Directions of our research

	Experimental phase diagrams of geomaterials		Thermodynamic modeling of the Earth's interior
	Kimberlites, mantle xenoliths, and diamonds		High-pressure crystallography
	Nanodiamonds and nanomaterials		High-pressure minerals in meteorites and impact craters

	Ab initio design of geomaterials

Experimental phase diagrams of geomaterials

Experimental study on the phase relations and phase stability under high-pressures and temperatures is the basis for Earth and planetary science. These data are used to interpret Earth’s layered structure and to reconstruct past geological events at the time of Magma Ocean and metallic core separation, plate formation, subduction, back-arc and plume magmatism. The knowledge of the relevant phase diagrams is necessary for design of experiments on physical properties measurements such as density, viscosity, elasticity, electrical and thermal conductivity as well as experiments on element diffusivities, trace element partitioning, reaction kinetics, and single crystal growth.

We must admit that basic systems modeling silicate mantle and Fe-Ni core are very well studied at least to pressures of 25-50 GPa and in some cases down to core-mantle boundary and core conditions. Nevertheless, presence of minor components, which impact on geodynamics and Earth’s physical properties is largely recognized, requires further experimental studies. Therefore, our research focuses on systems containing volatile and alkali elements and includes following topics:

• Subsolidus and melting phase relations in K-, Na-, Fe-, Ca-, Mg-bearing carbonate systems at 6 GPa.
• Carbonate-silicate reactions controlling alkalis redistribution and solidus temperatures of mantle rocks.
• Mechanism and kinetics of enstatite and olivine resorption in alkali-bearing Ca and Ca-Mg carbonate melts (dry and hydrous) under mantle conditions.
• Experimental reconstruction of primary kimberlite melt composition by means of study melting phase relations in unaltered kimberlite with adding the lost volatiles at 6.5 GPa.
• Study of P-T-X stability ranges of djerfisherite and K-bearing sulfides at pressures of 1-6 GPa.
• Relative reactivity (stability) of iron hydrates, carbides, sulfides, nitrides and metallic iron under mantle pressures.
• Mechanism and kinetics of reactions between metallic iron and carbonates and hydrous carbonate melt.

Phase relations in some carbonate systems at pressure of 6 GPa. K₂ = K₂CO₃, K₂Mg = K₂Mg(CO₃)₂, Mgs = magnesite, Na₂ = Na₂CO₃, Na₂Mg = Na₂Mg(CO₃)₂, Na₄Ca = Na₄Ca(CO₃)₃, Na₂Ca₃ = Na₂Ca₃(CO₃)₄, Na₂Ca₄ = Na₂Ca₄(CO₃)₅, Art = ordered or disordered aragonite, ss = limited solid solutions, L = liquid.

Thermodynamic modeling of the Earth's interior

Thermodynamics deals with energy, and is therefore fundamental to many branches of science. In the Erath’s sciences, thermodynamic models might be used to understand the physical and thermal state of minerals and rocks. It is difficult to create thermodynamic models, and to compile enough basic data to enable them to work, but it becomes increasingly difficult to know how well they work. Data estimation is now based on simultaneous evaluation of all possible reactions to extract an internally consistent set of pressure–temperature–volume relationships (equations of state), values of ΔH° (enthalpy of formation from the elements), S° (entropy), and C_p (heat capacity) compliant with calorimetry and phase equilibrium experiments.

Determination of the equation of state (EoS) is crucial to allowing the P‒V and deformation energy contributions to the Gibbs energy, which plays a central role in fixing the stability conditions for an assemblage at given P and T. Experimental studies of the minerals at high-P and high-T using synchrotron X-ray diffraction provide data to develop the EoS and as a results submit information for the interior of the Earth.

The aim of our group is to set up equations of state for most important mantle and core substances by joint analysis of existing experimental measurements of isobaric heat capacity, bulk moduli, and thermal expansion vs. temperature at ambient pressure as well as volume at the 298 K isotherm and at elevated temperatures. The derived equations of state permit calculation of any thermodynamic functions depending on temperature and volume or temperature and pressure.

