Maya Kopylova
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Dissertations completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest dissertations.
My thesis puts forth new models for diamond formation that explain the difference between octahedral and fibrous diamond growth, as well as the difference between octahedral diamond growth in the lithospheric and the sublithospheric mantle. Diamond growth in the mantle involves reactions between carbon-bearing fluid and the host rocks it infiltrates. This fluid is sometimes included in diamond.Fluids in dendritically-grown, fibrous diamonds from Wawa, Superior craton, were analysed in a novel way, using transmission X-ray diffraction. The technique allows bulk analysis of daughter minerals within fluid inclusions. The mineralogy, major and trace elements, Sr isotopes, volatiles, and nitrogen characteristics of the hydrous saline–high-Mg carbonatitic fluid in these Archean diamonds strongly resemble those of Phanerozoic fibrous diamonds. This implies that some mantle processes, including the formation of fibrous diamonds, can be extended unvaryingly back to 2.7 Ga.Fluid equilibrated with octahedrally-grown diamonds from the Siberian, Kaapvaal, and Congo cratons is trapped in healed fractures in the diamonds. They contain anhydrous CO₂–N₂ fluid inclusions with 40±4 mol% N₂ and inclusions of former silicate melt that had an original N₂ content of ~0.1 wt%, as shown by Raman, electron microprobe, and microthermometry analyses. The liberation of N₂ from the convecting mantle is proposed to be controlled by increasing oxygen fugacity that destabilizes host phases.The observed distinct fluid compositions between hydrous fluids in fibrous and anhydrous fluids in octahedrally-grown diamond entail distinct processes of diamond formation that, ultimately, govern the growth habit. Water may trigger fibrous growth by inhibiting the expansion of {111} layers and lowering the interfacial energy between the diamond and fluid. Certain features in diamond fluids, such as Eu anomalies and potential carbonate–CO₂ isotopic fractionation, show that several mantle processes can produce geochemical signatures that may be mistaken as input from subducted materials. The finding of N₂ in diamond-forming fluids leads to an explanation for the characteristically low N content of sublithospheric diamonds. I propose this compositional trait is due to growth in a metal-saturated environment. Metallic Fe in the mantle below ~250 km should trap N and may be the largest mantle N reservoir.
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Diamonds can form in a number of different ways. Physical and chemical properties of diamonds classify them as formed below cratons (xenocrystal cratonic) or in a subducting slab followed by rapid exhumation (orogenic). I studied diamonds from a cratonic (Jericho kimberlite, Nunavut) and a synorogenic (calc-alkaline lamprophyres of Wawa, Ontario) setting to reconstruct the process of diamond formation. Diamonds from these two locations have been analysed for their morphology, nitrogen content and aggregation, cathodoluminescence, composition of mineral inclusions, and stable carbon isotopes. In addition, fluorescence and stable nitrogen isotopes were studied in Wawa diamonds. Mineral inclusions in Jericho diamonds were compared with diamondiferous and non-diamondiferous eclogitic Jericho xenoliths with respect to major and trace element compositions. The majority of Jericho diamonds belong to “eclogitic” (90% of the studied samples) and “websteritic” (7%) assemblages. The Jericho diamonds differ from “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. 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. Wawa diamonds show the following features typical for a cratonic origin: 1) weakly resorbed, octahedral morphology; 2) Low N content; 3) high N aggregation state; 4) the mantle signature of carbon isotopes. Other characteristics of the Wawa diamonds suggest a subduction-related origin, i.e. 1) the presence of peridotitic and eclogitic minerals within single diamonds in a mixed paragenesis also combining shallow and deep phases, 2) the crustal signature of nitrogen isotopes. The most viable model to explain the origin of Wawa diamonds involves early subduction of crustal carbon and nitrogen followed by the carbon-bearing mantle metasomatism and advection of the diamondiferous mantle to the shallow depth of the lamprophyre magmagenesis.
