Hfв€“W chronometry of core formation in planetesimals inferred from ...

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Geochimica et Cosmochimica Acta 99 (2012) 287–304

Hf–W chronometry of core formation in planetesimalsinferred from weakly irradiated iron meteorites

Thomas S. Kruijer a,b,⇑, Peter Sprung a,b, Thorsten Kleine b, Ingo Leya c,Christoph Burkhardt a, Rainer Wieler a

a ETH Zurich, Institute of Geochemistry and Petrology, Clausiusstrasse 25, 8092 Zurich, Switzerlandb Institut fur Planetologie, Westfalische Wilhelms-Universitat Munster, Wilhelm-Klemm-Strasse 10, 48149 Munster, Germany

c Space Research and Planetary Sciences, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland

Received 16 January 2012; accepted in revised form 5 September 2012; available online 25 September 2012

Abstract

The application of Hf–W chronometry to determine the timescales of core formation in the parent bodies of magmatic ironmeteorites is severely hampered by 182W burnout during cosmic ray exposure of the parent meteoroids. Currently, no directmethod exists to correct for the effects of 182W burnout, making the Hf–W ages for iron meteorites uncertain. Here we presentnoble gas and Hf–W isotope systematics of iron meteorite samples whose W isotopic compositions remained essentially unaf-fected by cosmic ray interactions. Most selected samples have concentrations of cosmogenic noble gases at or near the low-ermost level observed in iron meteorites and, for iron meteorite standards, have very low noble gas and radionuclide basedcosmic ray exposure ages (<60 Ma). In contrast to previous studies, no corrections of measured W isotope compositions arerequired for these iron meteorite samples. Their e182W values (parts per 104 deviations from the terrestrial value) are higherthan those measured for most other iron meteorites and range from �3.42 to �3.31, slightly elevated compared to the initial182W/184W of Ca–Al-rich Inclusions (CAI; e182W = �3.51 ± 0.10). The new W isotopic data indicate that core formation inthe parent bodies of the IIAB, IIIAB, and IVA iron meteorites occurred �1–1.5 Myr after CAI formation (with an uncer-tainty of �1 Myr), consistent with earlier conclusions that the accretion and differentiation of iron meteorite parent bodiespredated the accretion of most chondrite parent bodies. One ungrouped iron meteorite (Chinga) exhibits small nucleosyn-thetic W isotope anomalies, but after correction for these anomalies its e182W value agrees with those of the other samples.Another ungrouped iron (Mbosi), however, has elevated e182W relative to the other investigated irons, indicating metal–sil-icate separation �2–3 Myr later than in the parent bodies of the three major iron meteorite groups studied here.� 2012 Elsevier Ltd. All rights reserved.

1. INTRODUCTION

Magmatic iron meteorites are interpreted as fragmentsfrom the metal cores of small planetary bodies (Scott,1972; Scott and Wasson, 1975) that had formed early in so-lar system history (e.g., Chen and Wasserburg, 1990; Smo-liar et al., 1996; Kleine et al., 2005a). Obtaining a precise

0016-7037/$ - see front matter � 2012 Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.gca.2012.09.015

⇑ Corresponding author at: ETH Zurich, Institute of Geochem-istry and Petrology, Clausiusstrasse 25, 8092 Zurich, Switzerland.Tel.: +41 44 63 26423, +49 251 83 39708.

E-mail address: [email protected] (T.S. Kruijer).

chronology of iron meteorites, therefore, is of fundamentalimportance for constraining the early history of the solarsystem, and the accretion and differentiation history ofsome of the earliest planetesimals. The short-lived182Hf–182W system has proven uniquely useful for deter-mining the timescales of core formation in planetary bodies,because both Hf and W are refractory and exhibit very dif-ferent geochemical behaviour during metal–silicate separa-tion (e.g., Lee and Halliday, 1995; Harper and Jacobsen,1996; Kleine et al., 2009). The first comprehensive Hf–Wstudy on iron meteorites was performed by Horan et al.(1998), showing that the different iron meteorite parentbodies underwent core formation within �5 Myr of each


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288 T.S. Kruijer et al. / Geochimica et Cosmochimica Acta 99 (2012) 287–304

other. Later studies demonstrated that magmatic iron mete-orites derive from planetesimals that segregated their coresvery early (e.g., Kleine et al., 2005a; Schersten et al., 2006;Markowski et al., 2006b; Qin et al., 2008b), at about thesame time as Ca–Al-rich inclusions (CAI), the oldest yet da-ted objects that formed in the solar system (Gray et al.,1973; Amelin et al., 2010; Bouvier and Wadhwa, 2010).The chronological interpretation of the Hf–W data for ironmeteorites is severely complicated, however, by 182W burn-out due to capture of secondary thermal and epithermalneutrons produced during cosmic ray exposure of the ironmeteoroids (Masarik, 1997; Leya et al., 2000, 2003). Themost obvious manifestation of these neutron capture reac-tions is the decrease of 182W/184W ratios of iron meteoriteswith increasing cosmic ray exposure age, leading to spuri-ously old Hf–W model ages. Neutron capture on W iso-topes may be responsible for generating 182W/184W ratioslower than the initial W isotope composition of CAI (Burk-hardt et al., 2008, 2012) and may also be the sole cause of182W variations observed within individual iron meteoritegroups or even within a single iron meteorite. The reliableinterpretation of W isotopic data for iron meteorites interms of core formation timescales, therefore, requires thequantification of cosmic ray-induced shifts on W isotopecompositions.

The interaction of iron meteoroids with cosmic rays didnot only cause neutron capture reactions on W isotopes butalso led to the production of cosmogenic noble gases. Forthis reason, a number of studies focused on cosmogenic no-ble gases and published cosmic ray exposure ages to correctmeasured W isotope compositions for cosmic ray-inducedshifts (Kleine et al., 2005a; Schersten et al., 2006; Markow-ski et al., 2006a; Qin et al., 2008b). For instance, Markow-ski et al. (2006a) used 3He abundances in conjunction withindependently determined exposure ages to correct mea-sured W isotope compositions in Carbo (IID) and Grant(IIIAB). However, the corrected 182W/184W of these twoiron meteorites still are lower than the initial W isotopecomposition of CAI (Burkhardt et al., 2008, 2012). At facevalue this would indicate core formation of the respectiveparent bodies to predate CAI formation. However, morelikely this strongly suggests that the correction procedureemploying 3He did not fully account for cosmic ray-inducedW isotope shifts. Qin et al. (2008b) estimated lower andupper bounds of 182W/184W ratios for several groups ofmagmatic iron meteorites and modelled a maximum ex-pected cosmogenic W isotope effect for a given exposureage. For all iron meteorite groups the upper bounds on182W/184W were found to be higher than the presently ac-cepted initial 182W/184W of CAI (Burkhardt et al., 2012),solving the problem that some iron meteorites have mea-sured 182W/184W below the CAI initial. However, the differ-ence between the lower and upper bounds of 182W/184Wratios for individual iron meteorite groups remaining afterthis correction corresponds to apparent 2–5 Myr intervalsof metal segregation (e.g., for IC, IIAB, IID, IIIF, IVA ir-ons), reflecting the inherent uncertainty of current correc-tion procedures employing noble gas systematics.

The major problem when using cosmogenic noble gasesand/or exposure ages to correct cosmic ray-induced shifts in

182W/184W is that cosmogenic noble gas production ratesreach their maximum at a shallower depth than that corre-sponding to the maximum fluence of thermal and epither-mal secondary neutrons. Cosmogenic noble gases are thusnot a perfect proxy for the fluence of slow neutrons.Obtaining a precise Hf–W chronometry of iron meteoritesthus requires the development of a direct neutron dosimeterfor iron meteorites, or the identification of specimens thatremained largely unaffected by (epi)thermal neutrons.While recent studies have shown that Os (Walker andTouboul, 2012; Wittig et al., 2012) and Pt isotopes (Kruijeret al., 2012) may be suitable neutron dosimeters for ironmeteorites, we here will focus on identifying iron meteoritespecimens with minor to absent (epi)thermal neutronfluences. Such samples do not require any correction onmeasured W isotope ratios and thus present key samplesfor establishing a precise Hf–W chronology of ironmeteorites.

Although cosmogenic noble gases do not allow a preciseand accurate correction for cosmic ray-induced neutroncapture effects on W isotopes, they do provide very usefulconstraints on the cosmic ray exposure history of meteor-ites. In some cases noble gas data allow recognising meteor-ites with a very low exposure age (for iron meteoritestandards). Such meteorites will have experienced a verylow (epi)thermal neutron fluence irrespective of the sampleposition within the meteoroid. Therefore, in this contribu-tion we use concentrations of cosmogenic He, Ne, and Arin samples of magmatic iron meteorites from differentgroups as major selection criteria to identify specimens withW isotopic compositions largely unaffected by cosmic rayeffects. New precise W isotope measurements on these sam-ples that define 182W/184W values devoid of cosmic ray ef-fects are then used to obtain improved constraints on thetiming and duration of core formation in iron meteoriteparent bodies.

2. THEORY AND APPROACH

Interactions of highly energetic (primary) cosmic rayparticles (�0.1–�10 GeV) with target atoms in meteoroidsgenerate a cascade of nuclear reactions. Among other prod-ucts this yields secondary high-energy protons and neutronsin the energy range of a few to a few hundred MeV. The pri-mary and secondary high-energy particles produce cosmo-genic nuclides, mostly by spallation-type reactions.Important for this study is the production of cosmogenicnoble gas isotopes, whose concentrations are a measurefor the fluence of medium- to high-energy particles in thesample. At the same time, the secondary cosmic ray parti-cles are slowed down, either due to electronic stopping inthe case of secondary protons or by elastic scattering inthe case of secondary neutrons. The latter are thus moder-ated to epithermal (�0.025 eV to a few keV) and eventuallyto thermal (�0.025 eV at 293 K) energies with increasingdistance from the meteoroid surface (Lingenfelter et al.,1972; Leya et al., 2000). Tungsten isotopes are predomi-nantly affected by neutron capture reactions at epithermalenergies (Masarik, 1997; Leya et al., 2000, 2003), andpossess different abilities to capture epithermal neutrons,

T.S. Kruijer et al. / Geochimica et Cosmochimica Acta 99 (2012) 287–304 289

expressed as distinct resonance integrals (e.g., at T = 300 K:182W �600 barns, 183W �355 barns, 184W �16 barns, 186W�520 barns; ENDFB-VI.8 300K library, 0.5 eV to1 � 105 eV). The maximum fluence of thermal and epither-mal neutrons occurs at larger depth than the maximum pro-duction of noble gases. Therefore, cosmogenic noble gasesare not a direct measure for neutron capture-induced shiftson W isotopes.

