9:45 AM - 10:00 AM
[SVC28-22] Tephrostratigraphy of drilled core C9010E off the Boso Peninsula: Estimated source volcano for 9th century eruption events
Keywords:tephrostratigraphy, Izu islands, marine sediment
We have been studying tephra correlations intervening in the sediment core off the Boso Peninsula, on the basis of the geologic, petrographic and geochemical characters of tephra samples collected at the reference sites on land. The sedimentary core C9010E was drilled at the site (34°33.46'N, 139°53.38'E, water depth: 2027.25 m) of about 40 km south of Boso Peninsula by the D/V Chikyu in 2009. The length of all core is 190.38 m. We collected 354 samples for tephra analysis from this core, including 161 tephra samples described (Tsuchiya et al., 2009). The major-element chemistry of volcanic glass shards in the fraction of 63-125 µm for all samples have been determined by EPMA of the Center for Advanced Marine Core Research, Kochi University. Aoki et al. (2019a) showed the core C9010E included tephras from Mukaiyama tephra (AD886) to Akasakimine tephra series at ca.15-20 ka of the Niijima Volcano origin (Aoki et al., 2019b). Furthermore, Aoki et al. (2020) presented that the tephra bed of the 22 cm thickness at 32.965-33.185 m was correlated to Iz-Tos2 tephra which was provided from unknown volcanic source (Takahashi et al., 2020) and the tephra spot at the 41.21-41.23 m was correlated to Asama Itahana Kasshoku (Brown) tephra group which provided during 26-28 ka (As-BP; Arai, 1962; Aramaki et al., 1998; Takahashi et al., 2003, Takahashi et al., 2018; Shitaoka et al., 2020).
Today, we present on a source volcano of a scoria patch between Niijima Mukaiyama tephra (AD886) and Kozushima Tenjyosan tephra (AD838) in the sedimentary core C9010E. The tephra bed at the depth of 1.15-1.17 m, which was correlated to Niijima Mukaiyama tephra, includes white pumice grains and black scoria grains of the maximum diameter 2 mm. Particles smaller than 250 µm consist of mostly white to clear colored volcanic glass shards which are various forms like as pumiceous, fibrous, bubble-wall, and chunky types. Biotite, orthopyroxene, hornblende, cummingtonite, plagioclase and quartz are observed. The tephra bed at the depth of 1.63-1.66 m, which was correlated to Kozushima Tenjyosan tephra, includes white pumice grains and black scoria grains of the maximum diameter 1 mm. Particles smaller than 250 µm consists of mostly white to clear colored pumiceous and bubble-wall type volcanic glass shards, a small amount of black scoria grains and rarely brown colored glass shards. Observed minerals are mainly plagioclase and quartz, and rarely biotite.
Between above two tephra bed, tephra patch intervened at the depth of 1.51-1.53 m. Particles larger than 250 µm includes black and dark gray scoria grains of the maximum diameter 2.5 mm, white pumice grains of the maximum diameter 1.5 mm, quartz, beta-type quarts, plagioclase, and a small amount of orthopyroxene and cummingtonite/hornblende. Particles smaller than 250 µm consist of large amount of white to clear colored pumiceous and chunky type, rarely bubble-wall type volcanic glass shards, quartz, beta-type quarts, plagioclase, rarely orthopyroxene. Major-element composition of volcanic glass shards in tephra patch at the depth of 1.51-1.53 m is rhyolite which similar to geochemistry of Niijima Mukaiyama tephra and Kozushima Tenjyosan tephra. However, the contents of CaO and K2O is different from them, rhyolite glass shards in tephra patch at the depth of 1.51-1.53 m would be correlated to a series of tephras provided from Niijima Kudamaki/Atsuchi event (Kobayashi et al., 2020). Furthermore, the contents of Na2O and MgO of tephra patch at the depth of 1.51-1.53 m are different from N1, N2, N3 and N4 scoria layers from 6 cent. to 9 cent.( Kawanabe, 2012) at Izu-Oshima and Fuji Hoei scoria (AD1707). Therefore, the origin of scoria grains in tephra patch at the depth of 1.51-1.53 m is presumed as basaltic magma of Niijima volcano. According to Kobayashi et al. (2020), Niijima Kudamaki/Atsuchi event started from eruptions of basaltic magma, and changed to eruption of rhyolitic magma. So, tephra patch at the depth of 1.51-1.53 m is likely correlated to tephra of Niijima Kudamaki/Atsuchi event.
