Isotopic characteristics of subduction fluids in an intra-oceanic setting,Izu-Bonin Arc, Japan

New radiogenic isotope and trace element data are presented for the volcani c sequences along 600 km of the active Izu-Bonin are, the Oligocene Izu are , and their associated rift basins. As with many intra-oceanic island arcs, the Pliocene-Recent Izu-Bonin frontal-are 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 ep isode of melting Ratios between these elements (such as Nb/Zr and La/Sm), a s well as Nd-143/Nd-144, do not vary significantly along the length of the frontal-are. These parameters suggest that each of the are volcanoes is der ived from similar melt fractions of the mantle wedge. However, Ba/Zr, Ba/Rb and Sr-87/Sr-86 increase along the frontal-are to the north. This leads us to propose that a variable enrichment in Ba and radiogenic Sr is superimpo sed on the mantle wedge. Sr-Nd and Ph-Nd isotope variation indicate that bo th Sr and ph become more radiogenic after fluid addition. However, Pb isoto pe ratios do not correlate with increases in Pb concentration or ratios suc h as Ba/Zr and Nb/Pb. In other words, the Pb isotopic composition of the ar e lavas appears to be independent of the amount of Pb introduced by subduct ion fluids into the mantle source. This buffering of Pb isotopes along the frontal-are means that the isotopic composition of the lavas is indistingui shable from that of the fluid. Isotopic mixing models presented for the are are only illustrative of the many plausible combinations of components and quantities. Despite this, we are able to determine that the mantle wedge h as isotopic characteristics similar to Indian Ocean MORE, and that the subd uction-fluid solute is primarily derived from subducted oceanic basalt with a <2% contribution from subducted sediment. Lavas in the Oligocene Izu are and fore-are basin were derived from a mantle wedge of similar composition to the active are. Despite levels of Pb enrichment comparable to those of the modem are, the Pb isotopes Df the Oligocene volcanics indicate a lower sediment input into the melting region. (C) 1998 Elsevier Science B.V. All rights reserved.