Fiss'i6h-track d 

Ages of Natural Glasses 

Fission-track and K-Ar 

Ages of Natural Glasses 

By C. W. NAESER, G. A. IZETT, and J. D. OBRADOVICH 

GEOLOGICAL SURVEY BULLETIN 1489 

A study of the problems associated 

with fission-track dating of glass 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1980 
UNITED STATES DEPARTMENT OF THE INTERIOR 

CECIL D. ANDRUS, Secretary 

GEOLOGICAL SURVEY 

H. William Menard, Director 

Library of Congress Cataloging in Publication Data 

Naeser, Charles W. 

Fission-track and K-Ar ages of natural glasses. 

(Geological Survey Bulletin 1489) 

Bibliograplw: p. 13 

Supt. of Docs. no.: L 2.3:1489 

1. Obsidian. 2. Tektite 3. Radioactive dating. 

1928- I. lzett, Glen Arthur, joint author. II. Obradovich, John D., joint author. 

III. Title. IV. Series: United States Geological Survey Bulletin 1489. 

QE75.B9 no. 1489 [QE462.028] 557.3s [552'.2] 79-607123 

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

Washington, D.C. 20402 

Stock No. 024-001-03306-2 
CONTENTS 

Page 

Abstract----------------------------------------- 1 

Introduction------------------------------------ 1 

Fission-track annealing------------------------------- 2 

Dating of nonhydrated natural glasses---------------------- 3 

Dating hydrated glass shards------------------------- 5 

Plateau annealing--------------------------------- 7 

Discussion of zircon fission-track ages--------------------- 10 

Conclusions----------------------------------- 13 

Acknowledgments----------------------------------- 13 

References cited---------------------------------- 13 

ILLUSTRATIONS 

Page 

FIGURES 1-5. Diagrams showing relationships between: 

1. K-Ar ages and fission-track ages for a group of 

natural glasses------------------- 4 

2. Fission-track glass and zircon fission-track ages of 

hydrated glass shards---------------- 6 

3. K-Ar glass age or zircon fission-track age and 

natural state or plateau annealing fission-track 

glass age------------------------ 8 

4. Apparent fission-track glass ages of selected 

samples after heating and the K-Ar glass or 

fission-track zircon ages-------------- 9 

5. Ratio of induced fission tracks in muscovite to 

those in zircon and angle between polished surface and optic axis of zircon grains--------- 12 

TABLES 

[Tables are in back of report] 

Page 

TABLE 1. Fission-track and potassium-argon ages of upper Cenozoic natural 

glasses from 24 localities in the Western United States, Peru, 

Canada, and Czechoslovakia------------------- 18 

III 
IV CONTENTS 

Page 

TABLE 2. Descriptions and localities of samples listed in table 1------- 21 

3. K-Ar analytical data for obsidian samples of table 1 not previously 

listed in published reports--------------------- 23 

4. Fission-track ages of cogenetic glass shards and zircon microphenocrysts separated from upper Cenozoic ash beds from the 

Western United States------------------------ 24 

5. Descriptions and localities of samples listed in table 4------- 26 

6. Fission-track plateau annealing ages of natural glasses from the 

Western United States, Peru, and Czechoslovakia-------- 28 
FISSION-TRACK AND K-Ar AGES OF 

NATURAL GLASSES 

By C. W. NAESER, G. A. IZETT, and J. D. OBRADOVICH 

ABSTRACT 

Considerable experimental evidence points to the conclusion that fission-track ages of 

natural glasses are either equal to or less than K-Ar or fission-track zircon ages of the same 

sample. To further test the hypothesis that fission-track ages of glasses are minimum ages, 

owing to track fading or annealing at ambient temperatures during geologic time, we dated 

38 samples of upper Cenozoic natural glasses for which K-Ar or zircon fission-track ages are 

available. Of the 24 samples of non hydrated glass we dated (23 obsidians and one tektite), 12 

have fission-track glass ages that are concordant (2-sigma level) with K-Ar ages, and 12 have 

fission-track ages that are markedly younger than the K-Ar ages. In contrast, only one of 14 

fission-track ages of hydrated glass shards from samples of upper Cenozoic ash beds is 

concordant (2-sigma level) with fission-track ages of coexisting zircon rnicrophenocrysts; 

the remaining 13 samples have fission-track glass ages significantly younger than fissiontrack zircon ages from coexisting zircon microphenocrysts. 

The plateau-annealing technique developed by D. Swrzer and G. Poupeau in 1973 was 

used to correct some of the anomalously young annealed fission-track glass ages. The 

plateau fission-track glass ages of 10 of the samples were found to be near the K-Ar or zircon 

fission-track ages. Plateau-annealing data for two of the non hydrated glasses (one obsidian 

and one tektite) show that they have not been annealed. 

The geometry factor used in the fission-track dating of zircon using a muscovite detector 

was found to be 0.5, which is in accord with previous experimental determinations of this 

factor by A. j. W. Gleadow and J. F. Lovering in 1977. 

INTRODUCTION 

Considerable experimental evidence points to the conclusion that 

whereas some fission-track ages of natural glasses are concordant with 

isotopic ages, most are younger. This information is well known among 

most fission-track geochronologists; however, many potential users of 

fission-track glass ages may not be aware of this problem, and it is to users 

of fission-track glass ages that this paper is directed. Moreover, a few 

fission-track geochronologists who only date glass shards may not be 

attuned to the problem of track annealing, inasmuch as they do not 

compare their fission-track ages with those determined on other 

materials. It is then, the chief purpose of this paper to present additional 

experimental evidence that many nonhydrated natural glasses, including 

1 
2 FISSION-TRACK AND K-AR AGES OF NATURAL GLASSES 

obsidians and tektites, yield fission-track ages that are significantly 

younger than K-Ar ages of the same material. In addition, fission-track 

ages of nearly all samples of hydrated glass shards from upper Cenozoic 

ash beds are significantly younger than fission-track ages of zircon 

microphenocrysts separated from the ash beds. The young fission-track 

ages of the glasses result from track fading (annealing) under ambient 

conditions during geologic time. 

Another purpose of this paper is to show that some naturally annealed 

glasses can give corrected fission-track ages using the plateau-annealing 

technique of Storzer and Poupeau (1973). Finally, we will briefly review a 

few of the critical factors that affect the fission-track dating of zircons. 

FISSION-TRACK ANNEALING 

One of the first materials to be successfully dated using the fission-track 

method (Fleischer and Price, 1964a) was natural glass, which was 

preceded only by mica (Price and Walker, 1963). A number of different 

types of natural glasses, including tektites and impact glasses, have been 

dated since the first work in 1964 (Fleischer and Price, 1964 b; Fleischer 

and others, 1965a; Gentner and others, 1967, 1969). Fission tracks have 

also been used to date obsidians, pitchstones, and basaltic glasses 

(Fleischer and Price, 1964b; Storzer, 1970; Suzuki, 1970; McDougall, 1976, 

and Wagner and others, 1976). In addition, silicic pumice and volcanic 

glass shards have been dated (Fleischer and Price, 1964a; Seward, 1974). 

The fission-track ages reported by Fleischer and Price ( 1964a) and in 

other papers are generally equal to or less than the age of natural glass as 

determined by other isotopic dating methods, principally K-Ar. The fact 

that fission-track ages of glass (usually) and other minerals (occasionally) 

are younger than K-Ar ages from the same rock led to the discovery 

(Fleischer and others, 1965b) that fission tracks anneal during heating of 

a glass or mineral in the laboratory and, by inference, also in natural 

situations. Fleischer and others ( 1965b) found that heat, rather than 

pressure, is the chief factor producing fission-track fading or annealing; 

pressure of 1 million kilopascals had only a minor effect on the annealing 

properties of a tektite they studied. 

Annealing under natural conditions has been documented in a number 

of cases; usually annealing is detected when a fission-track age is 

significantly younger than expected, based on geologic criteria. Storzer 

and Wagner (1969) first reported that physical evidence of track fading 

could be detected by comparing the diameters of fossil tracks to the 

diameters of neutron-induced tracks in the same material. Where track 

fading has occurred, the diameters of fossil tracks are appreciably smaller 

than diameters of induced tracks. In several hydrated glasses we have 

studied, fl marked difference occurs between the diameters of fossil and 

induced tracks. Even if the difference in diameters is barely detectable, the 

fission-track age will be very much less than quench age of the glass. 
DATING OF NONHYDRATED NATURAL GLASSES 3 

Storzer and Wagner (1969) developed an empirical curve that relates 

fossil- and induced-track diameters in partially annealed glasses in order 

to correct the age of formation of the glass. This method is time 

consuming and tedious. 

A far more satisfactory method for correcting fission-track ages of 

partially annealed glass was developed by Storzer and Poupeau (1973). 

Their method consists of heating, at progressively higher temperatures, a 

pair of samples consisting of a natural-state glass and an irradiated glass. 

