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Direct FE-SEM observation of crystallization in a rhyolitic silicate melt

Mayumi Mujin , Michihiko Nakamura, Megumi Matsumoto

Abstract

Nanocrystals in volcanic rock have recently attracted attention because they may influence eruption dynamics by increasing magma viscosity and serving as heterogeneous nucleation sites for bubbles. Hence, it is important to understand the crystal nucleation and initial growth processes in magmas. In the more commonly used heat and quench experiment, nucleation or nanocrystal crystallization may occur during the initial melting or quenching processes. Therefore, we attempted an in situ observation experiment that allowed continuous recording of the temporal evolution from heating to quenching. We succeeded in high-temperature in situ observation of crystallization in rhyolitic melt using field emission-scanning electron microscopy (FE-SEM). In our previous experiments under high-vacuum conditions, which was the first report of a non-classical growth pathway in magma, we observed metallic iron formation due to the low oxygen partial pressure and its growth by oriented attachment. In addition, in this study, attachment growth of iron oxide minerals, monomer-by-monomer growth, Ostwald ripening, and ultrananolite crystallization were observed under low-vacuum conditions adjusted by dry-air gas. Under sub-solidus conditions, we found that nanoscale liquid-phase immiscibility may promote the nucleation of ultrananolites. Understanding the occurrence of non-classical crystal growth pathways, depending on the composition and conditions of the magma, may contribute to better interpretations of the crystal size distribution and better understandings of the physical property evolution of ascending magmas.