GREAUX STEEVE GEORGI (グレオ　スティーブ　ジョルジュ)

The thermoelastic properties of K0.7Na0.3AlSi3O8 hollandite and NaAlSi2O6 jadeite, synthesized from a (K, Na)-felspar (microcline), were investigated by a combination of in situ energy dispersive synchrotron X-ray radiation and multi-anvil techniques at high pressure (P) and temperature (T) up to 21 GPa and 1700 K. The second-order phase transformation was found to occur in hollandite at ~16 GPa from tetragonal I/4m (hollandite-I) to monoclinic I2/m (hollandite-II), which confirms the previous report that the incorporation of Na in the hollandite structure decreases the transformation pressure. Fitting the pressure–volume–temperature data to the Birch–Murnaghan equation of state yielded estimates of the thermoelastic parameters for jadeite as well as the K0.7Na0.3AlSi3O8 hollandite-I and -II phases, which indicate that the incorporation of Na is likely to decrease the bulk moduli of both hollandite phases. The obtained thermoelastic parameters were combined with those of other mantle minerals reported previously to estimate the density of continental materials along an average mantle geotherm. Based on our results, continental crust and sediment become, respectively, 11% and 15% denser than the pyrolitic mantle at pressure >10 GPa, suggesting that once pulled down to the critical depth of ~300 km, the continental portions of the slab can subduct further into the deep mantle, down to the lowermost part of the mantle transition region.

SPring-8 Annual Report 2019

© 2017 China University of Geosciences (Beijing) and Peking University. We investigated phase relations, mineral chemistry, and density of lunar highland anorthosite at conditions up to 125 GPa and 2000 K. We used a multi-anvil apparatus and a laser-heated diamond-anvil cell for this purpose. In-situ X-ray diffraction measurements at high pressures and composition analysis of recovered samples using an analytical transmission electron microscope showed that anorthosite consists of garnet, CaAl4Si2O11-rich phase (CAS phase), and SiO2phases in the upper mantle and the mantle transition zone. Under lower mantle conditions, these minerals transform to the assemblage of bridgmanite, Ca-perovskite, corundum, stishovite, and calcium ferrite-type aluminous phase through the decomposition of garnet and CAS phase at around 700 km depth. Anorthosite has a higher density than PREM and pyrolite in the upper mantle, while its density becomes comparable or lower under lower mantle conditions. Our results suggest that ancient anorthosite crust subducted down to the deep mantle was likely to have accumulated at 660-720 km depth without coming back to the Earth's surface. Some portions of the anorthosite crust might have circulated continuously in the Earth's deep interior by mantle convection and potentially subducted to the bottom of the lower mantle when carried within layers of dense basaltic rocks.

© 2016. American Geophysical Union. All Rights Reserved.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.

Elastic wave velocities of synthetic Fe3Al2Si3O12 almandine have been determined at simultaneous high pressure and temperature up to 19 GPa and 1700 K by the ultrasonic technique in conjunction with in situ synchrotron X-ray diffraction in a multi-anvil apparatus. Velocities of almandine are found substantially lower than those of other major end-member garnets such as pyrope, grossular, and MgSiO3 majorite, while their pressure and temperature derivatives are comparable to those of the latter garnets. The observed density, and compressional (V-P) and shear (V-S) velocities were combined and fitted to functions of the Eulerian strain EoS, yielding a adiabatic bulk modulus K-S0 = 174.2 (12) GPa and a shear modulus G(0) = 94.9 (7) GPa, and their pressure and temperature derivatives partial derivative K-S/partial derivative P = 4.61 (14), partial derivative G/partial derivative P = 1.06 (6), partial derivative K-S/partial derivative T = -2.67 (7) x 10(-2) GPa K-1, and partial derivative G/partial derivative T = -1.31 (8) x 10(-2) GPa K-1. The pressure derivative of the bulk modulus of almandine is similar to those of other garnet end-members, which is in contrast to the substantially higher value (partial derivative K-S/partial derivative P = 6.2 (5)) reported for pure almandine in an earlier study based on experiments up to 3 GPa. The present new results combined with those of pyrope, grossular, and MgSiO3 majorite are successfully used to reproduce the sound velocities of majoritic garnet in the pyrolite composition. (C) 2015 Elsevier B.V. All rights reserved.

