Mantle Structure Beneath the SW Slave Craton, Canada: Constraints from Garnet Geochemistry in the Drybones Bay Kimberlite

This study describes the petrography of peridotite xenoliths,and the major and trace element geochemistry of garnets in bothxenoliths and coarse concentrate from the Drybones Bay kimberlite.The temperature and depth of equilibration of clinopyroxeneand garnet show that the mantle lithosphere beneath the SW marginof the Slave Province was at least 160 km thick at the timeof kimberlite emplacement (     

Major element composition of concentrate garnets in Proterozoic kimberlites from the Eastern Dharwar Craton,India: Implications on sub-continental lithospheric mantle

This study examines the major element composition of mantle-derived garnets recovered from heavy mineral concentrates of several Proterozoic kimberlites of the diamondiferous Wajrakarur Kimberlite Field (WKF) and the almost barren Narayanpet Kimberlite Field (NKF) in the Eastern Dharwar Craton of southern India. Concentrate garnets are abundant in the WKF kimberlites, and notably rare in the NKF kimberlites. Chemical characteristics of the pyropes indicate that the lithology of the sub-continental lithospheric mantle (SCLM) beneath both the kimberlite fields was mainly lherzolitic at the time of kimberlite eruption. A subset of green pyropes from the WKF is marked by high CaO and Cr₂O₃ contents, which imply contribution from a wehrlitic source. The lithological information on SCLM, when studied alongside geobarometry of lherzolite and harzburgite xenoliths, indicates that there are thin layers of harzburgite within a dominantly lherzolitic mantle in the depth interval of 115–190 km beneath the WKF. In addition, wehrlite and olivine clinopyroxenite occur locally in the depth range of 120–130 km. Mantle geotherm derived from xenoliths constrains the depth of graphite–diamond transition to 155 km beneath the kimberlite fields. Diamond in the WKF thus could have been derived from both lherzolitic and harzburgitic lithologies below this depth. The rarity of diamond and garnet xenocrysts in the NKF strongly suggest sampling of shallower (<155 km depth) mantle, and possibly a shallower source of kimberlite magma than at the WKF.     

Cr-rich megacrysts of clinopyroxene and garnet from Lac de Gras kimberlites,Slave Craton,Canada – implications for the origin of clinopyroxene and garnet in cratonic lherzolites

Kimberlites from the Diavik and Ekati diamond mines in the Lac de Gras kimberlite field contain abundant large (>1 cm) clinopyroxene (Cr-diopside) and garnet (Cr-pyrope) crystals. We present the first extensive mineral chemical dataset for these megacrysts from Diavik and Ekati and compare their compositions to cratonic peridotites and megacrysts from the Slave and other cratons. The Diavik and Ekati Cr-diopside and Cr-pyrope megacrysts are interpreted to belong to the Cr-rich megacryst suite. Evidence for textural, compositional, and isotopic disequilibrium suggests that they constitute xenocrysts in their host kimberlites. Nevertheless, their formation may be linked to extensive kimberlite magmatism and accompanying mantle metasomatism preceding the eruption of their host kimberlites. It is proposed that the formation of megacrysts may be linked to failed kimberlites. In this scheme, the Cr-rich megacrysts are formed by progressive interaction of percolating melts with the surrounding depleted mantle (originally harzburgite). As these melts percolate outwards, they may contribute to the introduction of clinopyroxene and garnet into the depleted mantle, thereby forming lherzolite. This model hinges on the observation that lherzolitic clinopyroxenes and garnets at Lac de Gras have compositions that are strikingly similar to those of the Cr-rich megacrysts, in terms of major and trace elements, as well as Sr isotopes. As such, the Cr-rich megacrysts may have implications for the origin of clinopyroxene and garnet in cratonic lherzolites worldwide.

