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Peridotitic diamonds from the Slave and the Kaapvaal

cratonssimilarities and differences based on a

preliminary data set

Thomas Stachela,*, Jeff W. Harrisb, Ralf Tapperta,c, Gerhard P. Brey c

aDepartment of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada T6G 2E3bDivision of Earth Sciences, University of Glasgow, Glasgow G12 8QQ, UK

cInstitut fur Mineralogie, Universitat Frankfurt, 60054 Frankfurt, Germany

Abstract

A comparison of the diamond productions from Panda (Ekati Mine) and Snap Lake with those from southern Africa shows

significant differences: diamonds from the Slave typically are un-resorbed octahedrals or macles, often with opaque coats, and

yellow colours are very rare. Diamonds from the Kaapvaal are dominated by resorbed, dodecahedral shapes, coats are absent

and yellow colours are common. The first two features suggest exposure to oxidizing fluids/melts during mantle storage and/or

transport to the Earths surface, for the Kaapvaal diamond population.

Comparing peridotitic inclusions in diamonds from the central and southern Slave (Panda, DO27 and Snap Lake

kimberlites) and the Kaapvaal indicates that the diamondiferous mantle lithosphere beneath the Slave is chemically lessdepleted. Most notable are the almost complete absence of garnet inclusions derived from low-Ca harzburgites and a generally

lower Mg-number of Slave inclusions.

Geothermobarometric calculations suggest that Slave diamonds originally formed at very similar thermal conditions as

observed beneath the Kaapvaal (geothermal gradients corresponding to 4042 mW/m 2 surface heat flow), but the diamond

source regions subsequently cooled by about 100150 jC to fall on a 3738 mW/m2 (surface heat flow) conductive geotherm,

as is evidenced from touching (re-equilibrated) inclusions in diamonds, and from xenocrysts and xenoliths. In the Kaapvaal, a

similar thermal evolution has previously been recognized for diamonds from the De Beers Pool kimberlites. In part very low

aggregation levels of nitrogen impurities in Slave diamonds imply that cooling occurred soon after diamond formation. This

may relate elevated temperatures during diamond formation to short-lived magmatic perturbations.

Generally high Cr-contents of pyrope garnets (inside and outside of diamonds) indicate that the mantle lithosphere beneath

the Slave originally formed as a residue of melt extraction at relatively low pressures (within the stability field of

spinelperidotites), possibly during the extraction of oceanic crust. After emplacement of this depleted, oceanic mantlelithosphere into the Slave lithosphere during a subduction event, secondary metasomatic enrichment occurred leading to strong

re-enrichment of the deeper (>140 km) lithosphere. Because of the extent of this event and the occurrence of lower mantle

diamonds, this may be related to an upwelling plume, but it may equally just reflect a long term evolution with lower mantle

diamonds being transported upwards in the course of normal mantle convection.

D 2003 Elsevier B.V. All rights reserved.

Keywords: Diamond; Inclusion; Peridotite; Craton; Kaapvaal; Slave; Geotherm

0024-4937/$ - see front matterD 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0024-4937(03)00127-0

* Corresponding author.

E-mail address: [email protected] (T. Stachel).

www.elsevier.com/locate/lithos

Lithos 71 (2003) 489 503

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1. Introduction

The predominantly peridotitic mantle roots of cra-

tonic cores such as the Kaapvaal or the Slave originatein the early (>3 Ga) to late Archean (deWit, 1998;

Carlson et al., 1999b; Griffin et al., 1999a; Pearson,

1999; Bleeker, 2003). Then, as they remained open

systems, they are expected to reflect a complex

history, involving stages of cooling and heating

(plume events), partial melting and infiltration by

melts and fluids. In consequence, xenoliths of cratonic

peridotites are probably unreliable witnesses to the

early stages of the formation of mantle lithosphere.

Diamonds, which formed in these cratonic garnet

peridotites, are also assumed to be generally of

Archean age (Richardson et al., 1984, 2001; Pear-

son and Shirey, 1999) some exceeding 3 Ga. In

contrast to xenoliths, the syngenetic inclusions in

such diamonds have been encapsulated in a closed

system for that entire period. Therefore, they repre-

sent important samples with which to study the

early evolution of the subcratonic mantle lithosphere

through time. In addition, diamonds can record the

PT-conditions at the time of their formation through

non-touching (i.e. separated) inclusions, which have

remained chemically isolated and thus unable to

adjust chemical equilibria to changing physicalconditions. Therefore, comparing diamonds and

their inclusions from the Slave and the Kaapvaal

cratons will supplement and clarify the information

collected using mantle xenoliths and will aid to

clearly identify Archean signatures as opposed to

later overprint.

