Zr isotope anomalies in chondrites and the presence of Nb-92 in the early solar system

The presence of Zr isotope anomalies in the early solar system is demonstra ted with the identification of Zr-92 excesses and Zr-96. deficits in severa l chondrites and the CAI Allende inclusions. The isotopic composition of Zr in carbonaceous, enstatite, and ordinary chondrites, along with four SNC m eteorites, was analyzed by plasma source mass spectrometry. Most chondrite samples show negative Zr-96 anomalies, which indicate the presence of a pre -solar nucleosynthetic component. Six of them also display a distinct negat ive Zr-92 anomaly, reaching down to -2.7 +/- 0.8 epsilon units for Forest V ale (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, als o show negative epsilon Zr-92 Of -2.4 +/- 0.5. Although a substantial fract ion of the Zr isotope variability may be due to pre-solar nucleosynthetic p rocesses, part of the Zr-92 excess must result from the decay of the now ex tinct Nb-92. (C) 2000 Elsevier Science B.V. All rights reserved.