Semi-quantitative analysis of bulk chondritic material using X-Ray fluorescence spectroscopy

73rd Annual Meeting of the Meteoritical-Society, New York, NY, July 26-30, 2010International audienceSynchrotron radiation X-ray fluorescence (SR-XRF) is a method of choice to analyze fragile, unique meteoritic samples, requiring no sample preparation. It is a nondestructive, multielemental, quantitative method, easily coupled to diffraction and speciation for a detailed sample characterization. The composition of samples thicker than a few microns is however difficult to obtain due to the high attenuation of the characteristic X-rays resulting in non-detection of low-Z elements (Z≤ 14)

Hyperspectral non-destructive analyses of Martian return samples under quarantine

International audienceIn preparation for the upcoming sample return missions containing potential biohazards which may have withstood the rigors of space travel we present a hyperspectral method of in-situ analysis of grains combining several non-destructive imaging diagnostics, performed in BSL4 quarantine conditions. This offers an alternative to the analyses in facilities at large, using optimized experimental setups while keeping the samples in conditions of quarantine. Our methodology was tested during analyses of meteorites and cometary and interstellar grains from the recent NASA Stardust mission

Shock-induced compaction, melting, and entrapment of atmospheric gases in Martian meteorites.

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Formation et transformations de la matière extraterrestre (grains comètaires et météorites choquées)

Au cours de cette thèse, nous avons pu participer à l'étude chimique et minéralogique des grains de la mission Stardust comètaires par des techniques basées sur le rayonnement synchrotron. L'importance de ces études tient au fait que les comètes sont parmi les objets les plus primitifs et les plus intacts de notre système solaire. Les météorites font aussi partie des premières briques du système solaire. Nous avons pu étudier les transformations qui se produisent au cours du choc dans ces roches. Nous avons notamment montré comment une partie de l'atmosphère martienne peut être capturée par les météorites lors de leur éjection mais aussi les différentes transformations qui peuvent se produire au cours du choc. Pour ceci, nous nous sommes intéressés à trois des grands systèmes minéralogiques qui existent : les olivines, les pyroxènes et le carbone. Nous avons ainsi pu mettre en évidence différents modes et mécanismes de transition de phase au sein de ces ensembles et montrer que le passage par un intermédiaire liquide dans deux des trois cas change la vision que nous avons des transitions de basse pression en polymorphe de haute pression dans les météorites. Par ailleurs, nous avons pu mettre en évidence l'existence d'un nouveau polytype du diamant et d'un nouvelle phase carbonée ultra dure dans les ureilites. Enfin, nous avons aussi étudié le comportement des feldspaths potassiques à haute pression par cellule à enclume de diamant couplée à la diffraction des rayons X et montré que la structure de haute pression, la hollandite, bien que subissant une transition de phase que nous avons découverte, semble stable jusqu'à la base du manteau terrestre.During this Ph. D, we were member of the Preliminary Examination Team which studied the chemical ans mineralogical compositions of Stardust mission cometary grains using synchrotron radiation techniques. These studies are of great importance since comets are the most primitives and unmodified bodies of our Solar System. Apart from comets, meteorites are also part of the first solar system stones. Using a very wide range of analytical techniques, we studied the different transformations which take place during a shock event undergone by these rocks. We particulary studied how part of the martian atmosphere was trapped in martian meteorites during their ejection from their parental body. We also studied the different phase transformations during the shock events on three different mineralogical system : olivines, pyroxenes and carbon. We showed that different transformation mechanisms were active in these systems and that a melting step in two of the three cases greatly change our vision in the transformation mechanism from low pressure polymorphs to high pressure polymorphs in meteorites. Moreover, we described a new diamond polytype and a new ultra-hard phase of carbon in ureilites. Finally , we also made in situ studies of the behavior of K-feldspar at high pressure using diamond anvil cell coupled with X-ray diffraction techniques. We showed that the high pressure polymorph, K-hollandite, over goes a phase transition which we discovered and that it seems to be stable as deep as the bottom of the Earth mantle.LYON-ENS Sciences (693872304) / SudocSudocFranceF

Carbon polymorphism in shocked meteorites: Evidence for new natural ultrahard phases

International audienceA slice of the Haverö meteorite which belongs to the ureilite class known to contain graphite and diamond was cut and then polished as a thin section using a diamond paste. We identified two carbonaceous areas which were standing out by more than 10 µm in relief over the surface of the silicate matrix suggesting that the carbonaceous phases were not easily polishable by a diamond paste and would therefore imply larger polishing hardness. These areas were investigated by reflected light microscopy, high-resolution Field Emission SEM (FESEM), energy-dispersive X-ray (EDX) analysis, Raman spectroscopy, and were subsequently extracted for in situ synchrotron microbeam X-ray fluorescence (XRF), imaging and X-ray diffraction (XRD). We report here the natural occurrences of one new ultrahard rhombohedral carbon polymorph of the R3m space group which structure is very close to diamond but with a partial occupancy of some of the carbon sites. We also report the natural occurrence of the theoretically predicted 21R diamond polytype with lattice parameters very close to what has been modelized. These findings are of great interests for better understanding the world of carbon polymorphs and diamond polytypes giving new natural materials to investigate. These natural samples demonstrate that the carbon system is even more complex than what is currently thought based on ab initio static lattice calculations and high-pressure experiments since this new ultrahard polymorph has never been predicted nor synthesized

