Schilling Et Al

Journal of Geodynamics 38 (2004) 57–83

Volcanism on the Eggvin Bank (Central Norwegian-Greenland Sea, latitude ∼71◦N): age, source, and relationship to the Iceland and putative Jan Mayen plumes

Dieter F. Mertz a,b,c,∗, Warren D. Sharp c, Karsten M. Haase d

a Johannes Gutenberg-Universität, Institut für Geowissenschaften, 55099 Mainz, Germany b Max Planck-Institut für Chemie, Postfach 3060, 55020 Mainz, Germany c Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709, USA d Institut für Geowissenschaften der Christian-Albrechts-Universität zu Kiel, Olshausenstr. 40, 24118 Kiel, Germany Received 8 December 2003; received in revised form 1 March 2004; accepted 5 March 2004

Abstract

The Eggvin Bank (Central Norwegian-Greenland Sea, latitude ∼71◦N) is a topographically anomalous shallow area with scattered volcanic peaks extending between the island of Jan Mayen and East Greenland and straddling the northern segment of the mid-Atlantic Kolbeinsey Ridge axis. Basalts dredged from the Eggvin Bank range from variably depleted, tholeiitic, near-axis lavas to enriched, transitional-to-alkaline, off-axis seamount lavas. In terms of normalised incompatible element patterns, the most depleted, near-axis tholeiite is similar to neighbouring Kolbeinsey Ridge basalts, whereas the off-axis, transitional-to-alkaline lavas are similar to other alkaline basalts occurring close to the Eggvin Bank region, e.g., those of Jan Mayen. 40Ar/39Ar step heating data indicate that the off-axis seamount lavas are coeval with other alkaline lavas erupted in the Central Norwegian-Greeland Sea at ca. 0.6–0.7 Ma. In contrast, the Eggvin near-axis tholeiites are <0.1 Ma. Volcanic peaks west and north of Jan Mayen show no indication of a systematic age progression. Therefore, the Jan Mayen hot spot hypothesis is not supported by the available radiometric age data. Sr, Nd, and Pb isotope compositions of near-axis and off-axis Eggvin Bank lavas are distinct, implying differences in their mantle sources. Isotope ratios of the off-axis basalts (87Sr/86Sr = 0.70344–0.70352, 143Nd/144Nd = 0.51283–0.51288, 206Pb/204Pb = 18.82–18.85) resemble those of neighbouring alkali basalt occurrences, however, isotope ratios of the near-axis tholeiites correspond to lavas erupting in the south-eastern volcanic zone of Iceland, e.g., at Vestmannaeyjar. The near-axis tholeiites are generated by an unusual source with highly radiogenic Pb (206Pb/204Pb = 18.95) together with relatively radiogenic Nd (143Nd/144Nd = 0.51295) and low-radiogenic Sr (87Sr/86Sr = 0.70314), respectively, representing an unique composition in the mantle north of central Iceland. The overlap in isotope

∗ Corresponding author. Tel.: +49-6131-3922857. E-mail address: [email protected] (D.F. Mertz).

0264-3707/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jog.2004.03.003 58 D.F. Mertz et al. / Journal of Geodynamics 38 (2004) 57–83 compositions between Eggvin Bank near-axis tholeiites and south-east Iceland alkaline lavas could be an indication that the Eggvin Bank tholeiite source was derived from the Iceland plume and that it was emplaced in the upper mantle by the original Iceland plume head during the Early Tertiary as suggested by Trønnes et al. [Trønnes, T., Planke, S., Sundvoll, B., Imsland, P., 1999. Recent volcanic rocks from Jan Mayen: low degree melt fractions of enriched north-east Atlantic mantle. J. Geophys. Res. 104, 7153–7167]. Isotope and trace element data indicate an abrupt change in source composition along the Kolbeinsey Ridge axis at latitude ca. 70.6◦N, apparently reﬂecting a boundary between two chemically distinct mantle domains with limited interaction. Based on Pb versus Pb isotope diagrams, no dispersion of enriched material is observed adjacent to the hypothetical Jan Mayen/Jan Mayen Platform plume, neither to the north-east along the Southern Mohns Ridge nor to the south along the Central Kolbeinsey Ridge. © 2004 Elsevier Ltd. All rights reserved.

1. Introduction

In 1983, Schilling et al. (1999) published a comprehensive major and trace element data set on Mid-Atlantic ridge samples dredged from latitude 29◦Nto73◦N providing an overview on the geochemistry of the North Atlantic basalts from the ridge segments south of the Azores via Iceland up to the north of Jan Mayen. Recently, the high-latitude part of this data set comprising the ridge segments north of Iceland, i.e., essentially Kolbeinsey, Mohns and Knipovich Ridges (Fig. 1) has been completed by isotope and additional trace element (Hanan et al., 2000) measurements. Together with new geochemical and isotope data on Iceland (e.g., Hanan and Schilling, 1997; Chauvel and Hémond, 2000) and Jan Mayen (e.g., Trønnes et al., 1999) volcanism, a consistent, large-scale picture of the composition of the high-latitude North Atlantic mantle has emerged. In contrast, the nature of the mantle sources, their interactions and the causes of magma generation for Eggvin Bank–Jan Mayen intraplate volcanism located between the northern Kolbeinsey Ridge and the southern Mohns Ridge segments (Central Norwegian-Greenland Sea; Fig. 1) remains controversial. Jan Mayen volcanism has been interpreted as recent hotspot activity (e.g., Johnson and Campsie, 1976; Morgan, 1983; Schilling et al., 1983; Vink, 1984) and the topographically anomalous shallow Eggvin Bank west of Jan Mayen (Fig. 1) with discontinuous volcanic peaks is thought to represent the Jan Mayen hot spot track (Morgan, 1981). Mohns Ridge spreading axis lavas occurring north of Jan Mayen—with radiogenic 87Sr/86Sr and Pb isotope ratios, relatively unradiogenic 143Nd/144Nd compositions, and incompatible element enrichment relative to normal-type mid-ocean ridge basalt (N-type MORB)—are regarded as the result of contamination of their asthenospheric source by material from a hot mantle plume underneath Jan Mayen or the Jan Mayen Platform (Schilling et al., 1983, 1999; Neumann and Schilling, 1984). In contrast, seismic, tectonic, petrological, and geochemical data are interpreted to indicate that no anomalously hot mantle underlies the Jan Mayen region (e.g., Imsland, 1980; Saemundsson, 1986; Havskov and Atakan, 1991; Haase et al., 1996). For example, Haase et al. (1996, 2003) concluded that Jan Mayen magmas are generated by melting of volatile-enhanced, passively up- welling mantle inﬂuenced by the adjacent Mohns Ridge spreading axis (Fig. 1). Moreover, they suggested that the shallow bathymetry of the Eggvin Bank is caused by an iron-depleted mantle, which is less dense than its surrounding mantle. Trønnes et al. (1999) proposed that Jan Mayen magma originates from low-degree partial melts of enriched material emplaced in the NE Atlantic mantle by the ancestral Ice- land Plume at about 60 Ma. D.F. Mertz et al. / Journal of Geodynamics 38 (2004) 57–83 59

Fig. 1. Map of the Eggvin Bank–Jan Mayen region (Central Norwegian-Greenland Sea) based on Perry (1986) showing major tectonic and bathymetric features as well as K–Ar (in italics) and 40Ar/39Ar ages (Fitch et al., 1965; Mertz and Renne, 1995; Upton et al., 1995; this work) on volcanic rocks. The inset presents the high-latitude North Atlantic with the Eggvin Bank–Jan Mayen region. Numbers in brackets indicate sampling locations (see Table 1). Depth contours in meter, JMFZ: Jan Mayen Fracture Zone (indicated by hatching), SFZ: Spar Fracture Zone (ca. 69◦N), TFZ: Tjörnes Fracture Zone (ca. 67◦N), JMP: Jan Mayen Platform, JMR: Jan Mayen Ridge, JM: Jan Mayen Island, MR: Mohns Ridge, CKR: Central Kolbeinsey Ridge (ca. 69◦–70.6◦N, also termed Middle Kolbeinsey Ridge in other papers), NKR: North Kolbeinsey Ridge (ca. 70.6◦–72◦N, other authors apply the acronym NKR to the segment 69◦–72◦N), OSC: Overlapping Spreading Center (ca. 70.6◦N). Reykjanes Ridge extends south of Iceland approximately at latitude 64◦–55◦N.

Only limited geochemical and geochronological data are available for Eggvin Bank lavas (Pedersen et al., 1976; Schilling et al., 1983, 1999; Campsie et al., 1990) and their relationship to lavas of Jan Mayen island as well as those of neighbouring Kolbeinsey and Mohns Ridge is not clear. During FS Polarstern expedition ARK VII/1 volcanic rocks from the Eggvin Bank region were dredged. Here we present new 40Ar/39Ar step heating data, geochemical compositions and Sr, Nd, Pb isotope ratios on these rocks in order to establish their ages, characterise their mantle sources, and evaluate source interactions.

