Evidence for homogeneous distribution of osmium in the protosolar nebula

Earth and Planetary Science Letters 351–352 (2012) 36–44

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Evidence for homogeneous distribution of osmium in the protosolar nebula Richard J. Walker n Isotope Geochemistry Laboratory, Department of Geology, University of Maryland College Park, MD 20742, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 22 February 2012 Received in revised form 11 June 2012 Accepted 17 July 2012 Editor: T. Elliot

Separate s-, r-, and possibly p-process enriched and depleted components have been shown to host Os in low metamorphic grade chondrites, although no measureable Os isotopic anomalies have yet been discovered for bulk chondrites. Here, iron meteorites from groups IAB, IIAB, IIIAB, IVA and IVB, as well as the main group pallasites are examined. Many of these meteorites show well-resolved anomalies in e190Os, e189Os and e186Osi. The anomalies, however, differ from those observed in chemically extracted components from chondrites, and are interpreted to reflect long-term exposure of the meteorites to cosmic rays, rather than nucleosynthetic effects. A neutron capture model is presented that can well account for observed isotopic variations in 190Os and 189Os. The same model predicts greater enrichment in 186Osi than is observed for at least one iron, suggesting as yet unaccounted for effects, or failings of the model. Despite the variable anomalies resulting from cosmic ray exposure, each of the major meteorite groups examined contains at one member with normal Os isotopic compositions that are unresolved from chondritic compositions. This indicates that some domains within these meteorites were little affected by cosmic rays. These domains are excellent candidates for application of the 182Hf–182W system for dating metal–silicate segregation on their parent bodies. The normal Os also implies that Os was homogeneously distributed throughout the protosolar nebula on the scale of planetesimal accretion, within the current level of analytical resolution. The homogeneity in Os contrasts with isotopic heterogeneity present for other siderophile elements, including Mo, Ru and W. The contrast in the scale of anomalies may reflect a late stage-injection of s- and p- process rich material into the coalescing nebula. Alternately, nebular thermal processing and destruction of some presolar host phases of Mo, Ru and W may also be responsible. & 2012 Elsevier B.V. All rights reserved.

Keywords: Nucleosynthetic anomaly Osmium isotopes Iron meteorites Pallasites Nebular mixing Cosmic-ray exposure

1. Introduction Isotopic anomalies in early solar system materials are normally defined as deviations from terrestrial isotopic abundances that have resulted from processes other than either radioactive decay or mass dependent fractionations caused by kinetic effects. Isotopic anomalies are important because of the information they may convey about the provenance of protosolar nebular materials, processes that occurred in the nascent nebula, as well as processes that occurred subsequent to the formation of earliest solids. Large isotopic anomalies for various elements, such as C and Si, have long been known to be present in presolar grains, and chemically concentrated fractions extracted from bulk samples of primitive meteorites that are selectively enriched in certain presolar components (Anders and Zinner, 1993; Huss et al., 1997). Anomalies in these materials have been fruitfully studied to decipher the types of nucleosynthetic processes that have contributed matter to our solar system (e.g., Zinner, 1998).

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As a complement to information provided by individual presolar grains and chemically separated concentrates of presolar materials, isotopic anomalies in bulk planetary materials, such as chondrites and achondrites, including iron meteorites, may reflect the incorporation of varying proportions of diverse presolar materials into early-formed planetary bodies (e.g., Andreasen and Sharma, 2006; Dauphas et al., 2002; Regelous et al., 2008; Trinquier et al., 2009). Isotopic anomalies in bulk materials could record large-scale heterogeneities in the distribution of presolar components within the protosolar nebula (e.g., Steele et al., 2011), selective destruction of thermally labile carrier phases (Trinquier et al., 2009), or changes in the distributions of carrier phases with time or location (Dauphas et al., 2010). Consequently, the presence or absence of anomalies in bulk materials is important with regard to understanding initial nebular heterogeneity (Quitte´ et al., 2010), the creation of localized heterogeneities (Trinquier et al., 2009), and possible late injections of new material into the nebula (e.g., Bizzarro et al., 2007; Dauphas et al., 2010). It is also possible that secondary processes, such as aqueous alteration, could potentially selectively remove isotopically distinct components from a domain within a parent body (e.g., Palguta et al., 2010; Yokoyama et al., 2011), as is achieved by chemical leaching

