International audienceSilica-rich hydrous magmas are commonly stored in crustal reservoirs, but are also present at mantle depths in subduction contexts as a result of slab melting in the presence of considerable amounts of water and other vol-atile species. Magnetotelluric surveys frequently identify highly conductive zones at crustal or mantle depths possibly revealing the presence of such silica-rich melts and this can be used to trace the cycling of water in sub-duction zones and its relationship with arc-magmatism. The achievement of such a purpose is impeded by poor knowledge of the electrical conductivity of both dry and hydrous silica-rich melts at pressure. To fill this gap, we performed in situ electrical conductivity measurements on a dacitic melt using a 4-wire set up to 1300 °C, 3.0 GPa and H 2 O content up to 12 wt.%. Melt conductivity is strongly correlated with its water content, and we reveal a complex effect of pressure being relatively small at low water contents and major at high water contents: with increasing water content, the activation volume ranges between 4 (dry) and 25 cm 3 /mol (H 2 O = 12 wt.%) and the activation energy decreases from 96 kJ (dry) to 62 kJ (12 wt.% H 2 O). By comparison with diffusivity data, so-dium appears to be the main charge carrier, even at high (12 wt.%) water content. A T–P–[H 2 O] model predicting the conductivity of dacitic melts shows that crustal and mantle wedge conductive bodies can be interpreted by the presence of silica-rich, hydrous, partially crystallized magma

Electrical impedance measurements were performed on two types of partial molten samples with basaltic and carbonatitic melts in a Kawai-type multi-anvil apparatus in order to investigate melt fractionconductivity relationships and melt distribution of the partial molten mantle peridotite under high pressure. The silicate samples were composed of San Carlos olivine with various amounts of mid-ocean ridge basalt (MORB), and the carbonate samples were a mixture of San Carlos olivine with various amounts of carbonatite. High-pressure experiments on the silicate and carbonate systems were performed up to 1600 K at 1.5 GPa and up to at least 1650 K at 3 GPa, respectively. The sample conductivity increased with increasing melt fraction. Carbonatite-bearing samples show approximately one order of magnitude higher conductivity than basalt-bearing ones at the similar melt fraction. A linear relationship between log conductivity (σ bulk ) and log melt fraction (Φ) can be expressed well by the Archie's law (Archie, 1942) (σ bulk /σ melt =CΦ n ) with parameters C=0.68 and 0.97, n=0.87 and 1.13 for silicate and carbonate systems, respectively. Comparison of the electrical conductivity data with theoretical predictions for melt distribution indicates that the model assuming that the grain boundary is completely wetted by melt is the most preferable melt geometry. The gradual change of conductivity with melt fraction suggests no permeability jump due to melt percolation at a certain melt fraction. The melt fraction of the partial molten region in the upper mantle can be estimated to be 1~3% and ~0.3% for basaltic melt and carbonatite melt, respectively.

Geophysical, laboratory conductivity and petrological experiments reveal that deep electrical conductivity anomalies beneath the Central Andes, Cascades and Taupo Volcanic Zone image the ponding of super-hydrous andesitic melts that contributes to the growth of continental crust.