RECEIVED AUGUST 7, 1995; REVISED TYPESCRIPT ACCEPTED MARCH 18, 1998 and these dunites as zones of focusing of this melt in which pyroxene Ocean Drilling Program Leg 125 recovered serpentinized harzhas preferentially been dissolved from the harzburgite protolith. burgites and dunites from a total of five sites on the crests and In contrast, harzburgites from Torishima Forearc Seamount give flanks of two serpentinite seamounts, Conical Seamount in the calculated oxygen fugacities between FMQ +

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... and FMQ + Mariana forearc and Torishima Forearc Seamount in the Izu-Bonin 1·6, similar to those calculated for other subduction-zone related forearc. These are some of the first extant forearc peridotites reported peridotites and similar to those calculated for the dunites (FMQ in the literature and they provide a window into oceanic, supra-+ 1·2 to FMQ + 1·8) from the same seamount. In this case, subduction zone (SSZ) mantle processes. Harzburgites from both we interpret both the harzburgites and dunites as linked to mantle seamounts are very refractory with low modal clinopyroxene (<4%), melting (20-25% fractional melting) in a supra-subduction zone chrome-rich spinels (cr-number = 0·40-0·80), very low inenvironment. The results thus indicate that the forearc is underlain compatible element contents, and (with the exception of amphiboleby at least two types of mantle lithosphere, one being trapped or bearing samples) U-shaped rare earth element (REE) profiles with accreted oceanic lithosphere, the other being lithosphere formed by positive Eu anomalies. Both sets of peridotites have olivine-spinel subduction-related melting. They also demonstrate that both types equilibration temperatures that are low compared with abyssal of mantle lithosphere may have undergone extensive interaction with peridotites, possibly because of water-assisted diffusional equisubduction-derived magmas. libration in the SSZ environment. However, other features indicate that the harzburgites from the two seamounts have very different origins. Harzburgites from Conical Seamount are characterized by calculated oxygen fugacities between FMQ (fayalite-KEY WORDS: mantle peridotites; mantle melting; supra-subduction zone; magnetite-quartz) -1·1 (log units) and FMQ + 0·4 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 * Corresponding author. Present address: reviews by Pearce et al. (1992) and Taylor (1992)]. The be composed of serpentinite mud (Fryer et al., 1990b). pre-subduction basin (the West Philippine Basin) was Two flank sites (Sites 778 and 779) and one summit site trapped to the west of this 'proto-arc'. The arc-basin (Site 780) were drilled and a variety of clasts including system then evolved through a series of arc construction, serpentinized harzburgites and dunites were recovered arc rifting and back-arc spreading events to give the from the serpentinite mud matrix. Cold, dense, high present-day geometry depicted in Fig. 1 . The rocks pH (~11) fluids are currently seeping from aragonite associated with the inception of subduction are thus now chimneys on the summit of the seamount, and geosplit, with one part located in the forearc terranes to the chemical studies indicate that a component of these fluids east (where the peridotites crop out) and one part located is derived from dehydration reactions in the downgoing at the eastern margin of the West Philippine Basin to Pacific plate (Mottl, 1992) . the west. The northerly seamount, Torishima Forearc Sea-Information obtained from dredging of the forearc mount, is located at 30°55′N, 141°47′E, 30 km east of terranes at the eastern margin indicates that they underthe Bonin (Ogasawara) trench axis and 20 km above the went a complex tectonic history. A wide variety of rocks underlying subducting Pacific plate (Fig. 1, inset b) . The has been recovered, including mantle peridotite, various seamount is 1400 m high and 20 km in diameter (Ishii, cumulate rocks and volcanic rocks with boninite, island 1985; Kobayashi, 1989) . It is mantled by a 1 km thick arc tholeiite, mid-ocean ridge basalt (MORB) and ocean sedimentary sequence which has no internal reflectors island basalt (OIB) affinities (Bloomer, 1983; Bloomer & and is thought to be composed of serpentinite muds Hawkins, 1983). The MORB and OIB samples are overlying a substrate with strong internal reflectors (Hothought to represent mainly accreted material from the rine et al ., 1990). Gravity models for the Izu-Bonin outer Pacific plate, the MORB from Pacific oceanic crust and forearc indicate that the seamount must be underlain the OIB from seamounts, although some may represent by low-density material (<2690 kg/m 3 ) down to the West Philippine Basin crust that formed before subdécollement and that this material is serpentinized ultraduction initiation (Bloomer, 1983; Johnson & Fryer, mafic rock (Horine et al., 1990). ODP Leg 125 drilled 1990). The island arc tholeiite and boninite series rocks two sites on this seamount, an upper flank site (Site 783) are thought to represent mainly the crust of the Eocene and a lower flank site (Site 784) (see Fig. 1, inset b) . 'protoarc'. In contrast to Conical Seamount, Torishima Forearc The peridotites discussed in this paper could therefore Seamount is not actively venting fluids and is not therefore have three possible origins. They could represent: (1) at present undergoing hydration from the underlying mantle that existed before subduction initiation and subduction zone. formed part of the basement of the Eocene 'protoarc'; (2) the mantle residue from Eocene boninite or island arc tholeiite magmatism within the 'protoarc'; or (3) Whole-rock analysis upper mantle from the Pacific plate accreted into the forearc terrane during Eocene-Recent subduction. These Representative samples from the various drill holes in alternative origins will be evaluated in this paper. the two seamounts were crushed in an agate Tema mill. One aliquot was analysed for major and some trace elements (Sc, Ti, V, Cr, Co, Ni, Cu, Zn and Sr) by X-ray fluorescence (XRF) using a Philips PW 1500 SAMPLING AND ANALYSIS spectrometer at the University of Durham. Extended Sampling count times were used for trace elements because of their low concentrations in the peridotites. Replicate PCC-1, As noted above, the peridotite samples discussed in this DTS-1 and in-house ultramafic standards show that both paper come from two serpentinite seamounts drilled precision and accuracy are about ±2% for MgO and during ODP Leg 125. The southerly seamount, Conical SiO 2 ; about ±1% for other major elements, Cr and Ni; Seamount, is located at 19°35′N, 146°40′E, 80 km west of and about ±5% for Ti, V, Co, Zn and Sr. Cu is the Mariana trench axis and 30 km above the underlying commonly below the detection limit of 3 ppm. subducting Pacific plate (Fig. 1, inset a) . It is 1500 m A second aliquot was analysed for~40 elements, high and 20 km in diameter and has a conical shape. including Ti, Ga, Rb, Sr, Cs, Ba, Y, the rare earth Seismic studies of the seamount and surrounding areas elements (REE), Zr, Hf, Nb, Ta, Pb, Th and U by indicate that the seamount is located at the intersection inductively coupled plasma mass spectrometry (ICP-MS) of two fault zones (Fryer et al., 1990b). Several small on a VG Elemental Plasmaquad II at the Research horsts and graben are also located near the seamounts, School of Earth Sciences (RSES), Australian National and this indicates that the area is under active extension. University (ANU). The technique has been described by Reflection profiles indicate that the seamount is draped with thin flows, which have been shown from drilling to Eggins et al. (1997). Another study by the first author