Kimberlite Magma Research Articles

New constraints on archetypal South African kimberlite petrogenesis from quenched glass-rich melt inclusions in olivine megacrysts

The evaluation of primary kimberlite compositions is hindered by significant melt modifications during ascent through the lithosphere by entrainment of xenolithic material, volatile degassing, and near surface alteration. Consequently, hypabyssal kimberlite emplaced in the upper crust may not provide a true reflection of the primary kimberlite magma. This contribution places new constraints on kimberlite melt composition by providing an assessment of quenched glass-rich polymineralic melt inclusions hosted in olivine megacrysts from the Monastery kimberlite, South Africa. Melt inclusions predominantly contain variable proportions of euhedral or skeletal grains of calcite, phlogopite, spinel, perovskite, serpentine, and fresh to devitrified glass. Estimates of the bulk compositions of melt inclusions, and the compositions of crystalline phases present therein, are compatible to those of hypabyssal kimberlites worldwide and show a volatile-rich (CO2 + H2O ~10–17 wt%) carbonated silicate (SiO2 ~27–41 wt%) composition. The glass component has a Si-Mg-Fe-rich and largely CaO-, K2O- and TiO2-free major element composition and is REE-depleted. It also contains approximately 10 wt% H2O but is CO2−free. The glass represents a residual melt that existed after crystallization of the observed mineral assemblage. From some, but not all melt inclusions, apophyses radiate outwards. These fractures typically contain partially devitrified glass that is compositionally identical to the fresh residual glass within the melt inclusions, indicating fracture formation during decompression of the hosting megacryst and at a stage after the melt had evolved. These features are consistent with a trapping of the melt inclusions at high pressure, prior to kimberlite ascent to the surface, in the SCLM at a depth corresponding to 4.5–6 GPa. Textures and compositions of phases within the melt inclusions represent stages of the kimberlite melt and magma evolution. They provide evidence in support of high-pressure experimental studies suggesting a carbonated silicate primary melt rather than a carbonatite. Furthermore, the composition of fresh glass in the melt inclusions, which is compositionally similar to serpentine, suggests that much groundmass serpentine in hypabyssal kimberlites may have formed from similar silicate melt or devitrified glass.

Petrogenesis of lamprophyres synchronous to kimberlites from the Wajrakarur kimberlite field: Implications for contrasting lithospheric mantle sources and geodynamic evolution of the eastern Dharwar Craton of southern India

Kimberlite field is an example of widespread Mesoproterozoic intracontinental magmatism. Recent studies have identified deep subcontinental lithospheric mantle as a source region of the kimberlite magmatism while timing, origin, and processes responsible for the generation of coeval lamprophyres remain poorly constrained. Here, we present and discuss new petrological and geochemical data for two lamprophyre dykes from the Wajrakarur kimberlite field and assess their petrogenetic relation to the kimberlite occurrences. Based on mineral compositional and whole‐rock geochemical characters, it is suggested that lamprophyres are formed through low degrees of partial melting of “enriched” lithospheric mantle that was modified and metasomatized by melts derived from recycled crust. This differs from geochemical imprints found in coeval kimberlites, where a crustal source component appears to be absent and is more consistent with rock derivation from “depleted” lithosphere which has experienced interaction with asthenosphere‐derived melts. An apparent lack of garnet in the mantle sources of lamprophyres is suggestive of melting at comparatively shallow depth (~100 km) relative to the kimberlites. Hence, these geochemically contrasting rocks, although have formed at the same time, are derived from vertically heterogeneous lithospheric mantle sources and can be explained through and linked with a thermal anomaly in the underlying convective asthenosphere. We suggest that the deeper mantle source region of the kimberlites was more pristine and devoid of subduction‐related signatures, whereas the shallower mantle source region of the lamprophyres seems to have preserved imprints of plate convergence and subduction associated with the evolution of the Dharwar Craton.

