Compositional gradients surrounding spherulites in obsidian and their relationship to spherulite growth and lava cooling
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Spherical masses of crystal fibers (spherulites) crystalize from rhyolitic melt/glass mainly in response to significant undercooling while lava cools. Spherulite growth should induce compositional gradients in the surrounding glass from expulsion of incompatible constituents and diffusion of those constituents away from the spherulite. Finite-difference numerical modeling of one-dimensional diffusion, in which diffusivities are allowed to vary with temperature, is used to investigate how compositional gradients reflect spherulite growth and lava cooling. Overall, three forms of gradients are identified. Elements that diffuse quickly are expelled from the spherulite but then migrate away too quickly to become enriched at the boundary of the spherulite. Elements that diffuse slowly are trapped within the growing spherulite. Between those endmembers are elements that are not trapped, yet diffuse slow enough that they become enriched at the contact. Their slow diffusion away then elevates their concentrations in the surrounding glass. How enriched those elements are at the spherulite-matrix interface and how far their enrichments extend outwards into the glass reflect how spherulites grow and thermal conditions during growth. Concentrations of H2O, Rb, F, Li, Cl, Na, K, Sr, Cs, Ba, and Be were measured in and around spherulites in obsidian from a 4.7 ± 1 km3 rhyolite lava dome erupted from Tequila volcano, Mexico. Measurable concentration gradients are found for H2O, Rb, and F. Attributes of those gradients and the behaviors of the other elements are in accord with their experimentally constrained diffusivities. Spherulites appear to have grown following radial, rather than volumetric, growth. The observed gradients (and lack of others) are more consistent with growth mainly below the glass transition, which would necessitate the dome cooling at ca. 10−5 to 10−7 °C s−1. Such slow cooling is consistent with the relatively large volume of the dome.
KeywordsObsidian Spherulite Tequila volcano Diffusion Crystallization
This work was partially supported by a grant from the National Science Foundation (EAR-1049829) to J.E.G. and analytical fee support from the Jackson School of Geosciences to K.S.B. Critical reviews by Dr. Hugh Tuffen and an anonymous reviewer greatly improved the manuscript.
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