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到大陆地壳的净物质转移发生的地方，在伸展脊处产生的物质又返回到地幔。洋中脊的火山作用中，熔体成分有限，熔体机制相对简单，而弧中的火山作用则要复杂得多：熔体成分众多，如下沉的板块、地幔楔块、上覆的地幔和大陆岩石圈、流体和沉积物。因此，弧岩浆在短距离内表现出元素和同位素组成的丰富多样性。人们普遍认为，流体的加入在俯冲带的熔融过程中起着重要作用，很可能是通过使地幔楔水化和降低固体温度来实现的。在某些弧（如喀斯喀特俯冲带）中，流体加入的时间和流体参与岩浆形成的程度尚不清楚。Cascade弧代表了全球弧系统的一个缓慢收敛和温暖的末端成员。经过近一个世纪的地球物理和地质研究，它也是最具特征的弧形之一。以铀系为基础的研究在这条弧线上非常适合回答一些悬而未决的问题。其中包括：1)熔炼和动态熔炼在弧中，特别是在喀斯喀特山脉中的相对重要性；2)解释了喀斯喀特熔岩喷发前估计的高含水量（高达8% H2O），但在喀斯喀特熔岩中缺乏关键流体指示元素和同位素比率的富集；3)以及喀斯喀特熔岩中地壳成分的数量。该研究将利用230Th-238U、226Ra-230Th、231Pa-235U、Sr、Nd、Pb和Os同位素数据来解决上述及相关问题。虽然这个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工作。该项目将作为培训两名基于分析的地球化学和岩石学研究生的场所。该实验室不仅供pi小组使用，还供其他各种小组使用，包括来自小型本科学院的学生，如爱荷华州康奈尔学院和华盛顿州普吉特海湾大学。新墨西哥大学是一个为少数民族服务的机构，目前的提案与现有的斯隆基金会资助的PI相结合，以支持少数民族博士生。该研究区是国家火山灾害关注区，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.