Earth and Planetary Science Letters 418 (2015) 27–39

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Earth and Planetary Science Letters

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Ancient mantle metasomatism recorded in subcalcic garnet

xenocrysts: Temporal links between mantle metasomatism,diamond growth and crustal tectonomagmatism

Qiao Shu a,b,c,∗, Gerhard P. Brey b

a State Key Laboratory of Geological Processes and Mineral Resources, and School of Earth Science and Mineral Resources, China University of Geosciences,Beijing 100083, Chinab Institut für Geowissenschaften, FE Mineralogie, Goethe Universität Frankfurt, Altenhöferallee 1, 60438 Frankfurt, Germanyc Department of Earth and Atmospheric Sciences, University of Alberta, Alberta, Canada

a r t i c l e i n f o a b s t r a c t

Article history:Received 9 October 2014Received in revised form 22 February 2015Accepted 23 February 2015Available online xxxxEditor: B. Marty

Keywords:Lu–Hf and Sm–NdArchean mantlecarbonatite metasomatismsubcalcic garnetdiamond

We have identified carbonatitic melts as the main agent for metasomatism in cpx-free garnet harzburgites from the Kaapvaal craton. Substantial overlap of the Ti/Eu and Zr/Hf ratios of subcalcic garnets from xenoliths and garnet inclusions in diamonds corroborate previous findings that mantle metasomatism and the growth of diamonds are connected processes. The key process involves the interaction of a carbonatitic melt with depleted garnet harzburgite leading to dissolution and regrowth of the constituent minerals and the growth of diamonds by redox reactions. Model calculations show that only small amounts of a carbonatite melt, between 0.3 to 3%, are needed to generate the range of sinusoidal Rare Earth Element patterns in the garnets from harzburgites and the inclusions in diamonds. The εHf values of the xenolith garnets range from extremely positive (∼ +1000) to extremely negative values (−65) with accompanying εNd values varying from +38 to −41. It can be shown that the negative εHf and εNd values correspond to the initial ratios of the metasomatizing agent. They can only stem from a very old (early Archean or Hadean) crustal component. Very negative εNd values have been found previously in subcalcic garnets from xenoliths and inclusions in diamonds. They yield early Archean model ages for the growth of diamonds. In these studies, very high, unsupported 87Sr/86Sr ratios were found as well. The combined evidence suggests that the metasomatic agents are derived from an old source with a high Rb/Sr ratio, typical for pelitic crustal rocks. Metasomatism in the mantle, the formation of diamonds and tectonomagmatic events in the crust occurred contemporaneously at least within the time span between the early Archean and the middle Proterozoic as can be seen from the comparison of crustal ages, Lu–Hf isochron ages from subcalcic garnets and Sm–Nd and Re–Os ages of inclusions in diamonds.

© 2015 Elsevier B.V. All rights reserved.

1. Introduction

Peridotitic mantle xenoliths entrained by kimberlites are mostly residues of high degrees of partial melting which were subse-quently metasomatized (e.g. Boyd and Mertzman, 1987 and more recent Simon et al., 2007 and Gibson et al., 2008). These processes are expressed in high abundances of both incompatible and com-patible trace elements, i.e. cryptic metasomatism (Dawson, 1984). Modal metasomatism is distinguished from cryptic metasomatism when new mineral phases, such as clinopyroxene (cpx), rutile and phlogopite are introduced into previously depleted host rocks

* Corresponding author at: Department of Earth and Atmospheric Sciences, Uni-versity of Alberta, Alberta, Canada.

E-mail address: qshu1@ualberta.ca (Q. Shu).

http://dx.doi.org/10.1016/j.epsl.2015.02.0380012-821X/© 2015 Elsevier B.V. All rights reserved.

(Harte, 1983). Much effort has been put into deciphering various enrichment processes and agents within the mantle e.g. by study-ing the compositions of bulk rocks. This approach may or may not provide unambiguous results for incompatible elements and isotope ratios because of commonly occurring contamination and interaction of the host kimberlite magma with the xenoliths. As an alternative bulk rock compositions can be reconstructed from the chemical and isotope composition of the constituent primary minerals and their modal abundances (e.g. Simon et al., 2007) and the contamination problem avoided. A further approach is that of Jacob et al. (1998), Klein-BenDavid and Pearson (2009), Lazarov et al. (2009) and Shu et al. (2013) who looked into the chemical and isotope inventory of single grain subcalcic garnets. They stem from cpx-free harzburgites and dunites (Gurney and Switzer, 1973;Sobolev et al., 1973), are common as inclusions in diamonds and

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Fig. 1. Structural units of the Kaapvaal craton after Eglington and Armstrong (2004)with interpretations after Jacobs et al. (2008). Color coded fields depict exposed Paleo- to Neoarchean crystalline basement, volcanics and sedimentary rocks and the Proterozoic Bushveld complex. The locations of the diamond mines Newlands, Kimberley, Finsch, Roberts Victor and Lace are shown as blue stars. Except for Kim-berley (∼85 Ma old group I kimberlites), all other kimberlites belong to the older group II kimberlites and have eruption ages close to 120 Ma.

serve as proxies for the bulk rock composition (see Discussion be-low). Their study is a focus on the most depleted members of the peridotite suite which means that their major and trace element compositions record the high degrees of depletion of their host rocks and also of subsequent metasomatism. The garnets have low modal abundances in a pre-metasomatized residue and relatively high partition coefficients for many incompatible trace elements (relative to olivine and opx), so that subcalcic garnets are sensitive tracers of metasomatism, displaying a continuous range of Rare Earth Element (REE) patterns, from sinusoidal to “normal” garnet patterns (Stachel et al., 1998). This variability in chemical composi-tion displayed by a single lithology – cpx-free harzburgite – offers the possibility to study a continuum of processes of partial melting and metasomatic overprinting.

Metasomatism of the depleted subcratonic lithospheric mantle is ubiquitous (e.g. Dawson, 1984). This can be ancient and con-nected to processes early in the Earth’s history (e.g. Richardson et al., 1984) but it can also be young and connected to the host kimberlite magmatism shortly before eruption (e.g. Zhang et al., 2000). Archean to Proterozoic ages were obtained for metasoma-tism in the Kaapvaal subcratonic lithospheric mantle by Lazarov et al. (2009, 2012) and Shu et al. (2013) through the application of the Sm–Nd and the Lu–Hf isotope systems to single grain sub-calcic garnets from the Finsch, Roberts Victor and Lace diamond mines (Fig. 1). Based on their trace element compositions these garnets were divided into subgroups that expressed the relative degree of metasomatism (Fig. 2). The Lu–Hf isochrons from these garnets date 3.3 Ga old metasomatism of a depleted lithospheric mantle precursor at Lace on the East-block, 2.9 Ga old metasoma-tism at Roberts Victor on the suture zone between the E- and the W-blocks and 2.6 and 1.9 Ga old metasomatic events at Finsch on the West-block. In contrast to the well-defined Lu–Hf isochron cor-relations, the Sm–Nd isotope system yields only errorchrons which likely date further metasomatism at around 1.3 and 0.9 Ga.

