3He evidence for fluid transfer and continental hydration above a flat slab

The transfer of large volumes fluid to the overriding lithosphere during flat-slab subduction should drastically alter physical and chemical properties continental margins. However, this process is poorly understood without active magmatism, direct evidence for has remained elusive in modern systems. New helium isotope ratios (3He/4He) gas species abundances thermal springs above Peruvian flat slab South America demonstrate widespread transit mantle-derived volatiles through thick (>45 km) crust. An unambiguous mantle signature present all 52 investigated over a 200,000 km2 region, with 3He/4He ranging from 0.26 2.52 RC/RA (3He/4He ratio relative air ratio, RA, 1.39×10?6, corrected air-derived helium). In absence recent observed are best explained by slab-derived fluids mobilizing subcontinental lithospheric (SCLM). As slab- migrate crust, reduced SCLM values (6.1±2.1 RA) ingrowth radiogenic 4He mixing crustal fluids. highest region probable tear Nazca Plate along Cordillera Blanca Detachment fault, implying that these features promote faster transport Our observations require persistent flux move lithospheric-mantle-derived into providing hydration system. Similar results located backarc south indicate below northern Altiplano experiencing dehydration and/or partial melting resulting mid-Cenozoic episode subduction.