www.elsevier.com/locate/lithos

Lithos 74 (2004) 149–165

Geochemistry of the Uintjiesberg kimberlite, South Africa:

petrogenesis of an off-craton, group I, kimberlite

Megan Harrisa, Anton le Roexa,*, Cornelia Classb

aDepartment of Geological Sciences, University of Cape Town, Private Bag, Rondebosch 7701, South AfricabLamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964, USA

Received 30 July 2003; accepted 2 February 2004

Abstract

The Uintjiesberg kimberlite diatreme occurs within the Proterozoic Namaqua–Natal Belt, South Africa, approximately 60

km to the southwest of the Kaapvaal craton boundary. It is a group I, calcite kimberlite that has an emplacement age of f 100

Ma. Major and trace element data, in combination with petrography, are used to evaluate its petrogenesis and the nature of its

source region. Macrocryst phases are predominantly olivine with lesser phlogopite, with very rare garnet and Cr-rich

clinopyroxene. Geochemical variation amongst the macrocrystic samples (Mg# 0.85–0.87, SiO2 = 27.0–29.3%, MgO=26.1–

30.5%, CaO= 10.9–13.5%) is shown to result from f 10% to 40% entrainment and partial assimilation of peridotite xenoliths,

whereas that shown by the aphanitic samples (Mg# 0.80–0.83, SiO2 = 19.1–23.0%, MgO= 17.9–23.9%, CaO = 16.5–23.7%)

is consistent with f 7–25% crystal fractionation of olivine and minor phlogopite. Changing trajectories on chemical variation

diagrams allow postulation of a primary magma composition with f 25% SiO2, f 26% MgO, f 2.3% Al2O3, f 5%H2O,

f 8.6% CO2 and Mg# = 0.85.

Forward melting models, assuming 0.5% melting, indicate derivation of the primary Uintjiesberg kimberlite magma from a

source enriched in light rare earth elements (LREE) by f 10� chondrite and heavy REE (HREE) by 0.8–2� chondrite, the

latter being dependent on the proportion of residual garnet. Significant negative Rb, K, Sr, Hf and Ti anomalies present in the

inferred primary magma composition are superimposed on otherwise generally smooth primitive mantle-normalized trace

element patterns, and are inferred to be a characteristic of the primary magma composition. The further requirement for a source

with chondritic or lower HREE abundances, residual olivine with high Fo content (f Fo94) suggests derivation from a mantle

previously depleted in mafic melt but subsequently enriched in highly incompatible elements prior to kimberlite genesis. These

requirements are interpreted in the context of melting of continental lithospheric mantle previously enriched by metasomatic

fluids derived from a sublithospheric (plume?) source.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Kimberlite; Petrogenesis; Geochemistry; Off-craton

0024-4937/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.lithos.2004.02.001

* Corresponding author. Fax: +27-21-650-3783.

E-mail address: aleroex@geology.uct.ac.za (A. le Roex).

1. Introduction

Despite their very small volumetric proportion of

the Earth’s crust, kimberlites are of interest because of

M. Harris et al. / Lithos 74 (2004) 149–165150

the great depth at which they are thought to originate

and because they provide the transport medium for

diamonds and a suite of deep mantle xenoliths to be

brought to the surface. There have been a considerable

number of experimental studies aimed at understand-

ing the petrogenesis of kimberlite magma (e.g., Dalton

and Presnall, 1998; Eggler, 1987; Kesson et al., 1994;

Ringwood et al., 1992; Wyllie, 1987), but compara-

tively little work has focused on the bulk rock major

and, particularly, trace element geochemistry of kim-

berlite itself (e.g., Clement, 1982; Fraser et al., 1985/

86; le Roex et al., 2003; Smith, 1983; Tainton and

McKenzie, 1994). The seminal isotopic study of

southern African kimberlites by Smith (1983) recog-

nised the occurrence of two isotopically distinct

kimberlite types—Group I (unradiogenic Sr; radio-

genic Nd and Pb) and Group II (radiogenic Sr;

unradiogenic Nd and Pb), although a relatively minor,

transitional group does exist (e.g., Skinner, 1989).

Off-craton kimberlites have attracted even less atten-

tion than on-craton kimberlites because they are

barren of diamonds.

The Uintjiesberg kimberlite pipe, situated in the

Northern Cape Province, South Africa, is located about

60 km from the southwestern boundary of the Kaap-

vaal craton (Fig. 1). It is an off-craton, Group I

Fig. 1. Map of southern Africa showing location of the Uintjiesberg kimbe

boundary, Namaqua–Natal mobile belt, and Cape Fold belt.

kimberlite, with a Rb–Sr age of 100 Ma (Smith,

1983). This is a similar age to other Cretaceous Group

1 kimberlites in South Africa emplaced at 80–114 Ma

(Smith, 1983). The kimberlite intrudes granulites of the

late Precambrian Namaqua–Natal metamorphic mo-

bile belt at depth (Robey, 1981), and a Permian aged

cover sequence consisting largely of shales, mudstones

and siltstones, with minor sandstone and tillite compo-

nents of the Ecca Group and the Karoo Supergroup.

Previous studies conducted on the Uintjiesberg

kimberlite include (i) Robey (1981), who described

the kimberlite occurrence and its xenolith suite, (ii)

De Bruin (2000), who conducted a geochemical

study on the megacryst/marid suite, (iii) and Janney

et al. (1999), who studied aspects of the isotope

geochemistry of the mantle xenoliths. The bulk rock

geochemistry of the Uintjiesberg kimberlite has not

previously been studied. The objectives of this study

are to use the major and trace element geochemistry

of a suite of 10 kimberlite samples to place some

constraints on the petrogenesis of this kimberlite

intrusion. An attempt will be made to constrain post

melting processes that may have modified the geo-

chemical composition of the primary magma en

route to the surface before considering the likely

source region composition and melting processes.

rlite diatreme, and the approximate location of the Kaapvaal Craton

M. Harris et al. / Lithos 74 (2004) 149–165 151

2. Sampling and analytical techniques

Both hypabyssal and diatreme facies kimberlite are

exposed within the Uintjiesberg kimberlite pipe, the

former being remarkably fresh and including both

aphanitic and macrocrystic varieties. Ten samples of

the most visibly fresh, hypabyssal kimberlite were

selected for analysis; the samples were split using a

hydraulic splitter and then crushed using a jaw crush-

er. Fragments without any visible xenolithic material,

veining or external weathering were then picked from

the jaw crush and powdered in a Sieb swing mill using

a carbon-steel vessel.

