CONTRASTING PROCESSES IN SILICIC MAGMA CHAMBERS: EVIDENCE FROM VERY LARGE VOLUME IGNIMBRITES

Several very large (volumes greater than 1000 km³) crystal-rich dacitic ignimbrites lack the vertical zonation found in many other tuffs (e.g., Fish Canyon Tuff from Colorado and the Cottonwood Wash, Wah Wah Springs, and Lund Tuffs of the Great Basin in western USA). Apparently, their magma chambers were modestly heterogeneous but not systematically zoned from top to bottom. These ignimbrites have 40% to 50% phenocrysts set in a high-silica rhyolite glass. Mineral assemblages and mineral compositions suggest pre-eruption temperatures were 730° to 800° C at relatively high water fugacities. We have speculated that these very large-volume ignimbrites are unzoned because crystallization and convection in slab-shaped magma chambers inhibited separation of crystals from liquids and resulted in a compositionally heterogeneous chambers that lacked systematic chemical zonation. However, many other very large-volume rhyolitic ignimbrites are vertically zoned (e.g., tuffs related to the Yellowstone hotspot) suggesting that their parent magma chambers were also vertically zoned. And yet these large Yellowstone-type rhyolites also must be derived from chambers that are sill-like—their calderas are 40 to as much as 70 km across, implying that the subjacent magma chamber was approximately this size as well. Thus, factors other than chamber shape must be important for establishing the nature of zonation. These zoned rhyolitic tuffs typically have strong imprints of fractional crystallization, few phenocrysts, anhydrous mineral assemblages, and crystallization temperatures of 850° to 950° C. Calculated magma viscosities are several orders of magnitude lower than for the cooler, wet crystal-rich dacites. Perhaps these water-poor and consequently hot rhyolites had low enough viscosities to allow efficient crystal-liquid separation—probably by sidewall crystallization and rise of fractionated, less dense magma to the top of large chambers. In contrast, crystallization of the water-enriched, phenocryst-rich magmas occurred at lower temperatures where magma viscosity was significantly higher. This inhibited crystal-liquid separation, hindered development of systematic vertical zonation, and promoted quasi-equilibrium crystallization in small domains within large heterogeneous magma chambers.