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Geochimica et Cosmochimica Acta 251 (2019) 73–86

Cosmogenic effects on chromium isotopes in meteorites

Jia Liu a, Liping Qin a,b,⇑, Jiuxing Xia a, Richard W. Carlson c, Ingo Leya d

Nicolas Dauphas e, Yongsheng He b

aCAS Key Laboratory of Crust – Mantle Materials and Environments and CAS Center for Excellence in Comparative Planetology, School

of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, ChinabState Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China

cDepartment of Terrestrial Magnetism, Carnegie Institution for Science, 5241 Broad Branch Road, NW, Washington, DC 20015, USAdSpace Research and Planetology, University of Berne, Sidlerstrasse 5, 3012 Berne, Switzerland

eOrigins Laboratory, Department of the Geophysical Sciences and Enrico Fermi Institute, The University of Chicago, 5734 South Ellis

Avenue, Chicago, IL 60637, USA

Received 2 May 2018; accepted in revised form 16 January 2019; available online 29 January 2019

Abstract

The 53Mn-53Cr short-lived radionuclide decay system is a powerful tool to investigate the timescales of early solar systemprocesses. A complication arises, however, from the fact that spallation and thermal/epithermal neutron capture processesinduced by cosmic rays can significantly alter 53Cr/52Cr ratios in solar system objects that have long exposure ages and highFe/Cr ratios. Quantifying these cosmogenic effects helps constrain the cosmic ray exposure history of extraterrestrial samples.The isotopic shifts produced by cosmic ray irradiation also need to be corrected before the Cr isotope systematics can be usedas a dating tool and as a tracer of nucleosynthetic provenance. To investigate the impact of cosmogenic production on Cr, theCr isotopic compositions of 25 samples from 16 iron meteorites belonging to nine different chemical groups were measured.The measurements show that exposure to cosmic rays can cause large coupled excesses in e53Cr (up to +268.29 ± 0.14; 2SE)and e54Cr (up to +1053.78 ± 0.72; 2SE) with a best fit line of e54Cr = (3.90 ± 0.03) � e53Cr. The magnitude of Cr isotopeproduction is controlled by various factors including the exposure age, the chemical composition (i.e., Cr concentrationand Ni/Fe ratio) and shielding conditions. Nevertheless, the correlation of e53Cr and e54Cr is independent of these factors,which provides an effective method to evaluate the cosmogenic contribution to 53Cr by monitoring the cosmogenic variationsin e54Cr in meteoritic irons. The results are compared with modeling results that yield a slightly shallower slope of 3.6 ± 0.2.Modeling results for the olivine in stony meteorites yield a higher slope (�5.4). However, the previous estimated results forlunar samples (stony targets for comic ray irradiation) exhibit an observably shallower slope (�2.62). The reason for the dif-ferent slopes is that the production rates of different cosmogenic Cr isotopes in iron meteorites and lunar samples are in dif-ferent proportions. The differences may not be completely controlled by the higher thermal and epithermal neutron fluenciesin lunar samples than in iron meteorites, but instead may largely reflect different radiation geometry between the two. Morestudies are needed to solve this open question.� 2019 Published by Elsevier Ltd.

Keywords: Chromium isotopes; Iron meteorites; Cosmogenic effects

https://doi.org/10.1016/j.gca.2019.01.032

0016-7037/� 2019 Published by Elsevier Ltd.

⇑ Corresponding author at: CAS Key Laboratory of Crust –Mantle Materials and Environments and CAS Center for Excel-lence in Comparative Planetology, School of Earth and SpaceSciences, University of Science and Technology of China, Hefei230026, China.

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

1. INTRODUCTION

Short-lived radiochronometers play an important rolefor establishing the timescales of the processes that shaped

74 J. Liu et al. /Geochimica et Cosmochimica Acta 251 (2019) 73–86

the solar system (e.g. Lugmair and Shukolyukov, 1998;Kleine et al., 2009; Dauphas and Chaussidon, 2011; Davisand McKeegan, 2014). The 53Mn-53Cr system is amongthe most widely used short-lived dating systems for high-resolution chronometry of the early solar system (e.g.Rotaru et al., 1992; Lugmair and Shukolyukov, 1998;Trinquier et al., 2008; Nyquist et al., 2009; Qin et al.,2010). Manganese is more volatile, more incompatible,and less siderophilic than Cr (Lodders, 2003), so that the53Mn-53Cr system is well suited to establish the timescalesof volatility-related processes and igneous differentiation.The half-life of 53Mn is 3.7 ± 0.4 Myr (Honda andImamura, 1971), which corresponds to the timescale ofaccretion and differentiation of planetesimals and embryos(e.g. Birck et al., 1999; Kleine et al., 2009; Dauphas andChaussidon, 2011). Another virtue of this system is thatMn and Cr are present in relatively high abundances insolar system bodies such that the 53Mn-53Cr system canbe applied to a wide variety of materials (e.g. Lugmairand Shukolyukov, 1998; Nyquist et al., 2001;Shukolyukov and Lugmair, 2004; Trinquier et al., 2008;Qin et al., 2010).

While transiting through space, meteorites can sufferfrom bombardment by cosmic rays of both solar and galac-tic origins. Compared to the galactic cosmic ray (GCR)particles, solar cosmic ray (SCR) particles are less impor-tant because of their relatively low energy (e.g. Wieler,2002; Ammon et al., 2009). The highly energetic galacticcosmic ray primary particles, which consist of 87% proton,12% a particle, and 1% heavier nuclei, produce a cascade ofsecondary particles within the meteoroid, mostly protonsand neutrons (Ammon et al., 2009). The interactions of pri-mary and secondary cosmic ray particles produce newnuclides (cosmogenic nuclides) that can modify the chemi-cal and isotopic compositions of the meteorites. The pres-ence of large amounts of cosmogenic Cr (compared toradiogenic Cr) is the main obstacle to the use of the53Mn-53Cr system for samples with long cosmic ray irradi-ation histories (e.g. Trinquier et al., 2008; Qin et al., 2010).Thus, devising an appropriate method for correcting theseeffects is essential.

Noble gases in meteorites have long been used to studythe exposure ages of meteorites and the shielding conditionsof the studied samples (e.g. Voshage and Feldmann, 1979;Voshage et al., 1983; Voshage, 1984). More recent studiesof cosmogenic nuclides have focused on the thermal andepithermal neutron capture reactions of elements with largeneutron capture cross sections and resonance integrals suchas Sm, Gd, Os, Pt and W (e.g. Hidaka et al., 2000; Kruijeret al., 2013; Wittig et al., 2013; Qin et al., 2015). In partic-ular, Sm, Gd and Hf are often used to monitor the neutronfluences of extraterrestrial rock samples (Eugster et al.,1970; Lugmair and Marti, 1971; Hidaka et al., 2000; Leyaet al., 2003; Kruijer et al., 2015) to correct neutron captureeffects on other elements such as Ti (Zhang et al., 2012) andW (Kruijer et al., 2015). Osmium, Pt, and W are siderophileelements that have been used recently as sensitive neutrondosimeters in iron meteorites (Wittig et al., 2013; Kruijeret al., 2013; Qin et al., 2015).

