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Geochimica et Cosmochimica Acta 107 (2013) 1–11

Peridotitic and websteritic diamondites provide newinformation regarding mantle melting and metasomatism

induced through the subduction of crustal volatiles

S. Mikhail a,b,⇑, G. Dobosi c, A.B. Verchovsky b, G. Kurat d,1, A.P. Jones a

a Department of Earth Science, University College London, WC1E 6BT, UKb Planetary and Space Sciences, The Open University, Milton Keynes, MK7 6BJ, UK

c Research Centre for Astronomy and Earths Sciences, Institute for Geological and Geochemical Research, Hungarian Academy of Sciences,

H-1112 Budapest, Hungaryd Naturhistorische Museum Wien, Burgring 7, 1010 Vienna, Austria

Received 31 January 2012; accepted in revised form 19 December 2012; available online 8 January 2013

Abstract

Diamondites are mantle xenoliths comprised of polycrystalline diamond intergrown with garnet and minor clinopyroxene.Diamondites have some geochemical characteristics distinct from monocrystalline diamonds. Examples include 13C-depletionwith modal d13C value at �18& containing a high abundance of websteritic garnets (73%). This is in contrast to monocrys-talline diamonds that show a strong mean d13C value at �5& and low abundance of websteritic inclusions (2%). At present,geochemical studies focusing on diamondites are lacking, relative to coated and monocrystalline diamond. As a consequence,there exists a substantial volume of abundant mantle material that has been largely overlooked. We have determined the cou-pled d13C–d15N values and N-concentrations for 20 samples of mantle diamondite. Although their provenance is uncertain,these diamondites are thought to originate from Southern Africa because the major and rare Earth element (REE) compo-sitions for the garnets are consistent with other Southern African diamondites.

The coupled d13C–d15N values, N-concentrations in the diamond and REE patterns for the garnets we conclude that thesource of the 13C-depleted carbon and 15N-enriched nitrogen is crustal in origin. This is by way of recycling subducted oceaniclithosphere beneath a stable craton, possibly the Kaapvaal craton in southern Africa. The peridotitic and websteritic garnetintergrowths have REE patterns similar to eclogitic garnets and we propose their petrogenesis due to mixing between a vol-atile saturated eclogitic melt and mantle peridotite. We propose that diamondites represent distinct diamond-forming event(s)related to mantle melting in the sub-cratonic mantle. Diamondite-formation events are proposed to be unrelated to mostmonocrystalline and coated diamonds formed by metasomatic processes involving little to no mantle melting.� 2013 Elsevier Ltd. All rights reserved.

1. INTRODUCTION

The stable isotope composition of carbon and nitrogen inmantle diamond coupled with the geochemistry of the asso-ciated syngenetic sulphide and silicate inclusions provide an

0016-7037/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.gca.2012.12.036

⇑ Corresponding author. Present address: Geophysical Labora-tory, 5251 Broad Branch Road NW, Washington, DC 20015, USA.

E-mail address: smikhail@ciw.edu (S. Mikhail).1 Deceased.

unparalleled insight into the geodynamic carbon cycle overgeologic time (Gurney et al., 2010). Mantle diamonds can bedivided into three main morphological groups; polycrystal-line, monocrystalline and the fluid-rich group termed‘coated or fibrous’ diamond. Polycrystalline diamonds arenot single mineral phases, but are in-fact monolithic mantlexenoliths. They consist chiefly of diamond, with garnet andminor clinopyroxene intergrowths, but never olivine (Kuratand Dobosi, 2000). The petrological difference betweenpolycrystalline diamonds (diamondites), monocrystallineand coated diamonds are stark. A recent study by Jacob

2 S. Mikhail et al. / Geochimica et Cosmochimica Acta 107 (2013) 1–11

et al. (2011) has shown that diamondites contain micro- andnano-inclusions that are distinct from fibrous and mono-crystalline diamonds. Diamondites crystalised from COHN-and Si-rich melts (Rege et al., 2008; Dobosi and Kurat,2010; Jacob et al., 2011), coated diamonds crystalised fromCOHN-rich and Si-poor fluids (Navon et al., 1988) andmonocrystalline diamonds are precipitates from metaso-matic fluids within solid eclogite (Palot et al., 2009) or peri-dotite (Thomassot et al., 2007). These observations suggestthat diamondites are formed in distinct diamond-formationevents.

The geochemistry of syngenetic silicate and sulphidemineral inclusions in mantle diamonds has long been recog-nised (Meyer and Boyd, 1972; Sobolev, 1977) and used togroup diamonds according to their inclusion paragenesis(nature of the host rock in which the diamonds are thoughtto have formed). The most abundant are the peridotitic andeclogitic suites (P- and E-Type) that show geochemical sim-ilarities to peridotites and eclogites. There does exist a 3rdgroup termed websteritic (W-Type) that can be describedas chemically transitional between the P- and E-Type suites(Gurney, 1984). The petrogenesis of the W-Type group isless well detailed in the literature (Stachel and Harris,2008) because W-Type inclusions make up only 2% of themonocrystalline diamond database (from 2844 analyses).However, W-Type garnets make up 73% of the diamonditedatabase (from 63 analyses) and clearly suggest a link to theformation of websteritic garnet and diamondite. Monocrys-talline diamond and diamondite also show an invertedabundance of P-Type silicates, where P-Type inclusionsdominate monocrystalline diamond inclusions (65%) butmake up only 23% of diamondite intergrowths (shown inFig. 1).

