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1.
《International Geology Review》2012,54(11):1652-1656
Inasmuch as kimberlites prove to be porphyritic rocks of effusive habit, upon review of published ideas and facts, the general classification of effusive rocks is fully applicable to kimberlites. Consequently superfluous adjectives such as “massive,” “igneous,” “basaltic,” etc., and use of the term “kimberlite” itself should be discouraged.  相似文献   

2.
Summary The Mesoproterozoic diamondiferous Majhgawan pipe of central India is re-examined in the light of new and recently published petrological, geochemical and isotope data. This investigation reveals that its tectonic setting is similar to that of lamproites and orangeites (Group II kimberlite of southern Africa) and not that of a typical kimberlite. The petrography and mineralogy are comparable to lamproite and to some extent to orangeite, whereas the major element geochemistry is more akin to that of kimberlite. Trace element geochemistry is closer to that of lamproite but Nd isotope systematics are atypical of lamproite or orangeite. The inferred petrogenesis of the Majhgawan pipe is also similar to that of other such potassic metasomatised mantle magmas without any strong affinity to a particular clan/group.It is demonstrated in this study that the Majhgawan pipe shares the petrological, geochemical and isotope characteristics of all three rock types. It is therefore suggested to constitute a transitional kimberlite–orangeite (Group II kimberlite)–lamproite rock. The existence of such transitional magmas in space and time in other cratons, outside India, is also highlighted. The name majhgawanite is proposed for this rock – keeping in mind the antiquity of the Majhgawan pipe, its intriguing petrological and geochemical characteristics and also on the basis of Indias legacy for introducing diamond to the world – to designate such mafic potassic-ultrapotassic transitional rock types so as to distinguish them from the classical kimberlite, lamproite or orangeite.It is concluded that the correlations between kimberlite petrography, geochemistry and isotopic types (viz., Group I and II), as established for kimberlites in southern Africa, need not be necessarily valid elsewhere. Hence, the recommendations of I.U.G.S. on classification of kimberlite, orangeite and lamproite are clearly inadequate when dealing with the transitional mafic potassic ultrapotassic rocks. It is further stressed that mineralogical, geochemical and isotopic aspects of mafic potassic-ultrapotassic rocks need to be considered in unison before assigning any name as the nomenclature of such exotic and rare alkaline rock types invariably implies economic and tectono-magmatic (regional) significance.  相似文献   

3.
We report the occurrence of garnierite (a general term referring to Ni-Mg bearing hydrous silicates in laterites) from the crater-facies Tokapal kimberlite of the Bastar craton, Central India. Garnierite occurs as discrete ovoid or amoeboid segregations (up to 200 mm) or as veinlets with up to 18.1 wt% NiO and high iron contents (up to 36.2 wt% FeOT). Chemical composition of the garnierite implies its derivation from a magnesium-rich protolith. Extensive lateritisation of the large crater-facies (～2.5 km diameter) saucer-shaped kimberlite under tropical weathering conditions, aided by suitable topography, drainage and favourable structural set-up, are the factors inferred to be responsible for the formation of garnierite in the Tokapal system. As lateritic nickel ores constitute significant resources for nickel exploration, the perspective of the Tokapal kimberlite as a nickel prospect needs to be investigated.  相似文献   

4.

The Letšeng Diamond Mine comprises two ~91 Ma kimberlite pipes. An update of the geology is presented based on the 2012–2017 detailed investigation of open pit exposures and all available drillcores which included mapping, logging and petrography. Each of the steep-sided volcanic pipes comprises a number of phases of kimberlite with contrasting diamond contents which were formed by the emplacement of at least four batches of mantle-derived magma. The resulting range of textures includes resedimented volcaniclastic kimberlite (RVK), Kimberley-type pyroclastic kimberlite (KPK), coherent kimberlite (CK) and minor amounts of hypabyssal kimberlite (HK). The pipes are compared with KPK occurrences from southern Africa and worldwide. Many features of the Letšeng pipes are similar to KPK infilled pipes particularly those of the widespread Cretaceous kimberlite province of southern Africa. The differences displayed at Letšeng compared to other large KPK pipe infills described from around the world are attributed to the marginal or melnoitic nature of the magma and the upper diatreme to crater setting of the Letšeng pipes, where processes become extrusive. It is concluded that the pipes comprise a variant of Kimberley-type pyroclastic kimberlite emplacement. The classification of many of the Letšeng rocks as KPK is important for developing the internal geology of the pipes as well as for predicting the distribution of diamonds within the bodies.