Diamond destruction and growth during mantle metasomatism: An experimental study of diamond resorption features

Most diamonds found in kimberlites show complex patterns of growth and dissolution (resorption) surface features. Populations of diamonds from within single kimberlite bodies commonly contain a large diversity of diamond surface forms, some of which are a result of dissolution in kimberlite magma and others are inherited from the mantle. Morphological studies of natural diamonds differentiated features produced during dissolution in kimberlite magma and during mantle metasomatism. The former features were experimentally reproduced at 1–3 GPa and used to infer the presence and composition of magmatic fluid in different kimberlites. However, the mantle-derived resorption features have not been reproduced experimentally and the composition and origins of their formative solvents are unknown. Here we report the results of diamond dissolution experiments conducted in a multi-anvil apparatus at 6 GPa and 1200 to 1500°C in synthetic CaO–MgO–SiO2–CO2–H2O system. The experiments produced very different diamond resorption morphologies in COH fluid, in silicate-saturated fluid, and in silicate and carbonate melts. Dissolution in SiO2-free COH fluid developed rounded crystal forms with shallow negative trigons, striations and hillocks, which are commonly observed on natural diamonds and are similar in 6 GPa and in 1–3 GPa experiments. However, silicate-saturated fluid produced very different resorption features that are rarely observed on natural diamonds. This result confirms that natural, SiO2-poor fluid-induced resorption develops under the comparatively low-pressures of kimberlite ascent, because at mantle pressures the high content of SiO2 in fluids would produce features like those from the silicate-saturated experiments. Comparison of the experimental products from this study to natural diamond resorption features from the literature suggests that natural diamonds show no record of dissolution by fluids during mantle metasomatism. Diamond resorption morphologies developed in experiments with silicate–carbonate melts closely resemble many of the mantle-derived resorption features of natural diamonds, whose diversity can result from variable SiO2 concentration in carbonatitic melts and temperature variation. The experimental results imply that metasomatism by fluids does not dissolve diamond, whereas metasomatism by melts is diamond-destructive. The repetitive growth-dissolution patterns of natural diamonds could be due to diamond growth from fluids in harzburgitic lithologies followed by its dissolution in partial melts.

Composition and emplacement of the Benfontein kimberlite sill complex (Kimberley, South Africa): Textural, petrographic and melt inclusion constraints

The Benfontein kimberlite is a renowned example of a sill complex and provides an excellent opportunity to examine the emplacement and evolution of intrusive kimberlite magmas. We have undertaken a detailed petrographic and melt inclusion study of the Benfontein Upper, Middle and Lower sills. These sills range in thickness from 0.25 to 5 m. New perovskite and baddeleyite U/Pb dating produced ages of 85.7 ± 4.4 Ma and 86.5 ± 2.6 Ma, respectively, which are consistent with previous age determinations and indicate emplacement coeval with other kimberlites of the Kimberley cluster.The Benfontein sills are characterised by large variations in texture (e.g., layering) and mineral modal abundance between different sill levels and within individual samples. The Lower Sill is characterised by carbonate-rich diapirs, which intrude into oxide-rich layers from underlying carbonate-rich levels. The general paucity of xenogenic mantle material in the Benfontein sills is attributed to its separation from the host magma during flow differentiation during lateral spreading. The low viscosity is likely responsible for non-explosive emplacement of the Benfontein sills, while the rhythmic layering is attributed to multiple magma injections.The Benfontein sills are marked by the excellent preservation of olivine and groundmass mineralogy, which is composed of monticellite, spinel, perovskite, baddeleyite, ilmenite, apatite, calcite, dolomite along with secondary serpentine and glagolevite [NaMg6[Si3AlO10](OH,O)8•H2O]. This is the first time glagolevite is reported in kimberlites. Groundmass spinel exhibits atoll-textures and is composed of a magnesian ulvöspinel – magnetite (MUM) or chromite core, surrounded by occasional pleonaste and a rim of Mg-Al-magnetite. We suggest that pleonaste crystallised as a magmatic phase, but was resorbed back into the residual host melt and/or removed by alteration.Analyses of secondary inclusions in olivine and primary inclusions in monticellite, spinel, perovskite, apatite and interstitial calcite are largely composed of Ca-Mg carbonates and, to a lesser extent, alkali-carbonates and other phases. These inclusions probably represent the entrapment of variably differentiated parental kimberlite melts, which became progressively more enriched in carbonate, alkalis, halogens and sulphur during crystal fractionation. Carbonate-rich diapirs from the Lower Sill contain more exotic phase assemblages (e.g., Ba-Fe titanate, barite, ancylite, pyrochlore), which probably result from the extreme differentiation of residual kimberlite melts followed by physical separation and isolation from the parental carbonate-rich magma. It is likely that any alkali or halogen rich minerals crystallising in the groundmass were removed from the groundmass during syn−/post-magmatic alteration, or in the case of Na, remobilised to form secondary glagolevite. The Benfontein sill complex therefore provides a unique example of how the composition of kimberlites may be modified after magma emplacement in the upper crust.

