Terceiro artigo de uma série de 3 novos estudos publicados na revista Nature de 9 de Abril de 2015 sobre as novas teorias da formação da Lua.

1. LETTER doi:10.1038/nature14355
Tungsten isotopic evidence for disproportional late
accretion to the Earth and Moon
Mathieu Touboul1{
, Igor S. Puchtel1
& Richard J. Walker1
Characterization of the hafnium–tungsten systematics (182
Hf
decaying to 182
W 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 observa-
tions were interpreted to reflect formation of the Moon and crys-
tallization of the lunar magma ocean after 182
Hf 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 182
W excess of 20.6 6 5.1 parts per million
(62 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 dispropor-
tional 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.
Early applications of Hf–W isotopic system to lunar rocks were
hampered by the effects of cosmic rays on 182
W, particularly due to
production of 182
W from 181
Ta via neutron capture5
. Consequently,
more-recent studies have focused on the analysis of Ta-free metals
extracted fromlunar basalts and impact-melt rocks1,6
. Here, wepresent
W isotopic data for metals separated from two KREEP-rich Apollo 16
impact-melt rocks, obtained using new high-precision analytical
methods7
(KREEP indicates rocks rich in potassium (K), rare-earth
elements (REE) and phosphorus (P)). In addition to W isotopic com-
positions and abundances, we determined abundances of Hf and the
highly siderophile elements (HSE), as well as 187
Os/188
Os ratios, in
order to assess the Hf/W ratio 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 about 3.9 Gyr ago8
. 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 probably partially or wholly equilibrated with
melt or vapour during the impact9
.
Metal separates from impact-melt rocks 68115,114, 68815,394 and
68815,396 (see Methods for nomenclature) have m182
W values (where
m182
W is the deviation in p.p.m. of the 182
W/184
W ratio of the sample
from that of the modern terrestrial mantle) of 123.3 6 3.8 (n 5 3,
2 s.d.), 118.1 6 2.5 and 120.4 6 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.
The positive W isotopic offset between the Moon and the 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 182
W in a high-Hf/W domain within the mantle of the Moon or of
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 of the
isotopic offset. The Ta/Hf ratio of ,0.11 for KREEP10
, 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 samples11
, a maximum effect of
approximately 20.1 p.p.m. 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 abun-
dances are highly enriched in chemically evolved, KREEP-rich rocks12
,
from which these impact-melt rocks were derived. Further, based on
HSE abundances, basin-forming impactors with chondritic bulk com-
positions contributed no more than ,2–3% of their mass to impact-
melt rocks13
. Thus, if the concentrations of W in the Apollo 16 target
rocks and the impactor are assumed to have been ,1,500 p.p.b.,typical
of KREEP-rich basalts12
, and,180p.p.b.,typical of chondritic meteor-
ites14
, 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 m182
W value
of 2200, the indigenous lunar W isotopic composition would have
been lowered by only ,0.8 p.p.m. (Extended Data Table 1).
The separated metals from 68815 are characterized by supra-
chondritic 187
Os/188
Os, Ru/Ir, Pt/Ir and Pd/Ir ratios (Table 2; Fig. 2),
features that are common in chemically evolved iron meteorites15
.
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 rocks16
.
Bushman Land is rich in siderophile elements, with 1,200 p.p.b. Ir and
460 p.p.b. W (ref. 15). 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 p.p.b. present in most
Apollo 16 impact-melt rocks16
. Incorporation of 1 wt% of an iron
1
Isotope Geochemistry Laboratory, Department of Geology, University of Maryland, College Park, Maryland 20742, USA. {Present address: Laboratoire de Ge´ologie de Lyon, Ecole Normale Supe´rieure de
Lyon, Labex LIO, Universite´ Lyon 1, 46 Alle´e d’Italie 69364 Lyon Cedex 7, France.
G2015 Macmillan Publishers Limited. All rights reserved
0 0 M O N T H 2 0 1 5 | V O L 0 0 0 | N A T U R E | 1

2. meteorite with the same W concentration and with m182
W 5 2330,
comparable to IVA irons17
, would have lowered the W isotopic com-
positions of the impact-melt rocks by only ,1 p.p.m., relative to the
indigenous lunar signature (Extended Data Table 1).
