[S-VC33] Mechanism of volcanic eruptions

[SVC33-01] Effects of nanometer scale chemical and structural heterogeneities in silicate melt on magma crystallization

*Kazuhisa Matsumoto¹, Satoshi Okumura¹, Jun Uzuhashi², Tadakatsu Ohkubo², Akira Miyake³ (1.Tohoku University , 2.National Institute for Materials and Science, 3.Kyoto University)

Keywords:decompression-induced crystallization, super heating, pre-crystal cluster, atom probe tomography

The crystallization of silicate melts induces the diversity of eruption style and intensity by changing the physical properties of magma. Numerous previous studies demonstrated that the degree of undercooling controls crystallization kinetics. Recently, Matsumoto et al. (2023) experimentally investigated the effect of superheating (i.e., heating above the liquidus temperature for a finite period) on crystallization kinetics and found that low superheating is insufficient to homogenize a melt resulting in the formation of pre-clusters of crystals, which could work on the site of heterogeneous nucleation. However, the identity of the pre-clusters remains unclear. In this study, we investigated, on the sub-nano to nanometer scale, the chemical heterogeneity in glassy samples, which were synthesized at two different temperatures (940 and 1300°C). In the magma studied here, the liquidus temperature is estimated to be 920°C. The glassy samples investigated in this study showed different crystallization kinetics during the decompression, that is, the sample heated at 940 °C showed a high crystal nucleation rate, whereas no crystallization occurred in the sample heated at 1300 °C under the same degree of undercooling. The atom probe tomography (APT) and scanning transmission electron microscopy (STEM) with energy-dispersive X-ray spectroscopy (EDS) were applied to the chemical analyses for the two samples. In both samples, the APT analyses showed that the regions with high Al and Ca content but low Si content are distributed in a reticulate pattern on the ~5 nm scale. Additionally, hydrogen clusters with a few nanometer size were found. The STEM-EDS analyses showed that Na migration during beam scanning occurred easily. We infer that a high Na migration rate originates from the well-connected non-bridging oxygens (NBOs) because the transition energy for Na migration is low along with NBOs. As the crystallization rate during the decompression was significantly high in the sample heated at 940 °C, the NBO-rich regions, where the melt is strongly depolymerized, may be the location of the pre-crystal clusters. In nature, the formation and dissolution of crystals occur due to temperature fluctuations in the magma reservoir and NBO-rich regions could be formed through these processes.