Major elements were analysed by X-ray fluores-

cence (XRF) using a low dilution fusion technique

(Willis, 1999) and a Philips X’Unique wavelength

spectrometer. The trace elements Cu, Ni, Co, Cr, V,

Zn and Nb were also analysed by XRF using pressed

powder briquettes. Associated errors and detection

limits are similar to those reported in le Roex et al.

(1981). A suite of 26 trace elements was analysed by

ICP-MS using a Perkin Elmer ELAN 6000 instrument

and multielement artificial standards. Relative preci-

sion was generally better than 3% (1r R.S.D.), and

accuracy can be gauged from the results for interna-

tional standard BHVO-1 reported in le Roex et al.

(2001). Ta abundances in samples UB-03 and UB-05

and UB-05B are suspect because it was not possible to

dissolve all ilmenite. For this reason, Nb data were

obtained by XRF for all samples, rather than by ICP-

MS. The CO2 content of the samples was determined

using a Karbonat bombe, following the technique

described by Birch (1981). The estimated precision

for this technique is better than 3% relative.

3. Petrography

In the following discussion, the term macrocryst is

used to refer to crystals larger than 2 mm (or obvious

anhedral fragments), the term phenocryst for largely

euhedral crystals between 0.5 and 1.5 mm, and the

term microphenocryst for crystals smaller than 0.5

mm in size.

The hypabyssal Uintjiesberg kimberlite includes

both macrocrystic and aphanitic textural varieties.

The latter are extremely fine grained, although some

contain rare macrocrysts, whereas the macrocrystic

samples are coarsely porphyritic and characterised by

20–50% macrocrystic phases set in a fine-grained

matrix. In some samples, calcite segregations lend a

segregationary texture to the groundmass; in others, it is

homogenous and fine-grained. The groundmass of all

samples consists predominantly of calcite, and the

Uintjiesberg kimberlite can thus be classified as a

calcite kimberlite (following Clement and Skinner,

1979). Other common groundmass phases include

phlogopite, ilmenite, perovskite and serpentine, with

secondary chlorite. Ilmenite and perovskite are ubiqui-

tous, occurring disseminated through the groundmass

or in clusters. Visually estimated modal abundances of

the major constituent phases in the Uintjiesberg kim-

berlite are provided in Table 1.

Olivine is the most abundant macrocryst phase,

ranging between 2 and 10 mm in size and is typically

rounded and anhedral. It is generally pervasively

altered and can be completely serpentinised or calci-

tised; despite extensive alteration around rims and

along fractures, fresh olivine cores are preserved in

some samples. Phlogopite macrocrysts, often with

corroded or altered rims, seldom constitute more than

5% of the kimberlite but can reach sizes of up to 8 mm.

Some phlogopite crystals exhibit classic kink banding,

indicative of deformation prior to incorporation into

the kimberlite magma. Very rare garnet, ilmenite,

rutile and red-brown Cr-spinel macrocrysts occur in

a few samples.

Olivine is the most common phenocryst phase,

with crystals generally euhedral and ranging between

0.75 and 1.5 mm in size. In some samples, they have

been completely serpentinised, but in others, fresh

cores are present. In most samples, olivine is also

present as a microphenocryst phase. Phlogopite is a

rare ( < 5%) phenocryst phase in all samples.

4. Bulk rock geochemistry

Ten samples of hypabyssal kimberlite from the

Uintjiesberg kimberlite pipe were analysed for major

and trace elements. The data are reported in Table 1.

4.1. Major element geochemistry

Selected major element variations are shown in

Fig. 2. The aphanitic samples are least MgO-rich

Table 1

Bulk rock analyses of Uintjiesberg kimberlite samples: LOI: loss on ignition; Mg#: atomic Mg/Mg+ Fe2 + with Fe2O3/FeO= 0.1

Sample # UB-01 UB-01B UB-02 UB-03 UB-04 UB-05 UB-05B UB-06 UB-07 UB-08 UB-PM

Aphanitic Macrocrystic Macrocrystic Aphanitic Macrocrystic Aphanitic Macrocrystic Macrocrystic Macrocrystic Macrocrystic Primary

