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Tracking continental-scale modification of the Earth’s mantle using zircon megacrysts

J. Woodhead1,

1School of Earth Sciences, The University of Melbourne, VIC 3010, Australia

J. Hergt1,

1School of Earth Sciences, The University of Melbourne, VIC 3010, Australia

A. Giuliani1,2,

1School of Earth Sciences, The University of Melbourne, VIC 3010, Australia
2Department of Earth and Planetary Sciences, Macquarie University, North Ryde, NSW 2019, Australia

J. Hergt1,

1School of Earth Sciences, The University of Melbourne, VIC 3010, Australia

R. Maas1

1School of Earth Sciences, The University of Melbourne, VIC 3010, Australia

Affiliations  |  Corresponding Author  |  Cite as  |  Funding information

Woodhead, J., Hergt, J., Giuliani, A., Phillips, D., Maas, R. (2017) Tracking continental-scale modification of the Earth’s mantle using zircon megacrysts. Geochem. Persp. Let. 4, 1–6.

Research funded by the research group.

Geochemical Perspectives Letters v4  |  doi: 10.7185/geochemlet.1727
Received 08 December 2016  |  Accepted 22 May 2017  |  Published 10 July 2017
Copyright © 2017 European Association of Geochemistry

Keywords: mantle metasomatism, zircon megacryst, hafnium isotopes, kimberlite



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Abstract


Metasomatism, the chemical alteration of rocks by a variety of melts and fluids, has formed a key concept in studies of the Earth’s mantle for decades. Metasomatic effects are often inferred to be far-reaching and yet the evidence for their occurrence is usually based upon individual hand specimens or suites of rocks that display considerable heterogeneity. In rare cases, however, we are offered insights into larger-scale chemical modifications that occur in the mantle. Here we utilise the Lu–Hf systematics of zircon megacrysts erupted in kimberlite magmas to discern two temporally and compositionally discrete metasomatic events in the mantle beneath southern Africa, each having an influence extending over an area exceeding one million km2. These data provide unambiguous evidence for metasomatic processes operating at continental scales and seemingly unperturbed by the age and composition of the local lithospheric mantle. The most recent of these events may be associated with the major Jurassic-Karoo magmatism in southern Africa.

Figures and Tables

Figure 1 Schematic map of southern Africa showing kimberlite localities for zircon megacrysts analysed in this and previous studies. The major tectonic domains are also included.

Figure 2 Age and isotopic composition data for zircon megacrysts. (a) Zircon Hf-isotope data showing a natural compositional subdivision into two distinct groups, further illustrated by Kernel Density estimates. Note the remarkable isotopic homogeneity within each group despite large variations in geographic location and age. All data from this study unless marked: N = Nowell et al. (2004), G = Griffin et al. (2000). (b) Inset showing a statistically significant correlation between zircon 176Hf/177Hf composition and age. Only zircons for which precise U-Pb Concordia ages are available are used to construct this plot. Literature data with less precise age determinations (greyed out in 2a) are excluded. (c) An equivalent plot to 2b for 143Nd/144Nd isotope variations. (d) A comparison between kimberlite U-Pb perovskite ages, widely used to estimate the timing of magmatism, and U-Pb zircon megacryst ages from the same intrusion. Megacryst ages clearly approximate those of the kimberlite host, at least within the resolution of the perovskite technique.

Figure 3 A three-step process, summarised in panel (a), proposed to explain the variations within the Nd- and Hf-isotope arrays for zircon megacrysts. (i) A source with prolonged depletion in Lu/Hf and Sm/Nd evolves to highly negative εNd and εHf compositions, depicted here as the source of various lamproites (from Davies et al., 2006 and references therein) (ii) A carbonate melt, with an isotopic composition similar to OIB pervades this source, preserving the Hf isotopic composition of the source owing to its low Hf content, but overprinting the source rocks with a Nd isotope composition more typical of OIB. In addition to displacing the isotopic composition of the source rocks off the mantle array, this metasomatic process stabilises high Lu/Hf phases. (iii) With time, the isotopic compositions of the source evolve along steep trajectories (shown by the dashed blue arrow in panel (b) owing to their now elevated Lu/Hf and Sm/Nd ratios. Parental melts to zircon megacrysts (red circles for Group A zircons, blue circles represent the possible locations of initial εHfNd for Group B zircons, calculated for a range of potential ages) tap this source periodically, as it evolves to higher εNd and εHf with time. Mantle array line from Vervoort et al. (1999).

Table 1 Summary data. U-Pb age, Hf and Nd-isotope data for megacryst zircons. All data from this study unless otherwise noted: G = Griffin et al. (2000), N = Nowell et al. (2004). Where multiple solution analyses are shown from the same kimberlite body these represent different zircon megacrysts.

Figure 1 Figure 2 Figure 3 Table 1

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Supplementary Figures and Tables

Figure S-1 U-Pb Concordia plots for zircons analysed in this study. Kimberlite megacryst zircons for Group A possess ‘well behaved’ U-Pb systematics allowing the calculation of Concordia ages (upper panel). In contrast all of the Group B zircon megacrysts (lower panel) show evidence of disturbance in the U-Pb system. This implies they are older and have been variably reset during entrainment into the host kimberlite.

Table S-1 U-Pb isotope and trace element data for zircon samples used in this study. See Materials and Methods for analytical details and Figure S-1 for corresponding Concordia plots.

Table S-2 Hf-isotope data for megacryst zircons. All data from this study unless otherwise noted: Griffin et al. (2000) or Nowell et al. (2004). Given the relatively small datasets involved, a Tukey Biweight robust mean has been employed to determine the central tendency in the Hf-isotope ratios for each suite. Parent daughter ratios are unsuited to this approach due to their inherently more scattered nature, the result of magmatic zonation: here a simple arithmetic mean is employed.

Table S-3 Calculated temperatures based on Ti-in-zircon using the formulation of Ferry and Watson (2007). See Materials and Methods and Figure S-2 for further information and our interpretation of these results.

Figure S-2 Calculated temperature intervals for zircon megacrysts. Temperature values calculated at variable pressure using the Ti-in-zircon thermometer of Ferry and Watson (2007) and Ti concentrations of megacrysts in southern African archetypal kimberlites. Note that, if the Ti-in-zircon temperatures truly reflect the equilibration conditions of zircon megacrysts and assuming a typical cratonic mantle geotherm of 40–42 mW/m2, the zircon megacrysts could only be in equilibrium with the ambient mantle at P of 2.5–3.0 GPa.

Figure S-1 Table S-1 Table S-2 Table S-3 Figure S-2

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Introduction


Metasomatism is an important process generating regions of mantle enriched in volatile and incompatible elements that may subsequently melt, giving rise to a range of magma types. The spatial extent of metasomatic processes is poorly understood because geographically extensive studies of relevant metasomatic minerals with known ages are rare. Zircon megacrysts, an uncommon, large (cm-sized) and somewhat unusual mineral occurrence, recovered during the processing of kimberlites to extract diamonds, may fill this gap. Their trace element patterns (Valley et al., 1998

Valley, J.W., Kinny, P.D., Schuklze, D.J., Spicuzza, M.J. (1998) Zircon megacrysts from kimberlite: oxygen isotope variability among mantle melts. Contributions to Mineralogy and Petrology 133, 1–11.

, Belousova et al., 2002

Belousova, E.A., Griffin, W.L., O’Reilly, S.Y., Fischer, N.I. (2002) Igneous zircon: trace element composition as an indicator of source rock type. Contributions to Mineralogy and Petrology 143, 602–622.

) and low δ18O (Page et al., 2007

Page, F.Z., Fu, B., Kita, N.T., Fournelle, J., Spicuzza, M.J., Schultz, D.J., Vijoen, F., Basei, M.A.S., Valley, J.W. (2007) Zircons from kimberlite: new insights from oxygen isotopes, trace elements, and Ti in zircon thermometry. Geochimica et Cosmochimica Acta 71, 3887–3903.

) indicate that they are not of crustal origin, but crystallised within the mantle and experienced only minimal chemical interaction with the host magmas that transported them to the surface. While details of their petrogenesis (and the origin of megacryst suites more broadly) remain a subject of active research, there is agreement that zircon megacrysts are produced by metasomatic melts in some way related to kimberlite magmas (e.g., Kinny et al., 1989

Kinny, P.D., Compston, W., Bristow, J.W., Williams, I.S. (1989) Archean mantle xenocrysts in a Permian kimberlite: two generations of kimberlitic zircon in Jwaneng DK2, southern Botswana. Geological Society of Australia Special Publication 14, 833–842.

; Nowell et al., 2004

Nowell, G.M., Pearson, D.G., Bell, D.R., Carlson, R.W., Smith, C.B., Kempton, P.D., Noble, S.R. (2004) Hf isotope systematics of kimberlites and their megacrysts: new constraints on their source regions. Journal of Petrology 45, 1583–1612.

; Page et al., 2007

Page, F.Z., Fu, B., Kita, N.T., Fournelle, J., Spicuzza, M.J., Schultz, D.J., Vijoen, F., Basei, M.A.S., Valley, J.W. (2007) Zircons from kimberlite: new insights from oxygen isotopes, trace elements, and Ti in zircon thermometry. Geochimica et Cosmochimica Acta 71, 3887–3903.

). They record precise U-Pb ages and initial 176Hf/177Hf isotope ratios providing important constraints on the age and nature of the metasomatic events occurring in their mantle sources. We present the first geographically-extensive survey of Hf-isotope and U-Pb age distributions for zircon megacrysts in southern African kimberlites, representing widely spaced intrusions spanning both cratonic (Kaapvaal, Zimbabwe) and non-cratonic settings (Fig. 1). We also report the first Nd-isotope data for zircon megacrysts.


Figure 1 Schematic map of southern Africa showing kimberlite localities for zircon megacrysts analysed in this and previous studies. The major tectonic domains are also included.
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Results


Zircons have very low Lu/Hf ratios and thus preserve the initial 176Hf/177Hf of their source metasomatic melts (Table 1; full data in Tables S-1 and S-2). Our results reveal an entirely unexpected first order observation; that is, remarkable large-scale isotopic homogeneity among southern African zircon megacrysts (Fig. 2a) across lithospheric domains with widely differing ages (e.g., Pearson and Wittig, 2014

Pearson, D.G., Wittig, N. (2014) The formation and evolution of cratonic mantle lithosphere – evidence from mantle xenoliths. In: Holland, H., Turekian, K. (Eds.) Treatise on Geochemistry. Second Edition, Elsevier, Amsterdam, Oxford, Waltham, 255–292.

). Although a restricted isotopic range in Hf-isotopes has been noted previously in a much smaller dataset of kimberlite megacrysts from this area (Griffin et al., 2000

Griffin, W.L., Pearson, N.J., Belousova, E., Jackson, S.E., van Achterburgh, E., O’Reilly, S.Y., Shee, S.R. (2000) The Hf isotope composition of cratonic mantle: LAM-MC-ICPMS analysis of zircon megacrysts in kimberlites. Geochimica et Cosmochimica Acta 64, 133–147.

), our analyses show near identical isotopic compositions in samples derived from numerous intrusions distributed across a region of >1 million km2.

The data form two homogeneous yet distinct compositional groups, which we term A and B (Fig. 2a); a distinction also mirrored in the new Nd-isotope data (Table 1). Some kimberlite pipes contain both zircon groups (e.g., Wesselton, Koffiefontein), as previously reported for the Orapa and Jwaneng kimberlites (Kinny et al., 1989

Kinny, P.D., Compston, W., Bristow, J.W., Williams, I.S. (1989) Archean mantle xenocrysts in a Permian kimberlite: two generations of kimberlitic zircon in Jwaneng DK2, southern Botswana. Geological Society of Australia Special Publication 14, 833–842.

, Griffin et al., 2000

Griffin, W.L., Pearson, N.J., Belousova, E., Jackson, S.E., van Achterburgh, E., O’Reilly, S.Y., Shee, S.R. (2000) The Hf isotope composition of cratonic mantle: LAM-MC-ICPMS analysis of zircon megacrysts in kimberlites. Geochimica et Cosmochimica Acta 64, 133–147.

