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

Journal Article · Tue Dec 07 04:00:00 EST 2010 · T. Roy. Soc. Edin.-Earth

OSTI ID:1008751

Gualda, G; Cook, D L; Chopra, R; Qin, L; Anderson, A T; Rivers, M ^[1]

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1. UC

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 {micro}m); (2) microscopy of 1 mm{sup 3} fragments (preferable for crystals <100 {micro}m); and (3) computerised X-ray microtomography of {approx}1 cm{sup 3} 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{sup -3}), high-crystal content (14.4-15.3 wt.%) and abundant large crystals (>800 {micro}m); 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{sup -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.

• Research Organization: Advanced Photon Source (APS), Argonne National Laboratory (ANL), Argonne, IL (US)
• Sponsoring Organization: USDOE
• OSTI ID: 1008751
• Journal Information: T. Roy. Soc. Edin.-Earth, Journal Name: T. Roy. Soc. Edin.-Earth Journal Issue: 02, 2004 Vol. 95; ISSN TRSSDZ; ISSN 0263-5933
• Country of Publication: United States
• Language: ENGLISH

Related Subjects

58 GEOSCIENCES
BUBBLES
CRUSHING
DISTRIBUTION
FRAGMENTATION
GLASS
MAGMA
MICROSCOPY
NUCLEATION
TUFF