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

Abstract

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 244 Pu 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 81 Kr-Kr exposure age of 168 ± 8 Myr for the silicate pebble is distinctly longer than the age of 139 ± 8 Myr for the bulk samples. The precursor of the pebble had been irradiated on the surface of the VMPB for more than 60 Myr (first stage irradiation), with subsequent incorporation into bulk materials approximately 4 Gyr ago. The Vaca Muerta meteorite was excavated from the VMPB 140 Myr 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 3.8 Gyr ago.

Neutron capture records of mesosiderites and an iron meteorite

“…These results suggest a two-stage irradiation occurred for the silicates of Vaca Muerta in the parent body (>50 Ma) before the formation of the mesosiderite and during its transit to earth (138 Ma). Our model is consistent with the results of the 81 Kr-Kr CRE ages of the Vaca Muerta pebble (Bajo and Nagao, 2011), which were estimated to be >60 Ma in the first CRE age before the second-stage irradiation.…”

“…On the other hand, at least a CRE age of 50 Ma (u < 4.3 n cm À2 s À1 and W = 6.7 Â 10 15 n cm À2 ) on the surface of a eucritic crust is required to amount to the thermal neutron fluence of 6.7 Â 10 15 n cm À2 as the first-stage irradiation for the silicates of Vaca Muerta. This is consistent with the 81 Kr-Kr CRE age of the basaltic pebble having an initial irradiation duration of >60 Ma (Bajo and Nagao, 2011).…”

“…Bajo and Nagao (2011) found two 81 Kr-Kr CRE ages of 168 ± 8 and 139 ± 8 Ma in the metal-silicate mixture and the eucritic clasts, respectively, in Vaca Muerta. This result suggests an initial irradiation of the pebble on the parent body in addition to space irradiation in later stage, although the sample used in this study was not related to that of Bajo and Nagao (2011).…”

“…The burial depth of the eucritic silicates of Vaca Muerta at 2p irradiation in the parent body can be estimated using the thermal neutron fluence of 6.7 Â 10 15 n cm À2 obtained in this study and an initial irradiation duration of >60 Ma supported by 81 Kr-Kr age data (Bajo and Nagao, 2011). Fig.…”

Calibration of cosmogenic noble gas production in ordinary chondrites based on 36Cl‐36Ar ages. Part 1: Refined produced rates for cosmogenic 21Ne and 38Ar

“…; Miura et al. ; Bajo and Nagao ), a rigorous testing of the correlation obtained from lunar samples is still lacking for meteorite samples of different sizes and chemistries. One earlier study aiming to calibrate the 81 Kr‐Kr dating system for H chondrites using independent 36 Cl‐ 36 Ar ages suffered from large uncertainties in the cosmogenic Kr concentrations as the Kr composition of these samples was dominated by large amounts of trapped atmospheric and primordial Kr (Leya et al.…”