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

langmuir / Sims, 07322448；[0732480]研究人员将测量来自东劳扩散中心（Ridge 2000综合研究地点）的样品的U， Th和Ra同位素，重点是ABE喷口区，这是该地区指定的“靶心”。这些数据将在具有现有Pb、Sr、Nd和He同位素、主元素、微量元素和含水量的样品上获得。由此产生的数据集将成为弧后盆地研究的基准，将解决洋脊和辐合边缘科学的主要问题，并为Lau ISS提供必要的年龄约束。U系列测量有可能解决该地区三个重要的科学问题：(1)俯冲流体U含量高，导致会聚边缘熔岩中238U过量。这些过量随时间衰减，反映了流体在离火山锋不同距离上通过地幔楔输送的时间尺度。我们将测试238U过剩（即更长的运输时间）随着距离海沟的增加而减少的假设。(2)弧后盆地和大洋扩张中心对源区水的响应差异很大，弧后扩张中心没有明显的石榴石影响，也没有深部低度熔体影响的证据。Langmuir等人（2006）将这些差异归因于活跃的角流和弧后深冷板的影响。由于石榴石控制着开放洋脊的U-Th不平衡，我们预测与开放洋脊相比，后弧的230度过剩较小。北部ELSC样品几乎没有俯冲特征。这些样本特别允许对这一假设进行检验。(3)许多与洋脊形成有关的问题需要时间尺度的知识。年龄限制是将火山活动与构造、下伏岩浆房反射物的变化和熔岩成分的时间变化联系起来的必要条件。它们是火山系统与热液和生物活动之间的重要联系，这些活动受到火山作用时空波动的支持和影响。u系列是量化年轻脊状火山作用时间限制的唯一手段。我们提出的工作将增加对该地区脊过程研究的时间维度。更广泛的影响：这项拟议的工作是多学科努力的一部分，旨在了解从地幔熔体产生到火山作用和海底生命的海洋脊系统。收敛边缘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。哈佛大学本科生在Langmuir的实验室例行工作，并将通过这个项目接触到u系列分析。Langmuir和Sims还将准备一套u系列教学模块。这些模块将成为Langmuir的“岩石学入门”和Sims的“同位素化学入门”的一部分。基于网络的模块将放在他们的个人网站上，并作为西姆斯同位素化学课程的一部分整合到麻省理工学院开放课程中。

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.