HR: 1340h
AN: T53A-1399    [Abstracts]
TI: Co-existing wet and dry basaltic magmas at Torishima volcano, 100 km south of Sumisu caldera, Izu-Bonin arc; implications for arc magma genesis and crustal evolution
AU: * Tamura, Y
EM: tamuray@jamstec.go.jp
AF: IFREE, JAMSTEC, 2-15 Natsushima-cho, Yokosuka, 237-0061 Japan
AU: Tani, K
EM: kentani@jamstec.go.jp
AF: IFREE, JAMSTEC, 2-15 Natsushima-cho, Yokosuka, 237-0061 Japan
AU: Chang, Q
EM: qchang@jamstec.go.jp
AF: IFREE, JAMSTEC, 2-15 Natsushima-cho, Yokosuka, 237-0061 Japan
AU: Shukuno, H
EM: shukuno@jamstec.go.jp
AF: IFREE, JAMSTEC, 2-15 Natsushima-cho, Yokosuka, 237-0061 Japan
AU: Kawabata, H
EM: hiroshik@jamstec.go.jp
AF: IFREE, JAMSTEC, 2-15 Natsushima-cho, Yokosuka, 237-0061 Japan
AB: Basalts erupted from Sumisu caldera, Izu-Bonin arc, include both highly depleted, wet basalts and moderately depleted, dry basalts, which were defined as low-Zr basalt and high-Zr basalt, respectively (Tamura et al., 2005). Higher degrees of partial melting of the mantle source are closely related to higher water contents of the basalts. Tamura et al. (2005) suggested that within a single volcanic complex, the water content in the mantle wedge is heterogeneous, resulting in different degrees of partial melting of this source. This was the first documented example of co-existing wet and dry basalts in a single volcano from the Izu-Bonin arc, and it remains unclear if this feature is unique to, or even common within, the Izu-Bonin arc. Here, we propose that Torishima Island is a further example of both wet and dry basalts occurring in a single arc volcano, and suggest that this may be a common feature of the Izu-Bonin arc. Torishima (translated into English as bird-island) is only a small island (3 km in diameter), however it is the subaerial exposure of a large submarine volcano (the 466 km³ Torishima volcano), built from depths of > 1000 m below sea level. The 1902 eruption of Torishima killed all 125 inhabitants, and the island has been uninhabited since 1965, when the weather station was evacuated due to an earthquake swarm in 1965 and 1966. Torishima volcano is situated approximately 100 km south of Sumisu caldera, with no arc volcanoes between the two. Furthermore, ODP Site 788, located on the volcanic front between the two volcanoes, found no proximal lithologies such as lava flows or agglutinates that indicate any submarine volcanic centres at least back to Pliocene times. Thus, a comparative study of Sumisu and Torishima volcanoes is important to determine the temporal dimension of along-strike variations in mantle composition and crust production rates, and to clarify the role that hot fingers (Tamura et al., 2002) might play in the production of arc magmas and crustal evolution. Torishima Island is comprised of three groups of basaltic lavas, distinguished by small, but clear gaps in SiO₂ content (< 50 wt. %, ~51-53 wt. %, and ~54.5 wt. % SiO₂). Using FeO*/MgO as proxy for differentiation, the basalts form two trends distinguished by their different K₂O and Zr contents and Sr/Zr ratios at the same FeO*/MgO. Torishima Island basalts with SiO₂ content < 50 wt %, and basaltic rocks with > 50 wt % SiO₂ are comparable to the low-Zr and high-Zr basalts of Sumisu caldera (Tamura et al., 2005), respectively. All Torishima volcano basalts are strongly depleted in light rare earth elements (LREE), compared with middle and heavy REE. Importantly, low-Zr basalts at Torishima Island are more LREE depleted than high-Zr basalts, a feature also observed at Sumisu caldera. Our results for Torishima Island basalts suggest that wet and dry basalts co-existing within a single volcanic complex could be a common feature of Izu-Bonin arc volcanoes . A mantle diapir model (Tamura et al., 2005) could explain the petrogenesis of both high-Zr and low-Zr basalts at both Torishima Island and Sumisu Caldera.
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1037 Magma genesis and partial melting (3619)
DE: 1038 Mantle processes (3621)
DE: 1065 Major and trace element geochemistry
SC: Tectonophysics [T]
MN: Fall Meeting 2005