Experimental investigation of FeCO3 (siderite) stability in Earth's lower mantle using XANES spectroscopy

We studied FeCO₃ using Fe K-edge X-ray absorption near-edge structure (XANES) spectroscopy at pressures up to 54 GPa and temperatures above 2000 K. First-principles calculations of Fe at the K-edge in FeCO₃ were performed to support the interpretation of the XANES spectra. The variation of iron absorption edge features with pressure and temperature in FeCO₃ matches well with recently reported observations on FeCO₃ at extreme conditions, and provides new insight into the stability of Fe-carbonates in Earth's mantle. Here we show that at conditions of the mid-lower mantle, similar to 50 GPa and similar to ~2200 K, FeCO₃ melts and partially decomposes to high-pressure Fe₃O₄. Carbon (diamond) and oxygen are also inferred products of the reaction. We constrained the thermodynamic phase boundary between crystalline FeCO₃ and melt to be at 51(1) GPa and similar to ~1850 K. We observe that at 54(1) GPa, temperature-induced spin crossover of Fe²⁺ takes place from low to high spin such that at 1735(100) K, all iron in FeCO₃ is in the high-spin state. A comparison between experiment and theory provides a more detailed understanding of FeCO₃ decomposition observed in X-ray absorption spectra and helps to explain spectral changes due to pressure-induced spin crossover in FeCO₃ at ambient temperature.