Fluid Geochemistry of El Chichón Volcano-Hydrothermal System

Abstract

El Chichón volcano hosts an intense hydrothermal system with surface manifestations consisting of an acid lake, steam vents, steam-heated boiling pools, mud pools and boiling springs in the crater, as well as several hot springs located on the outer slopes. This chapter reviews previous studies of the El Chichón volcano-hydrothermal system and proposes a conceptual model of the aquifer structure based on more than 15 years of fluid geochemical monitoring (major and rare-earth elements, δ18O- δD, 87Sr/86Sr). This model contains two aquifers: (1) Aquifer 1, located beneath the crater in the volcanic deposits, produces a total thermal water discharge of 220 L/s and feeds the flank ‘Agua Caliente-Agua Tibia’ spring group; (2) Aquifer 2, much deeper and with a lower total discharge of 7 L/s, is located in the evaporite-limestone basement and feeds the flank ‘Agua Salada-Agua Salada new’ spring group. The deep waters from Aquifer 2 have a much higher salinity than Aquifer 1 waters (25,000 vs. 2,200 mg/L Cl) and can be associated with oil-field brines. The crater lake chemistry and dynamics are mainly controlled by the steam condensation from Aquifer 1 waters and by the activity of the Soap Pool springs. Their chemical and isotopic composition can be associated with the volcanic Aquifer 1 water by a model of a single step liquid-vapor separation. Finally, El Chichón volcano is located in a non-classic volcanic arc and rather peculiar local and regional tectonic setting, as supported by CO2 flux surveys and He and C isotope systematics of emitted gases.