Diffusion of Sr and Ba in plagioclase: New experimental data and application to ignimbrites and calderas

Constraining the temperature evolution of magma reservoirs constitutes an important scientific and societal challenge in order to mitigate future volcanic hazards. In the last decades, diffusion chronometry emerged as a valuable tool to track the timescales of magmatic processes and is now routinely applied on erupted volcanic products to infer crystal residence times and mixing-to-eruption timescales. As such, calibrating the diffusion rates of Sr and Ba in plagioclase, the most abundant mineral in the Earth’s crust, is critical to place timing constraints on different magmatic systems. However, discrepancies between previous studies investigating the diffusion rates of divalent cations in plagioclase persist (1–6). Here we report diffusion experiments that aim to constrain the diffusivities of Sr and Ba in oligoclase and labradorite at 1 atm pressure, between 900 and 1200 °C, and as a function of the crystallographic orientation and aSiO₂. The experimental products were analysed by SIMS depth profiling and LA-ICP-MS line scanning. The analysed annealed crystals reveal no resolvable dependence of Sr and Ba diffusion in plagioclase upon aSiO₂ or crystal orientation. However, Sr and Ba diffusivities are found to vary with plagioclase anorthite content. The diffusion rate of Sr in plagioclase determined in this study is ~1.5–2 orders of magnitude slower than previously determined, whereas Ba diffusion is similar to previous studies. This is likely due to Ba-feldspar stability at the experimental conditions employed by previous studies, whereas Sr-feldspar was absent from their source powder assemblage. By applying the Sr and Ba diffusivities determined in this study to plagioclase crystals from the Cerro Galán ignimbrite (Argentina) and Santorini caldera (Greece), we find timescales of ~10⁵ years, with a good agreement between results from Sr and Ba diffusion modelling. Therefore, our data reconcile experimental diffusion data with measured Sr and Ba profiles in plagioclase and suggest that, at least regarding the Cerro Galán ignimbrite and Santorini caldera, plagioclase records the time needed to differentiate magma reservoirs and assemble large volumes of eruptible magma.

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