Low Thermal Conductivity of Epidote and Its Cooling Effect on the Oceanic Crust

Abstract Epidote is a potential water carrier in subducting oceanic crust, and it is involved in temperature records from blueschist and eclogite exhumed from subduction zones. Thermal conductivity ( κ ) and thermal diffusivity ( D ) of epidote were measured at 303–1,473 K and 0.5–3.0 GPa using the transient plane‐source method. κ and D values decreased with temperature before dehydration, but increase anomalously with temperature after dehydration, and finally decreased with crystallization of the decomposition products. Thermal conductivity and thermal diffusivity of epidote exhibited a positive pressure dependence. The obtained κ and D values of epidote were significantly smaller than those of nominally anhydrous minerals and other hydrous minerals. X‐ray computed tomography analysis revealed that the pores inside the recovered samples were partially connected. κ and D values were dominated by conductive heat transport before dehydration, while the anomalous increase in the thermal parameters after dehydration may have been caused by the fluid inside the connected pores. Using variable thermal conductivity obtained in this study, the thermal structures of warm subducting slabs associated with epidote‐blueschist and epidote‐eclogite facies were modeled. Our estimations revealed that the temperatures of warm slabs below the forearc with the epidote‐blueschist and epidote‐eclogite facies may be lower 7–10 and ∼40–50 K compared to those determined using a constant thermal conductivity value. This implies that the presence of epidote‐blueschist and epidote‐eclogite could reduce the temperature of the slab, which may alter the dehydration depth of the slab.