Geochemistry and radiogenic isotope ratios of the volcanic sequences from Izu-Bonin Arc, Japan

New radiogenic isotope and trace element data are presented for the volcanic sequences along 600 km of the active Izu-Bonin arc, the Oligocene Izu arc, and their associated rift basins. As with many intra-oceanic island arcs, the Pliocene-Recent Izu-Bonin frontal-arc lavas are highly depleted in Zr, Nb and the rare-earth elements relative to typical mid-ocean ridge basalt (MORB), indicating that the mantle wedge source has undergone a previous episode of melting. Ratios between these elements (such as Nb/Zr and La/Sm), as well as 143Nd/144Nd, do not vary significantly along the length of the frontal-arc. These parameters suggest that each of the arc volcanoes is derived from similar melt fractions of the mantle wedge. However, Ba/Zr, Ba/Rb and 87Sr/86Sr increase along the frontal-arc to the north. This leads us to propose that a variable enrichment in Ba and radiogenic Sr is superimposed on the mantle wedge. Sr-Nd and Pb-Nd isotope variation indicate that both Sr and Pb become more radiogenic after fluid addition. However, Pb isotope ratios do not correlate with increases in Pb concentration or ratios such as Ba/Zr and Nb/Pb. In other words, the Pb isotopic composition of the arc lavas appears to be independent of the amount of Pb introduced by subduction fluids into the mantle source. This buffering of Pb isotopes along the frontal-arc means that the isotopic composition of the lavas is indistinguishable from that of the fluid. Isotopic mixing models presented for the arc are only illustrative of the many plausible combinations of components and quantities. Despite this, we are able to determine that the mantle wedge has isotopic characteristics similar to Indian Ocean MORB, and that the subduction-fluid solute is primarily derived from subducted oceanic basalt with a <2% contribution from subducted sediment. Lavas in the Oligocene Izu arc and fore-arc basin were derived from a mantle wedge of similar composition to the active arc. Despite levels of Pb enrichment comparable to those of the modern arc, the Pb isotopes of the Oligocene volcanics indicate a lower sediment input into the melting region.本文报道了沿活动伊豆-小笠原弧（active Izu-Bonin arc）600km范围的火山岩系、渐新世伊豆弧（Oligocene Izu arc）及其相关裂谷盆地（rift basins）的全新放射成因同位素（radiogenic isotope）与微量元素（trace element）数据。与多数洋内岛弧（intra-oceanic island arcs）一致，上新世-现代（Pliocene-Recent）的伊豆-小笠原前缘弧熔岩（frontal-arc lavas）相较于典型洋中脊玄武岩（mid-ocean ridge basalt, MORB），其Zr、Nb与稀土元素（rare-earth elements）含量显著亏损，指示地幔楔（mantle wedge）源区曾经历过一期熔融事件。上述元素比值（如Nb/Zr与La/Sm）以及143Nd/144Nd比值沿前缘弧延伸方向无显著变化，这些参数表明，各弧火山岩均源自地幔楔中相近的熔融程度（melt fractions）。然而Ba/Zr、Ba/Rb比值与87Sr/86Sr比值沿前缘弧向北逐渐升高，据此我们提出，地幔楔中叠加了Ba与放射成因Sr的可变富集作用。Sr-Nd与Pb-Nd同位素的变化特征表明，俯冲流体（subduction fluids）加入后，Sr与Pb的放射成因程度均有所升高。但Pb同位素比值与Pb浓度或Ba/Zr、Nb/Pb等比值的升高并无相关性，换言之，弧熔岩的Pb同位素组成似乎与俯冲流体带入地幔源区的Pb总量无关。前缘弧上的Pb同位素缓冲效应意味着，熔岩的同位素组成与俯冲流体的同位素组成难以区分。本文针对该弧提出的同位素混合模型仅能展示端元与量值的多种可行组合。尽管如此，我们仍可确定：地幔楔的同位素特征与印度洋洋中脊玄武岩相似，且俯冲流体溶质主要源自俯冲洋壳玄武岩，俯冲沉积物的贡献占比小于2%。渐新世伊豆弧与弧前盆地（fore-arc basin）的熔岩源自与活动弧成分相近的地幔楔。尽管其Pb富集程度与现代弧相当，但渐新世火山岩的Pb同位素特征指示其熔融区域的沉积物输入（sediment input）量更低。