conditions of formations of slightly enriched tholeiites in the northern knipovich ridge

ISSN 0016�7029, Geochemistry International, 2010, Vol. 48, No. 4, pp. 321–337. © Pleiades Publishing, Ltd., 2010.Original Russian Text © N.M. Sushchevskaya, A.A. Peyve, B.V. Belyatsky, 2010, published in Geokhimiya, 2010, Vol. 48, No. 4, pp. 339–356.

321

INTRODUCTION

The Knipovich Ridge is the northernmost part ofthe general spreading system of the Atlantic Ocean,developing at slow spreading rates. It extends for550 km from the Molloy Fracture Zone in the north tothe joining with the Mohns Ridge in the south and, incontrast to the majority of spreading ridges, is notintersected by transform faults over its entire length.The anomalous structure of the bottom of the Norwe�gian–Greenland Basin, which is manifested in theasymmetry of the ridge flanks and the discontinuousand poorly identified character of magnetic anomalies[1, 2], is related to the specific geodynamic features ofits formation. The development of the basin was notuniform: the spreading axes underwent displacements,the main of which occurred in the Neogene, when themodern structure of the Knipovich Ridge near thewestern margin of Spitsbergen Island was formed [2].The eastward displacement of the spreading axis of theKnipovich Ridge and the subsequent stage of oceanic

crust formation in this region were synchronous (atapproximately 20 Ma) with the magmatic activity ofthe Svalbard Archipelago, which produced mainlybasalt flows [3–6]. It was shown that the magmatismof the spreading zone of the ridge belongs to the shal�lowest tholeiite type (Na�TOR) [7, 8]. However, theabsence of compositional data for the flanks of theridge hampered a comparison of magmatism at theearly and recent stages of its development.

A combined geological and geophysical expeditionof the R/V “Akademik Nikolay Strakhov” was per�formed in 2006 (supervised by A.V. Zaionchek) withinthe project of the International Polar Year “LateMesozoic–Cenozoic Tectonomagmatic Evolution ofthe Barents Sea Shelf and Continental Slope as a Keyto Paleogeodynamic Reconstructions in the ArcticOcean.” One of the main purposes of the cruise wasthe investigation of the flank structures of the northernKnipovich Ridge, as a key region whose geologic his�tory is essential for the understanding of the geody�

Conditions of Formations of Slightly Enriched Tholeiitesin the Northern Knipovich Ridge

N. M. Sushchevskayaa, A. A. Peyveb, and B. V. Belyatskyc

a Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, ul. Kosygina 19, Moscow, 119991 Russia

e�mail: [email protected] Geological Institute, Russian Academy of Sciences, Pyzhevsky per. 7, Moscow, 119017 Russia

e�mail: [email protected] All�Russia Research Institute of Geology and Mineral Resources of the World Ocean (VNIIOkeanologiya), Angliisky pr. 1,

St. Petersburg, 190121 Russiae�mail: [email protected] September 10, 2008

Abstract—New petrological and geochemical data were obtained for basalts recovered during cruise 24 of theR/V “Akademik Nikolay Strakhov” in 2006. These results significantly contributed to the understanding ofthe formation of tholeiitic magmatism at the northern end of the Knipovich Ridge of the Polar Atlantic.Dredging was performed for the first time both in the rift valley and on the flanks of the ridge. It showed thatthe conditions of magmatism have not changed since at least 10 Ma. The basalts correspond to slightlyenriched tholeiites, whose primary melts were derived at the shallowest levels and were enriched in Na anddepleted in Fe (Na�TOR type). The most enriched basalts are typical of the earlier stages of the opening andwere found on the flanks of the ridge in its northernmost part. Variations in the ratios of Sr, Nd, and Pb iso�topes and lithophile elements allowed us to conclude that the primary melts generated beneath the spreadingzone of the Knipovich Ridge were modified by the addition of the enriched component that was present bothin the Neogene and Quaternary basalts of Spitsbergen Island. Compared with the primitive mantle, theextruding magmas were characterized by positive Nb and Zr anomalies and a negative Th anomaly. The for�mation of primary melts involved melting of the metasomatized depleted mantle reservoir that appeared dur�ing the early stages of opening of the Norwegian–Greenland Basin and transformation of the paleo�Spitsber�gen Fault into the Knipovich spreading ridge, which was accompanied by magmatism in western Spitsbergenduring its separation from the northern part of Greenland.

DOI: 10.1134/S0016702910040014

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GEOCHEMISTRY INTERNATIONAL Vol. 48 No. 4 2010

SUSHCHEVSKAYA et al.

namic evolution of the Arctic segment of the NorthAtlantic and the western part of the Barents Sea conti�nental margin. In addition, the sampling of the rift val�ley and its slopes in the northern part of the KnipovichRidge allowed us to refine the boundaries of the mod�

ern structures of the rift zone, determine its geologicstructure, and estimate the extent and character of vol�canic activity within the segments distinguished on thebasis of morphological and tectonic characteristics(Fig. 1).

Е

Molloy Fracture Zone

Hovgard Ridge

78°30′

N

S2432

S2433

S2438S2420

S2434

EN 25D

S2445

Dr 12

S2430

S2443

2° E 5° 8° E

1

2

3

4

5

6

7

78°

77°30′

Fig. 1. Tectonic map of the northern segment of the Knipovich Ridge and location of dredging stations during cruise 24 of theR/V “Akademik Nikolai Strakhov” and sampling sites of the Knipovich 2000 expedition. The map was compiled by Peyve andChamov [9] using the results of the cruise.(1) Faults (mainly, strike�slip faults), (2) normal and transtensional faults, (3) strike of main positive structures, (4) contours ofuplifts, (5) axis of the rift valley, (6) direction of movement along strike�slip faults, and (7) axis of the paleorift valley.


