Sill and lava geochemistry of the mid-Norway and NE Greenland conjugate margins



[1] This paper presents major, trace-elements, and Sr-Nd isotopes for two prominent sills formed during the opening of the North Atlantic, sampled by the Utgard borehole (6607/5-2) in the Vøring Plateau. The Utgard sills are compared to opening-related lavas recovered from ODP Leg 104 Hole 642E farther west on the Vøring Plateau and on the NE Greenland conjugate margin. The Utgard sills (3.6–5.9 wt % MgO) are enriched in strongly relative to moderately incompatible trace elements and have 87Sr/86Sr and 143Nd/144Nd ratios of 0.70380–70387 and 0.51292–0.51293, respectively, in the Upper Utgard Sill, and 0.70303–0.70306 and 0.51297–0.51299 in the Lower Sill. Alteration is minor. The Utgard melts originated by partial melting of an asthenospheric, depleted mantle source (DMM or Iceland Rift Zone, IRZ, type) with chemical characteristics similar to the source that gave rise to NE Greenland lavas. The Utgard magmas underwent extensive fractional crystallization in the lower crust (Upper Sill: >70%; Lower Sill: >55%) with removal mainly of olivine and pyroxenes, accompanied by ≤1% assimilation of crustal melts. This crystallization formed significant masses of dense cumulates (∼3.25 g/cm3) (underplating). Assuming an areal extent similar to that of the two sills, we estimate a composite layer of ultramafic cumulates mixed with less dense country rocks to be >320 m thick beneath the two Utgard sills and >8.8 km beneath the thickest part of the Vøring Plateau lavas. Opening-related cumulates may thus account for a significant part of the lower crustal high-velocity, high-density bodies (average density 3.1 g/cm3) along the Norwegian margin.

1. Introduction

[2] The opening of the NE Atlantic was accompanied by anomalously high breakup related magmatic activity forming the North Atlantic Large Igneous Province (Figure 1) [e.g., Talwani and Eldholm, 1977; Eldholm et al., 1989]. Along the Norwegian margin, the volcanism includes seaward dipping reflectors, sills, and dykes, mainly in the Møre and Vøring basins [e.g., Planke and Eldholm, 1994; Eldholm et al., 1989; Eldholm, 1991; Berndt et al., 2000; Planke et al., 2000, 2005]. In contrast to Greenland, there are no volcanic exposures on the Norwegian margin; petrological-geochemical information is therefore sparse. The conjugate east Greenland margin shows various types of shallow intrusions in addition to large volumes of flood basalts at the Blosseville Kyst, Jameson Land Basin, Hold with Hope, and Wollaston Forland [e.g., Brooks et al., 1976; Upton et al., 1984; Hald and Tegner, 2000; Brooks, 2011] (Figure 1). Seismic surveys have also identified extensive lower crustal high-velocity, high-density bodies (LCBs) beneath both the Norwegian (Figure 2) and east Greenland margins and the adjacent oceanic lithosphere [e.g., Planke et al., 1991; Skogseid et al., 1992; Mjelde et al., 2002, 2009; Voss and Jokat, 2007; White et al., 2008; Reynisson et al., 2010].

Figure 1.

(a) Distribution of volcanic rocks on the Vøring and the conjugate central-east Greenland margin reconstructed to breakup time (55.8 Ma). The sills discussed in this paper were recovered from borehole 6607/5-2 on the Utgard High. Reconstruction and main structural elements are based on Faleide et al. [2010]. Distribution of volcanic rocks from Berndt et al. [2001], Escher and Pulvertaft [1995], Planke et al. [2005], and new seismic mapping. The landward limit of the LCB is from Mjelde et al. [2009] on the Vøring Margin and Voss and Jokat [2007] and Voss et al. [2009] on the Greenland margin. Volcanic features: Blosseville Kyst (BK), Faroe-Shetland Escarpment (FSE), Thetis Escarpment (TE), Vøring Escarpment (VE), and Vøring Marginal High (VMH). Sedimentary basins: Danmarkshavn Basin (DB), Halten Terrace (HT), Jameson Land Basin (JB), Thetis Basin (TB), Trøndelag Platform (TP), and Vøring Basin (VB). Structural highs: Danmarkshavn Ridge (DR), Gjallar Ridge (GR), Jan Mayen Ridge (JMR), Liverpool Land (LL), Nordland Ridge (NR), Nyk High (NH), Rån Ridge (RR), Utgard High (UH), and Utrøst High (UR). Geographical names: Hold-with-Hope (HwH), Shannon (S), Traill Ø (TØ), and Wollaston Forland (WF). Other abbreviations are LM: lineament and LCB: lower crustal body with high seismic velocities (>7 km/s). Black dots with and without numbers: wells. (b) Simplified lithological log of the Utgard 6607/5-2 borehole showing the two studied sill intrusions. The log is based on the well completion report available at

Figure 2.

Crustal transect across the central part of the Vøring Margin. The transect landward of the COB is located in Figure 1. Seismic reflection interpretation of profiles VBT-94-06 and HV-96-08 constrained by Skogseid et al. [2000], Planke et al. [2000, 2005], Berndt et al. [2001], and Faleide et al. [2008] and depth converted using stacking and wide-angle velocities. Deep crustal geometries derived from nearby ocean-bottom seismometer profiles L7–92 and L1–99 [Mjelde et al., 1997a, 1997b, 2005] and expanded spread profiles [Planke et al., 1991]. COB: Continent-ocean boundary; LCB: Lower crustal body with high seismic velocities (>7 km/s); SDR: Seaward-dipping reflections; and K: Lower reflection of SDR. Borehole 642 is located on the Vøring Marginal High, south of the transect (Figure 1).

[3] On the East Greenland margin, the timing and chemical characteristics of the breakup magmatism have been studied both on land and in drill cores of the seaward-dipping reflectors (ODP legs 152 and 163). It has been shown that flood basalts and intrusions were emplaced over tens of millions of years (∼61–14 Ma) with a short peak less than 300,000 years long, coinciding with continental rupture ∼56 million years ago [Larsen and Tegner, 2006; Storey et al., 2007b]. The regional, voluminous magmatism before and during continental breakup has been linked to melting of the Iceland mantle plume and records secular changes in melting conditions, mantle sources, and crustal assimilation controlled by lithosphere tectonics [Thirlwall et al., 1994; Fram and Lesher, 1997; Tegner et al., 1998a, 2008; Fitton et al., 2000; Peate et al., 2008; Voss and Jokat, 2007]. So far chemical data on the breakup magmatism on the Norwegian margin is restricted to the lava series recovered in a single drill core, ODP Leg 104 Hole 642E in the outer part of the Vøring Plateau and Deep Sea Drilling Programme (DSDP) Sites 338, 342, and 343 on the northern Vøring Margin (Figure 1) [e.g., Viereck et al., 1988, 1989; Parson et al., 1989; Meyer et al., 2009].

[4] This paper presents new data on two extensive sills penetrated by borehole 6607/5-2 located at the Utgard High in the Vøring Basin (Figure 1). The aim is to (1) establish the geochemical character and evolutionary history of the Utgard sills; (2) compare the Utgard sills to lavas in ODP Leg 104 Hole 642E on the Vøring Plateau and to outcrops of flood basalt at Hold with Hope and Wollaston Forland on the conjugate NE Greenland margin in order to examine regional variations with respect to crustal contamination, melting processes, and mantle sources; and (3) discuss possible relationships to the LBC in the deep crust beneath the Vøring Basin (Figure 2).

2. Geological Setting

[5] The thick sequence of lavas forming seaward dipping reflectors covering the outer part of the Møre and Vøring margins and adjacent oceanic crust exceeds 6 km in composite thickness and extruded during the last stages of rifting and earliest stages of seafloor spreading [e.g., Skogseid and Eldholm, 1987, 1989; Planke and Eldholm, 1994]. ODP Leg 104 Hole 642E (hereafter referred to as 642E) was drilled on the Vøring Plateau on continental lithosphere in the innermost part of the lavas that form the seaward dipping reflectors (Figures 1 and 2) [Eldholm et al., 1989].

[6] The Vøring and Møre basins east of the seaward dipping reflectors contain voluminous magmatic complexes of dominantly subhorizontal sheets (sills) that intruded Cretaceous sedimentary rocks during opening of the northeast Atlantic [e.g., Berndt et al., 2000; Planke et al., 2005]. Sill intrusions and the Mesozoic basins cover >85,000 km2 offshore mid-Norway (Figures 1 and 2). Seismic reflection data and amplitude modeling imply 6–7 sills with thicknesses of up to 100 m for individual sills [Berndt et al., 2000]. The sills have been penetrated by a few industrial boreholes on structural highs, but detailed geochemical studies of the sills have not yet been reported. Hydrothermal vent complexes are also abundant, more than 700 craters up to 12 km in diameter are mapped on the Paleocene-Eocene paleo-seafloor by seismic 2-D and 3-D imaging. Also the large number of craters and their size reflect the great volumes of magma emplaced as sills in the area during a short-time interval. Furthermore, these craters are interpreted to have formed during violent release of gas generated within the contact aureoles around the sills, possibly contributing to the global warming event at the Paleocene-Eocene boundary [Svensen et al., 2004, 2010; Planke et al., 2005; Storey et al., 2007a].

