Experimental Constraints on the Impact of Shear Strain Rate on Early Crystallization Dynamics and Residual Glass Composition: Implication for Magma Evolution in Natural Systems

Crystallisation processes in magmatic plumbing systems strongly influence magma rheology and eruptive styles. The presence of a deformation field in magma storage regions and transport systems can greatly affect crystal nucleation and growth. Here we present new experimental results on the effect of shear strain rates (ϓ̇) on texture, crystal zoning patterns, mineral phase proportion (plagioclase, clinopyroxene, olivine, and oxides), and residual glass composition. The experiments, five in total, were carried out using natural trachybasalts under controlled temperature conditions at atmospheric pressure. The main experiment has been initially conducted at 1130°C under a shear strain rate gradient (1 s−1 – 0 s−1, from the rotating spindle to crucible walls) using a Concentric Cylinder Apparatus. Then, the deformation has been removed and minerals continued to evolve under temperature oscillations (1170–1130°C). The main rationale behind our approach is to demonstrate the impact of deformation on early crystallization and the subsequent evolution of the system, even after deformation ceases to be effective. In natural settings, such conditions may arise during conduit dynamics, lava flow emplacement and during the development of a shallow magmatic system. Major element analyses and elemental maps were analysed using custom-built unsupervised and supervised machine learning algorithms (e.g. Hierarchical Clustering and Random Forest) to quantify how the area proportions of different chemical zoning patterns vary with inferred ϓ̇. The effect of ϓ̇ on nucleation, growth and mineral phase proportions was quantitatively investigated through shape and crystallographic preferred orientation analysis. The experimental results demonstrate how a small increase in ϓ̇ can lead to a significant increase in nucleation rate and thus in crystal number density. While this general relationship has been observed in previous studies (Vona & Romano, 2013; Kolzenburg et al., 2017; Vetere et al., 2017; Mollo et al., 2024), our results demonstrate how these changes directly influence growth competition among different mineral phases, leading to measurable variations in their growth rates, final mineral phase proportions, and residual melt composition. The application of our results to the February–April 2021 eruptive sequence of Mt. Etna reveals that the chemical variability observed in our experiments is in the same range as that observed at Mt. Etna during the studied eruptive sequence. This underlines how different ϓ̇ (e.g. different magma ascent rates) can provide a major contribution to the chemical variability of the erupted products.