As the majority of iron meteorites have been exposed tocosmic rays for longer than 100 Myr (and up to 2 Gyr) (e.g.,Voshage, 1978, 1984; Wieler, 2002; Eugster, 2003), the ex-pected cosmic ray effects on W isotopes will be more pro-nounced than in stony meteorites (e.g., Leya et al., 2000,2003). Moreover, the iron-dominated matrix promotes neu-tron energy spectra that are biased towards epithermalenergies (Kollar et al., 2006; Sprung et al., 2010) at whichW isotopes are most susceptible to neutron capture. Instony meteorites and lunar rocks W isotope ratios are pre-dominantly affected by the neutron capture reaction181Ta(n,c)182Ta(b�)182W, resulting in elevated 182W/184Wratios (e.g., Leya et al., 2000, 2003). This reaction is respon-sible for generating large 182W excesses in lunar rocks (Leeet al., 2002; Kleine et al., 2005b). Since Ta is not present inFe–Ni metal, neutron capture reactions that have W iso-topes as their target (e.g., 182W(n,c)183W, 183W(n,c)184W,184W(n,c)185W(b�)185Re, 186W(n,c)187W(b�)187Re) domi-nate in iron meteorites (Masarik, 1997; Leya et al., 2000).Neutron capture reactions in iron meteorites induce a neg-ative shift in 182W/184W and a positive shift in 183W/184W.The correction for instrumental mass discrimination (usingeither 186W/183W or 186W/184W), however, tends to largelycancel out the effect on 183W/184W ratios, while magnifyingthe effect on 182W/184W. Internal corrections for cosmicray-induced neutron capture effects (i.e., using W isotopesonly) are thus not possible.

Given the difficulty in quantifying cosmic ray-inducedneutron capture effects on W isotopes, the pre-exposure182W/184W ratio of magmatic iron meteorites would ideallybe determined on samples largely unaffected by such neu-tron capture reactions. Cosmogenic noble gases – althoughnot suitable for correcting neutron capture effects on W iso-topes – are useful to identify such samples. First, in somecases the combination of a noble gas and a radionuclide al-lows the determination of the production rate of the noblegas nuclide at the sample position and hence a reliableshielding-corrected exposure age of the meteorite. Two ofthe meteorites in this study (Braunau, IIAB, and Gibeon,IVA) were selected according to their exposure ages beingbelow 60 Ma as determined by previous workers with thisapproach (e.g., Cobb, 1966; Chang and Wanke, 1969; Hon-da et al., 2009). As demonstrated in Section 5, such sampleswill have very small if any neutron capture effects on theirW isotopic composition. A third meteorite studied here(Cape York, IIIAB) has a slightly higher exposure age ofabout 90 Ma (Mathew and Marti, 2009), but we will dem-onstrate that the W isotope compositions of the Cape Yorksamples investigated here also remained essentially unaf-fected by cosmic ray effects. Overall, iron meteorites withshielding-corrected very low exposure ages are rare,however.

Second, noble gases in iron meteorites provide informa-tion about meteoroid size and sampling depth, as ratios like4He/21Ne or 3He/4He depend on these parameters (e.g.,Signer and Nier, 1960). In the present study, such parame-ters must be used with caution, however, because the verylow noble gas concentrations (sometimes close to blank lev-els) often lead to large analytical uncertainties. Neverthe-less, for most of the specimens studied here, the He, Ne,and Ar concentration and isotopic composition analysespermit semi-quantitative estimates on the pre-atmosphericsize, sample location in the meteoroid and maximum possi-ble cosmic ray exposure age.

3. ANALYTICAL METHODS

3.1. Sample selection

Magmatic iron meteorites whose reported cosmogenicnoble gas concentrations are consistently at the lower endof the range observed in iron meteorites (Schultz andFranke, 2004) were selected for this work. The samplesinvestigated here include iron meteorite specimens fromfour different magmatic groups (IIAB, IIIAB, IVA, IIG)and two ungrouped iron meteorites (Tables 1 and 3). Noblegas concentrations were re-measured for all of the meteoritespecimens investigated in this study in material that hadbeen in immediate contact to that used for W isotopeanalysis.

3.2. Noble gas measurements

Extraction and measurement of He, Ne and Ar concen-trations and isotope compositions were performed at theUniversity of Bern. Analytical procedures are outlined inAmmon et al. (2008, 2011). Iron meteorite samples (50–160 mg) were cut using a diamond saw, cleaned ultrasoni-cally in ethanol (5 min), and wrapped in commercial Ni-foil. Extraction of He, Ne and Ar was performed by heatingthe samples at 1700–1800 �C in a Mo-crucible for 35 min. Aboron-nitride liner placed inside the Mo crucible helped toavoid corrosion of the Mo crucible. Extracted gases werecleaned using various Ti-getters and activated charcoal.The Ar fraction was separated from He and Ne using acti-vated charcoal (at �196 �C). Helium and Ne were measuredin a 90� sector field mass spectrometer and Ar in a tandemmass spectrometer with two 90� magnetic sector fields. Bothnon-commercial mass spectrometers have Nier-type ionsources and are equipped with a Faraday collector and anelectron multiplier working in analogue mode. Since the no-ble gas amounts of the studied samples are low, only elec-tron multiplier measurements were used. Noble gasconcentrations were determined by peak height comparisonto calibrated standard gases that are isotopically similar toatmospheric composition (only 3He is elevated relative toatmospheric composition). Calibration gases were mea-sured before or after each sample analysis. All 20Ne signalswere corrected for interferences from H2

18O. The sensitivityof the mass spectrometers used in this study varies non-lin-early as a function of total gas pressure. The measured Heand Ne amounts were corrected for this effect using

Tab

le1

Co

smo

gen

icn

ob

lega

sco

nce

ntr

atio

ns

of

iro

nm

eteo

rite

ssa

mp

les

inve

stig

ated

inth

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ud

y.

Met

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ten

ame

ID3H

e[±

1SD

]4H

e[±

1SD

]21N

e co

s[±

1SD

]38A

r co

s[±

1SD

]4H

e/21N

e co

s[±

1SD

]3H

e/4H

e[±

1SD

][1

0�8

cm3S

TP

/g]

[10�

8cm

3S

TP

/g]

[10�

8cm

3S

TP

/g]

[10�

8cm

3S

TP

/g]

[10�

8cm

3S

TP

/g]

[10�

8cm

3S

TP

/g]

IIA

Bir

on

sE

dm

on

ton

(Can

ada)

A02

0.29

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0.02

93.

25±

0.39

0.01

1±

0.00

60.

057

±0.

006

na

0.09

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01B

rau

nau

M03

2.41

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1524

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0.17

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645

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048

150

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425

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574

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Cap

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141

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320

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02

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56±

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434

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270

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Tab

le2

Tu

ngs

ten

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top

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for

the

terr

estr

ial

stan

dar

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e182/1

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e184W

e182/1

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e182/1

84W

e182/1

84W

(6/3

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s.a

(6/4

) mea

s.a

(6/3

) mea

s.a

(6/4

) mea

s.a

(6/3

) mea

s.a

(6/3

) co

rr.a

(6/4

) co

rr.a

(6/3

) co

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±2r

±2r

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cN

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0.11

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150.

06±

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0.10

�0.

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0.07

�0.

16±

0.21

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B07

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0.11

±0.

10�

0.11

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060.

02±

0.04

�0.

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0.05

�0.

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0.03

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08±

0.12

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11±

0.06

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0.05

C07

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0.15

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0.07

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0.05

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0.05

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D07

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0.16

±0.

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02±

0.08

0.09

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0.10

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0.07

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0.02

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0.27

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130.

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0.06

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Plu

s2

0.21

±0.

060.

04±

0.20

0.13

±0.

15�

0.10

±0.

21�

0.07

±0.

140.

07±

0.29

0.04

±0.

200.

06±

0.20

P09

cN

eptu

ne

Plu

s3

0.18

±0.

120.

05±

0.12

0.13

±0.

12�

0.08

±0.

04�

0.05

±0.

030.

08±

0.13

0.05

±0.

120.

08±

0.12

S04

Nep

tun

eP

lus

50.

02±

0.07

0.01

±0.

050.

01±

0.05

�0.

01±

0.04

�0.

01±

0.03

0.02

±0.

070.

01±

0.05

0.01

±0.

05

Mea

nN

eptu

ne

Plu

s(±

2SD

,n

=9)

0.02

±0.

080.

03±

0.08

0.02

±0.

080.

03±

0.08

Mea

nN

IST

129c

All

(±2S

D,

n=

15)

0.01

±0.

100.

01±

0.13

0.01

±0.

100.

01±

0.10

Alf

aA

esar

solu

tio

nst

and

ard

dN

eptu

ne

Plu

s5

0.22

±0.

06�

0.04

±0.

060.

10±

0.09

�0.

19±

0.03

�0.

12±

0.02

�0.

03±

0.07

�0.

04±

0.06

�0.

02±

0.09

NIS

T31

63so

luti

on

stan

dar

dN

eptu

ne

Plu

s5

0.01

±0.

03�

0.02

±0.

030.

00±

0.03

�0.

01±

0.03

�0.

01±

0.02

0.01

±0.

03�

0.02

±0.

030.

00±

0.03

aN

orm

aliz

edto

186W

/184W

=0.