Compared to the Kaapvaal, where diamonds have

been mined for more than a century, mining activity

on the Slave only just commenced with the opening of

the Ekati Mine (Fig. 1) in 1998. As a consequence,

only few systematic studies on diamonds and theirinclusions from the Slave craton have been carried out

so far. Davies et al. (1999) studied inclusions in

diamonds from the southeast side of Lac de Gras

(pipe DO27 of the Tli Kwi Cho kimberlite complex,

see Doyle et al., 1999) and Chinn et al. (1998)

examined the production of the Misery, Sable and

Jay diatremes, which are part of the Ekati Mine,

located about 25 km north of Lac de Gras (see Carlson

et al., 1999a). In addition to these published data for

the central Slave, we present new analyses from the

Panda diatreme of the Ekati mine. For the Southern

Slave (Fig. 1), Pokhilenko et al. (2001) studied

inclusions in diamonds from the Snap Lake kimberlite

dike (near Camsell Lake, 220 km NE of Yellowknife);no inclusion data are currently available for the

Northern Slave.

1.1. Database

We examined 132 inclusions from 88 Panda dia-

monds and observed inclusion abundances are given

in Table 1. For the purpose of the present study,

diamonds containing eclogitic, sub-lithospheric (low-

er mantle) or epigenetic inclusions are not considered,

excluding less than 20% of the Panda diamonds (see

Table 1). A preliminary Slave data base was

established by addition of published analyses of

peridotitic inclusions from DO27 (12 inclusions from

11 diamonds, Davies et al., 1999) and Snap Lake (20

inclusions from 14 diamonds, Pokhilenko et al.,

2001).

The data set for the Kaapvaal comprises inclusion

analyses from Bellsbank, Bultfontein, De Beers Pool,

Finsch, Jagersfontein, Koffiefontein, Letlakane, Let-

seng-la-Terai, Monastery, Premier, Roberts Victor and

Wesselton (Deines et al., 1984, 1987, 1989, 1991,

1997; Deines and Harris, 1995; Gurney et al., 1979,1984, 1985; McDade and Harris, 1999; Moore and

Gurney, 1989; Phillips and Harris, 1995; Rickard et

al., 1989; Wilding et al., 1994).

1.2. Analytical techniques

Inclusions were released by breakage of the host

diamond in a steel crusher. The inclusions were

embedded in AralditeR and polished on a PbSb

plate using 0.25 Am diamond powder. Major and trace

element compositions (Table 2) were obtained byelectron microprobe analysis (Jeol JXA-8900 RL) at

20 kV gun potential and 20 nA beam current using

silicate, oxide and metal standards. Count times

ranged between 30 and 100 s, and three analytical

points were averaged to ensure detection limits of 100

ppm or better for all oxides except Na2O (200 ppm).

A spectrometer with small Rowland circle (100 mm

radius) and a LIF crystal with Johannson type geom-

etry was used for Mn, Fe, Ni and Zn, resulting in ca.

three times higher count rates compared to a standard

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Fig. 1. Location of sample sources (Ekati, Tli Kwi Cho and Snap Lake) in the Slave Province. Subdivisions for the Slave are from Grutter et al.

(1999) and are based on compositional differences of xenocrystic garnet derived from the lithospheric mantle. The diamondiferous Jericho

kimberlite (Northern Slave) is shown with reference to the geothermobarometric work of Kopylova et al. (1999) discussed in the text.

Boundaries between the Slave Craton (light grey) and Proterozoic mobile belts and Proterozoic and Phanerozoic cover sequences (hatched) are

taken from Bleeker (2003).

T. Stachel et al. / Lithos 71 (2003) 489503 491

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configuration. To improve the analytical precision for

Zn in spinel, peak count times were also applied for

background measurements. Accuracy of major ele-

ment analyses was checked against secondary stand-

ards and is better than 1% (relative). Analytical

precision is mainly controlled by counting statistics

(which, for a given oxide species, depend on theconcentration) and therefore strongly declines for

minor and trace elements. At an oxide concentration

of 0.10 wt.%, the two sigma errors range between 4%

and 15% (relative).

Nitrogen concentrations and aggregation levels

were determined on diamond cleavage chips (i.e.

not through the diamond surface) by Fourier trans-

form infrared spectroscopy (FTIR) using a Nicolet

Magna IR 550 coupled to a Spectra-Tech IR-micro-

scope. After conversion to absorption coefficients,

the spectra were deconvoluted into the A, B and Dcomponents (e.g. Boyd et al., 1995) using least

square techniques (program by David Fisher). Ni-

trogen concentrations (atomic ppm) were calculated

using the absorption strength at 1282 cm 1 for the

A-centre (Boyd et al., 1994, 16.5F 1) and the B-

centre (Boyd et al., 1995, 79.4F 8). Detection

limits and errors strongly depend on the quality of

the cleavage chip, but typically range between 10

20 ppm and about 10 20% of the concentration

respectively.

2. Diamond characteristics

A comparison between the diamonds from the

Panda and Snap Lake kimberlites in the Slave cratonand the general southern African (Kaapvaal) produc-

tions shows that throughout common sizes ranges

diamonds from Canada are far less resorbed. This

difference means that the general dominance of pri-

mary crystal shapes of octahedra and the spinel twin

form (triangular macle), in the productions from the

Slave, is not matched on the Kaapvaal, where an

overwhelming predominance of secondary diamond

forms like dodecahedra and flattened dodecahedral

macles occurs.

