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Neon-E in CM-2 chondrite LEW90500 and collisional history of CM-2 chondrites, Maralinga, and other CK chondrites
Geochimica et Cosmochimica Acta, Vol. 62, No. 14, pp. 2573–2582, 1998 Copyright © 1998 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/98 $19.00 1 .00
Pergamon
PII S0016-7037(98)00168-9
Neon-E in CM-2 chondrite LEW90500 and collisional history of CM-2 chondrites, Maralinga, and other CK chondrites O. EUGSTER, P. EBERHARDT, CH. THALMANN, and A. WEIGEL Physikalisches Institut, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland (Received December 11, 1997; accepted in revised form April 20, 1998)
Abstract—We present He, Ne, and Ar data on stepwise heating experiments for the CM-2 chondrite LEW90500 and on noble gas isotopic abundances of the anomalous CK chondrite Maralinga. In LEW90500 we observe at least 0.20 3 1028 cm3STP/g Ne-E probably originating from the decay of presolar 22Na. Planetary He is characterized by 4He/3He 5 6500. For planetary-type trapped noble gases we obtain 20 Netr 5 13.4 3 1028 cm3STP/g and (36Ar/20Ne)tr 5 5.6. The cosmic-ray exposure age, based on 21Nec is 0.24 Ma. We review the literature data of the other CM-2 chondrites and find a Ne-E component in most of them; the largest concentration is observed in Mighei (0.53 3 1028 cm3STP/g). The exposure age distribution of the CM-2 chondrites confirms previous studies that report generally young ages (,6.5 Ma). A cluster of four meteorites is observed around 0.28 Ma. Maralinga (anomalous CK-4) contains no solar gases and relatively low amounts of planetary trapped gases. The cosmic-ray exposure age is 6.1 Ma. This is the lowest age of the six known CK or CK-like chondrites. Three of them lie in the range of 38 – 45 Ma. Copyright © 1998 Elsevier Science Ltd Alais and the CM-2 chondrites Cold Bokkeveld and Nogoya. Herzog and Anders (1974), Eberhardt (1978), Eberhardt et al. (1981), and Jungck and Eberhardt (1985) characterized the Ne-E isotopic composition for Orgueil mineral separates. Evidence for essentially pure 22Ne was reported for another CM-2 chondrite, Murchison (Meier et al., 1980). So far, Ne-E was detected in only one noncarbonaceous chondrite, the unequilibrated ordinary H-3,4 chondrite Dimmitt (Niederer and Eberhardt, 1977). Black (1972) suggested that Ne-E(H) is not radiogenic 22Ne from 22Na decay but primary, nucleosynthetic Ne from AGB-star He shells. For the origin of Ne-E(L) a major contribution from 22Na is their preferred explanation.
1. INTRODUCTION
In the framework of our study of recently found meteorites we investigated two carbonaceous chondrites that are remarkable in certain respects. LEW90500 from the Lewis Cliff area in Antarctica was first tentatively classified as a C-1 chondrite (AMNL, 1991) but reclassified as CM-2 (AMNL, 1992). It’s recovered weight is 294.7 g. The main purpose of our study was the investigation of the Ne-E component that is present in many CM-2 chondrites. The second meteorite is Maralinga (3.38 kg), found 1974 in the Nullarbor Plain, Australia, but recognized as meteorite in 1989. It is an anomalous CK-4 chondrite. Our aim was to derive it’s cosmic-ray exposure history. In the following we briefly summarize the relevant characteristics of the two investigated meteorites.
1.2. Maralinga 1.1. Lewis Cliff 90500 and Ne-E Member of the CK group of the carbonaceous chondrites are Karoonda, Maralinga, Ningqiang, and several small antarctic finds (Kallemeyn et al., 1991). Arguments for a genetic relationship of these carbonaceous chondrites were presented by Geiger and Spettel (1990), Kallemeyn et al. (1990), Geiger and Bischoff (1990), Kallemeyn et al. (1991), and Keller et al. (1992). The CK meteorites are metamorphosed and show petrographic grades from 4 to 6. Noble gases were studied in Karoonda (cf. Mazor et al., 1970) Y-69003 (Shima et al., 1973), ALH82135 and PCA82500 (Wieler et al., 1985), and Ningqiang (Eugster et al., 1988). The exceptionally high cosmic-ray exposure ages calculated from these data of about 20 –50 Ma are evidence for a common history of the CK chondrites. In this paper we report the results of stepwise heating experiments of LEW90500 and of bulk measurements of LEW90500 and Maralinga. Preliminary data regarding the cosmic-ray exposure history of these meteorites were presented in abstracts (Eugster and Weigel, 1992; Polnau et al., 1996).
There exist only few studies of this meteorite: Hanowski and Brearley (1996) carried out a detailed chondrule analysis and Nishiizumi et al. (1993) found relatively low radionuclide activities indicating a cosmic-ray exposure age of ,1 Ma. Low concentrations of cosmogenic Ne are favorable for deriving the Ne-E component. Ne-E is essentially pure 22Ne; it may be the in situ decay product of 22Na (half-life 2.6 years) that was produced in red giants, novae, or supernovae and was ejected in matter under conditions where solid grains can condense within a few months, long before 22Na has decayed (Black and Pepin, 1969; Black, 1972; Eberhardt et al., 1981). Essentially two carriers were identified: Ne-E (L), probably a carbonaceous phase, density , 2.3 g/cm3, releasing Ne-E at 500 to 700°C with 20Ne/22Ne , 0.01 and 21Ne/22Ne , 0.001. Ne-E (H), probably spinel and SiC, density 3–3.5 g/cm3, degassing at 1200 –1400°C with 20Ne/22Ne , 0.2 and 21Ne/22Ne , 0.003 (Jungck and Eberhardt, 1979). Ne-E was first observed by Black and Pepin (1969) in the Renazzo CR-2 chondrite and then by Black (1972) in the CI chondrites Ivuna, Orgueil, and 2573
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2. EXPERIMENTAL TECHNIQUE AND RESULTS
