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TY  - JOUR
T1  - North Atlantic hotspot-ridge interaction near Jan Mayen Island
JO  - Geochemical Perspectives Letters
VL  - 2
SP  - 55
EP  - 67
PY  - 2016
AU  - L.J. ElkinsAU  - C. HamelinAU  - J. Blichert-ToftAU  - S.R. ScottAU  - K.W.W. SimsAU  - I.A. YeoAU  - C.W. DeveyAU  - R.B. PedersenDO  - https://doi.org/10.7185/geochemlet.1606
UR  - https://www.geochemicalperspectivesletters.org/article1606
AB  - At slow to ultraslow spreading rates along mid-ocean ridges, thicker lithosphere typically impedes magma generation and tectonic extension can play a more significant role in crustal production (Dick et al., 2003). The source of anomalously high magma supply thus remains unclear along ridges with ultraslow-spreading rates adjacent to Jan Mayen Island in the North Atlantic (Neumann and Schilling, 1984; Mertz et al., 1991; Haase et al., 1996; Schilling et al., 1999; Trønnes et al., 1999; Haase et al., 2003; Mertz et al., 2004; Blichert-Toft et al., 2005; Debaille et al., 2009). Here we show that Jan Mayen volcanism is likely the surface expression of a small mantle plume, which exerts significant influence on nearby mid-ocean ridge tectonics and volcanism. Progressive dilution of Jan Mayen geochemical signatures with distance from the hotspot is observed in lava samples from the immediately adjacent Mohns Ridge, and morphological indicators of enhanced magma supply are observed on both the Mohns Ridge and the nearby Kolbeinsey Ridge, which additionally locally overlies a highly heterogeneous, eclogite-bearing mantle source. These morphological and geochemical influences underscore the importance of heterogeneous mantle sources in modifying melt supply and thus the local expression of tectonic boundaries.

North Atlantic hotspot-ridge interaction near Jan Mayen Island

L.J. Elkins^1,2,

¹Department of Geology, Bryn Mawr College, 101 North Merion Avenue, Bryn Mawr, Pennsylvania 19010, USA
²Department of Earth and Atmospheric Sciences, University of Nebraska Lincoln, Lincoln, NE 68588, USA

C. Hamelin³,

³Center for Geobiology, University of Bergen, Allegaten 41 5007, Bergen, Norway

J. Blichert-Toft⁴,

⁴Laboratoire de Géologie de Lyon, Ecole Normale Superieure de Lyon, 46 Allée d’Italie, 69007 Lyon, France

S.R. Scott⁵,

⁵Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071, USA

K.W.W. Sims⁵,

⁵Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071, USA

I.A. Yeo⁶,

⁶GEOMAR, Helmholtz Center for Ocean Research Kiel, Wischhofstraße 1-3 24148, Kiel, Germany

C.W. Devey⁶,

⁶GEOMAR, Helmholtz Center for Ocean Research Kiel, Wischhofstraße 1-3 24148, Kiel, Germany

R.B. Pedersen³

³Center for Geobiology, University of Bergen, Allegaten 41 5007, Bergen, Norway

Affiliations | Corresponding Author | Cite as

Geochemical Perspectives Letters v2, n1 | doi: 10.7185/geochemlet.1606
Received 17 August 2015 | Accepted 21 December 2015 | Published 22 January 2016
Copyright © 2016 European Association of Geochemistry

Keywords: mid-ocean ridge, lead, strontium, neodymium, radiogenic isotopes, Jan Mayen, Kolbeinsey Ridge, Mohns Ridge, hotspot, hotspot-ridge interaction



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Abstract

Abstract | Letter | Acknowledgements | References | Supplementary Information

Dick, H.J.B., Lin, J., Schouten, H. (2003) An ultraslow-spreading class of ocean ridge. Nature 426, 405-412.

). The source of anomalously high magma supply thus remains unclear along ridges with ultraslow-spreading rates adjacent to Jan Mayen Island in the North Atlantic (Neumann and Schilling, 1984

Neumann, E.R., Schilling, J.G. (1984) Petrology of Basalts from the Mohns-Knipovich Ridge - the Norwegian-Greenland Sea. Contributions to Mineralogy and Petrology 85, 209-223.

; Mertz et al., 1991

Mertz, D.F., Devey, C.W., Todt, W., Stoffers, P., Hofmann, A.W. (1991) Sr-Nd-Pb Isotope Evidence against Plume Asthenosphere Mixing North of Iceland. Earth and Planetary Science Letters 107, 243-255.

; Haase et al., 1996

Haase, K.M., Devey, C.W., Mertz, D.F., Stoffers, P., Garbe-Schönberg, D. (1996) Geochemistry of lavas from Mohns ridge, Norwegian-Greenland Sea: Implications for melting conditions and magma sources near Jan Mayen. Contributions to Mineralogy and Petrology 123, 223-237.

; Schilling et al., 1999

Schilling, J.G., Kingsley, R., Fontignie, D., Poreda, R., Xue, S. (1999) Dispersion of the Jan Mayen and Iceland mantle plumes in the Arctic: A He-Pb-Nd-Sr isotope tracer study of basalts from the Kolbeinsey, Mohns, and Knipovich Ridges. Journal of Geophysical Research-Solid Earth 104, 10543-10569.

; Trønnes et al., 1999

Trønnes, R.G., Planke, S., Sundvoll, B., Imsland, P. (1999) Recent volcanic rocks from Jan Mayen: Low-degree melt fractions of enriched northeast Atlantic mantle. Journal of Geophysical Research-Solid Earth 104, 7153-7168.

; Haase et al., 2003

Haase, K.M., Devey, C.W., Wieneke, M. (2003) Magmatic processes and mantle heterogeneity beneath the slow-spreading northern Kolbeinsey Ridge segment, North Atlantic. Contributions to Mineralogy and Petrology 144, 428-448.

; Mertz et al., 2004

Mertz, D.F., Sharp, W.D., Haase, K.M. (2004) Volcanism on the Eggvin Bank (Central Norwegian-Greenland Sea, latitude similar to 71 degrees N): age, source, and relationship to the Iceland and putative Jan Mayen plumes. Journal of Geodynamics 38, 57-83.

; Blichert-Toft et al., 2005

Blichert-Toft, J., Agranier, A., Andres, M., Kingsley, R., Schilling, J.G., Albarède, F. (2005) Geochemical segmentation of the Mid-Atlantic Ridge north of Iceland and ridge-hot spot interaction in the North Atlantic. Geochemistry Geophysics Geosystems 6, doi: 10.1029/2004GC000788.

; Debaille et al., 2009

Debaille, V., Trønnes, R.G., Brandon, A.D., Waight, T.E., Graham, D.W., Lee, C.-T.A. (2009) Primitive off-rift basalts from Iceland and Jan Mayen: Os-isotopic evidence for a mantle source containing enriched subcontinental lithosphere. Geochimica et Cosmochimica Acta 73, 3423-3449.

). Here we show that Jan Mayen volcanism is likely the surface expression of a small mantle plume, which exerts significant influence on nearby mid-ocean ridge tectonics and volcanism. Progressive dilution of Jan Mayen geochemical signatures with distance from the hotspot is observed in lava samples from the immediately adjacent Mohns Ridge, and morphological indicators of enhanced magma supply are observed on both the Mohns Ridge and the nearby Kolbeinsey Ridge, which additionally locally overlies a highly heterogeneous, eclogite-bearing mantle source. These morphological and geochemical influences underscore the importance of heterogeneous mantle sources in modifying melt supply and thus the local expression of tectonic boundaries.

