Geochemical constraints on supercritical fluids in geothermal systems

Abstract

Supercritical fluids with temperatures of ~400–500 °C have been reported from several active geothermal fields worldwide. Although the utilization of such fluids may multiply power production from new and already exploited geothermal systems, the fluid origin and chemical controls on their composition remain unclear. We performed flow-through high-temperature (400–420 °C) experiments at 34–69 bar to study the chemical and mineralogical changes associated with supercritical fluid formation upon boiling of subcritical geothermal fluids of varying chemical composition. Based on geochemical modeling and laboratory results, we propose that an important mechanism of supercritical fluid formation is conductive heating and boiling of subcritical geothermal groundwater by a magmatic intrusion. Such supercritical fluids will display low concentrations of mineral-forming elements (Si, Na, K, Ca, Mg, Al), with their concentrations being controlled by the solubility of salts, oxides, and aluminum silicates in high-temperature (>400 °C) and low-density (ρ < 0.3 g cm−3) fluids. In contrast, supercritical fluids will show elevated concentrations of volatile elements (C, S, B) of crustal and/or mantle origin with their concentrations often being similar to those of subcritical geothermal fluids. Associated mineral deposition, dominated by quartz, aluminum silicates, and salts, may form in the vicinity of the intrusion. Comparison of the modeling and laboratory results with observed chemical composition of natural supercritical fluid discharges indicates that conductive heating and boiling of subcritical geothermal groundwater may indeed be the formation mechanism of such fluids observed for example at Krafla (Iceland), Menengai (Kenya), Los Humeros (Mexico), and Larderello (Italy) with an addition of volcanic gases in many cases. Metal and salt-rich supercritical fluids, for example, at Kakkonda (Japan), may also exist in geothermal systems. However, such supercritical fluids are considered to have been trapped upon crystallization of the magmatic intrusion.