Crystallization kinetics of rhyolitic melts using oxygen isotope ratios

Abstract

Crystals provide the means to understand igneous systems, but natural constraints on crystallization kinetics are rare because thermal conditions and crystallization timescales are typically unknown. Oxygen isotope ratios in quartz and alkali feldspar crystals in spherulites provide a natural record of the temperature interval of crystallization and crystal growth rates in rhyolitic melts. Oxygen isotope compositions in both phases change progressively with position from the spherulite core to rim. Quartz δ18O increases from 5.0 ± 0.3‰ in the core to 5.6 ± 0.3‰ at the rims, whereas alkali feldspar decreases from 3.7 ± 0.4‰ in the core to 2.7 ± 0.9‰ at the rims. Fractionation therefore increases from 1.3 ± 0.7‰ in the cores to 2.9 ± 1.1‰ at the rims. Oxygen isotope thermometry tracks crystallization temperature with position. Spherulites nucleate at 578 ± 160°C and continue to grow until 301 ± 88°C. The in situ analyses demonstrate that spherulites self-contain a record of their thermal history and that of the host lava.