     

Ultra-high pressure (P > 6 GPa) garnet peridotites in Western Norway: exhumation of mantle rocks from > 185 km depth

We report here for the first time the occurrence of relics of majoritic garnet within orogenic garnet peridotites from Otrøy, Western Gneiss Region, Norway. The microstructural evidence consists of two-pyroxene exsolution from garnet. Majoritic garnets are only stable at depths greater than 150 km. Estimates of the initial composition of the majoritic garnets imply pressures of 6–6.5 GPa indicating that the Otrøy peridotites were derived from depths > 185 km.
  Mineral-chemical data indicate that the present mineral compositions equilibrated at mantle conditions around 805 ± 40 °C and 3.2 ± 0.2 GPa.
  Estimates of the initial pressure temperature (PT) conditions and PTtime ( t ) path are consistent with a multistage, multiorogenic exhumation history with upwelling of hot asthenosphere up to ≈ 100 km in the Pre-Cambrian followed by subsequent crustal emplacement and exhumation during the Caledonian orogeny.     

The discovery of garnets closely related to diamonds in the Finsch pipe,South Africa

Magnesium-rich, calcium-poor, lilac coloured garnets have been found in the heavy mineral concentrate of the Finsch kimberlite pipe. Some of these garnets contain sufficient chromium to place them within the compositional field of the garnets previously only reported as inclusions in diamonds.These lilac garnets are considered to have formed in equilibrium with the minerals found as inclusions in diamond and hence with the diamond itself. Their presence in the kimberlite should be diagnostic of the presence of diamond, but it is not known if there is any quantitative relationship. The garnets are considered to have a deeper provenance than the magnesian garnets commonly found as xenocrysts in kimberlite and in garnet peridotite xenoliths. The mantle composition at their depths of origin must be more refractory in nature than the peridotite xenoliths. The garnets having a higher magnesium and chromium content, a higher Mg/Fe ratio and lower calcium, aluminium and titanium than those found in the xenoliths.     

Phlogopite in mantle xenoliths and kimberlite from the Grib pipe,Arkhangelsk province,Russia:Evidence for multi-stage mantle metasomatism and origin of phlogopite in kimberlite

We present petrography and mineral chemistry for both phlogopite,from mantle-derived xenoliths(garnet peridotite,eclogite and clinopyroxene-phlogopite rocks)and for megacryst,macrocryst and groundmass flakes from the Grib kimberlite in the Arkhangelsk diamond province of Russia to provide new insights into multi-stage metasomatism in the subcratonic lithospheric mantle(SCLM)and the origin of phlogopite in kimberlite.Based on the analysed xenoliths,phlogopite is characterized by several generations.The first generation(Phil)occurs as coarse,discrete grains within garnet peridotite and eclogite xenoliths and as a rock-forming mineral within clinopyroxene-phlogopite xenoliths.The second phlogopite generation(Phl2)occurs as rims and outer zones that surround the Phil grains and as fine flakes within kimberlite-related veinlets filled with carbonate,serpentine,chlorite and spinel.In garnet peridotite xenoliths,phlogopite occurs as overgrowths surrounding garnet porphyroblasts,within which phlogopite is associated with Cr-spinel and minor carbonate.In eclogite xenoliths,phlogopite occasionally associates with carbonate bearing veinlet networks.Phlogopite,from the kimberlite,occurs as megacrysts,macrocrysts,microcrysts and fine flakes in the groundmass and matrix of kimberlitic pyroclasts.Most phlogopite grains within the kimberlite are characterised by signs of deformation and form partly fragmented grains,which indicates that they are the disintegrated fragments of previously larger grains.Phil,within the garnet peridotite and clinopyroxene-phlogopite xenoliths,is characterised by low Ti and Cr contents(TiO_21 wt.%,Cr_2 O_31 wt.% and Mg# = 100 × Mg/(Mg+ Fe)92)typical of primary peridotite phlogopite in mantle peridotite xenoliths from global kimberlite occurrences.They formed during SCLM metasomatism that led to a transformation from garnet peridotite to clinopyroxene-phlogopite rocks and the crystallisation of phlogopite and high-Cr clinopyroxene megacrysts before the generation of host-kimberlite magmas.One of the possible processes to generate low-Ti-Cr phlogopite is via the replacement of garnet during its interaction with a metasomatic agent enriched in K and H_2O.Rb-Sr isotopic data indicates that the metasomatic agent had a contribution of more radiogenic source than the host-kimberlite magma.Compared with peridotite xenoliths,eclogite xenoliths feature low-Ti phlogopites that are depleted in Cr_2O_3 despite a wider range of TiO_2 concentrations.The presence of phlogopite in eclogite xenoliths indicates that metasomatic processes affected peridotite as well as eclogite within the SCLM beneath the Grib kimberlite.Phl2 has high Ti and Cr concentrations(TiO_22 wt.%,Cr_2O_31 wt.% and Mg# = 100× Mg/(Mg + Fe)92)and compositionally overlaps with phlogopite from polymict brecc:ia xenoliths that occur in global kimberlite formations.These phlogopites are the product of kimberlitic magma and mantle rock interaction at mantle depths where Phl2 overgrew Phil grains or crystallized directly from stalled batches of kimberlitic magmas.Megacrysts,most macrocrysts and microcrysts are disintegrated phlogopite fragments from metasomatised peridotite and eclogite xenoliths.Fine phlogopite flakes within kimberlite groundmass represent mixing of high-Ti-Cr phlogopite antecrysts and high-Ti and low-Cr kimberlitic phlogopite with high Al and Ba contents that may have formed individual grains or overgrown antecrysts.Based on the results of this study,we propose a schematic model of SCLM metasomatism involving phlogopite crystallization,megacryst formation,and genesis of kimberlite magmas as recorded by the Grib pipe.     

