Petrogenesis of the diamondiferous Pipe-8 ultramafic intrusion from the Wajrakarur kimberlite field of Southern India and its relation to the worldwide Mesoproterozoic(～1.1 Ga) magmatism of kimberlite and related rocks

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.     

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 Québec. 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.

     

The temporal evolution of North American kimberlites

North American kimberlite magmatism spans a period of time in excess of 1 billion years from Mesoproterozoic kimberlites in the Lake Superior and James Bay Lowlands region of Ontario to Eocene kimberlites in the Lac de Gras field, N.W.T. Based on a compilation of more than 150 robust radiometric age determinations, several distinct kimberlite emplacement patterns are recognized. In general, the temporal pattern of kimberlite emplacement in North America can be broadly subdivided into five domains: (1) a Mesoproterozoic kimberlite province in central Ontario, (2) an Eocambrian/Cambrian Labrador Sea Province in northern Québec and Labrador, (3) an eastern Jurassic Province, (4) a central Cretaceous corridor and (5) a western mixed domain that includes two Type-3 kimberlite provinces (i.e. multiple periods of kimberlite emplacement preserved in the Slave and Wyoming cratons). For some provinces the origin of kimberlite magmatism can be linked to known mantle heat sources such as mantle plume hotspots and upwelling asthenosphere attendant with continental rifting. For example, the timing and location of Mesoproterozoic kimberlites in North America coincides with and slightly precedes the timing of 1.1 Ga intracontinental rifting that culminated in the Midcontinent Rift centered in the Lake Superior region. Many of the kimberlites in the Eocambrian/Cambrian Labrador Sea province were emplaced soon after the opening of the Iapetus Ocean at about 615 Ma and may also be linked to mantle upwelling associated with continental rifting. The eastern Jurassic kimberlites record an age progression where magmatism youngs in a southeast direction from the 200 Ma Rankin Inlet kimberlites to the 155–126 Ma Timiskaming kimberlites. The location of several kimberlite fields and clusters in Ontario and Québec lie along a continental extension of the Great Meteor hotspot track and represents one of the best examples in the world of kimberlite magmatism triggered by mantle plumes. The central Cretaceous (103–94 Ma) corridor extends for more than 4000 km from Somerset Island in northern Canada through the Fort à la Corne field in Saskatchewan to the kimberlites in central USA. This is the first recognized corridor of kimberlite magmatism of this magnitude. The possible westward younging of Cretaceous to Eocene corridors of kimberlite magmatism could reflect major changes in plate geometry during subduction of the Kula–Farallon plate.     

Geology of the Renard 65 kimberlite pipe,Québec,Canada

Renard 65, a diamondiferous pipe in the Neoproterozoic Renard kimberlite cluster (Québec, Canada), is a steeply-dipping and downward-tapering diatreme comprised of three pipe-filling units: kimb65a, kimb65b, and kimb65d. The pipe is surrounded by a marginal and variably-brecciated country rock aureole and is crosscut by numerous hypabyssal dykes: kimb65c. Extensive petrographic and mineralogical characterization of over 700 m of drill core from four separate drill holes, suggests that Renard 65 is a Group I kimberlite, mineralogically classified as phlogopite kimberlite and serpentine-phlogopite kimberlite. Kimb65a is a massive volcaniclastic kimberlite dominated by lithic clasts, magmaclasts, and discrete olivine macrocrysts, hosted within a fine-grained diopside and serpentine-rich matrix. Kimb65b is massive, macrocrystic, coherent kimberlite with a groundmass assemblage of phlogopite, spinel, perovskite, apatite, calcite, serpentine and rare monticellite. Kimb65c is a massive, macrocrystic, hypabyssal kimberlite with a groundmass assemblage of phlogopite, serpentine, calcite, perovskite, spinel, and apatite. Kimb65d is massive volcaniclastic kimberlite with localized textures that are intermediate between volcaniclastic and coherent, with tightly packed magmaclasts separated by a diopside- and serpentine-rich matrix. Lithic clasts of granite-gneiss in kimb65a are weakly reacted, with partial melting of feldspars and crystallization of richterite and actinolite. Lithic clasts in kimb65b and kimb65d are entirely recrystallized to calcite + serpentine/chlorite + pectolite and display inner coronas of diopside-aegirine and an outer corona of phlogopite. Compositions are reported for all minerals in the groundmass of coherent kimberlites, magmaclasts, interclast matrices, and reacted lithic clasts. The Renard 65 rocks are texturally classified as Kimberley-type pyroclastic kimberlites and display transitional textures. The kimberlite units are interpreted to have formed in three melt batches based on their distinct spinel chemistry: kimb65a, kimb65b and kimb65d. We note a strong correlation between the modal abundances of lithic clasts and the textures of the kimberlites, where increasing modal abundances of granite/gneiss are observed in kimberlites with increasingly fragmental textures.

