Other
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.508g&link_type=abstract
Meteoritics, vol. 30, no. 5, page 508
Other
5
Anomalies, Isotopic, Chondrites, Eh, Dust, Interstellar, Meteorites, Indarch, Nucleosynthesis
Scientific paper
We report results for the EH(4) Indarch. Earlier work [1] found 20 micrometers clumps of sub-micron SiC whose presolar nature was inferred from step-wise combustion, noble gas [2], and ion probe isotopic measurements. Our results indicate that the clumps were an artifact of sample preparation. Our sample was first cleaned using 6N HCl, and water and isopropanol rinses, then powdered and reacted with HCl-HF/HCl, KOH, and H3BO3-HCl/HCl giving a C-rich residue 1.14 wt.% of the original. X-ray mapping showed SiC grains and 5x as many Si3N4 grains, but no fine-grained clumps. Large (6 micrometers to 20 micrometers) C-rich spheroids were also present. The sample was further treated with KOH/HNO3 and NH3H2O; attempts to do density-separates were unsuccessful. An aliquot was treated with perchloric acid and separated into <1 micrometers and >1 micrometer fractions. SEM-EDS measurements of 73 (<1 micrometer) grains showed 44 SiC, 19 Si3N4, 4 C only, and 6 C with minor Si (both the C and Si in these particles are isotopically normal). A similar distribution of species was found for 37 (>1 micrometer) grains with the addition of 2 spinel and one Al2O3 grains. The whole rock concentration of SiC was 5.8 ppm, higher than previous determinations [1,3,9]. Confirming earlier suggestions [1,2], we find that SiC in Indarch is much finer-grained than in Murchison; about 2/3 of the mass is in grains <=0.3 micrometers compared to only about 4% for Murchison. This may represent size-sorting in the nebula or selective destruction of fine-grained material. Ion probe measurements of 22 (1-3 micrometers) grains gave isotopic results in the range previously measured for Murchison SiCs [4]. Several normal Si3N4 grains (>1 micron) were measured; probably exsolution products similar to those in Qingzhen [7]. Ion mapping was used to search for presolar oxide grains using previously developed techniques [5]. Seven candidate grains out of ~1000 were found. Multiple imaging confirmed an ^(16)O/^(18)O anomaly in one spinel grain - the first presolar oxide found in an E chondrite. Although the proportion of oxide grains relative to SiC is smaller, the fraction of anomalous oxide grains is not strikingly different in Indarch than in Murchison (1/1000) or Tieschitz (1/300). Prior ion probe measurements of clumped sub-micron grains [1] gave isotopic compositions closer to normal than the average of single >1 micrometer SiC grains, suggesting possible size-dependent effects. To investigate this we used 28Si/30Si maps to measure the fraction of X-type SiC in the <1 micron fraction to compare with the 1% fraction measured in larger grains [6]. The mapping also locates presolar Si3N4 grains since their Si isotopes are similar to those of X-SiC [8]. Although there is overlap from closely adjacent grains, our preliminary analysis of a fine-grained mount indicates similar fractions of X-type SiC. No presolar nitrides were found. The prior isotopic results in grain clumps [1] can be attributed to admixtures of isotopically normal submicron Si3N4 and C-rich grains. In summary, contrary to previous suggestions [1,9] the properties of measured presolar components in Indarch, with the exception of grain size, are similar to those measured in other meteorites. References: [1] Russell S. S. et al. (1993) Meteoritics, 28, 425. [2] Ott U. et al. (1991) Meteoritics, 26, 382. [3] Huss G. R. and Lewis R. S. (1995) GCA, 59, 115. [4] Hoppe P. et al. (1994) Astrophys. J., 430, 870. [5] Nittler L. R. et al. (1994) Nature, 370, 443. [6] Nittler et al. (1995) LPS XXVI, 1037. [7] Alexander C. M. O'D. et al. (1994) Meteoritics, 29, 79. [8] Hoppe P. et al. (1994) LPS XXV, 563. [9] Stone J. et al. (1991) Earth and Planet. Sci., 107, 570.
Gao XingHui
Nittler Larry R.
Swan Patrick D.
Walker Rober M.
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