JOURNAL OF PETROLOGY
VOLUME 43
NUMBER 1
PAGES 129–142
2002
Mantle Structure Beneath the SW Slave
Craton, Canada: Constraints from Garnet
Geochemistry in the Drybones Bay
Kimberlite
G. B. CARBNO AND D. CANIL∗
SCHOOL OF EARTH AND OCEAN SCIENCES, UNIVERSITY OF VICTORIA, 3800 FINNERTY ROAD, PO BOX 3055,
VICTORIA, B.C., CANADA V8W 3P6
RECEIVED OCTOBER 13, 2000; REVISED TYPESCRIPT ACCEPTED JULY 17, 2001
This study describes the petrography of peridotite xenoliths, and the
major and trace element geochemistry of garnets in both xenoliths
and coarse concentrate from the Drybones Bay kimberlite. The
temperature and depth of equilibration of clinopyroxene and garnet
show that the mantle lithosphere beneath the SW margin of the
Slave Province was at least 160 km thick at the time of kimberlite
emplacement (>450 Ma). The garnet population is dominated by
grains with sinuous light rare earth element (LREE)-enriched trace
element patterns many of which are ultra-depleted in Zr, Y and
Ti. Normal heavy REE (HREE)-enriched garnets make up a
small proportion of the garnet population. The spectrum in garnet
geochemistry may be explained by heating and percolation of a
metasomatic fluid from through an ultra-depleted mantle protolith
before kimberlite eruption. A shallow ultra-depleted layer of mantle
lithosphere recognized in garnets from the central Slave Province
may exist beneath the SW margin of the craton, but the layer has
been modified or overprinted by the metasomatic front, which has
caused enrichment in Ca and incompatible elements in garnets. The
changes in garnet chemistry in the Drybones Bay garnets are
coincident with a unique seismic discontinuity exhibiting multiple
layer anisotropy commencing at 110 km depth >50 km north of
Drybones Bay; correlation of the two features is, however, tenuous.
Seismic tomography and teleseismic methods show evidence for a cool mantle root beneath the Archean Slave
Province (Bostock, 1998). The Drybones Bay kimberlite
is located along the SW margin of the Slave Province
(62°15′N, 114°00′W; Fig. 1) and is of particular interest
because it contains mantle xenoliths for comparison with
a larger petrological database for mantle samples from
the central regions (Boyd & Canil, 1997; Kopylova et al.,
1998; Griffin et al., 1999a; MacKenzie & Canil, 1999;
Kopylova & Russell, 2000). Those studies confirm the
existence of a deep lithospheric mantle root beneath the
Slave Province but the extent and characteristics of the
root towards the externides of the craton, as reflected in
surface geology, are not known. This information is
central to models of the deep structure of cratons, diamond exploration and the overall evolution of Archean
continents. Furthermore, the Drybones Bay kimberlite
pipe is in close proximity to the YKA teleseismic array
(Bostock, 1998), electromagnetic ( Jones et al., 2001) and
deep seismic reflection surveys (Cook et al., 1998), and
the only published surface heat flow measurement for
the Slave Province (Lewis & Wang, 1992) (Fig. 1). For
these reasons, the Drybones Bay kimberlite affords an
unprecedented opportunity to compare petrological and
geochemical information from mantle material with a
diverse array of nearby geophysical measurements of the
lithosphere beneath an Archean craton.
This study describes the petrography of peridotite
xenoliths from the Drybones Bay kimberlite, and the
major and trace element geochemistry of peridotitic
garnets and clinopyroxenes in both the xenoliths and
Extended dataset can be found at http://www.petrology.
oupjournals.org
∗Corresponding author. Telephone: 250 472 4180. Fax: 250 721
6200. E-mail: [email protected]
 Oxford University Press 2002
KEY WORDS:
Canada; craton; garnet; mantle; trace elements
INTRODUCTION
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Fig. 1. Geological map of the Slave Province showing the locations of the Drybones Bay and other kimberlite pipes mentioned in the text (Ο)
[adapted from MacKenzie & Canil (1999)]. Also shown are the locations of the YKA teleseismic array (Bostock, 1998), the LITHOPROBE
SNORCLE line 1 transect (Cook et al., 1998) and the Pb isotope boundary recognized in galenas from crustal rocks (Thorpe et al., 1992).
coarse concentrate. Garnets are the focus of the study
because they show no sign of alteration in the Drybones
Bay kimberlite. Lithospheric thickness and structure are
deduced, and some questions bearing on the protolith
and processes involved in the formation of lithosphere
are addressed in the geochemical patterns recorded by
the garnets. These data provide some constraints on
lateral and vertical changes in the cratonic lithosphere
toward the edge of the Slave Province, and may also
furnish a means to compare geophysical and petrological interpretations of lithosphere beneath an Archean
craton.
