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

• Published in: Earth and environmental science transactions of the Royal Society of Edinburgh 2004-03, Vol.95 (1-2), p.375-390
• Main Authors: Gualda, Guilherme A. R., Cook, David L., Chopra, Rahul, Qin, Liping, Anderson, Alfred T., Rivers, Mark
• Format: Article
• Language: English
• Subjects: Bubbles; Crystal Content; Crystal growth; Crystal Size Distribution; Crystallization; Crystals; Density; Fragmentation; Fragments; Magma; Magmatic Processes; Nucleation; Pumice; Pumice Density; Rhyolite; Rhyolites; Silica; Size distribution; Stratigraphy; Tuff; X ray microtomography
• ISSN: 1755-6910; 0263-5933; 1755-6929; 1473-7116
• DOI: 10.1017/S0263593300001139

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 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