MgSiO3 Perovskite Research Articles

Thermal conductivity of CaGeO3perovskite at high pressure

[1] The pressure dependence of thermal conductivity () in Earth's lower mantle is poorly understood, resulting in very uncertain values in the D″. Using orthorhombic CaGeO3 perovskite as an analogue for MgSiO3 perovskite, we measured in a multi-anvil apparatus using the Angstrom method at pressures of 8, 10, 14 and 18 GPa, and temperatures of 573–1073 K. We find the pressure dependence of thermal conductivity for orthorhombic perovskite shows a somewhat higher slope than that predicted by Debye theory, and account for this by relating phonon velocities to the bulk modulus, rather than to the Debye temperature. From this relation, we estimate the thermal conductivity of MgSiO3 perovskite at the top and bottom of the D″ to be 3.3 W/m K and 2.2 W/m K, corresponding to values of 6.4 W/m K and 4.5 W/m K for the mantle.

The melting curve of MgSiO3 perovskite from molecular dynamics simulation

The high-pressure melting curve of MgSiO3 perovskite is simulated by using the constant temperature and pressure molecular dynamics method combined with effective pair potentials. The simulated structural properties of MgSiO3 perovskite at ambient conditions reproduce the experiments and agree well with other theoretical works. The calculated equation of state is very successful in reproducing accurately the recent experimental data over wide pressure ranges. The predicted high-pressure melting curve is in good agreement with the recent experimental and the latest theoretical ones, and the melting curve up to the core–mantle boundary pressure, being very steep at lower pressures, rapidly flattens on increasing pressure. The present results also suggest the validity of the experimental data of Zerr and Boehler (1993 Science 262 553) and Shen and Lazor (1995 J. Geophys. Res. 100 17699).

鉱物結晶学の多様化：新たな広がりと挑戦 １．高圧鉱物（地球深部物質）ＭｇＳｉＯ３‐ＦｅＡｌＯ３系ペロブスカイトの固溶メカニズムと結晶化学

MgSiO3 dominant perovskite is believed to be the most abundant constituent mineral in the Earth’s lower mantle. Generally minerals form solid solutions and their nature should affect on physical properties of minerals. In this paper, we will introduce our recent studies about incorporation mechanism of FeAlO3 component into MgSiO3 perovskite and its crystal chemistry.

Silicon self‐diffusion of MgSiO3 perovskite by molecular dynamics and its implication for lower mantle rheology

The pressure effect of silicon self‐diffusion of MgSiO3 perovskite was investigated by molecular dynamics (MD). The viscosity variation of MgSiO3 perovskite in the lower mantle was derived by the Nabarro‐Herring (Herring‐Nabarro) model. For the MD calculation, the spontaneous jumping of atoms by self‐diffusion was reproduced without using artificial forces, and the consistency of migration enthalpy (Hm*) with experimental data was improved. The results showed that migration enthalpy increases monotonically with increasing pressure. The viscosity of MgSiO3 perovskite in the lower mantle increases monotonically with increasing depth. The obtained depth profile is distinguishable from that of MgO periclase and can be utilized to determine which mineral dominates the lower mantle rheology. Depending upon the assumed shape of the depth profile for lower mantle viscosity, we considered the dominant mineral as (1) MgSiO3 perovskite for the monotonic shape case or (2) MgO periclase for the hill shape case that has the highest‐viscosity zone in the middle of the lower mantle.

Solid solution behaviour of CaSiO3 and MgSiO3 perovskites

Using density functional simulations within the generalized gradient approximation and projector-augmented wave method together with thermodynamic modelling, the reciprocal solubilities of MgSiO3 and CaSiO3 perovskites were calculated for pressures and temperatures of the Earth’s lower mantle from 25 to 100 GPa and 0 to 6,000 K, respectively. The solubility of Ca in MgSiO3 at conditions along a mantle adiabat is found to be less than 0.02 atoms per formula unit. The solubility of Mg in CaSiO3 is even lower, and most important, the extent of solid solution decreases with pressure. To dissolve CaSiO3 perovskite completely in MgSiO3 perovskite, a solubility of 7.8 or 2.3 mol% would be necessary for a fertile pyrolytic or depleted harzburgitic mantle, respectively. Thus, for any reasonable geotherm, two separate perovskites will be present in fertile mantle, suggesting that Ca-perovskite will be residual to low degree melting throughout the entire mantle. At the solidus, CaSiO3 perovskite might completely dissolve in MgSiO3 perovskite only in a depleted mantle with <1.25 wt% CaO. These implications may be modified if Ca solubility in MgSiO3 is increased by other major mantle constituents such as Fe and Al.

