Understanding the effect of H2O accumulation at boundary layer on dendritic quartz crystallization and melt inclusion formation via numerical modeling

Melt inclusions record magmatic compositions at depth, providing valuable information for understanding the storage, transport, and compositional evolution of host magma. However, the formation mechanisms of melt inclusions in various settings are still unclear. This lack of knowledge diminishes the reliability of using melt-inclusion compositions to infer volcanic processes, because of the uncertainty in formation conditions, and whether their compositions can reflect the larger-scale host melt.

The goal of this study is to understand the effect of latent heat and H2O release in crystallization, and crystal-melt relative motion on varying the local growth rate of quartz via direct numerical simulations of crystal growth. We hypothesize that the effect of latent heat is insignificant because of the high diffusivity of heat, while the synergy of H2O release and crystal motion can lead to locally heterogeneous growth rate. The heterogeneous growth can result in dendritic growth and formation of melt inclusions.

To test our hypothesis, we developed a numerical model of the interface growth of a quartz crystal immersed in magmatic melt with temperature and H2O concentration controlling the undercooling of the melt. Our model allows for flow within the domain and solves an advection-diffusion equation for temperature and H2O concentration. We follow published experimental results to construct the relationship between undercooling and temperature or H2O concentration, and the relationship between growth rate and undercooling.

Our preliminary results suggest that under the typical growth rate of quartz, latent heat is unlikely to cause a significantly heterogeneous temperature field around individual crystals, because heat diffuses much faster than crystal grows. In contrast, H2O released in crystallization can change the local H2O concentration in initially H2O-undersaturated melt, resulting in a change in the liquidus and thus degree of undercooling. This different behavior from latent heat is due to the much smaller diffusivity of H2O than heat. The resultant heterogeneous growth rates cause dendritic growth and initiation of melt inclusion formation under medium to high degree of undercooling. Crystal-melt relative motion leads to additional heterogeneity, resulting in asymmetric crystal shapes.