Research ArticleGEOLOGY

Oxygen isotopic evidence for accretion of Earth’s water before a high-energy Moon-forming giant impact

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Science Advances  28 Mar 2018:
Vol. 4, no. 3, eaao5928
DOI: 10.1126/sciadv.aao5928
  • Fig. 1 Oxygen isotope composition of terrestrial and lunar samples shown in relation to various meteorite groups that plot close to the TFL.

    Enstatite chondrites (EH and EL groups) (26) are shown as circular filled and unfilled fields. Aubrites, HEDs (howardites, eucrites, diogenites), angrites, and Martian meteorites are shown as bars to represent the δ18O range and mean average Δ17O value for each group (Δ17O calculated as per samples in this study). Data sources: terrestrial and lunar samples, this study and Starkey et al. (19); aubrites, Barrat et al. (20); other groups, Greenwood et al. (42).

  • Fig. 2 Oxygen isotopic composition of terrestrial and lunar samples and aubrites.

    Aubrites are the meteorites with the closest oxygen isotope composition to terrestrial and lunar rocks and, until the study of Barrat et al. (20), were thought to be indistinguishable from them. TWR, terrestrial whole rock analyses; LWR, lunar whole rock analyses.

  • Fig. 3 Average Δ17O values of lunar and terrestrial samples measured in this study.

    Aubrites are also shown (20). Blue ruled box ±2 SEM variation for terrestrial basalts (n = 20); brown ruled box ±2 SEM variation for lunar whole rocks (n = 17). n = number of samples.

  • Fig. 4 Diagram showing calculated oxygen isotopic composition of lunar rocks assuming an impact scenario defined by the canonical model (1).

    The oxygen isotope composition of the impactor is assumed to be that of the aubrites (table S1) and the composition of Earth following the giant impact is given by the terrestrial basalt samples analyzed in this study. These calculations indicate that the observed 4 ppm difference between terrestrial basalts and lunar rocks would require the Moon to be composed of between 25 and 28% impactor-derived material. This is significantly less than predicted by low-energy impact models in the absence of post-impact equilibration (1, 27). The SE of the 4 ppm difference is ±3 ppm (2 SE), which would indicate that the range of permissible impactor contributions to the Moon is between 13 and 39%. Note that apart from the aubrite (impactor) and terrestrial basalt data (Earth proxy), all other points on the diagram are calculated. These calculations can only predict Δ17O values (shown in italics) and not the measured δ17O and δ18O values, which, for natural samples, will be influenced by a variety of mass-dependent fractionation processes.

  • Fig. 5 Diagram showing how the addition of various chondritic components as part of the late veneer would alter the average Δ17O value of pristine terrestrial mantle.

    The Δ17O composition of pristine terrestrial material just after the giant impact is taken to be equivalent to that of present-day lunar basalts. A late-stage addition of any single chondritic component in an amount equivalent to the late veneer mass would result in a Δ17O isotopic shift greater than that defined by the 2 SEM error envelope of the terrestrial basalts (purple vertical band). An appropriate mix of chondritic components such as CI and CM would not violate the constraints imposed by our data. However, such a high percentage of chondritic material would violate Ru isotope constraints (31). A late veneer addition of 0.5% terrestrial mass of enstatite chondrite material (EL) (short black line) would add little water and result in only a very slight deviation to the Δ17O value of the lunar basalts. A CM late veneer fraction of approximately 20% (horizontal blue dashed line) can account for the 3 to 4 ppm Δ17O shift measured in the terrestrial basalts, while maximizing the amount of water delivered to Earth and minimizing the CC component within the late veneer, as required by Ru isotope systematics (31). An input of about 20% CV3 (Vigarano-type carbonaceous chondrite) would also satisfy the Δ17O constraints, but reduce the amount of water delivered by the late veneer. On the basis of the lowest estimates of Earth’s water content (33), the late veneer, at best, could only have supplied about one-third of our planet’s water. Higher global water estimates (32) would decrease this to about 5% (see the Supplementary Materials). Dots on the lines refer to 0.1% of terrestrial mass. OC, ordinary chondrite; LV, late veneer.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/3/eaao5928/DC1

