Periclase (MgO) is the second most abundant mineral after bridgmanite in the Earth's lower mantle, and its melting behaviour under pressure is important to constrain rheological properties and ...melting behaviours of the lower mantle materials. Significant discrepancies exist between the melting temperatures of MgO determined by laser-heated diamond anvil cell (LHDAC) and those based on dynamic compressions and theoretical predictions. Here we show the melting temperatures in earlier LHDAC experiments are underestimated due to misjudgment of melting, based on micro-texture observations of the quenched samples. The high melting temperatures of MgO suggest that the subducted cold slabs should have higher viscosities than previously thought, suggesting that the inter-connecting textural feature of MgO would not play important roles for the slab stagnation in the lower mantle. The present results also predict that the ultra-deep magmas produced in the lower mantle are peridotitic, which are stabilized near the core-mantle boundary.
Transparent ceramics are important for scientific and industrial applications because of the superior optical and mechanical properties. It has been suggested that optical transparency and mechanical ...strength are substantially enhanced if transparent ceramics with nano-crystals are available. However, synthesis of the highly transparent nano-crystalline ceramics has been difficult using conventional sintering techniques at relatively low pressures. Here we show direct conversion from bulk glass starting material in mutianvil high-pressure apparatus leads to pore-free nano-polycrystalline silicate garnet at pressures above ∼10 GPa in a limited temperature range around 1,400 °C. The synthesized nano-polycrystalline garnet is optically as transparent as the single crystal for almost the entire visible light range and harder than the single crystal by ∼30%. The ultrahigh-pressure conversion technique should provide novel functional ceramics having various crystal structures, including those of high-pressure phases, as well as ideal specimens for some mineral physics applications.
The partitioning of carbon between the core and mantle during the formation of terrestrial planets may have controlled the distribution of carbon in terrestrial planets. However, the abundance of ...carbon in the Earth's mantle is higher than a prediction based on previous metal‐silicate partitioning experiments of carbon at carbon‐saturated conditions by more than an order of magnitude. Here, we report new metal‐silicate partitioning experiments of carbon at carbon contents of 0.25–0.5 wt%. We show that the metal‐silicate partition coefficient of carbon (
DCmet/sil) strongly correlates with nonbridging oxygen per tetrahedral cations (nbo/t) of silicate melts at
fO2 conditions where C‐H species are stable. Moreover, the results suggest that
DCmet/sil at carbon‐undersaturated conditions may be lower than that at carbon‐saturated condition. Thus,
DCmet/sil at low carbon concentrations is essentially important to investigate the distribution of carbon in the Earth during the core formation.
Plain Language Summary
Carbon, one of the most abundant volatile elements in the Earth's mantle, affects planetary climate and volcanism. Therefore, elucidating when and how carbon in the Earth was distributed is important for understanding the habitability and chemical evolution of the Earth. Core‐mantle partitioning of carbon is thought to be a key process for determining the distribution of carbon in terrestrial planets. Here, we experimentally investigated carbon partitioning between metal droplets and silicate melts under high‐pressure conditions (4–12 GPa) and 0.25–0.5 wt% carbon concentration. Our results suggest that carbon may not be highly siderophile than previously thought. Therefore, metal‐silicate partitioning experiments on carbon at low concentrations are necessary to constrain the distribution of carbon in the Earth during its formation.
Key Points
Liquid metal‐silicate partitioning of carbon at carbon‐undersaturated conditions (i.e., 0.25–0.5 wt%) was experimentally investigated
The metal‐silicate partition coefficient of carbon in our study was lower than that of previous studies using a graphite capsule, at least for experiments using basalt
Metal‐silicate partitioning experiments of carbon at low carbon concentrations are necessary to constrain the distribution of carbon in terrestrial planets during their formation stages
New High‐Pressure Forms of Al2SiO5 Zhou, Y.; Irifune, T.; Ohfuji, H. ...
Geophysical research letters,
28 August 2018, Letnik:
45, Številka:
16
Journal Article
Recenzirano
Odprti dostop
Phase relations of Al2SiO5 have been studied by multianvil experiments at pressures of 13–23 GPa and temperatures of 2000–2900 K. Al2SiO5 kyanite was found to transform into two new high‐pressure ...forms of Al2SiO5 (kyanite II and III) at temperatures exceeding 2300–2500 K and pressures of 14–23 GPa: The first phase transition occurs near 14 GPa, and the second occurs near 17 GPa. The new Al2SiO5 phases have triclinic and monoclinic crystal symmetries with zero‐pressure densities of 3.876(2) and 3.982(1) g/cm3, respectively, which are significantly denser than kyanite (ρ0 = 3.666 g/cm3) but less dense than the isochemical mixture of Al2O3 corundum and SiO2 stishovite (ρ0 = 4.036 g/cm3). The exceptionally high stability temperatures of the new Al2SiO5 phases suggest that they are unlikely to form in the present mantle but may be found in some impact craters and shocked meteorites and act as important indicators of pressure and temperature for the shock events.
