Regional patterns in the paragenesis and age of inclusions in diamond, diamond composition, and the lithospheric seismic structure of Southern Africa

The Archean lithospheric mantle beneath the Kaapvaal–Zimbabwe craton of Southern Africa shows ±1% variations in seismic P-wave velocity at depths within the diamond stability field (150–250 km) that correlate regionally with differences in the composition of diamonds and their syngenetic inclusions. Seismically slower mantle trends from the mantle below Swaziland to that below southeastern Botswana, roughly following the surface outcrop pattern of the Bushveld-Molopo Farms Complex. Seismically slower mantle also is evident under the southwestern side of the Zimbabwe craton below crust metamorphosed around 2 Ga. Individual eclogitic sulfide inclusions in diamonds from the Kimberley area kimberlites, Koffiefontein, Orapa, and Jwaneng have Re–Os isotopic ages that range from circa 2.9 Ga to the Proterozoic and show little correspondence with these lithospheric variations. However, silicate inclusions in diamonds and their host diamond compositions for the above kimberlites, Finsch, Jagersfontein, Roberts Victor, Premier, Venetia, and Letlhakane do show some regional relationship to the seismic velocity of the lithosphere. Mantle lithosphere with slower P-wave velocity correlates with a greater proportion of eclogitic versus peridotitic silicate inclusions in diamond, a greater incidence of younger Sm–Nd ages of silicate inclusions, a greater proportion of diamonds with lighter C isotopic composition, and a lower percentage of low-N diamonds whereas the converse is true for diamonds from higher velocity mantle. The oldest formation ages of diamonds indicate that the mantle keels which became continental nuclei were created by middle Archean (3.2–3.3 Ga) mantle depletion events with high degrees of melting and early harzburgite formation. The predominance of sulfide inclusions that are eclogitic in the 2.9 Ga age population links late Archean (2.9 Ga) subduction-accretion events involving an oceanic lithosphere component to craton stabilization. These events resulted in a widely distributed younger Archean generation of eclogitic diamonds in the lithospheric mantle. Subsequent Proterozoic tectonic and magmatic events altered the composition of the continental lithosphere and added new lherzolitic and eclogitic diamonds to the already extensive Archean diamond suite.     

Integrated models of diamond formation and craton evolution

Two decades of diamond research in southern Africa allow the age, average N content and carbon composition of diamonds, and the dominant paragenesis of their syngenetic silicate and sulfide inclusions to be integrated on a cratonwide scale with a model of craton formation. Individual eclogitic sulfide inclusions in diamonds from the Kimberley area kimberlites, Koffiefontein, Orapa and Jwaneng have Re–Os isotopic ages that range from circa 2.9 Ga to the mid-Proterozoic and display little correspondence with the prominent variations in the P-wave velocity (±1%) that the mantle lithosphere shows at depths within the diamond stability field (150–225 km). Silicate inclusions in diamonds and their host diamond compositions for the above kimberlites, Finsch, Jagersfontein, Roberts Victor, Premier, Venetia, and Letlhakane show a regional relationship to the seismic velocity of the lithosphere. Mantle lithosphere with slower P-wave velocity relative to the craton average correlates with a greater proportion of eclogitic vs. peridotitic silicate inclusions in diamond, a greater incidence of younger Sm–Nd ages of silicate inclusions, a greater proportion of diamonds with lighter C isotopic composition, and a lower percentage of low-N diamonds. The oldest formation ages of diamonds support a model whereby mantle that became part of the continental keel of cratonic nuclei first was created by middle Archean (3.2–3.3 Ga or older) mantle depletion events with high degrees of melting and early harzburgite formation. The predominance of eclogitic sulfide inclusions in the 2.9 Ga age population links late Archean (2.9 Ga) subduction–accretion events to craton stabilization. These events resulted in a widely distributed, late Archean generation of eclogitic diamonds in an amalgamated craton. Subsequent Proterozoic tectonic and magmatic events altered the composition of the continental lithosphere and added new lherzolitic and eclogitic diamonds to the already extensive Archean diamond suite. Similar age/paragenesis systematics are seen for the more limited data sets from the Slave and Siberian cratons.     

Diamondiferous lithospheric roots along the western margin of the Kalahari Craton—the peridotitic inclusion suite in diamonds from Orapa and Jwaneng

The Orapa and Jwaneng kimberlites are located along the western margin of the Kalahari Craton and the prevalence of eclogitic over peridotitic diamonds in both mines has recently been linked to lower P-wave velocities in the deep mantle lithosphere (relative to the bulk of the craton) to suggest a diamond formation event prompted by mid-Proterozoic growth and modification of preexisting Archean lithosphere (Shirey et al. 2002). Here we study peridotitic diamonds from both mines, with an emphasis on the style of metasomatic source enrichment, to evaluate their relationship with this major eclogitic diamond formation event. In their major element chemistry, the peridotitic inclusions compare well with a world-wide database but reveal differences to diamond sources located in the interior of the Western Terrane of the Kaapvaal block, where the classical mines in the Kimberley region are located. The most striking difference is the relative paucity of low-Ca (<2 wt% CaO in garnet) harzburgites and a low ratio of harzburgitic to lherzolitic garnets (2:1). This suggests that lithospheric mantle accreted to the rim of the Zimbabwe and Kaapvaal blocks was overall chemically less depleted. Alternatively, this more fertile signature may be assigned to stronger metasomatic re-enrichment but the trace element signature of garnet inclusions is not in favor of strong enrichment in major elements. For both mines the majority of lherzolitic and harzburgitic garnet inclusions are characterized by moderately sinusoidal REE_N patterns and low Ti, Zr and Y contents, indicative of a metasomatic agent with very high LREE/HREE and low HFSE. This is consistent with metasomatism by a CHO-fluid or, as modeled by Burgess and Harte (2003), a highly fractionated, low-volume silicate melt from the MORB-source. In both cases, changes in the major element chemistry of the affected rocks will be limited. In a few garnets from Orapa preferential MREE enrichment is observed, suggesting that the percolating fluid/melt fractionated a LREE-phyllic phase (such as crichtonite). The overall moderate degree of metasomatism reflected by the inclusion chemistry is in stark contrast to lithospheric sections for Orapa and Jwaneng based on mantle xenocrysts and xenoliths, revealing extensive mantle metasomatism (Griffin et al. 2003). This suggests that the formation of peridotitic diamonds predates the intensive modification of the subcratonic lithosphere during Proterozoic rifting and compression, implying that diamonds may survive major tectonothermal events.Editorial responsibility: J. Hoefs     

