Changing Mantle Sources and the Effects of Crustal Passage on the Steens Basalt, SE Oregon: Chemical and Isotopic Constraints

Continental flood basalts are more prone to compositional modification from passage through thicker and (or) more felsic crust in comparison to their oceanic counterparts. The Steens Basalt in southeast Oregon (~17 Ma) is among the oldest and most mafic members of the Columbia River Basalt Group and provides a record of the early stages of flood basalt volcanism. We evaluate the balance of mantle sources in time during the onset of Columbia River Basalt Group magmatism and assess the effect of crustal passage using stratigraphically controlled Sr, Nd, Pb, Hf, Os, and O isotopic compositions, as well as whole rock major and trace element data. Mixing models indicate that depleted and enriched mantle sources identified by previous workers contribute in varying proportions during the life of the magmatic system, with the greatest contribution by depleted mantle when eruption rate and presumed intrusion rate increase. During waxing, enrichment of δ18O in some flows signals cryptic deep fractionation of abundant clinopyroxene followed by shallow fractionation of olivine ± clinopyroxene ± plagioclase. Os concentrations are among the highest worldwide at a given MgO (0.29–0.86 ppb at 6.0 to 10.9 wt.%). We argue that high Os results from scavenging of sulfides by recharging magmas passing through earlier crystallized magmas. Elevated 87Sr/86Sr in the latest stage supports modest assimilation of partial melts from mafic accreted terranes, facilitated by thermal priming of crust by persistent magmatism. This work provides a more detailed schematic view of the Steens Basalt magmatic system, from mantle origin through crustal staging. Plain Language Summary Continental Flood Basalts are voluminous outpourings of basaltic lava from dozens to hundreds of fissures over a widespread area. Basalt is the type of magma that is formed from mantle melting and records chemical properties of the mantle from which it melted. Magmas that erupt at the surface that have undergone minimal changes during their traverse from mantle to crust retain many of the chemical properties of the mantle from which they were generated, in particular radiogenic isotope ratios, as these do not change during cooling and crystallization of magma. We present new radiogenic and stable isotope compositions for the Steens Basalt, among the earliest members of the Columbia River Flood Basalt Group, western United States, that gives insight into the mantle and crustal sources, and assess the magmatic proces