The Oxford Lake—Knee Lake greenstone belt, northwestern Superior Craton, Canada hosts an unconventional diamond occurrence within Neoarchean ultramafic volcaniclastic rocks. Unusual features of the diamond occurrence include its ancient emplacement age (2.73 Ga), nature of the host rocks and high microdiamond counts (> 11,500 recovered from 1800 kg of drill core). The diamonds show unusual physical characteristics relative to typical kimberlite-hosted diamonds, having small dimensions, a restricted size range, prevalent cuboid primary shapes and low levels of nitrogen aggregation. These characteristics, together with a statistically significant correlation between shape and nitrogen aggregation, point to a single, rapid growth event in a cool mantle substrate for most of the Knee Lake diamonds, driven by C-oversaturation of the growth medium. Pressure–temperature estimates of diamond formation from a non-touching garnet-olivine inclusion pair indicate the lithospheric nature of the diamonds and formation at conditions equivalent to a modern-day reference model geotherm of 38–39 mW/m2. This lithospheric column also contained eclogite, indicated by an omphacite inclusion in one diamond. The diamonds resided for a short time in a cool lithospheric mantle environment, possibly lower than 1050 °C at 5.5 GPa. These cool P–T conditions are difficult to achieve within > 150 km thick lithosphere without invoking slab stacking, a process supported by the C isotope composition of some of the diamonds. The Knee Lake diamond suite likely records cool transient thermal conditions inherited from subduction stacking of lithospheric slabs to form the lithosphere of the western Superior Craton.

Diamonds mined from the Victor kimberlite (Ontario, Canada) grew at 720 Ma in lherzolite-dominated substrates that themselves were generated ca. 400 My earlier by metasomatic overprinting of refractory harzburgites-dunites with proven Archean heritage. Since diamonds from Victor represent a restricted mantle sample (5.7 ± 0.2 GPa and 1129 ± 16 °C), we reanalyzed major- and trace-element compositions of 196 Cr-diopside xenocrysts from Victor to investigate the depth range, style and extent of 1100 Ma-old overprinting in the lithospheric section. Our approach uses down-the-geotherm projection and visualization of geochemical variability. We find that a Si-Al-Ca-Na enriched andesitic to dacitic metasomatic agent, likely derived from eclogite, interacted with previously depleted high Cr/Al harzburgites-dunites, driving their bulk compositions towards lherzolites and pyroxenites. Our data provides support for (i) substantive preservation of (garnet-absent) refractory Cr-spinel harzburgites-dunites at T < 740 °C, (ii) profound overprinting of peridotitic substrates, thereby establishing a refertilized garnet ± spinel lherzolite ± pyroxenite assemblage over the lithospheric section from 600 to 1240 °C, (iii) relicts of partially overprinted refractory mantle at T 1010 °C and T 1100–1150 °C, the latter coincident with lherzolite-hosted diamond mineralization, (iv) a discrete carbonatitic geochemical signal uniquely associated with diamond mineralization that resides, in part, in garnet wehrlite with refractory heritage, and (v) maximum metasomatic agent:substrate ratios over the 1000–1200 °C interval, producing high Na-Al, low-Cr clinopyroxene-garnet megacrysts. We infer that + 10 mm high-Cr clinopyroxene-ilmenite megacryst assemblages result from lower metasomatic agent:substrate ratios in the interval 860–1000 °C.

Thermobarometry of composite peridotitic mineral inclusions in De Beers Pool diamonds (Kimberley, South Africa) has yielded puzzling results. Most non-touching inclusions record higher temperatures than touching inclusions, but both types record conditions colder than the Kimberley xenolith geotherm. Scenarios previously proposed to explain this discrepancy (lithosphere cooling after diamond formation, cooling of discrete diamond-growth pathways by slab-derived fluids, and diamond formation under various thermal regimes) fail to fully account for the observed thermobarometric outcomes. We propose an alternative scenario based on elastic theory of inclusion–host systems, which reconciles the contrasting pressure–temperature (P–T) estimates. Forward model calculations show that P–T conditions similar to those estimated for the touching inclusions can result from the development of overpressures on the inclusions. Our model requires initial diamond formation under conditions colder than a 35-mW/m2 geotherm, followed by mantle uplift ( 60 km, possibly multi-stage) and reequilibration on the Late Cretaceous xenolith geotherm ( 40 mW/m2). The initial cold conditions could be promoted by foundering of shallow lithospheric materials. Consequent development of exsolution textures could favor entrapment of composite orthopyroxene–garnet inclusions in these early forming diamonds. The subsequent large uplift may be the result of Archean and possibly, in part, later tectonic events. The diamonds with the ‘warmer’ non-touching inclusions belong to one or more generations of uncertain age, which formed on a relaxed geotherm that was distinctly colder ( 37 mW/m2) than the xenolith geotherm. Our proposed scenario may offer a generic explanation for sporadic cases of ‘cold’ touching inclusions reported at other localities.

