PETROLOGICAL STUDY ON MAGMA STORAGE AND MOVEMENT IN THE UPPER CRUST

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This research focuses on magmatic processes in magma reservoir and conduit.Porphyritic volcanic rocks contain glomerocrysts and the fragments as phenocrysts. This suggests their entrapment from crystal mush zone in the reservoir margins. Dacite of the latest eruption at Unzen is an example of such porphyritic volcanic rocks, which were mainly studied in this research. Texture of the phenocrysts and the chemical relationship between phenocrysts and groundmass including Sr and Nd isotopic ratios indicate their disequilibrium with the surrounding melt before and during eruption. At Unzen, phenocrysts probably represent crystals from the mushy zone of the magma reservoir, however, where the wall (intrusive rock) rocks were melted due to heating rather than crystallization of the melt occurred due to cooling. Distribution of isotope ratios in individual phenocrysts, with micro isotopic analysis near future, should prove the petrological model.During ascent of hydrous felsic magma in the conduit, melt's crystallization occurs due to degassing. The smaller the magma ascent rate, the more complete both water-loss from the melt and the increase in crystallinity. These facts cause the apparent viscosity of magma increase extremely. As a result, magma became too viscous to move in the top of the conduit. Fragmentation and compaction of foamy and brittle magma may occur easily, and the top of the conduit would be sealed with the compacted lava like a plug. Then, eruption stops. If such sealing is complete, successive crystallization below the plug builds up excess magmatic pressure, and eruption would resumes. The petrological characteristics of the Unzen dacite support this model. The higher the crystallinity of groundmass, the lower the rate of lava effusion. Fragmentation and compaction of foamy magma and the accompanied escape of volcanic gas may have generated low-frequency earthquakes that had occurred in the conduit top during eruption.