Isotopic variations in primitive meteorites

Abstract Oxygen isotopic variations in carbonaceous chondrites and in ordinary chondrites can each be interpreted as mixtures of two isotopically different reservoirs, one consisting of solids, enriched in 16O , the other of a gas, depleted in 16O relative to terrestrial abundances. The data suggest a common source of the solids for each of the two classes of meteorites, but a different gas reservoir for each. These conditions might prevail in gaseous protoplanets. Radiogenic 26Mg is variable in abundance among some classes of Allende inclusions, implying either nebular heterogeneity with respect to 26A1/27Al ratios, or time differences of crystal formation of 1 or 2 x 106 a. The presence of excess 107Ag from decay of extinct 107Pd supports the evidence from 26Mg for a time interval of at most a few million years between the last nucleosynthetic event and accretion of substantial bodies in the Solar System. The widespread small excess of 50Ti in Allende inclusions is tantalizing, but unexplained. An exceptional hibonite-rich inclusion from Allende contains strongly fractionated isotopes of oxygen and calcium, but isotopically normal magnesium. Its trace elements imply association with a hot, oxidized gas. Among the volatile elements, neon-E has been shown to be essentially pure 22Ne, and appears to be the decay product of extinct 22Na. If so, condensation of some stellar ejecta must take place on a time scale of a year or so. The problem of reconciling the 26A1 time scale of about 106 years between nucleosynthesis and Solar System condensation with the 108 year scale implied by the decay of 129I to 129Xe and fission of 244Pu requires that at most a small fraction of the 129I and 244Pu be formed in the most recent event. Progress has been made in establishing the carrier phases of isotopically anomalous xenon and krypton. The apparent location of anomalous xenon and 14N-rich nitrogen in identical carriers supports the notion that nucleosynthetic anomalies in nitrogen are also present in Allende.