MINERALOGY, REFLECTANCE SPECTRA, AND PHYSICAL PROPERTIES OF THE CHELYABINSK LL5 CHONDRITE — INSIGHT INTO SHOCK-INDUCED CHANGES
IN ASTEROID REGOLITHS
Tomas Kohout1, 2, Maria Gritsevich3, 4, 5, Viktor Grokhovsky6, Grigoriy Yakovlev6, Jakub Haloda7, 8, Patricie Halodova7, Radoslaw Michallik9, Antti Penttilä1, Karri Muinonen1,3
1.
2.
3.
4.
5.
6.
7.
8.
9.
Department of Physics, University of Helsinki, P.O. Box 64, 00014 Helsinki University, Finland
Institute of Geology, Academy of Sciences of the Czech Republic, Rozvojová 269, 16500 Prague 6, Czech Republic
Finnish Geodetic Institute, Geodeetinrinne 2, P.O. Box 15, FI-02431 Masala, Finland
Institute of Mechanics, Lomonosov Moscow State University, Michurinsky prt., 1, 119192, Moscow, Russia
Russian Academy of Sciences, Dorodnicyn Computing Centre, Department of Computational Physics, Vavilova ul. 40, 119333 Moscow, Russia
Ural Federal University, Ekaterinburg, Russia
Czech Geological Survey, Geologická 6, 152 00 Praha 5, Czech Republic
Oxford Instruments NanoAnalysis, High Wycombe, Bucks, United Kingdom
Department of Geosciences and Geography, University of Helsinki, P.O. Box 64, 00014 Helsinki University, Finland
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. A large-sized object with a relatively low mass-loss rate and shallow atmospheric entry angle led to a very long trail. The event
was recorded by numerous video cameras from the ground and was also imaged from space by the Meteosat and Fengyun satellites. 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. In total three lithologies, the light-colored, dark-colored, and impact melt lithologies,
were found within the recovered meteorites.
28
Mineralogy
26
Bulk and grain density and porosity of both light- and dark-colored lithology match values typical for LL chondrites. Magnetic susceptibility,
• The light colored lithology is a LL5 ordinary chondrite shocked to S4 level.
however, is in intermediate L/LL range. Thus, compared to a typical LL chondrite, Chelyabinsk meteorites are richer in metallic iron. No
• The dark colored lithology is of identical LL5 composition. However, it is shocked to higher level (shock-darkened).
significant difference related to shock-darkening is observed among light- and dark-colored lithology or impact melt rich clasts. Thus, shock does
The silicate grains are mechanically crushed. Partial melting occurred and is limited mainly to metal and sulfides.
not have a significant effect on the material physical properties.
Molten metal and sulfide-rich melt formed a dense network of fine veins impregnating the inter- and intra-granular
The reflectance spectrum of the light-colored lithology is typical for an LL ordinary chondrite with the presence of 1 and 2 µm olivine and
pore space within crushed silicate grains. Thus, high pressure loads and moderate temperature rise is characteristic
Light
Dark
Impact melt
pyroxene absorption bands. Such a spectrum is similar to fresh S or Q type asteroids. In contrast, the shocked dark-colored or impact melt
for shock darkening
Mole % Fs in low-Ca px
24
L
20
18
H
16
lithologies are darker and the silicate absorption bands are almost invisible. Thus, the shock history affects the reflectance and the depth of the
• The impact melt lithology is a breccia of crushed mineral grains with various amounts of silicate melt. It derived
14
absorption bands of chondrite material in a way similar to space weathering. However, unlike space weathering (or at least unlike its lunar type
from similar LL5 source material and is present as inter-granular veins within both light-colored and dark-colored
12
form with the presence of nanophase iron in the surface coatings of silicate minerals) there is no change in the spectral slope observed due to the
lithologies, or as a matrix supporting larger ~mm to ~ cm-sized clasts in brecciated meteorites. In comparison to
increased shock.
shock darkening the impact melting requires higher temperatures to reach melting point of the silicates.
