187Os–186Os systematics of Os–Ir–Ru alloy grains from southwestern Oregon

More than 100 Os–Ir–Ru alloy grains from southwestern Oregon, presumably originating in the Mesozoic Josephine ophiolite, were examined for major and minor element compositions and 187Os/188Os isotopic compositions. Sixteen grains spanning the range of 187Os/188Os ratios present in the larger suite were analyzed for high-precision 186Os/188Os ratios. The ranges of major and minor element abundances of individual grains were, in general, similar to compositions previously reported for such grains worldwide. Within-suite compositional variations were quite large. Most grains were likely disaggregated from larger intergrown clusters, and were subdivided based on texture into matrix and lamellae. Composition variations within individual matrix grains were limited, whereas variations within individual lamellae grains were considerably larger. The 187Os/188Os ratios of individual grains were isotopically homogeneous at the ±0.1% level of resolution. The 187Os/188Os ratios of the entire suite of grains ranged from 0.1173 to 0.1468. The average ratio for all grains combined was 0.1241, similar to the composition of three chromitites sampled from the Josephine Creek, and consistent with formation in the Mesozoic upper mantle. The average 186Os/188Os ratio of the subset of 16 grains was 0.119838. All grains but one had isotopic ratios within ±20 ppm of that ratio. Results indicate that 187Os/188Os variations on a scale of mm3 may be quite large within the upper mantle, mimicking heterogeneities observed in both hand-and outcrop-scale samples of upper mantle materials. Highly depleted compositions relative to ambient upper mantle with chondritic 187Os/188Os require that some of the heterogeneities must be >1 Ga. The homogeneity of the 186Os/188Os ratios lend no support to the contention that isotopic heterogeneities within the upper mantle may explain coupled 186Os–187Os enrichments that have been observed in putative plume derived lavas.