Izu-Bonin-Mariana boninites - natural laboratory to study mantle melting and the evolution of magma plumbing systems in early stages of subduction

点击翻译按钮获取中文摘要

The IODP Expedition 352 conducted in 2014 along the Izu-Bonin-Mariana arc is a unique opportunity to understand the magmatic processes during the initiation of subduction. During the Expedition, fore-arc basalts (FABs) and different types of boninites have been collected. The transition from FABs to boninites is interpreted to result from a change from pure decompression-driven mantle melting (with production of FABs) to subduction-related hydrous flux melting (with production of boninite) with progressively increasing subduction-related components. The compositional evolution of boninite magmas from low silica boninites (LSB), formed during early stages of subduction, to high silica boninites (HSB), formed during late stages of subduction, is related to the progressive mantle reorganization during subduction initiation. In this project, petrological investigations of boninites, complemented by high pressure experiments, are used to trace the evolution of mantle melting conditions and magma plumbing systems during subduction initiation (transition from LSB to HSB). The investigation of the conditions of magma generation, magma storage and magma differentiation (especially melting reactions, pressure P, temperature T, melt H2O concentrations, oxygen fugacity) will be based on the analyses of minerals and glasses (major and trace element concentrations) in IODP samples from different boninitic series, representative of different stages during subduction initiation. Melt and mineral compositions (especially olivine) will be used to apply recently calibrated thermometers and oxy-barometers. The petrological approach will be complemented by constraints obtained from high pressure experiments. Melting experiments and near-liquidus experiments will be conducted to constrain melting reactions and P-T-conditions leading to the formation of LSB and HSB primary melts generated from refractory harzburgite. Crystallization experiments will be performed at different pressures with LSB and HSB compositions representative of different differentiation stages. Natural liquid lines of descent and mineral compositions (olivine, clinopyroxene, orthopyroxene) will be compared to these experimental results to constrain the conditions at which differentiation occurred and, therefore, magma storage conditions along the plumbing system. Decompression experiments will be performed to constrain the role of orthopyroxene and clinopyroxene in the fractionation processes. The results of this complementary geochemical and experimental approach will provide crucial quantitative information (P,T, volatile fugacities) to model the reorganization of the lithosphere during initiation of subduction.