Assessment of nebular versus parent body processes on presolar components present in chondrites: Evidence from osmium isotopes

Earth and Planetary Science Letters 305 (2011) 115–123

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Earth and Planetary Science Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e p s l

Assessment of nebular versus parent body processes on presolar components present in chondrites: Evidence from osmium isotopes Tetsuya Yokoyama a,⁎, Conel M.O'D. Alexander b, Richard J. Walker c a b c

Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Ookayama, Tokyo 152–8551, Japan Department of Terrestrial Magnetism, Carnegie Institution of Washington, DC 20015, USA Department of Geology, University of Maryland, College Park, MD 20742, USA

a r t i c l e

i n f o

Article history: Received 30 November 2010 Received in revised form 22 February 2011 Accepted 25 February 2011 Editor: T. Spohn Keywords: Os isotopes presolar grains acid resistant residues nebular process aqueous alteration thermal metamorphism

a b s t r a c t We report Os isotope compositions of acid residues separated from six carbonaceous chondrites that were subjected to varying degrees of aqueous alteration on their parent bodies. The residues from three enstatite chondrites were also investigated in order to evaluate the effects of nebular and parent body processing on the survival of presolar grains in meteorites formed under redox conditions that were markedly reduced, compared to those under which carbonaceous and ordinary chondrites formed. All acid residues from CM and CR chondrites show enrichments in s-process Os isotopes relative to the Solar System average recorded in bulk chondrites. The extent of the anomaly present positively correlates with the degree of aqueous alteration of the host chondrites. This correlation was probably caused by selective destruction/modification of presolar grains carrying r-process-enriched Os during progressive aqueous alteration on the parent bodies. The r-process-enriched component was likely either presolar silicates formed in Type II supernova ejecta, or other unidentified reduced presolar phases such as metal alloys, carbides and silicides. Acid residues from enstatite chondrites have Os isotope anomalies that are much more enriched in the s-process components, relative to the residues from carbonaceous and ordinary chondrites that experienced the same grade of thermal metamorphism. This most likely reflects the selective destruction of s-process-enriched presolar phases that occurred under the more oxidized conditions experienced by carbonaceous and ordinary chondrites, either while components formed in the nebula, or on their parent bodies. To account for the uniform, terrestrial Os isotopic composition in all types of bulk chondrites, it is required that r-process or s-process-enriched Os, released from presolar phases during nebular or parent body processing, was re-incorporated into a new phase(s) which was not lost from the location where the bulk meteorites were derived. However, because parent body processing might have acted differently on other elements (e.g., open system behavior of fluid mobile elements during aqueous alteration), recent findings of isotopic heterogeneities in bulk meteorites should be evaluated not just by invoking nebular heterogeneities, but by also considering the effects of parent body processing. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Chondrites and differentiated meteorites at the bulk meteorite scale commonly show small but resolvable mass-independent isotopic deviations from the terrestrial values in refractory elements such as Cr, Ni, Ti, Mo, Ru, Ba, Nd and Sm (Andreasen and Sharma, 2006; Carlson et al., 2007; Chen et al., 2010; Dauphas et al., 2002; Qin et al., 2010; Regelous et al., 2008; Trinquier et al., 2007, 2009; Yin et al., 2002). At least some of these anomalous isotopic compositions are nucleosynthetic in origin, and have been interpreted to reflect incomplete mixing of isotopically diverse presolar materials in the protosolar nebula, or even late injection of isotopically distinct

⁎ Corresponding author. E-mail address: [email protected] (T. Yokoyama). 0012-821X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2011.02.046

materials into the nebula (Bizzarro et al., 2007). In contrast, uniform, terrestrial isotopic compositions in bulk chondrites have been reported for some elements including Os, one of the most refractory elements (Yokoyama et al., 2007, 2010). The contrasting isotopic characteristics of diverse elements may, thus, provide important clues to how Solar System precursor solids formed, were transported, and mixed in the nebula, ultimately constituting the building blocks of the rocky planets. Differences in the magnitudes of anomalous isotopic compositions in bulk meteorites also exist not only across but also within chondrite classes (carbonaceous, ordinary and enstatite). Of these, anomalies in carbonaceous chondrites are most prominent. For example, enrichment of r-process Ba isotopes and deficits in p-process Sm and Nd isotopes, with respect to the terrestrial values, are observed only in carbonaceous chondrites (Andreasen and Sharma, 2006; Carlson et al., 2007). A systematic variation in 54Cr/52Cr ratios, which may correlate

