Crystallization experiments in rhyolitic systems: The effect of temperature cycling and starting material on crystal size distribution

One of the various problems faced in experimental petrology is the fact that most experimental products obtained by crystallization experiments are too small, making their accurate identification by electron microprobe and laser ablation analyses very difficult. This problem is magnified when a highly polymerized starting material is used for experiments at low temperature (e.g., 700–800 °C). In this study, we present the results of crystallization experiments performed using a rhyolitic starting glass in which we test the potential of temperature cycling and pre-hydrated starting material to increase crystal size and discuss the effect of those variables on the attainment of chemical equilibrium. Experiments were performed at different temperatures (725 to 815 °C) and pressures (1 and 2 kbar), under water-saturated conditions (aH2O= 1; with aH2Obeing the water activity). During the experiments, temperature was either constant or cycled to ±15 °C around the target temperature during the first half of the runs. We used either a pre-hydrated (7 wt% H2O) rhyolitic glass or a dry rhyolitic glass to which 7 wt% H2O was added during capsule preparation.