TUNGSTEN ISOTOPIC EVIDENCE FOR DISPROPORTIONAL LATE ACCRETION TO THE EARTH AND MOON

Abstract

Characterization of the ¹⁸²Hf-¹⁸²W systematics (¹⁸²Hf → ¹⁸²W + β⁻; t_½ = 8.9 Myr) of the lunar mantle is important for constraining the timescale and processes involved in the current giant impact paradigm for the formation and evolution of the Moon, and testing the late accretion hypothesis. Here, we present W isotope data for three lunar samples that are ≥ a factor of four more precise than those previously reported¹. The new data reveal that the lunar mantle has a well-resolved ¹⁸²W excess of 20.6 ± 5.1 (2σ SD) parts per million (ppm), relative to the modern terrestrial mantle. The offset between the Moon and silicate Earth is best explained by assuming that the W isotopic compositions of the two bodies were identical immediately following formation of the Moon, then diverged as a result of disproportional late accretion to the Earth and Moon^{2, 3}. One ramification 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 (HSE) on its way to merge with the terrestrial core, requiring a substantial, but still poorly-defined level of metal-silicate equilibration.

Early applications of W isotopes to lunar rocks were hampered by the effects of cosmic rays on ¹⁸²W, particularly as a result of production of ¹⁸²W from ¹⁸¹Ta via neutron capture⁴. Consequently, more recent studies have focused on the analysis of Ta-free metals extracted from lunar basalts and impact melt rocks^{1, 5}. Here, we present W isotopic data for metals separated from two KREEP-rich Apollo 16 impact-melt rocks, obtained using new high-precision analytical methods⁶. In addition to W isotopic compositions and abundances, Hf and HSE abundances, as well as ¹⁸⁷Os/¹⁸⁸Os ratios, were determined in order to assess the Hf/W of the metal, examine the chemical signature of the impactor that produced the melt rocks, and evaluate potential contributions from meteoritic W. These impact melt rocks were generated by basin-forming events, possibly during a period of late heavy bombardment at ~3.9 Ga⁷. The metal present in these rocks may have been derived from either the crustal target rocks, or the impactor that created the melt rocks. In either case, siderophile elements in the metals likely partially or wholly equilibrated with melt or vapor during the impact⁸.

Metal separates from impact melt rocks 68115,114, 68815,394 and 68815,396 have μ¹⁸²W values (where μ¹⁸²W is the deviation in ppm of the ¹⁸²W/¹⁸⁴W ratio of the sample from that of the modern terrestrial mantle) of +23.3 ± 3.8 (n=3, 2σ SD), +18.1 ± 2.5 and +20.4 ± 2.9, respectively, which are identical within analytical uncertainty (Table 1). The new data are consistent with the previously published data for the same samples¹, but are considerably more precise (Fig. 1). Of greatest importance, the W isotopic compositions of the metals are now well resolved from the isotopic composition of the silicate portion of the modern Earth.

Table 1:

Tungsten isotopic compositions and W and Hf abundances of metals separated from lunar impact melt rocks.

Samples	W (ppm)	Hf (ppm)	μ182W
68115,114 metal	32.7 ± 0.3	2.23 ± 0.05	+25.3 ± 4.6
			+21.5 ± 2.6
			+23.0 ± 1.7
68115,114 metal average (± 2σ SD)			+23.3 ± 3.8
68815,394 metal	22.8 ± 0.3	1.41 ± 0.02	+18.1 ± 2.5
68815,396 metal	36.3 ± 0.5	0.27 ± 0.01	+20.4 ± 2.9
Bulk lunar mantle (n=3, ± 2σ SD)			+20.6 ± 5.1

Fig. 1:

