Rare gases in lavas from the ultraslow spreading Lena Trough, Arctic Ocean


  • F. Nauret,

    1. Equipe de Géochimie et Cosmochimie, Institut de Physique du Globe de Paris, UMR 7154, Université Paris Diderot, CNRS, Paris, France
    2. Max-Planck-Institut für Chemie, Mainz, Germany
    3. Now at Laboratoire Magmas et Volcans, Clermont Université, Université Blaise Pascal, CNRS, IRD, BP 10448, F-63000 Clermont-Ferrand, France.
    Search for more papers by this author
  • M. Moreira,

    1. Equipe de Géochimie et Cosmochimie, Institut de Physique du Globe de Paris, UMR 7154, Université Paris Diderot, CNRS, Paris, France
    Search for more papers by this author
  • J. E. Snow

    1. Max-Planck-Institut für Chemie, Mainz, Germany
    2. Now at Department of Earth and Atmospheric Sciences, University of Houston, Houston, Texas 77204-5007, USA.
    Search for more papers by this author


[1] Mid-ocean ridge basalts (MORB) from the Arctic Ocean have been much less studied than those from the Indian, Atlantic, and Pacific due to the difficulty of access related to ice cover. In 2001 and 2004 the Arctic ridges (Gakkel Ridge and Lena Trough) were intensively sampled. In this study we present the first helium, neon, and argon concentrations and isotopic ratios in a suite of samples from the ultraslow spreading Lena Trough (∼0.75 cm/yr effective full rate). Central Lena Trough (CLT) lavas display 4He/3He between 89,710 and 97,530 (R/Ra between 7.4 and 8.1), similar to the mean MORB ratio of 90,000 ± 10,000 (R/Ra = 8 ± 1). In a three neon isotope diagram, the samples fall on the MORB line, without showing any excess of nucleogenic 21Ne. The 40Ar/36Ar ratios vary from 349 to 6964. CLT samples have a typical MORB He and Ne isotopic composition. Rare gases do not indicate any mantle heterogeneities or contribution of subcontinental lithospheric mantle, although this has been suggested previously on the basis of the Sr-Nd and Pb isotopic systems. Based on noble gas systematics, a DUPAL-like anomaly is not observed in the Arctic Ocean. We propose two possible models which reconcile the rare gases with these previous studies. The first is that the Lena Trough mantle has a marble cake structure with small-scale heterogeneities (<1 km), allowing rapid diffusion and homogenization of rare gases compared to elements such as Sr, Nd, and Pb. The second model proposes that the recycled component identified by other isotopic systems was fully degassed at a recent date. It would therefore have a negligible mass budget of rare gases compared to other isotopic systems. This would suggest that the mantle enrichment beneath Lena Trough was generated by rift-forming processes and not by recycling.

1. Introduction

[2] Lena Trough is part of the obliquely spreading segment of the North American–Eurasian plate boundary that connects the Knipovitch and Gakkel Ridges (Figure 1). The rifting of Lena Trough began in the Miocene (∼10 Ma) [Crane et al., 2001], and is the final and most recent event in the separation of the North American and Eurasian continent. The spreading rate between the two plates in this region is one of the slowest in the world (13 mm full rate, ∼7.5 mm/yr effective rate accounting for its 55° obliquity). Lena Trough is considered to be a young nonvolcanic continental rift and details about the geological setting are given elsewhere [Hellebrand and Snow, 2003]. Based on Os isotope data obtained from fresh peridotites, the Arctic mantle is depicted as being heterogeneous, composed of refractory and fertile veins in a peridotite matrix [Liu et al., 2008].

Figure 1.

Geographical map of the Arctic and North Atlantic oceans where Gakkel Ridge and Svalbard archipelago are reported. The Lena Trough is located in the white square, between Greenland and Svalbard archipelago.