Isopycnicity of cratonic mantle restricted to kimberlite provinces

The isopycnicity hypothesis states that the lithospheric mantle of ancient platforms has a unique composition such that high density due to low lithosphere temperature is nearly compensated by low-density composition of old cratonic mantle. This hypothesis is supported by petrological studies of mantle xenoliths hosted in kimberlite magmas. However, the representativeness of the kimberlite sampling may be questioned, given that any type of magmatism is atypical for stable regions. We use EGM2008 gravity data to examine the density structure of the Siberian lithospheric mantle, which we compare with independent constraints based on free-board analysis. We find that in the Siberian craton, geochemically studied kimberlite-hosted xenoliths sample exclusively those parts of the mantle where the isopycnic condition is satisfied, while the pristine lithospheric mantle, which has not been affected by magmatism, has a significantly lower density than required by isopycnicity. This discovery allows us to conclude that our knowledge on the composition of cratonic mantle is incomplete and that it is biased by kimberlite sampling which provides a deceptive basis for the isopycnicity hypothesis.

The evolution of the crustal stress state of the Catoca kimberlite pipe area (northeastern Angola)

This paper presents the first results of the geostructural and tectonophysical studies of the crustal stress state in the Catoca kimberlite pipe area at the southwestern flank of the Kasai Shield in the northeasternAngola. In the evolution of the crustal stress state, six main stages are distinguished by analyzing the displacements of markers, fold hinges, long axes of boudins, granite dikes of various intrusion phases and kimberlites, as well as fractures with striations. For each of these stages, a dominating horizontal tectonic stress and its orientation is identified. During stage 1 (NW extension and shearing) and at the beginning of stage 2 (NW compression), structures formed in the host rocks in brittle-plastic conditions. The replacement of plastic deformation by faulting could occur about 530–510 Ma ago, when the continental crust ofAfricahad completely formed. Stage 3 (radial, mainly NW extension) and stage 4 (shearing, NW extension, and NE compression) were the most important for kimberlite occurrence: in the Early Cretaceous, radial extension was replaced by shearing. Both stages are related to opening of the central segment of theSouth Atlantic. The main kimberlite magmas occurred during the break-up of the Angola-Brazilian segment of Gondwana. In the course of all the four stages, stress was mainly released by the NE- and E-NE-striking faults and, to a lesser extent, by the NW-striking and latitudinal faults. The initial stage of kimberlite magmatism is associated with the NE- and E-NE-striking faults due to the presence of the Precambrian zones of flow and schistosity, which facilitated the NW-trending subhorizontal extension. Stage 5 (NE compression) began in the second half of the Cretaceous and possibly lasted until the end of the Paleogene, and compression occurred mainly along the NW-striking faults. Regionally, it corresponds to two stages of inversion movements in the southern regions of Africa, during which theAngoladome-shaped uplift emerged and the shoulders of the East African rifts began to take shape. Stage 6 (horizontal extension, mainly in the N-NE direction) is related to the processes that took place in the southern segment of theTanganyikarift and the eastern coast of theAtlantic. Based on the results of our studies, it became for the first time possible to get an idea of the main stages in the evolution of the studied region. Further geostructural measurements and dating of the host rocks will provide for a more precise definition of the proposed stages.

Type IIa Diamond Formation

The petrological model of type IIa diamond formation is proposed. By the model these diamonds were formed from fluids on the last stages of kimberlite magma formation at 600–700°C and in the range of 20–30 kbar. The model explains the main specific of the type IIa diamonds. They are the enrichment of light carbon, low nitrogen content, the absence of silicate inclusions, the large size and cleanliness, a high degree of desorption.

Transformation of the chemical composition of surface waters in the area of the exploited Lomonosov diamond deposit (NW Russia).