Left: The perovskite (Prv) into post-perovskite (PPrv) transition, according to experimental and computational data. Right: P–V–T relations of Na-containing majorite garnet from our study.

Kimberlites, mantle xenoliths, and diamonds

The problem of characterization of composition and structure of the Еarth’s interior in connection to the diamond genesis and the kimberlite formation is one of the major directions of activity of many famous scientists and research groups worldwide. Mineral and chemical compositions of deep-seated xenoliths display the history and properties of Earth’s upper mantle – composition, volatile content, heterogeneity, Р-Т-fO₂ conditions, metasomatic events etc. Diamonds are another important source of information about deep-seated associations. Due to their inert properties diamonds can capture and preserve the high pressure phases for many billions years. Recent geochemical studies of deep-seated associations have opened the perspectives for detailed characterization of diamond-forming media and determination of geochemical indicators of diamond-forming processes at the deep levels of the upper mantle or even lower mantle. The studies on mineralogy and geochemistry of kimberlites also provide valuable information about composition and melting regime of the Earth’s upper mantle. The urgency of these researches is following from that the knowledge about diamond-forming processes and kimberlite formation may be potentially used for development of methods for exploration and prospecting of new diamond mines.

The microinclusions in fibrous diamonds from the Snap Lake area in the eastern part of the Slave Craton (Canada) are indicative of their crystallization during partial melting of peridotites and eclogites at the base of a thick lithospheric mantle at depths below 300 km.

In our laboratory a series of important results on studies of kimberlites, mantle xenoliths and diamonds were done during several past years. Geochemical characteristics of superdeep diamonds and their mineral inclusions have been examined in details to document the compositions of sublithospheric mantle and to interpret the evolution of diamond-forming media. The analysis of subduction P-T trends of metamorphic complexes and xenoliths from kimberlites shows that analogs of hypothesized cold subduction were not observed in nature. Distribution of water among the main rock-forming nominally anhydrous minerals has been studied in both peridotitic and eclogitic mantle xenoliths. New data on the compositions of microinclusions in fibrous diamonds from several localities at Siberian craton (Russia) and Slave craton (Canada) suggest that the they have formed as a result of interaction between carbonate-silicate melts and peridotitic wallrocks at the base of a thick lithospheric mantle. High-pressure melting experiments on unique exceptionally fresh Udachnaya-East kimberlite from Siberian craton shows that the kimberlite melt has an alkali-carbonatite composition in the source region at the base of craton; during ascent this primary melt evolved toward a carbonate–silicate composition as a result of contamination by xenogenic matter.

A proposed model for kimberlite formation. This model suggests that some fibrous diamonds, which have low estimated mantle residence time, may precipitate from kimberlite-related melt at pre-eruption stage via its partial reduction by mantle wallrocks.

High-pressure crystallography

The crystalline state predominates under high-pressure conditions of the Earth’s and planetary interiors, and high-pressure crystallography is the most powerful tool to understand the structure and properties of materials under high pressures and high temperatures. We use X-ray diffraction method to study the crystal structure of high-pressure phases. The study can be carried out on samples recovered from HP-HT to room conditions, or even at actual high-pressure and high temperature (‘in situ’). The latter requires such equipment as high-pressure diamond anvil cells, and sources of high-brilliance X-ray – synchrotrons.

The main areas of our investigations are:

•  High-pressure crystal chemistry of water-containing solids. This topic is closely related to one of the most significant problems of modern geochemistry – a problem of water transport into the mantle. Together with our Japanese colleagues we study crystal structures of hydrous phases stable under mantle conditions and their stability limits.
•  Chemical bonding under high pressure. The research on this topic is based on a fruitful collaboration with Alex Goncharov’s team from Carnegie Geophysical Laboratory (Washington, DC). The aim is to understand how high and ultra-high pressure can influence on such characteristics of solid materials as optical spectra, magnetic and electrical properties and chemical bonding. Obtained results show how unconventional the high-pressure chemistry can be.