2. Geological setting

Fig. 1 is a simpliﬁed map of the Eggvin Bank–Jan Mayen region showing relevant bathymetric and tectonic features. Jan Mayen is located in a topographically anomalous area at the northern end of the Jan Mayen Ridge, which is at least in part a continental fragment (e.g., Grønlie et al., 1979; Myhre et al., 1984). With the northward propagation of the mid-Atlantic Kolbeinsey Ridge about 43 Ma ago, this fragment was split off from Greenland and drifted to its present position (Nunns, 1982). Vesterisseamount is an isolated volcanic ediﬁce located about 350 km north-west of Jan Mayen that was built on Middle Eocene oceanic crust about 3000 m below sea level. To the west of Jan Mayen, the northern segment of the Kolbeinsey Ridge penetrates the Eggvin Bank, a shallow region with seamounts reaching up to few tens of meters below sea level (Fig. 1). East Greenlandic basalts at latitude 72◦–75◦N (e.g., Upton et al., 60 D.F. Mertz et al. / Journal of Geodynamics 38 (2004) 57–83

1984) occur in the western extension of the Eggvin Bank–Jan Mayen shallow region. 40Ar/39Ar dating of these basalts yields eruption ages of 57–58 Ma as well as later volcanic activity at ca. 32 Ma (Upton et al., 1995). East Greenland basalts south of latitude 72◦N yield Paleogene K–Ar total rock ages with the oldest Paleocene ages of ca. 55–60 Ma being interpreted as geologically meaningful (Beckinsale et al., 1970). The mid-Atlantic spreading axis is offset along the Jan Mayen Fracture Zone about 200 km from the Kolbeinsey Ridge to the Mohns Ridge. Torske and Prestvik (1991) suggested that the Jan Mayen Fracture Zone is an old lithospheric fault system that provided pathways for volatiles and melts leading to alkaline magmatism. North of the Mohns Ridge the Knipovich segment forms the Mid-Atlantic spreading axis. The Mohns Ridge directly north of the Jan Mayen Fracture Zone runs into the so-called Jan Mayen Platform (Neumann and Schilling, 1984), an approximately 60 km wide bank opposite Jan Mayen. The North Kolbeinsey Ridge is separated from the Central Kolbeinsey Ridge by an overlapping spreading Center at ca. 70.6◦N. The Spar Fracture zone at latitude ca. 69◦N separates the Central Kolbeinsey Ridge from the South Kolbeinsey segment, which penetrates Iceland south of latitude ca. 67◦N. More detailed tectonic, bathymetric, and geological descriptions of the Eggvin Bank–Jan Mayen region are given by, e.g., Johnson and Campsie (1976), Bungum and Husebye (1977), and Saemundsson (1986). The plate tectonic evolution of the Norwegian-Greenland Sea is presented in Talwani and Eldholm (1977) and Eldholm et al. (1990).

3. Sampling

The samples were recovered by four dredge hauls. Detailed sampling data are given in Table 1, the geographical position of the dredge hauls is presented in Fig. 1. FS Polarstern dredges 21860 and 21861-3 sampled Eggvin Bank near-axis seamounts while dredge 21862 sampled an Eggvin Bank off-axis seamount. In addition to the Eggvin Bank samples, a Mohns Ridge near-axis alkali basalt from the

Table 1 Sampling data of basalts from the Eggvin Bank–Jan Mayen region (Norwegian-Greenland Sea) Dredge# Start End Location Latitude Longitude Latitude Longitude Water depth [m] Cruise 21860 70◦59.27N13◦58.39W70◦59.25N13◦59.01W 404–367 Eggvin Bank Polarstern ARK VII/1 21861-3 70◦56.23N13◦01.05W70◦56.25N13◦00.95W 38–35 Eggvin Bank Polarstern ARK VII/1 21862-B 71◦19.33N11◦08.92W71◦19.51N11◦09.42W 542–464 Eggvin Bank Polarstern ARK VII/1 23295-3 71◦08.90N05◦54.89W71◦09.29N05◦53.74W 1879–1773 Jan Mayen Platform Meteor 7/3 D.F. Mertz et al. / Journal of Geodynamics 38 (2004) 57–83 61

Jan Mayen Platform recovered by dredge 23295-3 of FS Meteor cruise 7/3 has been dated. During the dredge hauls sudden releases of the dredge wire tension indicated tearing off and sampling of in situ rocks rather than material transported by ice drift. Recovered rocks in each case were angular pillow fragments, sometimes with glassy rims. The freshest-looking samples from each dredge were selected for major and trace element as well as for radiogenic isotope analyses. Thin section investigations revealed that the holocrystalline groundmasses of some samples show only minor alteration. Therefore, groundmass separates of these rocks (tholeiites 21861-3-A, 21860-D, alkali basalts 21862-E, 23295-3) were processed for radiometric age dating.

4. Analytical methods

Major elements were analysed by X-ray fluorescence (whole rocks) and electron microprobe (glass and minerals) at the Mineralogical Institute in Kiel following the procedures described by Haase et al. (1996). The trace element concentrations were determined by Inductively coupled plasma mass spec- trometry (ICP-MS) at the Geological Institute in Kiel on representative samples following the methods of Garbe-Schönberg (1993). Results on standards analysed concurrently with the Eggvin Bank samples are given in Haase et al. (1996). 40Ar/39Ar analyses samples, weighting ∼200 mg, were loaded in Cu foil packages and irradiated in the cadmium shielded port (CLICIT) at the Triga reactor at University of Oregon and analysed at Berkeley Geochronology Center. Values of J (a measure of neutron dosage) were determined from the mean of 10 single-crystal laser fusion analyses of co-irradiated Fish Canyon sanidine, using an age of 28.02 Ma (Renne et al., 1998). Corrections for interfering nucleogenic isotopes of Ar were determined from analyses of irradiated CaF2 and a synthetic K-bearing glass (KFeSiO4) using the values given by Renne (1995). The samples were baked out at 250 ◦C for 10 h prior to analysis. The furnace is a double vacuum type with a Ta resistance heater and a Mo crucible. Extraction line operation, including sample heating, is fully automated. Blanks for the extraction line and furnace were measured and blank correction was applied as described by Sharp et al. (1996). Reactive components, such as H2O, CO2, CO, and N2, were removed from the sample gas in two sequential stages using SAES GP-50 Zr–Al getters operated at 400 ◦C. The purified gas was analysed using a Mass Analyser Products 215–50 mass spectrometer, configured for a resolution of 450. Mass discrimination was determined from repeated analyses of air Ar using an on-line pipette, yielding a mean value of 1.006 ± 0.0020 per atomic mass unit (amu) during the course of this study. The decay constants and isotopic ratios used are those given by Steiger and Jäger (1977). Uncertainties for ages are given at the 2σ level and include errors arising from irradiation and Ar analysis but do not include errors in decay constants or isotopic abundances of K and atmospheric Ar. Sr, Nd, and Pb isotope analyses were carried out on hand-selected grains which were leached for 2 h with hot 6 N ultrapure HCl prior to dissolution. Sr and the rare earth element (REE) fractions were separated using conventional cation exchange procedures. Nd of the REE fraction was then separated on a second column containing Teflon powder coated with di-2-ethyl-hexyl orthophosphoric acid. Pb was separated using the technique described by Manhès et al. (1978). The Sr, Nd, and Pb isotope compositions were measured at the Max Planck-Institut Mainz on a Finnigan MAT 261 mass spectrometer with static multi- collection. Sr and Nd isotope ratios are normalised to 86Sr/88Sr = 0.1194 and to 146Nd/144Nd = 0.7219. Pb isotope ratios are relative to values of 206Pb/204Pb = 36.738, 207Pb/204Pb = 17.159 and 208Pb/204Pb = 36.744 for NBS standard 982 and are corrected of 0.13% per amu. Total blank contributions were <0.4‰ 62 D.F. Mertz et al. / Journal of Geodynamics 38 (2004) 57–83 for Pb, <0.2‰ for Nd and <0.008‰ for Sr, and are considered not to be significant. NBS standard SrCO3 987 and the La Jolla Nd standard have been measured, yielding 87Sr/86Sr = 0.710241 ± 0.000011 (2σ) and 143Nd/144Nd = 0.511842 ± 0.000009 (2σ), respectively. The sample isotope ratios given are not normalised to the recommended standard values of 87Sr/86Sr = 0.710248 and 143Nd/144Nd = 0.511850.

5. Results

5.1. Petrography and mineral composition

The samples of dredge hauls 21860 and 21861-3 from Eggvin Bank near-axis seamounts show a large variation in petrography ranging from aphyric samples to lavas containing less than 10% phenocrysts. Phenocrysts are generally plagioclase and olivine with rare clinopyroxene, e.g., in sample 21860-D. In contrast, the Eggvin Bank off-axis lavas of dredge haul 21862 contain more than 30% phenocrysts of olivine, plagioclase and abundant clinopyroxene up to 4 mm in diameter. Olivine is relatively forsterite-rich with Fo82 to Fo89. In hand specimen the large clinopyroxenes of some samples are dark green and these rocks are similar in petrography to Jan Mayen ankaramites (e.g., Imsland, 1980). The clinopyroxene is a zoned Ti–augite which in general shows higher MgO contents at the rim (ca. 15.6 wt.%) compared to the core (ca. 12.7 wt.%, sample 21826-B). The cores of some augites are resorbed and rounded xenocrysts occur.

