ISSN 1028334X, Doklady Earth Sciences, 2015, Vol. 462, Part 2, pp. 586–591. © Pleiades Publishing, Ltd., 2015. Original Russian Text © S.G. Skublov, N.S. Guseva, S.L. Presnyakov, X.H. Li, Yu.B. Marin, S.A. Sergeev, N.G. Berezhnaya, N.V. Tyuleneva, V.I. Alekseev, 2015, published in Doklady Akademii Nauk, 2015, Vol. 462, No. 4, pp. 461–466.

GEOCHEMISTRY

U–Pb Age of Zircon and the History of Impact Transformations of the Chelyabinsk Meteorite S. G. Skublova, b, N. S. Gusevac, S. L. Presnyakovd, X.H. Lie, Corresponding Member of the RAS Yu. B. Marinb, S. A. Sergeevd, f, N. G. Berezhnayad, N. V. Tyulenevag, and V. I. Alekseevb Received January 26, 2015

DOI: 10.1134/S1028334X15060069

The fall of the Chelyabinsk meteorite LL5 (Febru ary 15, 2013), the largest in the past century, caused a wide resonance in the scientific world and initiated the complex investigation of meteorite material. The results of such study have been reported in many pub lications ([1, 2] and others). In discussing the results of the isotope–geochemical study of this meteorite, we should primarily mention the local U–Pb dating of apatite in situ, which was performed independently by X.H. Li in Beijing, China (4452 ± 21 Ma [2]) and by K. Terada in Hiroshima, Japan (4433 ± 110 Ma [3]). Both ages are consistent within the error and, in opin ion of the authors [2, 3], correspond to the largest impact event which was registered by apatite in other meteorites as well (e.g., Novato L6 [4]) and highly likely this impact event was connected with the forma tion of the Moon. The older age (4538 ± 2 Ma) obtained by A. Bouvier [5] who used the isochron method for leached meteorite glass is correlated by the author with the impact event, rather than with cooling of the parental body of the meteorite after its forma tion. Based on the results of the study of rockforming minerals (olivine, orthopyroxene, and troilite) of the Chelyabinsk meteorite, the Sm–Nd isochron with an age of 3733 ± 110 Ma was plotted [6]. This is the age of an impact event, which resulted in melting of meteor ite material and reequilibration of the Sm–Nd iso tope system. Independent dating of the meteorite by

a

Institute of Precambrian Geology and Geochronology, Russian Academy of Sciences, St. Petersburg, Russia b National Mineral Resources University (Mining University), St. Petersburg, Russia c OOO Maiskoe GoldMine Company, Pevek, Russia d Karpinskii Russian Geological Research Institute, St. Petersburg, Russia e State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China f St. Petersburg State University, St. Petersburg, Russia g Odessa National University, Odessa, Ukraine email: [email protected]

the Sm–Nd method at the Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sci ences, was not performed for monomineral samples and did not provide a precise isochronous depen dence. A linear trend (“geochron”) corresponding to an age of ~290 Ma was obtained [1]. The age of frag mentation of the parental body of the Chelyabinsk meteorite (the age of exposure) was estimated by the concentration of cosmogenic nuclides as ~1.2 Ma [7]. One zircon grain (no. 1 in Tables 1 and 2) was found at the laboratory of the Institute of Precambrian Geology and Geochronology, Russian Academy of Sciences, during extraction of monofractions of rock forming minerals for Sm–Nd dating from samples with a total weight of 20 g by a methodology which excludes a laboratory contamination [6]. With account for the importance of the find, zircon extraction was repeated from the sample with a weight of ~70 g at the laboratory of the ZAO “NATI” by the “ppmmineral ogy” technology [8] (www.natires.com). This technol ogy was primarily worked out for extraction and study of heavy ore minerals, especially gold and PGEbear ing phases, at a low (<1 ppm) concentration of these phases in rock. Later this technology was adapted for extraction of zircon, apatite, and monazite. Distinct groups of heavy minerals` concentrates with a big range of densities were obtained and studied and this approach allows to achieve required mineralogical sensitivity. The process of gravitational fractionation was carried out on a hydroseparator designed in the ZAO “NATI.” To prevent pollution, the applied appa ratus was preliminarily washed, a small portion of meteorite material was ground, and then it was washed again. New sieves were used in the study. The meteor ite sample was ground stepwise with separation of the –2.0 mm class after each stage of grinding. The ground material was passed through a 0.250mm sieve without preliminary grinding; the remnant on the sieve was ground stepbystep on a LDI85 grinder up to a 100% output of the –0.250 mm class. Such a scheme of grinding provides maximal preservation of grain completeness. The ground material with a size of –0.250 mm was sifted on sieves of 0.125, 0.071, and

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Table 1. Results of the U–Pb and δ18О local analysis of zircon from the Chelyabinsk meteorite 232

Point of analysis

206

Pbc, %

U, ppm

1 2 3 4 2re

0.63 0.12 0.09 1.97 b.d.l.

