28. Umino, S., 1996. Proposal for scientific drilling on the Bonin Islands. Proceedings of VIIIth International Symposium on the Observation of the Continental Crust Through Drilling, Tsukuba, Japan, 121 - 126.

The objectives of the scientific drilling on the Bonin Islands are:

(1) The nature of pre-boninite igneous basement and the timing and transition to subduction-related volcanism.

(2) The temporal variation in geochemistry of the magmas through the pre-boninite and boninite volcanism.

(3) The plutonic equivalents of boninite-series volcanics to reveal magmatic processes in the boninitic magma chambers.

The boninitic volcanics are formed by melting of hydrous mantle in a unusually shallow (<24 km), high temperature (1250-1350 oC) regime and are extensively known from the outer-arc highs and frontal arc highs along the Izu-Bonin-Mariana arc. Such an extensive occurrence and required P-T-X conditions of boninite magmas in conjunction with the paleomagnetic and tectonostratigraphic data have led to tectonic reconstructions of the Philippine Sea in Eocene-Oligocene time, which are relevant to the initiation of an arc development from an oceanic environment. Continued subduction-related magmatism since then formed the present oceanic Izu-Mariana arcs, the northernmost part of which has well developed continental crust, now colliding as Izu Peninsula to the Honshu arc.

The Bonin Islands located on an Eocene frontal arc high are the only subaerial and the most developed exposures of the boninitic volcanoes in the Izu forearc region. In addition to the whole sequence of the boninite-series volcanic strata, the uppermost level of the dyke complex is exposed on land, which makes it easier to penetrate through the hidden dyke and plutonic complex down to the pre-boninite basement. Drilling on this site will give us clues to understand the transition from the oceanic to the subduction-related environment and the incipient stage of development of the continental crust.