Left: the so-called 'ten angstrom phase', Mg₃Si₄O₁₀(OH)₂·H₂O, – a hydrous magnesium silicate with layered structure stable up to 10 GPa. Middle: 'phase H', MgSiO₂(OH)₂, another hydrous magnesium silicate with extreme pressure stability, and a possible water carrier down to the core-mantle boundary! Right: magnesium peroxide MgO₂, found to be very stable at high pressure.

Nanodiamonds and nanomaterials

Nano-polycrystalline diamond (NPD) synthesized through direct conversion from graphite at P > 12 GPa and T > 1500°C consists of 10-100 nm particles and has higher mechanical strength than conventional diamonds indicating its potential in science and engineering applications. Despite of great progress in synthesis of large (over 10 mm) NPD specimens, the effect of impurities on the NPD physical properties left unattended. At the same time, the composition, structure and concentration of impurity centers, incorporated into the diamond lattice, have pronounced effect or completely determine the physical properties of diamond and its applicability for the high technology. The maximum concentrations of impurities centers in diamonds grown from carbon solutions in variable solvent-catalysts at 5.5-7.5 GPa are limited by partitioning coefficient and achieve 0.1-0.5 % for B, N, and P, 30-50 ppm for Ni and Co, and ≤ n ppm for Si and Ge.

In our study, we use a direct diamond synthesis at P > 13-15 GPa from graphite + impurity mixture which allows overcoming the partitioning problem at least for boron. Furthermore, progressive increase of impurity in starting mixtures allows estimating temperature dependence of impurity solubility in the diamond lattice by the appearance of an additional phase out of diamond solid solution field. Using these samples, we study influence of doping level on lattice parameters and Raman shift. This data have a prime importance for characterization of heavily doped CVD diamonds, where concentration of impurities centers remains largely uncertain.

The most of experimental studies on diamond synthesis and annealing are limited by 2300°C. Nevertheless immobility of boron and some other atoms in the diamond lattice results in sluggish kinetic of centers transformation and aggregation. In our experiments, we use the B-doped diamond heater, which provides unique opportunity of diamond synthesis and annealing at temperatures exceeding 3000°C and pressure of 17 GPa. For diamond characterization we employ a variety of methods including Raman spectroscopy, optical absorption in the UV to mid-IR spectral range, luminescence spectroscopy (photo and X-ray-excited luminescence and thermoluminescence), electrical conductivity and photoconductivity.

Characterization of NPD diamond specimens synthesized by direct conversion at 17 GPa above 1500°C. Upper left: Room temperature Raman spectra with a 488 nm excitation of the boron-doped diamond. Spectra of boron free natural diamond are also shown. Lower left: Comparison of X-ray diffraction patterns of boron-doped diamond with different boron contents. The X-ray pattern of pure diamond synthesized under the same conditions is also shown. Diamond and boron carbide characteristic peaks are marked by ‘Dm’ and ‘B₄C’, respectively. Cell parameter (upper right) and characteristic Raman mode frequency (lower right) of diamond versus boron atomic concentration, C_B = B/(C+B), in the system.

High-pressure minerals in meteorites and impact craters

High-pressure phase transformations of minerals occur in the deep interior of terrestrial planets or in shock-metamorphosed rocks (including meteorites and impact craters) induced by the collisions of planetary bodies. Knowledge of the mechanisms of high-pressure phase transformations for rock-forming minerals is important to understand the dynamics and processes of planetary interiors, such as slab subduction, mantle convection, mantle rheology, and the genesis of deepfocus earthquakes. High-pressure and high–temperature experiments on the major constituent minerals of the Earth (e.g., olivine, pyroxene, and feldspar) have been carried out to clarify the kinetics of their phase transformations. The observation and description of naturally transformed samples are necessary to ascertain whether the transformation mechanism observed in the experiments could actually occur in nature. The opportunity for the investigation of natural high-pressure phase transformations is limited to a few samples, such as diamond inclusions derived from ultra-deep mantle, or rocks naturally shocked in planetary impacts. Among these, heavily shocked meteorites are unique and important samples because they contain many different high-pressure minerals such as wadsleyite, ringwoodite, jadeite, lingunite, majorite, akimotoite and bridgmanite.

