Cosmic ray exposure ages of Rumuruti chondrites from North Africa

Chemie der Erde 71 (2011) 135–142

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Cosmic ray exposure ages of Rumuruti chondrites from North Africa Nadia Vogel a,∗ , Heinrich Baur a , Addi Bischoff b , Rainer Wieler a a b

ETH Zurich, Institute for Geochemistry and Petrology, Clausiusstrasse 25, 8092 Zurich, Switzerland Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany

a r t i c l e

i n f o

Article history: Received 20 December 2010 Accepted 25 February 2011 Keywords: Rumuruti R chondrites Exposure ages Cosmogenic noble gases Pairing Source-pairing NWA Brecciation

a b s t r a c t We analyzed noble gases and determined 3 He, 21 Ne, and 38 Ar cosmic ray exposure ages (CREAs) of Rumuruti chondrites from North West Africa (NWA) to rule on potential pairings and/or source pairings of North Africa R chondrite samples. The 21 Ne exposure ages range between 10 and 74 Ma, with NWA 2897 and 1668 having the highest known exposure ages among R chondrites. We also include other R chondrites from North Africa (Schultz et al., 2005) and, based on their noble gas characteristics and their 21 Ne CREAs, propose pairings of the following samples: NWA 2198, 5069, 755, 4615, 845, 851, 978, 1471, and possibly DaG 013 belonging to one fall with a CREA of ∼10 Ma, and NWA 753, 4360, 4419, 5606, 1472, 1476, 1477, 1478, and 1566 representing one fall with a CREA of ∼14 Ma. NWA 2821, 2503, 2289, 3364, 3146, 4619, 4392, 3098, and 2446 seem to belong to one single fall with a CREA of ∼20 Ma, and NWA 2897 and 1668 seem to be paired and show a common CREA of ∼66 Ma. Overall, all R chondrite samples from North Africa analyzed for noble gases so far represent ∼16 individual falls. Comparing falls from North Africa to literature CREAs of R chondrites worldwide, it seems possible that a significant number of all R chondrite falls studied for noble gases were ejected from the R chondrite parent body during one large collisional event between 15 and 25 Ma ago. However, the database is still too small to draw definitive conclusions. The large portion of brecciated R chondrites in collections suggests severe impact brecciation of the R chondrite parent body. © 2011 Elsevier GmbH. All rights reserved.

1. Introduction The Rumuruti (R) chondrites, previously described as the “Carlisle Lakes-type” chondrites (Binns and Pooley, 1979; Rubin and Kallemeyn, 1989; Weisberg et al., 1991), are named after the only R chondrite fall, Rumuruti. They are a well-characterized chondrite group distinct from carbonaceous, ordinary, and enstatite chondrites (Bischoff et al., 1994; Kallemeyn et al., 1996; Rubin and Kallemeyn, 1994; Schulze et al., 1994). The R chondrite group comprises at present 106 samples. Nearly all of them are finds from hot and cold deserts (Bischoff et al., 2011). Therefore, special attention has to be paid to the problem of potential pairing of these samples in order to not misjudge the number of actual R chondrite falls, which in turn would lead to wrong assumptions about the size and nature of the R chondrite parent body (e.g., Bischoff and Schultz, 2004). The problem of pairing has its origin in the fact that during atmospheric entry, stony meteoroids in particular often break into a few to many pieces, which can then be found in more or less close proximity to each other on Earth. For example, more than 3400 individual meteorite pieces belonging to the L6 chondrite Jid-

∗ Corresponding author. Tel.: +41 44 6326615. E-mail address: [email protected] (N. Vogel). 0009-2819/$ – see front matter © 2011 Elsevier GmbH. All rights reserved. doi:10.1016/j.chemer.2011.02.008

dat al Harasis (JaH) 073 were recovered from a strewnfield area of ∼60 km2 in the Sultanate of Oman (Gnos et al., 2009). However, it is not always straightforward to recognize such “paired” meteorites, in particular if find coordinates are unknown or deliberately withheld by the finders as it is often the case for samples from North Africa. In those cases pairing (i.e., the allocation of samples to common falls) has to be postulated based on classification criteria such as mineralogical and petrographic properties, or noble gas contents. The situation is particularly difficult for R chondrites from North Africa, as many are breccias (e.g., Kallemeyn et al., 1996; Schultz et al., 2005), i.e., one single fall might host very variable lithologies with different noble gas contents. In those cases it can be helpful to determine CREAs of the samples in question. These define the time span an ∼m-sized meteoroid travelled in space after ejection from its parent asteroid until it finally reaches Earth. During this time, energetic particles of the galactic cosmic radiation (GCR) penetrate the meteoroid. The interaction of the primary and secondary GCR particles with the meteoroid’s target atoms results, for example, in the production of cosmogenic noble gas nuclides (see, e.g., Wieler (2002) for a comprehensive review of the production of cosmogenic noble gases and the determination of CREAs in meteorites). CREAs are generally identical for all samples from one single fall, unless single breccia pieces had experienced different degrees of pre-irradiation on their respective parent bodies.

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Of course, different falls can also have identical CREAs if they were released during the same collisional event from their common parent body (“source-paired” meteorites). For example, ∼45% of all H chondrites in collections were released from their parent body during a collisional event 7–8 Ma ago (e.g., Graf and Marti, 1995). While source-paired meteorites can also show a range of different lithologies and/or variable noble gas contents, one major difference may help to discriminate paired and source-paired samples: in contrast to paired meteorites, source-paired ones should be distributed all around the globe. If, for example, a significant number of R chondrite samples from different find locations in the world show the same cosmic ray exposure age, they are probably source-paired, i.e., were released during one single collisional event from the R chondrite parent body. For example, major collisional events between 7 and 40 Ma ago have been identified for the L, LL, and H chondrite parent bodies (e.g., Graf and Marti, 1994, 1995; Wieler, 2002), and Schultz et al. (2005) suggested a major breakup event for the R chondrite parent body ∼17 Ma ago. The goal of this study was to determine noble gas CREAs for 22 R chondrites from Northwest Africa (NWA) and thus enlarge considerably the small database on R chondrite CREAs (e.g., Schultz et al., 2005). The new data will be compared to published CREAs of R chondrites in order to draw conclusions about potential pairing and/or source-pairing among Rumuruti chondrites and on the nature of the R chondrite parent body.