Elasticity of AlFeO3and FeAlO3perovskite and post-perovskite from first-principles calculations

[1] We use state-of-the-art ab initio calculations based on the generalized gradient approximation of the density functional theory in the planar augmented wavefunction formalism to determine the elastic constants tensor of perovskite and post-perovskite with formulas AlFeO3 and FeAlO3 in which Fe or Al respectively occupy only octahedral sites, for the stable magnetic configurations. The phase transition between perovskite and post-perovskite is associated with a site exchange, during which Fe from the inter-octahedral site in perovskite moves into the octahedral site in post-perovskite. Following this transition path the elastic moduli show positive jumps, considerably larger than for MgSiO3. The phase transition is marked by a positive jump of 0.04 km/s (0.33%) in the velocity of the compressional waves and by a negative jump of −0.15 km/s (−1.87%) in shear wave velocity. We find that the effects of the Mg + Si Al + Fe substitution on the seismic properties of MgSiO3 perovskite and post-perovskite depend on the crystallography of the substitution, namely the position the exchanged cations take in the structure.

Application of the cBΩ model for the calculation of oxygen self-diffusion coefficients in minerals

The cBΩ model, which suggests the defect Gibbs energy is proportional to the isothermal bulk modulus and the mean volume per atom, is first introduced to predict self-diffusion coefficients of oxygen in various silicate and oxide minerals in terms of available elastic data. We develop a new approach to determine constant c in the cBΩ model on the basis of the observed compensation effect between the activation energies and pre-exponential factors, which is critical to the diffusivity prediction. Under anhydrous conditions, the validity of this model is tested by the experimentally determined oxygen self-diffusion coefficients. Our results show that the absolute oxygen diffusion rates derived from the cBΩ model are in agreement with experimental data in a variety of rock-forming minerals including olivine, MgSiO3 perovskite, spinel, and zircon.

New structure and spin state of iron-rich (Mg,Fe)SiO3 post-perovskite

There was a discrepancy between the seismic tomography and the elastic property of MgSiO3 perovskite at near the D" zone and core boundary. Since a discovery of post-perovskite (ppv) of MgSiO3, many investigations have made to explain the presence of low seismic velocity of the lower mantle and D" zone. However, precise experimental structure analysis of ppv-(Mg1-xFex)SiO3 has never been reported because of the experimental difficulty. Fe and Mg cation distribution and ordering in ppv-(Mg,Fe)SiO3 in consideration of spins states are significant subject in lower mantle electronic and magnetic states. The present experiment aims X-ray emission study and structure analysis by Rietveld profile fitting of ppv-(Mg0.6,Fe0.4)SiO3 by the precise powder diffraction measurement. Monte Carlo calculation proposed the reliable structures of iron-rich phase of ppv-(Mg,Fe)SiO3: Pmmn, Pmma, and Cm2m and Cmcm proposed by. The best-fit structure model with the highest reliability in the Rietveld fitting of ppv-(Mg0.6Fe0.4)SiO3 is the structure of space group Pmma, in which Fe and Mg occupy two different sites of M1 and M2: the site occupancies are (Fe0.25Mg0.75) in the larger M1 site and (Fe0.55, Mg0.45) in the smaller M2 site. The two-site model is consistence with the previous results of X-ray emission and X-ray Mössbauer experiments.

