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

Jörg Hermann,Shayne Lakey

doi:10.1130/g48658.1

Abstract

AbstractConstraining 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 °C) using two different natural ultramafic compositions. Up to 5.7 GPa, 740 °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 Å 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.

Highlights

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 °C) using two different natural ultramafic compositions
Recycling of water to the deep layers of Earth along subduction zones impacts a number of key plate-tectonics processes such as arc volcanism and intermediate-depth earthquakes that are triggered by dehydration reactions (Schmidt and Poli, 1998; Hacker et al, 2003)
Our study shows that deep-water recycling is sensitive to four key variables: ultramafic rock composition, distribution of these rocks in the slab, degree of hydration with depth, and slab geotherms

Summary

Introduction

Recycling of water to the deep layers of Earth along subduction zones impacts a number of key plate-tectonics processes such as arc volcanism and intermediate-depth earthquakes that are triggered by dehydration reactions (Schmidt and Poli, 1998; Hacker et al, 2003). 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.

Results

Conclusion