Textural characterization, major and volatile element quantification and Ar–Ar systematics of spherulites in the Rocche Rosse obsidian flow, Lipari, Aeolian Islands: a temperature continuum growth model

Abstract

Spherulitic textures in the Rocche Rosse obsidian flow (Lipari, Aeolian Islands, Italy) have been characterized through petrographic, crystal size distribution (CSD) and in situ major and volatile elemental analyses to assess the mode, temperature and timescales of spherulite formation. Bulk glass chemistry and spherulite chemistry analyzed along transects across the spherulite growth front/glass boundary reveal major-oxide and volatile (H2O, CO2, F, Cl and S) chemical variations and heterogeneities at a ≤5 μm scale. Numerous bulk volatile data in non-vesicular glass (spatially removed from spherulitic textures) reveal homogenous distributions of volatile concentrations: H2O (0.089 ± 0.012 wt%), F (950 ± 40 ppm) and Cl (4,100 ± 330 ppm), with CO2 and S consistently below detection limits suggesting either complete degassing of these volatiles or an originally volatile-poor melt. Volatile concentrations across the spherulite boundary and within the spherulitic textures are highly variable. These observations are consistent with diffusive expulsion of volatiles into melt, leaving a volatile-poor rim advancing ahead of anhydrous crystallite growth, which is envisaged to have had a pronounced effect on spherulite crystallization dynamics. Argon concentrations dissolved in the glass and spherulites differ by a factor of ~20, with Ar sequestered preferentially in the glass phase. Petrographic observation, CSD analysis, volatile and Ar data as well as diffusion modeling support continuous spherulite nucleation and growth starting at magmatic (emplacement) temperatures of ~790–825 °C and progressing through the glass transition temperature range (T g ~ 750–620 °C), being further modified in the solid state. We propose that nucleation and growth rate are isothermally constant, but vary between differing stages of spherulite growth with continued cooling from magmatic temperatures, such that there is an evolution from a high to a low rate of crystallization and low to high crystal nucleation. Based on the diffusion of H2O across these temperature ranges (~800–300 °C), timescales of spherulite crystallization occur on a timescale of ~4 days with further modification up to ~400 years (growth is prohibitively slow <400 °C and would become diffusion reliant). Selective deformation of spherulites supports a down-temperature continuum of spherulite formation in the Rocche Rosse obsidian; indeed, petrographic evidence suggests that high-strain zones may have catalyzed progressive nucleation and growth of further generations of spherulites during syn- and post-emplacement cooling.