5.2. Major and trace element geochemistry

Major and trace element analyses are presented in Table 2. Based on the TAS classiﬁcation (Fig. 2)of Le Maitre et al. (1989) Eggvin Bank near-axis lavas (dredges 21860 and 21861-3) are tholeiitic basalts comparable to mid-ocean ridge basalts erupting on the neovolcanic zone of the neighbouring Kolbeinsey Ridge (e.g., Schilling et al., 1983; Endres, 1992; Haase et al., 2003). In contrast, the Eggvin Bank off-axis lavas (dredge 21862) represent nepheline-normative, mildly alkaline to transitional basalts—termed alkali basalts herein for brevity—that are more typical for intraplate or off-axis seamounts (e.g., Cousens, 1996; Niu et al., 1999). The major and trace element composition of Jan Mayen Platform alkali basalt 23295-3

Fig. 2. Total alkalis versus silica (TAS) classiﬁcation for Eggvin Bank volcanic rocks. All analysed samples plot in the basalt ﬁeld (grey). Stippled line from MacDonald and Katsura (1964) distinguishes between alkaline and tholeiite lava series based on volcanic rocks from Hawaii. Open triangles: Eggvin Bank off-axis mildly alkaline to transitional basalts, open circles: Eggvin Bank near-axis tholeiites. D.F. Mertz et al. / Journal of Geodynamics 38 (2004) 57–83 63

Table 2 Chemical compositions of recovered glass (21860-B) and basalts (other samples) from the Eggvin Bank Sample 2186-3-A 21861-3-B 21860-A 21860-B 21860-D 21860-F 21862- B 21862-C 21862-E 21862-F

SiO2 49.29 49.93 49.80 47.31 50.81 49.99 47.02 47.54 47.41 47.06 TiO2 1.25 1.24 0.92 0.65 1.48 0.98 1.84 2.80 1.92 1.41 Al2O3 15.09 15.33 13.91 16.02 14.47 14.82 13.41 17.26 10.13 14.55 T Fe2O3 11.05 11.06 12.85 10.06 13.11 11.87 10.02 11.75 9.95 9.99 MnO 0.17 0.17 0.21 0.12 0.20 0.18 0.16 0.19 0.16 0.13 MgO 7.35 7.32 8.32 9.83 5.81 7.98 12.65 4.75 13.78 8.11 CaO 11.77 11.76 12.70 13.12 10.61 12.02 12.35 10.96 13.08 15.46 Na2O 2.21 2.09 1.82 1.85 2.37 2.16 1.74 2.57 1.46 1.50 K2O 0.62 0.61 0.09 0.05 0.54 0.10 1.11 1.28 1.33 0.50 P2O5 0.24 0.25 0.19 n.d. 0.27 0.18 0.36 0.61 0.35 1.21 Total 99.04 99.76 100.81 99.01 99.67 100.28 100.66 99.71 99.57 99.92 Mga 60.8 60.7 60.1 69.5 50.8 61.0 74.6 48.5 76.4 65.4 Sc 40.1 42.2 41.4 39.1 51.9 37.4 29.7 42.9 Cr 42.3 77.7 362 11.3 92.3 731 56.8 847 Co 45.2 52.5 45.6 51.1 48.8 52.3 36.0 53.4 Ni 60.8 58.1 212 25.7 51.7 232 44.0 246 Cu 122 92.7 106 87.0 99.2 57.0 95.0 93.3 Zn 75.6 61.4 56.8 91.1 84.7 65.5 86.9 66.9 Y 22.2 19.8 12.8 23.1 24.2 14.7 24.8 15.7 Rb 14.4 1.82 1.15 11.2 0.82 25.3 23.7 33.3 Cs 0.04 0.03 0.04 0.28 0.01 0.27 0.21 0.34 Sr 172 63.9 92.6 180 64.0 479 538 407 Ba 166 14.2 21.9 199 40.3 399 486 461 Zr 82.9 36.2 27.7 81.2 38.3 125 201 138 Hf 2.14 1.26 0.72 2.04 1.18 3.26 5.11 3.25 Nb 20.1 6.18 4.69 35.9 5.10 47.8 64.4 45.0 Ta n.d. n.d. n.d. n.d. n.d. 4.44 5.49 4.14 Pb 0.92 0.37 0.35 1.07 0.37 1.48 2.31 1.61 Th 1.31 0.25 0.11 1.36 0.13 3.36 4.37 2.98 U 0.38 0.10 0.05 0.47 0.13 0.86 0.87 0.82 La 12.4 1.72 2.54 13.90 2.04 28.4 38.9 27.8 Ce 25.0 4.65 5.80 28.70 5.24 55.0 85.6 56.6 Pr 3.17 0.77 0.80 3.55 0.84 6.40 10.1 6.90 Nd 12.8 4.51 3.81 14.20 4.50 24.5 40.0 26.1 Sm 2.99 1.80 1.22 3.30 1.79 4.57 7.79 5.03 Eu 1.02 0.71 0.53 1.15 0.73 1.41 2.35 1.53 Gd 3.38 2.70 1.75 3.71 2.68 4.15 6.67 4.46 Tb 0.56 0.54 0.32 0.63 0.52 0.63 1.00 0.66 Dy 3.65 3.85 2.23 4.04 3.68 3.16 5.29 3.43 Ho 0.77 0.85 0.49 0.86 0.80 0.62 1.01 0.65 Er 2.28 2.57 1.43 2.55 2.42 1.67 2.75 1.72 Tm 0.33 0.37 0.22 0.37 0.35 0.21 0.35 0.22 Yb 2.22 2.57 1.40 2.45 2.41 1.38 2.31 1.41 Lu 0.33 0.38 0.21 0.35 0.35 0.20 0.33 0.20 (La/Sm)N 2.68 0.62 1.34 2.72 0.74 4.01 3.23 3.56 a = × 2+ 2++ 2+ = T × Mg 100 Mg /(Mg Fe ) (atomic) and assuming FeO Fe2O3 0.85. La/Sm normalised using C1 chondrite of Sun and McDonough (1989); n.d.: not determined. 64 D.F. Mertz et al. / Journal of Geodynamics 38 (2004) 57–83

Fig. 3. Chondrite-normalised La/Sm ((La/Sm)N, using C1 chondrite from Sun and McDonough, 1989) vs. MgO for lavas from the region north of Iceland indicating mantle heterogeneities. Data from Sun et al. (1979), Schilling et al. (1983), Neumann and Schilling (1984), Maaløe et al. (1986), Devey et al. (1994), Endres (1992), Haase et al. (1996, 2003) and Trønnes et al. (1999), this work. has been published previously (Haase et al., 1996). Based on their chondrite-normalised La/Sm ratios [(La/Sm)N] varying from about 0.6–2.7 (Fig. 3), the Eggvin Bank near-axis tholeiites range from depleted [=(La/Sm)N < 1; N-type lava] to enriched [=(La/Sm)N > 1; E-type lava] similar to basalts from the Mohns Ridge. The Eggvin Bank off-axis alkali basalts (Pedersen et al., 1976; this work) resemble Jan Mayen (Maaløe et al., 1986; Trønnes et al., 1999) and Jan Mayen Platform (e.g., Neumann and Schilling, 1984) lavas in terms of incompatible element enrichment with (La/Sm)N of3to4(Fig. 3). Also, Eggvin Bank alkali basalts range in MgO from 4.7 to 13.8 wt.%, indicating that shallow level fractionation processes are comparable in importance to Jan Mayen magmas.

5.3. 40Ar/39Ar data

Three samples were analysed using the 40Ar/39Ar incremental heating technique. The Ar results are presented in Table 3, the corresponding age spectra are shown in Fig. 4. Samples 21861-3-A and 21862-E yielded plateau ages of 96 ± 34 and 697 ± 30 ka, respectively. A duplicate analysis of 21861-3-A yielded an age of 99 ± 28 ka, in good agreement with the earlier determination. The plateau steps for samples 21861-3-A and 21862-E yielded linear arrays on isochron diagrams (not shown) with mean square of weighted deviations (MSWD) of 1.0 and 0.7, 40Ar/36Ar initial ratios of 293.3 ± 4.2 and 294.3 ± 7.8, and ages of 113 ± 32 and 700 ± 48 ka, respectively. 40Ar/36Ar initial ratios are within 1σ error of the atmospheric 40Ar/36Ar ratio of 295.5, indicating that no excess 40Ar was detected. Considering analytical errors, isochron and plateau ages of each sample are indistinguishable. Sample 23295-3 yielded a discordant spectrum. The six “plateau” steps in Fig. 4c, however, contain >60% of the total 39Ar and their ages scatter only slightly more than allowed by the plateau criteria of Fleck et al. (1977). The mean age of these steps, weighted by their uncertainties, is 680 ± 83 ka. In an isochron diagram, these steps scatter more than expected from analytical error (MSWD = 8.2). Nevertheless, the data deﬁne a trapped 40Ar/36Ar ratio of 285 ± 15, indicating that there is no excess Ar related to this sample. The value of 680 ± 83 ka is considered the best available estimate for the age of this lava. Conventional K–Ar dating D.F. Mertz et al. / Journal of Geodynamics 38 (2004) 57–83 65

