81Kr-Kr age and multiple cosmic-ray exposure history of the Vaca Muerta mesosiderite

Abstract

Noble gas isotopic compositions were measured for a eucritic pebble and bulk material of a silicate-metal mixture from the Vaca Muerta mesosiderite as well as pyroxene and plagioclase separated from the eucritic pebble by total melting and stepwise heating methods. Trapped noble gases were degassed completely by a high-temperature thermal event, probably at the formation of the Vaca Muerta parent body (VMPB). The presence of fissiogenic Xe isotopes from extinct 244Pu in the bulk samples might be a result of rapid cooling from an early high-temperature metamorphism. High concentrations of cosmogenic noble gases enabled us to determine precise isotopic ratios of cosmogenic Kr and Xe. Spallogenic Ne from Na and unique Ar isotopic compositions were observed. The 81Kr-Kr exposure age of 168±8Myr for the silicate pebble is distinctly longer than the age of 139±8Myr for the bulk samples. The precursor of the pebble had been irradiated on the surface of the VMPB for more than 60Myr (first stage irradiation), with subsequent incorporation into bulk materials approximately 4Gyr ago. The Vaca Muerta meteorite was excavated from the VMPB 140Myr ago (second stage irradiation). Relative diffusion rates among the cosmogenic Ar, Kr, and Xe based on data obtained by stepwise heating indicate that Kr and Xe can be partially retained in pyroxene and plagioclase under the condition that resets the K-Ar system. This result supports the presence of fission Xe and of excess concentration of cosmogenic Kr, which could have survived the thermal event approximately 3.8Gyr ago. © The Meteoritical Society, 2011.