[7] The Utgard dolerite sills are 91 m (Utgard Upper Sill; located at 3792–3883 m depth) and >50 m (Utgard Lower Sill; upper contact at 4650 m depth), emplaced in Upper Cretaceous mudstones and sandstones (Figure 2) [Berndt et al., 2000]. Drilling terminated 50 m into the lower sill, thus its total thickness remains unknown. The sills are well imaged on seismic profiles and can be followed for more than 100 km westward into the deeper part of the basin (Figure 2). Recent radiometric dating gave U-Pb zircon ages of 55.6 ± 0.3 Ma for the Upper Utgard Sill and 56.3 ± 0.4 Ma for the Lower Utgard Sill [Svensen et al., 2010], showing that the sills were emplaced during the early stages of the breakup-related volcanism [Storey et al., 2007b]. The dolerite fragments forming each sample represent a visually homogenous population regarding color and grain size. Margin samples contained fragments of country rock which were carefully removed from the analyzed fraction.

3. Methods

3.1. Petrography and Compositional Variations

[8] The Utgard sills consist mainly of plagioclase (∼50%), clinopyroxene (∼42%), olivine (∼3%), and magnetite (∼5%), with small amounts of apatite. In the Upper Sill, olivine grains are <0.5 mm in diameter, rounded, and commonly enclosed by clinopyroxene. Clinopyroxene partly forms large (≤3 mm in diameter) subhedral grains that may enclose plagioclase grains along their boundaries, partly it forms smaller, subhedral to anhedral grains (<0.5–3 mm in diameter) interlocked with, or enclosed by, plagioclase. Also plagioclase shows a wide range in size (<0.5–3 mm long) and is mainly subhedral. Magnetite is up to 1 mm in diameter, the larger grains are rounded. The Lower Sill shows even grain size (<1 mm in diameter). Clinopyroxene is mostly anhedral and may locally enclose several plagioclase grains, plagioclase is mainly subhedral. Both sills show minor alteration expressed by alteration rims on olivine and minor amounts of biotite and chlorite.

[9] Major and trace element data are given in Table 1 and shown in Figures 3-6. In the total alkalis versus silica (TAS) system [Le Bas et al., 1986], the sills classify as basalts and fall in the subalkaline/tholeiitic field of MacDonald [1968] and Kuno [1968] (Figure 1a in supporting information).1 CIPW norm calculations show small amounts of normative quartz (0.04–7.4 mole percent). The rocks have low contents of MgO (3.6–5.9 wt%), Ni (35–71 ppm), and Cr (22–99 ppm), and high concentrations of total iron (12.3–15.4 wt % Fe2O3) and incompatible trace elements (e.g., 0.6–1.9 ppm Th, 6–18 ppm La, and 28–43 ppm Y; Table 1) relative to primitive tholeiitic basalts. Samples from the Lower Sill are, on average, slightly more MgO-rich than those from the Upper Sill (Figure 3).

Table 1. Major Element (Weight %) and Trace Element (ppm) Data on Samples From the Utgard Sillsa
 Upper Sill (US)
Depth (m)38553858386438673873387938824647465046744683
 Upper Sill (US)Lower Sill (LS)
Depth (m)38553858386438673873387938824647465046744683
  1. a

    Sample numbers show depths in m.

La26.422.322.118.920.119.6 8.889.729.811.8
Ce59.051.950.145.948.149.3 21.321.322.531.2
Nd36.833.132.729.631.731.8 15.414.915.721.7
Sm9. 4.574.564.776.49
Eu2.972.852.782.452.572.75 1.711.591.712.09
Gd10. 5.775.295.637.64
Tb1.631.381.561.281.471.29 0.910.900.981.25
Dy8.408. 5.625.435.907.74
Yb3.943.703.963.343.823.41 2.942.843.143.84
Lu0.550.550.530.530.710.58 0.410.410.430.53
Figure 3.

Major element compositions of the Utgard sills. The sills show trends of increasing concentrations in TiO2, Al2O3, and Na2O, and decreasing CaO with decreasing MgO. This is compatible with fractional crystallization with removal of olivine + clinopyroxene ± Fe-Ti-oxides. 642E US: “uncontaminated” ODP Hole 642E Upper Series lavas on the Vøring Plateau (data from Meyer et al. [2009]); NEG LS: “uncontaminated” Lower Series lavas from Hold with Hope and Wollaston Forland, NE Greenland (data from Thirlwall et al. [1994]). See text for discussion.

[10] The sills show considerable scatter with respect to major elements, but Al2O3, Na2O, and Sr increase with decreasing MgO, whereas CaO and particularly Ni, Sc, and Cr, decrease (Figures 3 and 4). The Lower Sill also shows increasing mg# and decreasing concentrations of incompatible elements with stratigraphic height, whereas no such relationships are seen in the Upper Sill (Figure 7 and Table 1). Samples from each sill show essentially parallel REE-patterns strongly enriched in light rare earth elements (LREE), relative to heavy REE (HREE; Figure 6). The Upper Sill is enriched in strongly incompatible elements relative to the Lower Sill (e.g., Upper Sill: ThN = 17–22, [Ce/Yb]N = 3.0–3.3, Lower Sill: ThN = 7.6–12.9, (Ce/Yb)N = 1.7–1.9), but both sills have negative K-, Sr-, and P-anomalies. There is little variation in ratios of incompatible elements in each sill, but incompatible element ratios are distinct in the two sills (e.g., Th/Y, Nb/Y, and La/Y; Figure 6), indicating distinct origins.

Figure 4.

The Utgard sills show low concentrations in Ni, Sc, and Cr which decrease with decreasing MgO, compatible with removal of olivine + clinopyroxene ± spinel. The arrows in the Ni-MgO diagram indicate two stages of fractional crystallization: Stage 1 at deeper levels in the crust, Stage 2 after intrusion into the upper crust. 642E US: “uncontaminated” ODP Hole 642E Upper Series (data from Viereck et al. [1988, 1989] and Meyer et al. [2009]); NEG LS: “uncontaminated” NE Greenland lavas (data from Thirlwall et al. [1994]) are shown for comparison. See text for discussion.

[11] Sr-Nd isotope data on selected Utgard samples are listed in Table 2 and shown in Figure 8. The Upper Sill samples gave 143Nd/144Nd ratios almost within analytical error (0.512917 ± 0.000004 to 0.512932 ± 0.000005), whereas the range in 87Sr/86Sr ratios is significant (0.703801 ± 0.000012 to 0.703872 ± 0.000011). The highest 87Sr/86Sr ratios were obtained from samples close to the contacts against sedimentary wall rocks (Figure 8), suggesting mild local contamination. The two Lower Sill samples show significantly different Sr and Nd isotopic ratios and have slightly higher Nd and lower Sr isotopic ratios than the Upper Sill. Both sills have higher 87Sr/86Sr and lower 143Nd/144Nd than mid-ocean ridge basalts (MORB) and most Icelandic compositions [e.g., Thirlwall et al., 2004; Kokfeldt et al., 2006] (Figure 9a).

Table 2. Sr-Nd Isotope Data on Selected Samples From the Utgard Sillsa
  1. a

    Sample numbers show depths (m).

Upper Sill
Lower Sill

4. Discussion

4.1. Correlation of Conjugate Margins

[12] The Utgard sills and lavas on the Vøring Plateau (642E) and on the conjugate NE Greenland margin (Hold with Hope and Wollaston Foreland) represent opening-related magmatism at different distances from, and different sides of, the continent-ocean boundary (Figures 1 and 2). Chemical similarities and differences between the magmatic rocks at the three locations provide important information about their evolutionary histories. Below we therefore summarize the petrology and geochemistry of 642E and NE Greenland lavas, followed by a discussion of the evolutionary history of breakup volcanism at the conjugate margins of the Greenland-Norwegian Sea.

[13] Lavas collected in the 642E core on the Vøring Plateau (Figure 1a) consist of an Upper Series (US) of tholeiitic E-MORB-type basalts, and a very heterogeneous Lower Series (LS) that comprises rhyolitic ignimbrites, tholeiitic basalts, basaltic andesites, and dacites [Viereck et al., 1988, 1989; Parson et al., 1989]. Based on trace elements and Sr-Nd isotope data, Viereck et al. [1988, 1989], Parson et al. [1989], and Meyer et al. [2009] concluded that the 642E US lavas are mostly uncontaminated and have essentially preserved the trace element and isotopic signature imprinted by a mantle source somewhat more radiogenic than the depleted MORB mantle (DMM). The LS assimilated significant proportions of anatectic melts formed from upper crustal rocks. The pattern of contamination seen in the Vøring Plateau succession is very similar to that seen on the SE Greenland margin and onshore East Greenland [e.g., Fitton et al., 1998, 2000].

