Earth and Planetary Science Letters 153 ( 1997) 18 1- 196
EPSL
Rift relocation - a geochemical and geochronological investigation of a palaeo-rift in northwest Iceland
B.S. Hardarson a,b,*, J.G. Fitton a, R.M Ellam b, M.S. FYingle b
a Department of Geology and Geophysics, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, UK b Scottish Unioersities Research and Reactor Centre. Scottish Enterprise Technology Park, Rankine Avenue. East Kilbride, G75 OQF, UK
Received 21 March 1997; revised 28 July 1997; accepted 1 August 1997
Abstract
A dominant process in the evolution of Iceland is the repeated eastward relocation of the spreading axis in response to westward migration of the plate boundary relative to the plume centre. Two major former rifts can be identified in western Iceland: the Snaefellsnes rift zone, which last erupted tholeiitic lavas at about 7 Ma, and an older spreading system, lava flows from which can be traced some 100 km along a SW-NE strike in the extreme northwest of Iceland. The extinction of the latter is marked by a 14.9 Ma unconformity with a late&e-lignite horizon representing a maximum 200 k.y. hiatus in the lava succession. Lavas below the unconformity dip northwest towards the older axis from which they were erupted, whereas lavas above the unconformity dip southeast towards their source in the younger Snaefellsnes axis. Thus, two nearly complete rift relocation cycles are preserved in western Iceland, each lasting about 8 m.y. as measured between rift extinction events, and for around 12 m.y. from initial propagation to extinction. In this paper we present major- and trace-element analyses, Sr, Nd and Pb isotope data, and “‘Ar/ 3gAr dates on basalt samples from above and below the unconformity in northwest Iceland. The Icelandic Tertiary and Quaternary plateau basalts are remarkably homogeneous in composition, in contrast to the much more diverse compositions found in the presently active rift zone. However, basaltic lava flows beneath the unconformity in northwest Iceland show a wider range of incompatible element and radiogenic isotope ratios than do the younger plateau basalts. At least two mantle components, one depleted and the other less depleted with respect to bulk Earth, are required to explain the composition of post-15 Ma Icelandic basalt. The depleted end-member is chemically and isotopically distinct from the N-MORB source. Basalt from the northwest palaeo-rift, however, contains a significant North Atlantic N-MORB component, suggesting that depleted upper mantle can influence the composition of Icelandic basalt in a dying rift that is too far from the plume centre to be dominated by plume mantle. This may account for the periods of low magma productivity represented by troughs between the V-shaped ridges on the Reykjanes Ridge. We suggest that temporal variation in the composition of Icelandic basalt is better explained by crustal accretion and rift relocation processes than by variations in plume composition and temperature. 0 1997 Elsevier Science B.V.
Keywords: Iceland; mantle plumes; basalts; mid-ocean ridge basalts; mid-ocean ridges: rift zones
1. Introduction
* Corresponding author. Tel.: +44 131 650 8511. Fax: +44
Numerous studies have demonstrated the value of Iceland as a natural laboratory for the investigation
131 668 3184. E-mail: [email protected] of magmatic processes. The unique - at least at the
0012-821X/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SOOl2-821X(97)00145-3
182 B.S. Hardarson et al./Earth and Planetary Science Letters 153 (1997) 181-196
B.S. Hardarson et al. /Earth and Planetary Science Letters 153 (19971 181-196 183
present day - coincidence of a mantle plume with
the Mid-Atlantic ridge combines the two fundamen- tal factors that promote magmatism; elevated mantle potential temperature (the plume) and adiabatic de-
compression in response to spreading at the ridge. One of the most intriguing aspects of Icelandic geol- ogy concerns the temporal variation in the location
of active magmatism (Fig. 1). The central axis of the
mantle plume is thought to be located below south- east Iceland [3]. Today, the Mid-Atlantic Ridge is
represented on land by the western (WVZ) and
northern (NVZ) volcanic zones (Fig. 1). The WVZ
and NVZ are offset along a region known as the Mid-Iceland Volcanic Zone (MVZ) which may be
viewed as a ‘leaky’ transform fault [4]. The Eastern Volcanic Zone (EVZ) is currently propagating to the
south of the Vestmann Islands in southeast Iceland.
Eventually, a ridge jump is expected, whereupon the focus of extension in southern Iceland will transfer
from the WVZ to the EVZ [2,4,5].
From the time when the Mid-Atlantic ridge mi- grated over the Iceland plume during magnetic
anomaly 6, about 24 m.y. ago [6], the plume has
repeatedly refocused the location of spreading with the necessary adjustments being accommodated by
transform displacements of the ridge. Relocation of
the spreading axis through ridge jumping is a promi- nent process in the evolution of Iceland. Previous rift axes are identified in the Icelandic geological record
by their gentle synclinal structure, produced by the loading of volcanic rocks. The consequent sagging of
crust in the rift zone causes the volcanic successions
on the flanks to dip towards the rift axis [2,4,5,7,8]. Identification of the synclinal structures as former
rift axes is important because it allows the geometric relationships between rifts to be incorporated into
tectonic models. Taking the synclines to be indica-
tive of former rifts suggests that successive rift axes may be regularly spaced [9,10]. Rift jumping is clearly related to the general WNW drift of the plate
system as a whole with respect to the plume 161. The ridge axis is continually migrating away from the
plume centre, and relocation of active magmatism may simply be a response to this migration. How- ever, what is unclear is whether relocation is trig- gered by an episodic increase in magmatic productiv- ity from the plume centre [9,11,12] (the ‘pulse’ or ‘blob’ hypothesis) or if the thermal structure gener-
ated by plume-ridge interaction militates against
continued magmatism remote from the plume centre
(the ‘steady-state’ plume hypothesis). Two major palaeo-rifts occur in western Iceland:
in the extreme northwest of the island and on the Snaefellsnes peninsula (Fig. 1). Our primary interest
is in the first example where lava flows from the old
rift axis occur beneath an unconformity marked by
late&e-lignite deposits and overlain by later basalts. Lava flows below the unconformity dip about 5” to
the NW, towards the rift axis from which they
originated, whereas flows above the unconformity dip about 5” to the SE, towards the younger Snaefell-
snes axis. The unconformity seemingly records a rift
relocation, with the lignite representing a volcanic hiatus. Lava flows from the older palaeo-rift, which
we shall refer to as the NW Iceland rift, can be
traced for almost 100 km along a zone striking
SW-NE (Fig. 1). In this paper, we present new
geochemical and geochronological data from the NW
Iceland palaeo-rift and discuss the causes of the
compositional variation shown by these lavas.
2. Analytical techniques
Major- and trace-element concentrations were measured using X-ray fluorescence spectrometry at the University of Edinburgh. Details of the analytical
techniques, including instrument settings and analyti- cal precision estimates are given in [ 131.
