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The transport properties of Earth’s upper mantle materials : insights from in situ HP-HT experiments

Abstract : The transport properties of mantle rocks are key parameters to qualitatively and quantitatively interpret direct and indirect geophysical information such as seismic velocities, heat fluxes and electromagnetic profiles across Earth’s and planetary interiors. The origins of upper mantle geophysical anomalies such as the Low Velocity Zone (70-150 km deep) and the Low Velocity Layer (350-410 km deep) are poorly known and require experimental constraints. In this PhD thesis, we have explored the electrical, seismic and thermal properties of realistic solid and partially molten peridotites via the development of geophysical in situ techniques. Performed at high pressures and temperatures in multi-anvil apparatus, our experiments allowed the characterization of the effect of melting on these different physical properties at mantle conditions. We performed the first experimental combination of electrical conductivity and sound wave velocity in a single multi-anvil experiment. Thanks to this technique, we reconciled electrical and seismic estimations of the melt fraction implied in the LVZ with 0.3-0.8 Vol.% of partial melting. The textural equilibration between melt and solid phases was found to be crucial for the comparison of laboratory estimations. We then realized the first reproduction of the dehydration melting process during the ascend of hydrous peridotites from the mantle transition zone to the upper mantle, between 12 and 14 GPa. Measurements during partial melting gave acoustic and electrical signals comparable to geophysical observations favoring partial melting explanation of the LVL anomaly. The implied melt fractions at upper mantle base were quantified to be moderate (<2 Vol.%). The chemical composition of produced melts confirmed the role of chemical filter of this melt layer located between upper and deep mantle. The calculated density confirmed the neutral buoyancy of the melt layer, making it a stable feature over geological times. Volatiles analyses and hydrogen transfer modeling confirmed this layer as a potential deep water reservoir and favored a bottom-up hydration of Earth’s upper mantle. Thermal diffusivity characterization techniques (Angström and pulse heating methods) were adapted to the LMV multi-anvil apparatus. Improved treatment procedures were elaborated for thermal transfer characterization under HP and HT conditions. The first thermal diffusivity characterization of glasses and melts at realistic mantle conditions were performed. In addition, thermal diffusivities of various samples (periclase, olivine, peridotite) were investigated with different structures (solid, solid+melt etc.) using Angström method.
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Damien Freitas. The transport properties of Earth’s upper mantle materials : insights from in situ HP-HT experiments. Earth Sciences. Université Clermont Auvergne, 2019. English. ⟨NNT : 2019CLFAC058⟩. ⟨tel-02515882⟩

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