Cosmogenic radionuclides and mineralogical properties of the Chelyabinsk (LL5) meteorite: What do we learn about the meteoroid?

Abstract

On February 15, 2013, after the observation of a brilliant fireball and a spectacular airburst over the southern Ural region (Russia), thousands of stones fell and were rapidly recovered, bringing some extremely fresh material for scientific investigations. We undertook a multidisciplinary study of a dozen stones of the Chelyabinsk meteorite, including petrographic and microprobe investigations to unravel intrinsic characteristics of this meteorite. We also study the short and long‐lived cosmogenic radionuclides to characterize the initial meteoroid size and exposure age. Petrographic observations, as well as the mineral compositions obtained by electron microprobe analyses, allow us to confirm the classification of the Chelyabinsk meteorite as an LL5 chondrite. The fragments studied, a few of which are impact melt rocks, contain abundant shock melt veins and melt pockets. It is likely that the catastrophic explosion and fragmentation of the Chelyabinsk meteoroid into thousands of stones was in part determined by the initial state of the meteoroid. The radionuclide results obtained show a wide range of concentrations of 14C, 22Na, 26Al, 54Mn, 57Co, 58Co, and 60Co, which indicate that the pre‐atmospheric object had a radius >5 m, consistent with other size estimates based on the magnitude of the airburst caused by the atmospheric entry and breakup of the Chelyabinsk meteoroid. Considering the observed 26Al activities of the investigated samples, Monte Carlo simulations, and taking into account the 26Al half‐life (0.717 Myr), the cosmic‐ray exposure age of the Chelyabinsk meteorite is estimated to be 1.2 ± 0.2 Myr. In contrast to the other radionuclides, 14C showed a very large range only consistent with most samples having been exposed to anthropogenic sources of 14C, which we associate with radioactive contamination of the Chelyabinsk region by past nuclear accidents and waste disposal, which has also been confirmed by elevated levels of anthropogenic 137Cs and primordial 40K in some of the Chelyabinsk fragments.

Mineralogy, petrology, chronology, and exposure history of the Chelyabinsk meteorite and parent body

“…; Povinec et al. ), we use modeled production rates given by Leya and Masarik () for spheres of radius 500 cm (5 m). According to this model, the maximum production rates occur at depths of ~12 cm beneath the surface of the modeled sphere.…”

The Chelyabinsk event

“…Its reflectance spectrum can mimic carbonaceous material. The cosmic ray exposure age was measured to be 1.2 Myr, one of the lowest among LL chondrites (Popova et al 2013;Povinec et al 2015).…”

The history of the Tissint meteorite, from its crystallization on Mars to its exposure in space: New geochemical, isotopic, and cosmogenic nuclide data

“…The pre-atmospheric radius of the Tissint meteorite was estimated using the following considerations. The 60 Co has been used as a depth indicator for fragments within the meteorite body, as well as for the estimation of its radius (e.g., Povinec et al 2013Povinec et al , 2015aPovinec et al , 2015b. It has mainly been produced in the meteorite by the capture of thermal neutrons on 59 Co nuclei.…”