Composition and origin of peculiar pyropes from lherzolites: evidence for the evolution of the lithospheric mantle of the Siberian Platform

Russian Geology and Geophysics 49 (2008) 225–239 www.elsevier.com/locate/rgg

Composition and origin of peculiar pyropes from lherzolites: evidence for the evolution of the lithospheric mantle of the Siberian Platform N.S. Tychkov *, N.P. Pokhilenko, S.S. Kuligin, E.V. Malygina Institute of Geology and Mineralogy, Siberian Branch of the RAS, 3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia Received 16 April 2007; receiver in revised form 15 November 2007; accepted 16 Novenber 2007

Abstract More than 1000 pyropes from the Muza (J3) and Ivushka (D–C) (northeastern Siberian Platform, and Khorkich (Mz) (southwestern part of the platform) kimberlite pipes, alluvial deposits of the Muna-Markha area, and granular peridotites of the Udachnaya pipe have been analyzed for major and some minor elements. As a result, a group of pyropes was distinguished whose composition is not typical of the lherzolite paragenesis (LAC pyropes). They are predominant in the Muza pipe and are widespread over the world. This group is described as a separate paragenetic type. In all known cases, LAC pyropes belong to granular clinopyroxene-bearing harzburgites, and in situ conditions for this suite are typically below 50 kbar and 1000 °C. Our own and literature data suggest that LAC pyropes may appear when the magmas corresponding to the high-degree melting of the primary magma affect the depleted peridotites of the lithosphere mantle. The character of paleogeotherm and distribution of LAC pyropes in kimberlites and secondary collectors of varying age that occur on the Siberian Platform indicate that the lithosphere mantle was considerably thinner in the northeast and the rocks characterized by LAC pyropes played an increasingly important role in this region in the period from Paleozoic to Mesozoic and that these rocks were abundant in the lithosphere mantle of the platform’s interior. These facts as well as a considerable change in the rock composition in the lithospheric mantle and in the southwestern part of the platform in the same range of time suggest that the effect of the Permian-Triassic Siberian plume on the lithospheric mantle of the platform considerably changed its composition and structure in its separate parts. © 2008, IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: Siberian Platform; kimberlites; LAC pyropes; lherzolites; pyroxenites; lithospheric mantle; structure

Introduction Many years of prospecting on the Siberian Platform revealed kimberlite bodies in its northeastern part that belong to different time cycles of magmatism. There are young kimberlite bodies of Late Jurassic age as well as older bodies, of pre-Permian age, intruded most likely in D–C time. Three cycles of active kimberlite magmatism are known to occur on the Siberian Platform: D3–C1 (367–345 Ma), T (245–215 Ma), and J3 (160–149 Ma) (Brakhfogel, 1984; Griffin et al., 1999; Kinni et al., 1997). The cycle of tectonothermal activity on the Siberian Platform, which took place about the time of the Permian-Triassic transition, manifested itself as an intense but rather short cycle of trap magmatism on the vast territory within the platform, with the maximum intensity at 245– 250 Ma. This large-scale event had an effect on and, possibly, considerably transformed the lithospheric mantle of the craton

* Corresponding author.

(Griffin et al., 1999; Pokhilenko and Sobolev, 1998; Pokhilenko et al., 1999, 2002). Most kimberlites of Mesozoic age bear no diamonds, while the mantle-derived minerals and xenoliths entrained by them are richer than the mantle nodules from the Paleozoic kimberlites (Pokhilenko, 1990). On the basis of comparative study of about 20,000 pyropes from more than 100 kimberlite pipes of the Siberian Platform, Pokhilenko with coauthors (Pokhilenko et al., 1999) draw the conclusion that by the Late Jurassic the lithosphere of the northeastern part of the Siberian Platform changed in composition and became thinner. The data on the changes in the lithosphere thickness and heat flow are inferred from the geotherms (Ryan et al., 1996) plotted for garnets from kimberlite pipes of varying age and pre-Mesozoic diamondiferous placers of the region. According to these data, the thickness of the lithosphere was 180–230 km in the Paleozoic and about 130– 150 km in the Mesozoic, and the heat flow was 37 and 40–41 mW/m2, respectively. In 1999, Griffin et al. (1999) repeated these results, refining the position of the lower boundary of the lithosphere on the garnet isotherm by the level

E-mail address: [email protected] (N.S. Tychkov) 1068-7971/$ - see front matter D 2008, IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. doi:10.1016/j. rgg.2007.11.009

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of the maximum temperature of partinioning of Y-poor garnets representing the depleted lithosphere. Pokhilenko et al. (1999) proved that from Paleozoic to Mesozoic the lithosphere composition changed there, which was expressed in the change of the averaged composition of pyropes from the kimberlites, namely in a decrease of average content of Cr2O3, increase in average content of FeO, etc. The same trend was earlier stressed when analyzing average contents of pyropes from pipes in separate areas of the Yakutian Diamond Province (Sobolev et al., 1978). More detailed studies (Tychkov, 2004a,b, 2006a,b; Tychkov et al., 2007) showed that the changes in the average composition of pyropes are due, in particular, to the fact that the kimberlite concentrate of the Mesozoic pipes in the northeastern Siberian Platform abounds in pyropes whose composition is abnormal for the lherzolite paragenesis. The abnormal character of composition of these pyropes is expressed, for instance, in a distinct trend of partitioning of their compositions on the Cr2O3–CaO discrimination diagram (Sobolev, 1971, 1974) dismatching the common “lherzolite trend”. The discrepancy stems from a different ratio of Cr2O3 and CaO contents, owing to which the high-Cr part of the trend falls on the field of garnets of wehrlite paragenesis (Fig. 1). The purpose of this study is to clear up the genesis of pyropes of anomalous composition as well as causes of a drastic increase in the amount of these pyropes in the Mesozoic kimberlites of the northeastern Siberian Platform relative to the Paleozoic ones. The following tasks were formulated: to describe the typical features of the above-mentioned pyropes, to study their mineral composition, structure, conditions of existence and, finally, the origin of the rocks

that contain pyropes of anomalous composition, to describe the pattern of distribution of these pyropes among kimberlites of varying age, in intermediate collectors and recent alluvium deposits of the Siberian Platform.