Sample LEW90500,22 was obtained from the NSF/NASA Meteorite Working Group. We prepared three bulk samples by crushing the material in a stainless steel mortar to a grain size of ,750 mm. A 5.11 mg sample (bulk 1) was used for a total gas extraction at 1600°C with a 1740°C step for the control of the completeness of gas release. A 11.01 mg sample (bulk 2) was used for a first stepwise heating experiment up to 1740°C in five steps. A 104.93 mg sample (bulk 3) was heated in twenty steps covering the range from 500°C to 1740°C. The samples were kept at the corresponding extraction temperature for 35 min. A 20.31 mg sample of the Maralinga CK chondrite was crushed to ,750 mm and analyzed in two fractions, first He and Ne and then Ar. Extractions and analyses of the noble gases were performed using our mass spectrometer system B. For details of system, procedure, background, and blank corrections see Eugster et al. (1993). The analytical results are given in Table 1–3. All errors correspond to a 95% confidence level (2s errors). 2. NOBLE GAS COMPONENTS IN LEW90500 2.1. Helium
Helium is a mixture of trapped (tr), radiogenic (r), and cosmogenic (c) gases. The temperature release pattern of the 4 He/3He ratios in bulk 3 (Fig. 1) shows a distinct plateau at an average value of 6500. This ratio is close to that found for trapped He in the main silicate phase of Orgueil: Eberhardt (1978) found (4He/3He)tr 5 6850. The total amount of trapped 4 He can be estimated with the following assumptions: an upper limit for 4Her of 810 3 1028 cm3STP/g is calculated adopting a 4He gas retention age of 2670 Ma (K-Ar age, see below), 11 ppb U, and 40 ppb Th (Wasson and Kallemeyn, 1988). The average of total 3He for the three bulk samples is 0.72 3 1028 cm3STP/g. Thus, adopting (4He/3He)c 5 5.2 (Heymann, 1967), cosmogenic 4He is #4 3 1028 cm3STP/g. Adopting (4He/ 3 He)tr 5 6500 in LEW90500, as found in the temperature release experiment leaves an average amount of 2790 3 1028
cm3STP/g 4Hetr and 0.29 3 1028 cm3STP/g 3Hec for the three bulk samples. The degassing patterns for 4He (Fig. 2) and 3He (Fig. 3) are similar but a difference in the quantity of gas released at #600°C is observed: whereas in this range 11% of 4 He is released we obtain 24% of total 3He. The release of cosmogenic 21Ne (Fig. 4) clearly indicates that a large fraction of 21Nec (58%) is released at #600°C. We conclude that the enhanced 3He release relative to that of 4He in this range is due to cosmogenic He. 2.2. Neon
Figure 5 shows the three isotope diagram of the Ne temperature steps for bulk 3. As already demonstrated in Fig. 4 21Ne in the 500°C fraction is dominated by spallation Ne. Between 700°C and 1000°C Ne-A (Ne-A1: 20Ne/22Ne 5 8.86 6 0.09, Ne-A2: 20Ne/22Ne 5 8.46 6 0.12, Tang and Anders, 1988b) and between 1100°C and 1200°C Ne-E (20Ne/22Ne # 0.15) are released. Inspection of the temperature release data for bulk 2 yields essentially the same pattern but with less extreme values because the temperature resolution is not as fine as that for bulk 3. The release pattern for the different Ne components for LEW90500 is very similar to that of bulk Orgueil (Black, 1972) and of the Ne-E rich phase G4j of Orgueil (Eberhardt, 1978). In order to derive the isotopic composition of trapped Ne and the absolute amounts of 20Netr, 21Nec, and 22Ne-E the following procedure was applied: In a 20Ne/22Ne vs. 21Ne/22Ne plot the data point for the total bulk meteorite lies on a mixing line with trapped Ne and cosmogenic Ne as endmembers. In Fig. 6 the average neon isotope ratios for the three bulk samples are used for plotting the data point (1) for LEW90500. The other data shown represent CM-2 chondrites that will be discussed later. The cosmogenic endpoint for carbonaceous chondrites is defined by (21Ne/22Ne)c 5 0.91 and (20Ne/22Ne)c 5 0.82 (average value adopted by Black and Pepin, 1969 and Mazor et al., 1970). The projection to the trapped Ne composition with (20Ne/22Ne)tr 5 7.67 indicates that it is a mixture of trapped Ne (consisting of Ne-A, SEP-Ne, and/or solar wind Ne) and Ne-E.
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The following 20Ne/22Ne and 21Ne/22Ne ratios for the known reservoirs in the solar system were adopted and are shown in Fig. 6. Solar wind Ne: 13.8 6 0.1 and 0.0328 6 0.0005, respectively (Benkert et al., 1993). Solar energetic particles (SEP) Ne: 11.2 6 0.2 and 0.0295 6 0.0005, respectively (Benkert et al., 1993). Ne-A2: 8.46 6 0.12 and 0.0348 6 0.0012, respectively (Tang and Anders, 1988b). Ne-E: #0.15 and #0.0022 (Eberhardt et al., 1981). Concluding that Ne in LEW90500 is composed of trapped Ne, Ne-E, and cosmogenic Ne we can derive the amounts of these components in each temperature step. Figs. 4, 7, and 8 show the release pattern of 21Nec, 20Ne-A, and 22Ne-E, respectively. As discussed before 21Nec is mainly released at 500°C
Fig. 1. 4He/3He release curve for LEW90500 as a function of the fraction of 4He released. The error band represents 2s level.
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Fig. 2. Release of 4He as a function of temperature.
with a minor peak around 1000°C. The degassing pattern of Ne-A strongly differs from that of Ne-E, clearly indicating that these two components originate from different sites in the meteoritic material. Ne-A yields a strong peak at 750°C, whereas Ne-E shows a bimodal pattern: between 500°C and 700°C Ne-E (l) is released from the l-carrier phase with poor noble gas retention properties. This phase seems to have low density and is compatible with graphite, amorphous carbon, carbines, as well as polymers (Eberhardt et al., 1981). Ne-E (H) in LEW90500 is degassed between 900°C and 1250°C. In a study of the Murchison CM-2 chondrite Alaerts et al. (1980) proposed an association of Ne-E (H) with spinel, whereas Jungck and Eberhardt (1985) found strong enrichment of this Ne component in an Orgueil separate rich in apatite. For a more detailed discussion of the origin of Ne-E(H) we refer to the review of Anders and Zinner (1993).
Fig. 3. Release of 3He as a function of temperature.
Fig. 4. Release of cosmic-ray produced perature.
