Transformations and Decomposition of MnCO3 at Earth's Lower Mantle Conditions

Eglantine Boulard,Mohamed Mezouar,Yijin Liu,Wendy L Mao,Julien Stodolna,Guillaume Morard,Ai L Koh,Mary M Reagan

doi:10.3389/feart.2016.00107

Abstract

Carbonates have been proposed as the principal oxidized carbon-bearing phases in the Earth’s interior. Their phase diagram for the high pressure and temperature conditions of the mantle can provide crucial constraints on the deep carbon cycle. We investigated the behavior of MnCO3 at pressures up to 75 GPa and temperatures up to 2200 K. The phase assemblage in the resulting run products was determined in situ by X-ray diffraction (XRD), and the recovered samples were studied by analytical transmission electron microscopy (TEM) and X-ray absorption near edge structure (XANES) imaging. At moderate temperatures below 1400 K and pressures above 50 GPa, MnCO3 transformed into the MnCO3-II phase, with XANES data indicating no change in the manganese oxidation state in MnCO3-II. However, upon heating above 1400 K at the same pressure conditions, both MnCO3 and MnCO3-II undergo decomposition and redox reactions which lead to the formation of manganese oxides and reduced carbon.

Highlights

Carbonates represent the main oxidized carbon-bearing phases which are transported into the mantle during subduction
To clarify the high pressure and high temperature behavior of MnCO3, we combined in situ XRD using laser-heated diamond anvil cells and ex situ analyses using analytical TEM and X-Ray Absorption Near Edge Structure (XANES) tomography for conditions up to ∼75 GPa and 2200 K, the results of which are reported in this work
When heating at relatively low temperature (∼1300 K) only MnCO3-I is observed; upon heating above 1400 K, the XRD patterns reveal the presence of another phase which does not correspond to the high pressure phase of MnCO3, MnO, cubic αMn2O3, or Mn3O4 manganese oxides

Summary

Introduction

Carbonates represent the main oxidized carbon-bearing phases which are transported into the mantle during subduction. The stability of carbonates vs their decomposition and melting provides critical constraints for understanding the global carbon cycle For all these reasons, the thermodynamic properties and phase diagrams for relevant carbonate compositions are needed down to core-mantle boundary conditions, i.e., megabar pressures and temperatures up to 3000 K. With a Mn2+ cation size that lies between those of Mg2+ and Ca2+, rhodochrosite (MnCO3) represents a potential model compound for understanding the differences in the high-pressure behavior of the two main carbonate compositions (Mg and Ca carbonates) The interplay between these two species has been the subject of many studies (e.g., de Capitani and Peters, 1981; Wang et al, 2011). To clarify the high pressure and high temperature behavior of MnCO3, we combined in situ XRD using laser-heated diamond anvil cells and ex situ analyses using analytical TEM and XANES tomography for conditions up to ∼75 GPa and 2200 K, the results of which are reported in this work

Methods

Results

Discussion

Conclusion