Jericho Kimberlite Research Articles

Asthenospheric kimberlites: Volatile contents and bulk compositions at 7 GPa

During ascent, kimberlites react with the lithospheric mantle, entrain and assimilate xenolithic material, loose volatiles and suffer from syn- and post-magmatic alteration. Consequently, kimberlite rocks deviate heavily from their primary melt. Experiments at 7 GPa, 1300–1480 °C, 10–30 wt% CO2 and 0.46 wt% H2O on a proposed primitive composition from the Jericho kimberlite show that saturation with a lherzolitic mineral assemblage occurs only at 1300–1350 °C for a carbonatitic melt with <8 wt% SiO2 and >35 wt% CO2. At asthenospheric temperatures of >1400 °C, where the Jericho melt stays kimberlitic, this composition saturates only in low-Ca pyroxene, garnet and partly olivine.We hence forced the primitive Jericho kimberlite into multiple saturation with a lherzolitic assemblage by adding a compound peridotite. Saturation in olivine, low- and high-Ca pyroxene and garnet was obtained at 1400–1650 °C (7 GPa), melts are kimberlitic with 18–29 wt% SiO2 + Al2O3, 22.1–24.6 wt% MgO, 15–27 wt% CO2 and 0.4–7.1 wt% H2O; with a trade-off of H2O vs. CO2 and temperature. Melts in equilibrium with high-Ca pyroxene with typical mantle compositions have ≥2.5 wt% Na2O, much higher than the commonly proposed 0.1–0.2 wt%.The experiments allow for a model of kimberlite origin in the convective upper mantle, which only requires mantle upwelling that causes melting at the depth where elemental carbon (in metal, diamond or carbide) converts to CO2 (at ∼250 km). If primary melts leading to kimberlites contain a few wt% H2O, then adiabatic temperatures of 1400–1500 °C would yield asthenospheric mantle melts that are kimberlitic (>18 wt% SiO2 + Al2O3) but not carbonatitic (<10 wt% SiO2 + Al2O3) in composition, carbonatites only forming 100–200 °C below the adiabat. These kimberlites represent small melt fractions concentrating CO2 and H2O and then acquire part of their chemical signature by assimilation/fractionation during ascent in the subcratonic lithosphere.

Tectonic significance and redox state of Paleoproterozoic eclogite and pyroxenite components in the Slave cratonic mantle lithosphere, Voyageur kimberlite, Arctic Canada

Mantle-derived eclogite and pyroxenite xenoliths from the Jurassic Voyageur kimberlite on the northern Slave craton in Arctic Canada were studied for garnet and clinopyroxene major and trace element compositions, clinopyroxene Pb and garnet O isotopic compositions, and garnet Fe3+/ΣFe contents. The Voyageur xenoliths record a wide range of pressures, but are cooler compared to mantle xenoliths derived from the nearby, coeval Jericho kimberlite. The CaO, TiO2 and Zr contents of Voyageur eclogites increase with depth, which is also observed in northern Slave peridotite xenoliths, demonstrating ‘bottom-up’ metasomatic processes within cratonic mantle lithosphere.The Voyageur eclogites have positive Eu anomalies, flat HREEN patterns, and major element compositions that are consistent with ultimate origins from basaltic and gabbroic protoliths within oceanic lithosphere. Clinopyroxene Pb isotope ratios intercept the Stacey-Kramers two-stage terrestrial Pb evolution curve at ca. 2.1Ga, and form an array towards the host kimberlite, indicating isotopic mixing. The 2.1Ga eclogite formation age broadly overlaps with known Paleoproterozoic subduction and collision events that occurred along the western margin of the Slave craton. Unlike the eclogites, the Voyageur pyroxenites contain garnet with distinctive fractionated HREEN, sinusoidal REE patterns of calculated bulk rocks, and clinopyroxene with 206Pb/204Pb ratios that intercept the Stacey-Kramers curve at 1.8Ga. This suggests a distinct origin as Paleoproterozoic high-pressure mantle cumulates. However, the pyroxenite Pb isotope ratios fall within the eclogite array and could also be explained by protoliths formation at ca. 2.1Ga followed by minor isotopic mixing during mantle metasomatism. Thus, an alternative scenario involves pyroxenite formation within the mantle section of Paleoproterozoic oceanic lithosphere followed by variable metasomatism after incorporation into cratonic mantle lithosphere. This model allows for a linked petrogenesis of the Voyageur eclogites (crust) and pyroxenites (mantle) as part of the same subducting oceanic slab.Oxygen fugacity determinations for one pyroxenite and ten eclogite xenoliths show a range of 3 log units, from −4.6 to −1.6 ΔFMQ, similar to the range observed for nearby Jericho and Muskox eclogites (ΔFMQ −4.2 to −1.5). Importantly, the northern Slave eclogite and pyroxenite mantle components are highly heterogeneous in terms of redox state provided that they range from reduced to oxidized relative to Slave peridotite xenoliths.Moreover, the Voyageur eclogites do not exhibit any trend between oxidation state and equilibration depth, which contrasts with the downward decrease in fO2 shown by Slave and worldwide cratonic peridotite xenoliths. Our investigation of mantle eclogite and pyroxenite fO2 reinforces the important influence of recycled mafic components in upper mantle processes, because their high and variable redox buffering capacity strongly controls volatile speciation and melting relations under upper mantle conditions.

