Geochemical Modeling of a Shallow Submarine Hydrothermal System at BahÌa ConcepciÛn Baja California Sur, Mexico

Abstract

Shallow submarine hydrothermal activity has been reported in BahÌa ConcepciÛn at the Gulf coast of the Baja California Peninsula. It is located along faults, which are probably related to the Tertiary extensional tectonics of the Gulf of California region. Diffuse and focused hydrothermal venting of water and gas occurs in the intertidal and shallow submarine areas at 15 mbsl along a NW-SE trending fault. Temperatures in the submarine vents vary from 50?C at the sea bottom up to 87?C 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 with respect to seawater. The observed chemical and isotopic composition of vent water agrees with a simple mixing model between local seawater and a thermal end-member, where the thermal end-member component is about 40% of this mixture. The temperature calculated using chemical geothermometers points to a deep reservoir temperature of approximately 200?C, and a shallow equilibrium temperature of about 120?C.
Chemical modeling of the hydrothermal solutions and deposited minerals suggests that the interaction of deep circulating thermal water with underlying sediments and mixing with seawater constrain the composition of the vent fluids.