). Remarkably, the subtle variations in 176Hf/177Hf and 143Nd/144Nd in zircons of the larger Group A correlate with age and may reflect radiogenic ingrowth in the source of the metasomatic zircon parent melts (Fig. 2b, 2c). Although the 176Hf/177Hf – age correlation is largely defined by the off-craton samples that show the greatest range of ages, it remains true that the cluster of on-craton samples also lies along this array. All results from this study plot below the Nd-Hf isotope mantle array (Fig. 3).

Group A zircons yield precise and concordant U-Pb ages which generally approximate the (usually less precise) age estimates of their kimberlite hosts (Table 1, Figs. 2d and S-1). In contrast, U-Pb systematics for Group B zircons are disturbed (Fig. S-1), precluding accurate dating, and suggesting a more protracted history.


Figure 2 Age and isotopic composition data for zircon megacrysts. (a) Zircon Hf-isotope data showing a natural compositional subdivision into two distinct groups, further illustrated by Kernel Density estimates. Note the remarkable isotopic homogeneity within each group despite large variations in geographic location and age. All data from this study unless marked: N = Nowell et al. (2004)

Nowell, G.M., Pearson, D.G., Bell, D.R., Carlson, R.W., Smith, C.B., Kempton, P.D., Noble, S.R. (2004) Hf isotope systematics of kimberlites and their megacrysts: new constraints on their source regions. Journal of Petrology 45, 1583–1612.

, G = Griffin et al. (2000)

Griffin, W.L., Pearson, N.J., Belousova, E., Jackson, S.E., van Achterburgh, E., O’Reilly, S.Y., Shee, S.R. (2000) The Hf isotope composition of cratonic mantle: LAM-MC-ICPMS analysis of zircon megacrysts in kimberlites. Geochimica et Cosmochimica Acta 64, 133–147.

. (b) Inset showing a statistically significant correlation between zircon 176Hf/177Hf composition and age. Only zircons for which precise U-Pb Concordia ages are available are used to construct this plot. Literature data with less precise age determinations (greyed out in 2a) are excluded. (c) An equivalent plot to 2b for 143Nd/144Nd isotope variations. (d) A comparison between kimberlite U-Pb perovskite ages, widely used to estimate the timing of magmatism, and U-Pb zircon megacryst ages from the same intrusion. Megacryst ages clearly approximate those of the kimberlite host, at least within the resolution of the perovskite technique.
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Figure 3 A three-step process, summarised in panel (a), proposed to explain the variations within the Nd- and Hf-isotope arrays for zircon megacrysts. (i) A source with prolonged depletion in Lu/Hf and Sm/Nd evolves to highly negative εNd and εHf compositions, depicted here as the source of various lamproites (from Davies et al., 2006

Davies, G.R., Stolz, A.J., Mahotkin, I.L., Nowell, G.M., Pearson, D.G. (2006) Trace element and Sr-Pb-Nd-Hf isotope evidence for ancient, fluid-dominated enrichment of the source of Aldan Shield lamproites. Journal of Petrology 47, 1119–1146.

and references therein) (ii) A carbonate melt, with an isotopic composition similar to OIB pervades this source, preserving the Hf isotopic composition of the source owing to its low Hf content, but overprinting the source rocks with a Nd isotope composition more typical of OIB. In addition to displacing the isotopic composition of the source rocks off the mantle array, this metasomatic process stabilises high Lu/Hf phases. (iii) With time, the isotopic compositions of the source evolve along steep trajectories (shown by the dashed blue arrow in panel (b) owing to their now elevated Lu/Hf and Sm/Nd ratios. Parental melts to zircon megacrysts (red circles for Group A zircons, blue circles represent the possible locations of initial εHfNd for Group B zircons, calculated for a range of potential ages) tap this source periodically, as it evolves to higher εNd and εHf with time. Mantle array line from Vervoort et al. (1999)

Vervoort, J.D., Patchett, P.J., Blichert-Toft, J., Albarède, F. (1999) Relationships between Lu-Hf and Sm-Nd isotopic systems in the global sedimentary system. Earth and Planetary Science Letters, 168, 79–99.

.
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Table 1 Summary data. U-Pb age, Hf and Nd-isotope data for megacryst zircons. All data from this study unless otherwise noted: G = Griffin et al. (2000)

Griffin, W.L., Pearson, N.J., Belousova, E., Jackson, S.E., van Achterburgh, E., O’Reilly, S.Y., Shee, S.R. (2000) The Hf isotope composition of cratonic mantle: LAM-MC-ICPMS analysis of zircon megacrysts in kimberlites. Geochimica et Cosmochimica Acta 64, 133–147.

, N = Nowell et al. (2004)

Nowell, G.M., Pearson, D.G., Bell, D.R., Carlson, R.W., Smith, C.B., Kempton, P.D., Noble, S.R. (2004) Hf isotope systematics of kimberlites and their megacrysts: new constraints on their source regions. Journal of Petrology 45, 1583–1612.

. Where multiple solution analyses are shown from the same kimberlite body these represent different zircon megacrysts.

IN SITU ANALYSESSOLUTION ANALYSES
HostU/Pb age2 sigma176Hf/177Hf2 sigmaSm ppmNd ppm143Nd/144Ndm143Nd/144NdiEpsilon Ndi176Hf/177Hfm176Hf/177HfiEpsilon Hfi

(Ma)










Mukurob57.620.570.2828100.0000240.7340.5650.5130230.5127273.300.2828380.2828373.11

57.62


0.7130.5470.5130600.5127664.050.2828330.2828332.95
Deutche Erde67.940.720.2827700.000033







Silvery Home79.600.240.2827030.000007







Wesselton86.360.450.2827400.000063







Wesselton - Group Bunknown
0.2822200.000011







De Beers87.260.690.2826370.0000101.3321.0870.5130710.5126492.420.2826950.282695-1.23
Du Toitspan87.801.200.2827470.000010







Monastery88.690.500.2827210.000007








88.69
0.282725N0.000006








88.69
0.282703G0.000004







Lethlakane92.010.670.2827320.000008







Bultfontein93.960.490.2826890.0000080.3140.2940.5129830.5125871.480.2827000.282700-0.92

93.96


0.4460.4770.5129520.5126051.780.2826890.282689-1.32

93.96


0.3710.3060.5130450.5125951.560.2826960.282696-1.06
Koffiefontein94.160.530.2827100.0000110.3160.2170.5131550.5126152.020.2827380.2827380.44

94.16


0.6250.4380.5132040.5126733.160.2827580.2827581.13
Koffiefontein - Group Bunknown
0.2822700.0000470.2320.2790.513226unknownunknown0.282281unknownunknown
Orapa - Group A96.550.730.2827250.000010








96.55
0.28275N0.000007








96.55
0.28271G0.000013







Orapa - Group Bunknown
0.2823200.000031








unknown
0.28233N0.000021








unknown
0.282254G0.000006







Uintjesberg103.420.740.2826600.000023







Frank Smith114.480.830.2826000.000011







Kaalvalie

0.282751N0.000009







Kamfersdam

0.282721N0.000010







Mothae

0.28257N0.000049







Gansfontein

0.282709N0.000005







Leicester

0.28257G0.000049







Download in Excel

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Discussion


Isotopic constraints. Our results provide a consistent picture of megacryst parental melts which tapped an isotopically homogeneous source extending over hundreds of kilometres, and encompassing a time interval of nearly 70 Myr, the range of U-Pb ages (114–56 Ma) recorded by the zircons. The apparent 176Hf/177Hf – age relationship defined by the Group A zircons places important constraints on the nature and evolution of their mantle source(s). To produce such a correlation these source rocks must have been relatively homogeneous initially and subsequently evolved rapidly with a strongly super-chondritic 176Lu/177Hf ratio (~0.16, Fig. 2b). The initial 143Nd/144Nd values for Group A zircon megacrysts also correlate with age (Fig. 2c), consistent with a source that evolved with a moderate-high 147Sm/144Nd ratio of ~0.53 (although both parent-daughter ratios are poorly defined, based on the paucity of the data). Importantly, prior to rapid radiogenic ingrowth, the initial source rock composition must have been located off the mantle-array, displaced to lower 176Hf/177Hf for a given 143Nd/144Nd (Fig. 3). This also provides important insights into both the nature of the original mantle source rocks and the metasomatic fluid that modified them.

We postulate that the mantle source rocks originally had a protracted history of unusually low Lu/Hf and Sm/Nd and developed initial 176Hf/177Hf and 143Nd/144Nd that are low relative to MORB mantle (i.e. ‘enriched mantle’, Fig. 3). Subsequent metasomatism of these source rocks not only drastically raised Lu/Hf to drive rapid 176Hf ingrowth for at least ~70 Myr (Fig. 2b) but must also have had a) low Hf contents to preserve the original unradiogenic 176Hf/177Hf signature of the protolith but b) sufficient Nd to modify the 143Nd/144Nd to values more typical of OIB. Metasomatism therefore decoupled Hf from Nd (and presumably Sr) isotope compositions to generate source rocks, and ultimately zircon megacrysts, with compositions to the right of the Nd-Hf mantle array (Fig. 3).

The lack of precise age control precludes a similar assessment for the much smaller Group B zircon dataset. Nevertheless, the fact that zircons sharing such similar isotopic characteristics were erupted across broad areas of southern Africa (i.e. Wesselton and Koffiefontein in the Kimberley area, South Africa, and Orapa in Botswana), supports the existence of a second widespread event in the mantle beneath the southern African sub-continent.

Towards a genetic model. The potential link between carbonate metasomatism and kimberlite/megacryst genesis has been made often but typically based upon petrographic or experimental evidence (e.g., Giuliani et al., 2012

Giuliani, A., Kamenetsky, V.S., Phillips, D., Kendrick, M.A., Wyatt, B.A., Goemann, K. (2012) Nature of alkali-carbonate fluids in the sub-continental lithospheric mantle. Geology 40, 967–970.

; Russell et al., 2012

Russell, J.K., Porritt, L.A., Lavallée, Y., Dingwell, D.B. (2012) Kimberlite ascent by assimilation-fuelled buoyancy. Nature 481, 352–356.

). Carbonate melts are the least viscous of known terrestrial magma types (Dobson et al., 1996

Dobson, D.P., Jones, A.P., Rabe, R., Sekine, T., Kurita, K., Taniguchi, T., Kondo, T., Kato, T., Shimomura, O., Urakawa, S. (1996) In-situ measurement of viscosity and density of carbonate melts at high pressure. Earth and Planetary Science Letters 143, 207–215.

) and may thus have the ability to pervade large regions of the mantle. The work of Bizimis et al. (2003)

Bizimis, M., Salters, V.J., Dawson, J.B. (2003) The brevity of carbonatite sources in the mantle: evidence from Hf isotopes. Contributions to Mineralogy and Petrology 145, 281–300.

also suggests that carbonate fractions of carbonatites have low Hf contents, high Lu/Hf and decoupled Nd-Hf isotope systematics. Accordingly, we explore a model in which a carbonate melt infiltrates mantle with compositions at the low εHfNd (enriched) end of the Hf-Nd mantle array, similar to the source of lamproite magmas which originate in enriched lithospheric mantle (Nowell et al., 1998

Nowell, G.M., Pearson, D.G., Kempton, P.D., Irving, A.J., Turner, S. (1998). A Hf isotope study of lamproites: Implications for their origins and relationships to kimberlites.In: Gurney, J.J., Gurney, J.L., Pascoe, M.D., Richardson, S.H. (Eds.) 7th International Kimberlite Conference, Extended Abstracts. Red Roof Design, Capetown, 637–639.

, 2004

Nowell, G.M., Pearson, D.G., Bell, D.R., Carlson, R.W., Smith, C.B., Kempton, P.D., Noble, S.R. (2004) Hf isotope systematics of kimberlites and their megacrysts: new constraints on their source regions. Journal of Petrology 45, 1583–1612.

