Mechanisms of water incorporation into enstatite with Al substitution: Density functional theory-based analysis

Water (or, more precisely, hydrogen) has a profound influence on the physical properties of the Earth’s mantle, including its viscosity, melting temperature, seismic-wave propagation, and electrical conductivity. The primary hosts for water are nominally anhydrous minerals, including olivine, pyroxene, and garnet, which constitute the main mineral components of the upper mantle. To understand the effects of water on the physical properties of nominally anhydrous minerals, it is important to investigate the incorporation of hydrogen into these minerals. Orthopyroxene, a major constituent of upper mantle, can significantly influence mantle rheology, and its water solubility increases with increasing Al2O3 content. However, it remains unclear how Al2O3 alters the mechanism of hydrogen incorporation and affects water solubility. Here, we focused on the effects of Al2O3 on the mechanism of hydrogen incorporation into orthopyroxene under low-water conditions. The vibrational spectra of hydroxyl group in Al-free enstatite (the Mg end-member of orthopyroxene) and of Al-bearing enstatite were calculated from first-principles using density functional theory to identify plausible hydrogen-incorporation mechanisms. Stable hydrogen positions were constrained by comparing the calculated infrared spectra with the those obtained for samples recovered from high-pressure experiments. In the Al-free enstatite, the dominant hydrogen incorporating substitutions were (4H)SiX and (2H)MgX, whereas in the Al-bearing enstatite, the dominant substitutions were (Al+H)2MgX and (4H)SiX+(Al+H)SiX. Hydrogen was thus preferentially incorporated with Al3+ together. By comparing our results with previous experimental findings on water-saturated, high-water-content Al-bearing enstatites, we found that the dominant hydration mechanism differs between the low- and high-water conditions. At low water content, the dominant substitution is (Al+H)2MgX, whereas at high water content, the substitution (Al+H)SiX structurally destabilizes adjacent Si atoms, promotes extensive (4H)SiX substitution. This synergetic defect formation result in a sharp increase in water content. This likely explains why the incorporation of Al into enstatite substantially enhances its water solubility. Furthermore, the formation of these Al-assisted hydrogen defects, which introduce hydrated Si vacancies, provides an atomic-scale basis for understanding the influence of water on upper mantle rheology.