1. Top of page
  2. Abstract
  3. Introduction
  4. Samples and analytical techniques
  5. Results and discussion
  6. Conclusions
  7. References

Abstract– We measured cosmogenic radionuclides and noble gases in the L3–6 chondrite breccia Northwest Africa (NWA) 869, one of the largest meteorite finds from the Sahara. Concentrations of 10Be, 26Al, and 36Cl in stone and metal fractions of six fragments of NWA 869 indicate a preatmospheric radius of 2.0–2.5 m. The 14C and 10Be concentrations in three fragments yield a terrestrial age of 4.4 ± 0.7 kyr, whereas two fragments show evidence for a recent change in shielding, most likely due to a recent impact on the NWA meteoroid, approximately 105 yr ago, that excavated material up to approximately 80 cm deep and exposed previously shielded material to higher cosmic-ray fluxes. This scenario is supported by the low cosmogenic 3He/21Ne ratios in these two samples, indicating recent loss of cosmogenic 3He. Most NWA samples, except for clasts of petrologic type 4–6, contain significant amounts of solar Ne and Ar, but are virtually free of solar helium, judging from the trapped 4He/20Ne ratio of approximately 7. Trapped planetary-type Kr and Xe are most clearly present in the bulk and matrix samples, where abundances of 129Xe from decay of now extinct 129I are highest. Cosmogenic 21Ne varies between 0.55 and 1.92 × 10−8 cm3 STP g−1, with no apparent relationship between cosmogenic and solar Ne contents. Low cosmogenic (22Ne/21Ne)c ratios in solar gas free specimens are consistent with irradiation in a large body. Combined 10Be and 21Ne concentrations indicate that NWA 869 had a 4π cosmic-ray exposure (CRE) age of 5 ± 1 Myr, whereas elevated 21Ne concentrations in several clasts and bulk samples indicate a previous CRE of 10–30 Myr on the parent body, most probably as individual components in a regolith. Unlike many other large chondrites, NWA 869 does not show clear evidence of CRE as a large boulder near the surface of its parent body. Radiogenic 4He concentrations in most NWA 869 samples indicate a major outgassing event approximately 2.8 Gyr ago that may have also resulted in loss of solar helium.


  1. Top of page
  2. Abstract
  3. Introduction
  4. Samples and analytical techniques
  5. Results and discussion
  6. Conclusions
  7. References

Northwest Africa (NWA) 869 represents one of the largest meteorite finds from the Sahara known so far. It consists of thousands of fusion-crusted individuals ranging in mass from less than 1 g up to more than 20 kg. The total weight of the shower is at least 2000 kg (Connolly et al. 2006), but is estimated to be in the order of 7000 kg (Metzler et al. 2011). Unfortunately, the strewn field is undocumented, as is the provenance of much of the material that has been distributed worldwide. On the basis of cosmogenic radionuclides and noble gas measurements, we will verify that these meteorites all represent a single large object, as was done for other large chondrite showers (Kring et al. 2001; Welten et al. 2006; Huber et al. 2008; Welten et al. 2010). This meteorite was initially classified as an L4–6 fragmental breccia, but was recently reclassified as an L3–6 chondrite breccia (Metzler et al. 2011). Noble gas measurements revealed that NWA 869 contains solar gases (Osawa and Nagao 2006) and hence is a regolith breccia. As the abundance of regolith breccias among L chondrites is much lower than for H chondrites (3% versus 15%; Crabb and Schultz 1981; Bischoff et al. 2006), NWA 869 represents a special and unusually large sample of the lithified regolith of the L-chondrite parent body. The petrology, chemical, and oxygen isotopic composition, and Ar-Ar ages of individual clasts, fragments, and matrix of NWA 869 are discussed in a companion paper (Metzler et al. 2011).

Northwest Africa 869 is not only interesting because it is a regolith breccia but also because it represents one of the largest, if not the largest, stony meteorite finds. With an estimated mass of approximately 7000 kg, NWA 869 must have had a minimum preatmospheric radius of 75–80 cm, but may have been much larger. Previous studies of large chondrites, such as Jilin, Gold Basin, and Jiddat al Harasis 073 (Begemann et al. 1985; Welten et al. 2003; Huber et al. 2008) have shown that large objects often have complex exposure histories, with a first-stage exposure on the parent body, followed by a second stage exposure as a meter-sized object in space. We investigate here whether NWA 869 follows this trend. In addition, regolith breccias often contain individual components that were exposed to galactic cosmic rays (GCR) before they were incorporated in the breccia (e.g., Goswami et al. 1984; Wieler et al. 1989). It is often difficult to constrain when the regolith breccias were compacted into solid rock and when the individual components were exposed to solar wind and GCR (e.g., Pellas 1972). By studying the cosmic-ray exposure (CRE) history and radiogenic gas retention ages of multiple samples from different locations in this large meteoroid, we investigate the timing of the different events that formed this regolith breccia.

In this article, we present concentrations of noble gases and cosmogenic radionuclides in bulk samples and separated lithologies to obtain information on the irradiation history (transit time, shielding depths) of the meteoroid during transit from parent body to Earth and the irradiation history of its breccia components on the parent asteroid prior to lithification. In addition, we used 14C and 10Be measurements to determine the terrestrial age of this meteorite. The NWA 869 meteoroid and its components show an interesting exposure history with evidence of exposure of individual components in a regolith and a recent impact on the NWA 869 meteoroid while traveling in space.

Samples and analytical techniques

  1. Top of page
  2. Abstract
  3. Introduction
  4. Samples and analytical techniques
  5. Results and discussion
  6. Conclusions
  7. References


We selected six individual specimens of the NWA 869 shower, including MB-13, M-05-38-1, M-05-38-2, SM-03-1, MS-04-1, and #D; these meteorites are described in detail by Metzler et al. (2011). Noble gases were measured in 10 samples, including three samples of clastic matrix, a clast of petrologic type 4, two clasts of petrologic type 5 or 6, two shock-darkened clasts, and an impact melt rock clast. For details on the individual clasts and matrix samples, we refer to table 1 of Metzler et al. (2011).

For radionuclide analysis, we crushed bulk samples of 2–4 g in an agate mortar and separated the magnetic (metal) fraction from the nonmagnetic (stone) fraction in each of the six fragments. We cleaned the metal fraction by ultrasonic agitation in 0.2N HCl (15 min) and concentrated HF (20–30 min) to remove attached troilite and silicates, respectively. The bulk metal contents (after HCl leaching) range from 4.4 to 9.9 wt%, consistent with L-chondrite composition. After HF leaching, which dissolved 30–50% of the metal, the remaining metal still contained many silicates. To reduce silicates that are mainly attached to the larger metal grains, we sieved the metal samples using a 40-mesh sieve and selected metal grains <400 μm that passed through the sieve. This fraction was then leached for another 10–15 min with concentrated HF to further reduce silicates. Despite repeated leaching in HF, the purified metal samples still contained significant amounts of silicate contamination. The measured Mg concentrations in aliquots of the dissolved metal samples indicate silicate contaminations of 0.6–2.1 wt% (Table 1).

Table 1.   Concentrations (in wt%) of major elements in stone and metal fraction of NWA 869 samples used for radionuclide analysis. The amount of silicate contamination (%sil, in wt%) in the metal fraction is estimated from the measured Mg concentration, assuming an average Mg content of 15.4 wt% in silicates. The recovered mass of each individual meteorite and the sample mass allocated for radionuclide analysis is given in grams, whereas the dissolved aliquots of the stone and metal fraction are given in milligrams.
Sample IDTotal mass (g)Alloc. mass (g)StoneMetal
Mass (mg)MgAlKCaMnFeNiMetal (wt%)Mass (mg)Mg%silCoNi

Cosmogenic Radionuclides

We dissolved 40–80 mg of purified metal along with a carrier solution containing 1–2 mg of Be, Al, and Ca and 2–4 mg of Cl in 20 mL 1.5 N HNO3. After dissolution, we took a small aliquot of the dissolved sample for chemical analysis (Fe, Ni, Co) by atomic absorption spectrometry (AA) using a Perkin Elmer AA3300. The elevated Ni concentrations of 19–29 wt% and Ni/Co ratios of 32–70 (relative to a Ni/Co ratio of approximately 20 for unleached metal) are consistent with preferential kamacite dissolution during HF leaching. For radionuclide analysis in the stone fraction, we dissolved 120–140 mg along with 4–5 mg of Be and Cl carrier in a mixture of HF/HNO3. After dissolution, we separated 36Cl as AgCl and removed Si as SiF4. We took a small aliquot of the remaining solution for chemical analysis by AA and separated Be, Al, and Ca using ion exchange and acetyl-acetone extraction techniques described previously (e.g., Welten et al. 2003).

The AMS measurements of 10Be, 26Al, and 36Cl were carried out at PRIME Laboratory (Sharma et al. 2000). The measured ratios were corrected for background and normalized to 10Be, 26Al, and 36Cl AMS standards (Nishiizumi 2004; Nishiizumi et al. 2007; Sharma et al. 1990). The concentrations were converted to activities (in dpm kg−1) by adopting half-lives of 1.36 Myr for 10Be, 0.705 Myr for 26Al, and 0.301 Myr for 36Cl. Measured 10Be and 26Al concentrations in the metal fraction were corrected for contributions of 10Be and 26Al from silicate contamination; corrections are 2–10% for 10Be and 8–30% for 26Al (Table 2). Finally, we normalized the measured 36Cl concentrations in the metal fraction to average H chondrite metal composition (90.5% Fe, 9.0% Ni, 0.5% Co) based on measured Ni concentrations in the metal and the assumption that the elemental production rate of 36Cl from Ni is approximately 25% lower than from Fe. This normalization procedure increases the 36Cl concentrations by 2.7–5.4%.