A fission-track age calculated from the fossil- and induced-track counts 

after each heating step shows that the apparent age of the glass gradually 

increases until a constant ratio of fossil tracks to induced tracks results 

(plateau age). After the plateau is reached, the age remains constant with 

increased temperature until at a critical temperature range all tracks are 

annealed. We have found this to be a satisfactory method for correcting 

partially annealed glasses. McDougall ( 1976) reported thanhese methods 

did not give satisfactory results when applied tp oceanic basaltic glasses. 

DATING OF NONHYDRATED NATURAL GLASSES 

We dated 24 samples of nonhydrated natural glass (obsidian and 

tektite) by the fission-track method, and their ages and associated 

analytical data are listed in table 1. Descriptions of the localities where the 

glasses were collected are given in table 2, and table 3 reports K-Ar 

analytical data for samples not previously reported on. These natural 

glasses were chosen from a large group in our collection because they 

appear very fresh and because they are essentially free of microlites and 

crystallites, which can make fission-track dating impossible. Many 

obsidians are crowded with microlites and crystallites (gobulites and 

trichites), and these form fission-track-like etch pits following etching 

with hydrofluoric acid. The etch pits of the microlites and crystallites are 

difficult to separate from real fission tracks formed from the spontaneous 

decay of 238U, and accordingly, calculated ages based on counts including 

the microlite and crystallite etch pits are not reliable. The ages of the 

natural glasses range from about 30 m.y. to about 0.1 m.y. Among these 

glasses are obsidians from 21 widely scattered localities in Western North 

America, a vitrophyre of a welded tuff from Wyoming, an obsidian from 

South America, and a tektite (moldavite) from Czechoslovakia. 

K-Ar age determinations are reported in table 1. The K-Ar ages were 

calculated or recalculated using the constants for the radioactive decay 

and abundance of 4°K recently recommended by the International Union 

of Geological Sciences Subcommittee on Geochronology (Steiger and 

Jager, 1977). Of the 24 natural glasses of table 1, 12 have fission-track ages 

that are significantly younger than the K-Ar ages of the same material. 

(The ages are discordant at the 2-sigma level.) A plot of the fission-track 

and K-Ar ages is shown in figure I. 
4 FISSION-TRACK AND K-AR AGES OF NATURAL GLASSES 

30 

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K-Ar GLASS AGE (X 1 06 YEARS) 

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30 

FIGURE I.-Diagram showing the relationship between the K-Ar ages and the fission-track 

for a group of natural glasses numbered and listed in table I. 

Special comments are required concerning the age of several of the 

glasses: specifically, the obsidians from Glass Buttes, Oreg. (table I, no. 

14), Beaver Creek, Colo. (table 1, no. 22), and Big Sand Spring Valley, 

Nevada (table I, no. 23). These samples are probably the best examples 

illustrating why fission-track ages of glass tend to be discordantly 

younger than K-Ar ages. Two different samples of glass from the obsidian 

flow of Glass Buttes, Oreg. (table 1, no. 14), were dated. One was a surface 

sample collected by Charles Vitaliano of Indiana University, and the 

other was a subsurface sample collected by E. H. McKee of the U.S. 

Geological Survey from about 1 meter below the surface. The surface 

sample has an apparent fission-track age of 1.5 ± 0.2 m. y. In contrast, the 

subsurface sample has a fission-track age of 4.4 ± 0.3 m.y., which is 

concordant with the K-Ar age of this obsidian (5.04 ± 0.73 m.y.; McKee 

and others, 1976). Another glass (table 1, no. 23; table 3, DKA 3317) that 

has discordant fission-track and K-Ar ages was collected from surface 
DATING HYDRATED GLASS SHARDS 5 

exposures in the Big Sand Spring Valley of Nevada, a region of the 

country known for very hot summers and sparse vegetation 'o provide 

shade. The discordancy between the fission-track and K-Ar age is about I' 

m.y. The fission-track and K-Ar ages of the Beaver Creek obsidian flow 

(table I, no. 22) are concordant at about 21-22 m.y. This sample came 

from surface exposures high in the San juan Mountains of Colorado 

(about 3,470 m above sea level), from a north-facing slope (Lipman and 

others, 1969). Based on the above data, it appears that solar radiation 

impinging on a black obsidian in hot, dry climates can raise the 

temperature of the glass enough to cause track fading. It is therefore 

necessary, when collecting glass for fission-track dating, to minimize 

solar-induced track fading by quarrying back. 

DATING HYDRATED GLASS SHARDS 

Fission-track ages of 14 samples of hydrated glass shards were 

determined, and the ages and analytical data are listed in table 4. The 

samples were taken from volcanic ash beds of late Cenozoic age in the 

Western United States, and information concerning their localities is 

given in table 5. These 14 samples of volcanic ash were chosen for several 

reasons: (I) The glass shards of the samples have large platy bubble walls 

that make track counting efficient; (2) their stratigraphic and 

paleontologic age is known; and (3) fission-track zircon ages are available 

for all the samples (table 2). 

To prepare the glass shards for fission-track counting, they were all 

etched in 1:1 HF and H20 at room temperature for 45 seconds, and the 

obsidians and tektites were etched for varying times between 10 and 15 

seconds in HF (48 percent). In preparing the glass shards for dating, the 

sample was sieved to provide the coarsest shards possible, and the glass 

was separated from its crystal content by use of bromoform and 

bromoform-acetone mixtures. The sample was then split into two parts. 

One part was mounted and used for the fossil-track density, and the other 

fraction was irradiated for the induced count. The obsidians and tektites 

were processed in a slightly different way. Large pieces of glass were 

crushed, and the coarse-sand-size fraction was used for dating. This was 

done to minimize any heterogeneity of the uranium content of the glass. 

After sizing, the sample was split; several hundred grains were mounted 

for the fossil count, and several hundred other grains were placed in 

aluminum foil packages for irradiation. 

After irradiation, the glasses were mounted in epoxy and polished. The 

mounts containing the grains for the fossil-track and induced-track 

counts were then etched together back to back in an HF and H20 

solution. After etching, the fission-track densities of the glasses were 

determined by counting the tracks in transmitted light at magnifications 

of 500 (Naeser) or 787 (lzett). The fossil tracks were not removed by 
6 FISSION-TRACK AND K-AR AGES OF NATURAL GLASSES 

heating prior to irradiation. Heating of the hydrated glass shards would 

change the water content of the glass, which would have a marked effect 

on the c:tching characteristics and the natural-state age of the glass. The 

induced-track density was determined by subtracting the fossil-track 

density from the total track density in the irradiated samples. 

The percentage of samples showing annealing effects is greater for the 

hydrated glass shards shown in table 4 than for the nonhydrated 

obsidians of table 1. Over 90 percent ( 13 of 14) of the volcanic ash samples 

studied have fission-track glass ages younger than the fission-track ages 

of coexisting zircon microphenocrysts. The results of these studies for the 

fission-track dating of the shards, shown in table 4 and figure 2, are not all 

that surprising in light of the studies of Lakatos and Miller (1972 a, b). 

They showed that when obsidian is hydrated in the laboratory, the 

temperatures at which annealing takes place are markedly lower than in 

the nonhydrated glass. Their data show that a significant percentage of 

the tracks present in a glass containing 2.2 percent H20 would be 

annealed at ambient surface temperatures. Most glass shards from upper 

Cenozoic ash beds have water contents in excess of 4 percent, which 

means that some annealing might be expected in glass shards from beds 

of this age. 

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FISSION-TRACK ZIRCON AGE (X106 YEARS) 

FIGURE 2.-Diagram showing relationship between glass fission-track age and zircon 

fission-track age of hydrated glass shards from upper Cenozoic ash beds in the Western 

United States. 
DATING HYDRATED GLASS SHARDS 7 

PLATEAU ANNEALING 

As mentioned, two procedures exist whereby partially annealed glasses 

can be treated in order to correct apparent fission-track ages for track 

annealing. One technique is to measure a large number of diameters of 

fossil and induced tracks of a sample and, by relating the average track 

diameters of the fossil and induced tracks, correct the apparent age. In this 

technique, it is critical that the materials used for measuring the 

diameters of the fossil and induced tracks be etched under identical 

conditions (temperature, time, and acid concentration). 

A second technique is the plateau annealing method developed by 

Storzer and Poupeau (1973), and because it is less time consuming and 

less tedious, we selected 12 samples from tables I and 4 to date by this 

technique. Two of the samples have concordant fission-track glass and 

K-Ar ages at the 2-sigma level, and the other I 0 have fission-track glass ages 

that are markedly discordant with either K-Ar or fission-track zircon ages. 

Plateau annealing ages and their associated analytical data for the 12 

samples are listed in table 6. The samples that have discordant fissiontrack glass ages are characterized by having fossil-track diameters that are 

much smaller than diameters of induced tracks for samples that were 

etched under identical conditions. These smaller fossil-track diameters 

were noted for both obsidians and glass shards that have discordant 

fission-track ages. 

For the plateau-annealing experiments, we crushed the glass to pass a 

425- p. m sieve and prepared two 0.1-g splits. One of the splits was 

irradiated and, following irradiation, was again split into four or five 

parts, and each part was placed into a small aluminum foil package. An 

aluminium foil package of the nonirrc~diated glass was also prepared. The 

two packages were placed in a tube furnace and he-c~ted for I hour at a 

temperature between 1500 and 2000C. This heating process was repeated 

with another pair of samples at a higher temperature. We used 

temperatures between 1500 and 350°C for samples I, 9, 10, II, and 12 of 

table 6, and the other samples of table 3 were heated to only about 200°C. 

