• undercooling
• pyroxene
• volcanology
• basalt
• dynamic crystallization
• experiments
• petrology
• Stromboli

Magma crystallization is a fundamental process driving the evolution of magmas in the crust and influencing the style of volcanic eruptions. Crystallization occurs through nucleation and growth of mineral phases. These are complex processes that significantly affect the textural evolution of cooling magmas and, in turn, their rheological behaviour and eruptive dynamics. The degree of undercooling, expressed as the difference between the liquidus and crystallization temperatures (∆T = Tliq – Txll), represents the thermodynamic force controlling magma crystallization and can be interpreted as the delay of a crystallizing melt with respect to a theoretical amount of crystallization expected at a given temperature. By investigating the effect of undercooling it is possible to model the crystallization of basaltic melts under dynamic conditions that are characteristic of open conduit volcanoes. Stromboli volcano is a reference example for these types of volcanic systems, being a worldwide famous example of persistent activity volcano, and periodically experiencing changes of eruptive style and abrupt shifts in explosion energy (paroxysms). Nonetheless, only a few experimental studies have been carried out to investigate the crystallization dynamics of its high-K basaltic magmas.
In this thesis, I performed dynamic crystallization experiments using a high-K basaltic magma from Stromboli (the low porphyritic - LP - pumice erupted at Stromboli volcano during the paroxysm of 5th April 2003) in order to examine the effect of undercooling on the crystallization mechanisms of this magma. Experiments were performed at 350 to 500 MPa, 1050 to 1150 °C, and at both anhydrous and hydrous (2 wt.% H2O) conditions. Under such conditions, the degree of undercooling imposed to the system ranges from 10 °C to 162 °C. The textural and compositional features of the experimental samples were analyzed using synchrotron radiation X-ray tomography (for the 3D reconstruction of crystal morphologies) and electron microscopy (FE-SEM and EPMA). Clinopyroxene is the most abundant mineral formed in the experiments and the most relevant in terms of textural and compositional changes. At increasing degree of undercooling the clinopyroxene morphology evolves from prevalently skeletal to dendritic, and the composition becomes enriched in incompatible elements, such as Ti and Al (Al in tetrahedral coordination), with a simultaneous decrease of Si and Mg. The effect of undercooling on the clinopyroxene composition constrained by dynamic crystallization experiments is then used to calibrate by least square regression an equation that allows to predict the undercooling of basaltic magmas erupted at Stromboli volcano.
In parallel to this thesis, I worked to an experimental project aimed at determining the timescales of LP and HP magmas interaction at Stromboli volcano. This project is part of my internship and was aimed at developing a new experimental technique to measure the dissolution rates of plagioclase in hydrous basaltic melts. A summary of the results is reported in the appendix.