HE AND CO2 FLUX FROM THE PERUVIAN FLAT SLAB ESTIMATED FROM HOT SPRING GEOCHEMISTRY

Geochemical exchanges between the crust and mantle occurs predominantly, but not exclusively, near plate boundaries, especially those associated with magmatism. This includes CO₂ and He, both critical for understanding the tectonic and climatic implications of volatile cycling. Quantifying the fluxes of these key volatiles is most often accomplished utilizing direct measurements and geochemical studies at hot springs, fumaroles, diffuse gas emission, and the rock record. At convergent margins, these estimates are best constrained at active arcs, and the flux from magmatic gaps associated with flat-slab subduction are cryptic. Slab devolatilization during past periods of flat-slab subduction beneath North and South America is attributed to hydration, and density and chemical modifications of the sub-continental mantle lithosphere (SCLM) and lower crust, leading to subsequent widespread magmatism after slab rollback and arc re-initiation. It follows that this flux of fluids through the lithosphere should include other volatiles such as CO₂ and He. Using thermal spring isotope and gas geochemistry, we estimate the volatile flux from the present-day Peruvian flat slab region and the adjacent back-arc region that experienced prior flat-slab subduction. Helium isotope ratios indicate widespread mantle volatiles (3 to 25%) in the flat-slab region. The back arc that has experienced recent magmatism exhibits mantle contributions up to 32%. Simple 1-D flux estimates are based on ³He/⁴He and CO₂/³He ratios in springs, published crustal thickness estimates, and the assumption that mantle volatiles are mobilized from the SCLM (flat slab) or asthenosphere (back arc). The ³He flux above the flat slab and back arc are estimated from 0.01 to 3.2 and 0.004 to 1.4 atoms/cm²/s, respectively. Surprisingly, the flat slab ³He flux is on average similar to or higher than the back arc, and consistent with observations at settings such as continental rifts with magmatism and much thinner crust. Assuming 10 % of the Peruvian flat-slab region is degassing to the atmosphere, the associated CO₂ emissions range from 1 X 10⁹ to 5 X 10¹⁰ mol/yr, or up to ~3 % of global subaerial emissions. These estimates suggest that flat-slab subduction contributes a modest, yet unaccounted for, flux of CO₂ and He, playing a role in global volatile cycling.