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with Δ17O values, is present among carbonaceous chondrite groups (e.g. CI, CM, CR, CO, CV) (Trinquier et al., 2007). These observations may reflect a radial isotopic heterogeneity in the early solar nebula at the time of planetesimal formation. Previous studies have attributed such isotopic heterogeneities mainly to nebular processes, such as differential sorting of physically different dust grains (Regelous et al., 2008), a late supernova injection (Bizzarro et al., 2007), and/or selective destruction of thermally labile presolar grains in the nebula (Trinquier et al., 2009). Processes that occur on parent bodies can also potentially affect the isotopic compositions of bulk chondrites and bear further investigation. For example, Huss and Lewis (1995) demonstrated that presolar SiC and graphite did not survive to higher degrees of thermal metamorphism on chondrite parent bodies. Losses of volatile presolar components during thermal metamorphism, if present, could potentially modify the isotopic compositions of a parent body from the original values at the time of planetesimal accumulation. Further, signs of aqueous alteration are widespread in carbonaceous chondrites. Although the timing, location and style of aqueous alteration are not unambiguously constrained, some studies have proposed that aqueous alteration was caused by fluid flow that resulted in open system material transport within parent bodies (Palguta et al., 2010; Young et al., 1999). If aqueous fluid in a parent body preferentially dissolves some presolar phases and carries away their constituent elements, it can potentially lead to formation of an isotopically heterogeneous parent body. In such a case, chondritic meteorites would not necessarily record the isotopic compositions of their source region or parent body. For these reasons, it is important to understand how much secondary processes have modified the initial population of presolar phases in chondritic parent bodies. Isotopic investigation of acid residues from primitive chondrites may shed light on this problem, because the resides are enriched in various types of presolar grains and, thus, possess internal isotopic diversities that, more or less, control the isotopic composition of bulk chondrites. Osmium is one of the more promising elements for tackling these problems. It has seven isotopes produced by stellar nucleosynthesis of the p-, s- and r-processes. The solar relative Os abundances can be estimated by averaging eighteen chondrite measurements that were treated by an alkali fusion total digestion technique (Yokoyama et al., 2010: 184 Os = 0.017%, 186 Os = 1.59%, 187 Os = 1.68%, 188 Os = 13.27%, 189 Os = 16.19%, 190Os = 26.33%, 192Os = 40.92%). Osmium-184 is a pure p-process isotope. Two isotopes (189Os and 192Os) have large (N90%) contributions from the r-process, while 186Os and 187Os are mainly produced by the s-process, with radiogenic production from 190 Pt and 187Re, respectively. The remaining two isotopes, 188Os and 190 Os, are mainly r-process but have moderate (10–20%) contributions from the s-process. A detailed discussion regarding the contribution of individual nucleosynthetic processes on the solar Os isotopic composition is given in Reisberg et al. (2009). The diversity in isotope production mechanisms makes the Os isotope composition sensitive to the addition/loss of a specific nucleosynthetic component to/from the solar component. We have previously measured Os isotopes in acid residues from nine carbonaceous and one ordinary chondrites, and found that they were enriched in Os isotopes produced by the s-process, of which the extent of anomaly varied across chondrite groups (CI N Tagish Lake N CM N CR N Adelaide N CV ≈ OC). The observed variation was presumably created during parent body processing, however, it remains unclear how individual processes have acted on the modification of initial presolar grain populations. In this study, we present evidence recorded in the Os isotope compositions of acid residues from carbonaceous chondrites with a variety of petrologic grades, including four CMs (CM1–CM2) and three CRs (CR1–CR2), that aqueous alteration in the meteorite parent bodies has preferentially destroyed some presolar phases. We also

report Os isotopic compositions in acid residues from three enstatite chondrites. The parent bodies of enstatite chondrites formed under extremely reducing conditions in the early Solar System. Thus, they might have sampled or preserved a different population of presolar grains compared to carbonaceous and ordinary chondrites. A closer look at the acid residues from enstatite chondrites can, therefore, provide useful information regarding, not only material distribution and transport mechanisms in the solar nebula, but also the fate of presolar grains during parent body processing under different conditions. 2. Experimental 2.1. Acid residue samples Acid residues analyzed in this study were prepared from six carbonaceous chondrites (Meteorite Hills (MET) 01070, CM1; Alan Hills (ALH) 83100, CM1/2; Murchison, CM2; Queen Alexandra Range (QUE) 97990, CM2; Grosvenor Mountains (GRO) 95577, CR1; Tagish Lake, C2) and three enstatite chondrites (MacAlpine Hills (MAC) 02837, EL3; Elephant Moraine (EET) 87746, EH3; Indarch, EH4). We followed a CsF/HF leaching technique (Cody and Alexander, 2005; Cody et al., 2002) for the preparation of the acid residues, which is a relatively mild process compared to the conventional HF/HCl method (e.g. Lewis et al., 1975). Powdered chondrites were first treated with 2 M HCl at room temperature to dissolve metal phases (e.g. Fe, Ni), carbonates, sulfides, amorphous materials, and HCl soluble silicates. The residue was treated with a mixture of dioxane and CS2 to remove elemental S. Then the residue was repeatedly treated with two immiscible solutions, dioxane and aqueous CsF/HF to extract insoluble organic matter (IOM), and was subsequently cleaned using sequentially 1 M HCl/9 M HF, 2 M HCl, H2O and dioxane. Typically, the IOMrich residues contain 5–15 wt% of non-combustible material. Examination of residues from OC, CM, CR and CI chondrites by scanning electron microscope (SEM) and ion microprobe show that this ‘ash’ is dominated by chromite with minor spinel, corundum, hibonite and presolar SiC. Other types of presolar grains (e.g., graphite, nanodiamonds, and oxides) are also present in IOM. The abundances and isotopic compositions of light elements (e.g., H, C, N and O) in IOM have been previously investigated (Alexander et al., 2007, 2010). For consistency with our prior report (Yokoyama et al., 2010), we hereafter refer to these resides as “IOMR” (IOM rich residues). Of these samples, measurement of Murchison IOMR (AR5) acts as a replicate of our earlier report (Yokoyama et al., 2010). The residue of Tagish Lake (AR2) was prepared in the same study, where only the 2 M HCl leachate of the bulk rock obtained during residue preparation was analyzed. 2.2. Sample digestion and mass spectrometry The details of sample digestion are described in Yokoyama et al. (2010). In brief, IOMR samples (2–5 mg) were first combusted at 1000 °C in quartz Carius tubes, and then digested with a 1:2 mixture of concentrated HCl and HNO3. At this point, a ~5% aliquot of the digested solution was saved and dedicated to the determination of highly siderophile element concentrations (HSE: Re, Os, Ir, Ru, Pt and Pd) via isotope dilution mass spectrometry, with the addition of appropriate amounts of the spikes. The Os concentration was measured by negative thermal ionization mass spectrometry (N-TIMS) using a Thermo-Fisher TRITON at the University of Maryland (UMd). The concentrations for the rest of the HSE were measured using either a sector-type ICP-MS (Thermo-Fisher ELEMENT2) at UMd or a quadrupole-type ICP-MS (Thermo-Fisher X-series II) at Tokyo Institute of Technology (Titech). From the rest of the digested solution (~ 95%), Os was extracted by CCl4 and subsequently purified using a microdistillation technique

