Electrical conductivity and partial melting of mafic rocks under pressure

We demonstrate the importance of electric conductivity measurements of partially molten mafic rocks by examining of Oman gabbro, Karelia olivinite, Ronda and Spitzbergen peridotites. The electrical conductivities of these rocks were estimated using the impedance spectroscopy at temperatures between 800°C and 1450°C and at pressures between 0.3 and 2 GPa in experiments performed in a piston cylinder apparatus. At temperatures below and above melting, samples were equilibrated during durations on the order of 200 h. Our results show that a jump in electrical conductivity can be correlated with the temperature range slightly above the solidus, due to the delayed formation of an interconnected melt phase. Thin sections of quenched samples were used to estimate volume fractions and chemical compositions of the partial melts. The increase of the electrical conductivity compares well with the connectivity of melt in partially molten samples. Above the solidus, the electrical conductivity increases by ∼1 to 2 orders of magnitude in comparison with the conductivity of non-melted rock below solidus. When a complete melt connectivity is established, the charge transport follows the network of the formed melt films at grain boundaries. Durations of up to ≃200 h are required in order to reach a steady state electrical resistance in a partially molten rock sample. The experimental results were compared with the conductivity data obtained from magnetotelluric (MT) and electromagnetic (EM) measurements in the Northern part of the mid-Atlantic ridge where a series of axial magma chambers (AMC) are presumably located. There is good agreement between the measured electric conductivity of gabbroic samples with a melt fraction of 10 to 13 vol.% and the conductivity estimated at AMC, beneath the central part of Reykjanes ridge, as well as between the conductivity of partially molten peridotites and the source zone beneath the mid-Atlantic ridge at ≃60 km.