Peridotite and pyroxenite xenoliths from the Muskox kimberlite, northern Slave craton, Canada

We present petrography, mineralogy, and thermobarometry for 53 mantle-derived xenoliths from the Muskox kimberlite pipe in the northern Slave craton. The xenolith suite includes 23% coarse peridotite, 9% porphyroclastic peridotite, 60% websterite, and 8% orthopyroxenite. Samples primarily comprise forsteritic olivine (Fo 89–94), enstatite (En 89–94), Cr-diopside, Cr-pyrope garnet, and chromite spinel. Coarse peridotites, porphyroclastic peridotites, and pyroxenites equilibrated at 650–1220 °C and 23–63 kbar (1 kbar = 100 MPa), 1200–1350 °C and 57–70 kbar, and 1030–1230 °C and 50–63 kbar, respectively. The Muskox xenoliths differ from xenoliths in the neighboring and contemporaneous Jericho kimberlite by their higher levels of depletion, the presence of a shallow zone of metasomatism in the spinel peridotite field, a higher proportion of pyroxenites at the base of the mantle column, higher Cr2O3 in all pyroxenite minerals, and weaker deformation in the Muskox mantle. We interpret these contrasts as representing small-scale heterogeneities in the bulk composition of the mantle, as well as the local effects of interaction between metasomatizing fluid and mantle wall rocks. We suggest that asthenosphere-derived pre-kimberlitic melts and fluids percolated less effectively through the less permeable Muskox mantle, resulting in lower degrees of hydrous weakening, strain, and fertilization of the peridotitic mantle. Fluids tended to concentrate and pool in the deep mantle, causing partial melting and formation of abundant pyroxenites.

A Record of Paleoproterozoic Subduction Preserved in the Northern Slave Cratonic Mantle: Sr–Pb–O Isotope and Trace-element Investigations of Eclogite Xenoliths from the Jericho and Muskox Kimberlites

Paleoproterozoic eclogite xenoliths from the Jericho and Muskox kimberlites located in the northern Slave craton, Canada, display geochemical and isotopic evidence of a recycled oceanic lithosphere origin. The eclogites comprise a diverse suite of kyanite- and diamond-bearing and bimineralic xenoliths that have a wide range of geochemical compositions. Calculated whole-rock eclogite compositions are broadly basaltic to picritic, ranging from 5 to 13 wt % MgO. Calculated whole-rock trace-element compositions for the eclogites show three overall REE patterns: the first pattern resembles that of oceanic gabbros with prominent positive Eu anomalies and flat heavy REE; the second pattern is similar to the first but displays marked light REE depletion that is interpreted to record partial melt loss; the third pattern lacks Eu anomalies and instead displays both gradually increasing REE contents from La to Lu similar to boninitic melts and ‘humped’ middle REE patterns similar to those observed in eclogites formed by reaction with subduction fluids. Several eclogites have garnet and/or clinopyroxene δ 18 O values >6‰, which coupled with the trace-element systematics of all the eclogites is interpreted to reflect derivation from recycled oceanic lithosphere protoliths. Lead isotope data for eclogite clinopyroxenes form a broad array that extends from highly radiogenic isotope compositions similar to the host Jericho kimberlite to an intersection with the Stacey–Kramers terrestrial two-stage Pb evolution curve at c. 2·2 Ga. The slope of this data array defines a secondary Pb isochron with an associated age of 1·7 ± 0·3 Ga. Although it is possible that the Pb isotope array is actually a mixing line between the eclogite protolith and the host Jericho kimberlite, both the secondary isochron age and the c. 2·2 Ga Stacey–Kramers model age are consistent with Paleoproterozoic protolith oceanic crust formation. Eclogite xenoliths and eclogitic diamond inclusions from the Slave craton almost exclusively yield c. 2 Ga ages, which are broadly compatible with formation during subduction associated with the c. 1·9–1·8 Ga Wopmay orogeny at the western margin of the Slave craton. Trends in the Sr–Pb isotopic compositions of the eclogite clinopyroxene show that seawater alteration of the oceanic crust protoliths was an important process during protolith formation, and cryptic metasomatism further increased the Pb and Sr isotope values. This metasomatism is probably associated with the Jurassic kimberlite magmatism; however, contributions from the Mesoproterozoic Mackenzie event cannot be completely excluded.

In situ analysis of garnet inclusion in diamond using single-crystal X-ray diffraction and X-ray micro-tomography

A single crystal of garnet enclosed in a diamond from the Jericho kimberlite (Slave Craton, Canada) has been investigated using X-ray diffraction and X-ray micro-tomography. The novel experimental approach allowed us to determine the crystal structure of the garnet. The unit-cell edge a and fractional atomic coordinates of oxygen were used to determine the composition via an updated Margules model for garnets. The composition is Pyr0.41(5)Alm0.36(7)Gro0.22(1)Uva0.01(1), which is indistinguishable from the eclogitic garnets found in other Jericho diamonds. We also demonstrated that residual pressures on the inclusion of up to 1 GPa do not affect significantly the determination of the garnet composition by structure refinement.