Re-Os isotopic composition of peridotitic sulphide inclusions in diamonds from Ellendale, Australia: Age constraints on Kimberley cratonic lithosphere

Sulphide-bearing diamonds recovered from the ∼20 Ma Ellendale 4 and 9 lamproite pipes in north-western Australia were investigated to determine the nitrogen aggregation state of the diamonds and Re-Os isotope geochemistry of the sulphide inclusions. The majority of diamond studies have been based on diamonds formed in the sub-continental lithospheric mantle (SCLM) below stable cratons, whereas the Ellendale lamproites intrude the King Leopold Orogen, south of the Kimberley craton. The sulphide inclusions consist of pyrrhotite-pentlandite-chalcopyrite assemblages, and can be divided into peridotitic and eclogitic parageneses on the basis of their Ni and Os contents. A lherzolitic paragenesis for the high-Ni sulphide inclusions is suggested from their Re and Os concentrations. Regression analysis of the Re-Os isotope data for the lherzolitic sulphides yields an age of 1426 ± 130 Ma, with an initial ¹⁸⁷Os/¹⁸⁸Os ratio of 0.1042 ± 0.0034. The upper limit of the uncertainty on the ¹⁸⁷Os/¹⁸⁸Os initial ratio gives a Re depletion age of 2.96 Ga, indicating the presence of SCLM beneath Ellendale since at least the Mesoarchaean, with the lherzolitic diamond-forming event much younger and unrelated to the craton keel stabilisation. The nitrogen aggregation state of the diamonds and calculated mantle residence temperatures suggest an origin and storage of the Ellendale diamonds in a stable cratonic SCLM, consistent with the King Leopold Orogen being cratonised by about 1.8 Ga. The diamonds do not show evidence for pervasive deformation or platelet degradation, which suggests that the diamonds had a relatively undisturbed 1.4 billion year mantle storage history.     

Kimberlite-hosted diamond deposits of southern Africa: A review

Following the discovery of diamonds in river deposits in central South Africa in the mid nineteenth century, it was at Kimberley where the volcanic origin of diamonds was first recognized. These volcanic rocks, that were named “kimberlite”, were to become the corner stone of the economic and industrial development of southern Africa. Following the discoveries at Kimberley, even more valuable deposits were discovered in South Africa and Botswana in particular, but also in Lesotho, Swaziland and Zimbabwe.A century of study of kimberlites, and the diamonds and other mantle-derived rocks they contain, has furthered the understanding of the processes that occurred within the sub-continental lithosphere and in particular the formation of diamonds. The formation of kimberlite-hosted diamond deposits is a long-lived and complex series of processes that first involved the growth of diamonds in the mantle, and later their removal and transport to the earth's surface by kimberlite magmas. Dating of inclusions in diamonds showed that diamond growth occurred several times over geological time. Many diamonds are of Archaean age and many of these are peridotitic in character, but suites of younger Proterozoic diamonds have also been recognized in various southern African mines. These younger ages correspond with ages of major tectono-thermal events that are recognized in crustal rocks of the sub-continent. Most of these diamonds had eclogitic, websteritic or lherzolitic protoliths.In southern Africa, kimberlite eruptions occurred as discrete events several times during the geological record, including the Early and Middle Proterozoic, the Cambrian, the Permian, the Jurassic and the Cretaceous. Apart from the Early Proterozoic (Kuruman) kimberlites, all of the other events have produced deposits that have been mined. It should however be noted that only about 1% of the kimberlites that have been discovered have been successfully exploited.In this paper, 34 kimberlite mines are reviewed with regard to their geology, mantle xenolith, xenocryst and diamond characteristics and production statistics. These mines vary greatly in size, grade and diamond-value, as well as in the proportions and types of mantle mineral suites that they contain. They include some of the world's richest mines, such as Jwaneng in Botswana, to mines that are both small and marginal, such as the Frank Smith Mine in South Africa. They include large diatremes such as Orapa and small dykes such as those mined at Bellsbank, Swartruggens and near Theunissen. These mines are all located on the Archaean Kalahari Craton, and it is apparent that the craton and its associated sub-continental lithosphere played an important role in providing the right environment for diamond growth and for the formation of the kimberlite magmas that were to transport them to the surface.     

Diamonds and their mineral inclusions from the A154 South pipe, Diavik Diamond Mine, Northwest territories, Canada

Mineral inclusions recovered from 100 diamonds from the A154 South kimberlite (Diavik Diamond Mines, Central Slave Craton, Canada) indicate largely peridotitic diamond sources (83%), with a minor (12%) eclogitic component. Inclusions of ferropericlase (4%) and diamond in diamond (1%) represent “undetermined” parageneses.

Compared to inclusions in diamonds from the Kaapvaal Craton, overall higher CaO contents (2.6 to 6.0 wt.%) of harzburgitic garnets and lower Mg-numbers (90.6 to 93.6) of olivines indicate diamond formation in a chemically less depleted environment. Peridotitic diamonds at A154 South formed in an exceptionally Zn-rich environment, with olivine inclusions containing more than twice the value (of  52 ppm) established for normal mantle olivine. Harzburgitic garnet inclusions generally have sinusoidal rare earth element (REE_N) patterns, enriched in LREE and depleted in HREE. A single analyzed lherzolitic garnet is re-enriched in middle to heavy REE resulting in a “normal” REE_N pattern. Two of the harzburgitic garnets have “transitional” REE_N patterns, broadly similar to that of the lherzolitic garnet. Eclogitic garnet inclusions have normal REE_N patterns similar to eclogitic garnets worldwide but at lower REE concentrations.

