Search

The origin and evolution of diamondiferous lithospheric mantle sampled by the Neoproterozoic Renard kimberlites (eastern Superior Province, Quebec) is constrained based on mantle-derived microxenoliths and xenocrysts. The dataset illustrates the wealth of knowledge that can be gleaned from small samples (1·2 mg–2·2 g) through an integration of multiple, mainly single mineral based approaches. Our samples document the presence of an ∼200 km thick lithosphere with a ‘cold’ (38 mW m−2 surface heat flow) model geotherm at the time of kimberlite emplacement (c. 632 Ma), resulting in a large diamond window from 130 to 200 km (42–60 kbar). On the basis of the mantle xenolith and xenocryst record and excluding megacrysts, the lithospheric mantle beneath Renard was dominated by peridotite (91%), composed of lherzolite (72% of samples), harzburgite (24%) and wehrlite (5%), with minor eclogitic (3%) and websteritic (6%) portions. Comparatively abundant harzburgite probably establishes the principal diamond source, but elevated Na contents in eclogitic garnet suggest the additional presence of diamond-stable eclogites. A number of events have modified the lithospheric mantle underlying the eastern Superior Province, including the following: (1) evolving ‘kimberlitic’ melts pervasively re-fertilized the originally strongly depleted lithospheric mantle with respect to highly and moderately incompatible trace elements; (2) less pervasive fluid style metasomatism is indicated by selective re-enrichment of highly incompatible elements that occurred within a depth range of 125–170 km. In situ Pb isotope data obtained for clinopyroxenes suggest a model age of ∼2·7 Ga for the protolith(s) of the cratonic lithospheric mantle beneath Renard. This age coincides with a major phase of continental crust generation within the Superior Province and throughout the Laurentia supercontinent (e.g. Greenland). [ABSTRACT FROM AUTHOR]

Abstract: The diamondiferous Carolina kimberlite (Rondônia State, Brazil) is located within Proterozoic basement rocks (1.8 to 1.2 Ga) of the Amazon Craton. This “unconventional” post-Archean setting is consistent with a lack of harzburgitic (G10) garnets in heavy media concentrate from the kimberlite. Diamonds from Carolina have high nitrogen contents and in part highly negative carbon isotopic values suggesting derivation predominantly from eclogitic portions of the underlying lithospheric mantle. This is consistent with the abundance and chemistry of eclogitic garnet xenocrysts, which make up 13% of the garnets analysed: just over half of the eclogitic garnets classify as Group I (>0.07 wt.% Na2O), which is considered to be an indication of good diamond potential. Based on nitrogen contents and aggregation states, the majority of the Carolina diamonds indicate time averaged residence temperatures between 1100 and 1150 °C (at 1.5 Ga mantle residence). Platelet degradation was noted in the majority of diamonds, suggesting that their mantle source was affected by a transient heating event. Geothermobarometry on clinopyroxene grains derived from both surficial samples and kimberlite core indicates two distinct model geotherms: a hot “Somerset Island type” geotherm (44 mW/m2), and a colder “Slave type” geotherm (38 mW/m2). Grains from the kimberlite drill core exclusively reflect the lower model geotherm, whereas clinopyroxenes from surficial samples depict both gradients. Given the Triassic age (230 Ma, Rb–Sr model age on phlogopite) of the Carolina kimberlite, it is speculated that a younger generation of Cretaceous–Tertiary kimberlites in the Pimenta Bueno area may represent the source of “hotter” mantle xenocrysts seen in surficial samples. The implied change in geotherm reflects a large scale, possibly plume related, heating episode occurring between the two kimberlite events (i.e. between the Jurassic and Cretaceous) that may relate to the opening of the South Atlantic. [Copyright &y& Elsevier]

