How carbon is stored in Earth’s deep interior remains one of the key open questions in Earth science. Due to its abundance in crustal and mantle rocks and its structural stability at high pressures and temperatures, iron-bearing magnesite, formally ferromagnesite [(Mg,Fe)CO3], likely represents a major deep-carbon carrier in Earth’s deep mantle. High-pressure studies of the physical and chemical properties of ferromagnesite can thus provide crucial constraints on our understanding of the carbon-bearing phase in Earth’s mantle, as well as the mantle’s role in the global carbon cycle. In the January 2015 issue of Scientific Reports, a research team from the University of Texas at Austin (USA), the Center for High Pressure Science and Technology Advanced Research (China), and the University of Chicago (USA) reports a new discovery that the rhombohedral ferromagnesite, stable at ambient conditions, transforms into an orthorhombic high-pressure phase at pressure-temperature conditions comparable to the lower parts of the lower mantle .
The researchers propose that the newly discovered high-pressure phase of ferromagnesite is induced by electronic high-spin to low-spin transition of iron in ferromagnesite at approximately 45 GPa because the spin transition is associated with a volume reduction of approximately 8 percent, which in term stablizes the high-pressure phase at lower pressures. The research team, funded by the Sloan Foundation’s Deep Carbon Observatory and the US National Science Foundation, used advanced synchrotron X-ray diffraction and laser-heated diamond anvil cell techniques at GSECARS Advanced Photon Source to investigate the crystal structure of ferromagnesite at high pressure and temperature conditions relevant to the deep lower mantle. They found that ferromagnesite is stable in the orthorhombic structure with ferrous iron ions in the low-spin state in the lattice at pressures above 85 GPa and temperatures above 2200 K. The structural transition is driven by the spin transition of iron accompanied by volume collapse, showing that the low-spin orthorhombic ferromagnesite can exist in mid to lowermost lower-mantle conditions. They propose that the orthorhombic ferromagnesite can act as a stable deep-carbon host at conditions below 1900 km deep in the lower mantle. Low-spin orthorhombic ferromagnesite as a potential carbon carrier displays unique physical and chemical properties distinct from other previously known carbonates at ambient conditions. The discovery of this new carbonate phase thus calls for further investigations to provide a clearer picture of the role of carbon in the planet’s interior.
Image: Schematic phase relations of ferromagnesite at relevant pressure and temperature conditions of the lower mantle. Blue and magneta: the low-pressure rhombohedral ferromagnesite (phase I) at the high- and low-spin states, respectively; red: the high-pressure orthorhombic ferromagnesite (phase II); black solid curve: an expected lower-mantle geotherm. Credit: Afu Lin