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Diffusion in K-feldspar: Sr, Ba, Ti and P diffusion experiments in sanidine

Aurore Toussaint1, Anne-Sophie Bouvier1, Florent Plane1, and Othmar Müntener1

Abstract

Diffusion is a key to understand the timescales of magma dynamics, and thus the evolution of igneous systems. K-feldspar is a major rock-forming mineral in felsic plutonic rocks occasionally forming megacrysts, investigating the diffusion behaviour of trace elements in K-feldspar provides valuable information on magma reservoir dynamics. To provide better constraints on the timing of magmatic processes, there is a need for accurate diffusion coefficients. Therefore, we performed diffusion experiments in sanidine (Or98) at 1 atm pressure, between 825 and 1050 °C, for diffusion normal to (010) at controlled aSiO2, from times ranging from 1 month to 6 months. Sources of diffusant were SrO-, BaO-, TiO2- and P2O5-doped cristobalite or SrO- and BaO-doped cristobalite (prepared using the sol-gel method) mixed with ground sanidine (1:1), or with ground sanidine and apatite (1:1:1). The experimental products were then analysed by SIMS depth profiling. Sr diffusion rate is ~2 orders of magnitude slower than previously determined, while Ba, Ti and P did not show any measurable diffusion profiles, indicating that Ba and Ti likely diffuse more slowly than previously determined. Zoning in Ba in natural K-feldspar megacrysts thus predominantly records crystal growth processes that are weakly affected by diffusion. To further investigate Ba diffusion, we annealed a natural sanidine megacryst with complex Ba zonation (Taápacá, Chile, Rout et al. 2020) at 925 °C for 3 months at controlled aSiO2.Ba profile (before and after annealing) were measured with EPMA and LA-ICP-MS to potentially retrieve diffusion coefficient for Ba.