High-pressure Experimental Studies Research Articles

Superconducting ternary hydrides: progress and challenges.

Since the discovery of the high-temperature superconductors H3S and LaH10 under high pressure, compressed hydrides have received extensive attention as promising candidates for room-temperature superconductors. As a result of current high-pressure theoretical and experimental studies, it is now known that almost all the binary hydrides with a high superconducting transition temperature (T c) require extremely high pressure to remain stable, hindering any practical application. In order to further lower the stable pressure and improve superconductivity, researchers have started exploring ternary hydrides and had many achievements in recent years. Here, we discuss recent progress in ternary hydrides, aiming to deepen the understanding of the key factors regulating the structural stability and superconductivity of ternary hydrides, such as structural motifs, bonding features, electronic structures, electron-phonon coupling, etc. Furthermore, the current issues and challenges of superconducting ternary hydrides are presented, together with the prospects and opportunities for future research.

Pressure-induced superconductivity in the nonsymmorphic topological insulator KHgAs

Recently, topological insulators (TIs) KHgX (X = As, Sb, Bi) with hourglass-shaped dispersion have attracted great interest. Different from the TIs protected by either time-reversal or mirror crystal symmorphic symmetry tested in previous experiments, these materials were proposed as the first material class whose band topology relies on nonsymmorphic symmetries. As a result, KHgX shows many exotic properties, such as hourglass-shaped electronic channels and three-dimensional doubled quantum spin Hall effects. To date, high-pressure experimental studies on these nonsymmorphic TIs are minimal. Here, we carried out high-pressure electrical measurements up to 55 GPa, together with high-pressure X-ray diffraction measurements and high-pressure structure prediction on KHgAs. We found a pressure-induced semiconductor-metal transition between ~16 and 20 GPa, followed by the appearance of superconductivity with a Tc of ~3.5 K at approximately 21 GPa. The superconducting transition temperature was enhanced to a maximum of ~6.6 K at 31.8 GPa and then slowly decreased until 55 GPa. Furthermore, three high-pressure phases within 55 GPa were observed, and their crystal structures were established. Our results showed the high-pressure phase diagram of KHgAs and determined the pressure-induced superconductivity in nonsymmorphic TIs. Thus, our study can be used to facilitate further research on superconductivity and topologically nontrivial features protected by nonsymmorphic symmetries.

Sustainability of crystallographic phase of α-Glycine under dynamic shocked conditions

High pressure experimental studies on amino acids have gained incredible attention over the past few years in variety of research areas such as materials science, geophysics, pharmaceutical sciences, and cosmo-chemistry. On en-route to the current experimental work, one of the elegant amino acid samples of α-Glycine is preferred in order to reveal its nature of crystallographic phase stability as and when exposed to dynamic shock waves thereby subjecting it to diffraction and microscopic analyses. Considering the observed results from the various analytical techniques, it is authenticated that the title sample does not undergo any kind of structural transition under shock wave exposed situations. On the other hand, the test sample has generated a few new shoulder peaks and diffraction peaks under shocked conditions. The outcome of the attained phase stability is well substantiated by the previous static high-pressure reports.

Experimental constraints on truly conjugate alkaline silicate – carbonatite melt pairs

Physical separation of immiscible carbonatites from CO2-bearing alkaline silicate melts is frequently invoked to explain the common presence of carbonatites in alkaline rock suites. We present new isobaric 1.0 GPa crystallization and liquid immiscibility experiments between 1250 and 925°C on chemically complex CO2-bearing subvolcanic basanitic, nephelinitic and nepheline-syenitic compositions from the intrusive complex on Brava Island, Cape Verdes. In the experiments, carbonatites exsolve from silicate melt along the basanite–nephelinite melt fractionation pathway throughout an unmixing temperature window from 1100 to 950°C. Liquid immiscibility is suppressed by 950°C by the onset of alkali-feldspathoid crystallization, which deflects the residual silicate melt away from the two-liquid field. While evolving along the two-liquid field boundary, silicate melt increases in SiO2 from 46 to 51 wt% and decreases in CaO / (Na2O + K2O) (by weight) from 0.83 to 0.12, reproducing the observed compositional array of alkaline silicate subvolcanic rocks from Brava. The conjugate carbonatite melts are moderately alkaline Ca-carbonatites with 5 to 9 wt% SiO2 and CaO / (Na2O + K2O) that decreases from 5.64 at 1100°C to 1.60 at 950°C. These coexisting silicate and carbonatite melt compositions show that previous high-pressure experimental studies have not yielded truly conjugate melt pairs, mainly due to the use of modified starting materials resulting in melt compositions too enriched in alkalis to match the natural alkaline silicate rock record.

