Would iron spin state affect the Earth’s deep carbon cycle?

Phys. Rev. B 103, 054401 (2021)

Han Hsu, Christian P. Crisostomo, Wenzhong Wang, and Zhongqing Wu

Anomalous thermal properties and spin crossover of ferromagnesite (Mg,Fe) CO 3

一般相信,含鐵碳酸鎂 (Mg,Fe)CO3 是下地函的主要攜碳礦物,且此礦物中的鐵在下地函的溫度壓力下會發生自旋態轉變。物理系徐翰教授研究團隊近期的計算結果顯示:伴隨著鐵自旋態轉變,含鐵碳酸鎂的種種熱力學性質 (密度、熱膨脹、彈性、比熱、熱傳導等) 會發生劇烈的異常變化,可能進一步影響地球深部碳循環。

Ferromagnesite (Mg,Fe)CO3 is believed to enter the Earth’s lower mantle via subduction and is considered a major carbon carrier in the Earth’s lower mantle, playing a key role in the Earth’s deep carbon cycle. Experiments have shown that ferromagnesite undergoes a pressure-induced spin crossover in the lower-mantle pressure range. A recent computational work done by Prof. Han Hsu’s group has indicated that nearly all thermal properties of ferromagnesite are drastically altered by iron spin crossover, including anomalous reduction of volume, anomalous softening of bulk modulus, and anomalous increases of thermal expansion, heat capacity, and the Grüneisen parameter. Remarkably, the anomaly of heat capacity remains prominent at high temperature without smearing out, which suggests that iron spin crossover may significantly affect the thermal properties of subducting slabs and the Earth’s deep carbon cycle. See further details in Hsu et al., Phys. Rev. B 103, 054401 (2021).