Search

International audience; Xenoliths enclosed in Lavas of the Nyos volcano (Cameroon Volcanic Line, continental sector) range from fertile lherzolites to harzburgites. One spi- nel-free wehrlite has been also sampled. The occurrence of phlogopites and pargasites in some harzburgites together with specific textural rock-type (lher- zolites transitional porphyroclastic to equigranular), including major and trace element compositions both in peridotites bulk rocks and minerals point out interactions between the mantle and basaltic magmas responsible for the for- mation of wehrlites beneath the Nyos volcano. Hydrous minerals (phlogo- pites and pargasites) and metasomatic events are their main petrogeochemi- cal signatures different from group 1 samples which are characterized by spoon-shaped REE patterns. Later on, hydrous phases, Ti-rich Cpx, CaO rich Ol, Ti, and V rich Ol wehrlite precipitated from melt enrichments due to the percolation of the mantle by basaltic magmas of alkaline affinity. The meta- somatic liquid which percolates the Nyos mantle column was a dense alkaline silicate rich in volatile, displaying low HFSE abundances in the metasomatic hydrous melts compared to the LILE. It is suggested that Nyos mantle perido- tites have experienced: 1) variable metasomatic events related to the perco- lating of the depleted mantle by a alkaline silicate liquid, 2) the spinel-free wehrlite is a group 2 sample corresponding to a cumulate of a similar melt, 3) amphibole may be a potassium-bearing mineral instead of or in addition to phlogopite at shallower levels of Nyos upper mantle and 4) transitional tex- tural rock facies express also the fingerprint of rising mantle plume which were percolated by alkaline magma during their transit to the surface.

We report the synthesis, at 7 GPa and 923 K, and the thermoelastic characterization, up to 16 GPa and 850 K, of a single crystal of Mg-sursassite, Mg5Al5Si6O21(OH)7. In situ high-pressure and high-temperature single-crystal diffraction allowed the study of structural variation at non-ambient conditions and the determination of bulk elastic properties. The refined parameters of a second-order Birch-Murnaghan equation of state (BM-II EoS) are V0 = 446.02(1) Å3 and KT0 = 135.6(7) GPa. The thermal expansion coefficients of a Berman-type EoS are α0 = 3.14 (5) × 10−5 K−1, α1 = 2.50(16) × 10−8 K−2, and V0 = 445.94(3). For comparison, the P-V EoS is determined for a natural sursassite sample, ideally Mn4Al6Si6O22(OH)6. The refined parameters of BM-II EoS [V0 = 470.2(3) Å3, KT0 = 128(4) GPa] indicate that composition has a minimal effect on elastic properties. The similarity of density and bulk properties of Mg-sursassite if compared to olivine and other anhydrous mantle minerals suggests that this phase could be overseen by geophysical methods.

Hydrous minerals within the subducting oceanic slab are important hosts for water. Clarification of the stability field of hydrous minerals helps to understand transport and distribution of water from the surface to the Earth’s interior. We investigated the stability of brucite, a prototype of hydrous minerals, by means of electrical conductivity measurements in both open and closed systems at 3 GPa and temperatures up to 1300 K. Dramatic increase of conductivity in association with characteristic impedance spectra suggests that partial dehydration of single-crystal brucite in the open system with a low water fugacity occurs at 950 K, which is about 300 K lower than those previously defined by phase equilibrium experiments in the closed system. By contrast, brucite completely dehydrates at 1300 K in the closed system, consistent with previous studies. Partial dehydration may generate a highly defective structure but does not lead to the breakdown of brucite to periclase and water immediately. Water activity plays a key role in the stability of hydrous minerals. Low water activity (aH2O) caused by the high wetting behavior of the subducted oceanic slab at the transition zone depth may cause the partial dehydration of the dense hydrous magnesium silicates (DHMSs), which significantly reduces the temperature stability of DHMS (this mechanism has been confirmed by previous study on super hydrous phase B). As a result, the transition zone may serve as a ‘dead zone’ for DHMSs, and most water will be stored in wadsleyite and ringwoodite in the transition zone.

Quasielastic neutron scattering is applied to analyse the atomic scale hydrogen transport processes occurring within the crystal lattices of hydrous minerals. Two types of transport processes were observed in Mg(OH)2, which has a prototypical layered hydrogen lattice structure., It is demonstrated that quasielastic neutron scattering is a novel and effective method to analyse atomic scale hydrogen transport processes occurring within a mineral crystal lattice. The method was previously characterized as sensitive for analysing the transport frequency and distance of highly diffusive hydrogen atoms or water molecules in condensed matter. Here are shown the results of its application to analyse the transport of much slower hydrogen atoms which are bonded into a crystal lattice as hydroxyls. Two types of hydrogen transport process were observed in brucite, Mg(OH)2: a jump within a single two-dimensional layer of the hydrogen lattice and a jump into the next nearest layer of it. These transport processes observed within the prototypical structure of brucite have direct implications for hydrogen transport phenomena occurring within various types of oxides and minerals having layered structures.