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5.
New geochemical data of the crater-facies Tokapal kimberlite system sandwiched between the lower and upper stratigraphic horizons of the Mesoproterozoic Indr avati Basin are presented.The kimberlite has been subjected to extensive and pervasive low-temperature alteration.Spinel is the only primary phase identifiable,while olivine macrocrysts and juvenile lapilli are largely pseudomorphed(talc-serpentinecarbonate alteration).However,with the exception of the alkalies,major element oxides display systematic fractionation trends; likewise,HFSE patterns are well correlated and allow petrogenetic interpretation.Various crustal contamination indices such as(SiO2+Al2O3+Na2O)/(MgO+ K2O) and Si/Mg are close to those of uncontaminated kimberlites.Similar La/Yb(79-109) of the Tokapal samples with those from the kimberlites of Wajrakarur(73-145) and Narayanpet(72-156),Eastern Dharwar craton,southern India implies a similarity in their genesis.In the discriminant plots involving HFSE the Tokapal samples display strong affinities to Group II kimberlites from southern Africa and central India as well as to ‘transitional kimberlites' from the Eastern Dharwar craton,southern India,and those from the Prieska and Kuruman provinces of southern Africa.There is a striking similarity in the depleted-mantle(TDM) Nd model ages of the Tokapal kimberlite system,Bastar craton,the kimberlites from NKF and WKF,Eastern Dharwar craton,and the Majhgawan diatreme,Bundelkhand craton,with the emplacement age of some of the lamproites from within and around the Palaeo-Mesoproterozoic Cuddapah basin,southern India.These similar ages imply a major tectonomagmatic event,possibly related to the breakup of the supercontinent of Columbia,at 1.3-1.5 Ga across the three cratons.The ‘transitional'geochemical features displayed by many of the Mesoproterozoic potassic-ultrapotassic rocks,across these Indian cratons are inferred to be memories of the metasomatising fluids/melts imprinted on their source regions during this widespread event.  相似文献   

6.
《地学前缘(英文版)》2020,11(3):793-805
Detailed mineralogy,bulk rock major,trace and Sr-Nd isotope compositions,and ~(40)Ar/~(39)Ar dating of the Pipe-8 diamondiferous ultramafic intrusion in the Wajrakarur cluster of southern India,is reported.Based on the presence of Ti-rich phlogopite,high Na/K content in amphibole,Al-and Ti-rich diopside,a titanomagnetite trend in spinel and the presence of Ti-rich schorlomite garnet and carbonates in the groundmass,the Pipe-8 intrusion is here more precisely classified as an ultramafic lamprophyre(i.e.,aillikite).An aillikite affinity of the Pipe-8 intrusion is further supported by the bulk rock major and trace element and Sr-Nd isotope geochemistry.Sr-Nd isotope data are consistent with a common,moderately depleted upper mantle source region for both the Pipe-8 aillikite as well as the Wajrakarur kimberlites of southern India.A phlogopite-rich groundmass ~(40)Ar/~(39)Ar plateau age of 1115.8±7.9 Ma(2σ) for the Pipe-8 intrusion falls within a restricted 100 Ma time bracket as defined by the 1053-1155 Ma emplacement ages of kimberlites and related rocks in India.The presence of ultramafic lamprophyres,carbonatites,kimberlites,and olivine lamproites in the Wajrakarur kimberlite field requires low degrees of partial melting of contrasting metasomatic assemblages in a heterogeneous sub-continental lithospheric mantle.The widespread association of kimberlite and other mantle-derived magmatism during the Mesoproterozoic(ca.1.1 Ga) have been interpreted as being part of a single large igneous province comprising of the Kalahari,Australian,West Laurentian and Indian blocks of the Rodinia supercontinent that were in existence during its assembly.In India only kimberlite/lamproite/ultramafic lamprophyre magmatism occurred at this time without the associated large igneous provinces as seen in other parts of Rodinia.This may be because of the separated paleo-latitudinal position of India from Australia during the assembly of Rodinia.It is speculated that the presence of a large plume at or close to 1.1 Ga within the Rodinian supercontinent,with the Indian block located on its periphery,could be the reason for incipient melting of lithospheric mantle and the consequent emplacement of only kimberlites and other ultramafic,volatile rich rocks in India due to comparatively low thermal effects from the distant plume.  相似文献   

