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

2014年沿伊豆-波宁-马里亚纳弧进行的IODP 352考察是了解俯冲开始期间岩浆过程的独特机会。考察期间，采集了弧前玄武岩（FABs）和不同类型的博长岩。从FABs到boninite的转变被解释为由纯粹的减压驱动的地幔熔融（与FABs的产生有关）到与俯冲相关的含水通量熔融（与boninite的产生有关）的变化，与俯冲相关的成分逐渐增加。从俯冲早期形成的低硅质博长岩（LSB）到俯冲晚期形成的高硅质博长岩（HSB），其岩浆成分演化与俯冲起始阶段的渐进式地幔重组有关。本项目通过岩石学研究，结合高压实验，对俯冲起始（从LSB向HSB过渡）过程中地幔熔融条件和岩浆管道系统的演化进行了研究。岩浆生成条件、岩浆储存条件和岩浆分异条件（特别是熔融反应、压力P、温度T、熔体H2O浓度、氧逸度等）的研究将基于对俯冲起始不同阶段代表的不同bonntic系列IODP样品中矿物和玻璃（主微量元素浓度）的分析。熔体和矿物成分（特别是橄榄石）将用于最近校准的温度计和氧气压计。岩石学方法将得到高压实验约束条件的补充。将进行熔炼实验和近液相实验，以限制熔炼反应和p - t条件，从而形成由难熔铝辉石生成的LSB和HSB初级熔体。结晶实验将在不同的压力下进行，LSB和HSB组成代表不同的分化阶段。将与这些实验结果（橄榄石、斜辉石、正辉石）进行比较，以限制分异发生的条件，从而限制岩浆沿管道系统的储存条件。减压实验将约束正辉石和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、挥发性逸度），以模拟俯冲开始时岩石圈的重组。

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.