μ¹⁸²W of lunar metals separated from KREEP-rich impact melts 68115,114 (red circles), 68815,394 (red diamond) and 68815,396 (red square) analyzed by N-TIMS in this study, compared to previously reported average data for lunar metals (open symbols) from Touboul et al.¹ measured by multi-collector ICP-MS. Error bars for our analysis show internal precision of one single measurement, for which the 2σ SD external reproducibility is ~4.5 ppm, as demonstrated by replicated standard measurements over the two year period. The red dotted circle corresponds to the average of the 3 replicated analysis of 68115,114 metal and error bars show the 2σ SD of these data. Error bars for the two KREEP-rich samples analyzed by Touboul et al.¹ show the 2σ SE external reproducibility of one single sample measured 4 times. Error bars of the low-Ti basalts (triangle) and high-Ti basalts (inverted triangle) show the 2σ SE external reproducibility for 4 and 5 different samples, respectively, measured once by Touboul et al. (2007). The pink area and red dashed line indicates the average μ¹⁸²W = +20.6 ± 5.1 (2σ SD, n=3) of the three splits of Apollo 16 impact melts analyzed here. The gray area and black dashed line indicates the average μ¹⁸²W = +9 ± 10 (2σ SE, n=15) of lunar metals from Touboul et al.¹ The blue dashed line corresponds to the W isotope composition of the modern terrestrial mantle.

The positive W isotopic offset between the Moon and silicate Earth can be attributed to one of several possible causes, including: 1) cosmogenic exposure effects, 2) contribution of W from the basin-forming impactor that created the melt rocks, 3) radiogenic ingrowth of ¹⁸²W in a high Hf/W domain within the mantles of the Moon or the giant impactor, or 4) disproportional late accretion to the Earth and Moon. These possibilities are considered below.

Cosmic ray exposure effects can be excluded as the cause for the isotopic offset. The Ta/Hf ratio of ~0.11 for KREEP⁹, coupled with the measured Hf/W ratios of the metal separates (Table 1), gives a calculated Ta/W average of ~0.008 for the metals. Based on this ratio and the ~2 Myr exposure ages of the samples¹⁰, a maximum effect of approximately −0.1 ppm on the W isotopic compositions of the separated metals is estimated. This is well below our current level of analytical uncertainty, so no exposure corrections were applied, and the isotopic compositions of the metal separates are interpreted to have pre-exposure W isotopic compositions that are identical, within our long-term external analytical reproducibility.

Mass balance calculations indicate that the basin-forming impactor that led to the creation of the melt rocks contributed little W to the metals, and did not play a significant role in generating the isotopic offset. As a strongly incompatible element in silicate systems, W abundances are highly enriched in chemically evolved, KREEP-rich rocks¹¹, from which these impact melt rocks were derived. Further, based on HSE abundances, basin-forming impactors with chondritic bulk compositions contributed no more than ~2–3% of their mass to impact melt rocks¹². Thus, if the concentrations of W in the Apollo 16 target rocks and the impactor are assumed to have been ~1500 ppb, typical of KREEP-rich basalts¹¹, and ~180 ppb, typical of chondritic meteorites¹³, respectively, a 3 wt.% contribution of chondritic impactor mass to the melt rocks would have added only ~0.4% of the total W present in the impact melt rock. If the impactor had a chondritic μ¹⁸²W value of −200, the indigenous lunar W isotopic composition would have been lowered by only ~0.8 ppm (Table S1).

The separated metals from 68815 are characterized by supra-chondritic ¹⁸⁷Os/¹⁸⁸Os, Ru/Ir, Pt/Ir, and Pd/Ir (Table 2; Fig. 2), features common in chemically evolved iron meteorites¹⁴. Based on similar HSE characteristics in bulk samples of Apollo 16 impact melt rocks, it has been argued that a chemically evolved iron meteorite, comparable to the group IVA iron meteorite Bushman Land, was involved in the generation of Apollo 16 impact melt rocks¹⁵. Bushman Land is rich in siderophile elements with 1200 ppb Ir and 460 ppb W¹⁴. Addition of no more than 0.8% of comparable material would be required to generate impact melt rocks with the observed average Ir concentrations of <10 ppb present in most Apollo 16 impact melt rocks¹⁵. Incorporation of 1 wt.% of an iron meteorite with the same W concentration and a μ¹⁸²W = −330, comparable to IVA irons¹⁶, would have lowered the W isotopic compositions of the impact melt rocks by only ~2 ppm, relative to the indigenous lunar signature (Table S1).

