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Helium isotope signature of lithospheric mantle xenoliths from the
Permo-Carboniferous magmatic province in Scotland - no evidence for
a lower-mantle plume
L. A. K I R S T E I N 1'2, T. J. D U N A I 1, G. R. D A V I E S 1, B. G. J. U P T O N : &
I. K. N I K O G O S I A N 1
1Department of Earth Sciences, Vrije Universiteit, 1085 De Boelelaan, 1081 H V Amsterdam,
the Netherlands (e-mail." [email protected])
2School of GeoSciences, Grant Institute, University of Edinburgh EH9 3JW, UK
Abstract: Noble gas studies of well-characterized spinel-peridotite-facies lithospheric mantle
xenoliths and garnet megacrysts from Scottish Permo-Carboniferous dykes, sills and vents
demonstrate that the mantle beneath Scotland during the late Palaeozoic was more radiogenic
than the source of mid-ocean ridge basalts (MORB). The samples were collected from the
Northern Highland Terrane and the Midland Valley Terrane, which vary from ArchaeanProterozoic to Proterozoic-Palaeozoic in age. Helium isotope ratios of between 3Ra and 6 Ra
(Ra = atmospheric ratio) indicate that there has been time-integrated U-Th enrichment of the
subcontinental mantle. This enriched mantle was preferentially melted following the transition
from early Palaeozoic compression to late Palaeozoic extensional tectonics. Helium isotope
ratios provide no clear evidence for the presence of undegassed plume-type mantle beneath this
part of Scotland during the Permo-Carboniferous. The measured helium ratios do not discount
the presence of a low-helium plume similar to those of the European Cenozoic volcanic
province. A passive origin, however, is preferred for the Permo-Carboniferous magmatism due
to the protracted activity, relatively small-extruded volumes of mafic magma and the lowhelium isotope ratios measured.
Permo-Carboniferous magmatism was widespread across northern Europe, with the later
stages of activity coinciding with Variscan
orogenesis (Fig. 1) (Timmerman 2004). In Scotland, Carboniferous lavas are preserved primarily in the Clyde Plateau, spread over an area of
3000 km 2 with an estimated present-day volume
of 6000km 3 (Tomkeieff 1937; Upton et al.
2004). This is a minimum estimate of the total
volume of melt produced as only the areal extent
of present-day isolated exposures of lower
Carboniferous lavas are included. A much
greater volume (29 200 km 3) of magmatic rocks
is estimated to have been extruded, and
intruded, in the Oslo region (Ramberg & Larsen
1978). Basaltic rocks make up 17% of this
29200km 3 total, i.e. approximately 5000km 3.
In addition, intrusions in the form of sills, dykes
and plutons were widespread during the PermoCarboniferous in Europe with the result that it is
difficult to estimate the total volume of melt
produced (Neumann et al. 2004; Upton et al.
2004).
The duration of igneous activity in the
Permo-Carboniferous across Europe is considered to be close to 100 Ma (Upton et al. 2004).
During this period the magmatism involved
phenomena ranging from extensive lava flows
to numerous high-level intrusions. The main
lava series in Scotland and Norway were
extruded over a short period ( < 5 M a ) in two
distinct pulses at c. 340 and c. 300 Ma, respectively (Timmerman & Wilson 1999; Timmerman
2004).
Mantle plume activity is often invoked in
models to explain the genesis of large volume
(1 x 106 km 3) igneous provinces that are rapidly
emplaced (<5 Ma). Plume impact in the PermoCarboniferous continues to be the subject of
debate (Smedley 1988; Ernst & Buchan 1997;
Neumann et al. 2004; Upton et al. 2004). The
Scottish Permo-Carboniferous basalts have
ocean island basalt (OIB)-like compositions,
that is they are more enriched in incompatible
elements relative to mid-ocean ridge basalts
(MORB), suggesting the involvement of a
From: WILSON, M., NEUMANN, E.-R., DAVIES,G.R., TIMMERMAN, M.J., HEEREMANS,M. & LARSEN, B.T.
(eds) Permo-CarboniferousMagmatism and Rifting in Europe. Geological Society, London, Special Publications,
223, 243-258. 0305-8719/04/$15 9 The Geological Society of London 2004.
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244
L.A. KIRSTEIN E T AL.
Fig. 1. Location map of the British Isles showing the
xenolith and megacryst localities: R, Rinnibar; SC,
Streap Com'laidh; EN, Elie Ness; F, Fidra. The main
tectonic boundaries between Laurentia and Avalonia
are included together with the Iapetus suture. The
Variscan front marks the northernmost deformation
zone associated with the Permo-Carboniferous mountain-building event to the south. The star surrounded
by a black circle indicates the proposed position of the
'Jutland' plume (Ernst & Buchan 1997). Grey shading
reflects terrane boundaries of accreted Laurentia
terrane (after Pharaoh 1999): dark grey, Laurentia
with Lewisian basement; medium grey, Northern
Highlands Terrane with Lewisian inliers in Caledonides; pale grey is divided into the Grampian Highlands between the Great Glen Fault (GG) and the
Highland Boundary Fault (HB), and the Midland
Valley between the HB Fault and the Southern
Uplands Fault (SUF) with early Proterozoic basement
(Brown 1991). AMF, Ardross-St. Monans Fault.
plume-like mantle source component (Smedley
1988). Ernst & Buchan (1997) traced dyke
swarms across northern Europe and proposed
the presence of a large mantle plume, termed the
'Jutland' plume, beneath the Skagerrak.
In searching for a novel way to resolve the
long-standing debate regarding the origin of
contemporaneous Permo-Carboniferous magmatism across northern Europe we have studied
the helium isotope composition of mantle
xenoliths entrained within the magmas. The
helium technique has recently been used successfully to detect mantle plume involvement in
ancient large igneous provinces, including
Siberia (250 Ma), Deccan (65 Ma) and Ethiopia
( < 4 2 M a ) (Basu et al. 1993, 1995; Marty et al.
1996). The ability to separate fresh olivine
phenocrysts from ancient lavas has been key to
the success of these studies. Unfortunately, fresh
olivine is rarely preserved in the Permo-Carboniferous volcanic rocks of northern Europe;
consequently, fresh olivine from mantle xenoliths was utilized instead. In doing so we assume
that the mantle xenoliths are likely to record the
He isotope signature of the host magma, as
demonstrated in a study of mantle xenoliths and
lavas from the Massif Central, France by Dunai
& Baur (1995).
We report the results of a He isotope study of
Permo-Carboniferous
lithospheric
mantlederived xenoliths from Scotland to assess if
there is any evidence that large-scale mantle
plume activity was responsible for the PermoCarboniferous magmatism. Ideally, we would
have liked to study a comparative suite of
xenoliths from the Oslo Rift of Norway;
unfortunately, no mantle xenoliths have been
located in the Oslo Rift and thus the distribution
of our sample locations is geographically
restricted. This study should thus be regarded
as a pilot study of the helium isotope signature
of the Permo-Carboniferous of northern Europe.