9276

7(6

/4)

or

186W

/183W

=1.

9859

(6/3

)u

sin

gth

eex

po

nen

tial

law

.b

Hig

hsu

lph

ur

stee

l(N

IST

129c

)d

op

edw

ith

add

itio

nal

Wto

mat

chco

nce

ntr

atio

ns

of

sam

ple

san

dsu

bse

qu

entl

yp

roce

ssed

thro

ugh

full

chem

ical

sep

arat

ion

.c

Aff

ecte

db

yan

dco

rrec

ted

for

mas

s-in

dep

end

ent

effec

t(S

ecti

on

4.2)

.d

Alf

aA

esar

solu

tio

nst

and

ard

pro

cess

edth

rou

ghfu

llch

emic

alse

par

atio

n.


Tab

le3

Wis

oto

pe

com

po

siti

on

so

fir

on

met

eori

tes

inve

stig

ated

inth

isst

ud

y.

Met

eori

teID

So

urc

eN

be1

82

/18

3W

(6/3

) mea

s.a

e18

2/1

84W

(6/4

) mea

s.a

e18

2/1

84W

(6/3

) mea

s.a

e18

3/1

84W

(6/4

) mea

s.a

e18

3/1

84W

(6/3

) mea

s.a

MC

-IC

PM

Sd

e18

2/1

83W

(6/3

) co

rr.a

e18

2/1

84W

(6/4

) co

rr.a

e18

2/1

84W

(6/3

) co

rr.a

±2r

±2r

±2r

±2r

±2r

±2r

±2r

±2r

Low

exposu

reage

(<

60

Myr)

and/o

reff

ecti

vesh

ield

ing

IIA

Bir

on

s

Ed

mo

nto

n,

Can

adac

A02

Un

iv.

Alb

erta

10�

3.29

±0.

08�

3.38

±0.

07�

3.34

±0.

07�

0.08

±0.

05�

0.05

±0.

03N

ept.

�3.

39±

0.10

�3.

38±

0.07

�3.

39±

0.08

Bra

un

auc

M03

Vie

nn

a9

�3.

22±

0.07

�3.

40±

0.05

�3.

31±

0.04

�0.

13±

0.05

�0.

09±

0.03

Nu

�3.

40±

0.09

�3.

40±

0.05

�3.

40±

0.05

Mea

nII

AB

iro

ns

�3.

39±

0.08

�3.

39±

0.08

�3.

39±

0.08

�3.

40±

0.08

IIIA

Bir

on

s

Cap

eY

ork

‘CY

01’c

A01

UC

San

Die

go8

�3.

22±

0.11

�3.

36±

0.12

�3.

29±

0.11

�0.

14±

0.04

�0.

09±

0.03

Nu

�3.

40±

0.12

�3.

36±

0.12

�3.

38±

0.11

Cap

eY

ork

‘CY

02’c

G02

JNM

CZ

uri

ch3

�3.

24±

0.05

�3.

39±

0.17

�3.

34±

0.06

�0.

09±

0.14

�0.

06±

0.09

Nep

t.�

3.36

±0.

19�

3.39

±0.

17�

3.40

±0.

11

Mea

nII

IAB

iro

ns

�3.

37±

0.10

�3.

38±

0.18

�3.

37±

0.14

�3.

39±

0.12

IVA

iro

ns

Gib

eon

‘102

’C

04/N

02T

ok

yo4

�3.

22±

0.15

�3.

31±

0.08

�3.

22±

0.10

�0.

02±

0.10

�0.

01±

0.07

Nep

t.�

3.22

±0.

15�

3.31

±0.

08�

3.22

±0.

10

Gib

eon

‘Rai

lway

’cA

03/S

01S

enck

enb

erg

5�

3.19

±0.

07�

3.42

±0.

08�

3.30

±0.

09�

0.17

±0.

06�

0.11

±0.

04N

ept.

�3.

41±

0.11

�3.

42±

0.08

�3.

41±

0.10

Mu

on

ion

alu

stac

A04

/S03

ET

HIR

-16

5�

3.23

±0.

03�

3.33

±0.

07�

3.29

±0.

07�

0.08

±0.

05�

0.05

±0.

03N

ept.

�3.

33±

0.07

�3.

33±

0.07

�3.

34±

0.07

Mea

nIV

Air

on

s(e

xcl.

Gib

eon

‘Rai

lway

’)�

3.32

±0.

08�

3.28

±0.

08�

3.32

±0.

08�

3.28

±0.

08

Un

gro

up

edir

on

s

Ch

inga

c,e

M01

ET

HIR

-54

4�

3.21

±0.

10�

2.86

±0.

13�

3.04

±0.

100.

26±

0.09

0.17

±0.

08N

ept.

�3.

30±

0.10

�3.

30±

0.17

�3.

29±

0.14

Mb

osi

M02

Sen

cken

ber

g4

�3.

05±

0.10

�3.

09±

0.03

�3.

06±

0.07

�0.

04±

0.08

�0.

03±

0.05

Nep

t.�

3.05

±0.

10�

3.09

±0.

03�

3.06

±0.

07

Hig

her

exposu

reage

(>

100

Myr)

and/o

rlo

wer

shie

ldin

g

IIA

B

Sik

ho

teA

lin

‘SA

01’

B01

ET

HIR

-27

7�

3.69

±0.

16�

3.58

±0.

24�

3.61

±0.

160.

08±

0.28

0.06

+0.

18N

u�

3.69

±0.

16�

3.58

±0.

24�

3.61

±0.

16

Mb

osi

M02

Sen

cken

ber

g4

�3.

05±

0.10

�3.

09±

0.03

�3.

06±

0.07

�0.

04±

0.08

�0.

03±

0.05

Nep

t.�

3.05

±0.

10�

3.09

±0.

03�

3.06

±0.

07

IIA

B

Sik

ho

teA

lin

‘SA

02’c

G03

JNM

CZ

uri

ch2

�3.

71±

0.14

�3.

80±

0.12

�3.

76±

0.11

�0.

05±

0.13

�0.

03+

0.08

Nep

t.�

3.77

±0.

21�

3.80

±0.

12�

3.79

±0.

14

Mb

osi

M02

Sen

cken

ber

g4

�3.

05±

0.10

�3.

09±

0.03

�3.

06±

0.07

�0.

04±

0.08

�0.

03±

0.05

Nep

t.�

3.05

±0.

10�

3.09

±0.

03�

3.06

±0.

07

IIG T

om

big

bee

Riv

erc

K04

AM

NH

2�

3.38

±0.

08�

3.60

±0.

08�

3.50

±0.

09�

0.16

±0.

04�

0.11

+0.

02N

ept.

�3.

60±

0.09

�3.

60±

0.08

�3.

61±

0.09

IVA G

ibeo

n‘E

gg’c

G01

/S02

JNM

CZ

uri

ch5

�3.

38±

0.13

�3.

44±

0.09

�3.

40±

0.06

�0.

05±

0.10

�0.

04±

0.07

Nep

t.�

3.45

±0.

19�

3.44

±0.

09�

3.44

±0.

09

IVA G

ibeo

n‘9

9’c

C06

To

kyo

6�

3.50

±0.

22�

3.63

±0.

20�

3.59

±0.

22�

0.04

±0.

16�

0.03

+0.

11N

u�

3.56

±0.

31�

3.63

±0.

20�

3.62

±0.

25

All

Wis

oto

pe

com

po

siti

on

sar

ere

po

rted

ase-

un

itd

evia

tio

ns

rela

tive

toth

eb

rack

etin

gso

luti

on

stan

dar

d(A

lfa

Aes

ar):

((18i W

/18j W

) sam

ple

/(18i W

/18j W

) sta

nd

ard�

1)�

104.

Un

cert

ain

ties

rep

rese

nt

95%

con

fid

ence

lim

its

of

the

mea

nan

dar

eca

lcu

late

du

sin

g(S

D�

t 0.9

5,N�

1)/p

Nif

N>

4.F

or

sam

ple

sw

her

eN

<4

the

un

cert

ain

tyw

ases

tim

ated

usi

ng

the

aver

age

2SD

of

mu

ltip

lean

alys

eso

fth

ete

rres

tria

lm

etal

stan

dar

d(N

IST

129c

)d

uri

ng

the

sam

ese

ssio

n.

aC

orr

ecte

dfo

rin

stru

men

tal

mas

sb

ias

thro

ugh

inte

rnal

no

rmal

isat

ion

to186W

/183W

(‘6/

3’)

or

186W

/184W

(‘6/

4’)

usi

ng

the

exp

on

enti

alla

w.

bN

=n

um

ber

of

solu

tio

nre

pli

cate

s.c

Aff

ecte

db

yan

dco

rrec

ted

(‘co

rr’)

for

mas

s-in

dep

end

ent

effec

t(S

ecti

on

5.2)

.d

MC

-IC

PM

Su

sed

;‘N

uP

lasm

a’:

Nu

Inst

rum

ents

Pla

sma

MC

-IC

PM

S(E

TH

Zu

rich

);‘N

eptu

ne

Plu

s’:

Th

erm

oS

cien

tifi

cN

eptu

ne

Plu

sM

C-I

CP

MS

(Un

iver

sity

of

Mu

nst

er).

eC

orr

ecte

dfo

rs-

pro

cess

defi

cits

usi

ng

Arl

and

ini

etal

.(1

999)

and

the

rela

tio

ns

des

crib

edin

Bu

rkh

ard

tet

al.

(201

2).



relations between measured He and Ne isotope concentra-tions and gas amounts that have been determined in dilu-tion series experiments.

Blanks were measured with the same extraction and puri-fication protocol as that used for the samples. At least oneblank was measured after each sample. Subtracted blank sig-nals are the average of multiple blanks (n = 25). For the Heisotopes blanks were subtracted directly from raw signals.Measured Ne and Ar isotope signals were first correctedfor trapped atmospheric noble gases using a two-componentde-convolution based on assumed atmospheric and cosmo-genic noble gas isotope ratios (20Ne/22NeCOS = 0.83;21Ne/22NeCOS = 1; 36Ar/38ArCOS = 0.65, 36Ar/38ArATM

= 5.32) (Wieler, 2002). Cosmogenic 21Ne and 38Ar amountswere subsequently corrected for average cosmogenic contri-butions in the blanks, stemming from previous samples.

