JOURNAL OF PETROLOGY
VOLUME 39
NUMBER 5
PAGES 819–839
1998
Crustal Processes: Major Controls on
Reykjanes Peninsula Lava Chemistry,
SW Iceland
M. A. M. GEE1∗, M. F. THIRLWALL1, R. N. TAYLOR2, D. LOWRY1
AND B. J. MURTON3
1
DEPARTMENT OF GEOLOGY, ROYAL HOLLOWAY, UNIVERSITY OF LONDON, EGHAM, SURREY TW20 0EX, UK
2
UNIVERSITY OF SOUTHAMPTON, SOUTHAMPTON OCEANOGRAPHY CENTRE, EMPRESS DOCK, EUROPEAN WAY,
SOUTHAMPTON SO14 3ZH, UK
3
SOUTHAMPTON OCEANOGRAPHY CENTRE, EMPRESS DOCK, EUROPEAN WAY, SOUTHAMPTON SO14 3ZH, UK
RECEIVED APRIL 7, 1997; REVISED TYPESCRIPT ACCEPTED JANUARY 14, 1998
Three hundred stratigraphically constrained samples from the Reykjanes Peninsula, SW Iceland, provide the basis for this study. This
area is an elevated section of mid-ocean ridge influenced by the
Iceland Plume. Selected chemical, Sr, Nd and laser-assisted fluorination oxygen isotope data are presented. The dataset is subdivided
into groups based on criteria which are independent of degree of
fractionation and petrography. Two of these groups, Depleted and
Stapafell, include high-MgO aphyric samples with d18Oolivine values
in equilibrium with normal peridotite mantle. Depleted group samples
have high 143Nd/144Nd, low Nb/Zr and low incompatible element
abundances compared with the dataset as a whole, the reverse of
the Stapafell group. The majority of the remaining samples have
radiogenic isotope ratios, and incompatible element concentrations
and ratios intermediate between the Depleted and Stapafell groups.
Some samples, however, define a range in 87Sr/86Sr and d18Oolivine
at constant 143Nd/144Nd, and others possess positive Sr anomalies
when normalized to primitive mantle values. We explore the
possibility that these and other chemical characteristics have been
produced by shallow crustal processes, including assimilation of
xenocrysts, cumulates and hydrothermally modified crust. We conclude that although these processes are important, the major crustal
process acting to modify characteristics indicative of mantle heterogeneity is magma mixing. Chemical variation previously thought to
be a consequence of dynamic melting is more readily explained by
magma mixing.
∗Corresponding author. Telephone: 44(0)1784 443581. Fax: 44(0)1784
471780. e-mail: [email protected]
KEY WORDS:
assimilation; Iceland; oxygen isotopes; radiogenic isotopes;
basalts
INTRODUCTION
Volcanic activity in Iceland takes place primarily through
crust younger than 0·7 Ma along neovolcanic zones
regarded as the present location of the Mid-Atlantic
Ridge (Fig. 1). Although tholeiitic basalts are dominant
in these regions of crustal generation, there is a higher
incidence of acid and intermediate rocks than is usually
associated with mid-ocean ridges (MOR). Silicic lavas
are concentrated in central Iceland and decrease towards
the northern and southwestern extremities of the rift
zone. The remaining Icelandic crust away from the axial
rift system, the marginal zone, is between 0·7 and 15
Ma old (Moorbath et al., 1968).
The ~3000 m elevation above normal mid-ocean
ridges, and increased thickness (~15–35 km) of Icelandic
crust compared with normal oceanic crust (e.g. Bott,
1988; Bjarnasson et al., 1993; White et al., 1996), regional
broadband seismic data (Wolfe et al., 1997), and particular
aspects of the lava chemistry are all cited as evidence for
a mantle plume centred beneath central–east Iceland
(Sigvaldason et al., 1974; Tryggvason et al., 1983). There
 Oxford University Press 1998
JOURNAL OF PETROLOGY
VOLUME 39
NUMBER 5
MAY 1998
Fig. 1. Sketch map of the Reykjanes Peninsula. The three most southwesterly geothermal fields are shown as shaded ellipses. Locations of
samples for which data are presented in Table 1 are circled and labelled with the appropriate sample number. Inset: sketch map of Iceland.
Shaded areas represent the main neovolcanic zones. Vatnajökull approximates to the centre of the Iceland Plume.
is a long history of research on Iceland regarding aspects
of the mantle plume (e.g. Jakobsson, 1972; Schilling,
1973; O’Nions et al., 1977; Jakobsson et al., 1978; Zindler
et al., 1979; Imsland, 1983; Hemond et al., 1993). Studies
based on the chemical and isotopic composition of the
basalts have focused on the thermal, compositional and
dynamic structure of the Iceland Plume, its contribution
to the depleted upper-mantle reservoir over the past ~60
my, and its role in the evolution of the North Atlantic
Basin.
The isotopic and chemical heterogeneity observed in
axial basalts, however, need not necessarily be a true
reflection of mantle sources and processes. Crustal processes such as assimilation and fractional crystallization
are capable of extensive modification of mantle characteristics. Both are processes more usually associated
with the evolved basalts and silicic rocks of central
volcanoes; for example, Macdonald et al. (1987, 1990) and
Furman et al. (1991, 1992) required varying contributions
from crustal melting or assimilation and fractional crystallization in their studies of central volcanoes in Iceland.
Similar processes have been proposed to account for the
range of basalts erupted in the neovolcanic zones (e.g.
Óskarsson et al., 1982, 1985; Steinthórsson et al., 1985).
In addition, the elements Rb, Sr, U, K, Na, Ba and Pb
may be variably mobilized in the different hydrothermal
and metamorphic facies through which the crust passes
as it subsides and moves away from the ridge axis. This
could also give rise to substantial heterogeneity in Sr
isotope ratios, especially in those areas where seawater
is present in the hydrothermal systems. Óskarsson et al.
(1985) proposed that variable degrees of metamorphism
in association with lower-crustal melting would give rise
to chemical stratification in the crust, more specifically,
elevated large ion lithophile element concentrations in
the upper layers. Periodic ridge jumps eastwards towards
the plume centre could result in rifting and volcanism
initially taking place in the older, thicker, chemically
stratified crust. Óskarsson et al. (1985) concluded that
assimilation of old, variably hydrothermally altered crust
by melts from a depleted mantle source could produce
the range of isotope and trace element ratios observed
on Iceland today.
More recently, Hemond et al. (1993) proposed that a
crustal component was responsible for some of the chemical ‘anomalies’ in the lavas from the neovolcanic zones.
820
GEE et al.
REYKJANES PENINSULA LAVA CHEMISTRY
They suggested that positive Sr, Ba and Rb anomalies
(normalized to primitive mantle) in some primitive Icelandic rocks require addition of variably hydrothermally
altered crustal material. The hydrothermal component
in this contaminant was thought to be responsible for
the elevated Sr, Ba and Rb, increased 87Sr/86Sr ratios
and low d18O values of some Reykjanes Peninsula lavas.
However, they concluded that ‘these processes are secondary and of lesser importance than variations in mantle
source compositions’.
A crustal component has thus been invoked to explain
a wide variety of chemical features in Icelandic lavas.
These same lavas are currently being used to explore the
composition and origin of the mantle source regions
beneath Iceland and associated MORs. Consequently, it
is very important to be aware of any non-mantle component in these lavas. High 87Sr/86Sr at a given 143Nd/
144
Nd is a characteristic of many Icelandic lavas and may
be a consequence of mixing between mantle sources with
normal and elevated Sr/Nd ratios (e.g. Taylor et al.,
1997). If, however, the positive Sr anomalies (i.e. high
Sr/Nd) in some Reykjanes Peninsula lavas are the result
of intracrustal processes (Hemond et al., 1993), then the
characteristic high 87Sr/86Sr in Iceland may also reflect
crustal processes.
The aim of this paper is to explore the chemistry of a
suite of lavas from the Reykjanes Peninsula, to determine
which chemical characteristics, if any, have been influenced by crustal processes. Previous studies (e.g. Condomines et al., 1983; Elliott et al., 1991; Hemond et al.,
1993) have used small sample suites that may be biased
towards certain petrographic types or particular time
periods, particularly Recent, or post-glacial, lavas. This
is not the case in this study. We have, for the most part,
restricted comparison of chemistry and isotope ratios to
our sample suite to avoid complications arising from any
spatial change in mantle composition under Iceland, and
any inter-laboratory variations in analyses.
Geology of the Reykjanes Peninsula
The Reykjanes Peninsula, SW Iceland (Fig. 1), is the onland continuation of the Reykjanes Ridge section of the
Mid-Atlantic Ridge. Volcanism takes place mainly within
five fissure systems ( Jakobsson, 1972; Pàlmason &
Sæmundsson, 1974). A high degree of obliquity between
the ridge axis at ~090°E and the plate spreading direction
of ~110°E induces a right-stepping, en-echelon arrangement of the fissure systems. The only occurrence
of acid lavas on the Reykjanes Peninsula is at Hengill, the
most northeasterly fissure system, which is not included in
our study.
Highly permeable rock formations, tectonism, low
elevation and high precipitation combined with high heat
flow from the ridge axis result in active high-temperature
hydrothermal systems. These systems are localized at the
surface by spreading-direction-parallel fractures, producing alteration which varies from ‘spotting’ of the
basalt to complete replacement by clay minerals. There
are four main geothermal fields on the Reykjanes Peninsula (Arnórsson, 1978), which show a decrease in fluid
salinity with distance from the SW of the area. This may
result from a decreasing seawater contribution to the
geothermal fluids, or a decrease in the evaporation rate
of the fluid (Sveinbjornsdottir et al., 1986). Elderfield &
Greaves (1981) reported a range in 87Sr/86Sr of 0·7037–
0·7042 for geothermal fluids and hydrothermally altered
rocks from the two southwestern geothermal fields, significantly higher than the maximum of 0·7033 in the
lavas studied here.
