Are seismic waves robust enough to detect the presence of water in the lower part of the mantle transition zone?

The mantle transition zone (MTZ) is known to be potentially hydrated as laboratory
experiments have shown that the two major mineral phases namely wadsleyite (β-M₂SiO₄ ;
M: Mg, Fe) and ringwoodite (γ-M₂SiO₄; M: Mg, Fe) can accommodate significant amounts of
water in the form of hydroxyl ions in their crystal structure. Direct in-situ evidence of the
MTZ being (at least locally) hydrated has been derived from natural diamonds containing
hydrous ringwoodite inclusions (Pearson 2014, Nature). Hence, in this study, we have
investigated the crystal structure and the thermoelastic properties of Fe-bearing ringwoodite
as a function of temperature, pressure, and water content (0 wt%, 1.56 wt%, 3.3 wt%) using a
combination of first-principles density functional theory (DFT) and quasi-harmonic
approximation (QHA). Our calculation reveals that hydration in general causes a reduction in
the sound wave velocity of ringwoodite. However, the ‘reduction’ brought in by hydration is
significantly suppressed at pressures corresponding to the lower part of the MTZ.
Consequently, the sound wave velocities for the 1.56 wt% water-containing ringwoodite
model is found to become very similar to the sound wave velocities of the anhydrous
ringwoodite. However, when the water concentration is increased further to ~3.3 wt%, the pressure-induced
suppression at lower MTZ pressures though present is not significant. These findings indicate that though seismic
waves may not be able to precisely decipher the state of hydration of the lower part of MTZ
when the water concentration is less than 1.56 wt%, it is still robust enough to locate regions
of very high water concentration ~ 3.3 wt%.