Article
Volume 14, Number 9
20 September 2013
doi: 10.1002/ggge.20224
ISSN: 1525-2027
Sill and lava geochemistry of the mid-Norway and
NE Greenland conjugate margins
Else-Ragnhild Neumann and Henrik Svensen
Physics of Geological Processes, University of Oslo, P.O. Box 1048 Blindern, 0316 Oslo, Norway
(e.r.neumann@geo.uio.no)
Christian Tegner
Department of Geoscience, Aarhus University, Aarhus, Denmark
Sverre Planke
Physics of Geological Processes, University of Oslo, P.O. Box 1048 Blindern, 0316 Oslo, Norway
Volcanic Basin Petroleum Research, Oslo Innovation Park, Oslo, Norway
Matthew Thirlwall
Department of Earth Sciences, Royal Holloway University of London, Egham, UK
Kym E. Jarvis
Hamilton Building Imperial College, Ascot, UK
[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 Vïring Plateau. The
Utgard sills are compared to opening-related lavas recovered from ODP Leg 104 Hole 642E farther west
on the Vïring 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
143
Nd/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 Vïring 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.
Components: 11,456 words, 13 figures, 5 tables.
Keywords: Norwegian margin; Vïring Basin; sill intrusions; breakup magmatism; underplating.
Index Terms: 1065 Major and trace element geochemistry: Geochemistry; 1036 Magma chamber processes: Geochemistry; 1040 Radiogenic isotope geochemistry: Geochemistry; 1020 Composition of the continental crust: Geochemistry;
3618 Magma chamber processes: Mineralogy and Petrology.
Received 15 February 2013; Revised 8 July 2013; Accepted 9 July 2013; Published 20 September 2013.
© 2013. American Geophysical Union. All Rights Reserved.
3666
NEUMANN ET AL.: UTGARD SILLS
10.1002/ggge.20224
Neumann, E.-R., H. Svensen, C. Tegner, S. Planke, M. Thirlwall, and K. E. Jarvis (2013), Sill and lava geochemistry of the
mid-Norway and NE Greenland conjugate margins, Geochem. Geophys. Geosyst., 14, 3666–3690, doi:10.1002/ggge.20224.
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 sea-
ward dipping reflectors, sills, and dykes, mainly in
the Mïre and Vïring 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
Figure 1. (a) Distribution of volcanic rocks on the Vïring 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 Vïring 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), Vïring Escarpment (VE), and Vïring
Marginal High (VMH). Sedimentary basins: Danmarkshavn Basin (DB), Halten Terrace (HT), Jameson Land
Basin (JB), Thetis Basin (TB), Trïndelag Platform (TP), and Vïring 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 Utrïst 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 www.npd.no.
3667
NEUMANN ET AL.: UTGARD SILLS
10.1002/ggge.20224
Figure 2. Crustal transect across the central part of the Vïring 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 oceanbottom 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 Vïring Marginal High, south of the transect (Figure 1).
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 highvelocity, 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].
[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 Vïring Plateau and Deep Sea Drilling Programme (DSDP)
Sites 338, 342, and 343 on the northern Vïring 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 Vïring 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 Vïring 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 Vïring Basin
(Figure 2).
2. Geological Setting
[5] The thick sequence of lavas forming seaward
dipping reflectors covering the outer part of the
3668
NEUMANN ET AL.: UTGARD SILLS
Mïre and Vïring 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 Vïring 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 Vïring and Mïre 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 2D 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 6 0.3
Ma for the Upper Utgard Sill and 56.3 6 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 homoge-
10.1002/ggge.20224
nous 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).
[10] The sills show considerable scatter with
respect to major elements, but Al2O3, Na2O, and
1
Additional supporting information may be found in the online
version of this article.