CONDITIONS OF FORMATIONS OF SLIGHTLY ENRICHED THOLEIITES 323

GEOLOGIC ENVIRONMENT OF THE SAMPLING SITES AND SHORT

DESCRIPTION OF BEDROCKS

During the cruise, the structures of the northern�most end of the rift valley and ridge flanks weredredged. The region studied is characterized by thepresence of a thick sedimentary cover overlying mostof the structures, which strongly hindered dredging. Inaddition to various ice�transported rocks, lithifiedsediments accounted for a considerable fraction of therecovered material, which is a distinctive feature of theregion directly adjoining the continental structures ofSvalbard. The stations where fragments of pillowbasalts were obtained are shown in the tectonicscheme (Fig. 1) [9], which was compiled on the basisof the results of cruise 24. Basalt samples with partlypreserved quench glasses on their surfaces were recov�ered in the rift valley at stations S2420, S2427, S2429,S2430, and S2432 and in the ridge flanks at stationsS2433, S2434, S2438, S2443, and S2445 (Fig. 1). Itshould be noted that glasses from stations S2438 andS2443 are completely palagonitized.

Oblique structures in the rift valley were sampled atstations S2420 and S2432. A number of large blocksand fragments of superficially weathered aphyric and,occasionally, olivine and plagioclase�phyric basaltswere dredged from depths of 3400–2900 m. Dredgingat Station S2430 was carried out in the rift valley alongtwo volcanic mountains, no more than 200 m in size.A single fragment of fresh aphyric basalt was found inthe dredge. The basalt fragment showed a well pre�served quenched glassy zone, up to 0.5 cm thick. Freshglass fragments were also obtained within the rift valleyusing tube corers.

In contrast to the rift valley, the material from sta�tions located on rises in the western (stations S2434,S2438, and S2443) and eastern (Station S2445) flanksof the Knipovich Ridge contained much more abun�dant sedimentary rocks, accounting for 20–80%.Dredging at Station S2434 was performed in the centralpart of rift mountains west of the rift valley axis at72°42′ N. Approximately 100 kg of diverse rock and clayfragments were recovered from depths of 2400–1400 m.The solid rock fragments include large pieces andangular chips along sectorial jointing of stronglyweathered mainly aphyric basalts (approximately25%) with locally preserved fresh areas of quenchglass.

Approximately 50 kg of plastic yellowish gray clay(accounting for approximately 20%) and lithified rockfragments of various shapes, sizes, and compositionswere recovered from the southeastern slope of the cen�tral of three mountains composing a high centered at77°50′ N and 5° E (Station S2438, depth range 2170–1800 m). Most of the fragments (approximately 45%)are rocks of glacial deposition (rounded fragmentswith glacial striations), basalts (15% or 25 kg, two largesamples), and thick ferromanganese crusts (approxi�

mately 20%). Glass on the surface of aphyric basalts isalmost completely palagonitized, whereas the centralparts of the samples are essentially fresh.

The sampling of a high centered at 77°15′ N and 2° E,which is located farthest to the west of the axis of theKnipovich rift valley (Station S2434), recoveredapproximately 100 kg of large rock fragments androunded pebbles. The dredge contained approxi�mately 40% of ferromanganese crusts and 15% ofstrongly altered basalts with completely palagonitizedglass and individual glass fragments.

Approximately 200 kg of blocks and fragments ofbasalts and dolerites (90% of the material) wereobtained from a high centered at 77°20′ N and 9° E onthe eastern flank of the Knipovich Ridge (StationS2445) at depths of 2030–1590 m.

PETROCHEMICAL AND MINERALOGICAL CHARACTERISTICS OF BASALTS

The dredged basalts are mostly aphyric and weakly(olivine) plagioclase�phyric varieties, which are typi�cal volcanic rocks of a rift valley. Rare olivine phenoc�rysts, up to 0.3 mm in size, were observed in all of thesamples. Plagioclase�phyric varieties containing up to15% phenocrysts were recovered at stations S2438 andS2443 on the ridge flanks. Plagioclase occurs as bothmegacrysts, massive and often resorbed crystals up to5 mm in size, and phenocrysts, which are usuallyintergrown with smaller olivine crystals. Clinopyrox�ene is characteristic of the groundmass only. The rela�tionships between the crystallizing phases indicate thecrystallization sequence olivine–plagioclase–cli�nopyroxene, which is typical of oceanic tholeiites.Fresh glass fragments corresponding to the composi�tions of melts erupting on the seafloor were selectedfor this study. Their analyses are shown in Table 1 andin the covariation diagrams of elements versus MgO(Fig. 2). The basalts of the remotest station (S2443)were covered by completely palagonitized glass.

It can be noted that the MgO content of the glassesranges from 6.8 to 7.8 wt %. The compositions ofglasses from the flank and rift areas are almost identi�cal. Glasses from stations S2427, S2429, and S2434show relatively high K2O contents. The most magne�sian glasses were found in the rift valley at StationS2420 (Fig. 1). In general, all of the compositions fallwithin the field of glasses from the Knipovich Ridgereported in previous studies [8, 10].

It can be seen in the Na8–Fe8 diagram (content ofelements in glass recalculated to 8 wt % MgO [11])(Fig. 3) that the compositions of cruise 24 glasses fallwithin the field of previously analyzed glasses from77° N and are different from the compositions ofglasses from more southerly areas of the Knipovich,Mohns, and Kolbeinsey ridges in lower Fe8 (5.5–7.0)and higher Na8 values (2.7–3.0). These observationssupport the presence in this ridge segment of tholeiites(Na�TOR) whose primary melts were enriched in Na

324


SUSHCHEVSKAYA et al. T

able

1.

Com

posi

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gla

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and

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, wt %

Sam

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SiO

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MgO

CaO

Na 2

OK

2OC

r 2O

3T

otal

K2O

/TiO

2N

WD

epth

, m

S24

20/3

077°47

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07°41

.65′

3473

51.5

01.

3316

.83

8.33

0.15

7.63

11.2

72.