Strontium, neodymium and lead isotope ratios
were determined at the Scottish Universities Re- search and Reactor Centre (SURRC). The samples
were leached for 2 h in hot 6 M HCl and subse-
quently washed in deionised water before Sr separa-
tion. Sr and Nd fractions were separated from
whole-rock powders using techniques described in
[14]. For some samples a modified procedure was used for the separation of Ba from the REE. The
dried-down bulk REE fraction collected from the cation exchange columns [14] was dissolved in 3 M
HNO,, and loaded onto a column containing 2 ml of Eichrom Sr Spec resin (IOO- 150 pm particle size). The REE were eluted with 3 M HNO,, whereas Ba was retained on the columns. All samples were run on a VG Sector 54-30 thermal ionisation mass spec- trometer and data were acquired in multi-dynamic
Tab
le
1 C
hem
ical
an
d is
otop
ic
data
fo
r ba
salti
c la
vas
from
no
rthw
est
Icel
and
Sam
ple”
SU
C.5
5 W
C27
su
c20
WC
15
SUC
I su
4 SU
8 su
12
SU17
T
F2
SU18
SN
8 SB
X3
TF6
$
Bel
ow
unco
nfor
mity
A
bove
un
conf
orm
ity
g : 4 (w
t%)
49.5
2 x
13.6
2 3
14.5
4 4.
79
; m
19.6
3 a
3.15
0.
50
5 2 2.
95
3 0.
29
2
0.47
z
SiO
z 48
.16
Als
o,
13.7
7
Fe&
13
.67
MS0
7.
21
CaO
11
.91
Na,
O
2.11
W
0.17
TiO
, 1.
76
MnO
0.
22
R,O
S 0.
15
Tot
al
100.
28
(ppm
) N
b 7.
8
Zr
92.5
Y
29.6
Sr
163.
5
Rb
2.8
Zn
87
cu
187
Ni
84
Cr
182
48.4
2 48
.76
47.7
8 48
.65
49.2
5 41
.96
47.7
0 47
.41
47.7
3 49
.46
48.2
6 48
.32
13.6
5 14
.33
14.2
3 13
.37
14.4
4 14
.41
16.1
4 15
.72
13.1
0 13
.11
15.5
5 14
.58
14.0
8 13
.01
13.1
2 15
.52
14.2
1 13
.10
11.8
1 12
.60
16.5
8 16
.30
13.4
5 12
.15
6.96
7.
13
1.57
6.
10
6.56
7.
49
6.77
7.
86
5.55
4.
67
5.06
7.
77
11.5
5 11
.92
11.5
0 10
.92
11.2
1 12
.03
12.1
6 11
.06
9.80
9.
22
11.1
0 12
.9
2.38
2.
41
2.19
2.
67
2.60
2.
31
2.12
2.
00
2.76
3.
31
2.78
2.
19
0.17
0.
17
0.20
0.
17
0.19
0.
17
0.18
0.
24
0.23
0.
47
0.41
0.
20
2.04
1.
76
1.74
2.
35
2.00
1.
78
1.72
1.
72
3.13
3.
15
2.09
1.
88
0.22
0.
22
0.20
0.
25
0.2
1 0.
21
0.18
0.
19
0.26
0.
27
0.19
0.
18
0.17
0.
16
0.14
0.
2 1
0.18
0.
16
0.16
0.
18
0.37
0.
44
0.26
0.
18
100.
04
99.9
8 99
.91
100.
11
100.
85
100.
15
99.8
4 10
0.09
99
.26
100.
04
99.1
6 10
0.36
99
.46
I LT
h 10
.2
10.0
6.
3 9.
5 6.
0 7.
6 8.
0 10
.1
19.3
23
.6
18.2
17
.5
30.8
11
3.5
141.
9 10
1.1
150.
8 11
2.6
102.
5 10
4.7
103.
6 21
9.7
199.
7 13
2.4
94.7
25
2.6
35.3
38
.0
30.4
41
.1
33.2
29
.2
26.8
26
.4
56.0
51
.4
32.6
2 3.
7 54
.9
173.
I 19
4.3
181.
6 21
2.7
199.
1 21
3.2
230.
5 20
9.6
208.
7 27
1.6
296.
1 29
9.0
276.
9 1.
4 0.
9 5.
4 0.
7 2.
3 0.
7 1.
1 2.
7 2.
6 9.
7 6.
7 3.
1 8.
9 10
0 93
92
II
I 10
1 91
82
88
14
0 14
6 10
6 90
13
8 17
1 12
8 97
II
I 16
0 93
11
3 13
2 14
4 15
9 13
3 12
6 17
0
83
72
84
45
65
87
66
99
41
26
41
92
30
219
171
139
57
106
209
203
114
68
24
40
286
33
V
361
419
382
346
420
405
348
308
296
525
422
359
329
395
Ba
45
34
52
27
44
24
31
50
49
59
82
76
56
108
SC
34
46
47
45
44
53
43
40
35
44
37
38
46
36
La
5 7
10
5 8
7 I
I 7
14
16
16
12
24
Ce
19
24
28
16
26
23
19
24
22
42
50
33
31
59
Nd
13
16
19
13
20
16
15
15
15
30
31
20
17
37
“Sr/
0.
7032
64
0.70
3256
0.
7030
83
0.70
3 12
0 0.
7030
26
0.70
3072
0.
7032
52
0.70
3326
0.
7034
07
0.70
3229
0.
7034
18
0.70
3402
0.
7034
65
0.70
3455
86
Sr
2se
11
17
15
18
17
16
18
17
16
17
16
15
17
17
14’N
d/
0.51
3073
0.
5130
72
0.51
3105
0.
5131
10
0.51
3144
0.
5131
59
0.51
3093
0.
5130
67
0.51
3051
0.
5130
71
0.51
3043
0.
5130
27
0.51
2999
0.
5130
25
14’N
d
2se
6 5
7 6
9 9
6 7
6 6
7 6
7 8
“‘Pb
/ 18
.468
18
.500
18
.117
18
.091
18
.017
18
.179
18
.235
18
.305
18
.283
18
.580
18
.715
18
.700
18
.618
*04P
b
2se
17
11
13
8 15
7
12
31
12
21
7 20
11
‘07P
b/
15.4
64
15.4
50
15.4
42
15.4
70
15.3
97
15.4
20
15.4
79
15.4
49
15.4
30
15.5
18
15.4
64
15.4
20
15.4
66
*“Pb
2se
18
9 10
6
20
8 14
34
18
26
8
20
II
*“Pb
/ 38
.064
38
.010
37
.695
37
.688
37
.500
37
.638
37
.861
37
.838
37
.809
38
.138
38
.288
38
.120
38
.193
204P
b
2se
50
19
28
17
35
21
30
93
44
51
20
50
22
“SU
C
= Su
gand
afjo
rdur
dr
ill
core
: SU
an
d T
F
see
Fig.
bTot
al
Fe a
s Fe
,O,.
2;
SN8
and
SBX
3 ar
e hi
gher
in
pro
file
SU
th
an
sect
ion
in F
ig.
2.
---_
1
I,.
., ”
,I ,..
I.
