Metamorphic fluids, faults, and shear zones are carriers of carbon from the deep Earth to shallower reservoirs. Some of these fluids are reduced and transport energy sources, like H₂and light hydrocarbons. Mechanisms and pathways capable of transporting these deep energy sources towards shallower reservoirs remain unidentified. Here we present geological evidence of failure of mechanically strong rocks due to the accumulation of CH₄-H₂-rich fluids at deep forearc depths, which ultimately reached supralithostatic pore fluid pressure. These fluids originated from adjacent reduction of carbonates by H₂-rich fluids during serpentinization at eclogite-to-blueschist-facies conditions. Thermodynamic modeling predicts that the production and accumulation of CH₄-H₂-rich aqueous fluids can produce fluid overpressure more easily than carbon-poor and CO₂-rich aqueous fluids. This study provides evidence for the migration of deep Earth energy sources along tectonic discontinuities, and suggests causal relationships with brittle failure of hard rock types that may trigger seismic activity at forearc depths.

Over the years, FIB-SEM tomography has become an extremely important technique for the three-dimensional reconstruction of microscopic structures with nanometric resolution. This paper describes in detail the steps required to perform this analysis, from the experimental setup to the data analysis and final reconstruction. To demonstrate the versatility of the technique, a comprehensive list of applications is also summarized, ranging from batteries to shale rocks and even some types of soft materials. Moreover, the continuous technological development, such as the introduction of the latest models of plasma and cryo-FIB, can open the way towards the analysis with this technique of a large class of soft materials, while the introduction of new machine learning and deep learning systems will not only improve the resolution and the quality of the final data, but also expand the degree of automation and efficiency in the dataset handling. These future developments, combined with a technique that is already reliable and widely used in various fields of research, are certain to become a routine tool in electron microscopy and material characterization.

X‐ray microscopy (XRM) is a non‐destructive characterization technique that provides quantitative information regarding the morphology/composition of the specimen and allows to perform multiscale and multimodal 2D/3D experiments exploiting the radiation‐matter interactions. XRM is particularly suitable to afford in situ images of inner parts of a battery and for the early diagnosis of its degradation in a non‐invasive way. Since traditional characterization techniques (SEM, AFM, XRD) often require the removal of a component from the encapsulated device that may lead to non‐desired contamination of the sample, the non‐destructive multi‐scale potential of XRM represents an important improvement to batteries investigation. In this work, we present the advanced technical features that characterize a sub‐micron X‐ray microscopy system, its use for the investigation of hidden and internal structures of different types of batteries and to understand their behavior and evolution after many charge/discharge cycles.