APPENDIX 1
Description of mineral major and trace element composition
Mineral major elements were determined by electron microprobe at the Vrije Universiteit
Amsterdam using a Jeol JXA-8800 and the analytical protocol described in Simon et al.
(2003). Values for average cores and rims obtained from 5-20 individual core and rim
analyses per mineral per sample are presented in Table 2.
Trace element contents in garnet, cpx and opx were measured by laser ablation ICP-MS
at the University of Utrecht (most samples) or SIMS (K18) at DTM, Carnegie Institution
of Washington. Analytical procedures are described in detail in Simon et al. (2003).
No trace element zonation could be detected with the laser ablation ICP-MS in most
cases. The concentrations are therefore average values of several ablation craters (1 - 5)
of cores and rims on different grains (1 – 5) in one sample. Where core and rim
compositions differ significantly (e.g. in some opx), mineral core and rim compositions
are presented separately (Table 3).
Olivine
Olivines in the Kimberley samples are generally homogeneous in major element
compositions. Minor variations in Fe/Mg, Ca, Ni, Ti, Al and Cr may occur in the
outermost rims (50 μm). All samples but K3 have mean and median Mg# (= molar
(Mg/(Mg+Fe)) × 100) in olivine of 92.4, within error of average cratonic mantle (median
92.1, Pearson et al., 2003), reflecting their highly depleted major element composition.
Ni contents are relatively constant but might decrease slightly with Mg#. This would not
be expected if the contents of these elements are governed entirely by partial melting
processes.
Orthopyroxene
Orthopyroxenes show slightly more variation between core and rim compositions than do
olivines. In the majority of samples, however, the variations are minor and/or restricted to
the outermost rims (50 μm). The opx from all samples of this suite (except K3) cover a
range in Mg# from 91.2 (group II) to 94.4 (group I). The opx mean and median Mg#
(93.2 and 93.3, respectively) are usually slightly higher than in the corresponding
olivines.
Trace element concentrations in opx could not be analyzed in all samples due to either
lack of fresh opx, exsolution lamella in opx, amphibole or phlogopite films on cleavage
plains, or too low trace element concentrations. We carefully selected spots for analyses
and monitored the changes in concentrations of critical elements (Ba, Sr, Nb, Ca) during
laser ablation, but some of the analyses were possibly affected by contamination. Due to
the very low trace element concentrations in Kimberley opx, HREE are very close to, or
even below, detection limits in many opx. Nevertheless, REE patterns will be
qualitatively discussed here. The errors are large, however, and variations in middle and
heavy REE (Eu-Lu), Ba, Y and sometimes Hf are not statistically significant. LREE and
LILE variations generally exceed analytical errors, but might suffer from contamination.
Therefore, interpretation of opx trace element data is hampered by these analytical
shortcomings.
C1-normalized opx REE patterns form two distinct groups (Fig. 2): (1) sinusoidal
patterns with maxima at Nd-Sm and minima at Dy-Er; (2) patterns with maxima at La-Ce
and La/LuN > 1. The first pattern is more common, but the second also occurs, often
within the same sample and even within (the rim of) the same opx grain (e.g. in K17).
Both compositions are depleted in all REE relative to chondrite, particularly in HREE.
Sm/NdN vary between 0.5 in opx with LREEN maxima between La and Nd, and max. 1.4
in opx with a REEN maximum at Sm (Fig. 2, Table 3). Interestingly, the samples with
superchondritic Sm/NdN also have positive Hf anomalies. Lu/HfN ratios can only be
estimated from HREE/Hf ratios since Lu could not be analyzed in most cases.
Extrapolated Lu/HfN ratios are subchondritic in all opx and vary between ~ 0.1 and ~0.5.
All opx show a strong positive Nb-Ta anomaly and a negative Y anomaly and variable
positive and negative Zr, Hf and Ti anomalies (Fig. 3).
