Helium Isotopes and Hydrogeochemistry of Thermal Waters from Central Mexico: Geologic and Geothermal Implications

Mexico is one of the world's top producers of geothermal energy, half of its territory exhibits an anomalously high heat flow. The heat source in the current production fields corresponds to active or recent magmatism associated with subduction or lithospheric thinning. High-enthalpy geothermal areas have been thoroughly studied and concentrate the majority of physicochemical and isotopic analysis of hydrothermal manifestations. As for the rest of the country, where geothermal waters locate in areas devoid of recent volcanism, data is limited, and the geologic framework and geothermal potential remain unclear. Low to high-enthalpy manifestations (up to 98°C in springs and more than 100°C in geothermal wells) are distributed in central and northern Mexico. Here, we present new physicochemical and isotopic data of groundwater, dissolved and free gas for springs, water wells, and geothermal wells from the Juchipila, Santiago Papasquiaro, Querétaro, Celaya, and San Juan del Río basins. The study areas locate in the southern Sierra Madre Occidental and Mesa Central geologic provinces and the northernmost Trans-Mexican Volcanic Belt (TMVB), where volcanism was active in the Miocene. Our study focuses particularly on Helium isotope analysis as they are widely used to distinguish fluid sources, which combined with geologic and hydrogeochemical data can be a powerful tool to identify tectonic and geologic processes occurring within the crust and mantle. In contrast with the well-known geothermal fields of Mexico, the primary heat source of these thermal waters is the convection of meteoric water through permeable faults. In some places, high heat production due to enrichment in radioelements in the granitic felsic upper crust accounts for a secondary source. Helium composition (up to 3.25 Rc/Ra) and flux are strongly controlled by the mixing with shallow groundwater and by the crustal structures that bound the geologic provinces, namely the Taxco-San Miguel de Allende, Chapala-Tula, and San Luis-Tepehuanes fault systems. The high mantellic Helium signatures and flux estimations imply that mantle volatile degassing is enhanced by crustal fault systems within central Mexico. Our study provides a contrasting view of geothermal processes and volatile degassing from archetypal subduction-related volcanic regions.