Helium isotope signature of mantle plumes
Helium isotope ratios are generally accepted as
providing a powerful way of determining the
presence of a deep, undegassed mantle plume
component in the source of basaltic volcanism
(Courtillot et al. 2003 and references therein).
Mid-ocean ridge basalts (MORB) are characterized by 3He/4He of 8 + l Ra (Ra = atmospheric
ratio of 1.39 x 10 -~ and are thought to be
derived from a degassed upper mantle source
(Kurz et al. 1982; Hilton et al. 2000). Magmas
enriched in 3He relative to MORB are inferred
to originate by partial melting of less degassed,
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HELIUM ISOTOPE STUDIES
deep-mantle sources, which may also include a
lower-mantle component. Such magmas, with
3He/4He of up to 38 Ra, include those erupted on
Loihi (Hawaii) and Iceland (Hilton et al. 2000)
and seem unequivocally plume-related (Courtillot et al. 2003). Thus, high 3He/4He ratios
(>8_+ 1 Ra) are thought to be evidence of
magmas derived from deep-seated mantle
plumes, possibly originating from thermal
instabilities at the core-mantle boundary. Such
reasoning has been used to detect regional
plume-related inputs to large igneous provinces
including the Permo-Triassic Siberian traps, the
Tertiary Ethiopian flood basalts and the North
Atlantic Volcanic Province (NAVP) (Basu et al.
1995; Marty et al. 1996; Stuart et al. 2000).
Ocean islands with 3He/4He lower than
MORB, such as St. Helena and Tristan da
Cunha, are considered to originate from a more
enriched mantle source region in the upper
mantle containing a recycled oceanic crustal
component (Hanyu et al. 1999). These hot-spot
islands are clearly plume-related, with associated
hot-spot tracks and a flood basalt province in
the case of Tristan da Cunha (Courtillot et al.
2003). The exact depth of origin of the thermal
instabilities responsible for ocean island volcanism is the subject of debate, with authors arguing
for recycled material being stored both at
670km and the core-mantle boundary (Hofmann & White 1982; Sekine et al. 1986;
Courtillot et al. 2003). Ratios of 4-8 Ra are
commonly measured in basalts with HIMU or
enriched mantle (EM) signatures, and are
consistent with their derivation from mantle
source regions with high Th and U or mixing
with enriched mantle lithosphere (Hanyu et al.
1999; Hilton et al. 2000).
245
rapid exchange of noble gases between the
xenolith and host magma at magmatic temperatures (Dunai & Baur 1995). Melt inclusions have
not been seen in the samples analysed.
In this study we present helium isotope data
from mantle xenoliths from the Midland Valley
of Scotland, the Scottish Highlands and Orkney
(Fig. 1). The helium isotope ratios can provide
important information regarding the subcontinental mantle (SCM) beneath Scotland during
the Permo-Carboniferous, and have the potential to establish whether a large, deep-source
mantle plume was involved in the petrogenesis
of the magmatic rocks; the 'Jutland event' of
Ernst & Buchan (1997). The sample localities lie
peripheral to the proposed focus of PermoCarboniferous magmatic activity and span two
basement terranes - the Northern Highlands
Terrane and the Midland Valley Terrane (Pharaoh 1999; Upton et al. 2001) (Fig. 1). It has
recently been shown that the 3He anomaly
associated with present-day mantle plumes, e.g.
Iceland, correlates with the region of thermal
uplift, and is therefore more pervasive than the
Sr, Pb and Nd isotope signature of the plume
(Taylor et al. 1997). Thus, samples from the
periphery of any plume-related magmatic province should potentially record plume-related
helium ratios, if any are present. In this study we
compare information regarding the nature and
setting of the Permo-Carboniferous magmatism
with that of the Tertiary, high-3He plumerelated North Atlantic Volcanic Province (Stuart
et al. 2000), with the aim of determining whether
a similar deep plume, or more regional tectonic
activity, caused the Permo-Carboniferous magmatism. The helium isotope results are also
discussed in terms of the more recent Cenozoic
volcanism of Europe.
Why study mantle xenoliths?
At the outset of this investigation it was
established that the occurrence of fresh phenocrysts of olivine in lavas of the Permo-Carboniferous of northern Europe was negligible.
However, a feature of late Palaeozoic magmatism in the British Isles, as distinct from mainland Europe, is the occurrence of silicaundersaturated magmas containing abundant
mantle xenoliths, some of which contain fresh
olivine (Upton et al. 1983). These xenoliths are
transported within dykes, sills and lavas to
shallow levels. The noble gases trapped in
subcontinental mantle xenoliths are mainly
contained in fluid and melt inclusions, and
most probably preserve more information about
the source of the host magma that entrained the
xenoliths than the xenolith source, due to the
Tectonic setting
The lithosphere of Scotland is made up of a
number of terranes brought together by strikeslip movements (Brown 1991). These faultbounded terrane blocks can be divided into
several major upper-crustal age provinces (e.g.
Menzies & Halliday 1988; Pharaoh 1999) (Fig.
1). These vary from Archaean to palaeoProterozoic domains west of the Moine Thrust
in the NW, through Moinian and Dalradian
domains to the north of the Highland Boundary
Fault and Proterozoic and Palaeozoic domains
in the SE (Pharaoh 1999) (Fig. 1). The collision
of Laurentia and Avalonia, through subduction
and closure of the Iapetus Ocean during the
Ordovician and Silurian, culminated in extensional magmatism and basin formation in the
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246
L. A. KIRSTEIN ET AL.
Early Palaeozoic (Thirlwall 1986; Pharaoh
1999). Thus, the Permo-Carboniferous magmas
ascended through different lithospheric domains
en route to the surface. The samples analysed
here are from two distinct terranes - the
Northern Highlands Terrane and the Midland
Valley Terrane.
The Northern Highlands Terrane is bounded
to the north by the Moine Thrust and to the
south by the Great Glen Fault (Fig. 1). Two
samples are located on this terrane; Streap
Com'laidh near Glenfinnan in the SW, and
Rinnibar, Orkney in the NW. The crust of the
Northern Highlands Terrane is approximately
27km thick and is composed of Caledonian
metamorphic rocks (6km) and pre-Caledonian
crust (18 km) (Bamford et al. 1977). Accretion of
the Northern Highlands Terrane to the Lewisian
Complex occurred in the early Proterozoic
(Dickin & Bowes 1991). The upper crust at
Streap Com'laidh is Moinian, while at Rinnibar
it is Upper Palaeozoic (Halliday et al. 1993). The
age of the basement complex of Orkney is not
known, although outcrops in the western islands
have been compared with Lewisian inliers found
in the Moine (Mykura 1976).
