The Deep Fault Drilling Project borehole. Credit: John Townend, Victoria University, NZ The Deep Fault Drilling Project borehole. The tea...
The influence of oxygen isotope exchange between CO 2 and H 2 O in natural CO 2 -rich spring waters: Implications for geothermometry
- Citation data:
Applied Geochemistry, ISSN: 0883-2927, Vol: 84, Page: 173-186
- Publication Year:
- Environmental Science; Earth and Planetary Sciences
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Oxygen isotope ratio (δ 18 O) value deviations from the Meteoric Water Line with no significant change in the hydrogen isotope (δ 2 H) composition have been reported in naturally occurring CO 2 -rich waters from around the world. Here we review the effects of oxygen isotope exchange with CO 2, high temperature equilibration with bedrock minerals and mineral dissolution and precipitation reactions on the CO 2 -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 CO 2 equilibration to the 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 δ 18 O values in natural CO 2 springs can be simply explained by equilibrium fractionation between water and free phase CO 2. Traditionally, the interaction of CO 2 and water in a natural CO 2 -rich groundwater setting has only been associated with water 18 O depletion and this is the first study to consider 18 O enrichment. We establish that in a natural setting, CO 2 and water equilibration can result in water 18 O depletion or enrichment, and that the change in the oxygen isotope composition ultimately depends on the initial CO 2 and water δ 18 O values. Our new conceptual model therefore provides a mechanism to explain water 18 O enrichment at ambient temperatures. This finding is critical for the use of δ 18 O 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 CO 2 at ambient temperatures.