Metasomatic diamond growth: A multi-isotope study (13C, 15N, 33S, 34S) of sulphide inclusions and their host diamonds from Jwaneng (Botswana)

Earth and Planetary Science Letters 282 (2009) 79–90

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Metasomatic diamond growth: A multi-isotope study (13C, 15N, inclusions and their host diamonds from Jwaneng (Botswana)

33

S,

34

S) of sulphide

E. Thomassot a,b,⁎, P. Cartigny a, J.W. Harris c, J.P. Lorand d, C. Rollion-Bard e, M. Chaussidon e a

Laboratoire de Géochimie des Isotopes Stables de l'Institut de Physique du Globe de Paris, Université Paris 7, UMR CNRS 7154, 2 Place Jussieu, T54-64 E1, 75251 Paris, Cedex 05, France Stable Isotope Laboratory, Earth and Planetary Science Department, McGill University, 3450 University, Room 238, Montréal, Québec, Canada H3A 2A7 Department of Geographical and Earth Science, Gregory Building, University of Glasgow, G12 8QQ, UK d Laboratoire de Minéralogie et Cosmochimie du Muséum National d'Histoire Naturelle (CNRS UMR 7202), CP 52, 61 Rue Buffon, 75005, Paris, France e Centre de Recherches Pétrologiques et Géochimiques, 15 rue Notre Dame de pauvres, BP 20, Vandoeuvre-les-Nancy, France b c

a r t i c l e

i n f o

Article history: Received 11 August 2008 Received in revised form 27 February 2009 Accepted 2 March 2009 Available online 5 April 2009 Editor: R.W. Carlson Keywords: suphide-bearing diamond isotopes (carbon, nitrogen, sulfur) metasomatic diamond growth mass-independent fractionation of sulphur isotopes

a b s t r a c t The formation of diamond through metasomatic events, from volatiles-enriched fluids/melts brought into preexisting mantle rocks, has been suggested from a series of independent studies. However, the link between these hypothetical volatile-rich fluids and deep-seated mineral inclusions (mostly silicate and sulphides) entrapped in diamonds remains unclear, yet the relationship between these two species may provide the key to our understanding of diamond crystallization. In order to address the relationship between the origin and formation of diamonds and their mineral inclusions, we carried out the first coupled stable isotopic study (δ13C, δ15N and multiple S-isotopes as δ34S, δ33S, Δ33S) of diamonds containing sulphide inclusions, using samples from the Jwaneng kimberlite as our model system. Sulphides extracted from the present collection of 55 diamonds belong to either eclogitic (E-type, 52 diamonds) or peridotitic (P-type, 3 diamonds) paragenetic suites, as attested by their Cr and Ni content (Cr b 0.03 wt.% for Etypes and Cr N 0.18 wt.% and NiN 14 wt.% for P-types). Sulphur isotopic compositions have been measured in-situ by multicollector secondary ion mass spectrometry and reveal that peridotitic sulphide inclusions in diamonds (n = 4) lie on the terrestrial fractionation line whereas Mass Independent sulphur isotope Fractionations (MIF) are preserved inside eclogitic sulphides inclusions (−0.5‰ b Δ33S b +0.9‰, n = 33). Such large MIF values, extended here to the negative values recorded in Archean sediments, are thought to be produced through photochemical reactions in the Archean atmosphere, which implies that sulphides inclusions contain an Archean sedimentary sulphur component that was transferred to the Earth's mantle. In contrast to this isotopic dichotomy between E-type and P-type sulphides, the geochemical characteristics of their host diamonds (δ13C, δ15N, N-content, N-speciation) are largely homogeneous throughout the population, and differ from silicate-bearing diamonds previously studied in the same locality. Sulphide-bearing diamonds appear to define a distinct population, which might reflect a different crystallization process characterized by carbon and nitrogen isotopic compositions falling within the range of unfractionated mantle values. The lack of a distinct isotopic signature of recycled sedimentary nitrogen and carbon in sulphide-bearing diamonds leads us to infer that sulphide-bearing Jwaneng diamonds were not derived from the same chemical reservoir as their inclusions. One possible formation mechanism that is compatible with our observations is diamond crystallization from a mantle-derived carbon-bearing fluid associated with pre-existing sedimentary sulphide minerals. This model has important implications for the interpretation of diamond ages obtained from sulphide inclusions using Re–Os systematics. Here we suggest that the incorporation of a significant amount of mantlerelated sulphur during a diamond growth event into the original sulphide could lead to a re-equilibration of the Re–Os isotope composition in any inclusion, and accordingly, account for some scatter on Re–Os isochrons. © 2009 Elsevier B.V. All rights reserved.

1. Introduction

⁎ Corresponding author. Stable Isotope Laboratory, Earth and Planetary Science Department, McGill University, 3450 University, Room 238, Montréal, Québec, Canada H3A 2A7. E-mail address: [email protected] (E. Thomassot). 0012-821X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2009.03.001

Diamonds are one of the rare samples formed at great depths in the mantle (from 150 to N700 km). They are not affected by decompression during ascent, by chemical alteration by their host kimberlite, or subsequent weathering. Moreover, diamond is a very robust container for materials enclosed during its crystallization, such as silicate and sulphide inclusions (Harris and Gurney, 1979) native metals (Gorshkov et

