mineralization history of gold deposits of Alaska 

(Goldfarb and others, 1991, 1993, 1997; Miller and others, 1994, 

1995; Ford and Snee, 1996). Muscovite argon geochronology 

also has been used on early Paleozoic and Precambrian gold 

Some Minerals Useful for 40Ar/39Ar Geochronology 17 
40Ar/39Ar AGE SPECTRUM FOR BIOTITE SFQZ

120 

110 

Figure 13. Typical age spectrum for a biotite sample that 

contains some chlorite. Typical form taken by a biotite 

age spectrum when chlorite is interlayered showing 

anomalously older apparent ages in middle temperature 

steps. Older apparent ages likely result from 39Ar recoil 

from the chlorite and out of the sample or into the lowerT biotite sites. 

Older age "hump" 

APPARENT AGE (Ma) 100 

90 

800 10 20 30 40 50 60 70 80 

PERCENT 39ArK RELEASED 

deposits (Onstott and others, 1989; Fortes and others, 1997; 

Feng and others, 1992; Hanes and others, 1992; Kerrich, 1994; 

Kerrich and Cassidy, 1994; Kerrich and Kyser, 1994; Kent and 

McDougall, 1996; Perkins and Wyborn, 1998). 

Brown Micas 

The brown micas are the most commonly dated group of 

minerals by the conventional K-Ar method and remain a popular 

and important group used in modern 40Ar/39Ar studies, despite 

numerous problems with their argon systematics. Solid solution 

exists between the Mg-end member, phlogopite, and the Fe-end 

member, annite, with the term phlogopite generally reserved for 

Mg/Fe > 2, and biotite used for the others. Like the white micas, 

the brown micas are high in potassium and widespread in the 

geologic environment. As previously noted, argon retentivity of 

biotite has been shown by numerous field studies to be slightly 

less than that of muscovite; nonetheless, biotite is highly useful 

for understanding cooling in the 300 °C geologic realm. Harrison and others (1985) experimentally showed that for cooling 

rates ranging from 100 °C/Ma to 1°C/Ma, biotite argon closure 

temperature ranges from 345° to 280 °C. Harrison and others 

(1985) also showed argon diffusivity increases in brown micas 

with increasing Fe/Mg ratio, a result consistent with numerous 

empirical observations. 

Despite biotite’s popularity for geochronologic studies, the 

fact is that it is highly susceptible to alteration to chlorite. The 

presence of even minor amounts of chlorite seems to have 

serious deleterious effects on biotite’s argon systematics. Disturbance to a biotite’s age spectrum is generally manifested by a 

hump shape (convex upward) with anomalously high apparent 

ages for intermediate temperature steps of about 850° to 1,050°C 

(fig. 13). This temperature range of release corresponds to the 

majority of the temperature range of biotite argon release and 

thus strongly influences the biotite age spectrum. 

Most experimental studies have shown that this effect is 

likely due to 39Ar recoiled into surrounding biotite or completely 

out of the sample from intergrown fine-grained low-retentivity 

chlorite (Hess and others, 1987; Lo and Onstott, 1989; Hess and 

90 100 

Lippolt, 1986). Lo and Onstott (1989) demonstrated that this 

kind of disturbance to a biotite age spectrum will result with as 

little as 1 percent chlorite present in a biotite lattice. 

Despite this serious problem with biotite, valuable results 

can be obtained on carefully selected samples. In mineral 

deposit studies, biotite is less common than muscovite as a primary alteration phase. Its greater utility is in its use in definition 

of the thermal history of host rocks. Phlogopite generally seems 

to be more reliable than biotite. Some additional studies that 

show the usefulness of brown micas in both vacuum furnace and 

laser probe approaches include Snee (1982a), Gaber and others 

(1988), Ruffet and others (1991), Kelley and others (1997), 

Pickles and others (1997), Cheilletz and others (1993), and 

Phillips (1991). 

Illite 

Illite is structurally similar to the micas; most illites are dioctahedral like muscovite. Illite, however, differs chemically 

from muscovite in having more silicon and less potassium and 

having clay-size (<2μm) particles. The most common polymorph of illite has a disordered, unexpandable, one-layered 

monoclinic cell (1M), but other polytypes, including 3T and 

2M1, also are known. Illite commonly is interlayered with smectite, which is an expandable Ca, Na-clay. Illites and illite/smectites are common in sedimentary rocks and are formed by 

diagenetic and low-grade metamorphic processes. As temperature of process increases, grain size increases (perhaps by a 

process such as Ostwald ripening; Eberl and others, 1990), 

expandability decreases, and the ratio of 2M to 1M polytype 

increases. Illite also forms in the hydrothermal environment, 

and the common term “sericitization” is used for the process of 

alteration of feldspar to illite, illite/smectite, and (or) white mica 

(muscovite and paragonite). For some low-temperature hydrothermal mineral deposits, illite may be the only datable argon 

geochronometer available (Halliday, 1978; Ilchik, 1995; Folger 

and others, 1996; Phinisey and others, 1996; Hofstra and others, 

1999). 

18 Argon Thermochronology of Mineral Deposits 
APPARENT AGE (Ma) APPARENT AGE (Ma) 

Figure 14. Age spectra for size fractions of illite from Jer500	

Unaltered Roberts Mountains Formation 

1250 

1250 1250 

1250 

600 

20 - 40 384

600 

300 2 - 5 308

600 

350 

0.1 - 0.5 262

600 

257 

 0 

 16 

 20 

< 0.1 22 

Recoil 

Size (μm) 

40Ar/39Ar AGE SPECTRA FOR JERRITT CANYON ILLITES 

%39 Tg Ar-rec 

ritt Canyon, Nev. A, One sample of unaltered Roberts 

Mountains Formation sedimentary rock metamorphosed to 

low grade. Size fractions less than 5 μm show as much as 

400	 22 percent 39Ar recoil. B, One sample of altered Roberts 

Mountains Formation sedimentary rock metamorphosed to 

low grade and subsequently altered. Size fractions less 

than 5 μm show as much as 28 percent 39Ar recoil (% 39Ar300	 rec in figure). In both cases, the recoiled argon was captured in vacuum-evacuated vials. Apparent argon-loss 

spectra result from mixing of argon reservoirs of different 

200	 ages (Folger and others, 1996). True age of gold mineralization is approximately 40 Ma. Mineralization event was 

not hot enough to completely reset illite. Size (μm), grain100 size range of analyzed illite fraction; Tg, total-gas age of 

analyzed fraction; % 39Ar-rec, percent 39Ar recoiled from 

sample and captured in break-seal volume. Heating temperature of each fraction is indicated above or below cor- 0

0 10 20 30 40 50 60 70 80 90 100 responding bar. 