990.

350.

0610

0.44

0.26

S24

20/3

577°47

.39′

07°41

.65′

3473

51.9

51.

1916

.97

7.76

0.15

7.67

11.7

42.

960.

270.

0910

0.75

0.23

S24

20/3

677°47

.39′

07°41

.65′

3473

51.9

61.

2016

.90

7.77

0.16

7.73

11.7

12.

890.

270.

0910

0.66

0.22

S24

2777°50

.29′

07°40

.35′

3473

51.4

51.

5616

.84

7.99

0.15

7.37

10.7

13.

140.

670.

0899

.96

0.43

S24

2977°52

.17′

07°42

.6′

3473

51.5

41.

5517

.28

7.49

0.13

7.07

11.0

03.

070.

800.

0610

0.00

0.52

S24

30/1

77°54

.05′

07°28

.12′

3300

51.8

71.

3816

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7.88

0.18

7.32

11.1

73.

070.

380.

0610

0.25

0.28

S24

32/1

77°24

.31′

07°27

.75′

3354

51.7

51.

4116

.48

8.53

0.18

7.55

10.9

03.

040.

350.

1110

0.33

0.25

S24

32/2

77°24

.31′

07°27

.75′

3354

51.9

31.

4316

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8.59

0.18

7.39

10.9

23.

090.

370.

0710

0.51

0.26

S24

32/3

77°24

.31′

07°27

.75′

3354

51.7

31.

4316

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8.54

0.16

7.45

10.7

93.

030.

380.

0610

0.02

0.26

S24

32/4

77°24

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07°27

.75′

3354

51.8

21.

4016

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8.59

0.17

7.51

10.9

02.

870.

380.

1110

0.30

0.27

S24

32/5

77°24

.31′

07°27

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3354

51.8

21.

4616

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8.68

0.16

7.37

10.7

73.

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360.

0910

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0.25

S24

32�a

77°24

.31′

07°27

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3354

51.9

61.

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8.65

0.15

7.49

10.8

53.

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350.

0910

0.61

0.24

S24

32�b

77°24

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07°27

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3354

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8.59

0.16

7.48

10.8

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070.

340.

0810

0.39

0.24

S24

32/6

77°24

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3354

52.0

21.

4316

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8.52

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7.27

11.0

33.

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360.

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S24

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77°42

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71.

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53.

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6.96

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91.

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7.61

10.8

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and Si and depleted in Fe [8]. The distribution of morethan 150 glass analyses [8, 10] in the latitude–Fe8 andlatitude– K2O/TiO2 diagrams (Fig. 4) also reflectsminor differences between the melts generated in the76.5° and 77.5° N ridge segments. There is a tendency

of the occurrence of relatively enriched tholeiitesmainly in the northern segment of the KnipovichRidge: their K2O/TiO2 is up to 0.3 in contrast to theregion of 76° N, where this ratio averages 0.25. Inaddition, lower Fe contents in primary melts are, on

54.0

53.5

53.0

52.5

52.0

51.5

51.0

50.5

50.08.58.07.57.06.56.05.5

SiO2

MgO

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.28.58.07.57.06.56.05.5

K2O

MgO

18.0

17.5

17.0

16.5

16.0

15.5

15.0

14.5

14.08.58.07.57.06.56.05.5

Al2O3

MgO

11

10

9

8

7

68.58.07.57.06.56.05.5

FeO

MgO

3.5

3.3

3.1

2.9

2.7

2.58.58.07.57.06.56.05.5

Na2O

MgO

1 2 3 4 5 6

Fig. 2. Comparison of variations of major elements (wt %) in glasses from various stations in the Knipovich Ridge.Glasses were recovered at flank stations (1) S2434 and (2) S2445 and in the rift valley at stations (4) S2420–S2430 and (5) S2432(Table 1). Also shown are the compositions of glasses from (3) the Knipovich Ridge [8] and (6) from the Mohns and KnipovichRidge [10].

326



average, typical of the northern segment. Composi�tions with low Fe content were previously reportedfrom the northernmost region [8], but such composi�tions were not found in the glass collection of cruise 24of the R/V “Akademik Nikolay Strakhov”.

Of special significance for the understanding of thegenesis of basaltic magmas are geochemical variationsin the compositions of olivine, which is the main liq�uidus phase of the crystallizing primary magmas. Theanalysis of more than 240 olivine phenocrysts sepa�rated mainly from the rift basalts (stations S2420 andS2427) and basalts of the western flank (StationS2438) showed that the crystallizing olivine rangesfrom Fo 85 to Fo 89.5 (Fig. 5) with a stable maximumat Fo 88.5, which comprises more than 70% of thephenocrysts. This reflects the early stage of primarymagma crystallization. The most magnesian olivinesare characteristic of basalts from Station S2420 in therift valley, which indicates the low degree of magmadifferentiation and eruption of melts approaching theprimary magmas.

GEOCHEMICAL CHARACTERISTICSOF MAGMAS

The analysis of element variations (Fig. 6, Table 2)in glasses from the northern end of the KnipovichRidge showed that the behavior of lithophile elementsis in general controlled by crystal fractionation, andthey are systematically accumulated in the melts dur�ing this process (MgO–Zr diagram). The lack of sig�nificant difference in lithophile element ratiosbetween the melts of the rift valley and its flank seg�ments suggests similar sources of the respective mag�

12

11

7

6

5

43.22.72.21.71.2

Fe8

Na8

10

9

8

76° Ν Knipovich

77° Ν Knipovich

Kolbensey

P, FMohns

Fig. 3. Variations of Na8 and Fe8 values [11] in glasses from various ridges of the North and Polar Atlantic according to [7, 8, 10,12, 13]. The arrow indicates changes in Na and Fe contents related to a decrease in the depth and degree of melting of the oceanicmantle.