[14] On the conjugate margin in NE Greenland (Figure 1a), Thirlwall et al. [1994] divided the lavas into three main groups based on Sr-Nd-Pb isotope and trace element compositions. Most Hold with Hope (HwH) and Wollaston Forland (WolF) Lower Series (LS) lavas are largely uncontaminated quartz tholeiites. One HwH group, termed “normal,” have (La/Yb)N ratios of about 5.5, whereas a group with (La/Yb)N ratios of 7.6–26 was termed “anomalous” (this group is disregarded below). The HwH Upper Series (US) lavas are heterogeneous and have isotope and trace element compositions that indicate significant assimilation of (upper) crustal material combined with fractional crystallization (AFC processes), and may also contain a small component from the subcontinental lithospheric mantle [Thirlwall et al., 1994]. Thirlwall et al. [1994] found the NE Greenland basalt sequence to be produced from the Iceland plume source, with no component from the DMM. They also concluded that the lavas are generated by mixing between relatively small degree melts from garnet facies with relatively high degree melts from spinel facies mantle.

[15] We have adopted the terms “contaminated” and “uncontaminated” used in the discussion of the Vøring Plateau (642E) and in NE Greenland (HwH + WolF) lavas. “Uncontaminated” lavas are those that have 87Sr/86Sr below 0.7037 and 143Nd/144Nd above 0.5129 (Figure 9). With respect to Sr-Nd isotopes, both Utgard sills fall in the “uncontaminated” group and share similarities in particular with the NE Greenland LS lavas. The NE Greenland lavas are, on average, richer in MgO than the Utgard sills, and the highest MgO contents are found in the 642E lavas. The Lower Utgard Sill has many compositional features common with the NE Greenland lavas. In major and trace element diagrams, the NE Greenland lavas define trends that coincide with, or pass close to, the Lower Utgard Sill, the two rock groups overlap with respect to trace element patterns and have similar ratios between strongly and moderately incompatible elements (Figures 3-6). The Upper Utgard Sill has lower concentrations of compatible elements, higher concentrations of strongly incompatible elements, and higher ratios between strongly and moderately incompatible elements. The 642E lavas, in contrast, show considerable scatter with respect to major elements and incompatible trace elements and most samples are less evolved (higher concentrations of MgO, Ni, Sc, and Cr than the other groups; Figures 3 and 4). The 642E lavas also differ from the Utgard sills and the NE Greenland lavas by having relatively flat HREE patterns, whereas the Utgard and the NE Greenland samples exhibit regularly decreasing enrichment factors from La to Yb (Figure 5). In the Sr-Nd isotope diagram, the NE Greenland LS (NEGLS) and Utgard Upper Sill define a common trend with the Eastern Mohns Ridge (EMR) falling at the low-87Sr/86Sr, and the Upper Utgard Sill at the high-87Sr/86Sr end (Figure 9b). This trend lies within the field of Iceland basalts. The Lower Utgard Sill falls slightly above this trend. The 642E US likewise plots above the EMR-NEGLS-Upper Utgard Sill trend with uniform 143Nd/144Nd ratios and 87Sr/86Sr ratios extending almost to 0.7060. Finally, the 642E US lavas have much wider ranges in Nb/Th and Nb/La ratios than the Utgard sills and NE Greenland LS (Figures 9b and 10), and they show no correlation between 87Sr/86Sr and Nb/Th or Nb/La ratios.

Figure 5.

(a) PM-normalized trace element patterns [PM: Primordal Mantle after McDonough and Sun, 1995] for the Utgard sills compared to “uncontaminated” lavas in 642E US and NE Greenland LS (data sources as in Figure 5). The Utgard Upper Sill is somewhat more enriched than the Lower Utgard Sill and the two lava series. Ba is not shown because the samples may be contaminated by Ba-rich drilling liquid. (b) REE diagrams for the Utgard Sills.

Figure 6.

Selected incompatible trace elements plotted against Y (ppm), compared to data on “uncontaminated” lavas from ODP Hole 642E Upper Series lavas (642 US) and NE Greenland Lower Series lavas (NEG LS), the Mid-Atlantic Ridge (MAR; Zr-Y data not available from the sources used), and average E-MORB and OIB. Samples from the same sill have similar ratios between, but different concentrations in, most incompatible elements, strongly suggesting that they are related through different degrees of fractional crystallization. Different ratios for Th/Y, Nb/Y, and so on imply that the Upper and the Lower Utgard sills are derived from different parent magmas. Data sources are as follows: Sun and McDonough [1989], Thirlwall et al. [1994], Michael [1995], Schilling et al. [1999], Hannigan et al. [2001], Meurer et al. [2001], and Meyer et al. [2009]. US: Utgard Upper Sill and LS: Utgard Lower Sill. See text for further discussion.

Figure 7.

Variations in 87Sr/86Sr and mg# (Mg/[Mg+Fe2+] assuming Fe2+ = 0.8*Fetotal) with depth in the Utgard Sills. The highest Sr isotope ratios in the Upper Sill are found toward the roof and floor of the sill.

Figure 8.

143Nd/144Nd versus 87Sr/86Sr relationships in the Utgard sills, showing 2se internal error on each analysis, and 2sd standard reproducibility. The Upper Sill samples fall within a limited range in 143Nd/144Nd ratios but show significantly different 87Sr/86Sr ratios. The Lower Sill samples are somewhat more depleted than those from the Upper Sill and show significantly different ratios.

Figure 9.

(a) Sr-Nd isotope data on the Utgard sills compared to domains covered by “contaminated” lavas from the Vøring Plateau (642E LS) and Hold with Hope in NE Greenland (HwH US), and to crustal rocks of different compositions from the Western Gneiss Region (WGR). AFC trends are estimated on the basis of the EC-RAFC model of Spera and Bohrson [2004]. Arrows indicate increasing degree of crustal contamination. Solid lines: initial temperature of the assimilant is 600°C and the solidus of the assimilant is 700°C; dashed line: temperatures of 600 and 750°C, respectively. Trends assuming different assimilants are shown in different colors. The choice of end-members is discussed in the text, their compositions, references, and other EC-RAFC parameters are given in Tables 3 and 4. ACC: average continental crust; SCLM: average subcontinental lithospheric mantle; trend1: AFC trend based on Figure 10b; trend2: trend representing second stage of contamination in the 642E LS, fitted visually to the data points. (b) The gray box in Figure 10a shown in greater detail, including data points for 642E US, and HwH and WolF from NE Greenland. “Uncontaminated” lavas are shown as filled symbols, “contaminated” lavas as open symbols and marked by * in the legend. The sample numbers of different assimilants are indicated in italics. The Upper Utgard Sill appears to fall at the high-87Sr/86Sr end of a trend defined by the Mohns Ridge basalts (MR) (at the low-87Sr/86Sr end) and “uncontaminated” lavas from NE Greenland (HwH LS and WolF LS) along its middle part. trend1: trend representing a compromise between the EC-RAFC modeling trends that show the best fit to the Utgard and NE Greenland LS data, that is Melt1 and Melt2 combined with assimilant FAR-31. The trend1 trend is proposed to represent the AFC processes that have affected the Utgard sills (and NE Greenland lavas). The Utgard Lower Sill fall slightly to the high 87Sr/86Sr side of the trend1 trend and most of the “uncontaminated” 642E US lavas. Percentages indicate proportion of assimilated melt relative to the initial magma body. Trend3 is fitted visually to the NE Greenland US. (c) EC-RAFC trends based on 642E US sample 045R3/128 (Melt3) as primary melt. Data sources: 642E: Meyer et al. [2009]; HwH and WolF: Thirlwall et al. [1994]; 465 Icelandic basalts (GEOROC database.; WGR: Austrheim et al. [2003]; basaltic lavas from Jan Mayen: Mertz et al. [2004]; EMR: Schilling et al. [1999]. Gran. gn.: granitic gneiss; monz. gn.: monzonitic gneiss; dior. gn.: dioritic gneiss; and xen.: xenolite. See text for discussion.

Figure 10.

