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Over the last decade experimental studies have shown a large B isotopefractionation between materials carrying boron incorporated in trigonally andtetrahedrally coordinated sites, but the mechanisms responsible for producingthe observed isotopic signatures are poorly known. In order to understand theboron isotope fractionation processes and to obtain a better interpretation ofthe experimental data and isotopic signatures observed in natural samples, weuse first principles calculations based on density functional theory inconjunction with ab initio molecular dynamics and a new pseudofrequencyanalysis method to investigate the B isotope fractionation between B-bearingminerals (such as tourmaline and micas) and aqueous fluids containing H_3BO_3and H_4BO_4- species. We confirm the experimental finding that the isotopefractionation is mainly driven by the coordination of the fractionating boronatoms and have found in addition that the strength of the produced isotopicsignature is strongly correlated with the B-O bond length. We also demonstratethe ability of our computational scheme to predict the isotopic signatures offluids at extreme pressures by showing the consistency of computedpressure-dependent beta factors with the measured pressure shifts of the B-Ovibrational frequencies of H_3BO_3 and H_4BO_4- in aqueous fluid. Thecomparison of the predicted with measured fractionation factors betweenboromuscovite and neutral fluid confirms the existence of the admixture oftetrahedral boron species in neutral fluid at high P and T foundexperimentally, which also explains the inconsistency between the variousmeasurements on the tourmaline-mica system reported in the literature. Ourinvestigation shows that the calculated equilibrium isotope fractionationfactors have an accuracy comparable to the experiments.

Ab initio prediction of equilibrium boron isotope fractionation between minerals and aqueous fluids at high P and T