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Deep mantle heterogeneities formed through a basal magma ocean contaminated by core exsolution

Nature Geoscience volume 18, pages 1056–1062 (2025)Cite this article

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Abstract

Earth’s lowermost mantle harbours two large low-velocity provinces with patches of ultralow-velocity zones. These seismic anomalies may retain geochemical signatures distinct from the surrounding mantle. Yet, their origin remains enigmatic. One proposed explanation is the differentiation of an early-formed basal magma ocean. However, the presence of an excessively thick layer of iron-rich ferropericlase in the crystallized basal magma ocean conflicts with seismic tomography models. Here we use combined thermodynamic and geodynamic modelling to investigate the crystallization of a basal magma ocean continuously contaminated by oxide exsolved from the core, termed the basal exsolution contaminated magma ocean. We find suppression of ferropericlase crystallization. Geodynamic modelling demonstrates that the solidified contaminated magma ocean mantle can lead to the formation of deep mantle structures consistent with large low-velocity provinces and ultralow-velocity zones. In addition, diapirs of core exsolution entrained into the solid mantle may cause small-scale scattering. The basal exsolution contaminated magma ocean inherits the silicon, tungsten and helium isotope compositions from the core and exhibits trace element enrichments, suggesting its possible role as a source material for ocean island basalts that may sample the large low-velocity provinces, pointing to a unified mechanism for forming deep mantle heterogeneities.

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Fig. 1: Schematic illustrating the formation and evolution of a BECMO.
Fig. 2: Mineralogical, compositional and density profiles of solidified BMO and BECMO.
Fig. 3: High-resolution geodynamic modelling results of the solidified BECMO.

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Data availability

All data related to this Article are presented in Supplementary Table 1. The complete thermodynamic modelling results for BECMO, BMO and unfractionated mantel as well as the Supplementary Table 1 are available via the Open Science Framework at https://osf.io/y9g67/ (ref. 104). The helium and tungsten isotopic data of OIB samples are taken from ref. 10 and references therein.

Code availability

Both the Citcom code used in this study and the thermodynamic modelling code are available via the Open Science Framework at https://osf.io/y9g67/ (ref. 104).

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Acknowledgements

We thank J. Badro, K. Hirose, C. Boukaré and M. Li for discussions on the phase diagram and mantle structures. Z.D. expresses thanks for the funding from National Natural Science Foundation of China and Strategic Priority Research Program (B) of the Chinese Academy of Sciences (grant nos. NSFC-42394112, 42150102 and XDB0840101). J.D. acknowledges the National Science Foundation EAR-2242946. Q.Y. acknowledges support from the NSF EAR‐2330810. Q.Y. acknowledges use of the Anvil supercomputer at Purdue University supported by the NSF ACCESS programme TG‐EAR160027.

Author information

Authors and Affiliations

  1. Department of Geosciences, Princeton University, Princeton, NJ, USA

    Jie Deng

  2. Department of Earth and Planetary Sciences, Rutgers University, Piscataway, NJ, USA

    Yoshinori Miyazaki

  3. Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA

    Qian Yuan

  4. Department of Geology and Geophysics, Texas A&M University, College Station, TX, USA

    Qian Yuan

  5. State Key Laboratory of Deep Earth Processes and Resources, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, China

    Zhixue Du

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  1. Jie Deng
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  2. Yoshinori Miyazaki
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Contributions

J.D. conceived and coordinated the entire project. Y.M., Q.Y. and J.D. performed calculations and modelling. J.D. wrote the first draft. All authors contributed to the discussion and revision of the manuscript.

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Correspondence to Jie Deng, Yoshinori Miyazaki, Qian Yuan or Zhixue Du.

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Nature Geoscience thanks Maxim Ballmer, George Helffrich and Hanika Rizo for their contribution to the peer review of this work. Primary Handling Editor: Alison Hunt, in collaboration with the Nature Geoscience team.

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Extended data

Extended Data Fig. 1 Dissolution of exsolution in a convective BECMO.

(a) dissolution rate and (b) survival timescale as a function of the grain size of exsolution. The shaded vertical lines indicate the size of MgO (red) and SiO2 (green) exsolution.

Extended Data Fig. 2 Ternary phase diagram of the MgO-FeO-SiO2 system.

(a) 90 GPa. (b) 125 GPa. The blue region represents compositions where ferropericlase crystallizes at the liquidus, while the white region indicates where bridgmanite crystallizes first. The gray region is where seifertite is the first liquidus phase. The isotherms are shown in dashed lines as they are not well calibrated for these compositions due to a lack of experimental constraints. The blue line shows a typical compositional path of BMO solidification, while the red line illustrates that of BECMO with the exsolution rate of 7.3 × 10−5 K−1. BMO evolves along the cotectic valley between bridgmanite (brg) and ferropericlase (fpc) stability fields, but the actual path deviates from the valley shown in the figure as the cotectic composition changes with pressure. Green and orange vectors show the compositional changes resulting from the removal of bridgmanite (brg rm) and the addition of MgO/SiO2 (SiO2 exsolution) exsolution, respectively. In this model, the BMO/BECMO starts with a thickness of 350 km and an initial composition of MgO: 47.3 wt%, FeO: 10.3 wt%, and SiO2: 42.4 wt%. The final compositions of the lowermost mantle are as follows, BMO: MgO: 22.4 wt%, FeO: 74.4 wt%, SiO2: 3.2 wt%; BECMO: MgO: 20.8 wt%, FeO: 30.7 wt%, SiO2: 48.5 wt%. The modeled compositional path of BECMO (red line in b) gradually approaches the eutectic, and in reality, fractionation should continue until the BECMO composition becomes eutectic. However, due to the limited resolution in the final grid cell, the SiO2 concentration continues to increase until complete solidification, rather than capturing continued fractionation. This limitation has minimal impact on the results, as the layer in question is sufficiently thin.