Table 3 Step heating 40Ar/39Ar data (J = 0.0001355) of basalts from the Eggvin Bank–Jan Mayen region Temperature [◦C] 40Ar[Mol] 40Ar/39Ar 38Ar/39Ar 37Ar/39Ar 36Ar/39Ar 40Ar∗/39Ar 40Ar∗[%] Age [Ma] ±2σ

21861-3-A 600 2.17E-16 205.540 2.0189 3.138 1.1518 −134.877 −65.5 −33.276 478.598 700 7.90E-14 46.512 0.0403 5.362 0.1583 0.156 0.3 0.038 0.110 750 3.67E-14 13.681 0.0180 3.994 0.0458 0.471 3.4 0.115 0.041 775 2.75E-14 10.061 0.0159 4.472 0.0343 0.270 2.7 0.066 0.035 800 1.65E-14 6.549 0.0142 4.411 0.0220 0.399 6.1 0.097 0.030 835 2.00E-14 5.888 0.0139 4.198 0.0194 0.478 8.1 0.117 0.024 865 1.26E-14 5.657 0.0150 3.884 0.0188 0.416 7.3 0.102 0.032 900 8.62E-15 7.160 0.0142 3.675 0.0226 0.770 10.7 0.188 0.056 1000 2.05E-14 8.865 0.0168 1.997 0.0300 0.161 1.8 0.039 0.035 1100 4.39E-14 34.458 0.0361 28.262 0.1247 −0.149 −0.4 −0.037 0.108 1200 2.76E-14 33.762 0.0310 65.701 0.1298 0.693 1.9 0.169 0.141 1400 2.40E-14 68.186 0.0445 87.895 0.2549 −0.123 −0.2 −0.030 0.382 21861-3-A (duplicate) 600 3.38E-13 5120.777 3.2699 4.802 17.2513 23.502 0.5 5.736 58.471 700 2.12E-12 351.335 0.2340 6.272 1.1829 2.287 0.6 0.559 0.680 750 1.62E-15 11.790 0.0183 7.867 0.0546 −3.748 −31.6 −0.917 1.304 775 3.79E-15 11.378 0.0199 7.110 0.0465 −1.800 −15.7 −0.440 0.583 800 5.25E-14 16.482 0.0213 5.887 0.0557 0.483 2.9 0.118 0.074 835 2.51E-14 4.686 0.0150 5.115 0.0158 0.440 9.4 0.108 0.040 865 4.16E-14 4.768 0.0143 4.377 0.0160 0.396 8.3 0.097 0.026 900 3.19E-14 3.619 0.0138 3.651 0.0117 0.451 12.4 0.110 0.025 1000 3.83E-14 3.573 0.0143 2.466 0.0114 0.410 11.4 0.100 0.020 1100 3.14E-14 5.411 0.0162 2.197 0.0179 0.288 5.3 0.070 0.038 1200 1.12E-13 46.884 0.0488 92.772 0.1848 −0.340 −0.7 −0.083 0.172 1400 1.14E-13 280.536 0.1979 119.446 0.9902 −2.776 −0.9 −0.679 1.419 23295-3 600 5.22E-16 413.481 0.7720 0.000 0.9374 136.473 33 33.056 101.709 650 5.95E-15 105.620 0.1387 0.663 0.3443 3.921 3.7 0.958 1.328 700 1.16E-13 32.457 0.0376 0.888 0.0981 3.539 10.9 0.865 0.067 730 1.13E-13 15.900 0.0254 1.362 0.0422 3.517 22.1 0.860 0.031 760 7.02E-14 13.237 0.0074 2.319 0.0315 4.112 31 1.005 0.030 800 1.12E-13 13.163 0.0224 3.673 0.0356 2.940 22.3 0.718 0.027 840 8.25E-14 11.700 0.0226 5.796 0.0328 2.467 21 0.603 0.025 900 5.99E-14 12.537 0.0243 7.045 0.0344 2.930 23.2 0.716 0.031 950 3.86E-14 16.728 0.0273 5.890 0.0472 3.252 19.3 0.795 0.048 1000 4.02E-14 18.752 0.0324 5.125 0.0569 2.327 12.4 0.569 0.056 1100 1.09E-13 39.262 0.0477 24.037 0.1279 3.371 8.4 0.824 0.087 1200 6.07E-14 55.167 0.0446 142.908 0.2078 5.368 8.5 1.312 0.222 1300 1.49E-14 153.945 0.0816 148.434 0.5273 10.940 6.2 2.672 1.382 21862-E 600 9.98E-16 24.435 −0.0078 1.269 0.0963 −3.915 −16 −0.957 3.804 700 1.83E-13 9.106 0.0189 1.484 0.0215 2.867 31.4 0.701 0.015 750 1.25E-13 14.311 0.0199 1.245 0.0392 2.842 19.8 0.695 0.028 775 7.79E-14 14.044 0.0194 1.563 0.0379 2.961 21.1 0.724 0.034 800 6.38E-14 15.602 0.0197 1.889 0.0433 2.948 18.9 0.721 0.044 66 D.F. Mertz et al. / Journal of Geodynamics 38 (2004) 57–83

Table 3 (Continued ) Temperature [◦C] 40Ar[Mol] 40Ar/39Ar 38Ar/39Ar 37Ar/39Ar 36Ar/39Ar 40Ar∗/39Ar 40Ar∗[%] Age [Ma] ±2σ 835 4.00E-14 15.098 0.0204 1.904 0.0428 2.613 17.3 0.639 0.059 865 2.40E-14 13.006 0.0185 1.751 0.0361 2.483 19.1 0.607 0.079 900 2.58E-14 14.426 0.0204 1.843 0.0415 2.312 16 0.565 0.083 1000 7.81E-14 24.651 0.0278 2.747 0.0771 2.101 8.5 0.513 0.065 1100 1.35E-13 28.688 0.0318 21.844 0.0942 2.650 9.1 0.648 0.064 1200 2.51E-14 40.369 0.0348 35.088 0.1398 1.901 4.6 0.464 0.267 1400 2.26E-14 90.008 0.0558 65.226 0.3118 3.241 3.4 0.792 0.723

(a)

(b)

(c)

Fig. 4. 40Ar/39Ar age spectra showing incremental heating results on (a) Eggvin Bank near-axis tholeiite 21861-3-A, (b) Eggvin Bank off-axis alkali basalt 21862-E and (c) Jan Mayen Platform near-axis alkali basalt 23295-3. Errors are 2σ˜. on sample 21860-D yielded an age of 0.08 ± 0.04 Ma (H.J. Lippolt, personal communication). The ages of 96 and 80 ka of samples 21861-3-A and 21860-D, respectively, represent Upper Pleistocene, the ages of 697 and 680 ka of samples 21862-E and 23295-3, respectively, represent Lower Middle Pleistocene.

5.4. Sr, Nd, Pb isotope composition

Sr, Nd, Pb isotope ratios from Eggvin Bank basalts are presented in Table 4. Additional Sr, Nd, and Pb isotope ratios from Eggvin Bank near-axis and off-axis basalts are published by Mertz and Haase (1997) and Schilling et al. (1999) and are compiled in Fig. 5 together with our new data. In all isotope diagrams Eggvin Bank off-axis alkali basalts and near-axis tholeiites deﬁne different data ﬁelds indicating distinct mantle sources for both rock groups. In the 87Sr/86Sr versus 143Nd/144Nd diagram (Fig. 5a) Eggvin Bank alkali basalts plot close to Jan Mayen volcanic rocks whereas Eggvin Bank tholeiites form a different data D.F. Mertz et al. / Journal of Geodynamics 38 (2004) 57–83 67

Table 4 Isotope compositions of Eggvin Bank basalts Sample 21860-D 21862-B 21862-C 87Sr/86Sr 0.703140 0.703442 0.703521 ±2σ 0.000011 0.000013 0.000014 143Nd/144Nd 0.512948 0.512875 0.512828 ±2σ 0.000009 0.000009 0.000011 206Pb/204Pb 18.948 18.821 18.854 ±2σ 0.015 0.017 0.013 207Pb/204Pb 15.541 15.525 15.521 ±2σ 0.013 0.016 0.018 208Pb/204Pb 38.723 38.661 38.694 ±2σ 0.036 0.044 0.048

ﬁeld with less radiogenic Sr and higher radiogenic Nd compared to the alkali basalt group, and with a minor overlap with Jan Mayen Platform lavas. Eggvin Bank tholeiites resemble Vestmannaeyjar alkaline and Hekla transitional lavas from the south-eastern volcanic zone of Iceland (Furman et al., 1991). There are also similarities to Icelandic lavas from Snaefell (Hards et al., 1995) and Torfajökull (Stecher et al., 1999)in143Nd/144Nd, however, the Icelandic rocks are more radiogenic in Sr. In the 206Pb/204Pb versus 208Pb/204Pb (Fig. 5b) and 206Pb/204Pb versus 207Pb/204Pb (Fig. 5c) diagrams both Eggvin Bank groups either overlap with or plot close to the highest-radiogenic Jan Mayen Platform lavas. As is the case for Sr and Nd isotopes, the Eggvin Bank tholeiites Pb isotope variation is also similar to the Icelandic Vestmannaeyjar and Hekla Pb.