First principles study of thermodynamics and phase transition in low‐pressure (P21/c) and high‐pressure (C2/c) clinoenstatite MgSiO3

Using first principles quasi‐harmonic theory, we have investigated the vibrational and thermodynamic properties of P21/c and C2/c MgSiO3 clinoenstatites. Very good agreement between experimentally measured thermodynamic properties, such as equation of state, and those predicted by the local density approximation (LDA) functional has been found. The level of agreement found in these chain silicates is comparable to that found in framework silicates such as MgSiO3 perovskite. The phase boundaries calculated by the LDA and the generalized gradient approximation (GGA) bracket the experimentally measured ones following the established trend that the GGA functional overestimates the transition pressure while the LDA functional underestimates it. The calculated Clapeyron slope is 2.9 MPa K−1 and is used to constrain the position of the 3‐pyroxene (P21/c, C2/c, and Pbca) triple point in MgSiO3, with the aid of room temperature experimental hysteresis in the P21/c to C2/c transition.

Multi-Mbar Phase Transitions in Minerals

MgSiO3 perovskite is the most abundant mineral in the Earth’s lower mantle. A structural phase transition in this phase to a CaIrO3-type polymorph was discovered in 2004 (Murakami et al. 2004; Oganov and Ono 2004; Tsuchiya et al. 2004). This new polymorph, the so-called post-perovskite (PPV) phase, was produced at pressures and temperatures close to those expected at the core-mantle boundary, 125 GPa and 2,500 K (Murakami et al. 2004). In the Earth, the PPV phase is the final form of MgSiO3. This surprising discovery invited a new question: what is the next polymorph of MgSiO3? MgSiO3 PPV consists of SiO3 layers intercalated by magnesium (Fig. 1⇓). Therefore, it is natural to expect still other pressure induced transitions to more isotropic close-packed looking structures. This question has acquired further importance since the discovery of terrestrial-type exoplanets: the Super-Earth planet with ~7 Earth masses, GJ876d (Rivera et al. 2005), and the Saturn-like planet with a massive dense core with ~67 Earth masses, D149026b (a dense-Saturn) (Sato et al. 2005). Many others have been found since then. Pressures and temperatures in the mantle of these planets are much higher than in the Earth. There is also a pressing need to understand and model matter in the core of the giants, particularly the solar ones, Jupiter, Saturn, Uranus, and Neptune. In GJ876d, pressure and temperature at its core-mantle boundary was roughly estimated to be ~1 TPa (10 Mbar) and ~4,000 K (Valencia et al. 2006). The gas giants, Jupiter and Saturn, and the icy giants, Uranus and Neptune, have small dense cores surrounded by hydrogen/helium and ice, respectively. Pressures and temperatures at the core-envelope boundaries of these planets have been estimated to be 40 Mbar and 15,000~20,000 K in Jupiter, 10 …

Spin state of ferric iron in MgSiO3 perovskite and its effect on elastic properties

Abstract Recent studies have indicated that a significant amount of iron in MgSiO 3 perovskite (Pv) is Fe 3+ (Fe 3+ /ΣFe = 10–60%) due to crystal chemistry effects at high pressure ( P ) and that Fe 3+ is more likely than Fe 2+ to undergo a high-spin (HS) to low-spin (LS) transition in Pv in the mantle. We have measured synchrotron Mossbauer spectroscopy (SMS), X-ray emission spectroscopy (XES), and X-ray diffraction (XRD) of Pv with all iron in Fe 3+ in the laser-heated diamond-anvil cell to over 100 GPa. Fe 3+ increases the anisotropy of the Pv unit cell, whereas Fe 2+ decreases it. In Pv synthesized above 50 GPa, Fe 3+ enters into both the dodecahedral (A) and octahedral (B) sites approximately equally, suggesting charge coupled substitution. Combining SMS and XES, we found that the LS population in the B site gradually increases with pressure up to 50–60 GPa where all Fe 3+ in the B site becomes LS, while Fe 3+ in the A site remains HS to at least 136 GPa. Fe 3+ makes Pv more compressible than Mg-endmember below 50 GPa because of the gradual spin transition in the B site together with lattice compression. The completion of the spin transition at 50–60 GPa increases bulk modulus with no associated change in density. This elasticity change can be a useful seismic probe for investigating compositional heterogeneities associated with Fe 3+ .