,,I
186 B.S. Hardarson et al./ Earth and Planetan, Science Letters 153 (1997) 181-196
Northeast SU Southwest
KE
I Flow no. I
I I
Aow no. Cande& Kent(1995)
32
31
30
29
u\
2;’
27
26
25
19
18
17
16
15
14
13
12
11
10
9
8
7
6
4
3
2
1
‘00 m
U
I
m
31
30
29
28
27
26
25
24
23
22
21
20
19
18*
'\ 17
16
15*
14
13
12
11
10
9
8
T,,,' /,'6
5
4
3A
3
2
KE = Skalavik
SU = Sugandafjordur
TF = Toarfjall, Dyrafjordur
U = Unconformity
no* = Dated lava flow
TF
j Flow no.
; i
I 14.178 C5ADn
14.612
14.800 CSBn,ln
14.888
15.034 C5Bn,2n
15.155
/’
I I /’
,’
m
I 16.014 C5Cn,ln
16.293
:
16.327 C5Cn,2n
16.488
16.556 C5C n,3n
16.726 I I I I
I I I
a Normal polarity
Reverse polarity
Fig. 2. Palaeomagnetic stratigraphy of the Skllavik, Sfigandafjiirdur and Dgrafjardur profiles (A, B and C on Fig. 1, respectively). Polarity
time scale from [18]. Lava flow numbers correspond with sample numbers.
B.S. Hardarson et al. /Earth and Planetary Science Letters 153 (19971 181-196 187
mode. *7Sr/ 86Sr was corrected for mass fractiona-
tion using 86Sr/ 88 Sr = 0.1194. Repeated analysis of
NBS987 gave 87Sr/ “Sr = 0.710236 + 19 (2 sd). 143Nd/ ‘44Nd was corrected for mass fractionation
using ‘46Nd/ ‘44Nd = 0.7219. During the course of
this study the SURRC Johnson Matthey Nd standard
gave ‘j3Nd/ ‘44Nd = 0.5 11500 f 10 (2 sd). Pb was
separated using standard HBr-HCl anion exchange
techniques. Analytical blanks were < 1 ng. The samples were loaded with H,PO, and silica gel on single Re filaments, and the lead isotopes were
determined using a VG 54E thermal ionisation mass
spectrometer. The data were corrected for mass frac-
tionation of 0.1% amu _ ’ based on replicate analysis
of the NBS981 standard. External reproducibility of
the Pb isotopic ratios is 0.2% (2 sd).
Radiometric dates were obtained by 40Ar/ 39Ar
incremental-heating analysis of holocrystalline
whole-rock cores. Approximately 1 mm thick, 5 mm diameter disks weighing 50-100 mg were stacked in
quartz vials and irradiatiated for 14 h in the CLICIT facility of the Oregon State University TRIGA reac-
tor. The irradiation flux factor, J, was monitored
every w 8 mm with the USGS standard sanidine
85GOO3 (27.92 Ma) and is known to better than 0.3% relative at any given vertical position. The
whole-rock cores were analysed by step heating in a double vacuum resistance furnace. Each step was
held at temperature for 15 min, including cleanup
with a N 400°C SAES Zr-Al getter, and then ex-
posed to a second N 400°C SAES Zr-Al getter for a
further 10 min before introduction into a MAP 2 15
rare-gas mass spectrometer for analysis. In order to aid in cleaning up H,O liberated from alteration
products, some sampies were also exposed to 500
mg of degassed 10 A molecular sieve in the first cleanup stage. All “Ar/ 39Ar errors are reported as
the standard deviation of analytical precision. Reliability of the Ar-Ar ages is assessed using
the following criteria, each of which involves a
rigorous statistical test [ 15,161. We accept an appar- ent 40Ar/39Ar age as an accurate estimate of the crystallization age of a volcanic rock only if: (1) a
well defined, high temperature age spectrum plateau is formed by at least three concordant, contiguous steps representing at least 50% of the “9Ar released; (2) a well defined isochron exists for the plateau points; (3) the plateau and isochron ages are concor-
0 5 10 15 20 Palaeomagnetic age (Ma)
Fig. 3. Variation of Zr/Nb (BSH and JGF unpubl. data) with
palaeomagnetic age in basalts (MgO > 5 wt%) from Iceland.
Basaltic lavas from below the unconformity KJ in inset) display
both a wide range of values and a rapid oscillation of Zr/Nb.
Open triangles on the inset diagram identify samples from above
the unconformity. Palaeomagnetic age is calibrated to the geomag-
netic polarity time scale [ 181.
dant; and (4) the isochron 40Ar/ 36Ar intercept is not
significantly different from the atmospheric composi-
tion.
3. Results
We have sampled in detail three profiles across
the unconformity in northwest Iceland at Skalavik,
Sugandafjbrdur and Dyrafjijrdur (A, B and C, respec-
tively, Fig. 1). The magnetic polarity of the lavas
was established in the field by using a fluxgate magnetometer. Careful measurements in the field on
3-4 samples will usually give a consistent polarity [ 171. In addition a 450 m drill core (Hole 3) consist-
ing of more than 50 lava flows, from Sudureyri
(Stigandafjordur), was made available by the Na-
tional Energy Authority in Reykjavik. The complete radiogenic isotope data set and the chemical analyses
for those samples are given in Table I ; the complete whole-rock geochemical data set are available as an
EPSL Online Background Dataset ’ and are avail- able from the authors on request. The magnetic polarity of the composite profiles is shown in Fig. 2.
The magnetostratigraphy and geochronology of
I http://www.elsevier.nl/locate/epsl, mirror site:
http://www.elsevier.com/locate epsl.
188 B.S. Hardarson et al./ Earth and Planetary Science Letters 153 (1997) 181-196
northwest Iceland, including the Skllavik section and the subaerial part of the Sigandafjiirdur section, has been described in detail by McDougall et al. [19].
Temporal variation of Zr/Nb in Icelandic basalt is shown in Fig. 3. Zr/Nb is chosen as an index of relative depletion (high Zr/Nb) and enrichment be- cause it is generally immune to the effects of low pressure fractional crystallization and low tempera- ture alteration. Exposed Tertiary Icelandic basalts have Zr/Nb = 8.9 f 1.2 (1 g) [20], which is very uniform compared with the much greater range of Zr/Nb (5-38) found in basalt from the neovolcanic rift zones (Fig. 3). Rocks from the NW Iceland rift also show a substantial range of Zr/Nb (10-20). During the final stages of the NW Iceland rift Zr/Nb oscillated with time, culminating in a marked Zr/Nb decrease up-section towards the unconformity.
A similar pattern is seen in Sr- and Nd-isotope ratios (Fig. 41, which are relatively constant in the Tertiary lava pile but show a much greater range in the neovolcanic zone and in rocks from the NW Iceland rift. Lavas from below the unconformity range in s7Sr/s6Sr from 0.70303 to 0.70341 and in ‘43Nd/ ‘j4Nd between 0.5 1305 and 0.5 13 16 (Fig. 4). Across the unconformity, there is a considerable overlap in 87Sr/ 86Sr but lavas from below the un- conformity consistently have higher 143Nd/ ‘44 Nd (Fig. 4). During the final stages of the NW Iceland rift the Sr- and Nd-isotope ratios show a continuous increase and decrease, respectively, correlating with the decreasing Zr/Nb. Pb-isotope ratios also show a
0.5132 NW Teltiary
(below unconformity)
ettialy
A NW Tertkwy (above ““c. A NW Tertiary (below unc. anic
0.5129
0.7026 0.7030 0.7032 0.7034 0.7036
“‘SrB”Sr
Fig. 4. Variation in “‘Nd/ 14’Nd and s7Sr/s6Sr in lavas from
northwest Iceland compared to other areas of the Iceland Tertiary (BSH and RME, unpubl. data) and the Iceland neovolcanic zone
[21-261.
