17:15 〜 19:15
[SVC35-P04] 栗駒火山の火山熱水系における岩石―火山性流体の反応に関する地球化学的研究
キーワード：地球化学、火山熱水系
Volcanoes with frequent phreatic eruptions are often associated with geothermal fields relating to volcanic hydrothermal systems (Ohba et al., 2022). In these geothermal fields, there are volcanic lake waters related to volcanic fluid, hot springs, and fumaroles. Kurikoma volcano, this study's filed, is one of the volcanoes with frequent phreatic eruptions (e.g. Doi, 2018; Ito et al, 2016). The volcano shows many geothermal manifestations.
This study aims to clarify the pathways of hydrothermal fluids from the core of Kurikoma volcano, focusing on the reactions between rock and volcanic fluids. We analysed unaltered and altered rock samples and water samples from a crater lake and hot springs. Minerals in the rocks are identified using XRD, and the whole rock chemistry were determined using XRF. Sulfur isotopic compositions (δ34S) of some rocks were measured with mass spectroscopy. Water chemistry was determined with ion chromatography, and bicarbonate ions and dissolved silica were measured separately. pH and temperatures were measured onsite.
Unaltered rocks contain abundant plagioclase and pyroxene with minor quartz and olivine. Altered rocks from the proximity of the hot springs and crater lakes are classified into three types according to the alteration minerals: opal-alunite type from the rocks around the crater lake and its nearby hot springs, smectite type from the locations 3 km southeast from the summit and a hot spring 10 km northeast of the summit, zeolite-type from a hot spring 10-15 km northwest of the summit. The altered rocks are silicified with higher SiO2 contents than the unaltered rocks. Coexisting sulfate and sulfide minerals show distinctly different sulfur isotopic compositions, suggesting hydrolysis rather than the original magmatic composition.
The pH values of the water samples indicate that the waters near the summit are acidic and the alkalinity increases with distance from the mountain. The distance and pH are poorly correlated with temperature. By plotting trilinear diagram, the hydrothermal waters are classified into three types: high-SO4 type (steam-heated), SO4-Cl type (volcanic gas origin),and SO4-Cl-HCO3 type (deep thermal water).
Three types of hydrothermal pathways are identified. Type I is a hydrothermal fluid path near the summit. Acidic to strong acidic hydrothermal fluids were distributed at the crater lake closest to the summit and the second nearest hot spring, accompanying opal-alunite alteration type. Near the summit, volcanic gas is mixed with groundwater shallow underground, resulting in acid hydrothermal water effusion. The fractionated sulfur isotope compositions of sulfate and sulfide indicate hydrolysis by gas-groundwater mixing. Type II is a hydrothermal pathway 10 km northeast of the summit. The hydrothermal fluids are neutral to weakly alkaline, accompanying smectite alteration. Type III is a hydrothermal pathway leading 15 km northwest from the summit. This hydrothermal fluid is alkaline containing minor HCO3, accompanying zeolite alteration. Types II and III originate from hydrothermal fluids, boiled during ascent, produced SO4-rich and HCO3-rich fluids, and mixed near the surface to form various hydrothermal fluids.
This study aims to clarify the pathways of hydrothermal fluids from the core of Kurikoma volcano, focusing on the reactions between rock and volcanic fluids. We analysed unaltered and altered rock samples and water samples from a crater lake and hot springs. Minerals in the rocks are identified using XRD, and the whole rock chemistry were determined using XRF. Sulfur isotopic compositions (δ34S) of some rocks were measured with mass spectroscopy. Water chemistry was determined with ion chromatography, and bicarbonate ions and dissolved silica were measured separately. pH and temperatures were measured onsite.
Unaltered rocks contain abundant plagioclase and pyroxene with minor quartz and olivine. Altered rocks from the proximity of the hot springs and crater lakes are classified into three types according to the alteration minerals: opal-alunite type from the rocks around the crater lake and its nearby hot springs, smectite type from the locations 3 km southeast from the summit and a hot spring 10 km northeast of the summit, zeolite-type from a hot spring 10-15 km northwest of the summit. The altered rocks are silicified with higher SiO2 contents than the unaltered rocks. Coexisting sulfate and sulfide minerals show distinctly different sulfur isotopic compositions, suggesting hydrolysis rather than the original magmatic composition.
The pH values of the water samples indicate that the waters near the summit are acidic and the alkalinity increases with distance from the mountain. The distance and pH are poorly correlated with temperature. By plotting trilinear diagram, the hydrothermal waters are classified into three types: high-SO4 type (steam-heated), SO4-Cl type (volcanic gas origin),and SO4-Cl-HCO3 type (deep thermal water).
Three types of hydrothermal pathways are identified. Type I is a hydrothermal fluid path near the summit. Acidic to strong acidic hydrothermal fluids were distributed at the crater lake closest to the summit and the second nearest hot spring, accompanying opal-alunite alteration type. Near the summit, volcanic gas is mixed with groundwater shallow underground, resulting in acid hydrothermal water effusion. The fractionated sulfur isotope compositions of sulfate and sulfide indicate hydrolysis by gas-groundwater mixing. Type II is a hydrothermal pathway 10 km northeast of the summit. The hydrothermal fluids are neutral to weakly alkaline, accompanying smectite alteration. Type III is a hydrothermal pathway leading 15 km northwest from the summit. This hydrothermal fluid is alkaline containing minor HCO3, accompanying zeolite alteration. Types II and III originate from hydrothermal fluids, boiled during ascent, produced SO4-rich and HCO3-rich fluids, and mixed near the surface to form various hydrothermal fluids.