Table 2:

Highly siderophile element contents (in ppb), and Os isotopic compositions of metals separated from lunar impact melt rocks.

Samples	Re	Os	Ir	Ru	Pt	Pd	187Os/188Os	±2σmean
68115,113 metal							0.13825	0.00007
							0.13837	0.00004
68815,394 metal	6.750	57.59	61.99	144.5	167.2	163.6	0.13720	0.00011
68815,396 metal	128.6	1256	1188	2479	3497	2347	0.13480	0.00006

Fig. 2:

HSE abundances (normalized to Ir and CI chondrite abundances³¹) of metal separates from sample 68815,394 (red diamonds) and 68815,396 (red squares). Data for IVA irons (gray dashed lines) and Apollo 16 impact melts (dark gray stars) from McCoy et al.¹⁴ and Fischer-Gödde & Becker¹⁵, respectively, are also shown for comparison.

As further evidence that the W isotopic compositions of the metal separates are nearly completely derived from the lunar target rocks, we note that the metals from the two pieces of 68815 are characterized by considerably different absolute and relative abundances of HSE, as well as ¹⁸⁷Os/¹⁸⁸Os, yet their W isotopic compositions are identical within uncertainties. The differences in HSE likely reflect the incorporation of different proportions of HSE from two impactors into the two pieces. Incorporation of HSE from more than one impactor is common in lunar impact melt rocks¹⁷. If our assumptions about the mass balance of W among target rocks and impactors are grossly incorrect, and significant but variable proportions of the W present in the metal separates was derived from different impactors, it is very likely they would also have different W isotopic compositions. This is not observed. We conclude that modifications to the indigenous lunar W isotopic composition by contamination from basin-forming impactors were minor, and that the average μ¹⁸²W value of +20.6 ± 5.1 (2σ SD) for the three metal separates provides the current best estimate of the W isotopic composition of their parental mantle KREEP domain.

The observed isotopic offset between the Moon and the silicate Earth might also reflect in situ decay of ¹⁸²Hf in a high Hf/W domain, formed as a consequence of the characteristics of the materials from which the Moon coalesced, fractionation of the two elements during core-mantle segregation of the Moon, or crystallization of the lunar magma ocean (LMO). Although the Hf/W ratio of the bulk lunar mantle likely increased slightly as a result of lunar core formation and extraction of an unknown proportion of the siderophile W into the core, recent studies have concluded that the silicate portions of the Earth and Moon have nearly identical Hf/W (e.g., ¹⁸). Consequently, if the Moon formed while ¹⁸²Hf was still extant, and the silicate portions of the Earth and Moon had identical W isotopic compositions at the time of formation, the isotopic compositions of W would not have evolved to the different compositions observed.

By contrast, fractional crystallization of the LMO almost certainly led to the creation of mantle domains with both higher and lower Hf/W ratios, compared to the bulk lunar mantle. This is due to the more highly incompatible nature of W in silicate systems, compared with Hf¹⁹. Crystal-liquid fractionation would, therefore, have led to the creation of ¹⁸²W-enriched and depleted domains in the mantle, if LMO crystallization was rapid while ¹⁸²Hf was extant. The comparatively large amount of W needed to make sufficiently high precision measurements, coupled with sample mass limitations for Apollo samples, prevented us from making isotopic measurements on rocks derived from lunar mantle domains with different Hf/W from the KREEP source. Two observations, however, suggest that radiogenic ingrowth inside the Moon was not the cause of the ¹⁸²W-enriched nature of the metals examined here. First, the coupled ^146,147Sm-^142,143Nd systematics of crustal rocks derived from the lunar mantle indicate that late stages of LMO crystallization occurred more than 100 Myr after Solar System formation²⁰, well after ¹⁸²Hf was extinct. Second, regardless of the timing of LMO crystallization, the mantle source of KREEP was likely a low Hf/W reservoir, given the W-enriched nature of KREEP. Thus, if the KREEP mantle source rapidly formed during the lifetime of ¹⁸²Hf, it would have developed a ¹⁸²W deficit relative to the Earth-Moon system, rather than the observed enrichment (Fig. S1), assuming the mantles of both bodies were in isotopic equilibrium at the time of Moon formation. We conclude that the average W isotopic composition of the Apollo 16 metals is representative of the bulk lunar mantle.