Fewer studies have focused on cosmogenic productionof the transition metal nuclides produced by spallationreactions, in which a light projectile (e.g. proton, neutronetc.) with high kinetic energy (several hundreds of MeVto several GeV) interacts with a heavy nucleus and causesthe breaking of the heavy nucleus into a lighter nucleusand a large number of nucleons (protons and neutrons).Iron (four stable isotopes with mass number 54, 56, 57,and 58) and nickel (five stable isotopes with mass number58, 60, 61, 62, and 64) are the main target elements forthe spallation production of Cr (four stable isotopes withmass number 50, 52, 53, and 54) (Shima and Honda,1966, Birck and Allegre, 1985). Accordingly, the largestspallogenic Cr isotope shifts should be found in samplesfrom small meteoroids and/or shallow shielding depths(where the highest production rates for cosmogenic nuclidescan be typically achieved, e.g., Ammon et al., 2009) withlong exposure ages, and high Fe/Cr and Ni/Cr ratios. Sincesizes of meteorites and shielding conditions are a priori notknown, iron meteorites with extremely high Fe/Cr, Ni/Crratios, as well as long exposure ages are prime targets forthe study of cosmogenic Cr production. Previous studiesof cosmogenic Cr focused on better characterizing the pro-duction rates. Shima and Honda (1966) analyzed cosmo-genic Cr in eight iron meteorites and the metal phase ofan ordinary chondrite. They found a marked enrichmentof cosmogenic Cr in most of the iron meteorites and esti-mated the cosmogenic production rates of50Cr:52Cr:53Cr:54Cr to be approximately 0.2:1:1:1 respec-tively. Despite the high uncertainty (�1% for isotopic anal-ysis), the ratio corresponds to a correlation ofe54Cr = 4 � e53Cr, where e54Cr and e53Cr values are the rel-ative deviations in parts per 10,000 of the internally normal-ized 54Cr/52Cr and 53Cr/52Cr ratios (corrected for massfractionation assuming normal 50Cr/52Cr of 0.051859(Shields et al., 1966) using the exponential law (Russellet al., 1978; Marechal et al., 1999)) from that of the stan-dard. Such an internal correction for mass fractionation isessential for TIMS measurements to obtain high-precisione54Cr and e53Cr values. Birck and Allegre (1985) investi-gated many cosmogenic nuclides, including cosmogenicCr, in the iron meteorites Grant and Carbo. They also con-cluded that the production rates of cosmogenic 53Cr and54Cr were very similar to each other and estimated produc-tion rates of 53Cr and 54Cr of �2.9 � 1011 atoms/(g. Myr) inthe iron meteorite Grant. Production rates of cosmogenicnuclides, however, significantly depend on the shieldingconditions (e.g. Ammon et al., 2009; Leya and Masarik,2013). Leya et al. (2003) modeled the cosmogenic effectson different short-lived nuclide systems, including the Mn-Cr system, for different shielding conditions. They obtaineda correlation between cosmogenic 53Cr and 54Cr in ironmeteorites of e54Cr = 5.85 � e53Cr. The slope of the mea-sured e54Cr and e53Cr correlation is higher than the slopecalculated using the production rates determined byShima and Honda (1966) as mentioned above. One expla-nation for the steeper slope might be that this modeling cal-culation did not include any contributions from spallationon Ni.

J. Liu et al. /Geochimica et Cosmochimica Acta 251 (2019) 73–86 75

In a later study, Qin et al. (2010) analyzed 3 differentpieces of the IID iron meteorite Carbo and found variableexcesses in e54Cr from 86e to 140e and a positive correlationbetween e54Cr and e53Cr (e54Cr = 3.95 � e53Cr � 0.70)among these samples. Bonnand and Halliday (2018) founda similar correlation (e54Cr = 3.8 � e53Cr � 0.6) amonganother five chemical groups of iron meteorites. The rela-tionship between cosmogenic e53Cr and e54Cr in iron mete-orites provides a potential method to correct for thepresence of spallation produced e53Cr if the pre-exposuree54Cr is known. For most differentiated meteorite groups,the pre-exposure e54Cr is well constrained (Trinquieret al., 2007; Qin et al., 2010).

To further examine the effects of various factors on thespallation production of Cr isotopes, such as shielding con-ditions and chemical composition, we analyzed the Cr iso-topic composition of 25 samples from nine differentchemical groups (IAB, IIAB, IID, IIE, IIIAB, IIICD, IIIF,IVA, and IVB), including 10 pieces of Carbo (IID) fromdifferent but known sampling locations from a well charac-terized slice. The measured Cr isotopic compositions arecompared with model predictions.

2. SAMPLES AND METHODS

2.1. Iron meteorite samples

The 25 samples analyzed are from nine different chemi-cal groups, including IAB (Woodbine, Persimmon Creek,and Campo del Cielo), IIAB (Old Woman, Negrillos, Brau-nau, and Ainsworth), IID (Carbo), IIE (Watson), IIIAB(Gundaring), IIICD (Nantan), IIIF (Clark County), IVA(Gibeon and Huizopa), and IVB (Tawallah Valley and Tla-cotepec). The selected iron meteorites cover a wide range ofexposure ages and chemical compositions (Table 1).

The 10 pieces of Carbo are from the same locations thatwere previously sampled to characterize W and Os isotopiccompositions (Qin et al., 2015). The positions on the sliceare shown in Fig. 1 and the distances from the pre-atmospheric center (which is the center of a meteoroidand is deduced by noble gas concentrations or isotopicratios, Ammon et al., 2008) are given in Table 2.

2.2. Chemical procedure

Iron meteorites were cut into small pieces (0.1–0.5 g)using a diamond saw blade. The iron meteorite pieces werewashed twice with methanol in an ultrasonic bath for30 min. The samples were then leached twice in concen-trated HCl (�11 N) for 10 min in order to remove anypotential terrestrial contamination. The acid leachingremoved 10–20 wt% of each sample. The cleaned iron mete-orites were dissolved in an HCl-HNO3 (2:1) mixture on ahot plate.

For each specimen of Carbo, 0.1 g was dissolved and asmall aliquot (10%) was taken for Ni/Fe ratio measure-ment; the remaining 90% was saved for Cr isotopic analysis.The 58Ni and 57Fe intensities of the samples were measuredsimultaneously using an Axiom high-resolution magnetic

sector ICP-MS in the Department of Terrestrial Magnetism(DTM) at the Carnegie Institution for Science (CIS). TheNi/Fe ratios in the samples were determined by construct-ing a calibrating curve for Ni/Fe ratios based on two gravi-metrically prepared Ni-Fe mixed standards with Ni/Fe = 1/20 and 1/10 (by weight), respectively. The estimatedaccuracy on the Ni/Fe ratios is better than 5%.

The purification protocol developed for meteoritic metalin Qin et al. (2010) was used to purify Cr. The dissolvedsample was first passed through an anion exchange columnto remove most of the Fe, followed by a two-step cationexchange column procedure. The total procedure yield is�80% and the blank is <10 ng. The amount of Cr in oursamples was usually >1 lg, thus the isotopic shifts inducedby laboratory contaminations are negligible. The final Crcollection was dried twice in concentrated HNO3 in orderto remove organic compounds left-over from the resin toimprove the ionization efficiency and to minimize theorganic background during the measurements.

2.3. Mass Spectrometry

The chromium isotopic compositions of the iron mete-orites were analyzed on a Thermo Finnigan Triton multi-collector thermal ionization mass spectrometer (TIMS)equipped with nine Faraday detectors in static mode atDTM using the method described in Qin et al. (2010).Approximately 1–2 lg of Cr was loaded on single degassedRe filaments with Si-gel and boric acid. The intensity of the52Cr+ beam was typically in the range of 1–10 � 10-11 A forall analyses. The 53Cr/52Cr and 54Cr/52Cr ratios were cor-rected for mass-fractionation by internal normalization toa 50Cr/52Cr ratio of 0.051859 (Shields et al., 1966) usingthe exponential law. Chromium isotopic ratios areexpressed using the e-notation, which is the relative devia-tion in parts per 10,000 of the Cr isotopic ratio of the sam-ple from that of the NIST SRM 3112a standard:

eiCr ¼ ðð iCr= 52CrÞsample=ð iCr= 52CrÞNIST3112a � 1Þ� 10; 000ði ¼ 53; 54Þ ð1Þ

2.4. Modeling of cosmogenic effects

The rates of production and burnout of Cr isotopes havebeen calculated using the model described in detail by Leyaand Masarik (2013) (also see Leya et al., 2003, Ammonet al., 2009). Briefly, the production rates are calculatedusing the excitation functions of the relevant nuclear reac-tions and the particle spectra for primary and secondaryparticles. For modeling we consider proton- and neutron-induced spallation reactions on Cr, Fe, and Ni to producethe various Cr isotopes. Since no experimental cross sec-tions are available for these reactions, the excitation func-tions were calculated using the TALYS-1.8 code system(Koning et al. 2015). Unfortunately, TALYS is limited toprojectile energies below 240 MeV and the predictions areoften relatively uncertain even above �200 MeV. To coverthe full energy range needed for modeling, we used the newversion of INCL (not official but it will be labelled INCL6)

Table 1Chromium isotopic compositions of the analyzed iron meteorites in this study.