Eclogite formation beneath the northern Slave craton constrained by diamond inclusions: Oceanic lithosphere origin without a crustal signature

We report the geochemical and oxygen isotope compositions for eclogitic mineral inclusions in diamonds hosted by high-MgO eclogite xenoliths from the Jericho kimberlite, Canada. These data are used to constrain the nature and evolution of the eclogite protolith. The garnet and clinopyroxene diamond inclusions (DIs) are compositionally different than their host eclogite counterparts. In particular, garnet DIs have much lower Mg-numbers (54 vs. 82) and Cr2O3 contents (0.1 vs. 0.6wt.%) and higher CaO contents (7.6 vs. 4.3wt.%) than host eclogite garnet. DI and host eclogite clinopyroxenes are more similar but differences include lower Mg-numbers (78–81 vs. 93) and higher Na2O contents (2.3 vs. 1.8wt.%) in the DIs. The DIs lack typical shallow oceanic crust signatures such as strong positive Eu and Sr anomalies, and oxygen isotope compositions that deviate significantly from the pristine mantle average. On the contrary, both the Jericho DIs and host eclogite garnets have small negative Eu and Sr anomalies, fractionated HREE patterns ((LuN/GdN)~3–5) and pristine mantle-like δ18O values of 5.2–6.0‰, indicating that shallow, plagioclase-rich oceanic crust protoliths are unlikely.The eclogitic DI trace-element characteristics require that both garnet and plagioclase were present in the protolith, which likely crystallized in the shallow upper mantle. DI-based reconstructed whole-rock eclogite compositions have higher Mg-numbers and lower Al2O3 contents than found in typical basaltic or gabbroic oceanic crust, and are similar to pyroxenitic veins found in orogenic peridotite massifs. Due to the lack of clear oceanic crust signatures and the mantle-like δ18O values of the studied DIs, we propose that the Jericho diamond eclogites originally crystallized as pyroxenite cumulates that formed veins within the oceanic mantle lithosphere. Following partial melt extraction, the eclogite protoliths were subducted into the diamond stability field beneath the evolving Slave craton. Hence, the Jericho DIs and host high-MgO eclogites may represent an example of eclogite formation in an oceanic setting without the diagnostic ‘crustal signatures’ that are typically observed in cratonic eclogite xenolith suites worldwide.

Transmission X-ray diffraction as a new tool for diamond fluid inclusion studies

AbstractTransmission X-ray diffraction is demonstrated as a new tool for examining daughter minerals within sub-micrometre-size fluid inclusions in fibrous diamond. In transmission geometry, the X-ray beam passes through the sample, interacting with a volume of material. Fibrous diamonds from Mbuji-Mayi. Democratic Republic of Congo; the Wawa area, Ontario, Canada; and the Panda kimberlite, Ekati Mine, Northwest Territories and the Jericho kimberlite, Nunavut, Canada were analysed using X-rays from a high-brilliance lab source and a synchrotron source. Daughter minerals present include the mica-group mineral celadonite, sylvite, halite, dolomite and other carbonates. This represents the first positive identification of halide minerals in fibrous diamond. Mineral inclusions such as forsteritic olivine and pyrope garnet were also found. Unexpectedly, daughter minerals were identified in only ten of the 38 diamonds analysed, despite their concentrations being greater than experimentally proven detection limits. The presence of significant amounts of amorphous or dissolved material appears unlikely, but cannot be ruled out. Alternatively, the results may indicate a wide variety of related daughter minerals, such that most phases fall below the detection limits. Transmission X-ray diffraction should be applied cautiously to the study of fibrous diamond, as it provides an incomplete account of the fluid-inclusion mineralogy.

Diamond growth from oxidized carbon sources beneath the Northern Slave Craton, Canada: A δ 13C–N study of eclogite-hosted diamonds from the Jericho kimberlite

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 δ 13C values. While diamonds from high-MgO eclogites have low nitrogen contents (5–82 ppm) and extremely low δ 13C values clustering at ∼−40‰, diamonds from the low-MgO eclogites have high nitrogen contents (>1200 ppm) and δ 13C 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 δ 13C values and nitrogen contents. Some of these diamonds have secondary rims with increasing δ 13C values from −40‰ to ∼−34‰, which suggests secondary diamond growth occurred from an oxidized growth medium. The extreme negative δ 13C 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 δ 13C values to −60‰ are known in the organic sedimentary record at ca. 2.7 and 2.0 Ga, 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 δ 13C values coupled with decreases in nitrogen contents, and a rim with pronounced alternating growth zones. The coupled δ 13C-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 δ 13C–N yielded a partition coefficient ( K N) value of 5, indicating that nitrogen is strongly compatible in diamond relative to the growth medium. δ 13C values of diamond cores (−4‰) dictate the growth medium had higher δ 13C 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.