Subject of investigation and methods This work is based on our new data on the contents of major elements (including Ti, Mn, and Na) in more than 940 grains of pyrope from the Muza (J3) and Ivushka (D–C) pipes situated in the northeastern part of the platform, from the Khorkich pipe (Taigikun-Nemba kimberlite field — MZ) in the southwest, and from the alluvium deposits of the MunaMarkha interfluve (central-eastern part of the Siberian Platform). We have also used data on the mineral composition of 275 granular lherzolites from the Udachnaya pipe (Malygina, 2002) as well as the data we have obtained from an additional study of 34 granular lherzolites from the Udachnaya pipe (Daldyn kimberlite field — PZ) and nonpublished data kept on deposit in the Laboratory of High-Pressure Minerals and Diamond Deposits (IGM, Novosibirsk) about the composition of pyropes from the concentrate of some Mesozoic pipes and intermediate collectors of Middle Paleozoic age in the northeastern and southwestern Siberian Platform (Fig. 2). Emphasis was placed on the pyropes typical of the Muza pipe and some pipes of the Siberian Platform and other regions whose composition is atypical of the depleted lherzolites of intracratonic regions as well as on their host rocks. No xenoliths have still been found in the Muza kimberlite pipe, but our studies of the xenoliths from the Udachnaya pipe, which also contains pyropes of this kind, as well as works of

Fig. 1. Paragenetic discrimination diagram Cr2O3–CaO (Sobolev, 1974). A: 1 — pyropes from xenoliths of harzburgite-dunites (HD) from the Udachnaya pipe, 2 — pyropes from xenoliths of sheared peridotites (SP) from the Udachnaya pipe, 3 — pyropes of lherzolite anomalous composition (LAC) from different pipes; B: LAC pyropes from xenoliths of granular peridotites (GP) from different pipes: 4 — Udachnaya, 5 — Jerico, 6 — pipes of the East Finland kimberlite province, 7 — Taba-Putsa. Dashed line — fields of compositions of genetic types of pyropes.

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227

Fig. 2. Schematic location of kimberlite fields and some secondary IMK collectors on the Siberian Platform. 1, 2 — field of occurrence on the surface of effusive (1) and intrusive (2) traps; 3, 4 — kimberlite fields of Paleozoic age (3) (I — Upper Muna, II — Daldyn, III — Alakit, IV — Nakyn, V — Mirny) and Mesozoic age (4) (VI — Taigikun-Nemba); 5 — secondary IMK collectors (A — conglomerates of the Kyutyungde graben PZ2, B — recent alluvium of the Elietibiye River, C — recent alluvium of the Muna-Markha interfluve, D — Tychany diamond region PZ); 6 — kimberlite pipes; 7 — dike complex of basites.

other authors (Kopylova et al., 1999; Malygina, 2002; Nixon and Boyd, 1973; Peltonen et al., 1999) show that they belong to garnet and garnet-spinel granular peridotites (GP). Though actually pyropes belong to the lherzolite paragenesis, their trend on the Cr2O3–CaO diagrams considerably differs from the common “lherzolite” trend (see Fig. 1). For the sake of brevity, the pyropes under study will be called “lherzolite pyropes of anomalous composition” (LAC pyropes). The composition of pyrope garnets and associated minerals was studied by means of CAMEBAX MICRO probe in the Laboratory of Electron Microprobing and Electron Microscopy (IGM, Novosibirsk). The geochemical characteristics of minerals were analyzed by the LA-ICP-MA method (Kuligin et al., 2000).

Results Specific composition of LAC pyropes. In addition to the anomalous position on the paragenetic discrimination diagram Cr2O3–CaO (higher content of Ca), the LAC pyropes differ from pyropes of other genetic types in having a higher content of FeO, a lower content of MgO, virtually no TiO2 and Na2O, and a higher content of MnO (Fig. 3, Tables 1 and 2). It is known that the content of Mn increases with Ca in coexisting clinopyroxene (Delaney et al., 1979). The content of Ca in lherzolites is controlled by the enstatite component in clinopyroxene and depends on temperature (Davis and Boyd, 1966). According to experimental data (Brey and Kohler, 1990a), the content of MnO considerably increases as

temperature drops (from 0.25 to 0.5 wt.% with T decreasing from 1200 to 900 °C, at 30 and 40 kbar). The LAC pyropes are mostly enriched in MnO (usually >0.4 wt.%, see Tables 1 and 2), and coexisting clinopyroxenes, in CaO, which evidently indicates a relatively low-temperature character of the association (Fig. 4). Lower contents of TiO2, Na2O, and, according to literature data (Kopylova et al., 2000; Kuligin et al., 2000), other trace incompatible elements such as Y, Zn, Zr, and Sr in LAC pyropes is not typical of Fe- and Ca-enriched varieties of this mineral. The LAC pyropes have lower contents of REE and an S-shaped profile of REE pattern of distribution, which is typical of depleted lherzolites and harzburgites (see Fig. 5) (Peltonen et al., 1999; Kuligin et al., 2000; Carbno and Canil, 2002). Specific composition of pyropes from the Khorkich pipe. On the Cr2O3–CaO diagram, the field of Khorkich pyrope compositions completely coincides with the LAC pyrope field. The average content of FeO (7.2 wt.%) in the Khorkich pyropes is considerably lower than in LAC pyropes and even in pyropes from sheared peridotites (SP). In contents of other components, the Khorkich pyropes are similar to the pyropes typical of SP (Fig. 6, see Tables 1 and 2). Occurrence of LAC pyropes. In the northeastern Siberian Platform, LAC pyropes abound in kimberlite pipes of Mesozoic age (J3) (Muza, Irina, D’yanga, Mary, Gobi, Vodorazdel’naya, and other pipes) and in the recent alluvium (Elietibiye and other rivers). In much lower amounts they occur in pipes of Paleozoic age (Ivushka pipe) and Lower Carboniferous conglomerates of the Kyutyungde graben (Sobolev et al.,

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Fig. 3. Composition of LAC pyropes of different types in the projections MnO–TiO2, MnO–Mg/(Mg+Fe)⋅100. 1 — from xenoliths of GR from the Udachnaya pipe, 2 — from kimberlite concentrate of the Muza pipe, xenoliths of GP from the Jerico pipe, pipes of the East Finland province as compared with the compositions of pyropes from: 3 — HD xenoliths from Udachnaya, 4 — SP xenoliths from Udachnaya; 5–7 — composition of pyropes from Jerico xenoliths: 5 — LAC pyropes from GP xenoliths, 6 — pyropes of normal lherzolite composition from GP xenoliths, 7 — pyropes from SP xenoliths (after: Kopylova et al., 1999).

