OSMIUM ISOTOPIC AND PLATINUM GROUP ELEMENT GEOCHEMISTRY OF PERIDOTITE XENOLITHS FROM THE SIERRA NEVADA ARC AND THE BASIN AND RANGE

Platinum group elemental concentrations were determined by isotope dilution ICP-MS on ten peridotite xenoliths from the Big Creek diatreme and Oak Creek flow in the Sierra Nevada batholith and the Cima volcanic field in the southern Basin and Range. These data were combined with published Re and Os isotopic analyses. Big Creek peridotites have low total PGE (8-44 ppb), high S (32-134 ppm), and near-chondritic 187Os/188Os (excluding one very radiogenic sample); Oak Creek peridotites have low total PGE (25-53 ppb), low S (26-67 ppm), and near-chondritic 187Os/188Os; and Cima peridotites have high total PGE (16-103 ppb), low sulfur (24-85 ppm), and chondritic to extremely unradiogenic 187Os/188Os (0.113-0.131). Nine samples have near-chondritic relative abundances of Ru, Ir, and Pt, but slight to significant depletions in Pd, Os and Re. Re/Os and Pd/Ir are roughly negatively correlated with degree of melting while Os/Ir is positively correlated with S content. These observations suggest that Ru, Ir, and possibly Pt are compatible to slightly incompatible during melting, whereas Pd and Re are incompatible. Os depletions may be related to sulfide decomposition during or after eruption, as suggested by Handler and Bennett (1999). The anomalous sample, 1026V, is characterized by radiogenic but unsupported Os (187Os/188Os=0.1507; 187Re/188Os=0.40), super-chondritic Pd/Ir (16) and Os/Ir (1.9), the presence of chalcopyrite (in addition to monosulfide solid solution and millerite), and the highest S (230 ppm) and Os (~9 ppb) contents of the entire suite of samples studied. This suggests that Os, Pd, and Pt can be transported together in secondary sulfides, and that disturbance of Os isotopes by sulfide metasomatism can be assessed by examining Pd/Ir ratios. If so, the chondritic to subchondritic Pd/Ir ratios in all other samples suggests little to no sulfide metasomatism. The origin of the radiogenic Os added to sample 1026V, however, is unknown. One possibility might be sulfides precipitated from a partial melt of the abundant olivine-bearing garnet-websterite xenoliths, which are characterized by high Re/Os and 187Os/188Os. However, this source may not be satisfactory because of the low Os contents in these rocks (60 ppt).