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The additional work we have done using our new laser heating in the diamond anvil cell system since the publication of Andrault et al. (Phys Chem Mineral 47(2), 2020) leads us to the conclusion that there was a systematic bias in the determination of temperature. First, the temperature of the W-lamp used for the calibration of the optical system was overestimated by ~ 22 K at 2273 K. Then, we made the assumption that hot SiO2 was a grey-body (constant emissivity ε(λ)), while the available measurements suggest instead that ε(λ) of SiO2 is similar to that of tungsten. Applying these two corrections lowers the SiO2 melting temperatures significantly. In LMV, we performed a new experimental determination of the SiO2 melting temperature, at 5000 (200) K and ~ 70 (4) GPa, which is well compatible with the amplitude of the correction proposed. The reevaluation of the melting temperature profile does not affect largely the interpretations or the main conclusions presented in Andrault et al. (Phys Chem Mineral 47(2), 2020). Within the stability field of stishovite, the melting curve still presents a relatively sharp change of slope at P–T recalculated as ~ 40 GPa and ~ 4800 K. It is related to a change of the melt structure. At higher pressures, the melting curve is almost flat up to the subsolidus transition from stishovite to the CaCl2-form around 85 GPa, where the slope of the melting curve increases again up to ~ 120 GPa. We present corrected figures and tables of the original publication. [ABSTRACT FROM AUTHOR]

As evidenced by isotope geochemistry, the persistence of primitive reservoirs indicates that the earth's lower mantle is likely to be heterogeneous. Such heterogeneity could be a legacy from magma‐ocean (MO) solidification. The viscosity of MO is a key parameter to constrain the solidification type of MO. Here we directly measure the viscosity of peridotite (an analog of MO composition) melt at the pressure‐temperature conditions of the deep mantle, using the in situ falling sphere method. The viscosity of peridotite melt along liquidus is in the range of 38–17 mPa s at pressures from 7 to 25 GPa, which is 0.9–0.4 times of the estimation based on the viscosity of endmember compositions. Low viscosity favors fractional solidification and chemically layering of the early mantle at least to the top lower mantle, which could be a source of heterogeneity for the present mantle. Plain Language Summary: The earth experiences a large‐scale melting and forms a deep magma ocean in its early history. The viscosity of peridotite melt is a key parameter for understanding the solidification type of magma ocean, which leads to the primitive mantle structure. This study directly measured the viscosity of peridotite melt to deep mantle conditions and revealed that peridotite melt has a lower value of viscosity than expected. The low viscosity of peridotite melt suggests a fractional solidification of the magma ocean, which supports a heterogeneous primitive mantle. Key Points: The viscosity of peridotite melt has been measured down to lower‐mantle conditions by in situ falling sphere viscometryPeridotite melt has a lower viscosity than expected from the viscosity of the endmembersThe low viscosity of peridotite melt suggests a fractional solidification in the magma ocean, supporting a heterogeneous primitive mantle [ABSTRACT FROM AUTHOR]

As evidenced by isotope geochemistry, the persistence of primitive reservoirs indicates that the earth's lower mantle is likely to be heterogeneous. Such heterogeneity could be a legacy from magma‐ocean (MO) solidification. The viscosity of MO is a key parameter to constrain the solidification type of MO. Here we directly measure the viscosity of peridotite (an analog of MO composition) melt at the pressure‐temperature conditions of the deep mantle, using the in situ falling sphere method. The viscosity of peridotite melt along liquidus is in the range of 38–17 mPa s at pressures from 7 to 25 GPa, which is 0.9–0.4 times of the estimation based on the viscosity of endmember compositions. Low viscosity favors fractional solidification and chemically layering of the early mantle at least to the top lower mantle, which could be a source of heterogeneity for the present mantle. Plain Language Summary: The earth experiences a large‐scale melting and forms a deep magma ocean in its early history. The viscosity of peridotite melt is a key parameter for understanding the solidification type of magma ocean, which leads to the primitive mantle structure. This study directly measured the viscosity of peridotite melt to deep mantle conditions and revealed that peridotite melt has a lower value of viscosity than expected. The low viscosity of peridotite melt suggests a fractional solidification of the magma ocean, which supports a heterogeneous primitive mantle. Key Points: The viscosity of peridotite melt has been measured down to lower‐mantle conditions by in situ falling sphere viscometryPeridotite melt has a lower viscosity than expected from the viscosity of the endmembersThe low viscosity of peridotite melt suggests a fractional solidification in the magma ocean, supporting a heterogeneous primitive mantle [ABSTRACT FROM AUTHOR]

