Electrical conductivity during incipient melting in the oceanic low-velocity zone

International audienceThe low-viscosity layer in the upper mantle, the asthenosphere, is a requirement for plate tectonics1. The seismic low velocities and the high electrical conductivities of the asthenosphere are attributed either to subsolidus, water-related defects in olivine minerals2, 3, 4 or to a few volume per cent of partial melt5, 6, 7, 8, but these two interpretations have two shortcomings. First, the amount of water stored in olivine is not expected to be higher than 50 parts per million owing to partitioning with other mantle phases9 (including pargasite amphibole at moderate temperatures10) and partial melting at high temperatures9. Second, elevated melt volume fractions are impeded by the temperatures prevailing in the asthenosphere, which are too low, and by the melt mobility, which is high and can lead to gravitational segregation11, 12. Here we determine the electrical conductivity of carbon-dioxide-rich and water-rich melts, typically produced at the onset of mantle melting. Electrical conductivity increases modestly with moderate amounts of water and carbon dioxide, but it increases drastically once the carbon dioxide content exceeds six weight per cent in the melt. Incipient melts, long-expected to prevail in the asthenosphere10, 13, 14, 15, can therefore produce high electrical conductivities there. Taking into account variable degrees of depletion of the mantle in water and carbon dioxide, and their effect on the petrology of incipient melting, we calculated conductivity profiles across the asthenosphere for various tectonic plate ages. Several electrical discontinuities are predicted and match geophysical observations in a consistent petrological and geochemical framework. In moderately aged plates (more than five million years old), incipient melts probably trigger both the seismic low velocities and the high electrical conductivities in the upper part of the asthenosphere, whereas in young plates4, where seamount volcanism occurs6, a higher degree of melting is expected

Ultramafic Carbonated Melt‐ and Auto‐Metasomatism in Mantle Eclogites: Compositional Effects and Geophysical Consequences

The mineralogy, chemical composition, and physical properties of cratonic mantle eclogites with oceanic crustal protoliths can be modified by secondary processes involving interaction with fluids and melts, generated in various slab lithologies upon subduction (auto‐metasomatism) or mantle metasomatism after emplacement into the cratonic lithosphere. Here we combine new and published data to isolate these signatures and evaluate their effects on the chemical and physical properties of eclogite. Mantle metasomatism involving kimberlite‐like, ultramafic carbonated melts (UM carbonated melts) is ubiquitous though not pervasive, and affected between ~20% and 40% of the eclogite population at the various localities investigated here, predominantly at ~60–150 km depth, overlapping cratonic midlithospheric seismic discontinuities. Its hallmarks include lower jadeite component in clinopyroxene and grossular component in garnet, an increase in bulk‐rock MgO ± SiO2, and decrease in FeO and Al2O3 contents, and LREE‐enrichment accompanied by higher Sr, Pb, Th, U, and in part Zr and Nb, as well as lower Li, Cu ± Zn. This is mediated by addition of a high‐temperature pyroxene from a UM carbonated melt, followed by redistribution of this component into garnet and clinopyroxene. As clinopyroxene‐garnet trace‐element distribution coefficients increase with decreasing garnet grossular component, clinopyroxene is the main carrier of the metasomatic signatures. UM carbonated melt‐metasomatism at >130–150 km has destroyed the diamond inventory at some localities. These mineralogical and chemical changes contribute to low densities, with implications for eclogite gravitational stability, but negligible changes in shear‐wave velocities, and, if accompanied by H2O‐enrichment, will enhance electrical conductivities compared to unenriched eclogites.Plain Language Summary: Oceanic crust formed at spreading ridges is recycled in subduction zones and undergoes metamorphism to eclogite. Some of this material is captured in the overlying lithospheric mantle, where it is exhumed by passing magmas. Having formed in spreading ridges, these eclogites have proven invaluable archives for the onset of plate tectonics, for the construction of cratons during subduction/collision, as probes of the convecting mantle from which their precursors formed, and as generators of heterogeneity upon recycling into Earth's convecting mantle. During subduction and until exhumation, interaction with fluids and melts (called metasomatism) can change the mineralogy, chemical composition, and physical properties of mantle eclogites, complicating their interpretation, but a comprehensive study of these effects is lacking so far. We investigated mantle eclogites from ancient continents (cratons) around the globe in order to define hallmarks of metasomatism by subduction‐related fluids and small‐volume ultramafic carbonated mantle melts. We find that the latter is pervasive and occurs predominantly at midlithospheric depths where seismic discontinuities are detected, typically causing diamond destruction and a reduction in density. This has consequences for their gravitational stability and for the interpretation of shearwave velocities in cratons.Key Points: Exploration of metasomatic effects during subduction of ancient oceanic crust and after its emplacement into cratonic lithospheric mantle. Metasomatism by kimberlite‐like ultramafic melt affected between 20% and 40% of mantle eclogite suites worldwide, mostly at 2–5 GPa. Metasomatism lowers FeO, hence density in eclogite; no significant effect on shearwave velocities.German Research Foundation http://dx.doi.org/10.13039/501100001659National Research Foundation (NRF) http://dx.doi.org/10.13039/501100001321Wilhelm and Else Heraeus Foundation http://dx.doi.org/10.13039/501100011618Deutsche Forschungsgemeinschaft (DFG, INSTresearc

