AbstractIn this study, we concentrate on the seismic signature of subducted sediments and suggest the formation of the L‐discontinuity beneath the ancient craton related to migrated sediment dehydration. We first determined the single‐crystal elasticity of topaz, the product of sediment dehydration, at high pressures and temperatures by Brillouin scattering. Using the derived elastic parameters, we establish the velocity and density profiles of subducted sediments in the upper mantle. According to our modeling results, 8.5–17.5 vol.% sediments intruding into the upper mantle will induce a 2%–4% low‐VS anomaly at 210–260 km. Meanwhile, continuous heating will lead to the dehydration of phengite in sediments. The dehydration of this amount sediments can generate a 3%–6% ISS with negative Clapeyron slopes, satisfying the observed L‐discontinuity in northern Finland and northern America without the anisotropy changes but accompanied by low‐velocity anomalies. Our study thus provides new insights into the origin of the L‐discontinuity.

The phase transition of AF(2) difluorides strongly depends on pressure, temperature, and cationic radius. Here, we have investigated the phase transition of three difluorides, including MgF2, CaF2, and BaF2, at simultaneously high pressures and temperatures using Raman spectroscopy and X-ray diffraction in externally heated diamond anvil cells up to 55 GPa at 300-700 K. Rutile-type difluoride MgF2 with a small cationic radius undergoes a transition to the CaCl2-type phase at 9.9(1) GPa and 300 K, to the HP-PdF2-type phase at 21.0(2) GPa, and to the cotunnite-type phase at 44.2(2) GPa. The phase transition pressure to the HP-PdF2 and cotunnite structure at 300 K for our single crystal was found to be higher than that in previous studies using polycrystalline samples. Elevating the temperature increases the transition pressure from rutile- to the CaCl2-type phase but has a negative influence on the transition pressure when MgF2 transforms from the HP-PdF2- to cotunnite-type phase. Meanwhile, the transition pressure from the CaCl2- to HP-PdF2-type phase for MgF2 was identified to be independent of the temperature. Raman peaks suspected to belong to the alpha-PbO2-type phase were observed at 14.6-21.0(1) GPa and 400-700 K. At 300 K, difluorides CaF2 and BaF2 in the fluorite structure with larger cationic radii transform to the cotunnite-type phase at 9.6(3) and 3.0(3) GPa at 300 K, respectively, and BaF2 further undergoes a transition to the Ni2In-type phase at 15.5(4) GPa. For both CaF2 and BaF2, elevating the temperature leads to a lower transition pressure from fluorite- to the cotunnite-type phase but has little influence on the transition to the Ni2In structure. Raman data provide valuable insights for mode Gruneisen parameters. We note that the mode Gruneisen parameters for both difluorides and dioxides vary linearly with the cation radius. Further calculations for the mode Gruneisen parameters at high pressures for MgF2, CaF2, and BaF2 yield a deeper understanding of the thermodynamic properties of the difluorides.

Orthopyroxene is one of the dominant minerals in the Earth's upper mantle. In this study, we used Raman spectroscopy to investigate the lattice vibration and phase transition of orthopyroxene with four different compositions using diamond-anvil cells up to 34 GPa at 300 K. Our orthopyroxene samples contain 0 (En(100)), 9% (En(91)Fs(9)), 11% (En(86)Fs(11)), and 21% (En(74)Fs(21)) Fe. At ambient conditions, the Raman modes exhibit a negative dependence on the Fe content, with the exception of the modes at similar to 850 and 930 cm(-1). In contrast, these two Raman modes increase with increasing the Fe content. The phase transition from metastable alpha- to beta-phase was observed at 12.9-15 GPa for samples with <21 mol% Fe and varying Fe content has a minor effect on the phase transition pressure. Besides Fe, incorporation of 2-24 mol% Al can cause an increase in the phase transition pressure from 10-13 to 14-16 GPa. At 29-30.1 GPa, we observed the second apparent change in the Raman spectra for all four investigated samples. For Fe-bearing orthopyroxene, this change in the Raman spectra and frequency shift is associated with the phase transition from beta- to gamma-phase, whereas for En(100), it should be caused by the change of coordination number of Si from 4 to 6 or the presence of alpha-popx phase. Using the obtained Raman frequency shifts, we calculated the Gruneisen parameters at high pressures. These parameters are useful for understanding the thermoelastic properties of orthopyroxene at high pressures.

Magnesite (MgCO3) entering the lower mantle together with the subducted oceanic crust is an important carbon carrier. The reaction between magnesite and mantle minerals has been documented, but its influence on the density and velocity profiles of lower mantle remains unexplored. To decipher the deep carbon transportation and its associated effect, here we determined the thermal equations of state of magnesite up to 120 GPa and 2600 K using X-ray diffraction in laser-heated diamond anvil cells. The obtained thermal elastic parameters of magnesite facilitated a comprehensive understanding on the influence of magnesite-SiO2 reaction, variation of carbon and SiO2 content, and temperature on the origin of lower-mantle scatterers at 1,000-1,800 km depth. Our modeling revealed that the depth of the lower-mantle V-S scatterers is mainly controlled by the Al2O3 content in SiO2 , while its magnitude depends on the SiO2 content. Along normal geotherm, the magnesite-SiO2 reaction would occur before the post-stishovite transition, consuming substantial SiO2 in the subducted oceanic crust. Depending on the amount of residual SiO2 , the post-stishovite transition can produce a 2.5-5.2 (2)% V-S reduction, compatible with the observed seismic scatterers in Izu-Bonin and Mariana subduction zones. Along slab geotherm, this reaction occurs after the post-stishovite transition, generating a greater VS reduction of 4.4-6.4 (4)%. We thus propose that the reaction between sinking MgCO3 and SiO2 in the slab is one of the potential factors influencing the magnitude of the lower-Vs scatterers at 1,000-1,900 km depth. Our results provide new insights into the deep-mantle carbonate transportation influencing regional geophysics. Plain Language Summary Carbonates could enter the deep Earth with the subducted oceanic crust. Magnesite is the most likely carbonate to exist stably in the lower mantle. During the subduction process, magnesite would interact with subducted oceanic crust and influence the density and velocity structure of the lower mantle. Here we determined the thermal equations of state (EoS) of magnesite up to 120 GPa and 2600 K. With the obtained EoS parameters, we discussed the influence of magnesite-SiO2 reaction, variation of carbon, SiO2 content, and temperature on the velocity and density profiles across the post-stishovite phase transition of SiO2 and explain the origin of lower-mantle VS scatterers at 1,000-1,900 km depth. Our findings reveal that the depth and magnitude of lower-mantle VS scatterers depend on Al2O3 content in SiO2 and SiO2 content, respectively. With the variation of temperature and residual SiO2 content of the reaction, the post-stishovite phase transition of SiO2 can generate a VS reduction in the range of 2.5%-6.4%. Our findings are important in understanding deep-mantle carbonate transportation and its implications for regional Geophysics.

