Mineralogy, reflectance spectra, and physical properties of the Chelyabinsk LL5 chondrite – Insight into shock-induced changes in asteroid regoliths

doi:10.1016/j.icarus.2013.09.027

Icarus

Volume 228, 15 January 2014, Pages 78-85

https://doi.org/10.1016/j.icarus.2013.09.027 Get rights and content

Highlights

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Chelyabinsk meteorite is an LL5 chondrite.
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Light-colored, dark-colored, and impact-melt lithologies are distinguished.
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Shock darkening does not affect the material physical properties.
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Impact melting and shock darkening cause suppression of the silicate absorption bands.
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Some dark asteroids can be composed of shock-darkened chondritic material.

Abstract

The mineralogy and physical properties of Chelyabinsk meteorites (fall, February 15, 2013) are presented. Three types of meteorite material are present, described as the light-colored, dark-colored, and impact-melt lithologies. All are of LL5 composition with the impact-melt lithology being close to whole-rock melt and the dark-colored lithology being shock-darkened due to partial melting of iron metal and sulfides. This enables us to study the effect of increasing shock on material with identical composition and origin. Based on the magnetic susceptibility, the Chelyabinsk meteorites are richer in metallic iron as compared to other LL chondrites. The measured bulk and grain densities and the porosity closely resemble other LL chondrites. Shock darkening does not have a significant effect on the material physical properties, but causes a decrease of reflectance and decrease in silicate absorption bands in the reflectance spectra. This is similar to the space weathering effects observed on asteroids. However, compared to space-weathered materials, there is a negligible to minor slope change observed in impact-melt and shock-darkened meteorite spectra. Thus, it is possible that some dark asteroids with invisible silicate absorption bands may be composed of relatively fresh shock-darkened chondritic material.

Graphical abstract

Introduction

On February 15, 2013, at 9:22 am, an exceptionally bright and long-duration fireball was observed by many eyewitnesses in the Chelyabinsk region, Russia (Meteorite Bulletin No. 102, in preparation; Galimov et al., 2013). A large-sized object with a relatively low mass-loss rate and shallow atmospheric entry angle led to a very long trail. The fireball was observed in the Chelyabinsk, Kurgan, Orenburg, Tyumen, Ekaterinburg, Kostanay, and Aktobe regions, and in the Republic of Bashkortostan, Russia, and in Kazakhstan. The event was recorded by numerous video cameras from the ground and was also imaged from space by the Meteosat and Fengyun satellites (Proud, 2013). A strong shock wave associated with the fireball caused significant damage including broken windows and partial building collapses in Chelyabinsk and the surrounding territories.

Two days later the first fragments of the Chelyabinsk meteorite were reported to be found around Pervomaiskoe, Deputatsky, and Yemanzhelinka, located approximately 40 km south of Chelyabinsk. Successful search campaigns in this area were organized by the scientists from the Ural Federal University as well as by the Laboratory of Meteoritics of the Vernadsky Institute, Russian Academy of Science. Numerous meteorite samples were also recovered and collected out of snow by local residents before a subsequent extensive snowfall. The majority of fragments are composed of relatively small pieces, with the largest officially reported being 3.4 kg as of this publication. The largest piece in the Ural Federal University collections is a 1.8 kg sample no. A50 (Fig. 1). A large amount of additional material was collected on the ground along the fireball track after the snow melted at the end of April and in May 2013. The total collected mass of the Chelyabinsk meteorites exceeds 100 kg.

As more meteorite samples were collected, it became apparent that multiple types of material are present. The recovered meteorites are composed of either light-colored or dark-colored (shock-darkened) lithology (Meteorite Bulletin No. 102, in preparation). Additionally, dark impact-melt is being present in varying amounts within light- and dark-colored stones. Impact-melt is distinct in appearance from the dark-colored (shock-darkened) lithology and is distinguished as a third lithology type. Some meteorites contain significant portion of this impact-melt lithology (Fig. 2). However, the light- and dark-colored (shock-darkened) lithologies were (up to our knowledge) not found together within one meteorite.

According to the computations by Yeomans and Chodas (2013), the initial diameter of the meteoroid (or a small asteroid) was approximately 17–20 m with a mass of approximately 11 × 10⁶ kg. This object is the largest observed celestial body colliding with the Earth since the Tunguska event in 1908.

In this study, we report physical properties (bulk and grain density, porosity, and magnetic susceptibility) of 49 individual Chelyabinsk meteorites from the Ural Federal University collections, Ekaterinburg, Russian Federation. Detailed mineralogical and spectral characterization of two samples (representing the light-colored and dark-colored lithologies, both with impact-melt veins) is presented to determine and explain the differences between the lithologies. Finally, the origin of the Chelyabinsk meteorites and their parent body history are briefly discussed.

Section snippets

Mineralogy and chemistry

The mineral composition was determined at the Department of Geosciences and Geography, University of Helsinki, with a JEOL JXA-8600 Superprobe using standard-calibrated energy-dispersive spectrometry (EDS) on a nitrogen-cooled Si(Li) detector. Two samples (the light-colored and dark-colored lithologies) were polished and carbon-coated prior to the measurements. Matrix correction was performed with the XMas software using the PAP correction method. A combination of natural and synthetic silicate

Mineralogy and shock characterization

Fig. 3 shows the overall optical and detailed SEM images of the light-colored (no. VG1) and dark-colored (no. VG4b) lithology meteorites. The computed olivine fayalite (Fa) content and orthopyroxene ferrosilite (Fs) content determined by means of EDS (Fig. 4) from the main meteorite compounds (matrix and chondrules, excluding melt veins) reveal that the light-colored and dark-colored lithologies are almost identical in composition and, based on the classification scheme in Brearley and Jones

Discussion

The Chelyabinsk meteorite fragments represent fresh LL5 type chondrite material originating from a small, ∼20 m sized meteoroid (small asteroid). The meteorite samples are of petrographic type 5 and were subjected to recrystallization at approx. 700–750 °C. This thermal metamorphism evidence suggests that their parent body must have been originally much larger. Therefore the meteoroid, which collided with the Earth and delivered the Chelyabinsk meteorites, was only a fragment of the LL chondrite

Conclusions

The Chelyabinsk fireball with an almost instant recovery of a large number of fresh meteorites is one of the most spectacular meteorite fall events in the recent history. The recovered meteorite material was characterized as brecciated LL5 ordinary chondrite. Based on the magnetic susceptibility being in the intermediate range between LL and L chondrites, the Chelyabinsk meteorites are richer in metallic iron as compared to other LL chondrites. The measured bulk and grain densities and the

Acknowledgments

The work was supported by Academy of Finland Projects Nos. 257487 and 260027 (SIRONTA), Ministry of Education, Youth and Sports Czech Republic Grant LH12079, Federal Grant-in-Aid Program GC No. 14.740.11.1006, and the ERC Advanced Grant No. 320773 (SAEMPL). Authors would like to thank to Ilya Weinstein for help with the measurements arrangements, to Pasi Heikkilä, Alevtina Maksimova, Razilya Gizzatullina, Albina Zainullina, and Anastasia Uryvkova for help with the laboratory work, to Hanna

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