Stability of iron-bearing carbonates in the deep Earth’s interior

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 FeCO(3) partially dissociates to form various iron oxides. At higher pressures FeCO(3) forms two new structures—tetrairon(III) orthocarbonate Fe(4)(3+)C(3)O(12), and diiron(II) diiron(III) tetracarbonate Fe(2)(2+)Fe(2)(3+)C(4)O(13), both phases containing CO(4) tetrahedra. Fe(4)C(4)O(13) 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.