Plain Language Summary
Aluminosilicate in the mantle is an important issue for Earth science. Previous studies argued the existence of a high‐pressure form of Al2SiO5 in the lower mantle, in addition to those of the well‐known polymorphs, andalusite, sillimanite, and kyanite, found in shallow metamorphic rocks. However, the existence of such a high‐pressure form of Al2SiO5 has been controversial among previous studies, due to the large discrepancies in its stability region and crystal structure. Here we figure out the phase relations of Al2SiO5 at mantle‐transition‐region pressures (~13–23 GPa, corresponding to ~410–660 km in depth) and temperatures of 2000–2900 K via multianvil high‐pressure experiments and show that kyanite transforms into two new high‐pressure forms of Al2SiO5 at temperatures exceeding 2300–2500 K in the pressure range of 14–23 GPa: The first phase transition occurs near 14 GPa, and the second occurs near 17 GPa. Although the extremely high stability temperatures of the new Al2SiO5 phases suggest their absence in the present mantle, they may be found in some impact craters and shocked meteorites and act as important indicators of pressure and temperature for the shock events.
Key Points
We investigated the phase relations of Al2SiO5 by multianvil experiments at pressures of 13‐23 GPa and temperatures of 2000‐2900 K
We confirmed the existence of two new high-pressure forms of Al2SiO5 at pressures of 14‐23 GPa and temperatures exceeding 2300‐2500 K
Meteoritic impact may cause potential natural formation of the new Al2SiO5 phases
Abstract
Iron oxides are among the major constituents of the deep Earth’s interior. Among them, the epsilon phase of Fe
2
O
3
is one of the less studied polymorphs and there is a lack of information ...about its structural, electronic and magnetic transformations at extreme conditions. Here we report the precise determination of its equation of state and a deep analysis of the evolution of the polyhedral units under compression, thanks to the agreement between our experiments and
ab-initio
simulations. Our results indicate that this material, with remarkable magnetic properties, is stable at pressures up to 27 GPa. Above 27 GPa, a volume collapse has been observed and ascribed to a change of the local environment of the tetrahedrally coordinated iron towards an octahedral coordination, finding evidence for a different iron oxide polymorph.
Nickel is the second most abundant element in the Earth's core. However, the properties of Fe‐Ni alloys are still poorly constrained under planetary cores conditions, in particular concerning the ...effect of Ni on the melting curve of Fe. Here we show that Ni alloying up to 36 wt% does not affect the melting curve of Fe up to 100 GPa. However, Ni strongly modifies the hexagonal‐closed‐packed/face‐centered‐cubic (hcp/fcc) phase boundary, pushing the hcp/fcc/liquid triple point of Fe‐20wt%Ni to higher pressures and temperatures. Our results allow constraining the triple point for Fe‐10wt%Ni, a composition relevant for the Earth interior, and point out a decrease of the melting temperature at core‐mantle boundary by 400 K with respect to pure Fe. A lower amount of light elements than previously predicted is thus required to reduce the crystallization temperature of core materials below that of a peridotitic lower mantle, in better agreement with geochemical observations.
Plain Language Summary
The Earth's core is believed to be composed of Fe alloyed with Ni and several lighter elements. In this paper, we investigate the effect of Ni alloying on the Fe phase diagram. The main effect of Ni addition is to enlarge the pressure/temperature stability domain of the face‐centered‐cubic (fcc) phase with respect to the hexagonal‐closed‐packed (hcp) phase and to shift the hcp/fcc/liquid triple point to higher pressures and temperatures. This implies a depression of the melting curve of Fe‐Ni alloys by around 400 K at mantle boundary conditions, at Ni concentrations pertinent for the Earth interior. This consequently decreases the temperature of the liquidus for Fe alloys constituting the Earth's core, in turn implying a reduced amount of light elements than previously predicted.