Figures and Tables

Figure 1 (a) Multibeam bathymetric map of the NKR, showing the Eggvin Bank and numbered dredge locations for samples analysed in this study. (b) Regional bathymetric map showing distribution of labelled seafloor features and Jan Mayen Island, with sample locations for this study from Jan Mayen Island (red), NKR (colours as in panel a), and SMR (orange). (c) Map with highlighted areas showing the proposed zones of underlying mantle melt generation and migration (blue: Kolbeinsey-type; purple: Eggvin-type; orange: Mohns-type; and red circle: Jan Mayen-type mantle).	Table 1 Radiogenic isotope compositions measured by ICP-MS.*	Figure 2 (a) ε_Nd vs. ε_Hf, (b) ε_Nd vs. ²⁰⁶Pb/²⁰⁴Pb, (c) ²⁰⁷Pb/²⁰⁴Pb vs. ε_Hf, and (d) ²⁰⁷Pb/²⁰⁴Pb vs. ²⁰⁶Pb/²⁰⁴Pb diagrams for lavas from the Jan Mayen region and Iceland (Sun and Jahn, 1975; Zindler et al., 1979; Óskarsson et al., 1982; Hemond et al., 1993; Nowell et al., 1998; Salters and White, 1998; Schilling et al., 1999; Chauvel and Hémond, 2000; Kempton et al., 2000; Stracke et al., 2003; Blichert-Toft et al., 2005; Elkins et al., 2011; Sims et al., 2013; Elkins et al., 2014) (Tables 1, S-2). Curves show calculated binary mixing trajectories between hypothesised geochemical compositions for Jan Mayen- (red box), Mohns- (yellow), Kolbeinsey- (blue) and Eggvin- (green) type melt endmembers, where tickmarks show percentage contributions of a pure Jan Mayen- or Eggvin-derived magma to a mixture. The Jan Mayen endmember, based on the most extreme enriched measurements for the island (Tables 1, S-2) has ε_Hf= +10.5, ε_Nd = +4.7, ²⁰⁶Pb/²⁰⁴Pb = 18.85, ²⁰⁷Pb/²⁰⁴Pb = 15.517, and Hf, Nd, and Pb concentrations of 6.9, 38.7, and 3.7 ppm, respectively. The hypothesised Mohns endmember, extrapolated to values that best explain available SMR samples as binary mixtures of Jan Mayen-Mohns Ridge lavas, has ε_Hf = +24, ε_Nd = +10.1, ²⁰⁶Pb/²⁰⁴Pb = 17.9, ²⁰⁷Pb/²⁰⁴Pb = 15.41, and Hf, Nd, and Pb concentrations of 5.6, 30, and 0.7, ppm, respectively; this composition is reasonable compared to published measurements from the Mohns Ridge (Schilling et al., 1983; Schilling et al., 1999; Blichert-Toft et al., 2005; Elkins et al., 2014). The Kolbeinsey endmember, based on depleted values from a suite of published MKR measurements (Schilling et al., 1983; Blichert-Toft et al., 2005; Elkins et al., 2011) and NKR sample POS436 246DR-2, has ε_Hf = +19.2, ε_Nd = +10, ²⁰⁶Pb/²⁰⁴Pb = 18.0, ²⁰⁷Pb/²⁰⁴Pb = 15.43, and Hf, Nd, and Pb concentrations of 0.5, 3, and 0.3 ppm, respectively; mixtures of Jan Mayen and Kolbeinsey endmembers cannot fully explain NKR lava compositions. The Eggvin-type component was extrapolated to values that best explain NKR basalts as mixtures between Kolbeinsey and an unknown enriched component, with ε_Hf = +11, ε_Nd = +5, ²⁰⁶Pb/²⁰⁴Pb = 18.96, ²⁰⁷Pb/²⁰⁴Pb = 15.528, ²⁰⁸Pb/²⁰⁴Pb = 38.72, and Hf, Nd, and Pb concentrations of 3, 22, and 11 ppm. Note that the high Pb content of the Eggvin-type endmember is necessary to generate a sufficiently hyperbolic mixing trajectory to account for NKR basalts.	Figure 3 Geochemical indicators vs. along-axis distance for the NKR and SMR, with the position of Jan Mayen Island projected westward onto the NKR using a geographic contour that runs parallel to the Jan Mayen Fracture Zone. (a) (Sm/Yb)_N, sensitive to the presence of garnet in, and the trace element makeup of, the source. The variation between Jan Mayen Island/SMR and the NKR likely reflects a heterogeneous mantle source. (b) α_Sm-Nd; because Sm is always more compatible than Nd during melting, values less than unity reflect the degree of melting of the model source, while values greater than unity (e.g., MKR basalts; Salters, 1996; Elkins et al., 2011) require a different source composition and/or younger age than recorded by radiogenic isotopes.
Figure 1	Table 1	Figure 2	Figure 3

View all figures and tables

Supplementary Figures and Tables

Figure S-1 (a) Chondrite-normalised (McDonough and Sun, 1995) REE concentrations and (b) N-MORB (Hofmann, 1988) normalised trace element concentrations for samples from this study (Table S-2). NKR basalts have elevated Pb and HREE compared to Jan Mayen Island and the MKR, indicating that they cannot be simple mixtures of Kolbeinsey-type and Jan Mayen-type magmas. High (²³⁰Th/²³⁸U) ratios measured in NKR lavas also require the presence of garnet in the melt source, indicating that the trace element compositions in Eggvin Bank basalts is principally controlled by mantle source composition.	Figure S-2 ε_Hf vs. ²⁰⁶Pb/²⁰⁴Pb for the Jan Mayen region, with symbols, mixing trajectories, and references as in Figure 2.	Figure S-3 (La/Sm)_N vs. FeO* for basalt samples from the Kolbeinsey Ridge and the NKR, using data from Haase et al. (2003) and C. Devey, M. Wieneke, and K. Haase (unpub. data). Linear best-fit regression for Kolbeinsey Ridge samples suggests a slight positive relationship between FeO* and (La/Sm)_N, likely controlled by degree of melting. Basalt rocks from the NKR are restricted to generally higher (La/Sm)_N and lower FeO* values than the rest of the Kolbeinsey Ridge, best explained by an eclogite-bearing, incompatible element-enriched mantle source beneath the Eggvin Bank.	Figure S-4 (a) ⁸⁷Sr/⁸⁶Sr vs. Pb, and (b) ε_Nd vs. Pb. for the Jan Mayen region, with symbols and references as in Figure 2. The data support lithologically and isotopically heterogeneous mantle source compositions for the Jan Mayen region and corroborate the existence of an Eggvin Bank end-member distinct from Jan Mayen mantle.
Figure S-1	Figure S-2	Figure S-3	Figure S-4

Table S-1 Location information for new submarine samples analyzed in this study.	Table S-2 Trace element abundance measured by ICP-MS.	Table S-3 Major element composition results.
Table S-1	Table S-2	Table S-3

View all supplementary figures and tables

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Letter

Abstract | Letter | Acknowledgements | References | Supplementary Information

The normal accretion process along divergent plate boundaries can be notably altered in hotspot-ridge interaction settings, where elevated mantle temperature anomalies enhance mantle melting, generating unusually thick oceanic crust (e.g., Schilling et al., 1985

Schilling, J.G., Thompson, G., Kingsley, R., Humphris, S. (1985) Hotspot-migrating ridge interaction in the South Atlantic. Nature 313, 187-191.

; Schilling, 1991

Schilling, J.G. (1991) Fluxes and excess temperatures of mantle plumes inferred from their interaction with migrating mid-ocean ridges. Nature 352, 397-403.

; Gale et al., 2013

Gale, A., Dalton, C.A., Langmuir, C.H., Su, Y., Schilling, J.G. (2013) The mean composition of ocean ridge basalts. Geochemistry Geophysics Geosystems 14, 489-518.

, 2014

Gale, A., Langmuir, C.H., Dalton, C.A. (2014) The global systematics of ocean ridge basalts and their origin. Journal of Petrology 55, 1051-1082.

). Jan Mayen and its immediate environs in the North Atlantic (Fig. 1) include an intraplate, volcanically-active island or hotspot (Jan Mayen Island), positioned at the northern terminus of a small, rifted microcontinent (Jan Mayen Ridge; Johnson and Heezen, 1967

Johnson, G.L., Heezen, B.C. (1967) Arctic Mid-Oceanic Ridge. Nature 215, 724-728.

; Kodaira et al., 1997

Kodaira, S., Mjelde, R., Gunnarsson, K., Shiobara, H., Shimamura, H. (1997) Crustal structure of the Kolbeinsey Ridge, North Atlantic, obtained by use of ocean bottom seismographs. Journal of Geophysical Research-Solid Earth 102, 3131-3151.

; Gaina et al., 2009

Gaina, C., Gernigon, L., Ball, P. (2009) Palaeocene-Recent plate boundaries in the NE Atlantic and the formation of the Jan Mayen microcontinent. Journal of the Geological Society of London 166, 601-616.

) and adjacent to two second-order ultraslow-spreading (Dick et al., 2003

Dick, H.J.B., Lin, J., Schouten, H. (2003) An ultraslow-spreading class of ocean ridge. Nature 426, 405-412.

) ridge segments, the Northern Kolbeinsey Ridge (NKR) and Southern Mohns Ridge (SMR), and the Jan Mayen Fracture Zone, a major fracture zone with ~200 km of transform offset. Although different in key ways, broad geochemical similarities between Jan Mayen Island and Icelandic lavas have suggested the influence of a mantle plume (either a unique Jan Mayen plume or emplaced Icelandic material) on mantle melting beneath Jan Mayen Island (Schilling et al., 1999

; Trønnes et al., 1999

; Debaille et al., 2009

). The absence of a clear hotspot track has led to conflicting, alternate interpretations for Jan Mayen’s high magma production rate and enriched chemistry (Imsland, 1986

Imsland, P. (1986) The volcanic eruption on Jan Mayen, January 1985: Interaction between a volcanic island and a fracture zone. Journal of Volcanology and Geothermal Research 28, 45-53.

; Maaløe et al., 1986

Maaløe, S., Sørensen, I. B., Hertogen, J. (1986) The trachybasaltic suite of Jan Mayen. Journal of Petrology 27, 439-466.

; Thy et al., 1991

Thy, P., Lofgren, G.E., Imsland, P. (1991) Melting relations and the evolution of the Jan Mayen magma system. Journal of Petrology 32, 303-332.

): cold edge effects near the fracture zone (Mertz et al., 1991

; Haase et al., 1996

), variably melting source heterogeneities (Mertz et al., 1991

; Haase et al., 2003

; Mertz et al., 2004

), upwelling along a mantle chemical discontinuity (Blichert-Toft et al., 2005

), or a locally wet mantle (Haase et al., 2003

; Mertz et al., 2004

). Jan Mayen thus presents a useful case study for 1) exploring the mechanisms by which hotspot volcanism can influence ultraslow-spreading ridge morphology, behaviour, and volcanism, 2) determining the relationships between hotspot volcanism and ambient variations in mantle geochemistry, and 3) exploring the disputed origins of local volcanic activity.

