Cosmic‐ray exposure history of two Frontier Mountain H‐chondrite showers from spallation and neutron‐capture products

Abstract— We measured the concentrations of ¹⁰Be, ²⁶Al, ³⁶Cl, ⁴¹Ca and ¹⁴C in the metal and/or stone fractions of 27 Antarctic chondrites from Frontier Mountain (FRO), including two large H‐chondrite showers. To estimate the pre‐atmospheric size of the two showers, we determined the contribution of neutron‐capture produced ³⁶Cl (half‐life = 3.01 times 10⁵ years) and ⁴¹Ca (1.04 times 10⁵ years) in the stone fraction. The measured activities of neutron‐capture ³⁶Cl and ⁴¹Ca, as well as spallation produced ¹⁰Be and ²⁶Al, were compared with Monte Carlo‐based model calculations. The largest shower, FRO 90174, includes eight fragments with an average terrestrial age of (100 ± 30) × 10³ years; the neutron‐capture saturation activities extend to 27 dpm/kg stone for ³⁶Cl and 19 dpm/kg stone for ⁴¹Ca. The concentrations of spallation produced ¹⁰Be, ²⁶Al and ³⁶Cl constrain the radius (R) to 80–100 cm, while the neutron‐capture ⁴¹Ca activities indicate that the samples originated from the outer 25 cm. With a pre‐atmospheric radius of 80–100 cm, FRO 90174 is among the largest of the Antarctic stony meteorites. The large pre‐atmospheric size supports our hypothesis that at least 50 of the ～150 classified H5/H6‐chondrites from the Frontier Mountain stranding area belong to this single fall; this hypothesis does not entirely account for the high H/L ratio at Frontier Mountain. The smaller shower, FRO 90001, includes four fragments with an average terrestrial age of (40 ± 10) × 10³ years; they contain small contributions of neutron‐capture ³⁶Cl, but no excess of ⁴¹Ca. FRO 90001 experienced a complex exposure history with high shielding conditions in the first stage (150 < R < 300 cm) and much lower shielding in the second stage (R < 30 cm), the latter starting ～1.0 million years (Ma) ago. Based on the measured ¹⁰Be/²¹Ne and ²⁶Al/²¹Ne ratios, the cosmic‐ray exposure ages of the two showers are 7.2 ± 0.5 Ma for FRO 90174 and 8 ± 1 Ma for FRO 90001. These ages coincide with the well‐established H‐chondrite peak and corroborate the observation that the exposure age distribution of FRO H‐chondrites is similar to that of non‐Antarctic falls. In addition, we found that corrections for neutron‐capture ³⁶Ar (from decay of ³⁶Cl) result in concordant ²¹Ne and ³⁸Ar exposure ages.     

Annama H chondrite—Mineralogy,physical properties,cosmic ray exposure,and parent body history

The fall of the Annama meteorite occurred early morning (local time) on April 19, 2014 on the Kola Peninsula (Russia). Based on mineralogy and physical properties, Annama is a typical H chondrite. It has a high Ar‐Ar age of 4.4 Ga. Its cosmic ray exposure history is atypical as it is not part of the large group of H chondrites with a prominent 7–8 Ma peak in the exposure age histograms. Instead, its exposure age is within uncertainty of a smaller peak at 30 ± 4 Ma. The results from short‐lived radionuclides are compatible with an atmospheric pre‐entry radius of 30–40 cm. However, based on noble gas and cosmogenic radionuclide data, Annama must have been part of a larger body (radius >65 cm) for a large part of its cosmic ray exposure history. The ¹⁰Be concentration indicates a recent (3–5 Ma) breakup which may be responsible for the Annama parent body size reduction to 30–35 cm pre‐entry radius.     

Cosmogenic nuclides in the solar gas‐rich H3–6 chondrite breccia Frontier Mountain 90174