Carbon isotopic values (δ¹³C) range from − 10.5‰ to + 0.7‰, with 94% of diamonds falling between − 6.3‰ and − 4.0‰. Nitrogen concentrations range from below detection (< 10 ppm) to 3800 ppm and aggregation states cover the entire spectrum from poorly aggregated (Type IaA) to fully aggregated (Type IaB). Diamonds without evidence of previous plastic deformation (which may have accelerated nitrogen aggregation) typically have < 25% of their nitrogen in the fully aggregated B-centres. Assuming diamond formation beneath the Central Slave to have occurred in the Archean [Westerlund, K.J., Shirey, S.B., Richardson, S.H., Gurney, J.J., Harris, J.W., 2003b. Re–Os systematics of diamond inclusion sulfides from the Panda kimberlite, Slave craton. VIIIth International Kimberlite Conference, Victoria, Canada, Extended Abstracts, 5p.], such low aggregation states indicate mantle residence at fairly low temperatures (< 1100 °C). Geothermometry based on non-touching inclusion pairs, however, indicates diamond formation at temperatures around 1200 °C. To reconcile inclusion and nitrogen based temperature estimates, cooling by about 100–200 °C shortly after diamond formation is required.     

Diamond formation episodes at the southern margin of the Kaapvaal Craton: Re–Os systematics of sulfide inclusions from the Jagersfontein Mine

Sulfide inclusions in diamonds from the 90-Ma Jagersfontein kimberlite, intruded into the southern margin of the Kaapvaal Craton, were analyzed for their Re–Os isotope systematics to constrain the ages and petrogenesis of their host diamonds. The latter have δ¹³C ranging between −3.5 and −9.8‰ and nitrogen aggregation states (from pure Type IaA up to 51% total N as B centers) corresponding to time/temperature history deep within the subcontinental lithospheric mantle. Most sulfides are Ni-poor ([Ni + Co]/Fe = 0.05–0.25 for 15 of 17 inclusions), have elevated Cu/[Fe + Ni + Co] ratios (0.02–0.36) and elemental Re–Os ratios between 0.5 and 46 (12 of 14 inclusions) typical of eclogitic to more pyroxenitic mantle sources. Re–Os isotope systematics indicate two generations of diamonds: (1) those on a 1.7 Ga age array with initial ¹⁸⁷Os/¹⁸⁸Os (¹⁸⁷Os/¹⁸⁸Os_i) of 0.46 ± 0.07 and (2) those on a 1.1 Ga array with ¹⁸⁷Os/¹⁸⁸Os_i of 0.30 ± 0.11. The radiogenic initial Os isotopic composition for both generations of diamond suggests that components with high time-integrated Re–Os are involved, potentially by remobilization of ancient subducted oceanic crust and hybridization of peridotite. A single sulfide with higher Os and Ni content but significantly lower ¹⁸⁷Os/¹⁸⁸Os hosted in a diamond with less aggregated N may represent part of a late generation of peridotitic diamonds. The paucity of peridotitic sulfide inclusions in diamonds from Jagersfontein and other kimberlites from the Kaapvaal craton contrasts with an overall high relative abundance of diamonds with peridotitic silicate inclusions. This may relate to extreme depletion and sulfur exhaustion during formation of the Kaapvaal cratonic root, with the consequence that in peridotites, sulfide-included diamonds could only form during later re-introduction of sulfur.     

Mineral inclusions in diamonds from the River Ranch kimberlite, Zimbabwe

More than 99% of mineral inclusions in diamonds from the River Ranch pipe in the Late Archean Limpopo Mobile Belt (Zimbabwe), are phases of harzburgitic paragenesis, namely olivine (Fo_92–93), orthopyroxene (Mg# = 93), G10 garnets and chromites. The diamond inclusion (DI) chemistry demonstrates a limited overlap with River Ranch kimberlite macrocrysts: the DI garnets are more Ca-undersaturated, and DI spinel and garnet are more Mg-rich. Most River Ranch diamond inclusions were equilibrated at T = 1080–1320 °C, P = 47–61 kbar, and f _O2 between IW and WM buffers. The P/T profile beneath the Limpopo Mobile Belt (LMB) is consistent with a paleo-heat flow of 41–42 mW/m², similar to calculations for Roberts Victor, but hotter than for the Finsch, Kimberley, Koffiefontein and Premier Mines. This is ascribed to the younger tectonothermal age of the LMB and its proximity to Late Archean oceans. Like diamond inclusions from all other kimberlites studied, the River Ranch DI have a lithospheric affinity and therefore indicate that an ancient, chemically depleted, thick (at least 200 km) mantle root existed beneath the Limpopo Mobile Belt 530–540 Ma ago. The mantle root might have developed beneath the continental Central Zone of the LMB as early as the Archean, and could be alien to the overthrust allochthonous sheet of the Limpopo Belt. Oxygen fugacity estimates for diamond inclusions at River Ranch are similar to other diamondiferous harzburgites beneath the Kaapvaal craton, indicating that the Kaapvaal mantle as a whole was well buffered and homogeneous with respect to f _O2 at the time of peridotitic diamond crystallization. Received: 11 January 1995 / Accepted: 10 June 1997     

The evolution of refertilized lithospheric mantle beneath the northeastern Siberian craton: Links between mantle metasomatism,thermal state and diamond potential

