Mechanisms controlling the Mg isotope composition of hydromagnesite-magnesite playas near Atlin, British Columbia, Canada

The alkaline playas near Atlin, British Columbia, Canada are likely one of the few surface environments on Earth where contemporaneous formation hydromagnesite and magnesite occurs at temperatures that do not exceed 15 °C. This environment offers a unique opportunity to examine impact different mechanisms Mg isotope compositions Mg-carbonate minerals low temperature. In this study, we report composition ultramafic bedrock, sediments, both ground waters in geological setting. suggests Rayleigh-type distillation effect fluid ratios unsaturated sediment above water table. Through mechanism formation, is progressively depleted 24Mg obtaining ?26Mg values as high +1.14‰ surface. contrast, characterized by enrichment solid phase 24Mg. apparent fractionation factor during ~10 °C ranges between -0.7 ± 0.1‰ -1.8 0.1‰. distinct comparison supports occurring precipitation from fluid, or dissolution-reprecipitation, rather than solid-phase transformation hydrous precursor. Overall, results provide insights temperature mineral has implications for long-term storage CO2.