Kimberlite Melt Research Articles

A Find of Coesite in Diamond-Bearing Kyanite Eclogite from the Udachnaya Kimberlite Pipe, Siberian Craton

A find of coesite in a kyanite graphite–diamond-bearing eclogite xenolith from the Udachnaya–Vostochnaya kimberlite pipe is described in this paper. The coesite relics were found in intensely fractured garnet indicating some influence of the kimberlite melt, which is supported by the typical secondary mineral assemblage around this inclusion. These data indicate that shallower diamond-free coesite-grade rocks (2.7 GPa) underwent metamorphism distinct from diamond-bearing coesite eclogites (≥4 GPa). The metasomatic alteration of rock as a result of the C–O–H fluid-rock interaction during diamond crystallization may be another possible reason for the absence of coesite in diamond-bearing xenoliths.

Silicate Melt Inclusions in Diamonds of Eclogite Paragenesis from Placers on the Northeastern Siberian Craton

New findings of silicate-melt inclusions in two alluvial diamonds (from the Kholomolokh placer, northeastern Siberian Platform) are reported. Both diamonds exhibit a high degree of N aggregation state (60–70% B) suggesting their long residence in the mantle. Raman spectral analysis revealed that the composite inclusions consist of clinopyroxene and silicate glass. Hopper crystals of clinopyroxene were observed using scanning electron microscopy and energy-dispersive spectroscopic analyses; these are different in composition from the omphacite inclusions that co-exist in the same diamonds. The glasses in these inclusions contain relatively high SiO2, Al2O3, Na2O and, K2O. These composite inclusions are primary melt that partially crystallised at the cooling stage. Hopper crystals of clinopyroxene imply rapid cooling rates, likely related to the uplift of crystals in the kimberlite melt. The reconstructed composition of such primary melts suggests that they were formed as the product of metasomatised mantle. One of the most likely source of melts/fluids metasomatising the mantle could be a subducted slab.

Geochemistry of garnet megacrysts from the Mir kimberlite pipe (Yakutia) and the nature of prothokimberlite melt

The paper presents the results of major and trace elements composition study of garnet megacrysts from Mir kimberlite pipe. On the major elements composition those garnets classified as low Cr and high Ti pyropes. Concentrations of TiO2 show a negative correlation with MgO и Cr2O3 contents in megacrysts composition. Fractional crystallization modeling indicates that the most appropriate melt to reproduce the garnet trace elements signatures is the melt of picritic composition. Composition of garnets crystallized from kimberlite melt do not correspond to observed natural garnets composition. Kimberlites contain less of Ti, Zr, Y and heavy REE (rare earth elements) but more of very incompatible elements such as light REE, Th, U, Nb, Ba then the model melt composition that necessary for garnet crystallization.

Polymineralic inclusions in kimberlite-hosted megacrysts: Implications for kimberlite melt evolution

Megacrysts are large (cm to >20 cm in size) mantle-derived crystals, which are commonly entrained by kimberlite magmas, comprising of olivine, orthopyroxene, clinopyroxene, phlogopite, garnet, ilmenite and zircon as common phases. Numerous studies have shown megacrysts to contain polymineralic inclusions, which have been interpreted to represent entrapped kimberlite melt. To constrain the origin of these inclusions in megacrysts and their relationship to kimberlite magmatism, we present a detailed petrographic and geochemical study of clinopyroxene and olivine megacrysts and their hosted inclusions from the Diavik, Jericho, Leslie (Slave Craton, Canada) and Udachnaya-East (Siberian Craton, Russia) kimberlites. The studied megacrysts are between 1 and 3 cm in size and representative of both the Cr-rich and Cr-poor suites. Megacrysts contain two types of inclusions:i.Large (<0.5–5 mm in size) round-to-irregular shaped polymineralic inclusions, which are composed of minerals similar to the host kimberlite groundmass, and consist of olivine, calcite, spinel, perovskite, phlogopite and apatite (± serpentine, alkali-carbonates, alkali-chlorides, barite).ii.Swarms/trails of ‘micro melt inclusions’ (MMI; <1–5 μm in size), which surround polymineralic inclusions, veins and fractures, thereby forming a ‘spongy’ texture. MMIs generally contain multiphase assemblages similar to polymineralic inclusions as well as various additional phases, such as alkali-carbonates or alkali-chlorides, which are typically absent in polymineralic inclusions and the surrounding kimberlite groundmass.Textural and geochemical evidence suggests that polymineralic inclusions in megacrysts crystallised from kimberlite melt, which infiltrated along fracture/vein networks. The polymineralic inclusion assemblages resulted from disequilibria reactions between the host megacryst and infiltrating kimberlite melt, which was likely enhanced by rapidly changing conditions during magmatic ascent. The connectivity of polymineralic inclusions to the kimberlite groundmass via network veins/fractures suggests that they are susceptible to infiltrating post-emplacement fluids. Therefore, the vast majority of polymineralic inclusions are unlikely to represent ‘pristine’ entrapped kimberlite melt. In contrast, MMIs are isolated within megacrysts (i.e. not connected to fractures/veins and therefore shielded from post-magmatic fluids) and probably represent entrapped remnants of the variably differentiated kimberlite melt, which was more enriched in alkalis-Cl-S-CO2 than serpentinised polymineralic inclusions and the host rocks exposed at Earth's surface as kimberlites.

