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Magnetic sources in the Earth’s mantle

Abstract

Since the 1970s, ferromagnetic minerals were believed to be absent in the Earth’s mantle and, even if present, the temperatures were considered too high for such phases to carry magnetic remanence. However, new experimental data, measurements on mantle xenoliths and an improved understanding of long-wavelength features in aeromagnetic data require that the magnetization of the mantle be revisited. In this Review, we examine mantle magnetism through the xenolith record, evaluate the latest experimental advances, assess detection methods of deep-seated mantle sources and identify salient, unsolved questions about magnetic sources in the Earth’s mantle. Critically, magnetic data on a worldwide collection of mantle xenoliths have revealed that pure magnetite is common in the uppermost mantle (<150 km), particularly in subduction zones and cratons. Furthermore, experiments on haematite and its polymorphs suggest that they could carry a magnetic remanence down to ~600 km, for example, in cold, subducted slabs. Finally, modern spectral analysis of aeromagnetic data confirms that a magnetized layer is present below the crust–mantle boundary in multiple tectonic settings. Future work needs to explore the magnetic minerals in the deepest available mantle xenoliths (150–660 km), in conjunction with experiments on mantle materials at pressures corresponding to these depths.

Key points

  • The old view of a globally non-magnetic mantle should be revisited.

  • Some iron oxides are stable and carry a magnetic remanence down to 660 km.

  • Magnetite is ubiquitous in upper mantle xenoliths.

  • New spectral methods show that some magnetic sources lie within the mantle.

  • Low-geotherm regions are prime locations for mantle magnetic sources.

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Fig. 1: Earth’s lithospheric magnetic field variations.
Fig. 2: Magnetic records from mantle xenoliths.
Fig. 3: Stability and magnetic properties of the Fe3O4 system at upper mantle pressure and temperature conditions.
Fig. 4: Depth to base of magnetization derived from analysis of magnetic data.
Fig. 5: The magnetic mantle.

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Acknowledgements

This work was supported by the National Science Foundation (grants EAR-0521558 and EAR-1345105 to E.C.F. and EAR-1246921 to D.R.), the National Aeronautics and Space Administration (grants NNX16AN51G and 80NSSC19K0014 to D.R.) and internal funding from the University of Münster.

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All authors contributed equally to drafting the figures, supplementary table and text.

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Correspondence to Eric C. Ferré.

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Swarm Anomaly Model: https://www.spacecenter.dk/files/magnetic-models/LCS-1/

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Supplementary information

Glossary

Curie temperature

Temperature above which magnetization is randomized and destroyed, resulting in the material becoming paramagnetic.

Thermoremanent magnetization

Permanent or remanent magnetization acquired by a ferromagnetic mineral (sensu lato) when cooled through its blocking temperature in the presence of a DC magnetic field, most commonly, the Earth’s magnetic field.

Blocking temperatures

Below a set temperature, the magnetic moments become blocked, owing to the lower thermal energy.

Mantle xenoliths

Deep-seated fragments of the mantle that ascend to the surface in basaltic or kimberlitic magma.

Curie depth

The depth below the Earth’s surface where ferrimagnetic and ferromagnetic minerals abruptly lose their spontaneous magnetization as a result of exceeding their Curie temperature.

Serpentinization

Geochemical reaction of peridotite with aqueous fluids to form serpentine.

Verwey transition

Magnetic-phase transition shown by magnetite (Fe3O4) at Tv ~120 K (at ambient pressure); upon heating, magnetite gradually transforms from a crystallographic monoclinic into an inverse spinel structure.

Alt-S%

Quantifies the degree of sulfide alteration; the total Fe-hydroxide area divided by the total sulfide area (including Fe-hydroxide).

Loss on ignition

High-temperature (~1,000 °C) heating experiment in inert atmosphere or vacuum designed to quantify the percentage of volatile elements in a rock.

Low-field magnetic susceptibility

Slope of the induced magnetization versus applied field curve in magnetic hysteresis when the applied field is sufficiently low to allow reversible magnetization.

Saturation remanent magnetization

Maximum remanent magnetization that can be reached in a high applied field experiment.

Magnetic coercivity

Magnetic intensity required to reduce the magnetization to zero in a fully magnetized specimen.

Fourier amplitude spectrum

Amplitude of each wavelength for a spatial or time variation, which is arranged according to their wavelength, where both wavelengths and amplitudes are obtained through Fourier analysis.

De-fractaling

Removal of power law (or fractal) characteristics from an observed spectra.

De-fractaled amplitude spectra

Amplitude spectrum of magnetic anomalies where the power law dependence is removed.

Fractal magnetization

Fractal relationship of the magnetization distribution.

Fractal parameter

The exponent associated with the power law that governs the spatial characteristics of magnetization or magnetic anomalies.

Cratons

Large, geologically stable regions in continental areas that are typically considered to be tectonically inactive.

Bouguer gravity anomaly

The gravity anomaly corrected for the height of the measurement and the influence of the underlying topography.

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Ferré, E.C., Kupenko, I., Martín-Hernández, F. et al. Magnetic sources in the Earth’s mantle. Nat Rev Earth Environ 2, 59–69 (2021). https://doi.org/10.1038/s43017-020-00107-x

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