The isotopic similarity between the Earth and Moon for lithophile elements, such as O, Cr, Si and Ti, is well established^{21, 22}, and has led to multiple hypotheses including the possibility that the giant impact that created the Moon involved an impactor comprised of genetically similar materials to the Earth²¹, that the Moon was constructed mainly from terrestrial materials rather than the impactor²³, or that the impact and coalescence processes somehow led to thorough isotopic mixing between the two bodies²⁴. The recent report of a small offset in the Δ¹⁷O composition of the Moon, compared to Earth²¹, leaves open the possibility that there were small differences in the isotopic compositions of elements in addition to O. Thus, it is possible that the W isotopic difference between the Earth and Moon is a result of the Moon forming from a mixture of terrestrial and impactor W, assuming there was no isotopic equilibration following metal segregation. Mixing of W between the Earth’s mantle and impactor mantle, plus varying proportions of impactor metal, could have raised the μ¹⁸²W of the Earth’s mantle by as much as ~100 ppm, if there was negligible equilibration between W from the core of the impactor and the Earth’s mantle²⁵, or lowered it by as much as ~200 ppm, if both the impactor core and mantle equilibrated with the Earth’s mantle²⁶. Thus, even if the mantles of the Earth and impactor had similar W isotopic compositions at the time of impact, plausible mixtures of terrestrial and impactor W could have produced a Moon with a broad range of W isotopic compositions, well beyond the observed offset.

The most parsimonious explanation for the W isotopic offset between the Earth and Moon is disproportional late accretion to the two bodies. Late accretion is the process whereby substantial mass of materials with chondritic bulk compositions is added to a planetary mantle after core segregation ceases². Late accretion would have led to decreases in the μ¹⁸²W values of the mantles of the Earth and Moon, because materials with chondritic bulk properties have much higher W concentrations and strongly negative μ¹⁸²W values, compared to the terrestrial and lunar mantles. The observed W isotopic offset is consistent with estimates that late accretion added between 0.3 to 0.8 wt. %, and ~0.05 wt. % to the masses of the Earth and Moon, respectively^{2, 3}. The addition of proportionally much greater mass to the Earth, relative to the Moon, has been explained as a consequence of stochastic processes²⁷. If these estimates for late accretion are accurate, then the μ¹⁸²W value of the Earth’s mantle, prior to late accretion, was 10 to 30 ppm higher than at present, compared with only 1 to 3 ppm higher for the lunar mantle (Fig. 3). Because the W isotopic composition of the Moon falls within this very narrow range of isotopic compositions predicted by disproportional late accretion, we conclude that it is the most likely cause of the isotopic offset.

Fig. 3:

μ¹⁸²W versus total HSE content relative to the present-day mantle (in %). This is based on the assumption that prior to late accretion, the mantle was HSE-free and had a μ¹⁸²W of +10 to +30 ppm, assuming total contributions of late accretion to be between 0.3 to 0.8% of mass of the mantle, as determined from HSE abundances in the Earth’s mantle² and W contents of 200 ppb and 13 ppb for chondrites¹³, and the current mantle³², respectively. With the addition of chondritic materials, the total HSE abundances present in the mantle increase and the W isotopic composition decrease 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 is taken from Day et al.³. The error bar for the lunar mantle is shown as the 2σ SD of the data from the three rocks examined.

