Metasomatism during high-pressure metamorphism: Eclogites and blueschist-facies rocks

Abstract

High-pressure (HP) and ultrahigh-pressure (UHP) metamorphic rocks, which represent parts of ancient subducted slabs, are the obvious target for obtaining constraints on the P-T conditions, composition, and type of slab-derived fluids responsible for metasomatism in subduction zone environments since they are the only direct witnesses of such processes. Usually, solute-poor, aqueous fluids, derived by prograde HP metamorphic devolatilisation reactions in subducted slabs, are made responsible for the trace element transfer between the slab and mantle wedge, both of which may have been metasomatised (modification of bulk rock composition) in due process. At deeper levels in subduction zones at UHP conditions, dense, solute-rich transitional fluids (intermediate between hydrous silicate melts and aqueous fluids), or hydrous silicate melts are thought to become more important with regards to element mobilization and transport. Nonetheless, when considering the petrological and fluid inclusion evidence, it seems reasonable to generally assume that solute-poor aqueous fluids (solute