Li diffusion in zircon

Diffusion of Li under anhydrous conditions at 1 atm and under fluid-present elevated pressure (1.0-1.2 GPa) conditions has been measured in natural zircon. The source of diffusant for 1-atm experiments was ground natural spodumene, which was sealed under vacuum in silica glass capsules with polished slabs of zircon. An experiment using a Dy-bearing source was also conducted to evaluate possible rate-limiting effects on Li diffusion of slow-diffusing REE+3 that might provide charge balance. Diffusion experiments performed in the presence of H2O-CO2 fluid were run in a piston-cylinder apparatus, using a source consisting of a powdered mixture of spodumene, quartz and zircon with oxalic acid added to produce H2O-CO2 fluid. Nuclear reaction analysis (NRA) with the resonant nuclear reaction Li-7(p,gamma)Be-8 was used to measure diffusion profiles for the experiments. The following Arrhenius parameters were obtained for Li diffusion normal to the c-axis over the temperature range 703-1.151 degrees C at 1 atm for experiments run with the spodumene source: D-Li = 7.17 x 10(-7) exp(-275 +/- 11 kJmol(-1)/RT)m(2)s(-1) Diffusivities are similar for transport parallel to the c-axis, indicating little anisotropy for Li diffusion in zircon. Similar Li diffusivities were also found for experiments run under fluid-present conditions and for the experiment run with the Dy-bearing source. Li diffusion is considerably faster than diffusion of other cations in zircon, with a smaller activation energy for diffusion. Although Li diffusion in zircon is comparatively rapid, zircons will be moderately retentive of Li signatures at mid-crustal metamorphic temperatures, but they are unlikely to retain this information for geologically significant times under high-grade metamorphism.