Table 2.   Cosmogenic radionuclide concentrations (in dpm kg−1) in stone (s) and metal (m) fraction of NWA 869 samples. Measured concentrations of 10Be and 26Al in the metal fraction were corrected for contributions from silicates, based on the amount of silicate from Table 1, whereas 36Cl concentrations in the metal were normalized to an average metal composition of 9 wt% Ni. The corrected values are indicated by an asterisk (*). The last two columns show the estimated contributions of 36Cl in the stone fraction produced by spallation, 36Cl(sp), and by neutron capture, 36Cl(nc), respectively.
M-05-38-118.1 ± 0.43.69 ± 0.163.54 ± 0.1656.8 ± 1.02.88 ± 0.182.32 ± 0.1811.8 ± 0.314.9 ± 0.315.7 ± 0.36.3 ± 0.55.5 ± 0.6
MB-1312.8 ± 0.33.01 ± 0.132.96 ± 0.1335.9 ± 0.82.12 ± 0.111.94 ± 0.119.5 ± 0.211.4 ± 0.311.7 ± 0.34.7 ± 0.44.8 ± 0.4
MS-04-111.1 ± 0.31.85 ± 0.071.67 ± 0.0838.0 ± 0.81.78 ± 0.201.08 ± 0.2021.7 ± 0.67.1 ± 0.27.4 ± 0.23.1 ± 0.218.6 ± 0.6
M-05-38-29.0 ± 0.31.77 ± 0.071.70 ± 0.0828.8 ± 0.71.44 ± 0.101.16 ± 0.1011.7 ± 0.36.8 ± 0.17.1 ± 0.13.1 ± 0.28.7 ± 0.3
#D3.4 ± 0.10.50 ± 0.030.47 ± 0.0313.0 ± 0.312.5 ± 0.63.0 ± 0.13.1 ± 0.11.4 ± 0.111.1 ± 0.6
SM-03-11.9 ± 0.10.21 ± 0.010.20 ± 0.017.1 ± 0.35.0 ± 0.11.7 ± 0.11.8 ± 0.10.9 ± 0.14.1 ± 0.1

Cosmogenic 14C

Bulk samples were crushed to a powder and treated with 100% phosphoric acid to remove carbonates, presumed to be products of terrestrial weathering. After the residues were washed with distilled water and dried, aliquots ranging in size from 45 to 100 mg were mixed with approximately 5 g of iron chips and placed in an alumina crucible. The crucible was placed in an oven at 500 °C to remove most low-temperature contaminants. The sample was heated in a RF induction furnace to 1400 °C in a flow of oxygen, and all carbonaceous gases were converted to CO2. We measured the volume of CO2 from its partial pressure at room temperature, and used it to determine the carbon content of the samples. Finally, the CO2 was converted to graphite to measure the 14C/12C ratio at the NSF-Arizona AMS facility together with graphite blanks and NIST oxalic acid I and II standards (Donahue et al. 1990). The measured ratio was corrected for AMS background and normalized to a 1950 AD graphite standard (14C/C = 1.17 × 10−12). The measured 14C concentration was then corrected for an average extraction background of (5.7 ± 3.4) × 105 atoms 14C following the procedure of Jull et al. (1989). The background corrections are 5–14% of the measured amounts of 14C atoms. The corrected 14C concentrations (in dpm kg−1) are reported in Table 3. For sample #D, a second extraction was performed on a duplicate sample of fragment #D, which confirmed the initial value within experimental uncertainty.

Table 3.   Concentrations of cosmogenic 14C and 10Be (in dpm kg−1) and bulk C (in ppm) in NWA 869 samples. Cosmogenic 10Be concentrations are bulk values, derived from measured 10Be concentrations in stone and metal fractions (Table 2) and bulk metal contents of Table 1. Cosmogenic 14C concentrations are based on measured 14C/12C ratios (in percent modern carbon, pmC) of carbon extracted from bulk samples weighing 44–101 mg. The amount of cosmogenic 14C atoms in each sample, N(14C)cos, was derived from the measured amount of 14C atoms N(14C)m by correcting for an average extraction blank of (5.7 ± 3.4) × 105 atoms.
SampleWeight (mg)10Be dpm kg−1CO2 (cc STP)C (ppm)14C/12C (pmC)N(14C)m (105 at)N(14C)cos (105 at)14C dpm kg−114C/10Be ratio
M-05-38-195.317.4 ± 0.40.177910231.2 ± 2.8117.9 ± 1.4112.2 ± 3.727.1 ± 0.91.56 ± 0.06
M-05-38-294.69.0 ± 0.30.079410273.7 ± 4.662.3 ± 1.056.6 ± 3.514.2 ± 0.91.58 ± 0.10
MB-1382.512.3 ± 0.30.092550239.4 ± 3.763.4 ± 1.057.8 ± 3.516.1 ± 1.01.31 ± 0.09
#D-a100.73.3 ± 0.10.2811370125.8 ± 1.4101.8 ± 1.196.2 ± 3.522.0 ± 0.86.5 ± 0.4
#D-b44.50.073800202.6 ± 3.442.6 ± 0.735.1 ± 3.419.1 ± 1.8
SM-03-191.51.9 ± 0.10.043230329 ± 1040.8 ± 0.756.6 ± 3.58.5 ± 0.94.5 ± 0.4

The measured carbon contents in most NWA 869 samples range from 0.42 to 1.37 mg g−1, consistent with typical carbon contents in type 4–6 ordinary chondrites (Makjanic et al. 1993), whereas the impact melt sample, SM-03-1, has a lower carbon content of 0.22 mg g−1. In addition, the measured delta 13C values of −30.2 to −30.7‰ for the extracted carbon exclude significant contributions from atmospheric CO2 (−7‰) or marine carbonates (0‰), indicating that terrestrial contamination was removed completely.

Noble Gases

Noble gases were measured on samples of 75–176 mg by standard techniques using procedures similar to those of Scherer et al. (1998). A deviation from this procedure was that Kr and Xe were cryogenically separated from each other, and separately analyzed to obtain their isotopic composition. Gas extraction was in two steps, at 600 °C (to preferentially release adsorbed terrestrial Ar, Kr, Xe) and 1800 °C. Blanks were run under the same conditions, both with and without empty Ni foils. Typical blanks (in cm3 STP) are 4He = 2 × 10−11, 20Ne = 2.5 × 10−13, 36Ar = 4 × 10−12, 84Kr = 6 × 10−14, and 132Xe = 2.5 × 10−14. Results are summarized in Table 4 (He, Ne, Ar) and Table 5 (trapped Ar, Kr, and Xe abundances plus isotopic ratios 129Xe/132Xe and 136Xe/132Xe). Data reported in the tables are corrected for both blank and interference on masses 3, 20, 22, 36, and 38. Errors include uncertainties in the corrections as well as errors in sensitivity and mass discrimination inferred from variations of standards.