The specific objective of this study was to investigate the changes in the chemical composition of river waters during the exploitation of the Lomonosov diamond deposit and the danger of these changes for the ichthyofauna. It was found that the Ca-HCO3 composition of river water both upstream and downstream from the quarry was almost identical before discharge of the drainage waters into the river. In subsequent years, the water downstream from the quarry acquired a Na-HCO3 composition, and then a Na-HCO3-Cl composition and TDS increased by 2.5 times. With respect to Fe, Mn, and Mo, concentrations that are above the maximum permissible concentrations (MPCs) for fishery rivers are apparent. At the same time, elevated Fe and Mn concentrations are associated with the natural composition of river water. The negative influence of drainage waters is manifested only with respect to the high concentrations of Mo. An important role in increasing Mo concentrations in drainage waters is played by the processes of hydrolysis of sodium aluminosilicates and mixing of fresh water with salt water. The concentrations of Sr, B, Ba, V, and Cr in drainage waters are higher than those in surface waters. However, they generally do not exceed the concentrations of the current MPCs. The source of Cr, Ba, Ni, and V in the drainage waters can be the products of the kimberlite magmatism. The possible impacts of metals effects on fish are presented.

Kimberlite emplacement and mantle sampling through time at A154N kimberlite volcano, Diavik Diamond Mine: lessons from the deep

The Diavik Diamond Mine in the NWT of Canada has produced in excess of 100 million carats from 3 kimberlite pipes since mining commenced in 2002. Here, we present new findings from deep (>400 m below surface) mining, sampling and drilling work in the A154N kimberlite volcano that require a revision of previous geological and emplacement models and provide a window into how the sub-continental lithospheric mantle (SCLM) below Diavik was sampled by kimberlite magmas through time. Updated internal geological models feature two volcanic packages interpreted to represent two successive cycles of explosive eruption followed by active and passive sedimentation from a presumed crater-rim, both preceded and followed by intrusions of coherent kimberlite. Contact relationships apparent among the geological units allow for a sequential organization of as many as five temporally-discrete emplacement events. Representative populations of mantle minerals extracted from geological units corresponding to four of the emplacement events at A154N are analyzed for major and trace elements, and provide insights into the whether or not kimberlites randomly sample from the mantle. Two independent geothermometers using clinopyroxene and garnet data indicate similar source depths for clinopyroxenes and G9 garnets (130–160 km), and suggest deeper sampling with time for both clinopyroxene and garnets. Harzburgite is limited to 110–160 km, and appears more prevalent in early, low-volume events. Variable ratios of garnet parageneses from the same depth horizons suggest random sampling by passing magmas, but deeper garnet sampling through time suggests early preferential sampling of shallow/depleted SCLM. Evaluations of Ti, Zr, Y and Ga over the range of estimated depths support models of the SCLM underlying the central Slave terrane.

Punctuated, long-lived emplacement history of the Renard 2 kimberlite, Canada, revealed by new high precision U-Pb groundmass perovskite dating

Kimberlites are rare volatile-rich ultramafic magmas thought to erupt in short periods of time (<1 Myr) but there is a growing body of evidence that the emplacement history of a kimberlite can be significantly more protracted. In this study we report a detailed geochronology investigation of a single kimberlite pipe from the Renard cluster in north-central Quebec. Ten new high precision ID-TIMS (isotope dilution – thermal ionization mass spectrometry) U-Pb groundmass perovskite dates from the main pipe-infilling kimberlites and several small hypabyssal kimberlites from the Renard 2 pipe indicate kimberlite magmatism lasted at least ~20 Myr. Two samples of the main pipe-infilling kimberlites yield identical weighted mean 206Pb/238U perovskite dates with a composite date of 643.8 ± 1.0 Myr, interpreted to be the best estimate for main pipe emplacement. In contrast, six hypabyssal kimberlite samples yielded a range of weighted mean 206Pb/238U perovskite dates between ~652-632 Myr. Multiple dates determined from these early-, syn- and late-stage small hypabyssal kimberlites in the Renard 2 pipe demonstrate this rock type (commonly used to date kimberlites) help to constrain the duration of kimberlite intrusion history within a pipe but do not necessarily reliably record the emplacement age of the main diatreme in the Renard cluster. Our results provide the first robust geochronological data on a single kimberlite that confirms the field relationships initially observed by Wagner (1914) and Clement (1982); the presence of antecedent (diatreme precursor) intrusions, contemporaneous (syn-diatreme) intrusions, and consequent (post-diatreme) cross-cutting intrusions. The results of this detailed U-Pb geochronology study indicate a single kimberlite pipe can record millions of years of magmatism, much longer than previously thought from the classical viewpoint of a rapid and short-duration emplacement history.

