Lithos 71 (2003) 47 – 80
www.elsevier.com/locate/lithos
Paleogene volcanic ash layers in the Danish Basin: compositions
and source areas in the North Atlantic Igneous Province $
Lotte M. Larsen a,b,*, J. Godfrey Fitton c, Asger K. Pedersen b,d
a
Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark
b
Danish Lithosphere Centre, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark
c
School of GeoSciences, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, UK
d
Geological Museum, Øster Voldgade 5 – 7, DK-1350 Copenhagen K, Denmark
Received 26 March 2003; accepted 24 July 2003
Abstract
The Paleogene mo clay basin in Denmark contains ca. 200 layers of mostly well-preserved volcanic ash. Of these, ca. 77
layers have been analysed for major and trace elements by X-ray fluorescence spectrometry (XRF), 22 for rare-earth elements
(REE), and 11 for Sr and Nd isotopes. The ‘negative’ ash series (layers
39 to
1) comprise tholeiitic basalt, crustally
contaminated trachyte, and rhyolite; alkaline basalt, trachybasalt, trachyte, and rhyolite; and Ti-rich nephelinite and phonolite.
The ‘positive’ ash series (layers + 1 to + 140) comprise enriched tholeiitic ferrobasalt and two rhyolite layers. The ferrobasalts
form one comagmatic group; however, oscillations are seen up-section between less-enriched and more-enriched compositions,
as indicated by variations in Zr/Nb, REE contents, and isotope compositions, suggesting heterogeneities in the mantle source.
Two samples of Eocene ash from Greifswalder Oie in northern Germany are identical to the positive series ashes.
By comparison of the ash compositions with other rocks from the North Atlantic Igneous Province, probable source areas
can be identified. Four stages of deposition can be distinguished. In stage 1 (layers 39 to 22), basalts and rhyolites were
sourced from centres on the NW European shelf. In stage 2 (layers 21b to 15), trachytes and rhyolites were still sourced
from centres on the shelf, whereas the strongly alkaline layers could all have originated in the Gardiner Complex in East
Greenland. In stage 3, alkali basalts (layers 13 to 11) may be the products of a failed or propagating part of the opening
oceanic rift. In stage 4 (layers + 1 to + 140), we suggest that the comagmatic suite of voluminous ferrobasalts were sourced
from a gigantic volcanic system representing the nascent Proto-Iceland within the opening ocean. The cataclysmic character of
stage 4 can be understood if the areas of extremely high magma production associated with the Proto-Icelandic mantle plume,
which had until then produced the large subaerial flood basalts in Greenland, at this time moved away from the continent and
into the sea-covered opening rift, thus switching the bulk of volcanism from effusive to explosive. When Proto-Iceland finally
emerged, the explosive activity abated again.
D 2003 Elsevier B.V. All rights reserved.
Keywords: Denmark; North Atlantic Igneous Province; Paleogene; Tephra; Volcanic ash
$
Supplementary data associated with this article can be found
at doi:10.1016/j.lithos.2003.07.001.
* Corresponding author. Geological Survey of Denmark and
Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark.
Tel.: +45-38142252; fax: +45-38142050.
0024-4937/$ - see front matter D 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.lithos.2003.07.001
1. Introduction
Layers of volcanic ash, which were erupted in
association with continental break-up and the start of
48
L.M. Larsen et al. / Lithos 71 (2003) 47–80
Fig. 1. (a) Map of the North Atlantic and northern Europe with locations mentioned in the text. Position of Greenland at 53 Ma after Skogseid
et al. (2000). (b) Sample localities in northern Denmark: (1) Knudeklint, (2) Stolleklint, (3) Østklint, (4) Hanklit, and (5) Skarrehage mo clay pit.
(c) Simplified geological log of the ash-bearing succession at the type locality of Knudeklint with the most important ash layers indicated; after
Pedersen and Surlyk (1983).
L.M. Larsen et al. / Lithos 71 (2003) 47–80
formation of the North Atlantic Ocean, are widespread in the North Atlantic region. Volcanic ash
layers have been reported from the Paleocene –Eocene sedimentary succession in northwestern and
central Europe (Gagel, 1907; Bøggild, 1918; Egger
et al., 2000), the North Sea (e.g. Knox and Morton,
1988), and the NW European shelf from the Vøring
Plateau (ODP Hole 642, Viereck et al., 1988) over
the Faroes shelf (Waagstein and Heilmann-Clausen,
1995) to the Goban Spur (DSDP Hole 550, Knox,
1984, 1985) (Fig. 1). The ash layers are diagnostic
parts of the extensive Sele and Balder Formations in
the drill cores of the North Sea and the Faroe –
Shetland Basin (e.g. Knox et al., 1997; Ahmadi et
al., 2003).
The ash layers are over large areas interbedded
with clays and are heavily altered. Major-element
analyses of bulk ash samples often show hydration
and leaching to such an extent that the original
chemical composition of the ash is unrecognizable
(Madirazza and Fregerslev, 1969; Malm et al., 1984;
Morton and Knox, 1990). The best major-element
data come from microprobe analyses of fresh ash
particles (Pedersen et al., 1975; Morton et al.,
1988a) and glass inclusions in feldspar crystals (Morton and Knox, 1990). On the other hand, useful data
on immobile elements such as Ti, P, Zr, Nb, and Y
may still be obtained from analysis of bulk ashes
(Malm et al., 1984; Morton and Evans, 1988; Morton
and Knox, 1990; Haaland et al., 2000).
Volcanic ash layers are present in the Paleogene
sediments in the Danish Basin. Where they are in
normal clay facies, the ash layers are completely
altered and only recognizable as varicoloured stripes
in the clay. However, in northwestern Denmark, the
ash-bearing succession consists of diatomite (mo clay)
formed in a local embayment (Pedersen and Surlyk,
1983), and in this environment, many of the ca. 200
ash layers are fairly well preserved. Bulk ash analyses
by Bøggild (1918) and Morton and Evans (1988)
show relatively low losses on ignition down to 2 – 4
wt.%; however, only few ash layers were analysed in
these studies.
From the original descriptions of the Danish ash
layers in the mo clay areas by Bøggild (1918) and
Gry (1940) and the microprobe and bulk ash data of
Pedersen et al. (1975) and Morton and Evans
(1988), it is clear that the lower part of the ash
49
series (the ‘negative’ series, see below) is variable,
with ash compositions ranging from basaltic and
possibly nephelinitic to peralkaline trachytic, phonolitic, and rhyolitic. In contrast, the upper part of the
ash series (the ‘positive’ series) is basaltic except for
two layers and is alleged to be very uniform,
although this was based on analytical data from
only nine layers.
The location of the volcanoes from which the ashes
were erupted is disputed. Suggested areas range from
the Skagerrak area some 100 km NW of the mo clay
areas (Bøggild, 1918; Andersen, 1937; Norin, 1940;
Pedersen et al. 1975) to the basalt areas in East
Greenland (Knox and Morton, 1988; Morton and
Knox, 1990; Heister et al., 2001) which at that time
was situated around 1100 km NW of the mo clay areas
(e.g. Larsen et al., 1999).
We have systematically sampled the whole stratigraphic range of the Danish ash layers and subjected
them to bulk chemical analysis for major and trace
elements. The aim of the work was to examine in
more detail than was previously done the compositional range present and the development up-section
of the magma compositions, particularly with respect to the possible source areas and their variation
with time. Another aim was to establish a stratigraphic variation profile for the immobile elements
which is useful for correlation to other areas, particularly drill cores in the North Sea and the Faroe –
Shetland area.
2. Geology
The ash-bearing mo clay succession is exposed in
coastal cliffs around Limfjorden in northwestern Denmark (Fig. 1). It is referred to the Fur Formation with
type section at Knudeklint on the north coast of the
island of Fur (Pedersen and Surlyk, 1983).
The volcanic ash layers were described in detail by
Bøggild (1918). Around 180 individual ash layers
were assigned numbers on a scale that is open in both
ends, with a zero level that divides the succession into
two parts with distinctly different visual appearances:
a lower part with ash layers of the ‘negative series’
and an upper part with ash layers of the ‘positive
series’ (Fig. 1c). The lower part is ca. 30 m thick and
dominated by diatomite, with widely spaced, thin,
50
L.M. Larsen et al. / Lithos 71 (2003) 47–80
inconspicuous, mostly light-coloured ash layers with
numbers 39 to 1 and an aggregate ash thickness
of only 0.6 m. The upper part is ca. 25 m thick and
conspicuously striped, the light-coloured diatomite
containing closely spaced black basaltic ash layers
numbered + 1 to + 140. Only ash layers + 13 and
+ 19 are grey and rhyolitic. The aggregate ash thickness of the positive series is ca. 3.5 m, and thus, if the
deposition rate of the diatomite was the same in the
two series, the volcanic activity increased dramatically
from the negative to the positive series. The succession containing the negative series is correlated with
the Sele Formation and that of the positive series with
the Balder Formation in the North Sea (e.g. Neal,
1996).
The ash layers vary in thickness from around 1
mm and barely discernible to 19 cm for the rhyolitic
layer + 19 and the basaltic layer + 118. Some thin ash
layers that were not included in the original numbered
sequence have been assigned letters, e.g. layers 19a
and
19b are two individual ash layers that are
situated between layers 19 and 20 (Gry, 1940).
All the ash layers are normally graded and a few of
them are composite, showing evidence of two or
more ashfall events in one layer. Grain sizes vary
between layers; most of the basaltic layers have grain
sizes up to 100– 200 Am in the basal part but up to
500-Am particles occur (Bøggild, 1918). Carbonate
cementation (‘cement stone’) is occasionally seen,
particularly at the + 101 to + 102 level. This cementation took place before compaction (Pedersen and
Buchardt, 1996) and has protected the enclosed ash
against later alteration. The succession contains a
sufficient amount of characteristic details that most
ash layers can easily be identified from the detailed
logs of Bøggild (1918) and Gry (1940). Most difficulties with identification occur in the negative series
between layers 17 and 33.
Coeval ash-bearing deposits are also found in
northern Germany where the ash layers are up to 10
cm thick and much more fine grained than the Danish
deposits (Gagel, 1907). Good outcrops occur in coastal cliffs at Greifswalder Oie 400 km SSE of Fur
(Elbert and Klose, 1903), and two cement stone
blocks from this locality were also included in this
study. The fist-size blocks consist almost entirely of
well-cemented, well-preserved, very fine-grained volcanic ash that shows signs of re-deposition.
3. Sampling and analytical methods
The major part of the ash layers were sampled in
the type section at Knudeklint. Additional samples
from the negative series are from Stolleklint and
Østklint also on the north coast of Fur, and from
Skarrehage mo clay pit on Mors (Fig. 1). The layers
+ 101 to + 118 were also sampled at Hanklit on
Mors (Fig. 1). Ash preserved in carbonate-cemented
layers and lenses was sampled where possible. A
loose block from Thy provided the easily recognizable interval + 26 to + 31 in cemented facies, and of
these, layers + 27 and + 28 were analysed. Cementation of the negative ash series is very rare, but
layer
35 has been reported in cemented facies
(Bøggild, 1918); the original sample is now nearly
exhausted and the layer has not been re-found. However, small slabs of cemented ash are occasionally
found washed out of the negative series at the foot of
Stolleklint, presumably from the interval 22 to 34
exposed there. Three such slabs are included in this
study and are named HM1, HM2, and HM3. The two
blocks from Greifswalder Oie are named Greif1 and
Greif2.
In the laboratory, the uncemented samples were
passed through a 0.4-mm sieve and washed in several
rounds in de-mineralised water in ultrasonic bath.
Between each rinse, the wet samples were rubbed
and smeared between fingers and against the beaker
sides to loosen clay particles which were subsequently
drained away. Most samples lost around half of their
mass during the washing process, indicating a considerable clay content in the ash.
Carbonate-cemented samples (Thy layers + 27,
+ 28; Knudeklint 1998 layers + 101, + 102, + 130;
HM1, HM2, HM3, Greif1, and Greif2) were cut
clean, comminuted in a steel cylinder, and passed
through a 0.5-mm sieve. They were then treated with
10% hydrochloric acid under constant stirring, and
the carbonate matrix was dissolved in about 20 min.
The samples were then washed as described above.
The content of clay in the carbonate-cemented samples is much less than the non-cemented samples,
and the grain size distribution is different with many
more small particles because these have not been
turned into clay. The carbonate-cemented samples
lost around two-thirds of their mass during the
preparation.
