Experimental constraints on the conditions of formation of highly calcic plagioclase microlites at the Soufrire Hills Volcano, Montserrat

High-pressure and -temperature experiments on a bulk-rock composition representative of the groundmass of the Soufriere Hills Volcano andesite have allowed the phase equilibria of the system to be determined; these are then compared with the natural samples. Experimental conditions varied from 825 to 1100degreesC and from 5 to 225 MPa; the main phases observed were clinopyroxene, crystalline silica, amphibole and plagioclase. A relationship between plagioclase microlite size and anorthite content is identified in samples of the natural andesite. Large crystals (>60 mum(2) in area) have cores of An(60-75), whereas small crystals (<60 mum(2) in area) have cores of An(40-60). Experimental results show that if the magma is heated to >950degreesC the high-anorthite microlite crystals can form at magma chamber pressures without any need for a change in bulk composition. It is proposed that convective self-mixing occurs within the magma chamber. Geothermometry of coexisting plagioclase-amphibole pairs confirms the complex crystallization history of the natural samples. Analysis of natural glass samples has identified compositional variations that can be related to the crystallinity of the sample and also the groundmass plagioclase composition. Rapidly erupted pumice samples have high glass contents, lower SiO2 glass compositions and plagioclase microlites that are large in size (>60 mum(2)) and have a high anorthite content (>An(60)). Slowly erupted dome samples are highly crystalline and contain numerous plagioclase microlites of variable size and composition.