Abstract— We re‐evaluated the cosmic‐ray exposure history of the H3‐6 chondrite shower Frontier Mountain (FRO) 90174, which previously was reported to have a simple exposure history, an irradiation time of about 7 Ma, and a pre‐atmospheric radius of 80–100 cm (Welten et al. 2001). Here we measured the concentrations and isotopic compositions of He, Ne, and Ar in 8 aliquots of 6 additional fragments of this shower, and ¹⁰Be and ²⁶Al in the stone fractions of seven fragments. The radionuclide concentrations in the stone fractions, combined with those in the metal fractions, confirm that all samples are fragments of the FRO 90174 shower. Four of the fragments contain solarwind‐implanted noble gases with a solar ²⁰Ne/²²Ne ratio of ?12.0, indicating that FRO 90174 is a regolith breccia. The concentrations of solar gases and cosmogenic ²¹Ne in the samples analyzed by us and by Welten et al. (2001) overlap with those of the FRO H‐chondrites from the 1984 season, suggesting that many of these samples are also part of the large FRO 90174 chondrite shower. The cosmogenic ²¹Ne concentrations in FRO 90174 show no simple correlation with ¹⁰Be and ²⁶Al activities. We found ²¹Ne excesses between 0.3‐1.1 × 10^?8cm³STP/g in 6 of the 17 samples. Since excess ²¹Ne and trapped solar gases are not homogeneously distributed, i.e., we found in one fragment aliquots with and without excess ²¹Ne and solar ²⁰Ne, we conclude that excess ²¹Ne is due to GCR irradiation of the regolith before compaction of the FRO 90174 object. Therefore, the chondrite shower FRO 90174 did not simply experience an exposure history, but some material was already irradiated at the surface of an asteroid leading to excess ²¹Ne. This excess ²¹Ne is correlated to implanted solar gases, clearly indicating that both processes occurred on the regolith.     

Noble gases and cosmogenic radionuclides in the Gold Basin L4 chondrite shower: Thermal history,exposure history,and pre‐atmospheric size

Abstract— We measured the concentrations of the cosmogenic radionuclides ¹⁰Be, ²⁶Al, ³⁶Cl, and ⁴¹Ca in the stone and metal fractions of 15 fragments of the Gold Basin L4 chondrite shower, as well as noble gases in 18 Gold Basin fragments. A comparison of ¹⁰Be, ²⁶Al, and ⁴¹Ca concentrations with calculated production rates from two different models indicates that the Gold Basin samples came from depths of about 10 cm to more than 150 cm in an object with a radius of 3–5 m. As was predicted by recent model calculations, the noble gases show a reversal of the ²²Ne/²¹Ne ratio at very high shielding. The ²¹Ne/¹⁰Be and ²¹Ne/²⁶Al ratios in most samples are constant and correspond to a 4π exposure age of 18 ± 2 Myr. However, three Gold Basin samples show a 30–120% excess of ²¹Ne implying that they were previously exposed close to the surface of the parent body, whereas the other samples were buried several meters deeper. Concentrations of neutron‐capture ³⁶Ar in most samples are consistent with measured concentrations of neutron‐capture ³⁶Cl and an exposure age of 18 Myr. Large excesses of neutron‐capture ³⁶Ar were found in those samples with an excess of ²¹Ne, providing additional evidence of a first‐stage exposure on the parent body. The excess of spallation‐produced ²¹Ne and neutron‐capture‐produced ³⁶Ar in these samples indicate a first‐stage exposure of 35–150 Myr on the parent body. The radiogenic ⁴He and ⁴⁰Ar concentrations indicate a major impact on the parent body between 300 and 400 Myr ago, which must have preceded the impacts that brought the Gold Basin meteoroid to the surface of the parent body and then expelled it from the parent body 18 Myr ago.     

Exposure history and terrestrial ages of ordinary chondrites from the Dar al Gani region,Libya