L.M. Larsen et al. / Lithos 71 (2003) 47–80
The washed and dried samples were inspected
under a binocular microscope and some remaining
impurities removed before grinding to powder.
The samples were analysed for major and trace
elements by X-ray fluorescence spectrometry (XRF).
Major elements were analysed at the Geological
Survey of Denmark and Greenland (GEUS) on fused
glass discs except for Na2O (by atomic absorption
spectrometry) and FeO (by titration), as described by
Kystol and Larsen (1999). Trace elements were analysed at the Department of Geology and Geophysics,
University of Edinburgh, on pressed powder pellets as
described by Fitton et al. (1998). A subset of samples
was analysed for rare-earth elements (REE) on a
Perkin-Elmer Elan 6100 quadrupole-ICP-MS at
GEUS using a modification of the method described
by Turner et al. (1999), with calibration on a combination of synthetic REE solutions and international
reference materials. A smaller subset was analysed for
Sr and Nd isotopes at the Geological Institute, University of Copenhagen, on a VG Sector 54-IT instrument and with chemical procedures as described by
Frei et al. (1999); run statistics are given in the data
table.
4. Results
In total, 104 samples from ca. 77 different
Danish ash layers and two samples from Germany
were analysed. Several layers have been sampled at
two localities or at one locality on two occasions.
This double or triple sampling serves to illustrate
both the consistent individuality of layers of extreme composition such as layers
21 and
21a,
and also the inherent variations in some layers, e.g.
layers
19 and
19b. They also show that in
some cases the major elements in a sample may be
strongly altered while many trace elements are still
preserved. Analyses of one of each of the sampled
layers are presented in Table 1, and the complete
data set is available in the online version of this
paper.1
1
paper.
See Supplementary data Table in the online version of this
51
Fig. 2. Volatile contents in the analysed bulk ash samples. Lines
connect analyses from the same ash layer; some layer numbers are
indicated. The volatiles are calculated as the loss on ignition
corrected for oxygen uptake during ignition. For a few samples
where FeO could not be determined due to the presence of pyrite,
the oxidation ratio was assumed to be that of the average positive
series basalt.
4.1. Alteration effects
The analysed ashes have been subjected to seafloor weathering and other alteration, and many have
high volatile contents (Fig. 2). In general, the negative
ash series is extensively altered, with volatile contents
up to 6 – 14 wt.%. The positive series is much less
altered, with volatile contents of 1 – 4 wt.%, and the
carbonate-cemented samples are the least altered, with
volatile contents around 1 wt.%.
The kind and magnitude of alteration may be
assessed by comparison of the bulk ash analyses with
microprobe analyses of fresh glass from the same ash
layer (Pedersen et al, 1975) (Fig. 3). In the positive
series, data for five basaltic layers can be compared.
The bulk ash analyses show low volatile contents, only
1.2– 2.1 wt.%, and TiO2, K2O, and P2O5 are virtually
identical in bulk and glass analyses. The bulk ash
analyses deviate systematically from the corresponding glass analyses by having higher SiO2 (0.2 –
2%) and Al2O3 (0.6 –0.7%), and lower FeO* (0.4 –
1.3%), MgO (0.2 – 0.5%), CaO (0.7 –1.3%), and Na2O
(0.1 – 0.4%) (wt.% calculated volatile-free). The two
carbonate-cemented bulk ash samples ( + 28 and
+ 102) show the least differences to the glasses. Nota-
52
Table 1
Chemical composition of bulk samples of the Danish Palaeogene ash layers
Stolle
2000
HM1
ca. 2
Stolle
2000
HM2
ca. 2
Stolle
2000
HM3
0.7
Knude
1997
34
1.5
SiO2
TiO2
Al2O3
Fe2O3
FeO
MnO
MgO
CaO
Na2O
K2O
P2O5
Volat.
Total
50.27
2.22
16.18
11.27
n.a.
0.17
4.31
9.56
2.68
0.44
0.14
2.18
99.41
48.86
2.09
15.74
12.64
n.a.
0.19
4.55
10.15
2.68
0.40
0.16
2.05
99.51
48.77
2.04
15.73
13.26
n.a.
0.19
4.98
9.77
2.84
0.45
0.18
0.24
98.45
46.75
2.67
17.59
13.35
n.a.
0.11
1.64
4.32
2.26
0.31
0.12
10.38
99.50
12
119
22
312
5.3
6
18
14
186
303
130
260
301
263
36
20
70
14
263
3.3
11
28
13
86
281
64
226
170
129
17
Trace elements (ppm)
Nb
12
12
Zr
130
128
Y
20
22
Sr
311
298
Rb
6.3
5.7
La
5
6
Ce
15
18
Nd
12
13
Zn
155
187
Cu
202
209
Ni
98
108
Cr
212
157
V
320
306
Ba
375
390
Sc
37
37
Knude Stolle Stolle
1997
1980
1980
33
29a
28
14
0.5
2.6
70.56
0.18
13.00
0.74
1.00
0.00
0.24
0.36
3.75
3.26
0.47
5.85
99.42
28
55
0
25
155
0
18
6
155
19
6
23
12
261
0
58.04
1.42
12.54
2.31
1.64
0.07
0.88
6.05
2.05
0.54
0.21
10.16
95.92
15
102
16
223
15
16
58
36
84
102
56
55
121
266
6
53.11
2.63
16.54
4.11
2.48
0.06
2.10
7.12
2.43
0.43
0.17
6.33
97.51
20
126
19
270
8
38
79
32
66
78
27
204
248
239
28
Stolle
1980
27
0.3
61.57
1.73
13.20
5.01
0.91
0.03
1.63
1.62
1.50
1.20
0.13
9.72
98.24
15
145
23
87
45
58
108
36
59
51
15
202
418
311
50
Stolle
1980
26
4.3
57.97
3.20
18.58
2.80
1.85
0.05
2.58
4.91
2.07
0.30
0.11
5.15
99.55
17
153
16
227
5.4
10
38
19
67
59
28
370
454
283
54
Stolle
1980
24
0.9
59.79
3.18
16.67
2.61
2.14
0.05
2.80
4.26
1.65
0.31
0.11
5.51
99.08
18
152
14
203
9.2
12
37
19
54
61
31
424
428
276
49
Stolle
1980
22
0.4
57.08
1.85
15.05
1.26
4.94
0.10
3.05
7.57
2.37
0.47
0.14
3.49
97.36
14
109
18
242
15
13
36
21
79
104
47
174
214
195
20
Knude Knude Knude Stolle
1980
1980
1980
1980
21b
21a
21
20
1
4
1
0.5 – 1
63.87
0.77
11.71
6.45
0.95
0.04
1.08
1.18
1.69
2.27
0.10
8.61
98.71
17
261
31
102
71
50
96
43
95
36
20
70
168
418
17
52.09
5.21
13.46
3.58
8.11
0.15
3.13
6.52
2.25
0.91
0.54
3.56
99.52
53
372
49
410
25
40
106
60
143
199
37
60
406
262
26
69.99
0.74
12.18
2.90
1.29
0.04
0.93
0.67
1.59
2.07
0.09
6.53
99.01
14
247
22
66
65
34
79
34
76
55
14
57
121
561
20
62.35
3.93
15.49
3.61
0.49
0.02
1.19
0.24
0.36
0.90
0.22
9.60
98.40
53
240
19
43
39
30
74
34
85
63
22
127
288
146
27
Stolle
1980
19c
0.3
67.39
2.45
9.47
4.09
0.99
0.02
0.99
0.22
0.43
1.16
0.20
10.54
97.95
27
207
11
49
56
26
57
23
82
79
26
114
191
240
22
Knude
1997
19b
1
64.22
8.14
7.79
2.94
0.83
0.07
1.23
1.50
0.73
1.80
0.49
7.36
97.11
450
1377
95
270
59
182
408
190
86
68
43
75
279
2137
4
Knude
1980
19a
1.5
56.21
6.89
8.36
15.15
0.66
0.03
0.66
0.47
0.52
0.90
0.38
8.78
99.00
108
385
34
106
34
43
111
56
65
90
35
172
219
144
21
L.M. Larsen et al. / Lithos 71 (2003) 47–80
Locality
Year
Layer
cm
Locality Knude Skarreh Skarreh Knude
Year
1997
2000
2000
1997
Layer
19
18a
18
17
cm
3
0.5
0.5
4
53.14
2.90
12.01
15.83
n.a.
0.11
0.82
2.69
2.80
1.76
0.76
6.58
99.39
68.27
0.31
13.13
1.24
1.90
0.11
0.07
0.53
3.08
3.27
0.03
6.52
98.45
Trace elements (ppm)
Nb
96
79
Zr
580
978
Y
50
91
Sr
316
4
Rb
44
97
La
49
87
Ce
115
187
Nd
57
84
Zn
131
150
Cu
53
16
Ni
8
1
Cr
28
4
V
109
3
Ba
676
71
Sc
4
12
69.35
0.28
11.43
4.87
0.85
0.03
0.42
0.25
2.16
2.99
0.07
6.38
99.08
71.57
1.49
8.97
5.32
0.22
0.03
0.25
0.27
0.71
1.05
0.20
7.81
97.90
41
448
51
12
86
50
113
48
152
32
5
33
105
44
14
582
1913
27
438
25
71
164
55
38
24
5
26
12
901
0
64.07
2.65
3.62
16.24
0.27
0.02
0.36
0.47
0.30
0.19
0.37
9.02
97.58
7.4
151
0
10
5.2
2
13
4
26
64
8
81
509
62
9
51.81
3.85
12.28
6.49
4.61
0.13
4.03
8.31
1.99
0.90
0.38
4.70
99.47
54
298
22
458
20
27
76
42
109
136
57
97
339
460
32
52.92
4.13
12.15
6.55
4.41
0.12
3.11
6.93
2.18
1.01
0.47
5.38
99.36
61
336
25
531
21
34
82
47
122
81
40
61
315
429
22
54.09
4.17
11.78
4.39
5.28
0.15
3.83
7.68
2.42
1.29
0.47
4.29
99.84
68
370
27
506
25
45
105
56
121
108
51
129
315
433
25
52.07
3.26
13.27
3.58
8.04
0.21
4.59
9.39
2.50
0.63
0.39
1.77
99.68
30
237
37
373
14
17
48
32
176
214
60
101
377
297
38
Knude Knude Knude Knude Knude Østklint
1997
1997
1997
1997
2000
2001
+1
+3
+9
+ 12
+ 13
+ 14 bottom
5
5
7
4
1
0.5
Østklint
2001
+ 14 top
3.5
50.65 50.43
3.52
3.47
13.00 12.91
4.38
5.43
9.00
7.82
0.19
0.17
4.58
4.38
8.95
8.46
2.37
2.17
0.68
0.57
0.36
0.36
2.21
2.98
99.89 99.14
50.62
3.33
13.44
2.66
9.54
0.20
5.16
9.92
2.49
0.59
0.33
1.28
99.56
26
259
40
292
11
17
49
32
139
213
52
82
405
205
41
26
253
36
260
12
15
50
30
126
218
58
80
444
172
42
50.91
3.53
12.79
4.84
8.53
0.18
4.58
8.46
2.33
0.62
0.36
2.47
99.61
28
259
37
268
13
15
45
30
134
196
59
82
447
163
45
51.20
3.59
13.01
4.81
7.69
0.18
4.29
8.64
2.41
0.66
0.38
2.54
99.39
29
269
37
299
13
19
52
32
161
204
54
83
415
231
36
64.99
0.41
14.25
1.84
2.23
0.11
0.13
1.33
3.66
3.50
0.07
7.17
99.70
22
395
61
36
106
73
154
74
137
23
1
6
3
191
9
52.34
3.41
13.34
2.93
8.27
0.17
4.21
8.98
2.51
0.66
0.33
2.28
99.44
30
281
35
334
12
16
48
30
177
185
54
103
370
290
36
27
255
38
319
12
15
49
31
159
194
72
153
403
260
39
Østklint
2001
+ 14a
1
53.57
2.81
16.63
4.39
4.29
0.12
2.58
8.57
3.19
0.68
0.34
2.45
99.61
24
208
24
438
12
12
37
20
183
181
34
74
277
429
26
Østklint
2001
+ 14b
0.2
47.58
3.00
12.79
16.15
n.a.
0.29
5.18
9.83
2.31
0.58
0.40
0.46
98.56
21
226
44
328
14
15
43
28
162
291
72
119
365
728
40
L.M. Larsen et al. / Lithos 71 (2003) 47–80
SiO2
TiO2
Al2O3
Fe2O3
FeO
MnO
MgO
CaO
Na2O
K2 O
P2O5
Volat.