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(Birck et al., 1997). High precision Os isotope analyses were carried out by NTIMS using the TRITON at the UMd. The data were normalized to 189 Os and corrected for mass fractionation using 192 Os/ 189 Os = 2.527411, the isotopic pair that has the least contribution from s-process nucleosynthesis to the solar system average (189Os: 4.2%, 192Os: 1.1% Reisberg et al., 2009). The Os isotope ratios are reported in εOs units (ε184Os, ε186Osi, ε188Os and ε190Os), which represent relative deviation (parts per 104) from the average of bulk chondrite analyses (‘solar values’) reported in Yokoyama et al. (2010). The 186Os/189Os ratio has been time-corrected for 190Pt decay over 4.56 Ga, hence, the use of ε186Osi. Here, in contrast to our prior work, we attempted precise determination of 184Os/189Os ratios in the samples. Osmium-184 is a pure p-process nuclide of which the abundance is extremely low in chondrites (~0.02% of total Os). To do this, we corrected for potential mass interferences on 184Os16O3 from Pt (198Pt16O18O) and 183 184 W16O17 W16O3) by monitoring m/e = 230 W (182W16O18 2 O, 2 O and 196 16 18 198 16 180 16 18 Pt O O + Pt O 2 + W O 2 O + 182 W 16 O 3 ) and 231 ( 183 16 O + W O3) using an electron multiplier (198Pt16O17O + 182W16O17 2 at the beginning of every block in individual isotope runs (1 run = 20 ratios/block × 18 blocks). In the repeated analyses of our laboratory standard (Johnson Matthey Os), the interferences from Pt and W (negligible) corresponded to b2% of the 184Os16O3 intensities. The correction significantly improved the reproducibility of 184Os/189Os measurement from 1.1% to 0.26% (2σ: n = 15), a reflection of the improvement to the accuracy of our technique. The interference correction was generally 1–2% for the acid residue runs, excluding two samples which required ~ 5% corrections owing to the low Os intensities (192Os16O3 b1 V). 3. Results 3.1. HSE concentrations in IOMR The HSE abundances in IOMR are presented in Table 1 as absolute concentrations in IOMR and normalized values relative to the CI chondrite abundances reported in Horan et al. (2003).Also presented in this table is the mass percent of IOMR yield, relative to the original meteorites. As previously reported, the concentrations reported in this table may contain as much as ~ 20% uncertainties due to imperfect recovery of HSE (~ 90%) during sample decomposition and the uncertainties in the determination of IOM recovery yield (Yokoyama et al., 2010). It should be noted, however, that uncertainties in the relative abundances of these elements, hence HSE ratios, are much smaller (b10%). All IOMR have HSE abundances more than two times greater than CI, in general agreement with our previous observation for acid

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residues from carbonaceous and ordinary chondrites (Yokoyama et al., 2010). The Pt/Os ratios of the residues are also higher than chondritic ratios in most cases. For IOMR from CM chondrites, the Os abundances are relatively homogeneous and do not show any variations that correlate with the degree of aqueous alteration of the host. This is observed as well in IOMR from CR chondrites (Os = 13.0 ppm in EET 92042; Os = 7.63 ppm in GRA 95229; Yokoyama et al., 2010). Although the abundances of some HSE (e.g., Ir and Ru) in the CM residues are variable compared to Os, their correlation with degree of alteration seems to be weak. In contrast, the IOMR from enstatite chondrites are rich in HSE, relative to those from CM and CR chondrites. However, their abundances are within the range of the IOMR from type 3 carbonaceous and ordinary chondrites (CV3 and LL3.05; Yokoyama et al., 2010).

3.2. Osmium isotope compositions in IOMR Osmium isotope data for the IOMR are listed in Table 2 and plotted on ε186Osi versus ε188Os (Fig. 1) and ε190Os versus ε188Os (Fig. 2) diagrams, along with data from other carbonaceous and ordinary chondrite residues reported by Yokoyama et al. (2010). All the IOMR measured in this study are characterized by positive ε186Osi, ε188Os and ε190Os values that are resolvable from the solar (terrestrial) component. Because the solar component has a large contribution from s-process 186Os and moderate contributions from s-process 188 Os and 190Os, relative to r-process dominant 189Os, the positive εOs values are suggestive of the enrichment of Os isotopes produced by the s-process. Excluding one sample (EET 87746), anomalies for ε184Os were not observed at the current level of analytical precision (±26ε units). There are significant variations in εOs values (excluding ε184Os) across IOMR from CM chondrites. The magnitude of the positive Os isotope anomalies in the IOMR of the CM1/2 chondrite, ALH 83100 (ε188Os = +3.10) is nearly twice as large as those present in residues from the CM2 chondrite, Murchison (ε188Osaverage = +1.66). Consistent with this, the IOMR of MET 01070 (CM1) has εOs values (ε188Os = +2.31) that are larger than those of the Murchison residues, whereas that from the CM2.6 chondrite (QUE 97990) has the smallest deviation in εOs values from zero among all CM chondrites (ε188Os = + 0.95). This is the first direct comparison of Os isotope anomalies in IOMR from a single chondrite group with different petrologic types. Variations in the magnitude of the εOs values are also observed in IOMR from CR chondrites, where the residue from the CR1 chondrite (GRO 95577) showed the largest positive Os isotope anomalies (ε188Os = + 2.64), and are comparable with CM1 chondrites. Overall, the results for IOMR from carbonaceous chondrites are consistent with our previous observation that the magnitude of the

Table 1 Concentrations of highly siderophile elements and Pt/Os ratios in acid-resistant residues. Sample (Class)

Absolute concentrations in residues (ppm)

Mass%

Re

Os

Ir

Ru

Pt

Pd

Pt/Os

Carbonaceous chondrites Tagish Lake AR2 (C2) MET 01070 (CM1) ALH 83100 (CM1/2) Murchison AR5 (CM2) QUE 97990 (CM2.6) GRO 95577 (CR1)

0.300 0.766 0.745 0.598 0.533 0.485

4.50 7.75 10.7 11.6 10.3 8.17

0.976 8.75 13.3 3.47 1.62 5.10

1.27 7.98 19.3 4.70 3.16 4.25

21.8 44.2 46.6 30.3 19.4 70.5

12.6 24.8 n.d. 3.79 3.61 33.2

4.84 5.71 4.35 2.61 1.89 8.62

Enstatite chondrites EET 87746 (EH3) Indarch (EH4) MAC 02837 (EL3)

0.651 0.952 0.368

27.6 37.8 23.9

15.2 33.0 17.7

53.9 81.0 63.3

191 114 130

n.d. n.d. n.d.