1981). In the central part of the platform, the LAC pyropes are found in the xenoliths from the Udachnaya pipe (PZ2) and in higher amounts in the recent alluvium of the drainage system of the Muna-Markha interfluve (see Fig. 2). The LAC pyropes are also documented in the Slave craton (Canada) from the Drybones Bay kimberlite pipes (PZ1) (Carbno and Canil, 2002), Jerico (MZ2) (Kopylova et al., 1999), within the Buffalo Head terrain in the Buffalo Hills kimberlites (MZ3) (Aulbach et al., 2004), within the Baltic Shield in the kimberlites of the East Finland kimberlite province (Peltonen et al., 1999), in the kimberlite pipes of North Lesotho (South Africa) (Nixon and Boyd, 1973). Everywhere the LAC pyropes retain the whole group signature in all chemical characteristics. To study the distribution of LAC pyropes over the Siberian Platform, we took pyropes of granular lherzolites from the Udachnaya pipe, from the concentrate of the Muza, Ivushka, and Khorkich kimberlite pipes as well as pyropes from the alluvium deposits of the Muna-Markha interfluve. We also

used unpublished data on the composition of pyropes from the Mesozoic pipes and heterochronous secondary reservoirs in the northeast of the platform. Of 34 granular pyrope peridotites from the Udachnaya pipe, twenty contain LAC pyropes (58.8%). At the same time, of 139 pyropes randomly taken from the concentrate of the same pipe, only three belong to the LAC type (2.2%). Of 111 pyropes from the Muza pipe, as little as 3.6% can be referred to the SP group, 5.4% to the pyrope group of the originally depleted granular lherzolites, and 91.0% to the group of LAC pyropes. In other Mesozoic pipes of the region, LAC pyropes make up: 92% in Irina, 39% in Vodorazdel’naya, 37% in Gobi, and 19% in D’yanga. Of 102 pyropes from the Ivushka pipe, 24.6% belong to low-Cr varieties of the so-called megacryst association, 68.5% to the GP and SP groups of pyropes, and only 6.9% to the LAC group. Of 609 Cr-pyropes from recent alluvium deposits in the Muna-Markha interfluve (the channel and tributaries of the Tyung River), more than 20% can be referred to the LAC paragenesis. Among the pyropes from the recent alluvium of

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N.S. Tychkov et al. / Russian Geology and Geophysics 49 (2008) 225–239 Table 1 Representative analyses of examined pyropes (wt.%) Specimen

SiO2

TiO2

Al2O3

Cr2O3

MnO

FeO

MgO

CaO

Na2O

Total

1 2 3 4 5 6 7 8 9 10 11* 12* 13* 14* 15* 16* 17* 18* 19* 20* 21* 22* 23* 24 25 26 27 28 29 30* 31 32 33 34 35* 36 336/89* 336/89* 304/89* 304/89* 50/93* 50/93* 50/93* 19/91* 19/91 423/86* 423/86* 149/89* 149/89* 149/89*

41.1 40.8 41.2 40.8 41.4 41.2 41.7 41.6 41.2 41.5 42.1 41.9 41.9 42.1 41.8 42.0 41.7 41.0 42.2 42.3 41.9 41.5 41.8 41.2 42.3 42.5 42.6 42.0 41.4 41.8 42.4 41.8 42.4 41.8 42.0 42.4 40.8 40.8 41.1 41.0 41.3 41.3 40.9 41.8 41.4 42.0 41.6 41.5 41.3 41.0

0.20 0.50 0.40 0.55 0.34 0.65 0.30 0.52 0.07 0.14 0.18 0.10 0.03 0.04 0.01 0.03 0.11 0.05 0.01 0.01 0.01 0.01 0.01 0.70 0.63 0.56 0.67 0.57 0.16 0.01 0.69 0.13 0.20 0.50 0.02 0.33 0.00 0.00 0.00 0.00 0.03 0.03 0.05 0.04 0.00 0.07 0.01 N.d. N.d. 0.02

17.9 19.1 20.3 19.7 20.0 19.7 21.0 21.8 18.9 19.6 21.8 21.3 21.4 20.9 20.5 20.3 19.9 19.9 20.0 19.8 19.7 19.0 18.3 21.5 21.4 20.8 20.4 20.7 20.8 20.8 19.2 18.7 19.1 16.9 21.4 20.1 17.3 17.0 19.9 18.4 20.2 20.0 18.9 19.7 15.6 19.2 21.1 19.6 19.0 16.9

6.6 5.0 3.5 4.0 4.3 4.4 2.4 1.5 6.0 4.8 1.7 2.6 2.7 3.4 3.6 4.2 4.6 4.6 4.7 5.2 5.3 5.9 6.8 0.4 1.5 2.4 2.6 3.0 3.5 3.8 4.9 6.1 6.2 8.1 3.0 4.6 7.8 8.2 4.4 6.3 3.7 4.2 5.4 5.4 11.3 5.6 3.3 5.8 7.1 8.9

0.32 0.35 0.37 0.34 0.37 0.33 0.27 0.29 0.32 0.32 0.46 0.45 0.47 0.54 0.53 0.58 0.49 0.58 0.52 0.57 0.61 0.48 0.50 0.34 0.28 0.26 0.30 0.32 0.36 0.59 0.28 0.32 0.30 0.34 0.55 0.31 0.48 0.51 0.55 0.56 0.50 0.53 0.50 0.50 N.d. N.d. N.d. N.d. N.d. N.d.

