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Mantle fluid evolution—a tale of one diamond
Lithos 77 (2004) 243 – 253 www.elsevier.com/locate/lithos
Mantle fluid evolution—a tale of one diamond Ofra Klein-BenDavid a,*, Elad S. Izraeli a, Erik Hauri b, Oded Navon a b
a Institute of Earth Sciences, The Hebrew University of Jerusalem, Jerusalem 91904, Israel Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015, USA
Received 27 June 2003; accepted 12 November 2003 Available online 25 May 2004
Abstract Microinclusions analyzed in a coated diamond from the Diavik mine in Canada comprise peridotitic minerals and fluids. The fluids span a wide compositional range between a carbonatitic melt and brine. The diamond is concentrically zoned. The brine microinclusions reside in an inner growth zone and their endmember composition is K19Na25Ca5Mg8Fe3Ba2Si4Cl32 (mol%). The carbonatitic melt is found in an outer layer and its endmember composition is K11Na21Ca11Mg26Fe7Ba2Si10Al3P2Cl5. The transition in inclusion chemistry is accompanied by a change in the carbon isotopic composition of the diamond from 8.5x in the inner zone to 12.1x in the outer zone. We suggest that this transition reflects mixing between already evolved brine and a freshly introduced carbonatitic melt of different isotopic composition. The compositional range found in diamond ON-DVK-294 is the widest ever recorded in a single diamond. It closes the gap between brine found in cloudy octahedral diamonds from South Africa and carbonatitic melt analyzed in cubic diamonds from Zaire and Botswana. Thus, all microinclusions analyzed to date fall along two arrays connecting the carbonatitic melt composition to either a hydrous-silicic endmember or to a brine endmember. This connection suggests that many diamonds are formed from fluids derived form a mantle source not significantly influenced by local heterogeneities. D 2004 Elsevier B.V. All rights reserved. Keywords: Inclusion; Brine; Carbonatitic melt; Peridotitic; Coated diamond; Fibrous diamond
1. Introduction Diamond inclusions provide a unique opportunity to investigate the mantle rocks where diamonds are formed and the fluids from which they crystallize. The strength of the diamond and its low reactivity in a silicate environment ensure that the trapped substances remain shielded from their changing environment. Mineral inclusions are widespread in diamonds and * Corresponding author. Fax: +972-2-5662581. E-mail address: [email protected] (O. Klein-BenDavid). 0024-4937/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2004.04.003
range from less than a micrometer to millimeter size. They consist of silicates, oxides, sulfides, and rare carbonate and phosphate minerals and provide information on host rock lithology and on the temperature and pressure of diamond crystallization (Meyer, 1987). Most mineral inclusions indicate peridotitic and eclogitic host rocks at temperatures of 900 – 1300 jC and depth of 150 – 200 km (Meyer, 1987; Harris, 1992). Based on the internal structure of diamonds (Milledge et al., 1984; Bulanova, 1995), the low density of crystallographic dislocations (Sunagawa, 1984) and the association of diamonds with
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cracks within their host xenoliths (Taylor et al., 2000), it is believed that most natural diamonds grow from fluids. Fluid inclusions are the best source of information about the nature and chemical composition of the diamond’s growth medium, and shed light on its evolution. Fluid inclusions are found in fibrous diamonds, in the fibrous coat of coated diamonds and as internal clouds in octahedral diamonds (Navon, 1999). The inclusion-bearing zones are populated by millions of submicrometer inclusions separated from each other and enclosed in the diamond matrix. TEM analyses of individual inclusions identified a multiphase assemblage of apatite, quartz, mica and carbonate surrounded by material of low electron density (Lang and Walmsley, 1983; Guthrie et al., 1991). The common interpretation is that the inclusions trapped a fluid and that the minerals are secondary phases that grew during cooling. Water and carbonates detected by infrared (IR) spectroscopy (Chrenko et al., 1967; Navon et al., 1988) are probably the main volatile components. Electron probe microanalysis of the bulk composition of microinclusions reveals a wide range of compositions among three endmembers (Schrauder and Navon, 1994; Izraeli et al., 2001): (a) hydroussilicic melt rich in water, Si, Al, and K; (b) carbonatitic melt rich in carbonate, Mg, Ca, Fe, K and Na and (c) brine-rich in Cl, K and Na. Intermediate compositions between carbonatitic and hydrous-silicic melts were found in fibrous diamond (Navon et al., 1988; Schrauder and Navon, 1994). Limited mixing between carbonatitic melt and brine were found in cloudy diamonds (Izraeli et al., 2001). No intermediate compositions between brine and hydrous-silicic melt were detected. All compositions are rich in K and many other incompatible elements and are characterized by steep REE patterns similar to those in kimberlites and lamproites (Schrauder et al., 1996; Raga et al., 2003). Halogens and Ar are also enriched and the halogen abundance ratios are similar to those of MORB (Johnson et al., 2000; Burgess et al., 2002). In spite of the high abundance of sulfide minerals in diamonds, such inclusions were never found in fluid-bearing diamonds, reminding us that diamonds may grow from other media as well, e.g., sulfide melts (Bulanova, 1995; Sobolev et al., 1997; Bulanova et
al., 1998) or reduced carbon-bearing fluids (Tomilenko et al., 1997). It is interesting to note that Bulanova et al. (1998) and Klein-BenDavid et al. (2003) suggested involvement of carbonatitic melt in the formation of sulfide-bearing diamonds. The carbon isotopic composition of fibrous diamonds from localities worldwide ranges over relatively narrow d13C values, between 5x and 8x. Their nitrogen isotopic composition ranges between 0x and 10x. These values lie close to those of carbon in MORB and kimberlites (Galimov, 1991; Boyd et al., 1992; Cartigny et al., 1998, 2003). Fluid inclusions are rarely found together with mineral inclusions. Tal’nikova (1995) reported the finding of omphacite along with silicic melt in a Siberian diamond. Izraeli et al. (2001, in press) reported microinclusions bearing peridotitic and eclogitic minerals as well as hydrous minerals residing together with brine inclusions in cloudy diamonds from Koffiefontein. This, together with the evidence for metasomatism in diamondiferous xenoliths suggest that the fluids penetrated the peridotitic or eclogitic host rock and took part in diamond formation in both rock types. Here we report the composition of 167 microinclusions in one concentrically zoned diamond from the Diavik mine in Lac de Gras, Slave Craton, Canada. Microinclusions in this diamond carry peridotitic minerals and fluids with a broad compositional range. In all previous reports (Navon et al., 1988; Schrauder and Navon, 1994; Izraeli et al., 2001), intradiamond variation was limited and the range was spanned because of the variation between different diamonds. Recently, Shiryaev et al. (2003) reported the finding of a wide compositional range in individual diamond from Brazil. Here we present the first detailed study of a diamond with such a wide compositional range. This, and the clear radial compositional zoning along profiles, allowed us to examine in details the formation of the diamond. The bridging between brine and carbonatitic endmember composition constrains the possible relation among the three endmembers and their evolution.
2. Lac de Gras diamonds The Diavik mine is composed of four high-grade kimberlites (A154 North, A154 South, A418 and
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A21). The kimberlites are located just off East Island beneath the shallow waters of Lac de Gras. The kimberlites are small (< 2000 m2), steep sided, and are hosted in a complex of Archean granitoids and micaceous metasediments of the Slave Craton (Graham et al., 1999). Griffin et al. (1999) suggested that the lithospheric mantle underneath the central Slave Craton consists of an ultradepleted, olivine-rich upper layer (140 – 150 km) and a less depleted lower layer where eclogites are more abundant (200 – 220 km). Over 150 pipes have been identified in the Lac de Gras area, most of those are Cretaceous to Paleocene age (Heaman and Kjarsgaard, 2000; Creaser et al., 2003). A quarter of the kimberlites carry macrodiamonds. Color and morphology was determined for diamonds from the neighboring Ekati kimberlites in the Lac de Gras vicinity. The three most common groups are white octahedra, brown octahedra and fibrous diamonds (Gurney et al., 2003). The inclusion population found in pipes in the Lac de Gras area consists of peridotitic, eclogitic and super deep paragenesis (ferropericlase and MgSi and CaSi perovskite). However the abundance of the different suits varies between pipes. For example, Tappert et al. (2003) and Chinn et al. (1999) found dominance of peridotitic inclusions in diamonds from Panda, Misery, Sable and Jay pipes, while Davies et al. (1999, 2003) recorded high abundance of eclogitic mineral inclusions in DO-27, A154, A418, A 21, Ranch Lake, DO18 and DD17 pipes.