The Midland Valley Terrane is the low-lying
central part of Scotland bounded by the Highland Boundary Fault to the north and the
Southern Uplands Fault to the south (Fig. 1).
The samples from Elie Ness and Fidra crop out
on the east coast of this terrane. The Midland
Valley Terrane is thought to represent Proterozoic crust with a buried Palaeozoic arc and
fore-arc, and has 20-25 km of basement, which
dramatically thins at the Southern Uplands
Fault (Cameron & Stephenson 1985).
Table 1. Selected microprobe analyses illustrating the
compositional variation in olivine from Fidra (Fid 311,
Fid311) and Streap Com'laidh (StP 2, StP 3). Data for
Rinnibar (CN2) (M. Coltorti pers. comm).
Sample
SiO2
FeO
MgO
CaO
NiO
Fo
Fid 311
Fid
StP 2
StP 3
CN2
40.6
11.3
48.4
0.05
0.37
88.3
39.0
11.4
48.0
0.08
0.33
88.2
41.4
9.2
48.1
0.08
0.36
90.3
40.7
9.4
48.6
0.04
0.37
90.2
40.9
9.7
49.3
0.04
0.37
89.7
by subduction-related processes during the
Proterozoic and Palaeozoic (Menzies & Halliday
1988; Halliday et al. 1993; Pharaoh 1999; Upton
et al. 1999, 2001).
Four localities in Scotland were sampled for
either olivine or garnet - Fidra and Elie Ness on
the east coast of the Midland Valley Terrane;
and Streap Com'laidh and Rinnibar, Orkney
from the Northern Highlands Terrane (Fig. 1).
Olivine-bearing spinel lherzolites were selected
from Fidra, Streap Com'laidh and Rinnibar,
while pyrope megacrysts were collected from the
Elie Ness vent. The CaO and Cr203 concentrations in the olivine compositions reported in
Table 1 clearly indicate that these samples
represent mantle xenoliths rather than cumulates
(Hervig et al. 1986). Xenoliths from these
localities have been previously studied in some
detail, providing information on geochemistry,
Sr-Nd isotope composition, mineralogy and
petrography (Chapman 1976; Praegel 1981;
Menzies & Halliday 1988; Halliday et al. 1993;
Upton et al. 1999; Downes et al. 2001; Bonadiman et al. 2002).
Sample locations and host-rock compositions
Ultramafic xenoliths in continental basalts may
be derived from variable depths in the lithosphere and record information about the nature
of the subcontinental lithospheric mantle
(SCLM), or represent fragments of cumulate
bodies in deep-crustal magma chambers. A
fundamental observation is that the high xenolith load precludes significant shallow-level
residence for the host magmas, otherwise the
xenoliths would have been lost through gravitational settling and resorption. This is important
in understanding the origin of any radiogenic
helium present in the analysed samples. Crustal
and mantle xenoliths entrained within the
Permo-Carboniferous magmas of Scotland
have been interpreted as crustal cumulates,
fragments of a magmatic underplate, and
samples of mantle lithosphere enriched partly
N o r t h e r n H i g h l a n d s Terrane
Rinnibar, South Ronaldsay, Orkney. The 1 mwide xenolith-bearing dyke sampled at Rinnibar,
South Ronaldsey is one of a series of
monchiquite dykes intruded in the area during
the Permo-Carboniferous. The dykes are
primarily volatile-rich alkali olivine basalts
with kaersutite _+ biotite (Upton et al. 1992).
The Rinnibar dyke is composed largely of
xenoliths, predominantly of spinel lherzolite,
but also of harzburgites, wehrlites and
clinopyroxenites with little interstitial host
basalt (Bonadiman et al. 2002). The wholerock trace-element contents vary from slightly to
strongly light rare earth element (LREE)enriched (Bonadiman et al. 2002). Olivine up
to l m m in size was separated from spinel
lherzolites for analysis. Representative olivine
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HELIUM ISOTOPE STUDIES
247
analyses are provided in Table 1; CaO contents
are less than 0.1 wt% consistent with a mantle
xenolith origin (Hervig et al. 1986).
1984). Inclusions in the garnets are rare and no
melt inclusions were identified under the
microscope.
Streap Com'laidh. Walker & Ross (1954) were
the first to describe this xenolithic dyke locality
on the south side of the corrie at Streap
Com'laidh, near Glenfinnan; the dyke intrudes
Moine granulites. The xenoliths comprise spinel
lherzolite, websterite, pyroxenite, and Moine
quartz-bearing granulites and schists (Walker
& Ross 1954; Praegel 1981). The xenoliths are
< 10 cm in size, and are locally concentrated in
channels possibly by flow differentiation
(Praegel 1981). The spinel lherzolites contain
olivine, orthopyroxene, clinopyroxene and
spinel. Grain size varies from 0.1 to 5mm.
Olivine, up to 1 mm in size, was separated for
helium analysis. Table 1 reports microprobe
data for olivine from the spinel lherzolites, the
Fo contents of which are marginally higher than
those reported by Praegal (1981). The data are
consistent with a restitic mantle rather than
cumulate origin due to their low CaO contents
<0.1 wt% (Hervig et al. 1986). The host dyke is a
fine-grained alkali basalt with euhedral
phenocrysts of olivine (now altered) and
clinopyroxene.
The
radiogenic
87Sr/86Sr
(<0.7107) and low 143Nd/144Nd (<0.51268)
measured by Menzies & Halliday (1988) for
the Streap Com'laidh xenoliths suggests
derivation from enriched mantle beneath the
Scottish Highlands. Bonadiman et al. (2002),
however, report less LREE enrichment in
xenoliths from Streap Com'laidh than in those
from Rinnibar.
Fidra. Fidra is a basanite sill containing spinel
lherzolite, wehrlite, pyroxenite and granulite
xenoliths (Hunter et al. 1984; Downes et al.
2001) (Fig. 1). The locality is along strike from
the main Southern Uplands Fault (Fig. 1)
(Cameron & Stephenson 1985). The olivine
compositions of the spinel lherzolites suggest a
restitic mantle origin (Hervig et al. 1986) (Table
1). Texturally, the samples are porphyroclastic
with orthopyroxene, olivine and clinopyroxene.
The xenoliths have low 87Sr/S6Sr (<0.7038), high
143Nd/144Nd (< 0.51309) and high MgO, and are
considered to be derived from shallow
lithospheric mantle. The Sr-Nd isotope
compositions suggest that they do not represent
ancient enriched lithospheric mantle and there is
little evidence of crustal contamination of the
host basanite (Downes et al. 2001).
M i d l a n d Valley Terrane
Elie Ness. Numerous Permo-Carboniferous vent
localities are concentrated in the Midland
Valley, many of which are close to the Fife
coast, with others close to the Southern Uplands
Fault Zone (Upton et al. 1999, 2004). Elie Ness
is a vent characterized by hydrous pyroxenite
and anorthoclasite xenoliths, and high-pressure
alkali feldspar, garnet and kaersutite megacrysts
(Chapman 1976; Donaldson 1984; Upton et al.