    Supplementary Text

    fig. S1. Linearized oxygen isotope plot for all terrestrial and lunar samples analyzed in this study (n = 68), including the terrestrial olivines (19).

    fig. S2. Linearized oxygen isotope plot for all filtered lunar samples (n = 24) analyzed in this study.

    fig. S3. Linearized oxygen isotope plot for all filtered terrestrial basalt samples (n = 18) analyzed in this study.

    fig. S4. Diagram showing how addition of various chondritic components would alter the isotopic and water abundance compositions of the post–giant impact Earth.

    fig. S5. Oxygen isotope composition of eclogitic garnets.

    fig. S6. Obsidian standards data collected during this study compared to data collected during the earlier terrestrial olivine study (19).

    fig. S7. Kernel estimate densities from samples’ average values.

    fig. S8. Kernel estimate densities from raw data (all replicates run for all samples).

    table S1. Full listing of oxygen isotope data.

    table S2. Results of Student t tests for unpaired data with equal variance.

    table S3. Whole rock terrestrial samples.

    table S4. Analyses of eclogitic garnet samples.

    table S5. Oxygen isotope analyses and water content values used to construct Fig. 5 and fig. S4 (Excel file).

    table S6. Late veneer end member calculations used to construct Fig. 5 and fig. S4 [Excel files (×6)].

    table S7. Water in Earth: Calculations used to construct Fig. 5 and fig. S4 (Excel file).

    model S1. A spreadsheet that calculates the oxygen isotopic composition of the Moon using a mix defined by our analyses of terrestrial basalts and aubrites (20) (Excel file).

    model S2. A spreadsheet that calculates the oxygen isotopic composition of the Moon using a mix defined by our analyses of olivines and aubrites (20) (Excel file).

    Compilation of all replicate analyses run in this study (Excel file).

    Compilation of obsidian standards data 2012–2014 (Excel file).

    References (4959)

  • Supplementary Materials

    This PDF file includes:

    • Supplementary Text
    • fig. S1. Linearized oxygen isotope plot for all terrestrial and lunar samples analyzed in this study (n = 68), including the terrestrial olivines (19).
    • fig. S2. Linearized oxygen isotope plot for all filtered lunar samples (n = 24) analyzed in this study.
    • fig. S3. Linearized oxygen isotope plot for all filtered terrestrial basalt samples (n = 18) analyzed in this study.
    • fig. S4. Diagram showing how addition of various chondritic components would alter the isotopic and water abundance compositions of the post–giant impact Earth.
    • fig. S5. Oxygen isotope composition of eclogitic garnets.
    • fig. S6. Obsidian standards data collected during this study compared to data collected during the earlier terrestrial olivine study (19).
    • fig. S7. Kernel estimate densities from samples’ average values.
    • fig. S8. Kernel estimate densities from raw data (all replicates run for all samples).
    • table S1. Full listing of oxygen isotope data.
    • table S2. Results of Student t tests for unpaired data with equal variance.
    • table S3. Whole rock terrestrial samples.
    • table S4. Analyses of eclogitic garnet samples.
    • References (49–59)

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    Other Supplementary Material for this manuscript includes the following:

    • table S5. Oxygen isotope analyses and water content values used to construct Fig. 5 and fig. S4 (Excel file).
    • table S6. Late veneer end member calculations used to construct Fig. 5 and fig. S4 Excel files (×6).
    • table S7. Water in Earth: Calculations used to construct Fig. 5 and fig. S4 (Excel file).
    • model S1. A spreadsheet that calculates the oxygen isotopic composition of the Moon using a mix defined by our analyses of terrestrial basalts and aubrites (20) (Excel file).
    • model S2. A spreadsheet that calculates the oxygen isotopic composition of the Moon using a mix defined by our analyses of olivines and aubrites (20) (Excel file).
    • Compilation of all replicate analyses run in this study (Excel file).
    • Compilation of obsidian standards data 2012–2014 (Excel file).

    Files in this Data Supplement:

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