Combined Li–He isotopes in Iceland and Jan Mayen basalts and constraints on the nature of the North Atlantic mantle

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Geochimica et Cosmochimica Acta 75 (2011) 922–936 www.elsevier.com/locate/gca

Combined Li–He isotopes in Iceland and Jan Mayen basalts and constraints on the nature of the North Atlantic mantle T. Magna a,b,c,⇑, U. Wiechert a,d, F.M. Stuart e, A.N. Halliday a,f, D. Harrison g,1 a

Institute of Geochemistry and Petrology, ETH Zu¨rich, Clausiusstr. 25, CH-8092 Zu¨rich, Switzerland b Institut fu¨r Mineralogie, Universita¨t Mu¨nster, Corrensstr. 24, D-48149 Mu¨nster, Germany c Czech Geological Survey, Kla´rov 3, CZ-118 21 Prague 1, Czech Republic d Institut fu¨r Geologische Wissenschaften, Freie Universita¨t Berlin, Malteserstr. 74-100, Haus B204, D-12249 Berlin, Germany e Isotope Geosciences Unit, SUERC, G75 0QF East Kilbride, United Kingdom f Department of Earth Sciences, University of Oxford, Parks Road, OX1 3PR Oxford, United Kingdom g SEAES, University of Manchester, Oxford Road, M13 9PL Manchester, United Kingdom Received 4 September 2009; accepted in revised form 10 November 2010; available online 16 November 2010

Abstract Lithium (Li) isotopes are thought to provide a powerful proxy for the recycling of crustal material, affected by low temperature alteration, through the mantle. We present Li isotope compositions for basaltic volcanic rocks from Hengill, Iceland, and Jan Mayen in order to examine possible links between ocean island volcanism and recycled oceanic crust and to address recent suggestions that mantle 3He/4He is also related to recycling of ancient slabs. Basaltic glasses spanning a range of chemical enrichment from the Hengill fissure system define an inverse correlation between d7Li (3.8–6.9&) and 3He/4He (12–20 RA). The high-3He/4He basalts have low d18O as well as excess Eu and high Nb/U, but carry no Li isotope evidence of being the product of recycling of altered slab or wedge material. In fact, there is no clear correlation between Li or He isotopes on the one hand and any of the other fingerprints of recycled slab components. The low-3He/4He samples do have elevated Nb/ U, Sr/Nd, positive Eu anomalies and high d7Li (6.9&), providing evidence of a cumulate-enriched source that could be part of an ancient altered ocean floor slab. Basalts from Jan Mayen are characterized by large degrees of enrichment in incompatible trace elements typical of EM-like basalts but have homogeneous d7Li typical of depleted mantle (3.9–4.7&) providing evidence for a third mantle source in the North Atlantic. It appears that oceanic basalts can display a wide range in isotope and trace element compositions associated with recycled components whilst exhibiting no sign of modern surface-altered slab or wedge material from the Li isotope composition. Ó 2010 Elsevier Ltd. All rights reserved.

1. INTRODUCTION Lithium (Li) isotope fractionation in nature is mainly the result of low-temperature processes near the Earth’s surface (Chan et al., 1992, 2002). Consequently, fraction⇑ Corresponding author at: Institut fu ¨ r Mineralogie, Universita¨t

Mu¨nster, Corrensstr. 24, D-48149 Mu¨nster, Germany. Tel.: +49 251 8333461; fax: +49 251 8338397. E-mail address: [email protected] (T. Magna). 1 Present address: PGNiG Norway AS, Vestre Svanholmen 4, N4313 Sandnes, Norway. 0016-7037/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2010.11.007

ation of Li isotopes accompanying aqueous alteration of oceanic crust and its subsequent dehydration during subduction has the potential to be used to trace subducted material in the convecting mantle (Tomascak et al., 2002; Zack et al., 2003; Kobayashi et al., 2004; Elliott et al., 2006; Magna et al., 2006b; Marschall et al., 2007b; Agostini et al., 2008; Chan et al., 2009; Halama et al., 2009; Vlaste´lic et al., 2009). Dehydration may eventually lead to an enrichment of 7Li in the overlying mantle wedge with substantial loss of Li from the subducted slab into the fluid phase (Zack et al., 2003). However, a significant proportion of the Li inventory is still retained in the residual slab even after

Lithium isotopes in the North Atlantic mantle

large-degree dehydration (Marschall et al., 2007a) and may perhaps retain a diagnostic d7Li signature (Vlaste´lic et al., 2009). Hydrothermally altered gabbros that were equilibrated subsequently with seawater tend to be depleted in Li relative to unaltered seafloor basalts and are isotopically heavy (d7Li up to 14&; Chan et al., 1992, 2002) whereas less altered portions of underlying basaltic oceanic crust hold d7Li that is only slightly elevated over unaltered basalts (up to 7.4&; Nishio et al., 2005). The role of recycled materials in the formation of ocean island basalts (OIB) has been the focus of considerable debate (Hofmann and White, 1982; Halliday et al., 1995; Hofmann, 1997; Niu and O’Hara, 2003). Elliott et al. (2004) proposed Li isotopes as a diagnostic tool for testing the presence of recycled components in the source of OIB. The Li isotope composition of OIB source mantle is likely to be variable, governed by the range of d7Li within the oceanic crust pile and by the proportion that each component contributes to the melt. The distribution of the variability in Li isotope compositions in unaltered mid-ocean ridge basalts (MORB) does not display any provinciality in the Mid-Atlantic Ridge, Southeast Indian Ridge and East Pacific Rise, nor does it co-vary with other geochemical tracers (e.g., K2O/TiO2; Chan et al., 1992; Elliott et al., 2006; Tomascak et al., 2008). The range in d7Li for MORB is similar to that observed in many OIB, for example Hawaii (2.5–5.7&; Tomascak et al., 1999b; Chan and Frey, 2003; Kobayashi et al., 2004) and Iceland (3.1–5.1&; Pistiner and Henderson, 2003; Ryan and Kyle, 2004; Jeffcoate et al., 2007; Schuessler et al., 2009) whereas basaltic lavas from Polynesia with extremely radiogenic Pb isotope signatures yield d7Li > 5& which has been explained as a result of derivation from recycled altered oceanic crust that retained much of its original Li (Nishio et al., 2005). Recent results from the Cook–Austral islands (Chan et al., 2009; Vlaste´lic et al., 2009) confirm the view of moderately elevated d7Li (up to 8&) in HIMU lavas (l  238U/204Pb, HIMU lavas have 206Pb/204Pb > 20.5; see Zindler and Hart (1986) for details). The range of Li isotope compositions in other types of enriched mantle remains less constrained (Tomascak, 2004) whereas normal mantle has a defined d7Li = 3–4& (Seitz et al., 2004, 2007; Elliott et al., 2006; Magna et al., 2006a, 2008; Jeffcoate et al., 2007). Here we report Li isotope data for basalts from the Western Rift Zone of Iceland and Jan Mayen island in order to test the role of recycled components in the mantle beneath the North Atlantic Ocean and whether or not recycling of ancient, hydrothermally altered crust is a viable process to account for an observed negative correlation between 7 Li/6Li and 3He/4He. We build on a previous investigation of Burnard and Harrison (2005) who hypothesized that low-d18O source of Iceland basalts might have been modified by surficial alteration. 2. SAMPLES AND GEOLOGIC BACKGROUND 2.1. Iceland Icelandic volcanism has long been attributed to the presence of a mantle plume, a hypothesis supported by seismic