Reduced methane-bearing fluids as a source for diamond

Diamond formation in the Earth has been extensively discussed in recent years on the basis of geochemical analysis of natural materials, high-pressure experimental studies, or theoretical aspects. Here, we demonstrate experimentally for the first time, the spontaneous crystallization of diamond from CH4-rich fluids at pressure, temperature and redox conditions approximating those of the deeper parts of the cratonic lithospheric mantle (5–7 GPa) without using diamond seed crystals or carbides. In these experiments the fluid phase is nearly pure methane, even though the oxygen fugacity was significantly above metal saturation. We propose several previously unidentified mechanisms that may promote diamond formation under such conditions and which may also have implications for the origin of sublithospheric diamonds. These include the hydroxylation of silicate minerals like olivine and pyroxene, H2 incorporation into these phases and the “etching” of graphite by H2 and CH4 and reprecipitation as diamond. This study also serves as a demonstration of our new high-pressure experimental technique for obtaining reduced fluids, which is not only relevant for diamond synthesis, but also for investigating the metasomatic origins of diamond in the upper mantle, which has further implications for the deep carbon cycle.

Discovery of nanoscale dissolution structure of monoclinic pyrite in jadeite quartzite from mesoscopic perspective

The mineral dissolution structure preserves information on the physicochemical conditions of the early parent minerals. Studying it helps to understand the source depth of the host rock and the dynamic evolution process of decompression and reentry. The directional quartz precipitates in the monoclinic pyroxene are formed by dissolution, and most of the quartz solution is formed in the α quartz stable region. High temperature and high pressure experimental studies show that the formation of vacancies in monoclinic pyroxene and the content of calcium Escola components are all affected by a variety of factors such as chemical composition, pressure, and temperature. Mesogen is a breakthrough for in-depth exploration of mineral structure and properties from the nanometer scale. Mesogen is the product of a non-classical crystallisation process. It uses nanoparticles as the basic building unit and possesses the properties and macro size of nanoparticles. The study of the monoclinic pyroxene nanostructure, formation mechanism and related dynamic processes in jadeite quartzite can not only limit the depth of continental crust subduction, but also help to understand the dynamic process and mechanism of continental deep subduction-reversion.

New constraints on archetypal South African kimberlite petrogenesis from quenched glass-rich melt inclusions in olivine megacrysts

The evaluation of primary kimberlite compositions is hindered by significant melt modifications during ascent through the lithosphere by entrainment of xenolithic material, volatile degassing, and near surface alteration. Consequently, hypabyssal kimberlite emplaced in the upper crust may not provide a true reflection of the primary kimberlite magma. This contribution places new constraints on kimberlite melt composition by providing an assessment of quenched glass-rich polymineralic melt inclusions hosted in olivine megacrysts from the Monastery kimberlite, South Africa. Melt inclusions predominantly contain variable proportions of euhedral or skeletal grains of calcite, phlogopite, spinel, perovskite, serpentine, and fresh to devitrified glass. Estimates of the bulk compositions of melt inclusions, and the compositions of crystalline phases present therein, are compatible to those of hypabyssal kimberlites worldwide and show a volatile-rich (CO2 + H2O ~10–17 wt%) carbonated silicate (SiO2 ~27–41 wt%) composition. The glass component has a Si-Mg-Fe-rich and largely CaO-, K2O- and TiO2-free major element composition and is REE-depleted. It also contains approximately 10 wt% H2O but is CO2−free. The glass represents a residual melt that existed after crystallization of the observed mineral assemblage. From some, but not all melt inclusions, apophyses radiate outwards. These fractures typically contain partially devitrified glass that is compositionally identical to the fresh residual glass within the melt inclusions, indicating fracture formation during decompression of the hosting megacryst and at a stage after the melt had evolved. These features are consistent with a trapping of the melt inclusions at high pressure, prior to kimberlite ascent to the surface, in the SCLM at a depth corresponding to 4.5–6 GPa. Textures and compositions of phases within the melt inclusions represent stages of the kimberlite melt and magma evolution. They provide evidence in support of high-pressure experimental studies suggesting a carbonated silicate primary melt rather than a carbonatite. Furthermore, the composition of fresh glass in the melt inclusions, which is compositionally similar to serpentine, suggests that much groundmass serpentine in hypabyssal kimberlites may have formed from similar silicate melt or devitrified glass.

