Natural spherulite crystallization kinetics in rhyolitic melts

Wednesday, 17 December 2014
Kenneth S Befus, University of Texas at Austin, Jackson School of Geosciences, Austin, TX, United States, Kouki Kitajima, University of Wisconsin, Madison, WI, United States and John W Valley, Univ Wisconsin Madison, Madison, WI, United States
Crystals and crystal textures contain a wealth of information concerning their host igneous systems (e.g., P-T conditions, age, dynamics). Crystal nucleation and growth occurs in response to supersaturation caused by either cooling or changes in melt composition. Because natural constraints on such kinetic information is rare and difficult to extract, much of our understanding comes from theory and experiments. Using techniques to draw parallels to natural systems has limitations, thus kinetic information extracted from natural systems is noteworthy. Recent work using diffusion modeling to replicate trace element gradients outside of spherulites may provide one method to naturally assess crystallization kinetics in rhyolitic melts. That work constrains spherulite nucleation to an interval of 650 to 500 °C with growth ongoing to temperatures as cool as ~400 °C. Max growth rates were estimated to be ~1 μm hr-1, which exponentially decreased with time. To test those results, we measured the 18O/16O ratios of quartz and alkali feldspar crystals using WiscSIMS (2 S.D. precision was 0.2 to 0.4‰) at targeted positions across spherulites. The spherulites are 500 to 5000 μm in diameter and are internally comprised of radiating, intergrown alkali feldspar and quartz crystals, which decrease in size from core to rim. Isotopic fractionation between quartz and alkali feldspar (∆Qtz-Kfs) is temperature dependent, and is predicted to be ~1‰ at magmatic temperatures and increases with decreasing temperature. If quartz and sanidine indeed crystallized at less than 700 °C, then ∆Qtz-Kfs values should be greater than ~1‰ and increase in crystals nearer spherulite rims. Measured ∆Qtz-Kfs in phenocrysts is 0.6±0.2‰, whereas spherulite cores have ∆Qtz-Kfs of 1.4±0.4‰, which increases to 1.8±0.4‰ near the midpoint of transects, and finally to 2.4±0.3‰ at the spherulite rims (uncertainty is 1 S.D. of Qtz and Kfs populations). Assuming equilibrium fractionation, those values indicate nucleation occurred at 570±100 °C and growth continued to temperatures as cold as 360±50 °C. These estimates imply nucleation was unable to initiate until the melt was undercooled by ~500 °C. Furthermore, crystal growth was able to continue until undercoolings of ~700 °C were reached, at which point slow diffusion likely prohibited growth.