For this study, we present comprehensive geochemical analyses (major and trace element concentrations and ⁸⁷Sr/⁸⁶Sr, ¹⁴³Nd/¹⁴⁴Nd, ¹⁷⁶Hf/¹⁷⁷Hf, ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb compositions) for a suite of submarine volcanic rocks from the NKR, the SMR, and Jan Mayen Island (Tables 1, S-1, S-2, S-3). These geochemical results are interpreted in the context of an enhanced geologic perspective, thanks to new high-resolution bathymetry of the volcanic and tectonic submarine morphology (Fig. 1). All submarine samples were retrieved during recent research cruises in combination with new multibeam bathymetry (Pedersen et al., 2010

Pedersen, R.B., Thorseth, I.H., Nygård, T.E., Lilley, M.D., Kelley, D.S. (2010) Hydrothermal activity at the Arctic mid-ocean ridges. In: Rona, P.A., Devey, C.W., Dyment, J., Murton, B.J. (Eds.) Diversity of Hydrothermal Systems on Slow Spreading Ocean Ridges. AGU, 67-89.

; Devey, 2012

Devey, C. (2012) RV Poseidon Cruise Report 436 [POS436]: North Kolbeinsey Ridge - geochemistry and volcanology, 06.07.2012 (Kiel) - 31.07.2012 (Akureyri). GEOMAR, Kiel, Germany, doi: 10.3289/CR_POS_436.

). Three additional, subaerial alkali basalts from Jan Mayen Island are included for literature comparison (Maaløe et al., 1986

Maaløe, S., Sørensen, I. B., Hertogen, J. (1986) The trachybasaltic suite of Jan Mayen. Journal of Petrology 27, 439-466.

**Figure 1** **(a)** Multibeam bathymetric map of the NKR, showing the Eggvin Bank and numbered dredge locations for samples analysed in this study. **(b)** Regional bathymetric map showing distribution of labelled seafloor features and Jan Mayen Island, with sample locations for this study from Jan Mayen Island (red), NKR (colours as in panel a), and SMR (orange). **(c)** Map with highlighted areas showing the proposed zones of underlying mantle melt generation and migration (blue: Kolbeinsey-type; purple: Eggvin-type; orange: Mohns-type; and red circle: Jan Mayen-type mantle).

Table 1 Radiogenic isotope compositions measured by ICP-MS.*

Sample	Location**	⁸⁷Sr/⁸⁶Sr	¹⁷⁶Hf/¹⁷⁷Hf	¹⁴³Nd/¹⁴⁴Nd	²⁰⁶Pb/²⁰⁴Pb	²⁰⁷Pb/²⁰⁴Pb	²⁰⁸Pb/²⁰⁴Pb
Submarine samples:
POS436 242DR-2ba	NKR	0.703151(5)	0.283175(5)	0.513006(6)	18.8926	15.5093	38.6157
POS436 246DR-2a	NKR	0.702961(6)	0.283255(4)	0.513083(5)	18.4553	15.4547	38.0857
POS436 235DR-1aa	NKR	0.703187(5)	0.283177(4)	0.513008(5)	18.8756	15.5177	38.5990
POS436 253DR-E2a	NKR	0.703195(7)	0.283175(4)	0.513015(5)	18.8899	15.5211	38.6184
POS436 253DR-6a	NKR	0.703203(7)	0.283183(4)	0.513019(5)	18.8881	15.5185	38.6109
POS436 232DR-1a	NKR	0.703047(7)	0.283217(4)	0.513044(5)	18.7881	15.5004	38.4908
POS436 209DR-2aa	NKR	0.703034(6)	0.283231(4)	0.513051(6)	18.7699	15.5003	38.4689
POS436 222DR-1a	NKR	0.703040(7)	0.283217(4)	0.513043(6)	18.8150	15.5047	38.5277
POS436 215DR-1a	NKR	0.703047(7)	0.283203(4)	0.513036(4)	18.8538	15.5114	38.5652
SM01-DR-24-14b	JM	0.703368(8)	-	0.512910(5)	18.8331	15.5057	38.5979
SM01-DR-23-3b	JM	0.703456(6)	0.283088(7)	0.512931(5)	18.8494	15.5070	38.6082
SM01-DR-5-5b	JM	0.70343(8)	0.283090(4)	0.512914(5)	18.8149	15.5061	38.5865
SM01-DR-60-43b	JM	0.703431(8)	0.283083(4)	0.512918(5)	18.8095	15.5051	38.5795
SM01-DR-100-01b	SMR	0.703395(8)	0.283233(5)	0.512978(5)	18.7946	15.4979	38.5077
CGB-2011-D17-2aa	SMR	0.703339(6)	0.283265(4)	0.512991(6)	18.7206	15.4949	38.4695
SM01-DR70-1a	SMR	0.703391(5)	0.283236(4)	0.512979(5)	18.7409	15.4995	38.4923
SM01-DR67-4b	SMR	0.703417(8)	0.283196(4)	0.512983(5)	18.8285	15.5012	38.5407
SM01-DR-91-13b	SMR	-	0.283314(5)	-	-	-	-
Subaerial samples (samples from Maaløe et al., 1986 Maaløe, S., Sørensen, I. B., Hertogen, J. (1986) The trachybasaltic suite of Jan Mayen. Journal of Petrology 27, 439-466. ):
JM-192a	JM	0.703490(7)	0.283083(4)	0.512880(6)	18.7648	15.5167	38.6121
JM-71a	JM	0.703454(6)	0.283068(4)	0.512901(5)	18.8186	15.5170	38.6310
JM-84a	JM	0.703453(7)	0.283087(4)	0.512903(6)	18.8404	15.5090	38.6229

* Values in parentheses indicate 2s uncertainty for the last digit expressed.
** NKR: Northern Kolbeinsey Ridge; JM: Jan Mayen Island; SMR: Southern Mohns Ridge.
a 206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb, 176Hf/177Hf, and 143Nd/144Nd measured by MC-ICP-MS (Nu Plasma HR) at the Ecole Normale Supérieure de Lyon. Strontium isotopes were analyzed at the University of Wyoming by MC-ICP-MS (ThermoFinnigan NeptunePlus). See Supplementary

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In agreement with previous work (Trønnes et al., 1999

; Debaille et al., 2009

), Jan Mayen Island lavas are “enriched” with relatively high ⁸⁷Sr/⁸⁶Sr, ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb and low e_Hf and e_Nd (e.g., ⁸⁷Sr/⁸⁶Sr = 0.703368-0.703490) (Table 1), and with trace element abundances resembling other ocean island basalts (Table S-2, Fig. S-1). While similar, Jan Mayen area lavas exhibit a distinct geochemical composition from Icelandic lavas (e.g., higher ⁸⁷Sr/⁸⁶Sr and Pb isotope ratios, lower ¹⁴³Nd/¹⁴⁴Nd and ¹⁷⁶Hf/¹⁷⁷Hf, normal MORB ³He/⁴He, and distinct ¹⁸⁷Os/¹⁸⁸Os on Jan Mayen Island; Schilling et al., 1999

; Hanan et al., 2000

Hanan, B.B., Blichert-Toft, J., Kingsley, R., Schilling, J.G. (2000) Depleted Iceland mantle plume geochemical signature: artifact of multicomponent mixing? Geochemistry Geophysics Geosystems 1, doi: 10.1029/1999GC000009.

; Blichert-Toft et al., 2005

; Debaille et al., 2009

), suggesting an enriched source discrete from the Icelandic hotspot source, possibly entraining subcontinental lithospheric mantle (SCLM) (Debaille et al., 2009

). The submarine samples from Jan Mayen Island appear relatively evolved compared to the most magnesian subaerial samples of this study (MgO = 5.1-6.45 vs. 10.6-11.1 wt. %; Table S-3), but as previously observed, there are no systematic trace element or isotopic variations correlating with differentiation, arguing against detectable crustal assimilation (Trønnes et al., 1999

) (Tables 1, S-2, S-3).

The Mohns Ridge is an ultraslow-spreading ridge (17 mm yr^-1 full-spreading rate; Mosar et al., 2002

Mosar, J., Lewis, G., Torsvik, T.H. (2002) North Atlantic sea-floor spreading rates: implications for the Tertiary development of inversion structures of the Norwegian-Greenland Sea. Journal of the Geological Society of London 159, 503-515.

; Dick et al., 2003

Dick, H.J.B., Lin, J., Schouten, H. (2003) An ultraslow-spreading class of ocean ridge. Nature 426, 405-412.

) north of Jan Mayen Island with relatively thin crust (~4 km; Klingelhofer et al., 2000

Klingelhofer, F., Géli, L., White, R.S. (2000) Geophysical and geochemical constraints on crustal accretion at the very-slow spreading Mohns Ridge. Geophysical Research Letters 27, 1547-1550.

; Okino et al., 2002

Okino, K., Curewitz, D., Asada, M., Tamaki, K., Vogt, P., Crane, K. (2002) Preliminary analysis of the Knipovich Ridge segmentation: influence of focused magmatism and ridge obliquity on an ultraslow spreading system. Earth and Planetary Science Letters 202, 275-288.

; Ljones et al., 2004

Ljones, F., Kuwano, A., Mjelde, R., Breivik, A., Shimamura, H., Murai, Y., Nishimura, Y. (2004) Crustal transect from the North Atlantic Knipovich Ridge to the Svalbard margin west of hornsund. Tectonophysics 378, 17-41.

; Kandilarov et al., 2008

Kandilarov, A., Mjelde, R., Okino, K., Murai, Y. (2008) Crustal structure of the ultra-slow spreading Knipovich Ridge, North Atlantic, along a presumed amagmatic portion of oceanic crustal formation. Marine Geophysical Researches 29, 109-134.