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This dissertation describes the volcanic facies, petrology and economic aspects of the diamondiferous Victor North kimberlite complex (Northern Ontario, Canada) using detailed drill core logging, petrographic observations, electron microprobe analysis, and physical volcanological calculations. This research project is aimed at improving our understanding of kimberlite emplacement models, as fragmentation and eruption mechanisms for these deposits are controversial.The results of this study show that the youngest kimberlite pipe (Victor Northwest) is filled by two similar eruption cycles. Each cycle starts with explosive crater-excavation forming predominantly pyroclastic deposits, followed by crater-filling with dark and competent rocks, and ends with volcanic quiescence resulting in formation of pipe wall collapse breccias and minor resedimented volcaniclastic kimberlite. Textural observations and eruption duration calculations suggest that the second crater-excavating eruption is phreatomagmatic in nature. This is based on the presence of fine-grained, well-mixed country rock fragment-rich, broken olivine-rich pyroclastic deposits containing small variably vesicular irregularly shaped juvenile pyroclasts as well as clastic pyroclasts. The crater-excavation stage is followed by formation of spatter-fed dark and competent clastogenic rocks. Evidence for a clastogenic origin includes the deposit morphology, presence of remnant pyroclasts, angular broken olivines, as well as the gradational nature of contacts with the enveloping pyroclastic units. Estimated eruption durations for each cycle range from days to months. The cross-cutting kimberlite pipe (Victor Main) comprises two macroscopically similar country rock fragment-poor pyroclastic kimberlites that have contrasting macro diamond sample grades. This study explains the variation in diamond grade within Victor Main by differential sampling of mantle material (incl. diamond) by two different magma batches that formed the high- and low-grade domains. Victor Main lacks textures indicative of phreatomagmatism, and the relatively long calculated phreatomagmatic eruption duration suggests that magmatic eruptions are most likely responsible for the formation of these deposits.This study concludes that, despite the generally more extreme range of physical properties of kimberlite melt, kimberlites erupt in a similar fashion as common basaltic-rhyolitic volcanoes and display a similarly diverse range of fragmentation processes and deposition styles. The geological and emplacement models presented here have broad economic implications for kimberlite exploration and mining.
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Master's Student Supervision
Theses completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest theses.
Research at the Kelvin kimberlite, NWT, is helping to define surface exploration practices and is testing new host rock lithogeochemical exploration tools that will result in reduced costs and improved discovery success. In regions where recent glaciation has buried kimberlites under glacial sediments, surface geochemical detection methods are best interpreted when coupled with a comprehension of the landscape formation processes. The glacial, post-glacial, and cryoturbation processes that have affected the landscape have, in turn, affected the dispersal of geochemical signatures in the till that can be detected and exploited by detailed surface mapping, sampling, and geochemical analysis.The Kelvin kimberlite is an inclined pipe that subcrops from metaturbidite country-rock beneath a lake. No indicator mineral train has been detected at Kelvin by traditional indicator mineral methods. Relative uniformity of surficial material (1km down-ice. Dry sonic sieving and four acid digestion results provide interpretations of geochemical partitioning and the ideal size fraction for geochemical sampling. Trace elements demonstrate systematically elevated concentrations in the fine and very fine silt fractions; however, background is higher and anomalous-to-background contrast is not enhanced compared to bulk -180 μm ratios. Elevated pathfinder concentrations in the fine to very fine sand fraction are attributed to fine kimberlite indicator minerals and their fragments, and display the best anomalous-to-background contrast.Additional research aims to detect mineralogical and metasomatic alteration signatures in the country-rock surrounding the Kelvin kimberlite in an attempted to develop exploration vectors. Hyperspectral scanning of drill core shows that the emplacement of the kimberlite did not produce pervasive mineralogical alteration of the surrounding country-rock. Evidence of alteration is limited to breccias and is vein-controlled, only extending a few meters from pipe contacts. Hyperspectral imaging and partial- and whole-rock lithogeochemical results do not detect evidence of pervasive alteration and preclude the identification of mineralogical or geochemical exploration vectors.