Mantle-like oxygen isotopes in kimberlites determined by in situ SIMS analyses of zoned olivine

Kimberlites are the deepest melts produced on Earth that are erupted at the surface and can therefore provide unique insights into the composition and evolution of the mantle. Radiogenic isotopes provide ambiguous evidence for the occurrence of recycled crustal material in kimberlite sources. Oxygen isotopes can fractionate significantly only in the shallow crust, and thus represent a powerful tracer of subducted material in the sources of kimberlite. To constrain the oxygen isotope composition of kimberlite melts, we have examined olivine grains in eleven Cretaceous to Eocene archetypal kimberlites from southern Africa, Lac de Gras (Canada) and Alto Paranaiba (Brazil), which exhibit radiogenic isotope evidence for recycled crustal material in their sources including highly radiogenic Pb isotopes and Nd-Hf isotope compositions deviating below the mantle array. Olivine grains are commonly zoned between a mantle-derived xenocrystic core and one or more magmatic overgrowths, i.e. occasional internal zones, ubiquitous rims and rare rinds (moving outward from the core). The oxygen isotope composition of different olivine zones was determined in situ within separated olivine grains by secondary ion mass spectrometry (SIMS) after point selection using back-scattered electron (BSE) images combined with major and minor element analyses.With the exception of a few cores, the δ18O values of different olivine zones do not deviate from typical mantle olivine values of 5.18 ± 0.28‰ (Mattey et al., 1994). There are no correlations between oxygen isotopes and major/minor element compositions for internal zones and rims from individual localities or in the entire dataset. This indicates that the oxygen isotope composition of kimberlite melts is not affected by melt differentiation to the point of olivine rim crystallisation. However, olivine rinds from the Koala kimberlite (Canada) display an inverse correlation between δ18O and Mn-Ca concentrations, with δ18O values extending below the mantle range, which is probably due to carbonate fractionation, CO2 degassing and/or assimilation of serpentine-rich material after kimberlite emplacement in the upper crust. The mantle-like δ18O composition of olivine internal zones and rims suggests that assimilation of mantle material and liberation of a CO2-rich phase during ascent in the mantle do not significantly modify the original δ18O signature of kimberlite melts. Modelling of oxygen isotope fractionation shows that up to 15 wt% of CO2 can be lost by kimberlites en route to the upper crust. Our results combined with mass balance calculations indicate that only a limited amount (<5–10 wt%) of recycled crustal material could occur in the source of kimberlites from southern Africa, Lac de Gras and Alto Paranaiba, or that the recycled material had an oxygen isotope composition similar to the mantle.

Evidence for subduction-related signatures in the southern African lithosphere from the N-O isotopic composition of metasomatic mantle minerals