Reconstruction of a multi-vent kimberlite eruption from deposit and host rock characteristics: Jericho kimberlite, Nunavut, Canada

The Jericho kimberlite (173.1 ± 1.3 Ma) is a small (~ 130 × 70 m), multi-vent system that preserves products from deep (> 1 km?) portions of kimberlite vents. Pit mapping, drill core examination, petrographic study, image analysis of olivine crystals (grain size distributions and shape studies), and compositional and mineralogical studies, are used to reconstruct processes from near-surface magma ascent to kimberlite emplacement and alteration. The Jericho kimberlite formed by multiple eruptions through an Archean granodiorite batholith that was overlain by mid-Devonian limestones ~ 1 km in thickness. Kimberlite magma ascended through granodiorite basement by dyke propagation but ascended through limestone, at least in part, by locally brecciating the host rocks. After the first explosive breakthrough to surface, vent deepening and widening occurred by the erosive forces of the waxing phase of the eruption, by gravitationally induced failures as portions of the vent margins slid into the vent and, in the deeper portions of the vent (> 1 km), by scaling, as thin slabs burst from the walls into the vent. At currently exposed levels, coherent kimberlite (CK) dykes (< 40 cm thick) are found to the north and south of the vent complex and represent the earliest preserved in-situ products of Jericho magmatism. Timing of CK emplacement on the eastern side of the vent complex is unclear; some thick CK (15–20 m) may have been emplaced after the central vent was formed. Explosive eruptive products are preserved in four partially overlapping vents that are roughly aligned along strike with the coherent kimberlite dyke. The volcaniclastic kimberlite (VK) facies are massive and poorly sorted, with matrix- to clast-supported textures. The VK facies fragmented by dry, volatile-driven processes and were emplaced by eruption column collapse back into the volcanic vents. The first explosive products, poorly preserved because of partial destruction by later eruptions, are found in the central-east vent and were formed by eruption column collapse after the vent was largely cleared of country rock debris. The next active vent was either the north or south vent. Collapse of the eruption column, linked to a vent widening episode, resulted in coeval avalanching of pipe margin walls into the north vent, forming interstratified lenses of country rock-rich boulder breccias in finer-grained volcaniclastic kimberlite. South vent kimberlite has similar characteristics to kimberlite of the north vent and likely formed by similar processes. The final eruptive phase formed olivine-rich and moderately sorted deposits of the central vent. Better sorting is attributed to recycling of kimberlite debris by multiple eruptions through the unconsolidated volcaniclastic pile and associated collapse events. Post-emplacement alteration varies in intensity, but in all cases, has overprinted the primary groundmass and matrix, in CK and VK, respectively. Erosion has since removed all limestone cover.

The origin of high-MgO diamond eclogites from the Jericho Kimberlite, Canada

A unique suite of diamond-rich eclogites from the Jericho kimberlite, Nunavut, Canada, has a uniform but unusual geochemical and isotopic composition that is unlike any other eclogite suite worldwide. Compared to other eclogite suites, garnets from Jericho diamond-bearing eclogites have high MgO (19.6–21.2 wt.%), Cr 2O 3 (0.28–0.60 wt.%), Sc (88–113 ppm), and Zr (32–36 ppm) contents and are characterized by consistent highly fractionated chondrite-normalized HREE patterns ([Lu/Gd] N = 4.0–6.6). Na-poor clinopyroxene has a uniform LREE-enriched pattern and radiogenic 87Sr/ 86Sr values between 0.7057 and 0.7061, higher than other eclogite groups at Jericho and the Jericho kimberlite itself. The distinct geochemical and isotopic compositions of the Jericho diamond eclogites are not directly compatible with either the subducted oceanic crust or high-pressure mantle melt models commonly invoked for the origin of mantle eclogites. We propose a multi-stage origin that involves multiple metasomatic events coupled with hybridization between basaltic eclogite and mantle peridotite. Emplacement of basaltic eclogites in the diamond stability field was followed by metasomatism by a carbon-bearing, LREE-enriched metasomatic fluid or melt as recorded by clinopyroxene and diamond inclusions in garnet. Subsequent or concurrent partial melting of eclogite produced melts that facilitated diffusional elemental exchange between residual eclogite and ambient peridotite. This process enabled the eclogites to attain their distinct high-Mg and Cr composition. Finally, carbonatite-like modal metasomatism grew phlogopite, carbonate and apatite and potentially also facilitated additional diamond growth, producing the extreme diamond enrichments found in these eclogites.

