Laurent Gautron

A new natural high-pressure (Na, Ca)-hexaluminosilicate [(CaxNa1-x)Al3+xSi3-xO11] in shocked Martian meteorites

A (Ca,Na)-hexaluminosilicate, whose Ca end member was previously synthesized in numerous high-pressure experiments, has been identified by Raman spectroscopy in heavily shocked Martian meteorites. This mineral has a structural formula close to (CaxNa1-x)Al3+xSi3-xO11 and is similar to the calcium aluminum silicate phase previously synthesized in high-pressure experiments performed on anorthite and rocks of basaltic composition. This new mineral occurs in shock melt pockets in two distinct settings and is intimately intergrown with SiO2-stishovite. The first setting, encountered in Zagami, consists of idiomorphic equant crystals overgrown by acicular stishovite that crystallized from a melt of labradorite composition. The second setting contains the (Na, Ca)-hexaluminosilicate phase intergrown with stishovite and hollandite and was formed during partial melting at high pressures. The mineralogical association (Na,Ca)-hexaluminosilicate+stishovite was observed in shock melt pockets, which have distinct bulk compositions in seven Martian shergottites. This new mineral represents, after majorite, the second natural occurrence of a silicate mineral with silicon in both four and six coordination. The assemblage stishovite+(Na,Ca)-hexalummosilicate sets constraints on the pressure and temperature conditions that prevailed during shock in some of the studied meteorites. The (Na, Ca)-hexaluminosilicate mineral is a potential carrier of Al and Na during subduction of oceanic crust in the lower mantle of the Earth

Uranium in the Earth's lower mantle

[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.

A study of the stability of anorthite in the PT conditions of Earth's transition zone

Abstract The laser-heated diamond anvil cell has been used to synthesize anorthite samples under the PT conditions of the transition zone. Samples recovered from high-pressure and high-temperature runs have been studied by X-ray diffraction (XRD) and analytical transmission electron microscopy (ATEM). Below 17.5 GPa, the structure of the anorthite remains stable. The lattice parameters of anorthite recovered at ambient pressure and temperature have been determined by lattice parameter refinement, whereby it is shown that the distortion of the structure increases with increasing pressure of synthesis. Above 17.5 GPa, anorthite decomposes into three phases, one with the composition Ca 1.33 Al 1.33 Si 2.33 O 8 and a hollandite-type structure, a phase with the composition Al 2 SiO 5 with a kyanite structure, and an amorphous phase with the approximate composition (Ca 0.8 ,Al 0.2 )(Si 0.8 ,Al 0.2 )O 3 . This last phase is assumed to be the result of the amorphization of Ca-perovskite on pressure release. The geophysical implications of the stability of these high-pressure aluminocalcic phases in the PT conditions of the transition zone are discussed.

Structural characterization of natural UO2 at pressures up to 82 GPa and temperatures up to 2200 K

Abstract Uranium is one of the main heat sources in the Earth, as about 25% of the total heat is produced by the radioactive decay of U. The location of U in the deep mantle is then essential for a better understanding of the geodynamics and thermal behavior of the Earth. For the first time, the crystal structure of natural simple dioxide UO2 uraninite has been studied by X-ray diffraction with synchrotron radiation (ESRF, Grenoble, France), in situ in a laser-heated diamond-anvil cell at pressures and temperatures relevant to the deep Earth’s mantle. Fluorite-type UO2 displays a new sequence of phase transitions at high P and T, with a cubic modified fluorite Pa3 observed at 18 GPa, and an orthorhombic Pbca structure from 33 GPa up to 82 GPa. Using a second-order Birch-Murnaghan equation of state, we calculated room-pressure bulk modulus K0 = 166(7) GPa with pressure derivative K′0 = 4.0 for the Pa3̄ structure, and K0 = 225(8) GPa with K′0 = 4 for the Pbca structure. The expected Pnma cotunnite structure was not observed but is not excluded at pressures higher than 82 GPa. Since UO2 displays a Pbca structure stable up to 82 GPa and presents a density much higher than the average density of the surrounding mantle, UO2 could be a host of U in the deep lower mantle.

X-ray absorption near edge structure (XANES) study of the speciation of uranium and thorium in Al-rich CaSiO3 perovskite

Abstract X-ray absorption spectroscopy was used to investigate the oxidation state of uranium in various Uand Th-bearing Al-rich CaSiO3 perovskite samples synthesized at high-pressure and high-temperature using a multi-anvil press apparatus. X-ray absorption near edge spectroscopy (XANES) spectra collected at the U LIII- and Th LIII-edges using both micro- and macro-focused beams show U4+ in the Al-rich CaSiO3 perovskite. The structure of the U- and Th-bearing Al-rich CaSiO3 perovskite samples have been cross-checked by XANES spectra collected at the Ca K-, Al K-, and Si K-edges. Al K and Si K spectra suggest that Al incorporates exclusively on the Si site of the CaSiO3 perovskite. Ca K spectra of the (U,Th)-bearing Al-rich CaSiO3 perovskite samples were succesfully compared to FEFF8.2 ab initio models of a tetragonal CaSiO3 perovskite with space group P4/mmm. Our results confirm previous assumptions of the coupled substitution of CaSi2 by UAl2 in CaSiO3 perovskite and that U and Th can be incorporated separately or together in CaSiO3 perovskite by means of this mechanism. The possible occurrence of the U- and Th-bearing Al-rich CaSiO3 perovskite are discussed as a potential candidate to locally host a large amount of actinides in the Earth’s deep mantle. The study of a phase that can act as a storage mineral for heat-producing actinide elements such as uranium and thorium is fundamental to the understanding of the geodynamics and thermal behavior of Earth.

Pb in naturally irradiated monazites and zircons

Pb-LIII edge XANES and μ-XAFS experiments (bulk and μ-focalised) were performed to evaluate the speciation of radiogenic Pb in naturally irradiated monazites (ideally LaPO4) and zircons (ideally ZrSiO4) minerals after ca. one billion years. XANES spectra show that Pb is divalent in all the samples investigated. But large variation in the speciation of Pb has been evidenced from monazites to zircons. Ab-initio calculations (FEFF8.4) were preliminary tested in a set of Pb-sulfide, oxide, carbonate, phosphate and silicate minerals and converged using Dirac-Hara exchange potentials. In monazite, XANES spectra calculations results are consistent with Pb substituting to La.