CI Chondrite Average (ppm) Ivuna and Orgueil

0.0378

0.449

0.431

0.636

0.856

0.555

CI normalized values Re

Os

Ir

Ru

Pt

Pd

3.11 1.52 1.25 1.13 1.65 1.56

7.94 20.3 19.7 15.8 14.1 12.8

10.0 17.3 23.9 25.9 22.9 18.2

2.27 20.3 30.8 8.05 3.77 11.9

2.00 12.5 30.3 7.39 4.98 6.68

25.5 51.7 54.4 35.5 22.7 82.4

22.7 44.8

6.91 3.03 5.42

0.24 0.39 0.33

17.2 25.2 9.72

61.5 84.1 53.3

35.2 76.7 41.2

84.8 127 99.6

223 134 152

1.91

Horan et al. (2003)

6.83 6.50 59.9

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Table 2 Osmium isotopic compositions of chondritic acid residues. Sample (Class)

ε184Os

ε186Os

ε186Osi

187

Os/189Os

ε188Os

ε190Os

Carbonaceous chondrites Tagish Lake AR2 (C2) MET01070 (CM1) ALH83100 (CM1/2) Murchison AR5 (CM2) QUE97990 (CM2.6) GRO95577 (CR1)

− 39 ± 13 10 ± 19 43 ± 54 − 17 ± 15 − 29 ± 23 13 ± 24

11.5 ± 0.2 12.1 ± 0.2 18.2 ± 0.6 7.3 ± 0.2 4.0 ± 0.3 15.1 ± 0.3

10.00 10.13 16.96 6.90 3.99 11.69

0.1036320 0.1041700 0.1033730 0.1000724 0.1008081 0.1012557

(18) (28) (63) (20) (30) (27)

2.04 ± 0.06 2.31 ± 0.07 3.10 ± 0.16 1.42 ± 0.07 0.95 ± 0.08 2.64 ± 0.08

1.38 ± 0.03 1.54 ± 0.04 2.10 ± 0.08 0.88 ± 0.04 0.50 ± 0.05 1.79 ± 0.05

Enstatite chondrites EET87746 (EH3) Indarch (EH4) MAC02837 (EL3)

−93 ± 30 −6±5 1 ± 12

13.9 ± 0.4 1.4 ± 0.1 1.9 ± 0.2

11.33 0.79 0.06

0.1023294 (35) 0.1034491 (9) 0.1004891 (17)

2.26 ± 0.11 0.29 ± 0.03 0.50 ± 0.05

1.53 ± 0.06 0.22 ± 0.02 0.32 ± 0.03

The numbers in parentheses after 187Os/189Os ratios indicate analytical uncertainties in the final digits (2σmean). For the calculation of non-age corrected ε186Os, the chondrite average 186Os/189Os determined in Yokoyama et al. (2010) was used. The Pt/Os ratios used for the determination of 186Osi/189Os ratios were measured in this study (Table 1).

positive εOs values in the residues are in the order of petrologic grade of the host meteorites (type1 N type 2 N type 3). In comparison, the IOMR from enstatite chondrites do not conform to this ordering. Several points should be noted. First, the εOs values in the residue for EET 87746 (EH3) are fairly large (ε188Os = + 2.26), which are at the upper end of those in IOMR from type 2 carbonaceous chondrites. Note that the IOMR from type 3 carbonaceous and ordinary chondrites have only marginal positive anomalies (ε188Os b 0.5). Second, the residue from Indarch (EH4) is characterized by positive εOs anomalies (ε188Os = +0.29) that are identical to those present in a residue from the OC QUE 97008 (L3.05). This is consistent with the observation that the whole rock of Indarch shows negative εOs values when incompletely digested by the mixture of HNO3 and HCl (Brandon et al., 2005), while this meteorite actually has the solar Os isotopic composition when decomposed by the alkali fusion total digestion method (Yokoyama et al., 2010). It should be also noted that 30 25

(a) CI,Tagish Lake

4. Discussion 4.1. Osmium isotope variation in IOMR and petrologic grades of CM and CR chondrites The εOs values for IOMR from CM and CR chondrites generally correlate with the petrologic grades of the host (Figs. 1–2). For a single IOMR analysis, we removed small portions (2–5 mg) of the sample from the IOMR fraction (N10 mg) prepared from individual meteorite pieces. Therefore, due to a sampling problem caused by the heterogeneous distribution of isotopically anomalous presolar grains in the IOMR, one would expect that the εOs values determined do not 30

s-process

25 Orgueil (CI1)

20 15 10

Tagish Lake (C2)

5

Solar

15 10

QUE 97008 (L3.05) Adelaide (C2/3)

0

-5

Leoville (CV3red) Allende (CV3ox)

-5

r-process

-10

-10 30

(b) CM, CR

20

20

ALH 83100 (CM1/2)

ε186Osi

15 10

GRO 95577 (CR1) MET 01070 (CM1)

GRA 95229 (CR2)

Murchison (CM2) EET 92042 (CR2) QUE 97990 (CM2.6)

5 0

15 10

EET 87746 (EH3)

5 0

MAC 02837 (EL3) Indarch (EH4)

-5

-5 -10 -1

(d) EC

25

25

ε186Osi

CV, Adelaide, OC

5

0

30

(c)

20

Ivuna (CI1)

ε186Osi

ε186Osi

the IOMR from Allende, which has a petrologic grade of N3.6 (Bonal et al., 2006), has the smallest deviation in εOs values from solar in all IOMR we have analyzed (ε188Os= +0.10 ± 0.06; Yokoyama et al., 2010).

