TRACING VOLATILE AND ISOTOPE GEOCHEMISTRY WITHIN MANTLE- AND SLAB-DERIVED FLUIDS IN A FLAT-SLAB SUBDUCTION ZONE, PERU

Thermal springs along an active flat-slab subduction zone, Peru provide geochemical evidence of mantle-derived fluid transfer to continental lithosphere. Springs were strategically targeted on the east and northeast side of the Cordillera Blanca mountain range, along the Cordillera detachment fault (western edge of Cordillera Blanca), and in the Cordillera Huayhuash range to test the role of deeply penetrating faults as a fluid conduit. Water and gas samples were acquired from springs in all three zones and analyzed for a suite of natural chemical and isotopic tracers. Springs yielded δ¹⁸O_SMOW (−14.7 to −4.9 ‰) and δD_SMOW (−112.9 to −74.3 ‰), forming a linear trend to values higher than local meteoric water. With spring temperatures up to 88°C, this deviation may be due to mixing with a deep-seated source, or evaporation processes. δ³⁷Cl_SMOC values from springs along the Cordillera detachment range (-0.5 to 1 ‰), with majority of springs yielding negative values. This range is indicative of a mantle-derived source fluid mixed with subducted marine sediments, similar to what is observed in active volcanic arc settings (e.g. Cascadia). He/Ar and N₂/He ratios provide evidence for a significant component of volatiles derived from the crust/mantle, and further interpretation of this data must be made in conjunction with helium isotope data, which is currently being processed. Given evidence for continued hydration of the lithosphere during flat-slab subduction, and mantle-sourced volatiles in springs along the detachment, we suggest that slab-derived hydrothermal fluids mobilize mantle volatiles, and structural controls influence their distribution.