F. Guyot

PHASE-CHANGES AND THERMODYNAMIC PROPERTIES OF CATIO3 - SPECTROSCOPIC DATA, VIBRATIONAL MODELING AND SOME INSIGHTS ON THE PROPERTIES OF MGSIO3 PEROVSKITE

The effect of pressure (up to 21 GPa at room temperature) and temperature (up to 1570 K at room pressure) on the Raman spectrum of CaTiO3 is presented. No significant changes, which could be attributed to a major structural change, are observed in the spectra up to 22 GPa. The pressure shifts of the Raman modes can be related to a significant compression of the Ti-O bond. Discontinuous changes in the spectra upon heating may be related to phase changes observed by calorimetry and X-ray diffraction. The important temperature shifts of some low-frequency modes can be related to an increase in the Ti-O-Ti angle in agreement with the X-ray data showing a decrease of the structural distortion with increasing temperature. These data are compared to those available for MgSiO3-perovskite and show that CaTiO3 is a good structural analogue for MgSiO3-perovskite. The present spectroscopic data are used to calculate the specific heat and entropy of CaTiO3. The role of the low frequency modes in the calculations is emphasized. Good agreement is observed between calculated and experimentally determined values in the 0–1300 K temperature range. A similarly defined model is proposed for MgSiO3-perovskite. It is found that the entropy lies between 57 and 64 J/mol/K at 298 K and between 190 and 200 J/mol/K at 1000 K in agreement with the values inferred from experimental equilibrium data. Finally we briefly discuss the values of the Grüneisen parameters of both perovskites inferred from macroscopic and microscopic data.

Experimental investigation of the stability of Fe-rich carbonates in the lower mantle

The fate of carbonates in the Earths mantle plays a key role in the geodynamical carbon cycle. Although iron is a major component of the Earths lower mantle, the stability of Fe-bearing carbonates has rarely been studied. Here we present experimental results on the stability of Fe-rich carbonates at pressures ranging from 40 to 105 GPa and temperatures of 1450-3600 K, corresponding to depths within the Earths lower mantle of about 1000-2400 km. Samples of iron oxides and iron-magnesium oxides were loaded into CO2 gas and laser heated in a diamond-anvil cell. The nature of crystalline run products was determined in situ by X-ray diffraction, and the recovered samples were studied by analytical transmission electron microscopy and scanning transmission X-ray microscopy. We show that Fe-(II) is systematically involved in redox reactions with CO2 yielding to Fe-(III)-bearing phases and diamonds. We also report a new Fe-(III)-bearing high-pressure phase resulting from the transformation of FeCO3 at pressures exceeding 40 GPa. The presence of both diamonds and an oxidized C-bearing phase suggests that oxidized and reduced forms of carbon might coexist in the deep mantle. Finally, the observed reactions potentially provide a new mechanism for diamond formation at great depth.

Electron energy-loss spectroscopy of silicate perovskite-magnesiowüstite high-pressure assemblages

Abstract Silicate perovskite-magnesiowüstite assemblages synthesized from natural olivine in the multianvil press and diamond-anvil cell were studied by electron energy-loss spectroscopy (EELS). Spectra of crystalline silicate perovskite, and its post-amorphization phase, as well as magnesiowüstite were collected at the Fe and Si L2,3 edge, and in the low loss (<50 eV) domain. The technique of line spectra ensuring very low beam doses allows good quality spectra to be collected from crystalline perovskite prior to amorphization and permits characterization of coexisting crystals of perovskite and magnesiowüstite. Spectra at the Si L2,3 edge show that the beam-induced amorphization of silicate perovskite is accompanied by a change from sixfold to fourfold oxygen coordination of silicon atoms. Spectra at the Fe L2,3 edge show that Fe2+ is the major form of Fe in olivine, ringwoodite, and magnesiowüstite, whereas Fe3+ is dominant in crystalline silicate perovskite and its amorphization products. In magnesiowüstite and silicate perovskite observed in contact in these samples, Fe3+ is strongly partitioned into the silicate phase. Careful experimental substraction of zero-loss peak by off Bragg acquisition of electron energyloss spectra allows good quality low loss spectra to be collected from crystalline silicate perovskite and magnesiowüstite. In magnesiowüstite, interband transitions are well characterized, leading to a measured gap of 7.8 eV, in agreement with previous theoretical calculations. Interband transitions at 10 eV and 12.5 eV are also well resolved in crystalline silicate perovskite, leading to a gap of about 9.5 eV.

Cristobalite inclusions in the Tatahouine achondrite: Implications for shock conditions

Abstract The mineralogy of the Tatahouine diogenite was investigated by optical microscopy, Raman micro-spectrometry, and scanning and transmission electron microscopies. Inclusions of α-cristobalite in orthopyroxenes, locally in symplectic association with chromites, or associated with metal, have been characterized for the first time in a diogenite. Mosaicism of the orthopyroxenes indicates shock effects in the meteorite. The shock history of the meteorite must be consistent with the presence of vein-like structures containing inclusions of well-crystallized cristobalite, a low-pressure, high-temperature phase. Several possible mechanisms to account for these observations are discussed. The simplest one, consistent with all observations, is that a shock event would have occurred in a hot orthopyroxenite, either before extensive cooling of the asteroid, or in materials heated by previous impacts and maintained hot under an ejecta blanket.

