Mantle to Surface Fluid Transfer Above a Flat Slab Subduction Zone: Isotopic Evidence from Hot Springs in the Cordillera Blanca, Peru

Abstract:

Thermal springs in the Cordillera Blanca, Peru, provide geochemical evidence for deeply circulated hydrothermal fluids that carry significant mantle-derived helium. The Cordillera Blanca is a ~200 km-long NNW-SSE trending mountain range in the Peruvian Andes located above an amagmatic flat-slab subduction segment. The west side of the range is bounded by the Cordillera Blanca detachment that preserves a progression of top to the west ductile shear to brittle normal faulting since ~5 Ma. We report aqueous and stable isotope geochemical results from fluid and gas samples collected in 2013 and 2015 from 13 hot springs emanating from the Cordillera Blanca detachment and associated hanging wall faults. Most springs are vigorously bubbling (degassing), and range in temperature, pH, and conductivity from 17-89 °C, 5.95-8.87, and 0.17-21.5 mS, respectively. The hottest springs issue directly from the northern segment of the detachment. Geochemically, springs are CO₂-rich, alkaline-chloride to alkaline-carbonate waters, with elevated trace metal contents including Fe, Cu, As, Zn, Sb, and Tl. Notably, As contents are ≤11 ppm, indicating that thermal waters may be adversely impacting local water quality. Water δ¹⁸O and δD, trends in elemental chemistry, and cation geothermometry collectively demonstrate mixing of hot (200–260 °C) saline fluid with cold meteoric recharge along the fault. Helium isotope ratios (³He/⁴He) for dissolved gases in the hot springs range from 0.62 to 1.98 R_C/R_A, indicating the presence of ~25% mantle–derived helium, assuming mixing of an asthenospheric end-member with the crustal helium reservoir. CO₂/³He and carbon stable isotope ratios indicate a carbon source derived from mixing of crustal sources with minor mantle carbon. Overall, the volatile signature overlaps with orogen-wide datasets where crustal overprinting has modified mantle contributions at active arc volcanoes. Given the long duration since active magmatism in the Cordillera Blanca region, we suggest that mantle helium was mobilized from the continental mantle-lithosphere by metasomatic fluids derived from slab dehydration. These spring data thus reveal a mantle to surface connection and highlight the role of detachment faults in compressional orogens for fluid transfer in the crust.