Geochemistry of supercritical fluids in active geothermal systems

Abstract

Supercritical fluids have been reported from both rift and subduction-related geothermal systems. They typically form in the vicinity of magmatic intrusions at the roots of geothermal systems upon conductive heating and boiling of the subcritical geothermal reservoir fluids to supercritical conditions and/or from gases released from the magmatic body. However, the origin and chemistry of these supercritical fluids are not yet fully understood as their chemical composition can be easily overprinted by mixing with subcritical reservoir fluids at lower temperatures and shallower depths. This study aims to link the hydrology in active intrusion-related geothermal systems with fluid chemistry and associated secondary mineralogy. The origin and formation of supercritical fluid were investigated by combining geochemical modeling and flow-through experiments and comparing modeling and experimental results with natural data reported from supercritical environments. Experimental and modeling work performed in this thesis revealed that supercritical fluids formed upon conductive heating and the boiling of subcritical geothermal reservoir fluids are characterized by low concentrations of non-volatile elements (Si, Na, K, Ca, Mg, Fe, Al, Cl) and similar concentrations of volatile elements as in the subcritical fluids (B, CO2, H2S). This process is predicted to be accompanied by mineral depositions dominated by silica, aluminum silicates and, in some cases, salts. Similar trends in fluid chemistry and mineralogy occur upon supercritical fluid formation in geothermal systems associated with rift and subduction zones. The results of the modeling and experiments compared well with the chemical composition for supercritical fluid discharges from the IDDP-1 well at Krafla (Iceland). Other geothermal systems where supercritical fluid temperatures have been reported and display similar chemical characteristics include Menengai (Kenya), Los Humeros (Mexico), Larderello (Italy), and The Geysers (USA).