Osmium and lead isotopes of rare OsIrRu minerals: derivation from the core-mantle boundary region?

We report exceptionally high Os-186/Os-188 for rare crystals of OsIrRu foun d in unconsolidated sediment near Port Orford, SW Oregon (USA), that are he ld to be derived from peridotite: in the nearby Klamath Mountains. These Os IrRu samples differ in composition from OsIr alloys thought to have formed in the upper mantle. They have resided in equilibrium with an evolved and h ighly supra-chondritic Pt/Os reservoir. We explore these data in the contex t of the core-mantle mixing model relevant to some deep-seated plume system s, and find the investigated alloys to exhibit an apparent coupled Os-186/O s-188 and Os-187/Os-188 (gamma Os(t = 1.4Ga) = +6 to +10) enrichment relati ve to the chondritic mantle at similar to 1.4 Ga, Our interpretation is tha t these samples crystallized at similar to 1.4 Ga in a long-term Pt/Os nod Re/Os enriched region, similar to that expected for Earth's outer core or t he core-mantle transition zone (D " layer). Subsequently, these OsIrRu samp les were transported from the core-mantle boundary region to the lithospher e as xenoliths in a mantle plume/convection system. Lead isotopes positivel y exclude a crustal origin of these OsIrRu crystals and support our estimat ed time of crystallization. (C) 1999 Elsevier Science B.V. All rights reser ved.