Polygenetic Sources of Kimberlites, Magma ... - Springer Link

ISSN 0869-5911, Petrology, 2009, Vol. 17, No. 6, pp. 606–625. © Pleiades Publishing, Ltd., 2009. Original Russian Text © O.A. Bogatikov, V.A. Kononova, A.A. Nosova, A.V. Kargin, 2009, published in Petrologiya, 2009, Vol. 17, No. 6, pp. 651–671.

Polygenetic Sources of Kimberlites, Magma Composition, and Diamond Potential Exemplified by the East European and Siberian Cratons O. A. Bogatikov, V. A. Kononova, A. A. Nosova, and A. V. Kargin Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry (IGEM), Russian Academy of Sciences, Staromonetnyi per. 35, Moscow, 119017 Russia; e-mail: [email protected] Received July 15, 2009

Abstract—The petrological and geochemical characteristics of kimberlites from two Russian provinces of the northern East European craton (EEP) and the Siberian craton (SC) (especially the Yakutian diamondiferous province, YDP), and aphanitic kimberlites from the Jericho pipe (Canada) were compared for the elucidation of some aspects of the genesis of these rocks. The comparison of the EEP and YDP showed that they comprise identical rock associations with some variations in kimberlite composition between particular fields and regions, which are clearly manifested in the TiO2–K2O, TiO2–(Y, Zr, HREE), SiO2–MgO, SiO2–Al2O3, MgO– Ni, MgO–CO2, and MgO–H2O diagrams and in variations in light element ratios (Li/Yb, Be/Nd, and B/Nb). The compositions of YDP kimberlites are confined mainly to quadrant III; i.e., their source was mainly the depleted mantle, whereas the compositions of EEP kimberlites fall within all four quadrants in the fields of both enriched and slightly depleted mantle reservoirs. The initial (143Nd/144Nd)i ratio of kimberlites from the Yakutian collection is 0.5121–0.5126. The lead isotopic characteristics of the EEP and YDP kimberlites are similar to mantle values: 206Pb/204Pb of 16.19–19.14, 207Pb/204Pb of 15.44–15.61, and 208Pb/204Pb of 34.99–38.55. In the 207Pb/204Pb–206Pb/204Pb diagram, part of the kimberlites, including those from the Botuobiya pipe, fall within the lower part of the field of group I kimberlites from southern Africa near the Pb isotopic composition of the depleted mantle. It was shown that the chemical compositions of the aphanitic kimberlites of the Jericho pipe (supposedly approaching the composition of primary magmas) are similar to those of some individual kimberlite samples from the YDP and EEP. It was supposed that the initial kimberlite melt arrived from the asthenosphere and was enriched in water and other volatile components (especially CO2). During its ascent to the surface, the melt assimilated mantle components, primarily MgO; as a result, it acquired the compositional characteristics observed in kimberlites. Subsequent compositional modifications were related to diverse factors, including the type of mantle metasomatism, degree of melting, etc. We emphasized the importance of petrological and geochemical criteria (low contents of HREE and Ti in the rocks and a kimberlite source similar to BSE or EMI) for the estimation of the diamond potential of rocks. DOI: 10.1134/S0869591109060071

INTRODUCTION During the past decades, new kimberlite provinces and pipes and new types of kimberlites have been discovered. In Russia, these are the new East European province (EEP) of kimberlites, which is characterized by wide variations in rock age (from the Proterozoic to the Devonian) and composition (Archangelsk Diamondiferous…, 1999; Kononova et al., 2007; Bogatikov et al., 2007). In the Siberian craton (SC), kimberlite magmatism is confined mainly to Yakutia (Yakutian Diamondiferous Province, YDP), where new kimberlite regions and fields were also discovered (e.g., Nakyn field). Their age ranged from the Middle Paleozoic to the Paleogene, and the composition of kimberlites was also variable (Golubeva et al., 2006; Kargin et al., 2008; etc.). The rare oldest (Proterozoic) occurrences of kimberlite magmatism were found in the southwest of the SC (Ingashinskoe field). Until recently, the comparative

analysis of the kimberlite magmatism of the EEP and SC was carried out only in a few studies, which focused mainly on the comparison of data on the geology, geomorphology, and mineral composition of kimberlites (Khar’kiv, 1992; Milashev and Sokolova, 2000; etc.). In a recent fundamental study, Belov et al. (2008) did not pay adequate attention to the comparative analysis of the EEP and SC kimberlites. Another debatable topic is the composition of primary kimberlite melt. The problem is related primarily to the fact that there are practically no rapidly quenched kimberlite lavas, and unfractionated kimberlites that are periodically described in the literature provide no compelling evidence for the retention of their primary composition. Kimberlite magmas are usually contaminated by crustal materials, mantle xenoliths, and xenocrysts; in addition, they are strongly altered owing primarily to the serpentinization or saponitization of oliv-

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ine. Fine-grained autoliths (Fergusson et al., 1975) and selvages of aphanitic kimberlite dikes and sills (Price et al., 2000; Le Roex et al., 2003; etc.) were sometimes interpreted as initial melts. Recent finding of a quenched contact zone in the Jericho kimberlite pipe of the Slave craton in Canada (Price et al., 2000) was interpreted most reliably as a primary kimberlite; these rocks are used here as a reference material for a comparison with the kimberlites of the northern EEP and YDP. In contrast to previous studies, our efforts (especially during recent years) have focused on the comprehensive geochemical and isotope geochemical investigations of kimberlites, which provided a basis for the solution of key petrological issues. In order to eliminate methodical errors, kimberlite samples were analyzed using a consistent procedure described by Bogatikov et al. (2001) in a few well-known laboratories (Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry of the Russian Academy of Sciences; Institute of Precambrian Geology and Geochronology of the Russian Academy of Sciences; Geological Institute of the Russian Academy of Sciences; and Institute of Mineralogy and Geochemistry of Rare Elements of the Ministry of Natural Resources). It should be pointed out that samples of the same type were selected for investigations, primarily porphyritic kimberlites (PK) and autolithic kimberlite breccias (AKB). Unfortunately, our collection included only a few samples of aphanitic kimberlites, which, according to recent investigations, are of genetic significance. The samples were analyzed using the ICP MS technique, and the least contaminated varieties with a contamination index (CI) of lower than 1.5 (Clement, 1982; Taylor et al., 1994) were selected for isotopic investigations and comparative analysis. STUDY OBJECTS We studied kimberlites from two Russian provinces, the EEP and YDP, where more than 1000 kimberlite pipes are known, even without sills and smaller bodies. In one of the earliest comparative studies of the geology and composition of kimberlites from the EEP and YDP, Milashov and Sokolova (2000) pointed out some systematic distinction, including the difference in the composition of the enclosing sequences (carbonates in the YDP and terrigenous in the EEP) and the character of postmagmatic alterations (serpentine in the YDP and saponite in the northern EEP). With respect to the size of provinces, the number of known kimberlite bodies, the number of diamondiferous diatremes, and diamond content and resources, Yakutia is far superior to the EEP. Khar’kiv et al. (2003) evaluated the characteristics of kimberlites from the northern EEP and pointed out the simple interior structure of pipes, the highly magnesian compositions of rocks, and the low contents of kimberlite indicator minerals and deep-derived nodules. Khar’kiv et al. (2003) proposed an explanation for PETROLOGY

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the formation of saponite instead of serpentine (which is predominant in most other provinces of the world) in the EEP kimberlites. Saponite is a more silicic mineral than serpentine, and it is formed in the EEP kimberlites owing to interaction with SiO2-rich postmagmatic solutions produced by exchange processes with quartzbearing xenogenic materials. Our investigations of kimberlites from these two provinces (northern EEP and YDP) and the analysis of published data on the aphanitic kimberlites of the Jericho pipe were aimed mainly at comparing their petrochemical and geochemical (including isotopic) characteristics. When possible, analytical data for each particular field of each province were used. Similar to the majority of the world’s kimberlite provinces, Russian kimberlites are represented by several major morphostructural varieties. The kimberlites of the intrusive phase include autolithic kimberlite breccias and porphyritic kimberlites. Aphanitic kimberlites were rarely reported in Russian publications and were almost never analyzed. The magmatic material of PK and AKB is dominated by strongly altered olivine of generations I and II, and approximately 2% of the rocks are crystal clasts of orange garnet, pyrope, picroilmenite, phlogopite, and occasionally chrome diopside. The statistical processing of the tremendous amount of data accumulated up to date on compositionally widely variable kimberlites resulted in distinguishing a number of varieties (up to 12 petrochemical varieties according to Vasilenko et al., 1997), which complicates their comparison. Therefore, we restricted ourselves to a comparison of analyses classified on the basis of TiO2 content, which is one of the main petrochemical characteristics of kimberlites. It was taken into account that, according to experimental investigations (Kogarko et al., 1988; Ryerson et al., 1987), an increase in TiO2 content indicates an increase in temperature. Bogatikov et al. (2004) evaluated the behavior of trace elements and tentatively distinguished three groups of samples, which were referred to as low-titanium (TiO2 < 1.1 wt %), medium-titanium (TiO2 from 1.1 to 2.5 wt %), and high-titanium (TiO2 > 2.5 wt %) kimberlite varieties. Table 1 shows the number of samples analyzed and used in this study for each field and province. The analytical data are presented in diagrams (Figs. 1–8), and representative analyses of several samples are given in Table 2. Of course, in addition to a dominant kimberlite type, individual samples of other types of these rocks may be found in some fields, but only data on the dominant kimberlite type were used for the comparison. Kimberlites of the Northern East European Province (Northern EEP) Several regions were distinguished in the northern EEP: Prionezhskii (Kimozero), Zimnii Bereg (Zolotitsa, Kepino, and Chernoozerskoe fields), Terskii Bereg, Kandalaksha, Timan, and Finland, which were formed in the Late Paleoproterozoic, Neoproterozoic,