GEOLOGY
130
The Archean Slave Province is composed of granites,
gneisses and supracrustal rocks. The supracrustal sequences are dominated by metasedimentary and lesser
metavolcanic rocks of the Yellowknife Supergroup dated
at 2·71–2·61 Ga. Several generations of granitoid plutons
(2·5–2·8 Ga) cover >65% of the region (Padgham &
Fyson, 1992).
The Drybones Bay kimberlite pipe is located beneath
Drybones Bay on the north shore of Great Slave Lake
(Fig. 1). The pipe has U–Pb zircon age of 442–485 Ma (L.
CARBNO AND CANIL
GARNET GEOCHEMISTRY AND MANTLE STRUCTURE
Heaman, personal communication, 1997) and intrudes
plutons of the Archean Defeat Plutonic Suite (2620 Ma).
The kimberlite includes crater, reworked crater (epiclastic) and diatreme facies, the extent and geometry
of which are only defined by very limited drilling (U.
Kretschmar, personal communication, 1997).
SAMPLES
A total of 17 mantle-derived xenoliths [seven garnet
peridotites; three spinel peridotites; two spinel–garnet
peridotites; one pyroxenite; one Mica–Amphibole–
Rutile–Ilmenite–Diopside (MARID); three eclogites]
were recovered from >100 m of 8 cm drill core. Only
the peridotite samples are the subject of this study. In
addition, hundreds of garnet grains and seven garnet–
clinopyroxene pairs were also recovered in heavy mineral
coarse concentrate of kimberlite drill core from several
locations within the pipe.
PETROGRAPHY
Petrographic details of the samples are provided in the
Appendix. In the studied section, the peridotite xenoliths
from the Drybones Bay kimberlite range in area from
0·7 to 8·5 cm2.
With the exception of garnet and spinel, all other
primary minerals are completely altered to serpentine or
carbonate. Certain characteristics of the pseudomorphs
evident in plane-polarized light were found to be useful
in distinguishing the original mineralogy and textures.
Serpentinized grains that have fractures and no cleavage
are pseudomorphs of olivine, whereas grains with relict
cleavage are pseudomorphs after pyroxenes (Fig. 2).
In a few heavily carbonatized samples, however, grain
boundaries are not well preserved and differentiating
pyroxene and olivine was impossible. In some samples,
carbonate replaces grains that are small, and it could
be assumed that clinopyroxene was the primary phase,
because orthopyroxene is usually coarse in cratonic xenoliths (Boyd & Mertzman, 1987). For modal analysis, all
grains with relict cleavage were considered simply as
pyroxene, because of the lack of confidence in applying
grain size to differentiate ortho- from clinopyroxene. All
of the peridotite xenoliths have a coarse texture (Harte,
1977) with an average grain size >2 mm. Many of the
xenoliths have secondary phlogopite and opaque minerals
in fractures, fissures or as mantles on garnet grains.
Modal data for nine samples were obtained by point
counting 1500–7500 points, depending upon sample size.
An appropriate grid spacing was chosen to minimize
counting errors (Solomon, 1963) estimated to be 15%
relative (at 2) in modal abundances of olivines, pyroxene
and garnet. The modal data reveal positive and negative
correlations of modal olivine and garnet, respectively,
with sample size (Fig. 3). This correlation has been
recognized in a detailed petrographic study of other
coarse, cratonic mantle xenoliths and signifies a close
spatial relationship of garnet and pyroxene in an olivinerich matrix (Cox et al., 1987). Cox et al. (1987) recommended modal estimations with minimum areas of
>7–8 cm2 to eliminate this spatial bias in typical coarse,
cratonic xenoliths. Extrapolation of the trend for the
Drybones Bay xenoliths to this minimum area would
suggest a mantle mineralogy with greater than >75%
olivine beneath Drybones Bay, but this value is obviously
not well constrained by the relatively few xenoliths available for study in the Drybones Bay pipe.
ANALYTICAL METHODS
Major and minor element compositions for garnet and
clinopyroxene were determined by wavelength-dispersive
analysis using a CAMECA SX-50 electron microprobe
at the University of British Columbia (UBC) and a JEOL
JXA 8900 electron microprobe at the University of
Alberta (U of A). Mineral grains were analysed at an
accelerating voltage of 15 kV and a 20 nA beam current
with peak-counting times of 20 s for both major and
minor elements. Instrument calibration was performed
on natural and synthetic standards. Previous work has
shown identical results for these two microprobe laboratories (MacKenzie & Canil, 1999). In xenolith samples,
two or four grains of each mineral (garnet or spinel) were
analysed, and on each of the grains two to four points
(depending on grain size) were analysed to determine
homogeneity on both a mineral and xenolith scale. In
coarse concentrates two to three points for each garnet
or clinopyroxene grain were analysed. Average mineral
compositions are given in Electronic Appendix Table 1,
as all garnets and pyroxenes show no sign of significant
major element heterogeneity. This table may be downloaded from the Journal of Petrology Web site at http://
www.petrology.oupjournals.org.