Crystallization of megacrysts from protokimberlitic fluids: Geochemical evidence from high-Cr megacrysts in the Jericho kimberlite

We report petrological, geochemical and isotopic data for low-Cr and high-Cr suite megacrysts from the Jericho kimberlite (N. Canada). The megacrysts comprise garnet, clinopyroxene, orthopyroxene and olivine; ilmenite is present only in the low-Cr suite. The megacrysts occur as individual crystals or intergrown grains of the same or different megacryst minerals showing partial recrystallization. Compositional zoning, textures of polycrystalline megacrysts and a lack of extended MgO or SiO 2 trends suggest megacrysts formed metasomatically shortly before the kimberlite emplacement. Fractionated REE patterns and radiogenic Hf compositions of Cr-rich megacrysts, as compared to low-Cr megacrysts elsewhere, imply a lower fluid/wall rock ratio than that characteristic of magmatic processes. Rb–Sr, Sm–Nd and Lu–Hf isochron ages for the megacrysts are similar to the eruption age of the kimberlite and represent blocking of the isotope systems at the eruption. Trace element patterns of melts buffered by megacrystal clinopyroxene and garnet were computed using experimental partition coefficients for silicate-carbonate high-pressure high-temperature melts. Our data suggests that the melt equilibrated with the megacrysts was kimberlitic as it has trace elements and Hf and Nd isotopic compositions of the host kimberlite. Sr isotopic range of the Jericho kimberlite can be modelled by 5–10% contamination of local crust such as limestone or granite, into a magma that had an initial unradiogenic Sr isotope composition similar to that of the megacrysts.

Diamonds and eclogites of the Jericho kimberlite (Northern Canada)

We studied diamonds and barren and diamondiferous eclogite xenoliths from the Jericho kimberlite (Northern Slave craton). The majority of the diamonds are non-resorbed octahedral crystals, with moderately aggregated N (IaB < 50%, N < 300 ppm) and δ13C = −5 to −41‰. The diamonds belong to “eclogitic” (90% of the studied samples), “websteritic” (7%) and “peridotitic” (3%) assemblages. The Jericho diamonds differ from the majority of “eclogitic” diamonds worldwide in magnesian compositions of associated minerals and extremely light C isotopic compositions (δ13C = −24 to −41‰). We propose that metasomatism triggered by H2O fluids may have been involved in the diamond formation. Multiple episodes of the metasomatism and associated melt extraction of various ages are evident in Jericho eclogite xenoliths where primary garnet and clinopyroxene have been recrystallized to more magnesian minerals with higher contents of some incompatible trace elements and to hydrous secondary phases. The model is supported by the general similarity of mineral compositions in diamondiferous eclogites to those in diamond inclusions and to secondary magnesian garnet and clinopyroxene in recrystallized barren eclogites. The ultimate products of the metasomatism could be “websteritic” diamond assemblages sourced from magnesian eclogites.

Petrology and textural classification of the Jericho kimberlite, northern Slave Province, Nunavut, Canada

The paper presents data on petrology, bulk rock and mineral compositions, and textural classification of the Middle Jurassic Jericho kimberlite (Slave craton, Canada). The kimberlite was emplaced as three steep-sided pipes in granite that was overlain by limestones and minor soft sediments. The pipes are infilled with hypabyssal and pyroclastic kimberlites and connected to a satellite pipe by a dyke. The Jericho kimberlite is classified as a Group Ia, lacking groundmass tetraferriphlogopite and containing monticellite pseudomorphs. The kimberlite formed during several consecutive emplacement events of compositionally different batches of kimberlite magma. Core-logging and thin-section observations identified at least two phases of hypabyssal kimberlites and three phases of pyroclastic kimberlites. Hypabyssal kimberlites intruded as a main dyke (HK1) and as late small-volume aphanitic and vesicular dykes. Massive pyroclastic kimberlite (MPK1) predominantly filled the northern and southern lobes of the pipe and formed from magma different from the HK1 magma. The MPK1 magma crystallized Ti-, Fe-, and Cr-rich phlogopite without rims of barian phlogopite, and clinopyroxene and spinel without atoll structures. MPK1 textures, superficially reminiscent of tuffisitic kimberlite, are caused by pervasive contamination by granite xenoliths. The next explosive events filled the central lobe with two varieties of pyroclastic kimberlite: (1) massive and (2) weakly bedded, normally graded pyroclastic kimberlite. The geology of the Jericho pipe differs from the geology of South African or the Prairie kimberlites, but may resemble Lac de Gras pipes, in which deeper erosion removed upper facies of resedimented kimberlites.

Lu–Hf, in-situ Sr and Pb isotope and trace element systematics for mantle eclogites from the Diavik diamond mine: Evidence for Paleoproterozoic subduction beneath the Slave craton, Canada

