Massive chromite in the Brenham pallasite and the fractionation of Cr during the crystallization of asteroidal cores

Large (greater than or equal to 2 mm) chromite grains are present in IIIAB iron meteorites and in the main-group pallasites (PMG). closely related to high-An IIIAB irons. Pallasites seem to have formed by the intrusion of a h ighly evolved metallic magma from a IIIAB-like core into fragmented olivine of the overlying dunite mantle. High Cr contents are commonly encountered during the analyses of metallic samples of high-Au IIIAB irons and main-gro up pallasites, an indication that Cr contents were high in the intruding li quid and that Cr behaved as an incompatible element during the crystallizat ion of the IIIAB magma, contrary to expectations based on the negative IIIA B Cr-Ni and Cr-Au trends among low-Au IIIAB irons. In a region about 10 cm across in the Brenham main-group pallasite massive chromite fills the inter stices between olivine grains, the site normally occupied by metal in Brenh am and other pallasites. The massive chromite may have formed as a late cum ulus phase; because Fe-Ni was also crystallizing, its absence in the chromi te-rich region suggests a separation associated with differences in liquid buoyancy. The coexisting chromite and olivine are zoned; in the olivine FeO is highest in pallasitic (olivine-metal) regions, lowest in rims adjacent to chromite, and intermediate in the cores of these olivines. Chromite show s the opposite zoning, with the highest FeO contents at grain edges adjacen t to olivine. The observed gradients are those expected to form by Fe-Mg ex change between olivine and chromite during slow cooling at subsolidus tempe ratures. Compared to normal Brenham, contents of phosphoran olivine and pho sphates are higher in the chromitic pallasitic region. We also report data for large-to-massive chromites present in PMG Molong and in high-Au IIIAB B ear Creek that, like Brenham, formed from a highly evolved magma. The Bear Creek chromite has a much lower Mg content than that in the pallasites, imp lying that, in the PMG, the Mg was extracted from the olivine during high-t emperature reaction with the precipitating chromite. There are other circum stantial arguments indicating that Cr was incompatible in the metal during the crystallization of the IIIAB magma, with the concentration in the resid ual magma rising from an initial value of about 300 mu g/g to a value aroun d 700 mu g/g when Bear Creek and Brenham were formed. We consider possible explanations for these negative Cr-Au and Cr-Ni trends and find the most pr obable one to be that they reflect sampling artefacts resulting from analys ts avoiding visible chromite (and the commonly associated phase FeS) when c hoosing metal samples. Copyright (C) 1999 Elsevier Science Ltd.