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al., 1997) and deep-seated fluid inclusions (Schrauder and Navon, 1993). Thus the study of diamonds and of their encapsulated inclusions provides key information about mantle chemistry and mineralogy, deep crystallization processes and allows determination of such parameters as pressure, temperature and the age(s) of diamond growth. Nevertheless, the crucial question of how diamond forms remains actively debated. To explain this crystallization process the genetic relationship between diamonds and their mineral inclusions is of primary importance. Diamond host rocks are peridotite and eclogite derived from the Earth's mantle and found as xenoliths in diamondiferous kimberlites or lamproites (Taylor and Anand, 2004). Whereas peridotites are commonly described as specifically mantle-related, the genesis of eclogites is more controversial. Some samples characterized by a high MgO contents are thought to have a cumulate origin (Barth et al., 2002), but the large majority of eclogites have been designated as remnants of altered oceanic crust that was partially melted during subduction (Barth et al., 2001; Jacob, 2004). A principal piece of evidence for a recycled origin is the large range in eclogite δ18O values, from 3 to 8‰ (Jacob et al.,1994; Snyder et al., 1995), which is best accounted for through ocean floor hydrothermal alteration, compared to the narrow range observed in mantle peridotites (δ18Osmow = 5.5±0.4‰, (Eiler, 2001; Mattey et al., 1994)). For eclogitic diamonds therefore, the origin of the carbon involved in their formation (i.e., mantle-related vs sedimentary/organic) is of particular interest. A number of crustal signatures have been detected from different types of minerals entrapped in eclogitic diamonds, the main isotopic indicators being (1) the hydrothermal signature of oxygen isotopes in silicate inclusions (Lowry et al., 1999; Schulze et al., 2003); (2) the wide δ34S range recorded in sulphides inclusions (Chaussidon et al., 1987; Eldridge et al., 1991; Rudnick et al., 1993; Westerlund et al., 2004) and more recently, (3) the presence of massindependent fractionations of sulphur in sulphide inclusions from Orapa (Farquhar et al., 2002). Set against the subduction related evidence, carbon and nitrogen multi-isotopic studies of the host diamond provide equally strong evidence for a purely mantle-related origin for eclogitic diamonds, (e.g., negative δ15N ranges which are incompatible with the average positive δ15N of recycled sedimentary nitrogen, Cartigny et al., 1998a,b). To investigate whether or not sedimentary-related inclusions occur within mantle-derived diamonds, we present in this paper, carbon, nitrogen and sulphur isotope data from a suite of sulphidebearing diamonds, from Jwaneng, in order to consider in detail the above paradox about the origin of eclogitic diamonds. 2. Geological context The Jwaneng kimberlite belongs to a cluster of 11 pipes, located in southeastern Botswana, along the Kaapvaal–Zimbabwe craton margin. This very profitable diamondiferous mine erupted 250±17 Ma ago (Kinny et al., 1989) and produces a large variety of diamond types, from polycrystalline to fibrous diamonds and gem quality twins, octahedra and their resorbed forms. The majority of diamonds (55%) with sulphide or silicate inclusions are eclogitic (Deines et al.,1997; Gurney et al.,1995) and have been largely studied with respect to their carbon and nitrogen isotopic composition, N-content and aggregation state (Deines et al., 1997, Cartigny et al., 1998a). Diamonds from Jwaneng kimberlite belong to at least two different generations. A first Sm–Nd isochron age of 1540 ±20 Ma has been measured on silicate inclusions (Richardson et al., 1999), whereas Re–Os isochrons from sulphide inclusions show more scatter, but define at least two diamond ages at 2.9 and 1.5 Ga (Richardson, 2004), with a third possible isochron at 2.0 Ga. 3. Samples description, preparation Fifty-five samples, recovered from three different extraction campaigns (1994, 1995 and 2000) were selected for the present study. These colourless to pale brown sulphide-bearing sub-dodecahedral diamonds

each weighed from 40 to 160 mg (i.e., from 0.2 to 0.8 carats). The sulphide inclusions usually occurred as octahedra or cubo-octahedra, with faces parallel to major planes within the diamond. The inclusions varied in size from 10 to 200 μm, although irregular flakes resulted when the inclusion broke during extraction. Initial examination showed that the inclusions were usually surrounded by a black rosette fracture, resulting from the differential expansion between sulphide and its host diamond during their ascent through the lithosphere. Particular attention was paid to these fractures to verify that each inclusion was isolated both from the surface of the diamond (closed fracture) and the other inclusions in the same diamond (non-touching inclusions). In this way the inclusion once trapped had no opportunity to re-equilibrate. 4. Analytical techniques 4.1. Sulphur isotopic compositions measurements Most physical or chemical processes fractionate isotopes according to the mass difference between these isotopes (i.e., two atomic mass units (amu) between 32S and 34S; 1 amu between 32S and 33S), with the result that 34S/32S values of most modern terrestrial samples are two times greater than their associated 33S/32S values. This covariation can be used to establish a Terrestrial Fractionation Line (TFL), set by theoretical calculations as δ33S ≈ 0.515 δ34S (Bigeleisen and Mayer, 1947; Hulston and Thode, 1965). Samples that do not lie on the TFL possess mass-independent fractionation (MIF), corresponding to nonzero Δ33S values: 33

33

33

Δ S = δ Smeasured − δ Spredicted : Conventionally δ33Spredicted has been defined according to measured δ34S values and either a linear reference fractionation law (δ33Spredicted = 0.515 ×δ34Smeasured) or an exponential reference fractionation law ((δ33Spredicted = 1000 × (1+δ34S / 1000)0.515). Here we use a series of standards (chosen to be mass dependent), covering a large range of isotopic composition (δ34S ranging from −0.32 to +16.73‰ vs Canyon Diablo Troilite, CDT), as described below. Sulphur isotopic compositions were obtained by secondary ion mass spectrometry using a CAMECA ims 1270 in the CRPG (Centre de Recherches Pétrographiques et Géochimiques) facilities in Nancy (France). Simultaneous measurements of 32S−, 33S− and 34S− ion beams were obtained working in a multi-collection mode using three off-axis Faraday cups (L'2, C and H1). The gains of Faraday cups were inter-calibrated at the beginning of each analytical session. A typical analysis of individual sulphides was made by bombardment with Cs+ primary beam (10 nA intensity) of around 20-µm in diameter, with a mass resolution (M/DM) of about 5000 (using slit #2 in multicollection). No isobaric interferences of 32SH− over 33S− and of 33SH− over 34S− peaks are present at this resolution. After a pre-sputtering of 2 min, typical ion intensities between 6 and 10 × 108 counts per second (cps) were obtained on 32S−, the intensities depending on the sulphide mineral analyzed. As a result, an internal 1σ error of less than ±0.1‰ was reached on δ34S after a few minutes of counting. Since instrumental fractionation can be influenced by the chamber vacuum, the chamber pressure was kept below 1 × 10− 8 Torr during all the sessions using a