     

Results of <Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar dating of phlogopites from kelyphitic rims around garnet grains (Udachnaya-Vostochnaya kimberlite pipe)

⁴⁰Ar/³⁹Ar dating of phlogopite from kelyphitic rims around garnet grains from the Udachnaya–Vostochnaya kimberlite pipe in the Sakha (Yakutia) Republic (Russia) revealed that when this mineral has contact with a kimberlite melt its age corresponds (within error limits) to that of the formation of the kimberlite pipe, thus indicating that the method may be used for dating kimberlites and related rocks. In mantle xenoliths, kelyphitic phlogopites rimming garnet grains partially lose radiogenic Ar, which results in a complex age spectrum. Rejuvenation of the K/Ar system in them is determined by the thermal impact of the kimberlite melt on captured rocks.     

Geology and resource development of the Kelvin kimberlite pipe,Northwest Territories,Canada

The early Cambrian to late Neoproterozoic Kelvin kimberlite pipe is located in the southeast of the Archean Slave Craton in northern Canada, eight km northeast of the Gahcho Kué diamond mine. Kelvin was first discovered in 2000 by De Beers Canada. Subsequent exploration undertaken by Kennady Diamonds Inc. between 2012 and 2016 resulted in the discovery of significant thicknesses of volcaniclastic kimberlite that had not previously been observed. Through extensive delineation drilling Kelvin has been shown to present an atypical, steep-sided inclined L-shaped pipe-like morphology with an overall dip of 15 to 20°. With a surface expression of only 0.08 ha Kelvin dips towards the northwest before turning north. The body (which remains open at depth) has been constrained to a current overall strike length of 700 m with varying vertical thickness (70 to 200 m) and width (30 to 70 m). Detailed core logging, petrography and microdiamond analysis have shown that the pipe infill comprises several phases of sub-horizontally oriented kimberlite (KIMB1, KIMB2, KIMB3, KIMB4, KIMB7 and KIMB8) resulting from multiple emplacement events. The pipe infill is dominated by Kimberley-type pyroclastic kimberlite or “KPK”, historically referred to as tuffisitic kimberlite breccia or “TKB”, with less common hypabyssal kimberlite (HK) and minor units with textures transitional between these end-members. An extensive HK sheet complex surrounds the pipe. The emplacement of Kelvin is believed to have been initiated by intrusion of this early sheet system. The main pipe-forming event and formation of the dominant KPK pipe infill, KIMB3, was followed by late stage emplacement of additional minor KPK and a hypabyssal to transitional-textured phase along the upper contact of the pipe, cross-cutting the underlying KIMB3. Rb-Sr age dating of phlogopite from a late stage phase has established model ages of 531 ± 8 Ma and 546 ± 8 Ma. Texturally and mineralogically, the Kelvin kimberlite is similar to other KPK systems such as the Gahcho Kué kimberlites and many southern African kimberlites; however, the external morphology, specifically the sub-horizontal inclination of the pipe, is unique. The morphology of Kelvin and the other kimberlites in the Kelvin-Faraday cluster defines a new type of exploration target, one that is likely not unique to the Kennady North Project area. Extensive evaluation work by Kennady Diamonds Inc. has resulted in definition of a maiden Indicated Mineral Resource for Kelvin of 8.5 million tonnes (Mt) of kimberlite at an average grade of 1.6 carats per tonne (cpt) with an average diamond value of US$ 63 per carat (ct).

