The influence of oxygen isotope exchange between CO2 and H2O in natural CO2-rich spring waters : implications for geothermometry

Oxygen isotope ratio (δ18O) value deviations from the Meteoric Water Line with no significant change in the hydrogen isotope (δ2H) composition have been reported in naturally occurring CO2-rich waters from around the world. Here we review the effects of oxygen isotope exchange with CO2, high temperature equilibration with bedrock minerals and mineral dissolution and precipitation reactions on the CO2-rich water isotopic composition. We present two case studies from Daylesford (Australia) and Pah Tempe (Utah, USA) mineral springs, where we use a numerical geochemical modelling approach to resolve the influence of low temperature water-rock interactions and CO2 equilibration on the observed oxygen isotope ranges observed in the mineral waters. In both cases, we find that mineral dissolution – precipitation reactions are unlikely to have a significant effect on the groundwater isotopic compositions, and that the observed δ18O values in natural CO2 springs can be simply explained by equilibrium fractionation between water and free phase CO2. Traditionally, the interaction of CO2 and water in a natural CO2-rich groundwater setting has only been associated with water 18O depletion and this is the first study to consider 18O enrichment. We establish that in a natural setting, CO2 and water equilibration can result in water 18O depletion or enrichment, and that the change in the oxygen isotope composition ultimately depends on the initial CO2 and water δ18O values. Our new conceptual model therefore provides a mechanism to explain water 18O enrichment at ambient temperatures. This finding is critical for the use of δ18O in groundwater geothermometry and for the interpretation of natural water circulation depths: we argue that in some cases, natural waters previously interpreted as geothermal based on their oxygen isotope composition may actually have acquired their isotopic signature through interaction with CO2 at ambient temperatures