[3] During the cruise ARK-XX/2 of the PFS Polarstern (Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany) undertaken in the summer of 2004, rare basalts were dredged from the central part of Lena Trough. These basalts (CLT) have been characterized in terms of major-trace elements and Sr-Nd-Pb-Hf isotope systematics (F. Nauret et al., Geochemical composition of Lena Trough lavas (Arctic Ocean), submitted to Journal of Petrology, 2010). A Southern Hemisphere DUPAL-like signature is recognized, e.g., radiogenic strontium for low 206Pb/204Pb isotopic ratios [Dupré and Allègre, 1983; Hart, 1984]. Consequently, Lena Trough lavas share similar, but much more extreme, geochemical characteristics with lavas from the westernmost Gakkel Ridge. The latter have been interpreted as reflecting the contribution of ancient Spitzbergen subcontinental lithospheric mantle [Goldstein et al., 2008], which is thought to account for the DUPAL-like nature of many Gakkel Ridge basalts. However, this interpretation seems to be in contradiction with recent Os isotope data obtained from mantle peridotites from Lena Trough, where the contamination by an ancient subcontinental lithospheric mantle (SCLM) component is not observed [Lassiter and Snow, 2009]. In order to test for the presence of veins (fertile or refractory) in a mantle source derived from a SCLM or from a deeply recycled component, we have analyzed the noble gas compositions of Lena Trough lavas to constrain the origin of the Southern Hemisphere DUPAL-like geochemical anomaly observed in the Arctic Ocean.

[4] In terms of noble gases, contribution from a SCLM (i.e., R/Ra = 6.1 ± 0.9 [Gautheron, 2002]) should provide more radiogenic 4He/3He isotope ratios for Lena Trough samples compared to samples derived from a mid-ocean ridge basalt (MORB) source (R/Ra = 8 ± 1) [Day et al., 2005; Gautheron, 2002]. Similarly, any continental crustal influence or recycled component, such as old oceanic crust ± sediment, might also generate high 4He/3He isotope ratios and thus R/Ra < 8 ± 1 [Ballentine and Burnard, 2002; Hanyu and Kaneoka, 1997].

2. Sample Descriptions and Analytical Procedures

[5] We measured helium, neon, argon, krypton and xenon concentrations and the He-Ne-Ar isotopic ratios for seven samples from Lena Trough. Samples were glassy rims of basaltic samples from dredges 261 and 262 during the cruise ARK-XX/2 of the PFS Polarstern. These lavas have been collected in a geographically restricted area in the central part of Lena Trough (dredge 261: −2.525°E longitude, 80.924°N latitude, depth 3570 m and dredge 262: −2.507°E, 80.907°N latitude, depth 3545 m) and sample 217-1 at the transition zone between Lena Trough and Gakkel Ridge (−6.147°E longitude, 82.857°N latitude). These samples are highly vesicular (up to 25%) despite water depths of over 3500 m (Figure 2) [Snow et al., 2007]. These samples have also been characterized for major and trace elements, and Sr-Nd-Pb-Hf isotopes (Nauret et al., submitted manuscript, 2010).

Figure 2.

(a) Total 4He/3He (or R/Ra) versus 4He. The helium isotopic ratios are very homogeneous compared to other segments from the mid oceanic ridges [Allègre et al., 1995; Moreira et al., 2008]. (b) Comparison between studied samples and MORB from the North Atlantic Ridge [Macpherson et al., 2005; Schilling et al., 1999].

[6] The analytical procedure is the same as that described by Moreira et al. [1995]. Glassy chips were cleaned in deionized water for 15 min in an ultrasonic bath to remove potential contaminants (like seawater or dust, as examples). Cleaned glassy rock chips (1 to 2 g) were loaded in to a crusher and baked overnight at ∼150°C under vacuum. The samples were crushed in order to extract the gas. Each step of crushing constituted 15 to 30 strokes. The gas extracted from each step of crushing was purified with two hot Ti getters and a SAES getter at room temperature. The rare gases were trapped at a temperature of 10° K on charcoal using a cryogenic system. Helium was separated from neon at 25° K and neon from the other noble gases at 70 K. Ar, Kr and Xe were desorbed together at 320° K. Rare gases were introduced successively into the ARESIBO II glass mass spectrometer. Typical blanks were 5 × 10−9, 4 × 10−13 and 2 × 10−12 ccSTP for 4He, 22Ne and 36Ar, respectively, with atmospheric isotopic compositions.

