Experimental evidence supports mantle partial melting in the asthenosphere

The low-velocity zone (LVZ) is a persistent seismic feature in a broad range of geological contexts. It coincides in depth with the asthenosphere, a mantle region of lowered viscosity that may be essential to enabling plate motions. The LVZ has been proposed to originate from either partial melting or a change in the rheological properties of solid mantle minerals. The two scenarios imply drastically distinct physical and geochemical states, leading to fundamentally different conclusions on the dynamics of plate tectonics. We report in situ ultrasonic velocity measurements on a series of partially molten samples, composed of mixtures of olivine plus 0.1 to 4.0 volume % of basalt, under conditions relevant to the LVZ. Our measurements provide direct compressional (VP) and shear (VS) wave velocities and constrain attenuation as a function of melt fraction. Mantle partial melting appears to be a viable origin for the LVZ, for melt fractions as low as ∼0.2%. In contrast, the presence of volatile elements appears necessary to explaining the extremely high VP/VS values observed in some local areas. The presence of melt in LVZ could play a major role in the dynamics of plate tectonics, favoring the decoupling of the plate relative to the asthenosphere

Electronic Band Transitions in γ-Ge3N4

This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018 and 2019-2020 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission. Support from Estonian Research Council grant PUT PRG 619 is gratefully acknowledged. The multi-anvil experiments at LMV were supported by the French Government Laboratory of Excellence initiative no ANR-10-LABX-0006, the Région Auvergne and the European Regional Development Fund (ClerVolc Contribution Number 478).Electronic band structure in germanium nitride having spinel structure, γ-Ge3N4, was examined using two spectroscopic techniques, cathodoluminescence and synchrotron-based photoluminescence. The sample purity was confirmed by x-ray diffraction and Raman analyses. The spectroscopic measurements provided first experimental evidence of a large free exciton binding energy De≈0.30 eV and direct interband transitions in this material. The band gap energy Eg = 3.65 ± 0.05 eV measured with a higher precision was in agreement with that previously obtained via XES/XANES method. The screened hybrid functional Heyd–Scuseria–Ernzerhof (HSE06) calculations of the electronic structure supported the experimental results. Based on the experimental data and theoretical calculations, the limiting efficiency of the excitation conversion to light was estimated and compared with that of w-GaN, which is the basic material of commercial light emitting diodes. The high conversion efficiency, very high hardness and rigidity combined with a thermal stability in air up to ~ 700 °C reveal the potential of γ-Ge3N4 for robust and efficient photonic emitters. © 2021, The Korean Institute of Metals and Materials. Published under the CC BY license.Euratom research and training programme 2014-2018 633053; Eesti Teadusagentuur ANR-10-LABX-0006, PUT PRG 619; ERDF; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART2

Melting phase relations in the systems Mg2SiO4-H2O and MgSiO3-H2O and the formation of hydrous melts in the upper mantle

High-pressure and high-temperature melting experiments were conducted in the systems Mg2SiO4–H2O and MgSiO3–H2O at 6 and 13 GPa and between 1150 and 1900 °C in order to investigate the effect of H2O on melting relations of forsterite and enstatite. The liquidus curves in both binary systems were constrained and the experimental results were interpreted using a thermodynamic model based on the homogeneous melt speciation equilibrium, H2O + O2− = 2OH−, where water in the melt is present as both molecular H2O and OH− groups bonded to silicate polyhedra. The liquidus depression as a function of melt H2O concentration is predicted using a cryoscopic equation with the experimental data being reproduced by adjusting the water speciation equilibrium constant. Application of this model reveals that in hydrous MgSiO3 melts at 6 and 13 GPa and in hydrous Mg2SiO4 melts at 6 GPa, water mainly dissociates into OH− groups in the melt structure. A temperature dependent equilibrium constant is necessary to reproduce the data, however, implying that molecular H2O becomes more important in the melt with decreasing temperature. The data for hydrous forsterite melting at 13 GPa are inconclusive due to uncertainties in the anhydrous melting temperature at these conditions. When applied to results on natural peridotite melt systems at similar conditions, the same model infers the presence mainly of molecular H2O, implying a significant difference in physicochemical behaviour between simple and complex hydrous melt systems. As pressures increase along a typical adiabat towards the base of the upper mantle, both simple and complex melting results imply that a hydrous melt fraction would decrease, given a fixed mantle H2O content. Consequently, the effect of pressure on the depression of melting due to H2O could not cause an increase in the proportion, and hence seismic visibility, of melts towards the base of the upper mantle