Total
Knude Knude Knude Knude Østklint
1997
1997
1997
1997
2001
15
13
12
11
10
1.5
8
5
4
2
(continued on next page)
53
54
Table 1 (continued)
Locality
Year
Layer
cm
50.87
3.90
12.95
3.37
9.29
0.21
4.48
9.32
2.50
0.72
0.42
1.60
99.63
50.16
3.42
13.22
3.14
9.26
0.20
5.21
9.92
2.41
0.57
0.35
1.39
99.24
Trace elements (ppm)
Nb
31
25
Zr
310
255
Y
50
42
Sr
327
300
Rb
15
10
La
21
17
Ce
60
52
Nd
38
33
Zn
181
147
Cu
240
198
Ni
51
67
Cr
97
128
V
367
403
Ba
216
170
Sc
36
38
Knude
1997
+ 19
19
Knude
1997
+ 20
1
65.23
0.83
13.33
1.77
2.59
0.18
0.52
1.02
4.39
3.35
0.13
6.19
99.53
51.70
3.47
12.97
6.47
5.68
0.17
4.57
9.13
2.47
0.66
0.34
1.87
99.50
127
669
44
178
85
89
198
87
162
10
5
16
11
663
0
28
244
35
306
12
19
47
30
155
195
59
126
391
305
40
Knude Knude Thy
1997
1997
2000
+ 22
+ 23
+ 27
8
2
1
51.08 50.95
3.52
2.94
13.02 13.69
3.72
3.41
9.77
9.23
0.19
0.19
4.21
4.82
8.40
9.60
2.44
2.48
0.76
0.62
0.43
0.34
2.08
1.74
99.61 100.00
30
297
46
287
14
26
64
43
144
252
46
60
363
236
35
23
235
37
292
12
20
49
33
131
195
49
50
359
223
37
48.48
3.45
13.09
2.53
11.54
0.21
5.18
9.88
2.54
0.61
0.38
1.11
99.02
27
262
42
312
11
20
53
33
139
259
61
67
413
203
33
Thy
2000
+ 28
0.8
48.25
3.45
13.04
1.97
12.27
0.21
5.17
9.93
2.57
0.58
0.37
1.19
99.00
27
265
43
302
11
17
51
34
134
263
57
70
399
151
35
Knude
1997
+ 30
2.5
50.45
3.71
12.57
4.81
9.05
0.19
4.55
8.66
2.34
0.62
0.40
2.43
99.77
28
273
40
290
11
20
51
33
153
207
54
74
432
271
41
Knude Knude Knude Knude Knude Knude
1997
1997
1997
1997
1998
1997
+ 35
+ 45
+ 46
+ 51
+ 54
+ 55
6
3
1
9
3.5
3
49.31
3.49
12.41
6.60
8.71
0.31
4.44
8.45
2.35
0.62
0.43
2.28
99.39
27
253
40
281
11
18
53
34
137
234
49
52
467
192
37
49.81
3.61
13.16
4.49
8.93
0.19
4.26
8.92
2.54
0.76
0.43
2.21
99.32
31
283
43
336
13
25
59
36
157
219
43
63
414
319
41
50.72
4.02
12.53
4.07
9.80
0.19
3.91
8.21
2.56
0.83
0.43
2.32
99.58
34
297
44
318
14
22
57
37
170
217
39
62
452
325
32
49.97
3.31
12.93
3.60
10.33
0.21
4.77
9.17
2.58
0.69
0.39
1.73
99.66
26
264
48
291
12
21
56
37
146
233
47
65
407
210
43
49.52
2.91
13.49
4.11
9.04
0.20
5.12
9.80
2.52
0.67
0.29
1.82
99.48
21
217
36
299
10
17
44
28
149
229
59
76
391
190
40
48.96
2.84
13.16
6.94
7.75
0.24
4.29
8.49
2.62
0.66
0.38
2.88
99.20
20
209
37
279
12
16
44
29
134
207
44
46
414
212
40
Knude
1998
+ 60
6
50.68
3.37
13.43
4.48
8.48
0.19
4.67
8.41
2.45
0.72
0.41
2.51
99.79
26
262
43
283
12
23
55
37
146
234
59
72
412
201
48
Knude
1998
+ 62
13
49.71
3.39
13.56
3.22
9.71
0.20
4.92
9.51
2.56
0.66
0.33
1.84
99.60
26
244
40
321
10
19
48
32
169
239
57
77
410
219
37
Knude
1998
+ 63
3.5
49.89
3.61
13.30
5.08
8.77
0.19
4.33
8.66
2.58
0.73
0.39
2.48
99.99
28
256
38
305
11
17
50
32
166
257
51
68
428
254
40
L.M. Larsen et al. / Lithos 71 (2003) 47–80
SiO2
TiO2
Al2O3
Fe2O3
FeO
MnO
MgO
CaO
Na2O
K2O
P2O5
Volat.
Total
Knude Knude
1997
1997
+ 16
+ 18
3
3.5
Knude Knude
1998
1998
+ 72
+ 75
4
2.5
Knude
1998
+ 79
13
Knude
1998
+ 80
6
SiO2
TiO2
Al2O3
Fe2O3
FeO
MnO
MgO
CaO
Na2O
K2O
P2O5
Vol
Total
50.20 50.83
3.25
3.37
13.48 13.47
4.67
3.63
9.00
8.87
0.19
0.18
4.27
4.21
8.69
8.84
2.70
2.73
0.66
0.74
0.37
0.36
2.16
2.11
99.63 99.33
49.38
3.24
13.42
5.03
9.04
0.21
4.69
8.59
2.48
0.65
0.41
2.58
99.71
50.15
3.39
13.24
3.90
10.41
0.21
4.49
8.78
2.58
0.70
0.37
1.75
99.96
24
252
46
264
11
23
61
40
150
273
58
72
412
173
50
25
263
45
278
12
21
56
37
154
272
53
46
419
179
37
Trace elements (ppm)
Nb
26
28
Zr
249
265
Y
38
40
Sr
288
305
Rb
12
13
La
16
18
Ce
43
52
Nd
31
33
Zn
148
177
Cu
224
233
Ni
48
48
Cr
59
70
V
419
407
Ba
191
271
Sc
38
36
Knude Knude Knude Knude Knude
1998
1997
1998
1998
1998
+ 83
+ 90
+ 92
+ 94
+ 99
4
8
4.5
3
2
49.85
3.43
13.18
4.44
9.04
0.20
4.77
9.07
2.51
0.72
0.34
2.17
99.72
25
254
39
301
10
20
47
31
153
256
57
92
414
273
47
52.31
3.81
11.71
4.56
8.09
0.18
4.20
7.83
2.08
0.62
0.41
3.71
99.50
28
264
37
295
12
15
48
31
131
248
55
104
399
264
41
49.36
4.51
13.06
3.59
9.40
0.20
4.58
8.90
2.54
0.89
0.49
1.97
99.47
37
352
50
339
16
25
63
41
181
317
72
155
412
363
40
49.49
3.79
13.22
2.95
10.81
0.21
4.35
8.83
2.63
0.88
0.48
1.70
99.34
33
333
51
345
16
27
67
43
175
257
46
53
408
358
36
50.02
3.21
13.79
3.72
8.50
0.19
4.91
9.53
2.64
0.72
0.30
1.95
99.46
23
236
36
317
11
16
39
27
156
205
62
129
390
317
37
Knude Knude Knude Knude Knude Knude
1998
1998
1998
1998
1998
1997
+ 101 + 102 + 105 + 110 + 114 + 118
14
7
4.5
4.5
13
16
Knude
1998
+ 130
6
48.72
3.80
13.16
14.72
n.a.
0.22
5.00
9.76
2.56
0.69
0.38
0.36
99.36
48.37
3.49
13.24
15.71
n.a.
0.23
5.04
9.94
2.60
0.57
0.35
0.00
99.13
47.61
3.69
12.44
16.79
n.a.
0.26
4.67
9.15
2.56
0.67
0.38
0.16
98.05
33
281
42
342
14
21
53
34
182
303
68
74
437
434
39
25
255
44
293
12
17
49
31
152
296
62
69
417
298
36
49.68
3.56
13.39
5.72
7.42
0.18
4.23
8.18
2.52
0.86
0.33
3.33
99.39
27
259
35
317
11
21
47
31
181
239
65
137
402
435
40
50.96 50.36
3.41
3.66
13.37 12.91
3.23
4.37
9.89 10.40
0.20
0.21
4.35
4.19
8.72
8.23
2.68
2.62
0.67
0.75
0.32
0.40
1.84
2.12
99.63 100.22
25
258
41
271
13
16
46
30
159
242
49
61
426
201
40
28
297
53
264
15
21
56
41
174
260
54
51
436
207
43
50.63
3.74
13.03
5.03
8.97
0.30
3.64
7.65
2.40
0.81
0.49
2.83
99.51
32
328
66
282
18
36
94
61
186
227
89
40
413
352
41
29
277
44
327
15
19
48
30
187
271
66
73
419
328
35
Greif
2000
Greif1
>6.5a
Greif
2000
Greif2
>10a
49.01 49.11
3.34 3.34
12.71 12.68
2.76 2.17
11.79 12.48
0.21 0.21
4.96 5.02
9.37 9.32
2.40 2.35
0.58
0.60
0.37
0.37
1.21
1.40
98.71 99.06
26
269
45
290
10
20
50
33
151
258
63
70
420
159
38
26
267
44
308
10
16
52
34
147
253
55
57
420
163
43
L.M. Larsen et al. / Lithos 71 (2003) 47–80
Locality
Year
Layer
cm
Analyses by XRF except Na2O (by AAS) and FeO (by titration). Volat.: loss on ignition corrected for oxygen uptake during ignition. n.a.: not analysed. Locality: Stolle: Stolleklint,
Fur; Knude: Knudeklint, Fur; Østklint, also Fur; Skarreh: Skarrehage Mo clay pit, Mors; Greif, Greifswalder Oie, N. Germany. Year: Year of collection. cm: ash layer thickness in cm.
a
The ashes in the Greifswalder Oie samples are redeposited.
55
56
L.M. Larsen et al. / Lithos 71 (2003) 47–80
Fig. 3. Comparison of bulk ash analyses with microprobe analyses of fresh glass (from Pedersen et al., 1975; L. Heister, unpublished data (layer
17, inclusion in sanidine); and this work). All data recalculated to 100% volatile-free. Lines connect bulk ash and glass analyses from the
same layer; layer numbers are indicated where possible. The altered basaltic layers 22 to 34 may be compared to the bulk analyses of the
carbonate-cemented samples HM1, HM2, and HM3 from the same interval and to the microprobed glass from HM2 and layer
35. For
comparison, data is also shown for layer *44 in the Balder Formation: a bulk ash analysis with 1.4% MgO and a microprobe analysis of a glass
inclusion in plagioclase with 5.4% MgO (Morton and Knox, 1990).
bly, the relative differences between individual layers
are preserved. The bulk – glass differences are
smaller than the total compositional range within
the basaltic positive series, and we conclude that the
major element bulk ash analyses from the basaltic
positive series approximate the original igneous
compositions reasonably well, although they may
show some gain of SiO2 and Al2O3 and loss of
iron, CaO, and MgO.
In the negative series, microprobe analyses of fresh
basaltic glasses in carbonate-cemented layers are
available for layer
35 which has not been bulk
analysed (Pedersen et al., 1975) and for the loose slab
HM2 from the interval 22 to 35. The glasses and
the three bulk samples HM1 to HM3 all have apparently fresh basaltic compositions and their volatile
contents are low, ca. 2 wt.%. In contrast, the uncemented ash layers from this interval have volatile
contents of 4 – 10 wt.% and are quite variable in
composition as shown in Fig. 3. Compared to the
glass and the cemented samples, the uncemented
samples of the negative series basalts have considerably higher SiO2, high and variable Al2O3, and
distinctly lower FeO*, MgO, and CaO. The elements
TiO2, Na2O, K2O, and P2O5 are at similar levels but
show significant scatter. The basalts of the negative
L.M. Larsen et al. / Lithos 71 (2003) 47–80
ash series are thus severely altered, and of the major
elements, only TiO2 and P2O5, and probably K2O,
resemble the original values.