3. Results

[7] The abundances and isotopic ratios of He, Ne and Ar released by multiple crushing of Lena Trough glasses are given in Tables 1 and 2. Abundances of 84Kr and 130Xe are also reported in Table 1. Samples were step crushed between 1 and 3 times. Total 4He concentrations show a limited range of variation from 1.2 × 10−5 to 3.6 × 10−7 ccSTP/g, similar to MORB concentrations (Table 1). 4He/3He varies from 89,710 to 97,350 (R/Ra between 7.4 and 8.1 where R is the 3He/4He ratio and Ra the atmospheric ratio: 1.384 × 10−6). Helium isotopic ratios are similar to MORB values (90,000 ± 10,000; R/Ra∼8 ± 1) (Figure 2). The mean ratio (93,290: R/Ra = 7.75) is similar to Pacific MORB sampled away from hot spots on a ridge with a high spreading rate (4He/3He = 99,725 ± 2720: R/Ra = 7.3 ± 0.2 [Moreira et al., 2008]). Measured 4He/3He compositions are similar to NMORB reference values and thus slightly more radiogenic compared to MORB from the North Atlantic Ridge (NMAR). So far, MORB from NMAR (Knipovich and Mohns Ridges (72–78°N)) have been described as having high 4He/3He ratios with 3He/4He ratios (R/Ra = 6.1–8.1) lower than 8 ± 1 [Macpherson et al., 2005; Schilling et al., 1999] (Figure 2). However, this comparison with MORB from the Knipovich and Mohns Ridges must be treated with caution because helium isotopes can show very large local variations [Graham, 2002; Hilton et al., 2000]. Thus, while the samples studied were collected within a geographically restricted area, the MORB samples from the Knipovich and Mohns Ridges cover a distance of more than 1000 km [Macpherson et al., 2005; Schilling et al., 1999].

Table 1. Depth, Helium, Neon, and Argon Concentrationsa
SampleDepth (m)Weight (g)4He22Ne36Ar84Kr130Xe40Ar*
  • a

    Concentrations are given in ccSTP/g. Bold values are sums of the amount of gas measured during different steps of crushing.