A high temperature and high pressure experimental study on re-bearing capability of sulfide

High temperature (1 270–1 550 oC) and high pressure (1.0 GPa) experimental studies on Re-bearing capabilities of pyrite, galena and sphalerite from typical Pb-Zn ore deposits were performed on a six-anvil apparatus. We observed microstructures of the quenched sulfides using scanning electron microscope (SEM) and analyzed compositions of the run products using both energy disperse spectroscopy (EDS) and electron probe microanalyzer (EPMA). The results show that pyrite melt can dissolve much more metallic Re than galena and sphalerite melts, forming scattered acicular ReS2 in the quenched matrix of pyrrhotite (Fe1-xS). The quenched matrixes of Fe1-xS, PbS and ZnS generally contain less than 1.0 wt.% of Re and their Re-bearing capabilities seem to range as Fe1-xS>PbS>ZnS. However, Re partition coefficients between them are difficult to estimate, because Re distribution is inhomogeneous in the quenched sulfide matrixes.

Thermal equation of state of a natural kyanite up to 8.55 GPa and 1273 K

The thermal equation of state of a natural kyanite has been investigated with a DIA-type, cubic-anvil apparatus (SAM85) combined with an energy-dispersive synchrotron X-ray radiation technique up to 8.55 GPa and 1273 K. No phase transition was observed in the studied pressure-temperature (P-T) range. The Le Bail full profile refinement technique was used to derive the unit-cell parameters. By fixing the bulk modulus K0 as 196 GPa and its pressure derivative K0′ as 4, our P-V (volume)-T data were fitted to the high temperature Birch–Murnaghan equation of state. The obtained parameters for the kyanite are: V0 = 294.05(9) Å3, α = 2.53(11) × 10−5 K−1 and (∂K/∂T)P = −0.021(8) GPa∙K−1. These parameters have been combined with other experimentally-measured thermodynamic data for the relevant phases to calculate the P-T locus of the reaction kyanite = stishovite + corundum. With this thermodynamically constrained phase boundary, previous high-pressure phase equilibrium experimental studies with the multi-anvil press have been evaluated.

Prediction of new high pressure structural sequence in thorium carbide: A first principles study

In the present work, we report the detailed electronic band structure calculations on thorium monocarbide. The comparison of enthalpies, derived for various phases using evolutionary structure search method in conjunction with first principles total energy calculations at several hydrostatic compressions, yielded a high pressure structural sequence of NaCl type (B1) → Pnma → Cmcm → CsCl type (B2) at hydrostatic pressures of ∼19 GPa, 36 GPa, and 200 GPa, respectively. However, the two high pressure experimental studies by Gerward et al. [J. Appl. Crystallogr. 19, 308 (1986); J. Less-Common Met. 161, L11 (1990)] one up to 36 GPa and other up to 50 GPa, on substoichiometric thorium carbide samples with carbon deficiency of ∼20%, do not report any structural transition. The discrepancy between theory and experiment could be due to the non-stoichiometry of thorium carbide samples used in the experiment. Further, in order to substantiate the results of our static lattice calculations, we have determined the phonon dispersion relations for these structures from lattice dynamic calculations. The theoretically calculated phonon spectrum reveal that the B1 phase fails dynamically at ∼33.8 GPa whereas the Pnma phase appears as dynamically stable structure around the B1 to Pnma transition pressure. Similarly, the Cmcm structure also displays dynamic stability in the regime of its structural stability. The B2 phase becomes dynamically stable much below the Cmcm to B2 transition pressure. Additionally, we have derived various thermophysical properties such as zero pressure equilibrium volume, bulk modulus, its pressure derivative, Debye temperature, thermal expansion coefficient and Gruneisen parameter at 300 K and compared these with available experimental data. Further, the behavior of zero pressure bulk modulus, heat capacity and Helmholtz free energy has been examined as a function temperature and compared with the experimental data of Danan [J. Nucl. Mater. 57, 280 (1975)].