A PERCENT 39ArK RELEASED 

40Ar/39Ar AGE SPECTRA FOR JERRITT CANYON ILLITES 

500 

400 

300 

200 

100 

0

0 10 20 30 40 50 60 70 80 90 100

1250 

1250 

1250 

1250 

600 

20 - 40 324 0 

600 

300 5 - 20 284 8 

600 

300 

1 - 2 246 20

600 

300 

< 0.1 149 28 

Recoil 

Altered Roberts Mountains Formation 

Size (μm) %39 Tg Ar-rec 

B PERCENT 39ArK RELEASED 

Illite, in its broadest definition, has been used in both K-Ar 

and 40Ar/39Ar geochronology with mixed to poor success. The 

reasons for this are clear from the preceding paragraph—illite 

has a wide range of compositions, structural types, grain sizes, 

and modes of formation. Very fine grained illite is affected by 39Ar recoil as high as 30–80 percent in 40Ar/39Ar geochronology 

(Foland and others, 1984; Hess and Lippolt, 1986; Folger and 

others, 1996; Kapusta and others, 1997). Conventional K-Ar 

analysis has been used to avoid the problems from recoil, but 

this approach suffers from being unable to evaluate argon systematics (Halliday, 1978). Because illite and illite/smectite are 

formed within a growth continuum that extends into muscovite 

and phengite (Hunziker and others, 1986), equally troublesome 

is the potential for the incorporation of inherited argon from 

older grains that may serve as the cores for growth of larger 

grains. 

A vacuum encapsulation approach has been developed and 

routinely is used to capture recoiled 39Ar (Foland and others, 

1992; Smith, Evensen, and York, 1993; Folger and others, 1996; 

fig. 14). This process results in total-gas dates that are identical 

to conventional K-Ar dates, and it allows evaluation of the 

sample’s argon systematics. This approach has great promise for 

studies on uncomplicated illite. However, with respect to mineral-deposit geochronologic studies, Folger and others (1996) 

showed conclusively that in low-temperature Carlin-type deposits, illite that formed during mineralization can use older illite or 

white mica grains as crystallization nuclei. If the mineralization 

process did not degas these nuclei, inherited argon will be incorporated in the sample. In the case of Carlin-type deposits, if this 

risk is not recognized, erroneous interpretations may result 

(Arehart and others, 1993; Morris and Tooker, 1996; Wilson and 

Parry, 1995, 1996, 1997; Mako, 1997). Recent studies using 

laser degassing of illite grains (Dong and others, 1995, 1997; 

Kapusta and others, 1997; Onstott and others, 1997) are promising a valuable new approach for exclusively releasing argon for 

pure young-growth illite. 

Alkali Feldspar 

This diverse group has extensive applicability in 40Ar/39Ar 

thermochronologic studies. Potassium feldspar has high 

Some Minerals Useful for 40Ar/39Ar Geochronology 19 
potassium content, widespread abundance in nature, anhydrous 

crystal character, and well-documented argon retention characteristics. Potassium feldspar remains stable during heating to 

high temperature in the extraction system, so it lends itself to 

being used in volume diffusion modeling. These models are 

then used to develop comprehensive thermal models of the natural environment. Because the thermal character of the geologic 

environment has a profound effect on both the alkali feldspar 

species and the argon systematics of the species in that 

environment, for purposes of this discussion, I will subdivide 

alkali feldspars into three species: low-temperature potassium 

feldspar, sanidine, and adularia. 

Low-Temperature Potassium Feldspar 

Low-temperature potassium feldspar is the premier example of a mineral that had been deemed virtually useless for conventional K-Ar dating but has become one of the most widely 

applied minerals in 40Ar/39Ar thermochronology. In K-Ar studies it was recognized early that orthoclase and microcline potassium feldspar dates were commonly much younger than those of 

any other dated coexisting minerals, leading to the conclusion 

that potassium feldspar easily lost radiogenic argon in the natural environment. Largely through the work of Harrison and his 

associates, and others, such as Foland (1994), the argon 

systematics of low-temperature potassium feldspar have been 

comprehensively unraveled. (See, among others, Harrison and 

McDougall, 1982; Harrison, 1990; Foster and others, 1990; 

Harrison and others, 1991; Fitz Gerald and Harrison, 1993; Lovera, 1992; Lovera and others, 1989, 1991, 1993, 1996; Harrison 

and Be, 1983.) As this work is well described in McDougall and 

Harrison (1999), I will not attempt to duplicate that description. 

However, based on their work, low-temperature potassium feldspar normally exhibits argon release behavior that can be best 

modeled assuming a multi-diffusion domain approach. This 

method results in the derivation of a range of argon diffusion 

temperatures from carefully analyzed potassium feldspar. These 

data, in turn, provide comprehensive understanding of the thermal conditions of the geologic environment. Commonly the 

model temperatures range from as high as 350 °C to as low as 

150 °C or lower, a range that is exceedingly valuable for understanding low-temperature crustal activity such as cooling and 

uplift. However, low-temperature potassium feldspar also is a 

valuable mineral for dating rapidly cooled magmatic systems 

(Geissman and others, 1992). 

Sanidine and Anorthoclase 

Sanidine, the high-temperature potassium feldspar common 

in intermediate to felsic-composition volcanic rocks, and anorthoclase, the alkali feldspar more common in sodium-rich volcanic rocks, are additional minerals that met with mixed success in 

conventional K-Ar studies. Because in many cases sanidine did 

not release all of its argon when heated in the extraction system, 

anomalously young K-Ar apparent ages routinely resulted. With 

the move from K-Ar dating to 40Ar/39Ar geochronology, sanidine 

has risen in favor because it is no longer necessary to extract all 

argon from the mineral to derive meaningful age information. In 

fact it is our experience that of the common material dated from 

volcanic rocks, including sanidine, biotite, hornblende, and 

whole rocks, sanidine is the most reliable and the least likely to 

produce disturbed argon age spectra. When more than one phase 

from a particular sample is analyzed, sanidine is the likeliest to 

produce the best analytical precision and the greatest accuracy. 

Because of the high precision and accuracy of sanidine argon 

dates, sanidine is very valuable in defining detailed extrusive history of volcanic fields (Brooks and others, 1995; Scott and others, 1995; Snee and Rowley, 2000; Rowley and others, 2001; 

E.A. duBray and others, unpub. data, 2002) and the mineralization history of volcanic complexes (Yambrick and Snee, 1989; 

Rowley and others, 1992, 2001; Setterfield and others, 1992; 

Shubat and Snee, 1992; Henry and others, 1997; Snee and Rowley, 2000). 