The compression of argon is measured between 10 K and 296 K up to 20 GPa and and up to 114 GPa at 296 K in diamond anvil cells. Three samples conditioning are used: (1) single crystal sample directly compressed between the anvils, (2) powder sample directly compressed between the anvils, (3) single crystal sample compressed in a pressure medium. A partial transformation of the face-centered cubic (fcc) phase to a hexagonal close-packed (hcp) structure is observed above 4.2–13 GPa. Hcp phase forms through stacking faults in fcc-Ar and its amount depends on pressurizing conditions and starting fcc-Ar microstructure. The quasi-hydrostatic equation of state of the fcc phase is well described by a quasi-harmonic Mie–Grüneisen–Debye formalism, with the following 0 K parameters for Rydberg-Vinet equation: V 0 = 38.0 Å 3 /at, K 0 = 2.65 GPa, K 0 ′ = 7.423. Under the current experimental conditions, non-hydrostaticity affects measured P–V points mostly at moderate pressure (≤ 20 GPa). [ABSTRACT FROM AUTHOR]

Properties of liquid silicates under high-pressure and high-temperature conditions are critical for modeling the dynamics and solidification mechanisms of the magma ocean in the early Earth, as well as for constraining entrainment of melts in the mantle and in the present-day core–mantle boundary. Here we present in situ structural measurements by X-ray diffraction of selected amorphous silicates compressed statically in diamond anvil cells (up to 157 GPa at room temperature) or dynamically by laser-generated shock compression (up to 130 GPa and 6,000 K along the MgSiO3 glass Hugoniot). The X-ray diffraction patterns of silicate glasses and liquids reveal similar characteristics over a wide pressure and temperature range. Beyond the increase in Si coordination observed at 20 GPa, we find no evidence for major structural changes occurring in the silicate melts studied up to pressures and temperatures exceeding Earth’s core mantle boundary conditions. This result is supported by molecular dynamics calculations. Our findings reinforce the widely used assumption that the silicate glasses studies are appropriate structural analogs for understanding the atomic arrangement of silicate liquids at these high pressures. [ABSTRACT FROM AUTHOR]

Fine structure of the oxygen K edge was investigated for water at ambient pressure, 0.16, 0.21, 0.27, 0.47, and 0.60 GPa using x-ray Raman scattering spectroscopy (XRS). Similarity in near-edge structures at 0.16 and 0.60 GPa suggests little difference in the electronic state of oxygen in the low-pressure and high-pressure forms of water. Yet, we observed significant variation of preedge structure of the XRS spectra with compression. The intensity of the preedge peak at 535.7 eV has a minimal value at around 0.3 GPa, indicating that the number of hydrogen bonding increases first and then decreases as a function of pressure. [ABSTRACT FROM AUTHOR]

Thermochemical heterogeneities detected today in the Earth's mantle could arise from ongoing partial melting in different mantle regions. A major open question, however, is the level of chemical stratification inherited from an early magma-ocean (MO) solidification. Here we show that the MO crystallized homogeneously in the deep mantle, but with chemical fractionation at depths around 1000 km and in the upper mantle. Our arguments are based on accurate measurements of the viscosity of melts with forsterite, enstatite and diopside compositions up to ~30 GPa and more than 3000 K at synchrotron X-ray facilities. Fractional solidification would induce the formation of a bridgmanite-enriched layer at ~1000 km depth. This layer may have resisted to mantle mixing by convection and cause the reported viscosity peak and anomalous dynamic impedance. On the other hand, fractional solidification in the upper mantle would have favored the formation of the first crust. Following the impact of the protoplanet Theia, planet Earth likely transformed into a magma ocean. New high temperature and pressure experiments by Xie et al. suggest that a layer enriched in bridgmanite formed during the magma ocean phase of Earth–remnants of this ancient layer today may be responsible for the viscosity peak between 660 and 1500 km in present solid mantle. [ABSTRACT FROM AUTHOR]

