Earth's mantle could be more magnetic than once thought

06-06-2019

The Earth’s mantle has long been considered non-magnetic, due to high temperatures at depth. An international team of scientists used ID18 to study the iron oxide hematite (Fe2O3), a strongly magnetic mineral, at temperatures and pressures found down to the Earth’s lower mantle. Their study, published in Nature, provides evidence that hematite retains magnetic properties at the depth of the transition zone between the upper and lower mantle at certain temperatures and could therefore be a source of magnetic anomalies there.

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Scientists have traditionally considered the Earth’s mantle to be non-magnetic due to its elevated temperatures being too high to retain any magnetism in the constituting minerals. However, satellite and aeromagnetic data provide evidence for magnetic anomalies in the mantle, particularly around cooler areas such as subduction zones (tectonic plate boundaries where one plate is forced underneath another). The source of the anomalies remains largely unknown, but iron oxides are considered a likely source due to their high critical temperatures. Of these, hematite (Fe2O3) is the dominant iron oxide at depths of around 300 – 600 km below the Earth’s surface – a transition zone between the upper and the lower mantle.

A team of researchers from the Universities of Münster and Bayreuth in Germany, Rutgers University in the USA, the Technical University of Denmark and the ESRF used samples of synthetic hematite in single crystal and powdered form. They compressed them in diamond anvil cells while laser-heating the samples, exposing them to extreme pressures and temperatures found in the depth of our planet – up to 90 GPa and 1300 K, respectively.

The scientists then applied Synchrotron Mössbauer Source (SMS) spectroscopy at nuclear resonance beamline ID18 to investigate the magnetic transitions and critical temperatures of the hematite and its high-pressure phases. “Synchrotron-based Mössbauer spectroscopy at the ESRF is currently one of the few – if not the only – method allowing the investigation of magnetism in situ at high pressures and high temperatures,” said Ilya Kupenko, assistant professor at the Institut für Mineralogie at the University of Münster in Germany and lead author of the paper. Moreover, one of the advantages of using SMS spectroscopy on laser-heated materials is that the temperature of the sample can be calculated from any collected Mössbauer spectra, meaning that this ‘internal thermometer’ feature provides a valuable back-up to temperature measurements taken at the sample surface.

Fig 1.jpg

Evolution of SMS spectra of Fe2O3 polymorphs with pressure and temperature.

ac, SMS spectra of α-Fe2O3 (haematite) at 19.4(4) GPa and at the temperatures indicated. The spectrum in a was collected after the heating cycle. The single line component (orange) is attributed to minor Fe-C alloying owing to the reaction of the sample surface with the diamonds. The single magnetic sextet displayed by α-Fe2O3 at room temperature (a) progressively broadens upon increasing temperature (b) until the long-range magnetic order collapses above the Néel temperature, TN, into a paramagnetic doublet (c). df, Characteristic SMS spectra of Fe2O3 high-pressure phases at room temperature and at the pressures indicated. A minor residual contribution from untransformed α-Fe2O3 (blue) is observed in d. The room-temperature SMS spectrum of the Rd2O3(II)-structure ι-Fe2O3 consists of a single magnetic component, while both the distorted perovskite-structure ζ-Fe2O3 and the post-perovskite-structure η-Fe2O3 display two Mössbauer components – one magnetically ordered (cyan in e and dark red in f) and one non-magnetic (yellow in e and light green in f). The spectrum of η-Fe2O3 displays the residue of the untransformed orthorhombic q-Fe2O3 phase and a novel mixed-valence iron oxide phase resulting from the partial decomposition of Fe2O3 upon laser heating at high temperature. Solid lines show the theoretical fits, the percentage bars indicate the relative absorptions, and the residuals of the fits are indicated below each spectrum.

 

The scientists found that hematite retains magnetism at pressures corresponding to the Earth’s transition zone at relatively cold temperatures, such as those found at subduction zones, and may cause deep magnetic anomalies. The team then calculated the temperature profiles of cross-sections of the Earth’s subduction zones and found a band of zones in the West Pacific region with plate temperatures low enough for hematite to remain ferromagnetic.

Not only does this suggest that the Earth’s mantle might not be as magnetically inactive as previously considered, but the research could also have implications for other branches of science, such as paleomagnetism. The location of the Earth’s geomagnetic poles can be traced in rocks that record the Earth’s magnetic field, including periods in history when the field reversed. Magnetic anomalies in the band of deep magnetised rock in the West Pacific region correlate with one of the preferred paths of the geomagnetic poles during reversals. Thus, this path could appear in rock records due to the presence of magnetic sources such as hematite in the transition zone, rather than reflect the real geometry of the magnetic field during reversals, suggesting the paleomagnetic records may need to be carefully evaluated for influence from the deep sources.

“Our key finding indicates that the Earth’s mantle is not as electromagnetically ‘dead’ as it has been considered,” explains Kupenko. “The potential deep magnetic sources can contribute to the signal we observe on the surface and should be taken into account when analysing the satellite geomagnetic data of our planet, and especially in studies of planetary bodies with only an ancient dynamo, such as Mars.”

Reference
Kupenko I. et al, Nature 570102106 (2019), doi: 10.1038/s41586-019-1254-8