Univesita Federico II, Italy

Serpentinised spinel harzburgites to orthopyroxene-rich spinel dunites recovered during the Ocean Drilling Program (ODP) Leg 195 on top of the South Chamorro Seamount (southern sector of the Mariana forearc, West Pacific Ocean), along with additional spinel harzburgites from Conical and Torishima Seamounts (northern Mariana and Izu-Bonin forearc, respectively), previously collected during the ODP Leg 125, have been investigated to shed light on the nature and evolution of forearc mantle in the intra-oceanic supra-subduction environment. All the samples show a marked heterogeneity in terms of petrographic, mineralogical and geochemical features that suggests a complex, multistage evolution involving, at variable extent, partial melting, reactive porous flow melt migration and subsolidus metamorphic re-equilibration under decreasing T and open system conditions. Geochemical evidence of the interaction between peridotites and various melts/fluids is the ubiquitous enrichment in highly incompatible elements, such as Large Ion Lithophile Elements (LILE). As for the high-T evolution of these peridotites, a three-stages-model is proposed, involving: 1) a former depletion event, during which the IBM forearc peridotites experienced 20-25% polybaric fractional melting during adiabatic upwelling; 2) a second depletion event characterised by a marked impoverishment in modal orthopyroxene, related to the upraise migration of ultra-depleted melts; 3) a late interaction between a relatively small volume of residual melts and the refractory mantle sequence. Oxidation state of the mantle minerals meanly decreases from north (Torishima Seamount) to south (South Chamorro), according to significant different contributions coming from the subducted Pacific Plate. In particular, the absence of a marked oxidation in South Chamorro peridotites suggests that the percolating melts during Stage 2 had not significant slab-derived component. This observation lead us to conclude that a thermal anomaly in the western Pacific mantle rather than the injection of hydrous components must be the "engine" determining the extreme depletion of the oceanic forearc peridotites and the arc formation. In this frame, it is proposed that IBM peridotites during Stage 1 underwent decompression partial melting and contributed to arc volcanism as actual mantle source. Successively, they were emplaced at relatively shallow levels (Stage 2), constituting the top of a strongly refractory mantle column and being percolated by melts produced by plumbing sources of the arc volcanism.