1999). Garnet occurs as discrete, equant
megacrysts or crystal fragments. Although the
crystals collected ranged up to a maximum of
3 ram, garnets up to 25mm in size have been
reported (Chapman 1976). The garnets are
optically homogenous and are thought to have
been brought to the surface by a rapidly
ascending water-bearing alkali basalt melt with
minimal interaction with the crust (Donaldson
Petrography of fluid inclusions
Three samples were subjected to microthermometry studies of fluid inclusions; two Fidra
samples from the Midland Valley Terrane and
one sample from Streap Com'laidh in the
Northern Highlands Terrane. Inclusions are
observed in olivine, clinopyroxene, orthopyroxene and spinel. All three samples contain fluid
inclusions, the vast majority of which are singlephase (liquid) inclusions at room temperature.
Low-temperature microthermometry on fluid
inclusions was performed at the Vrije University
Amsterdam on a Linkam TP/91-THMS 600
stage, cooled by liquid nitrogen. Temperature
measurements were calibrated using a standard
fluid inclusion sample, which has a phase
transition at - 5 6 . 6 ~ known to an accuracy
of better than _ 0.2 ~ Liquid-vapour transitions on cooling are observed in the range from
-10 to 45 ~ Less commonly bi-phase (liquidvapour) and single-phase (gas-rich) fluid inclusions are observed at room temperature. The
shapes of the fluid inclusions are subspherical or
approaching negative crystals, with sizes varying
from less than 2 up to 15/~m for high-density
inclusions and up to 50#m for gas-rich, lowdensity inclusions. Most inclusions occur as
trails with tens of inclusions near the border of
mineral grains (Group 1). Randomly distributed
inclusions also occur, sometimes in groups, away
from grain boundaries (Group 2) (Fig. 2). Spinel
crystals contain isolated single-phase fluid inclusions or mixed inclusions that contain crystalmelt +fluid.
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248
L.A. KIRSTEIN ET AL.
Fig. 2. Photomicrographs of primary, high-pressure
CO2 inclusions in olivine from Streap Com'laidh (a)
and Fidra (b). Note the lack of trapped aqueous-rich
inclusions or evidence of extensive metasomatism.
There is generally no observable difference in
the range of fluid inclusion densities in different
minerals within an individual sample. The
exception is that ultra-pressure fluids have only
been found in olivine crystals from the Streap
Com'laidh sample. The range of results for each
sample and the different types of inclusions are
summarized in Table 2. The observed range of
freezing (solid-vapour) temperatures ( - 6 3 to
- 9 8 ~ and solid-phase melting temperatures
( - 56.7 __+0.5 ~
rarely until - 58 ~
imply
that the fluid inclusions are mostly pure COe.
In rare cases the inclusions contain minor other
gases. Homogenization temperatures (Th, to
liquid) range from -2.5 to - 3 5 ~ for highdensity inclusions and from 9 to 30.2 ~ for lowdensity inclusions. These Th values correspond
to a range of CO2 densities from 0.34 to
1.1 g cm -3. In addition to low- and high-density
inclusions, ultra-pressure, super-dense fluid inclusions are observed. These inclusions have the
following phase transitions: liquid-vapour transition at temperatures from -71 to - 7 8 ~
freezing (solid-liquid) at temperatures from - 81
to - 9 9 ~ solid-phase melting (renewed gasphase origin) at temperatures from - 5 8 to
- 5 6 . 6 ~ and rapid homogenization to liquid
at temperatures from - 56.6 to - 54.2 ~ These
inclusions have densities from 1.17 to
1.18 g cm -3. Similar super-dense fluid inclusions
have previously only been recorded in peridotite
xenoliths from Oahu, Hawaii and SE Australia
(see review by Andersen & Neumann 2001).
Estimated densities and entrapment pressures
of the studied fluid inclusions have been
calculated based on the equation of state
following Holloway (1981) and an assumed
average equilibrium temperature of 1150 ~
(Table 2). These pressure estimates are therefore
considered very conservative because: (i) temperatures of the host alkali basalts/basanites
were probably greater than 1150 ~ and (ii)
most inclusions will have been deformed and
partially decrepitated during transport to the
surface resulting in re-equilibration at lower
pressures (see review by Andersen & Neumann
2001). It is assumed that the fluids in the fluid
inclusions are derived from degassing of the host
magma. This implies that the fluids were derived
from greater depth and that entrapment only
occurs after sufficient vapour pressure had been
established.
In respect of the He isotope studies discussed
here, it is important to constrain the proportion
of fluids trapped at different depths. The
calculated pressures shown in Table 2 imply
minimum entrapment depths between 4 and
50km. The Streap Com'laidh sample contains
three groups of inclusion, two of which were
entrapped at subcrustal levels. The low-pressure
group of inclusions only represent a small
proportion (< c. 20%) of the entire inclusion
population. The Fidra samples contain a greater
proportion of inclusions trapped at crustal levels
(c. 30%), but the majority of inclusions are of the
higher-pressure type. Coupled with the denser
nature of the high-pressure inclusions, this
conclusion implies that the entire fluid inclusion
suite predominantly contains fluids derived from
the mantle.
Age constraints
Mantle xenolith-bearing dykes, sills and vents in
Scotland were initially considered to be entirely
of upper Palaeozoic age (Upton et al. 1983;
Gallagher & Elsdon 1990). However, following
recent programmes of dating of a number of
xenolith localities including Inver, Co. Donegal,
Ireland and Loch Roag, Isle of Lewis, some
of these intrusions have been shown to be
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HELIUM ISOTOPE STUDIES
249
Table 2. Fluid inclusion data.
Sample
(number of measurements)
Tm
(~
(~
Th
(to liquid)
Densit~r
(g cm- ')
Pressure
(kbar)
Depth
(km)
0.3443.69
0.84~.94
0.94-l.07
1.2-3.4
5.1-5.5
6.8-10.2
4-11
17-18
22-34
0.94-1.00
6.8-8.2
22-27
4-5
8.2-11.2
14,8-15.3
13-17
27-37
49-50
Fid-I (n = 105)
Low pressure
Middle pressure
High pressure
( - 56.2) - (- 58)
- 56.2
( - 56.1) - ( - 57.3)
(+ 26) - (+ 30.2)
9-12
(- 2.5) - ( - 28.5)
- 56.6
(- 2.7) - ( - 14)
- 56.6
(- 56.2) - ( - 56.7)
(- 56.6) - (- 58.2)
(+ 15) - (+ 19)
( - 14) - (- 35)
(- 54.2) - (- 56.6)
Fid-311 (n = 30)
High pressure
StP (n = 75)
Low pressure
High pressure
Ultra pressure
0.78~).82
1.00-1.1
1.17-1.179
n, number of measured fluid inclusions; Tin, temperature of solid melting; Th, homogenization temperature;
depth, has been calculated as 3.3 km kbar- J; Fid, Fidra; StP, Streap Com'laidh.
