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

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.