Table 1 Standards used in this study to calibrate the mass discrimination line. Name

Mineralogy

δ34S

δ33S

34

S/32S

33

S/32S

OPM Enon LTB CAR123 CAR111 KA8

Cp (CuFeS2) Po (Fe1 − xS) Po (Fe1 − xS) Py (FeS2) Py (FeS2) Pn ((Fe, Ni)9S8)

2.29 0.90 −0.32 1.41 16.73 2.21

1.17 0.46 − 0.16 0.72 8.61 1.13

0.04426 0.04420 0.04414 0.04422 0.04490 0.04426

0.007886 0.007880 0.007875 0.007882 0.007945 0.007886

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liquid nitrogen cold-trap. Several sulphide standards for SIMS sulphur analyses have been analyzed in this study: pentlandite, pyrrhotite, pyrite and chalcopyrite (see Table 1 for detailed standard values). The sulphur in these standards is taken to be mass dependent and accordingly, they were used: (1) to determine the instrumental mass fractionation for each sulphide minerals; and (2) to determine a reference mass discrimination line (Supplementary Table 1 and Supplementary Fig. 1), equivalent to the TFL (gap between these two lines is small and due to a greater uncertainty on 33S counting statistic compare to 34S one). Mass-independent fractionation, which corresponds to the gap to the TFL, has been measured directly as the distance to this line, assuming that mass discrimination law (Farquhar et al., 2000):  0:515 33 33 34 −1: Δ S = δ S − 1000 × 1 + δ S= 1000 Potential fractionations created during a measurement do not affect Δ33S measurement because all the modern isotopic effects are mass dependent and do not modify the intrinsic Δ33S of a sample. Then, two sigma uncertainties are ±0.13‰ for single Δ33S. However, supplementary corrections have been applied to our δ34S measurements in order to take account of the instrumental mass fractionation, which is mainly sensitive to the chemical composition of the spot analyzed. Consequently, every area analyzed for δ34S values, was also measured for Fe–Cu–Ni abundances using a CAMECA SX100 electron microprobe (see below). Each analyzed area corresponds to a mixing between three phases (mainly pentlandite, chalcopyrite and pyrrhotite) and then the instrumental fractionation for each spot was calculated following: Δδinst = A
Δδinst ðpoÞ + B
Δδinst ðpnÞ + C
Δδinst ðcpÞ where A, B and C are the respective proportions of pyrrhotite (po), pentlandite (pn) and chalcopyrite (cp) (with A + B + C = 1) determined by electron microprobe, using a micron step cartography of the ion probe's spot, and Δδinst(x) the instrumental mass fractionations for each mineral (defined as: Δδinst(x) = δinst(x)measured − δinst(x)). Given the mineralogical heterogeneity of sulphides at the micro scale, we conclude that the use of the standard error on the mean is not warranted. Therefore, we report individual measurements with their individual instrumental error, derived for their specific mineralogical composition. Two sigma uncertainties are better than ±0.7‰ on single δ34S analyses. In order to evaluate our procedure, we also re-analyzed some sulphide inclusions from Orapa previously measured by Farquhar et al. (2002) at UCLA (Supplementary Table 2). The mean reproducibility on Δ33S measurements was better than ± 0.08‰ as illustrated in Supplementary Fig. 2. For the present work, we measured the sulphur isotopic compositions (71 measurements) of 38 individual inclusions from 22 diamonds. 4.2. Petrological observations and electron microprobe analyses of sulphide In order to define the E-type vs P-type parageneses of the sulphide inclusions, each fragment was studied using reflected light microscopy at the Museum National d'Histoire Naturelle (MNHN, Paris). Photomicrographs of a large panel of textures were taken at magnifications ranging from ×20 to ×1000, using a camera adaptor and video capture software on the microscope. Observations were complemented by electron microprobe analyses (EMPA), and were required to correct isotopic composition measurements from matrix effects (as explained in the above paragraph). Major elements of the sulphides were determined using a CAMECA SX100 electron microprobe in Centre d'Analyse par Microsonde-PARIS (CAMPARIS). The analyses were performed with an accelerating voltage of 15 kV and a current of 40 nA. The focused beam leads to an analysis

