Volcanic History of the Izu Bonin Mariana Arc Prior to the First Arc Rift from Volcaniclastic Sediments of DSDP Site 296 and IODP Site 1438

Abstract

The Izu-Bonin Mariana (IBM) island arc in the western Pacific basin is a volcanic chain formed by subduction of the Pacific lithospheric plate beneath the Philippine Sea plate. Worldwide, subduction causes high-magnitude earthquakes and explosive volcanic activity in addition to forming the Earth’s continental crust and major ore deposits, thus its understanding is important to human concerns. In the IBM, volcanic activity spans 50 million years to the present and the arc has undergone extension and periodic rifting, preserving remnants of its early history. The Kyushu Palau ridge (KPR), an inactive Oligocene remnant of the rifted IBM system, is a unique window to understanding the early subduction processes. In this work, contemporaneous volcaniclastic sediments drilled at Deep Sea Drilling Project (DSDP) Site 296, located in a basin at the crest of the northern KPR, and volcaniclastic sediments drilled at IODP (International Ocean Discovery Program) Site 1438, in the nearby Amami-Sankaku basin, were studied and compared to understand the early volcanic history of the IBM, from the early Oligocene to arc rifting and opening of the Shikoku basin in the early Miocene. Grains of feldspar, pyroxene and amphibole, together with enclosed melt inclusions, glass grains and lithic fragments were separated from the sediment at intervals along the drilled cores and analyzed for major and trace elements and radiogenic isotopes. In-situ analysis on minerals, glass grains and melt-inclusions was performed using electron probe microanalysis (EPMA) and laser-ablation inductively coupled plasma mass-spectrometry (LA-ICPMS). Lithic fragments were dissolved and analyzed for elemental abundances by solution introduction ICP-MS and for isotopes by multi-collector-ICP-MS. Mineral compositions were used to calculate equilibrium magma compositions using major element matched partition coefficients, to compare with glass grains and lithic fragments. Findings show that magma compositions of the arc became progressively more water-rich with time, with periods of explosive eruption evidenced by pumice layers, stabilization of amphibole and Ca-rich plagioclase feldspar. Interspersed incompatible element-depleted mafic magmas at intermediate depths in the Site 296 core probably represent the initial intrusions associated with arc rifting which may have begun contemporaneously with arc volcanism.