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

Abstract

Olivine is an important component of Earth's upper mantle and one that controls many of its properties. The presence of extrinsic defects/elements can affect these properties, however, and thus these need to be understood. One element that could be present on defective sites and that could have large effects is boron. The effect of boron on the properties of olivine has been previously ignored due to the fact that boron is largely incompatible in olivine. Most recovered/natural olivine samples have very low concentrations (<0.5 wt ppm) of boron (Kent & Rossman, 2002;Ottolini et al., 2004) and the general boron concentration of the bulk silicate earth (<0.19) and primitive (<0.19 wt. ppm) and depleted mantle (<0.07 wt. ppm) is low (Marschall et al., 2017). Boron partitions heavily from olivine into melts (K ∼ 0.001-0.05) (Ottolini et al., 2009) and so partial melting will deplete olivine of boron. Despite this olivines with very high boron concentrations (23-2,100 wt. ppm) have been found (

Mantle wedge olivine modifies slab-derived fluids: Implications for fluid transport from slab to arc magma source

“…The difference in  11 B between cores and rims/neoblasts would be consistent with a difference in depth of several tens of kilometers (Scambelluri and Tonarini, 2012). Antigorite has somewhat higher  11 B than olivine rims, which may suggest that it formed at even shallower depth, but equilibrium B isotope fractionation between antigorite and olivine is uncertain due to their poorly constrained site occupations (Muir et al, 2022).…”

The Oxidation States of Iron in Dry and Wet Olivine: A Thermodynamic Model

“…Reaction 18 produces $${\left\{{\text{Fe}}_{\text{Mg}}^{\cdot }{(3\mathrm{H})}_{\text{Si}}^{\prime }\right\}}^{\times }$$ and Equation 19 is its dissociation reaction while Equation 20 produces a Ferric iron on a silicon site with a hydrogen interstitial. Attempting to place the Ferric iron and the hydrogen on the same Si site in the manner of boron (Muir, Chen, et al., 2022) produced a very unstable structure due to the size of the iron atom. Generally our model predicts $${\left\{{\text{Fe}}_{\text{Mg}}^{\cdot }{\mathrm{H}}_{\text{Mg}}^{\prime }\right\}}^{\times }$$ to be the most favorable species but the other iron hydrogen species can be produced in non‐negligible quantities particularly as the pressure increases.…”

“…Si } × and Equation 19 is its dissociation reaction while Equation 20 produces a Ferric iron on a silicon site with a hydrogen interstitial. Attempting to place the Ferric iron and the hydrogen on the same Si site in the manner of boron (Muir, Chen, et al, 2022) produced a very unstable structure due to the size of the iron atom. Generally our model…”