Tetrahedrally coordinated carbonates in Earth's lower mantle

Carbonates are the main species that bring carbon deep into our planet through subduction. They are an important rock-forming mineral group, fundamentally distinct from silicates in Earth's crust in that carbon binds to three oxygen atoms, while silicon is bonded to four oxygens. Here, we present experimental evidence that under the sufficiently high pressures and high temperatures existing in the lower mantle, ferromagnesian carbonates transform to a phase with tetrahedrally coordinated carbons. Above 80 GPa, in situ synchrotron infrared experiments show the unequivocal spectroscopic signature of the high-pressure phase of (Mg,Fe)CO$_3$. Using ab-initio calculations, we assign the new IR signature to C-O bands associated with tetrahedrally coordinated carbon with asymmetric C-O bonds. Tetrahedrally coordinated carbonates are expected to exhibit substantially different reactivity than low pressure three-fold coordinated carbonates, as well as different chemical properties in the liquid state. Hence this may have significant implications on carbon reservoirs and fluxes and the global geodynamic carbon cycle

Stabilité de phases carbonatées en conditions mantelliques : implications pour le cycle géodynamique du carbone

Carbon is recycled into the deep Earth mainly as carbonates, therefore stability of carbonates during the subduction in the deep mantle plays a key role in the geodynamic carbon cycle. The goal of this thesis is to provide new experimental information concerning constraints on the stability of carbonates at pressure and temperature conditions relevant to the Earth's lower mantle. In this experimental study, samples were synthesized at high-pressure and high-temperature in diamond anvil cells and analysed in situ by X-ray diffraction. Once quenched to room pressure (P) and temperature (T), samples were prepared using the focused ion beam (FIB) method for ex situ analyses: transmission electron microscopy and transmission X-ray microscopy. This study shows a high stability of carbonates versus decarbonatation and provides evidence of two new high-pressure polymorphs of carbonates: (1) a new high-pressure phase described in the case of a FeCO3 starting material. This phase is observed for pressure and temperature above 40 GPa-1500K (corresponding to depth within the Earth of about 1000 km). (2) A second phase described in the case of (Fe,Mg)CO3 solid solutions for P-T conditions above 80 GPa-2000 K, in agreement with previous theoretical studies. Ex situ analyses show that carbon in these two new high-pressure phases is present as CO4 tetrahedral groups and iron in its oxidized form Fe(III). The presence of Fe(II) in starting materials induces redox reactions from which Fe(II) is oxidized and a part of the carbon is reduced. This leads to an assemblage of magnetite, diamonds, and carbonates or their Fe(III) - bearing high-pressure polymorphs. Our results show the possibility for carbon to be recycled in the lowermost mantle and provide evidence of a possible coexistence of reduced and oxidized carbon at lower mantle conditions. This latter result might be important for better modelling redox state and melting in the Earth's lowermost mantle.Le carbone est recyclé dans le manteau terrestre majoritairement sous forme de carbonates. La stabilité des carbonates lors de la subduction dans le manteau profond joue donc un rôle majeur sur le cycle géodynamique du carbone. Ce travail de thèse apporte de nouvelles contraintes expérimentales sur la stabilité des carbonates pour des conditions de pression et température pertinentes pour le manteau inférieur terrestre. Pour ce faire, les échantillons ont été synthétisés à haute-pression et hautetempérature en cellule à enclumes en diamants et analysés de manière in situ par diffraction de rayons X. Une fois ramenés à pression et température ambiante, les échantillons ont été préparés par faisceau d'ions focalisés (focused ion beam-FIB) afin d'être analysés ex situ par microscopie électronique à transmission et microscopie de rayons X en transmission. Cette étude a permis de montrer la grande stabilité des phases carbonatées par rapport à la décarbonatation et de mettre en évidence deux nouvelles phases de haute-pression des carbonates : (1) une phase obtenue dans le cas d'une composition de départ FeCO3 dès 40 GPa-1500 K (équivalent à ~1000 km de profondeur). (2) une deuxième phase décrite dans une solution solide (Fe,Mg)CO3 pour des conditions supérieures à 80 GPa-2000 K en accord avec certaines prédictions théoriques. Dans ces deux nouvelles phases de haute-pression, les études ex situ montrent la présence de carbone sous la forme de groupements tétraédriques CO4 et le fer sous la forme oxydée Fe(III). La présence de fer dans la composition des carbonates induit, en effet, des réactions d'oxydoréduction dans lesquelles le fer est oxydé et le carbone partiellement réduit. Il en résulte alors un assemblage à haute-pression de magnétite, diamant et de carbonate ou leurs polymorphes de haute-pression. Nos résultats montrent donc la possibilité pour le carbone d'être recyclé jusqu'à la base du manteau et prouvent que la coexistence de carbone oxydé et carbone réduit dans les conditions du manteau inférieur est possible. Ce résultat est essentiel pour de futures modélisations de l'état d'oxydoréduction et de la fusion du manteau profond

