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Morard G.,Institute for Study of the Earths Interior | Morard G.,Institute Of Mineralogie Et Of Physique Des Milieux Condenses | Katsura T.,Institute for Study of the Earths Interior
Geochimica et Cosmochimica Acta | Year: 2010

Multi-anvil press experiments were performed using a single cell assembly containing six different compositions. This set-up allows a careful sampling of the miscibility gap for given P-T conditions. Shrinking of the miscibility gap in the Fe-S-Si system has been studied from 4 to 12. GPa up to 2200. K, demonstrating a stable immiscible zone up to 4. GPa and 2200. K and its closure at higher pressures. Presence of both S and Si in the Earth's core is suggested by chondritic models. Therefore, its composition is inherited from processes at pressures higher than 4. GPa. This evolution of the Fe-S-Si miscibility gap is linked with the change in the local short-range order in Fe and Fe-S liquids. Our results indicate that core formation under reducing conditions would be affected by immiscibility for planetesimals up to size of the Moon. Furthermore, due to the difference in wetting properties between the two immiscible liquid phases, the S-rich metal phase would control the chemical exchange between liquid metals and silicates during early differentiation in planetesimals. © 2010 Elsevier Ltd.

Ferre E.C.,Southern Illinois University Carbondale | Friedman S.A.,Southern Illinois University Carbondale | Martin-Hernandez F.,Complutense University of Madrid | Feinberg J.M.,University of Minnesota | And 3 more authors.
Tectonophysics | Year: 2014

Wasilewski et al. (1979) concluded that no magnetic remanence existed in the uppermost mantle and that even if present, such sources would be at temperatures too high to contribute to long wavelength magnetic anomalies (LWMA). However, new collections of unaltered mantle xenoliths indicate that the uppermost mantle could contain ferromagnetic minerals. 1. The analysis of some LWMA over cratons and forearcs suggest magnetic sources in the uppermost mantle. 2. The most common ferromagnetic phase in the uppermost mantle is stoichiometric magnetite. Assuming a 30. km-thick crust, and crustal and mantle geotherms of 15. °C/km and 5. °C/km, respectively, the 600. °C Curie temperature implies a 30. km-thick layer of mantle. 3. The uppermost mantle is cooler than 600. °C in Archean and Proterozoic shields (>. 350. °C), subduction zones (>. 300. °C) and old oceanic basins (>. 250. °C). 4. Recently investigated sets of unaltered mantle xenoliths contain pure magnetite inclusions in olivine and pyroxene formed in equilibrium with the host silicate. 5. The ascent of mantle xenoliths occurs in less than a day. Diffusion rates in olivine suggest that the growth of magnetite possible within this time frame cannot account for the size and distribution of magnetite particles in our samples. 6. Demagnetization of natural remanent magnetization (NRM) of unaltered mantle xenoliths unambiguously indicates only a single component acquired upon cooling at the Earth's surface. This is most easily explained as a thermoremanent magnetization acquired by pre-existing ferromagnetic minerals as xenoliths cool rapidly at the Earth's surface from magmatic temperatures, acquired during ascent. 7. Modern experimental data suggest that the wüstite-magnetite oxygen buffer and the fayalite-magnetite-quartz oxygen buffer extend several tens of km within the uppermost mantle. 8. The magnetic properties of mantle xenoliths vary consistently across tectonic settings. In conclusion, the model of a uniformly non-magnetic mantle should be revisited. © 2014 Elsevier B.V.