We determined from the stepwise heating experiment that heating a 

sample for I hour is generally sufficient to reset the apparent fission-track 

glass age to near the K-Ar or fission-track zircon age. Following heating, 

the samples were mounted in epoxy and polished. Each pair of samples 

was etched back to back for the same time in the same acid, and the tracks 

were counted at the same magnification. The results of the plateauannealing experiment are shown graphically in figure 3. 

The plateau fission-track ages of the natural glasses that have 

concordant fission-track and K-Ar ages (macusanite and moldavite; table 

6, nos. I and 9) remained unchanged after stepwise heating, although the 

fossil- and induced-track densities in both natural glasses were 50 percent 
8 FISSION-TRACK AND K-AR AGES OF NATURAL GLASSES 

EXPLANATION 

• K-Ar glass age vs natural state fission 

30 track glass age 

o K-Ar glass age vs plateau annealing 

fission track glass age 

~ Fission track zircon age vs plateau 

- 25 annealing fission track glass age (/) 

ICII::: • Fission track zircon age vs natural <( 

w state fission track glass age 9 >-

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OL-----------~---------------~---------------~---------------~---------------~----------~ 0 5 1 0 15 20 25 30 

K-Ar GLASS AGE OR ZIRCON FISSION TRACK AGE 

(X 1 06 YEARS) 

FIGURE 3.-Plateau annealing diagram showing relationship between K-Ar glass age or 

zircon fission-track age (x axis) and natural state or plateau annealing fission-track glass 

age (y axis). Dashed lines show amount of age resetting accomplished during plateau 

annealing procedure. 

lower than in the natural-state material after the final heating step. These 

results indicate there had been no track fading in the natural-state 

material in the two samples. 

The samples that have natural-state discordant ages show a pattern of 

increasing age with increasing temperature, as shown in figure 4. 

Counting fission tracks in the hydrated glass shard samples that were 
PLATEAU ANNEALING 

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GLASS SHARDS (HYDRATED) 

CHIMMNEY ROCK, NEBRASKA 

Sample 1 2, table 6 

MOLDAVITE, CZECHOSLOVAKIA 

Sample 9 , table 6 

GLASS SHARDS (HYDRATED) 

SHEEP CREEK. NEBRASKA 

Sample 10, table 6 

MACUSANITE, PERU 

Sample 2, table 6 

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TEMPERATURE (° CELSIUS) 

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FIGURE 4.-Fission-track plateau annealing diagram for selected samples of table 6, 

showing the apparent fission-track glass ages of the samples after heating at various 

temperatures for I hour, compared with the K-Ar glass age or fission-track zircon age of 

the sample. Error bars indicate + one standard deviation. 

heated above 200° is very difficult. The glass shards fracture badly during 

dehydration and are crazed to such an extent that finding flat, clean 

surface areas to count is difficult. We ascribe the decrease in the apparent 
10 FISSION-TRACK AND K-AR AGES OF NATURAL GLASSES 

age at the highest temperatures of some samples (table 6, nos. 10 and 12) to 

this phenomenon. Of the 12 samples studied that have discordant 

natural-state fission-track glass ages, 10 have plateau-annealing ages 

concordant with K-Ar or fission-track zircon ages at the 2-sigma level. 

The obsidian from Cougar Mountain, Oreg., was heated at only 225°C for 

I hour; perhaps it should be heated at higher temperatures in order to 

reach a plateau age. MacDougall concluded that heating for I hour at 

200°C was enough to place basaltic glasses in the plateau region of the 

age-versus-temperature curve. The other glass that remained discordant 

after heating is the obsidian from Nevada. It is possible that because of the 

amount of initial discordance, 50 percent, some of the fossil tracks may 

have been totally annealed and, therefore, the age could not be corrected 

to the K-Ar age by the plateau method. 

DISCUSSION OF ZIRCON FISSION-TRACK AGES 

In fission-track dating, the investigator must make a choice among 

several possible numbers to substitute for "constants" in the fission-track 

age equation: 

Where 

1 p Ad u l t/) t=_1n s . + I 

Ad pi Af 

t =age in years, 

tfJ =neutron dose, 

Ad =total decay constant for usu, 

p 5 =spontaneous-track density, 

pi =induced-track density, 

u =thermal neutron fission cross section for 2ssu, 

A 1 =decay constant for spontaneous fission of 258U, 

I =ratio of 2ss U J2!l&U. 

The first of these decisions involves the value for the spontaneous fission 

of 258U (A1). Two decay constants are currently being used in fission-track 

dating-one at about 8.4XI0-17 yr-1 (Spadavecchia and Hahn, 1967; 

Wagner and others, 1975; and Thiel and Herr, 1976), and the second near 

7.0XI0-17 yr- 1• There is a difference of about 17 percent between these two 

constants. The second decision involves a choice between two calibration 

values (Cu or Au) used to determine the uranium content of a material to 

be dated. The U.S. National Bureau of Standards has an excellent set of 

calibrated glasses available for determining the neutron dose (Carpenter 

and Reimer, 1974) used for a particular dating experiment. These glasses 

were irradiated with Cu and Au foil dosimeters at the National Bureau of 

Standards. Because the copper values are 10 percent lower than the gold 

values, a choice must be made of which value to use. 

If we useAt-7.03X10-17 yr-1 (Roberts and others, 1968) arid the Cu 
ZIRCON FISSION-TRACK AGES 11 

values for the neutron dose calibration, we gt>t the best agreement with K-Ar 

agt•s. \\'e have dated 51 zircon st•paratt·s from subvokanic and vokanic 

rocks whose ages are between 0 .. 6 and 70 m. y. These rocks have been dated 

by the K-Ar method using one or more of the following minerals: biotite, 

hornblende, plagiolase, and sanidine. A least-squares analysis of the 

K-Ar and fission-track age data gives a slope of 0.996 and a correlation 

coefficient of 0.997. Other combinations of decay constant and neutron 

dose calibration constant yield ages as much as 20 percent too young 

(8.4xl0- 1' yr- 1,Cu) or 12 percent too young (8.4X10-17 yr-I,Au). This 

empirical method of calibration is not the most satisfactory procedure, 

but with the present controversy concerning fission-track decay 

constants, we feel that this is the best method to produce ages that are 

compatible with K-Ar ages and are therefore geologically useful. 

Another possible source of error that can affect the accuracy of a zircon 

fission-track age is the so-called "geometry factor." When a zircon is 

dated, the fossil tracks are counted on an internal surface of a zircon grain, 

and the induced tracks are counted on the cleavage surface of a lowuranium muscovite (<10 ppb U), which covered the zircon during 

neutron irradiation. A basic assumption is that half as many tracks are 

recorded from the fission of nsu in the muscovite detector as would be 

recorded in a zircon crystal. A geometry' factor of 0.5 indicates that the 

zircon and the muscovite have similar etching and counting efficiencies 

(Fleischer and others, 1975). Reimer and others (1970) questioned the 

validity of using a geometry factor of 0.5 for zircon in a muscovite 

detector. In their ex1x·riment they compared internal and external trat·ks in 

a crystal and dt•termirwd that the ratio was not 0.5. However, they did not 

compare the internal tracks with those in a muscovite detector. Their 

results raised some doubts about the validity of using a geometry factor of 

0.5, but Gleadow and Lovering (1977) determined the geometry factor as 

used in the external detector method to be 0.5 for apatite, sphene, and 

zircon. Using a zircon provided by Gleadow, from the Mud Tank 

Carbonatite of the Hart's Range, Northern Territory, Australia, we also 

have determined the geometry factor for the zircon-muscovite pair to be 

0.5. Carbonatite zircon ages determined by the population and external 

detector methods are equal and are as follows: 

Population method External detector method 

273 ± 14 m.y. 262±20 m.y. 

