Results of interlaboratory comparison of fission-track age standards: Fission-track workshop—1984

NucL Tracks, Vok 10, No. 3, pp. 383-391, 1985 Printed in Great Britain

0191-278X/85$3.00+ 0.00 Pergamon Press Ltd.

RESULTS OF INTERLABORATORY COMPARISON OF FISSION-TRACK AGE STANDARDS: FISSION-TRACK WORKSHOP--1984 DONALD S. MILLER,IAN R. DUDDY* Department of Geology, Rensselaer Polytechnic Institute, Troy, NY 12181, U.S.A., PAUL F. GREEN Department of Geology, University of Melbourne, Parkville, Victoria, 3052 Australia ANTHONYJ. HUREORD Isotope Geology Laboratory, University of Bern, Bern, CH-3012 Switzerland and CHARLESW. NAESER United States Geological Survey, Denver, CO 80225, U.S.A. (Received 17 October 1984; in revised form 11 January 1985)

Abstract--Five samples were made available as standards for the 1984 Fission Track Workshop held in the summer of 1984 (Rensselaer Polytechnic Institute, Troy, New York). Two zircons, two apatites and a sphene were distributed prior to the meeting to 40 different laboratories. To date, 24 different analysts have reported results. The isotopic ages of the standards ranged from 16.8 to 98.7 Myr. Only the statement that the age of each sample was less than 200 Myr was provided with the set of standards distributed. Consequently, each laboratory was required to use their laboratory's accepted treatment (irradiation level, etching conditions, counting conditions, etc.) for these samples. The results show that some workers have serious problems in achieving accurate age determinations. This emphasizes the need to calibrate experimental techniques and counting procedures against age standards before unknown ages are determined. Any fission-track age determination published or submitted for publication can only be considered reliable if it is supported by evidence of consistent determinations on age standards. Only this can provide the scientificcommunity with the background to build up confidence concerning the validity of the fission-track method.

INTRODUCTION As PART of the Fourth International Fission Track Dating Workshop held at Rensselaer Polytechnic Institute, Troy, New York, in August, 1984, five samples considered suitable for use as age standards for fission-track dating (FTD) were distributed to interested participants prior to the meeting. The five samples comprised two zircons, two apatites and one sphene. A previous interlaboratory comparison employing apatite and zircon from the Fish Canyon Tuff (Naeser et al., 1981) had given results from different

laboratories which were in fairly good agreement with the independently derived age of 27.8 Myr for this deposit. However in the previous study the independent age was made known to participants, and thus the exercise was not truly objective, Therefore in the comparison program reported in this contribution, no details were provided of the (known) ages of the circulated samples. Instead, participants were simply informed that all ages were less than 200 Myr. Participants were required to analyze each sample using techniques (sample preparation, etching, irradiation, counting etc.) regarded as standard

*Present address: Department of Geology, University of Melbourne, Parkville, Victoria, 3052 Australia. 383

Table I. Standards results for 1984 Fission-Track Workshop. (All ages in Myr, errors are one sigma) 84-1 * (zircon)

Isotopic age (details in text) Anlst 1 2

Techt Z~ta 7.03

3

7.03

4(a)

Zeta

4(b)

Zeta

5

7.03

6

7.03

7

7.03

8

Zeta

9

Zeta

84-2* (zircon)

84-3* (apatite)

84-4* (apa0te)

84-5* (sphene)

16.5 -4- I

27.77 + 0.04

27.77 -4- 0.04

98.7 -4- 0.3

98.7 -4-0.3

15.6 + 0.5 [U3] + 14.5 _+ 0.8 [613, Au] 19.4_+1.1[ [96 I, 71 16.71 -4- 0.7 [612, CNI, CN2] 16.7l -4- 1.0 [612, CNI, CN2] 16.0±1.0 [962, Cu] 39.7 ± 1.6!i•

28.1 + I).8 11-03] 25.4 ± 1. I [613, Au]

'9 " + 1.4 [612] 30.8 -4- 2,2 [613, Au]

94.4 +_ 4.6 [612] 88.3 -4- 6.6

ICNII

[613, Aul

[613, Au]

32.3 ± 1.5 [612, CNI, ('N2] 33.3_+2.4 [612, CNI, CN2] 28.1 ±0.9 [962, Cu]

24.5 + 3.1 [612, CN1, CN2] 25.1 ±3.0 [612, CN1. CN2]

68.0 ~ 7.8 [612, CNI, CN2] 73.2 ± 8.2 [612, CNI. CN2]

