Elasticity of AlFeO(3) and FeAlO(3) perovskite and post-perovskite from first-principles calculations

International audienceWe use state-of-the-art ab initio calculations based on the generalized gradient approximation of the density functional theory in the planar augmented wavefunction formalism to determine the elastic constants tensor of perovskite and post-perovskite with formulas AlFeO(3) and FeAlO(3) in which Fe or Al respectively occupy only octahedral sites, for the stable magnetic configurations. The phase transition between perovskite and post-perovskite is associated with a site exchange, during which Fe from the inter-octahedral site in perovskite moves into the octahedral site in post-perovskite. Following this transition path the elastic moduli show positive jumps, considerably larger than for MgSiO(3). The phase transition is marked by a positive jump of 0.04 km/s (0.33\%) in the velocity of the compressional waves and by a negative jump of -0.15 km/s (-1.87\%) in shear wave velocity. We find that the effects of the Mg + Si Al + Fe substitution on the seismic properties of MgSiO(3) perovskite and post-perovskite depend on the crystallography of the substitution, namely the position the exchanged cations take in the structure. Citation: Caracas, R. (2010), Elasticity of AlFeO(3) and FeAlO(3) perovskite and post-perovskite from first-principles calculations, Geophys. Res. Lett., 37, L20306, doi:10.1029/2010GL044404

The effect of a small amount of hydrogen in the atmosphere of ultrahot magma-ocean planets: atmospheric composition and escape

Here we investigate how small amounts of hydrogen (much smaller than the mass of the exoplanet) above a magma ocean on a rocky exoplanet may modify the atmospheric chemistry and atmospheric escape.We use a chemical model of a magma ocean coupled to a gas equilibrium code. An energy-limited model is used to compute atmospheric escape. The composition of the vapor above a magma ocean is drastically modified by hydrogen, even for very modest amounts of H ($\ll 10^{-6}$ planetary mass). Hydrogen consumes much of the O$_2$(g), which, in turn, promotes the evaporation of metals and metal oxides (SiO, Mg, Na, K, Fe) from the magma ocean. Vast amounts of H$_2$O are produced by the same process. At high hydrogen pressures, new hydrogenated species such as SiH$_4$ form in the atmosphere. In all cases, H, H$_2$, and H$_2$O are the dominant nonmetal-bearing volatile species. Sodium is the dominant atmospheric metal-bearing species at T$<$ 2000K and low H content, whereas Fe is dominant at high H content and low temperature, while SiO predominates at T>3000 K. We find that the atmospheric Mg/Fe, Mg/Si, and Na/Si ratios deviate from those in the underlying planet and from the stellar composition. As such, their determination may constrain the planet's mantle composition and H content. As the presence of hydrogen promotes the evaporation of silicate mantles, it is conceivable that some high-density, irradiated exoplanets may have started life as hydrogen-bearing planets and that part of their silicate mantle evaporated (up to a few $10 \%$ of Si, O, and Fe) and was subsequently lost owing to the reducing role of H. Even very small amounts of H can alter the atmospheric composition and promote the evaporation to space of heavy species derived from the molten silicate mantle of rocky planets.Comment: Accepted for publication in A&

Carbon Speciation and Solubility in Silicate Melts

To improve our understanding of the Earth's global carbon cycle, it is critical to characterize the distribution and storage mechanisms of carbon in silicate melts. Presently, the carbon budget of the deep Earth is not well constrained and is highly model-dependent. In silicate melts of the uppermost mantle, carbon exists predominantly as molecular carbon dioxide and carbonate, whereas at greater depths, carbon forms complex polymerized species. The concentration and speciation of carbon in silicate melts is intimately linked to the melt's composition and affects its physical and dynamic properties. Here we review the results of experiments and calculations on the solubility and speciation of carbon in silicate melts as a function of pressure, temperature, composition, polymerization, water concentration, and oxygen fugacity

Theoretical determination of the Raman spectra of MgSiO3 perovskite and post-perovskite at high pressure

We use the density functional perturbation theory to determine for the first time the pressure evolution of the Raman intensities for a mineral, the two high-pressure structures of MgSiO3 perovskite and post-perovskite. At high pressures, the Raman powder spectra reveals three main peaks for the perovskite structure and one main peak for the post-perovskite structure. Due to the large differences in the spectra of the two phases Raman spectroscopy can be used as a good experimental indication of the phase transition.Comment: 16 pages, submitted to Geophysical Research Letter

