Volatile concentrations in olivine-hosted melt inclusions from the Columbia River flood basalts and associated lavas of the Oregon Plateau: Implications for magma genesis

Abstract

The volatile budgets and elemental compositions of primary magmas and their sources can be estimated through study of olivine-hosted melt inclusions in basaltic lavas, thereby providing insights about melting processes in the mantle. We report data on melt inclusions in 58 host-olivine crystals from seven basalt samples from the Columbia River Basalt (CRB) group in eastern Oregon and Washington, which have been analyzed for their major-oxide, trace-element and volatile (H2O, S, F, and Cl) concentrations to constrain the origin of this large igneous province. A wide range in H2O concentrations has been discovered, with the highest amount of 4.2wt.% found in olivine grains from a lava flow in the Malheur Gorge area, which erupted several million years after the hypothesized initial impingement of the Yellowstone Hotspot against the lithosphere of the North American Plate. This H2O concentration and others in our Columbia River–Oregon Plateau sample suite are significantly higher than those observed in lavas from hotspots such as Hawaii (~0.8–0.9wt.%) or in Mid-Ocean Ridge Basalts (MORBs) (~0.2wt.%), and are comparable to those measured for the lavas in the Snake River Plain–Yellowstone volcanic corridor and in some island and continental arcs.Lavas considered to be of plume origin from the Picture Gorge basalts in the main-eruptive-stage of the CRB show H2O concentrations of up to 2.4 wt.%, also significantly higher than values published previously for any plume-related volcanism. The highest H2O concentrations have been found most often in the least differentiated melt inclusions, indicating that these volatiles are not the result of differentiation at shallow levels in the crust. The enrichment of Ba relative to Th in many of the melt inclusions may indicate that volatiles in the CRB-OPB large igneous province have a subduction zone origin, which underscores the importance of flux melting in the generation of at least some of Earth’s large igneous provinces.