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

朗缪尔/西姆斯，0732448; 0732480研究人员将测量东劳扩散中心（2000年山脊综合研究中心）整个长度上的样品中的U、Th和Ra同位素，重点是ABE喷口场，这是该地区指定的“靶心”。这些数据将从现有的Pb、Sr、Nd和He同位素、主要元素、微量元素和水含量的样品中获得。由此产生的数据集将成为弧后盆地研究的基准，将解决洋中脊和会聚边缘科学的主要问题，并为Lau国际空间站提供基本的年龄限制。 铀系测量有可能解决该地区的三个重要科学问题：（1）俯冲流体具有高U含量，导致会聚边缘熔岩中的238 U过剩。这些过剩衰减随时间的推移，并反映了时间尺度的流体输送通过地幔楔在不同的距离火山前。我们将检验238 U过量随离海沟距离的增加而减少（即更长的迁移时间）的假设。(2)弧后盆地和开放的海洋扩张中心有非常不同的反应，水的源，弧后扩张中心没有表现出明显的石榴石的影响，或证据的影响，深低度熔体。朗缪尔等人（2006年）将这些差异归因于活跃的角流和弧后深层冷板的影响。由于石榴石控制U-Th不平衡在开放的洋脊，我们预测较小的230 Th过剩在弧后相比，开放的洋脊。北方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方面的知识。年龄的限制是必不可少的火山活动的构造，在下面的岩浆房反射层的变化，并在熔岩成分的时间变化。它们是火山系统与受火山活动时空波动支持和影响的热液和生物活动之间的重要联系。U系列是唯一的手段来量化年轻的山脊火山作用的时间限制。我们拟议的工作将增加时间的维度，在这个区域的脊过程的研究。更广泛的影响：拟议的工作是一个多学科的努力，以了解洋中脊系统从地幔中的熔融生成的火山作用和海底的生命的一部分。收敛边缘的流体通量的限制将是相关的大社会感兴趣的俯冲过程，并NSF的边缘计划。哈佛大学的本科生在朗缪尔实验室进行日常工作，并将通过这个项目接触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。网络模块将被托管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被整合到麻省理工学院的开放式课程中。

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.