Kinetics of Mg partition and Mg stable isotope fractionation during its incorporation in calcite

Calcite growth experiments have been performed in the presence of aqueous Mg at 25 degrees C and 1 bar pCO(2) to quantify magnesium partition coefficient (D-Mg = (Mg/Ca)(solid)/(M-g/Ca)(fluid)) and Mg isotope fractionation between calcite and reactive fluid (Delta Mg-26(calcite-fluid)) as a function of calcite precipitation rate r(p) (mol m(-2) s(-1)). Mg partition coefficient, D-Mg, increases with calcite growth rate according to: Log D-Mg = 0.2517 (+/- 0.0150) x Log r(p) + 0.0944(+/- 0.0182); R-2 = 0.93, (10(-8.3) <= r(p) <= 10(-6.6) mol m(-2)s(-1)) Delta Mg-26(calcite-fluid) was found to depend heavily on calcite growth rate with preferential incorporation of Mg-24 in calcite and the extent of isotope fractionation decreasing with increasing calcite growth rate in accord with: The negative Delta Mg-26(calcite-fluid) values found in this study, with calcite overgrowths enriched in light Mg, are consistent with (i) recent experimental data on Mg isotope fractionation during low-Mg calcite homogeneous precipitation (Immenhauser et al., 2010) and magnesite growth (Pearce et al., 2012) and (ii) with theoretical values calculated for Mg-calcite by density-functional electronic structure models (Rustad et al., 2010). The deviation of the isotopic composition of precipitated Mg-calcite from the equilibrium mass fractionation line in a three isotope diagram is a linear function of calcite growth rate. The equilibrium Delta Mg-26(calcite-fluid) value at 25 degrees C derived from this linear extrapolation, Delta Mg-26(calcite-fluid) = -3.5 +/- 0.2 parts per thousand (2 sigma), is in good agreement with the theoretical value calculated by Rustad et al. (2010) for Mg-calcite (Delta Mg-26(calcite-fluid) = -3.6 parts per thousand; BP86 functional). A striking feature of the results of this study is the decrease of the extent of Mg isotope fractionation (from -3.16 parts per thousand to -1.88 parts per thousand) with the increase of calcite precipitation rate (from 10(-8.3) to 10(-6.6) mol m(-2)s(-1)) which is opposite to the variation of Ca, Ba and Sr isotope fractionation with calcite precipitation rate. This behavior likely stems from the strong free energy of hydration of the Mg2+ ion compared to Ca2+, Ba2+ and Sr2+ which leads, during fast calcite growth, to the entrapment in calcite overgrowths of hydrated Mg ions whose isotopic composition is close to that of aqueous Mg2+. The strong dependence of Mg isotope fractionation on calcite growth rate suggests that, using the three isotopes method, Mg isotopic signatures of calcite in association with those of other divalent metals (Zn2+, Cu2+) have the potential to reveal mineral precipitation rates and thus environmental conditions of the oceans over geological time. (C) 2013 Elsevier Ltd. All rights reserved.