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Low-Cr and high-Cr clinopyroxene, garnet, olivine, and ilmenite megacrysts from the Muskox kimberlite (Nunavut, Canada) have been analyzed for major and trace elements, as well as Sr, Nd and Pb isotopes in an attempt to constrain the debated origin of the kimberlitic megacryst suite. Samples display compositional overlap with respective phases in websterite, while clinopyroxene Sr-Nd-Pb isotope systematics reveal similarities with both websteritic clinopyroxene and metasomatic clinopyroxene in peridotites from the same kimberlite, in addition to whole-rock isotope data for the Muskox and neighboring Jericho kimberlites. All studied lithologies may represent the products of mixing between EM1 mantle, locally restricted relic Proterozoic enriched mantle and HIMU carbonatitic fluid. Equilibrium melts calculated using clinopyroxene trace element data reaffirm a carbonatitic affinity of the metasomatic agent. Thermodynamic modeling using the Deep Earth Water model shows that megacryst mineral compositions cannot be produced through metasomatism of mantle peridotite by H₂O-rich kimberlitic, asthenospheric or eclogitic fluids. The Sr-Nd-Pb isotope systematics argue against a strictly cognate relationship between Muskox megacrysts and the host kimberlite. Our findings rather suggest megacrysts and websterites represent products formed through regional metasomatism by carbonatitic HIMU fluids that predate kimberlitic magmatism.
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This study constrains the nature of the kimberlite-xenolith reactions, melt temperatures in the crust, and the origin of the fluids in the Renard 65 kimberlite, a Kimberley-type pyroclastic kimberlite (KPK) by focusing on the reaction textures and mineralogy assimilated felsic xenoliths. The kimberlite pipe is emplaced into granitoid and gneiss of the Superior Craton and is infilled by pyroclastic and hypabyssal kimberlite. The zonal mineralogical and textural changes that resulted from reactions with the host kimberlite were characterized by petrography, bulk composition, conserved element ratio analysis, and thermodynamic Perple_X modelling. To better replicate the observed mineral assemblages, the Perple_X database was extended to include new thermodynamic data on pectolite, calculated and measured using density functional theory methods. Theoretical models accurately reproduced the zoning at the kimberlite-xenoliths contacts and correctly predicted the distinct mineralogy in reacted granitoid and gneiss. The assimilation of xenoliths is a process that starts at high-temperatures (1200–600 ºC) with the formation of clinopyroxene and wollastonite, continues at 400–200 ºC with the further growth of clinopyroxene and the formation of garnet and phlogopite, and finishes at temperatures
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Diamonds and their inclusions from the Lace kimberlite (288 crystals ≤ 0.2 ct) are studied in order to constrain their origin as well as the thermal state and thickness of the ancient lithospheric mantle beneath the Lace kimberlite (Kaapvaal craton). I present the first study on diamonds from the Lace, a Cretaceous kimberlite that belongs to the Kroonstad Group II Kimberlite kimberlite cluster in South Africa. The study is based on optical microscopy, diamonds polishing, Raman and infrared spectroscopy, electron microscopy, U-Pb and Hf geochronology, analyses of C isotopes and trace elements.The studied Lace diamonds are colorless, yellow, brown and off-white and have dodecahedral (n=139), octahedral (n=125), undetermined (n=22) and hemimorphic (n=2) morphology. Infrared spectroscopy classified the diamonds into Type IIa (13) and Type I (73), with 13 to 1199 ppm N. B-centre N aggregation ranges from 2 to 100 % IaB for 