Magma Emplacement and Remobilization Timescales Beneath Montserrat: Insights from Sr and Ba Zonation in Plagioclase Phenocrysts

Abstract

Since 1995, the Soufrière Hills volcano on Montserrat, Lesser Antilles island arc, has erupted crystal-rich andesite magma that is geochemically very similar to the volcanic products of at least the last 174 ± 3 kyr (1σ). Nomarski images of plagioclase phenocrysts from six andesites erupted between 151 ± 4 ka (1σ) and AD 1999 reveal multiple crystal resorption events, and major shifts in anorthite content suggest that these are due to temperature variations. A model is developed that uses partitioning data for Ba and Sr to quantify the degree of intracrystalline disequilibrium of these elements with respect to anorthite content in 12 selected plagioclase phenocrysts. Intracrystalline diffusive equilibration of both Ba and Sr profiles is incomplete. However, local disequilibria do not consistently decrease towards crystal cores, i.e. potential differences between core and rim ages cannot be resolved, and so the timescale for crystal growth is considerably shorter than their residence time at magmatic temperatures. Using two-dimensional finite difference modelling of intracrystalline Sr diffusion as a first-order approximation of diffusion in three dimensions, it is possible to calculate bulk crystal residence times at the estimated Soufrière Hills magmatic temperature of 850°C. These range from ∼10 to ∼1200 years, independent of eruption age. Crystal residence times in the currently erupting andesite range from ∼15 to ∼320 years. Diffusion in three dimensions would result in shorter residence times. Such results can be explained by repeated intrusion of small volumes of andesitic material to upper-crustal levels, followed by rapid crystallization during degassing and rapid cooling. Remobilization of the intruded andesites by influx of hotter, more mafic magma, and amalgamation of andesites of different crystallization ages by associated convective processes, produced the currently erupting andesite that contains abundant inclusions of mafic magma. The short residence times of andesite magma in the upper crust, and the episodic character of volcanism at the Soufrière Hills with short periods of intense volcanism alternating with much longer periods of dormancy, are consistent with deep magma generation and storage and with episodic ascent to form ephemeral shallow chambers.