Contributors to chromium isotope variation of meteorites

Abstract

We report the results of a comprehensive, high precision survey of the Cr isotopic compositions of primitive chondrites, along with some differentiated meteorites. To ensure complete dissolution of our samples, they were first fused with lithium borate–tetraborate at 1050–1000 °C. Relative to the NIST Cr standard SRM 3112a, carbonaceous chondrites exhibit excesses in 54Cr/ 52Cr from 0.4 to 1.6 ε (1 ε = 1 part in 10,000), and ordinary chondrites display a common 54Cr/ 52Cr deficit of ∼0.4 ε. Analyses of acid-digestion residues of chondrites show that carbonaceous and ordinary chondrites share a common 54Cr-enriched carrier, which is characterized by a large excess in 54Cr/ 52Cr (up to 200 ε) associated with a very small deficit in 53Cr/ 52Cr (<2 ε). We did not find 54Cr anomalies in either bulk enstatite chondrites or in leachates of their acid-digestion residues. This either requires that the enstatite chondrite parent bodies did not incorporate the 54Cr anomaly carrier phase during their accretion, or the phase was destroyed by parent body metamorphism. Chromium in the terrestrial rocks and lunar samples analyzed here show no deviation from the NIST SRM 3112a Cr standard. The eucrite and Martian meteorites studied exhibit small deficits in 54Cr/ 52Cr. The 54Cr/ 52Cr variations among different meteorite classes suggest that there was a spatial and/or temporal heterogeneity in the distribution of a 54Cr-rich component in the inner Solar System. We confirm the correlated excesses in 54Cr/ 52Cr and 53Cr/ 52Cr for bulk carbonaceous chondrites, but the new data yield a steeper slope (∼6.6) than that reported in Shukolyukov and Lugmair (2006). The correlated excesses may affect the use of the Mn–Cr chronometer in carbonaceous chondrites. We could not confirm the bulk carbonaceous chondrite Mn–Cr isochron reported by Shukolyukov and Lugmair (2006) and Moynier et al. (2007), mostly because we find much smaller total variations in ε 53Cr (∼0.2). All bulk chondrites have small ε 53Cr excesses (up to 0.3) relative to the Earth, most likely reflecting the sub-chondritic Mn/Cr ratio of the Earth. The ε 53Cr variations in chondrites do seem to grossly correlate with Mn/Cr and yield an initial Solar System 53Mn/ 55Mn value of 5.4(±2.4) × 10 −6, corresponding to an absolute age of 4566.4 (±2.2) Ma. Nuclear interactions with cosmic rays result in coupled excesses in ε 54Cr and ε 53Cr with a ∼4:1 ratio in phases with high Fe/Cr. These are most dramatically demonstrated in the iron meteorite Carbo, showing excesses in ε 54Cr of up to 140 ε. These new results show that the Mn–Cr chronometer should be used with caution in samples/minerals with high Fe/Cr and long cosmic ray exposure ages.