Noble gases and nitrogen in the Almahata Sitta ureilite


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Abstract– A bulk sample (split from Almahata Sitta #36) and an acid resistant residue (from #44) have been analyzed for noble gases and nitrogen by step-wise combustion/pyrolysis. In the bulk sample, He and Ne are a mixture of cosmogenic and trapped components. Cosmic- ray exposure ages of 13.8 and 16.0 Ma are calculated based on 3He, and 21Ne, respectively. Except for a small amount of cosmogenic 3He, He, and Ne in the acid-resistant residue are not significantly above blank level. Ar, Kr, and Xe in both the bulk and residue are dominated by a trapped component, but the elemental ratios are different. While the ratios of 36Ar/132Xe and 84Kr/132Xe are about 400 and 1, respectively, in all the combustion steps of the residue, the bulk sample has about an order of magnitude more 132Xe in the corresponding combustion steps. It seems, an acid soluble phase is the host of this Xe-rich carrier and is different from a similar phase observed in the ureilite Allan Hills 82130. Nitrogen in the bulk sample and acid residue are 21.1 ppm (−36.8‰), and 249.5 ppm (−74.3‰), respectively. Peak release of C (monitored as CO + CO2), N, Ar, Kr, and Xe occurred at the 700 °C combustion step of the residue, confirming diamond as the principal carrier for these gases. In the residue, the isotopic ratio 38Ar/36Ar shows a monotonic increase with release temperature.