Petrology of highly aluminous xenoliths from kimberlites of Yakutia

Highly aluminous xenoliths include kyanite-, corundum- and coesite-bearing eclogites, grospydites and alkremites. These xenoliths are present in different kimberlites of Yakutia but have most often been found in Udachnaya and other pipes of the central Daldyn–Alakitsky region. Kimberlites of this field also contain eclogite-like xenoliths with kyanite and corundum that originate in the lower crust or the lower crust–upper mantle transition zone. Petrographic study shows that two rock groups of different structure and chemistry can be distinguished among kyanite eclogites: fine- to medium-grained with mosaic structure and coarse-grained with cataclastic structure. Eclogites with mosaic structure are characterized by the occurrences of symplectite intergrowths of garnet with kyanite, clinopyroxene and coesite; only in this group do grospydites occur. In cataclastic eclogites, coarse-grained coesite occurs, corresponding in size to other rock-forming minerals. Highly aluminous xenoliths differ from bimineralic eclogites in their high content of Al₂O₃ and total alkali content. Coesite-bearing varieties are characterized by low MgO content and higher Na/K and Fe²⁺/Fe³⁺ ratios, as well as high contents of Na₂O. Geochemical peculiarities of kyanite eclogites and other rocks are exhibited by a sloping chondrite-normalized distribution of rare earth elements (REE) in garnets and low Y/Zr ratio, in contrast to bimineralic rocks. Coesite is found in more than 20 kyanite eclogites and grospydites from Udachnaya. Grospydites with coesite from Zagadochnaya pipe are described. Three varieties of coesite in these rocks are distinguished: (a) subhedral grains with size of 1.0–3.0 mm; (b) inclusions in the rock-forming minerals; (c) sub-graphic intergrowths with garnet. The presence and preservation of coesite in eclogites indicate both high pressure of formation (more than 30 kbar) and set a number of constraints on the timing of xenolith cooling during entrainment and transport to the surface. Different ways of formation of the highly aluminous eclogites are discussed. Petrographic observations and geochemistry suggest that some highly aluminous rocks have formed as a result of crystallization of anorthosite rocks in abyssal conditions. δ¹⁸O-estimations and other petrologic evidence point out the possible origin of some of these xenoliths as the result of subduction of oceanic crust. Diamondiferous samples have been found in all varieties except alkremites. Usually these eclogites contain cubic or coated diamonds. However, two sample corundum-bearing eclogites with diamonds from the Udachnaya pipe contain octahedra that show evidence of resorption.     

Nature of the mantle roots beneath the North American craton: mantle xenolith evidence from Somerset Island kimberlites

The recently discovered Nikos kimberlite on Somerset Island, in the Canadian Arctic, hosts an unusually well preserved suite of mantle xenoliths dominated by garnet–peridotite (lherzolite, harzburgite, dunite) showing coarse and porphyroclastic textures, with minor garnet–pyroxenite. The whole rock and mineral data for 54 Nikos xenoliths indicate a highly refractory underlying mantle with high olivine forsterite contents (ave. Fo=92.3) and moderate to high olivine abundances (ave. 80 wt.%). These characteristics are similar to those reported for peridotites from the Archean Kaapvaal and Siberian cratons (ave. olivine Fo=92.5), but are clearly distinct from the trend defined by oceanic peridotites and mantle xenoliths in alkaline basalts and kimberlites from post-Archean continental terranes (ave. olivine Fo=91.0). The Nikos xenoliths yield pressures and temperatures of last equilibration between 20 and 55 kb and 650 and 1300°C, and a number of the peridotite nodules appear to have equilibrated in the diamond stability field. The pressure and temperature data define a conductive paleogeotherm corresponding to a surface heat flow of 44 mW/m². Paleogeotherms based on xenolith data from the central Slave province of the Canadian craton require a lower surface heat flow (40 mW/m²) indicating a cooler geothermal regime than that beneath the Canadian Arctic. A large number of kimberlite-hosted peridotites from the Kaapvaal craton in South Africa and parts of the Siberian craton are characterized by high orthopyroxene contents (ave. Kaapvaal 32 wt.%, Siberia 20 wt.%). The calculated modal mineral assemblages for the Nikos peridotites show moderate to low contents of orthopyroxene (ave. 12 wt.%), indicating that the orthopyroxene-rich mineralogy characteristic of the Kaapvaal and Siberian cratons is not a feature of the cratonic upper mantle beneath Somerset Island.     