0

1

2

3

ε188Os

4

5

6

-10 -1

0

1

2

3

4

5

6

ε188Os

Fig. 1. ε186Osi–ε188Os plots for acid residues from (a) CI chondrites and Tagish Lake, (b) CM and CR chondrites, (c) CV chondrites, Adelaide and ordinary chondrites and (d) enstatite chondrites. Data are from Yokoyama et al. (2007, 2010) and this study. Bold lines are regressions for acid residues from ten primitive chondrites obtained in (Yokoyama et al., 2010), which represent mixing lines between solar component and presumed s-process component.

T. Yokoyama et al. / Earth and Planetary Science Letters 305 (2011) 115–123

4

(a) CI, Tagish Lake

4

s-process Ivuna (CI1)

3

119

(c) CV, Adelaide, OC

3

2

ε190Os

ε190Os

Orgueil (CI1)

Tagish Lake (C2)

1

2

1

Adelaide (C2/3)

Solar 0

Leoville (CV3red) QUE 97008 (L3.05)

0

Allende (CV3ox)

r-process -1 4

-1 4

(b) CM, CR

3

ALH 83100 (CM1/2) GRO 95577 (CR1) MET 01070 (CM1)

2 GRA 95229 (CR2)

1

ε190Os

ε190Os

3

(d) EC

Murchison (CM2)

2 EET 87746 (EH3)

1

EET 92042 (CR2) QUE 97990 (CM2.6)

MAC 02837 (EL3)

0

0 Indarch (EH4)

-1 -1

0

1

2

3

4

5

6

-1

-1

0

1

2

ε188Os

3

4

5

6

ε188Os

Fig. 2. ε190Os–ε188Os plots for acid residues. Symbols are the same as Fig. 1. For bold lines, see Fig. 1.

serve as a new tool to evaluate the degree of aqueous alteration on their parent asteroids. 4.2. Enrichment of s-process Os isotopes and aqueous alteration IOM-rich residues are mixtures of isotopically anomalous components and phases with normal (solar) isotopic compositions. Our preliminary measurements revealed that high temperature components in chondrites, such as CAIs, chondrules and metal, have a uniform, solar Os isotopic composition (Yokoyama et al., 2009). Some of the high temperature components containing Os are thought to be acid resistant, which may survive in the IOMR fraction. The ordering of εOs values observed in CM and CR IOMR (Figs. 1b, 2b) is, however, not caused by a ‘dilution effect’, due to differing degrees of incorporation of solar Os from the high temperature components into the IOMR. This is because there are no correlations between εOs values and the absolute Os concentrations in the IOMR (Fig. 3). Osmium concentrations in the IOMR for CM and CR chondrites (5–13 ppm) do not vary significantly (cf. Os b 5 ppm for CI-IOMR; OsN 50 ppm for CV-IOMR). Also, the mass percent of 4 ALH 83100 (CM1/2)

3

ε188Os

represent the average of the entire IOMR fraction. However, four measurements of three Murchison IOMR (AR3, AR4 and AR5) prepared from different pieces of this meteorite show relatively homogeneous εOs values (ε188Os = 1.66 ± 0.18; 1σ) when compared to the entire range of the CM-IOMR analyzed (Fig. 1b). This indicates that the observed variation and ordering of εOs values against the petrologic grade of CM and CR chondrites are real. In CM chondrites, the largest positive anomaly was obtained from CM1/2 (ALH 83100), not from CM1 (MET 01070). Based on the mineralogical and textural properties, Rubin et al. (2007) assigned the petrologic subtypes for CM chondrites to range from 2.0 to 2.6. The least altered ones, including QUE 97990, were arbitrarily assigned to subtype 2.6. Murchison is relatively unaltered and designated as CM2.5. In contrast, the most aqueously altered CM1 chondrites, including MET 01070, were reclassified as subtype 2.0, because these chondrites contain abundant chondrule pseudomorphs, mainly composed of phyllosilicates, which are absent in CI1 chondrites. By following their definition, highly hydrated ALH 83100 can be assigned to CM2.1 (de Leuw et al., 2009). Thus, the εOs values in CM IOMR are in the order of CM2.1 N CM2.0 N CM2.5 N CM2.6. Though there is no such criteria for CR chondrites, completely hydrated GRO 95577 preserves the initial chondrule textures and chondrule-matrix boundaries (Weisberg and Huber, 2007), which is similar to the case of CM2.0 and 2.1. Meanwhile, like QUE 97990 and Murchison, the two CRs showing relatively low εOs values (GRA 95229 and EET 92042) are characterized by signs of low-degree alteration: i.e. the existence of anhydrous phenocrysts in chondrules (Weisberg and Huber, 2007). The reason for the larger εOs values for CM2.1 than CM2.0 is unclear at present. Of note, however, CM2.0 chondrites are only slightly more altered than CM2.1, the main difference being the existence of rare chondrules containing remnants of mafic silicate phenocrysts in CM2.1 s (Rubin et al., 2007). As will be discussed below, our observation suggests that Os isotopic compositions in the IOMR from CM and CR chondrites resulted from progressive aqueous alteration. Consequently, the εOs values in the IOMR can potentially

GRO 95577 (CR1) MET 01070 (CM1)

2 Murchison (CM2)

1

0

0

2

GRA 95229 (CR2)

4

6

QUE 97990 (CM2.6)

8

10

EET 92042 (CR2)

12

14

Os (ppm) Fig. 3. Relationship between absolute Os concentrations and ε188Os values in acid residues from CM and CR chondrites. Symbols are the same as Fig. 1.