21
Ne as a function of tem-
2.3. Argon
For the partitioning of Ar the following ratios were adopted: (36Ar/38Ar)tr 5 5.31, (36Ar/38Ar)c 5 0.65, (40Ar/36Ar)tr 5 2.9 3 1024, and (40Ar/38Ar)c 5 0.2 (for references see Eugster et al., 1993). The partitioning into radiogenic, trapped, and cosmogenic components shows that nonradiogenic 40Ar is negligibly little (less than one percent of total 40Ar). Figure 9 gives the release pattern of radiogenic 40Ar: all 40Arr is degassed below 1000°C with a major peak at 500°C and a minor one at 750°C. The K-bearing phase seems to have poor gas retention properties resulting in relatively low K-Ar ages of the CM-2 chondrites of #3400 Ma (Mazor et al., 1970). 3. CM-2 CHONDRITES: NOBLE GAS COMPONENTS AND EXPOSURE AGES
The method used for deriving the trapped neon isotopic composition of LEW90500 was applied to the other CM-2 chondrites for which Ne data were available in the literature. The data points for bulk samples are shown in Fig. 6. For each meteorite the projection to the solar wind-, SEP-, Ne-A, and Ne-E mixing lines yields the trapped Ne isotopic ratios. The method applied here yields the minimum content of Ne-E assuming that only those meteorites contain Ne-E whose (20Ne/ 22 Ne)tr , 8.46 (Ne-A), that is, assuming that their trapped Ne is a mixture of Ne-E and Ne-A without any solar Ne. Meteorites with (20Ne/22Ne)tr $ 8.46 contain Ne that is a mixture of solar Ne with Ne-A and/or Ne-E. Table 4 gives the concentrations of 21Nec, total 20Netr, and minimum 22Ne-E, as well as the ratio (20Ne/22Ne)tr. Multiple analyses of some meteorites reveal an inhomogeneous distribution of the trapped Ne components; e.g., Nogoya has a light/dark structure. Black (1972) analyzed two light samples, L1 and L2, indicating the presence of Ne-E. Sample L1 yields Ne-E only in the 1100°C temperature fraction with a ratio (20Ne/22Ne)tr 5 6.8 and 0.018 3 1028 cm3STP/g 22Ne-E. The list of CM-2 chondrites in Table 4 may not be complete but it gives information on the presence of Ne-E for more detailed
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Fig. 5. Neon three-isotope diagram with data from the stepwise heating experiment on a 104.93 mg sample of LEW90500. Extraction temperatures in units of 100°C are indicated.
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Fig. 6. Isotopic ratios of Ne from bulk samples of CM-2 chondrites. Ne in each sample is a mixture of trapped Ne and cosmogenic Ne. For adopted values see text. The isotopic composition of trapped Ne is given by the intersection of the mixing line with the near vertical line that connects the data points for solar wind-Ne, SEP-Ne, Ne-A, and Ne-E. The following meteorites are shown: 1 - LEW90500, 2 - Bells, 3 - Boriskino, 4 - Cold Bokkeveld I, 5 - Cold Bokkeveld II, 6 - Erakot, 7 - Haripura, 8 - Mighei I, 9 - Mighei II, 10 - Murchison, 11 - Murray I, 12 - Nawapali I, 13 - Nawapali II, 14 - Nogoya L2, 15 - Pollen, 16 - Santa Cruz. For references see Table 4.
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Fig. 9. Release of temperature.
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Ar from in situ K-decay as a function of
Fig. 7. Release of trapped Ne (mainly Ne-A) as a function of temperature.
studies. The largest excess of Ne-E is observed for Mighei. This meteorite might be a worthwhile target for the investigation of interstellar grains. For the calculation of cosmic-ray exposure ages we adopted a production rate of 0.262 3 1028 cm3STP/g, Ma 21Ne (Eugster, 1988). This value takes the average chemical composition of CM-chondrites into account but not shielding differences between individual meteorites because the shielding indicator (22Ne/21Ne)c could not be determined with sufficient precision. Table 5 gives the 21Ne exposure ages, T21, based on the average concentration of 21Nec where multiple analyses were available. 3Hec ages are not given as they are systematically
Fig. 8. Release of Ne-E as a function of temperature.
shorter than the 21Ne ages (Mazor et al., 1970). 38Arc is usually masked by trapped 38Ar. Nishiizumi et al. (1993) determined cosmic-ray exposure ages based on the radionuclides 10Be, 26 Al, 36Cl, and 53Mn and found general agreement with the 21 Ne ages. Hence, it appears that 21Nec is quantitatively retained. Caffee and Nishiizumi (1997) compared the radionuclide exposure ages of antarctic and nonantarctic C2 chondrites and found no significant difference. In their study all ages were below 2 Ma with a distinct cluster at 0.2 Ma. Figure 10 shows the cosmic-ray exposure ages of the CM chondrites. As noted by Mazor et al. (1970), Nishiizumi et al. (1993), and Caffee and Nishiizumi (1997) these ages are all relatively young compared to other stone meteorites. We obtain ages of ,7 Ma. LEW90500, Bells, Cold Bokkeveld, and Nawapali may be members of a meteorite cluster that were broken from their parent object 0.24 Ma– 0.31 Ma ago. All other CM-2 chondrites yield exposure ages quite evenly distributed in the range from 0.086 Ma (Nogoya) to 6.5 Ma (Santa Cruz). It is not clear whether these ages reflect the time when the collisional event responsible for the fragmentation of the parent body occurred. Some fraction of the cosmogenic 21Ne may have been produced during a pre-exposure of certain mineral phases. Most CM chondrites are breccias. Metzler (1997) found that 2.2% of the olivines in Cold Bokkeveld, Murchison, and Nogoya show a mean solar-cosmic-ray track density of about 2 orders of magnitude higher than that of the other grains. However, a pre-exposure is not indicated by the radionuclide activities that were reported by Nishiizumi et al. (1993) for some of the CM-2 chondrites. The 10Be activities are within errors consistent with the observed 21Ne ages. Table 5 also gives the K-Ar gas retention ages. Whenever available individual K concentrations were used; in all other cases we adopted a value of 400 ppm (Wasson and Kallemeyn, 1988). Based on the 40Arr data in Table 4 we obtain ages in the range from 1030 Ma (Murchison) to 3770 Ma (Boriskino) with LEW90500 at 2670 Ma. These ages reflect incomplete retention of radiogenic 40Ar and, thus, do not date a discrete event in the history of the meteoritic material (Mazor et al., 1970).
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4. MARALINGA AND OTHER CK CHONDRITES
The results obtained for the Maralinga chondrite are given in Table 3. For the partitioning of the noble gas components in all CK and CK-like chondrites analyzed for noble gases we assumed that all 3He is cosmogenic and 4He/3He 5 5.2. Trapped Ne was assumed to be Ne-A with 20Ne/22Ne 5 8.46 and 21 Ne/22Ne 5 0.035 (Tang and Anders, 1988b). For Ar the same ratios were adopted as for the CM chondrites. Table 6 gives the noble gas components. The (22Ne/21Ne)c ratios of all CK chondrites lie in the range of 1.146 –1.051 and reflect intermediate to relatively high shielding of the samples during cosmic-ray exposure. The concentrations of the target elements for cosmogenic nuclei in the CK chondrites (Kallemeyn et al., 1991) are very similar to those in CV chondrites (Wasson and Kallemeyn, 1988). Hence, we adopt 3He, 21Ne, and 38Ar production rates as proposed for CV chondrites by Eugster (1988).
