When and how Fe-oxide nanocrystals form: Insights from in situ FE-SEM and implications for eruption dynamics

Nanocrystals (nanolites and ultrananolites) in volcanic rocks have attracted considerable attention owing to their potential role in increasing magma viscosity and serving as heterogeneous nucleation sites for bubbles, thereby influencing eruption styles. However, the conditions, time scales, and mechanisms of their formation remain poorly constrained. In this study, we conducted high-temperature, in situ observations of crystallization in a rhyolitic silicate melt using field-emission scanning electron microscopy. Nucleation of fine-grained Fe-oxide crystals (≤100 nm in width and diameter) occurred at high number densities (∼1020−1021 m−3) under sub-solidus and oxidizing conditions, whereas their growth into coarser nanolites was not observed. The fastest case of beginning crystallization was ∼35 s after the melt reached the target temperature. Under above-solidus conditions, nucleation occurred at substantially lower number densities (≤∼1019 m−3) and crystal growth dominated with time. Nucleation under sub-solidus and oxidizing conditions was likely facilitated by spinodal-type liquid immiscibility between Fe-rich and Fe-poor melts. These results suggest that high-number-density ultrananolites formed through liquid immiscibility may serve as indicators of shallow magmatic processes but are unlikely to control the occurrence of Plinian eruptions in deep conduits. Rapid nucleation of Fe-oxide ultrananolites could allow their detection in pumice from explosive eruptions.