Simultaneous measurements of electrical conductivity and seismic wave velocity of partially molten geological materials: effect of evolving melt texture

Co-auteur étrangerInternational audienceComparison between geophysical observations and laboratory measurements yields contradicting estimations of the meltfraction for the partially molten regions of the Earth, highlighting potential disagreements between laboratory-based electricalconductivity and seismic wave velocity measurement techniques. In this study, we performed simultaneous acoustic wavevelocity and electrical conductivity measurements on a simplified partial melt analogue (olivine + mid oceanic ridge basalt,MORB) at 2.5 GPa and up to 1650 K. We aim to investigate the effect of ongoing textural modification of partially moltenperidotite analog on both electrical conductivity and sound wave velocity. Acoustic wave velocity (Vp and Vs) and EC aremeasured on an identical sample presenting the same melt texture, temperature gradient, stress field and chemical impurities.We observe a sharp decrease of acoustic wave velocities and increase of electrical conductivity in response to melting ofMORB component. At constant temperature of 1650 K, electrical conductivity gradually increases, whereas acoustic velocitiesremain relatively constant. While the total MORB components melt instantaneously above the melting temperature, themelt interconnectivity and the melt distribution should evolve with time, affecting the electrical conduction. Consequently,our experimental observations suggest that acoustic velocities respond spontaneously to the melt volume fraction for meltwith high wetting properties, whereas electrical conduction is significantly affected by subsequent melt texture modifications.We find that acoustic velocity measurements are thus better suited to the determination of the melt fraction of a partiallymolten sample at the laboratory time scale (~ h). Based on our estimations, the reduced Vs velocity in the major part of thelow velocity zone away from spreading ridges can be explained by 0.3–0.8 vol% volatile-bearing melt and the high Vp/Vsratio obtained for these melt fractions (1.82–1.87) are compatible with geophysical observations