Adularia 

Adularia is a low-temperature form of potassium feldspar 

that is regarded as a distinct variety because of its morphology 

and restricted paragenesis. Adularia has a high potassium content, ranging up to 17 percent K2O. It can form in epithermal 

mineral deposits and under low-temperature fluid-flow conditions, such as are present in marine saline basins. Adularia has 

proved effective in dating mineralization history (Halliday and 

Mitchell, 1976; Groff and others, 1997; Love and others, 1998, 

among others) and in some cases has been used to date fluid flow 

and potassium metasomatism along fault surfaces. Brooks and 

Snee (1996) used sanidine with adularia rims to date volcanic 

rocks in Nevada as well as the age of fluid flow along low-angle 

detachment fault surfaces that displaced them. In their study, 

Brooks and Snee (1996) used electron microprobe analysis of 

adularized sanidine to constrain interpretation of apparent argonloss spectra, concluding that detachment faulting occurred as 

long as 11 m.y. after volcanism (fig. 15). 

Because adularia in all occurrences commonly has a tendency to precipitate on older potassium feldspar, it is essential to 

carefully characterize the sample before analysis. Overgrowths 

of adularia on sanidine are not obvious under the microscope but 

can be resolved using elemental mapping in the electron microprobe or SEM. 

Plagioclase 

Plagioclase is a common mineral in volcanic rocks and is 

used to date basaltic rocks, which normally lack better argon 

geochronometers. Plagioclase has potassium contents ranging 

from 0.1 to more than 1 percent and has been shown to yield reasonable dates, especially when thermal-release spectra, isochrons, and K/Ca diagrams jointly are used to evaluate the distribution of argon in the sample. In some cases, especially for 

young, low-potassium plagioclase, excess argon may mask the 

low level of radiogenic argon, resulting in anomalously old 

apparent ages. Age-spectrum analysis generally will reveal the 

presence of the excess argon. 

20 Argon Thermochronology of Mineral Deposits 
Figure 15. Back-scattered electron image of adularia formed along cracks in sanidine. Sanidine has 12.4 

weight percent K2O and 3.0 weight percent Na2O; adularia along fracture has 16.7 weight percent K2O and 

0.1 weight percent Na2O (Brooks and Snee, 1996). 

Whole Rocks 

Volcanic rocks, low-grade metamorphic rocks, and volcanic 

glasses commonly can be used for K-Ar and 40Ar/39Ar geochronology, especially in cases in which a pure potassium-bearing 

phase of reasonable grain size is difficult to obtain. As noted, 

one of the strengths of the 40Ar/39Ar thermal-release method is 

the means to evaluate the distribution of argon within a single 

phase and to use that information to assess the geologic conditions when the argon was incorporated into the phase. An analysis of a multi-component sample, such as a whole rock, 

complicates this assessment. However, useful information on 

argon distribution and, in many cases, accurate age information 

can be derived from carefully selected samples. Numerous successful studies have been done on whole-rock basalt. However, 

Baker and others (1996) have recommended caution in interpretation even of apparently well behaved samples. Confidence in 

the results can be increased when independent geologic constraints are employed (Miggins and others, 2002). Low-grade 

metamorphic rocks, such as slates and phyllites, produce variable results (Reynolds and Muecke, 1978; Wintsch and others, 

1996); however, considering the implications from the prior discussion on illite, many obstacles must be overcome to produce 

meaningful results. Volcanic glass, in general, normally does 

not produce usable 40Ar/39Ar dates because of the deleterious 

effects from radiogenic 40Ar loss, recoiled 39Ar, and excess 40Ar. 

However, tektite, a melt glass produced from meteorite impact, 

does produce precise and accurate 40Ar/39Ar ages, as shown by 

Izett and others (1992), Dalrymple and others (1993), and 

Swisher and others (1992). 

Alunite and Jarosite 

Alunite and jarosite are common minerals in the epithermal 

environment, forming from the reaction with host rocks of 

hydrothermal sulfuric acid derived from the oxidation of sulfides. Alunite is hydrated potassium aluminum sulfate; jarosite 

is hydrated potassium iron sulfate. The habit of both is massive, 

and granular to dense; they are commonly mixed with quartz, 

kaolin, and iron oxide minerals. Both, but especially alunite, 

have been used in conventional K-Ar studies (for example, Ashley and Silberman, 1976). Recent attempts at 40Ar/39Ar dating 

using both vacuum furnace and laser-probe approaches have 

produced some success (Vasconcelos and others, 1994; Love and 

others, 1998). I expect much more work in the future on alunite 

and jarosite because, in many epithermal deposits, no other suitable potassium-bearing phase exists to provide potential geochronologic constraints. Recently in the Denver argon 

geochronology laboratory, we analyzed an alunite from a Precambrian epithermal gold deposit in the Brazilian shield and 

confirmed its apparent age of 1,834 Ma (fig. 16; Juliani and 

Some Minerals Useful for 40Ar/39Ar Geochronology 21 
ALUNITE FV3-01/16 — TAPAJOS GOLD PROVINCE  ́ 

2,000 

Plateau date = 1,834±11 Ma 

t max= 1,843±16 Ma 

APPARENT AGE (Ma) 1,800 

1,6000 50 100 

PERCENT 39ArK RELEASED 

Figure 16. 40Ar/39Ar age spectrum for 1,834 Ma alunite from Brazil. Age spectrum for magmatic hydrothermal alunite associated with high-sulfidation mineralization from the Tapajós Gold Province, 

Brazil, yields a plateau with an apparent age of 1,834+11 Ma defined by 93.4 percent of the released 39Ar. Although a plateau is exhibited, the gradual stepping-up in age may be a result of minor 40Ar 

loss over geologic time. The apparent age of this alunite is consistent with other geochronology 

and geology for this deposit. 

others, in press). Before this analysis, the oldest known alunite 

was 62 Ma (Bird and others, 1990). In a companion report to 

Juliani and others (in press), Landis and others (in press) evaluated noble gas systematics of a group of 10 alunites from the 

Brazilian gold deposit and derived an argon-closure temperature 

for alunite of 200° to 210 °C. 

Manganese Oxides 

Some manganese-oxide minerals, especially hollandite, 

cryptomelane, and others of the cryptomelane group, are gaining interest of argon geochronologists because they can contain 

5 percent or more potassium. These minerals form in the supergene and hydrothermal environments. Thus, they have potential 

for direct dating of both weathering processes and mineral 

deposits. Recently, Vasconcelos and others (1992, 1994, 1995), 

Lippolt and Hautmann (1995), and Vasconcelos (1999) have 

published studies on the application of manganese oxides to 

argon geochronology. Of particular interest for mineral deposit 

research are the 1.8-Ga and 950-Ma ages (Lippolt and Hautmann, 1995) determined on hollandite from mineral deposits in 

Sweden and India. Not only are these dates consistent with the 

known age of the deposits, but also they clearly show the retentivity of the manganese-oxide structure. More work needs to be 

done to characterize these minerals, but they are promising for 

argon geochronological studies on mineral deposit and surficial 

processes. 