Sub-glacial eruptions in Iceland are associated with
contained melt-water lakes (which may increase the meteoric contribution to hydrothermal systems), and result
in pillow mounds and associated hyaloclastite aprons;
the latter are commonly altered to palagonite. Eruptive
episodes have taken place at intervals of roughly
1000 years on the Reykjanes Peninsula, and the more
recent subaerial events have successively infilled volcanic
topography formed during glacial periods. However,
sinuous pillow and hyaloclastite ridges, and coalesced
pillow mounds remain as topographic highs, e.g. Núpshı́darháls, and the highest point, Langhóll, 391 m (Fig. 1).
Additionally, isolated conical tips of pillow mounds such
as Keilir still maintain an elevation of 200 m above the
surrounding lava plains. This variation in eruptive style
preserves older flows which, if all volcanism had been
subaerial, would have been buried as a result of crustal
subsidence at the ridge axis (Pàlmason, 1980, 1986).
Detailed field work, over two field seasons, involved
sampling all the main flows representative of the ranges
in age, petrography, eruptive morphology and type, and
has resulted in the most complete sample suite and
geochemical data set for the Reykjanes Peninsula to date.
Although individual flows are rarely laterally continuous,
field relationships identified both during this and previous
studies (e.g. Jakobsson, 1972; Sæmundsson & Einarsson,
1980) allow a reasonable relative stratigraphy to be
erected. This has been assisted by the identification of a
palaeomagnetic excursion correlated with the Las
Champs event ~46 000 years ago (Levi et al., 1990).
Approximately 80% of the samples are younger than this
excursion; the remainder are of excursion age, or their
age is uncertain but less than 700 000 years. This stratigraphically constrained suite of 300 samples forms the
basis of this study.
All lavas are tholeiitic basalts with olivine and plagioclase as the dominant phenocryst phases. All crystals
larger than the groundmass are referred to as phenocrysts,
but this does not necessarily mean that they are in situ
821
JOURNAL OF PETROLOGY
VOLUME 39
products of crystallization. Clinopyroxene is not seen as
a phenocryst, even in evolved basalts (<6 wt % MgO).
However, fragments of clinopyroxene macrocrysts are
seen in sample RP3 (Table 1). Xenoliths, which we
interpret as magmatic cumulates, are locally common,
and usually found in conjunction with highly phyric,
low-MgO basalts (~6 wt % MgO) containing >25%
phenocrystic plagioclase. We have included five gabbroic
xenoliths in the dataset. Troctolites and anorthosites are
less common, and are represented by a single sample of
each type. All the cumulate xenoliths are coarse grained
and there is no evidence of strained extinction in olivine,
plagioclase or clinopyroxene.
Where collection of a lava stratigraphy was possible,
flow-by-flow analyses of single edifices have been carried
out, including, to a lesser extent, Sr and Nd isotope
stratigraphy. The results show that individual flow units
from the same vent, where there is no visible evidence
for substantial time intervals between eruption, generally
have the same incompatible trace element and isotopic
ratios. There is a notable exception to this at Vatnsheidi
(Fig. 1), a group of associated craters ~3 km northeast
of Grindavı́k. The suite of lavas analysed from Vatnsheidi
shows a range in trace element and isotope ratios
(Table 1), which will be discussed further below.
GEOCHEMISTRY OF REYKJANES
PENINSULA LAVAS
Subdivisions of the Reykjanes Peninsula
samples
Previous workers on the Reykjanes Peninsula have divided samples into categories based on many criteria
including style and petrography of eruptive units, CIPW
norms, and mg-numbers (e.g. Jakobsson et al., 1978; Meyer
et al., 1985; Hemond et al., 1993). We have defined groups
by chemical characteristics which are independent of the
extent of fractional crystallization. It has already been
stated that the range in incompatible element concentrations in lavas from the Reykjanes Peninsula cannot
be generated solely by crystal fractionation (e.g. Wood,
1979; Hemond et al., 1993). Rather a variation in melting
processes and/or sources is required. This is illustrated
by the range of variation of Nb/Zr in Fig. 2a, where
samples with ~10 wt % MgO span the full range of Nb/
Zr. Whereas Nb/Zr does not vary systematically with
MgO content, there is a correlation between Nb/Zr and
incompatible element abundance; those samples with
higher Zr concentrations generally have the highest Nb/
Zr ratios (Fig. 2b). Similar relationships are observed for
other ratios of very incompatible to moderately incompatible elements. We focus on Nb and Zr as they
have high enough concentrations to be precisely analysed
by X-ray fluorescence (XRF).
NUMBER 5
MAY 1998
Samples with the lowest Nb/Zr ratios in the dataset
(Fig. 2b), 0·07 or less, with a mean of 0·037 ± 0·02
(2 SD, n = 19) at Zr concentrations between 10 and 30
ppm are referred to as the Depleted group. One of these
samples is BIR-1, the depleted basalt USGS standard.
Eighteen other samples come from 11 edifices across the
Reykjanes Peninsula, and are represented by RP95A,
RP95C and RP80D (data for all samples referred to by
sample numbers in this study are presented in Table 1).
Samples in this group are from volcanoes which do not
show a significant range in isotope or trace element
ratios, irrespective of the degree of differentiation or
crystal accumulation. This is demonstrated by two flows
from Làgafell (Fig. 1): RP95A is highly accumulative with
~35% olivine macrocrysts, but has the same trace element
and isotope ratios as RP95C, which has <1% olivine
phenocrysts (Table 1).
A suite of 14 samples from one large pillow mound,
Stapafell (Fig. 1), has a mean Nb/Zr ratio of 0·157 ±
0·004 (2SD, n = 14), at between 70 and 100 ppm Zr.
These samples, termed the Stapafell group, do not have
the highest Nb/Zr in the dataset, but they closely resemble
other high Nb/Zr samples (e.g. RP115J). This group has
been chosen to represent the high Nb/Zr samples in this
dataset because it includes both high-MgO cumulate
lavas, and aphyric high-MgO samples such as RP67E
(Fig. 2a). This allows a closer comparison with the highMgO rocks of the Depleted group without the need
for phenocryst dilution or enrichment corrections to be
applied.
In addition to the definition of these two sample groups
by trace element ratios, they also have distinctive Nd
and Sr isotope ratios. The Depleted group has 87Sr/86Sr
<0·70311 and 143Nd/144Nd >0·51307; the Stapafell group
has 87Sr/86Sr of 0·70317 and 143Nd/144Nd of 0·51301
(Fig. 3).
Samples from the Reykjanes Peninsula have featured
in many studies (e.g. Wood, 1979; Zindler et al., 1979;
Elliott et al., 1991) because the basalts have not undergone
extensive crystal fractionation. Some high-MgO lavas
have up to ~30 wt % MgO (Fig. 2a), but these highMgO lavas do not represent liquids. Instead they have
accumulated up to 40% olivine, or olivine and plagioclase.
However, as demonstrated by samples RP67E and
RP95C from the Stapafell and Depleted groups, respectively, not all magnesian flows are accumulative, and
aphyric or sparsely phyric flows (<1% phenocrysts) with
11–13 wt % MgO are not uncommon.
The only samples in this study that are specifically
identified because of their petrography or field relationships are the Cumulates and the Vatnsheidi suite.
Represented by RP63O in Table 1, the Cumulates have
Nb/Zr ratios which range from 0·08 to 0·13 (Fig. 2b),
at very low Zr abundances relative to the dataset as a
whole (six out of the seven samples have <8 ppm Zr).
822
Vatnsheidi
Stapafell
Others
RP82C
RP67E
RP56A
823
Svartsengi
Meradalur
Bæjarfell
Helgavik
Sandfellsheidi
Stapafell
Vatnsheidi
Vatnsheidi
Vatnsheidi
Vatnsheidi
Vatnsheidi
Halayjabunga
Lagafell
Lagafell
Grænavatn
Location
±0·25
48·82
46·92
49·52
46·21
47·74
46·56
48·21
47·68
48·00
47·55
45·22
47·09
48·02
44·65
48·05
SiO2
13·64
14·81
14·71
15·35
15·31
12·90
15·31
15·84
15·57
15·52
13·40
13·93
14·94
8·90
26·99
Al2O3
±0·007 ±0·11
1·968
1·760
1·856
1·830
1·270
1·467
0·865
0·707
0·662
0·493
0·363
0·394
0·481
0·266
0·739
TiO2
±0·10
14·64
12·67
14·37
13·39
11·75
12·42
10·76
10·18
9·83
9·08
9·82
9·18
9·03
9·98
5·33
Fe2O3
±0·08
6·89
9·70
5·50
9·20
9·12
13·87
10·00
11·17
11·22
12·41
18·25
16·29
11·59
27·83
1·85
MgO
±0·004
11·19
11·42
10·68
11·25
12·35
10·48
12·77
12·70
13·08
13·17
10·65
11·76
13·60
7·87
14·78
CaO
0·8%
161
215
139
219
148
173
93
81
84
75
55
52
47
26
230
Sr
12·5
13·9
14·6
10·2
5·9
13·3
3·5
2·6
1·9
1·1
0·5
0·5
0·8
0·5
3·2
Nb
1·5% ±0·1
105·3
90·1
143·2
94·9
60·2
84·3
39·4
31·5
26·6
17·9
11·6
14·3
17·0
9·6
30·9
Zr
0·8%
14
12·47
16·18
11·85
11·53
4·74
1·66
1·89
Nd
Sr/86Sr
0·703123±11
0·703295±16
0·703128±10
0·703223±11
0·703066±10
0·703177±09
0·703187±11
0·703251±12
0·703295±11
0·703194±11
0·703096±10
0·703078±12
0·702888±11
0·702888±08
0·703095±25
87
Nd/144Nd
0·513044±6
0·513008±5
0·513063±5
0·513027±5
0·513065±5
0·513011±4
0·513051±6
0·513094±5
0·513145±5
0·513159±5
0·513060±5
143
±0·1
4·61
3·88
3·68
5·00
4·94
4·64
4·57
4·95
4·92
5·08
5·07
4·44
‰ SMOW
d18Oolivine
aphyric
aphyric
2% P
aphyric
aphyric
aphyric
aphyric
2% O
2% O P
3% O P
30% O P C
20% O P
aphyric
35% O
phenocrysts
Total (%)
∗Total iron as Fe2O3.
