Dehydration of δ-AlOOH in Earth’s Deep Lower Mantle

Hélène Piet,Sang-Heon Shim,Eran Greenberg,Kurt D Leinenweber,Vitali B Prakapenka,Jacqueline Tappan,Peter R Buseck

doi:10.3390/min10040384

Abstract

δ -AlOOH has been shown to be stable at the pressure–temperature conditions of the lower mantle. However, its stability remains uncertain at the conditions expected for the lowermost mantle where temperature is expected to rise quickly with increasing depth. Our laser-heated diamond-anvil cell experiments show that δ -AlOOH undergoes dehydration at ∼2000 K above 90 GPa. This dehydration temperature is lower than geotherm temperatures expected at the bottom ∼700 km of the mantle and suggests that δ -AlOOH in warm slabs would dehydrate in this region. Our experiments also show that the released H 2 O from dehydration of δ -AlOOH can react with metallic iron, forming iron oxide, iron hydroxide, and possibly iron hydride. Our observations suggest that H 2 O from the dehydration of subducting slabs, if it occurs, could alter the chemical composition of the surrounding mantle and core regions.

Highlights

Hydrogen is known to significantly affect the chemical and physical properties of silicates and iron alloys at high pressure–temperature ( P-T ) [1,2]
The potential stability of δ-AlOOH in the lowermost mantle makes it an interesting phase to study together with metallic iron at the P-T conditions of the core-mantle boundary (CMB) to understand its possible interactions with the outer core
We investigate the dehydration of δ-AlOOH at pressures and temperatures relevant to the lower mantle in the laser-heated diamond anvil cell (LHDAC)

Summary

Introduction

Hydrogen is known to significantly affect the chemical and physical properties of silicates and iron alloys at high pressure–temperature ( P-T ) [1,2]. Recent high-pressure experiments have suggested that δ-AlOOH, which is isostructural with phase H and forms a solid solution with it [8], could be stable in the lower mantle [9,10]. The potential stability of δ-AlOOH in the lowermost mantle makes it an interesting phase to study together with metallic iron at the P-T conditions of the CMB to understand its possible interactions with the outer core. Minerals 2020, 10, 384 alloy at high P-T They reported the appearance of metal hydride at 500–700 K below the dehydration temperature of δ-AlOOH [10]. Because untransformed metastable diaspore starting material persists at high temperature in their experiments, it is unclear if this low-temperature appearance of metal hydride could occur with the dehydration of the more stable form of AlOOH, i.e., δ phase. We discuss the implications of our results for the Earth’s lower mantle and the core-mantle boundary (CMB)

Experimental Methods

Dehydration of δ-AlOOH at High P-T

Reaction between Iron Metal and H2 O Released from δ-AlOOH Dehydration

Findings

Implications