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Diffusion of Sr and Ba in plagioclase: New experimental data and consequences for volcanic timescales

Thomas Grocolas1, Elias Bloch2, Anne-Sophie Bouvier1, Othmar Müntener1

  • Affiliations: 1Institute of Earth Sciences, University of Lausanne, Switzerland; 2Lunar and Planetary Laboratory, University of Arizona, USA

  • Presentation type: Poster

  • Presentation time: Monday 16:30 - 18:30, Room Poster Hall

  • Poster Board Number: 75

  • Programme No: 1.2.20

  • Theme 1 > Session 2

Abstract

Strontium and barium diffusion chronometry in plagioclase is routinely applied to mafic and felsic magmatic systems. This technique can be used to determine the timescales of magma reservoir assembly and the cooling rates of plutons and volcanic rocks, which has emerged as a useful method to assess volcanic hazards. 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. There is 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 the 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 diffusivities determined in this study to plagioclase crystals from the Cerro Galán and Santorini calderas, we find timescales of ~105 years, with a good agreement between results from Sr and Ba diffusion modelling. Therefore, our data suggest that, at least regarding the Cerro Galán and Santorini calderas, plagioclase records the time needed to differentiate magma reservoirs and assemble large volumes of eruptible magma.