Fe3+ -hosting carbon phases in the deep Earth

Abstract

Iron-bearing carbonates play an important role in Earth's carbon cycle. Owing to their stability at mantle conditions, recently discovered iron carbonates with tetrahedrally coordinated carbon atoms are candidates for carbon storage in the deep Earth. The carbonates' iron oxidation and spin state at extreme pressure and temperature conditions contribute to the redox conditions and element partitioning in the deep mantle. By laser heating ${\mathrm{FeCO}}_{3}$ at pressures of about 83 GPa, ${\mathrm{Fe}}_{4}^{3+}{\mathrm{C}}_{3}{\mathrm{O}}_{12}$ and ${\mathrm{Fe}}_{2}^{2+}{\mathrm{Fe}}_{2}^{3+}{\mathrm{C}}_{4}{\mathrm{O}}_{13}$ were synthesized and then investigated by x-ray emission spectroscopy to elucidate their spin state, both in situ and temperature quenched. Our experimental results show both phases in a high-spin state at all pressures and over the entire temperature range investigated, i.e., up to 3000 K. The spin state is conserved after temperature quenching. A formation path is favored where ${\mathrm{Fe}}_{4}^{3+}{\mathrm{C}}_{3}{\mathrm{O}}_{12}$ forms first and then reacts to ${\mathrm{Fe}}_{2}^{2+}{\mathrm{Fe}}_{2}^{3+}{\mathrm{C}}_{4}{\mathrm{O}}_{13}$, most likely accompanied by the formation of oxides. Density functional theory calculations of ${\mathrm{Fe}}_{2}^{2+}{\mathrm{Fe}}_{2}^{3+}{\mathrm{C}}_{4}{\mathrm{O}}_{13}$ at 80 GPa confirm the experimental findings with both ferric and ferrous iron in high-spin state with antiferromagnetic order at 80 GPa. As the intercrystalline cation partitioning between the Fe-bearing carbonates and the surrounding perovskite and ferropericlase depends on the spin state of the iron, an understanding of the redox conditions prevalent in subducted slab regions in the lower mantle has to take the latter into account. Especially, ${\mathrm{Fe}}_{2}^{2+}{\mathrm{Fe}}_{2}^{3+}{\mathrm{C}}_{4}{\mathrm{O}}_{13}$ may play a key role in subducted material in the lower mantle, potentially with a similar role as silicate perovskite.