Abstract: The carbon isotopic composition, nitrogen content and aggregation state of 95 inclusion bearing diamonds, collected from the production of Damtshaa kimberlites, were investigated. The Damtshaa kimberlites belong to the group of southern African kimberlites showing 12C enrichment in eclogitic compared to peridotitic diamonds. The results were compared with similar studies of the nearby Orapa and Letlhakane kimberlites, providing the opportunity to examine variations in mantle composition on the kilometer scale. While Orapa, Letlhakane and Damtshaa diamonds may share some overall similarities in their δ13C distribution, there are significant differences among them. These differences support the concept that diamonds from different kimberlite pipes have distinctive features in their δ13C distribution and that this is true for kimberlite occurrences which are spatially separated by only a few tens of km. Compared to Orapa, peridotitic garnet inclusions from Damtshaa and Letlhakane are derived, in part, from highly depleted dunitic–harzburgitic reservoirs extending to greater depth. In all three kimberlites diamonds with eclogitic garnet inclusions displaying comparatively low Al2O3/Cr2O3 ratios, characteristic for mantle compositions, show a wide range in carbon isotopic composition from about −4 to −18 ‰ vs. PDB. The observed relationship between δ13C and Al2O3/Cr2O3 ratios indicates the presence of 13C depleted mantle sources that cannot be explained by subduction. Estimates of possible pressure/temperature conditions, based on the Fe/Mg distribution coefficients of coexisting eclogitic garnets and clinopyroxenes and the Ca mole fraction in garnets, lead to the conclusion that beneath the Orapa, Letlhakane, and Damtshaa kimberlites there is a mantle zone depleted in 13C, approximately 20–30 km thick and extending over an area of at least 200 km2. [Copyright &y& Elsevier]

Abstract: Xenocrysts or phenocrysts of olivine in kimberlite show a diversity of hydroxyl-related infrared absorption bands that reflect compositional variation of their mantle source and conditions of last equilibration. Interpretation of infrared spectra of olivine in terms of equilibration/crystallization conditions may help to isolate spectral features associated with good diamond carrying and diamond preservation potential. As an exploratory study we examine by FTIR the spectroscopic features of olivine inclusions in diamond, olivine coexisting with diamond in peridotite xenoliths and olivine from diamond-free peridotite xenoliths. The three environments are respectively characterized by spectroscopically anhydrous olivine, olivine with low water concentrations (H2O<60 wt. ppm) and olivine that may be fairly rich in water (H2O up to 260 ppm). Previous studies have shown that the majority of olivine macrocrysts in kimberlite contain >100 ppm H2O, with a significant proportion containing >250 ppm H2O. Our new results suggest that hydrogenation of olivine occurs in the mantle during equilibration with kimberlite-related hydrous fluids and/or during equilibration with, or crystallization from, kimberlite magmas during ascent. The association of gem diamonds with water-poor mantle olivine suggests that such samples experienced limited interaction with hydrous fluids/melts over a short time span, thus preventing diffusional equilibration. Future FTIR studies of olivine from kimberlites with widely variable diamond grades and reconciliation with diamond resorption features may permit development of new FTIR based procedures to assist in the evaluation of new kimberlite discoveries. [Copyright &y& Elsevier]

Sixty-eight alluvial diamonds from three placer deposits in Brazil (Arenapolis in the State of Mato Grosso, Boa Vista in the State of Roraima, and Canastra in the State of Minas Gerais) are characterized by similar crystal shapes, body colors, and surface textures, which are related to growth or resorption processes. The concentrations and the aggregation states of nitrogen impurities in the diamonds, as well as in their carbon isotope compositions, are also similar. A higher proportion of diamonds with radiation spots distinguishes the diamonds from Boa Vista from the other deposits. The majority of the diamonds from Arenapolis (∼70%) exhibit transport-related abrasion textures. In contrast, diamonds from Boa Vista and Canastra are characterized by the absence or a low abundance (<15%) of such abrasion textures, which indicates that the diamonds are derived from nearby kimberlitic sources. At this time, kimberlitic sources have been located only in the proximity of the Canastra placer deposits. The composition of the mineral inclusions is similar for diamonds from all three deposits. The diamonds formed in a strong to moderately depleted peridotitic mantle, with only minor involvement of eclogitic sources. Pressure mad temperature estimates for the diamonds from Boa Vista are similar to the estimates for diamonds from other deposits worldwide and are consistent with a geothermal gradient of 40 to 42 m W/m² surface heat flow. [ABSTRACT FROM AUTHOR]