Reported external uncertainties (1SD) of cosmogenicnoble gas concentrations represent the combined uncertain-ties of calibration, blank and sample measurements, to-gether with the uncertainties involved in correcting fornon-linear effects. For most of the samples these uncertain-ties are �5–10% for 4He, 5–10% for 3He, 4–25% for 21Nec,and 6–10% for 38Arc. However, some noble gas concentra-tions that barely exceed background levels, and thus re-quired large relative blank subtractions, have much largeruncertainties approaching �100% (Table 1). Therefore,4He/21Ne ratios are only reported in Table 1 if their uncer-tainties do not exceed 30% (1SD).

3.3. Tungsten isotope analyses

3.3.1. Sample preparation and chemical separation of W

Samples of �0.5–1 g from all selected specimens werecut using a hand saw, thoroughly cleaned with abrasive pa-per followed by ethanol and de-ionised water in an ultra-sonic bath. The outermost 15–25% of each sample wereremoved by leaching in 6 M HCl–0.06 M HF and a fewdrops of concentrated HNO3 on a hotplate (90–110 �C)for 20–30 min. Complete dissolution of the samples wasaccomplished in �15 ml 6 M HCl–0.06 M HF in Savillex�

vials at 130 �C on a hotplate overnight. After complete dis-solution, �5% solution aliquots were taken to determine Hfand W concentrations by isotope dilution using a mixed180Hf–183W isotope tracer (Kleine et al., 2004). As expected,all samples are virtually free of Hf and have 180Hf/184Wlower than �8 � 10�4.

Tungsten was separated from the sample matrix using amodified two-stage anion exchange chromatography afterHoran et al. (1998) and Kleine et al. (2002, 2005a). Thesamples were loaded onto pre-cleaned anion exchange col-umns (4 ml BioRad� AG1X8, 200–400 mesh size) in 75 ml0.5 M HCl–0.5 M HF. Most of the Fe–Ni matrix waswashed off using 0.5 M HCl–0.5 M HF, and W was elutedin 15 ml 6 M HCl–1 M HF. After the first chemical separa-tion, samples were dried down in HNO3–H2O2 severaltimes, and were then re-dissolved in 5 ml 0.5 M HCl–0.5 M HF. The second anion exchange chromatography issimilar to the first-stage separation, but uses one insteadof 4 ml anion exchange resin and includes an additionalelution step (8 M HCl–0.01 M HF) that rinses off other

trace elements (e.g., Ag). This step was found to be neces-sary when analysing the terrestrial metal standard (high-sulphur steel NIST 129c), which contains significantamounts of Ag. Silver, together with Ar and Cl, may formmolecular interferences in the W mass range, as verified byisotope measurements of the W standard with admixed Ag.Magmatic iron meteorites contain too little Ag to signifi-cantly affect the W isotope analyses, however. Nevertheless,for direct comparison to the terrestrial standard, all sampleswere processed through the exact same chemical purifica-tion procedure. After the second chromatographic step,the W cuts were dried again, treated with HNO3–H2O2 sev-eral times, and finally dissolved in a running solution(0.56 M HNO3–0.24 M HF) yielding analyte W concentra-tions of 100 ppb (Nu Plasma) or 40–100 ppb (Neptune).The combined W yield for the two-stage chemistry was typ-ically �60–80%. Total procedural blanks varied between�30 and 130 pg W and no blank corrections were neces-sary, given that �150–400 ng W were analysed for eachsample.

3.3.2. Tungsten isotope measurements

Tungsten isotope compositions were measured on aNu Plasma MC-ICPMS at ETH Zurich following previ-ously published procedures (Kleine et al., 2008) and ona ThermoScientific Neptune Plus MC-ICPMS at theWestfalische Wilhelms-Universitat Munster. On bothinstruments, samples were introduced into the mass spec-trometer using a Cetac Aridus II� desolvator. StandardNi cones were used for all W isotope measurements (typeB sampler + type A WA skimmer on the Nu Plasma; Hcones on the Neptune Plus). Total ion beam intensitiesfor W varied between 1 and 2 � 10�10 A at a 75 lL/min uptake rate (Nu Plasma, 100 ppb W) and were 1.5–5 � 10�10 A at a 50–100 lL/min uptake rate (NeptunePlus, 40–100 ppb W). Small isobaric interferences of Oson masses 184 and 186 were corrected by monitoringinterference-free 188Os. The Os interference correctionswere generally smaller than 20 ppm and insignificant.Only Chinga (UNG) required a larger interference correc-tion of �3 e-units. Instrumental mass bias was correctednormalising to either 186W/183W = 1.9859 or186W/184W = 0.92767 and using the exponential law.

The 182W/184W and 183W/184W of the samples weremeasured relative to a terrestrial solution standard pre-pared from pure W metal (Alfa Aesar; Kleine et al.,2002, 2004). Tungsten isotope composition measurementsconsisted of 40 cycles of 5 s (Nu Plasma), or 200 cycles of4.2 s integration time (Neptune Plus). Each measurementwas bracketed by analyses of the Alfa Aesar solutionstandard. Baselines were measured by deflecting the ionbeam using the electrostatic analyser (ESA) for 60–120 s. For analyses with relatively low ion beam intensi-ties, baselines were determined on-peak using an acidblank solution (0.56 M HNO3–0.24 M HF). Both meth-ods proved to yield identical results. A 3–5 min washoutusing 0.56 M HNO3–0.24 M HF between two successivemeasurements was sufficient to clean the sample introduc-tion system.

–0.4 –0.2 0.0 0.2 0.4–0.4

–0.2

0.0

0.2

0.4

–0.4 –0.2 0.0 0.2 0.4

–0.4

–0.2

0.0

0.2

0.4

ε 183/184W (6/4)

ε182/

184 W

(6/

4) 183W deficit

–0.4 –0.2 0.0 0.2 0.4–0.4

–0.2

0.0

0.2

0.4

ε 183/184W (6/3)

ε182/

183 W

(6/

3)

(a)

(b)

(c)

ε182/

184 W

(6/

3)

ε 183/184W (6/3)

183W deficit

183W deficit

Nu Plasma

Alfa Aesar (chem.)

NIST3163 (sol.)

NIST129c (chem.):

Neptune Plus

Fig. 1. Measured W isotope compositions for the terrestrial metalstandard (NIST129c) relative to bracketing analyses of the solutionstandard (Alfa Aesar). (a) e182/184W (6/4) vs. e183/184W (6/4), (b) e182/

183W (6/3) vs. e183/184W (6/3), and (c) e182/184W (6/3) vs. e183/184W (6/3), where ‘6/3’ indicates that measured W isotope ratios werecorrected for instrumental mass bias by internal normalisation using186W/183W, and ‘6/4’ by normalisation to 186W/184W. Also shown isthe expected correlation line for a deficit in 183W. Each data pointrepresents multiple solution replicates (n = 2–5) of a digestion thatwas processed through the full chemical separation. Distinguishedare samples measured with the Nu Plasma and ThermoScientificNeptune Plus MC-ICPMS. Also shown are the W isotope data forone aliquot of the Alfa Aesar solution standard that was processedthrough the full chemical separation (purple diamond), and for theNIST 3163 solution standard, which was not processed through thechemical separation (blue circle). Uncertainties are 95% conf. limitsof the mean. Although the uncertainties of e182W and e183W (6/i) arecorrelated, error ellipses were omitted in this diagram for clarity.(For interpretation of the references to colour in this figure legend,the reader is referred to the web version of this article.)


Measured 182W/184W and 183W/184W ratios of samplesare reported as e-unit (i.e., parts per 104) deviations relativeto the bracketing standard analyses. Reported are meane182W and e183W values of pooled solution replicates(n = 2–9) at an external reproducibility of <0.2 e-units(95% confidence limits of the mean) in most cases. Theaccuracy of the measurements was investigated throughanalysis of a terrestrial standard (high sulphur steelNIST129c) that was processed through the same purifica-tion procedure as the iron meteorite samples. Also, alterna-tive isotope ratios (182W/183W and 182W/184W) andnormalisation ratios for instrumental mass bias correction(186W/183W, ‘6/3’ and 186W/184W, ‘6/4’) were monitoredand are used to assess possible matrix effects or artefacts.

4. RESULTS

4.1. Cosmogenic noble gases

The cosmogenic noble gas concentrations for the samplesfrom this study are presented in Table 1. As anticipated, cos-mogenic noble gas concentrations (index ‘c’) in most sam-ples were found to be near the low end of the rangeobserved in iron meteorites (Schultz and Franke, 2004).About half of the samples analysed here have 21Nec concen-trations below <0.01 � 10�8 cm3STP/g, while anothergroup of samples shows slightly higher 21Nec values between0.01 and 0.7 � 10�8 cm3STP/g. In contrast, one of the Sikh-ote Alin samples (SA01) contains �2.6 � 10�8 cm3STP/g of21Nec. Since cosmogenic 22Ne cannot be determined directlywe cannot discuss cosmogenic 22Ne/21Ne ratios and, there-fore, cannot correct the 21Nec concentrations for contribu-tions from possible phosphorous and/or sulphurinclusions. This might in principle somewhat compromisethe quality of the 21Nec data and the discussion of pre-atmo-spheric sizes and exposure ages. However, given the consis-tency of the dataset (see below), we consider anycontributions to 21Nec from sulphur and/or phosphorusinclusions to be small for the meteorites studied here.