Current understanding of the fate of subducted material (and related fluids) in the deep Earth can be improved by combining major and trace element geochemistry with stable isotopic compositions of mantle rocks or minerals. Limited isotopic fractionation during high temperature processes means that significant deviations from mantle-like isotope ratios in mantle rocks probably result from recycling of surficial material. To determine the effects and origins of mantle metasomatic fluids/melts, new δ15N and δ18O data have been collected for thirteen mantle xenoliths – harzburgites, wehrlites, lherzolites, and MARID (Mica-Amphibole-Rutile-Ilmenite-Diopside) rocks – from the Bultfontein kimberlite (Kimberley, South Africa), which show varying degrees of metasomatism. The δ18O values of olivine and orthopyroxene in phlogopite-free harzburgites match the mantle composition (δ18Oolivine = +5.2 ± 0.3‰; δ18Oorthopyroxene = +5.7 ± 0.3‰; 2 s.d.), consistent with previous inferences that harzburgites were formed by interaction with ancient silica-rich melts unrelated to subduction processes. Wehrlite samples display mineral compositional characteristics (e.g., low La/Zr in clinopyroxene) resembling those of other products of kimberlite melt metasomatism, such as PIC (Phlogopite-Ilmenite-Clinopyroxene) rocks. The inferred interaction with kimberlite melts may be responsible for O isotopic disequilibrium between clinopyroxene and olivine (Δ18O = +0.2‰) in the wehrlites of this study. In contrast with broadly mantle-like δ18O values, the δ15N value of phlogopite in a wehrlite sample (+5.9‰) differs from the mantle composition (δ15N = −5 ± 2‰). This unusual N isotopic composition in kimberlite-related mantle products might indicate that a recycled crustal component occurred in the source of the Kimberley kimberlites, or was assimilated during interaction with the lithospheric mantle.Similar major and trace element characteristics in clinopyroxene from phlogopite-lherzolite and MARID samples suggest metasomatism by fluids of similar composition. Lherzolite and MARID clinopyroxene δ18O values (as low as +4.4‰) extend below those reported in mantle peridotites (i.e. δ18Oclinopyroxene = +5.6 ± 0.3‰; 2 s.d.), and strong negative correlations are found between mineral δ18O values and major element compositions (e.g., Na2O contents in clinopyroxene). Furthermore, phlogopite δ15N values (+4 to +7‰) in the studied lherzolite and MARID samples are higher than mantle values. Combined, the low δ18O-high δ15N isotopic signatures of MARID and lherzolite samples suggest progressive mantle metasomatism by a melt containing a recycled oceanic crust (eclogitic) component. This study demonstrates that progressive enrichment of the subcontinental lithospheric mantle may be inextricably linked to plate tectonics via recycling of subducted crustal material into the deep mantle.

Crustal and lithospheric mantle conductivity structure in the Dharwar craton, India

The vertical extension and structure of the sub-continental lithospheric mantle beneath the Archean Dharwar craton is the main attraction of the work presented here. To delineate the electrical conductivity structure of the Dharwar craton, a magnetotelluric study is carried out. This study comprises magnetotelluric data at 22 stations along a west-east slanting profile. Inter-station spacing is approximately 15 km. This magnetotelluric study is initiated from Dandeli (in the west) to Sindhanur (in the east side). The preferable geoelectric strike directions for the crust and lithospheric mantle are N3°E and N16°E respectively. A 2-dimensional (2-D) resistivity model derived by using the crustal and lithospheric mantle strike azimuths, identified conductive features in the stable continental Dharwar craton. In the crust, prominent conductors are present in the eastern and western part of the profile. A conducting feature is present in the deeper crust associated with the Chitradurga shear zone (CSZ). The study infers a thick lithosphere beneath Dharwar craton as a preserved cratonic nucleus on the eastern and a few conductive anomalies in the western part of the Dharwar craton. The model shows two separate conductors in the depth range of 110–250 km. This study shows, the possibility of presence of kimberlite melt in the western Dharwar craton in the depth range of 110–150 km.

Progressive metasomatism of the mantle by kimberlite melts: Sr–Nd–Hf–Pb isotope compositions of MARID and PIC minerals

