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We employed a “three-isotope” method to evaluate the Mg isotope exchange kinetics and approach to Mg isotope equilibrium between epsomite and saturated MgSO₄ solutions at 7, 20, and 40 °C. Magnesium isotope exchange was promoted by a recrystallization process, where fine-grained epsomite seed crystals were suspended in saturated MgSO₄ solutions and the epsomite crystals recrystallized into coarser grains via Ostewald ripening. In these experiments the concentration of the MgSO₄ solution did not change with time indicating that there was no net mass transfer of Mg.

The three-isotope method involves isotope exchange between two components, one of which is enriched in one of the isotopes. At isotopic equilibrium, the two components will lie along a secondary mass fractionation line, whose position on a δ-δ plot is dictated by the isotopic mass balance of the two components. In our experiments we used an enriched ²⁵Mg isotope tracer, where a ²⁵Mg-enriched MgSO₄ solution was reacted with epsomite with “normal” isotope composition (A series of experiments), as well as the reverse, where the MgSO₄ solution with “normal” isotope composition was reacted with ²⁵Mg-enriched epsomite (B series of experiments). Each experiment used 10 to 20 centrifuge tubes of identically prepared mixtures of crystals and solution, where individual tubes were sacrificed and sampled for isotopic analysis for time-series sampling.

Figure 1 shows the results for an experiment conducted at 7°C. Two centrifuge tubes were sacrificed at 1, 3, 7, and 15 days providing duplicate measurements at each time point. After 15 days the aqueous Mg and epsomite each plotted on the secondary mass fractionation line indicating that complete isotope exchange had occurred between the two phases. The difference in δ²⁶Mg between the epsomite and saturated Mg solution is interpreted to represent the equilibrium fractionation factor. The equilibrium fractionation factor measured using the A-series and B-series experiments at 7, 20, and 40°C are plotted on an Arrenhius plot in Figure 2 shows the temperature dependence of the fractionation factor is 1000lnα_epsomite-solution] = 0.0510 ±0.0026×10⁶/T² using the measured fractionation factors and extrapolating to zero fractionation at infinite temperature. Because the temperature dependence is small (0.004 ‰/°C), the epsomite-fluid fractionation would be a poor isotope thermometer, but the magnitude of this fraction (Δ²⁶Mg_eps-sol = 0.59 ‰ at 20°C) is large enough to produce significant Mg isotope variations in evaporite sequences, depending on the degree of closed-system behavior. Magnesium isotopes, therefore, could be used as an indicator of evaporative processes.