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 differentabsolute and relative abundances of HSE, as well
as 187
Os/188
Os, yet their W isotopic compositions are identical within
uncertainties. The differences in HSE are likely to reflect the incorp-
oration 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 rocks18
. 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 were 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 impac-
tors were minor, and that the average m182
W value of 120.6 6 5.1
(2 s.d.) 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 182
Hf 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 probably increased slightly as a result of lunar core formation
and extraction of anunknownproportion of the siderophileW into the
core, recent studies have concluded that the silicate portions of the
Earth and Moon had nearly identical Hf/W (see, for example, ref. 19).
Consequently, if the Moon formed while 182
Hf 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.
In 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 incompatible nature of W in silicate systems, compared with Hf
(ref. 20). Crystal–liquid fractionation would, therefore, have led to the
creation of 182
W-enriched and 182
W-depleted domains in the mantle,
compared to the 182
W of the bulk lunar mantle, if LMO crystallization
was rapid while 182
Hf was extant. The comparatively large amount of
W needed to make sufficiently high-precision measurements, coupled
with sample mass limitations for the 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 182
W-enriched nature of the metals
examined here. First, the coupled 146,147
Sm–142,143
Nd systematics of
crustal rocks derived from the lunar mantle indicate that late stages of
LMO crystallization occurred more than 100Myr after Solar System
formation21
, well after 182
Hf was extinct. Second, regardless of the
timing of LMO crystallization, the mantle source of KREEP was likely
to have been a low-Hf/W reservoir, given the W-enriched nature of
KREEP. Thus, if the KREEP mantle source rapidly formed during the
lifetime of 182
Hf, it would have developed a 182
W deficit relative to the
Earth–Moon system, rather than the observed enrichment (Extended
Data Fig. 1), assuming that the mantles of both bodies were in isotopic
equilibrium at the time of the Moon’s formation. We conclude that
µ182W
–10 0 10 20 30 40 50
68815,396
68115,114
Average 68115,114
68815,394
Figure 1 | Values of m182
W 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 m182
W 5 120.6 6 5.1 (2 s.d., n 5 3) of the
three metal separates from Apollo16 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 m182
W 5 0 corresponds to the
2 standard errors (s.e.) uncertainty for repeated analyses of the Alfa Aesar W
standard.
Table 2 | Highly siderophile element contents and Os isotopic compositions of lunar metals
Samples Re
(p.p.b.)
Os
(p.p.b.)
Ir
(p.p.b.)
Ru
(p.p.b.)
Pt
(p.p.b.)
Pd
(p.p.b.)
187
Os/188
Os 62smean
68115,113 metal 0.13825 0.00007
Replicate 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
2smean corresponds to 2 standard errors of an individual 187
Os/188
Os measurement.
Table 1 | Tungsten isotopic compositions and W and Hf abundances
of lunar metals
Samples W
(p.p.m.)
Hf
(p.p.m.)
m182
W
68115,114 metal 32.7 6 0.3 2.23 6 0.05 125.3 6 4.6
121.5 6 2.6
123.0 6 1.7
68115,114 metal average (62 s.d.) 123.3 6 3.8
68815,394 metal 22.8 6 0.3 1.41 6 0.02 118.1 6 2.5
68815,396 metal 36.3 6 0.5 0.27 6 0.01 120.4 6 2.9
Bulk lunar mantle (n 5 3, 62 s.d.) 120.6 6 5.1
RESEARCH LETTER
G2015 Macmillan Publishers Limited. All rights reserved
2 | N A T U R E | V O L 0 0 0 | 0 0 M O N T H 2 0 1 5

3. the average W isotopic composition of the metals from the Apollo 16
samples 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 established22,23
, and has led to
multiple hypotheses, including the possibility that the giant impact
that created the Moon involved an impactor composed of genetically
similar materials (that is, similar in terms of nebular components) to
the Earth22
, that the Moon was constructed mainly from terrestrial
materials rather than the impactor24
, or that the impact and coales-
cence processes somehow led to thorough isotopic mixing between the
two bodies25
. The recent report of a small offset in the D17
O composi-
tion of the Moon, compared to the Earth23
, leaves open the possibility
that there were small differences in the isotopic compositions of other
elements in addition to O. Thus, it is possible that the W isotopic
difference between the Earth and the Moon is a result of the latter
forming from a mixture of terrestrial and impactor W, assuming that
there was no isotopic equilibration following metal segregation.