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imaging of low-velocity material extending to 400 km depth (Wolfe et al., 1997). Such deep-sourced materials may originate in relatively undegassed lower mantle and are identified by 3He/4He in excess of the MORB value of 8 ± 1 RA (where RA denotes the measured 3He/4He normalized to the air value of 1.39  106) (Condomines et al., 1983; Hilton et al., 1999; Breddam et al., 2000; Breddam, 2002; Stuart et al., 2003; Brandon et al., 2007; Starkey et al., 2009). Modeling of He isotope evolution of the mantle suggests that high-3He/4He mantle has been isolated from depletion events for a few billion years (Porcelli and Elliott, 2008). Large variations in trace element patterns and lithophile radiogenic isotopes provide evidence for several components contributing to the observed variability in Icelandic basalts (e.g., Hemond et al., 1993; Chauvel and He´mond, 2000; Kempton et al., 2000; Fitton et al., 2003; Stracke et al., 2003a). The incorporation of ancient recycled oceanic crust in the Iceland plume has been proposed on the basis of incompatible trace elements (Chauvel and He´mond, 2000), Pb isotopes (Thirlwall et al., 2004), Os isotopes (Brandon et al., 2007), Sr–Nd–Os isotopes (Debaille et al., 2009), d18O-depletions (Skovgaard et al., 2001) or the combination thereof (Stracke et al., 2003a). Some high-3He/4He basalts show an imprint of recycled subducted oceanic crust, inferred from trace element and radiogenic isotope data (Macpherson et al., 2005). Complex multistage mixing scenarios have been modelled for Iceland lavas (Hanan and Schilling, 1997; Brandon et al., 2007; Kitagawa et al., 2008; Debaille et al., 2009), suggesting a source comprised of ancient recycled oceanic crust mixed with primitive mantle or incompletely degassed depleted mantle (or sub-continental lithospheric mantle) and subsequent isolation for 1–1.5 Gyr prior to mixing with present-day MORB-source mantle. Several possible origins for the recycled component have been postulated. (i) Some studies (e.g., Thirlwall et al., 2004) proposed heterogeneous basaltic oceanic crust. Lead isotopes are believed to reflect preferential hydrothermal addition of U to the Palaeozoic oceanic crust and Pb depletion during dehydration of the subducted slab (Thirlwall et al., 2004). (ii) A mixture of recycled basaltic oceanic crust and low-d18O Icelandic mantle is predicted on the basis of uniquely low d18O of Icelandic basalts that could be considered to reflect a contribution of recycled Ordovician crust with oxygen isotope compositions that are different from modern oceanic crust (Thirlwall et al., 2006). (iii) Both gabbroic and basaltic part of the oceanic crust are required to account for the mixing relationships observed in some locations (Chauvel and He´mond, 2000; Skovgaard et al., 2001). Excesses of Sr, Ba and Eu, (Sr/Nd)N > 1 and Zr–Hf deficiencies in the depleted basalts closely resemble plagioclase–clinopyroxene-rich gabbros (Chauvel and He´mond, 2000), coupled with other evidence for more ancient recycled oceanic crust, the isotope fingerprint for which develops through long-term storage in the mantle (Chauvel and He´mond, 2000; Skovgaard et al., 2001; Kokfelt et al., 2006; Brandon et al., 2007). (iv) Lower oceanic crust gabbros have been proposed to explain the compositions of depleted Icelandic lavas, representative of high-degree melts (Breddam, 2002). Foulger et al. (2005) argue that re-melting

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and Chaussidon, 2002; Burnard and Harrison, 2005) that may reflect either alteration by low-d18O aqueous fluids or an origin in old recycled oceanic lithosphere. A correlation between argon and oxygen isotopes is consistent with low d18O in the mantle source region rather than inherited from the local Icelandic crust (Burnard and Harrison, 2005). 3 He/4He increases from 12.7 to 20.2 RA along the fissure system (Burnard and Harrison, 2005). Samples with the highest 3He/4He are most extensively degassed which, however, does not exceed 30% (Harrison et al., 1999, 2003). An inverse correlation between 3He/4He and CaO/Al2O3 has been interpreted as reflecting assimilation of small amounts of lower crustal clinopyroxene (Burnard and Harrison, 2005).

of abyssal gabbros could explain major and trace element compositions as well as isotope signatures of primitive Icelandic tholeiites and that high 3He/4He may result from ancient time-integrated high-3He/(U + Th) mantle. The Icelandic basalt lavas studied here are olivine phenocryst- and Cr-diopside megacryst-bearing olivine tholeiites and picritic glasses from the 15 km long Mlifell– Miðfel fissure system (MMF; Fig. 1) in the Hengill area, 45 km east of Reykjavı´k (Harrison et al., 1999). The lavas erupted rapidly without significant crystallization within the crust (Trønnes, 1990; Hansteen, 1991), therefore a chemically evolved Icelandic crust (Sigmarsson et al., 1992; Jo´nasson, 1994; Gunnarsson et al., 1998; Burnard and Harrison, 2005; Martin and Sigmarsson, 2007) is rendered an improbable contaminant in these lavas. This is shown by the broadly uniform and light B isotope compositions in melt inclusions in olivine (average d11B = 11&; Gurenko and Chaussidon, 1997) that match “primitive mantle” values and that appear to represent <20% critical continuous melting (<4 wt.% of critical melt retained in the residue) of primitive or variously depleted mantle, or combination of both, from between 75 and 30 km depth and without significant interaction of such melts with local crustal material (Gurenko and Chaussidon, 1995). The MMF glasses have remarkably low d18O (4.6&; Gurenko

2.2. Jan Mayen Jan Mayen (71°N, 8°300 W) is situated south of the NW–SE trending Western Jan Mayen Fracture Zone which separates the Kolbeinsey and the Mohns Ridge. Volcanism is dominated by primitive alkali basalts that evolve from dominant ankaramites through alkali olivine basalts to trachytes (Maaløe et al., 1986). Partial melting of a spinel lherzolitic source at >1400 °C and 2  109 Pa

NRZ WRZ

Hrafnabjörg

DICE 9 L.Þingvallavatn

N

Ká

lfs

tin

da

r

ERZ

DICE 13 DICE 15

Miðfell DICE 10 DICE 11

5 km

DICE 8 DICE 7 MAE Hengill DICE 5 DICE 43

L.Úlfljótsvatn

Hyaloclastite and pillow lavas Basic postglacial lavas Basic interglacial lavas Sampling site Fault Fracture

Fig. 1. Schematic map of Hengill area (modified after Burnard and Harrison, 2005). Small inset shows major volcanic zones of Iceland (WRZ – Western Rift Zone, ERZ – Eastern Rift Zone, NRZ – Northern Rift Zone). Square denotes Hengill area with sampling locations for Icelandic lavas from this study, detailed in the main figure.

