Vent fluid chemistry in Bahia Concepcion coastal submarine hydrothermal system, Baja California Sur, Mexico

Shallow submarine hydrothermal activity has been observed in the Bahia Concepcion bay, located at the Gulf coast of the Baja California Peninsula, along faults probably related to the extensional tectonics of the Gulf of California region. Diffuse and focused venting of hydrothermal water and gas occurs in the intertidal and shallow subtidal areas down to 15 m along a NW-SE-trending onshore-offshore fault. Temperatures in the fluid discharge area vary from 50 degreesC at the sea bottom up to 87 degreesC at a depth of 10 cm in the sediments. Chemical analyses revealed that thermal water is enriched in Ca, As, Hg, Mn, Ba, HCO3, Li, Sr, B, I, Cs, Fe and Si, and it has lower concentrations of Cl, Na, SO4 and Br than seawater. The chemical characteristics of the water samples indicate the occurrence of mixing between seawater and a thermal end-member. Stable isotopic oxygen and hydrogen composition of thermal samples plot close to the Local Meteoric Water Line on a mixing trend between a thermal end-member and seawater. The composition of the thermal end-member was calculated from the chemistry of the Submarine samples data by assuming a negligible amount of Mg for the thermal end-member. The results of the mixing model based on the chemical and isotopic composition indicate a maximum of 40% of the thermal end-member in the submarine vent fluid. Chemical geothermometers (Na/Li, Na-K-Ca and Si) were applied to the thermal end-member concentration and indicate a reservoir temperature of approximately 200 degreesC. The application of K-Mg and Na/Li geothermometers for vent fluids points to a shallow equilibrium temperature of about 120 degreesC. Results were integrated in a hydrogeological conceptual model that describes formation of thermal fluids by infiltration and subsequent heating of meteoric water. Vent fluid is generated by further mixing with seawater. (C) 2004 Elsevier B.V. All rights reserved.