Intellectual merit. Ultramafic xenoliths provide direct information regarding the nature of the upper mantle. However, xenolith che geochemistry may be modified during transport due to reaction with the melts or fluids that carry them. Lithium may provide a window into the timescales and extent of this reaction due to its highly mobile nature at high temperatures. Mantle xenoliths often display inter-and intra-granular lithium isotopic fractionation. In particular, clinopyroxenes are often lighter and more variable than co-existing olivines, particularly in depleted xenoliths. The proposed experimental study aims to determine if this is an inherent characteristic of the Li isotope geochemistry of the mantle, or if this is reflective of processes occurring immediately prior to or during eruption. For instance, different diffusion rates for Li in olivine and clinopyroxene might explain the apparent fractionation of Li isotopes between these two phases, if one phase is able to equilibrate more fully with the host melt on the timescales of xenolith mobilization and transport. With the proposed study, we intend to address the following questions: 1) What is the equilibrium distribution of Li and its isotopes in mantle minerals?, 2) What is the rate of Li elemental diffusion in olivine and clinopyroxene?, and 3) To what extent are Li isotopes fractionated during solid-state diffusion?. The proposed research constitutes the first attempt to experimentally verify the presumed lack of equilibrium isotopic fractionation of Li at high temperatures, to determine Li diffusion rates in olivine, and to assess the degree to which Li isotopes are fractionated during solid-state diffusion. If the experimental results show that Li diffusion in olivine is slow relative to the timescales of xenolith mobilization (which may or may not be the case), the Li isotope composition of olivines in mantle nodules could be valuable tools for evaluating the Li isotope geochemistry of the upper mantle. At the same time, quantifying the rates of Li diffusion in clinopyroxene will allow us to assess the timescales of mantle-melt interaction prior to and during xenolith transport. The data generated during this experimental study will allow future researchers to assess the extent of Li isotope heterogeneity in the mantle, the timescales and extent of reactive alteration of mantle nodules immediately prior to and during transport, and cooling rates of olivine- and clinopyroxene- bearing lavas.

Broader Impacts. This project will enable the PI to establish research techniques and facilities. Collaborations with. David Eggler (Penn State) and James Brenan (U. of Toronto) will greatly facilitate establishment of a new experimental petrology lab at Penn State. The project will support one PhD student and senior thesis projects for several undergraduates at Penn State. Analytical work will be conducted in Japan (ISEI; Okayama U.) as part of an international educational and cultural excgange for the PhD student, and promotes East-West intellectual exchange for the geochemical community.

Project Report

Intellectual Merit Processes occurring in the Earth’s upper mantle are responsible for the generation of new oceanic crust, (often violent) volcanism at plate boundaries, and the establishment of stable continental cratons. Upper mantle processes are also very closely connected to heat flow at the Earth’s surface. The ways in which the upper mantle interacts with new and existing crust provide important clues regarding the large-scale dynamics of the Earth’s interior, and the record of these interactions is written in geochemical signatures of upper mantle products such as plutons and lavas. In order to best interpret these geochemical signatures, it is important that we understand the initial geochemical state of the mantle. One way of studying geochemical state of the upper mantle is by analyzing xenoliths, or pieces of the upper mantle that are carried to the surface in lava flows. However, determining the composition of the mantle by studying xenoliths presents us with a dilemma: the very fact that these mantle samples have been carried to the surface ensures that they are not directly representative of the in situ mantle. The geochemistry of mantle xenoliths may change shortly prior to and during transport due to reaction with the melts or fluids that carry them to the surface. Lithium is a light and highly mobile element that holds promise for interpreting the geochemical changes that may take place in mantle xenoliths during transport. At high temperatures in the mantle, it is expected that isotopic fractionation factors approach one – in other words, all minerals, melts, and fluids that are in equilibrium with each other should have very nearly the same isotope ratios. Nonetheless, it has been observed that olivine and clinopyroxene, the two most significant minerals in the upper mantle, often have different lithium isotope ratios in mantle xenoliths. In this project, we used high-pressure and high-temperature experiments to determine how lithium is distributed between olivine and clinopyroxene over a range of temperatures relevant to the upper mantle. It had previously been proposed (and largely accepted) that lithium is preferentially taken up by clinopyroxene relative to olivine as temperature decreases, resulting in redistribution of lithium and its isotopes by diffusion upon uplift and cooling of mantle xenoliths. Our study showed that there is no such relationship between temperature and lithium distribution. Lithium prefers olivine relative to clinopyroxene by a factor of two, regardless of temperature, over a range of temperatures between 700?C and 1400?C. This result suggests that some other process, for example direct interaction between mantle xenoliths and infiltrating melts or fluids, must be controlling the distribution of lithium and lithium isotopes in olivine and clinopyroxene. Broader Impacts This project has formed the foundation of two Master of Science (MS) theses at the Pennsylvania State University. Students have gained experience using the piston cylinder apparatus to conduct high-pressure experiments, as well as a wide variety of analytical techniques including scanning electron microscopy (SEM), electron probe micro-analysis (EPMA), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), and secondary ion mass spectrometry (SIMS). One student was trained to perform solid-state diffusion experiments at the University of Toronto, Canada, and the other was trained in analytical techniques at the University of Okayama at Misasa, Japan. The students have presented their research at national and international conferences, including a presentation that won a best student presentation award at the 2012 V.M. Goldschmidt Conference in Montreal. The first graduate student working on this project gained mentoring experience by training the second graduate student, and he in turn gained mentoring experience by training an undergraduate conducting senior thesis research in experimental petrology. This was the first funded research project for the PI, a junior female investigator. The project enabled her to establish, update, and personalize facilities for high-pressure experimental petrology and trace element analysis at the Pennsylvania State University. To date, this project has produced one published MS Thesis, one article published in a peer-reviewed scientific journal, and two published meeting abstracts. One more MS Thesis and at least one more peer-reviewed manuscript are anticipated in the near future.

National Science Foundation (NSF)
Division of Earth Sciences (EAR)
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Jennifer Wade
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Pennsylvania State University
University Park
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