Post-perovskite Transition Research Articles

Raman spectroscopic insights into the transition to the post-post-perovskite phase in NaZnF3

The phase transition sequence of perovskite materials is an important subject in numerous fields ranging from geophysics to material science. The fluoride NaZnF3, adopting the distorted perovskite (Pv) structure (GdFeO3-type), is a model system for studying such transitions. Up to date compression experiments on this compound, limited to pressures of 40 GPa, have identified a perovskite to post-perovskite (pPv, CaIrO3-type structure) transition to occur above 11 GPa followed by yet another transition commencing at 25 GPa and leading to a post-post-perovskite (ppPv) phase whose structure is yet to be identified. Here we present high-pressure Raman spectroscopic study aided by Density Functional Theory (DFT) calculations to explore the phase transition sequence of NaZnF3 at pressures reaching 70 GPa. We confirm that NaZnF3 undergoes a gradual phase transition between the GdFeO3 and CaIrO3 phases starting at 12 GPa. Our results also indicate that above 31 GPa, the CaIrO3 polymorph slowly transforms to the ppPv phase which we unambiguously confirm to adopt the metastable Sb2S3-type structure despite the ground state structure above 23 GPa being La2S3. The transition into Sb2S3-type structure type instead of the La2S3 polytype is most probably due to kinetic effects which also hinder the predicted decomposition of NaZnF3.

Anharmonic thermodynamic properties and phase boundary across the postperovskite transition in MgSiO3

To address the effects of lattice anharmonicity across the perovskite to post-perovskite transition in MgSiO$_3$, we conduct calculations using the phonon quasiparticle (PHQ) approach. The PHQ is based on \textit{ab initio} molecular dynamics and, in principle, captures full anharmonicity. Free energies in the thermodynamic limit ($N \rightarrow \infty$) are computed using temperature-dependent quasiparticle dispersions within the phonon gas model. Systematic results on anharmonic thermodynamic properties and phase boundary are reported. Both the local density approximation (LDA) and the generalized gradient approximation (GGA) calculations are performed to provide confident constraints on these properties. Anharmonic effects are demonstrated by comparing results with those obtained using the quasiharmonic approximation (QHA). The inadequacy of the QHA is indicated by its overestimation of thermal expansivity and thermodynamic Gr\"{u}neisen parameter and its converged isochoric heat capacity in the high-temperature limit. The PHQ phase boundary has a Clapeyron slope ($dP/dT$) that increases with temperature. This result contrasts with the nearly zero curvature of the QHA phase boundary. Anharmonicity bends the phase boundary to lower temperatures at high pressures. Implications for the double-crossing of the phase boundary by the mantle geotherm are discussed.

Ferric iron in bridgmanite and implications for ULVZs

The seismic velocities of ferric iron-enriched bridgmanite under core-mantle boundary (CMB) conditions were calculated using GGA + U ab initio molecular dynamics to probe whether ferric iron enriched mantle can explain the properties of ultra-low velocity zones (ULVZ). Under these conditions ferric iron demonstrated some unusual properties.The effect of ferric iron on the 0 K transition pressure of the bridgmanite (bdg) to postperovskite (ppv) transition with GGA + U has some dependence upon the value of Ueff that is set for B site ferric iron which is in contrast to most substances where Ueff has little effect on the properties. We find that ferric iron can both stabilise and destabilise the bdg phase relative to the ppv phase depending upon the value of Ueff. This is due to the spin state transition of B-site ferric iron and was not seen in ferrous or aluminous ferric bridgmanite which lack such a transition at mantle conditions. Due to the similar energies and pressure derivatives of the bdg and ppv phases very subtle energy changes (such as that of iron clustering or different theoretical or experimental setups) can have large effects on the relative phase stabilities at different pressures.The spin state of ferric iron in bdg demonstrates some novel behaviours. With large iron enrichment the spin state of the B site has a non-linear dependence on concentration and depends upon the local arrangement of iron. At typical lower mantle pressures, ferric iron is expected to be high spin in the A site and low spin at the B site but at the temperatures of the CMB thermal spreading of the electrons causes ferric iron at the B site to become high spin.This is not a typical “spin transition” however as it involves no structural rearrangement of the unit cell and is predicted to have no effect on the elasticity and thus the seismicity of the crystal.Our elasticity results show that at the CMB ferric iron-enriched bridgmanite has nearly identical Vp and Vs to ferrous iron-bearing bridgmanite, and thus bridgmanite containing large amounts of Fe2+ or Fe3+ can both explain ULVZ properties when mixed with ferropericlase. We find, however, that mixing of both ferric and ferrous iron in bridgmanite causes a large (up to 1 km/s) non-ideal decrease in Vp when there is middling amounts of ferric iron. This non-linear mixing should have significant effects throughout the lower mantle whenever such amounts of ferric iron are present though such a ferrous:ferric ratio would typically require an increase of the natural ferrous:ferric ratio (for example through very high oxidation fugacities or chemical doping). In the case of ULVZs this non-linear mixing significantly impairs the fitting to some ULVZ data sets (those with a dlnVs/dlnVp ~ 3), but greatly improves the fitting to other ULVZ data sets (those with a dlnVs/dlnVp ~ 1).The oxidation state of highly enriched iron in bridgmanite at CMB conditions has, therefore, a large effect on the ability of mantle mixtures that are highly enriched in iron to replicate ULVZ properties.