No. Name Type Exposure age (Myr)a Cr (ppm)b Ni (%)b e53Crc e54Crc k53.f53(d,R)d

1 Woodbine IAB 180 ± 54 73 9.7 7.93 ± 0.24 30.92 ± 0.54 3.3 ± 1.12 Persimmon Creek IAB – 32 13.78 27.50 ± 0.27 107.62 ± 0.48 –3 Campo del Cielo IAB 200 ± 60 40 7.13 36.39 ± 0.35 143.82 ± 0.89 7.4 ± 2.44 Old Woman IIB – 47 5.54 5.21 ± 0.35 20.50 ± 0.79 –5 Negrillos IIAB 30 ± 15 104 5.60 0.28 ± 0.19 1.71 ± 0.47 1.0 ± 0.86 Braunau IIAB 7:2þ4:6

�2:3 73 5.49 0.57 ± 0.19 3.38 ± 0.46 5.8 ± 4.27 Ainsworth IIAB 1280 ± 110 32 6.14 268.29 ± 0.14 1053.78 ± 0.72 6.8 ± 1.08 Carbo IID 850 ± 140 21 10.02 3.64 ± 0.19–188.04 ± 0.41 14.76 ± 0.39–735.79 ± 1.04 –9 Watson IIE 8 ± 2 – 8.21 0.96 ± 0.43 4.91 ± 1.10 –10 Gundaring IIIAB 685 ± 90 48 8.32 61.95 ± 0.31 237.62 ± 0.87 4.5 ± 0.811 Nantan IIICD 45 ± 40 31 7.03 6.46 ± 0.30 26.82 ± 0.90 4.5 ± 4.112 Clark County IIIF 1420 ± 55 231 7.50 15.93 ± 0.24 58.02 ± 0.57 2.6 ± 0.313 Gibeon IVA 45 ± 40 212 8.20 �0.04 ± 0.44 0.28 ± 0.72e �0.2 ± 2.114 Huizopa IVA 450 ± 90 326 8.00 4.84 ± 0.12 19.28 ± 0.33 3.6 ± 0.815 Tawallah Valley IVB 250 ± 85 54 18.21 10.54 ± 0.31 39.60 ± 0.81 2.5 ± 0.916 Tlacotepec IVB 945 ± 55 168 16.23 13.64 ± 0.07 53.67 ± 0.12 2.7 ± 0.3

a The exposure age of iron meteorites are from Chang and Wanke (1969), Voshage and Feldmann (1979), Niemeyer (1979), Olsen et al. (1994), Nishiizumi et al. (1995), Markowski et al. (2006),and Honda et al., (2012). The exposure age of Watson for its silicate phase is accepted because it is in good agreement with that for metal phase (38Ar exposure age of 7.7 Myr) (Olsen et al., 1994).The exposure age of Nantan is the average 21Ne-26Al exposure age of Nantan ‘‘A” and ‘‘S” reported in Nishiizumi et al. (1995). The exposure age of Campo del Cielo reported by Honda et al.(2012) (�200 Myr) is used instead of the earlier study (�70 Myr, Nagai et al., 1993). The uncertainty of the exposure age for Campo del Cielo was not reported in Honda et al. (2012). We assumedthis sample has an uncertainty (�30%) similar to the iron meteorites that have similar exposure ages (Woodbine and Tawallah Valley).b The Cr and Ni contents in the iron meteorites are from Lovering et al. (1957), Smales et al. (1967), Wasson (1969), Schaudy et al. (1972), Olsen et al. (1994), Choi et al. (1995), Wasson et al.

(2007), and Bonnand and Halliday (2018).c All uncertainties quoted for individual analyses are 2r standard errors, and are the larger of the external errors of the standard and sample measurements in the analytical sequence.d k53 f53(d,R) values are calculated using Eq. (3). The errors are the propagation error of the exposure age, Cr (10%) content, Ni (5%) content and Cr isotopic composition.

76J.

Liu

etal./

Geochim

icaet

Cosm

ochim

icaActa

251(2019)

73–86

Fig. 1. Section of the Carbo (IID) iron meteorite. The samplinglocations (the cross symbols) are the same as those studied by Qinet al. (2015). The sampled bars are labelled with capital letters fromA to Y. Samples are named after the location on the bar and thedistance from the reference line as shown in Table 2. The 3Hedistribution in this Carbo section is also shown (Fireman, 1958;Hoffman and Nier, 1959; Signer and Nier, 1962).

J. Liu et al. /Geochimica et Cosmochimica Acta 251 (2019) 73–86 77

for higher projectile energy. Thus, we used TALYS for thedescription of the reaction thresholds and INCL6.0/ABLA07 (Boudard et al., 2013) for all energies up to20 GeV. The cross sections of thermal and epithermal neu-tron capture reactions for the burnout and production ofCr isotopes were all taken from the JEFF-3.0A database.

The particle spectra for primary and secondary particlesare the same as described earlier (Leya and Masarik 2013).Briefly, the spectra were calculated using the LAHET codesystem (Prael and Lichtenstein, 1989) by following the tra-jectories of primary and secondary particles. For calculat-ing neutron capture rates we assume a primary GCRparticle flux of 2.99 cm-2 s�1 (Kollar et al., 2006). For spal-lation reactions we use a primary GCR spectrum of4.47 cm-2 s�1 (e.g. Ammon et al., 2009, see also Leya andMasarik, 2013). The reason for why these two values aredifferent is still unclear (e.g., Kollar et al., 2006; Leya andMasarik, 2013). The modeled production rates are a func-tion of pre-atmospheric radius of the meteoroid, which isassumed to be spherical, and the depth of the sample belowthe pre-atmospheric surface. Assuming that the target suf-fered a single-stage irradiation with a given exposure ageand chemical composition (i.e., Fe, Ni, and Cr concentra-tions), the 53Cr/52Cr and 54Cr/52Cr ratios for all consideredshielding conditions (with radii of 5, 10, 15, 25, 30, 32, 40,50, 60, 65, 85, 100, and 120 cm) are calculated and the

values are converted into the e-notation after internalnormalization.

3. RESULTS

3.1. Chromium isotopic compositions of the iron meteorites

The chromium isotopic compositions of the iron mete-orites are compiled in Tables 1 and 2 and shown inFig. 2. The samples display large variations in both e53Crand e54Cr, with e53Cr ranging from �0.04 ± 0.44 (IVAGibeon) to +268.29 ± 0.14 (IIAB Ainsworth) and e54Crvarying from +0.28 ± 0.72 (IVA Gibeon) to + 1053.78± 0.72 (IIAB Ainsworth). The excesses of e53Cr and e54Crexhibit a strong positive correlation, with a slope of 3.90± 0.03. This correlation is consistent with the resultsobtained by Qin et al. (2010) and Bonnand and Halliday(2018) for iron meteorites, which gave 3.95 and 3.8, respec-tively. The present work extends the validity of this linearcorrelation to iron meteorites from more chemical groupswith different exposure histories.