The carbon isotope geochemistry of coated and mono-crystalline diamond globally are similar with a mean d13Cvalue of �5 ± 3& (Cartigny, 2005). Coated diamond by

Fig. 1. Pie charts the abundance of the silicate paragenesis for diamonditeDobosi and Kurat (2010), McCandless et al. (1989), Gurney and Boyd (monocrystalline diamond were sourced from a review by Stachel and Ha

far shows the most homogeneous d13C distribution veryrarely falling outside of the mean mantle range (Boydet al., 1994). This is also true for monocrystalline diamondswith P-Type inclusions. Only 2% of P-type diamonds dem-onstrate d13C values <�10&, whereas 34% of E- and W-Type monocrystalline diamonds exhibit the herein termed‘low-d13C values’ (Stachel et al., 2009). Diamondites donot follow these C-isotopic trends set by their monocrystal-line counterparts. Globally, the d13C values of diamonditesare bimodal, with modes at ca. �5& and �20& (Maruokaet al., 2004 and shown in Fig. 2). However, data fordiamondites of known silicate paragenesis are sparse. TheP-Type diamondites analysed show 13C-depletion relativeto the mantle from 14 individual diamondites (Maruokaet al., 2004; Gautheron et al., 2005). Despite the limiteddatabase for P-Type diamondites this observation is signif-icant, because low-d13C values for peridotitic mantle mate-rials are extremely rare (<2%; Cartigny, 2005).

A common explanation for the low-d13C values, ob-served in some E- and W-Type monocrystalline diamondsis the recycling of crustal organic-carbon by subductioninto the mantle (Stachel et al., 2009). Alternatively, somestudies propose high-temperature stable isotope fraction-ation in the mantle as a viable mechanism (for monocrystal-line diamond see Cartigny et al., 2001 and for diamonditessee Maruoka et al., 2004). But controversy exists and nodefinitive conclusions can be drawn for carbon isotope dataalone. A useful tool in addressing the origin of 13C-deple-tion in mantle diamond is the corresponding 15N/14N com-position, because the mantle and crustal reservoirs havediffering nitrogen isotope characteristics. Mantle samplestypically exhibit negative d15N values and crustal samplestypically exhibit positive d15N values (Boyd and Pillinger,1994). There exists a vast database for the coupled carbonand nitrogen isotope compositions of mantle diamonds(>400 individual analyses) (Cartigny, 2005). Conversely,

s and monocrystalline diamonds. The data for diamondites are from1982), Kirkley et al. (1991a) and Jacob et al. (2000). The data forrris (2008).

Fig. 2. Histogram of the carbon isotope distributions for coateddiamond, diamondites and monocrystalline diamond samplesglobally (coated and monocrystalline data from Cartigny, 2005and diamondite data from Maruoka et al., 2004 and Cartigny,2010). The yellow bar represent the mean mantle. (For interpre-tation of the references to colour in this figure legend, the reader isreferred to the web version of this article.)

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the coupled carbon and nitrogen isotope database fordiamondites is tiny (n = 20) (Shelkov, 1997; Gautheronet al., 2005).

To summarise; Diamondites are distinct from coatedand monocrystalline diamonds based on the followingparameters: morphology, silicate paragenesis, fluid/meltinclusion compositions and carbon and nitrogen isotopiccompositions. Despite these petrological differences, geo-logic studies that focus on diamondites are lacking. Forexample, a recent review of diamond formation only detailspolycrystalline diamond in a proposed morphological clas-sification list, with no review/discussion regarding petro-genesis or temporal distribution (Gurney et al., 2010).Diamondites are not comparably rare with the other mor-phological groups, making up to 50% of the total yield ofOrapa diamond, one of the world’s most productive dia-mond mines (Deines et al., 1993; Heaney et al., 2005). Con-sequently, diamondites herald new information aboutcarbon cycling, sub-cratonic melting and metasomaticevents. For which studies focusing on the stable isotopecompositions of the diamonds where the geochemistry ofthe silicates are known will illuminate more clearly the ori-gin of the diamondite-forming carbon in the mantle.

We have determined the C–N isotope compositions for20 diamondites known to exhibit low-d13C values (Mar-uoka et al., 2004) of a known silicate paragenesis (Kuratand Dobosi, 2000; Dobosi and Kurat, 2002, 2010). We

aim to address the petrogenesis of mantle diamonditesand websteritic garnets using previously published trace ele-ment data for the garnets and clinopyroxenes (Dobosi andKurat, 2010) complimented with our new C–N isotope andN-concentration data from the diamonds.

