1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental
  5. Methods
  6. Results and Discussion
  7. Conclusions
  8. Acknowledgments
  9. References

Abstract– We have investigated the terrestrial ages, or residence times, of 78 meteorites (representing 73 discrete falls) recovered in Western Australia, and one from South Australia, using both 14C measurements and also 14C/10Be. The samples studied included two ureilites, one CK and one EL chondrite. We have included 10Be measurements from 30 meteorites, including some meteorites for which the 14C terrestrial age was previously determined. We find that the 14C/10Be terrestrial ages are more precise than 14C alone, as we can correct for shielding effects. In general, the two different age determinations age by 14C–10Be are precise to 0.5–1 ka and 14C alone within 1–2 ka. However, measurement of the 14C age alone gives good agreement with the 14C–10Be for most samples. The study of the terrestrial ages of meteorites gives us useful information concerning the storage and weathering of meteorites and the study of fall times and terrestrial age. We have compared the terrestrial ages to weathering, degree of oxidation (estimated from Mössbauer studies) and Δ17O. In this study, we found that weathering is not well correlated with terrestrial age for Nullarbor meteorites. However, there is a good correlation between degree of oxidation and Δ17O. The implications for the study of terrestrial ages and weathering from other desert environments will be discussed.


  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental
  5. Methods
  6. Results and Discussion
  7. Conclusions
  8. Acknowledgments
  9. References

Meteorites are recovered in significant quantities from many areas of the world, but are particularly easily found in desert and polar regions (Bevan 2006a). These are sometimes referred to as “hot” and “cold” deserts (e.g., Annexstad et al. 1986; Schultz et al. 1995, 2000; Chennaoui Aoudjehane and Jull 2006). Although the largest single source remains Antarctica (>50,000 individual specimens), there are increasingly large numbers of meteorites recovered from hot, arid, and semiarid regions. We shall refer to these as “desert meteorites.” This also means that the estimation of terrestrial ages of these meteorites has become increasingly important. Many researchers also now recognize the importance of terrestrial alteration on meteorite mineralogy and weathering (e.g., Al-Kathiri et al. 2005), as well as the more fundamental questions about infall rates of meteorites (Bland et al. 1996a, 1996b, 1998a, 1998b). The importance of desert meteorites has been highlighted by a number of workshops on this subject, held in Mainz (Annexstad et al. 1986), Nördlingen (Schultz et al. 1995), South Africa (Schultz et al. 2000) and Casablanca in 2006 (see Chennaoui Aoudjehane and Jull 2006).

Nullarbor Region Meteorites

The potential of the Nullarbor region for meteorite recovery was not fully realized until the late 1960s, when the Mundrabilla iron meteorite was recovered (see Bevan 2006b), even though previously two small irons had been recovered in 1911 by a railway surveyor (Simpson 1912). Bevan and Binns (1989a, 1989b) had noted the concentration of meteorites in the Nullarbor region of Australia and that this region was one of the first arid regions to be noted for its large collection of meteorites. Over 300 distinct meteorites have now been recovered from the Nullarbor Plain of Australia (Bevan and Binns 1989a, 1989b; Bevan et al. 1998; Bland et al. 2000) and many more remain to be characterized (Schultz et al. 2005). The Western Australian work was also strongly influenced by the observation of large concentrations of meteorites found in Roosevelt County, New Mexico, where a total of some 105 individuals have been recovered (e.g., Sipiera et al. 1987a, 1987b). In contrast to Roosevelt County, however, the Western Australian meteorites were found on stable desert surfaces, as opposed to Roosevelt County, where many meteorites had originally been discovered in blowouts of Late Quaternary cover-sands (Zolensky et al. 1990; Jull et al. 1991). Indeed, the first work on the terrestrial ages of Roosevelt County meteorites was critical to set the stage for the Western Australian study. In the first reported study of the terrestrial-age distribution of the Western Australian meteorites, Jull et al. (1995) discussed the 14C terrestrial ages of 22 meteorites from this region and this study was followed up by further studies by Schultz et al. (1995) and Bland et al. (2000).

Other Desert Meteorites

Since these initial discoveries, many meteorites have been recovered from North African deserts, which sparked an interested in the determination of their terrestrial residence time (Jull et al. 1990, 1995; Schlüter et al. 2002). Within the last decade, large numbers of meteorites have been recovered from the deserts of Oman and other regions of the Arabian Peninsula, and have been studied for their terrestrial age (Al-Kathiri et al. 2005; Gnos et al. 2009; Hofmann et al. 2009), although only a handful were known in 1995 (Franchi et al. 1995). In North America, Kring et al. (2001) reported the terrestrial age of the large strewnfield of the Gold Basin meteorite, where thousands of individual fragments have been recovered. Also, Verish et al. (2000) have discussed meteorites recovered from a dry lake-bed in California. In South America, the Atacama Desert has become an important collection area. Muñoz et al. (2007), Valenzuela et al. (2008) and Gattacceca et al. (2009) have also extended the study to meteorites collected from the Atacama Desert in Chile. The vast majority of meteorites recovered are ordinary chondrites; however, desert environments have also preserved many rare and valuable achondrites (e.g., Jull 2006).

