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Experimental Constraints on the Conditions of Formation of Highly Calcic Plagioclase Microlites at the Soufrire Hills Volcano, Montserrat 总被引：3，自引：0，他引：3
High-pressure and -temperature experiments on a bulk-rock compositionrepresentative of the groundmass of the Soufrière HillsVolcano andesite have allowed the phase equilibria of the systemto be determined; these are then compared with the natural samples.Experimental conditions varied from 825 to 1100°C and from5 to 225 MPa; the main phases observed were clinopyroxene, crystallinesilica, amphibole and plagioclase. A relationship between plagioclasemicrolite size and anorthite content is identified in samplesof the natural andesite. Large crystals (>60 µm2 inarea) have cores of An60–75, whereas small crystals (<60µm2 in area) have cores of An40–60. Experimentalresults show that if the magma is heated to >950°C thehigh-anorthite microlite crystals can form at magma chamberpressures without any need for a change in bulk composition.It is proposed that convective self-mixing occurs within themagma chamber. Geothermometry of coexisting plagioclase–amphibolepairs confirms the complex crystallization history of the naturalsamples. Analysis of natural glass samples has identified compositionalvariations that can be related to the crystallinity of the sampleand also the groundmass plagioclase composition. Rapidly eruptedpumice samples have high glass contents, lower SiO2 glass compositionsand plagioclase microlites that are large in size (>60 µm2)and have a high anorthite content (>An60). Slowly erupteddome samples are highly crystalline and contain numerous plagioclasemicrolites of variable size and composition. KEY WORDS: glass evolution; experiment; Montserrat; plagioclase; self-mixing 相似文献
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The Kinetics of Degassing-Induced Crystallization at Soufriere Hills Volcano, Montserrat 总被引：4，自引：0，他引：4
A series of decompression experiments at 875°C, with a 16h anneal period at 160 MPa, and depressurizations to a finalwater pressure (Pf) of 125–30 MPa, were run for between1 and 504 h. Using an experimentally derived plagioclase liquidus,the depressurizations are estimated to impose an undercooling( 相似文献
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