Schmidt, Angelika (2001) Temporal and Spatial Evolution of the Izu Island Arc, Japan in Terms of Sr-Nd-Pb Isotope Geochemistry. (PhD/ Doctoral thesis), Christian-Albrechts-Universität, Kiel, 81 pp.

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Abstract

The Izu Island arc is located south of Japan on one of the most active plate boundaries on earth. Since subduction began, phases of arc front volcanism alternated with phases of backarc spreading, rear arc volcanism and rifting. 40Ar/39Ar laserdating and step heating experiments from single crystal and glass particles from drilled forearc ash layers reveal volcanic activity in the Izu volcanic front for more than 40 million years. The Sr, Nd and Pb isotope ratios from the ash layers <17 Ma show nearly constant isotope ratios, while the isotope ratios of the older ash layers show larger variations and on average tend to less radiogenic Nd and Pb isotopic compositions with age. The spatial variations of the Izu arc system were investigated on lavas from the <17 Ma volcanic front, the active rift (<3 Ma), submarine rear arc volcanoes (13-3 Ma) and lavas and glass particles drilled in the Shikoku backarc basin (25-17 Ma). Trace element and isotope ratios reveal distinct geochemical signatures for each zone of the arc. Assuming a corner flow model for the streaming of mantle material, the isotope ratios can be explained by two component mixing between the underlying asthenospheric mantle wedge and a subduction component consisting of fluids from the subducted plate. The spatial geochemical variations reveal that both components must be heterogeneous across the arc. The temporal geochemical evolution shows that depletion of the mantle wedge beneath the volcanic front started when the Shikoku Basin began to spread about 25 Ma ago. With the exception of the state of enrichment or depletion of the mantle wedge and the subduction component, the input into the arc as well as the structure of the subduction zone are reflected in the arc output. The input into the arc is reflected in the temporal evolution and the Pb isotopes. These data possibly reflect a less radiogenic input in the Eocene and Oligocene due to the lesser age and smaller sediment carapace of the subducting plate.

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