3669
NEUMANN ET AL.: UTGARD SILLS
10.1002/ggge.20224
Table 1. Major Element (Weight %) and Trace Element (ppm) Data on Samples From the Utgard Sillsa
Upper Sill (US)
Depth (m)
3855
3858
3864
3867
3873
3879
3882
4647
4650
4674
4683
SiO2
TiO2
Al2O3
Fe2O3
MnO
MgO
CaO
Na2O
K2O
P2O5
LOI
SUM
Co
Cr
Cu
Ni
Sc
Sr
V
Zn
Zr
Ga
U
Th
Rb
Nb
Cs
Hf
Y
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Yb
Lu
Mo
Sn
Sb
47.45
3.17
13.92
15.31
0.22
4.33
9.66
2.66
0.57
0.33
0.78
98.40
178
44.9
309
47.4
36.1
269
498
131
191
23.2
0.44
1.38
8.6
17.3
0.01
5.1
36.9
20.5
47.3
6.48
31.4
7.40
2.52
8.43
1.34
8.10
1.56
4.33
3.55
0.59
2.49
1.33
0.07
46.13
3.13
13.71
14.96
0.21
3.91
9.16
2.62
0.59
0.35
0.89
95.65
103
39.3
300
42.6
32.1
264
474
128
197
23.1
0.48
1.47
8.9
20.4
0.08
4.9
37.4
20.7
49.4
6.73
31.6
8.17
2.56
9.46
1.30
7.18
1.47
4.28
3.66
0.53
1.92
1.47
0.06
48.21
2.97
14.27
14.70
0.21
4.32
9.85
2.72
0.61
0.32
1.58
99.76
144
52.6
320
48.0
35.1
275
464
158
210
23.7
0.49
1.48
9.0
21.1
0.20
5.7
37.7
18.5
43.7
6.19
27.8
7.18
2.33
7.90
1.27
7.22
1.49
3.99
3.39
0.44
3.06
1.24
0.08
49.28
2.44
14.75
13.12
0.19
4.11
9.92
2.83
0.63
0.34
1.22
98.84
123
42.8
327
40.6
33.6
281
352
119
212
23.2
0.51
1.54
9.5
18.5
0.01
5.6
38.8
21.1
49.6
6.82
31.7
8.08
2.61
9.36
1.38
7.60
1.58
4.24
3.95
0.55
2.31
0.91
0.20
48.21
2.80
14.51
14.16
0.20
4.03
9.67
2.80
0.64
0.32
0.32
97.66
156
34.1
313
40.7
33.9
285
402
126
192
23.1
0.49
1.52
9.2
18.7
0.08
5.4
38.2
20.1
45.4
6.50
29.5
7.34
2.37
8.00
1.26
7.31
1.51
3.88
3.05
0.48
3.43
1.27
0.09
48.41
2.77
14.81
14.36
0.20
4.01
9.85
2.80
0.60
0.32
0.67
98.80
155
37.1
307
40.3
34.5
280
416
128
189
23.2
0.48
1.38
8.6
17.7
n.d.
5.1
37.1
18.2
40.8
5.66
26.5
6.65
2.12
7.06
1.15
6.70
1.30
3.48
2.99
0.45
3.61
1.78
0.10
48.08
2.95
15.04
14.31
0.20
3.75
9.65
2.84
0.61
0.33
0.69
98.45
107
27.7
322
36.4
30.7
284
437
125
194
24.3
0.45
1.50
8.8
20.8
0.06
5.2
37.7
21.1
47.9
6.73
30.1
7.51
2.65
8.95
1.25
7.54
1.58
3.97
3.72
0.55
1.78
1.07
0.04
49.94
2.57
15.53
13.57
0.20
3.69
9.74
3.02
0.66
0.36
0.57
99.85
101
78.7
325
41.1
30.6
295
365
112
200
23.6
0.53
1.87
9.3
21.3
0.20
6.2
39.4
20.3
47.4
6.71
30.5
7.96
2.41
8.22
1.30
7.66
1.45
4.12
3.48
0.51
1.95
1.42
0.07
47.79
2.92
14.99
13.91
0.20
3.55
9.46
2.87
0.62
0.34
0.40
97.07
99.2
22.4
336
35.0
30.6
289
373
120
195
23.2
0.46
1.41
9.2
20.7
n.d.