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Table 1. Three types of kimberlites with different TiO2 contents (wt %) from the SC and EEP Number of analyzed samples and TiO2 variations in them Age (epoch)

PR1 êz

êz, Mz, Pg Pz, Mz Pz,Mz Mz-Pg T J J PR1 D

NP D PR1

Kimberlite field, region

2.5 wt % (high-titanium)

Southwestern Siberian craton 1 Northeastern Siberian province (Yakutia) Mirnyi 10 860* Nakyn 11 Daldyn–Alakit region 29 136* Upper Muna 6 Chomurdakh 2 2 Upper Motorchunskoe 1 Malaya Kounamka 2 Luchanskoe 3 Kuoika 3 4 Dyuken 1 Ary-Mastakh 1 Starorechenskoe 4 Orto-Yarginskoe 2 Kharamai 1 5 Northern East European province (northern EEP) Onega region (Kimozero) 25 8 Zolotitsa 27 3 Kepino 5 Chernoozerskoe (V. Grib pipe) 9 40 Terskii Bereg 1 1 Kandalaksha 1 Timan (Umba) 2 Finland (Kaavi–Kuopio) 20 Southern East European platform (southern EEP) Eastern Azov region Kirovograd Ingashinskoe

6 2 3 5 4 7 5 2 4

1

15 1

2 3 3 2

Note: In addition to our own data, the data of Vasilenko et al. (1997) were used.

and Paleozoic. The main types of kimberlites are briefly characterized below. Low-titanium kimberlites were first reported from the Zolotitsa field, and then individual samples were found in the Terskii Bereg and Cernoozerskoe fields (all of them are Devonian in age), and they are dominant among the Paleoproterozoic metakimberlites of Kimozero. Samples from the Zolotitsa area served as a prototype of low-titanium kimberlites. They show an age of 380.1 Ma (Pervov et al., 2005) and a number of specific petrochemical and geochemical features: TiO2 < 1.1 wt %, weak LREE enrichment to (La/Yb)n of 18–44, and neg-

ative εNd values (from –2.2 to –5.3) at relatively low εSr (mainly from –4.5 to +29). Compared with the kimberlites of the Kepino field, they have less radiogenic Pb isotopic characteristics: 206Pb/204Pb of 18.13–18.27, 207Pb/204Pb of 15.50–15.60, and 208Pb/204Pb of 37.69– 38.14. These geochemical characteristics probably indicate a considerable contribution of the ancient enriched mantle of the EMI type to the composition of these rocks. The model ages of the analyzed samples, í(Nd)DM, are 1.1–1.3 Ga. Among the Terskii Bereg kimberlites, there are diamond-bearing varieties. Their age ranges according to various authors from 337–384 Ma (Kalinkin et al., 1993) to 465 Ma (Bayanova et al., 2004). The bulk-rock PETROLOGY

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POLYGENETIC SOURCES OF KIMBERLITES, MAGMA COMPOSITION 6 (a)

609

(a) Nonmicaceous kimberlites (group IB) Olivine lamproites

TiO2, wt %

4

Micaceous kimberlites

2

(b) 0 6

n n Africa Souther ( s e it e ) Orang imberlites group II k Nonmicaceous kimberlites (group IA) 2 4 K2O, wt % (b) Nonmicaceous kimberlites (group IB)

Fig. 1. Petrographic features of kimberlites from the (a) Nachal’naya and (b) Daikovaya pipes of the YDP. (a) Fine-grained kimberlite; numerous phenocrysts of carbonate, partly replaced olivine, and scarce mica plates can be seen. (b) Kimberlite with olivine phenocrysts replaced by serpentine and rare mica prisms; the fine-grained and occasionally glassy groundmass comprises olivine and oxides. Magn. 10×.

1 2 3 4 5 6 7 8

TiO2, wt %

4

Olivine lamproites

6

2

Micaceous kimberlites

n n Africa Souther ( s e it e es) Orang kimberlit group II Nonmicaceous kimberlites (group IA) 2 4 K2O, wt %

kimberlite samples show εSr values from –9.4 to +1.2 and εNd from –1.4 to +3.2 (Beard et al., 1998; Bayanova et al., 2004).

0

The Paleoproterozoic diamondiferous metakimberlites of the Kimozero occurrence (northern Onega region) are dominated by low-titanium varieties (Table 1), although medium-titanium kimberlites were also documented. Their age is 1764 ± 125 Ma (Sm–Nd method) (Ushkov, 2001; Ushkov et al., 2008). The metakimberites are dominated by horizontally bedded lapilli tuffs strongly contaminated by the country rocks (high CI value).

Fig. 2. Variations in TiO2 and K2O in the kimberlites of Yakutia and the EEP and aphanitic kimberlite varieties from the Jericho pipe, Canada. (1)–(3) Kimberlites of the YDP, (4)–(6) kimberlites from the northern EEP; (1) and (4) high-titanium kimberlites; (2) and (5) medium-titanium kimberlites; (3) and (6) lowtitanium kimberlites; (7) aphanitic kimberlites from the Jericho pipe, Canada; and (8) field boundaries after Smith et al. (1985).

The emplacement of medium-titanium kimberlites was documented in the Paleoproterozoic, Neoproterozoic, and Paleozoic in the Kimozero and Chernoozerskoe fields. They are also widespread in Finland. Their prototypes are the Devonian kimberlites of the V. Grib

pipe, which also contains occasional low-titanium varieties (Table 1). They vary in composition depending on the morphology of occurrence: the vent facies of the pipe is made up of tuff and xenotuff breccias (phase I, 28% of the vent volume), and the major volume is built

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BOGATIKOV et al. 5

5

(a)

(b)

HREE

4

3 2 1

Zr, ppm

0 500 400 300 200

3 2 1 0 500 400 300 200 100 0

30

30 Y, ppm

100 0 Y, ppm

Zr, ppm

HREE

4

20 10

20 10

0

1

2

3 4 TiO2, wt %

5

0

6

1

2

3 4 TiO2, wt %

5

6

Fig. 3. Covariations of TiO2 with HREE, Zr, and Y in the kimberlites studied. Symbols are the same as in Fig. 2.

(a)

5 3

5 3 1

MgO, Ï‡Ò. %

1 MgO, wt %

(b)

7 Al2O3, Ï‡Ò. %

Al2O3, wt %

7

35 25

35 25 15

15 20

30

40

50

SiO2, wt %

20

30

40

50

SiO2, wt %

Fig. 4. Diagrams SiO2–Al2O3 and SiO2–MgO for the kimberlites studied. Symbols are the same as in Fig. 2.

up of kimberlites (phase II). The kimberlites of phase II are also heterogeneous and include kimberlitic autolithic breccias and porphyritic kimberlites. The diamond content of the phase II kimberlites is approximately three times that of the xenotuff breccias. The maximum diamond content is characteristic of AKB,

which is usually confined to the central parts of pipes. In terms of Nd isotope characteristics, the kimberlite samples (phase II) are similar to the Bulk Silicate Earth (BSE), and their εNd ranges from small negative (–1.0) to moderate positive (+1.5) values. However, their initial Pb isotope characteristics, 206Pb/204Pb of 18.03– PETROLOGY

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611

Ni, ppm

Ni, ppm (a)

2000

1500

1500

1000

1000

500

500 15

20

25 30 MgO, wt %

35

(b)

2000

40

15

20

25 30 MgO, wt %

35

40

Fig. 5. Diagram MgO–Ni for the kimberlites studied. Symbols are the same as in Fig. 2.

