ABSTRACT

In the Izu-Ogasawara arc, many Quaternary volcanoes are located in a line parallel to the Izu-Ogasawara Trench. ⁸⁷Sr/⁸⁶Sr ratios were determined for 65 samples of Quaternary volcanic rocks, 4 cognate inclusions and one sample of basement rock from 15 volcanic islands and 3 submarine volcanoes. In this arc, the ⁸⁷Sr/⁸⁶Sr ratios in several rocks from the same volcano agree with one another within 0.0002, regardless of the difference of the rock types and the eruptive stages. The ratios of the volcanoes along the volcanic front in the Izu-Ogasawara arc were constant over 1, 200 km and were all within the range between 0.7034 and 0.7039, which is significantly lower than those from the Northeast Japan arc (0.7038–0.7044) (NOTSU, 1983). In the northern region of the Izu-Ogasawara arc, across-arc ⁸⁷Sr/⁸⁶Sr ratio variation was found. The ratios decrease with increasing depth of the Wadati-Benioff zone. Even in the volcanic rocks distant from the volcanic front, the ratios (0.7033–0.7035) were slightly higher than those in normal-type MORB, so it is suggested that a small amount of oceanic sediment, altered basalt or sea water component that accompanies the subducting slab has been contributed to the source magma which originates from a partially melted, upper mantle wedge. This contribution becomes smaller with increasing distance from the volcanic front.