We extend their studies of subcalcic garnets to one further group each from Roberts Victor and Lace and to two further di-amond mines, Bellsbank and Bultfontein (Kimberley), and discuss

the new data together with those of Lazarov et al. (2009) and Shu et al. (2013). We focus on the nature and origin of the meta-somatizing agents in the subcratonic mantle as documented by these garnets and quantify the degree of metasomatism necessary to generate the various REE and trace element systematics of the subcalcic garnets. We discuss the striking similarities in chemical and Nd isotope compositions between subcalcic garnet xenocrysts and garnet inclusions in diamonds. We go on to evaluate the im-plications of extreme positive and negative εNd and εHf isotopic compositions with respect to the maturity and nature of the source of the metasomatic agent, finding clear evidence for a synchronic-ity between ancient metasomatism, the growth of diamonds and tectonomagmatic events in the crust.

2. Overview of the geology of the Kaapvaal craton and sample locations

The Kaapvaal craton is the continental nucleus of southern Africa (Fig. 1). It consists of the East-block with maximum crust formation ages of 3.6–3.7 Ga, sutured to the younger West-blockthat has maximum crust formation ages of 3.2 Ga (see summary by Eglington and Armstrong, 2004). The suturing occurred at around 2.88 Ga along the Colesberg magnetic lineament (Schmitz et al., 2004). The latest major magmatic event was the 2.6 to 2.8 Ga old Ventersdoorp magmatic activity which may have coincided with the final cratonization. Later, the 2.05 Ga old Bushveld Com-plex caused modifications within the East-block and the underly-ing lithospheric mantle. Accretion and reworking of the Archean margin occurred during the 1.8 to 2.1 Ga Kheis Magondi orogeny along the west–southwest margin of the Kaapvaal craton. Subse-quently, the Khibaran event occurred at around 1.2 Ga, forming the Namaqua–Natal unit. The Namaqua–Natal orogeny ended at around 900 Ma.

Three sample locations are situated on the West-block: The Bellsbank fissures and the Bultfontein diatreme (= the Boshof Road dump in Kimberley) lie in the centre of the West-block and the Finsch diatreme close to its western margin. The Lace diatreme is situated roughly in the middle of the East-block and the Roberts Victor mine sits on the Colesberg lineament, between the two blocks.

3. Analytical methods

Forty two single grains of subcalcic garnets from Roberts Vic-tor, Lace, Bellsbank and Kimberley were analyzed in this study for major and trace elements and the Sm–Nd and Lu–Hf isotope ratios after sorting about 200 grains >2.5 mm by μ-XRF. All analyzeswere carried out at Frankfurt University; the major elements by electron probe micro analysis (EPMA), the trace elements by laser ablation inductively coupled plasma mass spectrometry (LA ICP MS) and the Lu–Hf and Sm–Nd isotope systems by solution-mode multiple collector ICP MS (MC ICP MS). The detailed procedures and analytical errors are already published by Lazarov et al. (2009)and Shu et al. (2013). A summary is provided in the electronic ap-pendix.

4. Analytical results

4.1. Major elements and geothermobarometry

The major element compositions of the subcalcic garnets are given in Table A.1 and their CaO and Cr2O3 contents plotted in Fig. A1 together with the previous data from Roberts Victor, Lace and Finsch (Shu et al., 2013; Lazarov et al., 2009). The gar-nets show a wide range in Cr2O3 (2.2–12 wt%) and CaO contents

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Fig. 2. Primitive mantle normalized REE patterns of the subcalcic garnets from the various localities. The REE patterns were divided into subgroups according to their trace element abundances. (a) Three groups are distinguished at Lace: L1 and L2 garnets with sinusoidal REE patterns whereby L1 have lower middle to heavy REE contents compared to L2; L3 garnets have gently positive sloping REE patterns and highest heavy REE contents. (b) Three groups are also distinguished at Roberts Victor: a least enriched group RV1 and a more enriched group RV2 with higher middle REE. Both groups have pronounced sinusoidal REE patterns. The third group RV3 is the most enriched group with REE patterns ranging from sinusoidal to flat middle to heavy REE patterns. (c) The garnets from Bellsbank are divided into 2 groups, a more enriched group B2 with flat to negatively sloping REE patterns and a less enriched group B1 with weakly sinusoidal REE patterns. (d) The few garnets from Kimberley are tentatively divided into two groups K1 and K2. (e) The Finsch garnets were divided by Lazarov et al. (2009) into a less enriched F1 and a more strongly enriched F2 group, both with sinusoidal REE patterns. The elements Ba, Rb, Th, U, Nb and Ta have similar abundances in all groups and for all localities. All garnets have negative Ti-anomalies and negative Zr and Hf anomalies in the least enriched groups which disappear in the most enriched group.

(0.2–5.5 wt%). There is a substantial overlap in composition be-tween the single grain subcalcic garnets and the low-Ca garnet inclusions in diamonds (fields after Stachel and Harris, 2008, with a cut-off at 14 wt% Cr2O3). The CaO contents tend to be positively correlated with TiO2 and those with the highest Ti and Ca straddle the harzburgite–lherzolite boundary. The depth of derivation of the garnet xenocrysts was obtained by projecting the averaged temper-atures of the Ni-in-garnet thermometers of Griffin et al. (1989) and Canil (1999) onto the local geothermal gradients for each locality (Fig. A2). Our procedure yields lower pressures of 3.8–4.7 GPa for

Bellsbank, 4.4–5.6 GPa for Finsch and similar pressure ranges from 3.8–6.2 GPa for Lace, Kimberley and Roberts Victor.

4.2. Trace elements

Rare earth elements (Table A2) most commonly show sinusoidal chondrite-normalized patterns (Fig. 2); other garnets have LREE depleted patterns that are flat or negatively sloping from middle to heavy REE, with Lu and Yb contents higher than the sinusoidal garnets. Garnets were divided into groups at each locality depend-ing on the relative abundances of the middle and heavy REE and

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their Zr and Hf systematics (Lazarov et al., 2009 and Shu et al., 2013). L2 and L3 garnets from Lace are the only groups with posi-tive Zr, Hf and Ti anomalies; RV3 garnets from Roberts Victor have minor negative or no Zr–Hf anomalies. All other groups have pro-nounced negative Zr and Hf anomalies. All garnets have negative Sr anomalies. Abundances of Ba, Rb, Th, U, Nb, Ta and in general the LREE up to Nd overlap for all garnet groups, with a tendency that the groups with the higher total REE contents are lower in the more incompatible elements (e.g., RV1 ∼ RV2 > RV3; F1 > F2). There is no clear relationship between garnet trace element sys-tematics and major element compositions except that the extent of middle to heavy REE enrichment is positively correlated with CaO-content.