20 CO2, Ï‡Ò. %

(a)

15 10 5

10 5 0

15

15

10 5 0

15

20

25 30 MgO, %

35

40

(b)

15

0 H2O+, Ï‡Ò. %

H2O+, wt %

CO2, wt %

20

10 5 0

15

20

25 30 MgO, %

35

40

Fig. 6. Diagrams MgO–CO2 and MgO–H2O for the kimberlites studied. Symbols are the same as in Fig. 2.

18.08, 207Pb/204Pb of 15.49–15.52, and 208Pb/204Pb of 37.89–38.02, are shifted toward the low-titanium Zolotitsa kimberlites. The rocks show (La/Yb)n = 38– 87, i.e., transitional values between those of the Kepino and Zolotitsa kimberlites (Table 2). The Neoproterozoic group of kimberlite pipes and dikes at Kaavi–Kuopio in Finland is confined to the margin of the Karelian craton and has an age of 589– 626 Ma (U–Pb perovskite age; O’Brien and Tuni, 1999; PETROLOGY

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O’Brien et al., 2003). Twenty four kimberlite bodies were documented there. With respect to mineral composition, they are similar to the group I kimberlites of southern Africa, whereas their geochemical and isotopic characteristics are more similar to those of the Koidu kimberlites of western Africa (O’Brien and Tuni, 1999). Their isotopic systematics (εSr from –6.9 to +1.0 and εNd from 0 to +1.3 for an age of 600 Ma) indicate that the magma source could be a BSE-like reservoir or a mixture of sources similar to those of group I southern

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BOGATIKOV et al. 100

B/Nb

10

1

0.1

Be/Nd

0.01

0.1

0.01

Li/Yb

1000

100

10

1

0

5

10 Ga, ppm

15

20

Fig. 7. Variations in the Li/Yb, Be/Nd, and B/Nb ratios in the kimberlites studied. Symbols are the same as in Fig. 2.

African kimberlites and the lithospheric mantle of the Karelian craton. A dike of monticellite kimberlite with an age of 365 ± 16 Ma (K–Ar bulk rock age; Beard et al., 1998) was described in the Kandalaksha area of the Baltic shield. High-titanium kimberlites. The Devonian period was marked by the wide occurrence of alkali ultrabasic and kimberlite magmatism in the northern EEP: in the Baltic shield (Kola Peninsula and Karelia), in Archangelsk oblast, and in the Middle Timan. The Kepino field is a prototype for the development of high-titanium kimberlites. During the recent years, many new bodies have been discovered there (Bogatikov et al., 2007). They are dominated by xenotuff

breccias (Rozhdestvenskaya, Galina, and 495a pipes) with fragments of kimberlites, autoliths, and country rocks (mainly sandstones) and porphyritic kimberlites, including strongly altered varieties. Since the rocks of the pipes are altered to a varying degree, the contents of some elements probably do not exactly correspond to the initial values; nonetheless, most samples show high TiO2 contents (>2.5 wt %). Kimberlites from many magmatic bodies of the Kepino field are enriched in LREE and have (La/Yb)n values of 70–130; they show persistently positive εNd values (from +2.8 to +1.2) and high 206Pb/204Pb 18.46–19.03, 207Pb/204Pb 15.53–15.65 (15.53–15.65), and 208Pb/204Pb (38.43–38.77). Hence, they are similar in Sr, Nd, and Pb isotope characteristics to the group I kimberlites of southern Africa. PETROLOGY

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8 Group I kimberlites of southern Africa Yakutian kimberlites 4

εNd(T)

Botuobiya pipe

0

Kepino field

Nyurbinskaya pipe

Grib pipe

–4 Zolotitsa field Group II kimberlites of Southern Africa

–8

–20

0

20

40 εSr(T)

60

80

100

Fig. 8. Isotopic compositions of kimberlites from the EEP and YDP in the εNd(T)–εSr(T) diagram. The fields of southern African kimberlites are after Mitchell (1986). Symbols are the same as in Fig. 2.

Three pipes of the Middle Timan (Umba occurrence) were classified after some discussion as kimberlites (Kononova et al., 2000). Their age was determined as 390 ± 14 Ma by the K–Ar phlogopite method and 400 Ma by the U–Pb zircon method (Mal’kov and Kholopova, 1995). Their rock assemblages include both medium- and high-titanium varieties (Table 1). They show rather high positive εNd values from +2.8 to +3.0 (Kononova et al., 2002). The compositions of minerals and their assemblages in deep-derived xenoliths of garnet peridotites and eclogites from the Umba pipe correspond to the conditions of the diamond-free facies of mantle rocks (Mal’kov and Kholopova, 1995; Pervov et al., 2002). Kimberlites of the Siberian Province Bogatikov et al. (2004) established that, similar to the northern EEP, the kimberlites of the Yakutian kimberlite province include three petrochemical types (low-, medium-, and high-titanium). Each of the types is characterized by its specific petrochemical and geochemical features related to a number of factors: confinement to geoblocks (terranes) of different age in PETROLOGY

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the eastern part of the Siberian craton, characteristics of the mantle source, age and type of mantle metasomatism, character of plume–lithosphere interaction, etc. The most important criteria for the distinguishing of the petrochemical and geochemical types of kimberlites of the YDP are briefly discussed below. Low-titanium kimberlites are confined mainly to the recently discovered diamond-rich Nakyn field and show TiO2 < 1 wt %, HREE of 0.8–2.1, Y of 5–15 ppm, Zr of 18–160 ppm, and La of 9.31–119.45. Their εNd ranges from +2 (Botuobiya pipe) to –3 (Nyurbinskaya pipe), εSr is 20–50, 206Pb/204Pb is 17.70–19.14, 207Pb/204Pb is 15.36–15.59, and 208Pb/204Pb is 37.10– 38.54 (Table 2). The model age, TNd(DM), of the analyzed samples, which probably characterizes the time of their source formation, was estimated as 1.4–0.9 Ga, and older ages (1.4–1.2 Ga) were obtained for samples from the Nyurbinskaya pipe. These data should be probably accounted for during the interpretation of the genesis (and composition) of the kimberlites of the Nakyn field as products of plume–lithosphere interaction with the participation of fluid metasomatism and assimilation of lower crustal materials.

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Table 2. Chemical compositions of representative kimberlite samples from the EEP and SC Northern EEP Component

low-Ti

medium-Ti

Northeastern SC high-Ti

low-Ti

Pionerskaya pipe, V. Grib pipe, Zvezdochka pipe, Nyurbinskaya pipe, sample 58 AP sample 9ts-484 sample 41 AP sample 32-440

medium-Ti

high-Ti

Chomur pipe, sample 6

Lykhchan pipe, sample 21-05

SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O P2O5 CO2 H2O

36.80 1.06 2.11 4.69 4.12 0.16 35.24 4.54 0.96 1.80 n.d. 0.99 7.11

33.34 2.26 1.51 5.65 2.15 0.57 29.84 6.74 0.20 0.18 0.51 5.19 11.04

29.30 2.77 4.60 6.82 1.62 0.26 24.95 10.87 0.53 1.00 0.85 6.97 8.59

34.14 0.76 4.26 4.16 3.92 0.13 24.59 10.16 0.39 2.56 0.72 6.99 7.04

32.33 1.98 3.05 4.81 4.05 0.15 28.14 8.79 0.14 1.88 0.47 4.51 8.80

31.67 5.68 3.44 7.40 5.92 0.18 22.30 9.69 0.22 2.70 0.75 3.67 5.52

Total

99.58

99.18

99.13

99.82

99.10

99.14

116.8 3.74 1663 1278 64.8 502 11.3 77 33 490 0.57 4.57 22.75 2.34 4.68 24.7 2.8 1.31 85.1 +16 –0.2 1.2

37.09 2.43 1130 1637 72.7 877 26.6 220 176 1087 1.17 14.23 91.58 1.25 1.31 64.7 4.9 1.11 85.7 +13 +5 0.7

21.35 2.19 1390 1044 87.1 938 23.3 319 391 1251 1.06 15.33 107.43 0.81 0.66 97.7 6.2 1.28 76 –12 +4 0.5

Li Be Cr Ni Rb Sr Y Zr Nb Ba Yb Sm Nd Zr/Nb K/Ti La/Yb La/Sm ë.I. mg# εSr εNd TNd(DM), Ga

4.40 n.d. 1500 1300 37 383 15 85 36 697 0.80 4.00 38.00 2.36 2.35 31.9 5.9 1.08 88.3 26.2 –5.0 1.3

n.d. 150 2.49 n.d. 1452 1105 1125 746 13.7 40 536 631 9.6 13 64 259 85 146 728 787 0.48 0.85 5.96 9.67 38.68 77.5 0.75 1.77 0.11 0.50 70.8 141.72 5.2 18.46 1.16 1.28 88 85 –0.1 from –1.2 to –11.6* –0.1 and +1.3* from 1.2 to 1.7* 0.9 and 1.0* 0.8

Note: C.I. is contamination index, (SiO2 + Al2O3 + Na2O)/(2K2O + MgO) (Taylor et al., 1994), and mg# = MgO/(MgO + FeO). * The data were obtained for a series of samples.