The Earth's Lower Mantle and Core

More than 90 percent of the Earths mass is composed of iron, oxygen, silicon and magnesium, distributed among a metal-rich core, a silicate-rich mantle and more highly fractionated crustal rocks (less than 1% of the total). Mantle and core compositions can be approximated quite easily provided the bulk-Earth composition is assumed to be the same as that of appropriate meteorites. Critical mineral-physics data, some of which are reviewed in this article, are then needed to develop viable compositional and thermal Earth models, thus leading to a better knowledge of the deepest rocks in the Earth.

An experimental study of the external reduction of olivine single crystals

Abstract Single crystals of San Carlos olivine in contact with graphite were annealed at P = 1 bar, T = 1373 K, for studying the reaction of extraction of (Fe, Ni) metal. Scanning electron microscopy and transmission electron microscopy were performed on samples recovered after the experiments. Precipitates of (Fe, Ni) and thin amorphous layer of silica were identified, exclusively on the surface of the single crystals. Mass balance indicates that volatilization of Fe, Mg, and Si is negligible under these conditions. The reaction can be summarized as: Fe2SiO4 in olivine + 2Cgraphite = 2Fein metal + SiO2 amorphous + 2COin gas which occurs at the crystal surface without affecting the interior of the crystal, except for an Fe2+ and Mg2+ compositional profile in the olivine matrix. These chemical profiles are consistent with measured values of Fe2+-Mg2+ interdiffusion coefficients, in agreement with the fact that Si and O are relatively immobile in olivine under such conditions. This study shows that annealing at relatively moderate temperature under reducing conditions can cause surface modifications and thus probably can strongly influence the surface evolution of planetary objects exposed directly to space environments (regoliths, surfaces of asteroids, or interplanetary dust particles).

Study of the (010) olivine surface by Rutherford backscattering spectrometry in channeling geometry

Abstract The (010) surfaces of forsterite (Fo100) and natural forsteritic olivine (Fo90) single crystals have been studied by Rutherford backscattering spectrometry (RBS). Spectra were collected either in channeling geometry with the incident beam parallel to the [010] zone axis, or in a random orientation. In both materials, two surface preparations were examined: (1) mechanical polishing and (2) chemical etching by hydrofluoric acid after mechanical polishing. Composition profiles extending to several hundreds of nanometers below the surface were probed by RBS in random mode. Simulations of all the spectra indicated constant major element compositions, equal to the bulk compositions of the crystals. Characteristics of the few top atomic layers were probed by RBS in channeling mode. The crystalline quality of the surface of chemically etched samples has been evaluated quantitatively and is shown to be much better than that of mechanically polished samples. Given the energy resolution, we estimate that the bulk crystalline quality is perturbed for more than 40 nm below the surface in the case of mechanical polishing whereas disorder is limited to a topmost layer thinner than 1 nm in the case of chemical etching. On average, for the chemically etched samples, less than one atom per [010] atomic row is displaced from its mean crystallographic position. The bulk stoichiometry is preserved in the topmost layer of pure forsterite whereas a slightly higher Fe/Mg ratio is detected at the surface of chemically etched Fo90. A method for the quantitative assessment of surface quality of olivine is thus proposed, constituting a useful preliminary step before any study of surface modifications of olivine subjected to various geological conditions.

Laser-driven quasi-isentropic compression experiments and numerical studies of the iron alpha-epsilon transition in the context of planetology

The iron alpha-epsilon transition is one of the most studied solid-solid phase transition. However, for quasi-isentropic compression, the dynamic and the influences of this transition on the high-pressure states of iron are still unknown. We present experimental results and numerical simulations to study these effects. Experiments performed at LULI2000 and the Janus laser facility (LNLL), using two different ramp shapes and different compression rates allowed to study the dynamic of the alpha-epsilon transition. We have observed the transition at particle velocity ranging from 0.25 km/s to 0.52 km/s depending on the compression rate. Depending on the ramp, either a shock formation was observed (high compression rate) at the transition or a flat plateau whose duration is function of compression rate. Increasing the compression rate leads to a smaller plateau duration. These results are important for reproducing Earth and Super-earth core conditions (2-15Mbar, 5- 15000K) on laboratory where the quasi-isentropic compression is the most promising experimental scheme.

STUDY OF IRON UNDER HIGH PRESSURE CONDITIONS USING ISENTROPIC COMPRESSION

We use directly driven method to compress the iron along isentropic path. By adjusting the pulse intensity of laser to increase with time, we directly ablated the iron target with laser to achieve isentropic compression. Rear surface velocities of the iron targets were recorded by VISAR (Velocity Interferometer System for Any Reflector). The iron (bcc) to (hcp) phase transformation was clearly observed. Experiment results were compared with simulations

Dynamic fragmentation as a possible diagnostic for high pressure melting in laser shock-loaded iron

High pressure melting of iron conditions the understanding of the Earth core constitution. Shock compression has been used for many years to complement the data obtained under quasi‐static loading. Still, shock‐induced melting is not easy to detect. Here, we investigate how dynamic fragmentation of laser shock‐loaded iron can be affected by melting. Iron samples are irradiated by a high power pulsed laser. The motion of the fragments ejected from the free surface is recorded by transverse shadowgraphy and soft recovery of the ejecta is performed in a low density gel. At low laser intensity, spalled layers can be seen in the shadowgraphs and solid fragments of mm‐dimensions are recovered. At higher intensity, wide debris clouds are observed to expand from the free surface, and tiny spherical fragments are recovered in the gel. This evolution is qualitatively consistent with the predictions of one‐dimensional hydrodynamic simulations accounting for laser‐matter interaction and pulse decay during propagation...