In this study, we have determined the single‐crystal elasticity of clinohumite [Mg8.85Ti0.19Si3.93O16(OH1.11F0.89)] using Brillouin measurement up to 21 GPa at 300 K and 1 bar at 750 K, respectively. The elasticity of clinohumite was determined to be KS0 = 126.2(3) GPa, G0 = 79.7(2) GPa with pressure derivatives KS′ = 4.2(1), G′ = 1.3(1), pressure derivatives ∂KS/∂T = −0.024(1) GPa/K, and ∂G/∂T = −0.011(1) GPa/K). We comprehensively examined the effects of varying H2O, fluorine content and thermal states, on the velocity and density structures of the subducted harzburgite layer. Assuming a typical H2O content of 2 wt.% within harzburgite, our modeling has shown that hydrous harzburgite with clinohumite as the decomposition product of serpentine along a hot slab geotherm even has the VP and VS 0.4–0.8(6)% greater than it dry counterpart at 250–380 km depth. Yet in the top transition zone, the addition of H2O and F can effectively lower the sound velocities and density. The F‐bearing hydrous harzburgite has the VP and VS 1.1(5)–1.3(3)% lower than its dry counterpart, and only 0.6(5)% and 2.3(5)% greater than the pyrolitic mantle. Along cold slab geotherm, phase A will replace clinohumite as the dominant hydrous phase in the harzburgite, the VP and VS are 4.8(5)–5.3(3)% and 5.9(5)–6.0(3)% greater than the pyrolitic mantle in the upper mantle. In the top transition zone, the difference is approximately 3% in VP and 5% in VS. Our results provide crucial experimental evidence for future assessments of the seismic signals of subducted slabs with different hydrous minerals and thermal states.

Low-Velocity Structure of Subducted Oceanic Crust in the Upper Mantle: Insights from High Pressure and Temperature Elasticity Measurements of Aragonite.

The complex multi-discontinuity structure at 660-800 km depth is likely attributable to lateral mantle composition heterogeneities, which are closely related to the mid-ocean ridge basalts (MORB). To decipher the impact of varying composition on the seismic properties of MORB, detailed knowledge of the elasticity of candidate minerals is thus important. Here we employed Brillouin scattering coupled with diamond anvil cells to determine the single-crystal elasticity of corundum up to 14 GPa and 300 K. Using third-order finite strain equation of state, we calculate the pressure derivatives of adiabatic bulk modulus and shear modulus of corundum, which yields KS0' = 3.8(1), G0' = 1.8(1) with KS0 = 256(1) GPa and G0 = 163(1) GPa and rho 0 = 3.987(1) g/cm3. Combined with previous high-temperature data, the velocity and anisotropy of corundum have been calculated at 300 K or along normal geotherm. Our results are applied to model the density and velocity profiles of normal and alkalidepleted MORB. Our modeling demonstrates that varying the alkali content and temperature of MORB can significantly impact the discontinuity depth and velocity jump at 660-800 km depth. For normal MORB, reducing the temperature by 300 K from normal mantle geotherm results in a shift of the velocity jump from 670-710 to 650-710 km depth but hardly affects the magnitude of the velocity jump (5.8-6.8(3)% for VP and 10.0-10.8(5)% for VS). By contrast, in alkali-depleted MORB, the discontinuity will occur at a greater depth from 705-730 to 720-745 km depending on temperature with a VP jump of 3.8-4.6(2)% and VS jump of 6.3-7.4(4)%.

Single‐crystal elasticity of both α ‐ and β ‐orthopyroxene was determined up to 20 GPa and 300 K by Brillouin scattering. Using the derived full elastic moduli ( C ij ), we investigated the contribution of the metastable pyroxene to the seismically observed 3%–5% low‐velocity anomalies along the subducting slab in the top transition zone. Our modeled results show that a harzburgite wedge with a 1000‐K colder geotherm and metastable α ‐orthopyroxene and olivine displays compressional ( V P ) and shear‐wave ( V S ) velocities 3.0%–3.6(6)% and 2.0%–2.8(6)% lower than the surrounding mantle at 410–460 km depth, respectively. At deeper depth up to 520 km, V P and V S of this metastable wedge with β ‐orthopyroxene and olivine are 3.6%–4.4(6)% and 2.8%–4.3(6)% lower than the pyrolitic mantle, respectively. The presence of both metastable orthopyroxene and olivine instead of metastable olivine alone helps better explain the origin of the low‐velocity anomalies within the subduction slab in the top transition zone.