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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, suc...

Silica-rich coatings of up to 660 μm thickness were found on the juvenile volcanic lapilli and blocks of the 1783 CE pyroclastic flow deposits 1.9–3.6 km northeast of the Maekake crater of Asama volcano in Central Japan. These coatings are composed of fine-grained volcanic ash particles with silica-rich cement. Raman spectroscopic measurements revealed that the interstitial silica-rich phase is hydrous amorphous silica and is considered to be a reaction product of ash particles with acidic volcanic rain and fog like volcanic smog (vog) as it has been recorded in Hawaii. The formation of amorphous silica without jarosite and the preferential alteration of plagioclase compared with pyroxene indicate that the reaction occurred with a sulfuric acid solution of pH

The dihedral angle (θ) between olivine and aqueous fluid is a critical parameter in identifying the grain-scale fluid connectivity which controls the distribution and migration of aqueous fluids in subduction zones besides physical properties of mantle wedges. Recent magnetotelluric observations have suggested the occurrence of significant fluid circulation in deep fore-arc regions, which can be explained by infiltration of saline fluid with a low θ in the mantle wedge ( Huang et al., 2019 ). Along with the salt component, non-polarized gas such as CO2, is a crucial constituent of subduction zone fluids. CO2 is known to increase the olivine–fluid θ under conditions in which the olivine does not react with CO2, which is in contrast to the effect of NaCl on θ. For a better understanding of the connectivity of multicomponent fluid in the mantle wedge, we experimentally constrained θ in olivine + H2O–CO2 fluid and olivine + H2O–CO2–NaCl (multicomponent) fluid systems at 1–4 GPa and 800–1100 °C. For the H2O–CO2 system, we found that CO2 tends to increase θ at 1 GPa and 800–1100 °C and at 2 GPa and 1100 °C. In contrast, CO2 reduced θ even below 60° at relatively high-pressure (P) and low-temperature (T) conditions, in which the olivine partly reacts with CO2 to form magnesite and orthopyroxene (opx). The consumption of non-wettable CO2 components in aqueous fluid alone cannot explain θ lower than those in a pure H2O system. Additional experiments on olivine–magnesite + H2O and olivine–opx + H2O systems showed that the presence of magnesite or opx decreased the olivine–fluid θ, which implies that coexisting minerals affect the olivine–fluid interfacial energy by changing the fluid chemistry. The results of the multicomponent system showed that the effect of NaCl on θ is much more significant than that of CO2. Strikingly, θ was smaller than 60° in all the magnesite- and opx-bearing multicomponent systems. Our results suggest that slab-derived fluid can infiltrate into the deep fore-arc mantle wedge through an interconnected network even in a CO2-bearing multicomponent system at pressures above 2 GPa, which facilitates significant fore-arc fluid circulation. The contrasting effects of aqueous fluid and silicate melt on the seismic wave velocity in a wide condition may allow for the possibility of mapping partial melt in the mantle wedge.