7.0

8.0

9.0

4.078.077.577.076.576.075.5

6.0

5.0

Fe8

N

0.35

0.55

0.45

0.25

0.1578.077.577.076.576.075.5

K2O/TiO2

N 1 2 3

Fig. 4. Spatial distribution of Fe8 and K2O/TiO2 values inglasses from the Knipovich Ridge. There is an increase inthe degree of melt enrichment (increase in K2O/TiO2) inthe northernmost part of the ridge.Symbols show data from various sources: (1) this study, (2)[10], and (3) [8].



mas. All the new data are consistent with the variationtrends established previously for the melt composi�tions of the Knipovich Ridge [8]. One distinctive fea�ture of the tholeiites of the Knipovich Ridge is the ele�vated H2O contents in the erupted melts [8]. Figure 7compares the H2O contents and H2O/Ce ratios in theglasses studied here and typical depleted glasses fromthe 26° N (TAG) region [15]. It can be clearly seen thatthe glasses of the Knipovich Ridge are richer in H2O bya factor of 4–5 and have H2O/Ce ratios higher by afactor of 2–3. The H2O content is up to 0.5 wt % in theleast evolved varieties and increases up to 1 wt % inevolved melts (Fig. 7). There is a weak correlationbetween MgO and H2O contents, which indicates alow degree of melt degassing and its rapid eruptiononto the bottom.

The primitive mantle�normalized [16] lithophileelement patterns exhibit typical tholeiitic tendencieswith a slightly enriched character (Fig. 8). The(La/Sm)n ratio is close to 1. The diagram presents theresults obtained by two analytical techniques, ionmicroprobe analysis for glasses and laser ablation massspectrometry for basalts, mainly from the flank areas,where glasses were not found. There is good consis�tency for almost all elements, except for U, whosecontent in the basalts is higher than in the glasses. Thiscould be related to methodical errors; however, all ofthe basalts are rather altered, which could also beresponsible for the elevated U contents. On the otherhand, there is no U anomaly in two glass samples fromstations S2420 and S2430, and, in contrast, it ispresent in a relatively fresh basalt from StationS2434/1, which may indicate the compositional heter�ogeneity of the initial melts. The main distinctive fea�tures of the lithophile element distribution patterns ofrocks from the Knipovich Ridge are positive Nb,minor positive Zr, and negative Th anomalies. It isnoteworthy that the distribution patterns of basaltsample S2443/1 from Station S2443 exhibit a consid�erable enrichment in U and Sr.

Figure 9 shows the Nb/Th, Zr/Nb, Nb/Y, andZr/Y variations in the glasses of the Knipovich Ridgecompared with the enriched melts of the KolbeinseyRidge (closest to and affected by Iceland) and theNeogene basalts of Spitsbergen [8. 10]. The methodproposed by Condie [18] for the discrimination ofdepleted and enriched components on the basis of theelement ratios permitted to constrain the genesis ofmany volcanic provinces on earth. The tholeiites ofthe Knipovich Ridge show Nb/Th of 11–24, Zr/Nb of4–15, Nb/Y of 0.2–0.9, and Zr/Y of 0.5–0.8. Withrespect to these ratios, they fall within the field of oce�anic plateau basalts, similar to the rocks of the Ker�guelen Plateau and differ from typical depleted tholei�ites in lower Zr/Nb and higher Nb/Y values. It shouldbe noted that the tholeiites of the Knipovich Ridge aredifferent from the basalts of the Kolbeinsey Ridge,

which is probably affected by the Iceland plume andcharacterized by the presence of enriched tholeiites.The presented compositions of Neogene plateaubasalts of Spitsbergen Island have an enriched conti�nental source and are distinguished by higher Nb/Y,Zr/Nb, and Zr/Y values compared with the aforemen�tioned sources: an enriched mantle component (EM)and the upper continental crust (UC). The conven�tional character of the diagram of Fig. 9 does not allowus to unambiguously interpret the source of theenrichment of the Knipovich Ridge tholeiites. Itshould be emphasized that they are distinguished byenriched signatures from typical depleted TOR, andthe source of their enrichment is similar to the compo�nent contributing to the Neogene basalts of Spitsber�gen.

Figure 10 shows isotopic variations in the tholeiitesstudied in comparison with the isotopic characteristicsof the Quaternary and Neogene basalts of Spitsbergen[8, 10, 19, 20]. There is a clear trend between depletedmelts and an enriched component, which is similar tothe component contributing to the generation of theNeogene basalts of Spitsbergen. It was previouslynoted that the magmas of the Knipovich Ridge havevarying isotopic characteristics, primarily, with respectto the degree of enrichment in radiogenic Pb [8]. Newdata support these results (Table 3). In terms of the87Sr/86Sr ratio, two compositions fall into a moreenriched field, and glass S2430/1 plots in a less radio�genic field. The most depleted glasses show somewhatelevated 87Sr/86Sr (0.7029–0.7030) and 206Pb/204Pb val�ues (17.8–18.0) relative to the depleted source of theAtlantic, which has 87Sr/86Sr of 0.7022 and 206Pb/204Pbof 17.5. Especially enriched appeared to be sampleS2443/3 plotting on the continuation of the maintrend. However, its altered characteristics (presence ofU and Sr anomalies) do not allow us to correlate it

80

70

60

50

40

30

20

10

08988878483 85 86

Fre

quen

cy

Fo

Fig. 5. Histogram for the compositions of olivine frombasalts recovered during cruise 24 of the R/V “AkademikNikolay Strakhov”.

328



with the enriched component, the admixture of whichis evident in the enriched tholeiites of the KnipovichRidge. A more realistic component is similar in com�position to the Neogene flood basalts of Spitsbergen.Its elevated 87Sr/86Sr (0.7049), 207Pb/204Pb (15.55),206Pb/204Pb (18.60), and 207Pb/204Pb (38.60) and low143Nd/144Nd (0.5128) values reflect the crustal contam�ination of magmas during their eruption on Spitsber�gen.