15.7 o Neovolcanic 0 N Atlantic N-MORE
15.6 - * NW Tertiary (above uric.) q B 0
t
15.3
17.5 18.5 19.0
‘“Pb/mPb
(a)
19.5 20.0
40
o Neovokanic
D N Atlantic N-MORE
q
n
Ll 3g A NW Tetiiary (above uric.)
A NW Terliaiy (below uric.) a
k
a”38 - A
w
o.@dk 0 m Ib1
37 1 *-I
15.3 15.4 15.5
“‘PbPPb
15.6 15.7
Fig. 5. (a) *“Pb/ ‘04Pb versus ‘06 Pb/ 204Pb and(b) “*Pb/ 204Pb
versus “‘Pb/ ‘04Pb in lavas from NW Iceland. Data from other
Icelandic Tertiary volcanics (BSH and RME, unpubl. data), the
Iceland neovolcanic zone ([23,24,27-291, BSH and RME, unpubl.
data) and Atlantic N-MORB [30-331 are shown for comparison.
Analytical uncertainties (20 deviation of replicate analyses of
standards) are shown by crosses.
systematic variation. Rocks from below the uncon- formity are in general less radiogenic in ‘06Pb/ ‘04Pb, 207 Pb/ 204Pb and 2o8 Pb/ 204Pb than are lavas from above the unconformity (Fig. 5). Pb-isotope ratios are negatively correlated with Zr/Nb and ‘43Nd/ ‘44Nd and positively correlated with 87Sr/ 86 Sr.
We have dated in duplicate four samples from two locations across the northwest unconformity and one sample from the Stigandafjiirdur core @UC) by using @Ar/ 39Ar incremental-heating analysis of holocrystalline whole-rock cores (summarized in Table 2; complete data set available as an EPSL Online Background Data@ 2). The age spectra and the K/&of the samples analysed are shown in
2 http://www.elsevier.nl/locate/epsl,
http://www.elsevier.com/locate epsl.
mirror site:
Tab
le
2
@A
r,/
j9A
r in
crem
enta
l-he
atin
g da
ta
sum
mar
y fo
r ba
salt
sam
ples
fr
om
the
nort
hwes
t Ic
elan
d ri
fta
Sam
ple
K/C
a (t
otal
)
TF2
Toa
rfja
ll D
yraf
jord
ur
TF7
Toa
rfah
Dyr
atjo
rdur
suc3
4
Cor
e H
ole
3 Su
gand
afjo
rdur
su15
Bot
n
Suga
ndaf
jord
ur
SU18
Bot
n
Suga
ndaf
jord
ur
0.04
9
0.03
9
0.13
4
0.14
3
0.02
1
0.02
0
0.22
5
0.04
2
0.03
0
0.02
8
Age
pl
atea
u su
mm
ary
Isoc
hron
su
mm
ary
‘9A
r
(%)
60.9
58.7
46.3
57.0
97.3
100.
0
66.
I
47.7
78.5
74.9
No. of
ste
ps
Age
* 1
sd
(Ma)
MSW
D
5 of
11
14
.73
f 0.
07
0.78
6of
12
14.8
1 f0
.09
1.65
Wei
ghte
d M
ean
Age
: 14
.76
f 0.
06
0.49
5of1
3 14
.76k
0.14
12
.69
8of
12
14.9
4f0.
11
6.21
Wei
ghte
d M
ean
Age
: 14
.87
+ 0.
09
1.02
10
of
II
15.6
5f0.
13
1.54
12 o
f 12
15
.69f
0.12
0.
95
Wei
ghte
d M
ean
Age
: 15
.67k
O.0
9 0.
05
4of
9 15
.86
+ 0.
56
170.
70
Sofl
O
13.9
7 *
0.17
7.
65
Wei
ghte
d M
ean
Age
: 14
.13
f 0.
53
10.4
3
6of
II
14.4
6 k
0. I
9 4.
10
5ofl
O
14.8
0 f
0.24
6.
20
Wei
ghte
d M
ean
Age
: 14
.59
+ 0.
15
1.23
Age
+ 1
sd
(Ma)
Sum
s
N-2
14.7
4*0.
10
1.29
14.9
9f0.
11
0.88
14.8
5+0.
12
2.83
14.9
4t0.
12
8.99
15.1
1 *t
o.09
2.
93
15.0
5 kO
.08
1.28
15.6
2 +
0.23
1.
99
15.6
6kO
.19
1.22
15.6
4+0.
15
0.02
II.7
1 f
1.37
19
.17
16.0
0 f
0.27
0.
32
‘“A
r/
“Ar
+ 1
sd
inte
rcep
t
295k
5.6
285.
6 F
4.2
288.
9 k
2.2
290.
5 +
I .3
296.
2 +
2.6
296.
2 +
3.8
555.
5 1
82.5
-
89.9
+
49.2
_
_ 14
.64
+ 0.
21
3.42
28
7.9
* 5.
1
15.6
/k
0.27
2.
18
274.
1+
6.1
_ _
‘All
ages
ar
e re
lativ
e to
the
U
SGS
TC
R
san
8560
03@
27.9
2 is
ochr
on.
Num
bers
in
ita
lics
viol
ate
our
acce
ptan
ce
crite
ria
desc
ribe
d in
the
te
xt.
B.S. Hardarson et al./Earth and Planetary Science Letters I53 (1997) 181-196
22 20 18 16
14 12 10
. . . . . 0.20 : ..__. . . . .
‘... __.:-: l-iii ‘-7 : : 0.10 --_ ..I :
TF7
0.00
0 20 40 60 80 100 0 20 40 60 80 100 0 20 40 60 80 100
Cumulative % 39 Ar Age spectra and K/Ca of lava flows above and below the unconformity in northwest Iceland that met our acceptance criteria
described in the text. Solid and broken lines distinguish different experiments (Table 2).