Terrestrial-Age Determinations Using Different Methods and Chronometers

To determine the terrestrial age, some type of chronometer is needed and this is mostly easily provided by studying the remaining concentrations of cosmic-ray-produced radionuclides (Arnold et al. 1961). Originally, the use of 14C was shown by Suess and Wänke (1962) and also by Goel and Kohman (1962) to be feasible on large meteorite samples (10–100 g), using 14C β-decay counting. The first studies using accelerator mass spectrometry (AMS) for 14C terrestrial-age measurements demonstrated the possibilities of using much smaller sample sizes (0.1–0.7 g), which were begun by Brown et al. (1984) and Jull et al. (1984). Since the early 1980s, most radionuclide measurements have been studied using AMS, as opposed to the earlier methods (e.g., Jull et al. 1990, 1993, 1998; Nishiizumi 1995). The 14C literature concerning meteorites has been summarized in detail by Jull et al. (1998) as well as Jull (2001, 2006).

In general, 14C is the most useful for the terrestrial ages of stone meteorites from desert environments. Nishiizumi and Caffee (2001), as well as Nishiizumi et al. (2002) have observed some desert meteorites with terrestrial ages well beyond the limits of 14C, but these were achondrites which might be expected to survive longer, and their ages can be determined by 36Cl measurements. More recently, Welten et al. (2004) have found an ordinary chondrite in a desert environment with a terrestrial age of approximately 150 ka Dar al Gani (DaG) 343, and Levine et al. (2008) report on a terrestrial age of 410 ka for the eucrite Rio Cuarto 001, based on a combination of three nuclide measurements (26Al, 36Cl, and 41Ca). The scarcity of hot desert meteorites with terrestrial ages >40 kyr is in sharp contrast to the situation for Antarctic meteorites, where much longer terrestrial residence times of well over 100 ka can be observed from some locations (see Jull 2006).

In meteorites, the longer-lived nuclide 36Cl, with a half-life of 301 ka, was also first measured by decay counting (McCorkell et al. 1968; Chang and Wänke 1969). Chang and Wänke (1969) were the first to determine systematically terrestrial ages of iron meteorites using 36Cl and they also showed that some meteorites (particularly irons) could apparently survive for very long times on the surface of the Earth, the longest terrestrial age is that of iron meteorite Tamarugal (Chile) with a terrestrial age of approximately 2.7 Ma. This age remained the longest know terrestrial age, until two Antarctic iron meteorites, Inland Forts (ILD) 83500 and Lazarev, which have ages of 3.2 and 5 Myr, respectively (Nishiizumi et al. 1987). Several Antarctic ordinary chondrites, measured by Scherer et al. (1997a, 1997b) and Welten et al. (1997, 2006, 2008), showed terrestrial ages in excess of 2.0 Ma.

We would expect that the terrestrial age would be related to the weathering rates of meteorites and their eventual destruction. A direct relationship between weathering rates to the terrestrial survival times of meteorites was initially shown by Wlotzka et al. (1995) and later by Bland et al. (1996a, 1996b, 1998a, 1998b, 1998c). Gattacceca et al. (2009) assert a similar correlation for Chilean meteorites, although the weathering–age relationship is not always clear in other locations. In addition, weathering is expected to affect trace element and isotopic compositions of meteorites, this has been demonstrated in the recent work of Al-Kathiri et al. (2005), who showed the chemical exchange between the meteorite and the adjacent soil.

We have found that coupling 14C and 10Be measurements, in many cases, allows us to correct for shielding effects, as most ordinary chondrites have exposure ages >7–8 Ma (Jull 2006). Shielding effects can also be estimated from Ne isotopic ratios, but this also has limitations for very large or small meteorite falls (Welten et al. 2003; Schultz et al. 2005; Wieler and Graf 2001). The use of 14C in combination with 10Be has been used most effectively in large meteorite falls (e.g., Kring et al. 2001; Welten et al. 2004), but our initial studies indicate that it could also be applied to individual meteorite falls (Jull et al. 1994, 2005).

In this study, we explore the terrestrial ages of meteorites from the Nullarbor region of Australia, their distribution and examine correlations with weathering effects.