Biocompatibility study of europium doped crystalline hydroxyapatite bioceramics

International audienceFor the first time we presented the preliminary results of biocompatibility studies on bioceramic hydroxyapatite powders doped with europium Ca10-xEux(PO4)6(OH)2, with 0.01 = xEu = 0.2 prepared at low temperature by a simple coprecipitation approach. The X-ray diffraction (XRD) studies revealed the characteristic peaks of hydroxyapatite in each sample. No evidence for additional crystalline phases was found, proving the complete substitution of Eu in the HAp lattice in the whole range of concentrations. The scanning electron microscopy (SEM) observations suggest that these materials present a little different morphology, which reveals a homogeneous aspect of the synthesized particles for all samples. The PL investigations shown the PL intensity changed considerably by varying the xEu. The value of xEu in the hydroxyapatite formula has almost no effect on the wavelength of emission peaks. In order to test Hap_Eu biocompatibility, the effect of europium substituted hydroxyapatite nanocristalline powders with different xEu on cell viability and proliferation of HEK293 cell line were evaluated. The in vitro investigation showed no significant decrease of viability of HEK293 cell line and low levels of intracellular lipid peroxidation in the Hap_Eu treated cells. In conclusion, the results suggest that Eu doped HAp has low toxicity and exhibit a good biocompatibility

Geodynamics of melting in the Asthenosphere

International audienceAt geological time-scales, the mantle behaves as a high Rayleigh number fluid, i.e., thermal convection takes place and produces cells circulating at variable sizes and speeds. A lot of effort has been made to understand the upwelling part of these cells occurring underneath ridges and hotspots where they give birth to volcanoes. Nevertheless, local passive (adiabatic) sub-lithospheric mantle upwellings are likely to be more widespread and even common below oceanic plates. Just like under volcanoes, mantle is expected to undergo decompression melting in these concealed upwelling regions but the magma produced may be trapped and not have any volcanic expression. Here, we intend to discuss the fate of these deep melts and try to present a broad view of their geophysical and geochemical expressions. In our analyses, we model mantle melting that is favored by two critical parameters: high temperatures and/or elevated concentrations of H2O and CO2. It is frequently modeled as a chemical process in a static system, where thermodynamics is used to define the quantity of melts produced as a function of temperature and volatile contents. On the other hand, fluid mechanics tell us that the melt produced having low viscosity and low density tends to migrate away from its solid source at a rate depending on a variety of physical parameters; permeability and density/viscosity contrasts being the most influent. Combining thermodynamics and fluid mechanics, we show that CO2-H2O melts tend to focus at the lithosphere-asthenosphere boundary, where melt contents can reach 1-2%. This can easily explain many geophysical observations on the LVZ. The magnitude of the geophysical signal at the LVZ is related to convection (upwelling) in the asthenosphere; upwelling produces decompression-melting and the melt tends to accumulate below the impermeable lithosphere. The lithosphere-asthenosphere boundary must be featured by a strong and focused weakening where strain localizations enable decoupling between the plates and the asthenosphere. This geodynamic configurations is probably not always conceivable, particularly during the Archean, since temperatures was much hotter and melting much deeper

Médecine d’urgence : ce qui a changé en 2022 [Emergency medicine: what's new in 2022]

Emergency medicine is facing many challenges, particularly related to the consequences of the pandemic on the pressure of patient flows and the lack of human resources. More than ever, our discipline seeks to offer our patients quality care based on several recent studies, of which the following is a section: a) Gender effect in the administration of tranexamic acid; b) External validation of the Canadian Syncope Risk Score; c) Role of neuro-imaging in psychiatric decompensation; d) Choice of analgesia in renal colic; e) Use of carotid ultrasound for pulse control in cardiac arrest and f) The safetyness of performing simple sutures in non-sterile conditions