94.3 + 5.7 [612, CNI, ('N2 l 95.3 _+ 4,3 [612, CNI, CN2] 89.2 -4- 2,8

29.0 ± 0.5 [J R I, ?] 27.4 + 1.2' [CN2] 27.5+ 1.2 [U31 28.9 ± 1.4 [612, CNI. CN2] 27.4 -4- 0.9 [962, Cu]

30.7 ± 0.9 lJ R J, ?l 28.0 + 2.0 • [CN2] 30.1 -4- 16 1612l 28.9 ± 1.7 [612, CNI, CN2] 29.8 -4- 2.0 [963, Cu] 35.0 ± 2.O,} [963, Cu] 28.1 -4- 1.7§ [963, ('u]

100.3 -4- 2.5 98.5 ± 4.8

19o3, ('u]

1612, 71

10

Zeta

I l(a)

7.03

20.7 _+ 1,6g [CN2] 15.2±0.8 [U31 16.8 + 1.3 [612, CNI, CN2J 16.0 -4- 0.85 [962, Cu]

ll(b)

11(c) 12

7.03

13

7.03

14

7.03

15

8.46

16

7.03

17

7.03

18

7.03

19

Zeta

20(a)

7.03

20(b)

7.03

21

7.03

22

7.03

11.0_+ 1.0[ [612, Cu] 17.0 _+ 1.5 [612, Cu] 15.8 _+ 1.2 [962, Cu] 20.0-4- 1.6 [962a, Cu] 19.7,4,2.6 [962, Cu] 17.2 ± 0.6 [962. CN2, Cu]

15.5 ± 1.3 [U3] 14.9 ± 1.0 [962. Cu] 15.2±0.9 [612, Cu]

27.o ± I.O i [612, Cu] 24.3 -4- 2.5 [612, Cu] 19.5,4, 1.5 [962a, ('u] 27.8+ 1.5 [962, Cu] 27.8 -4- 0.8 [962, CN2, ('u] 28.8 -4- 0.8 [962, CNI, ('u] 27.7 + 1.2 Ill31 24.6 + 1.3 [962, Cu]

20.0 + 1.4 [612, Ca] 29.7 -4- 1.8 [962, ('u] 22.9±0.7 [962a, 963a, ('u] 34.2±3.1 [962, Cu] 27.0 + 1.0 [CN I, ('u] 26.3 ± 1.3 [CNI, ('u]

81.1 ~ ,~ [.1R 1. '.'1

86.3 ± 1.2 [J R 1. 71

959 2 4.~

100.7 ± 3.8 [C'N J] 93.7_+4.1 [612. CNI, ('N2J 95.9 _+ 4.5 (NaOHt [962, Cu] 96.7 + 5.6 (Acid) [962, ('u]

16121 94,7 4- 3.8 [612, CNI, ('N21 89.1 -4- 6.0 [963, Cu] 86.6 + 7.7§ [963, Cu] 100.0 + 11.5~ [963, Cu I 74.5 ± 4.1 [612, ('u]

54.5 ± 7.5 [962a, 963a, Cu] 64.9 + 5.t~ [962, Cu] 76.9 ± 2.~ [('nI. ('ul

~8.0 t 2.S [962a, Cu] 94,4 1- 2.9 [('N l. Cu] 95.5 ± 2.9

[('n I, Cul 95.8 + 4.6

I6121 27.5 ± 2.0 [963, ('ul 23.7 -2_2.3~ [962, Cu] 30.6 ± 2,3 [612, Cu] 92 ± 6 [CN I, '71

73.0 2 4.1 [963, ('u] 83.2 + 8.9~ [962. Cu] 91.4 ± 4.7 [612, Cu] I75 + 5 [CN l, '71

91,0 ± 5.5 [962, Cu]

197 ± 4 [CNJ, ?l

*External detector method used except where noted by §, [i or % +If analysis is not done by age calibration (Zeta), then ). is given in units of 10 ':~r ++Information in brackets is first, the glass standard identification [612, 613, 962, 963 are NBS standards, U3 is an exhausted Coming glass standard, CNI and CN2 are Corning glass standards, and J RI is a naturaI obsidian glass]: the second listing is the monitor (copper or gold foil) used for determining the neutron fluenee, if given. §Population method used. !!Re-etch method used, • Subtraction method used

