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 16 O , the other of a gas, depleted in 16 O 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 26 Mg is variable in abundance among some classes of Allende inclusions, implying either nebular heterogeneity with respect to 26 A1/ 27 Al ratios, or time differences of crystal formation of 1 or 2 x 10 6 a. The presence of excess 107 Ag from decay of extinct 107 Pd supports the evidence from 26 Mg 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 50 Ti 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 22 Ne, and appears to be the decay product of extinct 22 Na. If so, condensation of some stellar ejecta must take place on a time scale of a year or so. The problem of reconciling the 26 A1 time scale of about 10 6 years between nucleosynthesis and Solar System condensation with the 10 8 year scale implied by the decay of 129 I to 129 Xe and fission of 244 Pu requires that at most a small fraction of the 129 I and 244 Pu 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 14 N-rich nitrogen in identical carriers supports the notion that nucleosynthetic anomalies in nitrogen are also present in Allende.