Electrical resistivity is known as a good indicator for geo-fluid distribution especially in the crust and upper mantle. In this paper, we introduced physical concept of electrical resistivity in solid, liquid, and their mixing law, which can explain resistivity of crust and upper mantle. We also introduced magnetotelluric method, a common exploration method to image resistivity distribution in the earth, and modeling (inversion) method for resistivity distribution. Because resolution of inverted resistivity model from the magnetotelluric data depend on depth, resistivity, density of observation station and smoothness constraint, the model should be carefully interpreted. The magnetotelluric method has been applied for various tectonic settings. Many studies discovered low resistivity zones probably indicating fluid-rich area in or beneath the earthquake faults. In the volcanic zones, partial melt and hydrothermal areas were inferred based on three-dimensional modeling. Intensive MT surveys and newly developed interpretation techniques such as correction method of bathymetry effect and 3-D inversion method enable us to image resistivity of subduction slab and oceanic plate.

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  The slab-derived fluids and/or hydrous slab melts released from a subducted slab ascend into the mantle wedge, lower its melting temperature and thus induce generation of hydrous arc magmas. The estimation of H₂O concentration in primary arc magmas provides an important constraint on pressure and temperature conditions of magma generation at subduction zones. This paper gives an overview of the estimation of H₂O concentration in primary arc magmas by combining two petrological methods: experimental petrological studies and analyses of melt inclusions. Melting experiments of hydrous primary arc magmas have clarified that the P-T condition of magma generation shifts toward lower temperature and higher pressure with increasing H₂O concentration. Another experimental constraint is that only primary magmas with low H₂O (≤ 2 wt%) can erupt without modification of their primary composition by crystallization differentiation due to comparable dT/dP between olivine liquidus and basalt adiabat. However, this does not exclude presence of hidden H₂O-rich primary magmas at depths. Indeed, the H₂O concentrations in primary melt estimated from the analyses of primitive melt inclusions suggest wide variation (e.g., ~ 2 wt% at Kamchatka arc and ~ 4 wt% at Central American arc). H₂O-rich primary magmas may ascend and erupt after differentiation and/or supply volatiles to magmas at shallower level and cause so-called “excess degassing”. Analyses of melt inclusions also clarified that the H₂O concentration in primitive melt inclusions is almost constant or decrease from volcanic front to rear arc. This observation is opposite to a previous understanding that H₂O concentration in primary melt increases as well as incompatible K₂O across the arc.

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  Carbon dioxide fluxing is a recently proposed process in which a large amount of CO₂-rich vapour migrates in crustal magmatic systems. Such a proposal was derived from analytical studies on glass inclusions that have higher CO₂/H₂O ratios than those usually expected from a simple degassing process. The fluxing may be a universal phenomenon that occurs in various geological settings. However, its mechanisms, including the transport mode, duration and source, are still unclear. In this paper I review recent case studies on vapour transport in active volcanoes and discuss unsettled points and future studies.

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