VENT FLUID ORIGIN IN THE PUNTA MITA COASTAL SUBMARINE HYDROTHERMAL SYSTEM, MEXICO

Shallow submarine hydrothermal activity has been detected at Punta Mita, Mexico. Punta Mita is situated on the central Pacific coast of Mexico, at the western end of the Trans-Mexican Volcanic Belt. This region is characterised by a complex recent tectonic history, dominated by the subduction of the Rivera plate beneath western Mexico along the Jalisco Subduction Zone. Onshore geology in Punta Mita shows that basaltic and sedimentary units of Tertiary age discordantly overlay cretaceous rocks. The hydrothermal vents occur along the basalt-hosted "Las Coronas" fissure. Submarine hydrothermal vents are located approximately 400 m offshore at a depth of about 10m. Water and gas venting at a temperature of 85º C occurs in coarse sand and consists of both focused and diffuse discharge. Samples of hydrothermal fluid and gas were collected with a specifically designed 1 litre cylindrical bottle of two lines by flushing in thermal fluid until seawater total displacement. The chemistry and the isotopic composition reveal that the water is a mix of meteoric and seawater heated by the local high geothermal gradient. The discharge water is more dilute than seawater and its isotopic composition indicates that the predominant components are local meteoric water and seawater. The composition of the thermal end-member was calculated for Mg=0 using the linear regression of the concentration values vs magnesium content. The results of the end-member calculation show that Ca, Mn, Ba, I, Cs and Si are provided by the thermal component and Na, K, Cl, SO4, HCO3, and Br are added by the seawater. Strontium isotopic composition indicates interaction with basaltic rocks. The gas chemistry shows that there are no significant magmatic components, as the major components of the vented gas are nitrogen and methane, with small amounts of hydrogen sulphide, argon, helium and hydrogen. The rare gas data suggest that the gas composition may be the result of mixing between metasedimentary gas and air-saturated groundwater with an enrichment of crustal He. Isotopic data for carbon in methane yields a value of -42.8 per mil; this may be evidence of a thermogenic origin and the CH4 / CO2 ratio ranges from 105 to 230, which is typical of low temperature natural gas discharges.