Stability and Solid Solutions of Hydrous Alumino-Silicates in the Earth’s Mantle

Wendy R Panero,Razvan Caracas

doi:10.3390/min10040330

Abstract

The degree to which the Earth’s mantle stores and cycles water in excess of the storage capacity of nominally anhydrous minerals is dependent upon the stability of hydrous phases under mantle-relevant pressures, temperatures, and compositions. Two hydrous phases, phase D and phase H, are stable to the pressures and temperatures of the Earth’s lower mantle, suggesting that the Earth’s lower mantle may participate in the cycling of water. We build on our prior work of density functional theory calculations on phase H with the stability, structure, and bonding of hydrous phases D, and we predict the aluminum partitioning with H in the Al 2 O 3 -SiO 2 -MgO-H 2 O system. We address the solid solutions through a statistical sampling of site occupancy and calculation of the partition function from the grand canonical ensemble. We show that each phase has a wide solid solution series between MgSi 2 O 6 H 2 -Al 2 SiO 6 H 2 and MgSiO 4 H 2 -2 δ AlOOH + SiO 2 , in which phase H is more aluminum rich than phase D at a given bulk composition. We predict that the addition of Al to both phases D and H stabilizes each phase to higher temperatures through additional configurational entropy. While we have shown that phase H does not exhibit symmetric hydrogen bonding at high pressure, we report here that phase D undergoes a gradual increase in the number of symmetric H-bonds beginning at ∼30 GPa, and it is only ∼50% complete at 60 GPa.

Highlights

The storage and cycling of water in the Earth’s deep interior can be greatly enhanced if hydrous silicates are stable under the high pressures and temperatures of the Earth’s mantle
Synthesis experiments of phase D in the absence of aluminum demonstrates that it is stable at pressures greater than 16 GPa, with increasing thermal stability with increasing pressure, such that under pressures of the base of the Earth’s transition zone, phase D is stable to nearly 1700 K, suggesting it could even be stable in a mantle assemblage at moderately cool temperatures [3,4]
We focus on the stability, structure, and hydrogen bonding of hydrous phase D in the MgO-Al2 O3 -SiO2 -H2 O system, following the approach developed by [8]

Summary

Introduction

The storage and cycling of water in the Earth’s deep interior can be greatly enhanced if hydrous silicates are stable under the high pressures and temperatures of the Earth’s mantle. Phases D and H are likely candidates for the storage and transport of water in the transition zone and deep mantle (e.g., [1,2]). Synthesis experiments of phase D in the absence of aluminum demonstrates that it is stable at pressures greater than 16 GPa, with increasing thermal stability with increasing pressure, such that under pressures of the base of the Earth’s transition zone, phase D is stable to nearly 1700 K, suggesting it could even be stable in a mantle assemblage at moderately cool temperatures [3,4]. Walter et al [6] demonstrate that increasing silica

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