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IOM residues relative to the original meteorites are relatively uniform (1.0 ± 0.2% for most of CM and CR chondrites examined). An important finding of our earlier study was that bulk carbonaceous, ordinary and enstatite chondrites have uniform, solar Os isotopic compositions. This included the strongly hydrated CI1 chondrite, Ivuna (Yokoyama et al., 2010). Although the number of data is limited, it is expected that all bulk CM and CR chondrites examined in this study have a solar Os isotopic composition, irrespective of their degree of aqueous alteration. We propose that the isotopic variation in IOMR observed in Fig. 1b was caused by the destruction of presolar phases via progressive aqueous alteration on the CM/CR parent bodies, rather than any nebular processes creating heterogeneous distribution of presolar grains that predate the planetesimal formation. Some petrologic observations of CM chondrites point to the occurrence of aqueous alteration prior to the parent body accretion (Brearley, 2003). However, destruction of presolar phases during pre-accretionary alteration would result in the modification of Os isotopic compositions at the bulk meteorite scale, which is not evident at the current level of resolution. The enrichment of s-process Os isotopes in the IOMR from highly altered CM and CR chondrites implies that aqueous alteration on the parent body has preferentially destroyed the r-process-Os carrier(s) and modified it into an acid leachable phase in the chondrite matrices, while acid resistant s-process-rich grains, such as SiC and graphite, survived. This process, however, did not disturb the Os isotopic composition at bulk meteorite scale, which may suggest an extremely low solubility of Os in the aqueous fluids under physical conditions where parent-body alteration has occurred. A similar phenomenon has been observed in the case of thermal metamorphism: destruction of presolar grains during high grade (type N3.5) thermal metamorphism on the chondritic parent bodies did not fractionate the bulk Os isotopic compositions (Yokoyama et al., 2010). In contrast, oxygen isotopes provide conspicuous evidence for isotopic shifts of bulk meteorite compositions during aqueous alteration in the CM parent body. Bulk rocks of heavily altered CM chondrites (CM2.0–CM2.1) have larger Δ17O and δ18O values compared to those of the least altered CMs (CM2.5–2.6), while intermediate Δ17O and δ18O values are observed in moderately altered CMs (Clayton and Mayeda, 1999). The oxygen isotope variations have been interpreted to be the result of isotopic exchange between the primary rock and a fluid flowing thorough it during progressive aqueous alteration (Rubin et al., 2007; Young et al., 1999). More recently, a model of hydrothermal convection on a parent body scale has been proposed to account for the oxygen isotope compositions in carbonaceous chondrites (Palguta et al., 2010). A counterargument to the open-system behavior may arise from bulk compositional homogeneity of CM chondrites. For example, extremely fluid soluble elements, such as Ca, are relatively unfractionated from other refractory elements in bulk CM chondrites, irrespective of the degree of alteration, while Ca has been completely leached from chondrule mesostases of highly altered CMs (Brearley, 2003). This indicates that elemental transfer during aqueous alteration occurred, but also that it was highly localized (millimeter to centimeter scale). However, destruction of presolar phases would release isotopically anomalous components that are considerably different from the solar system average. The open system transportation of such components, though the total masses of material would be small, could change the bulk isotopic composition of the ambient rocks without modifying the bulk elemental abundances significantly. Further work is required to precisely determine the whole rock isotope compositions of fluid mobile elements in CM and CR chondrites as a function of the degree of aqueous alteration. 4.3. Destruction of r-process carriers during aqueous alteration Details regarding r-process nucleosynthesis are poorly understood compared to the s-process. Type II supernovae (SNeII) are recognized

to be potential sources for r-process nuclides (Wallerstein et al., 1997). Model calculations adopting a theory of nucleation and grain growth showed that the grain condensation in the SNeII depends on various parameters, such as supernova mass, radius, initial metallicity, and the kinetic energy of the explosion (Kozasa et al., 1989; Todini and Ferrara, 2001). These models predict that dust particles that condense in the ejecta of SNeII are typically Al2O3 (corundum), Fe3O4, Mg2SiO4 (forsterite), MgSiO3 (enstatite) and graphite grains, with sizes that would be much smaller than 1 μm. Some of these grains would subsequently have been destroyed or modified by the passage of reverse shocks through a supernova remnant after condensation. Nevertheless, excluding Fe3O4, the types of SNeII grains predicted above are actually found in carbonaceous and ordinary chondrites, as well as in interplanetary dust particles (IDPs), albeit often with grain sizes that are greater than 1 μm (Choi et al., 1998; Croat et al., 2003; Messenger et al., 2005; Nguyen and Zinner, 2004; Nittler et al., 1998). In addition, Type-X SiC presolar grains are almost certainly from SNeII (Zinner, 2003) and may be the carriers of r-process nuclides. For example, some Type-X SiC grains show large excesses in r-process components (e.g., 88Sr, 96Zr and 138Ba) relative to s-process dominated isotopes, such as 86Sr, 94Zr and 136Ba (δ88/86Sr = 1900–2700‰, δ96/94Zr = 700–4500‰ and δ138/136Ba = 1900–3800‰) (Pellin et al., 2000). These isotopic signatures, coupled with an unusual isotopic pattern of Mo (95Mo and 97Mo excesses) in the same grains, can be explained by a neutron burst in a supernova shell owing to the passage of the shock wave from the core (Meyer et al., 2000). Presolar grains found in meteorites are generally strongly acid resistant, excluding silicates. Forsterite is soluble in HF and HCl, and does not survive the IOMR preparation. However, low-Ca pyroxene, although it is a silicate, is moderately resistant to HF at room temperature (Yokoyama and Nakamura, 2002). Thus, presolar enstatite grains, if present in the meteorites, may survive in the IOMR. On the other hand, petrological observation of CM chondrites shows that fine-grained anhydrous silicates in the matrices are highly susceptible to aqueous alteration and form secondary phyllosilicates (Rubin et al., 2007), which are then leachable with relatively mild acids. For CM chondrites, it is also reported that parent-body aqueous alteration of refractory inclusions would have transformed melilite into phyllosilicate, producing melilitefree CM CAIs (Rubin, 2007). Therefore, the ordering of εOs values in CM and CR-IOMR (Figs. 1b and 2b) would suggest that the abundance of SNeII enstatite grains in the IOMR decreases as the degree of aqueous alteration on the parent body increases. The abundances of presolar silicate grains, deduced from the in situ searches using secondary ion mass spectrometry (SIMS), generally decrease with increasing level of aqueous alteration (Trigo-Rodriguez and Blum, 2009). However, it remains unknown whether Os is abundant in SNeII enstatite grains. A more likely candidate for the r-process Os carrier(s) would be some sort of reduced presolar phase(s) such as metal alloys, carbides and silicides which are unidentified in chondrites. Actually, nanoparticles of iron carbide and metallic Os have been found within presolar graphites from the Murchison meteorite, which are most likely to originate in supernova outflows (Croat et al., 2005). Many of these grains must contain much Os than would silicate minerals, and are thought to be susceptible to aqueous alteration, which tends to be quite oxidizing. In fact, the abundance of metallic Fe–Ni decreases during aqueous alteration as metal oxidizes (Rubin et al., 2007). The hypotheses above are further supported by the observation that the largest positive εOs values are found in IOMR from CI1 chondrites (Figs. 1a, 2a: Ivuna and Orgueil), implying higher levels of aqueous alteration on the CI parent body than CM and CR. It should be noted that the HCl leachates of bulk carbonaceous chondrites have negative εOs values, and the ordering of enrichment of r-process Os isotopes is CI1 N Tagish Lake (C2) N CM2 ≈ CR2 N CV3 (Yokoyama et al., 2010). Likewise, an r-process Zr signature (96Zr excess) is found in easily leachable fractions (acetic acid) of bulk carbonaceous chondrites (Schönbächler et al., 2003, 2005). These observations support

