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

Abstract

Calcite growth experiments have been performed in the presence of aqueous Mg at 25°C and 1bar pCO2 to quantify magnesium partition coefficient DMg=(Mg/Ca)solid(Mg/Ca)fluid and Mg isotope fractionation between calcite and reactive fluid (Δ26Mgcalcite–fluid) as a function of calcite precipitation rate rp (molm−2s−1). Mg partition coefficient, DMg, increases with calcite growth rate according to:LogDMg=0.2517(±0.0150)×Logrp+0.0944(±0.0182);R2=0.93,(10-8.3⩽rp⩽10-6.6molm-2s-1) Δ26Mgcalcite–fluid was found to depend heavily on calcite growth rate with preferential incorporation of 24Mg in calcite and the extent of isotope fractionation decreasing with increasing calcite growth rate in accord with:Δ26Mgcalcite-fluid=0.7918(±0.0452)×Logrp+3.2366(±0.3360);R2=0.97(10-8.3⩽rp⩽10-6.6molm-2s-1) The negative Δ26Mgcalcite–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 Δ26Mgcalcite–fluid value at 25°C derived from this linear extrapolation, Δ26Mgcalcite–fluid=−3.5±0.2‰ (2σ), is in good agreement with the theoretical value calculated by Rustad et al. (2010) for Mg-calcite (Δ26Mgcalcite–fluid=−3.6‰; BP86 functional). A striking feature of the results of this study is the decrease of the extent of Mg isotope fractionation (from −3.16‰ to −1.88‰) with the increase of calcite precipitation rate (from 10−8.3 to 10−6.6molm−2s−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.