378 617 438 368 453

2 2re 23 21 3 1 4

0.15 0.10 0.18 0.11 0.14 0.27 0.19

155 141 190 209 281 271 216

207

D, %

Pb  235 U

Th, ppm

Th  238 U

Zircon grain no. 1 413 1.13 657 1.10 406 0.96 474 1.33 581 1.33 Zircon grain no. 2 43 0.29 35 0.25 38 0.21 26 0.13 47 0.17 42 0.16 26 0.12

206

Pb*, ppm

44.5 73.8 50.7 44.8 53.9

Age Pb/238U, Ma

206

870 ± 7 877 ± 5 843 ± 4 550 ± 260 299 ± 43

Age Pb/206Pb, Ma

207

779 ± 38 803 ± 18 776 ± 23 804 ± 69 809 ± 23

74.9 69.1 96.2 97.2 128 115 95.1

2876 ± 23 2900 ± 22 2982 ± 28 2782 ± 28 2728 ± 22 2568 ± 18 2666 ± 25

2860 ± 11 2847 ± 10 2778 ± 13 2773 ± 13 2733 ± 10 2724 ± 10 2705 ± 13

Rho

δ18O, ‰

±, ‰

0.234 0.374 0.364 0.170 0.330

– – 4.69 4.25 –

– – 0.14 0.17 –

0.815 0.821 0.834 0.837 0.848 0.814 0.820

5.98 6.01 – – – 5.62 –

0.25 0.19 – – – 0.19 –

206

±, %

–5 –4 –5 –4 –3

1.223 1.264 1.207 1.260 1.262

1.9 0.9 1.2 3.3 1.2

–1 –2 –7 0 0 6 1

15.83 15.87 15.75 14.41 13.76 12.71 13.12

1.2 1.1 1.4 1.4 1.1 1.0 1.4

Pb  238 U

±, %

Zircon grain no. 1 0.13612 0.44 0.13906 0.35 0.13461 0.42 0.13866 0.57 0.13852 0.38 Zircon grain no. 2 0.5624 0.94 0.5684 0.91 0.5882 1.20 0.5396 1.20 0.5283 0.95 0.4906 0.82 0.5123 1.10

Pbc and Pb* are nonradiogenic and radiogenic lead, respectively. 1σ is the error of standard calibration (0.39% for grain no. 1 and 0.27% for grain no. 2). Isotope ratios are corrected by measured 204Pb. D, % is discordance: D = 100 · {[Age(207Pb/206Pb)]/[Age(206Pb/238Pb)] – 1}. Errors are given for the 1σ (U–Pb) and 2σ (δ18O) ranges. The dash means that the measurements were not performed; b.d.l.—below detecting level; re is a repeated measurement.

0.040 mm. The magnetic fraction was separated from each class. Subsequently, the material of each class which was previously cleaned from magnetic minerals was individually subjected to gravitational separation in order to obtain three concentrates. The concen trates were divided into several electromagnetic and nonmagnetic fractions. Finally, we obtained 24 heavy products with weights varied from 0.2 to 0.001 g. Each of the products was examined under a binocular microscope, and the grains were selected. As a result, DOKLADY EARTH SCIENCES

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we extracted twelve apatite and one zircon (no. 2 in Tables 1 and 2) grains from the meteorite sample with a weight of 70 g, which was confirmed on an electron microscope (Fig. 1). The local U–Pb dating of zircon was performed on a secondaryion SHRIMPII microprobe (Russian Geological Research Institute). The concentration of REEs and rare elements was analyzed on a Cameca IMS4f (Yaroslavl Branch, Physical–Technological

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Table 2. Distribution of rare elements (ppm) in zircon from the Chelyabinsk meteorite Zircon grain no. 1

Zircon grain no. 2

Component La Ce Pr Nd Sm Eu Gd Dy Er Yb Lu Li P Ca Ti Sr Y Nb Ba Hf Th U Th/U Eu/Eu* Ce/Ce* ΣREE ΣLREE ΣHREE LuN/LaN LuN/GdN SmN/LaN

11

12

21

22

23

221

0.67 35.7 0.85 9.82 19.4 1.40 99.9 433 1011 1709 267 3.50 481 10.0 4.67 1.91 5288 71.9 1.37 8436 599 528 1.13 0.10 11.4 3587 47.0 3519 3808 21.6 46.0

