Loïc Peiffer

New insights into the Kawah Ijen hydrothermal system from geophysical data

Abstract The magmatic–hydrothermal system of Kawah Ijen volcano is one of the most exotic on Earth, featuring the largest acidic lake on the planet, a hyper-acidic river and a passively degassing silicic dome. While previous studies have mostly described this unique system from a geochemical perspective, to date there has been no comprehensive geophysical investigation of the system. In our study, we surveyed the lake using a thermocouple, a thermal camera, an echo sounder and CO2 sensors. Furthermore, we gained insights into the hydrogeological structures by combining self-potential surveys with ground and water temperatures. Our results show that the hydrothermal system is self-sealed within the upper edifice and releases pressurized gas only through the active crater. We also show that the extensive hydrological system is formed by not one but three aquifers: a south aquifer that seems to be completely isolated, a west aquifer that sustains the acidic upper springs, and an east aquifer that is the main source of fresh water for the lake. In contrast with previous research, we emphasize the heterogeneity of the acidic lake, illustrated by intense subaqueous degassing. These findings provide new insights into this unique, hazardous hydrothermal system, which may eventually improve the existing monitoring system.

Fluid Geochemistry of El Chichón Volcano-Hydrothermal System

El Chichon 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 Chichon 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 Chichon 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.

Numerical modeling of cold magmatic CO2 flux measurements for the exploration of hidden geothermal systems

The most accepted conceptual model to explain surface degassing of cold magmatic CO2 in volcanic-geothermal systems involves the presence of a gas reservoir. In this study, numerical simulations using the TOUGH2-ECO2N V2.0 package are performed to get quantitative insights into how cold CO2 soil flux measurements are related to reservoir and fluid properties. Although the modeling is based on flux data measured at a specific geothermal site, the Acoculco caldera (Mexico), some general insights have been gained. Both the CO2 fluxes at the surface and the depth at which CO2 exsolves are highly sensitive to the dissolved CO2 content of the deep fluid. If CO2 mainly exsolves above the reservoir within a fracture zone, the surface CO2 fluxes are not sensitive to the reservoir size but depend on the CO2 dissolved content and the rock permeability. For gas exsolution below the top of the reservoir, surface CO2 fluxes also depend on the gas saturation of the deep fluid as well as the reservoir size. The absence of thermal anomalies at the surface is mainly a consequence of the low enthalpy of CO2. The heat carried by CO2 is efficiently cooled down by heat conduction and to a certain extent by isoenthalpic volume expansion depending on the temperature gradient. Thermal anomalies occur at higher CO2 fluxes (>37,000 g m−2 d−1) when the heat flux of the rising CO2 is not balanced anymore. Finally, specific results are obtained for the Acoculco area (reservoir depth, CO2 dissolved content, and gas saturation state).

GeoT User’s Guide, A Computer Program for Multicomponent Geothermometry and Geochemical Speciation, Version 2.1

Author(s): Spycher, Nicolas; Peiffer, Loic; Finsterle, Stefan; Sonnenthal, Eric | Abstract: GeoT implements the multicomponent geothermometry method developed by Reed and Spycher (1984, Geochim. Cosmichim. Acta 46 513–528) into a stand-alone computer program, to ease the application of this method and to improve the prediction of geothermal reservoir temperatures using full and integrated chemical analyses of geothermal fluids. Reservoir temperatures are estimated from statistical analyses of mineral saturation indices computed as a function of temperature. The reconstruction of the deep geothermal fluid compositions, and geothermometry computations, are all implemented into the same computer program, allowing unknown or poorly constrained input parameters to be estimated by numerical optimization using existing parameter estimation software, such as iTOUGH2, PEST, or UCODE. This integrated geothermometry approach presents advantages over classical geothermometers for fluids that have not fully equilibrated with reservoir minerals and/or that have been subject to processes such as dilution and gas loss.