Hard X-rays nano-imaging for Earth and Planetary Science samples

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The Isotope Geochemistry of Zn and Cr in Ureilites

International audienceWe measured the Cr and Zn stable isotope compositions of nine ureilite samples. Both Cr and Zn isotopic ratios are negatively correlated with their respective concentrations. These results may reflect isotopic fractionations during volatile loss

Volatilization induced by impacts recorded in Zn isotope composition of ureilites

International audienceUreilites are ultramafic achondrites formed by igneous processes early in the history of the Solar System. Ureilites are heavily depleted in volatile elements, probably by evaporation at the surface of the ureilite parent body; however the moderately volatile element Zn is an exception and its abundance is similar to CI. Volatilization can fractionate isotopes in a mass-dependent manner. Therefore, comparing the isotope composition of volatile elements in ureilites may better constrain the physical conditions during ureilite formation and the mechanism of origin of the variation of Zn content. For this study, we measured the Zn isotopic composition of 11 ureilites with a large range of Zn content. The elemental abundance of Zn is negatively correlated with δ66Zn, which may reflect that isotopic fractionation occurred by evaporation during the heating event on the ureilite parent body. Simple Rayleigh distillation predicts isotopic fractionations much larger than what is actually observed, therefore, such a model cannot account for the observed Zn isotope fractionation in ureilites. We propose that the observed isotopic fractionation can be produced by evaporation in a diffusion-limited regime. In addition, the isotopic composition of Zn is shown to be correlated with the shock grade of the ureilites. This may indicate that the heating events responsible for the depletion in light isotopes in some ureilites were impacts

Equation of state and phase transition in KAlSi3O8 hollandite at high pressure

The tetragonal hollandite structure (KAlSi3O8 hollandite) has been studied up to 32 GPa at room temperature using high-pressure in-situ X-ray diffraction techniques. A phase transformation from tetragonal I4/m phase to a new phase was found to occur at about 20 GPa. This transition is reversible on release of pressure without noticeable hysteresis and hence this new high-pressure phase is unquenchable to ambient conditions. The volume change associated with the transition is found to be small (not measurable), suggesting a second order transition. The diffraction pattern of the high-pressure phase can be indexed in a monoclinic unit cell (space group I2/m), which is isostructual with BaMn8O16 hollandite. The {gamma} angle of the monoclinic unit cell increases continuously above the transition. A Birch-Murnaghan equation of state fit to pressure-volume data obtained for KAlSi3O8 hollandite yields a bulk modulus K0 = 201.4 (7) GPa with K'0 = 4.0

Akimotoite in the Tenham meteorite: Crystal chemistry and high-pressure transformation mechanisms

International audienceThe transformation of pyroxene to its high-pressure polymorph akimotoite (MgSiO3 ilmenite type structure) is documented in the shock-induced melt veins of the L6 Tenham chondrite. Four textural relationships between pyroxene and akimotoite are observed in former pyroxene grains entrained in the shear melt vein and in pyroxene grains attached to the wall of the melt vein. In one of the entrained enstatite grains the transformation to akimotoite is partial. One third of the grain is transformed to a polycrystalline aggregate of akimotoite with a scalloped interface with enstatite. Akimotoite (Fe0,4Mg1,24Ca0.07Na0,12Al0,14)(Si1,963Al0,037)O6 is slightly enriched in Ca (CaO = 1.74%), Al and Na with respect to enstatite (CaO = 0.71%). Narrow bands of polycrystalline akimotoite with three crystallographic orientations and a chemical composition similar to the surrounding enstatite intersect the other part of the grain. In a second grain entrained in the shear melt vein, tablets of akimotoite are interwoven with a pyroxene glass with a lower Na and Fe content and a higher Mg content compared to the adjacent akimotoite. This pyroxene glass is probably the product of (Mg,Fe)SiO3-perovskite amorphization. Polycrystalline akimotoite is also formed at the expense of enstatite at the vein wall of the shear melt vein. In that case akimotoite is also enriched in Ca (CaO = 2.85%) Na (NaO = 1.72%) and Al (Al2O3 = 4.14%). All these observations suggest that akimotoite is mainly formed by solid-state transformation of former pyroxenes with subsequent diffusion of calcium, aluminum and sodium from the chondritic melt of the shear melt vein. Finally, the first complete X-ray diffraction pattern of natural akimotoite is presented