Bridgmanite, the dominant mineral in the Earth's lower mantle, crystallizes in the perovskite structure and transforms into post-perovskite at conditions relevant for the D ″ layer. This transformation affects the dynamics of the Earth's lowermost mantle and can explain a range of seismic observations. The thickness over which the two phases coexist, however, can extend over 100 km, casting doubt on the assignment of the observed seismic boundaries. Here, experiments show that the bridgmanite to post-perovskite transition in (Mg 0.86 ,Fe 0.14 )SiO 3 is fast on geological timescales. The transformation kinetics, however, affects reflection coefficients of P and S waves by more than one order of magnitude. Thick layers of coexisting bridgmanite and post-perovskite can hence be detected using seismic reflections. Morever, the detection and wave period dependence of D ″ reflections can be used to constrain significant features of the Earth's lowermost mantle, such as the thickness of the coexistence layer, and obtain information on temperature and grain sizes. The D ″ layer in the Earth's lower mantle involves a seismic discontinuity which is often assigned to a mineral phase transition to post-perovskite, however, as this phase transition occurs over broad region the assignment of seismic boundaries remains unclear. Here, the authors find that due to the kinetics of the bridgmanite to post-perovskite transformation, thick transition layers can be detected by seismic reflections, unlike previously thought. [ABSTRACT FROM AUTHOR]

Using large volume press, samples of bridgmanites (Bg) in equilibrium with both silicate melt and liquid Fe-alloy were synthesized to replicate the early period of core-mantle segregation and magma ocean crystallization. We observe that the Fe partition coefficient between Bg and silicate melt (DFeBg/melt) varies strongly with the degree of partial melting (F). It is close to 1 at very low F and adopts a constant value of ~0.3 for F values above 10 wt%. In the context of a partially molten mantle, a larger F (closer to liquidus) should yield Fe-depleted Bg grains floating in the liquid mantle. In contrast, a low F (closer to solidus) should yield buoyant pockets of silicate melt in the dominantly solid mantle. We also determined the valence state of Fe in these Bg phases using X-ray absorption near-edge spectroscopy (XANES). Combining our results with all available data sets, we show a redox state of Fe in Bg more complex than generally accepted. Under the reducing oxygen fugacities (fO2) of this study ranging from IW-1.5 and IW-2, the measured Fe3+ content of Bg is found moderate (Fe3+/ΣFe = 21 ± 4%) and weakly correlated with Al content. When fO2 is comprised between IW-1 and IW, this ratio is correlated with both Al content and oxygen fugacity. When fO2 remains between IW and Re/ReO2 buffers, Fe3+/ΣFe ratio becomes independent of fO2 and exclusively correlated with Al content. Due to the incompatibility of Fe in Bg and the variability of its partition coefficient with the degree of melting, fractional crystallization of the magma ocean can lead to important chemical heterogeneities that will be attenuated ultimately with mantle stirring. In addition, the relatively low-Fe3+ contents found in Bg (21%) at the reducing conditions (IW-2) prevailing during core segregation seem contradictory with the 50% previously suggested for the actual Earth's lower mantle. This suggests the presence of 1.7 wt% Fe3+ in the lower mantle, which reduces the difference with the value observed in the upper mantle (0.3 wt%). Reaching higher concentrations of trivalent Fe requires additional oxidation processes such as the late arrival of relatively oxidized material during the Earth accretion or interaction with oxidized subducting slabs. [ABSTRACT FROM AUTHOR]

