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

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.