Garnet and clinopyroxene grains from heavy mineral
concentrate and garnets from five xenoliths were analysed
for 26 trace elements by laser ablation inductively coupled
plasma mass spectrometry (LA-ICP-MS) using a Merchantek EO UV Nd:YAG laser coupled to a highsensitivity VG PQII S instrument at the University of
Victoria (UVic). Laser spot size ranged from 50 to 70 m
using a power output of 0·75–2·19 mJ and a pulse rate
of 10·0 Hz. Other operation details have been reported
elsewhere (Chen, 1999). For garnets in concentrate, one
to two spots for each grain were analysed. In each
peridotite xenolith, multiple garnets and often more than
six points per single grain were analysed. The data are
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Fig. 2. Photomicrograph of spinel–garnet peridotite xenolith in plane-polarized light. Spinel and garnet remain unaltered, whereas olivine and
pyroxene have been completely serpentinized. The relict cleavage in the serpentinized pyroxene grains that makes them distinguishable from
serpentinized olivine should be noted.
Fig. 3. Plot of xenolith size against combined modal percent of
pyroxene and garnet for mantle xenoliths from the Drybones Bay pipe
determined by detailed point counting. With increasing area of the
sample in thin section, modal pyroxene + garnet decrease and modal
olivine increases. Also shown for comparison are petrographic data for
mantle peridotites in the Grizzly and Torrie kimberlites (Boyd & Canil,
1997; MacKenzie & Canil, 1999).
given in Electronic Appendix Table 2, which may be
downloaded from the Journal of Petrology Web site at
http://www.petrology.oupjournals.org.
Data for all trace elements were collected with PQVision 4.36 Time Resolved Analysis software and exported offline for further processing using in-house
software (Chen, 1999; Chen et al., 2000). NIST 613
synthetic silicate glass (Chen et al., 1997) was used as
an external calibration standard (Electronic Appendix,
Table 2) with Ca as an internal standard to correct for
drift, matrix effect, changes in laser sampling yield and
transport efficiency during the analysis. Tests of precision
for this instrument over a 1 year period on BCR-2 glass
standard are 5% or better for all elements analysed.
Accuracy based on analysis of BCR-2 glass is better than
10% for all elements except Ti (10%) and Sc (20%).
Although the accuracy and precision of the LA-ICP-MS
technique used here has been tested on three glass
standards (Chen, 1999; Chen et al., 2000) it is also
important to compare results on mantle minerals.
Shimizu et al. (1997) and Shimizu (1999) reported secondary ion microprobe (SIMS) data on a mantle garnet
(UV417/89) that has been analysed in the UVic LAICP-MS laboratory. This garnet is homogeneous for
most trace elements except Sr. The SIMS and UVic LAICP-MS analyses compare within 20% for Zr, Y and
the REE (Electronic Appendix, Table 2). In addition, a
large homogeneous mantle garnet (Sample 21-6, Kopylova et al., 1998) with Ni determined by a proton
microprobe instrument (PMP) with accuracy for Ni that
is well demonstrated (Campbell et al., 1996) was used as
an external check. Garnet 21-6 was analysed for Ni
during every analytical run, and gave results within 10%
of the PMP value, thus showing further accuracy of the
LA-ICP-MS data for Ni in garnet. All elements reported
in Electronic Appendix Table 2 are above limits of
detection, defined by three standard deviations of the
background counts for each element. Detection limits for
LA-ICP-MS depend on sample quality, but ‘typical’
values obtained for mantle garnets in the UVic laboratory
are shown in Electronic Appendix Table 2.
MAJOR ELEMENT GEOCHEMISTRY
Garnet
A plot of the CaO and Cr2O3 contents of garnets from
Drybones Bay peridotite xenoliths and heavy mineral
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Fig. 4. Plot of the CaO vs Cr2O3 content of Drybones Bay garnets,
with fields labelled after Dawson & Stephens (1975). The Drybones
Bay garnets are primarily lherzolitic and wehrlitic, with a few samples
possibly being harzburgitic. Dashed line (‘sp–gt’) is the trend in garnet
compositions recognized in spinel–garnet peridotites (Kopylova et al.,
2000). It should be noted that all xenolith samples from this study plot
along the ‘sp–gt’ trend.
trace element patterns: garnets with normal heavy rare
earth element (HREE)-enriched patterns, herein termed
‘N-type’, and garnets with sinuous REE patterns, termed
‘S-type’ (Fig. 5). The N-type garnets show depletion in
the light REE (LREE), steady enrichment from SmN to
YbN (where the subscript N indicates chondrite normalized) and are less frequent in the entire garnet population (Fig. 5). The S-type pattern is recognized in the
vast majority of coarse concentrate garnets, and is characterized by enrichment in SmN over DyN, fairly flat
trends between the HREE, and (Nd/Y)N much greater
than unity (Pearson et al., 1998b). The S-type pattern is
commonly recognized in harzburgitic garnets [reviewed
by Stachel et al. (1998)]. Garnets from garnet–
clinopyroxene pairs generally have REE patterns that
are intermediate in shape to the N- and S-type end
members, but show enhanced levels of Sr, LREE, Ti, Zr
and Hf (Fig. 5b).