2. SAMPLES

The samples investigated here have been selected from acollection at the Naturhistorische Museum Vienna, Austria.Detailed descriptions of the samples are provided by Kuratand Dobosi (2000) and Dobosi and Kurat (2002). The sizeof the individual diamondite samples varies from 1 to2.5 cm with masses from 5 to 60 carats (Dia 059 is excep-tionally large, 103.5 carats). Fig. 3 shows some importantfeatures such as the variable optical appearance of this suiteof diamondites. In Fig. 3(a) the sample Dia063 has translu-cent and opaque regions. Sample Dia074 shown in Fig. 2(b)is almost completely translucent whereas sample Dia078,shown in Fig. 3(c) better resembles a rock (i.e. it is hardto recognise any diamond optically). However, using ascanning electron microscope, small octahedral, intergrowndiamonds are prevalent (polycrystalline growth) (Fig. 3(d)).Some samples contain a ‘mixed paragenesis’, with purple“peridotitic” garnet and orange “eclogitic/websteritic” gar-nets clearly visible (Fig. 3(d)). The intergrown nature ofgarnet and diamond that demonstrates their syngenesis isexemplified in Fig. 3(f).

Dobosi and Kurat (2010) previously catagorised thispopulation of diamondites into three main groups, P-Type,intermediate (I-Type) and E-type. Herein a simpler catagor-isation into P-Type and W-Type is favoured that uses themajor element geochemistry of the garnets (Fig. 4). Thegarnet discrimination diagram of Aulbach et al. (2002)was employed and shows that there are 12 websteritic and8 peridotitic samples in total, which appears to be a featureof southern African diamondites (Figs. 1 and 3). A garnetcan be assigned to the websteritic rather than the eclogiticsuite if the CaO contents in garnets fall <6 wt.% (Grutteret al., 2004). This criterion is followed here. The geograph-ical origin of this batch of diamondites samples is unknownin explicit terms (Kurat and Dobosi, 2000). However, thediamondite samples (purchased from a diamond dealer byone of the authors, Prof. G Kurat) were stated to be fromsouthern Africa (Kurat and Dobosi, 2000). Similar sampleswere described from the Orapa, Jwaneng (Botswana) andVenetia (South Africa) kimberlites (Gurney and Boyd,1982; McCandless et al., 1989; Kirkley et al., 1991a; Jacobet al., 2000); thus it can be presumed that our samples alsooriginated from one of these localities. Fig. 4 shows thatmost diamondites with low-Cr garnets plot as websteriticand not eclogitic. The available data from Southern Africandiamondites show that roughly 23% are peridotitic, 5%eclogitic with 73% websteritic. This is an entirely differentdistribution with the inclusion-bearing monocrystalline dia-monds where 65% are peridotitic, 33% are eclogitic andonly 2% are websteritic (Stachel and Harris, 2008 andshown graphically in Fig. 1). More data on the compositionof silicate intergrowths from diamondites is important toestablish the current association for diamondites with web-

Fig. 3. Representative images of diamondite from this study showing some important features. (a) Sample Dia063 shows translucent andopaque regions in the same sample where the translucent regions contain a lower abundance of silicate intergrowths and vice versa. (b) SampleDia074 is almost totally translucent whereas samples Dia078 (c) better resembles a rock in appearance. (d) Intergrown octahedral diamonds(polycrystalline) from sample Dia015 shown using secondary electron imagery (Dobosi and Kurat, 2002). (e) Sample Dia013 containing a‘mixed paragenesis’ with both orange ‘websteritic’ and purple ‘peridotitic’ garnets present (Dobosi and Kurat, 2010). (f) Shows garnets (grey)interstitial to diamonds (dark) in sample Dia016 using secondary electron imagery. Note the intergrown nature of garnet and diamond thatdemonstrates syngenesis (Kurat and Dobosi, 2000).

Fig. 4. A Cr2O3 vs. CaO discrimination diagram after Aulbachet al. (2002). The garnet data is from Dobosi and Kurat (2010).This demonstrates the websteritic nature of the samples termedeclogitic in previous works (Dobosi and Kurat, 2002, 2010; Kuratand Dobosi, 2000; Maruoka et al., 2004; Rege et al., 2008). Thedata for the garnet compositions for diamondites from Orapa,Botswana are from McCandless et al. (1989) and Gurney and Boyd(1982) and the data from Jwaneng, Botswana are from Kirkleyet al. (1991a).

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steritic garnets. Despite not knowing the location of thesamples within Southern Africa, the d15N-d13C values, N-concentrations and REE data will allow us to better con-strain the source for the observed 13C-depletion for perido-titic diamondites (Maruoka et al., 2004; Gautheron et al.,2005) and to constrain the origin of diamondite-formingcarbon.

3. ANALYTICAL TECHNIQUE

The d13C-, d15N-values and N-concentrations were ob-tained simultaneously on samples treated by a single tem-

perature programme using a custom made facility knownas the Finesse machine, housed at the Open University,UK (Verchovsky et al., 1998). This facility operates withthree fully automated static-mode mass spectrometers fedfrom a common extraction system under high vacuum(Verchovsky et al., 1998). All of the samples were too largeto fit within the combustion tube attached to the gas extrac-tion-line (inner diameter 4 mm). Because of this limitation,samples were mechanically fragmented using a custom builtstainless steel, hand held micro-vice. Because of the inter-grown nature of the diamond and silicates in diamondites(Fig. 3(a, c, e and f)) great care was taken to select frag-ments that did not contain observable inclusions or inter-growths of silicates, ergo only transparent or translucentfragments of samples were selected to avoid any contribu-tion of N from the inclusions/intergrowths to the analysisof the diamond. For the purpose of this study we only se-lected diamondites where the major and REE compositionsare available and those that contained distinct inclusions, soas to compare the different paragenesis. Samples with amixed paragenesis were excluded from the study (such asDia013 in Fig. 3(e)). For these reasons only 20 samples wereanalysed for C–N isotopes and N-concentrations from asuite of 53 individual diamondites.

