Diffusion-driven extreme lithium isotopic fractionation in country rocks of the Tin Mountain pegmatite

Lithium concentrations and isotopic compositions in the country rocks (amphibolites and schists) of the Tin Mountain pegmatite show systematic changes with distance to the contact. Both Li and δLi decrease dramatically along a ∼10m traverse from the pegmatite into amphibolite, with Li concentration decreasing from 471 to 68ppm and δLi decreasing from +7.6 to −19.9. Rubidium and Cs also decrease from the pegmatite contact into the country rock, but only within the first 2m of the contact, after which their concentrations remain constant. Neither mixing between pegmatite fluids and amphibolite, nor Li isotope fractionation by Rayleigh distillation during fluid infiltration is a likely explanation of these observations, due to the extremely light isotopic composition required for the amphibolite end-member in the mixing model (δLi=−20) and the similarly extreme isotopic fractionation required in a Rayleigh distillation model. Rather, these variations are likely due to isotopic fractionation accompanying Li diffusion from the Li-rich pegmatite (Li=450 to 730ppm) into amphibolites (Li=20ppm). The fact that other alkali element concentrations vary only within 2m of the contact reflects the orders of magnitude faster diffusion of Li relative to heavier elements. Quartz mica schists in contact with the pegmatite also show large variations in both Li and δLi as a function of distance from contact (∼1wt.% to ∼70ppm and +10.8 to −18.6, respectively), but over a longer distance of N30m. Lithium concentrations of the schist decrease from ∼1wt.% adjacent to the contact to ∼70ppm 300m from the contact; the latter is a typical concentration in metapelites. The nature of the δLi variations in the schists is different than in the amphibolites. Schists within the first 2m of the contact have nearly identical δLi of +10, which mimics that of the estimated bulk pegmatite (+8 to +11). At a distance of 30m the δLi reaches the lowest value in the schists of −18.6 (similar to the lowest amphibolite measured). At a distance of 300m the δLi climbs back to +2.5, which is within the range of δLi of other schists in the region and metapelites worldwide. The behavior of Li in the schists can also be modeled by Li diffusion, with the effective diffusion coefficient in the schist being ∼10 times greater than that in the amphibolite. The effective diffusion coefficients of Li in the amphibolite and schist are N2 orders of magnitude greater that those in minerals, which implicates the importance of fluid-assisted grain-boundary diffusion over solid-state diffusion in transporting Li through these rocks. © 2006 Elsevier B.V. All rights reserved.