INTERLABORATORY COMPARISON OF FT AGE STANDARDS

385

Table 2. List of analysts reporting data on the standards 84-1, 84-2, 84-3, 84-4 and 84-5 Amano, Yoshiyuki, Kadoma-Nishi Highschool, 29-1 Yanagida-Chyo, Kadoma City, Japan Bhattacharyya, P., Bajali College, Pathsala, PIN 781 325, India Carpenter, S., National Bureau of Standards, B108 Reactor Building, Gaithersburgh, MD 20859, U.S.A. Crnwley, Kevin D., School of Geology and Geophysics, University of Oklahoma, Norman, Oklahoma 73019, U.S.A. Evans, Ian, Bernard Price Institute, Geophysics, University of Witwatersrand, Johannesburg 2001, South Africa Gleadow, Andrew J. W., Department of Geology, University of Melbourne, Parkville, Victoria 3025, Australia Goswami, T. D., Department of Physics, Gauhati University, Gauhati, Assam 781014, India Green, Paul F., Department of Geology, University of Melbourne, ParkviUe, Victoria 3052, Australia Hansen, Kirsten, Institut for Petrologi, Copenhagen University, Oster Voldgrade 10, DK-1350, Copenhagen, Denmark Hayashi, Masao, Research Inst. of Industrial Science, Kyushu University, 86 Kasuga City, Fukuoka 816, Japan Hufford, Anthony J., Abt. fur Isotopengeologie, Univ. Bern, Erlachstrasse 9a, Berne, Ch-3012, Switzerland Kohn, B. P., Geology Department, Ben Gurion University, P.O. Box 653, Beer Sheva, Israel Kowallis, Bert, Department of Geology, Brigham Young University, Provo, Utah 84602, U.S.A. Matsuda, T., Department of Geology, Himeji Institute of Technology, Shosha 2167, Himeji, Japan, 671-22 Naeser, Charles, U.S. Gological Survey MS 424, Federal Center, Denver, Colorado 80225, U.S.A. Naeser, Nancy, U.S. Geological Survey MS 424, Federal Center, Denver, Colorado 80225, U.S.A. Nelson Eric, Department of Geology, Colorado School of Mines, Golden, Colorado 80401, U.S.A. Omar, G. I., Geology Department, University of Pennsylvania, Philadelphia, Pennsylvania 19104, U.S.A. Patgiri, K. K., Bajali College, Pathsala, PIN 781 325, India Suzuki, Masao, St. Paul's University, 34-1 Nishi-lkebukuro 3-chome, Toshima-ku, Tokyo 171, Japan Suzuki, Tatsuo, Department of Geology, Faculty of Education, Kagoshima University, Korimoto 1-20-6, Kagoshima 890, Japan Tagami, Takahiro, Department of Geology and Mineralogy, Kyoto University, Kyoto 606, Japan Tamanyu, Shiro, Geological Survey of Japan, 1-1-3 Higashi, Yatabe, Ibaraki 305, Japan Zimmermann, Robert, U.S. Geological Survey MS-423, Federal Center, Denver, Colorado 80225, U.S.A.

in their laboratory. A form was provided which requested participants to provide full details of the derivation of the fission-track ages, including such aspects as neutron dosimetry and choice of constants (Appendix I). This study represents the first truly objective interlaboratory comparison of fission-track dating techniques. SOURCE OF SAMPLES Zircon 84-1, showing uniform uranium distribution and few inclusions, was supplied by Anthony J. Hurford from an air fall tuff in Northern Kenya, with an independent K/Ar age of ~ 16.5 Myr (McDougall and Watkins, in preparation). The Fish Canyon Tuff standards made available by Charles W. Naeser several years ago (Naeser and

Cebula, 1978) were supplied and distributed as 84-2 and 84-3, zircon and apatite, respectively. The age of this deposit is well constrained by other techniques. K/Ar measurements by Steven et al. (1967) yield ages for co-existing sanidine, biotite, hornblende and plagioclase concordant at 27.9 + 0.7Myr. Similar measurements reported by Hurford and Green (1983) on the same four minerals also gave concordant results, with a mean of 27.4 + 0.5 Myr. A fifteen point 4°Ar/39Ar stepwise degassing measurement on biotite gave an excellent plateau with a total degassing age of 27.45 + 0.50 Myr (Hurford and Green, 1983). Recent high precision studies by the U.S.G.S. (reported verbally by John Sutter, during the workshop) are in good agreement with these earlier estimates, and the accepted age of this deposit is now taken to be 27.77 ± 0.08 Myr (obtained from sanidine 4°Ar/39Ar

