MANTLE-TO-CRUST FLUID TRANSFER ABOVE THE PERUVIAN FLAT SLAB: INSIGHT FROM HELIUM ISOTOPES IN THERMAL SPRINGS OF THE PERUVIAN ANDES

Thermal springs across the Peruvian Andes provide robust geochemical evidence of mantle-derived volatiles transiting through the continental lithosphere above an active flat-slab subduction zone. Elevated helium isotope ratios (³He/⁴He) of dissolved gasses in spring waters range from 0.26 to 2.52 R_A (R_A = air ³He/⁴He), indicating the presence of 3 to 32% mantle-derived helium. We investigated thermal springs from multiple regions of Peru and from a variety of structural settings, including across the transition from flat-slab subduction to the active back arc in the Peruvian Altiplano. As expected due to relatively recent magmatism, the highest mantle contributions (11 to 32%) occur in the back-arc region. However, mantle helium is also widespread above the flat slab (3 to 25%). Given the absence of recent magmatism in the flat-slab region, we suggest that mantle helium is mobilized from the sub-continental mantle lithosphere by metasomatic fluids released during slab dehydration. The largest helium isotope ratios above the flat slab (0.62 to 1.98 R_A) are observed along the 180 km long Cordillera Blanca detachment fault in central Peru, likely due to increased crustal permeability in the ductal lower crust, and/or more efficient fluid transport along deeper groundwater flow paths. Correlations between ³He/⁴He, δ¹³C, CO₂, and He concentrations are best explained by fractional degassing during ascent through the near surface hydrologic system, indicating that actual mantle volatile contributions are likely higher than we observe. Assuming that mantle helium is carried by a flux of slab-derived volatiles (mainly H₂O and CO₂), this data provides evidence for the widespread hydration of continental lithosphere above an active flat-slab subduction zone, and this has important tectonic implications. For example, lithospheric hydration is invoked as a likely mechanism for reducing the bulk density and mechanical strength of the North American lithosphere above the Farallon flat slab, thereby influencing the subsequent tectonic evolution of the Western Cordillera. The Peruvian flat slab serves as a modern analogue for understanding the rheological and tectonic evolution of other convergent margins that have experienced episodes of flat-slab subduction.