Key points of this research results
- The geochemical characteristics of the basaltic magmas of the Kita-Matsuura and Higashi-Matsuura in southwest Japan are similar to those of the ocean islands represented by the Hawaiian Islands.
- The geochemical characteristics of these magmas indicate that subducted oceanic crust and carbonate-bearing sediments were incorporated into the mantle beneath the continents.
- Model calculations using Pb isotopic ratios suggest that crustal components with low U/Pb ratios subducted about 2.2 and 1.8 billion years ago were incorporated into the mantle beneath the continents.
Outline
The geochemical characteristics of the Earth's mantle have been elucidated using magma produced by melting of the mantle. As a result, the geochemical characteristics of the mantle beneath the oceans is rather well understood, whereas that of the mantle beneath the continents is not yet well understood. This is because in continental regions, magma incorporates components of continental crust during its transformation to the surface, changing the geochemical characteristics of the magma. In this study, we investigated petrological and geochemical data of Cenozoic basalts in Kita-Matsuura and Higashi-Matsuura, southwestern Japan.
The results indicate that subducted carbonate-bearing sediments and oceanic crusts were incorporated into the mantle beneath southwest Japan. Furthermore, model calculations of lead isotopic compositions suggest that the uptake of the crustal materials with low U/Pb ratios into the mantle occurred at least twice during the Paleoproterozoic, about 2.2 billion years ago and 1.8 billion years ago, respectively. On the other hand, the geochemical characteristics of these samples are similar to those of basalts from Chinese continent and in a place called the petit spot on the Pacific plate near the Japan Trench. This suggests that mantle, containing low U/Pb crustal materials, is globally present not only beneath the Eurasian plate but also beneath the nearby Pacific plate.
This study reveals that crustal components subducted and incorporated into the mantle beneath the continents billions of years ago can retain independent chemical signatures in the mantle without being completely homogenized by mantle convection.

Fig. 1 Tectonic setting of the Southwest Japan area. The studied samples are from Cenozoic intraplate basalts on the Eurasian plate. The Philippine Sea Plate and the Pacific Plate is subducting below this area. (Modified after Dey et al., 2024)

Fig. 2. Schematic diagram for subduction recycling of oceanic crust and sediments in the East Asian region. (i) and (ii) show subduction of oceanic crust in the Paleoproterozoic and remnant of crustal fragments in the asthenospheric mantle. (iii) depict upwelling of hydrous mantle plume from the mantle transition zone and melting of the mantle with crustal remnants to produce basaltic magma at East Asian intraplate volcanoes. (Modified after Dey et al., 2024)
Paper Info
Dey, B., Shibata, T. & Yoshikawa, M. (2024). LOMU Type Alkali Basalts in East Asia Sourced from Subduction Recycling of Multiple Ancient Crustal Components. Journal of Petrology 65, 104.
DOI: 10.1093/petrology/egae104