A further distinctive feature is that a significant

proportion of both Panda and Snap Lake productions

are coated with impure diamond. In the larger sizes,

the coat may be so thin as to be almost transparent,

but more generally, it forms an opaque cover over a

clear diamond. In the smaller sizes, additionally, some

diamonds have nucleated as opaque cubes, interpen-

etrant cubes, or diamond with a more complex shape,

such as cubo-octa-dodecahedra. Coat colors at Panda

are dominated by grey to dark grey/black, throughout

all sizes and diamond shapes. Occasionally, some are

green-grey. At Snap Lake, some opaque coats are also

grey, but more are translucent/opaque yellow, with afew being green. Coated diamonds are rare in South

African productions. This rarity is probably related to

the general level of resorption, because occasionally,

as in the DeBeers Pool production for example,

vestiges of opaque coat are recorded on residual

octahedral faces of dodecahedra. It may be, therefore,

that some diamonds from the Kaapvaal were similarly

opaque, but subsequent resorption removed this ma-

terial. However, further north in Botswana, levels of

resorption are less than in South Africa and there is no

evidence of opaque coats. Boart, an opaque, oftenaggregated form of diamond, with no particular shape,

is not obviously present in the Canadian production.

Within southern Africa, however, this diamond form

is present in all mines.

On the Slave, the body colors of the diamonds

from the two mines are generally similar, especially

once the dark diamonds from these mines (consisting

of colorless diamond full of graphite inclusions) have

been removed. At Panda, brown is the dominant color

throughout all size ranges, followed by colorless. The

Table 1

Inclusion abundance table for the 88 diamonds (132 inclusions)

from Panda studied

Peridotitic (72 diamonds) (103 inclusions)

Pyrope garnet 30

Olivine 26

Cr-diopside 8

Enstatite 2

Chromite 18

NiFe-sulfide 19

Eclogitic (7 diamonds) (12 inclusions)

Pyr-alm garnet 8

Omphacitic cpx 1

Rutile 1

Fe-sulfide 2

Lower mantle (4 diamonds) (10 inclusions)

Ferropericlase 4

CaSi-perovskite 1Olivine 3

SiO2 1

MgAl2O4 1

Epigenetic (5 diamonds) (7 inclusions)


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brown color is linked to high levels of plastic defor-

mation (see Harris, 1992). Yellow diamonds areextremely rare in the production. At Snap Lake,

colorless dominates over brown plastically deformed

diamonds and yellow diamonds are extremely rare.

Additionally, at Snap Lake, diamonds with transparent

green coats are common. The green transparent coat is

quite distinct from the opaque or translucent coats

discussed above and results from radiation damage

(see Vance et al., 1973). Its absence in the Panda

production probably relates to the much younger age

of kimberlite emplacement (Snap Lake: 523 Ma,

Geospec Consultants, 1999; Panda: 53.2 Ma, Carlson

et al., 1999a).In Kaapvaal productions principal diamond colors

are colorless, yellow and brown. If mining is within

the upper oxidized part of a kimberlite, then diamonds

with transparent green coats are a common feature.

For a more detailed assessment of color in diamonds

from southern Africa, see Harris (1992) and referen-

ces therein.

Yellow in diamond results from the impurity

nitrogen, occurring in the so-called N3 configuration

(three nitrogens and a vacancy). Its virtual absence

Table 2

Electron microprobe analyses of peridotitic inclusions in diamonds from Panda

Sample

mineral

TOW79 (jC)

PA-1b (1)

garnet

1043

PA-1b (2)

olivine

PA-3a

garnet

PA-5a (1)

garnet

1076

PA-5a (2)

olivine

PA-5a (3)

opx

PA-13a

garnet

PA-13b

chromite

PA-19a (1)

garnet

1169

PA-19a

(2) opx

PA-19b

olivine

paragen.gt, gt-ol gt gt-ol-opx gt, chr gt-opx, ol

P2O5 0.01 V0.01 0.01 0.02 V0.01 V0.01 0.04 V0.01 0.05 V0.01 V 0.01

SiO2 41.31 41.11 40.79 41.71 41.13 57.90 41.02 0.20 41.80 57.94 40.93

TiO2 0.02 V0.01 0.03 0.08 V0.01 0.02 0.09 0.16 0.07 0.02 V 0.01

Al2O3 16.25 0.01 13.21 16.83 0.02 0.40 13.93 4.12 16.82 0.45 0.02

Cr2O3 10.45 0.12 13.66 9.63 0.21 0.34 12.69 65.72 9.08 0.38 0.05

FeO 6.37 6.87 5.59 6.45 7.08 4.13 6.71 16.68 6.04 3.79 7.19

MnO 0.31 0.09 0.28 0.32 0.10 0.10 0.37 0.28 0.29 0.09 0.11

NiO V0.01 0.39 0.01 0.01 0.40 0.11 0.02 0.10 0.01 0.14 0.35

MgO 20.79 50.75 21.67 21.45 50.40 35.57 19.36 12.84 20.50 35.95 50.65

CaO 4.67 0.03 3.71 3.93 0.04 0.34 6.14 V0.01 5.96 0.54 0.04

Na2O 0.31 0.23 0.02 V0.02 V0.02 V0.02 V0.02 V0.02 V 0.02 V0.02 V 0.02

K2O V0.01 V0.01 V0.01 V0.01 V0.01 V0.01 V0.01 0.08 V 0.01 V0.01 V 0.01

Total 100.50 99.60 99.00 100.42 99.37 98.89 100.37 100.15 100.60 99.30 99.33

Sample

mineral

TOW79 (jC)

