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

抄録

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, δ⁵⁴Cr values of Cr-rich grains were determined with a precision of ~30‰. 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‰.