4.2. Tungsten isotopes

4.2.1. Terrestrial standard

The W isotopic data for the terrestrial standard(NIST129c) are provided in Table 2 and Figs. 1 and 2. Thee18iW values are given for different W isotope ratios(182W/183W, 182W/184W, 183W/184W) and for different nor-malising ratios (186W/183W or 186W/184W, subsequentlymarked as ‘(6/3)’ and ‘(6/4)’, respectively). All eight measure-ments for NIST 129c – each of which represents the averageof 2–5 measurements of a single sample that was processedthrough the full chemical separation – show small butresolved positive e182/183W (6/3) (up to +0.27) and negativee183/184W (6/3) (down to �0.14) anomalies. These effectsare observed for measurements performed both on theNeptune Plus (WWU Munster) and on the Nu PlasmaMC-ICPMS (ETH Zurich). The average of all eight mea-surements of the NIST129c standard yields mean e182/183W(6/3) = +0.14 ± 0.05, e182/184W (6/3) = +0.08 ± 0.03, ande183/184W (6/4) = �0.10 ± 0.04 (SE 95% conf., n = 15).

–0.4

–0.2

0

0.2

0.4

–0.4

–0.2

0.0

0.2

0.4

–0.4

–0.2

0

0.2

0.4

–0.4

–0.2

0.0

0.2

0.4

–0.4

–0.2

0

0.2

0.4

–0.4

–0.2

0.0

0.2

0.4

ε182/

183 W

(6/

3)ε18

2/18

4 W (

6/3)

ε182/

184 W

(6/

4)

NIST129c:

ε182/183W (6/3)corr. = +0.01±0.13

(2SD, n=15)

NIST129c:

ε182/184W (6/3)corr. = +0.01±0.10

(2SD, n=15)

NIST129c:

ε182/184W (6/4)meas. = +0.01±0.10

(2SD, n=15)

(a)

(b)

(c)

measured

correctedNu Plasma

Neptune Plus

Fig. 2. External reproducibility of e182W for the terrestrial metal standard (NIST129c). (a) e182/183W (6/3), (b) e182/184W (6/3), and (c) e182/

184W (6/4). Each data point represents multiple solution replicates (n = 2–5) of a digestion that was processed through the full chemicalseparation. For normalizations involving 183W (i.e., those that are potentially affected by a mass-independent effect), both the corrected anduncorrected e182W values are shown. Error bars on individual data points are 95% conf. limits of the mean. The hashed area shows theexternal reproducibility of the NIST129c standard analyses (2SD).


Thus, while both 182W/183W (6/3) and 182W/184W (6/3) val-ues are elevated, the 183W/184W ratio is lower than that ofthe solution standard. The anomaly in e182/183W (6/3) is twicethat in e183/184W (6/3), while the anomalies in e182/184W (6/3)and e183/184W (6/3) are identical. At the same time no anom-aly is apparent in e182/184W (6/4) with an average value iden-tical to zero (e182/184W (6/4) = 0.01 ± 0.10, 2SD, n = 15).Thus, only W isotope ratios involving 183W yield inaccurateresults, while the 182W/184W ratio (6/4) is not affected. Thisindicates that the slight offset observed for the NIST129cdata is caused by a mass-independent W isotope fraction-ation of the odd isotope 183W from the even isotopes 182W,

184W and 186W. The same negatively correlated shifts ine182/184W (6/3) and e183/184W (6/3) relative to the unpro-cessed solution standard have been observed by Willboldet al. (2011) for W isotope measurements on terrestrialsilicate samples.

The observed 183W isotope effect in the terrestrial metalstandard may reflect an anomalous isotope composition ofthe bracketing solution standard (i.e., Alfa Aesar) relativeto the true terrestrial composition. However, an aliquot ofthe Alfa Aesar solution standard passed through thefull chemical separation produced identical anomalies tothose observed for the metal standard (NIST129c) (Fig. 1

–3.6

–3.4

–3.2

–3.0

–2.8

–2.6

–0.4 –0.2 0.0 0.2 0.4–4.0

–3.8

–3.6

–3.4

–3.2

–3.0

–2.8

–2.6

ε 183/184W (6/4)

ε182/

184 W

(6/

4)

183W deficit

ε182/

183 W

(6/

3)

(a)

(b)

s−de

ficit/r

−exc

ess

183W deficit

s−deficit/r−excess

Mbosi

Mbosi

Chinga

Chinga

SA01

SA02

SA01

GB ’99’

GB ’99’

TR

TR

GB ´Railway’


and Table 2). Furthermore, another solution standard(NIST3163) yields a W isotope composition identical to thatof the Alfa Aesar standard (Fig. 1). An anomalous Wisotope composition of the Alfa Aesar solution standard,therefore, cannot be the cause of the mass-independent183W effect observed for the NIST 129c metal standard. Thiseffect must rather be related to processes during samplepreparation and W purification and we concur withWillbold et al. (2011) that a mass-independent isotopefractionation between odd (i.e., 183W) and even W isotopes(i.e., 182W, 184W and 186W) associated with W-loss duringre-dissolution of purified W in Savillex vials is causing theobserved 183W deficits.

The mass-independent 183W effect on terrestrial stan-dards and meteorite samples can be corrected by using dif-ferent normalisation schemes for the W isotopemeasurements. For example, the terrestrial metal standardsanalysed in this study plot on a line with a slope of��2 thatis predicted for 183W deficits in e182/183W (6/3) vs. e183/184W(6/3) space (Fig. 1b). Likewise, the data plot on a line ofslope ��1 in e182/184W (6/3) vs. e183/184W (6/3) space, againconsistent with the predicted effect of a 183W deficit (Fig. 1c).For all samples, measured e182/183W (6/3) and e182/184W(6/3) values can thus be corrected using the measurede183/184W (6/3) and the relations e182/183W (6/3)corr. =e182/183W (6/3)meas. � (�2) � e183/184W (6/3) and e182/184W(6/3)corr. = e182/184W (6/3)meas. � (�1) � e183/184W (6/3)(Table 2). The corrected e182/183W (6/3) and e182/184W(6/3) values are identical to the measured e182/184W (6/4),indicating that the corrections are accurate (Fig. 2).

–0.4 –0.2 0.0 0.2 0.4–4.0

–3.8

ε 183/184W (6/3)

n−capture

SA02

Fig. 3. e182W–e183W variation for iron meteorite samples analysedin this study: (a) e182/184W (6/4) vs. e183/184W (6/4), (b) e182/183W (6/3) vs. e183/184W (6/3). Error bars indicate 95% conf. limits of themean (n = 2–9). Also shown are: (i) the expected correlation linefor the mass-independent effect (i.e., a deficit in 183W); (ii) theexpected correlation line for variations in s- and r-process Wisotopes calculated using the stellar model from Arlandini et al.(1999); and (iii) model arrays (small grey squares) for neutroncapture effects on W isotopes in iron meteoroids. The solid linesintersect at an ordinate value of e182W = �3.35, which is approx-imately the average value for the investigated iron meteoritesgroups. Although the uncertainties of e182/iW and e183/184W (6/i)are positively correlated, error ellipses were omitted in this diagramfor clarity.

4.2.2. Iron meteorites

Tungsten isotope compositions for the iron meteoritesamples are displayed in Table 3 and Figs. 3 and 4. Theco-variation of e182/184W (6/3) and e183/184W (6/3) mea-sured for the iron meteorites (Fig. 3) is similar to the corre-lated effects observed for the terrestrial metal standard(NIST129c) (Fig. 1). While many of the samples analysedhere have e183/184W indistinguishable from the terrestrialvalue (i.e., the Alfa Aesar solution standard), about halfof the samples exhibit small e183/184W deviations that are re-solved from the terrestrial value. These samples also showdifferences of similar magnitude in their e182W values calcu-lated using different normalisation schemes (Fig. 3).

For all iron meteorite samples investigated here(except Chinga) the difference between e182/184W (6/3) ande182/184W (6/4) is identical to the negative anomalyobserved for e183W (6/3) (Table 3). Similarly, the differencebetween e182/183W (6/3) and e182/184W (6/4) corresponds totwice the anomaly observed for e183W (6/3). These system-atic W isotope shifts are the same as those observed for theNIST129c standard, and are also in agreement with thepredicted effects for a deficit in 183W (Fig. 3). This stronglysuggests that the observed mass-independent 183W effects inthe NIST129c data and the iron meteorite samples have thesame origin. The iron meteorite data can thus be correctedusing the same approach employed above for the NIST129cstandard. The corrected e182/184W (6/3) and e182/183W (6/3)values are shown in Table 3, and are in excellent agreementwith the e182/184W (6/4) values directly measured for the

iron meteorites. This again demonstrates that, as for theNIST 129c data, the correction for the mass-independent183W effect is accurate. We nevertheless emphasise thatthe main conclusions of our study are not compromisedby the mass-independent 183W effect present in some ofthe sample measurements, because the chronological inter-pretation of the Hf–W data is entirely based on e182/184W(6/4) values (i.e., the W isotope ratio not involving 183W),which do not show this effect and, hence, do not requireany correction at all.

ε182W (6/4)

IIIAB irons

IIAB irons

IVA irons

CA

I ini

tial

IIG irons

Ungrouped irons

–4.2 –4.0 –3.8 –3.6 –3.4 –3.2 –3.0 –2.8

Mbosi

Chinga

Tombigbee River

Gibeon ’99’

Gibeon ’Egg’

Muonionalusta

Gibeon ’Railway’

Gibeon ’102’

Cape York ’CY02’

Cape York ’CY01’

Sikhote Alin ’SA02’

Sikhote Alin ’SA01’

Braunau

Edmonton, Canada

Fig. 4. e182/184W (6/4) for the iron meteorite samples investigatedin this study. Distinguished are samples with low exposure ageswhose W isotope compositions are unaffected by cosmic ray effects(filled symbols) and samples which have longer exposure times and/or whose e182W was lowered as a result of cosmic ray interactions(open symbols). For Chinga (UNG) the e182W corrected fornucleosynthetic isotope anomalies is shown. The vertical hashedarea represents the CAI initial of e182W = �3.51 ± 0.10 (Burkhardtet al., 2008, 2012). Error bars indicate 95% conf. limits of the mean.