7.
Mantle xenoliths from the Obnazhennaya kimberlite pipe, Yakutia, possess a large range of mineralogical and chemical compositions, from both group A and B eclogites. Major-element contents of the group A eclogites exhibit transitional features between the group B eclogites and peridotite. The Mg# of clinopyroxenes is 0.86–0.94, with 0.60–0.84 for garnets. Differences in concentration of LREEs exist between the Obnazhennaya group A and the well-studied group B eclogites from the Udachnaya kimberlite pipe. In general, garnets in the group A eclogites contain lower LREEs than those from the group B eclogites; however, the trend for clinopyroxene is reversed. High d 18O (5.46–7.81) values, and the positive Eu anomalies in the garnets and clinopyroxenes (Eu/Eu* 1.2–1.4) demonstrate the involvement of an oceanic crustal component in the formation of the group A eclogites. The group A eclogites formed between 21.0 and 37.6 kbar, and 711 and 923 °C, in a time interval of 1,071–1,237 Ma. An innovative model is proposed to explain the formation of the group A eclogites and websterites. It involves the reaction of a depleted mantle peridotite with TTG and carbonatite melts closely related to the subduction of oceanic crust.  相似文献   

8.
The results of study of rutile inclusions in pyrope from the Internatsionalnaya kimberlite pipe are presented. Rutile is characterized by unusually high contents of impurities (up to 25 wt %). The presence of Cr2O3 (up to 9.75 wt %) and Nb2O5 (up to 15.57 wt %) are most typical. Rutile inclusions often occur in assemblage with Ti-rich oxides: picroilmenite and crichtonite group minerals. The Cr-pyropes with inclusions of rutile, picroilmenite, and crichtonite group minerals were formed in the lithospheric mantle beneath the Mirnyi field during their joint crystallization from melts enriched in Fe, Ti, and other incompatible elements as a result of metasomatic enrichment of the depleted lithospheric mantle.  相似文献   

9.
International Journal of Earth Sciences - Glassy beads were found in the sand cover associated with known and suspected kimberlites on the Kalahari plateau, Botswana. Morphology and chemistry were...  相似文献   

10.
Mineral data from Yakutian kimberlites allow reconstruction of the history of lithospheric mantle.Differences occur in compositions of mantle pyropes and clinopyroxenes from large kimberlite pipes in the Alakit and Daldyn fields.In the Alakit field.Cr-diopsides are alkaline,and Stykanskaya and some other pipes contain more sub-calcic pyropes and dunitic-type diamond inclusions,while in the Daldyn field harzburgitic pyropes are frequent.The eclogitic diamond inclusions in the Alakit field are sharply divided in types and conditions,while in the Daldyn field they show varying compositions and often continuous Pressure-Temperature(P-T) ranges with increasing Fe~# with decreasing pressures.In Alakit,Crpargasites to richterites were found in all pipes,while in Daldyn,pargasites are rare Dalnyaya and Zarnitsa pipes.Cr-diopsides from the Alakit region show higher levels of light Rare Earth Elements(LREE)and stronger REE-slopes,and enrichment in light Rare Earth Elements(LREE),sometimes Th-U,and small troughs in Nb-Ta-Zr.In the Daldyn field,the High Field Strength Elements HFSE troughs are more common in clinopyroxenes with low REE abundances,while those from sheared and refertilized peridotites have smooth patterns.Garnets from Alakit show HREE minima,but those from Daldyn often have a trough at Yand high U and Pb.PTX/O2 diagrams from both regions show similarities,suggesting similar layering and structures.The degree of metasomatism is often higher for pipes which show dispersion in P-Fe~# trends for garnets.In the mantle beneath Udachnaya and Aykhal,pipes show 6-7 linear arrays of P-Fe~# in the lower part of the mantle section at 7.5-3.0 GPa,probably reflecting primary subduction horizons.Beneath the Sytykanskaya pipe,there are several horizons with opposite inclinations which reflect metasomatic processes.The high dispersion of the P—Fe~# trend indicating widespread metasomatism is associated with decreased diamond grades.Possible explanation of the differences in mineralogy and geochemistry of the mantle sections may relate to their tectonic positions during growth of the lithospheric keel.Enrichment in volatiles and alkalis possibly corresponds to interaction with subduction-related fluids and melts in the craton margins.Incorporation of island arc peridotites from an eroded arc is a possible scenario.  相似文献   

