Geological Survey of Denmark and Greenland Bulletin 13, 2007
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Review of Survey activities 2006, 45-48 |
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Exploration for diamonds in West Greenland has experi-
enced a major boost within the last decade following the
establishment of world-class diamond mines within the
nearby Slave Province of the Canadian Arctic. Numerous
companies have active programmes of diamond exploration
and increasingly larger diamonds have been discovered,
notably a 2.392 carat dodecahedral stone recovered by the
Canadian exploration company Hudson Resources Inc. in
January 2007. The Geological Survey of Denmark and
Greenland (GEUS) is currently carrying out several studies
aimed at understanding the petrogenesis of diamondiferous
kimberlites in Greenland and the physical and chemical
properties of their associated mantle source regions (e.g.
Hutchison 2005; Nielsen & Jensen 2005).
enced a major boost within the last decade following the
establishment of world-class diamond mines within the
nearby Slave Province of the Canadian Arctic. Numerous
companies have active programmes of diamond exploration
and increasingly larger diamonds have been discovered,
notably a 2.392 carat dodecahedral stone recovered by the
Canadian exploration company Hudson Resources Inc. in
January 2007. The Geological Survey of Denmark and
Greenland (GEUS) is currently carrying out several studies
aimed at understanding the petrogenesis of diamondiferous
kimberlites in Greenland and the physical and chemical
properties of their associated mantle source regions (e.g.
Hutchison 2005; Nielsen & Jensen 2005).
Constraint of the mantle geotherm, i.e. the variation of
temperature with depth for a particular mantle volume, is an
important initial step in assessing the likelihood of such a vol-
ume to grow diamonds and hence the diamond potential of
associated deep-sourced magmatic rocks occurring at surface.
Cool geotherms are often present within old cratonic blocks
such as West Greenland (Garde et al . 2000) and provide a
good environment for the formation of diamonds (Haggerty
1986). This study aims to constrain the mantle geotherm for
the southern extent of the North Atlantic Craton in
Greenland by applying three-phase geothermobarometry cal-
culations using chemical compositions of clinopyroxene,
orthopyroxene and garnet from four-phase kimberlite-hosted
lherzolite xenoliths.
important initial step in assessing the likelihood of such a vol-
ume to grow diamonds and hence the diamond potential of
associated deep-sourced magmatic rocks occurring at surface.
Cool geotherms are often present within old cratonic blocks
such as West Greenland (Garde et al . 2000) and provide a
good environment for the formation of diamonds (Haggerty
1986). This study aims to constrain the mantle geotherm for
the southern extent of the North Atlantic Craton in
Greenland by applying three-phase geothermobarometry cal-
culations using chemical compositions of clinopyroxene,
orthopyroxene and garnet from four-phase kimberlite-hosted
lherzolite xenoliths.
Xenoliths have been sampled from kimberlites from two
areas in South-West Greenland: Midternæs and Pyramide -
fjeld (Fig. 1). Kimberlites in the Pyramidefjeld area princi-
pally occur as sheeted sills hosted in the Pyramidefjeld granite
complex of Palaeoproterozoic Ketilidian age. In contrast,
Midternæs kimberlites occur as outcrops within a single,
extensive and undulating sill hosted within pre-Ketilidian
granodioritic gneiss and Ketilidian supracrustal rocks.
fjeld (Fig. 1). Kimberlites in the Pyramidefjeld area princi-
pally occur as sheeted sills hosted in the Pyramidefjeld granite
complex of Palaeoproterozoic Ketilidian age. In contrast,
Midternæs kimberlites occur as outcrops within a single,
extensive and undulating sill hosted within pre-Ketilidian
granodioritic gneiss and Ketilidian supracrustal rocks.