We investigated the properties of Al-bearing SiO2 (with 4 or 6 wt% Al2O3) at pressures and temperatures corresponding to the lowermost mantle, using laser-heated diamond-anvil cell coupled with synchrotron-based in situ X-ray diffraction. The phase transition from CaCl2-structured to α-PbO2- structured (seifertite) polymorphs occurs between 113 and 119 GPa at 2500 K. The range of pressure where the two phases coexist is small. There is a slight decrease of the transition pressure with increasing Al-content. We propose a tentative phase diagram reporting the minerals composition as a function of pressure in the SiO2-Al2O3 system. We also refine the P-V-T equation of state of Al-bearing seifertite based on volume measurements up to more than 160 GPa and 4000 K [V0 = 92.73(10) Å3, K0 = 304.2(3.0) GPa, K'0 = 4.59 (fixed), ɵD0 = 1130 K (fixed), γ0 = 1.61(3)]. At 300 K, the volume decrease at the CaCl2 to α-PbO2 transition is 0.5(1)%, a value slightly lower than the 0.6% reported previously for Al-free samples. At high temperature, the Grüneisen parameter of seifertite is found to be similar to that of stishovite. Nevertheless, the ▵ V/V across the CaCl2-form to seifertite transition is found to increase slightly with increasing temperature. Across the phase transition, volume changes can be translated into density changes only when the Al substitution mechanisms in both CaCl2-form and seifertite are defined. The analysis of all available data sets suggests different substitution mechanisms for the two SiO2 polymorphs. Al-substitution could occur via O-vacancies in the CaCl2-form and via extra interstitial Al in seifertite. This would result in a density increase of 2.2(3)% at 300 K for SiO2 in basaltic lithologies. Alternatively, the same Alsubstitution mechanism in both of the SiO2-dominated phases would yield a density increase of 0.5(1)%. [ABSTRACT FROM AUTHOR]

Confined H2O molecules act as local probes for depressurization phenomena during the pressure induced amorphisation of faujasite NaX at which the OH stretching frequency first decreases and then increases almost to its room pressure value upon further compression. Pair distribution function (PDF) analysis provides evidence that amorphisation corresponds to a collapse of the structure around hydrated sodium cations with strong distortion of the secondary building units (double six-membered rings, sodalite cages). Both the use of guest molecules as local probes in far- and mid-infrared spectroscopy, where we correlate intermolecular water H bonding vibrations and internal mode behaviour under confinement, and PDF analysis could be of great use to study the mechanical behaviour of other hydrated materials. [ABSTRACT FROM AUTHOR]

Compressibility of perovskite-structured CaAlSiO grossular (GrPv) was investigated at high pressure and high temperature by means of angle-dispersive powder X-ray diffraction using a laser-heated diamond anvil cell. We observed the Pbnm orthorhombic distortion for the pure phase above 50 GPa, whereas below this pressure, Al-bearing CaSiO perovskite coexists with an excess of corundum. GrPv has a bulk modulus ( K = 229 ± 5 GPa; $$K_{0}^{{\prime }}$$ fixed to 4) almost similar to that reported for pure CaSiO perovskite. Its unit-cell volume extrapolated to ambient conditions ( V = 187.1 ± 0.4 Å) is found to be ~2.5 % larger than for the Al-free phase. We observe an increasing unit-cell anisotropy with increasing pressure, which could have implications for the shear properties of Ca-bearing perovskite in cold slabs subducted into the Earth's mantle. [ABSTRACT FROM AUTHOR]

Wadsleyites with various iron contents were synthesized at ~12-14 GPa and 1400 °C under oxidizing and hydrous conditions in coexistence with enstatite. The samples were studied using micro-X-ray absorption near edge structure (XANES) and micro-Mössbauer spectroscopy to determine the ferric iron contents in polyphasic samples and secondary ion mass spectrometry (SIMS) to determine the water concentrations. XANES and Mössbauer analyses show that ferric iron content increases with increasing total iron content, and reaches a maximum of ~30% Fe3+/Fetotal. Two XANES results were cross-checked by Mössbauer analysis and both methods are in reasonable agreement. The use of Fourier transform infrared spectroscopy reveals a new protonation scheme in wadsleyite, with a significant proportion of protons associated with the high-frequency band at 3611 cm-1 and a new band located at 3500 cm-1. The intensity of these two bands is higher for Fe3+-rich wadsleyite. SIMS analyses show that water contents in wadsleyite vary from 4500 to 9400 ppm H2O by weight. Pyroxene water contents range from 790 to 1600 ppm wt H2O. The concentration of water in both phases decreases with increasing iron content. The partition coefficient of water between wadsleyite and pyroxene varies between 5 and 9 and increases with increasing Fe-number of wadsleyite [i.e., XFe/(XFe + XMg) x 100 ratio]. The divalent cation concentrations (i.e., Mg2+ + Fe2+), the Si as well as the H content in wadsleyite decrease with increasing Fe3+ content, indicating an incorporation mechanism via substitution into the metal (Me = Mg2+ and Fe2+) and Si sites with a ratio of 5/3 for (Fe3++H+):Me and of 5/1 for (Fe3++H+):Si, similarly as in the dry system. Thus, coupled substitution of Fe3+ and H+ does not affect the incorporation mechanism of Fe3+, but does affect the location of H+, which is partly incorporated at tetrahedral edges forming [FeSi'-(OH)o⋅]X neutral defects substituting for Si. In this model 25% of the ferric iron occupies the tetrahedral sites. [ABSTRACT FROM AUTHOR]

