Effect of stirring on crystallization kinetics of basalt: texture and element partitioning

Abstract

Isothermal crystallization experiments on basalt have been carried out using an infrared heating furnace to investigate the effect of stirring. When stirring was not applied (static experiment), the results agreed well with previous experiments. But when stirring was applied and a flow of Reynolds number=10^−3∼−4 was present (dynamic experiment), considerably different results were obtained, especially in respect to the nucleation rate and the morphology of crystals. At ΔT=25° C essentially similar results were obtained on the nucleation rates and morphologies of crystals in both static and dynamic experiments. However, at supercoolings larger than 45° C, nucleation density increased drastically in dynamic experiments reaching up to ten times as large as that in static experiments. Crystals of plagioclase and clinopyroxene were small and adapted acicular morphology regardless of ΔT in dynamic experiments, and hyalopilitic textures were formed. A TTT-diagram shows that the nucleation incubation time is shorter in dynamic experiments than in static experiments. No compositional difference in major elements was found in plagioclase and clinopyroxene produced in both static and dynamic experiments. However, minor element concentrations, e.g., Mg in plagioclase and Ti, Al in clinopyroxene, were found to increase with both ΔT and flow velocity. All these results imply that although chemical diffusion in the melts did not play an important role in the dynamic experiments, interface kinetics were important. It is suggested that hyalopilitic texture commonly seen in natural basalt is mainly due to flow in magma.