Variability of deep carbon sources in Mexican geothermal fluids

Abstract

A sampling campaign for collecting geothermal fluids was carried out between 2014 and 2016 at the Cerro Prieto (CP), Los Azufres (LA), Los Humeros (LH), and Las Tres Vírgenes (LTV) geothermal fields (GFs) in Mexico. The aim of this study was to determine the CO2 carbon origin and behavior in geothermal reservoirs. The LA-GF and LH-GF are situated in an arc magmatism context related to the subduction of the Cocos plate under the North American one. The CP-GF and LTV-GF are located in an extensional pull-apart context related to the San Andreas Fault System and the opening of Gulf of California. The most interesting observation of this study was that the measured isotopic composition of CO2 (13C/12C and 18O/16O ratios) is distinct and unique for each sampled geothermal field. The C isotopic variability is controlled mainly by its sources (i.e., δ13C is −6 ± 2‰ in the mantle; −20‰ in carbon organic matter-rich sediment; and 0 ± 2‰ in marine carbonates) and possibly secondary processes such as calcite precipitation. The δ13C in the CP-GF and LA-GF are compatible with a dominant mantle carbon source. LTV-GF and LH-GF show lower (−10.7 ± 0.5‰) and higher (−3.5 ± 0.9‰) values than that of the mantle respectively. The lower δ13C of carbon in LTV-GF fluids could be due to calcite precipitation (carbonate scaling), a well-known phenomenon in this field. However, calcite precipitation should lower the CO2/3He ratios from an initial mantle value of 2 × 109. Calculated CO2/3He ratios of 1.72 and 5.49 × 1010 in LTV-GF fluids suggest that the addition of sedimentary carbon rather than calcite precipitation might be the main process. Based on the isotopic composition of helium (3He/4He ratios, or R, from 3.2 to 7.9 times the atmospheric ratio, Ra = 1.384 × 10−6) and carbon (δ13C from −11 to −2‰ vs PDB) in the fluids, the δ13C variability could be caused by the addition of carbon organic matter-rich sediments from an ancient subducting plate in the LTV-GF and from leaching of marine carbonates located underneath the basement in the LH-GF. The oxygen isotopic variability is related to the degree of isotopic re-equilibration between steam (H2O) and CO2, which, in the case of fluids extracted from production wells, may be incomplete.