A cautionary tale of disequilibria in microlite-melt evolution driven by fast crystallization kinetics: Implications for modeling volcanic processes

• Increasing microlite content with more evolved melt towards the Hunga bomb interior. • Stronger microlite chemical-textural disequilibrium in bomb core due to fast growth. • Constant experimental crystal growth rate may not be applicable to natural processes. • Microlite growth creates melt compositional trend mimicking pre-eruptive processes. • Microlite-rich glass shards should be avoided in pre-eruptive process reconstruction. An intact submarine-quenched bomb collected from the globally impactful 2022 Hunga eruption served as a natural laboratory to investigate the role of syn- and post-eruptive magma cooling on microlite crystallization and, in-turn, melt composition in mafic-intermediate magmatic systems. We investigated chemical-textural gradients across the bomb and elucidated the crystallization kinetics and cooling history via thermal modelling. High crystal growth rates correlate with increasing chemical and textural disequilibrium of clinopyroxene and plagioclase microlites towards the bomb interior. These reflect unsteady crystallization kinetics and especially a transition from dominantly interface-limited to diffusion-limited growth regimes. These far-from-equilibrium processes are cause for caution in applying constant experimentally-derived crystal growth rates to explain natural fast-cooling processes, potentially leading to inaccurate magmatic timescales. In this single bomb, far-from-equilibrium crystallization processes drove compositional trends that could be mistaken for pre-eruptive fractional crystallization. If not thoroughly assessed, such compositional effects may impair our ability to reconstruct crystal-melt equilibrium pairs and retrieve accurate thermobarometric estimates. Our results suggest that before examining magmatic reservoir processes in mafic-intermediate systems samples should be checked for the overprinting influences of syn- and post-eruptive crystallization, particularly if textural examination reveals high microlite numbers and extreme crystal morphologies.