CO2-induced destabilization of pyrite-structured FeO2Hx in the lower mantle

Eglantine Boulard,Alexandre Corgne,Guillaume Fiquet,Jean-Philippe Perrillat,Anne-Line Auzende,François Guyot,Nicolas Menguy

doi:10.1093/nsr/nwy032

Abstract

Abstract Volatiles, such as carbon and water, modulate the Earth's mantle rheology, partial melting and redox state, thereby playing a crucial role in the Earth's internal dynamics. We experimentally show the transformation of goethite FeOOH in the presence of CO2 into a tetrahedral carbonate phase, Fe4C3O12, at conditions above 107 GPa—2300 K. At temperatures below 2300 K, no interactions are evidenced between goethite and CO2, and instead a pyrite-structured FeO2Hx is formed as recently reported by Hu et al. (2016; 2017) and Nishi et al. (2017). The interpretation is that, above a critical temperature, FeO2Hx reacts with CO2 and H2, yielding Fe4C3O12 and H2O. Our findings provide strong support for the stability of carbon-oxygen-bearing phases at lower-mantle conditions. In both subducting slabs and lower-mantle lithologies, the tetrahedral carbonate Fe4C3O12 would replace the pyrite-structured FeO2Hx through carbonation of these phases. This reaction provides a new mechanism for hydrogen release as H2O within the deep lower mantle. Our study shows that the deep carbon and hydrogen cycles may be more complex than previously thought, as they strongly depend on the control exerted by local mineralogical and chemical environments on the CO2 and H2 thermodynamic activities.

Highlights

Water (H2O) and carbon dioxide (CO2) both play an important role in the history of the Earth, as they strongly influence the chemical and physical properties of minerals, melts and fluids
In situ X-Ray diffraction (XRD) patterns showed a significant broadening of α-FeOOH main diffraction reflections characteristic of incipient amorphization
We measured a unit cell parameter of a = 4.367 Aat 107 GPa, which is significantly larger than that reported for FeO2 (a = 4.363 Aat 76 GPa) by Hu et al [37], but smaller than that reported for FeOOH (a = 4.386 Aat 109 GPa) in Nishi et al [35] (Fig. 2)

Summary

Introduction

Water (H2O) and carbon dioxide (CO2) both play an important role in the history of the Earth, as they strongly influence the chemical and physical properties of minerals, melts and fluids. As for the carbon cycle, carbonates preserved during subduction are estimated to account for a flux of 3.6 × 1012 mol/year of carbon being returned into the deep mantle [3,4,5]. This quantity accounts for 10–30 wt % of the carbon reservoir in the deep mantle [6]. ≈1.5 × 1013 mol/year) reaching depths exceeding 240 km This amount of H2O entering the deep mantle may not appear very large, it provides a mechanism for having significant amounts of water in the deep mantle. Part of the CO2 and H2O present in the deep mantle may originate from primitive mantle reservoirs [7], leading potentially to fairly large amounts of these volatiles in the deep mantle

Methods

Results

Conclusion