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Geochimica et Cosmochimica Acta 153 (2015) 91–104

Correlated cosmogenic W and Os isotopic variations in Carboand implications for Hf–W chronology

Liping Qin a,⇑, Nicolas Dauphas b, Mary F. Horan c, Ingo Leya d, Richard W. Carlson c

a CAS Key Laboratory of Crust-Mantle Materials and Environment, University of Science and Technology of China, Hefei, Anhui 230026, Chinab Origins Laboratory, Department of the Geophysical Sciences and Enrico Fermi Institute, The University of Chicago, 5734 South Ellis

Avenue, Chicago, IL 60637, USAc Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, NW, Washington, DC 20015, USA

d Space Research and Planetology, University of Berne, Sidlerstrasse 5, 3012 Berne, Switzerland

Received 29 April 2014; accepted in revised form 19 November 2014; available online 3 January 2015

Abstract

An obstacle for establishing the chronology of iron meteorite formation using 182Hf–182W systematics (t1/2 = 8.9 Myr) is tofind proper neutron fluence monitors to correct for cosmic ray modification of W isotopic composition. Recent studiesshowed that siderophile elements such as Pt and Os could serve such a purpose. To test and calibrate these neutron dosim-eters, the isotopic compositions of W and Os were measured in a slab of the IID iron meteorite Carbo. This slab has a well-characterized noble gas depth profile reflecting different degrees of shielding to cosmic rays. The results show that W and Osisotopic ratios correlate with distance from the pre-atmospheric center. Negative correlations, barely resolved within error,were found between e190Os–e189Os and e186Os–e189Os with slopes of �0.64 ± 0.45 and �1.8(+1.9/�2.1), respectively. TheseOs isotope correlations broadly agree with model predictions for capture of secondary neutrons produced by cosmic rayirradiation and results reported previously for other groups of iron meteorites. Correlations were also found between e182W–e189Os (slope = 1.02 ± 0.37) and e182W–e190Os (slope = �1.38 ± 0.58). Intercepts of these two correlations yield pre-exposuree182W values of �3.32 ± 0.51 and �3.62 ± 0.23, respectively (weighted average e182W = �3.57 ± 0.21). This value relies on alarge extrapolation leading to a large uncertainty but gives a metal–silicate segregation age of �0.5 ± 2.4 Myr after formationof the solar system. Combining the iron meteorite measurements with simulations of cosmogenic effects in iron meteorites,equations are presented to calculate and correct for cosmogenic effects on 182W using Os isotopes.� 2015 Elsevier Ltd. All rights reserved.

1. INTRODUCTION

Metal–silicate differentiation is an important process inthe development of planetary bodies. With a half-life of8.9 Myr (Vockenhuber et al., 2004), the short-lived decaysystem 182Hf–182W is the most often used and most versatilesystem for dating metal–silicate differentiation processes in

http://dx.doi.org/10.1016/j.gca.2014.11.015

0016-7037/� 2015 Elsevier Ltd. All rights reserved.

⇑ Corresponding author.E-mail address: lpqin@ustc.edu.cn (L. Qin).

the early solar system. Results from 182Hf–182W studies ofiron meteorites constrain core formation in small planetes-imals, up to a few hundreds of kilometers in radius (Chabotand Haack, 2006), to have occurred within �2 Myr afterthe condensation of calcium–aluminum-rich inclusions(CAIs) (Kleine et al., 2005, 2009; Schersten et al., 2006;Markowski et al., 2006b; Qin et al., 2008b, 2010;Burkhardt et al., 2012; Kruijer et al., 2012, 2013a, 2014a;Wittig et al., 2013). This suggests that iron meteorites couldbe the sole remnants of a generation of planetesimals thatwere present in the solar system before chondrites and otherplanetary bodies were formed. (Kleine et al., 2005, 2009;

92 L. Qin et al. / Geochimica et Cosmochimica Acta 153 (2015) 91–104

Schersten et al., 2006; Markowski et al., 2006b; Qin et al.,2008b; Dauphas and Chaussidon, 2011).

The ages deduced from Hf–W systematics, however,depend critically on the initial solar system abundance of182Hf and 182W, both deduced from measurements of CAIs.CAIs, however, also host tungsten nucleosynthetic anoma-lies (Burkhardt et al., 2012) whose consequences for theHf–W radiometric system must be properly accounted forin order to obtain an accurate age. Another complicatingfactor in the interpretation of Hf–W ages for iron meteoritesis the long exposure of many iron meteorites to galactic cos-mic rays. Model calculations and laboratory measurementsboth show that the W isotopic composition of meteorites(and lunar samples) can be significantly altered during theirextended exposure to galactic cosmic rays (GCR) throughsecondary neutron capture reactions on W isotopes andother nuclides such as 181Ta (Masarik, 1997; Leya et al.,2003; Markowski et al., 2006a; Qin et al., 2010; Leya andMasarik, 2013). The burnout and production effects on Wisotopes in stony meteorites can be quantified by monitoringthe isotopic composition of other elements, such as Gd, Sm,and Hf, that have large neutron capture cross sections (Leyaet al., 2003; Sprung et al., 2010, 2013; Kleine et al., 2014).However, most of the known neutron capture dosimetersare lithophile elements and are thus missing in iron meteor-ites. Using spallation products, such as 3He, to monitor cos-mic ray exposure effects results in large uncertainties on thecorrected 182W composition (Markowski et al., 2006a) and/or can only be applied to a limited sample set (Kruijer et al.,2012). This is largely due to the distinct production mecha-nisms of the two kinds of cosmogenic nuclides, with 3Hebeing produced primarily through spallation reactionscaused by primary cosmic ray particles and W isotopic vari-ations being altered almost exclusively through capture ofsecondary neutrons. The correlation between cosmogenic3He and 182W may therefore vary between meteorites of dif-ferent sizes and chemical compositions. Qin et al. (2008b)corrected for cosmogenic effects on W isotope compositionsfor individual groups of iron meteorites by applying themaximum of the modeled correction to each meteorite basedon its exposure age (Qin et al., 2008b), thus defining a lowerlimit on the pre-exposure 182W value; the uncorrected valueproviding an upper-limit. This approach gave good con-straints on the metal–silicate differentiation ages for individ-ual iron meteorite groups. In this approach, however,uncertainties on the ages depend on the number of meteor-ites studied for that group, its exposure age coverage and theaccuracy of the exposure ages. For example, these complica-tions in the use of exposure ages to correct W isotopic com-position resulted in a ±2.5 Myr uncertainty for thedifferentiation age of the IID iron meteorites (Qin et al.,2008b). A more precise correction scheme is needed to betteraccount for cosmogenic effects on the W isotopic composi-tions of iron meteorites, which is important for understand-ing the chronology of early solar system events.

Huang and Humayun (2008) first identified Os isotopevariations in group IVB iron meteorites. These variationsare consistent with cosmic ray irradiation effects (Huangand Humayun, 2008). Walker (2012) found similar co-var-iation in e190Os vs. e189Os as well as e186Os vs. e189Os in

major iron meteorite groups. Qin et al. (2010) found similarco-variation in e190Os vs. e189Os and identified correlatedcosmogenic effects between e182W and e190Os, which theyused to calculate a pre-exposure e182W value for the Carboiron meteorite of �3.3 ± 0.8. Wittig et al. (2013) use thee182W–e189Os and e182W–e190Os correlations to correctfor cosmogenic effects on W isotopic composition of IVBirons. Kruijer et al. (2013a,b, 2014a,b) and Wittig et al.(2013) found that Pt isotopes could also be used to tracesecondary neutrons and correct for cosmogenic effects onW isotopic composition in iron meteorites. Osmium andplatinum are highly siderophile elements, and thus theyare usually present at relatively high concentrations in ironmeteorites and have similar chemical behavior to W, mak-ing them useful tools for tracing neutron capture effects.