Fig. 6. Lavas from immediately below and directly
above the northwest unconformity at Toarfjall
(Dyrafjijrdur) gave weighted mean ages of 14.85 f 0.12 Ma and 15.05 f 0.08 Ma, respectively (Table
2). The lavas above the unconformity exhibit normal
magnetic polarity, whereas those below are reversed (Fig. 2). From our data it is apparent that the normal
polarity interval immediately above the unconform-
ity at Toatfjall correlates with CSBn,ln, or possibly
C5Bn,2n [ 181. Results from subaerial samples at Stigandafjijrdur
(SU, Table 2) did not meet our acceptance criteria and we do not have any 40Ar/ 39Ar data on the
Skalavik (KE) profile at present. We are therefore
currently unable to resolve the possibility or other-
wise of diachroneity along the strike of the uncon-
formity, although the rift relocation may have oc- curred gradually because the laterite-lignite horizon becomes thicker to the NE along the unconformity. Since ages from either side of the unconformity are within error of one another, and the stratigraphically younger lavas gave the older age, it is clear that the volcanic hiatus represented by the unconformity and associated laterite-lignite horizon at Toarfjall (TF) is short (i.e., less than 200 k.y. at the 95% confidence level). Previous conventional K-Ar dates on a lava flow resting a few flows above the unconformity at
Sugandafjordur gave the mean age of 12.90 + 0.18
Ma [19] which is young compared to our results.
McDougall et al. 1191 dated one flow below the lower boundary of the unconformity, which gave the
mean age of 15.32 f 0.17 Ma, and they suggested that the unconformity represented a significant break
in the stratigraphic succession. Our results, however,
strongly suggest that this break was very brief. The magnetic polarity of the Sugandafjiirdur core
(SUC, hole3) is ambiguous since it has not been
measured after demagnetisation, and fluxgate mea-
surements often gave anomalous results, probably
influenced by the nearby presence of dykes. Further- more, we do not have the rigorous stratigraphic
control possible in the subaerial profiles. Nonethe-
less, one sample from the core (SUC34 at - 294 m) met our analytical criteria and gave a mean age of 15.64 k 0.15 Ma, which is the oldest rock so far
analysed from Iceland. This sample showed anoma- lous fluxgate polarity but the age data suggest that the lava was erupted during the reversed polarity interval preceding C5Bn,2n [18]. A further 170 m lava pile (more than 20 flows) lies below the dated
lava flow from the Sugandafjordur core. For compar- ison, the oldest rock in eastern Iceland yet analysed is a normally magnetized lava flow from Gerpisfles
at Gerpir, giving an age of 13.69 + 0.05 Ma (BSH
B.S. Hardarson et al./ Earth and Planetaq Science Letters 153 (1997) 181-196 191
and MSP unpubl. data) and thus correlating with CSACn [18].
4. Discussion
4.1. Mantle sources
Many previous geochemical studies of Icelandic mafic magmatism (e.g. [2 1,24,26,27,34]) have em- phasized the role of mixing between different mantle reservoirs or components. In particular, Icelandic basalts reveal evidence for interaction between a relatively high- *’ Sr/ 86 Sr, low- 143 Nd/ ‘44 Nd and in- compatible-element-rich end-member, often equated with plume mantle, and a relatively low-” Sr/ 86 Sr, high- ‘43 Nd/ ‘44 Nd and incompatible-element-poor end-member similar to N-MORB and thus often identified as depleted upper mantle. Icelandic basalts and North Atlantic N-MORB have overlapping Sr- and Nd-isotope ratios which change systematically along the Reykjanes Ridge towards Iceland in a manner which can simply be attributed to increased mixing of plume material with ambient depleted upper mantle [27,34,35].
Pb-isotope data from Icelandic basalts show a wide range of values and define arrays consistent with two-component mixing, but neither end-mem- ber corresponds to any known North Atlantic N- MORB isotope composition [36]. Rather, the Pb isotope data for Iceland and Atlantic N-MORB form separate, parallel arrays on a plot of ‘O*Pb/ ‘04Pb against “‘Pb/ 204Pb (Fig. 5b). Low-““Pb/ lo4Pb samples from Iceland have lower “‘Pb/ 204Pb val- ues than any recorded North Atlantic N-MORB (Fig. 5a). These observations have been used to argue that the depleted component in the Iceland plume is distinct from N-MORB mantle [36] and may be an intrinsic plume component [37], although Mertz and Haase [38], using a more comprehensive data set, have challenged this conclusion.
Trace-element data provide further evidence in support of a distinction between the depleted Iceland plume component and the N-MORB source. An extensive high-precision data set from the whole of the Iceland neovolcanic zone defines a remarkably linear array on a logarithmic plot of Nb/Y vs. Zr/Y [13,39]. Primitive mantle [40] plots within the Ice-
10 1 _ A above ““cmfomlty : . b&w umnfomlilY
ICELAND ARRAY
t ’ P
0.1
1 ZrN
10
Fig. 7. Nb/Y and Zr/Y variation in basalt above and below the
unconformity in northwest Iceland. Parallel lines define the upper
and lower bounds of the Iceland array [13,39]. The N-MORB
array [39], depleted basalt (Zr/Nb > 15) from the neovolcanic
zone (BSH and JGF, unpubl. data). and the composition of
primitive mantle (PM [40]) are shown for comparison. Depleted
basalt from below the unconformity reflects mixing of plume and
N-MORB mantle sources, whereas that from the neovolcanic zone
reflects fractional melting of the plume-source mantle.
land array and N-MORB data define a parallel array below it (Fig. 7). The N-MORB and Iceland arrays neither overlap nor converge towards common end- members. Differences in degree and depth of partial melting, including source depletion through melt ex- traction, induce shifts parallel to the arrays and are unlikely to result in variations between them [39]. The two arrays are therefore likely to reflect funda- mental differences in the respective mantle sources [ 13,391. Basaltic samples from below the unconform- ity in northwest Iceland differ from all other Ice- landic samples in that they extend from the Iceland array towards the N-MORB array. These samples provide the first clear indication of North Atlantic N-MORB-source involvement in Icelandic magma- tism. By contrast, depleted basalt from the neovol- canic zone defines the low-Zr/Y end of the Iceland array (Fig. 7) and probably represents melts from a plume mantle source depleted through fractional melting 1391.
Deficiency or excess of Nb, relative to the lower bound of the Iceland array, may be expressed as:
ANb = 1.74 + log(Nb/Y) - 1.92log(Zr/Y)
such that Icelandic basalt has ANb > 0 and N-MORB has ANb < 0. ANb is a fundamental source charac- teristic and is insensitive to the effects of variable degrees of mantle melting and source depletion
192 B.S. Hardarson et al/Earth and Planetary Science Letters 153 (1997) 181-196
5 14.5
E
Lj 15.0
.j”
g 15.5
B E $ 16.0 m m a 16.5
-0.1 0 0.1 0.2 0.3 0.4 0.5
ANb
Fig. 8. ANb plotted against palaeomagnetic age across the uncon-
formity in northwest Iceland. ANb is the deviation, in log units,
fmm the lower bound of the Iceland array (Fig. 7). Positive values
of ANb indicate an Icelandic mantle source and negative values
an N-MORB mantle source [13,39].
through melt extraction [39]. Fig. 8 shows the tempo- ral variation in ANb in lavas below and above the unconformity in northwest Iceland. Lava flows be- low the unconformity display a scatter between N- MORB mantle (negative ANb) and Icelandic mantle (positive AI%) sources, whereas only positive values of ANb (similar to all other Tertiary basalt samples from Iceland; BSH and JGF, unpubl. data) are found above the unconformity.