2. Methods 2.1. Noble gas analyses Table 1 lists the samples analyzed in this study together with a classification and a short macroscopic description. For the noble gas analyses, one to several meteorite chips with total masses of 20–100 mg (Table 1) were wrapped in aluminum foil and loaded into a double walled furnace equipped with a molybdenum crucible, which is heated by electron impact. After sample preheating (120 ◦ C, 48 h) in vacuum to reduce the amount of adsorbed atmospheric gases, samples were heated to ∼1800 ◦ C for 20 min to quantitatively extract noble gases. The evolving gases were purified by admission to several getters, and Ar was separated from the He–Ne fraction using activated charcoal at −196 ◦ C. Both noble gas fractions were analyzed separately with a custom-built Nier-type mass spectrometer equipped with a Faraday cup and an electron multiplier. Sample gas amounts were corrected with those in socalled “aluminum blanks”, for which pieces of Al foil of the type used to wrap the samples were processed in the same way as real samples. Typical aluminum blanks for 3 He, 20 Ne, 21 Ne, and 38 Ar are (in 10−12 cm3 STP) ∼2, 10, 0.04, and 30, respectively. Blank contributions to sample gas amounts were always below 1% for He and Ne isotopes, but contributed in some cases up to several % to all Ar isotopes. Spectrometer sensitivity and mass discrimination were determined by frequent analyses of known amounts of pure calibration gases. These have essentially atmospheric isotopic compositions for Ne and Ar, but are strongly enriched in 3 He compared to the atmosphere (3 He/4 He = 0.0604). Uncertainties (1) of our standards are 2% for He, Ne, and Ar concentrations and the He isotopic ratio, and <0.1%/amu for Ne and Ar isotopic ratios (Heber et al., 2009). Gases to be analyzed are ionized with 100 eV electrons and at this electron acceleration voltage mass discrimination is ∼12%/amu for He, and 1–2%/amu for Ne and Ar on the multiplier favouring the light isotopes, respectively, and significantly less on the Faraday cup, which was predominantly used for analysis of 4 He and 40 Ar. Due to the mass resolution of the spectrometer of ∼100, HD interferes with 3 He; HF, (40 Ar)++ , and H2 18 O interfere with 20 Ne; and (44 CO)++ interferes with 22 Ne. Based on uniform H2 signals in

Fig. 1. Solar gas-bearing samples in a Ne-3-isotope plot. An error weighted linear fit through all data indicates a fractionated solar end-member with a 20 Ne/22 Ne of ∼11.6 and a cosmogenic 22 Ne/21 Ne ratio of ∼1.08. These endmember ratios are used for the correction for solar wind contributions to measured gas concentrations of all plotted samples. Solar Wind Ne isotopic compositions taken from Heber et al. (2009); primordial Ne-HL isotopic composition taken from Huss and Lewis (1994).

blanks and samples, HD and HF signals are assumed to be identical in blanks and samples, and interferences are corrected via blank subtraction. H2 O, 40 Ar, and CO2 are monitored throughout all analyses, and corrections ((40 Ar)++ /(40 Ar)+ = 0.14; (CO2 )++ /(CO2 )+ = 0.02, (H2 18 O)+ /(H2 16 O)+ = 0.002), are applied accordingly. The 40 Ar++ and (CO2 )++ corrections on 20 Ne and 22 Ne, respectively, are well below 1% for all samples, and water contributes even much less to the signal on mass 20. Tables 2 and 3 compile all measured data corrected for blanks, interferences, mass discrimination, and sensitivity variation (gas concentrations only). 2.2. Determination of cosmogenic 3 He, 21 Ne, and 38 Ar and cosmic ray exposure ages Noble gases in chondrites generally represent mixtures of a cosmogenic and one to several trapped components. In R chondrites, the dominant trapped component is often solar, as is also the case for many of the samples studied here. To calculate exposure ages, cosmogenic noble gas fractions have to be determined and reasonable production rates have to be selected. 2.2.1. 21 Ne Besides cosmogenic Ne, trapped components like solar and primordial Ne [mostly HL gases; see, e.g., Ott (2002)] can be present in the samples. These components are well distinguishable in a plot 20 Ne/22 Ne vs. 21 Ne/22 Ne (Fig. 1). Ten of the samples studied here contain essentially pure cosmogenic Ne (20 Ne/22 Ne < 0.9), in some cases with a very minor contribution of probably atmospheric Ne. This is corrected for using a cosmogenic 20 Ne/22 Ne ratio of 0.82 ± 0.02 [identical within uncertainties to the preferred value of 0.80 ± 0.03 for chondritic meteorites (Eugster et al., 2007)] and atmospheric 20 Ne/22 Ne and 21 Ne/22 Ne ratios of 9.80 (Eberhardt et al., 1965) and 0.02878 (Heber et al., 2009), respectively). The 20 Ne/22 Ne ratios of the remaining samples indicate the presence of a trapped component. In Fig. 1 these data points form a linear array (R = 0.998) between cosmogenic Ne and what we interpret as fractionated solar Ne. Its 20 Ne/22 Ne ratio of ∼11.6 is determined by an error weighted linear best fit line (dashed line in Fig. 1) and is considered valid for all solar gas bearing samples studied here. The ratio of 11.6 is in the range of 20 Ne/22 Ne ratios often observed in desert meteorite finds (e.g., Padia and Rao, 1986) and is close to the low end of ratios observed for fractionated solar