Fisher and Schmincke (1984, p. 327) noted that
silicic glasses are more resistant to alteration than
basaltic. Accordingly, the two rhyolitic layers
33
and + 19 show only small differences between bulk
and glass analyses (Fig. 3). Layer 33 is glass-rich
and the glass is fresh (Pedersen et al., 1975); for this
layer, the bulk ash faithfully represents the igneous
composition. Layer + 19 contains fresh glass which
is peralkaline (Pedersen et al., 1975); the bulk sample
is not peralkaline and in particular Na2O is lower in
the bulk (4.7 wt.%) than in the glass (6.9 wt.%
volatile-free). Though the bulk seems to have lost
alkalies, it also has slightly higher TiO2, MgO, and
CaO than the glass and may represent less fractionated material. In all, the + 19 bulk sample is not
heavily altered.
In the undersaturated phonolitic to trachytic layer
17, the glass is extensively leached and hydrated
(Pedersen et al., 1975), and fresh glass is only found
in melt inclusions in sanidine (L. Heister, personal
communication, 2003). Compared to both leached and
fresh glass (Fig. 3), the bulk has higher SiO2 (16 – 22
wt.% up) and lower Al2O3 (5 –9 wt.% down) and
alkalies (3 –11 wt.% down); the only reliable major
element appears to be TiO2. The interval
21b to
15 is the one comprising the largest range in
compositions, and unfortunately, the compositions of
the ashes in this interval must be treated with caution.
In particular, the total alkali –silica (TAS) classification diagram is of little use because undersaturated
alkaline magmas may classify as dacites and rhyolites
due to silica gain and alkali loss. Trace element
concentrations and mineralogy are better suited to
distinguishing such magmas.
Of the trace elements, those usually considered
least mobile are Nb, Zr, and Y, whereas Ba, Rb, and
Sr are known to be mobile (e.g. Larsen et al., 1998).
In the negative series ashes, there are clear examples
of both loss and gain of trace elements, particularly Y
and La, Ce, and Nd. As will be shown in later figures,
the best-preserved elements are Nb and Zr; however,
many other elements show significant and probably
original differences between groups of ash layers.
Although some secondary scatter of the data must
be accepted, with the effect of blurring of the original
57
igneous trends, several trace elements can, with caution, be used to characterise the original magmatic
compositions.
The samples from the upper part of the succession
at Hanklit show some features that prompted us to resample that part of the succession at Knudeklint. This
confirmed that all the ash samples from Hanklit have
gained significant amounts of yttrium, a feature that is
also known from the coeval ashes in bore holes in the
North Sea (Morton and Knox, 1990) and from the
broadly coeval weathered basalts from the top part of
the dipping reflector sequence on the SE Greenland
margin (Larsen et al., 1998). The same samples have
also gained P2O5, particularly layers + 101 and + 102
(>1% P2O5), together with significant amounts of La,
Ce, Nd, Sr, and Zn (see the online version of this
paper.2). This is probably the result of phosphoritisation processes which have re-distributed organic phosphorus and other elements in the sediment. Layers
20, + 118, and + 129 in Knudeklint also show
indication of such processes. In all samples, Zr and
Nb appear to be unaffected. The Hanklit samples are
not plotted in the geochemical diagrams except for Zr/
Nb in some layers in the interval + 106 to + 116 in
Fig. 6.
4.2. Major and trace elements
Major and trace element results for the analysed
ashes are shown in Figs. 4 and 5. The data confirm
that the chemical variation within the positive series is
much more limited than within the negative series.
Apart from two rhyolitic layers ( + 13 and + 19), the
positive series consists of basalts which form a densely clustered, well-defined trend with only moderate
chemical variations, TiO2 = 2.8– 4.5 wt.%, and Zr =
209– 352 ppm. In contrast, the negative series encompasses an extreme compositional range with TiO2 =
0.2 – 8.1 wt.% and Zr = 55 – 1913 ppm. Although
many elements are scattered by alteration, other elements such as TiO2, Nb, and Zr are not, and these
clearly indicate that a number of strongly different
magma types are present in the negative series.
2
paper.
See Supplementary data Table in the online version of this
58
L.M. Larsen et al. / Lithos 71 (2003) 47–80
Fig. 4. Major element composition of the Danish ash layers. All data recalculated to 100% volatile-free. Two samples from Greifswalder Oie are
indistinguishable from the positive series and are included in this group. Samples from the same ash layer are connected by a thin line. Some
layer numbers are indicated.
L.M. Larsen et al. / Lithos 71 (2003) 47–80
59
Fig. 5. Trace element composition of the Danish ash layers. All data recalculated to 100% volatile-free. Two samples from Greifswalder Oie are
indistinguishable from the positive series and are included in this group. Samples from the same ash layer are connected by a thin line. Some
layer numbers are indicated. Note the logarithmic scale in most diagrams.
60
L.M. Larsen et al. / Lithos 71 (2003) 47–80
Of the trace elements, Zr and Nb appear to be the
best preserved, and Fig. 6 shows a stratigraphic
variation profile for the Zr/Nb ratio. The very large
overall variation within the negative series (Zr/
Nb = 2 –20) is an original igneous feature and reflects
magma types ranging from peraluminous to peralkaline, and from basaltic to trachytic, rhyolitic, nephelinitic, and phonolitic, as also earlier identified from
glass chemistry and mineralogy by Pedersen et al.
(1975) and Rønsbo et al. (1977). The low Zr/Nb ratios
(2– 6) and high contents of Zr and Nb in layers 11,
12,
13,
17,
19,
19a,
19b, and
20
suggest that these layers are distinctly alkaline; several
other layers may be alkaline too. The low Zr content
Fig. 7. Zr/Nb vs. TiO2 for the Danish ash layers. Only basic rocks
that appear uncontaminated and have not fractionated Fe – Ti oxides
are plotted.
Fig. 6. Zr/Nb variations within the Danish ash layer succession.
Note that the y-axis is layer numbered which is independent of the
actual sedimentary thicknesses. Only one data point is shown for
each analysed layer; however, samples from the same layer have
closely similar Zr/Nb. Symbols for the negative series as in Figs. 4
and 5.
in layer
33 suggests zircon fractionation as the
cause of its low Zr/Nb ratio.
In the positive series, the two rhyolitic layers + 13
and + 19 stand out clearly in Fig. 6. The peralkaline
layer + 19 has a low Zr/Nb ratio of 5.3, in contrast to
layer + 13 with Zr/Nb = 17.8. The basalts of the
positive series show an apparently regularly oscillating evolution in Zr/Nb with height. The Zr/Nb ratios
in these basalts show an inverse correlation with TiO2
(Fig. 7, encircled), suggesting that these variations
reflect differences in the primary magma compositions of these basalts. A similar inverse correlation,
but at lower Zr/Nb, is apparent for uncontaminated
basic layers in the negative series (Fig. 7).
Rare-earth element (REE) analyses are given in
Table 2, and patterns of the different magma types are
shown in Fig. 8. The basalts of the positive series have
closely parallel patterns; in contrast, the basalts of the
negative series comprise four different types, with the
cemented HM samples having the lowest REE contents and relatively flat patterns, the uncemented
layers
26 and
22 having somewhat steeper
patterns, those from the interval 13 to 11 having
the steepest patterns, and layer
21a having the
highest REE contents, particularly the heavy REE.
The evolved layers from the positive series are both
indisputable rhyolites but have quite different REE
L.M. Larsen et al. / Lithos 71 (2003) 47–80
61
Table 2
Rare earth analyses of bulk samples of the Danish Palaeogene ash layers
Locality
Year
Layer
Stolle
2000
HM1
Stolle
2000
HM2
Stolle
2000
HM3
Knude
1997
33
Stolle
1980
26
Stolle
1980
22
Knude
1980
21a
Knude
1980
21
Knude
1980
19b
Skarreh
2000
18a
Knude
1997
13
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
6.9
17.5
2.4
11.1
3.2
1.23
3.6
0.57
3.41
0.69
1.87
0.27
1.69
0.24
7.6
19.1
2.6
12.4
3.5
1.37
4.1
0.64
3.81
0.77
2.05
0.29
1.80
0.26
8.0
20.2
2.8
13.3
3.7
1.39
4.0
0.65
3.79
0.78
2.12
0.29
1.80
0.27
3.1
9.1
1.2
4.5
2.2
0.42
1.8
0.21
0.58
0.07
0.18
0.02
0.14
0.02
13.1
35.9
4.7
20.2
4.8
1.67
4.6
0.69
3.93
0.76
2.12
0.29
1.80
0.27
15.4
37.4
5.1
21.5
4.7
1.58
4.5
0.66
3.83
0.75
2.13
0.29
1.81
0.27
43.0
104.7
14.2
60.2
13.7
4.06
13.3
2.01
10.71
2.00
5.34
0.68
4.12
0.58
34.8
74.0
8.8
33.3
6.5
1.41
6.2
0.87
4.91
0.98
2.92
0.40
2.63
0.41
83.8
185.7
23.9
91.1
17.5
4.52
15.2
1.96
9.45
1.68
4.69
0.62
3.86
0.55
83.6
159.9
19.9
74.9
15.6
0.60
15.6
2.56
15.18
3.22
9.66
1.43
9.33
1.43
30.0
70.9
9.5
39.3
8.2
2.62
7.4
1.00
5.05
0.91
2.42
0.30
1.82
0.26
Locality
Year
Layer
Knude
1997
12
Knude
1997
+1
Knude
2000
+ 13
Knude
1998
+ 18
Knude
1997
+ 19
Thy
2000
+ 28
Knude
1997
+ 55
Knude
1998
+ 79
Knude
1998
+ 92
Knude
1998
+ 102
Knude
1998
+ 130
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
35.6
78.4
11.1
46.4
9.4
2.95
8.5
1.16
5.81
1.03
2.71
0.33
2.01
0.28
20.3
49.8
7.1
32.1
8.1
2.56
8.1
1.29
7.33
1.43
3.92
0.53
3.24
0.47
68.1
141.2
17.0
63.8
13.1
0.85
11.8
1.79
10.21
2.06
5.84
0.83
5.41
0.82
19.9
49.4
7.3
33.4
8.5
2.75
8.8
1.38
7.90
1.55
4.19
0.57
3.48
0.50
76.5
156.7
20.7
77.0
13.6
3.24
12.3
1.68
8.35
1.53
4.21
0.54
3.34
0.49
20.9
52.3
7.3
32.8
8.5
2.59
8.4
1.32
7.54
1.45
3.86
0.51
3.25
0.48
18.6
43.4
6.3
28.1
7.0
2.33
7.3
1.16
6.71
1.36
3.77
0.51
3.30
0.48
24.5
64.3
8.9
40.8
10.2
3.10
10.4
1.65
9.49
1.89
5.21
0.71
4.43
0.65
30.1
71.0
10.2
46.1
11.3
3.46
11.5
1.80
10.05
1.97
5.38
0.73
4.46
0.65
20.7
51.6
7.5
35.0
8.9
2.87
9.3
1.48
8.53
1.69
4.68
0.63
3.83
0.56
20.8
51.6
7.4
34.0
8.8
2.77
9.5
1.49
8.40
1.66
4.61
0.62
3.89
0.57
Abbreviations as in Table 1.
patterns. The evolved layers from the negative series
represent a possible trachyte ( 21), two peralkaline
phonolitic or nephelinitic layers ( 17 and
19b,
Pedersen et al., 1975; Rønsbo et al., 1977; Heister et
al., 2001), and two rhyolitic layers of which 18a is
probably alkaline whereas
33 is subalkaline and
strongly incompatible-element depleted.
4.3. Isotopes
Eleven samples were analysed for Sr and Nd
isotopic compositions (Table 3). As the ash particles
have been exposed to sea water and sea floor alteration, the analyses are of reconnaissance character and
the samples were not leached. Despite this caveat, the
positive series basalts form a well-correlated negative
trend in the Nd – Sr isotope diagram (Fig. 9a), and the
isotope results are correlated with Zr/Nb ratios (Fig.
9b and c). Layers
12 and
13 continue the
isotopic trend towards higher 87Sr/86Sr and lower
143
Nd/144Nd, with correspondingly low Zr/Nb. The
isotopic trend lies in continuation of the trend for
recent basalts from Iceland but does not overlap with
it; however, the enriched component of the Proto-
62
L.M. Larsen et al. / Lithos 71 (2003) 47–80
Fig. 8. Chondrite-normalised REE patterns for the Danish ash layers. Data for layer 17 from L. Heister (personal communication, 2003). Grey
lines in diagrams b – d are the upper and lower bounds for the basalts of the positive series, shown for comparison. Chondrite normalisation
factors from McDonough and Sun (1995).
