Variations of Stable Isotope Ratios in Nature

Information about nonterrestrial isotopic abundances can be obtained from studies of meteoritic, lunar, or planetary dust materials from both space probes and from ground-based astronomical observations. From these different sources of information it has been established that many elements have cosmic isotopic compositions which are very different from those observed in terrestrial material. One example of such large isotope variations is observed for primitive meteorites which contain minute interstellar dust grains of diamond, silicon carbide, and graphite that survived the formation of the solar system. Diamonds are too small to be analyzed as single grains, but silicon carbide and graphite have diameters in excess of 1 µm and can be analyzed isotopically by ion microprobe mass-spectrometry (Amari et al. 1993; Ott 1993). Silicon carbide and graphite grains exhibit variations in 12C/13C ratios that range up to more than three orders of magnitude greater than terrestrial C-isotope variations. Such large variations can be only explained in terms of several types of different stellar sources.