HELIUM ISOTOPE GEOCHEMISTRY ACROSS THE CONTINENTAL U.S. LINKED TO MANTLE TOMOGRAPHIC IMAGES
Noble gases, including He isotopes, provide an ideal set of tracers for identifying the presence of mantle-derived gases and their crustal interactions. Recent works compared P-wave mantle tomography with the He isotopes in geothermal and carbonic springs in the western U.S. 3He/4He and CO2/3He data provide unequivocal evidence that mantle-derived volatiles are present in many groundwater systems, with contributions of 80-100% mantle-derived He in some geothermal areas and common values of ~10% in many cool springs. Data from these springs suggest a correlation between degree of mantle degassing and domains of low-velocity mantle and unexpectedly widespread, spatially partitioned, neotectonic mantle degassing along faults across the U.S.
Here, we test the relationship between mantle-derived gases and low-velocity anomalies in the central and eastern U.S. Our existing gas geochemistry database includes 3He/4He, CO2/3He, and δ13C-CO2 from a geographically and geologically diverse set of CO2-rich springs and natural gas wells across the U.S. Accumulating data show values of 0.5-0.7 RA (7-9% mantle helium) in springs and groundwaters in numerous areas of the eastern U.S., with highest values associated with the presence of underlying low-velocity mantle anomalies and/or zones of sharp velocity contrast. Thus, even in the relatively passive regions, flux of mantle volatiles, including degassing of CO2 and helium, is more active than previously considered.