(Table 1) Isotopic and trace-element composition of ODP Leg 127/128 basalts@en

The western Pacific includes many volcanic island arc and backarc complexes, yet multi-isotopic studies of them are rare. Basement rocks of the Sea of Japan backarc basin were encountered at Sites 794,795, and 797, and consisted of basaltic sills and lava flows. These rocks exhibit a broad range in isotopic composition, broader than that seen in any other western Pacific arc or backarc system: 87Sr/86Sr = 0.70369 to 0.70499, 143Nd/144Nd = 0.51267 to 0.51317, 206Pb/204Pb = 17.64 to 18.36. The samples form highly correlated arrays between very depleted mid-ocean ridge basalt (MORB) and the Pacific pelagic sediment fields on Pb-Pb plots. Similarly, on plots of Sr-Pb and Nd-Pb, the Sea of Japan samples lie on mixing curves between depleted mantle and enriched mantle (\"EM II\"), which is interpreted to be of average crustal or pelagic sediment composition. The source of these backarc rocks appears to be a MORB-like mantle source, contaminated by pelagic sediments. Unlike the Mariana and Izu arc/backarc systems, Japanese arc and backarc rocks are indistinguishable from each other in a Sr-Nd isotope plot, and have similar trends in Pb-Pb plots. Thus, sediment contamination of the mantle wedge appears to control the isotopic compositions of both the arc and backarc magmas. Two-component mixing calculations suggest that the percentage of sediments in the magma source varies from 0.5% to 2.5%.西太平洋分布有大量火山岛弧（volcanic island arc）与弧后盆地复合体（backarc complexes），但针对此类地质体的多同位素研究仍较为匮乏。日本海弧后盆地的基底岩系在794、795与797站位被钻遇，其岩性以玄武岩床（basaltic sills）与熔岩流（lava flows）为主。这套岩石的同位素组成跨度极广，远超西太平洋其他岛弧或弧后系统：87Sr/86Sr比值介于0.70369至0.70499之间，143Nd/144Nd比值介于0.51267至0.51317之间，206Pb/204Pb比值介于17.64至18.36之间。在铅-铅（Pb-Pb）同位素图解中，样品呈现出与极度亏损的洋中脊玄武岩（mid-ocean ridge basalt, MORB）及太平洋远洋沉积物域高度相关的线性分布趋势。类似地，在锶-铅（Sr-Pb）与钕-铅（Nd-Pb）同位素图解中，日本海样品落在亏损地幔与富集地幔（"EM II"）的混合曲线上，后者被认为具有典型地壳或远洋沉积物的同位素组成。此类弧后岩石的源区似乎为类洋中脊玄武岩的地幔源区，并受到远洋沉积物的混染作用改造。与马里亚纳及伊豆岛弧-弧后系统不同，日本岛弧与弧后岩石在锶-钕（Sr-Nd）同位素图解中难以区分，且在铅-铅图解中呈现相似的演化趋势。因此，地幔楔的沉积物混染作用似乎同时控制了岛弧与弧后岩浆的同位素组成。双端元混合计算结果显示，岩浆源区中沉积物的占比介于0.5%至2.5%之间。