Kinetics of crystallization: example of micro-crystallization in basalt lava

The kinetics of crystallization of basalt glass from Kilauea, Hawaii, was investigated to derive time and temperature dependence of growth rates, and to relate these results to crystallization characteristics that develop during in-place cooling of lava and isothermal re-heating in the lab. The choice of temperatures of re-heating was based on observations that pyroxene and Fe-Ti oxide production starts at about 850 °C and plagioclase above 920 °C. Crystallization parameters were determined by the crystal size distribution method (CSD), which allows one to derive growth (G) and nucleation (J) rates, if the residence time within the temperature interval of crystallization is known. For the in-place cooled lava, the residence time was determined from cooling rates that could be derived from literature data of temperature measurements in the upper 2 cm of lava lobes. Re-heating of basalt glass as a function of time shows a linear relationship between log G and log (time): % MathType!MTEF!2!1!+- % feaaeaart1ev0aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabiabaaGcbaGaciiBaiaac+ % gacaGGNbGaam4raiabg2da9iabgkHiTiaadggaciGGSbGaai4Baiaa % cEgacaWG0bGaeyOeI0Iaam4AamaaBaaaleaacaaIXaaabeaaaaa!42ED! logG = - alogt - k₁\log G = - a\log t - k_1 . This observation was possible because the time scale of crystallization was similar to the observational time scale, a condition usually not given for cooling processes. The time dependence of G is shown to correspond to a decrease in G (and J) from the original surface-air interface to the interior of the lava: a change that occurs concomitant with an increase in crystal size and number density. Hence, it is the time dependence of G that explains the hitherto well-observed relationship with cooling rate along a temperature gradient in magmatic bodies. Moreover, the log-linear relationship between time and growth rate is suggested to describe heterogeneous nucleation and growth, which is independent of the process, i.e. cooling or metamorphism. Isothermal re-heating extinguishes the high growth rate signatures of cooling. Temperature dependence of growth rates, carried out at 24 h, shows minimum temperatures of 900-950 °C, which, because of the time dependence of growth, refers to the nucleation and crystallization maximum, and is in agreement with results from differential scanning calorimetry.