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段规定的目标，即钻穿伊祖-博宁-马里亚纳(IBM)系统外弧外火山地层学。主要目的是为了更好地了解与俯冲开始有关的岩浆作用、构造和地壳增生过程。这反过来又意味着了解许多蛇绿岩的起源，这些蛇绿岩被认为是在这种背景下形成的，因此是检验超俯冲带蛇绿岩模型的好机会。博宁弧前的两个地点(Bon-1和Bon-2)形成了一对偏移钻探对，它们加在一起应该能穿透整个1.25公里的熔岩段。钻探这一段将有助于我们理解大洋内部会聚的板块边缘，因为它提供了：1)俯冲开始期间岩浆演化的高保真记录；2)“弧前玄武岩”(FAB)拉斑玄武岩位于博安岩之下的假设的检验；3)这些单元内部及其过渡过程中的化学梯度的记录；4)俯冲开始过程中地幔熔融过程的演化信息，从富饶的软流圈的早期减压熔融到亏损地幔的晚期熔融，为现实的俯冲开始地球动力学模型提供了关键的经验约束；5)对俯冲开始时形成的弧前岩石圈是俯冲带蛇绿岩的发源地这一假说的检验。申请人(A.H.F.罗伯逊)是一名沉积学家，因此有责任研究和解释任何沉积材料，从而尽可能为专家组的总体目标作出贡献。拟议的沉积学和地球化学研究应直接促进专家组的目标1、2、3和5，如下：目标1：在俯冲开始期间，在喷出序列内部和之上恢复的沉积物的化学记录与岩浆记录直接相关，因为这一信息可以表明钻探地点周围的化学变化，包括局部的(数百米)和更广泛的(公里到数十公里)；目标2：确定拉斑玄武岩和碳酸盐岩的成分将是重要的，因为这可能有助于揭示区域岩浆地层学；目标3：沉积物的组成将有助于确定原地火成岩中确定的化学梯度在区域尺度上是否一致。目标5：沉积物将被用来有效地绘制钻探地点周围的区域，从而有助于了解海底在超俯冲带背景下是如何发育的。此外，对基底上方较高层序的沉积物进行化学研究，将有助于了解伴随着俯冲带扩张阶段的最早弧岩浆活动的时间和成分。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)探测到的遥远火山弧。申请者可以为这项工作做出贡献，但这项工作将由另一位船上的沉积学家斯特凡·库特罗夫领导。必要的信息将通过主要在爱丁堡大学地球科学学院进行的化学、矿物学和沉积学观察和分析来获得。这项工作将包括使用XRF和ICPMS技术进行化学分析，使用X射线衍射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。该提案支持有英国主要参与的钻探探险，其中包括一名英国联合首席科学家，该探险建立在NERC在西南太平洋地区的投资以及多年来陆地上类似的诡辩背景的基础上。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·库特尔夫的合作。

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.