High-Pressure Transformations and Stability of Ferromagnesite in the Earth's Mantle

International audienceFerromagnesite (Mg,Fe)CO 3 plays a key role in the transport and storage of carbon in the deep Earth. Experimental and theoretical studies demonstrated its high stability at high pressure and temperature against melting or decomposition. Several pressure-induced transformations of ferromagnesite have been reported at conditions corresponding to depths greater than ~1030 km in the Earth's lower mantle. Although there is still no consensus on their exact crystallographic structures, evidence is strong for a change in carbon environment from the low-pressure planar CO 3 2-ion into carbon atoms tetrahedrally coordinated by four oxygens. High-pressure iron-bearing phases concentrate a large amount of Fe 3+ as a result of intra-crystalline self-redox reactions. These crystallographic par-ticularities may have significant implications on carbon reservoirs and fluxes in the deep Earth

Synchrotron x-ray computed microtomography for high pressure science

X-ray computed microtomography (XCT) has been a very promising and exciting technique for high pressure (HP) science since the introduction of the first HP setups optimized for tomography in the mid-2000s. Different experimental stations are now available using diamond anvil cells (DACs) or large volume presses, with their own benefits and limitations: access to very high pressures but at room temperature on one hand, high temperature (HT) at moderate pressures on the other, and slow acquisitions being an undesired common point between all techniques. We believe that we are at a turning point where current and future developments boost the interest of the technique for the HP community. Time-resolved experiments, with less than 1 s per tomogram, will become routinely available. Fast tomography will greatly reduce the problem of motion artifacts at HT, allowing new topics to be explored. Computing and data treatment issues must be taken into account to effectively exploit the large volumes of data produced. Foreseeable developments will allow higher pressures to be reached in larger volume presses and higher T in DACs. Furthermore, improved XCT resolution in large samples (several hundreds of μm in diameter) recorded in situ will offer to be an effective alternative to ex situ microscopy

Stabilité de phases carbonatées en conditions mantelliques (implications pour le cycle géodynamique du carbone)

Le carbone(C) est recyclé dans le manteau terrestre majoritairement sous forme de carbonates. Leur stabilité lors de la subduction joue donc un rôle majeur sur le cycle géodynamique du C. Afin d apporter de nouvelles contraintes sur la stabilité des carbonates pour des conditions de pression (P) et température (T) pertinentes pour le manteau inférieur, des échantillons ont été synthétisés à haute-pression (HP) et haute-température (HT) en cellule à enclumes en diamants et analysés de manière in situ par diffraction de rayons X. Une fois ramenés à P et T ambiante, les échantillons ont été préparés par faisceau d ions focalisés (FIB) afin d être analysés ex situ par microscopie électronique à transmission et microscopie de rayons X en transmission. Cette étude a permis de montrer la grande stabilité des phases carbonatées par rapport à la décarbonatation et de mettre en évidence deux nouvelles phases de HP des carbonates : (1) une phase obtenue dans le cas d une composition de départ FeCO3 dès 40 GPa-1500 K. (2) une phase décrite dans une solution solide (Fe,Mg)CO3 pour des conditions supérieures à 80 GPa-2000 K. Dans ces deux nouvelles phases de HP, le carbone est présent sous la forme de groupements tétraédriques CO4 et le fer sous la forme oxydée Fe(III). La présence de fer induit des réactions d oxydoréduction dans lesquelles il est oxydé et le C partiellement réduit. Il en résulte alors un assemblage de magnétite, diamant et de carbonate ou leurs polymorphes de HP. Nos résultats montrent donc la possibilité pour le C d être recyclé jusqu à la base du manteau et prouvent que la coexistence de C oxydé et C réduit dans les conditions du manteau inférieur est possible.Carbon is recycled into the deep Earth mainly as carbonates, therefore stability of carbonates during the subduction in the mantle plays a key role in the geodynamic carbon(C) cycle. The goal of this thesis is to provide new experimental information concerning constraints on the stability of carbonates at pressure(P) and temperature (T) conditions relevant to the Earth s lower mantle. In this experimental study, samples were synthesized at high-pressure (HP) and high-temperature (HT) in diamond anvil cells and analysed in situ by X-ray diffraction. Once quenched to room P and T, samples were prepared using the focused ion beam (FIB) method for ex situ analyses: transmission electron microscopy and transmission X-ray microscopy. This study shows a high stability of carbonates versus decarbonatation and provides evidence of two new HP polymorphs of carbonates: (1) a new phase described in the case of a FeCO3 starting material observed for P and T above 40 GPa-1500K. (2) A second phase described in the case of (Fe,Mg)CO3 solid solutions for P-T conditions above 80 GPa-2000 K, in agreement with previous theoretical studies. Ex situ analyses show that C in these two new high-pressure phases is present as CO4 tetrahedral groups and iron in its oxidized form Fe(III). The presence of Fe(II) in starting materials induces redox reactions from which Fe(II) is oxidized and a part of the C is reduced. This leads to an assemblage of magnetite, diamonds, and carbonates or their Fe(III) - bearing HP polymorphs. Our results show the possibility for C to be recycled in the lowermost mantle and provide evidence of a possible coexistence of reduced and oxidized C at lower mantle conditions.PARIS-BIUSJ-Sci.Terre recherche (751052114) / SudocSudocFranceF

