Mantle Structure Beneath Ultraslow-Spreading Mid-Ocean Ridges

作为全球地幔对流和岩石圈运动模式的一部分，大洋中脊是大洋板块相互分离的分界线。当地幔上升到山脊下方以取代横向移动的物质时，它会减压并部分融化。新形成的熔体粘性较小，密度较小，与地幔分离，浮力上升到表面，在那里形成新的洋壳。大洋扩张中心之下熔体的产生和输送可能是塑造地球的最重要的地质过程；它产生了全球地壳的三分之二以上，是地球上地球化学分化的主要手段；它还创造了巨大的海底热液系统，影响着海水化学，支持着巨大的生态系统。该项目旨在调查北极地区一系列超缓慢扩张的海脊。一个关键问题是异常缓慢的铺展速度如何影响熔体生产，因为这一过程严重影响了前述的脊峰过程。有两个需要检验的基本假设：(1)一个主要的假设预测，在非常缓慢的扩张条件下，海脊下的上地幔有有效的、深层次的冷却。这个凉爽的区域压低了熔融区的顶部，从而比沿着更快的扩张山脊更深地关闭了熔融。(2)另一种假设是，在浅层，熔体通过地幔的运输受到抑制：产生的熔体的很大一部分只是冻结在最浅的地幔中，地壳厚度并不代表发生的熔融总量。这两种假设都没有被地幔的实际测量所验证。地震波对地幔的热结构和熔体分布很敏感，因此提供了解决这些假设的最佳机会。我们将通过对北极圈以北超慢扩张海脊的地震面波和体波研究来检验地幔的热结构和熔体分布，以检验这些假设。我们将使用位于格陵兰-挪威海和北冰洋及其周围的永久和临时地震台站的数据，这些台站记录了10-15年或更长时间的公开可用数据。了解熔体对大洋中脊的供应是各种研究人员的基本关切，这类超低扩展脊正引起大洋中脊社区的极大兴趣。我们的研究将影响广泛的学科，从地幔化学家和地球动力学家，到研究山脊过程细节的人，如热液和生态系统。以研究生培训和本科生接触研究科学的形式开发人力资源也是一个重要的副产品。这项研究的社会影响是间接的，但对于社会对我们星球正在进行的演变以及我们海洋的化学和热平衡的全面了解很重要。我们将与RIDGE2000和SOEST教育和外联计划合作，将我们的成果传播给非专业人员，并与IRIS(联合地震研究所)合作，在夏威夷大学建造一个永久性的壁挂式博物馆式实时地震信息展示，以提高社区对地震学和地震危险的认识。我们还将建造一个或多个具有触摸屏功能的便携式显示器，借给夏威夷的K-12学校，用于建立地球科学课程。

ABSTRACTOCE-0648507As part of the Earth's global pattern of mantle convection and lithospheric motion, mid-ocean ridges mark the boundaries where oceanic plates separate from one another. As the mantle rises beneath the ridges to replace material that moves away laterally, it decompresses and partially melts. The newly formed melt, being less viscous and less dense, segregates from the mantle and buoyantly rises toward the surface, where it forms new oceanic crust. The generation and transport of melt beneath oceanic spreading centers is perhaps the most important geological process shaping the earth; it produces over two thirds of the global crust and is a primary means of geochemical differentiation in the Earth; it also creates vast seafloor hydrothermal systems that influence ocean water chemistry and support enormous ecosystems. This project aims to investigate a series of ultra-slow spreading ridges in the Arctic region. A critical issue is how the exceptionally slow spreading rate influences melt production, because this process heavily influences the aforementioned ridge crest processes. There are two basic hypotheses that are to be tested: (1) A leading hypothesis predicts that under very-slow spreading conditions there is efficient, deep cooling of the upper mantle beneath the ridge. This cool region depresses the top of the melting region, thereby shutting off melting much deeper than along faster spreading ridges.(2) An alternative hypothesis is that melt transport through the mantle is inhibited at shallow depths: a large proportion of the melt generated is simply frozen into the shallow-most mantle and crustal thickness does not represent the total amount of melting that occurs.Neither hypothesis has been tested by actual measurements of the mantle. Seismic waves are sensitive to the thermal structure of the mantle and melt distribution and thus offer the best opportunity to address these hypotheses. We will test these hypotheses by examining mantle thermal structure and melt distribution via a seismic surface and body wave study of the ultra-slow spreading ridges north of the Arctic Circle. We will use data from permanent and temporary seismic stations located in and around the Greenland-Norwegian Sea and Arctic Ocean that have recorded publicly available data for upwards of 10-15 years or more. Understanding melt supply to mid-ocean ridges is a fundamental concern of a wide variety of researchers and the class of ultraslow spreading ridges is generating considerable interest among the mid-ocean ridge community. Our research will influence a broad range of disciplines, from mantle chemists and geodynamicists to those who study the details of ridge crest processes, such as hydrothermal and ecological systems.The development of human resources in the form of graduate student training and exposure of undergraduates to research science is also an important outgrowth. Societal impact of the research is indirect, but important to society's overall understanding of the ongoing evolution of our planet and the chemical and thermal balance of our oceans. We will work with the RIDGE2000 and SOEST Education and Outreach programs to disseminate our results to the non-specialist and in collaboration with IRIS (Incorporated Research Institutes for Seismology) we will construct a permanent wall-mounted museum-style display at the University of Hawaii of real-time seismic information to generate greater community awareness of seismology and seismic hazards. We will also build one or more portable display units with a touch screen capability to loan out to Hawaiian K-12 schools for their use in building earth science curricula.