Search

Major- and trace-element and Sr^Nd^Hf isotopic compositions of garnet and clinopyroxene in kimberlite-borne eclogite and pyroxenite xenoliths were used to establish their origins and evolution in the subcontinental lithospheric mantle beneath the central Slave Craton, Canada. The majority of eclogites can be assigned to three groups (high-Mg, high-Ca or low-Mg eclogites) that have distinct trace-element patterns. Although post-formation metasomatism involving high field strength element (HFSE) and light rare earth element (LREE) addition has partially obscured the primary compositional features of the high-Mg and high-Ca eclogites, trace-element features, such as unfractionated middle REE (MREE) to heavy REE (HREE) patterns suggestive of garnet-free residues and low Zr/Sm consistent with plagioclase accumulation, could indicate a subduction origin from a broadly gabbroic protolith. In this scenario, the low P REE and small positive Eu anomalies of the high-Mg eclogites suggest more primitive, plagioclase-rich protoliths, whereas the high-Ca eclogites are proposed to have more evolved protoliths with higher (normative) clinopyroxene/plagioclase ratios plus trapped melt, consistent with their lower Mg-numbers, higher P REE and absence of Eu anomalies. In contrast, the subchondritic Zr/Hf and positive slope in the HREE of the low-Mg eclogites are similar to Archaean second-stage melts and point to a previously depleted source for their precursors. Low ratios of fluid-mobile to less fluid-mobile elements and of LREE to HREE are consistent with dehydration and partial melt loss for some eclogites. The trace-element characteristics of the different eclogite types translate into lower eNd for high-Mg eclogites than for low-Mg eclogites. Within the low-Mg group, samples that show evidence for metasomatic enrichment in LREE and HFSE have lower eNd and eHf than a sample that was apparently not enriched, pointing to long-term evolution at their respective parent^daughter ratios. Garnet and clinopyroxene in pyroxenites show different major-element relationships from those in eclogites, such as an opposite CaO^Na2O trend and the presence of a CaO^Cr2O3 trend, independent of whether or not opx is part of the assemblage. Therefore, these two rock types are probably not related by fractionation processes. The presence of opx in about half of the samples precludes direct crystallization from eclogite-derived melts. They probably formed from hybridized melts that reacted with the peridotitic mantle.

Major-element compositions of minerals in peridotite xenoliths from the Lac de Gras kimberlites provide constraints on the mode of lithosphere formation beneath the central Slave Craton, Canada. Magnesia contents of reconstructed whole rocks correlate positively with NiO and negatively with CaO contents, consistent with variable partial melt extraction. Alumina and Cr2O3 contents are broadly positively correlated, suggestive of melt depletion in the absence of a Cr–Al phase. Garnet modes are high at a given Al2O3 content (a proxy for melt depletion), falling about a 7 GPa melt depletion model. These observations, combined with high olivine Mg# and major-element relationships of FeO-poor peridotites ( 3 GPa (residual FeO content being a sensitive indicator of melt extraction pressure), and similar high pressures of last equilibration (∼4.2 to 5.8 GPa), provide multiple lines of evidence that the mantle beneath the central Slave Craton has originated as a residue from high-pressure melting, possibly during plume subcretion. Apparent low melt depletion pressures for high-FeO peridotites (>7.5 wt%) could suggest formation in an oceanic setting, followed by subduction to their depth of entrainment. However, these rocks, which are characterised by low SiO2 contents (

The Slave Province is a small Archaean fragment (600 × 400 km), bounded by Proterozoic mobile belts, in Arctic Canada. It hosts major diamond deposits in Paleozoic to Miocene kimberlites, which now are being mined. D etailed geochemical studies of mantle-derived xenoliths and xenocrysts have defined an unusual two-layered lithospheric mantle beneath the craton: a shallow, ultradepleted (low in basaltic components Ca, Al, Fe), olivine-rich layer interpreted commonly as oceanic or arc-related lithosphere emplaced during early tectonics, and a deeper, less depleted layer, interpreted as a frozen Archaean plume head. We have mapped variations in the relationship between gravity and topography across the Slave Province in terms of the effective elastic thickness (Te). Our results show that the northern part of the craton is characterised by a relatively weak lithosphere (Te 56 km). A N-S zone of low Te along the middle of the craton coincides approximately with the surface expression of the suture between the ancient continental block making up the western part of the craton, and the younger accreted terranes that make up the eastern part. The zone of maximum Te gradient coincides with an area of strongly conductive upper mantle, and with the Nd-isotope line which defines a major crustal boundary at depth. The Te gradient probably marks the deep expression of the major steep suture, and this lithosphere-scale structure has apparently guided kimberlite intrusion over approximately 400 Ma.

