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 found in unconsolidated sediment near Port Orford, SW Oregon (USA), that are held to be derived from peridotite: in the nearby Klamath Mountains. These OsIrRu samples differ in composition from OsIr alloys thought to have formed in the upper mantle. They have resided in equilibrium with an evolved and highly supra-chondritic Pt/Os reservoir. We explore these data in the context of the core-mantle mixing model relevant to some deep-seated plume systems, and find the investigated alloys to exhibit an apparent coupled Os-186/Os-188 and Os-187/Os-188 (gamma Os(t = 1.4Ga) = +6 to +10) enrichment relative to the chondritic mantle at similar to 1.4 Ga, Our interpretation is that 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 the core-mantle transition zone (D " layer). Subsequently, these OsIrRu samples were transported from the core-mantle boundary region to the lithosphere as xenoliths in a mantle plume/convection system. Lead isotopes positively exclude a crustal origin of these OsIrRu crystals and support our estimated time of crystallization. (C) 1999 Elsevier Science B.V. All rights reserved.