Mixing of W between the Earth’s mantle and the impactor mantle,
together with varying proportions of impactor metal, could have raised
the m182
W of the Earth’s mantle by as much as ,100 p.p.m., if there
was negligible equilibration between W from the core of the impactor
and the Earth’s mantle26
; or it could have lowered it by as much as
,200 p.p.m., if both the core and mantle of the impactor equilibrated
with the Earth’s mantle27
. Thus, even if the mantles of the Earth and the
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 possible W isotopic compositions, well
beyond the observed offset.
The most parsimonious explanation for the W isotopic offset
between the Earth and the 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 ceases28
. Late accretion would have led to
decreases in the m182
W values of the mantles of Earth and Moon,
because materials with chondritic bulk properties have much higher
W concentrations and strongly negative m182
W values, compared to
the terrestrial and lunar mantles. The observed W isotopic offset is
consistent with estimates that late accretion added 0.3–0.8 wt% to the
mass of the Earth, and ,0.05 wt% to the mass of the Moon3,4
. The
addition of proportionally much greater mass to the Earth, relative
to the Moon, has been explained as a consequence of stochastic
processes29
. If these estimates for late accretion are accurate, then the
m182
W value of the Earth’s mantle, before late accretion, was
10–30 p.p.m. higher than at present, compared with only 1–3 p.p.m.
higher for the lunar mantle (Extended Data Fig. 2). 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.
In addition to the requirement that the silicate portions of the Earth
and the Moon were the same at the time of the formation of the Moon,
an interpretation of disproportional late accretion requires the late
accretionary clocks for both the Moon and the Earth to have initiated
at the same time. Thus, if significant late accretion to the Earth
occurred before 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 experi-
ments and models of metal blobs falling through a magma ocean
predict that metallic cores of small impacting bodies efficiently equi-
librate with molten silicate,because of fragmentation of the metal cores
into droplets small enough to allow rapid metal–silicate masstransfer30
and turbulent mixing31
. 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 ele-
ments occurs more rapidly than isotopic equilibration, as has been
observed for the HSE Os (ref. 32). 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
before the giant impact is needed to estimate accurately 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.
Online Content Methods, along with any additional Extended Data display items
andSource Data, are available in the onlineversion of the paper; references unique
to these sections appear only in the online paper.
Received 20 August 2014; accepted 24 February 2015.
Published online 8 April 2015.
1. Touboul, M., Kleine, T., Bourdon, B., Palme, H. & Wieler, R. Late formation and
prolonged differentiation of the Moon inferred from W isotopes in lunar metals.
Nature 450, 1206–1209 (2007).
2. Touboul, M., Kleine, T., Bourdon, B., Palme, H. & Wieler, R. Tungsten isotopes in
ferroan anorthosites: implications for the age of the Moon and the lifetime of its
magma ocean. Icarus 199, 245–249 (2009).
3. Day,J.M.D., Pearson, D. G.& Taylor,L.A. Highlysiderophile elementconstraintson
accretion and differentiation of the Earth-Moon system. Science 315, 217–219
(2007).
4. Walker, R. J. Highly siderophile elements in the Earth, Moon and Mars: update and
implications for planetary accretion and differentiation. Chem. Erde 69, 101–125
(2009).
5. Leya, I., Wieler, R. & Halliday, A. N. Cosmic-ray production of tungsten isotopes in
lunar samples and meteorites and its implications for Hf–W cosmochemistry.
Earth Planet. Sci. Lett. 175, 1–12 (2000).
6. Kleine, T., Palme, H., Mezger, K. & Halliday, A. N. Hf–W chronometry of lunar metals
and the age and early differentiation of the Moon. Science 310, 1671–1674
(2005).
7. Touboul, M. & Walker, R. J. High precision measurement of tungsten isotopes by
thermal ionization mass spectrometry. Int. J. Mass Spectrom. 309, 109–117
(2012).