magma

Macrocrystic Ol – 40 39 5 35 30 14 40 45

Macrocrystic phl – 5 1 < 1 5 1 5 6 5 5

SiO2 22.98 27.69 27.31 19.10 29.08 19.21 27.61 26.99 27.90 29.33 24.99

TiO2 3.29 2.17 2.07 3.50 2.88 4.07 2.95 3.16 1.95 2.53 3.23

Al2O3 2.35 2.08 1.93 2.19 2.39 2.01 2.41 2.29 1.81 2.43 2.32

Fe2O3 10.31 9.71 9.55 10.09 9.76 9.70 9.60 10.22 9.59 9.99 10.26

MnO 0.20 0.17 0.17 0.19 0.16 0.21 0.17 0.17 0.16 0.17 0.19

MgO 23.94 29.11 29.58 22.81 26.93 17.88 25.93 28.24 30.49 26.08 26.09

CaO 16.51 11.07 11.97 21.18 11.08 23.69 12.84 13.47 10.94 10.93 14.99

Na2O < 0.04 < 0.04 < 0.07 0.07 < 0.01 < 0.18 < 0.03 0.06 < 0.04 < 0.03 0.06

K2O 1.12 0.88 0.94 0.44 1.44 0.86 1.28 1.01 0.88 1.29 1.06

P2O5 1.88 1.23 1.30 3.09 1.58 5.58 2.99 1.86 1.36 1.53 1.87

SO3 0.34 0.35 0.37 0.28 0.00 0.01 0.03 0.26 0.39 0.02 0.30

NiO 0.10 0.15 0.15 0.09 0.14 0.07 0.13 0.14 0.16 0.16 0.12

Cr2O3 0.22 0.18 0.21 0.18 0.19 0.18 0.21 0.22 0.20 0.20 0.22

H2O+ 0.26 0.32 0.17 0.17 0.55 0.44 0.18 0.27 0.20 0.27 0.26

LOI 16.04 14.30 13.92 15.98 13.71 16.07 13.59 11.06 13.34 14.12 13.55

Total 99.54 99.42 99.63 99.35 99.88 99.98 99.93 99.41 99.37 99.06 99.48

CO2 10.26 7.00 7.87 12.80 5.65 11.06 5.94 7.00 7.19 6.42 8.63

H2O+ 5.78 7.30 6.05 3.18 8.07 5.01 7.66 4.06 6.14 7.70 4.92

Mg# 0.83 0.87 0.87 0.83 0.86 0.80 0.85 0.86 0.87 0.85 0.84

XRF

Zn 72 73 85 73 83 63 76 74 74 59 73

Cu 64 53 61 65 112 93 116 57 55 72 60

Ni 966 1606 1839 759 1879 618 1383 1285 1717 1196 1126

Co 83 111 132 71 140 59 99 95 112 91 89

M.Harris

etal./Lith

os74(2004)149–165

152

Mn 1526 1391 1727 1371 1656 1504 1325 1469 1438 1140 1497

Cr 1501 1390 1958 1164 1913 1126 1564 1557 1561 1153 1529

V 141 112 161 149 217 326 248 195 133 171 168

Nb 258 155 148 241 155 259 163 174 143 157 216

ICP-MS

Rb 46.4 39.2 47.1 13.2 67.1 46.1 72.5 38.9 40.7 80.6 42.6

Ba 948 807 959 510 1807 1876 1601 1266 819 2060 1107

Sr 1232 844 1064 2047 1062 2459 1270 1465 943 1015 1349

Th 28.1 15.8 13.9 24.3 15.6 32.0 16.6 16.6 12.6 17.2 22.3

U 7.30 3.99 3.88 6.94 5.95 9.36 6.18 4.90 3.70 4.29 6.10

Pb 10.2 7.9 7.5 6.4 6.7 10.5 7.1 10.2 6.5 9.7 10.2

Ta 12.7 8.55 7.15 7.56 7.25 8.41 7.08 8.76 7.44 8.92 10.7

Hf 7.17 4.34 3.91 8.83 4.49 7.87 5.05 5.66 3.82 4.84 6.42

Zr 371 226 205 490 233 446 262 292 195 248 331

La 211 126 113 234 129 243 139 159 110 131 185

Ce 406 236 211 462 246 471 257 307 206 249 356

Pr 46.9 26.7 23.8 53.5 28.0 55.4 29.0 35.5 23.3 28.2 41.2

Nd 171 96.6 87.2 198 102 204 105 131 85 103 151

Sm 24.8 14.0 12.4 29.4 15.1 30.1 15.8 19.1 12.6 15.2 21.9

Eu 6.21 3.54 3.15 7.59 3.89 7.75 4.19 4.83 3.20 3.96 5.52

Gd 15.2 8.61 7.58 18.8 9.45 18.9 10.3 11.9 7.78 9.55 13.5

Tb 1.70 0.97 0.87 2.16 1.09 2.14 1.19 1.34 0.87 1.08 1.52

Dy 6.61 3.96 3.44 8.74 4.43 8.60 4.91 5.38 3.54 4.40 6.00

Ho 0.98 0.60 0.51 1.27 0.68 1.24 0.75 0.80 0.53 0.67 0.89

Er 1.97 1.20 1.05 2.45 1.38 2.43 1.53 1.59 1.06 1.38 1.78

Tm 0.22 0.14 0.13 0.28 0.16 0.26 0.18 0.18 0.12 0.16 0.20

Yb 1.02 0.74 0.63 1.30 0.80 1.21 0.91 0.88 0.62 0.80 0.95

Lu 0.13 0.10 0.078 0.15 0.10 0.15 0.12 0.11 0.075 0.10 0.12

U/Th 0.26 0.25 0.28 0.29 0.38 0.29 0.37 0.30 0.29 0.25 0.28

Zr/Nb 1.44 1.46 1.39 2.03 1.50 1.72 1.61 1.67 1.37 1.57 1.55

La/Ce 0.52 0.53 0.54 0.51 0.52 0.52 0.54 0.52 0.54 0.53 0.52

Zr/Hf 51.7 52.1 52.5 55.5 52.0 56.7 51.9 51.5 51.1 51.2 51.6

La/Nb 0.82 0.82 0.77 0.97 0.83 0.94 0.86 0.91 0.77 0.83 0.85

Proportions of olivine and phlogopite macrocrysts in vol. %. H2O+ by difference between CO2 and LOI.

M.Harris

etal./Lith

os74(2004)149–165

153

Fig. 2. Selected major element variations in Uintjiesberg kimberlite samples. Field of Group I Kimberley kimberlites from le Roex et al. (2003).

See text for comment on group-a and group-b macrocrystic kimberlite samples.

M. Harris et al. / Lithos 74 (2004) 149–165154

(17.9–23.9 wt.%), with the macrocrystic samples

having the highest MgO contents (25.9–30.5 wt.%).

When plotted against MgO, a number of elements

define coherent variations. SiO2 (19–29 wt.%) corre-

lates positively with MgO, with the macrocrystic

samples being most SiO2-rich (27–29 wt.%), whereas

TiO2 and CaO correlate negatively with MgO; TiO2

varies from 1.9 to 4.1 wt.% (sample UB-06 plots off

the trend towards higher TiO2 content, consistent with

the high perovskite and ilmenite content observed in

thin section), and CaO varies from 11 to 24 wt.% –

the aphanitic samples being most CaO-rich (>15%

CaO). Three macrocryst-rich samples, UB-04, -05B,

-08 deviate from the general MgO–SiO2 trend to-

wards higher SiO2 (Fig. 2a) and K2O contents (this

group is referred to as group-b and the remaining

macrocryst-rich samples as group-a). FeO* remains

broadly constant (7.9–8.8 wt.%), and Mg# [atomic

Mg/(Mg + Fe2 +)] ranges from 0.80 to 0.87, with the

aphanitic samples having values lower than 0.84.

The macrocrystic and aphanitic samples describe

separate trends when Al2O3 is plotted against

MgO – the macrocrystic samples correlate negative-

ly, whereas the aphanitic samples correlate positively

(Fig. 2c). K2O contents are scattered (more so for the

aphanitic samples) between 0.4 and 1.4 wt.%. LOI

ranges from 11 to 16 wt.%, with CO2 ranging from

5.6 to 12.8 wt.%, being highest in the aphanitic

samples (>10 wt.%; Table 1) and correlating posi-

tively with CaO.

4.2. Trace element geochemistry

Given their high MgO contents and generally

ultramafic nature, kimberlites are unusual in that they

exhibit both very high compatible and incompatible

trace element abundances. The Uintjiesberg kimberlite

is no exception, and the trace element abundances of

the analysed samples are reported in Table 1, and

selected variations are illustrated in Fig. 3.

