Geochemical compositions of the mid-ocean ridge basalt (MORB)-like basalts dredged from the trench slope of the Ogasawara Ridge and the Hahajima Seamount, southern Izu-Bonin fore arc are determined to obtain constraints on the tectonic evolution in the Izu-Bonin arc. The trench slope sample of the Ogasawara Ridge has a distinct enriched (E)-MORB chemistry unlike any other rocks previously sampled in the Izu-Bonin fore arc. The basalt is moderate TiO₂, low Zr/Nb, low Ba and is enriched in light rare-earth elements. This sample could represent a trapped remnant of the Philippine Sea plate on which the Izu-Bonin arc was built. On the other hand, two geochemical groups can be identified within the Hahajima Seamount basalts: Low-Ti type basalts and High-Ti type basalts. The Low-Ti type basalts are characterized by depleted in high-field strength (HFS) elements, lower ¹⁴³Nd/¹⁴⁴Nd ratio and high Zr/Nb compared with the High-Ti basalts. Trace element and isotopic compositions of the Low-Ti type basalts are similar to normal (N)-MORB from the West Philippine Basin. In contrast, the High-Ti type basalts show an overall Jurassic Pacific N-MORB signature, marked by high ¹⁴³Nd/¹⁴⁴Nd ratio and enriched in HFS elements. The available MORB-like basalts data show that the Low-Ti type basalts occur in the western side and the High-Ti type ones in the eastern trench side of the seamount. Geochemical signatures and occurrences of the two types of MORB-like basalts suggest that the Hahajima Seamount might compose an uplifted ophiolitic massif of the West Philippine Basin and an accreted piece of subducting Pacific Plate, which have been caused by collision and subduction of the Ogasawara Plateau on the Pacific Plate.

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  Aggregates composed of pentlandite, isocubanite and pyrrhotite occur in the basaltic andesite from Kasayama volcano, Hagi-city, Yamaguchi Prefecture, Japan. Fe content of pentlandite is slightly over the value of ideal formula (Fe_4.5Ni_4.5S)₈. Isocubanite is close to the ideal CuFe₂S₃. The aggregates of these sulfides may have been crystallized from sulfide melt having composition of pentlandite solid-solution in silicate melt. After that, pentlandite solid-solution exsolved to pentlandite and Cu-Fe-S intermediate solid solution (iss). The iss also exsolved to isocubanite and pyrrhotite.

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