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the idea that aqueous alteration on the chondritic parent bodies preferentially destroyed fine grained SNeII silicates, or other susceptible reduced minerals, that were enriched in r-process nuclides. 4.4. Osmium isotope anomalies in IOMR from enstatite chondrites Most prior studies of presolar grains have focused on carbonaceous and ordinary chondrites, and there has been comparatively little work done on presolar grains from enstatite chondrites (e.g., Ebata et al., 2008; Huss et al., 2003; Lin et al., 2002; Russell et al., 1997). As presented in Figs. 1d and 2d, the εOs values in IOMR from enstatite chondrites do not conform to the ordering observed in those from carbonaceous and ordinary chondrites. It has been reported that the relative abundances of various presolar grain types in enstatite chondrites are different from those in carbonaceous chondrites (Ebata et al., 2008; Lin et al., 2002). This may suggest a radially heterogeneous distribution of presolar grains in the solar nebula. For example, in Qingzhen (EH3) and Indarch (EH4), presolar Si3N4 appears to be more abundant, and oxide grains less abundant than in carbonaceous chondrites (Lin et al., 2002). Since Si3N4 and oxide grains are resistant to the CsF/HF digestion technique, they both are likely present in our IOMR. Although isotopic data for s- and r-process nuclides in these grains are not available at present, most presolar Si3N4 grains have C, N, Al and Si isotopic signatures similar to those of Type-X SiC grains that point to a supernova origin for Si3N4 grains, while most oxide grains likely formed in the stellar outflow of AGB stars. Assuming that presolar Si3N4 and oxide grains are rich in r-process and s-process Os isotopes, respectively, the enrichment of Si3N4 grains and deficit of oxide grains in the enstatite chondrite IOMR would not account for our observation that the IOMR from EET 87746 (EH3) is more enriched in s-process Os than those from type 2–3 carbonaceous chondrites. Thus, although some level of heterogeneity should exist, the relative enrichment of presolar Si3N4 and depletion of presolar oxide grains in enstatite chondrites must be masked by s-process-enriched phases that dominate the Os isotope anomalies in IOMR. These phases are, most likely, mainstream presolar SiC. Again, bulk carbonaceous, ordinary and enstatite chondrites have uniform “solar” Os isotope composition when dissolved by an alkali fusion, total digestion method (Yokoyama et al., 2007, 2010). This remains consistent with an interpretation of homogeneous distribution of presolar components in the early solar system, at least for Os isotopes, at the current level of precision. Alternately, nebular or parent body processes might have acted differently on presolar phases located in regions of the early solar system characterized by different redox conditions. For example, selective destruction/metamorphism of r-process-enriched presolar phases under highly reduced conditions (enstatite chondrites) could have led to s-process enrichments in the acid residues. Conversely, destruction of s-process-enriched phases under oxidizing conditions (carbonaceous and ordinary chondrites) could also account for the differences. All of the likely r-process carriers found in chondrites (e.g., silicates, graphite, Type-X SiC and Si3N4) are presumed to be stable under reduced conditions. Further, no signs of aqueous alteration that potentially destroys r-process carriers have been found in enstatite chondrites. Therefore, we infer that the relative enrichment of s-process Os in the acid residues from enstatite chondrites is the result of selective destruction of s-process-enriched presolar phases (e.g., graphite or SiC) that occurred under oxidized conditions in carbonaceous and ordinary chondrite parent bodies, or at their formation locations. Earlier studies have concluded that presolar SiC survives metamorphism better in the enstatite chondrites than carbonaceous and ordinary chondrites. Under oxidizing conditions, the presolar SiC is lost by the petrologic grade of 3.5–3.6 (Huss, 1990), probably because it forms silicates and CO2 (Alexander et al., 1990), but it survives much more severe metamorphism in the case of EH chondrites (Huss and Lewis, 1995). The existence of SiC in a type 4