Our new Pb isotope data also indicate the pres- ence of N-MORB-source mantle beneath the NW Iceland rift. Tertiary lavas from below the uncon- formity in northwest Iceland are quite different from other Icelandic basalts (Fig. 5). These samples plot between the fields of Icelandic basalt and Atlantic N-MORB (Fig. 5b), and are the first recorded Ice- landic basalts with N-MORB-like Pb. Furthermore, the northwest Iceland samples display strongly corre- lated *06 Pb/ ‘04Pb and ANb (Fig. 9). Thus, both Pb isotope composition and Nb-Zr-Y relationships can be used to distinguish the majority of Iceland basalts from Atlantic N-MORB, and both indicate an N- MORB contribution to the NW Iceland rift samples.
4.2. Timing of rift-relocation cycles
A noticeable anticline is found in the Borgarfjor- dur region, midway between the extinct Snaefellsnes rift zone and the presently active WVZ (Fig. 11, and on both sides of the anticline there are unconformi- ties overlain by thick sedimentary horizons [2]. Lavas
below the unconformity originate from the Snaefell- snes rift zone, whereas lavas above originate from the current WVZ. The oldest rocks from the core of the anticline have been dated at 13.2 Ma [41] and the youngest lavas at around 7 Ma [2,41]. The rocks immediately above the unconformity in northwest Iceland represent the oldest exposed lavas from the Snaefellsnes rift zone and these are around 15 m.y. old. We cannot date the initiation of the Snaefellsnes rift zone but activity had apparently slowed down by 9 Ma and the rift had died out by 7 Ma [2,42]. We have identified the final stages of the NW Iceland rift zone, which became extinct by 15 Ma, and therefore about 8 million years separate these two rift extinction events.
This estimate of 8 m.y. does not reveal the full duration of a rift-zone cycle from initial propagation to extinction because a new rift begins to propagate some time before the extinction of the old rift. The oldest rocks from above the unconformity in Borgar- nes date back to 7-8 Ma [2,43] and these are the oldest preserved rocks originating from the present WVZ. The age of the EVZ has been estimated to be at least 3 m.y., which gives a southward propagation rate of 3.5-5 cm/yr [5,10]. It is not possible to predict when and how the EVZ will link up with the mid-Atlantic Ridge, but the WVZ will probably be active for some time yet. If this overlap of at least 3 m.y. is typical of rift relocations, then it is likely that a complete rift zone cycle in Iceland lasts around 12 m.y. from initial propagation to extinction.
18.7 -A NW Tertiary (above uric.) A A
& NW Tertiary (below uric.) A A
it 18.5 AA
c. n 18.3 -
P
* A .
.
18.1 - AA *
17.9
-0.1 0 0.1 0.2 0.3 0.4 0.5
ANb
Fig. 9. ANb plotted against ‘06Pb/ *04Pb for lava flows above
and below the unconformity in northwest Iceland. The good
correlation for the older flows suggests mixing between plume
and N-MORB sources of mantle.
B.S. Hardarson et al. / Earth and Planetary Science Letters 153 (1997) 181-196 193
4.3. Pulsed plume versus steady-state plume
The observation that recent Icelandic basalt ex- tends to more depleted composition than Tertiary
basalt was noted by Schilling et al. [12], who used it as the basis of their ‘blob’ model. The discovery of
superficially similar depleted rocks in the NW Ice-
land rift (Fig. 3) might be seen as evidence in support of this model. Several lines of argument,
however, suggest that our data are more consistent
with a steady-state model in which chemical and isotopic variation in Icelandic basalt reflect shallow
tectonic processes rather than pulsing of the mantle
plume. Crustal accretion models for Iceland have impor-
tant implications for the Tertiary lava record. Ac-
cording to the model of Palmason [7,8] the kinemat- ics of crustal accretion on Iceland are such that material erupted at the rift axis is rapidly buried
beneath the large mass of basalt erupted in the rift
zone as spreading proceeds. Lava flows erupted clos- est to the rift axis will ultimately form the deepest
parts of the lava pile, and only those lavas which
flow, or are erupted, more than 25 km away from the axis are expected to remain in the upper 2 km of the
crustal section that is exposed by the glaciated to- pography of Iceland. Thus, the basalt record is heav- ily biased against rift axis samples. The effects of
this lack of preservation are clearly illustrated in the
available geochemical data from Iceland (Fig. 3). Accessible lava flows from the neovolcanic zone
exhibit a rather wide range of geochemical and
isotopic variation (e.g. [2 11). Chemically and isotopi- tally depleted basalt occurs as small flows erupted in
the rift axis, and these are not likely to be preserved close to the surface in the future plateau sequence
[ZOI. In contrast, the Tertiary lavas show a much
smaller range of elemental and isotopic compositions because these represent only the largest, and most homogeneous, lava flows [20]. Thus the apparent recent shift towards more depleted basalt composi-
tions is an artefact of preservation and not the effects of a pulsing plume.
The depletion shown by basalt from the NW Iceland rift reflects mixing of plume- and N-
MORB-source mantle (Fig. 7), and is fundamentally different from that seen in depleted basalt from the neovolcanic zone. There is no evidence that the latter
is due to involvement of N-MORB-source mantle
and is more likely the result of fractional melting of heterogeneous plume mantle [39]. An influx of de-
pleted upper mantle at N 15 Ma could result from a
pulsing plume but this would imply a very precise
causal link between rift relocation (represented by the unconformity) and fluctuations in plume activity.
This explanation leads to the implausible conclusion
that rift relocations were triggered by the influx of ambient mantle between pulses rather than by the
pulses themselves. Our data are more consistent with
a steady-state model in which rift relocation is re-
sponsible for the apparent change in mantle sources
across the unconformity.
The spreading axis in Iceland is fed with hot plume mantle which may well be channelled along
the low viscosity region immediately below the ac- cretion axis [9,11,34]. The productivity and position
of the rift zones seem to change systematically in
order to minimise the distance between the centre of
the plume and the spreading axis [4]. Rift relocation
would alter the pattern of mantle flow from the
plume into the rift, lowering the productivity of the
old spreading centre and transferring productivity to the new spreading centre. During this time, the old
spreading centre may become starved of plume man-
tle and, if the plume centre is sufficiently far away, its dwindling supply of plume mantle may become
contaminated with ambient upper mantle. Mean-
while, plume mantle is fed to the new spreading
centre and large flows of undepleted Icelandic basalt may extend to cover the eroded flanks of the dying
rift. The predicted result would be exactly what we see in northwest Iceland - a sequence of old flows
recording a fluctuating plume-N-MORB mantle
source, overlain unconformably by plume-derived
basalt flows from the new rift axis. Although the
NW Iceland and Snefellsnes rift zones are now about
150 km apart (Fig. 11, they would have been much closer at the time of initiation of the younger axis,
and large flows would easily have been able to reach the flanks of the old axis.
