Experimentally Determined Phase Relations in Hydrous Peridotites to 6.5 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-Al2O 3-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. ne 10 Å phase structure contains significant Al2O 3 (up to 10·53 wt%) deviating from the MgO-SiO 2-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. amphibole + olovine ± 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 Å phase structure contribute significantly to the water budget in subduction zones in the depth range relevant for are magmatism, whereas amphibole-related fluid release is restricted to the forearc region. Chlorite and Al-10 Å phase breakdoswns might explain the occurence of a double seismic zone by dehydration embrittlement.