Although the diamond potential of cratons is linked mainly to thick and depleted Archean lithospheric keels, there are examples of craton-edge locations and circum-cratonic Proterozoic terranes underlain by diamondiferous mantle. Here, we use the results of comprehensive major and trace-element studies of detrital garnets from diamond-rich Late Triassic (Carnian) sedimentary rocks in the northeastern Siberia to constrain the thermal and chemical state of the pre-Triassic mantle and its ability to sustain the diamond storage. The studied detrital mantle-derived garnets are dominated by low- to medium-Cr lherzolitic (～45%) and low-Cr megacrystic (～39%) chemistries, with a significant proportion of eclogitic garnets (～11%), and only subordinate contribution from harzburgitic garnets (～5%) with variable Cr₂O₃ contents (1.2–8.4 wt.%). Low-Cr megacrysts display uniform, “normal” rare-earth element (REE) patterns with no Eu/Eu* anomalies, systematic Zr and Ti enrichment (mainly within 2.5–5), which are evidence of their crystallization from deep metasomatic melts. Lherzolitic (G9) garnets exhibit normal or humped to MREE-depleted sinusoidal REE patterns and elevated Nd/Y (up to 0.33–0.41) and Zr/Y ratios (up to 7.62). Rare low- to high-Cr harzburgitic (G10) garnets have primarily “depleted”, sinusoidal REE-patterns, low Ti, Y and HREE, but vary significantly in Zr-Hf, Ti and MREE-HREE contents, Nd/Y (within 0.1–2.4) and Zr/Y (1.53–19.9) ratios. The observed trends of chemical enrichment from the most depleted, harzburgitic garnets towards lherzolitic (including high-Ti high-Cr G11-type) garnets and megacrysts result from either voluminous high-temperature metasomatism by plume-derived silicate melts or recurrent mobilization of less voluminous kimberlitic or related carbonated mantle melts, rather than the initially primitive, fertile nature of the Proterozoic SCLM. Calculated Ni-in-garnet temperatures (primarily within ～1150–1250 °C) indicate their derivation from at least ～220 km thick Cr-undersaturated lithosphere at the relevant Devonian to Triassic thermal flow of ～45 mW/m² or cooler. We suggest the existence of rare harzburgitic domains in the primarily lherzolitic diamond-facies SCLM beneath the northeastern Siberian craton at least by Triassic, whereas the abundance of eclogitic garnets, predominance of E-type inclusions in placer diamonds and specific morphologies argue for diamondiferous eclogites occurring within a ～50–65 kbar diamond window of the Olenek province by the same time.     

Diamonds from Jagersfontein (South Africa): messengers from the sublithospheric mantle

The diamond population from the Jagersfontein kimberlite is characterized by a high abundance of eclogitic, besides peridotitic and a small group of websteritic diamonds. The majority of inclusions indicate that the diamonds are formed in the subcratonic lithospheric mantle. Inclusions of the eclogitic paragenesis, which generally have a wide compositional range, include two groups of eclogitic garnets (high and low Ca) which are also distinct in their rare earth element composition. Within the eclogitic and websteritic suite, diamonds with inclusions of majoritic garnets were found, which provide evidence for their formation within the asthenosphere and transition zone. Unlike the lithospheric garnets all majoritic garnet inclusions show negative Eu-anomalies. A narrow range of isotopically light carbon compositions (δ¹³C −17 to −24 ‰) of the host diamonds suggests that diamond formation in the sublithospheric mantle is principally different to that in the lithosphere. Direct conversion from graphite in a subducting slab appears to be the main mechanism responsible for diamond formation in this part of the Earth’s mantle beneath the Kaapvaal Craton. The peridotitic inclusion suite at Jagersfontein is similar to other diamond deposits on the Kaapvaal Craton and characterized by harzburgitic to low-Ca harzburgitic compositions.     

Mineral inclusions in diamonds from the Sputnik kimberlite pipe, Yakutia

The Sputnik kimberlite pipe is a small “satellite” of the larger Mir pipe in central Yakutia (Sakha), Russia. Study of 38 large diamonds (0.7-4.9 carats) showed that nine contain inclusions of the eclogitic paragenesis, while the remainder contain inclusions of the peridotitic paragenesis, or of uncertain paragenesis. The peridotitic inclusion suite comprises olivine, enstatite, Cr-diopside, chromite, Cr-pyrope garnet (both lherzolitic and harzburgitic), ilmenite, Ni-rich sulfide and a Ti-Cr-Fe-Mg-Sr-K phase of the lindsleyite-mathiasite (LIMA) series. The eclogitic inclusion suite comprises omphacite, garnet, Ni-poor sulfide, phlogopite and rutile. Peridotitic ilmenite inclusions have high Mg, Cr and Ni contents and high Nb/Zr ratios; they may be related to metasomatic ilmenites known from peridotite xenoliths in kimberlite. Eclogitic phlogopite is intergrown with omphacite, coexists with garnet, and has an unusually high TiO₂ content. Comparison with inclusions in diamonds from Mir shows general similarities, but differences in details of trace-element patterns. Large compositional variations among inclusions of one phase (olivine, garnet, chromite) within single diamonds indicate that the chemical environment of diamond crystallisation changed rapidly relative to diamond growth rates in many cases. P-T conditions of formation were calculated from multiphase inclusions and from trace element geothermobarometry of single inclusions. The geotherm at the time of diamond formation was near a 35 mW/m² conductive model; that is indistinguishable from the Paleozoic geotherm derived by studies of xenoliths and concentrate minerals from Mir. A range of Ni temperatures between garnet inclusions in single diamonds from both Mir and Sputnik suggests that many of the diamonds grew during thermal events affecting a relatively narrow depth range of the lithosphere, within the diamond stability field. The minor differences between inclusions in Mir and Sputnik may reflect lateral heterogeneity in the upper mantle.     