Fig. 5. (a) 87Sr/86Sr vs. 143Nd/144Nd, (b) 206Pb/204Pb vs. 208Pb/204Pb, (c) 206Pb/204Pb vs. 207Pb/204Pb for Eggvin Bank volcanic rocks (alkaline/transitional basalts = filled triangles; tholeiites = filled circles) compared to lavas from Iceland (Snaefell = lavas closest to the suggested Iceland Plume center; Torfajökull = lavas representing high-radiogenic Pb of Iceland array; data fields of Hekla redrawn from Furman et al. (1995)) as well as from north of Iceland. Data from Ito et al. (1987), Furman et al. (1991), Mertz et al. (1991), Hards et al. (1995), Mertz and Haase (1997), Stecher et al. (1999), Schilling et al. (1999) and Trønnes et al. (1999)—open rhombuses of Jan Mayen field; DePaolo et al. (unpublished)—open squares of Jan Mayen field; this work. Eggvin Bank tholeiite TR139 27D-5g (Schilling et al., 1999) is not considered for the Eggvin Bank tholeiite field in (c) because of the substantial deviation in 207Pb/204Pb from the remaining Eggvin Bank tholeiites. 68 D.F. Mertz et al. / Journal of Geodynamics 38 (2004) 57–83

Fig. 5. (Continued ).

6. Geological discussion

6.1. Previous K–Ar and 40Ar/39Ar ages

In Fig. 1 K–Ar and 40Ar/39Ar ages on volcanic rocks from the region between Jan Mayen and east Greenland are shown. For the stratigraphically oldest Jan Mayen subaerial volcanic rock series, K–Ar total rock dating on olivine basalts yields an age of about 0.3–0.5 Ma (Fitch et al., 1965). The youngest volcanic eruptions on Jan Mayen island are of recent date (e.g., Saemundsson, 1986; Imsland, 1980). Ash layers in cores recovered from the Iceland Plateau show that volcanic activity at Jan Mayen may extend back to 3.3 Ma (Lacasse et al., 1996). For the isolated Vesteris seamount, laser step heating 40Ar/39Ar measurements on groundmass and mineral separates indicate two phases of alkaline volcanism at 30–60 ka and 0.5–0.7 Ma, respectively (Mertz and Renne, 1995). Thus, Jan Mayen and Vesteris volcanic activities overlap within the age range <0.7 Ma. In contrast to our 40Ar/39Ar data for Eggvin Bank–Jan Mayen Platform lavas, conventional K–Ar measurements on samples dredged close to the Jan Mayen Fracture Zone and in the northern extension of the Kolbeinsey Ridge (Campsie et al., 1990) do not show Quaternary ages. Clinopyroxenite yields ages of 304 and 529 Ma, which were interpreted as an indication that a large area of the northern Iceland Plateau is underlain by basement of Caledonian age. However, because the argon budget of pyroxene is commonly dominated by excess 40Ar (e.g., McDougall and Green, 1964; Schwartzman and Giletti, 1977) the geological signiﬁcance of these dates are questionable. Furthermore, K–Ar total rock ages on dolerite from the same dredge haul vary from about 7–19 Ma. This variation was interpreted to result D.F. Mertz et al. / Journal of Geodynamics 38 (2004) 57–83 69 from variable Ar loss in rocks of Oligocene or Miocene age (Campsie et al., 1990) caused by the intrusion of alkali-rich veins, which yield ages of 6.5 and 7.5 Ma, respectively. These ages may be ﬂawed because of several factors that cannot be resolved with the existing K–Ar data including the possible presence of excess 40Ar, K and/or Ar loss associated with alteration, and polymagmatic origins of some samples. Moreover, spurious apparent ages for Norwegian-Greenland Sea volcanic rocks resulting from K and Ar mobility or excess 40Ar have been noted previously (Mertz and Renne, 1995). Therefore, K–Ar ages indicating Tertiary intraplate magmatic activity in the Eggvin Bank–Jan Mayen region should be treated with caution.

6.2. Hypothetic hotspot track

Johnson and Campsie (1976) described a 300 km-wide seamount belt extending parallel to the Jan Mayen fracture zone and comprising the Eggvin Bank–Jan Mayen–Jan Mayen Platform region. This belt is considered to represent the track of the postulated Jan Mayen hotspot (Morgan, 1981). If so, an age progression in intraplate volcanic activity to the west of Jan Mayen would be expected. Although the East Greenlandic (Fig. 1) Lower Tertiary volcanism (e.g., Upton et al., 1995) would match this model, with the exception of the questionable Late Miocene conventional K–Ar ages on feldspar-rich veins in dolerites (compare 6.1.), measured ages within this seamount belt are exclusively <0.7 Ma.40Ar/39Ar plateau ages (reported herein) of 697 ± 30 and 680 ± 83 ka for Eggvin Bank sample 21862-E and Jan Mayen Platform sample 23295-3, respectively, together with a kaersutite plateau age of 640 ± 70 ka from a Vesteris seamount tephrite (Mertz and Renne, 1995) indicate broadly contemporaneous intraplate alkaline volcanic activity from 640 to 700 ka (Lower Middle Pleistocene) distributed within a radius of about 350 km north and west of Jan Mayen (Fig. 1). The plateau age of 96 ± 34 ka of Eggvin Bank tholeiite 21861-3-A, the conventional K–Ar age of 0.8 ± 0.4 Ma of Eggvin Bank tholeiite 21860-D (both reported herein), Vesteris seamount volcanic activity at 30–60 ka (Mertz and Renne, 1995), and Jan Mayen Holocene activity indicate a younger phase of off-axis volcanism in the age range <0.1 Ma (Upper Pleistocene). Intraplate volcanism forming seamounts can occur episodically over periods of 106–107 years (e.g., Pringle et al., 1991) and volcanoes from island chains are known to erupt lavas long after being transported over the location of the active hotspot (e.g., Clague and Dalrymple, 1988). Such rejuvenated-stage eruptions are generally alkalic. This type of volcanism, for example occurring in the Hawaiian island chain, is apparently related to rapid changes between uplift and subsidence because of the rapid motion of the Paciﬁc plate (e.g., Jackson and Wright, 1970; Clague and Dalrymple, 1987). Since the plates in the Norwegian-Greenland Sea move about 10 times slower (e.g., Vogt, 1986), the fast plate model probably does not apply to the high-latitude North Atlantic region. In this case, the alkaline 680 and 697 ka lavas of the Eggvin Bank–Jan Mayen region are difﬁcult to interpret as Hawaiian-type rejuvenated-stage alkaline volcanism of a hot spot track. Dredging of volcanic rocks between Eggvin Bank and East Greenland in order to test the hot spot track hypothesis for the western part of this region is not possible because of thick sedimentary cover. The volcanic activity in the Eggvin Bank–Jan Mayen region, however, shows no indication of a systematic age progression for a ca. 400 km distance (Fig. 1). Therefore, the hot spot track model is not supported by our geochronological data. It rather appears that Quaternary intraplate volcanism produced lavas during discrete time intervals at ca. 0.7 Ma and <0.1 Ma at various sites in the high-latitude Atlantic north of 70◦N. 70 D.F. Mertz et al. / Journal of Geodynamics 38 (2004) 57–83

6.3. Source of Eggvin Bank and neighbouring segments

6.3.1. Primitive mantle-normalised composition Ratios of highly incompatible elements can be useful for monitoring source compositions because the extent of fractionation from each other by partial melting and differentiation processes is relatively small. Primitive mantle-normalised incompatible elements of Eggvin Bank off-axis lavas produce subparallel patterns with prominent negative Pb and positive Nb anomalies, respectively, and comprising concentra- tion ranges, e.g., 44–62 times primitive mantle for the highly incompatible Rb or 12–20 times primitive mantle for the less incompatible Sm. The incompatible element patterns of Eggvin Bank off-axis, Jan Mayen Platform as well as Jan Mayen island alkaline lavas are similiar and there is a resemblance to HIMU (high ␮ (= 238U/204Pb))-type lavas (Fig. 6a). Although enriched mantle (EM)-type mantle sources occur within the high-latitude Atlantic mantle (e.g., Hanan and Schilling, 1997), they are not dominant in the central Norwegian-Greenland Sea region. Eggvin Bank off-axis, Jan Mayen Platform and Jan Mayen alkaline lavas do not show the EM-typical enrichment in the highly incompatible elements Rb and Ba.