To refine the use of Os isotopic composition as a neutrondosimeter in iron meteorites, we studied samples at varyingexposure depth from a single slab of the IID iron meteoriteCarbo to search for correlated stable isotope effects betweenW and Os isotopes. Our preliminary results were reported in(Qin et al., 2010). Carbo has a long exposure age of�850 Myr (Voshage and Feldmann, 1979). The slab selectedfor this study has well documented noble gas profiles(Fireman, 1958; Hoffman and Nier, 1959; Signer and Nier,1962; Ammon et al., 2008) and W isotopic ratios that havebeen found to correlate with the distance from the pre-expo-sure center and the cosmogenic 3He contents (Markowskiet al., 2006a; Qin et al., 2010). Thus, this Carbo slab is idealfor studying, first, if the Os isotopic variations in iron mete-orites reflect cosmic ray exposure effects and, second, if theseeffects correlate with those observed in W.

2. ANALYTICAL METHODS

2.1. Sample selection

For this study we selected 10 samples from known loca-tions on several bars from the Carbo slab-1 (Fig. 1). Toallow direct inter-laboratory comparison of the W isotopicmeasurements, many of the pieces studied here were sam-pled directly adjacent to the samples analyzed byMarkowski et al. (2006b). Three additional samples froma different Carbo slab were also analyzed. Noble gas con-centrations were not measured in this second slab.

2.2. Tungsten isotopic analyses

Chemical separation protocols are explained in detail in(Qin et al., 2007) and will only be briefly discussed here.Approximately 0.3–0.5 g samples were first cut from the3–4 g sub-specimens from known locations of the Carboslab #1 and slab #571A6. The rest of the material wassaved for Os and other isotopic analyses. The samples werefirst leached with 11 N HCl–1 N HF to remove surface-sited contamination from various sources. The cleanedsamples were dissolved in mixtures of HCl and HNO3 ina 2:1 ratio, evaporated, and then re-dissolved in as littleas possible 11 N HCl. Purification of W was achievedthrough several steps of ion exchange chromatography.The first step of the anion exchange focused on removing

Fig. 1. Sampling locations in Carbo slab_1. Capital letters Athrough Y represent bars cut from the slab. The cross symbolsdescribe the sampling location in this study; the dotted symbolsrepresent the sampling locations of Markowski et al. (2006a). Alsoshown is the 3He distribution (Fireman, 1958; Hoffman and Nier,1959; Signer and Nier, 1962).

L. Qin et al. / Geochimica et Cosmochimica Acta 153 (2015) 91–104 93

the major matrix elements Fe, Ni, and Co. After doing so,the samples, dissolved in 0.2 M HCl–0.3% H2O2 werepassed through pre-conditioned AG50 X8 200–400 meshcation exchange resin and W was eluted with 0.2 M HCl–0.3% H2O2. The W fraction from the cation exchange col-umn was loaded on pre-cleaned AG1 X8 200–400 meshresin in 1 M HCl–0.5 M HF. Matrix elements and potentialisobaric interferences were eluted with 0.5 M HCl–0.5 MHF, 0.5 mM HCl–0.5 mM HF, and 9 M HCl–0.01 M HF.Tungsten was eluted last using 7 M HCl–1 M HF. Thisanion exchange column step was repeated three times toensure complete removal of unwanted elements. Unlikeour previous study (Qin et al., 2007, 2008a,b), the final Wsolutions were treated with mixtures of HNO3 and H2O2

instead of HClO4 to remove organic compounds. We chan-ged this step because we observed more variability in mea-sured W isotopic composition when the final W cut wastreated with HClO4 than when the final cut was treated withHNO3–H2O2. Treating the W cut with HClO4 helps tobreak down organic compounds, decrease molecular inter-ferences, and helps the stability of the signal, but residue ofeven a small amount of HClO4 in the final measured solu-tion will generate noise in the signal.

The isotopic composition of the final cleaned W cut wasmeasured on a Neptune multi-collector ICP-MS at theUniversity of Chicago. The ion intensities of all W isotopes

(180, 182, 183, 184, 186) and of 179Hf, 181Ta, and 188Os weremeasured on Faraday detectors. The measured intensity ofthe 184W beam was typically in the 3–5 � 10�11 A range(measured as a 3–5 V signal with a 1011 X resistance). Theisobaric interferences from 180Hf and 180Ta on 180W, from184Os on 184W, and from 186Os on 186W were correctedfor by using the signals on (interference free) 179Hf, 181Ta,and 188Os, respectively. The ion intensities were integratedfor 15 s/cycle and each measurement comprised 20 cycles(ratios). The isotope ratios were internally normalized tothe 186W/183W or 186W/184W ratio by assuming an expo-nential mass fractionation law and a natural 186W/183Wratio of 1.98594 (Volkening et al., 1991a). For comparison,the ratios were also normalized to 186W/184W (0.927672). Inaddition to the Carbo samples, we also analyzed a NIST3163 W isotope standard solution. Within each analyticalsession, the measurements of the samples were bracketedby those of the standard to minimize the effect of instru-mental drift with time. The 182W/183W and 184W/183Wratios of the samples are expressed in e-unit, which is therelative deviation of the W isotopic ratio of the sample fromthe average of those of the bracketing standards times 104.Each sample was measured 8–10 times to achieve betterprecision. The W concentrations of the sample and stan-dard were always matched to within better than 3%, aswe previously found that a mismatch >3% in the W concen-tration between the standard and the sample can affect theaccuracy of the isotopic ratio measurement (Qin et al.,2007). The resulting external precision (2 s.e.) of �10repeats is typically 0.1–0.15 e-units for 182W and <0.1e-units for 184W.

2.3. Osmium isotopic analyses

Osmium has 7 stable isotopes (184, 186, 187, 188, 189,190, 192), among which 187Os and 186Os are the decayproducts of the long-lived radioactive isotopes 187Re(t1/2 = 41.6 Ga) and 190Pt (t1/2 = 490 Ga), respectively(Smoliar et al., 1996; Cook et al., 2004). Although the neu-tron capture probability of 187Os is high (Mughabghab,2003), resolving the cosmogenic from the radiogenic effectsto this isotope is difficult because the radiogenic contribu-tion is very large, and even a small uncertainty in Re/Osratio will result in a large uncertainty on the corrected187Os/188Os ratio. In contrast, for 186Os, only a smallfraction of 186Os results from the decay of 190Pt, thus theradiogenic effect can be reliably corrected.

For the Os isotope study, we selected samples directlyadjacent to those studied for W isotopic compositions(see above) and analyzed them using the following proce-dure. Samples of approximately 0.1 g were dissolved inCarius tubes with mixtures of HNO3 and HCl in a 2:1 ratio(reverse aqua regia). With this sample size, more than600 ng of Os was extracted for each sample, which is neces-sary for achieving the desired precision on Os isotopicratios. Chemical separation of Os followed the protocoldescribed by (Puchtel et al., 2005). Briefly, the Os dissolvedin reversed aqua regia was extracted in CCl4 (Cohen et al.,1996) and back-extracted into HBr. Further purificationwas achieved by micro-distillation (Birck et al., 1997).

94 L. Qin et al. / Geochimica et Cosmochimica Acta 153 (2015) 91–104

The purified Os was loaded on 2–3 Pt filaments and wasmeasured in negative ion mode on a Thermo Fisher Tritonmulti-collector TIMS at the Carnegie Institution of Wash-ington (CIW), with each filament being analyzed 1–2 times.Thus each sample data consists of a total of 2–6 measure-ments. All Os isotopes except 184Os were measured stati-cally in Faraday cups by integrating the ion intensity for8 s in each cycle, each measurement consisting of 540 cycles(ratios). The typical signal on 192OsO3

� was 1–3 � 10�11 Afor 70–200 ng of Os. 185ReO3

� was measured to correctthe isobaric interference from 187ReO3

� on 187OsO3�,

although in most cases no 187ReO3� was detected. Os iso-

tope ratios were corrected for 17O and 18O substituted oxi-des using the method described by Walker (2012), thennormalized to 192Os/188Os, assuming a natural ratio of3.08271 (Volkening et al., 1991b). For inter-laboratorycomparison, an Os standard from the University of Mary-land (UMD Johnson-Matthey laboratory standard) wasused to externally normalize the Os isotopic ratios. The pre-cisions on 189Os/188Os and 190Os/188Os ratios are typicallysmaller than 0.1 e-units.