For the Carbo samples, the Cr isotopic compositions,along with the distances from the pre-atmospheric center,and the Ni/Fe ratios (by weight) are summarized in Table 2.The ten Carbo samples exhibit a narrow range of Ni/Feratios, but large variations in e53Cr (from + 3.64 ± 0.19to + 188.04 ± 0.41) and e54Cr (from + 14.76 ± 0.39 to+ 735.79 ± 1.04). In Fig. 3a, the e53Cr values are plottedas a function of the distance from the pre-atmospheric cen-ter. No simple correlation between Cr isotopic compositionand the distance is observed.

3.2. Model results

Spallation reactions in iron meteorites produce stable Crisotopes with average relative production rates of�0.3:1.0:1.3:1.2 for 50Cr:52Cr:53Cr:54Cr, respectively (usingthe average Ni content of the analyzed iron meteorites inthis study (9%) and the average shielding conditions inthe model simulation (radii from 5 cm to 120 cm)). In addi-tion to the spallation reactions there is the capture of ther-mal and mostly epithermal neutrons that destroys 50Cr,52Cr, 53Cr, and 54Cr and simultaneously produces 53Crand 54Cr. The thermal neutron capture cross sections for50Cr, 52Cr, 53Cr, and 54Cr are �16, �0.7, �18, and �0.37barn, respectively (Schmidt et al., 2014). However, it hasbeen shown that neutrons in iron meteorites usually donot fully reach thermal energies and therefore that the res-onance integrals are a better measure for neutron captureeffects in iron meteorites. The resonance integrals for50Cr, 52Cr, 53Cr, and 54Cr are �7.4, �0.47, �8.6, and�0.18 barn, respectively (Schmidt et al., 2014). Due toextremely high Fe/Cr and Ni/Cr ratios, spallation usuallydominates the total cosmogenic effects. For example, pro-duction rates for spallation (atoms/(g of meteorite Myr)),and especially for spallation on Fe, are typically orders ofmagnitude higher than those of thermal/epithermal neutroncapture reactions. Thus, the modeled cosmogenic shifts inCr isotopic composition are clearly dominated by spallation

Tab

le2

Chromium

isotopic

compositionofCarbo(IID

)an

dcorrectede5

3Crva

lues.

No.

Nam

eNi/Fea

Distance(cm)a

e53Crb

e54Crb

Cr N

i/Fecal(ppm)c

Cr m

odel

cal(ppm)c

k53.f53(d,R)Ni/Fed

k53.f53(d,R)modeld

Carbo

1CarboA-210

0.11

232

.80

107.05

±0.18

415.17

±0.37

17.1

±1.8

16.11±

0.03

2.2±

0.6

2.1

2CarboD-127

0.11

521

.60

99.42±

0.14

388.47

±0.38

17.7

±1.8

15.33±

0.02

2.1±

0.6

1.8

3CarboG-48

0.11

511

.20

188.04

±0.41

735.79

±1.04

17.7

±1.8

7.59

±0.02

4.0±

1.1

1.7

4CarboJ+25

0.10

61.28

129.13

±1.79

487.55

±4.91

15.8

±1.7

10.40±

0.14

2.5±

0.7

1.6

5CarboJ-35

0.11

57.20

179.31

±0.61

699.38

±1.35

17.7

±1.8

7.42

±0.03

3.8±

1.1

1.6

6CarboJ-11

80.11

816

.00

126.30

±0.56

494.51

±1.39

18.3

±1.8

11.64±

0.05

2.8±

0.8

1.8

7CarboM-15

0.11

09.60

3.64

±0.19

14.76±

0.39

16.7

±1.7

379±

200.1±

0.1

1.7

8CarboP-71

0.10

819

.20

150.98

±0.26

588.90

±0.65

16.3

±1.7

9.87

±0.02

2.9±

0.8

1.8

9CarboV-71

0.11

731

.20

130.60

±0.31

509.42

±0.77

18.1

±1.8

12.83±

0.03

2.8±

0.8

2.0

10CarboY-82

0.11

839

.20

182.09

±0.80

697.90

±2.05

18.3

±1.8

10.08±

0.04

4.0±

1.2

2.2

aNi/Feratios(byweigh

t)aredetermined

byIC

P-M

Sat

DTM

andtheuncertainty

is5%

.Theinform

ationofdistancesfrom

thepre-atm

ospherecenterisfrom

Qin

etal.(201

5).

bAlluncertainties

quotedforindividual

analyses

are2r

stan

darderrors,an

darethelarger

oftheexternal

errors

ofthestan

dardan

dsample

measurements

inthesamean

alytical

session.

cCr N

i/Fecalcontents

arecalculatedusingthedecompositionas

showed

inSection4.2.3.

Cr m

odel

calcontents

arecalculatedto

match

themodelingresultwiththemeasureddata(see

text

for

details).Theuncertainties

ofCr N

i/Fecalan

dCr m

odel

calcontentarecalculatedby2stan

darddeviationofMonte

Carlo

simulation(N

=500,000).

dThek53f 53(d,R)Ni/Feva

lues

arecalculatedusingEq.(3)an

dtheerrors

arethepropag

ationerroroftheexposure

age,Cr N

i/Fecalan

dFecontents,an

dCrisotopiccomposition.Thek53f 53(d,

R)modelva

lues

arefrom

model

simulation.

78 J. Liu et al. /Geochimica et Cosmochimica Acta 251 (2019) 73–86

reactions on Fe, which is consistent with the earlier conclu-sions of Leya et al. (2003).

The modeled e54Cr and e53Cr values also display a closelinear correlation. The modeled slope of �3.6 is signifi-cantly lower than the previous prediction of 5.85 by Leyaet al. (2003), but is much closer to the measured slope of3.90. The reason for the different modeled slopes betweenthe earlier study by Leya et al. (2003) and the present workis that we use a much more reliable model code for the pre-diction of unknown cross sections. The TALYS code(Koning et al., 2015) is one of the most reliable codes topredict cross sections in the energy range up to 200 MeV.In addition, the new model predictions are based on particlespectra calculated using the LAHET code system whereasthe earlier calculations used particle spectra that were calcu-lated using the HERMES code system (e.g. Ammon et al.,2009). Naturally, using more reliable particle spectra andcross sections significantly improves the quality of themodel predictions (e.g. Leya and Masarik, 2009). Thoughthe model slightly underestimates the measured slope, thegeneral trend is well described and we can therefore usethe model predictions to study the relationships betweenmodeled cosmogenic effects on the Cr isotopic compositionwith exposure ages, native Cr concentrations, and Ni/Feratios. For the production of stable cosmogenic isotopes,the cosmogenic production is simply a linear function ofthe exposure age, therefore the slope in e54Cr -e53Cr is inde-pendent of the exposure age. Although excesses in e54Crand e53Cr scale with 1/Cr due to dilution effects, the slopebetween e54Cr and e53Cr is not affected. In addition, themodeling results reveal that the rates for the spallogenicproduction of Cr isotopes from Ni are much lower thanthose from Fe. Interestingly, when the Ni concentrationsvary from 5% to 25%, which covers the range of concentra-tions commonly encountered in iron meteorites, the mod-eled cosmogenic e53Cr variations decrease by �11%, andthe modeled slopes between e54Cr and e53Cr change slightlyfrom 3.48 to 3.75. The average of these modeled slopes is3.6 ± 0.2 (2SD). The finding that the slope in e54Cr ande53Cr is almost independent of the meteorite’s Ni/Fe ratiois consistent with the presented measurements, which allplot on a single linear correlation line despite their differentNi/Fe ratios (see Fig. 2).