Lithium isotopes in the North Atlantic mantle

(20 kbar) followed by late-stage gravitational settling of mafic phenocrysts in alkali basalt magmas at low pressure has been proposed to generate the variability of Jan Mayen basalts and trachytes (Maaløe et al., 1986). The trace elements and radiogenic isotope systematics of Jan Mayen and the adjacent platform differ markedly from the nearby Kolbeinsey and Mohns Ridges (Schilling et al., 1999; Trønnes et al., 1999; Mertz et al., 2004; Blichert-Toft et al., 2005) in that the former represent low-degree partial melts of enriched mantle reservoir (Trønnes et al., 1999). Trønnes et al. (1999) argued that a HIMU-like signature in Jan Mayen basalts, based on trace element data and Pb isotopes which, albeit low (206Pb/204Pb 18.6), could perhaps reflect a rather short isolation time (500 Myr) for the recycled oceanic crust source, consistent with model-based conclusions derived for Icelandic basalts (Thirlwall, 1997). On the other hand, Blichert-Toft et al. (2005) describe the source of basalts from the Jan Mayen platform as a mixture of depleted mantle (DM), component “C” (Hanan and Graham, 1996) and enriched mantle EM1. Most recently, Debaille et al. (2009) have proposed that incompatible-trace-element-enriched nature of the Jan Mayen alkali basalts is due to melting of sub-continental lithospheric mantle. We have analyzed eight clinopyroxene- and olivinephyric basalts (strictly ankaramites) from Jan Mayen. Samples with MgO > 8% were selected in order to avoid the effects of wall-rock assimilation; they show minimal post-eruption alteration. Previous observations have shown little variation in trace elements (Maaløe et al., 1986).

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3. ANALYTICAL Procedures for Li isolation and purification and mass spectrometry followed methods outlined elsewhere (Magna et al., 2004, 2006a). Lithium isotope compositions were measured with a Nu-1700 high-resolution MC–ICP-MS (Nu Instruments, Wrexham, UK), housed at the ETH Zu¨rich. Samples were measured against the L-SVEC reference standard (Flesch et al., 1973); the results are reported in d7Li notation (d7Li (&) = [(7Li/6Li)sample/(7Li/6Li)LSVEC  1]  1000). Reference basalts JB-2 (GSJ) and BHVO-2 (USGS) were measured together with unknown samples to assess reliability of the analytical approach and yielded d7Li = 4.84 ± 0.15& (2r, n = 3) and 4.58 ± 0.32& (2r, n = 4), respectively, in excellent agreement with previously published values (Tomascak et al., 1999a; Jeffcoate et al., 2004; Magna et al., 2004, 2008; Seitz et al., 2004). Strontium (Sr) and neodymium (Nd) were separated using the procedures of Horwitz et al. (1992) and Cohen et al. (1988), respectively. Note that the samples were not leached prior to analysis. Strontium and Nd isotope compositions were measured on a Nu Plasma MC–ICP-MS at the ETH Zu¨rich. Masses 83 and 85 were used to correct for Kr and Rb interferences, respectively. The instrumental mass bias was corrected using an exponential fractionation law. Measured 87Sr/86Sr ratios were corrected for mass bias using 88 Sr/86Sr = 8.3752 while measured 143Nd/144Nd ratios were corrected using 146Nd/144Nd = 0.7219 (Wasserburg et al., 1981). The 143Nd/144Nd ratios were further normalized to the JMC-Nd standard using a 143Nd/144Nd = 0.511833

Table 1 Lithium abundance and Li–He–O–Sr–Nd isotope compositions in Iceland and Jan Mayen lavas. Li (ppm)

d7Li (&)

2r

3

He/4Hea

Iceland DICE 5 DICE 7 DICE 10 DICE 11 MAE DICE 8 DICE 9 DICE 13 DICE 15 DICE 43

2.8 2.6 2.6 2.4 2.8 3.8 4.5 4.1 4.1 5.0

6.90 6.29 4.57 3.72 5.50 5.84 3.88 4.01 5.21 4.38

0.34 0.19 0.37 0.19 0.09 0.37 0.39 0.18 0.17 0.26

12.7 13.1 17.8 17.2 13.4 15.6 20.2 17.1 17.0 17.8

Jan Mayen JM 14 JM 44 JM 78 JM 207 JM 208 JM 16837 JM 16841 JM 16852

5.1 4.8 3.9 4.3 4.0 3.8 2.9 2.2

3.88 3.92 4.00 3.86 4.38 4.70 4.49 4.04

0.25 0.41 0.16 0.20 0.33 0.39 0.23 0.44

d18O (&)b

87

Sr/86Src

4.50 4.64 4.61 4.56 4.43 4.75 4.12 4.36 4.28 4.52

0.703040 0.703015 0.703089 0.703095 0.703007 0.703104 0.703141 0.703209 0.703120 0.703194

143

Nd/144Ndc

(14) (12) (14) (14) (13) (14) (17) (12) (11) (17)

0.513115 0.513115 0.513081 0.513083 0.513101 0.513066 0.513027 0.513023 0.513038 0.513019

(6) (9) (7) (6) (7) (6) (5) (6) (5) (7)

5.86 6.72 6.23 6.27 5.71 6.11 5.62 5.85

Lithium concentrations were measured with a quadrupole ICP-MS PQ 2 (VG Elemental) at the ETH Zu¨rich using beryllium as an internal standard. a Helium isotope compositions of Iceland lavas are from Harrison et al. (1999) and Burnard and Harrison (2005). b Oxygen isotope compositions are from Burnard and Harrison (2005). c Numbers in parentheses are 2r errors and refer to the last two digits (Sr) and the last digit (Nd), respectively.

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ger variation in d7Li (3.8–6.9&). There is no clear correlation between d7Li and 143Nd/144Nd, 87Sr/86Sr or trace element ratios. However, there is a quasi-linear negative correlation between d7Li and 3He/4He (Fig. 3) in which low d7Li values, typical of MORB-source mantle, are associated with high 3He/4He. Lithium contents in the Jan Mayen basalts are similar to those found for Icelandic basalts whereas d7Li is largely invariant (d7Li = 3.9–4.7&; Table 1). This is consistent with the homogeneous, though steep (LaN/SmN > 3.2) REE pattern (Fig. 2 and Table 3) and He isotope compositions of these samples (3He/4He = 5.3–6.3 RA; Table 1). Consequently, there is no clear co-variation with d7Li (Fig. 3). Unlike the Icelandic basalts, the Jan Mayen ankaramites show variable Pb depletions to enrichments whereas only minor depletions in Sr, Zr and Hf are observed (Fig. 2).