~,=7.03XIO-I7 yr-I 

± is 2 standard deviations 

During these geometry experiments, it became obvious that some of the 

zircons had many fewer tracks than other members of the population. A 

check of the tracks in the muscovite showed that the zircons all have the 

same amount of uranium. It was also noted that the etched surfaces of the 
12 FISSION-TRACK AND K-AR AGES OF NATURAL GLASSES 

zircons with low track densities had characteristics different from 

those of the zircons with high track densities, even though the shape of 

the tracks is identical. Twelve grains were chosen that showed divergent 

surface characteristics of the etched surface. These grains were counted 

for their induced-track density. After counting, they were mounted on the 

end of a wire needle. With the use of the Wilcox spindle stage (Wilcox, 

1959), the angle between the polished surface and the optic (c) axis of the 

zircon was measured. The results are shown in figure 5. Those grains 

whose polished surface was subparallel (±15°) to the optic axis had twice 

as many tracks as the muscovite detector. As the angle between the optic 

axis and the polished surface of the zircon increased, the track density 

decreased; and the geometry factor became greater than 0.5. Zircons, 

because of their general elongate shape, will invariably be mounted for 

counting with the c axis subparallel to the microscope stage. This 

coincidence minimizes the chance of counting a zircon with an unknown 

geometry factor. As was noted earlier, the surface texture showed 

differences as well. The grains that had the highest track densities also 

had the sharpest polishing scratches. As the track density decreased the 

scratches became almost invisible. Apparently, the etching rate is much 

greater normal to the c axis that parallel to the c axis. It should be noted 

that these effects were observed and measured on induced tracks in an 

annealed crystal. The behavior of fossil tracks may be different. If the 

Geometry Factor 

~ !t--+I---....... --....-{~~-------_[ ~~":_oo_:_ __ _ 

~ f ' ~ 20 ' 9 --lP~--------'-1 II! Pz = 0.727 

~ Geometry Factor 

z 

0 

~ 

N 10 

~ 

~ 

~ 

o~oo~---------,~oo~--------~20~0----------~3~00~------~~407o--~ 

ANGLE BETWEEN POLISHED SURFACE AND OPTIC AXIS OF ZIRCON 

FIGURE 5.-Diagram showing the change in the ratio of induced fission tracks (external 

surface) in muscovite (pm) to the induced fission tracks (internal surface) in zircon (p%) 

from the Mud Tank Carbonatite, Australia, as a function of the angle between a polished 

surface and the optic axis of zircon grains. Error bars are one standard deviation about the 

mean. X'l= Track density in muscovite, X2=2 times track density in muscovite. 
CONCLUSIONS 13 

surface to be counted is carefully chosen (that is, if it has sharp scratches 

and gives a flash figure), a geometry factor of 0.5 is valid and can be used 

with confidence. 

Figun· 5 shows that as the angle bt·twt·t·n the c axis and the polished 

surfan· approaches 45°, the geometry fanor approac·hes infinity. This 

suggests that the I 0 I or Ill fan·s art· far mort· soluble than the I 00. 

CONCLUSIONS 

The results of this study show that any fission-track glass age must be 

considered a minimum age. In the case of glass shards from upper 

Cenozoic volcanic ash beds, only 1 sample out of 14 studied provided 

concordant fission-track ages on glass and zircon. 

Track fading in samples collected at the surface of an outcrop is a real 

problem in fission-track dating of natural glasses. This result is in direct 

conflict with the findings of Boellstorff and Steineck (1975), who 

concluded that a reduction in diameter of the fossil tracks does not affect 

the age, and that annealing probably does not take place at ambient 

temperatures. 

Some glasses that have been affected by track fading can be corrected 

with the plateau-annealing procedure of Storzer and Poupeau (1973) to 

give concordant results. It should be noted that this procedure does not 

always yield concordant results (McDougall, 1976, and this study). 

Therefore, a glass fission-track age that has been "corrected" must also be 

considered a minimum age. 

We conclude that the geometry factor as used in the fission-track dating 

of zircon with muscovite detectors (0.5) is valid if the c axis of the crystal is 

subparallel (±15°) to the polished surface being counted. 

ACKNOWLEDGMENTS 

We wish to thank our colleagues listed in tables 2 and 5 for providing 

many samples of natural glasses. We also wish to thank G. T. Cebula for 

separating the zircons from some of the volcanic ash beds. 

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Aumento, F., and Souther, J. G., 1973, Fission-track dating of late Tertiary and Quaternary 

volcanic glass from the Mount Edziza Volcano, British Columbia: Canadian Journal 

of Earth Sciences, v. I 0, p. 1156-1163. 

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1970, Macusanite occurrence, age, and composition, Macusani, Peru: Geological 

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Boellstorff, J.D., and Steineck, P. L., 1975, The stratigraphic significance of fission-track 
14 FISSION-TRACK AND K-AR AGES OF NATURAL GLASSES 

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Dalrymple, G. B., and. Hirooka, Kimio, 1965, Variation of potassium, argon, and 

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Ekren, E. B., Quinlivan, W. D., Snyder, R. P., and Kleinhampl, F. J., 1974, Stratigraphy, 

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Fleischer, R. L., and Price, P. B., 1964a, Glass dating by fission fragment tracks: journal 

of Geophysical Research, v. 69. p. 311-339. 

--- 1964b, Fission-track evidence for the simultaneous origin of tektites and other 

natural glasses: Geochimica et Cosmochimica Acta, v. 28, p. 755-760. 

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tektites and other natural glasses: Geochimica et Cosmochimica Acta, v. 29, p. 161-166. 

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v. 70, p. 1497-1502. 

--- 1975, Nuclear tracks in solids: University of California, Berkeley Press, 605 p. 

Galusha, Ted, and Blick, J. C., 1971, Stratigraphy of the Santa Fe Group, New Mexico: 

American Museum of Natural History Bulletin, v. 144, 127 p. 

Gentner, W., Kleinmann, B., and Wagner, G. A., 1967, New K-Ar and fission-track ages of 

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related glasses: Geochimica et Cosmochimica Acta, v. 33, p. 1075...:1081. 

Gilbert, C. M., Christensen, M. N., Al-Rawi, Yehya, and Lajoie, K. R., 1968, Structural and 

volcanic history of Mono Basin, California-Nevada, in Studies in volcanology-A 

memoir in honor of Howell Williams; Geological Society of America Memoir 116, 

p. 275-329. 

Gleadow, A. J. W., and Lovering, J. F., 1977, Geometry factor for external detectors in 

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Hibbard, C. W., 1941, The Borchers fauna, a new Pleistocene interglaical fauna from 

Meade County, Kansas: Kansas University Science Bulletin 38, pt. 7, p. 197-220. 

Lakatos, Stephen and Miller, D. S., t~72a, Evidence for the effect ol water content on 

fission-track annealing in volcanic glass: Earth and Planetary Science Letters, v. 14, 

p. 128-130. 

___ 1972b, Fission-track stability in volcanic glass of different water contents: joumal 

of Gt'ophyskal Rt'St'iliTh, \'. 77. p. 6990-6993. 

Lipman, P. W., Christiansen, R. L., and Van Alstine, R. E., 1969, Retention of alkalis by 

calc-alkaline rhyolites during crystallization and hydration: American Mineralogist, 

v. 54. p. 286-291. 

Lipman, R. W., Steven, T. A., and Mehnert, H. H., 1970, Volcanic history of the San juan 

Mountains, Colorado, as indicated by potassium-argon dating: Geological Society 

of America Bulletin, v. 81, p. 2329...:2351. 

Lugn, A. L., 193~. Classification of the Tertiary system in Nebraska: Geological Society of 

America Bulletin, v. 50, p. 1245-1275. 
REFERENCES CITED 15 

Mankinen, E. A., and Dalrymple, G. B., 1972, Electron microprobe evaluation of 

terrestrial basalts for whole-rock K-Ar dating: Earth and Planetary Science Letters, 

v. 17, p. 89-'94. 

MacDougall, J. D., 1971, Fission-track dating of volcanic glass shards in marine sediments: 

Earth and Planetary Science Letters, v. 10, p. 403-406. 

___ 1976, Fission-track annealing and correction procedures for oceanic basalt glasses: 

Earth and Planetary Science Letters, v. 30, p. 19-26. 

McKee, E. H., MacLeod, N. A., and Walker, G. W .. 1976, Potassium-argon ages of late 

Cenozoic silicic volcanic rocks, southeast Oregon: Isochron/West, no. 15, p. 37-41. 

Naeser, C. W., Izett, G. A., and Wilcox, R. E., 1973, Zircon fission-track ages of Pearlette 

family ash beds in Meade County, Kansas: Geology, v. 1, p. 187-189. 

Price, P. B., and Walker, R. M., 1963, Fossil tracks of charged particles in mica and the age 

of minerals: journal of Geophysical Research, v. 68, p. 4847-4862. 

Reed, L. C., and Longnecker, 0. M., Jr., 1932, The geology of Hemphill County, Texas: 

Texas University Bulletin 3231, 98 p. 

Reimer, G. M., Storzer, D., and Wagner, G. A., 1970, Geometry factor in fission-track 

counting: Earth and Planetary Science Letters, v. 9, p. 401-404. 

Roberts, j. H., Gold, R., and Armani, R. J., 1968, Spontaneous fission decay constant of 

U-'238: Physical Review, v. 174, p. 1482-1484. 

Seward, Diane, 1974, Age of New Zealand Pleistocene substages by fission-track dating of 

glass shards from tephra horizons: Earth and Planetary Science Letters, v. 24, p. 

242-248. 

Skinner, M. F., Skinner, S. M., and Gooris, R. J., 1968, Cenozoic rocks and faunas of 

Turtle Butte, south-central South Dakota: American Museum of Natural History 

Bulletin, v. 138, art. 7, p. 381-436. 

Spadavecchia, A., and Hahn, B., 1967, Die rotationskammer und einige anevendungen: 

Helvt•tica Physica Acta, v. 40, p. I 063-1079. 

Steiger, R. H., and Jager, E., 1977, Subcommission of geochronology-Convention on the 

use of decay constants in geo- and cosmochronology: Earth and Planetary Science 

Letters, v. 36, p. 359-362. 