Metasomatic origin of garnet xenocrysts from the V. Grib kimberlite pipe,Arkhangelsk region,NW Russia

This paper presents new major and trace element data from 150 garnet xenocrysts from the V. Grib kimberlite pipe located in the central part of the Arkhangelsk diamondiferous province (ADP). Based on the concentrations of Cr₂O₃, CaO, TiO₂ and rare earth elements (REE) the garnets were divided into seven groups: (1) lherzolitic “depleted” garnets (“Lz 1”), (2) lherzolitic garnets with normal REE patterns (“Lz 2”), (3) lherzolitic garnets with weakly sinusoidal REE patterns (“Lz 3”), (4) lherzolitic garnets with strongly sinusoidal REE patterns (“Lz 4”), (5) harzburgitic garnets with sinusoidal REE patterns (“Hz”), (6) wehrlitic garnets with weakly sinusoidal REE patterns (“W”), (7) garnets of megacryst paragenesis with normal REE patterns (“Meg”). Detailed mineralogical and geochemical garnet studies and modeling results suggest several stages of mantle metasomatism influenced by carbonatite and silicate melts. Carbonatitic metasomatism at the first stage resulted in refertilization of the lithospheric mantle, which is evidenced by a nearly vertical CaO-Cr₂O₃ trend from harzburgitic (“Hz”) to lherzolitic (“Lz 4”) garnet composition. Harzburgitic garnets (“Hz”) have probably been formed by interactions between carbonatite melts and exsolved garnets in high-degree melt extraction residues. At the second stage of metasomatism, garnets with weakly sinusoidal REE patterns (“Lz 3”, “W”) were affected by a silicate melt possessing a REE composition similar to that of ADP alkaline mica-poor picrites. At the last stage, the garnets interacted with basaltic melts, which resulted in the decrease CaO-Cr₂O₃ trend of “Lz 2” garnet composition. Cr-poor garnets of megacryst paragenesis (“Meg”) could crystallize directly from the silicate melt which has a REE composition close to that of ADP alkaline mica-poor picrites. P-T estimates of the garnet xenocrysts indicate that the interval of ～60–110 km of the lithospheric mantle beneath the V. Grib pipe was predominantly affected by the silicate melts, whereas the lithospheric mantle deeper than 150 km was influenced by the carbonatite melts.     

