The postspinel phase boundary in Mg2SiO4 determined by in situ X-ray diffraction

Kawai-type multianvil ultrahigh-pressure technology

Since the large-volume press with a double-stage multianvil system was created by the late Professor Naoto Kawai, this apparatus (Kawai-type multianvil apparatus or KMA) has been developed for higher-pressure generation, in situ X-ray and neutron observations, deformation experiments, measurements of physical properties, synthesis of high-pressure phases, etc., utilizing its large sample volume and capacity in stable and homogeneous high temperature generation compared to those of competitive diamond anvil cells. These advancements in KMA technology have been made primarily by Japanese scientists and engineers, which yielded a wealth of new experimental data on phase transitions, melting relations, and physical characteristics of minerals and rocks, leading to significant constraints on the structures, chemical compositions, and dynamics of the deep Earth. KMA technology has also been used for synthesis of novel functional materials such as nano-polycrystalline diamond and transparent nano-ceramics, opening a new research field of ultrahigh-pressure materials science.

A New Approach Determining a Phase Transition Boundary Strictly Following a Definition of Phase Equilibrium: An Example of the Post-Spinel Transition in Mg2SiO4 System

The Clapeyron slope is the slope of a phase boundary in P–T space and is essential for understanding mantle dynamics and evolution. The phase boundary is delineating instead of balancing a phase transition’s normal and reverse reactions. Many previous high pressure–temperature experiments determining the phase boundaries of major mantle minerals experienced severe problems due to instantaneous pressure increase by thermal pressure, pressure drop during heating, and sluggish transition kinetics. These complex pressure changes underestimate the transition pressure, while the sluggish kinetics require excess pressures to initiate or proceed with the transition, misinterpreting the phase stability and preventing tight bracketing of the phase boundary. Our recent study developed a novel approach to strictly determine phase stability based on the phase equilibrium definition. Here, we explain the details of this technique, using the post-spinel transition in Mg2SiO4 determined by our recent work as an example. An essential technique is to observe the change in X-ray diffraction intensity between ringwoodite and bridgmanite + periclase during the spontaneous pressure drop at a constant temperature and press load with the coexistence of both phases. This observation removes the complicated pressure change upon heating and kinetic problem, providing an accurate and precise phase boundary.

Depressed 660-km discontinuity caused by akimotoite-bridgmanite transition

The 660-kilometre seismic discontinuity is the boundary between the Earth’s lower mantle and transition zone and is commonly interpreted as being due to the dissociation of ringwoodite to bridgmanite plus ferropericlase (post-spinel transition)^1–3. A distinct feature of the 660-kilometre discontinuity is its depression to 750 kilometres beneath subduction zones^4–10. However, in situ X-ray diffraction studies using multi-anvil techniques have demonstrated negative but gentle Clapeyron slopes (that is,  the ratio between pressure and temperature changes) of the post-spinel transition that do not allow a significant depression^11–13. On the other hand, conventional high-pressure experiments face difficulties in accurate phase identification due to inevitable pressure changes during heating and the persistent presence of metastable phases^1,3. Here we determine the post-spinel and akimotoite–bridgmanite transition boundaries by multi-anvil experiments using in situ X-ray diffraction, with the boundaries strictly based on the definition of phase equilibrium. The post-spinel boundary has almost no temperature dependence, whereas the akimotoite–bridgmanite transition has a very steep negative boundary slope at temperatures lower than ambient mantle geotherms. The large depressions of the 660-kilometre discontinuity in cold subduction zones are thus interpreted as the akimotoite–bridgmanite transition. The steep negative boundary of the akimotoite–bridgmanite transition will cause slab stagnation (a stalling of the slab’s descent) due to significant upward buoyancy^14,15.

X-ray diffraction experiments indicate that the depression of the Earth’s 660-kilometre seismic discontinuity beneath cold subduction zones is caused by a phase transition from akimotoite to bridgmanite, leading to slab stagnation.