262-84 step1−35451.263.04E-062.96E-105.66E-099.52E-116.11E-134.59E-07
262-84 step2  3.08E-061.07E-113.31E-101.05E-118.87E-144.31E-07
262-84 step3  8.60E-075.97E-121.29E-103.12E-127.98E-151.45E-07
262-84  6.98E-063.12E-106.12E-091.09E-107.07E-131.04E-06
262-21 step1−35451.17924.57E-061.97E-118.48E-122.30E-131.50E-147.74E-09
262-21 step1  3.06E-061.02E-111.48E-127.20E-145.80E-156.39E-09
262-21  7.63E-062.99E-119.96E-123.02E-132.08E-141.41E-08
261-1 step1−35701.077.97E-065.17E-11    
261-1 step2  4.13E-063.12E-124.18E-11  6.43E-08
261-1  1.21E-055.48E-114.18E-11  6.43E-08
261-1B  4.90E-064.84E-119.29E-101.89E-116.23E-139.94E-08
261-1  1.05E-051.40E-102.69E-095.34E-111.66E-121.94E-07
261-47 step1−35701.06083.52E-064.06E-111.25E-10  1.93E-08
261-47 step1  2.09E-061.02E-111.30E-10  5.31E-08
261-47  5.61E-065.08E-112.55E-10  7.24E-08
262-34 step1−35451.054.70E-062.08E-104.27E-098.90E-113.40E-125.47E-07
262-34 step2  1.97E-064.46E-123.66E-118.90E-132.30E-142.44E-07
262-34 step3  3.06E-072.80E-122.07E-114.60E-13 9.34E-08
262-34  6.98E-062.15E-104.33E-099.04E-113.42E-128.84E-07
261-5 step1−35701.071.05E-053.16E-114.81E-109.92E-122.75E-131.54E-07
261-5 step2  2.75E-064.16E-121.24E-119.92E-122.75E-133.99E-08
261-5  1.32E-053.57E-114.93E-101.98E-115.51E-131.94E-07
261-71 step1−35701.151.00E-058.12E-111.45E-092.94E-119.00E-131.43E-07
261-71 step2  3.95E-065.62E-126.15E-111.24E-126.55E-145.39E-08
261-71  1.40E-058.68E-111.51E-093.06E-119.66E-131.97E-07
217-1 step1−48001.122.75E-78.50E-135.56E-121.60E-131.90E-141.97E-9
217-1 step2  8.05E-82.21E-137.19E-13  2.23E-10
217-1  3.56E-71.07E-126.28E-12  2.21E-9
Table 2. Helium, Neon, and Argon Isotope Ratios
262-84 step16.60.69563014867.
262-84 step27.10.69984516347.22.913.110.680.040.03810.00070.18820.0005160081
262-84 step35.90.69631519067.51.512.010.300.050.03460.00100.18760.0005141789
262-21 step1590318988130438.02.615.710.460.020.03600.00070.18930.000710506
262-21 step14794010048732727.
261-1 step1  9157916577.91.737.19.980.030.03110.0004    
261-1 step264.211.09246915787.814.335.611.410.180.04910.00290.18480.00111832251
261-1B58.9 9059714528.00.722.79.920.020.03030.00050.18820.00053492
261-1B49.3 9377713577.
261-47 step1182109061429458.
261-47 step139.42.18832831128.22.312.510.540.030.03690.00060.18630.00047072
262-34 step18.60.59381614897.70.2
262-34 step28.10.49196026757.94.815.511.040.050.04340.00080.18790.0008696465
262-34 step33.30.29329021417.71.26.510.520.060.03850.00110.18780.0005481246
261-5 step168.03.490992.111459.97.93.639.710.200.030.03370.00040.18820.00046162
261-5 step268.93.588891.221339.58.17.439.610.600.070.03880.00090.19020.0006351437
261-71 step170.3 95469.01551.07.61.323.610.000.020.03110.00060.18770.00043942
261-71 step273.3 918841478.07.97.632.210.800.040.03960.00120.18850.000411727
217-1 step 1139.68.09431422337.663.452.810.100.160.03350.00200.18830.001065110
217-1 step 2339.7409701450597.453.817.110.680.440.03150.00690.18700.004062535

[8] 22Ne concentrations range from 3.1 × 10−10 to 1.1 × 10−12 ccSTP/g. Neon isotopic ratios vary from 10.08 to 11.04 for 20Ne/22Ne and from 0.029 to 0.0434 for 21Ne/22Ne (Table 2 and Figure 3). These ratios increase at the second step of crushing relative to the first one (Table 2). At the third step of crushing, 20Ne/22Ne and 21Ne/22Ne ratios decrease and tend toward atmospheric values but are still higher than values obtained at the first step of crushing (Table 2). Figure 3 shows that the samples fall on the MORB correlation defined by Sarda et al. [1988]. The 36Ar and 40Ar* concentrations (radiogenic argon), 40Ar/36Ar and 38Ar/36Ar isotopic ratios are reported in Tables 1 and 2. The 40Ar* concentrations range over more than 2 orders of magnitude from 6.4 × 10−9 to 5.5 × 10−7 ccSTP/g. The 40Ar/36Ar ratios vary from almost atmospheric (349) to radiogenic ratios of ∼7000, reflecting mixing between air and a mantle component (40Ar/36Ar up to 27,000 [Moreira et al., 1998; Raquin et al., 2008]). 38Ar/36Ar ratios are similar to the atmospheric value (0.1880). 40Ar/36Ar ratios increase with the number of steps of crushing (Table 2). Noble gas abundance ratios are also variable. For example, the 4He/40Ar* ratio varies from 3.28 to 590. Note that the mantle production ratio is ∼1.8, assuming a K/U of 12,700 [Jochum et al., 1983], or ∼1.2 if the MORB source has a K/U of 19,000 [Arevalo et al., 2009]. Total 84Kr concentrations range from 7.20 × 10−14 to 9.52 × 10−11 ccSTP/g. Total 130Xe concentrations vary by 3 orders of magnitude from 3.40 × 10−12 to 5.80 × 10−15 ccSTP/g (Table 1).