(cross-calibrated to La Jolla standard value 0.511858) and 87 Sr/86Sr were normalized to the ratio of 0.710245 for the NIST SRM 987. The He isotope compositions of gases released by in vacuo crushing of the Jan Mayen olivines were measured using established procedures at SUERC (Stuart et al., 2000). 4. RESULTS The Li abundance and d7Li of the Icelandic basalts are listed in Table 1 along with the pertinent stable and radiogenic isotope data. All Iceland samples display a restricted range of 87Sr/86Sr = 0.7030–0.7032 and 143Nd/144Nd = 0.51302–0.51312. Trace element data (Table 2) define two distinct patterns on the basis of LaN/SmN > 1.1 (enriched group) and <0.6 (depleted group). The depleted group has pronounced Zr and Hf depletions and excess Sr that are not observed in enriched lavas (Fig. 2). Marked Li and Pb depletion as well as Eu excess have been found in all samples. The d7Li values of the enriched group range between 3.9 and 5.8& whereas the depleted group shows slightly lar-

5. DISCUSSION Limited data exist for Li isotope compositions of Icelandic basalts (Fig. 4) and no systematic investigation has been

Table 2 Trace element concentrations and important elemental parameters in Iceland lavas.

Rb Cs Sr Ba Sc Y Zr Hf Nb Ta Co Ni Pb Th U La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu (Sr/Nd)N (La/Sm)N Eu/Eu*a

DICE 5

DICE 7

DICE 10

DICE 11

MAE

DICE 8

DICE 9

DICE 13

DICE 15

DICE 43

0.37 0.014 117 9.3 29 14 25 0.73 1.15 0.14 55 218 0.09 0.06 0.02 1.16 3.38 0.68 3.77 1.65 0.68 2.00 0.38 2.64 0.56 1.66 0.24 1.59 0.22 1.99 0.45 1.13

0.34 0.008 114 8.7 28 13 23 0.68 1.24 0.11 56 250 0.07 0.06 0.02 1.15 3.26 0.62 3.45 1.44 0.63 1.83 0.36 2.46 0.50 1.53 0.21 1.43 0.21 2.12 0.52 1.18

0.28 0.005 124 7.2 26 12 19 0.63 1.00 0.08 53 180 0.09 0.04 0.01 0.99 2.92 0.60 3.29 1.37 0.63 1.74 0.34 2.32 0.52 1.40 0.21 1.36 0.19 2.42 0.47 1.24

0.20 0.007 113 6.7 28 12 18 0.57 0.95 0.08 55 202 0.04 0.03 0.01 0.94 2.67 0.53 3.03 1.32 0.59 1.75 0.33 2.29 0.47 1.48 0.20 1.26 0.19 2.39 0.46 1.18

0.31 0.006 122 8.5 27 14 25 0.69 1.25 0.09 53 199 0.13 0.05 0.02 1.14 3.44 0.64 3.66 1.62 0.70 1.97 0.38 2.64 0.56 1.66 0.24 1.44 0.22 2.14 0.46 1.19

2.28 0.020 168 48 42 23 64 1.54 8.4 0.48 58 112 0.34 0.37 0.12 5.63 13.9 2.12 9.90 3.21 1.30 3.49 0.63 4.28 0.89 2.61 0.38 2.40 0.35 1.09 1.13 1.17

4.00 0.025 233 85 34 25 105 2.4 17 0.95 68 287 0.63 0.83 0.23 10.7 24.9 3.59 16.2 4.64 1.70 4.72 0.80 4.82 0.95 2.82 0.38 2.47 0.36 0.92 1.49 1.09

4.30 0.052 278 87 43 27 114 2.59 20 1.07 59 152 0.61 0.74 0.21 12.0 28.2 3.99 17.9 5.10 1.90 5.32 0.88 5.46 1.09 3.07 0.43 2.68 0.39 1.00 1.52 1.10

2.61 0.025 239 66 42 27 106 2.41 16 0.81 55 116 0.54 0.50 0.15 9.73 23.4 3.43 15.9 4.69 1.79 4.82 0.83 5.35 1.05 3.07 0.44 2.85 0.41 0.96 1.34 1.13

9.10 0.098 324 139 43 33 160 3.6 30 1.64 59 102 1.07 1.35 0.36 18.4 42.4 5.93 26.0 6.94 2.45 6.86 1.10 6.57 1.31 3.82 0.52 3.37 0.47 0.80 1.71 1.07

Trace element concentrations were measured with a quadrupole ICP-MS PQ-2 (VG Elemental) using rhodium as an internal standard and are given in ppm. The precision for most trace elements is better than 5% (2r), for Cs, Ta, Pb, Th and U better than 10% (2r). Trace element ratios are normalized to the primitive mantle (McDonough and Sun, 1995). a Eu/Eu* is calculated from the interpolated Eu* defined by the chondrite-normalized abundances of Sm and Gd (Anders and Grevesse, 1989).

Lithium isotopes in the North Atlantic mantle

927

1000

Rock/Primitive mantle

Jan Mayen

100

10 Enriched group

1 Depleted group

0.1 Cs Rb Ba Th U Nb Ta La Ce Pb Pr Sr Nd Sm Zr Hf Eu Gd Tb Dy Y Li Ho Er Tm Yb Lu

Fig. 2. Primitive mantle-normalized trace elements from Iceland and Jan Mayen. Trace element patterns for basaltic glasses from MMF system can be divided into the Enriched Group (open) and Depleted Group (black). Both lava groups are depleted in Li and Pb and enriched in Ba, Nb and Ta compared to similarly compatible elements. Depleted Group is also enriched in Sr and Zr- and Hf-depleted relative to neighbouring elements. A similar trace element pattern has been observed independently of our study in the Hengill picrite HEN5 and ascribed to recycled oceanic crust as one constituent for Icelandic plume (Chauvel and He´mond, 2000). Also, Kokfelt et al. (2006) have analyzed number of rocks from the south-western rift zone and Reykjanes Peninsula, observing pattern identical with that plotted here for depleted lavas. A small positive Eu anomaly has been detected for both lava groups. Trace element pattern of Jan Mayen ankaramites is shown in grey. It closely resembles that of Jan Mayen lavas (Trønnes et al., 1999). Note Li depletion and variable Pb depletion/enrichment relative to neighbouring elements. Enriched character of the lavas is documented by steepness of the primitive mantle-normalized pattern with high LaN/SmN values >3.2. Normalizing values are taken from McDonough and Sun (1995).