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Table 1 Samples of R chondrites analyzed in this study. Name

Abbreviation

Type

Bulk sample mass (g) Shock stage

Weathering grade

Classification (Bischoff et al., 2011)

Sample mass for noble gas analysis (g)

Optical characteristics of analyzed sample

Sample source

Northwest Africa 753 Northwest Africa 755 Northwest Africa 830 Northwest Africa 1668 Northwest Africa 2198 Northwest Africa 2289 Northwest Africa 2446 Northwest Africa 2503 Northwest Africa 2821 Northwest Africa 2897 Northwest Africa 3098 Northwest Africa 3146 Northwest Africa 3364 Northwest Africa 4360 Northwest Africa 4392 Northwest Africa 4419 Northwest Africa 4615 Northwest Africa 4619 Northwest Africa 4693 Northwest Africa 5069 Northwest Africa 5599 Northwest Africa 5606

NWA 753

R3.9

1200

S2

W2

R3–5, br

0.0869

Fresh

1

NWA 755

R3.7

352

S2

W4

R3–5, br

0.0871

Light brown

1

NWA 830

R5, br

55

S3

W3

br, sv

0.0864

2

NWA 1668

R5

710

R4–6, br, S3, W1

0.0762

Dark brown-black Fresh

NWA 2198

R4

38

NWA 2289

R3–6

NWA 2446

R3, br, cv

NWA 2503

160

3

S2

W1

0.0771

Fresh

2

Low

Low to moderate

0.0917

Dark brown

4

0.0704

Light brown

1 4 5

32

S2

R3–6

400

S3

W3/4

0.0896

NWA 2821

R3.8, br

384

S2

W3

0.0690

NWA 2897

R3–6

23.3

Low

Minimal

0.0780

Light to dark brown Light to dark brown Fresh

NWA 3098

R5

78.7

S3/4

W3

R4–5, br, S3

0.0799

Light brown

4

NWA 3146

R4, pm br

304

R4–6, br, S3

0.0623

Light brown

3

NWA 3364

R3–5

538

S2

W3

0.1038

Light brown

1

NWA 4360

R3.6

309

S4

Moderate

0.0959

Light brown

4

NWA 4392

R4

490

S2

W3

0.0880

Light brown

4

NWA 4419

R4

103.1

S2

Moderate

0.0479

6

NWA 4615

R3.4

10.5

S2

Moderate

0.0689

Light to dark brownish-grey Dark grey

NWA 4619

R3–5

704

S2

Moderate

0.0601

Light brown

4

NWA 4693

R3–6

866

S2

Moderate

0.0706

Light brown

4

NWA 5069

R3–5

1234

S2

W3

0.1025

1

NWA 5599

R4

360

S2

W1/2

0.0643

NWA 5606

R4

S2

W0

0.0742

Light to dark brown Dirty yellow-brown Fresh

79.4

R3–5

br

4

4

3 3

Official classification based on: Meteorite Bulletin Database (http://tin.er.usgs.gov/meteor/metbull.php). br = breccia, pm = polymict, sv = shock veins, cv = calcite veins. Sample source: (1) Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Münster, Germany; (2) Institute of Geophysics and Planetary Physics, UCLA, USA, (3) Department of Earth and Planetary Sciences, Washington University, Seattle, USA; (4) Museum für Naturkunde, Berlin, Germany; (5) Mineralogisches Museum, Universität Hamburg, Hamburg, Germany; (6) Museo di Scienze Planetarie, Prato, Italy.

wind Ne, which can be as low as 11.2 (e.g., Benkert et al., 1993; Grimberg et al., 2008). For some samples a fractionated solar wind 20 Ne/22 Ne endmember of 12.0 instead of 11.6 seems more realistic. We point out, however, that the respective shift in the 21 Ne exposure ages is at most 0.2 Ma, i.e. small compared to the uncertainties of the ages. It is not possible to exclude small primordial Ne contributions; however, these would be of little practical concern here. We calculate cosmogenic 21 Ne concentrations using the fractionated solar 20 Ne/22 Ne value of 11.6 and a corresponding fractionated solar 21 Ne/22 Ne ratio of ∼0.03 (Benkert et al., 1993). To calculate 21 Ne production rates we use the equation of Eugster (1988) for L chondrites adjusted to R chondrite chemistry (Schultz and Freundel, 1985; Schultz et al., 2005). Furthermore, using the published excel spreadsheets of Leya and Masarik (2009) one finds that cosmogenic 22 Ne/21 Ne ratios for average L chondrite chemistry [as given, e.g., by Lodders and Fegley (1989)] are systematically higher by ∼3% than those for average R chondrite chemistry [as given, e.g., by Schultz et al. (2005)]. We therefore increase the cosmogenic 22 Ne/21 Ne ratios of the solar gas free

samples by 3%. The corrected (i.e., increased by 3%) cosmogenic 22 Ne/21 Ne ratios are all ≥1.1. Thus, the equation of Eugster (1988), which is strictly valid only for L chondrites with a cosmogenic 22 Ne/21 Ne ≥ 1.08, can be safely applied. The shielding corrected 21 Ne production rates for the solar gas free samples range from 1.7 × 10−9 to 3.2 × 10−9 cm3 STP/g and are assigned an uncertainty of ∼15%. We point out that despite quite variable 22 Ne/21 Ne ratios and, hence, production rates, the ages in the age groups comprising the solar gas free samples are quite uniform, indicating internal consistency of our production rates. For the R chondrites analyzed here containing solar gas we adopt a uniform cosmogenic 22 Ne/21 Ne ratio of ∼1.08 which equals, adjusted to L chondrite chemistry, a 22 Ne/21 Ne ratio of 1.11 (cf., Table 3). The latter value is identical to the ratio adopted by Schultz et al. (2005) for solar gas rich R chondrites and corresponds to average shielding conditions in L chondrites. The resulting production rate for solar gas rich samples is 2.88 × 10−9 cm3 STP/gMa, similar to the one given by Schultz et al. (2005) of 2.94 × 10−9 cm3 STP/g Ma. We assign a 20% uncertainty to these 21 Ne production rates mainly because we

