EXPERIMENTAL DETERMINATION OF THE TEMPERATURE DEPENDENCE OF LITHIUM DIFFUSION IN FELDSPAR

Diffusion chronometry is an advancing methodology capable of quantifying the timescales of eruptive events. Feldspar solid solutions are ubiquitous across a variety of magma compositions; however, experimentally determined feldspar diffusion coefficients are limited. We ran a series of experiments to characterize the temperature dependence of Li diffusion in Mexican bytownite (An₆₀) to examine the potential for Li diffusion in feldspar to serve as a chronometer for rapid magmatic events. The bytownite was prepared for diffusion experiments by cutting samples perpendicular to the C-axis, polishing one face to a mirror finish, and kiln annealing to diffuse vacancies for 5 days. Individual vertical tube furnace experiments consisted of a single prepared bytownite surrounded by a powdered SiO₂ source containing 200 ppm Li. Experiments were executed at temperatures from 900 to 1100℃. The individual experiments were then mounted in epoxy, polished and analyzed with laser ablation inductively coupled plasma mass spectrometry to obtain core-to-rim Li concentration gradients. All Li concentration gradients show contributions from two diffusion modes (Garvey et al., this meeting), in contrast to the previous work of Giletti and Shanahan¹. Li diffusivities for both slow and fast diffusion paths were determined using a Python-based numerical diffusion model. Modeled profiles yield diffusion coefficients for the slow pathway that are approximately one order of magnitude less than the fast pathway. The diffusion coefficients were fit with an Arrhenius relationship to obtain activation energy and Log(D₀) values for both the slow and fast pathways. The fast pathway activation energy is smaller than the slow pathway activation energy. Giletti and Shanahan¹, however, identified a single Li diffusion pathway with an activation energy greater than our slow pathway. Our measured diffusion coefficients for both diffusion pathways are slower than previously calculated Li diffusivities¹ at all temperatures. Our new data enhance understanding of Li diffusion pathways in feldspar and, when paired with future experiments, may be applied to natural magmatic feldspars to more accurately quantify the timescales of volcanic eruptions.

¹Giletti, B.J., Shanahan, T.M. (1997) Chem Geol 139, 3-20.