Oxidation state of the lithospheric mantle beneath Diavik diamond mine, central Slave craton, NWT, Canada

Oxygen fugacity (fO₂) conditions were determined for 29 peridotite xenoliths from the A154-North and A154-South kimberlites of the Diavik diamond mine using the newly developed flank method modified specifically for measuring Fe³⁺ in mantle-derived pyropic garnets. The results indicate that the garnet-bearing lithospheric mantle beneath the central Slave craton is vertically layered with respect to oxidation state. The shallow (<140 km), “ultra-depleted” layer is the most oxidized section of garnet-bearing subcratonic mantle thus far measured, up to one log unit more oxidizing relative to the FMQ buffer [Δlog fO₂ (FMQ) + 1]. The lower, more fertile layer has fO₂ conditions that extend down to Δlog fO₂ (FMQ) − 3.8, consistent with xenolith suites from other localities worldwide. Based on trace element concentrations in garnets, two distinct metasomatic events affected the mantle lithosphere at Diavik. An oxidized fluid imparted sinusoidal chondrite-normalized REE patterns on garnets throughout the entire depth range sampled. In contrast, a reducing melt metasomatic event affected only the lower portion of the lithospheric mantle. The fO₂ state of the Diavik mantle sample suggests that diamond formation occurred by reduction of carbonate by fluids arising from beneath the lithosphere.     

Experimental studies At 5–12 GPa of the Ondermatjie hypabyssal kimberlite

Liquidus and sub-liquidus phase relationships are reported for melts formed from an aphanitic kimberlite composition crystallized at 5–12 GPa and 900–1400 °C. The liquidus phase over the pressure range investigated is forsteritic olivine. This is followed with decreasing temperature by olivine plus garnet as the initial sub-liquidus solid phase assemblage. Supra-solidus assemblages consist of olivine+garnet+clinopyroxene+Mg-ilmenite+liquid at 5–7 GPa or olivine+garnet+clinopyroxene+hematite–ilmenite solid solutions (+/−perovskite)+liquid at 8–12 GPa. Phlogopite forms as a near-solidus phase only at 900 °C and 6 GPa. Orthopyroxene does not form at any temperature and pressure. All garnets formed at 6–7 GPa are Ti-rich almandine–grossular–pyrope solid solutions and not Cr-pyrope, whereas garnets formed above 8 GPa are Ti- and Fe³⁺-rich and have no natural counterparts. Quenched liquids are represented by magnesite at 10–12 GPa and Mg–Ca-carbonates at lower pressures. In addition to forming discrete crystals, Mg-ilmenite and hematite–ilmenite solid solutions occur as lamellar intergrowths that are identical in texture to naturally occurring intergrowths. Mg-ilmenite compositions at 6–7 GPa are similar to those of the natural occurrences, whereas clinopyroxenes are richer in Ca. The effects of graphite versus platinum capsules on the oxygen fugacity of the experimental charges and the composition of the olivine, clinopyroxene, Fe–Ti-oxides and garnets formed are described. These experimental data are interpreted to indicate that kimberlite magmas are unlikely to be formed by very small degrees of partial melting of a simple homogeneous carbonated garnet lherzolite mantle. It is proposed that kimberlite magmas form by extensive partial melting of metasomatized mantle, i.e. mineralogically complex carbonate-bearing veins in a lherzolitic/harzburgitic substrate, and that lamellar ilmenite–clinopyroxene intergrowths represent the products of non-equilibrium growth in kimberlite magma.     

Decompression and unmixing of crystals included in diamonds from the mantle transition zone

A suite of exceptional mineral inclusions in diamonds from the São Luiz river, Juina province, Brazil, shows a wide range of garnet/majorite mineral compositions co-existing with clinopyroxene; the overall bulk compositions are eclogitic. The inclusions have a wide variety of textural arrangements, but crystallographic data obtained by EBSD shows that each inclusion consists of a single garnet with constant crystallographic orientation whilst clinopyroxene grains have preferred orientation with relation to garnet {110} and <111>. This suggests that the inclusions were originally single phase majoritic garnets, and that they preserve various states of progressive unmixing (exsolution) into lower pressure garnet and clinopyroxene compositions during transport of the host diamonds towards the Earth’s surface. On the basis of high pressure–temperature experimental data some of the original majoritic garnets must have come from depths of 450 km or more, and therefore resided in the transition zone and asthenospheric upper mantle. Particularly extensive re-equilibration of many inclusions took place at depths of ca 180–200 km (probably close to the base of the continental lithosphere). The partially unmixed state of the inclusions provides a unique opportunity for using mineral diffusion data to roughly estimate the rate of transport through the asthenospheric upper mantle, and within error this rate is found to be broadly compatible with expected transport rates by upper mantle convection or plume flow.     