The 186Os/188Os ratio of the UMD standard is moreradiogenic than that measured in chondrites. Thus, the solarsystem initial 186Os/188Os value of 0.11982736 was used tocalculate the e186Os values in the samples (Walker, 2012).The measured e186Os was then corrected for 4.56 Ga ofradiogenic ingrowth of 190Pt, using the Pt and Os concentra-tions in Table 3, and reported as e186Os(T) in Table 2 (thisvalue would be zero in absence of cosmogenic effects). ThePt and Os concentrations were measured by isotope dilu-

Table 1Tungsten isotopic compositions of specimens of Carbo.

Carbo specimen Distance (cm) e182W (6/3) e184W (6/3) e

Slab_1

A-210 32.8 �3.91 ± 0.10 �0.07 ± 0.06 �D-127 21.60 �4.14 ± 0.15 �0.10 ± 0.09 �G-48 11.20 �4.29 ± 0.12 �0.04 ± 0.06 �J-35 7.2 �4.21 ± 0.10 �0.09 ± 0.08 �J-118 16 �4.16 ± 0.16 �0.02 ± 0.11 �J25 1.28 �4.25 ± 0.13 �0.07 ± 0.12 �M-15 9.6 �4.24 ± 0.06 �0.02 ± 0.07 �P-71 19.20 �4.13 ± 0.15 �0.01 ± 0.10 �V-71 31.20 �4.05 ± 0.13 �0.09 ± 0.04 �Y-82 39.20 �3.96 ± 0.15 �0.07 ± 0.09 �

Slab_571A6

#1 �4.22 ± 0.11 �0.12 ± 0.07 �#2 �4.09 ± 0.17 �0.09 ± 0.05 �#3 �4.15 ± 0.11 �0.13 ± 0.07 �#4 �3.98 ± 0.15 �0.10 ± 0.11 �

Average �0.07 ± 0.07

e18iW (6/4) and e18iW (6/3): ratios normalized to 186W/184W and 186W/18

Subscript N denotes values corrected for potential nucleosynthetic effects(6/4)N = e182W (6/4) � 1.41*e183W (6/4)0 (Burkhardt et al., 2012; Kruijer eall measured e184W (6/3) or e184W (6/3) equal to �0.07 and 0.11, respecSampling locations in Slab_1 are shown in Fig. 1. The distance givendistribution of noble gases (Fireman, 1958; Hoffman and Nier, 1959; Sign present number of repeats consisted in one analysis (see Section 2.2).The error bars on individual analyses are 2 s.e. The error bars on the gr

tion, and analyzed on the Triton TIMS (for Os) and theCIW Nu Plasma MC-ICP-MS (for Pt).

2.4. Neutron-capture model predictions

For calculating the isotopic shifts on Os and W isotopeswe use the model by Leya and Masarik (2013). Comparedto Leya and Masarik (2013), we updated some of the inputdata needed for modeling. Briefly, the model is based on theparticle spectra for primary protons, secondary protons,and secondary neutrons, calculated via Monte-Carlo tech-niques and the cross sections for the relevant nuclear reac-tions. For calculating neutron capture rates we rely on crosssections compiled in the JEFF-3.0A database and weassume a primary GCR particle flux of 2.99 cm�2 s�1

(Kollar et al., 2006). In addition to the thermal and epither-mal neutron capture reactions, we also consider reactionsby fast particles, i.e., protons and neutrons with energiesabove a few MeV. Because for most of the important reac-tions no experimental cross sections exist, we calculated thedata using the TALYS code (Koning et al., 2004). Formodeling GCR induced effects for W isotopes, we consider

neutron capture reactions on 180W, 182W, 183W, 184W, 186W,181Ta, and 185Re and fast particle reactions on W, Re, andOs. However, the W isotopic shifts are mainly affected byneutron capture effects on the W isotopes.

To model cosmic ray effects for Os isotopes, we considerneutron capture effects on 184Os, 186Os, 187Os, 188Os, 189Os,190Os, 192Os, 191Ir, 185Re, and 187Re and fast particle reac-tions on Os, Ir, and Pt. Since neutron capture on 191Ir

182W (6/4) e183W (6/4) e182W (6/3)N e182W (6/4)N n

3.76 ± 0.14 0.11 ± 0.09 �3.90 ± 0.10 �3.91 ± 0.14 103.94 ± 0.11 0.15 ± 0.13 �4.13 ± 0.15 �4.09 ± 0.11 104.20 ± 0.10 0.07 ± 0.10 �4.28 ± 0.12 �4.35 ± 0.10 104.03 ± 0.12 0.14 ± 0.13 �4.20 ± 0.10 �4.19 ± 0.12 84.11 ± 0.18 0.03 ± 0.16 �4.15 ± 0.16 �4.27 ± 0.18 104.11 ± 0.20 0.10 ± 0.17 �4.24 ± 0.13 �4.27 ± 0.20 84.19 ± 0.13 0.04 ± 0.11 �4.23 ± 0.06 �4.35 ± 0.13 104.11 ± 0.11 0.01 ± 0.16 �4.12 ± 0.15 �4.27 ± 0.11 103.88 ± 0.12 0.13 ± 0.06 �4.04 ± 0.13 �4.04 ± 0.12 103.81 ± 0.15 0.11 ± 0.13 �3.95 ± 0.15 �3.96 ± 0.15 10

3.98 ± 0.12 0.18 ± 0.11 �4.21 ± 0.11 �4.13 ± 0.12 103.91 ± 0.18 0.13 ± 0.07 �4.08 ± 0.17 �4.07 ± 0.18 103.89 ± 0.15 0.19 ± 0.10 �4.14 ± 0.11 �4.05 ± 0.15 103.78 ± 0.11 0.15 ± 0.10 �3.97 ± 0.15 �3.94 ± 0.11 10

0.11 ± 0.11

3W, respectively.using e182W (6/3)N = e182W (6/3) � 0.11*e184W (6/3)0; and e182W

t al., 2014b), where e184W (6/3)0 and e183W (6/4)0 are the average oftively.is from the pre-atmospheric center as inferred from studying thener and Nier, 1962).

oup mean are 2 s.d.

Table 2Osmium isotopic compositions of specimens of Carbo.

Specimen 186Os/188Os 187Os/188Os 189Os/188Os 190Os/188Os e186Os(T)

Slab_1

A-210 0.119851 ± 3 0.126972 ± 3 1.219628 ± 8 1.983744 ± 90.119856 ± 2 0.126972 ± 2 1.219624 ± 6 1.983777 ± 70.119851 ± 3 0.126976 ± 3 1.219641 ± 8 1.983810 ± 100.119853 ± 3 0.126954 ± 4 1.219617 ± 7 1.983773 ± 100.119855 ± 5 0.126956 ± 6 1.219633 ± 3 1.983785 ± 60.119858 ± 8 0.126957 ± 7 1.219628 ± 8 1.983800 ± 7

Average 0.119854 ± 4 0.126964 ± 4 1.219628 ± 8 1.983781 ± 19eiOs 2.23 ± 0.20 1153.56 ± 1.71 �0.51 ± 0.05 0.31 ± 0.10 1.56 ± 0.20

D-127 0.119853 ± 2 0.126884 ± 2 1.219588 ± 6 1.983754 ± 80.119853 ± 2 0.126880 ± 2 1.219584 ± 6 1.983763 ± 80.119857 ± 3 0.126877 ± 3 1.219592 ± 9 a1.983801 ± 110.119850 ± 3 0.126884 ± 3 1.219587 ± 9 1.983769 ± 10

Average 0.119853 ± 3 0.126882 ± 3 1.219588 ± 3 1.983762 ± 7eiOs 2.15 ± 0.21 1150.35 ± 0.26 �0.56 ± 0.07 0.32 ± 0.07 1.48 ± 0.21

G-48 0.119849 ± 2 0.126919 ± 3 1.219573 ± 7 1.983761 ± 80.119855 ± 2 0.126924 ± 2 1.219580 ± 6 1.983788 ± 70.119848 ± 2 0.126918 ± 2 1.219574 ± 7 1.983773 ± 80.119852 ± 2 0.126919 ± 2 1.219563 ± 7 1.983792 ± 80.119865 ± 3 0.126924 ± 3 1.219582 ± 8 1.983816 ± 9

