Cosmogenic Noble Gas Nuclides In Zircons From The Estherville Mesosiderite

Zirconium 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 Kr-81 (t(1/2) = 2.29 x 10(5) years) was successfully detected in the zircons. The Kr-81-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 He-3 and Ne-21 (82 +/- 8 and 88 +/- 9 Ma, respectively) of the silicate part and the previously reported Cl-36-Ar-36 age of the metal part (77 +/- 9 Ma). The consistent CRE ages using different dating methods demonstrate that the Kr-81-Kr dating using meteoritic zircons is a new promising tool for determining the CRE age of meteorites. Moreover, based on the Kr-81-Kr age of the zircons, the production rates of cosmogenic He-3 and Ne-21 in a meteoritic zircon were estimated to be (15 +/- 2) x 10(-9) and (0.69 +/- 0.04) x 10(-9) cm(3) STP g(-1) Ma(-1), respectively.