Pyramidefjeld kimberlites have been shown to be
Mesozoic (Andrews & Emeleus 1971), and work is currently
being carried out tOFurther constrain the ages of these and
the Midternæs kimberlites and also xenoliths using modern
methods. No attempt is made herein to provide a correct
petrological classification of the rocks hosting the xenoliths;
however, the abundance of clinopyroxene reported by
Andrews & Emeleus (1971) suggests that further work may
being carried out tOFurther constrain the ages of these and
the Midternæs kimberlites and also xenoliths using modern
methods. No attempt is made herein to provide a correct
petrological classification of the rocks hosting the xenoliths;
however, the abundance of clinopyroxene reported by
Andrews & Emeleus (1971) suggests that further work may
more correctly conclude a classification as `orangeite' after
Mitchell (1995). Notwithstanding this, the term `kimberlite'
is employed throughout in order to be consistent with that
adopted by previous authors. The Precambrian Pyramide -
fjeld granite complex and adjacent Archaean grano dioritic
gneisses are host to several kimberlite sheets located at various
levels between 400 and 900 m elevation (Fig. 1A; Andrews &
Emeleus 1971, 1975). Kimberlites are mainly found as loose
Mitchell (1995). Notwithstanding this, the term `kimberlite'
is employed throughout in order to be consistent with that
adopted by previous authors. The Precambrian Pyramide -
fjeld granite complex and adjacent Archaean grano dioritic
gneisses are host to several kimberlite sheets located at various
levels between 400 and 900 m elevation (Fig. 1A; Andrews &
Emeleus 1971, 1975). Kimberlites are mainly found as loose
P-T
history of kimberlite-hosted garnet lherzolites from
South-West Greenland
South-West Greenland
Mark T. Hutchison, Louise Josefine Nielsen and Stefan Bernstein
Fig. 1. Location of sample sites with reference to context in South-West
Greenland (index map). Red dots indicate location of samples used for
pressure and temperature calculation; white dots indicate additional
samples.
A
: Sample localities in the Pyramidefjeld region. Geology sim-
plified from Henriksen (1966).
B
: Sample localities in the Midternæs
region. Geology simplified from Escher & Jensen (1972).
45
© GEUS, 2007.
Geological Survey of Denmark and Greenland Bulletin
13, 45-48. Available at:
www.geus.dk/publications/bull
blocks in scree; however, these are almost always sourced
locally from in situ bodies. Sheets can often be found deep
within overhanging clefts, particularly in granitic walls. The
kimberlite bodies are gently dipping, typically 20 degrees,
and with a range of strikes. The maximum thickness of sills is
approximately 2 m but thickness varies significantly over
short distances. In many instances, the occurrence of kim-
berlite is seen to be controlled locally by structures in the
country rocks. Field observations of the range of orientations
of intrusive bodies do not appear to suggest a particular focal
point which could be a likely location for an intrusive centre
such as a pipe. This observation is in line with what is seen
throughout West Greenland where kimberlite emplacement
appears as dykes and sills (Larsen & Rex 1992) rather than
the pipes and blows which are common in other world-wide
settings. The occurrence of xenoliths amongst Pyramidefjeld
kimberlites is highly variable with the most xenolith-rich
localities being in the vicinity of Safirsø (Fig. 1A). The major-
ity of xenoliths are dunites with occasional wehrlites and lher-
zolites (Emeleus & Andrews 1975). Of particular interest
from the point of view of thermobarometry is the occurrence
of garnet. This is rarely found, even in clinopyroxene-bearing
samples, and the two samples chosen for thermobarometry
(Fig. 1A) represent the majority of the garnet-bearing xeno-
liths identified within an estimated total population of 75
xenoliths collected.
locally from in situ bodies. Sheets can often be found deep
within overhanging clefts, particularly in granitic walls. The
kimberlite bodies are gently dipping, typically 20 degrees,
and with a range of strikes. The maximum thickness of sills is
approximately 2 m but thickness varies significantly over
short distances. In many instances, the occurrence of kim-
berlite is seen to be controlled locally by structures in the
country rocks. Field observations of the range of orientations
of intrusive bodies do not appear to suggest a particular focal
point which could be a likely location for an intrusive centre
such as a pipe. This observation is in line with what is seen
throughout West Greenland where kimberlite emplacement
appears as dykes and sills (Larsen & Rex 1992) rather than
the pipes and blows which are common in other world-wide
settings. The occurrence of xenoliths amongst Pyramidefjeld
kimberlites is highly variable with the most xenolith-rich
localities being in the vicinity of Safirsø (Fig. 1A). The major-
ity of xenoliths are dunites with occasional wehrlites and lher-
zolites (Emeleus & Andrews 1975). Of particular interest
from the point of view of thermobarometry is the occurrence
of garnet. This is rarely found, even in clinopyroxene-bearing
samples, and the two samples chosen for thermobarometry
(Fig. 1A) represent the majority of the garnet-bearing xeno-
liths identified within an estimated total population of 75
xenoliths collected.