Abstract— We measured the concentrations of noble gases in 32 ordinary chondrites from the Dar al Gani (DaG) region, Libya, as well as concentrations of the cosmogenic radionuclides ¹⁴C, ¹⁰Be, ²⁶Al, ³⁶Cl, and ⁴¹Ca in 18 of these samples. Although the trapped noble gases in five DaG samples show ratios typical of solar or planetary gases, in all other DaG samples, they are dominated by atmospheric contamination, which increases with the degree of weathering. Cosmic ray exposure (CRE) ages of DaG chondrites range from ?1 Myr to 53 Myr. The CRE age distribution of 10 DaG L chondrites shows a cluster around 40 Myr due to four members of a large L6 chondrite shower. The CRE age distribution of 19 DaG H chondrites shows only three ages coinciding with the main H chondrite peak at ?7 Myr, while seven ages are <5 Myr. Two of these H chondrites with short CRE ages (DaG 904 and 908) show evidence of a complex exposure history. Five of the H chondrites show evidence of high shielding conditions, including low ²²Ne/²¹Ne ratios and large contributions of neutron‐capture ³⁶Cl and ⁴¹Ca. These samples represent fragments of two or more large pre‐atmospheric objects, which supports the hypothesis that the high H/L chondrite ratio at DaG is due to one or more large unrecognized showers. The ¹⁴C concentrations correspond to terrestrial ages <35 kyr, similar to terrestrial ages of chondrites from other regions in the Sahara but younger than two DaG achondrites. Despite the loss of cosmogenic ³⁶Cl and ⁴¹Ca during oxidation of metal and troilite, concentrations of ³⁶Cl and ⁴¹Ca in the silicates are also consistent with ¹⁴C ages <35 kyr. The only exception is DaG 343 (H4), which has a ⁴¹Ca terrestrial age of 150 ± 40 kyr. This old age shows that not only iron meteorites and achondrites but also chondrites can survive the hot desert environment for more than 50 kyr. A possible explanation is that older meteorites were covered by soils during wetter periods and were recently exhumed by removal of these soils due to deflation during more arid periods, such as the current one, which started ?3000 years ago. Finally, based on the ²⁶Al/²¹Ne and ¹⁰Be/²¹Ne systematics in 16 DaG meteorites, we derived more reliable estimates of the ¹⁰Be/²¹Ne production rate ratio, which seems more sensitive to shielding than was predicted by the semi‐empirical model of Graf et al. (1990) but less sensitive than was predicted by the purely physical model of Leya et al. (2000).     

Cosmogenic nuclides in core samples of the Chico L6 chondrite: Evidence for irradiation under high shielding

Abstract— Core samples were obtained from various locations of the ～ 105-kg Chico, NM, L6 chondrite in order to study the effects of large shielding on the production rates of cosmic-ray-produced nuclides. Relations between measured abundances of cosmogenic nuclides (¹⁰Be, ²⁶Al, and stable isotopes of He, Ne, and Ar) and the cosmogenic ²²Ne/²¹Ne ratio were determined and compared with recent model predictions of production rates. The measured ²²Ne/²¹Ne ratios (1.06-1.08) and significant variations observed in concentrations of cosmogenic ²¹Ne and ³He suggest an ～40-cm shielding gradient across Chico and irradiation within a large object (> 100-cm radius). Noble gas data indicate that Chico experienced greater shielding than chondrites Knyahinya or Keyes and similar to Jilin. Values of ¹⁰Be (average = 20.7 dpm/kg) and ²⁶Al (average = 71.1 dpm/kg) are nearly constant, however, and show no correlation with either ²²Ne/²¹Ne or ²¹Ne. Activities of ¹⁰Be and ²⁶Al suggest irradiation in a smaller object (～40–80 cm radius). The ²⁶Al activity and the ²⁶Al/¹⁰Be ratio (average value = 3.42) are both significantly larger than values for most other chondrites. These results could indicate a two-stage irradiation with t₁ ～ 104 Ma and t₂ ～ 4 Ma and a second-stage body the size of Knyahinya. The single stage, ¹⁰Be/²¹Ne exposure age for Chico is 65 Ma. The ²²Ne/²¹Ne ratio apparently becomes insensitive to shielding for objects the size of Chico. No substantial evidence exists for chondrites with ²²Ne/²¹Ne ratios significantly less than ～ 1.055.     

Cosmogenic radionuclides and noble gases in Antarctic H chondrites with high and normal natural thermoluminescence levels

Abstract— We report noble gas data for 37 H chondrites collected from the Allan Hills by EUROMET in the 1988–1989 field season. Among these are 16 specimens with high levels (>100 krad) of natural thermoluminescence (NTL), originally interpreted as signaling their derivation from a single meteoroid with an orbit that became Earth‐crossin‐100 ka ago. One of these 16 is an H3 chondrite with a cosmic‐ray exposure age of ～33 Ma and clearly represents a separate fall. The other 15 H4–6 chondrites derive from three separate meteoroids, each of which is represented by a five or six member group. These groups have mean exposure ages of 3.7, 4.1, and 6.6 Ma: the middle‐group members all contain solar Ne. The two younger groups also seem to each include a few H chondrites with normal NTL levels. Measurements of cosmogenic ¹⁰Be (1.5 Ma), ²⁶AI (710 ka), and ³⁶CI (301 ka) in 14 of the high‐NTL chondrites indicate that all reflect a simple irradiation history. In contrast, many of a different (38 member) randomly selected suite of Antarctic H chondrites seem to have different cosmic‐ray irradiation histories. The 3.7 and 6.6 Ma groups from the 37 member Allan Hills suite come from about 5–30 and about 5–10 cm depths in 80–125 and 60–125 cm radius meteoroids, respectively.     

