Collaborative Research: Subduction Initiation and Development of the Izu-Bonin-Mariana arc: An Investigation of Samples from Cores from Recent Ocean Drilling

海洋板块在全球深海海沟的俯冲导致强烈地震、危险的火山喷发和重要的经济热液矿藏。这个项目将调查这个全球重要过程中一些最鲜为人知的方面：是什么导致它开始，俯冲开始后会产生什么样的地壳，以及俯冲板块最初下沉时会释放出哪些物质。国际海洋发现计划352号在西太平洋博宁前弧水深3.1-4.8公里处钻探了1.22公里的火山岩，以调查俯冲的开始。这项研究对钻探期间发现的弧前、海底、镁铁质、火山岩(即玄武岩和玻安岩)进行了全面的地球化学和岩石学表征，以更好地了解俯冲带火山作用是如何开始和演化的。在博宁前弧遇到的岩石序列与许多蛇绿岩(大陆上发现的洋壳和上地幔岩石)中发现的岩石序列非常相似，因此该项目也将为蛇绿岩的起源提供见解。该项目支持并扩大了在352次远征期间开展的广泛的国际科学合作，以包括更多的顶尖女科学家以及本科生和研究生研究人员。这个项目的结果将成为正在进行的NSF资助的教育项目支持的课堂资源的一部分。它还将为EPSCoR三个州(爱荷华州、犹他州和罗德岛州)的研究人员提供资金，并促进与澳大利亚科学家的国际合作。这项研究的首要目标是记录伊豆-博宁-马里亚纳弧系俯冲开始后产生的第一个地壳的性质。将对在国际海洋发现计划352至博宁前弧期间钻探的四个地点收集的样本进行分析。其中两个地点的钻芯主要回收了弧前玄武岩和两个回收的博尼岩。这项研究将与船上科学小组的其他成员合作，对回收的火山岩进行全面分析，提供有关火山玻璃的主要元素和微量元素组成的地球化学数据。它还将完成对火山玻璃中氧同位素、水和二氧化碳浓度的分析；辉石中水浓度的测量；玻璃中铁的比率；整个岩石和玻璃中可流动元素和B-Li同位素的浓度；整个岩石和玻璃中的放射性Re-Os同位素和PGE丰度；以及整个岩石的主要元素和微量元素组成。所得数据将用于限制熔融过程，并了解自俯冲开始以来，随着岩浆生成模式从减压熔融转变为熔剂熔融，通过新生地幔楔体的流体、固体和岩浆通量的变化。这些数据将被用来解决四个基本的科学问题：(1)在海底快速扩张过程中，在独特的高温、低压环境中减压熔融而产生的弧前玄武岩是否与俯冲开始有关；(2)弧前玄武岩和博尼岩的地幔来源是否异常亏损，它们是否显示出古代再循环地幔或大陆成分的特征；(3)是来自钻探地点U1439和U1442的低硅和高硅博安岩，它们是在弧前玄武岩产生后，随着扩张速度的降低和俯冲流体的参与而陆续产生的枯竭地幔；(4)岩浆的生成是否随着时间的推移而向陆地迁移，并向低压力迁移。这些问题与NSF资助的国际海洋发现计划的初始科学计划的主要目标一致，包括测试蛇绿岩模型的有效性和了解大陆地壳的初始起源。此外，这项工作将解决NSF资助的GeoPRISMS科学计划中的问题，这些计划侧重于控制俯冲带启动和弧光系统早期发展的物理和化学条件是什么。

The subduction of oceanic plates at deep-sea trenches around the globe leads to powerful earthquakes, dangerous volcanic eruptions, and economically important hydrothermal mineral deposits. This project will investigate some of the least understood aspects this globally important process: what causes it to start, what kind of crust is produced after subduction starts, and what materials are released from the subducting slab as it initially sinks. The International Ocean Discovery Program Expedition 352 drilled 1.22 kilometers of volcanic rocks at water depths of 3.1-4.8 kilometers in the Bonin fore-arc in the western Pacific Ocean to investigate the start of subduction. This research performs a complete geochemical and petrological characterization of drilled fore-arc, seafloor, mafic, volcanic rocks, (i.e., basalts and boninites) recovered during the drilling expedition to better understand how subduction zone volcanism begins and evolves. The rock sequences encountered in the Bonin fore-arc are remarkably similar to those found in many ophiolites (ocean crust and upper mantle rocks found on the continents), so this project will also provide insights into the origin of ophiolites as well. This project supports and expands the extensive international scientific collaborations developed during Expedition 352 to include additional top-flight female scientists as well as undergraduate and graduate researchers. Results from this project will become part of classroom resources supported by an ongoing NSF-funded education project. It will also provide funding to researchers at institutions in three EPSCoR states (Iowa, Utah, and Rhode Island) and foster international collaboration with Australian scientists.The overarching goal of this research is to document the nature of the first crust produced after subduction initiation in the Izu-Bonin-Mariana arc system. Samples collected from four sites drilled during the International Ocean Discovery Program Expedition 352 to the Bonin fore-arc will be analyzed. Drill cores from two of the sites primarily recovered fore-arc basalts and two recovered boninites. This research, in collaboration with that being done by other members of the shipboard scientific party, will comprehensively analyze the recovered volcanic rocks, providing geochemical data on the major and trace element compositions of volcanic glasses. It will also complete analyses of oxygen isotopes, water and CO2 concentrations in the volcanic glasses; measurements of water concentrations in pyroxenes; Fe ratios in glasses; concentrations of fluid-mobile elements and B-Li isotopes in whole rocks and glasses; radiogenic Re-Os isotopes and PGE abundances in whole rocks and glasses; and major element and trace element compositions of whole rocks. Resulting data will be used to constrain melting processes and understand variations in the fluid, solid, and magma fluxes through the nascent mantle wedge from subduction initiation onward as the mode of magma generation changes from decompression melting to flux melting. The data will be used to address four principle scientific questions: (1) are fore arc basalts generated by decompression melting in a unique high temperature, low-pressure environment during rapid sea-floor spreading related to subduction initiation; (2) are the mantle sources of fore arc basalts and boninites unusually depleted and do they exhibit signatures of ancient recycled mantle or continental components; (3) were low- and high-Si boninites from drilling Sites U1439 and U1442 generated in succession from depleted mantle left after generation of fore arc basalts as spreading rates decreased and the involvement of subducted fluids increased; and (4) did magma generation migrate landward with time and to lower pressures. These questions align with the principal objectives of the NSF-funded International Ocean Discovery Program's Initial Science Plan, including testing the validity of the ophiolite model and understanding the initial origins of continental crust. Additionally the work will address questions in the NSF-funded GeoPRISMS science plan that focus on what the physical and chemical conditions are that control subduction zone initiation and the early development of arc systems.