HELIUM ISOTOPE GEOCHEMISTRY ACROSS THE CONTINENTAL U.S. LINKED TO MANTLE TOMOGRAPHIC IMAGES

EarthScope’s USArray seismic network has produced unparalleled mantle velocity images that reveal novel insights into the heterogeneous mantle-seismic structure across the conterminous U.S. Some important findings include the identification of distinct zones of low-velocity anomalies in widespread and diverse geographic regions of the tectonically active western U.S. and the tectonically inactive cratonic core and ancient orogens of the eastern U.S. Sharp velocity contrasts reveal lithospheric-scale segmentation.

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 R_A (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.