Mantle Structure Beneath Ultraslow-Spreading Mid-Ocean Ridges

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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.