Spherulite formation in obsidian lavas in the Aeolian Islands, Italy

Bullock, Liam, Gertisser, Ralf and O'Driscoll, Brian (2017) Spherulite formation in obsidian lavas in the Aeolian Islands, Italy. Periodico di Mineralogia, 86, 37-54. (doi:10.2451/2017PM680).

Abstract

Spherulites in obsidian lavas of Lipari and Vulcano (Italy) are characterised by spatial, textural and geochemical variations, formed by different processes across flow extrusion and emplacement. Spherulites vary in size from <1 mm to 8 mm, are spherical to elongate in shape, and show variable radial interiors. Spherulites occur individually or in deformation bands, and some are surrounded by clear haloes and brown rims. Spherulites typically contain cristobalite (α, β) and orthoclase, titanomagnetite and rhyolitic glass, and grew over an average period of 5 days. Heterogeneity relates to formation processes of spherulite ‘types’ at different stages of cooling and emplacement. Distinct populations concentrate within deformation structures or in areas of low shear, with variations in shape and internal structure. CSD plots show differing size populations and growth periods. Spherulites which formed at high temperatures show high degree of elongation, where deformation may have triggered formation. Spherulites formed at mid-glass transition temperatures are spherical, and all spherulites are modified at vapour-phase temperatures. Enhanced undercooling, deformation and post-emplacement modification are therefore pivotal in the development spherulite heterogeneity in obsidian lavas. Key words: Spherulites; Obsidian; Aeolian Islands; Glass Transition; Lipari; Vulcano

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