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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 …