Section III-A is based upon a paper published in the journal Terra Nova in August 2019: Dusséaux C., Gébelin A., Boulvais P., Gardien V., Grimes S., Mulch A. (2019) Meteoric fluid-rock interaction in Variscan shear zones. Terra Nova, vol. 31, issue 4, p. 366-372. DOI: 10.1111/ter.12392. This thesis presents the first stable isotope quantification of paleoaltimetry for the Variscan Belt of Western Europe that integrates late-Carboniferous hydrogen isotope ratios of meteoric water in the internal zones in the Armorican Massif (AM) and the French Massif Central (FMC) with age-equivalent precipitation records in the foreland Bourbon l’Archambault basin (BA) and the Montagne Noire (MN). Combined microstructural, hydrogen and oxygen stable isotope, thermometry and geochronology data allow the calculation of the isotopic composition of meteoric water in the internal zones of the orogen where surface-derived fluids infiltrated the ductile segment of the Quiberon, Piriac (AM) and Felletin (FMC) detachment shear zones during high temperature deformation and post-orogenic extension. When compared to age-equivalent isotopic composition of surface-derived fluids based on freshwater shark remains found in the BA basin near sea level, the composition of meteoric water obtained from synkinematic muscovite in the detachment footwalls are consistent with a minimum elevation difference of 2500 ± 900 m in the AM and 3300 ± 1000 m for the FMC. This study suggests that the internal zones (AM and FMC) of the Variscan Belt of Western Europe were characterized by a mean elevation typical for medium-sized mountain belts. These results are in good agreement with the view of the Variscan belt as a hot orogen characterized by abundant syntectonic crustal melting and high-grade metamorphism that profoundly affected its tectonic evolution.

&delta, AlOOH has been shown to be stable at the pressure&ndash, temperature conditions of the lower mantle. However, its stability remains uncertain at the conditions expected for the lowermost mantle where temperature is expected to rise quickly with increasing depth. Our laser-heated diamond-anvil cell experiments show that &delta, AlOOH undergoes dehydration at &sim, 2000 K above 90 GPa. This dehydration temperature is lower than geotherm temperatures expected at the bottom &sim, 700 km of the mantle and suggests that &delta, AlOOH in warm slabs would dehydrate in this region. Our experiments also show that the released H 2 O from dehydration of &delta, AlOOH can react with metallic iron, forming iron oxide, iron hydroxide, and possibly iron hydride. Our observations suggest that H 2 O from the dehydration of subducting slabs, if it occurs, could alter the chemical composition of the surrounding mantle and core regions.

Near-Infrared spectroscopy (NIR) is a useful tool for direct and on-site identification of rock mineralogy in spite of the difficulties arising in spectral evaluation, due to limited availability of spectral libraries at the time. Especially in the field, a functional methodology for the identification and evaluation if possible, of the geologic materials, is of interest to many researchers. However, several different parameters (such as grain size, color, mineralogy, texture, water content etc.) can affect the spectroscopic properties of the samples resulting in spectral variability. The subject of the present work focuses in various lithotypes (monzodiorite, diorite, altered diorite, actinolite schist, cataclasite, slate) from Kos Island, Aegean Sea, in Greece, all bearing hydrous minerals in various amounts. The evaluation of the results obtained from NIR spectroscopy offered important qualitative information about the mineralogy of the lithotypes examined. The important asset of the method is that no sample preparation was necessary. From the reflectance spectra, the NIR-active minerals that were identified include chlorite, micas, amphiboles and epidotes. Petrographic and mineralogic analyses were also employed in order to confirm the NIR results and provide more detailed information about the mineralogy of the samples, the grain size and the orientation of the minerals. Correlation of wavelength positions at ~1400 nm with loss on ignition (LOI) values led us to relate the various lithotypes in terms of their petrological affinities. NIR spectroscopy was proved to be a useful tool, especially for the mineralogic identification of rocks underwent low- to medium grade metamorphism, from greenschist to amphibolite facies.