) and mainly characterised by highly oblique spreading expressed as a series of en echelon rift basins (Géli et al., 2012

Géli, L., Renard, V., Rommevaux, C. (2012) Ocean crust formation processes at very slow spreading centers: A model for the Mohns RIdge, near 72ºN, based on magnetic, gravity, and seismic data. Journal of Geophysical Research: Solid Earth 99, 2995-3013.

). In contrast, its southern segment (the SMR) has an orthogonal spreading direction and irregular off-axis crustal morphology, with a shallower ridge axis and thicker crust (~10 km; Kandilarov et al., 2012

Kandilarov, A., Mjelde, R., Pedersen, R.B., Hellevang, B., Papenberg, C., Petersen, C.J., Planert, L., Flueh, E. (2012) The northern boundary of the Jan Mayen microcontinent, North Atlantic determined from ocean bottom seismic, multichannel seismic, and gravity data. Marine Geophysical Research 33, 55-76.

) (Fig. 1). Recent mapping indicates the presence of large, partly eroded volcanic structures, often bisected by faulting (Pedersen et al., 2010

). We interpret these structural and morphological characteristics as indicative of magma supply considerably higher than along the rest of the Mohns Ridge, possibly reflecting the influence of a nearby mantle plume associated with enhanced melt production.

Typical Mohns Ridge MORB are characterised by relatively high incompatible element contents and enriched radiogenic isotope values (Schilling et al., 1999

; Elkins et al., 2014

Elkins, L.J., Sims, K.W.W., Prytulak, J., Blichert-Toft, J., Elliott, T., Blusztajn, J., Fretzdorff, S., Reagan, M., Haase, K., Humphris, S., Schilling, J.G. (2014) Melt generation beneath Arctic Ridges: Implications from U decay series disequilibria in the Mohns, Knipovich, and Gakkel Ridges. Geochimica et Cosmochimica Acta 127, 140-170.

), but with relatively high ²⁰⁸Pb/²⁰⁴Pb and ²⁰⁷Pb/²⁰⁴Pb for a given ²⁰⁶Pb/²⁰⁴Pb, akin to the so-called DUPAL anomaly observed in the southern oceans (Blichert-Toft et al., 2005

). The lavas are further characterised by unusually high ε_Hf for a given ε_Nd (Blichert-Toft et al., 2005

), best explained by ancient garnet in the mantle source, perhaps hosted by SCLM. Such a source could have originated as delaminated Greenland continental lithosphere during rifting of the relatively young Greenland basin. All SMR basaltic glasses analysed here are tholeiitic with geochemistry intermediate between typical Mohns Ridge MORB and lavas from Jan Mayen Island, readily explained as products of straightforward binary mixing between Mohns Ridge-type and Jan Mayen Island-type endmember magmas (Figs. 2, 3, S-1, S-2, Table 1).

**Figure 2** **(a)** ε_Nd vs. ε_Hf, **(b)** ε_Nd vs. ²⁰⁶Pb/²⁰⁴Pb, **(c)** ²⁰⁷Pb/²⁰⁴Pb vs. ε_Hf, and **(d)** ²⁰⁷Pb/²⁰⁴Pb vs. ²⁰⁶Pb/²⁰⁴Pb diagrams for lavas from the Jan Mayen region and Iceland (Sun and Jahn, 1975
Sun, S.S., Jahn, B. (1975) Lead and Strontium Isotopes in Postglacial Basalts from Iceland. *Nature* 255, 527-530.
; Zindler *et al.*, 1979
Zindler, A., Hart, S.R., Frey, F.A. (1979) Nd and Sr isotope ratios and rare earth element abundances in Reykjanes Peninsula basalts evidence for mantle heterogeneity beneath Iceland. *Earth and Planetary Science Letters* 45, 249-262.
; Óskarsson *et al.*, 1982
Óskarsson, N., Sigvaldason, G., Steinthorsson, S. (1982) A dynamic model of rift zone petrogenesis and the regional petrology of Iceland. *Journal of Petrology* 23, 28-74.
; Hemond *et al.*, 1993
Hemond, C., Arndt, N.T., Lichtenstein, U., Hofmann, A.W., Oskarsson, N., Steinthorsson, S. (1993) The Heterogeneous Iceland Plume - Nd-Sr-O Isotopes and Trace-Element Constraints. *Journal of Geophysical Research-Solid Earth* 98, 15833-15850.
; Nowell *et al.*, 1998
Nowell, G.M., Kempton, P.D., Noble, S.R., Fitton, J.G., Saunders, A.D., Mahoney, J.J., Taylor, R.N. (1998) High precision Hf isotope measurements of MORB and OIB by thermal ionisation mass spectrometry; insights into the depleted mantle. *Chemical Geology* 149, 211-233.
; Salters and White, 1998
Salters, V.J.M., White, W.M. (1998) Hf isotope constraints on mantle evolution. *Chemical Geology* 145, 447-460.
; Schilling *et al.*, 1999
Schilling, J.G., Kingsley, R., Fontignie, D., Poreda, R., Xue, S. (1999) Dispersion of the Jan Mayen and Iceland mantle plumes in the Arctic: A He-Pb-Nd-Sr isotope tracer study of basalts from the Kolbeinsey, Mohns, and Knipovich Ridges. *Journal of Geophysical Research-Solid Earth* 104, 10543-10569.
; Chauvel and Hémond, 2000
Chauvel, C., Hémond, C. (2000) Melting of a complete section of recycled oceanic crust: Trace element and Pb isotopic evidence from Iceland. *Geochemistry Geophysics Geosystems* 1, 1001.
; Kempton *et al.*, 2000
Kempton, P.D., Fitton, J.G., Saunders, A.D., Nowell, G.M., Taylor, R.N., Hardarson, B.S., Pearson, G. (2000) The Iceland plume in space and time: a Sr-Nd-Pb-Hf study of the North Atlantic rifted margin. *Earth and Planetary Science Letters* 177, 255-271.
; Stracke *et al.*, 2003
Stracke, A., Zindler, A., Salters, V.J.M., McKenzie, D., Blichert-Toft, J., Albarède, F., Gronvold, K. (2003) Theistareykir revisited. *Geochemistry Geophysics Geosystems* 4, doi: 10.1029/2001gc000201.
; Blichert-Toft *et al.*, 2005
Blichert-Toft, J., Agranier, A., Andres, M., Kingsley, R., Schilling, J.G., Albarède, F. (2005) Geochemical segmentation of the Mid-Atlantic Ridge north of Iceland and ridge-hot spot interaction in the North Atlantic. *Geochemistry Geophysics Geosystems* 6, doi: 10.1029/2004GC000788.
; Elkins *et al.*, 2011
Elkins, L.J., Sims, K.W.W., Prytulak, J., Mattielli, N., Elliott, T., Blichert-Toft, J., Blusztajn, J., Dunbar, N., Devey, C. W., Mertz, D.F., Schilling, J.G. (2011) Understanding melt generation beneath the slow spreading Kolbeinsey Ridge from ²³⁸U, ²³⁰Th, and ²³¹Pa excesses. *Geochimica et Cosmochimica Acta* 75, 6300-6329.
; Sims *et al.*, 2013
Sims, K.W.W., Maclennan, J., Blichert-Toft, J., Mervine, E. M., Blusztajn, J., Grönvold, K. (2013) Short length scale mantle heterogeneity beneath Iceland probed by glacial modulation of melting. *Earth and Planetary Science Letters* 379, 146-157.
; Elkins *et al.*, 2014
Elkins, L.J., Sims, K.W.W., Prytulak, J., Blichert-Toft, J., Elliott, T., Blusztajn, J., Fretzdorff, S., Reagan, M., Haase, K., Humphris, S., Schilling, J.G. (2014) Melt generation beneath Arctic Ridges: Implications from U decay series disequilibria in the Mohns, Knipovich, and Gakkel Ridges. *Geochimica et Cosmochimica Acta* 127, 140-170.
) (Tables 1, S-2). Curves show calculated binary mixing trajectories between hypothesised geochemical compositions for Jan Mayen- (red box), Mohns- (yellow), Kolbeinsey- (blue) and Eggvin- (green) type melt endmembers, where tickmarks show percentage contributions of a pure Jan Mayen- or Eggvin-derived magma to a mixture. The Jan Mayen endmember, based on the most extreme enriched measurements for the island (Tables 1, S-2) has ε_Hf= +10.5, ε_Nd = +4.7, ²⁰⁶Pb/²⁰⁴Pb = 18.85, ²⁰⁷Pb/²⁰⁴Pb = 15.517, and Hf, Nd, and Pb concentrations of 6.9, 38.7, and 3.7 ppm, respectively. The hypothesised Mohns endmember, extrapolated to values that best explain available SMR samples as binary mixtures of Jan Mayen-Mohns Ridge lavas, has ε_Hf = +24, ε_Nd = +10.1, ²⁰⁶Pb/²⁰⁴Pb = 17.9, ²⁰⁷Pb/²⁰⁴Pb = 15.41, and Hf, Nd, and Pb concentrations of 5.6, 30, and 0.7, ppm, respectively; this composition is reasonable compared to published measurements from the Mohns Ridge (Schilling *et al.*, 1983
Schilling, J.G., Zajac, M., Evans, R., Johnston, T., White, W., Devine, J.D., Kingsley, R. (1983) Petrologic and Geochemical Variations Along the Mid-Atlantic Ridge from 29-Degrees-N to 73-Degrees-N. *American Journal of Science* 283, 510-586.
; Schilling *et al.*, 1999
Schilling, J.G., Kingsley, R., Fontignie, D., Poreda, R., Xue, S. (1999) Dispersion of the Jan Mayen and Iceland mantle plumes in the Arctic: A He-Pb-Nd-Sr isotope tracer study of basalts from the Kolbeinsey, Mohns, and Knipovich Ridges. *Journal of Geophysical Research-Solid Earth* 104, 10543-10569.
; Blichert-Toft *et al.*, 2005
Blichert-Toft, J., Agranier, A., Andres, M., Kingsley, R., Schilling, J.G., Albarède, F. (2005) Geochemical segmentation of the Mid-Atlantic Ridge north of Iceland and ridge-hot spot interaction in the North Atlantic. *Geochemistry Geophysics Geosystems* 6, doi: 10.1029/2004GC000788.
; Elkins *et al.*, 2014
Elkins, L.J., Sims, K.W.W., Prytulak, J., Blichert-Toft, J., Elliott, T., Blusztajn, J., Fretzdorff, S., Reagan, M., Haase, K., Humphris, S., Schilling, J.G. (2014) Melt generation beneath Arctic Ridges: Implications from U decay series disequilibria in the Mohns, Knipovich, and Gakkel Ridges. *Geochimica et Cosmochimica Acta* 127, 140-170.
). The Kolbeinsey endmember, based on depleted values from a suite of published MKR measurements (Schilling *et al.*, 1983
Schilling, J.G., Zajac, M., Evans, R., Johnston, T., White, W., Devine, J.D., Kingsley, R. (1983) Petrologic and Geochemical Variations Along the Mid-Atlantic Ridge from 29-Degrees-N to 73-Degrees-N. *American Journal of Science* 283, 510-586.
; Blichert-Toft *et al.*, 2005
Blichert-Toft, J., Agranier, A., Andres, M., Kingsley, R., Schilling, J.G., Albarède, F. (2005) Geochemical segmentation of the Mid-Atlantic Ridge north of Iceland and ridge-hot spot interaction in the North Atlantic. *Geochemistry Geophysics Geosystems* 6, doi: 10.1029/2004GC000788.
; Elkins *et al.*, 2011
Elkins, L.J., Sims, K.W.W., Prytulak, J., Mattielli, N., Elliott, T., Blichert-Toft, J., Blusztajn, J., Dunbar, N., Devey, C. W., Mertz, D.F., Schilling, J.G. (2011) Understanding melt generation beneath the slow spreading Kolbeinsey Ridge from ²³⁸U, ²³⁰Th, and ²³¹Pa excesses. *Geochimica et Cosmochimica Acta* 75, 6300-6329.
) and NKR sample POS436 246DR-2, has ε_Hf = +19.2, ε_Nd = +10, ²⁰⁶Pb/²⁰⁴Pb = 18.0, ²⁰⁷Pb/²⁰⁴Pb = 15.43, and Hf, Nd, and Pb concentrations of 0.5, 3, and 0.3 ppm, respectively; mixtures of Jan Mayen and Kolbeinsey endmembers cannot fully explain NKR lava compositions. The Eggvin-type component was extrapolated to values that best explain NKR basalts as mixtures between Kolbeinsey and an unknown enriched component, with ε_Hf = +11, ε_Nd = +5, ²⁰⁶Pb/²⁰⁴Pb = 18.96, ²⁰⁷Pb/²⁰⁴Pb = 15.528, ²⁰⁸Pb/²⁰⁴Pb = 38.72, and Hf, Nd, and Pb concentrations of 3, 22, and 11 ppm. Note that the high Pb content of the Eggvin-type endmember is necessary to generate a sufficiently hyperbolic mixing trajectory to account for NKR basalts.