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

Inclusions of majoritic garnet in diamonds from the Jagersfontein kimberlite formed at unusually great depths of ∼250 to >500 km in the asthenosphere and transition zone. The original host rocks were derived from a much shallower, basaltic (eclogitic) source. The presence of negative Eu anomalies in all majoritic garnets requires a crustal origin, thereby linking these very deep diamond sources to subducting oceanic crust. The carbon isotope values (δ13C) of the host diamonds fall within a narrow range at ∼-20‰, which is fundamentally different from the broad range (-24‰ to -2‰) and bimodal distribution of carbon isotopes of Jagersfontein diamonds that formed in the shallower lithosphere. This indicates that majoritic garnet-bearing diamonds at Jagersfontein inherited their light carbon isotopic composition directly from organic matter contained in a subducting slab. These diamonds were likely formed by direct conversion from graphite, well within the diamond stability field. [ABSTRACT FROM AUTHOR]

Abstract: Diamonds from high- and low-MgO groups of eclogite xenoliths from the Jericho kimberlite, Slave Craton, Canada were analyzed for carbon isotope compositions and nitrogen contents. Diamonds extracted from the two groups show remarkably different nitrogen abundances and δ13C values. While diamonds from high-MgO eclogites have low nitrogen contents (5–82ppm) and extremely low δ13C values clustering at ∼−40‰, diamonds from the low-MgO eclogites have high nitrogen contents (>1200ppm) and δ13C values from −3.5‰ to −5.3‰. Coupled cathodoluminescence (CL) imaging and SIMS analysis of the Jericho diamonds provides insight into diamond growth processes. Diamonds from the high-MgO eclogites display little CL structure and generally have constant δ13C values and nitrogen contents. Some of these diamonds have secondary rims with increasing δ13C values from −40‰ to ∼−34‰, which suggests secondary diamond growth occurred from an oxidized growth medium. The extreme negative δ13C values of the high-MgO eclogite diamonds cannot be produced by Rayleigh isotopic fractionation of average mantle-derived carbon (−5‰) or carbon derived from typical organic matter (∼−25‰). However, excursions in δ13C values to −60‰ are known in the organic sedimentary record at ca. 2.7 and 2.0Ga, such that diamonds from the high-MgO eclogites could have formed from similar organic matter brought into the Slave lithospheric mantle by subduction. SIMS analyses of a diamond from a low-MgO eclogite show an outer core with systematic rimwards increases in δ13C values coupled with decreases in nitrogen contents, and a rim with pronounced alternating growth zones. The coupled δ13C-nitrogen data suggest that the diamond precipitated during fractional crystallization from an oxidized fluid/melt from which nitrogen was progressively depleted during growth. Model calculations of the co-variation of δ13C–N yielded a partition coefficient (K N) value of 5, indicating that nitrogen is strongly compatible in diamond relative to the growth medium. δ13C values of diamond cores (−4‰) dictate the growth medium had higher δ13C values than primary mantle-derived carbon. Therefore, possible carbon sources for the low-MgO eclogite diamonds include oxidized mantle-derived (e.g. protokimberlite or carbonatite) fluids/melts that underwent some fractionation during migration or, devolatilized subducted carbonates. [Copyright &y& Elsevier]