Kinetics and detectability of the bridgmanite to post-perovskite transformation in the Earth's D\u2033 layer

Bridgmanite, the dominant mineral in the Earth’s lower mantle, crystallizes in the perovskite structure and transforms into post-perovskite at conditions relevant for the D{}^{{primeprime} } layer. This transformation affects the dynamics of the Earth’s lowermost mantle and can explain a range of seismic observations. The thickness over which the two phases coexist, however, can extend over 100 km, casting doubt on the assignment of the observed seismic boundaries. Here, experiments show that the bridgmanite to post-perovskite transition in (Mg{}_{0.86},Fe{}_{0.14})SiO{}_{3} is fast on geological timescales. The transformation kinetics, however, affects reflection coefficients of P and S waves by more than one order of magnitude. Thick layers of coexisting bridgmanite and post-perovskite can hence be detected using seismic reflections. Morever, the detection and wave period dependence of D{}^{{primeprime} } reflections can be used to constrain significant features of the Earth’s lowermost mantle, such as the thickness of the coexistence layer, and obtain information on temperature and grain sizes.

Seismological Constraints on the Small‐Scale Heterogeneity in the Lowermost Mantle Beneath East Asia and Implication for Its Mineralogical Origin

AbstractPrevious seismological studies have focused on the large‐scale heterogeneity of the D″ layer beneath East Asia. Here, we use high‐frequency PcP precursors recorded at dense networks in China and Japan to constrain the small‐scale heterogeneity in this region. By forward modeling of the stacks of recorded PcP precursory signal, we find that the small‐scale heterogeneity in the D″ layer beneath East Asia is characterized by a P wave velocity perturbation of 0.1‐0.2% and a dominant length scale of 4‐20 km. Although the resolved length scale is similar to the dimension of the subducted oceanic crust, the weak velocity perturbation is difficult to explain by basaltic rock in the lowermost mantle. Postperovskite transition of the supposed slab might also not be viable for generating weak heterogeneity; thus, new explanations are needed from geodynamics and mineral physics to account for the different strength of the large‐ and small‐scale heterogeneity.

Thermoelasticity of Iron‐ and Aluminum‐Bearing MgSiO3 Postperovskite

AbstractWe present ab initio (LDA + Usc) studies of high‐temperature and high‐pressure elastic properties of pure as well as iron‐bearing (ferrous, Fe2+, and ferric, Fe3+) and aluminum‐bearing MgSiO3 postperovskite, the likely dominant phase in the deep lower mantle of the Earth. Thermal effects are addressed within the quasiharmonic approximation by combining vibrational density of states and static elastic coefficients. Aggregate elastic moduli and sound velocities for the Mg end members are successfully compared with scarce experimental data available. Effects of iron (Fe) and aluminum (Al) substitutions on elastic properties and their pressure and temperature dependence have been thoroughly investigated. At the observed perovskite to postperovskite transition (P = 125 GPa and T = 2,500 K), compressional and shear velocities increase by 0–1% and 1.5–3.75%, respectively. This observation is consistent with some seismic studies of the D′′ discontinuity beneath the Caribbean, which suggests that our robust estimates of elastic properties of the postperovskite phase will be very helpful to understand lateral velocity variations in the deep lower mantle region and to constrain its composition and thermal structure.

Post-perovskite Transition in Anti-structure.

