Tungsten isotopic evidence for disproportional late accretion to the Earth and Moon

Journal name:
Nature
Volume:
520,
Pages:
530–533
Date published:
DOI:
doi:10.1038/nature14355
Received
Accepted
Published online

Characterization of the hafnium–tungsten systematics (182Hf decaying to 182W and emitting two electrons with a half-life of 8.9 million years) of the lunar mantle will enable better constraints on the timescale and processes involved in the currently accepted giant-impact theory for the formation and evolution of the Moon, and for testing the late-accretion hypothesis. Uniform, terrestrial-mantle-like W isotopic compositions have been reported1, 2 among crystallization products of the lunar magma ocean. These observations were interpreted to reflect formation of the Moon and crystallization of the lunar magma ocean after 182Hf was no longer extant—that is, more than about 60 million years after the Solar System formed. Here we present W isotope data for three lunar samples that are more precise by a factor of ≥4 than those previously reported1, 2. The new data reveal that the lunar mantle has a well-resolved 182W excess of 20.6 ± 5.1 parts per million (±2 standard deviations), relative to the modern terrestrial mantle. The offset between the mantles of the Moon and the modern Earth is best explained by assuming that the W isotopic compositions of the two bodies were identical immediately following formation of the Moon, and that they then diverged as a result of disproportional late accretion to the Earth and Moon3, 4. One implication of this model is that metal from the core of the Moon-forming impactor must have efficiently stripped the Earth’s mantle of highly siderophile elements on its way to merge with the terrestrial core, requiring a substantial, but still poorly defined, level of metal–silicate equilibration.

At a glance

Figures

  1. Values of [mgr]182W of lunar metals separated from KREEP-rich impact melts analysed by negative thermal ionization mass spectrometry in this study.
    Figure 1: Values of μ182W of lunar metals separated from KREEP-rich impact melts analysed by negative thermal ionization mass spectrometry in this study.

    The data for 68115,114, 68815,394, and 68815,396 are shown as circles, diamond, and square respectively; error bars for our analysis show internal precision of one single measurement, for which the 2 standard deviations (s.d.) external reproducibility is ~4.5 ppm, as demonstrated by replicated standard measurements over the two year period. The white-dotted circle corresponds to the average of the three replicated analyses of 68115,114 metal; error bars show 2 s.d. of these data. The dark grey area and black dashed line indicates the average μ182W = +20.6 ± 5.1 (2 s.d., n = 3) of the three metal separates from Apollo 16 impact melt rocks analysed here. The light grey dashed line corresponds to the W isotope composition of the modern terrestrial mantle, and the light grey area at μ182W = 0 corresponds to the 2 standard errors (s.e.) uncertainty for repeated analyses of the Alfa Aesar W standard.

  2. HSE abundances (normalized to Ir and CI chondrite abundances) of metal separates from sample 68815,394 and 68815,396.
    Figure 2: HSE abundances (normalized to Ir and CI chondrite abundances33) of metal separates from sample 68815,394 and 68815,396.

    The symbols are the same as those used in Fig. 1. Data for IVA irons15 (grey dashed lines) and impact melts 60315 and 67935 (ref. 16, dark grey stars) are also shown for comparison. (To obtain data for this plot, all obtained element concentrations have been divided by their respective abundance in CI chondrites, then normalized so that they all plot on the same point for Ir.)

  3. Plot of [mgr]182W versus source 180Hf/184W ratio.
    Extended Data Fig. 1: Plot of μ182W versus source 180Hf/184W ratio.

    182W values shown as open symbols are weighted averages of the data obtained in ref. 1 for low-Ti (triangle down) and high-Ti (triangle up) mare basalts; error bars, 2 s.e. of the samples of each group. The red symbol corresponds to the average of our new high-precision data for metals separated from 68115 and 68815 impact melts; error bars, 2 s.d. of the data. Based on mineral–melt partition coefficients for minerals in a crystallizing magma ocean, significant Hf–W fractionations are expected among the products of the LMO6, 20, resulting in high Hf/W in the source of high-Ti mare basalts (>40), low Hf/W in KREEP (10 ± 10) and intermediate Hf/W (26.5 ± 1.1) in the source of low-Ti mare basalts. Reference isochrons (blue dashed lines) corresponding to different times after the start of the Solar System are shown.