Unlike the Mohns Ridge, the Kolbeinsey Ridge is overall characterised by orthogonal spreading at ultraslow rates (18 mm yr^-1; Mosar et al., 2002

; Dick et al., 2003

Dick, H.J.B., Lin, J., Schouten, H. (2003) An ultraslow-spreading class of ocean ridge. Nature 426, 405-412.

) and relatively thick ocean crust (7-10 km; Kodaira et al., 1997

). The NKR segment has a shallower ridge axis and therefore thicker crust than the neighbouring Middle Kolbeinsey Ridge (MKR). While ultraslow ridges are typically characterised by thin crust, tectonic spreading, and peridotite exposure, those features are not observed in the Jan Mayen region despite ultraslow full-spreading rates of 17-18 mm yr^-1 (Mosar et al., 2002

). Recent bathymetric mapping reveals that the Eggvin Bank in the centre of the NKR, in addition to being anomalously shallow, hosts fresh volcanic deposits indicative of high magma supply (e.g., sheet flows vs. monogenetic cones, a nearly subaerial volcanic edifice constructed atop the eastern axial flank wall, and fresh popping rocks) compared to the ends of the segment (Fig. 1). The large seamount lacks fresh fault scarps, suggesting elevated volcanic activity to maintain its height and cover active axial faulting. Regional bathymetry (Smith and Sandwell, 1997

Smith, W.H.F., Sandwell, D.T. (1997) Global sea floor topography from satellite altimetry and ship depth soundings. Science 277, 1956-1962.

) demonstrates the presence off-axis of shallow seafloor and highly segmented slopes persisting up to 30 km (~3 Ma) off-axis, further supporting a long-lived source of active volcanism. Bathymetry further reveals two parallel axial valleys to the south that both host fresh basalt (Fig. 1). This doubling of ridge axes suggests the segment is immature and can be explained by either active relocation of the segment towards the main, more easterly neovolcanic zone, or by simultaneously active, paired axial valleys as observed in Iceland. Either scenario suggests that NKR axial position is influenced by a long-lived source of enhanced magma supply.

Kolbeinsey Ridge basalts overall have notable depletions in incompatible trace elements and long-lived radiogenic isotope signatures, with high (²³⁰Th/²³⁸U) activity ratios, together suggesting high degrees of melting of a depleted garnet peridotite source (Elkins et al., 2014

). The abrupt change in purported mantle composition across the Jan Mayen Fracture Zone has been interpreted to indicate a sharp chemical discontinuity, perhaps reflecting a major mantle flow boundary (Haase et al., 1996

) (Fig. 3). Former work identified more enriched isotopic and trace element signatures on the Eggvin Bank and NKR than the MKR, generally attributed to the influence of the Jan Mayen hotspot (Schilling et al., 1999

; Haase et al., 2003

; Mertz et al., 2004

; Blichert-Toft et al., 2005

). Likewise, NKR α_Sm-Nd values (where α_Sm-Nd = (Sm/Nd)_sample / (Sm/Nd)_source, and (Sm/Nd)_source is calculated from ¹⁴³Nd/¹⁴⁴Nd_sample using a mantle model age of 1.8 Ga; DePaolo, 1988

DePaolo, D. (1988) Neodymium isotope geochemistry: An introduction.

; Sims et al., 1995

Sims, K.W.W., Depaolo, D.J., Murrell, M.T., Baldridge, W.S., Goldstein, S.J., Clague, D.A. (1995) Mechanisms of Magma Generation beneath Hawaii and Midocean Ridges - Uranium/Thorium and Samarium/Neodymium Isotopic Evidence. Science 267, 508-512.

; Salters, 1996

Salters, V.J.M. (1996) The generation of mid-ocean ridge basalts from the Hf and Nd isotope perspective. Earth and Planetary Science Letters 141, 109-123.

) are more typical of global MORB (<1.0), unlike other Kolbeinsey Ridge basalts with α_Sm-Nd > 1.0 (Salters, 1996

Salters, V.J.M. (1996) The generation of mid-ocean ridge basalts from the Hf and Nd isotope perspective. Earth and Planetary Science Letters 141, 109-123.

; Elkins et al., 2011

Elkins, L.J., Sims, K.W.W., Prytulak, J., Mattielli, N., Elliott, T., Blichert-Toft, J., Blusztajn, J., Dunbar, N., Devey, C. W., Mertz, D.F., Schilling, J.G. (2011) Understanding melt generation beneath the slow spreading Kolbeinsey Ridge from ²³⁸U, ²³⁰Th, and ²³¹Pa excesses. Geochimica et Cosmochimica Acta 75, 6300-6329.

), supporting a distinct mantle source beneath the NKR. While high (²³⁰Th/²³⁸U) activity ratios have suggested melting of a depleted garnet peridotite source for the MKR, NKR lavas have low (²³¹Pa/²³⁵U) activity ratios, likely the product of rapid melting of garnet-bearing eclogite (Elkins et al., 2011

, 2014

). We note that the basalt from the eastern axial valley resembles other NKR lavas, including geochemical indicators of enrichment, while the western axial valley basalt more closely resembles MKR basalts and presumably does not sample the enriched mantle component beneath the Eggvin Bank (Figs. 2, 3, S-1, S-2).