The 613±6 Ma Anuri kimberlite is a pipelike body comprising two lobes with a combined surface area of approximately 4–5 ha. The pipe is infilled with two contrasting rock types: volcaniclastic kimberlite (VK) and, less common, hypabyssal kimberlite (HK). The HK is an archetypal kimberlite composed of macrocrysts of olivine, spinel, mica, rare eclogitic garnet and clinopyroxene with microphenocrysts of olivine and groundmass spinel, phlogopite, apatite and perovskite in a serpentine–calcite–phlogopite matrix. The Ba enrichment of phlogopite, the compositional trends of both primary spinel and phlogopite, as well as the composition of the mantle-derived xenocrysts, are also characteristic of kimberlite. The present-day country rocks are granitoids; however, the incorporation of sedimentary xenoliths in the HK shows that the Archean granitoid basement terrain, at least locally, was capped by younger Proterozoic sediments at the time of emplacement. The sediments have since been removed by erosion. HK is confined to the deeper eastern parts of the Anuri pipe. It is suggested that the HK was emplaced prior to the dominant VK as a separate phase of kimberlite. The HK must have ascended to high stratigraphic levels to allow incorporation of Proterozoic sediments as xenoliths. Most of the Anuri kimberlite is infilled with VK which is composed of variable proportions of juvenile lapilli, discrete olivine macrocrysts, country rock xenoliths and mantle-derived xenocrysts. It is proposed that the explosive breakthrough of a second batch of kimberlite magma formed the western lobe resulting in the excavation of the main pipe. Much of the resulting fragmented country rock material was deposited in extra crater deposits. Pyroclastic eruption(s) of kimberlite must have occurred to form the common juvenile lapilli present in the VKs. The VK is variable in nature and can be subdivided into four types: volcaniclastic kimberlite breccia, magmaclast-rich volcaniclastic kimberlite breccia, finer grained volcaniclastic kimberlite breccia and lithic-rich volcaniclastic kimberlite breccia. The variations between these subtypes reflect different depositional processes. These processes are difficult to determine but could include primary pyroclastic deposition and/or resedimentation. There is some similarity between Anuri and the Lac de Gras kimberlites, with variable types of VK forming the dominant infill of small, steep-sided pipes excavated into crystalline Archean basement and sedimentary cover.

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 TNi=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–

Melt inclusions in clinopyroxenes from lherzolitic xenoliths from the deep lithospheric mantle beneath the Slave Craton (Lac de Gras area, Canada) reveal multiple origins for carbonatitic melts. One type of inclusions consists of a series of silicate–carbonate–silicate concentric layers, interpreted to have unmixed under disequilibrium conditions during rapid ascent to the surface. Bulk major- and trace-element compositions are typical of Group 1 kimberlites and quantitative nuclear microprobe imaging of the globules reveals fractionation of related elements (e.g. F–Br, Nb–Ta) between the silicate and carbonate components. The globules probably formed by partial melting of carbonated peridotite, consistent with results of melting experiments and some models for the generation of kimberlite magmas. They provide evidence for a genetic relationship between some carbonate-rich magmas and ultramafic silicate magmas, and for the possibility of unmixing processes of these melts during their evolution. The second inclusion type comprises carbonate-rich globules interpreted as samples of Mg-carbonatite melt that quenched on ascent to the surface. Bulk major- and trace-element compositions indicate that the melts were derived from a carbonate-rich source and oxygen, carbon, and strontium isotope data are consistent with the involvement of recycled crustal material and suggest that some mantle-derived carbonatites are unrelated to kimberlites.