4.3. Sm–Nd and Lu–Hf isotope compositions

Seven group RV3 garnets from Roberts Victor, five BB1 and six BB2 garnets from Bellsbank and three K1 and two K2 garnets from the Kimberley mines were analyzed for their Sm–Nd isotope ra-tios and the Bellsbank and Kimberley garnets for both Lu–Hf and Sm–Nd isotopes (Table 1 and Fig. 3). An accident in the clean lab-oratory prevented us from obtaining Lu–Hf isotope analyzes of the RV3 garnets.

The Sm/Nd ratios of the garnets from groups BB2, L3 and RV3 are higher than that of the primitive mantle (Blichert-Toft and Albarede, 1997) while those of the garnets from the less metaso-matized groups BB1 and from Kimberley are lower. The Kimberley samples have more scatter in their 143Nd/144Nd ratios than the BB1 garnets. No significant age correlation exists for the Sm–Nd iso-tope system. The 176Lu/177Hf ratios of the B1 garnets vary between 0.05 and 0.2 (higher than the PM value) and show a rough corre-lation with the 176Hf/177Hf ratios. The 176Lu/177Hf ratios of five of the B2 and the Kimberley garnets are lower than those of PM and the 176Hf/177Hf ratios are also very low. One Kimberley garnet has the lowest 176Hf/177Hf ratio of ∼0.28091. The L3 garnets from Lace have higher 176Lu/177Hf ratios than PM and scatter widely around the B1 correlation.

5. Discussion

Around 90% of the middle to heavy REE of a bulk cpx-free harzburgite or dunite reside in subcalcic garnets and around 70–80% of the LREE and HFSE (Fig. A5). As such, our selected trace elements and the Hf and Nd isotope ratios in single grains should corresponds closely to a bulk rock analysis.

5.1. The melting regime and composition of residual cratonic peridotite – the precursor to metasomatism

Two basic models exist for the origin of the residual subcratonic mantle: a) the mantle plume model where partial melting occurs at high pressures in the garnet stability field and b) a model of partial melting at spreading ridges in the spinel stability field (low pressure) followed by subduction. Summary discussions are found for example in Stachel et al. (1998), Pearson and Wittig (2008) and Gibson et al. (2008). Shu et al. (2013) summarized and extended the arguments for a low pressure origin. These authors also carried out partial melting model calculations for batch melting and non-modal batch and fractional melting at low and high pressures. They concluded that the trace element data can be best explained by non-modal batch melting at low pressures (1–3 GPa). Non-modal fractional melting in the presence of garnet would leave residues with extremely high Lu/Hf ratios which would lead, with time, to excessively high present day εHf values for Archean residual peri-dotites (see Section 5.2).

5.2. Quantitative modeling of metasomatism in residual peridotites – carbonatitic and kimberlitic compositions

5.2.1. Identification of the metasomatizing agent in single grain subcalcic garnets and comparison to garnet inclusions in diamonds

The degree of metasomatism in the subcalcic garnets must have been small because the majority plot in the “depleted peridotite field” of Griffin et al. (1999) in Y versus Zr space (Fig. 4a). All group 1 and group RV2, half of the F2 and L2 garnets and most inclusions in diamonds fall into the “depleted” field. Garnets from groups RV3, L3 and B2 fall into the “melt” or “fluid” metasoma-tized fields. For a finer distinction of metasomatism a more sensi-tive tracer must be applied. Rudnick et al. (1992) suggested high, superchondritic Zr/Hf ratios as indicators of carbonatitic metaso-matism and Yaxley et al. (1991) low, subchondritic Ti/Eu ratios. These suggestions are supported by the high pressure experiments of Sweeney et al. (1992) and Girnis et al. (2013) which show that garnets in equilibrium with carbonatitic melt fractionate Zr from Hf and Ti from Eu. The vast majority of the single grain subcalcic garnets (93 out of 103) have superchondritic Zr/Hf and subchon-dritic Ti/Eu ratios (lower than 2500; Fig. 4b), meaning that these garnets have equilibrated with a carbonatitic melt. A subgroup, the RV3 garnets from Roberts Victor, has only slightly elevated Zr/Hf ratios and Ti/Eu ratios between 2500 and chondritic (∼7830). This indicates that group RV3 garnets have interacted with a more silica rich, probably kimberlitic melt.

Chemical overlap between the subcalcic garnets and garnet in-clusions in diamonds is demonstrated in Fig. 4a, b. The inference is that harzburgitic garnets with sinusoidal REE patterns, garnet inclusions in diamonds and the diamonds themselves are the prod-ucts of the interaction of a carbonatitic melt with a residual, highly depleted peridotite. A small number of garnet inclusions in diamonds with low, subchondritic Zr/Hf ratios (<20) also ex-ists (Fig. 4b), which do not occur in the data set of our garnet xenocrysts and do not correspond to carbonatitic metasomatism. The five L3 garnets from Lace are different to all other garnets. They have Ti/Eu ratios higher than PM (McDonough and Sun, 1995), i.e., they are “depleted”, but have superchondritic Zr/Hf ra-tios. They have the lowest Mg# and the highest TiO2 contents and may be the products of Fe–Ti metasomatism on residual high Cr, low Ca mantle (e.g. Harte, 1983).

5.2.2. Open or closed system metasomatism, its distribution in the mantle column and the process of interaction

In the context of metasomatism, closed system behavior means complete crystallization of the melt within a defined mantle vol-ume, which results in modal metasomatism with the volatile com-ponents crystallizing as hydrous phases and carbonates. Models in-cluding the chromatographic fractionation of melts predict a strati-fied distribution of mineral compositions in a more than a 100 km thick mantle column, even for repeated infiltration. However, sinu-soidal and non-sinusoidal subcalcic garnets, expressed as (Eu/Gd)C1ratios, are randomly distributed within the mantle column. This can be seen in Fig. A4 where we use the Ni-content of the garnets as a proxy of the depth of derivation. The lack of any correlation of the REE patterns with depth points to similar sources of the metasomatic agents that are randomly distributed throughout the lithospheric keel. We therefore adopt a simple procedure for our modeling where we assume the localized equilibration of a perco-lating melt with the depleted mantle by a process of dissolution and re-precipitation (see further discussion below at 5.2.5).