conducted so far. The lower limit of d7Li values (3.7&) of the MMF suite appears to provide a plausible composition for the Icelandic mantle that is broadly consistent with the inferred d7Li of the upper mantle at <4&, derived from the weighted global average of modally unmetasomatized peridotites at 3.4& (Elliott et al., 2006) and from global MORB (Tomascak et al., 2008), and indiscernible from basaltic volcanic rocks from Hawaii (Tomascak et al., 1999b; Chan and Frey, 2003). This observation is of vital importance for constraining the global mantle Li isotope composition and implies broadly constant d7Li of the mantle regardless of incompatible trace element enrichment/ depletion. Considering high 3He/4He in OIB as a mixture between noble-gas-rich mantle and He from the depleted mantle (Ellam and Stuart, 2004), no Li isotope variability should be discernible given the large-scale homogeneity of Li isotopes in mantle rocks. 5.1. Significance of the Li–He isotope correlation in Iceland lavas – evidence for ancient recycling? The high 3He/4He of Iceland plume-derived basalts are commonly considered to reflect a mix of partially-degassed, and therefore 3He-rich, mantle and depleted upper mantle (Ellam and Stuart, 2004). The Icelandic basalts display an inverse and quasi-linear correlation between d7Li and 3 He/4He (Fig. 3a) where low d7Li values (4&), typical of primitive or MORB-source mantle, are associated with high 3He/4He (17 RA), and high d7Li values, common in

altered oceanic crust, are found for low-3He/4He lavas. This relationship is not clearly reflected in other trace element and radiogenic isotope ratios, although positive He–Os correlations have also been found for neovolcanic picritic lavas from Iceland. The He–Os data have been explained as a result of mixing between ancient recycled crust and primitive or incompletely degassed depleted mantle (Brandon et al., 2007). In contrast, Sr, Nd and O isotope compositions are relatively uniform for all of our samples, providing strong evidence that the source of all of these Iceland magmas is approximately similar. It has been documented that at high temperatures hydrous fluids may leach Li from the crust (Seyfried et al., 1984; Berger et al., 1988; Chan et al., 2002) whereas at lower temperatures smectite may form that can host significant amounts of Li (Vigier et al., 2008). The uppermost altered oceanic crust is therefore enriched in Li as a consequence of the deposition of Li leached from the lower parts of the crust. Fractionation into newly formed clay minerals yields d7Li up to 14& (Chan et al., 1992; Vigier et al., 2008). The deep mantle may carry high 3He/4He 50 RA (Stuart et al., 2003; Starkey et al., 2009) whereas crustal rocks are dominated by radiogenic 4He. Therefore, the simplest explanation for the observed Li–He isotope co-variation is that it represents mixing between a high-3He/4He mantle source (d7Li 3.7&; 3He/4He > 20 RA) and altered Icelandic crust (d7Li P 7.0&; 3He/4He < 12 RA). However, four lines of evidence suggest that this model is too simplistic.

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depleted lavas enriched lavas Jan Mayen

8.0

δ7Li (‰)

a 6.0 Mauna Kea

4.0 MORB

2.0

presumed Loihi source

7.0

b peridotites

δ18O (‰)

6.0

MORB OIB

5.0 4.0 3.0

hydrothermally altered oceanic crustal gabbros

125

c

Nb/U

100

75

50

MORB

OIB

25

5

10

15 3He/4He

20

25

(R/RA)

Fig. 3. Plot of He isotope variations vs. Li isotopes (a), O isotopes (b) and Nb/U (c), respectively. The near-linear correlation between Li and He isotopes is assumed to reflect mixing of at least three reservoirs (see text for further discussion). MORB Li range is taken from Tomascak et al. (2008), He range is from Graham (2003). Lithium isotope data for Mauna Kea and presumed Loihi source are from Chan and Frey (2003), He isotope data are from Kurz et al. (2004). In (b), broadly negative correlation of He and O isotopes may be observed, in particular obvious for Enriched group lavas and two high-3He/4He lavas from depleted group (see text for further discussion). Ranges in O isotope compositions in individual components are taken from Eiler (2001) and references therein. In (c), He isotopes are plotted against Nb/U, which is generally homogeneous in MORB and OIB reservoirs (Hofmann, 1997). Hence, high Nb/U is most likely inherited from depletion in U and residual enrichment in Nb during alteration of ocean floor and subsequent subduction. The Jan Mayen lavas lie away from the Icelandic lavas because their Nb/U ratios fall close to average value of 50 inferred for oceanic basalt suites (Hofmann, 1997). Black squares – Enriched group; Open diamonds – Depleted group; grey circles – Jan Mayen lavas.

(i) The d18O of all samples is lower than in most oceanic basalts. The depletion in 18O transcends the trace element enrichment/depletion and is common to the whole suite. Assimilation of meteoric fluids or material altered by meteoric fluids (Burnard and Harrison, 2005) is not considered

to be a likely explanation for the ubiquitous low-18O Icelandic volcanism. Instead, Kokfelt et al. (2006) have suggested that d18O reflects ancient hydrothermal alteration and that low d18O would require unrealistic degrees of assimilation and fractionation (AFC) when local altered Icelandic crust would be considered, consistent with independent models (Stracke et al., 2003a). It is more plausible that large-scale hydrothermal alteration with meteoric fluids provided source rocks with low d18O (Gautason and Muehlenbachs, 1998) that were formed in, for example, Palaeozoic ophiolites (Muehlenbachs et al., 2004) and these were subsequently recycled. (ii) Trace elements provide evidence for recycling and feldspar accumulation, even though none of the basalts analyzed contain feldspar as a phenocryst phase. Positive Eu anomalies have been found in all analyzed samples (Table 2) and correlate positively with Sr/Nd and Ba/Th (Fig. 5), consistent with a plagioclase accumulation signature. The Ce/Pb ratios scatter and show no correlation with Sr/Nd (cf. Chauvel and He´mond, 2000). Recycled gabbroic cumulates that have been converted to eclogite or pyroxenite could carry the positive Eu anomalies and high Sr/ Nd and Ba/Th imparted from an earlier cumulate history (Hofmann and Jochum, 1996). Ocean floor gabbros appear to have d7Li higher than unaltered MORB while maintaining Li contents that are lower than unaltered MORB (Chan et al., 2002). The samples that characterize the low-3He/4He high-d7Li end-member (DICE 5, DICE 7 and MAE) are rich in feldspathic cumulate xenoliths; petrologic studies show that all samples ascended rapidly through the Icelandic crust (Trønnes, 1990). The high Sr/Nd and Eu anomalies could, in principle, relate to interaction with feldspathic crust. (iii) High Nb/U (Fig. 3c) and negative Pb and Li anomalies (Fig. 2) are typical of all samples. High Nb/U in particular implicates U-loss during a prior subduction history (Chauvel et al., 1992) because partial melting does not fractionate Nb from U significantly. The exact age of this process is not well constrained by this study although Porcelli and Elliott (2008) propose an Archaean age for the Iceland plume. Niobium is effectively retained in the subducted residue (e.g., Rudnick et al., 2000) whereas U is removed into the mantle underlying arc and back-arc lavas (Elliott et al., 1997). Elevated d7Li values are indeed consistent with a prior modification through hydrothermal alteration and in particular, Li depletions (Fig. 2) hint to a former extraction of Li-rich arc magmas at convergent boundaries. Also, Fitton et al. (1997) suggest that particular Nb enrichment of Icelandic basalts rules out their origin in recent MORBsource mantle and instead, ancient recycling of subducted crust through a mantle plume has been invoked. Therefore, the Nb/U higher than in typical mantle (Hofmann, 1997) is characteristic of the enriched-mantle source component and does not result from secondary processes such as U mobilization by seawater or meteoric water and/or assimilation– fractional crystallization processes in the Icelandic crust (Stracke et al., 2003a,b). (iv) Simple mixing between altered Icelandic crust and deep mantle would require [He]/[Li] of the end-members to be broadly similar to maintain the quasi-linear relationship

Lithium isotopes in the North Atlantic mantle

929

Table 3 Trace element concentrations and important elemental parameters in Jan Mayen lavas.

