Potential mechanism for olivine to carry boron into the Earth's deep mantle

Olivine is an important component of Earth's upper mantle. The effect of boron on the properties of olivine has been previously ignored since boron is largely incompatible with olivine. Most recovered natural olivine samples have very low concentrations (<0.5 ppm) of boron.

However, olivines with very high boron (23–2100 ppm) have been found and forsterites enriched in boron (160–230 ppm) have been produced in wet conditions.

Dr. Joshua Muir, a postdoctoral researcher in the research team led by Prof. Zhang Feiwu from the Institute of Geochemistry of the Chinese Academy of Sciences (IGCAS) and their collaborators from Peking University investigated the stability of boron-hydrogen defects in olivine and predicted their effect on olivine properties.

Their results indicated that the compatibility of boron in olivine could be greatly increased by water through forming very stable [B-Si-H] complexes in the olivine lattice.

The study was published in Journal of Geophysical Research: Solid Earth on May 25.

The presence of H and B in olivine allowed the formation of B-H defects, which greatly increased the solubility of both B and H linearly in olivine regardless of pressure, temperature, SiO₂ activity, and Fe. This provided a potential mechanism for olivine to carry boron into the deep mantle.

Furthermore, the researchers revealed two stable B-H defects: an associated form (with bound H) stabled at lower temperature and a dissociated form (with unbound interstitial H) stabled at high temperature. The change in boron-hydrogen defect structure with temperature should affect the isotope partitioning by converting the favored boron coordination from 3-fold to 4-fold, which favors ¹⁰B over ¹¹B in the olivine. The isotopic fractionation of boron in olivine was suggested to be a strong function of temperature.

The researchers predicted a significant spike in H diffusion and conductivity in boron-enriched regions of the mantle through the production of interstitial hydrogen at high temperatures.