The virtually identical oxygen isotope compositions of the Earth and Moon revealed by Apollo return samples have been a challenging constraint for lunar formation models. For a giant impact scenario to explain this observation, either the precursors to the Earth and Moon had identical oxygen isotope values or extensive homogenization of the two bodies occurred following the impact event. Here we present high-precision oxygen isotope analyses of a range of lunar lithologies and show that the Earth and Moon in fact have distinctly different oxygen isotope compositions. Oxygen isotope values of lunar samples correlate with lithology, and we propose that the differences can be explained by mixing between isotopically light vapour, generated by the impact, and the outermost portion of the early lunar magma ocean. Our data suggest that samples derived from the deep lunar mantle, which are isotopically heavy compared to Earth, have isotopic compositions that are most representative of the proto-lunar impactor ‘Theia’. Our findings imply that the distinct oxygen isotope compositions of Theia and Earth were not completely homogenized by the Moon-forming impact, thus providing quantitative evidence that Theia could have formed farther from the Sun than did Earth.
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The authors declare that the necessary data supporting the findings of this study are available within the paper and its supplementary information files. All other datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.
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We are grateful to NASA and CAPTEM for approving our requests for Apollo samples used in this study. We thank F. Trusdell, M. Perfit, V. S. Kamenetsky, L. S. Crumpler, K. A. Smart, S. Tappe, S. C. Kruckenberg, B. Oller, G. Wörner, L. D. Ashwal and L. E. Borg for the collection and/or donation of sample material. Thank you to S. Locke for sharing his thoughts on post-giant impact dynamics. Thank you to S. Chakraborty and M. H. Thiemens for sharing additional insight regarding their work on mass-independent oxygen isotope fractionation in gas-phase SiO2 formation. We acknowledge support from NSF award 1903852.
The authors declare no competing interests.
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Extended Data Fig. 1 Plot of the change in the Δ′17O value (ΔΔ′17O) of the lunar vapour as a function of the amount lost during early deposition and incorporation into the bulk Moon (F).
If 60% of the vapour is removed into the mantle (F=0.4), the remaining vapour will have a Δ′17O value that is 0.75‰ lighter than the initial vapour (bulk Moon value). In order to change the outer 50 km of the ‘crust’ by 0.05‰, ~4×1023 g of vapour would have to be remixed into the outer crust. The effect on the δ18O value would be minimal.
Mineral separate data from the lunar crust samples are measured values and include plagioclase, pyroxene and/or olivine separates from samples 60016, 62237, 77215, and 78238. Mineral separate data for low-Ti Basalts (samples 12018, 12063, 15016, and 15426) are calculated from the whole-rock δ′18O measurements using simple mass-balance equations, modal mineral percentages, and expected isotope fractionations. The ‘theoretical’ mineral values for the VLT glass are calculated from modes estimated from the CIPW norm for this sample. Calculated δ18O mineral separate data, either from CIPW norm or measured modes agree to within <0.1‰. Mineral modes and bulk composition data used to calculate the CIPW norm values are from the Lunar Sample Compendium43. The isotope fractionations used in the mass balance equations are Δ′18OPl-Ol = 0.774, Δ′18OPl-Px = 0.35, Δ′18OPx-Ol = 0.424 (from Chiba et al.52) and assume isotopic equilibrium between mineral phases at 1200 °C.
Extended Data Fig. 3 Calculated Δ′17O values for Theia and Proto-Earth for varying degrees of mixing and initial proto-planet masses.
The plot illustrates the calculated Δ′17O values for Theia and Proto-Earth given varying initial Theia masses (indicated by different line styles) and the percentage of the Moon that is composed of material from Theia. For example, if Theia was initially 0.1 ME and 70% of the Moon is material from Theia, Theia’s initial Δ′17O value was about −0.028‰ and Proto-Earth’s was about −0.062‰. This assumes that the current values of the Moon and Earth are −0.038‰ and −0.060‰ respectively and the summed masses of Theia and Proto-Earth are equivalent to the total mass of the present Earth-Moon system. Increased mixing between Theia and Proto-Earth produce smaller amounts of Theia in the Moon.
Extended Data Fig. 4 Plot of Plot of Δ′17O versus δ′18O for the VLT green glass (15426), high-Ti orange/black glass (74220), and MORB glass (ALV2746-12).
The VLT green glass is represented by the green squares and high-Ti orange/black glass is represented by the yellow diamonds. The MORB glass values are black circles. The coloured in shapes highlight the range in oxygen isotope values for their corresponding data set. This illustrates how large the range seen in the VLT green glass is compared to that seen in homogeneous glass samples measured with identical methods. The green glass has a range over three times that of the high-Ti orange/black glass and the MORB sample, demonstrating that the isotopic heterogeneity in the green glass beads is not a product of the sample analysis and is real variation.
Extended Data Fig. 5 Plot of Δ′17O values for different fractions of very low-Ti green glass (left) with histogram (right).
Various fractions of VLT green glass (sample 15426) were separated by glass bead appearance and measured. Mixed fractions of the green glass containing a random assortment of glass beads ranged from −0.066‰ to −0.037‰, a spread far greater than what can be attributed to analytical error on measurements of a homogeneous sample. When separated by visual appearance, samples ranged from −0.057‰ to −0.025‰ with the dark rimmed fraction having the heaviest measured Δ′17O value in this study. Histogram bins are 0.005‰ wide and include both mixed and separated fractions combined to illustrate the frequency distribution of the measured values.
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Cano, E.J., Sharp, Z.D. & Shearer, C.K. Distinct oxygen isotope compositions of the Earth and Moon. Nat. Geosci. (2020). https://doi.org/10.1038/s41561-020-0550-0