@article {Gualda:February 2005:0263-5933:375, author = "Gualda, Guilherme A.R.", author = "Cook, David L.", author = "Chopra, Rahul", author = "Qin, Liping", author = "Anderson, Alfred T.", author = "Rivers, Mark", title = "Fragmentation, nucleation and migration of crystals and bubbles in the Bishop Tuff rhyolitic magma", journal = "Transactions: Earth Sciences", volume = "95", year = "February 2005", 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

m); (2) microscopy of sim

1 mm³ fragments (preferable for crystals <100

m); and (3) computerised X-ray microtomography of sim

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

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 ^-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.", pages = "375-390(16)", url = "http://www.ingentaconnect.com/content/rse/tes/2005/00000095/F0020001/art00029" }