Fig. 3. Selected trace element variations in Uintjiesberg kimberlite samples. Field of Group I Kimberley kimberlites from le Roex et al. (2003).

See text for comment on group-a and group-b macrocrystic kimberlite samples.

M. Harris et al. / Lithos 74 (2004) 149–165 155

Ferromagnesian trace elements correlate positively

with Mg#. Ni and Co define tightly constrained

correlations, and Cr a broader correlation. The apha-

nitic samples have lower concentrations of these

elements (Ni = 618–966 ppm, Co = 59–83 ppm,

Cr = 1126–1501 ppm) than the macrocrystic samples

(Ni = 1196–1879 ppm, Co = 91–140 ppm, Cr =

1153–1958 ppm).

High field strength elements (HFSE) and light rare

earth elements (LREE) are all high in abundance (e.g.,

La = 110–243 ppm, Zr = 195–446 ppm, Nb = 143–

259 ppm) and describe excellent mutual correlations,

with the aphanitic samples being most enriched and

the macrocrystic samples least enriched (Table 1; Fig.

3). These good correlations are reflected in uniform

interelement ratios, e.g., La/Ce = 0.53F 0.01, Zr/

Hf = 57F 2, Nb/Th = 9.84F 0.9, Zr/Nb = 1.7F 0.2.

Large ion lithophile element (LILE) concentrations

are more susceptible to modification by late stage,

deuteric alteration and weathering than the immobile

high field strength elements (HFSE). Ba, Rb and Pb

concentrations are high and variable (e.g., Ba = 510–

2060 ppm; Rb = 14–77 ppm) but show no correlation

with one another, nor with immobile elements such as

La or Zr. Sr concentrations (819–2040 ppm) do;

however, they show a very broad positive correlation

when plotted against light REE and other immobile

incompatible elements.

Chondrite-normalized REE patterns are shown in

Fig. 4a, where it is evident that all samples show

smooth, subparallel patterns and are strongly enriched

in light REE relative to heavy REE (HREE; La/

Smn = 5.53F 0.24; La/Ybn = 127.5F 11.9). Normal-

ized La abundances range between 466 and 1026�chondrite, whereas Lu ranges between 2.9 and 5.8�chondrite. The REE patterns for individual samples

are remarkably parallel, with the aphanitic samples

displaced to uniformly higher overall concentrations

relative to the macrocrystic samples.

Fig. 4b shows primitive mantle-normalized trace

element patterns which are generally subparallel and

strongly enriched in highly incompatible elements

(f 200 to 400� primitive mantle), with HREE

(Er to Lu) showing least enrichment (1–2� )

primitive mantle. The aphanitic samples have uni-

formly higher incompatible element abundances

than the macrocrystic samples. Samples UB-05

(aphanitic) and UB-05B (macrocrystic) differ from

the rest in showing distinct P enrichment (not

shown for clarity). All samples show negative

Fig. 4. (a) Chondrite-normalized rare earth element abundances in Uintjiesberg kimberlite samples. (b) Primitive mantle-normalized trace

element patterns of Uintjiesberg kimberlite samples. Note the negative anomalies in Rb, K, Sr, Hf and Ti in all samples. Normalizing values

from Sun and McDonough (1989).

M. Harris et al. / Lithos 74 (2004) 149–165156

anomalies (significant depletion relative to adjacent

elements) in Rb, (Ba), K, Sr, Hf and Ti, and the

aphanitic samples also have a slight negative Pb

anomaly. K shows the largest negative anomaly (K/

K* = 0.08–0.21, where K* is the interpolated value

assuming a smooth variation between Nb and Th),

with the aphanitic samples exhibiting a larger

anomaly (0.08) than the macrocrystic samples

(0.15 and 0.21). It is noteworthy that the magnitude

of the K anomaly increases uniformly with increas-

ing incompatible element abundances, and that

group-b macrocrystic kimberlite has higher K/K*

(0.21F 0.02; i.e., a lesser anomaly) than group-a

(K/K* = 0.16F 0.01). The aphanitic samples show a

larger Ti anomaly (Ti/Ti* = 0.47F 0.04) than the

macrocrystic samples (Ti/Ti* = 0.62F 0.04). In con-

trast, the size and variability of the Sr and Hf

anomalies is similar in both aphanitic and macro-

crystic samples (Sr/Sr* = 0.58F 0.08 and Hf/

Hf* = 0.52F 0.02).

M. Harris et al. / Lithos 74 (2004) 149–165 157

5. Petrogenesis

The petrogenesis of kimberlite magmas has largely

been the subject of experimental (e.g., Brey and

Ryabchikov, 1994; Dalton and Presnall, 1998; Girnis

et al., 1995; Kesson et al., 1994; Wyllie and Lee,

1999) or isotope geochemical studies (e.g., Fraser et

al., 1985/86; Smith, 1983; Taylor et al., 1994) – the

latter studies being more concerned with location and

isotopic composition of the source region than under-

standing details of the melting process (differences in

the source region mineralogy, volatile content and

degree of melting) or post melting processes (assim-

ilation, fractionation or contamination). To allow for

constraints to be placed on igneous petrogenetic

processes, it is necessary that magma compositions

are free of secondary (alteration, contamination)

effects, and if melting processes are to be considered,

then the magma compositions should be close to

primary in character (experimental work by Edgar et

al., 1988 attempted to address this by working on an

aphanitic Wesselton kimberlite sample to avoid the

macrocryst problem). These considerations are partic-

ularly relevant to kimberlite genesis, given the com-

mon abundance of crustal and mantle xenoliths, the

uncertain origin of the macrocryst assemblage and the

ease of deuteric alteration and weathering of kimber-

lite. Before the petrogenesis of the Uintjiesberg kim-

berlite is discussed, the extent of contamination of the

samples by crustal material and alteration by late stage

processes must be considered.

5.1. Crustal contamination

Kimberlites generally carry a significant xenolithic

load, a large portion of which is crustal (e.g., Mitchell,

1995). Although all visible crustal material was re-

moved as far as possible from the crushed samples

before powdering and analysis, the low-melting tem-

peratures of crustal materials makes it likely that some

assimilation might have occurred. In terms of Clem-

ent’s (1982) contamination index (C.I.), where

C.I.f 1 equates to an uncontaminated kimberlite,

the Uintjiesberg kimberlite samples are uncontaminat-

ed, with values ranging from 0.92 to 1.16. In addition

to raised SiO2 and Al2O3 contents, le Roex et al.