A classification scheme for mantle-derived garnets in kimberlite: a tool for investigating the mantle and exploring for diamonds

A new empirical method has been devised for classification of mantle-derived garnets in kimberlite. Simple chemical screens have been developed to distinguish between garnets from different parageneses, based on Mg, Fe, Ca, Cr, Ti and Na values of published analyses of garnets from >2000 ultramafic xenoliths in kimberlite. Although crustal garnets are typically uncommon as xenocrysts in kimberlite, the first step in the classification is to screen these from the mantle population, using data from >600 garnet-bearing crustal rocks. Such a screen may also prove useful in evaluating the source (crust vs. mantle) of garnet in kimberlite exploration samples. Subsequent steps divide mantle garnets into eclogite, peridotite and Cr-poor megacryst groupings, and sub-groups of the peridotite (lherzolite, harzburgite, wehrlite) and eclogite (Groups I and II and A, B, C and grospydite) populations. Important features of this classification include the fact that it is based on distinctions between groups of fundamental geological significance (e.g., peridotite vs. eclogite) and it is based on a large, well-documented and well-understood xenolith database. As it utilizes oxide values and molar ratios of major and minor elements, the rationale for the screens is readily understood and it is simple to use.     

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.     

Petrology and geochemistry of a diamondiferous lherzolite from the Premier diamond mine, South Africa

This paper reports on the petrology and geochemistry of a diamondiferous peridotite xenolith from the Premier diamond mine in South Africa.

The xenolith is altered with pervasive serpentinisation of olivine and orthopyroxene. Garnets are in an advanced state of kelyphitisation but partly fresh. Electron microprobe analyses of the garnets are consistent with a lherzolitic paragenesis (8.5 wt.% Cr₂O₃ and 6.6 wt.% CaO). The garnets show limited variation in trace element composition, with generally low concentrations of most trace elements, e.g. Y (<11 ppm), Zr (<18 ppm) and Sr (<0.5 ppm). Garnet rare earth element concentrations, when normalised against the C1 chondrite of McDonough and Sun (Chem. Geol. 120 (1995) 223), are characterised by a rare earth element pattern similar to garnet from fertile lherzolite.

All diamonds recovered are colourless. Most crystals are sharp-edged octahedra, some with minor development of the dodecahedral form. A number of crystals are twinned octahedral macles, while aggregates of two or more octahedra are also common. Mineral inclusions are rare. Where present they are predominantly small black rosettes believed to consist of sulfide. In one instance a polymineralic (presumably lherzolitic) assemblage of reddish garnet, green clinopyroxene and a colourless mineral is recognised.

Infrared analysis of the xenolith diamonds show nitrogen contents generally lower than 500 ppm and variable nitrogen aggregation state, from 20% to 80% of the ‘B’ form. When plotted on a nitrogen aggregation diagram a well defined trend of increasing nitrogen aggregation state with increasing nitrogen content is observed. Carbon isotopic compositions range from −3.6 ‰ to −1.3 ‰. These are broadly correlated with diamond nitrogen content as determined by infrared spectroscopy, with the most negative C-isotopic compositions correlating with the lowest nitrogen contents.

Xenolith mantle equilibration temperatures, calculated from nitrogen aggregation systematics as well as the Ni in garnet thermometer are on the order of 1100 to 1200 °C.

It is concluded that the xenolith is a fertile lherzolite, and that the lherzolitic character may have resulted from the total metasomatic overprinting of pre-existing harzburgite. Metasomatism occurred prior to, or accompanied, diamond growth.     

