Variations in the sulfur isotope composition of troilite from the Cañon Diablo iron meteorite

Variations in the sulfur isotope composition of troilite from the Cañon Diablo iron meteorite

Geochimica et Cosmochimica Acta,Vol.58, No. 19, pp. 4253-4255, 1994 Copyright 0 1994 Elsevier ScienceLtd Printed in theUSA. All rightsreserved 0016-...

376KB Sizes 0 Downloads 38 Views

Geochimica et Cosmochimica

Acta,Vol.58, No. 19, pp. 4253-4255, 1994

Copyright 0 1994 Elsevier ScienceLtd Printed in theUSA. All rightsreserved 0016-7037/94$6.00 + .OO

0016-7037(94)00187-l

LETTER

Variations in the sulfur isotope composition of troilite from the Cation Diablo iron meteorite GEORGES BEAUDOIN,“*

B. E. TAYLOR,’ D. RUMBLE III,’

and M. THIEMENS~

‘Geological Survey of Canada, 60 1 Booth Street, Ottawa, Ontario KI A OE8, Canada *Geophysical Laboratory, 525 1 Broad Branch Road NW, Washington, DC 200 15- 1305, USA 3Department of Chemistry, University of California San Diego, La Jolla, CA 92093-0356, USA (Received February 4, 1994; accepted in revisedform April 13, 1994)

Abstract-Cafion Diablo troilite (CDT), although accepted as the reference for the sulfur isotope scale, is not a Standard Reference Material manufactured for international distribution, and its sulfur isotope composition is not well characterized. We report high-precision sulfur isotope analyses of troilite from three different samples of Caiion Diablo meteorite, and interlaboratory comparison of one CDT sample and of a reference SF6 gas. The sulfur isotope composition of CDT displays a range in 634S values of 0.46, significantly larger than our analytical uncertainty (0.05%). CDT should not be used for calibration of the sulfur isotope scale. These results support the definition of a new sulfur isotope scale relative to a hypothetical Vienna-CDT, as recently proposed by the International Atomic Energy Agency. INTRODUCHON

results of high-precision sulfur isotope analyses of three different samples of CDT and an interlaboratory comparison of one CDT sample and a reference SF6 gas. The results indicate significant variations in the sulfur isotope composition of CDT of at least 0.4% which cannot be explained by different analytical methods and must result from sulfur isotope heterogeneity in CDT.

Meteoritic sulfide sulfur was determined early (MACNAand THODE, 1950) to have a limited range of isotope ratios, making it attractive as a reference for the sulfur isotope scale, particularly because its sulfur isotope ratios are approximately in the middle of the range of ratios for terrestrial sulfur. Caiion Diablo troilite (CDT) was adopted as the sulfur isotope scale reference and assigned a 32S/ 34Sratio of 22.220 during a National Science Foundation symposium held at Yale University in April 1962 ( AULT and JENSEN, 1962; JENSEN and NAKAI, 1962). It is convenient to report stable isotope ratios in per mil ( %o) variation relative to CDT: MARA

634sCDT

(34S/32Ssaample - 34w32scor 1* (34s/32scDr)

1ooo

=

.

INTERLABORATORY COMPARISON Aliquots of dry SFb, referred to as SFl, were prepared by passing Research Grade SF, (Matheson Gas Co.) from one bottle of gas twice over an ethanol-dry ice trap and sealed in Pyrex tubes. Interlaboratory comparison of SF1 was carried at the Geological Survey of Canada (GSC), the Geophysical Laboratory, Carnegie Institution (GL), and at the University of California San Diego (UCSD). Analyses of randomly selected aliquots of SF1 yield average 634SCorvalues of -6.54% at the GL and -6.95% at UCSD (Table 1) . The difference in 6 34ScDTvalues for SF 1 ( 0.4 1k ) is one order of magnitude larger than the standard deviation of the measurements, and is statistically significant (Table 1). A small standard deviation of 634S values (~0.005%, n = 8 ) is also obtained at the GSC for SF 1. We conclude that the aliquots of SF1 analysed are isotopically identical. The difference in mean 634Sc-T value from the GL and the UCSD laboratories must therefore be a result of different calibrations relative to CDT. This could imply that isotopically different aliquots of CDT were used to calibrate the sulfur isotope scales used in each laboratory.

