Noble gas measurements in the L/LL5 chondrite Knyahinya

Abstract

Abstract-The LLL5 chondrite Knyahinya had an approximately spherical shape, and! as it experienced a single stage exposure history, it represents a very interesting object to study depth profiles of cosmic-rayproduced nuclide concentrations. Such data are required to improve and to validate model calculations of production rates. We report Ne, Ar, Kr and Xe isotopic abundances in five bulk samples. The adopted procedure of noble gas extraction included two pyrolysis steps at 450 "C and 650 "C, respectively, followed by a combustion step in pure 0 2 at 650 "C before melting the sample. This procedure allows for the separation of a significant fraction of the trapped Kr and Xe, leading to an enrichment of the cosmic-ray produced component, which is released in the melting step. The isotopic composition of the trapped Xe component measured in the combustion step is found to be identical with the OC-Xe composition (Lavielle and Marti, 1992) and supports the suggestion that ordinary chondrites formed in a homogeneous trapped noble gas reservoir. Cosmic-ray produced Kr and Xe components and depth profiles were measured, including for the first time a 81Kr profile. The calculated exposure age of 39.5 2 1.0 Ma, based on the 81Kr-G method, is found to be in excellent agreement with previous determinations. The concentrations of trapped and fissiogenic noble gas components are clearly lower than those generally observed in type 5 ordinary chondrites and may suggest diffusion losses before a meter-sized object was exposed to the cosmic radiation.

Production rates and proton‐induced production cross sections of 129I from Te and Ba: An attempt to model the 129I production in stony meteoroids and 129I in a Knyahinya sample

“…The production rates of 129 I, which were calculated using an exposure age of 39.4 Myr (Leya et al 2000b), are also presented in Table 4. This age is in excellent agreement with the 81 Kr-Kr age of 39.5 Myr obtained by Lavielle et al (1997). The blank correction of the 129 I content amounted tõ 3%.…”

Cosmogenic nuclides in stony meteorites revisited

“…Maximum differences are 18% but most ages agree to within better than 10%. For the L/LL5 chondrite Knyahinya our age of 34.8 ± 1.4 also agrees reasonably well with 81 Kr-Kr ages of 39.5 ± 1.0 Ma and 41.8 ± 0.8 Ma determined by Lavielle et al (1997) and Leya et al (2004b), respectively.…”

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

“…; Eugster ; Lavielle et al. ; 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.…”

“…While the 81 Kr-Kr dating method is theoretically applicable to all meteorites, the Kr production rate ratios were deduced from Apollo 12 lunar samples, so strictly speaking, they are most valid for samples that have the same relative abundance of the major target elements for Kr production (Rb, Sr, Y, Zr, Nb) and were irradiated under similar shielding conditions as the lunar samples (Marti and Lugmair 1971). Although the 81 Kr-Kr dating technique has been applied successfully to different types of average-sized chondrites (e.g., Eugster et al 1967b;Finkel et al 1978;Eugster 1988;Lavielle et al 1997;Miura et al 1998;Bajo and Nagao 2011), 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 2004).…”