Heat, noble gases and CO2 sources in geothermal fields of Mexico

Abstract

The Trans-Mexican Volcanic Belt (TMVB) and the Gulf Extensional Province (Baja California) are the regions in Mexico with the highest heat flow and where the main geothermal energy resources are currently exploited. Here, previously published and newly collected heat and volatile species (He, Ar and CO2) data from 132 fluid samples of four geothermal areas belonging to TMVB (Los Azufres, Los Humeros, Acoculco and Cerritos Colorados) and two in Baja California (Cerro Prieto and Las Tres Vírgenes) were analyzed. The goal was to identify heat sources, understand how heat is transported into the geothermal reservoirs and how the multiple fluid sources (meteoric, magmatic and crustal) in the reservoir might impact the heat in each field. Distinct R/Ra ratios (where R is the measured 3He/4He ratio and Ra is the atmospheric ratio) are observed in the TMVB and Baja California geothermal fields, with those in the TMVB displaying significantly higher R/Ra values (7.14–7.27) than those in Baja California (1.21–6.62), indicating a higher contribution of mantle helium from possibly younger, active magmatic heat sources. Lower R/Ra values in Baja California might reflect local tectonic features, where reservoirs are primarily affected by crustal contributions from an old, subducted fossil slab related to the Farallon Plate. The analysis of heat to 3He ratios (or Q/3He) together with information provided by 4He/36Ar ratios suggest that heat and volatiles are transferred into the hydrothermal system by advection. The magma bodies beneath the geothermal areas of interest are sufficiently voluminous to provide aqueous fluids by exsolution to sustain this advective flow. The release of He and Ar from magma, during its degassing, appears to be controlled by their diffusivities, fractionating the pristine mantle 4He/36Ar ratio toward higher values, while advection transfers volatiles more rapidly than heat, fractionating the pristine mantle Q/3He ratios toward lower values. The Q/3He vs. 4He/3He relationship shows that boiling and dilution by meteoric water led to further fractionation of heat and volatiles, particularly in Los Azufres and Los Humeros fields, while Cerro Prieto has Q/3He ratios close to those at mid-ocean ridges or fractionated by circulation of low-heat, 4He-rich fossil waters. Distinct sources of heat and volatiles are also reflected in the CO2 content, the major dry gas component in these geothermal fields. Main sources of CO2 are mantle carbon (1%-51%) and limestone (34%–100%) from local sources and the subducting slab, although expected fractionation of the C isotopic composition and CO2/3He during magma ascent and degassing introduce uncertainties on the precise estimation of each source contributing C to the studied geothermal reservoirs. Yet, contribution of carbon from sediments might be minor (0%–35%), and mainly found in Cerro Prieto and Las Tres Virgenes fields located in Baja California, which are affected by the release of volatiles from the subducted Farallon plate and presence of connate waters in the reservoir of Cerro Prieto.