In situ discovery of graphite with interstellar isotopic signatures in a chondrule-free clast in an L3 chondrite

Optical and scanning electron microscopy of a chondrule-free clast in the unequilibrated L3 chondrite Khohar revealed a spherical object consisting of an aggregate of small (similar to 2-micrometer diameter), Ni-poor (0.5 to 2.89 weight percent) metal particles and fine-grained graphite (< 1-micrometer diameter). The graphite has large D and N-15 excesses (delta D similar to 1500 per mil and delta(15)N similar to 1300 per mil) with two isotopically distinct signatures: N rich with a high D/H ratio and N poor with a high N-15/N-14 ratio. These excesses are the largest D and N-15 excesses observed in situ in a well-characterized phase in a meteorite. The isotopic characteristics are suggestive of an interstellar origin, probably by ion-molecule reactions at low temperature in the interstellar molecular cloud from which the solar system formed. The structure and nonchondritic composition of the metal particles suggest they did not form under equilibrium conditions in the solar nebula.