References: Aoki et al. (2019a), JpGU2019, HQR05-05, Aoki et al. (2019b), JpGU2019, SVC38-P21, Aoki, K. (2020), Arai (1962), Aramaki et al., (1993), Kawanabe (2012), Kobayashi et al.(2020) , Takahashi, M., et al.(2003), Takahashi, M., et al. (2018), Takahashi, T., et al. (2020), Tsuchiya et al. (2009)
Today, we present on a source volcano of a scoria patch between Niijima Mukaiyama tephra (AD886) and Kozushima Tenjyosan tephra (AD838) in the sedimentary core C9010E. The tephra bed at the depth of 1.15-1.17 m, which was correlated to Niijima Mukaiyama tephra, includes white pumice grains and black scoria grains of the maximum diameter 2 mm. Particles smaller than 250 µm consist of mostly white to clear colored volcanic glass shards which are various forms like as pumiceous, fibrous, bubble-wall, and chunky types. Biotite, orthopyroxene, hornblende, cummingtonite, plagioclase and quartz are observed. The tephra bed at the depth of 1.63-1.66 m, which was correlated to Kozushima Tenjyosan tephra, includes white pumice grains and black scoria grains of the maximum diameter 1 mm. Particles smaller than 250 µm consists of mostly white to clear colored pumiceous and bubble-wall type volcanic glass shards, a small amount of black scoria grains and rarely brown colored glass shards. Observed minerals are mainly plagioclase and quartz, and rarely biotite.
Between above two tephra bed, tephra patch intervened at the depth of 1.51-1.53 m. Particles larger than 250 µm includes black and dark gray scoria grains of the maximum diameter 2.5 mm, white pumice grains of the maximum diameter 1.5 mm, quartz, beta-type quarts, plagioclase, and a small amount of orthopyroxene and cummingtonite/hornblende. Particles smaller than 250 µm consist of large amount of white to clear colored pumiceous and chunky type, rarely bubble-wall type volcanic glass shards, quartz, beta-type quarts, plagioclase, rarely orthopyroxene. Major-element composition of volcanic glass shards in tephra patch at the depth of 1.51-1.53 m is rhyolite which similar to geochemistry of Niijima Mukaiyama tephra and Kozushima Tenjyosan tephra. However, the contents of CaO and K2O is different from them, rhyolite glass shards in tephra patch at the depth of 1.51-1.53 m would be correlated to a series of tephras provided from Niijima Kudamaki/Atsuchi event (Kobayashi et al., 2020). Furthermore, the contents of Na2O and MgO of tephra patch at the depth of 1.51-1.53 m are different from N1, N2, N3 and N4 scoria layers from 6 cent. to 9 cent.( Kawanabe, 2012) at Izu-Oshima and Fuji Hoei scoria (AD1707). Therefore, the origin of scoria grains in tephra patch at the depth of 1.51-1.53 m is presumed as basaltic magma of Niijima volcano. According to Kobayashi et al. (2020), Niijima Kudamaki/Atsuchi event started from eruptions of basaltic magma, and changed to eruption of rhyolitic magma. So, tephra patch at the depth of 1.51-1.53 m is likely correlated to tephra of Niijima Kudamaki/Atsuchi event.
References: Aoki et al. (2019a), JpGU2019, HQR05-05, Aoki et al. (2019b), JpGU2019, SVC38-P21, Aoki, K. (2020), Arai (1962), Aramaki et al., (1993), Kawanabe (2012), Kobayashi et al.(2020) , Takahashi, M., et al.(2003), Takahashi, M., et al. (2018), Takahashi, T., et al. (2020), Tsuchiya et al. (2009)