Dataset Information

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.

SUBMITTER: Chevrel MO

PROVIDER: S-EPMC5012119 | biostudies-literature | 2015 Mar

REPOSITORIES: biostudies-literature

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Viscosity measurements of crystallizing andesite from Tungurahua volcano (Ecuador).

Chevrel Magdalena Oryaëlle MO Cimarelli Corrado C deBiasi Lea L Hanson Jonathan B JB Lavallée Yan Y Arzilli Fabio F Dingwell Donald B DB

Geochemistry, geophysics, geosystems : G(3) 20150325 3

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 th ...[more]

PMID: 27656114

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Project description:A series of pyroclastic density currents were generated at Tungurahua volcano (Ecuador) during a period of heightened activity in August 2006. Dense pyroclastic flows were confined to valleys of the drainage network, while dilute pyroclastic density currents overflowed on interfluves where they deposited isolated bodies comprising dune bedforms of cross-stratified ash exposed on the surface. Here, the description, measurement, and classification of more than 300 dune bedforms are presented. Four types of dune bedforms are identified with respect to their shape, internal structure, and geometry (length, width, thickness, stoss and lee face angles, and stoss face length). (1) "Elongate dune bedforms" have smooth shapes and are longer (in the flow direction) than wide or thick. Internal stratification consists of stoss-constructional, thick lensoidal layers of massive and coarse-grained material, alternating with bedsets of fine laminae that deposit continuously on both stoss and lee sides forming aggrading structures with upstream migration of the crests. (2) "Transverse dune bedforms" show linear crests perpendicular to the flow direction, with equivalent lengths and widths. Internally, these bedforms exhibit finely stratified bedsets of aggrading ash laminae with upstream crest migration. Steep truncations of the bedsets are visible on the stoss side only. (3) "Lunate dune bedforms" display a barchanoidal shape and have stratification patterns similar to those of the transverse ones. Finally, (4) "two-dimensional dune bedforms" are much wider than long, exhibit linear crests and are organized into trains. Elongate dune bedforms are found exclusively in proximal deposition zones. Transverse, lunate, and two-dimensional dune bedforms are found in distal ash bodies. The type of dune bedform developed varies spatially within an ash body, transverse dune bedforms occurring primarily at the onset of deposition zones, transitioning to lunate dune bedforms in intermediate zones, and two-dimensional dune bedforms exclusively on the lateral and distal edges of the deposits. The latter are also found where flows moved upslope. Elongate dune bedforms were deposited from flows with both granular-based and tractional flow boundaries that possessed high capacity and competence. They may have formed in a subcritical context by the blocking of material on the stoss side. We do not interpret them as antidune or "chute-and-pool" structures. The dimensions and cross-stratification patterns of transverse dune bedforms are interpreted as resulting from low competence currents with a significant deposition rate, but we rule out their interpretation as "antidunes". A similar conclusion holds for lunate dune bedforms, whose curved shape results from a sedimentation rate dependent on the thickness of the bedform. Finally, two-dimensional dune bedforms were formed where lateral transport exceeds longitudinal transport; i.e., in areas where currents were able to spread laterally in low velocity zones. We suggest that the aggrading ash bedsets with upstream crest migration were formed under subcritical flow conditions where the tractional bedload transport was less important than the simultaneous fallout from suspension. This produced differential draping with no further reworking. We propose the name "regressive climbing dunes" for structures produced by this process. A rapid decrease in current velocity, possibly triggered by hydraulic jumps affecting the entire parent flows, is inferred to explain their deposition. This process can in principle hold for any kind of particulate density current.

Dataset Information

Viscosity measurements of crystallizing andesite from Tungurahua volcano (Ecuador).

Publications

Viscosity measurements of crystallizing andesite from Tungurahua volcano (Ecuador).

Similar Datasets

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