Forearc basalt and boninite lineage: origin and evolution of magmas in the course of subduction initiation, Izu-Bonin Mariana arc

2014年沿着伊苏-博宁-马里亚纳(IBM)弧进行的IODP 352次考察是一个独特的机会，可以了解俯冲开始和随后的岛弧演化期间的地质过程。在考察过程中，采集了弧前玄武岩(FAB)和玻安岩，并解释为一种岩石类型向另一种岩石类型的转变是地幔熔融条件从减压驱动熔融(FAB)向水合熔剂熔融(玻安岩)转变的结果。俯冲启动过程中熔融状态的这种变化预计也会伴随着氧化还原条件的变化。俯冲启动过程中地幔重组过程中岩浆生成演化和岩浆分异条件是当前研究的中心问题。对4个钻探岩芯的岩石学研究将被用来阐明以下基本问题：(1)FAB和玻安岩原生岩浆产生的条件是什么；(2)FAB和玻安岩火山活动之间过渡阶段的岩浆储存和分异条件是什么(深度、fO2、挥发分状况)；(3)地幔熔融条件(特别是fO2、挥发分和熔融度)的变化如何控制氧化还原敏感的亲铁元素(例如Au、Ag)和亲铜元素(例如Cu、Zn)的预算和行为。将利用补充分析技术解决这些问题，包括微探针(玻璃和矿物的主要元素分析)、FTIR(玻璃中的挥发性浓度)和激光烧蚀技术(玻璃中的微量元素分析)、液体下降线的热力学模拟和阐明Izu-Bonin-Mariana Fab和玻安岩的形成和分异的高压实验研究。这一结果也将对确定超俯冲带蛇绿岩中预期的地球化学关键特征具有重要意义，这些特征被解释为与反映俯冲开始的阶段有关的地球动力学特征。

The IODP Expedition 352 conducted in 2014 along the Izu-Bonin-Mariana (IBM) arc is a unique opportunity to understand the geological processes during the initiation of subduction and the subsequent island arc evolution. During the expedition, both forearc basalts (FAB) and boninites have been collected and the transition from one to the other rock type is interpreted to result from a change of the mantle melting conditions from decompression-driven melting (FAB) to hydrous flux melting (boninite). This change of the melting regime during subduction initiation is also expected to be accompanied by changing redox conditions. The evolution of magma generation and magma differentiation conditions in the course of mantle reorganization during subduction initiation is the central issue of the current research project. A detailed petrological study of selected samples from 4 drilling cores will be used to clarify following fundamental questions: (1) what are the conditions of FAB and boninite primary magma generation, (2) what are the conditions of magma storage and differentiation at the transition between FAB and boninite volcanic activity (depth, fO2, regime of volatiles) and (3) how does the change of the mantle melting conditions (particularly fO2, volatiles and degree of melting) controls the budget and behavior of redox-sensitive siderophile (e.g. Au, Ag) and chalcophile (e.g. Cu, Zn) elements. These issues will be worked out using complementary analytical techniques, including microprobe (major element analyses of glasses and minerals), FTIR (volatile concentration in glasses) and Laser ablation techniques (trace element analyses in glasses), thermodynamic modeling of liquid lines of descent and high pressure experimental studies to clarify the formation and differentiation of Izu-Bonin-Mariana FAB and boninites. The results will also be of importance to establish the geochemical key characteristics expected in supra-subduction zone ophiolites, interpreted to be geodynamically related to a stage reflecting the initiation of subduction.