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Table 2 He, Ne, and Ar in bulk R chondrites. Sample

3

NWA 2198 NWA 5069 NWA 755 NWA 4615 NWA 4693 NWA 4360 NWA 753 NWA 4419 NWA 5606 NWA 2821 NWA 2503 NWA 3364 NWA 2289 NWA 3146 NWA 4619 NWA 4392 NWA 3098 NWA 2446 NWA 830 NWA 5599 NWA 2897 NWA 1668

12.98 11.58 12.59 12.78 13.08 17.33 16.73 17.16 17.08 16.02 15.74 11.73 21.63 14.56 16.73 23.70 24.24 21.48 29.14 50.62 104.01 83.83

4

He

He

1401 822 1172 1201 1213 1307 1310 1317 1426 13403 27386 1939 31652 9466 14919 31920 33355 29750 1068 1567 87715 2006

20

Ne

1.73 2.41 2.93 2.97 7.91 3.82 3.63 3.90 3.73 186.64 560.49 24.92 441.43 132.45 211.22 414.12 425.81 429.17 23.87 7.63 411.38 15.51

21

22

Ne

1.65 2.39 3.18 3.15 3.54 4.23 3.93 4.27 3.97 5.69 6.96 5.61 6.70 6.04 6.53 7.30 7.35 7.42 7.89 7.84 17.72 16.52

36

Ne

2.04 2.71 3.42 3.44 4.21 4.48 4.24 4.55 4.44 21.58 52.24 7.79 42.22 17.53 23.91 39.58 41.72 42.38 10.28 9.36 49.97 18.85

38

Ar

1.32 10.63 4.10 3.69 7.01 2.64 2.87 3.18 2.77 11.31 18.01 12.30 23.40 7.29 11.20 14.66 13.30 32.42 13.24 2.55 19.49 7.28

40

Ar

0.463 2.255 1.081 0.991 1.615 0.806 0.902 0.870 0.870 2.748 4.161 2.946 5.075 1.982 2.825 3.395 3.324 6.924 3.455 1.366 4.927 2.417

Ar

5733 5199 3343 3751 2790 1661 3787 3347 3846 3842 1974 4776 5185 2970 3945 3648 3208 2844 5389 5063 5431 5355

Table sorted by ascending T21 . Noble gas concentrations in 10−8 cm3 STP/g; uncertainties of gas concentrations (1): 3 He ≤ 1%; uncertainty of absolute calibration (1).

have no independent size and depth information for these samples. 2.2.2. 3 He Helium can be of cosmogenic, radiogenic, solar, and primordial origin. As He has only two isotopes, cosmogenic 3 He can only be reliably determined if non-cosmogenic 3 He is minor. To calculate cosmogenic 3 He, we subtract from the measured 4 He 1.5 × 10−5 cm3 STP/g (assigned uncertainty 20%) of radiogenic 4 He, an average amount for R chondrites (Schultz et al., 2005). For ten of the samples analyzed here, no other major sources of 4 He are present (except for a small amount of 4 Hecos ), and the measured 3 He is considered to be exclusively cosmogenic. Based on the Ne results we can safely assume that the non-radiogenic 4 He in all other samples is predominantly of solar origin. Also,

Pairs

Weathering grade

1 1 1 1 2 2 2

W1 W3 W4 Moderate Moderate Moderate W2 Moderate W0 W3 W3/4 W3 Moderate ? Moderate W3 W3 ? W3 W1/2 Minimal W1

2 3 3 3 3 3 3 3 3 3

4 4 21

Ne ≤ 2%;

38

Ar ≤ 1%; not included: 2%

4 He 20 non-rad / Ne

ratios in the range of 20–200 indicate fractionated solar gas. To subtract solar 3 He from the measured 3 He, we use a fractionated solar 4 He/3 He ratio of ∼4500 that, according to Benkert et al. (1993), goes along with a fractionated solar 20 Ne/22 Ne of 11.6. We assume the solar 4 He/3 He ratio to be uncertain by ∼20%. Contributions of solar 3 He to the total 3 He in solar He-bearing samples are between 12 and 37%. Based on the large uncertainty of the adopted fractionated solar 4 He/3 He ratio, 3 He exposure ages of samples that had to be corrected for solar 3 He by more than 15% are shown in italics in Table 3 and need to be viewed with caution. We calculated 3 He production rates using the equation of Eugster (1988) for L-chondritic chemistry (applying the R chondrite adjusted cosmogenic 22 Ne/21 Ne ratios). As the author shows, 3 He production rates are fairly similar for all chondrite classes. The average 3 He production rate of 1.61 × 10−8 cm3 STP/gMa for the

Table 3 Noble gas ratios and exposure ages (in Ma) of bulk R chondrites. Sample

4

He/3 He

NWA 2198 NWA 5069 NWA 755 NWA 4615 NWA 4693 NWA 4360 NWA 753 NWA 4419 NWA 5606 NWA 2821 NWA 2503 NWA 3364 NWA 2289 NWA 3146 NWA 4619 NWA 4392 NWA 3098 NWA 2446 NWA 830 NWA 5599 NWA 2897 NWA 1668