**Figure 3** Geochemical indicators vs. along-axis distance for the NKR and SMR, with the position of Jan Mayen Island projected westward onto the NKR using a geographic contour that runs parallel to the Jan Mayen Fracture Zone. **(a)** (Sm/Yb)_N, sensitive to the presence of garnet in, and the trace element makeup of, the source. The variation between Jan Mayen Island/SMR and the NKR likely reflects a heterogeneous mantle source. **(b)** α_Sm-Nd; because Sm is always more compatible than Nd during melting, values less than unity reflect the degree of melting of the model source, while values greater than unity (*e.g.,* MKR basalts; Salters, 1996
Salters, V.J.M. (1996) The generation of mid-ocean ridge basalts from the Hf and Nd isotope perspective. *Earth and Planetary Science Letters* 141, 109-123.
; Elkins *et al.*, 2011
Elkins, L.J., Sims, K.W.W., Prytulak, J., Mattielli, N., Elliott, T., Blichert-Toft, J., Blusztajn, J., Dunbar, N., Devey, C. W., Mertz, D.F., Schilling, J.G. (2011) Understanding melt generation beneath the slow spreading Kolbeinsey Ridge from ²³⁸U, ²³⁰Th, and ²³¹Pa excesses. *Geochimica et Cosmochimica Acta* 75, 6300-6329.
) require a different source composition and/or younger age than recorded by radiogenic isotopes.

While the above observations may suggest plume influence on NKR basalt production, the composition of the enriched endmember in the NKR/Eggvin mantle source differs notably from the Jan Mayen mantle component inferred from Jan Mayen Island- and SMR-derived lavas (Fig. 2). For example, the more enriched basalts collected from the Eggvin Bank exhibit lower (Sm/Yb)_N ratios than the Jan Mayen endmember (Table S-2, Figs. 3, S-1), which cannot be explained by a lack of residual garnet in the source, since NKR magmas are known to be products of melting in the presence of garnet from ²³⁰Th/²³⁸U > 1 (Elkins et al., 2011

, 2014

). Observed NKR trace element patterns thus likely reflect the composition of a distinct mantle source located beneath the Eggvin Bank. Although not as pronounced as DUPAL-type signatures to the north, this Eggvin-type mantle source also exhibits slightly elevated ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios for a given ²⁰⁶Pb/²⁰⁴Pb and higher e_Hf for a given e_Nd (Table 1, Figs. 2, S-2). Moreover, if generated by binary mixing, the isotopic compositions of Eggvin Bank basalts require a notably Pb-rich Eggvin endmember magma (Fig. 2). In addition to the ²³¹Pa/²³⁵U evidence for eclogite (Elkins et al., 2014

), partition coefficients for Pb, Si, Al, and Fe in eclogite support an eclogite-rich source contributing magmas with the relatively high Pb and SiO₂ and low FeO and Al₂O₃ observed in NKR MORB (Haase et al., 2003

; Pertermann and Hirschmann, 2003

Pertermann, M., Hirschmann, M. (2003) Partial melting experiments on MORB-like pyroxenite between 2 and 3 GPa: Constraints on the presence of pyroxenite in basalt source regions from solidus location and melting rate. Journal of Geophysical Research 108, doi: 10.1029/2000JB000118.

) (Tables S-2, S-3, Figs. S-2, S-3, S-4). Such an eclogite-bearing source is supported by correlations between Pb and radiogenic isotopes, with higher Pb contents associated with the most enriched isotopic signatures for the NKR (Fig. S-4). We thus infer that the most likely mantle source for the Eggvin-type signature in NKR basalts is an eclogite-rich mantle containing ancient, high-e_Hf garnet (Blichert-Toft et al., 2005

). Existing models suggest that garnet-bearing veins or blobs of SCLM are present in the North Atlantic mantle, likely having originated under Greenland prior to basin rifting by delamination (Blichert-Toft et al., 2005

); a concentrated pocket of such material may plausibly have been trapped beneath the NKR by the relocation of the active ridge axis to the Kolbeinsey Ridge from the Aegir Ridge at ~25 Ma (Fig. 1). While the more fusible eclogite can generate thickened crust without elevated mantle temperatures, the other morphological evidence (large near-axis seamounts and paired axial valleys) and extreme nature of the crustal thickening would also support the influence of a plume on mantle temperature beneath the NKR.

Jan Mayen and environs demonstrate the dramatic extent to which magmatism generated by heterogeneous mantle, possibly with a plume source, can influence the structure and behaviour of ultraslow mid-ocean ridges. Here, multiple mantle heterogeneities within a relatively small geographic area have significantly modified the accretionary process of two ridge segments, generating enhanced magmatic activity, variations in spreading direction, adjusted axial locations, and, where mantle flow permits, the direct addition of heterogeneous, possibly plume-derived magma. We hence assert that the distinct morphology and tectonically-dominated accretionary style typical of ultraslow spreading ridges (Dick et al., 2003

Dick, H.J.B., Lin, J., Schouten, H. (2003) An ultraslow-spreading class of ocean ridge. Nature 426, 405-412.

) is particularly sensitive to even modest increases in mantle temperature and magma supply, which cause the ridge to take on growth properties more typical of slow- or intermediate-spreading ridges. For comparison, the 17 ºS location on the East Pacific Rise is adjacent to a small hotspot but shows little geomorphological impact at fast spreading rates (Mahoney et al., 1994

Mahoney, J.J., Sinton, J.M., Kurz, M.D., Macdougall, J.D., Spencer, K.J., Lugmair, G.W. (1994) Isotope and trace element characteristics of a super-fast spreading ridge: East Pacific Rise, 13-23ºS. Earth and Planetary Science Letters 121, 173-193.

). This demonstrates that for ultraslow ridges, the control on accretionary mechanisms is principally magma supply, which is typically but, importantly, not solely controlled by spreading rate.

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Acknowledgements

Abstract | Letter | Acknowledgements | References | Supplementary Information

L.J.E. and K.W.W.S. acknowledge the Ocean Sciences Section of the National Science Foundation for supporting USA geochemical work and travel for this project. Geochemical analyses and field work were supported by the Norwegian Research Council in Norway to C.H. and R.P., and by the French Agence Nationale de la Recherche (ANR-10-BLAN-0603 M&Ms — Mantle Melting — Measurements, Models, Mechanisms) to J.B.T. I.A.Y. was supported by an A.v. Humboldt Fellowship. Jan Mayen Island samples from the Maaløe collection were supplied by D. DePaolo. Analyses at Boston University were performed by T. Ireland. We thank N. Augustin, M. Deutschmann, T. Laurila, K. Meisenhelder, E. Rivers, M. Rothenbeck, F. van der Zwan, and I. Yeo for field assistance on the F.S. Poseidon expedition in 2012; N. Augustin, I. Yeo, K. Meisenhelder, and R. Davis for assistance with bathymetric data; and E. Rivers, R. Davis, K. Meisenhelder, R. Chernow, Y. Ronen, S.H. Dundas, O. Tumyr and P. Telouk for assistance in the laboratory.

Editor: Graham Pearson

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References

Abstract | Letter | Acknowledgements | References | Supplementary Information

Sample number*	Location	Expedition^b	Year	Latitude (ºN)		Longitude (ºW)		Depth (m)
Sample number*	Location	Expedition^b	Year	Start	Stop	Start	Stop	Start	Stop
POS436 242DR-2b	NKR	R/V Poseidon Leg 436	2012	70.75997	70.76498	13.55035	13.54483	1559	1436
POS436 246DR-2	NKR	R/V Poseidon Leg 436	2012	70.78942	70.79472	13.75350	13.75400	1714	1630
POS436 235DR-1a	NKR	R/V Poseidon Leg 436	2012	70.91275	70.91237	13.12408	13.11248	485	381
POS436 253DR-E2	NKR	R/V Poseidon Leg 436	2012	70.94897	70.94742	13.03475	13.03770	207	175
POS436 253DR-6	NKR	R/V Poseidon Leg 436	2012	70.94897	70.94742	13.03475	13.03770	207	175
POS436 232DR-1	NKR	R/V Poseidon Leg 436	2012	71.05987	71.05655	12.95232	12.93983	622	578
POS436 209DR-2a	NKR	R/V Poseidon Leg 436	2012	71.31342	71.31592	12.70272	12.69447	1199	1205
POS436 222DR-1	NKR	R/V Poseidon Leg 436	2012	71.34700	71.34720	12.64333	12.62925	1139	1137
POS436 215DR-1	NKR	R/V Poseidon Leg 436	2012	71.47660	71.47597	12.39383	12.40617	1819	1703
SM01-DR-24-14a	JM	R/V Håkon Mosby. SM01	2001	-	71.1287	-	7.8082	-	738
SM01-DR-23-3	JM	R/V Håkon Mosby. SM01	2001	-	71.1022	-	7.7913	-	697
SM01-DR-5-5	JM	R/V Håkon Mosby. SM01	2001	-	71.1192	-	7.9187	-	47
SM01-DR-60-43	JM	R/V Håkon Mosby. SM01	2001	-	71.1645	-	7.9880	-	222
SM01-DR-100-01	SMR	R/V Håkon Mosby. SM01	2001		70.9855	-	6.4003	-	2493
CGB-2011-D17-2a	SMR	R/V G.O. Sars. CGB2011	2011	71.26167	71.2613	5.84300	5.8397	-	847
SM01-DR70-1	SMR	R/V Håkon Mosby. SM01	2001	-	71.2382	-	6.1102	-	953
SM01-DR67-4	SMR	R/V Håkon Mosby. SM01	2001	-	71.2188	-	6.1713	-	806
SM01-DR-91-13	SMR	R/V Håkon Mosby. SM01	2001	-	71.2715	-	5.8468	-	732