The Midternæs kimberlites are hosted in Archaean
gneisses and Proterozoic supracrustal rocks (Fig. 1B; Andrews
& Emeleus 1971, 1975). The style of kimberlite emplace-
ment and occurrence of garnet-bearing xenoliths are closely
similar to those of Pyramidefjeld. Contours of elevation be -
& Emeleus 1971, 1975). The style of kimberlite emplace-
ment and occurrence of garnet-bearing xenoliths are closely
similar to those of Pyramidefjeld. Contours of elevation be -
tween outcrops suggest that the kimberlites form parts of a
largely contiguous single body dipping at approximately 30
degrees to the west-south-west. Individual outcrops as in
Pyramidefjeld indicate that the body varies in thickness and
undulates in response to local structure. The south-western
portion of the body which outcrops near the glacier Sioralik
Bræ, is considerably thicker than elsewhere (Fig. 2) and in
some places is seen to have a true thickness in excess of 4 m.
Xenoliths are less abundant on average than in Pyramidefjeld
kimberlites, but a similar variety and proportion of rock types
and infrequent occurrence of garnet is observed.
largely contiguous single body dipping at approximately 30
degrees to the west-south-west. Individual outcrops as in
Pyramidefjeld indicate that the body varies in thickness and
undulates in response to local structure. The south-western
portion of the body which outcrops near the glacier Sioralik
Bræ, is considerably thicker than elsewhere (Fig. 2) and in
some places is seen to have a true thickness in excess of 4 m.
Xenoliths are less abundant on average than in Pyramidefjeld
kimberlites, but a similar variety and proportion of rock types
and infrequent occurrence of garnet is observed.
The kimberlites from both areas were intruded along
zones of platy jointing which likely were caused by degassing
of the magma and formed just prior to the kimberlite intru-
sion. In contrast to some kimberlites in other cratons, very
few xenoliths of local, lower crustal rock types have been
recognised in the kimberlites from Pyramidefjeld and Mid -
ternæs. The intrusions are therefore believed to have been of
a non-explosive nature, perhaps because of host-rock rheo l -
ogy or due to emplacement at relatively deep crustal levels.
of the magma and formed just prior to the kimberlite intru-
sion. In contrast to some kimberlites in other cratons, very
few xenoliths of local, lower crustal rock types have been
recognised in the kimberlites from Pyramidefjeld and Mid -
ternæs. The intrusions are therefore believed to have been of
a non-explosive nature, perhaps because of host-rock rheo l -
ogy or due to emplacement at relatively deep crustal levels.
Here we report on calculations of equilibrium pressure and
temperature using compositions of three-phase assemblages
of garnet, orthopyroxene and clinopyroxene from Midternæs
and Pyramidefjeld mantle xenoliths.
of garnet, orthopyroxene and clinopyroxene from Midternæs
and Pyramidefjeld mantle xenoliths.
Measurements
Polished thin sections of garnet-bearing mantle xenoliths
were prepared from fresh samples from both localities. Most
xenoliths have a coarse granular texture indicative of equilib-
rium growth amongst clinopyroxene, olivine, orthopyroxene
were prepared from fresh samples from both localities. Most
xenoliths have a coarse granular texture indicative of equilib-
rium growth amongst clinopyroxene, olivine, orthopyroxene
46
Fig. 2. Kimberlite sill of approximately 3.5 m
thickness intruded within pre-Ketilidian gneiss
and cross-cutting a vertical Gardar age (
c
. 1200
Ma) dolerite dyke. Located at Midternæs by
the glacier Sioralik Bræ (see Fig. 1B).
and garnet. Typically, triple junctions between mineral grains
are well defined. Mineral compositions were determined by
the JEOL 733 electron microprobe at the Department of
Geography and Geology, University of Copenhagen. Ana -
lyses were conducted using a 15 kV, 15 nA and 5 µm beam
for the elements Si, Ti, Al, Cr, Fe, Mn, Ni, Mg, K and Na.