108 71 93 94 93 75 78 77 83 837 1740 165 1464 650 892 1347 1376 1385 37 31 843 24

20

Ne/22 Ne

0.8452 0.8846 0.8562 0.8624 1.8687 0.8487 0.8527 0.8570 0.8430 8.6978 10.7290 3.2008 10.4558 7.6004 8.7826 10.4626 10.2063 10.1275 2.3282 0.8182 8.2330 0.8277

(22 Ne/21 Ne)cos

36

Ar/38 Ar

1.267 1.157 1.104 1.117 1.110 1.087 1.108 1.095 1.149 1.110 1.110 1.110 1.110 1.110 1.110 1.110 1.110 1.110 1.110 1.230 1.110 1.175

2.829 4.657 3.789 3.712 4.321 3.265 3.182 3.593 3.238 4.171 4.336 4.228 4.561 3.751 3.952 4.320 3.955 4.646 3.904 1.898 3.939 3.067

4

He/20 Ne

809 341 400 404 153 342 361 338 382 72 49 78 72 71 71 77 78 69 45 205 213 129

T3

T21

T38

Pairs

Weathering grade

8.4 7.3 7.8 7.9 8.1 10.7 10.4 10.6 10.7 8.3 6.2 7.2 9.3 7.9 8.5 10.5 10.6 9.4 18.1 32.4 52.6 52.8

9.8 10.1 10.7 11.3 12.3 13.1 13.5 13.8 16.2 18.1 19.2 19.3 19.3 19.8 20.8 21.7 21.7 22.0 27.3 41.9 58.0 74.1

8.1 8.1 8.8 8.6 6.1 8.6 10.4 7.6 10.6 14.0 16.1 14.0 11.3 15.1 16.8 13.3 19.1 12.7 23.2 31.1 29.6 33.4

1 1 1 1 2 2 2 2 2 3 3 3 3 3 3 3 3 3

W1 W3 W4 Moderate Moderate Moderate W2 Moderate W0 W3 W3/4 W3 Moderate ? Moderate W3 W3 ? W3 W1/2 Minimal W1

4 4

Table sorted by ascending T21 . Uncertainties for isotopic ratios corrected for blank and mass discrimination: 3 He/4 He, 20 Ne/22 Ne < 2%; 36 Ar/38 Ar ≤ 1%; not included are uncertainties of the absolute calibrations: 2% for He isotopic ratio; <0.1%/amu for Ne and Ar isotopic ratios (all values 1); uncertainties of exposure ages T3 , T21 , T38 ∼ 20%. T3 in italics: >15% of solar 3 He correction.

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samples studied here is identical to the one given by Schultz et al. (2005). The model of Leya and Masarik (2009) predicts a 3 He production rate ∼10–15% higher for depth and shielding conditions indicated by cosmogenic Ne. We assign an uncertainty of 20% to the 3 He production rates. 2.2.3. 38 Ar Argon usually represents a mixture of a cosmogenic and one or more trapped components (atmospheric, solar, primordial), the latter having all fairly similar 36 Ar/38 Ar ratios. For samples that do not contain solar Ne, we use as trapped endmember the 36 Ar/38 Ar ratio of 5.34 of phase Q (Busemann et al., 2000). For solar Ne-bearing samples we use the fractionated solar 36 Ar/38 Ar ratio of 5.0 that occurs, according to Benkert et al. (1993), together with a fractionated solar 20 Ne/22 Ne ratio of 11.6. The adopted uncertainty for this 36 Ar/38 Ar ratio is 5%. As did Schultz et al. (2005) we also use a cosmogenic 36 Ar/38 Ar ratio for R chondrites of ∼0.67. The corrections for trapped 38 Ar are in all cases substantial (on average ∼70%) and, thus, all 38 Ar ages need to be viewed with caution. Significant contributions of Ar originating from neutron capture on chlorine and subsequent ␤-decay are excluded, based on the fact that the measured 36 Ar/38 Ar ratios never exceed the value of the trapped endmembers. This would not be expected if significant amounts of ArCl with a (36 Ar/38 Ar)Cl ratio of ∼100 would be present. For calculating 38 Ar exposure ages we use the 38 Ar production rate equation of Schultz et al. (2005) which is based on the production rate for L chondrites of Eugster (1988) but is reduced by ∼13% (Schultz et al., 1991). Average abundances of target elements for cosmogenic Ar production in L and R chondrites are rather similar, thus, in general the production rates do not deviate by more than ∼2% for L and R chondrites (Leya and Masarik, 2009). Therefore, no additional correction for composition is necessary. For average shielding, the average production rate of 3.9 × 10−10 cm3 STP/gMa for the samples studied here is close to the one given by Schultz et al. (2005) of 4.12 × 10−10 cm3 STP/gMa. For size and shielding conditions indicated by the Ne data, the model of Leya and Masarik (2009) predicts again somewhat higher 38 Ar production rates in the range of 4.6 × 10−10 cm3 STP/gMa. A 20% uncertainty is assigned to the 38 Ar production rate. 3. Results: 3 He, 21 Ne, and 38 Ar exposure ages and age groups Fig. 2 shows the 3 He, 21 Ne, and 38 Ar CREAs of all R chondrites studied here. In all cases uncertainties are dominated by those of the production rates. Both 3 He and 38 Ar ages are often slightly and sometimes substantially lower than the respective 21 Ne ages. Small age deviations might be explained by somewhat incorrect assumptions on shielding conditions and, thus, production rates. Note that using the 3 He and 38 Ar production rates of Leya and Masarik (2009) would lead to even larger age differences. However, significantly low 3 He ages (compared to 21 Ne ages) are attributed to diffusive loss of He due to heating of the sample in space. 38 Ar ages, on the other hand, are highly composition-dependent, which for ≤100 mg samples becomes a major source of uncertainty if one has to rely on an average composition instead of a sample specific one. We assume that – apart from an imprecise correction for trapped Ar – this is the main reason for the sometimes large discrepancies between 21 Ne and 38 Ar CREAs. The target elements for cosmogenic 21 Ne production are generally much more homogeneously distributed throughout a meteoroid than those for cosmogenic 38 Ar production. Also, Ne is much less prone to diffusive losses than He. Therefore, the following discussion is primarily based on the 21 Ne exposure ages. Still, 3 He