Storzer, D., 1970, Fission-track dating of volcanic glasses and the thermal history of rocks: 

Earth and Planetary Science Letters, v. 8, p. 55-60. 

Storzer, D., and Poupeau, G., 1973, Ages-plateaux de mineraux et verres par Ia methode 

des traces de fission: Comptes Rendus Hebdomadaires des Seances de I' Academie des 

Sciences, p. 137-139. 

Storzer, D., and Wagner, G. A., 1969, Correction of thermally lowered fission-track ages 

of tektites: Earth and Planetary Science Letters, v. 5, p. 463-468. 

Stout, T. M., 1971, Geologic sections, in Rocks, real estate, and resources: Lincoln Neb., 

Nebraska Geological Survey: p. 29-65. 

Suzuki, M., 1970, Fission-track ages and uranium contents of obsidians: jinrui-Gaku 

Zasshi (Anthropoligical Society of Nippon journal), v. 78, p. 50-58. 

Thiel, K., and Herr, W., 1976, The mu spontaneous fission decay constant re-determined 

by fission tracks: Earth and Planetary Science Letters, v. 30, p. 50-56. 

Van Alstine, R. E., 1969, Geology and mineral deposits of the Poncha Springs NE quadrangle, Chaffee County, Colorado, with a section Fluorspar mines and prospects by 

R . .f.. Van Alstine and D. C. Cox: U.S. Geological Survey Professional Paper 626, 52 p. 

Wagner, G. A., Reimer, G. M., Carpenter, B.S., Faul, H., Vander Linden, R., and Gijbels, 

R., 1975, The spontaneous fission rate of U-238, and fission-track dating: Geochimica 

et Cosmochimica Acta, v. 39, p. 1279-1286. 

Wagner, G. A., Storzer, D., and Keller, J., 1976, Spaltspurendatierung quartarer 

Gesteinsglaser aus dem Mittelmeerraum: Neues jahrbuch fUr Mineralogie 

Monatshefte, 1976, p. 84-94. 

Wilcox, R. E., 1959, Use of the spindle stage for determination of principal indices of 

refraction of crystal fragments: American Mineralogist, v. 44, p. 1272-1293. 

TABLES 1-6 
18 FISSION-TRACK AND K-AR AGES OF NATURAL GLASSES 

TABLE I.-Fission-track and potassium-argon ages of upper Cenozoic natural glasses 

[Errors assigned to ages are 1 sigma about the mean. Fission-track age deby G.A. Izett. Fission tracks etched for about 10 seconds in 48 percent 

tracks counted at x720 by Izett and x625 by Naeser] 

Locality Neutron Spontaneous-track Induced-track 

No. (short name)l flux <+J density2 (p s) density2 (pi) 

(l015n em- ) (1o3 tracks cm-2) (1o3 tracks cm-2) 

1 Crystal Spring----- 0.53 0.246 (7) 58.5 (279) 

2 Obsidian Cliff----- 2.47 • 271 (9) 283 (593) 

3 Cougar Pass-------- .53 .912 (13) 143 (372) 

4 Big Southern Butte- 2. 72 .811 (27) 498 (569) 

5 Bearskin Butte----- .922 5.0373 (1) 125 (309) 

6 Mineral Range------ .53 • 789 (3) 44.0 (213) 

7 Owens River-------- .922 2.03 (29) 125 (521) 

8 West Queen Canyon-- 1.30 4.07 (110) 102 (784) 

1.17 3.80 (129) 83.5 (643) 

9 No Agua------------ 1.30 6.20 (47) 202 (300) 

10 Grassy Lake Reser- 1. 71 9.88 (84) 236 (489) 

voir quadrangle. 

11 Macusani----------- .922 23.3 (723) 296 (830) 

N .922 19.6 (476) 240 (1,893) 

12 Cougar Mountain---- 2.47 2.10 (20) 122 (471) 

2. 72 2.95 (84) 151 (732) 

13 Teton Pass 1. 30 5.94 (54) 99.1 (152) 

quadrangle. 

14 Glass Buttes------- 2. 72 4.58 (196) 119- (834) 

15 Mount Edziza------- 1.19 16.5 (505) 283 (3, 272) 

N 1.30 19.8 (96) 294 (339) 

16 Squaw Butte-------- 2.47 4.5 (214) 141 (416) 

2. 72 4.4 (84) 171 (667) 

17 Teton Pass 1. 71 5.90 (113) 102 (433) 

quadrangle. .53 6.38 (91) 28.5 (52) 

18 Horse Mountain----- 2. 72 7.08 (101) 242 (520) 

19 Drews Ranch-------- 2.47 5.22 (273) 184 (1,084) 

20 McComb Butte------- 2.47 2.46 (14) 167 (643) 

21 Czechoslovakia----- 2.69 9.12 (130) 99.8 (414) 

N 2.69 9.57 (242) 106 (327) 

22 Beaver Creek------- 2.53 63.9 (415) 491 (601) 

1.30 62.8 (705) 209 (490) 

1. 71 62.3 (296) 299 (306) 

23 Big' Sand Spring 1.30 67.3 (320) 390 (395) 

Valley. N 2.69 50.2 (401) 679 (787) 

1.30 54.7 (691) 308 (512) 

24 Nathrop------------ 1.30 114 (164) 335 (649) 

2.52 117 (211) 650 (277) 

See footnotes on following page. 
TABLES 

from 24 localities in the western United States, Peru, Canada, and Czechoslovakia 

terminations preceded by N were made by c. w. Naeser; all others were 

hydrofluoric acid; a few samples etched as long as 15 seconds. Fission 

No. 

1 

2 

3 

4 

5 

Fission-track 

glass age3 

(m.y.) 

0.13±0.05 

.14± .05 

.2Q± .06 

• 27± .05 6 .02 

K-Ar 

glass age4 

(m.y.) 

0.08 ±0.003 

.183± .003 

6.26 ± .06 

.31 i: .02 

.75 ± .1 

Table 3. 

Do. 

Do. 

Source of K-Ar age 

Armstrong and others (1975). 

H. H. Mehnert (written commun., 1975). 

Do. 

19 

6 

7 

8 

.59± .12 

.58-J: .30 

• 90% .17 

3.1 ± .31 

3.2 i: .30 6 2.4 ± .36 

4.3 ± .5 

.79 i: .08 

7 .92 ± .1 8 5.28 i: .22 

Gilbert and others (1968, P• 302, KA2081) • 

Table 3. 

9 

10 

11 

12 

13 

14 

15 

16 

17 

18 

19 

20 

9 4.3 ± .2 

4.5 ± .2 

6 2. s ± .6 6 3.2 ± .4 

4.7 ± .7 

4.4 i: .3 10 5.2 i: .3 

5.2 i: .6 6 4.7 ± .4 6 4.2 i: .5 

5.9 ± .6 

7.1 ±1.5 6 4.8 i: .5 

6 4. 2 i: • 3 

6 2.2 ± .6 

3.91 ± .27 

5.99 ± .06 

4. 3 ±1. 5 

4.42 ± .34 

8.48 ± .08 

5.04 i: .73 11 6.00 ± .06 

5.85 ± .67 

8.06 ± .08 

7.10 i: .14 

7.33 ± .34 

7.91 ± .09 

21 12 14.2 ±:1.4 13 15.1 ± .7 

14.5 ±:1.2 

22 19.4 ±1.2 22.4 i: .9 

23.3 ±1.4 

21.3 ±1.8 

23 613.4 ±1.0 26.4 ± 3 

6 11.9 ± .7 24.3 ± .3 

6 13.8 i: .8 

24 26.4 ±2.3 14 3o.o ± 1.s 

27.1 ±:2.5 

H. H. Mehnert (written commun., 1976). 

Table 3. 

Barnes and others (1970, p. 1543). 

McKee and others (1976). 

Table 3. 

McKee and others (1976). 

Table 3. 

McKee and others (1976). 

Table 3. 

McKee and others (1976). 

Do. 

Do. 

Gentner and others (1963). 

Lipman and others (1970, p. 2345). 

Ekren and others (1974, p. 605). 

Table 3. 

Van Alstine (1969, p. 18) 
20 FISSION-TRACK AND K-AR AGES OF NATURAL GLASSES 

TABLE I.-Fission-track and potassium-argon ages of upper Cenozoic natural glasses for 

24 localities in the western United States, Peru, Canada, and Czechoslovakia-Continued 

1see table 2 for complete name and description of locality. 

2Numbers in parentheses show total number of tracks counted in each sample. 

3Decay constant used: A£ • 7.03 x 10-17 yr-1• 

4All K-Ar ages calculated or recalculated using the following, recently 

revised decay constants: Ab • 4.962 x 10-10 yr-1; A£+ A£# • 0.581 x 10-10 

yr-1 ; 4 0K/K • 1.167 x 10-4 atom/atom (Steiger and Jager, 1977). 

5No fossil fission tracks seen; age calculated by assuming the presence 

of one track. 

6 sample had fossil fission-track annealing, as indicated by marked track 

diameter reduction. 

