担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Geothermics  

The geothermal water discharged from the production well at the Dieng geothermal power plant experiences various drastic changes depending on the specific structure. As a result, various siliceous scales were found along the movement of the geothermal water at the Dieng geothermal power plant. To elucidate the geochemical property relationship between siliceous scales and geothermal waters at different locations, both the scale and the geothermal water samples were collected from (1) the inside of the two-phase pipeline, (2) the inside of the brine pipeline, and (3) at the open canal. The saturation index of the minerals was calculated based on the properties of the geothermal waters at each location to predict minerals that are possible to form. By comparing the properties of the scale and the mineral saturation index, quantitative mineral assemblage at each location was calculated, and the scale formation can be explained as follows. (1) Scale in the two-phase pipeline was composed of (Al, K)- and Fe-rich silicate, amorphous silica, and sulfide minerals formed at high temperature and near-neutral pH. The (Al, K)-rich and the Fe2+-rich silicates were controlled by the oversaturation of clinoptilolite and minnesotaite, and the amorphous silica was formed. The sulfide minerals were formed due to the high sulfur fugacity (fs2). (2) Scale in the brine pipeline was mainly composed of amorphous silica containing a small amount of (Al, K)-rich silicate formed under low pH (4∼5) and high-temperature conditions. (Al, K)-rich silicates were still precipitated even after acidification, likely due to the oversaturation of clinoptilolite above pH ∼4, and then the amorphous silica was formed. (3) Scale at the open canal was mainly composed of amorphous silica with small particles of hydrous ferric oxide (HFO), formed under an oxidizing condition at low temperature and low pH (4∼5). HFO reacted with silicic acid in geothermal water to form Fe3+-rich silicate that chemically resembles nontronite. The mineral with the highest proportion was amorphous silica in all three siliceous scales. Although the saturation index of amorphous silica was small, precipitation of amorphous silica may be induced and accelerated due to the adsorption of silicic acid on the (Al, K)-rich silicate and the Fe2+-rich silicate at high temperature and on the Fe3+-rich silicate at low temperature because of high surface area. The adsorbed silicic acid polymerizes on the silicate minerals.

DOI： 10.1016/j.geothermics.2023.102717

DOI： 10.1016/j.geothermics.2023.102717

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Volcanology and Geothermal Research  

Whether the magma storage system is responsible for the formation of the 74 ka Youngest Toba Tuff (YTT) is still debatable. It is currently believed that the YTT super-eruption was formed either by a single voluminous chamber or by multiple magma chambers. This paper presents a detailed component analysis of eruption products, comprehensive stratigraphy, mineralogy, and geochemistry of phenocrysts and matrix glass to shed light on the issue. Based on the componentry, mineral assemblages, mineral and glass compositions, and vesicle texture, we identified four distinct pumice types: YtA–YtD. Of these, YtA is an amphibole-bearing pumice containing an average of 77.1 wt% SiO2. This type of pumice is characterized by abundant matrix vesicles, with plagioclase showing a wide range of anorthite content (An20–An90) and disequilibrium texture. The YtB and YtC types are the most evolved pumices, with 77.5 and 77.6 wt% SiO2, respectively. The plagioclase of these pumices commonly has an unzoned texture with low anorthite content (~An30). The YtB pumice is biotite-bearing, distinctively characterized by abundant pheno-vesicles; while the YtC pumice includes rare pheno-vesicles, skeletal-polyhedral quartz, and no biotite. The YtD type is a quartz- and sanidine-free pumice, with a less-evolved glass composition (76.3 wt% SiO2), and is characterized by dominant matrix-vesicles and crystal clots of plagioclase, amphibole, pyroxene, and biotite. The plagioclase of the YtD pumice shows a hollow texture and high anorthite content (~An50). In situ trace element analysis for the matrix glass of YtA, YtB, YtC, and YtD showed very distinct geochemical signatures, which correlated with the four pumice types. Bivariate plots (Ba vs. Y, Sr vs. Y, and Ba vs. Sr) showed that YtA pumice is characterized by medium Ba (400–875 ppm) and Sr (41–67 ppm), and variable Y (20–53 ppm) content. YtB and YtC pumices are characterized by low Ba (8–136 ppm) and Sr (13–37 ppm), and highly variable Y (27–77 ppm) content. YtD pumice is characterized by high Ba (1173–1340 ppm) and Sr (95–124 ppm), but a narrow range of Y (21–31 ppm) content. The trace-element geochemical signatures, occurrence and absence of certain minerals (i.e., biotite and quartz), and vesicle type from the distinct pumice type, suggest the presence of four distinct magma bodies before the eruptions, namely YtA, YtB, YtC, and YtD. The number of distinct glass shards and pumices with the same geochemical signatures represents the volume fraction of each magma type. The approximate total volume of the magma (5300 DRE km3) and the geographical distributions of each pumice type demonstrated that the voluminous YtA (3406 km3), medium YtB (1354 km3), and smallest YtD (168.5 km3) chambers in the northern direction, and the small YtC chamber (371.5 km3) in the southern caldera, were evacuated during the 74 ka YTT eruption. Eruptions from multiple magma chambers were initiated by a high supersaturation of pheno-vesicle-rich magma (YtB magma), triggering the subsequent eruptions of the YtA, YtB, YtC, and YtD magmas.

