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% MgO) and closes with highly variable basalts to trachybasaltic andesites. These compositional characteristics coupled with a change in average flow thickness from lower to upper Steens Basalt of <5 m to 5–10 m illustrate a shift to more silicic compositions and higher viscosity up section.
The chemical changes up section in other large igneous provinces record similar variations in differentiation processes through time, suggesting that these large volume systems share similar evolutionary histories: the earliest records suggest the magmatic systems are initially more ephemeral and compositionally variable as magma traverses relatively cool crust. With waxing, a transition to regimes of high thermal and mass input results in a stage where recharge outpaces crystal fractionation. Thermal priming of the crust during these events coupled with waning input yields magmas in which fractionation plus crustal assimilation dominates over recharge late in the system; pulses of later stage felsic magmatism in many large mafic provinces are consistent with this evolution. Using layered mafic intrusions as an analog for intrusive, cumulus-dominated residua of voluminous fractionation, as well as oceanic large igneous provinces as an analog for total magma volumes in continental flood basalt regimes, leads to the suggestion that 50%–85% of the total magma volume in a flood basalt remains in the crust, effectively remaking the crust in these regions.