Mg isotope fractionation during calcite precipitation: An experimental study

Abstract

Experimental precipitations of calcite and other carbonate minerals were performed under various conditions of pH, temperature and solution Mg/Ca to determine the Mg partition coefficient and Mg isotope fractionation. Fifteen experiments were performed at pH ranging from 7.41±0.07 to 8.51±0.39, temperature ranging from 16.2±0.7 to 26.5±0.3°C and Mg/Casolution ranging from 0.11 to 0.52mol/mol. The apparent Mg partition coefficient between calcite and solution (DMg) spans a large range of values from 0.018±0.014 to 0.15±0.11 and carbonate Mg isotope fractionation (Δ26Mg) ranges from −2.53±0.25‰ to −1.33±0.14‰ and does not correlate with either pH or temperature. The range in DMg and Δ26Mg suggests non-equilibrium partitioning controlled by the processes of calcite growth, i.e. mixing between calcite grown at equilibrium and fluid inclusions, and entrapment of a surface Mg-rich calcite layer in isotopic equilibrium with the solution. The equilibrium Mg isotope fractionation between inorganic calcite and solution is estimated to be −2.13±0.24‰. Additional Mg elemental and isotopic fractionations are observed to occur during biogenic formation of calcite due to variable removal of Mg by the organisms (high-Mg calcite corals, foraminifera) of seawater Mg from their calcification medium.