Radiogenic Os in primitive basalts from the northwestern U.S.A.: Implications for petrogenesis

Abstract

Chemically primitive late Cenozoic tholeiitic basalts from the northwestern U.S.A. have Os-isotopic compositions more radiogenic than observed for most basalts from the ocean basins. This result is inconsistent with the simple petrogenetic model that explains the geographically correlated Sr-, Nd- and Pb-isotopic variation in these basalts as resulting solety from melting of metasomatized lithospheric mantle peridotite of varying age across this area. A magma source composed of a mixture of peridotite and pyroxenite/eclogite also fails because at the high percentage of mafic component required to explain the observed Nd—Os isotope systematics, this mixed source would not produce melts that match the major- and trace-element compositions of the observed olivine tholeiites. A more likely explanation for the observed isotopic compositions involves interaction of sublithospheric primary melts that are similar in composition to mid-ocean ridge basalt (MORB) with high- 187Os/ 188Os materials in the lower crust or lithospheric mantle. Though not uniquely defined, mass-balance calculations suggest that the lithospheric component is mafic-potassic in composition, possibly a small-volume melt of mafic material in the lower crust or upper mantle. If the radiogenic Os is attributable to such interaction, the isotopic compositions of the tholeiites can be satisfied by 2–25% addition of the lithospheric component to a primitive MORB parental magma. These results show that the Os-isotope system in continental basalts can provide a clear distinction between magmas derived by melting isotopically evolved peridotitic lithosphere and those produced from a sublithospheric primary melt contaminated by mafic material in the lithosphere.