ADVECTION-DIFFUSION CONTROLLED LITHIUM ISOTOPIC DISTRIBUTION IN CONTACT AUREOLES: —A CASE STUDY FROM THE FLORENCE COUNTY PEGMATITES, WISCONSIN

Fluids can change the physical and chemical properties of the crust and characterizing their flow is a high priority in Earth sciences. Stable isotopes, (e.g., oxygen isotopes) are widely used as tracers of fluid-rock interactions in contact aureoles in the continental crust. Recent studies have shown that Li isotopes (7Li and 6Li) are also good tracers of fluid flow in a number of geological environments. To date, several case studies of contact aureoles have highlighted the very large Li isotopic fractionation that can be generated in these settings. However, these studies also show that the amount of isotopic fractionation and the distance to which Li diffuses into the country rocks varies greatly from place to place. We thus seek to identify the parameters that govern Li distribution, hence fluid flow, in contact aureoles using a combination of Li isotope analyses and 2-D advection-diffusion modeling for two country rock profiles adjacent to Li-rich pegmatite dikes from the Florence County pegmatite field, Wisconsin. The results show (1) Although less than ~3 m thick, the pegmatite sheets have had a large impact on the Li budget of their adjacent amphibolites and schists, with Li enriched up to a factor of twenty over regional values, (2) Li from the pegmatite has travelled more than 50 m into the country rocks, (3) The two country rock profiles show systematic Li isotopic fractionation with distance away from the contacts, which is consistent with diffusive fractionation. For example, the d7Li value in country rocks in one profile ranges from ~ +6 at the contact to ~ -7 at ~ 30 m from the contact. Both fluid infiltration, induced by increasing pore fluid pressure, and diffusion, caused by Li chemical potential gradients from the pegmatites into the country rocks, are responsible for the increased lithium content and isotopic distribution in contact aureoles, (4) Permeability structure and the Peclet number can play important roles in determining the Li distribution within the contact aureoles, (5) Lithium isotopes are sensitive tracers of fluid-rock interactions in contact aureoles.