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

西太平洋的伊豆-小笠原岛弧是世界上研究最充分的火山弧之一，铅 (Pb) 同位素是蚀变洋壳再循环的最重要的同位素示踪剂之一。人们普遍认为，沿俯冲带上方火山弧喷发的岩浆与通过深海海沟循环回地幔的俯冲蚀变洋壳之间存在着强烈的成分联系。也就是说，目前可获得的伊豆-小笠原岛弧岩浆和相关俯冲蚀变洋壳的铅元素和同位素数据与该模型不一致，这引发了人们的怀疑，即蚀变洋壳实际上可能不是弧岩浆的重要来源，这使我们对弧岩浆作用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本的地球化学分析（元素和同位素）来确定伊豆-博宁弧海沟系统中俯冲带输入和弧输出之间是否存在差异。使用UNOLS船，将沿着约900公里的横断面从海底挖掘玄武岩地壳样本，目标是沿着太平洋板块俯冲到伊豆-小笠宁海沟的活动垂直断层陡坡。该样线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约 125 Ma 时假设发生变化的区域的样本。将分析样品的化学成分，结果将用于确定板坯输入/电弧输出的质量平衡。这项工作还需要对磁时标进行校准。调查结果将为当前有关俯冲带地壳再循环的概念以及从地幔中提取地壳的模式提供重要的检验，其中碳的俯冲过程起着关键作用。这项研究的目标与 NSF 资助的综合海洋钻探计划 (IODP) 的科学计划直接相关，并将为即将进行的 4 次 IODP 钻探探险提供必要的数据输入（3 次 JOIDES 决议航段定于 2014 年进行，1 次 CHIKYU 探险正在等待中）。该项目的更广泛影响将促进研究人员之间的国内和国际合作，并将支持来自科学界代表性不足的群体的两名研究人员。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。项目结果将纳入由美国国家科学基金会 (NSF) 资助的加州海洋科学教育卓越中心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.