All analyses were determined at the Department of Geology, Royal Holloway University of London. The locations of these samples have been marked in Fig. 1.
All concentrations are reported on a volatile-free basis, totals are between 99·27 and 100·44, loss on ignition (at 1100°C) between 0 and –1%. XRF techniques are
as described by Thirlwall et al. (1997); but with extended count times for Sr, Zr and Nb. Sr and Nd isotopes, and Nd concentrations were measured on a VG354
mass spectrometer, using techniques described by Thirlwall (1991a, 1991b). Errors quoted on isotope ratios are within run 2 SE on the final quoted significant
figures. 87Sr/86Sr is expressed relative to SRM987 = 0·710248 and 143Nd/144Nd relative to La Jolla = 0·511856. External reproducability is better than ±0·000020
2SD for 87Sr/86Sr and better than ±0·000008 2SD for 143Nd/144Nd. Sr and Nd isotopes are normalized to 86Sr/88Sr = 0·1194 and 146Nd/144Nd = 0·7219, respectively.
All Sr isotope analyses are determined on the powder leached for 12 h in hot 6 M HCl. Oxygen isotopes are expressed as d18Oolivine relative to SMOW, determined
on olivine and clinopyroxene mineral separates and, where possible, glass. Mean oxygen yield was 100·1 ± 3·4 (2SD, n = 49). The Nd:YAG laser-fluorination
system and VG Isotech PRISM mass spectrometer are calibrated against V-SMOW and NBS30 biotite, and data for each day were corrected to d18O of +4·88‰
for San Carlos olivine (equivalent to NBS30 biotite of +5·10‰; Mattey & Macpherson, 1993), an internal standard run at least three times with 12 unknowns: this
correction was at most ±0·25‰. If total modal phenocryst content is less than ~1%, samples have been termed aphyric. Phenocryst phases: O, olivine; P,
plagioclase; C, clinopyroxene.
2SD error
Others
Vatnsheidi
RP82A
RP117A
Vatnsheidi
RP82D
Others
Vatnsheidi
RP82B
RP115J
Vatnsheidi
RP3
Others
Depleted
RP80D
RP65H
Depleted
RP95C
Others
Depleted
RP59F
Cumulate
RP95A
Group
RP63O
no.
Sample
Table 1: Selected major and trace elements, and isotope ratios of samples representative of the groups discussed in this paper
GEE et al.
REYKJANES PENINSULA LAVA CHEMISTRY
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VOLUME 39
NUMBER 5
MAY 1998
Fig. 2. This outlines the distinctions between the three groups, Depleted, Stapafell and Cumulates, the Vatnsheidi suite, and the rest of the
dataset, referred to in the legend as ‘Others’. All groups will be represented by the same symbols throughout this paper for ease of crossreferencing; the Vatnsheidi suite are individually identified by sample numbers. Error bars are indicated in (b) for the lowest concentrations in
the Depleted and Cumulate group; in all other samples errors are covered by the symbol or label size. (a) Nb/Zr vs MgO. Samples with >13
wt % MgO have accumulated olivine. Noteworthy features are the variation of Nb/Zr ratios at a similar MgO, e.g. 10 wt %, and the clustering
of the data at moderate Nb/Zr ratios and low MgO relative to the whole dataset. The Stapafell and Depleted reference samples are indicated.
(b) Nb/Zr vs Zr. Noteworthy features are the moderate Nb/Zr at low Zr concentrations in the Cumulates, and the general trend of increasing
Nb/Zr ratios with increasing Zr. Those samples defining a horizontal trend to higher Zr at constant Nb/Zr are generally increasingly evolved.
Olivine accumulation in the Depleted samples has no observable effect on Zr concentration.
In Fig. 2b the five samples of the Vatnsheidi suite are
identified in order of increasing Nb/Zr, V1 being the
lowest. If samples V1–V3 were not in this suite, they
would have been included in the Depleted group. Data
for the five samples are included in Table 1, where it
can be seen that the samples also display a range in Sr
and Nd isotope compositions (Fig. 3). Sample V1 is highly
accumulative, with a higher proportion of plagioclase
phenocrysts than Depleted group samples. It also has rare
824
clinopyroxene phenocrysts, again unlike the Depleted
group.
All of the remaining samples not included in any of
the groups outlined above are undifferentiated for the
purposes of this paper, and referred to as ‘Others’ in all
graphs. Except for ~10 samples with Nb/Zr >0·157,
these samples have incompatible trace element and Nd
isotopic ratios intermediate between the Stapafell and
Depleted groups, e.g. mean Nb/Zr 0·127 ± 0·038 (2SD,
GEE et al.
REYKJANES PENINSULA LAVA CHEMISTRY
Fig. 3. 143Nd/144Nd vs 87Sr/86Sr. This illustrates the Depleted group’s range in Nd isotopes. Bulk mixing lines are shown between the Depleted
and Stapafell reference samples, RP95C and RP67E, and hydrothermally altered crust with 300 ppm Sr, 10·6 ppm Nd, 87Sr/86Sr of 0·7036 and
143
Nd/144Nd of 0·51305. The dotted mixing line is between the Depleted and Stapafell reference samples plus 5% and 20% hydrothermally
altered crust, respectively. Error bars represent ±2SD standard reproducibility.
n = 250; Figs 2 and 3). Data for four of these samples
are presented in Table 1. RP56A is ~8000 years old and
comes from the same fissure system as the Depleted
samples RP95A and RP95C. RP59F is an old largevolume flow, now preserved off-axis. RP117A is an
evolved lava from the most recent eruptive cycle on the
Reykjanes Peninsula, ~1000. RP65H is an evolved
lava with moderate radiogenic isotope and trace element
ratios, and a low d18Oolivine value.
Overview of major and trace element data
RP95C and RP67E (Table 1) have been selected as
reference samples for the Depleted and Stapafell groups
primarily because of their aphyric, high-MgO nature.
This is doubly advantageous: in the Depleted group
incompatible element concentrations are as high and
therefore as precise as possible, and in both groups wholerock chemistry in a sparsely phyric sample is less likely
to be affected by any xenocryst or phenocryst content.
This may not be a problem where olivine is the accumulated phase, because of the low partition coefficients
of olivine for incompatible elements, but plagioclase could
exert a major control on Sr content and isotopic ratios
especially in the low abundance rocks (e.g. Hemond et
al., 1993).
The data presented have not been corrected for crystal
fractionation as this would require a series of assumptions
to be made about the sources and processes influencing
the lavas. For example, it should not be assumed that
the mantle source of the Depleted group is as fertile as
that of the Stapafell group. In this paper we demonstrate
the effects of crystal fractionation and accumulation on
a scale which ranges from intra-edifice to intra-flow
differentiation, the latter resulting in cumulate bases and
aphyric flow tops even in flows <20 cm thick. Olivine is
the main fractionating phase, and addition of up to 40%
and 20% olivine to the Depleted and Stapafell reference
samples, respectively, can account for the higher MgO
samples in these groups (and in the Vatnsheidi suite;
Fig. 4a). Plagioclase is cotectic with olivine from ~11 wt
% MgO (Fig. 4b). Clinopyroxene does not appear to
influence lava chemistry on the Reykjanes Peninsula.
Relative to the Stapafell group, the Depleted group has
higher Al2O3, SiO2 and CaO, and lower Fe2O3, TiO2,
Na2O, K2O and P2O5 at the same MgO (e.g. Fig. 4).
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Fig. 4. (a) Fe2O3 vs MgO. The lines represent simple addition of equilibrium olivine to the two (aphyric) reference samples from the Stapafell
and Depleted groups, RP67E and RP95C. The amount of olivine required to produce the range to high MgO in both groups is comparable
with the amount observed in the accumulative samples. (b) Al2O3 vs MgO, with mixing lines to both equilibrium olivine and plagioclase, with
20% increment tick marks. The tick marks with mixing quantities indicated are on the Stapafell trend. It should be noted that the undifferentiated
samples with the lowest MgO trend towards plagioclase, as a result of accumulation of up to 30% plagioclase in many of these samples. The
high Al2O3 of the Cumulate group is a function of high modal plagioclase.
The Vatnsheidi Suite follows the trends of the Depleted
group.
Elliott et al. (1991) and Hemond et al. (1993) observed
a correlation between petrographic type, e.g. picrite and
tholeiite, and incompatible element abundance. In their
datasets high-MgO samples have the lowest incompatible
element abundances, the least radiogenic Sr, and the
most radiogenic Nd. As can be seen in Figs 2, 3 and 4,
such simple relationships are not observed in this study,
and may be the result of unrepresentative sampling.
There are no low-MgO Depleted samples, but the same
is true of the Stapafell group, and other samples with
Nb/Zr >0·15 (Fig. 2a). Evolved lavas with less than ~9
wt % MgO have Nb/Zr lower than the Stapafell group
and higher than the Depleted group (Fig. 2a).
When elements are normalized to primitive mantle
concentrations (Fig. 5), all samples share a similar pattern
for the very incompatible elements: Nb is enriched relative
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GEE et al.
REYKJANES PENINSULA LAVA CHEMISTRY
Fig. 5. Primitive mantle normalized [values from Hofmann (1988)] incompatible element diagram. Order of elements as given by Hemond et
al. (1993). Rb and K in the Depleted group are subject to the errors indicated.
to La–K, and K and Rb are depleted relative to La, Ba
and U. Sr is anomalously high in the Depleted reference
sample, RP95C, and low in the evolved sample, RP65H.