Fig. 2. Plot of 3 He (T3 ), 21 Ne (T21 ), and 38 Ar (T38 ) exposure ages for all samples studied here. Samples are sorted according to their 21 Ne ages. Age groups indicate presumably paired meteorites; see text for more information.

and 38 Ar ages are very valuable, for example to identify potentially paired meteorites. The 21 Ne ages are between 10 and 74 Ma, well within the range covered by chondritic meteorites in general (e.g., Wieler, 2002). Published 21 Ne ages for R chondrites range from 0.2 to 45 Ma (Schultz et al., 2005). Thus, NWA 2897 and 1668 are currently the samples with the highest known exposure ages among R chondrites. Several exposure age “groups” can be distinguished in Fig. 2, separated by gaps. Four solar gas free samples show 21 Ne ages around 10 Ma. Their 3 He and 38 Ar ages are somewhat lower than, albeit within uncertainties identical to, the 21 Ne ages. Five samples have slightly higher 21 Ne ages around 14 Ma. In contrast to the 10 Ma group, the age discrepancies to the respective 3 He and 38 Ar ages are larger, i.e., the 38 Ar ages of the 14 Ma group are on average 35% lower than 21 Ne ages, compared to only 15% for the 10 Ma group. The youngest sample (NWA 4693) is also the only solar Ne-bearing sample of this group. Despite the similar 21 Ne CREA, this sample shows, e.g., a lower 4 He/20 Ne ratio and also larger differences between the 3 He, 38 Ar ages and the 21 Ne age than the other samples in this group. Therefore, we treat the sample as not paired with the other samples of the 14 Ma group. Despite the somewhat high 21 Ne age of NWA 5606 we add this sample to the 14 Ma age group, as its other characteristics like 3 He and 38 Ar ages as well as concentrations of trapped and radiogenic noble gases agree well with those of the other group members. Nine solar gas bearing samples show 21 Ne ages around 20 Ma, while all of their 3 He ages are substantially (∼50%) lower. This is assigned to heating of the samples within their meteoroid in space rather than, e.g., to wrong corrections for trapped noble gases, as solar gas contributions are highly variable (20 Ne/22 Ne ratios of 3–11), but the 3 He ages are quite uniformly low. Also the 38 Ar ages of this group are systematically lower than the respective 21 Ne ages, on average by ∼30%. It is possible that the actual sample composition systematically deviates from the “average R” composition. Also a systematically wrong correction for trapped noble gases cannot be excluded. NWA 830 and 5599 with 21 Ne ages of ∼27 and ∼42 Ma, respectively, cannot be grouped with any other samples. Finally, two samples, one with and one without solar gases, show 21 Ne ages around 66 Ma. Their 3 He ages are lower than the 21 Ne ages but not outside uncertainties. However, their 38 Ar ages are both only around 30 Ma, which is most probably due to

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wrong assumptions about their sample composition, as uncertainties in the production rates due to wrong assumptions about shielding conditions alone can clearly not account for such a large age discrepancy. Also, a wrong correction for trapped Ar would not change the resulting 38 Ar ages by more than a few Ma. It has been reported that terrestrial weathering, in particular in hot deserts, can alter the original noble gas composition of meteorite finds (e.g., Scherer et al., 1994). The authors report that cosmogenic 3 He, 21 Ne, and 38 Ar as well as radiogenic 4 He can be lowered by up to a factor of two in severely weathered meteorites. This probably happens indirectly by addition of terrestrial weathering products (e.g., formation of oxides from metal and sulphides, formation of phyllosilicates and oxides from mafic silicates), and thus the dilution of meteoritic material per g of sample. This effect would systematically lower all 3 He, 21 Ne, 38 Ar exposure ages, and also the deviation from average chemistry (as it is used here) would lead to the determination of wrong production rates. At the same time terrestrial 40 Ar (and the more so terrestrial Kr and Xe) is introduced via the alteration products into the meteorite. Stelzner et al. (1999) studied the effects of hot desert weathering on the noble gas composition on ordinary chondrites weathered to different degrees (W1–W5; see, e.g., Wlotzka (1993) for a definition of the weathering grades W0–W6). Stelzner et al. (1999) found that the 3 He, 21 Ne, and 38 Ar concentrations (and thus the exposure ages) did not vary significantly between the outermost, more weathered parts and the inner, less weathered parts of meteorite specimens. Therefore, the authors concluded that terrestrial weathering does not significantly affect cosmic ray exposure ages. Is there a significant influence of weathering on the noble gas compositions, and thus the exposure ages of our samples? In general, R chondrites are highly oxidized rocks (cf., Bischoff et al., 2011), therefore the addition of terrestrial alteration products via oxidation of metal is limited [although sulphide alteration can be an issue, e.g., Bischoff et al. (1994)]. Also, the samples studied here have weathering grades W0–W4 (see Tables 1–3), i.e., addition substantial amounts of alteration products via formation of phyllosilicates from olivines (weathering grade W5) or other mafic silicates (weathering grade W6) is not to be expected. The meteorite pairs defined here via their uniform exposure ages comprise in part samples with very different weathering grades (compare Tables 2 and 3). Finally, among those pairs, one could expect, e.g., a correlation between lowest weathering grade of a sample with highest radiogenic 4 He and lowest 40 Ar concentrations and vice versa, if weathering would play a major role for the light noble gas content of our samples, which is not the case. Based on this argumentation we exclude that terrestrial weathering has changed the original 3 He, 21 Ne, and 38 Ar exposure ages in the samples studied here to a significant degree.