Others 

Among other materials that have been used for argon geochronology are lepidolite, fuchsite, evaporate minerals, 

feldspathoids, glauconite, and pyroxene. I have analyzed lepidolite, a lithium-mica from chemically evolved pegmatites in 

southern California, Colorado, and Maine, and fuchsite, a chronium-mica from mineral deposits in Pakistan and Alaska, and 

have obtained results that are consistent with geologic constraints. Others (for example, Smith, Schandl, and York, 1993) 

also have successfully dated fuchsites. Most 40Ar/39Ar attempts 

at dating evaporates, glauconite, and pyroxene have met with 

minimal success. Evaporates and glauconite have potential for 

success if technical pitfalls can be avoided—such as problems 

caused by fine grain size and by radiogenic argon loss; pyroxene, 

on the other hand, should be avoided, because it has a very low 

potassium content but generally very high amounts of excess 

argon. 

Other materials that have been used for argon isotopic 

analysis or dating include sulfide minerals, magnetite, quartz, 

and fluid inclusions in quartz and muscovite. Of particular 

interest is work by Landis and Snee (1991) on the argon isotopic composition of irradiated amber. In 1988, Berner and Landis reported that gas bubbles in amber retained the 

compositional signature of the atmosphere at the time of formation. SEM elemental mapping of amber showed that some 

ambers contain quantities of potassium in the parts per million. 

22 Argon Thermochronology of Mineral Deposits 
To test the hypothesis of Berner and Landis (1988), we (Landis and Snee, 1991) irradiated vacuum encapsulated amber 

ranging in age from Cretaceous through Recent, to convert a 

portion of the small amount of incorporated 39K to 39Ar. 

Because of the chemically inert nature of argon, 39Ar proves to 

be an excellent tracer to evaluate diffusion processes. In the 

case of amber, we showed that 39Ar did not diffuse out of the 

irradiated amber, and we calculated that over the length of geologic time represented by the amber samples, gases trapped 

within the amber structure would retain their original chemical 

compositional signature. 

Standard Analytical Techniques 

Many components of a study must be carefully planned and 

executed to ensure a successful result. Success is not only a 

function of the use of the best, most highly sensitive and precise 

analytical equipment but also one of the proper selection and 

preparation of samples. The most important aspect of a geochronologic study is selection of samples best suited to answer 

the question being posed. From the preceding discussion, that 

many complications can arise during the course of a study is 

obvious; and the smarter the investigators are in selecting appropriate samples, the greater the probability is of their being able 

to interpret the results. 

The preparation of pure mineral separates is a critical 

aspect of any argon geochronologic study. Mineral separates for 40Ar/39Ar analysis are generally prepared by standard mineral 

separation techniques. Rock samples are crushed and sieved; 

for most samples the 80–120-mesh (180–125 μm) size fractions 

are used for mineral separation. Similar sizes are used for 

whole-rock samples, such as basalts. Initial mineral separations 

are done using heavy liquids, including methylene iodide, bromoform, lithium polytungstate, and sodium polytungstate in 

association with magnetic separation techniques. Whole-rock 

basalt samples may undergo removal of pyroxene, olivine, and 

plagioclase phenocrysts. After concentration, samples are normally hand-picked to near 100 percent purity. Before irradiation 

the samples are washed successively in acetone, ethanol, and 

deionized water in an ultrasonic cleaner. 

Depending on age and potassium content, between 1 and 

200 mg (milligrams) are loaded in aluminum capsules, which 

are stacked in silica-glass vials. Between every one to two samples and at the top and bottom of the glass vials, aluminumencapsulated neutron fluence monitors (that is, the standards) are 

placed. We use hornblende standard MMhb-1 (K-Ar age = 

520.4 Ma; Samson and Alexander, 1987; and recently modified 

to 523.1 Ma by Renne and others, 1998b) and sanidine standard 

FCT-3 (40Ar/39Ar age = 27.84 Ma calibrated in the Denver argon 

geochronology laboratory against MMhb-1; and recently modified to 28.02 Ma by Renne and others, 1998b) as the fluence 

monitors, depending on the expected age of samples in the irradiation vial. In addition, CaF2 and K2SO4 crystals are wrapped 

in aluminum capsules and included to monitor the production of 

interfering argon isotopes produced from Ca and K during irradiation. The vials are sealed under vacuum, and several vials are 

sealed in an aluminum irradiation canister. For some samples, 

which may be affected by 39Ar recoil, a sample is packaged in an 

aluminum capsule and then individually sealed within a vacuumevacuated, “breakseal,” silica glass vial; after irradiation, these 

vials are attached directly to the extraction line, the seal is broken under vacuum, and any recoiled contents are analyzed. 

Irradiations are done in the central thimble facility in the 

core of the U.S. Geological Survey TRIGA reactor (GSTR). 

Depending on age, samples are discontinuously irradiated for 

less than 1 hour to more than 100 hours; the younger the material, the shorter the irradiation, in order to minimize the effects 

from the production of interfering argon isotopes. Irradiation 

canisters are centered on the centerline of the reactor and rotated 

at 1 rpm throughout the period of irradiation to optimize neutron 

fluence distribution and to average out flux gradients. 

For argon isotopic measurement in the Denver argon laboratory, samples are heated and analyzed in an internal resistance 

furnace system similar in design to that described by Staudacher 

and others (1978). (Many laboratories use laser-probe extraction 

(for example, Kelley and others, 1994) in place of, or in association with, resistance-furnace extraction methods. Our laboratory 

also is employing laser-probe extraction methods, but discussion 

of this methodology is beyond the scope of this report.) This 

system consists of a low-blank, double-vacuum resistance furnace, an ultra-high vacuum extraction line, and a rare-gas mass 

spectrometer (Mass Analyzer Products 215) equipped with a 

Nier-type source and Faraday and electron-multiplier detectors. 