7Age of suspected source-area material. 

·

8 0bsidian reported to underlie basalt flows (Crowder and others, (1972). 

Basalt that caps Benton Range several kilometers southwest of this locality 

was dated 3.41±0.07 m.y. (average of 7 whole-rock K-Ar ages on samples spread 

along 3 km of outcrop) and 3.51±0.06 m.y. (average of 12 whole-rock K-Ar ages 

on age of 12 whole-rock K-Ar. ages on one hand sample) by Dalrymple and Hirooka 

(1965). Biotite from the basalt gave a K-Ar age of 3.25±0.30 m.y. Mankinin 

and Dalrymple (1972) reported an ~oAr_39Ar age of 3.45±0.48 m.y. for the basalt. 

9Fission-track glass age of 4•3t0.4 reported by Fleischer and Price 

(1964b). (Not recalculated with revised decay constant.) 

10Aumento and Souther (1973, P• 1162) report a fission-track glass age of 

3.1±0.1 m.y. (Not recalculated with revised decay constant). 

110bsidian is from a pebble in fluvial deposits overlying basalt flows 

dated by the K-Ar method at 4.1Q±0.6 m.y. (average of three whole-rock age 

determinations). 

12Gentner and others (1967) report a fission-track glass age for similar 

material of 14.1 m.y. (average of five determinations). Fleischer and Price 

(1964b) report fission-track glass ages of 15.3, 12.6, and 12.2 m.y. (Not 

recalculated with revised decay constants.) 

13Age for similar material. 

14K-Ar sanidine ages of 28.7±0.3 and 29.8±0.9 reported by Van Alstine 

(1969, P• 18). 
TABLE 2.-Descriptions and localities of samples listed in table 1. 

No. 

1. 

2. 

3. 

4. 

5. 

6. 

7. 

Locality 

Crystal Springs, Yellowstone National Park, 

Wyoming. 

Obsidian Cliff, Yellowstone National Park, 

Wyoming. 

Cougar Pass, Wyoming, sec. 14, T. 45 N., 

R. 107 w. (unsurveyed; Forest Service 

arbitrary grid). 

Big Southern Butte, Idaho (Armstrong and 

others, 1975). 

Bearskin Mountain, Mineral Range, Utah. 

NW 1/4 sec. 2U, T. 27 S., R. 8 W. 

Baily Ridge, Mineral Range, Utah. 

NE 1/4 SE 1/4 sec. 2, T. 27 S., R. 9 W. 

Along Owens River near Bishop, Inyo County, 

Calif. SW 1/4 NE 1/4 sec. 21, T. 6 s., 

R. 32 E. 

8. West Queen Canyon, Benton quadrangle, 

Mineral County, Nevada. SW 1/4 sec. 16, 

T. 1 N., R. 31 E. 

9. No Aqua, N. Mex. (Lipman and others, 1970). 

10. Grassy Lake Reservoir quadrangle, Teton 

County, Wyo. Center south line sec. 19, 

T. 47 N., 117 W. 

11. Macusani, Peru. (Barnes and othe{s, 1970). 

12. Cougar Mountain, Oregon (McKee and others, 

1976). 

Description Collector 

Obsidian from rhyolite flow---- R. L. Christiansen-

------do----------------------- ---do--------------

------do----------------------- F. s. Fisher-------

------do----------------------- W. P. Leeman-------

------do----------------------- P. W. Lipman-------

------do----------------------- ----do-------------

Obsidian pebbles in sedimentary G. A. Izett--------

rocks below the Bishop Tuff. 

Obsidian from rhyolite flow----

Obsidian from rhyolite dome----

Basal vitrophyre of Conant 

Creek Tuff. 

Obsidian pebbles in terrace 

deposits. 

Obsidian from rhyolite dome----

w. J. Carr---------

P. w. Lipman-------

J. D. Love---------

I. Friedman--------

G. W. Walker-------

Sample No. 

None. 

None. 

None. 

74-23 

75L56 

75L17 

75G46 

EWC-33-73 

66L234A 

None. 

None. 

M073-32 . 

~ = t""' 

1'11 

en 

too:) 

.... 
TABLE 2.-Descriptions and localities of samples listed in table i.-Continued 

No. Locality 

13. Teton Pass quadrangle, Teton County, Wyo. 

NE 1/4 sec. 36, T. 41 N., R. 118 W. 

14. Glass Buttes, Oregon (McKee and others, 

1976). 

15. Mount Edziza, British Columbia-----------

16. East of Squaw Butte, Oregon (McKee and 

others, 1976). 

17. McNeely Ranch, Teton County, Wyo. North 

of Mosquito Creek in SE 1/4 NW 1/4 sec. 

33, T. 41 N., R. 117 W. 

18. Horse Mountain, Oregon (McKee and others, 

1976). 

19. Drews Ranch, Oregon (McKee and others, 

1976). 

20. McComb Butte, Oregon (McKee and others, 

1976). 

21. Czechoslovakia----------------------------

22. Beaver Creek, San Juan Mountains, Colorado 

(Lipman and others, 1970). 

23. Big Sand Spring Valley, Nye County, Nev. 

Ten km north of Black Rock Summit, lat. 

38°35' N., long 115°54' W. 

24. Nathrop, Colo. North end of Ruby Mountain 

(Van Alstine, 1969). 

Description Collector 

Obsidian from rhyolite J. D. Love---------

intrusive. 

Obsidian from rhyolite dome---- G. w. Walker-------

Obsidian from rhyolite boulder R. Aumento---------

described by Aumento and 

Souther (1973). 

Obsidian from rhyolite dome---- N. S. MacLoud------

Obsidian from rhyolite J. D. Love---------

intrusive. 

Obsidian from rhyolite dome---- E. H. McKee--------

----do------------------------- ----do-------------

----do------------------------- ----do-------------

Tektite------------------------ H. Faul------------

Obsidian----------------------- P. W. Lipman-------

Apache tears from rhyolite P. P. Orkild-------

flow. 

Obsidian apache tears from P. w. Lipman-------

rhyolite flow. 

Sample No. 

None. 

M073-33 

DF596 

MS-70 

None. 

M073-41 

M073-43 

M073-39 

DF981 

65L161A 

0.15776 

66L2304 

~ 

~ 

"rl 

c;; 

rJ) 

0 

z 

~ 

n > "' 

~ 

> z 

t:::l 

~ 

> 

> "'

~ 

~ 

rJ) 

0 

"rl 

z 

> ..., 

c:: 

"' > r"' 

~ 

r"' 

> rJ) 

rJ) 

~ 

rJ) 
TABLE 3. -K -Ar analytical data for obsidian samples of table 1 not previously listed in published reports 

[All analyses by J. D. Obradovich. Decay constants: ~B • 4.962 x l0-10 yr-1 ; 

~£ + £; • 0.581 x 10-10 yr-1 ; 4 °K abundance: 4 °K = 1.167 x 10-10 atom/atom K] 

No. Locality Original 1<20 Radiogenic 4 0 Ar K-Ar age 

(short name) 1 Sample (weight In sample Percent of (m.y.) 

No. percent) (lo-11 g/g) total 40Ar 

1 Crystal Spring----------- DKA-2489 4.85 0.0558 30.7 0.08 ±0.003 

2 Obsidian Cliff----------- DKA-2466 4.98 .131 29.7 .183± .003 

8 West Queen Canyon-------- DKA-3355 4.14 3.79 12.1 5.28 ± .22 

10 Grassy Lake Reservoir DKA-3098 5.00 4. 27 70.8 5.99 ± .06 

quadrangle. ~ 15 Mount Edziza------------- DKA-3318 4.22 4.40 54.4 6.00 ± .06 = r 

3 Cougar Pass-------------- DKA-3330 3.46 3.76 82.4 6.26 ± .06 1'1'1 

CI'J 

17 Teton Pass quadrangle---- DKA-3328 3.25 4.56 75.3 8.06 ± .08 

13 Teton Pass quadrangle---- DKA-3331 3.47 5.12 79.8 8.48 ± .08 

23 Big Sand Spring Valley--- DKA-3317 4.23 18.0 96.9 24.3 ± .3 

1 See table 2 for complete name and description of locality. 

~ 
TABLE 4.-Fission-traclr. ages of cogenetic glass shards and zircon microphenocrysts separated from upper Cenozoic ash beds from the western tl:) 

United States .. [z • zircon; g • hydrated glass shards; (N)z • zircon age determined by c. W. Naeser; all others by 

G. A. Izett. Leaders (---) indicate data not available] .., 

Ci1 

Neutron Spontaneous-track Induced-track Fission- "' 0 No. Locality flux <+) density1 (ps) density1 (Pi) track age2 z 