4. Discussion In the following we compare our new NWA exposure ages to those presented for R chondrites in the literature, in order to rule on pairing and/or source-pairing among these samples. Cosmogenic noble gas concentrations and CREAs for R chondrites have been published by Bischoff et al. (1994), Nagao et al. (1999), and Schultz et al. (2005). The latter publication also contains a number of data previously published as abstracts only. All of the literature data (noble gas data and CREAs) including our new ones are compiled in Tables 15 and 16 of the review on R chondrites (Bischoff et al., 2011).

4.1. Pairing and source-pairing of R chondrites from North Africa based on CREAs As described above, it is not easy to discriminate between paired and source-paired meteorites. Therefore we will first compare our new NWA Rumuruti age groups with literature noble gas CREAs and other noble gas characteristics of R chondrites from North Africa (Schultz et al., 2005) in order to identify paired samples: Based on our noble gas results we defined above a 10 Ma group including NWA 2198, 5069, 755, and 4615. Also Schultz et al. (2005) found several NWA samples around this age, namely NWA 755, 845, 851, 978, and 1471. All eight samples show similar 20 Ne/22 Ne ratios (all ≤1), 4 He [(1.2 ± 0.2) × 10−5 cm3 STP/g] and 40 Ar [(4.1 ± 0.8) × 10−5 cm3 STP/g] concentrations, 4 He/20 Ne ratios mostly around 400, as well as uniform 21 Ne ages. Furthermore, they all show uniform discrepancies between their 21 Ne ages and their respective 3 He and 38 Ar ages (3 He and 38 Ar ages both are ∼20% lower than 21 Ne ages). This strongly suggests that all of these samples are paired and belong to one single fall. The 21 Ne exposure age of this fall is 10.3 ± 0.7 Ma. [Note that these numbers reflect the average of all nominal ages included in the group and the respective standard deviation, i.e. the scatter of the ages within the group. The uncertainties of the individual cosmic ray exposure ages (in the range of 20%) are not taken into account here.] This finding is in agreement with Schultz et al. (2005) who already suggested pairing of NWA 755, 845, 851, 978, and 1471 and gave a common age of 10.1 ± 0.7 Ma. DaG 013 with a 21 Ne age of 9.7 Ma (Schultz et al., 2005) also fits into this age range and its other noble gas characteristics are very similar to those of the eight NWA samples. Therefore it is possible that DaG 013 also belongs to the same meteorite shower. Since the find location of DaG 013 is well known, this result may indicate that the NWA samples in question were collected in the Dar al Gani area of Libya. The second age group (∼14 Ma) comprises samples NWA 753, 4360, 4419 and 5606. Based on their noble gas characteristics we assume that NWA 1472, 1476, 1477, 1478, and 1566 (Schultz et al., 2005) can be added to this age group. Ne in all samples is mainly cosmogenic. Their 4 He concentrations are on average slightly higher [(1.4 ± 0.1) × 10−5 cm3 STP/g] than those of the 10 Ma group, while their 40 Ar is slightly lower and much more variable [(3.0 ± 1.4) × 10−5 cm3 STP/g]. Their average 4 He/20 Ne ratio is ∼320 and all samples show a systematic discrepancy between their 21 Ne and 3 He ages on the one hand (∼30% lower) and their 38 Ar (∼40% lower) ages on the other. This strongly suggests that all nine samples belong to the same meteorite shower with a common 21 Ne exposure age of 13.7 ± 1.2 Ma. Note that Schultz et al. (2005) already assumed pairing for samples NWA 753, 1472, 1476, 1477, 1478, and 1566 and also gave a common exposure age of 13.7 ± 1.2 Ma. We further propose pairing for NWA 2821, 2503, 2289, 3364, 3146, 4619, 4392, 3098, and 2446. All contain variable amounts of solar gases (20 Ne/22 Ne ratios of 3–10) and form a well-defined mixing line in a Ne-3-isotope-plot (Fig. 1). Due to the solar contribution, their 4 He concentrations are higher and also more variable [(2 ± 1) × 10−4 cm3 STP/g] than in the case of the 10 and 14 Ma falls; however, 40 Ar is quite uniform in this group [(3.6 ± 1) × 10−5 cm3 STP/g]. All samples show rather low 4 He/ 20 Ne ratios of 50–80 and, most importantly, distinctly lower (up to 40%) 38 Ar ages compared to the 21 Ne ages and substantial (up to 60%) loss of 3 He. Therefore, we suggest that these nine samples belong to one single fall with a 21 Ne CREA of 20.2 ± 1.4 Ma. NWA 1583 (Schultz et al., 2005) has a slightly lower 21 Ne age (∼17 Ma) and shows no solar Ne and less pronounced 3 He loss than the samples analyzed by us. NWA 1585 has a 21 Ne age of ∼18 Ma, but very different 3 He (∼1 Ma) and 38 Ar (∼31 Ma) exposure ages. The North Africa sample DaG 417 has a 21 Ne exposure age identical to that of the above group (19.1–22.6 Ma) and contains solar noble gases,

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indicate a major collisional event on the R chondrite parent body within that time frame. This conclusion is in agreement with that of Schultz et al. (2005) who suggested, based on a smaller data set, a major collisional event ∼17 Ma ago on the R chondrite parent body. 4.2. Breccias and regolith breccias among R chondrites based on noble gas data