(a and b) Some of the EC-RAFC trends [Spera and Bohrson, 2004] involving Melt1 and Melt2 in Figure 10 shown in (Nb/Th)N-143Nd/144Nd and (Nb/La)N-143Nd/144Nd diagrams. The Melt2—FAR-31 trends pass through the Utgard sills in diagram Figure 10a, but not diagram Figure 10b. We therefore also show trends based on two eclogite (8815B and 8906A11) from the Western Gneiss Region as assimilants [Kylander-Clarck et al., 2007]. As Sr isotope data are not available on the eclogites, these are not shown in Figure 9. (c and d) Trends involving Melt3 do not pass though the Utgard sills. Trend lines have the colors of the assimilants and have the sample number of the assimilants in italics. 642E US and NE Greenland LS are shown as yellow and green fields, respectively. trend1: trend proposed to represent AFC processes in the Utgard (and NE Greenland LS); trend3: trend representing contamination in the NE Greenland US, visually fitted to the data points. The scatter of the 642E LS makes it impossible to indicate trend2 in these diagrams. End-member compositions, references, and other EC-RAFC parameters are listed in Tables 3 and 4. See text for discussion.

4.2. AFC Processes

[16] The only compositional evidence of in situ contamination is the slightly elevated Sr isotope ratios at the roof and floor of the Utgard Upper Sill, as compared to its central parts (Figure 8). The following discussion is therefore focused on samples from the interior parts of the sills.

[17] The low concentrations in MgO and other compatible elements (e.g., 3.9–5.9 wt % MgO, 30–75 ppm Ni; Table 1 and Figures 3 and 4) indicate that the magmas that gave rise to the Utgard sills have an extensive history of fractional crystallization. In Figure 4 (Ni-MgO diagram), we propose two stages of fractional crystallization. Stage 2 represented by the observed compositional range within each sill occurred after intrusion of the magmas into the upper crust. Stage 1 represented by the compositional difference between an assumed initial melt and the least evolved sample in each sill, took place before intrusion into the upper crust. Fractional crystallization is discussed in detail in supporting information.1 In summary, Stage 1 involved extensive removal of olivine, clinopyroxene, and possibly minor amounts of plagioclase. In the Lower Sill, Stage 1 may also have involved removal of minor amounts of magnetite. It was concluded that crystallization during Stage1 occurred in the lower crust. During Stage 2, the magmas crystallized the observed mineral assemblage, olivine + clinopyroxene + plagioclase + magnetite + apatite.

[18] Both Utgard sills have lower Nd and higher Sr isotopic ratios than many of the “uncontaminated” NE Greenland and 642E lavas (Figure 9) and the two sills have slightly different Sr-Nd isotope ratios. These differences suggest that, in addition to fractional crystallization, their parent magmas may have been subjected to crustal contamination. Minor contamination in the Utgard sills is supported by their Rb/Zr and K/Nb ratios (Figure 11). Both ratios are typically low in basalts derived from depleted mantle sources, and high in crustal rocks. As Zr and K have lower mineral/melt partition coefficients than Rb and Nb in basaltic systems [e.g., Sun and McDonough, 1989], fractional crystallization is expected to cause significant increases in the Rb/Zr ratio and mild decreases in the K/Nb ratio. Because of the high ratios in crustal rocks, assimilation of crustal melts will cause both ratios to increase. Both the 642E and NE Greenland lavas show trends from low toward high Rb/Zr and K/Nb ratios with the “uncontaminated” lavas (642E US and NE Greenland LS) at the low-ratio end (Figure 11); the highest K/Nb ratios are found among the “contaminated” 642E LS. The wide field shown by the Viereck et al. [1989] data on 642E US, as compared to the data of Meyer et al. [2009], is probably due to a less rigorous selection of unaltered rocks. The positions of the different series in Figure 11 are in perfect agreement with the conclusions that the 642E LS and NE Greenland US are contaminated [Parson et al., 1989; Thirlwall et al., 1994; Meyer et al., 2009]. The Utgard sills fall at the transition between “uncontaminated” and “contaminated” 642E and NE Greenland series. The relatively high K/Nb and Rb/Zr ratios of the Utgard sills (Figure 11) thus support crustal contamination in addition to fractional crystallization.

Figure 11.

Rb/Zr-K/Nb relationships in the Utgard sills compared to 642E and NE Greenland lavas [Parson et al., 1989; Viereck et al., 1989; Thirlwall et al., 1994; Meyer et al., 2009] and average compositions of N-MORB, OIB [Sun and McDonough, 1989], and upper-middle and lower crust [Rudnick and Gao, 2004]. M. et al.: Meyer et al. [2009] (Zr data are not given). The Utgard sills are located at the intersection between “uncontaminated” and “contaminated” 642E and NE Greenland lavas, strongly suggesting crustal contamination. See text for discussion.

[19] The relatively uniform compositions of each Utgard sill might be taken as evidence against contamination. If so, the Utgard sills, the 642E lavas and the NE Greenland lavas must be derived from a heterogeneous mantle source or different sources. In our opinion, the uniform compositions of each Utgard sill are not evidence against contamination. In a study of sills and dykes in the Golden Valley Complex in the Karoo Basin (South Africa), Neumann et al. [2011] found each sill and dyke to have uniform ratios between incompatible trace elements and uniform Sr and Nd isotope ratios. However, the units showed different ratios, so that such ratios could be used to “fingerprint” the magma batch that gave rise to a given unit. When the various units within a restricted area were compared, it became apparent that they fell on a common trend compatible with different degrees of assimilation and fractional crystallization (AFC processes) in the deep crust. Each batch of magma had clearly homogenized before intrusion into the upper crust. AFC processes during the evolution of the Utgard sills have been tested using the energy-constrained assimilation-fractional crystallization (EC-RAFC) model of Spera and Bohrson [2004].

[20] The EC-RAFC model requires assumptions about the compositions of the primitive melts and assimilants. Regarding the primary melt, we tested several candidates. The low 87Sr/86Sr and high 143Nd/144Nd ratios of the Utgard sills (≥0.7039 and ≤0.51292, respectively; Figure 9) indicate that the mantle source(s) of these sills must be depleted. Depleted MORB mantle, DMM, is the most common mantle source associated with sea-floor spreading. We therefore tested a primary melt, Melt1, with the Sr-Nd isotope composition of DMM [Workman and Hart, 2005] and the trace element concentrations of average N-MORB [Sun and McDonough, 1989]. As indicated above, the mantle source that gave rise to the NE Greenland lavas is thought to be the Iceland plume (termed IRZ: Iceland Rift Zone) [Debaille et al., 2009]. Melt2 has the Sr-Nd isotope compositions the mantle source IRZ [Debaille et al., 2009] and the trace element concentrations of a picritic lava from the Reykjanes Penninsula in Iceland with the same Sr and Nd istope ratios as IRZ. The source of the 642E lava series is proposed to be slightly more radiogenic than the DMM [Meyer et al., 2009], but details have not been given. Melt3 represents an attempt to find the chemical characteristics of a primary, uncontaminated source for the 642E lavas. Details on mantle sources and primary melts are given in supporting information.

[21] Data on the crustal basement beneath the Vøring Margin are not available. Rocks from the adjacent Norwegian mainland, the Western Gneiss Region (WGR), were therefore used as proxy. The choice of assimilants are discussed in detail in supporting information. Data for primary melts, assimilants, other parameters used in the modeling, and references are listed in Tables 3 and 4.

Table 3. Parameters Used in EC-RAFC Modeling
Parameter abbreviationsValues used
Tlm—Liquidus of magma1300°C
Tmo—Initial temperature of magma1300°C
Tla—Liquidus of assimilant1100°C
Tao—Initial temperature of assimilant600°C
Ts—Solidus (melt and assimilant)700°C (750°C)
Cpm—Specific heat of magma1495 J/kg K
Cpa—Specific heat of assimilant1400 J/kg K
Hcry—Heat of crystallization395,000 J/kg
Hfus—Heat of fusion354,000 J/kg

[22] The results of the EC-RAFC modeling, based on Sr-Nd isotope relationships, and Nb/Th and Nb/La ratios, are shown in Figures 9 and 10. The discussion in supporting information shows that the best fit to the Utgard data is provided by EC-RAFC trends involving assimilants with chemical characteristics typical of the lower crust. The best results are summarized in trend1, which is based on Melt2 as initial melt and a compromise between the Sr-Nd isotope composition of assimilant FAR-31 (monzonitic gneiss), and Nb/Th and Nb/La ratios somewhat higher then those of FAR-31 (ratios between those of FAR-31and eclogite 8815B). The low 87Sr/86Sr ratio (0.71212) of FAR-31 and relatively high Nb/Th and Nb/La of the assumed assimilant are typical of the lower crust. The EC-RAFC modeling thus gives strong support to the conclusion from the discussion based on fractional crystallization that Stage 1 (Figure 4) took place in the lower crust. The results are relatively similar for Melt1 and Melt2 as initial melts, whereas Melt3 does not give acceptable fits with any of the assimilants used in the modeling. Modeling based on the parameters listed in Tables 3 and 4 suggests that the extent of crustal contamination in the Utgard Sills is very minor (Figure 9). Using Melt2 (IFZ as mantle source) as initial melt, the estimates imply assimilation of less than 0.5% molten wall rock relative to the mass of the initial magma. The degree of fractional crystallization indicated by the EC-RAFC modeling is strongly dependent on the temperature difference between the initial temperature and the solidus of the assimilant (Tao and Ts, respectively, in Tables 3 and 4); modeling with Tao = 600 and Ts = 700°C gives 40% crystallization, whereas 600 and 750°C, respectively, gives 80%. An estimate of degree of fractional crystallization based on the niobium content in the least evolved Utgard Upper Sill sample (16.2 ppm Nb; Table 1), and Melt2 as initial melt (Nb = 1.47 ppm; Tables 3 and 4) proposes that ∼90% of the initial melt (F < 0.1) is removed by fractional crystallization, using bulk mineral/melt distribution coefficients for Nb in the range 0.01–0.1 (based on data by Green et al. [2000], Norman et al. [2005], and Severs et al. [2009]). The degree of fractional crystallization obtained for the least evolved Lower Utgard sample (Nb = 8.2 ppm) is somewhat lower, ∼80% (F ∼ 0.2). In situ crystallization (represented by the range 16.2–22.3 ppm Nb in the Upper Sill) indicates 4–6% crystallization relative to the initial melt.