(a)

(b)

Fig. 6. Primitive-mantle normalised (Hofmann, 1988) incompatible element plot for (a) Eggvin Bank off-axis (samples 21862-B, -C, -E; grey pattern; this work) alkali basalts in comparison to Jan Mayen Platform near-axis (sample 23295-3; Haase et al., 1996), Jan Mayen (sample 167; Trønnes et al., 1999) and St. Helena HIMU (Sun and McDonough, 1989) lavas as well as for (b) Eggvin Bank near-axis tholeiites (sample 21861-3-A with high La/Sm of 4.15 and sample 21860-A with low La/Sm of 0.96; Table 2) in comparison to N-type MORB (Hofmann, 1988) and Central Kolbeinsey MORB (grey pattern; Endres, 1992). D.F. Mertz et al. / Journal of Geodynamics 38 (2004) 57–83 71

Trønnes et al. (1999) suggested that there is a HIMU component in the North Atlantic upper mantle, which could represent recycled oceanic crust entrained in the ancestral Iceland plume and which has been distributed laterally by the ancestral plume head. Thirlwall (1995) has interpreted the Icelandic Pb isotope variation in terms of mixing between a single immature HIMU composition or a range of immature HIMU compositions, respectively, with an unradiogenic plume source represented by NE Icelandic picrites from Theistareykir. The Eggvin Bank off-axis incompatible element patterns conﬁrm that HIMU-inﬂuenced mantle domains are a common feature of the high-latitude North Atlantic mantle. Fig. 6b shows the incompatible element patterns of the Eggvin Bank near-axis tholeiites with the lowest (sample 21860-A) and highest La/Sm ratios (sample 21861-3-A) of our data set, compared to Central Kolbeinsey as well as to N-type MORB. The highly incompatible element concentrations Rb and Ba of the Eggvin Bank tholeiites are similar (21860-A) to or enriched (21861-3-A) by a factor of 27 relative to N-type MORB. In terms of the medium and heavy rare earth elements (Sm to Lu) and Y the Eggvin Bank tholeiites are depleted compared to N-type MORB and sample 21860-A matches average Central Kolbeinsey MORB. This suggests that the near-axis lavas probably were generated on the North Kolbeinsey spreading axis and represent old oceanic crust rather than recently formed intraplate volcanism. Consequently, the enriched material presently observed beneath the North Kolbeinsey spreading axis (Haase et al., 2003) must have been present for at least 100 ky beneath this part of the mid-Atlantic Ridge. The Eggvin Bank tholeiites show positive Nb as well as negative Pb anomalies as it is the case with Eggvin Bank alkaline lavas indicating similarities between off-axis and near-axis lavas.

6.3.2. Radiogenic isotope variation The diagrams 206Pb/204Pb versus 207Pb/204Pb (Fig. 5c) and 207Pb/204Pb versus 208Pb/204Pb (Fig. 7) demonstrate different Pb isotope ratios for the Eggvin Bank and Jan Mayen lavas indicating different mantle sources for both sites. The Eggvin Bank Pb is more radiogenic compared to Jan Mayen. In

Fig. 7. 207Pb/204Pb vs. 208Pb/204Pb for Eggvin Bank, Jan Mayen and Jan Mayen Platform volcanic rocks compared to MORB from the neighbouring spreading axes to the northeast (Southern Mohns Ridge, 71.5◦–72.5◦N) and to the south (Central Kolbeinsey Ridge, 69.0◦–71.5◦N). Symbols as in Fig. 5. Data from Ito et al. (1987), Mertz and Haase (1997), Schilling et al. (1999) and Trønnes et al. (1999), this work. 72 D.F. Mertz et al. / Journal of Geodynamics 38 (2004) 57–83

(a)

(b)

Fig. 8. (a) 206Pb/204Pb vs. 143Nd/144Nd and (b) 206Pb/204Pb vs. 87Sr/86Sr for Eggvin Bank volcanic rocks compared to lavas from Iceland as well as from north of Iceland. Symbols and references as in Fig. 5. Hatched straight lines separate mantle segments located north of central Iceland from those located south of central Iceland. See text for discussion of Eggvin Bank tholeiite data ﬁelds. Slopes for hatched straight lines are y =−3.6 × 10−4x + 0.51976 in (a) and y = 6.9 × 10−4x + 0.69031 in (b).

addition, the isotope diagrams (Figs. 5, 7 and 8) show that the neighbouring Central Kolbeinsey and Southern Mohns Ridge sources are also different from Eggvin Bank as well as from Jan Mayen/Jan Mayen Platform sources, respectively. In general, at a given 207Pb/204Pb ratio, the Central Kolbeinsey Ridge has less radiogenic 206Pb/204Pb and 208Pb/204Pb ratios than the Southern Mohns Ridge, whereas the Southern Mohns Ridge has less radiogenic 206Pb/204Pb and 208Pb/204Pb ratios than Jan Mayen/Jan Mayen Platform, resulting in a subparallel arrangement of Central Kolbeinsey Ridge, Southern Mohns Ridge as well as Jan Mayen/Jan Mayen Platform data ﬁelds (Figs. 5c and 7). On the basis of Sr, Nd, and Pb isotopic variations Mertz and Haase (1997) found a distinct large-scale pattern within the high-latitude North Atlantic mantle. At a given Pb isotope composition, ridge as well as intraplate lavas from the region of north of central Iceland up to the Arctic Ocean have more radiogenic Sr and less radiogenic Nd than lavas from south of central Iceland. This pattern is demonstrated in Fig. 8 using selected high-latitude mantle segments for comparison together with the Eggvin Bank samples. Whereas the Eggvin Bank alkali basalts correspond to the outlined large-scale isotope pattern, the Eggvin Bank tholeiites originate from a distinct source which is different from the “normal” mantle source north of central Iceland. The compositions of the Eggvin Bank tholeiites—high-radiogenic Pb and Nd, respectively, and low-radiogenic Sr—corresponds to lavas found in the south-eastern Icelandic volcanic zone (e.g., Vestmannaeyjar). D.F. Mertz et al. / Journal of Geodynamics 38 (2004) 57–83 73

6.4. Source heterogeneities and mantle domains

It has been shown previously using trace element (e.g., Schilling et al., 1983) and isotope compositions (Mertz and Haase, 1997; Schilling et al., 1999) that heterogeneous mantle sources feed modern volcanism in the North Atlantic north of latitude ca. 70.6◦N. Our new data show that these heterogeneities affected the North Kolbeinsey spreading axis since at least 100 ky (e.g., Figs. 3 and 5). The lavas from the Eggvin Bank region close to the Jan Mayen Fracture Zone are generally enriched in incompatible elements (Dittmer et al., 1975; Pedersen et al., 1976; Sun et al., 1979; Schilling et al., 1983; Neumann and Schilling, 1984). For example, these lavas show an increase in (La/Sm)N up to ca. 4 whereas further south the Eggvin Bank erupts lavas with (La/Sm)N < 1(Fig. 9b). In addition to the increase in ◦ (La/Sm)N approaching the Jan Mayen Fracture Zone, volcanic rocks north of latitude 70.6 N in general

(a)

(b)

(c)

◦ ◦ Fig. 9. (a) Variation of Na8.0, (b) (La/Sm)N and (c) Ba/La vs. latitude along the spreading axes between 66 and 78 N indicating a general northward decrease of the degree of melting and a more enriched mantle north of 70.6◦N. Data from Sun et al. (1979), Sigurdsson (1981), Schilling et al. (1983), Neumann and Schilling (1984), Melson and O’Hearn (1986), Waggoner (1989), Devey et al. (1994), Haase et al. (1996, 2003) and Trønnes et al. (1999), this work. 74 D.F. Mertz et al. / Journal of Geodynamics 38 (2004) 57–83 have higher Ba/La ratios of ca. 10–20 compared to ca. 5–10 of MORB from the central and southern Kolbeinsey Ridge segments south of latitude 70.6◦N(Fig. 9c). This compositional contrast is also demonstrated by substantial offsets in isotope compositions occurring at latitude 70.6◦N. Fig. 10a–d shows that 87 86 143 144 206 204 3 4 ◦ ◦ Sr/ Sr, Nd/ Nd, Pb/ Pb as well as He/ HeR/Ra of Kolbeinsey lavas (latitude 68.0 –70.6 N) are different from Eggvin Bank near-axis tholeiites and from Eggvin Bank off-axis alkali basalts (latitude >70.6◦N). For example, Central Kolbeinsey Ridge 87Sr/86Sr and 206Pb/204Pb ratios increase from approx-

(a)

(b)

(c)

