Modelling metamorphic rocks in complex systems : present-day developments in high pressure experimental petrology

Metamorphic processes at subduction zones are strictly related to the transfer and release of water via dehydration reactions from the subducting slab toward the mantle wedge. Results and present-day developments in high pressure experimental petrology in the laboratory of Milano are presented here, dealing with lithologies which are expected to be subducted, i.e. mainly mafics and ultramafics. Particular emphasis will be given to hydrate-bearing assemblages. Experiments performed in mafic rocks at pressures of up to 5 GPa show that phase relationships are controlled by amphibole s.s. to 2.5 GPa and by epidote group minerals and lawsonite at higher pressure. Phase relationships point out to strong sensitivity of assemblages from continuous reactions and therefore from bulk composition considered. In ultramafics, at temperatures between 680 and 800°C, amphibole and chlorite are the major hydrous phases able to transport water deep into the Earth's Mantle. At pressure > 4.8 GPa, a hydrous silicate with a 10Å phase structure forms at the expense of chlorite, providing a «carrier» by which water might be transported down to 200 km depth. The interaction between alkali-rich fluids and mantle peridotites lead to the occurrence of potassic hydrous phases such as phlogopite. Experiments on K-peridotite suggest a pressure dependent relevance of a «tale» component in phlogopites.