Key Points
Melting curve and phase diagram of Fe‐20wt%Ni and Fe‐36wt%Ni have been investigated by in situ X‐ray absorption up to 120 GPa and 3500 K
Ni alloying shifts the hcp/fcc/liquid triple point to higher pressures and temperatures
The triple point for Fe‐10wt%Ni is predicted to be around 135 GPa and 3800 K fixing new benchmarks for the Earth's core composition
Abstract
The discovery of superconductivity above 250 K at high pressure in LaH
10
and the prediction of overcoming the room temperature threshold for superconductivity in YH
10
urge for a better ...understanding of hydrogen interaction mechanisms with the heavy atom sublattice in metal hydrides under high pressure at the atomic scale. Here we use locally sensitive X-ray absorption fine structure spectroscopy (XAFS) to get insight into the nature of phase transitions and the rearrangements of local electronic and crystal structure in archetypal metal hydride YH
3
under pressure up to 180 GPa. The combination of the experimental methods allowed us to implement a multiscale length study of YH
3
: XAFS (short-range), Raman scattering (medium-range) and XRD (long-range). XANES data evidence a strong effect of hydrogen on the density of 4
d
yttrium states that increases with pressure and EXAFS data evidence a strong anharmonicity, manifested as yttrium atom vibrations in a double-well potential.
Solid krypton (Kr) undergoes a pressure-induced martensitic phase transition from a face-centered cubic (fcc) to a hexagonal close-packed (hcp) structure. These two phases coexist in a very wide ...pressure domain inducing important modifications of the bulk properties of the resulting mixed phase system. Here, we report a detailed in situ x-ray diffraction and absorption study of the influence of the fcc-hcp phase transition on the compression behavior of solid krypton in an extended pressure domain up to 140 GPa. The onset of the hcp-fcc transformation was observed in this study at around 2.7 GPa and the coexistence of these two phases up to 140 GPa, the maximum investigated pressure. The appearance of the hcp phase is also evidenced by the pressure-induced broadening and splitting of the first peak in the XANES spectra. We demonstrate that the transition is driven by a continuous nucleation and intergrowth of nanometric hcp stacking faults that evolve in the fcc phase. These hcp stacking faults are unaffected by high-temperature annealing, suggesting that plastic deformation is not at their origin. The apparent small Gibbs free-energy differences between the two structures that decrease upon compression may explain the nucleation of hcp stacking faults and the large coexistence domain of fcc and hcp krypton. We observe a clear anomaly in the equation of state of the fcc solid at ∼20 GPa when the proportion of the hcp form reaches ∼20%. We demonstrate that this anomaly is related to the difference in stiffness between the fcc and hcp phases and propose two distinct equation of states for the low and high-pressure regimes.
Water transportation to the deep lower mantle via plate subduction may induce a reaction between water and iron at the core‐mantle boundary. Recent experimental studies suggest that such a reaction ...may generate FeO2Hx‐rich domains, which can explain the seismic structures of the ultralow velocity zone in this region. In this study, the chemical reaction between metallic iron and a limited water supply at ~120 GPa was investigated using time‐resolved in situ synchrotron X‐ray diffraction measurements in combination with the laser‐heated diamond anvil cell technique. Contrary to the results of earlier studies, the formation of FeO instead of FeO2Hx without intermediate phases was observed. Considering the unlimited availability of iron in the core and the limited water supply resulting from mantle downflow, the FeO‐rich layers consisted of Fe‐bearing ferropericlase and postperovskite, which must have locally cumulated at the bottom of the mantle simultaneously with hydrogen incorporation into the core.
Plain Language Summary
Water strongly influences the structure, dynamics, and evolution of the deep Earth. Recent experimental studies suggest that hydrous phases play an important role as carriers of surface water to the deep mantle via the subduction of oceanic plates. Such deep‐water subduction processes may allow the surface water to reach the bottom of the mantle, where the mantle minerals are in direct contact with the iron at the core. Thus, the purpose of this study was to provide understanding regarding the behavior of water when it meets iron at the core‐mantle boundary. To investigate the reaction between water and iron at high pressures and temperatures, experiments were performed using in situ X‐ray diffraction measurements in combination with the diamond‐anvil cell technique. The results obtained confirmed the formation of FeO during the reaction. Thus, the deep water cycle may produce FeO‐rich layers at the core‐mantle boundary, which may explain the seismic characteristics of the bottom of the mantle and at the top of the outer core.
Key Points
The reaction between iron and a limited supply of water at ~120 GPa was studied via in situ X‐ray diffraction measurements
The deep water cycle may produce local FeO‐rich layers at the bottom of the mantle
Excess FeO contents may explain the seismic characteristics at the core‐mantle boundary
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