). At small degrees of metasomatic addition, the expected mixing trajectory is almost horizontal as the inferred carbonate melt has high Nd/Hf and eNd relative to the enriched mantle source (Fig. 3). The marked increase in Lu/Hf of this carbonate-metasomatised lithospheric mantle then drives a rapid rise in 176Hf/177Hf (producing very steep trends in Nd-Hf isotope space) with time. Garnet may have been among the newly-grown high-Lu phases important in establishing the high Lu/Hf ratio of the metasomatised source. As this source evolves and is sampled by kimberlite magmatism during the Jurassic and Cretaceous (producing zircon megacrysts), the isotope vs. age covariation is revealed (blue dotted arrow, Fig. 3).

Location and timing of metasomatism. The Hf isotope vs. age trend observed in the megacryst zircons is consistent with isotopic evolution under closed system conditions for ~70 Myr. While this could be readily achieved in the lithosphere, the observed trend crosses cratonic boundaries, and would therefore require that metasomatism efficiently overprinted any pre-existing compositional heterogeneity. A location at or below the lithosphere-asthenosphere boundary is also plausible, consistent with evidence that at least some initial kimberlite melts originate from sub-lithospheric depths (Tappe et al., 2013

Tappe, S., Pearson, D.G., Kjarsgaard, B.A., Nowell, G., Dowall, D. (2013) Mantle transition zone input to kimberlite magmatism near a subduction zone: origin of anaomalous Nd-Hf isotope systematics at Lac de Gras. Earth and Planetary Science Letters 371–372, 235–251.

; Pearson et al., 2014

Pearson, D.G., Brenker, F.E., Nestola, F., McNeill, J., Nasdala, L., Hutchinson, M.T., Matveev, S., Mather, K., Silvermit, G., Schmitz, S., Vekemans, B., Vincze, L (2014) Hydrous mantle transition zone indicated by ringwoodite included within diamond. Nature 507, 221–224.

). Our data do not preclude either possibility.

The occurrence of near-homogeneous 176Hf/177Hf in megacryst zircons across two cratons (Kaapvaal and Zimbabwe) and the surrounding Proterozoic requires the inferred metasomatic processes to postdate final tectonic assembly of these crustal domains. This suggests the source of Group A zircons postdates the ~1300 Ma amalgamation of the Kaapvaal craton and the Namaqua-Natal belt (Eglington, 2006

Eglington, B.M. (2006) Evolution of the Namaqua-Natal Belt, southern Africa – a geochonological and isotope geochemical review. Journal of African Earth Science 46, 93–111.

), the youngest terrane with Cretaceous kimberlites; a younger limit is provided by the age of the oldest host kimberlite, the 114 Ma Frank Smith pipe. Importantly, the rapid isotopic evolution of the modified mantle source required by the zircon data, make it unlikely that the metasomatic event occurred more than a few hundred million years ago.

Although the timing of Group B zircon formation is unknown (because their U–Pb systematics have been disturbed) some limits can be placed on their age (and hence the minimum age of metasomatism of their source), by calculating the initial εHfNd, for a range of hypothetical ages. Using the single Group B zircon for which we have Nd data (Koffiefontein), ages <250 Ma or >>500 Ma are highly improbable because the resultant zircon initial εHfNd would be unfeasible (Fig. 3). On this basis, we speculate an age for the Group B zircons of between 250 and 500 Ma, with metasomatic alteration of their mantle source being somewhat older.

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Concluding Remarks


Our new Hf-isotope data provide clear evidence for a discrete metasomatic event in the southern Africa mantle operating at a continent-wide scale between 114 Ma and several hundred million years ago, and subsequently sampled by separate kimberlite eruptions over a period of at least 70 Myr. The possibility of a link between such large-scale mantle metasomatism and formation of the Karoo large igneous province has previously been suggested (Konzett et al., 1998

Konzett, J., Armstrong, R.A., Sweeney, R.J., Compston, W. (1998) The timing of MARID metasomatism in the Kaapvaal mantle: an ion microprobe study of zircons from MARID xenoliths. Earth and Planetary Science Letters 160, 133–145.

; Ernst and Bell, 2010

Ernst, R.E., Bell, K. (2010) Large igneous provinces (LIPs) and carbonatites. Mineralogy and Petrology 98, 55–76.

), and would be consistent with the very large thermal and magmatic perturbation resulting from Karoo activity. New geochronological data for metasomatised mantle xenoliths from the Kimberley kimberlites also suggest a direct association of these events (Giuliani et al., 2014

Giuliani, A., Kamenetsky, V.S., Phillips, D., Kendrick, M.A., Wyatt, B.A., Goemann, K. (2014) LIMA U-Pb ages link lithospheric mantle metasomatism to Karoo magmatism beneath the Kimberley region, South Africa. Earth and Planetary Science Letters 401, 132–147.

). A link between widespread Karoo magmatism, modification of the southern African continental mantle, initiation of kimberlite magmatism, and megacryst formation therefore appears an intriguing possibility worthy of further study. A more disturbed and less sampled suite of zircon megacrysts supports the occurrence of a similar but older event.

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Acknowledgements


We thank DeBeers for provision of the zircon samples that were originally collected by Dr John Bristow. JW and AG acknowledge funding from the Australian Research Council. Alan Greig is thanked for technical assistance.

Editor: Graham Pearson

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References



Batumike, J.M., Griffin, W.L., Belousova, E.A., Pearson, N.J., O'Reilly, S.Y., Shee, S.R. (2008) LAM-ICPMS U-Pb dating of kimberlitic perovskite: Eocene-Oligocene kimberlites from the Kundelungu Plateau, D.R. Congo. Earth and Planetary Science Letters 267, 609–619.
Show in context

Figure 2
View in article


Belousova, E.A., Griffin, W.L., O’Reilly, S.Y., Fischer, N.I. (2002) Igneous zircon: trace element composition as an indicator of source rock type. Contributions to Mineralogy and Petrology 143, 602–622.
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Their trace element patterns (Valley et al., 1998, Belousova et al., 2002) and low δ18O (Page et al., 2007) indicate that they are not of crustal origin, but crystallised within the mantle and experienced only minimal chemical interaction with the host magmas that transported them to the surface.
View in article


Bizimis, M., Salters, V.J., Dawson, J.B. (2003) The brevity of carbonatite sources in the mantle: evidence from Hf isotopes. Contributions to Mineralogy and Petrology 145, 281–300.
Show in context

The work of Bizimis et al. (2003) also suggests that carbonate fractions of carbonatites have low Hf contents, high Lu/Hf and decoupled Nd-Hf isotope systematics.
View in article


Davies, G.R., Stolz, A.J., Mahotkin, I.L., Nowell, G.M., Pearson, D.G. (2006) Trace element and Sr-Pb-Nd-Hf isotope evidence for ancient, fluid-dominated enrichment of the source of Aldan Shield lamproites. Journal of Petrology 47, 1119–1146.
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Figure 3 [...] (i) A source with prolonged depletion in Lu/Hf and Sm/Nd evolves to highly negative εNd and εHf compositions, depicted here as the source of various lamproites (from Davies et al., 2006 and references therein)
View in article


Dobson, D.P., Jones, A.P., Rabe, R., Sekine, T., Kurita, K., Taniguchi, T., Kondo, T., Kato, T., Shimomura, O., Urakawa, S. (1996) In-situ measurement of viscosity and density of carbonate melts at high pressure. Earth and Planetary Science Letters 143, 207–215.
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Carbonate melts are the least viscous of known terrestrial magma types (Dobson et al., 1996) and may thus have the ability to pervade large regions of the mantle.
View in article


Eglington, B.M. (2006) Evolution of the Namaqua-Natal Belt, southern Africa – a geochonological and isotope geochemical review. Journal of African Earth Science 46, 93–111.
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This suggests the source of Group A zircons postdates the ~1300 Ma amalgamation of the Kaapvaal craton and the Namaqua-Natal belt (Eglington, 2006), the youngest terrane with Cretaceous kimberlites; a younger limit is provided by the age of the oldest host kimberlite, the 114 Ma Frank Smith pipe. Importantly, the rapid isotopic evolution of the modified mantle source required by the zircon data, make it unlikely that the metasomatic event occurred more than a few hundred million years ago.
View in article


Ernst, R.E., Bell, K. (2010) Large igneous provinces (LIPs) and carbonatites. Mineralogy and Petrology 98, 55–76.
Show in context

The possibility of a link between such large-scale mantle metasomatism and formation of the Karoo large igneous province has previously been suggested (Konzett et al., 1998; Ernst and Bell, 2010), and would be consistent with the very large thermal and magmatic perturbation resulting from Karoo activity.
View in article


Giuliani, A., Kamenetsky, V.S., Phillips, D., Kendrick, M.A., Wyatt, B.A., Goemann, K. (2012) Nature of alkali-carbonate fluids in the sub-continental lithospheric mantle. Geology 40, 967–970.
Show in context

The potential link between carbonate metasomatism and kimberlite/megacryst genesis has been made often but typically based upon petrographic or experimental evidence (e.g., Giuliani et al., 2012; Russell et al., 2012).
View in article


Giuliani, A., Kamenetsky, V.S., Phillips, D., Kendrick, M.A., Wyatt, B.A., Goemann, K. (2014) LIMA U-Pb ages link lithospheric mantle metasomatism to Karoo magmatism beneath the Kimberley region, South Africa. Earth and Planetary Science Letters 401, 132–147.
Show in context

New geochronological data for metasomatised mantle xenoliths from the Kimberley kimberlites also suggest a direct association of these events (Giuliani et al., 2014).
View in article


Griffin, W.L., Pearson, N.J., Belousova, E., Jackson, S.E., van Achterburgh, E., O’Reilly, S.Y., Shee, S.R. (2000) The Hf isotope composition of cratonic mantle: LAM-MC-ICPMS analysis of zircon megacrysts in kimberlites. Geochimica et Cosmochimica Acta 64, 133–147.
Show in context

Although a restricted isotopic range in Hf-isotopes has been noted previously in a much smaller dataset of kimberlite megacrysts from this area (Griffin et al., 2000), our analyses show near identical isotopic compositions in samples derived from numerous intrusions distributed across a region of >1 million km2.
View in article
Some kimberlite pipes contain both zircon groups (e.g., Wesselton, Koffiefontein), as previously reported for the Orapa and Jwaneng kimberlites (Kinny et al., 1989, Griffin et al., 2000).
View in article
Figure 2 [...] All data from this study unless marked: N = Nowell et al. (2004), G = Griffin et al. (2000).
View in article
Table 1 [...] All data from this study unless marked: N = Nowell et al. (2004), G = Griffin et al. (2000).
View in article


Griffin, W.L., Batumike, J.M., Greau, Y., Pearson, N.J., Shee, S.R., O’Reilly, S.Y. (2014) Emplacement ages and sources of kimberlites and related rocks in southern Africa: U-Pb ages and Sr-Nd of groundmass perovskite. Contributions to Mineralogy and Petrology 168, 1032.
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Figure 2
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Kinny, P.D., Compston, W., Bristow, J.W., Williams, I.S. (1989) Archean mantle xenocrysts in a Permian kimberlite: two generations of kimberlitic zircon in Jwaneng DK2, southern Botswana. Geological Society of Australia Special Publication 14, 833–842.
Show in context

While details of their petrogenesis (and the origin of megacryst suites more broadly) remain a subject of active research, there is agreement that zircon megacrysts are produced by metasomatic melts in some way related to kimberlite magmas (e.g., Kinny et al., 1989; Nowell et al., 2004; Page et al., 2007).
View in article
Some kimberlite pipes contain both zircon groups (e.g., Wesselton, Koffiefontein), as previously reported for the Orapa and Jwaneng kimberlites (Kinny et al., 1989, Griffin et al., 2000).
View in article


Konzett, J., Armstrong, R.A., Sweeney, R.J., Compston, W. (1998) The timing of MARID metasomatism in the Kaapvaal mantle: an ion microprobe study of zircons from MARID xenoliths. Earth and Planetary Science Letters 160, 133–145.
Show in context

The possibility of a link between such large-scale mantle metasomatism and formation of the Karoo large igneous province has previously been suggested (Konzett et al., 1998; Ernst and Bell, 2010), and would be consistent with the very large thermal and magmatic perturbation resulting from Karoo activity.
View in article