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volume smaller than 1 µm3. Counting times were 10 s on the peak and 5 s for the background on each side of the peak for each element. Raw data were converted into wt.% using either natural standards (pyrite for S) or synthetic metals for Fe, Ni, Cu, Co and Cr. The accuracy of the resulting concentrations is better than ±0.1 wt.%. The uncertainties given in Supplementary Table 3 are derived from counting statistics and the minimum detection limits are given at the 2σ level. 4.3. Nitrogen content and aggregation state Nitrogen is the main chemical impurity of diamonds (Kaiser and Bond, 1959). In the crystallographic network, this element is strongly bonded to carbon atoms initially through one-to-one substitution. Diffusion on the atomic scale results in the clustering of singlysubstitutioned nitrogen to form migration clusters. These comprise atomic pairs (IaA diamond) at the My scale because of a low activation energy, Ea ~5 eV (Evans and Qi, 1982) and, ultimately, 4 N atoms surrounding a vacancy (IaB diamond) at the Gy scale because of higher Ea of ~ 7 eV (Evans and Qi, 1982; Cooper, 1990; Taylor et al., 1990). This progression in defect formation (usually expressed as the %IaB) follows a second-order kinetic law dependent upon nitrogen content, residence time in the mantle and the temperature history of the particular diamond (Evans and Qi, 1982). The nitrogen content and aggregation state were obtained by Fourier Transform Infrared Spectroscopy (FTIR), using coefficient of 16.5 at. ppm cm− 1 for A (Boyd et al., 1994) and of 79.4 at. ppm cm− 1 for B (Boyd et al., 1995a). 4.4. Carbon and nitrogen isotopic compositions of host diamonds Carbon and nitrogen isotopic compositions of the total suite of Jwaneng sulphide-bearing diamonds were analyzed by gas source mass spectrometry on CO2 and N2 molecules obtained after combustion of a diamond fragment in an O2-rich atmosphere. For carbon isotopic compositions, a conventional dual-inlet mass spectrometer was used, with data being expressed in delta notation relative to the PDB standard, δ13C=(13C/12Csample/13C/12CPDB −1)×103, with an accuracy better than 0.1‰. Nitrogen isotopic compositions were measured with a triple collector static vacuum mass spectrometer directly connected to the extraction line (Boyd et al., 1995b). Results are expressed in delta notation relative to air, with accuracy better than 0.5‰ (δ15N=(15N/14Nsample/15N/ 14 Nair −1)×103). Nitrogen concentration was quantified using a capacitance manometer with accuracy better than 5%. 5. Results 5.1. Sulphide paragenesis In the diamond stability field, sulphides are expected to occur mostly as homogeneous mono-sulphide solid solutions (mss) (Tsai et al., 1979; Deines and Harris, 1995; Lorand and Gregoire, 2006) and references therein. When extracted from their host diamonds, sulphides appear as Cu–Fe–Ni-rich minerals, which have exsolved from pre-existing mss during sub-solidus re-equilibrations through cooling (Kullerud, 1963; Craig and Kullerud, 1969; Kullerud et al., 1969). In the present collection we identified two main types of textures: most sulphides (91%) are Nipoor pyrrhotite matrix (po, Fe1 − xS) containing pentlandite (pn, (Fe, Ni)9S8) exsolutions, surrounded by a rim of Cu-rich sulphide (either chalcopyrite, cp (CuFeS2) or cubanite, cb (CuFe2S3)). In two cases, fine intergrowths of two Ni-rich po-like phases show typical homogeneous emulsion-like textures of mss. Paragenetic affinity of sulphides is usually inferred from Ni and/or Os contents as shown in previous studies either on sulphides of mantlederived xenoliths (Lorand and Conquere, 1983; Guo et al., 1999; Lorand and Alard, 2001; Lorand and Gregoire, 2006) or on sulphide inclusions in diamonds (Tsai et al., 1979; Eldridge et al., 1991; Rudnick et al., 1993;

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Table 3). Fig. 1 shows that within the entire sample set there are four Ptype sulphides (extracted from two diamonds). Additionally, these four sulphides were the only ones characterized by an emulsion-like texture. In the absence of a chemical discriminator, this texture, therefore, may help to identify the paragenesis of sulphides in diamond. 5.2. Sulphur isotopic compositions The δ34S values measured for Jwaneng E-type sulphides (n = 34 extracted from 20 diamonds) range from −9.0‰ to +3.4‰ with a mean value of δ34S = 0.9‰ ± 2.4‰ (δ34S median = − 0.4‰), whereas P-type sulphides, (n = 4 recovered from 2 diamonds) display a narrow range (from −1.0‰ to +0.3‰) with an average value of −0.6‰ ± 0.5‰ (see Table 2). For both parageneses, δ34S measurements of Jwaneng sulphides are within the ranges reported by Eldridge et al. (1991) for sulphide inclusions from eight South African mines. Compared to recently reported δ34S measurements of sulphides from Orapa diamonds (ranging from −1.4 to +2.6‰, (Farquhar et al., 2002)), Jwaneng sulphide inclusions appear slightly depleted in heavy isotopes. However, the difference in δ34S might also lie in the fact that Farquhar et al. (2002) did not correct for instrumental mass fractionation according to the chemical composition of the investigated phase, but only considered the presence of pyrrhotite. Mass-independent fractionations (MIF) are observed in the sulphide population. E-type inclusions display a Δ33S range from −0.50‰ to +0.96‰ whereas P-type inclusions do not show any significant MIF (Δ33S values from −0.14‰ to − 0.08‰). We note that: (1) seven inclusions extracted from seven distinct diamonds show negative and significant Δ33S (≤−0.12‰); (2) resolvable Δ33S variability is present among inclusions released from some single Etype diamonds (Jw95-22, Jw95-23, Jw95-25 and Jw94-2) whereas there is no significant Δ33S variability measured in three inclusions extracted from a single P-type diamond (Δ33S variabilityb 0.02‰). All the present measurements are represented in Fig. 2. 5.3. Carbon and nitrogen isotopic compositions

Fig. 1. Plots of the sulphur/metal ratios vs (a) Ni (normalized to iron concentration) and (b) Cr (in wt.%). Grey circles represent analyses performed on P-type sulphides and black diamonds on E-type sulphides. The positions of chalcopyrite (cp), pyrrhotite (po) and pentlandite (Ni-poor end-member, pna, and Ni-rich one, pnb) are added for clarity.

Deines and Harris, 1995; Bulanova et al., 1996; Pearson et al., 1998). Ni is a compatible element, enriched in peridotites and sulphides that formed in equilibrium with olivine containing significant amount of Ni. Investigations of sulphide inclusions in Siberian diamonds indicated 12% Ni content as the lower limit for P-type sulphides (Yefimova et al., 1983; Bulanova et al., 1996). Nevertheless, further studies on African diamonds reveal that this limit is not sharp, and that Ni in sulphides alone does not allow a clear identification of some transitional samples (Deines and Harris, 1995; Pearson et al., 1998). This remark applies even more to spot analyses of pure exsolutions of pentlandite that are enriched in Ni, even if the surrounding matrix is an E-type Ni-poor pyrrhotite. When Os contents are measured, paragenetic distinction is much easier because E-types sulphides have markedly lower Os abundances (b700 parts per billion, ppb) compared with P-types sulphides (from 2000 to 20,000 ppb) (Pearson and Shirey, 1999). Nevertheless, conventional Os-concentration measurements are destructive and incompatible with later ion probe measurements. As recently suggested by Stachel and Harris (2008), chromium could be more discriminative for distinguishing P- and E-type sulphides. In fact, unlike Ni, Cr-content does not depend on the mineralogy of the sulphide (i.e., it is not sensitive to spot analyses of specific exsolution features) and presents a clear split between E-types (always near the detection limit, Crb 0.02%) and P-type (CrN 0.18 wt.%) sulphides (see Supplementary

