Unravelling the effect of undercooling on (dis)equilibrium textures and compositions of basaltic magmas

Undercooling and crystallization kinetics are recognized increasingly as important processes controlling the final textures and compositions of minerals as well as the physicochemical state of magmas during ascent and emplacement. Within a single volcanic unit, phenocrysts, microphenocrysts and microlites can span a wide range of compositions, develop complex zoning patterns, and show intricate textures testifying to crystallization far from equilibrium. These petrographic complexities are not associated necessarily with magma chamber processes such as mixing or mingling of distinctly different bulk compositions but, rather, may be caused by kinetic effects controlling the crystal growth and magma evolution. Heat-dissipation and decompression are the most effective driving forces of cooling and volatile loss that, in turn, exert a primary control on the solidification path of the bulk system (i.e., crystal and melt). Understanding these kinetic aspects over the temporal and spatial scales at which volcanic processes occur is therefore essential to interpret correctly the time-varying environmental conditions recorded in igneous minerals. This PhD thesis aims to summarize and integrate experimental and natural studies pertaining to the crystallization of magmas along kinetic or time-dependent pathways, where solidification is driven by changes in temperature, pressure and volatile concentration. Fundamental concepts examined in the last decades include the effect of undercooling on crystal nucleation and growth, as well as on the transition between interface- and diffusion-controlled crystal growth and mass transfer occurring after crystals stop growing. In this thesis, static and dynamic crystallization processes are investigated for natural trachybasaltic-trachyandesitic products typically of the magmatic activity at Mt. Etna volcano (Sicily, Italy). By decoding the textural and compositional information within crystalline phases, it is possible to place quantitative constraints on the crustal transport, ascent and emplacement histories of erupted magmas. Here, I present a collection of three papers that has been published in some of the most respected peer-reviewed journals for mineralogy, petrology, geochemistry and volcanology, i.e., Chemical Geology (first paper), Geochimica et Cosmochimica Acta (second paper) and Minerals (third paper). More specifically, magma crystallization under dynamic conditions has been assessed through isothermal time-series experiments on clinopyroxene, illustrated in the first paper reported in the following thesis. In this work a variety of departures from polyhedral growth, including morphologies indicating crystal surface instability, dendritic structures, sector zoning and growth twins are linked to the rate at which crystals grow. These have implications for the entrapment of melt inclusions and plausibility for interpreting the growth chronology of individual crystals. In the second paper, deviation from chemical equilibrium, developed in response to kinetically controlled cation redistributions and related to the partitioning of major and trace elements between clinopyroxene and melt, has been evaluated by the analysis of hourglass sector-zoned phenocrysts based on equilibrium and thermodynamic principles. In the third paper, the crystallization conditions of a sill-like intrusion at Mt. Etna volcano have been elucidated by integrating major cation exchanges in clinopyroxene, plagioclase and titanomagnetite, thereby providing decompression, degassing and geospeedometric constrains on the emplacement conditions of magma. All the aforementioned papers were carried out in team, but I have contributed either in designing and performing both the experiments and analyses, or in interpreting the acquired data and in writing the original manuscripts.