[SVC38-P21] Standard stratigraphy and characteristic properties of late Quaternary tephras on the Niijima-island: Report of geochemistry and refractive indices of volcanic glass shards, orthopyroxene, hornblende, and cummingtonite
Keywords:Izu, Niijima, tephrochronology, petrographic characters, major-element composition
At the Daisan-yama, Fujimi Touge, and Miyatsukayama on Niijima island, and also Jinaijima off 1.6km west of Niijima island, we have recognized 21 eruptive events at least, including some tephras derived from off-island (Kobayashi et al., 2019a). Kobayashi et al. (2019a) recognized as Niijima Origin tephra felsic magma events and mafic magma events on Nijjima islnad. On the island, as widespread marker tephra, AT and K-Ah tephra were detected. After tephra samples were washed by the ultrasonic cleaner, we collected the description of petrographic characters, major-element chemistry determined by EDS and refractive indices (RI) of volcanic glass shards, orthopyroxene, hornblende, and cummingtonite in each tephra.
Generally, felsic tephra derived from vents on the Niijima island include often a small amount scoria grain. Most felsic tephras consist of felsic volcanic glass shards, and including plagioclase, quartz (including β-quartz), orthopyroxene, hornblende, cummingtonite, and occasionally biotite. The characteristic fact of Niijima Origin tephra is that RIs of heavy minerals (orthopyroxene and hornblende) has broad variations. Especially, RIs of orthopyroxene in Akasakimine tephra series, Miyatsukayama tephra series, and Miyatsukayama Nannbu tephra series show the bimodal or trimodal histograms within the limit of 1.700-1.730, including high RIs over 1.730 slightly, considered as close to ferrosilite. Also, there is the tendency that RIs of hornblende in Akasakimine tephra series and Miyatsukayama tephra series show the bimodal histograms or broad variations within the limit of 1.663-1.688. On the other hand, RIs of cummingtonite in Niijima Origin tephra during 60 ka show the unimodal histograms within the limit of 1.658-1.669. RIs of felsic volcanic glass shards in Niijima Origin tephra show within the limit of 1.492-1.501. By comparison that common range of RIs on Quaternary tephra in Japan is 1.501-1.505 (Machida and Arai, 1992), RIs of Niijima Origin tephra would be classified to lower RI groups in Japanese Quaternary tephras.
The content of SiO2 in volcanic glass shards is 77.7-78.2 wt%, Na2O+K2O is 7.2-7.7 wt% generally, excluding some tephra which range is within the limit of 7.0-8.3 wt%. The content of FeO* is 0.7-1.0 wt% and high-SiO2 is harmonious to the lower RIs of volcanic glass shards (Furuta et al.,1986; Kikkawa, 1990).