Spin and Structural Transitions of Ferromagnesite in the Earth’s Lower Mantle

Abstract:

Physical and chemical properties of the potential deep-carbon carriers such as the deep-mantle carbonates can play a significant role in our understanding of the deep-carbon storage as well as the global carbon cycle of the planet. Iron-bearing carbonates especially ferromagnesite [(Mg,Fe)CO₃] has been commonly proposed to be a major carbon career in the Earth’s mantle. Previous studies have reported very different scenarios for the (Mg,Fe)CO₃ system in the deep-mantle conditions including the chemical dissociation and various structural transitions. Using synchrotron X-ray diffraction in a laser-heated diamond anvil cell, we have studied the spin transition diagram as well as the phase stability and compressional behavior of (Mg,Fe)CO₃ carbonates up to lower-mantle conditions of approximately 120 GPa and 2400 K. These studies focus on understanding the effects of the spin transition on the physical and chemical properties of the deep-mantle carbonates. Our high-pressure results show that an electronic spin crossover occurs in ferromagnesite at mid-lower mantle P-T conditions and that it then transforms into an orthorhombic high-pressure phase following the spin transition at deeper parts of the lower mantle pressure-temperature conditions. The high-pressure orthorhombic phase is likely in the low-spin state that can become a stable deep-carbon carrier at deeper parts of the lower mantle below 2000 km in depth. These findings suggest that deep-mantle carbonates can exhibit unique physical and chemical properties than that at shallower mantle conditions. Here we will address how the spin transition as well as the structural phase transition affects our understanding of the deep-mantle carbonate storage in the Earth’s interior.

References:

J. Liu, J. F. Lin, Z. Mao, and V. B. Prakapenka, Thermal equation of state and spin transition of magnesiosiderite at high pressure and temperature, Am. Miner., 99, 84-93, 2014.

J. F. Lin, J. Liu, C. Jacobs, and V. B. Prakapenka, Vibrational and elastic properties of ferromagnesite across the electronic spin-pairing transition of iron, Am. Miner., 97, 583-591, 2012.