Experimentally determined phase relations in hydrous peridotites to 6.5 GPa and their consequences on the dynamics of subduction zones

Fluid-saturated subsolidus experiments from 2.0 to 6.5 GPa, and from 680 to 800degreesC have been performed on three model peridotites in the system Na2O-CaO-FeO-MgO-Al2O3-SiO2-H2O (NCFMASH). Amphibole and chlorite coexist up to 2.4 GPa, 700degreesC. Chlorite persists to 4.2 GPa at 680degreesC. Starting from 4.8 GPa, 680degreesC a 10 Angstrom phase structure replaces chlorite in all compositions. The 10 Angstrom phase structure contains significant Al2O3 (up to 10.53 wt %) deviating from the MgO-SiO2-H2O 10 Angstrom phase (MSH 10 Angstrom phase). A mixed layered structure (chlorite-MSH 10 Angstrom phase) is proposed to account for aluminium observed. In the Tinaquillo lherzolite amphibole breakdown occurs via the reaction amphibole + olivine +/- H2O = clinopyroxene + orthopyroxene + chlorite. Thermal stability of chlorite (chlorite + orthopyroxene = forsterite + garnet + H2O) is shifted towards lower temperatures, compared with the system MASH. Furthermore, the chlorite thermal breakdown is also related to the degenerate reaction chlorite + clinopyroxene = olivine + garnet(+/- orthopyroxene) + H2O. Chlorite and the Al-10 Angstrom phase structure contribute significantly to the water budget in subduction zones in the depth range relevant for arc magmatism, whereas amphibole-related fluid release is restricted to the forearc region. Chlorite and Al-10 Angstrom phase breakdowns might explain the occurrence of a double seismic zone by dehydration embrittlement.