Noble gases and nitrogen in the Almahata Sitta ureilite

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.