Icelandic mantle plume suggested by Bernstein et al.
(2001) extends the Icelandic trend to overlap with the
positive series basalts. The alkaline layers 21a and
19b fall off the trend with relatively low 143Nd/
144
Nd, and the two rhyolitic layers + 19 and 33
have high 87Sr/86Sr ratios. There is a very good
overall correlation between 143Nd/144Nd and Zr/Nb
(Fig. 9c), suggesting that the Nd isotope ratios are
well preserved. An exception is the basaltic layer
22; as shown later, the uncemented basalts at this
level appear to have gained light and medium REE,
and the added Nd may have a lower 143Nd/144Nd
ratio.
5. Magma types and comparisons
In the following sections, the various magma
types in the Danish ash series are identified and
characterised, starting with the positive series where
both major and trace elements are relatively undisturbed, and proceeding down-section to more altered
layers where identification relies mostly on multielement diagrams. For each magma type, examples
of similar types are given for other parts of the North
Atlantic Igneous Province in order to investigate the
settings in which the types have been produced. We
will consider a much wider time interval than that of
L.M. Larsen et al. / Lithos 71 (2003) 47–80
63
Table 3
Sr and Nd isotope analyses of the Danish Palaeogene ash layers
Locality
Year
Knude
Skarreh
Knude
Knude
Knude
Knude
Knude
Knude
Thy
Knude
Knude
1997
1980
1980
1980
1997
1997
1998
1997
2000
1997
1998
Layer
33
22
21a
19b
13
12
+ 18
+ 19
+ 28
+ 55
+ 92
87
Sr/86Sr(m)
2SE
87
Sr/86Sr(54)
143
Nd/144Nd(m)
2SE
143
Nd/144Nd(54)
0.71991
0.70431
0.70348
0.70525
0.70476
0.70489
0.70378
0.70790
0.70376
0.70412
0.70426
2
1
1
1
1
1
2
1
1
1
1
0.70613
0.70417
0.70335
0.70461
0.70467
0.70480
0.70370
0.70684
0.70365
0.70402
0.70416
0.512488
0.512539
0.512733
0.512545
0.512727
0.512620
0.512918
0.512711
0.512910
0.512847
0.512824
21
18
8
9
18
9
9
11
11
11
13
0.512384
0.512492
0.512684
0.512504
0.512683
0.512576
0.512865
0.512674
0.512857
0.512793
0.512772
NBS 987 average 87Sr/86Sr = 0.710 244 F 7 during analysis (four determinations).
JM Nd standard average 143Nd/144Nd = 0.511115 F 13 during analysis (five determinations).
(m), as measured; (54) recalculated to 54 Ma.
Localities as in Fig. 1.
the ash-forming events, and thus the similarity of an
ash layer to an occurrence does not necessarily imply
a suggestion that the ash was sourced from that
occurrence.
6. Magma types in the positive ash series
6.1. Tholeiitic ferrobasalts
The basaltic ash layers in the positive series are
evolved, Fe – Ti-rich tholeiitic basalts. They are all
quartz normative, but as shown in Figs. 3 and 4,
the uncemented samples may have gained some
SiO2 and Al2O3 and lost Fe, MgO, and CaO.
However, the basalts evidently form one single
igneous suite, and this is true for both major and
trace elements (Figs. 4 and 5). The decrease in
CaO with decreasing MgO may be caused by both
alteration and fractionation (the rocks are plagioclase – clinopyroxene – phyric), and the concomitant
slight increase in incompatible elements indicates
some fractionation within the suite. With around
one third (50) of the positive ash layers analysed,
we consider it safe to conclude that the basaltic ash
layers in the positive series are comagmatic. Layer
+ 14a, with only 2.7 wt.% MgO, low TiO2, Zr, and
Nb, and high Al2O3, CaO, and Sr, is however
somewhat aberrant and may have an atypically high
plagioclase content.
Within this uniform basalt suite, some systematic
variations are evident. As shown in Fig. 6, the Zr/Nb
ratio oscillates regularly up-section between lows with
Zr/Nb of ca. 9 and highs with Zr/Nb of ca. 10.8. At
least four oscillations are seen below layer + 102;
above this level the pattern is indistinct. The low-Zr/
Nb layers (e.g. + 92 and + 101) are relatively
enriched not only in Nb but also in Ti, K, P, Sr, Zr,
and REE, whereas the high-Zr/Nb layers (e.g. + 18
and + 55) are relatively depleted in the same elements. The multi-element patterns (Fig. 10), as well as
the REE patterns (Fig. 8), are near-parallel, with lowZr/Nb layers in the upper part and high-Zr/Nb layers
in the lower part of the variation range. There is also a
good correlation between Zr/Nb and Sr and Nd
isotope ratios (Fig. 9b and c). The isotope results
suggest that differences in the degree of enrichment
are at least partly due to source heterogeneities and
not only to different degrees of melting. The regular
oscillations between highs and lows may be ascribed
to mixing in a replenished magma chamber.
The Fe –Ti-rich tholeiites are very similar to the
ferrobasalts in the Tertiary lava pile in Iceland (Wood,
1978; Flower et al., 1982). An unpublished large
analytical database (J.G.F. and B. S. Hardarson) contains several examples of Icelandic lavas almost
identical to the positive series basalts (including
SiO2) with very similar multi-element patterns, the
only exception being Ba which is significantly lower
in the Icelandic lavas (Ba = 110 – 176 ppm) than in the
64
L.M. Larsen et al. / Lithos 71 (2003) 47–80
Danish ashes (Ba = 150 – 944 ppm) (Fig. 10). Although the highest Ba values may be due to alteration,
the differences are most probably real.
The Tertiary flood basalt successions in East
Greenland and the Faroe Islands contain a few ferrobasalts which, however, are less enriched than the
Danish ashes (Larsen et al., 1989; Hald and Waagstein, 1984). The best likeness is found in lavas of the
youngest formation in East Greenland, the post-breakup Igtertivâ Formation (Fig. 10). Of the many dykes
cutting the lava plateaus, a few are ferrotholeiites
broadly similar to the Danish positive series ashes
(Fig. 10). Similar rocks are not known from the
British part of the province.
6.2. Rhyolites: layers +13 and +19
Fig. 9. (a) Nd – Sr isotopic composition of selected Danish ash
layers. (b) Zr/Nb vs. Sr isotopic compositions showing negative
correlation between the basalts. (c) Nd isotopic compositions vs. Zr/
Nb ratios showing overall positive correlation. Field of recent
Iceland after Stecher et al. (1998). Field of the enriched protoIcelandic mantle component after Bernstein et al. (2001).
The bulk major element composition of layer
+ 19 shows a sub-alkaline rhyolite. As noted above,
the bulk ash has lost alkalies, and microprobe
analysis of glass by Pedersen et al. (1975) clearly
indicates a peralkaline composition. This is confirmed by the high contents of incompatible trace
elements in layer + 19 (Figs. 5 and 10) and the low
Zr/Nb ratio of 5.3 (Fig. 6). Layer + 13 has a majorelement composition rather similar to layer + 19,
but judging from the smaller contents of incompatible elements and high Zr/Nb ratio of 17.8, it was
not peralkaline. The Ba – Nb trough in the trace
element pattern (Fig. 10) suggests that it may be
crustally contaminated. Deep troughs for Eu, Sr, P,
and Ti in the patterns for both rhyolites, deepest for
layer + 13, show that the magmas have fractionated
feldspar, apatite, and Fe – Ti oxide. Layer + 19
contains a variety of feldspar phenocrysts and
xenocrysts as well as ilmenite and lithic fragments
(Pedersen et al., 1975).
The central volcanic complexes in Iceland contain rhyolitic rocks with compositions and multielement patterns very similar to layer + 19, but none
have the Ba – Nb trough of layer + 13 (Fig. 10 and
unpublished data by J.G.F. and B. S. Hardarson).
Some acid rocks in Britain have similar patterns
with a Nb trough, e.g. dykes from St Kilda (Harding et al., 1984) and Lundy (Thorpe and Tindle,
1992). In East Greenland, rhyolites are found as
dykes in the Kangerlussuaq area (comendites,
Brooks and Rucklidge, 1976) and as lavas in the
L.M. Larsen et al. / Lithos 71 (2003) 47–80
65
Fig. 10. Multi-element patterns of ferrobasalts (a) and rhyolites (b) of the positive ash series with comparisons (c – d). Grey lines in (c) and (d)
are the upper and lower bounds of the positive basalt series. Primitive Mantle normalisation values from McDonough and Sun (1995). East
Iceland basalts (filled symbols) and rhyolites (open symbols): unpublished data, J.G.F. and B.S. Hardarson. Ferrobasalts from East Greenland:
83975, flood lava in the Skrænterne Formation, Blosseville Kyst (unpublished GEUS data); 116343, lava from the Igtertivâ Formation, Kap
Dalton, Blosseville Kyst (Larsen et al., 1989; Bernstein et al., 1998); 421549, dyke cutting the Skrænterne Formation, Gronau Nunatakker,
Blosseville Kyst (unpublished DLC data).
Myggbugta central complex, NE Greenland (Upton
et al., 1984); these rhyolites have no Nb troughs
(shown in Fig. 12).
7. Magma types in the negative ash series
The negative ash series presents the problem of
severe alteration combined with extreme compositions, and it is not always possible to decide on
original igneous compositions. The elements least
affected by alteration are Zr and Nb. However, in
general, the trace elements are much less affected by
alteration than the major elements, and in many cases,
multi-element plots show consistent patterns that
essentially reflect igneous processes.
During examination of analyses of several hundred
samples of lavas and dykes from East Greenland, it
was found that alkaline and tholeiitic rocks show
consistent differences in some element ratios. For
rocks that have not fractionated a Ti phase, tholeiitic
(hy normative) rocks have Zr/Nb>6 and (Ti/Y)N < 2.3.
Enriched tholeiites have Zr/Nb f 6 – 8 and (Ti/
Y)N f 2.3. Alkaline (ne normative) rocks have Zr/
Nb < 7 and (Ti/Y)N covering the tholeiitic interval plus
higher values up to 6. These ranges, though not
66
L.M. Larsen et al. / Lithos 71 (2003) 47–80
infallible, are useful when considering multi-element
diagrams of strongly altered ash layers.
As shown in Fig. 6, all except three ash layers in
the negative series have Zr/Nb ratios of 2– 12, suggesting many alkaline or enriched tholeiitic compositions. Layers 17 and 19b are known to contain
strongly alkaline mineral assemblages (Pedersen et al.,
1975; Rønsbo et al., 1977), and the whole interval
between layers
17 and
20 seems to comprise
mainly strongly alkaline layers with Zr/Nb ratios of
2– 6. In contrast, layers 15, 21, and 21b have
much higher Zr/Nb ratios of 16– 20.
The negative ash layers can be divided into a
number of groups, with the layers in each group
having similar or related multi-element patterns. The
groups are discussed below in mainly descending
stratigraphic order.
7.1. Alkali basalt: layers
Layer 10
11,
12, and
are much steeper, with (La/Lu)N of 12 –13. The multielement patterns are also steeper (Fig. 11a), and their
Ti– Y limbs are so steep ((Ti/Y)N f 4) that they are
only comparable with nepheline normative basalts in
East Greenland. The original melts for these ash layers
must have been alkali basalts. The trace element
features suggest that the parental melts were generated
at relatively deeper mantle levels than the parental
13.
Layers
11,
12, and
13 are in many sections the only black layers in the negative ash series
and are generally the least altered. They are 5– 8 cm
thick, situated within a 1-m interval, and are characteristic and easily identifiable (Bøggild, 1918). The
surrounding ash layers of the negative series ( 1 to
16) are thin, altered, greyish to brownish (Bøggild,
1918), and inconspicuous to absent in the sampled
sections.
In many major-element diagrams, layers
11,
12, and 13 appear to form a continuation of the
trend of the basalts of the positive series (Fig. 4);
however, they have distinctly higher SiO2 (55 – 57
wt.% volatile-free), making them basaltic andesites
(Le Maitre, 1989). This is possibly ascribable to an
alteration effect. The volatile contents of 4.3– 5.4 wt.%
are higher than in the positive series basalts (Fig. 2),
and similarly Ti-rich (ca. 4 wt.%) unaltered and
uncontaminated rocks with such high SiO2 contents
do not seem to exist in the North Atlantic igneous
province.
