We propose a new geothermometer (TCO2 Flux) based on soil diffuse CO2 flux and shallow temperature measurements made on areas of steam heated, thermally altered ground above active geothermal systems. The geothermometer is based on a previous gas (CO2) geothermometer. TCO2 Flux provides an additional exploration tool for estimating subsurface temperatures in high-temperature geothermal systems. The spatial distribution of geothermometry estimates matches the location of major upflow zones previously reported at the Rotokawa (New Zealand) and Wairakei (New Zealand) geothermal systems. Mean TCO2 Flux estimates fall within the range of deep drill hole temperatures at Wairakei, Rotokawa, Tauhara (New Zealand), Ohaaki (New Zealand), Reykjanes (Iceland) and Copahue (Argentina). TCO2 Flux was also evaluated at White Island (New Zealand) and Reporoa (New Zealand), where limited sub-surface data exists. Mode TCO2 Flux at White Island is the highest of the systems considered in this study (320 °C). However, the geothermometer relies on mineral-water equilibrium in neutral reservoir fluids, and this assumption would be violated in such an active and acidic environment. Mean TCO2 Flux at Reporoa (310 °C) is high, which suggests Reporoa is a separate system with a separate upflow from the nearby Waiotapu geothermal system.

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