We also evaluated the effect of shielding on cosmogenicCr production in Carbo assuming a single-stage irradiation(Shankar, 2011) with an exposure age of 850 Myr (Voshageand Feldmann, 1979), a pre-atmospheric radius of 65 cm(Ammon et al., 2008), an Fe content of 90 wt%, a Ni con-tent of 10 wt% (Wasson, 1969), and a Cr content of 7.5–16 ppm (in order to cover the range observed for most ofthe studied Carbo pieces in Fig. 3a). According to themodel predictions as described in Section 2.4, e53Cr valuesfirst increase close to exponentially with increasing distancefrom the meteoroid center, reach a maximum at a distanceof 53–56 cm, and then decrease with increasing distancefrom the center, i.e., getting close to the pre-atmosphericsurface (Fig. 3a). Compared to the modeling results, themeasurements show no clear correlation with distancethough the magnitude of the isotopic shifts are consistent,with the exception of Carbo M-15, with those predicted

Fig. 2. e54Cr vs. e53Cr for the analyzed iron meteorites from 9chemical groups. The open and filled symbols are from this studyand previous studies (Qin et al., 2010; Bonnand and Halliday,2018), respectively (the error bars are smaller than the symbol size).The black line shows the model-2 fit through the data for all ironmeteorites in this study using Isoplot 4.15 program (Ludwig, 2012).The calculated slope is 3.90 ± 0.03 (95% confidence). The insertedgraph is the enlarged part in the region of e53Cr = 0 to +50 ande54Cr = 0 to +200.

Fig. 3. Effects of shielding conditions. (a) Variation of cosmogenice53Cr with distance from the pre-atmospheric center for Carbo.The open squares represent the measured data. The dashed, solidand dotted lines are modeled correlations for Cr contents of 7.5, 10,and 16 ppm, respectively. (b) The relationship of estimatedproduction rate of cosmogenic 53Cr with distance from the pre-atmospheric center in the Carbo iron meteorite. The black solidcurve represents the results of the model calculation. The solidsquares represent the results calculated as described in theSection 4.3.

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by the model given reasonable ranges in Cr concentrationin the metal. The significant scatter of the measured dataaround the model predictions clearly indicates that otherparameters that are so far not accounted for are also ofimportance (which will be discussed in the discussionsection).

4. DISCUSSION

4.1. Origin of chromium isotopic variations in iron meteorites

Chromium in iron meteorites can in principle be a mix-ture of native, radiogenic, and cosmogenic Cr. To obtainthe isotopic composition of native Cr in iron meteorites,Trinquier et al. (2007, 2008) analyzed exsolved chromitegrains in two iron meteorites (St-Abin (IIIAB) and MontDieu (ungrouped)) and obtained an average e53Cr of�0.14e and e54Cr of �0.72e. The anomalous (non-zero)e54Cr values of these chromite grains may have a nucle-osynthetic origin (Qin and Carlson, 2016). Radiogenic53Cr is the daughter of 53Mn decay, and if a sample has ahigh Mn/Cr ratio when it formed in the early solar system(i.e. the first 10 Myr), this will result in significant 53Cringrowth. The Mn/Cr ratios of iron meteorites, however,are very low (e.g., �2 � 10-3 for the Gibeon iron meteorite,Sekimoto et al., 2007; Wasson et al., 1998; Qin et al., 2010),thus 53Cr ingrowth from 53Mn decay is negligible.Chromium-54 is not the decay product of any short-livednuclide, but can vary due to variable nucleosynthetic contri-butions to different meteorite groups (Trinquier et al., 2007;Qin et al., 2010). At the whole rock scale in meteorites, the

total range observed in e54Cr is �2.5e (Trinquier et al.,2007; Qin et al., 2010), with most of the variability seenin carbonaceous chondrites. Although the 54Cr nucleosyn-thetic anomalies in iron meteorites are not well constrained,we nevertheless use other correlated nucleosyntheticanomalies to evaluate the expected ranges in nucleosyn-thetic e54Cr. There is a broad correlation between e64Niand e54Cr in meteorites with a slope of �3 (Dauphas andSchauble, 2016; Dauphas, 2017). The total observed e64Nirange in iron meteorites is �0.7e (Steele et al., 2011), thusthe expected nucleosynthetic e54Cr range in iron meteoritesis �2e. Nucleosynthetic heritage thus cannot explain thelarge e54Cr variations documented in iron meteorites.

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The Cr isotope variations in iron meteorites were causedby cosmogenic effects that are superimposed on native Crpresent in the meteorite prior to space exposure (Shimaand Honda, 1966; Qin et al., 2010). Expectedly, suchmixing should follow a straight line, as it is indeed seenin the data (see Fig. 2). The new results that reveal a lin-ear correlation between e54Cr and e53Cr with a slope thatis consistent with model predictions for mixing betweencosmogenic and normal Cr clearly confirm this earlyassumption.

4.2. Factors controlling the production of cosmogenic Cr

Iron meteorites display large Cr isotopic variations (e.g.e53Cr from �0.04 ± 0.44 to +268.29 ± 0.14). Even within asingle iron meteorite such as Carbo, significant variationsare found (e.g. e53Cr from +3.64 ± 0.19 to +188.04± 0.41). Various factors can affect Cr cosmogenic isotopicvariations (e.g., Shima and Honda, 1966; Birck andAllegre, 1985; Ammon et al., 2009). Based on the modelpredictions, the cosmogenic Cr production rate for ironmeteorites as a function of Fe content and shielding condi-tion can be expressed as:

P ðiCRÞ / ð0:546� ½Fe� þ 0:481Þ � fiðd;RÞ ði ¼ 50; 52; 53;54Þð2Þ

where [Fe] is the Fe content of the sample and fi(d, R)

describes the shielding condition and is a function of thedistance d of the sample from the pre-atmospheric centerand the radius R of the meteorite. Based on Eq. (2), cosmo-genic effects on the Cr isotopic composition can beexpressed as:

e iCr¼ki �TCRE �1=½Cr� � ð0:546�½E�þ0:481Þ �fiðd;RÞ ði¼ 53;54Þð3Þ

where ki is a constant, TCRE is the exposure age of the sam-ple, [Cr] and [Fe] are the Cr and Fe contents in the sample,respectively, and fi(d, R) is again the shielding factor.

4.2.1. Exposure age

The amount of stable cosmogenic nuclides linearlyincreases with exposure age as long as the energy spectrumof incident galactic cosmic rays does not change signifi-cantly over time. Sloan and Wolfendale, (2013) used con-ventional assumptions of GCR parameters to concludethat the variation of GCR intensity is 10–20% over the past1000 Myr. We can therefore expect that the productionrates are constant with time and that thus Eq. (3) is validwithin an uncertainty of 20%.

4.2.2. Fe, Ni contents

Iron and Ni, whose abundances are complementary iniron meteorites (i.e., [Fe] + [Ni] � 100%), are the main tar-gets for the production of spallogenic Cr. In the numericalmodel, the spallation of Ni results in a significantly shal-lower slope (�1.4) in the e54Cr-e53Cr space than spallationon Fe (�3.8). However, because of the 2–7 times lower Crproduction rates from Ni than from Fe and because of the3–19 times lower Ni concentration than Fe concentration,

the slope of the e54Cr-e53Cr correlation is almost indepen-dent of the Ni/Fe ratio for typical iron meteoritecompositions.