The resulting exposure ages are given in Table 7. The histogram (Fig. 11) for the average values, Te(av), shows that these exposure ages are remarkably high considering the general trend for ages below about 10 Ma for C chondrites. Agreement among the three different methods for calculating the cosmic-ray exposure ages is generally good, except for ALH82135 that yields a relatively low 3He age. A possible scenario for 3He loss is discussed below. Table 7 also gives the U, Th-4He, and K-40Ar ages. We assumed that no solar 4He is present and adopted 17 ppb U and 60 ppb Th, as suggested for CV chondrites by Wasson and Kallemeyn (1988). The thermal history of the CK chondrites can be discussed based on a T3/T21 vs. T4/T40 diagram (Fig. 11). Ningqiang and Karoonda plot close to values of one for
Fig. 10. Histograms for cosmic-ray exposure ages of CK- and CM-2 chondrites. Data are from Tables 5 and 7.
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these ratios, indicating approximately the same gas retention age for 4He and 40Ar and no loss of 3He during the cosmic-ray exposure period after break-up of the parent body. Maralinga, Y-69003, and PCA82500 show a 4He loss relative to 40Ar of 27%, 30%, and 26% before cosmic-ray exposure and no 3He loss of the meteoroid in free space. For ALH82135 T3 is 65% lower than T21 and T4 is 64% lower than T40. We conclude that this He loss must have occurred during cosmic-ray exposure
Fig. 11. Ratio of cosmic-ray exposure ages T3/T21 2 vs. ratio of gas retention ages T4/T40 for chondrites. Meteorites following the broken line with slope 1 lost 3Hec and 4Her during the time they were exposed to cosmic rays. Meteorites lying between the horizontal broken lines lost 4Her before their exposure to cosmic rays, that is, at or before break-up of their parent body. A - ALH82135, K - Karoonda, M Maralinga, N - Ningqiang, P - PCA82500, Y - Y69003.
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after parent body break-up, perhaps due to an orbit of the meteoroid with small perihelion distance resulting in solar heating and diffusive loss of He and 3H. 5. SUMMARY AND CONCLUSIONS
CM-2 chondrite LEW90500 contains Ne-E, probably originating from the decay of 22Na in pre-solar grains, that is released in a bimodal pattern: Ne-E(H), the larger peak, is degassed around 1150°C. Ne-E(L) is released at about 600°C from a carbon phase. In addition to Ne-E LEW90500 contains solar, planetary and cosmic-ray produced noble gases. Planetary He is characterized by 4He/3He 5 6500. Planetary Ne, Ne-A, is the major Ne component and is released between the two Ne-E types around 750°C. An overview on noble gas characteristics of the CM-chondrites from literature data yields the lower limit of Ne-E based on bulk analyses, and where available, on stepwise heating experiments. For LEW90500 the sample bulk 3 contains 0.20 3 1028 cm3STP/g Ne-E. The highest concentration is found in Mighei (0.53 3 1028 cm3STP/g), indicating that this is a promising meteorite for the study of pre-solar grains. From cosmogenic 21Ne cosmic-ray exposure ages were calculated; all CM chondrites show ages at #6.5 Ma with a cluster of four meteorites at about 0.28 Ma. Maralinga and other CK or CK-like chondrites generally yield low concentrations of planetary-type trapped noble gases. Only Ningqiang, the only type 3 (all others are type 4 or 4/5) contains planetary type noble gases similar to the type 2 carbonaceous chondrites. The exposure age distribution for the CK chondrites shows that three meteorites lie in a range of 38 to 45 Ma; the three other ages are 6, 19, and 30 Ma, respectively. These ages are
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remarkably high considering that almost all other carbonaceous chondrites yield exposure ages of less than 10 Ma. ALH82135 has lost an equal fraction of 3Hec and 4Her possibly due to solar heating of the meteoroid in an orbit with small perihelion distance. Acknowledgements—We thank the NSF/NASA Meteorite Working Group for making available the LEW90500 sample. We also thank P. Guggisberg and M. Zuber for support in the noble gas analyses and B. Balsiger and R. Bu¨tzer for the preparation of the manuscript. This work was supported by the Swiss National Science Foundation. REFERENCES Alaerts L., Lewis R. S., Matsuda J., and Anders E. (1980) Isotopic anomalies of noble gases in meteorites and their origins. VI. Presolar components in the Murchison C2 chondrite. Geochim. Cosmochim. Acta 44, 189 –209. Anders E. and Zinner E. (1993) Interstellar grains in primitive meteorites: Diamond, silicon carbide, and graphite. Meteoritics 28, 490 – 514. Antarctic Meteorite Newsletter (1991) Vol. 14, No. 2 Johnson Space Center, Houston. Antarctic Meteorite Newsletter (1992) Vol. 15, No. 2 Johnson Space Center, Houston. Benkert J. P., Baur H., Signer P., and Wieler R. (1993) Helium, neon, and argon from the solar wind and solar energetic particles in lunar ilmenites and pyroxenes. J. Geophys. Res. 98, 13147–13162. Black D. C. (1972) On the origins of trapped helium, neon, and argon isotopic variations in meteorites: I. Gas-rich meteorites, lunar soil and breccia. Geochim. Cosmochim. Acta 36, 347–375. Black D. C. and Pepin R. O. (1969) Trapped neon in meteorites: II. Earth Planet. Sci. Lett. 6, 395– 405. Caffee M. W. and Nishiizumi K. (1997) Exposure ages of carbonaceous chondrites: II. Meteoritics Planet. Sci. 32, A26. Eberhardt P. (1978) A neon-E rich phase in Orgueil: Results of stepwise heating experiments. Proc. Lunar Planet. Sci. Conf. 9th, 1027– 1051. Eberhardt P., Jungck M. H. A., Meier F. O., and Niederer F. R. (1981) A neon-E rich phase in Orgueil: results obtained on density separates. Geochim. Cosmochim. Acta 45, 1515–1528. Eugster O. (1988) Cosmic-ray production rates for 3He, 21Ne, 38Ar, 83 Kr, and 126Xe in chondrites based on 81Kr-Kr exposure ages. Geochim. Cosmochim. Acta 52, 1649 –1662. Eugster O. and Weigel A. (1992) Exposure histories of lodranites, shergottite LEW88516, and CK-chondrites. Meteoritics 27, 219. Eugster O., Michel Th., and Niedermann S. (1988) Guangnan (L6) and Ningqiang (CV3): Exposure ages and radiogenic ages of two unusual chondrites. Meteoritics 23, 25–27. Eugster O., Michel Th., Niedermann S., Wang D., and Yi W. (1993) The record of cosmogenic, radiogenic, fissiogenic, and trapped noble gases in recently recovered Chinese and other chondrites. Geochim. Cosmochim. Acta 57, 1115–1142. Geiger T. and Bischoff A. (1990) The metamorphosed carbonaceous chondrites—a new meteorite group? 15th Symp. Ant. Met., Natl. Inst. Polar Res., Tokyo, 78 – 80.