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Medium-titanium kimberlites were documented in the commercially diamondiferous Mirnyi and Upper Muna fields in the Daldyn–Alakit region. They contain 1.1–2.7 wt % TiO2, 1.1–2.7 ppm HREE, 6–31 ppm Y, 51–457 ppm Zr, and 29.98–226.16 ppm La; their εNd ranges from 3.5 to 4.5, εSr is approximately 35, 206Pb/204Pb is 18.5–19.0, 207Pb/204Pb is 15.3, and 208Pb/204Pb is 38.0–38.1. The model age of the rocks, TNd(DM), ranges from 0.7 to 0.6 Ga (Table 2). Medium-titanium kimberlites occur within the Magan and the central part of the Anabar (boundary of the Markha and Daldyn terranes) geoblocks, whose crust was formed within 3.6–3.1 Ga. The compositions of mantle garnets and chromites from the kimberlites suggest that the lithosphere beneath these geoblocks is very similar to that of typical Archean cratons, for instance, the Kaapvaal craton of southern Africa. It is very thick and strongly depleted, contains a substantial fraction of harzburgites, and is characterized by a cold geotherm and low-temperature metasomatism. According to geochemical data, including Sr, Nd, and Pb isotopic systematics, this type of kimberlites is most similar to the group I kimberlites of southern Africa. High-titanium kimberlites occur mainly in the “northern” fields confined to the Olenek terrane. They show TiO2 > 2.7 wt %, 1.7–6.6 ppm HREE, 11–45 ppm Y, 119–690 ppm Zr, and 49.15–152.19 ppm La. Their εNd value (3.5–4.5) is approximately the same as in the medium-titanium kimberlites, εSr ranges from –20 to +30, 206Pb/204Pb is 17.6–18.8, 207Pb/204Pb is 15.4–15.5, and 208Pb/204Pb is 37.3–38.5. The mantle of the Olenek geoblock is thinner than that of the southern structures and is characterized by a hot geotherm, a smaller fraction of harzburgites, and some other specific features. The continental crust of the buried basement was formed in the Early Proterozoic (2.4–2.2 Ga). Kimberlite magmatism occurred there in two stages, Paleozoic (model Nd age of 0.7–0.8 Ga) and Mesozoic (model Nd age of 0.6–0.5 Ga). Hence, the latter was almost coeval with the emplacement of kimberlites in the southern regions of the province (Table 2). The hotter geotherm could result in a higher temperature in the source of kimberlite melts, which, in particular, could be responsible for the increase in titanium and zirconium contents in the kimberlites of the northern fields of the YDP. This suggestion is supported by experimental studies. The comparison of the EEP and YDP showed that their rock assemblages are identical, although the compositions of kimberlites are somewhat different between fields and regions, which is discussed below.

dominated by Cretaceous and Early Tertiary kimberlites cutting two-stage geologic sequences composed of Archean (approximately 3500 Ma) gneisses and granulites overlain by Proterozoic metasedimentary clastic, greenstone volcanic, and plutonic rocks of an age of 2160 Ma. More than 150 kimberlite bodies were identified in the Slave craton (Khar’kiv et al., 1998). They are made up of volcaniclastic and hypabyssal varieties. The Jericho pipe is a polyphase kimberlite body, which was formed in four stages: explosion with the formation of an open cavity (stage 1), filling of the cavity with pyroclastic material (stage 2), deposition of epiclastic material (stage 3), and kimberlite emplacement (stage 4). The most representative samples of the rocks of the pipe are aphanitic kimberlites with sparse microphenocrysts. The rocks are light gray in color, moderately loose, and significantly serpentinized. The contact zone of a dike accompanying the Jericho pipe attracted special attention. The structural and mineralogical characteristics of the border zone indicate its crystallization from a melt: the rock contains rare macrocrysts in a fine-grained homogeneous groundmass. Several parts of a single sample and several samples from the quenched selvage of the body display only minor chemical variations, which is expected for the products of melt crystallization. With respect to geochemical features, the aphanitic samples reflect the composition of the primary magma of the Jericho kimberlites: they contain 20–25 wt % MgO and have a high mg# value of 86–88 (Table 3, analysis 5). However, their mg# values are lower than those of samples with macrocrysts (89– 90), which increases owing to the entrainment of mantle olivines and other deep-seated macrocrysts. The quenched samples of the border zones of the Jericho kimberlites contain 1300–1900 ppm Cr and 800– 1400 ppm Ni; they are enriched in incompatible elements (Zr, Nb, and Y) relative to the bulk composition of the kimberlites with macrocrysts. A characteristic feature of the aphanitic kimberlites of the Jericho pipe is the high, although variable, content of volatiles (12– 19 wt % ëé2 and 5.3–7.5 wt % ç2é). The high ëé2 content is related to the abundance of primary calcite in the groundmass. This provides compelling evidence that, during transportation to the surface or between the emplacement and subsequent crystallization, the magmas did not lose volatiles. The presented analyses provide insight into the minimum contents of volatiles in the magmas of the Jericho kimberlites. Another argument for the primitive composition of these kimberlites was gained from the analysis of Os isotopes: the γOs values are close to zero and the Os content is 1.5 ppb (Price et al., 2000).

Aphanitic Kimberlites of the Jericho Pipe It was previously supposed that massive porphyritic kimberlites are closest to the primary melts, but this is not quite true, which is indicated by the composition of aphanitic kimberlites from the Jericho pipe of the Slave craton in Canada (Price et al., 2000). This region is

Among the analyses from our database (the whole database includes approximately 160 our own analyses) on the kimberlites of the EEP and YDP, there are approximately 100 samples from 14 fields (regions) of the YDP, and only two samples (or, less strictly, three samples) are comparable with the primary kimberlites of the Jericho pipe (Canada). From approximately the

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Table 3. Short petrochemical characteristics of aphanitic kimberlites Sample no.

Pipe, province

Short description

Nch-12

Nachal’naya, Fine-grained (aphanitic) kimberlite with very rare rounded Yakutia porphyritic phenocrysts of mica and numerous rounded phenocrysts of carbonate Mt-K Malyutka, Kimberlite with an aphanitic groundmass and numerous pheYakutia nocrysts of serpentinized olivine and calcite Dk-E2 Daikovaya, Rapidly crystallized olivine kimberlite consisting of a fineYakutia grained groundmass with individualized serpentinized olivine crystals and rare small mica scales 746746, EEP Aphanitic kimberlite grading locally to porphyritic kimber1a/235.3 lite with small completely altered olivine (serpentine, calcite, and opaque minerals) JD-82 Jericho, Light gray aphanitic kimberlite with scarce serpentinized oliCanada vine phenocrysts (Price et al., 2000)

same number of samples from the northern EEP, only one sample from the Kepino field is similar to the Jericho kimberlites. These kimberlite samples (Table 3), whose petrographic characteristics are shown in photomicrographs (Fig. 1), support the suggestion that primary kimberlites are practically not preserved and underwent various magmatic and postmagmatic processes (differentiation, contamination, etc.), which controlled to a large extent the geochemical diversity of the YDP and EEP kimberlites. RESULTS. COMPARATIVE PETROGEOCHEMISTRY OF THE OF THE EEP AND YDP KIMBERLITES AND APHANITIC KIMBERLITES FROM THE JERICHO PIPE, CANADA After many years of debate on kimberlite composition, a concept was developed that kimberlites are a large clan (group or series) of rocks including several types and varieties. Using two-dimensional diagrams, we compared the compositions of kimberlites from the YDP and the northern EEP with samples from the Jericho pipe in Canada. The majority of diagrams were constructed separately for the EEP and YDP to facilitate the comprehension of the data. The diagrams of Figs. 2–9 show that the compositions of the Jericho rocks correspond to low-titanium kimberlites and are similar to the YDP samples in having minor K2O contents (up to 0.5 wt %). With respect to all other ratios, they also approach or fall within the field of the YDP samples. High CO2 and relatively low H2O contents were detected in the samples from the Jericho pipe. High H2O contents (up to 12–15 wt %) were documented in the kimberlites of the northern EEP and YDP, whereas the samples from the Jericho pipe contained no more than 6–7 wt % H2O. It should be noted that our analysis was based mainly on two morphotextural varieties, massive por-