3. Analytical methods The diamond was polished and cleaned in HF (60%), HNO3 (69%) and ethanol and rinsed in distilled water in order to remove all organic and inorganic surface contamination prior to analysis. It was then carbon-coated and analyzed using a JEOL JXA 8600 electron probe micro analyzer (EPMA). Cathodoluminescence (CL) images of the diamond faces were collected using a Gatan MiniCL attached to the electron probe and were used as maps of the diamond internal structure. Individual, shallow, subsurface microinclusions were detected using backscattered electron imaging and were accurately related to specific diamond growth layers using the CL imaging. Individual inclusions
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were measured using a focused 15 keV, 10 nA electron beam. The inclusions were analyzed for 100 s using a Pioneer-Norvar energy depressive spectrometer (EDS). The beam interaction volume is about 4 Am3, and is larger then the common inclusion volume (< 1 Am3). The spectral data were reduced using the PROZA correction procedure supplied by Noran (Bastin and Heijligers, 1991). As the inclusion material comprises only a few percent of the total analyzed volume it was assumed that the difference to 100 wt.% is comprised of pure carbon. The small and variable totals obtained for the inclusions varied between 2 and 26 wt.%. As the total reflects mostly the size and depth of each inclusion, it made sense to renormalize all results to 100% (water and CO2-free basis). The original total is also reported. Izraeli et al. (in press) verified the accuracy of the measurements by analyzing olivine microinclusions of restricted composition. They concluded that the accuracy is better than 15% for the major elements in individual inclusions. Precision is even better, of the order of a few percent. Carbon isotopic composition was determined along a cross-section through one of the diamond faces (Face I) using a Cameca 6F secondary-ion mass spectrometer (SIMS) with Cs+ source (Hauri et al., 2002). The d13C values were determined using extreme energy filtering; the total uncertainty (accuracy + precision) of the result is F 0.4x. d13C values are calculated relative to a working standard with composition of 6.51 F 0.1xand reported relative to Pee Dee Belemnite (PDB) standard (Hauri et al., 2002). No infrared spectroscopic data are available for this diamond due to its opaque nature.
4. Description of the diamond Diamond ON-DVK-294 is 2.6 mm in diameter and weighs 22.8 mg. The diamond is opaque, gray in color and has an external octahedral morphology. Two parallel faces were polished to form a 1-mmthick slab (Fig. 1). One of the faces (Face I) intersected the central part of the diamond. It exposed a transparent core (A) surrounded by a white layer containing cavities of approximately 20 Am (B). A gray opaque zone (C) surrounds the hollowed layer. Under a microscope, the gray zone appears similar to the fibrous material of coated diamonds. It is sur-
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Fig. 1. Diamond ON-DVK-294. (a) A reflected light image of the polished Face I reveals concentric zoning: (A) transparent core, (B) internal cavity-filled layer, (C) internal gray layer, (D) external cavity-filled layer, (E) an external gray layer. (b) A reflected light image of polished face II exposing layer E only. (c) CL image of Face I. All layers visible by reflected light are apparent. The outermost zone (E) is divided into a darker inner zone (E1) and a brighter outer zone (E2), not visible in reflected light. (d) CL image of Face II. This face exposes the inner darker zone (E1) and an outer brighter zone (E2) (colors in the CL images were inverted to allow a better view).
rounded by another cavity-rich layer (D) and a gray fibrous rim (E) (Fig. 1a). Cathodoluminescence (CL) of the core reveals some brighter areas that may reflect pressure deformation and some resorption of the core margins. The CL image of the outer fibrous layer reveals that it consists of two zones (Fig. 1c): a thin inner darker zone (E1), and an outer brighter zone (E2). Face II exhibits a uniform gray opaque surface in normal light (Fig. 1b). The CL image of this face reveals an inner darker zone (E1) and an external brighter zone (E2) (Fig 1d). This transition is similar to the one recognized in the outer fibrous layer of Face I.
5. Results One hundred and sixty-seven microinclusions were analyzed in the fibrous zones exposed on both
diamond faces. Most inclusions are indicative of fluids with compositions varying between carbonatitic melt and brine, but 29 microinclusions represent peridotitic minerals. All mineral inclusions were detected on face I. No inclusions were detected in the cavity filled zones (Fig. 1a, zones B and D). 5.1. Mineral microinclusions All the mineral inclusions detected belong to the peridotitic suite. They are all smaller than 1 Am and their average total oxide content is 12 F 8 wt.%.
5.1.1. Cr-diopside Twenty-six inclusions were detected. The composition of one representative individual inclusion along with the average composition is presented in Table 1.
O. Klein-BenDavid et al. / Lithos 77 (2004) 243–253 Table 1 Composition of mineral inclusions in diamond ON-DVK-294 Phase (wt.%)
Cr-diopside
Inclusion number SiO2 TiO2 Al2O3 FeO MgO CaO BaO Na2O K2O P2O5 Cl Cr2O3 #Mg Analyzed totalc Formula units Si Ti Al Fe Mg Ca Ba Na K P Cl Cr
182b
Chromite
Average (11)a Standard deviation
52.3 1.34 1.98 1.55 14.3 17.2 n.d. 4.02 1.45 0.55 1.45 4.12 0.94 12.5
51.2 0.24 2.37 1.84 14.5 16.5 0.11 5.22 1.79 0.12 1.64 4.44 0.93 13.4
2.79 0.42 0.44 1.09 1.51 2.01 0.36 2.22 1.05 0.21 0.94 1.20 0.04 6.4
(6 oxygen) 1.94 0.04 0.09 0.05 0.79 0.68 n.d. 0.29 0.07 0.02 0.09 0.12
189
190
n.d. 0.25 8.28 17.1 13.1 0.27 n.d. 0.62 0.28 n.d. 0.11 60.0
1.52 n.d. 9.00 16.6 12.6 n.d. 2.26 0.96 0.98 n.d. n.d. 56.1
26.2
9.9
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5.1.3. Olivine One inclusion has Mg# of 0.93 but (Mg + Fe)/Si ratio of 2.2. High fluorine content indicates contamination during sample preparation. The inclusion probably contained an olivine crystal, but the contamination precludes accurate analysis.
(32 oxygen) 1.92 0.01 0.11 0.06 0.81 0.67 0.002 0.38 0.09 0.004 0.10 0.002
0.05 0.01 0.02 0.04 0.07 0.09 0.01 0.16 0.05 0.01 0.06 0.01
n.d. 0.05 2.58 3.78 5.15 0.08 n.d. 0.32 0.09 n.d. 0.05 12.54
0.40 n.d. 2.82 3.67 4.99 n.d. 0.24 0.49 0.33 n.d. n.d. 11.78
n.d.—not detected. a Average composition of 11 inclusions that have low Cl and K concentration (contain only a small amount of brine). b Representative analysis. c The original total contents of oxide and chlorine before renormalization to 100%.
The average Mg# is 0.94 F 0.04 and the Ca/(Ca + Mg + Fe) ratio is 0.43 F 0.05. The Cr2O3 content is 4.4 F 1.2 wt.%. Some of the diopside-bearing inclusions are high in K and Cl (up to 8% Cl), indicating the entrapment of some brine together with the mineral phase in a single inclusion. 5.1.2. Chromite Two inclusions share a Cr/(Cr + Al) ratio of 0.82 (Table 1).
Fig. 2. (A) Composition of the microinclusions in ON-DVK-294 and in diamonds from other localities. Hydrous silicic melt endmember composition falls close to the Si + Al apex. The carbonatitic melt falls close to the Ca + Mg + Fe + Na apex and the K + Cl apex represents the brine. D: carbonatitic melt inclusions in ON-DVK-294; o: brine-rich inclusions in ON-DVK-294. E: Koffiefontein brine (Izraeli et al., 2001); y: Koffiefontein melt (Izraeli, unpublished data); .: Botswanan melt (Schrauder and Navon, 1994). (B) A K, Cl and Ca + Mg + Fe + Na projection of the same data set.
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presents the location of the inclusions relative to the zones revealed by CL. Careful examination of the data showed that inclusions located in the inner growth zone are enriched in the brine component, while the inclusions in the outer zone are enriched in the
Fig. 3. Map of the analyzed inclusions on Face II. The different symbols represent analyses performed on different days.
5.2. Fluid microinclusions Microinclusions containing fluids were found on both sides of the diamond. On Face I they were detected in layers C and E (Fig. 1a). Fig. 2 presents the wide range of composition spanned by the inclusions between the brine and carbonatitic melt endmembers. 5.2.1. Spatial relation between the two components Face II was mapped in detail in order to detect any zoning in the composition of the inclusions. Fig. 3
Fig. 4. Mg (o) and Cl (n) variations along a profile across the diamond. The brine resides in the inner zone (high Cl and low Mg concentration) and the carbonatitic melt in the outer zone (low Cl and high Mg concentration).
Fig. 5. Variation diagrams of inclusions containing brine and carbonatitic melts. 5: Carbonatitic melt, y: brine.