Figure 3.

Plot of 20Ne/22Ne versus 21Ne/22Ne. Central Lena Trough lavas plot on the MORB reference line defined by Sarda et al. [1988], suggesting a binary mixing between DMM and air-like components. Note the extreme homogeneity of the samples. For comparison, we report MORB from the MAR (PetDB database), southern MAR [Sarda et al., 2000], and SEIR [Burnard et al., 2004]. We also show lines defined by the North Chile Ridge [Niedermann and Bach, 1998], Manu Basin [Shaw et al., 2001], and DUPAL line estimated from Ne isotope composition of southwest Indian MORB [Gautheron, 2002]. Similarly to helium, no DUPAL-like component can be observed for neon (i.e., with nucleogenic neon).

4. Discussion

4.1. Degassing and Initial Gas Content of CLT Magmas

[9] Most 4He/40Ar* ratios in CLT magmas are higher than reasonable production/accumulation values for a MORB source mantle (between 2 and 4 [Staudacher et al., 1989]). Degassing processes are illustrated in Figure 4. In such a diagram, open system degassing (distillation) is represented by straight lines, with a slope of SAr/SHe – 1, where SAr and SHe are the solubilities in basalt of argon and helium, respectively. Indeed, during open system degassing, the concentration ratio He/Ar can be written: equation image where f = 40Ar/40Ar0 is the fraction of argon remaining [Burnard et al., 2002; Moreira and Sarda, 2000]. SAr/SHe = 0.105 according to Jambon et al. [1986]. Therefore, in the logarithmic scale diagram of Figure 4, distillation processes are straight lines with a slope of ∼–0.9. However, the 40Ar content is sensitive to vesicle loss after eruption and during sample preparation. This could be the case for sample 217-1, although this sample probably also has a different source composition because it was dredged at a different location. If we exclude this sample, the slope of the regression line is ∼–0.99, close to the theoretical value for a distillation process. Therefore, these data could be consistent with a solubility-controlled Rayleigh (fractional) degassing being the dominant fractionating mechanism. However, as mentioned before, such a diagram may not be appropriate in the case of vesicle loss, which certainly occurred in these samples. A complementary diagram is a 3He/22Nec4He/40Ar* diagram, which distinguishes between open system and closed system vesiculation (Figure 5) [Moreira and Sarda, 2000; Sarda et al., 2000]. In this diagram, it appears that only three samples clearly suffered an open system vesiculation process (distillation trend, see Figure 5). The others show closed system vesiculation, although possibly with a kinetic effect in order to explain the high 3He/22Nec ratio [Aubaud et al., 2004]. Therefore, it appears that not all the samples underwent the same degassing process. Some magma vesiculated in a closed system, whereas some vesiculated in an open system. If we consider only the three samples that clearly show a distillation process, it is impossible to derive initial helium or argon concentrations.

Figure 4.

Plot of 4He/40Ar versus 40Ar*. Only totals of the steps of crushing are represented. Data from SEIR [Burnard et al., 2004], MAR [Sarda et al., 2000; Moreira and Sarda, 2000], EPR [Niedermann et al., 1997], and the Rodrigues Triple Junction (RTJ) [Kumagai and Kaneoka, 2005] are reported for comparison. Data show that the samples may have suffered a distillation-type degassing (slope of −0.99). Since there is no depth variation between the different samples, this degassing can be attributed to major volatile abundance variations. Note the 40Ar* abundance of the highly vesicular popping rock 2πD43 from 14°N in the Atlantic. Black dots are samples that clearly show a distillation process based on Figure 5.

Figure 5.

Plot of 3He/22Nec [Raquin and Moreira, 2009] versus 4He/40Ar*. Only samples with 20Ne/22Ne higher than 10.5 are reported. Figure 5 suggests that three samples show a distillation-type degassing (vesiculation in an open system). The other samples indicate a two-step closed system vesiculation loss. Another possible interpretation is that rare gas compositions reflect a vesiculation process with a kinetic effect [Aubaud et al., 2004], suggesting a fast vesiculation during eruption. The calculations were done using the solubilities of Jambon et al. [1986]. Mantle ratios are derived from Moreira et al. [1998].