Phase transformations in natural serpentine and chlorite have been studied at 25-50 GPa and 900-1280 degrees C by in-situ X-ray diffraction measurements using the multianvil apparatus with sintered diamond anvils. Antigorite was found to transform to a phase assemblage including phase H, at pressures above about 35-40 GPa corresponding to those in the upper part of the lower mantle. The zero pressure bulk modulus K-0 of phase H was determined to be 160 GPa, assuming its pressure derivative K' = 4. The thermal stability of phase H is significantly enhanced by a solid solution with a delta-AlOOH component, suggesting that aluminous phase H in cold slabs would deliver a certain amount of water into the deepest part of the lower mantle. (C) 2015 Elsevier B.V. All rights reserved.

The elastic wave velocities of polycrystalline Mj(80)Py(20) garnet along the majorite-pyrope system have been measured at pressures up to 21 GPa and temperatures up to 2,000 K using ultrasonic interferometry in conjunction with in situ X-ray diffraction techniques in a Kawai-type multi-anvil apparatus. The elastic moduli of Mj(80)Py(20) garnet and their pressure and temperature derivatives are determined by a two-dimensional linear fitting of the present experimental data, yielding: K (S) = 161.5 (7) GPa, a,K (S)/a,P = 4.42 (4), a,K (S)/a,T = -0.0154 (2) GPa/K, G = 86.2 (2) GPa, a,G/a,P = 1.28 (1), a,G/a,T = -0.0096 (5) GPa/K. The present results together with those of the studies on the majorite-pyrope solid solutions suggest the pressure and temperature derivatives of elastic moduli are insensitive to the majorite content in the majorite-pyrope system. The velocity gradients of the majoritic garnets in the majorite-pyrope system are 3 6 times lower than those required to account for the high seismic velocity gradients observed in the mantle transition zone.

Compressibility of perovskite-structured Ca3Al2Si3O12 grossular (GrPv) was investigated at high pressure and high temperature by means of angle-dispersive powder X-ray diffraction using a laser-heated diamond anvil cell. We observed the Pbnm orthorhombic distortion for the pure phase above 50 GPa, whereas below this pressure, Al-bearing CaSiO3 perovskite coexists with an excess of corundum. GrPv has a bulk modulus (K (0) = 229 +/- A 5 GPa; fixed to 4) almost similar to that reported for pure CaSiO3 perovskite. Its unit-cell volume extrapolated to ambient conditions (V (0) = 187.1 +/- A 0.4 (3)) is found to be similar to 2.5 % larger than for the Al-free phase. We observe an increasing unit-cell anisotropy with increasing pressure, which could have implications for the shear properties of Ca-bearing perovskite in cold slabs subducted into the Earth's mantle.

Elastic wave velocities of MgSiO3 akimotoite polycrystalline samples have been measured at pressures up to 25.7 GPa and temperatures to 1500 K by a combination of in situ X-ray diffraction and ultrasonic interferometry techniques in a large volume Kawai-type multianvil apparatus (KMA). The elastic moduli of akimotoite and their pressure and temperature dependences are determined by a 2D linear fitting analysis of the present data, yielding: K-S = 219.4(7) GPa, partial derivative K-S/partial derivative P = 4.62(3), partial derivative K-S/partial derivative T = -0.0228(4) GPa/K, G(0) = 132.1(7) GPa, partial derivative G/partial derivative P = 1.63(4), partial derivative G/partial derivative T= -0.0225(4) GPa/K. The bulk and shear moduli at ambient conditions are generally consistent with the result of a previous Brillouin study. However, significant nonlinear behaviors of the elastic moduli were observed at higher temperatures, indicating that the velocities derived from the linear fitting analysis are overestimated for the actual mantle conditions. Using the present new experimental data, we compared the elastic velocities of various high-pressure forms of MgSiO3 under the mantle conditions. The results demonstrate a large velocity difference between akimotoite and perovskite, which may be relevant to the complex seismic structures near the bottom of the mantle transition zone. (C) 2013 Elsevier B.V. All rights reserved.

