Water distribution in the lower mantle: Implications for hydrolytic weakening

Joshua M.R Muir,John P Brodholt

doi:10.1016/j.epsl.2017.11.051

Abstract

The presence of water in lower mantle minerals is thought to have substantial effects on the rheological properties of the Earth's lower mantle in what is generally known as “hydrolytic weakening”. This weakening will have profound effects on global convection, but hydrolytic weakening in lower mantle minerals has not been observed experimentally and thus the effect of water on global dynamics remains speculative. In order to constrain the likelihood of hydrolytic weakening being important in the lower mantle, we use first principles methods to calculate the partitioning of water (strictly protons) between mineral phases of the lower mantle under lower mantle conditions. We show that throughout the lower mantle water is primarily found either in the minor Ca-perovskite phase or in bridgmanite as an Al3+–H+ pair. Ferropericlase remains dry. However, neither of these methods of water absorption creates additional vacancies in bridgmanite and thus the effect of hydrolytic weakening is likely to be small. We find that water creates significant number of vacancies in bridgmanite only at the deepest part of the lower mantle and only for very high water contents (>1000 ppm). We conclude that water is thus likely to have only a limited effect on the rheological properties of the lower mantle.

Highlights

A growing number of studies are finding that incorporating a water dependent rheology into global mantle convection models has a strong effect on global dynamics
For instance the feedback between water recycling and rheology can control mantle cooling history, the efficiency and timescale of water recycling, the initiation of plate tectonics, continental growth, and the formation of dense chemical anomalies (Crowley et al, 2011; Korenaga, 2011; Sandu et al, 2011; Nakagawa et al, 2015; Honing and Spohn, 2016). These results are based on the assumption that water has a strong effect on the rheological properties of mineral and rocks and that this occurs throughout the convecting mantle
Due to its small grain size (Solomatov et al, 2002; Solomatov and Reese, 2008) and the lack of any significant anisotropy throughout most of the lower mantle (e.g. Karato, 1998), it is generally assumed that bridgmanite deforms by diffusion creep and so our results suggest that water would not produce hydrolytic weakening in bridgmanite

Summary

Introduction

A growing number of studies are finding that incorporating a water dependent rheology into global mantle convection models has a strong effect on global dynamics. For instance the feedback between water recycling and rheology can control mantle cooling history, the efficiency and timescale of water recycling, the initiation of plate tectonics, continental growth, and the formation of dense chemical anomalies (Crowley et al, 2011; Korenaga, 2011; Sandu et al, 2011; Nakagawa et al, 2015; Honing and Spohn, 2016) These results are based on the assumption that water has a strong effect on the rheological properties of mineral and rocks (hydrolytic weakening) and that this occurs throughout the convecting mantle. To approach this problem we have calculated the energetics of water partitioning between various lower mantle mineral sites using Density Functional Theory (DFT) which allows us to simulate the high pressure and high temperature conditions of the lower mantle that are hard to replicate experimentally

Methods

Results

Conclusion