Water in the Earth's mantle: a solid-state NMR study of hydrous wadsleyite

Abstract

Wadsleyite, β-(Mg,Fe)₂SiO₄, is the main component of the transition zone in the Earth's mantle, at depths of 410–530 km below the surface. This mineral has received considerable interest as a potential reservoir for the vast amount of hydrogen, as hydroxyl, referred to as water, that is thought to be contained within the mantle. However, the exact way in which water is incorporated into the structure of wadsleyite is not fully understood and has been the subject of considerable debate. In this work, ¹⁷O, ²⁵Mg, ²⁹Si, ¹H and ²H solid-state NMR spectra were obtained from isotopically enriched samples of anhydrous and hydrous β-Mg₂SiO₄. First-principles DFT calculations were also carried out for a range of model structures to aid interpretation of the experimental data. The results are consistent with a model for hydrous wadsleyite whereby hydrogen bonds to the O1 site to form hydroxyl groups that are charge balanced by cation vacancies on the Mg3 site. Structural models containing cation vacancies on the Mg2 site are found to be energetically less favourable and calculated NMR parameters show poor agreement with the experimental data. Disorder was also observed in the hydrous wadsleyite samples, and ¹H and ²H NMR are consistent with not only Mg–O1–H but also more strongly hydrogen-bonded Si–O–H environments. These silanol protons can be incorporated into the structure with only a small increase in energy. Two-dimensional ¹H–²⁹Si and ¹H–¹⁷O NMR correlation experiments confirm that the additional resonances do not correspond to Mg–OH protons and enable the identification of ²⁹Si and ¹⁷O species within the Si–OH groups. This assignment is also confirmed by first-principles DFT calculations of NMR parameters. Silanol protons within Mg3 vacancies could account for up to 20% of the protons in the structure.