Geiger T. and Spettel B. (1990) Maralinga—a new metamorphosed carbonaceous chondrite. Lunar Planet. Sci. XXII, 433– 434. Hanowski N. P. and Brearley A.J. (1996) Chondrule alteration in the CM carbonaceous chondrite, LEW90500. Lunar Planet. Sci. XXVII, 487– 488. Herzog G. F. and Anders E. (1974) Primordial noble gases in separated meteoritic minerals, II. Earth Planet. Sci. Lett. 24, 173–181. Heymann D. (1967) On the origin of hypersthene chondrites: Ages and shock effects of black chondrites. Icarus 6, 189 –221. Jungck M. H. A. and Eberhardt P. (1979) Neon-E in Orgueil density separates. Meteoritics 14, 439 – 441. Jungck M. H. A. and Eberhardt P. (1985) Ne-E in inclusions in apatite from Orgueil. Meteoritics 20, 677. Kallemeyn G. W., Rubin A. E., and Wasson J. T. (1990) The Karoonda (CK) group of carbonaceous chondrites: A new group found exclusively in Australia and Antarctica. Lunar Planet. Sci. XXII, 675. Kallemeyn G. W., Rubin A. E., and Wasson J. T. (1991) The compositional classification of chondrites: V. The Karoonda (CK) group of carbonaceous chondrites. Geochim. Cosmochim. Acta 55, 881– 892. Keller L. P., Clark J. C., Lewis C. F., and Moore C. B. (1992) Maralinga, a metamorphosed carbonaceous chondrite found in Australia. Meteoritics 27, 87–91. Levsky L.K. (1979) Rare gases in carbonaceous chondrites. Meteoritika 38, 27–36. Mazor E., Heymann D., and Anders E. (1970) Noble gases in carbonaceous chondrites. Geochim. Cosmochim. Acta 34, 781– 824. Meier F. O., Jungck M. H. A., and Eberhardt P. (1980) Evidence for pure neon-22 in Orgueil and Murchison. Lunar Planet. Sci. XI, 723–725. Metzler K. (1997) Preirradiated olivines in CM chondrites. Meteoritics Planet. Sci. 32, A91–A92. Niederer F. and Eberhardt P. (1979) A Neon-E rich phase in Dimmitt. Meteoritics 12, 327–331. Nishiizumi K. et al. (1993) Exposure ages of carbonaceous chondrites. Lunar Planet. Sci. XXIV, 1085–1086. Polnau E., Eugster O., Thalmann Ch., Weigel A., and Marti K. (1996) Noble gas exposure ages of some selected chondrites. Meteoritics Planet. Sci. 31, A109 –A110. Schultz L., Franke L., and Kruse H. (1996) Helium, Neon, and Argon in Meteorites, a Data Compilation. Max Planck Inst. Chem. Shima M., Shima M., and Hintenberger H. (1973) Chemical composition and rare gas content of four new detected antarctic meteorites. Earth Planet. Sci. Lett. 19, 246 –249. Tang M. and Anders E. (1988a) Isotopic anomalies of neon, xenon, and carbon in meteorites. II. Interstellar diamond and SiC: Carriers of exotic noble gases. Geochim. Cosmochim. Acta 52, 1235–1244. Tang M. and Anders E. (1988b) Isotopic anomalies of neon, xenon, and carbon in meteorites. III. Local and exotic noble gas components and their interrelations. Geochim. Cosmochim. Acta 52, 1245– 1254. Wasson J. T. and Kallemeyn G. W. (1988) Compositions of chondrites. Phil. Trans. Roy. Soc. Lond. A 325, 535–544. Wieler R., Baur H., Graf Th., and Signer P. (1985) Helium, neon, and argon in antarctic meteorites: Solar noble gases in an enstatite chondrite. Lunar Planet. Sci. XVI, 902–903. Zadnik M. G. (1985) Noble gases in the Bells (C2) and Sharps (H3) chondrites. Meteoritics 20, 245–257.
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Neon-E in CM-2 chondrite LEW90500 and collisional history of CM-2 chondrites, Maralinga, and other CK chondrites O. EUGSTER, P. EBERHARDT, CH. THALMANN, and A. WEIGEL Physikalisches Institut, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland (Received December 11, 1997; accepted in revised form April 20, 1998)
Abstract—We present He, Ne, and Ar data on stepwise heating experiments for the CM-2 chondrite LEW90500 and on noble gas isotopic abundances of the anomalous CK chondrite Maralinga. In LEW90500 we observe at least 0.20 3 1028 cm3STP/g Ne-E probably originating from the decay of presolar 22Na. Planetary He is characterized by 4He/3He 5 6500. For planetary-type trapped noble gases we obtain 20 Netr 5 13.4 3 1028 cm3STP/g and (36Ar/20Ne)tr 5 5.6. The cosmic-ray exposure age, based on 21Nec is 0.24 Ma. We review the literature data of the other CM-2 chondrites and find a Ne-E component in most of them; the largest concentration is observed in Mighei (0.53 3 1028 cm3STP/g). The exposure age distribution of the CM-2 chondrites confirms previous studies that report generally young ages (,6.5 Ma). A cluster of four meteorites is observed around 0.28 Ma. Maralinga (anomalous CK-4) contains no solar gases and relatively low amounts of planetary trapped gases. The cosmic-ray exposure age is 6.1 Ma. This is the lowest age of the six known CK or CK-like chondrites. Three of them lie in the range of 38 – 45 Ma. Copyright © 1998 Elsevier Science Ltd Alais and the CM-2 chondrites Cold Bokkeveld and Nogoya. Herzog and Anders (1974), Eberhardt (1978), Eberhardt et al. (1981), and Jungck and Eberhardt (1985) characterized the Ne-E isotopic composition for Orgueil mineral separates. Evidence for essentially pure 22Ne was reported for another CM-2 chondrite, Murchison (Meier et al., 1980). So far, Ne-E was detected in only one noncarbonaceous chondrite, the unequilibrated ordinary H-3,4 chondrite Dimmitt (Niederer and Eberhardt, 1977). Black (1972) suggested that Ne-E(H) is not radiogenic 22Ne from 22Na decay but primary, nucleosynthetic Ne from AGB-star He shells. For the origin of Ne-E(L) a major contribution from 22Na is their preferred explanation.
1. INTRODUCTION
In the framework of our study of recently found meteorites we investigated two carbonaceous chondrites that are remarkable in certain respects. LEW90500 from the Lewis Cliff area in Antarctica was first tentatively classified as a C-1 chondrite (AMNL, 1991) but reclassified as CM-2 (AMNL, 1992). It’s recovered weight is 294.7 g. The main purpose of our study was the investigation of the Ne-E component that is present in many CM-2 chondrites. The second meteorite is Maralinga (3.38 kg), found 1974 in the Nullarbor Plain, Australia, but recognized as meteorite in 1989. It is an anomalous CK-4 chondrite. Our aim was to derive it’s cosmic-ray exposure history. In the following we briefly summarize the relevant characteristics of the two investigated meteorites.
