The plagioclase as archive of ascent dynamics: the 2001-2006 eruptive period at Mount Etna

Abstract

Several studies are focused on textural and compositional features of plagioclase as an usefull tool to investigate magma chamber processes, ascent dynamics, and physico-chemical conditions. In particular water content, which plays a fundamental role in volcanic process, strongly affects plagioclase stability and, by conseguence, textural and compositional features. However, such reconstruction are usually biased by too many assumptions; particularly when dealing with past eruptions or remote volcanoes. Only few volcanoes provide an array of instrumental monitoring to constrain timing and modality of eruptive events. In this respect Mount Etna probably represents one of the most controlled volcano in the world and a great wealth of seismological and ground deformations data are available. In this work we present a textural and compositional study of plagioclases from lavas emitted during the 2001-2006 eruptive period on Mount Etna. Textural classification has been done on over 130 thin sections taking into account different portion of the crystals. This allow to recognize different types of core (ehuedral and rounded) and rims (dusty or with melt inclusion alignment) separated by oscillatory zoned overgrowth. Oxygen fugacity in magmas has been calculated using the method of [1] and results has been used to reequilibrate the melts to mantle equilibrium, adding back the appropriate quantity of fractionated material. Water content of the melt has been estimated using the hygrometer of [2]. These data were used in the MELT model to estimate the plagioclase stability field and to calculate theoretic composition at different water content. Results were integrated with monitoring data acquired during the entire period under study with the aim to reconstruct magma ascent and storage conditions, as well as the mechanism of eruption triggering. Results indicate the 2001-2006 eruptive period involved magmas with quite similar major element composition but different dissolved H2O. Complex zoning such as dusty areas and alignments of melt inclusions in outer portion of the phenocrysts suggest two different trigger mechanism respectively: i) magma input and mixing with a more basic and volatile-rich magma; ii) fracture migration that induce decompression of shallow magma batches