386

D O N A L D S. M I L L E R et al. their data as one submission) from 16 different laboratories. Results are tabulated from data received on a form shown in Appendix I (modified after one designed by Green and Hurford). Detailed information is available upon request from Donald S. Miller at RPI. Several methods ot" analysis were used: external detector method, population method, re-etch method and subtraction method (see Hurford and Green, 1982 for descriptions). Most analysis used the external detector method. When other methods were used it is noted in Table l. The error quoted in Table 1 is one sigma while the error bars in Figs 1 to 5 represent ± 2 sigma. Not all analysts reporting data used the same technique to report errors. Consequently, in several cases it was necessary to recalculate the data submitted to obtain a conventional Poisson error generally agreed, during the workshop, to be appropriate to the external detector method of fission-track dating. The error was calculated as

age relative to an accepted age of 519.4 Myr for the standard M M h b - I reported by Dalrymple e t a / . , 1981). Apatite 84-4 and sphene 84-5 are from the central banatite of the M o u n t Dromedary complex of New South Wales, Australia. McDougall and Roksandic (1974) report a K Ar age of 97.9 _+ 1.4 Myr on biotite from the outer monzonite phase of the complex. McDougall and Wellman (1976) reported ages of 9 6 _ + 2 M y r and 9 3 _ + 3 M y r for biotite and hornblende respectively from two marginal intrusive phases associated with the complex. Williams e t at. (1982) report a K / A r biotite age of 97,9 _+ 1.4 Myr and a Rb/Sr biotite age of 98.8___ 0.6 Myr for the outer monzonitic phase. From these results a preferred age of 98.7 ± 0.6 Myr is taken for the age of this complex. Samples of apatite and sphene from the central banatite were made available by the Fission Track Research G r o u p of the University of Melbourne. RESULTS

40

/i

a(T)--7

Analyses (Table 1 and Figs I 5) were reported b5 24 individual analysts (Table 2; 3 analysts submitted

i

i

, + + ~' N N, .\"r,

T ZIRCON

84

1

30 f i

T

i

f

,

10

i

; d

1

i 0

I

.

. 1

.

. 2

. 3

.

. 4

. 4

. 5

.

. 6

ANALYST

. 7

.

. 8

. 9

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

i

10 11 12 13 14 15 18 17 18 19 2 0 21 3 2

IDENTIFICATION

NUMBER

Fits. 1. Fission-track ages reported for 1984 Workshop Standard, Zircon 84-I (bars represent + 2 sigma). Dashed lines represent 2 sigma around a mean of 16.5.

(I)

I N T E R L A B O R A T O R Y C O M P A R I S O N OF FT A G E S T A N D A R D S

80 ZIRCON 84-2

60

GE 40

a)

20

1

2

3

a

b

4.

4

5

6

7

8

9

1 0 11 1 2 13 14 15 1 6 17 18 19 2 0 2 1 2 2

ANALYST IDENTIFICATION NUMBER FIG, 2. Fission-track ages reported for 1984 Workshop Standard, Zircon 84-2 (bars represent + 2 sigma).

I00 APATITE 84-3

80

6O AGE (Ma)

40

III

20

abc

ab 1 2

3

4- 4- 5 6

7

8

a

b

9 10 11 11 11 12 13 14 15 16 17 18 19 20 20 21 ;~2

ANALYST IDENTIFICATION NUMBER

FIG. 3. Fission-track ages reported for 1984 Workshop Standard, Apatite 84-3 (bars represent + 2 sigma).

387

388

D O N A L D S. M I L L E R et al.

3501

r I APATITE 8 4 - 4

200 t

I

1,

'i

AGE

i

i

(Ma)

f

!

i

!

t 0

i

E

1

a b 1

2

3

4

a b c

4

5

6

7

8

ANALYST

9

a

b

1

10111111121314151617181920202122

IDENTIFICATION

NUMBER

FIG. 4, Fission-track ages reported ('or 1984 Workshop Standard. Apatite 84-4 (bars represent + 2 sigma).

250 SPHENE 8 4 - 5 200

150 AGE (Ma)

100 ]

a

b

4

4

a

b

c

0 1

2

3

5

0

7

8

9 I 0 11 I I i i 12 13 14 15 16 17 18 19 20 21 22

ANALYST IDENTIFICATION

NUMBER

FIG. 5. Fission-track ages reported for 1984 Workshop Standard, Sphene 84-5 (bars represent +2 sigma).

