PHASE RELATIONS IN SILICIC SYSTEMS AT ONE-ATMOSPHERE PRESSURE

Abstract

An important control on magma rheology is the extent to which the magma crystallizes during ascent as a result of the effective undercooling created by volatile exsolution. To assess this undercooling, we need to know the final (anhydrous) one-atmosphere phase relations of silicic magmas. For this reason, we have performed one-atmosphere controlled-fO2 crystallization experiments on dacitic to rhyolitic melt compositions (67–78wt% SiO2) and determined equilibrium phase assemblages, melt fractions, and some phase compositions over a range of temperatures. Experiments were run at oxygen fugacities between NNO+1 and NNO+2 and temperatures of 1,000 to 1,250C. Constant phase compositions and sample crystallinities in runs longer than 3.5days suggest that these runs closely approached compositional equilibrium. Additionally, melting experiments with similar compositions yielded results closely resembling those obtained in crystallization experiments. All samples have liquidus temperatures between 1,250 and 1,200C, with plagioclase the liquidus phase for the two most mafic samples and quartz for the most silicic sample. When associated glass compositions are projected into the Qz-Ab-Or system they define a revised one-atmosphere quartz-feldspar cotectic 5–10% less quartz normative than previously estimated. Glass compositions from each sample plot along this cotectic between 1,100 and 1,000C, consistent with the plagioclase-quartz co-crystallization textures found in runs at these temperatures. This cotectic constrains glass compositions to a maximum silica content of 761wt% SiO2. Reported glass compositions in excess of 77wt% SiO2 in volcanic samples suggest non-equilibrium crystallization, perhaps a consequence of large melt undercoolings.