PA-23a

garnet

1214

PA-23b

olivine

PA-24a

chromite

PA-26a

chromite

PA-29a

chromite

PA-31b

chromite

PA-40a

olivine

1196

PA-40b

garnet

PA-42a

olivine

1118

PA-42b

garnet

paragen.gt, ol 2chr chr chr 2chr gt, ol gt, ol

P2O5 0.01 V0.01 V0.01 V0.01 V0.01 V0.01 V0.01 0.03 V 0.01 0.01

SiO2 41.84 40.87 0.12 0.10 0.09 0.10 40.85 42.40 40.87 42.12

TiO2 0.16 V0.01 0.04 0.10 0.22 0.10 V0.01 0.13 V 0.01 0.03

Al2O3 16.94 0.03 5.74 7.25 7.20 7.16 0.02 16.30 0.02 15.02

Cr2O3 8.14 0.07 65.77 63.90 63.73 63.99 0.06 8.59 0.04 10.74

FeO 6.10 7.44 16.23 15.01 14.87 15.02 7.13 5.95 7.57 6.61MnO 0.29 0.11 0.30 0.25 0.25 0.25 0.10 0.28 0.11 0.34

NiO 0.01 0.34 0.06 0.10 0.10 0.09 0.35 0.01 0.38 0.01

MgO 20.75 50.09 11.60 13.73 13.82 13.79 50.20 20.72 50.06 20.56

CaO 5.51 0.05 V0.01 V0.01 V0.01 V0.01 0.04 5.79 0.04 5.21

Na2O V0.02 V0.02 V0.02 V0.02 0.04 V0.02 V0.02 0.03 V 0.02 V0.02

K2O V0.01 V0.01 V0.01 V0.01 V0.01 V0.01 V0.01 V0.01 V 0.01 V0.01

Total 99.75 99.00 99.85 100.44 100.32 100.50 98.75 100.24 99.09 100.65

TOW79 refers to temperatures (at P= 5 GPa) calculated from the Mg Fe exchange between garnet and olivine (ONeill and Wood, 1979;

ONeill, 1980).


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in Canadian diamonds means that the N3 centre is

not a significant by-product of the more common

nitrogen aggregation from the A-centre (a nitrogen

pair) to the B-centre (four nitrogens and a vacancy)(Evans, personal communication, but also see Col-

lins, 2001, for a detailed discussion of color in

diamonds). Thus, at present, there is a clear color

distinction between diamonds from Canada and

southern Africa.

3. Inclusion compositions

Comparing the composition of pyrope garnet

inclusions from the southern and central Slave and

the Kaapvaal in a CaCr plot (Fig. 2) shows that a

low-Ca harzburgitic garnet population, which is

characteristic for the Kaapvaal, is absent for the

Slave samples. On average, Slave garnet inclusions

are higher in Ca and Cr and many harzburgitic

garnets plot close to the lherzolitic trend, which

suggests that their source rocks were near saturation

in clinopyroxene. A CaCr plot for garnet inclusions

from Misery, Sable and Jay in Chinn et al. (1998,

their Fig. 3) suggests compositional similarity to

Panda and Snap Lake but indicates lower Cr2O3(46 wt.%) for the lherzolitic inclusion paragenesis

observed at Sable and reveals the presence of a

minor low-Ca harzburgitic suite (2 out of 18 garnets

have Cr2O3 < 2 wt.%).

A comparison of the Mg-number of garnet inclu-

sions from Panda and Snap Lake with the Kaapvaal

and other diamond sources world-wide also indicates

a higher fertility for the Slave (Fig. 3). However, the

Mg-number of garnet is not only a function of bulk

composition but is also strongly controlled by the

temperature dependent Mg Fe exchange with oliv-

ine, orthopyroxene, clinopyroxene and Mg-chromite.

If the low Mg-number of Slave garnets only indi-

cates formation at unusually low temperatures, then

olivine and orthopyroxene should show elevated Mg-

numbers. However, Fig. 4 shows that the Mg-num-

ber of Slave olivine inclusions perfectly overlaps

with the world-wide data set and falls on the Mg-

poor side of the Kaapvaal distribution. Including 43

inclusions from Misery, Jay and Sable (Chinn et al.,

1998), olivine inclusions from the Slave show a

sharp mode in Mg-number in class 92 93, as

opposed to a mode in class 9495 for olivines fromthe Kaapvaal. Only nine orthopyroxene inclusions

have been recovered for Panda, DO27 and Snap

Lake but they all have a Mg-number < 94.5, which

is clearly distinct from the Kaapvaal (mode in Mg-

number in class 95 96). An interesting feature in

Fig. 4 is the clear chemical distinction between

Panda and DO27 olivine, with Panda being high

and DO27 being low in Ni. In contrast, Ni contents

of DO27 orthopyroxenes are normal (not illustrated).

