The pyroxene pallasites, Vermillion and Yamato 8451: Not quite a couple

Two pallasites, Vermillion and Yamato (Y)-8451, have been studied to obtain petrologic, trace element, and O-isotopic data. Both meteorites contain lo w-Ca and high-Ca pyroxenes (<2% by volume) and have been dubbed "pyroxene p allasites. Pyroxene occurs as large individual grains, as inclusions in oli vine and in other pyroxene, and as grains along the edges of olivine. Sympl ectic overgrowths, sometimes found in Main Group and Eagle Station pallasit es, are not seen in the pyroxene pallasites. Olivine compositions are Fa(10 -12), similar to those of Main Group pallasites. Siderophile trace element data show that metal in the two meteorites have significantly differing com positions that are, for many elements, outside the range of the Main Group and Eagle Station pallasites. These compositions also differ from those of IAB and IIIAB iron meteorites. Rare earth element (REE) patterns in merrill ite are similar to those seen in other pallasites, indicating formation by subsolidus reaction between metal and silicate, with the merrillite inherit ing its pattern from the surrounding silicates. The O-isotopic compositions of Vernillion and Y-8451 are similar but differ from Main Group or Eagle S tation pallasites, as well as other achondrite and primitive achondrite gro ups. Although Vermillion and Y-8451 have similar mineralogy, pyroxene compo sitions, REE patterns, and O-isotopic compositions, there is sufficient evi dence to resist formally grouping these two meteorites. This evidence inclu des the texture of Vermillion, siderophile trace element data, and the pres ence of cohenite in Vermillion.