Geochemical study on the seamounts along the Japan Trench

The Northwestern Pacific Plate is characterized by existence of a large number of seamounts (e.g., Koppers et al., 1998). The seamounts along the Japan trench, such as, Takuyo, Erimo and Ryofu-Daini Seamounts, and Joban Seamount Chain, are old seamounts on the Pacific Plate. These seamounts of Early to Late Cretaceous ages are on the Early Cretaceous Pacific Plat (Ozima et al. 1970; Takagi et al., 1989; Masalu et al., 1997; Nakanishi and Winterner 1998; Kobayashi et al., 1998). As many western Pacific Seamounts are linearly aligned, these seamounts are considered to be produced at the stationary hot spots. Although the Joban Seamounts produce a seamount chain, the chain was not in parallel with Pacific Plate motion during Jurassic to Cretaceous (Duncan and Clague, 1985), suggesting that the Joban seamount chain could not be produced by a stationary hot spot. In addition, the chain is neither parallel nor perpendicular to the magnetic lineation, suggesting that spreading ridge related processes can not explain the origin of the Joban seamount chain. Furthermore, other seamounts near the Japan Trench would not show any alignment. Therefore, the origin of these seamounts is still ambiguous and in controversy (e.g., Masalu et al., 1993, 1997). In order to evaluate the origin of these seamounts, we collected samples during Geological Survey of Japan (GSJ) Daini-Hakureimaru cruise 05GH. Drilling was conducted at 10 sites, i.e., Ryoufuu-Daini Seamounts (4 sites), Joban Seamount chain (6 site) including; Bousei Seamount (2 sites), Daisan-Kahisma Northwest Seamount (3 site) and Daisan Kashima Seamount (1 site). In addition, samples from the Daisan-Kahisma Northwest Seamount were also collected by dredging (2 sites). The collected samples are mainly alkali basalts and andesites. These lavas are porphyritic and poorly to moderately vesiculated. The phenocrysts are mainly plagioclase and pyroxene with/without hornblende. We will conduct a geochemical study on these new samples, including Pb, Nd and Sr isotopic and trace element analyses and constrain the origin of the seamounts along the Japan Trench.