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.     

Exploring for kimberlites in glaciated terrains using chromite in quaternary till—a regional case study from northern Finland

Prospecting for kimberlites and related rocks in till-covered terrains requires a methodology for recovering a few small grains within tens of kilograms samples, necessitating 1 ppb sensitivity or better. As part of reconnaissance survey for the kimberlite indicator minerals, i.e. pyrope garnet, picroilmenite, chromite and chromian diopside, the Geological Survey of Finland (GTK) developed such a system by significantly modifying and augmenting a 3″ Knelson Concentrator that accomplishes nearly complete recovery of moderately heavy minerals (>0.25 mm) from till samples.Diamondiferous kimberlites occur in the eastern Finland around the Kaavi–Kuopio and Kuhmo areas and much of the rest of the Karelian craton remains prospective based on the empirical evidence necessary for diamond preservation: thick (>200 km) lithospheric mantle, low heat flow and Archaean age rocks. A target area in Lapland, 20×50 km in size, was selected for a pilot study to test extraction of chromite for the (1) discrimination of regionally and locally derived populations, and (2) recognition of possible kimberlitic/lamproitic chromites. Area selection was based on the regional occurrence of a variety of mantle-derived rocks, the recovery of a chromian pyrope grain from till in 1996 and most importantly, the well-established Quaternary stratigraphy in the region. The sample material consisted of sixty-two 80-kg excavator and 40-kg shovel samples. Approximately 1000 chromite grains, almost exclusively 0.25–0.5 mm in diameter, were recovered and analysed by electron microprobe.Tills in the sampling area proved to contain at least two compositional populations of chromite. The first is present in almost every sample and is apparently derived from layered mafic intrusions distal to and up-ice from the study area. The second population consists of chromites with low Ti, high Cr and Mg similar to inclusions in diamond. It is present in approximately one third of the samples, concentrated in a couple of clusters within the target area and is therefore considered to be of more local derivation. Since no high-Ti, high-Cr chromites diagnostic for kimberlites and lamproites were present in the samples, the source for the low-Ti, high-Cr, high-Mg chromite grains remains uncertain, but is probably not kimberlitic. Although this apparently is a negative outcome for diamond exploration in the target area, the main goal of the study was realised by showing the applicability of the system to heavy mineral separation from Quaternary glacial deposits.     

Petrologic origin of exsolution textures in mantle minerals: evidence in pyroxenitic xenoliths from Yakutia kimberlites

Exsolution lamellae in pyroxene and garnet porphyroblasts in pyroxenite xenoliths from the Mir, Udachnaya, and Obnazhennaya kimberlites (Siberian Craton) reveal a diverse suite of exsolved phases, including oxides (spinels, ilmenite, rutile, and chromite), pyroxene, and garnet. Textural characteristics suggest that exsolved phases progressively increased in volumetric proportions, and in some cases, the bulk xenoliths transformed from a lithology dominated by coarse grains (i.e. > 2 cm; megacrystalline) to a significantly finer-grained texture (i.e. < 1 cm).

These exsolved lamellae are the result of a complex and protracted sub solidus history following magmatic crystallization. Equilibrium pressure–temperature estimates place these xenoliths at low-to-moderate pressure–temperature conditions (690–910°C and 2.0–4.5 GPa) in the lithospheric mantle at the time of entrainment in the kimberlite. However, reconstructed compositions of initial pyroxene and garnet crystals suggest that this suite of pyroxenites formed at considerably higher temperatures and pressures that, in some instances, may have approached the majorite stability field. Pyroxenites that do not contain primary garnet may have been derived from shallower depths.

Progressive exsolution in these pyroxenites is of importance inasmuch as such processes can permit localized changes in rheological properties and may also accommodate strain within portions of lithospheric mantle. Because most xenolith studies focus on peridotites and eclogites, the pyroxenite sample suite studied in this work represents an important contribution towards a greater understanding of the Siberian lithospheric mantle.     