The iron meteorite samples from groups IIAB, IIIABand IVA with the lowest noble gas concentrations displaya narrow range of e182/184W (6/4) values, from �3.42 to�3.31 (Table 3), slightly higher than the current best esti-mate for the initial W isotope composition of CAI[e182W = �3.51 ± 0.10; Burkhardt et al. (2012)]. In general,samples with higher noble gas concentrations tend to havelower e182W values (Tables 1 and 3). Among these samples,Sikhote Alin (IIAB, sample SA02) stands out by havinge182W significantly below the initial W isotope compositionof CAI. The two ungrouped iron meteorites analysed here(Chinga and Mbosi) have e182W significantly higher (upto �0.4 e-units) than those observed for samples from themajor groups of investigated magmatic iron meteorites(Figs. 3 and 4; Table 3). One of the ungrouped iron meteor-ites (Chinga) stands out by also having a positive e183Wanomaly resolved from zero.

5. DISCUSSION

5.1. Identifying iron meteorite samples with very low fluences

of (epi)thermal neutrons

Very low concentrations of cosmogenic noble gases in aniron meteorite sample may be the result of a low exposure

age or of a heavily shielded location in a large meteoroid.In the first case, W isotope compositions will essentially beunaffected by cosmic rays irrespective of meteoroid sizeand sample position. A sample from a large meteoroidmay also be unaffected by cosmic rays, regardless of itsexposure age, if it was located deep enough in the pre-atmo-spheric body such that not only the high energy particle flu-ence but also the epi(thermal) neutron fluence were stronglyattenuated. However, if a sample was irradiated at someintermediate depth where the high energy particle flux wasalready low but the (epi)thermal neutron flux was still high,noble gas concentrations in the sample may underestimatethe modification of its W isotopic composition by neutroncapture effects. Model calculations for neutron capture ef-fects on W isotope compositions suggest that for a sphericaliron meteoroid with an exposure age of 60 Ma, the maxi-mum cosmic ray induced decrease in e182W would be�0.09 e-units [calculated using the nuclide production mod-el described for noble gases and several radionuclides byAmmon et al. (2009) and Leya and Masarik (2009)], whichis close to our average analytical uncertainty (Tables 2 and3). This maximum cosmogenic effect would be reached inthe centre of an iron meteoroid with a radius of 85 cm. Asfor all other meteoroid sizes and sample depths the correc-tion would be – partly considerably – smaller than 0.09 e-units, we consider the measured W isotopic composition inmeteorites with exposure ages <60 Ma unaffected byneutron capture effects. In the following paragraphs wetherefore evaluate which of the meteorites studied here canbe assigned exposure ages below 60 Ma (Table 4). We will,at this stage, assume that all samples were subjected to a sin-gle stage exposure history to cosmic rays, i.e., we assumethat the entire fluence of (epi)thermal neutrons in any sam-ple was acquired at the same shielding and over the sametime interval as the entire fluence of the high energy particlesproducing cosmogenic noble gases. We will later discuss thevalidity of this assumption in light of the W isotope data.We furthermore assume a temporally constant primary cos-mic ray flux (cf. Wieler et al., 2011).

Braunau (IIAB, 39 kg) has a shielding-corrected verylow exposure age of approximately 8 Ma, determined withthe cosmogenic nuclide pairs 39Ar–38Ar, 36Cl–10Be and36Cl–36Ar, respectively (Cobb, 1966; Chang and Wanke,1969). According to the model calculations by Ammonet al. (2009), the noble gas concentrations of the investi-gated sample (e.g., 21Nec = 0.17; 38Arc = 0.65 � 10�8

cm3STP/g) indicate production rates as expected for ameteoroid with a radius of 615 cm, consistent with therecovered mass of Braunau. The very low 4He/21Ne ratioof �150, although not precise, also hints at a relativelysmall pre-atmospheric size. The low exposure age and smallpre-atmospheric size indicate that the W isotope composi-tion of Braunau certainly remained unaffected by cosmicray effects. The maximum shift in e182W predicted by ourneutron capture model for a meteorite having TCRE = 8 Macorresponds to ��0.01 e-units (Table 4).

Cape York (IIIAB) is a large (>58,000 kg; pre-atmo-spheric radius >1.2 m) and well-studied iron meteorite.Our three noble gas analyses on samples from two differentspecimens (CY01 and CY02) yield similar concentrations at

Table 4Summary of iron meteorite samples with minimal cosmic ray effects.

Meteorite ID Radiusa Cosmic-ray exposure age (TCRE)b Max.De182WGCR

cMeasurede182W

DtCAI

[cm] [Ma] Method (model) [±95% conf.] [Myr]

IIAB ironsBraunau M03 615 �8 39Ar–38Ar, 36Cl–10Be and 36Cl–36Ar (1) 6�0.01 �3.40 ± 0.05Edmonton, Canada A02 <120 <60 Production rates (4) 6�0.09 �3.38 ± 0.07Mean IIAB �3.39 ± 0.08 þ1:0þ1:1

�1:0

IIIAB ironsCape York (CY01) A01 P120 82 ± 7 129I–129Xe (2) 6�0.08 �3.36 ± 0.05Cape York (CY02) G02a P120 82 ± 7 129I–129Xe (2) 6�0.08 �3.39 ± 0.17Mean IIIAB �3.37 ± 0.14 þ1:2þ1:5

�1:4

IVA ironsGibeon ‘102’ C04 P90 <50 10Be–21Ne (3) 6�0.07 �3.31 ± 0.08Muonionalusta A04 <120 <6 Production rates + noble gases (4) 6�0.01 �3.33 ± 0.07Gibeon ‘Railway’ A03 P90 <50 10Be–21Ne (3) 6�0.07 �3.42 ± 0.08Mean IVA (excl. ‘Railway’) �3.32 ± 0.08 þ1:6þ1:1

�1:0

Ungrouped ironsChinga M01 660 <40 Production rates + noble gases (4) 6�0.05 �3.30 ± 0.17 þ1:8þ1:8

�1:7

Mbosi M02 <120 <6 Production rates + noble gases (4) 6�0.01 �3.09 ± 0.03 þ3:9þ0:9�0:9

Tombigbee River (IIG) is not shown as it likely had a complex exposure history and its W isotope composition was likely modified by cosmicrays (Section 5.3).

a Pre-atmospheric radius for a spherical meteoroid, based on either the total recovered mass (lower bound), or noble gas systematics (upperbound).

b Cosmic-ray exposure ages (or upper and lower bounds thereof) and the method(s) used for determining these ages. References: (1) Cobb(1966); Chang and Wanke (1969); (2) Mathew and Marti (2009); (3) Honda et al. (2009); (4) Ammon et al. (2009)/this study.

c Model prediction for maximum cosmic-ray effect on e182W for given TCRE (Ma), pre-atmospheric radius, and 4He/21Ne (if determined).


the very low end of the range of values observed in ironmeteorites (Table 1), in agreement with other Cape Yorkanalyses (Schultz and Franke, 2004). A shielding corrected129I–129Xe exposure age of 82 ± 7 Ma was determined byMathew and Marti (2009), slightly higher than the 60 Malimit mentioned above. However, according to the nuclideproduction model of Ammon et al. (2009) and Leya andMasarik (2009), in a r = 1.2 m meteoroid the maximumcosmic ray-induced decrease in 182W/184W for a 82 Maexposure age is �0.08 e182W, even marginally lower thanthe limit of 0.09 e182W tolerated here. We therefore includeCape York into our set of ‘low exposure age meteorites’(Table 4).

Gibeon (IVA) is a large iron meteorite (>26,000 kg; ra-dius >0.9 m). Reported noble gas concentrations are vari-able, yet again near the low end of the range observed foriron meteorites (Schultz and Franke, 2004). The Gibeonsamples analysed here also have varying noble gas concen-trations (Table 1). In a comprehensive study, Honda et al.(2009) report shielding-corrected 21Ne–10Be cosmic rayexposure ages for a large number of Gibeon specimens.These authors identified two groups of samples, one withonly loosely defined but low (�10–50 Ma) and another withhigh (�300 Ma) exposure ages. Honda et al. (2009) con-clude that Gibeon had a complex exposure history. Thesamples with low nominal exposure ages (<50 Ma) thuspresumably were largely shielded from highly energetic cos-mic rays prior to their second (meteoroid) exposure stage

and thus likely also had seen a considerably lower (epi)ther-mal neutron fluence than the samples of the other group.The 21Ne analyses for Gibeon specimens ‘99’, ‘102’ and‘Railway’ from Honda et al. (2009) are in agreement withour 21Ne results for exactly these samples (Table 1). Hondaet al. (2009) interpreted the samples ‘Railway’ and ‘102’ tobelong to the low exposure age group (<50 Ma). We thusexpect cosmic ray effects on W isotopes to be minimal forthese samples, assuming a low (epi)thermal neutron fluencefor the two samples during the first exposure stage (i.e., forthe low noble gas group the above assumption that the totalfluences of high energy- and (epi)thermal particles wereboth acquired at the same constant shielding can be ex-pected to be correct). The maximum negative shift ine182W predicted by the neutron capture model forTCRE = 50 Ma and r > 0.9 m corresponds to �0.07 e-units.As minor – albeit not resolved – variations might thus beexpected, the sample with the most elevated e182W, i.e.,Gibeon ‘102’, will be taken as representative for Gibeon be-fore exposure to cosmic rays. The other two Gibeon sam-ples analysed here (‘Egg’ and ‘99’) have higher noble gasconcentrations than the ‘Railway’ and ‘102’ specimens.Although in absolute terms the concentrations of ‘Egg’and ‘99’ are still low, these two specimens supposedly be-long to the second group recognised by Honda et al.(2009) with an exposure age of approximately 300 Ma.The W isotope compositions of these two samples may thushave been modified by cosmic ray effects.