Abstract: Diamonds in South Australia occur in kimberlites at Eurelia (Orroroo), and in placer deposits, which include the Springfield Basin and the historic Echunga goldfield. To identify the kimberlitic and mantle sources of the placer diamonds, and to determine any possible connections between the placer diamonds and the diamonds from the Eurelia kimberlites, we examined the physical and compositional characteristics, and the mineral inclusion content of 122 diamonds from the Springfield Basin and 43 diamonds from kimberlites at Eurelia. Additional morphological data for three Echunga diamonds are also given. Most of the diamonds from the Springfield Basin are similar to the diamonds from Eurelia with respect to their crystal shapes, surface textures, and colors. The diamond populations from both areas are characterized by a high abundance of low-nitrogen (<100 ppm) diamonds with variable nitrogen aggregation states. The stable carbon isotope compositions of the Springfield Basin diamonds are similar to the Eurelia diamonds with δ 13C values in the range −20.0 to −2.5‰, and a mode at −6.5‰. Ferropericlase inclusions in two diamonds from the Springfield Basin are consistent with ferropericlase-bearing mineral inclusion assemblages found in the Eurelia diamonds and indicate that part of the diamond population from both areas is of sublithospheric origin. One diamond from the Springfield Basin contained an inclusion of lherzolitic garnet. The overall similarities between the Springfield Basin and Eurelia diamonds indicates that the bulk of the Springfield Basin diamonds are derived from kimberlitic sources that are similar (or identical) to those at Eurelia. However, three diamonds from the Springfield Basin are markedly distinct. These have well-developed crystal shapes, large sizes, yellow body colorations, and brown irradiation spots. The brown irradiation spots and abrasion textures provide evidence that these diamonds are much older than the other diamonds in the Springfield Basin, and that they are derived from distal kimberlitic sources. The diamonds are most likely derived from Permian glacigene sediments and may ultimately be sourced from kimberlites on the East Antarctic craton. Abrasion textures and brown irradiation spots are also present on diamonds from Echunga. This provides a link to the three “old” Springfield Basin diamonds and other alluvial diamonds in Eastern Australia, and suggests that Permian glaciations caused a widespread distribution of diamonds over large parts of southern Australia, which at that time was part of the supercontinent Gondwana. [Copyright &y& Elsevier]

The recently recognised Sask Craton, a small terrane with Archean (3.3–2.5 Ga) crustal ages, is enclosed in the Paleoproterozoic (1.9–1.8 Ga) Trans Hudson Orogen (THO). Only limited research has been conducted on this craton, yet it hosts major diamond deposits within the Cretaceous (~106 to ~95 Ma) Fort à la Corne (FALC) Kimberlite Field. This study describes major, trace and platinum group element data, as well as osmium isotopic data from peridotitic mantle xenoliths (n = 26) from the Star and Orion South kimberlites. The garnet-bearing lithospheric mantle is dominated by moderately depleted lherzolite. Equilibration pressures and temperatures (2.7 to 5.5 GPa and 840 to 1250 °C) for these garnet peridotites define a cool geotherm indicative of a 210 km thick lithosphere, similar to other cratons worldwide. Many of the peridotite xenoliths show the major and trace element signatures of carbonatitic and kimberlitic melt metasomatism. The Re–Os isotopic data yield T RD (time of Re-depletion) model ages, which provide minimum estimates for the timing of melt depletion, ranging from 2.4 to 0.3 Ga, with a main mode spanning from 2.4 to 1.7 Ga. No Archean ages were recorded. This finding and the complex nature of events affecting this terrane from the Archean through the Palaeoproterozoic provide evidence that the majority of the lithospheric mantle was depleted and stabilised in the Palaeoproterozoic, significantly later than the Archean crust. The timing of the dominant lithosphere formation is linked to rifting (~2.2 Ga - 2.0 Ga), and subsequent collision (1.9–1.8 Ga) of the Superior and Hearne craton during the Wilson cycle of the Trans Hudson Orogen. • Sask Craton mantle keel is dominantly lherzolitic, no harzburgites were identified • Cool Cretaceous paleogeotherm (38 mW/m2), comparable to Slave and Superior cratons • Melt metasomatism affected mineral major and trace element chemistry • Diamondiferous cratonic mantle root was stabilised in the Paleoproterozoic [ABSTRACT FROM AUTHOR]