The thermoelastic parameters of synthetic Mn3Al2Si3O12 spessartine garnet were examined in situ at high pressure up to 13 GPa and high temperature up to 1,100 K, by synchrotron radiation energy dispersive X-ray diffraction within a DIA-type multi-anvil press apparatus. The analysis of room temperature data yielded K (0) = 172 +/- A 4 GPa and K (0) (') = 5.0 +/- A 0.9 when V (0,300) is fixed to 1,564.96 (3). Fitting of P-V-T data by means of the high-temperature third-order Birch-Murnaghan EoS gives the thermoelastic parameters: K (0) = 171 +/- A 4 GPa, K (0) (') = 5.3 +/- A 0.8, (a,K (0,T) /a,T) (P) = -0.049 +/- A 0.007 GPa K-1, a (0) = 1.59 +/- A 0.33 x 10(-5) K-1 and b (0) = 2.91 +/- A 0.69 x 10(-8) K-2 (e.g., alpha (0,300) = 2.46 +/- A 0.54 x 10(-5) K-1). Comparison with thermoelastic properties of other garnet end-members indicated that the compression mechanism of spessartine might be the same as almandine and pyrope but differs from that of grossular. On the other hand, at high temperature, spessartine softens substantially faster than pyrope and grossular. Such softening, which is also reported for almandine, emphasize the importance of the cation in the dodecahedral site on the thermoelastic properties of aluminosilicate garnet.

Compressional (V-p) and shear (V-s) wave velocities of polycrystalline MgAl2O4 spinel have been measured up to 14 GPa and 900 K using ultrasonic interferometry and in situ X-ray diffraction techniques. Here, we observed a weaker pressure dependence in shear modulus (G) for MgAl2O4 spinel, as compared to a stronger partial derivative G/partial derivative P for magnesium silicate/germanate counterpart. Our first-principles calculations show that the tetragonal shear modulus C-s = (C-11-C-12)/2 decrease with pressures, indicating acoustic mode softening, which further supports our observed experimental results. Using a finite strain equation of state approach the elastic bulk and shear moduli, as well as their pressure and temperature derivatives, are derived from the directly measured velocities and densities, yielding K-so =196.0(9) GPa, G(o) = 109.0(4) GPa, partial derivative K-s/partial derivative P = 4.60(9), and partial derivative G/partial derivative P = 0.58(3) independent of pressure calibration. The temperature derivatives for the bulk and shear moduli were tightly constrained from acoustic velocity measurements as partial derivative K-s/partial derivative T = -0.022(3) GPa/K and partial derivative G/partial derivative T = -0.014(1) GPa/K. In addition, the mechanism for the unusual pressure effect on the shear modulus in MgAl2O4 spinel has been addressed by the coupling between atomic displacements and shear strains, namely a better accommodation of the AlO6 octahedral distortion and shear strains, as well as the pressure-induced tilting/distortion and/or symmetry changes in MgAl2O4 spinel.

Thermoelastic properties of synthetic Mg3Al2Si3O12 pyrope garnet have been measured at high pressure and high temperature by using in situ energy-dispersive X-ray diffraction, using a Kawai-type multi-anvil apparatus. Measurements have been conducted up to 19 GPa and 1,700 K, equivalent to the P-T conditions of the middle part of mantle transition zone. Analyses of the room-temperature P-V data to a third-order Birch-Murnaghan (BM) equation of state (EoS) yields: V (0) = 1,500 +/- A 1 (3), K (0) = 167 +/- A 6 GPa and = 4.6 +/- A 0.3. When fitting the entire P-V-T data using a high-temperature Birch-Murnaghan (HTBM) EoS at a fixed = 4.6, we obtain V (0) = 1,500 +/- A 2 (3), K (T0) = 167 +/- A 3 GPa, (a,K/a,T) (P) = -0.021 +/- A 0.009 GPa K-1 and alpha (300) = (2.89 +/- A 0.33) x 10(-5) K-1. Fitting the present data to the Mie-Gruneisen-Debye (MGD) EoS with Debye temperature I similar to(0) = 806 K gives gamma(0) = 1.19 and 1.15 at fixed q = 1.0 and 1.5, respectively. Comparison of these fittings with two different approaches, we propose to constrain the bulk modulus and its pressure derivative to K (0) = 167 GPa and = 4.4-4.6, as well as the Gruneisen parameter to gamma (0) = 1.15-1.19.