6.5 7.6 8.5 8.3 6.3 7.1 8.4 7.8 6.5 6.7 9.5 8.8 8.8 8.7 8.6 8.8 7.6 8.4 8.3 8.0 8.0 7.8 8.0 11.8 8.8 7.8 7.6 7.4 7.6 8.4 6.6 6.4 5.8 6.0 8.4 6.2 7.4 7.4 8.0 7.9 7.8 7.8 7.8 7.9 6.6 7.7 8.4 8.1 7.7 7.7

19.7 19.1 19.5 19.9 20.9 20.6 20.0 20.8 20.4 20.7 19.3 18.6 19.3 19.0 19.1 18.6 19.8 18.6 18.8 19.0 19.0 18.1 18.4 18.7 20.5 21.1 21.1 20.3 20.7 18.5 21.2 20.2 23.3 20.4 19.3 21.7 18.6 18.9 19.7 18.3 19.9 20.0 19.5 19.0 22.4 20.9 19.2 18.6 18.1 19.1

6.5 6.2 5.4 5.6 5.3 5.8 5.1 4.9 6.0 5.5 4.5 5.3 5.1 5.3 5.6 6.0 5.2 5.9 6.0 6.0 6.1 6.5 6.7 4.7 4.8 4.7 4.9 5.1 5.0 6.2 5.1 5.9 3.2 5.9 5.5 4.9 7.5 7.5 6.1 7.5 5.7 5.9 6.5 6.2 2.5 3.6 5.4 6.5 6.9 5.5

0.02 0.05 0.04 0.04 0.03 0.05 0.04 0.05 0.02 0.01 0.05 0.03 0.02 0.02 0.03 0.02 0.04 0.01 0.03 0.02 0.03 0.02 0.01 0.12 0.07 0.09 0.07 0.08 0.05 0.02 0.14 0.02 0.08 0.06 0.04 0.07 0.02 0.03 0.01 0.02 0.04 0.04 0.04 0.01 N.d. 0.08 0.01 N.d. N.d. 0.02

98.8 98.6 99.1 99.2 99.0 99.8 99.2 99.2 99.5 99.2 99.6 99.2 99.6 100.0 99.7 100.6 99.3 99.0 100.5 100.9 100.6 99.2 100.4 99.3 100.1 100.2 100.3 99.6 99.5 100.2 100.4 99.6 100.6 100.1 100.1 100.6 99.8 100.3 99.7 99.9 99.1 99.8 99.5 100.5 99.8 99.1 99.0 100.1 100.1 99.1

Note. Pyropes from the kimberlite concentrate of the pipes Khorkich (1–10), Muza (11–23), and Ivushka (24–36) and GP xenoliths from the Udachnaya pipe (336/89, 304/89, 50/93, 19/91, 423/86, 149/89). N.d. — not determined. * LAC pyropes.

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Table 2 Averaged compositions of pyropes from different sources and pyropes of different genetic types (wt.%) Specimen Pipe, alluvium

Type

x, s

SiO2

TiO2

Al2O3

Cr2O3

FeO

MnO

MgO

CaO

Na2O

Total

T

1

LAC

x

41.9

0.04

20.4

3.98

8.47

0.51

18.9

5.67

0.03

99.9

MZ

s

0.43

0.03

0.86

1.12

0.48

0.04

0.48

0.57

0.01

—

x

41.9

0.02

21.0

3.23

8.79

0.56

18.8

5.74

0.03

100.1

s

0.21

0.01

0.55

0.77

0.49

0.05

0.33

0.45

0.01

—

Muza n = 97

2

Ivushka

LAC

n=7 3

Udachnaya

LAC

n = 34 4

Muna-Markha interfluve

LAC

n = 159 5

Jerico

LAC

n = 27 6

Udachnaya n=7

7

Muna-Markha interfluve

8

Ivushka

10

Jerico

Udachnaya Udachnaya n = 154

0.43

19.1

5.95

0.02

99.5

0.05

0.60

0.46

0.02

—

x

41.7

0.07

20.9

3.37

8.61

0.48

19.1

5.45

0.02

99.8

s

0.30

0.04

0.81

1.08

0.61

0.05

0.54

0.55

0.01

—

x

41.1

0.02

20.7

4.32

8.91

0.54

18.3

5.81

0.02

99.7

s

0.37

0.01

0.79

0.93

0.38

0.05

0.44

0.49

0.01

—

0.05

19.8

4.63

7.80

0.43

19.0

6.02

0.03

99.6

0.03

1.05

1.33

0.52

0.05

0.66

0.55

0.02

—

al.c.

x

41.9

0.35

19.5

4.65

6.91

0.31

20.9

4.90

0.05

99.5

s

0.45

0.23

1.36

1.82

0.77

0.06

0.91

0.70

0.03

—

x

42.2

0.48

20.2

3.52

7.28

0.30

20.9

4.96

0.08

100.0

s

0.43

0.23

1.15

1.67

0.84

0.04

0.70

0.54

0.03

—

k.c.

x

41.3

0.38

20.0

4.04

7.24

0.33

20.3

5.64

0.04

99.3

s

0.34

0.17

0.94

1.29

0.79

0.02

0.51

0.49

0.01

—

GP

x

41.4

0.20

21.1

3.78

8.33

0.38

19.7

4.69

0.05

99.6

s

0.24

0.12

1.21

1.64

0.74

0.04

0.50

0.40

0.02

—

SP

x

40.8

0.45

17.1

7.89

7.57

0.33

19.3

5.81

0.06

99.3

s

0.26

0.13

1.16

1.49

0.41

0.03

0.40

0.49

0.02

—

HD

x

41.5

0.08

16.3

9.47

7.09

0.41

21.7

2.98

0.04

99.6

s

0.59

0.09

1.78

2.27

0.34

0.04

1.37

1.60

0.02

—

x

41.7

0.51

18.3

5.38

7.54

0.35

20.2

5.37

0.06

99.4

s

0.72

0.32

2.32

2.86

0.97

0.08

0.97

0.97

0.02

—

k.c.

n = 30 13

7.66 0.35

41.8

n = 42 12

4.66 1.30

0.21

n = 11 Jerico

19.8 1.09

x

n = 149

11

0.02 0.03

s

n = 70 Khorkich

41.9 0.29

LAC (from OPxt)

n = 450

9

x s

SP

PZ PZ ?

MZ PZ

?