The complex exposure history of the Jiddat al Harasis 073 L‐chondrite shower

Abstract— We measured the concentrations and isotopic compositions of He, Ne, and Ar in 29 bulk samples from 11 different strewn field fragments of the large Jiddat al Harasis (JaH) 073 L6 chondrite shower, including 7 samples from known locations within the main mass. In addition, we measured the concentrations of cosmogenic ¹⁰Be, ²⁶Al, ³⁶Cl, and ⁴¹Ca in 10 samples. All fragments of this shower are characterized by low ¹⁰Be concentrations (7.6–12.8 dpm/kg), high ²⁶Al/¹⁰Be ratios (3.5‐5), large contributions of neutron capture ⁴¹Ca (200–1800 dpm/kgCa), low ³He/²¹Ne ratios (1.5‐3.0), large variations in cosmogenic ²¹Ne (1.2–12) × 10^?8cm³STP/g, and significant contributions of neutron‐capture ³⁶Ar. Stepwise heating experiments show that neutron‐capture produced ³⁶Ar is predominantly released between 1000–1200 °C. All these results are consistent with a first‐stage exposure of ?65 Ma within ?20 cm of the surface of the L‐chondrite parent body, followed by ejection of a 1.5‐2 m large object, which was then delivered to Earth within about 0.5 and 0.7 Ma. The cosmogenic nuclide data in JaH 073 thus corroborate the trend that many of the large chondrites studied so far experienced a complex exposure history. The observed ³He/²¹Ne ratios of 2.5‐3.0 in the most shielded samples (including those of the main mass) are lower than predicted by model calculations, but similar to the lowest values found in the large Gold Basin L‐chondrite shower. The Bern plot, which gives a linear correlation for ³He/²¹Ne versus ²²Ne/²¹Ne, is evidently not valid for very high shielding. Some of our measured ²²Ne/²¹Ne ratios in JaH 073 are lower than 1.06, which is not well understood, but might be explained by loss of cosmogenic neon from shocked sodium‐rich plagioclase during terrestrial weathering. The amount of trapped atmospheric argon in the JaH 073 fragments varies by almost two orders of magnitude and shows only a weak correlation with the size of the fragments, which range from <100 g to >50 kg. Finally, low concentrations of radiogenic ⁴He and ⁴⁰Ar indicate incomplete degassing < 1 Ga ago, probably at the main collision event on the L‐chondrite parent body ?480 Ma ago.     

Cosmic‐ray prdocution rates of helium,neon and argon isotopes in H chondrites based on chlorine‐36/argon‐36 ages

Abstract— We present the concentrations and isotopic compositions of He, Ne, and Ar for nonmagnetic fractions and bulk samples of 17 H chondrites which were recently investigated for their ³⁶Cl‐³⁶Ar cosmic‐ray exposure ages (Graf et al., 2001). All selected meteorites are observed falls with cosmic‐ray exposure ages close to the 7 Ma peak. The rare gas data are consistent with ¹⁰Be and ³⁶C1 production rates in the metal phase. Remarkably, only 1 out of the 17 H chondrites, Bath, shows clear indications for a complex exposure history. Based on rare gas concentrations and ³⁶Cl‐³⁶Ar exposure ages, ²¹Ne production rates as a function of ²²Ne/²¹ Ne and a mean ³⁸Ar production rate are determined. The results confirm model calculations which predict that the relationship between ²¹Ne production rates and ²²Ne/²¹Ne is ambiguous for high shielding. Besides the mean ³⁸Ar production rate we also give production rate ratios P(³⁸Ar from Ca)/P(³⁸Ar from Fe). They vary between 10 and 77, showing no significant correlation with ³⁸Ar concentrations or ²²Ne/²¹Ne. By investigating the metal separates, Graf et al. (2001) found significant ³He deficits for 6 out of the 17 meteorites. For the nonmagnetic fractions and bulk samples investigated here, the data points in a ³He/²¹Ne vs. ²²Ne/²¹Ne diagram plot in the area defined by most of the H chondrites. This means that ³He deficits in the metal phase are much more pronounced than in silicate minerals and we will argue that ³H diffusive losses in meteorites should be the rule rather than the exception. The ²¹Ne exposure ages, calculated on the basis of modeled ²¹Ne production rates, confirm the assumption by Graf et al. (2001) that the H5 chondrites with low ³He/³⁸Ar in the metal formed in a separate event than those with normal ³He/³⁸Ar ratios. The data can best be interpreted by assuming that the prominent 7 Ma exposure age peak of the H chondrites is due to at least two events about 7.0 and 7.6 Ma ago.     