The Siberian lithosphere traverse: mantle terranes and the assembly of the Siberian Craton

The kimberlite fields scattered across the NE part of the Siberian Craton have been used to map the subcontinental lithospheric mantle (SCLM), as it existed during Devonian to Late Jurassic time, along a 1000-km traverse NE–SW across the Archean Magan and Anabar provinces and into the Proterozoic Olenek Province. 4100 garnets and 260 chromites from 65 kimberlites have been analysed by electron probe (major elements) and proton microprobe (trace elements). These data, and radiometric ages on the kimberlites, have been used to estimate the position of the local (paleo)geotherm and the thickness of the lithosphere, and to map the detailed distribution of specific rock types and mantle processes in space and time. A low geotherm, corresponding approximately to the 35 mW/m² conductive model of Pollack and Chapman [Tectonophysics 38, 279–296, 1977], characterised the Devonian lithosphere beneath the Magan and Anabar crustal provinces. The Devonian geotherm beneath the northern part of the area was higher, rising to near a 40 mW/m² conductive model. Areas intruded by Mesozoic kimberlites are generally characterised by this higher, but still ‘cratonic' geotherm. Lithosphere thickness at the time of kimberlite intrusion varied from ca. 190 to ca. 240 km beneath the Archean Magan and Anabar provinces, but was less (150–180 km) beneath the Proterozoic Olenek Province already in Devonian time. Thinner Devonian lithosphere (140 km) in parts of this area may be related to Riphean rifting. Near the northern end of the traverse, differences in geotherm, lithosphere thickness and composition between the Devonian Toluopka area and the nearby Mesozoic kimberlite fields suggest thinning of the lithosphere by ca. 50–60 km, related to Devonian rifting and Triassic magmatism. A major conclusion of this study is that the crustal terrane boundaries defined by geological mapping and geophysical data (extended from outcrops in the Anabar Shield) represent major lithospheric sutures, which continue through the upper mantle and juxtapose lithospheric domains that differ significantly in composition and rock-type distribution between 100 and 250 km depth. The presence of significant proportions of harzburgitic and depleted lherzolitic garnets beneath the Magan and Anabar provinces is concordant with their Archean surface geology. The lack of harzburgitic garnets, and the chemistry of the lherzolitic garnets, beneath most of the other fields are consistent with the Proterozoic surface rocks. Mantle sections for different terranes within the Archean portion of the craton show pronounced differences in bulk composition, rock-type distribution, metasomatic overprint and lithospheric thickness. These observations suggest that individual crustal terranes, of both Archean and Proterozoic age, had developed their own lithospheric roots, and that these differences were preserved during the Proterozoic assembly of the craton. Data from kimberlite fields near the main Archean–Proterozoic suture (the Billyakh Shear Zone) suggest that reworking and mixing of Archean and Proterozoic mantle was limited to a zone less than 100 km wide.     

Statistical approaches to the discrimination of mantle- and crust-derived low-Cr garnets using major and trace element data

Diamond exploration focuses on geochemical analysis of indicator minerals that are more abundant than diamond itself. Among such indicators, low-Cr (Cr₂O₃ < 1 wt%) garnets from mantle eclogites are problematic since they overlap compositionally with many lower-crust-derived garnets also transported by kimberlite. Misclassification of these garnets may create “false positive” mantle signatures and possible misdirection of exploration efforts. Statistical solutions using major elements in low-Cr garnet (Hardman et al. in J Geochem Explor 186:24–35, 2018) provide improved error rates for the discrimination of low-Cr crustal and mantle garnets recovered from kimberlite. In this study we analysed a large suite of garnets (n = 571) from both crustal and mantle settings, already characterised for major elements, for a wide range of trace elements by laser ablation inductively-coupled plasma mass spectrometry and use these new data along with literature data (n = 169) to evaluate the effectiveness of adding trace elements to garnet-based diamond exploration programs. A new garnet classification scheme, initially using a major-element based filter, uses garnet Sr contents and Eu anomalies to help identify low-Cr garnets that are misclassified using major element methods. Combined with existing methods, our new trace element classifiers offer improvement in classification error rates for low-Cr, crustal and mantle garnets to as low as 4.7% for calibration data.

