187Re-187Os, 190Pt-186Os, 34S/32S Isotopic Systematics of the Subcontinental Mantle of the Western United States

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9804909Walker The timing of melt extraction in the subcontinental lithospheric mantle (SCLM) underlying regions of Proterozoic and Phanerozoic crust in the western United States is largely unknown. Distinctions between metasomatism of the SCLM via fluids and melts derived from subducting slabs, and the processes resulting in formation of OIB-type mantle domains, remain unclear. As a first step to alleviate some of the ambiguity, recent investigations have identified several compositional parameters that are unique to subduction-related metasomatism. These include high oxygen fugacities relative to oceanic and cratonic mantle, suprachondritic 187Os/188Os, and heavy 34S/32S. Some OIB-type mantle domains may have suprachondritic 186Os/188Os ratios. Therefore, these data combined with other geochemical parameters may discern between subduction zone metasomatism and OIB-type compositional enrichments within the SCLM. To address the timing of melting and the processes that modified the SCLM associated with Proterozoic and Phanerozoic crust, we will obtain 187Re-187Os, 190Pt-186Os and 34S/32S isotopic data on mantle xenoliths from the western United States. We will use negative thermal ionization mass spectrometry to obtain Os isotopic data. Bulk rock S isotopic data will be obtained using gas source mass spectrometry, and in situ S isotopes will be obtained by secondary ionization mass spectrometry.