11.

It has been demonstrated for the first time that the isotopic compositions of carbon (δ13C) in magmatic calcites from the Udachnaya–East pipe kimberlite groundmass varies from–2.5 to–1.0‰ (V-PDB), while those of oxygen (δ18O) range from 15.0 to 18.2‰ (V-SMOW). The obtained results imply that during the terminal late magmatic and postmagmatic stages of the kimberlite pipe formation, the carbonates in the kimberlite groundmass became successively heavier isotopically, which indicates the hybrid nature of the carbonate component of the kimberlite: it was formed with contributions from mantle and sedimentary marine sources.

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12.

The Renard 2 kimberlite pipe is one of nine diamondiferous kimberlite pipes that form a cluster in the south-eastern portion of the Superior Province, Québec, Canada and is presently being extracted at the Renard Mine. It is interpreted as a diatreme-zone kimberlite consisting of two Kimberley-type pyroclastic units and related country rock breccias, all cross-cut by coherent kimberlite dykes and irregular intrusives. Renard 2 has been the subject of numerous diamond drilling campaigns since its discovery in 2001. The first two geological models modelled kimberlite and country rock breccia units separately. A change in modelling philosophy in 2009, which incorporated the emplacement envelope and history, modelled the entire intrusive event and projected the pipe shape to depth allowing for more targeted deep drilling where kimberlite had not yet been discovered. This targeted 2009 drilling resulted in a > 400% increase in the volume of the Indicated Resource. Modelling only the kimberlite units resulted in a significant underestimation of the pipe shape. Current open pit and underground mapping of the pipe shape corresponds well to the final 2015 geological model and contact changes observed are within the expected level of confidence for an Indicated Resource. This study demonstrates that a sound understanding of the geological emplacement is key to developing a reliable 3D geological and resource model that can be used for targeted delineation drilling, feasibility studies and during the initial stages of mining.

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13.
《Applied Geochemistry》1995,10(6):725-738
The relationship between euhedral microdiamonds (<1 mm in maximum dimension) and macrodiamonds is problematic. A major paradox is that euhedral microdiamonds, by virtue of their small size, should be especially susceptible to resorption compared to macrodiamonds, whereas the opposite is commonly observed. We discuss difficulties with published mechanisms for euhedral microdiamond formation and eruption, most of which involve two separate, possibly unrelated, events: first, euhedral microdiamond crystallization (by a variety of possible processes), and second, later entrainment in erupting kimberlitic or lamproitic magma.We propose an alternative hypothesis based on published experimental studies showing there is a significant decrease in the C-bearing volatile content of kimberlite magma as pressure decreases. During ascent of the kimberlite magma, this may result in a pressure-related transition from conditions at depth at which diamond is unstable in fluid-undersaturated kimberlite magma to shallower-level conditions at which diamond becomes stable (or metastable) as the magma reaches fluid saturation. In this scenario macrodiamond xenocrysts (of an early generation), entrained at asthenospheric or deep lithospheric depths, may partially or totally dissolve into the undersaturated magma upon disaggregation of their host rocks; upon ascent of the magma to shallower depths, and under appropriate oxidation conditions, the lowered capacity of the magma to dissolve C-bearing volatiles may result in euhedral microdiamond precipitation, probably immediately before final eruption. Although framed in qualitative terms, our hypothesis has testable consequences which we discuss. In view of these considerations, and the multiple possible relationships within and between microdiamonds and macrodiamonds in individual deposits, we suggest that the use of microdiamond abundances from small samples as a guide to economic macrodiamond grade is problematic as an exploration strategy.  相似文献   

