Pressure-volume-temperature (P-V-T) data of synthetic Mg7Si2O8(OH)6 phase A were collected under P-T conditions up to ~10.5 GPa and 900 K by energy-dispersive X-ray diffraction using a cubic type multi-anvil apparatus, MAX80, located at the Photon Factory–Advanced Ring (PF-AR) at the High Energy Accelerator Research Organization (KEK). P-V EoS using only room-temperature data yielded V0 = 511.6(2) Å3, KT0 = 106.8(18) GPa, and pressure derivative KT = 3.88(38). These parameters were consistent with the subsequent equation of state (EoS) analysis. The compressibility of phase A was anisotropic, with its a-axis being ~26% more compressible than the c-axis, which is normal to the plane of the distorted close-packed layers. A fit of the present data to the high-temperature Birch-Murnaghan EoS yielded V0 = 511.7(3) Å3, K0 = 104.4(24) GPa, K′ = 4.39(48), (∂KT/∂T)P = –0.027(5) GPa K–1, and thermal expansion α = a + bT with values of a = 2.88(27) × 10–5 K–1 and b = 3.54(68) × 10–8 K–2. The lattice dynamical approach by the Mie-Grüneisen-Debye EoS yielded θ0 = 928(114) K, q = 2.9(10), and γ0 = 1.19(8). The isobaric heat capacity CP of phase A at 1 atm. was calculated based on the Mie-Grüneisen-Debye EoS fit of present P-V-T data. In addition, the density profiles of subducting slabs with different degrees of serpentinization were also calculated along the cold geotherm up to ~13 GPa. The serpentinization of subducting slab will significantly lower the density of slab at shallower depth; however, this effect becomes negligible when antigorite dehydrates to phase A. Because the phase A bearing subducting slab is supposed to be denser than the surrounding mantle, the water can transport into deeper parts of the upper mantle and the transition zone.

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