The pH values of subsurface hydrothermal fluids remain controversial and largely unknown, because of the difficulties of measuring them in situ. This paper presents a new method for evaluating the pH of hydrothermal fluids from analyses of halogen concentrations in hydrous minerals. This method is general and can be used to study water-rock interactions in a variety of geologic environments. An example of the method's application to the Salton Sea geothermal systems, California, is given.

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