MARID (Mica-Amphibole-Rutile-Ilmenite-Diopside) and PIC (Phlogopite-Ilmenite-Clinopyroxene) rocks occur as mantle-derived xenoliths in kimberlites and other alkaline volcanic rocks. Both rock types are alkaline and ultramafic in composition. The H2O and alkali metal enrichments in MARID and PIC rocks, reflected in abundant phlogopite, have been suggested to be caused by extreme mantle metasomatism. Radiogenic (Sr–Nd–Hf–Pb) isotope and trace element compositions for mineral separates from MARID (clinopyroxene and amphibole) and PIC (clinopyroxene only) samples derived from Cretaceous kimberlites (Kimberley) and orangeites (Newlands) from South Africa are used here to examine the source(s) of mantle metasomatism. PIC clinopyroxene (n=4) is relatively homogeneous, with narrow ranges in initial isotopic composition (calculated to the emplacement age of the host Bultfontein kimberlite; 87Sr/86Sri: 0.7037–0.7041; εNdi: +3.0 to +3.6; εHfi: +2.2 to +2.5; 206Pb/204Pbi: 19.72–19.94) similar to kimberlite values. This is consistent with PIC rocks representing peridotites modified by intense metasomatic interaction with kimberlite melts.The MARID clinopyroxene (n=9) and amphibole separates (n=11) studied here display broader ranges in isotope composition (e.g., 87Sr/86Sri: 0.705–0.711; εNdi: −11.0 to −1.0; εHfi: −17.9 to −8.5; 206Pb/204Pbi: 17.33–18.72) than observed in previous studies of MARID rocks. The Nd–Hf isotope compositions of kimberlite-derived MARID samples fall below the mantle array (ΔεHfi between −13.0 and −2.4), a feature reported widely for kimberlites and other alkaline magmas. We propose that such displacements in MARID minerals result from metasomatic alteration of an initial “enriched mantle” MARID composition (i.e., 87Sr/86Sri = 0.711; εNdi = −11.0; εHfi = −17.9; and 206Pb/204Pbi = 17.3) by the entraining kimberlite magma (87Sr/86Sr∼i0.704; εNd∼i+3.3; εHf∼i+2.3; 206Pb/204Pb∼i19.7). A model simulating the flow of kimberlite magma through a mantle column, thereby gradually equilibrating the isotopic and chemical compositions of the MARID wall-rock with those of the kimberlite magma, broadly reproduces the Sr–Nd–Hf–Pb isotope compositions of the MARID minerals analysed here. This model also suggests that assimilation of MARID components could be responsible for negative ΔεHfi values in kimberlites. The isotopic composition of the inferred initial MARID end-member, with high 87Sr/86Sr and low εNd, εHf, and 206Pb/204Pb, resembles those found in orangeites, supporting previous inferences of a genetic link between MARID-veined mantle and orangeites. The metasomatic agent that produced such compositions in MARID rocks must be more extreme than the EM-II mantle component and may relate to recycled material that experienced long-term storage in the lithospheric mantle.

Djerfisherite in kimberlites and their xenoliths: implications for kimberlite melt evolution

Djerfisherite (K6(Fe,Ni,Cu)25S26Cl) occurs as an accessory phase in the groundmass of many kimberlites, kimberlite-hosted mantle xenoliths, and as a daughter inclusion phase in diamonds and kimberlitic minerals. Djerfisherite typically occurs as replacement of pre-existing Fe–Ni–Cu sulphides (i.e. pyrrhotite, pentlandite and chalcopyrite), but can also occur as individual grains, or as poikilitic phase in the groundmass of kimberlites. In this study, we present new constraints on the origin and genesis of djerfisherite in kimberlites and their entrained xenoliths. Djerfisherite has extremely heterogeneous compositions in terms of Fe, Ni and Cu ratios. However, there appears to be no distinct compositional range of djerfisherite indicative of a particular setting (i.e. kimberlites, xenoliths or diamonds), rather this compositional diversity reflects the composition of the host kimberlite melt and/or interacting metasomatic medium. In addition, djerfisherite may contain K and Cl contents less than the ideal formula unit. Raman spectroscopy and electron backscatter diffraction (EBSD) revealed that these K–Cl poor sulphides still maintain the same djerfisherite crystal structure. Two potential mechanisms for djerfisherite formation are considered: (1) replacement of pre-existing Fe–Ni–Cu sulphides by djerfisherite, which is attributed to precursor sulphides reacting with metasomatic K–Cl bearing melts/fluids in the mantle or the transporting kimberlite melt; (2) direct crystallisation of djerfisherite from the kimberlite melt in groundmass or due to kimberlite melt infiltration into xenoliths. The occurrence of djerfisherite in kimberlites and its mantle cargo from localities worldwide provides strong evidence that the metasomatising/infiltrating kimberlite melt/fluid was enriched in K and Cl. We suggest that kimberlites originated from melts that were more enriched in alkalis and halogens relative to their whole-rock compositions.