R.J. Walker / Earth and Planetary Science Letters 351–352 (2012) 36–44

of bulk materials in laboratories. Towards these ends, considerable effort has been expended searching for and interpreting isotopic anomalies for elements such as Ba, Sm, Cr, Mo, W, Ni and Ru in bulk materials that may be isotopically representative of major reservoirs within various parent bodies (Andreasen and Sharma, 2007; Burkhardt et al., 2011; Carlson et al., 2007; Chen et al., 2010; Dauphas et al., 2002; Qin et al., 2008; Ranen and Jacobsen, 2006; Regelous et al., 2008; Trinquier et al., 2009). Comparison of isotopic variability for additional elements with different chemical characteristics and nucleosynthetic production pathways may help to elucidate the dominant processes involved in the generation of isotopic anomalies present among early-formed planetesimals. Osmium is an important element to add to this list as it is one of the most refractory elements (b. p.44000 1C), yet is highly volatile in oxidized forms (b. p. OsO4 ¼105 1C). Further, large anomalies, attributed to the presence of varying proportions of s-, rand possibly p-process nucleosynthesis products have been identified for Os in chemical residues and leachates of primitive chondrites of low metamorphic grade (Brandon et al., 2005; Reisberg et al., 2009; Yokoyama et al., 2007, 2010, 2011). Analysis of bulk chondrites using total digestion methods, as well as thermally metamorphosed chondrites and other primitive meteorites, however, has shown no evidence for resolvable Os isotopic heterogeneities (Brandon et al., 2005; Yokoyama et al., 2007; van Acken et al., 2011). This suggests that, although materials with highly different Os isotopic composition were present in the protosolar nebula, mixing processes homogenized the diverse materials to such an extent that at least some early-formed primitive bodies had isotopic compositions identical to the Earth, within the current level of measurement resolution. Here we use high precision thermal ionization mass spectrometric techniques to examine Os isotopes in five major iron meteorite groups, plus an anomalous iron, as well as four pallasites, three from the pallasite main group (PMG) and one, Eagle Station, from a separate grouplet (PES) with a distinct genetic heritage. Two lines of evidence suggest that irons and pallasites may be richer hunting grounds for Os isotopic anomalies in bulk materials than bulk chondrites. First, the application of the 182Hf–182W short-lived isotope system has demonstrated that the segregation of some metallic reservoirs generated as asteroidal cores or melt pools, occurred very early in solar system history, in some cases possibly within 1 Ma of the formation of calcium–aluminum rich inclusions (Harper and Jacobsen, 1996; Horan et al., 1998; Kleine et al., 2005; Lee and Halliday, 1996; Markowski et al., 2006; Sche´rsten et al., 2006; Schoenberg et al., 2002). Consequently, the assembly of the parent bodies of some irons and pallasites may predate that of the formation of bulk chondrites, when nebular heterogeneities were likely greater (e.g., Baker et al., 2005). Second, isotopic anomalies have been reported for several other elements with siderophile characteristics (e.g., Ni, W, Mo and Ru) in certain iron meteorites and pallasites, and at least some of these anomalies are likely to be nucleosynthetic in origin (Burkhardt et al., 2011; Chen et al., 2010; Dauphas et al., 2002; Qin et al., 2008; Regelous et al., 2008; Steele et al., 2011). Further, Huang and Humayun (2008) reported Os isotopic anomalies for some Group IVB irons. In that study, however, the cause of the observed isotopic effects was interpreted to be long-term cosmic ray exposure (CRE), rather than nucleosynthetic anomalies, consistent with the observation that many of the iron meteorites have experienced long-term exposure to cosmic rays (e.g., Herzog, 2005; Voshage and Feldmann, 1979; Voshage et al., 1983). Thus, in order to positively identify nucleosynthetic anomalies in some elements, observed anomalies must first be considered in light of possible secondary isotopic modification processes, such as by CRE (e.g., Markowski et al., 2006). If anomalies can be correlated with CRE, they may ultimately be useful for monitoring and correcting for the effects of neutron fluence, such as

37

would be useful for applying the 182Hf–182W isotopic system to constraining the timing of metal–silicate segregation (e.g., Kruijer et al., 2012; Wittig et al., 2012).

2. Samples and analytical methods All samples were obtained from the Smithsonian Institution’s National Museum of Natural History. Sample collection numbers are provided in Table 1. For this study, the pieces provided by the museum were cut into appropriate size sub-pieces using a Leco Varicut slow-speed saw using a diamond wafering blade (copper alloy substrate). Purified water was used as the coolant/lubricant during sawing. The blade was cleaned between samples by cutting carborundum. Prior to dissolution, all surfaces on all pieces of meteorite were subsequently ground and polished using sandpaper or carborundum. Both nucleosynthetic and CRE effects upon Os isotopes in irons and pallasites are likely to be small, where they exist at all. Consequently, we have refined our chemistry and mass spectrometric methods to obtain high precision measurements of most Os isotopes. Osmium was extracted from metal using conventional digestion and solvent extraction techniques (Cohen and Waters 1996; Shirey and Walker, 1995), and subsequently highly purified by microdistillation from a chromic acid solution into concentrated HBr (Birck et al., 1997). For most irons analyzed, Os was in sufficiently high concentrations that dissolution of o0.1 to 0.2 g of metal liberated 470 ng of Os that is necessary for the highest precision isotopic analysis. This quantity of metal was dissolved in sealed PyrexTM glass Carius tubes at 240 1C using 3 g of concentrated HCl and 6 g of concentrated HNO3 (Shirey and Walker, 1995). For meteorites with Os concentrations o200 ng/g, 1 to 4 g of the metal was necessary to liberate sufficient Os for highest precision analysis. Because of the inefficiency of oxidizing both Os and Fe for such large quantities of metal, larger samples were initially dissolved in an excess of 8 M HCl in a loosely-capped teflon beaker. The Fe in solution was allowed to auto-oxidize by exposure to air for 1–3 weeks, and then the Fe3þ was removed via solvent extraction into diethyl ether (Myers and Metzler, 1950). The residual acid solution was dried down, picked up in 6 g of concentrated nitric acid, combined with 3 g of concentrated HCl in a Carius tube, then sealed and processed like the other samples. Total analytical blanks for Os were always o3 pg, and were inconsequential to the measurements reported here. Approximately 70–300 ng of Os were loaded onto a filament per analysis. Osmium was analyzed as OsO3 via negative thermal ionization, using a Thermo-Fisher Triton thermal ionization mass spectrometer at the UMd. To enhance negatively-charged Os oxide production, all analyses were achieved by loading purified samples onto degassed Pt filaments in either concentrated HCl or HBr, completely dried to a sample salt, then covering the sample with a mixture of Ba(OH)2 and NaOH, added as the electron emitter (Birck et al., 1997; Creaser et al., 1991). In addition, a leak valve was used to bleed pure oxygen into the source region of the mass spectrometer so that an initial background pressure of  2  10  7 bar was established and maintained for every analysis. Over the course of the four years data were collected for this study, most meteorites were measured multiple times using slightly different mass spectrometric approaches. Data were normally collected with average signal intensities for 192Os (at mass 240) between 1 and 5 V. Initially, analyses were accomplished via a static measurement routine that simultaneously collected data on masses 231 (183WO3 ), 232 (184OsO3 ), 233 (185ReO3 ), 234 (186OsO3 ), 235 (187OsO3 ), 236 (188OsO3 ), 237 (189OsO3 ), 238 (190OsO3 ), and 240 (192OsO3 ), using the L4, L3, L2, L1, C, H1, H2, H3 and H4 Faraday cups, respectively (where L, C

