Abstract— Calcium‐aluminum‐rich inclusions (CAIs) were among the first solids in the solar system and were, similar to chondrules, created at very high temperatures. While in chondrules, trapped noble gases have recently been detected, the presence of trapped gases in CAIs is unclear but could have important implications for CAI formation and for early solar system evolution in general. To reassess this question, He, Ne, and Ar isotopes were measured in small, carefully separated and, thus, uncontaminated samples of CAIs from the CV3 chondrites Allende, Axtell, and Efremovka. The 20 Ne/ 22 Ne ratios of all CAIs studied here are <0.9, indicating the absence of trapped Ne as, e.g., Ne‐HL, Ne‐Q, or solar wind Ne. The 21 Ne/ 22 Ne ratios range from 0.86 to 0.72, with fine‐grained, more altered CAIs usually showing lower values than coarse‐grained, less altered CAIs. This is attributed to variable amounts of cosmogenic Ne produced from Na‐rich alteration phases rather than to the presence of Ne‐G or Ne‐R (essentially pure 22 Ne) in the samples. Our interpretation is supported by model calculations of the isotopic composition of cosmogenic Ne in minerals common in CAIs. The 36 Ar/ 38 Ar ratios are between 0.7 and 4.8, with fine‐grained CAIs within one meteorite showing higher ratios than the coarse‐grained ones. This agrees with higher concentrations of cosmogenic 36 Ar produced by neutron capture on 35 Cl with subsequent β − ‐decay in finer‐grained, more altered, and thus, more Cl‐rich CAIs than in coarser‐grained, less altered ones. Although our data do not strictly contradict the presence of small amounts of Ne‐G, Ne‐R, or trapped Ar in the CAIs, our noble gas signatures are most simply explained by cosmogenic production, mainly from Na‐, Ca‐, and Cl‐rich minerals.
Abstract— Cosmogenic He, Ne, and Ar were measured in the iron meteorites Grant (IIIAB) and Carbo (IID) to re‐determine their preatmospheric geometries and exposure histories. We also investigated the influence of sulphur‐ and/or phosphorus‐rich inclusions on the production rates of cosmogenic Ne. Depth profiles measured in Grant indicate a preatmospheric center location 117 mm left from the reference line and 9 mm below bar B, which is clearly different (˜10 cm) from earlier results (˜165 mm left from the reference line on bar F). For Carbo the preatmospheric center location was found to be 120 mm right of the reference line and 15 mm above bar J, which is in agreement with literature data. The new measurements indicate a spherical preatmospheric shape for both meteorites and, based on literature 36 C1 data, the radii were estimated to be about 32 cm and 70 cm for Grant and Carbo, respectively. We demonstrate that minor elements like S and P have a significant influence on the production rates of cosmogenic Ne. In our samples, containing on average 0.5% S and/or P, about 20% of 21 Ne was produced from these minor elements. Using measured 21 Ne concentrations and endmember 22 Ne/ 21 Ne ratios for Fe + Ni and S + P, respectively, we show that it is possible to correct for 21 Ne produced from S and/or P. The thus corrected data are then used to calculate new 41 K‐ 40 K exposure ages—using published K data—which results in 564 ± 78 Ma for Grant and 725 ± 100 Ma for Carbo. The correction always lowers the 21 Ne concentrations and consequently decreases the 41 K‐ 40 K exposure ages. The discrepancies between 36 Cl‐ 36 Ar and 41 K‐ 40 K ages are accordingly reduced. The existence of a significant long‐term variation of the GCR, which is based on a former 30–50% difference between 41 K‐ 40 K and 36 Cl‐ 36 Ar ages, may warrant re‐investigation.