Transformations and Decomposition of MnCO3 at Earth's Lower Mantle Conditions

Carbonates have been proposed as the principal oxidized carbon-bearing phases in the Earth’s interior. Their phase diagram for the high pressure and temperature conditions of the mantle can provide crucial constraints on the deep carbon cycle. We investigated the behavior of MnCO3 at pressures up to 75 GPa and temperatures up to 2200 K. The phase assemblage in the resulting run products was determined in situ by X-ray diffraction (XRD), and the recovered samples were studied by analytical transmission electron microscopy (TEM) and X-ray absorption near edge structure (XANES) imaging. At moderate temperatures below 1400 K and pressures above 50 GPa, MnCO3 transformed into the MnCO3-II phase, with XANES data indicating no change in the manganese oxidation state in MnCO3-II. However, upon heating above 1400 K at the same pressure conditions, both MnCO3 and MnCO3-II undergo decomposition and redox reactions which lead to the formation of manganese oxides and reduced carbon

Pressure-induced phase transition in MnCO3and its implications on the deep carbon cycle

International audienceThe high-pressure behavior of manganese-rich carbonate, rhodochrosite, has been characterizedup to 62GPa by synchrotron-based midinfrared spectroscopy and X-ray diffraction. Modifications in boththe infrared spectra and the X-ray diffraction patterns were observed above ~35GPa, indicating the presenceof a high-pressure phase transition at these pressures. We found that rhodochrosite adopts a structure closeto CaCO3-VI with a triclinic unit cell (a=2.87Å, b = 4.83 Å, c=5.49Å, α = 99.86°, β = 94.95°, and γ=90.95° at62 GPa). Using first-principles calculations based on density functional theory, we confirmed these observationsand assigned modes in the new infrared signature of the high-pressure phase. These results suggest thathigh-pressure metastable phase of calcite may play an important role in carbon storage and transport in thedeep Earth

Following the phase transitions of iron in 3D with X-ray tomography and diffraction under extreme conditions

International audienc

Quantitative 4D X-ray microtomography under extreme conditions: a case study on magma migration

International audienceX-ray computed tomography (XCT) is a well known method for three-dimensional characterization of materials that is established as a powerful tool in high-pressure/high-temperature research. The optimization of synchrotron beamlines and the development of fast high-efficiency detectors now allow the addition of a temporal dimension to tomography studies under extreme conditions. Presented here is the experimental setup developed on the PSICHE beamline at SOLEIL to perform high-speed XCT in the Ultra-fast Tomography Paris–Edinburgh cell (UToPEc). The UToPEc is a compact panoramic (165° angular aperture) press optimized for fast tomography that can access 10 GPa and 1700°C. It is installed on a high-speed rotation stage (up to 360° s −1 ) and allows the acquisition of a full computed tomography (CT) image with micrometre spatial resolution within a second. This marks a major technical breakthrough for time-lapse XCT and the real-time visualization of evolving dynamic systems. In this paper, a practical step-by-step guide to the use of the technique is provided, from the collection of CT images and their reconstruction to performing quantitative analysis, while accounting for the constraints imposed by high-pressure and high-temperature experimentation. The tomographic series allows the tracking of key topological parameters such as phase fractions from 3D volumetric data, and also the evolution of morphological properties ( e.g. volume, flatness, dip) of each selected entity. The potential of this 4D tomography is illustrated by percolation experiments of carbonate melts within solid silicates, relevant for magma transfers in the Earth's mantle

Liquid properties in the Fe-FeS system under moderate pressure: Tool box to model small planetary cores

International audienc