Table 4.   Helium, neon, and argon abundances and isotopic compositions of NWA 869 samples; concentrations of 3He, 22Ne, and 36Ar are in units of 10−8 cm3 STP g−1, those of 4He and 40Ar in 10−5 cm3 STP g−1. For each sample the results for both temperature steps (600 °C and 1800 °C) and the total abundances and isotopic ratios are shown.
M-05-38-1600 °C4.87 ± 0.280.97 ± 0.060.63 ± 0.0310.70 ± 0.220.195 ± 0.0070.26 ± 0.030.273 ± 0.0541.55 ± 0.15
 Bulk1800 °C8.51 ± 0.482.24 ± 0.1219.71 ± 1.0611.91 ± 0.270.099 ± 0.0214.52 ± 0.440.225 ± 0.0073.97 ± 0.38
 102 mgTotal13.38 ± 0.563.21 ± 0.1320.34 ± 1.0611.88 ± 0.260.102 ± 0.0214.78 ± 0.480.228 ± 0.0085.52 ± 0.41
#D-a600 °C1.72 ± 0.072.33 ± 0.102.45 ± 0.0811.81 ± 0.060.0609 ± 0.00090.30 ± 0.020.273 ± 0.0152.21 ± 0.10
 Matrix1800 °C0.78 ± 0.091.02 ± 0.0424.19 ± 0.7312.09 ± 0.070.0568 ± 0.000712.06 ± 0.590.194 ± 0.0021.51 ± 0.07
 176 mgTotal2.50 ± 0.113.34 ± 0.1026.64 ± 0.7312.06 ± 0.070.0572 ± 0.000612.37 ± 0.590.196 ± 0.0023.73 ± 0.13
MB-13-a600 °C3.52 ± 0.150.51 ± 0.020.27 ± 0.016.95 ± 0.050.445 ± 0.0030.46 ± 0.040.296 ± 0.0461.52 ± 0.08
 Dark1800 °C5.32 ± 0.241.00 ± 0.046.84 ± 0.219.74 ± 0.050.237 ± 0.0022.44 ± 0.140.235 ± 0.0122.64 ± 0.13
 99 mgTotal8.84 ± 0.281.51 ± 0.057.10 ± 0.219.63 ± 0.050.245 ± 0.0022.90 ± 0.140.245 ± 0.0134.16 ± 0.15
MB-13-c600 °C6.68 ± 0.250.80 ± 0.030.24 ± 0.010.81 ± 0.040.914 ± 0.0120.02 ± 0.025.2 ± 2.62.08 ± 0.10
 Type 5/61800 °C2.95 ± 0.110.21 ± 0.011.88 ± 0.060.95 ± 0.010.908 ± 0.0060.54 ± 0.040.61 ± 0.041.07 ± 0.05
 132 mgTotal9.63 ± 0.281.01 ± 0.342.12 ± 0.060.93 ± 0.010.909 ± 0.0060.55 ± 0.050.73 ± 0.163.15 ± 0.06
MB-13-d600 °C4.44 ± 0.161.65 ± 0.071.02 ± 0.0311.87 ± 0.080.1427 ± 0.00150.27 ± 0.030.464 ± 0.0252.07 ± 0.11
 Matrix1800 °C4.77 ± 0.172.12 ± 0.0933.00 ± 0.9711.79 ± 0.070.0746 ± 0.000410.72 ± 0.550.206 ± 0.0042.45 ± 0.12
 132 mgTotal9.21 ± 0.243.77 ± 0.1134.03 ± 0.9711.79 ± 0.070.0767 ± 0.000610.99 ± 0.550.212 ± 0.0044.51 ± 0.16
M-05-38-2-b600 °C1.75 ± 0.090.50 ± 0.020.04 ± 0.001.66 ± 0.070.888 ± 0.0280.19 ± 0.030.32 ± 0.071.57 ± 0.08
 Type 41800 °C3.73 ± 0.161.01 ± 0.040.73 ± 0.021.25 ± 0.010.904 ± 0.0060.05 ± 0.021.07 ± 0.623.08 ± 0.15
 126 mgTotal5.48 ± 0.181.51 ± 0.050.77 ± 0.021.26 ± 0.010.904 ± 0.0060.24 ± 0.030.47 ± 0.144.64 ± 0.17
M-05-38-2-c600 °C1.61 ± 0.100.23 ± 0.010.10 ± 0.005.61 ± 0.100.522 ± 0.0121.71 ± 0.100.207 ± 0.0104.75 ± 0.24
 Dark1800 °C3.34 ± 0.160.77 ± 0.030.72 ± 0.023.61 ± 0.030.699 ± 0.0050.19 ± 0.030.820 ± 0.0683.76 ± 0.19
 105 mgTotal4.95 ± 0.190.99 ± 0.030.82 ± 0.023.86 ± 0.030.678 ± 0.0051.90 ± 0.100.267 ± 0.0118.50 ± 0.30
M-05-38-2-d600 °C3.91 ± 0.160.91 ± 0.040.14 ± 0.010.84 ± 0.060.949 ± 0.0130.03 ± 0.033.2 ± 1.84.30 ± 0.20
 Type 5/61800 °C1.75 ± 0.100.22 ± 0.011.29 ± 0.040.86 ± 0.010.952 ± 0.0060.37 ± 0.050.66 ± 0.041.90 ± 0.09
 115 mgTotal5.66 ± 0.191.13 ± 0.041.42 ± 0.040.86 ± 0.010.952 ± 0.0060.41 ± 0.050.87 ± 0.21619 ± 0.22
M-05-38-2-e600 °C2.22 ± 0.131.25 ± 0.050.87 ± 0.0312.24 ± 0.070.0940 ± 0.00110.44 ± 0.030.250 ± 0.0261.64 ± 0.08
 Matrix1800 °C4.17 ± 0.172.53 ± 0.1029.53 ± 0.8812.25 ± 0.060.0634 ± 0.000810.08 ± 0.510.198 ± 0.0082.49 ± 0.12
 119 mgTotal6.39 ± 0.223.77 ± 0.1130.40 ± 0.8812.25 ± 0.060.0642 ± 0.000710.51 ± 0.510.199 ± 0.0084.13 ± 0.15
SM-03-1-1-a600 °C0.69 ± 0.091.20 ± 0.050.88 ± 0.0311.95 ± 0.170.080 ± 0.0020.05 ± 0.031.37 ± 0.2710.08 ± 0.47
 Impact melt1800 °C0.18 ± 0.050.46 ± 0.026.16 ± 0.2211.30 ± 0.070.131 ± 0.0061.03 ± 0.060.28 ± 0.0211.23 ± 0.53
 75 mgTotal0.87 ± 0.101.66 ± 0.057.03 ± 0.2211.38 ± 0.060.124 ± 0.0051.08 ± 0.070.34 ± 0.0421.30 ± 0.70
Table 5.   Trapped 20Ne and 36Ar abundances, as well as measured abundances of 84Kr and 132Xe and isotopic ratios 129Xe/132Xe and 136Xe/132Xe of NWA 869 samples; concentrations of Ar are in units of 10−8 cm3 STP g−1, those of Kr and Xe in 10−12 cm3 STP g−1. Concentrations of excess 129Xe (129Xe*, from decay of now-extinct 129I) were calculated assuming a primordial 129Xe/132Xe ratio of 1.0 (Drozd and Podosek 1976).
M05-38-1600 °C6.7 ± 0.30.24 ± 0.0328 ± 319 ± 21.10 ± 0.030.320 ± 0.0061.9 ± 0.5
 Bulk1800 °C234 ± 144.40 ± 0.4842 ± 736 ± 52.23 ± 0.070.320 ± 0.00445 ± 7
 102 mgTotal241 ± 194.64 ± 0.4870 ± 755 ± 51.85 ± 0.060.320 ± 0.00447 ± 5
#D600 °C29 ± 10.28 ± 0.03102 ± 784 ± 71.11 ± 0.020.332 ± 0.0079 ± 2
 Matrix1800 °C292 ± 2112.02 ± 0.8435 ± 3185 ± 151.91 ± 0.030.319 ± 0.006167 ± 14
 176 mgTotal321 ± 2112.30 ± 0.84137 ± 7269 ± 161.66 ± 0.020.323 ± 0.005176 ± 12
MB-13-a600 °C1.8 ± 0.20.43 ± 0.43208 ± 2179 ± 50.93 ± 0.030.321 ± 0.007−5 ± 2
 Dark1800 °C65 ± 52.35 ± 0.18113 ± 12143 ± 81.23 ± 0.030.319 ± 0.00733 ± 4
 99 mgTotal67 ± 52.78 ± 0.18321 ± 24222 ± 91.13 ± 0.020.320 ± 0.00528 ± 5
MB-13-c600 °C−0.04 ± 0.0640 ± 318 ± 11.01 ± 0.020.315 ± 0.009<1
 Type 5/61800 °C0.23 ± 0.080.37 ± 0.0427 ± 344 ± 41.51 ± 0.030.332 ± 0.00822 ± 2
 132 mgTotal0.23 ± 0.080.33 ± 0.0766 ± 462 ± 41.36 ± 0.020.327 ± 0.00622 ± 2
MB-13-d600 °C12 ± 10.21 ± 0.0231 ± 317 ± 11.17 ± 0.020.318 ± 0.0143.0 ± 0.5
 Matrix1800 °C387 ± 2810.58 ± 0.7690 ± 689 ± 71.65 ± 0.040.338 ± 0.00957 ± 6
 132 mgTotal399 ± 2810.80 ± 0.76121 ± 6106 ± 71.57 ± 0.030.335 ± 0.00860 ± 5
M05-38-2-b600 °C0.03 ± 0.010.17 ± 0.0379 ± 849 ± 20.98 ± 0.020.333 ± 0.007<1
 Type 41800 °C0.33 ± 0.040.02 ± 0.0214 ± 218 ± 11.05 ± 0.050.335 ± 0.011<1
 126 mgTotal0.36 ± 0.040.19 ± 0.0393 ± 967 ± 21.00 ± 0.020.333 ± 0.006<1
M05-38-2-c600 °C0.52 ± 0.041.69 ± 0.13491 ± 5067 ± 51.06 ± 0.020.337 ± 0.0104.2 ± 1.2
 Dark1800 °C2.15 ± 0.150.10 ± 0.0248 ± 562 ± 51.30 ± 0.030.346 ± 0.00918.5 ± 2.3
 105 mgTotal2.67 ± 0.161.79 ± 0.13539 ± 50129 ± 71.18 ± 0.020.341 ± 0.00722.6 ± 2.5
M05-38-2-d600 °C−0.04 ± 0.0664 ± 443 ± 40.99 ± 0.030.329 ± 0.011<1
 Type 5/61800 °C0.04 ± 0.050.24 ± 0.0428 ± 337 ± 31.12 ± 0.020.359 ± 0.0104.5 ± 0.8
 115 mgTotal0.04 ± 0.050.20 ± 0.0692 ± 580 ± 51.05 ± 0.020.343 ± 0.0074.2 ± 1.4
M05-38-2-e600 °C11 ± 10.42 ± 0.0466 ± 735 ± 21.19 ± 0.040.337 ± 0.0146.6 ± 1.4
 Matrix1800 °C361 ± 2210.02 ± 0.71129 ± 13110 ± 61.97 ± 0.040.316 ± 0.007107 ± 7
 119 mgTotal372 ± 2310.44 ± 0.71195 ± 15145 ± 61.78 ± 0.030.321 ± 0.007113 ± 6
SM-03-1600 °C10 ± 10.01 ± 0.0124 ± 438 ± 31.04 ± 0.020.336 ± 0.0111.4 ± 0.9
 Impact melt1800 °C69 ± 50.95 ± 0.08312 ± 2141 ± 31.22 ± 0.030.358 ± 0.0129.1 ± 1.4
 75 mgTotal79 ± 50.96 ± 0.08336 ± 2179 ± 51.13 ± 0.020.347 ± 0.00810.5 ± 1.6

