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The Grande Ronde Basalt lavas constitute ~63% of the Columbia River Basalt Group, a large igneous province in the NW United States. The lavas are aphyric or contain less than 5% phenocrysts of plagioclase, augite, pigeonite, and olivine (altered). Plagioclase hygrometry shows that the erupted lavas generally contained less than 0.3% dissolved H2O; however, the presence of rare disequilibrium An96 plagioclase phenocrysts suggests that some magmas may have originally had 4.5 wt% dissolved H2O at depth, but they all degassed during ascent and eruption. The size of plagioclase phenocrysts suggests an average plagioclase phenocryst residence...

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