ZR ISOTOPE ANOMALIES IN CHONDRITES AND THE PRESENCE OF 92NB IN THE EARLY SOLAR SYSTEM

Abstract

The presence of Zr isotope anomalies in the early solar system is demonstrated with the identification of 92Zr excesses and 96Zr deficits in several chondrites and the CAI Allende inclusions. The isotopic composition of Zr in carbonaceous, enstatite, and ordinary chondrites, along with four SNC meteorites, was analyzed by plasma source mass spectrometry. Most chondrite samples show negative 96Zr anomalies, which indicate the presence of a pre-solar nucleosynthetic component. Six of them also display a distinct negative 92Zr anomaly, reaching down to -2.7+/-0.8ε units for Forest Vale (H4). The CAI inclusions from Allende, which are among the oldest known igneous objects of the solar system and have the highest Zr/Nb ratios, also show negative ε92Zr of -2.4+/-0.5. Although a substantial fraction of the Zr isotope variability may be due to pre-solar nucleosynthetic processes, part of the 92Zr excess must result from the decay of the now extinct 92Nb.