Mineral inclusions in diamonds from the Kelsey Lake Mine, Colorado, USA: Depleted Archean mantle beneath the Proterozoic Yavapai province

Thirty-four silicate and oxide inclusions large enough for in situ WDS electron microprobe analysis were exposed by grinding/polishing of 19 diamonds from the Kelsey Lake Mine in the Colorado-Wyoming State Line Kimberlite district. Eighteen olivines, seven Cr-pyropes, four Mg-chromites, and one orthopyroxene in 15 stones belong to the peridotite (P) suite and three garnets and one omphacite in three stones belong to the eclogite (E) suite. The fact that this suite is dominated by the peridotite population is in stark contrast to the other diamond suites studied in the State Line district (Sloan, George Creek), which are overwhelmingly eclogitic. Kelsey Lake olivine inclusions are magnesian (17 of 18 grains in 9 stones are in the range Fo 92.7-93.1), typical of harzburgitic P-suite stones worldwide, but unlike the more Fe-rich (lherzolitic) Sloan olivine suite. Mg-chromites (wt% MgO = 12.8-13.8; wt% Cr₂O₃ = 61.4-66.6) are in the lower MgO range of diamond inclusion chromites worldwide. Seven harzburgitic Cr-pyropes in five stones have moderately low calcium contents (wt% CaO = 3.3-4.3) but are very Cr-rich (wt% Cr₂O₃ = 9.7-16.7). A few stones have been analyzed by SIMS for carbon isotope composition and nitrogen abundance. One peridotitic stone is apparently homogeneous in carbon isotope composition (δ¹³C_PDB = −6.2‰) but with variable nitrogen abundance (1296-2550 ppm). Carbon isotopes in eclogitic stones range from “normal” for the upper mantle (δ¹³C_PDB = −5.5‰) to somewhat low (δ¹³C_PDB = −10.2‰), with little internal variation in individual stones (maximum difference is 3.6‰). Nitrogen contents (2-779 ppm) are lower than in the peridotitic stone, and are lower in cores than in rims. As, worldwide, harzburgite-suite diamonds have been shown to have formed in Archean time, we suggest that the Kelsey Lake diamond population was derived from a block of Archean lithosphere that, at the time of kimberlite eruption, existed beneath the Proterozoic Yavapai province. The mixed diamond inclusion populations from the State Line kimberlites appear to support models in which volumes of Wyoming Craton Archean mantle survive buried beneath Proterozoic continental crust. Such material may be mixed with eclogitic/lherzolitic regimes emplaced beneath or intermingled with the Archean rocks by Proterozoic subduction.     

Diamonds from the Buffalo Head Hills,Alberta: Formation in a non-conventional setting

Kimberlite pipes K11, K91 and K252 in the Buffalo Head Hills, northern Alberta show an unusually large abundance (20%) of Type II (no detectable nitrogen) diamonds. Type I diamonds range in nitrogen content from 6 ppm to 3300 ppm and in aggregation states from low (IaA) to complete (IaB). The Type IaB diamonds extend to the lowest nitrogen concentrations yet observed at such high aggregation states, implying that mantle residence occurred at temperatures well above normal lithospheric conditions. Syngenetic mineral inclusions indicate lherzolitic, harzburgitic, wehrlitic and eclogitic sources. Pyropic garnet and forsteritic olivine characterize the peridotitic paragenesis from these pipes. One lherzolitic garnet inclusion has a moderately majoritic composition indicating a formation depth of ∼ 400 km. A wehrlitic paragenesis is documented by a Ca-rich, high-chromium garnet and very CaO-rich (0.11–0.14 wt.%) olivine. Omphacitic pyroxene and almandine-rich garnet are characteristic of the eclogitic paragenesis. A bimodal δ¹³C distribution with peaks at − 5‰ and − 17‰ is observed for diamonds from all three kimberlite pipes. A large proportion (∼ 40%) of isotopically light diamonds (δ¹³C < −10‰) indicates a predominantly eclogitic paragenesis.The Buffalo Head Terrane is of Lower Proterozoic metamorphic age (2.3–2.0 Ga) and hence an unconventional setting for diamond exploration. Buffalo Hills diamonds formed during multiple events in an atypical mantle setting. The presence of majorite and abundance of Type II and Type IaB diamonds suggests formation under sublithospheric conditions, possibly in a subducting slab and resulting megalith. Type IaA to IaAB diamonds indicate formation and storage under lower temperature in normal lithospheric conditions.     

Ar/Ar analyses of clinopyroxene inclusions in African diamonds: implications for source ages of detrital diamonds

Clinopyroxene inclusions in diamond contain elevated potassium contents and can potentially be dated by ⁴⁰Ar/³⁹Ar techniques. Previous ⁴⁰Ar/³⁹Ar studies of clinopyroxene inclusions contained in cleaved diamonds have suggested that argon, produced from the decay of potassium prior to eruption of the host kimberlite magma, diffuses to the diamond/clinopyroxene interface under mantle conditions. After intrusion and cooling below the closure temperature for argon diffusion, radiogenic argon is retained by the clinopyroxene inclusions. This behaviour complicates efforts to date diamond crystallisation events; however, extraction of inclusions from their host diamond should induce loss of all interface argon, thus raising the possibility of determining kimberlite emplacement ages. This possibility has important implications for constraining the source localities of detrital diamond deposits worldwide, with concomitant benefits to diamond exploration. To investigate this premise, ⁴⁰Ar/³⁹Ar laser probe results are presented for single clinopyroxene inclusions extracted from a total of fifteen gem-quality diamonds from the Mbuji-Mayi kimberlite in the Democratic Republic of Congo, and the Jwaneng and Orapa kimberlites in Botswana.Initial fusion analyses of clinopyroxene inclusions from Mbuji-Mayi diamonds yielded ages older than the time of host kimberlite intrusion, indicating partial retention of extraneous argon by the clinopyroxene inclusions themselves. Step-heating analyses of clinopyroxene inclusions from Orapa and Jwaneng diamonds produced older apparent ages from lower temperature steps and the ‘rim’ fragment of one Orapa inclusion. High temperature (fusion) analyses yielded younger apparent ages, commonly approaching the times of host kimberlite eruption. Total-gas integrated ⁴⁰Ar/³⁹Ar ages are mostly intermediate between the times of inferred diamond crystallisation and kimberlite eruption. Ca/K ratios for each sample are uniform across step-heating increments, indicating that age variations are not due to compositional, mineralogical or alteration effects. The favoured explanation for these results is partial retention of extraneous argon in primary and/or secondary fluid inclusions. This component is then preferentially outgassed in lower temperature heating steps, yielding older apparent ages.The partial retention of extraneous argon by clinopyroxene inclusions clearly restricts efforts to determine source ages for detrital diamond deposits. Results from individual samples must necessarily be interpreted as maximum source emplacement ages. Nonetheless, step-heating analyses of several clinopyroxene inclusions from a detrital diamond deposit may provide reasonable constraints on the ages of source kimberlites/lamproites; however minor age populations as well as those closely spaced in time, may be difficult to resolve.It is argued that the majority of older ⁴⁰Ar/³⁹Ar ages can be explained in terms of the partial retention of inherited argon, produced between the times of diamond crystallisation and kimberlite eruption. Although the presence of excess argon in some clinopyroxene inclusions cannot be excluded, available evidence (e.g. no excess argon in Premier eclogitic inclusions or potassium-poor inclusions) suggests that this is not a factor for most samples. Three possible mechanistic models are forwarded to account for the uptake of inherited (± excess) argon in fluid inclusions. The first envisages negligible interface porosity and diffusion of extraneous argon exclusively to primary fluid inclusions, which subsequently partially decrepitated during eruption, causing accumulation of argon at the diamond/clinopyroxene interface. The second model permits diffusive loss of extraneous argon to both the interface region and primary fluid inclusions. The third involves diffusion of extraneous argon to the interface region, with later entrapment of some interface argon in secondary fluid inclusions, produced by fracture/annealing processes active during eruption. The first model can account for all ⁴⁰Ar/³⁹Ar results, whereas the latter two mechanisms require the presence of an excess argon component to explain older integrated ages (up to 2.9 Ga) from two Jwaneng samples. Excess argon contamination would compromise efforts to determine diamond genesis ages using the ⁴⁰Ar/³⁹Ar dating technique. However, if the first model is valid, then the older ⁴⁰Ar/³⁹Ar integrated ages support previous Re-Os age results for the crystallisation of Jwaneng diamonds.     