4.16 69.7 2.77 23.5 45.6 4.35 153 719 1490 2591 395 6.50 610 61.6 8.28 2.91 8271 56.7 3.93 8970 1771 973 1.82 0.16 4.97 5499 100 5349 915 20.9 17.6

12.0 105 9.96 61.1 31.1 15.7 60.5 111 191 354 59.8 9.73 188 68.7 54.7 1.33 1252 44.0 3.19 8074 34.9 278 0.13 1.10 2.32 1012 188 777 47.8 7.99 4.13

3.66 37.1 2.79 16.9 8.59 2.82 30.9 101 213 412 70.4 12.3 223 30.1 28.9 1.14 1298 42.3 4.09 8144 44.0 264 0.17 0.53 2.81 899 60.4 827 185 18.4 3.76

10.5 74.5 7.11 35.9 14.9 4.72 42.2 135 266 482 79.5 20.5 219 94.5 72.0 1.79 1630 37.0 5.01 8310 60.9 374 0.16 0.57 2.09 1152 128 1005 73.3 15.2 2.29

12.7 126 10.84 73.6 39.3 17.6 75.0 155 255 472 72.6 15.4 131 94.5 67.9 1.97 1646 25.3 4.63 9105 38.2 425 0.09 0.99 2.61 1310 224 1030 55.2 7.84 4.97

The number of an analytical point corresponds to the grain and crater numbers within this grain (after hyphen).

Institute, Russian Academy of Sciences) by the stan dard methodology. The oxygen isotope composition of zircon was studied in the same areas on a Cameca IMS1280HR ion microprobe (Institute of Geology and Geophysics, Chinese Academy of Sciences) after repolishing of the sample. Zircon grain (no. 1) with a diameter of ~50 µm is a poorly fractured fragment of euhedral noncorroded crystal with clear faces and edges (Fig. 2a). The CL images of the crystal structure are characterized by alternation of light and dark zones parallel to the grain boundaries; the darkest zones with a thickness of <5 µm surround the grain. Grain no. 2 is a prismatic crystal with a thickness of ~40 µm and an elongation

coefficient not less than 1 : 3, crushed from one side (Fig. 2b). The zoned grain structure is observed on the CL image. The central part has a fine rhythmical oscil latory zoning closing in the center as a narrow (<5 µm in width) elongated zone (growth seed or inherited phase). The grain rim has an uneven thickness: it is thick (up to 10 µm) near the apex; it consists of CL dark and light zones as well (Fig. 2b). In addition, the BSE image of the rim shows uniform transverse fractures, which are not observed in the central part of the grain. The results of U–Pb dating of grain no. 1 by the SIMS method are given in Table 1 and Fig. 3a. Four points (except for point 3) provide a concordant age of DOKLADY EARTH SCIENCES

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cordia correspond to an age of 2744 ± 13 Ma. The lower crossing of this discordia (calculated by 9 points in both grains, Fig. 3c) based on four results of dating in grain no. 1 provides an age of 842 ± 6 Ma (Fig. 3c), which is consistent with the concordant age obtained for grain no. 1 (Fig. 3a). The age difference between the central and outer parts of zircon grain no. 2 is not less than 68 m. y., which is significant even if take into account the error. The central part of grain no. 2 is characterized by the U concentration in comparison with the other points (~150 and 200–280 ppm, respec tively) and the same Th concentration (~40 ppm, Table 1). The Th/U ratio is 0.25–0.29 in the central part of the grain; the average value for five other points is 0.16. Thus, the analytical data provide evidence for the same history of the formation and evolution of the studied crystals at least during the period of 2 b. y.

SE 16.7 µm

δ18O was analyzed at five points of both zircon grains (Table 1). The average values of δ18O are 4.47‰ (two analyses) for grain no. 1 and 5.87‰ (three anal yses) for grain no. 2. The oxygen isotope composition in the Chelyabinsk meteorite was previously analyzed using the laser fluoration system in the samples of three petrological types of meteorite with a weight of ~2 mg. Each sample is characterized by variable pro portions of the impact melt and shows signs of recrys tallization during the impact event [9]. However, the oxygen isotope compositions in these types do not dif fer from each other with an average δ18O value of ~5‰, which plots in the field of LL chondrite. Inde pendent study of the oxygen isotope composition in

Fig. 1. Secondaryelectron (SE) image of zircon grain extracted by the technology of ppmmineralogy [8] from the Chelyabinsk meteorite.

