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

New Luminescent 1D and 2D d10 Coinage Metal Organic Chalcogenolate Coordination Polymers

SSCI-VIDE+CDURABLE+SHA:ABH:ADMInternational audienceCoordination polymers (CPs) with chalcogenate ligands (-Ch-R = S-R, Se-R, Te-R) and d10 coinage metals (M(I) = Cu, Ag and Au ), are well-known for their photoluminescence properties and offer a sustainable opportunity to replace rare earth elements based materials that are considered as critical raw materials by the European union.[1,2] The photoemission of Metals Organic Chalcogenolates (MOCs) is mostly attributed to the presence of d10 coinage metals and their ability to display metallophilic interactions that result in the possibility to fine tune their photophysical properties to get a large palette of color emission with high quantum yields. [3,4]. Herein, we will show the effect of the synthesis protocol on the coordination polymer’s crystal structures and their physical properties. In fact, different exciting crystal structures features were identified from powder and single crystal X-ray diffraction data. Depending on the nature of the synthesis protocol and thiol linker, the structures of the copper MOCs could be centrosymmetric (P21/c) or non-centrosymmetric (Cc, Pca21). The rich variety of the crystal structures impacts directly the photophysical properties with a shift from red to green emission. Moreover, an interesting Second Harmonic Generation response for the non-centrosymmetric compounds was detected, opening a new field of optical applications. [1] H. Furukawa, K. E. Cordova, M. O'Keeffe, O. M. Yaghi, The chemistry and applications of metal-organic frameworks, Science 341 (2013) 1230444. [2] Report on critical raw materials for the EU, May 2014, https://ec. europa.eu/growth/sectors/raw.[3] C. Lavenn; L. Okhrimenko; N. Guillou; M. Monge; G. Ledoux; C. Dujardin; R. Chiriac; A. Fateeva; A. Demessence, J. Mater Chem. C, 2015, 3, 4115.[4] Lavenn; N. Guillou; M. Monge; D. Podbevšek; G. Ledoux; A. Fateeva; A. Demessence, Chem. Commun., 2016, 52, 9063

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

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

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

Ultrafast photoluminescence spectroscopy of exciton-exciton annihilation in oligoaniline films with nanoscale ordering

The exciton dynamics and optical characteristics of blue-emitting N,N'-diphenyl-1,4-phenylene-diamine oligoaniline films have been determined. Transient photoluminescence experiments are consistent with internal quantum yields of 5.3% and 10.1% measured for oriented and nonoriented films, respectively. The data indicate a drastic dependence on nanoscale ordering which promotes photoluminescence quenching, a large annihilation rate, and fast exciton diffusion. Therefore, the emission properties can be controlled by the textural morphologies of the layers

Ultrafast photoluminescence spectroscopy of exciton-exciton annihilation in oligoaniline films with nanoscale ordering

The exciton dynamics and optical characteristics of blue-emitting N,N'-diphenyl-1,4-phenylene-diamine oligoaniline films have been determined. Transient photoluminescence experiments are consistent with internal quantum yields of 5.3% and 10.1% measured for oriented and nonoriented films, respectively. The data indicate a drastic dependence on nanoscale ordering which promotes photoluminescence quenching, a large annihilation rate, and fast exciton diffusion. Therefore, the emission properties can be controlled by the textural morphologies of the layers

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