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 laboratoryexperiments 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 ofwater in the form of hydroxyl ions in their crystal structure. Direct in-situ evidence of theMTZ being (at least locally) hydrated has been derived from natural diamonds containinghydrous ringwoodite inclusions (Pearson 2014, Nature). Hence, in this study, we haveinvestigated the crystal structure and the thermoelastic properties of Fe-bearing ringwooditeas a function of temperature, pressure, and water content (0 wt%, 1.56 wt%, 3.3 wt%) using acombination of first-principles density functional theory (DFT) and quasi-harmonicapproximation (QHA). Our calculation reveals that hydration in general causes a reduction inthe sound wave velocity of ringwoodite. However, the ‘reduction’ brought in by hydration issignificantly suppressed at pressures corresponding to the lower part of the MTZ.Consequently, the sound wave velocities for the 1.56 wt% water-containing ringwooditemodel is found to become very similar to the sound wave velocities of the anhydrousringwoodite. However, when the water concentration is increased further to ~3.3 wt%, the pressure-inducedsuppression at lower MTZ pressures though present is not significant. These findings indicate that though seismicwaves may not be able to precisely decipher the state of hydration of the lower part of MTZwhen the water concentration is less than 1.56 wt%, it is still robust enough to locate regionsof very high water concentration ~ 3.3 wt%.