CRYSTAL SIZES IN EVOLVING SILICIC MAGMA CHAMBERS

Abstract

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 con-ditions 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-flow tuffs and smaller-volume volcanic units, and for fingerprinting different magma batches (layers) in products of the same eruption. INTRODUCTION Crystal sizes (phenocrysts and microlites) in igneous rocks are important parameters for interpreting magma cooling conditions and mechanisms (Cashman and Marsh, 1988; Hig-gins, 1998; Marsh, 1998). Silicic volcanic and plutonic rocks show significant variations in crystal sizes and proportions within the same unit, although these phenomena are poorly un-derstood. These variations have particular im-portance for studying zoned magma chambers with significant gradients in composition and crystal content. Most experimental and obser-vational work on crystal size distributions (CSDs) has been in mafic magma systems, i.e., lava lakes, sills, and plutons. Eutectoid silicic magmas are more complicated because of multiple saturation with as many as 10 min-erals and a fluid phase. The high viscosity of silicic melts, with its dependence on temper-ature (T) and f , causes complex nucleation H O