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R.J. Walker / Earth and Planetary Science Letters 351–352 (2012) 36–44

Table 1 Osmium isotopic data for irons and pallasites.

IAB Canyon Diablo IIAB Negrillos Bennett County IIIAB Costilla Peak Henbury Tamarugal IVA Charlotte Gibeon Jamestown Yanuitlan IVB Warburton Range Cape of Good Hope Hoba Tlacotepec Tawallah Valley

Sample ID

190

Pt/188Os

USNM 1530

0.002374a

n

e186Osi

2s

2sM

e189Os

2s

2 sM

e190Os

2s

2sM

e184Os

2s

5

0.25

0.81

0.35

0.00

0.17

0.07

0.03

0.11

0.05

n.d.

USNM 1222 USNM 1199

b

0.0005507 0.0005245b

12 11

0.22 0.30

0.22 0.31

0.07 0.09

0.00  0.03

0.08 0.15

0.03 0.05

 0.03 0.02

0.12 0.17

0.04 0.05

 0.7  5.2

19.6 52.1

USNM 702 USNM 6494 USNM 6680

0.0008841b 0.0008619a 0.02404b

4 5

0.09 0.60 n.d.

0.19 0.30

0.09 0.13

0.06  0.22  0.16

0.10 0.06 0.12

0.05 0.03

 0.02 0.18 0.11

0.12 0.18 0.12

0.06 0.09

n.d. 20.7 0.1

90.4 30.3

USNM USNM USNM USNM

577 2873 1046 459

0.00361c 0.002057c 0.001487c 0.001365c

4 5 5 2

 0.06 0.23 0.58 0.55

0.48 0.38 0.81 0.30

0.24 0.17 0.41

 0.02 0.00  0.16  0.22

0.07 0.07 0.09 0.12

0.03 0.03 0.04

 0.03 0.01 0.12 0.10

0.14 0.12 0.15 0.13

0.07 0.05 0.06

17.4 12.5  8.4 n.d.

38.5 128.7 10.6

USNM USNM USNM USNM USNM

5884 986 6506 872 1458

0.001908d 0.0005889d 0.0005979d 0.0006094d 0.001766d

7 6 5 5 4

0.31 0.47 0.11 0.79 0.30

0.34 0.37 0.33 0.24 0.42

0.13 0.15 0.15 0.10 0.21

 0.12  0.24  0.07  0.55  0.17

0.05 0.09 0.09 0.04 0.02

0.02 0.04 0.04 0.02 0.01

0.10 0.17 0.03 0.31 0.16

0.08 0.11 0.13 0.09 0.12

0.03 0.04 0.05 0.04 0.06

 25.5  5.7  7.1 8.1 8.4

138.5 13.7 6.9 38.5 61.7

Ungrouped Nedagolla

USNM 268

0.001439a

5

 0.05

0.48

0.22

0.02

0.15

0.07

0.07

0.24

0.11

n.d.

Pallasites Eagle Station Brenham Imilac Krasnojarsk

USNM USNM USNM USNM

0.001398a 0.12951a 0.01187a 0.01545a

4 2 1 1

 0.12 n.d. n.d. n.d.

0.40

0.20

0.03 0.02  0.17 0.02

0.02 0.03 0.03 0.03

0.01

 0.04 0.08 0.11  0.10

0.05 0.03 0.02 0.02

0.02

16.7  3.3  3.9  13.8

2752 266 872 499

n-number of separate filament loadings. n.d.-not determined due to large correction for radiogenic ingrowth of

61.8 8.1 10.5 31.3

186

Os.

a

Concentration data for Pt/Os from: SOM Table S3. b Concentration data for Pt/Os from: Cook et al. (2004). c Concentration data for Pt/Os from: McCoy et al. (2011). d Concentration data for Pt/Os from: Walker et al. (2008).

and H refer to low, center and high mass cups). Data were collected in 18 to 72 blocks of 20 cycles each (360 to 1440 ratios). Integrations times on peak were 17 s. Off-peak baselines (using beam deflection) were collected for 30 s to 2 m at the beginning of each block. An idle time of 4 s was used between baseline and onpeak data acquisition. Data were collected while utilizing the virtual amplifier rotation capability of the Triton to cancel potential amplifier biases. In order to improve the quality of 184Os/188Os measurements, midway through the analytical campaign we switched to a data collection routine whereby signals on masses 230 (198PtO2 ), 231 (183WO3 ), and 233 (185ReO3 ) were obtained at the beginning of each block of data for 2 s for each peak, using the central secondary electron multiplier. Faraday cups were then used to statically collect signal on masses 232 (184OsO3 ), 234 (186OsO3 ), 235 (187OsO3 ), 236 (188OsO3 ), 237 (189OsO3 ), 238 (190OsO3 ), and 240 (192OsO3 ). For this routine, signal data were collected for 17 s per cycle, and there were 20 Faraday cup cycles per block, again collecting background by beam deflection for 30 s per block. Using the calibrated efficiency factor between the Faraday cup and the multiplier (determined weekly during each analytical campaign), the calculated contributions from 198Pt and 183W were subtracted from the 184Os signal. Osmium is ionized from Pt filaments, so some Pt background signal is present for all analyses. Typical Pt background corrections to 184Os/188Os were  0.1%, but as much as 0.5%. We only report standard and meteorite 184 Os/188Os data for analyses accomplished using this correction. We have never observed contributions to 184Os or 186Os from W oxides, so corrections based on the 183WO3 signal were