Results and discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Samples and analytical techniques
  5. Results and discussion
  6. Conclusions
  7. References

14C-10Be Terrestrial Age

Measured 14C concentrations (14Cm) in five NWA 869 fragments range from 8.5 to 27 dpm kg−1 (Table 3), values that may be compared with an average saturation value (14Csat) for L chondrites of 51 ± 7 dpm kg−1 (Jull et al. 1993). For meteorites with average shielding, the terrestrial age (Tterr, in kyr) can be determined from Tterr = 8.27*ln(14Cm/14Csat), in which 14Csat is 51 dpm kg−1. The highest 14C concentration of 27 dpm kg−1 measured in sample M05-38-1 yields a terrestrial age of NWA 869 of 5.2 ± 1.2 kyr, but this must be regarded as an upper limit, as the 14C saturation value in large objects is lower than the value of 51 dpm kg−1 used for average-sized meteorites. We therefore determined a more precise terrestrial age using the measured 14C/10Be ratio, based on the assumption that the production rate ratio is relatively constant at approximately 2.5, as was previously done for the large Gold Basin L chondrite shower (Kring et al. 2001). Figure 1 shows that 14C in three of five NWA samples is linearly correlated to 10Be, yielding an average 14C/10Be ratio of 1.48 ± 0.12. This ratio corresponds to a terrestrial age of 4.4 ± 0.7 kyr, slightly lower than the 14C age of the least shielded sample. The short terrestrial age is consistent with the relatively low degree of weathering (W1 on the scale of Wlotzka 1993) of the NWA 869 shower (Metzler et al. 2011).


Figure 1.  Concentrations of cosmogenic 14C (a) and cosmogenic 14C/10Be ratios (b) versus 10Be concentration in five bulk fragments of the NWA 869 shower. The dashed line in panel (a) represents a linear fit through three samples (MB-13, M05-38-1, and M05-38-2), yielding an average 14C/10Be ratio of approximately 1.5, whereas the solid line represents an average 14C/10Be production ratio of 2.5 for chondrite falls. The 14C/10Be production ratio of 2.5 is also represented by the gray bar in panel (b). Two samples, SM-03-1 and #D, show elevated 14C/10Be ratios of 4.5–6.5, which may be explained by a recent change in shielding conditions (see text).

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The average 14C concentration of approximately 21 dpm kg−1 in two aliquots of sample #D yields a 14C/10Be ratio of approximately 6.5, a factor of 4 higher than other NWA 869 fragments, and a factor of approximately 2.5 above the typical production ratio of 2.5 in ordinary chondrites. Another meteorite fragment, SM-03-1, also yields an elevated 14C/10Be ratio of approximately 4.5. Interestingly, these two NWA 869 fragments with elevated 14C/10Be ratios have the lowest 10Be concentrations, indicating high shielding during the last few Myr of CRE. A possible explanation for the high 14C/10Be ratios in #D and SM-03-1 is that these fragments do not belong to the NWA 869 shower, but to one or two other large L chondrite falls with unusual exposure histories. However, this explanation contradicts the mineralogical, petrologic, and noble gas data, which provide strong evidence that these two samples are part of the large NWA 869 shower (Metzler et al. 2011).

An alternative explanation for the high 14C/10Be ratios in samples #D and SM-03-1 could be that the NWA 869 meteoroid experienced a recent collision during its exposure in space, which either removed a slab or excavated a crater on one side of the NWA 869 meteoroid, thus exposing #D and SM-03-1 close to the newly excavated surface, while leaving the rest of the meteoroid (including samples M05-38-1, M05-38-2, MS-04-1, and MB-13, which presumably were on the other side of the meteoroid) intact. These lower shielding conditions for samples #D and SM-03-1 result in much higher cosmogenic nuclide production rates. The relatively high 14C concentrations in sample #D and SM-03-1 indicate that the lower shielding conditions started at least 15–20 kyr before impact on Earth, i.e., long enough for 14C to adjust to the higher production rates that are associated with lower shielding conditions. At the same time, the very low 10Be concentrations in #D and SM-03-1 constrain the timing of this collision to less than a few hundred kyr ago, i.e., not long enough for 10Be to adjust to the new, much higher, production rates.

10Be, 26Al, and 36Cl Concentrations

The concentration of cosmogenic 10Be in the stone fraction of six NWA 869 fragments ranges from 1.9 dpm kg−1 in SM-03-1 to 18.1 dpm kg−1 in M-05-38-1, whereas 10Be in the metal ranges from 0.2 to 3.5 dpm kg−1 (Figure 2a). These large variations in 10Be indicate irradiation in a large object, in which M-05-38-1 represents the least shielded sample (closest to the surface), and SM-03-1 represents the most shielded sample (closest to the center). The concentrations of 26Al in the stone fraction show a similar range of 7.1 dpm kg−1 in SM-03-1 to 56.8 dpm kg−1 in M-05-38-1 and are correlated with those of 10Be. The 26Al and 10Be concentrations in the stone fraction of six NWA 869 fragments yield an average 26Al/10Be ratio of 3.4 ± 0.4, which is typical for large objects (Ferko et al. 2002; Garrison et al. 1992; Welten et al. 2003), and indicates a minimum 4π exposure age of approximately 5 Myr. However, Figure 2b shows that the 26Al/10Be ratio does not vary randomly, but is significantly higher in the two most shielded samples than in the four remaining samples, showing average ratios of 3.83 ± 0.12 and 3.14 ± 0.13, respectively. Although the elevated 26Al/10Be ratios in the two most shielded samples are partly due to the higher shielding conditions, 26Al/10Be ratios >3.5 cannot be explained by shielding alone, but either indicate a short exposure age or a recent change in shielding conditions. The latter scenario is consistent with the one concluded from the elevated 14C/10Be ratios in these samples.


Figure 2.  Relationship of 10Be and 26Al concentrations (a) and 26Al/10Be ratio (b) in the stone fraction versus 10Be concentrations in the metal fraction of NWA 869. The dashed lines in panel (a) represent power law fits to the measured data, whereas the solid lines in panel (b) represent the average 26Al/10Be ratios of approximately 3.8 for the two most shielded samples (open symbols) and approximately 3.1 for the four least shielded samples (closed symbols) of the NWA 869 shower, respectively.

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The 36Cl concentrations in the metal fraction of NWA 869 also show large variations (a factor of approximately 8), but significantly smaller than those of 10Be in the metal phase (which vary by a factor of approximately 18). Previous measurements of 10Be and 36Cl in iron meteorites have shown that the production rate of 36Cl in metal is slightly less shielding-dependent than that of 10Be in metal, causing the 36Cl/10Be ratio to increase from approximately 3.5 in small iron falls to approximately 6 in the interior of large irons (Lavielle et al. 1999). This shielding dependence of the 36Cl/10Be ratio in metal is expected from the slightly different energy dependence of the cross sections of Fe(p,x)10Be and Fe(p,x)36Cl, which are summarized in figure 6 of Nishiizumi et al. (2009). However, the observed variations in the 36Cl/10Be ratio in the metal fraction of the six NWA 869 meteorites are significantly larger than expected from the correlation between 36Cl/10Be and 10Be in iron meteorite falls and the metal phase of chondrite falls (Fig. 3). Although the four least shielded samples show relatively constant 36Cl/10Be ratios of 3.9–4.5, slightly below the ratios observed for falls, the two most shielded samples (#D and SM-03-1) show elevated ratios of 6.5–9.2, i.e., 10–50% above the maximum value of approximately 6 found in the metal fraction of chondrite, and iron meteorite falls with high shielding. These high 36Cl/10Be ratios in the two most shielded samples are best explained by a complex exposure history in which these samples (#D and SM-03-1) were recently exposed to a much higher cosmic-ray flux, most likely due to a significant impact on the NWA 869 meteoroid, which exposed the two previously and most heavily shielded samples closer to the surface of the meteoroid. The timing of this recent collision will be discussed in more detail in the Recent Collision on NWA 869 Meteoroid section.


Figure 3.  Relationship of cosmogenic 36Cl/10Be ratio versus 10Be concentration in the metal fraction of NWA 869 meteorites (symbols as in Fig. 2b) relative to the observed correlation (solid curve) of 36Cl/10Be versus 10Be in iron meteorite falls and the metal fraction of chondrite falls (Lavielle et al. 1999; Welten et al. 2006).

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Cosmogenic 26Al in the metal phase was only measured in the four least shielded fragments and thus only shows a factor of 2 variation, from 1.1 to 2.3 dpm kg−1 metal, similar to the variation observed in 10Be of these four metal samples. The average 26Al/10Be ratios of 0.65 ± 0.02 in the metal fraction of these four fragments overlaps with the saturation ratio of 0.70 ± 0.02 (Aylmer et al. 1988), indicating that the 26Al and 10Be concentrations were at or close to saturation, implying a minimum 4π exposure age of approximately 5 Myr.