DISCUSSION

The complex tectonic evolution of the Norwe�gian–Greenland Basin resulted in a substantial rear�rangement of plate motion at 33–25 Ma [21] and theseparation of Greenland and Spitsbergen at approxi�mately 20 Ma. This time period was accompanied byNeogene magmatism, which was confined to one ofthe faults of Spitsbergen (Andree Land), at 25–20 Ma

Th2.0

2.5

1.0

1000.5

250

014060 10080 120

Zr

Ba300

200

150

50

014060 10080 120

Zr

3.0

4.5 Er

5.0

4.0

3.5

2.5

2.014060 10080 120

Zr100

250

Sr300

200

150

14060 10080 120

Zr

100

130

Zr140

120

110

90

808.06.5 7.0 7.5

MgO0.4

H2O1.4

0.6

14060 10080 120

Zr

0.8

1.0

1.2

1 2 3

Fig. 6. Variations in the contents of lithophile trace elements (ppm) and Н2О (wt %) in the glasses of the Knipovich Ridge accord�ing to the data of Table 2.The compositions of glasses: (1) from the rift valley of the Knipovich Ridge, (2) from the flank structures, and (3) data of theR/V “Logachev” of 2000 [8].



Tab

le 2

. C

onte

nts

of li

thop

hilic

trac

e el

emen

ts (

ppm

) in

the

glas

ses

of th

e K

nipo

vich

Rid

ge a

ccor

ding

to th

e re

sult

s of

ion

mic

ropr

obe

anal

ysis

(an

alys

t S. S

imak

in)

Ele

men

t30

/30

20/3

520

/36

2729

30/1

32a

32/1

32/3

34/1

34/1

134

/12

34/1

5a45

/2

Ba

106.

3381

.25

78.8

819

8.46

244.

3713

6.99

114.

5111

5.06

113.

0918

8.43

180.

7618

0.92

169.

1111

6.26

Th

0.70

0.59

0.57

1.48

1.79

1.03

0.77

0.81

0.80

1.07

1.06

1.21

1.03

0.62

U0.

220.

170.

170.

420.

550.

310.

250.

310.

250.

390.

310.

330.

300.

13

Nb

11.6

39.

109.

4721

.43

25.9

613

.31

12.6

212

.79

12.6

717

.70

17.8

017

.41

17.7

211

.11

La

7.69

6.67

6.65

13.8

815

.41

9.66

8.87

8.82

9.02

11.4

611

.33

11.6

911

.26

7.66

Ce

19.3

516

.90

16.8

930

.96

33.5

723

.50

22.2

721

.57

22.2

828

.40

25.3

626

.06

25.7

519

.75

Sr

194.

7818

2.60

183.

3624

0.02

276.

7720

1.22

194.

8218

6.80

192.

3127

8.02

259.

2926

3.15

262.

2019

1.69

Nd

13.0

211

.92

11.3

019

.74

17.9

915

.48

15.6

415

.67

15.4

817

.78

16.9

917

.33

16.2

514

.14

Sm

3.66

3.70

3.50

5.60

4.75

4.14

4.66

4.42

4.50

4.73

4.64

4.69

4.25

4.58

Zr

99.6

995

.93

96.0

313

3.84

130.

2711

4.24

121.

0311

4.53

118.

2612

9.18

120.

3112

5.26

126.

2611

1.59

Hf

2.99

2.92

2.89

4.32

3.51

3.36

3.61

3.54

3.66

3.79

3.48

3.42

3.22

3.41

Eu

2.10

1.85

1.79

3.26

3.28

2.42

2.35

2.31

2.27

2.97

2.85

2.79

2.65

2.25

Ti

8166

7670

7657

1052

810

090

8731

9334

9084

9103

9563

9475

9501

9288

9054

Gd

4.91

4.26

4.25

6.42

4.40

5.31

4.73

5.21

5.54

5.84

5.84

5.27

4.82

n

.d.

Dy

4.81

4.58

4.67

6.12

4.85

5.18

5.52

5.80

5.79

5.63

5.30

5.56

5.22

5.77

Y28

.11

27.2

227

.61

33.9

328

.38

29.5

233

.06

32.2

032

.01

30.1

829

.02

29.8

228

.53

31.7

5

Er

3.70

3.39

3.45

4.47

3.72

3.93

4.13

4.17

4.45

4.19

3.71

3.94

3.87

4.30

Yb

3.92

3.56

3.73

5.03

3.83

4.34

4.74

4.83

4.67

4.67

4.00

4.29

4.11

4.51

F0.

010.

010.

010.

010.

010.

010.

010.

010.

010.

010.

010.

010.

010.

01

B1.

311.

311.

272.

212.

441.

861.

461.

471.

431.

661.

791.

841.

751.

40

Li

3.70

3.72

3.90

4.64

4.39

4.21

4.17

3.98

4.19

3.99

4.07

4.26

4.08

4.27

Be

0.52

0.54

0.50

0.70

0.76

0.59

0.59

0.63

0.60

0.68

0.72

0.68

0.69

0.62

H2O

, wt %

0.67

0.73

0.66

0.84

0.91

0.65

0.71

0.84

0.73

0.65

0.82

0.84

0.82

0.63

Zr/

Y3.

553.

523.

483.

954.

593.

873.

663.

563.

694.

284.

154.

204.

433.

52

Zr/

Nb

8.57

10.5

510

.14

6.25

5.02

8.58

9.59

8.96

9.33

7.30

6.76

7.19

7.13

10.0

4

Nb/

Y0.

410.

330.

340.

630.

910.

450.

380.

400.

400.

590.

610.

580.

520.

35

Nb/

Th

16.7

115

.43

16.6

614

.53

14.5

312

.94

16.4

215

.73

15.9

116

.52

16.8

714

.41

17.2

417

.98

330



and the beginning of the formation of the spreadingzone of the Knipovich Ridge.