  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental
  5. Methods
  6. Results and Discussion
  7. Conclusions
  8. Acknowledgments
  9. References

14C Studies

Samples of meteorites were crushed and treated with 85% phosphoric acid to remove weathering carbonates. In some early measurements, the acid-cleaning step was not used; however, the pretreatment of different samples is summarized in Table 1, including comparison of results from different treatments. The samples were then loaded into a ceramic crucible, to which we added 4–5 g of iron chips, which are used as a combustion accelerator. The crucible and contents were heated to 500 °C in air to remove most other combustible contaminants. Jull et al. (1989, 1993) have shown that heating to 500 °C does not release significant cosmogenic 14C (see also Jull 2006). Hence, this gives us a good way to separate the higher-temperature cosmogenic component from any low-temperature contamination. The cleaned crucible and sample was then loaded into an RF induction furnace, the sample is heated to the Fe melting-point in a flow of oxygen. As the Fe fuses with the sample, all carbon in the sample is released during this process, as was demonstrated by Jull et al. (1989). We used a Pt/CuO furnace at 450 °C to ensure conversion of any CO to CO2. The CO2 gas is collected from the oxygen gas stream. We then measured the volume of the CO2 and convert this to graphite over Fe at approximately 600 °C. The 14C concentrations in the graphite sample were measured by AMS as discussed by Donahue et al. (1990). To confirm our reproducibility, we studied the 14C yield from repeat measurements of a sample of Bruderheim, an L6 chondrite, in order to verify the reproducibility of the 14C extraction method. This Fig. 1 verifies that the mean value of 50.4 dpm kg−1, with a standard deviation of 6.0 dpm kg−1 (12%) and an error in the mean value of ±1.0 dpm kg−1 is consistent with the assumed long-term average value of the previously reported production rate of 14C in Bruderheim of 51 dpm kg−1 (e.g., Jull et al. 1998).

Table 1. 14C and 10Be measurements on Nullarbor meteorites.
Meteorite nameClassReference (3)Weathering gradeOxidation (%)1Δ17O (‰)Acid Etch14C (dpm kg−1)10Be (dpm kg−1)T14T14/10
  1. Note: NHM = Natural History Museum Catalogue of Meteorites (Grady, 2000); MB = Meteoritical Bulletin (MB54: Clarke 1976; MB68: Graham 1990; MB72, 74, 77: Wlotzka 1992, 1993, 1994; MB80: Grossman 1996; MB97: Weisberg et al. 2010.

  2. a—Terrestrial-age data from Jull et al. 1995; b—terrestrial-age data from Bland et al. 2000.

  3. 1Oxidation determined by Mössbauer spectroscopy (Bland et al. 2002).