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similarly (not shown). Potassium is present in both fluid compositions, but reaches its maximum concentration in the brine-rich inclusions. The sodium content is high in all fluid compositions. From these observations it is clear that compositions vary between two endmembers—The brineenriched composition: The average composition of the fluid in all the brine-bearing inclusions is K18Na24Ca6Mg10Fe4Ba3Si5PCl 26 (mol%) and the composition of five inclusions that show the strongest enrichment in Cl is K19Na25Ca5Mg8Fe3Ba2Si4Cl32 (Table 2). In these brine-rich inclusions, the most abundant cation is sodium, the average Na/Cl ratio is 0.8 F 0.1. The K/Cl ratio is 0.6 F 0.1 and the Mg/Cl and Ca/Cl ratios are 0.2 F 0.04 and 0.14 F 0.05 re-
carbonatitic melt component. Face II (Fig. 4) clearly presents the compositional difference between the two zones but the transition is not as sharp as exhibited by CL. Some inclusions with intermediate composition were analyzed in the contact area between the two zones. The inclusions detected on Face I of this diamond have similar chemical compositions and obey the same spatial relations. The inclusions in the outermost zone contain carbonatitic melt, while the inner zones contain brine inclusions. As an example a negative correlation between Mg and Cl is exhibited in Fig. 5. Magnesium decreases from 28 mol% in the carbonatitic composition to almost zero in the brine-rich compositions, while Cl rises from zero to more than 30%. Calcium behaves
Table 2 Composition of fluid inclusions in diamond ON-DVK-294 Brine
Carbonatitic melt a
Representative inclusions Inclusion number (wt.%) SiO2 TiO2 Al2O3 FeO MgO CaO BaO Na2O K2 O P2O5 Cl Analysed total (wt.%) (mol%) Si Ti Al Fe Mg Ca Ba Na K P Cl a
129
End Averagea Standard member deviation
End Average Standard Representative inclusions member deviation
66
70
71
153
5.8 1.1 0.4 7.6 9.9 8.7 13.1 12.8 21.1 2.5 19.3 7.6
4.4 n.d. 1.0 11.0 8.9 8.5 11.9 15.1 19.3 1.9 21.0 6.1
5.7 4.0 7.0 6.7 n.d. n.d. n.d. 1.0 n.d. n.d. 2.1 – 10.5 3.8 6.7 5.3 9.4 6.6 10.9 7.7 7.7 6.7 9.0 6.4 12.0 12.0 14.5 8.7 16.7 22.4 13.1 18.9 21.0 21.5 19.4 21.5 1.8 1.1 2.2 0.6 18.1 28.3 16.7 28.2 6.1 5.7 6.0
6.4 0.8 0.9 6.3 9.1 8.1 10.2 17.6 20.0 2.0 21.6 3.9
4.5 0.6 0.4 4.9 11.4 7.1 3.9 19.0 20.7 1.6 25.1
3.3 n.d. 0.9 6.9 9.9 6.8 3.5 21.8 18.4 1.2 26.6
4.3 2.7 5.4 4.5 n.d. n.d. n.d. 0.6 n.d. n.d. 1.9 – 6.6 2.1 4.3 3.0 10.4 6.7 12.6 7.8 6.2 4.8 7.5 4.7 3.5 3.2 4.4 2.4 24.2 29.2 19.7 24.7 20.1 18.5 19.2 18.5 1.2 0.6 1.4 0.4 23.0 32.3 21.9 32.4
4.6 0.4 0.8 3.8 9.8 6.3 2.9 24.4 18.4 1.2 26.1
87
92
143
145
180
2.4 1.5 1.4 3.0 2.4 2.4 4.6 4.7 4.6 1.8 5.1 2.0
12.3 1.2 2.5 8.2 19.4 15.2 4.3 12.0 12.2 3.8 9.0 7.0
9.8 7.4 n.d. n.d. 0.8 0.8 6.9 7.9 18.2 18.6 12.9 16.3 10.3 4.0 15.6 15.5 13.8 15.0 3.1 2.4 9.5 12.6 5.0 5.1
9.0 2.8 1.3 10.3 21.1 12.9 5.0 15.2 13.3 2.5 6.8 7.4
7.3 2.5 2.2 8.0 19.1 12.6 5.5 16.6 12.1 3.1 8.3 5.0
12.4 0.7 2.7 10.2 21.3 12.5 6.8 13.6 11.1 3.5 3.5
9.5 1.0 1.5 7.7 18.6 12.8 7.9 15.7 12.8 3.1 8.8 5.0
2.9 1.7 1.3 2.1 2.8 3.0 4.7 2.9 2.2 1.8 3.1 2.3
1.8 0.8 1.2 1.9 2.8 2.1 1.4 5.1 3.8 1.1 4.7
9.6 0.7 2.3 5.3 22.5 12.7 1.3 18.1 12.1 2.5 11.9
7.6 5.4 n.d. n.d. 0.8 0.7 4.5 4.9 21.1 20.5 10.7 12.9 3.1 1.2 23.5 22.2 13.6 14.1 2.0 1.5 12.5 15.7
7.0 1.6 1.2 6.7 24.3 10.7 1.5 22.7 13.1 1.6 9.0
5.6 1.4 2.0 5.1 22.0 10.4 1.6 24.7 11.9 2.0 10.8
10.1 0.4 2.5 6.9 25.6 10.8 2.2 21.3 11.4 2.4 4.8
7.4 0.6 1.4 5.0 21.5 10.6 2.4 23.6 12.6 2.0 11.4
2.3 1.0 1.2 1.4 3.2 2.6 1.5 4.4 2.1 1.2 4.0
Average composition of 71 brine inclusions and 67 carbonatitic melt inclusions with their standard deviation.
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spectively. On the average, the mono and divalent positive ions account for 80 positive charges of which only 32 are compensated by Cl ions, the rest may be compensated by carbonate ions (Izraeli et al., 2001). The carbonatitic melt enriched composition: The average composition of all the inclusions that carry carbonatitic melt is K13Na24Ca11Mg22Fe5Ba2Si7 AlP2Cl11 (mol%) and the composition of the five inclusions that are richest in carbonatitic melt is K11Na21Ca11Mg26Fe7Ba2Si10Al3P2Cl5 (Table 2). In this carbonatitic melt endmember sodium is also the most abundant cation. However the Cl concentration is much lower compared with the brine, The Na/Cl ratio of the carbonatitic melt is 5.1 F 2.5, the K/Cl ratio is 2.6 F 1.0 and the Mg/Cl ratio is 6.0 F 2.3. The average total content of the oxides and chloride for all fluid inclusions is 4.4 F 2.2 wt.% and is lower than that obtained for the mineral inclusions. A possible reason for that difference may be a smaller size of the fluid inclusions. Alternatively, if the size of the different microinclusions in this diamond is similar, the lower totals measured for the nonmineral inclusions may reflect a high concentration of light elements not measured by EPMA (Izraeli et al., 2001). We could not analyze the water and carbonate of the inclusions by InfraRed spectroscopy. However, in analogy with the Koffiefontein and Jwaneng diamonds (Schrauder and Navon, 1994; Izraeli et al., 2001), we suggest that water and carbonate are also present in the inclusions of ON-DVK-294.
Fig. 6. Variation in the C isotopic composition (d13C) along a line from the diamond core through its fibrous layers (error bars F 0.4x).
yielded values of 5.2x and 6x. The coat is depleted in 13C relative to the core. Three measurements in the inner zone (B to E1), where brine inclusions were found, had similar d13C of 8.1x to 8.9 x. Additional depletion in 13C was observed in the outer zone (E2), where the carbonatitic inclusions were found. The d13C measured in two points was 11.4x and 12.8x (Fig. 6). All five measurements in the fibrous zones revealed isotopic compositions that are more depleted than the worldwide range of fibrous diamonds (Cartigny et al., 1998, 2003).
