Subduction fluids and their interaction with the mantle wedge : a perspective from the study of high pressure ultramafic rocks

We review three case studies emphasizing the role of ultramafic rocks in the recycling of volatiles and trace elements at convergent plate margins. Serpentinites are major water carriers in subduction zones and their breakdown liberates large quantities of water at sub-arc depths. The incompatible elements incorporated during oceanic serpentinization are released into the fluid phase produced once antigorite dehydrates to olivine + orthopyroxene. Importantly, the antigorite breakdown can trigger either wet melting or production of supercritical fluids in altered basalts and sediments. The produced fluid phases incorporate substantial amounts of incompatible element, initially residing in the crustal reservoirs. The fluid phase which exits the slab is highly reactive with respect to the overlying, silica undersaturated, mantle rocks. This leads to formation of reactive (ortho)pyroxenite layers, which filter the uprising hydrous melt/supercritical fluid to produce aqueous, solute-rich solutions. This fluid has equilibrated with peridotites and is mobile in the mantle. A consequence of these subduction fluid/mantle reactions is that the mantle wedge domains overlying the slabs can be heterogeneous in composition and layered, due to the presence of reactive pyroxenite bodies. Another aspect regards the debate whether supercritical fluids or hydrous melts are effective media for trace element transport. Since both agents are saturated in silica, they will react with the silica-undersaturated mantle wedge peridotites to produce aqueous, incompatible trace element-rich residual fluids. Hence, while hydrous melt and/or supercritical fluids are important for scavenging incompatible elements from the slab, they may not be the agents that transfer the metasomatic subduction signature to the inner parts of the mantle wedges.