Evidence of adiabatic decompression melting and melt-solid reaction in the Izu-Bonin-Mariana forearc peridotites: insights on the onset and evolution of intra-oceanic arc magmatism

The mantle peridotites recovered from intra-oceanic forearc regions are of particular interest to place constraints on the origin and early evolution of Supra- Subduction Zones (SSZ). A new opportunity to document the petrologic features of forearc peridotites has been provided by drilling at site 1200 during Ocean drilling Program (OdP) Leg 195. In this site, the top of the South Chamorro Seamount, located in the Mariana forearc, was cored, and several fragments of serpentinised mantle peridotites were recovered. Although the degree of serpentinisation of these samples is rather large, the occurrence of relics of the primary and secondary mineral assemblages allowed us to document a series of re-crystallisation events at high-T, mantle conditions. In this work, the major and trace element composition of bulk rock, as well as the major and trace element mineral chemistry, of a suite of South Chamorro serpentinites along with that of a number of peridotite samples previously recovered during the OdP Leg 125 at Conical seamount (site 779a; mariana forearc) and Torishima seamount (site 784a; Izu-bonin forearc), respectively, will be addressed in order to assess: 1) the fashion and extent of partial melting process(es); 2) the occurrence of melt-peridotite reaction and, if it is the case, the geochemical characteristics of the migrating melt(s), discussing, in particular, the possible presence of slab-derived component; 3) the possible geodynamic scenarios in which these processes took place.