Accounting for the natural post-antigorite UHP hydrous phases in thermo-petrological models of subduction zones: implications for water retention into the deep upper mantle

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 3.4×10⁸ 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_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 700-800ºC 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˚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.