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

Abstract

Comparison between geophysical observations and laboratory measurements yields contradicting estimations of the melt fraction for the partially molten regions of the Earth, highlighting potential disagreements between laboratory-based electrical conductivity and seismic wave velocity measurement techniques. In this study, we performed simultaneous acoustic wave velocity 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 molten peridotite analog on both electrical conductivity and sound wave velocity. Acoustic wave velocity (Vp and Vs) and EC are measured 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 of MORB component. At constant temperature of 1650 K, electrical conductivity gradually increases, whereas acoustic velocities remain relatively constant. While the total MORB components melt instantaneously above the melting temperature, the melt 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 melt with 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 partially molten sample at the laboratory time scale (~ h). Based on our estimations, the reduced Vs velocity in the major part of the low velocity zone away from spreading ridges can be explained by 0.3–0.8 vol% volatile-bearing melt and the high Vp/Vs ratio obtained for these melt fractions (1.82–1.87) are compatible with geophysical observations.