What do aqueous geothermometers really tell us?

The application of chemical geothermometry to shallow groundwaters or spring discharge assumes that there is minimal mixing or re‐equilibration of water as it travels from depth to the surface. In this study, we examine the potential for mixing and re‐equilibration by examining heat and fluid flow along crustal‐scale faults in tectonic geothermal systems. Numerical modeling results indicate that maximum in situ temperatures could be under‐predicted by up to 30% due to mixing of fluids that enter the fault at different depths. This, coupled with the depression of isotherms by downward groundwater flow in the hanging wall, could cause underestimates of maximum circulation depth of greater than 80% in extreme cases. Kinetics does not favor re‐equilibration in the shallower portions of faults due to low temperatures and higher fluid velocities. However, in areas of deeper circulation or higher heat flow such reactions are possible.