Extended Data Fig. 3 Evolution of the compositions of magma ocean as a function of the core-mantle boundary temperature.

The upper panel is for BMO and the bottom is for BECMO. The fractions of components in wt% are shown in white (MgO), gray (FeO), and black (SiO2). As FeO enriches in the BMO/BECMO, the cotectic point along the (Mg,Fe)O-SiO2 system shifts towards a composition of lower SiO2 contents, allowing for continuous fractionation of bridgmanite even as SiO2 becomes increasingly depleted in the basal magma ocean. The addition of SiO2 deviates the bulk composition of magma ocean away from the cotectic valley of MgO-MgSiO3, and thus prolong the production of bridgmanite in BECMO.

Extended Data Fig. 4 Density anomalies of the solidified BMO (thin green) and BECMO (thick blue) relative to the unfractionated mantle (black line) with no chemical differentiation along the geotherm of ref. 18.

The black curve denotes no fractionation. Below (above) this curve the mantle is lighter (heavier) than the unfractionated mantle. The buoyancy number is calculated assuming a thermal expansivity of 1×10−5 K−1 and the temperature difference between the CMB and surface of 3000 K.

Extended Data Fig. 5 High-resolution geodynamic modelling results of a solidified BMO.

a–f, Snapshots of the temperature field (a,c,e) and effective buoyancy ratio field (b,d,f) of the geodynamic model. Note that a dense layer covers nearly the entire CMB even after ~5 Gyr.

Extended Data Fig. 6 Geodynamic modelling results of a solidified BECMO in spherical geometry.

The parameters, including density profiles, and approach used in this case are nearly identical to those in the Cartesian BECMO case in Fig. 3. (a,c) Snapshots of the temperature field and (b,d) the effective buoyancy ratio of the geodynamic model.

Extended Data Fig. 7 The results of the reference solidified BECMO case with effective buoyancy fields.

a–f, Snapshots of the temperature field (a,c,e) and the effective buoyancy ratio (b,d,f) of the geodynamic model.

Extended Data Fig. 8 High-resolution geodynamic modelling results of the solidified BECMO with a higher Rayleigh number.

a–f, Snapshots of the temperature field (a,c,e) and the residual buoyancy field (b,d,f) of the geodynamic model. The parameters of this case are the same as those of the reference BECMO case, except for a Rayleigh number that is 5 times higher (5 × 107).

Extended Data Fig. 9 Trade-off between exsolution rates and initial BMO thicknesses.

(a) Density profiles of the solidified BECMO along the geotherm18 as a function of the SiO2 exsolution rate and initial BMO thickness, including 7.3 ×10−5 K−1 with 350 km (Case I: blue solid line, also shown in Fig. 2f of the main text), 4 ×10−5 K−1 with 350 km (Case II: blue dashed line), and 8 ×10−5 K−1 with 780 km (Case III: red solid line). The density profile of the unfractionated model with no chemical differentiation is also shown for comparison (black solid line). The bottom density of the solidified BECMO as a function of (b) the SiO2 exsolution rate and (c) the normalized exsolution rate defined as the product of the exsolution rate and the core mass, divided by the initial BMO mass. The colorbar represents the initial BMO thickness for the given core mass of Earth. The MgO exsolution rate is fixed at 4 ×10−6 K−1 for all calculations. Our simulations show that cases that have the bottom density near and below ~6000 kg cm−3 (black dashed line in b and c) can generate LLVP and ULVZ-like structures. The three examples shown in (a) are also marked in other panels.

Extended Data Fig. 10 Mixing model showing correlations between 3He/4He ratios and μ182W of OIB samples.

(a) Mixing between ambient mantle, exsolution 1 (solid lines), and exsolution 2 (dashed lines). (b) Mixing between ambient mantle, dense piles (solid lines), and exsolution 2 (dash lines). Numbers denote the weight fraction of each component. OIB samples compiled by ref. 10 are also shown in filled circles with 2 SD uncertainties. Refer to ref. 10 and references therein for detailed discussion on the uncertainties of isotopic compositions of OIBs.

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Deng, J., Miyazaki, Y., Yuan, Q. et al. Deep mantle heterogeneities formed through a basal magma ocean contaminated by core exsolution. Nat. Geosci. 18, 1056–1062 (2025). https://doi.org/10.1038/s41561-025-01797-y

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