Clinopyroxene
concentrates (Fig. 4) shows that the suite is dominated
by wehrlite and lherzolite, with a few samples plotting
near the harzburgite ‘field’, although in detail the levels
of Ca saturation, and thus the boundaries of this classification scheme, can change with P, T and composition
(Brenker & Brey, 1997). Some harzburgites are known
to plot in the lherzolite field (e.g. Kopylova et al., 1998)
and two clinopyroxene-saturated garnets (from pairs in
this study) plot within the harzburgite field (Fig. 4). Many
of the garnets from the Drybones Bay kimberlite define
a unique compositional trend characterized by subdued
Cr levels for a given Ca abundance (Fig. 4), which is
thought to be indicative of garnets derived from spinel–
garnet peridotites (Kopylova et al., 2000). This inference
is also supported here by the fact that many of the garnets
from spinel–garnet peridotite xenoliths in the Drybones
Bay suite also plot along this vector. Some of the garnets
within the ‘spinel–garnet’ trend also plot within the
wehrlite field and are probably derived from orthopyroxene-free or orthopyroxene-poor lithologies.
Clinopyroxene
Clinopyroxenes in the garnet–clinopyroxene pairs from
heavy mineral concentrate are Cr diopsides (0·87–
4·2 wt % Cr2O3) with Mg/(Mg + Fe) ranging from
03·896 to 0·951, and contain from 1·1 to 3·8 wt %
Na2O.
TRACE ELEMENT GEOCHEMISTRY
Garnet
Two distinct types of garnets from Drybones Bay are
recognized on the basis of their chondrite-normalized
Clinopyroxenes from garnet–clinopyroxene pairs all have
LREE-enriched trace element patterns (Fig. 6) and depletions in Y that are complementary to the enrichment
of this element in coexisting garnets (Fig. 5b). They
also exhibit levels of LREE enrichment (10–100 times
chondrite) that have been suggested to have been caused
by metasomatism by carbonate melts (Ionov, 1998;
Norman, 1998).
THERMOBAROMETRY
With the exception of seven garnet–clinopyroxene pairs,
the absence of multiple phases coexisting with garnet
prevents the use of conventional peridotite geothermobarometers to estimate the pressure and temperature of origin for the Drybones Bay samples. A Niin-garnet geothermometer (TNi) calibrated by experiment
(Canil, 1990) is used to estimate temperatures for the
garnets (Electronic Appendix, Table 3). An empirical
version of this thermometer (TNi-Ryan) was also used (Electronic Appendix, Table 3). Olivine is altered in the
Drybones Bay samples but was assumed to have a Ni
content of 2900 ± 300 ppm, typical of many mantle
olivines (Ryan et al., 1996). The uncertainty in Ni in
olivine results in an error of ±30°C in TNi and the
uncertainty in the experimental calibration is ±70°C
(2) (Canil, 1999).
The Drybones Bay garnets have TNi values between
800 and 1200°C, and when projected to an average
Slave paleogeotherm these correspond to depths of equilibration of >100–190 km (Fig. 7). Paleogeotherms constructed from xenolith thermobarometry in the central
part of the Slave Province vary between localities, even
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Fig. 5. Representative chondrite-normalized trace element patterns for garnets from the Drybones Bay coarse concentrate (a) and from xenoliths
and garnet–clinopyroxene pairs (b). Specific samples are labelled as typical normal (N-type) and sinuous (S-type) patterns.
Fig. 6. Chondrite-normalized trace element abundances for clinopyroxenes from garnet–clinopyroxene pairs in Drybones Bay coarse concentrate.
134
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Fig. 7. Depth–temperature array for all Drybones Bay garnets calculated based on TNi and projected TNi to an ‘average’ Slave Province
paleogeotherm fitted to all published P–T data for Slave Province
mantle xenoliths (Boyd & Canil, 1997; Kopylova et al., 1998; MacKenzie
& Canil, 1999). Also shown are P–T points for garnet–clinopyroxene
pairs determined with the thermobarometer of Nimis & Taylor (2000)
showing 2 error bars for T Cr-in-cpx pairs.
Fig. 8. Temperature distribution for all Drybones Bay garnets calculated based on TNi (Canil, 1999).
within 100 km of each other (Kopylova et al., 1998;
Pearson et al., 1998a; MacKenzie & Canil, 1999), resulting
in considerable uncertainty in estimating the absolute
depth of the Drybones Bay garnets with this approach.
Furthermore, the distribution for TNi is not uniform, with
a dominant cluster of points between 900 and 1050°C.
One possibility is that the most of the garnets were
derived over a limited depth interval between 100 and
160 km (Fig. 7) and record heating associated with other
metasomatic effects described below.