The advent of continuous-flow isotope-ratio mass spectrometry (CF-IRMS) coupled with a high temperature conversion (HTC) system enabled faster, more cost effective, and more precise δ2H analysis of hydrogen-bearing solids. Accurate hydrogen isotopic analysis by on-line or off-line techniques requires appropriate isotopic reference materials (RMs). A strategy of two-point calibrations spanning δ2H range of the unknowns using two RMs is recommended. Unfortunately, the supply of the previously widely used isotopic RM, NBS 30 biotite, is exhausted. In addition, recent measurements have shown that the determination of δ2H values of NBS 30 biotite on the VSMOW-SLAP isotope-delta scale by on-line HTC systems with CF-IRMS may be unreliable because hydrogen in this biotite may not be converted quantitatively to molecular hydrogen. The δ2HVSMOW-SLAP values of NBS 30 biotite analyzed by on-line HTC systems can be as much as 21 mUr (or ‰) too positive compared to the accepted value of − 65.7 mUr, determined by only a few conventional off-line measurements. To ensure accurate and traceable on-line hydrogen isotope-ratio determinations in mineral samples, we here propose two isotopically homogeneous, hydrous mineral RMs with well-characterized isotope-ratio values, which are urgently needed. The U.S. Geological Survey (USGS) has prepared two such RMs, USGS57 biotite and USGS58 muscovite. The δ2H values were determined by both glassy carbon-based on-line conversion and chromium-based on-line conversion, and results were confirmed by off-line conversion. The quantitative conversion of hydrogen from the two RMs using the on-line HTC method was carefully evaluated in this study. The isotopic compositions of these new RMs with 1-σ uncertainties and mass fractions of hydrogen are: USGS57 (biotite) δ2HVSMOW-SLAP = − 91.5 ± 2.4 mUr (n = 24) Mass fraction hydrogen = 0.416 ± 0.002% (n = 4) Mass fraction water = 3.74 ± 0.02% (n = 4) USGS58 (muscovite) δ2HVSMOW-SLAP = − 28.4 ± 1.6 mUr (n = 24) Mass fraction hydrogen = 0.448 ± 0.002% (n = 4) Mass fraction water = 4.03 ± 0.02% (n = 4). These δ2HVSMOW-SLAP values encompass typical ranges for solid unknowns of crustal and mantle origin and are available to users for recommended two-point calibration.

International audience; We conducted ab initio molecular dynamics simulations of the d phase of the hydrous mineral aluminium oxide hydroxide (AlOOH) at ambient temperature and high pressure. Nuclear quantum effects were included through a Langevin dynamics in a bath of quantum harmonic oscillators. We confirm that under increasing pressure d-AlOOH undergoes a phase transition from a P2 1 nm structure with asymmetric and disordered O-H bonds to a stiffer Pnnm phase with symmetric hydrogen bonds, which should be stable within the pressure and temperature ranges typical for the Earth's mantle. The transition is initially triggered by proton tunneling, which makes the mean proton position to coincide with the midpoint of the O-O distance, at pressures as low as 10 GPa. However, only at much larger pressures, around 30 GPa as previously found by other calculations, the Pnnm phase with symmetric hydrogen bonds is stable from the classical point of view. The transition is also characterized through the analysis of the H-O stretching modes, which soften considerably and fade out around 10 GPa in the P2 1 nm structure, when thermal and nuclear quantum effects are taken into account in the simulations. At variance, the harmonic picture is not adequate to describe the highly anharmonic effective potential that is seen by the protons at the transition. Finally, we propose that the picture of a dynamical transition to the high-symmetry and proton-centered Pnnm phase, which is brought about by the onset of proton tunneling, could be confirmed by quasi-elastic neutron scattering and vibrational spectroscopy under pressure.

The newly described hydrous phase H, MgSiO4H2, is stable to the pressures and temperatures of the Earth's lower mantle, suggesting that the Earth's lower mantle may participate in the cycling of water. We present the results of ab initiocalculations on the stability of this phase within the phase H-delta AlOOH-SiO(2)system between 20 and 60GPa, exploring the wide pressure-and temperature stability field of this mineral. We find that hydrogen in phase H is bonded in a single well at all pressures, and only the pure phase results in pressure-induced symmetrization, while the solid solution maintains asymmetric bonding to high pressure. Substitutions on octahedral sites are locally charge-balanced by H. Furthermore, small amounts of Al + H in SiO(2)stabilize the CaCl2-structure of SiO(2)to pressures as low as 20GPa while phase H and delta AlOOH form an ideal mixture at all pressures considered. The resulting structure is stable relative to the anhydrous assemblage of the Earth's lower mantle by 16-19kJ/mol. In alumina-rich phase H this increases the stability of the mineral by similar to 800K relative to the pure substance, making it potentially stable under lower mantle temperatures along typical geotherms. (C) 2017 Elsevier B. V. All rights reserved.