(2003) have argued that crustally contaminated kim-

berlite typically shows raised HREE abundances rel-

ative to uncontaminated kimberlite and is relatively

enriched in Pb (low Ce/Pb ratio, < 15, relative to OIB

mantle values, 15–35; Hofmann, 1988). Although

group-b macrocrystic samples (UB-04, -05B and -08)

have slightly raised SiO2, none of the Uintjiesberg

kimberlite samples analysed in this study reflect either

relative enrichment in Pb (Ce/Pbn < 1) or raised HREE

abundances, and thus appear to be free of significant

crustal contamination.

5.2. Peridotite entrainment versus crystal

fractionation

The aphanitic and macrocrystic Uintjiesberg kim-

berlites have clearly experienced different evolution-

ary histories. The aphanitic varieties are likely to

represent liquid compositions, whereas the macro-

crystic varieties are more equivocal in this regard. It

is commonly accepted that the macrocryst population

of minerals in kimberlites represents disaggregated

mantle-derived xenoliths (e.g., Clement, 1982; Shee,

1985); that is, macrocrystic kimberlite does not rep-

resent a liquid composition but rather a partial cumu-

late of mantle-derived minerals, dominated by olivine;

this has been clearly shown by le Roex et al. (2003)

for the Kimberley Group I kimberlite cluster. The

Uintjiesberg kimberlite hosts a significant inventory

of mantle-derived xenoliths, including lherzolite, gar-

net lherzolite, phlogopite lherzolite and MARID

rocks, and also abundant megacryst minerals, includ-

ing dominant ilmenite, clinopyroxene, garnet, ortho-

pyroxene and phlogopite (e.g., De Bruin, 2000;

Robey, 1981), together providing an ample source of

such mantle-derived xenocrystic material. Major ele-

ment variations in the Uintjiesberg macrocrystic kim-

berlite samples are consistent with xenolith and

megacryst mineral entrainment (e.g., Fig. 5), and it

is evident that two trends can be distinguished – one

suggesting greater involvement of ilmenite (group-a),

the other, with slightly elevated SiO2, Al2O3, K2O,

Rb, and Ba, consistent with greater phlogopite in-

volvement (group-b). The latter requirement is well

illustrated in Fig. 6, where K/Nb ratios increase with

increasing K2O content in group-b samples but remain

constant with decreasing K2O content in group-a

macrocrystic samples. The compositions of the mac-

rocrystic kimberlites are thus inferred to reflect vari-

able entrainment of mantle lherzolite and megacryst

Fig. 5. Selected major element variations in Uintjiesberg kimberlite

samples illustrating effect of mantle entrainment in macrocrystic

kimberlite samples. Note the two trends described by group-a and

group-b macrocrystic samples, suggestive of different proportions

of entrained minerals. The inferred composition of the Uintjiesberg

primary kimberlite magma is shown for reference (see text for

explanation). Symbols as in Fig. 4.

Fig. 6. K/Nb ratio versus K2O content in Uintjiesberg kimberlite

samples. Aphanitic samples describe a trend consistent with olivine

plus phlogopite fractionation, whereas macrocrystic samples define

two distinct trends: group-a consistent with dilution by K-free

mantle minerals, group-b by mantle minerals including significant

phlogopite. The composition of the inferred Uintjiesberg primary

magma is shown for reference.

M. Harris et al. / Lithos 74 (2004) 149–165158

phases into a primary kimberlite magma, which would

be less MgO-rich than the least MgO-rich macro-

crystic sample.

The three aphanitic samples analysed show a range

in Mg#, and are all less MgO-rich than the macro-

crystic kimberlite samples. Because Ni and Mg# tend

to be buffered during mantle melting processes by

olivine and orthopyroxene, the strong positive corre-

lation between Mg# and Ni (Fig. 7b) and strong

negative correlation between Ni and incompatible

elements (Table 1) argue against variable degrees of

partial melting being responsible for the composition-

al variation shown by the aphanitic kimberlite samples

and are more consistent with crystal fractionation

processes. Rare olivine and phlogopite phenocrysts

occur in the aphanitic samples, and these are the most

likely phases to have been involved in any crystal

fractionation process (see discussion below). The

primary Uintjiesberg kimberlite magma composition

is therefore suggested to lie between the composition

of the most MgO-rich aphanitic sample (UB-01), and

that of the least MgO-rich macrocrystic sample (UB-

06), corresponding to the change in trajectory between

the aphanitic and macrocrystic samples on many

interelement variation diagrams (e.g., Fig. 2c). The

average of these two samples has been taken to most

closely represent the primary Uintjiesberg kimberlite

magma, containing f 25% SiO2, f 26% MgO,

Mg# f 0.85, f 1.1% K2O, f 8.6% CO2, f 5%

H2O and f 1100 ppm Ni (Table 1).

REE abundances in mantle-derived silicate miner-

als and ilmenite are negligible compared to those

present in a primary kimberlite magma, and entrain-

ment of such mantle mineral phases should do little

more than dilute the absolute REE abundances uni-

formly across the REE spectrum. Fig. 8 shows that

entrainment of 10–40% peridotitic mantle into the

inferred primary magma composition can thus ade-

Fig. 7. (a) Al2O3 versus MgO in Uintjiesberg kimberlite samples

showing change in trajectory between aphanitic and macrocrystic

samples. The compositional variation in the aphanitic samples is

consistent with up to 30% fractionation of olivine and phlogopite in

the proportion of 77:23 from an inferred primary kimberlite magma.

(b) Ni versus Mg# showing two distinct trajectories described by the

two groups (a and b) of macrocrystic kimberlite samples. The

aphanitic samples describe a trend consistent with up to 25% olivine

plus phlogopite fractionation. DolNi from Hart and Davis (1978);

DphlNi =Dol

Ni� 0.5 (Gregoire et al., 2003). Symbols as in Fig. 6.

M. Harris et al. / Lithos 74 (2004) 149–165 159

quately account for the variation in REE patterns

shown by the macrocrystic kimberlite samples.

5.3. Crystal fractionation

Having identified the approximate composition of

the primary Uintjiesberg kimberlite magma, it is

possible to consider the origin of the more evolved

aphanitic samples. Fig. 7a shows removal of up to

30% olivine plus phlogopite in the ratio 77:23 from

this primary magma composition which can broadly

explain the major element compositional variation of

the aphanitic kimberlite samples. Additional support

for this degree of fractionation is provided by the

Ni–Mg# variation in these samples, where the

change in Ni and Mg# can be accounted for by

f 25% fractionation of these two phases (Fig. 7b).

