Equilibrium and Kinetic Isotopic Fractionation of Lithium in Mantle Minerals

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.