Fitting Iron and Magnesium into the Hydrothermal Chlorite Geothermometer

Two versions of a useful chlorite geothermometer calibrated in the Los Azufres and Salton Sea geothermal fields have been published. The first applies the linear correlation between measured tetrahedral site aluminum (AlIVM) in the chlorites and the measured in situ temperatures. In the second, a correction for Fe/(Fe+Mg) is made before applying the AlIV-T°C correlation. However, while Fe and Mg do affect AlIVM independently of temperature and should be incorporated, the published Fe/(Fe+Mg) normalization was not isothermal. For an isothermal correction, the field data are contoured for temperature in AlIVM – Fe/(Fe+Mg) space, with the resulting isotherms being linear with consistent negative slopes. Except for two Salton Sea temperatures, data from both sets are compatible, supporting the isothermal contours. For structural formulae based on 14 oxygens, normalizing along the isotherms to Fe/(Fe+Mg) = 0 yields: AlIVC = AlIVM + 0.10 [Fe/(Fe+Mg)], and T°C = 318.5 AlIVC - 68.7. Temperatures from this isothermal Fe-Mg modified chlorite geothermometer are compatible with independent temperature estimates in ore deposits and with one geothermal field. Within its limits, it provides an alternative to existing theoretical and field-based versions. Detailed petrography for composition and paragenesis is necessary before the chlorite geothermometer can be applied with confidence.