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

We report exceptionally high 186Os/188Os 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 186Os/188Os and 187Os/188Os (γOs(t=1.4Ga) = +6 to +10) enrichment relative to the chondritic mantle at ∼1.4 Ga. Our interpretation is that these samples crystallized at ∼1.4 Ga in a long-term Pt/Os and Re/Os enriched region, similar to that expected for Earthʹs 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.