MgO, mg# wt %

Cr, ppm

Ni, ppm

CO2, wt %

20.21

88

1330

830

14.26

25.95

87

1528

1088

10.41

28.06

88

1562

1075

8.42

27.29

87

1353

957

8.86

23.83

86

1736

1351

11.42

phyritic kimberlites and autolithic kimberlite breccias. There are only a few samples of aphanitic kimberlites in our collection; they are usually strongly altered and were used for the comparison only in rare cases. Geochemistry of Major and Trace Elements The TiO2–K2O diagram is one of the most informative for kimberlites. It was used by many foreign (Taylor et al., 1994; Smith et al., 1985; Mitchell, 1995) and Russian authors (Bogatikov et al., 2004; Kononova et al., 2002). It can be seen in the diagram (Fig. 2) that the tendency of kimberlite separation with respect to titanium is retained, and the low-titanium kimberlites of the YDP include several orangeite samples. Another interesting feature is the lower TiO2 content in the Yakutian samples (Fig. 2b) compared with the samples from the northern EEP (Fig. 2a). The general similarity of the two Russian provinces is evident in Fig. 3. Nonetheless, each of them shows its distinctive features: (a) as was noted above, among the low-titanium varieties, lower TiO2 contents are characteristic of the Yakutian samples; and (b) most of the low-titanium kimberlites from the two Russian provinces show low contents of Y (consequently, garnet) and HREE. The analysis of REE distribution patterns in the kimberlites of the two provinces showed that the persistently moderate total of HREE (1.2–2.5 ppm) is an indicator for the diamond content of kimberlites, which was previously noted by Kononova et al. (2007). The SiO2–ågO and SiO2–Al2O3 diagrams (Fig. 4) characterize the extent of contamination with the enclosing sequences, which is clearly manifested in samples from the northern EEP (Fig. 3a), whose compositions are shifted in both diagrams to the right toward higher SiO2 contents (up to 49 wt %). This feature is correlated with the composition of the country rocks of the northern EEP, which are dominated by terPETROLOGY

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0

15.8

400

UC

800 1200 15.6

Group I kimberlites of Southern Africa

400

1600

207Pb/204Pb

800 1200

15.4 2000

400

1600

Group II kimberlites of Southern Africa

M

800

0

LC

1200

15.2 2000 1600 2400 15.0 2000 2400 14.8 14

16

18

20

206Pb/204Pb

Fig. 9. Diagram 207Pb/204Pb–206Pb/204Pb for the kimberlites of the EEP and YDP (fields are after Smith, 1983). Symbols are the same as in Fig. 2. Also shown are the model curve of Pb evolution in the mantle (M), lower crust (LC), and upper crust (UC). Numbers near the lines are ages in Ma.

rigenous material (sandstones and siltstones), whereas carbonate sequences are dominant in the YDP. In the MgO–Ni diagram (Fig. 5), the kimberlites of two Russian provinces form trends characteristic of differentiation processes. Since the samples for analysis were taken from different complexes (Table 1), they cannot have simple genetic relations and form clusters of points; nonetheless, the general tendency is retained. Noteworthy are the different directions of the ëé2 and ç2é trends (Fig. 6). Both in the Yakutian and Archangelsk samples, the ëé2 content increases with decreasing MgO. The same relation was noted for the primary aphanitic kimberlites: it is believed that the melt assimilates mantle material during its ascent. The content of CO2 is moderate in the Archangelsk kimberlites (only slightly higher than 10 wt %) and reaches 20 wt % in the Yakutian samples. High water contents (up to 12–15 wt %) were observed in the kimberlites of both provinces (Figs. 6a, 6b). The distribution of light lithophile elements (with atomic numbers of 3–5, i.e., Li, Be, and B) was studied in detail in the YDP by Kononova et al. (2005), whereas the contents of these elements were measured only in a PETROLOGY

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few samples from the EEP. The diagram of Fig. 7 shows variations in the Li/Yb, Be/Nd, and B/Nb ratios. High Li, Be, and B contents were noted in the kimberlites of the Nakyn field of the YDP. Samples from the Zolotitsa field of the EEP show elevated Be contents. Geochemistry of Radiogenic Isotopes The neodymium and strontium isotope characteristics (Fig. 8) vary significantly both between particular fields of a single province and between the EEP and YDP. In general, the YDP kimberlites fall mainly within quadrant III (i.e., their source was mainly the depleted mantle), whereas the EEP kimberlites occur in all four quadrants, within the fields of both the enriched and slightly depleted mantle. The kimberlites with different isotopic compositions show a zoned spatial distribution (concentric and linear) both within the EEP and YDP. The Nd–Sr isotopic systems of kimberlites were comprehensively studied in the YDP (in two stages for different kimberlite pipes and fields but in the same laboratory of the Institute of Precambrian Geology and Geochronology of the Russian Academy of Sciences,

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St. Petersburg) by Yu.Yu. Golubeva (e.g., Bogatikov et al., 2004) and A.V. Kargin (2009) with coauthors. The initial (87Sr/86Sr)0 ratios of the YDP kimberlites vary even within a single field between morphologically different kimberlites. For instance, AKB samples from the Lykhchan and Dama pipes (Luchakan field) show εSr similar to that of BSE, 0.7054–0.7056. In contrast, a PK sample from the Lykhchan pipe differs from the AKB sample in having a negative εSr value of –12. In most samples, (87Sr/86Sr)0 ranges within 0.7041–0.7072 and even up to 0.7081; i.e., the samples are somewhat enriched in radiogenic Sr owing probably to crustal contamination. Such variations in (87Sr/86Sr)0 are typical of kimberlites: 0.703–0.705 in the group I kimberlites of southern Africa (Taylor et al., 1994) and 0.7035–0.7071 in the Yakutian kimberlites (Kostrovitskii et al., 1999; Agashev et al., 2000). Samples from two pipes of the Nakyn field are significantly different in the Sr isotope ratio. The Botuobiya pipe is slightly enriched in radiogenic Sr (εSr from +42 to +55) compared with the Nyurbinskaya pipe (εSr = +16). The initial neodymium isotope ratio, (143Nd/144Nd)0, in the collection of Yakutian kimberlites ranges from 0.5121 to 0.5126. Only three samples (D’yanga, Mgrishnitsa, and Ruslovaya pipes of the Kuoika field) show (143Nd/144Nd)0 of 0.5126, which corresponds to the lower boundary for the group I kimberlites of southern Africa, whereas the samples from 12 other pipes (Kargin, 2009) have (143Nd/144Nd)0 values of 0.5123. Similar to Sr isotope characteristics, the (143Nd/144Nd)0 values of kimberlites from the two pipes of the Nakyn field are different. The εNd value of a sample from the Nyurbinskaya pipe (–0.2) is similar to that of BSE and even shifted toward the field of the EMI enriched mantle component. Samples from the Botuobiya pipe show slightly positive εNd values from 2.0 to 1.8, but are also close to BSE, differing significantly from all other pipes analyzed here. The lead isotope characteristics (Fig. 9) of the studied EEP and YDP kimberlites are similar to the mantle values: 206Pb/204Pb varies from 16.19 to 19.14, 207Pb/204Pb is 15.44–15.61, and 208Pb/204Pb is 34.99– 38.55. Figure 9 also shows model curves for the evolution of the isotopic composition of the mantle, upper, and lower crust constructed on the basis of the Stacey and Kramers (1975) model. It can be seen in the 207Pb/204Pb–206Pb/204Pb diagram that some kimberlite compositions, including those of the Botuobiya pipe, plot within the lower part of the filed of the group I kimberlites of southern Africa, near the Pb isotopic composition of the depleted mantle. DISCUSSION. SOME ASPECTS OF THE COMPOSITION AND GENESIS OF KIMBERLITE MAGMAS The available database on the petrological and geochemical characteristics of kimberlites from two Russian provinces allowed us to compare the kimber-