The 36Cl concentrations in the stone fraction range from 5 dpm kg−1 in SM-03-1 to 21.7 dpm kg−1 in MS-04-1. Cosmogenic 36Cl in the stone fraction of NWA 869 includes contributions from spallation reactions on Fe, K, and Ca, as well as contributions from low-energy reactions by thermalized neutrons on 35Cl. We estimated the spallation contributions in the NWA 869 samples from the 36Cl concentrations in the metal fraction, the Fe, Ca, and K concentrations in the stone fraction and elemental production rate ratios, P(36Cl)Ca/P(36Cl)Fe = 10 and P(36Cl)K/P(36Cl)Fe = 20. The spallation 36Cl contributions range from 0.7 dpm kg−1 in the most shielded sample to 5.5 dpm kg−1 in the least shielded sample of the NWA 869 shower (Table 3). By simple subtraction, this yields neutron-capture contributions of 4.4–19.0 dpm kg−1. These neutron-capture 36Cl values are in the same range as those observed in other large objects, such as Chico (Bogard et al. 1995), FRO 90174 (Welten et al. 2001), Ghubara (Ferko et al. 2002), and Gold Basin (Welten et al. 2003). The neutron-capture 36Cl contributions are quite variable, and do not show a simple correlation with the 10Be concentrations (Table 3). The scatter in 36Cl is probably due to variations in the concentration of Cl, which is the target element for the production of 36Cl by neutron-capture. As we did not measure the native Cl concentrations (which is difficult due to terrestrial contamination), the neutron-capture 36Cl contributions are not a reliable measure for preatmospheric size and depth. Future measurements of 41Ca in the stone fraction, which is also produced by neutron capture, may provide additional shielding information on these samples.

By comparing the measured 10Be concentrations in the stone and metal phase of NWA 869 with calculated production rates as a function of size and depth, we can constrain the preatmospheric size of the NWA 869 meteoroid and the preatmospheric depth of samples within this object. Although the calculated production rates of cosmogenic nuclides in large objects are somewhat dependent on which model one uses for these calculations (e.g., Welten et al. 2003; Leya and Masarik 2009), the semiempirical model of Honda (Nagai et al. 1993; Honda et al. 2002) appears the most reliable for calculating 10Be production rates in the metal and stone fraction of large chondrites (Welten et al. 2010). Figure 4a shows that 10Be production rates for a preatmospheric radius of 225 cm most closely reproduce the observed variations in 10Be in stone and metal fractions of NWA 869, even though the measured 10Be concentrations in the least shielded sample (M-05-38-1) are approximately 20% higher than calculated production rates. Some of the deviations between measured concentrations and calculated production rates may be due to a nonspherical shape of the meteoroid, while the calculations are based on spherical objects. With a radius of approximately 225 cm, NWA 869 is the second largest chondritic object recovered, after the Gold Basin L4–6 chondrite, which had a preatmospheric radius of 3–5 m (Welten et al. 2003). Adopting a radius of 225 cm and calculated 10Be depth profiles of Honda et al. (2002), the measured 10Be concentrations in the NWA 869 samples indicate preatmospheric depths ranging from <10 cm for MB-13 and M05-38-1 to approximately 140 cm for the most shielded sample, SM-03-1 (Figure 4b). It is likely that the respective shielding depths of sample #D and SM-03-1 are slightly underestimated, as the recent collision on NWA 869 meteoroid has increased the 10Be concentrations in these two samples by 10–20%.


Figure 4.  Constraining the preatmospheric size (a) and depth (b) of NWA 869 meteorites based on 10Be in the stone and metal fraction. Dashed and solid curves in panel (a) represent model calculations for 10Be in chondritic objects ranging from 85 cm to 225 cm in radius (as well as for 2π irradiation), based on the semiempirical model of Honda (Nagai et al. 1993; Honda et al. 2002). Estimated depths of NWA 869 samples (b) are based on model calculations of 10Be production rate in stone (dashed line) and metal (solid line) phase of a chondritic object with a radius of 225 cm. Uncertainties in derived depth of each sample are estimated at approximately 10 cm.

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Solar, Planetary, Radiogenic, and Cosmogenic Noble Gases

Ten separates of five different NWA 869 meteorite fragments were measured for He, Ne, Ar, Kr, and Xe isotopes. Solar wind implanted gases are evident in seven of the 10 measured samples, representing all five meteorite fragments. The presence of solar gases in all meteorite fragments confirms that NWA 869 is a regolith breccia, as was first concluded by Osawa and Nagao (2006). Although neon has only two components, a trapped (solar) and a cosmogenic one, helium and argon also have a radiogenic component, 4He and 40Ar, respectively.

Trapped Neon

In a three-isotope plot of 20Ne/22Ne versus 21Ne/22Ne for the individual temperature steps (Figure 5), the neon isotope data of NWA 869 plot on a mixing line between spallation and solar-type neon are similar in composition to Ne-B (Black 1972). A fit to the data, taking into account errors in both variables (Williamson 1968), yields a trapped (solar) 20Ne/22Ne ratio of 12.58 ± 0.05 (for 21Ne/22Ne = 0.03). Bulk and clastic matrix samples contain the highest amounts of solar Ne, whereas lower abundances are present in the impact melt and shock-darkened clasts. The clasts of petrologic type 4–6 are essentially free of solar gases.


Figure 5.  Neon 3-isotope plot for each of the two temperature extraction steps of NWA 869 samples, at 600 °C (open symbols) and 1800 °C (closed symbols), respectively. The crossed square represents the (solar) Ne-B composition (Black 1972) with 21Ne/22Ne and 20Ne/22Ne ratios of 0.03 and 12.58 ± 0.05, respectively.

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Cosmogenic Neon

The trapped Ne ratios together with a cosmogenic 20Ne/22Ne ratio of 0.83 ± 0.02 (Schultz et al. 1991) have been used to derive abundances of cosmogenic 21Ne and 22Ne, and thus of the cosmogenic (22Ne/21Ne)c ratio. Uncertainties in the assumed endmembers as given above have been propagated in these calculations; the results are listed in Table 6. No values are given for the shielding parameter for the bulk and the three matrix samples, because solar Ne is too dominant and the resulting (22Ne/21Ne)c too uncertain.

Table 6.   Abundances of cosmogenic 3Hec and 21Nec as well as cosmogenic (3He/21Ne)c and (22Ne/21Ne)c ratios. Abundances are in units of 10−8 cm3 STP g−1. The last column lists apparent shielding corrected 21Ne/26Al CRE ages (in Myr), assuming 4π exposure in a large L-chondritic object with a constant 21Ne/26Al production rate ratio of 2.5 atoms/atoms (Welten et al. 2003). The uncertainties in the CRE age include a 5% uncertainty in the 21Ne/26Al production rate ratio.
Sample3Hec21Nec(3He/21Ne)c(22Ne/21Ne)c26Al (bulk)21Ne/26AlT(21Ne/26Al)
M-05-38-112.9 ± 0.61.51 ± 0.098.5 ± 0.653.4 ± 1.014.1 ± 0.95.9 ± 0.5
D-a1.8 ± 0.20.76 ± 0.052.4 ± 0.311.8 ± 0.332.4 ± 2.313.5 ± 1.2
MB-13-a8.7 ± 0.31.58 ± 0.055.5 ± 0.21.126 ± 0.03033.2 ± 0.823.8 ± 0.99.9 ± 0.6
MB-13-c9.6 ± 0.31.92 ± 0.055.0 ± 0.21.091 ± 0.00733.2 ± 0.829.0 ± 1.012.1 ± 0.7
MB-13-d8.3 ± 0.31.66 ± 0.085.0 ± 0.333.2 ± 0.824.9 ± 1.310.4 ± 0.8
M05-38-2-b5.5 ± 0.20.69 ± 0.027.9 ± 0.41.067 ± 0.00726.8 ± 0.712.9 ± 0.55.4 ± 0.3
M05-38-2-c4.9 ± 0.20.55 ± 0.028.9 ± 0.41.108 ± 0.00926.8 ± 0.710.3 ± 0.54.3 ± 0.3
M05-38-2-d5.7 ± 0.21.35 ± 0.044.2 ± 0.21.048 ± 0.00726.8 ± 0.725.4 ± 1.010.6 ± 0.7
M05-38-2-e5.6 ± 0.21.07 ± 0.075.2 ± 0.426.8 ± 0.719.9 ± 1.48.3 ± 0.7
SM-03-10.7 ± 0.10.69 ± 0.041.0 ± 0.26.8 ± 0.350.4 ± 3.721.0 ± 1.9

Cosmogenic (22Ne/21Ne)c ratios for the type 4 clast (M-05-38-2-b) as well as type 5 or 6 clast M-05-38-2d are 1.07 and 1.05, respectively, indicative of exposure in a large body. The same may be true for the impact melt clast (SM-03-1), although the cosmogenic 22Ne/21Ne ratio has a large error. Type 5 or 6 clast MB-13-c as well as the shock-darkened clasts, appear to have been irradiated at slightly lower shielding conditions, on the other hand. Concentrations of cosmogenic 21Ne vary by a factor 3.5, between 0.55 × 10−8 cm3 STP g−1 and 1.92 × 10−8 cm3 STP g−1, with the extreme values provided by the two shock-darkened clasts. Abundances of cosmogenic 21Ne in the matrix samples vary by a factor of 2.2. Concentrations of cosmogenic 38Ar in NWA 869 samples are not reported because they are highly uncertain, due to large contributions of trapped (atmospheric and solar) Ar as well as unknown contributions of neutron-capture produced 36Ar (from decay of 36Cl).