Canada is host to at least six separate cratons that comprise a significant proportion of its crustal extent. Of these cratons, we possess knowledge of the cratonic lithospheric roots beneath only the Slave craton and, to a lesser extent, the Superior craton, despite the discovery of many new diamond-bearing kimberlites in Canada's North. Here we present the first age, composition and geothermal information for kimberlite-borne peridotite xenoliths from two localities within the central Rae craton: Pelly Bay and Repulse Bay. Our aim is to investigate the nature and evolution of the deep lithosphere in these regions and to examine how events recorded in the mantle may or may not correlate with the complex history of crustal evolution across the craton. Peridotite xenoliths are commonly altered by secondary processes including serpentinization, silicification and carbonation, which have variably affected the major element compositions. These secondary processes, as well as mantle metasomatism recorded in pristine silicate minerals, however, did not significantly modify the relative compositions of platinum-group elements (PGE) and Os isotope ratios in the majority of our samples from Pelly Bay and Repulse Bay, as indicated by the generally high absolute PGE concentrations and mantle-like melt-depleted PGE patterns. The observed PGE signatures are consistent with the low bulk Al 2 O 3 contents (mostly lower than 2.5%) of the peridotites, as well as the compositions of the silicate and oxide minerals. Based on PGE patterns and Os model ages, the peridotites from both localities can be categorized into three age groups: Archean (3.0–2.6 Ga overall; 2.8–2.6 Ga for Pelly Bay and 3.0–2.7 Ga for Repulse Bay), Paleoproterozoic (2.1–1.7 Ga), and “Recent” (<1 Ga, with model ages similar to the ca. 546 Ma kimberlite eruption age). The Archean group provides the first direct evidence of depleted Archean lithospheric mantle forming coevally with the overlying Archean crustal basement, indicating cratonization of the Rae during the Archean. The subtle difference in Os model ages between Pelly Bay and Repulse Bay coincides with the age difference between crustal basement rocks beneath these two areas, supporting the suggestion that the Rae craton was assembled by collision of separate two Archean blocks at 2.7–2.6 Ga. The Paleoproterozoic peridotites are interpreted to represent newly formed lithospheric mantle, most likely associated with regional-scale underplating during the 1.77–1.70 Ga Kivalliq-Nueltin event via removal of the lower portion of Archean lithospheric mantle followed by replacement with juvenile Paleoproterozoic lithospheric mantle. The existence of multiple age clusters in the lithosphere at each locality is consistent with the observation of present-day seismic lithospheric discontinuities ( Snyder et al., 2013, 2015 ) that indicate two or more layers of fossil lithospheric mantle fabric beneath this region. Our data define a shallow mantle lithosphere layer dominated by Archean depletion ages underlain by a layer of mixed Archean and Paleoproterozoic ages. This lithospheric mantle structure is probably a response to complex tectonic displacement of portions of the lithospheric mantle during Paleoproterozoic orogeny/underplating. The best equilibrated Archean and Paleoproterozoic peridotites at both Pelly Bay and Repulse Bay define a typical cratonic geotherm at the time of kimberlite eruption, with a ∼200 km thick lithospheric root extending well into the diamond stability field, in keeping with the diamondiferous nature of the kimberlites. Such thick lithosphere remains in place to the present day as suggested by seismic and magnetotelluric studies ( Snyder et al., 2013, 2015; Spratt et al., 2014 ). The metasomatically disturbed peridotites in the Rae lithospheric mantle, yielding model ages indistinguishable from kimberlite eruption, may represent parts of the Rae craton mantle root that show anomalous magnetotelluric signatures. [ABSTRACT FROM AUTHOR]