In addition to the requirement that the silicate portions of the Earth and Moon were the same at the time of the formation of the Moon, an interpretation of disproportional late accretion requires that the late accretionary clocks for both the Moon and Earth were initiated at the same time. Thus, if significant late accretion to Earth occurred prior to the giant impact, metal from the core of the giant impactor must have efficiently stripped the pre-existing HSE from the mantle. It is unknown how much metal-silicate equilibration would be needed to strip the mantle of HSE, or how this level of equilibration would affect W in the mantle. Fluid dynamics experiments and models of metal blobs falling through a magma ocean predict that metallic cores of small impacting bodies efficiently equilibrate with molten silicate, because of fragmentation of the metal cores into droplets small enough to allow rapid metal-silicate mass transfer²⁸ and turbulent mixing²⁹. The nature of merging cores during much larger impact events, particularly with regard to isotopic and elemental equilibration of siderophile elements, is less well understood. It is even possible that elemental metal-silicate equilibration for siderophile elements occurs more rapidly than isotopic equilibration, as has been observed for the HSE Os³⁰. Thus, it remains unknown how much the metal from the core of the impactor modified the W isotopic composition of the terrestrial mantle en route to the terrestrial core. Knowledge of the W isotopic composition of the terrestrial mantle prior to the giant impact is needed to accurately estimate the average core-mantle differentiation age of the Earth. Better experimental data on the rates of elemental versus isotopic metal-silicate equilibration will be needed for W in order to tighten existing constraints on the timing of primary Earth differentiation.

Methods

Samples 68115 and 68815 are glassy polymict breccias composed of a variety of impact melts, as well as relict aluminous plagioclase clasts. A 3.2 g sample split (68115,114), a 2.7 g sample split (68815,394) and a 3 g sample split (68815,396) appeared visually free of large clasts, and were selected for study. Each sample was crushed in an agate mortar and separated into several size fractions using nylon sieves. Magnetic fractions were separated using a hand-magnet, and further purified by repeated grinding, magnetic separation and ultrasonication in high purity water. 100 mg, ~40 mg, and ~20 mg of separated, high purity metal, were obtained for 68115, 114, 68815,394, and 68815,396, respectively.

After metal dissolution in 6M HCl, a ~0.5% aliquot of each sample was spiked for determination of Hf and W concentrations. Spike-sample mixtures were equilibrated in 7 mL screw-cap Teflon vials at 130°C for 2 days, then the solutions were dried down. Residues were then re-dissolved in 2ml of a HCl (0.5M)-HF (0.5M) mixture, and W and Hf were then purified using a previously established anion exchange chromatography technique³³.

Tungsten was separated from the remaining sample solution (99.5%) using anion exchange chromatography for determination of isotopic composition. After evaporation, residues were digested twice in concentrated HNO₃, with traces of H₂O₂, over ~24 hours at 120°C, and dried down. Residues were then converted into the chloride form by repeated dissolutions with 6M HCl and subsequent evaporations. The samples were finally dissolved in 2ml of a HCl (0.5M)-HF (0.5M) mixture and then purified using the 3-step anion exchange chromatography described in Touboul and Walker⁶. Approximately 2.5, 0.7 and 0.5 μg of W were ultimately harvested from the metal fractions of 68115,114, 68815,394 and 68815,396, respectively, with corresponding procedural yields of ~85%. These quantities of W allowed us to make three independent high-precision W isotope measurements for 68115,114, one for 68815,394 and one for 68815,396. The total procedural W blank was 1±0.5 ng, and was negligible. Tungsten isotope compositions were measured to <5 ppm precision level using a Thermo Triton TIMS at UMd, following our published analytical procedure⁹. A similar level of reproducibility for W separated from cosmochemical metals is demonstrated through duplicate and triplicate analyses similar quantities of W extracted from iron meteorites as reported in Kruijer et al.¹⁶ (Table S5).

