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

Abstract

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 mm 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.

Magmatic evolution and textural development of the 1739 CE Pietre Cotte lava flow, Vulcano, Italy

“…Spherulite nucleation may have had an early onset, based on microlite deflections around spherulites (Bullock et al, 2017). However, spherulite formation is predominantly a subsolidus process (Clay et al, 2013;Befus et al, 2015;Bullock et al, 2017) in the Pietre Cotte rhyolitic host, and the majority of spherulites overprint microlites (Bullock et al, 2017). Therefore, the presence of spherulites nucleating upon enclaves suggests that enclaves were at least semi-solid whilst interacting with the rhyolite host.…”

Crystallization kinetics of rhyolitic melts using oxygen isotope ratios

“…Hence, a conversion of temperature to time would be required to directly compare the natural crystallization kinetics with those determined experimentally. Differential scanning calorimetry and advection-diffusion modeling of diffusion profiles surrounding spherulites suggest that sample Y24 from Pitchstone Plateau cooled at 10 À5.2±0.3°C s À1 [Befus et al, 2015]. Assuming that cooling rate, the best fit for spherulite growth was approximately 0.1 μm h À1 and did not significantly vary with decreasing temperature.…”

“…Using those constraints, the closure temperature of oxygen diffusion in spherulites is greater than or equal to magmatic temperatures, which are estimated to have been 720–790°C [ Vazquez et al , ; Befus, ; Stelten et al , ]. Spherulite crystallization occurred at even cooler temperatures below the glass transition temperature (600–700°C) because spherulites do not deflect flow bands [ Befus et al , ]. In summary, oxygen was not modified by diffusion, and Δ 18 O Qtz‐Kfs can indeed be used for oxygen isotope thermometry.…”

“…The new estimate for spherulite growth is similar to the slower growth rates determined from experiments but is largely similar to results from advection‐diffusion modeling [ Fenn , ; Swanson , ; Baker and Freda , ; Castro et al , ; Arzilli et al , ; Befus et al , ]. Much of the discrepancy between the new rates and the faster rates determined from past experiments is simply a result of dissimilar materials.…”

“…Oxygen isotope fractionation between quartz and alkali feldspar plotted against temperature, using the experimental data from Clayton et al []. The arrowed domains show the magmatic temperature of Pitchstone Plateau flow [ Befus, ; Stelten et al, ], the anticipated window of spherulite growth from past numerical modeling [ Gardner et al, ; Befus et al, ], and these new results.…”