Résumé
The water flux at subduction zones is a crucial component of the Earth's water cycle. It partly controls the evolution of surficial liquid water that allowed the development of life. Previous modeling studies predicted a water retention in subduction zones into the deep upper mantle which may have reduced sea level by at least 300 m through the Phanerozoic [Parai and Mukhopadhyay, 2012]. Here, we use thermo-petrological models with new phase diagrams for mantle peridotite which combine thermodynamic calculations and experimental data at UHP conditions. Our revised calculations of water retention are 10% and 50% lower than that of previous studies and are more consistent with the hypothesis of a quasi-steady state sea level over hundreds of My, as proposed by studies based on observational constraints. To date, the most comprehensive study on subduction water fluxes indeed predicts 3.4X10 (super 8) Tg/My of water retained beyond post-arc depths, 30% of which occurs via the subduction of chemically-bound water to hydrous phases in the lithospheric mantle [van Keken 2011]. This study is based on modeled phase diagrams by the thermodynamic code Perple (sub X) [Connolly et al., 2009] and associated thermodynamic databases [e.g. Holland and Powell, 2011]. At UHP conditions, these phase diagrams suggest that bound-water transport in intermediate-to-cold subduction zones occur through the sequence antigorite-phase A-brucite. In particular, subductions for which the temperatures at the Moho are lower than 700-800 degrees C at 8-10 GPa, are predicted to hold voluminous amounts of water in the lithospheric mantle up to mantle-transition zone depths. However, laboratory experiments on natural peridotitic systems exhibit the formation of post-antigorite hydrous phases such as the aluminous-phase E, only stable below 800 degrees C above 8 GPa [Maurice et al., 2018]. We take into account the presence of natural phase assemblages in mantle peridotites at UHP conditions by building a phase diagram which combines thermodynamic calculations and experimental data at UHP conditions. Our novel thermo-petrological models suggest that all but the coldest oceanic plates are completely dehydrated by depths of 250-350 km in present-day subduction zones.