Spin Transition in Deep-Mantle Ferromagnesite

Studying the physical and chemical properties of carbonates at high pressures and temperatures can provide crucial constraints on the role and behavior of deep-carbon carriers in Earth’s mantle, and therefore the mantle’s role in the global carbon cycle.

Carbonates, such as magnesite, calcite, and dolomite, are important potential deep-carbon carriers in Earth’s deep mantle. Studying the physical and chemical properties of these carbonates at high pressures and temperatures can provide crucial constraints on the role and behavior of such deep-carbon carriers in Earth’s mantle, and therefore the mantle’s role in the global carbon cycle. One possible means of deep-mantle carbon storage involves an iron-bearing magnesite ((Mg,Fe)CO3) called ferromagnesite for the Mg-rich part of the solid solution system, in which approximately 20 mol% ferrous iron is dissolved in magnesite as carbonated peridotite and eclogite.

In the January 2014 issue of the American Mineralogist, a research team at the University of Texas at Austin and the University of Chicago report for the first time the occurrence of an electronic high-spin to low-spin transition of ferrous iron in lower-mantle ferromagnesite at pressures and temperatures of Earth’s mid-mantle [1]. The research team, with funding from the Deep Carbon Observatory, used an intense synchrotron X-ray diffraction beamline at the Advanced Photon Source to deduce the spin states of iron from X-ray diffraction patterns of a tiny ferromagnesite sample in a high pressure-temperature diamond anvil cell. Their results show that the spin transition of iron in ferromagnesite is strongly associated with a unit cell volume collapse from the high-spin to the low-spin state, resulting in significant changes in physical and potentially chemical properties of carbonate.

Since iron is the most abundant transition metal in deep Earth, the observed spin transition and abnormal thermodynamic behavior of ferromagnesite at high pressure and temperature help us understand the characteristics of this potential deep-carbon host. These new experimental results imply that low-spin ferromagnesite with distinct physical properties can become more stable than high-spin ferromagnesite and magnesite at pressures above approximately 50 GPa, providing a mechanism for (MgFe)-bearing carbonate to act as a major carbon host in Earth’s mid-lower mantle.

Image: Adapted from Figure 5 in reference [1]. The image depicts spin crossover of ferromagnesite at high pressure and temperature. The color bar on the right represents the fraction of the low-spin state of iron in the sample.

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