1.2. Maralinga 1.1. Lewis Cliff 90500 and Ne-E Member of the CK group of the carbonaceous chondrites are Karoonda, Maralinga, Ningqiang, and several small antarctic finds (Kallemeyn et al., 1991). Arguments for a genetic relationship of these carbonaceous chondrites were presented by Geiger and Spettel (1990), Kallemeyn et al. (1990), Geiger and Bischoff (1990), Kallemeyn et al. (1991), and Keller et al. (1992). The CK meteorites are metamorphosed and show petrographic grades from 4 to 6. Noble gases were studied in Karoonda (cf. Mazor et al., 1970) Y-69003 (Shima et al., 1973), ALH82135 and PCA82500 (Wieler et al., 1985), and Ningqiang (Eugster et al., 1988). The exceptionally high cosmic-ray exposure ages calculated from these data of about 20 –50 Ma are evidence for a common history of the CK chondrites. In this paper we report the results of stepwise heating experiments of LEW90500 and of bulk measurements of LEW90500 and Maralinga. Preliminary data regarding the cosmic-ray exposure history of these meteorites were presented in abstracts (Eugster and Weigel, 1992; Polnau et al., 1996).
There exist only few studies of this meteorite: Hanowski and Brearley (1996) carried out a detailed chondrule analysis and Nishiizumi et al. (1993) found relatively low radionuclide activities indicating a cosmic-ray exposure age of ,1 Ma. Low concentrations of cosmogenic Ne are favorable for deriving the Ne-E component. Ne-E is essentially pure 22Ne; it may be the in situ decay product of 22Na (half-life 2.6 years) that was produced in red giants, novae, or supernovae and was ejected in matter under conditions where solid grains can condense within a few months, long before 22Na has decayed (Black and Pepin, 1969; Black, 1972; Eberhardt et al., 1981). Essentially two carriers were identified: Ne-E (L), probably a carbonaceous phase, density , 2.3 g/cm3, releasing Ne-E at 500 to 700°C with 20Ne/22Ne , 0.01 and 21Ne/22Ne , 0.001. Ne-E (H), probably spinel and SiC, density 3–3.5 g/cm3, degassing at 1200 –1400°C with 20Ne/22Ne , 0.2 and 21Ne/22Ne , 0.003 (Jungck and Eberhardt, 1979). Ne-E was first observed by Black and Pepin (1969) in the Renazzo CR-2 chondrite and then by Black (1972) in the CI chondrites Ivuna, Orgueil, and 2573
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2. EXPERIMENTAL TECHNIQUE AND RESULTS
Sample LEW90500,22 was obtained from the NSF/NASA Meteorite Working Group. We prepared three bulk samples by crushing the material in a stainless steel mortar to a grain size of ,750 mm. A 5.11 mg sample (bulk 1) was used for a total gas extraction at 1600°C with a 1740°C step for the control of the completeness of gas release. A 11.01 mg sample (bulk 2) was used for a first stepwise heating experiment up to 1740°C in five steps. A 104.93 mg sample (bulk 3) was heated in twenty steps covering the range from 500°C to 1740°C. The samples were kept at the corresponding extraction temperature for 35 min. A 20.31 mg sample of the Maralinga CK chondrite was crushed to ,750 mm and analyzed in two fractions, first He and Ne and then Ar. Extractions and analyses of the noble gases were performed using our mass spectrometer system B. For details of system, procedure, background, and blank corrections see Eugster et al. (1993). The analytical results are given in Table 1–3. All errors correspond to a 95% confidence level (2s errors). 2. NOBLE GAS COMPONENTS IN LEW90500 2.1. Helium
Helium is a mixture of trapped (tr), radiogenic (r), and cosmogenic (c) gases. The temperature release pattern of the 4 He/3He ratios in bulk 3 (Fig. 1) shows a distinct plateau at an average value of 6500. This ratio is close to that found for trapped He in the main silicate phase of Orgueil: Eberhardt (1978) found (4He/3He)tr 5 6850. The total amount of trapped 4 He can be estimated with the following assumptions: an upper limit for 4Her of 810 3 1028 cm3STP/g is calculated adopting a 4He gas retention age of 2670 Ma (K-Ar age, see below), 11 ppb U, and 40 ppb Th (Wasson and Kallemeyn, 1988). The average of total 3He for the three bulk samples is 0.72 3 1028 cm3STP/g. Thus, adopting (4He/3He)c 5 5.2 (Heymann, 1967), cosmogenic 4He is #4 3 1028 cm3STP/g. Adopting (4He/ 3 He)tr 5 6500 in LEW90500, as found in the temperature release experiment leaves an average amount of 2790 3 1028
cm3STP/g 4Hetr and 0.29 3 1028 cm3STP/g 3Hec for the three bulk samples. The degassing patterns for 4He (Fig. 2) and 3He (Fig. 3) are similar but a difference in the quantity of gas released at #600°C is observed: whereas in this range 11% of 4 He is released we obtain 24% of total 3He. The release of cosmogenic 21Ne (Fig. 4) clearly indicates that a large fraction of 21Nec (58%) is released at #600°C. We conclude that the enhanced 3He release relative to that of 4He in this range is due to cosmogenic He. 2.2. Neon
Figure 5 shows the three isotope diagram of the Ne temperature steps for bulk 3. As already demonstrated in Fig. 4 21Ne in the 500°C fraction is dominated by spallation Ne. Between 700°C and 1000°C Ne-A (Ne-A1: 20Ne/22Ne 5 8.86 6 0.09, Ne-A2: 20Ne/22Ne 5 8.46 6 0.12, Tang and Anders, 1988b) and between 1100°C and 1200°C Ne-E (20Ne/22Ne # 0.15) are released. Inspection of the temperature release data for bulk 2 yields essentially the same pattern but with less extreme values because the temperature resolution is not as fine as that for bulk 3. The release pattern for the different Ne components for LEW90500 is very similar to that of bulk Orgueil (Black, 1972) and of the Ne-E rich phase G4j of Orgueil (Eberhardt, 1978). In order to derive the isotopic composition of trapped Ne and the absolute amounts of 20Netr, 21Nec, and 22Ne-E the following procedure was applied: In a 20Ne/22Ne vs. 21Ne/22Ne plot the data point for the total bulk meteorite lies on a mixing line with trapped Ne and cosmogenic Ne as endmembers. In Fig. 6 the average neon isotope ratios for the three bulk samples are used for plotting the data point (1) for LEW90500. The other data shown represent CM-2 chondrites that will be discussed later. The cosmogenic endpoint for carbonaceous chondrites is defined by (21Ne/22Ne)c 5 0.91 and (20Ne/22Ne)c 5 0.82 (average value adopted by Black and Pepin, 1969 and Mazor et al., 1970). The projection to the trapped Ne composition with (20Ne/22Ne)tr 5 7.67 indicates that it is a mixture of trapped Ne (consisting of Ne-A, SEP-Ne, and/or solar wind Ne) and Ne-E.
