Bidisha Dey , Tomoyuki Shibata, Masako Yoshikawa

Alkali basalts with distinctive time-integrated low U/Pb (low µ, LOMU) have been reported in East Asia from the arc, rear-arc, forearc and intraplate volcanoes in northeast China, Korea, Sea of Japan, and the Petit Spot near the Japan Trench. The origin of these alkali basalts in East Asia is controversial due to the complex geochemical and tectonic signatures reported from this region. We report new data on the petrology and geochemistry of the Higashi-Matsuura and Kita-Matsuura alkali basalts from southwest Japan, which confirm the presence of a LOMU-type mantle component below the Japanese islands. Petrological studies show that the Higashi-Matsuura alkali basalts (~3 Ma) were derived from a hydrous mantle source with ~1500 µg/g H2O, at a pressure of 1.9 to 2.1 GPa. These alkali basalts show 206Pb/204Pb values of 17.72 to 18.04 which are among the lowest values from southwest Japan. Relatively older (6 - 8 Ma) alkaline basalts from the Kita-Matsuura area showing similar physicochemical characteristics do not show LOMU-type isotopic trends. Trace element and Pb-Sr-Nd isotopic data indicate that the Higashi-Matsuura mantle component is similar to the extreme LOMU components reported from the Chinese and Korean alkali basalts, as well as the recently discovered Petit Spot volcanoes on the Pacific plate. Pressure estimates and geochemical signatures suggest that these basalts were formed by the melting of an enriched asthenospheric mantle showing LOMU-like isotope ratios and melt interaction with the MORB-like subcontinental lithospheric mantle. We model the origin of the LOMU signature from the lowest reported Pb isotope ratios in East Asia, from the Xiaogulihe volcano in northeastern China. Our model suggests that at least two separate subduction events of marine sediments, at 1.8 Ga and 2.2 Ga, are required to explain the observed Pb isotopic variation in the East Asian region. Other LOMU type basalts from East Asia, including southwest Japan and Petit Spot, define a linear trend between the Xiaogulihe basalts and lithospheric mantle xenoliths. This suggests that the LOMU array in East Asia may have been formed by mixing between multiple ancient, subducted sediment components accumulated at the mantle transition zone for over 2 billion years, and its recent upwelling due to dehydration of the stagnant Pacific slab and related melting of the metasomatized asthenospheric mantle.