The crystal structure of CaIrO3 post-perovskite revisited

Abstract The structure of CaIrO3 post-perovskite has been determined using CAD4 and Bruker SMART diffractometers for two different crystals. In both cases the structure was solved by Direct Methods which revealed all four atoms in the asymmetric unit. Anisotropic refinement of all atoms gave (CAD4/SMART): R 1 = 0.016/0.008, wR 2 = 0.032/0.009. There is excellent agreement between the structure models of the two studies. However, some notable differences were found between the structures determined here and that reported by Sugahara et al., (2008): most obviously, we observe no significant anisotropic displacements for any of the atoms. Anisotropic displacement parameters for Ca, Ir and O atoms are very similar to those of their counterparts in MgSiO3 perovskite. The close similarity of the structures of the two crystals determined here using two very different diffractometers suggests that the derived model structure reported here is better-constrained and more representative of the characteristic features of CaIrO3 post-perovskite than that obtained by Sugahara et al., (2008).

The MgSiO3system at high pressure: Thermodynamic properties of perovskite, postperovskite, and melt from global inversion of shock and static compression data

We present new equation‐of‐state (EoS) data acquired by shock loading to pressures up to 245 GPa on both low‐density samples (MgSiO3glass) and high‐density, polycrystalline aggregates (MgSiO3perovskite + majorite). The latter samples were synthesized using a large‐volume press. Modeling indicates that these materials transform to perovskite, postperovskite, and/or melt with increasing pressure on their Hugoniots. We fit our results together with existingP‐V‐Tdata from dynamic and static compression experiments to constrain the thermal EoS for the three phases, all of which are of fundamental importance to the dynamics of the lower mantle. The EoS for perovskite and postperovskite are well described with third‐order Birch‐Murnaghan isentropes, offset with a Mie‐Grüneisen‐Debye formulation for thermal pressure. The addition of shock data helps to distinguish among discrepant static studies of perovskite, and for postperovskite, constrain a value ofK′ significantly larger than 4. For the melt, we define for the first time a single EoS that fits experimental data from ambient pressure to 230 GPa; the best fit requires a fourth‐order isentrope. We also provide a new EoS for Mg2SiO4liquid, calculated in a similar manner. The Grüneisen parameters of the solid phases decrease with pressure, whereas those of the melts increase, consistent with previous shock wave experiments as well as molecular dynamics simulations. We discuss implications of our modeling for thermal expansion in the lower mantle, stabilization of ultra‐low‐velocity zones associated with melting at the core‐mantle boundary, and crystallization of a terrestrial magma ocean.

P‐V‐T relations of MgSiO3 perovskite determined by in situ X‐ray diffraction using a large‐volume high‐pressure apparatus

The volume of MgSiO3 perovskite has been precisely measured at pressures of 19 to 53 GPa and temperatures of 300 to 2300 K by means of in situ X‐ray diffraction in a multi‐anvil apparatus. The present results indicate the isothermal bulk modulus KT0 = 256(2) GPa and its pressure derivative K′T0 = 3.8(2). The fixed Debye temperature θ0 = 1030 K gives a Grüneisen parameter at ambient pressure γ0 = 2.6(1) and its logarithmic volume dependence q = 1.7(1). The pressure derivative of the isothermal bulk modulus, Anderson‐Grüneisen parameter and thermal expansion coefficient at ambient pressure are found to be (∂KT/∂T)P = −0.035(2) GPa/K, δT = 6.5(5), α0 = 2.6(1) × 10−5 + 1.0(1) × 10−8 (T − 300)/K. Thus the thermal expansion coefficient largely becomes smaller with increasing pressure. The adiabatic geotherm would be fairly large, such as 0.41 K/km at a 660 km depth, and becoming smaller with increasing depth. The temperature and adiabatic geothermal gradient at the bottom of the D′ layer would be 2400 K and 0.14 K/km. The buoyancy‐driven mantle convection could be very small in the lower part of the lower mantle.

The effect of Fe and Al substitution on the compressibility of MgSiO3-perovskite determined through single-crystal X-ray diffraction