Rb Cs Sr Ba Y Zr Hf Nb Ta Co Ni Pb Th U La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu (Sr/Nd)N (La/Sm)N Eu/Eu*a

JM 14

JM 44

JM 78

JM 207

JM 208

JM 16837

JM 16841

JM 16852

51 0.48 673 828 26 218 4.7 79 4.7 50 712 12 5.9 1.4 52 101 12 45 8.1 2.6 8.0 1.0 5.2 0.96 2.7 0.36 2.2 0.32 0.96 4.13 0.97

50 0.44 658 823 26 222 4.7 79 4.8 54 623 5.0 5.8 1.3 51 100 12 45 8.1 2.7 7.6 1.1 5.4 0.98 2.6 0.35 2.3 0.32 0.94 4.10 1.03

32 0.27 479 537 19 140 3.2 54 3.2 52 274 2.8 3.6 0.87 36 70 8.3 31 5.9 1.9 5.6 0.74 3.8 0.71 1.8 0.24 1.5 0.23 0.98 3.94 0.98

39 0.30 523 667 21 181 3.9 66 4.0 55 772 6.5 5.0 1.2 44 84 9.9 37 6.7 2.2 6.6 0.85 4.2 0.81 2.2 0.29 1.8 0.27 0.91 4.27 0.99

40 0.38 531 669 21 180 3.8 66 3.9 55 742 6.4 5.1 1.1 44 84 9.9 37 6.5 2.1 6.6 0.84 4.3 0.82 2.1 0.29 1.9 0.26 0.91 4.35 0.97

38 0.35 501 620 20 169 3.6 61 3.6 56 733 3.3 4.6 1.0 41 78 9.2 35 6.4 2.0 6.1 0.83 4.0 0.76 2.0 0.28 1.7 0.26 0.92 4.10 0.98

19 0.14 378 438 16 124 2.9 42 2.5 59 621 2.7 2.5 0.70 28 56 6.8 27 5.3 1.6 5.0 0.70 3.5 0.65 1.6 0.23 1.4 0.21 0.90 3.44 0.95

21 0.18 304 346 13 91 2.3 32 2.0 59 344 1.8 1.4 0.52 22 43 5.3 21 4.3 1.3 4.1 0.57 2.9 0.53 1.3 0.19 1.2 0.18 0.92 3.25 0.92

Trace element concentrations were measured with a quadrupole ICP-MS PQ-2 (VG Elemental) using rhodium as an internal standard and are given in ppm. The precision for most trace elements is better than 5% (2r), for Cs, Ta, Pb, Th and U better than 10% (2r). Trace element ratios are normalized to the primitive mantle (McDonough and Sun, 1995). a Eu/Eu* is calculated from the interpolated Eu* defined by the chondrite-normalized abundances of Sm and Gd (Anders and Grevesse, 1989).

found in Fig. 3. Local Icelandic crust has been excluded as an end-member (Kokfelt et al., 2006; Thirlwall et al., 2006; Debaille et al., 2009). This is in accordance with up to one order higher Li contents in more evolved rocks than those found in samples from this study, yet maintaining similar range in d7Li (Schuessler et al., 2009). Burnard and Harrison (2005) inferred mixing between depleted MORB-source mantle (DMM) and a hypothetical Icelandic mantle (ICE) where [He]DMM P 10[He]ICE. This mixing itself would entail a 10-fold difference in Li concentrations of the two respective mantle reservoirs that is unlikely given that the bulk distribution coefficient for Li in mantle melting is probably not far from unity. 5.1.1. A role of diffusive isotope fractionation? Recent work has highlighted the relative importance of diffusive fractionation of Li in mantle minerals and melts at limited spatial scales (Richter et al., 2003; Lundstrom et al., 2005; Jeffcoate et al., 2007; Rudnick and Ionov, 2007), triggered dominantly by large concentration gradients of tens to hundreds of ppm (Teng et al., 2006). This

is rather unrealistic for basalts having low Li contents, however. Theoretical considerations also predict a limited diffusive He isotope fractionation by assuming faster diffusion of 3He and, more specifically, by a positive correlation between [4He] and 3He/4He (Burnard et al., 2004; Harrison et al., 2004). This would likely result in a positive Li–He isotope correlation. Such a trend is not observed (Fig. 3) and it appears unlikely that diffusion of Li (and He) isotopes has generated the apparent correlation. 5.1.2. A role of residual light-Li mantle? Alternatively, the Li–He isotope correlation could result from binary mixing between altered crust of whatever origin and eclogites. The latter have been shown to carry a light Li signature under certain circumstances (Zack et al., 2003) and in general, they average d7Li < 0& (Marschall et al., 2007b). Provided that the recorded Li– He correlation were linear, extrapolating the highest 3 He/4He 37 RA and 43 RA measured in Iceland (Hilton et al., 1999; Harlou et al., 2004), would correspond to d7Li of 3& and 5&, respectively. Lithium isotope data from

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T. Magna et al. / Geochimica et Cosmochimica Acta 75 (2011) 922–936 10.0

1.3

depleted lavas

a

enriched lavas Jan Mayen

8.0

1.2

literature

Eu/Eu*

δ7Li (‰)

HIMU EM2

6.0

4.0

Enriched group Depleted group

1.1

1.0

EM1 PM

0.9

DM

Jan Mayen

2.0

Depleted Group

0.8

Enriched Group Jan Mayen

0.0 0

2

4

6

8

b

10 3.0

Fig. 4. Lithium systematics in Iceland and Jan Mayen lavas, in relation to major mantle reservoirs (PM, DM, HIMU, EM1 and EM2). The EM1 d7Li value is from Simons et al. (2008) whereas Li abundance has been estimated at 6 ppm, similar to EM2 lavas (Chan et al., 2009). Yet, the EM2 d7Li > 11& seems overestimated (Simons et al., 2008). Slightly elevated Li contents and d7Li have consistently been reported for lavas with HIMU affinity (Ryan and Kyle, 2004; Nishio et al., 2005; Chan et al., 2009). Literature data for Iceland (reversed triangles) are from Pistiner and Henderson (2003), Ryan and Kyle (2004), Jeffcoate et al. (2007) and Schuessler et al. (2009). Other lavas than basaltic were omitted.