[10] Sample 262-34 appears to be the least degassed sample, which appears contrary to its relatively low helium concentration (7 × 10−6 ccSTP/g). Such a helium concentration is relatively low compared to gas-rich popping rocks, which show He contents on the order of a few 10−5 ccSTP/g and can be considered as undegassed samples [Javoy and Pineau, 1991; Moreira et al., 1998; Sarda and Graham, 1990; Staudacher et al., 1989]. The low He content of sample 262-34 may reflect vesicle loss during or after eruption.

[11] It is important to note that overall the CLT magmas are not as rich in rare gases as might be expected from a low degree of partial melting (which should result in high noble gas contents) and their high vesicularity (which should indicate high gas contents). This probably reflects vesicle breaking during eruption or sample preparation. Although it is very hard to quantify, we suggest a loss of more than 90% of the vesicles in these samples. Alternatively, the low rare gas concentrations might be linked in a different way to the low degree of partial melting which has generated these Lena Trough lavas. In this scenario, low-degree partial melts are enriched in incompatible elements including H2O. H2O promotes the formation of the CO2 vapor phase noble gases follow CO2 into this vapor phase, so that an outgassing process during eruption of water-rich magma would result in lavas with lower gas contents when compared to MORB generated by a higher degree of partial melting erupted at similar depth [Dixon and Stolper, 1995]. Such relationships have been noted in back-arc lavas, for example from Manus Basin [Macpherson et al., 2005; Shaw et al., 2004].

4.2. Mantle Structure and Arctic DUPAL Anomaly

[12] The Lena Trough samples were dredged from a portion of the ridge with an extremely slow spreading rate (∼7.5 mm. yr−1 effective full rate). Due to the very low magmatic budget along ultraslow spreading ridges, erupted lavas generally display a larger chemical heterogeneity due to the preferential sampling of mantle heterogeneities [Hirschmann and Stolper, 1996; Sinton et al., 2002]. These mantle heterogeneities are usually interpreted as reflecting ancient recycled components (e.g., oceanic crust ± sediment, sublithospheric mantle). Therefore, U contained in these recycled components will produce 4He by radioactive decay, which results in a high 4He/3He. Consequently the 4He/3He ratio is a good tracer for the existence of mantle heterogeneities in MORB sources. Mantle heterogeneities have been observed along the Southwest Indian Ridge (SWIR), a slow spreading ridge where 4He/3He isotopic ratios are uniformly higher than the MORB value, with 4He/3He ratios ranging from 95,000 to 120,000 (R/Ra = 6.0 to 7.6) [Georgen et al., 2003; Mahoney et al., 1989]. Similar high 4He/3He isotopic ratios were also clearly identified along the SEIR where the measured R/Ra along the Australian-Antarctic Discordance (AAD) are as low as 6.2 [Graham, 2002]. These high 4He/3He ratios are classically interpreted as reflecting the presence of recycled crustal or lithospheric material [Ballentine and Burnard, 2002; Graham et al., 1992; Hanyu and Kaneoka, 1997; Hilton et al., 2000].

[13] The DUPAL anomaly [Hart, 1984] is Southern Hemisphere isotopic anomaly whose origin intensively debated in an attempt to identify the nature of recycled components, for example oceanic crust ± sediment, lithospheric mantle, or lower crust [Escrig et al., 2004; Hanan et al., 2004; Mahoney et al., 1992; Meyzen et al., 2005; Sarda et al., 2000]. In terms of noble gases, the DUPAL anomaly is characterized by highly radiogenic He (4He/3He > 120000 [Gautheron, 2002] and Ne, mildly radiogenic Ar and atmospheric-like Xe [Sarda et al., 2000]. The origin of the DUPAL material is described by time-integrated, relatively low 40K/36Ar ratios whereas He and Ne suggest elevated (U-Th)/He and (U-Th)/Ne ratios, which could represent lithospheric metasomatized mantle or lower continental crust.