8. Tera, F., Papanastassiou, D. A. & Wasserburg, G. J. Isotopic evidence for a terminal
lunar cataclysm. Earth Planet. Sci. Lett. 22, 1–21 (1974).
9. Misra, K. C. & Taylor,L. A. Characteristicsof metal particles in Apollo 16 rocks.Proc.
Lunar Sci. Conf. 6, 615–639 (1975).
10. Mu¨nker, C. et al. Evolution of planetary cores andthe Earth-Moon system from Nb/
Ta systematics. Science 301, 84–87 (2003).
11. Drozd, R. J., Hohenberg, C. M., Morgan, C. J. & Ralston, C. E. Cosmic-ray exposure
history at the Apollo 16 and other lunar sites: lunar surface dynamics. Geochim.
Cosmochim. Acta 38, 1625–1642 (1974).
12. Lee, D. C., Halliday, A. N., Snyder, G. A. & Taylor, L. A. Age and origin of the Moon.
Science 278, 1098–1103 (1997).
HSEabundancesnormalizedtoIrandCI
1
10
IVA irons
60315
67935
68815,396
Re Os Ir Ru Pt Pd
68815,394
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.)
LETTER RESEARCH
G2015 Macmillan Publishers Limited. All rights reserved
0 0 M O N T H 2 0 1 5 | V O L 0 0 0 | N A T U R E | 3

4. 13. Norman, M. D., Bennett, V. C. & Ryder, G. Targeting the impactors: siderophile
element signatures of lunar impact melts from Serenitatis. Earth Planet. Sci. Lett.
202, 217–228 (2002).
14. Kleine, T., Mezger, K., Mu¨nker, C., Palme, H. & Bischoff, A. 182
Hf-182
W isotope
systematics of chondrites, eucrites, and Martian meteorites: chronology of core
formation and mantle differentiation in Vesta and Mars. Geochim. Cosmochim.
Acta 68, 2935–2946 (2004).
15. McCoy, T. J. et al. Group IVA irons: new constraints on the crystallization and
cooling history of an asteroidal core with a complex history. Geochim. Cosmochim.
Acta 75, 6821–6843 (2011).
16. Fischer-Go¨dde, M. & Becker, H. Osmium isotope and highly siderophile element
constraints on ages and nature of meteoritic components in ancient lunar impact
rocks. Geochim. Cosmochim. Acta 77, 135–156 (2012).
17. Kruijer, T. S. et al. Protracted core formation and rapid accretion of protoplanets.
Science 344, 1150–1154 (2014).
18. Sharp, M. et al. Characterization of the dominant impactor signature for Apollo 17
impact melt rocks. Geochim. Cosmochim. Acta 131, 62–80 (2014).
19. Ko¨nig, S. et al. The Earth’s tungsten budget during mantle melting and crust
formation. Geochim. Cosmochim. Acta 75, 2119–2136 (2011).
20. Righter, K. & Shearer, C. K. Magmatic fractionation of Hf and W: constraints on the
timing of core formation and differentiation in the Moon and Mars. Geochim.
Cosmochim. Acta 67, 2497–2507 (2003).
21. McLeod,C. L., Brandon, A. D. & Armytage, R. M.G. Constraintson the formation age
and evolution of the Moon from 142
Nd-143
Nd systematics of Apollo 12 basalts.
Earth Planet. Sci. Lett. 396, 179–189 (2014).
22. Dauphas, N., Burkhardt, C., Warren, P. H. & Teng, F.-Z. Geochemical arguments for
anEarth-likeMoon-formingimpactor.Phil.Trans.R.Soc.A372,20130244(2014).
23. Herwartz, D., Pack, A., Friedrichs, B. & Bischoff, A. Identification of the giant
impactor Theia in lunar rocks. Science 344, 1146–1150 (2014).
24. C´uk, M.& Stewart,S. T.MakingtheMoon froma fast-spinning Earth:a giantimpact
followed by resonant despinning. Science 338, 1047–1052 (2012).
25. Canup, R. M. Forming a Moon with an Earth-like composition via a giant impact.
Science 338, 1052–1055 (2012).