Other approaches to estimate the P (depth) of single
garnets are the empirical Cr-in-garnet barometer (Ryan
et al., 1996) and the Ca-in-garnet barometer (Brenker &
Brey, 1997). The Cr-barometer gives a minimum P
but is based on TNi-Ryan, which is incompatible with
experimental calibration (Canil, 1999). The Ca-in-garnet
barometer is based on the Ca–Cr systematics of garnets
in equilibrium with ortho- and clinopyroxene (Brenker
& Brey, 1997). Using TNi as a temperature input in this
method produced very erratic P values, with some garnets
plotting near a reasonable P–T array, but many far
removed to extremely low and high pressures (0–14
GPa). This could result partly because the Ca-in-garnet
barometer is calibrated on only one fertile mantle composition, and cannot be extrapolated well to other compositions (Nimis & Trommsdorff, 2001) or from the fact
that many of the samples in this study were not in
equilibrium with orthopyroxene (Fig. 4).
Nimis & Taylor (2000) have formulated a thermobarometer (P–TCr-cpx) to estimate both P and T from
single Cr-diopsides assuming they were in equilibrium
with garnet. This method is based on Cr exchange
between garnet and Cr-diopside coexisting with orthopyroxene. The results for the two of the seven garnet–
clinopyroxene pairs that were probably in equilibrium
with orthopyroxene (Fig. 4) plot near an average Slave
paleogeotherm, whereas a third at 750°C plots significantly below it (Fig. 7). Two of these three garnet–
clinopyroxene pairs have P–TCr-cpx below that determined
by TNi (Electronic Appendix, Table 3) perhaps recording
heating of the garnets. Unfortunately, more garnet–
clinopyroxene pairs could not be found in the heavy
mineral concentrate.
It is also possible that the Cr-diopside thermobarometer
is not appropriate for the samples or produces erroneous
results. Nimis & Taylor (2000) showed, however, how
their calibration reproduces accurately the temperatures
and pressures of clinopyroxene from many independent,
diverse experimental datasets in simple and complex
systems, and compares very well with other conventional
mantle thermobarometers when applied to natural
mantle samples. Garnet–pyroxene pairs in this study
show no significant heterogeneity in major elements and
the partition coefficients (Dgt/cpx) of many trace elements
in four samples measured (Fig. 9) show patterns identical
to those for well-equilibrated Kakanui garnet pyroxenite
xenoliths (Zack et al., 1997). The differences in absolute
Dgt/cpx values between the Drybones Bay and Kakanui
samples for the LREE, Sr, Th and U are expected
because of the differences in mineral composition (Na,
Ca, Fe) that control these partitions (Blundy & Wood,
1994). The consistency in trace element partitioning is
further evidence for equilibrium between garnet and
clinopyroxene.
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Fig. 9. Range of Dgt/cpx values for trace elements from coexisting garnet–clinopyroxene pairs in this study compared with results for wellequilibrated Kakanui garnet pyroxenites (Zack et al., 1997).
Intercomparison of different geothermometers for the
garnet–clinopyroxene pairs is equivocal. Agreement
within uncertainty is found between TNi and Fe–Mg
exchange thermometry (Krogh, 1988) for four samples
measured (Electronic Appendix, Table 3). The latter
thermometer, however, cannot be used as a critical
test for equilibrium because it is prone to immense
uncertainties from the presence of substantial ferric iron
in both mantle garnet and clinopyroxene, and unknown
amounts of ferric iron in the calibrations (Canil & O’Neill,
1996). Therefore, there is no reason to believe that
garnet–clinopyroxene pairs are not in chemical equilibrium. In summary, the limited P–T information can
be interpreted to mean that the minimum lithospheric
thickness sampled by the Drybones Bay kimberlite at
>450 Ma was at least 160 km and possibly as much as
190 km thick (Fig. 7). The Drybones Bay kimberlite is
diamondiferous (U. Kretschmar, personal communication, 1997), making the depth estimates from TNi
reasonable.
DISCUSSION
Mantle protolith and process deduced from
garnet geochemistry
Mantle garnets from Drybones Bay form a spectrum
from ultradepleted, with Zr, Ti and Y abundances (Fig.
10a–c) that are among the lowest recognized in a large
database of mantle garnets (Griffin et al., 1999b), to
garnets with levels of these elements more typical of
Archean mantle, to some greatly enriched in Zr (Fig.
10a–c) and having higher Mg/(Mg + Fe) that correlates
with TNi of some garnets (Fig. 11a). Almost all of the
ultradepleted garnets are S-type, with significant LREE
enrichment, as measured by the ratio (Nd/Y)N much
greater than unity (Fig. 10d). The garnets with high Zr
(>40 ppm) are not found at TNi below >900°C, but the
division between the ultradepleted and Zr-rich garnets
is not abrupt, but rather is intermixed over a narrow
range of TNi.