西藏罗布莎橄榄岩与中国大陆科学钻探主孔(CCSD-MH)榴辉岩中金刚石的红外特征初探

大别苏鲁超高压变质带上的大陆科学钻探主孔(CCSD-MH)中榴辉岩中的金刚石形成于大陆深俯冲作用过程,西藏雅鲁藏布江缝合带罗布莎蛇绿岩铬铁矿中的金刚石来自深部地幔,两者的形成背景和机制可能不同.本文对两地的金刚石样品开展了傅立叶变换红外光谱(FT-IR)定性分析.结果表明,西藏罗布莎金刚石样品为ⅠaA型;而CCSD-MH金刚石为ⅠaAB型,既表明其杂质氮的聚集形式和演化路径上存在着差异.红外光谱特征不仅仅表明两者属天然金刚石常见类型,并且超高压变质带中的金刚石形成时间可能更久远.     

Lithospheric mantle beneath the south-eastern Siberian craton: petrology of peridotite xenoliths in basalts from the Tokinsky Stanovik

We provide petrographic, major and trace element data for over 30 spinel peridotite xenoliths from the Tokinsky Stanovik (Tok) volcanic field on the Aldan shield to characterize the lithospheric mantle beneath the south-eastern margin of the Siberian craton, which formed in the Mesoproterozoic. High equilibration temperatures (870–1,010°C) of the xenoliths and the absence of garnet-bearing peridotites indicate a much thinner lithosphere than in the central craton. Most common among the xenoliths are clinopyroxene-poor lherzolites and harzburgites with Al₂O₃ and CaO contents nearly as low as in refractory xenoliths from kimberlite pipes (Mir, Udachnaya) in the central and northern Siberian craton. By contrast, the Tok peridotites have higher FeO, lower Mg-numbers and lower modal orthopyroxene and are apparently formed by shallow partial melting (3 GPa). Nearly all Tok xenoliths yield petrographic and chemical evidence for metasomatism: accessory phlogopite, amphibole, phosphates, feldspar and Ti-rich oxides, very high Na₂O (2–3.1%) in clinopyroxene, LREE enrichments in whole-rocks.Electronic Supplementary Material Supplementary material is available for this article at     

Petrology and geochemistry of diamondiferous Mesoproterozoic kimberlites from Wajrakarur kimberlite field,Eastern Dharwar craton,southern India: genesis and constraints on mantle source regions

The petrology and geochemistry of some new occurrences of Mesoproterozoic diamondiferous hypabyssal-facies kimberlites from the Chigicherla, Wajrakarur-Lattavaram and Kalyandurg clusters of the Wajrakarur kimberlite field (WKF), Eastern Dharwar craton (EDC), southern India, are reported. The kimberlites contain two generations of olivine, and multiple groundmass phases including phlogopite, spinel, calcite, dolomite, apatite, perovskite, apatite and rare titanite, and xenocrysts of eclogitic garnet and picro-ilmenite. Since many of the silicate minerals in these kimberlites have been subjected to carbonisation and alteration, the compositions of the groundmass oxide minerals play a crucial role in their characterisation and in understanding melt compositions. While there is no evidence for significant crustal contamination in these kimberlites, some limited effects of ilmenite entrainment are evident in samples from the Kalyandurg cluster. Geochemical studies reveal that the WKF kimberlites are less differentiated and more primitive than those from the Narayanpet kimberlite field (NKF), Eastern Dharwar craton. Highly fractionated (La/Yb = 108–145) chondrite-normalised distribution patterns with La abundances of 500–1,000 × chondrite and low heavy rare earth elements (HREE) abundances of 5–10 × chondrite are characteristic of these rocks. Metasomatism by percolating melts from the convecting mantle, rather than by subduction-related processes, is inferred to have occurred in their source regions based on incompatible element signatures. While the majority of the Eastern Dharwar craton kimberlites are similar to the Group I kimberlites of southern Africa in terms of petrology, geochemistry and Sr–Nd isotope systematics, others show the geochemical traits of Group II kimberlites or an overlap between Group I and II kimberlites. Rare earth element (REE)-based semi-quantitative forward modelling of batch melting of southern African Group I and II kimberlite source compositions involving a metasomatised garnet lherzolite and very low degrees of partial melting demonstrate that (1) WKF and NKF kimberlites display a relatively far greater range in the degree of melting than those from the on-craton occurrences from southern Africa and are similar to that of world-wide melilitites, (2) different degrees of partial melting of a common source cannot account for the genesis of all the EDC kimberlites, (3) multiple and highly heterogeneous kimberlite sources involve in the sub-continental lithospheric mantle (SCLM) in the Eastern Dharwar craton and (4) WKF and NKF kimberlites generation is a resultant of complex interplay between the heterogeneous sources and their different degrees of partial melting. These observations are consistent with the recent results obtained from inversion modelling of REE concentrations from EDC kimberlites in that both the forward as wells as inverse melting models necessitate a dominantly lithospheric, and not asthenospheric, mantle source regions. The invading metasomatic (enriching) melts percolating from the convecting (asthenosphere) mantle impart an OIB-like isotopic signature to the final melt products.     

