Experimental calibration of Mg isotope fractionation between dolomite and aqueous solution and its geological implications

Hydrothermal experiments at 220, 160, and 130 °C were performed to calibrate the Mg isotope fractionation factor between dolomite and aqueous Mg. Hydrothermal experiments included synthesis of dolomite using different starting materials, as well as exchange experiments that used poorly-ordered proto-dolomite. The morphology of synthesized dolomite was dependent on starting mineralogy, suggesting that dolomite was synthesized by different pathways. Hydrothermally synthesized dolomite was initially fine-grained disordered or poorly-ordered dolomite that, with time, recrystallized to coarser-grained ordered dolomite. Isotopic exchange was monitored using 87Sr/86Sr ratios and 25Mg tracers, and these indicated near-complete isotope exchange between dolomite and aqueous solutions at the end of most hydrothermal experiments. The Mg isotope fractionation factor between dolomite and aqueous solution obtained from synthesis and exchange experiments converged with time and was independent of dolomite morphology, suggesting attainment of isotopic equilibrium. Combining results from synthesis and exchange experiments, the temperature dependent Mg isotope fractionation factor for ordered dolomite is:Δ26Mgdolo-aq=-0.1554(±0.0096)×106/T2where T is in Kelvin. In contrast, poorly-ordered dolomite has a Δ26Mgdolo-aq fractionation factor that is up to 0.25‰ lower than that of ordered dolomite, and this is attributed to longer Mg-O bonds in imperfectly ordered dolomite. The experimentally calibrated Δ26Mgdolo-aq fractionation factors lie between those calculated by Schauble (2011) and Rustad et al. (2010). The Δ26Mgdolo-aq fractionation factor extrapolated to lower temperatures using the Δ26Mg-T function of this study matches the Δ26Mgdolo-aq fractionation factor obtained by modeling of Mg isotope compositions of ODP drill core samples. This study shows that significant Mg isotope fractionation occurs during dolomite precipitation. These results collectively demonstrate that Mg isotopes in dolomite are a useful tool for studying Mg global cycling and dolomitization.