MMF lavas provide further insights into such a hypothesis as a consequence of the evident presence of recycled oceanic crust in the source of MMF lavas, based on trace element patterns as well as Li and O isotopes. The absence of very light or even negative d7Li signature is inconsistent with negative-d7Li residual oceanic crust in the mantle. This finding provides further evidence that the hypothesis of dehydrated oceanic crust with low or even negative d7Li in the mantle (Zack et al., 2003) cannot be applied in general (Marschall et al., 2007b), although some recent data may imply low d7Li in opportune subduction settings (Agostini et al., 2008). Furthermore, the loss of mantle He during subduction of oceanic crust, and the subsequent ingrowth of radiogenic He from U (and Th) decay will, in all likelihood, leave eclogites with extremely low 3He/4He making them unlikely candidates for the high-3He/4He (and also low-d7Li) reservoir in the Earth, unless U is drastically more incompatible than He (cf. Parman et al., 2005) and subduction creates a high He/U. More importantly, the fact that the lowest d7Li found in the MMF samples is the same as ‘normal’ mantle renders as extremely unlikely the notion that this composition is a fortuitous mixture of the exactly balanced light and heavy Li. 5.2. Isotope considerations of Li–He correlation Little is known about Li isotope fractionation by hydrothermal fluids (Chan et al., 2002; Foustoukos et al., 2004). Few analyses of ocean floor gabbros provide preliminary evidence that d7Li does not change significantly or slightly increases during alteration, while maintaining low Li contents (Chan et al., 2002). Seafloor alteration at low

(Ba/Th)N

Li (ppm)

2.0

1.0

0.0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

(Sr/Nd)N Fig. 5. The plot of Eu/Eu* and (Ba/Th)N vs. (Sr/Nd)N. Enriched group lavas have lower Sr/Nd ratios than depleted group lavas. Within each group a steep positive trend can be seen. The largely distinct Sr/Nd ratios require different sources. (Ba/Rb)N vs. (Sr/Nd)N plot (not shown) has very similar pattern as (Ba/Th)N vs. (Sr/Nd)N plot. Jan Mayen lavas mostly lie on the intercept of mantle values for (Sr/Nd)N and Eu/Eu* but two samples (16841, 16852) show remarkable Th depletion relative to other Jan Mayen lavas which results in their substantially higher (Ba/Th)N ratios.

and elevated temperatures generally fractionates 7Li into secondary minerals like smectites (Chan et al., 1992; Vigier et al., 2008). Strongly leached gabbroic xenoliths have Li abundances <1 ppm as a consequence of hydrothermallyconditioned mobility of Li (Seyfried et al., 1998) and also low d18O < 4& (Cocker et al., 1982). This is in the opposite sense to fresh gabbros with 3 ppm Li and usually high d18O 6& (e.g., Bach et al., 2001). From this we can envision that a few tens of percent of leached gabbros would be difficult to detect with Li isotopes but may produce a detectable shift in d18O. The lower oceanic crust is also depleted in Th and U, which may be due to elemental loss during subduction (see discussion in Section 5.1) or cumulate character of such a crust. Mixing this crust into undegassed, deep mantle will lower the 3He/4He with time by ingrowth of 4He and could produce a mantle source with 3 He/4He = 20 RA and ‘normal mantle’ d7Li. The low d18O values (4.1&) require several tens percent of low-d18O gabbros admixed to the source of MMF lavas. However, this does not seem to be the signature of modern oceanic crust. Low-d18O rocks are produced by seawater alteration of deep oceanic crust (gabbros) or large-scale hydrothermal alteration with meteoric fluids (Gautason and Muehlenbachs, 1998). Thus, the low d18O

Lithium isotopes in the North Atlantic mantle

infers recycled crustal material in the source of all MMF lavas. The Li–He isotope trend observed for the MMF glasses (Fig. 6) seems to provide evidence for a two-component mixing process. This may be a scaling problem due to sampling only local area of the Icelandic plume. In fact, many studies have shown that at least three components are required to account for observed trace element patterns and isotope variations (e.g., Hemond et al., 1993; Hanan and Schilling, 1997; Kempton et al., 2000; Thirlwall et al., 2004, 2006; Brandon et al., 2007; Kitagawa et al., 2008; Peate et al., 2010). Stuart et al. (2003) have shown that 5% of 3He-rich plume material derived from proto-Icelandic plume with 3He/4He 50 RA is sufficient to produce 3He/4He ratios as high as 20 RA by mixing with DMM having 3He/4He 8 RA. There is evidence that the DMM has d7Li 3.4& (Elliott et al., 2006; Tomascak et al., 2008) and the same will likely be true for the Icelandic mantle plume (cf. Hawaiian plume with the lowest d7Li at 3.5&, save for two anomalously light samples; Chan and Frey, 2003). Therefore, we may reasonably postulate that both mantle end-members have similar d7Li 3.5& identical to that postulated for the average terrestrial mantle. It can also be inferred that a limited range in Li isotope compositions of DM and PM will not have a detectable effect on the Icelandic enriched mantle (ICEM). Given the trace-element-enriched character of plume materials, the plume-dominated materials control the trace element inventory of the Enriched group lavas except for Li which has similar abundance in DMM and PM.

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The depleted Icelandic group lavas derived probably from a mantle segment that was more strongly affected by an oceanic crust component than the source of the enriched group lavas although the absolute magnitudes of Li depletion are similar (Fig. 2). Additional evidence for a greater proportion of crustal addition comes from elevated Sr/ Nd, Ba/Th and larger Eu excess in depleted group lavas (Fig. 5). Higher U (and Th) contents would have generated more 4He over time and result in lower 3He/4He. Nonetheless, the Li/He elemental ratios of all sampled mantle segments are similar as required by the inverse and quasilinear correlation between d7Li and 3He/4He. Therefore, the Icelandic depleted mantle (IDM) end member with 3 He/4He < 12 RA and d7Li  8& is generated by mixing of recycled crustal material (3He/4He < 4 RA; d7Li > 7&) with “normal” depleted mantle. This is consistent with modelled He systematics of Debaille et al. (2009) whereas Li data for recycled crust remain less constrained. 5.3. Implications for the origin of Jan Mayen basalts from Li isotopes The high incompatible trace element concentrations in Jan Mayen basalts (Table 3) combined with radiogenic Sr and strongly unradiogenic Nd and Hf (Blichert-Toft et al., 2005; Debaille et al., 2009) provide evidence for a particularly enriched mantle beneath Jan Mayen. Although the d7Li values for the enriched group of MMF basalts (3.9– 5.9&) overlap values recorded in the Jan Mayen ankaramites (3.9–4.7&), several lines of evidence are inconsistent with Jan Mayen and Iceland having the same source. First,

10.0 depleted lavas enriched lavas

Oceanic Crust

9.0 8.0

IDM

30

δ7Li (‰)

20

7.0

10 10

20

6.0

40

5

5.0

70

2

ICEM

1

4.0 3.0

Depleted Mantle

1

2

5

10

20

Primitive Mantle

2.0 0

10

20 3He/ 4He

30

40

50

(R/RA)

Fig. 6. The Li–He isotope mixing plot. The Li isotope compositions of primitive mantle (PM) and depleted mantle (DM) are the same whereas their He isotope compositions differ markedly. Not more than 5% of PM component is required for the Icelandic enriched mantle (ICEM) to develop high 3He/4He ratios whereas this amount of mixing has no effect on resulting d7Li. Ancient hydrothermally altered oceanic crust with marked Li depletion and elevated d7Li represents the oceanic crust member (OC) mixing of which with DM end member results in the generation of Icelandic depleted mantle (IDM). Certainly, modest variability in proportions of DM and OC is allowed without significantly affecting the ultimate IDM composition and, by inference, the IDM–ICEM mixing relationship.