Interaction of Kimberlite Magma with Diamonds Upon Uplift from the Upper Mantle to the Earth’s Crust

Interaction between a melt of kimberlite from the Nyurbinskaya pipe (Yakutia) and natural monocrystalline diamonds was studied experimentally at 0.15 GPa and 1200–1250°C in high-pressure and high-temperature Ar gas “bombs.” The loss of diamond weight with slight surface dissolution of diamonds in a Ca carbonate-bearing kimberlite melt over the course of 2 h (the period of kimberlite transport from upper-mantle diamond-forming chambers to the crustal cumulative centers) is 3–4.5%. In 4 and 7–8 days (under the conditions of crustal cumulative centers), the weight of diamond decreases with remarkable bulk dissolution by 13.5 and 24.5–27.5%, respectively. In the run at 0.15 GPa and 1200°C kimberlite and ilmenite (added) melts interact to produce perovskite melt. Both of the melts, rich in titanium minerals, are immiscible with kimberlite melt and therefore cannot influence the diamond dissolution kinetics in the kimberlite melt. The experimental results suggest that precisely the dissolution processes for thermodynamically metastable diamonds in silicate–carbonate kimberlitic magmas are responsible for the effective decrease in the diamond potential of kimberlite deposits. The paper discusses the physicochemical reasons for the decrease in the kimberlite diamond potential during the chemically active history of diamond genesis: from upper-mantle chambers to the explosive release of diamonds and kimberlite material from cumulative centers to the Earth’s surface. The data on experimental physicochemical studies of the origin, analytical mineralogy of inclusions, and isotope geochemistry of diamonds are correlated.

‘Premier’ evidence for prolonged kimberlite pipe formation and its influence on diamond transport from deep Earth

Volcanic pipes, or maar-diatreme volcanoes, form during explosive eruptions of mantle-derived magmas near Earth's surface. Impressive examples are the carrot-shaped, downward-tapering structures formed by kimberlite magmas. Kimberlites originate from >150 km depth within Earth's mantle beneath thick continental roots, away from tectonic plate margins. Kimberlite pipes can be significant diamond deposits, and the complex architecture revealed during exploration and mining is ascribed to repeated magma injections leading to multiple eruptions. Repeated magmatic pulses cause diatremes to widen and grow downward, forming kilometer-sized subterranean structures. However, the time-resolved evolution of kimberlite pipe systems is largely unknown. We present the first U/Pb perovskite ages for newly discovered kimberlite dikes (1139.8 +/- 4.8 Ma) that cut through the volcaniclastic infill of the Premier kimberlite pipe (1153.3 +/- 5.3 Ma) at Cullinan Diamond Mine, South Africa. The ages reveal that renewed kimberlite volcanic activity occurred, at a minimum, 3 m.y. after the main pipe formation. This finding suggests that the largest kimberlite pipes, and maar-diatreme volcanoes in general, may be magmatically active for several millions of years, which conflicts with this volcanism being described as monogenetic' at millennia time scales. Exemplified by Tier-1 diamond deposits on the Kaapvaal craton, long-lasting kimberlite volcanic activity may be an important factor in growing large diatremes, plus enabling effective transport of mantle cargo from the diamond stability field to Earth's surface.

Petrology and Sr-Nd isotope systematics of the Ahobil kimberlite (Pipe-16) from the Wajrakarur field, Eastern Dharwar craton, southern India