For determining Os isotopic compositions and HSE concentrations, 1.1 mg and 0.61 mg of metal from 68115 and 68815.396 in the form of HCl solution, respectively, and 1.1 mg of a mixture of silicate and metal separate from 68815.394, 5 mL of triple-distilled, concentrated HNO₃, 4 mL of triple-distilled, concentrated HCl, and appropriate amounts of mixed ¹⁸⁵Re-¹⁹⁰Os and HSE (⁹⁹Ru, ¹⁰⁵Pd, ¹⁹¹Ir, ¹⁹⁴Pt) spikes were sealed in double-cleaned, chilled 25 mL Pyrex borosilicate Carius tubes and heated to 270°C for at least 96 h. Osmium was extracted from the acid solution by CCl₄ solvent extraction³⁴, then back-extracted into HBr, followed by purification using microdistillation³⁵. Iridium, Ru, Pt, Pd, and Re were separated and purified using anion exchange chromatography.

The total analytical blanks in picograms were as follows: Re, 0.16; Os, 0.41; Ir, 0.31; Ru, 4.5; Pt, 95; Pd, 5.3. The abundances of Re and the PGE, and the ¹⁸⁷Os/¹⁸⁸Os ratios were corrected using the values for the blank measured with this set of samples.

Osmium isotopic measurements were accomplished by negative thermal ionization mass-spectrometry (NTIMS³⁶). All samples were analyzed using a secondary electron multiplier (SEM) detector of a Thermo Fisher Triton mass spectrometer at the Isotope Geochemistry Laboratory (IGL), University of Maryland. The measured isotopic ratios were corrected for mass fractionation using ¹⁹²Os/¹⁸⁸Os = 3.083. The internal precision of measured ¹⁸⁷Os/¹⁸⁸Os for all samples was better than 0.1% relative. The ¹⁸⁷Os/¹⁸⁸Os of 300–500 pg loads of the in-house Johnson-Matthey Os standard measured during the period of the analytical campaign averaged 0.11376±10 (2σ_stdev, N = 64). This value characterizes the external precision of the isotopic analysis (0.1%), which we use to calculate the true uncertainty on the measured ¹⁸⁷Os/¹⁸⁸Os ratio for each individual sample. The measured ¹⁸⁷Os/¹⁸⁸Os ratios further were also corrected for the instrumental bias relative to the average ¹⁸⁷Os/¹⁸⁸Os = 0.11379 measured for the Johnson-Matthey Os standard on the Faraday cups of the IGL Triton. The correction factor of 1.00026 was calculated by dividing this value by the average ¹⁸⁷Os/¹⁸⁸Os measured in the Johnson-Matthey Os standard on the SEM of the same instrument.

The measurements of Ru, Pd, Re, Ir, and Pt were performed at the IGL by inductively coupled plasma mass-spectrometry (ICP-MS) using a Nu Plasma instrument with a triple electron multiplier configuration in a static mode. Isotopic mass fractionation was monitored and corrected for by interspersing samples and standards. The accuracy of the data was assessed by comparing the results for the reference materials UB-N and GP-13 obtained during the ongoing analytical campaign³⁷ with the results from other laboratories. Concentrations of all HSE and Os isotopic compositions obtained at the IGL are in good agreement with the other laboratories. Diluted spiked aliquots of iron meteorites were run during each analytical session as secondary standards. The results from these runs agreed within 0.5% for Re and Ir, and within 2% for Ru, Pt, and Pd, with fractionation-corrected values obtained from measurements of undiluted iron meteorites using Faraday cups of the same instrument with a signal of >100 mV for the minor isotopes. Depending on the total amount of HSE aliquant processed for each sample, the uncertainties on the HSE concentrations varied between 0.5 and 2% for Re, 0.1 and 0.6% for Os, 4 and 50% for Pt, 2 and 3% for Pd, and were 0.5% for Ir, and 2% for Ru.