5.2
37.0
19.2
46.1
6.43
29.6
7.52
2.40
8.50
1.17
7.10
1.43
3.86
3.46
0.50
1.38
0.97
0.03
48.86
3.00
15.31
14.37
0.20
3.94
9.80
2.97
0.63
0.32
0.61
100.01
117
30.5
295
40.2
31.9
289
421
130
193
23.6
0.43
1.42
9.0
18.5
0.04
5.0
35.6
20.3
48.3
6.79
31.8
7.71
2.54
8.32
1.34
7.24
1.52
4.27
3.39
0.47
1.82
1.24
0.06
48.00
3.31
14.79
15.44
0.21
3.93
9.80
2.82
0.60
0.32
0.21
99.42
112
36.2
317
41.8
33.4
279
507
132
226
24.6
0.48
1.45
8.9
22.3
0.01
6.1
37.0
18.5
43.6
6.04
28.6
7.22
2.38
7.98
1.24
7.01
1.44
3.89
3.31
0.46
2.33
1.68
0.07
Upper Sill (US)
Lower Sill (LS)
Depth (m)
3855
3858
3864
3867
3873
3879
3882
4647
4650
4674
4683
SiO2
TiO2
Al2O3
Fe2O3
MnO
MgO
CaO
Na2O
K2O
P2O5
LOI%
SUM
Co
Cr
Cu
Ni
Sc
49.41
2.14
14.23
13.14
0.19
4.11
9.45
2.82
0.74
0.38
1.77
98.39
122
40.4
329
38.9
32.2
50.99
2.33
14.57
13.78
0.21
4.48
10.04
2.91
0.74
0.38
0.50
100.90
143
49.7
328
44.2
35.2
49.17
2.48
13.88
14.57
0.21
4.89
10.05
2.67
0.65
0.35
0.15
99.08
123
70.2
316
54.5
37.0
46.36
3.03
13.16
15.19
0.22
4.46
9.40
2.57
0.61
0.33
0.15
95.47
228
58.9
316
49.7
37.1
48.76
2.75
13.44
15.35
0.22
5.35
10.06
2.56
0.64
0.32
0.48
99.93
148
97.2
328
68.6
38.2
49.37
2.59
13.99
14.61
0.21
5.20
10.17
2.66
0.65
0.34
0.65
100.45
137
90.4
319
68.5
36.6
48.22
2.68
13.31
14.99
0.22
5.28
9.77
2.57
0.67
0.33
0.40
98.41
121
99.4
325
70.5
35.4
44.65
1.77
12.51
12.27
0.19
5.51
9.84
2.10
0.33
0.18
1.79
91.14
149
77.9
142
47.7
37.6
47.21
1.85
13.34
12.70
0.20
5.78
10.42
2.23
0.33
0.19
2.69
96.93
190
96.6
161
59.2
40.2
47.59
1.95
13.16
13.34
0.21
5.86
10.54
2.29
0.32
0.18
0.87
96.31
160
91.4
175
58.4
41.9
49.01
2.19
12.67
14.67
0.23
5.17
9.71
2.61
0.48
0.25
1.03
98.02
159
41.7
203
45.1
42.7
3670
NEUMANN ET AL.: UTGARD SILLS
10.1002/ggge.20224
Table 1. (continued)
Upper Sill (US)
Lower Sill (LS)
Depth (m)
3855
3858
3864
3867
3873
3879
3882
4647
4650
4674
4683
Sr
V
Zn
Zr
Ga
U
Th
Rb
Nb
Cs
Hf
Y
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Yb
Lu
Mo
Sn
Sb
279
325
116
238
22.9
0.57
1.75
10.6
18.5
n.d.
6.2
42.0
26.4
59.0
8.21
36.8
9.19
2.97
10.7
1.63
8.40
1.78
4.95
3.94
0.55
3.32
1.49
0.09
285
334
121
223
23.8
0.57
1.74
10.0
19.3
0.06
6.3
42.6
22.3
51.9
7.23
33.1
8.27
2.85
9.29
1.38
8.10
1.74
4.60
3.70
0.55
3.09
1.55
0.17
269
384
121
203
22.1
0.51
1.74
9.1
19.3
0.20
6.4
39.1
22.1
50.1
7.07
32.7
8.15
2.78
9.24
1.56
8.26
1.69
4.76
3.96
0.53
2.76
1.30
0.45
256
460
287
157
22.9
0.46
1.46
9.4
16.2
0.09
4.6
38.3
18.9
45.9
6.36
29.6
7.78
2.45
8.06
1.28
7.27
1.44
3.91
3.34
0.53
6.14
1.75
0.27
258
455
137
191
22.4
0.45
1.43
9.1
16.4
n.d.