No shielding-corrected exposure ages based on pairs of aradioactive and a stable cosmogenic nuclide are availablefor Edmonton, Canada (IIAB, pre-atmospheric mass>7 kg), Muonionalusta (IVA, >230 kg), Tombigbee River

(IIG, >43 kg), Chinga (UNG, >200 kg), and Mbosi

(UNG, >16,000 kg). However, it seems likely that all thesemeteorites owe their low noble gas concentrations primarilyto a low exposure age rather than a very high shielding. ForTombigbee River and Chinga a rough maximum pre-atmo-spheric radius of �60 cm (Ammon et al., 2009) can be esti-mated from the upper limit of the 4He/21Ne ratio of �300(Table 1). The minimum 21Ne production rate in the centreof meteoroids of this radius is �0.19 � 10�10 cm3STP/g Ma(Ammon et al., 2009). This yields maximum one stage cos-mic ray exposure ages of �40 Ma for both Tombigbee Riv-er and Chinga. For Edmonton, the uncertainty of the4He/21Ne ratio is too large to obtain an estimate of themaximum pre-atmospheric radius. However, assuming thatthe Edmonton sample originated from close to the centre ofan iron meteoroid with a pre-atmospheric radius of 120 cm,which is the largest object modelled by Ammon et al.(2009), yields an maximum possible one stage exposureage of �60 Ma. With the same assumption we arrive atupper limits of �6 Ma for the exposure ages of Muonion-alusta and Mbosi for which 21Necos was below detectionlimit, if we additionally assume these samples to have hada 21Necos concentration of 1 � 10�11 cm3STP/g, i.e., thelowest value we were able to measure in one of our samples(Gibeon ‘Railway’). Thus, for all these samples cosmic rayeffects on W isotopes can be expected to be minimal andsmaller than our analytical resolution for 182W/184W.

Sikhote Alin (27,000 kg; radius >0.9 m) has a 40K–Kexposure age of 430 Ma (Voshage, 1984). Hence, SikhoteAlin is the only meteorite in this study with an unequivo-cally high exposure age. The samples investigated here(SA01 and SA02) presumably derive from rather near thesurface of this large meteoroid, as suggested by their4He/21Ne ratios of �215 and 250 (Ammon et al., 2009).Also, sample SA01 has by far the highest noble gas concen-trations observed in this study and even contains an orderof magnitude more gas than the two SA02 samples. Signif-icant cosmic ray effects on W isotopes can thus be antici-pated in SA01 but possibly also in SA02.

In summary, the samples with shielding-corrected expo-sure ages <60 Ma as deduced by pairs of stable and radio-active nuclides [Braunau and Gibeon (‘102’ and ‘Railway’)],as well as Cape York with its 82 Ma exposure age are ex-pected to have a W isotopic composition not modified byepi(thermal) neutron capture reactions beyond the presentanalytical uncertainty of �0.1 e 182W (95% conf.), andany corrections should mostly be lower than this value (Ta-ble 4). Five other meteorites (Edmonton Canada, Muon-ionalusta, Tombigbee River, Chinga, and Mbosi) verylikely also have low noble gas based exposure ages andhence should be expected to have W isotope compositionsthat are unaffected by cosmic rays within our current ana-lytical resolution. We will discuss below, however, thatTombigbee River may have to be excluded from this group,because our assumption of a single stage exposure historyto cosmic rays does not seem to be valid for this sample.

The W isotope compositions of Sikhote Alin (samplesSA01 and SA02), and possibly also Gibeon (‘99’ and‘Egg’) will likely have been modified more strongly by neu-tron capture. We emphasise that for the following discus-sion we use only measured 182W/184W, without anyattempt to correct for neutron capture effects.

5.2. Core formation ages for samples with minimal cosmic

ray effects

A model time of metal–silicate separation (i.e., core for-mation) in iron meteorite parent bodies can be calculated asthe time of Hf–W fractionation from an unfractionated,chondritic reservoir. The model age of core formation isconveniently expressed as the time elapsed since formationof CAI and is calculated using the relation

DtCAI ¼ �1

klnðe182WÞSample � ðe182WÞChondrites

ðe182WÞSSI � ðe182WÞChondrites

( )ð1Þ

in which k is the 182Hf decay constant of0.078 ± 0.002 Myr�1 (Vockenhuber et al., 2004),e182WChondrites is the present-day e182W of chondrites(e182W = �1.9 ± 0.1) (Kleine et al., 2002, 2004; Schonberget al., 2002; Yin et al., 2002), e182Wsample is the e182W valueof an iron meteorite, and e182WSSI is the solar system initialW isotopic composition as determined from a Hf–W iso-chron for CAI (Burkhardt et al., 2008, 2012). The currentbest estimate for this value is e182WSSI = �3.51 ± 0.10(Burkhardt et al., 2012). Note that this value is lower thanthat used previously (Burkhardt et al., 2008), as a result ofcorrecting the CAI data for nucleosynthetic isotope anom-alies (Burkhardt et al., 2012).

As pointed out in Section 5.1 the fluence of (epi)thermalneutrons is expected to have been minimal for many of theinvestigated samples. This applies to the meteorites thathave shielding-corrected known low exposure ages (i.e.,Braunau, Gibeon ‘102’ and ‘Railway’, Cape York) as wellas further samples with most likely low exposure ages(Edmonton Canada, Muonionalusta, Tombigbee River,Chinga, and Mbosi). The ungrouped irons Chinga andMbosi will be discussed in Section 5.3. With the exceptionof Tombigbee River, all samples with low noble gas basedexposure ages indeed show e182W values identical to eachother within our analytical resolution (i.e., �0.1 e units),and slightly higher than the initial W isotope compositionof CAI (Fig. 4). In contrast, samples for which the noblegas systematics indicate exposure ages of several hundredMa (i.e., Sikhote Alin, and the two Gibeon specimens ‘99’and ‘Egg’ with a discernible long first exposure stage) dis-play more negative and more variable e182W values. SampleSA02 from Sikhote Alin has a e182W value significantlylower than the initial W isotope composition in CAI. Thus,the low values of both Sikhote Alin and the two Gibeonspecimens ‘99’ and ‘Egg’ are reasonably explained as the re-sult of 182W burnout resulting from cosmic ray-inducedneutron capture reactions.

The only data point in Fig. 4 not perfectly fitting theoverall picture is that of Tombigbee River, whose e182Wvalue is slightly lower than the CAI initial and significantly

644-

ε 182Wpre-exposure

Δ tCAI [Ma]

IIAB

IIIAB

IVA

(1)

(2)

Previous studies

This study

-2 0 2

(1)

CA

I ini

tial

(1)

IIAB

IIIAB

IVA

(1)

(2)

Previous studies

This study(1)

CC

CA

I ini

tial

CA

I ini

tial

CA

I ini

tial

(1)

–4.2 –4.0 –3.8 –3.6 –3.4 –3.2 –3.0 –2.8

Fig. 5. Mean e182W for samples with low cosmic ray exposure agesfor the IIAB, IIIAB, IVA iron meteorite groups. The upper axisshows the timing of core formation relative to the formation ofCAI [i.e., e182W = �3.51 ± 0.10; (Burkhardt et al., 2008, 2012)].Shown for comparison are the corrected e182W values obtained inprevious studies that employed cosmogenic noble gas systematicsor theoretical models of more strongly irradiated meteorites tocorrect for cosmic ray effects (2: Markowski et al., 2006a; 1:Qin et al., 2008b). Note that the uncertainty on the initialW isotope composition of CAI is excluded from the error barson the calculated ages, but instead is shown separately (hashedarea).


lower than values of any other sample for which the noblegas systematics indicate a low exposure age. Complexexposure histories are not uncommon among iron meteor-ites (e.g., Welten et al., 2008). One explanation for this dis-crepancy, therefore, is that – contrary to our assumption inSection 5.1 – Tombigbee River had a complex irradiationhistory, shielded in a first stage deeply enough to have seenonly a relatively small fluence of high-energy cosmic rayparticles but discernible amounts of (epi)thermal neutrons.However, for all other samples for which cosmic ray effectson W isotopes are expected to be minimal, the perfect con-sistency between W isotopic data on the one hand and cos-mogenic noble gas or radionuclide data on the otherstrongly indicates that our basic assumption here is valid:samples identified as having a low cosmic ray exposureage have experienced a negligible fluence of (epi)thermalneutrons. Among our low exposure age group of meteor-ites, Tombigbee River thus appears to be the only meteoritewith a complex exposure history.

The mean e182/184W (6/4) values of the investigated ironmeteorite samples with low exposure ages and negligiblecosmic ray effects are �3.39 ± 0.08, �3.37 ± 0.14, and�3.32 ± 0.08 (±95% conf.) for the IIAB, IIIAB, and IVAiron meteorite groups (Tables 3 and 4; Fig. 5). These valuesare in excellent agreement with results from a previousstudy that used theoretical models and noble gas systemat-ics to correct for cosmic ray-induced effects (i.e., IIAB:�3.30 ± 0.10; IIIAB: �3.34 ± 0.06; IVA: �3.36 ± 0.07;Fig. 5; Qin et al., 2008b), indicating that this procedurequantitatively corrected for the effects of 182W burnout inthe IIAB, IIIAB and IVA iron meteorites. Nevertheless,

in contrast to the samples investigated in previous studies,the weakly irradiated samples identified here require nocorrection for cosmic ray effects on their W isotopic compo-sitions. As such, these samples are of paramount impor-tance to establish a precise Hf–W chronometry of ironmeteorites.

Core formation ages deduced from the measured e182Wvalues of the IIAB, IIIAB, and IVA irons having negligiblecosmic ray effects are displayed in Fig. 5. The core forma-tion ages, calculated relative to a solar system initiale182W = �3.51 ± 0.10 (Burkhardt et al., 2012), correspondto þ1:0þ1:1

�1:0 Myr for the IIAB, þ1:2þ1:5�1:4 Myr for the IIIAB,

and þ1:6þ1:1�1:0 Myr for the IVA iron meteorite parent bodies.

