12/09/16 Geeth Manthilake (Clermont-Ferrand)
de 14:00 à 15:00
|S'adresser à||JP Perrillat|
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Sound wave velocity/electrical conductivity of melt and fluids
Seismological and magnetotelluric (MT) studies have been extremely useful in characterizing the hydration and partial melting in the mantle. For interpreting the seismological and MT results, laboratory measurements of Sound wave velocity (SV) elasticity and electrical conductivity (EC) of fluids and melt have been crucial.
In the first study we have investigated the electrical conductivity of aqueous fluids at high pressure and high temperature. Our measurements indicate that the fluids released at high pressure (> 4 GPa) are significantly conductivity than those occurred at low pressures. The low resistivity observed at 150–250 km depths in subduction zone settings such as NE Japan, northern, and central Chile could be explained by the presence of highly conductive fluid. However, at relatively shallower depths (40 to 100 km), dehydration-induced aqueous fluids are less conductive (<2×10−2 S/m) and the high electrical conductivity of up to 1 S/m observed in mantle wedge regions cannot be explained by presence of fluid. Our experimental measurements demonstrate that the development of an interconnected network of chemically impure magnetite during dehydration of chlorite could explain the anomalously high conductivity observed in the mantle wedge. Chlorite dehydration in the mantle wedge provides an additional source of aqueous fluid above the slab and could also be responsible for the fixed depth (120 ± 40 km) of melting at the top of the subducting slab beneath the subduction-related volcanic arc front.
In the second study, we performed simultaneous sound wave velocity (SV) and electrical conductivity (EC) measurements on a mixture of San Carlos olivine and hydrous mid oceanic ridge basalt (MORB) at 2.5 GPa and up to 1650 K. Our experimental observations suggest that (i) Vp and Vs are mostly sensitive to the melt volume fraction and are not significantly affected by the spatial distribution of melt, while (ii) EC is significantly affected by melt texture, can led to a underestimation of the overall electrical conductivity for a given melt fraction if melt interconnectivity is not completed. Velocity measurements are thus better suited for the determination of the melt fraction of a partially molten sample at the laboratory time scale. Based on our texture-equilibrated measurements, the low velocity and high conductivity (LV-HC) zone in the asthenosphere can be explained with 0.2 to 0.5 vol. % of hydrous basalt melt.
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