Experimentally Determined Phase Relations in Hydrous Peridotites to 6{middle dot}5 GPa and their Consequences on the Dynamics of Subduction Zones

Abstract

Fluid-saturated subsolidus experiments from 2·0 to 6·5 GPa, and from 680 to 800°C 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, 700°C. Chlorite persists to 4·2 GPa at 680°C. Starting from 4·8 GPa, 680°C a 10 Å phase structure replaces chlorite in all compositions. The 10 Å phase structure contains significant Al2O3 (up to 10·53 wt %) deviating from the MgO–SiO2–H2O 10 Å phase (MSH 10 Å phase). A mixed layered structure (chlorite–MSH 10 Å phase) is proposed to account for aluminium observed. In the Tinaquillo lherzolite amphibole breakdown occurs via the reaction \begin{eqnarray*}&&amphibole\ +\ olivine\ {\pm}\ H_{2}O\\&&=\ clinopyroxene\ +\ orthopyroxene\ +\ chlorite.\end{eqnarray*} 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 \begin{eqnarray*}&&chlorite\ +\ clinopyroxene\\&&=\ olivine\ +\ garnet\left({\pm}\ orthopyroxene\right)\ +\ H_{2}O.\end{eqnarray*} Chlorite and the Al-10 Å 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 Å phase breakdowns might explain the occurrence of a double seismic zone by dehydration embrittlement.