6. Discussion 5.3. Carbon isotopic composition 6.1. Comparison to other mantle derived fluids Table 3 presents C isotopic composition of the diamond matrix, as measured by SIMS. d13C was measured at seven points across Face I. The core (A) Table 3 Carbon isotopic composition of selected points across Face I of diamond ON-DVK-294 Point location Zone Zone Zone Zone Zone Zone Zone
A A B C D E2 E2
d13C (F 0.4x) 6.0 5.2 8.9 8.6 8.1 12.8 11.4
Carbonatitic melts and brine inclusions were previously reported for diamonds from Jwaneng (Schrauder and Navon, 1994) and Koffiefontein (Izraeli et al., in press). Partial analyses of chlorine-rich inclusions (0.2 –2 Am in size) were also reported by Chen et al. (1992) in Chinese diamonds. However, no full analysis was provided. Recently, Kamenetsky et al. (2002) reported Na – K –Cl and C-rich inclusions in olivine phenocrysts from the Udachnaya kimberlite. Fig. 2 presents the chemical composition of the fluids from Jwaneng and Koffiefontein along with the compositions detected in ON-DVK-294. It is clear that the microinclusions in this diamond bridge the compositional gap between the brine-rich inclusions from
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Koffiefontein and the carbonatitic melt endmember of the Jwaneng diamonds. The composition of brine microinclusions in diamond ON-DVK-294 is broadly similar to the brine detected in the Koffiefontein cloudy diamonds. The Canadian brine inclusions carry higher proportions of the carbonatitic endmember, but their K/Cl ratio (0.7 F 0.1) is similar to that of Koffiefontein (0.64 F 0.06). Thus, the fundamental components of the brine endmember are similar in diamonds from different cartons. The fact that the ON-DVK-294 brine was detected in a fibrous coat while the Koffiefontein brine was detected in inner clouds within octahedral diamonds indicates that diamonds of both habits can grow from similar solutions. The main difference between the two brines is the strong enrichment in Na concentration in the Diavik diamond. It is interesting that Izraeli et al. (2001) noted higher concentration of Na in diamonds carrying peridotitic inclusions. The Na/K ratio was 0.4 F 0.1 in two peridotitic diamonds compared with 0.12 F 0.06 in the brine of five eclogitic diamonds. ON-DVK-294 also contains peridotitic mineral inclusions, but its enrichment in Na (Na/K = 1.3) in the brine-rich inclusions is even higher that in any of the Koffiefontein peridotitic diamonds. The carbonatitic melt inclusions in ON-DVK-294 (Fig. 2) show similar characteristics to the carbonatitic melt endmember in the Jwaneng cubic diamonds (Schrauder and Navon, 1994) and to the carbonatitic melt detected recently in Koffiefontein (Izraeli et al., unpublished data). All the fluids are enriched in divalent ions such as Mg, Ca and Fe; however, the ON-DVK-294 carbonatitic melt is characterized by higher proportions of Mg. It is also unique in its high Na and notable Ba content. The similar nature of the Botswanan, Koffiefontein and ON-DVK-294 carbonatitic melts become is impressive considering the fact that the Botswanan melts align along the compositional range between the carbonatitic and the hydrous silicic melt, whereas the Diavik fluids represent mixtures along an array between carbonatitic melt and brine. The fact that both arrays meet at a similar endmember composition suggests that the carbonatitic melt is the link connecting all fluids and that all fluids along both arrays are genetically related. Johnson et al. (2000) and Burgess et al. (2002) studied the composition of Ar, Cl, Br, I, Ca and K in
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fibrous diamond from Africa, Russia and Canada. In most diamonds, they found uniform I/Cl and Br/Cl ratios that are similar to depleted astenospheric mantle ratios derived from MORB data. Only in some Canadian diamonds (from the Ekati kimberlites, Lac de Gras area) they detected much higher Br/Cl and I/Cl ratios that deviated significantly from the restricted range of all other diamonds. The analyses of all microinclusions in diamonds from Congo, Botswana and Siberia revealed only carbonatitic to hydrous silicic melts (Navon et al., 1988; Schrauder and Navon, 1994; Klein-BenDavid and Logvinova, unpublished data), but no brine. It is possible that the high I/Cl and Br/Cl ratios in some of the Canadian diamonds are related to the formation of the halogenrich brine similar to that found in ON-DVK-294 diamond. The high ratios may represent preferred fractionation of Br and I into the brine, whereas Cl, as a major element is enriched to a lesser amount. 6.2. Evolution of the fluid The similar compositions of carbonatitic melts associated with brine and with hydrous silicic melt suggest that both fluids are the products of a primary carbonatitic melt. If so, fluids should evolve from carbonatitic to brine-rich compositions, but the fluid inclusions in ON-DVK-294 show the reversed transition from brine-enriched fluids in the inner part of the diamond to carbonatitic melt at the diamond rim. However, the inclusions in ON-DVK-294 do not show a continuous, uniform evolution. The transition between the brine-rich central zones (C and E1) and the carbonatitic outer zone (E2) is abrupt with only a few inclusions close to the interface showing intermediate compositions (Fig. 4). This reflects a rapid but continues transition from brine-rich to carbonatitic composition. The observed compositional zoning is accompanied by an even sharper transition in the CL image (Fig. 1c and d) and a significant depletion in 13 C (Fig. 6) from the inner fibrous zones ( 8.5x) to the outer zone ( 12.1x). We suggest that the sharp compositional and isotopic transition reflects mixing between evolved brine and freshly introduced carbonatitic melt infiltrating into the area. Most probably, the change in the isotopic composition of the diamond reflects growth from a more 13C depleted carbonatitic melt. More data
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are needed before the source of the fluids can be inferred. 7. Conclusions The compositional range of microinclusions trapped in diamond ON-DVK-294 sheds light on fluid evolution in the diamond-forming environment in the mantle. The compositional gap between brine found in cloudy octahedral diamonds from South African and carbonatitic melt analyzed in cubic diamonds from Zaire and Botswana is now closed. The compositions of all microinclusions analyzed to date fall along two arrays connecting the carbonatitic melt composition to either a hydrous silicic endmember or to a brine endmember (Fig. 2). The connection between all fluids found in microinclusions in fibrous and cloudy diamonds from various cratons suggest that diamonds are formed from fluids derived form a mantle source not significantly influenced by local heterogeneities. The composition of microinclusions in diamond ON-DVK-294 changes from brine-rich to carbonatitic. This transition is associated with depletion in the carbon isotopic composition of the diamond. We suggest that this transition reflects mixing between an already evolved brine and a freshly introduced carbonatitic melt of different isotopic composition. References Bastin, G., Heijligers, J., 1991. In: Heinrich, K., Newbury, D. (Eds.), Electron Probe Quantitation, Workshop at the National Bureau of Standards, Gaithersburg, Maryland. Plenum, New York, pp. 145 – 161. Boyd, S.R., Pillinger, C.T., Milledge, H.J., Seal, M.J., 1992. CIsotopic and N-Isotopic composition and the infrared-absorption spectra of coated diamonds-evidence for the regional uniformity of CO2 – H2O rich fluids in lithospheric mantle. Earth and Planetary Science Letters 108 (1 – 3), 139 – 150. Bulanova, G.P., 1995. The formation of diamond. Journal of Geochemical Exploration 53 (1 – 3), 1 – 23. Bulanova, G.P., Griffin, W.L., Ryan, C.G., 1998. Nucleation environment of diamonds from Yakutian kimberlites. Mineralogical Magazine 62 (3), 409 – 419. Burgess, R., Layzelle, E., Turner, G., Harris, J.W., 2002. Constraints on the age and halogen composition of mantle fluids in Siberian coated diamonds. Earth and Planetary Science Letters 197 (3 – 4), 193 – 203.
Cartigny, P., Harris, J.W., Phillips, D., Girard, M., Javoy, M., 1998. Subduction-related diamonds? The evidence for a mantle-derived origin from coupled dC13 – dN15 determinations. Chemical Geology 147 (1 – 2), 147 – 159. Cartigny, P., Harris, J.W., Taylor, A., Davies, R., Javoy, M., 2003. On the possibility of a kinetic fractionation of nitrogen stable isotopes during natural diamond growth. Geochimica et Cosmochimica Acta 67 (8), 1571 – 1576. Chen, F., Guo, J., Chen, C., Liu, C., 1992. High-K and high-Cl inclusions in diamond and mantle metasomatism. Acta Mineralogica Sinica 12 (3), 193 – 198. Chinn, I., Gurney, J., Kyser, K., 1999. Diamonds and mineral inclusions from the NWT, Canada. 7th International Kimberlite Conference. Addendum to Extended Abstracts. Cape Town, South Africa. Chrenko, R., McDonald, R., Darrow, K., 1967. Infrared spectrum of diamond coat. Nature 214, 474 – 476. Creaser, R., Grutter, H., Carlson, J., Crawford, B., 2003. Macrocrystal phlogopite Rb – Sr dates for the Ekati property kimberlites, Slave province, Canada: evidence for multiple intrusive episodes in the Paleocene and Eocene. 8th International Kimberlite Conference, Extended Abstracts, Victoria, Canada. Davies, R., Griffin, W., Pearson, N., Andrew, A., Doyle, B., O’Reilly, S., 1999. Diamonds from the deep; Pipe DO-27, Slave Craton, Canada. Proceedings of the 7th International Kimberlite Conference. Red Roof Design Cc, Cape T, South Africa, pp. 148 – 155. Davies, R., Griffin, W., O’Reilly, S., Doyle, B., 2003. Geochemical characteristics of microdiamonds from kimberlites at Lac De Gras, central Slave craton. 8th International Kimberlite Conference, Extended Abstracts, Victoria, Canada. Galimov, E.M., 1991. Isotope fractionation related to kimberlite magmatism and diamond formation. Geochimica et Cosmochimica Acta 55 (6), 1697 – 1708. Graham, I., Burgess, J., Bryan, D., Ravenscroft, P., Thomas, E., Doyle, B., Hopkins, R., Armstrong, K., 1999. Exploration history and geology of the Diavik Kimberlites, Lac de Gras, Northwest Territories, Canada. Proceedings of the 7th International Kimberlite Conference. Red Roof Design, Cape Town, South Africa, pp. 262 – 279. Griffin, W.L., Doyle, B.J., Ryan, C.G., Pearson, N.J., O’Reilly, S.Y., Davies, R., Kivi, K., Van Achterbergh, E., Natapov, L.M., 1999. Layered mantle lithosphere in the Lac de Gras area, Slave Craton: composition, structure and origin. Journal of Petrology 40 (5), 705 – 727. Gurney, J., Hildebrand, P., Carlson, J., Dyck, D., Fedortchouk, F., 2003. Diamonds from the Ekati core and buffer zone Properties. 8th International Kimberlite Conference, Extended Abstracts, Victoria, Canada. Guthrie, G., Veblen, D., Navon, O., Rossman, G., 1991. Submicrometer fluid inclusions in turbid-diamond coats. Earth and Planetary Science Letters 105, 1 – 12. Harris, J., 1992. Diamond geology. In: Field, J. (Ed.), The Properties of Natural and Synthetic Diamonds. Academic Press, Oxford, UK, pp. 384 – 385. Hauri, E.H., Wang, J., Pearson, D.G., Bulanova, G.P., 2002. Microanalysis of delta C-13, delta N-15, and N abundances in dia-
O. Klein-BenDavid et al. / Lithos 77 (2004) 243–253 monds by secondary ion mass spectrometry. Chemical Geology 185 (1 – 2), 149 – 163. Heaman, L., Kjarsgaard, B., 2000. Timing of eastern North American kimberlite magmatism: continental extension of the Great Meteor Hotspot track? Earth and Planetary Science Letters 178, 253 – 268. Izraeli, E.S., Harris, J.W., Navon, O., 2001. Brine inclusions in diamonds: a new upper mantle fluid. Earth and Planetary Science Letters 187 (3 – 4), 323 – 332. Izraeli, E.S., Harris, J.W., Navon, O., 2003. Fluid-and mineral inclusions in cloudy diamonds from Koffiefontein, South Africa. Geochmica et Cosmochemica Acta (in press). Johnson, L., Burgess, R., Turner, G., Milledge, H., Harris, J., 2000. Noble gas and halogen geochemistry of mantle fluids: comparison of African and Canadian diamonds. Geochimica et Cosmochimica Acta 64 (4), 717 – 732. Kamenetsky, M., Sobolev, A., Kamenetsky, V., Maas, R., Danyushevsky, L., Sobolev, N., Pokhilenko, N., 2002. Origin, composition and fractionation of the Udachnaya pipe kimberlitic melts: constraints from melt, fluid and crystal inclusions in olivine. Goldschmidt Conference, Extended Abstracts, Davos, Switzerland. Klein-BenDavid, O., Logvinova, A.M., Izraeli, E., Sobolev, N.V., Navon, O., 2003. Sulfide melt inclusions in Yubileinayan (Yakutia) diamonds. 8th International Kimberlite Conference, Extended Abstracts, Victoria, Canada. Lang, A., Walmsley, J., 1983. Apatite inclusion in natural diamond coat. Physics and Chemistry of Minerals 9, 6 – 8. Meyer, H.O.A., 1987. Inclusions in diamond. In: Nixon, P.H. (Ed.), Mantle Xenoliths. Wiley, Chichester, UK, pp. 501 – 522. Milledge, H., Mendelssohn, M., Woods, P., Seal, M., Pillinger, C., Mattey, D., Carr, L., Wright, I., 1984. Isotopic variations in diamond in relation to cathodoluminescence. Acta Crystallographica. Section A, Foundations of Crystallography 40, 255. Navon, O., 1999. Formation of diamonds in the Earth’s mantle. Proceedings of the 7th International Kimberlite Conference. Red Roof Design, Cape Town, South Africa, pp. 584 – 604. Navon, O., Hutcheon, I.D., Rossman, G.R., Wasserburg, G.L., 1988. Mantle-derived fluids in diamond micro-inclusions. Nature 335, 784 – 789.