A partially molten mantle

International audienceA global analysis of seismic waves has identified a widespread sharp velocity anomaly at the base of the low seismic velocity zone that is consistent with partial melting, closing a decades-long debate about the origin of this zone

Electrical conductivity of hydrous silicate melts: Implications for the bottom-up hydration of Earth's upper mantle

International audienceThe upwelling of the hydrous mantle transition zone triggers dehydration-induced partial melting atop the 410-km discontinuity. Here we investigate the electrical conductivity of hydrous silicate melts in the 200-400 km depth range and explore whether melting at the 410-km depths is responsible for the hydration of the upper mantle. Our experimental electrical conductivity data demonstrate that the mantle at 180-350 km depths is mostly melt free, confirming the H 2 O under-saturated conditions. However, the residual mantle from partial melting atop the 410-km discontinuity may contain various possible amounts of water according to the initial mantle transition zone and melt concentrations. This residual H 2 O could contribute to the hydration of the upper mantle either through diffusion or material replacement by upwelling. Our calculations suggest that the diffusion may not be responsible for the hydration of the upper mantle to present H 2 O concentration of 50-200 ppm wt. Melting of the upwelling mantle transition zone with less than 1500 ppm wt. H 2 O produces residual peridotites with ∼ 200 ppm H 2 O at the 410-km discontinuity. Continuous upwelling of such hydrous residues would gradually replace the dry upper mantle with depleted residual hydrous peridotites in less than 260 Ma. In this study, we propose a bottom-up hydration mechanism for the Earth's upper mantle driven by dehydration-melting at the 410-km discontinuity. The hydrous partial melting at the top of the asthenosphere appears to be a consequence of H 2 O saturation in the upwelling residual peridotites

Deformation of olivine-orthopyroxene aggregates at high pressure and temperature: Implications for the seismic properties of the asthenosphere

Co-auteur étrangerInternational audienc

Insights on the deep carbon cycle from the electrical conductivity of carbon‑bearing aqueous fluids

Co-auteur étrangerInternational audienceThe dehydration and decarbonation in the subducting slab are intricately related and the knowledge of the physical properties of the resulting C–H–O fluid is crucial to interpret the petrological, geochemical, and geophysical processes associated with subduction zones. In this study, weinvestigate the C–H–O fluid released during the progressive devolatilization of carbonate-bearing serpentine-polymorph chrysotile, with in situ electrical conductivity measurements at high pressures and temperatures. The C–H–O fluid produced by carbonated chrysotile exhibits high electrical conductivity compared to carbon-free aqueous fluids and can be an excellent indicator of the migration of carbon in subduction zones. The crystallization of diamond and graphite indicates that the oxidized C–H–O fluids are responsible for the recycling of carbon in the wedge mantle. The carbonate and chrysotile bearing assemblages stabilize dolomite during the devolatilization process. This unique dolomite forming mechanism in chrysotile in subduction slabs may facilitate the transport of carbon into the deep mantle

Electrical conductivity of metasomatized lithology in subcontinental lithosphere

Co-auteur étrangerInternational audienceA plausible origin of the seismically observed mid-lithospheric discontinuity (MLD) in the subcontinental lithosphere is mantle metasomatism. The metasomatized mantle is likely to stabilize hydrous phases such as amphiboles. The existing electrical conductivity data on amphiboles vary significantly. The electrical conductivity data on hornblendite is much higher low modal proportion of amphiboles could only reduce the shear seismic wave velocity by 0.4-0.5%, which is significantly lower than the observed velocity reduction of 2-6%. Thus, it might be challenging to explain both seismic and magnetotelluric observations at MLD simultaneously