Multiple samples are loaded into a Pyrex-glass sidearm located 

above the resistance furnace, which comprises a tantalum crucible with molybdenum liner surrounded by an outer can cooled 

with a water jacket. Within the outer can and around the crucible 

is a tungsten heating element that is shielded from the outer can 

by a heat shield. The volume containing the heating elements is 

evacuated to less than 1×10-7 torr. The crucible is on-line with 

the extraction line. A furnace controller provides power to the 

heating elements and can drive the system to the specified temperature in 2 minutes or less. The temperature of the crucible is 

monitored by a thermocouple whose feedback to the controller 

ensures that the set temperature during any heating step is maintained to within +2 °C. The gas released during each heating 

step is cleaned with Zr-Al and Zr-V-Fe getters (alloys with high 

efficiency for capturing contaminant gases). The extraction system includes three compartments, each separated from the others 

by valves. Gas is transferred between compartments by opening 

an intervening valve and freezing the gas with liquid nitrogen 

onto charcoal within a finger in the adjacent compartment. The 

liquid nitrogen is contained in a vessel exterior to the charcoal, 

and the freezing process is an efficient pump that transfers the 

gas in entirety between compartments. The furnace, sidearm, 

and first two compartments of the extraction line are evacuated 

and baked to 250 °C after each new addition of samples. 

Standard Analytical Techniques 23 
Samples commonly undergo incremental heating analysis, 

during which each sample is progressively heated for 20 minutes 

per heating step; argon from standards is released in a single 20-

minute-long heating step. The gas from each step is transferred 

from the furnace to the getter section for 10 minutes, during 

which the furnace is turned off. After cleaning, the argon is 

expanded into the mass spectrometer and analyzed in static 

mode. Masses 40, 39, 38, 37, and 36 are analyzed and the data 

are collected and reduced on-line. Raw isotopic data are corrected for volume adjustments, mass discrimination, trap current 

settings, radioactive decay of 37Ar and 39Ar, and interfering 

argon isotopes. Measuring the 40Ar/36Ar ratio of atmospheric 

argon admitted to the system from an air pipette enables mass 

discrimination and atmospheric argon corrections. 

Argon Thermochronology Applied to 

Mineral Deposits 

40Ar/39Ar geochronology has enormous potential for mineral deposit research. Not only can the method provide direct 

information on the age of a mineral deposit but also it can be 

used to unravel duration, number of episodes, and temperature 

of mineralization. The age-spectrum method provides information about the distribution of argon in the analyzed mineral 

whether the mineral formed before, during, or after an alteration/ 

mineralization event. Even strongly altered wall-rock minerals 

can yield meaningful geochronology and provide constraints on 

the temperature of the mineralization event. Because the distribution of argon in the analyzed mineral is assessed by the agespectrum method, conclusions can be drawn on the extent to 

which preexisting minerals were affected by the mineralization 

event. 

General Strategy 

Deriving the most information with least ambiguity 

requires more than simply directly dating minerals that were 

formed or reset by the mineralization process, although this is a 

critical part of the geochronologic procedure. It is equally 

important to define the premineralization cooling history of host 

rocks to provide constraints on the temperature of the host rocks 

during mineralization. Understanding the geologic and thermal 

constraints during and after mineralization and alteration is also 

critical. For dated minerals that may have formed during 

mineralization or alteration (for example, muscovite, alunite, 

adularia), we must know whether or not host rocks at the time of 

mineralization were cooler than the argon retention temperatures 

of the minerals, in order to be able to uniquely interpret their 

apparent ages. For ubiquitous minerals such as white mica, 

which may form during alteration activity, knowing the textural 

relationships and the temperature of mineralization will provide 

valuable constraints on whether or not preexisting white mica 

contributed inherited argon to alteration-associated white mica. 

It is also valuable to understand the complexity of the mineralization process, including the possibility of multiple events and 

prolonged activity, to avail ourselves of the full potential of the 

geochronologic method. Finally, knowing the postmineralization history is essential for understanding potential effects of 

those events on argon systematics. 

Some Examples 

Three published reports and one ongoing study are 

described here to illustrate the application of 40Ar/39Ar geochronology in studies of the age and origin of mineral deposits and in the unraveling of the tectonic history of orogenic 

belts. 

Panasqueira, Portugal, Tin-Tungsten Deposit 

Snee and others (1988) was the first study to use high-precision 40Ar/39Ar thermochronology on a well-characterized mineral deposit. The study defined the isotopic age and thermal 

history of the Panasqueira, Portugal, tin-tungsten deposit, which 

is spatially associated with Hercynian plutons, by obtaining 1σ

analytical precisions between 0.7 percent (2.2 m.y. for single 

samples) and 0.3 percent (0.9 m.y. for populations) on muscovite 

associated with several stages of mineralization for this ≈300-

Ma deposit. 

The Panasqueira tin-tungsten deposit is located in northcentral Portugal, near Fundão, along the south edge of the Iberian Hercynian plutonic and metamorphic belt (fig. 17). The ore 

deposit consists of a swarm of near-horizontal hydrothermal 

veins that crosscut a sequence of tightly folded pelitic schists. 

The quartz veins are spatially associated with a granite cupola 

(fig. 18) and contain economically significant amounts of wolframite, cassiterite, and chalcopyrite. An unusual mass of 

quartz, the silica cap, lies at the apex of the cupola. 

Kelly and Rye (1979) worked out in detail the complex 

paragenesis of the tin-tungsten veins. They defined four stages 

of mineralization, which, from oldest to youngest, are the oxidesilicate, main sulfide, pyrrhotite alteration, and late carbonate 

stages. The economically important oxide-silicate stage (OSS) 

was subdivided into two substages, an earlier stage (I), represented by muscovite selvages that line the vein wall, and a later 

stage (II), consisting of intergrown coarse quartz, muscovite, 

wolframite, and arsenopyrite. Kelly and Rye (1979) originally 

thought that muscovite and quartz formed continuously during 

these two substages. The main sulfide stage (MSS) represents a 

period when sulfide deposition was dominant, but quartz and 

muscovite also formed. Pyrrhotite that formed during this stage 

was selectively replaced by marcasite and pyrite during the pyrrhotite alteration stage. Finally, carbonate minerals were widely 

deposited during the late carbonate stage. 

To determine both the age of the Panasqueira deposit and 

its mineralization history, we selected well-characterized samples from the two muscovite-bearing stages, the oxide-silicate 

and the main sulfide stages. We chose samples from both substages of the OSS (I and II). One of our samples (fig. 19) represented both of these substages, and muscovite from each was 

carefully separated. 

24 Argon Thermochronology of Mineral Deposits 
8° 

42° 

Lisboa 

Fundao Panasqueira X 

~ 

Hercynian 

granites 

P O R T U G A L 

0 100 KILOMETERS 

40° 

38° 

8° 

Figure 17. Sketch map of Portugal showing location of Panasqueira. The Panasqueira tin-tungsten deposit is located near Fundão 

along south edge of Hercynian plutonic and metamorphic belt. 

