Search

co-auteur étranger; International audience; 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.

International audience; The martensitic transformation is a fundamental physical phenomenon at the origin of important industrial applications. However, the underlying microscopic mechanism, which is of critical importance to explain the outstanding mechanical properties of martensitic materials, is still not fully understood. This is because for most martensitic materials the transformation is a fast process that makes in situ studies extremely challenging. Noble solids krypton and xenon undergo a progressive pressure-induced face-centered cubic (fcc) to hexagonal close-packed (hcp) martensitic transition with a very wide coexistence domain. Here, we took advantage of this unique feature to study the detailed transformation progress at the atomic level by employing in situ x-ray diffraction and absorption spectroscopy. We evidenced a four-stage pathway and suggest that the lattice mismatch between the fcc and hcp forms plays a key role in the generation of strain. We also determined precisely the effect of the transformation on the compression behavior of these materials.

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 ![Formula][1] 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 ![Formula][2] 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 ![Formula][3] is comprised between IW-1 and IW, this ratio is correlated with both Al content and oxygen fugacity. When ![Formula][4] remains between IW and Re/ReO2 buffers, Fe3+/ΣFe ratio becomes independent of ![Formula][5] 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. [1]: /embed/mml-math-1.gif [2]: /embed/mml-math-2.gif [3]: /embed/mml-math-3.gif [4]: /embed/mml-math-4.gif [5]: /embed/mml-math-5.gif

The high ability of the Al-rich CaSiO 3 perovskite to contain large amounts of uranium (up to 4 at.% U) has been studied up to 54 GPa and 2400 K, using laser-heated diamond anvil cell (LH-DAC) and up to 18 GPa and 2200 K using a multi-anvil press (MAP). Both latter HP-HT techniques proved to be complementary and gave similar results, in spite of different heating modes (laser and furnace). Chemical reactions were characterized and described by electron probe microanalysis and analytical scanning electron microscopy while associated structural changes were precisely characterized by synchrotron angle dispersive X-ray diffraction and by X-ray micro-diffraction. The diffusion of uranium into the CaSiO 3 matrix was measured as a function of run duration and temperature. We obtain diffusion coefficients with the same order of magnitude (about 10 −16 m 2 s −1 ) than for those found in the literature. After this work, coupled cationic substitutions of Ca by U and Si by Al are proposed to generate new interesting crystallographic features for a CaSiO 3 perovskite: a higher compressibility, a tetragonal distortion along the c -axis with c / a ratio >1, a different compression behaviour of c -axis relative to a -axis, and a perovskite structure quenchable to ambient P and T conditions. The tetragonal U-bearing aluminous CaSiO 3 perovskite is observed to remain stable at pressures up to 54 GPa, then in the ( P , T ) range of the upper part of the lower mantle. The influence of the present results, in terms of both uranium and aluminium partitioning related to the coexisting mineral phases as the (Mg,Fe)SiO 3 perovskite, is discussed. Uranium provides approximately 25% of the total energy generated within the deep Earth through its radioactive decay. The location of this source within the deep mantle is fundamental to the understanding of the geodynamics and thermal behaviour of our planet. Since the tetragonal structure of the U-bearing Al-rich CaSiO 3 perovskite is expected to remain stable towards the base of the Earth's mantle, this latter phase is proposed to be the main storage mineral for heat producing actinides of the lower mantle.

We used configurational entropy theory to model the viscosity (η) of hydrous melts of NaAlSi 3 O 8 , haplogranite (SiO 2 -KAlSi 3 O 8 -NaAlSi 3 O 8 ), and complex (natural) granite composition from available measurements and recently published configurational heat-capacity data. The equation log η = A e + B e / TS conf ( T ), where S conf is configurational entropy, reproduces viscosity data for individual samples as well as or better than the empirical three-parameter TVF equation (defined below), and has the advantage of being based on thermodynamic theory. The variables A e , B e , and S conf ( T g ), where T g is glass transition temperature, were parameterized as a function of water content for compilations of viscosity data for hydrous NaAlSi 3 O 8 , haplogranite, and peraluminous granite melts. With the simplest assumption of ideal mixing between silicate and water components, configurational entropy models with between 4 and 10 fitting parameters reproduce experimentally determined η- T - X H 2 O relationships significantly better than previous literature models based on empirical equations. Our preferred configurational entropy models have root-mean-square deviations of 0.26 log units for NaAlSi 3 O 8 ( n = 77), 0.16 log units for haplogranite ( n = 55), and 0.28 log units for peraluminous granites ( n = 79). The best statistical fits to the data sometimes require thermodynamically unlikely variations in A e , B e , and S conf ( T g ) as a function of water content, however, such that further calorimetry data are needed to extract accurate thermodynamic information from viscosity data sets for hydrous melts.