Nowell, G.M., Pearson, D.G., Kempton, P.D., Irving, A.J., Turner, S. (1998). A Hf isotope study of lamproites: Implications for their origins and relationships to kimberlites.In: Gurney, J.J., Gurney, J.L., Pascoe, M.D., Richardson, S.H. (Eds.) 7th International Kimberlite Conference, Extended Abstracts. Red Roof Design, Capetown, 637–639.
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Accordingly, we explore a model in which a carbonate melt infiltrates mantle with compositions at the low εHfNd (enriched) end of the Hf-Nd mantle array, similar to the source of lamproite magmas which originate in enriched lithospheric mantle (Nowell et al., 1998, 2004).
View in article


Nowell, G.M., Pearson, D.G., Bell, D.R., Carlson, R.W., Smith, C.B., Kempton, P.D., Noble, S.R. (2004) Hf isotope systematics of kimberlites and their megacrysts: new constraints on their source regions. Journal of Petrology 45, 1583–1612.
Show in context

While details of their petrogenesis (and the origin of megacryst suites more broadly) remain a subject of active research, there is agreement that zircon megacrysts are produced by metasomatic melts in some way related to kimberlite magmas (e.g., Kinny et al., 1989; Nowell et al., 2004; Page et al., 2007).
View in article
Figure 2 [...] All data from this study unless marked: N = Nowell et al. (2004), G = Griffin et al. (2000).
View in article
Table 1 [...] All data from this study unless marked: N = Nowell et al. (2004), G = Griffin et al. (2000).
View in article
Accordingly, we explore a model in which a carbonate melt infiltrates mantle with compositions at the low εHfNd (enriched) end of the Hf-Nd mantle array, similar to the source of lamproite magmas which originate in enriched lithospheric mantle (Nowell et al., 1998, 2004).
View in article


Page, F.Z., Fu, B., Kita, N.T., Fournelle, J., Spicuzza, M.J., Schultz, D.J., Vijoen, F., Basei, M.A.S., Valley, J.W. (2007) Zircons from kimberlite: new insights from oxygen isotopes, trace elements, and Ti in zircon thermometry. Geochimica et Cosmochimica Acta 71, 3887–3903.
Show in context

Their trace element patterns (Valley et al., 1998, Belousova et al., 2002) and low δ18O (Page et al., 2007) indicate that they are not of crustal origin, but crystallised within the mantle and experienced only minimal chemical interaction with the host magmas that transported them to the surface.
View in article
While details of their petrogenesis (and the origin of megacryst suites more broadly) remain a subject of active research, there is agreement that zircon megacrysts are produced by metasomatic melts in some way related to kimberlite magmas (e.g., Kinny et al., 1989; Nowell et al., 2004; Page et al., 2007).
View in article


Pearson, D.G., Wittig, N. (2014) The formation and evolution of cratonic mantle lithosphere – evidence from mantle xenoliths. In: Holland, H., Turekian, K. (Eds.) Treatise on Geochemistry. Second Edition, Elsevier, Amsterdam, Oxford, Waltham, 255–292.
Show in context

Our results reveal an entirely unexpected first order observation; that is, remarkable large-scale isotopic homogeneity among southern African zircon megacrysts (Fig. 2a) across lithospheric domains with widely differing ages (e.g., Pearson and Wittig, 2014).
View in article


Pearson, D.G., Brenker, F.E., Nestola, F., McNeill, J., Nasdala, L., Hutchinson, M.T., Matveev, S., Mather, K., Silvermit, G., Schmitz, S., Vekemans, B., Vincze, L (2014) Hydrous mantle transition zone indicated by ringwoodite included within diamond. Nature 507, 221–224.
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A location at or below the lithosphere-asthenosphere boundary is also plausible, consistent with evidence that at least some initial kimberlite melts originate from sub-lithospheric depths (Tappe et al., 2013; Pearson et al., 2014).
View in article


Russell, J.K., Porritt, L.A., Lavallée, Y., Dingwell, D.B. (2012) Kimberlite ascent by assimilation-fuelled buoyancy. Nature 481, 352–356.
Show in context

The potential link between carbonate metasomatism and kimberlite/megacryst genesis has been made often but typically based upon petrographic or experimental evidence (e.g., Giuliani et al., 2012; Russell et al., 2012).
View in article


Tappe, S., Pearson, D.G., Kjarsgaard, B.A., Nowell, G., Dowall, D. (2013) Mantle transition zone input to kimberlite magmatism near a subduction zone: origin of anaomalous Nd-Hf isotope systematics at Lac de Gras. Earth and Planetary Science Letters 371–372, 235–251.
Show in context

A location at or below the lithosphere-asthenosphere boundary is also plausible, consistent with evidence that at least some initial kimberlite melts originate from sub-lithospheric depths (Tappe et al., 2013; Pearson et al., 2014).
View in article


Valley, J.W., Kinny, P.D., Schuklze, D.J., Spicuzza, M.J. (1998) Zircon megacrysts from kimberlite: oxygen isotope variability among mantle melts. Contrib. Mineral. Petrol. 133, 1–11.
Show in context

Their trace element patterns (Valley et al., 1998, Belousova et al., 2002) and low δ18O (Page et al., 2007) indicate that they are not of crustal origin, but crystallised within the mantle and experienced only minimal chemical interaction with the host magmas that transported them to the surface.
View in article


Vervoort, J.D., Patchett, P.J., Blichert-Toft, J., Albarède, F. (1999) Relationships between Lu-Hf and Sm-Nd isotopic systems in the global sedimentary system. Earth and Planetary Science Letters, 168, 79–99.
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Figure 3 [...] Mantle array line from Vervoort et al. (1999).
View in article


Wu, F.-Y., Yang, Y.-H., Mitchell, R.H., Li, Q.-L., Yang, J.-H., Zhang, Y.-B. (2010) In situ U-Pb age determination and Nd isotopic analysis of perovskites from kimberlites in southern Africa and Somerset Island, Canada. Lithos 115, 205–222.
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Figure 2
View in article


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Supplementary information


Materials and Methods


U-Pb geochronology
Megacryst zircons were mounted in resin blocks, sectioned and diamond-polished to remove any surface scratches. Mounts prepared in this way were then cleaned by ultrasonic agitation in dilute nitric acid before rinsing in ultrapure water and drying.

U-Pb analyses were conducted using an ASI RESOlution 193nm excimer laser ablation system, coupled to an Agilent 7700 quadrupole ICPMS. Ablation was performed in helium and the ablated sample then rapidly entrained in argon before leaving the sample cell to improve aerosol transport efficiency. Laser spot sizes were varied between 90 and 120 mm to provide sufficient count rates to enable measurement of all relevant isotopes for the construction of U-Pb Wetherill Concordia plots (Table S-1). The laser was operated with a repetition rate of 5 Hz and energy density ~3 J cm-2.

Data deconvolution were undertaken in the Iolite (Paton et al., 2011) environment using the method of correcting for downhole elemental fractionation described in detail in Paton et al. (2010). Most samples produced concordant analyses (Fig. S-1) and thus common Pb corrections were deemed unnecessary. Concordia ages were calculated using the IsoplotEx software (Ludwig, 2012).

Analyses were conducted over a number of analytical sessions. During this time analyses of multiple zircon reference materials (Temora, Plesovice, 91500) provided ages accurate to within 2 % of accepted values (417 Ma, 337 Ma, 1065 Ma respectively).


