Mechanisms of isotope exchange between aqueous solutions and barite in low-temperature geochemical systems

The prevailing view that solid-state diffusion is negligible at low temperatures is challenged by rapid sulfur and barium isotope exchange between natural barite crystals and aqueous solutions in laboratory experiments. This assumption relies on diffusivities extrapolated from high-temperature experiments. Here, isotope exchange rates were measured in solutions enriched with 137Ba and 32S at 50 and 80 oC for less than 10,360 hours. SIMS depth profiles revealed 137Ba enrichment to ~75 nm, with shapes characteristic of classical diffusion. Isotope disequilibrium between the barite surface and aqueous solution implies a continuous supply from the interior. These results indicate that defects and vacancies in barite and similar low-temperature minerals enable effective solid-state diffusion, with profound implications for paleoenvironmental reconstructions, materials science, and engineering.