The discovery of the post-perovskite transition, which is the structural transition from the perovskite to post-perovskite structure in MgSiO3 under pressure, has aroused great interests in geosciences. Despite of previous extensive studies, key factors of the post-perovsktie transition are still under hot debate primarily due to the big difficulty in performing systematic experiments under extreme conditions. Hence, search for new materials showing the post-perovskite transition under ambient pressure has been highly expected. We here report a new-type of materials Cr3AX (A = Ga, Ge; X = C, N), which exhibits the post-perovskite transition as a function of “chemical pressure” at ambient physical pressure. The detailed structural analysis indicates that the tolerance factor, which is the measure of the ionic radius mismatch, plays the key role in the post-perovskite transition. Moreover, we found a tetragonal perovskite structure with loss of inversion symmetry between the cubic perovskite and orthorhombic post-perovskite structures. This finding stimulates a search for a ferroelectric state in MgSiO3.

Spin crossovers in iron-bearing MgSiO3 and MgGeO3: Their influence on the post-perovskite transition

MgGeO3-perovskite is known to be a low-pressure analog of MgSiO3-perovskite in many respects, but especially in regard to the post-perovskite transition. As such, investigation of spin state changes in Fe-bearing MgGeO3 might help to clarify some aspects of this type of state change in Fe-bearing MgSiO3. Using DFT+U calculations, we have investigated pressure induced spin state changes in Fe2+ and Fe3+ in MgGeO3 perovskite and post-perovskite. Owing to the larger ionic radius of germanium compared to silicon, germanate phases have larger unit cell volume and inter-atomic distances than equivalent silicate phases at same pressures. As a result, all pressure induced state changes in iron occur at higher pressures in germanate phases than in the silicate ones, be it a spin state change or position change of (ferrous) iron in the perovskite A site. We showed that iron state transitions occur at particular average Fe–O bond-length (i.e., ∼2.22(1)Å and ∼1.86(1)Å for Fe2+ and Fe3+ substitutions, respectively) irrespective of mineral composition (silicate or germanate) or exchange–correlation functionals used in the calculation (LDA+Usc or GGA+Usc). Ferrous iron substitution decreases the perovskite to post-perovskite (PPv) transition pressure while coupled ferric iron substitution increases it noticeably.

The stability of hydrous silicates in Earth's lower mantle: Experimental constraints from the systems MgO–SiO2–H2O and MgO–Al2O3–SiO2–H2O

We performed laser-heated diamond anvil cell experiments on bulk compositions in the systems MgO–SiO2–H2O (MSH) and MgO–Al2O3–SiO2–H2O (MASH) that constrain the stability of hydrous phases in Earth's lower mantle. Phase identification by synchrotron powder diffraction reveals a consistent set of stability relations for the high-pressure, dense hydrous silicate phases D and H. In the MSH system phase D is stable to ~50GPa, independent of temperature from ~1300 to 1700K. Phase H becomes stable between 35 and 40GPa, and the phase H out reaction occurs at ~55GPa at 1600K with a negative dT/dP slope of ~−75K/GPa. Between ~30 and 50GPa dehydration melting occurs at ~1800K with a flat dT/dP slope. A cusp along the solidus at ~50GPa corresponds with the intersection of the subsolidus phase H out reaction, and the dT/dP melting slope steepens to ~15K/GPa up to ~85GPa.In the MASH system phase H is stable in experiments between ~45 and 115GPa in all bulk compositions studied, and we expect aluminous phase H to be stable throughout the lower mantle depth range beneath ~1200km in both peridotitic and basaltic lithologies. In the subsolidus, aluminous phase D is stable to ~55GPa, whereas at higher pressures aluminous phase H is the stable hydrous phase. The presence of hydrogen may sharpen the bridgmanite to post-perovskite transition. The ambient unit cell volume of bridgmanite increases systematically with pressure above ~55GPa, possibly representing an increase in alumina content, and potentially hydrogen content, with depth. Bridgmanite in equilibrium with phases D and H has a relatively low alumina content, and alumina partitions preferentially into the hydrous phases. The melting curves of MASH compositions are shallower than in the MSH system, with dT/dP of ~6K/GPa. Phase D and H solid solutions are stable in cold, hydrated subducting slabs and can deliver water to the deepest lower mantle. However, hydrated lithologies in the lower mantle are likely to be partially molten at all depths along an ambient mantle geotherm.

