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The Earth’s crust–mantle boundary, the Mohorovičić discontinuity, has been traditionally considered to be the interface between the magnetic crust and the non-magnetic mantle. However, this assumption has been questioned by geophysical observations^, and by the identification of magnetic remanence in mantle xenoliths, which suggest mantle magnetic sources. Owing to their high critical temperatures, iron oxides are the only potential sources of magnetic anomalies at mantle depths. Haematite (α-Fe₂O₃) is the dominant iron oxide in subducted lithologies at depths of 300 to 600 kilometres, delineated by the thermal decomposition of magnetite and the crystallization of a high-pressure magnetite phase deeper than about 600 kilometres. The lack of data on the magnetic properties of haematite at relevant pressure–temperature conditions, however, hinders the identification of magnetic boundaries within the …