5.2.3. Modeling metasomatism of a highly depleted peridotiteHigh degrees of non-modal batch melting at low pressures

in a convecting mantle will generate residues with low, highly fractionated REE abundances characterized by very low LREE and

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Table 1Lu–Hf and Sm–Nd isotope compositions of subcalcic garnets from the Bellsbank and Kimberley mine. Abbreviations: ID – isotope dilution.

Fig. 3. Isochron diagrams for garnets from Bellsbank, Kimberley, Roberts Victor and Lace: (a) The Sm/Nd ratios of the garnets from the most strongly metasomatized groups BB2, L3 and RV3 are higher than that of the primitive mantle while those of the garnets from the less metasomatized groups BB1 and from Kimberley are lower. The 143Nd/144Nd ratios scatter widely and no significant age correlation exists. For orientation: the Sm/Nd isotope ratios of the six group BB1 garnets scatter around a correlation line with 1162 ± 720 Ma. (b) The 176Lu/177Hf ratios of the six BB1 garnets vary between 0.05 and 0.2 (higher than the PM value) and correlate with the 176Hf/177Hf ratios with an apparent age of 1108 ± 560 Ma. The 176Lu/177 ratios of five of the B2 garnets and of the Kimberley garnets are lower than those of PM and the 176Hf/177Hf ratios are also very low. One Kimberley garnet has the lowest 176Hf/177Hf ratio of ∼0.28091. The L3 garnets from Lace have higher 176Lu/177 ratios than PM and scatter widely around the BB1 correlation. Symbol sizes are larger than the analytical error.

low HREE. Chondrite normalized abundances of Hf and Zr would lie between those of Nd and Sm. Two of the Roberts Victor garnets (RV23 and RV93) come closest to this requirement, representing the least metasomatized of all the garnets (Fig. 2). They display si-nusoidal REE patterns with a minimum at Gd or Dy and with only slightly elevated LREE. We consider the HREE as being practically

unaffected by the metasomatism that generated the sinusoidal REE patterns. We estimate the pre-metasomatic compositions for the middle and light REE along with Hf and Zr (black dashed lines, Fig. 5) using the slope of the HREE from Dy to Lu and the relative experimental partition coefficients of Green et al. (2000). This ap-proach is similar to that taken by Stachel et al. (2004) and more

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Fig. 4. (a) Diagram of Zr versus Y of the subcalcic garnets of the present study (after Griffin et al., 1999) with fields for garnet inclusions after Stachel and Harris (2008). Most of our samples plot in the “depleted field” like the majority of the harzburgitic garnet inclusions, yet they are enriched in LREE and other highly incompatible elements. The little metasomatized garnet groups L1, L2, RV1, RV2, BB1, F1 and a number of F2 fall into the “depleted” field, the group L2, RV3, BB2 and the other F2 garnets into the metasomatic fields. (b) Zr/Hf vs. Ti/Eu diagram: These ratios were previously suggested to be indicative of carbonatite metasomatism (Yaxley et al., 1991; Rudnick et al., 1992;Sweeney et al., 1992). The “carbonatite-metasomatized” garnets with low and subchondritic Ti/Eu and high and superchondritic Zr/Hf ratios are shown by bluish symbols. The red shaded area comprises samples that were affected by kimberlitic melts. The green arrow is the depletion trend for residual garnets. The garnets from Lace with superchondritic Ti/Eu ratios are shown by black triangles. The compositions of peridotitic garnet inclusions in diamonds for worldwide occurrences are shown by small black dots (data set was kindly provided by Stachel; University of Alberta, Canada). The compositional spread is outlined by dashed lines in two fields.

recently by Ziberna et al. (2013). These compositions are similar to those of pre-metasomatic garnets proposed by Gibson et al.(2013). The calculated compositions for the two garnets are our best estimate of the pre-metasomatic residues. They are now used for modeling metasomatic interaction with a carbonatitic and kim-berlitic agent together with the experimentally determined garnet-melt partition coefficients of Girnis et al. (2013) for the range of carbonatitic to kimberlitic melts. They provide the most recent and most comprehensive data set for the partitioning of trace elements between peridotitic garnets and such melt compositions.

Carbonatites were first suggested as mantle metasomatic agents by Frey and Green (1974), being further advocated in many subse-quent studies, e.g. Yaxley et al. (1991) and Rudnick et al. (1992). They originate as near solidus melts of carbonated peridotites (lherzolites to harzburgites) and have a magnesio-dolomitic com-position over a very wide range of pressures (e.g. Wallace and Green, 1988; Dalton and Presnall, 1998). As products of very low degrees of partial melting and with garnet as the residual phase,

these carbonatitic melts have highly fractionated REE patterns with extremely high La and very low Lu contents. Such highly fraction-ated REE patterns are the prerequisite to explain the sinusoidal REE patterns of subcalcic garnets as a result of metasomatism (e.g. Stachel and Harris, 2008). We use for our modeling a hypotheti-cal carbonatitic melt composition with an extremely fractionated REE pattern and Hf and Zr contents corresponding to EuN or GdN

(Fig. 5a).Kimberlitic melts are the products of higher degrees of partial

melting of carbonated peridotites at pressures of more than 5 GPa (e.g. Dalton and Presnall, 1998; Girnis et al., 2005). Consequently, they have less fractionated REE patterns than magnesio-dolomitic carbonatites, with lower LREE and higher HREE contents. For our modeling we use the average of the Group II kimberlites of Le Roex et al. (2003), which has less fractionationed Zr/Hf ratios than car-bonatites and higher Ti/Eu ratios (Fig. 5b).

We equilibrate the estimated pre-metasomatic garnet composi-tions of RV23 and RV93 with the adopted carbonatite and kim-

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Fig. 5. Trace element modeling of metasomatism with carbonatitic and kimberlitic melts shown as extended REE patterns. We used for the modeling a hypothetical carbon-atitic melt composition (with a highly fractionated REE pattern and Hf and Zr contents corresponding to EuN or GdN) and the average composition of the Group II kimberlites as given by Le Roex et al. (2003). Two garnets from Roberts Victor (RV23 and RV93) appear to be the least metasomatized of all garnets. Their measured extended REE pat-terns were used to estimate the garnet compositions of the pre-metasomatized residues (black dashed lines). Garnet-melt partition coefficients are from Girnis et al. (2013)for carbonatitic and kimberlitic melts. The colored dashed lines are calculated patterns for varying degrees of equilibration relative to garnet. (a) Only 3% relative to garnet of a carbonatitic melt are needed to generate the REE patterns of garnet RV23 and (b) only 0.15% for RV93. Similarly small amounts of carbonatite generate the REE patterns of all group 1 garnets.

berlite compositions in various proportions (0.15, 1, 3 and 20% relative to garnet) using the average of the partition coefficients of Girnis et al. (2013) for carbonatitic to kimberlitic melts. The modeling results (Fig. 5) show clearly that interaction with car-bonatite generates sinusoidal REE patterns, and that the required relative amount of the metasomatic agent is very small. Assum-ing 5% modal garnet requires about 3 wt% melt for garnet RV23 and only 0.15% in the case of RV93. The HREE, Y, Zr and Hf abun-dances of the pre-metasomatic garnet are barely affected by such small amounts of melt/fluid, explaining why such metasomatized garnets can still plot in the “depleted” sector of the Zr–Y diagram of Fig. 4a. In the resulting REE patterns, the maximum for the LREE lies at Sm and the trough at Dy. Higher degrees of metasomatism increasingly add more light and middle REE and successively over-whelm the heavy REE and also Zr and Hf.

