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Vestiges of a lunar ilmenite layer following mantle overturn revealed by gravity data

Nature Geoscience volume 17, pages 361–366 (2024)Cite this article

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Abstract

The lunar crust and mantle formed through the crystallization of a magma ocean, culminating in a solid cumulate mantle with a layer of dense ilmenite-bearing cumulates rich in incompatible elements forming above less dense cumulates. This gravitationally unstable configuration probably resulted in a global mantle overturn, with ilmenite-bearing cumulates sinking into the interior. However, despite abundant geochemical evidence, there has been a lack of physical evidence on the nature of the overturn. Here we combine gravity inversions together with geodynamic models to shed light on this critical stage of lunar evolution. We show that the observed polygonal pattern of linear gravity anomalies that surround the nearside mare region is consistent with the signature of the ilmenite-bearing cumulates that remained after the global mantle overturn at the locations of past sheet-like downwellings. This interpretation is supported by the compelling similarity between the observed pattern, magnitude and dimensions of the gravity anomalies and those predicted by geodynamic models of the ilmenite-bearing cumulate remnants. These features provide physical evidence for the nature of the global mantle overturn, constrain the overturn to have occurred before the Serenitatis and Humorum basin-forming impacts and support a deep Ti-rich mantle source for the high-Ti basalts.

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Fig. 1: Global maps of lunar topography, surface abundance of TiO2 and Bouguer gravity gradient.
Fig. 2: Forward-modelled Bouguer gravity gradient anomalies associated with the IBCs.
Fig. 3: Results from the MCMC model runs.
Fig. 4: IBC thickness distribution over the PKT border anomalies from gravity analyses and geodynamic models.

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Data availability

The gravity field model from refs. 21,45 can be found at https://pds-geosciences.wustl.edu/grail/grail-l-lgrs-5-rdr-v1/grail_1001/shadr/, and the topography model from refs. 40,46 can be obtained at https://zenodo.org/records/3870924. The MCMC, gravity–topography inversion and IBC thickness results required to reproduce key figures in this Article are provided in a Zenodo repository (LiangBroquetIBC, https://zenodo.org/records/10389616).

Code availability

The Markov chain Monte Carlo model from ref. 26 can be found at https://zenodo.org/records/10622788. The topography and gravity inversion model from ref. 28 can be found at https://zenodo.org/records/10552129.

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Acknowledgements

This work was supported by grant 80NSSC22K1340 from the NASA Lunar Data Analysis Program to J.C.A.-H.

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Author notes

  1. These authors contributed equally: Weigang Liang, Adrien Broquet.

Authors and Affiliations

  1. Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA

    Weigang Liang, Adrien Broquet & Jeffrey C. Andrews-Hanna

  2. Key Laboratory of Orogenic Belts and Crustal Evolution, School of Earth and Space Sciences, Peking University, Beijing, China

    Nan Zhang

  3. Southern University of Science and Technology, Shenzhen, China

    Min Ding

  4. Department of Earth, Environmental and Planetary Sciences, Brown University, Providence, RI, USA

    Alexander J. Evans

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  1. Weigang Liang
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Contributions

W.L., A.B. and J.C.A.-H. conceptualized the work and methodology and wrote the manuscript. W.L. carried out the gravity analyses and Monte Carlo modelling. A.B. performed the gravity–topography inversions. N.Z. conducted the geodynamic modelling of mantle overturn. M.D. and A.J.E. contributed to editing of the manuscript and discussion of the work.

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Correspondence to Adrien Broquet.

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Nature Geoscience thanks Felipe Leitzke and the other, anonymous, reviewer(s) 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 Bouguer gravity gradients over the nearside mare region.

(a) Bouguer gravity gradient map computed using a low-pass filter at spherical harmonic degree 100 to isolate longer wavelength anomalies consistent with arising at the base of the crust. (b) Interpreted Bouguer gravity gradients map, with the border anomalies related to sub-crustal IBC (black) and anomalies internal to the PKT that are potentially related to sub-crustal IBC (grey). Major impact basins are shaded orange and labelled, including Imbrium (Im.), Serenitatis (Ser.), Humorum (Hum.), and Asperitatis (Asp.).

Extended Data Fig. 2 Predicted IBC thickness from the geodynamic model of mantle overturn.

The models shown assume a density contrast of 200 kg/m3 and viscosity ratios of 10−3 (a), 10−2 (b), and 10−1 (c), and density contrasts of 300 kg/m3 (d) and 400 kg/m3 (e) for a viscosity ratio of 10−2.

Extended Data Fig. 3 Forward modelled Bouguer gravity gradient anomalies associated with IBCs as derived from mantle overturn models.

The models shown assume density contrasts both during overturn and in the final state of (a) 300 and (b) 400 kg/m3, each assuming a viscosity ratio 10−2 between the IBCs and mantle.

Extended Data Fig. 4 Additional results from the gravity and topography inversion for the ilmenite-bearing cumulate (IBC) layer thickness.

Shown are the initial and post-interpolation mare (a,b) and crustal (c,d) thicknesses as calculated during the first and second steps of the gravity and topography inversion for IBC thickness.

Extended Data Fig. 5 Nearside Bouguer anomaly contour map (a) and zoom-in panels of the (b) Humorum and (c) Serenitatis–Asperitatis regions.

Our mapped paths of the PKT border anomalies are shown as white lines, with the projected paths where erased by impact basins shown as transparent white dashed lines. The black boxes in (a) indicate the location of (b) and (c). These maps are generated using a high-pass filter at spherical harmonic degree 8, a low-pass filter at degree 150, and a Bouguer correction density of 3100 kg/m3. Both gravity gradient and Bouguer contour maps were used in mapping anomalies, with the latter being particularly useful for showing the lack of features on the basin floors.

Extended Data Table 1 Compilation of results from all Markov chain Monte Carlo model runs
Full size table

Supplementary information

Supplementary Information

Supplementary Figs. 1–5.

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Liang, W., Broquet, A., Andrews-Hanna, J.C. et al. Vestiges of a lunar ilmenite layer following mantle overturn revealed by gravity data. Nat. Geosci. 17, 361–366 (2024). https://doi.org/10.1038/s41561-024-01408-2

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