Stand ard isation was achieved against natural and synthetic
standards.
are well defined. Mineral compositions were determined by
the JEOL 733 electron microprobe at the Department of
Geography and Geology, University of Copenhagen. Ana -
lyses were conducted using a 15 kV, 15 nA and 5 µm beam
for the elements Si, Ti, Al, Cr, Fe, Mn, Ni, Mg, K and Na.
Stand ard isation was achieved against natural and synthetic
standards.
Geothermobarometry
Estimates of the temperatures and pressures within the Earth
are essential for the understanding of many geological
processes and in particular, in the case of kimberlites, for dia-
mond prospectivity. Geothermobarometry is based on the
study of mineral assemblages in chemical and physical equi-
librium. As mantle xenoliths often retain information about
the physical conditions at the time of formation, they are widely
used for such estimates. In this study temperature-pressure cal-
culations are based on the two-pyroxene thermometer and
the aluminium-in-orthopyroxene / garnet barometer of Brey
& Köhler (1990). Results are presented in the context of
standard cratonic mantle geotherm models in Fig. 3. Pressure
and temperature estimates from Pyramide fjeld range from
909°C and 3.29 GPa to 975°C and 3.50 GPa. These pres-
sures correspond to a depth range of 106-113 km. Peak
assemblages using cores of touching grains for samples from
Pyramidefjeld fall on a smooth curve. The Mid ternæs sample
reflects equilibrium conditions of 1087°C and 3.73 GPa cor-
responding to 120 km depth and therefore deeper than the
Pyramidefjeld samples. All values show a similarity with a
warm mantle geotherm based on a surface heat flow of 44
mW/m
are essential for the understanding of many geological
processes and in particular, in the case of kimberlites, for dia-
mond prospectivity. Geothermobarometry is based on the
study of mineral assemblages in chemical and physical equi-
librium. As mantle xenoliths often retain information about
the physical conditions at the time of formation, they are widely
used for such estimates. In this study temperature-pressure cal-
culations are based on the two-pyroxene thermometer and
the aluminium-in-orthopyroxene / garnet barometer of Brey
& Köhler (1990). Results are presented in the context of
standard cratonic mantle geotherm models in Fig. 3. Pressure
and temperature estimates from Pyramide fjeld range from
909°C and 3.29 GPa to 975°C and 3.50 GPa. These pres-
sures correspond to a depth range of 106-113 km. Peak
assemblages using cores of touching grains for samples from
Pyramidefjeld fall on a smooth curve. The Mid ternæs sample
reflects equilibrium conditions of 1087°C and 3.73 GPa cor-
responding to 120 km depth and therefore deeper than the
Pyramidefjeld samples. All values show a similarity with a
warm mantle geotherm based on a surface heat flow of 44
mW/m
2
after Pollack & Chapman (1977). However, the
location of pressure and temperature points more closely fol-
lows the trends in the steady-state geotherm of McKenzie et
al . (2005) although at an average temperature elevated by
approximately 50°C (Fig. 3). Furthermore, the Midternæs
sample may reflect the same type of high-T inflection evident
under similar conditions from Lesotho kimberlite-hosted
xenoliths (Finnerty 1989) although further data are required
to confirm this inference.
lows the trends in the steady-state geotherm of McKenzie et
al . (2005) although at an average temperature elevated by
approximately 50°C (Fig. 3). Furthermore, the Midternæs
sample may reflect the same type of high-T inflection evident
under similar conditions from Lesotho kimberlite-hosted
xenoliths (Finnerty 1989) although further data are required
to confirm this inference.
Discussion and conclusions
The apparent coincidence of pressure and temperature values
for xenoliths from Pyramidefjeld and Midternæs along the
McKenzie et al . (2005) geotherm suggests that the thermal
conditions of the mantle sampled from the two localities sep-
arated on the ground by 14 km are largely the same. The
McKenzie et al . (2005) model takes account of lower radi-
ogenic heating in the cratonic crust than previously accepted
for xenoliths from Pyramidefjeld and Midternæs along the
McKenzie et al . (2005) geotherm suggests that the thermal
conditions of the mantle sampled from the two localities sep-
arated on the ground by 14 km are largely the same. The
McKenzie et al . (2005) model takes account of lower radi-
ogenic heating in the cratonic crust than previously accepted
and shows good correlation with pressure and temperature
estimates from kimberlite-hosted mantle xenoliths from
northern Canada and central Siberia (McKenzie et al . 2005
and references therein). Results from Pyramidefjeld and
Midternæs xenoliths are hence also consistent with kimber-
lite-hosted xenoliths from elsewhere whilst at the same time
elevated temperatures observed in this study suggest that the
geotherm was slightly warmer in South-West Greenland than
in the northern Canada and central Siberian diamond-bear-
ing mantle.
