[1] Recent diving with the JAMSTEC Shinkai 6500 manned submersible in the Mariana fore arc southeast of Guam has discovered that MORB-like tholeiitic basalts crop out over large areas. These "fore-arc basalts" (FAB) underlie boninites and overlie diabasic and gabbroic rocks. Potential origins include eruption at a spreading center before subduction began or eruption during near-trench spreading after subduction began. FAB trace element patterns are similar to those of MORB and most Izu-Bonin-Mariana (IBM) back-arc lavas. However, Ti/V and Yb/V ratios are lower in FAB reflecting a stronger prior depletion of their mantle source compared to the source of basalts from mid-ocean ridges and back-arc basins. Some FAB also have higher concentrations of fluid-soluble elements than do spreading center lavas. Thus, the most likely origin of FAB is that they were the first lavas to erupt when the Pacific Plate began sinking beneath the Philippine Plate at about 51 Ma. The magmas were generated by mantle decompression during near-trench spreading with little or no mass transfer from the subducting plate. Boninites were generated later when the residual, highly depleted mantle melted at shallow levels after fluxing by a water-rich fluid derived from the sinking Pacific Plate. This magmatic stratigraphy of FAB overlain by transitional lavas and boninites is similar to that found in many ophiolites, suggesting that ophiolitic assemblages might commonly originate from near-trench volcanism caused by subduction initiation. Indeed, the widely dispersed Jurassic and Cretaceous Tethyan ophiolites could represent two such significant subduction initiation events.

[1] We present a comprehensive major and trace element dataset establishing ODP Site 801 as a geochemical reference for altered oceanic crust. The composition of old crustal sequences like those at Sites 801 and 1149 are critical to developing models of crustal aging and seawater chemistry evolution and to understanding the fate of crust consumed at subduction zones. Our estimate of the bulk composition of oceanic crust at Site 801 comprises ICP-AES and ICP-MS analyses of 117 discrete samples, 14 mixed composites and 5 glasses from the upper 500 m of Jurassic Pacific crust. Comparing the 801 ''supercomposite'' with glass reveals enrichment of U (5x), Li (2x), K 2 O (4x), Rb (9x), and Cs (7x), similar to DSDP Sites 417/418, but little to no enrichment in Ba or Pb. The data also demonstrate good ($10%) agreement between U measured on discrete samples and natural gamma logs, suggesting logging data is a reliable means of establishing bulk geochemical characteristics of oceanic crust. Data reported here serve to link other geochemical and mineralogical measurements on Site 801 and 1149 samples. We also document Boston University sample preparation procedures and instrument parameters for ICP-MS and ICP-AES analyses, and provide comparisons with other laboratories and techniques. We present new techniques for basaltic glass analyses using the Boston University 213 nm Nd-YAG LA-ICP-MS system, and show the data agree well with both solution-ICP-MS (5-10%) and ion probe measurements ($10%).

Mantle oxygen fugacity exerts a primary control on mass exchange between Earth's surface and interior at subduction zones, but the major factors controlling mantle oxygen fugacity (such as volatiles and phase assemblages) and how tectonic cycles drive its secular evolution are still debated. We present integrated measurements of redox-sensitive ratios of oxidized iron to total iron (Fe 3+ /SFe), determined with Fe K-edge micro-x-ray absorption near-edge structure spectroscopy, and pre-eruptive magmatic H 2 O contents of a global sampling of primitive undegassed basaltic glasses and melt inclusions covering a range of plate tectonic settings. Magmatic Fe 3+ /SFe ratios increase toward subduction zones (at ridges, 0.13 to 0.17; at back arcs, 0.15 to 0.19; and at arcs, 0.18 to 0.32) and correlate linearly with H 2 O content and element tracers of slab-derived fluids. These observations indicate a direct link between mass transfer from the subducted plate and oxidation of the mantle wedge. P late tectonics leads to a two-way geochemical exchange between Earth's interior and exterior. This process is driven by the formation of new oceanic crust by mantle melting at mid-ocean ridges, hydration and oxidative alteration of oceanic crust as it transits the seafloor, and the subsequent return of hydrated oxidized oceanic crust to the deep Earth at subduction zones (Fig. 1A) (1, 2). How this exchange has affected the oxygen fugacity of the mantle spatially (3) and over time (2, 4, 5) remains unclear. Many lines of evidence point to oxidizing conditions in arc peridotites and magmas (1, 6), but a quantitative link between oxidation state and the subduction process, although intuitive, has not been established. Here we provide coupled measurements of the redox-sensitive Fe 3+ /SFe ratio and magmatic H 2 O concentrations at the same spatial resolution in a global suite of undegassed basaltic glasses, in order to determine the current oxidation condition of the mantle as a function of tectonic regime.The ratio of oxidized iron to total iron [Fe 3+