Palaeogene in age (Menzies et al. 1987; Kirstein
& Timmerman 2000; M. J. Timmerman pers.
comm.). Constraining the age of the host
magmas is essential if the tectonic significance
of the magmatism is to be understood.
The Elie Ness vent has been dated at
311 _.+ 10 Ma by the A r - A r technique (Timmerman 2003 volume). The Fidra basanitic sill was
recently suggested to be Permian (264 _+ 10 Ma)
following a summary of available age data by
Downes et al. (2001). Streap Com'laidh was
assigned an age of 280Ma by Menzies &
Halliday (1988) in their isotopic investigation
of the Scottish lithosphere. Finally, although the
Rinnibar dyke has not been dated, there is good
reason to assume that it is Permian or PermoCarboniferous in age. Reported ages from
samples of camptonite-monchiquite dykes
from Orkney vary from 235 to 292 Ma (Speight
& Mitchell 1979). These age variations are
consistent with the observation that much of
the magmatism during the Silesian and Permian
in Scotland was alkalic (Upton et al. 2004). No
dykes younger than Permian have yet been
found on Orkney (Mykura 1976).
H e f i u m i s o t o p e s in m a n t l e x e n o f i t h s
Helium isotopes are sensitive tracers of magma
origins and also of higher-level crustal processes
including crustal assimilation (e.g. Hanyu et al.
1999; Hilton et al. 2000). Ultramafic xenoliths
may, therefore, inherit a composite rare gas
signature of melts derived from the asthenosphere, from the lithospheric mantle and, if
residence times of the host magmas are long
enough, the continental crust.
Helium isotope ratios in minerals can be
affected by radiogenic ingrowth from the decay
of U and Th. This presents the greatest problem
in obtaining an accurate measurement of the
magmatic 3He/4He, as with time this process
reduces the 3He/4He. Olivine has low distribution coefficients for U (0.00001) and Th
(0.00001) (Beattie 1993), and therefore has low
U and Th contents, as well as exhibiting the
lowest 4He diffusion into fluid inclusions (Hart
1984; Trull & Kurz 1993). The garnet meltpartition coefficient is greater for U (0.01) than
Th (0.00 I) (Beattie 1993); however, both are still
relatively low. Consequently, in order to minimize the effect of > 2 5 0 M a of radiogenic
ingrowth (minimum age constraint from Fidra),
these minerals were separated for helium isotope
analyses.
The concentration of 3He in a mineral can be
increased either by thermal neutrons (6Li (n, ~),
3H (/?-) 3He) (Mamyrin & Tolstikhin, 1984) or
cosmic ray spallation (Lal 1987). A step-wise
crushing technique, whereby gases from fluid
and melt inclusions are liberated in the first
incremental steps, followed by an increasing
contribution from the lattice component during
more prolonged crushing, was employed to limit
the release of these post-eruption He components. Only after extreme crushing are the
lattice-hosted radiogenic and cosmogenic products released (Kirstein & Timmerman 2000;
Scarsi 2000).
Continental crust has low 3He/4He (c. 0.01
Ra). Crustal assimilation thus tends to lower the
3
He/ 4 He ratios in a magma (Hilton et al. 2000).
Consequently, crustal contamination and posteruptive radiogenic ingrowth of 4He must be
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250
L. A. KIRSTEIN E T AL.
ruled out if the helium isotope ratios measured
in the Scottish xenoliths are to provide further
information about the nature of the mantle
beneath Scotland during the Permo-CarboniferOUS.
Sample preparation and helium isotope
analysis
Olivine was separated from spinel lherzolites
from three locations and garnet megacrysts
from Elie Ness. For details of sample preparation see Kirstein & Timmerman (2000). Olivines
and garnets from the 500-1000/~m-fraction
were hand-picked to ensure >99% purity. Gases
were extracted from samples by crushing in
vacuum at the Vrije Universiteit, Amsterdam.
The crusher consists of an Inconel tube with a
fiat bottom and a stainless steel piston that is
lifted by a solenoid coil outside the vacuum
system. Samples were loaded into a rotating
bucket above the crushing tube. The crushing
tube and sample holder were baked overnight
at 250 and 150~
respectively. Initially,
samples were crushed for 40 pulses and the
resultant gas released into the gas line. The
samples were crushed two further times for a
progressively greater number of pulses, 80 and
320 pulses, respectively. The separation and
cleaning of helium in the gas line at the Vrije
Universiteit, Amsterdam is outlined by van
Soest et al. (1998). The helium was measured
on a VG5400 rare gas mass spectrometer
equipped with a Johnson electron multiplier.
For each experiment the number of crushing
steps was varied according to the initial gas
content. Blank measurements over the duration
of the study were typically < 0.07 ncc STP 4He.
Blank 3He abundances were below the detection limit.
Results
An incremental crushing technique was designed
to separate the components produced during
post-eruption radiogenic ingrowth. The helium
isotope ratio (R) normalized to the atmospheric
ratio (R,) from each incremental crush is plotted
against the number of pulses in each crush in
Figure 3. The total He isotope results, rather
than incremental steps, are given in Table 3 and
Figure 4. The total helium isotope data record a
range of values from 0.97 Ra to 6.3 R,. Owing to
potential radiogenic ingrowth of 4He, these
values are minimum estimates of the 3He/4He
signature of the host magmas.
8
StC 2
2
~
~EN1
1
Fidra 1
0
i
0
50
J
i
~
,
,
i
100 150 200 250 300 350
Number of pulses
Fig. 3. The variation in 3He/4He (R) normalized to the
atmospheric ratio (Ra) v, number of pulses. Pulses per
crush vary from 20 to 320. The majority of the samples
were measured after 40, 80 and 320 pulses.
Discussion
The variation observed in the measured 3He/4He
ratios is potentially a function of radiogenic
ingrowth (~ particle production), which results
from in situ decay of U and Th and, to a much
lesser extent, Sm. This radiogenic 4He is
primarily released from crystal defects during
crushing, although some excess 4He may originate in fluid inclusions due to implantation of
particles (Stuart et al. 1995).
Radiogenic ingrowth has been calculated for
both olivine and garnet assuming that the system
has remained closed since it cooled. Closure is
assumed to have occurred instantaneously in
these rapidly cooled samples, so no diffusive loss
has occurred. Over a period of 300 Ma (average
sample age) ~ 1.2 x 10-9 cc STP [4He] could be
produced in olivine and ~ 2 . 9 x 10-7 cc STP
[4He] in garnet. These calculations have been
made using the average chemical composition
for the whole rock (U = 8.8 ppm; Th = 1.2 ppm)
and partition coefficients for olivine and garnet
from Beattie (1993) and Hauri et al. (1994),
respectively. This equates to 0.00001 ppm U and
0.00009ppm Th in olivine, and 0.0058ppm U
and 0.0087ppm Th in garnet. Standard decay
constants for U and Th are from Steiger & Jfiger
(1977). The low-helium isotope ratios of samples
EN 1, Fidra 1 and R1 suggest that these samples
may contain a significant radiogenic component.