Further evidence for the involvement of minor phlog-

opite in the fractionating assemblage is found in the

more pronounced negative K anomaly in the apha-

nitic samples, where K/Nb ratios decrease systemat-

ically with increasing incompatible elements (e.g.,

Zr) and decreasing K (Fig. 6). A similar, but less

pronounced, systematic decrease in Ti/Eu ratio (de-

creasing Ti/Ti*) in the aphanitic samples is consistent

with the involvement of phlogopite or very minor

ilmenite/perovskite.

The highly incompatible nature of all REE with

respect to olivine and phlogopite will result in a

uniform enrichment of the full REE spectrum during

crystal fractionation of these two phases. Fig. 8 shows

that f 25% fractionation of olivine and phlogopite

from the inferred primary magma composition ade-

quately accounts for the subparallel enrichment in

REE shown by the aphanitic kimberlite samples.

The similar degrees of fractionation shown by these

contrasting methods of evaluation lend credence to the

proposal that the aphanitic kimberlite samples reflect

variable degrees of crystal fractionation from a more

Mg-rich parental kimberlite magma.

5.4. Partial melting

Dalton and Presnall (1998) have shown that melt-

ing of Fe-free synthetic carbonated garnet lherzolite at

high pressures (P=~6 GPa) produces a range in melt

compositions uniformly varying from carbonatite-like

through to kimberlite-like, over the melting interval

0.3–1%. Given the off-craton location of the Uintjies-

berg kimberlite, these high pressures might not be

directly applicable. Nevertheless, the results allow

some indication of the likely degree of melting giving

rise to the proposed Uintjiesberg primary kimberlite

composition. Regression lines through the Dalton and

Presnall (1998) experimental data (melt composition

versus F) would suggest that the Uintjiesberg primary

kimberlite formed by some 0.5% melting (with re-

spect to SiO2, Al2O3 and MgO). However, both the

Fig. 8. Chondrite-normalized REE abundances in Uintjiesberg kimberlite samples and inferred primary magma composition (only selected

kimberlite samples are plotted to aid clarity). The range in absolute REE abundances in the macrocrystic samples is consistent with entrainment

of up to 40% mantle peridotite into the inferred primary magma composition, whereas the range in aphanitic samples is consistent with

fractionation of up to 25% olivine plus phlogopite from the inferred primary magma composition.

Fig. 9. Chondrite-normalized REE element pattern of the inferred

primary Uintjiesberg kimberlite magma. Source composition

calculated by forward modeling assuming that the primary magma

was formed by 1% melting. The effect of variable amount of

residual garnet is shown for two situations: 2% and 6% residual

garnet. Equilibrium batch melting assumed, with partition coef-

ficients taken from Spath et al. (2001). Residual mineralogy at 1%

melting: 0.63ol, 0.23opx, 0.12cpx and 0.02gt, and 0.63ol, 0.23opx,

0.08cpx and 0.06gt. Normalizing values from Sun and McDonough

(1989). Field of Kaapvaal craton garnet lherzolites from Gregoire et

al. (2003).

M. Harris et al. / Lithos 74 (2004) 149–165160

CaO and CO2 contents of the inferred primary magma

composition are significantly lower than that predicted

by the Dalton and Presnall (1998) experiments, per-

haps not surprising given the strongly CaO- and CO2-

rich nature of the synthetic starting material.

Fig. 9 shows the chondrite-normalized REE abun-

dances of the inferred Uintjiesberg primary magma.

Also shown is the field of calculated source composi-

tions for degree of melting varying from 0.5% to 2%,

with residual garnet varying from 2% to 6% (the

presence of garnet-bearing lherzolites xenoliths sup-

ports the derivation of the parental magma from depths

within the garnet stability field). It is evident that,

whereas the LREE are sensitive to degree of melting,

the HREE are sensitive only to the amount of residual

garnet. The calculated source composition is enriched

in LREE (f 10� chondrite), with HREE abundances

less than or slightly greater than chondritic, depending

on the amount of residual garnet present (Fig. 9). It is

noteworthy that the calculated source composition

falls within that found for garnet lherzolite xenoliths

from the Kaapvaal craton (Gregoire et al., 2003). Fig. 9

also shows supporting evidence that the aphanitic

kimberlites do not represent a suite of primary magmas

derived by different degrees of melting, as the effect of

variable F (at low F values) leads to a range of LREE

abundances at effectively fixed HREE abundances

(reflecting the high HREE partition coefficients for

garnet), which is inconsistent with the subparallel

chondrite-normalized REE patterns shown by the

aphanitic kimberlite samples.

M. Harris et al. / Lithos 74 (2004) 149–165 161

When normalized to primitive mantle values, the

inferred primary Uintjiesberg kimberlite magma has

distinct negative anomalies (depletion relative to ad-

jacent elements) in Rb, K, Sr, Hf and Ti (Fig. 10).

These same anomalies are present in both the macro-

crystic and aphanitic samples, with the aphanitic

samples also showing a slightly negative Pb anomaly.

To understand the overall petrogenetic evolution of

the Uintjiesberg kimberlite, it is instructive to consider

the possible causes of each of these elemental anoma-

lies in turn. The magnitudes of the Rb and K anoma-

lies are very large (e.g., K/K* = 0.2� 0.08) and show

coherent variations reflective of igneous processes,

suggesting that deuteric alteration is not the cause of

the anomalies. Fig. 6 shows the variation of K/Nb

ratio (which reflects the magnitude of the K anomaly)

with K, and it is clear that, relative to the inferred

primary magma, the aphanitic kimberlite samples

define a coherent decrease in K/Nb with decreasing

K attributable to combined olivine plus phlogopite

fractionation. The macrocrystic kimberlite samples

define two trends – one consistent with entrainment

of mantle minerals including phlogopite (increasing

K/Nb with increasing K), the other with entrainment

of mantle minerals excluding significant phlogopite

Fig. 10. Primitive mantle-normalized trace element pattern of the infer

calculated by forward modeling assuming that the primary magma was form

is shown for two situations: 2% and 6% residual garnet. Equilibrium batch

(2001). Residual mineralogy at 1% melting: 0.63ol, 0.23opx, 0.12cpx, and

from Sun and McDonough (1989). Field for Kaapvaal craton garnet lherz

(constant K/Nb with decreasing K). Despite some

scatter in the data, the coherency of these variations

argues for the parental magma to have had a negative

K anomaly. A primary magma without a K anomaly

would need to contain some 3.6% K2O and require

some 30% phlogopite-only fractionation to result in

the observed K2O content in the emplaced magmas;

combined olivine–phlogopite fractionation would in-

crease the amount of fractionation required. Although

phlogopite is a crystallising phase in the Uintjiesberg

kimberlite, it is a comparatively minor phase, and,

moreover, transport of dense mantle xenoliths to the

surface also conflicts with a model requiring extensive

phlogopite fractionation en route to the surface, unless

such fractionation took place prior to xenolith entrain-

ment. A negative K anomaly is therefore believed to

be a feature of the Uintjiesberg primary kimberlite

magma composition.