(1)

The d 34&,T value of Caiion Diablo troilite is defined as 0%. Although accepted as the reference and primary standard for the sulfur isotope scale, Cafion Diablo troilite is not a Standard Reference Material (SRM). A SRM is commonly designed and manufactured for a specific purpose, is well characterized, and is available internationally. Approximately 3 X 10’ g of Caiion Diablo iron meteorite fragments have been found in the vicinity of Baninger Meteor Crater in Arizona, USA ( MCEWING et al., 1983). The troilite forms blebs in massive octahedrite and must be separated from fragments obtained either from museum collections or commercial rock and mineral dealers. Samples of troilite from Cafion Diablo meteorite are available, but a SRM has not been manufactured from CDT for international distribution. In addition, the isotope composition of sulfur in troilite from Cafion Diablo meteorite is not well characterized. In this note, we report

ANALYSES OF CARON

DIABLO

TROILITE

Three different samples of powdered Caxion Diablo troilite (Appendix) were analysed using the laser-SF, technique at the GSC and at the GL (TAYLOR and BEAUDOIN, 1993; BEAUD~IN and TAYLOR, 1993, 1994; RUMBLEet al., 1993).

* Present address..Dkpartement de gkologie et de gtkie gt%logique, Universitt Laval, Qukbec G I K 7P4, Canada.

4253

G. Beaudoin et al.

4254 Table1. Summery of

SyS,

(%o) values for

Laboratoy

sys, x

f

of the mean 634SsF1value for each of the three samples of CDT analysed at the GSC (Table 2 ) .

SF1

(%a) n

1s

DISCUSSION - 6.54 f 0.03

GL’

6.95

UCSD’

f

12

0.02

12

1. Geophysical Labomtoty. 2. University of California San Diego

The samples analysed at the GSC weighed from 1.06 to 2.12 mg. The theoretical stoichiometric yield ranged from 103 to 110% for samples CDT1 and CDT2, but low yields, ranging from 58 to 80%, were obtained from sample CDT3. Samples of CDT3 weighting 3 to 4 mg were analysed at the GL and stoichiometric yields are low as well (38 to 45%). The low stoichiometric yields for sample CDT3 result from impurities of goethite and metallic Fe in that sample. The Fz pressure ranged from 39.3 to 165 mbar for all analyses and no variation of 634S values with F2 pressure was detected. The average 634S value for each CDT sample relative to the common reference SF6 (SF1 ) ranges from 6.54 to 6.9 1%O(Table 2). Student t tests suggest that the mean 634S value for CDT 1 is statistically different from the mean 6 34Svalues of CDT2 and CDT3. The precision of the analyses and the sulfur isotope homogeneity of the individual CDT samples is attested by standard deviations ( 1a) ranging from 0.04 to 0.170/w(Table 2). This analytical uncertainty includes errors from mass spectrometry and sulfur isotope variations in SFl, the reference SF6 (<0.03%0, Table 1). The larger standard deviation of 6 34Svalues for CDT3 (0.17%0 ) is probably a result of fluorination of goethite in that sample. Oxygen in goethite may combine with sulfur to form SOF2 and S02F2. Based on reduced partition function ratios (01 et al., 1985), partial sulfur isotope exchange between these sulfur species and SF6 could produce the larger standard deviation of sulfur isotope ratios from CDT3. Replicate, high-precision analyses of powdered troilite from Cailon Diablo meteorite commonly yield 6 34ScnT values with a low standard deviation (for example, 0.04%0; GAO and THIEMENS, 199 1). This indicates that either each sample of CDT is isotopically homogeneous or that powdering etficiently homogenizes the troilite sample, and that each CDT sample has different sulfur isotope composition from the others. Because no single sample of CDT can be selected as a reference, a 634Scur value of -6.75% is assigned to SF1 at the GSC. This G”S-ur value is based on the arithmetic average