Elastic wave velocities of synthetic polycrystalline Mg3Al2(SiO4)(3) garnet have been successfully measured to 20 GPa and temperatures up to 1700 K by ultrasonic interferometry combined with energy-dispersive synchrotron x-ray diffraction in a Kawai-type multi-anvil apparatus. Compressional (Vp) and shear (Vs) wave velocities as well as the adiabatic bulk (Ks) and shear (G) moduli exhibit monotonic increase with increasing pressure and decrease with increasing temperature, respectively. Two-dimensional (P-T) linear fittings of the present data yield the following parameters: K-S0 = 170.0(2) GPa, partial derivative Ks/partial derivative P = 4.51(2), partial derivative Ks/partial derivative T = -0.0170(1) GPa/K, G(0) = 93.2(1) GPa, partial derivative G/partial derivative P = 1.51(2), and partial derivative G/partial derivative T = -0.0107(1) GPa/K, which is in good agreement with the earlier results by Brillouin scattering and/or ultrasonic measurements at relatively low P-T conditions. The observed linear pressure and temperature dependence in both Vp and Vs is in contrast to the non-linear behavior of Vp and Vs for majorite garnet with the pyrolite composition, in particular for Vs. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4736407]

X-ray absorption spectroscopy was used to investigate the oxidation state of uranium in various U-and Th-bearing Al-rich CaSiO3 perovskite samples synthesized at high-pressure and high-temperature using a multi-anvil press apparatus. X-ray absorption near edge spectroscopy (XANES) spectra collected at the U L-III- and Th L-III-edges using both micro- and macro-focused beams show U4+ in the Al-rich CaSiO3 perovskite. The structure of the U- and Th-bearing Al-rich CaSiO3 perovskite samples have been cross-checked by XANES spectra collected at the Ca K-, Al K-, and Si K-edges. Al K and Si K spectra suggest that Al incorporates exclusively on the Si site of the CaSiO3 perovskite. Ca K spectra of the (U,Th)-bearing Al-rich CaSiO3 perovskite samples were succesfully compared to FEFF8.2 ab initio models of a tetragonal CaSiO3 perovskite with space group P4/mmm. Our results confirm previous assumptions of the coupled substitution of CaSi2 by UAI(2) in CaSiO3 perovskite and that U and Th can be incorporated separately or together in CaSiO3 perovskite by means of this mechanism. The possible occurrence of the U- and Th-bearing Al-rich CaSiO3 perovskite are discussed as a potential candidate to locally host a large amount of actinides in the Earth's deep mantle. The study of a phase that can act as a storage mineral for heat-producing actinide elements such as uranium and thorium is fundamental to the understanding of the geodynamics and thermal behavior of Earth.

The thermoelastic parameters of the CAS phase (CaAl4Si2O11) were examined by in situ high-pressure (up to 23.7 GPa) and high-temperature (up to 2,100 K) synchrotron X-ray diffraction, using a Kawai-type multi-anvil press. P-V data at room temperature fitted to a third-order Birch-Murnaghan equation of state (BM EOS) yielded: V (0,300) = 324.2 +/- A 0.2 (3) and K (0,300) = 164 +/- A 6 GPa for K' (0,300) = 6.2 +/- A 0.8. With K' (0,300) fixed to 4.0, we obtained: V (0,300) = 324.0 +/- A 0.1 (3) and K (0,300) = 180 +/- A 1 GPa. Fitting our P-V-T data with a modified high-temperature BM EOS, we obtained: V (0,300) = 324.2 +/- A 0.1 (3), K (0,300) = 171 +/- A 5 GPa, K' (0,300) = 5.1 +/- A 0.6 (a,K (0,T) /a,T) (P) = -0.023 +/- A 0.006 GPa K-1, and alpha(0,T) = 3.09 +/- A 0.25 x 10(-5) K-1. Using the equation of state parameters of the CAS phase determined in the present study, we calculated a density profile of a hypothetical continental crust that would contain similar to 10 vol% of CaAl4Si2O11. Because of the higher density compared with the coexisting minerals, the CAS phase is expected to be a plunging agent for continental crust subducted in the transition zone. On the other hand, because of the lower density compared with lower mantle minerals, the CAS phase is expected to remain buoyant in the lowermost part of the transition zone.

