Elasticity of single‐crystal superhydrous phase B at simultaneous high pressure‐temperature conditions

Abstract

We investigated the combined effect of pressure and temperature on the elasticity of single‐crystal superhydrous phase B (Shy‐B) using Brillouin scattering and X‐ray diffraction up to 12 GPa and 700 K. Using the obtained elasticity, we modeled the anisotropy of Shy‐B along slab geotherms, showing that Shy‐B has a low anisotropy and cannot be the major cause of the observed anisotropy in the region. Modeled velocities of Shy‐B show that Shy‐B will be shown as positive velocity anomalies at the bottom transition zone. Once Shy‐B is transported to the topmost lower mantle, it will exhibit a seismic signature of lower velocities than topmost lower mantle. We speculate that an accumulation of hydrous phases, such as Shy‐B, at the topmost lower mantle with a weight percentage of ~17–26% in the peridotite layer as subduction progresses could help explain the observed 2–3% low shear velocity anomalies in the region.

Elasticity of superhydrous phase B at the mantle temperatures and pressures: Implications for 800 km discontinuity and water flow into the lower mantle

“…At ambient pressure, and are −15.9 and −12.0 MPa/K, respectively. Pressure derivatives of elastic moduli are also comparable to experimental data except C 22 in Li et al []. C 22 obtained in Li et al [] shows a significant nonlinear dependence with pressure, and the first and second pressure derivatives were 7.8 and −0.462/GPa, respectively.…”

“…(a) Bulk modulus K and shear modulus G , (b) compressional wave velocity V p and shear wave velocity V s of ShyB at various temperatures. Solid points represent the experimental results [ Li et al ., ; Pacalo and Weidner , ; Rosa et al ., ].…”

“…To obtain pressure and temperature dependence of elastic moduli M ( M = C ij , K S , and G ), the first and second derivatives with respect to temperature and pressure are calculated and reported in Tables S5–S7. Elastic moduli show linear dependences with temperature at different pressures, and their temperature derivatives are in good agreement with experimental results [ Li et al ., ; Rosa et al ., ]. Notably, temperature effect on elastic constants slightly decreases with pressure as suggested by the decreasing trend of temperature derivatives with pressure.…”

“…The elasticity of ShyB at high pressure has also been investigated by first‐principles calculations [ Mookherjee and Tsuchiya , ] at static condition and Brillouin scattering spectroscopy [ Pacalo and Weidner , ; Rosa et al ., ] at room temperature. Li et al [] reported the elasticity of ShyB up to 12 GPa and 700 K. These previous studies confirmed that ShyB shows a much lower wave velocities than those of majorite, ringwoodite, ferropericlase, bridgmanite, and CaSiO 3 perovskite at the same pressure (P)‐temperature (T) conditions. Therefore, it is proposed that the accumulation of ShyB might be responsible for some observed local low‐velocity anomalies [ Li et al ., ; Rosa et al ., ].…”

“…Li et al [] reported the elasticity of ShyB up to 12 GPa and 700 K. These previous studies confirmed that ShyB shows a much lower wave velocities than those of majorite, ringwoodite, ferropericlase, bridgmanite, and CaSiO 3 perovskite at the same pressure (P)‐temperature (T) conditions. Therefore, it is proposed that the accumulation of ShyB might be responsible for some observed local low‐velocity anomalies [ Li et al ., ; Rosa et al ., ]. However, the elasticity of ShyB at the P‐T conditions of the transition zone and the uppermost lower mantle has not yet been studied.…”