Detailed mineralogical, bulk-rock geochemical and Sr-Nd isotopic data for the recently discovered Ahobil kimberlite (Pipe-16) from the Wajrakarur kimberlite field (WKF), Eastern Dharwar craton (EDC), southern India, are presented. Two generations of compositionally distinct olivine, Ti-poor phlogopite showing orangeitic evolutionary trends, spinel displaying magmatic trend-1, abundant perovskite, Ti-rich hydrogarnet, calcite and serpentine are the various mineral constituents. On the basis of (i) liquidus mineral composition, (ii) bulk-rock chemistry, and (iii) Sr-Nd isotopic composition, we show that Ahobil kimberlite shares several characteristic features of archetypal kimberlites than orangeites and lamproites. Geochemical modelling indicate Ahobil kimberlite magma derivation from small-degree melting of a carbonated peridotite source having higher Gd/Yb and lower La/Sm in contrast to those of orangeites from the Eastern Dharwar and Bastar cratons of Indian shield. The TDM Nd model age (∼2.0 Ga) of the Ahobil kimberlite is (i) significantly older than those (1.5–1.3 Ga) reported for Wajrakarur and Narayanpet kimberlites of EDC, (ii) indistinguishable from those of the Mesoproterozoic EDC lamproites, and (iii) strikingly coincides with the timing of the amalgamation of the Columbia supercontinent. High bulk-rock Fe-Ti contents and wide variation in oxygen fugacity fO2, as inferred from perovskite oxybarometry, suggest non-prospective nature of the Ahobil kimberlite for diamond.

Viscosity of haplokimberlitic and basaltic melts at high pressures: Experimental and theoretical studies

Only limited data are available at present on the viscosity of kimberlite magmas. We investigate viscosity of synthetic carbonate-bearing (silicate82 + carbonate18, wt%, 100NBO/T = 313) anhydrous haplokimberlite melts theoretically and in experiments. We use new experimental data on viscosity of anhydrous haplokimberlite melts and a physical-chemical model (Persikov and Bukhtiyarov 2009; Persikov et al. 2015) to compare basic viscosity features in kimberlitic and basaltic melts (100NBO/T = 56). Viscosity of melts is determined by the falling sphere quenching method in a large range of temperatures from 1300 to 1950 °C and pressures up to 7.5 GPa. We use two types of high-pressure apparatuses: a high gas pressure apparatus and a high pressure split-sphere multi-anvil apparatus to study the viscosity of melts at moderate (100 MPa CO2 pressure) and high (5.5 GPa and 7.5 GPa) pressures, respectively. The measured viscosity ranges for anhydrous haplokimberlite melts are from 1.5 (±0.45) to 0.11(±0.03) Pa s. The temperature dependence of the viscosity of haplokimberlite and basaltic melts is consistent with the theoretical Arrhenian equation. At a constant temperature, viscosity of anhydrous haplokimberlite melts increases exponentially about ten-fold as pressure increases from 100 MPa to 7.5 GPa. The activation energy of viscous flow increases linearly with pressure increase from 100 MPa to 7.5 GPa for anhydrous haplokimberlite melts but decreases in the case of basaltic melts, with the minimum at ~5.5 GPa. At a moderate pressure (100 MPa), haplokimberlite melts are about twenty times less viscous than basaltic melts, but are about four times more viscous at a high pressure (7.5 GPa), the temperature being 1800 °C in both cases. The experimentally observed behavior of the viscosity of anhydrous haplokimberlite melts is consistent with predictions of the physical-chemical model within the range of uncertainties in both experimental and calculated data (±30% rel.). Thus, the physical-chemical model is used to discuss possible effects of volume percentages of crystals and bubbles on viscosity of kimberlitic and basaltic magmas at different pressures and temperatures during their origin, evolution, and ascent.

On the origin of large type IIa gem diamonds

The processes of formation of some diamond types still raise contentious issues, mainly on the origin of the largest diamond crystals recovered from kimberlites. These diamonds constitute less than 2% of worldwide resources and correspond to rare type IIa. They possess some peculiar features: (i) silicate and oxide inclusions are extremely rare, (ii) their δ13C ranges from −17 to −21‰. The detailed estimation of the Premier pressure-temperature-oxygen fugacity parameters and the physic-chemical modeling of diamond growth-dissolution processes suggest that extra-large diamonds have multiple origins. Their formation may occur from lower mantle to crustal depths. Their main building-up takes place from fluids in the pegmatitic veins solidified along the contacts of kimberlite magma at a crustal depth. The model explains the main features of the largest kimberlitic diamonds, i.e. their great sizes, light δ13C signatures, low nitrogen contents, high degree of resorption, absence of mantle-derived mineral inclusions and their occurrence in the form of rare isolated crystals in the host kimberlite.