Mineral inclusions and geochemical characteristics of microdiamonds from the DO27, A154, A21, A418, DO18, DD17 and Ranch Lake kimberlites at Lac de Gras, Slave Craton, Canada

A mineral inclusion, carbon isotope, nitrogen content, nitrogen aggregation state and morphological study of 576 microdiamonds from the DO27, A154, A21, A418, DO18, DD17 and Ranch Lake kimberlites at Lac de Gras, Slave Craton, was conducted. Mineral inclusion data show the diamonds are largely eclogitic (64%), followed by peridotitic (25%) and ultradeep (11%). The paragenetic abundances are similar to macrodiamonds from the DO27 kimberlite (Davies, R.M., Griffin, W.L., O'Reilly, S.Y., 1999. Diamonds from the deep: pipe DO27, Slave craton, Canada. In: Gurney, J.J., Gurney, J.L., Pascoe, M.D., Richardson, S.H. (Eds.), The J. B. Dawson Vol., Proc. 7th Internat. Kimberlite Conf., Red Roof Designs, Cape Town, pp. 148–155) but differ to diamonds from nearby kimberlites at Ekati (e.g., Lithos (2004); Tappert, R., Stachel, T., Harris, J.W., Brey, G.P., 2004. Mineral Inclusions in Diamonds from the Panda Kimberlite, S. P., Canada. 8th International Kimberlite Conference, extended abstracts) and Snap Lake to the south (Dokl. Earth Sci. 380 (7) (2001) 806), that are dominated by peridotitic stones.

Eclogitic diamonds with variable inclusion compositions and temperatures of formation (1040–1300 °C) crystallised at variable lithospheric depths sometimes in changing chemical environments. A large range to very ¹³C-depleted C-isotope compositions (δ¹³C=−35.8‰ to −2.2‰) and an NMORB bulk composition, calculated from trace elements in garnet and clinopyroxene inclusions, are consistent with an origin from subducted oceanic crust and sediments. Carbon isotopes in the peridotitic diamonds have mantle compositions (δ¹³C mode −4.0‰). Mineral inclusion compositions are largely harzburgitic. Variable temperatures of formation (garnet T_Ni=800–1300 °C) suggest the peridotitic diamonds originate from the shallow ultra-depleted and deeper less depleted layers of the central Slave lithosphere. Carbon isotopes (δ¹³C av.=−5.1‰) and mineral inclusions in the ultradeep diamonds suggest they formed in peridotitic mantle (670 km). The diamonds may have been entrained in a plume and subcreted to the base of the central Slave lithosphere.

Poorly aggregated nitrogen (IaA without platelets) in a large number of eclogitic (67%) and peridotitic (32%) diamonds, with similar nitrogen contents, indicates the diamonds were stored in the mantle at low temperatures (1060–<1100 °C) following crystallisation in the Archean. Type IaA diamonds have largely cubo-octahedral growth forms, and Type II and Type IaAB diamonds, with higher nitrogen aggregation states, mostly have octahedral morphologies. However, no correlation between these groups and their mineral inclusion compositions, C-isotopes, and N-contents rules out the possibility of unique source origins and suggests eclogitic and peridotitic diamonds experienced variable mantle thermal states. Variation in mineral inclusion chemistries in single diamonds, possible overgrowths of ¹³C-depleted eclogitic diamond on diamonds with peridotitic and ultradeep inclusions, and Type I ultradeep diamond with low N-aggregation is consistent with diamond growth over time in changing chemical environments.     

