PETROLOGICAL STUDY ON MAGMA STORAGE AND MOVEMENT IN THE UPPER CRUST

重点研究了岩浆储层和岩浆通道中的岩浆作用过程。斑状火山岩中含有球晶和斑晶碎片。这表明它们来自于储层边缘的结晶糊状带。云仙最新喷发的英安岩就是这类斑岩火山岩的一个例子，本研究主要对其进行研究。斑晶的结构和斑晶与大地的化学关系（包括Sr和Nd同位素比值）表明它们在喷发前和喷发期间与周围熔体的不平衡。在云禅，斑晶可能代表岩浆储层糊状带的结晶，然而，在那里，壁（侵入岩）岩石是由于加热而熔化的，而不是由于冷却而熔化的结晶。个别斑晶的同位素比值分布，在不久的将来进行微量同位素分析，将证明岩石学模型。在管道中含水长英质岩浆上升过程中，熔体因脱气而发生结晶。岩浆上升速率越小，熔体失水和结晶度增加越完全。这些因素使岩浆的表观粘度急剧增加。结果，岩浆变得过于粘稠，无法在管道顶部移动。泡沫和脆性岩浆很容易破碎和压实，而管道的顶部会被压实的熔岩像塞一样封住。然后，喷发停止。如果这种密封完成了，岩塞下面的连续结晶就会形成过量的岩浆压力，火山喷发就会重新开始。云仙英安岩的岩石学特征支持这一模式。地面物质的结晶度越高，熔岩流出的速率越低。泡沫岩浆的破碎和压实以及伴随而来的火山气体的逸出可能在喷发期间在导管顶部发生了低频地震。

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.