Global Eocene tectonic unrest: Possible causes and effects around the North American plate

Many of our planet's “crises” were the result of sudden changes in plate tectonic configuration or catastrophic outbursts of volcanism caused by mantle plume impingement at the base of the lithosphere. At the Paleocene-Eocene boundary and in the Early Eocene several mantle plumes, continental collision and mid-ocean ridge subduction triggered a series of changes in seafloor spreading dynamics. We have constructed a detailed global model of oceanic lithosphere age and spreading rates for the 60 to 35 Ma interval. We revise evidence for changes in seafloor spreading direction in the North Atlantic, Arctic and NE Pacific oceans. At least two periods of spreading rate highs, which are separated by sharp value decrease, occurred along the entire eastern North American plate boundary from C25 to C18 time (c. 57 to 40 Ma). The collision and incipient subduction of the Early Eocene Siletzia oceanic LIP may have caused the sharp decrease in spreading rate at C23 time in the Labrador Sea and north of Charlie-Gibbs fracture zone. The post C23 rapid Farallon slab-break-off and subsequent upper mantle flow upwelling may have led to further variations in North Atlantic spreading rates at C22-21 time. Eastward Pacific subduction may have resumed at c. 43 Ma as indicated by a steady NE Pacific seafloor-spreading regime which resumed at or shortly after C21. The North Atlantic realm shows a delayed response to tectonic events west of North America, with an increase in spreading rate south of Charlie-Gibbs fracture zone from C20 to C18 time, followed by a steady decrease until the Oligocene. North American Late Paleocene-Early Eocene kimberlite magma that erupted more than 1000 km from its western plate boundary constitutes additional evidence that tectonic stresses due to changes in the mantle-lithosphere interactions may have affected the entire plate, and therefore also its eastern boundaries.

Characterisation of primary and secondary carbonates in hypabyssal kimberlites: an integrated compositional and Sr-isotopic approach

Carbonates in fresh hypabyssal kimberlites worldwide have been studied to understand their origin [i.e. primary magmatic (high T) versus deuteric (‘low T’) versus hydrothermal/alteration (‘low T’)] and identify optimal strategies for petrogenetic studies of kimberlitic carbonates. The approach presented here integrates detailed textural characterisation, cathodoluminescence (CL) imaging, in situ major- and trace-element analysis, as well as in situ Sr-isotope analysis. The results reveal a wide textural diversity. Calcite occurs as fine-grained groundmass, larger laths, segregations, veins or as a late crystallising phase, replacing olivine or early carbonates. Different generations of carbonates commonly coexist in the same kimberlite, each one defined by a characteristic texture, CL response and composition (e.g., variable Sr and Ba concentrations). In situ Sr isotope analysis revealed a magmatic signature for most of the carbonates, based on comparable 87Sr/86Sr values between these carbonates and the coexisting perovskite, a robust magmatic phase. However, this study also shows that in situ Sr isotope analysis not always allow distinction between primary (i.e., magmatic) and texturally secondary carbonates within the same sample. Carbonates with a clear secondary origin (e.g., late-stage veins) occasionally show the same moderately depleted 87Sr/86Sr ratios of primary carbonates and coexisting perovskite (e.g., calcite laths-shaped crystals with 87Sr/86Sr values identical within uncertainty to those of vein calcite in the De Beers kimberlite). This complexity emphasises the necessity of integrating detailed petrography, geochemical and in situ Sr isotopic analyses for an accurate interpretation of carbonate petrogenesis in kimberlites. Therefore, the complex petrogenesis of carbonates demonstrated here not only highlights the compositional variability of kimberlites, but also raises concerns about the use of bulk-carbonate C-O isotope studies to characterise the parental melt compositions. Conversely, our integrated textural and in situ study successfully identifies the most appropriate (i.e. primary) carbonates for providing constraints on the isotopic parameters of parental kimberlite magmas.

