Timescales of spherulite crystallization in obsidian inferred from water concentration profiles

• Published in: American Mineralogist
• Main Authors: Castro, J. M., Beck, Pierre, Tuffen, H., Nichols, Alexander R.L., Dingwell, Donald B., Martin, Michael C.
• Other Authors: Department of Mineral Sciences Washington, Smithsonian Institution, Laboratoire de Planétologie de Grenoble (LPG), Université Joseph Fourier - Grenoble 1 (UJF)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS), Department of Environmental Science Lancaster, Lancaster University, Institute for Research on Earth Evolution Yokosuka (IFREE), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Department of Earth and Environmental Sciences Munich, Ludwig Maximilian University Munich = Ludwig Maximilians Universität München (LMU), Advanced Light Source LBNL Berkeley (ALS), Lawrence Berkeley National Laboratory Berkeley (LBNL)
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2008
• Subjects: Spherulite • diffusion • obsidian • crystallization; Iceland
• Online Access: https://insu.hal.science/insu-00363340; https://doi.org/10.2138/am.2008.2904

International audience We determined the kinetics of spherulite growth in obsidians from Krafla volcano, Iceland. We measured water concentration profiles around spherulites in obsidian by synchrotron Fourier transform infrared spectroscopy. The distribution of OH– groups surrounding spherulites decreases exponentially away from the spherulite-glass border, reflecting expulsion of water during crystallization of an anhydrous paragenesis (plagioclase + SiO2 + clinopyroxene + magnetite). This pattern is controlled by a balance between the growth rate of the spherulites and the diffusivity of hydrous solute in the rhyolitic melt. We modeled 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 of post-growth diffusion is incorporated in the model. Comparisons between models and data constrain the average spherulite growth rates for different temperatures and highlight size-dependent growth among a small population of spherulites.