Carbon isotopic compositions of the E-type diamonds range from δ13C values of − 8.38‰ to −3.09‰ (Table 3), with one extreme single value at − 18.73‰. This range defines a Gaussian distribution around a median value of − 5.43‰ (±1.08‰ in 2σ). Moreover, duplicate measurements on five samples reveal a very small internal variability in δ13C values (b0.6‰). The three P-types specimens show δ13C values (−4.50, − 4.57 and − 5.29‰), typical of the mantle canonical value. Measured δ15N values range from −10.1‰ to +4.9‰, with one extremely positive value at +8.3‰). The median is −5.4‰ with 85% of the values being negative. Samples are apparently heterogeneous with respect to their nitrogen isotopic compositions, and reveal 1.6 to 6.3‰ of internal variation in 10 duplicated samples (Table 3). Within the dataset, diamond Jw00 19, characterized by a low δ13C of −18.73‰ has one of the few positive nitrogen isotopic compositions, (δ15N = +0.9‰). The N-content measured by manometry displays a wide range (from 11 to 1899 ppm) around a median value of 879 ppm. Three samples are nitrogen-free (Type II diamonds) and 85% of the total suite contains more than 400 ppm nitrogen. Despite their high N-contents, Jwaneng sulphide-bearing diamonds are poorly aggregated (12% of IaB defect on average, 46% being pure IaAdiamonds, see Table 3). The present data set is summarized in Fig. 3 as histograms. 6. Discussion 6.1. Sulphide-bearing diamonds as a separate diamond population The over-abundance of sulphide inclusions in diamonds led several authors (Meyer,1987; Deines et al.,1997) to suggest an individual S-type paragenesis. In such a model, sulphide-bearing and silicate-bearing

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Table 2 Sulphur isotope measurements of sulphide-bearing diamonds from Jwaneng.

Data in gray highlight refer to peridotitic samples.

diamonds from the same kimberlite should provide distinct information. For most other authors, sulphide and silicate-bearing diamonds sample the same part(s) of the continental lithosphere, and accordingly, both would yield similar information. Accordingly, it becomes important to see if there are differences between these diamond types at Jwaneng. Previous studies (Deines et al.,1997; Cartigny et al.,1998a) dedicated to silicate-bearing diamonds from Jwaneng showed that eclogitic diamonds display a wide range of δ13C values (−21.13 to −2.71‰), N-content (0 to 1528 ppm) and N-aggregation (0 to 98% of IaB defects). The δ15N values of E-type silicate-bearing diamond are strictly negative and range from

−10.1 to −1.1‰ (Cartigny et al., 1998a). As illustrated in the comparative histograms of Fig. 3, Jwaneng sulphide-bearing eclogitic diamonds are characterized by smaller variability in δ13C, higher N-contents and lower N-aggregation state, and as such are distinct for a major part of Jwaneng silicate-bearing diamonds. Comparison of these characteristics to sulphide-bearing diamonds from the Panda (Westerlund et al., 2006), and Robert Victor (Deines et al., 1987) shows complete compatibility. At Koffiefontein, whereas the δ13C range is more scattered, N-content and aggregation state of sulphide-bearing diamonds are also consistent with present results (Deines and Harris, 1995; Thomassot et al., unpub.). Thus,

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aggregation state data (Table 3, Fig. 4), sulphides-bearing diamonds from Jwaneng showing a lower average N-aggregation state (16% inferred from both the study made by (Deines et al. (1997) and from the present work) than the average N-aggregation state in silicatebearing diamonds (45% IaB, for E-types, Deines et al., 1997). Given the dependence of nitrogen diffusion on N-concentration, residence time but most importantly on temperature (Evans and Harris, 1989), the Naggregation state particularly reflects the relative temperature of diamond formation. In the present situation, the 12% IaB implies a relatively low temperature (b1000 °C for most of the samples, for a residence time of 1.26 Ga, Fig. 4). This result would not actually change significantly if different diamond ages are considered. 6.2. Separate isotopic signatures of eclogitic diamonds and their sulphide inclusions

Fig. 2. Plot of the MIF (Δ33S) vs δ34S for Jwaneng sulphides. Grey circles represent P-type inclusions and black diamonds are E-type sulphides. (a) Individual measurements; (b) Average value measured for each inclusion. Error bars represent absolute deviations. Black dashed lines link several inclusions from one unique diamond.

whilst a major portion of Jwaneng sulphide-bearing eclogitic diamonds are clearly distinct from Jwaneng silicate-bearing diamonds, the global picture may indicate that for both major parageneses, this silicate/sulphide difference is a worldwide characteristic, although we acknowledge that data for Siberian, Western-African or even Australian diamonds are missing. At Jwaneng at least, the difference may be explained by the fact that diamond ages indicate distinct crystallization events, however, multiple diamond generations are less supported by the narrow variable C-isotope compositions, N-contents and N-aggregation states. The isotopic characteristics of the Jwaneng sulphide-bearing eclogitic diamonds enable some important constraints to be placed on their formation mechanism. Relative to the broad δ13C distribution there is a much narrower mantle-like signature for sulphide-bearing diamonds. The narrow range implies either that the sulphide-bearing diamonds are carbon sourced differently, or that if the carbon source is the same, then the isotopic fractionation accompanying their crystallization is smaller than that observed for E-type silicate-bearing diamonds. Such a difference could be due to a more rapid diamond formation, the high nitrogen content (Fig. 3) of sulphide-bearing diamonds being further support for this view. The high nitrogen content could reflect diamond formation from a methane-bearing fluid, in which case, nitrogen would behave as a compatible element with respect to diamond (see Thomassot et al., 2007). The hypothesis that sulphide-bearing eclogitic diamonds from Jwaneng had a distinct formation history is further supported by N-

