Fluid-mantle interaction in an intra-oceanic arc: constraints from high-precision Pb isotopes

We present new isotopic and trace element data, including high-precision double-spike Ph isotope measurements, for back-arc lavas from the Izu-Bonin arc. Systematic along-arc isotopic variation of lavas has been identified in the Izu-Bonin arc that is coherent between the volcanic front and back-arc. The Sr isotopes are more radiogenic in the north, while Ph isotopes are less radiogenic in the north compared to the central part of the arc. This is particularly apparent in the back-arc seamounts. Decoupled variation of Pb and Sr isotopes cannot be explained by changes in the amount of a single subduction component. Almost parallel but distinct trends on Pb-Pb isotope plots imply differing mantle sources in the northern and central parts of the arc. The decoupling of Sr and Pb isotopic variation for both volcanic front and back-arc can be explained by the presence of two mantle components: a MORB source observed in the back-arc basins of the Philippine Sea Plate and a Pacific MORB-like source. An internally consistent model which explains along- and across-arc isotopic trends can be obtained by assuming mixing between the two mantle sources, but a lesser contribution of Pacific MORB-type source in the northern part of the arc. The source mantle of the rifting-related volcanism is distinct from the back-arc seamount chain volcanism and has a much stronger signature of Pacific MORB. A strong correlation between Sr isotopes and fluid-mobile element enrichment in the volcanic front lavas imply a significant contribution of slab-derived fluid to the source of volcanic front. In contrast, back-arc seamounts and rifting-related volcanism show a more limited influence of a fluid phase contribution to their source. Instead, high Delta7/4 and low Nd-143/Nd-144 associated with high Th/Ce imply that subducted bulk sediment is an important component in the back-arc. By assuming the regional mantle end-members, the relative contribution of a subduction-related component can be estimated. The magmatism in the back-arc seamounts is estimated to have a 0.2-0.3% bulk sediment addition to the source for both the northern and central parts of the arc. Minimal addition of fluid from altered oceanic crust is also predicted. For the rifting-related magmatism, a lower sediment contribution is predicted. Volcanic front magmatism is compatible with having no direct bulk sediment input, but is likely to involve the contribution of 2-2.5% of a fluid derived from altered oceanic crust and sediment in a mixing ratio of about 99:1. (C) 2003 Elsevier Science B.V. All rights reserved.