Development of age determination method using U-Th isotope disequilibrium for the prediction of volcanic eruption

为了估算岛弧岩浆作用的时间尺度，我们发展了新的非线性分离方法和高精度的U、Th、Ra同位素分析。这些技术首先应用于三宅岛火山岩的综合岩石学和地球化学研究，使用主要和微量元素组成和B-Sr-Nd-Pb同位素组成。本文总结了三宅岛下楔状地幔中俯冲板片岩浆的成因和迁移机制：<238><230>三宅岛火山岩中~（238）U大量过剩的~（238）U-~（238）Th不平衡现象表明亏损地幔通过与流体有关的交代作用，微量元素和B-Sr-Nd-Pb同位素体系也表明了这一点。在等值线图中，在第1和第2阶段观察到的趋势表明两个不同的等时线，具有共同的初始^<230>Th/^<232>Th比值。获得了单个液体释放的时间间隔， ...更多信息 第一阶段和第二阶段之间为15千美元，第二阶段和第三阶段之间为3千美元。在<226><230>某些熔岩中观察到的^ Ra-^ Th不平衡表明，流体释放的最小间隔约为1 kyr，表明楔状地幔中流体释放和随后的熔体生成是在1-15 kyr内间歇发生的幕式事件。^<226>Ra-^<230>Th的不平衡限制了板片组分的上升速率&gt;45 m/yr，导致楔状地幔中存在裂纹。与此相反，第3和第4阶段的样品描绘了混合的趋势，在古老的安山质岩浆和新注入的原始岩浆之间的等值线图。从第三阶段开始，原始岩浆的参与程度逐渐降低，表明原始岩浆系统是一个近封闭系统。分异岩浆在地壳中的停留时间约为10 kyr，比理论计算结果长一倍。这可能是由于邻近岩浆注入的持续热量供应。少

In order to estimate the time scale of magma processes beneath island arc, we have developed new chimerical separation methods and highly precise isotope analyses for U, Th and Ra. These techniques were first applied to lavas from Miyakejima Island together with comprehensive petrological and geochemical investigations using major and trace element compositions and B-Sr-Nd-Pb isotopic compositions. The inferred genesis and transportation mechanism of magma with subduction component derived from descending slab in the wedge mantle beneath Miyakejima is summarized as follows:^<238>U-^<230>Th disequilibrium observed in Miyakejima lavas with large 238U excess implies metasomatism of depleted mantle by fluid-related processes, as also suggested by the trace element and B-Sr-Nd-Pb isotope systematics. In the equiline diagram, trends observed in Stages 1 and 2 indicate two different isochrones with common initial ^<230>Th/^<232>Th ratios. The interval of individual fluid-release is obtained a … More s 15 kyr between Stages 1 and 2, and 3 kyr between Stages 2 and 3. ^<226>Ra-^<230>Th disequilibrium observed in some lavas suggests that the minimum interval of the fluid-release is approximately 1 kyr, indicating that the fluid-release and subsequent melt generation in the wedge mantle are episodic events intermittently occurring within 1-15 kyr. The ^<226>Ra-^<230>Th disequilibrium constraints the ascending rate of slab components as >45 m/yr, leading to the existence of crack in the wedge mantle. In contrast, Stages 3 and 4 samples depict a mixing trend in the equiline diagram between old andesitic magma and newly injected primitive magma. The involvement of primitive magma generally decreases from Stage 3 towards present, implying that primitive magma system is nearly closed system. Residence time of the differentiated magma in the crust is approximately 10 kyr, which is factor of two longer than theoretically obtained result. This may be due to the continuous heat supply from neighboring magma injection. Less