Major elements, trace elements including highly siderophile elements (HSE) and Re–Os isotope ratios were analysed in situ on individual sulfide grains and one Fe metal grain enclosed in fresh xenocrystic olivine and pyroxenes from kimberlites in the Lac de Gras area, central Slave Craton. Reconstruction of low-temperature assemblages to high-temperature precursor sulfides shows that there are two major sulfide populations: nickeliferous monosulfide solid solution (mss), and a distinct group of S-deficient Ni- and Co-rich sulfides ((Ni,Co,Fe)3−xS2). The inclusion suite is unusual because all (Ni,Co,Fe)3−xS2, a subgroup of mss, and Fe metal inclusion have high W concentrations (up to 3400, 2600 and 24,000 ppm, respectively) and because (Ni,Co,Fe)3−xS2 have higher Ni and Co concentrations (mean 47 and 6.8 wt.%, respectively) combined with higher metal/sulfur (mean Me/S=1.1) than the great majority of mantle-derived sulfides reported in the literature. Ca-in-olivine temperatures indicate that the host olivine and included sulfides are derived from the deep layer of the stratified lithosphere beneath Lac de Gras. Re–Os isotope data were collected for 24 mss inclusions. 187Re/188Os ranges from 0.006 to 2.71, and 187Os/188Os from 0.1002 to 0.4732, corresponding to γOs from −21 to 272. The sulfides show a major mode of TRD at ∼2.8 Ga, extending to 3.9 Ga, and of TCHUR at ∼2.9 Ga. Eleven samples define an isochron at t=3.27±0.34 Ga, with an enriched initial 187Os/188Os at t of 0.10725±0.00014 (γOs,i=2.53±0.13; MSWD=0.75). Weak covariations of Os contents and 187Re/188Os in mss with Mg/(Mg+Fe) of host olivine are consistent with variable degrees of melt depletion of sulfide-bearing peridotite. The incoherent behaviour of incompatible element abundances, the unrelated enrichment in the fluid-mobile elements W, As and Zn, and the unsupported radiogenic Os and recent Re-enrichment in some sulfides suggest repeated metasomatic processes subsequent to melting. The formation of the unusual (Ni,Co,Fe)3−xS2 could be related to metasomatism or alteration of mantle sulfide. However, the major element compositions of mss and (Ni,Co,Fe)3−xS2 would also be consistent with an igneous origin, by fractionation from extremely Ni-rich sulfide melts that unmix from S-bearing metal melts expected in the lower mantle. Despite the evidence for interaction with fluids, some sulfides were able to preserve ancient melt-depletion signatures. Model ages and the isochron age in the deep lithosphere beneath Lac de Gras exceed the age of the overlying crust of the Contwoyto terrane. This “age paradox” may be reconciled if the lithospheric mantle beneath the adjacent older Central Slave Basement Complex (CSBC) was thrust beneath the younger Contwoyto terrane during collision of these two entities, resulting in a translithospheric northeast-dipping suture.

The composition of the subcontinental lithospheric mantle (SCLM) varies in a systematic way with the age of the last major tectonothermal event in the overlying crust. This secular evolution in SCLM composition implies quasi-contemporaneous formation (or modification) of the crust and its underlying mantle root, and indicates that crust and mantle in many cases have remained linked through their subsequent history. Archean SCLM is distinctively different from younger mantle; it is highly depleted, commonly is strongly stratified, and contains rock types (especially subcalcic harzburgites) that are essentially absent in younger SCLM. Some, but not all, Archean SCLM also has higher Si/Mg than younger SCLM. Attempts to explain the formation of Archean SCLM by reference to Uniformitarian processes, such as the subduction of oceanic mantle (“lithospheric stacking”), founder on the marked differences in geochemical trends between Archean xenolith suites and Phanerozoic examples of highly depleted mantle, such as abyssal peridotites, island-arc xenolith suites and ophiolites. In Archean xenolith suites, positive correlations between Fe, Cr and Al imply that no Cr–Al phase (i.e. spinel or garnet) was present on the liquidus during the melting. This situation is in direct contrast to the geochemical patterns observed in highly depleted peridotites from modern environments, which are controlled by the presence of spinel during melting. It is more likely that Archean SCLM represents residues and/or cumulates from high-degree melting at significant depths, related to specifically Archean processes involving major mantle overturns or megaplumes. The preservation of island-arc like SCLM at shallow levels in some sections (e.g. Slave Craton, E. Greenland) suggests that this specifically Archean tectonic regime may have coexisted with a shallow regime more similar to modern plate tectonics. Preliminary data from in situ Re–Os dating of sulfide minerals in mantle-derived peridotites suggest that much Archean SCLM may have formed in a small number of such major events >3.0 Ga ago. The survival of Archean crust may have been critically determined by the availability of large plugs of very buoyant SCLM (a “life-raft model” of craton formation). Many Archean SCLM sections have been strongly affected by Proterozoic and Phanerozoic metasomatism, and much of the observed secular evolution in SCLM composition, at least through Proterozoic time, may reflect the progressive modification of relict, buoyant Archean lithosphere.

