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Timescales of spherulite crystallization in obsidian inferred from water concentration profiles

¹ Department of Mineral Sciences, Smithsonian Institution, MRC-119, Washington, D.C. 20013, U.S.A.
² Laboratoire de Planetologie de Grenoble, 122 rue de la Piscine, Grenoble 38041, France
³ Environmental Science Department, Lancaster University, Bailrigg, Lancaster LA1 4YW, U.K.
⁴ Institute for Research on Earth Evolution (IFREE), Japan Agency for Marine Earth Science and Technology (JAMSTEC), 2-15 Nasushima-cho, Yokosuka, Kanagawa 237-0061, Japan
⁵ Earth and Environmental Sciences, LMU-University of Munich, Theresienstrasse 41/III, 80333 Munich, Germany
⁶ Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, U.S.A.

Correspondence: ^* E-mail: castroj{at}si.edu

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.

Key Words: Spherulite • diffusion • obsidian • crystallization

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