     

Diamondiferous peridotitic microxenoliths from the Diavik Diamond Mine, NT

Twenty-five diamonds recovered from 21 diamondiferous peridotitic micro-xenoliths from the A154 South and North kimberlite pipes at Diavik (Slave Craton) match the general peridotitic diamond production at this mine with respect to colour, carbon isotopic composition, and nitrogen concentrations and aggregation states. Based on garnet compositions, the majority of the diamondiferous microxenoliths is lherzolitic (G9) in paragenesis, in stark contrast to a predominantly harzburgitic (G10) inclusion paragenesis for the general diamond production. For garnet inclusions in diamonds from A154 South, the lherzolitic paragenesis, compared to the harzburgitic paragenesis, is distinctly lower in Cr content. For microxenolith garnets, however, Cr contents for garnets of both the parageneses are similar and match those of the harzburgitic inclusion garnets. Assuming that the microxenolith diamonds reflect a sample of the general diamond population, the abundant Cr-rich lherzolitic garnets formed via metasomatic overprinting of original harzburgitic diamond sources subsequent to diamond formation, conversion of original harzburgitic diamond sources occurred in the course of metasomatic overprint re-fertilization. Metasomatic overprinting after diamond formation is supported by the finding of a highly magnesian olivine inclusion (Fo₉₅) in a microxenolith diamond that clearly formed in a much more depleted environment than indicated by the composition of its microxenolith host. Chondrite normalized REE patterns of microxenolith garnets are predominantly sinusoidal, similar to observations for inclusion garnets. Sinusoidal REE_N patterns are interpreted to indicate a relatively mild metasomatic overprint through a highly fractionated (very high LREE/HREE) fluid. The predominance of such patterns may explain why the proposed metasomatic conversion of harzburgite to lherzolite appears to have had no destructive effect on diamond content. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

http://www.sciencedirect.com/science/article/pii/S1674987115000663

Mantle xenocrysts from early Triassic kimberlite pipes from Kharamai,Ary-Mastakh and Kuranakh fields in the Anabar shield of Siberia revealing similar compositional trends were studied to estimate the superplume influence on the subcratonic lithosphere mantle(SCLM).Pressure-temperature(PT) reconstructions using monomineral thermobarometry for 5 phases show division of the SCLM beneath the Kharamai field into 6 units:pyroxenitic Fe-rich(1-2 GPa) and Mg-rich(2-3 GPa) layers;middle with two levels of Gar-Sp pyroxenites at ~3 and 4-5 GPa;Gar-dunite-harzburgites ~4.5-6.5 GPa subjected to Ilm-Px vein metasomatism;and a Mg-rich dunite lower part.In the Anabar shield(Ary-Mastakh,Dyuken and Kuranakh fields) mantle lithosphere is composed of three large units divided into two parts:upper part with amphiboles and phlogopite;two levels of pyroxenites and eclogites at 3 and 4 GPa,and a lower part composed of refertilized dunites.Diagrams showing P-Fe~#Gar clusters for garnets and omphacites illustrate the differences between SCLM of these localities.Differences of Triassic SCLM from Devonian SCLM are in simple layering;abundance of Na-Cr-amphiboles and metasomatism in the upper SCLM part,thick pyroxenite-eclogite layer and lower part depletion,heated from SCLM base to 5.0 GPa.Kharamai mantle clinopyroxenes represent three geochemical types:(1) harzburgitic with inclined linear REE,HFSE troughs and elevated Th,U;(2) lherzolitic or pyroxenitic with round TRE patterns and decreasing incompatible elements;(3) eclogitic with Eu troughs,Pb peak and high LILE content.Calculated parental melts for garnets with humped REE patterns suggest dissolution of former Cpx and depression means Cpx and garnets extraction.Clinopyroxenes from Ary-Mastakh fields show less inclined REE patterns with HMREE troughs and an increase of incompatible elements.Clinopyroxenes from Kuranakh field show flatter spoon-like REE patterns and peaks in Ba,U,Pb and Sr,similar to those in ophiolitic harzburgites.The PT diagrams for the mantle sections show high temperature gradients in the uppermost SCLM accompanied by an increase of P-Fe~#Ol upward and slightly reduced thickness of the mantle keel of the Siberian craton,resulting from the influence of the Permian-Triassic superplume,but with no signs of delamination.     

