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Accurate characterization of the magmatic compositions of flood basalt ­lavas is fundamental to interpretations of magma genesis, stratigraphy, and correlation across these extensive provinces. Analysis of the geochemistry of the Sentinel Bluffs Member of the Grande Ronde Basalt, Columbia River Basalt Group (northwestern USA), demonstrates that a mass-based methodology, similar to those routinely used in studies of weathering and soil formation, enables the identification of subtle and previously unrecognized low-temperature alteration, and the determination of primary magmatic geochemical characteristics in rocks modified by secondary processes. This methodology, here termed mass analysis, employs concentrations and ratios of immobile elements, which are not transported by low-temperature alteration processes, to show that alteration has resulted in loss of rock mass due to mineral dissolution in anoxic groundwater. Immobile element abundances corrected for mass loss permit the identification and province-wide correlation of individual flows and flow packages, even for rocks that have undergone nearly 50% mass loss. The methodology developed with Sentinel Bluffs lavas is applicable to other lavas of the Columbia River flood basalt province, and most likely to other volcanic provinces in which lavas have undergone long-term interaction with groundwater.

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