Highly Siderophile Elements and <SUP>187</SUP>Re-<SUP>187</SUP>Os Isotopic Systematics of the Allende Meteorite: Evidence for Primary Nebular Processes and Late-stage Alteration

Abstract

The abundances of highly siderophile elements (HSE) Re, Os, Ir, Ru, Pt, and Pd, as well as 187Re-187Os isotopic systematics were determined for calcium-aluminum-rich inclusions (CAIs), chondrules, and matrix, separated from the CV3 carbonaceous chondrite Allende. Consistent with prior studies, CAIs are characterized by significant depletions in Pd relative to the other HSE, while the other HSE are in generally bulk chondritic relative abundances. The depletions in Pd can be linked with initial formation of CAIs via condensation, or subsequent processing by evaporative processes. Chondrules generally have relative HSE patterns similar to CAIs, although they have lower absolute abundances. Palladium depletions in chondrules may reflect solid metal-liquid metal fractionation at the time of formation, or alternatively, be the result of processes that acted on precursor materials. Matrix samples have nearly chondritic absolute abundances of all HSE measured. Consequently, matrix is the only major chondritic component examined here that shows no relative depletion in Pd. Mass balance suggests the existence of an unidentified Pd-rich carrier, although it is possible that the dataset presented here is too limited to represent typical HSE abundances of some chondritic components (e.g., chondrules). The 187Re-187Os isotopic systematics of only six out of twenty-four Allende chondritic components analyzed plot within uncertainties of a 4568 Ma primordial reference isochron. The deviations from the expected isochron most likely reflect late-stage, open-system behavior within the last 2 billion years, and, in some cases, could even have resulted from terrestrial alteration. The open-system behavior is most readily observed in small, millimeter-size sub-samples of Allende, consistent with Re and/or Os mobility on that scale.