Whether or not these three layers are more differentiated than the basalts of the positive series, they are
distinctly more enriched in TiO2, Nb, Sr, and K, and
lower in Cu, Sc, Y, and the heavy REE. Their Zr/Nb
ratios of 5.5 are significantly lower than those of the
positive series basalts, and their REE patterns (Fig. 8)
Fig. 11. Multi-element patterns of alkali basalts of the negative ash
series (a) with comparisons (b). Grey lines in (a) and (b) are the
upper and lower bounds of the positive basalt series. Primitive
mantle normalisation values from McDonough and Sun (1995).
Comparative data: 98768, basanite dyke, Gåsefjord, Blosseville
Kyst, East Greenland (Larsen et al., 1989, and unpublished GEUS
data); 29928/4, alkali basalt dyke, Gardiner Complex, East Greenland (Nielsen, 1994 and unpublished data); SNB103, alkali basalt
lava, Snaefellsnes, Iceland (unpublished data, J.G.F. and B.S.
Hardarson); 429297, alkali basalt lava, Prinsen af Wales Bjerge
Formation, East Greenland (Peate et al., 2003); 437936, enriched
tholeiite dyke, Borggraven, Blosseville Kyst, East Greenland
(unpublished DLC data).
L.M. Larsen et al. / Lithos 71 (2003) 47–80
melts of the positive series, leaving a more distinct
signature of residual garnet in the low HREE, Y, and
Sc. The degree of melting may have been lower, but
as shown by the high Sr and low Nd isotope ratios
(Fig. 9), the mantle source was also more enriched
than for the positive series.
Compared to other rocks in the North Atlantic
region, layers 11 to 13 show similarities to alkali
basalts from both East Greenland and Iceland. They
are more enriched than enriched tholeiites and less
enriched than basanites (Fig. 11b).
Layer 10 is in most respects very similar to the
basaltic ash layers of the positive series, but its Zr/Nb
67
ratio of 7.9 is intermediate between the positive series
and layers 11 to 13 (Fig. 6).
7.2. Strongly alkaline to peralkaline rocks: layers
17 to 20
This interval contains several thin, inconspicuous
ash layers that were not observed by Bøggild (1918)
and therefore have later been assigned a, b, and c
suffixes (Gry, 1940). Most of these layers are only a
few millimetres thick. The high volatile contents of all
the layers (6 –13 wt.%) indicate that the major-element analyses should be treated with great caution.
Fig. 12. Multi-element patterns of evolved alkaline layers 17, 18 and 18a from the negative ash series (a and c) with comparisons (b and
d). 421564A, alkaline tuff, Gronau Nunatakker, Blosseville Kyst, East Greenland (Heister et al., 2001 and personal communication, 2003);
98770.1, alkaline trachyte dyke, Gåseland, East Greenland (Larsen et al., 1989, and unpublished data); 228262, rhyolite lava, Myggbugta
complex, NE Greenland (Upton et al., 1984); 29905, peralkaline trachyte dyke, Gardiner complex, East Greenland (Nielsen, 1994 and
unpublished data); 436076, peralkaline microsyenite dyke, Sortebræ, Blosseville Kyst, East Greenland (unpublished DLC data); LUN 69,
68
L.M. Larsen et al. / Lithos 71 (2003) 47–80
The multi-element patterns indicate the presence of a
number of types.
7.2.1. Evolved alkaline rocks: layers 17, 18, and
18a
The deep Ti troughs in the multi-element patterns
(Fig. 12) show that the magmas were evolved and had
fractionated Fe – Ti oxide. Feldspar fractionation is
indicated by deep troughs for Sr in layers 18 and
18a and for Eu in layer
18 (Fig. 8); it is less
evident for layer 17 with no Eu trough.
Layer
17 contains crystals of sanidine, Mg –
kataphorite, titan-aegirine, ilmenite, perovskite, and
sphene alongside lithic fragments and granulite-facies
orthopyroxenes, and is thus polymict (Pedersen et al.,
1975; Rønsbo et al., 1977). These authors judged the
erupted magma to be a peralkaline phonolite or trachyte that had picked up earlier solidified rocks in a
high-level volcanic complex of nephelinitic affinity.
The multi-element pattern (Fig. 12a) and the low Zr/Nb
of 3.3 suggest a peralkaline phonolite. The pattern has
unusually high peaks for Nb and Zr, and there may be a
mineralogical effect of accumulated crystals in leached
glass, but at least the pattern is consistent over large
areas because similar high peaks for Nb and Zr have
been found in ash layer Sele 70, which is correlated to
the Danish layer 17, in bore hole BGS 81/46a in the
North Sea (Morton and Knox, 1990). The only other
North Atlantic rocks with similarly high Nb and Zr
peaks are from East Greenland and comprise a few
dykes in the Gardiner complex, and a dyke and a tuff
layer in the flood basalts in northern Blosseville Kyst
(Fig. 12b). Heister et al. (2001) correlated the East
Greenland tuff layer directly with layer
17 and
suggested that both originated in the Gardiner complex. Our sample of layer 17 has lower La/Nb than
the Gardiner rocks and the tuff layer (Figs. 12 and 13),
and it has significantly lower contents of REE and Y
than L. Heister’s sample of layer 17 shown in Fig. 8.
Our sample has presumably lost REE and Y.
Layers 18 and 18a contain sanidine and Mgkataphorite (Pedersen et al., 1975). They contain less
Nb and Zr than layer
17 and have higher Zr/Nb
ratios of 10– 12. Both layers appear to be more fractionated than layer 17. The multi-element patterns
are different from the evolved dykes in the Gardiner
Complex which have no Sr trough (Fig. 12b). Most
probably, the magmas were alkaline rhyolites and the
Fig. 13. Multi-element patterns of Ti-rich alkaline layers 19a and
19b from the negative ash series, and layer
17 repeated for
comparison (a) with other comparisons (b). 429289, nephelinite
lava, and 429295, basanite lava, both Prinsen af Wales Bjerge
Formation, East Greenland (Peate et al., 2003); 29906, peralkaline
nephelinite dyke, and 29929/3, alkali basalt dyke, both from the
Gardiner complex, East Greenland (Nielsen, 1994 and unpublished
data); 200146, nephelinite from the Nunatak Zone, NE Greenland
(Brooks et al., 1979).
present compositions are close to the original. Rhyolitic lavas from the Myggbugta Complex, NE Greenland, have very similar trace element patterns. Rhyolite
dykes from Lundy, Britain, show remarkable similarities to layer 18, including relatively low Nb suggestive of some crustal contamination (Fig. 12d).
7.2.2. Ti-rich, strongly alkaline rocks: layers 19a
and 19b
The two samples of layer 19a and three samples
of layer 19b show a fair amount of internal varia-
L.M. Larsen et al. / Lithos 71 (2003) 47–80
tion, which may be due to crystal accumulation effects
because these layers are crystal-rich. Layer 19b is
polymict and contains plagioclase, anorthoclase, sanidine, diopside, titan-augite, titan-aegirine, kaersutite,
biotite, aenigmatite, sphene, perovskite, ilmenite, and
lithic fragments (Pedersen et al., 1975). The mineralogy of layer
19a was not investigated but is
expected to be similar.
Both layers are extremely rich in TiO2, 6.8 – 9.1
wt.% volatile-free (Fig. 4). They are also highly
enriched in all incompatible elements, but with a deep
Sr trough (Fig. 13a) that may be caused by clinopyroxene fractionation superimposed on a Sr trough in the
primary magma. The REE pattern of layer 19b (Fig.
8) has no Eu anomaly and is steep, with (La/Lu)N f 16,
suggesting melting in garnet-facies mantle. With high
contents of Ti and Nb and very low Zr/Nb ratios of 2.4–
4.7, the magmas must have been mafic – alkaline,
basanitic to nephelinitic. The magmas appear to be
related to the phonolitic layer
17 and could have
originated from the same volcanic complex.
In East Greenland, similar rocks form dykes in the
Gardiner complex, lavas of the Prinsen af Wales
Bjerge Formation, Kangerlussuaq, and lavas and
plugs in the Nunatak areas in NE Greenland (Fig.
13b). Similar rocks are not known from the European
side of the North Atlantic. The Vestbrona nephelinites,
offshore Kristiansund, Norway, do not have high Ti
contents (Fig. 14b).
7.2.3. Intermediate alkaline rocks: layers 19, 19c,
and 20
Bøggild (1918) described layer
19 as basaltic,
with a few mineral grains of labradorite, and layer
20 as intermediate between basaltic and andesitic,
with many mineral grains of plagioclase, quartz,
amphibole, augite, alkali feldspar, and muscovite.
Some of these minerals may be xenocrystic, probably
basement fragments.
Of the three analysed samples of layer 19, two
from Knudeklint are mutually similar while one from
Stolleklint deviates considerably, either because of
alteration or because of a mineralogical effect (Fig.
4: more Ti-oxide, less apatite). Of the two highly
altered samples of layer 20, the one from Knudeklint has considerably higher REE and Y than that
from Stolleklint, which could be due to either gain or
loss of elements. The Zr and Nb contents are still
69
Fig. 14. Multi-element patterns of intermediate alkaline layers 19,
19c, and 20 from the negative ash series (a) with comparisons
(b). 29928/1, alkali basalt dyke, Gardiner complex, East Greenland
(Nielsen, 1994 and unpublished data); 215613, K-rich tholeiitic
basalt dyke, Blosseville Kyst, East Greenland (Larsen et al., 1989
and unpublished data); 75 – 6/06, nephelinite from Vestbrona,
offshore Kristiansund, Norway (Prestvik et al., 1999).
consistent within the individual layers (Fig. 5), illustrating the robustness of these elements.
The common feature of the multi-element patterns
of these three ash layers is flat Rb-to-Ce segments with
no or only small K troughs; they have deep Sr troughs,
small P troughs, and no Ti troughs (Fig. 14). Layer
19c, with low Nb, may be somewhat contaminated.
Layer
19 has a less pronounced Sr trough and a
small Ti trough suggesting some Ti-oxide fractionation; it is more enriched than the other two layers, and
it has higher P than any other ash layer in the data set.
Judging by the low Zr/Nb ratios around 4 –6 (7.6
for layer 19c due to low Nb) and high (Ti/Y)N of
70
L.M. Larsen et al. / Lithos 71 (2003) 47–80
5– 6 in layers
19c and
20, the magmas were
alkaline. The deep Sr trough may be a feature of the
primary magma but is accentuated by fractionation of
feldspar or clinopyroxene. The magmas had not, or
only just, reached saturation with apatite and Ti-oxide
and were probably trachybasaltic to trachyandesitic or
tephriphonolitic. Because of the lack of a K trough,
similar rocks in the North Atlantic are scarce; none
was found which shows convincing similarities (Fig.
14b), and loss of REE and Y cannot be excluded.
7.3. Contaminated trachytes or dacites: layers
21, and 21b
15,
Two samples of layer 21 and one of layer 21b
have almost identical compositions, suggesting that
the two layers originated from the same volcano. The
multi-element patterns (Fig. 15a) have pronounced Nb
troughs, clearly suggestive of crustal contamination,
and consequent high Zr/Nb ratios of 15.7 –17.7 (Fig.
6). Deep troughs for Sr, P, and Ti may be due to both
contamination and fractionation. Similar though
somewhat more fractionated patterns are found for
ne-normative trachyte lavas (benmoreites) from Mull,
as well as for Q-normative trachyte dykes from Lundy
(Fig. 15b). A very similar though slightly less fractionated pattern is found in dacite lavas from the
Darwin complex. Trachyandesites from Iceland have
similar patterns but with no Nb trough.
Two samples of layer
15 have Zr/Nb ratios
around 20, suggesting a similar origin. However, both
samples are so thoroughly altered that their trace
element patterns are unreliable.
7.4. Alkali basalt: layer
Fig. 15. Multi-element patterns of contaminated trachyte layers
15,
21, and
21b from the negative ash series (a) with
comparisons (b). DX30, tholeiitic trachyandesite, East Iceland
(unpublished data, J.G.F. and B. S. Hardarson); LUN61, trachyte
dyke, Lundy, SW Britain (Thorpe and Tindle, 1992); BM60,
benmoreite lava, Mull (Kerr et al., 1999); C7.1, dacite, drill hole
163/6-1A, Darwin Complex, NW Europe margin (Morton et al.,
1988b).
21a
Three samples of this layer show fairly good
mutual coherence for both major and trace elements.
The magma was alkali basaltic, or perhaps more
evolved, with very high contents of TiO2, 5.4 –5.8
wt.% volatile-free (Fig. 4). The multi-element pattern
(Fig. 11a) is similar to those of the alkali basaltic
layers
11 to
13, but is systematically tilted to
lower values in the Rb end and higher values in the
Y end, yielding higher Zr/Nb ratios around 7 (Fig.