     

The timing of kimberlite magmatism in North America: implications for global kimberlite genesis and diamond exploration

Based on a compilation of more than 100 kimberlite age determinations, four broad kimberlite emplacement patterns can be recognized in North America: (1) a northeast Eocambrian/Cambrian Labrador Sea province (Labrador, Québec), (2) an eastern Jurassic province (Ontario, Québec, New York, Pennsylvania), (3) a Cretaceous central corridor (Nunavut, Saskatchewan, central USA), and (4) a western mixed (Cambrian-Eocene) Type 3 kimberlite province (Alberta, Nunavut, Northwest Territories, Colorado/Wyoming). Ten new U–Pb perovskite/mantle zircon and Rb–Sr phlogopite age determinations are reported here for kimberlites from the Slave and Wyoming cratons of western North America. Within the Type 3 Slave craton, at least four kimberlite age domains exist: I-a southwestern Siluro-Ordovician domain (450 Ma), II-a SE Cambrian domain (540 Ma), III-a central Tertiary/Cretaceous domain (48–74 Ma) and IV-a northern mixed domain consisting of Jurassic and Permian kimberlite fields. New U–Pb perovskite results for the 614.5±2.1 Ma Chicken Park and 408.4±2.6 Ma Iron Mountain kimberlites in the State Line field in Colorado and Wyoming confirm the existence of at least two periods of pre-Mesozoic kimberlite magmatism in the Wyoming craton.

A compilation of robust kimberlite emplacement ages from North America, southern Africa and Russia indicates that a high proportion of known kimberlites are Cenozoic/Mesozoic. We conclude that a majority of these kimberlites were generated during enhanced mantle plume activity associated with the rifting and eventual breakup of the supercontinent Gondwanaland. Within this prolific period of kimberlite activity, there is a good correlation between North America and Yakutia for three distinct short-duration (10 my) periods of kimberlite magmatism at 48–60, 95–105 and 150–160 Ma. In contrast, Cenozoic/Mesozoic kimberlite magmatism in southern Africa is dominated by a continuum of activity between 70–95 and 105–120 Ma with additional less-prolific periods of magmatism in the Eocene (50–53 Ma), Jurassic (150–190) and Triassic (235 Ma). Several discrete episodes of pre-Mesozoic kimberlite magmatism variably occur in North America, southern Africa and Yakutia at 590–615, 520–540, 435–450, 400–410 and 345–360 Ma. One of the surprises in the timing of kimberlite magmatism worldwide is the common absence of activity between about 250 and 360 Ma; this period is even longer in southern Africa. This >110 my period of quiescence in kimberlite magmatism is likely linked to relative crustal and mantle stability during the lifetime of the supercontinent Gondwanaland.

Economic diamond deposits in kimberlite occur throughout the Phanerozoic from the Cambrian (Venetia, South Africa; Snap Lake and Kennady Lake, Canada) to the Tertiary (Mwadui, Tanzania; Ekati and Diavik in Lac de Gras, Canada). There are clearly some discrete periods when economic kimberlite-hosted diamond deposits formed globally. In contrast, the Devonian event, which is such an important source of diamonds in Yakutia, is notably absent in the kimberlite record from both southern Africa and North America.     

Kimberlites of the Manchary pipe: a new kimberlite field in Central Yakutia

This paper reports new petrographic and mineralogical data on the Manchary kimberlite pipe, which was discovered south of Yakutsk (Central Yakutia) in 2007–2008, 100 km. The pipe breaks through the Upper Cambrian carbonate deposits and is overlain by Jurassic terrigenous rock masses about 100 m thick. It is composed of greenish-gray kimberlite breccia with a serpentine-micaceous cement of massive structure. The porphyry texture of kimberlite is due to the presence of olivine, phlogopite, and picroilmenite phenocrysts. The SiO₂ and Al₂O₃ contents of the groundmass are indicative of typical noncontaminated kimberlites. The groundmass has a significant content of ore minerals: Fe- and Cr-spinels, perovskite, magnetite, and, less commonly, magnesian Cr-magnetite. Pyropes occur in kimberlites as sharp-edged fragments and show uneven distribution. Chemically, they belong to lherzolite, wehrlite, or nondiamondiferous dunite–harzburgite parageneses. Garnets corresponding to lherzolites of anomalous composition make up 8%; this is close to the garnet content of Middle Paleozoic kimberlites from the Yakutian kimberlite province. The pyropes from the new pipe are compositionally similar to those from diamond-poor Middle Paleozoic kimberlites in the north of the Yakutian diamondiferous province. Chemically, pyropes from the Manchary pipe and those from the modern alluvium of the Kengkeme and Chakyya Rivers differ substantially. Consequently, the rocks of the pipe could not be a source of pyropes for this alluvium. They probably occured from other sources. This fact along with numerous “pipelike” geophysical anomalies, suggest the existence of a new kimberlite field in Central Yakutia.     