Prominent V-shaped topography and gravity
anomalies on the Reykjanes Ridge, created by lo- cally thickened crust, have long been known to exist [l 11. The ridges are diachronous and are believed to record the passage of hotter-than-normal astheno- sphere, or a melt-rich front, from the mantle plume
I94 B.S. Hardarson et al/Earth and Planetary Science Letters I53 (1997) 181-196
at intervals of 5- 10 m.y. [9,11,44]. Alternatively, the V-shaped ridges could result from periodic rift relo- cations rather than from variations in the temperature of the plume. The three prominent V-shaped ridges on the Reykjanes Ridge are separated by troughs originating in Iceland at about 15 and 5 Ma [44]. These correspond respectively to the approximate times at which spreading relocated from the NW Iceland rift to the Snaefellsnes rift, and from the latter to the present spreading centre. We suggest that these troughs represent times at which the Reyk- janes Ridge was starved of plume mantle, just before a ridge relocation. Therefore the troughs, not the ridges, are the significant features of the Reykjanes Ridge. The initiation of the oldest ridge, at around Anomaly 6 (24 Ma), marks the time when the spreading centre first became located over the Ice- land plume.
5. Conclusions
The main focus of spreading in Iceland was lo- cated in the NW Iceland rift zone until N 15 m.y. ago, at which time a new rift zone (Snaefellsnes) had been evolving as a propagating rift to the east. The mantle plume axis was probably to the east of the Snaefellsnes zone, just as the plume centre is now located east of the presently propagating EVZ (East- em Volcanic Zone). Spreading in the Snaefellsnes zone increased to its maximum rate as spreading on the waning NW Iceland rift declined. Subsequently, the Snaefellsnes zone became the main focus of magmatism, although two parallel rift axes were active for some time while rifting in northwest Ice- land persisted, as in Iceland today.
As the Snaefellsnes rift zone became fully active, the main outflow from the mantle plume was fed into this new accretionary axis. Consequently, the mantle beneath the NW Iceland rift became less dominated by plume-derived material and increas- ingly contaminated with N-MORB-source mantle as rifting continued. The final demise of the NW Ice- land rift occurred because spreading gradually ceased there and was taken up by the Snaefellsnes zone. The mixing of N-MORB and plume sources in the wan- ing NW Iceland rift is represented by the oscillating geochemical signatures observed in basaltic lava
flows beneath the unconformity. We cannot say for certain whether this influx of depleted upper mantle is a general feature of dying rifts or is unique to the NW Iceland rift. There is no evidence for such an influx in the present WVZ (Western Volcanic Zone) but this rift has not yet reached its final stage, nor have we yet found any evidence for an N-MORB- source influence in the most recent lavas of the Snaefellsnes rift zone.
While the NW Iceland rift zone was in its declin- ing stages, the lava successions on its flanks were eroded, and later&e-lignite deposits were formed. However, since the Snaefellsnes rift zone had been active for some time, large lava flows from it had occasionally spread over the flat Tertiary landscape to cover the eroded NW-dipping flows on the flanks of the dying NW Iceland rift zone. The products of the two rifts are consequently separated by an uncon- formity representing only a short volcanic hiatus of less than 200 k.y. The flows beneath the unconform- ity still dip offshore towards the NW Iceland rift zone, while those above dip SE towards the now-ex- tinct Snaefellsnes rift zone.
Our data from basalt lavas erupted in the NW Iceland rift zone provide the first clear evidence for the involvement of North Atlantic N-MORB mantle in Icelandic magmatism. The northwest rift basalts therefore strengthen the conclusion [36] that the At- lantic N-MORB source has not contributed signifi- cantly to Icelandic magmatism in more recent times. Compositional variation observed elsewhere in Ice- land, and particularly in the neovolcanic zone, ap- pears to result from mixing between depleted and less depleted components, but the former is not ambient (i.e. Atlantic N-MORB) upper mantle. Both end-members more probably reside within the plume as it impinges on the lithosphere beneath Iceland. We cannot rule out the possibility that the influx of N-MORB-source mantle at N 15 m.y. is due to fluctuations in the output of the Iceland plume (the pulse model), but our observations are better ex- plained by rift relocation caused by the spreading axis drifting away from a steady-state plume.
Acknowledgements
We are grateful to Anne Kelly and Vincent Gal- lagher, who provided expert technical assistance at
B.S. Hardarson et al. / Earth and Planetap Science L.etters 153 (1997) 181-196 195
SURRC. Kristjan Saemundsson at the National En- ergy Authority in Iceland kindly supplied us with core material from Sudureyri. Discussions with M. Nony, N. Oskarsson and K. Saemundsson are much appreciated. We thank S. Steinthorsson, Karl Griinvold, Rex Taylor and B. Hanan for their thoughtful and constructive reviews of the manuscript. This project was funded by NERC (GST/02/673 and support to the Isotope Geology Unit at SURRC), and the Icelandic Research Council (96 1850096). [CL]
References
[I] S.P. Jakobsson, Petrology of recent basalts of the Eastern
volcanic zone, Iceland, Acta Nat. Isl. 26 (1979) 103.
[2] H. Johannesson, Evolution of rift zones in western Iceland
(in Icelandic with an English summary). Nattumfraedin-
gurinn 50 (1980) 13-31.
[3] K. Tryggvason. ES. Huseby, R. Stefansson, Seismic image
of the hypothesized Icelandic hot spot. Tectonophysics 100
(1983) 97-l 18.
[4] N. Oskarsson. S. Steinthorsson, G.E. Sigvaldason. Iceland
geochemical anomaly: origin, volcanotectonics, chemical
fractionation and isotope evolution of the crust, J. Geophys.
Res. 90 (1985) 1001 l-10025.
[5] K. Saemundsson. Outline of the geology of Iceland, Jokull
29 (1980) 7-28.
[6] GE. Vink, A hotspot model for the Iceland and Voting
plateau, J. Geophys. Res. 89 (1984) 9949-9959.
[7] G. Palmason. Crustal rifting and related thermo-mechanical
processes in the lithosphere beneath Iceland, Geol. Rundsch.
70 (1981) 244-260.
[8] G. Palmason, Model of crustal formation in Iceland and
application to submarine mid-ocean ridges. in: M. Talwani,
C.G. Harrison, D.E. Hayes (Eds.). Deep Drilling Results in
the Atlantic Ocean: Ocean Crust, Maurice Ewing Series 2,
American Geophysical Union, 1986. pp. 43-65.
[9] P.R. Vogt. The Iceland mantle plume: status of the hypothe-
sis after a decade of new work, in: M.H.P. Bott, S. Saxov,
M. Talwani, J. Thiede (Eds.). Structure and Development of
the Greenland-Scotland Ridge. New Methods and Concepts,
Plenum. New York, 1983, pp. 191-213.
[ 101 P. Einarsson, Earthquakes and present-day tectonics in Ice-
land, Tectonophysics I89 (1991) 261-279.
[l I] P.R. Vogt, Asthenosphere motion recorded by the ocean
floor south of Iceland. Earth Planet. Sci. Lett. 13 (1971)
153-160.
1121 J.-G. Schilling, P.S. Meyer, R.H. Kingsley, Evolution of the
Iceland hotspot, Nature 296 (1982) 3 13-320.