However, R1 and the remaining samples (StC I StC 3) contain at least 10 times more 3He than
the Fidra sample, strongly implying that these
samples retain their pre-eruptive He isotope
ratios. This conclusion is supported by the
multiple samples analysed from Streap Com'laidh. The lack of variation in the helium isotope
ratios with helium content demonstrates that the
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HELIUM ISOTOPE STUDIES
251
Table 3. Helium isotope data and 2~ concentrations for the Scottish xenolith samples. Neon isotopic compositions
are not reported here as all measured ratios were within error o f atmospheric composition.
Sample name
StC 1"
StC 2
StC 3
RI
EN 1
Fidra 1
Mineral
[4He]
(10-8cc STP/g -l)
O1
O1
O1
O1
Gnt
O1
10.6
63.3
9.26
2.78
2.57
2.62
3He/4He
(R/R~) • 1 ~
6.01 +
6.11 +
6.33 +
2.84 +
0.97 +
1.08 +
0.17
0.21
0.21
0.29
0.05
0.11
2~
(10 -ll cc STP)
Weight (mg)
2.64
9.0
7.77
11.2
1.64
1.52
250
250
250
150
500
250
Note: Errors are lo- and include the analytical uncertainty as well as the reproducibility of the calibration
measurements throughout the experiments.
StC, Streap Com'laidh; EN, Elie Ness; R, Rinnibar.* Indicates final step omitted due to a large decrease in 3He.
O1, olivine phenocrysts from spinel lherzolite; Gnt, high-pressure garnet megacrysts.
cc STP, cubic centilitres at standard temperature and pressure.
samples have not been significantly modified by
radiogenic helium addition (Fig. 3).
In theory, the initial crush data should
provide the best estimates of the source
3He/4He of the host basalts, as with p r o l o n g e d
crushing an increased a m o u n t o f 4He is released
from the mineral lattice and so the 3He/aHe ratio
is decreased. The low 3He/aHe o f the samples
Fig. 4. Averaged helium isotope data for the Scottish xenoliths and megacrysts. Different grey shades represent
different mantle reservoirs including MORB and enriched mantle (Hanyu et al. 1999; Hilton et al. 2000). Error
bars are 2a error.
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252
L.A. KIRSTEIN ET AL.
suggest that, compared to MORB, the mantle
source region of the magmas is characterized by
a greater time integrated U + T h / H e ratio, or
that higher-level processes such as mixing,
sediment addition or crustal interaction have
introduced radiogenic helium. The measured
3He/4He ratios, even when allowing for radiogenic ingrowth, are more radiogenic than North
Atlantic MORB (Hilton et al. 2000).
The analytical approach is designed so that
the initial crush should provide the best estimates of the source 3He/4He of the host basalts.
Prolonged crushing is expected to release 4He
from the mineral lattice and so the 3He/4He ratio
is decreased. The Streap Com'laidh data are an
internally consistent dataset with three separate
xenolith samples having 3He/4He of 6 _+ 0.5 R~
at variable [~He] (Fig. 4). These results clearly
demonstrate the consistency of our analytical
approach and that radiogenic 4He production is
not significant in these samples. The Rinnibar
sample (R1) gave a lower ratio, and is more
difficult to interpret as it may have suffered from
addition of radiogenic 4He. Nevertheless, the
entire dataset appears to indicate that the
magma sources were characterized by U + Th/
He enrichment relative to MORB source mantle.
Interestingly, both Streap Com'laidh and Rinnibar are situated on the Archaean-Proterozoic
Northern Highlands Terrane, while the samples
that may be compromised most by 4He addition
are from the Proterozoic-Palaeozoic Midland
Valley Terrane.
Source enrichment, crustal contamination
a n d sediment addition
The helium isotope ratio typical of continental
crust is c. 0.01 R,. Consequently, any crustal
contamination of the host magma(s) is liable to
lead to the xenoliths inheriting a low He isotope
ratio. The chemical compositions of the host
basalts, however, indicate that little crustal
contamination occurred (Praegal 1981; Upton
et al. 1999; Downes et al. 2001). For example, at
Streap Com'laidh the host rock contains
10.3wt% MgO and has OIB-like trace-element
ratios (high Nb/La (1.5), Ti/Y (670) and low Zr/
Nb (3.3)). Thus, a large amount of high-level
crustal contamination (>2%) is unlikely. The
transportation of helium by fluids circulating
from crustal wall rocks is also a potentially
important process. In this case, however, fluid
circulation into the magmas is not considered
significant for a number of reasons. First, the
rapid ascent of the xenolith-bearing host magmas allows little time for mature hydothermal
systems to be established. Secondly, in other
locations where the helium isotope ratio of
mantle xenoliths has been used as a proxy for
the helium signature of the source of the host
magma, high 3He/4He ratios have been preserved, e.g. Inver, Co. Donegal (Kirstein &
Timmerman 2000); in addition, any fluids would
need to be at temperatures in excess of 600 ~ to
affect the helium isotope ratios by diffusion.
Such high-temperature fluids would result in
severe metasomatism of the xenoliths, yet little
metasomatism is observed and there is no
evidence of trapped aqueous-rich fluid inclusions
(Fig. 2).
Sediment entering the mantle at subduction
zones varies markedly in composition, with U
and Th contents also varying considerably. For
example, U contents can vary between 0.3 and
2.4 ppm, and Th from 1 to 15 ppm (Plank &
Langmuir 1998). As a result the helium isotope
ratio of the subducted sediment also varies. In
the Lesser Antilles arc system the sediment has
been shown to vary from <0.02 Ra to 2 Ra (van
Soest 2000). As sediment is subducted, however,
the helium is expelled in the fore-arc environment; thus, little, if any, is transported to depth.
Time-integrated source enrichment is an
alternative and geologically realistic means to
produce low 3He/4He. Decay of U and Th in
oceanic crust and sediment recycled into the
mantle will result in a radiogenic helium isotope
signature. The incorporation of such a recycled
component in the source of some of the PermoCarboniferous magmas could, over time,
increase the concentration of 4He and lower
the original helium isotope ratio.
Bulk addition of U and Th to the mantle
source has been modelled, with the amount of
radiogenic ingrowth of 4He from the time of
enrichment calculated (Table 4). Such calculations highlight the potential for changing the
3He/4He of a MORB-source mantle reservoir.