Sample	Ce	Pr	Nd	Sm	Eu	Gd	Tb	Dy	Ho	Er	Yb	Lu	Hf	Pb	U	Th
Submarine samples:
POS436 242DR-2b^a	16.3	2.1	8.9	2.3	0.8	3.0	0.5	3.4	0.7	2.3	2.5	0.4	1.5	0.7	0.21	0.76
POS436 246DR-2^a	6.1	0.9	4.6	1.6	0.6	2.5	0.5	3.1	0.7	2.2	2.4	0.4	1.1	0.4	0.04	-
POS436 235DR-1a^a	33.0	4.2	17.8	4.5	1.5	5.4	0.9	5.9	1.3	4.0	4.1	0.6	3.1	1.3	0.44	1.60
POS436 253DR-E2^a	34.4	4.1	17.4	4.3	1.4	5.0	0.9	5.4	1.1	3.6	3.7	0.6	3.4	1.4	0.49	1.54
POS436 253DR-6^a	32.4	3.7	15.3	3.7	1.2	4.4	0.8	4.7	1.0	3.2	3.3	0.5	3.3	1.7	0.48	1.49
POS436 232DR-1^a	11.0	1.5	7.0	2.1	0.8	2.9	0.5	3.5	0.8	2.4	2.6	0.4	1.4	0.6	0.13	0.37
POS436 209DR-2a^a	12.3	1.7	8.1	2.6	0.9	3.7	0.7	4.4	1.0	3.1	3.4	0.5	1.6	0.7	0.14	0.46
POS436 222DR-1^a	12.7	1.7	7.8	2.3	0.9	3.3	0.6	3.9	0.8	2.7	2.8	0.4	1.5	0.7	0.16	0.50
POS436 222DR-1 replicate^a	12.9	1.8	8.1	2.4	0.9	3.3	0.6	3.9	0.9	2.7	2.9	0.4	1.5	0.7	0.16	0.54
POS436 215DR-1^a	15.4	2.0	8.5	2.3	0.8	3.1	0.6	3.7	0.8	2.5	2.7	0.4	1.7	0.9	0.21	0.62
SM01-DR-24-14^b	144.2	16.2	60.1	10.2	3.1	7.8	1.0	6.1	1.1	3.0	2.6	0.4	6.5	-	2.31	8.56
SM01-DR-23-3^b	184.6	20.9	75.4	12.3	3.8	9.6	1.3	7.4	1.4	3.9	3.4	0.5	9.1	-	2.75	10.39
SM01-DR-5-5^b	100.9	11.9	45.3	8.0	2.4	6.8	0.9	5.2	1.0	2.6	2.1	0.3	5.7	-	1.46	5.97
SM01-DR-60-43^b	123.2	14.0	52.7	9.0	2.8	7.3	1.0	5.5	1.0	2.7	2.3	0.3	6.2	-	1.91	7.34
SM01-DR-100-01^b	65.1	8.2	31.4	6.2	2.0	5.6	0.8	5.4	1.1	3.0	3.0	0.4	4.0	-	0.95	0.95
CGB-2011-D17-2a^a	60.9	7.1	27.7	5.7	1.8	5.6	0.9	5.0	1.0	3.0	3.0	0.5	4.0	1.7	0.93	3.44
SM01-DR70-1^a	58.0	5.6	21.3	3.9	1.2	3.4	0.5	2.9	0.6	1.7	1.6	0.2	4.0	1.9	0.99	2.46
SM01-DR67-4^a	85.5	10.7	39.8	7.6	2.2	6.7	1.0	5.9	1.1	3.2	2.9	0.4	5.3	2.4	4.86	1.29
SM01-DR-91-13^b	45.3	5.7	23.6	5.5	1.8	5.5	0.8	5.6	1.2	3.3	3.3	0.5	3.6	-	0.64	2.44
Subaerial samples (samples from Maaløe et al., 1986 Maaløe, S., Sørensen, I.B., Hertogen, J. (1986) The trachybasaltic suite of Jan Mayen. Journal of Petrology 27, 439-466. ):
JM-192^a	78.8	10.1	39.5	7.7	2.5	6.8	1.0	5.1	0.9	2.7	2.3	0.3	6.9	2.2	0.81	4.45
JM-71^a	80.1	10.1	37.9	6.9	2.1	5.7	0.8	4.2	0.8	2.2	2.0	0.3	6.9	2.5	1.14	5.08
JM-84^a	154.6	20.8	76.2	13.1	3.9	10.6	1.5	7.7	1.4	4.1	3.7	0.5	10.4	3.7	2.06	12.22
Rock standard:
BHVO-2	36.1	5.4	23.8	6.0	2.0	5.9	0.9	5.2	0.9	2.4	1.9	0.3	4.3	1.6	1.23	0.41

Sample	Location	SiO₂	Al₂O₃	TiO₂	CaO	MnO	MgO	FeO	Fe₂O₃	Na₂O	K₂O	Cr₂O₃	P₂O₅	Cl	Total
Submarine samples:
POS436 242DR-2b	NKR	50.69(8)	15.25(4)	1.04(2)	13.02(4)	-	8.37(5)	8.66(5)	-	1.70(1)	0.267(4)	0.09(1)	0.144(7)	0.032(2)	99.4(1)
POS436 246DR-2	NKR	50.73(9)	15.02(4)	0.85(3)	13.64(6)	-	8.95(3)	9.16(7)	-	1.57(1)	0.074(3)	0.07(1)	0.094(7)	0.012(1)	100.3(1)
POS436 235DR-1^a	NKR	52.27(9)	13.06(4)	2.25(5)	8.68(5)	-	4.54(3)	14.60(6)	-	2.76(2)	0.551(4)	0.031(9)	0.22(1)	0.141(3)	99.3(2)
POS436 253DR-E2	NKR	52.5(2)	11.9(9)	2.0(1)	9.8(6)	-	6.2(1.3)	13.6(2)	-	2.5(3)	0.45(5)	0.03(1)	0.20(2)	0.14(2)	99.5(2)
POS436 253DR-6	NKR	52.70(8)	12.8(5)	2.06(7)	9.1(2)	-	5.4(7)	13.66(7)	-	2.7(1)	0.50(3)	0.034(9)	0.22(1)	0.149(8)	99.5(2)
POS436 232DR-1	NKR	50.90(7)	14.83(3)	1.00(3)	13.16(5)	-	8.30(3)	9.30(6)	-	1.777(9)	0.192(2)	0.07(1)	0.074(5)	0.028(1)	99.8(1)
POS436 209DR-2^a	NKR	52.2(1)	13.03(4)	2.28(3)	8.68(3)	-	4.57(2)	14.70(7)	-	2.75(2)	0.568(2)	0.05(1)	0.233(8)	0.143(2)	99.4(2)
POS436 222DR-1	NKR	51.4(1)	14.58(4)	1.10(2)	12.44(5)	-	7.56(7)	10.22(5)	-	1.95(3)	0.230(3)	0.06(1)	0.095(7)	0.026(1)	99.9(1)
POS436 215DR-1	NKR	52.24(7)	14.24(4)	1.25(3)	11.09(6)	-	6.64(3)	11.50(7)	-	2.088(8)	0.309(3)	0.027(7)	0.121(8)	0.038(2)	99.7(1)
SM01-DR-24-14	JM	47.8(1)	15.11(5)	4.42(3)	8.94(6)	-	3.70(2)	12.20(6)	-	3.26(2)	3.10(5)	0.009(5)	0.75(1)	0.118(3)	99.6(2)
SM01-DR-23-3	JM	56.8(1)	17.14(4)	2.14(4)	4.56(4)	-	2.19(1)	6.11(4)	-	3.7(2)	3.93(2)	0.020(6)	0.453(8)	0.191(3)	97.5(2)
SM01-DR-5-5^a	JM	48.01	15.1907562756	3.29	10.31	0.20	6.45	-	12.1	2.98	2.48	-	0.59	0.06	101.7
SM01-DR-60-43^a	JM	48.17	16.1320050686	3.51	10.41	0.20	5.05	-	12.8	3.02	2.67	-	0.62	0.12	102.7
CGB-2011-D17-2^a	SMR	51.4(2)	14.57(7)	2.44(2)	9.77(6)	-	5.22(2)	11.57(7)	-	2.90(1)	1.003(8)	0.015(7)	0.367(9)	0.152(2)	99.6(3)
SM01-DR70-1	SMR	50.37(7)	14.74(4)	2.37(3)	10.17(5)	-	5.62(2)	10.5(1)	-	2.75(2)	0.984(6)	0.026(9)	0.354(8)	0.124(2)	98.2(2)
SM01-DR67-4	SMR	51.18(9)	14.35(8)	2.86(3)	9.08(4)	-	4.61(3)	11.06(9)	-	2.85(4)	1.40(3)	0.029(9)	0.476(9)	0.109(3)	98.2(2)
SM01-DR-91-13	SMR	52.1(2)	14.17(4)	2.34(3)	9.8(2)	-	5.2(1)	12.0(1)	-	2.5(2)	0.75(1)	0.025(7)	0.298(8)	0.069(2)	99.4(2)
Subaerial samples (samples from Maaløe et al., 1986 Maaløe, S., Sørensen, I.B., Hertogen, J. (1986) The trachybasaltic suite of Jan Mayen. Journal of Petrology 27, 439-466. ):
JM-192^a	JM	47.39	12.92	2.52	12.45	0.17	11.13	-	11.1	2.20	1.13	-	0.43	-	101.4
JM-71^a	JM	46.41	12.84	2.48	12.11	0.17	10.61	-	11.0	2.16	1.15	-	0.42	-	99.3
JM-84^a	JM	47.65	16.52	3.26	9.88	0.19	5.02	-	11.1	3.59	2.05	-	0.68	-	100.0
Rock standard:
BHVO-2		48.80	13.30	2.74	11.20	0.16	7.17	-	12.1	2.11	0.50	-	0.27	-	98.4

