Sound velocities of aluminum‐bearing stishovite in the mantle transition zone

Abstract

The elasticity of Al‐bearing stishovite with 1.0, 3.3, and 4.5 wt % Al2O3 was investigated in the multianvil apparatus at high pressures and temperatures up to 21 GPa and 1700 K, by ultrasonic interferometry in conjunction with in situ X‐ray techniques. The moduli KS and G are found to decrease with increasing Al2O3 content, while their pressure and temperature derivatives do not change in a significant manner for 1.0 and 3.3 wt % Al2O3. The temperature derivatives for 4.5 wt % Al2O3, however, are larger, which may result from a change in the Al substitution mechanism at high Al2O3 content. It is shown that acoustic velocities of any mid‐ocean ridge basalt are lower by −0.4% than those calculated from pure stishovite data. Velocity perturbations up to −3.4% (VP) and −4.2% (VS) in subducted slabs are explained by the combination of the thermal equilibration (ΔT ~ 600 K) of the slab and Al enrichment in stishovite.

The Electrical Conductivity of Liebermannite: Implications for Water Transport Into the Earth's Lower Mantle

“…However, the presence of liebermannite alone cannot account for the positive velocity anomaly observed in the upper mantle beneath the Philippine Sea (Tarits & Mandéa, 2010) and northeastern China (Kelbert et al, 2009) because the elastic wave velocities measured in liebermannite are comparable with those of common mantle mineral phases (Caracas & Boffa Ballaran, 2010; Mookherjee & Steinle‐Neumann, 2009). Instead, the seismic wave velocities of Al‐poor stishovite, another principal mineral phase stable in subducting continental sediments, are notably faster than those of mantle mineral phases (Gréaux et al, 2016). The stishovite coexisting with liebermannite in our samples is noticeably devoid of aluminum (Table 2).…”

Elasticity of Hydrated Al‐Bearing Stishovite and Post‐Stishovite: Implications for Understanding Regional Seismic VS Anomalies Along Subducting Slabs in the Lower Mantle

“…ν jumps across the transition. Elasticity data for different compositions in the literature are also plotted for comparison (Gréaux et al., 2016; Lakshtanov et al., 2007a; Zhang et al., 2021).…”

“…On the other hand, thermoelastic parameters for CaPv are obtained by refitting combined elasticity data sets from Gréaux et al (2019) and Sun et al (2016) (Table S10 in Supporting Information S1). We should note that the CaPv data by Thomson et al (2019) are not used here because they measured sound velocities at a nearly constant pressure such that some thermoelastic parameters cannot be reliably constrained such as pressure derivatives of K S and μ. Mie-Grüneisen EOS and finite-strain theory are then used to calculate ρ, K S , and μ of each mineral phase in the MORB mineralogy along a cold subducting slab based on the following equations (Stixrude & Lithgow-Bertelloni, 2005): (Gréaux et al, 2016;Lakshtanov et al, 2007a;Zhang et al, 2021). We should note that the finite-strain model cannot be applied to evaluate the shear softening feature across the post-stishovite ferroelastic transition at high P-T conditions.…”

“…The Al 3+ and/or H + incorporation in stishovite can reduce the post‐stishovite transition pressure to the depth range more consistent with the seismic observations of the regional V S anomalies in the shallow lower mantle (Lakshtanov et al., 2007b; Umemoto et al., 2016). Although full elasticity of pure stishovite and post‐stishovite and the effect of Al on the post‐stishovite transition pressure have been relatively well investigated (Asahara et al., 2013; Karki et al., 1997; Lakshtanov et al., 2007b; Li et al., 1996; Shieh et al., 2002; Yang & Wu, 2014; Zhang et al., 2021), elasticity data of hydrated Al‐bearing stishovite across the post‐stishovite transition remain largely unexplored (Bolfan‐Casanova et al., 2009; Gréaux et al., 2016; Lakshtanov et al., 2007a). This is mainly due to the technical difficulty in measuring sound velocities and reliably deriving full elastic moduli ( C ij ) of the stishovite crystal at high pressure (Zhang et al., 2021).…”

Experimental elasticity of Earth’s deep mantle

“…data are plotted for ringwoodite 13,58,64 , wadsleyite 50,60 , majorite 55 , ferropericlase 53,65 , bridgmanite 5 , NALphase 151 . Polycrystalline data are for wadsleyite 152 , majoritic garnet 153 , ferropericlase (magnesiowüstite) 6,32 , bridgmanite 6 , stishovite 81,154 , CaPv 7,11,82 , post-perovskite 77 (measured up to 172 GPa). The scribbled regions shows the approximate range of PT-conditions that have been reached using the respective high-PT apparatus, but where few or no elasticity measurements have been performed to-date.…”