Mechanochemical Enhancement of Siderite‐To‐Magnetite Conversion in Faults

Abstract Magnetic phases in fault rocks offer potential “geothermometers” for estimating frictional heating during fault slip. However, the thermochemical kinetics of magnetic minerals subjected to shear deformation in hydrothermal fluids‐conditions typical of natural faults ‐ remains poorly understood, hindering accurate temperature estimates. We examined the influence of mechanical shearing on magnetic and thermochemical changes of siderite through low‐velocity friction and compaction experiments, complemented by rock magnetic, thermogravimetric, microscopic and X‐ray diffraction (XRD) analyses. Results show that mechanical shearing can significantly lower the thermal stability of siderite, likely through crystallite refinement and/or lattice distortion. This facilitates thermochemical reactions of siderite and lowers the onset temperature to ∼269°C for its conversion to magnetite in fault zones. Therefore, such an effect should be considered not only when estimating frictional heating via magnetic assemblages, but also in thermochemical studies of carbonates beyond siderite, such as investigations of the fate of carbonates in subduction zones.