Tabte 2. Summary

of S”s,,

(%) values for samples of Canon Diablo troilite

GL’

GSC’

Sample x

f

1s



CDT1

6.91

f

0.05

6

CDT2

6.73

f

0.04

4

CDT3

6.63

f

0.17

12

x

f

1s

6.54

f

0.11



-

1. Geological Survey of Canada. 2.Geophysical Laboratory.

3

The accepted absolute 34S/ 32Sratio for Canon Diablo troilite has been assigned an error equivalent to 0.2% by HOEI% ( 1987). This error is based on the analytical uncertainty quoted by JENSEN and NAKAI ( 1962) for isotopic analyses of sulfur in troilite from iron meteorites, including one sample of Caiion Diablo troilite. Sulfur isotope ratios were measured on SO2 evolved from combustion of FeS or AgZSwith oxygen. Silver sulfide was prepared from troilite by HCl leaching or using the Parr bomb (JENSENand NAKAI, 1962). MACNAMARA and THODE ( 1950) reported four analyses of troilite from Cafion Diablo which varied within analytical error (0.56). It is not certain from their Table 1 if the analyses are from different samples, or from aliquots of one sample. KAPLAN and HULSTON ( 1966) reported the only analyses clearly noted to represent more than one sample of Caiion Diablo troilite: the two samples analysed varied in 6 34Svalues by 0.1 %O(i.e., within analytical uncertainty). The foregoing represents the bulk of the published data that defines the sulfur isotope composition of Caiion Diablo troilite. It is obvious that the sulfur isotope composition of Caiion Diablo troilite has not been well characterized. Although the sulfur isotope homogeneity of Canon Diablo troilite has not been investigated thoroughly, MCEWING et al. ( 1983) showed that metallic spheroids in soils surrounding the Barringer crater had 6 34ScDr values ranging from 0.14 to 0.74%0 for small diameter spheroids and from 0.04 to 0.53%0 for larger diameter. These variable 634ScoT values were interpreted to result from post-impact, low temperature weathering (MCEWING et al., 1983). It is possible that part of the variation in 634Scur values for the spherules results from a different initial sulfur isotope composition or from variable sulfur isotope fractionations during impact. The range in 634S values (0.37%0) for different samples of Canon Diablo troilite is almost one order of magnitude larger than typical analytical uncertainty (0.05%0), indicating measurable differences in sulfur isotope ratios of the primary reference for the sulfur isotope scale. CDT3 yields similar 6 34S values at the GSC and GL laboratories (Table 2): the values are at the low end of the range determined both by the analyses of CDT samples (Table 2) and by the interlaboratory comparison of the isotopically homogeneous SF, reference gas (Table 1). It is thus justifiable to use the mean 634S value for CDT3 to define the range of sulfur isotope ratios in Canon Diablo troilite. Interlaboratory comparison of an isotopically homogeneous SF6 reference gas (SF1 ) also yields a significant range in 6 34S values (0.4 1%O), which could result from different sulfur isotope compositions of the CDT samples used to calibrate the sulfur isotope scale in each of the laboratories. Large ranges of averaged 634Scor values ( 1.1 to 1.9%0) are also reported for sulfur isotope intercomparison samples by several laboratories in the world (HUT, 1987). A significant part of these variations could result from sulfur isotope vatiations in the samples of Caiion Diablo troilite used to calibrate the sulfur isotope scale in each of these laboratories. It is noteworthy, however, that recent high-precision sulfur isotope

