Recent advances in geofluid-related studies have created a new organically-bound research filed. High-resolution magnetotelluric (MT) observations revealed that interconnected conductive networks present almost everywhere in the Japan arc crust; this stimulated a broad range of geoscience community to reconsider the fluid distribution in the crust and the role of fluids in generation of earthquakes. This special issue, a part of which is going to be published in the March issue, contains reviews of some recent studies including the molecular simulation of bulk and grain boundary water and brines, thermodynamics of interfacial energy that gives an interpretation of experimentally-obtained dihedral angles, constraints on chemical and isotopic signatures of subducted slab-derived fluids from geochemistry of igneous rocks and high-pressure experiments, the basic theory of MT method and its applications, the water solubility in arc magmas, and the “CO₂-fluxing” phenomena recently found in world-wide volcanic systems.

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  We demonstrate the usefulness of molecular simulations for understanding the physical properties of geofluids in the Earth's crust and mantle. Classical molecular dynamics (MD) methods are powerful tools to investigate the equation of state, electric conductivity, dielectric constant, and interfacial tension of highly-concentrated salt solutions over the wide range of temperature and pressure conditions, which are difficult to be studied by experiments. These properties are necessary to interpret the observations by seismic tomography and MT (Magneto-Telluric) method in terms of the distribution of geofluids, since physical properties of the fluid/mineral interfaces affect the bulk properties of fluid-bearing rocks. The experimental data on the mineral surfaces have been limited almost to those in the ambient conditions; they should be investigated over the wide ranges of temperature and pressure by both experimental and theoretical approaches.

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  The connectivity of grain-edge fluid channels in the lower crust and mantle is controlled mainly by the solid-liquid dihedral angle. To explain the change in equilibrium dihedral angle at elevated temperature and pressure, as observed in laboratory experiments, we developed two kinds of statistical thermodynamic models: a lattice-like model based on the Gibbs theory of adsorption, and a model based on the Cahn-Hilliard theory of non-uniform systems. The models perform well in explaining experimental data on dihedral angles in the forsterite-H₂O system. The complicated temperature dependence of dihedral angle in the quartz-H₂O system is possibly explained by the occurrence of multilayered adsorption.

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  The slab-derived fluid (slab-fluid) liberated from subducted materials, such as sediments and altered oceanic crust (AOC) in the subducting plate, is in general abundant in hydrophile elements. Many geological features occurred in subduction zones such as volcanism, earthquake, diastrophism and metamorphism are thought to be affected by the contribution of slab-fluid. The physical and chemical properties of slab-fluid depend on the compositions of AOC and sediment consisting of the subducting oceanic plate, and the condition of pressure, temperature and mobility of elements where the slab-fluid is generated. In this article, I overview how trace element and isotopic ratio are constrained on the basis of the compositions of volcanic rocks and the high-pressure experiments on element mobility, then estimate the geochemical characters of slab-fluid and hydrous mantle involving magma genesis beneath the Japan arcs as an example.

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