Average 0.119854 ± 6 0.126921 ± 2 1.219575 ± 6 1.983786 ± 17eiOs 2.21 ± 0.47 1153.80 ± 0.26 �0.67 ± 0.07 0.45 ± 0.09 1.54 ± 0.47

J-35 0.119863 ± 11 0.126809 ± 12 1.219590 ± 24 1.983803 ± 280.119857 ± 7 0.126823 ± 6 1.219595 ± 16 1.983802 ± 20

Average 0.119860 ± 6 0.126816 ± 14 1.219592 ± 6 1.983802 ± 0eiOs 2.75 ± 0.51 1140.54 ± 1.71 �0.80 ± 0.05 0.42 ± 0.05 2.08 ± 0.51

J-118 0.119856 ± 3 0.126917 ± 3 1.219584 ± 7 1.983774 ± 90.119855 ± 2 0.126920 ± 2 1.219584 ± 6 1.983790 ± 80.119857 ± 3 0.126921 ± 3 1.219591 ± 8 1.983804 ± 10

Average 0.119856 ± 1 0.126919 ± 3 1.219587 ± 5 1.983789 ± 17eiOs 2.39 ± 0.20 1153.67 ± 26 �0.58 ± 0.07 0.46 ± 0.09 1.72 ± 0.20

M-15 0.119843 ± 5 0.126768 ± 3 a1.219502 ± 0.17 1.983826 ± 110.119857 ± 2 0.126782 ± 2 1.219600 ± 5 1.983814 ± 70.119856 ± 2 0.126783 ± 2 1.219602 ± 6 1.983814 ± 8

Average 0.119852 ± 9 0.126777 ± 9 1.219601 ± 2 1.983818 ± 8eiOs 2.05 ± 0.77 1137.13 ± 1.71 �0.74 ± 0.03 0.50 ± 0.05 1.38 ± 0.77

V-71 0.119849 ± 3 0.126941 ± 3 1.219618 ± 8 1.983783 ± 100.119857 ± 2 0.126782 ± 2 1.219600 ± 5 1.983814 ± 70.119856 ± 2 0.126942 ± 2 1.219614 ± 5 1.983793 ± 60.119861 ± 2 0.126940 ± 2 1.219618 ± 6 1.983801 ± 80.119854 ± 2 0.126942 ± 2 1.219607 ± 5 1.983768 ± 7

Average 0.119855 ± 4 0.126909 ± 1 1.219611 ± 7 1.983792 ± 16eiOs 2.34 ± 0.33 1148.73 ± 5.03 �0.65 ± 0.06 0.37 ± 0.08 1.67 ± 0.33

Y-82 0.119847 ± 3 0.126944 ± 3 1.219596 ± 7 1.983741 ± 90.119850 ± 2 0.126941 ± 2 1.219603 ± 6 1.983741 ± 7

Average 0.1198488 ± 2 0.1269424 ± 2 1.2195994 ± 5 1.9837410 ± 0eiOs 1.79 ± 0.20 1155.70 ± 0.26 �0.47 ± 0.07 0.22 ± 0.07 1.12 ± 0.20

Slab_571A6#1 0.119859 ± 1 0.126947 ± 1 1.219613 ± 5 1.983779 ± 6

0.119858 ± 1 0.126946 ± 1 1.219622 ± 5 1.983793 ± 50.119860 ± 2 0.126946 ± 2 1.219615 ± 5 1.983797 ± 60.119859 ± 2 0.126949 ± 2 1.219615 ± 6 1.983817 ± 80.119860 ± 2 0.126948 ± 2 1.219618 ± 5 1.983820 ± 60.119858 ± 2 0.126948 ± 2 1.219624 ± 6 1.983819 ± 7

(continued on next page)

L. Qin et al. / Geochimica et Cosmochimica Acta 153 (2015) 91–104 95

Table 3Pt, Os concentration in Carbo.

Specimen Pt (ppm) Os (ppm) Pt/Os (weight)

J-35 20.14 15.68 1.284J-118 20.64 15.92 1.296

Fig. 2. Direct comparison of e182W results from Markowski et al.(2006a; measured with a Nu Plasma) and this study (measured witha Neptune) for adjacent samples.

Table 2 (continued)

Specimen 186Os/188Os 187Os/188Os 189Os/188Os 190Os/188Os e186Os

Average 0.119859 ± 1 0.126948 ± 1 1.219618 ± 4 1.983804 ± 14eiOs 2.64 ± 0.07 1155.92 ± 0.26 �0.61 ± 0.03 0.38 ± 0.07 1.97 ± 0.07

#3 0.119853 ± 1 0.126882 ± 1 1.219614 ± 4 1.983763 ± 50.119855 ± 1 0.126883 ± 2 1.219623 ± 5 1.983797 ± 60.119859 ± 2 0.126883 ± 2 1.219615 ± 5 1.983814 ± 80.119861 ± 2 0.126934 ± 2 1.219619 ± 5 1.983796 ± 70.119856 ± 2 0.126929 ± 2 1.219618 ± 6 1.983809 ± 7

Average 0.119857 ± 3 0.126902 ± 24 1.219618 ± 3 1.983796 ± 18eiOs 2.45 ± 0.22 1151.92 ± 1.89 �0.62 ± 0.03 0.33 ± 0.09 1.78 ± 0.22

UMD Os stdSession #1 0.119857 ± 1 0.113794 ± 3 1.219693 ± 4 1.983730 ± 9Session #2 0.119855 ± 2 0.113833 ± 19 1.219690 ± 4 1.983719 ± 10Session #2 0.119853 ± 2 0.113792 ± 3 1.219657 ± 9 1.983698 ± 14

Sampling locations in Slab_1 are shown in Fig. 1.The error bars on the sample Os isotopic ratios are the internal error bars of individual runs, which are the two standard errors of up to 540ratios. The error bars on the UMD Os standard isotopic ratios are the external reproducibility for the UMD Os standard during oneanalytical session, which is 2 times the standard error of 5–7 runs within an analytical session. A total of three analytical sessions were run forall Os measurements.The eiOs (i = 186, 187, 189, 190) values of the samples were calculated relative to the average of the standard measurements in the sameanalytical session times 10,000, except for e186Os. The uncertainty on eiOs is the 2 standard error of the sample or standard measurements,whichever is larger.The 186Os/188Os value of the UMD standard is higher than chondrites. Thus the solar system initial 186Os/188Os value of 0.11982736 was usedto calculate the e186Os value in the samples (Walker, 2012). e186Os(T) value represents the e186Os value corrected for 4.56 Ga of radiogenic in-growth using measured Pt/Os ratios (Table 3).

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and 185Re significantly contributes to the cosmic rayinduced Os isotope shifts, the modeled effects are not onlya function of shielding and exposure age but also a functionof Re/Os and Ir/Os elemental ratios. In contrast, effects dueto neutron capture on 181Ta and 185Re are only very minorfor the cosmic ray effects on W isotopes, therefore theirshifts are simply expressed as a function of exposure age,pre-atmospheric size, and shielding depth. For more detailson the model calculations see Leya and Masarik (2013).

3. RESULTS

3.1. Tungsten isotopic results

The W isotopic ratios normalized to both 186W/183Wand 186W/184W ratios are reported and compared in Table 1.We found limited but resolvable variations (from �4.29 to�3.91) in e182W values normalized to 186W/183W among theCarbo pieces studied (Table 1). The e182W values normal-ized to 186W/184W also show a range but are less negativethan those normalized to 186W/183W. The reason will befurther explored in the next section. To test the accuracyof our analytical method, the e182W (186W/183W) valuesobtained in this study are compared with those reportedby Markowski et al. (2006a,b) for adjacent samples. The

two datasets are in excellent agreement (Fig. 2). In all thefigures where e182W values are plotted, the e182W valuesnormalized to 186W/183W were used, and the reason willbe discussed in the next section. Fig. 3 shows a plot ofe182W as a function of distance from the pre-atmosphericcenter for samples from Carbo slab #1. A trend is observedwith e182W values increasing with distance from the pre-atmospheric center, although the e182W values largely over-lap. Our results fall on the same trend as that defined by thedata of Markowski et al. (2006a,b) (Fig. 3).