effectively nil, consistent with the conclusions of Brandon et al. (1999) and Luguet et al. (2008). One potential impediment to obtaining high precision data for Os via thermal ionization is that Os isotope intensities must be corrected for contributions from 17O– and 18O–substituted oxides (e.g., Brandon et al., 1999; Creaser et al., 1991; Ireland et al., 2011; Luguet et al., 2008). After flushing the gas lines and source with the isotopically modified gas, an Os standard was analysed in the normal way. Surprisingly, the modified gas composition had little impact on the measured Os oxide ratios. From mass balance, it was determined that the O bleed gas provides only about 2% of the total O incorporated into the Os oxides that form (see SOM for additional details). During the second half of our analytical campaign, we periodically monitored O isotopic compositions for standards and samples using a peak-hopping routine and measuring 18O/16O via the deconvolution of signals present on masses 240 (192Os16O3 ) and 242 (192Os18O16O2 ). Oxide signals were integrated over the course of an analysis and used to generate an average 18O/16O ratio for each analysis where this routine was used. The average 18 O/16O for our campaign period was 0.002034 70.000016 (2s), with no systematic change in the ratio with time (Table S1). This average ratio is identical, within analytical uncertainties, to that reported by Luguet et al. (2008). Although this ratio is nearly 0.5% lower than the Nier (1950) ratio of 0.0020439, we used the Nier (1950) O composition for oxide corrections because we did not have a complete record of O isotopic variations over the course of the first half of our four year data acquisition period, and because of our laboratory history of using the Nier values in prior studies.

R.J. Walker / Earth and Planetary Science Letters 351–352 (2012) 36–44

In practice, the offset of the measured O isotopic composition from the values used in the correction result in no changes in the relative differences among the isotopic composition comparisons reported. As noted by Luguet et al. (2008) variations in oxygen isotopic compositions within runs, or over longer periods of time can result in small variations in corrected Os isotopic ratios. Luguet et al. (2008) reported o10 ppm effects on the 186Os/188Os ratio, despite greater variations in O isotopic composition than we observed. Consequently, we conclude that the minor variations in O isotopic composition we observed had no significant effect on that ratio. It is likely that some of the variations in these ratios among standard runs (see below) were induced by periodic, longterm variations in the integrated O isotopic composition. This conclusion is based on the fact that 189Os/188Os and 190Os/188Os ratios are measured to a higher precision than 186Os/188Os, and also that the oxide corrections are larger for the heavier mass isotopes (only oxide contributions from the very minor 184Os affect 186Os). The effects of these changes were largely removed from our meteorite analyses by re-normalizing all measurements to the appropriate averages for the standard obtained during individual periods of sample analysis. In anticipation of finding CRE effects, thermal and epithermal neutron capture probabilities of the isotopes of Os, as well as the isotopes of Re, Ir and W that will contribute to Os via burnout, were considered when choosing the isotopes to use for fractionation correction. Neutron capture cross sections and resonance integrals for relevant isotopes of Os, Re, Ir and W follow: 184Os (3000 b; 601 b), 186Os (80 b; 280 b), 187Os (245 b; 500 b), 188Os (5 b; 152 b), 189Os (25 b; 674 b), 190Os (13 b; 22.1 b), 192Os (2 b; 4.6 b), 185Re (112 b; 1717 b), 187Re (76.4 b; 300 b), 191Ir (954 b; 3500 b), 184W (1.7 b; 14.7 b) and 186W (38.5 b; 485 b) (Mughabghab et al., 1984; Mughabghab, 2003) (Table 2). Because of the limited neutron capture cross sections for 192Os and 188Os, for this study an oxide corrected 192Os/188Os ratio of 3.08271 (Alle gre and Luck, 1980) was used to correct for instrumental mass fractionation, rather than the 192Os/189Os ratio used in our previous studies of Os isotopic anomalies in primitive meteorites (e.g., Yokoyama et al., 2007). For our laboratory, this 192Os/188Os ratio is equivalent to the 192Os/189Os ratio of 2.527431 used by Yokoyama et al. (2007) for normalization. Corrections for mass fractionation were achieved by applying the exponential law. Data for our UMd Johnson–Matthey laboratory standard are reported in Table S2 and Fig. S1a–c. These data were collected with average signal intensities for 192Os (at mass 240) between

Table 2 Natural abundances and corresponding neutron capture cross sections for relevant isotopes. Cross sections from Mughabghab (2003). Isotope Isotope Abundance (%)

Cross section, thermal resonance integral (b)

Cosmogenic reactions

184

1.7, 14.7 38.5, 485 112, 1717 76.4, 300 3000, 601 80, 280 245, 500 4.7, 152 25, 674 13.1, 22.1 3.1, 4.6 954, 3500

184

W W 185 Re 187 Re 184 Os 186 Os 187 Os 188 Os 189 Os 190 Os 192 Os 191 Ir 186

30.64 28.43 37.40 62.60 0.01739 1.59718 1.26789 13.3286 16.2569 26.4400 41.0921 37.30

