Mantle source lithologies for the Columbia River flood basalt province

The Columbia River flood basalts in the northwestern United States are recognized as part of the greater Yellowstone hotspot province, the products of a mantle plume operating at least since the Miocene. Prior geochemical and isotopic studies have identified contributions to flood lavas from plume mantle, depleted asthenospheric mantle, subcontinental lithospheric mantle, subduction-modified mantle, and young and old continental crust. Here, we review major and trace element compositions of lavas and present new analyses of whole rocks and olivines from the early, least evolved Columbia River basalts of the Imnaha, Steens and Picture Gorge formations to identify mantle source lithologies. These lavas exhibit trace element enrichments and depletions, correlated with highly variable Ni contents of olivine phenocrysts that vary systematically by formation and lava chemical type. The variations are attributed to a range in basalt source lithologies from pyroxenite to peridotite. Among Imnaha Basalt flows, these differences correspond to a long-recognized contrast between American Bar and Rock Creek chemical types. The Rock Creek mantle source was richer in pyroxene than that of the American Bar lavas. Olivines in Imnaha lavas show distinct flow-by-flow trends in Ni–Mg-number space, recording distinct mantle source compositions and melting events supplying individual flows during the ramp-up stage of the province. Only later, during production of the voluminous Grande Ronde lavas, were mantle melts processed through a large integrated crustal magma system. The influence of pyroxenite mantle sources decreased with time throughout the flood basalt sequence. The pyroxenite signature is not accompanied by radiogenic isotope indicators of long-term enrichment, hence is constrained to be young. However, its origin is uncertain. Possible contributors to enrichment include entrained, recently subducted basalt derived from the Farallon slab, and pyroxenite produced within and adjacent to the rising plume by melting and freezing during basalt genesis.