Speciation and Effect of Water on Hydrous Peridotitic Glasses Quenched by a Novel Rapid-Quench Multi-Anvil Technique

The dissolution of water into the melt changes its chemical and physical properties. The effect of water dissolution depends on the bulk H2O content and dissolution mechanisms. Previous research mainly focused on relatively polymerized glasses, whereas water dissolution mechanisms in highly depolymerized glasses have not been well studied due to the poor glass-forming ability of highly depolymerized hydrous melts.

The novel rapid-quench multi-anvil technique [Bondar et al., 2020; 2021] enabled us to quench peridotitic (KLB-1) glasses with H2O contents from 0 to 5 wt.% at high pressures up to 4 GPa for the first time [Bondar et al., 2022]. The synthesized glasses are transparent, optically isotropic, and chemically homogeneous. Raman spectroscopy revealed no long-range ordering in the atomic structure of the peridotitic glasses.

Infrared spectroscopy on these glasses has provided three important observations. The first is the dramatic decrease in the fraction of the H2O molecules, shown by the ~5200 cm-1 band, to the bulk H2O with increasing depolymerization (Fig. 1a). The second is the appearances of the (Mg,Ca)OH peak at 4320-4180 cm-1, next to the SiOH peak, at high CaO and MgO contents (Fig. 1b). The third is the shift of the SiOH peak to lower wavenumbers with increasing depolymerization.

Raman spectroscopy demonstrated a shift of the high-wavenumber envelope to higher wavenumbers with increasing H2O content in peridotitic glasses. It may be interpreted as an increasing polymerization of the silicate network. In the case of polymerized melts, water depolymerizes them by breaking the Si-O-Si linkages and forming the SiOH species. On the other hand, a 29Si NMR spectroscopic study observed no effect of water on melt polymerization in relatively depolymerized glasses in the simple CaO-MgO-SiO2 system [Xue and Kanzaki, 2004]. Based on this observation, they hypothesized that water dissolved as MgOH and CaOH in depolymerized melts, polymerizes them. The results of our Raman spectroscopy support Xue and Kanzaki's [2004] hypothesis. An alternative explanation may be a change in Fe3+/Fetot. ratio, as it can alter the spectral feature in the 800-1200 cm-1 region.

These findings provide fertile ground for developing new studies of the structure/physical properties relationships of extremely depolymerized melts.