Magma reservoir beneath Azumayama Volcano, NE Japan, as inferred from a three-dimensional electrical resistivity model explored by means of magnetotelluric method

An electrical resistivity model beneath Azumayama Volcano, NE Japan, is explored using magnetotelluric method to probe the magma/hydrothermal fluid distribution. Azumayama is one of the most concerning active volcanoes capable of producing a potential eruption triggered by the 2011 Tohoku-Oki Earthquake. The three-dimensional resistivity model reveals a conductive magma reservoir (< 3 Ωm) at depths of 3–15 km below sea level (bsl). The 67% and 90% confidence intervals of resistivity are 0.2–5 Ωm and 0.02–70 Ωm, respectively, for the magma reservoir. We assumed dacitic melt + rock at a shallow depth of 4 km bsl and andesitic melt + rock at a greater depth of 9 km bsl. The confidence interval of resistivity cannot be explained by using dacitic melt + rock condition at a depth of 4 km bsl. This suggests that very conductive hydrothermal fluids coexist with dacitic melt and rock in the shallow part of the magma reservoir. For the depth of 9 km bsl, the 67% confidence interval of resistivity is interpreted as water-saturated (8.0 weight %) andesitic melt–mafic rock complex with melt volume fractions greater than 4 volume %, while the shear wave velocity requires the fluid and/or melt volume fraction of 6–7 volume % at that depth. Considering the fluid and/or melt volume fraction of 6–7 volume %, the conductive hydrous phase is likewise required to explain the wide range of the 67% confidence interval of resistivity. The Mogi inflation source determined from geodetic data lies on the resistive side near the top boundary of the conductive magma reservoir at a depth of 2.7 or 3.7 km bsl. Assuming that the resistivity of the inflation source region is above the upper bound of the confidence interval of resistivity for the conductive magma reservoir and that the source region is composed of hydrothermal fluid + rock, the resistivity of the source region is explained by a hydrothermal fluid volume fraction below 5 volume %, which is the percolation threshold porosity in an effusive eruption. This indicates that the percolation threshold characterizes the inflation source region.

The Temporal Variation of Magma Plumbing System of the Kattadake Pyroclastics in the Zao Volcano, Northeastern Japan

The geologic and petrologic study of the Kattadake pyroclastics (around 10 ka) from the Zao volcano (NE Japan) revealed the structure of the magma plumbing system and the mixing behavior of the shallow chamber. The Kattadake pyroclastic succession is divided into lower and upper parts by a remarkable discontinuity. All rocks belong to medium-K, calc-alkaline rock series and correspond to ol-cpx-opx basaltic-andesite to andesite with 20–28 vol% phenocrystic modal percentage. All rocks were formed by mixing between andesitic magma and near aphyric basalt. The petrologic features of andesites of lower and upper parts are similar, 59–61 wt% SiO2, having low-An plagioclase and low-Mg pyroxenes, with pre-eruptive conditions corresponding to 960–980 °C, 1.9–3.5 kb, and 1.9–3.4 wt% H2O. However, the basalts were ca. 49.4 wt% SiO2 with Fo~84 olivine in the lower part and 51.8 wt% SiO2 with Fo~81 olivine and high-An plagioclase the in upper one. The percentage of basaltic magma in the mixing process was lower, but the temperature of the basalt was higher in the lower part than the upper one. This means that the shallow magma chamber was reactivated more efficiently by the hotter basalts and that the mixed magma with a 70–80% of melt fraction was formed by a smaller percentage of the basaltic magma.

Simultaneous replacement of plagioclase by albite and K-feldspar: natural evidence and hydrothermal experiments

&lt;p&gt;Replacement of feldspars occurs ubiquitously during fluid-rock interaction in crusts, and the formation of micro- to nano- pores along with the replacement potentially provides significant impacts on hydrological properties within the crust (e.g. Pl&amp;#252;mper et al., 2017; Yuguchi et al., 2019). In this contribution, we report the novel texture of the plagioclase replacement by K-feldspar and albite and showed the conditions of such replacement. The mafic schists near the pegmatitic quartz diorite within the Kinkasan Island, NE Japan show extensive feldspar alteration at various stages, involving Na-rich and K-rich fluids, respectively. Interestingly, during the later K-rich fluid infiltration at 400-570 &amp;#730;C at 0.3&amp;#8211;0.45 GPa, plagioclase (An35-60) was replaced by K-feldspar (An0Ab1Or99) and albite (An4Ab94Or2) intergrowth, meaning that simultaneous K-feldspathization and albitization, and nano- to microscale pore network developed preferentially along with albite, resulting in an increase of the bulk rock porosity up to 1.34&amp;#177;0.14%.&lt;/p&gt;&lt;p&gt;To understand the relationship between K-feldspar and albite formations within the same plagioclase grain, we conducted the hydrothermal experiments on the feldspar replacement by using different pairs of starting minerals (anorthite, An96Ab4; labradorite, An66Ab33Or1; albite, An1Ab99) and fluid compositions (2M KCl and/or NaCl aqueous solutions) for 4-8 days. AIn all runs, the replacement processes of feldspars developed the distinct reaction front and pores formation close to the reaction front with porosity up to ~7%. In the experiments with KCl solution, the reaction front migrated twice faster than those with the mixture of KCl and NaCl. The most intense replacement occurred in the run of Labradorite-KCl solution, where large cavities were formed in the center of the labradorite grain with developing albite exsolution, and homogenous rim of K-feldspar precipitation. Such occurrences are similar to the replacement texture observed in the mafic schist within the Kinkasan Island and suggest the preferential removal of Ca and the fixed Na during K-feldspar formation. Our experimental results indicate the primary controls of the fluid composition on the replacement texture, pore formation, and the reaction rate.&lt;/p&gt;&lt;p&gt;Keywords feldspar replacement, micropores, fluid transport, hydrothermal experiment, Kinkasan&lt;/p&gt;