44 diamonds, while the remaining 29 stones do not contain B-aggregated N. Lace diamonds (n=19) formed from mantle carbon (– 9.1 to – 2.5 ‰ δ¹³C), with an exception of one eclogitic diamond (– 19.2 ‰) which originated from subducted organic carbon. The eclogitic diamond paragenesis (n=19, 44%) includes garnet, omphacite, kyanite, coesite, K-feldspar and zircon; the peridotitic paragenesis (n=15, 35%) includes forsterite and enstatite and the websteritic paragenesis (n=4, 9%) comprises orthopyroxene, omphacite, garnet and K-feldspar. The difference between the parent diamondiferous rocks below adjacent pipes Lace and Voorspoed kimberlites suggest that diamond formation below this Kaapvaal block of the Kaapvaal craton of the Kaapvaal craton was a spatially restricted process. Equilibration temperatures of Lace peridotitic diamonds were 940 – 1359 °C at 46 – 80 kbar. Lace eclogitic diamonds formed at 1000-1358 °C at 51-79 kbar and websteritic diamonds at 1000-1190 °C at 51-66 kbar. The thermobarometry constrained the maximum heat flow under the Lace kimberlite at 38 mW/m² and the minimum lithospheric thickness of 240 km. The zircon inclusion ²⁰⁶Pb/²³⁸U geochronology yielded an age of 188±37 Ma and a Hf model age of 2-2.6 Ga which constrains the Archean eclogitic formation age and a potential later metasomatic event in the Mesozoic.
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I studied the petrography, major element chemistry, geothermobarometry, and trace element chemistry of 19 eclogite xenoliths from Chidliak kimberlites (Baffin Island, Nunavut, Canada). These granoblastic, partially melted, mostly bi-mineralic eclogites are composed of pyrope and omphacite, with accessory orthopyroxene, kyanite, and rutile, as well as secondary chlorite, serpentine, phlogopite, amphibole, and spinel. The Chidliak eclogites are classified in the Coleman’s groups A, B, and C based on the garnet composition, and into groups A and B based on the clinopyroxene composition. Parameters of the eclogite formation were calculated by projecting clinopyroxene-garnet temperatures onto a geotherm constrained using Chidliak peridotite xenoliths. Thermobarometry of Chidliak eclogites yields temperatures of 855-1390°C and pressures of 45 70 kbar, i.e. in the diamond stability field. The temperatures may have been overestimated by 50-120°C due to the inability to account for Fe³⁺ in clinopyroxene and garnet. Chidliak eclogites come from the depths similar to those for eclogites of the Slave craton. Whole rock major and trace element composition was reconstructed based on the compositions of clinopyroxene, garnet, and accessory minerals analyzed on the electron microprobe and laser ablation inductively coupled plasma mass spectrometer. Whole rock major element composition of the eclogites suggests that their protoliths could be Archean basalts or oceanic gabbros, modified by partial melting and carbonatitic mantle metasomatism. Metasomatism was responsible for the introduction of Mg into Chidliak eclogites with MgO>16.5 wt. % and the high Zr/Hf values that correlate with the high MgO bulk content. Based on REE and HFSE concentrations, Chidliak eclogites were divided into several groups that show evidence for: 1) the origin in a shallow, low-P, garnet-free source protolith, 2) partial melting and extraction of arc melts, 3) cumulation of plagioclase, and 4) ancient mantle metasomatism by alkali and carbonatitic fluids. I propose two possible tectonic settings for the formation of the studied eclogites: 1) metamorphosed mid-ocean ridge gabbros subducted under the Hall Peninsula Block during the 1.8-1.9 Ga Trans-Hudson orogeny, or alternatively 2) oceanic plateau basalts metamorphosed to Archean greenstone belts of the North Atlantic Craton that sank to the mantle via delamination of the lower crust.