4.2.3. Shielding condition

Qin et al. (2010) suggested that the observed Cr isotopicvariation among different pieces in Carbo could reflect dif-ferences in shielding conditions. To further investigateshielding effects on cosmogenic Cr in iron meteorites, weanalyzed the Cr isotopic compositions of 10 Carbo piecesfrom known sampling locations (Fig. 1 and Table 2). Asshown in Fig. 3a, the measured data cannot be describedby a single modeling curve (corresponding to different Crconcentrations). Most of the measured data points lie inthe area between the two modeling curves for Cr contentsof 7.5 ppm and 16 ppm, except the data for Carbo M-15that will be discussed later. The lack of correlation betweene53Cr values and shielding depth is likely due to sampleheterogeneities (i.e. the variations of Fe, Ni, and Cr con-tents), which may have overprinted the shielding effect.The two main constitutes of iron meteorites (kamaciteand taenite) have very different Ni and Cr contents, andtaenite has much higher Ni and Cr contents than kamacite(e.g. Nichiporuk, 1958, Romig and Goldstein, 1980;Rasmussen et al., 1988). Although �0.1 g of sample wasdissolved for each Carbo specimen, the different dissolvedpieces may have contained different proportions of taeniteto kamacite, resulting in different Ni/Fe ratios and differentCr concentrations. The Ni/Fe ratios (by weight) in the 10pieces of Carbo display a variation of 0.106–0.118 (i.e.90.4–89.4% for Fe contents). According to Eq. (3), the max-imum adjustment for varying Ni/Fe ratios on e53Cr is only1.0e, which is trivial compared to the magnitude of thee53Cr variations in Carbo specimens. However, heteroge-neous Cr contents among the Carbo pieces might lead todifferent dilution factors for cosmogenic Cr (see also Eq.(3)). Because the amounts of samples dissolved (�0.1 g)were typically sufficient to avoid sample heterogeneity, theCr contents in Carbo subsamples were assumed, incor-rectly, to be constant, or at least insignificantly variable.Thus, direct determination of Cr contents in Carbo sub-samples were not attempted. Considering that the Ni/Feratio is an approximate index for the relative proportionsof taenite to kamacite, one way to evaluate the Cr contentsis via the Ni/Fe ratios of the analyzed samples. Using the Niand Cr contents in taenite (31.1 wt% Ni and 71 ppm Cr)and kamacite (8.1 wt% Ni and 12 ppm Cr) of Carbo mea-sured by Shima and Honda (1966) (the uncertainty of Niand Cr is 5%), the initial Cr concentration in the differentCarbo pieces can be estimated based on measured Ni/Feratios assuming that taenite and kamacite are the main/onlysources of Fe, Ni, and Cr in the samples (Table 2). The cal-culated Cr concentrations of the different Carbo piecesrange from 15.8 ± 1.7 ppm to 18.3 ± 1.8 ppm. Accordingto this finding, any variation in e53Cr due to variable Crconcentrations should be below a factor of �1.16, whichis �33% smaller than the range (a factor of �1.73) of theobserved Cr isotopic compositions that were corrected forthe shielding conditions (using the model) for the Carbosamples (except M-15). A notable exception is sample M-

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15 showing �50 times smaller excesses in e53Cr and e54Crcompared to the other Carbo samples. Consequently, thisapproach cannot explain the variations seen in the mea-sured data. The reason for this discrepancy is that the Crbudget may be also affected by the presence of chromite(or daubreelite) grains which cannot be properly indexedusing the Ni/Fe ratio. Turning the argument around, ifwe assume that the differences between measured isotopicdata and modeling predictions at a given shielding depthis nevertheless due to differences in Cr contents, the Cr con-tents in the model can be calculated using Eq. (3). The thuscalculated Cr contents for the studied Carbo pieces varybetween 7.4 and 16.1 ppm (an exception is again sampleCarbo M-15 (see Table 2)). This range is �3.5 times therange of Cr concentration calculated above via Ni/Feratios. The range in Cr concentration is not unreasonablefor different splits of iron meteorites (due to the low Cr con-tent in kamacite, a large uncertainty could exist on the con-centration measurement by Shima and Honda (1966)), sothe variable Cr concentrations could be responsible forthe poor match of the data to the predicted shielding depen-dency. For M-15, two possibilities could have caused themuted cosmogenic effects on Cr isotopes. First, Crextracted from the sample could be overwhelmed by terres-trial Cr originating from the laboratory processing blank.Our procedural blank is typically less than 10 ng, which isnegligible compared to the total Cr in the sample (usually>1 lg), so the extremely small e53Cr excess is unlikely tobe the result of laboratory contamination. The second pos-sibility is that the cosmogenic effect is reduced by a signifi-cantly higher native Cr content of the sample, perhapsreflecting the presence of exsolved chromite or daubreelitegrains. The signal intensity for sample M-15 was indeedseveral times higher than those for other iron meteorites,which is consistent with this sample having an unusuallyhigh native Cr concentration.

For most of the analyzed iron meteorites, we have noknowledge of the shielding conditions of the studied sam-ples. The term ki�fi(d, R) in Eq. (3) may provide empiricallya first-order evaluation for the shielding conditions of theanalyzed samples if the Cr isotopic compositions, exposureages, and Fe and Cr concentrations are known. The empir-ically determined k53�f53(d, R) values range between �0.2± 2.1 and 7.4 ± 2.4 (Tables 1 and 2). In comparison, themodeled k53�f53(d, R) values are in the range of 0.2–3.5for meteorites with radii from 5 cm to 120 cm. Most ofthe empirically determined values are in the range givenby the model predictions. Only the k53�f53(d, R) values ofCampo del Cielo (7.4 ± 2.4) and Ainsworth (6.8 ± 1.0)are resolvably higher than the upper limit given by themodel (3.5), which are likely due to the unexpectedly lowCr concentrations of these special samples compared tothose reported by previous studies (Lovering et al., 1957;Choi et al., 1995; Wasson et al., 2007).

To sum up and most importantly, despite differences inexposure ages, chemical composition (i.e., Cr contents andNi/Fe ratios), and shielding conditions that all affect thecosmogenic Cr isotopic variations, the e54Cr/e53Cr ratiosare independent of these factors.

4.3. Prediction of the production rate of cosmogenic Cr in the

Carbo meteorite

Apart from the model predictions, we can also calculatethe production rate of cosmogenic Cr using the measureddata. Since native Cr acts as a dilutant for cosmogenicCr, the measured e53Cr values are linear mixtures betweenthe cosmogenic and the native endmember. Therefore, thetotal cosmogenic Cr contribution can be calculated fromthe measured Cr isotopic composition (i.e., e53Cr) and thetotal Cr content. For example, the exposure age of Carbois 850 Myr (Voshage and Feldmann, 1979). The (total) cos-mogenic Cr abundances of the Carbo samples can be calcu-lated using the measured e53Cr values and the Cr contentsthat are inferred from Ni/Fe ratios. Next, the cosmogenicproduction rates of each Cr isotope in the different Carbopieces can be evaluated using the results from Shima andHonda (1966), i.e., 50Cr:52Cr:53Cr:54Cr = 0.2:1:1:1. Thethus determined production rates for Carbo samples aregiven in Table 3 and are shown in Fig. 3b. The 53Cr produc-tion rates are in the range (2.1–4.0) � 1011 atoms/(g ofmeteorite �Myr), except for Carbo M-15 which exhibits aremarkably lower value of 7.0 � 109 atoms/(g of mete-orite �Myr). As mentioned earlier, this sample could becompromised by high native Cr concentrations due toexsolved chromite or daubreelite grains (see above).Except for sample Carbo M-15, the production ratesfor cosmogenic 53Cr exhibit no correlation with the dis-tance from pre-atmospheric center as the model pre-dicted. Besides, the average of the determinedproduction rates are �35% higher than that of the modelpredictions (Table 3). This discrepancy could either bedue to an overestimate of the Cr content of the studiedsample and/or due to an underestimate of the Cr produc-tion rates by the model calculations.

As shown in Eq. (2), the production rate is a function ofthe Fe content and the shielding condition. For Carbo spec-imens, the Fe contents are normalized to 90% (Wasson,1969) and the shielding conditions (i.e. modeled f53(d,R)

values) are normalized to the pre-exposure center via Eq.(2). The expression is:

Pð 53CrÞnorm ¼ ð0:546� 90% þ 0:481Þ � f 53ð0� 2; 65Þmodelð0:546� ½Fe� þ 0:481Þ � f 53ðd; RÞmodel

� Pð 53CrÞð4Þ

The normalization method is appropriate when themeteorite suffered a single-stage irradiation, which is likelythe situation for Carbo and Grant (Ammon et al., 2008;Shankar, 2011). The average normalized production rateis (2.3 ± 0.5) � 1011 atoms/(g of meteorite Myr) (CarboM-15 is excluded; 95% confidence, N = 9). The large uncer-tainty of the average production rate is mainly due to thelarge uncertainty of Cr contents calculated on the basis ofthe Fe-Ni approximation used here. Such production ratesare comparable with the modeled production rate at thepre-exposure surface (1.6 � 1011 atoms/(g of mete-orite Myr) and the estimated production rates

82 J. Liu et al. /Geochimica et Cosmochimica Acta 251 (2019) 73–86

(1.3 � 1011–2.2 � 1011 atoms/(g of meteorite Myr), normal-ized to Fe = 90% and pre-exposure surface via Eq. (4)based on the results from Shima and Honda (1966)).