This suggests that the low Ni contents in olivine

inclusions probably reflect the modal ol/opx ratiorather than bulk Ni contents, with the source regions

of DO27 being orthopyroxene poor compared to

Panda.

The composition of chromite inclusions is shown

in Fig. 5 and again Panda and Snap Lake (no

chromites have been observed at DO27) on average

fall on the Fe-rich side of the Kaapvaal data. In terms

of ferric iron content the available chromite data

appear to fall into two groups, one (five chromites

from Panda, ferric iron number of 6.7 9.2) being

Fig. 2. Diagram of CaO versus Cr2O3 (wt.%) in garnet with the

lherzolite field (shaded area) as defined by Sobolev et al. (1973). As

in all following diagrams, data for DO27 and Snap Lake are taken

from Davies et al. (1999) and Pokhilenko et al. (2001), respectively,

references for Kaapvaal diamonds are given in the text and sources

for the data base on inclusions from mines world-wide are given in

Stachel and Harris (1997) and Stachel et al. (2000).


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similar to the reduced Mg-chromites from the Kaap-

vaal, the other (15 inclusions from Panda, 5 inclusions

from Snap Lake, ferric iron number of 16.0 24.5)

being slightly more oxidized.

For Snap Lake, Pokhilenko et al. (2001) report twoknorringite rich (Cr2O3 of 11.8 and 12.8 wt.%) garnet

inclusions with a significant majorite component (6.23

and 6.33 cations Si at [O] = 24). From the experimen-

tal work of Irifune (1987), this may be interpreted to

suggest a continuation of the subcratonic lithospheric

mantle beneath the southern Slave to about 300350

km depth, at least up to the emplacement of the Snap

Lake dike in the Cambrian. Apart from this unique

feature, the inclusion compositions show that dia-

monds from the Slave are derived from a chemical

environment that is chemically less depleted than the

Fig. 3. Histograms showing the Mg-number (molar Mg/(Mg + Fe*))

of garnet inclusions in diamonds from the Slave, Kaapvaal and other

cratonic areas. The effect of Ca on the MgFe partitioning between

garnet and olivine was eliminated by recalculating all garnets to a

Ca-free composition. From the data of ONeill and Wood (1979), it

may be estimated that at constant temperature the Mg-number of

garnet decreases by 2.0 per cation Ca (based on a formula with 24

oxygens) for the temperature range (11001200 jC, 5 GPa) and

olivine composition (Mg-number around 93) relevant here.

Fig. 4. NiO (wt.%) versus molar Mg-number for olivine inclusions

in diamonds.


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Kaapvaal but very similar to other diamond sourcesworld-wide.

4. Geothermobarometry

Equilibration temperatures based on the Mg Fe

exchange between olivine and garnet (ONeill and

Wood, 1979; ONeill, 1980) were obtained for eight

Panda diamonds (see Table 2). Calculated for an

assumed pressure of 5 GPa, temperatures range from

1060 to 1210 jC, with an average of 1140 jC (Fig. 6).

Garnetolivine temperatures of 1180 jC reported byChinn et al. (1998) for two more diamonds from the

Ekati mine (Misery, Sable or Jay) and of 1130 jC

calculated for diamond SL3-31/00 from Snap Lake

Fig. 5. Cr-number versus Fe-number (molar ratios) for Mg-chromite

inclusions. Fe2 + is estimated from total iron contents (EPMA) using

the equation ofDroop (1987). For crystallo-chemical reasons, Fe/Mg

and Cr/Al ratios in spinel are positively correlated. The fact that a

number of inclusions from Panda and Snap Lake plot below the

world-wide trend thus implies a source with elevated Fe/Mg (as

unusually low temperatures may be excluded).

Fig. 6. Histograms showing calculated equilibration temperatures (at

P= 5 GPa) for coexisting garnet and olivine (ONeill and Wood,

1979; ONeill, 1980) inclusions in diamonds from the Slave (only

Panda), Kaapvaal and other cratonic areas. The two Panda

diamonds, which contain touching pairs of garnetolivine, fall on

the low-temperature end and yield temperatures of about 1070 jC.


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(Pokhilenko et al., 2001) are in good agreement with

the Panda data. These results for the Slave are similar

to the Kaapvaal (29 diamonds with an average of

1130 jC) and also to data from world-wide sources

(N= 115), which form a normal distribution with a

mode and average of approximately 1150 jC (Fig. 6).

The Zn content of Mg-chromite inclusions indi-

cates very similar temperatures (calculated after Ryan

et al., 1996), resulting in averages of 1150 jC for bothPanda (see Table 3) and world-wide sources. From all

these estimates, it appears that diamond formation at

the Slave and Kaapvaal cratons and in other cratonic

areas world-wide took place in a very similar geo-

thermal regime.

