Geochemical and sedimentological evidence used to characterise supra-subduction spreading processes in the Izu-Bonin-Mariana forearc (IODP leg 352)

计划中的工作直接符合IODP第352条规定的目标，即钻探伊扎-博宁-马里亚纳（IBM）系统外前弧的火山地层。主要目的是为了更好地了解与俯冲起始有关的岩浆活动、构造和地壳增生过程。反过来，这对理解许多蛇绿岩的起源有意义，这些蛇绿岩被认为是在这个环境中形成的，因此是检验超俯冲带蛇绿岩模型的好机会。Bonin前弧的两个地点（BON-1和BON- 2）形成了一个偏移钻井对，它们一起应该能穿透一个完整的1.25公里熔岩剖面。该剖面的钻探将有助于我们对洋内辐合板块边缘的认识，提供：1)俯冲起始期岩浆演化的高保真记录；2)对“弧前玄武岩”（FAB）拉斑岩位于博美岩下假说的检验；3)这些单元内及其过渡期间的化学梯度记录；4)从早期富软流圈的减压熔融到晚期衰竭地幔的通量熔融，揭示了俯冲起始过程中地幔熔融过程的演化过程，为实际俯冲起始地球动力学模型提供了关键的经验约束；5)对俯冲起始时形成的弧前岩石圈是超俯冲带蛇绿岩诞生地假说的验证。申请人（A.H.F. Robertson）以沉积学家的身份航行，因此有责任研究和解释任何沉积材料，从而尽可能地为该项目的总体目标做出贡献。拟建的沉积学和地球化学研究应直接有助于本研究的目标1、2、3和5，具体如下：目标1：在挤出演化带内部和上面回收的沉积物的化学记录与俯冲开始时的岩浆记录直接相关，因为这些信息可以表明钻探地点周围的化学变化，包括局部（数百米）和更广泛（公里至数十公里）；目的2：确定拉斑岩和波闪岩的组成，特别是任何熔岩间沉积物的组成，将是重要的，因为这些可能有助于揭示区域岩浆地层学；目的3：沉积物组成将有助于确定在原位火成岩中确定的化学梯度在区域尺度上是否一致。目标5：沉积物将被用来绘制钻探地点周围区域的地图，从而有助于了解海底在超俯冲带环境下的发育情况。此外，对基底以上层序较高位置的沉积物进行化学研究，将有助于了解在超俯冲带扩张阶段之后最早的弧岩浆活动的时间和组成。也可能有来自更偏远地区的输入；例如，可以通过分析空降火山玻璃（tephra）来探测到的遥远的火山弧。申请人可以对这项工作作出贡献，但这项工作将由船上的另一位沉积学家斯蒂芬·库特洛夫领导。必要的资料将通过化学、矿物学和沉积学的观察和分析获得，这些观察和分析将主要在爱丁堡大学地球科学学院进行。这项工作将包括使用XRF和ICPMS技术进行化学分析，使用XRD和电子探针进行矿物分析以及使用离子探针进行火山玻璃分析。该提案支持由英国主要参与的钻探考察，包括英国联合首席科学家，并建立在NERC在西南太平洋地区的投资和多年来陆地上可比较的蛇绿岩环境的基础上。这项工作将涉及与船上同事的合作，特别是与另一位沉积学家斯特凡·库特洛夫（Stefan Kuttterolf）。

The planned work, which is directly in line with stated aims of IODP Leg 352, is to drill a section through the volcanic stratigraphy of the outer forearc of the Izu-Bonin-Mariana (IBM) system. The main aims are to better understand the processes of magmatism, tectonics and crustal accretion associated with subduction initiation. This, in turn, has implications for understanding the origin of the many ophiolites that are believed to form in this setting and so is a good opportunity to test the supra-subduction zone ophiolite model. Two sites in the Bonin forearc (BON-1 and BON- 2) form an offset-drilling pair that together should penetrate a full c.1.25 km lava section. Drilling this section will contribute to our understanding of intra-oceanic convergent plate margins by providing: 1) A high-fidelity record of magmatic evolution during subduction initiation; 2) A test of the hypothesis that "Fore-arc Basalts" (FAB) tholeiites lie beneath boninites;3) A record of the chemical gradients within these units and across their transitions; 4) Information on how mantle melting processes evolve during subduction initiation from early decompression melting of fertile asthenosphere to late flux melting of depleted mantle, providing key empirical constraints for realistic subduction initiation geodynamic models; and 5) A test of the hypothesis that forearc lithosphere created during subduction initiation is the birthplace of supra-subduction zone ophiolites. The applicant (A.H.F. Robertson) is sailing as a sedimentologist and, therefore, has responsibilities to study and interpret any sedimentary materials and so contribute to the overall objectives of the leg as far as possible. The proposed sedimentological and geochemical research should contribute directly to objectives 1,2,3 & 5 of the leg, as follows:Objective 1: A chemical record of the sediments recovered within and above the extrusive succession is directly relevant to the magmatic record during subduction initiation because this information can indicate chemical variability around the drill site, both locally (hundreds of metres) and more widely (kms to tens of kms);Objective 2: It will be important to determine the tholeiitic vs boninitic composition, in particular, of any interlava sediments as these may help to reveal the regional magmatic stratigraphy;Objective 3: The sediment composition will help determine whether the chemical gradients that are determined within the in situ igneous rocks are consistent on a regional scale.Objective 5: Sediments will be used to, in effect, map the region around the drill site and so contribute to an understanding of how sea floor develops in a supra-subduction zone setting. In addition, chemical study of sediments higher in the sequence above the basement will contribute information about the timing and composition of the earliest arc magmatism that follows the supra-subduction zone spreading phase. There may also be input from more remote locations; e.g. distant volcanic arc that may be detected by analysis of airfall volcanic glass (tephra). The applicant may contribute to this work, which will, however, be lead by the other shipboard sedimentologist, Stefan Kutterolf. The necessary information will be obtained by chemical, mineralogical and sedimentological observations and analyses which will mainly be carried out at the School of GeoSciences, University of Edinburgh. This work will involve chemical analysis by the XRF and ICPMS techniques, mineral analysis using XRD and electron probe and also volcanic glass analysis using the ion probe. The proposal supports a drilling expedition in which there is major UK involvement, including a UK Co-chief scientist and which builds on NERC investment in the SW Pacific region and comparable ophiolitic settings on land over many years. The work will involve collaboration with shipboard colleagues, especially Stefan Kuttterolf, the other sedimentologist.