Spherulites Record Crystallization, Degassing, and Oxidation-reduction Mechanisms in Obsidian Flows
Résumé
Insight into the formation conditions of obsidian lavas may be gained from studying crystals that grow in their interiors during eruption and cooling. We analyzed spherulites in an emergent obsidian dike from Hrafntinnuhryggur, near Krafla volcano, Iceland, in order to quantify their crystallization rates, the relations between spherulites and lithophysae, and how the growth of spherulites may affect the oxidation state of the magma. We measured water concentration profiles around spherulites in rhyolitic obsidian with synchotron Fourier Infrared spectroscopy. The concentration of OH- groups in the glass is elevated by up to 50% above the background level near the spherulite-glass border, and decreases radially away from the spherulite. This pattern reflects expulsion and diffusion of water away from the spherulites as they crystallized an anhydrous assemblage of albite+magnetite+quartz. We modeled the advective and diffusive transport of the water away from the growing spherulites by numerically solving the diffusion equation with a moving boundary. Numerical models fit the natural data best when a small amount (~10%) of post-growth diffusion is incorporated in the model. Comparisons between models and natural data constrain the spherulite growth rate to between approximately 0.1 to 0.3 mm/day for model eruption temperatures of 800 and 850°C. The expulsion of water during spherulite growth may affect the oxidation state of the melt, specifically by causing local reduction of melt adjacent to the spherulite. Optical evidence of this reduction includes clear glass zones enclosing the spherulites; outside of these halos the glass is uniformly brown. Near-infrared spectra and Micro- XANES measurements of the Fe3+/Fe-total ratio in these different color zones confirms that there is an enrichment of ferrous iron in the colorless glass adjacent to the spherulite. We suggest that reduction is driven by the release of water during growth of anhydrous minerals however the exact reduction mechanism remains unclear. A nearly one-to-one correspondence between the molar concentrations of hydroxyl groups and ferric iron that was reduced to ferrous iron in the clear glass zones suggests that the reduction may be due to an interconversion reaction involving molecular water and hydroxyl groups bound in the silicate glass.