Thermal Conductivity Anomaly in (Fe0.78Mg0.22)CO3 Siderite Across Spin Transition of Iron

Abstract Siderite was proposed to be an important mantle carbon‐hosting mineral in Earth's deep carbon cycle. It undergoes a pressure‐induced spin transition of iron around 40–55 GPa, through which many physical properties change drastically. Thermal conductivity of mantle minerals is key to control temperature profiles and thermal evolution in the mantle and subducting slabs. However, the lattice thermal conductivity of iron‐bearing carbonates under relevant mantle conditions has never been investigated. Here we combined high‐pressure diamond‐anvil cell, Raman spectroscopy, and ultrafast optical pump‐probe method to measure the lattice thermal conductivity of siderite to 67 GPa at room temperature. We found that during the spin transition the thermal conductivity varies significantly with the extent of low‐spin state, which is largely different from that of the lower‐mantle ferropericlase. Our results further suggest that when the siderite is transported to depths of 1,400–1,800 km, such thermal conductivity anomaly within a narrow pressure range may induce anomalies in local thermochemical profiles and alter the distribution fields of subducting minerals, which in turn would influence the fate of carbonates in global carbon cycle.