Park Forest (L5) and the asteroidal source of shocked L chondrites

The Park Forest (L5) meteorite fell in a suburb of Chicago, Illinois (USA) on March 26, 2003. It is one of the currently 25 meteorites for which photographic documentation of the fireball enabled the reconstruction of the meteoroid orbit. The combination of orbits with pre‐atmospheric sizes, cosmic‐ray exposure (CRE), and radiogenic gas retention ages (“cosmic histories”) is significant because they can be used to constrain the meteoroid's “birth region,” and test models of meteoroid delivery. Using He, Ne, Ar, ¹⁰Be, and ²⁶Al, as well as a dynamical model, we show that the Park Forest meteoroid had a pre‐atmospheric size close to 180 g cm^?2, 0–40% porosity, and a pre‐atmospheric mass range of ~2–6 tons. It has a CRE age of 14 ± 2 Ma, and (U, Th)‐He and K‐Ar ages of 430 ± 90 and 490 ± 70 Ma, respectively. Of the meteorites with photographic orbits, Park Forest is the second (after Novato) that was shocked during the L chondrite parent body (LCPB) break‐up event approximately 470 Ma ago. The suggested association of this event with the formation of the Gefion family of asteroids has recently been challenged and we suggest the Ino family as a potential alternative source for the shocked L chondrites. The location of the LCPB break‐up event close to the 5:2 resonance also allows us to put some constraints on the possible orbital migration paths of the Park Forest meteoroid.     

Cosmogenic nuclides in the Košice meteorite: Experimental investigations and Monte Carlo simulations

Results of nondestructive gamma‐ray analyses of cosmogenic radionuclides (⁷Be, ²²Na, ²⁶Al, ⁴⁶Sc, ⁴⁸V, ⁵⁴Mn, ⁵⁶Co, ⁵⁷Co, ⁵⁸Co, and ⁶⁰Co) in 19 fragments of the Ko?ice meteorite are presented and discussed. The activities varied mainly with position of fragments in the meteoroid body, and with fluxes of cosmic‐ray particles in the space affecting radionuclides with different half‐lives. Monte Carlo simulations of the production rates of ⁶⁰Co and ²⁶Al compared with experimental data indicate that the pre‐atmospheric radius of the meteoroid was 50 ± 5 cm. In two Ko?ice fragments, He, Ne, and Ar concentrations and isotopic compositions were also analyzed. The noble‐gas cosmic‐ray exposure age of the Ko?ice meteorite is 5–7 Myr, consistent with the conspicuous peak (or doublet peak) in the exposure age histogram of H chondrites. One sample likely contains traces of implanted solar wind Ne, suggesting that Ko?ice is a regolith breccia. The agreement between the simulated and observed ²⁶Al activities indicate that the meteoroid was mostly irradiated by a long‐term average flux of galactic cosmic rays of 4.8 particles cm^?2 s^?1, whereas the short‐lived radionuclide activities are more consistent with a flux of 7.0 protons cm^?2 s^?1 as a result of the low solar modulation of the galactic cosmic rays during the last few years before the meteorite fall.     

Exposure history of the Torino meteorite

Abstract— We determined He, Ne, Ar, ¹⁰Be, ²⁶Al, ³⁶Cl, and ¹⁴C concentrations, as well as cosmic-ray track densities and halogen concentrations in different specimens of the H6 chondrite Torino, in order to constrain its exposure history to cosmic radiation. The Torino meteoroid had a radius of ～20 cm and travelled in interplanetary space for 2.5–10 Ma. Earlier, Torino was part of a larger body. The smallest possible precursor had a radius of 55 cm and a journey through space longer than ～65 Ma. If the first-stage exposure took place in a body with a radius of >3 m or in the parent asteroid, then it lasted nearly 300 Ma. The example of Torino shows that it is easy to underestimate first-stage exposure ages when constructing two-stage histories.     