6). The REE pattern is less steep than for layers 11
to
13 (Fig. 8), suggesting less garnet influence
during melting. The multi-element pattern is very
similar in overall shape to patterns of alkali basaltic
dykes in the Gardiner complex and lavas in the
Prinsen af Wales Bjerge Formation, East Greenland
(Fig. 11b).
7.5. Basalt: layers 22,
29a, 34, and 35
24,
26, ( 27),
28,
Bøggild (1918) described these ash layers as basaltic (with labradorite), except for layer 27 which
was described as acid. Despite heavy alteration of the
major elements (Fig. 4), the minor and trace elements
L.M. Larsen et al. / Lithos 71 (2003) 47–80
show some common traits. These older basalts are less
enriched than the younger basaltic ash layers, as
evident from their relatively low TiO2 (1.8 – 3.4
wt.% volatile-free), K2O ( V 0.5 wt.%), P2O5 ( < 0.2
wt.%), Nb, Zr, and most other incompatible trace
elements (Figs. 4 and 5). The contents of Cr are
higher than in all the other ash layers, mostly above
200 ppm. The Zr/Nb ratios range from 3.6 to 9.7,
highest in the acid layer 27, and perhaps some of
the magmas were crustally contaminated.
The multi-element patterns of these layers are
somewhat variable (Fig. 16a). In accordance with
Bøggild’s (1918) description of layer
27 as acid,
71
this layer has the most fractionated multi-element
pattern with the highest Rb, Ba, K, REE, Zr, and Y,
lowest Sr, and low P and Ti, and it may be characterised as dacitic. Also layers 29a and 28 appear
to be more evolved than the other layers. Layers 22,
24, and
26 have mutually consistent patterns
which, except for high Ba and Ti, are subparallel with
the other basalt patterns from the whole succession,
but at lower levels.
No rocks within the North Atlantic Igneous Province have been found that match the multi-element
patterns of layers 22 to 34, viz. with K, Sr, and P
troughs and Ba and Ti peaks, with steep Ti –Y limbs
Fig. 16. Multi-element patterns of basalts of the low negative ash series (a) and (b), with comparisons (c) and (d). Grey lines in (b), (c), and (d)
are the upper and lower bounds of the positive basalt series. Primitive mantle normalisation values from McDonough and Sun (1995).
Comparative data: FX3, basalt lava, E Iceland (unpublished data, J.G.F. and B. S. Hardarson); BM70, basalt lava, Mull (Kerr et al., 1999); B2,
basalt lava, East Iceland (unpublished data, J.G.F. and B. S. Hardarson); BCH6, basalt lava, Mull (Kerr et al., 1999); JHS37, basalt lava, Skye
(Scarrow and Cox, 1995); C4.7, basalt lava, drill hole 163/6-1A, Darwin Complex, NW Europe margin (Morton et al., 1988b).
72
L.M. Larsen et al. / Lithos 71 (2003) 47–80
(Fig. 16a). The ‘closest’ matches are a basalt from
Mull, which has no Sr trough, and a basalt from East
Iceland, which has no Ba peak (Fig. 16c).
Because of the evident possibility that the multielement patterns of layers 22 to 34, despite their
mutual consistency, are grossly changed by alteration,
the three carbonate-cemented loose slabs HM1 to
HM3 were analysed. They represent at least two
different layers of ca. 2 cm (HM1 and HM2) and
0.7 cm (HM3) thickness and could be layers
26,
30, or
35 (ca. 2 cm), and layer
31 (0.5 cm)
(logs from Bøggild, 1918). Layer
35 is the only
layer known to occur in cemented facies, but the thin
section of this layer is significantly more glass-rich
than either of HM1 and HM2, and the glass analyses
of 35 and HM2 are also not identical (Fig. 3). The
glass analyses do, however, have the same character,
with high MgO, low TiO2, and low K2O. The three
HM samples are fresh and very nearly identical, and
their chemical character confirms that they are related
to layers 22 to 35. They have similarly low TiO2,
K2O, and P2O5 (Fig. 4), low Nb and Zr, and in
particular, high Cr as characteristic of layers
22
to
34 (Fig. 5). However, they have significantly
lower REE (but not Y) and higher Ba and Sr. They
have Zr/Nb ratios of 10– 11 and are tholeiitic, hy
normative, as are the microprobed glasses. The
multi-element patterns (Fig. 16b) have a distinct Ba
peak, and, due to low REE, a Sr peak in contrast to the
Sr trough in the uncemented layers.
Multi-element patterns with Ba and Sr peaks and
relatively low degrees of enrichment are found in
several North Atlantic rocks from the NW European
margin, viz. Skye, Mull, the Darwin complex, and the
Faroe – Shetland sill complex (Fig. 16d). A group of
lavas from the ‘continental’ Lower Series in ODP
Hole 917A on the SE Greenland margin (the low-Zr/
Nb group) has, in principle, similar patterns but in
detail different element ratios (Fitton et al., 1998). The
type is not known from central East Greenland.
7.6. Peraluminous rhyolite: layer
33
Layer
33 is a subalkaline rhyolite. It has been
described by Bøggild (1918) and Pedersen et al.
(1975) as very fine-grained, homogeneous, crystalpoor, and relatively fresh, and accordingly, there is
good correspondence between the bulk and glass
Fig. 17. Multi-element patterns of the peraluminous rhyolitic layer
33 in the negative ash series and the G3 phase of the
peraluminous Lundy granite (Stone, 1990).
major-element analyses (Fig. 3). Both are peraluminous, with molecular Al2O3/(CaO + Na2O + K2O) =
1.20 – 1.26. The magma was extremely evolved with
very low TiO2, MgO, and CaO, and the multi-element
pattern shows an unusual combination of low Sr, Zr,
La, Ce, and Y, and high P (Fig. 17). These are exactly
the characteristics of many S-type granites (Chappell,
1999). The layer shows striking resemblances to the
Lundy granite which is also an S-type granite: it is
peraluminous and has only 15– 75 ppm Zr and 9– 40
ppm Ce (Stone, 1990; Thorpe et al., 1990). Some
peraluminous rhyolites from Antrim may also be
similar (Meighan et al., 1984); however, full trace
element analyses of these are not available.
8. Timing
The age of the Danish ash layers forms an important calibration point for the international time scale.
Two layers have been dated by the 39Ar – 40Ar method: layer 17 at 54.52 F 0.05 Ma, and layer + 19 at
54.04 F 0.14 Ma (Chambers et al., 2003), results
which are closely comparable to earlier datings by
C. Swisher (cited in Berggren et al., 1995, p. 135).
Older and younger ash layers are thus only confined
biostratigraphically.
Knox and Morton (1988) and Knox (1997) distinguished three phases of ash deposition in the North
Sea. Phase 1 is early, nannoplankton zones NP5 –
NP6, 60 –58 Ma after Berggren et al. (1995), and
L.M. Larsen et al. / Lithos 71 (2003) 47–80
synchronous with the main volcanism in the British
Isles from where the ashes were probably derived.
Coeval ash layers in Denmark should be situated in
the Holmehus Clay Formation and the Kerteminde
Marl (Heilmann-Clausen et al., 1985) and they probably exist. The few ash layers that have been found
below the level of the negative ash series are so
inconspicuous that still-older layers may well have
been overlooked (C. Heilmann-Clausen, personal
communication, 2000).
Phase 2 comprises several sub-phases. Phase 2a (or
2.1 and 2.2a), zones NP9 – NP10, 56– 54.5 Ma, comprises the negative ash series in Denmark and the Sele
Formation in the North Sea. Phase 2b (or 2.2b), zone
NP10, ca. 54.5 –54.0 Ma, comprises the positive ash
series in Denmark and the Balder Formation in the
North Sea. Phase 2b is the phase of paroxysmal activity.
Phase 2c (or 2.2c) and phase 2d (or 3), zones NP10 –
NP12, 54– ca. 50 Ma, comprise sporadic ash layers in
the younger sediments and are also represented in
Denmark (e.g. Heilmann-Clausen et al., 1985).
Thus, in search for eruption sites for the Danish
ashes investigated here, the approximate time window
56 –54 Ma is of relevance. This is the time of formation of the major part of the flood basalt succession in
East Greenland and the middle and upper lava formations in the Faroes (Storey et al., 1996; Larsen et al.,
1999, Waagstein et al., 2002). There was also volcanism on the Vøring Plateau (Sinton et al., 1998). The
basaltic activity in the British Isles was extinct by this
time (Chambers and Pringle, 2001), whereas some
acid centres were active, among these the Mourne
Mountains and Slieve Gullion in Ireland (around 56
Ma, Gamble et al., 1999), and Lundy, St. Kilda, and
Rockall (all around 55 Ma, Mussett et al., 1988;
Ritchie et al, 1999). On the shelf, the igneous centres
of Darwin and Erlend were active, Darwin at 55 – 56
Ma (Sinton et al., 1998) and Erlend probably around
the same time (Ritchie et al., 1999; Jolley and Bell,
2002). There are more than 20 igneous centres on the
shelf and some of these were probably active, but for
several there are no data.
9. Source areas for the Danish ashes
Because of the thickness (up to 19 cm) and
coarseness (grains up to 0.5 mm) of the ash layers,
73
most early workers inferred that the eruption sites lay
at most a few hundred kilometres away (Bøggild,
1918; Andersen, 1937; Norin, 1940; Pedersen et al.,
1975; Pedersen and Jørgensen, 1981). An exception is
Gagel (1907) who, based on the observation that the
ash layers in northern Germany are finer grained than
the Danish ashes and show no appreciable variations
throughout Germany, suggested that the sources were
distant and ‘‘within the sea-covered parts of the
Atlantic’’. Grain size variations of individual ash
layers throughout Denmark indicate that the source
areas lay to the northwest. Andersen (1937) presented
contour maps for several individual ash layers that
showed different source directions to the north and
northwest, but data on which the contours are based
are far too few to warrant the detailed contours, and
only the general conclusion of a northwestern source
can be upheld.
The buried volcano in the Skagerrak south of
Norway, indicated by old geophysical surveys and
suggested by Pedersen et al. (1975) as a possible
source, has not been confirmed by later investigations.
Furthermore, ash layers coeval with the Danish ashes
have since then been found widespread in drill holes
in the North Sea and the Faroe – Shetland platform
where they are included in the Sele and Balder
Formations (e.g. Knox et al., 1997). These ash layers
increase in thickness to the NW, and Knox and
Morton (1988) and Morton and Knox (1990) suggested that the sources lay as far away as in East
Greenland and in the opening North Atlantic Ocean
between Greenland and the Faroes. In concert with
this, Waagstein and Heilmann-Clausen (1995) considered a ca. 400-m-thick unit of partly reworked tuffaceous sediments on the Faroes platform, the oldest
sediments post-dating the Faroes basalt plateau, to be
proximal deposits of the Balder Formation tuffs.
There is thus circumstantial evidence pointing to
sources for the Danish ashes in the Faroe – Greenland
area. This requires transport distances of at least 1100
km to Fur, 1200 –1600 km to the deposits in SE
England and northern Germany, and about 2000 km
to deposits in Austria. At Anthering in Austria, 23
bentonite layers with immobile trace element contents
very similar to the Danish positive series ashes, and
including a 3-cm-thick possible equivalent to layer
+ 19, have been described by Egger et al. (2000). In
comparison to these distances, younger ash layers
74
L.M. Larsen et al. / Lithos 71 (2003) 47–80
from the whole age range Eocene to Holocene in drill
and piston cores in the North Atlantic have been
interpreted as sourced from Iceland, even in cores at
distances of up to 1500 km from Iceland (Sigurdsson
and Loebner, 1981; Werner et al., 1996, 1998; Clift
and Fitton, 1998). In recent times, ash from the 1875
eruption of Askja in central Iceland fell in a 0.5-cmthick layer in Stockholm, 1700 km away (Thoroddsen, 1925, pp. 208– 209). Thus, it seems realistic that
some of the Paleogene ash deposits could be sourced
from as far away as East Greenland.
Pedersen et al. (1975) recognized four stages of
volcanic activity in the Danish ashes. The data presented here allow consideration of these stages in
more detail and with slight revision, and probable
source areas can now be identified (Fig. 18).