The samples were treated by pyrolysis at 1100 �C to re-move surficial contaminants. In the case of low nitrogensamples (ca. <50 ppm), the mass of nitrogen as a contami-nant is more than the mass of N by weight within the dia-mond (where the mass of diamond was <0.3 mg). This isfollowed by pyrolysis at 500 �C to ensure that blank levelshave been retained within the system. The blank levels fornitrogen and carbon are better than 0.5 and 10 ng respec-tively per combustion step above 800 �C, this equates toroughly 5 ppm nitrogen and 100 ppm carbon for a given

Fig. 5. Comparative d13C values for samples that feature in thisstudy and in Maruoka et al. (2004). The deviation away from a 1:1trend is consistent with the deviation observed during multiple runsof samples from the same samples in Maruoka et al. (2004) and byobserving the deviation in d13C values at different stages of steppedcombustion.

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combustion step where the amount of sample combustedwas 0.1 mg in weight (Mikhail, 2011). The main combus-tion sequence is from 800 to 1400 �C with 100 �C incre-ments for 30 min at each temperature. Carbon andnitrogen stable isotope compositions (13C/12C and15N/14N) are expressed in delta notation relative to thePDB (for C) and Air (for N) standards using the followingequation (carbon is shown as an example);d13C = ((13C/12C sample)/(13C/12C standard) � 1) � 1000.The N/C ratio is expressed as N ppm = (N/C) � 1 � 106.The analytical accuracy is ±< 0.5& for both d13C andd15N values and <10% for the N-concentrations. The preci-sion for the d15N values given in Table 1 considers the ef-fects of blank correction on the precision, and thereforeincreases with decreasing nitrogen content (Table 1).

4. RESULTS

The range for the d13C values in this study is �28.5& to�4.6& for diamondites with P-Type garnets, and �22.9&

to �5.5& for diamondites with W-Type garnets. The over-all mean d13C value is �18& for the whole sample set(mean = �18.1 ± 6.1&), and no discrepancy is observedbetween d13C values and garnet compositions (Table 1).These observations are consistent with the previous d13Cdata for these samples by Maruoka et al. (2004). Thereare 19 of the 20 samples in this study that are featured inMaruoka et al. (2004), this provides a good opportunityto test the homogeneity of the diamondites used for thisstudy. The d13C data corroborate well (Fig. 5) where thedeviation from 1:1 trend is the same as the variability seenfor multiple runs of the same sample in Maruoka et al.(2004). This demonstrates that the C-isotope data presentedhere is representative of the sample set.

Table 1Carbon and nitrogen isotope data and the nitrogen concentrations for theblank corrected because the blank constituted to <1% of the measured cablank corrected. The precision for the nitrogen concentrations is <10%.

Paragenesis Intergrowth Sample N ppm

Peridotitic Garnet Dia050 314Peridotitic Garnet Dia074 59Peridotitic Garnet Dia066 412Peridotitic Garnet Dia005 449Peridotitic Garnet Dia054 130Peridotitic Garnet Dia078 76Peridotitic Garnet Dia006 28Peridotitic Garnet Dia019 207Websteritic Garnet Dia052 34Websteritic Garnet Dia059 13Websteritic Garnet Dia018 15Websteritic Garnet Dia030 8Websteritic Garnet Dia073 3635Websteritic Garnet Dia022 1635Websteritic Garnet Dia053 56Websteritic Garnet Dia061 1428Websteritic Garnet Dia063 462Websteritic Garnet Dia068 266Websteritic Garnet Dia001 54Websteritic Garnet Dia020 437

The diamondites show a range in d15N values from �6.1to +22.6& with a mean +8.7 ± 8.2&. The diamonditeswith the highest d15N values belong to the P-Type suite.The samples in this study extend the range for the d15N val-ues of diamondites by almost +8&, from +15 to +22.5&.The mean N-concentration for the diamondites in thisstudy is 495 ppm with a large dispersion (r = 865) from 8to 3635 ppm. This range is larger than what has previously

samples in this study. The errors on the d13C values have not beenrbon dioxide gas, whereas the errors on the d15N values shown are

d13C ±& d15N ±&

�24.1 0.5 21.5 0.8�4.6 0.2 5.2 1.4�24.6 0.4 21.7 0.6�21.6 0.3 21.8 1.3�16.5 0.4 12 0.5�28.5 0.3 6.6 0.5�19.3 0.5 �0.2 2.0�19.1 0.4 8.6 0.5�15.9 0.3 4.8 2.5�22.9 0.2 5.3 6.1�22.2 0.4 6.4 2.8�16.3 0.4 6.4 3.9�17.6 0.2 6.9 0.5�21.3 0.2 8.6 0.5�21.1 0.2 2 0.7�13.9 0.5 15.3 0.5�19.1 0.4 22.5 0.5�5.5 0.3 2.7 0.5�18.9 0.3 �5.7 2.8�9.8 0.3 10.7 0.7

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been reported for diamondites with a sample set of equalsize (Shelkov, 1997; Gautheron et al., 2005), and is as largerthan the range for coated (n = 165) and monocrystallinediamonds of P- and E-Types (where the sample set is 3 or-ders of magnitude larger (n = >2500) (reviewed by Carti-gny, 2005).