Fig. 3. 21 Ne exposure age histogram of all pairing-corrected R chondrites listed in Table 15 in Bischoff et al. (2011). Besides our data, the plot includes exposure ages published by Bischoff et al. (1994), Nagao et al. (1999), and Schultz et al. (2005). Light grey bars represent falls from North Africa; dark ones represent those from other regions in the world. The arrow indicates a potential major collision on the R chondrite parent body.

however, other parameters like the 4 He/20 Ne ratio (280) or the 3 He exposure age disagree with those of the 20 Ma group. Therefore, none of the last three samples can be clearly assigned to the 20 Ma group, and thus are not considered to be paired with the 9 samples above. NWA 2897 and 1668 show within uncertainties identical 21 Ne ages of ∼66 Ma. Although one sample is solar gas rich (NWA 2897) and the other is solar gas free, we consider them as paired based – besides the identical 21 Ne ages – on their basically identical 40 Ar concentrations and their identical discrepancies between 3 He, 21 Ne, and 38 Ar ages. The latter are about 50% lower than the 21 Ne ages, which is almost certainly due to uniformly wrong assumptions about the sample composition, while the 3 He ages are somewhat lower but identical within uncertainties to the respective 21 Ne ages. Thus, we presume that NWA 1668 represents a solar gas free clast of the same meteoroid the regolith breccia NWA 2897 was derived from and propose a common 21 Ne exposure age for this fall of 66 ± 11 Ma. Finally, the three samples Sahara 99527, 99531, and 99537 studied by Schultz et al. (2005) show 21 Ne ages around 31 Ma and were proposed to be paired by these authors. Six of the NWA samples [NWA 053, 1583, 1585 (Schultz et al., 2005); NWA 830, 4693, and 5599 (this study)] cannot unequivocally be allocated to any age group and are therefore treated as individual falls here. The same is true for five other samples from North Africa (DaG 417, HaH 119, Ouzina, Sahara 98248, Acfer 217). Therefore, the 43 R chondrite samples from North Africa analyzed for cosmogenic noble gases represent, according to the pairing proposed above, 16 individual falls. In Fig. 3, these 16 falls from North Africa (light grey bars) are compared to published (pairing-corrected) noble gas exposure ages of R chondrites found in different areas in the world (dark grey bars) (Bischoff et al., 1994; Nagao et al., 1999; Schultz et al., 2005). A significant peak in this diagram could indicate a major collisional event on the R chondrite parent body and, thus, source-pairing of the samples forming the peak. We emphasize that the total number of known individual R chondrite falls of ∼27 worldwide is certainly too low to make definite conclusions about potential source-pairing. Indeed, a striking clustering of 21 Ne ages is not detectable in Fig. 3. However, a significant number of falls show 21 Ne CREAs between 15 and 25 Ma (see arrow in Fig. 3). This might

About half of the pairing-corrected NWA samples studied here contain solar gases, i.e., were exposed to the solar wind on the surface of their parent body and, thus, are regolith breccias. This is in agreement with Schultz et al. (2005) who report 48% of solar gas bearing samples among NWA and non-NWA R chondrite samples. Compared to ordinary chondrites and achondrites this fraction is rather high, whereas for several carbonaceous chondrite classes (e.g., CI, CM, CR chondrites), the fraction of solar gas bearing samples increases up to 100% (Bischoff and Schultz, 2004). We point out that the individual pieces of the two predominantly solar gas free brecciated falls (i.e., the 10 Ma pair and the 14 Ma pair) consist of quite variable petrologic types (Table 1). Based either on the classification as a breccia or the presence of solar wind noble gases (or both), we find that more than two thirds of the 27 R chondrite falls studied here are breccias. This may indicate that the R chondrite parent body is severely brecciated. Also the existence of highly metamorphosed clasts in most R chondrite breccias (cf., Bischoff et al., 2011) indicates large scale brecciation and mixing of clasts from different depths during the evolution of the R chondrite parent body. 5. Summary We have measured He, Ne, and Ar in 22 individual samples of Rumuruti chondrites from Northwest Africa. Including North Africa R chondrites compiled by Schultz et al. (2005), several 21 Ne exposure age groups (∼10, ∼14, ∼20, ∼31, and ∼66 Ma) exist. These groups are interpreted as representing individual meteorite showers. The North Africa R chondrites analyzed so far for noble gases represent ∼16 individual falls. Taking into account all pairingcorrected R chondrites (analyzed for noble gases) worldwide, more than one third of the individual falls were released from the Rumuruti parent body between 15 and 25 Ma ago. Although no clear exposure age peak could be identified, it is possible that these samples were released in one major collisional event within that time frame. Based either on the classification as a breccia or the presence of solar wind noble gases (or both), about two thirds of the ∼27 R chondrite falls worldwide (analyzed for noble gases) appear to be breccias. This might indicate that the R chondrite parent body has experienced severe brecciation and thorough mixing. Acknowledgements For providing samples for noble gas analysis we thank Alan Rubin (UCLA, USA), Jochen Schlüter (Universität Hamburg, Germany), Ansgar Greshake (Museum für Naturkunde, Berlin), Tony Irving (University of Washington, Seattle), and Vanni Moggi Cecchi (Prato Ricerche Foundation, Museo di Scienze Planetarie, Prato, Italy). Constructive reviews of K. Keil, H. Palme, and L. Schultz are greatly appreciated. We also thank K. Keil for the editorial handling of this manuscript. References Benkert, J.-P., Baur, H., Signer, P., Wieler, R., 1993. He, Ne, and Ar from the solar wind and solar energetic particles in lunar ilmenites and pyroxenes. J. Geophys. Res. 98, 13147–13162. Binns, R.A., Pooley, G.D., 1979. Carlisle Lakes (a): a unique oxidized chondrite. Meteoritics 14, 349–350 (abstract).

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