Collaborative Research: Geochemical variation of the Pacific crust subducting beneath the Izu-Bonin arc and its implications for the generation of arc magmas

伊豆-小笠原弧是世界上研究最多的火山弧之一，铅同位素是研究蚀变洋壳再循环的重要同位素示踪剂之一。人们普遍认为，在俯冲带上方沿沿着火山弧喷发的岩浆与通过深海沟循环回到地幔的俯冲蚀变洋壳之间存在着很强的成分联系。也就是说，目前可用的Pb元素和同位素数据伊豆小笠原弧岩浆和相关的俯冲蚀变洋壳与这个模型不一致，提出了猜测，即蚀变洋壳可能不是，事实上，弧岩浆的一个重要来源，这使我们对弧岩浆活动的整体理解产生了疑问。这项研究将利用新的和已经收集的洋壳样品的地球化学分析（元素和同位素），以确定在伊豆-小笠原岛弧-海沟系统中俯冲带输入和岛弧输出之间是否存在差异。将利用联合国海洋法办事处的一艘船舶，沿沿着一条约900公里的断面，以太平洋板块沿着活动的垂直断层陡坎为目标，从海底挖掘玄武岩壳样品，该板块正在向伊豆-小笠海沟俯冲。该断面允许对一系列地壳年龄的岩石进行采样，其中包括来自中生代太平洋地壳成分在~125 Ma发生假设变化的地区的样本。将分析样品的化学成分，并将结果用于确定板输入/电弧输出的质量平衡。这项工作还将需要校准磁时标。调查结果将提供一个关键的测试，目前的概念，地壳再循环在俯冲带和模式的地壳提取地幔，其中俯冲过程中的碳起着关键作用。这项研究的目标与NSF资助的综合大洋钻探计划（IODP）的科学计划直接相关，并将为即将到来的四次IODP钻探探险提供必要的数据输入（计划于2014年进行的三次JOIDES Resolution腿和一次CHIKYU探险）。该项目的更广泛影响将促进研究人员之间的国家和国际合作，并将支持来自科学界代表性不足的团体的两名研究人员。该项目还将支持研究生和本科生，他们将完全融入该项目的科学发现方面。项目成果将通过NSF资助的加州海洋科学教育卓越中心，纳入为圣地亚哥联合学区教师举办的为期1天的研讨会。

The Izu-Bonin arc in the western Pacific Ocean is one of the best-investigated volcanic arcs in the world and lead (Pb) isotopes are one of the foremost isotopic tracers of the recycling of altered ocean crust. It is widely accepted that there is a strong compositional link between magmas erupted along volcanic arcs above subduction zones and subducted altered oceanic crust that is cycling back down into the mantle via deep ocean trenches. That said, currently available Pb elemental and isotopic data for Izu-Bonin arc magmas and the associated subducting altered ocean crust are at odds with this model, raising the spector that altered ocean crust may not, in fact, be an important source of arc magmas, which throws into question our understanding of arc magmatism as a whole. This research will use geochemical analyses (elemental and isotopic) of new and already collected samples of ocean crust to determine whether there is a discrepancy between subduction zone input and arc output in the Izu-Bonin arc-trench system. Using a UNOLS ship, samples of basaltic crust will be dredged from the seafloor along a ~900 km transect targeting active, vertical fault scarps along the Pacific Plate which is subducting into the Izu-Bonin Trench. This transect allows sampling of rocks of a range of crustal ages, which will include samples from the area where there is a hypothesized change in the composition of Mesozoic Pacific crust at ~125 Ma. Samples will be analyzed for chemical composition and the results will be used to determine mass balances of slab input/arc output. The work will also entail a calibration of the magnetic timescale. Results of the investigation will provide a crucial test of current concepts on crustal recycling at subduction zones and modes of crustal extraction from the mantle in which the subduction processing of carbon plays a pivotal role. Goals of this research are linked directly to the science plan of the NSF-funded Integrated Ocean Drilling Program (IODP) and will provide essential data input to four upcoming IODP drilling expeditions (three JOIDES Resolution legs scheduled for 2014 and one CHIKYU expedition that is pending). Broader impacts of the project will foster national and international collaboration among the investigators and will support two investigators from groups under-represented in the sciences. The project also will support graduate and undergraduate students who will be fully integrated into the science discovery aspect of the project. Project results will be incorporated into a 1-day workshop for teachers from the San Diego Unified School District through the NSF-funded Center for Ocean Science Education Excellence in California.