A compressibility study on three well characterized single crystals of magnesium silicate perovskite containing variable proportions of iron and aluminium has been carried out using X-ray diffraction in a diamond anvil cell in order to access the effect of varying chemical composition on perovskite elastic properties. The single crystals were synthesized using a multianvil apparatus at 25GPa and 1800–2000°C. It was found that the substitution of Fe2+ for Mg2+ in Al-bearing perovskite increases the ambient pressure unit-cell volume to a larger extent than when the same substitution occurs in Al-free perovskite. Equation of state measurements were performed up to pressures of 9GPa at room temperature. With increasing Fe and Al incorporation perovskite exhibits a modest increase in compressibility compared to MgSiO3-perovskite over the pressure range investigated. A 3rd order Birch–Murnaghan equation of state fit to the compression data indicates that the bulk modulus (K0), decreases from 243 to 234GPa while K′ increases from 5.0 to 6.3 as the Fe and Al contents increase from approximately 5 to 17 and from 2 to 8wt.%, respectively. The high values of K′ will result in Fe–Al-bearing perovskites becoming less compressible than MgSiO3 perovskite at pressures >10GPa. Extrapolating these results to lower mantle conditions indicates a pronounced effect of composition on density in the upper portions of lower mantle, with perovskites formed in subducted oceanic crust, i.e. with higher Fe and Al contents, being substantially denser than those formed within a primitive mantle composition. This density contrast diminishes, however, with increasing pressure, which also contributes to a stronger dependence of bulk sound velocity on composition towards the base of the lower mantle.

POST-PEROVSKITE: THE LAST MANTLE PHASE TRANSITION.: K. Hirose, J. Brodholt, T. Lay, D. Yuen, Eds. (2007) American Geophyscial Union, Geophysical Monograph Series, 174. 350 pages, hardbound. ISBN 13: 978-0-87590-439-9. $104.00 (AGU Member Price: $72.80).

We live in interesting times. It used to be that mantle phase transitions were so few in number that even a child could become an expert—but my childhood is long past. Now even seismologists invoke obscure phase transitions to elicit favorable publishing decisions whenever an unexpected wiggle arises in a seismogram, and even mineral physicists routinely discuss seismic anisotropy. (Note to self: good reason not to drop optical mineralogy from a syllabus.) This surprising broadening of knowledge arose from the singular alliance of large-scale facilities provided by synchrotron light sources, supercomputers, and seismological data facilities like IRIS. Kei Hirose, John Brodholt, Thorne Lay, and David Yuen marshal the forces of this effort in their collection of papers, Post-perovskite, The Last Mantle Phase Transition, which highlights the knowns and unknowns of the Pbnm to Cmcm transition in MgSiO3 perovskite (pv) to post-perovskite (ppv). One known is that the seismic features in D″ are largely attributable to it. Oh, D″? That’s the bottom of the Earth’s mantle, a place where Birch’s famous warning applies: “Language undergoes a transformation to high pressure forms.” Now reread the prepenultimate sentence. Possible hype notwithstanding, the editors provide a nice disciplinary packaging of the papers: experimental and theoretical mineral physics, seismology, and dynamics. The experimental section begins with two histories of high-pressure experimentation, a more general one by Yagi on mantle phase transitions, and a more ppv-centric one by Hirose. Their thrill of discovery is evident. While reading, I kept wondering what Ringwood would say. “You mean, you didn’t investigate analogues first ?” was my surmise (but I imagine less politely put). Graff Jewelers (Bond Street, London) report that wealthy Russian shoppers prefer yellow diamonds (source: International Herald Tribune ). This is good …

Peierls dislocation modelling in perovskite (CaTiO3): comparison with tausonite (SrTiO3) and MgSiO3 perovskite

We present here a numerical modelling study of dislocations in perovskite CaTiO3. The dislocation core structures and properties are calculated through the Peierls–Nabarro model using the generalized stacking fault (GSF) results as a starting model. The GSF are determined from first-principles calculations using the VASP code. The dislocation properties such as collinear, planar core spreading and Peierls stresses are determined for the following slip systems: [100](010), [100](001), [010](100), [010](001), [001](100), [001](010), $$ [001](\bar{1}10), $$ $$ [\bar{1}10](001) $$ and $$ [110](\bar{1}10). $$ All dislocations exhibit lattice friction, but glide appears to be easier for $$ [\bar{1}10](001), $$ [100](010) and [010](100). $$ [110](\bar{1}10), $$ $$ [\bar{1}10](001), $$ $$ [001](\bar{1}10), $$ [001](010) and [001](100) exhibit collinear dissociation. Comparing Peierls stresses among tausonite (SrTiO3), perovskite (CaTiO3) and MgSiO3 perovskite demonstrates the strong influence of orthorhombic distortions on lattice friction. However, and despite some quantitative differences, CaTiO3 appears to be a satisfactory analogue material for MgSiO3 perovskite as far as dislocation glide is concerned.

