The role of low-temperature 18O exchange in the isotopic evolution of deep subsurface fluids

Abstract

Ca-Na-Cl fluids with high concentrations of dissolved reduced gases reside within fractures in crystalline Precambrian rocks around the world, and have been most intensively studied within South Africa, Fennoscandia and the Canadian Shield. In contrast to surface waters, shallow groundwaters, sedimentary basin brines and metamorphic fluids, the δ18O and δ2H values for these Ca-Na-Cl fluids typically plot to the left/above of the Global Meteoric Water Line (GMWL). To date, most interpretive frameworks for these fracture fluids have focused on their production via water-rock alteration reactions that affect both δ18O and δ2H values, resulting in co-variation of water isotope values above the GMWL. Such alteration processes include silicate hydration coupled with formation of secondary minerals, radiolytic H2 formation, isotopic exchange with a H2-rich gas, or isotope exchange with O and/or H-bearing minerals. This study presents the first compiled global isotopic dataset for these types of fluids, integrating a large amount of unpublished data with the previously published literature in order to investigate these fracture fluid systems on a global scale. Importantly this global perspective allows differentiation between fluids impacted by late-stage mixing with meteoric waters, from fluids that reflect the most saline end-members stored in the host rocks in hydrogeologic isolation from the surface hydrologic cycle. The most saline fluids are shown to occupy a more restricted range of δ18O-δ2H space than previously recognised, with end-member fluids from all of the Precambrian rock settings investigated occupying a range of δ2H δ18O isotope space within which there is no co-variation in these values. These findings suggest a set of common processes may define the isotopic signatures of these most saline end-member fluids in Precambrian settings around the world – creating common signatures identifiable in these fluids, despite differences in geologic setting. This study identifies the important role of oxygen isotopic exchange between primary fluids (associated with hydrothermal/metamorphic activity) and the host rocks, taking place under low temperature, low-volume, water-rock ratios over long (Ma) geologic timescales. This process results in progressive 18O depletion in the fluids over time, while δ2H values remain less affected. For each site the specific isotopic signature of the fracture fluid end-member depends on initial hydrothermal/metamorphic fluid composition, rates of isotopic exchange, water-to-rock ratios, and in-situ residence times. We suggest the often-observed co-variation in both δ18O-δ2H above the GMWL primarily results from late stage mixing of the fracture fluid end-members with (paleo)-meteoric water, resulting in isotopic regression back towards the GMWL, with end-points defined by the local meteoric-climatic conditions for each site.