Cosmogenic noble gas nuclides in zircons from the Estherville mesosiderite

Abstract

AbstractZirconium produces cosmogenic Kr through spallation reactions with cosmic rays. Meteoritic zircons (ZrSiO4) therefore possibly contain a significant amount of cosmogenic Kr in addition to other cosmogenic nuclides. Detection of cosmogenic nuclides from meteoritic zircons would make it possible to determine precise cosmic ray exposure (CRE) ages without knowing the whole rock chemistry because of the robust nature of zircons and limited target elements that produce cosmogenic nuclides in a zircon crystal. Herein, we report the noble gas compositions of zircons separated from the Estherville mesosiderite in addition to those of the silicate and metal parts. The zircons contain cosmogenic noble gas nuclides, and more importantly, cosmogenic 81Kr (t1/2 = 2.29 × 105 years) was successfully detected in the zircons. The 81Kr‐Kr exposure age of the zircons was calculated to be 76 ± 5 million years (Ma). This age corresponds to the CRE ages obtained from cosmogenic 3He and 21Ne (82 ± 8 and 88 ± 9 Ma, respectively) of the silicate part and the previously reported 36Cl‐36Ar age of the metal part (77 ± 9 Ma). The consistent CRE ages using different dating methods demonstrate that the 81Kr‐Kr dating using meteoritic zircons is a new promising tool for determining the CRE age of meteorites. Moreover, based on the 81Kr‐Kr age of the zircons, the production rates of cosmogenic 3He and 21Ne in a meteoritic zircon were estimated to be (15 ± 2) × 10−9 and (0.69 ± 0.04) × 10−9 cm3 STP g−1 Ma−1, respectively.