Evidence for a Stratified Upper Mantle Preserved Within the South Pole‐Aitken Basin

Moriarty, D. P.; Clegg-Watkins, R. N.; Valencia, S. N.; Kendall, J. D.; Evans, A. J.; Dygert, Nick; Petro, N. E.

doi:10.1029/2020je006589

Cited by 40 publications

(75 citation statements)

References 129 publications

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“…2, first row). All numerical convection simulations in this study (see Supplementary Text) begin with a global layer of ilmenite-bearing late-stage cumulates overlying the lunar mantle at the time of the SPA impact, in accordance with recent remote sensing analyses ( 14 ). Reference scenario 1 (RS1) (Fig.…”

Section: Introductionsupporting

confidence: 78%

A South Pole–Aitken impact origin of the lunar compositional asymmetry

et al. 2022

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The formation of the largest and most ancient lunar impact basin, South Pole–Aitken (SPA), was a defining event in the Moon’s evolution. Using numerical simulations, we show that widespread mantle heating from the SPA impact can catalyze the formation of the long-lived nearside-farside lunar asymmetry in incompatible elements and surface volcanic deposits, which has remained unexplained since its discovery in the Apollo era. The impact-induced heat drives hemisphere-scale mantle convection, which would sequester Th- and Ti-rich lunar magma ocean cumulates in the nearside hemisphere within a few hundred million years if they remain immediately beneath the lunar crust at the time of the SPA impact. A warm initial upper mantle facilitates generation of a pronounced compositional asymmetry consistent with the observed lunar asymmetry.

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Section: Introductionsupporting

confidence: 78%

A South Pole–Aitken impact origin of the lunar compositional asymmetry

et al. 2022

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“…The most reliable result was obtained for the upper mantle consisting mostly of olivine-bearing pyroxenite, whose dominant mineral is low-Ca pyroxene but not olivine. This result is compatible with the inversion of Apollo seismic data (Lognonné et al, 2003;Kuskov et al, 2019aKuskov et al, , 2019b and with petrological (Ringwood and Essene 1970;Moriarty et al, 2021) and spectral observation according to data of the Kaguya and Chang'E missions (Li et al, 2019;Lemelin et al, 2019).…”

Section: Al Bulk Fe Bulk D D M Mantlesupporting

confidence: 89%

Physical Properties and Internal Structure of the Central Region of the Moon

et al. 2021

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One of the pivoting problems of the geochemistry and geophysics of the Moon is the structure of its central region, i.e., its core and adjacent transition layer located at the boundary between the solid mantle and liquid or partially molten core. The chemical composition of the mantle and the internal structure of the central region of the Moon were simulated based on the joint inversion of seismic, selenophysical, and geochemical parameters that are not directly interrelated. The solution of the inverse problem is based on the Bayesian approach and the use of the Markov chain Monte Carlo algorithm in combination with the method of Gibbs free energy minimization. The results show that the radius of the Moon’s central region is about 500–550 km. The thickness of the transition layer and the radii of the outer and inner cores relatively weakly depend on the composition models of the bulk silicate Moon with different contents of refractory oxides. The silicate portion of the Moon is enriched in FeO (12–13 wt %, FeO ~ 1.5 × BSE) and depleted in MgO (Mg# 79–81) relative to the bulk composition of the silicate Earth (BSE), which is in conflict with the possibility of the formation of the Moon from the Earth’s primitive mantle and does not find an adequate explanation in the current canonical and non-canonical models of the origin of the Moon. SiO2 concentrations in all zones of the lunar mantle vary insignificantly and amount to 52–53 wt %, and the predominant mineral of the upper mantle is low-Ca orthopyroxene but not olivine. With respect to Al2O3, the lunar mantle is stratified, with a Al2O3 content higher in the lower mantle than in all overlying shells. The partially molten transition layer surrounding the core is about 200–250 km thick. The radii of the solid inner core are within 50–250 km, and the most probable radii of the liquid outer core are ~300–350 km. The physical characteristics of the lunar core are compared with experimental measurements of the density and speed of sound of liquid Fe(Ni)–S–C–Si alloys. If the seismic model of the liquid outer core with VP = 4100 ± 200 m/s (Weber et al., 2011) is reasonably reliable, then this uncertainty range is in the best agreement with the VP values of 3900–4100 m/s of liquid Fe(Ni)–S alloys, with sulfur content up to ~10 wt % and a density of 6200–7000 kg/m3, as well as with the inverted values of density and velocity of the outer core. The VP values of liquid Fe–Ni–C and Fe–N–Si alloys at 5 GPa exceed seismic estimates of the speed of sound of the outer lunar core, which indicates that carbon and silicon can hardly be dominant light elements of the lunar core. The inner Fe(Ni) core (possibly with an insignificant content of light elements: sulfur and carbon) is presumably solid and has a density of 7500–7700 kg/m3. The difference in density between the inner and outer cores Δρ ~ 500–1000 kg/m3 can be explained by the difference in their composition.

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“…As shown in Figure 6, the K/Th ratio is the same on the near and far sides. There is more noise in the K/Th ratio on the far side owing to lower concentrations of K and Th, except for a small region around the South Pole-Aitken (Moriarty et al 2021;Figure 6, bottom panel;Figures 7(a), (b)), but the K/Th ratio is the same. Lunar meteorites, which should be less biased than Apollo missions in their sampling, also show uniformly low K/U ratios relative to BSE (Figure 7(c)).…”

Section: Lamentioning

confidence: 95%

The Extent, Nature, and Origin of K and Rb Depletions and Isotopic Fractionations in Earth, the Moon, and Other Planetary Bodies

Dauphas¹,

Nie²,

Blanchard³

et al. 2022

Planet. Sci. J.

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Moderately volatile elements (MVEs) are depleted and isotopically fractionated in the Moon relative to Earth. To understand how the composition of the Moon was established, we calculate the equilibrium and kinetic isotopic fractionation factors associated with evaporation and condensation processes. We also reassess the levels of depletions of K and Rb in planetary bodies. Highly incompatible element ratios are often assumed to be minimally affected by magmatic processes, but we show that this view is not fully warranted, and we develop approaches to mitigate this issue. The K/U weight ratios of Earth and the Moon are estimated to be 9704 and 2448, respectively. The 87Rb/86Sr atomic ratios of Earth and the Moon are estimated to be 0.072 5 and 0.015 4, respectively. We show that the depletions and heavy isotopic compositions of most MVEs in the Moon are best explained by evaporation in 99%-saturated vapor. At 99% saturation in the protolunar disk, Na and K would have been depleted to levels like those encountered in the Moon on timescales of ∼40–400 days at 3500–4500 K, which agrees with model expectations. In contrast, at the same saturation but a temperature of 1600–1800 K relevant to hydrodynamic escape from the lunar magma ocean, Na and K depletions would have taken 0.1–103 Myr, which far exceeds the 1000 yr time span until plagioclase flotation hinders evaporation from the magma ocean. We conclude that the protolunar disk is a much more likely setting for the depletion of MVEs than the lunar magma ocean.

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Evidence for a Stratified Upper Mantle Preserved Within the South Pole‐Aitken Basin

Cited by 40 publications

References 129 publications

A South Pole–Aitken impact origin of the lunar compositional asymmetry

A South Pole–Aitken impact origin of the lunar compositional asymmetry

Physical Properties and Internal Structure of the Central Region of the Moon

The Extent, Nature, and Origin of K and Rb Depletions and Isotopic Fractionations in Earth, the Moon, and Other Planetary Bodies

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