Mathematics – Logic
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.283r&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 283
Mathematics
Logic
11
Scientific paper
Presolar grains have now been studied in a considerable number of primitive meteorites so that it can be readily shown that the diamond/silicon carbide ratio is not constant (Fig. 1). To highlight some of the distinctions: the enstatite chondrite Indarch appears to be particularly enriched in SiC compared to its diamond content, whereas the CV3s are relatively SiC poor. The abundance of SiC content in CV3s, however, seems to depend strongly on the oxidation state; the highly oxidized Allende has much less SiC than the more reduced Vigarano. The differences seen in Fig. 1 imply either heterogeneity in the solar nebula, i.e., preferential inclusion of one of the components into different meteorite parent bodies or different destruction mechanisms for the two components. Alexander et al. (1990) and Huss (1990) noted that abundance of both diamond and silicon carbide in primitive chondritic meteorites declines with increasing petrologic type, perhaps indicating that these components are destroyed during metamorphism. In addition to the above observations, diamond and silicon carbide from different meteorite classes can be distinguished. The nitrogen content of the diamond varies considerably in a way that might be petrologic type dependent (Russell et al., 1991a). The combustion temperature of SiC in different samples is widely variable and the delta^13C measured for SiC from the CV3 meteorites is isotopically much lighter. The similarity in average delta^13C of SiC in the lowest petrologic type carbonaceous chondrites, Andrar 003 and Indarch (when it is known from ion probe studies that individual SiC grains are extraordinarily variable in ^12C/^13C), suggests that the interstellar mineral was well mixed in the parent body forming regions of the solar nebula (Russell et al., 1991b). Clearly understanding all these apparently unrelated facts is vital to unraveling the history of primitive parent bodies and their formation. Metamorphism must be involved after accretion and the role of participating reactions may be considered qualitatively if not quantitatively. Alexander et al. (1990) suggested that, in ordinary chondrites, interstellar grains could be destroyed by oxidation with magnetite. This could also be the dominant oxidation reaction in the carbonaceous chondrites. In the enstatite chondrites, which are free of magnetite, interstellar grains must be destroyed by reaction with another phase. A likely candidate is olivine although this may be less efficient than magnetite. Nevertheless, olivine has advantages over magnetite for oxidizing diamond relative to SiC, explaining the comparatively high abundance of the latter in the enstatite chondrite group. Whatever metamorphic processes are occurring to determine the fate of diamond and silicon carbide, secondary effects such as the preferential destruction of defective crystals must also be considered (Fisenko et al., 1992). References: Alexander et al. (1990), EPSL 99, 220-229. Fisenko A. V., Russell S. S., Semjenova L. F., Verchovsky A. B., and Pillinger C. T. (1992) LPSC XXIII, 365-366. Huss G. R. (1990) Nature 347, 159-162. Russell S. S., Arden J. W., and Pillinger C. T. (1991a) Science 2654, 1118-1191. Russell S. S., Ash R. D., Pillinger C. T., and Arden J. W. (1991b) Meteoritics 26, 390.
Arden John W.
Pillinger Colin T.
Russell Stuart
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