26. Halliday,A.N.Mixing, volatilelossandcompositional changeduringimpact-driven
accretion of the Earth. Nature 427, 505–509 (2004).
27. Halliday, A. N. A young Moon-forming giant impact at 70–110 million years
accompaniedbylate-stagemixing,coreformationanddegassingoftheEarth.Phil.
Trans. R. Soc. A 366, 4163–4181 (2008).
28. Chou, C.-L.Fractionation ofsiderophileelementsinthe Earth’suppermantle.Proc.
Lunar Planet. Sci. Conf. 9, 219–230 (1978).
29. Bottke, W. F., Walker, R. J., Day, J. M. D., Nesvorny, D. & Elkins-Tanton, L.
Stochastic late accretion to Earth, the Moon, and Mars. Science 330,
1527–1530 (2010).
30. Stevenson, D. J. in Origin of the Earth (eds Newsom, H. E. & Drake, J. H.) 231–249
(Oxford Univ. Press, 1990).
31. Deguen, R., Landeau, M. & Olson, P. Turbulent metal–silicate mixing,
fragmentation, and equilibration in magma oceans. Earth Planet. Sci. Lett. 391,
274–287 (2014).
32. Yokoyama, T., Walker, D. & Walker, R. J. Low osmium solubility in silicate at high
pressures and temperatures. Earth Planet. Sci. Lett. 279, 165–173 (2009).
33. Horan, M. F., Walker, R. J., Morgan, J. W., Grossman, J. N. & Rubin, A. E. Highly
siderophile elements in chondrites. Chem. Geol. 196, 27–42 (2003).
Acknowledgements This work was supported by NASA Cosmochemistry grant
NNX13AF83G. We thank the Lunar Sample Laboratory Facility at Johnson Space
Center for the provision of appropriate samples for this study.
Author 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.
Author Information The data presented here can be found in the EarthChem library
(http://www.earthchem.org/library/browse/view?id5849). Reprints and permissions
information is available at www.nature.com/reprints. The authors declare no
competing financial interests. Readers are welcome to comment on the online version
of the paper. Correspondence and requests for materials should be addressed to M.T.
(mathieu.touboul@ens-lyon.fr), I.S.P. (ipuchtel@umd.edu) or R.J.W.
(rjwalker@umd.edu).
RESEARCH LETTER
G2015 Macmillan Publishers Limited. All rights reserved
4 | N A T U R E | V O L 0 0 0 | 0 0 M O N T H 2 0 1 5

5. 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 puri-
fied by repeated grinding, magnetic separation and ultrasonication in high-purity
water. About 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 6 M HCl, a ,0.5% aliquot of each sample was spiked
for determination of Hf and W concentrations. Spike-sample mixtures were equi-
librated in 7 ml screw-cap Teflon vials at 130uC for 2 days, then the solutions were
evaporated to dryness. Residues were then re-dissolved in 2 ml of an HCl (0.5 M)–
HF (0.5 M) mixture, and W and Hf were then purified using a previously estab-
lished anion exchange chromatography technique1,2
.
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 h at 120uC, and evaporated to dryness. Residues were then
converted into the chloride form by repeated dissolution in 6 M HCl and sub-
sequent evaporation. The samples were finally dissolved in 2 ml of an HCl
(0.5 M)–HF (0.5 M) mixture and then purified using the three-step anion
exchange chromatography described in ref. 7. Approximately 2.5, 0.7 and 0.5 mg
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 6 0.5ng, and was negligible. Tungsten isotope
compositions were measured to ,5 p.p.m. precision level using a Thermo Triton
thermalionization mass spectrometerat the University ofMaryland, followingour
published analytical procedure7
. A similar level of reproducibility for W separated
from cosmochemical metals is demonstrated through duplicate and triplicate
analyses of similar quantities of W extracted from iron meteorites as reported in
supplementary table 5 in ref. 17.
