Arc foundations and the Initiation of subduction in the Izu-Bonin forearc
Our understanding of the lower crust in the ocean basins has been inferred from geophysical studies of mid-ocean ridges (MOR), volcanic arcs, and ophiolites. Many studies of ophiolites suggest that although they exhibit characteristics of MOR’s, they may originate in supra-subduction zone (SSZ) settings. It is also suggested that the foundations and lower crust of SSZ arcs are created by a MOR, possibly in a trench-ridge-trench triple junction setting. Although many samples of the lower crust have been collected along the world’s MOR’s, few samples have been retrieved from SSZ arcs. Fifty ultramafic and gabbroic samples recovered by dredges 31 and 42 of the KH07-02 dredging cruise along the inner trench wall of the Izu-Bonin Arc have been characterized as lower crustal rocks related to MOR-like basalts (fore-arc basalts or FAB). Major element analyses of Cr-spinels indicate two distinct compositional trends. Group M consists of wehrlites and gabbros with medium Cr# (45-60) and high Al2O3 and TiO2 (12-30 & 0.1-2.25 wt. %) which reacted with MORB-like melts. Group B consists of spinels solely from dunites and peridotites with high Cr# (65-94) and low Al2O3 and TiO2 (3-21 & 0-0.12 wt. %) which reacted with boninitic melts. Boninites from the Bonin Ridge are known to be younger (44-48 Ma) than FABs (50-52 Ma). This suggests that the majority of the lower crust was related to subduction initiation. Major and trace element analyses of clinopyroxenes associated with Group M spinels indicates highly depleted compositions characteristic of high degree partial melts or boninites. A lack of orthopyroxene and association with Group M spinel suggests that D31 clinopyroxenes evidence singly depleted, un-aggregated melt fractions. This suggests that FABs are the result of mixing between a highly depleted mantle source and a MORB-like liquid. Modeling results in 5 to 7% fractional melts of DMM refertilized by the addition of ~12% N-MORB reproducing such a mixing array. This model is consistent with models for subduction initiation which suggest that asthenosphere will upwell into the void created between the down-going and overriding plates during initiation and begin to undergo decompression melting.