The age of continental roots

Determination of the age of the mantle part of continental roots is essential to our understanding of the evolution and stability of continents. Dating the rocks that comprise the mantle root beneath the continents has proven difficult because of their high equilibration temperatures and open-system geochemical behaviour. Much progress has been made in the last 20 years that allows us to see how continental roots have evolved in different areas. The first indication of the antiquity of continental roots beneath cratons came from the enriched Nd and Sr isotopic signatures shown by both peridotite xenoliths and inclusions in diamonds, requiring isolation of cratonic roots from the convecting mantle for billions of years. The enriched Nd and Sr isotopic signatures result from mantle metasomatic events post-dating the depletion events that led to the formation and isolation of the peridotite from convecting mantle. These signatures document a history of melt– and fluid–rock interaction within the lithospheric mantle. In some suites of cratonic rocks, such as eclogites, Nd and Pb isotopes have been able to trace probable formation ages. The Re–Os isotope system is well suited to dating lithospheric peridotites because of the compatible nature of Os and its relative immunity to post-crystallisation disturbance compared with highly incompatible element isotope systems. Os isotopic compositions of lithospheric peridotites are overwhelmingly unradiogenic and indicate long-term evolution in low Re/Os environments, probably as melt residues. Peridotite xenoliths from kimberlites can show some disturbed Re/Os systematics but analyses of representative suites show that beneath cratons the oldest Re depletion model ages are Archean and broadly similar to major crust-forming events. Some locations, such as Premier in southern Africa, and Lashaine in Tanzania, indicate more recent addition of lithospheric material to the craton, in the Proterozoic, or later. Of the cratons studies so far (Kaapvaal, Siberia, Wyoming and Tanzania), all indicate Archean formation of their lithospheric mantle roots. Few localities studied show any clear variation of age with depth of derivation, indicating that >150 km of lithosphere may have formed relatively rapidly. In circum-cratonic areas where the crustal basement is Proterozoic in age kimberlite-derived xenoliths give Proterozoic model ages, matching the age of the overlying crust. This behaviour shows how the crust and mantle parts of continental lithospheric roots have remained coupled since formation in these areas, for billions of years, despite continental drift. Orogenic massifs show more systematic behaviour of Re–Os isotopes, where correlations between Os isotopic composition and S or Re content yield initial Os isotopic ratios that define Re depletion model ages for the massifs. Ongoing Sr–Nd–Pb–Hf–Os isotopic studies of massif peridotites and new kimberlite- and basalt-borne xenolith suites from new areas, will soon enable a global understanding of the age of continental roots and their subsequent evolution.     

Mineral chemistry and thermobarometry of inclusions from De Beers Pool diamonds, Kimberley, South Africa

Silicate and oxide mineral inclusions in diamonds from the geologically and historically important De Beers Pool kimberlites in Kimberley, South Africa, are characterised by harzburgitic compositions (>90%), with lesser abundances from eclogitic and websteritic parageneses. The De Beers Pool diamonds contain unusually high numbers of inclusion intergrowths, with garnet+orthopyroxene±chromite±olivine and chromite+olivine assemblages dominant. More unusual intergrowths include garnet+olivine+magnesite and an eclogitic assemblage comprising garnet+clinopyroxene+rutile. The mineral chemistry of the De Beers Pool inclusions overlaps that of most worldwide localities. Peridotitic garnet inclusions exhibit variable CaO (<5.8 wt.%) and Cr₂O₃ contents (3.0–15.0 wt.%), although the majority are harzburgitic with very low calcium concentrations (<2 wt.% CaO). Eclogitic garnet inclusions are characterised by a wide range in CaO (3.3–21.1 wt.%) with low Cr₂O₃ (<1 wt.%). Websteritic garnets exhibit intermediate compositions. Most chromite inclusions contain 63–67 wt.% Cr₂O₃ and <0.5 wt.% TiO₂. Olivine and orthopyroxene inclusions are magnesium-rich with Mg-numbers of 93–97. Olivine inclusions in chromite exhibit the highest Mg-numbers and also contain elevated Cr₂O₃ contents up to 1.0 wt.%. Peridotitic clinopyroxene inclusions are Cr-diopsides with up to 0.8 wt.% K₂O. Eclogitic and websteritic clinopyroxene inclusions exhibit overlapping compositions with a wide range in Mg-numbers (66–86).

Calculated temperatures for non-touching inclusion pairs from individual diamonds range from 1082 to 1320 °C (average=1197 °C), whereas pressures vary from 4.6 to 7.7 GPa (average=6.3 GPa). Touching inclusion assemblages are characterised by equilibration temperatures of 995 to 1182 °C (average=1079 °C) and pressures of 4.2–6.8 GPa (average=5.4 GPa). Provided that the non-touching inclusions represent equilibrium assemblages, it is suggested that these inclusions record the conditions at the time of diamond crystallisation (1200 °C; 3.0 Ga). The lower average temperatures for touching inclusions are attributed to re-equilibration in a cooling mantle (1050 °C) prior to kimberlite eruption at 85 Ma. Pressure estimates for touching garnet–orthopyroxene inclusions are also skewed towards lower values than most non-touching inclusions. This apparent difference may be an artefact of the Al-exchange geobarometer and/or the result of sampling bias, due to limited numbers of non-touching garnet–orthopyroxene inclusions. Alternatively pressure differences could be caused by differential uplift in the mantle or possibly variations in thermal compressibility between diamond and silicate inclusions. However, thermodynamic modelling suggests that thermal compressibility differences would cause only minor changes in internal inclusion pressures (<0.2 GPa/100 °C).     

A study of eclogitic diamonds and their inclusions from the Finsch kimberlite pipe, South Africa

Previous studies of diamonds from Finsch have shown that eclogitic inclusions are rare at Finsch and that the eclogitic garnet and clinopyroxenes are iron and manganese-rich. In order to expand the current database of information, 93 eclogitic diamonds were selected for this study. Eight diamonds were polished into plates for cathodoluminescence studies and infrared examination of diamond growth and 31 diamonds were cracked to retrieve inclusions. The eclogitic garnets analysed in this study are enriched in Fe and are relatively depleted in Ca and Mg relative to worldwide data. FeO contents for garnet range from 15 to 27 wt.% and MnO contents reach a maximum value of 1.6 wt.%. The eclogitic clinopyroxenes have relatively high FeO contents, up to 14.8 wt.% and K₂O contents are low (<0.4 wt.%). Three non-touching garnet–clinopyroxene mineral pairs produce equilibration temperatures of 1138–1179 °C at an assumed pressure of 50 kb. No Type II diamonds were found during this study, all diamonds are of Type IaAB. Total nitrogen contents of Type IaAB diamonds range from 11 to 1520 ppm, with variable aggregation states (up to 84% nitrogen aggregated as B-defects). Distinct infrared characteristics suggest that the Finsch kimberlite sampled either more than one mantle source region of similar age but differing temperature, or two different populations of diamonds with different ages. The diamonds provide evidence of changing mantle conditions during crystallisation. Continuous diamond growth is illustrated by the presence of regular octahedral growth zones, although in some diamonds cubic growth is noted. One diamond shows evidence of platelet degradation, suggesting exposure to high temperatures and/or shearing stresses.     