Understanding of the crystal chemistry of the Earth's deep mantle has evolved rapidly recently with the gradual acceptance of the importance of the effect of minor elements such as aluminium on the properties of major phases such as perovskite1,2,3. In the early Earth, during its formation and segregation into rocky mantle and iron-rich core, it is likely that silicate liquids played a large part in the transport of volatiles to or from the deep interior. The importance of aluminium on solubility mechanisms at high pressure has so far received little attention, even though aluminium has long been recognized as exerting strong control on liquid structures at ambient conditions4,5,6. Here we present constraints on the solubility of argon in aluminosilicate melt compositions up to 25 GPa and 3,000 K, using a laser-heated diamond-anvil cell. The argon contents reach a maximum that persists to pressures as high as 17 GPa (up to 500 km deep in an early magma ocean), well above that expected on the basis of Al-free melt experiments. A distinct drop in argon solubility observed over a narrow pressure range correlates well with the expected void loss in the melt structure predicted by recent molecular dynamics simulations7,8,9. These results provide a process for noble gas sequestration in the mantle at various depths in a cooling magma ocean. The concept of shallow partial melting as a unique process for extracting noble gases from the early Earth, thereby defining the initial atmospheric abundance, may therefore be oversimplified.

As measured in air above the glass transition range, the viscosity of an alkali basalt increases markedly with time by about two orders of magnitude in 12 h. This effect is essentially physical and due to the presence of microcrystals although partial crystallization of the melt into spinel and an SiO2-poor pyroxene leads to a considerable enrichment in silica of the residual liquid. Partial crystallization depends strongly on the initial redox state of samples in that the presence of ferrous iron is required for spinel crystals to form and for pyroxene to nucleate and grow around them. Other measurements show that the viscosity of the crystal-free liquid decreases slightly with increasing ratios r=Fe2+/∑Fe because the differences between samples with r=0.16 and 0.83 amount to about 1.5 and 0.3 log-units at 950 and 1400 K, respectively. Comparisons of the viscosities of the residual liquid matrix and of the initially crystal-free basalt show that physical effects caused by the presence of microcrystals begin to be observed at a low crystal fraction of 5 vol%. Finally, a model of viscosity calculation is developed for the melts which reproduces all data obtained in this work to better than 10%.

High-pressure and high-temperature experiments have been conducted with a laser-heated diamond-anvil cell (LHDAC) to determine the partition coefficients for Ni and Co up to 42 GPa and around 2500 K. Comparison of the present experimental data with those of multi-anvil devices shows a good agreement between the different exchange partitioning coefficients. The agreement suggests conditions in LHDAC experiments can reproduce those of multi-anvil experiments in the pressure range studied. Up to the maximum pressure reached in our work, Ni and Co become less siderophile with increasing pressure, as already observed in previous studies at lower pressures. Our data, combined with lower-pressure results, suggest a magma ocean would have extended to as much as 45 GPa (near 1200 km in depth) in order to obtain homogeneous equilibrium between core-forming metals and the silicate mantle in the early Earth.

The heat capacities of three sodium and potassium titanosilicate glasses and melts have been determined between room temperature and 1800 K. For all melts, the heat capacity decreases with increasing temperatures after a 30% to 53% rise at the glass transition. For both glasses and melts, the heat capacity is an additive function of composition, which indicates that the calorimetric anomalies are an intrinsic feature of the TiO 2 component that depends neither on the relative abundances of SiO 2 and Na 2 O or K 2 O, nor on the nature of the alkali cation. The viscosities measured for these and two other melts indicate that, as embodied by the Adam–Gibbs theory of relaxation processes, the considerable decrease of the melt configurational entropy results in a dramatic viscosity increase near the glass transition range. Consistent with the major influence of configurational entropy, structural relaxation in the glass transition range proceeds much more slowly for Ti-bearing than for Ti-free melts. These various features are attributed to important temperature-induced changes in short-range order around oxygen atoms, with strongly decreasing substitution of Ti for Si at lower temperatures constituting probably the main factor.