Nature of diamonds in Yakutian eclogites: views from eclogite tomography and mineral inclusions in diamonds

We have performed dissections of two diamondiferous eclogites (UX-1 and U33/1) from the Udachnaya kimberlite, Yakutia in order to understand the nature of diamond formation and the relationship between the diamonds, their mineral inclusions, and host eclogite minerals. Diamonds were carefully recovered from each xenolith, based upon high-resolution X-ray tomography images and three-dimensional models. The nature and physical properties of minerals, in direct contact with diamonds, were investigated at the time of diamond extraction. Polished sections of the eclogites were made, containing the mould areas of the diamonds, to further investigate the chemical compositions of the host minerals and the phases that were in contact with diamonds. Major- and minor-element compositions of silicate and sulfide mineral inclusions in diamonds show variations among each other, and from those in the host eclogites. Oxygen isotope compositions of one garnet and five clinopyroxene inclusions in diamonds from another Udachnaya eclogite (U51) span the entire range recorded for eclogite xenoliths from Udachnaya. In addition, the reported compositions of almost all clinopyroxene inclusions in U51 diamonds exhibit positive Eu anomaly. This feature, together with the oxygen isotopic characteristics, is consistent with the well-established hypothesis of subduction origin for Udachnaya eclogite xenoliths. It is intuitive to expect that all eclogite xenoliths in a particular kimberlite should have common heritage, at least with respect to their included diamonds. However, the variation in the composition of multiple inclusions within diamonds, and among diamonds, from the same eclogite indicates the involvement of complex processes in diamond genesis, at least in the eclogite xenoliths from Yakutia that we have studied.     

A subsurface criterion for predictive exploration of kimberlites from Bouguer gravity in the eastern Dharwar craton,India

The Archaean-Proterozoic Dharwar craton has many recorded occurrences of diamondiferous kimberlites. Reports of kimberlite emplacement in parts of the tectonically complex eastern Dharwar craton and a significant density contrast between kimberlites and the host peninsular gneisses motivated us to conduct gravity studies in the Narayanpet-Irladinne area of the eastern Dharwar craton. This region is contiguous with the Maddur-Narayanpet kimberlite that lies to its north, while the river Krishna lies to its south. From observed association of reported kimberlites in the Maddur-Narayanpet field with subsurface topography of the assumed three-layer earth section obtained by Bouguer gravity modelling, we developed a subsurface criterion for occurrence of kimberlites in the present study area. Using this criterion, five potential zones for kimberlite localization were identified in the Narayanpet-Irladinne region, eastern Dharwar craton.     

