An investigation on the effects of continental weathering on magnesium isotope systematics in various hydrologic settings

Magnesium isotopic compositions of water and bedrock samples from distinctive silicate and carbonate settings were analyzed by high-precision MC-ICPMS to better understand how Mg isotopes behave during continental weathering. Stable isotope ratios in rivers are controlled by mixtures of waters with distinct isotopic compositions that are primarily controlled by bedrock compositions, but also by fractionation during weathering reactions. Terrestrial delta 26Mg values are highly heterogeneous, but demonstrate distinct ranges based on rock composition. The terrestrial samples for this study have delta 26Mg values that range from -2.85 to +0.12‰ and have an average of -1.27‰. Silicate rocks display a limited range from -0.80 to +0.12‰, while carbonate rocks are much lighter and have a broader range from -2.85 to -1.82‰. As weathering progresses, Mg isotopes are fractionated, mobilizing light Mg to the hydrosphere and leaving the heavy Mg in the residual weathered products. Water samples from all terranes display similar heterogeneity with a delta26Mg range from -2.02 to +0.42‰. Waters draining silicate rocks generally display delta26Mg values lighter than the silicate rocks. For waters in a carbonate setting, the water/rock delta26Mg relationship is opposite of that observed in a silicate terrane. The carbonate waters have delta26Mg values, on average 0.73‰ heavier than the carbonate lithology.