The seismology is the most effective method to explore the Structure of subduction zones to great depth. The distinguishing feature of the mantle in the subduction regions is the presence of hydrated phases, which transport water into the Earth's interior and release it with dramatic local consequences, triggering earthquakes and melting. The seismological detection of these hydrous phases and geo-dynamic interpretation of flow in the hydrous mantle depend on knowledge of the anisotropic elastic properties and the characteristics of the wave propagation in anisotropic media. In this paper we briefly recall the distinguishing features of anisotropic wave propagation and the observable parameters. We suggest the Vp/Vs ratio is a physically sound parameter than can be observed by seismology in anisotropic regions of the Earth, whereas the Poisson's ratio, which is often quoted, is not directly observable and does not correspond to the characteristics of wave propagation in an anisotropic or isotropic medium. We report for the first time the ratios of Vp/Vs1 and Vp/Vs2, where Vs1 is the fastest and Vs2 slowest S-wave velocities of an anisotropic media. We present the current knowledge of the anisotropic seismic properties of hydrous minerals in the upper mantle, transition zone, and lower mantle that are stable along low temperature geotherms associated with subduction, and identify which minerals are likely to influence seismological observations because they have very high volume fractions, or very high anisotropies, or both of these.;In the upper mantle antigorite and talc are exceptionally anisotropic (Vp 71% Vs 68% and Vp 65%, Vs 68%, respectively) and chlorite is also very anisotropic for S-waves (Vp 35%, Vs 76%). Comparatively less anisotropic are hornblende (Vp 27%, Vs 31%), used as proxy for tremolite in subduction zones, and clinohumite (21.8%, Vs 15.9%). Brucite at 4 GPa (Vp 26.5%, Vs 30.9%) and the dense hydrogen magnesium silicate (DHMS) phase A at 9GPa (Vp 9.3%, Vs 17.6%) are the only hydrous minerals stable in the upper mantle that have had their elastic properties measured at in situ mantle pressures. Except for the phase A, all these minerals are more anisotropic than olivine for at least one parameter (Vp, Vs, Vp/Vs1, or Vp/Vs2). In the transition zone the major phases hydrous wadsleyite and ringwoodite C have moderate to weak anisotropies (Vp 16.3%, Vs 16.5%, and Vp 1.9%, Vs 4.4%, respectively). The DHMS Superhydrous B has a moderate anisotropy (Vp 6.9%, Vs 11.6%). At greater depth the DHMS phase D Lit 24 GPa (Vp 10.8%, Vs 18.0%) is the only hydrous phase that can transport hydrogen from the transition zone into the lower mantle. The phase D is more anisotropic for S than P waves like many of the hydrous phases. From these data it is clear that hydrous phases are in general very anisotropic. However, pressure can play a strong role in reducing anisotropy. It is the case for brucite and talc, in which increasing pressure from ambient to 4 GPa reduces the anisotropy by about 50% for both P and S waves. In contrast, the anisotropy of the DHMS phase A does not change significantly with pressure.;Our picture of the seismic anisotropy of hydrated minerals remains incomplete; the elastic properties of many have not been measured even at ambient conditions (e.g., 10 angstrom phase and phase E) or not measured in their true elastic symmetry (e.g., clinochlore). The majority of hydrous minerals have not been measured at high pressure. and none related to hydrated mantle have been measured at elevated temperature. We have shown in the few cases where hydrous minerals have been measured as a function of pressure, that this variable has an important effect oil the velocity distribution and in most cases reduces the degree of anisotropy, hence we Would expect seismic anisotropy to play a key role in the determination of the shallow structure of subduction zones in the upper mantle.

