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Fragmentation, nucleation and migration of crystals and bubbles in the Bishop Tuff rhyolitic magma

Published online by Cambridge University Press:  26 July 2007

Guilherme A. R. Gualda
Affiliation:
Department of the Geophysical Sciences, The University of Chicago, 5734 S. Ellis Avenue, Chicago, IL 60637, USA.
David L. Cook
Affiliation:
Department of the Geophysical Sciences, The University of Chicago, 5734 S. Ellis Avenue, Chicago, IL 60637, USA.
Rahul Chopra
Affiliation:
Department of the Geophysical Sciences, The University of Chicago, 5734 S. Ellis Avenue, Chicago, IL 60637, USA.
Liping Qin
Affiliation:
Department of the Geophysical Sciences, The University of Chicago, 5734 S. Ellis Avenue, Chicago, IL 60637, USA.
Alfred T. Anderson Jr
Affiliation:
Department of the Geophysical Sciences, The University of Chicago, 5734 S. Ellis Avenue, Chicago, IL 60637, USA.
Mark Rivers
Affiliation:
Consortium for Advanced Radiation Sources, The University of Chicago, Building 434A, 9700 South Cass, Argonne, IL 60439, USA.

Abstract

The Bishop Tuff (USA) is a large-volume, high-silica pyroclastic rhyolite. Five pumice clasts from three early stratigraphic units were studied. Size distributions were obtained using three approaches: (1) crushing, sieving and winnowing (reliable for crystals >100 μm); (2) microscopy of ∼1 mm3 fragments (preferable for crystals <100 μm); and (3) computerised X-ray microtomography of ∼1 cm3 pumice pieces.

Phenocryst fragments coated with glass are common, and the size distributions for all crystals are concave-upward, indicating that crystal fragmentation is an important magmatic process.

Three groups are recognised, characterised by: (1) high-density (0·759–0·902 g cm−3), high-crystal content (14·4–15·3 wt.%) and abundant large crystals (>800 μm); concave-downward size distributions for whole crystals indicate late-stage growth with limited nucleation, compatible with the slow cooling of a large, gas-saturated, stably stratified magma body; (2) low-density (0·499 g cm −3), low-crystal content (6·63 wt.%) and few large crystals; the approximately linear size distribution reveals that nucleation was locally important, perhaps close to the walls; and (3) intermediate characteristics in all respects.

The volumetric fraction of bubbles inversely correlates with the number of large crystals. This is incompatible with isobaric closed-system crystallisation, but can be explained by sinking of large crystals and rise of bubbles in the magma

Type
Research Article
Copyright
Copyright © Royal Society of Edinburgh 2004

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