Abstract
The Steens Basalt, southeast Oregon, USA, initiated at 17 Ma as the earliest pulse of the Columbia River Flood Basalt of the northwestern USA. New and existing stratigraphically controlled data reveal temporal changes in lava flow character, and whole-rock and mineral compositions, which we use to evaluate how the balance of magma differentiation processes change in time. Temporal petrochemical variations in the Steens Basalt are analogous to the transition from Imnaha Basalt to Grande Ronde Basalt units of the Columbia River Flood Basalt and have parallels to the temporal evolution of the Deccan and Siberian traps, in India and Russia, respectively, as well as to the stratigraphic sequences of the Bushveld, of South Africa, and Stillwater, in southern Montana, USA, layered mafic intrusions. The excellent stratigraphic control from the Steens Basalt provides a detailed record for comparison across this variety of large mafic systems, providing ability to focus on commonalities among differentiation processes in time.
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