W(n,g)185W(b  )185Re W(n,g)187W(b  )187Re 185 Re(n,g)186Re(b  )186Os 187 Re(n,g)188Re(b  )188Os 184 Os(n,g)185Os(e.c.)185Re 186 Os(n,g)187Os 187 Os(n,g)188Os 188 Os(n,g)189Os 189 Os(n,g)190Os 190 Os(n,g)191Os(b  )191Ir 192 Os(n,g)193Os(b  )193Ir 191 Ir(n,g)192Ir(e.c., 4.76%)192Os 186

39

1 and 5 V, comparable to run conditions established for the meteorite analyses. The average 186Os/188Os, 189Os/188Os and 190Os/188Os ratios and 2s (SD) uncertainties for standard analyses over the entire four year period of analysis (n¼ 43) were 0.1198504740 (33 ppm), 1.219710720 (16 ppm) and 1.983735730 (15 ppm), respectively. For these statistics, data for multiple standard runs from a single filament were averaged and counted as a single run. These ratios are identical, within uncertainties, to those reported for the same standard when re-normalized to the ratios used for correction of instrumental mass fractionation by the other studies (Brandon et al. 2005; Luguet et al., 2008). The 2s uncertainties for 186Os/188Os, 189 Os/188Os and 190Os/188Os ratios are 10 to 30% higher than those reported by Brandon et al. (2005) and Luguet et al. (2008). We attribute this to the longer period over which the standards were measured for this study. Factors that can lead to changes in ratios over time include long-term changes in Faraday cup efficiencies as well as long term changes in the O isotopic composition. The 2s (SD) for 186Os/188Os, 189Os/188Os and 190Os/188Os ratios for standard analyses for the much shorter 1 to 2 week periods during which samples were periodically analyzed (where nZ2; and n is the number of filaments loaded) ranged from 16 ppm to 32 ppm, 4 to 11 ppm, and 3 ppm to 11 ppm, respectively. Changes in measured ratios for standards over time were monitored by comparison of averaged standard analyses for individual periods of analysis. As noted above, some of these variations were likely due to modest changes in O composition. The nominal external uncertainties during the shorter analytical periods were 22, 9 and 9 ppm, respectively, for 186 Os/188Os, 189Os/188Os and 190Os/188Os, and these uncertainties are shown in Fig. 2a–f, unless 4 or more analyses of a sample permitted reporting 2sM uncertainty that is better (smaller uncertainties), or if data were collected during a period when standards showed greater variance (larger uncertainties). The average 184Os/188Os ratio for 13 standard analyses utilizing the SEM correction for Pt, over a two year period, was 0.00130377 36 (72740 ppm or 727 e units) (2s; Table S2). This ratio is in good agreement with previously published thermal ionization data (Brandon et al., 2005; Luguet et al., 2008; Reisberg et al., 2009). Measurements for individual meteorite samples normally consisted of multiple analyses using the same filament loading, as well as multiple separate loadings for most samples (n), and give comparable 2s variations, and with 2sM precisions as low as 3 ppm for repeated measurements of some ratios (where n 44). Osmium isotopic measurements for 186Os were mathematically corrected for ingrowth from 190Pt (190Pt -186Osþ 4He; t1/2 ¼ 488 Ga). To do this, all concentrations were determined in our lab by isotope dilution using conventional techniques (e.g., Walker et al., 2008). Most of the Pt and Os concentration data were taken from previous studies conducted by our group for adjoining pieces of the meteorite (e.g., Cook et al., 2004; McCoy et al., 2011; Walker et al., 2008) (Table 1), although several of the concentration pairs were determined especially for this study (Pt and Os concentrations for all samples are provided in Table S3). For most meteorites, this correction is relatively minor ( o60 ppm) and introduces little additional error. However, four meteorites examined here, the IIIAB iron Tamarugal, and the PMG meteorites Brenham, Imilac and Krasnojarsk have such high Pt/Os that corrections ranging from 700 (Imilac) to 48000 ppm (Brenham) are required for calculating initial 186Os/188Os. Such large corrections, coupled with the possibility of minor open-system behavior means that the initial 186Os data for these meteorites cannot be meaningfully considered here, and are, therefore, not reported. Corrections for radiogenic ingrowth from 187Re on 187Os for all meteorites are much larger than for ingrowth of 190Pt on 186Os. As with the meteorites characterized by very high Pt/Os, these corrections are of sufficient magnitude to result in insufficient

40

R.J. Walker / Earth and Planetary Science Letters 351–352 (2012) 36–44

precision in constraining initial 187Os/188Os such that the exploration of variations in initial 187Os is not possible. Effects of CRE on 187 Os, however, are discussed below. Deviations from terrestrial values are reported for the meteorites as e units for e189Os and e190Os, where e1xxOs is the part per 104 deviation from bulk chondritic/terrestrial ratios (Table 1). The 186 Os/188Os of the UMd Os standard is elevated relative to chondrites, so the meteorite sample data are reported as e186Osi, which is the part per 104 deviation of the calculated initial 186Os/188Os of a meteorite from the initial ratio of the solar system. The solar system initial ratio of 0.11982736 was calculated from our running average for bulk chondrites of 0.1198387 (Yokoyama et al., 2011), corrected for 4.56 Ga of ingrowth. 3. Results Results of Os isotope analyses for 18 iron meteorites and four pallasites are provided in Table 1 and shown in Fig. 1a–f. There are well-resolved isotopic anomalies for 186Osi, 189Os and 190Os among many of the irons and pallasites analyzed. Anomalous e186Osi and e190Os values show negative, linear correlations with e189Os. The magnitudes of anomalies vary among samples and among the major magmatic iron groups and pallasites. The e189Os and e190Os results for IVB iron meteorites are generally consistent with those reported by Huang and Humayun (2008), with the largest Os isotopic anomalies observed in the IVB iron Tlacotepec. The e186Osi for Tlacotepec (not reported by Huang and Humayun, 2008) is also the largest of the suite measured here ( þ0.79). None of the samples examined for 184Os have e184Os values that, within the measurement resolution of 727 e units, are enriched or depleted relative to our laboratory standard, despite its large neutron capture cross section and the possibility of nucleosynthetic heterogeneity (Fig. 2).