Over a long time period, Spitsbergen moved rela�tive to Greenland along a continental fault zone(paleo�Spitsbergen Fault). This was accompanied byalternating compression and extension stages, whicheventually resulted in the formation of the KnipovichRidge and Spitsbergen separation from Greenland(Fig. 11) [21].

Spreading in the Knipovich Ridge began in theEarly Paleogene in its southern part, in the Neogene inthe central part, and only approximately 5 Ma ago inthe north [23, 9]. The paleo�Spitsbergen Faultbetween the Mohns Ridge in the south and the GakkelRidge in the north was transformed into the Knipovichspreading ridge during the northward propagation ofthe spreading zone.

The results obtained during cruise 24 of the R/V“Akademik Nikolay Strakhov” showed that the ridge isprobably amagmatic north of 78° N. According to thedredging data, basalts occur south of 78° N, both in theflanks and in the rift valley of the Knipovich Ridge.

These facts suggest that the development of the north�ern Knipovich Ridge corresponds to the stage of riftingof thinned continental crust with local occurrence ofrift magmatism [9]. An important result of the cruisewas the discovery of modern tectonic activity in thewestern slope of the ridge, where a series of steep east�dipping normal faults was documented. The structuraland morphological characteristics of a system ofdepressions, which is located 40 km west of the rift val�ley axis and is 10–15 km wide, suggest that they couldbe confined to a paleorift zone which joined a north�west�trending paleofault at the southern margin of ahigh centered at 78°20′ N and 5°40′ E. During thattime, the main dextral strike�slip fault passed alongthis paleofault [23] (Fig. 1). Previously, Crane et al.[24] proposed the model of a spreading axis jump fromthe central part of the Greenland Sea toward its east�ern margin and further northward propagation of thespreading zone. The possibility of the spreading axisjump is supported by the fact that mudstones in con�tact with tholeiitic basalts were previously (in 2000)recovered in the northern segment (77°50′ N) from a

H2O

, w

t %

1.4

1.2

1.0

0.8

0.6

0.4

0.2

09.08.58.06.0 7.06.5 7.5

MgO

H2O

1.4

1.2

1.0

0.8

0.6

0.4

0.2

03.53.01.0 2.0 2.5

Na8

H2O

/Ce

0.075

0.065

0.055

0.045

0.035

0.025

0.0151159515 5535 75

Zr

H2O

1.4

1.2

1.0

0.8

0.6

0.4

0.2

011105 98

Fe8

6 7

1 3 4

wt %

2

Fig. 7. Variations in the contents of lithophile trace elements and Н2О in the glasses of the Knipovich Ridge.The glasses of the Knipovich Ridge are enriched in H2O and B compared with the depleted melts from the 26° N segment of theMid�Atlantic Ridge (TAG) in the region of the occurrence of typical depleted tholeiites [14]. The symbols show the compositionsof glasses from the Knipovich Ridge at (1) 76° N and (2) 77° N and from the Mid�Atlantic Ridge region of 26° N according to(3) [15] and (4) this study.



depth of 3000 m. The ages of the mudstones estimatedon the basis of plankton and benthic foraminifera fallwithin two intervals, Middle Jurassic–Late Creta�ceous and Oligocene [25]. The Oligocene timing ofthe supposed spreading axis jump is consistent withavailable seismostratigraphic constraints (anomaly 7,

Late Oligocene) [26]. Since that time, the rift zone ofthe Knipovich Ridge has been formed along the mar�gin of Spitsbergen cutting the earlier oceanic crust.

The eastward movement of the spreading axis of theKnipovich Ridge coincided with the activation ofmagmatism in Svalbard (approximately 20 Ma),

Y YbHf Ti Gd Dy ErEu

100

10

1ZrSmNdSrLaUBa NbTh Ce

Sam

ple/

Pri

mit

ive

man

tle

Glasses

Y YbHf Ti Gd Dy ErEu

1000

100

0,1ZrSmNdSrLaUBa NbTh Ce

Sam

ple/

Pri

mit

ive

man

tle

Rocks

1

10

20/30

30/145/2

20/35

S2430/134/11

20/3632a34/12

2732/334/15a

2932/1

S2434/5S2443/5

S2434/6S2445/3

S2434/7S2445/6

S2438/1S2445/11

S2438/2S2432/1

S2443/134/1

Fig. 8. Distribution of lithophilic elements in the basalts and glasses from the Knipovich Ridge.The compositions are normalized to the contents of elements in the primitive mantle [16]. The compositions of glasses from theKnipovich Ridge are given in Table 2.

332



which usually produced basalt flows (Fig. 1). Accord�ing to the absolute dating of these magmatic com�plexes by Prestvik [3], the process had continued up toan age of 10 Ma. During the Quaternary, approxi�mately 1 Ma ago, this process resulted in the formationof three volcanoes at the northern end of the Breibo�gen Fracture Zone. The spreading activity in the Nor�wegian–Greenland Basin could stimulate magmaticactivity in the Svalbard continental margin. Duringthe opening of the Arctic Ocean, the existing andnewly formed faults in the western part of Spitsbergencould be repeatedly activated and serve as conduits foralkaline melts. These melts were derived from theenriched fluid�saturated continental mantle and werechemically similar to the strongly enriched meltsdetected as veins in mantle nodules from the Quater�nary volcanoes of Spitsbergen [19, 20]. They couldmigrate not only within the continental mantle butalso into the apical parts of the ancient depleted(asthenospheric) oceanic mantle. The possible migra�tion of low�degree melts through poorly dense serpen�tinized rocks was noted by Whitmarsh et al. [27]. Sucha process occurred in pre�Miocene time and resultedin the formation of the suboceanic enriched mantle.