4.4. Correction of cosmogenic effects on e53Cr in meteorites

Here we expand the linear correlation between e53Crand e54Cr established by Qin et al. (2010) from 3 Carbomeasurements to different chemical groups of iron mete-orites with different exposure ages, chemical composi-tions, and shielding conditions. If for a studiedmeteorite the pre-exposure 54Cr nucleosynthetic anomalyis known, the inferred amount of cosmogenic e54Cr canbe used to correct for the cosmogenic effects on e53Cr.Luckily, nucleosynthetic anomalies in Cr are usually veryconstant among the different groups, exceptions are car-bonaceous chondrites and some ungrouped achondrites(Trinquier et al., 2007; Qin et al., 2010; Sanborn et al.,2013). The most important result of our study is thatthe linear correlation is robust, i.e., the correctionmethod for cosmogenic effects on e53Cr does not requirethe knowledge of the exposure age, Fe/Cr ratio, and/orshielding condition.

The pre-exposure 53Cr/52Cr ratios in iron meteorites arethe sum of the initial solar system 53Cr/52Cr ratio and theradiogenic contribution to 53Cr/52Cr ratio induced by53Mn. Because iron meteorites have extremely low Mn/Crratios (e.g. the 55Mn/52Cr ratio in the IVA Gibeon is2.4 � 10-3, Wasson et al., 1998; Sekimoto et al., 2007; andcould be even lower in IVBs, Scott, 1972), radiogenicingrowth should be negligible after the formation of ironmeteorites. Thus assuming iron meteorite parent bodieshad chondritic 55Mn/52Cr and only experienced a singleevent of Mn/Cr fractionation (silicate-metal segregation),the pre-exposure 53Cr/52Cr of iron meteorites can be writtenas:

ð 53Cr= 52CrÞiron ¼ ð 53Cr= 52CrÞssiþ ð 53Cr= 52CrÞCHUR;ingrowth ð5Þ

ð 53Cr= 52CrÞCHUR;ingrowth ¼ ð 53Mn= 52MnÞssi � ð1� e�kTÞ� ð 55Mn= 52CrÞCHUR ð6Þ

where (53Cr/52Cr)ssi is the solar system initial 53Cr/52Crratio (e53Cr = �0.20 ± 0.09, Trinquier et al., 2008),(53Cr/52Cr)CHUR, ingrowth is the radiogenic ingrowth of53Cr/52Cr when iron meteorites formed in the chondriticuniform reservoir (CHUR), (53Mn/55Mn)ssi is the solar sys-tem initial 53Mn/55Mn ratio ((6.28 ± 0.66) � 10-6, Trinquieret al., 2008), (55Mn/52Cr)CHUR is the 55Mn/52Cr ratio ofCHUR (0.74 ± 0.22; Trinquier et al., 2008; Qin et al.,2010), k is the decay constant of 53Mn, and T is the forma-tion time of iron meteorites after CAI formation. The for-mation ages of IVA and IVB irons are 1.5 ± 0.6 Myr and2.8 ± 0.7 Myr after CAI formation, respectively (Kruijeret al., 2017). From Eqs. (1), (5), and (6), the pre-exposuree53Cr values of IVA and IVB irons are �0.10 ± 0.10 and�0.03 ± 0.11, respectively.

We can also estimate the pre-exposure e53Cr values ofIVA and IVB irons using the cosmogenic e54Cr-e53Cr corre-

lation. According to the broad nucleosynthetic e64Ni-e54Crcorrelation in meteorites (the correlation with a slope of3.06 ± 0.52 and an intercept of 0.00 ± 0.13 is calculatedusing Isoplot 4.15 model 2-fit (Ludwig, 2012)), and e64Niof �0.30 ± 0.05 for IVA irons and +0.13 ± 0.07 for IVBirons (Dauphas and Schauble, 2016; Dauphas, 2017), weused Monte-Carlo simulation (i.e., n = 500,000) to calcu-late the pre-exposure e54Cr values in IVA and IVB irons.The average pre-exposure e54Cr values in IVA and IVBirons are �0.92 ± 0.25 (2r) and +0.40 ± 0.26 (2r), respec-tively. The cosmogenic effects on e53Cr in Gibeon (IVA),Huizopa (IVA), Tawallah Valley (IVB), and Tlacotepec(IVB) in this study are corrected with the cosmogenice54Cr-e53Cr correlation and pre-exposure e54Cr values asmentioned above. Their pre-exposure e53Cr values are�0.48 ± 0.83, �0.47 ± 0.70, +0.35 ± 0.78, and �0.16± 0.69, respectively. Taking weighted average, we estimatedthe pre-exposure e53Cr values of �0.47 ± 0.52 (2r) in IVAirons and +0.06 ± 0.51 (2r) in IVB irons, which are similarto the expected values (�0.10 ± 0.10 for IVA irons and�0.03 ± 0.11 for IVB irons) despite the large uncertainties.We also corrected the cosmogenic effect for e53Cr in IVAirons reported by Bonnand and Halliday (2018) using thesame method and obtained their weighted average of�0.29 ± 0.46 (2r, N = 9), which is also similar to the valueobtained in this study and the expected value.

To evaluate the applicability of the correction methodfor olivines in stony meteorites, which have high Mn/Cr(e.g. �106 for an angrite Angra dos Reis, Lugmair andShukolyukov, 1998) and Fe/Cr ratios and hence radiogenicand cosmogenic Cr, we also modeled the production of cos-mogenic Cr in an olivine target composition (the bulkchemical composition is from an olivine in an ordinarychondrite Ste. Marguerite (H4) measured by Trinquieret al., 2008). Modeling reveals a correlation between cosmo-genic e54Cr and e53Cr of a slope �5.4, i.e., higher than theone predicted for iron meteorites (�3.6), which may be dueto the difference in chemical composition (i.e., Fe/Cr ratios)between olivine (�54, Trinquier et al., 2008) and iron mete-orites (typically > 500, e.g. Wasson et al., 1998). In addi-tion, compared to iron meteorites, stony meteoritesexhibit a significantly higher contribution from thermaland epithermal neutron capture because, first, thermaland epithermal neutron fluencies are higher in stony thanin iron meteorites and, second, stony meteorites have muchhigher Cr/Fe ratios (0.01–0.02 for chondrites, Wasson andKallemeyn, 1988), i.e., a higher ratio of neutron capturetargets relative to spallation targets. Note that isotopicshifts in Cr isotopes produced by neutron capture reactionsare different from those induced by spallation reactions.The modeling results also reveal that the slope of e53Cr–e54Cr correlation decreases as the contribution from ther-mal and epithermal neutron capture drops (i.e. Fe/Cr ratioincreases) in stony targets. Mougel et al. (2018) reportedthat lunar samples exhibited a notable variation in Cr iso-topic composition (�2e in e54Cr), which was interpretedas a result of cosmogenic effects. The lunar samples plotalong a line in the e53Cr–e54Cr space with a slope of�2.62, lower than the modeling result for olivine target instony meteorites (�5.4). The explanation for different

Table 3The production rates of cosmogenic Cr in iron meteorites.