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the projection of Icelandic mantle plume material to the north of Iceland is spatially restricted to several hundreds of kilometres northwards (Thirlwall et al., 2004). Second, Sr–Nd isotope trends in Iceland do not approach those in Jan Mayen (Mertz et al., 1991; Debaille et al., 2009). Third, He isotopes in Jan Mayen lavas (<7 RA) show no sign of the high values typical of the Iceland plume. It has been speculated that the source region of Jan Mayen basalts contains a HIMU-like component (Thirlwall, 1997; Trønnes et al., 1999). High d7Li values (d7Li > 5&) have been reported for HIMU basalts (Ryan and Kyle, 2004; Nishio et al., 2005; Chan et al., 2009; Vlaste´lic et al., 2009), generated by partial melting of recycled oceanic crust that has been hydrothermally depleted in Pb (Chauvel et al., 1992; Hofmann, 1997; Willbold and Stracke, 2006). However, this is not reflected in Li isotope systematics; the Jan Mayen ankaramites display also variable Pb depletions/ enrichments and marked Th–U depletions (Fig. 2). Both the trace element pattern of Jan Mayen ankaramites and trace element ratios (e.g., Rb/Sr, Th/U, Ce/Pb, U/Pb, Ba/ La and Rb/La) more closely resemble those of the EM1/ EM2 component (Workman et al., 2004; Willbold and Stracke, 2006). Detailed Pb isotope study of basalts from the Jan Mayen platform lead Blichert-Toft et al. (2005) to propose an origin in mixing of the depleted mantle and mantle with an EM1-like signature. The Li isotope composition of the latter component is not well constrained but available Li data for basalts derived from non-HIMU enriched mantle largely precludes exotic compositions (Fig. 4 and Nishio et al., 2004, 2005; Chan et al., 2009). An origin in recycled metasomatized oceanic lithosphere has been proposed for the EM2 end-member (Workman et al., 2004); however, an involvement of terrigenous sediments resembling the upper continental crust and mixed with recycled oceanic lithosphere is required by trace elements and radiogenic isotope compositions (Willbold and Stracke, 2006; Jackson et al., 2007). The upper continental crust has a rather narrow average of d7Li (0 ± 2&) and high average Li content of 35 ppm (Teng et al., 2004). This limits the maximum volume of material from the upper continental crust to <5%, consistent with previous findings (Jackson et al., 2007). Jan Mayen basalts may thus represent a mixture of normal depleted mantle with an enriched mantle component, reminiscent of the model of Blichert-Toft et al. (2005); in this case the d7Li would be dominated by a Li contribution from the depleted mantle (Fig. 4). While ancient sub-continental lithospheric mantle (SCLM) modified by metasomatic enrichments following after an earlier episode of melt depletion has been proposed as a reasonable explanation for radiogenic isotope signatures in Jan Mayen (Debaille et al., 2009), there is insufficient data to better characterize the Li isotope composition of SCLM (Magna et al., 2006a; Jeffcoate et al., 2007; Ionov and Seitz, 2008) which would allow testing hypotheses of Debaille et al. (2009). 6. CONCLUSIONS We have presented Li isotope data for two suites of basaltic lavas from the North Atlantic. Lithium isotope compositions of lavas from the MMF system (Hengill,

Iceland) show an inverse and near-linear correlation with He isotopes. The high-3He/4He lavas show a rather narrow range of d7Li averaging 4.3& whereas low-3He/4He lavas possess isotopically heavier Li. The simple interpretation of the observed relationship as being a mixture between mantle and local altered Icelandic crust is rendered unlikely. Instead, recycled gabbroic oceanic crust is required in the source of MMF lavas from coupled Li isotopes and trace element systematics. Although exact qualification and mixing chronology of these end members is not straightforward (Stracke et al., 2003b), the observed ubiquitous Eu enrichment hints to an old recycled component, efficiently mixed into the source of depleted lavas. Data from this study do not support diffusive fractionation of Li and He isotopes, nor do they provide evidence of contamination by silicic volcanic rocks. The Jan Mayen ankaramitic lavas are homogeneous in terms of Li isotopes and trace elements that show remarkable enrichments in LREE. A possible HIMU origin of the enriched component of Jan Mayen lavas is not easily reconciled with the data and we suggest these lavas may include an EM1/EM2-like component instead. Lithium isotope compositions of North Atlantic lavas from this study imply homogeneous pristine mantle d7Li of 3.5& which is broadly constant and independent of chemical enrichment/depletion and evidence of recycling. Further, it largely prohibits the existence of large mantle domains with a significantly lighter Li isotope signature that could result from subduction-associated processes. This is corroborated by Jan Mayen basalts that show a remarkable homogeneity of d7Li despite distinctively different trace element pattern and He isotope compositions of the basalts. Therefore, Li seems relatively homogeneous, at the level of <±2&, over a large range of 3 He/4He. This may be explained by the buffering effect of a dominant mantle component to a great extent even after entraining material with different trace element patterns and isotope ratios. The fact that He isotopes vary greatly in North Atlantic lavas all displaying the same mantle-like Li isotope composition, 18O-depletion and variable trace element enrichments reflecting recycled cumulates is consistent with recycled lower oceanic crust with variable (U + Th)/He in the source region. The fact that Li isotopes are correlated quasi-linearly with He isotopes in Iceland provides evidence of a recycled altered oceanic crustal component with different (U + Th)/He ratio but the same He/Li. ACKNOWLEDGMENTS We thank Marie-Theres Ba¨r, Heiri Baur, Donat Niederer, Bruno Ru¨tsche, Urs Menet and Andreas Su¨sli for the electronics and mechanical workshop services, Irene Ivanov-Bucher for help with sample preparation, Marcus Gutjahr and Martin Frank for assistance by Sr–Nd isotope analyses and Felix Oberli for maintenance and troubleshooting of Nu Plasma 1700. We are grateful to Harry Becker, Mark Rehka¨mper, Andreas Stracke, Paul Tomascak and Helen Williams for comments and Francis Albare`de, Janne Blichert-Toft, Karsten Haase and Kaj Hoernle for helpful information. Insightful journal reviews by Paul Tomascak and one anonymous reviewer as well as thorough editorial comments by Frederick Frey

Lithium isotopes in the North Atlantic mantle have clarified the manuscript significantly. ETH and Swiss National Fonds (SNF) supported this work.

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