Sample	Location	Li	Sc	V	Cr	Co	Ni	Cu	Zn	Rb	Sr	Zr	Y	Mo	Ba	La
Submarine samples:
POS436 242DR-2b^a	NKR	4.9	47.8	274.7	404.1	45.6	127.1	88.0	79.2	7.5	145.5	54.8	24.7	0.5	99.0	8.0
POS436 246DR-2^a	NKR	4.5	46.9	275.3	246.0	49.4	85.7	110.2	72.4	1.6	56.4	36.5	23.9	0.2	-	2.3
POS436 235DR-1a^a	NKR	7.7	43.7	424.4	18.1	43.8	20.1	49.9	104.2	12.0	150.0	118.6	39.6	0.9	172.2	15.4
POS436 253DR-E2^a	NKR	6.8	36.9	409.0	14.6	42.9	174.6	49.1	105.8	11.6	140.4	128.2	34.6	1.0	175.7	15.1
POS436 253DR-6^a	NKR	6.6	30.2	402.3	19.4	43.2	38.8	73.8	160.4	11.4	138.1	132.4	32.6	1.2	173.2	13.7
POS436 232DR-1^a	NKR	4.9	47.9	280.4	376.3	46.2	106.3	92.3	73.8	4.2	99.2	46.8	25.5	0.4	52.7	5.2
POS436 209DR-2a^a	NKR	6.7	52.9	329.2	173.9	47.6	55.7	77.4	89.9	6.4	103.2	52.4	33.2	0.4	66.4	5.8
POS436 222DR-1^a	NKR	5.5	47.4	302.1	226.0	47.0	74.3	88.4	83.2	4.8	110.3	50.6	27.7	0.5	72.3	6.0
POS436 222DR-1 replicate^a	NKR	5.4	49.2	311.1	233.2	48.5	74.4	91.6	87.3	5.2	114.6	52.7	28.6	0.5	75.1	6.2
POS436 215DR-1^a	NKR	5.8	38.3	332.2	87.6	47.4	51.6	76.0	98.7	6.6	100.1	61.8	25.6	0.6	103.4	7.2
SM01-DR-24-14^b	JM	-	-	-	17.0	-	31.8	-	-	76.8	764.4	285.5	31.3	-	1192.4	71.6
SM01-DR-23-3^b	JM	-	-	-	3.7	-	-	-	-	90.5	876.6	404.6	39.6	-	1727.2	94.9
SM01-DR-5-5^b	JM	-	-	-	39.2	-	23.2	-	-	54.6	556.9	231.9	26.8	-	832.2	48.6
SM01-DR-60-43^b	JM	-	-	-	20.0	-	22.8	-	-	69.7	674.7	261.5	28.7	-	1051.6	56.5
SM01-DR-100-01^b	SMR	-	-	-	7.6	-	29.8	-	-	29.4	315.9	173.2	32.8	-	454.7	0.0
CGB-2011-D17-2a^a	SMR	6.3	31.4	367.8	23.4	41.8	28.3	54.8	103.8	24.5	326.2	162.1	30.8	2.0	453.7	28.8
SM01-DR70-1^a	SMR	4.0	18.3	361.5	24.4	42.2	33.5	63.9	95.3	20.9	316.8	165.1	19.3	2.1	423.8	23.2
SM01-DR67-4^a	SMR	7.0	25.4	423.8	26.9	44.4	22.3	55.5	107.6	39.7	403.5	227.6	30.4	-	639.1	45.0
SM01-DR-91-13^b	SMR	-	-	-	9.0	-	15.5	-	-	21.0	244.6	135.1	32.5	-	319.3	20.3
Subaerial samples (samples from Maaløe et al., 1986 Maaløe, S., Sørensen, I.B., Hertogen, J. (1986) The trachybasaltic suite of Jan Mayen. Journal of Petrology 27, 439-466. ):
JM-192^a	JM	5.1	40.4	284.7	671.1	49.6	167.5	34.0	87.1	33.0	820.8	279.2	30.5	1.8	620.1	40.5
JM-71^a	JM	4.8	35.3	364.3	533.0	51.9	188.1	100.0	81.4	41.6	650.7	287.5	25.3	2.3	814.7	44.1
JM-84^a	JM	7.7	27.1	347.7	55.9	33.9	38.6	51.9	102.8	101.8	1156.9	457.5	46.0	4.0	1546.7	90.8
Rock standard:
BHVO-2		4.4	29.1	315.3	327.6	48.4	122.9	132.2	104.0	8.0	365.8	158.7	24.5	-	128.79298	14.6

Sample	Location	Li	Sc	V	Cr	Co	Ni	Cu	Zn	Rb	Sr	Zr	Y	Mo	Ba	La
Submarine samples:
POS436 242DR-2b^a	NKR	4.9	47.8	274.7	404.1	45.6	127.1	88.0	79.2	7.5	145.5	54.8	24.7	0.5	99.0	8.0
POS436 246DR-2^a	NKR	4.5	46.9	275.3	246.0	49.4	85.7	110.2	72.4	1.6	56.4	36.5	23.9	0.2	-	2.3
POS436 235DR-1a^a	NKR	7.7	43.7	424.4	18.1	43.8	20.1	49.9	104.2	12.0	150.0	118.6	39.6	0.9	172.2	15.4
POS436 253DR-E2^a	NKR	6.8	36.9	409.0	14.6	42.9	174.6	49.1	105.8	11.6	140.4	128.2	34.6	1.0	175.7	15.1
POS436 253DR-6^a	NKR	6.6	30.2	402.3	19.4	43.2	38.8	73.8	160.4	11.4	138.1	132.4	32.6	1.2	173.2	13.7
POS436 232DR-1^a	NKR	4.9	47.9	280.4	376.3	46.2	106.3	92.3	73.8	4.2	99.2	46.8	25.5	0.4	52.7	5.2
POS436 209DR-2a^a	NKR	6.7	52.9	329.2	173.9	47.6	55.7	77.4	89.9	6.4	103.2	52.4	33.2	0.4	66.4	5.8
POS436 222DR-1^a	NKR	5.5	47.4	302.1	226.0	47.0	74.3	88.4	83.2	4.8	110.3	50.6	27.7	0.5	72.3	6.0
POS436 222DR-1 replicate^a	NKR	5.4	49.2	311.1	233.2	48.5	74.4	91.6	87.3	5.2	114.6	52.7	28.6	0.5	75.1	6.2
POS436 215DR-1^a	NKR	5.8	38.3	332.2	87.6	47.4	51.6	76.0	98.7	6.6	100.1	61.8	25.6	0.6	103.4	7.2
SM01-DR-24-14^b	JM	-	-	-	17.0	-	31.8	-	-	76.8	764.4	285.5	31.3	-	1192.4	71.6
SM01-DR-23-3^b	JM	-	-	-	3.7	-	-	-	-	90.5	876.6	404.6	39.6	-	1727.2	94.9
SM01-DR-5-5^b	JM	-	-	-	39.2	-	23.2	-	-	54.6	556.9	231.9	26.8	-	832.2	48.6
SM01-DR-60-43^b	JM	-	-	-	20.0	-	22.8	-	-	69.7	674.7	261.5	28.7	-	1051.6	56.5
SM01-DR-100-01^b	SMR	-	-	-	7.6	-	29.8	-	-	29.4	315.9	173.2	32.8	-	454.7	0.0
CGB-2011-D17-2a^a	SMR	6.3	31.4	367.8	23.4	41.8	28.3	54.8	103.8	24.5	326.2	162.1	30.8	2.0	453.7	28.8
SM01-DR70-1^a	SMR	4.0	18.3	361.5	24.4	42.2	33.5	63.9	95.3	20.9	316.8	165.1	19.3	2.1	423.8	23.2
SM01-DR67-4^a	SMR	7.0	25.4	423.8	26.9	44.4	22.3	55.5	107.6	39.7	403.5	227.6	30.4	-	639.1	45.0
SM01-DR-91-13^b	SMR	-	-	-	9.0	-	15.5	-	-	21.0	244.6	135.1	32.5	-	319.3	20.3
Subaerial samples (samples from Maaløe et al., 1986 Maaløe, S., Sørensen, I.B., Hertogen, J. (1986) The trachybasaltic suite of Jan Mayen. Journal of Petrology 27, 439-466. ):
JM-192^a	JM	5.1	40.4	284.7	671.1	49.6	167.5	34.0	87.1	33.0	820.8	279.2	30.5	1.8	620.1	40.5
JM-71^a	JM	4.8	35.3	364.3	533.0	51.9	188.1	100.0	81.4	41.6	650.7	287.5	25.3	2.3	814.7	44.1
JM-84^a	JM	7.7	27.1	347.7	55.9	33.9	38.6	51.9	102.8	101.8	1156.9	457.5	46.0	4.0	1546.7	90.8
Rock standard:
BHVO-2		4.4	29.1	315.3	327.6	48.4	122.9	132.2	104.0	8.0	365.8	158.7	24.5	-	128.79298	14.6