Optical access to multi-anvil apparatus with ultrasonic method under high-pressure environment

Optical methods have widely been utilized in high-pressure high-temperature experiments for the past several decades. However, optical investigations using the visible spectrum in large volume press have not been well explored. In this study, we incorporate optical access into a multianvil apparatus (MAA) to perform optical experiments. Furthermore, by acquiring the optical image of the sample under high pressure, we have realized the thickness measurement of the sample and further applied it to the existing ultrasonic measurement. We report the optical method from the aspects of apparatus modification, ruby scale, and the demonstrations of the feasibility of the optical access in the MAA.

Complete agreement of the post-spinel transition with the 660-km seismic discontinuity

The 660-km seismic discontinuity, which is a significant structure in the Earth's mantle, is generally interpreted as the post-spinel transition, as indicated by the decomposition of ringwoodite to bridgmanite + ferropericlase. All precise high-pressure and high-temperature experiments nevertheless report 0.5-2 GPa lower transition pressures than those expected at the discontinuity depth (i.e. 23.4 GPa). These results are inconsistent with the post-spinel transition hypothesis and, therefore, do not support widely accepted models of mantle composition such as the pyrolite and CI chondrite models. Here, we present new experimental data showing post-spinel transition pressures in complete agreement with the 660-km discontinuity depth obtained by high-resolution in situ X-ray diffraction in a large-volume high-pressure apparatus with a tightly controlled sample pressure. These data affirm the applicability of the prevailing mantle models. We infer that the apparently lower pressures reported by previous studies are experimental artefacts due to the pressure drop upon heating. The present results indicate the necessity of reinvestigating the position of mantle mineral phase boundaries previously obtained by in situ X-ray diffraction in high-pressure-temperature apparatuses.

A new high-pressure form of Mg2SiO4 highlighting diffusionless phase transitions of olivine

High-pressure polymorphism of olivine (α-phase of Mg₂SiO₄) is of particular interest for geophysicists aiming to understand the structure and dynamics of the Earth’s interior because of olivine’s prominent abundance in the upper mantle. Therefore, natural and synthetic olivine polymorphs have been actively studied in the past half century. Here, we report a new high-pressure polymorph, the ε*-phase, which was discovered in a heavily shocked meteorite. It occurs as nanoscale lamellae and has a topotaxial relationship with the host ringwoodite (γ-phase of Mg₂SiO₄). Olivine in the host rock entrapped in a shock-induced melt vein initially transformed into polycrystalline ringwoodite through a nucleation and growth mechanism. The ringwoodite grains then coherently converted into the ε*-phase by shear transformation during subsequent pressure release. This intermediate metastable phase can be formed by all Mg₂SiO₄ polymorphs via a shear transformation mechanism. Here, we propose high-pressure transformations of olivine that are enhanced by diffusionless processes, not only in shocked meteorites but also in thick and cold lithosphere subducting into the deep Earth.

First-principles molecular dynamics calculations of the equation of state for tantalum

The equation of state of tantalum (Ta) has been investigated to 100 GPa and 3,000 K using the first-principles molecular dynamics method. A large volume dependence of the thermal pressure of Ta was revealed from the analysis of our data. A significant temperature dependence of the calculated effective Grüneisen parameters was confirmed at high pressures. This indicates that the conventional approach to analyze thermal properties using the Mie-Grüneisen approximation is likely to have a significant uncertainty in determining the equation of state for Ta, and that an intrinsic anharmonicity should be considered to analyze the equation of state.

Mineral phases of the Earth´s mantle

Our knowledge of the structure of the Earth´s interior has been obtained by analysing seismic waves that travel in the Earth, and the reference Earth global models used by geophysicists are essentially seismological. Depth profiles of the seismic waves velocities reveal that the deep Earth is divided in several shells, separated by velocity and density discontinuities. The main discontinuity located at a depth of 2900 km corresponds to the transition between the mantle and the core. The Earth´s mantle can be further divided into the upper mantle and the lower mantle, with a transition zone characterised by two prominent increases in velocities observed at 410- and 660-km depths. This article will be focused on the mineral phases of the Earth´s mantle. The interpretation of seismological models in terms of chemical composition and temperature relies on the knowledge of the nature, structure and elastic properties of the candidate materials. We will describe to what extent recent advances in experimental mineral physics and X-ray diffraction have yielded essential knowledge on the structure and high-pressure high-temperature behaviour of pertinent materials, and major improvements in our understanding of the chemical and mineralogical composition of the Earth´s mantle.