lites of the two provinces with each other and with the composition of aphanitic kimberlites from the Jericho pipe (Canada). Using these data and the results of our previous studies (Kononova et al., 2005; Bogatikov et al., 2007), some genetic aspects of these rocks and the conditions of the generation of kimberlite magmas will be discussed. We will focus on the controversial problems of the composition of initial kimberlite magmas, the role of mantle metasomatism in the formation of kimberlites, and its influence on the composition and spatiotemporal distribution of kimberlites. Initial Kimberlite Magmas According to Le Roex et al. (2003), the near-primary group I kimberlites of southern Africa were formed by the partial melting (0.4–1.5%) of a preliminarily metasomatized source (garnet lherzolite) enriched in strongly incompatible elements (by a factor of 2–5) and depleted in HREE (approximately 0.3 of the primitive mantle level, depending on the amount of residual garnet). The low HREE content, high mg#, and high Ni content indicate that the garnet lherzolite source was depleted before the metasomatism. The compositional characteristics of the Jericho kimberlites are similar to those of aphanitic kimberlites from the Wesselton pipe (Pearson et al., 2003). According to some authors (Edgar et al., 1988; Edgar and Charbonneau, 1993), the latter can be regarded as unfractionated kimberlites. Similar to the aphanitic rocks from Canada, they contain only minor amounts of olivine macrocrysts, xenoliths, and xenocrysts, and their chemical composition shows high MgO (27 wt %), mg# of 83–84, low SiO2 (25.6 wt %), high Ni (810 ppm), and high Cr (2410 ppm) and corresponds to some other unfractionated kimberlite magmas (Arima and Inoue, 1995; Mitchell, 1995). Despite the compositional similarity, the Wesselton kimberlites have much lower CO2 contents (5 wt %); perhaps, the Jericho kimberlites are closer to the primary kimberlite magma composition. The contents of ëO2 and H2O in the Jericho samples correspond to the values characteristics of primary magmas. This conclusion is supported by experimental results on the partial melting of carbonated lherzolite (supposed analog of the mantle source of kimberlites) at pressures and temperatures of the uppermost asthenosphere of the Slave craton (Price et al., 2000). The content of ëé2 in the samples is only slightly lower than in the experiments; it was suggested that the earliest hypabyssal phases of the Jericho kimberlites retained most of the initial volatile components. Based on the recent study of Sparks et al. (2008), it can be suggested that the initial kimberlite melt arriving from the asthenosphere was enriched in water and other volatile components (especially ëé2). During its ascent to the surface, it assimilated mantle components (primarily, MgO) and approached the observed kimberlite compositions. PETROLOGY

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Mantle Sources of Kimberlites and the Role of Mantle Metasomatism Variations in the geochemical and isotopic characteristics of kimberlites indicate differences in their mantle sources. Their compositional interpretation is consistent with the model of the emplacement of a mantle plume ascending from the core–mantle boundary and promoting the penetration of the deep material into the upper mantle (Ringwood et al., 1992; Taylor et al., 1994). The ascending lower mantle plume may form a dome structure more than 1000 km in diameter. Many authors invoked at least three main sources, DM, EM1, and EM2, the combination of materials from which is responsible for variations in the isotopic compositions of Nd, Sr, Pb, and Hf, as well as in a number of trace elements and their ratios. Kimberlites rarely correspond to the primary mantle melts, because their composition is modified under the influence of various physicochemical processes. Of special significance is the difference in the behavior of trace elements, i.e., weather they are mobile (LIL group elements: Cs, Sr, K, Rb, and Ba) or immobile (HFS group elements: REE, Th, U, Ce, Zr, Ti, Nb, etc.) in aqueous fluids (Pearce, 1983; Fraser et al., 1985). The former are transported mainly by aqueous fluids, and the latter, by melts. The elevated contents of Li, Be, and B in some kimberlitic rocks allow us to suggest that their formation involved the fluid metasomatism of the mantle. For instance, such metasomatism could be dominant in the petrogenesis of kimberlites from the Nakyn (Yakutia) and Zolotitsa (EEP) fields (Table 2). It was related to subduction processes that occurred in the ancient geologic history of the regions and were accompanied by the submergence of geoblocks enriched in hydrous minerals, which resulted in the generation of intense fluid flows. This suggestion may be supported by the proportions of trace elements in kimberlites. As an example, Table 2 presents the K/Ti and Zr/Nb indicator ratios. These parameters decrease sharply in the rocks formed under the influence of melt metasomatism, which was discussed in detail by Pearce (1983). The obtained data show that the kimberlites of both provinces were formed under the influence of mantle metasomatism caused by both fluids in the case of kimberlites from the Nakyn (YDP) and Zolotitsa (EEP) fields and melts in the case of kimberlites from the Kepino field (EEP) and some fields from the central YDP. Three Petrogeochemical Types of Kimberlites and the P–T Conditions of Their Formation In the previous section we emphasized the role of various mantle sources. Next, we will consider the influence of P–T conditions on the composition of kimberlites. During the last years, considerable variations have been established in the composition of rocks that were previously assigned to kimberlites. Smith et al. PETROLOGY

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(1985) distinguished two groups with different Sr and Nd isotopic characteristics among the kimberlites of southern Africa. Subsequently, Taylor et al. (1994) recognized two new types of these rocks at Koidu (Sierra Leone, Africa) and Aries (Australia). Mitchell (1995) proposed to use the term kimberlite only for the group I kimberlites of southern Africa. Since the compositions of rocks and minerals from southern African group II kimberlites show some similarity to lamproites (presence of REE-rich phosphates, perovskite, and Krichterite in evolved varieties), the term orangeite was proposed for these kimberlites. During the last decade, the number of kimberlite types has increased: of special importance is the new type of diamondiferous kimberlites that was called Zolotitsa by Bogatikov et al. (2001) after the Zolotitsa occurrence of the EEP. Approximately at the same time, Pokhilenko et al. (2004) proposed the term anomalous for geochemically very similar diamondiferous kimberlites from the Nakyn field of Yakutia and the Slave craton of Canada. In our opinion, this term is not informative. We believe that the content of TiO2 can be used as a criterion for distinguishing kimberlite varieties, taking into account, first, the role of titanium as an indicator of temperature conditions and, second, the methodical approaches used for the classification of lamproites (Bogatikov et al., 1991). In addition, low titanium contents are characteristic of kimberlites formed at high pressures (Vasilenko et al., 2005). We proposed to distinguish three petrogeochemical types of kimberlites, which were conventionally termed low-titanium (2.5 wt %). Note that as the data have been accumulated, the initial boundary at 1 wt % was shifted to 1.1 wt % for the more adequate reflection of the ranges of TiO2 contents in natural samples. In addition to low titanium, the lowtitanium kimberlites show low contents of the majority of trace elements. They have been found up to now mainly in northern continents, including the Russian kimberlite provinces of the EEP and YDP. This geochemical feature of kimberlite magmatism may reflect a global heterogeneity in the Earth’s mantle. Among the rocks discussed here, several samples from the northern EEP and YDP (Internatsional’naya, Botuobiya, Nyurbinskaya, and other pipes) have peculiar compositional characteristics: their points fall within or at the boundary of the orangeite field. However, only rocks with a certain mineral composition can be classified as orangeites: they contain K–Ba titanates, zirconium silicates, and, occasionally, groundmass sanidine and K-richterite (Mitchell, 1995). The absence of these minerals in the samples from the aforementioned pipes and some geochemical features (e.g., K2O < 3 wt %, Zr < 75 ppm, and La < 21 ppm) suggest that their classification with orangeites or group II kimberlites of southern Africa is incorrect. High-titanium kimberlites with 4–7 wt % TiO2 are sometimes termed madjgawanites after the Madjgawan kimberlite pipe in India (Lapin et al., 2004). Samples with high

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ä2é/Na2é ratios were found among the kimberlites of the Pionerskaya pipe and autoliths from the Karpinskaya pipe (northern EEP). This is indicative of the presence of normative leucite or nepheline, and these rocks were identified as lamproites. Each of the two kimberlite provinces of Russia discussed here contains all of the three petrogeochemical types of kimberlites, which are most conveniently distinguished by the TiO2 content: low-titanium (2.5 wt %) (Table 1). Zoning (Concentric and Linear) in the Spatial Distribution of Kimberlite Types Ilupin (1971) was the first to note the spatial zoning of kimberlites in the Daldyn region of Yakutia. Subsequently, a model of the symmetrically zoned structure of this part of the Yakutian province was proposed, and it was shown that the deepest and highest temperature melts are confined to its periphery (Vaganov, 2000). Recently, Kargin et al. (2008) studied in detail the spatial distribution of kimberlites with particular compositional (major and trace elements and Rb–Sr and Sm–Nd isotopic systems) characteristics within the Daldyn– Alakit region. They distinguished four areas with concentric distribution of kimberlites of different (primarily, isotopic) compositions. The central parts of such areas (Fig. 10) comprise kimberlites with high CaO, CO2, Rb, Sr, and Ba, whereas high contents of MgO, TiO2, Cr, Ni, and a number of other indicator elements were observed in the periphery. From the center to the periphery, the εNd and εSr values decrease and the abundance of diamonds in the rocks increases. The zoned spatial distribution of kimberlites was reported for the Slave province in Canada (Dowall et al., 2003), where isotopic heterogeneity was detected in the kimberlites of central and peripheral objects. The investigation of the geochemical and isotopic characteristics of rocks from the Slave craton showed that kimberlites with low negative εNd values, which were probably derived from an enriched mantle domain, are located at the periphery or outside of the craton. Zoning, both concentric and linear, in the distribution of kimberlites of different compositions was described in the EEP (mega-concentric zoning) by Kononova et al. (2007) and in the Zimnii Bereg field of the EEP by Sablukov et al. (2008). In particular, megaconcentric zoning (though not very distinct) was detected in the EEP: the mantle sources of kimberlites from the northern and southern peripheries of the craton (Kepino, Kandalaksha, and Umba fields in the north and eastern Azov and Kirovograd regions in the south) are weakly depleted relative to CHUR (εNd up to +3 to +4) and are similar to the group I kimberlites of southern Africa. On the other hand, there is an area shifted toward the center of the craton where kimberlites were derived from mantle reservoirs with the isotopic char-