Trapped Helium

Figure 6 reveals that concentrations of trapped 4He (corrected for cosmogenic He assuming (4He/3He)= 6.1; Alexeev 1998) are linearly correlated with those of trapped solar 20Ne for most (eight of 10) of the samples. The slope of the correlation line yields a trapped 4He/20Ne ratio of approximately 7, much lower than values of 300–600 for other solar gas–rich H chondrites, such as Weston, Fayetteville, and Acfer 111 (Schultz et al. 1972; Wieler et al. 1989; Pedroni and Begemann 1994), but similar to typical ratios of 2–11 in solar gas–rich lunar meteorites (Lorenzetti et al. 2005), and in two L-chondrite regolith breccias, Ghubara (Ferko et al. 2002) and Itawa Bhopji (Bhandari et al. 2002). As trapped helium is released at lower temperatures than trapped Ne (e.g., Schultz et al. 1971; Osawa and Nagao 2006), the low 4He/20Ne ratios in NWA 869 are most likely due to loss of solar He, either during (1) heating by impact events in the regolith, (2) compaction of the regolith into solid rock, (3) ejection of the meteorite from the parent body (e.g., Lorenzetti et al. 2005; Schwenzer et al. 2008), or (4) solar heating in low-perihelion orbits, i.e., during close encounters with the Sun. A general relationship between solar gas content and the relative abundance of interstitial melt (Bischoff et al. 1983) suggests that shock melting (presumably during the lithification process) reduces solar gas abundances by several orders of magnitude, but does not affect the ratio of trapped 4He to 20Ne, thus ruling out scenario (2). The concentrations of radiogenic 4He and 40Ar as well as the ratio of cosmogenic 3He/21Ne in the Jilin meteorite suggest that the relative loss of noble gases during or after ejection from the parent body is a function of distance from the preatmospheric surface (Begemann et al. 1985, 1996). As such a relationship is not observed for NWA 869, and as most NWA 869 samples show no evidence of cosmogenic 3He loss, explanations (3) and (4) seem unlikely. It thus seems plausible that most of the solar helium in NWA 869 was lost during thermal events on the L chondrite parent body before compaction of NWA 869 into solid rock.


Figure 6.  Relationship between noncosmogenic 4He and trapped (solar) 20Ne in NWA 869 samples. Measured 4He concentrations have been corrected for the cosmogenic contribution using (4He/3He)= 6.1 (Alexeev 1998). The dashed line represents a linear fit through the solid data points, taking into account errors in both variables (Williamson 1968). The intercept yields an inferred average radiogenic 4He concentration of (1000 ± 20) × 10−8 cm3 STP g−1, whereas the two samples represented by open symbols yield a higher radiogenic 4He concentration of approximately 1445 (same units). The slopes of both lines correspond to a 4He/20Ne ratio of 7.3 for the trapped solar wind component.

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Radiogenic Helium

The intercept of the correlation in Fig. 6 (at 20Nesol = 0) yields a radiogenic 4He concentration of (1000 ± 20) × 10−8 cm3 STP g−1, with a slope of 7.27, which is the inferred 4He/20Ne ratio of the solar component. Assuming typical U and Th concentrations of 13 and 43 ppb (Wasson and Kallemeyn 1988), this radiogenic 4He concentration corresponds to a U,Th-He age of 2.83 ± 0.05 Gyr. The 4He and 20Ne concentrations in two other samples, M05-38-1 and a type 4 clast, M05-38-2b, can be fit by a line with a similar slope, but a significantly higher radiogenic 4He concentration of (1440 ± 40) × 10−8 cm3 STP g−1 (average value). Assuming typical U and Th concentrations of 13 and 43 ppb, the 4He value yields a U,Th-He age of 3.65 ± 0.07 Gyr. The observation that the NWA 869 breccia contains clasts with different 4He retention ages of approximately 2.8 and approximately 3.7 Gyr, respectively, implies that lithification of the components occurred ≤2.8 Gyr ago. If we also include the Ar-Ar age of approximately 1.8 Gyr for impact melt clast SM-03-1 (Metzler et al. 2011), then we have to conclude that compaction of NWA 869 occurred ≤1.8 Gyr ago.

Cosmogenic Helium

With trapped solar 4He concentrations up to approximately 2700 × 10−8 cm3 STP g−1, some fraction of 3He must be of solar wind origin. We made corrections for the solar wind helium contribution, assuming (4He/3He)solar = 3300 ± 300, which is based on the correlation between 3He/4He and 20Ne/22Ne in lunar regolith as determined by Benkert et al. (1993) and the trapped 20Ne/22Ne ratio of 12.58 ± 0.05 measured in NWA 869. Corrections for solar 3He range from 0% for solar-gas free samples to 30% for matrix sample D-a. In a plot of cosmogenic 3He/21Ne versus 22Ne/21Ne ratios (Fig. 7), half of the samples plot in a position consistent with the well-established correlation between cosmogenic 3He/21Ne and 22Ne/21Ne ratios, which is often referred to as the “Bern line” (Eberhardt et al. 1966), whereas the impact melt (even before correction for solar He) and matrix sample #D-a plot clearly below this correlation line, indicative of cosmogenic 3He loss. The observation that #D and SM-03-1 lost cosmogenic 3He is especially significant, as we proposed (based on radionuclide data) that these two samples may have been affected by a recent impact on the NWA 869 meteoroid approximately 105 yr ago. The fact that the two samples that were closest to this impact event show that loss of cosmogenic 3He supports our hypothesis.


Figure 7.  Relationship between cosmogenic 3He/21Ne and 22Ne/21Ne ratios (a) and cosmogenic 3He versus 21Ne concentrations (b) in NWA 869 samples. Solid symbols represent solar gas–rich samples (20Ne > 200 × 10−8 cm3 STP g−1), open symbols represent solar-gas-free samples (20Ne < 3 × 10−8 cm3 STP g−1), and gray symbols represent intermediate solar-gas containing samples. For solar gas–rich samples, we assume a cosmogenic 22Ne/21Ne ratio of 1.07 ± 0.03. Measured 3He concentrations have been corrected for a trapped solar He contribution based on trapped solar 4He as identified in Fig. 6 and assuming (4He/3He)trapped = 3300 (Benkert et al. 1993). The solid line in (a) represents the correlation of cosmogenic 3He/21Ne versus 22Ne/21Ne ratios in chondrites (Nishiizumi et al. 1980), whereas the dashed line in (b) corresponds to a 3He/21Ne ratio of 5. The two gray bars highlight the constant 3He concentrations of (5.4 ± 0.3) × 10−8 cm3 STP g−1 in four aliquots of fragment M05-38-2 and of (8.9 ± 0.7) × 10−8 cm3 STP g−1 in three aliquots of fragment MB-13.

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The elevated 3He/21Ne ratios of 7.9–9 of a bulk sample, M-05-38-1, and two clasts of M-05-38-2, which fall significantly above the Bern line, are more puzzling (Fig. 7). We do not have a good explanation for this excess 3He. A possible, although speculative, idea might call for the presence of solar flare He. Normal solar wind helium has been thoroughly lost, yielding a trapped 4He/20Ne ratio of approximately 7 (Fig. 6). More energetic and hence more deeply (more retentively?) sited He from solar flares, which are often high in 3He (e.g., Mason 2007), may have made a noticeable contribution to 3He in such cases. Interestingly, the high 3He/21Ne ratios in the two clasts of M-05-38-2 correspond to the samples with the lowest 21Ne concentrations (Table 6), whereas the cosmogenic 3He concentrations in the four samples of M-05-38-2 are relatively constant at (5.3 ± 0.3) × 10−8 cm3 STP g−1 (Fig. 7b). This could suggest that cosmogenic 3He from the first-stage irradiation was redistributed within this meteorite fragment, whereas 21Ne was not, thus increasing the 3He/21Ne ratios of the samples with the lowest 21Ne concentrations. Both explanations are speculative, and so the occurrence of high 3He/21Ne ratios will require further investigation.

Trapped Ar, Kr, Xe; Excess 129Xe

Trapped 36Ar is most abundant in the three matrix samples, with concentrations in the order of 10 × 10−8 cm3 STP g−1, yielding ratios of solar 20Ne to trapped 36Ar (Table 5) in the range of 20–50. These 20Ne/36Ar ratios are similar to those of solar gas–rich meteorites (Wieler et al. 1989a), indicating that most of the trapped 36Ar in these gas-rich NWA 869 samples is also of solar wind origin. However, on the basis of the significant amounts of trapped 132Xe (1–3 × 10−10 cm3 STP g−1) in some of the gas-rich NWA 869 samples, and the correlation between planetary Ar and Xe (Marti 1967), we estimate that 10–20% of the trapped Ar is of planetary origin. On the other hand, samples with low trapped Ne and Ar seem to be dominated by planetary and/or atmospheric 36Ar, which is generally <1 × 10−8 cm3 STP g−1 (Fig. 8). The high trapped 20Ne/36Ar ratio of approximately 80 in SM-03-1 is clearly an outlier, which may be due to the impact melt nature of this sample.