6.2.1. Origin of mass-independent fractionation in sulphide inclusions in diamonds At Orapa, MIF has been measured in four sulphides (Farquhar et al., 2002), with Δ33S values up to +0.6‰. Thermodynamic theory dictates that isotopic fractionation at high temperatures is negligible, effectively ruling out mantle processes in the genesis of such abnormal isotopic ratios (Farquhar et al., 2002). To account for the Δ33S values, these authors developed a model in which the anomalies were produced by ultraviolet photolysis of S-bearing gases in the Archean atmosphere after which the photolytic products were trapped in surface sediments, transferred through subduction into the mantle and eventually became encapsulated in diamonds. It is worth noting that several alternatives exist to produce significant deviations from TFL. The first one, known as mass conservation effect, relies on (i) open-system fractionations and (ii) mixing processes (Farquhar et al., 2007). Theses two processes, however, are not pertinent here because the first one only concerns huge |δ34S| or strongly fractionated δ34S values, whilst the later leads to only negative and too small Δ33S (b0.1‰). Significant variations can also be produced by isotope exchange by equilibria (Deines, 2003). The mechanism, previously described in by Oi et al. (1985), (see also Kotaka et al., 1992; Skaron and Wolfsberg, 1980) requires two specific molecules at a very precise cross over temperature. However, for sulphur, Oi et al. (1985) indicated that these effects only concern extremely rare molecules (OCS, CS2, SCF2 SCCl2, SPF and SPCl for H2S, and SOBr and SOCl for SO2). Hyperfine effects are also known to have a specific consequence on odd-numbered isotopes. Because of the covariance of Δ33S with Δ36S in Archean sediments, this effect is again not likely to be significant (Farquhar et al., 2000). Finally, based on theoretical calculations, Lasaga et al. (2008) predicted that MIF would be produced during their absorption on organic molecules. Although their calculations suffer from a series of simplifications and lack proper experimental confirmation, the MIF production would require sediment extremely rich in organic matter, siderite and clays together with sulphur-bearing species (such as aqueous SO2− 4 , H2S, gypsum/anhydrite, native sulphur, iron sulfides) and both at elevated temperatures. These starting conditions are unlikely to occur in an Archean environment and therefore do not constitute a realistic alternative explanation of the non-zero Δ33S occurrence in Archean sediments. Thus, in our view, the UV photolysis of SO2 in an oxygen-poor atmosphere remains the only mechanism to account for non-zero Δ33S observed in the Earth's mantle, as developed earlier by Farquhar et al. (2002). In the present work, we have extended the range of MIF up to +0.9‰ in mantle-related samples, which is even more incompatible with an intra-mantle process. We note that, only some E-type sulphide inclusions contain non-zero Δ33S whereas, albeit scarce (n = 4) P-type sulphides plot only on the terrestrial fractionation line. If MIF were not a

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Table 3 N-contents (determined by Fourier Transform Infrared Spectroscopy, FTIR, and capacitance manometer after bulk combustion, Mano), percentage of the nitrogen B-defects, δ13C and δ15N of sulphide-bearing diamonds from Jwaneng.

Data in gray highlight refer to peridotitic diamonds.

surface signature, being related to a high-T process, similar Δ33S among eclogitic and peridotitic samples would be expected. This observation is thus further evidence for sedimentary inherited Δ33S. And finally, we report the first significant negative Δ33S values in sulphides included in diamond (down to −0.5‰). Although negative anomalies have been reported in Archean sediments (Farquhar et al. (2000, 2007), and

references therein) they are rarer than the positive ones and their maximum amplitudes are four times smaller. These MIF characteristics are illustrated in Fig. 2 in both frequency and amplitude. Since MIF is chemically conservative in the mantle environment, the only process able to diminish the amplitude of an anomaly is physical mixing, either between sulphur sources with Δ33S values of

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Fig. 3. A set of histograms, which compare the geochemical characteristics (δ13C, N-content and δ15N) of sulphide-bearing diamonds (top histograms, the present study) with E-type silicate-bearing diamonds from Jwaneng (bottom histograms, Deines et al., 1997). The two P-type sulphide-bearing diamonds are represented in black.

opposite sign, or between MIF sulphur and sulphur that does not exhibit MIF. In our view, the diminished amplitudes of the Δ33S signature in sulphide inclusions relative to the sedimentary Δ33S

Fig. 4. Plot of N-aggregation state (% IaB) vs N-content (at. ppm) for sulphide-bearing diamonds from Jwaneng (black squares). Errors on IaB defects and N-content are respectively ± 5% and ± 10% in 2σ. Hollow and grey circles represent respectively the Etype silicate-bearing and sulphide-bearing diamonds of Deines et al. (1997). Isotherms are inferred from the equation of nitrogen diffusion (Evans and Qi, 1982), for a residence time of 1.26 Ga (a minimum genesis age of 1.5 Ga (Richardson et al., 2004) and an eruption age of 240 Ma). Note that the assumed residence time does not significantly affect the resulting temperature.

