Subduction zones are one of the primary manifestations of plate tectonics, occuring where two of Earth's plates collide and one slips beneath the other. Subduction of one plate beneath another functions as the primary mechanism for mass transfer between the surface and interior of the Earth. An understanding of fluid-driven chemical cycling in subduction zones provides us with the tools to discover valuable natural resources such as metal ores and natural gas and oil reserves, to develop new resources such as geothermal energy, and to understand the geologic hazards of subduction zones such as earthquakes and volcanic eruptions. This study focuses on the distribution of trace elements in fluids produced in the down-going plate during subduction zone metamorphism. These fluids are responsible for generating the magmas that produce arc volcanoes. In particular, high-pressure metamorphic rocks such as eclogite (subducted oceanic crust) are valuable resources for studying fluid production and transfer within the deeper parts of the subduction zone. In order to track the mobility of trace elements in subduction fluids, this study will examine the geochemistry of individual minerals found in eclogite from the Ring Mountain locality of the Franciscan Complex in California, and the Erzgebirge (Ore Mountains) region of Germany and the Czech Republic. In particular, garnet and rutile will be analyzed in detail to examine the behavior of rare earth and high field strength elements during the production and transport of fluids in eclogitic rocks.

The preferred method for analyzing a broad spectrum of trace elements in individual mineral grains is laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Complementary LA-ICP-MS facilities exist at Pennsylvania State University in the US and the University of Mainz in Germany. Faculty at Mainz have developed cutting edge methods that make use of information encoded in minor mineral phases in metamorphic rocks, such as Zr-in-rutile thermometry and U-Pb geochronology. Together these methods can be used to develop a temperature-time history for our samples to help us interpret the conditions under which fluid transport occurred. Penn State PhD student Alicia Cruz-Uribe will spend six weeks at the LA-ICP-MS laboratory in Mainz learning how to implement these specialized analytical techniques from her host and mentor, Dr. Thomas Zack. Cruz-Uribe will return to the US with her knowledge of these techniques to be implemented at the LA-ICP-MS facility at Penn State, under the guidance of advisor Dr. Maureen Feineman. The student will benefit both by direct exposure to new methods and new ways of thinking as well as by teaching those methods to other students upon her return. Both institutes benefit by virtue of the collaborative international research project. This project is funded jointly by the Office of International Science and Engineering and the Division of Earth Sciences.

Project Report

Intellectual Merit Subduction zones are sites of intense geologic activity. Subduction of one tectonic plate beneath another functions as the primary transport mechanism for geological materials such as rock and water between the surface and interior of the Earth. As a consequence of this action, subduction zones are responsible for some of the Earth’s most devastating natural disasters, including violent volcanic eruptions, earthquakes, and tsunamis. During subduction, minerals in rocks react to form new minerals as the rocks undergo metamorphism in response to increasing heat and pressure. This study focused on the reactions that produce water-rich fluids during subduction zone metamorphism. These fluids are responsible for generating the magmas that produce arc volcanoes, and may also be responsible for triggering intermediate-depth earthquakes. In particular, we investigated the rates at which reactions occur in subducted rocks, which is important in assessing the location and amount of fluids released in subduction zones worldwide. Our data provide some of the first quantitative estimates of reaction rates in subduction settings, and provide a basis for assessing how and when fluids are released during metamorphic reactions in subduction zones. In order to investigate the rates of reaction in subducted rocks, we used laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to analyze trace element concentration profiles across grains of rutile (TiO2) that had partially reacted to form rims of titanite (CaTiSiO5) by reaction with hydrous Ca-bearing minerals, silicate melts, or aqueous fluids. These grains were found in rocks from ancient subduction zones in California, Norway, and China. Niobium (Nb) is a high field strength element that is more compatible in the crystalline structure of rutile than in that of titanite. This means that when rutile reacts to form titanite, much of the Nb is rejected by the titanite rim and diffuses back into the rutile grain. We analyzed the Nb concentrations in high-resolution linear traverses across rutile-titanite pairs, and using known rates of Nb diffusion in rutile we were able to model the rate at which Nb was re-incorporated into rutile relative to the moving grain boundary with titanite. By combining the rate of grain boundary movement with the measured thickness of the titanite rim, we were able to calculate the rate at which the reaction from rutile to titanite occurred in these subduction zone rocks. Broader Impacts This collaboration supported two young female scientists: one a graduate student from an underrepresented group (Cruz-Uribe), and the other a junior faculty member (Feineman). This project fostered a strong relationship between scientists at The Pennsylvania State University and the University of Mainz, Germany. Cruz-Uribe traveled to the University of Mainz over the course of two summers, which provided her with international exposure and the opportunity to collaborate with researchers at a world-leading institution for international subduction zone research and LA-ICP-MS technology. Cruz-Uribe has been able to implement the techniques learned in Mainz to analyze minerals at the LA-ICP-MS facility at Penn State at smaller spatial scales than was possible prior to this collaboration. Collaborators at the University of Mainz also provided Cruz-Uribe with pieces of mineral standards, which make it possible to analyze new minerals at Penn State that could not be analyzed previously due to the lack of appropriate standards. In addition, Cruz-Uribe had the opportunity to present the findings of this study at international conferences in 2010-2012.

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