Clinopyroxene
Clinopyroxenes show a much larger spread in Mg# (84.3-95.9) than olivine and opx
(Table 2). Petrographic observations, however, show that there are several generations of
cpx present in these samples. Large (usually >2 mm up to 1 cm) cpx porphyroblasts
usually have homogeneous cores, but often have thin rims with distinct compositions. In
most cases, this zonation involves increasing Al and decreasing Na contents in the rims,
occasionally accompanied by increasing Ti and Ca contents (Table 2). Secondary cpx
form thin interstitial veins. Vein cpx have highly variable compositions, but are
commonly characterized by high Ti, low Al, and Na higher than in the cpx granublast
rims. The vein cpx are believed to have formed by interaction with the host kimberlite
(Carswell, 1975; Simon et al., 2003). Cpx can also be found in the kelyphite rims around
garnet. The latter are believed to originate from breakdown of garnet during late stage
processes and are not further discussed.
A wide compositional variation is also found in symplectitic cpx (interpreted as
exsolution from opx). Cpx clearly identified as symplectitic intergrowth with spinel and
opx have very high CaO (23.5 wt%) and low Na2O (0.4 – 0.6 wt%) and TiO2 (0 – 0.04
wt%) (Supplementary Table A). Group I cpx cores have very low TiO2 (<0.05 wt%). K18
shows some deformation (subgrain formation in olivine and opx) and has the highest
jadeite component in cpx. This might indicate higher equilibration temperatures and/or
pressures for this sample, or a secondary origin of cpx. All group II cpx are characterized
by elevated Ti and Fe contents and low Cr2O3, CaO and MgO. Group III samples have
cpx with very high Cr2O3, low Al2O3 and relatively high Na2O.
All cpx are significantly LREE enriched (Table 3, Fig. 4, LaN = 10 – 80) and HREE
depleted, leading to subchondritic Sm/Nd (Sm/NdN = 0.4 – 0.8) and Lu/Hf (Lu/HfN = 0 –
0.13) ratios. The cpx REE patterns of group I samples are similar, especially in LREE
concentrations (LaN = 10 – 20), and have extremely low HREE (LuN ~ 0.1). They have
humped patterns with maxima at Pr-Nd (30 – 80 x C1). Group II samples K19 and K21
have cpx with REE maxima at Ce (CeN ~ 80). They also have the highest LREE, Ba, Th
and Nb-Ta and HREE concentrations of all cpx. Sr, Zr and Hf concentrations are
superchondritic in all cpx (Fig. 5). All cpx have negative HFSE anomalies, but group II
and III samples have higher Zr-Hf and Ti contents.
Garnet
No significant zonation in major elements was observed in Kimberley garnets, with the
exception of deformed sample K18 (Supplementary Table A). Mg# in garnets (80.4-87.8,
mean = 84) correlate positively with Cr# (6.4-15.5, mean = 12). The lowest Cr (<3.1 wt%
Cr2O3), highest Al (>21.5 wt% Al2O3) and relatively low CaO in garnet (<4.6 wt%) are
found in group II samples K3, K19 and K21 (and GP402), consistent with their differing
textures and pyroxene major element chemistry. Only K3 (Group II) and K18 (Group I,
deformed, high Cr# and Mg#) contain garnets with a significant amount of Ti and Na
(0.3-0.4 wt% and 0.05-0.06 wt%, respectively). Garnets from GP402 and K15 are the
only ones from Kimberley that have below 4.2 wt% CaO. All others have clearly
lherzolitic Ca-Cr relations (Sobolev et al., 1973).
Group I garnet REE patterns (C1-normalized) show LREE contents that increase
markedly from La to Nd (La/NdN ~ 0.05 – 0.015). The patterns are characterized by a
maximum between Nd and Eu (6.5 – 18 x C1), a decrease in C1-normalized
concentrations from Eu to Dy-Tm and Dy-Tm/LuN < 1, resulting in sinusoidal REE
patterns (Table 3, Fig. 6). Group II display very distinct LREE depleted (LaN = 0.1 – 0.3)
and HREE enriched patterns (Nd/LuN ~ 0.2 – 0.08).
All garnets have sub-chondritic Ba, Sr and Rb, whereas Hf and Zr are super-chondritic
(Fig. 7). TiN is ≤1 for all group I and group I-II garnets, but garnets in group II have
super-chondritic Ti concentrations. All garnets except K1, K3 and K17 have negative HfZr (and Ti) anomalies and Zr and Hf concentrations similar to or lower than a primitive
mantle garnet (Simon, 2004), but higher than C1. All samples but GP402 (Sm/NdN =
0.85) have superchondritic Sm/Nd ratios. Group I garnets (except GP402gt: Lu/HfN =
2.7) have Lu/Hf << C1, whereas group II garnets (and GP402gt) have (Lu/Hf)N > 1.
