Extremely Stable, Highly Conductive Boron-Hydrogen Complexes in Forsterite and Olivine

Boron is an important tracer in the mantle but its fate during deep subduction is poorly constrained. In this work we use density functional theory and a thermodynamic model to study the stability of boron-hydrogen defects in forsterite and olivine and predict their effect on olivine properties. In the absence of water/hydrogen we predict that in all conditions B3+ replaces Si4+ charge balanced by an oxygen vacancy and thus pure boron increases oxygen diffusion rates. In the presence of hydrogen two boron-hydrogen defects are found to be stable, an associated boron-silicon hydrate (B-Si-H) with planar BO3 at low temperatures and an attached hydrogen and a dissociated B-Si-H with tetrahedral BO4 and interstitial free hydrogen at high temperatures. These B-Si-H complexes are extremely stable compared to non-hydrous boron and form to the exclusion of all other water defects regardless of pressure, temperature, SiO2 activity and Fe# thus forsterite and olivine should behave identically. The stability of B-Si-H defects against pressure and temperature suggests that dissolved boron will remain dissolved deep into the mantle and that boron should induce a near linear increase in the solubility of water in olivine. We predict that at high temperatures water associated with boron in olivine will not have FTIR signals in the standard measurement range 3,000-3,600 cm(-1) and that instead a large rise in conductivity will be seen. The change in the coordination of boron in B-Si-H with temperature will impact upon its fractionation.

Automated summary

In this study, the fate of boron during deep subduction was investigated using density functional theory and a thermodynamic model. The results show that boron-hydrogen defects have a significant impact on the stability and properties of olivine, and may influence water solubility and conductivity.