acteristics of the enriched mantle (EMI): εNd up to –6 and low Sr isotopic ratios (Zolotitsa kimberlites). Similar are kimberlites with transitional Nd isotopic characteristics plotting in the εNd−εSr diagram near BSE (Kononova et al., 2007; Nosova et al., 2008). These kimberlites usually show lower εNd compared with the group I kimberlites of southern Africa (kimberlites from the V. Grib pipe, Terskii Bereg, and the Kaavi– Kuopio fields in Finland). Sablukov et al. (2008) described a zonal distribution of εNd values and Ta and some other elements in the Zimnii Bereg field of the EEP. Kononova et al. (2007) reported evidence for the linear changes in kimberlite composition within the Zimnii Bereg field of the EEP. In an approximately N–S direction, in the sequence Zolotitsa–Chernoozerskoe (previously Verkhotina)–Kepino fields, the content of TiO2 and the La/Yb ratio increase, the diamond content decreases (Table 2), and the initial Nd isotopic ratio increases considerably (εNd increases from –6 to +3). There is, probably, a positive correlation between εNd and the degree of rock enrichment in trace elements (in particular, light REE and HFSE). A similar linear (regional) zoning was described by Yu.Yu. Golubeva (Kononova et al., 2005) in the YDP, where low-titanium kimberlites are changed from south to north by medium-titanium and high-titanium rocks (Table 2). Exceptions are the Starorechenskoe and Orto–Yarginskoe fields in the extreme north, where low-titanium kimberlite again becomes dominant. The low-titanium kimberlites of the YDP from the Nyurbinskaya (εNd up to –3) and Botuobiya (εNd up to +2) pipes of the Nakyn filed were derived from a slightly enriched (EMI) or a slightly depleted source. In contrast, the source of the other kimberlite fields of the SC is located in the depleted mantle (DM) similar to the group I kimberlites of southern Africa. Zoning in the spatial distribution of kimberlites both within a province (linear and regional) and within a pipe cluster or region (concentric) is primarily related to the characteristics of the mantle source, including the type of mantle metasomatism (caused by melts or fluids). Nd Model Age (Time of Primary Melt Generation) of the SC and EEP Kimberlites and Time of Their Emplacement The comprehensive investigation of kimberlite magmatism in the EEP and SC established the ages (epochs) of its occurrence (the main data for the occurrences discussed here are given in Table 1). The EEP kimberlites were formed in four stages from the Proterozoic to the Devonian: Late Paleoproterozoic, Mesoproterozoic, Neoproterozoic (Vendian), and Devonian. The Mesoproterozoic stage of magmatism is characterized by inclusions in diamonds from Ural placers (data published in the abstract volume of the 9th InternaPETROLOGY

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N 4.1/12.7 4.2/41.5 4.0/36.1 3.6/33.7

3.2/27.4 3.9/1.3 2.8/22.3 3.7/32.3

4.2/39.7

a

b

1 2

5.6/36.0

3

3.2/11.7 3.2/11.7

4 5

2.9/24.8 0

10

20 km

Fig. 10. Zoned spatial distribution of kimberlites in the Daldyn–Alakit region (Kargin, 2009). (1) Kimberlite bodies of (a) clusters 1–4 and (b) clusters 5 and 6; (2) area of the occurrence of kimberlite bodies of clusters 1–4 (central part of the distinguished areas); (3) area of the occurrence of kimberlite bodies of clusters 5 and 6 (periphery of the distinguished areas); (4) εNd(T) in the numerator and εSr(T) in the denominator; and (5) diamondiferous kimberlites containing more than 0.5 ct/t of diamonds.

tional Kimberlite Conference by Laiginhas et al., 2008). The products of Precambrian magmatism were preserved only locally, mainly at the periphery of the craton. The most extensive occurrences of kimberlite magmatism correspond to the Devonian stage. The searches of many years for younger kimberlite magmatism in the Russian platform were unsuccessful. According to various authors (Agashev et al., 2004; Altukhova and Zaitsev, 2006; Lepekhina et al., 2008; etc.), the Yakutian province was formed in at least five stages, and the oldest epoch (Late Proterozoic) is known only in the southwestern SC. The occurrences of kimberlite magmatism of the YDP change from the southwest to the northeast in the sequence Middle Paleozoic, Early Mesozoic (Triassic), Late Jurassic, Cretaceous, and Paleogene (Altukhova and Zaitsev, 2006). The time of the formation of particular fields lengthens significantly. For instance, the Late Paleozoic, Middle– Late Triassic, Jurassic, Cretaceous, and, locally, Paleogene epochs of kimberlite magmatism were docuPETROLOGY

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mented in the Kuonapka zone of the eastern slope of the Anabar shield (Table 1), and kimberlites of several epochs may be found within particular fields. The minor occurrences of Proterozoic kimberlite magmatism are separated by a vast time interval from the later extensive kimberlite magmatism that occurred over the whole Yakutian province from the Devonian to the Paleogene. The formation of the mantle source of EEP kimberlite magmas (Nd model age of kimberlites corresponding to the time of metasomatic mantle enrichment) occurred within the following age ranges: 2.9, 2.1, 1.3– 1.1, and 0.8 Ga. Within the Zimnii Bereg region, the oldest Nd model ages are characteristic of the kimberlites of the Zolotitsa and Chernoozerskoe fields (1.0– 1.3 Ga), and the formation of the source of the kimberlites of the Kepino field was accomplished much later (0.8 Ga). It is evident that commercial diamond concentrations are formed/retained in kimberlites related to ancient sources.

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The model ages of the Yakutian kimberlites, TNd(DM), form several groups within 1.2–0.5 Ga. The oldest model ages (Middle–Late Riphean, 1.2–0.9 Ga) were obtained for the source of kimberlites from the pipes of the Nakyn field. The ages of the formation of kimberlite sources were estimated as the Late Riphean (0.7–0.8 Ga) for the Chomurdakh and Ogoner–Yuryakh fields, 0.7–0.5 Ga for the Mirnyi field, the Daldyn–Alakit region, and the Mesozoic Luchakan field, and 0.5 Ga for the Mesozoic Kuoika field (Kononova et al., 2007). Thus, both the EEP and SC show distinct trends in the time of emplacement and the metasomatic enrichment of mantle sources (generation of kimberlite melts). The emplacement kimberlites terminated in the Devonian in the EEP and in the Paleogene in the SC. Precambrian occurrences are limited to the Proterozoic dikes in the extreme southwest of the SC; the kimberlitic magmatism of Yakutia started only in the Devonian and terminated in the Paleogene. Perhaps, these spatiotemporal trends in the distribution of kimberlite magmatism were controlled by the trajectories of continental plates (Baltica and Siberia) relative to longlived stationary plumes. CONCLUSIONS The comparative petrochemical and geochemical investigation of EEP and SC kimberlites showed that the nature and the general trend of the formation of polygenetic kimberlite magmatism were in general identical in the two cratons. On the other hand, the general tendencies are often complicated both within particular provinces and within fields or even cluster of pipes. (1) It was shown that undifferentiated kimberlites approaching primary melts in composition can be found in the EEP and SC provinces of Russia. Since there are no glassy kimberlites, aphanitic kimberlites, whose composition approaches that of kimberlite melts, can be used for this purpose. Several samples from hundreds analyzed by us approach the primitive (primary) kimberlite composition, which is indicated by the contents of indicator elements and oxides (Mg, Cr, Ni, and CO2). (2) The geochemical composition of kimberlites within a province varied from one field to another, and any field was characterized by the dominant petrological and geochemical type of kimberlites accompanied by minor amounts of other rock type(s). There are three such types of rocks differing in titanium content: low-, medium-, and high-titanium. The two kimberlite provinces of Russia considered here comprise the three petrological and geochemical types of kimberlites. The conclusions on regional and/or global relationships are based on the dominant kimberlite type occurring in the given area.