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.     

Emplacement of Deep Upper-Mantle Rocks into Cratonic Lithosphere by Convection and Diapiric Upwelling

Rocks containing breakdown products of majoritic garnet, derivedfrom the deep upper mantle, occur in kimberlite xenoliths andin orogenic peridotites from Otrøy in Norway. The Otrøyperidotites are banded harzburgites and dunites with similarcompositions to mantle xenoliths from Precambrian cratons andPhanerozoic supra-subduction-zone peridotites. Pressure–temperature(P–T) paths deduced for the Otrøy peridotites andkimberlite xenoliths from South Africa are consistent with emplacementof deep mantle peridotites into cratonic lithosphere by asthenospherediapirism. Numerical thermo-convection models provide insightinto the possible P–T histories of deep upper-mantle rocks.In the models, material from the base of the convecting systemis transported to depths of 60–100 km by convection andsmall (50–100 km) diapirs. Diapir intrusion induces small-scaleconvection in the low-viscosity deeper part of the thermochemicallydefined lithosphere. Small-scale convection in the craton rootcan produce complex P–T paths, complex recurrent meltinghistories and complex compositional structure in the craton.P–T paths derived from the numerical models for asthenospherediapirism in a hot upper mantle are consistent with the sequenceof sub-solidus P–T conditions deduced for the cratonicperidotites. KEY WORDS: asthenosphere diapirs; cratonic lithosphere; deep upper mantle; majoritic garnet     

The Kharamai kimberlite field, Siberia: modification of the lithospheric mantle by the Siberian Trap event

The kimberlites of the Kharamai field intruded through the Siberian Traps shortly after their eruption in Permo-Triassic time. The composition and thermal state of the subcontinental lithospheric mantle (SCLM) beneath the Kharamai field in lower Triassic time have been reconstructed using major- and trace-element analyses of 345 Cr-pyrope garnet xenocrysts from six of the kimberlites, supplemented by a small suite of mantle-derived peridotite xenoliths. The data define a geotherm lying near a 38 mW/m² conductive model to a depth of ca 170 km, where the base of the depleted lithosphere is defined by a marked increase in melt-related metasomatism and by an inflected geotherm. Compared to the SCLM sampled by Devonian (pre-Trap) kimberlites in the same and adjacent terranes, the Kharamai SCLM in Triassic time was warmer and was cooling from a previous thermal high. It was also thinner than the SCLM beneath the Daldyn and Alakit kimberlite fields, and had been strongly metasomatised. The metasomatism lowered the mean Fo content of olivine (from ≥Fo₉₃ to Fo₉₂), greatly reduced the proportion of subcalcic harzburgites, and increased the proportion of fertile lherzolites, especially in the depth range of 80–130 km. The overall pattern of metasomatism is similar to that observed in the SCLM sampled by the Group I kimberlites of the SW Kaapvaal Craton, and inferred to be related to the Karoo thermal event. These observations suggest that events such as the eruption of the Karoo basalts and Siberian Traps change the composition of the SCLM, but do not necessarily destroy it, at distances of several hundred kilometres from the main eruption centres.     