High-pressure phase behaviors of ZnTiO3: ilmenite–perovskite transition, decomposition of perovskite into constituent oxides, and perovskite–lithium niobate transition

High-pressure high-temperature phase transitions of ZnTiO3 ilmenite were examined using multianvil apparatus up to 25.5 GPa and 1,500 °C and diamond anvil cell to 26.5 GPa and about 2,000 °C. Combined results of the multianvil quench experiments and in situ diamond anvil cell experiments indicated that at about 10 GPa and 1,200 °C ZnTiO3 ilmenite transforms to orthorhombic perovskite which is converted to lithium niobate phase on release of pressure. The boundary of the ilmenite–provskite transition is expressed by P(GPa) = 15.9 − 0.005T (°C). The high-pressure experiments also indicated that at 20–24 GPa and 1,000–1,400 °C ZnTiO3 orthorhombic perovskite dissociates into rocksalt-type ZnO + baddeleyite-type TiO2 which are recovered, respectively, as wurtzite-type ZnO and α-PbO2-type TiO2 at 1 atm. The boundary of the perovskite dissociation is expressed by P(GPa) = 8.7 + 0.011T (°C). Molar volume changes of ZnTiO3 at ambient conditions were estimated as −4.7 % for the ilmenite–perovskite transition and −3.5 % for the perovskite decomposition into the oxides. The absence of CaIrO3-type postperovskite in ZnTiO3 is consistent with that dissociation of ZnTiO3 perovskite into the oxides has the larger molar volume change than −1 to −2 % of the perovskite–postperovskite transition in various ABO3 compounds and with previous data that ABO3 perovskites with relatively ionic B–O bonds do not transform to the postperovskite. The transition behaviors of ZnTiO3 are similar to those of MnTiO3 and FeTiO3, but ZnTiO3 perovskite dissociates into the constituent oxides.

Postperovskite phase transition of ZnGeO3: comparative crystal chemistry of postperovskite phase transition from germanate perovskites.

The postperovskite phase of ZnGeO3 was confirmed by laser heating experiments of the perovskite phase under 110-130 GPa at high temperature. Ab initio calculations indicated that the phase transition occurs at 133 GPa at 0 K. This postperovskite transition pressure is significantly higher than those reported for other germanates, such as MnGeO3 and MgGeO3. The comparative crystal chemistry of the perovskite-to-postperovskite transition suggests that a relatively elongated b-axis in the low-pressure range resulted in the delay in the transition to the postperovskite phase. Similar to most GdFeO3-type perovskites that transform to the CaIrO3-type postperovskite phase, ZnGeO3 perovskite eventually transformed to the CaIrO3-type postperovskite phase at a critical rotational angle of the GeO6 octahedron. The formation of the postperovskite structure at a very low critical rotational angle for MnGeO3 suggests that relatively large divalent cations likely break down the corner-sharing GeO6 frameworks without a large rotation of GeO6 to form the postperovskite phase.

Effect of Fe-enrichment on seismic properties of perovskite and post-perovskite in the deep lower mantle

Recent experimental measurements of the equation of state of perovskites and post-perovskites in the (Mg,Fe)SiO3 and (Mg,Fe,Al)(Fe,Al,Si)O-3 systems over a wide range of iron contents are used to constrain the effects of Fe and Al on density and bulk modulus of these phases at deep mantle pressures. The density of Fe-bearing perovskite follows a linear relationship with Fe-content at a representative mid-mantle depth of 1850 km (80 GPa): (80) (g cm(- 3)) = 5.054(1) + 1.270(3)X-Fe. The bulk modulus of silicate perovskite is not sensitive to Fe-content and follows the relationship, K-80 (GPa) = 546(2) + 12(25)X-Fe. The velocity heterogeneity parameter, partial derivative ln V-B/partial derivative X-Fe, determined by experimental values for the bulk sound speed is 0.10(1), in agreement with theory and the behaviour of other Fe-bearing silicates. Near the core-mantle boundary, Fe-rich post-perovskite is observed to be more compressible than the Mg-end-member, in contrast to theoretical predictions. From experimental data, the densities of perovskite and post-perovskite at 125 GPa (2700 km depth) are (125,Pv) (g cm(-3)) = 5.426(11) + 1.38(4)X-Fe and (125,pPv) (g cm(-3)) = 5.548(1) + 1.41(3)X-Fe. The density contrast across the post-perovskite transition is similar to 2 per cent, irrespective of Fe-content, but the contrast in bulk sound speed increases with Fe-content. Al-rich silicates exhibit no significant differences in density or compressibility relative to Al-free silicates, but may be responsible for seismic heterogeneities due to differences in the depth and width of the post-perovskite transition. Observations of increased densities in large low shear velocity provinces and ultra-low-velocity zones may be consistent with local iron enrichment from Mg#90 to Mg# 78-88 and Mg# < 50, respectively.