4. Discussion The e190Os versus e189Os and e186Osi trends defined by the iron groups and pallasites do not have the same slopes as trends defined by isotopic variations recorded by chemical separates from chondrites (when re-normalized for fractionation using 192 Os/189Os) that have been attributed to nucleosynthetic effects (e.g., Humayun and Brandon, 2007) (Fig. S2). Moreover, it has been long recognized that iron meteorites within a chemical group likely formed from the same, isotopically homogeneous magmas in closed systems (e.g., Schaudy et al., 1972). The goodness of fit of 187Re-187Os isotopic data to primordial isochrons for major iron groups, coupled with modeling of the trace element behavior of Os in iron systems (e.g., Cook et al., 2004; Horan et al., 1992; McCoy et al., 2011; Shen et al., 1996; Smoliar et al., 1996; Walker et al., 2008) support the contention of closed-system crystal–liquid fractionation, at least for Os. Thus, it is very unlikely that the isotopic variations within a chemical group could be due to genetic differences. Instead, the dominant isotopic variations are most likely attributable to CRE effects. 4.1. Cosmic ray exposure model. To investigate the effects of CRE on isotopes of Os in iron meteorites and pallasites, a simple model is presented here. The modification of isotopic abundances of a given element, such as Os, can result from the capture of secondary neutrons generated by interactions between the meteorite and cosmic rays, both solar and galactic, (e.g., see discussions by Markowski et al., 2006; Masarik, 1997; Sprung et al., 2010). Those isotopes with large neutron capture cross sections, or isotopes that are produced from

the burnout of other isotopes with large neutron capture cross sections, are most strongly affected. The magnitude of burnoutþ production for individual isotopes reflects the neutron fluence (flux integrated over the period of exposure) experienced by the portion of the meteorite studied, as well as the distribution of the secondary neutron energies produced. Neutron fluence and energies are established by the period of exposure of a meteorite to cosmic rays, coupled with the long-term integrated types and energies of the cosmic rays and the secondary neutrons that are produced. The nature of the neutrons produced is affected by the composition of the meteorite and the depth of the sample from the surface of the meteoroid in space. Thus, although cosmic ray exposure ages have been previously reported for some of the meteorites examined here, it is impossible to convert these ages to estimates of neutron fluence. At present the Os data cannot be used to quantitatively assess the CRE history of the meteorites examined here. When exposed to cosmic rays for long periods of time, the isotopic abundances of Os in a sample are significantly affected by Os burnout, as well as by production of Os isotopes via the burnout of Re, Ir and to a lesser extent, W. The major cosmogenic reactions involved are provided in Table 2. For the model calculations, burnout modification to the abundances of each isotope of Os, Re, Ir and W was assessed using: NiBurnout ¼ ðNi nAnsi Þ þðN i nBnIi Þ where NiBurnout is the total number of atoms of target isotope i converted by burnout per gram of sample, Ni is the number of atoms of isotope i present in 1 g of sample prior to exposure to neutron capture, si is the thermal neutron capture cross section of isotope i (barns), Ii is the epithermal resonance integral of isotope i (barns), A is the neutron fluence of thermal neutrons (cm  2), and B is the neutron fluence of epithermal neutrons (cm  2) (Masarik, 1997; Mughabghab, 1984). Because the thermal and epithermal neutron fluences for specific samples are unknown, the two variables are treated as free parameters and were varied to achieve viable fits to the observed Os measurements. A final accounting of burnout and production effects on the isotopes of Os was obtained using a spreadsheet that is supplied in the SOM. Because 188Os and 192 Os are affected by their own burnout as well as by burnout of 187 Os, 187Re and 191Ir, the isotopic compositions of Os in the model were calculated following the accounting and renormalization to 192 Os/188Os¼3.08271. For the variables used in the models, burnout and production effects on this ratio were  8 ppm, leading to modest corrections of  2 ppm per amu on the other isotop ratios. Because the IVB iron meteorite Tlacotepec exhibits the largest Os isotopic shifts from terrestrial abundances of the suite examined here, modeling efforts were focused on attaining similar Os isotopic compositions to it. Tlacotepec had a very long history of exposure to cosmic rays. Voshage and Feldmann (1979) reported a cosmic ray exposure age of 945 755 Ma for it. Using the model described above, it is discovered that the isotopic enrichment in 190 Os and corresponding depletion in 189Os observed in Tlacotepec, cannot be achieved solely by invoking the effects of thermal neutrons. This is because for all thermal neutron fluences, production of 188Os from 187Os and 187Re burnout is greater than for burnout of 188Os, and cumulative burnout and production of 189 Os and 190Os. Thus, negative e values for both 189Os and 190Os obtain for all neutron fluences (Fig. 1). Epithermal neutron capture can better account for the 189Os–190Os co-variations, albeit when the model is optimized to match e189Os to the value observed for Tlacotepec, the e190Os is  0.1 unit too high (Fig. 1). To achieve the best match of model results to the meteorite data, contributions from both thermal and epithermal neutrons were necessary in the calculations. A good match to the Tlacotepec data was achieved using neutron fluence values of 4.0  10  7 cm  2

