EXPERIMENTAL CHARACTERIZATION OF MULTI-PATHWAY DIFFUSION OF LITHIUM IN FELDSPAR: IMPLICATIONS FOR LI-IN-FELDSPAR DIFFUSION CHRONOMETRY

Lithium is a relatively fast diffusing, moderately volatile element and therefore has the potential to quantify the timescales of processes such as magma recharge, degassing, and decompression which can occur minutes to days before an eruption. The experimental calibration of Li diffusion in feldspar provides a method to quantify these rapid timescales in a ubiquitous mineral phase and aid in eruption forecasting. We ran Li diffusion experiments in vertical gas mixing tube furnaces on An₆₀ feldspar in open top alumina crucibles with powdered spodumene (LiAlSi₂O₆) as our Li source. The spodumene was mixed with powdered quartz at the 20, 200, and 2000 ppm levels to test the influence of source concentration on Li diffusivity. Time series experiments at the 20 ppm level produce diffusion profiles that are difficult to resolve within the uncertainties of modern LA-ICP-MS analyses. In contrast to previous results¹, longer time series experiments at both the 200 and 2000 ppm levels produce diffusion profiles that typically do not conform to that of the constant surface concentration solution and require a multi-path diffusion finite difference model to obtain relevant diffusivities for Li. Preliminary results show that Li diffuses in feldspar (and other materials e.g., olivine²) via a slow vacancy mechanism and a fast interstitial mechanism, governed by the reaction Li_Interstitial + Vacancy → Li_Metal. Time series results show the interstitial mechanism operates roughly one order of magnitude faster than the vacancy mechanism, though this difference reduces with increased time. This suggests a greater contribution from the slower vacancy mechanism over longer periods of time. Experiments run with 200 ppm Li source show Li diffusivities decrease by more than an order of magnitude as experiment time increases from one hour to five days. Our preliminary data suggests that previous timescales constrained by Li-in-feldspar diffusion chronometry may be longer than suggested by the data of Giletti and Shanahan¹. However, more experiments are needed to better understand the multi-path diffusion behavior of Li in feldspar and to produce a meaningful Arrhenius relationship (Larouche et al., this meeting).

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

²Dohmen et al. (2010) Geochim Cosmochim Acta 74, 274-292.