[23] Despite the numerous assumptions in the AFC model, we find it possible to identify realistic melt—wall rock combinations that reproduce the relationships between Sr-Nd isotope ratios and Nb-Th-La of the Utgard sills. During Stage 1, the Lower Utgard Sill may have been contaminated by crustal rocks with slightly higher 87S/86Sr ratio than the assimilant that contaminated the Upper Sill. An alternative possibility is that this difference is inherited from a heterogeneous mantle source.

[24] In contrast to the Utgard sills, the 642E US lavas show wide ranges in both 87Sr/86Sr, (Nb/Th)N and (Nb/La)N ratios (Figures 9 and 10) (data from Meyer et al. [2009]). Some of the samples fall to the high-87Sr/86Sr side of the Iceland basalt field. However, the ranges of the 642E US overlap the compositions of the Utgard sills. The scatter in 87Sr/86Sr, (Nb/Th)N and (Nb/La)N ratios in the 642E US lavas must be due to contamination by rocks with other chemical characteristics than those tested by EC-RAFC modeling in this study. The origin of the 642E lavas is unclear, but it remains possible that these lavas have a mantle source similar to that of the Utgard sills with respect to Sr-Nd isotopes. The “contaminated” 642E LS appears to follow a kinked trend in the Sr-Nd isotope diagram (Figure 9), essentially following the afc1 trend in the high-143Nd/144Nd domain, whereas samples at lower 143Nd/144Nd trend toward high 87Sr/86Sr ratios (trend2). In Figure 10, the 642E LS series does not define clear trends, but show lower (Nb/Th)N ratios, and, on average, lower (Nb/La)N ratios than the 642E US. Very low (Nb/Th)N and (Nb/La)N ratios are typical of the upper continental crust. This suggests two stages of contamination caused by crustal rocks with different chemical signatures or interaction with a heterogeneous column of crustal rocks during ascent.

[25] The short trend formed by the NE Greenland LS in Figure 9b (along trend1) suggests that minor contamination has indeed taken place also in these lavas, although, as concluded by Thirlwall et al. [1994] the contamination is not significant. The US, in contrast, was clearly affected by AFC processes involving significant assimilation of melts formed from crustal rocks (trend3). These lavas appear to have formed from a mantle source similar to that of the Utgard sills. The crustal contaminants that fit the Utgard sills and the “contaminated” Vøring Margin lavas (at the high-87Sr/86Sr end of trend1 and trend2, respectively); however, cannot easily reproduce the NE Greenland trend. It thus seems likely that the crustal rocks that interacted most strongly with the NE Greenland magmas are different from those that affected the Norwegian margin.

4.3. Partial Melting

[26] We have tested partial melting in the mantle on the basis of (Ce/Sm)N-(Sm/Yb)N relationships (Figure 12). (Ce/Sm)N and (Sm/Yb)N ratios are very sensitive to degree of partial melting, in addition to temperature, pressure, and the phase assemblage in the source rock, but are not significantly affected by moderate degrees of fractional crystallization. The Utgard sills, however, have undergone extensive fractional crystallization. The partition coefficient clinopyroxene/melt is lower for Ce than for Sm and that for Yb may be slightly higher than, or similar to, that for Sm [e.g., Green et al., 2000]. Extensive fractional crystallization will therefore cause some increase in the Ce/Sm ratio, whereas the Sm/Yb ratio will stay constant or increase. The positions of the Utgard sills in Figure 12 are therefore likely to have shifted somewhat in opposite direction to that caused by partial melting. The estimated trends shown in Figure 12 also depend on the trace element composition of the source rock, partition coefficients, and, to some degree, the proportions in which the different phases go into the melt (melting mode). Because of the many uncertainties, we have chosen not to indicate degrees of partial melting (F) along the trends; however, different Fs may be inferred from the arrows which point toward increasing degrees of partial melting. The PM source was used for modeling; ideally, we would have used IRZ (Iceland Rift Zone), but REE data other than Nd are not available. The IRZ source has a somewhat higher Nd concentration (1.61 ppm) [Debaille et al., 2009] than the PM source (1.25 ppm) [McDonough and Sun, 1995] but this difference does not preclude using PM to represent the general features of the trends depicted in Figure 12. Partial melting in the stability field of spinel peridotite produces melts in which (Ce/Sm)N ratios decrease with increasing degree of melting, whereas their (Sm/Yb)N ratio stays about the same (trend A). Partial melting in the garnet stability field produces melts that are more strongly enriched in (Sm/Yb)N than in (Ce/Sm)N. Furthermore, the degree of enrichment in (Ce/Sm)N relative to (Sm/Yb)N decreases with increasing degree of partial melting, increasing proportion of garnet in the source, increasing pressure, and decreasing fertility of the source [e.g., Walter, 1998; Simon et al., 2007]. The effects of increasing proportions of garnet in the source are shown by the trends B, C, and D (3, 6, and 10% modal garnet, respectively), the difference between partial melts formed from a depleted relative to a fertile source is reflected in the differences between solid and dotted lines. The parameters used in the calculations are listed in Table 5.

Figure 12.

(Ce/Sm)N − (Sm/Yb)N relationships among the Utgard sills and “uncontaminated” magmatic rocks from the Vøring Plateau and NE Greenland conjugate margins. Trend A: partial melting of depleted MORB mantle [DMM; Workman and Hart, 2005] in the spinel stability field. Trends B, C, and D: partial melting in the garnet stability field (at 3 GPa), assuming 3, 6, and 10% modal garnet, respectively, in an undepleted PM-type mantle source (dotted lines) [McDonough and Sun, 1995] and enriched DMM (EDMM; full lines) [Workman and Hart, 2005]. Arrows show increasing degree of partial melting. The parameters used in the calculations and references are listed in Table 5. NEG LS: NE Greenland Lower Series lavas (“uncontaminated”) (data from Thirlwall et al. [1994]); EMR: Eastern Mohns Ridge (data from Schilling et al. [1999]); UC, MC, and LC: upper, middle, and lower crust as given by Rudnick and Gao [2004]; WGR: Western Gneiss Region (data from Austerheim et al. [2003]). See text for discussion.

Table 4. Compositions of End-Members Used in EC-RAFC Modelling With References
 Sr (ppm)Nd (ppm)Th (ppm)Nb (ppm)La (ppm)87Sr/86Sr143Nd/144Nd
  1. a

    N-MORB: Sun and McDonough et al. [1989].

  2. b

    DMM: depleted MORB-mantle: Workman and Hart [2005].

  3. c

    Iceland Rift Zone basalt with IRZ Sr-Nd isotope ratios: sample 9372 in the GEOROC database (

  4. d

    IRZ: Debaille et al. [2009].

  5. e

    642E US sample 045R3/128: Meyer et al. [2009].

  6. f

    Estimated on the basis of partition coefficients given by: Francalanci [1989]; Nielsen et al. [1992], and Salters and Longhi [1999].

  7. g

    WGR, granitic gneiss: Austrheim et al. [2003].

  8. h

    ACC: average continental crust: Rudnick and Gao [2004].

  9. i

    ACC: Allégre et al. [1996].

  10. j

    WGR, quartz monzonitic gneiss: Austrheim et al. [2003].

  11. k

    WGR, monzodioritic gneiss: Austrheim et al. [2003].

  12. l

    WGR, doleritic gneiss: Austrheim et al. [2003].

  13. m

    WGR, eclogite: Kylander-Clark et al. [2007].

  14. n

    SCLM: McDonough [1990].

  15. o

    SCLM: subcontinental mantle lithosphere: Lee et al. [1994].

