矿床地球化学国家重点实验室知识库

Equilibrium Mg isotope fractionation properties between aqueous Mg²⁺ and minerals are the key for the applications of Mg isotopes in geochemistry. This study conducts first-principles molecular dynamics (FPMD) simulations for aqueous Mg²⁺ based on the density functional theory (DFT). Thirty-five snapshots are extracted from the FPMD trajectories after equilibration for the calculations of the reduced partition function ratio (β factor or 10³lnβ). Combining with the β factors of minerals, we found that the β factor decreases in the sequence of lizardite > brucite > aqueous Mg²⁺ > dolomite > magnesite > calcite > aragonite. Our calculations also confirm the effect of Mg concentration on the β factor of calcite, and further show that the concentration effect is negligible when Mg/(Mg + Ca) (atomic ratio hereafter) is lower than 1/32.

Our results depict significant equilibrium Mg isotope fractionations between minerals and aqueous Mg²⁺ (10³lnα_{minerals-Mg_aq}), which are dominantly controlled by the average force constant of Mg in these phases. Compared to aqueous Mg²⁺, carbonates are enriched in light Mg isotopes but brucite and lizardite show enrichment in heavy Mg isotopes. Among all minerals, 10³lnα_{aragonite-Mg_aq} is the largest, which is up to −14‰ at 300 K. Notably, 10³lnα_{calcite-Mg_aq} is not only controlled by temperature, but also significantly affected by Mg content in calcite. 10³lnα_{calcite-Mg_aq} is negatively correlated with Mg content in calcite. The experimentally measured Mg isotope fractionations between minerals (dolomite, magnesite, brucite) and solution at equilibrium are consistent with our predicted results, showing the accurate description of FPMD simulations for aqueous Mg²⁺ and the reliability of our calculations. Overall, the calculated equilibrium Mg isotope fractionations between minerals and aqueous Mg²⁺ provide a guideline for applications of Mg isotopes in aqueous geochemical processes.