R.J. Walker / Earth and Planetary Science Letters 351–352 (2012) 36–44

1.0

0.4 Production + burnout model

0.8

0.3

0.4

ε186Osi

ε190Os

Production + burnout model

0.6

0.2 0.1

0.2

0.0

0.0

-0.1 -0.2 -0.6

-0.2 -0.4

-0.2

-0.4 -0.6

0.0

Warburton Range (IVB) Cape of Good Hope (IVB) Hoba (IVB) Tlacotepec (IVB) Tawallah Valley (IVB)

-0.4

ε189Os

Production + burnout model

0.8

ε186Osi

ε190Os

0.1 0.0

Production + burnout model

0.4 0.2 0.0

-0.1

-0.2 -0.4

-0.2

-0.4 -0.6

0.0

Negrillos (IIAB) Bennett County (IIAB) Costilla Peak (IIIAB) Henbury (IIIAB) Tamarugal (IIIAB)

-0.4

-0.2

0.0

ε189Os

ε189Os 0.4

1.0 Burnout + production model

0.8

0.3

Production + burnout model Charlotte (IVA)

0.6 ε186Osi

0.2 ε190Os

0.0

0.6

0.2

-0.2 -0.6

-0.2 ε189Os

1.0

0.4 0.3

41

0.1 0.0

Gibeon (IVA) Jamestown (IVA)

0.4

Yanhuitlan (IVA) Nedagolla (ungrouped)

0.2 0.0

-0.1

-0.2

-0.2 -0.6

-0.4 -0.6

Canyon Diablo (IAB) Eagle Station (PES) Brenham (PMG) Imilac (PMG) Krasnojarsk (PMG)

-0.4

-0.2

0.0

ε189Os

-0.4

-0.2 ε189Os

0.0

Fig. 1. (a–f) Plots of e189Os vs. e186Osi (corrected for radiogenic ingrowth from 190Pt as discussed in text), and e190Os vs. e189Os for various iron groups and pallasites. The CRE production model is discussed in the text. Error bars shown are the 2sM of multiple analyses of each meteorite where nZ 4. If fewer than 4 measurements were made for a given meteorite, the 2s external uncertainty for repeated standards analysis for the corresponding analytical period, or the 2sM internal statistics, whichever was larger, is shown. The IIIAB iron Tamarugal, and the pallasites Brenham, Imilac and Krasnojarsk have such high Pt/Os that sufficiently precise correction for radiogenic ingrowth is not possible to permit assessment of 186Os anomalies. Hence, no e186Osi are plotted for these meteorites. The stippled boxes show the extent of external precision for analyses of the terrestrial standard (189Os/188Os and 190Os/188Os) and bulk chondrites (for 186Os/188Os, from Yokoyama et al., 2007).

and 1.6  10  7 cm  2 for variables A and B, respectively (Fig. 1). Other mixtures of thermal and epithermal neutron fluence values are capable of producing isotopic compositions similar to those observed for Tlacotepec, so this model does not provide a unique constraint on the neutron fluence experienced by this portion of the meteorite. The results of this model appear similar to the cosmic ray, neutron capture burnout model of Huang and Humayun (2008). The results of this model are also broadly consistent with the offsets for e189Os–e190Os present in meteorites from the other iron groups and pallasites (Fig. 1c and e), so it is concluded that CRE has similarly affected those other meteorites with non-normal

Os, albeit with diminished neutron fluences, as compared with Tlacotepec. Compared to 189Os and 190Os, CRE effects on 186Os are more difficult to model because, in addition to burnout and production, 186 Os is a radiogenic daughter product of 190Pt. Consequently, the effects of long-term 186Os ingrowth must be accurately removed via calculation in order to gauge CRE or nucleosynthetic effects for a particular meteorite. Tlacotepec is characterized by a relatively low Pt/Os (Table 1), so the difference between measured e186Os and calculated e186Osi for this meteorite is small, only 0.33 e units, and modest uncertainties in Pt/Os or the decay constant for

R.J. Walker / Earth and Planetary Science Letters 351–352 (2012) 36–44

100

ε184Os

50

0

-50

-100 -0.6

-0.4

-0.2

0.0

0.2

ε189Os Fig. 2. Plot of e189Os vs. e184Os for irons and pallasites. No deviations from the terrestrial value are observed beyond the 7 27 e unit analytical uncertainties for standards (dashed lines). Symbols are the same as in Fig. 1. Note that only data for which Pt corrections have been made on 184Os are shown. 190

