MG DIFFUSION IN ALKALI FELDSPAR; APPLICATIONS FOR GEOSPEEDOMETRY IN MAGMATIC SYSTEMS

The diffusion of elements and isotopes in crystals is now recognized as a powerful chronometer that enables the determination of rates of magmatic processes. Due to the variability in the duration of magmatic processes (millennia to hours), it is important to develop a range of geospeedometers that are sensitive to a range of timescales. Feldspar is a ubiquitous phase in magmatic systems and hosts both major and trace elements with variable diffusivities. Mg diffusion in Na- and Ca-rich feldspar, for example, has been shown to diffuse relatively fast compared to Ba and Sr (Cherniak 2010), such that it can record processes that occur over timescales as short as years to days at magmatic temperatures (Van Orman et al. 2014). What remains unknown is the diffusivity of Mg in K-bearing feldspar such as sanidine, a ubiquitous phase in dacites to rhyolites. Here we present the results of a series of 1-atm diffusion experiments in order to determine the diffusivity of Mg in sanidine for geospeedometry applications. Polished sections of Eifel sanidine (Or₇₃ and Or₈₃) were surrounded by source powder of MgO, SiO₂, Al₂O₃ mixed with ground sanidine of the same composition in Pt capsules and annealed in air. Experiments were conducted at 800–1150°C (in increments of 50°C) for times ranging from 30 minutes to 6 weeks and analyzed via depth profiling on the Secondary Ion Mass Spectrometry (SIMS) at Arizona State University. For the range of orthoclase contents investigated, it appears there is no major-element compositional dependence on Mg diffusivity and is a couple of orders magnitude slower than Mg diffusion in plagioclase. The Mg diffusivities in sanidine are within error of one another for the two crystallographic orientations investigated (|| to (001) and (010)). Overall rates of Mg diffusion in sanidine are comparable to the rates of Ba diffusion. This may be due to Mg diffusing via a coupled substitution with the more compatible Ba and/or Sr, when these large cation trace elements are at sufficient concentrations. Mg in sanidine thus may be most useful as a geospeedometer for longer magmatic timescales (decades to millennia) when sanidine contains common concentrations of Ba and Sr.

References Cherniak (2010), RMG 72, 691–733., Van Orman et al. (2014), EPSL 385, 79–88.