Lu–Hf, Sm–Nd and in-situ clinopyroxene Sr and Pb isotope systematics, and mineral major and in-situ trace element compositions were obtained for a suite of non-diamond and diamond-bearing eclogites from the Diavik kimberlites (A154; 55 Ma old), Slave craton (Canada). Temperature estimates of last equilibration in the lithosphere for the non-diamond-bearing Diavik eclogites define two groups; low-temperature (800–1050 °C) and high-temperature eclogites (1100–1300 °C). Most diamond–eclogites indicate temperatures similar to those of the high-temperature eclogites. Isotopic and major and trace element systematics for the non-diamond- and diamond-bearing eclogites indicate overlapping chemical compositions suggesting similar rock formational histories. Calculated whole rock major and trace element abundances using chemical and modal abundances for constituent minerals exhibit broad similarities with mafic cumulates from ophiolite sequences. Most importantly the calculated whole rock eclogite compositions display positive Sr and Eu anomalies, typically interpreted as the result of plagioclase accumulation in cumulate rocks of oceanic crust sequences. Initial whole rock Hf isotopic values and in-situ Sr isotope data from clinopyroxene grains provide evidence that the eclogites were derived from precursor rocks with depleted mantle isotope characteristics. These combined results support the interpretation that the eclogites from Diavik represent remnants of subducted oceanic crust. Lu–Hf isotope systematics indicate that the oceanic protolith for the eclogites formed in the Paleoproterozoic at ∼2.1 Ga, which is in agreement with the in-situ Pb isotope data from clinopyroxene. This result also corroborates the ∼2.1 Ga Lu–Hf model ages recorded by mantle zircons from eclogite found within the Jericho kimberlite in the northern Slave Province (∼200 km northwest of Diavik). The results from both studies indicate a link between eclogite formation and Paleoproterozoic subduction of oceanic lithosphere along the present-day western margin of the Archean Slave craton.

Multi-Stage Modification of the Northern Slave Mantle Lithosphere: Evidence from Zircon- and Diamond-Bearing Eclogite Xenoliths Entrained in Jericho Kimberlite, Canada

The Jericho kimberlites are part of a small Jurassic kimberlite cluster in the northern Slave craton, Canada. A variety of dating techniques were applied to constrain the nature and age of two Jericho kimberlites, JD-1 (170·2 ± 4·3 Ma Rb–Sr phlogopite megacrysts, 172·8 ± 0·7 Ma U–Pb eclogite rutile, 178 ± 5 Ma U–Pb eclogite zircon lower intercept) and JD-3 (173 ± 2 Ma Rb–Sr phlogopite megacryst; 176·6 ± 3·2 Ma U–Pb perovskite), and all yielded identical results within analytical uncertainty. As there is no discernible difference in the radiometric ages obtained for these two pipes, the composite Rb–Sr phlogopite megacryst date of 173·1 ± 1·3 Ma is interpreted as the best estimate for the emplacement age of both Jericho pipes. The initial Sr isotope composition of 0·7053 ± 0·0003 derived from phlogopite megacrysts overlaps the range (0·7043–0·7084) previously reported for Jericho whole-rocks. These strontium isotope data, combined with the radiogenic initial 206Pb/204Pb ratio of 18·99 ± 0·33 obtained in this study, indicate that the Jericho kimberlites are isotopically similar to Group 1 kimberlites as defined in southern Africa. The Jericho kimberlites are an important new source of mantle xenoliths that hold clues to the nature of the Slave craton subcontinental mantle. A high proportion (30%) of the Jericho mantle xenolith population consists of various eclogite types including a small number (2–3%) of apatite-, diamond-, kyanite- and zircon-bearing eclogites. The most striking aspect of the Jericho zircon-bearing eclogite xenoliths is their peculiar geochemistry. Reconstructed whole-rock compositions indicate that they were derived from protoliths with high FeO, Al2O3 and Na2O contents, reflected in the high-FeO (22·6–27·5 wt %) nature of garnet and the high-Na2O (8·47–9·44 wt %) and high-Al2O3 (13·12–14·33 wt %) character of the clinopyroxene. These eclogite whole-rock compositions are highly enriched in high field strength elements (HFSE) such as Nb (133–1134 ppm), Ta (5–28 ppm), Zr (1779–4934 ppm) and Hf (23–64 ppm). This HFSE enrichment is linked to growth of large (up to 2 mm) zircon and niobian rutile crystals (up to 3 modal %) near the time of eclogite metamorphism. The diamond-bearing eclogites on the other hand are characterized by high-MgO (19·6–21·3 wt %) garnet and ultralow-Na2O (0·44–1·50 wt %) clinopyroxene. Paleotemperature estimates indicate that both the zircon- and diamond-bearing eclogites have similar equilibration temperatures of 950–1020°C and 990–1030°C, respectively, corresponding to mantle depths of 150–180 km. Integration of petrographic, whole-rock and mineral geochemistry, geochronology and isotope tracer techniques indicates that the Jericho zircon-bearing eclogite xenoliths have had a complex history involving Paleoproterozoic metamorphism, thermal perturbations, and two or more episodes of Precambrian mantle metasomatism. The oldest metasomatic event (Type 1) occurred near the time of Paleoproterozoic metamorphism (∼1·8 Ga) and is responsible for the extreme HFSE enrichment and growth of zircon and high-niobian rutile. A second thermal perturbation and concomitant carbonatite metasomatism (Type 2) is responsible for significant apatite growth in some xenoliths and profound light rare earth element enrichment. Type 2 metasomatism occurred in the period 1·0–1·3 Ga and is recorded by relatively consistent whole-rock eclogite model Nd ages and secondary U–Pb zircon upper intercept dates. These eclogite xenoliths were derived from a variety of protoliths, some of which could represent metasomatized pieces of oceanic crust, possibly linked to east-dipping subduction beneath the Slave craton during construction of the 1·88–1·84 Ga Great Bear continental arc. Others, including the diamond-bearing eclogites, could be cumulates from mafic or ultramafic sill complexes that intruded the Slave lithospheric mantle at depths of about 150–180 km.

