Other
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28..465y&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 465
Other
1
Cm Chondrites, Siderophile Elements, Sulfides, Trace Elements
Scientific paper
Five sulfide-coated, rounded lumps (240-440 micrometers in largest dimension) were recovered from the dense fraction after freeze-thaw disaggregation of the Murchison CM2 chondrite. Each was split into a fraction studied by SEM and EPMA and a fraction analyzed by INAA. Petrographic study shows that S1 and S5 are composed predominantly of FeS enclosing ~10% olivine. In S1, olivine is Fa38 and occurs isolated or in clusters of 20-50-micrometer, rounded grains, some with glass inclusions. S6 consists mostly of 50-100-micrometer forsterite crystals, lesser enstatite, interstitial anorthite, and minor, 10-micrometer blebs of low-Ni metal. The entire object is surrounded by a 5-20-micrometer- thick rind of FeS and permeated by a series of subparallel veins of FeS. Both S3 and S4 contain silicates enclosed in massive troilite, with lesser pentlandite in the case of S4. In S4, the silicates include euhedral, zoned olivine (Fa18-40) crystals up to 30 micrometers in size; polycrystalline, masses of finer-grained (2-5 micrometers) olivine (Fa45) and Ca-pyroxene. S3 has subhedral forsterite crystals up to 20 micrometers across and irregular, fine-grained clasts up to 100 micrometers in size that resemble the Murchison matrix. Sulfide proportions in the samples studied by INAA were estimated by comparing their Mg contents with EPMA data for silicates in their splits: 86%, 63%, 51%, 83%, and 28% in S1, S3, S4, S5, and S6 respectively. Refractory siderophiles (W, Re, Os, Ir, Mo, and Ru) are unfractionated in S6, each having a concentration within 20% of C1 chondrites. In S1, S4, and S5, Ru and Ir abundances are 1.8-3.1 x C1 and 0.024-0.25 x C1, respectively, leading to C1- normalized Ru/Ir ratios of 115 +- 7, 7.0 +- 2.6, and 117 +- 13, respectively, as also seen in Allende pentlandite [1]. S3, which is depleted in both Ru and Ir, also has a superchondritic Ru/Ir ratio, 1.8 +- 0.4. Ru and Mo are strongly positively correlated in all samples. Au, As, Ga, Zn, Ni, and Co are close to or below C1 levels in S1, S3, S5, and S6, except that the C1-normalized enrichment factor for Ga in S3 is 3.8 +- 0.2 and that for Co in S1 is 2.1. S4 is quite distinct, however, with As at 5.4 +- 0.2 x C1, Ni at 5.5 +- 0.2 x C1, and Co at 4.7 +- 0.1 x C1. Except for S6, Se is above C1 levels in all samples, with enrichment factors of 4.3-5.3. As suggested by groups IIAB and IIIAB irons, fractional crystallization of metal from a low-S liquid containing chondritic proportions of siderophiles leads to residual liquids with Ru/Ir and Mo/Ir ratios that are greater than chondritic [2]. Formation of sulfides with these characteristics in S1, S3, and S5 from metal made by this process seems unlikely, however, as it would also lead to C1 chondrite-normalized Au/Ir ratios that are greater than those of Ru/Ir, while the Au/Ir ratios in S1, S3, and S5, 0.5-1.2 x C1, are far less than Ru/Ir ratios. Mo and Ru are the most chalcophile of the refractory siderophiles studied here, as seen from experimentally determined partition coefficients between solid metal and solid FeS for Mo (1.4 [3]), Ru (0.1-13 [4]) and Ir (12-240 [4]), and from the fact that Ru/Ir and Mo/Ir ratios in troilite nodules in Odessa and Canyon Diablo are factors of 2.4-4.0 and 42-55, respectively, higher than in coexisting metal [5]. Thus, if an alloy enriched in refractory siderophiles were equilibrated with FeS and then segregated from the FeS, the resulting sulfide would have many of the chemical features of S1, S3, and S5. Fremdlinge in CAIs are interpreted as high-temperature condensate alloys (e.g., [6,7]). In many of these objects, Ru and Mo are depleted relative to other siderophiles compared to C1 chondrites, though not in concert, due to volatility and phase separation effects accompanying condensation. Furthermore, some entire CAIs in Leoville and Efremovka are strongly depleted in both Mo and Ru relative to Ir compared to C1 chondrites [8], implying that the processes that made Fremdlinge resulted in significant amounts of Ru and Mo being left in the gas phase after formation of CAIs in some nebular regions. Thus, another possibility is that the sulfides seen here formed from metal that condensed from such a fractionated gas before Au condensed. References: [1] Davis A. M. et al. (1978) Meteoritics, 13, 438-439. [2] Scott E. R. D. (1972) GCA, 36, 1205-1236. [3] Jones J. H. et al. (1993) GCA, 57, 453-460. [4] Fleet M. E. et al. (1991) JGR, 96, 21949-21958. [5] Hermann F. et al. (1971) Mikrochim. Acta, 1971, 225-240. [6] Bischoff A. and Palme H. (1987) GCA, 51, 2733-2748. [7] Sylvester P. J. et al. (1990) GCA, 54, 3491-3508. [8] Sylvester P. J. et al. (1993) GCA, in press.
Grossman Lawrence
Hsu Allen
Simon Steven B.
Sylvester Paul J.
Yoneda Shigekazu
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