[14] A DUPAL anomaly signature has been recognized on the basis of heavy radiogenic isotopes Sr-Nd-Pb and Hf in the Arctic Ocean [Goldstein et al., 2008; Mühe et al., 1993, 1997] although the Δ208Pb/204Pb is less elevated in the Arctic Ocean. Therefore, similar noble gas signatures to those of the Southern Hemisphere DUPAL-like anomaly were expected to be measured in Arctic MORB (along Gakkel ridge and Lena Trough). However, these high 4He/3He ratios are not found in our samples. The mean 4He/3He ratio in Lena Trough lavas is 93,290 (R/Ra = 7.75). The Lena Trough He isotope ratios (4He/3He = 88,300 to 100,500 or 7.2–8.2 Ra) are less radiogenic than typical values given for Indian MORB with a DUPAL anomaly (4He/3He > 120,000; R/Ra < 6.02) [Gautheron, 2002; Sarda et al., 2000] and instead are similar to typical MORB values (4He/3He = 90,000; R/Ra = 8.0). Therefore, the Lena Trough He isotope compositions do not show a typical DUPAL anomaly signature. Moreover, the Lena Trough lavas plot on the MORB line in the three Ne isotope diagrams [Sarda et al., 1988]. The scatter of the data around the MORB line is limited compared to that displayed by SWIR MORB [Gautheron, 2002]. Some SWIR MORB values display more radiogenic 21Ne/22Ne at a given 20Ne/22Ne than the MORB reference line (Figure 3). This was interpreted as reflecting the contribution of metasomatized SLCM, which might have generated the Southern Hemisphere DUPAL anomaly [Gautheron, 2002]. Alternatively, such high 21Ne/22Ne at a given 20Ne/22Ne is also observed along the North Chile Ridge [Niedermann and Bach, 1998] and in the Manus Basin where no SLCM is identified [Shaw et al., 2001] (Figure 3). In these cases, this feature has been interpreted as resulting from a prior melting/degassing event that has fractionated the U/Ne ratio in the residual mantle [Niedermann and Bach, 1998; Shaw et al., 2001]. A high 21Ne/22Ne at a given 20Ne/22Ne is not observed in Lena Trough lavas, although a SCLM component might have been expected, taking into account the proximity of the continental margin (80 km away), the Sr-Nd-Pb isotopic composition of MORB from the Arctic Ocean [Goldstein et al., 2008; Mühe et al., 1993, 1997] and the recent opening of the ocean basin in the Lena Trough area.

[15] Thus, taking into account the helium isotopic composition and the data distribution in the three Ne isotope diagram, Lena Trough samples are more like MORB away from a hot spot. A SCLM or previously melted or degassed residual mantle is not observed. In conclusion, the measured He-Ne isotope compositions do not display characteristics shown by the Southern Hemisphere DUPAL signature. Therefore the Southern Hemisphere DUPAL and the Arctic DUPAL-like samples are clearly distinct in terms of noble gas compositions, which indicates a different origin or processes generating this anomaly. This difference between southern and northern DUPAL anomalies could indicate a long-term separation between the two hemispheres [Jackson et al., 2007]. In this case, a mechanism is required to preserve these distinct anomalies since mantle convective stirring tends to attenuate mantle heterogeneities [van Keken et al., 2002]. Alternatively, this difference might be linked to the abundance, size, relative proportions and variable origins of these mantle heterogeneities [Meibom and Anderson, 2004].