Isotope composition of S in Cafion Diablo troilite

analyses of intercomparison materials from the IAEA and China has yielded identical 6 34ScoT values, within analytical uncertainty, to those accepted for these materials (GAO and THIEMENS, 1993). CONCLUSION The sulfur isotope composition of Cafion Diablo troilite is variable as shown by high-precision analyses of three different samples and by an interlaboratory comparison. The present range in S34S values is 0.4%0, but more samples of different troilite inclusions from the Cafion Diablo meteorite should be analysed. Because Caiion Diablo troilite is not a SRM it should not be used for calibration of the sulfk isotope scale. A new SRM (IAEA-S-l ) and other intercomparison materials are proposed by the IAEA to define a new sulfur isotope scale relative to a hypothetical V-CDT (Vienna Cation Diablo Troilite; R. Gonfiantini and W. kichler, pers. commun., January 1994). This proposal should remove ambiguities about the origin of the present sulfur isotope scale. Acknowledgments-Reviews by E.M. Cameron and two Geochimica and Cosmochimica Acta referees are greatly appreciated. GB acknowledges a Visiting Fellowship in Canadian Government Laboratories at the Geological Survey of Canada. This is Geological Survey of Canada Contribution 11094.

Editorial handling: J.D. Macdougall

REFERENCES AUNT W. U. and JENSENM. L. ( 1962) Summary of sulfur isotopic standards. In Biogeochemistry of Sulfur Isotopes (ed. M. L. JENSEN), NSF Symposium, Yale University, pp. 16-29. BEAUDOING. and TAYLORB. E. ( 1993) MILES laser microprobe. Part 2: Preliminary assessment of precision and accuracy of sulphur isotope analysis. In Current Research, Part D; Geological Survey of Canada, Paper 93-10, 199-204. BEAUDOING. and TAYLORB. E. ( 1994) High precision and spatial

4255

resolution sulfur isotope analysis using MILES laser microprobe. Geochim. Cosmochim. Acta 58 (in press). GAO X. and THIEMENSM. H. ( 1991) Systematic study of sulfur isotopic composition in iron meteorites and the occurrence of excess “S and ‘%. Geochim. Cosmochim. Acta 55,267 l-2619. GAO X. and THIEMENSM. H. ( 1993) Isotopic composition and concentration of sulfur in carbonaceous chondrites. Geochim. Cosmochim. Acta 57,3 159-3 169. HOEFSJ. ( 1987) Stable Isotope Geochemistry, 3rd ed. Springer-Verlag. HUT G. ( 1987) Consultants group meeting on stable isotope reference samples for geochemical and hydrological investigations. IAEA Special Report 200. JENSENM. L. and NAKAI N. ( 1962) Sulfur isotope meteorite standards results and recommendations. In Biogeochemistry of Suljiir Isotopes (ed. M. L. JENSEN), NSF Symposium, Yale University, pp. 30-35. KAPLAN 1. R. and HULSTONJ. R. ( 1966) The isotopic abundance and content of sulfur in meteorites. Geochim. Cosmochim. Acta 30,479-496. MACNAMARAJ. and THODEH. G. ( 1950) Comparison of the isotopic constitution of terrestrial and meteoritic sulfur. Phvs. Rev. 78.307308. MCEWING C. E., REES C. E., and THODE H. G. ( 1983) Sulphur isotope ratios in the Canyon Diablo metallic spheroids. Meteorifics 18, 171-178. 01 T., TAGUMAN., and KAKIHANAH. ( 1985) Calculations of thermodynamic sulfur isotope effect. J. Nucl. Sci. Tech. 22,8 18-832. RUMBLE D., HOERINGT. C., and PALIN J. M. (1993) Preparation of SF, for sulfur isotope analysis by laser heating sulfide minerals in the presence of F2 gas. Geochim. Cosmochim. Acta 57,44994512. TAYLOR B. E. and BEAUDOING. (1993) MILES laser microprobe. Part I: System description. In Current Research, Part D; Geological Survey of Canada, Paper 1993-10, 191-198. APPENDIX: DESCRIPTION OF SAMPLES OF TROILITE FROM CARON DIABLO METEORITE CDT 1 was prepared about 7 years ago by BET from sample #030310 of the Geological Survey of Canada collection and a description of the initial material is not available. CDT2 was powdered by GB from a fragment of troilite from sample #44Oh of the Harvard University collection. CDT3 was prepared from sample #USNM 3270 of the Smithsonian Institution. CDT3 contains metallic Fe impurities and minor weathering of troilite grains by goethite.