a1 Department of the Geophysical Sciences, The University of Chicago, 5734 S. Ellis Avenue, Chicago, IL 60637, USA.
a2 Consortium for Advanced Radiation Sources, The University of Chicago, Building 434A, 9700 South Cass, Argonne, IL 60439, USA.
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
(Received September 02 2003)
(Accepted September 09 2004)