Fluid Geochemistry of El Chichón Volcano-Hydrothermal System

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.