  Over the last decade experimental studies have shown a large B isotope
fractionation between materials carrying boron incorporated in trigonally and
tetrahedrally coordinated sites, but the mechanisms responsible for producing
the observed isotopic signatures are poorly known. In order to understand the
boron isotope fractionation processes and to obtain a better interpretation of
the experimental data and isotopic signatures observed in natural samples, we
use first principles calculations based on density functional theory in
conjunction with ab initio molecular dynamics and a new pseudofrequency
analysis method to investigate the B isotope fractionation between B-bearing
minerals (such as tourmaline and micas) and aqueous fluids containing H_3BO_3
and H_4BO_4- species. We confirm the experimental finding that the isotope
fractionation is mainly driven by the coordination of the fractionating boron
atoms and have found in addition that the strength of the produced isotopic
signature is strongly correlated with the B-O bond length. We also demonstrate
the ability of our computational scheme to predict the isotopic signatures of
fluids at extreme pressures by showing the consistency of computed
pressure-dependent beta factors with the measured pressure shifts of the B-O
vibrational frequencies of H_3BO_3 and H_4BO_4- in aqueous fluid. The
comparison of the predicted with measured fractionation factors between
boromuscovite and neutral fluid confirms the existence of the admixture of
tetrahedral boron species in neutral fluid at high P and T found
experimentally, which also explains the inconsistency between the various
measurements on the tourmaline-mica system reported in the literature. Our
investigation shows that the calculated equilibrium isotope fractionation
factors have an accuracy comparable to the experiments.
