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

2014年沿着伊豆-小笠原-马里亚纳岛弧进行的IODP第352次考察是了解俯冲开始期间岩浆过程的独特机会。在考察期间，收集了弧前玄武岩（FAB）和不同类型的玻安岩。从FABs玻安岩的过渡被解释为从纯减压驱动的地幔熔融（与生产FABs）的俯冲相关的含水通量熔融（与生产玻安岩）与逐渐增加的俯冲相关的组件的变化的结果。玻安岩岩浆从俯冲早期的低硅玻安岩（LSB）到俯冲晚期的高硅玻安岩（HSB）的成分演化，与俯冲开始时的地幔重组有关。在这个项目中，玻安岩的岩石学研究，辅以高压实验，被用来跟踪地幔熔融条件和岩浆管道系统在俯冲启动（从LSB到HSB的过渡）的演变。对岩浆生成、岩浆储存和岩浆分异条件（特别是熔融反应、压力P、温度T、熔体H2O浓度、氧逸度）的研究将基于对来自不同玻安岩系列的IODP样品中的矿物和玻璃（主要和微量元素浓度）的分析，这些样品代表俯冲开始期间的不同阶段。熔体和矿物成分（特别是橄榄石）将用于应用最近校准的温度计和氧气压计。岩石学方法将通过从高压实验中获得的约束来补充。将进行熔化实验和近液相线实验，以限制导致由耐火材料哈氏硬度产生的LSB和HSB初级熔体形成的熔化反应和P-T条件。结晶实验将在不同压力下进行，LSB和HSB组成代表不同分化阶段。天然液体下降线和矿物成分（橄榄石，单斜辉石，斜方辉石）将这些实验结果进行比较，以限制分化发生的条件，因此，岩浆储存条件沿着管道系统。将进行减压实验以限制斜方辉石和单斜辉石在分馏过程中的作用。这种互补的地球化学和实验方法的结果将提供关键的定量信息（P，T，挥发逸度），以模拟俯冲开始过程中岩石圈的重组。

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.