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[SVC33-P08] A white pumice found in the Omuro scoria deposit of Fuji Volcano
Keywords:Fuji Volcano, Omuro scoria deposit, White pumice
Most of the volcanic products of Fuji Volcano are composed of basalt. However, some evidence suggests that differentiated SiO2-rich magma exists beneath. There is a discussion that SiO2-rich magma evolved in shallow magma reservoirs and mixed with differentiated basalt magma in deep magma reservoirs (e.g., Kaneko et al., 2010). During the AD1707 Hōei eruption and the 2.8 ka Zunasawa eruption, SiO2-rich pumice was erupted, and it has been revealed that a shallow magma reservoir existed at a depth of several kilometers during the Hōei eruption (Fujii et al., 2013). In addition, there has been a recent discussion about the state of shallow magma and the generation process based on the analysis of gabbro xenoliths found in other eruption products (e.g., Yasuda et al., 2019). In this poster, we report on the occurrence and petrological characteristics of white pumice found in the eruption products of Omuroyama, a scoria cone located on the northwestern foot of Fuji Volcano.
This white pumice was collected from an outcrop of the Omuro scoria deposit, approximately 3 km from Omuroyama. It is oval-shaped, with a major axis of ~8 cm and a minor axis of ~5 cm, and is larger than the surrounding scoriae (1–3 cm in diameter). Basaltic scoria is slightly adhered to the pumice. This scoria contains plagioclase with ~1 mm and olivine with ~0.1 mm diameter, and the matrix is brown glass, consistent with the petrological characteristics of the Omuro scoria. The white pumice contains 1–2 mm diameter plagioclase phenocrysts , and the matrix is well-vesiculated glass. The boundary between the pumice and the scoria is gradual. The whole-rock SiO2 content of the white pumice is 76.9 wt.%, K2O content is 1.5 wt. % and MgO content is 0.4 wt. %.
This white pumice has the highest SiO2 content of all the whole-rock compositions reported from eruption products of Fuji Volcano. It is comparable to the matrix glass composition of the pumice of the Hōei eruption (Yoshimoto et al., 2004) and the intergranular melt composition of the gabbroic xenoliths (Ishibashi et al., 2020). The white pumice’s matrix is a vesiculated glass, suggesting that the shallow mush magma was partially melted and vesiculated by basaltic magma or the melting and vesiculating shallow magma got trapped. Although the Omuro scoria deposit appears to be a homogeneous scoria deposit at the outcrop, the whole-rock SiO2 content decreases from the bottom to the top, and plagioclase phenocrysts’ core with low An values are present, as it is interpreted to be antecrysts derived from the residual magma of a previous eruption (Endo et al., 2021). The gradual transition between the pumice and the scoria may suggest that the remaining magma that formed such antecrysts did not mix in completely.
Acknowledgments: We thank Dr. Tomohiro Kubo, Dr. Takashi Uchiyama, and Ms. Sumire Nozawa of the Mount Fuji Research Institute for the sample.
This white pumice was collected from an outcrop of the Omuro scoria deposit, approximately 3 km from Omuroyama. It is oval-shaped, with a major axis of ~8 cm and a minor axis of ~5 cm, and is larger than the surrounding scoriae (1–3 cm in diameter). Basaltic scoria is slightly adhered to the pumice. This scoria contains plagioclase with ~1 mm and olivine with ~0.1 mm diameter, and the matrix is brown glass, consistent with the petrological characteristics of the Omuro scoria. The white pumice contains 1–2 mm diameter plagioclase phenocrysts , and the matrix is well-vesiculated glass. The boundary between the pumice and the scoria is gradual. The whole-rock SiO2 content of the white pumice is 76.9 wt.%, K2O content is 1.5 wt. % and MgO content is 0.4 wt. %.
This white pumice has the highest SiO2 content of all the whole-rock compositions reported from eruption products of Fuji Volcano. It is comparable to the matrix glass composition of the pumice of the Hōei eruption (Yoshimoto et al., 2004) and the intergranular melt composition of the gabbroic xenoliths (Ishibashi et al., 2020). The white pumice’s matrix is a vesiculated glass, suggesting that the shallow mush magma was partially melted and vesiculated by basaltic magma or the melting and vesiculating shallow magma got trapped. Although the Omuro scoria deposit appears to be a homogeneous scoria deposit at the outcrop, the whole-rock SiO2 content decreases from the bottom to the top, and plagioclase phenocrysts’ core with low An values are present, as it is interpreted to be antecrysts derived from the residual magma of a previous eruption (Endo et al., 2021). The gradual transition between the pumice and the scoria may suggest that the remaining magma that formed such antecrysts did not mix in completely.
Acknowledgments: We thank Dr. Tomohiro Kubo, Dr. Takashi Uchiyama, and Ms. Sumire Nozawa of the Mount Fuji Research Institute for the sample.