14.
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15.
New mineralogical and bulk-rock geochemical data for the recently recognised Mesoproterozoic(ca.1100 Ma) and late Cretaceous(ca.90 Ma) kimberlites in the Timmasamudram cluster(TKC) of the Wajrakarur kimberlite field(WKF),Eastern Dharwar Craton,southern India,are presented.On the basis of groundmass mineral chemistry(phlogopite,spinel,perovskite and clinopyroxene),bulk-rock chemistry(SiO_2.K_2O,low TiO_2.Ba/Nb and La/Sm),and perovskite Nd isotopic compositions,the TK-1(macrocrystic variety) and TK-4(Macrocrystic variety) kimberlites in this cluster are here classified as orangeites(i.e.Group Ⅱ kimberlites),with geochemical characteristics that are very similar to orangeites previously described from the Bastar Craton in central India,as well as the Kaapvaal Craton in South Africa.The remaining kimberlites(e.g.,TK-2,TK-3 and the TK-1 microcrystic variant),are more similar to other 1100 Ma,Group Ⅰ-type kimberlites of the Eastern Dharwar Craton,as well as the typical Group Ⅰkimberlites of the Kaapvaal Craton.Through the application of geochemical modelling,based on published carbonated peridotite/melt trace element partition coefficients,we show that the generation of the TKC kimberlites and the orangeites results from low degrees of partial melting of a metasomatised,carbonated peridotite.Depleted mantle(T_(DM)) Nd perovskite model ages of the 1100 Ma Timmasamudram kimberlites show that the metasornatic enrichment of their source regions are broadly similar to that of the Mesoproterozoic kimberlites of the EDC.The younger,late Cretaceous(ca.90 Ma) TK-1(macrocrystic variant)and TK-4 kimberlites,as well as the orangeites from the Bastar Craton,share similar Nd model ages of1100 Ma,consistent with a similarity in the timing of source enrichment during the amalgamation of Rodinia supercontinent.The presence of late Cretaceous diamoncliferous orangeite activity,presumably related to the location of the Marion hotspot in southern India at the time,suggests that thick Iithosphere was preserved,at least locally,up to the late Cretaceous,and was not entirely destroyed during the breakup of Gondwana,as inferred by some recent geophysical models.  相似文献   