Exposure History of H5 Chondrite Gujargaon

Abstract— Cosmic ray produced tracks, He and Ne isotopes and radionuclides have been studied in the recently fallen H5 chondrite Gujargaon. The results indicate an exposure age of about 7 Ma. The high track production rates of 0.25 to 0.69 × 10⁶ cm^?2 Ma^?1 suggest that the Gujargaon meteoroid had a small size (Re = 9–10 cm) in space and suffered 1–3 cm ablation in the atmosphere. The conclusion about the meteoroid size is supported by the low activity of neutron capture isotope ⁶⁰Co and high spallogenic ²²Ne/²¹Ne ratio of about 1.25. The data on long lived isotopes ¹⁰Be, ⁵³Mn and ²⁶Al are used to derive production rates of these isotopes in a rock having a radius of 9 cm and the activity levels of the short lived isotopes ²²Na and ⁵⁴Mn are used to estimate the effect of modulation of galactic cosmic rays at the time of solar maximum of 1982.     

Radiogenic isotope investigation of the St‐Robert H5 fall

Abstract— The St‐Robert H5 chondrite yields a mineral/whole‐rock Pb‐Pb age of 4565 ± 23 Ma (2σ) comparable to the accepted age of most chondrites. The regression of chondrule data give a similar age of 4566 ± 7 Ma (2σ). These results imply that no major perturbation affected the Pb‐Pb systematics of this meteorite's parent body within the first few billion years following its accretion. Re and Os concentrations along with Os isotopic compositions of whole‐rock fragments, surface fusion crusts and metal phases are also reported. The whole rock measurements for this ordinary chondrite are characterized by high Re/Os ratio coupled with relatively high ¹⁸⁷Os/¹⁸⁸Os (compared to average ordinary chondrites), that we interpret as a long term Re enrichment. As for most chondrites, no precise geochronological information could be extracted from the Re/Os systematics, although most data plot near the IIIAB reference isochron (Smoliar et al. 1996). From the fusion crust results, we rule out the possibility that atmospheric entry caused the perturbations in the Re‐Os system, since melted crust analysis yields among the most concordant data points. Evidence from metal phases suggests that a very recent process perturbed the isochron, relocating Re from kamacite toward troilite.     

Light noble gases in 12 meteorites from the Omani desert,Australia, Mauritania,Canada, and Sweden

We measured the concentrations and isotopic compositions of He, Ne, and Ar in 14 fragments from 12 different meteorites: three carbonaceous chondrites, six L chondrites (three most likely paired), one H chondrite, one R chondrite, and one ungrouped chondrite. The data obtained for the CV3 chondrites Ramlat as Sahmah (RaS) 221 and RaS 251 support the hypothesis of exposure age peaks for CV chondrites at approximately 9 Ma and 27 Ma. The exposure age for Shi?r 033 (CR chondrite) of 7.3 Ma is also indicative of a possible CR chondrite exposure age peak. The three L chondrites Jiddat al Harasis (JaH) 091, JaH 230, and JaH 296, which are most likely paired, fall together with Hallingeberg into the L chondrite exposure age peak of approximately 15 Ma. The two L chondrites Shelburne and Lake Torrens fall into the peaks at approximately 40 Ma and 5 Ma, respectively. The ages for Bassikounou (H chondrite) and RaS 201 (R chondrite) are approximately 3.5 Ma and 5.8 Ma, respectively. Six of the studied meteorites show clear evidence for ³He diffusive losses, the deficits range from approximately 17% for one Lake Torrens aliquot to approximately 45% for RaS 211. The three carbonaceous chondrites RaS 221, RaS 251, and Shi?r 033 all have excess ⁴He, either of planetary or solar origin. However, very high ⁴He/²⁰Ne ratios occur at relatively low ²⁰Ne/²²Ne ratios, which is unexpected and needs further study. The measured ⁴⁰Ar ages fit well into established systematics. They are between 2.5 and 4.5 Ga for the carbonaceous chondrites, older than 3.6 Ga for the L and H chondrites, and about 2.4 Ga for the R chondrite as well as for the ungrouped chondrite. Interestingly, none of our studied L chondrites has been degassed in the 470 Ma break‐up event. Using the amount of trapped ³⁶Ar as a proxy for noble gas contamination due to terrestrial weathering we are able to demonstrate that the samples studied here are not or only very slightly affected by terrestrial weathering (at least in terms of their noble gas budget).     

