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Izu-Bonin弧是世界上研究最多的火山弧之一，铅(Pb)同位素是洋壳蚀变再循环的最重要的同位素示踪之一。人们普遍认为，沿着俯冲带上方的火山弧喷发的岩浆与俯冲的蚀变洋壳之间存在着强烈的成分联系，这些洋壳正通过深海海沟循环回到地幔。也就是说，目前可用的伊祖-博宁弧状岩浆和与之相关的俯冲蚀变洋壳的铅元素和同位素数据与这一模型不一致，这增加了Spector的观点，即蚀变洋壳实际上可能不是弧状岩浆的重要来源，这使我们对弧状岩浆作用的整体理解产生了疑问。这项研究将利用新收集的和已经收集的洋壳样品的地球化学分析(元素和同位素)来确定Izu-Bonin弧沟系统中俯冲带输入和弧输出之间是否存在差异。利用联合国全球海洋观测系统的一艘船，将沿着一条约900公里的横断面从海底挖出玄武岩结壳样品，目标是沿着太平洋板块俯冲到伊豆-博宁海沟的活跃的垂直断层崖。这一断面允许对不同地壳年龄的岩石进行采样，其中将包括中生代太平洋地壳成分在~125 Ma发生假设变化的地区的样品。将对样品进行化学成分分析，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性时间刻度进行校准。调查结果将对目前关于俯冲带地壳再循环的概念和从地幔中提取地壳的方式提供关键的检验，其中碳的俯冲过程起着关键作用。这项研究的目标与美国国家科学基金会资助的综合大洋钻探计划(IODP)的科学计划直接相关，并将为即将进行的四次IODP钻探探险提供必要的数据输入(三个JOIDES决议段计划于2014年进行，一个Chikyu探险正在进行中)。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。项目成果将通过加州国家科学基金会资助的海洋科学教育卓越中心，为圣地亚哥联合学区的教师举办为期一天的研讨会。

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.