Comparison of the calculated and natural garnet REE + Hf +Zr patterns shows that the main features in the patterns of most of the subcalcic garnets from the five investigated localities can be explained by variable degrees of carbonatite metasomatism of highly depleted peridotite (see also Gibson et al., 2013). The group 1 garnets in each locality were less enriched by such an agent than the group 2 garnets and the transition between the two groups may be continuous at least for Finsch, Roberts Victor and Lace. The BB2 group from Bellsbank appears to be the most enriched. Similar modelling with a kimberlitic metasomatic agent generates only weakly sinusoidal patterns and only for very low degrees of metasomatism. Higher degrees of metasomatism produce LREE de-pleted patterns with only slightly increasing middle to heavy REE (Figs. 5c, d). These modeled patterns are very similar to those of the group RV3 garnets (compare Fig. 2b). Our modeling is consis-tent with the identification of a kimberlitic agent for these garnets from their relatively low Zr/Hf and high Ti/Eu ratios (see Fig. 4b).

5.2.4. The role of high density fluids in fibrous diamondsHigh density fluids occur in fibrous diamonds. They range in

major element composition from saline to carbonatitic to silicic

and have highly fractionated REE patterns as a common feature (e.g. Schrauder and Navon, 1994; Tomlinson and Mueller, 2009; Klein-BenDavid et al., 2014; Rege et al., 2010). These decrease steeply from La to Dy and then level off towards Lu (Fig. A6). Very similar patterns arise from model calculations of melts/flu-ids (using the partition coefficients of Girnis et al., 2013) which were in equilibrium with the subcalcic garnets of groups 1 and 2 (Fig. A6). The leveled HREE show that these melts were in equi-librium with the garnets at the end of the metasomatic process. Correspondingly, we consider the high density fluids as residual after metasomatism, i.e. diamond precipitation and reaction with preexisting silicates including garnet. Witnesses are high density fluids and garnets that coexist in individual fibrous diamonds (e.g. Tomlinson and Mueller, 2009). The high density fluids are there-fore not the primary metasomatic agents and we did not use them for modeling metasomatism.

5.2.5. Growth of diamonds by redox reactions during carbonatite metasomatism

Diamond growth and metasomatism are related to redox re-actions between residual mantle and percolating carbon-bearing fluids and melts (e.g. Richardson et al., 1984; Stachel and Har-ris, 1997; Luth and Stachel, 2014) and high pressure experiments show that diamond precipitation is especially favorable from (al-kali)-carbonate bearing systems (e.g. Litvin et al., 1997; Sokol et al., 2000; Pal’yanov et al., 2002). Because of the substantial chemi-cal and isotopic overlap between garnet inclusions in diamond and subcalcic garnets from harzburgites we assume that the metasom-atized garnets in the harzburgite grew by the same process as that occurring during diamond crystallization. We envisage dissolution and re-precipitation during metasomatism because inclusions in diamonds are generally syngenetic, i.e. garnets and diamonds have grown simultaneously (although some recent work demonstrated that, in some cases the inclusions could be protogenetic, Nestola et al., 2014). The envisaged process is depicted in Fig. 6: A metaso-matic agent liberated from a deeper seated, carbonate and hydrous

34 Q. Shu, G.P. Brey / Earth and Planetary Science Letters 418 (2015) 27–39

Fig. 6. A schematic drawing to illustrate the consanguinity of metasomatism and growth of diamonds. A residual garnet harzburgite with an intrinsic oxygen fugacity lower than the EMOD buffer (enstatite–magnesite–olivine–diamond) is infiltrated by a carbonatitic agent which leads to equilibration between melt and silicate minerals via a dissolution and re-precipitation process and precipitation of diamond via redox reactions. Residual melt/fluid continues to migrate upwards. The metasomatized peridotite may subsequently coarsen by recrystallization and Ostwald ripening.

Fig. 7. Present day εHf of subcalcic garnets plotted against εNd values for: (a) Bellsbank, Finsch and Kimberley and (b) Roberts Victor and Lace. The εNd data of group RV3 garnets are plotted at εHf = 0 because 176Hf/177Hf could not be analyzed for these samples. (c) Present day εNd and εHf values of residua calculated for different degrees of modal batch melting in the spinel stability field 2, 3 and 4 Ga ago. The diagrams (a), (b) and (c) are divided into four quadrants I, II, III and IV. Schematic REE patterns + Hf are presented in (a) and (b) as a guide to visualize the effect of partial melting and degrees of subsequent metasomatism on element ratios and of time on the development of the Hf- and Nd-isotopes. The patterns in quadrant (I) depict the residues of partial melting with high Sm/Nd and Lu/Hf ratios that will result in a fast growth of the radiogenic isotopes. In quadrant (IV), pronounced sinusoidal REE patterns with Hf abundances between Nd and Sm are the result of high degrees of partial melting at shallow depths and small degrees of metasomatic overprint. The small degrees affect the Hf and HREE abundances only very little, Lu/Hf remains high and 176Hf/177Hf will grow, whereas Sm/Nd becomes low to very low with the result that 143Nd/144Nd grows only very little or not at all since the time of metasomatism. In the latter case, the 143Nd/144Nd ratios are the initials of the metasomatizing agent. In quadrant (III), high degrees of metasomatism led to high Hf abundances and increasingly affect the HREE. This resulted in low Lu/Hf and Sm/Nd ratios and little to no growth of the radiogenic isotopes. Quadrant (II) depicts the rare case where overwhelming metasomatism (probably with a more kimberlitic agent) led to superchondritic Lu/Hf and subchondritic Sm/Nd ratios with the result that the 143Nd/144Nd ratios grow and 177Hf/176Hf only very little.

phase containing component by partial melting, percolates and re-acts with the silicates of the overlying, highly depleted and rela-tively reduced lithospheric mantle and precipitates diamonds. Di-amonds precipitate by reduction from the oxidized carbon species and the inclusions grow by a dissolution and precipitation process. This merges the signatures of previous partial melting events and of the metasomatizing agent. The residual melt from the meta-somatic interaction, if containing sufficient thermal energy, would continue moving upwards in the mantle column, causing further metasomatism at shallower depth. The amount of diamonds that can be precipitated by the envisaged process can only be small because it is determined by the redox potential of the depleted, reduced harzburgite (Luth and Stachel, 2014). This is indicated by

the limited overlap of the εNd values of the pooled garnet inclu-sions in diamonds and the subcalcic garnet xenocrysts as seen in Fig. 8a. The εNd values of the garnet inclusions are restricted to a field at the boundary of the subcalcic garnet xenocryst array where the garnets with the least metasomatic overprint plot.