5. DISCUSSION

The origin of the diamondite-forming carbon can beconstrained from the d13C–d15N data presented here. Mar-uoka et al. (2004) previously demonstrated that thesediamondites show mean d13C values of �18& with no iso-topic discrimination between P- and W-Type suites forwhich our data consistent (Fig. 6). These data presentedhere show diamondites do not exhibit negative d15N values,argued to be a defining characteristic of mantle nitrogen(Cartigny et al., 2009). Noteworthy, no diamondites toour knowledge, exhibit ‘mantle-like’ coupled d13C–d15Nvalues centred at �5 ± 3& (Fig. 6). We can state that thesource material for the diamondites in this study was dom-inated by high d15N values and a low d13C values, relativeto the mantle. Solely based on the C–N stable isotope data,a subducted component appears to dominate the diamon-dite-forming fluid(s) (Fig. 6). However, there also exist sev-eral models in the literature to explain 13C-depletion indiamond without the need for subducted 13C-depleted crus-tal carbon (Kirkley et al., 1991b; Bulanova et al., 2010) andthey are reviewed herein. These models can be divided intothree main groups; primordial heterogeneities (Javoy et al.,1986; Deines, 2002), high-temperature stable isotope frac-tionation (Cartigny et al., 2001 and references therein)and the subduction of 13C-depleted crustal-organic carbon.

Fig. 6. A variation diagram for various mantle and crustal reservoirs wfollowing: mean mantle (Cartigny, 2005), eclogitic diamonds (Argyle, Aus1998; Kimberly pool, South Africa Cartigny et al., 1999), other diamond(Fuxian, China Cartigny et al., 1997; Orapa, Botswana Cartigny et al.,pelagic organic materials (Thomazo et al., 2009).

A detailed discussion on primordial heterogeneities is notwarranted for this dataset. Diamondites most likely formedin an open system and contain a distinctively websteriticcomponent. As such, they are highly unlikely to retain pri-mordial C- or N-isotopic signatures.

5.1. High-temperature stable isotope fractionation

A previous study of the C-isotope compositions forthese samples proposed a solely mantle origin for thediamondite-forming carbon, the 13C-depletion in thesediamondites was explained using open-system stable iso-tope fractionation (Rayleigh-type) in the mantle prior todiamondites-formation (Maruoka et al., 2004). The modelinvolved the loss of 13C-rich CO2 relative to diamondite-forming 13C-depleted CH4 (Maruoka et al., 2004). TheCO2 generation was through the partial oxidation of mantleCH4 (with a d13C value of �5&). This resulted in a mixtureof 13C rich CO2 and 13C depleted CH4. The relatively 13C-rich CO2 reacts with olivine to form magnesite and clinopy-roxene; the magnesite is subsequently removed from thesystem and the diamondites precipitate from the 13C-de-pleted CH4 (Maruoka et al., 2004). However, this modelcannot explain the observed N-concentrations or the d15Nvalues for these diamondites (Fig. 6). There is no overallcorrelation for d13C- or d15N values with N-concentrations(Fig. 7). There is also no clear relationship for d13C vs. d15Nas would be predicted by a coupled stable isotope fraction-ation process (Fig. 8). A model for stable isotope fraction-ation with an initial 13C-content akin to the mean mantlerequires progressive or instantaneous 13C depletion and15N-enrichment as a function of the fraction removed (seethe caption in Fig. 8 for parameters used in the model).

ith the samples from this study. The fields are sourced from thetralia Boyd and Pillinger, 1994; Jwaneng, Botswana Cartigny et al.,ites (Gautheron et al., 2005; Shelkov, 1997), peridotitic diamonds

1999; Alluvial diamonds from Namibia Cartigny et al., 2004) and

Fig. 7. On the left is shown the variation diagram for d13C values (&) vs. N concentrations for the diamondites in this study. The dashed linelabelled the limit sector curve and the yellow band that represents the mantle is from Cartigny et al. (2001) and the blue box that representspelagic sediments and organic materials is from Hoefs (2009). About 90% of monocrystalline mantle diamonds plot below the limit sectorcurve shown. On the right is shown the variation diagram for d15N values (&) vs. N concentrations for the diamondites in this study. Theyellow band that represents the mantle is from Cartigny et al. (2001) and the blue box that represents pelagic sediments and organic materialsis from Thomazo et al. (2009). The symbols are the same as in Fig. 5. (For interpretation of the references to colour in this figure legend, thereader is referred to the web version of this article.)