16.
We present major and trace element compositions of mineral concentrates comprising garnet xenocrysts,ilmenite,phlogopite,spinel,zircon,and uncommon minerals(titanite,calzirtite,anatase,baddeleyite and pyrochlore)of a newly discovered Late Cretaceous kimberlite(U-Pb zircon age 90.0±1.3 Ma;2σ)named Osvaldo Franca 1,located in the Alto Paranaíba Igneous Province(APIP),southeastern Brazil.Pyrope grains are lherzolitic(Lherz-1,Lherz-2 and Lherz-3),harzburgitic(Harz-3)and wehrlitic(Wehr-2).The pyrope xenocrysts cover a wide mantle column in the subcratonic lithosphere(66–143 km;20–43 kbar)at relatively low temperatures(811–875°C).The shallowest part of this mantle is represented by Lherz-1 pyropes(20–32 kbar),which have low-Cr(Cr2O3=1.74–6.89 wt.%)and fractionated middle to heavy rare earth elements(MREE-HREE)pattern.The deepest samples are represented by Lherz-2,Lherz-3,Harz-3,and Wehr-2 pyropes(36–43 kbar).They contain high-Cr contents(Cr2O3=7.36–11.19 wt.%)and are characterized by sinusoidal(Lherz-2 and Wehr-2)and spoon-like(Lherz-3 and Harz-3)REE patterns.According to their REE and trace elements,pyrope xenocrysts have enriched nature(e.g.,Ce and Yb vs.Cr2O3),indicating that the cratonic lithosphere has been affected by a silicate melt with subalkaline/tholeiite composition due to their low Zr,Ti and Y concentrations.Besides minerals with typical kimberlitic signatures,such as ilmenite and zircon,the exotic compositions of phlogopite and ulv?spinel suggest an enriched component in the magma source.The formation of rare mineral phases with strong enrichment of light-REE(LREE)and high field strength elements(HFSE)is attributed to the late-stage kimberlitic melt.We propose a tectonic model where a thermal anomaly,represented by the low-velocity seismic anomaly observed in P-wave seismic tomography images,supplied heat to activate the alkaline magmatism from a metasomatized cratonic mantle source during the late-stages of Gondwana fragmentation and consequent South Atlantic Ocean opening.The metasomatism recorded by mineral phases is a product of long-lived recycling of subducted oceanic plates since the Neoproterozoic(Brasiliano Orogeny)or even older collisional events,contributing to the exotic character of the Osvaldo Fran?a 1 kimberlite,as well as to the cratonic lithospheric mantle.  相似文献   

17.
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, Tirich 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 fO_2, as inferred from perovskite oxybarometry, suggest non-prospective nature of the Ahobil kimberlite for diamond.  相似文献   

18.
Experimental studies of melting relations in the system ilmenite–K–Na–Mg–Fe–Ca carbonatite–carbon at 8 GPa and 1600°C provide evidence for the effect of liquid immiscibility between ilmenite and carbonatite melts. It is shown that the solubility of ilmenite in carbonatitic melts is negligible and does not depend on its concentration in experimental samples within 25–75 wt %. However, carbonatite–carbon melts are characterized by a high diamond-forming efficiency. This means that the correlation between the concentration of TiO2 and diamond content is problematic for mantle chambers and requires further, more complex, experimental studies.  相似文献   

19.

The Renard 2 pipe is currently the deepest-drilled and most extensively studied kimberlite body in the Renard cluster, central Québec, 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.

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20.
The Let?eng-la-Terae kimberlite (Lesotho), famous for its large high-value diamonds, has five distinct phases that are mined in a Main and a Satellite pipe. These diatreme phases are heavily altered but parts of a directly adjacent kimberlite blow are exceptionally fresh. The blow groundmass consists of preserved primary olivine with Fo86?88, chromite, magnesio-ulvöspinel and magnetite, perovskite, monticellite, occasional Sr-rich carbonate, phlogopite, apatite, calcite and serpentine. The bulk composition of the groundmass, extracted by micro-drilling, yields 24–26 wt% SiO2, 20–21 wt% MgO, 16–19 wt% CaO and 1.9–2.1 wt% K2O, the latter being retained in phlogopite. Without a proper mineral host, groundmass Na2O is only 0.09–0.16 wt%. However, Na-rich K-richterite observed in orthopyroxene coronae allows to reconstruct a parent melt Na2O content of 3.5–5 wt%, an amount similar to that of highly undersaturated primitive ocean island basanites. The groundmass contains 10–12 wt% CO2, H2O is estimated to 4–5 wt%, but volatiles and alkalis were considerably reduced by degassing. Mg# of 77.9 and 530 ppm Ni are in equilibrium with olivine phenocrysts, characterize the parent melt and are not due to olivine fractionation. 87Sr/86Sr(i)?=?0.703602–0.703656, 143Nd/144Nd(i)?=?0.512660 and 176Hf/177Hf(i)?=?0.282677–0.282679 indicate that the Let?eng kimberlite originates from the convective upper mantle. U–Pb dating of groundmass perovskite reveals an emplacement age of 85.5?±?0.3 (2σ) Ma, which is significantly younger than previously proposed for the Let?eng kimberlite.  相似文献   

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