Compressional pathways of α-cristobalite, structure of cristobalite X-I, and towards the understanding of seifertite formation

In various shocked meteorites, low-pressure silica polymorph α-cristobalite is commonly found in close spatial relation with the densest known SiO2 polymorph seifertite, which is stable above ∼80 GPa. We demonstrate that under hydrostatic pressure α-cristobalite remains untransformed up to at least 15 GPa. In quasi-hydrostatic experiments, above 11 GPa cristobalite X-I forms—a monoclinic polymorph built out of silicon octahedra; the phase is not quenchable and back-transforms to α-cristobalite on decompression. There are no other known silica polymorphs, which transform to an octahedra-based structure at such low pressures upon compression at room temperature. Further compression in non-hydrostatic conditions of cristobalite X-I eventually leads to the formation of quenchable seifertite-like phase. Our results demonstrate that the presence of α-cristobalite in shocked meteorites or rocks does not exclude that materials experienced high pressure, nor is the presence of seifertite necessarily indicative of extremely high peak shock pressures

Elasticity and lattice dynamics of enstatite at high pressure

The behavior of synthetic-powdered ^(57)Fe-enriched enstatite (Mg_(0.980)Fe_(0.020(5)))(Mg_(0.760)Fe_(0.240))Si_2O_6 has been explored by X-ray diffraction (XRD) and nuclear resonant inelastic X-ray scattering (NRIXS). The Pbca-structured enstatite sample was compressed in fine pressure increments for our independent XRD measurements. One structural transition between 10.1 and 12.2 GPa has been identified from the XRD data. The XRD reflections observed for the high-pressure phase are best matched with space group P2_1/c. We combine density functional theory with Mössbauer spectroscopy and NRIXS to understand the local site symmetry of the Fe atoms in our sample. A third-order Birch-Murnaghan (BM3) equation of state fitting gives K_(T0)=103±5 GPa and K'_(T0)=13±2 for the Pbca phase. At 12 GPa, a BM3 fitting gives K_(T12)=220±10 GPa with K'_(T12)=8±4 for the P2_1/c phase. NRIXS measurements were performed with in situ XRD up to 17 GPa. The partial phonon density of states (DOS) was derived from the raw NRIXS data, and from the low-energy region of the DOS, the Debye sound velocity was determined. We use the equation of state determined from XRD and Debye sound velocity to compute the isotropic compressional (V_P) and shear (V_S) wave velocities of enstatite at different pressures. Our results help constrain the high-pressure properties of Pbca-structured enstatite in the Earth's upper mantle. We find that candidate upper mantle phase assemblages containing Pbca-structured enstatite are associated with shear velocity gradients that are higher than the average Earth model PREM but lower than regional studies down to about 250 km depth

The influence of carbon on the seismic properties of solid iron

International audienceThe mechanical properties of C-doped hexagonal close-packed (hcp) iron are studied at high pressure from first-principles calculations. The energy required for doping with C as an interstitial impurity is 246meV/1wt%C at 120GPa for one unit cell of hcp Fe and increases almost linearly with pressure. The density deficit of the inner core can be matched for 1 to 2.5wt% in hcp Fe, depending on the thermal profile. Carbon doping in hcp iron increases the compressional seismic wave velocity, decreases the shear wave velocity, while increasing the shear wave splitting and seismic anisotropy. In general, the presence of C in the inner core helps in explaining the observed seismic properties, though it cannot be considered the only light element

Elasticity of (K,Na)AlSi(3)O(8) hollandite from lattice dynamics calculations

International audienceWe compute the elastic constants tensor and the seismic properties of KAlSi(3)O(8) and K(0.8)Na(0.2)AlSi(3)O(8) up to the ferroelastic transition using density-functional theory and density-functional perturbation theory in the ABINIT implementation. We observe a softening of the tetragonal shear with pressure that precedes the ferroelastic transition. The Reuss shear moduli become negative at respectively 23 GPa and 13 GPa for the two compositions considered in here. The ferroelastic transition is associated with a strong decrease of the horizontal shear wave velocities and a corresponding increase of the seismic anisotropy. The presence of Na enhances these features. (C) 2010 Elsevier B.V. All rights reserved