High-pressure and high-temperature phase transformations of Ca3Al2Si3O12 grossular garnet were examined at 19-26 GPa and 700-2000 K using Kawai-type multi-anvil apparatus coupled with in situ X-ray diffraction (XRD). Recovered samples were analyzed by a combination of micro-focused X-ray diffraction (mu-XRD) and transmission electron microscopy (TEM). The results show that grossular garnet gradually transforms to an Al-rich CaSiO3 perovskite at 22-26 GPa and 1000-1400K. The transition boundary can be expressed as P (GPa) = -0.0082 X T (K) + 33.05. When the garnet completely disappears, we observed orthorhombic CaSiO3 perovskite with a grossular composition. At 20-24 GPa and temperatures above 1500 K the CAS phase with the composition CaAl4Si2O11 appears to accommodate excess Al from the perovskite along with two distinct populations of Al-bearing CaSiO3 perovskites, with Al content of 3.7 and 10.0 wt% Al2O3, respectively. The pressure and temperature of these transitions correspond to the lowermost part of the transition zone and therefore it suggests that Ca-rich aluminosilicates could provide alternative candidates to explain multiple seismic reflections near the 660 km depth discontinuity. (C) 2011 Elsevier B.V. All rights reserved.

The thermoelastic parameters of synthetic Ca3Al2Si3O12 grossular garnet were examined in situ at high-pressure and high-temperature by energy dispersive X-ray diffraction, using a Kawai-type multi-anvil press apparatus coupled with synchrotron radiation. Measurements have been conducted at pressures up to 20 GPa and temperatures up to 1,650 K: this P, T range covered the entire high-P, T stability field of grossular garnet. The analysis of room temperature data yielded V-0,V-300 = 1,664 +/- 2 angstrom(3) and K-0 = 166 +/- 3 GPa for K'(0) fixed to 4.0. Fitting of our P-V-T data by means of the high-temperature third order Birch-Murnaghan or the Mie-Gruneisen-Debye thermal equations of state, gives the thermoelastic parameters: (partial derivative K-0,K-T/partial derivative T)(P) = -0.019 +/- 0.001 GPa K-1 and alpha(0,T) = 2.62 +/- 0.23 x 10(-5) K-1, or gamma(0) = 1.21 for fixed values q(0) = 1.0 and theta(0) = 823 (Isaak et al. Phys Chem Min19:106-120, 1992). From the comparison of fits from two different approaches, we propose to constrain the bulk modulus of grossular garnet and its pressure derivative to K-T0 = 166 GPa and K'(T0) = 4.03-4.35. Present results are compared with previously determined thermoelastic properties of grossular-rich garnets.

Simultaneous ultrasonic elastic wave velocity and in situ synchrotron X-ray measurements on grossular garnet were carried out up to 17 GPa and 1 650 K. P- and S-wave velocities and bulk and shear modulus showed linear pressure and temperature dependence. These data yielded a pressure derivative of the bulk modulus of 4.42(7) and a shear modulus of 1.27(3), which are in good agreement with those of garnets with variable chemical compositions. Temperature dependence of the bulk modulus of grossular (-1.36x10(-2)GPa/K) is also similar to that of other garnets, while the temperature dependence of the shear modulus of grossular (-1.11x10(-2)GPa/K) is higher than those of magnesium end-member garnets and pyrolitic garnet.