5.1
38.1
20.1
48.1
6.88
31.7
8.33
2.57
8.59
1.47
7.54
1.63
4.65
3.82
0.71
3.41
1.52
0.22
268
424
125
204
23.1
0.50
1.49
8.9
18.3
0.03
5.4
38.9
19.6
49.3
6.82
31.8
8.16
2.75
8.93
1.29
8.10
1.60
4.17
3.41
0.58
2.31
1.66
0.09
255
426
127
213
22.0
0.46
1.44
9.6
19.4
0.05
5.7
37.8
183
322
59.3
98.4
16.8
0.22
0.79
5.1
9.3
0.73
3.3
27.8
8.88
21.3
3.15
15.4
4.57
1.71
5.77
0.91
5.62
1.17
3.22
2.94
0.41
1.08
0.45
0.06
203
356
125
105
18.6
0.23
0.63
4.0
8.6
0.25
2.8
30.6
9.72
21.3
3.00
14.9
4.56
1.59
5.29
0.90
5.43
1.14
3.19
2.84
0.41
2.63
0.90
0.11
187
374
126
106
18.4
0.21
0.60
3.9
8.2
0.24
3.0
31.2
9.8
22.5
3.03
15.7
4.77
1.71
5.63
0.98
5.90
1.21
3.52
3.14
0.43
2.27
1.00
0.10
200
387
113
149
20.1
0.33
1.03
6.7
12.0
0.37
4.4
39.0
11.8
31.2
4.52
21.7
6.49
2.09
7.64
1.25
7.74
1.78
4.46
3.84
0.53
2.26
1.17
0.10
2.08
1.26
0.06
a
Sample numbers show depths in m.
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 ¼ 17–
22, [Ce/Yb]N ¼ 3.0–3.3, Lower Sill: ThN ¼ 7.6–
12.9, (Ce/Yb)N ¼ 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.
[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 6
0.000004 to 0.512932 6 0.000005), whereas the
range in 87Sr/86Sr ratios is significant (0.703801 6
0.000012 to 0.703872 6 0.000011). The highest
87
Sr/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
87
Sr/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).
4. Discussion
4.1. Correlation of Conjugate Margins
[12] The Utgard sills and lavas on the Vïring 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
3671
NEUMANN ET AL.: UTGARD SILLS
10.1002/ggge.20224
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
Vïring Plateau succession is very similar to that
seen on the SE Greenland margin and onshore
East Greenland [e.g., Fitton et al., 1998, 2000].
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 þ clinopyroxene 6 Fe-Ti-oxides. 642E US:
‘‘uncontaminated’’ ODP Hole 642E Upper Series lavas on the
Vïring 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.
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
Vïring 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
[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 Vïring Plateau (642E) and in NE Greenland
(HwH þ WolF) lavas. ‘‘Uncontaminated’’ lavas
are those that have 87Sr/86Sr below 0.7037 and
143
Nd/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
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NEUMANN ET AL.: UTGARD SILLS
10.1002/ggge.20224
Figure 4. The Utgard sills show low concentrations in Ni, Sc, and Cr which decrease with decreasing MgO,
compatible with removal of olivine þ clinopyroxene 6 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.
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 EMRNEGLS-Upper Utgard Sill trend with uniform
143
Nd/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.
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)
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NEUMANN ET AL.: UTGARD SILLS
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.
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. 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 þ clinopyroxene þ plagioclase þ
magnetite þ apatite.
10.1002/ggge.20224
[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.
[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
3674
NEUMANN ET AL.: UTGARD SILLS
10.1002/ggge.20224
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.
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
143
Nd/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 sea3675
NEUMANN ET AL.: UTGARD SILLS
10.1002/ggge.20224
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 7. Variations in 87Sr/86Sr and mg# (Mg/[MgþFe2þ]
assuming Fe2þ ¼ 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.