Calibration of cosmogenic noble gas production in ordinary chondrites based on 36Cl‐36Ar ages. Part 1: Refined produced rates for cosmogenic 21Ne and 38Ar

We measured the concentrations and isotopic compositions of He, Ne, and Ar in bulk samples and metal separates of 14 ordinary chondrite falls with long exposure ages and high metamorphic grades. In addition, we measured concentrations of the cosmogenic radionuclides ¹⁰Be, ²⁶Al, and ³⁶Cl in metal separates and in the nonmagnetic fractions of the selected meteorites. Using cosmogenic ³⁶Cl and ³⁶Ar measured in the metal separates, we determined ³⁶Cl‐³⁶Ar cosmic‐ray exposure (CRE) ages, which are shielding‐independent and therefore particularly reliable. Using the cosmogenic noble gases and radionuclides, we are able to decipher the CRE history for the studied objects. Based on the correlation ³He/²¹Ne versus ²²Ne/²¹Ne, we demonstrate that, among the meteorites studied, only one suffered significant diffusive losses (about 35%). The data confirm that the linear correlation ³He/²¹Ne versus ²²Ne/²¹Ne breaks down at high shielding. Using ³⁶Cl‐³⁶Ar exposure ages and measured noble gas concentrations, we determine ²¹Ne and ³⁸Ar production rates as a function of ²²Ne/²¹Ne. The new data agree with recent model calculations for the relationship between ²¹Ne and ³⁸Ar production rates and the ²²Ne/²¹Ne ratio, which does not always provide unique shielding information. Based on the model calculations, we determine a new correlation line for ²¹Ne and ³⁸Ar production rates as a function of the shielding indicator ²²Ne/²¹Ne for H, L, and LL chondrites with preatmospheric radii less than about 65 cm. We also calculated the ¹⁰Be/²¹Ne and ²⁶Al/²¹Ne production rate ratios for the investigated samples, which show good agreement with recent model calculations.     

Cosmogenic and trapped noble gases in individual chondrules: Clues to chondrule formation

Abstract— We studied the elemental and isotopic abundances of noble gases (He, Ne, Ar in most cases, and Kr, Xe also in some cases) in individual chondrules separated from six ordinary, two enstatite, and two carbonaceous chondrites. Most chondrules show detectable amounts of trapped ²⁰Ne and ³⁶Ar, and the ratio (³⁶Ar/²⁰Ne)_t (from ordinary and carbonaceous chondrites) suggests that HL and Q are the two major trapped components. A different trend between (³⁶Ar/²⁰Ne)_t and trapped ³⁶Ar is observed for chondrules in enstatite chondrites indicating a different environment and/or mechanism for their formation compared to chondrules in ordinary and carbonaceous chondrites. We found that a chondrule from Dhajala chondrite (DH‐11) shows the presence of solar‐type noble gases, as suggested by the (³⁶Ar/²⁰Ne)_t ratio, Ne‐isotopic composition, and excess of ⁴He. Cosmic‐ray exposure (CRE) ages of most chondrules are similar to their host chondrites. A few chondrules show higher CRE age compared to their host, suggesting that some chondrules and/or precursors of chondrules have received cosmic ray irradiation before accreting to their parent body. Among these chondrules, DH‐11 (with solar trapped gases) and a chondrule from Murray chondrite (MRY‐1) also have lower values of (²¹Ne/²²Ne)_c, indicative of SCR contribution. However, such evidences are sporadic and indicate that chondrule formation event may have erased such excess irradiation records by solar wind and SCR in most chondrules. These results support the nebular environment for chondrule formation.     

The L3–6 chondritic regolith breccia Northwest Africa (NWA) 869: (II) Noble gases and cosmogenic radionuclides

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.     

Noble gas record,collisional history,and pairing of CV,CO, CK,and other carbonaceous chondrites