  4. Plot of [mgr]182W versus total HSE content relative to the present-day mantle, PM.
    Extended Data Fig. 2: Plot of μ182W versus total HSE content relative to the present-day mantle, PM.

    This is based on the assumption that before late accretion, the mantle was HSE-free and had a μ182W of +10 to +30 p.p.m., assuming total contributions of late accretion to be between 0.3% and 0.8% of the mass of the mantle (see labels), as determined from HSE abundances in the Earth’s mantle4 and using W contents of 200 p.p.b. for chondrites14 and 13 p.p.b. for the current mantle38. With the addition of chondritic materials, the total HSE abundances present in the mantle increase and the W isotopic composition decreases to present-day values. Evolution of the mantle composition by late accretion, or mixing between pre-late accretionary mantle and current accessible mantle, are represented by the grey field. Estimate for the HSE content of the lunar mantle (red circle) is taken from ref. 3; error bars, 2 s.d. of the data from the three rocks examined. Diamond symbol indicates the composition of the present-day mantle.

Tables

  1. Tungsten contents and isotopic compositions of mixing components
    Extended Data Table 1: Tungsten contents and isotopic compositions of mixing components

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Author information

  1. Present address: Laboratoire de Géologie de Lyon, Ecole Normale Supérieure de Lyon, Labex LIO, Université Lyon 1, 46 Allée d’Italie 69364 Lyon Cedex 7, France.

    • Mathieu Touboul

Affiliations

  1. Isotope Geochemistry Laboratory, Department of Geology, University of Maryland, College Park, Maryland 20742, USA

    • Mathieu Touboul,
    • Igor S. Puchtel &
    • Richard J. Walker

Contributions

M.T. conducted the W isotopic measurements and was involved in both the interpretations and the writing of the manuscript. I.S.P. conducted the measurements of the highly siderophile elements and Os isotopes, and was involved in both the interpretations and the writing of the manuscript. R.J.W. was involved in both the interpretations and the writing of the manuscript.

Competing financial interests

The authors declare no competing financial interests.

Corresponding authors

Correspondence to:

The data presented here can be found in the EarthChem library (http://www.earthchem.org/library/browse/view?id=849).

Author details

Extended data figures and tables

Extended Data Figures

  1. Extended Data Figure 1: Plot of μ182W versus source 180Hf/184W ratio. (51 KB)

    182W values shown as open symbols are weighted averages of the data obtained in ref. 1 for low-Ti (triangle down) and high-Ti (triangle up) mare basalts; error bars, 2 s.e. of the samples of each group. The red symbol corresponds to the average of our new high-precision data for metals separated from 68115 and 68815 impact melts; error bars, 2 s.d. of the data. Based on mineral–melt partition coefficients for minerals in a crystallizing magma ocean, significant Hf–W fractionations are expected among the products of the LMO6, 20, resulting in high Hf/W in the source of high-Ti mare basalts (>40), low Hf/W in KREEP (10 ± 10) and intermediate Hf/W (26.5 ± 1.1) in the source of low-Ti mare basalts. Reference isochrons (blue dashed lines) corresponding to different times after the start of the Solar System are shown.

  2. Extended Data Figure 2: Plot of μ182W versus total HSE content relative to the present-day mantle, PM. (42 KB)

    This is based on the assumption that before late accretion, the mantle was HSE-free and had a μ182W of +10 to +30 p.p.m., assuming total contributions of late accretion to be between 0.3% and 0.8% of the mass of the mantle (see labels), as determined from HSE abundances in the Earth’s mantle4 and using W contents of 200 p.p.b. for chondrites14 and 13 p.p.b. for the current mantle38. With the addition of chondritic materials, the total HSE abundances present in the mantle increase and the W isotopic composition decreases to present-day values. Evolution of the mantle composition by late accretion, or mixing between pre-late accretionary mantle and current accessible mantle, are represented by the grey field. Estimate for the HSE content of the lunar mantle (red circle) is taken from ref. 3; error bars, 2 s.d. of the data from the three rocks examined. Diamond symbol indicates the composition of the present-day mantle.

Extended Data Tables

  1. Extended Data Table 1: Tungsten contents and isotopic compositions of mixing components (79 KB)

Additional data