Billygoat DongaL6NHM 23.61.07No etch20.5 ± 0.3  20.0 ± 0.2 7.6 ± 1.3  7.7 ± 1.3 
Billygoat DongaL6NHM 23.6 Acid etch14.1 ± 0.720.2 ± 0.3410.4 ± 1.410.6 ± 0.9
Boorabie 001H4–5MB77W2–341.70.50No etch41.7 ± 0.315.39 ± 0.340.9 ± 1.3−0.7 ± 0.8
BurnabbieH5NHM 59.10.47No etch2.85 ± 0.2 23.1 ± 1.4 
Camel Donga 007L5MB77W222.3 Acid etch2.58 ± 0.36 24.7 ± 1.7 
Camel Donga 011L/LL6MB80W3  Acid etch2.70 ± 0.3421.2 ± 0.524.3 ± 1.724.7 ± 1.3
Camel Donga 041L5MB97W17.61.16Acid etch20.42 ± 0.2515.1 ± 0.27.6 ± 1.35.1 ± 0.8
Camel Donga 042H3.8MB97W2–339.6 Acid etch1.24 ± 0.4910.9 ± 0.229.9 ± 3.325.7 ± 3.4
Camel Donga 043L/LL6MB97W437.5 Acid etch1.66 ± 0.30 28.3 ± 2.0 
Camel Donga 045H4MB97W233.4 Acid etch2.15 ± 0.3416.13 ± 0.2625.4 ± 1.324.3 ± 1.5
Camel Donga 047H4–5MB97W446.40.46Acid etch15.38 ± 0.28 9.1 ± 1.3 
Camel Donga 050L/LL6MB97W440.1 Acid etch1.24 ± 0.34 30.7 ± 0.26 
Camel Donga 052H3.1–3.2MB97W357.70.49Acid etch0.93 ± 0.25 32.3 ± 2.6 
Carlisle Lakes 002H4/5MB77W2–338.80.53No etch32.9 ± 0.315.96 ± 0.342.8 ± 1.31.6 ± 0.8
CocklebiddyH5NHMW116.20.93No etch36.9 ± 0.1 1.9 ± 1.3 
Colville Lake 001H5MB77W2  No etch21.7 ± 0.9 7.1 ± 1.3  
Colville Lake 001H5    Acid etch22.75 ± 0.28 6.7 ± 1.3 
CoonanaH4NHM   Acid etch0.86 ± 0.10 33.0 ± 1.6 
Deakin 001 (a)LL3 (anom)MB68W337.9 No etch1.9 ± 0.1 27.1 ± 1.4  
Deakin 001LL3 (anom) W337.9 Acid etch2.4 ± 0.419.44 ± 0.4525.3 ± 1.925.0 ± 1.6
Dingo Pup DongaUreiliteMB68   Acid etch3.2 ± 1.0 23.7 ± 2.9 
Forrest 007 (a)H4MB68W244.6 No etch30.9 ± 0.314.83 ± 0.243.4 ± 1.31.5 ± 0.8
Forrest 010 (a)L4–5MB68W123.41.11No etch5.8 ± 0.1 17.9 ± 1.3 
Forrest 010 (a)L4–5 W123.41.11Acid etch1.4 ± 0.3   3.5 ± 0.911.57 ± 0.2229.5 ± 2.2 22.1 ± 2.411.8 ± 1.9
Forrest 016H5/6MB77W3  Acid etch0.34 ± 0.18 41.4 ± 4.5 
Field no. 249/92 Forrest 021L6 W234.5 Acid etch7.72 ± 0.21 15.6 ± 1.3 
Field no. 291/92 Forrest 021L6MB77W267.5 Acid etch0.71 ± 0.19 35.4 ± 2.5 
Forrest 026H5/6MB77W3  Acid etch1.01 ± 0.22 32.4 ± 2.2 
Forrest 027L5/6MB77W3  Acid etch0.56 ± 0.19 37.4 ± 3.1 
Forrest 028L7MB77W3  Acid etch0.17 ± 0.30 47 
Kybo 001 (a)LL5MB77W26.91.08No etch42.5 ± 0.321.24 ± 0.342.2 ± 1.31.9 ± 0.8
Loongana 002H6MB77W3  Acid etch1.1 ± 0.2 31.7 ± 2.1 
Lynch 001L5/6MB77W2  Acid etch22.8 ± 0.6 6.7 ± 1.3 
Mulga (north) (a)H6NHM 25.40.66No etch33.4 ± 0.2  34.4 ± 1.017.2 ± 0.82.7 ± 1.3  2.5 ± 1.32.1 ± 0.9
      Acid etch38.3 ± 0.5 1.6 ± 1.31.0 ± 0.9
Mulga (south) (a)H4NHM 31.30.82No etch4.1 ± 0.2 20.0 ± 1.3 
Mundrabilla 002 (a)H5MB68W341.30.60No etch2.46 ± 0.09 24.3 ± 1.3 
Mundrabilla 005 (a)H5MB68W237.70.53No etch52.2 ± 0.613.72 ± 0.60Recent fallRecent fall (excess 14/10)
Mundrabilla 015H5MB77W2  Acid etch19.97 ± 0.21 7.8 ± 1.3 
Mundrabilla 017H5MB77W2  Acid etch1.19 ± 0.48 31.1 ± 3.6 
North Forrest (a)H5MB54W345.30.57No etch10.2 ± 0.916.4 ± 0.212.0 ± 1.311.5 ± 1.1
North Forrest (a)H5 W345.30.57Acid etch11.9 ± 0.3 11.2 ± 1.3 
North HaigUreiliteNHM   Acid etch10.49 ± 1.00 13.8 ± 1.5 