Here we report the nitrogen characteristics and composition of high-density fluid (HDF) trapped in micro inclusions in a suite of fibrous diamonds from the-142 Ma Chidliak CH-7 kimberlite pipe, the Hall Peninsula, southern Baffin Island, Nunavut. Within these diamonds, we observe three populations based on the chemistry of the encapsulated HDFs, the diamond's nitrogen aggregation states, and the diamond color. 'Chidliak C' diamonds contain highly silicic HDFs, have nitrogen in A-and C-centers (with 5-20% in C-centers), and a characteristic intense yellow color. 'Chidliak A' diamonds contain silicic to low-Mg carbonatitic HDFs, carry nitrogen solely in A-centers, and are mostly colorless. A third population, 'Chidliak B', has grey color and distinctive low-K2O silicic to low-Mg carbonatitic HDF compositions and overall smoother and less fractionated trace element pattern relative to 'Chidliak C' and 'Chidliak A' diamonds; they carry nitrogen in A-and B-centers (with-15% in B centers) and are characterized by a grey hue. An eclogitic paragenesis of all diamonds is evident by the HDF compositional variation as well as the presence of omphacitic clinopyroxene inclusions. The appearance of a diamond with A-and B-centers in its octahedral core and A-and C-centers in its coat suggests formation at two distinct events at a similar depth. Combined with pressure and mantle residence estimates based on nitrogen aggregation considerations, we argue that the three diamond populations formed at the relatively shallow region of the lithosphere (likely < 180 km) during distinct metasomatic events in the North Atlantic Craton (NAC) since the Proterozoic. The youngest event by silicic HDFs took place close in time to kimberlite activity at 142-157 Ma, as evident by the preservation of nitrogen C-centers in 'Chidliak C' diamonds. A link between this event and the mid-lithosphere discontinuity (MLD) in eclogitic portions of the cratonic lithosphere in Chidliak is plausible. The timing of 'Chidliak A' diamonds formation by more carbonatitic HDFs is less well constrained, but can be related to Ca-rich metasomatism observed in local peridotite xenoliths and/or alkaline magmatism between 610 and 550 Ma. A possible link between the formation of 'Chidliak B' diamonds and the timing of Mesoproterozoic olivine lamproite magmatism ca. 1400 Ma is suggested based on the HDF trace element composition and the aggregated nature of nitrogen in these diamonds. The nitrogen systematics and eclogitic source of the fibrous diamonds are comparable with those observed for previously studied gem-quality diamonds from Chidliak. We suggest that these similarities show a temporal connection and mutual crystallization of the two diamond types. This strengthens the involvement of HDFs in the formation of gem-quality diamonds.

More than 800 kimberlite bodies are known in Canada. Among these >70 kimberlites were recently discovered at Chidliak, a new diamond district on the Hall Peninsula of southern Baffin Island, Nunavut. Here we report the compositional variation of microinclusions carrying high-density supercritical fluids (HDFs) in a suite of diamonds from the ~142 Ma Chidliak CH-7 kimberlite pipe. The HDFs vary within the silicic to low-Mg carbonatitic composition array, which was attributed to an eclogitic mantle source. Within the studied diamonds, we observe three populations based on the chemistry of the encapsulated HDFs, the diamond’s nitrogen aggregation states, and their color. ‘Group Ib/IaA’ diamonds contain highly silicic HDFs, have nitrogen in A-and C-centers (with 5-20% C-center) and a characterizing intense yellow color. ‘Group IaA’ diamonds contain silicic to low-Mg carbonatitic HDFs, carry nitrogen solely in A-centers, and are mostly translucent. The third population, ‘Group IaAB’, has distinguished low-K 2 O silicic to low-Mg carbonatitic compositions and overall smoother and less fractionated primitive mantle normalized trace element pattern relative to Group Ib/IaA and IaA diamonds; they carry nitrogen in A-and B-centers (with ~15% B-centers) and are characterized by a gray hue. These differences indicate diamond formation during different metasomatic events, which we correlate with the various lithospheric extension episodes in the North Atlantic Craton (NAC) since the Proterozoic. Based on major and trace element similarities, we suggest

