Osmium isotopic compositions of Os-rich platinum group element alloys from the Klamath and Siskiyou Mountains

[1] We present new measurements of Os-186/Os-188 and Os-187/Os-188 in 10 Os-rich platinum group element (PGE) alloys from placer deposits formed by the mechanical erosion of peridotite-bearing ophiolites in the Klamath and Siskiyou Mountains in northern California and southwestern Oregon. These data nearly double our database of high-precision Os-186/Os-188 measurements on such samples. Together with previously published data, our new results reinforce the conclusion that the radiogenic Os-186/Os-188 compositions of these PGE alloys are very difficult to reconcile with a derivation of their Os from the outer core. Such a model requires extremely early growth of the inner core to its present size, within several hundred million years after accretion of the Earth, which is geophysically implausible. Collectively, our data suggest instead that partial melting or metasomatic processes in the upper mantle play a primary role in controlling the Os isotopic systematics of these Os-rich PGE alloys and suggest the existence of upper mantle components characterized by radiogenic Os-186/Os-188 ratios. Pyroxene-rich lithologies are possible candidates.