5.3. Effects of partial melting on coupled Hf–Nd isotope systematics – the pre-metasomatic precursor

The age of partial melting and metasomatism may be deter-mined from Lu–Hf and Sm–Nd isochrons and errorchrons (e.g. Lazarov et al., 2009, 2012; Shu et al., 2013), TRD and TMA Re–Os model ages of peridotites and sulfides, and sulfide Re–Os isochrons

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Fig. 8. (a)–(d): A further evaluation of the Sm–Nd and Lu–Hf isotope systems. (a) shows a comparison of our new data (colored symbols) with previous work on subcalcic garnet xenocrysts (= black dots; from Richardson et al., 1984; Pearson et al., 1995; Jacob et al., 1998) and garnet inclusions in diamonds (= diamonds from Richardson et al., 1984; Richardson and Harris, 1997) in a εNd (0) versus 147Sm/144Nd diagram. This kind of “isochron” diagram shows (i) the very negative εNd (0) that are common to subcalcic garnet xenocrysts and garnet inclusions in diamonds, (ii) that multiple overprints affected the majority of the subcalcic garnets (large spread to less negative and positive εNd) whereas the inclusions have only negative values and (iii) reliable age information may be obtained only from individual localities. (b) shows that the Lu–Hf isotope system has retained defined age information on the earliest periods of metasomatism around 2.9 ± 0.3 Ga and reemphasizes the ancient crustal signature of the metasomatizing agent through the presence of very negative εHf values. The age of this crustal component within the subcratonic mantle is 3.6 Ga at minimum as can be seen from the oldest model ages in (c) and (d). (c) is a diagram of 143Nd/144Nd ratios versus the age of the Earth. It shows the evolution line of the primitive mantle for the Nd isotope system in green. The blue lines connect the 143Nd/144Nd ratios of the group 1 and 2 garnets from this study with their Nd model ages (Ga) and the red dashed lines for group 3 garnets. The black dashed lines are those for garnet inclusions in diamonds (Richardson et al., 1984; Richardson and Harris, 1997). The light grey band represents the range of Nd isotope ratios of bulk rocks from the Kaapvaal crust which are projected back to the zircon ages from these rocks (Schoene et al., 2009). (d) is a corresponding diagram for 176Hf/177Hf. It shows the evolution line for the primitive mantle. The dashed red lines connect the 176Hf/177Hf ratios of group 3 garnets with their Hf model ages (Ga). Black crosses are the Hf isotope ratios and the U–Pb ages of detrital zircons from the Kaapvaal craton (Zeh et al., 2011) The present-day 176Hf/177Hf ratios for potential bulk rocks were calculated assuming a derivation either from (i) mafic rocks with a 176Lu/177Hf ratio of 0.02 (yellow circles on the y-axis) or (ii) a felsic crust with a 176Lu/177Hf ratio of 0.113 (grey circles) or (iii) a TTG crust with a 176Lu/177Hf ratio of 0.005 (green circles; Zeh et al., 2011). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

(e.g. see Pearson and Wittig, 2008 for a summary). Partial melt-ing, the age of metasomatism and the ancestry of the agent are also reflected in the present day εNd and εHf values of the sub-calcic garnets (Fig. 7a, b, c). Ancient residues of partial melting should have correlated, highly positive present-day εHf and εNd values. Calculated present-day εNd and εHf values for residues of different degrees of non-modal batch partial melting in the spinel stability field of a primitive mantle and for times of 2, 3 and 4 Ga ago are shown in Fig. 7c. The isotopic ratios increase as a function of the degree of partial melting and time of depletion. The calcu-lated values are minima because partial melting in nature is more fractional than batch. The isotopic compositions grow to εHf values of 1200 and εNd values of 120 for 40% of partial melting two Ga ago, through to εHf = 2500 and εNd = 250 for melting at four Ga ago. Even though likely minima, these values are extremely high

compared to values from any known peridotitic rock or mineral. Any fractional partial melting model, especially one with garnet as a residual phase would triple or quadruple these values. The parent–daughter element ratios of the residua will be modified by subsequent metasomatism and the isotope ratios will grow into different quadrants of Fig. 7, depending on the composition of the metasomatizing agent and the degree of metasomatism.

The highest εHf in our data set of +991 is from sample RV31 from Roberts Victor which is, to our knowledge, the highest value of any peridotitic garnet worldwide. Even this value is anomalously low and cannot reflect an undisturbed residue from 30 to 40% par-tial melting that generated highly depleted Archean harzburgites. The sinusoidal REE pattern of this garnet and its position in quad-rant IV of Fig. 7 shows that it is metasomatically overprinted by an amount sufficient to increase Hf and hence lower Lu/Hf, retard-

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ing slightly the ingrowth of the daughter 176Hf. Garnet RV31 is a member of the Lu–Hf isochron that defines an age of 2.95 Ga and is interpreted by Shu et al. (2013) as the age of partial melting of a slightly depleted mantle. However, the εHf values of the gar-nets on this isochron are vertically aligned in Fig. 7c, displaying the most negative εNd values (−40). Their εHf values vary from +991 down to +79. Because all of these garnets are sub-calcic and high in Cr the range in εHf values cannot be explained by modest (23%) to low (9%) melt extraction during partial melting to form the lithospheric mantle. Since the Lu–Hf isochron correlation is not a mixing line (see Shu et al., 2013), the more likely interpre-tation of the isochron is that it dates a metasomatic event at high pressure.

5.4. The ancient character of the metasomatic agent and its source properties – a crustal link?

Metasomatism decouples the Lu–Hf and Sm–Nd isotope sys-tems and εHf and εNd values can become positive or negative independently, the extent being dependent on time. The present day εHf and εNd values of the subcalcic garnets from Lazarov et al. (2009), Shu et al. (2013) and the present work are plotted in Fig. 7a (Finsch, Bellsbank and Kimberley), Fig. 7b (Roberts Victor and Lace) and combined in Fig. 7c. Schematic REE patterns and Hf are presented in Figs. 7a, b as a guide to visualize the effects of partial melting and degree of metasomatism on element ra-tios and of time of the development of the Hf- and Nd-isotopes. None of our samples plot in the calculated residue field in quad-rant I which demonstrates that all samples are metasomatically overprinted. About 70% plot into quadrant IV with superchondritic Hf- and subchondritic Nd-isotope ratios that result from equilibra-tion with small amounts of carbonatitic melt. Moderate degrees of carbonatitic metasomatism (3–5% relative to garnet) lead to an in-creased addition of Hf with the result that the samples plot into quadrant III. The samples in quadrant II experienced either higher degrees of carbonatitic overprint (samples BB2) or had equilibrated with a kimberlitic melt (samples RV3).