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The following 20Ne/22Ne and 21Ne/22Ne ratios for the known reservoirs in the solar system were adopted and are shown in Fig. 6. Solar wind Ne: 13.8 6 0.1 and 0.0328 6 0.0005, respectively (Benkert et al., 1993). Solar energetic particles (SEP) Ne: 11.2 6 0.2 and 0.0295 6 0.0005, respectively (Benkert et al., 1993). Ne-A2: 8.46 6 0.12 and 0.0348 6 0.0012, respectively (Tang and Anders, 1988b). Ne-E: #0.15 and #0.0022 (Eberhardt et al., 1981). Concluding that Ne in LEW90500 is composed of trapped Ne, Ne-E, and cosmogenic Ne we can derive the amounts of these components in each temperature step. Figs. 4, 7, and 8 show the release pattern of 21Nec, 20Ne-A, and 22Ne-E, respectively. As discussed before 21Nec is mainly released at 500°C
Fig. 1. 4He/3He release curve for LEW90500 as a function of the fraction of 4He released. The error band represents 2s level.
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Fig. 2. Release of 4He as a function of temperature.
with a minor peak around 1000°C. The degassing pattern of Ne-A strongly differs from that of Ne-E, clearly indicating that these two components originate from different sites in the meteoritic material. Ne-A yields a strong peak at 750°C, whereas Ne-E shows a bimodal pattern: between 500°C and 700°C Ne-E (l) is released from the l-carrier phase with poor noble gas retention properties. This phase seems to have low density and is compatible with graphite, amorphous carbon, carbines, as well as polymers (Eberhardt et al., 1981). Ne-E (H) in LEW90500 is degassed between 900°C and 1250°C. In a study of the Murchison CM-2 chondrite Alaerts et al. (1980) proposed an association of Ne-E (H) with spinel, whereas Jungck and Eberhardt (1985) found strong enrichment of this Ne component in an Orgueil separate rich in apatite. For a more detailed discussion of the origin of Ne-E(H) we refer to the review of Anders and Zinner (1993).
Fig. 3. Release of 3He as a function of temperature.
Fig. 4. Release of cosmic-ray produced perature.
21
Ne as a function of tem-
2.3. Argon
For the partitioning of Ar the following ratios were adopted: (36Ar/38Ar)tr 5 5.31, (36Ar/38Ar)c 5 0.65, (40Ar/36Ar)tr 5 2.9 3 1024, and (40Ar/38Ar)c 5 0.2 (for references see Eugster et al., 1993). The partitioning into radiogenic, trapped, and cosmogenic components shows that nonradiogenic 40Ar is negligibly little (less than one percent of total 40Ar). Figure 9 gives the release pattern of radiogenic 40Ar: all 40Arr is degassed below 1000°C with a major peak at 500°C and a minor one at 750°C. The K-bearing phase seems to have poor gas retention properties resulting in relatively low K-Ar ages of the CM-2 chondrites of #3400 Ma (Mazor et al., 1970). 3. CM-2 CHONDRITES: NOBLE GAS COMPONENTS AND EXPOSURE AGES
The method used for deriving the trapped neon isotopic composition of LEW90500 was applied to the other CM-2 chondrites for which Ne data were available in the literature. The data points for bulk samples are shown in Fig. 6. For each meteorite the projection to the solar wind-, SEP-, Ne-A, and Ne-E mixing lines yields the trapped Ne isotopic ratios. The method applied here yields the minimum content of Ne-E assuming that only those meteorites contain Ne-E whose (20Ne/ 22 Ne)tr , 8.46 (Ne-A), that is, assuming that their trapped Ne is a mixture of Ne-E and Ne-A without any solar Ne. Meteorites with (20Ne/22Ne)tr $ 8.46 contain Ne that is a mixture of solar Ne with Ne-A and/or Ne-E. Table 4 gives the concentrations of 21Nec, total 20Netr, and minimum 22Ne-E, as well as the ratio (20Ne/22Ne)tr. Multiple analyses of some meteorites reveal an inhomogeneous distribution of the trapped Ne components; e.g., Nogoya has a light/dark structure. Black (1972) analyzed two light samples, L1 and L2, indicating the presence of Ne-E. Sample L1 yields Ne-E only in the 1100°C temperature fraction with a ratio (20Ne/22Ne)tr 5 6.8 and 0.018 3 1028 cm3STP/g 22Ne-E. The list of CM-2 chondrites in Table 4 may not be complete but it gives information on the presence of Ne-E for more detailed
LEW90500 (cm) and CK chondrites
Fig. 5. Neon three-isotope diagram with data from the stepwise heating experiment on a 104.93 mg sample of LEW90500. Extraction temperatures in units of 100°C are indicated.
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Fig. 6. Isotopic ratios of Ne from bulk samples of CM-2 chondrites. Ne in each sample is a mixture of trapped Ne and cosmogenic Ne. For adopted values see text. The isotopic composition of trapped Ne is given by the intersection of the mixing line with the near vertical line that connects the data points for solar wind-Ne, SEP-Ne, Ne-A, and Ne-E. The following meteorites are shown: 1 - LEW90500, 2 - Bells, 3 - Boriskino, 4 - Cold Bokkeveld I, 5 - Cold Bokkeveld II, 6 - Erakot, 7 - Haripura, 8 - Mighei I, 9 - Mighei II, 10 - Murchison, 11 - Murray I, 12 - Nawapali I, 13 - Nawapali II, 14 - Nogoya L2, 15 - Pollen, 16 - Santa Cruz. For references see Table 4.
LEW90500 (cm) and CK chondrites
Fig. 9. Release of temperature.
