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

Rūta Karolytė,Sascha Serno,Gareth Johnson,Stuart M.V Gilfillan

doi:10.1016/j.apgeochem.2017.06.012

Abstract

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 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 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.

Highlights

The stable isotope ratios in groundwater are useful indicators of a wide range of geological conditions associated with groundwater reservoirs and mineral springs
We have developed a simple geochemical modelling approach to study the influence of low temperature water-rock reactions on oxygen isotope changes in subsurface waters
Numerical modelling can be applied to assess the water-rock interaction influence on oxygen isotope ratios in other saline natural waters or CO2 storage sites where oxygen isotopes are used as natural tracer of the injected CO2 plume

Summary

Introduction

The stable isotope ratios in groundwater are useful indicators of a wide range of geological conditions associated with groundwater reservoirs and mineral springs. Applications include interpretation of water origin (Harris et al, 1997; Ziegler, 2006; Demlie and Titus, 2015), residence times (Vuataz and Goff, 1986; Hearn et al, 1989), migration pathways and mixing trends (Hearn et al, 1989; Siegel et al, 2004; Wilkinson et al, 2009; Delalande et al, 2011), temperature and circulation depths (Ziegler, 2006; Nelson et al, 2009), fault and fracture permeability to fluids (Ceron et al, 1998; Losh et al, 1999; Lions et al, 2014), local rainfall variations (Burns and Matter, 1995) and paleoclimates (Hays and Grossman, 1991) These geological interpretations rely on the identification of fundamental natural processes controlling kinetic and equilibrium fractionation of water stable oxygen and hydrogen isotopes (Fig. 1).

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