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 236km 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 20km. Between 20 and 189km, δ²⁶Mg values remain nearly constant at -1.40‰, and a final sample at 236km 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 20km. This value remains unchanged for the next 169km 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.