Fluid-rock interaction experiments at supercritical conditions: a tool to investigate geothermal systems

In the framework of Integrated Methods for Advanced Geothermal Exploration (IMAGE) project, fluid-rock interaction experiments were performed with the aim to better understand the mineralogical reactions occurring in different geothermal systems at supercritical conditions. Experiments considered different geological environments represented by two fossil geothermal systems: 1) Elba Island (Italy), assumed an exposed proxy of the high-temperature hydrothermal system active below the presently exploited reservoir of the Larderello-Travale geothermal field; 2) the Geitafell (Iceland) fossil hydrothermal system resembling the supercritical conditions occurring around the intrusive rocks of actively rifting neo-volcanic zones of Iceland. Experiments evaluated either a metamorphic (Elba) or a magmatic (Iceland) rock reacting with aqueous fluids (with variable salinities and pH) under upper crustal conditions (400-600 °C, 50-130 MPa). They were performed using sealed gold capsules in an externally heated pressure vessel for duration of around 1 week. These P-T conditions were chosen based on microthermometric measurements on fluid inclusions found in rocks of both fossil systems. Furthermore, some experiments were planned in order to sample the liquid phase, besides to solid, at the end of the experiments. In these cases, long capsules were utilised and, at the end of the experiments, the liquid inside them was extracted and diluted with milli-Q water. Solids before and after experiments were characterized through XRD, SEM and EMPA whereas liquids were analysed through LC and ICP-OES. Experiments results indicate mica and plagioclase from Elban micaschist as the major suppliers to the tourmaline crystallisation when aqueous fluids containing boric acid react with a micaschist. Moreover, mineral chemistry shows a compositional overlapping between natural and synthetic crystals (mainly dravites) in the experiments. Chamosite of micaschist disappears after experiments and its' role in tourmaline formation is currently under investigation. It is thus expectable to find tourmaline-rich rocks in the deep reservoir of Larderello-Travale geothermal system. The quartz-tourmaline veins fragments erupted from San Pompeo 2 geothermal well, which encountered a deep high-temperature pressurized fluid, support this assertion. Experiments using a basalt produced a palagonitization only at 400°C whereas modest spilitization and amphibole formation developed mainly at higher temperatures using pure water as a fluid reactant. The high sulphate contents found in the liquids after the experiments disclose the significant S content in glass of the basalt. The utilization of acid fluids and/or Ag2C2O4 (as a CO2 source) significantly increased the rate of metamorphic reactions but other common alteration minerals, extensively found in active and fossil geothermal fields (e.g. chlorite, garnet, hedenbergite,...) were still not detected through XRD, probably due to kinetic reasons.