The extremely negative εNd(0) values recorded by the garnets from quadrants III and IV mean that the carbonatitic agent was derived from a very old source in the lithosphere with very low Sm/Nd ratios which prevented the growth of 143Nd/144Nd. The high Nd contents and high Nd/Sm ratios of the carbonatitic agent overwhelmed the 143Nd/144Nd isotope ratios of the residual peri-dotite and reverted their high Sm/Nd ratios to much lower values. Similarly negative εNd values to those found in this study were measured in subcalcic garnets from other localities (Udachnaya – Siberia; Pearson et al., 1995; Jacob et al., 1998 and Newlands; Klein-BenDavid and Pearson, 2009) as well as from pooled garnet inclusions in diamonds from Kimberley and Finsch (Richardson et al., 1984; Fig. 8a).

The very low εNd(0) garnets show maxima in their REE pat-terns at Ce or Nd. Increasing amounts of a carbonatitic agent rel-ative to the residue lead to a shift of the REE maxima to Sm and eventually Eu during equilibration, i.e. an increase of the Sm/Nd ra-tios and therefore, with time, of 143Nd/144Nd and less pronounced negative εNd values. If Sm/Nd ratios become super-chondritic, a substantial growth of 143Nd/144Nd occurs with time. This effect is best seen for the group B2 garnets with a maximum at Eu (quad-rant II) from Bellsbank. Strong metasomatism resulted in the addi-tion of Hf in such amounts that the 176Hf/177Hf isotope signature of the metasomatic agent becomes dominant during the equilibra-tion process and the Lu/Hf ratios are lowered to the extent that the growth of 176Hf practically stops. The present day εHf values are then extremely negative (as low as −58 for BB2) and reflect the initial ratio of the metasomatic agent. Further samples from Kim-berley and Lace have similar, extremely negative εHf values (insert

Fig. 8b). One garnet from Lace has εNd = −39 and εHf = −65 (Shu et al., 2013), an extremely low 176Lu/177Hf ratio of 0.0070, a sub-chondritic 147Sm/144Nd ratio of 0.1209 and high Hf (1.01 ppm) and Nd (4.48 ppm) contents. The very negative εHf and εNd values require an early enrichment of the LREE and of Hf in a depleted mantle. The time of this enrichment is inferred by the 3.3 Ga Lu–Hf errorchron from the Lace garnets.

The combined database for Nd and Hf isotopes indicates some coherence between the most negative εNd and εHf values of the subcalcic garnets and those of the Archean crust of the Kaapvaal craton (Fig. 8c, d). The lowest mantle garnet 143Nd/144Nd ratios overlap with the range of Nd isotope ratios of crustal rocks from the Kaapvaal craton projected back to their zircon ages. Present-day 176Hf/177Hf ratios of the oldest zircons from the Kaapvaal cra-ton (Fig. 8d) calculated for different possible parental rocks, such as mafic crust (assumed Lu/Hf ratio = 0.02), felsic crust (assumed Lu/Hf ratio = 0.01) and for TTG’s (assumed Lu/Hf ratio = 0.015) also show a clear overlap in the model age lines with mantle gar-nets with the lowest 176Hf/177Hf. We infer from this overlap in both Nd and Hf isotopic compositions that the enriching agents af-fecting the mantle lithosphere were ultimately sourced from an old crustal component whose age may lie between 3.65 and 3.99 Ga, as indicated by the Hf and Nd model ages of garnet L2 (Figs. 8c and d).

An additional characteristic of subcalcic garnets with very low εNd values, both in peridotite xenoliths and diamond inclusions, is that they tend to have radiogenic 87Sr/86Sr ratios that are un-supported by their measured Rb contents (Richardson et al., 1984;Pearson et al., 1995; Jacob et al., 1998; Klein-BenDavid and Pear-son, 2009). Furthermore, very unradiogenic Nd isotope ratios cou-pled to very high, unsupported 87Sr/86Sr ratios also seem to be an important feature of fluid inclusions in fibrous diamonds (Klein-BenDavid et al., 2014), apparently reflecting the involvement of ancient crustal materials in the source of diamond-forming flu-ids. The carbonatitic melt, which metasomatized the host harzbur-gites containing subcalcic garnets and generated the inclusions in diamond, must stem from a source with low Sm/Nd and high Rb/Sr ratios. Sufficient time must have elapsed to develop high 87Sr/86Sr ratios before this source was tapped early in the Earth’s history to provide the metasomatizing agent. The source may have been subducted carbonated pelites or carbonated peridotite, with magnesite and alkali carbonates derived from melts sub-ducted carbonated pelites (Foley, 2010; Grassi and Schmidt, 2011;Bulatov et al., 2014) or from seafloor altered basalts (carbonate bearing eclogites). Any of these sources would provide the requi-site parent–daughter isotope fractionations and observed isotopic compositions.

5.5. Synchronous metasomatism, diamond growth and crustal events

The estimated growth ages of diamonds have been summa-rized by Stachel and Harris (2008), Pearson and Wittig (2008)and Gurney et al. (2010) and can be correlated with craton accre-tion, craton edge subduction and mantle metasomatism. Indicators of growth ages of diamonds are Sm–Nd model ages from pooled subcalcic garnet inclusions, two-point isochron ages from pooled garnet and clinopyroxene inclusions of lherzolitic and eclogitic par-ageneses, and Re–Os model and isochron ages for eclogitic and peridotitic sulfides. Maxima in the frequency distribution of sul-phide TMA ages correlate with diamond growth ages and with tectonomagmatic events in the crust within and around the edges of the Kaapvaal craton (for a summary see Pearson and Wittig, 2008).