The occurrence of garnets with very low Y (<5 ppm),
a very compatible element in this mineral (Green, 1994),
might at first appear difficult to account for by any
equilibrium mantle process. A full consideration of the
behaviour of Y between melt and bulk residue aids in
explaining the presence of these ‘ultradepleted’ garnets
in both the Drybones Bay suite and elsewhere in the
Slave Province. Figure 12a show the covariation of Y
with Al2O3 for a large database (n >400) of both on- and
off-craton spinel and garnet peridotites [see Canil &
Fedortchouk (2000) and references therein]. This plot
shows the depletion trend of peridotite residues away
from primitive mantle compositions. The change in slope
of the depletion trend for off-craton samples at >2 wt %
Al2O3 represents exhaustion of clinopyroxene from the
source during partial melting. Most cratonic peridotites
have Al2O3 levels of beyond clinopyroxene exhaustion
and were originally harzburgites (Boyd et al., 1993) and
their Y abundances are similar to or above those of offcraton peridotites (Fig. 12a).
Partial melting calculations based on isobaric, equilibrium melting reactions along the peridotite solidus at
2, 3 and 7 GPa and partition coefficients DYcrystal/liq for
olivine, orthopyroxene, clinopyroxene, garnet and spinel
were used to estimate the change in Y with melt depletion
and thereby to examine the partitioning of this element
into garnet from a bulk peridotite residue (Fig. 12b). The
topologies of the calculated residue trends are very similar
to that exhibited by natural peridotite residues, but
the large change in slope representing clinopyroxene
exhaustion in the melting interval occurs at higher Al2O3
than in the natural residue trend (Fig. 12). This latter
feature arises because partial melting in nature is polybaric and fractional, whereas the calculations, being based
on experiments, are for isobaric equilibrium conditions
(Canil & Fedortchouk, 2000).
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GARNET GEOCHEMISTRY AND MANTLE STRUCTURE
Fig. 10. Plots of TNi against (a) Zr, (b) Y, (c) TiO2 and (d) (Nd/Y)N (in log units) for Drybones Bay garnets from concentrate, garnet–clinopyroxene
pairs and xenoliths. Grey shaded bars are median or average values for garnets from ‘shallow’ and ‘deep’ mantle layers thought to exist in the
lithosphere beneath the central Slave Province (Griffin et al., 1999a).
The levels of Y that are expected to partition into the ultradepleted levels of Y (and by similar arguments
garnet from melting residues at each pressure are also Zr and Ti) in garnets from both Drybones Bay and the
shown in Fig. 12b, and are estimated assuming that central Slave Province are not disequilibrium features,
garnet contains all the Y in the residue and comprises but can be reasonably accounted for by extreme levels
only 5% of the mode. Modal garnet varies with degree of melt depletion of the bulk peridotite residue from
of melting along the mantle solidus from being absent at which they were derived.
2 GPa, to being exhausted after only 5% melting at 3
One interpretation of the spectrum of mantle garnets
GPa, to being stable even after 20% melting at 7 GPa from the Drybones Bay kimberlite is that they are derived
(Kinzler & Grove, 1992; Walter, 1998). The assumption from a mantle protolith that has been ultradepleted by
of only 5% modal garnet, however, was used in this melt extraction, but have since been affected by some
exercise to maximize the amount of Y that will partition chemical enrichment process attended by heating. The
into garnet from the bulk residue. Most natural garnet high Cr2O3 and extremely low Y and Sc/Y in these
peridotites, even harzburgites, contain >5% garnet (Boyd garnets (Figs 10b and 11c) are indicative of a protolith
et al., 1993). This fact, as well as the use of lower D that has suffered extensive melt extraction as was quantified
values in the calculations, or consideration of minor in Fig. 12, but this signature contrasts with Ca–Cr sysamounts of Y that will partition into coexisting olivine and tematics, which shows they are almost exclusively lherpyroxenes, will serve to reduce the level of U estimated in zolitic or wehrlitic (Fig. 4). The ultradepleted garnets could
garnets crystallizing from these residues.
have once been harzburgitic, but have been overprinted
The calculations indicate that all garnets crystallizing by a process that has added Ca, LREE, Zr and other
from harzburgite residues will have <10 ppm Y, and incompatible elements, making most of them lherzolitic
many from rocks with very high degrees of depletion and wehrlitic. This transition from harzburgite to lherzolite
would contain <1 ppm. These figures are upper limits has been recognized in mantle garnets from suites in
in the calculations. These arguments make it clear that southern Africa (Schulze, 1995; Griffin et al., 1999c).
137
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S-type pattern in mantle garnets making up the majority
of the Drybones Bay suite thus remains enigmatic [see
also Stachel et al. (1998)].