Fig. 8. A variation diagram for various mantle and crustalreservoirs with the samples from this study and coupled isotopicfractionation modelled trends shown as dashed lines to fit the dataof Jwaneng and Orapa. The fields are sourced from the following;mean mantle (Cartigny, 2005), Orapa eclogitic diamonds (Cartignyet al., 1999), Jwaneng (Cartigny et al., 1998), other diamondites(Gautheron et al., 2005; Shelkov, 1997) and pelagic organicmaterials (Thomazo et al., 2009). The models are open systemRayleigh fractionation models using equations outlined in Cartignyet al. (2001) where DC � �3.5& for diamond-CO2 (Bottinga, 1969)and DN � +2& for diamond-N2 (inferred using empirical data inThomassot et al., 2007) where both fractionation factors are at ca.1100 �C. The symbols are the same as in Fig. 5.

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The random spread of data points and the overlap for thefield that defines crustal organic carbon for these diamon-dites is viewed here as evidence against a 13C-the fraction-ation model to explain these data (Fig. 6). Equally, thisdata does not fit an alternative open system progressivefractionation model of Cartigny et al. (1998, 2001; plottedon Fig. 8).

5.2. Subduction of 13C-depleted crustal organic material

There is a strong clustering and overlap for the obtaineddiamondite data with the C–N isotopic field for crustal or-ganic carbon (Fig. 6). Contributions of both mantle andcrustal organic carbon are evident with organic (crustal)carbon as the dominant component (Figs. 6 and 7). Themantle contribution is estimated from samples Dia074 (P-Type) and Dia068 (W-Type) that show d13C values of�4.6& and �5.5& and d15N values of +5.2& and+2.7& (Fig. 6). A numerical model was implemented tofit some calculated binary-mixing lines to these data(Fig. 9). The model used end-members that are a good fitfor these data and the mantle–crustal reservoirs (Fig. 9).This model demonstrates that a simple binary mixing modelcannot fit all of these data, but does fit the majority, wherethe crustal reservoir dominates the mantle reservoir as theC–N source of the diamondite-forming carbon.

There are two main arguments that oppose the contribu-tions of crustal/lithospheric carbon to diamond petrogene-sis. These are outlined, and ruled out, herein. The first isthat eclogitic diamonds with low-d13C values are 15N-de-pleted, whereas crustal materials are 15N-enriched (Carti-gny, 2005). This is not true for diamondites; the majorityshow crustal like 15N-enrichment (Fig. 6). Secondly, most

Fig. 9. The results of a binary mixing model where the mantle endmember C/N ratio fixed to 300 (ca. 3000 ppm nitrogen) and varyingthe C/N ratio of the subducted component. The C/N ratio wasvaried from sedimentary values of ca. 100 to mantle values of 1000–3000. The large range was used because the C/N ratio of the crustalorganic carbon would probably evolve towards higher values (moremantle-like) during devolatalisation of the subducting sedimentarypackage. This is because the degree of carbon removal from slabs isthought be around 50% (Wallace, 2005), whereas for nitrogen it ispotentially >90% (Fischer et al., 2002); ergo, the nitrogen in thecrustal organic material would be fractionated to a higher degreerelative to carbon during subduction thus increasing the C/N ratio.The C and N isotope values are from the sample with the highestd13C value and the d15N value is the median value for the range ofpelagic organics shown in the figure. The field for pelagic organicsis from Hoefs (2009) and Thomazo et al. (2009). The symbols arethe same as in Fig. 5.

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monocrystalline diamonds with low-d13C values also havelow N-concentrations, whereas crustal sediments and or-ganic compounds typically have high N-concentrations;where crustal C/N ratios are <100 relative to mantle C/Nratios >1000 (Cartigny et al., 1998). Three W-Type diamon-dites (Dia073, Dia022 and Dia061) all have N-concentra-tions above 1000 ppm and plot in the field for pelagicorganic materials (in terms of both nitrogen contents andC–N isotopic values). Most samples show N-concentrationbelow the fields that define pelagic organic materials(Fig. 7) and we explain these observations in two ways.Firstly, the degree of carbon removal during devolatiliza-tion of subducted materials during decent into the mantlewedge is through to be around 50% (Wallace, 2005),whereas for nitrogen it is >90% (Fischer et al., 2002). There-fore the nitrogen in the crustal organic material will be frac-tionated relative to carbon during subduction. Thisincreases the C/N ratio of the subducted component consid-erably to lower N-concentrations (relative to C). Secondly,not all of the diamondite-forming carbon and nitrogen is ofa crustal origin. Fig. 9 shows that the spread of these datacan be partially explained by incomplete mixing betweencrustal and mantle end-members. Our modelling presentedin Fig. 9 varies the C/N ratio from sedimentary values (ca.100) to mantle values (1000–3000). The large range was