The uncertainty on these ages includes the uncertainty onthe mean e182W for each iron group, and the uncertaintieson the CAI initial and the present-day e182W of carbona-ceous chondrites Eq. (1). The core formation ages for theIIAB, IIIAB and IVA iron meteorite parent bodies areindistinguishable from each other and indicate that metalsegregation in these bodies occurred within less than�1 Myr of each other. It is noteworthy that unlike the neg-ative model ages reported in several previous studies, thecore formation ages obtained here are positive. This doesnot only reflect the fact that the iron meteorite samples usedhere have only negligible cosmic ray effects, but is also dueto the recent downward revision of the initial e182W of CAI(Burkhardt et al., 2012).

The new Hf–W results from this study confirm earlierconclusions that accretion and core formation in the par-ent bodies of at least the IIAB, IIIAB, and IVA ironmeteorites predated chondrule formation and the accre-tion of chondrite parent bodies (cf. Kleine et al.,2005a). From the new Hf–W ages and using the modelfor 26Al heating of planetesimals from Qin et al.(2008b), we can infer accretion time scales for the IIAB,IIIAB, and IVA iron meteorite parent bodies, indicatingthat these bodies most likely formed within less than�1 Myr after CAI formation and no later than�1.5 Myr after CAI formation. In contrast, the parentbodies of chondrites appear to have formed later, as con-strained by age differences between CAI and chondrulesbased on U–Pb and Al–Mg isotope systematics (Kitaet al., 2000; Amelin et al., 2002; Kunihiro et al., 2004;Rudraswami and Goswami, 2007; Kurahashi et al.,2008). Note that chondrule formation must have pre-dated accretion of chondrite parent bodies, so that chon-drule formation ages correspond to the earliest possibleaccretion time for a given chondrite parent body. Avail-able Al–Mg ages for chondrules from ordinary chondrites(L, LL) as well as CO and CR chondrites (Kita et al.,2000; Kunihiro et al., 2004; Kurahashi et al., 2008; Hut-cheon et al., 2009; Kita and Ushikubo, 2012) indicatethat at least these chondrite parent bodies accreted morethan �2 Myr after CAI formation, i.e., �1 Myr laterthan the IIAB, IIIAB and IVA iron meteorite parentbodies. It is noteworthy that some chondrules separatedfrom the CV chondrite Allende appear to be as old asCAI (Connolly, 2011), suggesting that some chondriteparent bodies may have accreted as early as the ironmeteorite parent bodies.


5.3. Tungsten isotope anomalies and Hf–W chronometry of

ungrouped iron meteorites

Apart from cosmic ray-induced shifts, the W isotopiccomposition of iron meteorites may vary due to the pres-ence of nucleosynthetic isotope anomalies. Nucleosyn-thetic heterogeneity among bulk iron meteorite sampleshas been reported for several elements like Ni, Ru andMo (Dauphas et al., 2002; Regelous et al., 2008; Chenet al., 2010; Burkhardt et al., 2011). Bulk nucleosyntheticW isotope variations have so far only been identified inCAI from Allende (Burkhardt et al., 2008, 2012) and insome members of the IVB group and ungrouped mag-matic iron meteorites (Qin et al., 2008a). These anomaliescould either be explained by a deficit in s-process or anexcess in r-process W isotopes. Such nucleosynthetic het-erogeneity could significantly affect e182W and producescorrelation lines in diagrams of e182W vs. e183W, whoseslopes depend on the normalisation used (Burkhardtet al., 2012) (Fig. 3).

The two ungrouped iron meteorites studied here(Chinga and Mbosi) show elevated e182W (up to �0.4e-units) in comparison with samples from the major groupsof investigated magmatic iron meteorites (Figs. 3 and 4;Table 3). In addition, Chinga also displays elevated e183Wof +0.26 (Fig. 3). The higher e182W in this meteorite maythus be the result of a nucleosynthetic W isotope anomalyrather than a relatively young core formation age of itsparent body. The e182/184W and e183/184W (6/3) valuesof Chinga correlate as expected for s-process deficits(or r-process excesses) relative to the terrestrial composition(Fig. 3) and are consistent with nucleosynthetic effects inW isotopes observed for one other ungrouped ironmeteorite (Deep Springs; Qin et al., 2008a). Correctingthe measured e182/184W (6/4) of Chinga using the e182W–e183W relation for a variable distribution of s- and r-processisotopes, yields e182Wcorr = �3.30 ± 0.17, in close agree-ment with the measured values of the IIAB, IIIAB andIVA iron meteorite groups (Fig. 4). Thus, there appearsto be no resolvable age difference between Chinga andsamples from the major magmatic iron meteorite groups(Table 4).

In contrast to Chinga, Mbosi does not show evidence fornucleosynthetic isotope variation as no anomalies are ob-served in e183W. The measured e182W of Mbosi, therefore,indicates that this iron meteorite is slightly younger(DtCAI = +3.9 ± 0.9 Myr) than the other meteorites studiedhere. If Mbosi derived from the metallic core of a differen-tiated parent body, then core formation in this object oc-curred �2–3 Myr later than in the IIAB, IIIAB and IVBiron meteorite parent bodies. In this case, core formationin the iron meteorite parent bodies occurred over a longertime interval than given by the Hf–W ages for the majorgroups of magmatic irons. Alternatively, Mbosi may haveformed by a more localised metal–silicate separation eventakin to those recorded in the non-magmatic iron meteor-ites. The metal–silicate separation age for Mbosi would inthat case be consistent with ages reported for some non-magmatic iron meteorites (e.g., Kleine et al., 2005a; Schulzet al., 2012).

6. CONCLUSIONS

This study demonstrates that the cosmogenic noblegases He, Ne, and Ar are suitable for identifying iron mete-orite specimens with likely minimal cosmic ray effects on Wisotopes, especially if meteorites with low cosmic ray expo-sure ages (i.e., <60 Ma) can be identified. Based on noblegas data we selected several magmatic iron meteorite sam-ples (including IIAB, IIIAB, IVA and IIG, as well as twoungrouped iron meteorites) that were expected to showno resolvable cosmic ray-induced effects on their W isotopecompositions. With the exception of the IIG meteoriteTombigbee River all these samples have e182W values iden-tical to each other within our analytical resolution of �0.1 eunits. All these values are slightly higher than the CAI ini-tial, reflecting radiogenic ingrowth of 182W (correspondingto �0.1–0.2 e-units) in a time interval between CAI forma-tion and core formation in iron meteorite parent bodies. Incontrast, samples from two meteorites with exposure ageson the order of several hundred Ma (but still with low con-centrations of cosmogenic noble gases) show e182W valuesscattering towards more negative values, some of whichare lower than the CAI initial. This demonstrates that cos-mic ray-induced neutron capture reactions on W isotopeseven affected iron meteorite samples having low concentra-tions of cosmogenic noble gases, if these samples stem fromrelatively well shielded positions in large meteoroids withlong exposure times.

The W isotopic compositions of iron meteorites withnegligible cosmic ray effects indicate that core formationin the IIAB, IIIAB and IVA iron meteorite parent bodiesoccurred þ1:0þ1:1

�1:0 Myr, þ1:2þ1:5�1:4 Myr, and þ1:6þ1:1

�1:0 Myrafter CAI formation, respectively, and that metal segrega-tion in these bodies occurred within a brief time intervalof less than �1 Myr of each other. In contrast, one un-grouped iron meteorite (Mbosi) derives from a parent bodythat underwent metal–silicate separation 2–3 Myr laterthan the major magmatic iron meteorite groups. The newHf–W results are consistent with conclusions of earlierstudies that accretion and core formation of the parentbodies of magmatic iron meteorites predated accretion ofchondrite parent bodies. However, unlike the sometimesnegative model ages obtained in some previous Hf–W stud-ies on magmatic iron meteorites, the ages obtained here areall positive and consistent with the results of a previousstudy that used physical models to correct for cosmic rayeffects on W isotope compositions of iron meteorites. Themajor advance of the present study, however, is the identi-fication of samples that remained essentially unaffected bycosmic rays, and thus require no correction on their mea-sured e182W at all.

While this study demonstrates that iron meteorites withlow cosmic ray exposure ages are well suited to determineW isotopic compositions essentially unmodified by cosmicray interactions, such samples are rare. Furthermore, Tom-bigbee River likely shows neutron capture effects on its Wisotope composition in spite of very low concentrations ofcosmogenic noble gases. In such cases, even a short secondstage exposure derived from a low noble gas concentration(perhaps in conjunction with a cosmogenic radionuclide


analysis) may not guarantee a negligible (epi)thermal neu-tron fluence during a first irradiation at larger shielding.The comprehensive application of Hf–W chronometry toall known groups of iron meteorites, therefore, requiresthe development of a direct neutron dosimeter that will per-mit the full quantification of neutron capture-induced Wisotope shifts in iron meteorites with longer exposure ages(>100 Ma). Initial results suggest that Pt isotopes may bea suitable neutron dosimeter for iron meteorites and anappropriate proxy for neutron capture-induced shifts inW isotopes. Nevertheless, future studies employing such adirect neutron dosimeter will profit from analysis of theweakly irradiated samples identified in this study. As suchsamples essentially require no correction, they will be keysamples to firmly establish the Hf–W chronology of ironmeteorites.

ACKNOWLEDGEMENTS

F. Brandstatter (Naturhistorisches Museum Wien), D. Ebel andJ. Boesenberg (American Museum of Natural History, New York),B. Hofmann (Naturhistorisches Museum Bern), M. Honda (To-kyo), A. Locock and C. Herd (Univ. Alberta, Edmonton), K. Mar-ti (Univ. California, San Diego), J. Nauber (JNMC, Zurich), and J.Zipfel (Forschungsinstitut und Naturmuseum Senckenberg, Frank-furt) are gratefully acknowledged for providing samples. ThomasKruijer thanks F. Oberli, L. Huber, N. Dalcher and M. Cosarinskyfor advice. The very detailed and constructive comments by LarryNyquist are gratefully acknowledged. This paper greatly benefitedfrom constructive reviews by Nicolas Dauphas and Larry Nyquistand the editorial efforts of Greg Herzog. This study was supportedby the Schweizerische Nationalfonds (SNF Grant PP00P2_123470to T. Kleine).

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Associate editor: Gregory F. Herzog

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