Experimentally reversed quartz solubilities at 250°C and at 250, 500 and 1000 bars yield values of the logarithm of the molality of aqueous silica of -2.126, -2.087 and -2.038, respectively. Extrapolation of quartz solubility to the saturation pressure of water at 250°C results in a log molality of aqueous silica of-2.168. These solubility determinations and analyses of fluid pressures in geothermal systems indicate that pressure is significant when calculating quartz equilibrium temperatures from silica concentrations in waters of deep thermal reservoirs. The results of this investigation, combined with other reported quartz solubility measurements, yielded a pressure-sensitive "silica geothermometer" for fluids that have undergone adiabatic steam loss of t°C = 874 - 0.156P (log mSi(OH)4 · 2H2O)2 + 411 log mSi(OH4 · 2H2O + 51 (log mSi(OH)4 · 2H2O)2 where P is the fluid pressure in bars and mSi(OH)4 · 2H2O represents the molality of aqueous silica measured in surface samples. The geothermometer is applicable to solutions in equilibrium with quartz from 180°C to 340°C and fluid pressures from H2O saturation to 500 bars.
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Acknowledgements-The research reported here represents part of the senior author’s Ph.D. dissertation at North-western University. We wish to thank Drs. S. Arnorsson, R. 0. Fournier and an anonymous reviewer for critically reviewing the paper. We would like to acknowledge Cheril Cheverton for drafting figures and especially for her limitless love and support. This project was supported by NSF grants EAR79-04892 and EAR80-24146 and the Margaret Fuller Boos fellowship fund.