Fig. 3. e182W depth profile for Carbo Slab_1. Also shown areresults by Markowski et al. (2006a). The lines represent differentmodel simulation results for neutron capture. The modeling resultsfrom this study (black line) is based on a pre-atmospheric radius of65 cm (Ammon et al., 2008), as compared to a value of 50 cm usedin our previous model simulations (dashed lines) (Qin et al., 2008b).JEF and KASKAD stand for two different nuclear reactionlibraries.

Fig. 4. exOs (x = 186, 189, 190) depth profile for Carbo Slab_1.e186Os(T) values are corrected for radiogenic ingrowth of 190Ptusing a decay constant of 4.90 Gyr (see Table 2 for details). Thelines represent model predictions for neutron-capture.

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All Carbo pieces display systematic small negative e184W(6/3) values, with an average of �0.07 ± 0.07 (2 s.d.). Sim-ilarly, e183W (6/4) values are all systematically slightly posi-tive, averaging at 0.11 ± 0.11 (2 s.d.). The magnitudes ofthese negative and positive anomalies are not quite statisti-cally resolved from zero. There is a difference between thee182W (6/4) and e182W (6/3) values of �0.1 to 0.2 e. Thereason of this discrepancy will be further discussed in Sec-tion 4.

Small mass-independent fractionation in 183W intro-duced either by the mass spectrometry (Shirai andHumayun, 2011) or during sample preparation have beenpreviously documented (Willbold et al., 2011; Kruijeret al., 2012, 2013a), causing spurious isotope data. How-ever, such effects are usually inconsistent from sample tosample (Kruijer et al., 2013a), and are unlikely to be thereason of the seemingly systematic negative e184W (6/3) orpositive e183W (6/4) observed in this study.

The 188Os signals during W measurements in all samplesare slightly higher than those in the standard with188Os/186W ratios on the order of 10�5, corresponding tonegligible corrections on e184W values of <0.01 e. Neverthe-less, possible interferences from 186Os and 184Os werealways corrected in all measurements. Thus the small nega-tive e184W values are also not likely caused by an under- orover-correction of interferences. Small molecular isobaricinterferences on relevant W peaks are also unlikely, becausethey are not expected to influence the 184W data for varioussamples similarly. Possible reasons for the apparent deficitsin 184W will be further explored in Section 4.1.

3.2. Osmium isotopic results

The final Os fractions of sample J25 from Slab_1 andsample #2 from Slab_571A6 were contaminated withCrO3 that was used in the micro-distillation step to oxidize

Os, thus no valid data were collected for the two samples.This happened when the mixture of CrO3 and sampleplaced in the cap of the inverted conical vial was heatedto distill Os as OsO4 vapor, a tiny amount of CrO3 wassplashed onto the wall of the vial and was then accidentlypicked up along with the reducing trapping HBr drop atthe bottom of the vial. The Cr-contaminated Os samplesfailed to run in the mass spectrometer. Nevertheless, allother Carbo pieces show deficits in e189Os ranging from�0.47 to �0.80, accompanied by excesses in e190Os of+0.22 to +0.50 (Table 2). Both e190Os and e189Os valuesbroadly correlate with distance from the pre-atmosphericcenter, with e190Os values increasing and e189Os valuesdecreasing with increasing depth in the meteorite, i.e., fromthe surface to the center (Fig. 4). The e190Os and e189Os val-ues are linearly correlated (Fig. 5a). Using Probability_2 fitof Isoplot (Ludwig, 2003), we calculate a slope of�0.64 ± 0.45 (95% C.I., MSWD = 2.7) for this correlation(Fig. 5a). Unless otherwise indicated, all the slopes belowwere calculated using Isoplot, model-2 fit, and the errorbars on the slopes are 95% C.I.

Larger deviations from the terrestrial standard areexpected for e186Os due to the large neutron capture crosssection and resonance integral of 186Os (Mughabghab,2003). As discussed above, quantifying cosmogenic effectson e186Os requires correction of the radiogenic effects from190Pt decay, which requires knowledge of the Pt/Os ratio.The concentrations of Pt and Os were determined for twoCarbo specimens, J-35 and J-118. The results are given inTable 3. The Pt/Os ratios for the two specimens are 1.284and 1.296, respectively. Using an average Pt/Os of 1.290,a half-life for 190Pt of 490 Ga (Cook et al., 2004), andassuming a core crystallization age of �4560 Ma, the calcu-lated radiogenic contribution to the e186Os values is 0.67 e.After radiogenic corrections, all Carbo specimens still showa systematic positive shift in e186Os ranging from+1.12 ± 0.20 to +2.08 ± 0.50 relative to the solar systeminitial. A correlation with distance from the pre-atmo-spheric center is also observed, although measurementuncertainties are relatively large. A rough negative linear

a b

Fig. 5. (a) e190Os vs. e189Os, and (b) e186Os vs. e189Os for Carbo, IID. The Os isotope data from Walker (2012), Wittig et al. (2013) for otheriron meteorite groups were also shown for comparison. The s- and r-mixing line (dotted line) in (a) was calculated as in (Dauphas et al., 2004,2014) using a s-process abundances provided by (Reisberg et al., 2009); The s- and r-mixing line in (b) was calculated using the s-processabundances calculated in (Reisberg et al., 2009), which used the MACs values in (Mosconi et al., 2006) for light Os isotopes through 188Os,and the MACs values in (Bao et al., 2000) for heavy Os isotopes. The solar system abundances are from (Lodders, 2003). The gray solid linesshow neutron capture simulation results modeled for mean iron meteorite composition. The black solid line and the dashed lines show linearregressions to the data from this study and to all the data, respectively.

a b

Fig. 6. (a) e182W vs. e189Os, and (b) e182W vs. e189Os for Carbo specimens, in comparison with data for IVBs (Wittig et al., 2013) and forRodeo, IID. The e182W value for Rodeo was from Kruijer et al. (2013a). The e189Os and e190Os values of this sample are assigned to 0, as thissample shows no cosmogenic effect on Pt isotopic composition (Kruijer et al., 2013a). The linear regression line (dashed line) is for thecombined datasets, as both groups show similar pre-exposure e182W (Kruijer et al., 2013a,b; Wittig et al., 2013) and similar correlationsbetween W and Os isotopic composition. Combining the datasets provide a better constraint on our model simulation.

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correlation with a slope of �1.8(+1.9/�2.1) (Isoplot, robustregression) is observed between e186Os and e189Os (Fig. 5b).

All studied Carbo specimens have much more radio-genic 187Os/188Os ratios than the standard. However, con-tributions from the radiogenic ingrowth of 187Re are toolarge to be properly corrected for (i.e., a small uncertaintyin Re/Os ratio will result in large uncertainty in radiogeniccorrection). Therefore, 187Os will not be discussed further.

Fig. 6 demonstrates that e182W values linearly correlatewith e189Os and e190Os. The slopes for data produced in thisstudy are 1.02 ± 0.37 for e182W vs. e189Os and �1.38 ± 0.58for e182W vs. e190Os.

3.3. Results of the model calculations for neutron capture

production of Os and W isotopes

As shown in Fig. 3, the current model predictions givevery similar depth-dependent neutron capture profiles fore182W to our previous modeling results, which were basedon the nuclear libraries KASKAD and JEF (Qin et al.,2008b). Note that our previous modeling results are for ameteoroid with a pre-atmospheric radius of 50 cm (Qinet al., 2008b). Based on a recent study, the pre-atmosphericradius of Carbo might be larger (�65 cm) (Ammon et al.,2008), and the present model uses that radius. Our model

Fig. 7. e182W vs. e184W for all Carbo specimens in this study (blackdiamonds) and from previous studies (gray circles) (Markowskiet al., 2006a; Qin et al., 2008b; Kruijer et al., 2013a).

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results for e182W are consistent with the measurements,showing a large depletion relative to the pre-exposuree182W value. Although the modeled cosmogenic effects on184W are big, the mass bias calculation procedure cancelsout most of the effects on e184W.

