I. J. Parkinson

quartz) -11 (log units) and FMQ + 04 which ODP Leg 125; Izu-Bonin-Mariana forearc overlap those of mid-ocean ridge basalt (MORB) peridotites. Dunites from Conical Seamount contain small amounts of clinopyroxene, orthopyroxene and amphibole and are light REE (LREE) enriched. Moreover, they are considerably more oxidized than the harzburgites

Abstract Understanding mantle melting above subduction zones requires an evaluation of the behaviour of elements for which the mantle contribution greatly exceeds any subduction contribution. In this paper we present a compilation of partition coefficients for a suite of these elements (Nb, Zr, Y, Yb, Ca, Al, Ga, V, Sc, Fe, Mn, Co, Cr, Mg, Ni) for temperatures of 1200–1300°C, oxygen fugacities of QFM ± 1 and sub-alkaline compositions. These coefficients yield good-fit mantle depletion trends for abyssal, orogenic and trench-wall peridotites. Modelling of pooled melts from mantle melting columns, presented as FMM (fertile MORB mantle) normalized patterns, give signatures of the composition and degree of melting of the mantle wedge that are generally independent of the subduction component. In particular, patterns formed from melting of fertile mantle exhibit normalized element abundances in the order VHI > HI > MI (VHI = very highly incompatible, HI = highly incompatible and MI = moderately incompatible) at low degrees of melting, becoming VHI = HI = MI at high degrees of melting. With derivation from progressively depleted sources, the patterns for moderate degrees of melting change to VHI < HI = MI at moderate degrees of depletion and VHI < HI < MI at high degrees of depletion. The details, but not the principles, can be varied by changing the shape of the melting column, the porosity of the mantle during melting, the potential temperature of the mantle, and the temperature and depth of initiation of melting. Bivariate plots of elements of contrasting compatibilities (Cr-Yb, Sc-Yb, Nb-Yb) can be contoured according to the degree of depletion or enrichment of the mantle and the degree of melting, with selected plots also emphasizing the role of garnet (Ti-Yb) and oxygen fugacity (V-Yb). Evaluation of data from present-day volcanic arcs suggests that: (1) intra-oceanic arcs with associated active backarc basins are derived principally from fertile MORB mantle that has lost up to about 3% melt in a previous melting event; (2) this depletion takes place in spinel lherzolite facies, supporting models that relate it to backarc basin melting events; (3) oceanic arcs with no associated backarc basins are derived principally from fertile MORB mantle, though enriched sources can be important locally; (4) intra-continental arcs are commonly derived from enriched mantle, probably because of the involvement of sub-continental lithosphere; (5) degrees of melting are probably high (in the order of 25–30%) in intra-oceanic arcs on thin crust, decreasing to 150r less in areas of thicker lithosphere; (6) some 10% melting can be explained by volatile addition to the mantle, the remainder by decompression.

Holes drilled into the volcanic and ultrabasic basement of the Izu-Ogasawara and Mariana forearc terranes during Leg 125 provide data on some of the earliest lithosphere created after the start of Eocene subduction in the Western Pacific. The volcanic basement contains three boninite series and one tholeiite series. (1) Eocene low-Ca boninite and low-Ca bronzite andesite pillow lavas and dikes dominate the lowermost part of the deep crustal section through the outer-arc high at Site 786. (2) Eocene intermediate-Ca boninite and its fractionation products (bronzite andesite, andesite, dacite, and rhyolite) make up the main part of the boninitic edifice at Site 786. (3) Early Oligocene intermediate-Ca to high-Ca boninite sills or dikes intrude the edifice and perhaps feed an uppermost breccia unit at Site 786. (4) Eocene or Early Oligocene tholeiitic andesite, dacite, and rhyolite form the uppermost part of the outer-arc high at Site 782. All four groups can be explained by remelting above a subduction zone of oceanic mantle lithosphere that has been depleted by its previous episode of partial melting at an ocean ridge. We estimate that the average boninite source had lost 10-15 wt% of melt at the ridge before undergoing further melting (5-10%) shortly after subduction started. The composition of the harzburgite (<2% clinopyroxene, Fo content of about 92%) indicates that it underwent a total of about 25% melting with respect to a fertile MORB mantle. The low concentration of Nb in the boninite indicates that the oceanic lithosphere prior to subduction was not enriched by any asthenospheric (OIB) component.