Fig. 10. Variation of Sr, Nd, Pb and He isotope ratios versus latitude along the spreading axes between 68◦ and 73◦N indicating a depleted source with little variation south of an overlapping spreading center at ca. 70.6◦N where as the source north of 70.6◦Nis enriched and shows a large variation except of He. Grey ranges highlight major trends in the isotope composition of rocks from the segment south of 70.6◦N in contrast to the Eggvin Bank near-axis and off-axis volcanic rocks. Data from Poreda et al. (1986), Ito et al. (1987), Mertz et al. (1991), Macpherson et al. (1997), Mertz and Haase (1997), Trønnes et al. (1999) and Schilling et al. (1999), this work. SFZ: Spar Fracture Zone at latitude ca. 69◦N, OSC: Overlapping Spreading Center at latitude ca. 70.6◦N, JMFZ: Jan Mayen Fracture Zone at latitude ca. 72◦N (at longitude ca. 12◦W). D.F. Mertz et al. / Journal of Geodynamics 38 (2004) 57–83 75 imately 0.7027–0.7030 and 17.9–18.1, respectively, to Eggvin Bank tholeiite ratios of 0.7031–0.7032 and 18.8–19.0, respectively, and to Eggvin Bank alkaline lava ratios of 0.7034–0.7035 and 18.8–18.9, respectively. There is a single site at ca. 70.3◦N where Kolbeinsey lavas tap a more enriched mantle, e.g., with an 206Pb/204Pb ratio of 18.34 (Ito et al., 1987), relative to the otherwise unradiogenic Kolbeinsey mantle. The change in source composition at latitude 70.6◦N correlates geographically with overlapping spreading Center segments of the Central and North Kolbeinsey Ridges (Appelgate, 1997). A similar correlation is known from the northern Juan de Fuca Ridge (Cousens, 1996; Karsten et al., 1986, 1990). Schilling et al. (1999), based on the interpretation of geographical variations and trends in He–Pb–Nd–Sr isotope space, suggested that there is a boundary in the vicinity of the Spar Fracture Zone at latitude ca. 69◦N between the zones of inﬂuence of a postulated Jan Mayen plume characterised by low 3He/4He ratios and the Iceland plume with high 3He/4He ratios. However, since the isotopic compositions along the Kol- beinsey Ridge up to latitude 70.6◦N are unusually homogeneous (Fig. 10), we infer a boundary between two mantle domains beneath the Kolbeinsey Ridge at 70.6◦N. There is a general resemblance between alkaline Eggvin Bank, Jan Mayen Platform and Jan Mayen lavas north of 70.6◦N in terms of normalised incompatible element patterns (Fig. 6) or high time-integrated U/Pb and Th/Pb element ratios producing highly radiogenic Pb compared to most other high-latitude North Atlantic volcanic rocks (Mertz and Haase, 1997). However, the isotope ratios vary between the three sites with substantial differences especially between Eggvin Bank and Jan Mayen lavas (Figs. 5 and 7), indicating that the magma source composition is heterogeneous not only with latitude but also with longitude between 7◦ and 15◦W (at latitude ca. 71◦N).

6.5. Source interactions and mantle ﬂow

A prominent mantle model for the Mohns Ridge-Jan Mayen Platform-Kolbeinsey Ridge region suggests an enriched mantle plume located below the Jan Mayen Platform (Schilling et al., 1999) or below Jan Mayen Island (e.g., Schilling et al., 1983) which is dispersing outward at shallow depth and is progressively diluted by mixing with the surrounding depleted asthenosphere. This model of binary mixing is supported by geochemical trends based on certain isotope or trace element ratios as shown in Fig. 11. However, for evaluating such source mixing processes it is more useful to apply isotope diagrams using the same x-axis and y-axis denominators (e.g., Pb versus Pb isotope diagrams in Figs. 5 and 7) because in this type of diagram binary mixing should result in linear arrays and is therefore simply to identify. In the case of central Norwegian-Greeland Sea volcanism, these Pb versus Pb isotope diagrams show subparallel data ﬁelds for Jan Mayen/Jan Mayen Platform on one hand and Southern Mohns Ridge as well as Central Kolbeinsey Ridge, respectively, on the other hand in 206Pb/204Pb versus 207Pb/204Pb (Fig. 5c) and 207Pb/204Pb versus 208Pb/204Pb (Fig. 7) spaces with a slightly shallower slope for the Central Kolbeinsey Ridge in the latter diagram. The subparallel arrangement indicates that Jan Mayen/Jan Mayen Platform does not mix with the neighbouring ridge segments, i.e., enriched Jan Mayen/Jan Mayen Platform material disperses neither to the north-east along the Southern Mohns Ridge nor to the south along the Central Kolbeinsey Ridge. Thus, the curves presented in Fig. 11 cannot be interpreted to represent trends caused by binary source mixing. Systematic errors in Pb isotope measurements can occur if instrumental mass fractionation is not appropriately corrected. The effect of 0.1% per atomic mass unit mass fractionation is demonstrated by trajectories in the Pb versus Pb isotope diagrams shown. The true Pb isotope composition of any sample lies along a line plotted through the measured composition with similar length as and parallel to the 76 D.F. Mertz et al. / Journal of Geodynamics 38 (2004) 57–83

206 204 ◦ ◦ Fig. 11. Pb/ Pb vs. (La/Sm)N for segments between latitude 69 and 73 N. Roman figures indicate hypothetic binary mixing trends between depleted Southern Mohns Ridge and enriched Jan Mayen sources (I) or depleted Central Kolbeinsey Ridge and enriched Jan Mayen Platform sources (II). However, based on Pb vs. Pb isotope diagrams these hypothetic mixing trends can be excluded. See text for discussion. Data from Schilling et al. (1983), Neumann and Schilling (1984), Mertz et al. (1991), Endres (1992), Devey et al. (1994), Haase et al. (1996), Mertz and Haase (1997) and Trønnes et al. (1999), this work. instrumental mass fractionation trajectory. In Fig. 5c it would theoretically be possible to create a more or less linear Central Kolbeinsey Ridge-Southern Mohns Ridge-Jan Mayen/Jan Mayen Platform array by assuming not only extreme but also contrasting instrumental mass fractionation effects for the Kolbeinsey data on one hand and the Jan Mayen data on the other hand. However, replicate measurements of selected Kolbeinsey samples using a Pb double spike indicates that uncorrected instrumental mass fractionation effects are insignificant (M. Thirlwall, personal communication). We therefore assume that this is also the case for the other compiled Pb isotope data. Furthermore, the mass fractionation trajectories lies approximately parallel to the data fields (Fig. 7). Thus, the subparallel arrangement of the data fields is not an analytical artifact but appears to be geologically meaningful. In cases where mantle plumes are thought to feed enriched material into neighbouring spreading axes, regular geochemical and isotope gradients are observed along the ridge axes, for example, the Galapagos or Easter plumes (Verma et al., 1983; Hanan and Schilling, 1989; Fontignie and Schilling, 1991). Such a pattern, however, is not seen along the Eggvin Bank-Kolbeinsey Ridge segment. As shown on Fig. 10, the geographically discrete offset in isotope ratios at latitude 70.6◦N does not support the assumption of significant interaction between a hypothetical Jan Mayen/Jan Mayen Platform plume-type mantle and the Central Kolbeinsey MORB mantle. 87 86 Mohns Ridge MORB with (La/Sm)N > 1, Ba/La > 10 (Fig. 9) and Sr/ Sr of 0.7029 (Schilling et al., 1999) occur as far north as ca. 77◦N indicating the presence of an enriched source more than 600 km north of Jan Mayen. Considering the Jan Mayen/Jan Mayen Platform plume model of Schilling et al. (1983, 1999) would mean that a far-reaching unilateral pollution of the upper mantle to the north by the suggested plume occurred. However, no clear gradient in incompatible element ratios (Fig. 9)orinSr,Nd, and He isotope ratios (Fig. 10) exists along the Mohns Ridge axis. Instead, a large variation in compositions of the basalts is observed close to Jan Mayen while MORB further north show slightly less variation and the most enriched lavas are lacking. As an alternative to the model of recent asthenospheric contamination by plume material, we suggest that the observed incompatible element and isotope ratio variation could arise from older small-scale heterogeneities in the upper mante, possibly caused by variable mixing of D.F. Mertz et al. / Journal of Geodynamics 38 (2004) 57–83 77 small-degree melts from enriched parts of the mantle with large-degree melts from relatively depleted mantle peridotite. In all Pb versus Pb isotope diagrams (Figs. 5 and 7) Jan Mayen, Jan Mayen Platform and Eggvin Bank lavas are linearly arrayed with the Jan Mayen Platfom rocks lying between the less radiogenic Jan Mayen and the more radiogenic Eggvin Bank data fields. In contrast, in 87Sr/86Sr versus 143Nd/144Nd space (Fig. 5a) Jan Mayen lavas have more enriched signatures and Eggvin Bank near-axis tholeiites have less enriched signatures than lavas of the Jan Mayen Platform whereas the Eggvin Bank off-axis alkali basalts are similar to Jan Mayen/enriched Jan Mayen Platform volcanic rocks. The above Jan Mayen-Jan Mayen Platform-Eggvin Bank array can be explained if most of the Jan Mayen Platform lavas result from mixing between Jan Mayen alkaline lava and Eggvin Bank tholeiite sources. This process requires that the influence of the Jan Mayen mantle component extend for several tens of km in a north-west and north-east direction, and that the influence of the Eggvin Bank mantle component extend up to a few 100 km to the east–northeast. In this case, source interactions between different segments would mainly occur in the mantle below the Jan Mayen Platform and along the suggested mantle flow paths below the topographically shallow region extending ca. 400 km E–W from the Eggvin Bank to the Jan Mayen Platform between latitude ca. 70.6◦–72◦N and longitude ca. 5◦–15◦W, respectively, rather than to the south along the Kolbeinsey Ridge or to the north-west along the Mohns Ridge.

