3:45 PM - 4:00 PM
[SCG46-07] Acidic alteration process of andesite in the Unzen Jigoku, Nagasaki prefecture, Japan: based on the occurrence of alteration minerals and whole-rock composition
Keywords:Hydrothermal alteration, Unzen Jigoku, Raman spectroscopy, Whole-rock composition
[Introduction] Unzen Jigoku steaming field is a geothermal manifestation area located southwest of Unzen volcano on the Shimabara peninsula. There are active fumaroles and hydrothermal alteration zones in the area, acid alteration associated with geothermal activity (especially hot spring water). The wall rock of the altered rocks is hornblende andesite (NEDO, 1988). The temperature, pH, and chemical composition of the fluid affecting to the acid alteration process can be directly measured from hot spring water. This study aims to clarify the dissolution and formation processes of minerals in the acid alteration process associated with H-SO4 type hot spring water at the Unzen Jigoku. For this purpose, we investigated the occurrence of alteration minerals with micro scale. In addition, we analyzed the whole-rock composition and mineral assemblages of altered rock to reveal the migration of elements during the acidic alteration process.
[Samples and Analysis] Some of the breccias collected at Unzen Jigoku had different degrees of alteration between the inner and the outer side. It is thought that the outer parts were subjected to acidic alteration process by the hot spring water since the breccias were collected close to the hot spring pool. In this study, such breccia samples were sealed in epoxy resin and processed into thin sections and polished slabs. The microstructure of the altered breccias was observed under microscope for the thin sections and by SEM-EDS for the polished slabs. The mineral assemblages of groundmass and phenocryst were identified by Raman microspectroscopy (Renishaw inVia Raman microscope) for the polished slabs of the inner and outer parts of breccias. For analysis of whole-rock composition and mineral assemblages, the altered breccia samples were separated into the inner side part (weakly-altered part: W) and the outer side part (acidic-altered part: A). They were analyzed by XRF (Rigaku Primus Ⅳ) and XRD (Rigaku Ultima Ⅳ). To compare the degree of alteration, the wall rock of the altered rock, which is unaltered, was also analyzed by XRF and XRD.
[Occurrence of alteration minerals] Under microscope, groundmass of altered samples indicated homogeneity in both the W and A parts. Phenocrysts (plagioclase and hornblende) were also altered in the A parts, however, those were remained in the W parts. In the result of Raman analysis, cristobalite and anatase were identified within groundmass. On the other hand, hydrous amorphous silica (likely opal) and alunite were detected in the pseudomorph of phenocryst within A part. Therefore, the acid alteration associated with hot spring water can be divided into a two-step process, in which plagioclase and hornblende were dissolved first, and then hydrous amorphous silica and alunite were replaced.
[Whole-rock composition] To consider the behavior of major and trace elements during the acid alteration, whole-rock compositions of A part were plotted on Isocon diagram (Grant, 1986) with the W part as the reference. In the consideration, TiO2 and Zr were set as immobile elements. As a result of the consideration, most of the elements decreased, while the SiO2 content remined unchanged, and Ba content increased. In the result of XRD analysis, cristobalite, alunite and anatase were identified by A part. The peaks of alunite were weakly. In addition, the A part, K and Al contents of the part, which are major constituent elements of alunite, were decreased to the same extent as other elements. From this, a very small amount of alunite was contained in A part. Therefore, the migration of elements during the acid alteration process from the W to A part is explained by the leaching of most elements by the dissolution of plagioclase and hornblende and the residue of SiO2 by the formation of silica minerals.
[Samples and Analysis] Some of the breccias collected at Unzen Jigoku had different degrees of alteration between the inner and the outer side. It is thought that the outer parts were subjected to acidic alteration process by the hot spring water since the breccias were collected close to the hot spring pool. In this study, such breccia samples were sealed in epoxy resin and processed into thin sections and polished slabs. The microstructure of the altered breccias was observed under microscope for the thin sections and by SEM-EDS for the polished slabs. The mineral assemblages of groundmass and phenocryst were identified by Raman microspectroscopy (Renishaw inVia Raman microscope) for the polished slabs of the inner and outer parts of breccias. For analysis of whole-rock composition and mineral assemblages, the altered breccia samples were separated into the inner side part (weakly-altered part: W) and the outer side part (acidic-altered part: A). They were analyzed by XRF (Rigaku Primus Ⅳ) and XRD (Rigaku Ultima Ⅳ). To compare the degree of alteration, the wall rock of the altered rock, which is unaltered, was also analyzed by XRF and XRD.
[Occurrence of alteration minerals] Under microscope, groundmass of altered samples indicated homogeneity in both the W and A parts. Phenocrysts (plagioclase and hornblende) were also altered in the A parts, however, those were remained in the W parts. In the result of Raman analysis, cristobalite and anatase were identified within groundmass. On the other hand, hydrous amorphous silica (likely opal) and alunite were detected in the pseudomorph of phenocryst within A part. Therefore, the acid alteration associated with hot spring water can be divided into a two-step process, in which plagioclase and hornblende were dissolved first, and then hydrous amorphous silica and alunite were replaced.
[Whole-rock composition] To consider the behavior of major and trace elements during the acid alteration, whole-rock compositions of A part were plotted on Isocon diagram (Grant, 1986) with the W part as the reference. In the consideration, TiO2 and Zr were set as immobile elements. As a result of the consideration, most of the elements decreased, while the SiO2 content remined unchanged, and Ba content increased. In the result of XRD analysis, cristobalite, alunite and anatase were identified by A part. The peaks of alunite were weakly. In addition, the A part, K and Al contents of the part, which are major constituent elements of alunite, were decreased to the same extent as other elements. From this, a very small amount of alunite was contained in A part. Therefore, the migration of elements during the acid alteration process from the W to A part is explained by the leaching of most elements by the dissolution of plagioclase and hornblende and the residue of SiO2 by the formation of silica minerals.