Kimberlite AT-56: a mantle sample from the north central Superior craton, Canada

Kimberlite AT-56, discovered in February 2001, represents the most recent addition to the Attawapiskat kimberlite cluster, located in the James Bay Lowlands of Ontario, Canada. AT-56 is a small kimberlite body with a surface diameter of approximately 40 m and a steep southeastern plunge. It consists of a medium to coarse-grained matrix supported kimberlite with abundant olivine, clinopyroxene, garnet, ilmenite and mica macrocrysts in a green-black to orange-black matrix. The kimberlite is classified as a hypabyssal facies sparsely macrocrystic calcite kimberlite. Heavy mineral concentrates from two representative samples of AT-56 have been analyzed to characterize the mantle sampled by the kimberlite. Both samples yielded large heavy mineral concentrates comprised of roughly equal proportions of Mg-ilmenite, Cr-diopside, high-Cr garnet and low-Cr garnet. Mg-chromite is also present in quantities an order of magnitude less than the other constituents.

The high-Cr peridotitic garnet macrocrysts are only slightly more abundant than the low-Cr varieties, the population being dominated by G9 (lherzolitic) types with only a few (less than 10%) weakly sub-calcic G10 (probable harzburgitic) garnets present. Ni thermometry results for a representative selection of G9 and G10 garnets indicate that the majority equilibrated at temperatures ranging from 1000 to 1250 °C. A significant proportion of the low-Cr garnet population derived from AT-56 is characterized by relatively low-Ti (0.2 to 0.4 wt.% TiO₂) and elevated Na (0.07 to 0.13 wt.% Na₂O) contents characteristic of Group 1, diamond inclusion type eclogite garnets. These sodic garnets have elevated Cr₂O₃ contents (typically 1 to 2 wt.% Cr₂O₃), suggesting they may be websteritic in origin rather than eclogitic. Comparison of AT-56 garnet compositions with published data available for other Attawapiskat kimberlites suggests websteritic mantle has also been sampled by kimberlite bodies elsewhere in the Attawapiskat cluster and it may be an important diamond reservoir in this area.     

Mantle heterogeneity,plume-lithosphere interaction at rift controlled ocean-continent transition zone: Evidence from trace-PGE geochemistry of Vempalle flows,Cuddapah Basin,India

This study reports major, trace, rare earth and platinum group element compositions of lava flows from the Vempalle Formation of Cuddapah Basin through an integrated petrological and geochemical approach to address mantle conditions, magma generation processes and tectonic regimes involved in their formation. Six flows have been identified on the basis of morphological features and systematic three-tier arrangement of vesicular-entablature-colonnade zones. Petrographically, the studied flows are porphyritic basalts with plagioclase and clinopyroxene representing dominant phenocrystal phases.Major and trace element characteristics reflect moderate magmatic differentiation and fractional crystallization of tholeiitic magmas. Chondrite-normalized REE patterns corroborate pronounced LREE/HREE fractionation with LREE enrichment over MREE and HREE. Primitive mantle normalized trace element abundances are marked by LILE-LREE enrichment with relative HFSE depletion collectively conforming to intraplate magmatism with contributions from sub-continental lithospheric mantle(SCLM) and extensive melt-crust interaction. PGE compositions of Vempalle lavas attest to early sulphur-saturated nature of magmas with pronounced sulphide fractionation, while PPGE enrichment over IPGE and higher Pd/Ir ratios accord to the role of a metasomatized lithospheric mantle in the genesis of the lava flows. HFSEREE-PGE systematics invoke heterogeneous mantle sources comprising depleted asthenospheric MORB type components combined with plume type melts. HFSE-REE variations account for polybaric melting at variable depths ranging from garnet to spinel lherzolite compositional domains of mantle. Intraplate tectonic setting for the Vempalle flows with P-MORB affinity is further substantiated by(i) their origin from a rising mantle plume trapping depleted asthenospheric MORB mantle during ascent,(ii) interaction between plume-derived melts and SCLM,(iii) their rift-controlled intrabasinal emplacement through Archeane Proterozoic cratonic blocks in a subduction-unrelated ocean-continent transition zone(OCTZ). The present study is significant in light of the evolution of Cuddapah basin in the global tectonic framework in terms of its association with Antarctica, plume incubation, lithospheric melting and thinning, asthenospheric infiltration collectively affecting the rifted margin of eastern Dharwar Craton and serving as precursors to supercontinent disintegration.