DOI： 10.1016/j.jvolgeores.2023.107804

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担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Geothermics  

This study investigated the precipitation behavior of silicic acid (Si) and iron (Fe) when acidic and weakly alkaline geothermal waters were mixed at various mixing ratios (R) (the proportion of the acidic geothermal water, which ranged from 1 to 9) at 85 ˚C under an oxidizing atmosphere at the Hatchobaru Geothermal Power Plant in Japan. The pH and total Fe concentration of the acidic and weakly alkaline geothermal waters were 2.8 and 8.7 at 25 ˚C and 6.5 and 0.5 ppm, respectively. The pH of the mixed geothermal waters at 85 ˚C ranged from 8.11 to 3.30. Although Si precipitated after they were mixed at R of 1 to 4, Fe precipitated immediately after mixing, independent of R. The amount of Fe precipitated also increased with increasing R (thus increasing the Fe initial concentration). The precipitates were divided into two groups based on their chemical composition: those with a high Si/Fe atomic ratio in the range of 100 to 150, found at R of 1 to 4, and those with a Si/Fe atomic ratio lower than 10, found at R of 5 to 9. Silicic acid polymerized after they were mixed at R between 1 and 3 and the polymerization was not observed at R above 4. Fe was present as colloidal hydrous ferric oxide (HFO) and hydrolyzed species of Fe3+ in the acidic geothermal water. From these results, it may be concluded that the precipitates with a high Si/Fe atomic ratio were formed due to the aggregation of polysilicic acid particles with Fe at R from 1 to 3, whereas the precipitates with low Si/Fe atomic ratios were formed owing to the aggregation of HFO bonding monosilicic acid because of the sudden increase in pH at all R. There was a large amount of precipitate (the sum of Fe and Si mmol) at R from 1 to 4, indicating that delaying the polymerization of the silicic acid is key in preventing large precipitation of Si and Fe. Judging from the saturation indices for minerals commonly found in geothermal fields, the amorphous precipitates with high and low Si/Fe atomic ratios may be precursors of silica polymorph and nontronite as a Fe(III) silicate because they can be formed at R from 1 to 9.

DOI： 10.1016/j.geothermics.2022.102470

DOI： 10.1016/j.geothermics.2022.102470

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.17014/IJOG.7.1.1-14

開催年月日： 2024年11月

記述言語：英語   会議種別：口頭発表（一般）  

開催地：Auckland, New Zealand   国名：ニュージーランド  

開催年月日： 2024年3月

記述言語：日本語   会議種別：公開講演，セミナー，チュートリアル，講習，講義等  

開催地：岐阜県岐阜市   国名：日本国  

開催年月日： 2024年2月

記述言語：英語   会議種別：シンポジウム・ワークショップ パネル（公募）  

国名：アメリカ合衆国  

開催年月日： 2024年1月

記述言語：英語   会議種別：シンポジウム・ワークショップ パネル（公募）  

国名：日本国  

開催年月日： 2023年9月

記述言語：英語   会議種別：公開講演，セミナー，チュートリアル，講習，講義等  

国名：インドネシア共和国