A role for subducting clays in the water transportation into the Earth's lower mantle

Abstract

Subducting sedimentary layer typically contains water and hydrated clay minerals. The stability of clay minerals under such hydrous subduction environment would therefore constraint the lithology and physical properties of the subducting slab interface. Here we show that pyrophyllite (Al2Si4O10(OH)(2)), one of the representative clay minerals in the alumina-silica-water (Al2O3-SiO2-H2O, ASH) system, breakdowns to contain further hydrated minerals, gibbsite (Al(OH)(3)) and diaspore (AlO(OH)), when subducts along a water-saturated cold subduction geotherm. Such a hydration breakdown occurs at a depth of similar to 135 km to uptake water by similar to 1.8 wt%. Subsequently, dehydration breakdown occurs at similar to 185 km depth to release back the same amount of water, after which the net crystalline water content is preserved down to similar to 660 km depth, delivering a net amount of similar to 5.0 wt% H2O in a phase assemblage containing delta-AlOOH and phase Egg (AlSiO3(OH)). Our results thus demonstrate the importance of subducting clays to account the delivery of similar to 22% of water down to the lower mantle.