No. Name Position/distancefrom center (cm)

Cosmogenic 53Cr(ppb)

Exposure age(Myr)

Production rate(atoms/(g of meteorite �Myr)

Normalized production rate(atoms/g of meteorite �Myr)

Ref. A Grant surface 695 ± 65 2.9 � 1011

Ref. B Grant interior 13.8 695 ± 65 2.3 � 1011 –Treysa surface 12.6 620 ± 60 2.3 � 1011 –

Ref. C Grant Q-76 32 18.7 ± 0.9 695 ± 65 (3.1 ± 0.3) � 1011 (2.2 ± 0.2) � 1011

Grant E-77 7.3 18.8 ± 0.9 695 ± 65 (3.1 ± 0.3) � 1011 (1.5 ± 0.2) � 1011

Grant I-81 12.5 16.7 ± 1.1 695 ± 65 (2.7 ± 0.3) � 1011 (1.3 ± 0.2) � 1011

Carbo (cc) 40.8 20.5 ± 4.6 850 ± 140 (2.7 ± 0.8) � 1011 (2.0 ± 0.6) � 1011

Carbo

1 Carbo A-210 32.80 16.6 ± 1.7 850 ± 140 (2.2 ± 0.4) � 1011 (1.7 ± 0.3) � 1011

2 Carbo D-127 21.60 15.9 ± 1.6 (2.1 ± 0.4) � 1011 (1.9 ± 0.4) � 1011

3 Carbo G-48 11.20 30.1 ± 3.0 (4.0 ± 0.8) � 1011 (3.8 ± 0.7) � 1011

4 Carbo J + 25 1.28 18.5 ± 2.0 (2.5 ± 0.5) � 1011 (2.5 ± 0.5) � 1011

5 Carbo J-35 7.20 28.7 ± 2.9 (3.8 ± 0.7) � 1011 (3.8 ± 0.7) � 1011

6 Carbo J-118 16.00 21.0 ± 2.1 (2.8 ± 0.5) � 1011 (2.5 ± 0.5) � 1011

7 Carbo M-15 9.60 0.5 ± 0.1 (7.0 ± 1.4) � 109 (6.6 ± 1.3) � 109

8 Carbo P-71 19.20 22.2 ± 2.4 (3.0 ± 0.6) � 1011 (2.7 ± 0.5) � 1011

9 Carbo V-71 31.20 21.4 ± 2.1 (2.9 ± 0.6) � 1011 (2.3 ± 0.4) � 1011

10 Carbo Y-82 39.20 30.2 ± 3.0 (4.0 ± 0.8) � 1011 (2.9 ± 0.6) � 1011

Weighted average* 20.5 ± 3.8 (2.7 ± 0.5) � 1011 (2.3 ± 0.5) � 1011

Modeling results 0–2 (2.0 ± 0.6) � 1011 1.6 � 1011

References A, B and C are Birck and Allegre (1985), Shimamura et al. (1986), and Shima and Honda (1966), respectively. The distances from the center for the samples in Ref. C are induced fromthe calibrated sections of Carbo and Grant iron meteorites which were described in Ammon et al. (2008). The concentration of cosmogenic 53Cr is from the corresponding references and this study.The exposure ages are from Voshage and Feldmann (1979). The chemical effects and shielding effects on the production rates are normalized for the samples in Ref. C. The Fe content for Grant(90.8%; Voshage and Feldmann, 1979) are normalized to that for Carbo (90.0%; Wasson, 1969) via Eq. (4). The shielding conditions are normalized to the pre-exposure center of Carbo ironmeteorites (f53(0–2, 65) through Eq. (4).The production rates of Carbo pieces were calculated as shown in Section 4.3 and we also normalized the production rate to the no shielding condition and Fe = 90.0% (see text for details).* The weighted average (excluding M-15) is calculated by Isoplot 4.15 (Ludwig, 2012) and the uncertainty is 95% confidence.

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slopes is that the production rates of different cosmogenicCr isotopes in lunar samples and olivine target in stonymeteorites are in different proportions. The irradiationgeometry of lunar samples can be regarded as irradiationon an infinitely large flat body (2p irradiation), which is dif-ferent from that for other stony meteorites (4p irradiation,irradiation on a relatively small spherical body). A previousstudy (Leya et al., 2003) showed that the slope for lunarsamples (2p irradiation) was shallower than that for otherstony meteorites (4p irradiation). Thus, the radiation geom-etry can potentially affect the e53Cr–e54Cr slope for stonytargets. Further Cr isotope measurements on heavilycosmic-ray-irradiated stony meteorites or their rock-forming minerals and the updated model simulation forlunar samples are needed for a better understanding ofthe cosmogenic production of Cr isotopes in stony targets.

5. CONCLUSIONS

(1) Exposure to galactic cosmic rays induced large Crisotopic variations in iron meteorites with e53Cr rang-ing from �0.04 ± 0.44 to +268.29 ± 0.14 and e54Crranging from +0.28 ± 0.72 to +1053.78 ± 0.72. Thevariability in isotopic composition is caused by thedifferent proportions of cosmogenic to native Cr,which is ultimately controlled by the exposure age,Cr content, Fe content (or Ni/Fe ratio), and shieldingcondition for any given sample. Modeling resultsreveal that the observed excesses in e53Cr and e54Crdecrease with increasing native Cr content, and arepositively correlated with the exposure age and Fecontent. The modeling results for Carbo show thatthe cosmogenic e53Cr first increases with distancefrom the pre-atmospheric center, reaches a maximumat the distance of 53–56 cm, and then decreases withincreasing shielding. The measured Cr isotope effectsroughly follow the modeled curve but show signifi-cant scatter, probably reflecting the sensitivity ofthe Cr isotopic composition to variations in theabundance of native Cr between different subsam-ples. All the controlling factors can be expressed asa governing equation (Eq. (3)) for cosmogenic Cr.

(2) The cosmogenic e53Cr and e54Cr values of the ironmeteorites in this study exhibit a robust positive cor-relation with a slope of 3.90 ± 0.03, which agreeswith those given by previous studies (Qin et al.,2010; Bonnand and Halliday, 2018) and is slightlyhigher than the slope predicted by physical modelcalculations (3.6 ± 0.2). This correlation is indepen-dent of the exposure age, chemical composition,and shielding condition of iron meteorites. The mod-el calculations show that such a correlation can alsobe found for stony targets. Though the slope willbe different due to differences in the ratios of thermaland epithermal neutrons relative to fast neutrons andtherefore to differences in the production rate ratio ofcapture reactions relative to spallation reactions. Inaddition, highly variable Fe/Cr ratios will also affectthe slope in stony target. However, the demonstrated

procedure can be used to correct e53Cr for cosmo-genic effects using variations in e54Cr in meteoriticmaterials of various chemical compositions if thepre-exposure e54Cr is independently known. A lowerslope in lunar samples (�2.6) than that in iron mete-orites (�3.9) may not be due to higher contributionfrom thermal and epithermal neutron capture inlunar samples, but instead may largely reflect differ-ent radiation geometry between lunar samples (2pirradiation) and iron meteorites (4p irradiation).More studies are needed to solve this openquestion.

(3) The production rate of cosmogenic Cr is a functionof the Fe content and the shielding conditions. Inthe case of the iron meteorite Carbo, the calculatedproduction rate ((2.3 ± 0.5) � 1011 atoms/(g of mete-orite.Myr)) agrees with the modeling results and withprevious estimated after adjusting for difference in Feconcentration and shielding condition.

ACKNOWLEDGMENTS

We thank Mary Horan and Timothy Mock for help in the cleanlab and the TIMS lab. We also thank the Smithsonian Institutionfor providing the majority of the meteorite samples analyzed. Thiswork was funded by the Key Research Program (Grant No.XDPB11) and Strategic Priority Research Program (B) (GrantNo. XDB18000000) of the Chinese Academy of Sciences, NationalNatural Science Foundation of China (Grant No. 41473066,41625013, 41721002), and the ‘‘111” project.

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