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New noble gas data of primitive and differentiated achondrites includingNorthwest Africa 011 and Tafassasset Abstract-This work reports on the noble gas inventory of 3 new acapulcoites, 3 brachinites, 2 new eucrites from the Dar al Gani region in Libya, the unique achondrite Dar al Gani (DaG) 896 from the same locality, the new eucrite-like achondrite Northwest Africa (NWA) 011, and the controversial sample Tafassasset. We determined cosmic ray exposure and gas retention ages, evaluated shielding conditions, and discuss the trapped noble gas component of the specimens. All exposure ages are within the known range of stony meteorites and partly confirm previously established age clusters. Shielding conditions vary, suggesting substantial shielding for all 3 brachinites and Tafassasset. We cannot exclude, however, that the Mg-rich composition of brachinites simply simulates heavy shielding. Regarding the trapped component, we found Q-like compositions only for the acapulcoite Thiel Mountains (TIL) 99002. The brachinite Elephant Moraine (EET) 99402 yields a high, subsolar 36 Ar/ 132 Xe ratio of ~400 along with a slightly elevated 84 Kr/ 132 Xe ratio, indicating minor atmospheric contamination. All the other samples, particularly the eucrite DaG 983, are characterized by clearly elevated Ar/Kr/Xe ratios due to significant terrestrial alteration.Tafassasset exhibits noble gas parameters that are different from those of CR chondrites, including a relatively high cosmic ray exposure age, the absence of a solar component, low 132 Xe concentrations, a low trapped 36 Ar/ 132 Xe ratio of ~30, and a noticeable amount of radiogenic 129 Xe. Similar attributes have been observed for some primitive achondrites. These attributes are also consistent with the metamorphic character of the sample. We, therefore, consider Tafassasset's noble gas record to be inconclusive as to its classification (primitive achondrite versus metamorphosed CR chondrite).

Abstract-We measured cosmic-ray products-noble gases, radionuclides, thermoluminescence, and nuclear tracks-and trace element contents and mineralogy of samples ofthree orthogonal and mutually intersecting cores (41-46 em long) of a 101.6 kg Ghubara individual (1958,805) at The Natural History Museum, London. The xenoliths, like the host, have high concentrations of trapped solar gases and are heavily shocked. While contents of noble gases and degree of shock-loading in this individual and three others differ somewhat, the data indicate that Ghubara is a two-generation regolith breccia. Contents of cosmogenic 26A I and lOBe and low track densities indicate that the Ghubara individuals were located more than 15 em below the surface of an 85 em met eoroid. Because of its large size, Ghubara's cosmic-ray exposure age is poorly defined to be 15-20 Ma from cosmogenic nuclides. Ghubara's terrestrial age , based on l4C data, is 2-3 ka . Not only is Ghubara the first known case of a two-generation regolith breccia on the macroscale , it also has a complicated thermal and irradiation history.

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 14 C, 10 Be, 26 Al, 36 Cl, and 41 Ca 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 22 Ne/ 21 Ne ratios and large contributions of neutron-capture 36 Cl and 41 Ca. 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 14 C 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 36 Cl and 41 Ca during oxidation of metal and troilite, concentrations of 36 Cl and 41 Ca in the silicates are also consistent with 14 C ages <35 kyr. The only exception is DaG 343 (H4), which has a 41 Ca 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 26 Al/ 21 Ne and 10 Be/ 21 Ne systematics in 16 DaG meteorites, we derived more reliable estimates of the 10 Be/ 21 Ne 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).

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.