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Raga, S., Davies, R.M., Griffin, W.L, Jackson, S., O’Reilly, S.Y., 2003. Trace element analysis of diamond by LAM ICPMS: preliminary results. 8th International Kimberlite Conference, Extended Abstracts, Victoria, Canada. Schrauder, M., Navon, O., 1994. Hydrous and carbonatitic mantle fluids in fibrous diamonds from Jwaneng, Botswana. Geochimica et Cosmochimica Acta 58 (2), 761 – 771. Schrauder, M., Koeberl, C., Navon, O., 1996. Trace element analyses of fluid-bearing diamonds from Jwaneng, Botswana. Geochimica et Cosmochimica Acta 60 (23), 4711 – 4724. Shiryaev, A., Izraeli, E., Hauri, E.H., Galimov, E.M., Navon, O., 2003. Fluid inclusions in Brazilian coated diamonds. 8th International Kimberlite Conference, Extended Abstracts, Victoria, Canada. Sobolev, N., Kaminsky, F., Griffin, W., Yefimova, E., Win, T., Ryan, C., Botkunov, A., 1997. Mineral inclusions in diamonds from the Sputnik kimberlite pipe, Yakutia. Lithos 39 (3 – 4), 135 – 157. Sunagawa, I., 1984. Morphology of natural and synthetic diamond crystals. Materials Science of the Earth’s Interior, Terra Sci. Publ. Co., 303 – 330. Tal’nikova, S.B., 1995. Inclusions in natural diamonds of different habits. 6th International Kimberlite Conference, Extended Abstracts, Novosibirsk, Russia, pp. 603 – 605. Tappert, R., Stachel, T., Harris, J., Brey, G., 2003. Mineral inclusions in diamonds from Panda kimberlite, Slave province (Canada). 8th International Kimberlite Conference, Extended Abstracts, Victoria, Canada. Taylor, L.A., Keller, R.A., Snyder, G.A., Wang, W.Y., Carlson, W.D., Hauri, E.H., McCandless, T., Kim, K.R., Sobolev, N.V., Bezborodov, S.M., 2000. Diamonds and their mineral inclusions, and what they tell us: a detailed ‘‘pull-apart’’ of a diamondiferous eclogite. International Geology Review 42 (11), 959 – 983. Tomilenko, A.A., Chepurov, A.I., Palyanov, Y.N., Pokhilenko, L.N., Shebanin, A.P., 1997. Volatile components in the upper mantle (from data on fluid inclusions). Geologiya I Geofizika 38 (1), 276 – 285.
Mantle fluid evolution—a tale of one diamond Ofra Klein-BenDavid a,*, Elad S. Izraeli a, Erik Hauri b, Oded Navon a b
a Institute of Earth Sciences, The Hebrew University of Jerusalem, Jerusalem 91904, Israel Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015, USA
Received 27 June 2003; accepted 12 November 2003 Available online 25 May 2004
Abstract Microinclusions analyzed in a coated diamond from the Diavik mine in Canada comprise peridotitic minerals and fluids. The fluids span a wide compositional range between a carbonatitic melt and brine. The diamond is concentrically zoned. The brine microinclusions reside in an inner growth zone and their endmember composition is K19Na25Ca5Mg8Fe3Ba2Si4Cl32 (mol%). The carbonatitic melt is found in an outer layer and its endmember composition is K11Na21Ca11Mg26Fe7Ba2Si10Al3P2Cl5. The transition in inclusion chemistry is accompanied by a change in the carbon isotopic composition of the diamond from 8.5x in the inner zone to 12.1x in the outer zone. We suggest that this transition reflects mixing between already evolved brine and a freshly introduced carbonatitic melt of different isotopic composition. The compositional range found in diamond ON-DVK-294 is the widest ever recorded in a single diamond. It closes the gap between brine found in cloudy octahedral diamonds from South Africa and carbonatitic melt analyzed in cubic diamonds from Zaire and Botswana. Thus, all microinclusions analyzed to date fall along two arrays connecting the carbonatitic melt composition to either a hydrous-silicic endmember or to a brine endmember. This connection suggests that many diamonds are formed from fluids derived form a mantle source not significantly influenced by local heterogeneities. D 2004 Elsevier B.V. All rights reserved. Keywords: Inclusion; Brine; Carbonatitic melt; Peridotitic; Coated diamond; Fibrous diamond
1. Introduction Diamond inclusions provide a unique opportunity to investigate the mantle rocks where diamonds are formed and the fluids from which they crystallize. The strength of the diamond and its low reactivity in a silicate environment ensure that the trapped substances remain shielded from their changing environment. Mineral inclusions are widespread in diamonds and * Corresponding author. Fax: +972-2-5662581. E-mail address: [email protected] (O. Klein-BenDavid). 0024-4937/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2004.04.003
range from less than a micrometer to millimeter size. They consist of silicates, oxides, sulfides, and rare carbonate and phosphate minerals and provide information on host rock lithology and on the temperature and pressure of diamond crystallization (Meyer, 1987). Most mineral inclusions indicate peridotitic and eclogitic host rocks at temperatures of 900 – 1300 jC and depth of 150 – 200 km (Meyer, 1987; Harris, 1992). Based on the internal structure of diamonds (Milledge et al., 1984; Bulanova, 1995), the low density of crystallographic dislocations (Sunagawa, 1984) and the association of diamonds with
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cracks within their host xenoliths (Taylor et al., 2000), it is believed that most natural diamonds grow from fluids. Fluid inclusions are the best source of information about the nature and chemical composition of the diamond’s growth medium, and shed light on its evolution. Fluid inclusions are found in fibrous diamonds, in the fibrous coat of coated diamonds and as internal clouds in octahedral diamonds (Navon, 1999). The inclusion-bearing zones are populated by millions of submicrometer inclusions separated from each other and enclosed in the diamond matrix. TEM analyses of individual inclusions identified a multiphase assemblage of apatite, quartz, mica and carbonate surrounded by material of low electron density (Lang and Walmsley, 1983; Guthrie et al., 1991). The common interpretation is that the inclusions trapped a fluid and that the minerals are secondary phases that grew during cooling. Water and carbonates detected by infrared (IR) spectroscopy (Chrenko et al., 1967; Navon et al., 1988) are probably the main volatile components. Electron probe microanalysis of the bulk composition of microinclusions reveals a wide range of compositions among three endmembers (Schrauder and Navon, 1994; Izraeli et al., 2001): (a) hydroussilicic melt rich in water, Si, Al, and K; (b) carbonatitic melt rich in carbonate, Mg, Ca, Fe, K and Na and (c) brine-rich in Cl, K and Na. Intermediate compositions between carbonatitic and hydrous-silicic melts were found in fibrous diamond (Navon et al., 1988; Schrauder and Navon, 1994). Limited mixing between carbonatitic melt and brine were found in cloudy diamonds (Izraeli et al., 2001). No intermediate compositions between brine and hydrous-silicic melt were detected. All compositions are rich in K and many other incompatible elements and are characterized by steep REE patterns similar to those in kimberlites and lamproites (Schrauder et al., 1996; Raga et al., 2003). Halogens and Ar are also enriched and the halogen abundance ratios are similar to those of MORB (Johnson et al., 2000; Burgess et al., 2002). In spite of the high abundance of sulfide minerals in diamonds, such inclusions were never found in fluid-bearing diamonds, reminding us that diamonds may grow from other media as well, e.g., sulfide melts (Bulanova, 1995; Sobolev et al., 1997; Bulanova et
al., 1998) or reduced carbon-bearing fluids (Tomilenko et al., 1997). It is interesting to note that Bulanova et al. (1998) and Klein-BenDavid et al. (2003) suggested involvement of carbonatitic melt in the formation of sulfide-bearing diamonds. The carbon isotopic composition of fibrous diamonds from localities worldwide ranges over relatively narrow d13C values, between 5x and 8x. Their nitrogen isotopic composition ranges between 0x and 10x. These values lie close to those of carbon in MORB and kimberlites (Galimov, 1991; Boyd et al., 1992; Cartigny et al., 1998, 2003). Fluid inclusions are rarely found together with mineral inclusions. Tal’nikova (1995) reported the finding of omphacite along with silicic melt in a Siberian diamond. Izraeli et al. (2001, in press) reported microinclusions bearing peridotitic and eclogitic minerals as well as hydrous minerals residing together with brine inclusions in cloudy diamonds from Koffiefontein. This, together with the evidence for metasomatism in diamondiferous xenoliths suggest that the fluids penetrated the peridotitic or eclogitic host rock and took part in diamond formation in both rock types. Here we report the composition of 167 microinclusions in one concentrically zoned diamond from the Diavik mine in Lac de Gras, Slave Craton, Canada. Microinclusions in this diamond carry peridotitic minerals and fluids with a broad compositional range. In all previous reports (Navon et al., 1988; Schrauder and Navon, 1994; Izraeli et al., 2001), intradiamond variation was limited and the range was spanned because of the variation between different diamonds. Recently, Shiryaev et al. (2003) reported the finding of a wide compositional range in individual diamond from Brazil. Here we present the first detailed study of a diamond with such a wide compositional range. This, and the clear radial compositional zoning along profiles, allowed us to examine in details the formation of the diamond. The bridging between brine and carbonatitic endmember composition constrains the possible relation among the three endmembers and their evolution.
