Bidisha Dey , Tomoyuki Shibata, Masako Yoshikawa

The origin of alkali basalts with distinctive time-integrated low U/Pb (low µ, LOMU) from 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, similar to East Asian intraplate volcanoes. We use whole rock geochemistry and mineral chemistry to determine mantle melting, magma evolution conditions and isotopic evolution of the LOMU mantle source. Petrological studies show that the Higashi-Matsuura alkali basalts (~3 Ma) were derived from a hydrous mantle source with ~950 µg/g H2O, at a pressure of 1.9 to 2.3 GPa at a mantle potential temperature of ~1300 C. Trace element modelling concludes that these basalts may have been generated through the hydrous upwelling and partial melting of the asthenospheric mantle containing ~15% dehydrated eclogitic component. 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) alkali 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 northeast China alkali basalts, as well as the recently discovered Petit Spot volcanoes on the Pacific plate. 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 about 2 billion years, and its recent upwelling due to dehydration of the stagnant Pacific slab and related melting of the metasomatised asthenospheric mantle.