North West Forrest (1)EL6MB54W3  No etch31.1 ± 0.3 1.7 ± 1.3 
Nullarbor 002 (b)H6MB77W2  Acid etch17.6 ± 0.2 8.0 ± 1.3 
Nullarbor 011 (b)L6MB77W222.8 Acid etch39.4 ± 0.4 2.1 ± 1.3 
Nullarbor 012 (b)L6MB77W139.3 Acid etch1.2 ± 0.2 31.2 ± 1.8 
Nullarbor 013 (b)L6MB77W227.90.95Acid etch3.55 ± 0.26 22.1 ± 1.4 
Nullarbor 015 (b)L6MB77W212.21.18Acid etch1.1 ± 0.14 31.8 ± 1.7 
Nullarbor 016H4/5MB77W248.4 Acid etch7.90 ± 0.23 14.6 ± 1.3 
Nullarbor 017L6MB77W237.8 Acid etch0.96 ± 0.26 32.9 ± 2.6 
Gunnadorah 011L/LL6 W334.51.00Acid etch2.42 ± 0.26 25.2 ± 1.6 
Nurina 002 (b)H5MB77W231.20.46No etch46.9 ± 0.2 Recent fall 
Nurina 003 (b)LL5MB77W229.2 Acid etch1.75 ± 0.10 28.5 ± 1.4 
Nurina 004 (b)L6MB77W340.4 Acid etch0.9 ± 0.2 33.4 ± 2.3 
Nurina 006L6MB97W117.81.04Acid etch8.47 ± 0.28 14.9 ± 1.3 
Nurina 007H3MB97W338.6 Acid etch0.46 ± 0.19 38.1 ± 3.6 
Nurina 008H4MB97W456.5 Acid etch1.05 ± 0.30 31.3 ± 2.7 
Nurina 009L4MB97W235.8 Acid etch16.39 ± 0.23 9.4 ± 1.3 
Nyanga Lake 001 (a)H3MB74W356.20.49No etch21.1 ± 0.2 6.5 ± 1.3 
Nyanga Lake 003H5MB77W3  No etch44.6 ± 0.214.0 ± 0.20.3 ± 1.3Recent fall (high 14/10)
OakL5MB54   Acid etch3.24 ± 0.05 22.8 ± 1.3 
Old Homestead 002 (b)L5/6MB77W112.31.05Acid etch6.5 ± 0.3 16.3 ± 1.4 
Reid 006 (a)H5MB68W340.20.54No etch33.8 ± 0.518.5 ± 0.82.6 ± 1.32.6 ± 0.9
      No etch32.9 ± 0.2 2.8 ± 1.3 
      Acid etch36.9 ± 0.5 1.9 ± 1.3 
Reid 007 (a)L6MB68W225.01.04No etch23.8 ± 0.214.7 ± 0.36.3 ± 1.33.6 ± 0.8
Reid 010 (a)H6MB68W128.00.65No etch39.3 ± 0.314.7 ± 0.31.4 ± 1.3−0.6 ± 0.8
Reid 011 (a)H3–6MB68W233.90.61No etch39.5 ± 0.316.6 ± 0.31.3 ± 1.30.4 ± 0.8
Sleeper Camp 007L4MB77W3  Acid etch5.00 ± 0.2417.4 ± 0.319.2 ± 1.418.0 ± 0.9
Sleeper Camp 009L6MB77W3  Acid etch1.70 ± 0.2017.0 ± 0.328.0 ± 1.326.7 ± 1.3
      Acid etch1.70 ± 0.20   
      Acid etch1.60 ± 0.20   
Sleeper Camp 009L6MB97W399.0 Acid etch1.69 ± 0.19 28.2 ± 1.6 
Sleeper Camp 009L6 W330.90.94Acid etch1.71 ± 0.2417.5 ± 0.328.1 ± 1.726.9 ± 1.4
Sleeper Camp 009L6 W334.7 Acid etch1.58 ± 0.21 28.7 ± 1.7 
Sleeper Camp 011L6MB77W2  Acid etch0.53 ± 0.3017.4 ± 0.437.8 ± 4.936.6 ± 4.7
Sleeper Camp 013LL5–7MB80W2–3  Acid etch5.25 ± 0.2718.3 ± 0.719.5 ± 1.418.0 ± 1.0
Sleeper Camp 018H6MB97W567.20.49Acid etch1.19 ± 0.6416.1 ± 0.430.3 ± 4.429.2 ± 4.5
Sleeper Camp 019CK4–5MB97W146.7 Acid etch5.16 ± 0.558.65 ± 0.2218.9 ± 1.711.9 ± 1.2
Sleeper Camp 020LL3.9–4MB97W351.8 Acid etch1.05 ± 0.4718.0 ± 0.332.8 ± 3.931.2 ± 3.7
Sleeper Camp 021LL5MB97W439.6 Acid etch0.11 ± 0.3118.0 ± 0.4>40>49
Sleeper Camp 022L5MB97W440.2 Acid etch6.14 ± 0.18 17.5 ± 1.3 
Sleeper Camp 023H4MB97W239.1 Acid etch15.91 ± 0.2310.3 ± 0.78.8 ± 1.34.0 ± 1.0
Sleeper Camp 009L6MB97W3  Acid etch1.69 ± 0.19 28.2 ± 1.6 
Sleeper Camp 024L5MB97W17.61.09Acid etch7.83 ± 0.20 15.5 ± 1.3 
Thylacine Hole 001 (b)H4/5MB77W3  No etch0.82 ± 0.66 >28.5 
Virginia 001 (b)L6MB77W24.0 Acid etch6.55 ± 0.26 17.0 ± 1.3 

