Quantifying Rare Earth (REE) and High Field Strength (HFSE) Element Mobility in Fluids at Conditions Appropriate for Forearc to Subarc Cold and Hot Subduction Zones

知识价值：海洋岩石圈和上覆沉积物在这些物质俯冲时发生变质脱水（即压力和温度升高）。这一过程被假设用来解释弧岩浆中不同于大洋中脊（MORB）和大洋岛屿（OIB）的独特地球化学特征。潜在的过程涉及在流体-岩石反应中溶解流体选择性分馏元素的能力。研究认为，该流体富集大离子亲石元素（LILE: K、Rb、Cs、Sr、Ba）相对于高场强元素（HFSE: Nb、Zr、Ta、Hf、Ti），富集轻稀土元素（LREE）相对于中、重稀土元素（MREE、HREE）。目前，量化这一过程所需的数据主要用于纯或稀水溶液，并且覆盖了相对较小的P-T空间区域，其中在弧前和弧次环境中发生了递进变质脱水。在俯冲带条件下，稀土元素和HFSE在含水流体中的行为随流体化学、压力和温度的变化而变化。例如，铌在金红石和流体之间的分配，La和Ce在独居石和流体之间的分配，Y在xenotime和流体之间的分配，将在一系列流体体组成中确定，包括H2O-NaCl和H2O-NaF体系，石英用于确保流体含有相对于自然预期的足够的Si。因此，这项工作将能够评估溶解的Cl、F和Si对实际含水流体中微量元素溶解度的影响。将使用两种互补的实验技术：1)在1 - 3gpa和300-600°C的水热金刚石砧细胞（HDAC）中，使用同步加速器x射线荧光（SXRF）原位测量Nb， La， Ce和Y的浓度；2)在1和1.8 GPa和700-900°C下的活塞缸装置，其中流体中的微量元素浓度将通过使用质量损失技术结合回收晶体的LA-ICP-MS分析来测量，以评估固相的一致溶解性质。通过测量在5 GPa的HCl-H2O流体中的xenotime溶解度（即Y丰度），证明了实验技术的成功开发和校准（并在本提案中进行了描述）。新的结果将大大增加有限的矿物-流体划分数据库，使地质学家能够更准确地模拟俯冲带环境中存在流体的变质作用和岩浆作用的影响。更广泛的影响：该项目涉及三位UNLV教师之间的合作，他们结合自己的专业知识，每年指导一名女博士生Elizabeth Tanis女士和至少两名UNLV本科生。该博士生已经在这个项目上工作，并在两次AGU会议上展示了数据，并提交了一份等待修改的手稿。所有学生都将接受基于同步加速器的尖端实验技术、活塞-气缸质量损失实验、分析化学和地球化学数据集解释的培训，重要的是，他们将通过与UNLV、纽芬兰纪念大学和先进光子源的光束线科学家的合作，与不同的科学家群体建立研究关系。pi定期参加社区外展活动，为K-12学生提供讲座和实践地质示范。西蒙和伯恩利定期访问K-12班，西蒙是UNLV-Clark县学区“暑期学院”的地球科学成员，该学院每年夏天都会带60名K-5教师到UNLV进行密集的科学课程教育，同时他也是2011-2012年Clark Magnate高中科学奥林匹克队的地球科学内容导师。研究结果将直接纳入pi教授的课程，并将在适当的科学会议上展示，并在同行评议的期刊上发表。

Intellectual Merit: Ocean lithosphere and overlying sediments undergo metamorphic dehydration as these materials are subducted (i.e., increasing pressure and temperature). This process has been hypothesized to account for distinctive geochemical signatures in arc magmas as opposed to those from mid-ocean ridges (MORB) and ocean islands (OIB). The underlying process involves the ability of exsolved fluids to selectively fractionate elements during fluid-rock reactions. The fluid is proposed to enrich arc magmas in the large ion lithophile elements (LILE: K, Rb, Cs, Sr, Ba) relative to high field strength elements (HFSE: Nb, Zr, Ta, Hf, Ti), as well as enriching the magmas in light-REE (LREE) relative to the middle- and heavy-REE (MREE, HREE). At present, data needed to quantify such processes are mostly available for pure or dilute aqueous solutions and cover a relatively small area in the P-T space wherein prograde metamorphic dehydration occurs in the forearc and subarc environments. Experimental studies are proposed to expand significantly our knowledge of the behavior of the REE and HFSE in aqueous fluid as a function of fluid chemistry, pressure and temperature at subduction zone conditions. For example, the partitioning of Nb between rutile and fluid, La and Ce between monazite and fluid, and Y between xenotime and fluid will be determined for a range of fluid bulk compositions, including H2O-NaCl and H2O-NaF systems, with quartz used to ensure the fluid contains sufficient Si relative to that expected in nature. This work will thus allow assessment of the effects of dissolved Cl, F and Si on trace element solubilities in realistic aqueous fluids. Two complementary experimental techniques will be used: 1) a hydrothermal diamond anvil cell (HDAC) at 1 - 3 GPa and 300-600°C, wherein Nb, La, Ce and Y concentrations will be measured in situ by using synchrotron X-ray fluorescence (SXRF); and 2) a piston-cylinder apparatus at 1 and 1.8 GPa and 700-900°C, wherein trace element concentrations in fluid will be measured by using the mass-loss technique combined with LA-ICP-MS analysis of recovered crystals to assess the nature of congruent dissolution of the solid phases. Successful development and calibration of the experimental technique was demonstrated (and described in this proposal) by measuring xenotime solubility, hence Y abundance, in a HCl-H2O fluid to 5 GPa. New results will add significantly to the limited mineral-fluid partitioning data base over pressure-temperature regime of interest, and allow geologists to model more accurately the effects of fluid-present metamorphism and magmatism in the subduction zone environment. Broader Impacts: This project involves collaboration between three UNLV faculty members who combine their expertise to mentor one female PhD student, Ms. Elizabeth Tanis, and at least two UNLV undergraduate students per year. The PhD student is already working on this project and has presented data at two AGU meetings as well as one submitted manuscript that is accepted pending revision. All students will be trained in cutting-edge synchrotron-based experimental techniques, piston-cylinder mass loss experiments, analytical chemistry and interpretation of geochemical data sets, and, importantly, will develop research relationships with a diverse group of scientists through collaboration with faculty at UNLV, Memorial University of Newfoundland and beamline scientists at the Advanced Photon Source. The PIs participate regularly in community outreach, presenting lectures and hands-on geology demonstrations to K-12 students. Simon and Burnley regularly visit K-12 classes, and Simon is the Geoscience member of a UNLV-Clark County School District "summer institute" that brings 60 K-5 teachers to UNLV each summer for intensive science-curriculum education as well as the Geoscience content mentor for the Clark Magnate High School science Olympiad team in 2011-2012. Results will be incorporated directly into courses taught by the PIs, and results will presented at appropriate science meetings and published in peer-reviewed journals.