record likely reflect dilution of the surface sulphur source with massdependent sulphur derived from the primary mantle. Comparison of the maximum Δ33S values in sulphide inclusions (+0.9‰) with the maximum Δ33S values from Archean sediments (~ + 10‰, see Domagal-Goldman et al., 2008), we infer that eclogitic sulphides contain no more than ~90% of primary mantle sulphur. This value is an upper limit, as a potential implication of sulphur derived from recycled sulphates (i.e. with negative Δ33S) was neglected in the above calculation. 6.2.2. Origin of the diamond-forming source Carbon isotopic compositions at Jwaneng for E-type silicate-bearing diamonds define an asymmetric distribution with compositions highly depleted in heavy isotopes (δ13C = −19‰, Deines et al., 1997). These low δ13C values have lead some authors to suggest the presence of organic carbon (δ13C ≈ −25‰), in a source (Kirkley et al., 1991; Tappert et al., 2005), although at Jwaneng, the range was suggested to reflect either primordial heterogeneity (Deines et al., 1997) or a hightemperature fractionation process (Cartigny et al., 1998a). In the present case, our results (δ13Cmedian = −5.43‰, with a standard deviation of 1.08‰) reveal that carbon isotopic compositions are homogeneous and within the range of typical mantle-related materials like MORB (Javoy and Pineau, 1991), fibrous diamonds (Boyd and Pillinger, 1994; Cartigny et al., 2003), carbonatitic melts (Deines, 1988) and carbonates from kimberlites (Deines and Gold, 1973). This unequivocal mantle isotopic signature inferred from Cisotopes is strongly supported, in our view, by nitrogen isotopic compositions measured on the same samples, which are dominantly in the range of canonical mantle values (average δ15N = −5 ± 3‰). Nitrogen is a useful tracer for subducted sediments/metasediments in the Earth's mantle because it is initially associated with organic matter as a result of biological fixation and shows 15N-enriched isotopes compositions (see below). From available data, early Archean sediments have average δ15N = 0‰, ranging from −6.2 up to +35.8‰ (Beaumont

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Fig. 5. (a) Plot of δ13C (∝13C/12C) vs N-content (in at. ppm, i.e. ∝14N/12C). (b) Plot of 1/N (∝ 12C/14N) vs δ15 N (∝ 15N/14N) for sulphide-bearing diamonds, from Jwaneng. These diagrams test a potential geochemical mixing between two sources, in this case mantle-related and subducted. If there is a mixing in such diagrams with same denominators, one would expect trends between end members.

and Robert, 1999; Kerrich and Jia, 2004; Ueno et al., 2004; Shen et al., 2006; Thomazo et al., in press for review). Through time, organic matter likely shows a slight δ15N secular evolution, from positive δ15N of +3‰ during the Neo-Archean (Beaumont and Robert, 1999; Thomazo et al., in press; Garvin et al., 2009) to ubiquitous positive δ15N during and after the Proterozoic (average from +2 to +8‰). It is worth noting that early Archean δ15N data were obtained from cherts that are often associated with hydrothermal systems. Pinti et al. (2001) mentioned that 15N-depleted values measured in Archean kerogens might thus correspond to inorganic sources of N fixed by chemolithoautotrophs living at proximity to the hydrothermal vents (characterized by negative δ15N) and not represent the isotope signal of the open-ocean Archean sediments (see Cartigny and Ader, 2003 for discussion). But when the same authors looked at pristine N (in iron bands of 3.5 Ga cherts, Pinti et al., 2007) they measured δ15N values of +6‰, suggesting that diversification of metabolic pathways was possibly acting at that time producing localized pools of both negative and positive d15N in specific Archean sediments (Shen et al., 2006). In addition, during metamorphism/subduction, the loss of 14N by devolatilization further increases the original δ15N value of ~3‰ to ~6‰ (Bebout and Fogel, 1992; Jia, 2006). Assuming a high geothermal gradient during the Archean, the strongest 15N-enrichment (≥6‰) must be considered (Busigny et al., 2003). Strong enrichment in 15Nisotope with increasing metamorphism might be recorded in Archean muscovites showing strictly positive δ15N-values (average ~+13‰; Jia and Kerrich, 2004, see also Papineau et al., 2005). This is also consistent with the δ15N values of metamorphic diamonds formed from N1.8 to N0.5 Ga, in crustal rocks subducted to ultra-high pressures, which range −1.8 to +12.4 (average ~+6‰, Cartigny et al., 2004 and reference therein). If the subducted oceanic crust is now considered in its entirety, rather than the associated superficial sediments, all but one sample reported so far are positive with δ15N values ranging from − 1‰ (Halama et al., 2008) to +10.3‰ (Busigny et al., 2005). Considering both a corrected statistic of available data for Archean sediments and the isotopic effects of subduction on nitrogen, we conclude that the average δ15N range of recycled Archean metasediments is positive, with only scarce negative δ15N. In this respect, these cannot match both the average and the δ15N-range displayed by mantle samples (diamonds and MORB vesicles). The present δ15N distribution (δ15Naverage = − 4.5‰, 48 negative values from 55 samples), therefore, likely points to a mantle-related nitrogen source

for most of our diamonds. This hypothesis is strongly supported by the lack of any correlation between neither δ13C (≈ 13C/12C) and Ncontent (≈ 14N/12C), nor between δ15N (≈ 15N/14N) and 1 / N-content (≈ 12C/14N) as shown in Fig. 5. In such diagrams, mixing between subducted organic matter and mantle-related material would produce linear arrays (see caption in Fig. 5). The few positive δ15N values within our present dataset could correspond either to subordinate diamond sources, (perhaps reflecting final vestiges of an intra-mantle fractionation process) or, most likely, reflect an isotopic fractionation during diamond crystallization, as previously observed in a Cullinan (formally Premier) diamond population (Thomassot et al., 2007). 6.2.3. Evidence of a decoupled origin for eclogitic diamonds and their sulphide inclusions The sulphur isotopic evidence presented here indicates that E-type sulphide is partly sourced from sulphur that passed through the Earth's near-surface environment. In contrast, carbon- and nitrogen-

Fig. 6. Plot of Δ33S from sulphide inclusions vs δ15N from their host diamonds. The grey areas represent the domains of sedimentary recycled signatures (δ15N N 0‰ and nonzero Δ33S). Note that only two samples (hollow diamonds) plot in these areas (see text).