(3) In both cratons, kimberlite magmatism occurred over considerable time intervals (up to 2 Gyr) in several cycles separated by time gaps of up to 1 Gyr. The kimberlite magmatism of the EEP started in the Paleoproterozoic and terminated in the Devonian. In the SC, the kimberlite formation finished by the Paleogene and started from small Paleoproterozoic dikes, although the major volume of YDP kimberlites was emplaced in the Devonian. (4) Geochemical zoning (concentric and linear) is a characteristic feature of the spatial distribution of kimberlites of different compositions within fields/regions as well as in a whole province. The spatial distribution of kimberlites is correlated with their diamond content and depends primarily on the chemical characteristics of the mantle source rocks. (5) The compositions of kimberlites only rarely correspond to primary mantle melts owing to alteration by various physicochemical processes. The composition of melts is mainly influenced by the character of the metasomatic agent (melt or fluid) modifying the mantle source. The processes of mantle metasomatism are mainly responsible for the diversity of the parental melts of low-titanium (formed at the active participation of fluid metasomatism), medium-titanium, and high-titanium (formed under the active influence of melt metasomatism) kimberlites. (6) Some practical recommendations can be proposed to supplement the previously known geological and mineralogical criteria: —using new data on the kimberlites of the northern EEP and YDP, it is recommended to account for the spatial distribution of pipes and their concentric and linear zoning, and —geochemical (low contents of Ti, HREE, etc.) and isotopic criteria are helpful in estimating the prospects of diamond content in kimberlite bodies: (a) it is recommended to use the low TiO2 content of kimberlites as an indicator favorable for the discovery of high diamond contents; (b) among the kimberlites of a single province, the highest diamond content is characteristic of varieties whose sources approach BSE (εNd(i) and εSr(i) are close to 0) or the enriched EMI mantle component; within the EEP, such objects are the Zolotitsa kimberlites and the V. Grib pipe, and in the YDP, such kimberlites are those of the Nakyn field, Mir pipe, and diamondiferous pipes of the Daldyn–Alakit region; (c) noteworthy is the character of rare earth element patterns, namely the low content of the heavy rare earth elements (HREE); in particular, the kimberlites of the V. Grib pipe show the lowest HREE contents among the samples from the northern EEP, and this pipe displays the highest diamond content in the Archangelsk province; the dominance of low HREE contents is typical of a number of kimberlites from the central diamondiferous fields of the YDP, the Nakyn kimberlites of the YDP, and the Jericho pipe. PETROLOGY

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(7) The polygenetic character of kimberlites can be explained within the model of plume formation accompanied by the formation of a number of diapirs with compositional variations between different areas affected by melt or fluid metasomatism. This controls spatial variations in kimberlite composition reflecting deep changes: the content of TiO2 and some other elements increases with decreasing model age of the formation of the primary kimberlite melt. Thus, variations in the composition and diamond content of kimberlites are correlated with variations in the composition of the mantle and kimberlite sources. These variations are manifested even within relatively small areas, for instance, in the Zimnii Bereg filed of the EEP. The prospects of the diamond potential of particular objects must be estimated using a series of indicators. Currently, in addition to the previously known tectonic and mineralogical criteria, the most informative are petrological, petrochemical, and geochemical indicators.

10. 11.

12.

13.

14.

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24. A. V. Kargin, Yu. Yu. Golubeva, and V. A. Kononova, “Regularities of Spatial Dstribution of Compositional Characteristics of the Kimberlites of the Daldy–Alakit District (Yakutia),” in Proceedings of Conference on 40th Anniversary of YaNIGP TsNIGRI AK “ALROSA” on Problems of Prediction and Exploration of the Diamond Deposits at the Closed Territories, Yakutsk, Russia, 2008 (Izd-Vo YaNTs SO RAN, Yakutsk, 2008), pp. 68–75 [in Russian]. 25. A. D. Khar’kiv, “Signs of Similarity and Difference between Kimberlite Rocks of the Northern Siberian Platform and Other Regions,” Geol. Geofiz., No. 7, 91–99 (1992). 26. A. D. Khar’kiv, N. N. Zinchuk, and A. I. Kryuchkov, Geological–Compositional Models of the Diamondiferous Magmatic Rocks. Problems of Prediction, Exploration, and Study of Mineral Resources on the Turn of 21th Century (Voronezh. Gos. Univ., Voronezh, 2003) [in Russian]. 27. A. D. Khar’kiv, N. N. Zinchuk, and A. I. Kryuchkov, Bedrock Diamond Deposits of the World (OAO Izdatel’stvo “Nedra”, Moscow, 1998) [in Russian]. 28. L. N. Kogarko, L. N. Lazutkina, and L. D. Krigman, Conditions of Zircon Accumulation in the Magmatic Processes (Nauka, Moscow, 1988) [in Russian]. 29. V. A. Kononova, L. K. Levskii, V. A. Pervov, et al., “Pb– Sr–Nd Isotopic Systematics of Mantle Sources of Potassic Ultramafic and Mafic Rocks in the North of the East European Platform,” Petrologiya 10 (5), 493–509 (2002) [Petrology 10, 433–447 (2002)]. 30. V. A. Kononova, V. A. Pervov, and I. P. Ilupin, “Geochemical and Mineralogical Correlation of Kimberlites from Timan and Zimnii Bereg,” Dokl. Akad. Nauk 372 (3), 364–368 (2000) [Dokl. Earth Sci. 372, 724–727 (2000)]. 31. V. A. Kononova, Yu. Yu. Golubeva, O. A. Bogatikov, et al., “Geochemical Diversity of Yakutian Kimberlites: Origin and Diamond Potential (ICP-MS Data and Sr, Nd, and Pb Isotopy),” Petrologiya 13 (3), 227–252 (2005) [Petrology 13, 205–228 (2005)]. 32. V. A. Kononova, Yu. Yu. Golubeva, O. A. Bogatikov, and A. V. Kargin, “Diamond Resource Potential of Kimberlites from the Zimny Bereg Field, Arkhangel’sk Oblast,” Geol. Rudn. Mestorozhd. 49 (6), 483–505 (2007) [Geol. Ore Dep. 49, 421–441 (2007)] 33. S. I. Kostrovitskii, T. Morikiyo, N. V. Vladykin, et al., “Sr and Nd Isotope Systematics of Kimberlites and Related Rocks in the North Yakutian Province,” Dokl. Akad. Nauk 369 (3), 371–374 (1999) [Dokl. Earth Sci. 369, 1281–1284 (1999)]. 34. F. Laiginhas, D. G. Pearson, D. Phillips, et al., “Re–Os and Ar–Ar Isotope Measurements of Inclusions in Diamonds from the Ural Mountains: Constraint on Diamond Genesis and Eruption Ages,” in Proceedings of the 9th International Kimberlite Conference, Frankfurt, Germany, 2008 (Frankfurt, 2008), No. 91KC-A-00147, pp. 1–3. 35. A. V. Lapin, A. V. Tolstov, and D. V. Lisitsin, Kimberlites and Convergent Rocks. Formational Petrogeochemical Criteria (IMGRE, Moscow, 2004) [in Russian]. 36. E. N. Lepekhina, A. Ya. Rotman, A. V. Antonov, et al., SHRIMP U–Pb Dating of Perovskite from Kimberlites

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POLYGENETIC SOURCES OF KIMBERLITES, MAGMA COMPOSITION 49. S. E. Price, J. K. Russel, and M. G. Kopylova, “Primitive Magma from the Jericho Pipe, NWT, Canada: Constraints on Primary Kimberlite Melt Chemistry,” J. Petrol. 41 (6) 789–808 (2000). 50. F. E. Ringwood, S. E. Kesson, W. Hibberson, and N. Ware, “Origin of Kimberlites and Related Magmas,” Earth Planet. Sci. Lett. 113, 521–538 (1992). 51. F. J. Ryerson and E. B. Watson, “Rutile Saturation in Magmas: Implications for Ti–Nb–Ta Depletion in Island-Arc Basalts,” Earth Planet. Sci. Lett. 86 (2–4), 225–239 (1987). 52. S. M. Sablukov and L. I. Sablukova, “Astenospheric Effect on the Mantle Substrate and Diversity of Kimberlite Rocks in Zimni Bereg (Arkhangel’sk province),” in Proceedings of Extended Abstract of the 9th International Kimberlite Conference, Frankfurt, Germany, 2008 (Frankfurt, 2008), No. 91KC-A-00162. pp. 1–3. 53. C. B. Smith “Pb, Sr, and Nd Isotopic Evidence for Sources of Southern African Cretaceous Kimberlites,” Nature 304, 51–54 (1983). 54. C. B. Smith, J. J. Gurney, E. M. Skinner, et al., “Chemical Character of Southern African Kimberlites: a New Approach Based on Isotopic Constraints,” Trans. Geol. Soc. S. Afr. 88 267–280 (1985). 55. R. S. J. Sparks, R. A. Brooker, R. J. Brown, et al., “The Nature of Kimberlite Melts, Rocks and Magmas,” in Extended Abstracts of 9th International Kimberlite Conference, Frankfurt, Germany, 2008, (Frankfurt, 2008), No. 91KC-A-00260, pp. 1–4.

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