(1015n cm-2) (1o3 tracks cm-2 ) (lo6 tracks cm-2 ) (m.y.) ~ 

1 Borchers locality, z 3 1.96±0.6 > "' ---- ----- ---- n 

Kansas. g 1.16 2.94 (70) 0.162 (297) 1.3 -;1:0.17 ~ 

> z 

2 Dalton, Nebr.-------- z 1.15 529 (88) 7.33 (910) 5.0 -;1:0.25 0 

g 1.18 12.5 (15) .193 (371) 4. 5 ±1. 2 ~ 

> 

3 Hemphill County, 1.09 1160 (313) 11.1 (2,728) 6.o8 ±0.2 "' z >Tex. g 1.10 16.8 (449) .227 440) 4.9 ±0.33 ~ 

l"'l 

4 Balcom Canyon, (N)z 1.18 573 (130) 5.16 (585) 7.8 ±0.4 "' 0 

Calif. g 5.03 18.0 (13) 1. 03 (380) 5. 3 ±1. 5 "''l 

z 

5 Espanola basin, (N)z 1.10 1350 (153) 9.47 (493) 9.4 ±0.9 > ...; 

New Mexico. g 1.78 14.9 (157) .233 (347) 6.2 ±0.6 c:: 

"' > 

6 Browns Park, Colo.--- z 1.28 766 (227) 6.10 (1,248) 9.8 ±0.4 r-' 

(N)z 1.02 1140 (163) 7.01 (503) 9.9 ±0.4 ~ 

r-' 

g 1.09 21.6 (183) .196 (638) 7.2 ±0.6 ~ 

"' 7 Cherry County, Nebr.- 1.22 776 (291) 5.48 (1, 824) 10.3 ±0.3 l"'l z Vl 

g 1.16 24.9 (108) .250 (541) 6.9 ±0.7 

8 Espanola basin, (N)z 1.10 1190 (132) 6.88 (382) 11.4 -;1:1.1 

New Mexico. g 2.09 21.2 (55) .331 (228) 8.0 ±1. 2 
TABLE 4.-Fission-track ages of cogenetic glass shards and zircon microphenocrysts separated from upper Cenozoic ash beds from the westhn 

1 United States-Continued 

Neutron Spontaneous-track Induced-track FissionNo. Locality flux <+) density1 (ps) density1 (Pi) track age2 

(lolSn cm-2) (103 tracks cm-2 ) (106 tracks cm-2) (m. y •) 

9 Middle Park, Colo.--- z 1.22 943 (201) 5.97 (1, 284) 11.5 ±0.9 

g 1.18 22.2 (210) .182 (699) 8.6 ±0.7 

10 ------do------------- z 1.23 567 (55) 3.08 (108) 13.5 ±1. 1 

(N)z 1.07 1440 (173) 6.93 (417) 13.3 ±0.6 

g 1.18 25.1 (411) .169 (280) 10.5 ±0.8 

11 Sioux County, Nebr.-- z 1.32 3680 (434) 16.6 (1, 804) 17.5 ±0.5 

(N)z 1.33 1200 (288) 5. 77 (695) 16.5 ±0.6 

g 2.47 20.0 (291) .313 (889) 9.4 ±0.6 

12 Helvas Canyon, Nebr.- z 1.23 4140 (605) 11.0 (1, 578) 27.6 ±0.6 

(N)z 1.23 4150 (577) 10.6 (737) 28.8 ±0.8 

g 1.16 35.5 (77) .210 (177) 11. 7 %1.8 

13 Chimney Rock, (N)z 1.06 4550 (548) 10.6 (641) 27.2 ±0.9 

Nebr. g 1. 71 41.3 ( 107) .362 (436) 11.7%1.3 

14 Uinta County, Wyo.-- (N)z .977 7450 (1,439) 12.62 (1,219) 34.4 %0.7 

g 1.95 66.9 (191) .399 (403) 19.5 ±1.7 

1Numbers in parentheses show total number of tracks counted in each determination. 

2Error shown is ±1 sigma. Af - 7.03x1o-17 yr-1 • 

3Fission-track age reported by Naeser, Izett, and Wilcox (1913). 

..., 

> t:l:l 

r" 

~ 

Vl 

~ 

01 
No. 

1. 

2. 

3. 

4. 

5. 

6. 

7. 

TABLE 5.-Descriptions and localities of samples listed in table 4. 

Locality 

Borchers locality, in NW 1/4 NE 1/4 

sec. 21, T. 33 S., R. 28 W., 

Meade County, Kans. 

Dalton, Nebr. Roadcut along State 

Highway 383 about 4 km north of 

Dalton, in NW 1/4 SW 1/4 

sec 9, T. 17 N., R. 49 W., 

Morrill County, Nebr. 

Hemphill County, Tex., in 

SE 1/4 NE 1/4 NE 1/4 sec. 59, 

blk. A-2 (locality 20 of 

Reed and Longnecker, 1932). 

Balcom Canyon, in NE 1/4 SE 1/4 

sec. 16, T. 3 N., R. 20 w., 

Ventura County, Calif. 

Espanola basin, in SE 1/4 

sec. 31, T. 20 N., R. 9 E., 

Espanola 7 1/2-minute quadrangle, 

Santa Fe County, N. Mex. 

Browns Park. Roadcut along State 

Highway 318 in SW 1/4 NW 1/4 

sec. 21, T. 9 N., R. 101 W., 

Moffat County, Colo. 

Cherry County, Nebr., in 

NE 1/4 SE 1/4 sec. 15, 

T. 32 N., R. 70 W. 

Description 

Silver-gray volcanic ash (type B Pearlette 

ash) about 0. 6 m thick underlying upper 

Neogene sedimentary rocks that contain 

the Borchers fauna of Hibbard (1941). 

Silver-gray volcanic ash about 1.0 m tQick 

interlayered in sedimentary rocks of the 

uppermost part of the Miocene Ogallala 

Formation (Kimball Formation of Stout, 

1971). 

Silver-gray volcanic ash about 1.0 m thick 

interlayered in Miocene sedimentary rocks 

rocks of the Ogallala Formation that contain the type Coffee Ranch Hemphill fauna. 

Silver-gray volcanic ash about 0.1 m thick 

interlayered in Miocene sedimentary rocks 

of the Modelo Formation. 

Silver-gray volcanic ash about 2 m thick 

interlayered with the Miocene Pojoaque 

Member (Tesque Formation) of Galusha 

and Blick (1971). 

Silver-gray volcanic ash about 8 m thick 

interbedded in sedimentary rocks of the 

uppermost part of the Browns Park Formation (Miocene). 

Silver-gray vol~nic ash in lower part of 

the Caprock Member (Skinner and others, 

1968) of the Ash Hollow Formation of Lugn 

(1939) (equivalent to the Miocene Ogallala 

Formation) about 15 m above the Burge fossil mammal quarry. 

Collector 

G. A. Izett and 

R. E. Wilcox. 

G. A. Izett-------

G. A. Izett, 

G. R. Schultz, 

and 

P. W. Lambert. 

J. D. Obradovich--

G. A. Izett and 

T. Galusha. 

G. A. Izett-------

M. R. Skinner-----

Sample No. 

72G100 

73G200 

68G67 

71-0-3 

74G327 

68G86 

73G223 

~ 

"!j 

{;; 

Vl 

0 

z 

.:., 

n > "' 

~ 

> z 

0 

~ 

> 

> "'C1 

~ 

en 

0 

"!j 

z 

> 

~ 

c: 

"' > t""' 

C1 

t""' 

> Vl 

Vl 

~ 

en 
No. 

8. 

9. 

10. 

11. 

12. 

13. 

14. 

TABLE 5.-Descriptions and localities of samples listed in table 4.-Continued 

Locality 

Espanola basin, in SE 1/4 

sec. 36, T. ~0 N., R. 8 E., 

Espanola 7 1/2-minute quadrangle, 

Santa Fe County; N. Mex. 

Middle Park, in SW 1/4 SE 1/4 

sec. 32. T. 2 N., R. 80 W., 

Grand County, Co. 

Middle Park, NE 1/4 SE 1/4 

sec. 24, T. 1 N., R. 80 W., 

Grand County, Colo. 

Sioux County, Nebr. Head of 

Merychippus Draw. 

Helvas Canyon, in NW 1/4 NW 1/4 

sec, 6, T. 20 N., R. 55 w •• 

Scotts Bluff County, Nebr. 

Chimney Rock. South of Chimney 

Rock on the main butte in the 

SW 1/4 SW 1/4 ~ec. 17, T. 20 N., 

R. 52 w •• Morrill County, Nebr. 

Uinta County, ~7o., in 

SW 1/4 NE 1/4 SE 1/4 sec. 17, 

T. 16 N., R. 120 W. 

Description 

Olive-gray volcanic ash about 1 m thick in 

the Miocene Pojoaque Member (of Galusha 

and Blick, 1971). 

Silver-gray volcanic ash bed about 1.0 m 

thick interlayered in the Miocene 

Troublesome Formation. 

Olive-gray volcanic ash bed about 1.0 m 

thick interlayered in the upper part 

of the Miocene Troublesome Formation. 

Olive-gray volcanic ash bed about 1.0 m 

thick interbedded in the upper patt of 

the Ogallala Formation (Sheep Creek 

Formation of Lugn, 1939). 

Chalky-white volcanic ash bed about 1.2 m 

thick above the base of the Gering Sandstone of the Arikaree Group (Miocene). 