CUPOLA 

Schist 

Flat-lying veins 

0 50 METERS 

Silica cap 

Schist footwall 

Topaz + Muscovite I (207A) 

Muscovite II (207B) 

Apatite 

Quartz 

0 1 CENTIMETER 

Figure 19. Drawing of sample 207 exhibiting 

the two substages of the oxide-silicate stage. 

Drawing shows relationship between older OSS 

I alteration muscovite (sample 207A) and younger OSS II late-stage muscovite (sample 207B). 

300 

OSS I 

OSS II 

207A 

503 207B 

279 

40Ar/39Ar AGE SPECTRA FOR OSS MUSCOVITES 

290


280


270


0 10 20 30 40 50 60 70 80 90 100


PERCENT 39ArK RELEASED 

Figure 20. Composite 40Ar/39Ar age-spectrum diagrams for muscovite from OSS I and OSS II. Samples 207A and B represent separates 

from substage I and II respectively and were separated from the 

sample illustrated in figure 19. 

OXIDESILICATE 

STAGE 

MAIN 

SULFIDE 

STAGE 

GREISEN & LATE 

MUSCOVITES 

IN SILICA CAP 

ARGON LOSS 

EVENT 

296.3 ± 0.6 292.9 ± 0.8 

295.8 ± 0.6 293.5 ± 0.8 

294.5 ± 0.8 

292.1 ± 0.4 

4.2 ± 0.5 

o o 

o o

o 

o 

≤ 292 to ≥ 274 

PYRRHOTITE ALTERATION 

MINIMUM STAGE

DURATION 

APPARENT AGE (Ma) 

298 296 294 292 290 288 286 284


AGE (Ma) 

Figure 18. Diagrammatic cross section of the Panasqueira 

granite cupola that is host to the Sn-W deposit, showing silica Figure 21. Paragenesis of the Panasqueira deposit, showing 

cap and near-flat-lying quartz veins. detailed results of the 40Ar/39Ar geochronology. 

Argon Thermochronology Applied to Mineral Deposits 25
5° 4° 

▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ 

G 

H 

LE 

T 

C 

CM 

CB 

SA 

B 

G 

D 

H 

Plutons 

Lands End 

Tregonning-Godolphin 

Carnmenellis 

Carn Marth 

Carn Brea 

St. Austell 

Bodmin 

Gunnislake 

Dartmoor 

Hemerdon 

CB 

20 

kilometers 

0 

Carboniferous 

Lizard Complex 

Tertiary sediments 

Permo-Triassic red beds 

Devonian 

Argillites and turbidites 

Carboniferous 

Undifferentiated sediments and volcanics 

Pre-batholith metamorphic rocks 

Intrusive rocks 

Mafic volcanics 

Coarse-grained granites (CGBG) 

Outline of the Cornubian batholith 

T 

CM 

D 

Medium-fine-grained granites 

LE C 

SA 

B 

50° 

Figure 22. Geologic sketch map of southwest England, showing Cornubian batholith. Generalized geology modified from Chesley 

and others (1993) and Willis-Richards and Jackson (1989); line with sawteeth, thrust fault; dashed line, extent of geophysical expression of the Cornubian batholith. 

The study resolved age differences between the oxidesilicate and main sulfide stages, defined two periods of oxide-silicate activity (fig. 20)—one of which was younger than the main 

sulfide stage, and showed that geisenization and younger oxidesilicate activity were contemporaneous (fig. 21). Minor argon 

loss from all dated samples occurred during later thermal 

activity during the pyrrhotite alteration stage. Beyond these 

geologic implications, because of the comprehensive crosscutting relationships and fluid-inclusion homogenization temperatures defined by Kelly and Rye (1979), Ar-retention temperature 

for 2M1 muscovite was defined to be between 325° and 270 °C, 

depending on cooling rate. 

Cornubian Batholith and Associated Mineral Deposits, 

Southwest England 

Chesley and others (1993) built on the methods defined by 

Snee and others (1988) by combining high-precision 40Ar/39Ar 

thermochronology on muscovite, biotite, and hornblende with 

U-Pb monazite and xenotime and Sm-Nd fluorite geochronology. The metalliferous ore deposits associated with Hercynianage biotite-muscovite granites of the Cornubian batholith 

(southwest England) provide a well-defined geologic framework 

for applying multiple high-precision methods to unravel the 

details of granite magmatism and the superposed mineralization 

and cooling history (fig. 22). 

26 Argon Thermochronology of Mineral Deposits 

The Cornubian batholith was emplaced at the end of the 

Carboniferous to Permian Hercynian orogeny, which affected 

much of Europe and central Asia. The batholith has five major 

plutons and many smaller satellites and is primarily peraluminous in chemical character. Approximately 90 percent of 

exposed plutonic rock is coarse-grained biotite-muscovite granite. Four later episodes of granite magmatism are grouped under 

the label “medium-fine-grained granites” in figure 22 and are 

represented by fine-grained biotite granite, megacrystic lithiummica granite, fine-grained lithium-mica granite, and fluorite 

granite. Crosscutting some of the major plutons are vertical to 

near vertical felsic porphyry dikes, called elvans. Mineralization 

is associated with multiple, steeply dipping fractures and lodes 

exhibiting evidence of multiple episodes of mineralizing fluid 

flow. Four stages of mineralization are observed and are summarized in table 3. Stages 1 through 3 have been directly attributed to intrusion and cooling of the Cornubian batholith. The 

origin of Stage 4 mineralization is unclear because of its notably 

low temperature of formation (100°–170°C). 

The results of the geochronology study are shown in figure 

23. The U-Pb emplacement ages show that granite magmatism 

across the batholith occurred over a protracted period extending 

from ≈300 Ma to ≈275 Ma with no major hiatus, and that magmatism within any single pluton occurred over periods of as 

much as 4.5 m.y. The U-Pb date on monazite (280.4+1.2 Ma) 

for the Dartmoor granite is statistically identical to the 40Ar/39Ar 

date (280.3+1.0 Ma; calibrated against MMhb-1 hornblende 
300 

290 

AGE (Ma) 

? 

? 

