Other
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.237i&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 237
Other
3
Scientific paper
Graphite spherules from the Murchison meteorite have been identified as interstellar because they carry Ne-E(L) and Kr-S, extreme ^12C/^13C variations, excess ^26Mg, and large anomalies in ^18O/^16O (Lewis and Amari, 1992; Amari et al., 1990; Hoppe et al., 1992a,b; Nichols et al., 1992). Surprisingly, only a few graphite spherules were found to contain anomalous Si although the uncertainties were large because of low Si concentrations. Here we report Si- isotopic compositions measured to higher precision with the SHRIMP ion microprobe and results of a feasibility study of determining Ti isotopic compositions. For the Si-isotopic measurements the instrumental mass fractionation was corrected by concurrent measurements of terrestrial kaersutite. All graphite grains analyzed are normal within analytical uncertainty (1 sigma errors range from less than 10 per mil to ~30 per mil). However, the grains with anomalous Si analyzed by Hoppe et al. (1992) were not analyzed in this work in order to preserve them for other measurements. Titanium has been found in graphite spherules as TiC at a level of ~10^-5 (Bernatowicz et al., 1991). Ti-isotopic measurements at this level have been obtained on SHRIMP for some SiC particles (Ireland et al., 1991). However, the graphite separate had been treated with chromic acid, which resulted in extreme interference of ^50Cr on ^50Ti. We analyzed only one particle (KFB1-301), a grain with the highest Al/C ratio and therefore an expected high Ti concentration. Even in this grain, the ^50Cr correction is 460 per mil, making a definitive measurement of the ^50Ti abundance difficult. The Ti isotope pattern is shown in the figure after correction for a 15 per mil/amu instrumental mass fractionation. The grain has solar ^46Ti and ^47Ti relative to ^48Ti , but both ^49Ti and ^50Ti are depleted. Normal isotopic ratios for Cr were used for the interference correction since no mass fractionation of Cr was detected in a variety of terrestrial standards. Since instrumental mass fractionation tends to favor the light isotopes, the ^50Cr correction is a lower limit, which means that the ^50Ti would be even more negative if fractionated Cr abundances were used. Furthermore, the ^50Ti depletion is accompanied by a ^49Ti depletion, as is commonly observed in meteoritic hibonites (Ireland, 1990). Still, some caution in overinterpreting this effect is probably required. A way to alleviate the problem is to produce graphite separates without exposure to chromic acid and then Ti compositions should be more readily measurable. References: Amari S., Anders E., Virag A., and Zinner E. (1990) Nature 345, 238-240. Bernatowicz T. J., Amari S., Zinner E. K., and Lewis R. S. (1991) Astrophys. J. 373, L73- L76. Hoppe P., Amari S., Zinner E., and Lewis R. S. (1992a) Lunar Planet. Sci. (abstract) 23, 553-554. Hoppe P., Amari S., Zinner E., and Lewis R. S. (1992b) Meteoritics (abstract), this volume. Ireland T. R. (1990) Geochim. Cosmochim. Acta 54, 3219-3237. Ireland T. R., Zinner E. K., and Amari S. (1991) Astrophys. J. 376, L53-L56. Lewis R. S. and Amari S. (1992) Lunar Planet. Sci. (abstract) 23, 775- 776. Nichols R. H. Jr., Hohenberg C. M., Hoppe P., Amari S., and Lewis R. S. (1992) Lunar Planet. Sci. (abstract) 23, 989-990.
Amari Sachiko
Hoppe Peter
Ireland Trevor R.
Lewis Reed S.
Zinner Emst
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