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

全球俯冲过程，即地球构造板块下沉到其他板块之下，伴随着引发自然灾害的物理和化学过程，如地震、岩浆生成和火山喷发。它也是许多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知识至关重要。这项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。样本来自在西太平洋伊豆-博宁-马里亚纳地区钻探的岩芯，这是2014年国际海洋发现计划(IODP)协调钻探考察的一部分。考察队发现了超过1600米的岩芯，保存了从俯冲板块的玄武岩基础延伸到覆盖的火山弧沉积物的海底地层。这项研究的目的是利用基岩的地球化学成分和火山沉积过程来了解俯冲作用、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日本、瑞士和英国的研究人员合作。此外，牵头机构是一个大型的公共、拉美裔和少数群体服务机构，它提供了在研究项目中吸收代表人数不足的少数族裔学生参与研究项目的潜力。其他影响包括对EPSCoR州(南卡罗来纳州)的一所机构的支持，以及对佛罗里达州迈阿密K-12公立学校的外联。该项目的一项成果将是对IODP考察队第351次考察队在1438号场址发现的火山岩进行全面的地球化学分析，该考察队位于伊祖-博宁马里亚纳(IBM)弧源所在地九州-帕劳山脊的西侧。初步研究结果表明，ODP1438站位的大洋基底与构成IBM弧前序列基础的弧前玄武岩的年龄接近。在考察之前，人们认为在俯冲边缘形成的岩浆是拉斑玄武岩性质的“弧前玄武岩”，与大洋中脊玄武岩相似但不完全相同，其次是玻安岩。初步研究结果表明，这种最初的岩浆作用可能并不局限于弧前，而是在俯冲和弧初的过程中发生在板块边缘的大片区域。这项研究提供了1438点、1201点和447点基性玄武岩的全岩常量元素、微量元素和Hf-ND同位素地球化学数据。对1438号遗址基底上覆的火山碎屑沉积岩分离出的碎屑进行了类似的研究，将表征弧形层状火山的最早产物及其成分随时间的演变。拟议的研究将建立弧前玄武岩和基底玄武岩之间的全面地球化学相似程度。它还将记录可能在1438号站点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开始期间岩浆产生的假设的修正。1438遗址上覆的火山碎屑段为研究岛弧的化学演化提供了前所未有的机会，从俯冲开始到弧的生长和成熟，再到由于裂谷作用和弧后盆地形成而导致的弧火山活动的减弱/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.