Applying the relationship between crustal heat

production and heat transported from the mantle as

established by Pollack and Chapman (1977), steady

state palaeo-geotherms of about 4042 mW/m2 (sur-

face heat flow) have been calculated for the formation

of diamonds by a number of authors (e.g. Boyd andGurney, 1986; Griffin et al., 1992; Stachel and Harris,

1997). Kopylova et al. (1999) and Russell and Kopy-

lova (1999) demonstrated that crustal heat production

in the Slave exceeds the values observed in other

cratons (including the Kaapvaal), leading to signifi-

cantly higher surface heat flow. However, to compare

palaeo-geotherms in the lithospheric mantle, it is

common practice to ignore differences in crustal heat

production and to look at apparent surface heat flow

values instead, with a fixed amount of 60% (Pollack

and Chapman, 1977) of the surface heat flux coming

from the mantle. The very good agreement of the

geothermometers applied, indicates that diamond for-

mation beneath the Slave took place at a similargeothermal gradient as in the Kaapvaal, corresponding

to about 4042 mW/m2 surface heat flow. This

palaeo-geotherm is hotter than the gradients of about

38 mW/m2 derived from the study of xenoliths and

xenocrysts for the deeper (>140 km) mantle litho-

sphere at Lac de Gras (Pearson et al., 1999; Griffin et

al., 1999a; Cretaceous to Tertiary kimberlites activity)

and beneath the Jurassic Jericho kimberlite pipe

(Kopylova et al., 1999).

Four diamonds from Snap Lake and Panda contain

inclusions of garnet and orthopyroxene, a paragenesis

from which pressure and temperature can be calculat-

Table 3

Zn contents in Mg-chromite inclusions and corresponding temper-

atures (Ryan et al., 1996)

Sample ZnO (ppm) T-Zn (jC)

PA-13 433 1287

PA-20 429 1293

PA-24 821 962

PA-25 423 1303

PA-26 498 1202

PA-27 505 1194

PA-28 713 1021

PA-29 458 1252

PA-30 500 1200

PA-31 551 1146

PA-32 550 1147

PA-33 715 1020

PA-34 630 1079

PA-35 595 1107Average 1158

Fig. 7. Pressure and temperature estimates for coexisting inclusions

of garnet and orthopyroxene in diamonds from the Slave (Panda and

Snap Lake), Kaapvaal and other cratonic areas. Geothermalgradients are calculated after Pollack and Chapman (1977). The

diamond graphite transition is from Kennedy and Kennedy (1976).

Data points for the De Beers Pool kimberlites are shown separately

from the rest of the Kaapvaal data, because they reveal a cooling

history from a geotherm corresponding to 40 mW/m2 surface heat

flow (non-touching inclusion pairs) to about 38 mW/m2 for

touching pairs (Phillips and Harris, 1995). The two Panda diamonds

containing touching pairs of garnetorthopyroxene also indicate a

lower geotherm than deduced from the two non-touching pairs from

Snap Lake and garnet olivine and Zn in spinel thermometry. Thus,

re-equilibration of touching inclusions during cooling subsequent to

diamond formation is also assumed for the two Panda diamonds.


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ed simultaneously, based both on the Al (Brey and

Kohler, 1990) and MgFe exchange (Harley, 1984)

between these two phases. For Snap Lake, the two

data points calculated from analyses SL3-12/00 andSL5-8/00 (Pokhilenko et al., 2001, their Tables 2 and

3) plot on a 40 mW/m2 surface heat flow geotherm

(Fig. 7), consistent with the results of garnetolivine

thermometry. The PT-conditions derived for Panda

from two touching pairs of garnet orthopyroxene

indicate a lower geothermal gradient corresponding

to about 37 mW/m2 surface heat flow. This result is

within error of Jurassic to Tertiary palaeo-geotherms

determined from xenolith studies by Kopylova et al.

(1999) and Griffin et al. (1999a). Fig. 7 shows the

results obtained by Phillips and Harris (1995) for

touching and non-touching garnet and orthopyroxene

inclusions in diamonds from the De Beers Pool

kimberlites. Phillips and Harris (1995) found that

touching pairs generally indicate lower temperatures

than isolated inclusions and suggested that this is due

to re-equilibration of the touching parageneses. Sim-

ilarly, our data for the Lac de Gras area (Figs. 6 and 7)

suggest that diamond formation was followed by

significant (about 100150 jC) cooling of the dia-

mond source regions in the deep lithosphere.