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isotope measurements reveal a clear mantle-related source for the diamonds that host these inclusions. These arguments are further resumed through Fig. 6, where the majority of the sulphides carrying a sedimentary sulphur signature are entrapped in diamonds that fall into the domain of mantle-related nitrogen. Two samples (Jw95 10, 26) plot into the area expected when a diamond and its inclusion would derive from a single recycled source. Considering the entire sample set, our study supports a decoupled origin for diamonds and their inclusions, a proposal previously made by Spetsius et al. (2002) or Taylor and Anand (2004). 6.3. Implications of the decoupled origins of diamond and their sulphide inclusions 6.3.1. Metasomatic diamond crystallization The uncoupled signature presented above has direct implications concerning our understanding of diamond formation with the principal question being how these two minerals can be unified. From a large number of independent indicators, it is now generally agreed that diamonds could crystallize from a metasomatic C–H–O–N–S or carbonatitic fluid (Westerlund et al., 2004; Thomassot et al., 2007, and reference therein). Our study supports the hypothesis where pre-existing sedimentary sulphides are captured later by diamonds crystallized from a metasomatic fluid. One of the unexpected observations of the present study is the significant heterogeneity in the measured amplitudes of the MIF between distinct (non-touching) inclusions entrapped in the same host diamond. Such observation illustrates, for the first time, an isotopic heterogeneity at the mm scale in the immediate region in which the diamond crystallized. Over geological time at mantle temperature, this heterogeneity is not expected to survive. Whilst no experimental work exists on diffusion rate of sulphur through silicates at P/T domains comparable to those were diamonds are formed, diffusion measurements through garnet for oxygen are available (Connolly and Muehlenbachs, 1988). At 1000 °C and over 1 Ga, oxygen diffuses ~1 m. On the micron to millimeter scale, therefore, heterogeneity inherited from the slab and preserved in the continental lithosphere is not convincing. An alternative explanation for two isotopically distinct inclusions in the same diamond is episodic diamond growth, a mechanism which would also be consistent with the multiple diamond ages reported at Jwaneng that call for several generations of sulphides (Richardson, 2004). However, the striking homogeneity of the carbon isotopic composition suggests only very slight variations during diamond growth, making this proposal unlikely. Because the heterogeneity of MIF within one diamond inclusion population cannot be primordial, it has to be acquired by mixing. For this heterogeneity to be preserved, the mixing has to occur during or just before diamond crystallization, which closes the system and prevents S diffusion. The amplitude of any Δ33S would reflect the quantity of mass-dependent sulphur added by the diamond-forming fluid, to the mass-independently fractionated sulphur. 6.3.2. The syngenetic relationship observed between diamond and mineralogical inclusion One of the commonly accepted hypotheses about a diamond and its inclusion is that they are syngenetic, having crystallized simultaneously (jointly) from the same proto-chemical environment at the same pressure and temperature. This proposal explains imposed cubic morphologies on many silicate inclusions that do not have cubic symmetry, such as diopside, which belongs to the monoclinic system. These negative forms are thought to be imposed by the host crystal during a corresponding crystallization due to a diamond's higher form energy (Harris and Gurney, 1979). The crystallographic observation constitutes the main argument cited to justify the syngenetic hypothesis (see for example Meyer, 1985 or Pearson et al., 1998). Considering the present results, it appears that the negative crystal forms of sulphides could be acquired during the diamond crystallization from a fluid that

partially re-crystallizes pre-existing minerals and incorporates mantle sulphur, as suggested by the variable Δ33S within a single diamond. 6.3.3. Implication for diamonds dating This study is relevant to the interpretation of the diamond ages that have been obtained on sulphide inclusions using Re–Os systematics. This system dates the closure of the Re–Os couple in a sulphide and isolated from the mantle by diamond growth. Previous works have produced direct age regressions that range from 3.5 to 1 Ga. In the present paper, we document an important variability in the sulphur isotope composition of the sulphides that can only be explained if the sulphides were introduced into the mantle, by subduction of the sedimentary part of a slab, in the solid state. The model describes diamond growth around those sulphides from a volatile-rich fluid (C–H–O–N–S) that was introduced independently and after the sulphides. Because the sulphides clearly have some prehistory before their incorporation in diamond, the model provides a way to explain radiogenic initial Os isotope compositions seen in some sulphide suites (Panda, Kimberley, Orapa and Jwaneng) and may also explain the limited scatter noted on some Re–Os isochrons. 7. Conclusions • Eclogitic sulphide inclusion-bearing diamonds show specific geochemical characteristics in respect of N-content, N-speciation, δ13C and δ15N. These gem diamonds represent a distinct diamond population, thought to result from different growth conditions largely separate from silicatebearing diamonds from the same locality. As such, results obtained on sulphides-bearing diamonds, like genesis ages for example, should not be generalized to the entire eclogitic diamond population. • Some of these sulphides preserve a mass-independent fractionation of S-isotopes indicative of an initial origin from recycled oceanic crust, which is transported to the upper mantle by subduction. • We show that sulphide-bearing diamonds likely crystallized from mantle-derived volatiles-rich fluid as suggested by their C- and Nisotope composition. • Heterogeneity in Δ33S between different sulphide inclusions extracted from the same host is best explained by localized melt mixing of the original sulphur isotope characteristics with sulphur carried by the introduced metasomatic volatile-rich fluid. Because of the preservation of this heterogeneity at high T, we propose that this fluid also leads to the diamond crystallization, which enclosed the isotope signature and preserved it. • The distinct characteristics of both the diamonds and their sulphide inclusions at Jwaneng indicate formation at a specific point in a recrystallization sequence within a subducting slab in the upper mantle. Our model reconciles morphological observation (such as oriented and faceted inclusions) and geochemical evidences of the pre-existence of inclusions with respect to their host diamond. • This study has direct implications for the interpretation of Re–Os measurement and the determination of diamond ages, specifically the presence of scatter on some Re–Os isochrons. Acknowledgements This work benefited from constructive discussions with many colleagues, including S. Shirey. D. Rumble, G. Pearson, J. Farquhar, B. Wing and D. Pinti. Technical assistance was provided by F. Couffignal, M. Fialin (Electron microprobe facility at CAMPARIS), M. Champenois, D. Mangin (ion probe group of CRPG-Nancy), M. Girard and J.J Bourrand (Stable Isotope Lab-IPGP). We wish to acknowledge financial support from CNRS (through the Dyeti Program) and a donation from De Beers to support the maintenance of our experimental equipment. The manuscript also benefited from constructive reviews by S. Aulbach and an anonymous reviewer. IPGP contribution 2488.

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