834 ± 7 Ma. The concentration of Th and U varies from 370 to 660 ppm, and the Th/U ratio ranges within 1.0–1.3 (Table 1). Two analyses in the central part of grain no. 2 (points 2 and 2re, Table 1) provide a concordant age of 2861 ± 15 Ma (Fig. 3b). Five other points from the peripheral part of the grain form a subconcordant cluster of discordia with an upper crossing with con

(а)

3

2

1

4 20 µm СL

20 µm

BSE

20 µm (b)

3 4 23

21 2 1

20 µm СL

20 µm

BSE

20 µm

Fig. 2. CL and BSE images of zircon grains from the Chelyabinsk meteorite. (a) Grain no. 1; (b) grain no. 2. The points of the isotope–geochemical study on an ion microprobe are shown. The crater diameter is ~20 μm. DOKLADY EARTH SCIENCES

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206

Pb/238U 0.142

Zircon/Chondrite 105 850

(а)

0.140

104

840

0.138

103

830

0.136

820

Grain no. 1 4 points (except for point 3) Concordant age 834 ± 7 Ma MSWD = 5.8

810

0.134 800

0.132 1.14 0.64

1.18

1.22

1.26

1.30

Grain no. 2 5 points (except for points 2 and 2re) 0.60 Upper crossing of discordia 2600 2744 ± 34 Ma 0.56 2840 MSWD = 2.7

1.34

11 12 23 22 21 221

102

101

1.38 (b)

1

La Ce Pr Nd

Sm Eu Gd

Dy

Er

Yb Lu

Fig. 4. Spectra of REE distribution in zircon from the Che lyabinsk meteorite. Points of analyses (after grain no.) for zircons of magmatic origin correspond to those in Table 1 and Fig. 2.

2920 2880

2800 2760 2720

0.52

Grain no. 2 2 points (2 and 2re) Concordant age 2861 ± 15 Ma MSWD = 4.3

2680 2640 2600

0.48 0.44 11.5

12.5

13.5

14.5

15.5

16.5 (c) 3000

0.6 2600 2200

0.4

Grain nos. 1, 2, 9 points Upper crossing of discordia 2744 ± 13 Ma Lower crossing of discordia 842 ± 6 Ma MSWD = 1.6

1800 1400

0.2 1000

0

4

8

12

16

20

207Pb/235U

Fig. 3. Diagram with concordia for zircon from the Chely abinsk meteorite.

bulk samples of the Chelyabinsk meteorite provided an average δ18O value of 4.95 ± 0.9 (1σ) ‰ (11 analy ses) [10]. The δ17О and δ18O values plot in the fields plot in the area of LL chondrites with an average δ18O of 5.02 ± 0.21 (1σ)‰ for LL5 chondrite [11]. How

ever, as is evident from [11], δ18O in LL chondrites without isotope equilibrium ranges within 5.1–6.1‰ with an average of 5.60 ± 0.33 (1σ)‰. As a whole, our data on the δ18O value in zircon from the Chelyabinsk meteorite plot in this range. The REE distribution in zircon from the Chelyab insk meteorite is differentiated (Fig. 4), which is typi cal for zircons of magmatic origin. Grain no. 1 is char acterized by a more differentiated character of the spectrum (LuN/LaN are 915 and 3808) in comparison with the values for grain no. 2 (55–185, Table 2), as well as clear Ce and Eu anomalies. Grain no. 2 differs from grain no. 1 by the lower Th concentration, Th/U (<0.17), and an REE content with a slightly higher LREE portion (Table 2). The positive Ce anomaly in grain no. 2 is strongly reduced (Ce/Ce* = 2.1–2.8); the negative Eu anomaly is either reduced or absent (Table 2, points 1 and 21). The size of a thin rim in grain no. 2 (Fig. 2b) does not allow us to study its com position accurately. However, judging from analysis 21 covering this rim in a half of the field of analysis, the rim is enriched with Hf and REEs and depleted in P in comparison with the central part of the grain (Table 2). Zircon from chondrite that may be compared with our data is almost not studied in relation to the distribution of rare elements [12]. The limited data on the compo sition of zircon from achondrites (eucrites) show the typical absence of the positive Ce anomaly, the com pulsory presence of the negative Eu anomaly (Eu/Eu* <0.2), and the lower total level of REEs in comparison with that in zircon from the Chelyabinsk meteorite ([13] and others). Comparison of the composition of zircon from the Chelyabinsk meteorite with that from terrestrial rocks provides evidence for significant sim DOKLADY EARTH SCIENCES