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
Table 2. Sr-Nd Isotope Data on Selected Samples From the
Utgard Sillsa
87
Sr/86Sr
Upper Sill
3807
0.703872
3810
0.703853
3822
0.703848
3834
0.703801
3846
0.703844
3855
0.703802
3864
0.703812
3873
0.703811
3879
0.703825
3882
0.703869
Lower Sill
4674
0.703755
4683
0.703728
a
2se
143
Nd/144Nd
2se
eNd
0.000011
0.000010
0.000010
0.000012
0.000012
0.000012
0.000013
0.000010
0.000010
0.000011
0.512920
0.512926
0.512922
0.512925
0.512927
0.512920
0.512922
0.512928
0.512917
0.512932
0.000004
0.000004
0.000006
0.000004
0.000004
0.000004
0.000005
0.000004
0.000004
0.000005
5.50
5.62
5.54
5.60
5.64
5.50
5.54
5.66
5.44
5.74
0.000015
0.000012
0.512992
0.512970
0.000005
0.000008
6.91
6.48
Sample numbers show depths (m).
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
Vïring 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 3a and 3b.
[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 ECRAFC 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 FAR31 (ratios between those of FAR-31and eclogite
3676
NEUMANN ET AL.: UTGARD SILLS
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 3a and 3b suggests that the extent
of crustal contamination in the Utgard Sills is very
minor (Figure 9). Using Melt2 (IFZ as mantle
10.1002/ggge.20224
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 ECRAFC modeling is strongly dependent on the temperature difference between the initial temperature
and the solidus of the assimilant (Tao and Ts,
respectively, in Tables 3a and 3b); modeling with
Tao ¼ 600 and Ts ¼ 700 C gives 40% crystallization, whereas 600 and 750 C, respectively, gives
Figure 9. (a) Sr-Nd isotope data on the Utgard sills compared to domains covered by ‘‘contaminated’’ lavas from the
Vïring 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 3a and 3b. 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 ECRAFC 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) ECRAFC 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. http://georoc.mpch-mainz.gwdg.de);
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.
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NEUMANN ET AL.: UTGARD SILLS
Table 3a. Parameters Used in EC-RAFC Modeling
Parameter abbreviations
Values used
Tlm—Liquidus of magma
Tmo—Initial temperature of magma
Tla—Liquidus of assimilant
Tao—Initial temperature of assimilant
Ts—Solidus (melt and assimilant)
Cpm—Specific heat of magma
Cpa—Specific heat of assimilant
Hcry—Heat of crystallization
Hfus—Heat of fusion
1300 C
1300 C
1100 C
600 C
700 C (750 C)
1495 J/kg K
1400 J/kg K
395,000 J/kg
354,000 J/kg
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 ¼ 1.47
ppm; Tables 3a and 3b) 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 ¼ 8.2 ppm) is
10.1002/ggge.20224
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
Table 3b. Compositions of End-Members Used in EC-RAFC Modelling With References
Sr (ppm)
Mantle Melts
Melt1
90a
Melt2
91c
e
Melt3
219
Bulk D0f upper crust
1.5
f
Bulk D0 lower crust
0.2
Assimilant Upper Crust
FAR-5g
340
ACC
320h
f
Bulk D0
1.5
Assimilants Lower Crust
FAR-24j
1137
FAR-31k
776
l
FAR-37B
316
m
8906A11
137
8815Bm
300
Bulk D0f
0.3
Assimilant Lithospheric Mantle
SCLM
20n
f
Bulk D0
0.15
Nd (ppm)
Th (ppm)
Nb (ppm)
La (ppm)
7.3a
4.0c
17.3
0.15
0.15
0.12a
0.0628c
0.55
0.1
0.1
2.33a
1.47c
17.0
0.1
0.1
2.5a
1.36c
8.0e
0.15
0.15
22.7
20h
0.15
40.43
39.65
32.11
14.9
12.4
0.15
2n
0.15
10.4
5.6h
0.1
11
8h
0.1
30.1
30.9h
0.15
5.4
13
72.7
0.7
0.6
0.1
8.8
5.8
0.1
7.76
9.2
0.15
0.22n
0.1
2.7n
0.1
0.77n
0.15
87
Sr/86Sr
143
Nd/144Nd
0.70219b
0.702891e
0.70346
0.51326b
0.513132e
0.51302
0.735889
0.7123i
0.511637
0.5115i
0.706649
0.712119
0.707270
0.511759
0.511823
0.512260
0.512197
0.512306
0.7041o
0.512617o
a
N-MORB: Sun and McDonough et al. [1989].