INTERLABORATORY COMPARISON OF FT AGE STANDARDS where N, = total spontaneous tracks counted, N~ = total induced tracks counted, Nn = total tracks counted in determining a neutron fluence. Several papers discussing or reporting various aspects of standards are given in Appendix II. These references include information on various glass standards and mineral standards, as well as articles related to interlaboratory calibrations. DISCUSSION The results of this interlaboratory comparison show that whereas fission-track ages obtained by most workers are in good agreement with the independent ages, some results are grossly in error. The overall spread in results is from 0.55 to 3.1 times the independent age. Of all analyses returned, 26 of 93 lie outside two standard deviations of the independent age. Individual standards show proportions that are outside 2-sigma error limits, 2 out of 20 for 84-1; 5 out of 18 for 84-2; 5 out of 21 for 84-3; 9 out of 19 for 84-4; and 3 out of 15 for 84-5. Thus, roughly a third of the returned results are in disagreement with the known independent age, although results for 84-1 and 84-5 are somewhat better. There may be special problems in the analyses of standard 84-4 (apatite) which show more discrepancies than do the other standards with many ages being younger than the accepted age. It is noteworthy that assigned errors do not give appropriate assessment of the reliability of an age determination, presumably due to the presence of systematic errors which remain unrecognized. Thus, sixteen of the determinations are equal to, or greater than, three sigma from the accepted value. A more detailed study of the data already submitted will be made in an effort to reveal the cause(s) of this result. Clearly, several analysts are prone to errors in fission-track age determination of which they are unaware. Such errors could arise from poor sample preparation, insufficient etching, inadequate microscope observation methods, inexperienced counting, erroneous combination of constants, inappropriate neutron dosimetry techniques, or a variety of other sources. The results stress the need for extensive and continued use of age standards in fission-track dating, so that the analyst successfully calibrates his experimental procedures and counting ability. Before progressing to dating samples of unknown age, the analyst must display an acceptable degree of consistency on a number of age standards. This simple

389

scientific principle seems to have been overlooked in most fission-track studies. It is essential that in any publication of a fission-track dating study evidence should be presented of this consistency. If results on age standards are not included in the publication, they should be referenced. Referees are urged to recommend acceptance of a manuscript only if this condition is met. Only through such measures should fission-track dating results be widely accepted by the scientific community at large. Standards are available through the suppliers mentioned earlier in the section entitled "Source of Samples". Acknowledgements--PFG acknowledges support through the award of a Melbourne University Fellowship, AJH acknowledges support from the Swiss National Science Foundations for the promotion of ScientificResearch, IRD and DSM acknowledgesupport from Texaco, Inc. We wish to thank our colleagues who cooperated with this study by providing the analytical data for comparison. Our thanks to Dr. J. Peter Watt for producing the computer program to plot figures one through five. The provision of the new Corning glass standards, CN1 and CN2, by Dr. Jan Schreuers is greatly appreciated by the fission-track community.

REFERENCES

Dalrymple G. B., Alexander C. E. Jr., Lanphere M. A. and Kraker G. P. (1981) Irradiation of samples for 4°Ar/39Ar dating using Geological Survey Triga reactor. USGS Prof. Paper 1176, p. 55. Hurford A. J. and Green P. F. (1982) A users' guide to fission track dating calibration. Earth Planet. Sci. Lett. 59, 343-354. Hurford A. J. and Green P. F. (1983) The zeta age calibration of fission-track dating, lsotope Geosci. 1, 285-317. McDougall I. and Roksandic Z. (1974) Total Fusion 4°Ar/39Arages using HIFAR reactor. J. Geol. Soc. Am. 21, 81-89. McDougall I. and Watkins, in preparation. McDougall I. and Wellman P. (1976) Potassium-argon ages for some Australian Mesozoic rocks. J. Geol. Soc. Aust. 23, 1-9. Naeser C. W. and Cebula G. T. (1978) Fission-track dating of apatite and zircon: an interlaboratory comparison. U.S.G.S. Open-File Report 78-107. Naeser C. W., Zimmerman R. A. and Cebula G. T. (1981) Fission-track dating of apatite and zircon: an interlaboratory comparison. Nucl. Tracks 5, 65-72. Steven T. A., Mehnert H. H. and Obradovich J. D. (1967) Age of volcanic activity in the San Juan Mountains, Colorado. Geological Survey Prof. Paper 575-D, D47-D55. Williams I. S., Tetley N. W., Compston W. and McDougall I. (1982) A comparison of K-Ar and Rb--Sr ages of rapidly cooled igneous rocks: 2 points in the Paleozoic time scale re-evaluated. Geol. Soc. Lond. 139, 557-568.

