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Single‐crystal equation of state of phase D to lower mantle pressures and the effect of hydration on the buoyancy of deep subducted slabs

An understanding of the physical properties of the hydrous magnesium silicate phase D is important for the interpretation of the seismic anomalies observed in subducted slabs and to evaluate the effect of hydration on slab dynamics. Here we report the equation of state of phase D (Mg1.1Si1.8H2.5O6) up to 65 GPa obtained from high‐precision single‐crystal X‐ray diffraction. A single‐crystal of phase D was loaded in a diamond anvil cell using helium as pressure transmitting medium to ensure quasi‐hydrostatic conditions during the entire data collection. The volume of phase D decreases smoothly over the entire pressure range, without the anomalies in the compressibility reported at 40 GPa in previous powder diffraction studies. If existing in phase D, a hydrogen bond symmetrization transition as predicted by first‐principles calculation is therefore not associated with anomalies in the volume compression behavior. The isothermal bulk modulus KT and its pressure derivative K′T obtained from the fitting of the unit cell volumes using a third‐order Birch‐Murnaghan equation of state are KT = 151.4 ± 1.2 GPa and K′T = 4.89 ±0.08, respectively. This bulk modulus is in good agreement with a recent single‐crystal Brillouin scattering experiment. The presence of 16 vol. % of phase D in hydrous peridotite lithologies reduces the density by up to 2.6% at upper most lower mantle pressure‐temperature conditions (1273 K and 30 GPa), phase D is thus a potential candidate to influence the buoyancy of hydrated stagnant slabs below the transition zone.