THOROUGHLY ANOMALOUS CHROMIUM IN ORGUEIL

Stepwise dissolution of bulk Orgueil reveals that all of the Cr in the whole rock is isotopically anomalous, with an anomaly pattern that is thus far unique. Most of the Cr (along with other major and minor cat ions) is dissolved by acetic and nitric acids; it is deficient in Cr-5 4 by similar to 5 epsilon. Subsequent treatment with hydrochloric acid dissolves a small fraction of the Cr with positive Cr-54 anomalies, u p to similar to 210 epsilon. Mass balance indicates that whole rock Cr is isotopically normal within analytical uncertainties. The least ext ravagant interpretation of these results is that some mineral phase is enriched in a heavy-Cr nucleosynthetic component, while most of the C r is a homogenized mixture of diverse nucleosynthetic components that would be normal except for lack of the postulated heavy Cr carrier. Th e carrier is likely, but not necessarily, presolar interstellar grains . Its identity is unknown and constrained only circumstantially: it mu st be relatively rich in Cr, it is substantially soluble in hydrochlor ic acid, and it is not magnetite or spinel/chromite. Scanning electron microscope (SEM) examination of Orgueil reveals candidate Cr-rich oxi des, silicates, sulfides and phosphides, but none of these can be iden tified yet as the heavy Cr carrier. Whether presolar or not, the carri er is not chemically resistant and likely not thermally refractory, th ereby differing from most other phases known to host isotopic anomalie s. Its survival (or production) thus establishes constraints on a diff erent regime of nebular history.