Melting Processes and Crust-Mantle Interaction in the Cascade Arc: Constraints from U-Series, Nd, Os, Pb and Sr Isotopic Data

地球上的俯冲带系统是从地幔向大陆地壳进行净物质转移的地方，也是在扩张山脊产生的物质返回地幔的地方。与大洋中脊的火山作用不同，大洋中脊的火山作用熔融成分有限，熔融机制要简单得多，而弧形火山作用则复杂得多：熔体成分繁多，如下降板、地幔楔形、上覆地幔和大陆岩石圈、流体和沉积物。因此，弧状岩浆在短距离内表现出丰富的元素和同位素组成的多样性。人们普遍认为，流体的加入在俯冲带熔融中起着重要作用，很可能是通过使地幔楔体水化和降低固相线温度来实现的。流体加入的时间和流体参与某些弧区岩浆形成的程度尚不清楚，例如瀑布俯冲带。级联弧既代表着全球弧系的一个缓慢收敛的终端成员，也代表着一个温暖的终端成员。考虑到近一个世纪的地球物理和地质研究，它也是最具特征的弧线之一。以铀系列为基础的这一弧形研究非常适合回答一些悬而未决的问题。这些问题包括：1)熔剂熔融和动态熔融在总体上，特别是在瀑布中的相对重要性；2)对某些喀斯特熔岩估计的高喷发前水含量(高达8%H2O)和关键流体指示元素和同位素比值缺乏的矛盾的解释；3)喀斯特熔岩中地壳成分的数量。这项拟议的研究将利用230Th-238U、226Ra-230Th、231PA-235U、Sr、ND、Pb和Os年轻熔岩和矿物分离的同位素数据来解决上述和相关问题。尽管该项目的目的是回答基本的科学问题，但它将支持新墨西哥大学的人类发展和基础设施建设努力。该项目将为两名研究生提供分析地球化学和岩石学方面的培训。该实验室不仅被PIS小组使用，还被其他各种小组使用，包括来自爱荷华州康奈尔学院和华盛顿州普吉特湾大学等小型本科学院的学生。新墨西哥大学是一所为少数族裔服务的机构，目前的提议与斯隆基金会现有的一项拨款相结合，以支持少数族裔博士生。该研究区是国家关注火山灾害的地区，该项目的结果可能有助于了解火山喷发周期。

The subduction zone system on Earth is where net material transfer from the mantle to the continental crust occurs, and where material that is created at spreading ridges is returned to the mantle. Unlike volcanism in mid-ocean ridges, where melting components are limited and melting mechanism are considerably simpler, volcanism in arcs is considerably more complex: components of melt are numerous, such as the down going slab, the mantle wedge, the overlying mantle and continental lithospheres, fluids and sediments. As a result, arc magmas exhibit rich diversity in elemental and isotopic composition over short distances. It is generally agreed that fluid addition plays an important role in melting in subduction zones, most likely by hydrating the mantle wedge and lowering the solidus temperature. The timing of fluid addition and the extent of fluid involvement in generation of magmas in certain arcs, such as the Cascade subduction zone, are not clear. The Cascade arc represents both a slow converging and warm end-member of the global arc system. It is also one of the best characterized arcs, given almost century long geophysical and geologic studies. A uranium-series based study in this arc is ideally suited to answer a number of the outstanding questions. These include, 1) the relative importance of flux melting and dynamic melting in arcs in general and the Cascades in particular, 2) the explanation for the paradox of the estimated high pre-eruptive water content (up to 8% H2O) inferred for some Cascade lavas and yet the lack of enrichment in key fluid indicating elements and isotopic ratios, 3) and the amount of crust component in Cascade lavas. The proposed study will utilize 230Th-238U, 226Ra-230Th, 231Pa-235U, Sr, Nd, Pb and Os isotopic data on young lavas and mineral separates to address the above and related questions.Although this project's aim is answering basic scientific questions, it will support the human development and infrastructural building efforts at the University of New Mexico. The project will serve as a venue for training two graduate students in analytically-based geochemistry and petrology. The lab is used not only by the PIs group but also by a variety of other groups, including students from small undergraduate colleges, such as Cornell College, Iowa, and University of Puget Sound, Washington. The University of New Mexico is a minority serving institution and the current proposal is coupled to an existing Sloan Foundation grant to the PI to support minority PhD students. The study area is a National area of concern for volcanic hazards, and results from this project may contribute towards understanding of volcanic eruption cycles.