Simple time-integrated ingrowth of 4He following the addition of, for example, 10% degassed
terrigenous sediment would change the 3 H e /4H e
from 8.5 Ra to 6.5 Ra in 70Ma. The highest
measured helium ratio is 6.5 _+ 0.5 Ra and 70 Ma
is the approximate time difference between the
end of subduction (c. 410Ma) and start of
extensional magmatism (c. 340 Ma) in Scotland
(Smedley 1988).
Streap Com'laidh and Rinnibar are located
on the Northern Highlands Terrane and so the
mantle source of the magmas is unlikely to have
been affected by Caledonian (late SilurianLower Devonian) subduction-related sediments
and fluids. Enrichment of the subcontinental
lithospheric mantle in the northern Highlands is,
however, apparent from studies of mantle
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HELIUM ISOTOPE STUDIES
253
Table 4. Model calculations of source enrichment at different times using standard decay constants for U and Th
( Steiger & Jfiger, 1977), and model parameters after Dunai & Baur (1995). M O R B source mantle after Kurz et al.
(1982) and Jochum et al. (1983).
Location
Streap Com'laidh
Rinnibar
Time since contamination
(Ma)
Added subducted component
to decrease 3He/4He (%)
70
100
1900
2800
70
100
1900
2800
10
7
0.25
0.12
40
30
2
1.25
Note." UMORRsource= 1.38 X 10-la mo1 g-l, ThMoR~ source= 3.45 X 10-l~ mol g-l; UcRusv = 1,176 • 10-8 tool
g-l; ThcRt:sT=4.586 x 10-8 mol g-l.
xenoliths (Menzies & Halliday 1988). If the
subcontinental lithospheric mantle has remained
coupled with the overlying crust, which is
Lewisian (c. 2.8Ga), then the amount of
U + T h enrichment required to reduce the
helium isotope ratio from MORB-like values at
Streap Com'laidh would be < 0.15% (Table 4).
The Streap Com'laidh alkali basalt dyke is
considered to have originated by small-degree
melting of an enriched upper-mantle source
(Praegel 1981). The radiogenic Sr and Nd
isotope signatures of the host magma reported
by Halliday et al. (1993) are consistent with an
ancient enrichment hypothesis for the measured
helium isotope ratios. If the trace-element
enrichment of the source of Streap Com'laidh
is related to subduction, then the enrichment
process must have occurred more than 300 Ma
prior to the Permo-Carboniferous magmatic
event, and possibly much earlier in association
with the accretion of the Northern Highlands
Terrane to Laurentia.
It is possible to model the measured helium
ratio for the Rinnibar sample using a similar
approach (Table 4). The calculations indicate
that if the enrichment of the source region
occurred during the Phanerozoic, only 2% of a
subducted component is required to decrease the
3He/4He of the xenolith sufficiently (Table 4).
At Fidra and Elie Ness it is apparent that
radiogenic ingrowth and/or mantle metasomatism are the principal factors governing the
measured helium isotope ratios. The Midland
Valley Terrane was undoubtedly affected by the
closure of the Iapetus Ocean and Caledonian
subduction. In order to decrease the helium
isotope ratio from 8 Ra to less than 1 Ra, U + Th
enrichment of the mantle source during Caledonian subduction would have to have been in the
order of 40%. The geochemical compositions of
the alkali basalts do not support such large
amounts of enrichment. Cryptic metasomatism,
however, combined with radiogenic ingrowth,
could potentially be responsible for the high
[4He] and low helium isotope ratios.
The, admittedly, limited dataset shows a
general regional control in the helium isotope
ratios that can be related to the age of the
underlying lithosphere and the nature of the
inferred mantle metasomatism (Fig. 1). The
mantle xenoliths from Streap Com'laidh and
Rinnibar on the Scottish Highlands Terrane
represent ancient enriched lithospheric mantle
(Menzies & Halliday 1988). Enrichment of the
Rinnibar xenoliths in LREE and large ion
lithophile elements (LILE) (including Rb, Ba,
Sr) is greater than at Streap Com'laidh. This
enrichment has been considered as evidence of
variable metasomatism of the source of the
mantle xenoliths in the NE of the Northern
Highlands Terrane by hydrous melts (Upton et
al. 2001; Bonadiman et al. 2002).
The xenoliths at Fidra represent shallow,
relatively young, lithospheric mantle with
depleted and enriched domains. The variation
in REE is considered to be related to cryptic
metasomatism of the mantle lithosphere
(Downes et al. 2001). The Elie Ness locality
contains a profusion of hydrated ultramafic
xenoliths (Chapman 1976) supporting this suggestion.
P l u m e v. no plume
Scotland has experienced three major magmatic
events in the last 350 Ma: at c. 340 and c. 295 Ma
in the Permo-Carboniferous and at c. 60 Ma in
the Tertiary. The characteristics and chemical
signatures of each are very different. The two
periods of volcanic activity in the Permo-
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254
L.A. KIRSTEIN E T AL.
Carboniferous are distinctive, with the early
Carboniferous characterized by a large number
of lava flows together with alkali basalt intrusions. The later activity at c. 295Ma is predominantly intrusive in the form of high-level
sills, dykes and plugs intruding Carboniferous
sediments. Both alkaline and tholeiitic magmatism is recorded. Sporadic activity continued
thereafter until the early Permian, a period of
over 70 Ma (Smedley 1988).
The Tertiary North Atlantic Volcanic Province (NAVP) in contrast was erupted in a much
shorter time-span of 4 Ma (Saunders et al. 1997),
although there is evidence of small magma
pulses and dyke emplacement during the Palaeogene (Kirstein & Timmerman 2000; O'Connor et
al. 2000). The regional high 3He/4He isotope
signature of the large Tertiary NAVP (Stuart et
al. 2000) exhibits a wide range, up to 31 Ra,
consistent with a deep-mantle component rich in
primordial 3He. It has therefore been argued
that the magmas were derived from the protoIceland mantle plume (Stuart et al. 2000). The
absence of high 3He/4He in the analysed PermoCarboniferous xenoliths provides no unequivocal evidence for the involvement of a similar,
regional, deep-seated high-3He plume in this
region during the Permo-Carboniferous.
The highest 3He/4He ratios measured at
Streap Com'laidh are similar to those of basalts
and mantle xenoliths from the European Cenozoic volcanic province with helium isotope
signatures of 6 . 0 _ 1 Ra (Dunai & Baur 1995).
The helium isotope signature of lithospherederived xenocrysts from Europe, Australia and
Africa varies from 3.5 Ra to 8.3 R~ (Gautheron &
Moreira 2002 and references therein). The
measured helium isotope ratios are less than
those of MORB-source mantle because the
subcontinental mantle has been enriched by
fluids, either from the asthenosphere or from
slab dehydration during subduction episodes. If
the host magmas were generated in the lithosphere, their helium isotope composition would
reflect the helium isotope signature of the
subcontinental lithospheric mantle (Gautheron
& Moreira 2002). If, however, the host magmas
were generated from a mantle plume in the
asthenosphere, it is likely that the associated
helium signature of the mantle plume would
have swamped the helium signature of the
lithospheric mantle (Dunai & Baur 1995). It is
therefore possible to discuss the measured
helium isotope ratios in terms of possible endmember mixing models. If we discount highlevel crustal contamination, then there are five
possible models: (1) mixing MORB source
mantle (c. 8R~) with a high-He plume (c.
