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

Abstract

[1] 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 (Mg 1.1 Si 1.8 H 2.5 O 6 ) 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 K T and its pressure derivative K′ T obtained from the fitting of the unit cell volumes using a third-order Birch

Two‐stage spin transition of iron in FeAl‐bearing phase D at lower mantle

“…Hushur et al [] observed an 18% increase in K 0 in a high‐pressure X‐ray powder diffraction experiment of FeAl‐free phase D (MgSi 1.7 H 3.0 O 6 ) by fitting the P‐V data in different pressure ranges (0.0001–30 GPa versus 40–55.8 GPa) and proposed the presence of the hydrogen bond symmetrization at 40 GPa. However, this change was not reported in Rosa et al [], as well as in this study. In addition, previous infrared spectroscopy characterization does not support the existence of hydrogen bond symmetrization of FeAl‐free phase D up to 42 GPa at room temperature [ Shieh et al , ].…”

“…At lower pressure, the axial compressibility of phase D is found to be anisotropic with the c axis being twice as compressible as the a axis, but at higher pressure, the c / a ratio becomes almost constant or pressure independent (Figure S3b). The critical value of pressure is in the range of 15–40 GPa as found in a series of prior experiments and this study [ Litasov et al , ; Shinmei et al , ; Hushur et al , ; Chang et al , ; Rosa et al , ]. High‐precision data only from single‐crystal XRD clearly show that the c / a ratio becomes constant at 25–30 GPa for FeAl‐free phase D [ Rosa et al , , this study], ~37 GPa for the FeAl‐bearing sample with ∑Fe 3+ /Fe = 0.40 (this study), and ~40 GPa for the FeAl‐bearing sample with ∑Fe 3+ /Fe = 0.94 [ Chang et al , ] (Figure S3b).…”

“…Phase D, a classic DHMS with a wide pressure‐temperature ( P‐T ) stability range [ Shieh et al , ; Shinmei et al , ; Tsuchiya , ; Nishi et al , ; Ghosh and Schmidt , ; Pamato et al , ], may carry water from the upper to lower mantle. Thus, the properties of phase D, including structural stability, sound velocity, electronic conductivity, texture, and melting, have been used, for example, in discussions of water recycling, buoyancy of deep‐subducted slabs, and seismic velocity anomalies in the uppermost‐to‐middle lower mantle [ Lawrence and Wysession , ; Chang et al , ; Rosa et al , , , ; Ghosh and Schmidt , ].…”

Single-crystal elasticity of iron-bearing phase E and seismic detection of water in Earth's upper mantle

“…Among the DHMS, phase A has a density most similar to phase E and comparably low bulk and shear moduli, K S = 106(1) GPa; G = 61 GPa (Sanchez-Valle et al 2006, 2008. Other DHMS phases such as superhydrous phase B (Rosa et al 2015) and phase D (Rosa et al 2013) have more dense-packed structures and both their elastic moduli and pressure derivatives differ substantially from those of phase E. Therefore, we assume that the shear modulus of phase E shows a similar response to compression as the one of phase A. Consequently, we used the pressure derivative of the shear modulus of iron-bearing phase A, G′ = 1.8 (Sanchez-Valle et al 2008) together with a K 0 ′ of 7.1, consistent with the re-evaluated compression studies of Crichton and Ross (2000) and Shieh et al (2000) on phase E, to compute sound wave velocities of phase E at mantle pressures. We found that at 13 GPa and 300 K, phase E will have compressional and shear wave velocities of 7.33 and 4.88 km/s, respectively, which are about 8% and 16% lower compared to hydrous wadsleyite at the same pressure (Mao et al 2011).…”

Elasticity of superhydrous phase B, seismic anomalies in cold slabs and implications for deep water transport

“…Anisotropic fabrics of perovskite could serve as a potential explanation of ULM anisotropy but its deformation mechanisms and elastic properties are not sufficiently constrained for a detailed analysis. Recently, experimental studies of the mechanical properties of phase D have shown that LVZ, shear splitting delay times as well as the splitting geometry V SH > V SV observed in the northern part of the Tonga subduction zone at ULM mantle depths could be explained by the presence of 16 vol.% of textured phase D in a hydrous subducted peridotite containing 1.2 wt.% of water (Rosa et al, 2013a(Rosa et al, , 2013b(Rosa et al, , 2012. Interestingly, the sub-slab anisotropy below Taiwan has been ascribed to anisotropic fabrics of hydrous phases (Hu et al, 2012) and particularly, ShyB has been pointed as a candidate to explain sub-slab scatterers below the Tonga slab at TZ depths (Kaneshima, 2013).…”