Pt have very minor effects upon the calculated e186Osi. Nevertheless, model results for 186Os in Tlacotepec are less satisfying than for 190Os–189Os. Model runs biased towards either thermal or epithermal neutrons all yield e186Osi values that are  2.5 times too high, relative to e189Os (Fig. 1b). Model runs with both thermal and epithermal neutrons fare little better. For example, using the preferred values for neutron fluence discussed above for 189 Os and 190Os in Tlacotepec, an e186Osi value of þ 2.05 is calculated, as compared to the measured value of þ0.79 for the meteorite. The difference is well above possible uncertainties in measurement or correction for radiogenic ingrowth. No explanation for the discrepancy with regard to Tlacotepec is offered here. The observation may indicate that the simple model used here is inadequate to account for all nuclear effects. Nevertheless, this model provides a somewhat better fit to the data for IIAB, IIIAB and IVA irons (Fig. 1d and f), although in the case of the IVA irons, the errors are sufficiently large that there is little discrimination among different possible trends. Two other observations relating to the model are noted. First, using the same values for neutron fluence as above, the model offset in e187Osi is -0.5 units. However, the propagated uncertainty of the calculated initial e187Osi is 74.5 units for Tlacotepec, assuming 70.15% uncertainty for 187Re/188Os. Hence, the effects of CRE cannot be resolved for this isotope. Second, the model offset for e184Os is 6.7 e units, which is well below the measurement precision of 727 e units, so CRE effects cannot currently be resolved for this isotope as well. Conversely, the lack of resolved anomalies for this isotope means that the large p-process anomalies, recorded in e184Os values of þ350 to þ460 in some Murchison leachates (Reisberg et al., 2009) are not manifested in these irons and pallasites In summary, we currently attribute all resolved Os isotopic anomalies in our sample suite to CRE, consistent with the conclusions of Huang and Humayun (2008) regarding IVB irons. This means that Os may be useful as a sensitive monitor for neutron fluence in metal phases with long-term CRE. If Os isotopic offsets can be calibrated with known neutron fluence dosimeters, such as 3He (e.g., Markowski et al., 2006), it may eventually be possible to use Os to estimate neutron fluence, and Os may prove useful for correcting for the effects of CRE when studying other systems, such as the 182Hf–182W system.

beyond analytical uncertainties, from bulk chondritic/terrestrial compositions. The iron meteorites with normal Os are Canyon Diablo (IAB), Negrillos (IIAB), Costilla Peak (IIIAB) Charlotte (IVA) and Hoba (IVB). Krasnojarsk of the PMG, Eagle Station of the PES grouplet, and the ungrouped iron Nedagolla all have normal Os isotopic compositions. In light of the strong genetic relationships between meteorites within groups, we infer that nucleosynthetic effects are not present for Os in the parent bodies of the irons and pallasite meteorites at the 410 ppm level for 189Os and 190Os. Further, we infer there is no heterogeneity at the 430 ppm level for 186Os in the major iron groups, or the PES grouplet. The level of homogeneity for 186Os in the PMG cannot currently be constrained because of the lack of sufficiently precise initial 186Os/188Os data. The isotopic homogeneity of Os among bulk materials in the early solar system is in contrast to the isotopic variability recorded in the HSE Ru, and the moderately siderophile elements Mo (Fig. 3) and W (e.g., Burkhardt et al., 2011; Chen et al., 2010; Dauphas et al., 2002; Qin et al., 2008). The difference between Os and these other siderophile elements could mean that the magnitude of isotopic variability among presolar precursor materials is greater for those elements, than for Os. In such a case, partial mixing may have reduced Os isotopic heterogeneities in the nebula below our current level of resolution, while not as much for the other elements. This possibility can be tested by analyzing the same chondritic leachates and residues for all of these elements. If this interpretation is correct, it might be expected that Os isotopic variations would be more muted than for the other elements in the same chemical fractions. Limited existing data for leachates of the CM2 chondrite Murchison, however, do not appear to indicate particularly larger anomalies for Mo or W than for Os (Burkhardt ¨ et al., 2012; Fischer-Godde et al., 2012). It is also possible that the isotopic disparities are the result of a late-stage injection of presolar materials to the coalescing nebula (e.g., Bizzarro et al., 2007; Dauphas et al., 2010). The r-process rich nature of Os contrasts with Ru and Mo that are dominated by sand p-process nucleosynthesis. Consequently, a late-stage injection of s-process enriched material, such as from an asymptotic giant branch star, could have led to the creation of variable isotopic anomalies for Mo and Ru, depending on timing and the geometry of the injection. The same injection would have generated much smaller collateral effects for Os. This option would be supported if correlations can be established between enrichments of certain isotopes of each element that can be linked to a specific stellar process.

2.0 1.5 ε92Mo

42

1.0

IAB IIAB IIIAB IVA IVB PMG

0.5 0.0 -0.5

-0.1

0.0

0.1

0.2

ε189Os 4.2. Implications for mixing of nucleosynthetic components in the protosolar nebula. Of the five iron groups examined, there is one meteorite from each group whose Os isotopic compositions are not resolved,

Fig. 3. Plot of e189Os vs. e92Mo for irons and pallasites. Molybdenum isotopic data are from Burkhardt et al. (2011). No deviations from the terrestrial value are observed for e189Os beyond the 2s 7 0.08 e unit analytical uncertainty for standards (dashed lines). In contrast, Mo isotopic variations range over  2 e units, which is well beyond analytical uncertainties.

R.J. Walker / Earth and Planetary Science Letters 351–352 (2012) 36–44

Finally, it has been argued that selective destruction of thermally labile carrier phases in the protosolar nebula can lead to isotopic heterogeneities, such as for Ti isotopes (Trinquier et al., 2009). Consequently, it is possible that the destruction of certain presolar host phases for Ru, Mo and W led to isotopic heterogeneity, as compared to Os. Although the carrier phases of presolar Os are not yet well constrained, Croat et al. (2005) reported the presence of metallic Os in presolar graphites. Due to the extremely refractory nature of Os, such grains are likely to be much more resistant to thermal processing than grains containing Ru and Mo.

Acknowledgements This work was supported by the NASA Cosmochemistry grant #NNX10AG94G, which is gratefully acknowledged. This study benefitted from discussions with D. Cook, J. Day, G. Herzog, and T. Yokoyama, as well as technical support from I. Puchtel. Construc¨ tive and thorough reviews by M. Fischer-Godde, M. Humayun and editor T. Elliott are also gratefully acknowledged.

Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.epsl.2012.07.026.

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