Local and exotic components of primitive meteorites, and their origin

Most of the material of chondrites was heavily reprocessed in the early Solar System, and hence retains only a dim memory of its interstellar origin even in the least altered meteorites. Opaque matrix, the most primitive material, seems to have taken up Fe2+ and changed its mineralogy and texture. Chondrules and Ca, Al-rich inclusions have been further altered by melting, oxidation or reduction, loss of volatiles, etc. None the less, small amounts of exotic components have survived, as indicated by isotopic anomalies. A dust component enriched in 16O is the most abundant and widespread. It survives in Ca, Al-rich inclusions as anomalous spinel grains, but is recognizable even in highly evolved meteorites and planets from variations in bulk oxygen isotopic composition. A few inclusions show small nucleosynthetic anomalies for many elements (Si, Ca, Ti, Cr, Sr, Ba, Nd, Sm), always coupled with mass fractionation of several of these elements, as well as O and Mg. Seven extinct radionuclides (26Al, 41Ca, 53Mn, 107Pd, 129I, 146Sm, and 244Pu) have been recognized from their decay products, and provide clues to the chronology and nucleosynthetic sources of the early Solar System. (author)