Heavily shocked meteorites contain pervasive shock melt veins (SMVs) or melt pockets, where many high-pressure minerals have been discovered. SMVs are formed by localized melting of materials during an impact due to concentration of stress, compaction of pores, or frictional heating. In SMVs, both high-pressure and high-temperature conditions can be achieved simultaneously under shock, where constituent minerals can transform to their high-pressure polymorphs. Mineralogy of SMVs in a meteorite reflects P–T–t (pressure–temperature–time) conditions during an impact. The reconstruction of P–T–t conditions is one of the aim of high-pressure mineral investigations in the shocked meteorites and impact craters. The last reconstruction was done for Chelyabinsk meteorite at collaboration with Japanese colleagues. High-pressure mineral jadeite was found in the SMV. The investigation showed that the pressure and temperature in the SMV reached 3-12 GPa and 1700-2000°С, respectively, at the impact event and duration of shock pressure was longer than 70 ms. The estimated shock pressure and its duration correspond to a scenario that an impactor larger than 0.15–0.19 km collided with a parent body of Chelyabinsk meteorite with a relative speed of 0.4–1.5 km/s.

Back-scattered electron images of Chelyabinsk meteorite samples. (a) A shock-melt vein (SMV) cutting through the host-rock. The two white dotted lines represent the boundaries between them; (b) Needle-like and skeletal-rhombic crystals of jadeite (Jad) occur with feldspathic glass (Gl).

Ab initio design of geomaterials

The main goals of our ab-initio group is a theoretical study of matter under extreme conditions (high pressures and temperatures), based on density functional theory (VASP and PHONOPY packages), molecular dynamic and crystal structure prediction algorithms (USPEX package). This study is of interest for both modeling of Earth’s interiors (core and mantle) and production of new materials. Three main direction are presented:

• Investigation of compounds, consisting of metal (mainly iron or nickel) and light elements (sulfur, carbon, nitrogen) under pressures and temperatures of the Earth’s core. In this direction the intermediate compounds in the system Fe-S, Fe-N, Ni-Si were determined. The series of new phases, explaining earlier experimental results were predicted.

• Alkaline and alkaline-earth carbonates: K₂CO₃, Na₂CO₃, CaCO₃, SrCO₃. For alkaline carbonates prediction of crystal structures reveals several new phases of AlB₂ structural type. High-pressure experiments with diamond anvil cell and large-volume apparatus carried out in our laboratory have confirmed the correctness of this results. For CaCO₃ crystal structure prediction as well as molecular dynamics calculations were performed. In addition to revealing of two new phases this results let to explain some features of  phase diagram of CaCO₃ which earlier have no explanation.

• Elements. This direction is a new one in our group. On the moment, crystal structure prediction of sulfur has been carried out. Several new phases, which are on the common structural trend of sulfur phases, were predicted.

Recently, the modern technique of topological analysis (TOPOS package) were added to the list of the methodologies used in our group, allowing to analyze predicted high pressure phases more rigorously and swiftly, and find their places in the common crystall-chemical trends.

The predicted high-pressure phases of K₂CO₃ (our results) and experimentaly determined high-pressure phase of Li₂CO₃. It can be clearly seen that all these structures are of AlB₂-type, i.e. can be presented as hexagonal prism of atom B (ideal or deformed) centered by atom A, which reflects a common crystal-chemical trend of all alkaline carbonates.