The Stapafell reference sample, RP67E, has no Sr anomaly. The Vatnsheidi sample, RP82C (V5), has an
almost flat rare earth pattern, but still maintains the same
relationships between the very incompatible elements
seen in the other samples, and has a positive Sr anomaly.
These Sr anomalies are very unlikely to be the result of
surface alteration processes, as Gurenko et al. (1991) and
Sobolev et al. (1994) reported the presence of ultradepleted melt inclusions with positive Sr anomalies in
olivines taken from some Reykjanes Peninsula picrites.
Relative to Sm and Eu, Zr is anomalously high in the
Stapafell reference sample, RP67E, and the evolved
sample RP65H, and low in the Depleted reference sample
RP95C.
This dataset does not show any examples of the Rb
enrichment observed by Hemond et al. (1993), but instead
shows Rb depletion, which like the K depletion is present
in all sample groups, and is not merely a function of low
concentrations in the Depleted group.
high Nb/Zr samples (Fig. 6a, b). It is noticeable that
samples with higher 87Sr/86Sr ratios than the Stapafell
group do not have correspondingly lower 143Nd/144Nd
ratios (Figs 3 and 6a). The range in the Vatnsheidi suite
is best seen in Fig. 6a, where the samples follow a trend
towards higher 87Sr/86Sr ratios at lower Nb/Zr ratios
than the bulk of the data. Sample V3 has the highest
87
Sr/86Sr ratio of the whole sample suite, but a low Nb/
Zr that is not even the highest of the Vatnsheidi suite.
Samples V1 and V5 maintain the same relationship in
both Sr and Nd isotope ratios relative to Nb/Zr (Fig. 6).
The Cumulates have Sr and Nd isotope ratios typical of
the evolved samples, and they have moderate Nb/Zr
ratios, ~0·11 (Fig. 6).
The general correlations between Nb/Zr and both Sr
and Nd isotope ratios (Fig. 6) also exist for other ratios
of more incompatible/less incompatible elements, such
as Ba/Zr (not shown). Major element ratios, for instance,
Al2O3/TiO2 and CaO/Al2O3 (not shown), also show
good correlations with radiogenic isotopes.
Overview of oxygen isotope data
Sr and Nd isotope data
As all samples are effectively zero-age, no corrections for
radiogenic in-growth have been applied. 143Nd/144Nd
ranges from 0·513151 to 0·513008, and broadly correlates
with the range in 87Sr/86Sr from 0·70289 to 0·70330
(Fig. 3). The scatter on this correlation is significantly
outside analytical error. The Depleted group covers a
wider range in both Sr and Nd isotope ratios than the
Whole-rock d18O values for tholeiites in Iceland vary
from ~+3 to +6·25 (e.g. Muehlenbachs et al., 1974;
Hemond et al., 1993). In the dataset of Hemond et al.
(1993) lavas with 87Sr/86Sr > ~0·703150 do not have
whole-rock d18O values in equilibrium with normal peridotite mantle (+5·5 to +6·0‰ whole rock). There is a
correlation between degree of evolution, increasing 87Sr/
86
Sr and decreasing d18O values. However, there is a
noticeable absence of the expected negative correlation
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Fig. 6. Mixing curves between the Depleted and Stapafell group reference samples are shown in both graphs. (a) 87Sr/86Sr vs Nb/Zr. Although
there is a general positive correlation, it should be noted that high 87Sr/86Sr is present over a wide range in Nb/Zr. Also noteworthy is the Nb/
Zr ‘gap’ between the Depleted group and the majority of the remaining samples. (b) 143Nd/144Nd vs Nb/Zr. The ‘scatter’ to high 87Sr/86Sr in
(a) is not accompanied by scatter to low 143Nd/144Nd, except in the Depleted group.
between 143Nd/144Nd and d18O, considering the correlation between 87Sr/86Sr and 143Nd/144Nd.
We report in Table 1 new oxygen isotope data on
mineral separates from Reykjanes Peninsula lavas obtained by laser fluorination methods (Mattey & Macpherson, 1993). Data are presented as equivalent d18Oolivine
values, as >95% of these analyses are on olivines. Clinopyroxene data are mainly from the Cumulates and have
been corrected using measured clinopyroxene–olivine
fractionation. d18O values of +5·0 to +5·5‰ have been
obtained by both laser fluorination and conventional
methods for olivine from mantle peridotites, mid-ocean
ridge basalts (MORB) and lunar rocks (Muehlenbachs
& Clayton, 1972; Clayton et al., 1973; Mattey et al., 1994;
Eiler et al., 1997). The range in d18Oolivine of basalts from
the Reykjanes Peninsula is between +3 and +5·2‰,
with a mean of +4·58 ± 0·26 (av. dev., n = 34). In
Fig. 7 these data are plotted against Nb/Zr and a
calculated MgO where olivine accumulation has been
estimated and the MgO content of the lavas adjusted
accordingly. There is a very broad trend of decreasing
MgO with decreasing d18Oolivine suggesting the operation
of AFC-type processes (Fig. 7a). However, samples with
~10 wt % MgO have d18Oolivine values which span almost
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REYKJANES PENINSULA LAVA CHEMISTRY
Fig. 7. (a) d18Oolivine vs MgO (calculated). Values of +5·0‰ to +5·5‰ are in equilibrium with lithosphere peridotites and both MORB and
ocean island mantle (Mattey et al., 1994; Eiler et al., 1997; Thirlwall et al., 1997). MgO has been recalculated to an aphyric value.
Assimilation–fractional crystallization (AFC) processes explain the trend towards the Cumulates at lower MgO and low d18Oolivine. (b) d18Oolivine
vs Nb/Zr. Bulk mixing lines are between RP67E and RP95C and average crust having 9 ppm Nb, 78 ppm Zr and d18Oolivine equivalent of
+2‰. It should be noted that the majority of the samples (Others) lie in a triangular field defined by the two mixing lines and mantle oxygen
values. The same errors for low-Nb samples apply as those shown in Fig. 6.
the whole range in the dataset. Cumulates have d18Oolivine
values similar to the majority of the lavas.
Figure 8a shows a broad trend of decreasing d18Oolivine
with increasing 87Sr/86Sr. This is not mirrored by the
relationship between 143Nd/144Nd and d18Oolivine (Fig. 9b),
which is much more scattered. Both the Depleted and
Stapafell groups, which have high and low 143Nd/144Nd
ratios, respectively, have d18Oolivine values of ~5‰. Lavas
with the lowest d18Oolivine values have 143Nd/144Nd, 87Sr/
86
Sr and trace element ratios close to the mean of the
whole dataset (Figs 7 and 8).
DISCUSSION
Two mechanisms of crustal interaction which may have
influenced the mantle-derived magma compositions are:
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Fig. 8. Mixing lines in both graphs are to the same average hydrothermally altered crust as in Figs 3 and 7b, with 87Sr/86Sr of 0·7036 and
143
Nd/144Nd of 0·51305. (a) d18Oolivine vs 87Sr/86Sr. The trend of the Depleted group does not fit the mixing line above ~10% assimilation. Either
a lower 87Sr/86Sr assimilant (~0·7034) is required specially for this group, or assimilation accompanies magma mixing which increases both the
Sr content and isotopic ratio of the magma, producing a trend similar to that between Stapafell and altered crust. (b) d18Oolivine vs 143Nd/144Nd.
Most data points are confined to a similar triangular field to that in Fig. 7b. The same limit on mixing, ~10% still applies to the Depleted
group.
(1) crystal fractionation and assimilation of cumulates or
xenocrysts; (2) assimilation of hydrothermally altered
crust.
We discuss the expected consequences of these processes
and then test whether they may be responsible for any
significant component of the chemical and isotopic variation outlined above. Then we will consider other crustal
influences on the Reykjanes Peninsula lavas. Finally, as it is
not the aim of this paper to fully categorize the composition
and melting regime of the mantle beneath Iceland, only to
indicate which characteristics of the lavas could be ascribed
to the mantle, we will suggest possible mantle origins of
some of those characteristics more usually thought of as
resulting from crustal processes.
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REYKJANES PENINSULA LAVA CHEMISTRY
Fig. 9. Mixing lines between the Depleted and Stapafell reference samples and plagioclase in equilibrium with the Stapafell group (S-plag) and
with RP115J (E-plag); errors shown in Fig. 3 apply. To avoid confusion not all mixing increments are indicated.
Crystal fractionation and assimilation of
cumulates or xenocrysts
Crystal fractionation has affected all lavas on the Reykjanes Peninsula, ~90% of which contain olivine and/or
plagioclase phenocrysts. The important aspects of crystal
fractionation relating to this study are outlined below.
Initially, MgO is reduced because of the dominance of
olivine as a fractionating phase (Fig. 4). Later, Sr and
Al2O3 are depleted in the remaining liquid because of
the removal of plagioclase, e.g. RP65H (Figs 4 and 5).
Conversely, accumulation of olivine dilutes incompatible
831
element concentrations and increases MgO content (e.g.
RP95A). Plagioclase accumulation results in higher than
magmatic Sr and Al2O3, best demonstrated by the Cumulates, where plagioclase forms the majority of the rock
(Fig. 4b). Elements which are incompatible in plagioclase
are present in lower abundances in the Cumulates than
the lavas, for example Zr (Fig. 2b). However, ratios of
very incompatible/moderately incompatible elements are
similar to average lava ratios, e.g. Nb/Zr (Fig. 2b).
Cumulates also have similar oxygen isotope ratios to
lavas with comparable Nb/Zr and isotopic characteristics
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VOLUME 39
(Figs 7 and 8). d18Oolivine (calculated from clinopyroxene)
of the Cumulate group is ~+4·5‰, close to the dataset
mean, +4·58 ±0·26 (av. dev., n = 34).