Figure 1.  Map of the Nullarbor region of South and Western Australia, showing meteorite recovery locations.

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10Be Measurements

We selected 30 meteorites randomly from the larger group for 10Be measurements.

Samples of approximately 0.1 g meteorite were dissolved in HF–HNO3, and a carrier of 0.3 mg Be as Be(OH)2 was added. The Be was separated by a combination of acetyl–acetone extraction and ion chromatography (see McHargue et al. 1995; Kring et al. 2001). 10Be measurements were normalized to the currently accepted half-life of 1.36 Ma (Nishiizumi et al. 2007) and we used the NIST standard with the reported value of 2.68 × 10−11 10Be/9Be.

Stable-Isotope Measurements

The oxygen stable-isotopic composition of some of the meteorites was determined at the Open University using standard techniques for oxygen isotopes. Isotope ratio measurements are given using the standard δ notation, so that

  • image

and similarly for δ17O using the 17O/16O ratio. Values are expressed as per mil (‰) deviation from the international reference standard VSMOW (Vienna-Standard Mean Ocean Water). On an oxygen three-isotope diagram, in which δ18O is plotted against δ17O, samples related to each other by mass-dependent fractionation processes define linear arrays, with a slope of approximately 0.52 (Clayton 1993; Clayton and Mayeda 1996). Thus, terrestrial silicate rocks form an array that is generally referred to as the Terrestrial Fractionation Line (TFL) (Clayton and Mayeda 1996; Rumble et al. 2007). In meteorite studies, the TFL is a useful reference for assessing the degree to which extraterrestrial material departs from the isotopic composition of the Earth’s oxygen reservoir (Clayton and Mayeda 1996; Rumble et al. 2007). The term Δ17O is used to quantify the extent to which a sample deviates from the TFL and is defined as Δ17O = δ17O − 0.52δ18O (Clayton and Mayeda 1996). In the context of this article, we use Δ17O as an indication of the degree of terrestrial alteration of the meteoritic oxygen-isotopic composition, similar to the approach of Tyra et al. (2007). Typical errors in Δ17O are ±0.05‰.

57Fe Mössbauer Spectroscopy Measurements

Samples were taken from the outer portion of meteorite stones, but interior to any surviving fusion crust. This material (∼2 g) was crushed, and approximately 0.3 g taken for analysis and sandwiched between adhesive tape (∼1.25 cm2) in a lead holder. 57Fe Mössbauer spectra were recorded at 298 K with a microprocessor controlled Mössbauer spectrometer using a 57Co/Rh source. Drive velocity was calibrated with the same source and a metallic iron foil. The Mössbauer spectra were fitted with a constrained nonlinear least squares fitting program of Lorentzian functions.

In a study of the limitations of Mössbauer spectral fitting, Dollase (1975) found that for spectra with partially overlapped peaks parameter uncertainty partly depends on component peak separation. The analysis of complex geological materials by 57Fe Mössbauer spectroscopy, therefore, involves a compromise between considering enough components to enable a fit, and not exceeding an amount beyond which the parameters of individual components become meaningless. Our study concentrates on determining the proportion of Fe3+ in meteorite samples, and whether this Fe3+ is bound in paramagnetic or magnetically ordered species: thus, deriving well-constrained Mössbauer parameters for individual components is not vital.


  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental
  5. Methods
  6. Results and Discussion
  7. Conclusions
  8. Acknowledgments
  9. References

Age Calculation and Production Rates

With measurement of more than one radionuclide, we can correct for shielding effects (Kring et al. 2001; Welten et al. (2001, 2004)). At the Arizona laboratory, we make measurements of 14C and 14C/10Be to estimate the terrestrial age. Jull et al. (1994) compared the values of 14C with 10Be from Knyahinya, showed that in the case of Knyahinya, that 14C and 10Be tracked well and the production ratio remained relatively constant from different depths in the Knyahinya, which had a preatmospheric radius approximately 45 cm. These measurements gave 14C saturation values of 37 dpm kg−1 at the surface to 58 dpm kg−1 at the center of the meteorite (Jull et al. 1994).

Wieler et al. (1996) have discussed the variation in production rates for 10Be and 14C at different depths in meteorites of different sizes. Recent falls generally show activities of 14C equivalent in a range of production rates from about 38–58 atoms min−1 kg−1, depending on classification, with an average value of 51 ± 1 dpm kg−1 for L chondrites (see Jull 2001, 2006). Wieler et al. (1996) showed that in H chondrites with preatmospheric radii from 20 to 45 cm, the saturated activity (or production rate) varied from 38 to 52 dpm kg−1. We expect that smaller objects have lower production rates of 14C. In ordinary chondrites and achondrites, nearly all 14C is produced from spallation of oxygen, only about 3% produced from Si (Sisterson et al. 1994). Hence, we estimate the saturated activity for a given class of meteorite by normalizing the mean value of the 14C content of Bruderheim (51 ± 1 dpm kg−1) to the oxygen content of the meteorite determined from bulk chemistry or from average compositions (see Jull et al. 1998).

The terrestrial age is then calculated from the equation:

  • image(1)

where λ14 is the decay constant for 14C of 1.21 × 10−4, Nm is the measured amount of 14C (in activity, dpm kg−1; or atoms g−1) and Nsat is the saturated activity (Jull et al. 1989; Jull 2006).