DMM: depleted MORB-mantle: Workman and Hart [2005].
c
Iceland Rift Zone basalt with IRZ Sr-Nd isotope ratios: sample 9372 in the GEOROC database (http://georoc.mpch-mainz.gwdg.de).
d
IRZ: Debaille et al. [2009].
e
642E US sample 045R3/128: Meyer et al. [2009].
f
Estimated on the basis of partition coefficients given by: Francalanci [1989]; Nielsen et al. [1992], and Salters and Longhi [1999].
g
WGR, granitic gneiss: Austrheim et al. [2003].
h
ACC: average continental crust: Rudnick and Gao [2004].
i
ACC: Allegre et al. [1996].
j
WGR, quartz monzonitic gneiss: Austrheim et al. [2003].
k
WGR, monzodioritic gneiss: Austrheim et al. [2003].
l
WGR, doleritic gneiss: Austrheim et al. [2003].
m
WGR, eclogite: Kylander-Clark et al. [2007].
n
SCLM: McDonough [1990].
o
SCLM: subcontinental mantle lithosphere: Lee et al. [1994].
b
3678
NEUMANN ET AL.: UTGARD SILLS
10.1002/ggge.20224
Table 4. 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
PM (spinel)
PM (10% garnet)
PM (6% garnet)
PM (3% garnet)
Spinel stability field
PC mineral/melt
PC mineral/melt
PC mineral/melt
Garnet stability field
PC mineral/melt
PC mineral/melt
PC mineral/melt
PC mineral/melt
PC mineral/melt
ol
Source/Melt
Sourceb
Meltc
Sourced
Melte
Sourcef
Melte
Sourcef
Melte
Element
Ce
Sm
Yb
Element
Ce
Sm
Yb
Nb
Th
opx
cpx
sp
0.60
0.22
0.55
0.12
0.55
0.12
0.55
0.12
0.28
0.38
0.20
0.67
0.20
0.67
0.20
0.67
0.10
0.71
0.15
1.25
0.19
1.25
0.22
1.25
0.02
0.13
0.0060g
0.0067g
0.045g
0.009g
0.020g
0.1033g
0.092g
0.445g
0.542g
0.0005g
0.0009g
0.0045g
0.0060g
0.0067h
0.045g
0.0001i
0.0001i
0.0016h
0.015h
0.22h
0.002j
0.0004j
0.12h
0.58h
1.2h
0.018j
0.007j
Garnet
PMa (co ppm)
0.10
0.30
0.06
0.30
0.03
0.30
1.675
0.406
0.441
0.0029h
0.18h
6.5h
0.01j
0.005j
1.675
0.406
0.441
0.658
0.0795
a
McDonough and Sun [1995].
Workman and Hart [2005].
Baker and Stolper [1994].
d
Johnson et al. [1990].
e
Presnall et al. [2002].
f
Johnson et al. [1990], with modified cpx-gar relationship.
g
Niu and Hekinian [1997].
h
Green et al. [2000].
i
No data available.
j
Salters and Longhi [1999].
b
c
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’’ Vïring 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
3679
NEUMANN ET AL.: UTGARD SILLS
10.1002/ggge.20224
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 3a and 3b. See
text for discussion.
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
3680
NEUMANN ET AL.: UTGARD SILLS
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.
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 4.
10.1002/ggge.20224
Figure 12. (Ce/Sm)N (Sm/Yb)N relationships among the
Utgard sills and ‘‘uncontaminated’’ magmatic rocks from the
Vïring 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 4. 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.
[27] Figure 5 shows that the Utgard sills have
inclined patterns in the HREE domain [Dy/
Yb]N ¼ 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 ¼ 1.2–1.4), whereas most
642E US lavas fall in the range (Dy/Yb)N¼1.0–
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
3681
NEUMANN ET AL.: UTGARD SILLS
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 6 0.3 and 56.3 6 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 Mïre and Vïring
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
10.1002/ggge.20224
by dense minerals (olivine þ pyroxenes þ 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 þ 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 þ pyroxenes þ spinel forming
dense olivine pyroxenite cumulates (Stage 1a), followed by removal of 6 olivine þ pyroxenes þ plagioclase 6 spinel forming gabbros (Stage 1b). The
evolutionary sequence outlined above is shown
schematically in Figure 13.
[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], highgrade 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].
3682
NEUMANN ET AL.: UTGARD SILLS
10.1002/ggge.20224
Table 5. 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/Lava
Cumulate Crystals
Utgard Upper Sill
Proportions of initial melt (%)
40
60
Density (g/cm3)
2.75c
3.37
Volume prop.
