DEGASSING AND MAGMA EVOLUTION IN THE COLUMBIA RIVER FLOOD BASALTS DURING THE ERUPTION OF THE IMNAHA AND GRANDE RONDE FORMATIONS

The Imnaha and Grande Ronde Basalts of the Columbia River Basalt Group (CRBG) form the early parts main phase of the province and provide clues to the evolution of the system. Many vents show phreatomagmatic characteristics caused by eruption through water, both surficial and potentially subsurface. Maar complexes may be in excess of 1 km and contain considerable basement lithic material. Smaller vent structures include low-angle cones with almost no perceptible lithic material. Both types of vent produce quenched lapilli in addition to variable levels of palagonite within the tephra. While the glassy lapilli have experienced variable Na leaching they are otherwise pristine and contain phenocrysts with glassy lapilli. Melt inclusions may contain undegassed glass, allowing for sampling of the pre-eruptive magma chamber.

Here we analyze lapilli and inclusion glasses from several locations throughout northeastern Oregon and compare the chemistry of the evolving magma chamber. We focus on degassing of S, as well as looking at H2O and CO2 and their relationship to magma chemistry and examine the connection between the early phases of the eruption and the climatological impact of flood basalt eruptions.