MANTLE CONTRIBUTIONS TO GROUNDWATER IN THE OREGON CASCADES AND EASTERN OREGON INFERRED FROM HELIUM ISOTOPES

Heat flow as well as noble gas and other magmatic volatile fluxes are often used to infer the extent of magma intrusions, groundwater flow patterns, and advective heat transfer in magmatically active regions. In this study, we measure helium (He) concentrations and isotopic ratios in springs in the Oregon Cascades volcanic arc and in eastern Oregon, including the north-western extent of the Basin and Range. We show that in order to estimate mantle (magmatic) He contributions to groundwaters, it is critical to simultaneously consider 1) He isotopic ratios, 2) He concentrations, and 3) mixing of He components from the crust, mantle, and air-saturated water. Our results suggest that mantle fractions (F) of excess (crustal + mantle) He contributions to cold springs that are fed by deep groundwater flow beneath the eastern flanks of the Oregon Cascades are comparable to F of up to 0.7 in thermal springs on the western flanks. Similarly, we find excess He isotopic ratios of up to 5.3 Ra on either side (east and west) of the Oregon Cascades, where Ra is the He isotopic ratio of the atmosphere. Furthermore, while these mantle He contributions are largest in deep groundwaters near the Cascades crest, greater mantle excess He fractions (F) than would be inferred from He isotopic ratios alone are present in all (deep) groundwaters including those at larger distances (>70 km) from the volcanic arc. We also suggest that excess He and heat discharge without dilution by air-saturated water may be restricted to spring discharge along faults.