Water transfer to the deep mantle through hydrous, Al-rich silicates in subduction zones

Abstract

Constraining deep-water recycling along subduction zones is a first-order problem to understand how Earth has maintained a hydrosphere over billions of years that created conditions for a habitable planet. The pressure-temperature stability of hydrous phases in conjunction with slab geotherms determines how much H2O leaves the slab or is transported to the deep mantle. Chlorite-rich, metasomatic rocks that form at the slab-mantle interface at 50-100 km depth represent an unaccounted, H2O-rich reservoir in subduction processes. Through a series of high-pressure experiments, we investigated the fate of such chlorite-rich rocks at the most critical conditions for subduction water recycling (5-6.2 GPa, 620-800 degrees C) using two different natural ultramafic compositions. Up to 5.7 GPa, 740 degrees C, chlorite breaks down to an anhydrous peridotite assemblage, and H2O is released. However, at higher pressures and lower temperatures, a hydrous Al-rich silicate (11.5 angstrom phase) is an important carrier to enable water transfer to the deep mantle for cold subduction zones. Based on the new phase diagrams, it is suggested that the deep-water cycle might not be in secular equilibrium. Show more