Chalk-white volcanic ash bed about 0.8 m 

thick interbedded in the lowest part of 

the Gering Sandstone (of the Miocene 

Arikaree Group) about 0.3 m above the 

base of the Gering. 

Ash bed ia Gooseberry(?) Member of Fowkes(?) 

Formation. 

Collector Sample No. 

G. A. lzett------- 74G326 

----do------------ 68G105 

----do------------ None. 

G. A. lzett 0 73 

M. R. Skinner, 

and J. G. Honey. 

G. A. lzett------- 65 

----do------------ 70G15 

James Honey------- None. 

1-j 

> 

= r'" 

!'PI 

tiJ 

~ 

.....::1 
28 FISSION-TRACK AND K-AR AGES OF NATURAL GLASSES 

TABLE 6.-Fission-track plateau annealing ages of natural glasses 

[Fission-track determinations prefixed by an N made by C. W. Naeser; all other 

is one standard deviation abou·t the 

Furnace Neutron Spontaneous-track 

No. Locality Material temp. 1 flux+ density2 (p, (oC) (lolsn C11l-2) (lo3 tracks c ) 

1 Macusani, Peru--- Macusanite 0.922 23.3 (723) 

(obsidian) N .922 19.6 (476) 

N 2.09 19.0 (213) 

200 2.09 20.4 (97) 

N 200 2.09 19.4 (145) 

250 2.09 12.8 (61) 

N 250 2.09 12.0 (152) 

300 2.09 9.89 (47) 

N 300 2.09 9.34 (118) 

2 Cougar Mountain, Obsidian---- 2.47 2.10 (20) 

Oreg. 225 2. 72 Z.95 (84) 

3 Glass Buttes, ---do------ 2.47 1. 42 (137) 

Oreg. 2. 72 4.58 (196) 

225 2.47 2. 2 (187) 

4 West Queen ---do------ 1.17 . 3.80 (129) 

Canyon, Nev. 220 1. 78 2.90 (124) 

5 Drews Rancll, ---do------- 2.47 5.22 (273) 

Oreg. 225 2.47 4.36 (145) 

6 Cherry County, Glass shards 1.16 24.9 (108) 

Nebr. 210 3.33 19.6 (117) 

7 Middle Park, ---do---- 1.18 22.2 (210) 

Colo. 210 3.33 11.9 (38) 

8 Middle Park, ---do------ 1.18 25.1 (411) 

Colo. 210 3.33 19.9 (53) 

9 Czechoslovakia- Moldavite-- 2.69 9.12 (130) 

N 2.69 9. 57 (242) 

198 2.69 9.40 (134) 

N 198 2.69 8.8 (223) 

N 275 2.69 7.48 (189) 

350 2.69 5.26 (100) 

N 350 2.69 4.82 (122) 
TABLES 29 

from the western United States, Peru, and Czechoslovakia 

fission-track determinations by G. A. lzett. Precision of fission-track ages mean. Af • 7.03 x 10-17 yr-•) 

Induced-track Fission- Other 

No. density2 (pi) track age reported Reference 

(m.y.) (m.y.) ages (m.y.) 

296 (830) 4. 3±0. 2 4.3 ±1-5 K-Ar age reported by Barnes and 

240 (1, 893) 4. stO. 2 others (1970, P• 1543). 

51.6 (721) 4.6±0.3 

613 (411) 4. 2±0.5 

518 (755) 4.7-J;0.3 

360 (242) 4.4%0.6 

316 (460) 4. 7-J:O. 4 

276 (309) 4. sto. 1 

239 (349) 4.9:0.5 

2 122 (471) 2. sto. 6 4.42-J;O. 34 K-Ar age reported by McKee and 

151 (732) 3.2-J;0.4 others (1976). 

3 140 (386) 1. sto. 2 5.04%0.73 Do. 

170 (834) 4.4-J;0.3 1.5-J;0.2 Fission-track glass ~ge. 3 

66.8 (252) 4. 9-J;O. 5 

4 83.5 (643) 3.2-J;0.30 5.28±0.22 K-Ar age from table 3 (no. 8). 

63.3 (301) 5.1-J;O. 31 

5 184 (1,084) 4.2±0.3 7.33±0.34 K-Ar age reported by McKee and 

88.9 (443) 7.2-J;0.7 others (1.976). 

6 250 (541) 6.9±0.8 10.3±0.6 Fission-track zircon age from 

361 (453) 10. 9±1.1 table 4 (no. 7). 

7 182 (699) 8.6-J;0.6 11.5±1.8 Fission-track zircon age from 

348 (323) 11.4-J;2.0 table 4 (no. 9). 

8 169 (280) 9.4-J;0.7 13.5±2.2 Fission-track zircon ages from 

274 (421) 14. 5±2. 1 13. 3±1. 1 table 4 (no. 10). 

9 99.8 (414) 14. 7±1.4 15.1±0.7 K-Ar age reported by Gentner 

106 (327) 14. 5±1. 2 and others (1967). 

98.6 (412) 15.3±1.5 

99.2 (306) 14. 3±1. 2 

78.3 (241) 15.4%1.5 

54.1 (395) 15. 6t1. 7 

49.8 (307) 15.6±1.6 
30 FISSION-TRACK AND K-AR AGES OF NATURAL GLASSES 

No. Locality 

10 Sioux County, 

Nebr. 

11 Big Sand Spring 

Valley, Nev. 

12 Chimney Rock, 

Nebr. 

TABLE 6.-Fission-track plateau annealing ages of natural glasses 

Material 

Glass shards 

N 

N 

Apache tears 

N 

N 

N 

Glass shards 

N 

N 

N 

N 

N 

N 

Furnace 

temp. 1 

(oC) 

149 

149 

204 

204 

225 

225 

250 

200 

200 

250 

250 

300 

300 

153 

153 

179 

179 

204 

204 

227 

227 

250 

250 

Neutron 

flux; 

( 1015n cm-2 ) 

2.47 

2.47 

2.47 

2.47 

2.47 

2.47 

2.47 

2.47 

2.47 

1.30 

2.69 

1.30 

2.09 

2.09 

2.09 

2.09 

2.09 

2.09 

1. 71 

1. 71 

1. 71 

1. 71 

1. 71 

1. 71 

1. 71 

1. 71 

1. 71 

1. 71 

1. 71 

1. 71 

Spontaneous-track 

densityl (p s) 

(103 tracks cm-2 ) 

20.0 

22.0 

21.9 

21.0 

20.5 

22.2 

21.1 

15.0 

15.4 

67.3 

50.2 

54.7 

55.5 

61.4 

38.3 

37.1 

28.2 

21.2 

41.3 

36.2 

46.3 

41.2 

48.6 

43.2 

46.7 

38.6 

54.0 

36.3 

31.3 

22.4 

(292) 

(185) 

(290) 

(177) 

(268) 

(187) 

(183) 

(126) 

(23) 

(320) 

(401) 

(691) 

(116) 

(776) 

(182) 

(469) 

(134) 

(268) 

(107) 

(195) 

(120) 

(222) 

(126) 

(216) 

(121) 

(207) 

( 140) 

(121) 

(61) 

(121) 

1samples were heated at the indicated temperature for one hour prior to 

2Numbers in parentheses show total number of tracks counted in each de3Determination on a sample collected by Charles Vitaliano. 
TABLES 31 

from the western United States, Peru, and Czechoslovakia-Continued 

Induced-track Fission- Other 

No. density2 (Pi) track age reported References 

(103 tracks cm-2) (m.y.) ages (m.y.) 

10 313 (889) 9.4±0.7 15. 1 K-Ar age reported by Evernden 

285 (861) 11.4±0.9 and others (1964, KA891). 

210 (846) 15.3±1.0 

223 (685) 13.9±1. 2 

171 (1, 455) 17. 7±1. 2 

181 (570) 18. H:1. 5 

176 (505) 17. 7±1. 5 

134 (418) 16.5±1.6 

139 (266) 16. 4±3. 4 

11 390 (395) 13. 4±1. 0 26.4±3.0 K-Ar age reported by Eckren and 

679 (787) 11. 9±0. 7 others (1974, P• 605). 

308 (512) 13.8±0.8 24. 3±0. 3 K-Ar age from table 3 (no. 23). 

342 (258) 20.3±2.4 

337 (560) 22.8%1.3 

242 (302) 19.8%1.9 

215 (354) 21.6±1.5 

155 (238) 22.7-tZ.5 

122 (201) 21.7±2.1 

12 362 (436) 11.7%1.2 30.2 K-Ar age reported by Evernden 

331 (991) 11. 2±0. 9 and others (1964, KA981). 

278 (351) 17.0%1.8 

224 (715) 18.8%1.4 

197 (424) 25.2±2.6 

171 (569) 25.8%1.9 

168 (415) 28.4%3.0 

174 (572) 22. 7±1. 8 

176 (597) 31. 3±2. 9 

136 (466) 27.3±2.3 

156 (247) 20.5%2.9 

97.9 (329) 23.4%2.5 

analysis. Leaders <---) indicate samples that were not heated. 

termination. 

tr U.S. GOVERNMENT PRINTING OFFICE, 188Q-677-1211/311