Figure S-1 U-Pb Concordia plots for zircons analysed in this study. Kimberlite megacryst zircons for Group A possess ‘well behaved’ U–Pb systematics allowing the calculation of Concordia ages (upper panel). In contrast all of the Group B zircon megacrysts (lower panel) show evidence of disturbance in the U–Pb system. This implies they are older and have been variably reset during entrainment into the host kimberlite.
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Table S-1 U-Pb isotope and trace element data for zircon samples used in this study. See Materials and Methods for analytical details and Figure S-1 for corresponding Concordia plots.
207Pb/235U2 se206Pb/238U2 seErr. Corr.U ppmTh ppmPb ppmU/Th
Mukurob
0.06410.00500.00890.00030.1713.765.950.132.64
0.06400.00520.00920.00030.0113.336.190.132.54
0.06340.00500.00910.00030.0513.055.920.122.59
0.06480.00780.00900.00030.0641.1614.590.382.96
0.06200.00840.00890.00030.0134.0511.890.263.06
0.06010.00700.00880.00030.0239.0314.070.293.01
0.05880.00720.00890.00030.0539.8314.710.322.97
0.05530.00800.00860.00030.1025.678.260.143.42
0.05570.01120.00900.00040.0517.498.030.162.40
0.05880.01060.00900.00040.1020.6510.680.232.13
0.05940.00980.00910.00040.0121.949.580.222.49
Deutche Erde
0.07240.00480.01040.00030.0215.9511.640.261.86
0.07040.00420.01060.00030.0917.1311.420.281.91
0.06970.00380.01050.00030.1220.9613.570.331.86
0.07190.00560.01080.00040.0420.3611.540.341.83
0.07290.00480.01050.00030.2528.0015.610.491.90
0.07140.00460.01060.00030.1620.4711.090.341.99
0.07570.00440.01050.00030.0925.5013.820.372.02
Silvery Home
0.08300.00140.01250.00010.20345.10375.0014.960.83
0.08330.00130.01250.00010.25406.00430.6018.070.81
0.08330.00190.01240.00020.17157.00127.505.500.99
0.08350.00220.01240.00020.03155.90117.805.291.01
0.08200.00190.01230.00020.21186.3097.004.381.39
0.08040.00260.01230.00020.0979.5045.401.521.88
0.08250.00200.01250.00020.31146.60152.805.871.01
0.08150.00460.01230.00030.0827.636.780.254.25
0.08170.00320.01240.00010.07172.50165.305.791.11
0.08050.00380.01230.00020.06135.40103.033.511.41
0.08250.00300.01240.00020.15202.80173.406.051.26
0.08000.00380.01220.00020.11153.20163.125.581.01
0.07800.00440.01220.00020.05104.1058.191.951.94
0.07960.00480.01250.00020.0586.1552.271.761.77
0.08100.00520.01250.00020.0279.8649.941.651.71
0.08200.00280.01250.00020.06291.30345.7012.380.89
0.08100.00300.01260.00020.13216.70178.406.351.27
0.08360.00320.01240.00020.21260.40255.509.161.06
0.08320.00340.01260.00020.09218.30211.807.561.06
0.08170.00360.01230.00020.16182.90190.606.810.98
0.08020.00440.01230.00020.25143.70125.894.551.16
0.08070.00360.01230.00020.14208.90217.607.710.97
Wesselton - Group B
0.11660.00300.01660.00010.18162.6075.544.112.26
0.11240.00360.01520.00010.25101.3545.672.322.42
0.11520.00240.01510.00010.18209.10102.415.232.33
0.10010.00420.01470.00020.0659.0672.413.140.97
0.10850.00260.01510.00010.16210.30138.516.561.92
0.09900.00340.01450.00020.07109.9087.503.551.69
0.10140.00240.01410.00010.11207.50538.7021.630.54
0.08940.00340.01390.00020.0680.53272.6010.050.43
0.11590.00340.01450.00010.13140.40324.2013.040.64
0.10490.00280.01430.00010.02184.80183.107.371.46
0.10050.00400.01480.00020.0660.0070.062.980.93
0.11940.00400.01600.00020.38186.10104.045.201.99
0.11250.00300.01590.00020.27165.0481.204.092.31
0.10290.00400.01550.00020.1082.4137.531.772.49
0.11860.00340.01580.00020.17134.4060.513.172.50
0.11450.00220.01610.00020.25259.40144.507.541.97
0.13970.00540.01500.00020.31132.67118.616.341.20
0.09840.00360.01460.00020.1088.9695.524.310.98
Wesselton - Group A
0.10150.01720.01350.00050.026.961.850.095.14
0.09350.01640.01390.00050.015.601.170.055.55
0.09500.01880.01370.00050.043.340.510.025.64
0.08810.01080.01380.00040.056.350.920.045.85
0.09310.00380.01340.00010.0648.2421.080.921.92
0.09190.00400.01350.00020.0934.4613.380.582.16
0.08920.00800.01350.00030.0210.091.600.075.28
0.10030.00520.01370.00020.1325.176.040.313.46
0.08990.00520.01360.00020.1330.688.210.383.73
0.08930.00360.01330.00020.1054.0724.561.042.22
0.09080.00660.01360.00030.0116.833.190.135.36
0.08890.00760.01350.00030.0112.252.130.095.90
0.08890.00640.01350.00030.0814.822.860.115.40
0.08770.00420.01330.00020.1433.559.770.433.60
0.08820.00360.01340.00020.0841.4713.930.603.16
0.08890.00320.01340.00020.1147.5420.410.892.55
DeBeers
0.09140.00600.01340.00040.189.743.310.113.63
0.09060.00680.01340.00040.118.953.130.083.55
0.09090.00600.01350.00040.2111.533.970.103.54
0.08960.00580.01310.00040.1112.324.040.143.54
0.09130.00600.01340.00040.189.773.310.113.63
0.09090.00680.01340.00040.118.983.140.083.55
0.09090.00600.01350.00040.2111.553.970.103.55
0.09210.00540.01380.00020.1774.9337.071.372.12
0.09110.00980.01420.00040.0526.4113.500.482.14
0.09790.01140.01420.00040.0224.3412.810.482.18
0.08240.01160.01320.00040.0721.308.390.213.40
0.09200.01080.01310.00040.0531.1612.200.293.46
0.09400.01300.01320.00050.0218.567.380.193.31
0.09600.01140.01330.00040.0425.059.290.293.33
0.08520.00800.01390.00040.0420.926.330.243.37
0.08720.00860.01380.00040.1222.086.700.283.36
0.09060.01140.01410.00040.0217.975.280.253.47
0.08670.01280.01390.00050.0115.274.520.173.46
0.08970.01360.01390.00050.0216.394.780.193.49
0.08370.00960.01350.00040.0315.804.730.203.41
0.09590.01360.01390.00040.0110.623.180.123.39
0.08320.01200.01380.00050.0210.643.210.133.35
0.08280.01460.01360.00060.0212.343.740.153.33
Dutiotspan
0.09950.01100.01340.00070.034.131.490.033.16
0.09180.01220.01360.00070.014.321.510.063.20
0.10440.01380.01320.00070.093.831.370.043.09
0.09900.01080.01320.00060.024.291.540.043.02
0.09280.01180.01360.00070.034.291.330.043.36
0.10900.06000.01360.00070.044.271.440.072.98
0.09000.04400.01400.00130.083.771.310.052.89
0.09300.04400.01360.00140.124.011.360.062.95
0.08800.02800.01410.00090.064.111.410.052.93
0.10600.05000.01420.00110.004.361.470.062.97
0.08340.01920.01400.00070.054.841.620.063.03
0.10200.02600.01380.00070.034.681.570.063.00
0.09700.02400.01390.00060.045.051.670.063.03
0.08400.02000.01380.00060.014.801.600.052.99
0.07900.02600.01380.00090.015.131.690.073.01
Monastery
0.09990.00820.01370.00060.056.723.220.092.72
0.10080.00640.01370.00040.0710.755.250.152.77
0.09600.00640.01380.00050.059.955.100.142.69
0.09210.00620.01360.00040.1010.485.320.142.72
0.08430.01320.01360.00040.015.461.350.054.11
0.08680.01420.01380.00050.075.801.470.054.08
0.09820.01280.01380.00040.066.652.160.082.83
0.08630.01100.01380.00040.026.191.970.072.78
0.08410.01360.01400.00050.016.691.660.084.06
0.08830.01540.01400.00050.005.461.380.053.99
0.09930.01920.01400.00070.035.251.400.054.00
0.08230.01740.01380.00060.145.141.120.054.86
0.08700.01500.01370.00050.014.691.050.044.30
0.09030.01620.01390.00050.004.300.900.044.58
0.10000.02000.01400.00060.023.060.600.034.91
0.09280.01700.01400.00050.024.070.930.044.28
0.09270.01340.01390.00050.066.431.620.054.03
0.09820.01260.01360.00040.038.123.070.122.63
0.09860.01700.01400.00050.065.011.230.054.15
0.09490.01500.01400.00050.045.491.210.064.58
0.09630.01380.01380.00050.026.171.470.074.18
0.09410.01060.01380.00040.068.323.330.132.51
0.09590.01500.01420.00050.065.941.460.054.11
0.09120.01540.01400.00050.055.981.480.074.06
0.09650.01700.01420.00050.005.181.200.054.35
0.10050.01620.01400.00050.055.391.350.064.00
0.09670.01660.01410.00050.014.951.210.054.14
0.08990.01520.01370.00050.015.391.290.064.26
0.09040.01240.01380.00050.026.321.750.073.69
0.09310.01180.01400.00050.007.481.930.083.92
0.09010.00960.01350.00040.045.912.070.093.17
0.08500.00980.01380.00040.045.151.930.082.99
0.08650.00940.01380.00040.027.072.630.112.81
Lethlakane
0.09630.00460.01460.00040.2722.936.670.253.93
0.09650.00520.01450.00040.0917.944.870.154.18
0.09910.00580.01450.00040.0117.014.600.164.07
0.09820.00540.01440.00040.0516.634.180.144.26
0.09100.01000.01420.00040.0225.486.480.264.00
0.08730.01200.01420.00050.1917.714.090.154.41
0.08710.01040.01430.00040.0224.626.530.263.84
0.08740.00920.01450.00040.0030.778.900.373.58
0.09070.00980.01440.00040.0127.217.540.293.77
0.08790.01080.01440.00040.0322.165.680.244.03
0.08920.00840.01430.00040.0035.7010.740.403.45
0.09190.00840.01430.00040.0135.6810.690.433.43
0.09300.01020.01440.00040.0629.157.280.304.10
Bultfontein
0.09880.00280.01460.00020.1374.9034.001.622.03
0.09840.00280.01470.00020.2880.9042.101.821.82
0.09740.00360.01480.00030.2349.8018.010.872.63
0.09660.00500.01470.00020.0481.0038.071.712.06
0.09770.00800.01480.00030.1023.106.430.293.48
0.09360.00560.01460.00030.1348.5716.170.742.84
0.09450.00500.01450.00030.1254.4419.860.932.60
0.09630.00640.01450.00030.0560.1623.221.042.46
0.09810.00380.01480.00020.0294.3049.762.191.79
0.10030.00460.01480.00020.0673.7031.251.352.25
Koffiefontein - Group B
0.12830.00980.01740.00040.0811.356.920.391.56
0.11610.00820.01670.00030.0311.526.750.361.61
0.11510.00860.01550.00040.0010.215.300.281.81
0.12920.00960.01910.00040.049.985.380.341.73
0.10520.00840.01570.00030.0010.735.710.291.76
0.11690.00860.01580.00030.0810.503.110.183.18
0.10910.00720.01540.00030.0416.274.550.243.36
0.10860.00640.01530.00030.0316.694.870.263.25
0.12680.00760.01780.00030.1016.304.490.273.44
0.11010.00620.01530.00020.0417.195.040.263.24
0.12510.01980.01590.00070.0010.094.950.272.02
0.09810.01640.01530.00060.0610.134.400.212.29
0.10790.01820.01500.00060.0612.194.980.212.45
0.14460.01900.02130.00080.0411.334.920.332.31
0.09590.01540.01560.00060.0212.345.320.232.33
0.11200.01740.01650.00060.1111.275.840.271.94
0.10060.01780.01530.00060.0010.805.510.201.99
0.08870.01520.01480.00050.0512.524.780.192.68
0.08960.01440.01540.00060.1113.725.650.222.49
0.09720.01340.01540.00050.0918.6411.590.431.65
Koffiefontein - Group A
0.11600.01180.01510.00040.0610.494.900.192.46
0.10780.00860.01490.00030.0417.158.230.312.45
0.11560.01160.01520.00040.0211.655.430.212.54
0.10810.00900.01500.00030.0216.507.660.292.56
0.11870.01020.01510.00030.0618.605.370.274.05
0.10770.00780.01520.00030.0320.475.840.253.92
0.10350.00700.01500.00030.0321.656.430.263.68
0.10570.00720.01480.00030.0721.478.740.362.62
0.10070.00480.01480.00020.0940.5517.680.692.38
0.11010.01020.01510.00030.0311.844.430.212.73
0.07860.01500.01440.00060.039.503.450.152.64
0.08900.01420.01450.00060.0311.894.220.182.68
0.09580.01660.01460.00060.019.513.350.142.72
0.09640.01700.01440.00060.0110.263.580.122.75
0.08880.01580.01450.00060.0210.763.240.133.20
0.09200.01540.01470.00060.0010.503.180.113.21
Orapa - Group A
0.09610.00600.01440.00040.1621.833.560.144.31
0.10750.01140.01520.00040.066.981.490.074.70
0.10630.01220.01520.00040.027.531.610.084.70
0.10320.00680.01490.00030.0119.094.900.233.89
0.10370.01260.01520.00040.068.181.910.094.32
0.11240.00820.01500.00030.0814.854.360.203.41
0.10670.01280.01510.00050.038.051.380.075.83
0.10960.01000.01490.00040.0312.403.490.163.59
0.11170.00840.01500.00030.0318.045.850.273.10
0.11140.01700.01550.00050.115.841.160.065.08
0.11060.01380.01510.00040.037.811.860.104.23
0.10510.00860.01510.00030.0315.063.680.184.12
0.10430.01200.01500.00040.029.742.120.114.63
0.11820.01180.01520.00040.119.291.980.144.70
0.10760.01340.01480.00040.017.781.840.094.25
0.10940.01140.01510.00040.0710.851.800.096.07
0.10490.01140.01490.00030.1110.561.720.076.18
0.10440.01020.01500.00040.037.062.080.112.92
0.10970.01180.01490.00040.046.812.030.092.92
Orapa - Group B
0.09680.00500.01430.00040.0829.736.010.274.22
0.11580.00920.01700.00030.0814.974.230.223.55
0.13050.01100.01880.00040.0614.363.880.223.73
0.11670.01020.01590.00040.0015.334.060.203.79
0.12080.00920.01550.00030.0215.734.530.243.50
0.15540.01080.02100.00040.1614.064.280.283.28
0.11740.00860.01570.00030.0114.294.300.213.36
0.16310.01120.01580.00030.1917.439.020.551.95
0.25600.02400.01700.00040.4811.3710.170.701.13
0.11180.00920.01550.00030.0512.287.160.301.73
0.16000.01020.02290.00040.1417.694.720.303.79
0.29050.01240.03780.00070.3216.584.590.453.64
0.15120.01020.02130.00040.0114.074.180.243.43
0.10280.00540.01500.00020.0530.6417.150.701.81
0.10440.00540.01520.00020.0627.8712.500.502.26
0.11550.00720.01640.00030.0918.367.450.342.51
Uintjesberg
0.10730.00520.01620.00040.1217.026.760.263.01
0.11040.01060.01630.00070.004.491.170.044.53
0.10920.00460.01630.00030.1529.7012.610.582.48
0.10640.00440.01590.00040.1333.2014.750.692.33
0.10780.00580.01610.00040.1219.466.010.263.33
0.11410.01300.01640.00090.036.901.990.073.53
0.14480.01960.01640.00080.328.691.950.144.50
0.10880.01480.01600.00060.0810.303.340.162.77
0.10660.01360.01600.00050.0111.493.730.222.74
0.10950.01480.01610.00050.0510.953.370.182.84
0.10140.01400.01620.00050.0110.883.470.202.72
0.09980.01360.01590.00050.0611.683.880.232.61
0.09310.01780.01630.00070.046.722.100.112.78
0.11000.02000.01650.00080.066.741.900.103.12
0.10430.01320.01640.00050.0413.524.460.242.70
0.12290.01840.01660.00060.1911.953.110.193.48
0.13300.01640.01630.00060.1012.164.300.252.62
0.09800.02400.01610.00080.075.001.410.063.37
0.10730.01940.01590.00070.108.492.300.103.60
Frank Smith
0.11570.00540.01770.00050.0921.074.470.254.42
0.11970.00660.01750.00050.0715.683.250.184.35
0.12420.00720.01760.00050.0616.603.180.204.57
0.12060.00680.01750.00060.0718.183.350.224.58
0.11330.01220.01780.00050.0513.003.050.134.26
0.12030.01140.01790.00040.0815.963.790.214.23
0.11890.01460.01820.00060.0412.562.960.164.23
0.12820.01260.01800.00050.0113.753.260.164.23
0.12100.01420.01830.00050.0111.252.670.134.22
0.13200.02000.01820.00050.0113.773.300.194.19
0.12350.01460.01790.00050.1013.013.080.174.20
0.11230.01480.01830.00060.0415.443.600.204.26
0.11780.01240.01790.00050.0113.323.120.164.26
0.12240.01600.01810.00060.0210.612.500.114.25
Download in Excel

Hf-isotope analyses
The same resin blocks and laser system described above were employed for Hf-isotope analyses except that, in this case, the laser was connected to a Nu Plasma MC-ICPMS. Laser spot sizes were varied between 70 and 90 mm in diameter and the laser was operated with a repetition rate of 5 Hz and energy density ~3 J cm-2.