Mantle Melts
Bulk D0f upper crust1.  
Bulk D0f lower crust0.  
Assimilant Upper Crust
Bulk D0f1.  
Assimilants Lower Crust
FAR-24j113740.43   0.7066490.511759
FAR-37Bl31632.11   0.7072700.512260
8906A11m13714. 0.512197
8815Bm30012. 0.512306
Bulk D0f0.  
Assimilant Lithospheric Mantle
Bulk D0f0.  
Table 5. Modal Relationships and Trace Element Compositions of Mantle Source, Modal Proportions in Melt and Partition Coefficients Used to Calculate Melting Trends in Figure 13
Mantle Source olopxcpxspGarnetPMa (co ppm)
  1. a

    McDonough and Sun [1995].

  2. b

    Workman and Hart [2005].

  3. c

    Baker and Stolper [1994].

  4. d

    Johnson et al. [1990].

  5. e

    Presnall et al. [2002].

  6. f

    Johnson et al. [1990], with modified cpx-gar relationship.

  7. g

    Niu and Hekinian [1997].

  8. h

    Green et al. [2000].

  9. i

    No data available.

  10. j

    Salters and Longhi [1999].

PM (spinel)Sourceb0.600.280.100.02  
PM (10% garnet)Sourced0.550.200.15 0.10 
 Melte0.12−0.671.25 0.30 
PM (6% garnet)Sourcef0.550.200.19 0.06 
 Melte0.12−0.671.25 0.30 
PM (3% garnet)Sourcef0.550.200.22 0.03 
 Melte0.12−0.671.25 0.30 
Spinel stability fieldElement      
PC mineral/meltCe0.0060g0.009g0.092g0.0005g 1.675
PC mineral/meltSm0.0067g0.020g0.445g0.0009g 0.406
PC mineral/meltYb0.045g0.1033g0.542g0.0045g 0.441
Garnet stability fieldElement      
PC mineral/meltCe0.0060g0.0016h0.12h 0.0029h1.675
PC mineral/meltSm0.0067h0.015h0.58h 0.18h0.406
PC mineral/meltYb0.045g0.22h1.2h 6.5h0.441
PC mineral/meltNb0.0001i0.002j0.018j 0.01j0.658
PC mineral/meltTh0.0001i0.0004j0.007j 0.005j0.0795

[27] Figure 5 shows that the Utgard sills have inclined patterns in the HREE domain [Dy/Yb]N = 1.4–1.5 and 1.3 in the Upper and Lower Sills, respectively. This implies garnet in the mantle source. The same is true for the NE Greenland LS lavas ([Dy/Yb]N = 1.2–1.4), whereas most 642E US lavas fall in the range (Dy/Yb)N=1.0–1.3. In Figure 12, both the 642E US and the NE Greenland LS define trends of strongly increasing (Sm/Yb)N with mildly increasing (Ce/Sm)N, but they also have off-shoots toward higher (Ce/Sm)N ratios. This is in agreement with the conclusion of Thirlwall et al. [1994] that the NE Greenland LS lavas formed by mixing between partial melts from both garnet peridotite and spinel peridotite (trend E; Figure 12). A similar model is compatible with the 642E US trend, but the low (Dy/Yb)N ratios in many samples suggest a larger proportion of spinel-facies melts on the Norwegian side. The Lower Utgard Sill may have formed by a higher degree of partial melting than the Upper Sill, and/or contains a larger proportion of spinel-facies melts (melting at a somewhat shallower level). It is unlikely that the minor amount of crustal contamination estimated by the EC-RAFC modeling has significant effects on the Ce/Sm and Sm/Yb ratios.

[28] The Upper Utgard Sill has a higher (Sm/Yb)N ratio than the NE Greenland and 642E lavas (Figure 12). This may be explained by either of the following models: (i) partial melting in the garnet stability field only or (ii) mixing between melts formed from both garnet and spinel peridotites with a higher proportion of garnet-facies melts than in 642E and NE Greenland lavas. Both models imply partial melting at greater depths in the mantle. Melting at deeper levels is in agreement with the timing and position of the Upper Utgard Sill landward relative to the 642E lavas. The ages of the Utgard sills (55.6 ± 0.3 and 56.3 ± 0.4 Ma) [Svensen et al., 2010] imply emplacement during the early stages of the breakup-related volcanism, and the location of the Utgard borehole (6607/5-2) is at about 190 km from the breakup zone. Melting at relatively shallow levels to form the 642E magmas is in agreement with the general view that the lavas on the outer part of the Møre and Vøring margins extruded during the last stages of rifting and earliest stages of seafloor spreading when the lithosphere in the area had been stretched and thinned [e.g., Skogseid and Eldholm, 1987, 1989; Meyer et al., 2009]. We therefore surmise that the lithosphere in the area of the Utgard borehole was considerably thicker than that at the 642E location (Figures 1 and 2). A thicker lithosphere implies a lower geothermal gradient and thus less melting at shallow depths.

4.4. Amount of Lower Crustal Cumulates and Underplating

[29] The discussion above and in supporting information indicates that fractional crystallization and assimilation during Stage 1 must have taken place in magma chambers in the lower crust. This means that the magmas stayed in the lower crustal magma chambers long enough to heat the country rocks to solidus temperatures, and that this was accompanied by cooling and crystallization in the magmas. Experimental studies of crystallization sequences in different types of basaltic melts have shown that plagioclase (low density) is an early crystallizing phase at low pressures; at high pressures (0.8–1.0 GPa), crystallization is dominated by dense minerals (olivine + pyroxenes + spinel), whereas plagioclase only starts crystallizing at temperatures near the solidus; at pressures >1.0 GPa spinel forms instead of plagioclase [e.g., Green and Ringwood, 1967; Presnall et al., 1978, 2002; Villiger et al., 2004; Falloon et al., 2008]. The Upper Utgard Sill shows trends of strongly decreasing concentrations in Ni, Sc, and Cr with decreasing MgO (Figure 4). This is evidence of extensive fractionation of olivine + pyroxenes, and may have involved Cr-spinel (but not magnetite). The Upper Utgard Sill is also significantly richer in Sr than the NE Greenland and 642E lavas (Figure 4). This means that although the Utgard sills show small negative Sr and Eu anomalies (Figure 5) crystallization of plagioclase must have been insignificant before intrusion into the upper crust where the petrography shows plagioclase to be a major crystallizing phase. We conclude that a large portion of the fractional crystallization that gave rise to the final Utgard magmas occurred in the lower crust (Stage 1). In experiments on tholeiitic melts at 1.0 GPa, Villiger et al. [2004] found plagioclase to start forming only after >50% crystallization (equilibrium or fractional) of olivine, clinopyroxene, orthopyroxene, and spinel. In transitional magmas, like the Utgard melts, crystallization of clinopyroxene is even more dominant than in tholeiitic melts. Based on these experiments, we propose that crystallization in the Utgard melts took place partly in the lower crust, Stage 1, and partly in situ in the upper crust, Stage 2. Crystallization in the lower crust comprises two steps, first removal of olivine + pyroxenes + spinel forming dense olivine pyroxenite cumulates (Stage 1a), followed by removal of ± olivine + pyroxenes + plagioclase ± spinel forming gabbros (Stage 1b). The evolutionary sequence outlined above is shown schematically in Figure 13.

Figure 13.

Schematic presentation of the evolutionary history of the Utgard magmas. The figure is not to scale. Stage 1 comprises extensive fractional crystallization (>70%) and minor assimilation (<1%) of crustal melts in lower crustal magma chambers. Dense cumulates left by the fractional crystallization in these deep magma chambers (Stage 1a), mixed with less dense, old continental crust form part of the lower crustal high-velocity, high-density body (LCB) identified by seismic surveys. In Stage 1b, plagioclase is among the fractionating phases, giving rise to less dense gabbroic cumulates. Stage 2 represents an additional stage of fractional crystallization in situ in the upper crust with removal of ±olivine (ol)+ clinopyroxene (cpx)+plagioclase (plag)+Fe-Ti-oxides. Opx: orthopyroxene; sp: spinel; Post.-Pal. sed.: Post-Paleocene sediments; Cret.+Pal. sed. rocks: Cretaceous and Paleocene sedimentary rocks; SCLM: subcontinental lithospheric mantle; DMM: depleted MORB mantle; and IRZ: Iceland Rift Zone mantle [Debaille et al., 2009]. See text for discussion.