Растворение макрокристов оливина в кимберлитовом расплаве при высоких Р–Т-параметрах

Растворение макрокристов оливина в кимберлитовом расплаве при высоких Р–Т-параметрах

Природа разнообразия алмазов в кимберлитах

The one-way evolution of inclusions in diamonds and its ultra-long duration (more than 2.8 Ga) provide evidence that the crystallization of diamonds took place from the fractionated peridotitic layer in the global magmatic ocean. Diamonds were formed as a result of accumulation of carbon in residual kimberlitic melts. The increment of SiO 2 concentration in the melts caused increase in its viscosity, decrease of diffusion velocity and degree of carbon oversaturation. These resulted that the layered diamond crystallization mode was changed by the radial one, diamond octahedrons transformed into rhombic dodecahedrons and cubes, varied sculptures were created on diamond crystals faces.

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.

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.

Slab–mantle interaction, carbon transport, and kimberlite generation in the deep upper mantle

Low-degree partial melts from deeply subducted, carbonated ocean crust are carbonatite liquids with ∼35–47 wt% CO2. Their reactions with the overlying mantle regulate the slab–mantle interaction and carbon transport in the deep upper mantle but have not been investigated systematically. Here we present new multi-anvil experiments and parameterized phase relation models to constrain the fate of slab-derived carbonatite melts in the upper mantle. The experiments were conducted at 7 GPa/1400°C and 10 GPa/1450°C, and used starting compositions mimicking the ambient mantle infiltrated by variable carbonatite fluxes (0–45 wt%) from the slab surface. Kimberlitic melts (CO2 = 14–32 wt%, SiO2 = 15–33 wt%, and MgO = 20–29 wt%) were produced from experiments with 5.8–25.6 wt% carbonatite influxes. Experimental phase relations demonstrate a reactive melting process in which the carbonatite influx increases in proportion by dissolution of olivine, orthopyroxene, garnet and precipitation of clinopyroxene. This manifests a feasible mechanism for slab-derived carbonatite melts to efficiently transport in the ambient mantle through high-porosity channels. The melt and mineral fractions from this study and previous phase equilibria experiments in peridotite + CO2±H2O systems were empirically parameterized as functions of temperature (900–2000°C), pressure (3–20 GPa), and bulk compositions (e.g., CO2 = 0.9–17.1 wt% and Na2O + K2O = 0.27–2.51 wt%). Applications of the phase relation models to prescribed melting processes indicate that reactive melting of a carbonatite-fluxed mantle source could produce kimberlitic melts with diverse residual lithologies under various melting conditions. However, reactive melting at the slab–mantle interface can only commence when the slab-released carbonatite melt conquers the carbonation freezing front, i.e., the peridotite solidi suppressed by infiltration of CO2-rich melts in an open system. Depending on temperatures and local influxes, reactive melting and carbonation/redox freezing can occur simultaneously above the slab–mantle interface, yielding heterogeneous lithologies and redox conditions as well as various time-scales of carbon transport in Earth's mantle.

Zircon Macrocrysts from the Drybones Bay Kimberlite Pipe (Northwest Territories, Canada): A High-Resolution Trace Element and Geochronological Study

Zircon macrocrysts in (sub)volcanic silica-undersaturated rocks are an important source of information about mantle processes and their relative timing with respect to magmatism. The present work describes variations in trace element (Sc, Ti, Y, Nb, lanthanides, Hf, Ta, Pb, Th, and U) and isotopic (U-Pb) composition of zircon from the Drybones Bay kimberlite, Northwest Territories, Canada. These data were acquired at a spatial resolution of ≤100 µm and correlated to the internal characteristics of macrocrysts (imaged using cathodoluminescence, CL). Six types of zircon were distinguished on the basis of its luminescence characteristics, with the majority of grains exhibiting more than one type of CL response. The oscillatory-zoned core and growth sectors of Drybones Bay zircon show consistent variations in rare-earth elements (REE), Hf, Th, and U. Their chondrite-normalized REE patterns are typical of macrocrystic zircon and exhibit extreme enrichment in heavy lanthanides and a positive Ce anomaly. Their Ti content decreases slightly from the core into growth sectors, but the Ti-in-zircon thermometry gives overlapping average crystallization temperatures (820 ± 26 °C to 781 ± 19 °C, respectively). There is no trace element or CL evidence for Pb loss or other forms of chemical re-equilibration. All distinct zircon types are concordant and give a U-Pb age of 445.6 ± 0.8 Ma. We interpret the examined macrocrysts as products of interaction between a shallow (&lt;100 km) mantle source and transient kimberlitic melt.

