Crystal size distribution of plagioclase and amphibole from Soufrière Hills Volcano, Montserrat: Evidence for dynamic crystallization-textural coarsening cycle

Abstract

The crystal size distributions (CSDs) of plagioclase and amphibole were determined from andesites of the Soufrière Hills volcano, Montserrat. Plagioclase occurs as separate crystals and as chadocrysts in large amphibole oikocrysts. The chadocrysts represent an earlier stage of textural development, preserved by growth of the oikocryst. Seventeen rock and eight chadocryst plagioclase CSDs are considered together as a series of samples of textural development. All are curved, concave up, and coincident, differing only in their maximum crystal size. Three amphibole CSDs have a similar shape and behaviour, but at a different position from the plagioclase CSDs. A dynamic model is proposed for the origin of textures in these rocks. Crystallization of plagioclase started following emplacement of andesite magma at a depth of at least 5km. A steep, straight CSD developed by nucleation and growth. This process was interrupted by the injection of mafic magma into the chamber, or convective overturn of hotter magma. The magma temperature rose until it was buffered, initially by plagioclase solution and later by crystallization. During this period textural coarsening (Ostwald ripening) of plagioclase and amphibole occurred: small crystals dissolved simultaneously with the growth of large crystals. The CSD became less steep and extended to larger crystal sizes. Early stages of this process are preserved in coarsened amphibole oikocrysts. Repetitions of this cycle generated the observed family of CSDs. Textural coarsening followed the 'Communicating Neighbours' model. Hence, each crystal has its own, unique growth-solution history, without appealing to mixing of magmas that crystallized in different environments.