9.1. Stage 1. Basalts and peraluminous rhyolite,
layers ( 39?) 35 to 22: sources on the NW
European shelf
As described above, the heavily altered uncemented
basaltic ash layers in this interval have no compositional equivalents in the North Atlantic, and their
compositions may be thoroughly corrupted. The
multi-element patterns of the fresh, cemented ash
layers HM1 to HM3, with Ba and Sr peaks and no K
troughs, do not resemble any basalts from East Green-
Fig. 18. Changing source areas for the Danish Palaeogene ash layers around 56 – 54 Ma. Possible source areas are shown with grey shading.
Igneous centres on the NW European margin are shown as dots. Stages 1 – 3 correspond to the Sele Formation, and stage 4 to the Balder
Formation in the North Sea and Faroe – Shetland area. Note the suggested end of flood volcanism in East Greenland at the onset of stage 4.
L.M. Larsen et al. / Lithos 71 (2003) 47–80
land or the Faroes, which almost invariably have no Ba
peaks and troughs for K and Sr. However, similar
patterns with Ba and Sr peaks, with or without K and
Nb troughs, are common in basalts from the NW
Europe margin (Fig. 16) and are found in rocks both
older (Mull, Skye) and of similar age (Darwin);
although in detail, the HM samples are not identical
to any of these. We suggest that the basalts and more
evolved rocks of this interval were sourced from one or
more volcanic centres on the NW European margin;
centres such as Judd, S. Westray, and Sigmundur have
not been sampled (Ritchie et al., 1999).
The rhyolitic ash layer 33 is an S-type rhyolite.
The available analyses of the Scottish and Irish granites and felsites (Thompson, 1982; Bell, 1983; 1985;
Meighan et al., 1984) do not show these characteristics. The Lundy Granite is the only British granite
with S-type character (Stone, 1990; Thorpe et al.,
1990). Its age of around 55 Ma is appropriate though
not very precise. In any case, the source volcano for
layer
33 has to be situated within a sedimentary
basin where the sediments have contributed a significant melt fraction to the magma. Peraluminous dacites
with a significant shale component are known from
both the Darwin and Erlend complexes (Morton et al.,
1988b; Kanaris-Sotiriou et al., 1993) and also from the
Vøring Plateau (Viereck et al., 1988, 1989), but no
rhyolites are known. The Lundy centre is an obvious
possible source for layer
33, but this layer could
also have originated from one of the central complexes
on the NW European shelf.
9.2. Stage 2. Trachytes, rhyolites, alkali basalts,
nephelinites, and phonolite, layers 21b to 15:
sources on the NW European shelf and in East
Greenland
The compositional range covered by these ash
layers is very large, and they must represent a number
of different eruption sites.
The crustally contaminated trachytes or dacites
(layers 21b, 21, 15?) have no known equivalents in East Greenland as lavas or dykes, and the
central complexes in Greenland with evolved rocks are
too young. The similarity of the trace element patterns
of layers 21b and 21 to those of some benmoreites
from Mull (Kerr et al., 1999), a trachyte dyke from
Lundy (Thorpe and Tindle, 1992), and dacites from the
75
Darwin complex (Morton et al., 1988b) is considerable
(Fig. 15); however, the Mull volcano had long been
extinct. The rhyolitic layers 18a and 18 also have
equivalents on Lundy but not in Darwin. Both Lundy
and the Darwin complex have about the right age. The
Myggbugta complex in NE Greenland is too young
(34 – 28 Ma, Upton et al., 1984). Lundy is an interesting
possibility as a common source for the trachytic to
rhyolitic ash layers in this interval; however, the ashes
seem to be less contaminated than most of the Lundy
rocks, and the ashes may more likely have been sourced
from one of the volcanic complexes on the shelf.
Judged from sparse available data from St. Kilda and
Rockall, these do not seem to be good candidates.
The strongly alkaline Ti-rich nephelinitic ( 19b,
19a) and phonolitic ( 17) ash layers, as well as the
intermediate ( 20, 19c, 19) and alkali basaltic
layer 21a, in this interval must have other sources.
The Vestbrona nephelinite centres off Norway are
unsuitable because they are Ti-poor (Prestvik et al.,
1999). Heister et al. (2001) correlated the phonolitic
polymict layer 17 to the Gardiner complex in East
Greenland based on age, mineralogy, and bulk REE
patterns. The Gardiner complex is ultramafic alkaline
and consists mainly of melilitolites, pyroxenites, and
other ultramafic cumulate rocks (Nielsen, 1979, 1980);
the associated fine-grained dykes cover a range from
melanephelinitic to alkali basaltic and peralkaline
trachytic and phonolitic compositions (Nielsen,
1994). This range is remarkably similar to that seen
in the alkaline ash layers 21a to 17, and this could
suggest that all of these ash layers originated in the
Gardiner complex. If this is so, then the ash layers have
lost substantial amounts of REE and Sr. The Gardiner
nephelinitic and phonolitic dykes have very high REE
contents so that LaN and CeN are both greater than
NbN, and the Sr troughs are small in the nephelinites
and absent in the phonolites (Figs. 12 – 14). These
features are in strong contrast to the patterns of the
alkaline Danish ash layers. Only the alkali basaltic
patterns from Gardiner and Denmark (layer 21a) are
fairly similar. Other nephelinites and basanites in
Greenland (Brooks et al., 1979; Bernstein et al.,
2000; Peate et al., 2003) and off Norway (Prestvik et
al., 1999) have Nb/La ratios more similar to the
Gardiner rocks than to the ash layers (Figs. 13 and
14). Considering the strong indications of alteration of
the basaltic ash layers 34 to 22 (REE addition to
76
L.M. Larsen et al. / Lithos 71 (2003) 47–80
depleted rocks), it is very probable that the alkaline ash
layers are altered (REE extraction from enriched
rocks). Their silica-undersaturated character would
have made them prone to alteration in the silica-rich
diatomite environment, in contrast to the much more
robust siliceous trachytes and rhyolites (Figs. 2 and 3).
It is still an attractive theory that the nephelinitic to
phonolitic ashes originated in the Gardiner complex. If
not, then one or more similar large nephelinitic complexes must have existed but are unknown.
9.3. Stage 3. Alkali basalts, layers
a source near the opening rift
13,
12,
11:
These three characteristic black layers have such
similar chemical compositions that they would have
originated from one volcanic system. The Prinsen af
Wales Bjerge volcanoes were active at this time (Peate
et al., 2003), but although their alkali basalts have
broadly similar multi-element patterns (Fig. 11), their
major-element compositions are significantly different
from those of the ash layers (lower Al2O3, higher
MgO). The multi-element patterns and major-element
compositions of layers 13 to 11 show affinities
with those of alkali basalts from the Snaefellsnes and
SE rift zones in Iceland (Fig. 11), and we speculate
that they were produced in association with the
opening rift but within a failed, offset, or propagating
rift zone setting similar to the Kangerlussuaq rift
(Brooks, 1973) or the Snaefellsnes and SE rift zones
in Iceland. Layer 10 heralds the advent of stage 4;
however, layers
10 to
1 are of insignificant
volume.
9.4. Stage 4. Tholeiitic ferrobasalts and rhyolites,
layers +1 to +140: the emergence of Proto-Iceland
Ferrobasalts similar to those in the positive ash
series are unknown on the NW European margin
except for the Faroes; they are rare in the Faroes and
East Greenland, but they are common rocks in Iceland.
Moreover, a bimodal distribution of rock types between ferrobasalts and rhyolites, such as seen in the
positive ash series, is a characteristic feature of the
Tertiary lava plateaus of Iceland (Wood, 1978; Flower
et al., 1982) and even more so of the widespread ash
deposits in the North Atlantic inferred to be sourced in
Iceland (Sigurdsson and Loebner, 1981; Werner et al.,
1996, 1998; Clift and Fitton, 1998; Lacasse et al.,
1998). Pedersen and Jørgensen (1981) concluded, on
the basis of tephra morphology, that both the basaltic
and acid layers of the Danish positive series were
erupted into shallow water, leading to violent phreatomagmatic activity. We suggest that the positive ash
series was produced from the nascent Proto-Iceland in
the widening rift between the continental margins of
central East Greenland and the Faroes. As earlier
noted, the ferrobasalts are so uniform and show such
coherent variations that we consider that they all
originated within a single volcanic mega-system. The
two acid layers are not co-magmatic and could have
been produced in one or two central volcanoes. The
volcano sourcing layer + 13 was situated on continental crust, or a remnant of such crust, and could be
situated in the rift or on the NW European shelf. The
volcano sourcing the cataclysmic eruption of layer
+ 19 would have been situated in the rift.
The particularly voluminous, presumably plume
derived, volcanism in the sector around central East
Greenland and the Faroes led to the formation of the
thick successions of subaerial flood basalts in these
parts. The alkaline tuff layer in East Greenland that
has been correlated to the Danish ash layer
17 is
situated in the uppermost voluminous lava formation,
the Skrænterne Formation (Heister et al., 2001). At
this stage, the volcanism on the Faroes side had
stopped (Larsen et al., 1999), leaving the surface very
near sea level (Ellis et al., 2002). We speculate that the
start of the cataclysmic stage 4 is linked to the end of
the formation of the subaerial lava plateau in East
Greenland, the common cause being that the main
production areas for the large magma volumes moved
away from beneath the continental margin and into the
widening rift. Stage 4 then represents a short time
interval during the transition in which the magma
production rate in the rift was picking up but was not
high enough to sustain the rift above sea level, or to
allow eruption of unfractionated magmas. When the
transition was accomplished and Proto-Iceland emergent, the volcanism became more effusive and the
phreatomagmatic activity abated again. Alternatively,
Proto-Iceland may at all times have been emergent
except for a certain part in which the phreatomagmatic
activity was confined; however, this does not account
for the apparent time relations, the uniqueness of the
event, and the colossal volumes involved.
L.M. Larsen et al. / Lithos 71 (2003) 47–80
77
10. Conclusions
Acknowledgements
With analyses available now of more than half the
layers of the negative ash series and more than one
third of the positive series, the compositional range of
the Eocene ash layers in Denmark is most probably
covered in all essentials. The negative ash series
shows a large compositional variation ranging from
an S-type peraluminous rhyolite over tholeiitic basalts
and crustally contaminated trachytes and rhyolites to
alkali basalts, trachyandesites, alkali rhyolites, and
strongly alkaline Ti-rich nephelinites and phonolite.
In contrast, the positive ash series is compositionally
bimodal and consists of a comagmatic suite of
voluminous, enriched tholeiitic ferrobasalts and two
rhyolite layers. The ash deposits on Greifswalder Oie
in northern Germany are identical to the positive
series ashes.
Four stages of ash deposition with changing sources can be identified. The earliest ash layers of basalt
and rhyolite (stage 1, layers
39 to
22) were
sourced from centres on the NW European shelf such
as the Lundy (peraluminous rhyolite), Darwin, Erlend,
or non-analysed complexes. During stage 2 (layers
21b to
15), centres on the shelf continued to
source some crustally contaminated trachytes and
rhyolites, whereas the suite of strongly alkaline layers,
despite the large compositional variation, all could
have originated from a nephelinitic volcanic complex
such as the Gardiner igneous centre in East Greenland
where a similar range is present. In stage 3, three
alkali basaltic layers ( 13 to
11) may be the
products of a failed or propagating part of the opening
rift. In the cataclysmic stage 4 (layers + 1 to + 140),
the tholeiitic ferrobasalts, and at least one of the two
rhyolite layers, were probably sourced from a gigantic
volcanic system representing the nascent Proto-Iceland within the opening ocean.
The formation of the voluminous subaerial lava
plateau in East Greenland took place concomitantly
with stage 1– 2 and probably stage 3. The cataclysmic character of stage 4 can be understood if the
areas of extremely high magma production at this
time moved away from the continent and into the
sea-covered opening rift, thus switching the bulk of
volcanism from effusive to explosive. When ProtoIceland finally emerged, the explosive activity abated
again.
We are grateful to Henrik Madsen from Skarrehage
Molermuseum, Mors, for the samples of cemented
negative series basalts (the HM samples) and for
general help and discussions. Gunver Krarup Pedersen
and Stig Schack Pedersen collected the samples from
Greifswalder Oie and provided cheerful discussions on
the diatomite environment. Patricia Thompson prepared many of the ash samples for analysis. Robert
Frei analysed the Sr and Nd isotopes, Stefan Bernstein
microprobed the HM2 glass, and Lara Heister and
Troels Nielsen provided unpublished data. Discussions
with Claus Heilmann-Clausen are appreciated. The
paper is published with permission from the Geological Survey of Denmark and Greenland.
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