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.

Preservation of systematic Ni and Cr heterogeneity in otherwise homogeneous mantle olivine: Implications for timescales of post-metasomatism re-equilibration

The flux of elements into Earth's sub-continental lithospheric mantle is facilitated by the passage of small-fraction melts that either crystallise new phases or react with pre-existing minerals.Metasomatised peridotite records the end product of this exchange but rarely captures the process in the act due to subsolidus re-equilibration. We present the results of a systematic investigation of a metasomatic melt channel preserved in a mantle peridotite from the Late Cretaceous Bultfontein kimberlite (Kaapvaal craton) that shows rare direct evidence of the melt-rock reaction processes. We show that the metasomatic proto-kimberlite melt underwent variable crystallisation of clinopyroxene, sulfides, phlogopite, spinel and zircon together with interaction and diffusive exchange with the surrounding olivine-rich mantle.Element profiles across large olivine porphyroclasts (Fo88) show significant core-to-rim variations in Ni (NiO = 0.18–0.32 wt%) and Cr (Cr = 35–60 ppm), while concentrations of all other elements (e.g. Mg, Fe, Mn, Co, V)are remarkably homogeneous. Electron backscatter diffraction analysis shows that the disequilibrium of Ni and Cr is greatest where the crystal contains large components of the [100] and [010] axes. The disequilibrium is preserved in certain orientations because diffusion of Ni and Cr in olivine is more anisotropic than Fe-Mg and Mn, and slower in the [100] and [010] directions. We present the first observations of Ni and Cr decoupling from other elements in mantle olivine and suggest that this is a consequence of: (i)changing mineral-melt concentration gradients associated with the reactive percolation of a precursory kimberlite melt; and (ii) late-stage sulfide and spinel precipitation.We use the diffusion limited re-equilibration of Ni in olivine to quantify the timing of metasomatism prior to xenolith entrainment by the host kimberlite. Our modelling indicates that reactive percolation occurred on the order of 103–105 years prior to entrainment; this provides an additional line of support for the hypothesis that a period of metasomatism by proto-kimberlite melts precedes the final kimberlite ascent to the surface. The broader implication of our finding of variable rates of minor element diffusion in natural olivine is that it highlights the importance of anisotropy and the impact of changing local concentration gradients during subsolidus re-equilibration.

Geochemistry and origin of the Mirny field kimberlites, Siberia

Here we present new data from a systematic Sr, Nd, O, C isotope and geochemical study of kimberlites of Devonian age Mirny field that are located in the southernmost part of the Siberian diamondiferous province. Major and trace element compositions of the Mirny field kimberlites show a significant compositional variability both between pipes and within one diatreme. They are enriched in incompatible trace elements with La/Yb ratios in the range of (65–300). Initial Nd isotope ratios calculated back to the time of the Mirny field kimberlite emplacement (t = 360 ma) are depleted relative to the chondritic uniform reservoir (CHUR) model being 4 up to 6 ɛNd(t) units, suggesting an asthenospheric source for incompatible elements in kimberlites. Initial Sr isotope ratios are significantly variable, being in the range 0.70387–0.70845, indicating a complex source history and a strong influence of post-magmatic alteration. Four samples have almost identical initial Nd and Sr isotope compositions that are similar to the prevalent mantle (PREMA) reservoir. We propose that the source of the proto-kimberlite melt of the Mirny field kimberlites is the same as that for the majority of ocean island basalts (OIB). The source of the Mirny field kimberlites must possess three main features: It should be enriched with incompatible elements, be depleted in the major elements (Si, Al, Fe and Ti) and heavy rare earth elements (REE) and it should retain the asthenospheric Nd isotope composition. A two-stage model of kimberlite melt formation can fulfil those requirements. The intrusion of small bodies of this proto-kimberlite melt into lithospheric mantle forms a veined heterogeneously enriched source through fractional crystallization and metasomatism of adjacent peridotites. Re-melting of this source shortly after it was metasomatically enriched produced the kimberlite melt. The chemistry, mineralogy and diamond grade of each particular kimberlite are strongly dependent on the character of the heterogeneous source part from which they melted and ascended.

