Rubén Bernard Romero

In this work, we report new chemical and isotopic data (³He/⁴He, δ¹³C_CO2, δ¹³C_CH4, and δD_CH4) from poorly or previously unstudied hydrothermal and magmatic gases that are emitted along the eastern coast of the Baja California Peninsula (BCP). High ³He/⁴He values (up to ~7 Ra) characterize the magmatic gases, while lower ratios (≤1.6 Ra) characterize hydrothermal springs. We infer that the mantle beneath the BCP could be Mid‐ocean‐ridge basalt (MORB)‐likes, as in the rift within the Gulf of California, or it may reflect contamination from C‐rich sediment during paleo‐subduction of the Farallon plate. During their ascent, through the crust, mantle/magmatic gases mix with CO₂‐ and ⁴He‐rich fluids, thus forming CO₂‐rich hydrothermal gases. These hydrothermal gases undergo partial dissolution of CO₂ in shallow waters under different temperature and pH conditions, which further modifies their composition. Thermogenic and possibly abiogenic sources of methane are present only in magmatic gases from the BCP. Secondary methane oxidation (microbial/inorganic) processes are proposed for some hydrothermal gases, which are extremely enriched in heavy isotopes. Finally, we argue that the hydrothermal gases that are emitted from the BCP have variable percentages of mantle contribution, indicating the presence of lithospheric faults enhancing the rise of mantle fluids also in areas where volcanism is absent.

Chemical and isotopic analyses of waters from 4 thermal springs of the Guerrero Pacific coast, the most seismically active area in Mexico, were performed weekly during a period of 1.5 years (October 2002–March 2004). Within the same time interval more than 200 earthquakes with 3.8 ≤ M ≤ 5.3 occurred in the area. The data display several anomalies in Cl⁻, SO₄²⁻, δD and δ¹⁸O, always immediately after an event, with a relaxation time of 3–4 weeks. The responses occurred only to earthquakes with the estimated epicenters very close to the location of a spring. These results indicate that: 1) at least for earthquakes with M < 5.3 within the Guerrero “seismic gap” there were no precursors in the chemical (ionic) and isotopic composition of thermal waters (on a weekly basis); 2) two groups of springs near Acapulco, Dos Arroyos and Paso Real, are sensitive to seismic activity and therefore further monitoring of these springs may help to unravel the mechanisms of the “hydro‐seismo‐interaction” in the area.