[16] Several hypotheses can be formulated to explain the “normal” helium isotope ratio observed at Lena Trough. One is the absence of any mantle heterogeneity, although this goes against the evidence for an enriched, highly potassic mantle source evidenced by the trace element characteristics of the lavas (Nauret et al., submitted manuscript, 2010). A second possibility is that the mantle heterogeneity signal is not recorded in the lavas due to recent gas loss from the enriched component, which then does not, contribute to the noble gas budget during melting. For Hawaii [Sobolev et al., 2005, 2008] a proposed binary melt mixing model involves a melt derived from a MORB source and a melt derived from a second, enriched component, like a SCLM or a recycled component. A similar binary mixing scenario is often proposed for the MAR and Arctic Ridges based on Pb-Pb isotope linear correlations [Blichert-Toft et al., 2005; Goldstein et al., 2008; Nauret et al., submitted manuscript, 2010]. Taking into account, first, that fertile and enriched component melt prior to and deeper than peridotite mantles, and second that this melt might react with surrounding peridotite to form a new pyroxene-rich lithology [Sobolev et al., 2005], it is likely that a melt-vapor fractionation (degassing) occurs during melt formation or melt-rock reaction. Consequently, a melt derived from the enriched component is strongly depleted in volatile elements. Therefore, the signal of the mantle heterogeneity is lost before these two different melts mix to form the sampled lavas. The advantage of this scenario is that it explains the strong decoupling between noble gases and trace element and Sr-Nd-Pb-Hf isotopes which have been interpreted as binary mixing involving more than 50% of enriched component (Nauret et al., submitted manuscript, 2010). The main question in this scenario is why a degassing process occurs at Lena Trough whereas a recycled component in Hawaii or plume derived melts have high 3He/4He and 20Ne/22Ne for a given 21Ne/22Ne [Coltice and Ricard, 2002; Honda et al., 1993; Moreira et al., 2001; Trieloff et al., 2000]. As a preliminary answer, we propose that an enriched component melts and forms a volatile incompatible element enriched melt that reacts with the surrounding mantle at a shallow depth than a plume setting. Such shallow depth, low-pressure melting and reaction within Lena Trough mantle allows for melt-vapor fractionation (degassing), which erases the mantle heterogeneity (SCLM or recycled component) signature.

[17] Our preferred explanation is that the mantle heterogeneities (SCLM or veins formed by reaction between a high Si-rich melt derived from a recycled component and peridotite mantle) are volumetrically extremely minor (few km3) and that the helium and neon have been equilibrated by diffusion between the lherzolitic part of the mantle and the recycled component. Therefore the helium and neon isotopic ratios were homogenized, as suggested by Hart et al. [2008] for helium. This is possible if the thickness of the heterogeneity is in the order of 1 km and if the time is longer than 1.5 Ga [Hart et al., 2008]. Considering that the lithospheric mantle beneath Svalbard was formed roughly at the same time as the formation of the oldest known crustal rock (up to 2.8 Ga [Hellman and Witt-Nilsson, 1999]), we consider our explanation to be a likely scenario. A low melt volume agrees with the observation that no volcanic edifice is observable in bathymetric data.

[18] Lena Trough is thus an example that confirms that spreading rate and noble gas compositions are not related [Graham, 2002] and contradicts the hypothesis that slow spreading ridges should have systematically more radiogenic helium compared to faster spreading ridges [Lupton et al., 1993].

5. Conclusion

[19] We have reported the first elemental and isotopic compositions of noble gases in lavas collected along the Lena Trough (Arctic Ocean). These lavas appear to have been affected by a strong degassing process, as suggested by the very high 4He/40Ar*, although they have been collected at great depth (3500 m deep). In contrast to other isotopic systems (Sr-Nd-Pb-Hf), these rare gas compositions do not reflect the presence of long-lived or deep-seated mantle enrichment, which is surprising when taking into account the proximity of the continental margin and the observed geochemical enrichment of the basalts. From the evidence of noble gases, lavas from the Central Lena Trough are generated from a typical MORB source and not from an Arctic DUPAL anomaly. To reconcile the interpretation based on noble gases and radiogenic isotopes, we suggest that mantle heterogeneities are present in the Lena Trough mantle, but their volume are small (<1 km), allowing diffusion of the noble gases, and subsequent homogenization. Another possibility could be that the recycled component has been degassed and thus plays a minor role in the noble gas mass balance of the Central Lena Trough mantle source.


[20] The authors wish to thank the captain, crew, and other scientific participants in PFS Polarstern ARK XX-2 and the Alfred Wegener Institute for access to ship time on PFS Polarstern. J.E.S. was supported by grant OCE 0648567 from the National Science foundation and by the Deutsche Forschungsgeminschaft and the Max Planck Society. Thoughtful and thorough reviews by P. Burnard and anonymous reviewer greatly improved the manuscript. This is IPGP contribution 3017.