EXPERIMENTAL WATER-ROCK INTERACTION - SILICA GEOTHERMOMETERS IN THE TEMPERATURE-RANGE 100-350-DEGREES-C

The reliability of silica geothermometers for different natural mineral assemblages and fluid compositions bas been experimentally assessed in the P,T range 185-435 bar, 100-350-degrees-C using the Barnes hydrothermal kinetic apparatus. A series of metasediments (two phyllosilicate-rich schists and one dolomitic carbonate rock) of the Latium-Tuscany geothermal area (central Italy) and fluids of pure and sea-water compositions were used as starting materials. Experiments with the carbonate rock indicate that silica geothermometry is poorly applicable, probably because of the finite character of silica in the starting material. Experiments with feldspar-free, phyllosilicate-rich schists indicate that both the rock chemistry (mainly the redox ratio) and the fluid composition affect silica solubility at any given P-T. Oxidized assemblages in general favour higher silica solubilities. The effect of rock composition is greatly diminished when saline fluids are used. The observed solubility-temperature relationships are assessed against commonly used experimental and empirical geothermometers, giving a best approximation to the chalcedony geothermometer, followed by the quartz - no steam loss geothermometer. Three new silica geothermometers were calibrated : two for oxidized and reduced silicate assemblages and pure water fluids (EST-o and EST-r, respectively) and one for a variety of silicate assemblages interacting with saline (marine) fluids (EST-m). These geothermometers are effective for the whole experimentally investigated range (100-350-degrees-C). The new geothermometers have been tested for various geothermal areas and found to give accurate results. Combined equations allow a reliable chemical geothermometry over the temperature range 25-350-degrees-C, provided the redox conditions of the reservoir and the composition of the initial fluid are approximately known.