Dartmoor

St. Aus

Bodmin

Gunnislake

tell

Carnmenellis

Lands End 

Haytor 

Yellowmeade Farm 

Hemerdon W-Mine 

Wheal Jane Mine 

Rosemanowas deep drill core 

Carn Marth 

South Crofty Mine 

Castle-an-Dinas 

CGBG 

Geevo Miner 

Fine- grained granite dikes 

South Crofty elvans 

Fluorite granite 

Wheal Remfry Breccia 

Goonbarrow Pit 

Gunnislake Mine 

CGBG 

CGBG 

CGBG 

CGBG 

CGBG 

CGBG 

Hemerdon 

WSW ENE Pendarves Mine 

Tamar Valley Crosscourse 

Menhenniot Crosscourse 

Emplacement age (monazite or xenotime U-Pb) 

Cooling age (muscovite 40Ar/

39Ar) Stage-III vein mineralization (fluorite, Sm-Nd) 

Elvans (inferred age muscovite 40Ar/

39Ar) 

Stage-I skarn (hornblende 40Ar/

39Ar) 

Stage-III vein mineralization (fluid inclusion, Rb-Sr) 

Stage-IV crosscourse mineralization (Rb-Sr, Sm-Nd) 

280 

270 

260 

250 

240 

230 

Stage-II vein mineralization (muscovite 40Ar/

39Ar) 

Figure 23. Summary geochronology for Cornubian batholith and associated mineral deposits. All geochronology data are from Chesley and 

others (1993) and Chesley (1999). σ error in age; 

Table 3. Stages of mineralization of the Cornubian batholith, southwest England. 

[Th , fluid-inclusion homogenization temperature; gnt, garnet; px, pyroxene; tourm, tourmaline; amph, amphibole; qz, quartz; fsp, feldspar; 

musc, muscovite; chl, chlorite; hem, hematite; fluor, fluorite; bar, barite; dol, dolomite; calc, calcite] 

Vertical width of symbols (error bars) indicates 1 CGBG, coarse-grained biotite granite. 

Stage 1 Stage 2 Stage 3 Stage 4 

Style Skarns Pegmatitic 

deposits 

Sheeted 

greisen 

bordered veins. 

Tourmalinebearing veins. 

Sn-bearing 

polymetallic 

fissure veins. 

Predominantly 

east-west. 

Late polymetalic 

sulfide veins. 

“Crosscourse” 

Predominantly 

north-south. 

Economic 

metals 

Fe, Cu, Sn Sn, W +Mo Sn, W Sn, Cu, Pb, As, Fe Pb, Zn, Ag, Fe, Sb, U 

Gangue 

minerals 

Gnt, px, 

tourm, 

amph 

Qz, fsp, 

musc, tourm 

Qz, musc, tourm Qz, fsp, chl, hem, fluor Qz, bar, dol, calc, fluor 

Th (o

C) 375o–450o 300o–500o 300o–500o 200o–400o 100o–170o 

with a date of 520.4 Ma based on Samson and Alexander, 1987) systems in this study. Elvan emplacement was synchronous with 

for hornblende from metamorphic skarn at its contact. This con- granite emplacement and continued to as young as 270 Ma. 

cordance confirms proper crosscalibration of the U-Pb and Ar Cooling rates derived from both the U-Pb and the 40Ar/39Ar dates 

Argon Thermochronology Applied to Mineral Deposits 27 
Silver Plume Granite 

63° 

66° 

65° 

D U 

offset dike 

offset 

dike 

WOODS CREEK FAULT

VASQUEZ PASS FAULT 

COLORADO 

Denver 

Red Mtn 

Climax 

N 

X 

Porphyry 

of Red Mountain 

Porphyry 

of East 

Knob 

Square 

Quartz 

porphyry 

Red Mountain 

border 

0 400 METERS 

Figure 24. Sketch map of the Red Mountain intrusive center. Generalized surface geology of 

Red Mountain is modified from Wallace and others (1978), and Geraghty and others (1988). Two 

faults, the Woods Creek and the Vasquez Pass, cut across the Middle Proterozoic granite of 

the area. Vasquez Pass fault offsets a dike and indicates down to the east movement of the 

Red Mountain porphyry system. Only a few stocks of the Red Mountain system are exposed at 

surface. Numbers show dip of dike and faults; U, upthrown side; D, downdropped side of fault. 

are unrelated to emplacement age and show a decrease from 

southwest to northeast from ≈210 °C/ m.y. to ≈60 °C/ m.y., probably reflecting the effect from the heat added to the crust by the 

many pulses of magmatism. Stage 1, skarn mineralization, was 

directly related to intrusion of granites. Between 275 and 265 

Ma, chemically evolved granites were emplaced. High-temperature tin and tungsten oxide-silicate mineralization (Stage 2) was 

broadly synchronous with emplacement of the granite magmas 

and was as much as 25 m.y. older than the main episode of economic mineralization represented by polymetallic sulfide-fluorite veins (Stage 3). Later formation of polymetallic veins, the 

crosscourses (Stage 4), is much younger (Chesley, 1999) and 

commenced after emplacement and cooling of the batholith 

were completed. 

Red Mountain Intrusive System and Associated Urad-Henderson 

Molybdenum Deposits, Colorado 

Geissman and others (1992) acquired paleomagnetic and 40Ar/39Ar data from most stocks of the Red Mountain intrusive 

system and associated Urad-Henderson molybdenum deposits 

and alteration zones. The Red Mountain intrusive system 

within the Colorado mineral belt is located in the Front Range 

of Colorado. The system is of Tertiary age and intrudes Middle Proterozoic (1.4 Ga) Silver Plume Granite (fig. 24) and 

comprises 15 stocks and 4 igneous breccias (Lovering and 

Goddard, 1950; Wallace and others, 1978; Carten and others, 

1988; Geraghty and others, 1988). Only four of the stocks and 

breccias are exposed at the surface; the remainder are below 

the surface and were discovered during the course of exploration and underground mining. The system has been a major 

molybdenum resource. The purpose of Geissman and others’ 

study was better understanding of the emplacement and cooling history of the system, the relationship between the intrusive events and mineralization, and the postemplacement 

structural modifications to the complex. 40Ar/39Ar geochronology on potassium feldspar, biotite, and 

muscovite from the intrusive rocks and alteration zones (figs. 25 

and 26) indicates that the thermal activity responsible for the Red 

Mountain intrusive system started at or before 29.9+0.3 Ma and 

ended at 26.95+0.09 Ma—more than 3 m.y. of nearly continuous 

thermal activity. The porphyry of Red Mountain, one of the oldest stocks in the system, was emplaced before 29.9 Ma and 

28 Argon Thermochronology of Mineral Deposits 
40Ar/

39Ar AGE SPECTRA FOR RED MOUNTAIN SYSTEM 

40 

30 

20 

40 

30 

20 

40 

30 

20 

40 

30 

20 

0 10 20 30 40 50 60 70 80 90 100 

PERCENT 39ArK RELEASED 

Figure 25. Composite 40Ar/39Ar age-spectrum diagram for argon samples from Red 

Mountain intrusive system and Urad-Henderson mineral deposit. One sigma analytical 

errors for data for most statistically significant temperature steps are between 0.06 and 

0.10 m.y. A, Age spectrum for orthoclase from porphyry of Red Mountain and potassium 

feldspar from a rhyolite dike emplaced into Precambrian host rocks. B, Six (five orthoclase and one biotite) age spectra for minerals from the Urad, Seriate, Henderson, Ute, 

and Vasquez stocks, all of which are only exposed underground. C, Six age spectra for 

biotites from Henderson, Ute, Vasquez, and Seriate stocks within the central part of the 

Red Mountain intrusive system. D, Three age spectra for muscovite from the magnetitesericite alteration zone around the Seriate stock. 

