Proton substitution mechanism in forsterite

Protons are incorporated into olivine crystal structures in either Mg site or Si site. Since different incorporation mechanisms may change the point defect chemistry of olivine differently, understanding the incorporation mechanisms is essential for investigating dynamics and geochemical circulation in the upper mantle. However, the question about which site dominates for protons in olivine still remains and been under debate for decades. High-pressure experiments indicate that the water solubility in olivine is nearly proportional to the water fugacity in the system (Bai and Kohlstedt, 1992; Bali et al., 2008; Kohlstedt et al., 1996), suggesting that protons mainly occupy Mg sites by the reaction,
2H2O + Mg2SiO4 = (4H)SiO4 + 2MgO           (1)

In contract, the main proton components are attributed to Si sites substituted by four protons from nuclear magnetic resonance spectroscopy (NMR) and first-principle calculation (e.g., Balan et al., 2011; Crepisson et al., 2014; Ingrin et al., 2013; Xue et al., 2017) by the following the reaction:
2H2O + Mg2SiO4 = Mg2(4H)O4 + SiO2          (2)
Measurements of Mg/Si atomic ratio as a function of water content and comparison with the ideal chemical formula can provide constraints for understanding proton substitution mechanisms in minerals. However, owing to the small water solubility in olivine, the deviation of Mg/Si ratio from the ideal formula is very small even when saturated with water and conclusive results are thus difficult to obtain.

In this study, we carefully compare the Mg/Si ratio in dry and hydrous Fe-free olivine by electron microprobe. Combined with infrared spectroscopy analysis, our results show that the Mg/Si ratio in hydrous olivine is clearly higher than dry samples, therefore, demonstrates that protons in olivine are mostly stored in Si sites.

Mg/Si atomic ratios in the samples measured by EPMA. Although the data points are largely scattered due to the resolution of EPMA, the sample with 2800 ppm water has clearly higher Mg/Si ratio than the lower water content or dry samples.

Pressure dependence of water solubility in Fe-free forsterite and Fe-bearing olivine. The blue and red symbols represent data points under SiO2 rich and MgO rich, respectively. Only the data points from the literatures that clearly mentioned the SiO2 or MgO buffering states (e.g., starting material rich in MgO, brucite, talc, or enstatite; with (Mg,Fe)/Si ratio; or run product coexisting with orthopyroxene or ferropericlase) were plotted in the figure. All experimental temperature conditions were 1200-1300 oC and water contents adjusted to Withers et al. (2012) FTIR calibration. Fo-MgO: Fe-free forsterite under MgO rich conditions. Fo-SiO2: Fe-free forsterite under SiO2 rich conditions. Ol-MgO: Fe-bearing olivine under Mg(Fe)O-rich conditions. Ol-SiO2: Fe-bearing olivine under SiO2-rich conditions.

Fei, H., Katsura, T., Pressure dependence of proton incorporation and water solubility in olivine. Journal of Geophysical Research: Solid Earth 125, e2019JB018813, 2020.