Melting processes in the deep mantle have important implications for the origin of the deep-derived plumes believed to feed hotspot volcanoes such as those in Hawaii. They also provide insight into how the mantle has evolved, geochemically and dynamically, since the formation of Earth. Melt production in the shallow mantle is quite well understood, but deeper melting near the core-mantle boundary remains controversial. Modelling the dynamic behaviour of deep, partially molten mantle requires knowledge of the density contrast between solid and melt fractions. Although both positive and negative melt buoyancies can produce major chemical segregation between different geochemical reservoirs, each type of buoyancy yields drastically different geodynamical models. Ascent or descent of liquids in a partially molten deep mantle should contribute to surface volcanism or production of a deep magma ocean, respectively. We investigated phase relations in a partially molten chondritic-type material under deep-mantle conditions. Here we show that the iron partition coefficient between aluminium-bearing (Mg,Fe)SiO3 perovskite and liquid is between 0.45 and 0.6, so iron is not as incompatible with deep-mantle minerals as has been reported previously. Calculated solid and melt density contrasts suggest that melt generated at the core-mantle boundary should be buoyant, and hence should segregate upwards. In the framework of the magma oceans induced by large meteoritic impacts on early Earth, our results imply that the magma crystallization should push the liquids towards the surface and form a deep solid residue depleted in incompatible elements. [ABSTRACT FROM AUTHOR]

High pressure melting behavior of three Fe-alloys containing 5 wt% Ni and (1) 10 wt% Si, (2) 15 wt% Si or (3) 12 wt% S was investigated up to megabar pressures by in situ X-ray diffraction and laser-heated diamond anvil cell techniques. We observe a decrease in melting temperature with increasing Si content over the entire investigated pressure range. This trend is used to discuss the melting curve of pure Fe. Moreover, our measurements of eutectic melting in the Fe-FeS system show a change in slope around 50 GPa concomitant with the fcc-hcp phase transition in pure solid iron. Extrapolations of our melting curve up to the core-mantle boundary pressure yield values of 3,600-3,750 K for the freezing temperature of plausible outer core compositions. [ABSTRACT FROM AUTHOR]

Uranium is one of the main heat sources in the Earth, as about 25% of the total heat is produced by the radioactive decay of U. The location of U in the deep mantle is then essential for a better understanding of the geodynamics and thermal behavior of the Earth. For the first time, the crystal structure of natural simple dioxide UO2 uraninite has been studied by X-ray diffraction with synchrotron radiation (ESRF, Grenoble, France), in situ in a laser-heated diamond-anvil cell at pressures and temperatures relevant to the deep Earth's mantle. Fluorite-type UO2 displays a new sequence of phase transitions at high P and T, with a cubic modified fluorite Pa3¯ observed at 18 GPa, and an orthorhombic Pbca structure from 33 GPa up to 82 GPa. Using a second-order Birch-Murnaghan equation of state, we calculated room-pressure bulk modulus K0 = 166(7) GPa with pressure derivative K′0 = 4.0 for the Pa3¯ structure, and K0 = 225(8) GPa with K′0 = 4 for the Pbca structure. The expected Pnma cotunnite structure was not observed but is not excluded at pressures higher than 82 GPa. Since UO2 displays a Pbca structure stable up to 82 GPa and presents a density much higher than the average density of the surrounding mantle, UO2 could be a host of U in the deep lower mantle. [ABSTRACT FROM AUTHOR]