Geodynamics of melting in the Asthenosphere

International audienceAt geological time-scales, the mantle behaves as a high Rayleigh number fluid, i.e., thermal convection takes place and produces cells circulating at variable sizes and speeds. A lot of effort has been made to understand the upwelling part of these cells occurring underneath ridges and hotspots where they give birth to volcanoes. Nevertheless, local passive (adiabatic) sub-lithospheric mantle upwellings are likely to be more widespread and even common below oceanic plates. Just like under volcanoes, mantle is expected to undergo decompression melting in these concealed upwelling regions but the magma produced may be trapped and not have any volcanic expression. Here, we intend to discuss the fate of these deep melts and try to present a broad view of their geophysical and geochemical expressions. In our analyses, we model mantle melting that is favored by two critical parameters: high temperatures and/or elevated concentrations of H2O and CO2. It is frequently modeled as a chemical process in a static system, where thermodynamics is used to define the quantity of melts produced as a function of temperature and volatile contents. On the other hand, fluid mechanics tell us that the melt produced having low viscosity and low density tends to migrate away from its solid source at a rate depending on a variety of physical parameters; permeability and density/viscosity contrasts being the most influent. Combining thermodynamics and fluid mechanics, we show that CO2-H2O melts tend to focus at the lithosphere-asthenosphere boundary, where melt contents can reach 1-2%. This can easily explain many geophysical observations on the LVZ. The magnitude of the geophysical signal at the LVZ is related to convection (upwelling) in the asthenosphere; upwelling produces decompression-melting and the melt tends to accumulate below the impermeable lithosphere. The lithosphere-asthenosphere boundary must be featured by a strong and focused weakening where strain localizations enable decoupling between the plates and the asthenosphere. This geodynamic configurations is probably not always conceivable, particularly during the Archean, since temperatures was much hotter and melting much deeper

Anti-Stokes Raman Scattering and Luminescence in Carbon Nanotube Nanostructures

International audienceIn this paper, we present recent results obtained on single-walled carbon nanotubes (SWNTs) and carbon nanotube/conjugated polymer composites by using resonant Raman scattering and Surface Enhanced Raman Scattering (SERS). Besides the characterization of these materials, we report on peculiar properties observed in the anti-Stokes Raman branch of the Raman spectra. They consist in an abnormal anti-Stokes Raman emission which is explained by a mechanism reminiscent of a Coherent anti-Stokes Raman Scattering (CARS) emission. It results from a wave mixing process between the incident laser light and Stokes Raman light, generated by the SERS mechanism. In a parallel way, we have investigated in details the resonance effects which also induce anomalies in the anti-Stokes/Stokes intensity ratios, as a function of several parameters including the observation temperature, the environmental conditions, the dilution in solvents, etc. Studies extended to composites based on carbon nanotubes and conjugated polymers reveal also interesting properties. In the case of poly(bithiophene) (PBTh), one observes a strong amplification of the 1450 cm−1 Raman line in the anti-Stokes branch, generated by the plasmon excitation of metallic tubes. This phenomenon occurs in several other conjugated polymers such as PEDOT and PPV for modes located around 1500 cm−1. The role of metallic SWNTs is discussed. Finally, an anti-Stokes luminescence excited in the low energy tail of the absorption band of PPV and PPV/SWNTs composites has even been observed for the first time, explained through a phonon-energy up-conversion mechanism

A plate tectonic origin of kimberlites on a cooling Earth

International audienceDuring the past 20 years it has become fashionable to link global kimberlite magmatism to large igneous provinces, hotspot tracks, and mantle plumes. We reappraise the evidence used to propose these connections and find that compelling cases of cause-and-effect relationships between thermal anomalies in the deep mantle and kimberlite eruptions on thick continental lithospheres are rare if not absent. A new integrated analysis of emplacement ages, petrologic phase equilibria, and Nd-Hf-W isotopic compositions of kimberlites from Africa through time suggests that these CO2-H2O-rich high-Mg magmas represent low-degree partial melting products of rather 'normal' fertile peridotite beneath the thickest portions of relatively 'cold' continental lithospheres. Near the LAB beneath cratonic regions, volatile-fluxed incipient mantle melting dominates over a major melting regime; only the latter leads to production of large basaltic magma volumes. Importantly, upper mantle melting by volatile fluxing gained significance only after 2 Ga, when the ambient mantle potential temperature had dropped to 2 conditions directly beneath cratons explain the strong link to kimberlite melt formation after 2 Ga. We acknowledge that global kimberlite magmatism between 250 and 50 Ma appears to be attracted to the surface projection of the western margin of the African LLSVP. However, a new geodynamic reconstruction, in which we combine African plate velocities and kimberlite eruption incidents, demonstrates a link between plate tectonic motions and volatile-rich mantle-derived magmatism. This diverse approach suggests that global kimberlite magmatism, as recorded at Earth's surface, does not necessarily represent plume-related melting events, but rather melt drainage events through thick continental lithospheres that have been repeatedly under significant tectonic stresses while moving on a cooling Earth (Tappe et al., 2018). Tappe et al., 2018, Geodynamics of kimberlites on a cooling Earth. Earth and Planetary Science Letters 484, 1-14