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 δ⁷Li decrease dramatically along a ∼10 m traverse from the pegmatite into amphibolite, with Li concentration decreasing from 471 to 68 ppm and δ⁷Li 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 2 m 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 (δ⁷Li = -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 730 ppm) into amphibolites (Li = 20 ppm). The fact that other alkali element concentrations vary only within 2 m 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 δ⁷Li as a function of distance from contact (∼1 wt.% to ∼70 ppm and + 10.8 to -18.6, respectively), but over a longer distance of > 30 m. Lithium concentrations of the schist decrease from ∼1 wt.% adjacent to the contact to ∼70 ppm 300 m from the contact; the latter is a typical concentration in metapelites. The nature of the δ⁷Li variations in the schists is different than in the amphibolites. Schists within the first 2 m of the contact have nearly identical δ⁷Li of +10, which mimics that of the estimated bulk pegmatite (+8 to +11). At a distance of 30 m the δ⁷Li reaches the lowest value in the schists of -18.6 (similar to the lowest amphibolite measured). At a distance of 300 m the δ⁷Li climbs back to +2.5, which is within the range of δ⁷Li 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 >2 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.