DFT study of migration enthalpies in MgSiO3 perovskite

The effect of pressure on ionic diffusion in orthorhombic MgSiO3 perovskite has been investigated using density functional theory. An intensive investigation of possible silicon pathways revealed new positions of the saddle-points and an enthalpy of migration at 26.2 GPa of 4.7 eV that is in fair agreement with the experimental values of about 3.5 eV at 25 GPa. This is much lower than found in previous studies (~9 eV) and removes the need to explain silicon diffusion by a complicated process involving coupled oxygen vacancies, as has been previously proposed. Our migration enthalpies for oxygen and magnesium are in excellent agreement with experiments. We find that oxygen diffusion occurs via a chain of several inequivalent jumps along the octahedron edges, and that magnesium occurs via two inequivalent [110] jumps and one [001] jump. We also present activation volumes for all three species at 25 and 135 GPa.

Mantle‐wide sequestration of carbon in silicates and the structure of magnesite II

The participation of the deep mantle in the global carbon cycle and its ability to sequester carbon over billion‐year time scales depends upon the mineralogical host for carbon. Density‐functional theory calculations for MgCO3‐magnesite and structures with tetrahedrally coordinated carbon reveal the stability of magnesite up to ∼80 GPa, with a bulk modulus of 110 (±2) GPa. Magnesite undergoes a structural transition to a pyroxene‐like structure at ∼80–100 GPa, with a density increase of 4.5–7.1%. Combined with thermodynamic models for the MgSiO3—MgCO3 system, the inter‐solubility of MgCO3 with MgSiO3 orthoenstatite and perovskite constrains the carbon content in the silicates to an upper bound of 4 and 20 ppm (wt), respectively. The carbon content in lower mantle silicates is estimated to be no more than 1% of the mantle's total carbon budget for degassed regions, such that in even the mantle's most depleted regions, most carbon must be stored in carbonates or diamond.

Discovery of Post-Perovskite and New Views on the Core-Mantle Boundary Region

Research Article| June 01, 2008 Discovery of Post-Perovskite and New Views on the Core-Mantle Boundary Region Kei Hirose; Kei Hirose 1Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro Tokyo 152-8551, Japan E-mail: kei@geo.titech.ac.jp Search for other works by this author on: GSW Google Scholar Thorne Lay Thorne Lay 2Department of Earth and Planetary Sciences, University of California Santa Cruz, Santa Cruz, CA 95064, USA E-mail: thorne@pmc.ucsc.edu Search for other works by this author on: GSW Google Scholar Author and Article Information Kei Hirose 1Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro Tokyo 152-8551, Japan E-mail: kei@geo.titech.ac.jp Thorne Lay 2Department of Earth and Planetary Sciences, University of California Santa Cruz, Santa Cruz, CA 95064, USA E-mail: thorne@pmc.ucsc.edu Publisher: Mineralogical Society of America First Online: 09 Mar 2017 Online ISSN: 1811-5217 Print ISSN: 1811-5209 © 2008 by the Mineralogical Society of America Elements (2008) 4 (3): 183–189. https://doi.org/10.2113/GSELEMENTS.4.3.183 Article history First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Kei Hirose, Thorne Lay; Discovery of Post-Perovskite and New Views on the Core-Mantle Boundary Region. Elements 2008;; 4 (3): 183–189. doi: https://doi.org/10.2113/GSELEMENTS.4.3.183 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyElements Search Advanced Search Abstract A phase transition of MgSiO3 perovskite, the most abundant component of the lower mantle, to a higher-pressure form called post-perovskite was recently discovered for pressure and temperature conditions in the vicinity of the Earth's core-mantle boundary. This discovery has profound implications for the chemical, thermal, and dynamical structure of the lowermost mantle called the D” region. Several major seismological characteristics of the D” region can now be explained by the presence of post-perovskite, and the specific properties of the phase transition provide the first direct constraints on absolute temperature and temperature gradients in the lowermost mantle. Here we discuss the current understanding of the core-mantle boundary region. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.