The T distribution in Fig. 8 may not be a steady-state
feature but probably represents heating of the SW Slave
Province mantle before entrainment of these samples in
their host kimberlite at >450 Ma. This scenario could
involve penetration of metasomatizing fluid into the
mantle causing the geochemical enrichment of previously
ultradepleted lithosphere, and the heating now recorded
in TNi (Fig. 7) and the Mg/(Mg + Fe) of garnets
(Fig. 11a). This metasomatic event would have shortly
preceded entrainment of the samples in the Drybones
Bay kimberlite, so as to prevent any thermal relaxation
after interaction with melt.
Mantle structure in the SW Slave Province
Fig. 11. Plots of TNi against (a) Mg/(Mg + Fe), (b) Cr2O3, and (c)
(Sc/Y)N for Drybones Bay garnets. Fewer points are plotted in (c)
because not all garnets were analysed for Sc. Grey-shaded bars as in
Fig. 10.
The abundance of wehrlitic garnets in the Drybones
Bay garnet population is explicable by carbonatite metasomatism, where clinopyroxene forms from orthopyroxene as a result of the interaction of carbonate melt
with harzburgite (Yaxley et al., 1998). The sinuous REE
pattern in mantle garnets, such as those at Drybones
Bay, was originally thought to arise from disequilibrium
effects that cause the LREE to adjust faster than the
HREE to an introduced carbonatitic melt, possibly because of differences in diffusion rates (Hoal et al., 1994).
This process requires larger LREE cations to diffuse
faster than smaller HREE in garnet, which is not observed
by experiment (Ganguly et al., 1998). The origin of the
Teleseismic data only >50 km north of Drybones Bay
reveal a seismologically unique discontinuity at 110 km
depth (‘X’ in Fig. 13) marked by pronounced anisotropy
changes in multiple layers over a 30 km depth interval
(Bostock, 1998). The location and characteristics of the
X discontinuity have a striking correlation with the onset
of order-of-magnitude variations in garnet chemistry seen
at the same inferred depth in the Drybones Bay garnet
suite (Fig. 13). Correlation of the two features is tenuous,
but if real, would require the X discontinuity to be older
than 450 Ma, the age of the Drybones Bay kimberlite.
Indeed, Bostock (1998) believes the X discontinuity to
be of structural origin, resulting from the subduction
and subsequent stacking of different slabs of lithosphere
beneath the craton in Archean or early Proterozoic time.
In the central Slave Province, Griffin et al. (1999a)
recognized a ‘shallow’ mantle layer, characterized by
garnets with low levels of Zr, Y and Ti, separated from
a ‘deeper’ layer with higher levels of these elements more
typical of Archean mantle. The ultradepleted Drybones
Bay garnets match fairly well with several geochemical
parameters (median Cr2O3, Zr, Y, Ti, Sc/Y, Nd/Y)
that characterize the ‘shallow’ layer in the central Slave
Province (Figs 10 and 11) but they are much more calcic.
The implication is that the ‘shallow’ layer structure
recognized in the central Slave Province may also appear
at its SW margin, but it has been severely overprinted
by subsequent heating and metasomatism that is now
recorded by several geochemical parameters in the
Drybones Bay garnets, such as elevated Ca, Zr and
LREE (Fig. 10a). If the ultradepleted ‘shallow’ layer
extends from the central Slave Province to its SW margin,
and is partly imaged seismically as the X discontinuity
(Fig. 13), it is probably a regional, long-lived feature of
structural origin, still preserved in a deep mantle root.
Grütter et al. (1999) examined a large database of
mantle garnets from till samples throughout the Slave
138
CARBNO AND CANIL
GARNET GEOCHEMISTRY AND MANTLE STRUCTURE
Fig. 12. (a) Covariation of Y with Al2O3 in a large database of natural spinel and garnet peridotites [see Canil & Fedortchouk (2000) for data
sources]. The trend is attributed to melt extraction from primitive upper mantle (PUM, McDonough & Sun, 1995) with the change in slope at
>2 wt % Al2O3 signifying clinopyroxene exhaustion in the source during partial melting to form harzburgite. (b) Calculated covariation of Y
with Al2O3 based on isobaric, equilibrium melting reactions along the peridotite solidus at 2, 3 and 7 GPa (Kinzler & Grove, 1992; Walter,
1998) and a maximum DYcrystal/liq measured by experiment for olivine, orthopyroxene, clinopyroxene, garnet and spinel in basaltic or ultramafic
systems (Green, 1994). The calculated residue trends (grey lines) have a similar topology to that of natural residues (grey shaded field) but are
shifted to higher Al2O3 because melting in nature is polybaric and fractional (not equilibrium and batch as assumed here). The maximum Y
contents of garnets that crystallize from the bulk residues at various pressures (continuous lines) are estimated assuming all Y partitions into this
mineral and that it makes up only 5% of the mode. The trends show how garnets with <1 ppm Y are expected in very depleted harzburgite
residues.