The magnitude of the neutron capture effects on Os iso-topic composition, as well as the predicted slopes of thee190Os–e189Os and e186Os–e189Os correlations, dependstrongly on the Re/Os and weakly on the Ir/Os elementalratios of the studied samples. Given the limited range ofvariations in Re/Os and Ir/Os in iron meteorites (Morganet al., 1995; Shen et al., 1996; Walker et al., 2008), the slopesof e190Os–e189Os correlations from neutron capture for alliron meteorites should be between �0.76 to �0.52. Largervariations are possible for the slope of e186Os–e189Os(�1.4 to �3.5) as a result of direct production of 186Os byneutron capture on 185Re.

Using a Re/Os ratio of 0.08 and an Ir/Os ratio of 0.85(Petaev and Jacobsen, 2004) and a pre-atmospheric radiusof 65 cm, the model predicts maximum neutron captureeffects on e189Os and e190Os of �0.57 and +0.41 e-units,respectively. We sampled only the innermost �40 cm, andthe model predicts variation among the studied samplesof �0.28 e-units and +0.20 e-units for e189Os and e190Os,respectively. The isotopic shifts in 189Os predicted by themodel are slightly lower than the measured anomalies(Fig. 4). The variation of e190Os with depth agrees withthe model prediction, while the e186Os data sit above thepredicted depth profile. Interestingly, the modeled slopesfor the e190Os–e189Os and e186Os–e189Os correlations(�0.703 and �1.93) are very similar to the slopes obtainedfrom the Carbo measurements (�0.64 ± 0.45 and�1.8(+1.9/�2.1)).

4. DISCUSSION

4.1. Causes of tungsten isotopic variations

Carbo has among the most negative e182W valuesobserved so far in iron meteorites (e.g., Kleine et al.,2005, 2009; Schersten et al., 2006; Markowski et al.,2006a,b; Qin et al., 2008a,b, 2010; Burkhardt et al., 2012;Kruijer et al., 2013a,b, 2014a,b; Wittig et al., 2013).Although not resolvable with error, the e182W valuesamong different Carbo pieces decrease with distance fromthe pre-atmospheric surface, which is consistent with modelpredictions of neutron capture effects on W isotopes(Fig. 3). Thus the variations in e182W values can be attrib-uted to exposure to galactic cosmic rays. The data fit apolynomial regression and extrapolating the data to thepre-atmospheric surface, where the thermal and epithermalneutron concentrations are expected to be the lowest(though not zero), yields a pre-exposure e182W value of ��3.3.

Our numerical results indicate that the maximum cos-mogenic effects should be at the pre-atmospheric center,with a negative shift in e184W (6/3) of �0.14. Negativee184W (6/3) values are indeed found in Carbo, but thesedo not correlate with e182W (6/3) as would be expected ifcosmogenic effects were present on both isotopes, casting

doubt on the possibility that the barely resolvable negativeeffects on e184W are due to neutron capture (Fig. 7). Notethat the absolute cosmogenic effect on 184W is significant,but cosmogenic effects on the isotopes involved in the inter-nal normalization (183W and 186W) pair are propagated to184W, which counteracts the direct cosmogenic effect on184W and fortuitously cancels out most of the effect onthe internally normalized e184W value.

A more plausible explanation for the 184W deficits is thepresence of small nucleosynthetic anomalies. Our previouswork revealed W isotopic heterogeneity (negative e184Wvalues) in IVB iron meteorites both with and without signif-icant exposure effects on e182W, pointing to a nucleosyn-thetic origin (Qin et al., 2008a). This result was confirmedby Kruijer et al. (2013a,b and Wittig et al. (2013). Kruijeret al. (2013a) also reported the presence of small deficitsin e184W for Carbo and another IID iron meteorite Rodeo,after correcting for mass-independent 183W fractionationintroduced in the sample preparation procedure. The onlyCarbo measurement in our previous study (Qin et al.,2008a) showed no resolvable nucleosynthetic anomaly,but is consistent with the current data within error. Nucle-osynthetic W isotope anomalies also have been reported inCAIs and acid leachates of chondrites (Burkhardt et al.,2012). Thus the slightly negative e184W (6/3) and positivee183W (6/4) values of Carbo may reflect the presence ofsmall nucleosynthetic variability on 184W relativeabundance.

Assuming that the anomalous e184W (6/3) and e183W(6/4) values are the result of nucleosynthesis, the effectson e182W (6/3) and e182W (6/4) values can be correctedfor according to equations from Burkhardt et al. (2012)and Kruijer et al. (2014a) (Table 1), using average e184W(6/3) and e183W (6/4) values of �0.07 ± 0.07 and0.11 ± 0.11, respectively. The resulted corrections toe182W (6/3) and e184W (6/4) are 0.01 and �0.15, respec-tively. The correction to e182W(6/3) is minimal comparedto measurement uncertainty, but is significant for e182W(6/4). After applying these corrections, the differencebetween e182W (6/3) and e184W (6/4) is very small, within0.1 for most samples.

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Kruijer et al. (2013a) suggest that mass-independentfractionation on 183W can only increase e184W (6/3) anddecrease e183W (6/4) for some unknown causes. In the pres-ent study, several samples (J-115, M-15, P-71) show lessnegative e184W (6/3) values and less positive e183W (6/4)values than the rest, but even these overlap with the othersamples within analytical uncertainties. Although thesesamples also show slightly larger discrepancy betweennucleosynthetic-effect-corrected e182W (6/3) and e182W (6/4), again the difference is within the analytical uncertainty.More importantly, we did not find a correlation betweeninternally normalized e182W (6/3) or e182W (6/4) with rawratios of 183W/184W (relative to bracketing standards).Thus, we conclude that the mass-independent fractionationof 183W in Carbo is not resolvable at the current analyticaluncertainty, and so prefer not to correct our e182W (6/3)values, except for nucleosynthetic effects.

4.2. Causes of Os isotopic variations

4.2.1. Nucleosynthetic effect?

Although, there is some discrepancy between the dataand model predictions, the changes in e186Os, e189Os, ande190Os as a function of depth from the pre-atmospheric sur-face (Fig. 4) are in the same direction as those predicted bymodeling. The model-predicted e190Os–e189Os and e186Os–e189Os slopes are consistent with those defined by Carbomeasurements. Besides cosmogenic effects, nucleosyntheticanomalies have been reported for Os (Brandon et al.,2005; Yokoyama et al., 2007, 2010; Reisberg et al., 2009).Selective dissolution of primitive carbonaceous and ordin-ary chondrites revealed Os isotopic heterogeneity withinthese meteorites, and the correlation line between e190Osand e189Os is in good agreement with a mixing line betweens- and r-components predicted by AGB stellar models(Brandon et al., 2005; Yokoyama et al., 2007, 2010;Reisberg et al., 2009).

Two lines of evidence suggest that the variations in Osisotopic composition measured in Carbo do not have anucleosynthetic origin. Most importantly, any nucleosyn-thetic anomalies in the IID parent body should have beenhomogenized in its molten core, and should be constantamong different IID meteorites derived from that core.Our data for Carbo, by contrast, show correlated variationsin e186Os, e189Os, and e190Os with distance from the pre-atmospheric center (Fig. 4), which would not be expectedif these variations reflected nucleosynthetic anomaliesalone. Second, the observed e190Os–e189Os, and e186Os–e189Os correlations for Carbo are in good agreement withcorrelations in other groups of iron meteorites, and distinc-tive from s- and r-mixing lines (Fig. 5) (Walker, 2012;Wittig et al., 2013).