The heat capacities of a soda-lime silicate and four borosilicate glasses and liquids have been determined from drop-calorimetry measurements made between 450 and about 1700 K. Between 850 and 1000 K, the glass transition takes place when the glass specific heat, Cp, approaches the Dulong-Petit limit. At the glass transition, the heat capacity of liquids is from 17 to 31% higher than that of glasses, the increase being greater for melts with smaller SiO2 contents. The heat capacity of borosilicate liquids has a markedly non-linear composition dependence and shows either positive or negative temperature dependences. The extreme case is that of a melt with 5 wt% Na2O and 16 wt% B2O3, whose configurational heat capacity decreases by 50% between 800 and 1800 K. These anomalous variations do not correlate with temperature-induced coordination changes of boron above the glass transition and could indicate instead some mixing of SiO4 and BO4 units in the liquid whose rate would decrease progressively at higher temperatures.

A simple, rapid, and inexpensive heating‐wire technique is used for physical observations at high temperatures. The upper limit is 2000 K in air with platinum‐iridium or platinum‐rhodium wires and 2700 K with iridium; temperatures up to 3600 K can be achieved under an inert atmosphere with tungsten wires. Raman spectroscopy measurements made up to 1900 K by this technique suggest that the high‐temperature harmonic vibrational behavior of corundum (α‐Al2O3) results from the cancellation of anharmonic effects. Powder x‐ray diffraction experiments with synchrotron radiation show that perovskite (CaTiO3) changes from orthorhombic symmetry to cubic between 1330 and 1530 K, with an intermediate tetragonal phase likely, consistent with λ‐type transitions recorded by recent calorimetric measurements. Finally, observations of CaAl2Si2O8 polymorphism has shown the existence of a new metastable phase.

The present state of the Earth evolved from energetic events that were determined early in the history of the Solar System. A key process in reconciling this state and the observable mantle composition with models of the original formation relies on understanding the planetary processing that has taken place over the past 4.5 Ga. Planetary size plays a key role and ultimately determines the pressure and temperature conditions at which the materials of the early solar nebular segregated. We summarize recent developments with the laser-heated diamond anvil cell that have made possible extension of the conventional pressure limit for partitioning experiments as well as the study of volatile trace elements. In particular, we discuss liquid–liquid, metal–silicate (M–Sil) partitioning results for several elements in a synthetic chondritic mixture, spanning a wide range of atomic number—helium to iodine. We examine the role of the core as a possible host of both siderophile and trace elements and the implications that early segregation processes at deep magma ocean conditions have for current mantle signatures, both compositional and isotopic. The results provide some of the first experimental evidence that the core is the obvious replacement for the long-sought, deep mantle reservoir. If so, they also indicate the need to understand the detailed nature and scale of core–mantle exchange processes, from atomic to macroscopic, throughout the age of the Earth to the present day.

[1] The distribution of the radiogenic heat sources strongly influences the geodynamics and thermal behaviour of the Earth. About 11 TW is produced by the radioactive decay of uranium (25% of the total heat flux at Earth surface), and 55% of this energy comes from the lower mantle. Here we report the first experimental evidence that aluminous CaSiO 3 perovskite is the major, or even the only, host of uranium in the Earth lower mantle, since such a phase is able to incorporate up to 35 wt% UO 2 (or 4 at% of U). The aluminous Ca-perovskite could be the main U-bearing constituent of a dense and radiogenic reservoir proposed in a recent model and located in the bottom half of the lower mantle.

The effect of water on heat capacity has been determined for four series of hydrated synthetic aluminosilicate glasses and supercooled liquids close to albite, phonolite, trachyte, and leucogranite compositions. Heat capacities were measured at atmospheric pressure by differential scanning calorimetry for water contents between 0 and 4.9 wt % from 300 K to about 100 K above the glass transition temperature (Tg). The partial molar heat capacity of water in polymerized aluminosilicate glasses, which can be considered as independent of composition, is C p ¯ H 2 O = - 122.319 + 341.631 × 10 - 3 T + 63.4426 × 10 5 / T 2 (J/mol K). In liquids containing at least 1 wt % H2O, the partial molar heat capacity of water is about 85 J/mol K. From speciation data, the effects of water as hydroxyl groups and as molecular water have tentatively been estimated, with partial molar heat capacities of 153 ± 18 and 41 ± 14 J/mol K, respectively. In all cases, water strongly increases the configurational heat capacity at Tg and exerts a marked depressing effect on Tg, in close agreement with the results of viscosity experiments on the same series of glasses. Consistent with the Adam and Gibbs theory of relaxation processes, the departure of the viscosity of hydrous melts from Arrhenian variations correlates with the magnitude of configurational heat capacities.