There are some characteristics of the groups within this
dataset which could be caused by crystal accumulation,
notably the high Al2O3, SiO2 and CaO, and low Fe2O3,
TiO2 and P2O5 of the Depleted group. For instance,
addition of ~12 % of plagioclase to the Stapafell reference
sample, RP67E, produces Al2O3 and MgO concentrations similar to the Depleted group reference
sample, RP95C (Fig. 4b). However, crystal accumulation
is incapable of affecting the radiogenic isotope ratios in
a closed system. Hence the correlations within the dataset
between ratios affected by plagioclase accumulation, e.g.
Al/Ti, those unaffected by plagioclase accumulation, e.g.
Nb/Zr, and radiogenic isotopes, are not compatible with
this model. However, the crystals and/or cumulates
produced have the potential to change element concentrations and radiogenic isotope ratios of non-cognate
magmas, especially when those magmas have the low
incompatible element concentrations typical of the Depleted group.
Elevated 87Sr/86Sr and positive Sr anomalies (Fig. 5,
relative to primitive mantle) have already been proposed
as resulting from some form of crustal interaction (e.g.
Hemond et al., 1993). So, to estimate the effects of one
of the proposed crustal processes, plagioclase assimilation,
we have calculated Sr and Nd concentrations for two
plagioclase compositions, using partition coefficients of 2
and 0·09, respectively. ‘S-plag’ is in equilibrium with the
Stapafell group reference sample, and ‘E-plag’ is in
equilibrium with RP115J, a sample with a higher Sr
concentration and Sr isotopic ratio than the Stapafell
group. A plagioclase in equilibrium with the Depleted
group is not included in this model, as lavas with Depleted
characteristics tend to be high-MgO and account for
<5% of the Reykjanes Peninsula crust. Also, the low
concentration of Sr in such a ‘D-plag’ would require
assimilation of >40% ‘D-plag’ to significantly affect the
Sr isotope ratios of average lavas.
Mixing lines between Sr and 87Sr/86Sr of calculated
plagioclase compositions and the reference samples,
RP67E and RP95C, are shown in Fig. 9a. There are
some similarities between the mixing lines and trends
within the dataset, noticeably the variation within the
Depleted group. This can be produced by addition of
up to 40% ‘S-plag’ or 20% ‘E-plag’ to the Depleted
reference sample, RP95C. However, a much greater
amount of ‘S-plag’ or ‘E-plag’ is required to reproduce
the range in Nd isotopes of the Depleted group (Fig. 9b).
Although there are some indications that assimilation
of ~20% plagioclase could produce a few Depleted group
samples with high 87Sr/86Sr at a given 143Nd/144Nd, such
samples do not have elevated Al or depleted Mg relative
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to other Depleted group samples. Furthermore, to influence the Stapafell group the contaminating plagioclase
must have fractionated from a magma with a higher Sr
concentration and 87Sr/86Sr than any samples in this
dataset. Also, the strongly curved mixing lines in Sr–Nd
isotope space (Fig. 9b) do not resemble any trends in
the dataset. Consequently, creation of the positive Sr
anomalies by plagioclase assimilation alone can be discounted.
To assess the influence of cumulate assimilation on the
lavas of the Reykjanes Peninsula we have used a gabbroic
composition representative of the Cumulate group as a
mixing end-member. Mixing curves for Sr and Sr isotope
ratios between this calculated cumulate and the two
reference samples from the Depleted and Stapafell groups
are shown in Fig. 10a. A third curve between the ‘average
cumulate’ and RP115J has also been included. The
resulting curves are a good match for the general trend
in the dataset, with the offset of high 87Sr/86Sr samples
to higher Sr being explicable by higher than modelled
Sr contents in the end-members. In the cumulates, Sr
concentration is a function of the nature of the cumulate,
with a troctolite having lower Sr than an anorthosite
from the same magma.
Cumulate assimilation is a better model than plagioclase assimilation in Sr–Nd isotope space (Fig. 10b). The
mixing line between the Depleted reference sample,
RP95C, and the ‘calculated cumulate’ is a reasonable fit
to the range and trend in the Depleted group, but up to
60% cumulate assimilation is required. This is not feasible
in these small-volume high-MgO melts, nor is it compatible with element concentrations and ratios in the
Depleted group, such as Zr and Nb/Zr (Fig. 2). We
conclude that whereas the mixing lines generated by
both assimilation models do show some similarity to
trends within the dataset, crystal accumulation or bulk
assimilation cannot be significant factors controlling these
trends.
Hydrothermal alteration and assimilation
of hydrothermally altered crust
The four hydrothermal systems on the Reykjanes Peninsula are all classified as high temperature, in that the
water is >200°C at <1 km. The origin of the hydrothermal fluid may be either seawater or meteoric water,
or a mixture of the two. Seawater has a d18O value of
0‰ relative to SMOW, whereas meteoric water falling
on Iceland is strongly negative. In central Iceland values
of –11·9‰ reflect the meteoric input to the hydrothermal
systems (Sveinbjornsdottir et al., 1986). Hydrothermal
fluid from the most southwesterly system on the Reykjanes
Peninsula, Reykjanes (Fig. 1), has d18O values of –1‰,
whereas local precipitation is –6·5‰ (Arnórsson, 1978).
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REYKJANES PENINSULA LAVA CHEMISTRY
Fig. 10. Bulk mixing models for addition of an average cumulate, 161 ppm Sr, 1·3 ppm Nd, 87Sr/86Sr of 0·70313 and 143Nd/144Nd of 0·51360,
to the Stapafell and Depleted reference samples, and to the higher Nb/Zr composition RP115J; errors shown in Fig. 3 apply. (a) Sr vs 87Sr/
86
Sr. (b) 143Nd/144Nd vs 87Sr/86Sr.
Analyses of bulk-rock fragments from drill samples vary
from +2·8 to +5·6‰ depending upon the intensity of
alteration (Sveinbjornsdottir et al., 1986). There does not
appear to be any consistent variation with depth over
the 1800 m of basalt, hyaloclastite breccia and sediment
drilled, as fresh and altered basalt is present at all depths.
Elderfield & Greaves (1981) reported Sr, Rb and K
concentrations of ~9 ppm, ~3·5 ppm and ~1300 ppm,
respectively, in the Reykjanes and Svartsengi hydrothermal systems. However, element concentrations vary
across the Reykjanes Peninsula; e.g. Sveinbjornsdottir et
al. (1986) presented K concentrations which show an
increase from 13·8 ppm in the northeast to 1480 ppm
in the southwest. Elderfield & Greaves (1981) also reported 87Sr/86Sr ratios of palagonite, ~0·7039, which
show isotopic equilibration with the hydrothermal fluid
whereas those of associated altered basalt, ~0·7038, are
only semi-equilibrated, reflecting their differing permeability and porosity. Hyaloclastite, because of its high
glass content and fragmental nature, is apparently more
susceptible to secondary alteration than is lava. Elderfield
& Greaves (1981) concluded that ~75% of the leached
Sr is included in the alteration minerals epidote and
chlorite, rather than calcite. Therefore minor assimilation
(p10%) of shallow crust with a high palagonite:basalt
ratio containing these secondary alteration minerals (e.g.
selective contamination) could substantially modify Sr,
Rb and K concentrations and 87Sr/86Sr. However, it
would be more difficult to change oxygen isotope ratios
inherited from the mantle, because of the low volume of
material assimilated and the relatively small difference
in d18O values between them, ~3·0‰.
Rare earth elements and Nd isotopes are unlikely to
be significantly affected by hydrothermal alteration or
radiogenic in-growth in crust beneath the Reykjanes
Peninsula. Depleted group lavas do have high Sm/Nd
ratios, ~0·45 (Table 1), but these lavas constitute <5%
of the area sampled for this study. The majority of the
lavas have lower Sm/Nd, ~0·2. Additionally, as this is a
mature spreading ridge segment, the crust beneath the
axis will effectively be zero age and will not have had
sufficient time to develop a distinctive Nd isotope ratio.
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The mean Nb/Zr of all lavas analysed in this study is
0·117 ± 0·03 (2SD, n = 292), increasing to 0·127 ±
0·02 (2SD, n = 254) when the Depleted and Stapafell
groups are excluded from the mean. Lavas with extreme
chemical compositions constitute <10% of the flows at
the surface of the Reykjanes Peninsula. There is no
reason to suppose that this is not representative of the
buried crust. Consequently, the dominant upper-crustal
signature on the Reykjanes Peninsula, and therefore the
one most likely to be assimilated, has ‘average’ chemistry
with Nb/Zr of ~0·11 and 143Nd/144Nd of ~0·51305.
As 143Nd/144Nd and Nb/Zr should be unaffected in
hydrothermally altered crust these ratios will be used as
constraints for this model.
If we assume that the mantle beneath Iceland is homogeneous with respect to oxygen isotope ratios then any
samples with lower ratios must result from some assimilation of hydrothermally altered material, which will
have a higher 87Sr/86Sr than unaltered lava. This is
reflected in the d18Oolivine value of +2·0‰ and 87Sr/86Sr
of 0·7036 for the ‘average crust’ in our model. These
isotopic characteristics are in broad agreement with those
reported by Elderfield & Greaves (1981) and
Sveinbjornsdottir et al. (1986).
In the model presented, the mixing end-members are
the reference samples, RP95C and RP67E, and average
crust with the modified Sr and O isotopes outlined above.
Mixing relationships between the d18Oolivine values of these
three end-members and Nb/Zr, 143Nd/144Nd and 87Sr/
86
Sr are plotted in Figs 7b and 8. Interestingly, it is the
chemical and isotopic characteristics in the dataset that
are expected to be unmodified by hydrothermal alteration, Nb/Zr and 143Nd/144Nd, which lie within a field
defined by the mixing lines, albeit requiring 40–60%
crustal assimilation.