High-Pressure Experimental and DFT-D Structural Studies of the Energetic Material FOX-7

This work reports the hydrostatic compression of the perdeuterated α-form of FOX-7 using neutron powder diffraction to follow the structural changes up to 4.58 GPa at room temperature. The equation of state for the hydrostatic compression of the α-form over the range 0–4.14 GPa has been determined, and a phase transition was observed over the pressure range 3.63–4.24 GPa. On the basis of dispersion-corrected density functional theory (DFT-D) calculations performed on the γ-form over a range of pressures, the high-pressure form observed in the neutron diffraction experiments can unambiguously be identified as being different from the γ-form and should therefore be denoted as the ε-form. Based on similarities between the simulated and experimental powder diffraction patterns of the γ- and ε-forms, it is suggested that the ε-form adopts a planar, layered structure. The structural responses to pressure of the α-form observed experimentally are reproduced by DFT-D calculations, but in-depth analysis of the bond lengths, angles, dihedrals, and vibrational frequencies calculated in the DFT-D simulations identified a very subtle second-order phase transition at 1.9 GPa. This corroborates results obtained from previous far- and mid-IR vibrational spectroscopic studies. These very small changes in molecular geometry do not manifest themselves in either the measured or calculated lattice parameters or unit-cell volumes and are much smaller than can be detected by diffraction experiments. The results of phonon calculations were compared with experimental inelastic neutron scattering measurements and were used to investigate the effect of pressure on the heat capacities of α-FOX-7. The simulations predict very weak pressure dependencies (approximately −1 J K–1 mol–1 GPa–1), in accordance with the conclusions reached in our previous studies of the energetic material RDX.

High-pressure experimental studies on geo-liquids using synchrotron radiation at the Advanced Photon Source

We review recent progress in studying silicate, carbonate, and metallic liquids of geological and geophysical importance at high pressure and temperature, using the large-volume high-pressure devices at the third-generation synchrotron facility of the Advanced Photon Source, Argonne National Laboratory. These integrated high-pressure facilities now offer a unique combination of experimental techniques that allow researchers to investigate structure, density, elasticity, viscosity, and interfacial tension of geo-liquids under high pressure, in a coordinated and systematic fashion. Experimental techniques are described, along with scientific highlights. Future developments are also discussed.

Pressure-driven reentrance phenomena in a binary mixture of achiral liquid crystals: role of inverse layer spacing

The occurrence of pressure-driven reentrant phenomena in a binary mixture of liquid crystals exhibiting the phase sequence on cooling is observed in high-pressure experimental studies has been explained within the framework of Landau de Gennes theory. In this phenomenological theory, the free energy density is written as an expansion series containing terms involving powers of nematic and smectic () order parameters and the () and () coupling terms. The most important feature of the work is that the role of inverse layer spacing , which mimics an order parameter, couples with the order parameters. The influence of concentration on reentrant behaviour is also discussed by varying the coupling between the concentration variable and the order parameters. We found that the order parameters coupling terms and inverse of layer spacing play an important role and that the present analysis is in good qualitative agreement with the available experimental results.

Hot mantle geotherms stabilize calcic carbonatite magmas up to the surface

The eruption of calciocarbonatites at Earth’s surface is at odds with them being equilibrated with the mantle at depth because high-pressure experimental studies predict that significant magnesium contents should be expected. Here we report on new high-pressure experiments that demonstrate extreme calcium enrichment of carbonatites en route to the surface. We have monitored the decompression of partially molten carbonated peridotite using a multianvil apparatus coupled to synchrotron radiation. The experimental charge was molten at high pressure and high temperature, before being decompressed along a path that avoided the so-called “carbonate ledge” (a boundary that prevents carbonatitic melts from reaching the surface). Reaction with clinopyroxene yields calcium enrichment and magnesium depletion. The resulting Ca/(Ca + Mg) of the quenched melt reaches 0.95, which compares well with the composition of erupted calcic carbonatites [Ca/(Ca + Mg) ∼0.96–0.99] and of calcic melts trapped in mantle xenoliths from ocean islands [Ca/(Ca + Mg) ∼0.84–0.97]. Our results demonstrate that it is possible to bring carbonatites very close to the surface, without breakdown, and therefore without catastrophic CO2 release. Such occurrence appears to be favored by hot geotherms, meaning that higher temperatures tend to stabilize carbonatitic melts at shallow mantle pressure. Carbonatitic magmas are usually associated with low temperatures, because of the assumed low melting degree or low eruption temperature of the only active carbonatite volcano (i.e., Oldoinyo Lengai, Tanzania). Here we show that emplacement of carbonatites at or near the surface necessitates a hot environment.