[1] Subduction zone magmas are characterized by high concentrations of H 2 O, presumably derived from the subducted plate and ultimately responsible for melting at this tectonic setting. Previous studies of the role of water during mantle melting beneath back-arc basins found positive correlations between the H 2 O concentration of the mantle (H 2 O o ) and the extent of melting (F), in contrast to the negative correlations observed at mid-ocean ridges. Here we examine data compiled from six back-arc basins and three mid-ocean ridge regions. We use TiO 2 as a proxy for F, then use F to calculate H 2 O o from measured H 2 O concentrations of submarine basalts. Back-arc basins record up to 0.5 wt % H 2 O or more in their mantle sources and define positive, approximately linear correlations between H 2 O o and F that vary regionally in slope and intercept. Ridge-like mantle potential temperatures at back-arc basins, constrained from Na-Fe systematics (1350°-1500°C), correlate with variations in axial depth and wet melt productivity ($30-80% F/wt % H 2 O o ). Water concentrations in back-arc mantle sources increase toward the trench, and back-arc spreading segments with the highest mean H 2 O o are at anomalously shallow water depths, consistent with increases in crustal thickness and total melt production resulting from high H 2 O. These results contrast with those from ridges, which record low H 2 O o (<0.05 wt %) and broadly negative correlations between H 2 O o and F that result from purely passive melting and efficient melt focusing, where water and melt distribution are governed by the solid flow field. Back-arc basin spreading combines ridge-like adiabatic melting with nonadiabatic mantle melting paths that may be independent of the solid flow field and derive from the H 2 O supply from the subducting plate. These factors combine significant quantitative and qualitative differences in the integrated influence of water on melting phenomena in back-arc basin and mid-ocean ridge settings.

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[1] Recent examinations of the chemical fluxes through convergent plate margins suggest the existence of significant mass imbalances for many key species: only 20-30% of the to-the-trench inventory of large-ion lithophile elements (LILE) can be accounted for by the magmatic outputs of volcanic arcs. Active serpentinite mud volcanism in the shallow forearc region of the Mariana convergent margin presents a unique opportunity to study a new outflux: the products of shallow-level exchanges between the upper mantle and slab-derived fluids. ODP Leg 125 recovered serpentinized harzburgites and dunites from three sites on the crests and flanks of the active Conical Seamount. These serpentinites have U-shaped rare earth element (REE) patterns, resembling those of boninites. U, Th, and the high field strength elements (HFSE) are highly depleted and vary in concentration by up to 2 orders of magnitude. The low U contents and positive Eu anomalies indicate that fluids from the subducting Pacific slab were probably reducing in nature. On the basis of substantial enrichments of fluid-mobile elements in serpentinized peridotites, we calculated very large slab inventory depletions of B (79%), Cs (32%), Li (18%), As (17%), and Sb (12%). Such highly enriched serpentinized peridotites dragged down to depths of arc magma generation may represent an unexplored reservoir that could help balance the input-output deficit of these elements as observed by Plank and Langmuir (1993, 1998) and others. Surprisingly, many species thought to be mobile in fluids, such as U, Ba, Rb, and to a lesser extent Sr and Pb, are not enriched in the rocks relative to the depleted mantle peridotites, and we estimate that only 1-2% of these elements leave the subducting slabs at depths of 10 to 40 km. Enrichments of these elements in volcanic front and behind-the-front arc lavas point to changes in slab fluid composition at greater depths.

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