Formation of Paleoproterozoic eclogitic mantle, Slave Province (Canada): Insights from in-situ Hf and U–Pb isotopic analyses of mantle zircons

In-situ Hf isotope analyses and U–Pb dates were obtained by laser ablation-MC-ICP-MS for a zircon-bearing mantle eclogite xenolith from the diamondiferous Jericho kimberlite located within the Archean Slave Province (Nunavut), Canada. The U–Pb zircon results yield a wide range of ages (∼2.0 to 0.8 Ga) indicating a complex geological history. Of importance, one zircon yields a U–Pb upper intercept date of 1989 ± 67 Ma, providing a new minimum age constraint for zircon crystallization and eclogite formation. In contrast, Hf isotope systematics for the same zircons display an intriguing uniformity, and corresponding Hf depleted mantle model ages range between 2.1 ± 0.1 and 2.3 ± 0.1 Ga; the youngest Hf model age is within error to the oldest U–Pb date. The Jericho eclogites have previously been interpreted as representing remnants of metamorphosed oceanic crust, and their formation related to Paleoproterozoic subduction regimes along the western margin of the Archean Slave craton during the Wopmay orogeny. Hf isotope compositions and U–Pb results for the Jericho zircons reported here are in good agreement with a Paleoproterozoic subduction model, suggesting that generation of oceanic crust and eclogite formation occurred between 2.0 and 2.1 Ga. The slightly older Hf depleted mantle model ages (∼2.1 to 2.3 Ga) may be reconciled with this model by invoking mixing between ‘crustal’-derived Hf from sediments and more radiogenic Hf associated with the oceanic crust during the ∼2 Ga subduction event. This results in intermediate Hf isotope compositions for the Jericho zircons that yield ‘fictitiously’ older Hf model ages.

A Re–Os isotope and PGE study of kimberlite-derived peridotite xenoliths from Somerset Island and a comparison to the Slave and Kaapvaal cratons

The concentrations of platinum-group elements (PGE; Os, Ir, Ru, Pd and Pt) and Re, and the Os isotopic compositions were determined for 33 lithospheric mantle peridotite xenoliths from the Somerset Island kimberlite field. The Os isotopic compositions are exclusively less radiogenic than estimates of bulk-earth ( 187Os/ 188Os as low as 0.1084) and require a long-term evolution in a low Re–Os environment. Re depletion model ages ( T RD) indicate that the cratonic lithosphere of Somerset Island stabilised by at least 2.8 Ga, i.e. in the Neoarchean and survived into the Mesozoic to be sampled by Cretaceous kimberlite magmatism. An Archean origin also is supported by thermobarometry (Archean lithospheric keels are characterised by >150 km thick lithosphere), modal mineralogy and mineral chemistry observations. The oldest ages recorded in the lithospheric mantle beneath Somerset Island are younger than the Mesoarchean (>3 Ga) ages recorded in the Slave craton lithospheric mantle to the southwest [Irvine, G.J., et al., 1999. Age of the lithospheric mantle beneath and around the Slave craton: a Rhenium–Osmium isotopic study of peridotite xenoliths from the Jericho and Somerset Island kimberlites. Ninth Annual V.M. Goldschmidt Conf., LPI Cont., 971: 134–135; Irvine, G.J., et al., 2001. The age of two cratons: a PGE and Os-Isotopic study of peridotite xenoliths from the Jericho kimberlite (Slave craton) and the Somerset Island kimberlite field (Churchill Province). The Slave–Kaapvaal Workshop, Merrickville, Ontario, Canada]. Younger, Paleoproterozoic, T RD model ages for Somerset Island samples are generally interpreted as the result of open system behaviour during metasomatic and/or magmatic processes, with possibly the addition of new lithospheric material during tectono-thermal events related to the Taltson–Thelon orogen. PGE patterns highly depleted in Pt and Pd generally correspond to older Archean T RD model ages indicating closed system behaviour since the time of initial melt extraction. Younger Proterozoic T RD model ages generally correspond to more complex PGE patterns, indicating open system behaviour with possible sulfide or melt addition. There is no correlation between the age of the lithosphere and depth, at Somerset Island.