Origin of complex zoning in olivine from diverse, diamondiferous kimberlites and tectonic settings: Ekati (Canada), Alto Paranaiba (Brazil) and Kaalvallei (South Africa)

Olivine in kimberlites can provide unique insights into magma petrogenesis, because it is the most abundant xenocrystic phase and a stable magmatic product over most of the liquid line of descent. In this study we examined the petrography and chemistry of olivine in kimberlites from different tectonic settings, including the Slave craton, Canada (Ekati: Grizzly, Koala), the Brasilia mobile belt (Limpeza-18, Tres Ranchos-04), and the Kaapvaal craton, South Africa (Kaalvallei: Samada, New Robinson). Olivine cores display a wide range of compositions (e.g., Mg# = 78–95). The similarity in olivine composition, resorption of core zones and inclusions of mantle-derived phases, indicates that most olivine cores originated from the disaggregation of mantle peridotites, including kimberlite-metasomatised lithologies (i.e. sheared lherzolites and megacrysts). Olivine rims typically show a restricted range of Mg#, with decreasing Ni and increasing Mn and Ca contents, a characteristic of kimberlitic olivine worldwide. The rims host inclusions of groundmass minerals, which implies crystallisation just before and/or during emplacement. There is a direct correlation between olivine rim composition and groundmass mineralogy, whereby high Mg/Fe rims are associated with carbonate-rich kimberlites, and lower Mg/Fe rims are correlated with increased phlogopite and Fe-bearing oxide mineral abundances. There are no differences in olivine composition between explosive (Grizzly) and hypabyssal (Koala) kimberlites. Olivine in kimberlites also displays transitional zones and less common internal zones, between cores and rims. The diffuse transitional zones exhibit intermediate compositions between cores and rims, attributed to partial re-equilibration of xenocrystic cores with the ascending kimberlite melt. In contrast, internal zones form discrete layers with resorbed margins and restricted Mg# values, but variable Ni, Mn and Ca concentrations, which indicates a discrete crystallization event from precursor kimberlite melts at mantle depths. Overall, olivine exhibits broadly analogous zoning in kimberlites worldwide. Variable compositions for individual zones relate to different parental melt compositions rather than variations in tectonic setting or emplacement mechanism.

Fossilized Melts in Mantle Wedge Peridotites

The shallow oxidized asthenosphere may contain a small fraction of potassic silicate melts that are enriched in incompatible trace elements and volatiles. Here, to determine the chemical composition of such melt, we analysed fossilized melt inclusions, preserved as multiphase solid inclusions, from an orogenic garnet peridotite in the Bohemian Massif. Garnet-poor (2 vol.%) peridotite preserves inclusions of carbonated potassic silicate melt within Zn-poor chromite (<400 ppm) in the clinopyroxene-free harzburgite assemblage that equilibrated within the hot mantle wedge (Stage 1, > 1180 °C at 3 GPa). The carbonated potassic silicate melt, which has a major element oxide chemical composition of K2O = 5.2 wt.%, CaO = 17 wt.%, MgO = 18 wt.%, CO2 = 22 wt.%, and SiO2 = 20 wt.%, contains extremely high concentrations of large ion lithophile elements, similar to kimberlite melts. Peridotites cooled down to ≅800 °C during Stage 2, resulted in the growth of garnet relatively poor in pyrope content, molar Mg/(Mg + Fe + Ca + Mn), (ca. 67 mol.%). This garnet displays a sinusoidal REE pattern that formed in equilibrium with carbonatitic fluid. Subsequently, subduction of the peridotite resulted in the formation of garnet with a slightly higher pyrope content (70 mol.%) during the Variscan subduction Stage 3 (950 °C, 2.9 GPa). These data suggest the following scenario for the generation of melt in the mantle wedge. Primarily, infiltration of sediment-derived potassic carbonatite melt into the deep mantle wedge resulted in the growth of phlogopite and carbonate/diamond. Formation of volatile-bearing minerals lowered the density and strength of the peridotite. Finally, phlogopite-bearing carbonated peridotite rose as diapirs in the mantle wedge to form carbonated potassic silicate melts at the base of the overriding lithosphere.