5. Nitrogen contents and aggregation levels

The concentration and aggregation of nitrogen

impurities in peridotitic diamonds from Panda (this

study) and DO27 (Davies et al., 1999) was determined

by micro-FTIR and is shown in Fig. 8. As already

noted by Chinn et al. (1998) for Misery, Sable and

Jay, the nitrogen data are very variable, ranging in

concentration from 0 (Type II) to over 1000 atomic

ppm and showing a range in nitrogen aggregation

from pure Type IaA to pure Type IaB.The dependence of nitrogen aggregation on mantle

residence time and temperature was derived by Evans

and Harris (1989) and Taylor et al. (1990). Under

mantle conditions, the aggregation of nitrogen from

single substitutional atoms to pairs (A-centre)

occurs so fast (Taylor et al., 1996) that it may be

ignored in a thermo-chronometric analysis. The sec-

ond step, the aggregation from the A-centre to groups

of four nitrogens surrounding a vacancy (B-centre),

occurs at a much lower rate and thus, in principle, it is

possible to constrain either the mantle residence time

(given the residence temperature) or the residence

temperature (given the residence time) of a diamond

from the concentration and aggregation of nitrogen

impurities. Estimates of mantle residence tempera-

tures are fairly insensitive to the choice of residence

time (based on values generally >1 Ga) and normally

show satisfactory agreement with independent esti-mates based on inclusion geothermobarometry (e.g.

Taylor et al., 1995; Leahy and Taylor, 1997).

However, plastic deformation is common in dia-

monds and for other minerals has been shown to

increase diffusion rates significantly (see Cole and

Chakraborty, 2001 for a recent review). This renders

mantle residence temperatures and residence times

based on nitrogen data maximum estimates. Consid-

ering the very strong control of temperature on

nitrogen aggregation, introduced errors will probably

Fig. 8. Aggregation level (percentage of the higher aggregate

B-centre relative to the A-centre) versus concentration (atomic ppm)

of nitrogen impurities in peridotitic diamonds from the Slave (Panda

and DO27) and world-wide sources (including Kaapvaal data). The

grey sector indicates mantle residence times of 050 Ma assuming a

residence temperature of 1150 jC, which is consistent with the

results of garnetolivine and Zn in spinel thermometry for Panda

inclusions. The 50 Ma envelope and the residence time of 176 Ga for

a Type IaB diamond (top of diagram) were calculated using theactivation energy for the conversion of the A- to the B-centre derived

by Taylor et al. (1990).


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be within the accuracy of conventional geothermobar-

ometry (as indicated by the overall agreement of both

methods) but the application of nitrogen data as a

chronometer may be seriously hampered. However,maximum residence times may be safely calculated

using this method. Some of the Slave diamonds show

medium to high nitrogen concentrations at very low

aggregation levels (grey sector in Fig. 8). Maximum

residence times forthese diamonds are in the range of

10 50 Ma (Fig. 8), if the average temperature of 1150

jC derived from various geothermometers (see above)

is used for the calculations. If, however, fairly rapid

cooling after diamond formation by about 100 jC is

assumed, residence times in excess of 1 Ga are

calculated. This is in good agreement with the cooling

of the diamond source regions by about 100150 jC

derived from garnet orthopyroxene geothermobar-

ometry and may indicate that diamond formation

was accompanied by transient thermal perturbations

(magmatic intrusions). Spatially confined lithospheric

heating through magmatic pulses may also be indi-

cated by an unusually high paleo-geotherm (equiva-lent to 42 mW/m2 surface heat flow) derived by

MacKenzie and Canil (1999) for the late Creta-

ceousearly Paleocene Torrie kimberlite, which is in

stark contrast to the 38 mW/m2 (at T>900 jC)

observed by Griffin et al. (1999a) for xenocrysts and

xenoliths from penecontemporaneous kimberlites at

nearby Lac de Gras. Compared to the Lac de Gras

area, the elevated paleo-geotherm at Torrie is also

accompanied by a highly re-fertilized mantle litho-

sphere (Griffin et al., 1999c) providing further evi-

dence for a locally confined magmatic event.

Some Panda diamonds show very high aggregation

levels which result in un-reasonably long residence

times at 1150 jC (Fig. 8). These diamonds obviously

Fig. 9. (a) Pressure (gt-opx) and temperature (cpx-opx) estimates (calculated after Brey and Kohler, 1990) for garnet peridotites occurring as

xenoliths in Kaapvaal kimberlites (for details, see Brey, 1989). The majority of xenoliths define a geotherm of about 42 mW/m2 surface heat flow

(Pollack and Chapman, 1977). Between about 5 and 6.5 GPa, a second array of xenoliths exists that indicates significantly hotter conditions. (b)

Using Ti in garnet as a measure of chemical fertility, a positive correlation between pressure (i.e. depth) and chemical enrichment beco mes

apparent. This relationship is particularly prominent for garnets from the xenoliths defining the hot array in (a). As firstly suggested by Brey

(1989), these hot enriched xenoliths are likely to reflect the infiltration of asthenospheric melts into the base of the subcratonic lithosphere.


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were exposed to higher temperatures, possibly imply-

ing formation at greater depth or very close to

magmatic intrusions.

