Exploring crystallization kinetics in natural rhyolitic melts using high resolution CT imagery of spherulites

Abstract:

Little of our understanding of crystallization kinetics has been directly derived from studies of natural samples. We examine crystallization of rhyolitic melts by quantifying spherulite sizes and number densities in obsidian collected from Yellowstone caldera using high-resolution x-ray computed tomography (CT) imagery. Spherulites are spherical to ellipsoidal masses of intergrown alkali feldspar and quartz in a radiating, fibrous structure. They are thought to form in response to relatively rapid crystallization of melt in response to large amounts of undercooling. Recent research using compositional gradients that form outside of spherulites has suggested that they nucleate at 700 to 500 ˚C and their growth slows exponentially until it eventually ceases at ~400 ˚C. By quantifying spherulite textures, and using those temperature constraints, we derive new kinetic information regarding crystallization in natural rhyolitic systems. We find that spherulites range from 0.2 to 12.3 mm in diameter, and are 0.004 to 49.5 mm³ in volume. Such values generate number densities of 70 to 185 spherulites cm^-3. Histograms of size display positively skewed distributions indicating small spherulites are far more abundant than larger ones. Those distributions imply nucleation rates change as a function of temperature. At higher temperatures where the melt is undercooled by 400-500 ˚C, nucleation is rare and growth is favored. With decreasing temperature, nucleation rates increase rapidly until cold enough temperatures are reached that diffusion limits crystallization and causes it to cease (undercoolings of ~650 ˚C). Assuming a cooling rate for the host obsidian of 10^-5 ˚C s^-1, then overall spherulite nucleation rates are 0.01 to 0.03 spherulites cm^-3 hour^-1.