Figure 8.  Trapped Ne, Ar, Kr, and Xe in NWA 869 samples. The two dashed lines in (a) represent 20Ne/36Ar ratios of 20 and 50, respectively. Samples with trapped 20Ne/36Ar ratios in the range of 20–50 contain trapped Ne and Ar of almost purely solar origin, whereas lower ratios indicate the admixture of atmospheric and/or planetary-type Ne and Ar. Panel (b) shows trapped 36Ar and 84Kr versus 132Xe in NWA 869, with solid symbols representing samples with low solar-gas contents (20Ne < 100 × 10−8 cm3 STP g−1) and half-filled symbols representing samples with high solar-gas contents (20Ne > 200 × 10−8 cm3 STP g−1). The two shaded areas in panel (b) show typical concentrations of planetary Ar, Kr, and Xe in ordinary chondrites of petrologic type 4–6 (Marti 1967).

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Krypton and xenon are mixtures of atmospheric contamination (often dominant in the 600 °C steps of the clasts) and typical planetary-type Kr and Xe, as shown in Fig. 8b. The presence of planetary Xe is also indicated by, e.g., the 136Xe/132Xe ratio (Ott 2002). The concentrations of trapped 132Xe in all NWA 869 samples (including the clast of petrologic type 4) are in the range for chondrites of petrologic type 5 and 6 chondrites (Marti 1967). Note that matrix sample #D shows the highest 132Xe/84Kr ratio and thus seems least affected by atmospheric contamination (which has a low Xe/Kr ratio). Like planetary Xe, excess 129Xe from decay of now-extinct 129I is most abundant in the three matrix samples. Assuming a primordial 129Xe/132Xe ratio of 1.0 (Drozd and Podosek 1976), the amount of excess 129Xe in matrix samples of NWA 869 ranges from 0.6 × 10−10 cm3 STP g−1 in sample MB-13-d to 1.76 × 10−10 cm3 STP g−1 in #D (Table 5).

Radiogenic 40Ar

Concentrations of 40Ar range from (3.1–21.3) × 10−5 cm3 STP g−1. The highest values were found in impact melt sample SM-03-1 and shock-darkened clast M05-38-2c, for which Metzler et al. (2011) reported anomalously high K concentrations of 4480 and 2290 ppm. The remaining samples show 40Ar concentrations of 3.1–6.2 × 10−5 cm3 STP g−1, with an average of (4.5 ± 1.0) × 10−5 cm3 STP g−1, and no correlation with trapped 36Ar. Assuming that 40Ar is dominated by the radiogenic component, and adopting an average K concentration of 800 ppm, we derive an average K-Ar age of 3.85 ± 0.35 Gyr with individual ages ranging from 3.3 to 4.4 Gyr. The most plausible explanation for the range in K-Ar ages is that they are due to a mixture of clasts having 39Ar-40Ar ages around 4.4 Gyr and clasts having ages as young as 1.8–2.2 Gyr, as discussed in more detail by Metzler et al. (2011).

Regolith Exposure

Samples of regolith breccias containing solar wind and having experienced GCR preirradiation on their parent body often show a relation between the contents of trapped solar wind and spallation neon, as was observed for matrix samples of Fayetteville (Wieler et al. 1989; Wieler 2002) and to some extent for the solar gas–rich samples of the Frontier Mountain 90174 shower (Leya et al. 2009), two H-chondrite regolith breccias. In the case of NWA 869, there is no evidence for such a correlation (Fig. 9), suggesting that the irradiation history of NWA 869 on the L-chondrite parent body is more complex. A similar complex irradiation history of individual components in the regolith was previously proposed for the Ghubara L5-chondrite regolith breccia (Ferko et al. 2002).


Figure 9.  Relationship between cosmogenic 21Ne and solar 20Ne in matrix, clasts, bulk, impact melt, and shock-darkened samples of NWA 869 fragments.

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CRE History

The cosmogenic 21Ne concentrations in 10 NWA 869 samples range from (0.55–1.92) × 10−8 cm3 STP g−1. Due to the large preatmospheric size of the NWA 869 L chondrite, 21Ne production rates in our samples cannot be determined reliably using the formalism of Eugster (1988), which is only valid for objects <50 cm in radius (e.g., Graf et al. 1990; Leya and Masarik 2009). A more reliable method for calculating 21Ne production rates in large objects uses 26Al as an internal shielding parameter (Graf et al. 1990). The method is based on a relatively constant 21Ne/26Al production rate ratio of approximately 2.5 atom per atom (Graf et al. 1990; Welten et al. 2003). Assuming that both 21Ne and 26Al in each sample were entirely produced during 4π irradiation, this method yields consistent CRE ages independent of shielding, as was shown for the majority of samples from the very large Gold Basin L-chondrite shower (Welten et al. 2003).

The 21Ne/26Al ratios in NWA 869 vary by a factor of 5, yielding CRE ages ranging from 4.3 to 21 Myr (Fig. 10). Three of the samples show a relatively constant CRE age of 5 ± 1 Myr, which most probably represents the 4π exposure time in space, whereas the higher apparent CRE ages are due to production of 21Ne during a previous exposure of the samples on the L-chondrite parent body, either as individual components in a regolith (e.g., Wieler et al. 1989; Leya et al. 2009) and/or as part of a larger object on the surface of the parent body (Welten et al. 2003). The challenge is to distinguish between these two scenarios, which is difficult for regolith breccias, and is especially difficult for regolith breccias that produced large meteorite showers, for which the relative locations of the samples within the preatmospheric object are unknown. A recent study on the exposure history of the Frontier Mountain 90174 H-chondritic regolith breccia represents such a complex exposure scenario (Leya et al. 2009).


Figure 10.  Correlation between cosmogenic 21Ne and 26Al in NWA 869 fragments. The three lines correspond to CRE ages of 5 Myr (solid), 10 Myr (semidashed), and 20 Myr (dashed), based on a 21Ne/26Al production rate ratio of 2.5 atom per atom (Welten et al. 2003). Symbols are the same as in Figure 7.

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Figure 10 shows that the cosmogenic 21Ne concentrations vary by a factor of 2.5 among clasts and matrix samples from a single meteorite fragment (M05-38-2) of only approximately 5 cm in diameter. Such large variations of cosmogenic 21Ne on a centimeter scale cannot be explained by variations in shielding during 2π or 4π exposure in a large meteoroid, and thus clearly indicate that the first-stage exposure must have occurred in the individual components before compaction into the NWA 869 meteorite. Such a scenario is often observed for individual components of regolith breccias (e.g., Schultz et al. 1972; Wieler et al. 1989a). The most likely site of this CRE is in the regolith of the L-chondrite parent body. Most of the NWA 869 samples have the same radiogenic U,Th-He retention age of approximately 2.8 Gyr and the same low trapped 4He/20Ne ratios, suggesting they were in close proximity during a major heating event that led to loss of radiogenic and trapped solar 4He. It is plausible that the first-stage irradiation of individual components took place before this main heating event, i.e., more than 2.8 Gyr ago. After the CRE exposure of individual samples in the regolith, the samples were heated together (presumably due to a major impact event), releasing solar and radiogenic He, before being compacted into a lithified regolith breccia.

The excess of cosmogenic 21Ne from the first-stage irradiation ranges from <0.1 to 1.1 × 10−8 cm3 STP g−1. Assuming a maximum 2π21Ne production rate of approximately 0.18 × 10−8 cm3 STP g−1 Myr−1 for L chondrite composition (Leya and Masarik 2009), these 21Ne concentrations correspond to minimum 2π exposure ages ranging from 0.5 Myr to approximately 6 Myr. These ages are lower than minimum regolith exposure ages of up to approximately 25 Myr found for solar gas–rich matrix samples of the Fayetteville H-chondrite breccia (Wieler et al. 1989), but similar to those found in samples of the FRO 90174 H chondrite regolith breccia (Leya et al. 2009).

Due to the presence of excess 21Ne from CRE of individual components (matrix and clasts) of NWA 869 in a regolith, it is very difficult to assess whether or not NWA 869 was also exposed as a large boulder on the parent body immediately before ejection. Although we cannot completely rule out a recent 2π exposure on the parent body, the present study provides no evidence that the NWA 869 meteoroid experienced such an exposure. We thus conclude that NWA 869 does not follow the trend of several other large chondrites (such as Jilin, JaH 073, and Gold Basin).

Why Are L Chondrite Regolith Breccias So Rare?

It is well known that L chondrite regolith breccias are much less common than H chondrite regolith breccias (Crabb and Schultz 1981; Bischoff et al. 2006). This difference is presumably related to the catastrophic disruption of the L chondrite parent body approximately 0.5 Gyr ago, which resulted in loss of radiogenic noble gases (e.g., Heymann 1967; Haack et al. 1996). As solar gases are more loosely bound than radiogenic gases, it has been suggested that the catastrophic disruption of the L chondrite parent body may have led to complete loss of solar gases from most of the gas-rich L chondrites, whereas an alternative explanation for the lack of L-chondrite regolith breccias is that the disruption simply yielded more interior asteroid fragments that were too small to develop regoliths (Crabb and Schultz 1981). Both explanations are consistent with the lack of solar gas–rich samples among L chondrites in the 500 Myr peak (Marti and Graf 1992).