Province and recognized three NE-trending domains in
the mantle. Their southernmost domain, represented in
their study only by garnets from the SE portion of
Slave Province, has a significant population of high-Cr
harzburgitic garnets (>8 wt % Cr2O3) that is clearly not
recognized at Drybones Bay. This southernmost domain
does not, therefore, appear to extend to the SW portion
of the Slave Province, and may be truncated by some
other lithospheric-scale structure, perhaps represented at
the crustal level by the north-trending ‘Pb’ isotopic line
(Fig. 1). The latter feature is a robust goechemical division
distinguishing the source regions for crustal rocks from
the east and west Slave Province, and separates Drybones
Bay in the SW from the garnet populations in the SE
considered by Grütter et al. (1999). Further geophysical
imaging and better sampling of mantle materials between
Drybones Bay and the interior of the craton will be able
to test these and other regional inferences of its mantle
structure.
ACKNOWLEDGEMENTS
139
We sincerely thank U. Kretschmar and D. Smith for
access to drill core, and for permission to publish our
results. Analytical assistance was provided by M. Raudsepp (UBC), L. Shi (U of A) and Z. Chen (UVic). We
JOURNAL OF PETROLOGY
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NUMBER 1
JANUARY 2002
Fig. 13. Present-day cross-section [adapted from Bostock (1998)] of the SW Slave craton drawn by merging geophysical interpretation of deep
seismic reflections on SNORCLE line 1 (shaded terranes on left; Cook et al., 1998) with a high-resolution teleseismic image below station YKA
(shadowgram to right of cross-section; Bostock, 1998). Locations of the SNORCLE line and station YKA are shown in Fig. 1. It should be
noted how the location of the ‘X’ discontinuity (dark ribbon-like feature labelled in the shadowgram) beginning at a depth of >110 km in the
YKA profile coincides with the onset of order-of-magnitude changes in the trace element geochemistry in Drybones Bay garnets, shown here
by Zr in garnet over a depth interval of <50 km (see also Figs 10 and 11).
thank M. Bostock for discussions on the YKA teleseismic
study, and M. Kopylova for donation of garnet 21-6.
The comments of W. Griffin, D. Ionov, R. Rudnick and
M. Kopylova on a previous version of our paper, and
journal reviews by A. Peslier, J. K. Russell and T. Stachel
are appreciated. This study was supported by NSERC
LITHOPROBE grants to D.C. The UVic LA-ICP-MS
facility is funded with NSERC Major Equipment and
Major Facility Access grants. This paper is LITHOPROBE Contribution 1240.
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APPENDIX: PETROGRAPHIC
DESCRIPTIONS
Garnet peridotites
Xenoliths of garnet peridotite are composed of subhedral
to anhedral olivine (0·5–2 mm), pyroxene (1–4 mm) and
garnet (1–7 mm). In most cases the garnets have a reaction
rim (or corona) of optically unresolvable kelyphite. Carbonate and serpentine are also seen invading some of
the garnets along fractures. Phlogopite occurs as mantles
(<0·1 mm) on some of the garnets and in fractures.
Secondary subhedral to euhedral spinel (<1 mm) is also
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present in most samples within relict olivine and pyroxene
grains. Other minor phases such as ilmenite and sulfide
(sphalerite and chalcopyrite) are also found in varying
amounts.
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olivine and pyroxene, whereas with the spinel, the boundaries are more irregular as they usually are bounded by
larger grains.
Heavy mineral concentrate
Spinel–garnet peridotites
Xenoliths of spinel–garnet peridotite are composed of
subhedral to anhedral olivine (0·5–2 mm), pyroxene
(1–4 mm), garnet (2–3 mm) and primary subhedral to
anhedral spinel (<1 mm), sometimes possessing altered
margins. Petrographically these rocks are similar to those
described in the garnet peridotite suite, but garnets have
a smaller corona of kelyphite (<1 mm), and the grain
size is generally smaller.
Spinel peridotite
Anhedral olivine and pyroxene, with an average grain
size of 2–3 mm, dominates the one spinel peridotite
xenolith examined. Anhedral spinel is present and
averages <1 mm in size. This sample has a larger grain
size than in other peridotites in this study. The grain
boundaries are straight to slightly curved between the
Over 100 garnet grains from heavy mineral concentrate
were studied under a stereomicroscope and show no
signs of alteration, good clarity and no reaction rims
(kelyphite). A large range in colour is also apparent, from
orange–yellow to deep purple. Garnets with light red to
deep purple–violet colour were chosen for analysis because they represented the most pyropic of the garnets
in the concentrate, as shown by later chemical analysis.
Seven (physically attached) Cr-diopside–garnet pairs
were also recognized in the coarse concentrate. The
diopside grains show no sign of alteration and all have
a distinctive bright emerald green colour. It is not clear
why clinopyroxene grains in concentrate were not carbonatized and/or serpentinized, whereas those in xenoliths consistently were. The garnet–diopside pairs are
angular with poor to moderate sphericity, implying that
these grain pairs are probably of xenolithic origin. The
diopside grains in particular are more angular because
of fracturing on well-developed cleavage planes.
142