hf∓w chronometry of core formation in planetesimals inferred from
TRANSCRIPT
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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
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
isst
ud
y.
Met
eori
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
0±
0.02
93.
25±
0.39
0.01
1±
0.00
60.
057
±0.
006
na
0.09
±0.
01B
rau
nau
M03
2.41
±0.
1524
.8±
1.5
0.17
±0.
030.
645
±0.
048
150
±29
0.10
±0.
01S
ikh
ote
Ali
n‘S
A01
’B
0113
3±
956
1±
382.
61±
0.20
14.5
±0.
9521
5±
220.
24±
0.02
Sik
ho
teA
lin
‘SA
02’
G03
a12
.7±
0.68
58.3
±3.
10.
22±
0.03
1.22
±0.
079
265
±40
0.22
±0.
02S
ikh
ote
Ali
n‘S
A02
’G
03b
13.7
±0.
7663
.6±
3.5
0.25
±0.
041.
12±
0.07
425
5±
450.
22±
0.02
IIIA
Bir
on
sC
ape
Yo
rk‘C
Y01
’A
010.
574
±0.
043
3.47
±0.
310.
006
±0.
004
0.04
2±
0.00
4n
a0.
17±
0.02
Cap
eY
ork
‘CY
02’
G02
a0.
229
±0.
019
2.02
±0.
170.
005
±0.
001
0.01
7±
0.01
141
0±
104
0.11
±0.
01C
ape
Yo
rk‘C
Y02
’G
02b
0.22
0±
0.02
11.
57±
0.15
0.00
5±
0.00
10.
025
±0.
003
320
±91
0.14
±0.
02
IVA
iro
ns
Gib
eon
‘102
’N
02<
Det
ecti
on
<D
etec
tio
n<
Det
ecti
on
0.00
03±
0.00
03n
an
aM
uo
nio
nal
ust
aA
04<
Det
ecti
on
6.03
±0.
50<
Det
ecti
on
0.00
6±
0.00
1n
an
aG
ibeo
n‘R
ailw
ay’
A03
0.06
±0.
010.
58±
0.19
0.00
1±
0.00
10.
002
±0.
002
na
na
Gib
eon
‘Egg
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01a
5.20
±0.
2924
.2±
1.4
0.09
±0.
020.
454
±0.
030
270
±49
0.21
±0.
02G
ibeo
n‘E
gg’
G01
b5.
56±
0.37
26.0
±1.
70.
10±
0.02
10.
434
±0.
031
250
±55
0.21
±0.
02G
ibeo
n‘9
9’C
061.
61±
0.10
18.3
±1.
20.
06±
0.01
0.33
8±
0.02
830
0±
730.
088
±0.
008
IIG
iro
ns
To
mb
igb
eeR
iver
K04
3.34
±0.
3621
.4±
1.3
0.08
±0.
020.
421
±0.
031
270
±54
0.16
±0.
02T
wan
nb
erg
C01
4.90
±0.
2823
.4±
1.4
0.09
±0.
020.
483
±0.
035
270
±51
0.21
±0.
02
Un
gro
up
edir
on
sC
hin
gaM
015.
23±
0.35
23.8
±1.
60.
08±
0.02
0.40
0±
0.03
231
0±
850.
22±
0.02
Mb
osi
M02
0.03
2±
0.00
70.
2±
0.1
<D
etec
tio
n0.
005
±0.
003
na
na
Tis
ho
min
goC
0243
.6±
2.5
198
±11
0.71
±0.
10n
a28
0±
420.
22±
0.02
290 T.S. Kruijer et al. / Geochimica et Cosmochimica Acta 99 (2012) 287–304
Tab
le2
Tu
ngs
ten
iso
top
eco
mp
osi
tio
ns
for
the
terr
estr
ial
stan
dar
ds
anal
ysed
inth
isst
ud
y.
MC
-IC
PM
SN
e182/1
83W
e182/1
84W
e182/1
84W
e183W
e184W
e182/1
83W
e182/1
84W
e182/1
84W
(6/3
) mea
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
rr.a
±2r
±2r
±2r
±2r
±2r
NIS
T12
9cb
A07
cN
uP
lasm
a5
0.11
±0.