6.6. Melt generation

Klein and Langmuir (1987) suggested that variation of the fractionation-corrected Na2O (Na8.0) concentrations in MORB reflects the degree of mantle melting which in turn depends largely on temperature variations. That is, low Na8.0 values indicate high degrees of partial melting (and high temperatures) while high Na8.0 values indicate the opposite, though some dependence on source composition has also been shown (Niu et al., 2001). Nevertheless, the variation of Na8.0 versus latitude along the Kolbeinsey Ridge between 66◦ and 78◦N(Fig. 9a) indicates that melting temperature probably exerts the dominant effect since Na8.0 content increases relatively smoothly towards the north despite more complex changes in source enrichment as indicated by variable (La/Sm)N (Fig. 9b). The high Na8.0 north of Jan Mayen implies lower degrees of partial melting, which is consistent with the relatively thin crust of 4.0 ± 0.5km of the Mohns Ridge at latitude 72.0◦–72.5◦N measured by refraction seismic (Klingelhöfer et al., 2000). The low degree of melting beneath the Mohns Ridge and its thin crust adjacent to Jan Mayen indicate that the underlying mantle is comparatively cold, which is inconsistent with the influence of the putative Jan Mayen/Jan Mayen Platform plume. Moreover, ridge crust thought to be influenced by the Iceland plume is thicker by a factor of about 2 than the crust of Mohns Ridge. For example, seismic experiments show that crust on the Reykjanes Ridge axis near latitude 62◦N is ca. 8–10 km thick (Smallwood et al., 1995), and that ∼1 Ma old crust on the east flank of the Kolbeinsey Ridge at latitude 70◦N is about 9 km thick (Kodaira et al., 1997). Basalts from the Mohns and Knipovich spreading axes have a high average H2O/Ce of 287 ± 33 (Michael, 1995), probably implying high water contents in their mantle sources. Based on experimental data, Stolper and Newman (1994) and Hirose and Kawamoto (1995) suggested that the addition of 0.1% H2O to mantle peridotite increases the degree of partial melting by about 6%. Accordingly, we suggest ◦ that the high H2O contents in the North Atlantic mantle north of 70.6 N together with the setting of Jan Mayen opposite an active spreading axis may be responsible for the excess melting in this intraplate 78 D.F. Mertz et al. / Journal of Geodynamics 38 (2004) 57–83 region. Smaller intraplate volcanoes like the Eggvin Bank off-axis seamount may form close to the spreading axis when the enriched parts of the mantle begin to melt during mantle ascent close to the axis as envisaged by the model of Davis and Karsten (1986).

6.7. Nature of Eggvin Bank mantle

One part of the mantle domain north of 70.6◦N tapped by the Eggvin Bank alkali basalts is related to the source of the enriched Jan Mayen Platform lavas. The other part tapped by the Eggvin Bank tholeiites is similar to the Icelandic mantle based on consistent Sr–Nd–Pb isotope compositions of lavas from Vestmannaeyjar/south-east Iceland and Eggvin Bank (Figs. 5 and 8). However, it is difficult to explain how the feeding of the Eggvin Bank mantle by Vestmannaeyjar mantle plums at shallow depth could work. A lateral dispersion of Icelandic plume material by channelling along the Kolbeinsey Ridge (e.g., Yale and Phipps Morgan, 1998) or by radially symmetric (Ito et al., 1996) mantle flow has been suggested. In this case, since no enriched south-eastern Iceland plume-type Vestmannaeyjar source with highly-radiogenic Pb is tapped along the entire Kolbeinsey Ridge from the Tjörnes Fracture Zone at latitude 67◦ up to the overlapping spreading Center at 70.6◦N(Fig. 1), the hypothetical north-directed Iceland plume flow must lie deeper than the solidus along the Kolbeinsey Ridge as suggested by Mertz et al. (1991). This scenario is based on the model of, e.g., Batiza and Vanko (1984) assuming that enriched plums occur in a matrix of depleted asthenosphere. On the other hand, the transport of plums mainly depends on the regional direction of mantle flow (e.g., Zhang and Tanimoto, 1992). Considering the plate tectonic setting of Vestmannaeyjar within the active southeast volcanic zone of Iceland together with the interpretation that south-pointing V-shaped bathymetric morphologies along the Reykjanes Ridge south of Iceland indicate asthenosphere flow to the south (e.g., Vogt, 1971), hypothetic Vestmannaehyjar plums most probably can be regarded as components of the regional mantle flow regime to the south rather than being transported to the north. For example, Yale and Phipps Morgan (1998) modelled flow rates to the south along the Reykjanes Ridge as high as 30 cm/y for a low viscosity asthenosphere. Therefore, in contrast to the assumption that the Eggvin Bank tholeiite source represents a mantle component recently derived from the presently stationary Iceland plume, we concur with Trønnes et al. (1999) that this source was emplaced in the upper mantle by the original Early Tertiary plume head. This relationship between Eggvin Bank and southeast Iceland plume sources is indicated by a substantial overlap in Sr–Nd–Pb isotope compositions of samples from both segments and on the assumption that the isotope composition of the Iceland plume essentially has been constant during its life time (e.g., Thirlwall et al., 1994) from the plume head impinging on the lithosphere to the present on-axis setting.

7. Conclusions

Basalts dredged from the Eggvin Bank include tholeiitic, near-axis lavas and transitional-to-alkaline, off-axis seamount lavas. The Eggvin Bank tholeiites are variably enriched, with their chondrite-normalised La/Sm ratios ranging from about 0.6 to 2.7. The most depleted tholeiite is geochemically similar to basalts from the neighbouring Kolbeinsey Ridge. The incompatible element patterns of Eggvin Bank off-axis lavas are similiar to other alkaline basalts occurring close to this region (e.g., Jan Mayen, Jan Mayen Platform) and are derived from a HIMU-type source. As suggested previously, our data conﬁrm that there are large chemical heterogeneities in the mantle north of latitude ca. 70.6◦N. D.F. Mertz et al. / Journal of Geodynamics 38 (2004) 57–83 79

Sr–Nd–Pb isotope diagrams allow differentiation between mantle sources of Eggvin Bank near-axis and off-axis lavas. The Eggvin Bank off-axis basalts resemble neighbouring alkali basalts in both isotope and incompatible element patterns indicating a distinctive mantle source for a region extending over at least 100 km. In contrast, the near-axis tholeiites resemble lavas from the southeastern volcanic zone of Iceland (e.g., Vestmannaeyjar). These differences show that in the Norwegian-Greenland Sea the magma sources are heterogenous not only with latitude but also with longitude, at least in the Eggvin Bank–Jan Mayen region between ca. 7◦ and 15◦W. The inferred source of Eggvin Bank off-axis lavas is consistent with the large-scale isotope pattern within the high-latitude North Atlantic mantle that has previously been described. The Eggvin Bank near-axis tholeiites, however, are generated by an distinct source characterised by highly-radiogenic Pb and Nd and relatively low-radiogenic Sr. This compositional pattern is unique in the mantle north of central Iceland. Based on the similiar Sr–Nd–Pb isotope compositions of Eggvin Bank tholeiites and southeast Iceland Vestmannaeyjar alkaline lavas, we conclude that the Eggvin Bank tholeiite source could be related to the Icelandic plume mantle. Following the model of Trønnes et al. (1999), we suggest that the Eggvin Bank tholeiite source was emplaced in the upper mantle by the original Iceland plume head during the Early Tertiary. Isotopic and trace element data for near-axis tholeiites indicate an abrupt change in source compositions at ca. 70.6◦N. This compositional change coincides geographically with an overlap of the northern and central segments of the Kolbeinsey Ridge and apparently reflects a boundary between two chemically distinct mantle domains. Trace element and isotope data show no evidence for interactions between the different mantle sources north and south of 70.6◦N. Therefore, the existence of an enriched Jan Mayen/Jan Mayen Platform plume feeding the Kolbeinsey ridge axes to the south is questionable. Lavas of the Jan Mayen/Jan Mayen Platform, the Southern Mohns Ridge, and the Central Kolbeinsey Ridge, define subparallel arrays on Pb versus Pb isotope diagrams. This indicates that the Jan Mayen/Jan Mayen Platform source has not mixed with the sources of the neighbouring ridge segments, i.e., enriched Jan Mayen/Jan Mayen Platform material disperses neither to the north-east beneath the Southern Mohns Ridge nor to the south beneath the Central Kolbeinsey Ridge. It appears that source interactions mainly occur in the Jan Mayen Platform mantle possibly between Eggvin Bank tholeiite and Jan Mayen alkali basalt sources. 40Ar/39Ar step heating data indicate that approximately contemporaneous intraplate alkaline volcanism was broadly distributed within a ca. 400 km radius north and west of Jan Mayen at ca. 0.6–0.7 Ma as well as younger, near-axis tholeiitic and intraplate alkaline activity at <0.1 Ma, respectively. If previously published Miocene K/Ar ages for alkaline basalts dredged close to the Jan Mayen Fracture Zone are accurate, it appears that the Eggvin Bank–Jan Mayen intraplate region produced significant volumes of alkaline melts intermittently from the Miocene to recent. The available age data do not support the Jan Mayen hot spot track hypothesis, since volcanic peaks west and north of Jan Mayen show no indication of a systematic age progression.

Acknowledgements

Captains, ofﬁcers, and crews of FS Polarstern and FS Meteor are thanked for their skillful assistence with dredge operations. Thorough and helpful reviews by B.L. Cousens, K. Hoernle and an anonymous reviewer 80 D.F. Mertz et al. / Journal of Geodynamics 38 (2004) 57–83 as well as constructive discussions with R. Kraus are highly appreciated. We thank P. Koppenhöfer for help with drawing the ﬁgures and W. Jacoby for his patient editorial handling with an earlier version of the paper. D. DePaolo made unpublished data on Jan Mayen accessible. The Polarstern and Meteor cruises were funded by the Bundesministerium für Bildung und Forschung (BMBF).

References

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