High-pressure transitions in NaZnF3 and NaMnF3 perovskites, and crystal-chemical characteristics of perovskite–postperovskite transitions in ABX3 fluorides and oxides

Phase transitions in NaZnF3 and NaMnF3 were examined up to 24GPa and 1100°C using a multianvil apparatus. NaZnF3 perovskite transforms to postperovskite above 11–16GPa at 600–1000°C, and the postperovskite is quenchable at ambient conditions. The NaZnF3 perovskite–postperovskite transition boundary is expressed as P (GPa)=4.9+0.011T (°C). At 8–11GPa and 900–1100°C, NaMnF3 perovskite dissociates into two phases of Na3Mn2F7 and MnF2. The latter phase is suggested to have the structure of orthorhombic-I type ZrO2 or cotunnite. Using available experimental data on the perovskite–postperovskite transitions in thirteen compounds of A2+B4+O3 and A+B2+F3, several crystal-chemical characteristics of the transition are elucidated as follows. In the transition, the volume change is between −1% and −2%, and the Clapeyron slope of the boundary is 10–17MPa/°C. These support reliability of recently determined Clapeyron slope of 13MPa/°C in MgSiO3 which suggests that the perovskite–postperovskite boundary intersects the temperature profile twice in the D″ layer. Postperovskites of ABX3 whose enthalpies are higher by more than 70kJ/mol relative to the phase stable at 1atm are unquenchable, while those by less than 15kJ/mol are quenchable to ambient conditions. Structure refinements indicate that A+B2+F3 postperovskites quenched at 1atm are more similar to that of MgSiO3 postperovskite at high pressure, than those of quenched A2+B4+O3 postperovskites. With increasing pressure, octahedral tilt angles of both A2+B4+O3 and A+B2+F3 perovskites increase, resulting in transition to postperovskite at the angle of about 26°, and fluoride perovskites are more rapidly distorted with pressure than oxide perovskites. Covalent character of B–X bonds of ABX3 postperovskite is suggested to be favorable for stabilization of the postperovskite structure. All these features suggest that NaNiF3 is a good quenchable, low-pressure analogue compound to MgSiO3 to investigate the perovskite–postperovskite transition.

Stability, metastability, and elastic properties of a dense silica polymorph, seifertite

Dense silica polymorphs with sixfold coordinated Si have been found in SNC and lunar meteorites and may be important minerals for silica‐rich components in the lower mantle. However, the stable crystal structure in the lower mantle and properties of dense silica remain controversial. Under stable heating and quasi‐hydrostatic stress conditions, we found that the CaCl2 type undergoes a phase transition to the α‐PbO2type (seifertite) at 130–140 GPa and 2500 K. Our data suggest that this phase transition occurs at a greater depth than the perovskite → postperovskite transition in the lowermost mantle. The molar volume measured at 1 bar is the smallest among the reported silica polymorphs, therefore having the highest calculated density and in excellent agreement with recent first‐principles calculations. The greater molar volume of seifertite found in the shergottite meteorite and previous experiments supports a metastable synthesis of the phase outside its stability field. Our data combined with the Hugoniots of silica polymorphs also rule out the possibility of the formation of seifertite in the meteorite within its stability field. We found very little change in bulk sound speed across the CaCl2‐type → seifertite transition. If shear wave velocity decreases at the transition to seifertite as suggested by some computational studies, this silica transition may provide an alternative explanation for the discontinuities with a shear wave velocity decrease found at depths greater than the D″discontinuity.

Nucleation mechanism and kinetics of the perovskite to post-perovskite transition of MgSiO3 under extreme conditions

The atomistic mechanism of the perovskite→post-perovskite transformation of MgSiO3 is explored from molecular simulation mimicking extreme pressure and temperature conditions akin to planet Earth’s interior (D″ region). The nucleation process is highly anisotropic and initiated by column-wise rotation of SiO6 octahedra around the [001] direction. The post-perovskite transition is found to be a rare event (i.e. one requiring substantial activation energy, ∼15eV for the critical nucleus, but, once started, propagates very fast (103–104m/s). The most stable phase front, the (010) interface, propagates at only 10% of the speed of the (001) phase front.

