Collaborative Research: U-series constraints on the ages and petrogenesis of Lau Basin Lavas

朗缪尔/西姆斯，0732448；0732480调查人员将在东刘扩散中心全长的样品上测量U、Th和Ra同位素，该中心是2000年山脊综合研究场地，重点是阿贝喷口，这是该地区被指定的“靶心”。这些数据将从现有的铅、锶、钕和氦同位素、常量元素、微量元素和水分含量的样品中获得。由此产生的数据集将成为弧后盆地研究的基准，将解决大洋脊和收敛边缘科学的主要问题，并为LAU ISS提供必要的年龄限制。铀系列测量有可能解决该地区的三个重要科学问题：(1)俯冲流体铀含量高，导致会聚边缘熔岩中238U过剩。这些过剩随着时间的推移而衰减，并反映了流体通过离火山前缘不同距离的地幔楔体输送的时间尺度。我们将检验随着距离海沟的增加而减少238U过剩(即更大的运输时间)的假设。(2)弧后盆地和开阔海洋扩张中心对源区水的响应截然不同，弧后扩张中心没有明显的石榴石影响，也没有证据表明深部低度熔融的影响。Langmuir等人。(2006)将这些差异归因于弧后活动的角流和深冷板的影响。由于石榴石控制着开阔洋脊的U-Th不平衡，我们预测，与开阔洋脊相比，在后弧处的230度过剩较小。北部ELSC样品很少或根本没有俯冲特征。这些样本尤其允许对这一假设进行检验。(3)许多与大洋海脊形成有关的问题都需要时间尺度的知识。年龄限制是将火山活动与构造、下伏岩浆室反射层的变化以及熔岩成分的时间变化联系起来的关键。它们是火山系统与受火山活动时间和空间波动支持和影响的热液和生物活动之间的重要联系。U系列是量化年轻山脊火山活动时间限制的唯一手段。我们拟议的工作将增加该地区海脊过程研究的时间维度。波及影响：拟议的工作是多学科努力的一部分，目的是了解从地幔熔体生成到火山活动和海底生命的大洋脊系。对收敛边缘的流体通量的限制将与对俯冲过程感兴趣的大型社区和NSF边缘计划相关。哈佛大学的本科生经常在朗缪尔的实验室工作，通过这个项目，他们将接触到U系列分析。朗缪尔和西姆斯还将准备一套U系列教学模块。这些模块将是朗缪尔的《岩石学导论》和西姆斯的《同位素化学导论》的一部分。基于网络的模块将托管在他们的个人网站上，并作为Sims同位素化学课程的一部分整合到麻省理工学院开放式课程中。

AbstractLangmuir/ Sims, 0732448; 0732480The investigators will measure U, Th and Ra isotopes on samples from throughout the length of the East Lau Spreading Center, a Ridge 2000 Integrated Study Site, with an emphasis on the ABE vent field, which is the designated "bulls-eye" for this region. These data will be obtained on samples with existing Pb, Sr, Nd and He isotopes, major elements, trace elements and water contents. The resulting data set will be a benchmark for backarc basin studies, will address major problems of ocean ridge and convergent margin science, and provide essential age constraints for the Lau ISS. U-series measurements have the potential to address three important scientific problems in this region:(1) Subduction fluids have high U contents, causing 238U excesses in convergent margin lavas. These excesses decay with time, and reflect the time scale of fluid transport through the mantle wedge at varying distances from the volcanic front. We will test the hypothesis ofdecreasing 238U excess (i.e. greater transport time) with increasing distance from the trench.(2) Back-arc basin and open ocean spreading centers have very different responses to water in the source, with back-arc spreading centers showing no apparent garnet influence, or evidence of effects of deep low degree melts. Langmuir et al. (2006) attribute these differences to the effects of active corner flow and a deep cold slab in the back-arc. Since garnet controls U-Th disequilibria at open ocean ridges, we predict smaller 230Th excesses at back-arcs compared to open ocean ridges. The northern ELSC samples have little or no subduction signature. These samples in particular permit a test of this hypothesis.(3) A host of problems related to ocean ridge formation require knowledge of time scales. Age constraints are essential to relate volcanic activity to tectonics, variations in the underlying magma chamber reflector, and temporal changes in lava compositions. They are an essential link between the volcanic system and the hydrothermal and biological activities that are supported and impacted by temporal and spatial fluctuations in volcanism. U-series are the sole means to quantify time constraints for young ridge volcanism. Our proposed work will add the dimension of time to the study of ridge processes in this region.Broader Impacts: The proposed work is part of a multi-disciplinary effort to understand the ocean ridge system from melt generation in the mantle to volcanism and life on the seafloor. The constraints on fluid fluxes at convergent margins will be pertinent for the large community interested in subduction processes, and to the NSF MARGINS program. Harvard undergraduate students work routinely in Langmuir's lab, and will be exposed to U-series analysis through this project. Langmuir and Sims will also prepare a set of U-series teaching modules. These modules will be part of Langmuir's 'Introduction to Petrology' and Sims' 'Introduction to Isotope Chemistry'. The web-based module will be hosted on their personal web sites, and integrated into the MIT OpenCourseWare as part of Sims' Isotope Chemistry course.