The trace element composition of silicate inclusions in diamonds: a review

On a global scale, peridotitic garnet inclusions in diamonds from the subcratonic lithosphere indicate an evolution from strongly sinusoidal REE_N, typical for harzburgitic garnets, to mildly sinusoidal or “normal” patterns (positive slope from LREE_N to MREE_N, fairly flat MREE_N–HREE_N), typical for lherzolitic garnets. Using the Cr-number of garnet as a proxy for the bulk rock major element composition it becomes apparent that strong LREE enrichment in garnet is restricted to highly depleted lithologies, whereas flat or positive LREE–MREE slopes are limited to less depleted rocks. For lherzolitic garnet inclusions, there is a positive relation between equilibration temperature, enrichment in MREE, HREE and other HFSE (Ti, Zr, Y), and decreasing depletion in major elements. For harzburgitic garnets, relations are not linear, but it appears that lherzolite style enrichment in MREE–HREE only occurs at temperatures above 1150–1200 °C, whereas strong enrichment in Sr is absent at these high temperatures. These observations suggest a transition from melt metasomatism (typical for the lherzolitic sources) characterized by fairly unfractionated trace and major element compositions to metasomatism by CHO fluids carrying primarily incompatible trace elements. Melt and fluid metasomatism are viewed as a compositional continuum, with residual CHO fluids resulting from primary silicate or carbonate melts in the course of fractional crystallization and equilibration with lithospheric host rocks.

Eclogitic garnet inclusions show “normal” REE_N patterns, with LREE at about 1× and HREE at about 30× chondritic abundance. Clinopyroxenes approximately mirror the garnet patterns, being enriched in LREE and having chondritic HREE abundances. Positive and negative Eu anomalies are observed for both garnet and clinopyroxene inclusions. Such anomalies are strong evidence for crustal precursors for the eclogitic diamond sources. The trace element composition of an “average eclogitic diamond source” based on garnet and clinopyroxene inclusions is consistent with derivation from former oceanic crust that lost about 10% of a partial melt in the garnet stability field and that subsequently experienced only minor reenrichment in the most incompatible trace elements. Based on individual diamonds, this simplistic picture becomes more complex, with evidence for both strong enrichment and depletion in LREE.

Trace element data for sublithospheric inclusions in diamonds are less abundant. REE in majoritic garnets indicate source compositions that range from being similar to lithospheric eclogitic sources to strongly LREE enriched. Lower mantle sources, assessed based on CaSi–perovskite as the principal host for REE, are not primitive in composition but show moderate to strong LREE enrichment. The bulk rock LREE_N–HREE_N slope cannot be determined from CaSi–perovskites alone, as garnet may be present in these shallow lower mantle sources and then would act as an important host for HREE. Positive and negative Eu anomalies are widespread in CaSi–perovskites and negative anomalies have also been observed for a majoritic garnet and a coexisting clinopyroxene inclusion. This suggests that sublithospheric diamond sources may be linked to old oceanic slabs, possibly because only former crustal rocks can provide the redox gradients necessary for diamond precipitation in an otherwise reduced sublithospheric mantle.     

A subduction wedge origin for Paleoarchean peridotitic diamonds and harzburgites from the Panda kimberlite, Slave craton: evidence from Re–Os isotope systematics

An extensive study of peridotitic sulfide inclusion bearing diamonds and their prospective harzburgitic host rocks from the 53 Ma Panda kimberlite pipe, Ekati Mine, NWT Canada, has been undertaken with the Re–Os system to establish their age and petrogenesis. Diamonds with peridotitic sulfide inclusions have poorly aggregated nitrogen (<30% N as B centers) at N contents of 200–800 ppm which differs from that of chromite and silicate bearing diamonds and indicates residence in the cooler portion of the Slave craton lithospheric mantle. For most of the sulfide inclusions, relatively low Re contents (average 0.457 ppm) and high Os contents (average 339 ppm) lead to extremely low ¹⁸⁷Re/¹⁸⁸Os, typically << 0.05. An age of 3.52 ± 0.17 Ga (MSWD = 0.46) and a precise initial ¹⁸⁷Os/¹⁸⁸Os of 0.1093 ± 0.0001 are given by a single regression of 11 inclusions from five diamonds that individually provide coincident internal isochrons. This initial Os isotopic composition is 6% enriched in ¹⁸⁷Os over 3.5 Ga chondritic or primitive mantle. Sulfide inclusions with less radiogenic initial Os isotopic compositions reflect isotopic heterogeneity in diamond forming fluids. The harzburgites have even lower initial ¹⁸⁷Os/¹⁸⁸Os than the sulfide inclusions, some approaching the isotopic composition of 3.5 Ga chondritic mantle. In several cases isotopically distinct sulfides occur in different growth zones of the same diamond. This supports a model where C–O–H–S fluids carrying a radiogenic Os signature were introduced into depleted harzburgite and produced diamonds containing sulfides conforming to the 3.5 Ga isochron. Reaction of this fluid with harzburgite led to diamonds with less radiogenic inclusions while elevating the Os isotope ratios of some harzburgites. Subduction is a viable way of introducing such fluids. This implies a role for subduction in creating early continental nuclei at 3.5 Ga and generating peridotitic diamonds.Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users.