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Ar from in situ K-decay as a function of
Fig. 7. Release of trapped Ne (mainly Ne-A) as a function of temperature.
studies. The largest excess of Ne-E is observed for Mighei. This meteorite might be a worthwhile target for the investigation of interstellar grains. For the calculation of cosmic-ray exposure ages we adopted a production rate of 0.262 3 1028 cm3STP/g, Ma 21Ne (Eugster, 1988). This value takes the average chemical composition of CM-chondrites into account but not shielding differences between individual meteorites because the shielding indicator (22Ne/21Ne)c could not be determined with sufficient precision. Table 5 gives the 21Ne exposure ages, T21, based on the average concentration of 21Nec where multiple analyses were available. 3Hec ages are not given as they are systematically
Fig. 8. Release of Ne-E as a function of temperature.
shorter than the 21Ne ages (Mazor et al., 1970). 38Arc is usually masked by trapped 38Ar. Nishiizumi et al. (1993) determined cosmic-ray exposure ages based on the radionuclides 10Be, 26 Al, 36Cl, and 53Mn and found general agreement with the 21 Ne ages. Hence, it appears that 21Nec is quantitatively retained. Caffee and Nishiizumi (1997) compared the radionuclide exposure ages of antarctic and nonantarctic C2 chondrites and found no significant difference. In their study all ages were below 2 Ma with a distinct cluster at 0.2 Ma. Figure 10 shows the cosmic-ray exposure ages of the CM chondrites. As noted by Mazor et al. (1970), Nishiizumi et al. (1993), and Caffee and Nishiizumi (1997) these ages are all relatively young compared to other stone meteorites. We obtain ages of ,7 Ma. LEW90500, Bells, Cold Bokkeveld, and Nawapali may be members of a meteorite cluster that were broken from their parent object 0.24 Ma– 0.31 Ma ago. All other CM-2 chondrites yield exposure ages quite evenly distributed in the range from 0.086 Ma (Nogoya) to 6.5 Ma (Santa Cruz). It is not clear whether these ages reflect the time when the collisional event responsible for the fragmentation of the parent body occurred. Some fraction of the cosmogenic 21Ne may have been produced during a pre-exposure of certain mineral phases. Most CM chondrites are breccias. Metzler (1997) found that 2.2% of the olivines in Cold Bokkeveld, Murchison, and Nogoya show a mean solar-cosmic-ray track density of about 2 orders of magnitude higher than that of the other grains. However, a pre-exposure is not indicated by the radionuclide activities that were reported by Nishiizumi et al. (1993) for some of the CM-2 chondrites. The 10Be activities are within errors consistent with the observed 21Ne ages. Table 5 also gives the K-Ar gas retention ages. Whenever available individual K concentrations were used; in all other cases we adopted a value of 400 ppm (Wasson and Kallemeyn, 1988). Based on the 40Arr data in Table 4 we obtain ages in the range from 1030 Ma (Murchison) to 3770 Ma (Boriskino) with LEW90500 at 2670 Ma. These ages reflect incomplete retention of radiogenic 40Ar and, thus, do not date a discrete event in the history of the meteoritic material (Mazor et al., 1970).
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4. MARALINGA AND OTHER CK CHONDRITES
The results obtained for the Maralinga chondrite are given in Table 3. For the partitioning of the noble gas components in all CK and CK-like chondrites analyzed for noble gases we assumed that all 3He is cosmogenic and 4He/3He 5 5.2. Trapped Ne was assumed to be Ne-A with 20Ne/22Ne 5 8.46 and 21 Ne/22Ne 5 0.035 (Tang and Anders, 1988b). For Ar the same ratios were adopted as for the CM chondrites. Table 6 gives the noble gas components. The (22Ne/21Ne)c ratios of all CK chondrites lie in the range of 1.146 –1.051 and reflect intermediate to relatively high shielding of the samples during cosmic-ray exposure. The concentrations of the target elements for cosmogenic nuclei in the CK chondrites (Kallemeyn et al., 1991) are very similar to those in CV chondrites (Wasson and Kallemeyn, 1988). Hence, we adopt 3He, 21Ne, and 38Ar production rates as proposed for CV chondrites by Eugster (1988).
The resulting exposure ages are given in Table 7. The histogram (Fig. 11) for the average values, Te(av), shows that these exposure ages are remarkably high considering the general trend for ages below about 10 Ma for C chondrites. Agreement among the three different methods for calculating the cosmic-ray exposure ages is generally good, except for ALH82135 that yields a relatively low 3He age. A possible scenario for 3He loss is discussed below. Table 7 also gives the U, Th-4He, and K-40Ar ages. We assumed that no solar 4He is present and adopted 17 ppb U and 60 ppb Th, as suggested for CV chondrites by Wasson and Kallemeyn (1988). The thermal history of the CK chondrites can be discussed based on a T3/T21 vs. T4/T40 diagram (Fig. 11). Ningqiang and Karoonda plot close to values of one for
Fig. 10. Histograms for cosmic-ray exposure ages of CK- and CM-2 chondrites. Data are from Tables 5 and 7.
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these ratios, indicating approximately the same gas retention age for 4He and 40Ar and no loss of 3He during the cosmic-ray exposure period after break-up of the parent body. Maralinga, Y-69003, and PCA82500 show a 4He loss relative to 40Ar of 27%, 30%, and 26% before cosmic-ray exposure and no 3He loss of the meteoroid in free space. For ALH82135 T3 is 65% lower than T21 and T4 is 64% lower than T40. We conclude that this He loss must have occurred during cosmic-ray exposure
Fig. 11. Ratio of cosmic-ray exposure ages T3/T21 2 vs. ratio of gas retention ages T4/T40 for chondrites. Meteorites following the broken line with slope 1 lost 3Hec and 4Her during the time they were exposed to cosmic rays. Meteorites lying between the horizontal broken lines lost 4Her before their exposure to cosmic rays, that is, at or before break-up of their parent body. A - ALH82135, K - Karoonda, M Maralinga, N - Ningqiang, P - PCA82500, Y - Y69003.
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after parent body break-up, perhaps due to an orbit of the meteoroid with small perihelion distance resulting in solar heating and diffusive loss of He and 3H. 5. SUMMARY AND CONCLUSIONS
CM-2 chondrite LEW90500 contains Ne-E, probably originating from the decay of 22Na in pre-solar grains, that is released in a bimodal pattern: Ne-E(H), the larger peak, is degassed around 1150°C. Ne-E(L) is released at about 600°C from a carbon phase. In addition to Ne-E LEW90500 contains solar, planetary and cosmic-ray produced noble gases. Planetary He is characterized by 4He/3He 5 6500. Planetary Ne, Ne-A, is the major Ne component and is released between the two Ne-E types around 750°C. An overview on noble gas characteristics of the CM-chondrites from literature data yields the lower limit of Ne-E based on bulk analyses, and where available, on stepwise heating experiments. For LEW90500 the sample bulk 3 contains 0.20 3 1028 cm3STP/g Ne-E. The highest concentration is found in Mighei (0.53 3 1028 cm3STP/g), indicating that this is a promising meteorite for the study of pre-solar grains. From cosmogenic 21Ne cosmic-ray exposure ages were calculated; all CM chondrites show ages at #6.5 Ma with a cluster of four meteorites at about 0.28 Ma. Maralinga and other CK or CK-like chondrites generally yield low concentrations of planetary-type trapped noble gases. Only Ningqiang, the only type 3 (all others are type 4 or 4/5) contains planetary type noble gases similar to the type 2 carbonaceous chondrites. The exposure age distribution for the CK chondrites shows that three meteorites lie in a range of 38 to 45 Ma; the three other ages are 6, 19, and 30 Ma, respectively. These ages are
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