More than 800 kimberlite bodies are known in Canada. Among these, >70 kimberlites were recently discovered at Chidliak, a new diamond district on the Hall Peninsula of southern Baffin Island, Nunavut. Here we report the compositional variation of microinclusions carrying high-density fluids (HDFs) in a suite of diamonds from the ~142 Ma Chidliak CH-7 kimberlite pipe. The HDFs reflect diamond growth in mantle eclogite substrates and range in composition from silicic to low-Mg carbonatitic. Within the studied diamonds, we observe three populations based on the chemistry of the encapsulated HDFs, the diamond’s nitrogen aggregation states, and their color. ‘Chidliak C’ diamonds contain highly silicic HDFs, have nitrogen in A- and C-centers (with 5-20% in C-centers) and a characterizing intense yellow color. ‘Chidliak A’ diamonds contain silicic to low-Mg carbonatitic HDFs, carry nitrogen solely in A-centers, and are mostly colorless. The third population, ‘Chidliak B’, has distinctive low-K 2 O silicic to low-Mg carbonatitic HDF compositions and overall smoother and less fractionated primitive mantle normalized trace element pattern relative to ‘Chidliak C’ and ‘Chidliak A’ diamonds; they carry nitrogen in A- and B-centers (with ~15% in B-centers) and are characterized by a gray hue. These differences indicate episodic diamond formation during different metasomatic events, which we correlate with lithospheric extension episodes in the North Atlantic Craton (NAC) since the Proterozoic. Correlations of diamond host characteristics with gem-quality diamonds from the Hall Peninsula further strengthen the case for episodic diamond growth events, which we relate to the regional volcanic history. Based on major and trace element similarities, we suggest a genetic relation between the formation of ‘Chidliak B’ diamonds and ca. 1400 Ma metasomatism relating to olivine lamproites volcanism. The event in which ‘Chidliak A’ diamond formed could take place ca. 600 Ma, preceding aillikite/carbonatitic volcanism (590–555 Ma) and can be related to Ca-rich metasomatism that is also observed in local peridotite xenoliths. ‘Chidliak C’ diamonds likely grew during a metasomatic event close in time to kimberlite activity between 142-157 Ma. Considering available petrographic and experimental data, a link between this last event and the formation of a mid-lithosphere discontinuity boundary in the NAC is plausible.

Mark R. Muller, Alan G. Jones, Stuart Fishwick, Chris Hatton, Herman Grütter, Rob L. Evans, Xavi Garcia, Marion P. Miensopust, Mark P. Hamilton and the SAMTEX Team Dublin Institute for Advanced Studies, 5 Merrion Square, Dublin 2, Ireland University of Leicester, Department of Geology, University Road, Leicester, LE1 7RH, U.K. MSA Geoservices, Johannesburg, South Africa BHP Billiton, 6 Hollard Street, Johannesburg 2001, South Africa Woods Hole Oceanographic Institution, Department of Geology and Geophysics, Clark South 172, 360 Woods Hole Road, Woods Hole, Massachusetts, 02543-1542, U.S.A. EMGS, Stiklestadveien 1, N-7041 Trondheim, Norway

New Rb–Sr age determinations using macrocrystal phlogopite are presented for 27 kimberlites from the Ekati property of the Lac de Gras region, Slave Province, Canada. These new data show that kimberlite magmatism at Ekati ranges in age from at least Late Paleocene (∼61 Ma) to Middle Eocene time (∼45 Ma). Older, perovskite-bearing kimberlites from Ekati extend this age range to Late Cretaceous time (∼74 Ma). Within this age range, emplacement episodes at ∼48, 51–53, 55–56 and 59–61 Ma can be recognized. Middle Eocene kimberlite magmatism of the previously dated Mark kimberlite (∼47.5 Ma) is shown to include four other pipes from the east-central Ekati property. A single kimberlite (Aaron) may be younger than the 47.5 Ma Mark kimberlite. The economically important Panda kimberlite is precisely dated in this study to be 53.3±0.6 Ma using the phlogopite isochron method, and up to six additional kimberlites from the central Ekati property have Early Eocene ages indistinguishable from that of Panda, including the Koala and Koala North occurrences. Late Paleocene 55–56 Ma kimberlite magmatism, represented by the Diavik kimberlite pipes adjacent to the southeastern Ekati property, is shown to extend onto the southeastern Ekati property and includes three, and possibly four, kimberlites. A precise eight-point phlogopite isochron for the Cobra South kimberlite yields an emplacement age of 59.7±0.4 Ma; eight other kimberlites from across the Ekati property have similar Late Paleocene Rb–Sr model ages. The addition of 27 new emplacement ages for kimberlites from the Ekati property confirms that kimberlite magmatism from the central Slave Province is geologically young, despite ages ranging back to Cambrian time from elsewhere in the Slave Province. With the available geochronologic database, Lac de Gras kimberlites with the highest diamond potential are currently restricted to the 51–53 and 55–56 Ma periods of kimberlite magmatism.

