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 Os-187/Os-188 isotopic compositions. Sixteen grains spanning the range of Os-187/Os-188 ratios present in the larger suite were analyzed for high-precision Os-186/Os-188 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 Os-187/Os-188 ratios of individual grains were isotopically homogeneous at the +/-0.1% level of resolution. The Os-187/Os-188 ratios of the entire suite of grains ranged from 0.1173 to 0.1468. The average ratio for all grains combined was 0.?241, similar to the composition of three chromitites sampled from the Josephine Creek, and consistent with formation in the Mesozoic upper mantle. The average Os-186/Os-188 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 Os-187/Os-188 variations on a scale of mm(3) 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 Os-187/Os-188 require that some of the heterogeneities must be >1 Ga. The homogeneity of the Os-186/Os-188 ratios lend no support to the contention that isotopic heterogeneities within the upper mantle may explain coupled Os-186/Os-187 enrichments that have been observed in putative plume derived lavas. (C) 2004 Elsevier B.V. All rights reserved.