Stability of iron-bearing carbonates in the deep Earth\u2019s interior

Valerio Cerantola,Jeroen Jacobs,Rudolf Rüffer,Leyla Ismailova,Maxim Bykov,Elena Bykova,Sylvain Petitgirard,Catherine Mccammon,Clemens Prescher,Innokenty Kantor,Ilya Kupenko,Michael Hanfland,Vitali B Prakapenka,Alexandr I Chumakov,Leonid Dubrovinsky,Volodymyr Svitlyk,Marco Merlini,Mohamed Mezouar

doi:10.1038/ncomms15960

Abstract

The presence of carbonates in inclusions in diamonds coming from depths exceeding 670 km are obvious evidence that carbonates exist in the Earth’s lower mantle. However, their range of stability, crystal structures and the thermodynamic conditions of the decarbonation processes remain poorly constrained. Here we investigate the behaviour of pure iron carbonate at pressures over 100 GPa and temperatures over 2,500 K using single-crystal X-ray diffraction and Mössbauer spectroscopy in laser-heated diamond anvil cells. On heating to temperatures of the Earth’s geotherm at pressures to ∼50 GPa FeCO3 partially dissociates to form various iron oxides. At higher pressures FeCO3 forms two new structures—tetrairon(III) orthocarbonate Fe43+C3O12, and diiron(II) diiron(III) tetracarbonate Fe22+Fe23+C4O13, both phases containing CO4 tetrahedra. Fe4C4O13 is stable at conditions along the entire geotherm to depths of at least 2,500 km, thus demonstrating that self-oxidation-reduction reactions can preserve carbonates in the Earth’s lower mantle.

Highlights

The presence of carbonates in inclusions in diamonds coming from depths exceeding 670 km are obvious evidence that carbonates exist in the Earth’s lower mantle
We performed an experimental investigation of the high-pressure high-temperature behaviour of synthetic iron carbonate (FeCO3)
high-pressure high-temperature (HPHT) experiments were performed in laser-heated diamond anvil cells (DACs)

Summary

Introduction

The presence of carbonates in inclusions in diamonds coming from depths exceeding 670 km are obvious evidence that carbonates exist in the Earth’s lower mantle Their range of stability, crystal structures and the thermodynamic conditions of the decarbonation processes remain poorly constrained. Iron can radically change the thermodynamic stability of carbonate phases, thereby preserving them from breaking down This behaviour may be a direct consequence of pressure-induced spin crossover[17,18,19,20,21], which has been observed to occur at B43 GPa at room temperature to over 50 GPa at B1,200 K Our single-crystal X-ray diffraction data unambiguously establish the existence of at least one carbonate with a unique structural type (not known for silicates or other tetrahedral anion-bearing compounds), and demonstrate that the conditions in the Earth’s lower mantle do not lead to full decomposition of Fe-based carbonates due to self-oxidation-reduction reaction(s)

Objectives

Methods

Results

Conclusion