Ilmenite as a recorder of kimberlite history from mantle to surface: examples from Indian kimberlites

Five compositional-textural types of ilmenite can be distinguished in nine kimberlites from the Eastern Dharwar craton of southern India. These ilmenite generations record different processes in kimberlite history, from mantle to surface. A first generation of Mg-rich ilmenite (type 1) was produced by metasomatic processes in the mantle before the emplacement of the kimberlite. It is found as xenolithic polycrystalline ilmenite aggregates as well as megacrysts and macrocrysts. All of these ilmenite forms may disaggregate within the kimberlite. Due to the interaction with low-viscosity kimberlitic magma replacement of pre-existing type 1 ilmenite by a succeeding generation of geikielite (type 2) along grain boundaries and cracks occurs. Another generation of Mg-rich ilmenite maybe produced by exsolution processes (type 3 ilmenite). Although the identity of the host mineral is unclear due to extensive alteration and possibility includes enstatite. Type 4 Mn-rich ilmenite is produced before the crystallization of groundmass perovskite and ulvospinel. It usually mantles ilmenite and other Ti-rich minerals. Type 5 Mn-rich ilmenite is produced after the crystallization of the groundmass minerals and replaces them. The contents of Cr and Nb in type 2, 4 and 5 ilmenites are highly dependent on the composition of the replaced minerals, they may not be a good argument in exploration. The highest Mg contents are recorded in metasomatic ilmenite that is produced during kimberlite emplacement, and cannot be associated with diamond formation. The higher Mn contents are linked to magmatic processes and also late processes clearly produced after the crystallization of the kimberlite groundmass, and therefore ilmenite with high Mn contents cannot be considered as a reliable diamond indicator mineral (DIM) and kimberlite indicator mineral (KIM).

Geology of the Renard 2 pipe to 1000 m depth, Renard Mine, Québec, Canada: insights into Kimberley-type pyroclastic kimberlite emplacement

The Renard 2 pipe is currently the deepest-drilled and most extensively studied kimberlite body in the Renard cluster, central Quebec, Canada, forming the major component of the Mineral Resource of Stornoway Diamond Corporation’s Renard Mine. Renard 2 is infilled with two distinct kimberlite units that exhibit Kimberley-type pyroclastic kimberlite and related textures. Hypabyssal kimberlite also occurs as smaller cross-cutting sheets and irregular intrusions. The units are distinguished by their rock textures, groundmass mineral assemblages, olivine macrocryst size distributions and replacement products, mantle and country rock xenolith contents, whole rock geochemical signatures, bulk densities and diamond grades. These differences are interpreted to reflect different mantle ascent and near-surface emplacement processes and are here demonstrated to be vertically continuous from present surface to over 1000 m depth. The distinctive petrological features together with sharp, steep and cross-cutting internal contact relationships, show that each unit was formed from a separate batch of mantle-derived kimberlite magma, and was completely solidified before subsequent emplacement of the later unit. The mineralogy and textures of the ultra-fine-grained interclast matrix are consistent with those described at numerous Kimberley-type pyroclastic kimberlite localities around the world and are interpreted to reflect rapid primary crystallization during emplacement of separate kimberlite magmatic systems. The units of fractured and brecciated country rock surrounding the main kimberlite pipe contain kimberlite-derived material including carbonate providing evidence of subsurface brecciation. Together these data show that Renard 2 represents the deeper parts of a Kimberley-type pyroclastic kimberlite pipe system and demonstrates that their diagnostic features result from magmatic crystallisation during subsurface volcanic emplacement processes.

The role of eclogites in the redistribution of water in the subcontinental mantle of the Siberian craton: results of determination of the water content in minerals from the Udachnaya pipe eclogites

A comprehensive study of 26 mafic mantle xenoliths from the Udachnaya kimberlite pipe was carried out. The contents of major and trace elements, equilibrium temperature parameters, and water content in the rock-forming minerals were determined. The temperatures of formation of the studied rocks are estimated at 800–1300 °C. According to IR spectroscopy data, the water content in clinopyroxenes from the studied eclogites varies from values below the detection limit to 99 ppm. The IR spectra of garnets lack bands of water. The water content in clinopyroxene and orthopyroxene from garnet websterite is 72 and 8 ppm, respectively. The water content in the average rock, calculated from the ratio of the rock-forming minerals, varies from a few to 55 ppm. No relationship among the water content, equilibrium temperatures, and rock composition is established. The low water contents in the eclogites are close to the earlier determined water contents in peridotites from the same pipe and are, most likely, due to the re-equilibration of the eclogites with the rocks of the peridotitic lithospheric mantle. The dehydration of the protolith during its subduction and the partial melting of eclogites before their removal by kimberlitic magma to the surface might be an additional cause of the low water contents in the mantle eclogite xenoliths.