Mantle-to-Surface Fluid Transit Times Estimated from Helium Isotopes in Thermal Springs above the Peruvian Flat Slab: Implications for Tectonic Controls during Flat Slab-to-Lithosphere Fluid Transfer

Abstract:

Mantle-derived helium in thermal springs across the Peruvian volcanic gap in South America is best explained by persistent flat slab-to-crust fluid transfer mobilizing helium from the subcontinental lithospheric mantle (SCLM). Helium isotope ratios (³He/⁴He) above the flat slab range from 0.26 to 2.40 R_C [³He/⁴He ratio normalized to air ratio (R_A) of 1.4x10^-6, corrected for air-contamination], and are lowered from SCLM endmember values as a function of the time-dependent in situ ingrowth of radiogenic ⁴He during fluid ascent through the thick Andean crust. By assuming initial isotopic ratios of the SCLM (~6 R_A) and calculating initial helium concentrations from our least degassed samples, helium isotope ratios can be utilized to estimate mantle-to-surface fluid transit times. Helium isotope ratios are highest in hot springs along the Cordillera Blanca Detachment fault (CBD; 1.95 R_C) and above a recently described tear in the subducting Nazca Plate (2.4 R_C), with estimated fluid transit times of up to 97 ka (515 mm/yr) and 70 ka (710 mm/yr), respectively. Elsewhere above the flat slab, the lowest observed helium isotope ratio of 0.26 R_C corresponds to a fluid transit time of 1.03 Ma (49 mm/yr). Our estimates do not account for mixing with ⁴He rich crustal fluids, and assume vertical fluid migration. Along the CBD fault, footwall exhumation of the Cordillera Blanca Batholith since ca. 5 Ma has produced the second highest topography in the Andes. We propose that exhumation has locally promoted retrograde metamorphism and accelerated hydration reactions in the lower crust, leading to a fluid pressure gradient that focuses a persistent fluid flux into and up through the crust below the Cordillera Blanca. To the south, where the Nazca slab is tearing, we propose that asthenosphere interacting with the slab tear has accelerated plate dehydration and has potentially led to partial melting of the upwelling asthenosphere or hydrated SCLM. In both cases, higher fluid supply to the base of the crust would promote faster fluid flow rates, leading to higher helium isotope ratios in surface springs. These results suggest that tectonic heterogeneities in both the upper and lower plate can influence slab-derived fluid fluxes and transit rates through the overlying continental crust.