The phase relations and equation of state of ZrO2 were investigated up to 100 GPa by means of in situ observation using laser heating in a diamond anvil cell and synchrotron radiation. A cotunnite (PbCl2)-type phase, which appears above 12.5 GPa, is stable to a pressure of 100 GPa and a temperature of 2500 K. No post-cotunnite phase was observed under the present experimental conditions. The unit-cell parameters and the volumes of the cotunnite-type ZrO2 were determined as a function of pressure at room temperature using a laser-annealing technique. The cotunnite-type ZrO2 shows rather isotropic compression. The bulk modulus calculated using the Birch–Murnaghan equations of state is 278 GPa, which suggests that high-density ZrO2 is a candidate for potentially very hard materials. In situ high-temperature experiments performed below 12.5 GPa revealed that a tetragonal fluorite (CaF2)-type phase is stable up to 3000 K, although a cubic fluorite-type phase has been assumed to exist in this high-temperature regime. The result suggests the possibility that stoichiometric ZrO2 does not show the cubic structure up to the melting temperature. [ABSTRACT FROM AUTHOR]

Presents information on a study which clarified the equation of state of stishovite after suspected systematic errors in previous experimental reports. Measurement of the stishovite equation of state; Consistency between the different data sets; Results of single crystal diffraction.

Investigates structural refinements of magnesite at very high pressure. Details of the experiment; Equation of state and high-pressure stability; Structural changes with pressure.

The last decade has seen the development of very bright third generation x-ray synchrotron sources that have significantly improved x-ray diffraction experiments at high pressure and high temperature. In the same time, focusing optics as well as detectors have been improved in such a way that x-ray diffraction experiments can be routinely carried out on samples in laser-heated diamond anvil-cell, i.e., under extreme static pressure and temperature conditions. It is now possible to obtain from a laser heated sample in a diamond anvil cell (DAC) very high quality powder patterns, needed for accurate in situ phase boundary and pressure-volume-temperature equation of state determinations, as well as for structural refinements. The setup for in situ x-ray investigation at simultaneous high pressure and temperature is complex, since perfect alignment of x-ray path, infrared-laser hot spot, and optical temperature measurement is required. It provides, however, the most accurate information on the structural behavior with step by step increases of pressure and temperature. In addition, a range of experiments can also be realized at room temperature after laser annealing of DAC samples, which has been shown very efficient in releasing stresses build on compression, and/or overcoming phase transitions kinetic barriers. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Presents information on a study on the structure analysis and stability field of beta-iron at high pressure and temperature. Experimental methods; Results and discussion; Conclusion.

The isostructural lithium (LiSiO) and sodium (NaSiO) metasilicates have been investigated from room temperature up to the melting point by single-crystal Raman spectroscopy and energy-dispersive X-ray powder diffraction. The unit-cell parameters and Raman frequencies of LiSiO vary regularly with temperature up to the melting point, which is consistent with the lack of premelting effects in calorimetric measurements. In contrast, NaSiO undergoes a transition at about 850 K from orthorhombic Cmc 2 symmetry, to a lower symmetry (possibly Pmc 2), and shows near 1200 K changes in the Raman spectra that correlate well with the premelting effects as determined from calorimetry observations. In both compounds, a high alkali mobility likely sets in several hundreds of degrees below the melting point. Premelting in NaSiO is associated with extensive deformation of the silicate chains as evidenced near the melting point by similarities in the Raman spectra of the crystalline and liquid phases. [ABSTRACT FROM AUTHOR]

Cubic BC[sub 2]N was synthesized from graphite-like BC[sub 2]N at pressures above 18 GPa and temperatures higher than 2200 K. The lattice parameter of c-BC[sub 2]N at ambient conditions is 3.642(2) Å, which is larger by 1.48% than would be expected based on ideal mixing between diamond and cubic boron nitride. The bulk modulus of c-BC[sub 2]N is 282 GPa which is one of the highest bulk moduli known for any solid, and is exceeded only by the bulk moduli of diamond and c-BN. The hardness of c-BC[sub 2]N is higher than that of c-BN single crystals which indicates that the synthesized phase is only slightly less hard than diamond. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]