Macrocrystal phlogopite Rb–Sr dates for the Ekati property kimberlites, Slave Province, Canada: evidence for multiple intrusive episodes in the Paleocene and Eocene

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

Mineralogy and geochemistry of Devonian ultramafic minor intrusions of the southern Kola Peninsula, Russia: implications for the petrogenesis of kimberlites and melilitites

Minor magmatic intrusions of kimberlite, melilitite and cpx-melilitite occur in the southern part of the Kola Peninsula, Russia, on the Terskii Coast and near the town of Kandalaksha. They yield K-Ar ages of 382 ± 14 Ma and 365 ± 16 Ma, similar to the magmatic rocks from the Kola Alkaline Province. The Terskii Coast kimberlites have mineralogical and geochemical affinities with group 1 kimberlites, whereas the Kandalaksha monticellite kimberlite more closely resembles calcite kimberlites. The lower Al₂O₃ content in the Kola kimberlites indicates a strongly depleted harzburgitic source, while higher Al₂O₃ in the melilitites suggests a lherzolitic source. The Terskii Coast kimberlites are anomalously potassic and significantly enriched in P and Ba compared to other group 1 kimberlites. In contrast, the melilitites are sodic and are anomalously depleted in P compared to worldwide melilitites. Trace element patterns of the Kola kimberlites and melilitites indicate the presence of K- and P-rich phases in the mantle source. To account for the K-troughs shown by both magma types, a K-rich phase such as phlogopite is thought to be residual in their sources; however, the anomalous K-enrichment in the Terskii Coast kimberlites may indicate that an additional metasomatic K-rich phase (e.g. K-richterite and/or a complex K-Ba-phosphate) existed in the kimberlite source. The P-depletion in the melilitites may suggest that a phosphate phase such as apatite remained residual in the melilititic source. However, anomalous P-enrichment in the kimberlites cannot be explained by complete melting of the same phase because the kimberlites are a smaller degree melt; thus, it is most likely that another metasomatic phosphate mineral existed in the source of the kimberlites. The Kola kimberlites and melilitites are all strongly LREE-enriched but the kimberlites have a steeper REE pattern and are significantly more depleted in HREE, indicating a higher proportion of garnet in their source. Higher Nb/Y ratios and lower SiO₂ values in the kimberlites indicate that they were a smaller degree partial melt than the melilitites. The presence of diamonds in the Terskii Coast kimberlites indicates a relatively deep origin, while the melilitites originated from shallower depth. The non-diamondiferous Kandalaksha monticellite kimberlite has lower abundances of all incompatible trace elements, suggesting a higher degree of partial melting and/or a less enriched and shallower source than the Terskii Coast kimberlites. The ⁸⁷Sr/⁸⁶Sr_i, ¹⁴³Nd/¹⁴⁴Nd_i and Pb isotope compositions confirm that the Terskii Coast kimberlites have close affinities with group 1 kimberlites and were derived from an asthenospheric mantle source, while the Kandalaksha monticellite kimberlite and Terskii Coast melilitites were derived from lithospheric mantle. Impact of a Devonian asthenospheric mantle plume on the base of the Archaean-Proterozoic lithosphere of the Kola Peninsula caused widespread emplacement of kimberlites, melilitites, ultramafic lamprophyres and other more fractionated alkaline magmas. The nature of the mantle affected by metasomatism associated with the plume and, in particular, the depth of melting and the stability of the metasomatic phases, gave rise to the observed differences between kimberlites and the related melilitites and other magmas. Received: 3 March 1997 / Accepted: 7 October 1997     

Kimberlites of the Man craton, West Africa

The Man craton in West Africa is an Archaean craton formerly joined to the Guyana craton (South America) that was rifted apart in the Mesozoic. Kimberlites of the Man craton include three Jurassic-aged clusters in Guinea, two Jurassic-aged clusters in Sierra Leone, and in Liberia two clusters of unknown age and one Neoproterozoic cluster recently dated at 800 Ma.

All of the kimberlites irrespective of age occur as small pipes and prolific dykes. Some of the Banankoro cluster pipes in Guinea, the Koidu pipes in Sierra Leone and small pipes in the Weasua cluster in Liberia contain hypabyssal-facies kimberlite and remnants of the so-called transitional-facies and diatreme-facies kimberlite. Most of the Man craton kimberlites are mineralogically classified as phlogopite kimberlites, although potassium contents are relatively low. They are chemically similar to mica-poor Group 1A Southern African examples.

