Spherulites as in-situ recorders of thermal history in lava flows

Spherulites in rhyolitic obsidian provide a record of the thermal history of their host lava during the interval of spherulite growth. We use trace element concentration profiles across spherulites and into the obsidian host from Yellowstone National Park (USA) to reconstruct the conditions that existed during spherulite formation. The measured transects reveal three behaviors: expulsion of the most diffusively mobile elements from spherulites with no concentration gradients in the surrounding glass (type 1); enrichment of slower-diffusing elements around spherulites, with concentration gradients extending outward into the glass (type 2); and complete entrapment of the slowest-diffusing elements by the spherulite (type 3). We compare the concentration profiles, measured by laser ablation-inductively coupled plasma-mass spectrometry and Fourier transform infrared spectroscopy, to the output of a spherulite growth model that incorporates known diffusion parameters, the temperature interval of spherulite growth, the cooling rate of the lava, and data on the temporal evolution of spherulite radius. Our results constrain spherulite nucleation to the temperature interval 700-550 °C and spherulite growth to 700-400 °C in a portion of lava that cooled at 10^{-5.2 ± 0.3} °C s^-1, which matches an independent experimental estimate of 10^-5.3 °C s^-1 measured using differential scanning calorimetry. Maximum spherulite growth rates at nucleation are on the order of 1 μm hr^-1 and are inferred to decrease exponentially with time. Hence, spherulites may serve as valuable in-situ recorders of the thermal history of lava flows.