- © The Mineralogical Society Of America
Isotopic anomalies are expected in primitive meteorites since astronomical observation and astrophysical modeling of stars predict a great variety of stellar processes. Protostellar clouds should partially preserve the memory of this diversity in solid grains. Since 1970, high precision mass spectrometry and high resolution ion probes have led to the discovery of numerous isotopic anomalies, which were rapidly associated with nucleosynthetic processes. A general rule is that small isotopic effects (parts in 103–104) are observed in centimeter size samples, whereas order of magnitude variations are observed at the micron scale in circumstellar grains.
Refractory materials in primitive meteorites were investigated first as they have the best chance of escaping homogenization in the early solar system. Inclusions in C3 carbonaceous chondrites exhibit widespread anomalies for oxygen and the iron group elements. Only a few members, dubbed “FUN” (for “Fractionated and Unknown Nuclear” effects), also display anomalous compositions for the heavy elements. Anomalies in inclusions have generally been connected with explosive or supernova nucleosynthesis.
Several types of presolar circumstellar grains have been separated from the matrix of chondrites: diamonds, silicon carbide, graphite, oxides. The isotopic ratios of the light elements (C-N-O) vary over several orders of magnitude in these grains. Only a few measurement have been performed for heavier elements with generally s-process signatures. AGB stars at different stages of their evolution are thought to be the sources of most circumstellar grains. Nevertheless grains with supernova signatures have also been found. For Cr and Mo in bulk primitive carbonaceous chondrites (C1, C2), large isotopic differences exist between the different major mineral phases of the bulk rock.
A number of now extinct radioactive isotopes have existed in the early solar system. This is shown by the variations that they induce in the abundances in their daughter nuclides. Their main use …