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ilarity with zircon from Archean anorthosite from the Kola Peninsula studied at the same laboratory of the Yaroslavl Branch, Physical–Technological Institute, Russian Academy of Sciences [14]. The major problem in the study of zircon is to understand the processes that resulted in the forma tion of individual zircon grains in the Chelyabinsk meteorite with the different concentrations of rare and rareearth elements, as well as with the different oxy gen isotope compositions. Moreover, these grains are characterized by a younger U–Pb age (Fig. 3) than apatite analyzed in situ [2, 3]. It appears that the for mation of zircon was controlled by repeated impacts of the meteorite parental body with other objects. The intensity of impact events in the evolution of the Che lyabinsk meteorite is evident from highpressure phases (jadeite, etc.) in veins of molten glass and from the polychronous character of impact glass veins (not less than three generations) [15]. Zircon crystalliza tion may be related to melting of the portions of the plagioclase melt enriched in incompatible elements during the impact events. This process was nonequi librium, which is evident from the isotope–geochem ical composition of grains. The appearance of a “pla gioclase” melt during the impact influence explains the similarity in the composition of the studied zircon grains to zircon from anorthosite [12]. Thus, the complex of isotope–geochemical meth ods applied for study of the Chelyabinsk meteorite allowed us to reconstruct the sequence of multiple impacts of the meteorite parental body with other cos mic bodies for the first time. Dating of apatite in situ by the U–Pb method provided an age of 4450– 4430 Ma [2, 3] falling behind the event of meteorite formation for more than 100 m. y. A strong impact event with significant melting of meteorite is registered from the Sm–Nd isochron by olivine, orthopyroxene, and troilite with an age of ~3730 Ma [6]. The local dat ing of two zircon grains by the U–Pb method (SIMS SHRIMP II) extracted by a special technology that prevents the laboratory contamination, provided the group of ages (~2860, 2740, and 830 Ma) correspond ing to melting of impact glass veins of different ages in the meteorite. ACKNOWLEDGMENTS The authors are grateful to V.V. Knauf (ZAO NATI) for help in zircon extraction and to

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S.G. Simakin, E.V. Potapov (Yaroslavl Branch, Physi cal Technical Institute), and O.L. Galankina (Institute of Precambrian Geology and Geochronology, Russian Academy of Sciences) for analytical investigations. This study was supported by the Ministry of Educa tion and Science of the Russian Federation (contract no. 5.2115.2014/K, 2014–2016). REFERENCES 1. E. M. Galimov, V. P. Kolotov, M. A. Nazarov, et al., Geochem. Int. 51, 522 (2013). 2. O. P. Popova, P. Jenniskens, V. Emel’yanenko, et al., Science 342 (6162), 1069 (2013). 3. M. Kamioka, K. Terada, H. Hidaka, et al., in LXII Annual Conf. on Mass Spectrometry (Osaka, 2014), P205. 4. Q.Z. Yin, Q. Zhou, Q.L. Li, et al., Meteoritics Planet. Sci. 49, 1426 (2014). 5. A. Bouvier, Large Meteorite Impacts and Planetary Evo lution (2013). 6. E. S. Bogomolov, S. G. Skublov, Yu. B. Marin, et al., Dokl. Earth Sci. 452 (2), 1034 (2013). 7. K. Nishiizumi, M. W. Caffee, and L. Huber, in LXXVI Annual Meteoritical Society Meeting (Edmonton, 2013), 5260. 8. V. V. Knauf, N. S. Guseva, and O. V. Knauf, in Digging Deeper: Proc. Biennial Meeting of the Society for Geology Applied to Mineral Deposits (Dublin, 2007), pp. 777– 779. 9. C. T. Pillindger, R. C. Greenwood, D. Jonson, et al., Geochem. Int. 51, 540 (2013). 10. A. I. Khanchuk, V. I. Grokhovskii, A. V. Ignat’ev, et al., Dokl. Earth Sci. 452 (1), 967 (2013). 11. R. N. Clayton, T. K. Mayeda, J. N. Goswami, and E. J. Olsen, Geochim. Cosmochm. Acta 55, 2317 (1991). 12. N. M. Kudryashov, S. G. Skublov, A. V. Mokrushin, and L. M. Lyalina, Vestn. MGTU 17, 314 (2014). 13. T. R. Ireland and F. Wlotzka, Earth Planet. Sci. Lett. 109, 1 (1992). 14. M. K. Haba, A. Yamaguchi, K. Hori, and H. Hidaka, Earth Planet. Sci. Lett. 387, 10 (2014). 15. S. V. Berzin, Yu. V. Erokhin, K. S. Ivanov, and V. V. Khiller, Litosfera, No. 3, 106 (2013).

Translated by A. Bobrov