In this work, we applied electronic structure calculations based on density functional theory to compute oxygen, hydrogen and silicon fractionation properties between several materials of interest in Earth Sciences. Understanding of such mechanisms is a main objective in geochemistry, since the isotopic content of minerals is a witness of geochemical events. Electronic structure calculations enables the determination of the complete vibrational properties of one material at the harmonic level, from which its fractionation properties can be deduced. Our goal is to validate an ab initio approach, i.e. free from any parameter coming from experiment, enaling the prediction of the fractionation between two minerals. We studied the materials: quartz, gas water, ice, kaolinite, lizardite, enstatite, forsterite, brucite and gibbsite. Some of them have been the subject of precise experimental studies of their various physical properties, which permits us to validate our approach, and all are of main interest in Earth Sciences, which permits us to propose new fractionation laws as a function of temperature, and to check the validity of laws existing in the litterature. This is the first time that such a systematic study, based on ab initio approaches, is realized on so many different systems. That is why we realized the possibly most complete study of the possible sources of error in our calculation. Moreover, this approach enables detailed analysis of the physical processes driving isotopic fractionation.; Dans ce travail, nous avons appliqué les méthodes de calcul électronique basés sur la théorie de la fonctionnelle de la densité pour calculer les propriétés de partage isotopique à l'équilibre de l'hydrogène, de l'oxygène et du silicium entre divers matériaux d'intérêt en sciences de la Terre. La compréhension de ces mécanismes de partage est un enjeu majeur en géochimie, car le contenu isotopique des minéraux constitue un témoin des événements géochimiques. Le calcul électronique permet de déterminer les propriétés vibrationnelles complètes d'un matériau au niveau harmonique,desquelles on déduit ses propriétés de partage. Notre but est de valider une approche ab initio, i.e. libre de tout paramètre issu de l'expérience, permettant la prédiction du partage entre deux matériaux. On s'est intéressé aux matériaux: quartz, vapeur d'eau, glace, kaolinite, lizardite, enstatite, forstérite, brucite et gibbsite. Certains ont fait l'objet d'études expérimentales précises de leurs différentes propriétés physiques, ce qui nous permet de valider notre approche, et tous sont de grand intérêt en sciences de la Terre, ce qui nous permet de proposer de nouvelles lois de partage en fonction de la température, et de vérifier la validité de celles déjà proposées. C'est la première fois qu'une telle étude systématique est réalisée à partir d'une approche ab initio, et sur autant de systèmes différents. On a donc procédé à une étude la plus complète possible des sources d'erreur susceptibles d'influencer le résultat final. Par ailleurs, cette approche nous permet d'analyser de façon plus détaillée les processus physiques à l'origine du partage isotopique dans ces différents matériaux.

In this work, we applied electronic structure calculations based on density functional theory to compute oxygen, hydrogen and silicon fractionation properties between several materials of interest in Earth Sciences. Understanding of such mechanisms is a main objective in geochemistry, since the isotopic content of minerals is a witness of geochemical events. Electronic structure calculations enables the determination of the complete vibrational properties of one material at the harmonic level, from which its fractionation properties can be deduced. Our goal is to validate an ab initio approach, i.e. free from any parameter coming from experiment, enaling the prediction of the fractionation between two minerals. We studied the materials: quartz, gas water, ice, kaolinite, lizardite, enstatite, forsterite, brucite and gibbsite. Some of them have been the subject of precise experimental studies of their various physical properties, which permits us to validate our approach, and all are of main interest in Earth Sciences, which permits us to propose new fractionation laws as a function of temperature, and to check the validity of laws existing in the litterature. This is the first time that such a systematic study, based on ab initio approaches, is realized on so many different systems. That is why we realized the possibly most complete study of the possible sources of error in our calculation. Moreover, this approach enables detailed analysis of the physical processes driving isotopic fractionation.; Dans ce travail, nous avons appliqué les méthodes de calcul électronique basés sur la théorie de la fonctionnelle de la densité pour calculer les propriétés de partage isotopique à l'équilibre de l'hydrogène, de l'oxygène et du silicium entre divers matériaux d'intérêt en sciences de la Terre. La compréhension de ces mécanismes de partage est un enjeu majeur en géochimie, car le contenu isotopique des minéraux constitue un témoin des événements géochimiques. Le calcul électronique permet de déterminer les propriétés vibrationnelles complètes d'un matériau au niveau harmonique,desquelles on déduit ses propriétés de partage. Notre but est de valider une approche ab initio, i.e. libre de tout paramètre issu de l'expérience, permettant la prédiction du partage entre deux matériaux. On s'est intéressé aux matériaux: quartz, vapeur d'eau, glace, kaolinite, lizardite, enstatite, forstérite, brucite et gibbsite. Certains ont fait l'objet d'études expérimentales précises de leurs différentes propriétés physiques, ce qui nous permet de valider notre approche, et tous sont de grand intérêt en sciences de la Terre, ce qui nous permet de proposer de nouvelles lois de partage en fonction de la température, et de vérifier la validité de celles déjà proposées. C'est la première fois qu'une telle étude systématique est réalisée à partir d'une approche ab initio, et sur autant de systèmes différents. On a donc procédé à une étude la plus complète possible des sources d'erreur susceptibles d'influencer le résultat final. Par ailleurs, cette approche nous permet d'analyser de façon plus détaillée les processus physiques à l'origine du partage isotopique dans ces différents matériaux.