Spinel
Spinels in the Kimberley xenoliths are of different origins that are not always easy to
distinguish. Almost all samples contain thin films of secondary interstitial spinel,
probably related to kimberlite infiltration. Garnet bearing samples usually show differing
extents of garnet breakdown and kelyphitization, involving spinel as a reaction product.
Some samples appear to contain primary spinel coexisting with garnet. The spinel-facies
peridotites are very depleted and do not contain large spinel granublasts. Spinels in these
samples occur as small single grains or symplectitic spinel intergrown with opx and/or
cpx. These primary spinels are magnesiochromites (Mg# = 40 – 65, Cr# ~ 50) with
homogeneous compositions and very low Ti contents (Supplementary Table A). Primary
spinels coexisting with garnet are generally more Cr-rich (e.g., Boyd & Nixon, 1975;
Boyd et al., 1999). Secondary interstitial spinel and spinel derived from garnet
breakdown reactions can be extremely zoned in major elements and display a wide
variation of Cr# and Mg#, ranging from Mg-rich spinel with Cr# < 20 to chromite (Cr# =
80 - 100) with a Mg# of 40 - 50. This whole compositional range is often displayed by a
single spinel crystal.
Mg# in spinel is positively correlated with Al2O3 content and decreases with increasing
Cr2O3. TiO2 in spinel varies hugely (0 – 2.8 wt%) and shows a negative correlation with
Mg# for TiO2 larger then ~0.2 wt%. Symplectitic spinels are restricted to very low TiO2
concentrations. High-Ti spinels are usually interpreted as ‘magmatic’ in origin, i.e. as
crystallized from a melt, in contrast to typical low-Ti ‘mantle’, i.e. residual, spinels.
Several spinels analyzed in this study have TiO2 > 1 wt% and can therefore be clearly
related to a magmatic origin. They probably precipitated from infiltrating kimberlite
(consistent with their generally small size and interstitial texture). Spinels with
intermediate TiO2 (0.1 – 1 wt% TiO2) can be explained by reaction between ‘mantle’
spinel and kimberlite melt. Spinels in kelyphite usually have intermediate Ti contents and
show a strong zonation towards more Ti- and Al-rich rims.
Phlogopite and amphibole
As for spinel and cpx, several generations of phlogopite and amphibole often coexist in
the samples (as interstitial thin films, within kelyphite rims and as primary, texturally
equilibrated porphyroblasts). The entire phlogopite population in the Kimberley samples
spans a range from 93 – 95 in Mg# and 4 – 93 in Cr#. Ti contents vary from 0.05 to 1.7
wt% TiO2 (Supplementary Table A). As with spinel compositions, the Ti content can
serve as an indicator for the involvement of kimberlite in phlogopite genesis. The Ti
contents in the Kimberley samples, however, are generally relatively low compared with
some of the phlogopites in peridotite xenoliths from Lesotho (up to 4.5 wt% TiO2 in
phlogopite rims, Simon et al., 2000; Simon et al., 2003).
Amphibole major element compositions also vary widely (Supplementary Table A) and
display Mg# from 0.87 to 0.99 and Cr# from 0.05 to 0.69. TiO2 varies from 0 to 1.35,
K2O from 0.35 to 4.6 and Na2O from 2.9 to 5.2 wt%.
Amphibole was analyzed for trace elements in group III sample K11 (Table 3, Fig. 8a
and b). Amphibole is LREE enriched (LaN = 16) with La/LuN ~ 40 and has a small
positive Eu anomaly. K11am is strongly enriched in Rb relative to Ba and Th (RbN = 300,
Rb/ThN ~ 100) and has positive Sr and HFSE anomalies (least pronounced for Nb-Ta),
HfN > ZrN and NbN > TaN. K11 amphibole has subchondritic Sm/NdN and Lu/HfN ratios
(0.56 and 0.02, respectively), but a high Rb/Sr ratio compared to primitive mantle
(Rb/SrPM = 7.2).