PZ MZ MZ MZ PZ PZ

Note. 1, 2 — LAC pyropes, 3, 5 — LAC pyropes from GP xenoliths (Kopylova, 1999), 4 — LAC pyropes from recent alluvium deposits, 6 — LAC pyropes from orthopyroxenite xenoliths; 7-9 — pyropes from different sources, without LAC pyropes taken into account; 7 — from recent alluvium deposits; 10–13 — pyropes from different genetic types of rocks: 10 — from xenoliths of common GP, after: Kopylova et al. (1999), 11 — from SP xenoliths (Kopylova et al., 1999), 12 — from xenoliths of harzburgite-dunites, 13 — from SP xenoliths. n — number of specimens, x — mean, s — standard deviation. T — time of pyrope exhumation; low-Cr pyropes (less than 1.0 wt.% Cr2O3) were ignored in calculations. k.c. — kimberlite concentrate, al.c. — alluvium concentrate.

the Elietibiye River, the LAC pyropes make up about 48%. In the Paleozoic conglomerates of this region the share of LAC pyropes does not exceed few per cent (Pokhilenko et al., 1999) (Fig. 7). Characteristics of the rocks hosting LAC pyropes. All the rocks bearing LAC pyropes belong to equigranular peridotites without traces of shearing. As a rule, the amount of clinopyroxene is low, not exceeding few per cent in volume (there are no rocks of this kind without clinopyroxene as a phase). As compared with other associated mantle ultrabasic rocks, the rocks containing LAC pyropes have the richest mineral composition (Fig. 8). Very often the rocks with LAC pyropes demonstrate the nonequilibrium of coexisting phases (xenoliths from the Udachnaya, Jerico, and Taba-Putsa pipes), significantly different compositions of minerals of one species within a xenolith, zoning of minerals in composition, including the so-called "inverse zoning" in pyrope (with Cr content

increasing from core to rim of a grain; xenoliths Uv-404/86 — Udachnaya pipe (Kuligin et al., 2000), 26-3 and 9-2 — Jerico pipe (Kopylova et al., 1999)). Figure 9 shows the calculated (Brey and Kohler, 1990b) temperatures and pressures for deep-seated rocks from the pipes where xenoliths with LAC pyropes have been found. Though these rocks exist in wide ranges of temperatures (650–1000 °C, seldom to 1100 °C) and pressures (20–50 kbar, seldom to 60 kbar), it is seen that they belong mostly to middle and upper horizons of the vertical section of the lithosphere mantle.

Discussion LAC pyropes on the Siberian Platform. The Muza and Ivushka kimberlite pipes are situated in the northeast of the

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Fig. 5. Distribution of REE in LAC pyropes. 1 — xenolith Uv-404/86, Udachnaya pipe (Kuligin et al., 2001); 2 — from xenoliths of the East Finland kimberlite province, averaged (Peltonen et al., 1999); 3 — from xenoliths of kmberlites from Drybones Bay, averaged (Carbno and Canil, 2002). Fig. 4. Ratio of contents of MnO in garnet and CaO in coexistent clinopyroxene from Jerico xenoliths (Kopylova et al., 1999). 1 — from GP xenoliths with LAC pyropes; 2 — from GP xenoliths with common lherzolite pyropes; 3 — from SP xenoliths.

Siberian Platform, 70 km apart from one the other (see Fig. 2). The Muza pipe was intruded at about 151 Ma (J3) (Griffin et al., 1999); the Ivushka pipe is of Paleozoic age (overlain by Upper Paleozoic terrigene rocks and traps of Early Triassic age (Pokhilenko and Sobolev, 1995). Though, at a medium scale of the lithosphere thickness, these pipes are very close, they significantly differ in distribution of compositions of concentrate-hosted pyropes: The share of LAC pyropes in the Ivushka pipe is much inferior to that in Muza (6.9% and 91.0, respectively) and in other Mesozoic pipes of the region. These data as well as the fact that in the Paleozoic conglomerates of this region the LAC pyropes do not exceed few per cent

(Pokhilenko et al., 1999) suggest that the amount of the rocks containing LAC pyropes in the lithosphere mantle of the region increases by the Mesozoic as compared with the Paleozoic. The Khorkich kimberlite pipe of Mesozoic age situated in the southwestern Siberian Platform at the southern margin of the Tunguska syneclise typically contain pyropes whose composition can be characterized as intermediate between SP and LAC types (see Results). Secondary collectors of kimberlite indicator minerals (KIM) of Paleozoic age have been documented from the same region (Tychany diamond district, see Fig. 2) (Afanas’ev et al., 2005). The distribution of pyrope compositions in them is generally typical of the Paleozoic kimberlites on the Siberian Platform and neither LAC pyropes nor pyropes typical of the Mesozoic pipes (Khorkich and

Fig. 6. Pyrope composition in the projections MnO–TiO2, MnO–Mg/(Mg+Fe)⋅100 from the Khorkich pipe (1) as compared with the composition of pyropes from other parageneses. For the rest of symbols see Fig. 3.

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Fig. 7. Distribution of compositions on the Cr2O3–CaO diagram for pyropes from kimberlite pipes and secondary collectors of the Siberian Platform. F: 1 — from spinel-pyrope GP, 2 — from pyrope GP; G: 1 — Tychany aureole, 2 — Tarydak aureole. Solid line — fields of compositions of genetic types of pyropes (Sobolev, 1974).

others) make up considerable shares (see Fig. 7, G, H). These data also indicate that the rock composition of the lithospheric mantle of this region also considerably changed from Paleozoic to Mesozoic. The kimberlites from the northeastern part of the platform cannot be the source for the LAC pyropes we have found in the alluvium deposits of the central part of the Siberian Platform. Among the nearest kimberlite fields (Nakyn, Markha, Alakit, Daldyn, Upper Muna, see Fig. 2) that could affect the KIM composition distribution of the recent alluvium, there are no kimberlite pipes having an appropriate amount of LAC pyropes (the average content of LAC pyropes in concentrates of known pipes from the central Siberian Platform is about few per cent (e.g., Udachnaya pipe — 2.2%). Thus, being an additional search indicator, the relative amount of LAC pyropes in the recent alluvium suggests that kimberlite bodies occur within the central Siberian Platform which remain to be found.