[30] This conclusion bears on the interpretation of the extensive high-velocity, high-density lower crustal body (LCB) identified in the deep crust beneath the mid-Norwegian margin (Figure 2) and frequently discussed. Proposed models include underplating by dense magmatic material during rifting and breakup [e.g., Mjelde et al., 2002; Voss and Jokat, 2007], stacked sill complexes in the lower continental crust [White et al., 2008], high-grade metamorphic rocks formed during the collapse of the Caledonian mountain range [Gernigon et al., 2003, 2004; Ebbing et al., 2006], older, high-pressure granulite/eclogite bodies [e.g., Gernigon et al., 2004, 2006; Mjelde et al., 2009], and serpentinized mantle [e.g., Reynisson et al. 2010]. A detailed review of the different hypotheses was recently presented by Reynisson et al. [2010].

[31] Our results imply that for each volume unit of sill or lava on the Norwegian margin, there is a large mass of cumulates left in the lower crust. Based on density data, we have made some very simplified estimates of the average thickness of cumulate bodies that may be associated with the Utgard sills and with other sills and lavas in the Vøring Basin, assuming that the cumulate bodies have the same areal extent as the sills/lava bodies with which they are associated. The parameters used are discussed below and the results listed in Table 6.

Table 6. Estimates, Based on Densities, of the Thicknesses Per Areal Unit (e.g., km2) of Cumulates in the Lower Crust Corresponding to the Thicknesses of Sills or Lavas Per Square Unit in the Upper Crust
 Sill/LavaCumulate CrystalsTrapped Liquid (20%)Average Cum.aCrustal BasementbLCB
  1. a

    Average cumulate: Cumulates mixed with trapped liquid.

  2. b

    The proportion of old crustal basement required to get an average density of 3.10 g/cm3 for a rock body (LCB) consisting of crustal basement intruded by cumulates.

  3. c

    Berndt et al. [2000].

  4. d

    Hyndman and Drury [1976].

  5. e

    Ebbing et al. [2006] and Reynisson et al. [2010].

  6. f

    Thickness per areal unit of sill intrusion or lava in the upper crust.

  7. g

    Berndt et al. [2000] identified 6–7 sills with a maximum thickness of 100 m in the Hel Graben. We show estimates assuming six sills each 50 m thick (Min. thick.), and seven sills each 100 m thick (Max.thick.).

  8. h

    Less evolved (Figure 4), therefore a different lava/cumulate proportion.

  9. i

    Vierick et al. [1989].

  10. j

    Upper Series 770 m thick, Lower Series 140 m thick.

  11. k

    Minimum value, see text for explanation.

  12. l

    Eldholm et al. [1987] and Skogseid and Eldholm [1987].

Utgard Upper Sill
Proportions of initial melt (%)4060    
Density (g/cm3)2.75c3.372.8d3.242.95e3.10e
Volume prop.11.280.2581.550.782.33
Thickness (m)f90e116f23f139f70f209f
Utgard Lower Sill
Proportions of initial melt (%)4555    
Density (g/cm3)2.75c3.342.8d3.232.95e3.10e
Volume prop.
Thickness (m)f>50c>61>12>73f>37f>110f
Utgard total>140>177>35>212>107>319
Hel Graben Sills
Proportions of initial melt (%)4060    
Min. thick. (m)g50 × 6c38777464232694
Max. thick. (m)g100 × 7c90318110845421626
ODP Leg 104 Hole 642E, Outer Vøring Plateau
Proportions of initial melt (%)h5050    
Density (g/cm3)2.8d3.232.8d3.232.95e3.10e
Thickness (m)g910i, j797159956443>1400k
Vøring Plateau (Max Thickness)
Proportions of initial melt (%)5050    
Max. thick. (m)>6000l>5010>1002>6012>2790>8,800

[32] We showed above that the amount of fractional crystallization varies in the models examined. The EC-RAFC modeling indicates that 70% of the original mass of the parent magma precipitated as cumulates (Mc) to produce the Utgard magmas (Stage 1 + Stage 2). Estimates for Mc depend on the temperature difference (ΔT) between the initial and the solidus temperature of the assimilant. We used ΔT = 10°C in the EC-RAFC modeling, higher ΔT gives more extensive fractional crystallization (higher Mc) before the onset of assimilation. Estimated degree of fractional crystallization based on Nb concentrations indicate that Stage 1 involves ∼90% fractional crystallization to form the least evolved Upper Sill sample and ∼80% for the least evolved Lower Sill. In the following calculations, we have used much more conservative degrees of fractional crystallization, 60 and 55% crystallization for Stage 1 in the Upper and Lower Utgard Sills, respectively.

[33] Fractional crystallization in the Utgard magmas during Stage 1 (see supporting information) was dominated by olivine + clinopyroxene ± orthopyroxene. Olivine and pyroxenes in the asthenospheric mantle typically have Mg/(Mg+Fe2+) ratios (mg#) of 0.85–0.91 [e.g., Simon et al., 2008]. The first olivine and pyroxene to form in primary mantle melts will have the same mg# ratio as the mantle host, but this will decrease during fractional crystallization. Assuming an average Mg/(Mg+Fe2+) ratio of 0.8 for olivine and pyroxenes in the Utgard cumulates, density data on mineral end members [Robie et al., 1966] give average densities for olivine, clinopyroxene, and orthopyroxene of 3.45, 3.33, and 3.40 g/cm3, respectively. The presence of Al-bearing end-members in clinopyroxene does not change the given average significantly. Experimental data [Presnall et al., 1978] imply that at 1.0 GPa mantle melts will have a first stage of olivine crystallization followed by coprecipitation of olivine + clinopyroxene in the proportion 20:80. We have chosen an average olivine:clinopyroxene ratio of 30:70 for these two stages and ignored orthopyroxene as the proportion of orthopyroxene is difficult to assess. Such an olivine-clinopyroxene mixture gives an average density of 3.37. Lower average Mg/(Mg+Fe2+) ratios, a significant proportion of orthopyroxene and/or the presence of titanomagnetite (∼5 g/cm3), Al-Cr spinel (3.6–4.4 g/cm3), and/or garnet (3.4–4.3 g/cm3) in the cumulate increase the average density. However, the cumulates will also contain gabbroic material formed from melts trapped in interstices between cumulate minerals. Hyndman and Drury [1976] report averages of 2.795 g/cm3 for basalts and 2.957 g/cm3 for gabbros in the oceanic crust. We have chosen the intermediate value of 2.8 g/cm3 for trapped material in the cumulates. Tegner et al. [2009] found densities of 3.0–3.3 g/cm3, and the proportion of trapped liquid to be 3–47% in low-pressure cumulates with high proportions of plagioclase (2.6–2.8 g/cm3) [Robie et al., 1966] and Fe-Ti-oxides in the Skaergaard intrusion (east Greenland). The density of a cumulate decreases with increasing proportion of trapped liquid. We have chosen 20% trapped liquid for our estimates. This suggests an average density of roughly 3.24 g/cm3 for lower crustal cumulates (cumulate minerals + interstitial material) produced from a primary magma that gave rise to the Utgard sills (Table 6).

[34] Berndt et al. [2000] found a mean density of 2.75 g/cm3 (and a mean velocity of 7.0 km/s) in the Upper Sill. The mass proportion of 40% dolerite with a density of 2.75 g/cm3 relative to 60% cumulates with an average density of 3.24 g/cm3 gives a volume proportion of 1.28 and 1.22 units (for the Upper and Lower sills, respectively) of cumulates relative to 1 unit sill (Table 6). Assuming the same areal extent, the formation of the two Utgard sills with a combined thickness >140 m must have given rise to a >177 m thick layer of cumulate crystals. Including 20% trapped liquid, the total thickness of cumulates becomes >212 m. Furthermore, as our estimated cumulate density is higher than the average density of 3.10 g/cm3 estimated by geophysics for the LCB beneath the Vøring Plateau [Ebbing et al., 2006; Reynisson et al., 2010], the cumulates could be mixed with significant volumes of rocks with densities <3.10 g/cm3, for example, old continental crust. Ebbing et al. [2006, and references therein] gave densities of 2.95–3.00 g/cm3 for the old continental crust in the Vøring Plateau; we chose the value of 2.95 g/cm3 (Table 6) for our estimates. With an average density of 3.25 g/cm3 for the Utgard cumulates, cumulates and old crustal basement would have to be mixed in the approximate proportion 2:1 in order to match a bulk density of 3.10 g/cm3. This implies that the Utgard sills may account for a >320 m thick layer of mixed cumulate + crustal basement in the LCB (Table 6). In addition, the fractional crystallization in Stage 1b has given rise to some gabbros.

[35] Seismic surveys have not found the LCB beneath the 6607/5-2 Hole in the Utgard High (Figure 2). However, this hole only penetrates the easternmost part of the Utgard sills, and the LCB extends eastwards to just west of the hole and is thus present beneath most of the E-W range of these sills. We therefore do not see any disagreement between the seismic data and our results.