Each analysis represents a different zircon grain and incorporated a 30 second baseline measurement followed by 60 seconds on peak. Mass bias and interference corrections were performed as described in detail in Woodhead et al. (2004). The weighted means of analyses for individual zircons were calculated using the IsoplotEx software (Table S-2).

Analyses were conducted over a number of analytical sessions. During this time analyses of multiple reference materials (Temora, Plesovice, BR266, QGNG, Mud Tank, Monastery) provided 176Hf/177Hf values within uncertainty of solution ICPMS values (Woodhead and Hergt, 2005).

Table S-2 Hf-isotope data for megacryst zircons. All data from this study unless otherwise noted: Griffin et al. (2000) or Nowell et al. (2004). Given the relatively small datasets involved, a Tukey Biweight robust mean has been employed to determine the central tendency in the Hf-isotope ratios for each suite. Parent daughter ratios are unsuited to this approach due to their inherently more scattered nature, the result of magmatic zonation: here a simple arithmetic mean is employed.
176Lu/177Hf2 seMean176Hf/177Hf2 seMean95 % conf.178Hf/177Hf2 seLu interferenceYb interferenceHf Beam


(unweighted)

(Robust-Tukey)


(ppm on 176)(ppm on 176)(volts)
Mukurob
0.0000880.00000050.0000630.2828160.0000340.2828100.0000241.4673690.0000643101365112.2
0.0000770.0000007
0.2827940.000029

1.4673770.0000532721167412.4
0.0001060.0000007
0.2828380.000034

1.4673630.0000613721668611.9
0.0000380.0000005
0.2827890.000031

1.4673770.000053133621312.3
0.0000370.0000006
0.2828170.000030

1.4674030.000055130602012.5
0.0000330.0000005
0.2827870.000036

1.4673840.000063118550912.4
Deutche Erde
0.0000290.00000050.0000310.2827620.0000280.2827700.0000331.4673470.000054103493715.5
0.0000350.0000004
0.2827720.000033

1.4673740.000060125604915.4
0.0000310.0000005
0.2827590.000037

1.4673340.000062111540415.2
0.0000410.0000004
0.2828010.000034

1.4674260.000066144711215.3
0.0000210.0000004
0.2827530.000032

1.4673710.00006174358915.1
Silvery Home
0.0001530.00000030.0001400.2827140.0000360.2827030.0000071.4675150.0000635312438014.3
0.0001320.0000004
0.2827080.000042

1.4674260.0000714592130012.0
0.0001350.0000003
0.2827060.000039

1.4674960.0000714702146212.4
0.0000510.0000002
0.2827140.000031

1.4674730.000057181940117.4
0.0001650.0000002
0.2826670.000035

1.4674790.0000645702788114.3
0.0000750.0000003
0.2826770.000039

1.4674530.0000732651169412.8
0.0001640.0000002
0.2826840.000035

1.4674840.0000605662857013.4
0.0001680.0000003
0.2826910.000037

1.4674550.0000665792984012.3
0.0002090.0000003
0.2827220.000032

1.4674460.0000627183556014.2
0.0000520.0000002
0.2827370.000033

1.4674510.000062184951417.2
0.0001400.0000005
0.2826640.000032

1.4674100.0000664872628014.7
0.0001640.0000005
0.2826890.000035

1.4674640.0000655682872013.7
0.0001910.0000005
0.2826820.000037

1.4674580.0000696573357214.0
0.0002650.0000005
0.2827090.000040

1.4674220.0000668974865014.1
0.0000400.0000006
0.2827050.000032

1.4674050.000057141652015.3
0.0001280.0000005
0.2827020.000032

1.4674400.0000584452302915.0
0.0001570.0000004
0.2826920.000033

1.4674250.0000565432906014.2
0.0001560.0000006
0.2826930.000030

1.4674240.0000545382871814.2
0.0001470.0000005
0.2827440.000031

1.4674120.0000635092774014.9
0.0002150.0000010
0.2827240.000035

1.4674250.0000657383776014.3
0.0000840.0000004
0.2827320.000032

1.4674060.0000592951580416.9
0.0000810.0000004
0.2827030.000032

1.4674210.0000592821512916.8
Wesselton - Group B
0.0002100.00000020.0001420.2822350.0000340.2822200.0000111.4673770.0000677243387014.8
0.0002100.0000002
0.2822220.000033

1.4674320.0000657243345016.5
0.0002180.0000003
0.2822350.000041

1.4674300.0000707503538116.0
0.0001480.0000002
0.2822010.000041

1.4673660.0000725142441613.0
0.0000410.0000002
0.2822010.000035

1.4673580.000068145628115.9
0.0001160.0000003
0.2822310.000033

1.4673370.0000644061835716.9
0.0001680.0000003
0.2822060.000042

1.4674540.0000745832694513.9
0.0000760.0000001
0.2822170.000035

1.4674150.0000692671178119.1
0.0001020.0000002
0.2822040.000034

1.4674180.0000653591588417.8
0.0001970.0000003
0.2822340.000038

1.4674560.0000686793125716.4
0.0001190.0000005
0.2822180.000032

1.4673640.0000604151840618.0
0.0001040.0000005
0.2822630.000037

1.4674460.0000673651676617.4
Wesselton - Group A
0.0000440.00000050.0000300.2827680.0000340.2827400.0000631.4674210.000066155702314.7
0.0000330.0000004
0.2827440.000031

1.4673310.000060118518315.4
0.0000130.0000004
0.2827160.000034

1.4673740.00006047209914.9
De Beers
0.0000490.00000040.0000400.2826180.0000310.2826370.0000331.4673450.000058173787114.5
0.0000410.0000007
0.2826390.000030

1.4673640.000062146650214.5
0.0000330.0000005
0.2826320.000031

1.4674000.000062117528913.7
0.0000310.0000004
0.2826490.000031

1.4673760.000056111498814.1
0.0000160.0000004
0.2826330.000033

1.4673770.00006357245914.8
0.0000300.0000005
0.2826550.000032

1.4673970.000060105448514.7
0.0000370.0000005
0.2826590.000031

1.4674010.000059131595011.8
0.0000710.0000006
0.2826180.000039

1.4673810.0000622501114011.1
0.0000510.0000006
0.2826330.000034

1.4673960.000060180815211.0
0.0000430.0000006
0.2826390.000038

1.4673980.000068152669911.5
DuToitspan
0.0000070.00000010.0000090.2827580.0000390.2827470.0000101.4674060.00007725117620.0
0.0000070.0000002
0.2827500.000031

1.4673960.00006625125118.3
0.0000080.0000002
0.2827560.000029

1.4674010.00005627129419.7
0.0000080.0000002
0.2827530.000037

1.4673960.00006827133318.1
0.0000080.0000002
0.2827670.000027

1.4673840.00005629140620.1
0.0000070.0000001
0.2827340.000035

1.4673810.00007224116220.8
0.0000070.0000002
0.2827730.000031

1.4674530.00006224115221.6
0.0000070.0000002
0.2827570.000034

1.4673730.00006824119318.6
0.0000080.0000002
0.2827500.000038

1.4673700.00006928139217.8
0.0000080.0000002
0.2827630.000035

1.4674120.00006529138617.7
0.0000070.0000003
0.2827320.000029

1.4673910.00005625128321.6
0.0000040.0000002
0.2827280.000031

1.4673490.0000631362521.1
0.0000190.0000003
0.2827180.000037

1.4673770.00006668320015.9
0.0000210.0000004
0.2827130.000032

1.4673540.00006175347615.6
0.0000100.0000003
0.2827430.000030

1.4673470.00006236180017.8
0.0000100.0000003
0.2827550.000033

1.4673660.00006237187017.3
Monastery
0.0000370.00000020.0000140.2827030.0000310.2827210.0000071.4674800.000057130625218.0
0.0000370.0000002
0.2827130.000035

1.4674670.000064132630918.1
0.0000360.0000002
0.2827000.000033

1.4674280.000061126619316.2
0.0000270.0000002
0.2827090.000026

1.4674790.00005995459418.0
0.0000210.0000002
0.2827040.000031

1.4674720.00005773352519.0
0.0000160.0000002
0.2827130.000027

1.4675190.00005658276719.8
0.0000050.0000002
0.2827390.000035

1.4674670.0000641887018.3
0.0000050.0000002
0.2827150.000031

1.4675260.0000521886121.2
0.0000130.0000002
0.2827400.000032

1.4675120.00005948227119.8
0.0000130.0000002
0.2827320.000031

1.4674790.00005546222618.5
0.0000060.0000003
0.2827210.000033

1.4674240.0000652091218.0
0.0000060.0000002
0.2827450.000032

1.4674420.00005822100319.6
0.0000050.0000002
0.2827220.000030

1.4674460.0000601989618.8
0.0000050.0000002
0.2827310.000037

1.4674040.0000721988116.7
0.0000050.0000002
0.2827120.000032

1.4673980.0000592090416.6
0.0000050.0000002
0.2827480.000036

1.4673930.0000661985518.6
0.0000050.0000003
0.2827020.000031

1.4673410.0000571885916.7
0.0000050.0000004
0.2827260.000030

1.4673930.0000582090716.7
0.0000060.0000002
0.2827230.000039

1.4675520.0000662099312.2
0.0000130.0000003
0.2827320.000040

1.4675950.00007645229712.1
Monastery - Nowell et al. (2004)
0.000006
0.0000090.2827240.0000060.2827250.000006




0.000007

0.2827350.000006






0.000010

0.2827280.000006






0.000001

0.2827130.000006






0.000001

0.2827160.000006






0.000005

0.2827370.000006






0.000011

0.2827230.000006






0.000010

0.2827300.000006






0.000020

0.2827180.000006






0.000022

0.2827230.000006






Monastery - Griffin et al. (2000)
0.000006
0.0000090.2827120.0000140.2827030.000004




0.000007

0.2827070.000011






0.000007

0.2826990.000017






0.000006

0.2827250.000014






0.000006

0.2827170.000012






0.000008

0.2827150.000013






0.000007

0.2827140.000013






0.000012

0.2826960.000015






0.000012

0.2827160.000015






0.000014

0.2826850.000018






0.000015

0.2827000.000019






0.000014

0.2826960.000020






0.000004

0.2826940.000020






0.000003

0.2826930.000018






0.000010

0.2827170.000012






0.000007

0.2826910.000015






0.000005

0.2827000.000016






0.000013

0.2827400.000015






0.000013

0.2827230.000015






0.000007

0.2827220.000022






0.000004

0.2827070.000019






0.000005

0.2826610.000020






0.000004

0.2826960.000012






0.000004

0.2826980.000015






0.000006

0.2826960.000010






0.000013

0.2827050.000030






0.000005

0.2826850.000017






0.000005

0.2826840.000019






0.000012

0.2827020.000017






0.000003

0.2826850.000024






0.000014

0.2826740.000017






0.000014

0.2827060.000016






0.000018

0.2827030.000017






0.000004

0.2827080.000022






0.000007

0.2827310.000015






0.000011

0.2827100.000022






0.000004

0.2827040.000013






0.000017

0.2827170.000024






0.000014

0.2826950.000020






0.000006

0.2827060.000017






0.000006

0.2826910.000017






0.000012

0.2826790.000014






Lethlakane
0.0000190.00000020.0000190.2827430.0000350.2827320.0000081.4673540.00007067292917.2
0.0000160.0000002
0.2827330.000042