Mantle events evident from garnet Lu–Hf isochron and model ages and Sm–Nd isochrons also broadly correlate with crustal events in cratons (Shu et al., 2013; Fig. 9). The new data pre-

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Fig. 9. Correlation of mantle metasomatism, diamond growth and crustal events. The ages for crustal tectonomagmatic events are taken from summaries by Eglington and Armstrong (2004) and Schmitz et al. (2004). Ages for mantle metasomatism are from the present work and previous work by Lazarov et al. (2009, 2012) and Shu et al.(2013). Ages for the growth of diamond are from summaries by Pearson and Wittig (2008), Stachel and Harris (2008) and Gurney et al. (2010). The frequency diagrams for mantle TRD and sulfide model ages are drawn after Pearson and Wittig (2008).

sented here augment this view. In particular, there are good tem-poral overlaps between crustal and mantle events (from Lu–Hf and Sm–Nd) and diamond formation ages in the time span between the middle Archean and early Proterozoic (1.9 Ga). A temporal cor-relation of crustal events with mantle and diamond ages is not as clear in the latter stages of Earth’s history. This may be because the latter two metasomatism ages are based on errorchrons with a larger uncertainty about the actual meaning of these numbers.

Pearson and Wittig’s (2008) interpretation of a temporal corre-lation between diamond growth ages and mantle events is based on a crude correlation of maxima in the frequency curve of sul-phide Re/Os model ages and ages from diamonds (Fig. 9). The growth of diamonds is connected with metasomatism as discussed above and in previous work so that we would argue that the sul-phide TMA’s also reflect the age of metasomatism. In fact, it is clear that any base metal sulfide existing in highly depleted cratonic peridotite must be metasomatic in nature (Pearson et al., 2004).

6. Implications and conclusions

We have identified carbonatitic to kimberlitic melts as the main agents for metasomatism in cpx-free garnet harzburgites using di-agnostic trace element ratios such as Ti/Eu and Zr/Hf together with REE patterns and abundances within subcalcic garnets. Substan-tial overlap of the Ti/Eu and Zr/Hf ratios in subcalcic garnets and garnet inclusions in diamonds corroborate previous findings that mantle metasomatism and the growth of diamonds are connected processes. The envisaged process of the interaction of a carbon-atitic melt with depleted garnet harzburgite is that of dissolution and regrowth of the constituent minerals and the growth of dia-monds by redox reactions. Model calculations show that only small amounts of a carbonatite melt in the range of 0.3 to 3% are needed to generate the range of sinusoidal REE patterns in the garnets from harzburgites and the inclusions in diamonds. The garnet εHf values range from extremely positive (almost +1000) to extremely negative values (−65) and the εNd values from +38 to −41. It can be shown that the highly unradiogenic Nd and Hf isotope

composition likely correspond to the initial ratios of the metaso-matizing agent. Such time-integrated isotopic systematics can only stem from a very old (early Archean or Hadean) crustal component subducted into the mantle. Very negative εNd values in subcalcic garnets, found previously in subcalcic garnets from xenoliths and inclusions in diamonds yield the Mesoarchean model ages for the growth of diamonds and high, unsupported 87Sr/86Sr ratios found in these garnets as well as in the fluids within diamonds indicate the derivation of the metasomatic agent from an old source with high Rb/Sr ratios, typical for pelitic sediments.

The presence of old crustal components along with the geo-chemical properties of the subcalcic garnets themselves require an origin of the subcratonic mantle by subduction (-like) processes in the early Archean as suggested e.g. by Helmstaedt and Schulze(1989). Subduction initially produces a mantle with different lithologies (depleted and metamorphosed peridotite, (carbonate-altered) ocean floor basalts, gabbros, cumulates and carbon-bearing sediments), with different oxidation states and with an inhomoge-neous thermal structure. Subsequently, thermal consolidation in the Archean will generate a stable thermal structure between a heat producing upper crustal lid and the convective asthenosphere at the basis of the “then to be lithospheric keel” (Michaut and Jaupart, 2007). Thermal consolidation results in a redistribution of heat and matter by conduction, melts and fluids. Low temperature melt fractions with high incompatible element contents will mi-grate upwards and precipitate new minerals in the intruded rocks, e.g. K–Mg-carbonates in peridotites (e.g. Brey et al., 2011). These carbonates are presumably also high in Rb and their 87Sr/86Sr ratios will therefore grow very fast. Alkali carbonates determine the position of the peridotite solidus which is lower than that of magnesite or dolomite bearing peridotite (e.g. Litasov et al., 2013). Alkali carbonates are therefore very easily reactivated ei-ther still during the course of thermal consolidation or during a subsequent destruction of the lower parts of an originally thicker lithospheric keel (e.g. Jordan, 1975). These melts migrate upwards to further metasomatize peridotite while the residual mantle is destroyed (Lazarov et al., 2012). The time of thermal consolidation

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and cratonization occurred before 2.5 Ga, probably in the time interval between 2.7 and 3.3 Ga as indicated by the age range for metasomatism from the Lu–Hf isotope system and the spread in Re–Os model ages (see Fig. 9). We consider this time inter-val as the major period of metasomatism and diamond formation. The subcratonic mantle cooled since that time at a rate of about 0.1 ◦C/Ma (e.g. Shu et al., 2014).

The carbon isotope ratios of peridotitic diamonds cluster around the δ13C mantle value of around −5�. This seems at odds with the idea of a sedimentary (subduction) origin of at least part of the diamond carbon. However, the present day εNd and εHf val-ues of the subcalcic garnets indicate an early Archean (Hadean?) crustal source of the metasomatic agent. At that time the fraction-ation of carbon isotopes by organic activity probably played only a very minor role because life was at its very beginning. CO2 was re-moved from the atmosphere mainly by carbonation of an oceanic basaltic protocrust (e.g. Sleep, 2010) and most of it was probably fast recycled into the Earth’s interior. Carbonation in the anorganic cycle fractionates carbon isotopes much less than the organic cycle and this smaller range may have been re-homogenized to mantle values during vigorous recycling processes.

Acknowledgements

The authors gratefully acknowledge the continuous help and support in the laboratory and through discussions by A. Gerdes, J. Heliosch, H. Hoefer, F. Kneissl, M. Lazarov, A. Neumann, J. Schas-tok and H.-M. Seitz, S. Weyer and A. Zeh. Jeff Harris was fan-tastic company in the field through his local knowledge, advice, discussions and social skills. He continued to support our work during his stays in Frankfurt. Steve West from diamond corp.plc is thanked for allowing access to the coarse concentrates from the Lace mine. Jim Davidson from Petra Diamonds made the ac-cess to the Bellsbank diamond mine possible. Jock Robey is a valued friend in Kimberley. His contacts made the access to var-ious dump reworking facilities in South Africa possible. Graham Pearson made numerous suggestions which greatly improved the manuscript. We also thank Maud Boyet for valuable advice in the reorganization and for constructive comments of an earlier ver-sion of the manuscript and F. Nestola and S. Gibson for helpful reviews. The project was supported by the Deutsche Forschungs-gemeinschaft (BR 1012/33-1 and BR 1012/37-1).

Appendix A. Supplementary material

Supplementary material related to this article can be found on-line at http://dx.doi.org/10.1016/j.epsl.2015.02.038.

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