2. Lac de Gras diamonds The Diavik mine is composed of four high-grade kimberlites (A154 North, A154 South, A418 and
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A21). The kimberlites are located just off East Island beneath the shallow waters of Lac de Gras. The kimberlites are small (< 2000 m2), steep sided, and are hosted in a complex of Archean granitoids and micaceous metasediments of the Slave Craton (Graham et al., 1999). Griffin et al. (1999) suggested that the lithospheric mantle underneath the central Slave Craton consists of an ultradepleted, olivine-rich upper layer (140 – 150 km) and a less depleted lower layer where eclogites are more abundant (200 – 220 km). Over 150 pipes have been identified in the Lac de Gras area, most of those are Cretaceous to Paleocene age (Heaman and Kjarsgaard, 2000; Creaser et al., 2003). A quarter of the kimberlites carry macrodiamonds. Color and morphology was determined for diamonds from the neighboring Ekati kimberlites in the Lac de Gras vicinity. The three most common groups are white octahedra, brown octahedra and fibrous diamonds (Gurney et al., 2003). The inclusion population found in pipes in the Lac de Gras area consists of peridotitic, eclogitic and super deep paragenesis (ferropericlase and MgSi and CaSi perovskite). However the abundance of the different suits varies between pipes. For example, Tappert et al. (2003) and Chinn et al. (1999) found dominance of peridotitic inclusions in diamonds from Panda, Misery, Sable and Jay pipes, while Davies et al. (1999, 2003) recorded high abundance of eclogitic mineral inclusions in DO-27, A154, A418, A 21, Ranch Lake, DO18 and DD17 pipes.
3. Analytical methods The diamond was polished and cleaned in HF (60%), HNO3 (69%) and ethanol and rinsed in distilled water in order to remove all organic and inorganic surface contamination prior to analysis. It was then carbon-coated and analyzed using a JEOL JXA 8600 electron probe micro analyzer (EPMA). Cathodoluminescence (CL) images of the diamond faces were collected using a Gatan MiniCL attached to the electron probe and were used as maps of the diamond internal structure. Individual, shallow, subsurface microinclusions were detected using backscattered electron imaging and were accurately related to specific diamond growth layers using the CL imaging. Individual inclusions
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were measured using a focused 15 keV, 10 nA electron beam. The inclusions were analyzed for 100 s using a Pioneer-Norvar energy depressive spectrometer (EDS). The beam interaction volume is about 4 Am3, and is larger then the common inclusion volume (< 1 Am3). The spectral data were reduced using the PROZA correction procedure supplied by Noran (Bastin and Heijligers, 1991). As the inclusion material comprises only a few percent of the total analyzed volume it was assumed that the difference to 100 wt.% is comprised of pure carbon. The small and variable totals obtained for the inclusions varied between 2 and 26 wt.%. As the total reflects mostly the size and depth of each inclusion, it made sense to renormalize all results to 100% (water and CO2-free basis). The original total is also reported. Izraeli et al. (in press) verified the accuracy of the measurements by analyzing olivine microinclusions of restricted composition. They concluded that the accuracy is better than 15% for the major elements in individual inclusions. Precision is even better, of the order of a few percent. Carbon isotopic composition was determined along a cross-section through one of the diamond faces (Face I) using a Cameca 6F secondary-ion mass spectrometer (SIMS) with Cs+ source (Hauri et al., 2002). The d13C values were determined using extreme energy filtering; the total uncertainty (accuracy + precision) of the result is F 0.4x. d13C values are calculated relative to a working standard with composition of 6.51 F 0.1xand reported relative to Pee Dee Belemnite (PDB) standard (Hauri et al., 2002). No infrared spectroscopic data are available for this diamond due to its opaque nature.
4. Description of the diamond Diamond ON-DVK-294 is 2.6 mm in diameter and weighs 22.8 mg. The diamond is opaque, gray in color and has an external octahedral morphology. Two parallel faces were polished to form a 1-mmthick slab (Fig. 1). One of the faces (Face I) intersected the central part of the diamond. It exposed a transparent core (A) surrounded by a white layer containing cavities of approximately 20 Am (B). A gray opaque zone (C) surrounds the hollowed layer. Under a microscope, the gray zone appears similar to the fibrous material of coated diamonds. It is sur-
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Fig. 1. Diamond ON-DVK-294. (a) A reflected light image of the polished Face I reveals concentric zoning: (A) transparent core, (B) internal cavity-filled layer, (C) internal gray layer, (D) external cavity-filled layer, (E) an external gray layer. (b) A reflected light image of polished face II exposing layer E only. (c) CL image of Face I. All layers visible by reflected light are apparent. The outermost zone (E) is divided into a darker inner zone (E1) and a brighter outer zone (E2), not visible in reflected light. (d) CL image of Face II. This face exposes the inner darker zone (E1) and an outer brighter zone (E2) (colors in the CL images were inverted to allow a better view).
rounded by another cavity-rich layer (D) and a gray fibrous rim (E) (Fig. 1a). Cathodoluminescence (CL) of the core reveals some brighter areas that may reflect pressure deformation and some resorption of the core margins. The CL image of the outer fibrous layer reveals that it consists of two zones (Fig. 1c): a thin inner darker zone (E1), and an outer brighter zone (E2). Face II exhibits a uniform gray opaque surface in normal light (Fig. 1b). The CL image of this face reveals an inner darker zone (E1) and an external brighter zone (E2) (Fig 1d). This transition is similar to the one recognized in the outer fibrous layer of Face I.
5. Results One hundred and sixty-seven microinclusions were analyzed in the fibrous zones exposed on both
diamond faces. Most inclusions are indicative of fluids with compositions varying between carbonatitic melt and brine, but 29 microinclusions represent peridotitic minerals. All mineral inclusions were detected on face I. No inclusions were detected in the cavity filled zones (Fig. 1a, zones B and D). 5.1. Mineral microinclusions All the mineral inclusions detected belong to the peridotitic suite. They are all smaller than 1 Am and their average total oxide content is 12 F 8 wt.%.
5.1.1. Cr-diopside Twenty-six inclusions were detected. The composition of one representative individual inclusion along with the average composition is presented in Table 1.
O. Klein-BenDavid et al. / Lithos 77 (2004) 243–253 Table 1 Composition of mineral inclusions in diamond ON-DVK-294 Phase (wt.%)
Cr-diopside
Inclusion number SiO2 TiO2 Al2O3 FeO MgO CaO BaO Na2O K2O P2O5 Cl Cr2O3 #Mg Analyzed totalc Formula units Si Ti Al Fe Mg Ca Ba Na K P Cl Cr
182b
Chromite
Average (11)a Standard deviation
52.3 1.34 1.98 1.55 14.3 17.2 n.d. 4.02 1.45 0.55 1.45 4.12 0.94 12.5
51.2 0.24 2.37 1.84 14.5 16.5 0.11 5.22 1.79 0.12 1.64 4.44 0.93 13.4
2.79 0.42 0.44 1.09 1.51 2.01 0.36 2.22 1.05 0.21 0.94 1.20 0.04 6.4
(6 oxygen) 1.94 0.04 0.09 0.05 0.79 0.68 n.d. 0.29 0.07 0.02 0.09 0.12
189
190
n.d. 0.25 8.28 17.1 13.1 0.27 n.d. 0.62 0.28 n.d. 0.11 60.0
1.52 n.d. 9.00 16.6 12.6 n.d. 2.26 0.96 0.98 n.d. n.d. 56.1
26.2
9.9
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5.1.3. Olivine One inclusion has Mg# of 0.93 but (Mg + Fe)/Si ratio of 2.2. High fluorine content indicates contamination during sample preparation. The inclusion probably contained an olivine crystal, but the contamination precludes accurate analysis.
