Determining the Effects of Cooling Rate on Magma Crystallisation Using a High Temperature Heating Stage

Abstract

In order to understand igneous rock textures and the history of erupted products and the dynamics of lava flow emplacement, it is necessary to understand how crystals grow and what effects different cooling histories have on their growth. Previous studies on crystal growth have often assumed constant crystal growth rates and focused on crystallisation over long timescales more appropriate to igneous intrusions. This work aims to quantify and describe crystal growth rates, morphological variations and textural development of crystals growing from natural lava flow samples, primarily focussing on plagioclase feldspar. High Temperature Heating Stage experiments were carried out at temperatures and cooling rates appropriate to basaltic lava flows, in which wafers of the glassy rind from Blue Glassy Pahoehoe were melted and re-crystallised. It was possible to directly observe and record crystal growth over time at controlled cooling rates, and to extract information from the quenched products. The experiments in this study grew crystals at very low undercoolings, maintaining an interface controlled growth regime and facetted crystal morphologies. Bulk
evolution of crystal growth indicated that growth rates were not constant over time, decaying as they grew. The morphology and aspect ratio of these crystals changed over time, with aspect ratio increasing as growth was significantly faster in the length direction during the observed period. The relationship between mean aspect ratio and crystallisation time proposed by Holness (2014) was experimentally verified. We also observed the ‘true’ crystallisation time of crystals, highlighting a need for better constraint on how crystal growth times are used to calculate growth rates. The results of this study will contribute to better future interpretations of magmatic histories and crystallisation conditions in natural basaltic lava flows, as well as refinement of Crystal Size Distribution studies.