Mineral solubility and aqueous speciation under hydrothermal conditions to 300 ^\circC–the carbonate system as an example
Résumé
Carbon is a major element in the Earth’s system and plays an important role in many geochemical processes including metamorphism, volcanism, oceanic systems and atmospheric evolution. Knowledge and understanding of chemical speciation, mineral solubility and reactivity involving carbon are very important in order to qualitatively and quantitatively understand these processes. Indeed, dissolved inorganic carbon is among the major components of natural geothermal fluids. It originates from various sources including magmatic degassing, rock dissolution and organic matter degradation (e.g., Giggenbach 1992; Giggenbach et al. 1993; Simmons and Cristenson 1994; Sanoa and Marty 1995). Dissolved CO2 is the most common form of dissolved inorganic carbon in these systems, though other forms exist like CH4 and CO but usually in much lower concentrations than CO2 (e.g., Chiodini and Marini 1998; Stefánsson and Arnórsson 2002). The concentrations of dissolved inorganic carbon (DIC) in geothermal fluids from active geothermal systems throughout the world are shown in Figure 1a together with the pH of the fluids. In general, the DIC concentration increases in geothermal fluids with increasing temperature from a few mmol per kg up to half a mole per kg of fluid. This increase is accompanied by a decrease in the fluid pH (Fig. 1b). Higher concentrations (not shown) are observed in fumarole fluids discharging volcanic gases (e.g., Chiodini et al. 1996). At Earth’s surface, CO2 also degasses through the soils in active geothermal systems (e.g., Chiodini et al. 1998).