The low trapped 4He/20Ne ratios in the L-chondrite regolith breccias Ghubura (Ferko et al. 2002), Itawa-Bhopji (Bhandari et al. 2002), and NWA 869 (this work) indicate that large impacts on the L-chondrite parent body indeed caused loss of solar helium. However, despite the large solar helium losses, most of the solar Ne and Ar is retained in these L-chondrite regolith breccias, even though at least one of them (Ghubara) was directly linked to the catastrophic disruption of the L-chondrite parent body approximately 470 Myr ago (Korochantseva et al. 2007). In addition, evidence from fossil meteorites found in Ordovician limestone indicates that at least one of the L chondrites (Ark 002) that arrived to Earth shortly after this large disruption contains small amounts of solar Ne (Heck et al. 2004). It therefore seems unlikely that the catastrophic disruption of the L-chondrite parent body led to complete outgassing of gas-rich L chondrites. We thus favor the alternative explanation that the catastrophic collision of the L-chondrite parent body excavated materials from the interior of the L-chondrite parent body that were never exposed in a regolith, and that these remaining parent body fragments were too small to develop regoliths (Crabb and Schultz 1981; Rubin et al. 1983).

Recent Collision on NWA 869 Meteoroid

As discussed above, the high 14C/10Be and 36Cl/10Be ratios in two fragments of the NWA 869 meteorite shower, #D and SM-03-1, suggest that the NWA 869 meteoroid experienced a recent collision in space, which removed enough material to expose these two samples to higher cosmic-ray fluxes during the last part of their CRE. On the basis of the elevated 14C/10Be ratio in sample #D, which is a factor of approximately 4 higher than in most other NWA 869 samples, and calculated 14C depth profiles in large objects (Leya and Masarik 2009), we estimate that at least 80 cm of material was removed from one side of the NWA 869 meteoroid. To constrain the timing of this collision from the elevated 36Cl/10Be ratios in the metal phase, we made the following assumptions (1) 10Be production rates for the first-stage 4π irradiation of #D and SM-03-1 were in the range between 0.1 and 0.5 dpm kg−1 metal, (2) the removal of 80 cm of material increased the production rate of 10Be in the metal fraction of #D and SM-03-1 by a factor of 5, and (3) 36Cl production rates follow the correlation between 36Cl/10Be and 10Be (Lavielle et al. 1999; Welten et al. 2006). Figure 11 shows the calculated 36Cl/10Be ratios and 10Be concentrations as a function of the time elapsed since the recent impact event. Although the 36Cl/10Be ratios in the two samples do not yield the same age, they are both consistent with a relatively recent collision event that occurred between 20 and 120 kyr before impact on Earth. As we do not know exactly how much material was removed and how far the two samples were located from this impact, we cannot constrain the timing more accurately. However, more important than the exact timing of this event, is the fact that this meteoroid shows evidence for a recent impact event during its travel as a meter-sized object in space.


Figure 11.  Constraining the timing of a recent collision on the NWA 869 meteoroid from the elevated 36Cl/10Be ratios in two NWA 869 meteorites, #D and SM-03-1. The curves show calculated 36Cl/10Be ratios as a function of the 10Be concentration 10–200 kyr after a collision that removed approximately 80 cm of material on one side of the meteoroid, near #D and SM-03-1. Assuming that the most recent 10Be and 36Cl production rates for these samples are five times higher than those in the first part of the 4π exposure, the elevated 36Cl/10Be ratios indicate recent exposure ages of 20 ± 15 kyr for #D and 120 ± 50 kyr for SM-03-1 since the collision.

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According to equations in Farinella et al. (1998), a chondritic object of 4–5 m in diameter is collisionally destroyed by a projectile with a diameter of 20–25 cm hitting at an average velocity of 5.8 km s−1. The impact on the NWA 869 meteoroid must have been significantly smaller, as the meteoroid survived the impact, which only removed material up to approximately 80 cm deep, whereas more than 95% of the original mass survived the impact. We speculate that the impactor must have been in the order of 5–15 cm in diameter. Based on average impact probabilities in the asteroid belt (Farinella et al. 1998), an object of 4–5 m in diameter is struck about once every 3–5 Myr by a projectile of 10 cm in diameter. Although this is a rough estimate, it does indicate that large meteoroids like NWA 869 are more likely to show evidence of recent collisions during their journey through space than small meteoroids.


  1. Top of page
  2. Abstract
  3. Introduction
  4. Samples and analytical techniques
  5. Results and discussion
  6. Conclusions
  7. References

The combined measurements of noble gases and cosmogenic radionuclides in samples of the large NWA 869 L3–6 chondrite shower lead to the following conclusions.

  • 1
     The long-lived radionuclides, 10Be, 26Al, and 36Cl, show large variations among the NWA 869 samples, indicating that the samples were irradiated at depths of <10 cm to 140–160 cm in an object with a preatmospheric radius of 225 ± 25 cm. This size corresponds to a mass of 120–230 metric tons, whereas the recovered mass of approximately 7 metric tons suggests that more than 90% of the mass was lost by ablation during atmospheric passage.
  • 2
     The average 14C/10Be ratio of 1.5 ± 0.1 in three NWA 869 samples yields a terrestrial age of 4.4 ± 0.7 kyr, consistent with the relatively low degree of weathering (W1).
  • 3
     Two NWA 869 meteorites show anomalously high 14C/10Be ratios in the bulk sample and high 36Cl/10Be ratios in the metal, which can be explained by a scenario in which a recent impact event on the NWA 869 meteoroid removed material up to approximately 80 cm deep, exposing previously heavily shielded samples to a much higher cosmic-ray flux during the past 20–120 kyr. This scenario is supported by the very low cosmogenic 3He/21Ne ratios in these two samples, indicating recent loss of cosmogenic 3He.
  • 4
     Radiogenic 4He concentrations in eight NWA 869 samples cluster around 1.0 × 10−5 cm3 STP g−1, whereas two NWA 869 samples show approximately 45% higher radiogenic 4He. Assuming average L-chondritic U and Th concentrations (Wasson and Kallemeyn 1988), these radiogenic 4He concentrations yield U,Th-He ages of 2.8 Gyr and 3.6 Gyr, respectively. Although the radiogenic 40Ar concentrations show considerable variations due to elevated K concentrations in a few samples, the majority of the samples yields K-Ar ages of 3.3–4.4 Gyr. The variable ages most likely represent mixtures of old clasts with ages of approximately 4.4 Gyr and young clasts with ages of 1.8–2.2 Gyr (Metzler et al. 2011).
  • 5
     Matrix and bulk samples of NWA 869 contain significant amounts of solar neon and argon, but are virtually free of solar He (solar 4He/20Ne approximately 7). The 4He/20Nesol ratios in NWA 869 are 1–2 orders of magnitude lower than in other solar gas–rich breccias, which indicates that trapped solar 4He has been lost—along with radiogenic 4He—during one or more impact events on the L-chondrite parent body 2–4 Gyr ago.
  • 6
     The low trapped 4He/20Ne ratios in NWA 869 are similar to ratios found in two other L-chondrite regolith breccias. These ratios indicate that large impacts on the L-chondrite parent body may have removed most of the solar He, but only a small fraction of solar Ne, and thus cannot explain the low abundance of regolith breccias among L chondrites. We thus favor the alternative explanation that the catastrophic disruption approximately 470 Myr ago produced many smaller asteroid fragments, which were not large enough or did not have enough time to develop significant regoliths.
  • 7
     Cosmogenic 21Ne concentrations range from 0.55 to 1.92 × 10−8 cm3 STP g−1 and show poor correlation with cosmogenic 10Be and 26Al. The combined 21Ne and 26Al concentrations in two of the five meteorite fragments indicate a 4π exposure age of approximately 5 Myr, consistent with the saturated 10Be and 26Al concentrations. Excess of 21Ne in some matrix and clast samples indicate minimum regolith exposure ages up to approximately 6 Myr, assuming 2π exposure at a depth of approximately 50 g cm−2 on the L-chondrite parent body. The cosmogenic nuclide data for the NWA 869 samples show no evidence that the large NWA 869 meteoroid was exposed as a large boulder on the surface of the L-chondrite parent body. In that respect, the large NWA 869 chondrite does not follow the trend of complex exposure histories found for other large chondrites, such as Jilin, Gold Basin, and JaH 073.
  • 8
     The heterogeneous distribution of cosmogenic and solar Ne opposed to the relative constancy of radiogenic 4He suggests that the regolith exposure occurred before the impacts that led to radiogenic He loss. The lack of correlation between cosmogenic 21Ne produced on the parent body versus trapped solar Ne suggests that the individual components of the NWA 869 breccia were exposed to solar wind and GCR during two different episodes of regolith exposure.

Acknowledgments–– This work was supported by NASA grants NNG06GF22G and NNX09AM75G and NSF grant EAR0929458. We thank Greg Herzog and Ingo Leya for constructive reviews and Tim Swindle for editorial comments, which improved the paper.

Editorial Handling–– Dr. Timothy Swindle


  1. Top of page
  2. Abstract
  3. Introduction
  4. Samples and analytical techniques
  5. Results and discussion
  6. Conclusions
  7. References
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