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.

The diversity of small shelly fossils (SSF) demonstrates that multicellular organisms underwent large-scale radiation at the beginning of the Cambrian, which is highlighted by the coexistence of various metazoans and the occurrence of their embryo fossils. However, little is known about early Cambrian eukaryotic multicellular algae, the primary producers that replaced oxygenic cyanobacteria and played a crucial ecological role in matter cycling and energy dynamics in marine ecosystems. In this study, hundreds of microscopic three-dimensionally preserved multicellular agglomerate fossils were obtained from the early Cambrian Kuanchuanpu Formation (535 Ma) in southern Shaanxi, South China, which consisted of several tightly-packed multicellular clusters encapsulated within a thin organic membrane. Synchrotron tomography analysis further revealed that the cells of the whole agglomerate, although partitioned into different subunits by a gelatinous membrane, were distinctly differentiated into an outer conical cell layer and an inner spherical-cell layer, thus suggesting of a cortex-medulla-like differentiation. These characteristics resemble those of multicellular algae (e.g. Wengania, Gremiphyca, and Thallophyca) from the Ediacaran Weng’an biota (South China) in morphology, size, and internal cell structure. Furthermore, a potential asexual life cycle for these membranous algae was proposed based on their morphological and structural characteristics. Our findings support an evolutionary continuity of the multicellular algae from the Ediacaran to the early Cambrian Period.