(32 oxygen) 1.92 0.01 0.11 0.06 0.81 0.67 0.002 0.38 0.09 0.004 0.10 0.002
0.05 0.01 0.02 0.04 0.07 0.09 0.01 0.16 0.05 0.01 0.06 0.01
n.d. 0.05 2.58 3.78 5.15 0.08 n.d. 0.32 0.09 n.d. 0.05 12.54
0.40 n.d. 2.82 3.67 4.99 n.d. 0.24 0.49 0.33 n.d. n.d. 11.78
n.d.—not detected. a Average composition of 11 inclusions that have low Cl and K concentration (contain only a small amount of brine). b Representative analysis. c The original total contents of oxide and chlorine before renormalization to 100%.
The average Mg# is 0.94 F 0.04 and the Ca/(Ca + Mg + Fe) ratio is 0.43 F 0.05. The Cr2O3 content is 4.4 F 1.2 wt.%. Some of the diopside-bearing inclusions are high in K and Cl (up to 8% Cl), indicating the entrapment of some brine together with the mineral phase in a single inclusion. 5.1.2. Chromite Two inclusions share a Cr/(Cr + Al) ratio of 0.82 (Table 1).
Fig. 2. (A) Composition of the microinclusions in ON-DVK-294 and in diamonds from other localities. Hydrous silicic melt endmember composition falls close to the Si + Al apex. The carbonatitic melt falls close to the Ca + Mg + Fe + Na apex and the K + Cl apex represents the brine. D: carbonatitic melt inclusions in ON-DVK-294; o: brine-rich inclusions in ON-DVK-294. E: Koffiefontein brine (Izraeli et al., 2001); y: Koffiefontein melt (Izraeli, unpublished data); .: Botswanan melt (Schrauder and Navon, 1994). (B) A K, Cl and Ca + Mg + Fe + Na projection of the same data set.
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presents the location of the inclusions relative to the zones revealed by CL. Careful examination of the data showed that inclusions located in the inner growth zone are enriched in the brine component, while the inclusions in the outer zone are enriched in the
Fig. 3. Map of the analyzed inclusions on Face II. The different symbols represent analyses performed on different days.
5.2. Fluid microinclusions Microinclusions containing fluids were found on both sides of the diamond. On Face I they were detected in layers C and E (Fig. 1a). Fig. 2 presents the wide range of composition spanned by the inclusions between the brine and carbonatitic melt endmembers. 5.2.1. Spatial relation between the two components Face II was mapped in detail in order to detect any zoning in the composition of the inclusions. Fig. 3
Fig. 4. Mg (o) and Cl (n) variations along a profile across the diamond. The brine resides in the inner zone (high Cl and low Mg concentration) and the carbonatitic melt in the outer zone (low Cl and high Mg concentration).
Fig. 5. Variation diagrams of inclusions containing brine and carbonatitic melts. 5: Carbonatitic melt, y: brine.
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similarly (not shown). Potassium is present in both fluid compositions, but reaches its maximum concentration in the brine-rich inclusions. The sodium content is high in all fluid compositions. From these observations it is clear that compositions vary between two endmembers—The brineenriched composition: The average composition of the fluid in all the brine-bearing inclusions is K18Na24Ca6Mg10Fe4Ba3Si5PCl 26 (mol%) and the composition of five inclusions that show the strongest enrichment in Cl is K19Na25Ca5Mg8Fe3Ba2Si4Cl32 (Table 2). In these brine-rich inclusions, the most abundant cation is sodium, the average Na/Cl ratio is 0.8 F 0.1. The K/Cl ratio is 0.6 F 0.1 and the Mg/Cl and Ca/Cl ratios are 0.2 F 0.04 and 0.14 F 0.05 re-
carbonatitic melt component. Face II (Fig. 4) clearly presents the compositional difference between the two zones but the transition is not as sharp as exhibited by CL. Some inclusions with intermediate composition were analyzed in the contact area between the two zones. The inclusions detected on Face I of this diamond have similar chemical compositions and obey the same spatial relations. The inclusions in the outermost zone contain carbonatitic melt, while the inner zones contain brine inclusions. As an example a negative correlation between Mg and Cl is exhibited in Fig. 5. Magnesium decreases from 28 mol% in the carbonatitic composition to almost zero in the brine-rich compositions, while Cl rises from zero to more than 30%. Calcium behaves
Table 2 Composition of fluid inclusions in diamond ON-DVK-294 Brine
Carbonatitic melt a
Representative inclusions Inclusion number (wt.%) SiO2 TiO2 Al2O3 FeO MgO CaO BaO Na2O K2 O P2O5 Cl Analysed total (wt.%) (mol%) Si Ti Al Fe Mg Ca Ba Na K P Cl a
129
End Averagea Standard member deviation
End Average Standard Representative inclusions member deviation
66
70
71
153
5.8 1.1 0.4 7.6 9.9 8.7 13.1 12.8 21.1 2.5 19.3 7.6
4.4 n.d. 1.0 11.0 8.9 8.5 11.9 15.1 19.3 1.9 21.0 6.1
5.7 4.0 7.0 6.7 n.d. n.d. n.d. 1.0 n.d. n.d. 2.1 – 10.5 3.8 6.7 5.3 9.4 6.6 10.9 7.7 7.7 6.7 9.0 6.4 12.0 12.0 14.5 8.7 16.7 22.4 13.1 18.9 21.0 21.5 19.4 21.5 1.8 1.1 2.2 0.6 18.1 28.3 16.7 28.2 6.1 5.7 6.0
6.4 0.8 0.9 6.3 9.1 8.1 10.2 17.6 20.0 2.0 21.6 3.9
4.5 0.6 0.4 4.9 11.4 7.1 3.9 19.0 20.7 1.6 25.1
3.3 n.d. 0.9 6.9 9.9 6.8 3.5 21.8 18.4 1.2 26.6
4.3 2.7 5.4 4.5 n.d. n.d. n.d. 0.6 n.d. n.d. 1.9 – 6.6 2.1 4.3 3.0 10.4 6.7 12.6 7.8 6.2 4.8 7.5 4.7 3.5 3.2 4.4 2.4 24.2 29.2 19.7 24.7 20.1 18.5 19.2 18.5 1.2 0.6 1.4 0.4 23.0 32.3 21.9 32.4
4.6 0.4 0.8 3.8 9.8 6.3 2.9 24.4 18.4 1.2 26.1
87
92
143
145
180
2.4 1.5 1.4 3.0 2.4 2.4 4.6 4.7 4.6 1.8 5.1 2.0
12.3 1.2 2.5 8.2 19.4 15.2 4.3 12.0 12.2 3.8 9.0 7.0
9.8 7.4 n.d. n.d. 0.8 0.8 6.9 7.9 18.2 18.6 12.9 16.3 10.3 4.0 15.6 15.5 13.8 15.0 3.1 2.4 9.5 12.6 5.0 5.1
9.0 2.8 1.3 10.3 21.1 12.9 5.0 15.2 13.3 2.5 6.8 7.4
7.3 2.5 2.2 8.0 19.1 12.6 5.5 16.6 12.1 3.1 8.3 5.0
12.4 0.7 2.7 10.2 21.3 12.5 6.8 13.6 11.1 3.5 3.5
9.5 1.0 1.5 7.7 18.6 12.8 7.9 15.7 12.8 3.1 8.8 5.0
2.9 1.7 1.3 2.1 2.8 3.0 4.7 2.9 2.2 1.8 3.1 2.3
1.8 0.8 1.2 1.9 2.8 2.1 1.4 5.1 3.8 1.1 4.7
9.6 0.7 2.3 5.3 22.5 12.7 1.3 18.1 12.1 2.5 11.9
7.6 5.4 n.d. n.d. 0.8 0.7 4.5 4.9 21.1 20.5 10.7 12.9 3.1 1.2 23.5 22.2 13.6 14.1 2.0 1.5 12.5 15.7
7.0 1.6 1.2 6.7 24.3 10.7 1.5 22.7 13.1 1.6 9.0
5.6 1.4 2.0 5.1 22.0 10.4 1.6 24.7 11.9 2.0 10.8
10.1 0.4 2.5 6.9 25.6 10.8 2.2 21.3 11.4 2.4 4.8
7.4 0.6 1.4 5.0 21.5 10.6 2.4 23.6 12.6 2.0 11.4
2.3 1.0 1.2 1.4 3.2 2.6 1.5 4.4 2.1 1.2 4.0
Average composition of 71 brine inclusions and 67 carbonatitic melt inclusions with their standard deviation.