The subsequent jump of the spreading axis in theMiocene from the Norwegian Basin to the westernmargin of Spitsbergen [26, 8] could result in theentrainment and melting of this suboceanic enrichedmantle reservoir and formation of melts with isotopic

signatures similar to those of the Spitsbergen melts.The similarity of lithophile element distribution pat�terns in the Neogene magmas and Quaternary alkalibasalts of Spitsbergen, on the one hand, and the riftbasalts of the Knipovich Ridge, on the other hand,support this suggestion [17]. Figure 12 shows isotopicvariations in the tholeiitic basalts of the rift zones ofthe North and Polar Atlantic (Kolbeinsey, Mohns, andKnipovich ridges) and basalts from continental areasaffected by plume activity. The latter are the basalts ofGreenland related to the initial activity of the TertiaryNorth Atlantic plume, Iceland, Jan Mayen Island, andSpitsbergen. It can be noted that, while the depletedcharacteristics of the tholeiites of these spreadingridges are similar to one another, the enriched compo�nents of the basalts of the Polar and North Atlantic aresomewhat different. For instance, the basalts of thepolar province straddle a trend whose component(PAEK) was more enriched in radiogenic Sr and Pb(87Sr/86Sr = 0.7055 and 207Pb/204Pb = 15.60) than thecomponent of the North Atlantic (NAEK: 87Sr/86Sr =0.704 and 207Pb/204Pb = 15.45). It was shown that theenriched component of the northern provinceinvolved in the generation of Jan Mayen and Green�land basalts was related to the contamination of mag�mas in the continental crust [17]. Lightfoot et al. [31]argued that the enrichment of West Greenland picriteswas caused by crustal contamination during parentmagma generation. The Iceland magmas developing

100

10

13025201050 15

OIB

DEM

DM

NMORB

EN

UC

Ocean plateau basalts

Nb/Th

Zr/Nb10

0.1

0.011000 10

Zr/Y

Nb/Y

1

1 2 3 4 5

Fig. 9. Variations in the Zr/Nb, Nb/Th, Nb/Y, and Zr/Y characteristic ratios in the tholeiites of the Knipovich and Kolbeinseyridges and the Neogene basalts of Spitsbergen Island.The fields correspond to the basalts of ocean islands (OIB), depleted oceanic tholeiites (N�MORB), and oceanic plateau basaltsaccording to [18].The large symbols show the compositions of end�members: source of depleted MORB (DM, star); upper continental crust (EM,circle); lower continental crust (UC, triangle); and deep depleted mantle (DEM, diamond).(1)–(3) Glasses from the Knipovich Ridge according to (1) Table 2, (2) [8], and (3) [10]; (4) Neogene basalts of Spitsbergen [17];and (5) enriched glasses from the Kolbeinsey Ridge [12, 13].



in a mid�ocean ridge environment are less affected bysuch an enriched component. At the same time,another trend can be distinguished for the Tertiarybasalts of West Greenland. It is related to the contribu�tion of an EM�1 type source showing low 206Pb/204Pband elevated 87Sr/86Sr values. This source could proba�bly be related either to the ancient continental crust orancient recycled sediments, but its presence has notbeen manifested during the subsequent developmentof the plume.

Within the polar province, the most enrichedbasalts of the Knipovich Ridge were found in its flanks(Station 43). It is important that the Neogene basalts ofSpitsbergen appeared to be more enriched in radiogenic

isotopes than the Quaternary alkali basalts, although thecharacter of enrichment of both of them indicates a con�tinental upper crustal source [17].

Figure 13 shows the calculated trend of the addi�tion of the enriched component and the enrichedbasalts of the Polar Atlantic in the 87Sr/86Sr–143Nd/144Nd diagram [17]. The basalts of the Arcticregion form a common trend from the oceanicdepleted mantle reservoir (DMM) [32] to an enrichedcomponent with high Sr and Nd isotopic ratios.Assuming that the characteristics of the enriched res�ervoir are identical to those of the Neogene basalts ofSpitsbergen, it can be shown that the admixture of the

15.7

15.6

15.5

15.4

39.2

38.8

38.4

38.0

37.218.818.618.418.218.017.817.6 19.0 19.2

37.6

206Pb/204Pb

206Pb/204Pb

208Pb/204Pb

207Pb/204Pb

206Pb/204Pb

87Sr/86Sr

18.818.618.418.218.017.817.6 19.0 19.2

18.818.618.418.218.017.8 19.0 19.2

0.7055

0.7045

0.7035

0.7025

0.7050

0.7040

0.7030

43/4

30/1

1 2 3 4 5 6

Fig. 10. Variations in 87Sr/86Sr, 208Pb/204Pb, 206Pb/204Pb, and 207Pb/204Pb in the basalts of the Arctic province of the Atlantic.(1) Veins in the Quaternary basalts of Spitsbergen [19], (2) Quaternary basalts of Spitsbergen [20], (3) glasses from the KnipovichRidge [8], (4) Quaternary basalts of Spitsbergen [17], (5) Neogene basalts of Spitsbergen [17], and (6) glasses and basalts of theKolbeinsey Ridge [12, 13].

334



enriched component was in general minor (approxi�mately 0.25%) in the tholeiites of the Knipovich Ridgeand significantly higher (several percent) in the Qua�

ternary basalts of Spitsbergen [17]. The enrichmentof the Neogene basalts could be due to the meltingof an enriched mantle reservoir with a high fraction

Table 3. Results of the isotopic analysis of basalts dredged on the Knipovich Ridge during cruise 24 of the R/V “Akademik Ni�kolay Strakhov”

