Insights into the relationship between mixing duration and Volcanic Explosivity Index (VEI): Pacaya and Fuego volcanoes, Guatemala

This study investigates whether the timing of magma mixing phenomena could be related to explosivity and style of volcanic activity. Lava flow and tephra samples, derived from observed, energetically-diverse eruptions, from Pacaya and Fuego volcanoes (Guatemala), were studied, the Volcanic Explosivity Index (VEI) was used as an indicator of eruption intensity. Polarized light microscopy and Scanning Electron Microscopy (SEM) techniques were used for textural and geochemical measurements. Results are interpreted as evolution of mixing through changes in textures of plagioclase and olivine crystals. These suggest that boxy-cellular plagioclase (high frequency in VEI 0 samples) is associated with brief mixing residence time prior to the eruption, while sieve and spongy textures in plagioclase (high frequency in VEI 1-2 samples) reflect heating by recharge events with a hotter melt, and reflect longer time in the magma chamber prior to the eruption. CSD data lead to a model to explain the increase in explosivity. Crystal size distributions with a significative step that derive from a single mixing event correlates with low intensity eruptions (VEI 0-1) because convection is hindered by the dissolution of the microcrysts, while multiple or prolonged events of mixing gives a more linear curve resulting in a high content of phenocrysts that make convection difficult, increasing the explosive potential (VEI 2). Besides, CSDs analysis show how the residence time in the shallow magma chamber increase from VEI 0 to VEI 2 samples. As a result, this study could assert that the number of mixing events that determine the final CSDs shape, and the magma mixing-to-eruption duration is proportional to the explosivity of the volcanic event.