Collaborative Research: Geochemistry of IODP Site 1438 and West Philippine Basin Volcanic Rocks: Constraints on Subduction Initiation and the Early Development of the Izu-Bonin-Mar

全球性的俯冲过程，即地球的构造板块下沉到其他板块之下，伴随着导致地震、岩浆生成和火山爆发等自然灾害的物理和化学过程。它也是许多具有经济价值的矿物形成的基础。详细了解俯冲过程对于保护人类利益、定位资源和建立对地球的基本认识至关重要。本研究的重点是了解俯冲的初始阶段，特别是从早期俯冲到自我维持的俯冲带和火山弧的地球化学过程。样品来自西太平洋Izu-Bonin-Mariana地区的一个岩心，该岩心是2014年国际海洋发现计划（IODP）协调钻探探险的一部分。探险队回收了1600多米的岩心，保存了海底地层学，从俯冲板块的玄武岩基础延伸到上覆的火山弧沉积物。研究的目的是利用基底岩的地球化学组成和火山沉积过程来了解俯冲起始、岩浆形成的原因以及火山弧喷发的时间和程度。这项工作的更广泛影响包括与来自日本、瑞士和英国的研究人员的合作。此外，牵头机构是一个大型的、公共的、为西班牙裔和少数族裔服务的机构，这为在研究项目中吸引代表性不足的少数族裔学生提供了潜力。其他影响包括支持EPSCoR州（南卡罗来纳州）的一个机构，并向佛罗里达州迈阿密的K-12公立学校提供服务。该项目的一个成果将是对IODP 351号探险队在1438号地点回收的火山岩进行全面的地球化学分析，该地点位于九州-帕劳山脊以西，是伊扎-波宁马里亚纳（IBM）弧的起始地点。初步结果表明，ODP Site 1438的洋基底与形成IBM前弧序列基底的前弧玄武岩年龄接近。在这次考察之前，人们认为在俯冲边缘形成的岩浆是具有拉斑岩特征的“前弧玄武岩”，与洋中脊玄武岩相似，但不完全相同，其次是博宁岩。初步研究结果表明，这种初始岩浆活动可能并不局限于弧前，而是在俯冲和弧起生期间发生在板块边缘的大片地区。本研究获得了1438、ODP站点1201和DSDP站点447的基底玄武岩的全岩主元素、微量元素和Hf-Nd同位素地球化学数据。对1438遗址基底上的火山碎屑沉积岩中分离出的碎屑进行类似的研究，将描绘出弧状层状火山最早的产物及其成分随时间的演变。建议的研究将全面确定弧前玄武岩与基底玄武岩的地球化学相似性。它还将记录1438遗址可能记录的成分随时间的变化。这项工作的结果可能会导致对目前关于俯冲起始岩浆产生的假设的修正。Site 1438上覆的火山碎屑剖面为研究岛弧的化学演化提供了前所未有的机会，从俯冲起始到弧的生长和成熟，再到裂谷和弧后盆地形成导致的弧火山作用的减弱/停止。

The global process of subduction, which is the sinking of Earth's tectonic plates beneath other plates, is accompanied by physical and chemical processes that cause natural hazards such as earthquakes, magma generation, and volcanic eruptions. It is also the basis of the formation of many economically valuable minerals. Understanding subduction processes in detail is of critical importance for protecting human interests, locating resources, and building a basic knowledge of the Earth. This research focuses on understanding the initial stages of subduction, specifically the geochemical processes, that lead from incipient subduction to a self-sustaining subduction zone and volcanic arc. Samples come from a core drilled in the Izu-Bonin-Mariana area in the western Pacific Ocean as part of a coordinated drilling expedition of the International Ocean Discovery Program (IODP) in 2014. The expedition recovered over 1600 meters of core, preserving seafloor stratigraphy extending from the basaltic foundation of the subducting plate through overlying volcanic arc sediments. Goals of the research are to use the geochemical composition of the basement rocks and volcanic sedimentary processes to understand subduction initiation, the causes of magma formation, and the timing and extent of volcanic arc eruptions. Broader impacts of the work include collaboration with researchers from Japan, Switzerland and the UK. In addition the lead institution is a large, public, Hispanic- and minority-serving institution, which provides the potential for engaging under-represented minority students in the sciences in the research project. Additional impacts include support of an institution in an EPSCoR state (South Carolina) and outreach to Miami, Florida K-12 public schools. One outcome of this project will be comprehensive geochemical analyses of volcanic rocks recovered on IODP Expedition 351, at Site 1438, which is located just west of the Kyushu-Palau Ridge, the site of Izu-Bonin Mariana (IBM) arc inception. Preliminary findings indicate that oceanic basement at ODP Site 1438 is close to the age of forearc basalts which form the base of the IBM forearc sequences. Prior to the expedition, it was thought that magmas formed at subducting margins were "forearc basalts" of tholeiitic character, similar but not identical, to mid-ocean ridge basalts, followed by boninites. Preliminary findings demonstrate that this initial magmatism may not have been limited to the forearc, but may instead have occurred over a broad area of the plate margin during subduction and arc initiation. This research produces whole-rock major element, trace element, and Hf-Nd isotope geochemical data for basement basalts at Site 1438, at ODP Site 1201, and at DSDP Site 447. Similar studies of clasts separated from volcaniclastic sedimentary rocks that overlie basement at Site 1438 will characterize the earliest products of arc stratovolcanoes and their compositional evolution through time. The proposed studies will establish the full extent of geochemical similarity between forearc and basement basalts. It will also document compositional changes with time that may be recorded at Site 1438. Results of this work will likely lead to a revision of current hypotheses about magma generation during subduction initiation. The overlying volcaniclastic section of Site 1438 provides an unprecedented opportunity to study the chemical evolution of an island arc, from subduction initiation through arc growth and maturity, through the waning/cessation of arc volcanism as the result of rifting and back-arc basin formation.