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spectively. On the average, the mono and divalent positive ions account for 80 positive charges of which only 32 are compensated by Cl ions, the rest may be compensated by carbonate ions (Izraeli et al., 2001). The carbonatitic melt enriched composition: The average composition of all the inclusions that carry carbonatitic melt is K13Na24Ca11Mg22Fe5Ba2Si7 AlP2Cl11 (mol%) and the composition of the five inclusions that are richest in carbonatitic melt is K11Na21Ca11Mg26Fe7Ba2Si10Al3P2Cl5 (Table 2). In this carbonatitic melt endmember sodium is also the most abundant cation. However the Cl concentration is much lower compared with the brine, The Na/Cl ratio of the carbonatitic melt is 5.1 F 2.5, the K/Cl ratio is 2.6 F 1.0 and the Mg/Cl ratio is 6.0 F 2.3. The average total content of the oxides and chloride for all fluid inclusions is 4.4 F 2.2 wt.% and is lower than that obtained for the mineral inclusions. A possible reason for that difference may be a smaller size of the fluid inclusions. Alternatively, if the size of the different microinclusions in this diamond is similar, the lower totals measured for the nonmineral inclusions may reflect a high concentration of light elements not measured by EPMA (Izraeli et al., 2001). We could not analyze the water and carbonate of the inclusions by InfraRed spectroscopy. However, in analogy with the Koffiefontein and Jwaneng diamonds (Schrauder and Navon, 1994; Izraeli et al., 2001), we suggest that water and carbonate are also present in the inclusions of ON-DVK-294.
Fig. 6. Variation in the C isotopic composition (d13C) along a line from the diamond core through its fibrous layers (error bars F 0.4x).
yielded values of 5.2x and 6x. The coat is depleted in 13C relative to the core. Three measurements in the inner zone (B to E1), where brine inclusions were found, had similar d13C of 8.1x to 8.9 x. Additional depletion in 13C was observed in the outer zone (E2), where the carbonatitic inclusions were found. The d13C measured in two points was 11.4x and 12.8x (Fig. 6). All five measurements in the fibrous zones revealed isotopic compositions that are more depleted than the worldwide range of fibrous diamonds (Cartigny et al., 1998, 2003).
6. Discussion 5.3. Carbon isotopic composition 6.1. Comparison to other mantle derived fluids Table 3 presents C isotopic composition of the diamond matrix, as measured by SIMS. d13C was measured at seven points across Face I. The core (A) Table 3 Carbon isotopic composition of selected points across Face I of diamond ON-DVK-294 Point location Zone Zone Zone Zone Zone Zone Zone
A A B C D E2 E2
d13C (F 0.4x) 6.0 5.2 8.9 8.6 8.1 12.8 11.4
Carbonatitic melts and brine inclusions were previously reported for diamonds from Jwaneng (Schrauder and Navon, 1994) and Koffiefontein (Izraeli et al., in press). Partial analyses of chlorine-rich inclusions (0.2 –2 Am in size) were also reported by Chen et al. (1992) in Chinese diamonds. However, no full analysis was provided. Recently, Kamenetsky et al. (2002) reported Na – K –Cl and C-rich inclusions in olivine phenocrysts from the Udachnaya kimberlite. Fig. 2 presents the chemical composition of the fluids from Jwaneng and Koffiefontein along with the compositions detected in ON-DVK-294. It is clear that the microinclusions in this diamond bridge the compositional gap between the brine-rich inclusions from
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Koffiefontein and the carbonatitic melt endmember of the Jwaneng diamonds. The composition of brine microinclusions in diamond ON-DVK-294 is broadly similar to the brine detected in the Koffiefontein cloudy diamonds. The Canadian brine inclusions carry higher proportions of the carbonatitic endmember, but their K/Cl ratio (0.7 F 0.1) is similar to that of Koffiefontein (0.64 F 0.06). Thus, the fundamental components of the brine endmember are similar in diamonds from different cartons. The fact that the ON-DVK-294 brine was detected in a fibrous coat while the Koffiefontein brine was detected in inner clouds within octahedral diamonds indicates that diamonds of both habits can grow from similar solutions. The main difference between the two brines is the strong enrichment in Na concentration in the Diavik diamond. It is interesting that Izraeli et al. (2001) noted higher concentration of Na in diamonds carrying peridotitic inclusions. The Na/K ratio was 0.4 F 0.1 in two peridotitic diamonds compared with 0.12 F 0.06 in the brine of five eclogitic diamonds. ON-DVK-294 also contains peridotitic mineral inclusions, but its enrichment in Na (Na/K = 1.3) in the brine-rich inclusions is even higher that in any of the Koffiefontein peridotitic diamonds. The carbonatitic melt inclusions in ON-DVK-294 (Fig. 2) show similar characteristics to the carbonatitic melt endmember in the Jwaneng cubic diamonds (Schrauder and Navon, 1994) and to the carbonatitic melt detected recently in Koffiefontein (Izraeli et al., unpublished data). All the fluids are enriched in divalent ions such as Mg, Ca and Fe; however, the ON-DVK-294 carbonatitic melt is characterized by higher proportions of Mg. It is also unique in its high Na and notable Ba content. The similar nature of the Botswanan, Koffiefontein and ON-DVK-294 carbonatitic melts become is impressive considering the fact that the Botswanan melts align along the compositional range between the carbonatitic and the hydrous silicic melt, whereas the Diavik fluids represent mixtures along an array between carbonatitic melt and brine. The fact that both arrays meet at a similar endmember composition suggests that the carbonatitic melt is the link connecting all fluids and that all fluids along both arrays are genetically related. Johnson et al. (2000) and Burgess et al. (2002) studied the composition of Ar, Cl, Br, I, Ca and K in
251
fibrous diamond from Africa, Russia and Canada. In most diamonds, they found uniform I/Cl and Br/Cl ratios that are similar to depleted astenospheric mantle ratios derived from MORB data. Only in some Canadian diamonds (from the Ekati kimberlites, Lac de Gras area) they detected much higher Br/Cl and I/Cl ratios that deviated significantly from the restricted range of all other diamonds. The analyses of all microinclusions in diamonds from Congo, Botswana and Siberia revealed only carbonatitic to hydrous silicic melts (Navon et al., 1988; Schrauder and Navon, 1994; Klein-BenDavid and Logvinova, unpublished data), but no brine. It is possible that the high I/Cl and Br/Cl ratios in some of the Canadian diamonds are related to the formation of the halogenrich brine similar to that found in ON-DVK-294 diamond. The high ratios may represent preferred fractionation of Br and I into the brine, whereas Cl, as a major element is enriched to a lesser amount. 6.2. Evolution of the fluid The similar compositions of carbonatitic melts associated with brine and with hydrous silicic melt suggest that both fluids are the products of a primary carbonatitic melt. If so, fluids should evolve from carbonatitic to brine-rich compositions, but the fluid inclusions in ON-DVK-294 show the reversed transition from brine-enriched fluids in the inner part of the diamond to carbonatitic melt at the diamond rim. However, the inclusions in ON-DVK-294 do not show a continuous, uniform evolution. The transition between the brine-rich central zones (C and E1) and the carbonatitic outer zone (E2) is abrupt with only a few inclusions close to the interface showing intermediate compositions (Fig. 4). This reflects a rapid but continues transition from brine-rich to carbonatitic composition. The observed compositional zoning is accompanied by an even sharper transition in the CL image (Fig. 1c and d) and a significant depletion in 13 C (Fig. 6) from the inner fibrous zones ( 8.5x) to the outer zone ( 12.1x). We suggest that the sharp compositional and isotopic transition reflects mixing between evolved brine and freshly introduced carbonatitic melt infiltrating into the area. Most probably, the change in the isotopic composition of the diamond reflects growth from a more 13C depleted carbonatitic melt. More data
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are needed before the source of the fluids can be inferred. 7. Conclusions The compositional range of microinclusions trapped in diamond ON-DVK-294 sheds light on fluid evolution in the diamond-forming environment in the mantle. The compositional gap between brine found in cloudy octahedral diamonds from South African and carbonatitic melt analyzed in cubic diamonds from Zaire and Botswana is now closed. The compositions of all microinclusions analyzed to date fall along two arrays connecting the carbonatitic melt composition to either a hydrous silicic endmember or to a brine endmember (Fig. 2). The connection between all fluids found in microinclusions in fibrous and cloudy diamonds from various cratons suggest that diamonds are formed from fluids derived form a mantle source not significantly influenced by local heterogeneities. The composition of microinclusions in diamond ON-DVK-294 changes from brine-rich to carbonatitic. This transition is associated with depletion in the carbon isotopic composition of the diamond. We suggest that this transition reflects mixing between an already evolved brine and a freshly introduced carbonatitic melt of different isotopic composition. References Bastin, G., Heijligers, J., 1991. In: Heinrich, K., Newbury, D. (Eds.), Electron Probe Quantitation, Workshop at the National Bureau of Standards, Gaithersburg, Maryland. Plenum, New York, pp. 145 – 161. Boyd, S.R., Pillinger, C.T., Milledge, H.J., Seal, M.J., 1992. CIsotopic and N-Isotopic composition and the infrared-absorption spectra of coated diamonds-evidence for the regional uniformity of CO2 – H2O rich fluids in lithospheric mantle. Earth and Planetary Science Letters 108 (1 – 3), 139 – 150. Bulanova, G.P., 1995. The formation of diamond. Journal of Geochemical Exploration 53 (1 – 3), 1 – 23. Bulanova, G.P., Griffin, W.L., Ryan, C.G., 1998. Nucleation environment of diamonds from Yakutian kimberlites. Mineralogical Magazine 62 (3), 409 – 419. Burgess, R., Layzelle, E., Turner, G., Harris, J.W., 2002. Constraints on the age and halogen composition of mantle fluids in Siberian coated diamonds. Earth and Planetary Science Letters 197 (3 – 4), 193 – 203.
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