estimates from kimberlite-hosted mantle xenoliths from
northern Canada and central Siberia (McKenzie et al . 2005
and references therein). Results from Pyramidefjeld and
Midternæs xenoliths are hence also consistent with kimber-
lite-hosted xenoliths from elsewhere whilst at the same time
elevated temperatures observed in this study suggest that the
geotherm was slightly warmer in South-West Greenland than
in the northern Canada and central Siberian diamond-bear-
ing mantle.
Additional samples are required to more closely constrain
the geotherm and also to assess the ranges of depths from
which xenoliths were sampled at the two locations; however,
the greater depth represented sOFar at Midternæs may have
significance. Midternæs is slightly closer to the central part of
the craton, and the greater depths represented in the xenolith
suite may thus reflect a thicker cratonic lithospheric root.
which xenoliths were sampled at the two locations; however,
the greater depth represented sOFar at Midternæs may have
significance. Midternæs is slightly closer to the central part of
the craton, and the greater depths represented in the xenolith
suite may thus reflect a thicker cratonic lithospheric root.
47
Fig. 3. Temperature-pressure diagram showing positions of equilibrium
conditions for garnet lherzolite xenoliths in this study in the context of
mantle geotherms. Diamond-graphite phase boundary after Kennedy &
Kennedy (1976) and steady-state mantle geotherm after McKenzie
et al
.
(2005). Mantle geotherms from Pollack & Chapman (1977) where fig-
ures represent surface heat flow for each model in mW/m
2
. Calculated
pressures and temperatures are for peak conditions (cores of mineral
grains).
Consequently the present data suggest that the Midternæs
kimberlite is closer to directly sampling mantle material
within the diamond stability field than Pyramidefjeld.
Although xenolith suites may be formed under different con-
ditions compared to diamonds found within the same kim-
berlites (Shee et al . 1982), it appears that Midternæs may
have a better diamond potential than Pyramidefjeld.
Midternæs kimberlites have not sOFar been tested for the
presence of diamonds, and the apparently shallower-sourced
kimberlites from Pyramidefjeld have yielded small numbers
of diamonds (unpublished company reports collated in
Jensen et al . 2004). Since Midternæs contains some of the
thickest outcropping kimberlite evident in Greenland, the
area may merit further attention by diamond prospectors.
kimberlite is closer to directly sampling mantle material
within the diamond stability field than Pyramidefjeld.
Although xenolith suites may be formed under different con-
ditions compared to diamonds found within the same kim-
berlites (Shee et al . 1982), it appears that Midternæs may
have a better diamond potential than Pyramidefjeld.
Midternæs kimberlites have not sOFar been tested for the
presence of diamonds, and the apparently shallower-sourced
kimberlites from Pyramidefjeld have yielded small numbers
of diamonds (unpublished company reports collated in
Jensen et al . 2004). Since Midternæs contains some of the
thickest outcropping kimberlite evident in Greenland, the
area may merit further attention by diamond prospectors.
Acknowledgements
Graham Pearson, Geoff Nowell and Nadine Wittig (University of
Durham) are acknowledged for support. MTH acknowledges the
European Community's 6th Framework Program for support under a
Marie Curie EIF Fellowship. Disclaimer: This publication reflects the
authors' views, and the European Community shall not be held liable for
any use of the information contained herein.
Durham) are acknowledged for support. MTH acknowledges the
European Community's 6th Framework Program for support under a
Marie Curie EIF Fellowship. Disclaimer: This publication reflects the
authors' views, and the European Community shall not be held liable for
any use of the information contained herein.
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Authors' addresses
M.T.H. & S.B.,
Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark.
E-mail:
mhutchis@lpl.arizona.edu
L.J.N.,
Department of Geography and Geology, University of Copenhagen, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark.