used because the C/N ratio of the crustal organic carbonwould probably evolve towards higher values (more man-tle-like) during devolatilization of the subducting materialas stated above. Such fractionated C/N ratios (<30 ppmnitrogen) in crustal organic-carbon are observed in graphitemicro-particles from >3700-Ma pelagic sediments (W.Greenland) (van Zuilen et al., 2005) and from metamorphicdiamonds (compiled in Cartigny, 2005). Both the graphitemicro-particles and the metamorphic diamonds formedvia conversion of organic carbon during metamorphism.These data show a large range for N-concentrations fromca. <20 to >9000 ppm (Cartigny, 2005 for diamond; vanZuilen et al., 2005 for graphite). Therefore it can be statedthat crustal organic carbonaceous materials do have highC/N ratios, but it is not true that diamond formed via recy-cling of this crustal organic carbonaceous material will alsohave low C/N ratios (i.e. high N-concentrations). The C–Nisotopic and N-concentration results presented here showboth crustal and mantle carbon-sources contributed todiamondite formation, where the bulk of the diamondite-forming carbon was crustal, organic carbonaceousmaterial.

5.3. A dynamic model

To summarise; the textual evidence demonstrates syn-chronous crystallization for both garnet (and pyroxene)with diamond (Fig. 3(f)). This is consistent with the compa-rable trace element patterns for the diamonds (Rege et al.,2008) and garnets (Dobosi and Kurat, 2010). The similarityfor the C- and N-isotopic compositions between both P-and W-Type diamondites is also evidence for a genetic linkbetween the different silicate suites (Fig. 6). The remainderof this discussion proposes a dynamic model to explain whydiamondites are olivine-absent, contain P- and W-Typegarnet intergrowths and show 13C-depletion and 15N-enrichment relative to the mantle; very rare characteristicsfor silicate bearing mantle diamonds.

The absence of olivine in diamondites, the most abun-dant mantle mineral, is strange for diamondites with P-Type garnets. This can be explained in two ways; carbon-ation of olivine and the melting regime in mantle peridotite.The consumption of olivine by carbonation reactions hasbeen proposed to explain the absence of olivine in diamon-dites (Kurat and Dobosi, 2000; Dobosi and Kurat, 2002,2010; Maruoka et al., 2004; Rege et al., 2008). The reactionproduces enstatite + dolomite from diopside + forste-rite + CO2 (Wendlandt and Mysen, 1980). Conversely, theabsence of olivine in diamondites could be a function ofthe melting regime of mantle peridotite. Each episode ofmantle melting under mantle pressures and temperaturestypically produces a more olivine-saturated residuum (My-sen and Kushiro, 1977). Progressive melting would result inthe evolution of lherzolite to harzburgite and further melt-ing produces a dunite (Mysen and Kushiro, 1977). At eachstage of melting an olivine-poor melt is produced. Thesedata shown in Fig. 4 demonstrates that the garnets in thediamondites are akin to lherzolite, harzburgite and webste-rite. Partial melting of volatile saturated lherzolite shouldproduce a melt where olivine is residual, and the melt is

S. Mikhail et al. / Geochimica et Cosmochimica Acta 107 (2013) 1–11 9

of a garnet ± clinopyroxene composition (Mysen and Kus-hiro, 1977). This assemblage, garnet ± minor clinopyroxeneis consistent for the silicate intergrowths in diamondites(Kurat and Dobosi, 2000; Dobosi and Kurat, 2002,2010). Finally, because olivine and/or primary Mg/Ca-car-bonate occurrences are absent in diamondites, explainingthe absence of olivine by melting of peridotites is favouredover the carbonation of olivine (Jacob et al., 2011 did notesome secondary carbonate in diamondites related to latestage oxidation).

The C–N stable isotope data suggests a link betweenwebsteritic garnet and diamondite formation. However,there are no comparative datasets available for the resultspresented here because prior to this study, investigationsusing both the silicate geochemistry and C–N isotope com-positions of diamondites are absent. Nonetheless, Deineset al. (1993) presented the 13C/12C compositions for variousmorphological diamond types and silicate paragenesis fromthe Orapa kimberlite, Botswana (the silicate paragenesis ofthe diamondites was not detailed). The d13C values fromDeines et al. (1993) show monocrystalline diamond withW-Type inclusions and diamondites share similar distribu-tions, and are distinct from the other monocrystalline dia-mond paragenesis (Fig. 10). Gurney and Boyd (1982) andMcCandless et al. (1989) analysed the major element com-position of 17 garnets from diamondites at Orapa andfound that 21 out of 27 have websteritic compositions;therefore we infer that the C-isotope correlation fordiamondites with monocrystalline W-Type diamonds atOrapa is evidence of genetic relationship. The petrogenesisof the websteritic garnets in diamondites was previously ex-

Fig. 10. The carbon isotope distribution of diamondites (a) withmonocrystalline diamonds containing websteritic (b), eclogitic (c)and peridotitic (d) inclusions from Orapa, Botswana (data fromDeines et al., 1993). Note the similarity for the distributions of d13Cvalues for diamondites with websteritic monocrystalline diamond.

plained by the removal of chromite–ilmenite–sulphide froma carbonatitic melt (peridotitic) that occurred synchro-nously with polycrystalline diamond formation (Dobosiand Kurat, 2010). Conversely, the co-existence of P- andW-Type garnets in the same diamondite is evidence of mix-ing between a peridotitic and an eclogitic melts (Dia013;Fig. 3e). The melting of eclogite is induced by the presenceof crustal fluids in the subducted material (H2O, CO2 andCH4) lowering the solidi of the host eclogite under mantleP–T conditions (Yaxley and Brey, 2004). Re-mobilisationof subducted eclogite infiltrates peridotitic mantle andcauses further redox melting, resulting in a volatile-rich,carbon over-saturated melt with P- and W-Type affinities.This melt partially mixes and precipitates P- and W-Typegarnets (Aulbach et al., 2002) synchronously with polycrys-talline diamond (Fig. 3(e)).