Sample [Rb], ppm

[Sr], ppm

[Sm], ppm

[Nd], ppm

87Rb/86Sr

87Sr/86Sr 2s, abs

147Sm/144Nd

143Nd/144Nd

2s, abs206Pb/204Pb

207Pb/204Pb

208Pb/204Pb

S2445/3 5.705 184.8 3.883 13.65 0.08929 0.70314 0.00002 0.17192 0.513097 0.000015 17.851 15.463 37.653

S2443/4 26.31 245.8 7.563 29.23 0.30965 0.70540 0.00001 0.15638 0.512806 0.000003 19.004 15.607 38.896

S2438/1 8.136 92.27 2.815 7.650 0.25503 0.70372 0.00002 0.22244 0.512988 0.000006 18.175 15.446 37.918

S2430/1 8.695 184.0 3.585 12.97 0.13667 0.70329 0.00001 0.16710 0.513039 0.000005 18.464 15.589 38.475

Note: The chemical separation of elements was performed by the chromatographic method using ion exchange columns and standard procedures.During the analytical investigations, blanks were no higher than 0.01 ng for Rb, 0.02 ng for Sr, and 0.05 ng for Sm and Nd. The contents of ele�ments were determined by isotope dilution with the addition of a calibrated isotope tracer. The isotopic compositions of elements were measuredusing a TRITON multicollector solid�phase mass spectrometer (Center of Isotopic Investigations of the Karpinskii All�Russia Institute of Geol�ogy) operating in a static mode. The values of 88Sr/86Sr = 8.375209 and 146Nd/144Nd = 0.7219 were used for normalization. The followingvalues were obtained for international standards: 143Nd/144Nd = 0.512106 ± 3 for JNdi�1; 87Sr/86Sr = 0.710222 ± 10 for NBS�987; and206Pb/204Pb = 16.937 ± 0.011, 207Pb/204Pb = 15.492 ± 0.017, and 208Pb/204Pb = 36.722 ± 0.017 for NIST�981.

Spitsbergen Archipelago

Northeast

1

2

34

18

18

Fig. 11. Incipient stage of the separation of the Svalbard Archipelago from Greenland at 36 Ma according to [21].1 is the Gakkel spreading ridge with the framing geochrones; 2 is the Hornsund Fault Zone, which migrated eastward from itsinitial position (3) in the Late Cretaceous; and 4 is the Donskoy basin. 18 is the number of magnetic anomaly corresponding toan age of approximately 40 Ma. The dashed lines show three lineaments of the Spitsbergen terrane, central, western, and easternand the arrows indicate the direction of movements of the lineaments, which resulted in the separation of Spitsbergen fromGreenland. The circle shows the possible extent of the hot spot responsible for the formation of the Morris Jesup Rise and theYermak Plateau. The diamonds denote the area of the Neogene and Quaternary magmatism of Spitsbergen [22].

Greenland



0.7055

0.7050

0.7045

0.7040

0.7035

0.7030

0.70250.51270.51280.51290.51300.51310.51320.5133

PAEK

NAEK

87Sr/86Sr

143Nd/144Nd

15.65

15.50

15.45

15.40

15.35

15.25

15.2019.018.518.017.517.0

207Pb/204Pb

206Pb/204Pb

15.30

15.55

15.60

1 2 3 4 5 6 9 107 8

Fig. 12. Differences of enriched components in the magmas of the Polar and North Atlantic with respect to 87Sr/86Sr and207Pb/204Pb.(1) Isotopic ratios in the Iceland basalts, (2) West Greenland, (3) glasses and basalts of the southern part of the Kolbeinsey Ridge,(4) glasses and basalts of the northern part of the Kolbeinsey Ridge, (5) Jan Mayen platform, (6) glasses of the Mohns Ridge,(7) Knipovich Ridge, (8) data of Table 3 for the Knipovich Ridge, (9) Neogene basalts of Spitsbergen, and (10) Quaternary basaltsof Spitsbergen. The data of [7, 17, 19, 20, 28–31] were used. The arrows show possible mixing trends between the depleted sourceand enriched materials characteristic of the polar (PAEK) and north Atlantic (NAEK).

of continental lower crustal component [32]. It isinteresting that the Sr and Nd isotopic characteristicsof metasomatic minerals from veins in mantle xeno�

liths are also similar and, probably, slightly lower thanthose of the Neogene basalts: 87Sr/86Sr = 0.7045 and143Nd/144Nd = 0.5129.

336



CONCLUSIONS

New data obtained during the petrochemical andgeochemical investigation of basalts recovered fromthe rift valley and flanks of the northern end of theKnipovich Ridge confirmed the development withinthis region of tholeiites whose primary melts wereenriched in Na and depleted in Fe (Na�TOR type) andwere derived at the sallowest mantle levels.

The geochemical character of the magmatism hasremained unchanged for at least 10 Ma. On the otherhand, more enriched basalts are characteristic of theflanks and the northernmost part of the ridge.

Compared with the primitive mantle [16], theerupted magmas show positive Nb and Zr anomaliesand a negative Th anomaly and are similar in thisrespect to the Neogene basalts of Spitsbergen.

Variations in the isotopic characteristics of Sr, Nd,and Pb show that the primary melts generated beneaththe spreading zone of the Knipovich Ridge wereaffected by the enriched component that was involvedin the formation of both the Neogene and Quaternarybasalts of Spitsbergen. This component is similar inisotopic characteristics to the Neogene magmas ofSpitsbergen.

The formation of primary melts beneath the riftzone of the Knipovich Ridge could be accompanied bythe entrainment and melting of the metasomatizeddepleted mantle formed during the early stages of theopening of the Norwegian–Greenland Basin andtransformation of the paleo�Spitsbergen fault into aspreading ridge. The beginning of ocean crust forma�tion was accompanied by magmatism in western Spits�bergen during its separation from northern Greenland.

ACKNOWLEDGMENTS

This study was financially supported by the RussianFoundation for Basic Research, project no. 06�05�64651; the Program for the Support of Leading Scien�tific Schools (NSh�150.2008.5); and Program no. 14of the Department of Earth Sciences of the RussianAcademy of Sciences “Physicochemical Conditionsof the Formation and Geodynamics of the ModernOceanic Crust of the Arctic Basin.”

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conditions of formations of slightly enriched tholeiites in the northern knipovich ridge

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