Origin of volatile species and aqueous fluids in the Los Humeros Geothermal Field, Mexico

Abstract The Los Humeros Geothermal Field, discovered in the late 1970s, is the last geothermal field in Mexico to be significantly developed in recent years. Located in a nested caldera system within the Central Mexican Volcanic Belt, the field is within an andesitic reservoir, with low porosity and permeability, limiting the recharge of the system. Here, a new dataset of noble gas and stable isotopes (δ2H and δ 18O), tritium (3H), CO2 contents, halogens (Cl content and δ37Cl-δ81Br), and isotopes of Sr (87Sr/86Sr) were analyzed and interpreted along with previously published noble gas data. The goal was to identify the fluid sources present in the Los Humeros reservoir. This study highlights a complex hydrogeological scenario, with multiple generations of meteoric waters. Furthermore, some volatile species of mantle origin (He, Ar, C) appear to be partially decoupled from the aqueous phase, providing only partial information on the origin and evolution of the waters present in the system. The northern sector of the field contains meteoric waters having interacted with the basalt-andesitic reservoir or deeper melts (87Sr/86Sr of 0.7040) and containing mantle He and Ar. Together with this fluid, a meteoric component containing tritium confirms the presence of local and rapid meteoric recharge in this sector. The central and southern sectors of the field contain meteoric waters having interacted with carbonates from the basement. However, the relationship between 87Sr/86Sr and CO2/3He suggests a possible decoupling between mantle C and He and the aqueous phase, transporting Sr. Of particular interest, limestone C seems to be sourced from subducting slab components from the northern sector of the field, while the often-cited C contribution from local basement metacarbonates could be negligible. Finally, chlorine is sourced by magmatic HCl and partitioned in the steam phase, causing acidic waters.