The negative Sr (Sr/Sr* = 0.58F 0.09) and Hf

anomalies (Hf/Hf* = 0.53F 0.02) remain constant

for both macrocrystic and aphanitic kimberlite varie-

ties, arguing against the cause being related to some

crystal fractionation process, and suggesting rather

that they are also a feature of the primary kimberlite’s

composition. In contrast, a variable size negative Pb

red primary Uintjiesberg kimberlite magma. Source composition

ed by 1% melting. The effect of variable amount of residual garnet

melting assumed, with partition coefficients taken from Spath et al.

0.02gt, and 0.63ol, 0.23opx, 0.08cpx and 0.06gt. Normalizing values

olites from Gregoire et al. (2003).

M. Harris et al. / Lithos 74 (2004) 149–165162

anomaly (Pb/Pb* = 0.59F 0.17) appears only in the

aphanitic kimberlite samples, suggestive of control by

crystal fractionation processes. Although Pb is known

to be mobile in the weathering environment, there is

no obvious reason why the aphanitic samples should

show a greater effect of weathering than the macro-

crystic samples. The size of the negative Ti anomaly

differs slightly between the macrocrystic kimberlite

samples (Ti/Ti* = 0.62F 0.04) and the aphanitic kim-

berlite (Ti/Ti* = 0.47F 0.04), consistent with the in-

fluence of phlogopite fractionation discussed earlier.

Minor ilmenite/perovskite fractionation would have a

similar effect but would decrease Nb and Ta, which

does not appear to be the case, suggesting that the

primary magma was relatively depleted in Ti.

Because K is a stoichiometric component of phlog-

opite, the K content of a melt in equilibrium with

residual phlogopite will be constant as long as phlog-

opite is residual, at a concentration dependent only on

the proportion of phlogopite entering the melt (Spath

et al., 2001). If mantle phlogopite contains f 9 wt.%

K2O (e.g., Gregoire et al., 2003) and has a melt mode

of about 30–50% (e.g., Greenough, 1988), the melt in

equilibrium with phlogopite should contain between

25,000 and 37,000 ppm K. The K content of the

inferred Uintjiesberg primary magma is approximate-

ly 9800 ppm, very low to have been in equilibrium

with residual phlogopite. It seems unlikely, therefore,

that the primary Uintjiesberg kimberlite could have

formed in equilibrium with residual phlogopite, but

rather that the negative K anomaly (and by inference,

Rb) must have been an intrinsic feature of the mantle

source.

Fig. 10 shows a forward model in which the likely

source composition giving rise to the primary Uint-

jiesberg kimberlite magma is calculated assuming 1%

melting of a garnet lherzolite, with 2% and 6%

residual garnet. It is clear that the source would be

strongly enriched in highly incompatible elements

(f 5� ) and depleted in HREE (f 0.3–0.5� ) rel-

ative to primitive mantle, not unlike garnet lherzolite

from the Kaapvaal craton (Gregoire et al., 2003). The

previously discussed negative anomalies (relative

depletions) in Rb, (perhaps Ba), K, Sr, Hf and Ti

are evident in the calculated source.

The Ti anomaly might be nothing more than a

reflection of the basaltic partition coefficients used

because Hill et al. (2000) have shown that DcpxTi

depends on the ivAl content in clinopyroxene, increas-

ing with increasing jadeite content, and Blundy and

Dalton (2000) have suggested that DcpxTi increases with

increasing carbonate content of the melt. Furthermore,

Baker et al. (1995) have shown that, at low melt

fractions just above the solidus, Ti clinopyroxene-melt

partition coefficients are larger than at high degrees of

melting. Because the inferred CO2 content of the

Uintjiesberg primary magma is f 8.6%, it is likely

that the partition coefficient of Ti in clinopyroxene

may be higher than the value of 0.2 used in this model.

A value of 0.4 would remove the need for any source

anomaly. Although a residual oxide phase (rutile,

lindsleyite, ilmenite) could clearly also hold back Ti,

the absence of any relative depletion in Nb (or Ta)

argues against their presence in the residue. It is

notable that many analysed garnet lherzolites from

the Kaapvaal craton have a negative Ti anomaly

(e.g., Gregoire et al., 2003), and so a Ti source

anomaly is not unreasonable.

Because Hf also is a high field strength element, its

partition coefficient, like that for Ti, is likely to be

affected by the carbonate content of the melt (Blundy

and Dalton, 2000), and a slightly higher value than the

one used would similarly negate the need for any

source anomaly. However, one might then expect the

same to apply to Zr which does not show a negative

anomaly. The negative Rb, K and Sr anomalies in the

Uintjiesberg kimberlite samples necessitate similar

magnitude anomalies in the source (Fig. 10). Many

Kaapvaal craton garnet lherzolites show a relative

depletion in K and Sr, compared to adjacent elements

of similar incompatibility when normalized to primi-

tive mantle abundances (Gregoire et al., 2003), and a

source anomaly does not therefore seem unreasonable.

In summary, semiquantitative forward modelling of

partial melting processes indicates that the mantle

source of the Uintjiesberg kimberlite was enriched

in incompatible elements (f 5� primitive mantle

values) and depleted in HREE (0.3–0.5� primitive

mantle values), with superimposed relative depletion

in at least Rb, K and Sr. Apparent relative depletion in

Hf and Ti might simply be a function of greater

compatibility of these elements in a carbonate-rich

melt system and might not reflect a characteristic of

the source region. The primary Uintjiesberg kimber-

lite magma is inferred to have had about 5% H2O and

8.6% CO2 (Table 1). Assuming that a degree of partial

M. Harris et al. / Lithos 74 (2004) 149–165 163

melting as low as 0.5–1% gave rise to the primary

kimberlite magma, the source was not strongly vola-

tile enriched, having 250–500 ppm H2O and 400–

900 ppm CO2.