4.2.2. Neutron capture effects

Although neutron capture reactions in meteorites areusually assumed to be in the thermal energy range, studieshave shown that neutron capture in the epithermal energyrange is also important for iron meteorites (Kollar et al.,2006; Sprung et al., 2010; Leya and Masarik, 2013;Kruijer et al., 2013b). Our measured Os isotope values

are broadly consistent with model-predicted neutron cap-ture effects for e190Os and e189Os. However a larger discrep-ancy was observed for e186Os: our model-predicted valuesare lower than observations. We note that there is a largevariation in the decay constant of 190Pt in the literaturedepending on the method used (Tavares et al., 2006). Tobring the radiogenic-corrected e186Os values closest to themodel depth profile, the half-life of 190Pt would have tobe �2.85 Gyr (Fig. 4). This half life is shorter than boththose inferred by geological comparison (4.90 Gyr) (Cooket al., 2004) and based on quantum mechanical consider-ations (3.7 ± 0.3 Gyr) (Tavares et al., 2006). One possibilityif that the model underestimates the effect on e186Os. Thecapture rates have an uncertainty in the range of 10–20%.Giving a proper estimate for model uncertainty is impossi-ble, but the total uncertainty should be <20% for the finalshifts on epsilon values. This is barely enough to explainthe difference we see here.

The co-variations of Os isotopic ratios in iron meteoritesfrom the two recent studies (Walker, 2012; Wittig et al.,2013) are compared to our Carbo data and the slopes pre-dicted by our model in Fig. 5a and b. Iron meteorites fromvarious groups (IIAB, IID, IIIAB, IVA, and IVB) plot onsimilar trends in the e190Os vs. e189Os diagram (Fig. 5a).No resolvable dependence on the chemical compositionwas found for the e190Os–e189Os correlation. This isexpected given the limited range of variations in Re/Osand Ir/Os ratios in iron meteorites and uncertainties onOs isotopic ratio measurements. Regression of all ironmeteorites has a slope of �0.60 ± 0.07, similar to the mod-eled value (�0.66) for a mean iron meteorite composition(approximately at Re/Os = 0.1, Ir/Os = 1). After fittingour model to these measurements, the e190Os–e189Os slopecan be approximately described as (see Appendix A fordetails):

k190=189 ¼ �0:92þ 3:6 ðRe=OsÞ � 4:78 ðRe=OsÞ2

þ 0:006 ðIr=OsÞ � 0:02 ðRe=OsÞðIr=OsÞ:

Because the above slope is only very weakly dependenton the Ir/Os ratio, for simplicity, the last two terms canbe omitted, and we have

k190=189 ¼ �0:92þ 3:6 ðRe=OsÞ � 4:78 ðRe=OsÞ2:

Regarding the e186Os–e189Os slope, the model-predictedslope for the Carbo iron meteorite (�1.93) also agrees withthat defined by Carbo measurements (�1.8 + 1.9/�2.1),although the latter is not precisely determined. A previousstudy (Walker, 2012) suggested that the e186Os–e189Os cor-relation may be different among different groups of ironmeteorites. However the first-order observation is that alliron meteorites form a single trend in the e186Os–e189Os plot(Fig. 5b), with a slope of �2.44 ± 0.34, consistent with themodeled value (�2.36) for a mean iron meteorite composi-tion. This relatively well-defined slope is at odds with modelpredictions that there should be large variations in thisslope among meteorites with different compositions. Thiscould be a result of sample bias (i.e., the slope is largelydetermined by Carbo measurements) and poor resolutionin the data (i.e., relatively large error bars on 186Os

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measurements). Similar to the e190Os–e189Os slope, thee186Os–e189Os slope can be also approximately describedas a function of Re/Os and Ir/Os ratios, and the terms withIr/Os can be omitted:

k186=189 ¼ 0:02� 28:4 ðRe=OsÞ þ 35:8 ðRe=OsÞ2:

4.3. Co-variations of W and Os isotopes, and implication for

Hf–W chronology in Carbo

4.3.1. Corrections for neutron capture effects on e182W based

on Os isotopic variations

The cosmogenic shifts in Os isotopic compositiondepend on the Re/Os and Ir/Os ratios of the target mate-rial, while cosmogenic shifts in W isotopes are relativelyinsensitive to the chemical compositions. Thus the e182W–e189Os and e182W–e190Os correlations could vary amongdifferent groups of iron meteorites and among different ironmeteorites from a single group. Our model simulation pre-dicts that the e182W–e189Os and e182W–e190Os slopes shouldshow less dependency on the Re/Os and Ir/Os ratios thanthe e190Os–e189Os and e186Os–e189Os slopes.

By combining measured e182W–e189Os or e182W–e190Osslopes with known Re/Os and Ir/Os ratios, and slopesobtained from our model, we can derive a simple set ofequations to predict the neutron capture effects on e182Wfor any iron meteorite with known Re/Os and Ir/Os ratiosand Os isotopic composition. We note that the Re/Os andIr/Os ratios in Carbo are close to those of IVBs, and thetwo groups have similar pre-exposure e182W (Kruijeret al., 2013a, 2014b). Therefore the W–Os correlations areexpected to be very similar and the data from both groupscan be combined to provide a better constraint on ourmodel. The W–Os co-variation for IVBs (Wittig et al.,2013) are compared with those of Carbo in Fig. 6. AnotherIID iron, Rodeo was measured by Kruijer et al. (2013a,b)for both Pt and W isotopic composition and shows noresolvable cosmogenic effects on Pt isotopic composition.We can simply assume that this sample has e189Os ande190Os values both equal to 0, and this sample was alsoplotted in Fig. 6. The measured data for the two groupsof iron meteorites yield combined slopes of 1.27 ± 0.19and �2.06 ± 0.36 for e182W–e189Os and e182W–e190Os cor-relations, respectively. The effect of Ir/Os is so small thatit can be neglected. The e182W–e189Os and e182W–e190Osslopes resulting from varying neutron capture can beparameterized as:

k182=189 ¼ 1:64� 5:28 ðRe=OsÞ þ 7:25 ðRe=OsÞ2; and

k182=190 ¼ �1:76� 2:84 ðRe=OsÞ � 13:53 ðRe=OsÞ2

and the corresponding cosmogenic – corrected e182W* valueis:

e182W� ¼ e182Wmeasured � k182=189e189Osmeasured; or

e182W� ¼ e182Wmeasured � k182=190e190Osmeasured:

These formulas are fits to complex numerical modelresults and can only be applied to iron meteorites. This isbecause the dominance of Fe in the target material leads

to a neutron energy spectra that is dominated by neutronsat epithermal and higher energies (Kollar et al., 2006;Sprung et al., 2010; Kruijer et al., 2013b). It has long beenknown that varying Fe content will change the neutronenergy spectra, but in iron meteorites the small spread inchemical compositions is not enough to change the neutronenergy spectra. As shown in Fig. 6, the e182W–e189Os corre-lations defined by our own data and by the two combineddatasets, respectively, are very similar. On the other hand,the variation in e190Os in our data and the combined data-sets is only about half of that displayed by e189Os, so whilethe combined datasets are consistent with a single slope, thescatter about the best fit line is larger than for e182W–e189Os. Thus, the e182W–e189Os correlation should be pre-ferred over the e182W–e190Os correlation to correct e182Wfor cosmogenic effects.

Equations for correction of cosmogenic effects on e182Wusing measured Pt isotopic compositions (Wittig et al.,2013; Kruijer et al., 2013a, 2014b) are also given here. Dif-ferent from the approaches above to derive the W–Os cor-

relations, the e182W–e196Pt (internally normalized with198Pt/195Pt ratio) correlation is linear and its slope doesnot change with the chemical composition of the samples.The e182W–e192Pt correlation is simply a power of Ir/Ptratio. Thus we have:

k182=196 ¼ �1:32

k182=192 ¼ �0:0194 ðIr=PtÞ�0:98

e182W� ¼ e182Wmeasured þ 1:32e196Ptmeasured

e182W� ¼ e182Wmeasured þ 0:0194 ðIr=PtÞ�0:98 e192Ptmeasured:

The slope between e182W–e196Pt is the same as that givenin (Kruijer et al., 2014b), which is based on the weightedmean of five iron meteorite groups. The constant slopebetween e182W–e196Pt also indicates the robustness of thiscorrection method. We also note that the e182W–e192Pt cor-relation is strongly dependent on the chemical compositionof the sample. The two previous studies (Kruijer et al.,2013a, 2014b; Wittig et al., 2013) give different slopes forIVBs and for consistency we used the slopes for IVB andIID given in (Kruijer et al., 2013a, 2014b) to constrainthe model.

