三宅島火山の連続脱ガスに伴う小噴火噴出物に含まれる玄武岩本質物から推測する火道内マグマプロセス

抄録

The Miyakejima volcano repeats small ash eruptions from the degassing vent opening inside the A.D. 2000 caldera. The juvenile glassy fragments in the tephra indicate that the basaltic magmas with primitive composition are ascending in the open conduit and degassing at the shallow depth. We examined the representative 12 tephra samples erupted between October 2001 and July 2010, and found the juvenile glassy fragments with basaltic character. Their fresh surfaces and free from alteration or weathering indicate that these glassy grains are juvenile materials. The glassy fragments occupy less than 1 vol.% of the tephra, that mainly consists of the fragments of lavas and pyroclastics derived from the previous edifice. The juvenile fragments consist of volcanic glass with pale-brownish color and euhedral crystals of plagioclase, clinopyroxene and olivine up to several tens micrometers across. Larger crystals of plagioclase and olivines more than 100 micrometers are also contained as microphenocryst. The range of the Fo contents of the olivine microphenocrysts in the glassy fragments is 73-83, which is higher than that of the phenocryst olivine in the basaltic bombs erupted during the August 18, 2000 eruption and similar to that of the olivine phenocrysts derived from the primitive basaltic magma, indicating that the continuous degassing since the A.D. 2000 eruption has been driven by the basaltic magmas with primitive character. The low sulfur and chlorine contents in the groundmass glass indicate the degassing under low pressure.