For determining Os isotopic compositions and HSE concentrations, 1.1 mg and
0.61mg of metal in the form of HCl solution from 68115,114 and 68815,396,
respectively, and 1.1mg of a mixture of silicate and metal separate from
68815,394, together with 5ml of triple-distilled, concentrated HNO3, 4ml of
triple-distilled, concentrated HCl, and appropriate amounts of mixed 185
Re–190
Os
and HSE (99
Ru, 105
Pd, 191
Ir, 194
Pt) spikes, were sealed in double-cleaned, chilled
25ml Pyrex borosilicate Carius tubes and heated to 270 uC 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 the highly siderophile elements,
and the 187
Os/188
Os ratios, were corrected using the values for the blank measured
with this set of samples.
Osmium isotopic measurements were accomplished by negative thermal ion-
ization mass spectrometry (NTIMS36
). All samples were analysed using a second-
ary electron multiplier 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 192
Os/188
Os5 3.083.
The internal precision of measured 187
Os/188
Os for all samples was better than
0.1% relative. The 187
Os/188
Os of 300–500 pg loads of the in-house Johnson-
Matthey Os standard measured during the period of the analytical campaign
averaged 0.113766 10 (2sstdev, N 5 64). This value characterizes the external pre-
cision of the isotopic analysis (0.1%), which we use to calculate the true uncertainty
on the measured 187
Os/188
Os ratio for each individual sample. The measured
187
Os/188
Os ratios further were also corrected for the instrumental bias relative
to the average 187
Os/188
Os50.11379 measured for the Johnson-Matthey Osstand-
ard on the Faraday cups of the IGL Triton. The correction factor of 1.00026 was
calculated by dividing this value by the average 187
Os/188
Os measured in the
Johnson-Matthey Os standard on the secondary electron multiplier 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 frac-
tionation-corrected values obtained from measurements of undiluted iron
meteorites using Faraday cups of the same instrument with a signal of
.100mV 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.
Sample size. We had prior data for W concentrations in similar metals, and these
data were used to define sample size.
34. Cohen, A. S. & Waters, F. G. Separation of osmium from geological materials by
solvent extraction for analysis by thermal ionisation mass spectrometry. Anal.
Chim. Acta 332, 269–275 (1996).
35. Birck, J. L., Roy-Barman, M. & Capman, F. Re-Os isotopic measurements at the
femtomole level in natural samples. Geostand. Newsl. 21, 19–27 (1997).
36. Creaser, R. A., Papanastassiou, D. A. & Wasserburg, G. J. Negative thermal ion
spectrometry of osmium, rhenium, and iridium. Geochim. Cosmochim. Acta 55,
397–401 (1991).
37. Puchtel, I. S., Walker, R. J., Touboul, M., Nisbet, E. G. & Byerly, G. R. Insights into
earlyEarthfromthe Pt-Re-Osisotopeandhighlysiderophileelementabundance
systematics of Barberton komatiites. Geochim. Cosmochim. Acta 125, 394–413
(2014).
38. Arevalo, R. & McDonough, W. F. Tungsten geochemistry and implications for
understanding the Earth’s interior. Earth Planet. Sci. Lett. 272, 656–665 (2008).
LETTER RESEARCH
G2015 Macmillan Publishers Limited. All rights reserved

6. Extended Data Figure 1 | Plot of m182
W versus source 180
Hf/184
W ratio.
182
W 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 theLMO6,20
, resulting
in high Hf/W in the source of high-Ti mare basalts (.40), low Hf/W in KREEP
(10 6 10) and intermediate Hf/W (26.5 6 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.
RESEARCH LETTER
G2015 Macmillan Publishers Limited. All rights reserved

7. Extended Data Figure 2 | Plot of m182
W 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 m182
W of 110 to 130 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 contentof thelunar 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.
LETTER RESEARCH
G2015 Macmillan Publishers Limited. All rights reserved

8. Extended Data Table 1 | Tungsten contents and isotopic compositions of mixing components
Component A corresponds to a typical KREEP-rich sample. The basin-forming impactor (component B) has either a chondritic composition or is composed of IVA iron meteorite material similar to the Bushman
Land meteorite15
as proposed in ref. 16. The calculated W isotopic compositions of the mixture demonstrate that the m182
W of Apollo 16 impact melts was not measurably affected by a contribution of W from
basin-forming impactor(s).
RESEARCH LETTER
G2015 Macmillan Publishers Limited. All rights reserved