Multi-mode magnesium diffusion in sanidine: Applications for geospeedometry in magmatic systems

The duration of magmatic processes can occur over a wide range of time from as long as millennia to as short as hours. It is therefore important to have a variety of geospeedometers that are sensitive to different timescales. The diffusivity of Mg in K-bearing feldspar such as sanidine has previously been a critical gap in the application of geospeedometry, as sanidine is a ubiquitous phase in dacites and rhyolites and also hosts a variety of major and trace elements with variable diffusivities. Here we present the results of a series of 1-atm diffusion experiments in order to constrain the diffusivity of Mg in sanidine. Two compositions of sanidine (Or₇₁ and Or₈₂) and crystallographic orientations (c-parallel and c-perpendicular) are investigated, showing Mg diffusion is isotropic with little to no resolvable major element compositional dependence. Additionally, Mg diffusion in sanidine simultaneously operates by a fast- and slow-path diffusion mechanism as suggested by irregular depth profile shapes that do not conform to an analytical solution to the diffusion equation. Therefore, we employed a model that considers multi-site diffusion reactions in order to determine diffusion coefficients. The following Arrhenius relationships are obtained for Mg diffusion in sanidine: D_Or71-Fast = 4.2 E-12 exp (−275 ± 14 kJ mol⁻¹/RT) m² s⁻¹, D_Or71-Slow = 1.4 E-05 exp (−369 ± 15 kJ mol⁻¹/RT) m² s⁻¹, D_Or82-Fast = 1.6 E-05 exp (−283 ± 14 kJ mol⁻¹/RT) m² s⁻¹, D_Or82-Slow = 3.9 E-04 exp (−405 ± 15 kJ mol⁻¹/RT) m² s⁻¹. Fast-path diffusivities for Mg in sanidine are similar to those of Mg in plagioclase determined by Van Orman et al. (2014). Slow-path diffusion in sanidine is at least a few orders of magnitude slower than fast-path diffusion. Slow-path diffusion may be rate-limited by exchange reactions between divalent cations such as Ba and Sr with K, and Mg with Al. Fast-path diffusion likely operates by interstitial and/or vacancy diffusion. Equipped with these new diffusivities, this study calculates timescales for magmatic rejuvenation that likely initiated the Lava Creek Tuff supereruption at Yellowstone Caldera to be as short as weeks but no more than a few decades. This work also demonstrates the complexity that may exist in experimentally-derived geospeedometers and the importance of careful application to natural systems.