Understanding water–rock interaction in crystalline shield fluids using calcium isotopes

Abstract

Calcium isotopes provide a potentially robust tool for understanding the evolution of crystalline shield fluids, but previous applications have focused on near surface groundwaters. The tendency of Ca isotopes to be affected by mass-dependent fractionation during processes such as water–rock interaction provides a powerful tool for studying the evolution and origin of groundwaters hosted in crystalline rocks. We report Ca isotope ratios (δ^44/40Ca) of deep fluids (>300 m) from across the Canadian Shield and integrate these with Sr and Br isotope values to understand long-term fluid–rock interactions in crystalline shield environments. Ca isotope ratios have a wide range of values from 0.07‰ to 0.86‰. At individual sites, δ^44/40Ca values are variable whereas ⁸⁷Sr/⁸⁶Sr ratios are relatively constant. Sr isotope ratios (⁸⁷Sr/⁸⁶Sr) have a negative relationship with the Ca vs. Na content of the fluid indicating different host-minerals contributing Sr to the fluid. Ca isotope fractionation was caused by metamorphic reactions and by the growth and dissolution of Ca-rich fracture-filling minerals. The δ^44/40Ca signatures of these processes are overprinted by radiogenic ingrowth of ⁴⁰Ca by decay of ⁴⁰K, which is expected to affect older and more K-rich rocks. At one site, δ^44/40Ca and δ⁸¹Br variability reflects gas-generating reactions in the fluid and/or water–rock interaction processes. These new results demonstrate the strength of combining multiple isotope analysis to elucidate the sources of groundwater salinity and decipher the complex long-term processes that occur in crystalline shield environments.