Abstract: We carried out a detailed study of sulphide minerals, a ubiquitous mineral group in lower crustal mafic to peraluminous granulite xenoliths from the Diavik kimberlites, to assess their use in constraining the origin and tectonothermal evolution of the deep crust, and to obtain additional data on the composition of lower crust beneath ancient continents. Sulphides are overwhelmingly pyrrhotite with minor Ni (0.7–3.9at.%), Co (0.1–0.7at.%), and Cu contents (0.4–3.9at.%). Sulphide modes in mafic granulites range from 0.14 to 0.55vol%, translating into bulk rock S contents from ∼600 to 2000ppm, similar to S contents in other mafic igneous rocks and indicating preservation of primary igneous S contents. In mafic granulites, Re and Os abundances in sulphides range from 42.5 to 726ppb and 3.2 to 180ppb, respectively, whereas those in peraluminous granulites are distinctly lower (36.1–282ppb and 1.8–7.2ppb, respectively), suggestive of Re and Os loss to fractionating sulphides in the more evolved precursors of these rocks. The significant within-sample variability of 187Os/188Os and correlation with 187Re/188Os indicates the preservation of primary Re–Os isotope systematics and time-integrated decay of the measured 187Re. Within the large uncertainties inherent in the nature of the samples and technique, sulphides in some granulites may record major tectonothermal events in the central Slave craton spanning several billion years of evolution. Multiple generations of sulphide can occur in a single sample. These data attest to the heterogeneous composition and complex history of the Slave craton lower crust. [ABSTRACT FROM AUTHOR]

Abstract: In order to address diamond formation and origin in the lithospheric mantle underlying the Central Slave Craton, we report N- and C-stable isotopic compositions and N-contents and aggregation states for 85 diamonds of known paragenesis (73 peridotitic, 8 eclogitic and 4 from lower mantle) from the Panda kimberlite (Ekati Mine, Lac de Gras Area, Canada). For 12 peridotitic and two eclogitic sulfide inclusion-bearing diamonds from this sample set, we also report multiple-sulfur isotope ratios. The 73 peridotitic diamonds have a mean δ 13C-value of −5.2‰ and range from −6.9 to −3.0‰, with one extreme value at −14.1‰. The associated δ 15N-values range from −17.0 to +8.5‰ with a mean value of −4.0‰. N-contents range from 0 to 1280ppm. The 8 eclogitic diamonds have δ 13C-values ranging from −11.2 to −4.4‰ with one extreme value at −19.4‰. Their δ 15N ranges from −2.1 to +7.9‰ and N-contents fall between 0 and 3452ppm. Four diamonds with an inferred lower mantle origin are all Type II (i.e. nitrogen-free) and have a narrow range of δ 13C values, between −4.5 and −3.5‰. The δ 34S of the 14 analyzed peridotitic and eclogitic sulfide inclusions ranges from −3.5 to +5.7‰. None of them provide evidence for anomalous δ 33S-values; observed variations in δ 33S are from +0.19 to −0.33‰, i.e. within the 2 sigma uncertainties of mantle sulfur (δ 33S=0‰). At Panda, the N contents and the δ 13C of sulfide-bearing peridotitic diamonds show narrower ranges than silicate-bearing peridotitic diamonds. This evidence supports the earlier suggestion established from eclogitic diamonds from the Kaapvaal that sulfide-(±silicate) bearing diamonds sample a more restricted portion of sublithospheric mantle than silicate-(no sulfide) bearing diamonds. Our findings at Panda suggest that sulfide-bearing diamonds should be considered as a specific diamond population on a global-scale. Based on our study of δ 34S, Δ33S, δ 15N and δ 13C, we find no evidence for subduction-related isotopic signatures in the mantle sampled by Panda diamonds. [Copyright &y& Elsevier]