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

点击翻译按钮获取中文摘要

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