Abstract— Concentration and isotopic composition of the light noble gases as well as of ⁸⁴Kr, ¹²⁹Xe, and ¹³²Xe have been measured in bulk samples of 60 carbonaceous chondrites; 45 were measured for the first time. Solar noble gases were found in nine specimens (Arch, Acfer 094, Dar al Gani 056, Graves Nunataks 95229, Grosnaja, Isna, Mt. Prestrud 95404, Yamato (Y) 86009, and Y 86751). These meteorites are thus regolith breccias. The CV and CO chondrites contain abundant planetary‐type noble gases, but not CK chondrites. Characteristic features of CK chondrites are high ¹²⁹Xe/¹³²Xe ratios. The petrologic type of carbonaceous chondrites is correlated with the concentration of trapped heavy noble gases, similar to observations shown for ordinary chondrites. However, this correlation is disturbed for several meteorites due to a contribution of atmospheric noble gases, an effect correlated to terrestrial weathering effects. Cosmic‐ray exposure ages are calculated from cosmogenic ²¹Ne. They range from about 1 to 63.5 Ma for CO, CV, and CK classes, which is longer than exposure ages reported for CM and CI chondrites. Only the CO3 chondrite Isna has an exceptionally low exposure age of 0.15 Ma. No dominant clusters are observed in the cosmic‐ray exposure age distribution; only for CV and CK chondrites do potential peaks seem to develop at ～9 and ～29 Ma. Several pairings among the chondrites from hot deserts are suggested, but 52 of the 60 investigated meteorites are individual falls. In general, we confirm the results of Mazor et al. (1970) regarding cosmic‐ray exposure and trapped heavy noble gases. With this study, a considerable number of new carbonaceous chondrites were added to the noble gas data base, but this is still not sufficient to obtain a clear picture of the collisional history of the carbonaceous chondrite groups. Obviously, the exposure histories of CI and CM chondrites differ from those of CV, CO, and CK chondrites that have much longer exposure ages. The close relationship among the latter three is also evident from the similar cosmic‐ray exposure age patterns that do not reveal a clear picture of major breakup events. The CK chondrites, however, with their wide range of petrologic types, form the only carbonaceous chondrite group which so far lacks a solar‐gas‐bearing regolith breccia. The CK chondrites contain only minute amounts of trapped noble gases and their noble gas fingerprint is thus distinguishable from the other groups. In the future, more analyses of newly collected CK chondrites are needed to unravel the genetic and historic evolution of this group. It is also evident that the problems of weathering and pairing have to be considered when noble gas data of carbonaceous chondrite are interpreted.     

Light noble gases and cosmogenic radionuclides in Estherville,Budulan, and other mesosiderites: Implications for exposure histories and production rates

Abstract— We report measurements of ²⁶AI, ¹⁰Be, ⁴¹Ca, and ³⁶Cl in the silicate and metal phases of 11 mesosiderites, including several specimens each of Budulan and Estherville, of the brecciated meteorite Bencubbin, and of the iron meteorite Udei Station. Average production rate ratios (atom/atom) for metal phase samples from Estherville and Budulan are ²⁶Al/¹⁰Be = 0.77 ± 0.02; ³⁶Cl/¹⁰Be = 5.3 ± 0.2. For a larger set of meteorites that includes iron meteorites and other mesosiderites, we find ²⁶Al/¹⁰Be = 0.72 ± 0.01 and ³⁶Cl/¹⁰Be = 4.5 ± 0.2. The average ⁴¹Ca/³⁶Cl production rate ratio is 1.10 ± 0.04 for metal separates from Estherville and four small iron falls. The ⁴¹Ca activities in dpm/(kg Ca) of various silicate separates from Budulan and Estherville span nearly a factor of 4, from <400 to >1600, indicating preatmospheric radii of >30 cm. After allowance for composition, the activities of ²⁶Al and ¹⁰Be (dpm/kg silicate) are similar to values measured in most ordinary chondrites and appear to depend only weakly on bulk Fe content. Unless shielding effects are larger than suggested by the ³⁶Cl and ⁴¹Ca activities of the metal phases, matrix effects are unimportant for ¹⁰Be and minor for ²⁶Al. Noble gas concentrations and isotopic abundances are reported for samples of Barea, Emery, Mincy, Morristown, and Marjalahti. New estimates of ³⁶Cl/³⁶Ar exposure ages for the metal phases agree well with published values. Neon‐21 production rates for mesosiderite silicates calculated from these ages and from measured ²¹Ne contents are consistently higher than predicted for L chondrites despite the fact that the mesosiderite silicates have lower Mg contents than L chondrites. We suggest that the elevation of the ²¹Ne production rate in mesosiderite silicates reflects a “matrix effect,” that is, the influence of the higher Fe content of mesosiderites, which acts to enhance the flux of low‐energy secondary particles and hence the ²¹Ne production from Mg. As ¹⁰Be production is relatively insensitive to this matrix effect, ¹⁰Be/²¹Ne ages give erroneously low production rates and high exposure ages. By coincidence, standard ²²Ne/²¹Ne based “shielding” corrections give fairly reliable ²¹Ne production rates in the mesosiderite silicates.