The high ability of the Al-rich CaSiO3 perovskite to contain large amounts of uranium (up to 4 at.% U) has been studied up to 54 GPa and 2400 K, using laser-heated diamond anvil cell (LH-DAC) and up to 18 GPa and 2200 K using a multi-anvil press (MAP). Both latter HP-HT techniques proved to be complementary and gave similar results, in spite of different heating modes (laser and furnace). Chemical reactions were characterized and described by electron probe microanalysis and analytical scanning electron microscopy while associated structural changes were precisely characterized by synchrotron angle dispersive X-ray diffraction and by X-ray micro-diffraction. The diffusion of uranium into the CaSiO3 matrix was measured as a function of run duration and temperature. We obtain diffusion coefficients with the same order of magnitude (about 10(-16) m(2) s(-1)) than for those found in the literature. After this work, coupled cationic substitutions of Ca by U and Si by Al are proposed to generate new interesting crystallographic features for a CaSiO3 perovskite: a higher compressibility, a tetragonal distortion along the c-axis with c/a ratio >1, a different compression behaviour of c-axis relative to a-axis, and a perovskite structure quenchable to ambient P and T conditions. The tetragonal U-bearing aluminous CaSiO3 perovskite is observed to remain stable at pressures up to 54 GPa, then in the (P, T) range of the upper part of the lower mantle. The influence of the present results, in terms of both uranium and aluminium partitioning related to the coexisting mineral phases as the (Mg,Fe)SiO3 Perovskite, is discussed. Uranium provides approximately 25% of the total energy generated within the deep Earth through its radioactive decay. The location of this source within the deep mantle is fundamental to the understanding of the geodynamics and thermal behaviour of our planet. Since the tetragonal structure of the U-bearing Al-rich CaSiO3 perovskite is expected to remain stable towards the base of the Earth's mantle, this latter phase is proposed to be the main storage mineral for heat producing actinides of the lower mantle. (C) 2008 Elsevier B.V. All rights reserved.

Uranium is one of the main heat sources in the Earth, as about 25% of the total heat is produced by the radioactive decay of U. The location of U in the deep mantle is then essential for a better understanding of the geodynamics and thermal behavior of the Earth. For the first time, the crystal structure of natural simple dioxide UO2 uraninite has been studied by X-ray diffraction with synchrotron radiation (ESRF, Grenoble, France), in situ in a laser-heated diamond-anvil cell at pressures and temperatures relevant to the deep Earth's mantle. Fluorite-type UO2 displays a new sequence of phase transitions at high P and T, with a cubic modified fluorite Pa (3) over bar observed at 18 GPa, and an orthorhombic Pbca structure from 33 GPa up to 82 GPa. Using a second-order Birch-Murnaghan equation of state, we calculated room-pressure bulk modulus K-0 = 166(7) GPa with pressure derivative K'(0) = 4.0 for the Pa (3) over bar structure, and K-0 = 225(8) GPa with K'(0) = 4 for the Pbca structure. The expected Pnma cotunnite structure was not observed but is not excluded at pressures higher than 82 GPa. Since UO2 displays a Pbca structure stable up to 82 GPa and presents a density much higher than the average density of the surrounding mantle, UO2 could be a host of U in the deep lower mantle.

Macro-, and microXANES and EXAFS spectra were collected at the U-L-3, Al-K, Si-K and Ca-K edges in high pressure/high temperature CaSiO3 perovskite that aim to simulate one potential reservoir for the Earth heat flow. In that perovskite, U is tetravalent and appears substituted to Ca thanks to a coupled substitution of Al for Si. A FEFF model of the Ca site occupied by U is consistent with that mechanism but the modeled EXAFS spectrum is markedly different to that measured in various samples at various scales (mm and mu m). Structural relaxation of the U site probably explains these discrepancies, suggesting that, if ionic rules can explain qualitatively cation substitutions, more complex mechanisms occur at the angstrom-scale.

The distribution of the radiogenic heat sources strongly influences the geodynamics and thermal behaviour of the Earth. About 11 TW is produced by the radioactive decay of uranium (25% of the total heat flux at Earth surface), and 55% of this energy comes from the lower mantle. Here we report the first experimental evidence that aluminous CaSiO3 perovskite is the major, or even the only, host of uranium in the Earth lower mantle, since such a phase is able to incorporate up to 35 wt% UO2 ( or 4 at% of U). The aluminous Ca-perovskite could be the main U-bearing constituent of a dense and radiogenic reservoir proposed in a recent model and located in the bottom half of the lower mantle.