Synthesis and equation of state of perovskites in the (Mg, Fe)3Al2Si3O12 system to 177 GPa

Natural and synthetic pyrope–almandine compositions from 38 to 100mol% almandine (Alm38–Alm100) were studied by synchrotron X-ray diffraction in the laser-heated diamond anvil cell to 177GPa. Single-phase orthorhombic GdFeO3-type perovskites were synthesized across the entire examined compositional range at deep lower mantle pressures, with higher Fe-contents requiring higher synthesis pressures. The formation of perovskite with Alm100 (Fe3Al2Si3O12) composition at 80GPa marks the first observation of a silicate perovskite in a Fe end-member. Fe-enrichment broadens and lowers the pressure range of the post-perovskite transition for intermediate compositions such as Alm54, but the more Fe-rich Alm100-composition perovskite remains stable to pressures as high as 149GPa. Volume compression data for the Alm54 and Alm100 compositions were fit to the Birch–Murnaghan equation of state. The compressibility of perovskites synthesized from compositions along the pyrope–almandine join is not strongly sensitive to Fe-content. The compression curves were smooth over the entire measured range, and no evidence for a volume anomaly associated with a spin transition was observed.

Thermal and transport properties of mantle rock at high pressure: Applications to super-Earths

In the present study, the temperature- and pressure-dependent transport and thermal properties, i.e., viscosity, phonon thermal conductivity, thermal expansivity and heat capacities, as well as electronic and radiative thermal conductivities, have been derived for the mantles of super-Earths. These properties are necessary to understand the interior dynamics and the thermal evolution of those planets. We assume that the mantles consist of MgSiO3 perovskite (pv), but we discuss the effects of the post-perovskite transition, and we elaborate on an addition of periclase MgO and incorporated Fe. However, MgO is found to only significantly influence the phonon thermal conductivity – the viscosities, heat capacities and thermal expansivities of pv and MgO remain comparable. We use the Keane theory of solids, which takes into account the behavior of solid matter at the infinite pressure limit, adopt the Keane equations of state, and adjust for pv and MgO by comparison with experimental high-pressure and high-temperature data. We find the theory of the infinite pressure limit of Keane to be in excellent agreement with recent ab initio studies and experiments. To calculate the melting curve, we further use the Lindemann–Stacey scaling law and fit it to available experimental data. The best data fitting melting temperature for pv reaches 5700K at 135GPa and increases to 20,000K at 1.1TPa, corresponding to the core-mantle boundary of a 10 Earth mass super-Earth (10MEarth). We find the pv adiabatic temperature (with a potential temperature of 1700K) to reach 2570K at 135GPa and 5000K at 1.1TPa. To calculate the pressure-and temperature-dependent viscosity, we use the semi-empirical homologous temperature scaling to relate enthalpy change, and hence viscosity, to the melting temperature. We find that the resulting activation volume of pv decreases from 2.8cm3/mol at 25GPa to 1.4cm3/mol at 1.1TPa-resulting in a viscosity increase by ∼15 orders of magnitude through the adiabatic mantle of a 10MEarth planet. Furthermore, the thermal expansivity (of pv and MgO) decreases by a factor of eight, and the total thermal conductivity (phonon, radiative and electronic) of an Earth-like pv/MgO composite increases by a factor of seven through an adiabatic mantle of a 10MEarth super-Earth. At higher temperatures, i.e., for super-adiabatic temperature profiles, the electronic and radiative thermal conductivities strongly increase and dominate the conductive heat transport. All findings indicate an increase of heat transfer solely by conduction in the lower mantles of super-Earths. Thus our results disagree with Earth-biased full-mantle convection assumptions made by previous models for super-Earths, and additionally raise questions about the differentiation of massive rocky exoplanets and their ability to generate magnetic fields or sustain plate tectonics.