Origin of LAC pyropes. The experimental data of Boyd (1970) suggest that the CaO content in pyropes from peridotites depends on paragenesis. The so-called lherzolite trend in CaO–Cr2O3 projection is formed by the compositions of the pyropes that belong to the lherzolite (or clinopyroxenebearing harzburgite) paragenesis. The amount of CaO in pyrope is controlled by a pair of coexisting pyroxenes and slightly regularly grows with Cr2O3 in the system (and in pyrope). Some features of pyrope composition depend not only on the rock composition but also on PT conditions of its existence. This follows from experimental data on natural systems (Brey and Kohler, 1990a). The data show that for the lherzolite paragenesis the amount of CaO in pyrope, which is actually in equilibrium with clinopyroxene, depends on temperature and pressure. Describing the lherzolites of the Kaapval craton, Brey (1991) wrote: “It can be demonstrated, however, from Ca-Cr relationship of garnets that garnet was

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Fig. 8. Triangular diagrams (Ca–Fe–Mg) of composition of minerals from xenoliths of mantle-derived rocks. A — Jerico pipe, B — Taba-Putsa pipe, C — xenoliths of East Finland kimberlite province, D — Udachnaya pipe. 1 — GP containing LAC pyropes, 2 — common GP, 3 — SP.

never in equilibrium with clinopyroxene except of the nodules experienced very high temperatures at one stage of their history (higher than calculated here from the two-pyroxene thermometer), and that the garnet compositions were frozen in at these conditions.” Thus, the experimental data show that in the Ca–Cr projection a distinct linear trend is formed by compositions of the pyropes that belong to the high-temperature peridotites or to the rocks warmed up to a degree sufficient for the pyrope to be re-equilibrated with coexisting clinopyroxene. The trend position in this case depends on the pressure and temperature at which pyrope is in equilibrium with clinopyroxene. Figure 10 shows Ca and Cr ratios in pyropes from several LAC-pyrope-bearing pipes. The dependence of pyrope composition on temperature and pressure is visible. For the reason indicated by Brey the rock temperature determined from two-pyroxene thermometer is somewhat underestimated there relative to experimental data. Changes in PT conditions of equilibrium and rock composition can be inferred from pyrope grains, which have acquired composition zoning as a result of these processes. Most frequently, zonal pyropes belong to sheared peridotites. The

observed zoning is of two kinds: a drastic increase in CaO with an insignificant decrease in Cr2O3 content (after: Burgess and Harte (1998) — type III) corresponding to the process of enrichment of pyrope and zoning with a slight decrease in CaO against a considerable change in Cr2O3 content (after: Burgess and Harte (1998) — type I) corresponding to the process of evolution of rock composition under isobaric cooling and fractional crystallization of pyrope (Brey and Kohler, 1990a; Burgess and Harte, 1998; Smith and Boyd, 1992) (Fig. 11). The zoning types are most illustratively expressed in pyropes from granular peridotites of the Udachnaya pipe (sp. Uv-105/89, Uv-4/76) (Pokhilenko et al., 1999). They are characterized by a significant composition gradient. Both types of zoning are expressed there, regularly alternating from core to rim. The same zoning is described for pyropes from the Mir kimberlite concentrate (sp. M49, M41) (Sobolev, 1974) (see Fig. 11). Thus, a distinct trend of lherzolite pyrope compositions is visible on the Cr2O3–CaO diagram as a single direction of variation of pyrope composition in the process of their fractional crystallization under certain conditions. This is often preceded by a change in pyrope composition from the

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Fig. 9. Calculated (Brey and Kohler, 1990b) temperatures (T) and pressures (P) for xenoliths from: A — Jerico pipe, B — East Finland kimberlite province, C — Taba-Putsa pipe, D — Udachnaya pipe. 1 — xenoliths of GP with LAC pyropes, 2 — xenoliths of common GP, 3 — xenoliths of SP, 4 — xenoliths of pyrope orthopyroxenites from the Udachnaya pipe; 5 — xenoliths with compositionally zoned pyropes, 6 — xenoliths whose parameters were calculated with the use of pyrope and clinopyroxene (Nimis and Taylor, 2000); 7 — graphite-diamond interface (Kennedy and Kennedy, 1976), 8 — geotherm 40 mW/m2.

composition typical of harzburgite-dunite and lherzolite depleted parageneses toward the trend, with the appearance of clinopyroxene in the assemblage and enrichment of pyrope. The LAC pyropes may form a distinct discrete trend on the Cr2O3–CaO diagram. They evidently are the result of the enrichment of pre-existing pyropes and subsequent evolution of their composition. A solid argument for this hypothesis is that zoned grains of pyropes exist within the trend (Kopylova et al., 2000; Smith and Boyd, 1992). The center-to-rim change of composition completely match the compositions of unzoned pyropes of the LAC group and can be related to the zoning typical of fractional crystallization (see Fig. 11). When examining xenoliths of LAC-pyrope-bearing granular lherzolites from the Udachnaya pipe, Malygina (2002) found about a dozen of xenoliths of granular garnet and garnet-spinel lherzolites containing pyropes homogeneous inside the grain but strongly different in composition within the xenolith. The change of composition of LAC pyropes in these xenoliths corresponds to both types of zoning (see Fig. 11). Thus, the LAC pyropes may appear as a result of the intense warming of mantle rocks by the intruding magma formed under a high degree of melting of the mantle matter followed by re-equilibration of pyrope and coexisting phases on cooling at a relatively low pressure. The inverse zoning in LAC pyropes observed in some specimens containing nonequilibrated mineral phases (Kopylova et al., 2000) also indicates that the

paragenesis existed at a relatively low temperature. Experimental data on crystallization of pyrope equilibrated with two pyroxenes in the CMASCr (CaO–MgO–Al2O3–SiO2–Cr2O3) system show that in the process of fractional crystallization on cooling at a constant pressure of 30 and 40 kbar, the content of Cr2O3 in pyrope begins to grow since temperatures of 1050 and 1100 °C, respectively (Smith and Boyd, 1992). The calculated (Brey and Kohler, 1990b) parameters of equilibrium for the specimens containing LAC pyropes with inverse zoning correspond to temperatures of 810, 830, and 940 °C. It remains unclear how important is the process of secondary enrichment for imparting a peculiar character to the LAC pyrope composition. The peculiar composition is possibly due only to the re-equilibration of pyrope with relatively low-temperature clinopyroxene. As to zonal pyropes from sheared peridotites and the above-mentioned pyropes with distinct zoning from GP of the Udachnaya pipe and concentrate from the Mir pipe (see Fig. 11), it is evident that along with considerable warming, the secondary enrichment also took place, which is inferred from a higher content of incompatible trace elements (Ti, Y, Sr, Zr) and the full suite of REE in rims of grains relative to the core. As the LAC pyropes are characterized by the minimum content of incompatible dispersed elements and distinct S-shaped profile of REE pattern of distribution (see Fig. 5) typical of depleted peridotites one can suppose that during the formation of LAC pyropes the

