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处的俯冲导致了强烈的地震、危险的火山爆发和具有重要经济意义的热液矿床。该项目将调查这一全球重要过程中一些最不为人所知的方面：是什么导致了它的开始，俯冲开始后产生了什么样的地壳，以及俯冲板块在最初下沉时释放了什么物质。国际海洋探索计划（ippc）第352远征队在西太平洋博宁弧前3.1 ~ 4.8公里的水深处钻了1.22公里的火山岩，调查了俯冲的开始。本研究对钻探考察中发现的弧前、海底、基性、火山岩（即玄武岩和博宁岩）进行了完整的地球化学和岩石学表征，以更好地了解俯冲带火山作用的开始和演化过程。在博宁弧前发现的岩石序列与许多蛇绿岩（在大陆上发现的海洋地壳和上地幔岩石）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。该项目支持并扩展了352远征队期间开展的广泛的国际科学合作，包括更多的顶级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由美国国家科学基金会资助的教育项目所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）机构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。本研究的首要目标是记录伊豆-博宁-马里亚纳弧系统俯冲开始后产生的第一地壳的性质。国际海洋探索计划第352次远征波宁弧前，从四个地点采集的样本将被分析。其中两个地点的钻孔岩心主要回收了弧前玄武岩和两个回收的博宁质。这项研究将与船上科学小组的其他成员合作，全面分析回收的火山岩，提供火山玻璃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；辉石中水浓度的测定；玻璃中的铁比；整个岩石和玻璃中流体流动元素和B-Li同位素的浓度；全岩和玻璃中放射性成因Re-Os同位素和PGE丰度；整块岩石的主量元素和微量元素组成。由此产生的数据将用于约束熔融过程，并了解从俯冲起始到岩浆生成模式从减压熔融到通量熔融的变化过程中，通过新生地幔楔的流体、固体和岩浆通量的变化。这些数据将用于解决四个主要的科学问题：(1)弧前玄武岩是在与俯冲起始相关的海底快速扩张过程中，在独特的高温低压环境中减压熔融形成的；(2)弧前玄武岩和boninites的地幔源是否异常枯竭，是否具有古地幔或大陆成分的再循环特征；(3) U1439和U1442钻孔点的低硅和高硅博长岩是在弧前玄武岩形成后的衰竭地幔中，随着扩张速率的降低和俯冲流体参与的增加而依次生成的；(4)岩浆的生成是否随着时间的推移和压力的降低而向陆地迁移。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，这项工作将解决美国国家科学基金会资助的地质科学计划中的问题，重点关注控制俯冲带起始和弧系统早期发展的物理和化学条件。

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.