1
1.28
Thickness (m)f
90e
116f
Utgard Lower Sill
Proportions of initial melt (%)
45
55
Density (g/cm3)
2.75c
3.34
Volume prop.
1.0
1.22
Thickness (m)f
>50c
>61
Utgard total
>140
>177
Hel Graben Sills
Proportions of initial melt (%)
40
60
Min. thick. (m)g
50 6c
387
Max. thick. (m)g
100 7c
903
ODP Leg 104 Hole 642E, Outer Vïring Plateau
h
Proportions of initial melt (%)
50
50
Density (g/cm3)
2.8d
3.23
Thickness (m)g
910i
797
Vïring Plateau (Max Thickness)
Proportions of initial melt (%)
50
50
Max. thick. (m)
>6000l
>5010
Trapped Liquid (20%)
Average Cum.a
Crustal Basementb
LCB
2.8d
0.258
23f
3.24
1.55
139f
2.95e
0.78
70f
3.10e
2.33
209f
2.8d
0.24
3.23
1.46
2.95e
0.73
3.10e
2.19
>110f
>319
>73f
>212
>12
>35
>1002
>37f
>107
77
181
464
1084
232
542
694
1626
2.8d
159
3.23
956
2.95e
443
3.10e
>1400k
>6012
>2790
>8,800
a
Average cumulate: Cumulates mixed with trapped liquid.
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.
c
Berndt et al. [2000].
d
Hyndman and Drury [1976].
e
Ebbing et al. [2006] and Reynisson et al. [2010].
f
Thickness per areal unit of sill intrusion or lava in the upper crust.
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.).
h
Less evolved (Figure 4), therefore a different lava/cumulate proportion.
i
Vierick et al. [1989].
j
Upper Series 770 m thick, Lower Series 140 m thick.
k
Minimum value, see text for explanation.
l
Eldholm et al. [1987] and Skogseid and Eldholm [1987].
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
Vïring 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 5.
[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 þ Stage 2). Estimates for Mc depend on
the temperature difference (DT) between the initial
and the solidus temperature of the assimilant. We
used DT ¼ 10 C in the EC-RAFC modeling, higher
DT 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 þ clinopyroxene 6
orthopyroxene. Olivine and pyroxenes in the
asthenospheric mantle typically have Mg/
(MgþFe2þ) 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
3683
NEUMANN ET AL.: UTGARD SILLS
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 6olivine (ol)þ
clinopyroxene (cpx)þplagioclase (plag)þFe-Ti-oxides. Opx:
orthopyroxene; sp: spinel; Post.-Pal. sed.: Post-Paleocene
sediments; Cret.þPal. 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.
during fractional crystallization. Assuming an average Mg/(MgþFe2þ) 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 endmembers 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 þ 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 olivineclinopyroxene mixture gives an average density of
3.37. Lower average Mg/(MgþFe2þ) 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 ma-
10.1002/ggge.20224
terial 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 þ interstitial material) produced from a primary magma that gave rise to the Utgard sills
(Table 5).
[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 5). 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
Vïring 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
Vïring Plateau; we chose the value of 2.95 g/cm3
(Table 5) 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 þ crustal basement in the LCB
(Table 5). 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
3684
NEUMANN ET AL.: UTGARD SILLS
(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 5 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
Vïring 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/(MgþFe) 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 Vïring Plateau is 1400 m. The thickest lava
sequence on the Vïring 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 Vïring
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 Vïring
Margin [Gernigon et al., 2006].
[38] The estimates summarized in Table 5 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
10.1002/ggge.20224
crystallization than indicated by EC-RAFC modeling and estimates based on Nb, high Mg/(MgþFe)
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 5 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 Vïring 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
Vïring 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
3685
NEUMANN ET AL.: UTGARD SILLS
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 Vïring 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
10.1002/ggge.20224
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
143
Nd/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
143
Nd/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 Vïring Plateau,
slightly higher than the estimate of Mjelde et al.
[2002] (Table 5). We thus argue that openingrelated 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.
Acknowledgments
[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
3686
NEUMANN ET AL.: UTGARD SILLS
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. Trïnnes, and Tod Waight.
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