RADIOGENIC OS IN PRIMITIVE BASALTS FROM THE NORTHWESTERN USA - IMPLICATIONS FOR PETROGENESIS

Chemically primitive late Cenozoic tholeiitic basalts from the northwe stern U.S.A. have Os-isotopic compositions more radiogenic than observ ed for most basalts from the ocean basins. This result is inconsistent with the simple petrogenetic model that explains the geographically c orrelated Sr-, Nd-and Pb-isotopic variation in these basalts as result ing solely from melting of metasomatized lithospheric mantle peridotit e of varying age across this area. A magma source composed of a mixtur e 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 ma tch the major-and trace-element compositions of the observed olivine t holeiites. A more likely explanation for the observed isotopic composi tions involves interaction of sublithospheric primary melts that are s imilar in composition to mid-ocean ridge basalt (MORE) with high-Os-18 7/Os-188 materials in the lower crust or lithospheric mantle, Though n ot uniquely defined, mass-balance calculations suggest that the lithos pheric component is mafic-potassic in composition, possibly a small-vo lume melt of mafic material in the lower crust or upper mantle. If the radiogenic Os is attributable to such interaction, the isotopic compo sitions of the tholeiites can be satisfied by 2-25% addition of the li thospheric component to a primitive MORE parental magma. These results show that the Os-isotope system in continental basalts can provide a clear distinction between magmas derived by melting isotopically evolv ed peridotitic lithosphere and those produced from a sublithospheric p rimary melt contaminated by mafic material in the lithosphere. (C) 199 7 Elsevier Science B.V.