Mineral saturation versus temperature for a geothermal water sample from Surprise Valley, California

Computed mineral saturation indices (log(Q/K)) as a function of temperature for a geothermal water sample from Surprise Valley, California, after correction for CO₂ loss due to degassing. Note that the mineral saturation points converge at 220 °C (428 °F) with a very small error of +/- 4 °C (7 °F). In contrast, classical geothermometers (in box at lower left) do not agree and range over 50 °C (90 °F) average 198 °C (388 °F) with an error of +/- 22 °C (40 °F), which calls into question both the accuracy and precision of the conventional geothermometers.  Adapted from Spycher and others, 2016 (https://pangea.stanford.edu/ERE/pdf/IGAstandard/SGW/2016/Spycher.pdf).