Kurat and Dobosi (2000) previously argued the lack ofEu and Ce anomalies in the W-Type garnets (Fig. 9) cou-pled with the presence of P-Type garnets (Fig. 3) was evi-dence against the role of subducted oceanic lithospherefor these diamondites. Noteworthy, only 4 of 8 P-Type gar-nets show mildly sinusoidal REE patterns that are charac-teristic for P-Type garnet inclusions in mantle diamonds(samples Dia074, Dia066, Dia050 and Dia005; Fig. 11).Three of these diamondites also show 13C-depletion thatis very uncharacteristic for P-Type diamond and more com-monly seen in E-Type diamond. These diamondites alsoshow extreme 15N-enrichment, uncharacteristic for mantlesamples in general (Boyd and Pillinger, 1994; Cartigny,2005). The d13C values are �24.6&, �24.1& and�16.3& and d15N values are +21.7&, +21.5& and+21.8& for samples Dia066, Dia050 and Dia005 respec-tively (Table 1). The remainder of the P-Type garnets lackMREE patterns, characteristic for P-Type garnet inclusions(i.e. a hump between Pr-Eu and/or a slope between Eu–Ho;Fig. 11). These garnets instead show REE patterns moreakin to eclogitic garnets. The W-Type garnets show LREEdepletion and progressive enrichment towards the HREE,trends that are more characteristic of eclogitic garnet inclu-sions in diamond (Fig. 11). These data demonstrate theneed for an eclogitic component.

We propose that southern African diamondites areformed from subduction derived COHN-fluids within a hy-brid melt of partially melted peridotite + eclogite. The min-or-proportion of mantle-derived carbon was derived fromthe surrounding peridotitic mantle, because the growth ofpolycrystalline diamond acted as nuclei for further dia-mond formation, consuming any surrounding mantle-de-rived carbon with a d13C value of �5 ± 3&. The eclogiticmelt is fluid-saturated and initiates melting of peridotite.The peridotitic and eclogitic melts partially mix and pro-duce peridotitic and websteritic garnet compositions thatcrystallise polycrystalline diamond intergrown with the sur-rounding poorly mixed silicate melt. The descriptive modelpresented above explains the occurrence of polycrystallinediamond with low d13C values, positive d15N values thatare intergrown with garnet (±cpx) of mixed olivine-freeP- and W-Type paragenesis. Our coupled d13C–d15N datademonstrates that the volatiles were mostly derived fromsubducted crustal material, not primary mantle volatiles.

Fig. 11. Chondrite-normalised trace element abundance patternsfor (a) peridotitic garnets and (b) websteritic garnets in diamon-dites. The CI chondrite data is from McDonough and Sun (1995)and the data for the garnet intergrowths in diamondites is fromDobosi and Kurat (2010). The average CI chondrite normalisedpatterns for eclogitic and peridotitic (lherzolitic and harzburgitic)garnet inclusions in diamond were sourced from Stachel et al.(2004).

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6. CONCLUSIONS

We have determined the coupled d13C–d15N values andN-concentrations for 20 samples of mantle diamondite, pre-sumably from Southern Africa. Based on the coupled d13C–d15N values and N-concentrations presented here we con-clude that the source of the 13C-depleted carbon and 15N-enriched and nitrogen is crustal in origin, by way of sub-ducted oceanic lithosphere beneath a stable craton, mostlikely the Kaapvaal craton. The P- and W-Type garnetsshow REE-patterns that are more akin to eclogitic garnets.This is explained by incomplete mixing between volatile sat-urated eclogitic and peridotitic melts, consistent with themodel for websteritic garnet petrogenesis of Aulbachet al. (2002). We propose that diamondite formation is clo-sely related to websteritic, monocrystalline diamond-forma-tion events. Future geochemical and geochronologicalinvestigations between websteritic diamonds and diamon-dites of known provenance are required to test the conclu-sion presented here.

ACKNOWLEDGEMENTS

This study is dedicated to the late Prof Gero Kurat whopassed away before its completion, but not before contributingconsiderably to every aspect of this study. S.M. would like to

thank the Engineering and Physical Sciences Research Council,the Diamond Trading Company and the Physical SciencesDepartment, the Open University for financial support duringhis time as a Ph. D. student at University College London.The support of OTKA (Hungarian Scientific Research Fund)T49176 grant for G.D. is gratefully acknowledged. This versionof the manuscript has benefited from critical, constructive andenthusiastic reviews by Dr. Adrian Van Rythoven, two anony-mous reviewers and the editorial handling of Dr. Steven B.Shirey.

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