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In the study of geochemical mass balances at subduction zones, the composition of the mantle wedge prior to additions from the slab is a critically important yet poorly constrained parameter. Deconvolving the influence of ancient versus modern enrichments is particularly difficult, especially when considering elements that are highly mobile. Here we provide an alternative approach, using less mobile elements, and a filter to remove the effects of recent slab additions. We provide new Hf isotope data for 30 Mariana Trough (MT) backarc basin lavas. Once filtered, Hf and Nd isotope ratios are highly correlated, of Indian mid-oceanic ridge basalt character, and display variations similar to ocean ridges of comparable lengths. The isotopic variability observed in this "ambient mantle" provides a new paradigm for the interpretation of the varied volcanic products of the arc. Thus, shoshonites associated with the northern termination of the backarc basin rift axis reflect the interaction of a subducted sediment melt with an isotopically enriched mantle source. In contrast, the large volcanoes of the Central Island province have a consistent offset in Nd isotope compositions from the MT array resulting from fluid addition. Existing data for smaller edifices in the submarine portion of the arc have larger variations resulting from fluid addition on a more local scale. We suggest that the similar characterization of ambient mantle elsewhere may help to resolve many conflicting geochemical observations in arc lavas worldwide. [ABSTRACT FROM AUTHOR]

Pagan is one of the largest volcanoes along the Mariana arc volcanic front. It has a maximum elevation of 570 m (Mt. Pagan), but its submarine flanks descend to 2000–3000 m below sea level, and are unexplored. Bathymetric mapping and ROV Hyper-Dolphin dives (HPD1147 and HPD1148) on the submarine NE and SW flanks of Pagan were carried out during cruise NT10-12 of R.V. Natsushima in July 2010. There are no systematic compositional differences between subaerial lavas reported in the literature and differentiated submarine lavas collected in HPD1148, with <7 wt % MgO, suggesting they are derived from the same magmatic system. However, these differentiated lavas show complexities including magma mixing; thus we concentrate on magnesian submarine lavas (>7 wt % MgO). Twenty least-fractionated basalts (48·5–50 wt % SiO2) collected during HPD1147 extend to higher MgO (10–11 wt %) and Mg# (66–70) than the subaerial lavas. Olivine (up to Fo94) and spinel (Cr# up to 0·8) compositions suggest that these Pagan primitive magmas formed from high degrees of mantle melting. Two basalt types can be distinguished based on their geochemistry at similar (10–11 wt %) MgO; these erupted recently, 500 m apart. Both contain clinopyroxene and olivine phenocrysts and are referred to as COB1 and COB2. Lower TiO2, FeO, Na2O, K2O, incompatible trace element abundances, and Nb/Yb suggest that COB1 formed from higher degrees of mantle melting. In addition, light rare earth element (LREE) enrichment and higher Th/Nb in COB2 contrast with LREE depletion and lower Th/Nb in COB1. Higher Ba/Th and Ba/Nb and lower Th/Nb indicate that the main subduction addition in COB1 was dominated by hydrous fluid, whereas that in COB2 was dominated by sediment melt. Sr–Nd–Pb–Hf isotopes are also consistent with this interpretation. These observations suggest that the subduction component responsible for the greater degree of melting of the COB1 source was mostly hydrous fluid. The origin of such different metasomatic agents resulted in different primary magmas forming in the same volcano. Both hydrous fluid and sediment melt components may have unmixed from an originally homogeneous supercritical fluid in or above the subducting slab below the volcanic front. These may have been added separately to the mantle wedge peridotite (mantle diapir) and resulted in two neighboring but completely different primary magmas from the same diapir. Moreover, these primitive lavas suggest that even for intra-oceanic arcs assimilation–fractional crystallization is inevitable when these magmas evolve in the crust and, in addition, that phlogopite is present in their mantle residue and thus played an important role in their genesis. [ABSTRACT FROM PUBLISHER]