5.5. Source region evolution

It has been argued above that the Uintjiesberg

kimberlite requires a trace element-enriched source

region. Incompatible element ratios shown by the

Uintjiesberg kimberlite (e.g., Ba/Nb = 7.2F 3; Nb/

U = 34F 5; Ce/Pb = 31F 4; Nb/Th = 9.8F 0.9; La/

Nb = 0.85F 0.07; Zr/Nb = 1.7F 0.2) are similar to

those characteristic of OIB in general, and South

Atlantic OIB in particular (e.g., Fig. 11). The

very limited available Sr-isotope data (87Sr/86Sr =

0.7043F 2, Smith, 1983) support this contention.

In contrast, the requirement for derivation from a

source depleted in HREE relative to primitive mantle

values (Fig. 10), and perhaps less than chondritic

values (Fig. 9), indicates a source that had experi-

enced a previous depletion event (see also le Roex et

al., in 2003; Tainton and McKenzie, 1994). Further-

more, the high Mg# of the inferred primary magma

(f 0.85) requires equilibration against highly mag-

nesian residual olivine (f Fo94), consistent with

derivation from a refractory source; a two-stage

model is therefore required by the data.

Fig. 11. La/Th and Nb/Th ratios in Uintjiesberg kimberlite magmas.

Fields for Karoo lavas from Marsh (unpublished data), MORB and

OIB from le Roex (unpublished) and Group I kimberlites from le

Roex et al. (2003). PM: primitive mantle of Sun and McDonough

(1989), average crust Rudnick and Fountain (1995).

Peridotite xenoliths from the sub-Gondwana litho-

sphere typically indicate depletion in basaltic compo-

nents (e.g., Boyd and Mertzman, 1987), indicating

that they have experienced a melt extraction event.

Such melt extraction would cause strong depletion in

incompatible elements, increase in olivine Fo content

and slight depletion in the HREE (being only moder-

ately incompatible during melting in the presence of

garnet). Subsequent enrichment of such a depleted

source by infiltration of metasomatic fluids would

replenish the highly incompatible elements to a great-

er degree than the less incompatible HREE. The Rb,

K, Sr and Ti anomalies shown to be a characteristic of

the source are interpreted to be features of this

metasomatic fluid. Wyllie (1987) has shown experi-

mentally that melts rising from sublithospheric depths

would, on crossing the asthenosphere-lithospheric

rheological boundary, likely stall in their upward

passage, and on cooling across the solidus would

crystallise phlogopite and carbonate, releasing fluids

that would metasomatize higher reaches of the low-

ermost lithospheric mantle. Late stage fluids released

from the crystallisation of these early melts would be

depleted in Rb, K and Ti (phlogopite crystallisation)

and Sr (carbonate crystallisation) and rise into the

overlying depleted continental lithospheric mantle,

leading to its metasomatism and enrichment in highly

incompatible elements. The relative depletion in Rb,

K, Sr and Ti would thus be transferred into the

lithospheric mantle. Subsequent low degrees of melt-

ing of this metasomatised mantle consequent to con-

vective heating of the base of the lithosphere would

lead to formation of kimberlite melt carrying a mixed

plume-depleted continental lithospheric mantle geo-

chemical signature. The Uintjiesberg kimberlite lies

on the paleo-position of the Shona mantle plume at

f 100 Ma (le Roex, 1986), which is suggested to be

the most likely source of the metasomatic fluids which

infiltrated the sub-Gondwana mantle prior to kimber-

lite formation.

The ability of very low F (f 0.5%) melts to

escape from the lithospheric mantle is of fundamental

relevance with regard to the proposed model and

considerable debate exists as to the likelihood of this

happening (e.g., Faul, 1997, 2001; Foley, 1992;

McKenzie, 1985, 1989). Support for the extraction

of such melts is provided by Faul (1997, 2001) who

has shown that whereas basaltic melt only becomes

M. Harris et al. / Lithos 74 (2004) 149–165164

mobile at a porosity above 1–3%, deep, volatile-rich

melt with low viscosity and density is mobile at 0.1%

porosity. We envisage that the early melts rising from

the upwelling plume and metasomatising the overly-

ing lithosphere were formed by perhaps 2–5% melt-

ing and were thus able to leave their source region. In

contrast, the very low F (f 0.5%) melts formed by

melting of the metasomatised lithosphere and giving

rise to the Uintjiesberg kimberlite magma were highly

volatile-rich (Table 1), and consequently were mobile

at porosities as low as f 0.1% (Faul, 2001).

6. Conclusions

Compositional variation shown by the Uintjiesberg

aphanitic kimberlite is consistent with up to 30%

fractionation of olivine and phlogopite, whereas that

shown by the macrocrystic kimberlite varieties is

consistent with up to 25% entrainment of mantle

lherzolite which, for some samples, includes phlogo-

pite. Inflections in geochemical variation trends allow

a primary magma composition to be defined interme-

diate between the aphanitic and macrocrystic kimber-

lites, having a Mg# of f 0.85 and containing f 25%

SiO2, f 26% MgO, f 15% CaO, f 2.3% Al2O3,

f 5%H2O, and f 8.6% CO2.

Forward modelling of mantle melting processes

suggests that the primary Uintjiesberg kimberlite

magma formed by f 0.5% melting of a depleted

garnet lherzolite source that had been enriched in

volatiles and highly incompatible elements prior to

melting. Depletion in Rb, K, Sr and Ti relative to

elements of similar incompatibility are interpreted to

be features of the primary kimberlite magma and the

source region. Incompatible trace element ratios sug-

gest that the fluids/melts that metasomatised the

refractory lithospheric mantle source, derived from

sublithospheric sources, inferred to be a mantle

plume, possibly the Shona plume, that was located

beneath this region of Gondwana at the time of

kimberlite eruption (le Roex, 1986).

Acknowledgements

The authors acknowledge financial support from

the NRF and the University of Cape Town (MH and

AlR), and NSF (EAR: 0207311 CC). Jock Robey is

thanked for logistic assistance in the field, and we

extend our appreciation to Flippie and Corina

Jacobs for their hospitality in allowing us access

to their farm land. The manuscript benefited from

comments by Bill Griffin, Hong-Fu Zhang and

Steve Foley.

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