Abstract We calibrated the 81 Kr‐Kr dating system for ordinary chondrites of different sizes using independent shielding‐corrected 36 Cl‐ 36 Ar ages. Krypton concentrations and isotopic compositions were measured in bulk samples from 14 ordinary chondrites of high petrologic type and the cosmogenic Kr component was obtained by subtracting trapped Kr from phase Q . The thus‐determined average cosmogenic 78 Kr/ 83 Kr, 80 Kr/ 83 Kr, 82 Kr/ 83 Kr, and 84 Kr/ 83 Kr ratiC(Lavielle and Marti 1988; Wieler 2002). The cosmogenic 78 Kr/ 83 Kr ratio is correlated with the cosmogenic 22 Ne/ 21 Ne ratio, confirming that 78 Kr/ 83 Kr is a reliable shielding indicator. Previously, 81 Kr‐Kr ages have been determined by assuming the cosmogenic production rate of 81 Kr, P( 81 Kr) c , to be 0.95 times the average of the cosmogenic production rates of 80 Kr and 82 Kr; the factor Y = 0.95 therefore accounts for the unequal production of the various Kr isotopes (Marti 1967a). However, Y should be regarded as an empirical adjustment. For samples whose 80 Kr and 82 Kr concentrations may be affected by neutron‐capture reactions, the shielding‐dependent cosmogenic ( 78 Kr/ 83 Kr) c ratio has been used instead to calculate P( 81 Kr)/P( 83 Kr), as for some lunar samples, this ratio has been shown to linearly increase with ( 78 Kr/ 83 Kr) c (Marti and Lugmair 1971). However, the 81 Kr‐Kr ages of our samples calculated with these methods are on average ~30% higher than their 36 Cl‐ 36 Ar ages, indicating that most if not all the 81 Kr‐Kr ages determined so far are significantly too high. We therefore re‐evaluated both methods to determine P( 81 Kr) c /P( 83 Kr) c . Our new Y value of 0.70 ± 0.04 is more than 25% lower than the value of 0.95 used so far. Furthermore, together with literature data, our data indicate that for chondrites, P( 81 Kr) c /P( 83 Kr) c is rather constant at 0.43 ± 0.02, at least for the shielding range covered by our samples ([ 78 Kr/ 83 Kr] c = 0.119–0.185; [ 22 Ne/ 21 Ne] c = 1.083–1.144), in contrast to the observations on lunar samples. As expected considering the method used, 81 Kr‐Kr ages calculated either directly with this new P( 81 Kr) c /P( 83 Kr) c value or with our new Y value both agree with the corresponding 36 Cl‐ 36 Ar ages. However, the average deviation of 2% indicates the accuracy of both new 81 Kr‐Kr dating methods and the precision of the new dating systems of ~10% is demonstrated by the low scatter in the data. Consequently, this study indicates that the 81 Kr‐Kr ages published so far are up to 30% too high.
Abstract We collected the published noble gas data of altogether 35 lunar meteorites. This compilation includes the stable isotopes of He, Ne, Ar, Kr, and Xe. We also give a summary of cosmogenic, trapped, and radiogenic noble gas components of lunar meteorites for which data are available in the literature.
Abstract We measured specific activities of the long‐lived cosmogenic radionuclides 60 Fe in 28 iron meteorites and 53 Mn in 41 iron meteorites. Accelerator mass spectrometry was applied at the 14 MV Heavy Ion Accelerator Facility at ANU Canberra for all samples except for two which were measured at the Maier‐Leibnitz Laboratory, Munich. For the large iron meteorite Twannberg (IIG), we measured six samples for 53 Mn. This work doubles the number of existing individual 60 Fe data and quadruples the number of iron meteorites studied for 60 Fe. We also significantly extended the entire 53 Mn database for iron meteorites. The 53 Mn data for the iron meteorite Twannberg vary by more than a factor of 30, indicating a significant shielding dependency. In addition, we performed new model calculations for the production of 60 Fe and 53 Mn in iron meteorites. While the new model is based on the same particle spectra as the earlier model, we no longer use experimental cross sections but instead use cross sections that were calculated using the latest version of the nuclear model code INCL. The new model predictions differ substantially from results obtained with the previous model. Predictions for the 60 Fe activity concentrations are about a factor of 2 higher, for 53 Mn, they are ~30% lower, compared to the earlier model, which gives now a better agreement with the experimental data.
Abstract— We have measured the concentrations of the cosmogenic radionuclides 10 Be, 26 Al and 36 Cl (half‐lives 1.51 Ma, 716 ka, and 300 ka, respectively) in two different laboratories by accelerator mass spectrometry (AMS) techniques, as well as concentrations and isotopic compositions of stable He, Ne and Ar in the Antarctic H‐chondrite Allan Hills (ALH) 88019. In addition, nuclear track densities were measured. From these results, it is concluded that the meteoroid ALH 88019 had a preatmospheric radius of (20 ± 5) cm and a shielding depth for the analyzed samples of between 4 and 8 cm. Using calculated and experimentally determined production rates of cosmogenic nuclides, an exposure age of ∼40 Ma is obtained from cosmogenic 21 Ne and 38 Ar. The extremely low concentrations of radionuclides are explained by a very long terrestrial age for this meteorite of 2 ± 0.4 Ma. A similarly long terrestrial age was found so far only for the Antarctic L‐chondrite Lewis Cliff (LEW) 86360. Such long ages establish one boundary condition for the history of meteorites in Antarctica.
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