International audienceWe have investigated six impact glass spherules from the K-Pg event bed at Beloc, Haiti, using optical and electron microscopy, electron microprobe and in situ laser ablation-mass spectrometry (LA-ICP-MS; 37 trace elements, spot size 90-35m), in order to understand geochemical changes during alteration. The mm-sized glass spherules are partly or totally altered to smectite, but original textural features are preserved. The average trace-element composition of glass matches that one of the upper continental crust. Hints for a meteoritic component are lacking (Ni/Cr<1.3; Pt below detection limit). Compared to this fresh glass, smectites are strongly depleted in trace elements, except for Li, Sc, V, Ni, Ga, Ge, and Ba. The chondrite-normalized REE distribution patterns are flat with subchondritic abundances, related to their very low degree of crystallinity. We observe a positive Eu and a strong negative Ce anomaly; the latter is explained by formation of an organic Ce4+-complex, soluble under reducing conditions. Zr/Hf of glasses and smectites is chondritic to superchondritic (35-40), whereas Nb/Ta in smectite is subchondritic (5-12) compared to Nb/Ta in the glass (similar to 14-18). The low Nb/Ta is due to the low Nb concentrations in the smectite. Using in situ techniques with high spatial resolution, we have documented for the first time the significant changes in diagnostic elemental ratios during alteration of glass spherules. This has to be taken into account in the interpretation of geochemical data of not only impact materials but also volcanic glass, especially if bulk rock methods are used

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Electron probe micro-analysis (EPMA) is an important method for a broad range of applications in nuclear sciences. One main target is to improve the safety of the nuclear fuel cycle, by studying the chemical and physical properties of spent nuclear fuel and its fission products, either solid, volatiles, or gases, after the irradiation (post-irradiation examinations) [1].As in every other micro-analytical method, quantification of elements can only be achieved when the new sample is referenced to a sample of known concentration (i.e., a standard). In this paper we investigate the options to prepare and use reference materials for the actinides Am and Cm.

Carbonate-rich melts are primary conveyors of carbon and other incompatible elements in the deep Earth. Their mobility in the mantle thus controls the exchanges of carbon (and H 2 O) between deep and surficial reservoir and ultimately, the recycling of carbon and the fluxes to the surface. However, a quantitative understanding of these processes has been largely limited by poor knowledge of their physical properties, i.e. density and viscosity, at relevant mantle conditions. Here we report density and viscosity data of dry and hydrous carbonate melts that mimic incipient melts from subducted metapelites and carbonated eclogites and peridotites in the upper mantle. Densities were determined by the synchrotron X-ray absorption method in a Paris-Edinburgh press to 4 GPa-1800 K, while viscosities were computed up to 12 GPa and 2000 K by classical MD simulations using optimized interaction potentials for carbonate species. Our results provide the first experimental EOS for carbonate melts at upper mantle conditions, constrain the effect of hydration on the density/mobility of carbonate melts and the compressibility of dissolved volatiles in the melt. Moreover, we report evidence for non-Arrhenian temperature dependence of the viscosity of mixed alkali/alkaline earth carbonate melts at upper mantle conditions, which may arise from the formation of low-dimensional structure in the carbonate network. Therefore, this behavior may be extensible to other carbonate melts regardless of the composition and should be taken into account when modelling their mobility in the upper mantle. Finally, we present a global density model for mantle-derived carbonate-rich melts in the system MgO-Ca-Nathe upper mantle calibrated by > 880 density data points from experiments and simulations. The model, based on a simple Murnaghan equation of state and the assumption of linear mixing of volume between the different components, permits density and sound velocity predictions down to the mantle transition zone, 2300 K and 30 GPa. The applications of the new data and model to constrain the timescales of carbonate melt extraction from subducting slabs, buoyancy relations between mantle lithologies and carbonaterich melts and their geodynamic signature within the deep carbon cycle will be discussed.

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