LAUSR

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