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Fig. 10. Cr and Ca ratios (f.u.) in pyropes from xenoliths of different pipes and calculated T (ºC) and P (kbar) (Brey and Kohler, 1990b): A — Jerico, B — Taba-Putsa, C — compositions of pyropes from SP xenoliths from Udachnaya (1) and LAC pyropes from different pipes (2). Lines show experimental data of pyrope compositions at different T and P (Brey, 1991).

secondary enrichment, if any, was rather insignificant. The contribution of secondary enrichment depends heavily on the composition of the agent affecting the mantle rocks, in particular, on the concentration of incompatible elements. By the example of pyropes from the Jerico pipe, Kopylova (Kopylova et al., 2000) states that the LAC-group pyropes belong to the spinel-bearing peridotites and, using the calculated spinel-garnet equilibrium in the system, argues that this fact is responsible for the abnormal position of the trend of these pyropes on the Cr2O3–CaO diagram. The main argument for this supposition is that in the Jerico pipe more than 90% of the pyropes from xenoliths of spinel-bearing garnet peridotites belong to the trend typical of the LAC pyropes. However, this rule does not always work for the LAC pyropes from kimberlites in other regions, e.g., in the Udachnaya pipe (see Fig. 7, E) or Buffalo Hills (Aulbach, 2004). On the basis of similarity between compositions of LAC pyropes and pyropes from pyroxenites and, in particular, from the similar position of the trend on the Cr2O3–CaO diagram, Pokhilenko et al. (1999) suppose a genetic relationship between pyroxenites and LAC-pyrope-bearing rocks. They call the LAC-pyrope-bearing rocks compositionally intermediate hybrid rocks. This hypothesis is confirmed by numerous findings of complex xenoliths containing lherzolites (clinopyroxene-bearing harzburgites) and pyroxenites in immediate

contact (Kopylova et al., 1999; Kuligin et al., 2000; Nixon and Boyd, 1973; Solov’ev et al., 1994). These xenoliths are especially abundant in the pipes of the northeastern Siberian Platform, e.g., in the Obnazhennaya pipe (Sobolev, 1974). Xenoliths of pyrope orthopyroxenites were found in the Udachnaya pipe, where more than a half of the examined granular peridotites contain LAC pyropes (Kuligin, 1997). In composition minerals of these pyroxenites are virtually identical to the corresponding minerals of the rocks bearing LAC pyropes (see Table 2). Despite the predominant orthopyroxene, other minerals of lherzolite paragenesis (olivine, clinopyroxene) are present in small amounts. As the pyrope composition is controlled just by the presence of mineral phases in the paragenesis (Boyd, 1970; Sobolev, 1974), it is not strange that the composition of LAC pyropes from lherzolites and orthopyroxenites is identical. On the other hand, only few pyroxenites with the lherzolite suite of minerals contain pyropes of the same composition. It is quite possible that, in addition to the accompanied mineral phases, other factors also have a certain effect. Among the xenoliths found in the Udachnaya pipe, there are complicated ones in which LAC-pyrope-bearing lherzolites are in direct contact with pyrope orthopyroxenites similar to those mentioned above (Solov’eva et al., 1994; Kuligin et al., 2000). The authors who described these xenoliths come to the

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Fig. 11. Change of pyrope compositions on Cr2O3–CaO and Cr2O3–TiO2 diagrams (Sobolev, 1971). 1, 2 — LAC pyropes from GP: 1 — Jerico, 2 — Taba-Putsa; 3, 4 — pyropes from SP xenoliths: 3 — Jagersfontain (Burgess and Harte, 1998), 4 — Jerico (Kopylova et al., 1999); 5 — pyropes of varying composition from GP xenoliths of Udachnaya (composition points of pyropes from the same specimen are connected by lines); 6 — zonal pyropes from GP of Udachnaya (sp. Uv-105/89) and from concentrate of Mir pipe (sp. M49, M41). In all zonal pyropes the content of Cr2O3 decreases from core to rim. Dotted line — fields of compositions of genetic types of pyropes (Sobolev, 1974).

conclusion that their orthopyroxenite part formed as a result of intrusion and subsequent crystallization (possibly, in the form of cumulate) of a high-temperature magma close to the komatiite magma in the content of major components (Solov’eva et al., 1994). The high-temperature character of the magma is inferred from evident signs of decomposition of high-temperature pyroxene which dropped garnet and clinopyroxene on progressive cooling. As follows from the pattern of distribution of trace and rare-earth elements in garnets from orthopyroxenite, garnet formed there exclusively as a product of decomposition of high-temperature pyroxene. Geochemical studies of complicated sp. Uv-404/86 (Kuligin et al., 2000) show that the garnets from orthopyroxenite are depleted as compared with the garnets from lherzolites in all REE, and for two garnets from the lherzolite part a slight zoning is observed in distribution of REE with a decrease in contents

throughout the spectrum from center to rim. Thus, REE distribution in rims of lherzolite garnets approaches that for the garnets from the orthopyroxenite part of the specimen, which proves the effect of the orthopyroxenite part on the lherzolite one. The same major-element composition of garnet and orthopyroxene from the lherzolite and orthopyroxenite parts of these specimens suggests that the minerals from both parts were recrystallized under conditions of chemical equilibrium (Solov’eva et al., 1994). The effect of orthopyroxenites depleted in dispersed incompatible elements can explain the abnormally low contents of these elements in minerals of the LAC-pyrope-bearing rocks, which are generally characterized by an enriched major-element composition as compared with common depleted peridotites. Thus, we can suppose that the specific composition of the LAC pyropes and related rocks is produced by the crystal-