Seismic velocity structures and detailed features of the D″ discontinuity near the core-mantle boundary beneath eastern Eurasia

Shear and compressional velocity structures and features of the D″ discontinuity near the core-mantle boundary (CMB) beneath eastern Eurasia are studied based on forward waveform modeling and differential-travel-time analysis of ScSH-SH, sScS-sS, PcP-P and pPcP-pP phases. High-quality differential-travel-time analysis reveals two high-velocity patches spanning a width of 3000 km and separated by a 300-km wide normal velocity gap. The maximum shear and compressional velocity perturbations reach 3.8% and 2.7%, respectively. Seismic data indicate that the D″ discontinuity emerges in the high velocity patches and disappears in the normal-velocity gap. Furthermore, the features of the D″ discontinuity, although constrained by rather sparse seismic data, vary from diffusive discontinuities in the eastern patch to sharp discontinuities in the western patch. The D″ discontinuities occur at about 270 km above the CMB in the eastern patch with a shear velocity jump of 3.0–5.0% and a transitional depth of at least 100 km, and at 250 km above the CMB in the western patch with an abrupt shear velocity jump of 5.0%. A diffusive P wave discontinuity is also observed in the eastern patch with the similar features of the S wave discontinuity. The inferred features of the D″ discontinuity can be explained by the post-perovskite transformation in the lower mantle. The observations that the D″ discontinuities occur only in the high-velocity patches may be due to the effect of the positive Clapeyron slope of the post-perovskite transition, and the varying transitional depths of the D″ discontinuity in the high-velocity regions may be caused by the compositional effects on the post-perovskite transition in the presence of strong variations of composition within the high-velocity anomalies. The high-velocity patches geographically correlate with the subduction of the Pacific and Indian plates in the past 30–60 Ma, and the normal-velocity gap falls into the subduction gap between the Pacific and Indian plates. These geographical correlations raise the possibility that the existence of the D″ discontinuity is intimately related to the presence of the past subduction material at the CMB, and varying features of the discontinuity further suggest existence of strong variations of composition within the subduction materials at the CMB.

Influence of initial CMB temperature and other parameters on the thermal evolution of Earth's core resulting from thermochemical spherical mantle convection

Here we present a wide‐ranging parameter study of the effects of initial core‐mantle boundary (CMB) temperature, concentration of radioactive potassium in the core, and density difference between harzburgite and mid‐ocean ridge basalt (MORB) in a coupled spherical model of thermochemical mantle convection and parameterized core heat balance. The initial CMB temperature is expected to be much higher than the solidus temperature of silicates at the base of the mantle. The results indicate that as with previous, purely thermal convection models, the final state of the system is only weakly dependent on initial CMB temperature unless the CMB becomes blanketed by a global layer of dense material. Fully 3‐D spherical cases have a very similar core evolution to cases in a 2‐D spherical annulus, giving confidence in the applicability of 2‐D spherical annulus geometry for modeling Earth's evolution. Obtaining a successful thermal evolution, in the sense of obtaining the correct present‐day inner core size and maintaining a geodynamo over geological time, is helped by the accumulation of piles of dense material at the CMB (subducted MORB in the present calculations) and a concentration of radiogenic K in the core in the range 400–800 ppm. The present‐day CMB heat flow is predicted to be around 9 TW. While this is lower than estimates based on calculating temperature gradients in regions where the postperovskite transition is seismically imaged, these tend to be areas of higher than average heat flux and thus likely overestimate the global heat flow.

On the interaction of the geotherm with a post-perovskite phase transition in the deep mantle

The energy balance in the presence of a perovskite (Pv) to post-perovskite (pPv) transition within Earth’s D” layer is examined in order to explore the relationship between changes in seismic velocity associated with this phase change, the extent of the two-phase Pv–pPv coexistence region, and the thermal structure of the deep mantle. This is motivated in part by the fact that discontinuities attributed to the Pv–pPv transition are inferred to be seismically sharper than permitted by some recent estimates of the pressure increment across the two-phase co-existence region. Here it is shown that sharp gradients in phase abundance may arise even when the two-phase loop is very broad, and therefore the pressure increment determined from thermodynamic stability alone is a poor proxy for predicting the sharpness of Pv–pPv related seismic discontinuities. The change in pPv fraction over the steepest gradients in phase can also be highly variable, which would lead to potentially complex variations in the total strength of seismic discontinuities. Latent heat plays an important role in the structure of the pPv phase change and its influence upon the geotherm. For the double-crossing scenario – in which a deeper reverse transformation from pPv to Pv occurs in a steep thermal boundary layer – latent heat release from the shallower Pv–pPv transition moderates the effects of latent heat absorption at the deeper reverse transition.