30Ra); (2) mixing MORB source mantle (c.
8 Ra) with a low-He plume (4-7 Ra); (3) mixing
MORB source mantle (c. 8 Ra) with slab-derived
fluids/subducted sediment (0-2Ra); (4) mixing
ancient mantle lithosphere (1 Ra) with a high-He
plume (c. 30 Ra); or (5) mixing ancient mantle
lithosphere (1 Ra) with a low-He plume (4-7 R~).
In each of the above hypothetical mixing
models ((1)-(5)), the magnitude of the effect on
the mantle source of the host magma will be
dependent on the initial He isotope ratios of the
mixing end-members (i.e. lithospheric mantle
and the mantle plume/subduction component).
It is apparent that models (1) and (2) cannot
produce the range in helium isotope ratios
measured in the Scottish xenoliths. Differentiating between models (3), (4) and (5) is, however,
more difficult on the basis of the current dataset.
A mantle source enriched in incompatible
trace elements is required to explain the geochemical characteristics of the Permo-Carboniferous North Sea volcanism (Heeremans et al.
2004); in addition, Sr and Nd isotope data for
lavas from the Oslo Rift suggest the involvement
of a PREMA (PREvalent MAntle) component
at 300 Ma (Neumann et al. 2004). PREMA is a
common source component in ocean island
basalt (OIB) petrogenesis (Neumann et al. 2004).
The helium isotope data obtained in this
study are clearly limited, and confined to regions
that are potentially far away from the centre of
any proposed Permo-Carboniferous mantle
plume impact (e.g. the Jutland plume of Ernst
& Buchan 1997). The inferred distances are,
however, comparable to the range over which a
high 3He/aHe isotope signature has been
recorded in the Tertiary NAVP (Stuart et al.
2000). Therefore, despite the limited dataset, the
lack of evidence for a deep-seated mantle plume
beneath Scotland during the Permo-Carboniferous appears to be geologically significant.
The helium isotope ratios of the Scottish
xenoliths are within the range of those reported
for HIMU (High Ub--Pb ratio, #) OIBs,
generally considered to be plume-related.
Although it is not possible to rule out the
involvement of such a plume source in the
petrogenesis of the host magmas of the xenoliths, the dispersed and widespread nature of the
volcanism across northern Europe could suggest
the involvement of numerous small uppermantle mantle plumes or diapirs (cf. the Tertiary
volcanic province in Europe; Wilson &Downes,
1991; Granet et al. 1995), rather than a single
large-scale plume. Low 3He/4He ratios (6 + 1
Ra) have been measured in both lavas and
mantle xenoliths from the French Massif Central; here Granet et al. (1995) interpreted the
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HELIUM ISOTOPE STUDIES
results of a local seismic tomography experiment
data as evidence of a finger-like upwelling within
the upper mantle beneath the main volcanic
fields. Low 3He/4He ratios may be a characteristic of such upper-mantle upwellings. Although
we cannot dismiss outright such an origin for the
Permo-Carboniferous magmatism, we favour a
non-plume, passive stretching model. This is
based on the protracted duration of the magmatic activity (c. 100 Ma), the large volume of
evolved magmas (c. 50% in the Oslo Graben)
that suggests residence of the parental magmas
in high-level magma chambers and the smalldegree melts that characterize much of the
magmatic activity. Metasomatized, relatively
fertile lithospheric mantle is readily melted and
we propose that enriched SCM could have
provided the dominant magma source beneath
Scotland during Permo-Carboniferous extension.
Additional geological observations argue
against the involvement of a mantle plume,
and in favour of lithospheric extension, in the
petrogenesis of the Permo-Carboniferous magmatism. Of the order of 6000 km 3 of magma was
produced in Scotland in the early Carboniferous. This volcanism mainly occurred in the
Midland Valley of Scotland and was distributed
over an area of 3000km 2. Based upon both
petrological studies of mantle xenoliths (Upton
et al. 1999) and seismic reflection and refraction
studies (Price & Morgan 2000), the lithosphere
beneath Scotland is considered to be 80-100 km
thick. Approximately 3% of the mantle lithosphere would be required to have melted if this
represents the sole source of the Permo-Carboniferous volcanism; this is entirely plausible. The
overall geochemical character of the magmatism
is, however, OIB-like suggesting the additional
involvement of an asthenospheric mantle source
component (e.g. Smedley 1988). Estimated
Permo-Carboniferous /~ (stretching) factors for
Scotland are < 1.3 (Price & Morgan 2000).
Given that thinning of the lithosphere would
give rise to decompression melting of the
underlying asthenosphere, it is probable that
both the lithospheric and asthenospheric mantle
would have partially melted.
Mantle xenoliths studies indicate that the
subcontinental lithospheric mantle beneath
Scotland has been variably metasomatized and
trace-element-enriched (Upton et al. 1999).
Geophysical data suggest the presence of a
remnant c. 1.9 Ga subducted slab in the lithospheric mantle under NW Scotland (Price &
Morgan 2000), providing evidence for Proterozoic subduction in addition to the geological
evidence for Early Paleozoic (Caledonide) sub-
255
duction further to the south. We suggest that
small melt-fractions from the asthenosphere
modified by mixing with partial melts from this
enriched lithospheric mantle source can explain
the petrogenesis of the Permo-Carboniferous
magmatism in Scotland without the need to
invoke a large, anomalously hot, chemically
distinct mantle plume beneath the region. The
enrichment of the mantle lithosphere is likely to
have occurred at various times, ranging from
Proterozoic in the north to early Paleozoic
(Caledonide) in the south.
We thank M. Coltorti (University of Ferrara) for
information about the Rinnibar dyke and olivine
composition data, J. Konig (VU) for assistance with
mineral separation and W. Lustenhouwer for microprobe analysis. W. Koot is thanked for help in the
preparation of sections for thin section and fluid
inclusion studies.
L. Kirstein gratefully acknowledges funding by the
European Commission TMR Network project 'PermoCarboniferous Rifting in Europe' (ERB FMRXCT96
0093). I. K. Nikogosian was supported by NWO
(ALW-810.31.002). The input of the network partners
is appreciated in discussing ideas. The authors
appreciated thorough reviews by D. Barfod and I.
Tolstikhin, and editorial comments by M. Wilson.
Acknowledgement of this funding source, co-workers
and referees does not imply that they concur with the
conclusions drawn.
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