Crystal sizes in evolving silicic magma chambers

Crystal size distributions (CSDs) of quartz and zircon phenocrysts in individual pumice clasts from several voluminous ash-flow tuffs provide a quenched snapshot view of conditions in preclimactic magma chambers. A common feature of these CSDs is a concave-down, lognormal shape, in contrast to the reported linear CSDs in more mafic systems. This feature is interpreted to be a general result of surface-controlled, size-dependent growth by a layer nucleation in silicic magmas at low supersaturation. Specific CSDs may be important for interpreting nucleation and crystal-growth conditions and mechanisms in magmas erupted as large ash-How tuffs and smaller-volume volcanic units, and for fingerprinting different magma batches (layers) in products of the same eruption.