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

2014年沿着伊扎-波宁-马里亚纳岛弧进行的IODP 352考察是一个独特的机会，可以了解俯冲开始和随后岛弧演化的地质过程。考察期间采集了前弧玄武岩（FAB）和博长岩（boninite）两种岩石类型，解释了一种岩石类型向另一种岩石类型转变的原因是地幔熔融条件从减压驱动熔融（FAB）转变为水通量熔融（boninite）。在俯冲起始阶段，这种熔融状态的变化也可能伴随着氧化还原条件的变化。俯冲起始期地幔重组过程中岩浆生成和岩浆分异条件的演化是本课题研究的核心问题。从4个钻孔岩心中选取样本进行详细的岩石学研究，以澄清以下基本问题：(1) FAB和boninite原生岩浆产生的条件是什么；(2)FAB和boninite火山活动过渡时期岩浆的储存和分化条件是什么（深度、fO2、挥发物状态）；(3)地幔熔融条件（特别是fO2、挥发物和熔融程度）的变化如何控制氧化还原敏感的亲铁元素（如Au、Ag）和亲铜元素（如Cu、Zn）的数量和行为。这些问题将利用互补的分析技术来解决，包括微探针（玻璃和矿物的主要元素分析）、红外光谱（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建模和高压实验研究，以澄清伊豆-博宁-马里亚纳FAB和博宁矿的形成和分化。研究结果也将对建立超俯冲带蛇绿岩的地球化学关键特征具有重要意义，并解释为与反映俯冲起始阶段的地球动力学相关。

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.