title: Viscosity measurements of crystallizing andesite from Tungurahua volcano (Ecuador).
abstract: Viscosity has been determined during isothermal crystallization of an andesite from Tungurahua volcano (Ecuador). Viscosity was continuously recorded using the concentric cylinder method and employing a Pt-sheathed alumina spindle at 1 bar and from 1400°C to subliquidus temperatures to track rheological changes during crystallization. The disposable spindle was not extracted from the sample but rather left in the sample during quenching thus preserving an undisturbed textural configuration of the crystals. The inspection of products quenched during the crystallization process reveals evidence for heterogeneous crystal nucleation at the spindle and near the crucible wall, as well as crystal alignment in the flow field. At the end of the crystallization, defined when viscosity is constant, plagioclase is homogeneously distributed throughout the crucible (with the single exception of experiment performed at the lowest temperature). In this experiments, the crystallization kinetics appear to be strongly affected by the stirring conditions of the viscosity determinations. A TTT (Time-Temperature-Transformation) diagram illustrating the crystallization "nose" for this andesite under stirring conditions and at ambient pressure has been constructed. We further note that at a given crystal content and distribution, the high aspect ratio of the acicular plagioclase yields a shear-thinning rheology at crystal contents as low as 13 vol %, and that the relative viscosity is higher than predicted from existing viscosity models. These viscosity experiments hold the potential for delivering insights into the relative influences of the cooling path, undercooling, and deformation on crystallization kinetics and resultant crystal morphologies, as well as their impact on magmatic viscosity.
authors.affiliation: Instituto de Geofísica, Departamento de Vulcanología, Universidad Nacional Autónoma de México México D.F Mexico; Department of Earth and Environmental Sciences Ludwig-Maximilians-Universität Munich Germany.
authors.forename: Magdalena Oryaëlle
authors.initials: MO
authors.lastname: Chevrel
authors.affiliation: Department of Earth and Environmental Sciences Ludwig-Maximilians-Universität Munich Germany.
authors.forename: Corrado
authors.initials: C
authors.lastname: Cimarelli
authors.affiliation: Department of Earth and Environmental Sciences Ludwig-Maximilians-Universität Munich Germany; Karlsruhe Institute of Technology, Institute of Applied Materials Eggenstein-Leopoldshafen Germany.
authors.forename: Lea
authors.initials: L
authors.lastname: deBiasi
authors.affiliation: Department of Earth Sciences University of Bristol Bristol UK.
authors.forename: Jonathan B
authors.initials: JB
authors.lastname: Hanson
authors.affiliation: Department of Earth, Ocean and Ecological Sciences University of Liverpool Liverpool UK.
authors.forename: Yan
authors.initials: Y
authors.lastname: Lavallée
authors.affiliation: Instituto Nazionale di Geofisica e Vulcanologia Sezione di Pisa Pisa Italy.
authors.forename: Fabio
authors.initials: F
authors.lastname: Arzilli
authors.affiliation: Department of Earth and Environmental Sciences Ludwig-Maximilians-Universität Munich Germany.
authors.forename: Donald B
authors.initials: DB
authors.lastname: Dingwell
keywords: andesite; concentric cylinder; crystallization kinetics; magma; viscosity