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[SCG54-10] Drifted pumice from Ioto found in the Nansei Islands
Keywords:EPMA analysis, Izu-Ogasawara arc, Kikai Island
Ioto, a volcanic island in the Izu-Ogasawara arc, is one of Japan's most active volcanoes and is located about 60 km north of Fukutoku-Oka-no-Ba (FOB), which caused a large-scale pumice drift due to a volcanic eruption in August 2021. Since July 2022, the first volcanic activities involving the magma eruption in the last 100 years have occurred off the southeast coast of Ioto, where rocks seemed to be generated by the eruptions are floating, and pumice with internal temperatures exceeding 120 degrees Celsius drifted ashore on Ioto (Japan Meteorological Agency (JMA), 2022). In October 2022 and October 2023, field surveys to sample the FOB pumice were conducted on Kikai, Amami Oshima, Okinawa, and Ishigaki Islands and drift pumice with a different appearance from the FOB pumice was discovered on the coasts. Then, the major element compositions of the volcanic glass shards in the pumice with a different appearance from the FOB pumice were conducted. As a result, the pumice clasts collected in Kikai Island in October 2023 showed similar major element compositions to that of pumice collected from Ioto by the JMA in August 2023.
The drift pumice sampled in the Nansei Islands analyzed was 7 samples collected on Kikai Island in October 2022 and October 2023, 5 samples collected on Amami Oshima Island in October 2022 and October 2023, 2 samples collected on Okinawa Island in October 2023, and 3 samples collected on Ishigaki Island in October 2023. Since the FOB eruption in 2021, according to JMA (2023), volcanic eruptions in Ioto have occurred off Okinahama on August 12, 23-26, September 2-3, 17 in 2021, near the Hyoryuboku coast on November 24, 2021, from July 11 to August 9, October 5-8, December 7-11 in 2022, and from June 15-24 in 2023 until October 2023 when the drift pumice was sampled in Nansei Islands. Moreover, FOB drift pumice surveys were conducted in October and December 2021, and May and June 2022, but no pumice similar to those analyzed in this study was found during the surveys.
The major element composition was analyzed for 63–120μm volcanic glass shards using an electron probe micro analyzer (EPMA: JEOL JXA-8200) at Marine Core Research Institute, Kochi University. The reference pumice used in this study (hereafter, 20230805IOTO) was collected by the JMA on August 5, 2023, off Okinahama in the southeast of Ioto. Based on the sample location and date, it is considered to be pumice erupted from the eruptions off Okinahama between July 2022 and June 2023. 20230805IOTO exhibited a black color and highly vesicular pumice. The average values of the major element compositions of volcanic glass shards in 20230805IOTO were SiO2=63.5 wt.%, Na2O+K2O=9.3 wt.%, and are characterized as rich in alkali. Although it is known that the rocks of Ioto and FOB in the Izu-Ogasawara arc are rich in alkali and have similar chemical compositions (e.g., Kosaka et al., 1990), a comparison of the major element compositions of volcanic glass shards in 20230805IOTO with that in FOB pumice of 1986 and 2021 showed clear differences in the contents of SiO2, TiO2, and Na2O.
As a result of EPMA analysis, three samples collected from Kikai Island in October 2023 have the same chemical composition as 20230805IOTO. Therefore, these pumice clasts are considered to originate from Ioto. Furthermore, two samples collected from Amami Oshima Island in October 2022 and eight samples collected from Ishigaki, Okinawa, and Amami Oshima Islands in October 2023 showed trends in chemical composition similar to those of 20230805IOTO rather than FOB. This suggests that these pumice clasts, although from a different eruption than the one that produced 20230805IOTO, could have been erupted by volcanic activity off Okinahama after July 2022.
No pumices with a composition similar to 20230805IOTO were found in the analysis of drift pumice collected before the 2021 FOB eruption from five locations on the Pacific side of the Japanese Islands (Amami Oshima, Miyazaki, Muroto, Shimoda, and Choshi) (Hiramine et al., 2023). This supports the conclusion that the drift pumice clasts identified as originating from Ioto in this study were erupted by volcanic activities on Ioto after July 2022.
The presence of drift pumice in the Nansei Islands, originating from recent eruptions in Ioto, suggests the possibility that pumice could have drifted to the Nansei Islands from similar eruptions in Ioto in the past. Therefore, if pumice that have drifted to the Nansei Islands in the past are estimated to originate from Ioto, they could provide information for the eruption history of Ioto.
The drift pumice sampled in the Nansei Islands analyzed was 7 samples collected on Kikai Island in October 2022 and October 2023, 5 samples collected on Amami Oshima Island in October 2022 and October 2023, 2 samples collected on Okinawa Island in October 2023, and 3 samples collected on Ishigaki Island in October 2023. Since the FOB eruption in 2021, according to JMA (2023), volcanic eruptions in Ioto have occurred off Okinahama on August 12, 23-26, September 2-3, 17 in 2021, near the Hyoryuboku coast on November 24, 2021, from July 11 to August 9, October 5-8, December 7-11 in 2022, and from June 15-24 in 2023 until October 2023 when the drift pumice was sampled in Nansei Islands. Moreover, FOB drift pumice surveys were conducted in October and December 2021, and May and June 2022, but no pumice similar to those analyzed in this study was found during the surveys.
The major element composition was analyzed for 63–120μm volcanic glass shards using an electron probe micro analyzer (EPMA: JEOL JXA-8200) at Marine Core Research Institute, Kochi University. The reference pumice used in this study (hereafter, 20230805IOTO) was collected by the JMA on August 5, 2023, off Okinahama in the southeast of Ioto. Based on the sample location and date, it is considered to be pumice erupted from the eruptions off Okinahama between July 2022 and June 2023. 20230805IOTO exhibited a black color and highly vesicular pumice. The average values of the major element compositions of volcanic glass shards in 20230805IOTO were SiO2=63.5 wt.%, Na2O+K2O=9.3 wt.%, and are characterized as rich in alkali. Although it is known that the rocks of Ioto and FOB in the Izu-Ogasawara arc are rich in alkali and have similar chemical compositions (e.g., Kosaka et al., 1990), a comparison of the major element compositions of volcanic glass shards in 20230805IOTO with that in FOB pumice of 1986 and 2021 showed clear differences in the contents of SiO2, TiO2, and Na2O.
As a result of EPMA analysis, three samples collected from Kikai Island in October 2023 have the same chemical composition as 20230805IOTO. Therefore, these pumice clasts are considered to originate from Ioto. Furthermore, two samples collected from Amami Oshima Island in October 2022 and eight samples collected from Ishigaki, Okinawa, and Amami Oshima Islands in October 2023 showed trends in chemical composition similar to those of 20230805IOTO rather than FOB. This suggests that these pumice clasts, although from a different eruption than the one that produced 20230805IOTO, could have been erupted by volcanic activity off Okinahama after July 2022.
No pumices with a composition similar to 20230805IOTO were found in the analysis of drift pumice collected before the 2021 FOB eruption from five locations on the Pacific side of the Japanese Islands (Amami Oshima, Miyazaki, Muroto, Shimoda, and Choshi) (Hiramine et al., 2023). This supports the conclusion that the drift pumice clasts identified as originating from Ioto in this study were erupted by volcanic activities on Ioto after July 2022.
The presence of drift pumice in the Nansei Islands, originating from recent eruptions in Ioto, suggests the possibility that pumice could have drifted to the Nansei Islands from similar eruptions in Ioto in the past. Therefore, if pumice that have drifted to the Nansei Islands in the past are estimated to originate from Ioto, they could provide information for the eruption history of Ioto.