METAL-ASSOCIATED CARBON IN PRIMITIVE CHONDRITES: STRUCTURE, ISOTOPIC COMPOSITION, AND ORIGIN

Abstract

Small (<1–20 μm) carbon inclusions are common in Fe-Ni metal grains of the LL3.1 chondrite Bishunpur, and are also commonly found in the metal of type 3 ordinary chondrites with total carbon content ≥0.2 wt.%. They are rare or absent in carbonaceous chondrite metal. Raman spectroscopy shows that these inclusions consist of very poorly graphitized carbon, similar to carbon-rich aggregates. The isotopic composition of C and N in individual inclusions were measured with an ion microprobe. In Bishunpur δ13C varies significantly from inclusion to inclusion, even within single metal grains, and ranges from −64 to −25‰, whereas most δ15N values are consistent with 0, although some variation cannot be excluded. Two similar inclusions in Khohar (L3.6) yield δ13C values within the range found in Bishunpur, but a unique inclusion in Kainsaz (CO3) has much heavier carbon with δ13C = 19‰. In Khohar, carbon veins at the periphery of metal grains have relatively uniform δ13C around −31‰, but δ15N ranges from 460 to 1135‰. We interpret Bishunpur-type inclusions as the remains of carbonaceous matter that escaped oxidation during chondrule formation and was not heated to sufficiently high temperatures and for sufficiently long times to graphitize. Their carbon isotopic heterogeneity most probably reflects that of the precursor material. However, the carbon isotopic range of the inclusions in Bishunpur is different from both that of the bulk chondrite and that of the organic matter in chondrites. It is suggested that this isotopic composition reflects that of the carbonaceous matter at the time the chondrules formed, before further evolution led to the carbon isotopic composition measured in bulk chondrites. In spite of their high 15N enrichment, carbon veins in Khohar are probably not interstellar matter. They possibly formed during a shock event by decomposition of organic matter and trapped 15N-enriched nitrogen, maybe of interstellar origin, after it had been mobilized by the shock together with other volatiles.