We previously also investigated 14C/10Be dating on the assumption that this production ratio should be reasonably constant at approximately 2.5–2.6 (Kring et al. 2001; Welten et al. 2003, 2004). As 10Be and 14C are produced by similar nuclear spallation reactions, we expect that the production ratio of these two nuclides is relatively constant. In this case, we can calculate the terrestrial age from the ratio of 14C/10Be, according to the following equation. The terrestrial age is then calculated from the equation:

  • image(2)

Previously, we based this age calculation on a fixed 14C/10Besat ratio of 2.5 ± 0.1 (Kring et al. 2001), although Welten et al. (2001) used a ratio of 2.65 ± 0.20.

Infall Rates

The infall rate of meteorites as a function of mass and time is known (Halliday et al. 1989) and we can therefore write the equation:

  • image(3)

where I is the infall rate in meteorites per area per year and W is the weathering rate of the meteorites as a function of time (in years). We can assume that a surface which continuously accumulates meteorites would eventually reach saturation in terms of the number of meteorites, i.e., that dN/dt = 0, as discussed by Bland et al. (1996a, 1996b).

In that case, the distribution of meteorites as a function of age will depend on

  • image(4)

where Na is the number of meteorites remaining in interval dt, a is the infall rate during dt and b is a weathering rate constant (Jull et al. 1998; Bland 2001).

Results and Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental
  5. Methods
  6. Results and Discussion
  7. Conclusions
  8. Acknowledgments
  9. References

In Table 1, we present the results of 14C studies of 77 individual meteorites from Western Australia, representing 72 discrete falls, and in addition, one meteorite from South Australia. We have previously reported some 14C measurements on 28 of these meteorites (Jull et al. 1995; Bland 2001). Further measurements on some of these meteorites are presented and for 30 samples, we have now also determined 10Be to assess any corrections to the terrestrial age. We then present the 14C terrestrial age (calculated from 14C alone and assumed saturated activities, as discussed above) and also, where possible, we quote the 14C/10Be terrestrial age. The ages are calculated according to Equations (1) and (2), respectively. Table 1 also indicates the weathering grade (Wlotzka et al. 1995), oxidation determined by Mössbauer spectroscopy (Bland et al. 2002) and Δ17O. We will discuss these other observations later.

In Fig. 2, we have plotted the 14C–10Be ages against the 14C ages for samples where both sets of data are available, and good agreement is observed. There are only three samples which are the exception to this observation. For Forrest 010 (L4-5), the discrepancy is due to a low exposure age of 4.7 Ma (Schultz et al. 2005) the values observed are below radioactive saturation for 10Be. For Sleeper Camp 019 (CK5), we note that Scherer et al. (1997a, 1997b) determined an exposure age of 18.1 Ma for a possibly related CK4 (Sleeper Camp 006), so we attribute the difference in the terrestrial-age calculation to high shielding and a different 14C/10Be for CK chondrites than for those plotted in Fig. 4. Two other meteorites also show differences in the 14C versus 14C/10Be age and low 10Be concentration, suggesting high shielding, Sleeper Camp 023 and Camel Donga 042. For a fifth sample, we have plotted the limit ages of Sleeper Camp 021 (LL5), so this is only due to the difference in maximum age that can be determined for this meteorite. Another anomaly is demonstrated by the two samples which have been named as Forrest 021. These are two different field samples, with field numbers 249/92 and 291/92 that were originally paired and we believe these results indicate from the cosmogenic nuclide data and also the difference in degree of oxidation, that there may be two different meteorites represented—however, further work needs to be carried out to verify this result.


Figure 2.  Comparison of 14C–10Be ages against 14C terrestrial ages. Two values shown plotting below the line are due to low 10Be exposure ages of two meteorites (Forrest 010 and Sleeper Camp 019), and one plots above (Sleeper Camp 021).

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Figure 4.  Calculated 14C/10Be ratios for different values of 10Be, as a function of size of the irradiated meteoroid in space and depth in the object.

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In Fig. 3, we show the distribution of terrestrial ages from Table 1, where the 14C–10Be result was available, we have preferred this value to the 14C age alone, except in the case of for Forrest 010 and Sleeper Camp 019, due to low 10Be exposure ages. In the other meteorites, 14C/10Be should correct for shielding effects. In this case, we assume the ratio of 2.5 ± 0.25, based on our observations in Fig. 4 for different values of 14C/10Be as a function of total 10Be. Although other values have been used in the literature (e.g., Welten et al. 2001). In an earlier study of the Knyahinya (L5) chondrite, the observed 14C/10Be ratio was closer to 2.1. These differences can be understood by some variability in the production ratio of 14C/10Be. Preliminary calculations by Jull et al. (2005) suggest that the 14C/10Be ratio can vary from about 2 to 3. In Fig. 4, we show the calculated 14C/10Be versus 10Be for different-sized objects irradiated in space. Our calculations indicate that most meteorites should fall within the 14C/10Be ratio from 2 to 3. We should note that the errors quoted in Table 1 include a 10% error in the 14C/10Be ratio, higher than in the earlier publications. However, very small objects likely have lower 14C/10Be and this warrants further investigation.


