Lawsonite metasomatism: A new route for water to the deep Earth

•We describe lawsonite metasomatism.•We model the effect of metasomatism on the bulk water content via pseudosection.•We discuss the role of this process for transferring water at subduction zones.

Hybrid rocks formed by fluid-rock interactions at high-pressure (HP) metamorphic conditions are active players in the recycling of volatiles in subduction zones. Such rocks include chlorite-talc-amphibole-rich (± carbonate) rocks formed by chemical and mechanical mixing of mafic, ultramafic and sedimentary protoliths. Recent discovery of widespread formation of lawsonite-rich hybrid rocks extends the composition range of these rocks and their significance for volatile transfer to the deep Earth. In this study we quantify the drastic water enrichment across a metasomatic aureole characterized by intense chemical resetting and massive lawsonite precipitation in Alpine Corsica (France). The chemical composition of the metasomatic product, which in many cases approaches the CASH system, favors (i) the precipitation of lawsonite and the unexpected reincorporation of free water at HP conditions, and (ii) the stability of lawsonite at higher temperature and at greater depth compared to the MORB+H2O system. We conclude that these hybrid rocks may contribute to transfer water to great depth in subduction, with implications for water cycling to the mantle.