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[SVC29-P02] The obsidian pyroclasts in 1895CE eruption products of Zao Volcano
Keywords:obsidian pyroclasts, volcanic glass, phreatic eruption, phreatomagmatic eruption, Okama, Zao
Zao Volcano locates in the middle part of the volcanic front of NE Japan. The present crater Okama started its activity at about 800 years ago. In the latest activity of 1894 to 1897CE, phreatic to phreatomagmatic eruptions were taken place. Recent study reported the scoriaceous bombs were included in the climatic eruption (1895CE) products of the latest activity. This study firstly presents the occurrence and petrologic features of pyroclastic obsidians in the 1895CE eruption products.
The obsidian pyroclasts are classified into 3 types. Type 1 is composed of only by the obsidian, types 2 and 3 include andesite and scoria in the core part, respectively. Type 1 is less than ca. 7 cm, whereas types 2 and 3 are up to ca. 30 cm in length. All types are vesiculated in various degrees. The vesicles are concentrated in the boundary parts of the obsidian and the core andesite/scoria. In type 2, more than two andesite fragments are sometimes included in the core part. In such case, the vesicles in the obsidian bonding andesite fragments tend to elongate near parallel to the boundary of the obsidian and the fragment. Chilled cracks are observed in the crust of the obsidian pyroclasts. These observations indicate that the obsidian pyroclasts are the juvenile materials.
Whole rock chemical compositions (57-58 wt% in SiO2 contents) of the core andesite and scoria are in the range of scoriaceous bombs in 1895CE eruption products. Whereas, those (63-68 wt% in SiO2 contents) of the obsidian parts differ from the scoriaceous bombs. Averaged SiO2 content of groundmass of the scoriaceous bombs is ca. 63 wt% , that falls in the range of the obsidian pyroclasts, but the other chemical compositions, such as CaO content, are out of the rangeof the obsidian pyroclasts, which excludes the possibility that the melts, squeezed from the scoriaceous bomb magma leaving phenocrystic minerals, became the magma for the obsidian.
The glasses in the obsidian are divided into colorless, beige, and brown colored ones. These three glasses are included in all types but beige or brown are not observed in several samples. Plagioclase microlites are occasionally included in the colorless and beige glasses but in the brown glass. Tiny cubic opaque minerals are observed in all glass types. The opaque minerals sometimes constitute aggregates. The beige glass tends to surround bobbles, while brown one patchily distributes in the colorless glass. The colorless and beige glasses have a curved boundary. The opaque minerals tend to concentrate in the boundary area. The three glasses have distinct chemical compositions. SiO2/Al2O3 contents of the colorless, beige, and brown glasses are 69-78 wt% /11-18 wt% , 59-67 wt% /19-32 wt% , 52-58 wt% /31-39 wt% , respectively. The colorless glasses are plotted on same trends in the SiO2 variation diagrams. Some of the beige glasses are plotted on the SiO2 poorer extension of the colorless glass trend, but the others are scattered distributed. The brown glasses are plotted on a trend that differs from the colorless glass trend. The colorless glass trend can be explained by the residual liquid trend subtracting plagioclase microlite components. The compositions of the scattered distributed beige glass and the brown glass cannot be explained by simple addition or subtraction processes of existing phases. The effect of secondary processes, such as a post emplacement alteration, should be considered.
The obsidian pyroclasts are classified into 3 types. Type 1 is composed of only by the obsidian, types 2 and 3 include andesite and scoria in the core part, respectively. Type 1 is less than ca. 7 cm, whereas types 2 and 3 are up to ca. 30 cm in length. All types are vesiculated in various degrees. The vesicles are concentrated in the boundary parts of the obsidian and the core andesite/scoria. In type 2, more than two andesite fragments are sometimes included in the core part. In such case, the vesicles in the obsidian bonding andesite fragments tend to elongate near parallel to the boundary of the obsidian and the fragment. Chilled cracks are observed in the crust of the obsidian pyroclasts. These observations indicate that the obsidian pyroclasts are the juvenile materials.
Whole rock chemical compositions (57-58 wt% in SiO2 contents) of the core andesite and scoria are in the range of scoriaceous bombs in 1895CE eruption products. Whereas, those (63-68 wt% in SiO2 contents) of the obsidian parts differ from the scoriaceous bombs. Averaged SiO2 content of groundmass of the scoriaceous bombs is ca. 63 wt% , that falls in the range of the obsidian pyroclasts, but the other chemical compositions, such as CaO content, are out of the rangeof the obsidian pyroclasts, which excludes the possibility that the melts, squeezed from the scoriaceous bomb magma leaving phenocrystic minerals, became the magma for the obsidian.
The glasses in the obsidian are divided into colorless, beige, and brown colored ones. These three glasses are included in all types but beige or brown are not observed in several samples. Plagioclase microlites are occasionally included in the colorless and beige glasses but in the brown glass. Tiny cubic opaque minerals are observed in all glass types. The opaque minerals sometimes constitute aggregates. The beige glass tends to surround bobbles, while brown one patchily distributes in the colorless glass. The colorless and beige glasses have a curved boundary. The opaque minerals tend to concentrate in the boundary area. The three glasses have distinct chemical compositions. SiO2/Al2O3 contents of the colorless, beige, and brown glasses are 69-78 wt% /11-18 wt% , 59-67 wt% /19-32 wt% , 52-58 wt% /31-39 wt% , respectively. The colorless glasses are plotted on same trends in the SiO2 variation diagrams. Some of the beige glasses are plotted on the SiO2 poorer extension of the colorless glass trend, but the others are scattered distributed. The brown glasses are plotted on a trend that differs from the colorless glass trend. The colorless glass trend can be explained by the residual liquid trend subtracting plagioclase microlite components. The compositions of the scattered distributed beige glass and the brown glass cannot be explained by simple addition or subtraction processes of existing phases. The effect of secondary processes, such as a post emplacement alteration, should be considered.