Elasticity of hydrous olivine at high pressure and seismic detectability of water in Earth's mantle

The dominant phases of Earth’s upper mantle are commonly referred to as nominally anhydrous minerals (NAMs) but may contain significant amounts of water. The colloquial term “water” in NAMs is related to the presence of hydroxyl-bearing (OH^-) point defects in their crystal structure, where hydrogen is bound to lattice oxygen and is charge-balanced by cation vacancies. For this reason, the incorporation of even small amounts of water may substantially affect the physico-chemical properties of NAMs, such as their elasticity, rheology, and melting temperature. Olivine is considered the most abundant phase of Earth’s upper mantle and constitutes about 60 vol.% of a primitive upper mantle (pyrolite) phase assemblage. Although natural olivine samples originating from the shallow upper mantle are relatively dry (maximum H₂O concentrations of about 400 ppm), a plethora of experimental data indicate that olivine water storage capacity significantly increases to 0.2-0.5 wt.% H₂O at deeper upper mantle conditions.
In this contribution, we investigated the effect of water on the elastic properties and sound wave velocities of hydrous Mg_1.8Fe_0.2SiO₄ (Fo90) olivine samples with realistic water contents for deep upper mantle conditions with the aim of interpreting both seismic velocity anomalies in potentially hydrous regions of Earth's upper mantle and the observed seismic velocity and density contrasts across the 410-km discontinuity between the upper mantle and the mantle transition zone. To do so, we performed simultaneous single-crystal X-ray diffraction and Brillouin scattering measurements at room temperature up to ~12 GPa on Fo90 olivine with ~0.20 wt.% H₂O to constrain its full elastic tensor. Results were complemented with a careful re-analysis of all the available single-crystal elasticity data from the literature for Fo90 olivine to re-determine the elastic behaviour of the anhydrous phase. Our new data show that the sound wave velocities of hydrous and anhydrous olivines are indistinguishable within uncertainties at pressures corresponding to the base of the upper mantle. Therefore, if amounts of water were to be incorporated into the crystal structure of Fo90 olivine, its elastic and seismic behaviour at high pressure would likely remain unchanged. This suggests that water in olivine is not seismically detectable, at least for contents consistent with deep upper mantle conditions. Moreover, the incorporation of water in olivine is unlikely to be a key factor in reconciling seismological observations at the 410-km discontinuity with a pyrolitic mantle, but rather corroborates previous evidence of a deep upper mantle that is less enriched in olivine than the pyrolite model.