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 delta Mg-26 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 delta Mg-26 values of - 1.08 and - 1.18 parts per thousand. Following recharge, groundwater delta Mg-26 values vary between - 1.10 and - 1.63 parts per thousand up to a distance of 20 km. Between 20 and 189 km, delta Mg-26 values remain nearly constant at - 1.40 parts per thousand, and a final sample at 236 km shows an increase to - 1.09 parts per thousand. Dolomite exhibits a wide range of delta Mg-26 values between - 2.21 and - 1.27 parts per thousand. Reactive-transport modeling and isotope mixing calculations employing previously published major ion mass-balances, Sr-87/Sr-86 ratios, and delta Ca-44 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 delta Mg-26 value to preferential uptake of Mg-24 during Mg-for-Na ion-exchange. Otherwise, we find little evidence of isotopic fractionation and observe instead that delta Mg-26 conservatively traces lithologic and hydrologic sources. Either isotope exchange between dolomite and water, with a fractionation factor of 0 parts per thousand, or mixing between different water sources establishes the delta Mg-26 value of - 1.40 parts per thousand at 20 km. This value remains unchanged for the next 169 km of water transport because dolomite adds Mg with an average delta Mg-26 value near - 1.40 parts per thousand, 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. (C) 2010 Elsevier B.V. All rights reserved.