Extreme enrichment of high field strength elements in Jericho eclogite xenoliths: A cryptic record of Paleoproterozoic subduction, partial melting, and metasomatism beneath the Slave craton, Canada

Research Article| June 01, 2002 Extreme enrichment of high field strength elements in Jericho eclogite xenoliths: A cryptic record of Paleoproterozoic subduction, partial melting, and metasomatism beneath the Slave craton, Canada Larry M. Heaman; Larry M. Heaman 1Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada Search for other works by this author on: GSW Google Scholar Robert A. Creaser; Robert A. Creaser 1Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada Search for other works by this author on: GSW Google Scholar Harrison O. Cookenboo Harrison O. Cookenboo 2Meridian Geoscience Ltd., 700-1285 West Pender Street, Vancouver, British Columbia V6E 4B1, Canada Search for other works by this author on: GSW Google Scholar Geology (2002) 30 (6): 507–510. https://doi.org/10.1130/0091-7613(2002)030<0507:EEOHFS>2.0.CO;2 Article history received: 17 Oct 2001 rev-recd: 03 Feb 2002 accepted: 12 Feb 2002 first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Larry M. Heaman, Robert A. Creaser, Harrison O. Cookenboo; Extreme enrichment of high field strength elements in Jericho eclogite xenoliths: A cryptic record of Paleoproterozoic subduction, partial melting, and metasomatism beneath the Slave craton, Canada. Geology 2002;; 30 (6): 507–510. doi: https://doi.org/10.1130/0091-7613(2002)030<0507:EEOHFS>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract An unusual suite of zircon- and rutile-bearing eclogite xenoliths from the 172 Ma Jericho kimberlite, Northwest Territories, Canada, displays a peculiar geochemistry highlighted by extreme enrichment of high field strength elements. Most profound are the large positive Nb and Zr anomalies that are linked to a high modal abundance of niobian rutile and zircon. This peculiar geochemistry, combined with results from U-Pb geochronology, requires a complex origin involving east-dipping Paleoproterozoic subduction and partial melting of oceanic crust beneath the Slave craton, one or more metasomatic overprints prior to kimberlite entrainment, and a Mesoproterozoic thermal disturbance in the Slave lithospheric mantle possibly linked to Mackenzie igneous events. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Chemical stratification of cratonic lithosphere: constraints from the Northern Slave craton, Canada

We describe the mineralogical and chemical composition of the Northern Slave mantle as deduced from xenoliths of peridotite within the Jericho kimberlite, Northwest Territories. Our data set includes modal, major, trace and rare earth element compositions of bulk samples of spinel peridotite, low-T and high-T garnet peridotite and minor pyroxenite. Compared to primitive upper mantle, Jericho peridotite shows depletion in the major elements and enrichment in incompatible elements (except for HREE). The Slave mantle is also uniquely stratified. Older, depleted spinel peridotite extends to a depth of 80–100 km and is underlain by garnet peridotite which shows a gradual decrease in Mg# with depth to 200 km. The youngest layer of fertile garnet peridotite, enriched in clinopyroxene and garnet, is underlain by a pyroxenite-rich horizon at the base of the petrological lithosphere. The Northern Slave is further distinguished from the Kaapvaal and Siberian upper mantle by a marked vertical stratification in Mg#, lower abundances of orthopyroxene and higher abundances of clinopyroxene. In addition, a deeper layer of garnet peridotite below Jericho shows less depletion than low-T peridotite from other cratons. The Northern Slave peridotite results from a series of chemical events that include: (i) high-degree melting of pyrolite at P>3 Gpa for low-T peridotite and lower pressure melting for high-T peridotite, (ii) enrichment of low-T spinel peridotite in orthopyroxene, and (iii) pervasive metasomatic enrichment in alkali and LREE’s by kimberlite-related fluids. The chemical stratification described for two of the three lithospheric domains of the Slave craton makes this craton an exception among cratons with commonly unstratified lithospheres. The gradual increase in fertility with depth below the Slave craton is related to age stratification and may have formed by incremental downward growth of mantle lithosphere with time, and/or later re-fertilization of deeper mantle horizons.

Primitive Magma From the Jericho Pipe, N.W.T., Canada: Constraints on Primary Kimberlite Melt Chemistry

We report the first estimates of primary kimberlite melt composition from the Slave craton, based on samples of aphanitic kimberlite from the Jericho kimberlite pipe, N.W.T., Canada. Three samples derive from the margins of dykes where kimberlite chilled against wall rock (JD51, JD69 and JD82) and are shown to be texturally consistent with crystallization from a melt. Samples JD69 and JD82 have geochemical characteristics of primitive melts: they have high MgO (20–25 wt %), high mg-numbers (86–88), and high Cr (1300–1900 ppm) and Ni (800–1400 ppm) contents. They also have high contents of CO2 (10–17 wt %). Relative to bulk macrocrystal kimberlite, they have lower mg-numbers and lower MgO but are enriched in incompatible elements (e.g. Zr, Nb and Y), because the bulk kimberlite compositions are strongly controlled by accumulation of mantle olivine and other macrocrysts. The compositions of aphanitic kimberlite from Jericho are similar to melts produced experimentally by partial melting of a carbonate-bearing garnet lherzolite. On the basis of these experimental data, we show that the primary magmas from the Jericho kimberlite could represent 0·7–0·9% melting of a carbonated lherzolitic mantle source at pressures and temperatures found in the uppermost asthenosphere to the Slave craton. The measured CO2 contents for samples JD69 and JD82 are only slightly lower than the CO2 contents of the corresponding experimental melts; this suggests that the earliest hypabyssal phase of the Jericho kimberlite retained most of its original volatile content. As such these samples provide a minimum CO2 content for the primary kimberlite magmas from the Slave craton.