Transportation of water into the deep Earth by Al-phase D
IMAGE: (A) SHEAR VELOCITY CONTRAST BETWEEN THE AL-RICH HYDROUS LAYER (INCLUDING AL-PHASE D) AND THE DRY MANTLE FOR TWO MODEL COMPOSITIONS: HYDROUS PYROLITE (H-PYROLITE) AND HYDROUS HARZBURGITE (H-HARZBURGITE). (B) HYPOTHETICAL MECHANISMS... view more
CREDIT: EHIME UNIVERSITY
Since the discovery of a water-bearing ringwoodite specimen trapped in a superdeep diamond from Brazil by Pearson et al., in 2014 (published in Nature), there is a regained interest for finding and characterizing the potential carrier and host minerals of water in the deep Earth's interior. Among the candidate minerals, Dense Hydrous Magnesium Silicates (DHMSs) are considered as primary water carriers from the shallow lithosphere to the deep mantle transition zone (MTZ; 410-660 km in depth), but because of their relative instability against pressure (P) and temperature (T), DHMSs were generally associated with the presence of water up to the middle-part of the MTZ.
An experimental study published also in 2014, in the journal Nature Geoscience however showed that when aluminum incorporates DHMSs, their stability against P and T is drastically improved, allowing those minerals to transport and host water up to depths of 1200 km in the lower mantle (Pamato et al., 2014). Their experiments indeed showed that the aluminum-bearing DHMS mineral called Al-phase D is likely to form at the uppermost lower mantle P and T conditions, from the recrystallization of hydrous melt at the boundary of the mantle and the subducted slab. Although this reaction was justified by laboratory experiments, there were no direct measurement of the sound velocities of Al-phase D and therefore it was difficult to associate the presence of Al-rich hydrated rocks to the seismic observations at the bottom of the MTZ and in the uppermost lower mantle.
The researchers at Ehime successfully measured the longitudinal (VP) and shear (VS)
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