The evolution of magma and the 2000AD eruption and caldera subsidence event of Miyakejima Volcano, Izu arc, Japan

在本次研究过程中，1.2000年9月至2003.2年1月期间，从直升机上对火山口壁和火山口生长进行了38次空中检查。2001.3年12月对这2000种产品进行了实地调查和样本采集。为了刻画岩浆的演化，分析了近10ka以来年轻物质的全岩和矿物成分。在2001-2002年期间，2000火山口的沉陷扩大幅度很小，但偶尔会发生不稳定墙的坍塌。对火山口壁的观察表明，较老的火山碎屑产物暴露在北半部，而较新的熔岩堆积在南壁上。最后10ka的岩浆生长历史和演化可分为以下四个喷发阶段：1)10-4ka的大船多阶段，以斑状玄武岩为特征；2)4-2.5ka的Tsubota阶段，以安山岩为特征；3)2.5ka至12世纪的Oyama阶段，以玄武岩-玄武岩为特征。这一阶段始于约0.37千米的排泄和随后的Hatchodaira火山口的形成。4Shinmio阶段，公元1469年至1983年。Mg#(=mg/(mg+Fe)*100)随喷发年龄的变化呈锯齿状变化，在2.5ka、1.3ka和0.5ka时比值突然增大，然后平缓减小。这种模式表明，相对未分化的岩浆偶尔会进入正在缓慢分化的岩浆管道系统。2000年8月18日发射的炸弹的散装成分显示，镁#明显增加。这意味着自15世纪以来一直工作的岩浆管道系统在2000年事件中破裂，并转移到一个新的喷发系统。

In the course of this research project,1. Thirty-eight aerial inspection flights for caldera wall and growth of caldera from a helicopter were carried out during September 2000 to January 2003.2. Field survey and sample collection of the 2000 products were carried out in December 2001.3. Whole-rock and mineral composition of the juvenile materials during the last 10ka and borehole samples were analyzed to depict the evolution of magma.During the period from 2001 to 2002, enlargement of the subsidence 2000 Caldera proceeded very little, but for occasional collapse of unstable wall. Observation of the caldera wall revealed that the older pyroclastic products are exposed on the northern half in contrast to the younger lavas are piled up on the southern wall.Growth history and evolution of magma during the last 10ka was divided into following four eruptive stages ;1) Ofunato Stage of 10-4ka, characterized by porphyritic basalts, 2) Tsubota Stage of 4-2.5ka, distinguished by andesitic products, 3) Oyama Stage of 2.5ka to the 12^<th> century, characterized by basalt to basaltic andesite. This stage began with discharge of ca. 0.37km^3 and subsequent formation of the Hatchodaira Caldera. 4) Shinmio stage, from 1469AD to 1983AD eruption. The variation of Mg# (=Mg/(Mg+Fe)*100) versus erupted age during the last 4ka show saw-teeth like change ; abrupt increase of the ratio at 2.5ka, 1.3ka, and 0.5ka followed by gentle decrease. This pattern suggests occasional input of relatively undifferentiated magma into the magma plumbing system, which was slowly differentiating. The bulk composition of bombs ejected on August, 18, 2000 shows clear increase in Mg#. This implies that the magma plumbing system having been working since 15^<th> century was failed by the 2000 event and shifted to a new eruptive system.