[13] J.G. Fitton, A.D. Saunders, L.M. Larsen, B.S. Hardarson,
M.J. Norry. Volcanic rocks from the southeast Greenland
margin at 63”N: composition, petrogenesis and mantle
sources, Proc. ODP Sci. Results 152 (in press).
[14] L. Barbero, C. Villaseca, G. Rogers, P.E. Brown, Geochemi-
cal and isotopic disequilibrium in crustal melting: an insight
from the anatectic granitoids from Toledo, Spain. J. Geo-
phys. Res. 100 (1995) 15745-15765.
[I51 B.S. Singer, MS. Pringle, Age and duration of the
Matuyama-Brunhes geomagnetic polarity reversal from
J’Ar/39Ar incremental heating analyses of lavas, Earth
Planet. Sci. Lett. 139 (1996) 47-61.
[16] M.S. Pringle, Age progressive volcanism in the Musicians
seamounts: a test of the hot spot hypothesis for the late
Cretaceous Pacific, in: M.S. Pringle. W.W. Sager. W.V.
Sliter, S. Stein (Eds.), The Mesozoic Pacific Geology, Tec-
tonics and Volcanism, Am. Geophys. Union Geophys.
Monogr. 77 (1993) 187-215.
[17] L. Kristjansson, A. Gudmundsson, H. Haraldsson. Stratigra-
phy and paleomagnetism of a 3-km thick Miocene lava pile
in the Mjoifjordur area, eastern Iceland, Geol. Rundsch. 84
(1995) 813-830.
1181 SC. Cande. D.V. Kent, Revised calibration of the geomag-
netic polarity timescale for the Late Creataceous and Ceno-
zoic, J. Geophys. Res. 100 (1995) 6093-6095.
[19] I. McDougall. L. Kristjansson, K. Saemundsson, Magne-
tostratigraphy and geochronology of northwest Iceland. J.
Geophys. Res. 89 (1984) 7029-7060.
[20] B.S. Hardarson, J.G. Fitton, Mechanisms of crustal accretion
in Iceland, Geology tin press).
[21] C. Htmond. N. Amdt, U. Lichtenstein, A.W. Hofmann, N.
Oskarsson, S. Steinthorsson. The heterogeneous Iceland
plume: Nd-Sr-0 isotopes and trace element constraints. J.
Geophys. Res. 98 (1993) 15833-15850.
[22] H. Nicholson. The magmatic evolution of Krafla. NE Ice-
land, Ph.D. Thesis, Univ. Edinburgh, 1990.
1231 T. Furman. F.A. Frey, K.-H. Park. Chemical constraints on
the petrogenesis of mildly alkaline lavas from Vestmannaey-
jar, Iceland: the Eldfell (1973) and Surtsey (1963-1967)
eruptions. Contrib. Mineral. Petrol. 109 (1991) 19-37.
[24] T.R. Elliott, C.J. Hawkesworth. K. Gronvold, Dynamic melt-
ing of the Iceland plume, Nature 35 I ( 1991) 201-206.
[25] B.S. Hardarson, Alkalic rocks in Iceland with special refer-
ence to the Snaefellsjokull volcanic system, Ph.D. Thesis.
Univ. Edinburgh, 1993.
1261 A. Zindler, S.R. Hart, F.A. Frey, S.P. Jakobsson. Nd and Sr
isotope ratios and rare element abundances in Reykjanes
peninsula basalts: evidence for mantle heterogeneity beneath
Iceland. Earth Planet. Sci. Lett. 45 (1979) 249-262.
1271 S.-S. Sun, M. Tatsumoto. J.-G. Schilling. Mantle plume
mixing along the Reykjanes ridge axis: Lead isotope evi-
dence, Science 190 (1975) 143-147.
[28] S.-S. Sun, B. Jahn, Lead and strontium isotopes in post-gla-
cial basalts from Iceland, Nature 255 (1975) 527-530.
1291 K.-H. Park, Sr, Nd and Pb isotope studies of ocean island
basalts. Constraints on their origin and evolution. Ph.D.
Thesis. Columbia Univ., 1990.
[30] R.S. Cohen, N.M. Evensen, P.J. Hamilton. R.K. O’Nions.
196 B.S. Hardarson et al./ Earth and Planetary Science Letters 153 (1997) 181-196
U-Pb, Sm-Nd and Rb-Sr systematics of mid-ocean ridge
basalt glasses, Nature 283 (19801 149-153.
[31] B. Dupre, C.J. Allegre, Pb-Sr-Nd isotopic correlation and
the chemistry of the North Atlantic mantle, Nature 286
(19801 17-22.
of the high-latitude North Atlantic mantle, Geology 25 (1997)
411-414.
[32] S.-S. Sun, Lead isotope study of young volcanic rocks from
mid-ocean ridges, ocean islands and island-arcs, Philos.
Trans. R. Sot. London A297 (1980) 409-445.
[33] E. Ito, W.M. White, C. Gopel, The 0, Sr, Nd and Pb isotope
geology of MORB, Chem. Geol. 62 (19871 157-176.
[34] J.-G. Schilling, Iceland mantle plume: Geochemical study of
the Reykjanes Ridge, Nature 242 (19731 565-57 I. [35] R.N. Taylor, B.J. Murton, M.F. Thirlwall, Petrographic and
geochemical variation along the Reykjanes Ridge, 57”N-
59”N, J. Geol. Sot. London 152 (19951 1031-1037.
[36] M.F. Thirlwall. Generation of the Pb isotopic characteristics
of the Iceland plume. J. Geol. Sot. London 152 (19951
992-996.
1391 J.G. Fitton, A.D. Saunders, M.J. Non-y, B.S. Hardarson, R.N.
Taylor, Thermal and chemical structure of the Iceland plume,
Earth Planet. Sci. Lett. 153 (19971, this issue.
1401 W.F. McDonough. S.-S. Sun, The composition of the Earth,
Chem. Geol. 120 (1995) 223-253.
[41] S. Moorbath, H. Sigurdsson, R. Goodwin, K-Ar ages of the
oldest exposed rocks in Iceland, Earth Planet. Sci. Lett. 4
(1968) 197-205.
[42] L. Kristjansson, G. Jonsson, Aeromagnetic results and the
presence of an extinct rift zone in western Iceland, J. Geo-
dyn. (in press).
[43] L. McDougall, K. Saemundsson, H. Johannesson, N.D.
Watkins, L. Kristjansson, Extension of the geomagnetic po-
larity time scale to 6.5. m.y. K-Ar dating, geological and
paleomagnetic study of a 3500 m lava succession in western
Iceland. Geol. Sot. Am. Bull. 88 (1977) l-15.
[37] A.C. Kerr, A.D. Saunders, J. Tamey, N.H. Berry, V.L. [44] R.S. White, J.W. Brown, J.R. Smallwood, The temperature
Hards, Depleted mantle-plume geochemical signatures: No of the Iceland plume and origin of outward-propagating
paradox for plume theories, Geology 23 (1995) 843-846. V-shaped ridges, J. Geol. Sot. London 152 (1995) 1039-
[38] D.F. Mertz, K.M. Haase, The radiogenic isotope composition 1045.