1.4673630.00007256247614.4
0.0000210.0000002
0.2827290.000033

1.4673760.00006676338315.8
0.0000210.0000002
0.2827560.000037

1.4674190.00006776338417.2
0.0000200.0000002
0.2827420.000036

1.4674130.00006972307817.5
0.0000210.0000003
0.2827580.000036

1.4673880.00006674324517.9
0.0000240.0000003
0.2827480.000035

1.4674280.00006684374817.5
0.0000200.0000002
0.2827550.000037

1.4673690.00006772315017.2
0.0000220.0000003
0.2827340.000038

1.4673800.00007478347515.8
0.0000260.0000003
0.2827370.000040

1.4673530.00007492436113.8
0.0000270.0000003
0.2827470.000041

1.4673970.00007496421612.4
0.0000170.0000004
0.2827320.000025

1.4674510.00005460263017.2
0.0000160.0000004
0.2827130.000030

1.4674050.00006056248317.4
0.0000170.0000004
0.2826960.000028

1.4674010.00005461275619.5
0.0000100.0000003
0.2827160.000027

1.4674550.00005337165820.1
0.0000140.0000004
0.2827280.000029

1.4673980.00005549219417.8
0.0000150.0000004
0.2827250.000030

1.4674580.00006354236117.9
Bultfontein
0.0000230.00000050.0000360.2826660.0000380.2826890.0000081.4674270.00007780356915.3
0.0000160.0000005
0.2826720.000033

1.4674330.00006358261615.9
0.0000460.0000006
0.2826700.000030

1.4673830.000060164731315.3
0.0000330.0000005
0.2826800.000028

1.4674260.000053118513415.4
0.0000150.0000005
0.2826900.000032

1.4673810.00006052230915.2
0.0000160.0000006
0.2826690.000031

1.4674250.00005958261315.0
0.0000300.0000002
0.2826850.000053

1.4673020.000092106472416.6
0.0000670.0000002
0.2826870.000053

1.4673300.0001002361122816.1
0.0000230.0000002
0.2826900.000059

1.4673100.00011081403017.0
0.0000130.0000002
0.2826850.000051

1.4673420.00009845208518.4
0.0000340.0000002
0.2827120.000046

1.4673940.000091120526017.5
0.0000620.0000003
0.2827300.000058

1.4672900.0001002191045617.8
0.0000590.0000003
0.2827070.000051

1.4673430.000095209997317.5
0.0000240.0000002
0.2826940.000043

1.4673740.00008786379016.3
0.0000160.0000002
0.2827070.000053

1.4673300.00011056263218.1
0.0000720.0000002
0.2826630.000057

1.4672800.0001002551184715.0
0.0000350.0000003
0.2826400.000062

1.4674200.000110125543116.2
0.0000100.0000003
0.2826670.000046

1.4673500.00009136164018.5
0.0000280.0000003
0.2826970.000049

1.4673710.00009099440817.3
0.0000640.0000003
0.2826690.000052

1.4673580.0000972251007615.7
0.0000440.0000003
0.2826880.000051

1.4673010.000099154697914.9
0.0000550.0000003
0.2826950.000049

1.4672960.000097194884814.6
0.0000090.0000002
0.2827390.000051

1.4673040.00009834180421.2
0.0000140.0000002
0.2827430.000054

1.4673490.00009549234218.4
0.0001130.0000003
0.2826770.000047

1.4673730.0000973951947316.1
0.0000490.0000003
0.2826990.000067

1.4673800.000120172795917.6
0.0000090.0000002
0.2827280.000073

1.4672500.00014032170022.0
0.0000380.0000003
0.2826800.000043

1.4672820.000079135618214.4
0.0000200.0000002
0.2826870.000042

1.4673590.00007872339017.5
0.0000360.0000003
0.2826720.000060

1.4673100.000110128591614.0
0.0000400.0000003
0.2827200.000054

1.4673030.000097142678815.7
0.0000410.0000003
0.2827070.000042

1.4673320.000082147678913.3
0.0000350.0000002
0.2826660.000046

1.4673730.000080124540615.5
0.0000460.0000002
0.2826620.000044

1.4673210.000086163741914.5
Koffiefontein - Group A
0.0000190.00000050.0000260.2827120.0000310.2827100.0000111.4673610.00005966303916.5
0.0000190.0000004
0.2827210.000031

1.4673770.00006069316416.3
0.0000130.0000004
0.2827060.000027

1.4674030.00005346202415.9
0.0000130.0000004
0.2827040.000032

1.4673710.00005848212315.9
0.0000130.0000002
0.2826930.000032

1.4674210.00005848210019.1
0.0000160.0000002
0.2826770.000035

1.4673840.00006658268821.6
0.0000350.0000002
0.2827260.000032

1.4674340.000052123545817.8
0.0000240.0000002
0.2826960.000032

1.4674200.00005784354218.4
0.0000160.0000002
0.2827200.000035

1.4674220.00006756244722.5
0.0000620.0000002
0.2826870.000037

1.4674050.000072219979118.5
0.0000540.0000002
0.2827220.000039

1.4674620.000069191837118.3
Koffiefontein - Group B
0.0000140.00000020.0000130.2823140.0000370.2822700.0000471.4674480.00007351201320.1
0.0000200.0000003
0.2823070.000055

1.4674400.00011071349113.6
0.0000170.0000002
0.2822700.000071

1.4674000.00013059331817.1
0.0000080.0000004
0.2822160.000030

1.4673690.00005627120118.3
0.0000070.0000004
0.2822470.000029

1.4673660.00005624109118.8
0.0000150.0000001
0.2822380.000032

1.4674220.00006252238022.8
Orapa - Group A
0.0000180.00000020.0000150.2827400.0000330.2827250.0000101.4674540.00006265274518.1
0.0000140.0000002
0.2827350.000036

1.4674430.00006450210916.3
0.0000190.0000002
0.2827670.000034

1.4675050.00006468285519.1
0.0000080.0000002
0.2827290.000032

1.4674490.00006329125117.3
0.0000090.0000002
0.2827230.000034

1.4673860.00006634147917.0
0.0000110.0000002
0.2827560.000034

1.4674950.00006238165417.5
0.0000150.0000002
0.2827600.000034

1.4674570.00006454235118.7
0.0000200.0000002
0.2827320.000032

1.4674360.00005973308419.5
0.0000130.0000003
0.2827420.000033

1.4674120.00005946195919.2
0.0000120.0000002
0.2827360.000035

1.4674210.00006343183118.9
0.0000150.0000004
0.2827080.000027

1.4673910.00005755254118.6
0.0000180.0000005
0.2827120.000031

1.4674070.00005963292017.8
0.0000090.0000004
0.2827140.000032

1.4674220.00006034155318.2
0.0000090.0000004
0.2827100.000029

1.4673590.00005431142518.4
0.0000240.0000004
0.2827020.000031

1.4673760.00006084384016.0
0.0000250.0000005
0.2826940.000030

1.4674130.00006290420415.3
0.0000030.0000005
0.2826670.000029

1.4673640.0000531156820.0
0.0000130.0000006
0.2827010.000032

1.4674260.00006046206617.9
0.0000190.0000004
0.2827040.000029

1.4673960.00005468303317.9
0.0000050.0000003
0.2826870.000027

1.4673650.0000501779418.8
0.0000150.0000004
0.2826880.000028

1.4673550.00005553231416.5
0.0000160.0000006
0.2827240.000030

1.4673880.00005555235012.6
0.0000090.0000001
0.2827300.000040

1.4674410.00007731133920.3
0.0000090.0000001
0.2827670.000038

1.4674490.00007031135724.8
0.0000270.0000001
0.2827280.000036

1.4674310.00006695416319.1
0.0000310.0000002
0.2827490.000034

1.4674280.000067110457420.2
0.0000280.0000002
0.2827220.000036

1.4674160.00006898446619.0
0.0000050.0000002
0.2827290.000037

1.4674560.0000691877920.9
0.0000140.0000002
0.2827700.000031

1.4674500.00006051220023.2
0.0000190.0000002
0.2827520.000034

1.4673980.00006468288517.5
0.0000060.0000002
0.2826950.000031

1.4674180.0000572390817.3
Orapa - Group B
0.0000120.00000020.0000110.2823000.0000340.2823200.0000311.4674590.00006243186620.1
0.0000100.0000003
0.2822540.000078

1.4674000.00013034183714.7
0.0000210.0000003
0.2823470.000066

1.4674200.00012074398614.1
0.0000170.0000002
0.2822630.000075

1.4674200.00014060320113.4
0.0000070.0000002
0.2823370.000069

1.4674700.00013026141215.2
0.0000040.0000003
0.2823460.000056

1.4674400.0001101470713.6
0.0000030.0000002
0.2823620.000041

1.4674790.0000751251623.8
0.0000050.0000001
0.2823810.000033

1.4674620.0000631982424.7
0.0000190.0000001
0.2823400.000034

1.4674260.00006466284420.6
0.0000080.0000001
0.2822950.000034

1.4674250.00005929127224.4
Orapa - Group A - Nowell et al. (2004)
0.000024
0.0000200.2827660.0000080.2827500.000007




0.000031

0.2827450.000010






0.000016

0.2827520.000011






0.000017

0.2827550.000008






0.000019

0.2827450.000014






0.000016

0.2827380.000012






0.000021

0.2827500.000006






0.000021

0.2827780.000008






0.000027

0.2827240.000010






0.000036

0.2827600.000007






0.000015

0.2827570.000012






0.000019

0.2827630.000012






0.000018

0.2827540.000019






0.000018

0.2827630.000014






0.000021

0.2827550.000011






0.000025

0.2827440.000013






0.000008

0.2827260.000013






0.000005

0.2827230.000010






0.000023

0.2827620.000014






0.000017

0.2827320.000011






Orapa - Group B - Nowell et al. (2004)
0.000007
0.0000090.2823660.0000100.2823300.000021




0.000006

0.2823610.000007






0.000008

0.2823430.000011






0.000014

0.2823590.000010






0.000007

0.2823030.000014






0.000011

0.2823050.000015






0.000008

0.2823140.000014






0.000006

0.2822930.000015






0.000009

0.2823390.000011






0.000010

0.2823540.000016






Orapa - Group A - Griffin et al. (2000)
0.000009
0.0000430.2827280.0000140.2827100.000013




0.000009

0.2827260.000024






0.000017

0.2827110.000018






0.000015

0.2827090.000024






0.000020

0.2826820.000022






0.000012

0.2827070.000034






0.000013

0.2827230.000015






0.000039

0.2827330.000017






0.000031

0.2825520.000022






0.000028

0.2825590.000030






0.000004

0.2826790.000020






0.000007

0.2827070.000019






0.000246

0.2831260.000030






0.000156

0.2830410.000024






Orapa - Group B - Griffin et al. (2000)
0.000007
0.0000150.2823470.0000240.2822540.000006




0.000006

0.2823280.000018






0.000027

0.2822590.000026






0.000027

0.2822490.000020






0.000014

0.2822510.000024






0.000009

0.2822560.000020






Uintjiesberg
0.0000130.00000020.0000280.2825490.0000320.2826600.0000231.4674160.00006147185016.3
0.0000060.0000002
0.2825570.000038

1.4674480.0000672393418.5
0.0000400.0000003
0.2826320.000040

1.4674030.000070143607711.0
0.0000430.0000004
0.2826410.000040

1.4673230.000070153745011.0
0.0000220.0000003
0.2826720.000037

1.4674440.00007076345011.8
0.0000760.0000006
0.2827000.000037

1.4674410.0000682661143014.0
0.0000100.0000003
0.2826860.000033

1.4674210.00006136151814.6
0.0000410.0000003
0.2826730.000042

1.4674290.000077144609311.9
0.0000440.0000003
0.2827020.000042

1.4673920.000076155726911.3
0.0000340.0000003
0.2826780.000033

1.4674090.000060122588813.5
0.0000370.0000006
0.2826830.000034

1.4674320.000063131624313.4
0.0000100.0000003
0.2826620.000036

1.4674170.00006736170918.3
0.0000210.0000004
0.2826940.000031

1.4674270.00005576365417.8
0.0000110.0000004
0.2826550.000029

1.4673860.00005240190918.5
0.0000120.0000004
0.2826750.000032

1.4674160.00006242205218.5
Frank Smith
0.0000260.00000030.0000270.2826000.0000390.2826000.0000111.4674070.00006892388613.3
0.0000250.0000002
0.2825930.000033

