Determining the Processes Responsible for Lithium Isotope Fractionation

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Intellectural Merit. Lithium isotopes are strongly fractionated by fluid-rock interactions and thus have the potential to be unique tracers of fluid transport processes within the Earth. However, the processes responsible for isotopic fractionations and the conditions under which these fractionations occur must be determined before this potential is fully realized. This project will investigate the causes of lithium isotope fractionation in rocks from the continental crust and uppermost mantle. Previous work has addressed lithium isotope fractionation accompanying weathering, subduction zone metamorphism, late stage granite differentiation and diffusion from a lithium-rich pegmatite into surrounding country rocks, and shows negligible lithium isotope fractionation accompanying prograde metamorphism at temperatures of 480-640 C and during granite differentiation. Preliminary work suggests that average lithium isotopic composition of granulite xenoliths differs from that for granulite terrains, but it is unclear whether this is a general result, and further work is proposed to better define the Li isotopic composition of the deep crust and, by inference, the total crust. It is also unclear whether lithium isotopes fractionate during the lowest grades of regional metamorphism and the extent to which diffusion drives isotopic fractionation at the high temperatures typical of the mantle. Understanding these processes is critical to evaluate variations in lithium isotopes accompanying subduction.These issues will be addressed through the following projects focussed on fluid- or meltrock interaction within the lithosphere: 1) Lithium isotopic fractionation at low grades of regional metamorphism in meta-sediments from the British slate belts and the Otago schist, New Zeland. Spatially-resolved chemical and lithium isotopic data should also yield insights into how regional fluid flow affects lithium in these systems. 2) The degree of lithium isotopic fractionation accompanying granulite-facies metamorphism in Archean to Paleozoic terrains that are highly depleted in large-ion lithophile elements. 3) Lithium diffusion at high temperatures and associated isotopic fractionation in contact aureoles developed around a carbonatite intrusion, and in veins of pyroxenite, glimmerite and hornblendeite from composite mantle xenoliths. These studies will allow for quantification of important variables (e.g., temperature, porosity, tortuosity, permeability, fluid composition) that control fluid assisted lithium diffusion. Broader Impacts. The Univ. of Maryland laboratory has become a center of lithium isotopic analyses for the community at-large, training visiting students and researchers from a number of institutions in the methods of lithium isotope geochemistry. This project will provide matching funds for a year's salary for Dr. Ralf Halama, a Feodor Lynen post-doctoral fellow. and will also support a full-time PhD student and several undergraduate research assitants.