Effects of fluid and melt density and structure on high-pressure and high-temperature experimental studies of hydrogen isotope partitioning between coexisting melt and aqueous fluid

Hydrogen isotope partitioning (as H2O and D2O) between silicate-saturated aqueous fluid and water-saturated aluminosilicate melt has been determined with vibrational spectroscopy (Raman and infrared) in situ with the samples at high temperature and pressure by using a hydrothermal diamondanvil cell (HDAC) for sample containment. To assess the effects of pressure and, therefore, different silicate speciation in fluids and melts, on the D/H partitioning behavior, two pressure/temperature experimental trajectories (450-800 °C/155-754 MPa, and 450-800 °C/562-1271 MPa) were used. In these temperature and pressure ranges, the fluid/melt partition coefficients are temperature (and pressure) dependent with the average enthalpy change, △H = -6.6 ± 15 kJ/mol and -10.3 ± 1.1 kJ/mol for H2O and D2O, respectively. The △H-values for the lower-pressure trajectory (and, therefore, lower fluid density) were 15-20% higher than for the higher-pressure (and higher fluid density) trajectory. The (D/H) ratios of fluids and melts, (D/H)fluid and (D/H)melt, are also temperature dependent with a small negative DH for (D/H)fluid (average: -2.4 ± 0.8 kJ/mol) and a positive △H-value for (D/H)melt (2.3 ± 1.4 kJ/mol). The (D,H) exchange equilibrium between fluid and melt is also temperature (and pressure) dependent so that for the low-density P/T trajectory, the △H = -4.2 ± 0.6 kJ/mol, whereas for the higher-density trajectory, △H = -5.4 ± 0.7 kJ/mol. The difference between the H2O and D2O fluid/melt partition coefficients and the temperature- and pressure-dependent D/H fractionation behavior in and between hydrous silicate melts and silicate-saturated aqueous fluid in part is because pressure increases with increasing temperature in the HDAC experiments and the volume difference between fluid and melt differ for H2O and D2O. In addition, the silicate speciation in fluids and melts are temperature and pressure dependent, which also leads to significantly temperature- and pressuredependent D/H fractionation within and between silicate melts and fluids at high temperature and pressure. In the Earth’s deep crust and upper mantle, hydrogen isotope partitioning between condensed phases and aqueous fluid can differ substantially from that between condensed phases and pure H2O because the aqueous fluid in the Earth’s interior is a concentrated silicate solution wherein the silicate speciation affects the isotope partitioning.

An experimental study of phase transformations in olivine under pressure and temperature conditions corresponding to the mantle transition zone

High-pressure polymorphs of olivine (wadsleyite and ringwoodite) are major minerals in the mantle transition zone (MTZ). Phase transformations in olivine are important for a series of geodynamic problems such as the mineralogical and evolutionary history of the mantle, mantle convection patterns, and deep focus earthquakes in subduction zones. In this study, we examine phase transformations in olivine with two compositions, namely Mg2SiO4 (Fo100) and (Mg0.9Fe0.1)2SiO4 (Fo90), at pressures between 14.1 and 20 GPa and a constant temperature of 1400°C, using the newly installed multi-anvil system at the Laboratory for Studies of the Earth’s Deep Interior (SEDI), China University of Geosciences (Wuhan). At 14.1 GPa, Fo90 transformed completely into the wadsleyite structure (β), while Fo100 remained as olivine (α). Between 14.8 and 15.6 GPa, both Fo100 and Fo90 transformed into the wadsleyite structure. Wadsleyite crystals were identified by two characteristic Raman peaks between 722 and 723 and 917 and 919 cm−1. They exhibit a bimodal grain size distribution: large-crystals with average grain sizes greater than 100 μm and microcrystals less than 10 μm. The population of microcrystals increased with pressure, apparently due to the increase in over-pressure (the difference between the experimental pressure condition and the equilibrium transformation pressure at 1400°C), which promotes nucleation and retards grain growth. All run charges contained large numbers of wadsleyite microcrystals, because of the low activation energy of the nucleation process. The experimentally observed microstructure may shed light on the morphology of wadsleyite observed in shocked meteorites. At 19.5 GPa, wadsleyite coexisted with ringwoodite (γ) in Fo100, but was absent in Fo90. At 20 GPa, both samples transformed completely into ringwoodite, which was characterized by the 798 and 840 cm−1 Raman lines. Ringwoodite crystals are euhedral grains (average grain size 10–20 μm), with well-developed triple junctions. The complex upper mantle structure in eastern China determined from seismological studies cannot be explained by the simple transformation sequence of the olivine system alone. Phase transformations in other pyroxene-normative components (including pyroxenes and garnets) and the interaction of these components with olivine may be responsible for the complex structure. High-pressure and high-temperature experimental studies on complex systems (e.g. olivine-pyroxene), combined with data from geophysical exploration, may help in establishing a more realistic geological-petrological model for eastern China and further our understanding of the possible physical mechanisms that are responsible for the complex structure. Such studies will have profound implications for understanding the dynamic processes in the deep Earth interior.