10
12
16
Dark-colored Lithology
Impact Melt Lithology
L
2000
Frequency (n=24)
Impact melt
breccia
6
4
27
28
29 30
31
Light-colored
Dark-colored
3.27 (s.d.0.08)
3.51 (s.d. 0.07)
3.42 (s.d. 0.10)
Impact melt
vein
SEM-BSE
Dark ol
SEM-BSE
8
6
4
6
4
19
6.0 (s.d. 3.2)
5.7 (s.d. 1.7)
4.49 (s.d. 0.07)
20
16
21
22
23
Dark px
24
25
26
= 22.52
Me = 22.53
= 0.39
n = 15
14
12
10
8
6
4
0
26
27
28
29 30
16
IM ol
31
32
19
14
12
20
21
22
23
24
25
26
Mole % Fs in low-Ca px
= 29.09
Me = 28.19
= 0.27
n =6
10
8
6
4
16
IM px
= 23.41
Me = 23.41
= 0.10
n= 6
14
12
10
8
6
4
2
0
0
24
25
26
27
28
29 30
Mole % Fa in ol
Magnetic susceptibility
(log in 10-9 m3/kg)
8
2
25
2
Porosity (%)
= 22.70
Me = 22.63
= 0.57
n = 20
Mole % Fs in low-Ca px
= 27.88
Me = 27.81
= 0.42
n = 24
10
24
SEM-BSE
32
10
32
0
Frequency (n=6)
Grain density (g/cm3)
26
2
Light-colored
lithology
clast
Light px
12
Mole % Fa in ol
3.32 (s.d. 0.09)
30
0
Wavelength (nm)
Bulk density (g/cm3)
28
2
25
12
2400
Impact melt
vein
26
14
Frequency (n=6)
1600
8
16
Sulphide and
metal-rich melt
vein
1200
= 27.96
Me = 27.90
= 0.52
n = 25
10
14
0
24
16
Mole % Fa in ol
3.6
800
22
Frequency (n=15)
Dark
4.4
Impact melt
vein
4
0.04
Light ol
12
24
LL
0.08
20
0
4.8
Light
(in 10-9 m3/kg)
0.12
18
2
Log
Refectance
5.2
Frequency (n=25)
Light-colored Lithology
5.6
H
0.16
16
14
0.2
Light-colored lithology
Dark-colored lithology
Impact melt lithology
14
Mole % Fa in ol
Magnetic Susceptibility
Reflectance Spectra
LL
22
Frequency (n=20)
Physical Properties
31
32
19
20
21
22
23
24
25
26
Mole % Fs in low-Ca px
4.52 (s.d. 0.15)
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. Three lithologies, light-colored, dark-colored, and impact melt, were found within the recovered meteorites. All three are of LL5 composition. The darkcolored lithology is being shocked to a higher level (shock darkening). The silicate grains are mechanically crushed and partial melting, limited to metal and sulfides, occurs. The impact melt lithology experienced higher temperatures during the shock and is made of crushed mineral grains with a silicate melt.
Based on the magnetic susceptibility, the Chelyabinsk meteorites are richer in metallic iron as compared to other LL chondrites (in the intermediate range between LL and L 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.
However, changes in reflectance spectra are observed in the highly shocked lithologies. A decrease in reflectance and suppression of the silicate absorption bands is observed in both shock-darkened and impact melt lithologies. This is similar to the space weathering effects observed on asteroids. However, no spectral slope
change similar to space weathering is observed. Thus, it is possible that some dark asteroids with invisible silicate absorption bands may be composed of relatively fresh shock-darkened or molten chondritic material. The main spectral difference between shock darkening and space weathering of asteroid surfaces is the
presence of the spectral slope change (reddening) in the latter case.
Acknowledgments
The project is supported by Academy of Finland, Ministry of Education, Youth and Sports, Czech Republic, and ERC Advanced Grant. Authors would like to thank to Ilya Weinstein for help with the measurements arrangements and to Alevtina Maksimova, Razilya Gizzatullina, Albina Zainullina, and Anastasia Uryvkova
for help with the laboratory work.