Mantle Structure Beneath Ultraslow-Spreading Mid-Ocean Ridges

作为地球地幔对流和岩石圈运动的全球格局的一部分，洋中脊标志着大洋板块相互分离的边界。当地幔在脊下上升以取代横向移动的物质时，它会减压并部分融化。新形成的熔体粘度和密度较低，从地幔中分离出来，浮力上升到地表，在那里形成新的海洋地壳。在海洋扩张中心之下，熔体的产生和移动也许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，影响着海水的化学成分，支持着巨大的生态系统。这个项目的目的是调查北极地区一系列超缓慢扩张的山脊。一个关键问题是异常缓慢的扩散速度如何影响熔体的产生，因为这一过程严重影响了上述脊峰过程。有两个基本的假设需要验证：(1)一个主要的假设预测，在非常缓慢的扩散条件下，脊下的上地幔会发生有效的深度冷却。这一较冷的地区压制了融化区域的顶部，从而阻止了比沿着快速扩张的山脊更深的融化。(2)另一种假设是，通过地幔的熔体运输在浅层深处受到抑制：产生的很大一部分熔体只是被冻结在最浅层的地幔中，而地壳厚度并不能代表发生的熔体总量。这两种假设都没有通过对地幔的实际测量得到验证。地震波对地幔的热结构和熔体分布很敏感，因此为解决这些假设提供了最好的机会。我们将通过对北极圈北部超慢扩展脊的地震表面和体波研究来检验地幔热结构和熔体分布，从而验证这些假设。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数据。了解洋中脊的融水供应是众多研究人员关注的一个基本问题，而超低速扩张脊的类别正在引起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那些研究脊顶过程细节的人，如热液和生态系统。以培养研究生和培养本科生从事科学研究为形式的人力资源开发也是一项重要成果。这项研究的社会影响是间接的，但对社会对我们星球的持续进化以及海洋的化学和热平衡的整体理解很重要。我们将与RIDGE2000和SOEST教育和推广项目合作，向非专业人士传播我们的研究成果，并与IRIS（地震学联合研究所）合作，我们将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显示器，以提高社区对地震学和地震危害的认识。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，供他们在建立地球科学课程时使用。

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.