1.4674300.00006387370614.1
0.0000250.0000003
0.2826110.000035

1.4673190.00006190383712.7
0.0000270.0000003
0.2825990.000038

1.4673590.00006997417311.9
0.0000240.0000003
0.2825630.000039

1.4673470.00007384360413.1
0.0000320.0000004
0.2826170.000043

1.4674380.000077114481913.2
0.0000290.0000003
0.2826320.000039

1.4674460.000065102432913.7
0.0000300.0000003
0.2826300.000037

1.4674350.000068107451113.7
0.0000300.0000003
0.2826020.000033

1.4674340.000066106443514.0
0.0000260.0000003
0.2826120.000038

1.4673880.00007494399113.6
0.0000320.0000006
0.2825760.000031

1.4673460.000056115496712.0
0.0000220.0000006
0.2825980.000031

1.4673980.00005679349912.3
0.0000280.0000006
0.2825880.000035

1.4673700.00006399432912.3
0.0000250.0000006
0.2825770.000031

1.4673520.00005888389912.3
0.0000270.0000005
0.2825990.000031

1.4673360.00006197424512.0
0.0000240.0000005
0.2825750.000032

1.4673180.00005985372912.3
Kaalvallie - Nowell et al. (2004)
0.000004
0.0000100.2827380.0000170.2827510.000009




0.000004

0.2827170.000017






0.000004

0.2827180.000018






0.000003

0.2827340.000015






0.000008

0.2827480.000022






0.000008

0.2827720.000024






0.000007

0.2827700.000020






0.000007

0.2827280.000021






0.000006

0.2827720.000025






0.000006

0.2827370.000024






0.000010

0.2827170.000025






0.000016

0.2827580.000004






0.000016

0.2827450.000004






0.000018

0.2827630.000003






0.000017

0.2827520.000004






0.000013

0.2827510.000004






0.000013

0.2827540.000005






0.000018

0.2827740.000005






0.000017

0.2827740.000006






0.000016

0.2827730.000004






0.000015

0.2827630.000003






0.000006

0.2827480.000004






0.000006

0.2827470.000004






Kamfersdam - Nowell et al. (2004)
0.000022
0.0000220.2827210.0000100.2827210.000010




Mothae pipe - Nowell et al. (2004)
0.000008
0.0000120.2827110.0000070.2827180.000008




0.000015

0.2827180.000007






0.000007

0.2827230.000007






0.000019

0.2827180.000007






Gansfontein - Nowell et al. (2004)
0.000009
0.0000120.2827010.0000060.2827090.000006




0.000010

0.2827100.000004






0.000019

0.2827270.000007






0.000015

0.2827120.000006






0.000015

0.2827150.000005






0.000010

0.2827050.000004






0.000008

0.2827040.000008






0.000010

0.2827020.000010






Leicester - Griffin et al. (2000)
0.000006
0.0000240.2825670.0000220.2825700.000049




0.000006

0.2825670.000022






0.000006

0.2825360.000022






0.000039

0.2826660.000022






0.000046

0.2826680.000024






0.000020

0.2826340.000028






0.000006

0.2824950.000026






0.000005

0.2825340.000017






0.000005

0.2825270.000019






0.000083

0.2824540.000026






0.000040

0.2826190.000026






Download in Excel

Nd-isotope analyses
The single greatest impediment to Nd-isotope analysis of megacryst zircons is the initial dissolution of large quantities of material with low U content (zircons with low lattice radiation damage are notoriously hard to dissolve). For each sample, around 100 mg of crushed and powdered zircon was subjected to leaching in hot 2 M HCl to remove any contaminant phases and then washed, dried and weighed into high-pressure Teflon digestion vessels. The zircons were dissolved gradually over a period of several weeks with combinations of HF, HCl and HNO3 acids at 220 ˚C. After periods of several days at this temperature, liquid (with some undissolved sample) was removed from each vessel, collected, and replaced with fresh acid. This procedure eventually produced complete dissolution of the entire powder and, for each sample, the various dissolution steps could be combined and processed further.

At this stage a small aliquot was taken for trace element analysis employing our Agilent 7700 quadrupole ICPMS, following techniques outlined in Eggins et al. (1997). This allowed determination of optimum spiking of the main solution with a 149Sm-150Nd mixed tracer. After a period of equilibration in a sealed Teflon vessel on the hotplate the sample plus tracer solution was dried down and Sm-Nd separated using conventional ion exchange procedures. Both elements were run on a Nu Plasma MC-ICPMS operating in static multi-collection mode with sample introduction using an Aridus desolvating unit and Glass Expansion OpalMist nebuliser operating at ~50 ml min-1 uptake. This produced Nd signals of 5–10 V for each sample. 143Nd/144Nd ratios were normalised to La Jolla = 0.511860, using reference material analyses interspersed with the samples. Typical internal (2 se) precisions of ≤±0.000012 and external (2sd) reproducibility of ±0.000020 were achieved. External reproducibility on the 147Sm/144Nd ratio was ±0.2 %. The 147Sm decay constant used in calculation of initial values was 6.54 10-12. Blank correction calculations produced shifts well within analytical uncertainty.

Calculation and Interpretation of Ti-in-Zircon Temperatures


Ti-in-zircon temperatures for the compositions of zircon megacrysts were calculated using the formulations of Ferry and Watson (2007), as in Table S-3. These formulations also require input of values for SiO2 and TiO2 activity and pressure. Our zircon megacrysts contain variable, but generally low concentrations of Ti (4 to 24 ppm with a single outlier showing 63 ppm; Table S-3). SiO2 activity was assumed to be buffered by olivine (Mg# = 84–88) – orthopyroxene (Mg# = 85–89) equilibrium in the ambient mantle (T = 1200–1400 °C; P ~ 5 GPa) surrounding the megacrysts before kimberlite entrainment, by using the formulation of O’Neill and Wall (1987). Compositions of olivine and orthopyroxene megacrysts and equilibration conditions of silicate megacrysts are based upon analyses from the Monastery (Gurney et al., 1979) and Jagersfontein (Hops et al., 1992) kimberlite. Modification of P, T, Mg#olivine and Mg#orthopyroxene input values in the calculation of SiO2 activity change the final calculated temperatures by <50 °C. The TiO2 activity was assumed buffered by crystallisation of ilmenite (i.e. aTiO2 ~ 0.9 ~ XTi in the tetrahedral site of ilmenite), which has previously been shown to co-precipitate with zircon in the kimberlite megacrysts suite (Moore et al., 1992). However, the dependence of calculated temperatures on TiO2 activity is minimal (<20 °C).

Table S-3 Calculated temperatures based on Ti-in-zircon using the formulation of Ferry and Watson (2007). See Materials and Methods and Figure S-2 for further information and our interpretation of these results.
SampleTi (ppm)log Ti (ppm)log aSiO2log aTiO2
~log XTi in rutile
Ferry and Watson (2007) extrapolated to P =




or ~log XTi in ilmenite 4+ site1 GPa2 GPa3 GPa4 GPa5 GPa6 GPa





T (°C)T (°C)T (°C)T (°C)T (°C)T (°C)



minimum aSiO2






Koffiefontein8.90.95-0.71-0.05611661711761811861
Koffiefontein15.61.19-0.71-0.05652702752802852902
Koffiefontein9.40.97-0.71-0.05614664714764814864
Mukurob4.10.61-0.71-0.05559609659709759809
Mukurob14.71.17-0.71-0.05648698748798848898
Bultfontein63.31.8-0.71-0.057758258759259751025
Bultfontein24.21.38-0.71-0.05688738788838888938
Bultfontein3.70.57-0.71-0.05552602652702752802
De Beers10.71.03-0.71-0.05624674724774824874














maximum aSiO2






Koffiefontein8.90.95-1.01-0.05565615665715765815
Koffiefontein15.61.19-1.01-0.05602652702752802852
Koffiefontein9.40.97-1.01-0.05568618668718768818
Mukurob4.10.61-1.01-0.05518568618668718768
Mukurob14.71.17-1.01-0.05598648698748798848
Bultfontein63.31.8-1.01-0.05711761811861911961
Bultfontein24.21.38-1.01-0.05634684734784834884
Bultfontein3.70.57-1.01-0.05512562612662712762
De Beers10.71.03-1.01-0.05577627677727777827














intermediate aSiO2






Koffiefontein8.90.95-0.86-0.05587637687737787837
Koffiefontein15.61.19-0.86-0.05627677727777827877
Koffiefontein9.40.97-0.86-0.05591641691741791841
Mukurob4.10.61-0.86-0.05538588638688738788
Mukurob14.71.17-0.86-0.05622672722772822872
Bultfontein63.31.8-0.86-0.05742792842892942992
Bultfontein24.21.38-0.86-0.05660710760810860910
Bultfontein3.70.57-0.86-0.05532582632682732782
De Beers10.71.03-0.86-0.05600650700750800850
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Pressure significantly affects the calculated Ti-in-zircon temperatures (~50 °C/GPa). Thermobarometry measurements of megacrysts in southern African and Canadian kimberlites have previously shown that silicate megacrysts (i.e. garnet, clinopyroxene, orthopyroxene and olivine) crystallised at pressures above 4.0–4.5 GPa – (Gurney et al., 1979, Hops et al., 1992) and, perhaps, up to 7.0 GPa (Kopylova et al., 2009). When P values of 4.0 GPa or higher are applied to calculate Ti-in-zircon temperatures for the southern African megacrysts, however, the resulting temperatures are much lower than the ambient temperatures at those depths given by typical cratonic geotherms of 40–42 mW/m2 (Fig. S-2). The difference is exacerbated if a hotter, non-cratonic continental geotherm is selected, which might be more representative for off-craton kimberlite megacrysts. The zircon megacrysts would reflect ambient cratonic mantle conditions only if they equilibrated at P of ~2.5–3.0 GPa, corresponding to calculated temperatures of ~650–750 °C. This range is very similar to the T interval calculated by Page et al. (2007) for zircon megacrysts from the Kaapvaal craton.


Figure S-2 Calculated temperature intervals for zircon megacrysts. Temperature values calculated at variable pressure using the Ti-in-zircon thermometer of Ferry and Watson (2007) and Ti concentrations of megacrysts in southern African archetypal kimberlites. Note that, if the Ti-in-zircon temperatures truly reflect the equilibration conditions of zircon megacrysts and assuming a typical cratonic mantle geotherm of 40–42 mW/m2, the zircon megacrysts could only be in equilibrium with the ambient mantle at P of 2.5–3.0 GPa.
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These results can be interpreted in two different ways. It is possible that the Ti-in-zircon temperatures reflect ‘true’ crystallisation (and equilibration) conditions for zircon megacrysts. However, this contrasts with the much higher equilibration temperatures recorded by other megacrysts from South-African kimberlite pipes (e.g., Monastery – Gurney et al., 1979, Jagersfontein – Hops et al., 1992), coupled with the interpretation that kimberlite megacrysts crystallise from magma batches that evolve and crystallise in situ (Moore et al., 1992, Page et al., 2007), i.e. without migrating over long distances. Alternatively, we note that Fu et al. (2008) compiled the Ti composition of ~500 terrestrial zircons from igneous and mantle rocks worldwide and demonstrated that the Ti-in-zircon thermometer largely underestimates the temperature of mafic igneous and mantle rocks provided by other independent geothermometers. Therefore, we prefer to conclude that the Ti-in-zircon megacrysts temperatures are not accurate in this particular setting and should be not considered in any petrogenetic model of zircon megacrysts in kimberlites.

Supplementary Information References


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Fu, B, Page, F.Z., Cavosie, A.J., Fournelle, J., Kita, N.T., Lackey, J.S., Wilde, S.A., Valley, J.W. (2008) Ti-in-zircon thermometry: applications and limitations. Contributions to Mineralogy and Petrology 156, 197–215.


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