from some highly depleted ophiolites from convergent-margin settings, Heavy-mineral concentrates (garnets, chromites) and xenoliths from and may have formed in a similar situation during the accretion of 21 Cretaceous–Tertiary kimberlite intrusions have been used to map the Hackett and Contwoyto terranes (magmatic arc and accretionary the lithospheric mantle beneath the Lac de Gras area in the central prism, respectively) to the ancient continental Anton terrane at part of the Slave Province. Analyses of Nickel Temperature ( TNi) 2·6–2·7 Ga. The deeper layer is interpreted as a plume head, and Zinc Temperature ( TZn) have been used to place garnet and which rose from the lower mantle and underplated the existing chromite xenocrysts, respectively, in depth context. Paleogeotherms lithosphere at 2·6 Ga; evidence includes a high proportion of the derived from both xenoliths and concentrates lie near a 35 mW/ superdeep inclusion assemblage (ferropericlase–perovskite) in the m conductive model at T Ζ 900°C, and near a 38 mW/m diamond population. This event could have provided heat for model at higher T, implying a marked change in conductivity and/ generation of the widespread 2·6 Ga post-tectonic granites. Proteroor a thermal transient. Plots of garnet composition vs TNi also zoic subduction from east and west may have modified the cratonic show a sharp discontinuity in mantle composition at 900°C. root, mainly by introduction of eclogites near its base. Garnets from

[1] We have mapped the deep structure of the Slave craton by combining analysis the effective elastic thickness (Te) with data on mantle samples from numerous kimberlites. Three-dimensional mapping of the subcontinental lithospheric mantle (SCLM), using mantle-derived xenoliths and xenocrysts in kimberlites, has shown that much of the craton is underlain by a strongly layered SCLM; a highly depleted upper layer (low in basaltic components Ca, Al, Fe) is separated from a relatively fertile lower layer by a sharp boundary. This boundary lies at 140–150 km depth in the Lac de Gras area and shallows to ≤100 km in the northern and southern parts of the Craton. Weak lithosphere (Te 56 km), in the younger eastern part of the craton, is separated from the older western part by a zone of steep Te gradient parallel to the major locus of kimberlite intrusion, which may map the deep extension of the boundary between the two domains. Another strong Te gradient across the Kilohigok Basin accompanies a marked compositional change in the upper layer of the lithospheric mantle; the Basin probably marks a major translithospheric fault. Correlations between Te and mantle composition suggest that Te is strongly influenced by the rheology of the upper mantle.

The Slave Craton in Canada, is a small Archaean fragment, bounded on the east by the Thelon magmatic arc (2.0-1.9 Ga) on the western edge of the Archaean Rae Province and on the west by the Great Bear magmatic arc of the Wopmay Orogen (1.88-1.84 Ga). The northern and northeastern part is overlapped by Late Proterozoic and Phanerozoic supracrustal rocks. On the south, the Slave Craton is separated by the Great Slave Lake Shear Zone from the Early Proterozoic Chinchaga and Buffalo Head terranes. We use robust geochemical methods based on mantle-derived xenoliths, heavy mineral concentrates from over 25 kimberlites, and representative diamond populations and their inclusions to construct sections that delineate the composition, structure and thermal state of the lithospheric mantle across the Slave Craton. This analysis reveals a distinct two-layered lithosphere beneath the craton: a shallow ultradepleted, olivine-rich layer and a deeper less depleted layer, interpreted as an Archaean plume head. We have mapped variations in the gravity/topography relationships across the Slave Province in terms of the effective elastic thickness (Te). The results show that the northern part of the craton is characterised by a relatively weak lithosphere (Te 56 km). A N-S zone of low Te along the middle of the craton may map the deep extension of the suture between the ancient continental block making up the western part of the craton, and the younger accreted terranes that make up the eastern part. The zone of low Te gradient coincides with an area of strongly conductive upper mantle, and with the Nd/Pb isotope lines which define a major crustal boundary at depth, and is a major locus of kimberlite intrusion.

Quenched carbonate-silicate inclusions in lherzolitic clinopyroxene macrocrysts, derived from 200 km beneath the Slave craton in northern Canada, are interpreted as natural samples of mantle carbonatites. Oxygen, carbon, and strontium isotope data provide evidence for the involvement of subducted crustal material in the origin of these carbonatites, supporting suggestions that carbon recycling by subduction is an important prerequisite for carbonatite magmatism. The compositional range of the inclusions suggests that the parent melt was decreasing in silica content as it was trapped in the host crystal, a trend that is predicted experimentally. Isotopic disequilibrium between the carbonatitic inclusions and the host clinopyroxene indicates that they were trapped shortly before kimberlite eruption, suggesting a temporal link between the entrapment of the carbonatite in the host and the Paleocene eruption of the kimberlite.