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 mantle1. However, this assumption has been questioned by geophysical observations2,3 and by the identification of magnetic remanence in mantle xenoliths4, which suggest mantle magnetic sources. Owing to their high critical temperatures, iron oxides are the only potential sources of magnetic anomalies at mantle depths5. 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 kilometres6. The lack of data on the magnetic properties of haematite at relevant pressure–temperature conditions, however, hinders the identification of magnetic boundaries within the mantle and their contribution to observed magnetic anomalies. Here we apply synchrotron Mössbauer source spectroscopy in laser-heated diamond anvil cells to investigate the magnetic transitions and critical temperatures in Fe2O3 polymorphs7 at pressures and temperatures of up to 90 gigapascals and 1,300 kelvin, respectively. Our results show that haematite remains magnetic at the depth of the transition zone in the Earth’s mantle in cold or very cold subduction geotherms, forming a frame of deep magnetized rocks in the West Pacific region. The deep magnetic sources spatially correlate with preferred paths of the Earth’s virtual geomagnetic poles during reversals8 that might not reflect the geometry of the transitional field. Rather, the paths might be an artefact caused by magnetized haematite-bearing rocks in cold subducting slabs at mid-transition zone depths. Such deep sources should be taken into account when carrying out inversions of the Earth’s geomagnetic data9, and especially in studies of planetary bodies that no longer have a dynamo10, such as Mars.
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We thank C. Finlay and A. Hirt for their comments and discussions in an early stage of this work. We thank E. Bykova for the selection of single crystals by X-ray diffraction and verification of the phases in laser-heated diamond anvil cells. This work was supported by the University of Münster, the German Research Foundation and the German Federal Ministry of Education and Research. We acknowledge the European Synchrotron Radiation Facility for the provision of synchrotron radiation facilities and J. Jacobs from the Sample Environment Service-HP laboratory for technical support.
Nature thanks Takaya Mitsui and the other anonymous reviewer(s) for their contribution to the peer review of this work.
Extended data figures and tables
Phase boundaries are defined according to ref. 7 with the exception of the α-Fe2O3 to ι-Fe2O3 boundary, which is taken from ref. 52. Filled symbols show the pressure–temperature conditions investigated in this study. Blue, α-Fe2O3; green, ι-Fe2O3; yellow, ζ-Fe2O3; red, η-Fe2O3; orange, θ-Fe2O3. Coexisting phases are indicated by two-coloured symbols. The crystallographic designation of the phases is provided in the Methods. The error bars are one standard error.
Temperature evolution of SMS spectra of α-Fe2O3 at 19.4(4) GPa (a) and at 24.5(2) GPa (b). Temperatures marked by asterisks were determined by spectroradiometry. The broadening of the absorption lines above 800 K is related to the enhanced sensitivity of Bhf to temperature gradients owing to the steeper temperature dependence close to TN. Source data
Temperature evolution of SMS spectra of ι-Fe2O3 at 40(1) GPa (a) and at 46(1) GPa (b). Heating of ι-Fe2O3 at 46(1) GPa above 500 K causes its transformation into ζ-Fe2O3. The ζ-Fe2O3 phase can be cooled down to around 500 K, but it transforms back to ι-Fe2O3 upon further cooling. Temperatures marked by asterisks and daggers were determined by spectroradiometry or estimated using the laser power53, respectively. Source data
Temperature evolution of SMS spectra of η-Fe2O3 together with the residue of an untransformed θ-Fe2O3 and the product of partial decomposition of the sample (see Fig. 1 for details) at 75.1(5) GPa (a) and at 90.7(5) GPa (b). Temperatures marked by daggers were estimated from the laser power. Source data
Temperature dependence of Bhf in ι-Fe2O3 (a) and η-Fe2O3 (b) at the indicated pressures. Filled symbols correspond to temperatures determined from the SMS internal thermometer (equation (2). Half-filled symbols correspond to temperatures estimated from the calibrated laser power and were excluded from the fit of the critical temperatures Tc. Lines are fits of the experimental data to equation (4) in the 0.5Tc < T < 0.99Tc range. The crystallographic designation of the phases is provided in the Methods. Data at one atmosphere are from ref. 16. The error bars are one standard error. Source data