Modeling clinopyroxene and plagioclase growth kinetics at Mt. Etna and Stromboli: a time-integrated, polybaric and polythermal perspective

Basaltic volcanoes (e.g., Mt. Etna, Stromboli, Hawaii, etc.) are characterized by a range of effusive to explosive activities with variable intensity, which can pose different type of threats to local populations. Challenges in modern volcanology and petrology involve the attempt to constrain pre-eruptive magmatic processes, which provide the basis for volcanic hazard assessment. Although the recent literature has reported constant advancements in this respect, several key questions remain unanswered. Understanding how magma is stored, migrates and feeds eruption is not a trivial task, requiring for renewed improvements over the years. In this context, both textural maturation and compositional variability of minerals crystallizing in basaltic systems represent valuable sources of information to quantify the physio-chemical conditions experienced by magmas upon the effect of changing and complex plumbing system dynamics. This study aims to provide new insights on the solidification behavior of mafic alkaline magmas erupted at Mt. Etna and Stromboli (Italy). Such open conduit volcanoes are characterized by the ubiquitous stability of clinopyroxene from mantle depths to shallow crustal levels. More evolved magmas are also saturated with plagioclase, especially at lower temperatures, melt-water contents, and pressures. Thus, clinopyroxene and plagioclase crystals represent powerful recorders of the intricate ascent dynamics explored by mafic alkaline magmas during their ascent paths towards the surface. By focusing on textural and chemical features of natural/synthetic clinopyroxene, plagioclase and coexisting glasses, I have provided new tools for interpreting polythermal-polybaric changes of magmas, together with several guidelines and a secure methodology to model pre- and syn-eruptive conditions. The temporal evolution of Etnean and Strombolian magmas has been also tracked via timescale modeling to better constrain the cooling-decompression paths of magmas rising and accelerating through the vertically extended, highly dynamic plumbing systems. In the first part of this PhD thesis, I have experimentally explored the role of supersaturation and relaxation phenomena on clinopyroxene nucleation and growth processes, which affect the final crystal cargo of variably undercooled magmas. A certain degree of undercooling is pivotal to promote the growth and textural maturation of crystals. With increasing crystallization time, however, the crystal growth rate decreases as the system approaches to near-equilibrium conditions that minimize the effect of melt supersaturation. By quantifying the textural features of synthetic and natural crystals it has been possible to parameterize clinopyroxene growth kinetics under a broad range of isothermal-isobaric, decompression, and cooling conditions representative of crystallization scenarios typically encountered in open-conduit volcanoes. This model parameterization has been combined with the textural analysis of natural clinopyroxene crystals erupted during lava fountain events at Mt. Etna allowing to unlock timescale of growth for microphenocryst and microlite populations. The retrieved temporal information has been used to develop a new conceptual model for the timescale of magma dynamics recorded by the (dis)equilibrium textural evolution of clinopyroxene and for the rapid acceleration of magma ascending within the volcanic conduit, immediately before eruption at the vent. A more comprehensive work, focusing on plagioclase textural and compositional features, characterized the second part of my PhD thesis with the aim to identify disparate aspects of plagioclase growth scenarios. Following the same approach discussed above, timescale of plagioclase crystallization from mafic alkaline magmas has been parameterized as a function of growth rate by integrating experimental (i.e., isothermal-isobaric, decompression, and cooling experiments) and natural textural data from literature. Timescales of eruptive processes at Mt. Etna and Stromboli volcanoes have been quantified by considering phenocryst/microphenocryst and microlite crystals growing during lava flow and explosive eruptions. Statistical methodologies have been employed to assess the correlation between plagioclase growth rate and other system parameters governing the crystallization process. Special attention has been paid to disambiguate the role of temperature and melt-H2O content on plagioclase chemical zoning patterns at Stromboli and Mt. Etna. By using plagioclase components and major cation substitution mechanisms, I have refined and readapted equilibrium, thermometric, and hygrometric models for future investigations.