CALIBRATION AND APPLICATION OF A SILICA-WATER SINGLE MINERAL THERMOMETER TO GEOTHERMAL SYSTEMS IN ICELAND AND CHILE

Triple oxygen isotope values of silica samples and formation water with tight temperature constraints from the Hellisheiði power plant in southwest Iceland and the natural Puchuldiza siliceous hot springs in northern Chile were used to evaluate potential fractionation effects of biogenic vs. abiogenic samples and silica samples of different crystallinity. Temperature estimates from the Hellisheiði power plant based on silica-water oxygen isotope thermometry are in excellent agreement with samples with measured temperatures, and lower for samples from within the heat exchanger where temperatures can only be estimated. The calculated θ values from this study are in close agreement with the calculated θ values from other studies and indicate a 0.00001 change in θ per degree Celsius at ~100°C. In a real-time silica precipitation experiment at the hot springs in Puchuldiza of northern Chile, silica only precipitated at the air-water interface. Amorphous silica and coexisting waters were collected in active hot springs ranging in temperatures from 63-84°C. In all cases, the calculated temperatures were less than the measured temperatures, ranging from 38-48°C. Recrystallized paleosinter record higher temperatures, 69.5°C and 89°C. All samples appear to be in equilibrium in the triple oxygen isotope system. We interpret all samples as recording the temperatures of their formation. The poorly crystalline modern samples likely precipitated at the air-water interface while the water was cooling, reaching saturation with respect to silica and preserving temperatures that are less than the measured geothermal water temperatures. In contrast, the more crystalline paleosinter samples record the temperature of hydrothermal recrystallization below the air-water interface. Silica re-precipitating from dissolved paleosinter in the presence of acidic fumarolic steam was not in equilibrium with the condensed steam water on the basis of the D'17O values, which was not indicated by the d