Aqueous fluids are one of the principal agents of chemical transport in Earth´s interior. The precise determination of fluid fractions is essential to understand bulk physical properties, such as rheology and permeability, and the geophysical state of the mantle. Laboratory-based electrical conductivity measurements are an effective method for estimating the fluid distribution and fraction in a fluid-bearing rock. In this study, the electrical conductivity of texturally equilibrated fluid-bearing forsterite aggregates was measured for the first time with various fluid fractions at a constant salinity of 5.0 wt.% NaCl at 1 GPa and 800 °C. We found that the electrical conductivity nonlinearly increases with increasing fluid fraction, and the data can be well reproduced by the modified Archie’s law. The three-dimensional (3-D) microstructure of the interstitial pores visualized by the high-resolution synchrotron X-ray computed micro-tomography (CT) shows a change in fluid distribution from isolated pockets at a fluid fraction of 0.51 vol.% to interconnected networks at fluid fractions of 2.14 vol.% and above due to grain anisotropy and grain size differences, accounting for the nonlinear increase in electrical conductivity. The rapid increase in conductivity indicates that there is a threshold fluid fraction between 0.51 and 2.14 vol.% for forming interconnected fluid networks, which is consistent with the 3-D images. Our results provide direct evidence that the presence of > 1.0 vol.% aqueous fluid with 5.0 wt.% NaCl is required to explain the high conductivity anomalies above 0.01 S/m detected in deep fore-arc mantle wedges.

Grain-scale pore geometry primarily controls the fluid distribution in rocks, affecting material transport and geophysical response. The dihedral angle (Θ) in the olivine–fluid system is a key parameter determining the pore fluid geometry in mantle wedges. Both curved and faceted olivine–fluid interfaces define Θ in the system, generating the faceted–faceted (FF), faceted–curved (FC), and curved–curved (CC) angles. However, the effect of faceting on Θ under various pressure and temperature (P–T) conditions and fluid compositions have not been constrained, and its mineralogical understanding is unresolved. This study evaluates the facet-bearing Θ and their proportions in olivine–multicomponent aqueous fluid systems. Our results show that 1/3 of olivine–fluid Θ are facet-bearing angles irrelative to the P–T conditions and fluid compositions. Faceting produces larger dihedral angles than the CC angles. The grain boundary plane (GBP) distribution reveals that the GBPs of faceted interfaces at triple junctions were subjected to low Miller Index faces ((100), (010), and (101)). Moreover, calculating the FF angles from two adjacent low Miller index planes highly reproduces measured angle values based on the olivine crystal habit. Therefore, our study suggests that the FF angle is strongly affected by olivine crystallography. The presence of faceting increases Θ and critical fluid fraction (Φc) for percolation, thus decreases the permeability. In the mantle wedge, where olivine crystallographic preferred orientation (CPO) is expected, increasing the FF angle proportion with associated changes in fluid pore morphology will lead to the permeability anisotropy and consequent geophysical anomalies.