[36] In the upper crust beneath the Hel Graben, Berndt et al. [2000] (Figure 2) have identified six to seven sills, each with a maximum thickness of 50–100 m. Table 6 shows two estimates for the amount of mixed cumulates and basement associated with these sills, one based on the assumption of six 50 m thick sills, the other assuming seven 100 m thick sills. Using the same chemistry and proportion of trapped melt as for the Utgard sills, the Hel Graben sills may account for a 700–1600 m thick mixed cumulate and basement layer in the LCB.

[37] The Upper and Lower 642E lava series on the Vøring Platform are about 770 and 140 m thick, respectively [Viereck et al., 1998, 1989]. The average MgO contents in the Upper Series are considerably higher than in the Utgard sills (Figure 4). We therefore assume a significantly lower average degree of fractional crystallization (50%) for the 642E lavas than for the Utgard sills, and consequently a somewhat higher Mg/(Mg+Fe) ratio for the mafic phases (0.85). The higher MgO content of the lavas and cumulates give a lower average cumulate density, 3.23 g/cm3, and a volume proportion of lavas:cumulates of 1:0.87. On this basis, the estimated thickness of cumulates mixed with basement rocks associated with the drilled volcanics of the Vøring Plateau is ∼1400 m. The thickest lava sequence on the Vøring Plateau, however, has been estimated to exceed 6 km [Eldholm et al., 1987; Skogseid and Eldholm, 1987]. Assuming the same parameters as for the lavas recovered in Hole 642E, the mixture of cumulates and old basement rocks beneath the thickest lava sequence on the Vøring Plateau must be at least 8.8 km thick. This result is slightly higher than that based on wide-angle seismic data indicating the LCB in this area is ≤8 km [e.g., Mjelde et al., 2002], and somewhat lower than a model based on structural observations and thermokinematic modeling which gave a maximum thickness of 11 km for the LCB beneath the Vøring Margin [Gernigon et al., 2006].

[38] The estimates summarized in Table 6 strongly depend on the choice of input parameters (degree of fractional crystallization, proportion of trapped melt in the cumulates, sill thicknesses, etc.). However, we have systematically chosen parameters, e.g., significantly lower degree of fractional crystallization than indicated by EC-RAFC modeling and estimates based on Nb, high Mg/(Mg+Fe) ratios, and a high proportion of trapped melt in the cumulate minerals that give low estimates of cumulate densities and mixed cumulate-basement complexes. In spite of the fact that the estimates given in Table 6 represent oversimplifications, we regard our results as evidence that significant parts of the LCB can be explained as a heterogeneous mixture of dense cumulates associated with the opening-related magmatism and less dense rocks, such as old continental basement.

[39] A model involving a mixture of rocks with contrasting physical properties (for example, opening-related cumulates and old continental crust) is in agreement with the large variations in VP (7.1–7.8 km/s) and relatively low VP/VS ratios (1.75–1.78) documented within the LCB by Mjelde et al. [2002]. Our model is also in agreement with the conclusions of Wangen et al. [2011]. They studied the nature of the LCB beneath the western Vøring Margin on the basis of three scenarios related to the extension history (a) only Caledonian crust; (b) half Caledonian crust and half magmatic underplating; (c) only magmatic underplating, and found model (b) to be most likely. However, our results indicate that the LBC includes opening-related gabbros, and do not exclude the possibility that the LCB beneath the Vøring Plateau also involves other rock types, such as older igneous material, high-grade metamorphic rocks (granulites or eclogites), and/or serpentinized mantle [e.g., Gernigon et al., 2003, 2004, 2006; Ebbing et al., 2006; Mjelde et al., 2009; Reynisson et al., 2010].

5. Evolutionary History

[40] Based on major and trace element and Sr-Nd isotope data, we propose the following evolutionary history for the Utgard sills (summarized in Figure 13) and their relationships with the 642E and NE Greenland lava series.

[41] The primary Utgard melts originated by partial melting of an asthenospheric mantle source depleted with respect to Sr-Nd isotope and trace element compositions. Its chemical characteristics appear to be similar to those of the source that gave rise to the NE Greenland lavas (IRZ). The wide ranges in 87Sr/86Sr, (Nb/Th)N, and (Nb/La)N ratios among the 642E Upper Series lavas prevent conclusions about their mantle source. However, a similar mantle source as for the Utgard sills and the NE Greenland lavas is possible and likely. The slightly different Sr-Nd isotope compositions of the two Utgard sills most likely reflect heterogeneities in the mantle source.

[42] The primary melts that gave rise to the Utgard sills formed either by partial melting in the garnet stability field or by mixing between melts from both garnet-facies and spinel-facies peridotites, similar to the model proposed for the NE Greenland lavas [Thirlwall et al., 1994]. In any case, the melting dynamics associated with the Upper Utgard Sill appears to have involved a higher garnet proportion in the source, showing that the Utgard magmas formed at greater depth than the 642E and NE Greenland lavas. The Utgard sills were emplaced at a great distance from the continent-ocean transition during the early stages of the breakup-related volcanism, whereas the 642E lavas extruded on the outer Vøring Plateau during the last stages of rifting and earliest stages of seafloor spreading. This indicates that at their times of emplacement the lithosphere was significantly thicker beneath the Utgard sills than beneath the 642E lavas.

[43] The magmas that gave rise to the Utgard sills appear to have ascended through the subcontinental lithospheric mantle (SCLM; Figure 13) without significant interaction with the wall rocks. When entering the less dense lower crust, the melts lost much of their buoyancy and ponded to form magma chambers where the melts were subjected to AFC processes (Stage 1). Our modeling indicates this stage involved extensive fractional crystallization in the lower crust, mainly involving removal of dense the phases olivine and pyroxenes. Fractional crystallization in the lower crust also gave rise to minor amounts of gabbros. The total extent of fractional crystallization during Stage 1 is estimated to be at least 80% relative to the initial magma mass. Furthermore, processes in the lower crust included minor assimilation of crustal melts (<0.5%). The anatectic crustal melts appear to have had 87Sr/86Sr ratios ≤0.715, (Nb/Th)N of ∼1 and (Nb/La)N between 0.5 and 1.0 (Figures 9 and 10). Rocks with similar properties are found in the West Norwegian Gneiss region.

[44] The evolved residual melts finally ascended to the upper crust where they formed the Utgard sills. A new stage of fractional crystallization (Stage 2: 4–6% relative to the original magma mass; Figure 13) occurred in situ.

[45] In the NE Greenland LS lavas fractional crystallization is, on average, less extensive than in the Utgard sills and crustal contamination is insignificant. The NE Greenland US is significantly contaminated by crustal rocks although different from those that contaminated the Utgard sills. The 642E LS lavas show a two-stage contamination history. The first stage involves strongly decreasing 143Nd/144Nd with moderately increasing 87Sr/86Sr, compatible with a lower crustal assimilant. The second stage involves typical upper crustal contaminants (high 87Sr/86Sr and moderately low 143Nd/144Nd, (Nb/Th)N<<1, (Nb/La)N<<1; Figures 9 and 10). The wide ranges in Sr isotope ratios and ratios between incompatible elements among the 642E lavas suggest contamination by assimilants with a variety of chemical characteristics.

[46] The extensive fractional crystallization that affected the parent melts of the Utgard sills imply that significant proportions of the parent magma was left as cumulates in the deep crust (underplating). Assuming an areal extent similar to that of the Utgard sills, these cumulates, with an estimated average density of 3.23–3.25 g/cm3, form a >210 m thick layer. In order to obtain the average density of 3.10 g/cm3 estimated for the LCB by geophysics, the cumulates may be mixed with less dense rocks. A mixture between cumulates and old continental lower crust (2.95 g/cm3) makes a >320 m thick layer beneath the Utgard sills with the average density of 3.10 g/cm3. Significant volumes of dense cumulates in the lower crust must also exist beneath other opening-related magmatic complexes on the Norwegian margin. A layer with an average density of 3.10 g/cm3, consisting of cumulates and old continental lower crust, would have a thickness between 700 and 1600 m beneath the Hel Graben, >1.4 km beneath Hole ODP Leg 104 Hole 642E, and >8.8 km beneath the thickest part of the lava sequence on the Vøring Plateau, slightly higher than the estimate of Mjelde et al. [2002] (Table 6). We thus argue that opening-related cumulates make up a significant part of the LCB. The LCB also includes gabbros formed during the last part of the crystallization in the lower crust (Stage 1b; Figure 13), and may comprise other rock types such as old sill complexes, eclogites, and/or serpentinized peridotites.


[47] We are indebted to Romain Meyer who gave us access to new geochemical data on lavas from ODP Leg 104 Hole 642E lavas before publication. We thank the Norwegian Petroleum Directorate for access to samples from the Utgard borehole. This work was financed by a YFF grant to H. Svensen and a SFF grant to PGP (Physics of Geological Processes), both from the Norwegian Research Council. The paper benefited from the constructive reviews of Godfrey Fitton, Reidar G. Trønnes, and Tod Waight.