Cosmic‐ray exposure ages of six chondritic Almahata Sitta fragments

Abstract

AbstractThe Almahata Sitta strewn field is dominated by ureilites, but contains a large fraction of chondritic fragments of various types. We analyzed stable isotopes of He, Ne, Ar, Kr, and Xe, and the cosmogenic radionuclides10Be,26Al, and36Cl in six chondritic Almahata Sitta fragments (EL6 breccia,EL6,EL3‐5,CB,LL4/5, R‐like). The cosmic‐ray exposure (CRE) ages of five of the six samples have an average of 19.2 ± 3.3 Ma, close to the average of 19.5 ± 2.5 Ma for four ureilites. The cosmogenic radionuclide concentrations in the chondrites indicate a preatmospheric size consistent with Almahata Sitta. This corroborates that Almahata Sitta chondrite samples were part of the same asteroid as the ureilites. However,MS‐179 has a lowerCREage of 11.0 ± 1.4 Ma. Further analysis of short‐lived radionuclides in fragmentMS‐179 showed that it fell around the same time, and from an object of similar size as Almahata Sitta, making it almost certain thatMS‐179 is an Almahata Sitta fragment. Instead, its lowCREage could be due to gas loss, chemical heterogeneity that may have led to an erroneous21Ne production‐rate, or, perhaps most likely,MS‐179 could represent the true 4π exposure age of Almahata Sitta (or an upper limit thereof), while all other samples analyzed so far experienced exposure on the parent body of similar lengths. Finally,MS‐179 had an extraordinarily high activity of neutron‐capture36Cl, ~600 dpm kg−1, the highest activity observed in any meteorite to date, related to a high abundance of the Cl‐bearing mineral lawrencite.