Behavior of Mg isotopes during dedolomitization in the Madison Aquifer, South Dakota

To constrain how Mg isotopes behave during chemical interactions and physical transport in carbonate-rich settings, we measured δ26Mg values of surface water, groundwater, and dolomite samples from the Madison Aquifer, South Dakota. Groundwater in the Madison Aquifer chemically evolves by dedolomitization during transport along a 236 km flow path. Surface streams recharging the aquifer have δ26Mg values of − 1.08 and − 1.18‰. Following recharge, groundwater δ26Mg values vary between − 1.10 and − 1.63‰ up to a distance of 20 km. Between 20 and 189 km, δ26Mg values remain nearly constant at − 1.40‰, and a final sample at 236 km shows an increase to − 1.09‰. Dolomite exhibits a wide range of δ26Mg values between − 2.21 and − 1.27‰. Reactive-transport modeling and isotope mixing calculations employing previously published major ion mass-balances, 87Sr/86Sr ratios, and δ44Ca values were used to determine whether dedolomitization reactions, namely dolomite dissolution, calcite precipitation, and Mg-for-Na ion-exchange, fractionate Mg isotopes. We tentatively attribute the final δ26Mg value to preferential uptake of 24Mg during Mg-for-Na ion-exchange. Otherwise, we find little evidence of isotopic fractionation and observe instead that δ26Mg conservatively traces lithologic and hydrologic sources. Either isotope exchange between dolomite and water, with a fractionation factor of 0‰, or mixing between different water sources establishes the δ26Mg value of − 1.40‰ at 20 km. This value remains unchanged for the next 169 km of water transport because dolomite adds Mg with an average δ26Mg value near − 1.40‰, and no other processes cause fractionation. Calcite precipitation is unimportant either because calcite is not a significant sink for Mg or because Mg uptake during calcite precipitation under conditions of chemical equilibrium does not fractionate Mg isotopes. This study suggests Mg isotopes undergo conservative transport in carbonate-rich settings where waters are in chemical equilibrium with respect to major sources and sinks of Mg.