Interstellar Grains in Primitive Meteorites: Diamond, Silicon Carbide, and Graphite

Abstract— Primitive meteorites contain a few parts per million (ppm) of pristine interstellar grains that provide information on nuclear and chemical processes in stars. Their interstellar origin is proven by highly anomalous isotopic ratios, varying more than 1000‐fold for elements such as C and N. Most grains isolated thus far are stable only under highly reducing conditions (C/O > 1), and apparently are “stardust” formed in stellar atmospheres. Microdiamonds, of median size ∼ 10 Å, are most abundant (∼ 400–1800 ppm) but least understood. They contain anomalous noble gases including Xe‐HL, which shows the signature of the r ‐ and p ‐processes and thus apparently is derived from supernovae. Silicon carbide, of grain size 0.2–10 μm and abundance ∼ 6 ppm, shows the signature of the s ‐process and apparently comes mainly from red giant carbon (AGB) stars of 1–3 solar masses. Some grains appear to be ≥10 9 a older than the Solar System. Graphite spherules, of grain size 0.8–7 μm and abundance <2 ppm, contain highly anomalous C and noble gases, as well as large amounts of fossil 26 Mg from the decay of extinct 26 Al. They seem to come from at least three sources, probably AGB stars, novae, and Wolf‐Rayet stars.