Analytical results for the material of the Chelyabinsk meteorite

Geochemistry International (Impact Factor: 0.53). 07/2013; 51(7). DOI: 10.1134/S0016702913070100

ABSTRACT This paper presents the results of the mineralogical, petrographic, elemental, and isotopic analysis of the Chelyabinsk meteorite and their geochemical interpretation. It was shown that the meteorite can be assigned to LL5-group ordinary chondrites and underwent moderate shock metamorphism (stage S4). The Chelyabinsk meteorite contains a significant fraction (approximately one-third by volume) of shock-melted material similar in composition to the main volume of the meteorite. The results of isotopic analysis suggest that the history of meteorite formation included an impact event approximately 290 Ma ago.

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    ABSTRACT: Cosmogenic radionuclides produced by galactic cosmic rays (GCR) in meteorites during their motion in space are natural detectors of the GCR intensity and variations along the meteorite orbits. On the basis of measured and calculated contents of cosmogenic radionuclides in the freshly fallen Chelyabinsk and Košice chondrites some peculiarities of generation of cosmogenic radionuclides of different half-lives in the chondrites of different orbits and dates of fall onto the Earth are demonstrated. Dependence of production rates of the radionuclides on the GCR variations in the heliosphere is analyzed. Using radionuclides with different half-lives it is possible to compare the average GCR intensity over various time periods. The measurement and theoretical analysis of cosmogenic radionuclides in consecutively fallen chondrites provide a unique information on the space-time continuum of the cosmogenic radionuclide production rates and their variations over a long time scale, which could be useful in correlative analyses of processes in the heliosphere. Some applications of cosmogenic radionuclide depth distribution in chondrites for estimation of their pre-atmospheric sizes are illustrated.
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    ABSTRACT: The Chelyabinsk meteorite (which fell on February 15, 2013) is a LL5 chondrite of shock stage S4, whose fragments are classified into light and dark lithologies. According to the intensity of their shock metamorphism, light lithology fragments are subdivided into two groups, which were affected by peak pressures within the ranges of 20-25 and 25-30 GPa, respectively. The material of the dark lithology was shocked at 25-30 GPa but was then annealed, which resulted in a decrease in the discernible degree of shock metamorphism. Black veins cutting across both the light and the dark lithologies and impact melt dikes in the dark lithology were produced by friction melting along boundaries of blocks that had been generated by fragmentation in a shock wave. The impact melt of the dikes is slightly enriched in Si, Al, Ca, Na, and K and has an oxygen isotopic composition similar to the chondrite matrix. It is thought that black vein melt started to crystallize in a rarefaction wave. Melt in the dikes and the central portions of the black veins crystallized after total pressure release. Heating of material hosting the melt dikes resulted in its blackening and annealing of its shock metamorphic features. The Hugoniot obtained for the Chelyabinsk meteorite was utilized to calculate the post-shock and shock temperatures within a broad pressure range. According to these evaluations, the meteorite was heated for 65-135 degrees during the impact event. The melting of the LL chondrite started at a load of approximately 100 GPa because of the high “equilibrium” post-shock temperatures, and a pressure of 140 GPa resulted in the complete melting of the material.
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    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.
    02/2015; 50(2). DOI:10.1111/maps.12419