Figure 3.  Distribution of terrestrial ages based on 14C or 14C–10Be measurements for Nullarbor meteorites. The distribution of L chondrites is shown in brown, H chondrites in orange and others (EL, CK, and ureilites) in yellow-green.

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The terrestrial-age distribution for the Western Australia meteorites generally shows an expected trend from a simple first-order exponential “decay” process (Equation 3 and Bland 2001), where infall of meteorites is offset by the removal of meteorites by weathering. The distribution can be compared to the Omani meteorite locations (Al-Kathiri et al. 2005), where similar processes appear to operate. However, an interesting effect observed by Al-Kathiri et al. (2005) is a deficit of meteorites with young terrestrial ages. The Nullarbor meteorite age distribution does not show such an effect. There is a deficiency of meteorites in the range of 10–15 ka, which was first suggested by Bland et al. (2000) to be due to climatic changes, perhaps due to a different weathering rate. Bland et al. (2000) noted a period of maximum aridity from 18 to 15 ka, and palynological records indicate more arid conditions from 10 to 20 ka (Martin 1973). We suggest therefore that meteorites which fell at this arid time were more susceptible to weathering in the following wetter conditions, after 10 ka. Another possibility is that an excess number of meteorites between 20 and 35 ka is due to unidentified pairings between meteorites. However, in the case of the Nullarbor meteorites, we believe this is unlikely, as all paired meteorites have already been identified by microscopy and oxygen-isotope studies. If we estimate a “weathering half-life” for the distribution shown, we obtain an apparent weathering half-life of about 25 ka, similar to the previous estimate of Bland et al. (1998a). We show this simple age–decay relationship as a line in Fig. 3.

We can also compare the terrestrial ages determined to the degree of Fe oxidation (Bland et al. 2002) and Δ17O (Clayton and Mayeda 1996). In contrast to the meteorites collected from Roosevelt County (Bland et al. 1996a, 1996b), Acfer (Wlotzka et al. 1995), and Oman (Al-Kathiri et al. 2005), but similar to observations of Bland et al. (2000) for Australian and Antarctic meteorites, we do not observe any correlation between degree of weathering, either in weathering grade (W1–W5 scale) or degree of oxidation. However, a good correlation is observed between oxidation and Δ17O, indicating that these two parameters do well represent the degree of weathering of the meteorite (Fig. 5). This is to be expected, as weathering of Fe in meteorites would incorporate terrestrial oxygen isotopes compositions, and increased terrestrial alteration would push values down on Fig. 5, toward 0‰Δ17O. What is striking therefore about the Nullarbor results is that degree of weathering is not correlated with terrestrial age in contrast to the other desert meteorite recovery areas from North Africa, Roosevelt County, and Oman.


Figure 5.  Comparison of degree of Fe oxidation from Mössbauer spectroscopy (Bland et al. 2002) and Δ17O. Black circles are H chondrites and open squares are L chondrites. L chondrites show generally less oxidation and high values of Δ17O.

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  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental
  5. Methods
  6. Results and Discussion
  7. Conclusions
  8. Acknowledgments
  9. References

We have shown that we can determine the terrestrial ages of Nullarbor meteorites and that the distribution of meteorites as a function of age correlates with the expected dependence. The degree of weathering of the meteorites, whether measured by weathering grade, degree of oxidation or Δ17O is not well correlated to their terrestrial age, in contrast to some other collections (e.g., Wlotzka et al. 1995; Al-Kathiri et al. 2005). The distribution of terrestrial ages of Nullarbor meteorites is a useful database for comparison to the growing collections of meteorites from other desert environments, such as North Africa, the Arabian Peninsula, and North and South American desert regions.


  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental
  5. Methods
  6. Results and Discussion
  7. Conclusions
  8. Acknowledgments
  9. References

Acknowledgments— We would particularly like to thank George Burr, Lori Hewitt, Alex Leonard, Dana Biddulph, and Richard Cruz from the AMS laboratory team for technical assistance. We also thank K. C. Welten and M. E. Zolensky for helpful reviews. This work was funded in part by NASA grants NAG5-11979, NNG06GC23G, NNX09AM57G, and NSF grant EAR06-22305.

Editorial Handling— Dr. Marc Caffee


  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental
  5. Methods
  6. Results and Discussion
  7. Conclusions
  8. Acknowledgments
  9. References
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