This amount of assimilation, 40–60%, has several
implications for the model. Either this level of crustal
contamination is possible because the Reykjanes Peninsula magmatic systems are able to assimilate greater
proportions of crust than is more usually observed, or
the d18O values used in the model are incorrect. The
assumption that the mantle beneath Iceland is homogeneous with respect to oxygen isotopes and has a d18O
composition typical of peridotite mantle may not be
correct. Melts from a mantle source with a lower d18O
value could be the reason why high Nb/Zr, low 143Nd/
144
Nd lavas tend to have d18O below 5·0‰ (Figs 7 and
8). Furthermore, it is possible that hydrothermally altered
crust has a lower d18O value than that used in this model.
This would reduce the amount of assimilation required
to, for example, between 30 and 40% if the d18O value
of the hydrothermally altered crust was 0·0‰.
Assimilation of hydrothermally altered material accompanied by fractional crystallization (AFC) would
result in a trend of decreasing d18Oolivine with decreasing
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MAY 1998
MgO, as a result of the dominance of olivine fractionation.
However, when the effects of olivine accumulation are
removed, MgO is only weakly correlated with d18Oolivine
in the whole dataset (Fig. 7a). Some low d18Oolivine evolved
samples may have been produced by AFC processes, but
samples with high MgO and low d18Oolivine indicate that
AFC was not the only mechanism of crustal interaction.
Further, evolved samples with relatively high d18Oolivine
indicate that in some magma chambers, fractionation
proceeded with relatively little assimilation of hydrothermally altered crust.
When this model is illustrated on an Nd–Sr isotope
plot (Fig. 3) much less assimilation of hydrothermally
altered crust is required to produce the variation in 87Sr/
86
Sr seen at a given 143Nd/144Nd, between 5% and 20%,
than is required to produce the range observed in oxygen
isotopes. Mixing between these contaminated magmas
would then produce a mixing line similar to the one
shown between the Depleted and Stapafell reference
samples, but displaced to higher 87Sr/86Sr. These two
mixing lines enclose the whole dataset. Therefore contamination by hydrothermally altered crust could explain
some of the observed ‘scatter’ in 87Sr/86Sr at a given
143
Nd/144Nd, but a heterogeneous mantle source is required to produce the variation in Nd isotope ratios.
Furthermore, most of the lavas which are included in
the ‘Others’ group lie between the assimilation lines in
Figs 7b and 8b, suggesting that magma mixing is also
important.
Magma mixing: crustal or mantle process
The majority of the dataset cluster about the mean, e.g.
Nb/Zr (Fig. 2), and have correspondingly ‘moderate’ Nd
(and Sr) isotope ratios intermediate between the Depleted
and Stapafell samples. The ensuing element–radiogenic
isotope correlations (e.g. Fig. 6) have been interpreted as
arising from mixing of ‘instantaneous’ melts in the mantle
(Elliott et al., 1991), but may be better explained by
crustal mixing.
If mixing was primarily taking place in the mantle,
eruption of Stapafell and Depleted lavas is more difficult
to explain than if magma chambers were the main sites
of homogenization. Almost all the data are contained
within triangular fields defined by mixing lines with
hydrothermally altered crust in d18O–Nb/Zr and d18O–
143
Nd/144Nd spaces (Figs 7b and 8b). If all mixing had
taken place in the mantle it might be expected that the
crustal process producing low d18O values would affect
Depleted and Stapafell samples as well as the evolved
low-MgO ‘average’ lavas. Further, the absence of highMgO ‘average’ lavas and low-MgO Depleted and
Stapafell samples indicates that mixing is usually accompanied by fractional crystallization, most probably
in crustal magma chambers.
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REYKJANES PENINSULA LAVA CHEMISTRY
If all magmas had mixed in robust, assimilating magma
chambers all samples might be expected to show some
reduction in their d18O values. The majority of samples
show d18O values substantially below peridotite mantle
values, implying that large-scale assimilation was common. However, several lavas exist with d18O values in
equilibrium with the mantle. Of those, the Depleted group
samples also have low incompatible element abundances,
which would be easily modified by mixing with magmas
with higher incompatible element concentrations and/
or by crustal assimilation. Therefore some magmas must
largely bypass assimilating magma chambers. RP56A,
an ‘average’ lava, also has d18O in equilibrium with the
mantle (Table 1), implying that mixing can occur with
little or no assimilation.
Magma mixing between end-members with the different Sr and Nd concentrations and ratios of the Depleted
and Stapafell groups (Table 1) would be expected to
produce a mixing gap. Parental magma to the Depleted
group would be converted to ‘average magma’ by the
addition of small volumes of anything except other Depleted magmas. Figure 7b illustrates that the addition of
15–20% of average hydrothermally altered crust takes
the Depleted samples beyond their group’s Nb/Zr limit
of 0·07. Addition of >10% of magma with higher incompatible element concentrations, e.g. Stapafell, or an
evolved residual magma, would have the same effect.
This 10% mixing limit for Depleted magmas is further
reduced when Nd and Sr isotope characteristics are
considered (Fig. 6; note that the Depleted end-member
used here is not RP95C, but the Depleted sample with
the highest d18Oolivine values). Consequently, incorporation
of any enriched component, by mixing or assimilation,
into a Depleted magma will tend to result in a sharp
increase in incompatible element concentrations and a
related jump in radiogenic isotope ratios, producing a
mixing gap between lavas from this group and the
average, homogenized product of the magma chambers
(Fig. 6). Mixing between RP95C and the Stapafell reference sample, RP67E, will produce a similar trend to
that shown in Fig. 6, but offset to lower Sr and higher
Nd isotope ratios, bypassing the majority of the dataset.
This reinforces the conclusion based on MgO and d18O
relationships that mixing is usually accompanied by assimilation and fractional crystallization.
The Vatnsheidi suite may represent incomplete mixing
between Depleted magma and a residual ‘average’
magma. This would result in increased trace element
concentrations, higher incompatible element ratios (e.g.
Nb/Zr), lower 143Nd/144Nd and d18O values but substantially elevated 87Sr/86Sr, all characteristics of the
Vatnsheidi suite. Melt inclusions from Vatnsheidi and
the same edifice as the Depleted group sample, RP80D,
have been analysed by Sobolev et al. (1994). The Vatnsheidi inclusions show the same range of chondrite-normalized rare earth patterns as our whole-rock Vatnsheidi
data. Those from the same edifice as RP80D all have
the same depleted pattern. Evidently, the RP80D magma
chamber (with mantle-like d18O values) was homogeneous
before eruption, unlike Vatnsheidi where the lavas show
a range in chemical and isotopic compositions.
Magmatic plumbing
As we have shown above, magma mixing coupled with
assimilation in the crust effectively filters out extreme
chemical signatures. The effectiveness of this type of
system depends upon a stable magma chamber, where
mixing can occur between incoming melts, evolved residual magma and perhaps also cumulates, which could
be genetically related to the residual evolved lava. Within
a magma mixing model, the Depleted and Stapafell
groups exist as a result of either interruption of a steadystate system in a robust magma chamber or the existence
of small magma chambers dominated by the appropriate
parental magmas.
The large, robust systems would also be capable of
assimilating crust. However, in an extensional environment this system would require a large magma
throughput. This may satisfactorily explain not only the
clustering of Reykjanes Peninsula lava flows at moderate
Nb/Zr and isotopic ratios, but also the relative lack
of variation in the large flows of the marginal zones.
Compared with the neovolcanic zones, lava flows in the
marginal zones tend to be larger volume, and have a
more restricted range in very incompatible/moderately
incompatible element ratios, e.g. Nb/Zr ratios of ~0·11
± 0·03 (n >100; Hardarson & Fitton, 1994), compared
with the wider range of Reykjanes Peninsula lavas, Nb/
Zr = 0·12 ± 0·06 (2SD, n = 294). The difference
between the neovolcanic and marginal zones has been
ascribed to a temporal change in the composition of the
mantle source for these lavas (Schilling et al., 1982), or
variation in depth of melt segregation (Meyer et al., 1985).
However, subsidence of the axial rift zone in response
to volcanic overloading (Pàlmason, 1980, 1986; Menke
& Levin, 1994) would lead to selective preservation of
large flows similar to RP59F (Table 1) off-axis. These
flows have an ‘average’ composition compared with the
range in chemical and isotopic characteristics observed
in the neovolcanic zone. Therefore, preservation of the
chemically and isotopically extreme samples off-axis, such
as the Depleted group, would not be expected. Indeed,
as already stated, these are rare in the neovolcanic zones,
<5% of the crust.
There is no evidence to suggest that Icelandic crust is
not analogous to oceanic crust in that both comprise
835
JOURNAL OF PETROLOGY
VOLUME 39
lavas, sheeted dykes and cumulates. However, Icelandic
crust is a factor of ~3 thicker than normal oceanic crust,
and borehole stratigraphies indicate a high palagonite to
lava ratio on the Reykjanes Peninsula as a result of subaqueous eruption at shallower depths than at normal
mid-ocean ridges (e.g. Sveinbjornsdottir et al., 1986; Bott,
1988; Bjarnasson et al., 1993; White et al., 1996). A
shallow magma chamber has been detected ~4 km
beneath Krafla, NE Iceland (Einarsson, 1978) equivalent
to depths of ~3 km below the seafloor in normal midocean ridges. Tryggvason (1986) proposed that additional
magma chambers exist beneath Krafla at ~2·6 km and
at the base of the crust. The deeper magma chambers
serve as ‘holding areas’ for melt, whereas the shallow
crustal magma chambers are associated with eruptive
events. There is no reason to suppose that this is not the
case beneath the Reykjanes Peninsula. The existence of
magma chambers at ~3 km beneath the Reykjanes
Peninsula can be inferred by the heat flow in geothermal
systems. These magma chambers may mark the transition
between the sheeted dyke and extrusive layers in the
crust, and as such will be able to interact with
hydrothermally altered crust, specifically the low-d18O
palagonite. The higher hyaloclastite:lava ratio in the
Reykjanes Peninsula crust may well be an important
factor in producing low-d18O lavas relative to MORB.