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lization of high-temperature pyroxenites at relatively high levels of the mantle section followed by the re-equilibration of the composition of their minerals with the minerals of the host, initially depleted, mantle rocks and by the change of pyrope composition from the common lherzolite-harzburgite to pyroxenite one. At higher levels of the mantle section, in the domain of development of spinel peridotites, the processes of this kind seem also to occur. The evidence comes from numerous findings of contacts, in complex xenoliths, between the garnet-free rocks of the pyroxenite series and spinel peridotites (Solov’eva et al., 1994) as well as from the nonequilibrated assemblages in spinel lherzolites we have found in xenoliths from the Udachnaya pipe. The intrusion of magmas of varying composition corresponding to different degrees of melting of the nondepleted mantle source leads to the appearance in the lithosphere mantle of numerous types of intrusive rocks and re-enriched rocks such as SP or LAC-pyrope-bearing rocks. The asthenospheric magmas, from which minerals of the so-called “megacryst assemblage” are crystallized (Burgess and Harte, 1998), are responsible for the formation of SP at the bottom of the lithosphere through the enrichment of the depleted mantle rocks. They are characterized by high concentrations of incompatible elements as they formed at relatively low degrees of melting of the nondepleted mantle matter and at temperatures of about 1200–1300 °C typical of the lithosphere bottom (Pokhilenko, 1990). Approximately the same concentrations of incompatible elements, melting degrees, and temperatures seem also to characterize some of the melts having an effect on granular peridotites, mantle rocks from higher levels of the section (xenoliths of these enriched rocks were described by Pokhilenko et al. (1999, 2002) from the Udachnaya pipe, see Fig. 11) and crystallizing in the form of the enriched types of pyroxenites (clinopyroxenites, websterites). The melts corresponding to higher temperatures (1300–1500 °C) and forming at higher degrees of melting of the mantle matter contain much lower amounts of incompatible elements. They can be compared with the intrusive orthopyroxenites and olivine websterites present at different levels of the section of the lithospheric mantle. It is likely that, rising to depths of 60 km and even higher, these melts affect the host rocks and form, as shown above, LAC-pyrope-bearing peridotites. These temperatures and melting degrees extremely high for the lithosphere bottom should evidently correspond to a certain tectonothermal activity in the lower lithosphere. Our studies as well as literature data (Pokhilenko, 1990; Pokhilenko and Sobolev, 1995, 1998; Pokhilenko et al., 1999, 2002) indicate that a part of the rocks characterized by LAC pyropes in the mantle section of the northeastern Siberian Platform considerably increases from Late Paleozoic to Middle Mesozoic. This is in agreement with the data on a greater portion of pyroxenite xenoliths in some Mesozoic pipes of this region, which was stressed earlier (Sobolev et al., 1978). A decrease in the lithosphere thickness in this region from Late Paleozoic to Mesozoic is inferred from the maximum pressures and temperatures of the existence of mantle rocks from the xenoliths of heterochronous pipes (Sobolev, 1974)

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and pyropes from the concentrate as well as from a considerable increase in heat flow (37–41 mW/m2) (Pokhilenko and Sobolev, 1998). This might proceed according to the mechanism proposed by Yuen and Fleitout (1985), which implies that the lower lithosphere is eroded by means of small-scale convection in the head of a rising mantle plume. Taking into account the proposed origin of LAC pyropes and the fact that they exist in the mantle within 170 km in depth, we can argue that the significant increase in their amount in the concentrate of the kimberlite pipes of the northeastern Siberian Platform from Late Paleozoic to Mesozoic is a consequence of two factors: a considerable increase in the amount of pyroxenites in the mantle section and a decrease in the thickness of the lithosphere mantle from 180–230 to 130–150 km. The two events might be related to the tectonothermal activity of the lithosphere of the Siberian Platform in the period between the Late Paleozoic and Middle Mesozoic cycles of kimberlite magmatism, which was expressed on the surface in extensive effusions of traps in Permo-Triassic time. Many authors (e.g., Dobretsov, 2005; Renne and Basu, 1991; Sheth, 1999) believe that the tectonothermal activity was triggered by the Siberian superplume, which affected the lower lithosphere mantle.

Conclusions The specific compositions of different types of pyropes appear when the rocks of the lithosphere mantle section experience the action of the rising asthenosphere melts characterized by different concentrations of incompatible elements. The specific composition of LAC-pyropes shows that they belong to the lherzolite paragenesis (i.e., coexist with olivine, orthopyroxene, and clinopyroxene) and reflects typical PT conditions of existence of this paragenesis. There is evidence that LAC pyropes might be produced by the magmas corresponding to high degrees of melting of nondepleted mantle matter, which affected the rocks of the upper mantle section, including the horizons where spinel peridotites occur. As the LAC pyropes exist in the mantle within narrow limits of temperature and pressure, their predominance in the concentrate of some kimberlite pipes suggests that the lithosphere thickness penetrated by these pipes most likely does not exceed 170 km. On the Siberian Platform the LAC pyropes occur in kimberlites of both Paleozoic and Mesozoic age. However, considerable concentrations of the LAC pyropes are recorded only in Mesozoic kimberlite bodies where trap magmatism is expressed (northwestern part). This fact is additional evidence that the LAC pyropes might appear in great amounts on the Siberian Platform when the Siberian superplume affected the lithosphere mantle. Small amounts of LAC pyropes brought to the surface by Paleozoic kimberlites on the Siberian Platform reflect similar, though less extensive, processes of enrichment of the lithosphere mantle occurring in the Paleozoic.

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A larger amount of LAC pyropes in the alluvial deposits of the Muna-Markha interfluve suggests that still unknown kimberlite bodies of Mesozoic age might exist in the central part of the platform. A considerable change in the composition of pyropes from the kimberlites in separate parts of the Siberian Platform (northeastern, southwestern, and possibly central) from Paleozoic to Mesozoic indicates that the action of the Permo-Triassic plume led to a considerable change in the composition and structure of the lithosphere mantle of the Siberian Platform in its separate parts.

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Editorial responsibility: N.V. Sobolev