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enstatite chondrite (Indarch) was directly confirmed via scanning electron microscopy (Russell et al., 1997). It is important to note that after the putative destruction of the s-process-enriched phases, the liberated Os must have been incorporated into new, acid leachable phases rather than lost at the bulk meteorite scale, thereby explaining the uniform Os isotope composition observed in all types of bulk chondrites. 4.5. Nebular vs parent body processing Chondritic meteorite precursors have experienced various physical and chemical processes in the nebula, as well as on their parent bodies. Petrological, mineralogical and chemical characteristics recorded in the chondrites reflect complicated histories. Consequently, the timing and location of the individual processes are still heavily debated. This includes the distribution of presolar grains in the protoplanetary nebula and their abundances in chondrites. For example, early measurements of noble gas abundances in chondritic acid residues were interpreted to suggest that presolar SiC and diamond abundances in chondrites vary significantly across chondrite groups and their petrologic types (Huss, 1990). Subsequent work resulted in the conclusion that such variations reflect, not only parentbody metamorphism, but also the history of heating and destruction of presolar grains to varying degrees in the solar nebula before chondrule formation and accretion (Huss and Lewis, 1995; Huss et al., 2003). Such processes would lead to heterogeneous distribution of presolar materials in the Solar System at the time of planetesimal formation, consistent with isotopic data for some elements in some meteorites (e.g., Trinquier et al., 2009). However, direct measurement of presolar grain abundances in acid residues by SIMS has shown that presolar SiC abundances in most primitive chondrites (petrologic grade ≤ 3.0) are relatively homogeneously distributed (Davidson et al., 2009). Specifically, those in CR chondrites are an order of magnitude more abundant than predicted by noble gas analysis. This may indicate that parent body processing caused degassing or destruction of a noble gas-rich sub-population of grains, without modifying the SiC abundances significantly (Davidson et al., 2009). The fact that CR chondrites, which are the most depleted in presolar grain-hosted noble gases, contain chemically and isotopically the most primitive, possibly interstellar IOM, argues against thermal destruction of the presolar grains in the nebula (Alexander, 2005; Alexander et al., 2007). At least for Os, heating of presolar grains and degassing of gaseous components in the nebula and/or during parent body processing did not fractionate the isotopic composition at bulk meteorite scale. Interestingly, no nucleosynthetic isotope anomalies have been found in moderately volatile elements, such as Cd and Te (Fehr et al., 2005, 2006; Wombacher et al., 2008), in carbonaceous chondrites. Even though bulk chondrites are isotopically homogeneous with respect to Os, the new results presented here suggest that parent body aqueous/metamorphic processes have acted on the distribution of Os among presolar phases. This in turn suggests that isotopic anomalies reported for some other elements that have been interpreted as reflecting nebular heterogeneities, may partly or wholly be the result of parent body processing. Presolar phases that are enriched in certain nucleosynthetic components could release these components upon destruction of the hosting phases. If the element is more highly soluble than Os, fluid transport could lead to the formation of isotopically modified bulk samples whose isotopic compositions are not representative of the parent body. In support of this interpretation, there is a clear correlation between ε54Cr values in the bulk carbonaceous chondrites and ε188Os in their IOMR (Fig. 4). It has recently been found that the acid resistant residue of Orgueil (CI1) contained some submicron-size (b200 nm) Cr-oxide grains with extreme 54Cr excesses (ε54Cr ~ 1.5 × 104), the grains that were presumably injected into the solar protoplanetary disk by a Type II supernova (Dauphas

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Acknowledgments We are grateful to the Smithsonian Institution's National Museum of Natural History for providing meteorite samples. We thank T. Spohn, A.E. Rubin and an anonymous reviewer for their editorial handling and constructive review comments. This research was supported by Grantin-Aid for Scientific Research by JSPS to TY (21740388), by a NASA Cosmochemistry program grants to CA (NNX08AH65G) and RJW (NNX10AG94G). References

Fig. 4. Plots for ε188Os in acid residues and ε54Cr values in their host carbonaceous chondrites. Chromium data are from (Qin et al., 2010; Yin et al., 2009). Symbols are the same as Fig. 1.

et al., 2010; Qin et al., 2011). A heterogeneous distribution of the 54 Cr-rich carrier in the disk may account for the entire ε54Cr variation (N2ε units) between different meteorites including chondrites, achondrites and irons (Qin et al., 2010; Trinquier et al., 2007). However, the correlation in Fig. 4 does not exclude the possibility that parent body processing created the observed ε54Cr variation, at least between bulk carbonaceous chondrites that span a 1ε unit difference that generally correlates with the petrologic type. The anomalous 54Cr-rich carriers, Cr-oxide grains, are most likely resistant to aqueous alteration. This implies the relative enrichment of such phases at bulk chondrite scale for type 1–2 chondrites, as a result of selective destruction of some Cr-bearing phases with normal or depleted ε54Cr signature during parent-body aqueous alteration, followed by, unlike Os, an open-system Cr removal via aqueous flow. To demonstrate this scenario it will be necessary to fully identify the carriers of isotopically anomalous Cr and Os in chondrites, as well as the variation of their abundances between different chondrites. Nevertheless, we conclude that isotopic heterogeneities among bulk chondrites should not automatically be interpreted as evidence for nebular heterogeneity. 5. Concluding remarks Based on the precise measurement of Os isotopes in acid resistant residues from carbonaceous and enstatite chondrites, we advocate that aqueous alteration and thermal metamorphism on the chondritic parent bodies acted to modify a homogeneous initial population of presolar grains. The aqueous alteration on the CM, CR and, presumably, CI parent bodies caused selective destruction of r-process-enriched grains such as presolar silicates from SNeII or, more likely, some reduced presolar phases (e.g., metal alloys, carbides and silicides) which have yet to be identified in chondrites. For meteorites with petrologic grades N3.5, thermal metamorphism under oxidizing conditions has destroyed most of presolar grains, while s-process-enriched presolar grains (e.g., SiC) survived severe metamorphism (type 4) under the highly reducing conditions that existed in the enstatite chondrite parent bodies. During parent body processing, r-process or s-process-enriched Os that was liberated from presolar grains must have been incorporated into secondary, acid leachable phases rather than lost to explain the uniform Os isotope compositions of bulk carbonaceous, ordinary and enstatite chondrites. Conversely, any pre-accretionary nebular processes would require a coincidental mixing of s-, r-, and p-process carriers in equal proportion to the initial presolar grain abundances, in order to preserve the solar Os isotopic composition for all types of chondrites. However, for the other elements, isotopic heterogeneities among bulk meteorites should be further evaluated by taking the effect of parent body processing into account, because parent body processing might have acted differently on the other elements.

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