390

DONALD

S. M I L L E R

et

al.

A P P E N D I X 1. S T A N D A R D R E P O R T FORM: W O R K S H O P L A B O R A T O R Y : ...........................................................

1984, T R O Y

S A M P L E 84-. ................................................................

Address: ..........................................................................

UNCORRECTED

Analyst: .........................................................................

E R R O R (1 S I G M A ) ..............................................

A G E ........................................ Myr Myr

(Please put X in appropriate boxes) (1)

[]

.............................. External Detector or

~]

or

~

.......................... Other (please specify*):

*[see

(2) Reactor: ..............................................

............... Population Method Nucl

Tracks

5, 3 14 (1981)]

Facility: ........................................................................................

(3) Cd ratio: ............................................. which monitor: .................................................................................... or nominal thermal and fast fluxes: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (4) Dosimetry determined by: (Answer I or 1|. X appropriate box.) I.

[]

Standard glass, I.D. n u m b e r .......................................................................................................................... (a)

were tracks counted in glass'? ............................................................................................

(b)

if not, detector used was

(c)

track density was ...............................................................................................................

(d)

n u m b e r of tracks counted was

(e)

neutron fluence from standard glass data was ...............................................................................................................................

(f) 1I.

[]

(yes or no)

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

tracks/cm-'

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

neutrons/cm:

H o w was neutron fluence calctdated from your standard data. Please describe in detail.

Method other than standard glass. Please describe in detail.

USE T H I S SPACE* T O A N S W E R E I T H E R I. (f) or ll., whichever applies.

(5) Decay constant used (-~3~2/). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

x 10 ~:yr '

(6) Why did you use this combination of neutron fluence and :~2 value? If any age standards were used in choosing the combination, please specify.

(7) H o w are the age error, quoted above, calculated?

(8) If you believe an age correction should be made based on track length or plateau methods, please state reason(s) and give corrected age:

INTERLABORATORY

COMPARISON

OF FT AGE STANDARDS

391

(9) DATA: On separate sheet, please supply a full list of following data: For population method: list individual ns and n~ values, together with total Ns, N~, p, and p~. For external detector method: list ns, n~, Ps, P~ for each crystal and total Ns, N~, Ps, P~. (where ns, n~ refer to individual crystal track counts, Ns and hr. refer to total numbers of tracks counted). *Please use this space if necessary to continue answers to questions 1 to 8.

APPENDIX II. REFERENCES CONTAINING INFORMATION ON STANDARDS Carpenter B. S. (1984) NBS Special Publ. 260-92, Standard Reference Materials, 1984, p. 12. Carpenter B. S. and Reimer G. M. (1974) NBS Special Publ. 260-49, Standard Reference Materials, p. 17. Hurford A. J. and Green P. F. (1982) Interlaboratory standardization of FTD system calibrations. Nucl. Tracks 6, 229. Hurford A. J. and Green P. F. (1983) The zeta age calibration of fission-track dating. Isotope Geosci. 1, 285-317. Hurford A. J. and Hammerschmidt K. (1984) 4°Ar/39Ar and K/Ar dating of the Bishop and Fish Canyon tufts: calibration ages for fission-track dating standard. Isotope Geoscience (submitted). NBS Special Publ. 260 (1984) Standard Reference Materials, p. 76, 77 and 80. Naeser C. W. and Cebula G. T. (1978) Fission-track dating of apatite and zircon: an interlaboratory comparison: Open-File Report 78-107, United States Department of the Interior Geological Survey, p. 12. Naeser C. W. and Fleischer R. L. (1975) Age of the apatite at Cerro de Mercado, Mexico: A problem for fission-track annealing corrections: Geophys. Res. Lett. 2, 67-70. Schreuers J. W. H., Friedman A. M., Rokop D. J., Hair M. W. and Walker R. M. (1971) Calibrated U-Th glasses for neutron dosimetry and determination of uranium and thorium concentration by the fission-track method. Radiat. Eft. 7, 231. Steven T. A., Mehnert H. H. and Obradovich J. D. (1967) Age of volcanic activity in the San Juan Mountains, Colorado: U.S. Geol. Survey Prof. Paper 575-D, p. D47-D55. Williams I. S., Tetley N. W., Compston W. and McDougall I. (1982) A comparison of K-Ar and Rb-Sr ages of rapidly cooled igneous rocks: 2 points in the Paleozoic time scale re-evaluated: Geol. Soc. Lond. 139, 557-568.