HIGH-PRESSURE BEHAVIOR OF 3.65 Å PHASE USING RAMAN SPECTROSCOPY

The 3.65 Å phase [MgSi(OH)₆] is a hydrous phase that has been experimentally observed in a simplified MgO-SiO₂-H₂O (MSH) ternary system which is representative of hydrated mantle peridotite. This phase is predicted to be stable along cold subduction zones at pressures exceeding 9 GPa and is likely to carry water into the Earth’s interior. The crystal structure of the 3.65 Å phase consists of Mg and Si octahedral units attached to the hydroxyl group that forms a hydrogen bond. Based on first principles simulation, the 3.65 Å phase is predicted to undergo pressure-induced symmetrization of the hydrogen bond is associated with stiffening of elasticity making them harder to detect in the deep Earth.

To test such theoretical predictions, we have explored the high-pressure behavior of the 3.65 Å phase using Raman spectroscopy. To identify the vibrational modes, we have used first-principles simulations based on density functional perturbation theory. Our results indicate that at high pressures the first derivative of the vibrational modes in the lattice region stiffens (dνⁱ_lattice/dP)>0, an expected behavior at high pressures as bonds stiffen. The high-pressure behavior of the hydroxyl region is in stark contrast to that of the lattice region with (dν_OH/dP)<0. This observation confirms the prior theoretical prediction of the strengthening of hydrogen bonding. However, we noticed a significant broadening of ν_OH, indicative of proton disorder. We did not find evidence for pressure-induced symmetrization of the hydrogen bonds up to 60 GPa. Using the pressure derivative of the vibrational modes we determined the ratio of the bulk moduli (K₀) and their pressure derivative (K'₀). Our results indicate that hydrous phases have smaller K₀ compared to the major mantle phases but this is compensated by significantly larger K'₀ for the hydrous phases. This difference between K₀/K'₀ for the major mantle phases and hydrous phases is likely to result in a significant reduction in the elasticity contrast between hydrous and major mantle phases. If such behavior is unaffected by modest temperatures that are likely in subduction zone settings, we predict that the detection of the degree of mantle hydration is likely to be challenging at greater depths.

Acknowledgements: We acknowledge funding from NSF (EAR 1753125 and 1638752) and computing resources from XSEDE/ACCESS facilities (GEO170003).