Chollet M.,CNRS Geological Laboratory of Lyon: earth, planets and environment | Daniel I.,CNRS Geological Laboratory of Lyon: earth, planets and environment | Koga K.T.,CNRS Magmas and Volcanoes Laboratory | Morard G.,European Synchrotron Radiation Facility | And 2 more authors.
Journal of Geophysical Research: Solid Earth | Year: 2011

Properties of serpentine minerals are thought to influence the occurrence and location of intermediate-depth seismicity in subduction zones, which is often characterized by two dipping planes separated by ∼30 km defining a double seismic zone. The seismicity of the lower plane is believed to be provoked by the dehydration of serpentine since the experimentally determined stability limit for antigorite matches hypocenter locations. This requires that the fluid produced by dehydration is released much faster than the typical time scale of ductile deformation mechanisms. Here we measured the kinetics of antigorite dehydration in situ at high pressure and high temperature by time-resolved synchrotron X-ray diffraction in a closed system. Antigorite dehydrates in two steps. During step 1 it partially breaks down into olivine and a hydrous phyllosilicate closely related to the 10 phase. The modal abundance of the intermediate assemblage is described by 66 wt % antigorite, 19 wt % olivine, 12 wt % 10 phase. During step 2 at higher temperature, the remaining antigorite and the 10 phase fully dehydrate. From the analysis of reaction progress data, we determined that the major release of aqueous fluid occurs during step 2 at a fast rate of 10-4 mfluid 3 mrock -3 s-1. This exceeds by orders of magnitude the typical time scale of deformation by ductile mechanisms of any mineral or rock in the subducting slab or in the overlying mantle wedge. These results suggest that the fast dehydration of antigorite may well trigger the seismicity of the lower plane of the double seismic zone. Copyright 2011 by the American Geophysical Union.

Hayashi T.,National Museum of Nature and Science | Ohno M.,Kyushu University | Acton G.,University of California at Davis | Guyodo Y.,Institute Of Mineralogie Et Of Physique Des Milieux Condenses | And 4 more authors.
Geochemistry, Geophysics, Geosystems | Year: 2010

Iceberg discharges from continental ice sheets are widely believed to have exerted a great influence on global climate, but an iceberg discharge regime in early glacial periods after intensification of Northern Hemisphere glaciation (NHG) remains largely unclear. Here we present high-resolution rock magnetic records during the period from 2.1 to 2.75 Ma after intensification of NHG, reconstructed from the subpolar North Atlantic. Although the establishment of the middle Pliocene chronology of North Atlantic sediments is often a serious problem, we overcame it based on findings concerning the properties of magnetic susceptibility and natural gamma radiation. Reconstructed rock magnetic records indicate that millennial-scale iceberg surges were dominant features in the early glacial periods. Additionally, the millennial-scale iceberg surges occurred within glacial stages during intervals when ratios of global oxygen isotope stack from benthic foraminifera (LR04 δ18O stack) surpassed approximately 3.5‰. These are comparable to the climatic and environmental changes in Pleistocene glacial periods as represented by last glacial Dansgaard-Oeschger cycles, suggesting that continental ice sheets have oscillated and calved icebergs in a similar manner since intensification of NHG. © Copyright 2010 by the American Geophysical Union.

Stia C.R.,National University of Rosario | Gaigeot M.-P.,University of Evry Val dEssonne | Gaigeot M.-P.,Institut Universitaire de France | Vuilleumier R.,Ecole Normale Superieure de Paris | And 3 more authors.
European Physical Journal D | Year: 2010

We present a series of ab initio density functional based calculations of the fragmentation dynamics of core-ionized biomolecules. The computations are performed for pure liquid water, aqueous and isolated Uracil. Core ionization is described by replacing the 1s 2 pseudopotential of one atom of the target molecule (C, N or O) with a pseudopotential for a 1s 1 core-hole state. Our results predict that the dissociation of core-ionized water molecules may be reached during the lifetime of inner-shell vacancy (less than 10 fs), leading to OH bond breakage as a primary outcome. We also observe a second fragmentation channel in which total Coulomb explosion of the ionized water molecule occurs. Fragmentation pathways are found similar for pure water or when the water molecule is in the primary hydration shell of the uracil molecule. In the latter case, the proton may be transferred towards the uracil oxygen atoms. When the core hole is located on the uracil molecule, ultrafast dissociation is only observed in the aqueous environment and for nitrogen-K vacancies, resulting in proton transfers towards the hydrogen-bonded water molecule. © 2010 EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg.

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