Pioneering experimental investigations in simplified model peridotite compositions (Boyd and England, 1964; MacGregor, 1974) spawned the use of pyroxene compositions to constrain mantle pressures and temperatures, and hence geotherms (Boyd, 1973). Thermobarometric expansion of the simple-system pyroxene-garnet relations to deal with complex naturally-occurring compositions has involved substantial further experimental investigation (e.g. Nickel, 1989; Brey et al., 1990; Taylor, 1998) and semi-empirical thermodynamic analysis (e.g. Mercier, 1980; Brey and Köhler, 1990; Taylor, 1998) that has been presented as reasonably successful by some authors (e.g. Finnerty and Boyd, 1987; Smith, 1999), or problematic by others (e.g. Carswell, 1991). Nimis and Taylor (2000) recently calibrated a semi-empirical thermobarometer for Cr-diopside coexisting with garnet that appears to satisfy extant experimental and phasecompatibility constraints to within acceptable error for a wide variety of peridotitic bulk compositions over a broad P-T range. This new calibration presents an opportunity to empirically compare and contrast clinopyroxene-garnet thermobarometric results with those of orthopyroxene-garnet, for coexisting mineral pairs occurring in the same xenoliths. This abstract documents the initial results of such an investigation for kimberlite-borne peridotite xenoliths from Canadian localities. The results are also briefly compared with thermobarometric results for the Gibeon, Kimberley and North Lesotho kimberlite provinces in Southern Africa.

The diamondiferous Carolina kimberlite (Rondonia 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 1 (>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 degrees 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/m(2)), and a colder "Slave type" geotherm (38 mW/m(2)). 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, Crown Copyright (C) 2009 Published by Elsevier B.V. All rights reserved.

The natural remanent magnetisation of twenty Lac de Gras kimberlites has been correlated with their age as determined by isotopic dating techniques, and placed in a refined temporal context by comparison to a welldetermined geomagnetic polarity time scale. The data show clustering of economic kimberlite magmatism at 55.4 ± 0.5 Ma and 53.1 ± 0.3 Ma, but do not preclude other potentially economic intrusive episodes within the age range of 75 to 45 Ma. The known economic intrusive episodes show distinct remanent magnetic characteristics that are evident at the scale of detailed aeromagnetic data sets.

Exploration for diamond-bearing kimberlites in the Chidliak project area by Peregrine Diamonds has generated a grid-like till sampling pattern across four discrete areas of interest totalling 402 km2 that is densely populated with research-grade compositional data for 10,743 mantle-derived Cr-pyrope garnets. The available dataset is well suited to statistical analysis, in part due to the relatively unbiased spatial coverage. Previous workers showed empirically that the TiO2 and Mn thermometry (Ti-TMn) attributes of Cr-pyrope populations at the Chidliak project may serve as source-specific “fingerprints”. In this work, we employ a simplified version of the multivariate Mahalanobis distance technique to formally examine the variability of, and differences between, Ti-TMn attributes of Cr-pyrope subpopulations recovered from a Laurentide-age glaciated terrain that also contains 30 known kimberlites within the four areas of interest. We show the simplified Mahalanobis distance approach enables accurate discrimination of Cr-pyrope subpopulations with subtly to distinctly different Ti-TMn attributes, and permits proper demarcation of their respective kimberlite source(s), specifically in areas with straightforward glacial histories. Redistribution and blending of Cr-pyrope subpopulations from known kimberlite sources is also observed, and typifies areas at Chidliak with complex late-glacial histories. Our results support <1 km horizontal scale subtle to obvious variability in the proportions of TiO2-rich and high-temperature (> 1100 °C) Cr-pyropes between closely spaced kimberlite source(s) and also between physically adjacent magma batches within single kimberlite pipes. The local scale variability is attributed to protokimberlite fluid or melt interacting with, and metasomatizing discrete conduits within, the ambient diamond-facies peridotitic mantle at times closely preceding eruption of kimberlite magma batches at Chidliak.