Petrology of two diamondiferous eclogite xenoliths from the Lahtojoki kimberlite pipe, eastern Finland

Diamondiferous Group A eclogites constitute a minor portion of the mantle-derived xenoliths in the eastern Finland kimberlites. They have been derived from the depth interval 150–230 km where they are inferred to occur as thin layers or small pods within coarse-grained garnet peridotites. The chemical and isotopic composition of minerals suggest that they represent (Proterozoic?) mantle-derived melts or cumulates rather than subducted oceanic lithosphere. During magma ascent and emplacement of the kimberlites, the eclogite xenoliths were mechanically and chemically rounded judging from the types of surface markings. In addition, those octahedral crystal faces of diamonds that were partially exposed from the rounded eclogite xenolith became covered by trigons and overlain by microlamination due to their reaction with the kimberlite magma. The diamonds bear evidence of pervasive plastic deformation which is not, however, evident in the eclogite host. This suggests that annealing at ambient lithospheric temperatures has effectively recrystallised the silicates while the diamond has retained its lattice imperfections and thus still has the potential to yield information about ancient mantle deformation. One of our samples is estimated to contain approximately 90,000 ct/ton diamond implying that some diamonds occur within very high-grade pods or thin seams in the lithospheric mantle. To our knowledge, this is one of the most diamondiferous samples described.     

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.     

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.     

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.     

Underground geotechnical and geological investigations at Ekati Mine–Koala North: case study

Since 1998, BHP Billiton has mined diamonds at the Ekati Diamond Mine™ near Lac de Gras in the Northwest Territories of Canada. Current operations are based on mining multiple pipes by the open-pit method, but as some pits deepen, converting to underground mining is being considered.

As a test of underground mining methods and to provide access to the lower elevations of the Panda and Koala pipes, the Koala North pipe is being developed for underground mining. Initially, the top 40 m of the pipe were mined as an open pit to provide grade information and a prepared surface for the transition to underground mining. Currently, Koala North is being developed as an open-benching, mechanized, trackless operation. Although the method was successfully used at several De Beers diamond operations in South Africa, it has never been tested in an Arctic environment.

This case study describes basic geology, mining method layout and ongoing geological and geotechnical investigation. From the beginning of underground development, geotechnical daily routines have been fully integrated within the technical services department, which supports the operation. Geotechnical, geological and structural information obtained from underground mapping and core logging is compiled, processed, reviewed and analyzed on site by the geotechnical staff. Conclusions and recommendations are implemented as part of the operations in a timely manner. This ongoing “live” process enables the operators to make the most efficient use of resources both for ground support and excavations as well as to address safety issues, which are the top priority.     

Ar–Ar age determinations of eclogitic clinopyroxene and garnet inclusions in diamonds from the Venetia and Orapa kimberlites

Ar–Ar age measurements are reported for selected eclogitic clinopyroxene and garnet inclusions in Orapa diamonds and clinopyroxene inclusions in Venetia diamonds. Laser drilling of encapsulated clinopyroxene inclusions within Venetia diamonds released a maximum of 3% of the total ⁴⁰Ar, indicating little diffusive transfer and storage of radiogenic ⁴⁰Ar at the diamond–inclusion boundary. Apparent ages obtained during stepped heating of three diamonds are consistent with diamond crystallisation occurring just prior to the kimberlite eruption 520 Ma ago. Stepped heating of three clinopyroxene-bearing Orapa diamonds gave ages of 906–1032 Ma, significantly above the eruption age, but consistent with previously determined isotopic ages. A few higher apparent ages hint at the presence an older generation of Orapa diamonds that formed >2500 Ma ago. Orapa garnets also contain measurable K contents, and record a range of ages between 1000 and 2500 Ma. The old apparent ages and lack of significant interface ⁴⁰Ar released by the laser probe, suggests that pre-eruption radiogenic ⁴⁰Ar and mantle-derived ⁴⁰Ar components are trapped in microinclusions within the pyroxene and garnet inclusions.