高温高圧実験によるマントル鉱物における水の影響に関する研究

高温高圧実験によるマントル鉱物における水の影響に関する研究

Recent advances of high-pressure generation in a multianvil apparatus using sintered diamond anvils

The tried and tested multianvil apparatus has been widely used for high-pressure and high-temperature experimental studies in Earth science. As a result, many important results have been obtained for a better understanding of the components, structure and evolution of the Earth. Due to the strength limitation of materials, the attainable multianvil pressure is generally limited to about 30 GPa (corresponding to about 900 km of the depth in the Earth) when tungsten carbide cubes are adopted as second-stage anvils. Compared with tungsten carbide, the sintered diamond is a much harder material. The sintered diamond cubes were introduced as second-stage anvils in a 6–8 type multianvil apparatus in the 1980s, which largely enhanced the capacity of pressure generation in a large volume press. With the development of material synthesis and processing techniques, a large sintered diamond cube (14 mm) is now available. Recently, maximum attainable pressures reaching higher than 90 GPa (corresponding to about 2700 km of the depth in the Earth) have been generated at room temperature by adopting 14-mm sintered diamond anvils. Using this technique, a few researches have been carried out by the quenched method or combined with synchrotron radiation in situ observation. In this paper we review the properties of sintered diamond and the evolution of pressure generation using sintered diamond anvils. As-yet unsolved problems and perspectives for uses in Earth Science are also discussed.

Diamond formation in the deep mantle: the record of mineral inclusions and their distribution in relation to mantle dehydration zones

AbstractStudies of the inclusions contained in natural diamonds have shown the occurrence of minerals which must have formed at depths below the lithosphere and which may be closely matched with the silicate mineral assemblages determined by high pressure and temperature experimental studies for depths of 300 to 800 km in the Earth's mantle. The inclusions come principally from two main depth zones: (1) the lower asthenosphere and upper transition zone; (2) the Upper Mantle/Lower Mantle (UM/LM) boundary region and the uppermost LM. The inclusions from zone 1 are very largely majoritic garnets (with or without clinopyroxene) which indicate bulk compositions of eclogitic/metabasic affinity. The minerals from zone 2 include Ca-Si and Mg-Si perovskites and ferropericlase and are dominantly of metaperidotitic bulk composition, but include some possible metabasite assemblages. In many of these natural assemblages, the tetragonal almandine pyrope phase occurs rather than the garnet found in experiments.As natural diamonds are believed to crystallize in fluids/melts, the hypothesis is developed that the restriction of diamonds and inclusions of particular compositions to the above two depth intervals is because they are controlled by loci of fluid/melt occurrence. Attention is focused on subduction zones because both suites of inclusions show some evidence of subducted protoliths. The lower zone (600–800 km) coincides with the region where dehydration may be expected for hydrous ringwoodite and dense hydrous Mg-silicates formed in subducted peridotites. The dehydration of lawsonite in subducted metabasites provides a particular location for melt formation and the inclusion of the shallower (~ 300 km) majoritic inclusions. For the deeper majoritic inclusions in the region of the upper transition zone, melt development may occur as a consequence of the hydrous wadsleyite-to-olivine transformation, and such melt may then interact with the upper crustal portion of a subducting slab. These suggestions offer an explanation of the depth restrictions and the compositional restrictions of the inclusions. The differences in δ13C values in the host diamonds for the two suites of inclusions may also be explained on this basis.

Pressure-driven reentrant phenomena in liquid crystals: the role of inverse layer spacing

The occurrence of pressure-driven reentrant phenomena observed in high-pressure experimental studies in some achiral mesogenic materials has been explained using a thermodynamic model based on Landau–de Gennes theory. In this approach, the free-energy is expanded in terms of nematic, smectic A order parameters and the couplings (cubic and biquadratic) between them. The basic theme here is that the ‘inverse layer spacing’, which mimics an order parameter, becomes coupled to the nematic and smectic A order parameters. Secondly, in addition to the order parameter couplings the N–SA metastable temperature (which appears due to elimination of inverse layer spacing from the free-energy density expression) becomes pressure dependent. The occurrence of a pressure-driven reentrant nematic phase is explained in terms of these three pressure-dependent parameters. They all show smooth but rapid variation at the critical pressure beyond which nematic reentrance appears.