15�
0.05
±0.
150.
06±
0.15
�0.
21±
0.10
�0.
14±
0.07
�0.
16±
0.21
�0.
05±
0.15
�0.
07±
0.17
B07
cN
uP
lasm
a5
0.11
±0.
10�
0.11
±0.
060.
02±
0.04
�0.
14±
0.05
�0.
09±
0.03
�0.
08±
0.12
�0.
11±
0.06
�0.
07±
0.05
C07
cN
uP
lasm
a5
0.15
±0.
230.
05±
0.09
0.09
±0.
22�
0.07
±0.
12�
0.05
±0.
080.
06±
0.28
0.05
±0.
090.
04±
0.23
D07
cN
uP
lasm
a5
0.16
±0.
080.
02±
0.08
0.09
±0.
07�
0.10
±0.
07�
0.07
±0.
050.
02±
0.13
0.02
±0.
080.
02±
0.09
E07
cN
uP
lasm
a4
0.27
±0.
130.
07±
0.07
0.17
±0.
07�
0.16
±0.
08�
0.10
±0.
050.
07±
0.16
0.07
±0.
070.
07±
0.09
F07
cN
uP
lasm
a4
0.16
±0.
14�
0.05
±0.
050.
07±
0.09
�0.
15±
0.09
�0.
10±
0.06
�0.
04±
0.19
�0.
05±
0.05
�0.
03±
0.11
Mea
nN
uP
lasm
a(±
2SD
,n
=6)
�0.
01±
0.14
�0.
02±
0.18
�0.
01±
0.14
�0.
01±
0.12
NIS
T12
9cG
07c
Nep
tun
eP
lus
40.
17±
0.07
0.04
±0.
160.
10±
0.10
�0.
08±
0.13
�0.
05±
0.09
0.06
±0.
190.
04±
0.16
0.05
±0.
13R
09N
eptu
ne
Plu
s2
�0.
02±
0.12
�0.
03±
0.09
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01±
0.11
0.03
±0.
030.
02±
0.02
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02±
0.12
�0.
03±
0.09
�0.
01±
0.11
M09
Nep
tun
eP
lus
50.
06±
0.08
0.04
±0.
070.
05±
0.06
�0.
01±
0.06
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±0.
040.
06±
0.08
0.04
±0.
070.
05±
0.06
L05
cN
eptu
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0.25
±0.
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±0.
110.
10±
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18±
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12±
0.03
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11�
0.01
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±0.
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0.06
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02�
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±0.
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100.
08±
0.14
0.06
±0.
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01±
0.09
0.00
±0.
060.
05±
0.10
0.08
±0.
140.
06±
0.12
O03
cN
eptu
ne
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.
T.S. Kruijer et al. / Geochimica et Cosmochimica Acta 99 (2012) 287–304 291
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).
292 T.S. Kruijer et al. / Geochimica et Cosmochimica Acta 99 (2012) 287–304
T.S. Kruijer et al. / Geochimica et Cosmochimica Acta 99 (2012) 287–304 293
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.)
294 T.S. Kruijer et al. / Geochimica et Cosmochimica Acta 99 (2012) 287–304
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).
T.S. Kruijer et al. / Geochimica et Cosmochimica Acta 99 (2012) 287–304 295
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’
296 T.S. Kruijer et al. / Geochimica et Cosmochimica Acta 99 (2012) 287–304
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.
T.S. Kruijer et al. / Geochimica et Cosmochimica Acta 99 (2012) 287–304 297
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).
298 T.S. Kruijer et al. / Geochimica et Cosmochimica Acta 99 (2012) 287–304
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.
T.S. Kruijer et al. / Geochimica et Cosmochimica Acta 99 (2012) 287–304 299
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).
300 T.S. Kruijer et al. / Geochimica et Cosmochimica Acta 99 (2012) 287–304
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.
T.S. Kruijer et al. / Geochimica et Cosmochimica Acta 99 (2012) 287–304 301
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
302 T.S. Kruijer et al. / Geochimica et Cosmochimica Acta 99 (2012) 287–304
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