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The Renard 65 pipe is located in the Otish Mountains, Quebec, Canada. It is one of nine diamondiferous kimberlite pipes in the ~ 640 Ma Renard cluster and is the largest of four pipes in the Renard Mine reserve. Detailed characterizations of the petrographic and compositional features of these pipe-infilling kimberlite rock types supports their classification into three geological units: Kimb65a, Kimb65b, and Kimb65d. These pipe-infilling kimberlites are interpreted to represent the solidified products of two separate magmatic events: Phase A containing Kimb65a, and Phase B containing Kimb65b and Kimb65d. This research demonstrates that the interclast matrix modal mineralogy (diopside + phlogopite + serpentine) in pyroclastic rock types in the Renard 65 kimberlites are inconsistent with origins by hydrothermal alteration involving hydrous meteoric fluids. Detailed investigation of the reactions between granitic and gneissic crustal xenolith lithologies and their host kimberlites, suggests that reactions occur at both magmatic and subsolidus temperatures involving significant volumetric proportions of xenoliths. The assimilation of crustal xenoliths, and contamination of the kimberlite magmas primarily by Si, are demonstrated to result in enhanced degassing of magmatic volatiles during emplacement and stabilization of the hybrid groundmass assemblage diopside + phlogopite + serpentine over the non hybrid groundmass assemblage calcite + phlogopite + serpentine. It is thus interpreted that the spatial distribution of transitional to Kimberley-type pyroclastic kimberlite rock types, which are characterized by diopside-rich and calcite-poor matrix assemblages as observed in the Renard 65 pipe and other similar pipes, is a function of crustal xenolith distribution in the magma during emplacement. This model not only accounts for the features of Kimberley-type pyroclastic kimberlite rock types, but also the spatial distribution of these rock types in numerous pipes which is often not consistent with lateral textural gradations as has been previously proposed. These results further indicate that the different mineralogy and textures of Fort-à-la-Corne-type pyroclastic kimberlites with respect to Kimberley-type pyroclastic kimberlites may be a consequence of not only the structural controls imparted by the host rock lithology with implications for emplacement-related processes, but also the absence of contamination of the magma by silicic crustal xenoliths. Supplementary video material is available at: http://hdl.handle.net/2429/60339
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Alluvial diamonds from the Kasai River, Katanga Province, Democratic Republic of the Congo (DRC), were studied in order to constrain the composition, thermal state, and diamond forming fluids of the ancient lithospheric mantle of the Congo craton. The diamonds originate from Cretaceous kimberlites of the Lucapa graben in northeastern Angola. We report carbon isotope compositions (δ¹³CVPDB), nitrogen concentrations ([N]), and nitrogen aggregation states of 138 diamonds, as well as compositions of mineral and fluid inclusions in the diamonds. Diamonds emplaced by kimberlites of the northeastern Lucapa graben and eroded into alluvials along the Kasai river contain 25–2900 ppm [N], show 0–88% N aggregation and δ¹³C isotopic compositions spanning -27‰ to -2‰ with a mode near mantle-like values. In situ cathodoluminescence (CL), secondary ion mass spectrometry (SIMS) and Fourier transform infrared spectroscopy (FTIR) reveal large heterogeneities in [N], N aggregation and δ¹³C, indicating diamonds grew episodically from fluids of distinct sources. Fluid inclusion compositions of fibrous diamonds analyzed by electron probe microanalysis are moderately to highly silicic, matching compositions of diamond-forming fluids from other DRC diamonds. Regional homogeneity of Congo fibrous diamond fluid inclusion compositions suggests spatially extensive homogenization of Cretaceous diamond forming fluids within the Congo lithospheric mantle. Electron probe microanalysis (EPMA) of trapped silicate inclusions revealed both peridotitic (Fo₉₁₋₉₅ and En₉₂₋₉₄, 78% of the suite) and eclogitic parageneses (Cr-poor pyrope and omphacite with 11–27% jadeite, 17% of the suite) within diamonds (11% remainder unknown). Clinopyroxene-garnet thermobarometry suggest diamond formation at 1350–1375 °C, whereas [N] aggregation thermometry yields diamond residence temperatures between 1000 and 1275 °C, if the assumed residence time is 0.9–3.3 Ga. Integrated geothermobaromtery indicates heat fluxes of 41–45 mW/m² during diamond formation and a shallow lithosphere-asthenosphere boundary (LAB) 175–189 km. The shallow LAB may result from a higher than average cratonic geotherms and the position of the Kasai block near the Congo cratonic margin. The hotter mantle may be attributable to contemporaneous rifting of the southern Atlantic, multiple post-Archean reactivations of the craton, and/or proximal Cretaceous plumes.