Toward an internally consistent pressure scale

Our ability to interpret seismic observations including the seismic discontinuities and the density and velocity profiles in the earth's interior is critically dependent on the accuracy of pressure measurements up to 364 GPa at high temperature. Pressure scales based on the reduced shock-wave equations of state alone may predict pressure variations up to 7% in the megabar pressure range at room temperature and even higher percentage at high temperature, leading to large uncertainties in understanding the nature of the seismic discontinuities and chemical composition of the earth's interior. Here, we report compression data of gold (Au), platinum (Pt), the NaCl-B2 phase, and solid neon (Ne) at 300 K and high temperatures up to megabar pressures. Combined with existing experimental data, the compression data were used to establish internally consistent thermal equations of state of Au, Pt, NaCl-B2, and solid Ne. The internally consistent pressure scales provide a tractable, accurate baseline for comparing high pressure-temperature experimental data with theoretical calculations and the seismic observations, thereby advancing our understanding fundamental high-pressure phenomena and the chemistry and physics of the earth's interior.

Simultaneous determination of mean pressure and deviatoric stress based on numerical tensor analysis: a case study for polycrystalline x-ray diffraction of gold enclosed in a methanol-ethanol mixture

It is known that the {100} and {111} planes of cubic crystals subjected to uniaxial deviatoric stress conditions have strain responses that are free from the effect of lattice preferred orientation. By utilizing this special character, one can unambiguously and simultaneously determine the mean pressure and deviatoric stress from polycrystalline diffraction data of the cubic sample. Here we introduce a numerical tensor calculation method based on the generalized Hooke's law to simultaneously determine the hydrostatic component of the stress (mean pressure) and deviatoric stress in the sample. The feasibility of this method has been tested by examining the experimental data of the Au pressure marker enclosed in a diamond anvil cell using a pressure medium of methanol-ethanol mixture. The results demonstrated that the magnitude of the deviatoric stress is ∼0.07 GPa at the mean pressure of 10.5 GPa, which is consistent with previous results of Au strength under high pressure. Our results also showed that even a small deviatoric stress (∼0.07 GPa) could yield a ∼0.3 GPa mean pressure error at ∼10 GPa.

Numerical study of pressure relationships between sample and calibrant inside the diamond anvil cell

We present isotropic, elastic-plastic finite element calculations detailing the pressure relationship between an inclusion and its surrounding matrix, subject to an externally imposed hydrostatic strain. In general, the inclusion and the matrix have different values of hydrostatic pressure, depending on their absolute and relative values of Young's modulus and Poisson's ratio. A series of finite element models was used to explore the parameter space of the elastic and plastic properties of an inclusion within a matrix. In all cases where there is insufficient relaxation of the nonhydrostatic stress, the material with the higher bulk modulus will also have a higher pressure, regardless of the shear moduli. The complete data set was subjected to a Pareto analysis to determine the main and secondary effects which influence the final result, expressed as the ratio of the pressure of the matrix to that of the inclusion. The four most important factors which determine the pressure ratio of an inclusion and matrix are the Young's modulus of the matrix, the interaction of the Young's modulus and the yield strength of the matrix material, the Young's modulus of the inclusion, and the interaction of the Young's modulus of the inclusion with the yield strength of the matrix material. The yield strength of the inclusion has a statistically insignificant effect on the results. This information provides guidelines for designing the most effective combinations of unknowns and material standards to minimize pressure errors in equation of state measurements.

Seismic evidence for olivine phase changes at the 410- and 660-kilometer discontinuities

The view that the seismic discontinuities bounding the mantle transition zone at 410- and 660-kilometer depths are caused by isochemical phase transformations of the olivine structure is debated. Combining converted-wave measurements in East Asia and Australia with seismic velocities from regional tomography studies, we observe a correlation of the thickness of, and wavespeed variations within, the transition zone that is consistent with olivine structural transformations. Moreover, the seismologically inferred Clapeyron slopes are in agreement with the mineralogical Clapeyron slopes of the (Mg,Fe)2SiO4 spinel and postspinel transformations.