Deeper magma chambers in the thickened Icelandic
crust may be less likely to assimilate material with a
hydrothermal component. In the case of the Depleted
group lavas, the shallow-level magma chambers were
probably bypassed, hence their relatively primitive, unmixed, high d18O nature.
Finally, we consider the rarity of the Stapafell-type
high-MgO lavas compared with Depleted group lavas,
Stapafell being the only occurrence of lavas with high
MgO, high Nb/Zr and low 143Nd/144Nd. The density
difference between Stapafell and Depleted lavas, 2·75 g/
cm3 and 2·70 g/cm3, respectively, at ~12·5 wt % MgO,
1300°C and volatile free, may be a factor. This density
difference would favour longer crustal residence times
for the Stapafell group, increasing the potential of the
magma to interact with the crust and other magmas,
and to evolve via fractional crystallization to lower MgO.
Composition of mantle source or sources
and melting processes within the mantle
It has been proposed that the variation in Fe in Reykjanes
Peninsula lavas (Fig. 4a) is a function of dynamic mantle
melting, and subsequent mixing of those melts in the
mantle (e.g. Elliott et al., 1991), whereas we have proposed
that mixing takes place mainly between magmas within
the crust. Crustal mixing involving assimilation of hydrothermally altered ‘average’ crust does require mantle
NUMBER 5
MAY 1998
heterogeneity to generate the observed range in Nd
isotopes (Fig. 3). However, this model only requires two
mantle sources to provide the low-143Nd/144Nd Depleted
group and high-143Nd/144Nd Stapafell group. As stated
above, there is no reason to suppose that these two
isotopically distinct sources have similar major and trace
element concentrations; for example, the low incompatible element concentrations and ratios, high Sm/
Nd and high 143Nd/144Nd (Figs 3 and 5) of Depleted
group lavas may well reflect a time-integrated depletion
event. This Depleted mantle might be expected to generate melts with lower Fe2O3 than those from a less
depleted source. Consequently, major element variations
between the Stapafell and Depleted groups (e.g. Fig. 4)
may be as much a result of magma mixing and assimilation as the range in isotope ratios.
Melting plagioclase peridotite
Menke & Levin (1994) presented geophysical evidence
that supports a crustal geothermal gradient in the neovolcanic zones of ~50°C/km, which would predict a
temperature of ~1000°C at 20 km. This is below the
peridotite solidus, but above that required for the spinel to
plagioclase peridotite transition in the mantle at ~24 km
(>900°C; Kushiro & Yoder, 1966). Consequently, the
shallow mantle under the Reykjanes Peninsula is in
plagioclase peridotite facies. In the spinel to plagioclase
peridotite reaction concentrations of Sr and Al are reduced in clinopyroxene whereas Zr is increased (e.g.
Rampone et al., 1993). In addition, as the spinel undergoes
sub-solidus recrystallization during the transition, its Al
content falls as Cr and Ti increase (Rampone et al., 1993),
the same effect as progressive melting in spinel facies.
These variations in concentration are caused by the
change in interphase element partitioning as a result of
plagioclase-in. Subsequent melting-out of the remaining
aluminous phase (plagioclase) could give rise to high
Al2O3/TiO2 melts, another characteristic of the Depleted
group (Al2O3/TiO2 ~20).
Melting plagioclase facies mantle would also change
the compatibility order of the moderately incompatible
elements. In particular, Sr would be more compatible,
which would agree with the relative incompatibility order
in Depleted group lavas deduced from element–element
plots. If Sr had a bulk D similar to Sm or Gd, and Zr
had one similar to Nd, this would remove both the
positive Sr anomaly and the negative Zr anomaly of
RP95C (Fig. 5). Positive Sr anomalies still exist in other
Depleted lavas, but they are much lower, and more
comparable with the plagioclase content of the rock.
836
CONCLUSIONS
Assuming mantle beneath Iceland has a similar oxygen
isotope ratio to MORB peridotite mantle, then samples
GEE et al.
REYKJANES PENINSULA LAVA CHEMISTRY
with oxygen isotopes in equilibrium with the mantle
must by definition have had the least interaction with
hydrothermally altered crust.
On this criterion alone, the Depleted group of lavas
(Nb/Zr <0·07) appears the least affected by crustal
interaction in this dataset. This is compatible with the
observation that if magmas parental to this group did
interact with a crustal component, their low incompatible
element abundances would be overprinted, given that
average crust is very likely to have higher incompatible
element concentrations similar to the average Reykjanes
Peninsula lavas. Thus for Depleted lavas to be erupted
they must avoid crustal residence, which accounts for
the absence of evolved lavas in this group, and mantlelike oxygen isotope ratios. Some of the most incompatible
element enriched samples in the suite, e.g. the Stapafell
group, also have high MgO and oxygen isotope ratios
close to mantle ratios. Therefore, these samples are also
interpreted as having had limited crustal residence.
The majority of the remaining samples (~90% of the
basalts sampled) show variable amounts of incorporation
of hydrothermally altered crust, typically 20–40% based
on an average crustal d18O value of +2‰. This amount of
assimilation may be viable because of the high geothermal
gradient at the ridge axis. Those lavas that are the most
evolved have the lowest d18Oolivine, suggesting operation
of AFC processes. These low-d18Oolivine evolved samples
have Nd isotopes that are intermediate between Stapafell
and Depleted group samples. They result from a homogenization process, between one or more of these two
extreme compositions and Icelandic crust, but frequently
have higher 87Sr/86Sr than either parental magma. It
seems that eruption of simple mixed magma is rare; more
likely are fractionated lavas in which assimilation of
hydrothermally altered crust has taken place. As a result
of this homogenization process the majority of the dataset
have ‘average’ chemical characteristics, similar to largevolume off-axis lavas.
The variation in crustal processes indicates that there
are several ‘tiers’ of magma chambers beneath the Reykjanes Peninsula. AFC processes occur in shallow-level
magma chambers sited in oceanic-type crust with a higher
hyaloclastite to lava ratio than normal oceanic crust.
This enhances the effects of any crustal assimilation on
oxygen and Sr isotope ratios.
Chemical characteristics in some lavas thought elsewhere to be the result of crustal processes, e.g. anomalously high Sr, may result from shallow melting of
mantle in plagioclase peridotite facies. The resulting
melts normally mix in the manner outlined above, but
occasionally bypass the shallow-level magma chambers
and are erupted.
Shallow-level crustal process have had considerable
effects on lavas from the Reykjanes Peninsula. However,
as these processes tend to produce similar results, distinguishing between them is not a simple task, especially
when, as in Iceland, the mantle source is isotopically and
compositionally heterogeneous. This study suggests that
caution should be exercised when small, or unrepresentative, sample suites are used to infer mantle
sources and processes.
ACKNOWLEDGEMENTS
M.G. would like to thank Kristjan Sæmundsson for his
practical and geological assistance in Iceland, and Bjorg
Traustadottir for her help with accommodation and
communication during both field seasons. The authors
would like to thank David Graham, Jon Davidson, Wendy
Bohrson and the two anonymous reviewers for their
thorough reviews, which greatly improved this manuscript. Giz Marriner and Gerry Ingram are also thanked
for their help in the XRF and radiogenic isotope laboratories, respectively. M.G. was supported by NERC–CASE
Studentship GT4/94/367/G during this study. XRF and
radiogenic isotope laboratories at Royal Holloway are
London University intercollegiate facilities.
837
REFERENCES
Arnórsson, S. (1978). Major element chemistry of the geothermal seawater at Reykjanes and Svartsengi, Iceland. Mineralogical Magazine
42, 209–220.
Bjarnasson, I. T., Menke, W., Flóvenz, Ó. G. & Caress, D. (1993).
Tomographic image of the mid-Atlantic plate boundary in SW
Iceland. Journal of Geophysical Research 98, 6607–6622.
Bott, M. P. H. (1988). A new look at the causes and consequences of
the Iceland hotspot. In: Morton, A. C. & Parson, L. M. (eds) Early
Tertiary Volcanism and the Opening of the NE Atlantic. Geological Society,
London, Special Publication 39, 15–23.
Clayton, R. N., Hurd, J. M. & Mayeda, T. K. (1973). Oxygen isotope
compositions of Apollo 15, 16, and 17 samples and their bearing
on lunar origin and petrogenesis. Proceedings of the 4th Lunar Science
Conference. Geochimica et Cosmochimica Acta Supplement 4, 1535–1542.
Condomines, M., Grönvold, K., Hooker, P. J., Muehlenbachs, K.,
O’Nions, R. K., Óskarsson, N. & Oxburgh, E. R. (1983). Helium,
oxygen, strontium and neodymium isotopic relationships in Icelandic
volcanics. Earth and Planetary Science Letters 66, 125–136.
Eiler, J. M., Farley, K. A., Valley, J. W., Hauri, E., Craig, H., Hart,
S. R. & Stolper, E. M. (1997). Oxygen isotope variations in ocean
island basalt phenocrysts. Geochimica et Cosmochimica Acta 61, 2281–
2293.
Einarsson, P. (1978). S-wave shadows in the Krafla caldera in NE
Iceland, evidence for a magma chamber in the crust. Journal of
Geophysics 47, 160–165.
Elderfield, H. & Greaves, M. J. (1981). Strontium isotope geochemistry
of Icelandic geothermal systems and implications for sea water
chemistry. Geochimica et Cosmochimica Acta 45, 2201–2212.
Elliott, T. R., Hawkesworth, C. J. & Grönvold, K. (1991). Dynamic
melting of the Iceland Plume. Nature 351, 201–206.