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Petrography, mineralogy and thermobarometry are reported for 53 mantle-derived xenoliths from the Muskox kimberlite pipe in the northern Slave craton. The xenolith suite includes 15% coarse spinel peridotite, 4% coarse spinel-garnet peridotite, 4% coarse garnet peridotite, 9% porphyroclastic peridotite, 60% websterite and 8% orthopyroxenite. Peridotites are composed primarily of forsteritic olivine (Fo 89-94), enstatite (En 89-94), Cr-diopside, Cr-pyrope garnet and chromite spinel. Pyroxenites are composed primarily of enstatite (En 90-92), Cr-diopside and Cr-pyrope garnet. Thermobarometric estimates were made using two-pyroxene, garnet-clinopyroxene and Ca-in-orthopyroxene thermometers and garnet-orthopyroxene barometer. Results suggest that coarse peridotites equilibrated at 650-1220 °C and 23-63 kbar; porphyroclastic peridotites equilibrated at 1200-1350 °C and 57-70 kbar; pyroxenites equilibrated at 1030-1230 °C and 50-63 kbar. Muskox xenoliths are compared with xenoliths recovered from the neighboring Jericho kimberlite, erupted 15 km away from and at the same time as Muskox. Contrasts in the characteristics of these two suites of mantle samples include: 1) higher levels of depletion throughout the Muskox mantle column based on the contents of MgO in olivine and orthopyroxene and Cr₂O₃ in garnet; 2) the presence of a shallow zone of metasomatism in the spinel stability field in the Muskox mantle; 3) a higher proportion of pyroxenitic versus peridotitic rock types at the base of the mantle column beneath the Muskox kimberlite and higher Cr₂O₃ in all minerals in pyroxenites; and 4) lower levels of 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 kimberlite formation and ascent. If percolation of asthenosphere-derived pre-kimberlitic fluids in the less permeable Muskox mantle was impeded, localization of this fluid may have resulted in higher proportions of pyroxenitic rock types here, as well as lower degrees of deformation of the peridotitic mantle.
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A total of 109 eclogite xenoliths from the Jericho and Muskox kimberlites (Nunavut, Canada) were studied petrographically and mineralogically to constrain their depth distribution within the Northern Slave mantle. The eclogites are dominated by pyrope-almandine and omphacite with accessory rutile, apatite and olivine. Garnet-clinopyroxene thermobaromtry suggests that Northern Slave eclogites formed at 670 -1300 °C and 25 – 70 kbar. Eclogites were classified into Group A, B, or C based on mineral composition and into massive and foliated textural types. Group A Northern Slave eclogites may have formed as cumulates from mantle mafic melts, whereas Group B and C eclogites are interpreted as modified subducted oceanic crust. All Northern Slave eclogites were subjected to partial melting and recrystallization, which produced secondary high-MgO garnet and clinopyroxene, phlogopite, amphibole carbonates and spinel group minerals. The recrystallization was caused by an influx of carbonatitic and hydrous hot fluid. The most recent heating event immediately predating kimberlite eruption resulted in garnet and clinopyroxene zoning. Diamondiferous eclogites from the Northern Slave are always massive and belong mostly to Group A. The majority of diamondiferous eclogites from the Northern Slave occur at shallower depths than those from the Central Slave craton. The criteria for distinguishing diamondiferous eclogites based on high Na₂O content in garnet and high K₂O content in clinopyroxenes can be applied only to Muskox eclogites. The high Mg content in both garnet and clinopyroxene best distinguishes the diamondiferous eclogites from Jericho. A model with multiple subducted slabs of oceanic crust below the Slave craton is proposed. The deepest subducted slab (190 – 210 km) dated at 1.88 – 1.84 Ga below the Central Slave extends to shallower depths of 170 – 185 km below the Northern Slave. Another slab (1.95 – 1.91 Ga) that occurs at 140 – 160 km below the Central Slave may extend to the north where it becomes progressively thicker from imbrication. The shallowest (120 – 130 km) and oldest (2.67 – 2.6 Ga) slab occurs only below the Northern Slave. Eclogites of mantle origin formed in mafic magma chambers, which existed only below the Northern Slave at 135 – 150 km depths.
