The three-stage petrochemical evolution of the Steens Basalt (southeast Oregon, USA) compared to large igneous provinces and layered mafic intrusions

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. Chemostratigraphic excursions and volcanological characteristics in the Steens Basalt record a three-stage history. A minimally exposed early stage preserved in the lower A Steens Basalt section is characterized by heterogeneity (3-8 wt% MgO) collapsing to homogeneity (∼5 wt% MgO), suggesting crystal fractionation outpaces recharge. Sparse weathering horizons indicate some time elapses between eruptions. The second stage, lower B Steens Basalt, is volumetrically dominant and represents waxing of the basaltic pulse. Flows are stacked immediately upon one another without evidence of weathering or intervening sedimentary horizons, indicating high-eruptive frequency. Compositions oscillate over a ΔMgO of ∼4-5 wt% between low- and high-MgO basalt, both of which become more magnesian up section, signaling a period dominated by recharge. This stage closes with declining oscillations to produce homogeneous compositions (6-8 wt% MgO). The waning stage of eruption is represented by the upper Steens Basalt section, where thin intercalated weathering horizons occur especially high in the section. The upper Steens Basalt is characterized by overall declining MgO and increasing incompatible element concentrations confirming the dominance of crystal fractionation accompanied by crustal assimilation. In detail, the upper Steens Basalt initiates with a small stack of heterogeneous flows (5-8 wt% MgO), followed by a period of relatively homogeneous flows (∼6 wt