Crystallization of Cpx in the Ab-Di System Under the Oscillating Temperature: Contrast Dynamic Modes at Different Periods of Oscillation

Abstract

The evolution of the texture of a partially crystallized melt at a temperature oscillating around liquidus has been experimentally investigated. The model pseudobinary albite—diopside system was used, in which the clino-pyroxene is the only phase formed under the experimental conditions P = 200 MPa, T = 1050–1150 °C, C_H2O = 3.3 wt%. All products of the experiments were characterized by the Crystal Size Distributions (CSD) measured by BSE image analysis. Due to the high nucleation rate of clinopyroxene and the slow cooling rate of about 100 °C/min a large volume of quenching crystals was formed with residual glass approaching pure albite in composition. The quench tails were identified, and parts of the CSD formed prior to quenching were restored. In the CSD recovery procedure, we used an estimate of the equilibrium volume of Cpx, based on an experimentally verified melting diagram. We also assume that the crystal growth rate at quenching can be approximated as a size-independent parameter. In the experiment yielding the largest crystals, the Cpx cores formed prior to quenching were identified using microprobe observations, and their sizes were estimated from BSE images. The measured size distribution of the cores of Cpx crystals turned out to be close to the restored CSD. In our experiments, the variable period of temperature oscillations in the same range of 1135 < T < 1155 °C was a parameter controlling the texture. With a large period of oscillation (about an hour), all crystals dissolve at the hot stage and reprecipitate in a cold one, which leads to the formation of log-linear CSDs. The shortest oscillation period of 4–10 min results in a gradual maturation of the texture with time with a decrease in the number density of crystals, and CSD evolved to a log-normal shape with a maximum. The growth rate independent of the crystal size is essential for effective ripening with temperature oscillations. Simple calculations show that with diffusion control of both dissolution and growth, only the first temperature oscillation affects CSD.