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Mineral inclusions in non-fibrous and fibrous diamonds from an Archean metaconglomerate deposit in Wawa, Ontario, Southern Superior craton were studied to characterize the compositional and thermal state of the lithospheric mantle from the Archean to present day. Electron microprobe analysis of Wawa non-fibrous diamonds shows large inclusions of Cr-pyrope, Mg-chromite, olivine, and enstatite indicating harzburgitic parent rock. Wawa fibrous diamonds host microinclusions of pyrope and olivine of predominantly lherzolitic assemblage. Thermobarometry calculations for non-fibrous diamonds yield temperatures and pressures consistent with formation in a cool, cratonic root reaching to a minimum depth of 190 km with a geotherm between 39-41 mW/m², located beneath the Southern Superior province during the Archean. Comparison to results from xenoliths in nearby post-Archean kimberlites, and to modern geophysics, indicates heating and thinning of the cratonic root. This effectively destroyed the diamondiferous portion of the lithospheric mantle, as early as 1.1 Ga in some areas of the Southern Superior, through tectonic erosion during amalgamation of terranes to the protocraton. Diamond inclusion analysis for Wawa fibrous diamonds and datasets for non-fibrous and fibrous diamonds from Diavik, Ekati (Panda kimberlite), and Koffiefontein (South Africa) reveal metasomatic trends of mantle rock evolution due to the influx of K-rich hydrous carbonatitic fluid related to fibrous diamond precipitation. Thermometry for fibrous diamond inclusions yields temperatures of 580-1030°C. Low formation temperatures, paired with the alkali-rich and hydrous nature of the metasomatic agent, result in subsolidus diamond growth in the absence of melting or thermal disturbance of the mantle. Fibrous diamond growth, previously linked to kimberlite generation, may be a temporally distinct and genetically independent event, as suggested by long mantle residence times for fibrous diamonds and contrasting chemistry of fibrous diamond fluid and kimberlites. This would make metasomatism associated with formation of fibrous diamonds a “cratonic root-friendly” process that would not have played any part in the destruction of the Southern Superior lithospheric root.
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I studied a diamondiferous Archean meta-conglomerate from Wawa (North Ontario), part of the Michipicoten Greenstone Belt (MGB) of the Superior Craton. Field observations determined the 170 m thick meta-conglomerate displays poorly sorted, matrix to clast supported and massive to bedded textures. Petrographic and SEM analyses determined it was metamorphosed in the greenschist facies.Twenty-four clast types were identified and classified into groups: igneous with subophitic texture; coarse-grained felsic; vesicular igneous; porphyritic mafic; porphyritic felsic; untextured volcanic; chert-like, unidentifiable clasts rich in chlorite, and opaques. The pebble - cobble sized clasts are derived from nearby meta-volcanic and meta-sedimentary rocks. Predominance of local volcanic clasts confirms the meta-conglomerate formed as a Timiskaming type deposit, between 2700.4±1.0 Ma and 2697.2±1.8 Ma. 383 diamonds (
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