PETROLOGICAL STUDY ON MAGMA STORAGE AND MOVEMENT IN THE UPPER CRUST

本研究主要研究岩浆储集层和输导通道中的岩浆作用。斑岩火山岩中含有球晶和斑晶碎片。这表明它们是从储集层边缘的结晶碎屑带圈闭的。温泉最近一次喷发的英安岩就是这种斑状火山岩的一个例子，本研究主要对其进行了研究。斑晶的结构和斑晶与基质之间的化学关系，包括锶、钕同位素比值，反映了它们在喷发前和喷发过程中与周围熔体的不平衡。在温岑，斑晶很可能是来自岩浆库糊状带的晶体，然而，在那里，围岩(侵入岩)由于加热而熔化，而不是由于冷却而发生熔体结晶。单个斑晶中同位素比值的分布，结合显微同位素分析，在不久的将来应该可以证明岩石学模型。在含水长英质岩浆在管道中上升的过程中，熔体因脱气而结晶。岩浆上升速率越小，熔体失水越彻底，结晶度越高。这些因素导致岩浆的表观粘度极大地增加。结果，岩浆变得太粘稠，无法在管道顶部移动。泡沫和脆性岩浆容易发生破碎和压实，管道顶部会被压实的熔岩像塞子一样密封。然后，喷发停止。如果这种封闭是完全的，塞子下面的连续结晶形成了过量的岩浆压力，喷发将重新开始。深成英安岩的岩石学特征支持这一模式。基质结晶度越高，熔岩流出速率越低。泡沫岩浆的破碎和压实以及伴随的火山气体的泄漏可能导致了在喷发期间在导管顶部发生的低频地震。

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.