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To determine the baseline Mg isotope fractionation factor for abiogenic carbonate precipitation, thirty free-drift synthesis experiments of carbonate were carried out at temperatures between 4 °C and 45 °C, using solutions with Mg:Ca molar ratio between 3:1 and 13:1, buffered at PCO₂ between 0.038% and 3%. Pure calcite seed crystals were used to promote heterogeneous growth of Mg-bearing calcite solid form solution and to reduce kinetic isotope effects associated with nucleation and rapid crystallization. Under such conditions, calcite overgrowths (confirmed by XRD) that contained 1.28-14.9 mole percent Mg precipitated onto the seed crystals over 1 to 58 days. The Mg isotope composition of Mg-bearing calcite and Mg-Ca solutions were measured to a precision of ±0.15‰ (2 standard deviation in 26Mg/24Mg) after careful purification of Mg from these Ca-rich materials using multiple ion-exchange separation techniques. The measured Mg isotope fractionation factors between Mg-calcite and solution show a systematic temperature dependence, changing from -2.26 ‰ at 45 °C to -2.76 ‰ at 4 °C in ²⁶Mg/²⁴Mg (Figure 1).

The fractionation factors are not correlated with experimental settings, Mg content of the Mg-calcite overgrowth, PCO₂, or the composition of the Mg-Ca solution. This study proposes a temperature-dependent function for Mg isotope fractionation during precipitation of inorganic Mg-bearing calcite: Δ²⁶/²⁴Mg_cal-sol = -2.82(±0.11) + 0.0119(±0.0038)×T, where T is temperature in Celsius.

Speleothem studies define similar temperature dependence for Mg isotope fractionation factor between Mg-calcite and companion aqueous Mg drip waters assuming an average speleothem formation occurred at the average annual cave temperature (Figure 1). In contrast to the excellent agreement between natural samples and our experiments, calculated calcite-solution Mg isotope fractionation factors do not agree, where calculations predict a larger magnitude fractionation as compared to experimental results (Figure 2).

It is uncertain if these differences reflect scaling problems between the calculated fluid and solid beta values (e.g. Beard et al., 2010), inaccuracies associated with difficulties in modeling of the hydration sphere of the aqueous cation (Rustad et al., 2010), or if the assumed crystallographic model for calcite with an infinitesimal amount of Mg does not accurately reflect natural lattice settings. Alternatively, this discrepancy may reflect experimental kinetic isotope effects associated with rapid precipitation. However, because our experimental study employed calcite seed crystals to decrease precipitation rates by minimizing possible crystallization nucleation problems and the fact that our experiments are in excellent agreement with natural speleothem studies, we believe that the experimental results accurately reflect Mg isotope fractionation associated with abiogenic precipitation of Mg-bearing calcite.