PERIDOTITES FROM THE IZU-BONIN-MARIANA FORE-ARC (ODP LEG-125) - EVIDENCE FOR MANTLE MELTING AND MELT-MANTLE INTERACTION IN A SUPRA-SUBDUCTION ZONE SETTING

Ocean Drilling Program Leg 125 recovered serpentinized harzburgites an d dunites from a total of five sites on the crests and flanks of two s erpentinite seamounts, Conical Seamount in the Mariana forearc and Tor ishima Forearc Seamount in the Izu-Bonin forearc. These are some of th e first extant forearc peridotites reported in the literature and they provide a window into oceanic, suprasubduction zone (SSZ) mantle proc esses. Harzburgites from both seamounts are very refractory with low m odal clinopyroxene (<4%), chrome-rich spinels (cr-number = 0.40-0.80), very low incompatible element contents, and (with the exception of am phibole-bearing samples) U-shaped rare earth element (REE) profiles wi th positive Eu anomalies. Both sets of peridotites have olivine-spinel equilibration temperatures that are low compared with abyssal peridot ites, possible because of water-assisted diffusional equilibration in the SSZ environment. However, other features indicate that the harzbur gites from the two seamounts have very different origins. Harzburgites from Conical Seamount are characterized by calculated oxygen fugaciti es between FMQ (fayalite-magnetite-quartz) - 1.1 (log units) and FMQ 0.4 which 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. Moreov er, they are considerably more oxidized than the harzburgites to which they are spatially related, with calculated oxygen fugacities of FMQ - 0.2 to FMQ + 1.2. Using textural and geochemical evidence, we interp ret these harzburgites as residual MORB mantle (from 15 to 20% fractio nal melting) which has subsequently been modified by interaction with boninitic melt within the mantle wedge, and these dunites as zones of focusing of this melt in which pyroxene has preferentially been dissol ved from the harzburgite protolith. In contrast, harzburgites from Tor ishima Forearc Seamount give calculated oxygen fugacities between FMQ + 0.8 and FMQ + 1.6, similar to those calculated for other subduction- zone related peridotites and similar to those calculated for the dunit es (FMQ + 1.2 to FMQ + 1.8) from the same seamount. In this case, we i nterpret both the harzburgites and dunites as linked to mantle melting (20-25% fractional melting) in a supra-subduction zone environment. T he results thus indicate that the forearc is underlain by at least two types of mantle lithosphere, one being trapped or accreted oceanic li thosphere, the other being lithosphere formed by subduction-related me lting. They also demonstrate that both types of mantle lithosphere may have undergone extensive interaction with subduction-derived magmas.