Red Mountain	 Figure 26. Three-dimensional block diagram of Red Mountain intrusive center, showing relative ages of stocks and 

cooling after emplacement; all ages are in million years. 

Dashed white line represents outer edge of part of system 

that cooled below biotite argon closure temperature at 

27.59+0.03 Ma. Circular dashed yellow line marks an area of 

magnetite-sericite alteration that formed between 27.51+0.03 

and 26.95+0.08 Ma at end of system cooling. This area overlaps the system core, but its associated thermal pulse did not 

reset older phases. 

APPARENT AGE (Ma) 

Magnetite-sericite alteration 

System core, cooling below 280° C 

Urad, Seriate, Henderson, Ute, 

and Vasquez stocks 

Porphyry of Red Mountain 

Rhyolite dike A 

B 

C 

D 

Silver Plume 

Granite 

Silver Plume 

Granite 

Urad 

porphyry 

dikes 

Seriate stock Henderson 

stock 

Vasquez 

Ute 

0 

200 

meters 

2000 

3000 m 

surface 

Porphyry 

of Red Mtn 

>29.9 

>28.7 

core of system

27.6 

28.7 

28.7 

28.5 

magnetite 

-sericite 

27-27.5 

Argon Thermochronology Applied to Mineral Deposits 29 
Australia 

Yilgarn 

Perth 

South Western 

Gneiss Terrain 

Southern 

Cross 

Province 

NorsemanWiluna Belt 

Eastern 

Goldfields 

Province 

Murchison 

Province 

North Western 

Gneiss Terrain 

N 

0 200 KILOMETERS 

EXPLANATION 

Metamorphic grade 

Granitoid and gneiss Proterozoic and younger units of gold deposit 

Greenstones Indian Ocean Subgreenschist 

Layered bodies Fault/shear zone Greenschist 

Gold-province boundaries Amphibolite-granulite 

Figure 27. Regional-scale geologic sketch map of Yilgarn-block gold deposits, Western Australia. 

possibly before 30.38+0.09 Ma. Nearby lamprophyre dikes were core of the system cooled below the biotite argon closure tememplaced at 29.8+0.1 Ma; rhyolite dikes intruded at 29.4+0.2 perature of about 300 °C at 27.59+0.03 Ma. The last period of 

Ma. The Urad and Seriate stocks intruded after 29.8 Ma but thermal activity involved pulses of magnetite-sericite alteration 

before emplacement of the Vasquez stock at 28.71+0.08 Ma. around the Seriate stock between 27.51+0.03 Ma and 

Based on six identical dates from potassium feldspar and biotite 26.95+0.08 Ma, and the activity did not thermally overprint 

from the Urad, Seriate, Henderson, Ute, and Vasquez stocks, the unaltered parts of the system. 

30 Argon Thermochronology of Mineral Deposits 
APPARENT AGE (Ga) 

U-Pb zircon 

U-Pb sphene 

U-Pb rutile 

Ar/Ar mica 

Sm-Nd garnet 

Pb-Pb pegmatite 

Re-Os molybdenite 

EXPLANATION 

2.68 2.66 2.64 2.62 2.60 2.58 2.56 

YOUNGEST ROCKS HOSTING GOLD 

GOLD DEPOSITS 

OLDEST ROCKS CROSSCUTTING GOLD DEPOSITS 

Eastern 

Goldfields 

Province 

Other 

provinces 

Figure 28. Summary of currently available geochronologic data for gold deposits in Western Australia 

(modified from Groves and others, 2000). 

The combined argon age-spectrum and paleomagnetic data 

also allow the conclusion that the Red Mountain intrusive system was tilted by subsequent structural activity. The magnitude 

of tilting was between 15° and 25° and occurred about an 

approximately north-northeast horizontal axis paralleling the 

orientation of the Woods Creek fault (fig. 24). 

Eastern Goldfields Province, Western Australia 

The Eastern Goldfields Province in the Archean-age Yilgarn Craton of Western Australia is one of the world’s most 

important gold provinces and one of the oldest. The gold deposits in the province have been well studied, but high-precision 

geochronological data on the age of mineralization are lacking, 

and existing data from several isotopic systems are in many 

cases contradictory. The purpose of an ongoing study (D.I. 

Groves, N.J. McNaughton, and L.W. Snee, work in progress) is 

to use a multi-isotope, multi-mineral approach in several key 

areas of the province to (1) assess its applicability to problems 

on Archean gold deposits and (2) provide precise and accurate 

ages that will allow a reinterpretation of the interrelationship 

among deformation, metamorphism, and gold mineralization in 

the Eastern Goldfields. 

The distribution of the gold deposits of southwestern Australia is shown on a regional-scale geologic sketch map of the 

Yilgarn Craton (fig. 27). The exposed geology is part of a craton 

that was formed in the Archean before about 2.6 Ga. Volcanic 

and sedimentary precursors of the greenstone metamorphic 

rocks in this classic granite-greenstone belt were deposited 

before 2.7 Ga. Four major compressive deformation events 

affected the craton, and granites intruded during and after deformation. Metamorphism varies across the region from subgreenschist to granulite facies, and the age of metamorphism lies 

between about 2.685 and 2.63 Ga. Most studies carried out on 

the gold deposit history indicate that gold was deposited late in 

the structural evolution of the craton and at peak or post-peak 

metamorphic conditions. 

Geochronologic data shown in figure 28 from numerous 

sources support a major period of gold mineralization between 

2.65 and 2.62 Ga. However, few well-constrained isotopic dates 

currently exist. The Denver argon geochronology laboratory 

will focus on providing high-quality 40Ar/39Ar geochronology on 

metamorphic rocks and alteration assemblages in an attempt to 

constrain the metamorphic cooling history and the age of the 

gold deposits. We discovered early in the study that the accepted 

age of the argon geochronology standards will be critical in 

comparison across isotopic techniques. As pointed out by 

Renne and others (1998a), age inaccuracies in studies of 

Archean geochronology are magnified to the extent that 25 m.y. 

inaccuracies could easily occur. In the case of the Eastern Goldfields, 25 m.y. inaccuracy in argon ages falls significantly outside the analytical precision of the method (approximately 6 

m.y., 2σ, at 2.6 Ga) and obviates comparisons with other isotopic methods until the inaccuracy can be resolved. 

31
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