The Jurassic kimberlites are considered to represent one province of kimberlites that track from older bodies in Guinea (Droujba 153 Ma) to progressively younger kimberlites in Sierra Leone (Koidu, 146 Ma and Tongo, 140 Ma). The scarcity of diatreme-facies kimberlites relative to hypabyssal-facies kimberlites and the presence of the so-called transitional-facies indicate that the pipes have been eroded down to the interface between the root and diatreme zones. From this observation, it is concluded that extensive erosion (1–2 km) has occurred since the Jurassic. In addition to erosion, the presence of abundant early crystallizing phlogopite is considered to have had an effect on the relatively small sizes of the Man craton kimberlites.     

Primary petrology,mineralogy and age of the Letšeng-la-Terae kimberlite (Lesotho,Southern Africa) and parental magmas of Group-I kimberlites

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 Fo_86?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.     

40Ar-39Ar age of carbonatite-alkaline magmatism in Sung valley, Meghalaya, India

⁴⁰Ar-³⁹Ar analyses of one alkali pyroxenite whole rock and two phlogopite separates of calcite carbonatites from the Sung Valley carbonatite-alkaline complex, which is believed to be a part of the Rajmahal-Bengal-Sylhet (RBS) flood basalt province, yielded indistinguishable plateau ages of 108.8 ± 2.0Ma, 106.4 ± 1.3Ma and 107.5 ± 1.4Ma, respectively. The weighted mean of these ages, 107.2 ± 0.8 Ma, is the time of emplacement of this complex. This implies that Sung Valley complex and probably other such complexes in the Assam-Meghalaya Plateau postdate the main flood basalt event (i.e., the eruption of tholeiites) in the RBS province by ∼10Ma.     

Petrology,genesis and geodynamic implication of the Mesoproterozoic–Late Cretaceous Timmasamudram kimberlite cluster,Wajrakarur field,Eastern Dharwar Craton,southern India

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.     

The Discovery of Phlogopite Exsolution Lamellae in Garnets of Eclogite Inclusions, Liaoning Province

In No. 50 kimberlite pipe of Fuxian County, Liaoning Province, an eclogite inclusion(nodule), which is extremely rare in kimberlites, was discovered and phlogopite exsolutionlamellae were found in garnets of the inclusion. Microscopic, TEM and energy spectral observa-tions and studies confirmed that these lamellae are phlogopite. They are colourless and acicularin section, generally 0.5-5μm in width and 10-100μm in length. Nevertheless, fine lamellae,0.05-0.1μm wide and 1-2μm long, are also well developed. Along [111] of the garnet, three setsof phlogopite lamellae show oriented arrangement approximately at angles of 60°-70°, indi-cating that these lamellae might be the product of exsolution from garnet as a result ofpressure-release when eclogite ascended from the relatively deep level to the relatively shallowlevel of the mantle. Tiny acicular exsolution minerals (or inclusions) are commonly found ingarnet and pyroxene in eclogite inclusions of kimberlites all over the world and it has been re-ported that the identified exsolution minerals include pyroxene and rutile. This is the first timethat phlogopite exsolution lamillae were found in eclogite inclusions in the world.     

Mesozoic lithospheric mantle of the northeastern Siberian craton (evidence from inclusions in kimberlite)