JOURNAL OF PETROLOGY
VOLUME 39
Furman, T., Frey, A. F. & Park, K.-H. (1991). Chemical constraints
on the petrogenesis of mildly alkaline lavas from Vestmannaeyjar,
Iceland: the Eldfell (1973) and Surtsey (1963–1967) eruptions. Contributions to Mineralogy and Petrology 109, 19–37.
Furman, T., Frey, A. F. & Meyer, P. S. (1992). Petrogenesis of evolved
basalts and rhyolites at Austurhorn, southeastern Iceland: the role
of fractional crystallization. Journal of Petrology 33, 1405–1445.
Gurenko, A. A., Sobolev, A. V. & Kononkova, N. M. (1991). Petrology
of the primary magma of the Reykjanes Peninsula rift tholeiites.
Geochemistry International 28, 58–71.
Hardarson, B. S. & Fitton, J. G. (1994). Geochemical variation in the
Tertiary basalts of Iceland. Mineralogical Magazine 58A, 372–373.
Hemond, C., Arndt, N. T., Lichtenstein, U. & Hofmann, W. (1993).
The heterogeneous Iceland Plume: Nd–Sr–O isotopes and trace
element constraints. Journal of Geophysical Research 98, 15833–15850.
Hofmann, A. W. (1988). Chemical differentiation of the Earth: the
relationship between mantle, continental crust, and oceanic crust.
Earth and Planetary Science Letters 90, 297–314.
Imsland, P. (1983). Biased chemical range of Icelandic and oceanic
basalt analyses: the result of different sampling methods and compositionally selective kinematic evolution within rift zones. Jökull 33,
33–38.
Jakobsson, S. P. (1972). Chemistry and distribution pattern of Recent
basaltic rocks in Iceland. Lithos 5, 365–386.
Jakobsson, S. P., Jónsson, J. & Shido, F. (1978). Petrology of the western
Reykjanes Peninsula, Iceland. Journal of Petrology 19, 669–705.
Kushiro, I. & Yoder, H. S. (1966). Anorthite–forsterite and anorthite–
enstatite reactions and their bearing on the basalt–eclogite transformation. Journal of Petrology 7, 337–362.
Levi, S., Audunsson, H., Duncan, R. A., Kristjansson, L., Gillot,
P.-Y. & Jakobsson, S. P. (1990). Late Pleistocene geomagnetic
excursion in Icelandic lavas: confirmation of the Laschamp excursion.
Earth and Planetary Science Letters 96, 443–457.
Macdonald, R., Sparks, R. S. J., Sigurdsson, H., Mattey, D. P.,
McGarvie, D. W. & Smith, R. L. (1987). The 1875 eruption
of Askja volcano, Iceland: combined fractional crystallization and
selective contamination in the generation of rhyolitic magma. Mineralogical Magazine 51, 183–202.
Macdonald, R., McGarvie, D. W., Pinkerton, H., Smith, R. L. &
Palacz, Z. A. (1990). Petrogenetic evolution of the Torfajökull
volcanic complex, Iceland I. Relationship between the magma types.
Journal of Petrology 31, 429–459.
Mattey, D. P. & Macpherson, C. G. (1993). High-precision oxygen
isotope microanalysis of ferromagnesian minerals by laserfluorination. Chemical Geology (Isotope Geosciences Section) 105, 305–
318.
Mattey, D. P., Lowry, D. & Macpherson, C. G. (1994). Oxygen isotope
composition of mantle peridotite. Earth and Planetary Science Letters
128, 231–241.
Menke, W. & Levin, V. (1994). Cold crust in a hot spot. Geophysical
Research Letters 21, 1967–1970.
Meyer, P. S., Sigurdsson, H. & Schilling, J.-G. (1985). Petrological and
geochemical variations along Iceland’s neovolcanic zones. Journal of
Geophysical Research 90, 10043–10072.
Moorbath, S. H., Sigurdsson, H. & Goodwin, R. (1968). K–Ar ages
of the oldest exposed rocks in Iceland. Earth and Planetary Science
Letters 4, 197–205.
Muehlenbachs, K. & Clayton, R. N. (1972). Oxygen isotope studies of
fresh and weathered submarine basalts. Canadian Journal of Earth
Sciences 9, 172–184.
Muehlenbachs, K., Anderson, A. T. & Sigvaldason, G. E. (1974). LowO18 basalts from Iceland. Geochimica et Cosmochimica Acta 38, 577–588.
NUMBER 5
MAY 1998
O’Nions, R. K., Hamilton, P. J. & Evensen, N. M. (1977). Variations
in 143Nd/144Nd and 87Sr/86Sr ratios in oceanic basalts. Earth and
Planetary Science Letters 34, 13–22.
Óskarsson, N., Sigvaldason, G. E. & Steinthórsson, S. (1982). A dynamic
model of rift zone petrogenesis and the regional petrology of Iceland.
Journal of Petrology 23, 28–74.
Óskarsson, N., Steinthórsson, S. & Sigvaldason, G. E. (1985). Iceland
geochemical anomaly: origin, volcanotectonics, chemical fractionation and isotope evolution of the crust. Journal of Geophysical
Research 90, 10011–10025.
Pálmason, G. (1980). A continuum model of crustal generation in
Iceland: kinematic aspects. Journal of Geophysics 47, 7–18.
Pàlmason, G. (1986). Model of crustal formation in Iceland, and
application to submarine mid-ocean ridges. In: Vogt, P. R. &
Tucholke, B. E. (eds) The Western North Atlantic Region. Boulder, CO:
Geological Society of America, pp. 87–97.
Pàlmason, G. & Sæmundsson, K. (1974). Iceland in relation to the
Mid-Atlantic Ridge. Annual Review of Earth and Planetary Sciences 2,
25–50.
Rampone, E., Picccardo, G. B., Vannucci, R., Bottazzi, P. & Ottolini,
L. (1993). Subsolidus reactions monitored by trace element partitioning: the spinel- to plagioclase-facies transition in mantle peridotites. Contributions to Mineralogy and Petrology 115, 1–17.
Sæmundsson, K. & Einarsson, S. (1980). Geological map of Iceland,
sheet 3, SW Iceland, 2nd edn. Reykjavik: Museum of Natural
History and the Iceland Geodetic Survey.
Schilling, J.-G. (1973). Iceland mantle plume: geochemical study of
Reykjanes Ridge. Nature 242, 565–571.
Schilling, J.-G., Meyer, P. S. & Kingsley, R. H. (1982). Evolution of
the Iceland hotspot. Nature 296, 313–320.
Sigvaldason, G. E., Steinthórsson, S., Óskarsson, N. & Imsland, P.
(1974). Compositional variation in recent Icelandic tholeiites and
the Kverkfjöll hot spot. Nature 251, 579–582.
Sobolev, A. V., Gurenko, A. A. & Shimizu, N. (1994). Ultra-depleted
melts from Iceland: data from melt inclusion studies. Mineralogical
Magazine 58A, 860–861.
Steinthórsson, S., Óskarsson, N. & Sigvaldason, G. E. (1985). Origin
of alkali basalts in Iceland: a plate tectonic model. Journal of Geophysical
Research 90, 10027–10042.
Sveinbjornsdottir, A. E., Coleman, M. L. & Yardley, B. W. D. (1986).
Origin and history of hydrothermal fluids of the Reykjanes and
Krafla geothermal fields, Iceland. Contributions to Mineralogy and Petrology
94, 99–109.
Taylor, R. N., Thirlwall, M. F., Murton, B. J., Hilton, D. R. & Gee,
M. A. M. (1997). Isotopic constraints on the influence of the Icelandic
plume. Earth and Planetary Science Letters 148, E1–E8.
Thirlwall, M. F. (1991a). Long-term reproducibility of multicollector
Sr and Nd isotope ratio analysis. Chemical Geology (Isotope Geosciences
Section) 94, 85–104.
Thirlwall, M. F. (1991b). High precision multicollector isotope analysis
of low levels of Nd as oxide. Chemical Geology (Isotope Geosciences Section)
94, 13–22.
Thirlwall, M. F., Jenkins, C., Vroon, P. Z. & Mattey, D. P. (1997).
Crustal interaction during construction of ocean islands: Pb–
Sr–Nd–O isotope stratigraphy of shield basalts of Gran Canaria.
Chemical Geology 135, 233–262.
Tryggvason, E. (1986). Multiple magma reservoirs in a rift zone volcano:
ground deformation and magma transport during the September
1984 eruption of Krafla, Iceland. Journal of Volcanology and Geothermal
Research 28, 1–44.
Tryggvason, K., Husebye, E. S. & Stefansson, R. (1983). Seismic image
of the Icelandic hotspot. Tectonophysics 100, 97–118.
838
GEE et al.
REYKJANES PENINSULA LAVA CHEMISTRY
White, R. S., McBride, J. H., Maguire, P. K. H., Brandsdóttir, B.,
Menke, W., Minshull, T. A., Richardson, K. R., Smallwood, J. R.,
Staples, R. K. & F. Group (1996). FIRE: Faeroe–Iceland Ridge
Experiment. EOS Transactions, American Geophysical Union 197, 200–
201.
Wolfe, C. J., Bjarnason, I. T., VanDecar, J. C. & Solomon, S. C.
(1997). Seismic structure of the Iceland mantle plume. Nature 385,
245–247.
Wood, D. A. (1979). Dynamic partial melting: its application to the
petrogenesis of basalts erupted in Iceland, the Faroe Islands, the Isle
of Skye (Scotland) and the Troodos Massif (Cyprus). Geochimica et
Cosmochimica Acta 43, 1031–1046.
Zindler A., Hart, S. R., Frey, F. A. & Jakobsson, S. P. (1979). Nd and
Sr isotope ratios and rare earth element abundances in Reykjanes
Peninsula basalts: evidence for mantle heterogeneity beneath Iceland.
Earth and Planetary Science Letters 45, 249–262.
839