6. Discussion

Stabilization of the Slave craton is presently dated

to about 2.58 Ga (cessation of terminal granite

bloom, Bleeker, 2003) and possibly relates to the

formation of a late Archean supercontinent (Scla-

via) including the Dharwar, Wyoming and Zim-

babwe cratons (Bleeker, 2003). Re Os model ages

on sulphide inclusions in olivine (Aulbach et al.,

2003) indicate that the diamondiferous mantle root

beneath the central Slave may be significantly older

(>3 Ga), thus implying tectonic emplacement of a pre-

existing root at around 2.6 Ga. Griffin et al. (1999a)

propose that the mantle lithosphere beneath the Lac de

Gras area consists of two layers, an ultra-depleted

upper layer ( < 145 km) and a less refractory lower

layer. Differences in thermal conductivity between the

two layers appear to be larger than expected from the

observed change in bulk composition and may be

related to the high electrical conductivity of the upper

layer, as mapped by Jones et al. (2001). The presence

of lower mantle diamonds (Davies et al., 1999) andthe sharp transition between the two layers led Griffin

et al. (1999a) to propose that the lower layer repre-

sents a frozen plume head.

However, this conclusion is not supported by the

high Cr contents of peridotitic garnet occurring as

inclusions in diamonds (Ekati Mine, DO27 and Snap

Lake), in peridotitic xenoliths (Pearson et al., 1999)

and as xenocrysts (Grutter et al., 1999). Experimental

evidence (summarized in Bulatov et al., 1991; Stachel

et al., 1998; Grutter, 2001) shows (i) that high Cr

garnets can only form in bulk rock compositions withhigh Cr/Al ratios (relative to primitive mantle) and (ii)

that melt extraction only produces residues with the

required high Cr/Al ratios, if partial melting occurs at

relatively low pressures (0.82.5 GPa) in the stability

field of spinel peridotite. This suggests that the

primary depletion event occurred at the Archean

equivalent of mid-ocean ridges or island arc settings

(see also Grutter and Anckar, this volume). These

depleted protoliths were subsequently transferred to

garnetfacies depths.

Based on this evidence, we propose that both the

upper and the lower layer of the mantle lithosphere

beneath Lac de Gras represent depleted former oce-

anic lithosphere, which became imbricated beneaththe Slave during Archean subduction of relatively

young and hot slabs (Helmstaedt and Schulze,

1989). The age mismatch between the stabilization

of the Slave craton and its deep mantle lithosphere in

the Lac de Gras area may imply a more complex

tectonic evolution: Aulbach et al. (this volume) sug-

gest thrusting of old mantle lithosphere attached to the

central Slave Basement Complex beneath the Con-

twoyto terrane during collision of these terranes. An

alternative explanation, which could reconcile the

occurrence of high Cr garnets with the plume model

of Griffin et al. (1999a), is derivation of the deep

lithosphere from an ascending megalith consisting of

accumulated old oceanic slabs (Ringwood, 1991).

However, typical Phanerozoic plume derived melts

(e.g. oceanic island or flood basalts) do not indicate

sources with such high Cr/Al ratios.

Fig. 10. Cr2O3 content in peridotitic garnet inclusions in diamonds

from Panda versus equilibration temperature (gt-ol thermometry:

ONeill and Wood, 1979; ONeill, 1980) as a measure of depth. The

two samples with touching inclusions of garnet olivine were

excluded from the regression and are indicated as re-equilibrated.

As shown by Griffin et al. (1999b), Cr in garnet may be employed

as a direct proxy for the bulk rock composition of lithospheric

garnet peridotites and thus decreasing Cr contents indicate that the

diamond sources become less depleted with depth.


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Our preferred model, therefore, is a similar (al-

though probably not contemporaneous) origin of the

upper and lower layer of the mantle lithosphere be-

neath Lac de Gras, with the less depleted character ofthe lower layer being in part a secondary feature, due to

metasomatic overprint. Increasing fertility of the sub-

cratonic lithosphere with depth is also observed for the

Kaapvaal. This is particularly prominent for xenoliths

falling on the hot side of the local palaeo-geotherm

which show a marked increase in Ti with increasing

pressure (Fig. 9). Taking temperature as a measure of

depth, similar co-variations are also determined for the

Slave, e.g. a significant decrease in the Cr content of

garnet (Fig. 10) towards the base of the lithosphere.

Re-fertilization of cratonic roots from beneath, through

melts or fluids ascending from the asthenosphere, is

also shown by Griffin et al. (1999b) for a number of

cratons, including the Kaapvaal. The strong metaso-

matic overprint observed for the deeper lithosphere in

the Slave may relate to a plume event, thereby explain-

ing the occurrence of lower mantle diamonds, but the

evidence for such an event remains inconclusive.

Acknowledgements

David Fisher (DTC Research Laboratories, Maid-enhead) is thanked for his help with the deconvolution

of the infrared spectra. The manuscript was consid-

erably improved through formal reviews by Dante

Canil, Maya Kopylova and Nick Pokhilenko, and

significant input from Herman Grutter as the respon-

sible editor. Funding through the German Research

Foundation (DFG), NSERC and the Canada Research

Chairs program (CRC) is gratefully acknowledged.

Additional support and co-operation was provided by

The Diamond Trading, a De Beers Group Company,

De Beers Consolidated Mines and BHP-Billiton.

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