4.3.2. Hf–W chronology in Carbo

The deviation of the Os isotopic composition of samplesof Carbo from a terrestrial standard can be used to correctfor cosmic ray irradiation effects on e182W. Linear extrapo-lation of the two correlation lines in Fig. 6 to e189Os = 0and e190Os = 0, yields intercepts of �3.31 ± 0.55 and�3.63 ± 0.23 for e182W, respectively. Because the twoe189Os and e190Os anomalies are affected by the same reac-tion (189Os + n� ! 190Os), assuming there is no systematicbias on the two anomalies, we can treat the two as individ-ual analyses of the same quantity. The weighted average forthe two pre-exposure e182W (�3.57 ± 0.21) is more negativethan the value of �3.15 ± 0.07 recently reported for IIDiron meteorites (Kruijer et al., 2014a), but relies on a largeextrapolation leading to a large uncertainty. IncludingRodeo, a IID iron that shows no cosmogenic effects on Pt

102 L. Qin et al. / Geochimica et Cosmochimica Acta 153 (2015) 91–104

isotopes (Kruijer et al., 2013a,b, 2014b), in our regressionby assuming both e189Os = 0 and e190Os = 0, yields pre-exposure e182W values of �3.27 ± 0.2 and �3.31 ± 0.14from the e182W vs. e189Os and e182W vs. e190Os correlations,respectively. The weighted average intercept of�3.29 ± 0.13 is more in line with (Kruijer et al., 2014b),which is expected given that the regression is largely lever-aged by this data point. Rodeo was used as an anvil at aforge in Mexico and its outer portion shows evidence forplastic flow from the use of heavy tools and a specimenwas described with a dent produced by a tool that had beenforced into the mass and broken (Buchwald, 1975) .WhileW contamination by the forging tools is unlikely, this sam-ple should be re-measured to ascertain its W isotopic com-position given its importance in defining the pre-exposuree182W composition of IID iron meteorites. Using the solarsystem initial e182W of �3.51 ± 0.10 (Burkhardt et al.,2012) and a simple two-stage model, the calculated pre-exposure e182W value of Carbo �3.29 ± 0.13 translates intoa time interval of 1.9 ± 2.0 Myr between metal–silicate dif-ferentiation in the Carbo parent body and the beginning ofthe solar system. Taking a pre-exposure value of�3.57 ± 0.21 translates into a time of metal-silicate differ-entiation of �0.5 ± 2.4 Myr after formation of the solarsystem. The first estimate depends on a single measurementwhile the second estimate relies on a large extrapolation, sofurther work will be needed to determine which value is cor-rect. Those two ages correspond to an early accretion age ofless than 1.3 Myr after solar system formation using thethermal model of Qin et al. (2008b).

5. CONCLUSIONS

(1) Variations in e182W are observed among differentpieces of the IID iron meteorite Carbo that correlatewith distance from the pre-atmospheric center. Thisresult confirms the depth profile of e182W reportedin Markowski et al. (2006a) and indicates exposureto cosmic ray irradiation is cause of this variation.

(2) Different Carbo specimens show variations in stableOs isotopic compositions. The variations of Os isoto-pic composition are also broadly correlated with dis-tance from the pre-atmospheric center. A negativecorrelation is barely resolved between e190Os ande189Os (�0.64 ± 0.45), but is unresolved betweene186Os and e189Os (�1.8(+1.9/�2.1)). The largeuncertainties with these slopes reflect the limitedrange of variations in Os isotopic compositionamong the analyzed samples. These values are inexcellent agreement with correlation lines observedfor iron meteorites from other groups (Wittig et al.,2013 and Walker, 2012) and model simulation ofneutron capture effects.

(3) Among Carbo specimens, e182W values are positivelyand negatively correlated with e189Os and e190Os,respectively (also see Qin et al., 2010). By extrapolat-ing the correlation lines to e189Os and e190Os valuesof zero, we obtain the pre-exposure e182W value of�3.31 ± 0.55 and �3.63 ± 0.23, respectively. After

including Rodeo (showing minimum exposureeffects) in the regression, we obtained an interceptof �3.29 ± 0.13, which is more in line with (Kruijeret al., 2014b). Because this value is largely leveragedby Rodeo data, the Os and W isotopic compositionneeds to be remeasured in the future. Using a two-stage model (Qin et al., 2008b), the best calculatedmetal–silicate differentiation age for Carbo is within2 Myr of the formation of the solar system, corre-sponding to an accretion age of within 1.3 Myr usingour previous thermal model (Qin et al., 2008b).

(4) Comparing the new results to the data from Walker(2012) and Wittig et al. (2013), the e190Os–e189Osand e186Os–e189Os correlations observed for Carboare close to those observed for groups IIAB, IIIAB,IVA, and IVB iron meteorites, as well as pallasites.Correlations of e182W–e189Os and e182W–e190Os forCarbo are very similar to those for IVB iron meteor-ites (Wittig et al., 2013), reflecting in part their similarRe/Os and Ir/Os ratios. General equations werederived to calculate neutron capture effects one182W for any iron meteorite with known Os isotopiccompositions and known Re/Os and Ir/Os ratios. Wealso extended this approach to the Pt isotopic system,since the latter was also used to quantify cosmogeniceffects on e182W (Wittig et al., 2013; Kruijer et al.,2013a, 2014b).

ACKNOWLEDGEMENTS

We gratefully acknowledge the associate editor Munir Huma-yun, Thornsten Kleine, Peter Sprung and one anonymous reviewerfor their thorough and constructive reviews. We thank RainerWieler, Carl A. Francis, and Stein B. Jacobsen for their help inmaking the Carbo samples available for this study; and to RichardJ. Walker for helpful discussions and providing an aliquot of the Osstandard used at the University of Maryland. This work is finan-cially supported by the Natural Science Foundation of China(41273076, 41473066), the “111” project and the FundamentalResearch Funds for the Central Universities to L.Q., and NSFPetrology and Geochemistry (EAR1144429) and NASA Cosmo-chemistry (NNX12AH60G, NNX14AK09G) programs to N.D.

APPENDIX A

According to our model simulation results, for any givenIr/Os and Re/Os ratio, the e190Os–e189Os slope k190/189 is aconstant, independent of the exposure age, depth from theexposure surface, and the pre-atmospheric radius and otherchemical properties of the meteorite. Thus, we can assumethat k190/189 is simply a function of Re/Os and Ir/Os ratios:g(Re/Os, Ir/Os). Using Taylor series expansion, we canwrite the function to the second order:

k190=189 ¼ aþ bðRe=OsÞ þ cðIr=OsÞ þ dðRe=OsÞ2

þ eðIr=OsÞ2 þ f ðRe=OsÞðIr=OsÞ;

where a, b, c, d, and e are all constants. We performed 6model calculations with variable Re/Os and Ir/Os ratios,

L. Qin et al. / Geochimica et Cosmochimica Acta 153 (2015) 91–104 103

which is sufficient to constrain the parameters of the Taylorseries expansion:

k190=189 ¼ �0:97þ 3:6 ðRe=OsÞ þ 0:006 ðIr=OsÞ

� 4:8 ðRe=OsÞ2 � 0:00008 ðIr=OsÞ2

� 0:02 ðRe=OsÞðIr=OsÞ:

To fit the model to the iron meteorite measurements, wesimply adjust the value of a. The combined datasets inFig. 5 potentially provide a more robust constraint thanthe data of individual iron meteorite groups, so by assum-ing a mean iron meteorite composition, the slope equalsto the one given in Fig. 5a yields a value of �0.92 for a;the e value is sufficiently small to be neglected. After rear-ranging the terms, we have:

The same approach was used to derive the equationsgiving the slopes between e182W–e189Os and e182W�e190Pt.To determine the equations giving the e182W�e196Pt ande182W�e192Pt slopes, the method is slightly different (seethe main text for details).

k190=189 ¼ �0:92þ 3:6 ðRe=OsÞ � 4:8 ðRe=OsÞ2

þ 0:006 ðIr=OsÞ � 0:02 ðRe=OsÞðIr=OsÞ:

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Associate editor: Munir Humayun