In situ measurement of interstellar silicon carbide in two CM chondrite meteorites

Abstract

SMALL crystals of silicon carbide, probably of interstellar origin, have previously been isolated from primitive meteorites by dissolution of the host meteorite with acid^1–8. Here we report the first in situ observations of isotopically anomalous SiC^9,10. The grains were found by X-ray mapping of polished sections of two chondritic (group CM) meteorites, Cold Bokkeveld and Murchison. Ion microprobe measurements showed ¹³C enrichments, δ¹³C, from 199 to 2,800‰, proving that the grains are indigenous. Calculations of the condensation of grains in circumstellar envelopes of different compositions and C/O ratios have suggested that other phases, such as metal carbides, nitrides and sulphides, could form in association with SiC^11,12. Etching methods may have destroyed any such phases, had they existed in association with SiC, but our direct detection reveals SiC grains only as isolated matrix particles. This rules out also the possibility that SiC grains were brought into the Solar System as inclusions in larger grains, which protected them from destruction in the solar nebula. Several of the SiC grains we have found are cracked, suggesting that the etching treatment may result in size distributions biased towards smaller grains.