Chemical control of phase transformation kinetics in periodic silica/surfactant composites

Control of phase stability is investigated through control of silica chemistry in ordered silica/surfactant composites under hydrothermal conditions. The composites were hydrothermally treated in pH 9 through pH 11 buffers while using in situ real time X-ray diffraction to follow a p6mm hexagonal-to-lamellar structural transition. The data were analyzed using both isothermal and nonisothermal (temperature-ramped) kinetics to determine activation energies. It was found that the most mildly basic conditions utilized (pH 9), which favor silica condensation, best inhibit the phase transition and thus produce the most kinetically stable composites. High-pH treatment, conversely, allows for the most facile rearrangements. Condensation occurring during composite synthesis rather than during hydrothermal treatment has a much smaller effect on phase stability, probably because much of the condensation that occurs during synthesis is random and not optimally coupled to the nanoscale architecture. Materials that start out poorly condensed, by contrast, can be extensively hydrothermally modified so that the final material has an inorganic framework with a highly uniform silica density; this provides the maximum resistance to transformation and the highest kinetic stability. In all cases, very good agreement is found between the results of isothermal and nonisothermal kinetic methods. The trends across pHs indicate that both isothermal and nonisothermal measurements are accurate and that differences between them are meaningful and represent physical differences in the transforming materials resulting from the different heating processes.

The post-spinel transformation in Mg2SiO4 and its relation to the 660-km seismic discontinuity

The 660-km seismic discontinuity in the Earth's mantle has long been identified with the transformation of (Mg,Fe)2SiO4 from gamma-spinel (ringwoodite) to (Mg,Fe)SiO3-perovskite and (Mg,Fe)O-magnesiowüstite. This has been based on experimental studies of materials quenched from high pressure and temperature, which have shown that the transformation is consistent with the seismically observed sharpness and the depth of the discontinuity at expected mantle temperatures. But the first in situ examination of this phase transformation in Mg2SiO4 using a multi-anvil press indicated that the transformation occurs at a pressure about 2 GPa lower than previously thought (equivalent to approximately 600 km depth) and hence that it may not be associated with the 660-km discontinuity. Here we report the results of an in situ study of Mg2SiO4 at pressures of 20-36 GPa using a combination of double-sided laser-heating and synchrotron X-ray diffraction in a diamond-anvil cell. The phase transformation from gamma-Mg2SiO4 to MgSiO3-perovskite and MgO (periclase) is readily observed in both the forward and reverse directions. In contrast to the in situ multi-anvil-press study, we find that the pressure and temperature of the post-spinel transformation in Mg2SiO4 is consistent with seismic observations for the 660-km discontinuity.

High-pressure polymorphs of olivine and the 660-km seismic discontinuity

It had long been accepted that the 400-km seismic discontinuity in the Earth's mantle results from the phase transition of (Mg,Fe)2-SiO4-olivine to its high-pressure polymorph beta-spinel (wadsleyite), and that the 660-km discontinuity results from the breakdown of the higher-pressure polymorph gamma-spinel (ringwoodite) to MgSiO3-perovskite and (Mg,Fe)O-magnesiowüstite. An in situ multi-anvil-press X-ray study indicated, however, that the phase boundary of the latter transition occurs at pressures 2 GPa lower than had been found in earlier studies using multi-anvil recovery experiments and laser-heated diamond-anvil cells. Such a lower-pressure phase boundary would be irreconcilable with the accuracy of seismic measurements of the 660-km discontinuity, and would thus require a mineral composition of the mantle that is significantly different from what is currently thought. Here, however, we present measurements made with a laser-heated diamond-anvil cell which indicate that gamma-Mg2SiO4 is stable up to pressure and temperature conditions equivalent to 660-km depth in the Earth's mantle (24 GPa and 1,900 K) and then breaks down into MgSiO3-perovskite and MgO (periclase). We paid special attention to pressure accuracy and thermal pressure in our experiments, and to ensuring that our experiments were performed under nearly hydrostatic, inert pressure conditions using a variety of heating methods. We infer that these factors are responsible for the different results obtained in our experiments compared to the in situ multi-anvil-press study.