Sound velocities of the 23 Å phase at high pressure and implications for seismic velocities in subducted slabs

Dense hydrous magnesium silicates (DHMS) are believed to play an important role in transporting water back into the deep interior of the Earth. Recently, a new Al-bearing hydrous Mg-silicate, named the 23 Å phase [ideal composition Mg ₁₂ Al ₂ Si ₄ O ₁₆ (OH) ₁₄ ], was reported (Cai et al., 2015), which could be another important hydrous phase in subducting slabs. In this paper, we report new measurements of the compressional (P) and shear (S) wave velocities of the 23 Å phase under pressures up to 14 GPa at room temperature, using a bulk sample with a grain size of less than 20 μm and density of 2.947 g/cm ³ . The acoustic measurements were conducted in a 1000-ton uniaxial split-cylinder multi-anvil apparatus using ultrasonic interferometric techniques (Li et al., 1996). The pressures were determined in situ by using an alumina buffer rod as the pressure marker (Wang et al., 2015). A dual-mode piezoelectric transducer enabled us to measure P and S wave travel times simultaneously, which in turn allowed a precise determination of the sound velocities and elastic bulk (K _S ) and shear (G) moduli at high pressures. The velocities of this 23 Å phase (ambient V _P = 7.53 km/s, V _S = 3.72 km/s) are lower than those of phase A, olivine, pyrope, etc. (especially for S waves), yet the V _P /V _S ratio (from 2.02 to 1.94, decreasing with increasing pressure) is much higher compared to those mantle minerals (1.7–1.8). A fit to the acoustic data using finite strain analysis combined with the Hashin-Shtrikman (HS) averaging scheme yields: K _{S 0} = 111.4(6) GPa, G ₀ = 41.0(2) GPa, and K _S ′ = 3.6, G′ = 1.8 for the bulk and shear moduli and their pressure derivatives. Using these data, the density and velocity profiles of the hydrous harzburgite and lherzolite were calculated along a warm slab P-T condition. The results suggest that a hydrous pyrolitic assemblage containing 23 Å phase should be distinguishable from a dry assemblage at the high pressure and temperature conditions relevant to Al-bearing subducted slabs.