In-situ chromium isotope measurement of chromium-rich fine grains in the Murchison CM2 chondrite

Abstract

Isotopic anomalies of 54Cr have been reported in bulk chondrites. Stepwise dissolution experiments have suggested the presence of a carrier of the 54Cr anomaly, probably of presolar origin. Although stepwise dissolution experiments and ion microprobe studies of Cr-rich grains have revealed, to some extent, the carrier of anomalous 54Cr, the nature of the carrier is not yet well understood. In this study, we attempted to detect the carrier phases. We performed an in-situ measurement of Cr isotopic compositions of small (∼1 μm or less in size) Cr-bearing grains in the Murchison CM2 chondrite using a NanoSIMS 50 ion microprobe. 54Cr has an isobaric interference of 54Fe, which makes the measurement of 54Cr/ 52Cr isotopic ratios difficult. In order to evaluate the contribution of 54Fe, the abundance of 56Fe was measured. Even after large corrections for 54Fe, δ 54Cr values of Cr-rich grains were determined with a precision of ∼30%c. No grain out of ∼200 Cr-rich grains was confirmed to be a presolar grain with a large 54Cr anomaly. If the carrier of the 54Cr anomaly is Cr-rich, this result implies a low abundance and a huge isotopic anomaly of the carrier, and/or its extremely fine grain size. In the former case, the maximum abundance of the presolar Cr-rich grains is roughly estimated to be ∼1 ppm. In that case the minimum isotopic anomaly would be ∼90%. Copyright © 2011 by The Geochemical Society of Japan.