Several thousand clinopyroxene, garnet, and phlogopite inclusions of mantle rocks from Jurassic and Triassic kimberlites in the northeastern Siberian craton have been analyzed and compared with their counterparts from Paleozoic kimberlites, including those rich in diamond. The new and published mineral chemistry data make a basis for an updated classification of kimberlite-hosted clinopyroxenes according to peridotitic and mafic (eclogite and pyroxenite) parageneses. The obtained results place constraints on the stability field of high-Na lherzolitic clinopyroxenes, which affect the coexisting garnet and decrease its Ca contents. As follows from analyses of the mantle minerals from Mesozoic kimberlites, the cratonic lithosphere contained more pyroxenite and eclogite in the Mesozoic than in the Paleozoic. It virtually lacked ultradepleted harzburgite-dunite lithologies and contained scarce eclogitic diamonds. On the other hand, both inclusions in diamond and individual eclogitic minerals from Mesozoic kimberlites differ from eclogitic inclusions in diamond from Triassic sediments in the northeastern Siberian craton. Xenocrystic phlogopites from the D’yanga pipe have ⁴⁰Ar/³⁹Ar ages of 384.6, 432.4, and 563.4 Ma, which record several stages of metasomatic impact on the lithosphere. These phlogopites are younger than most of Paleozoic phlogopites from the central part of the craton (Udachnaya kimberlite). Therefore, hydrous mantle metasomatism acted much later on the craton periphery than in the center. Monomineral clinopyroxene thermobarometry shows that Jurassic kimberlites from the northeastern craton part trapped lithospheric material from different maximum depths (170 km in the D’yanga pipe and mostly < 130 km in other pipes). The inferred thermal thickness of cratonic lithosphere decreased progressively from ~ 260 km in the Devonian-Carboniferous to ~ 225 km in the Triassic and to ~ 200 km in the Jurassic, while the heat flux (Hasterok-Chapman model) was 34.9, 36.7, and 39.0 mW/m², respectively. Dissimilar PT patterns of samples from closely spaced coeval kimberlites suggest different emplacement scenarios, which influenced both the PT variations across the lithosphere and the diamond potential of kimberlites.     

Sm-Nd,K-Ar and petrologic study of some kimberlites from eastern United States and their implication for mantle evolution

We provide new data on Sm-Nd systematics, K-Ar dating and the major element chemistry of kimberlites from the eastern United States (mostly from central New York State) and their constituent mineral phases of olivine, clinopyroxene, garnet, phlogopite and perovskite. In addition, we report Nd-isotopes in a few kimberlites from South Africa, Lesotho and from the eastern part of China. The major element compositions of the New York dike rocks and of their constituent minerals including a xenolith of eclogite are comparable with those from the Kimberley area in South Africa. The K-Ar age of emplacement of the New York dikes is further established to be 143 Ma.We have analyzed the Nd-isotopic composition of the following kimberlites and related rocks: Nine kimberlite pipes from South Africa and Lesotho, two from southern India; one from the U.S.S.R., fifteen kimberlite pipes and related dike rocks from eastern and central U.S. and two pipes from the Shandong Province of eastern China. The age of emplacement of these kimberlites ranges from 1300 million years to 90 million years. The initial Nd-isotopic compositions of these kimberlitic rocks expressed as _Nd ^I with respect to a chondritic bulk-earth growth-curve show a range between 0 and +4, with the majority of the kimberlites being in the range 0 to +2. This range is not matched by any other suite of mantle-derived igneous rocks. This result strengthens our earlier conclusion that kimberlitic liquids are derived from a relatively primeval and unique mantle reservoir with a nearly chondritic Sm/Nd ratio.     

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, 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 T_Dm 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.     

Some petrological aspects of the Prairie Creek diamond-bearing kimberlite diatreme,Arkansas

Based on modal and chemical composition, the rocks of the Prairie Creek diatreme situated 4 km SSE of Murfreesboro, Pike County, Arkansas, are classified as micaceous kimberlite. The K-Ar isotopic analysis of phlogopite from this diatreme yielded an age of 106 ± 3 m.y. (Albian) which is in agreement with stratigraphic relations. Electron beam probe data on minerals from kimberlite breccia, one of the three textural types, are presented. The breccia is considered as the potential source of the diamonds that have been mined at the diatreme. It contains phenocrysts of olivine (Fo_90–92) and serpentine pseudomorphs after olivine embedded in a groundmass of serpentine, minor calcite, chrome-diopside, phlogopite (Mg/Mg+Fe = 84.15%), perovskite, spinels, and pentlandite. Xenoliths of shales, sandstones, and mantle-derived ultramafic material are also present. Spinels are rich in Cr, Ti, and Fe and generally low in Al. Zoned spinels show enrichments in Ti and Fe towards their rims. A positive correlation between 100(Fe³⁺+Ti)/(Cr+Al+Fe³⁺+Ti) and 100 Mg/(Mg+Fe²⁺) ratios exists in these spinels and probably reflects an oxygen fugacity increase during magma crystallization. Occluded gases in diamonds and kimberlites corroborate the hypothesis that the parent magma of the Prairie Creek kimberlite was derived by partial melting of upper-mantle garnet lherzolite under volatile-rich conditions, primarily enriched in H₂O and CO₂.     

Nd,Sr, Pb,Ar, and 0 isotopic systematics of Sturgeon Lake kimberlite,Saskatchewan, Canada: constraints on emplacement age,alteration, and source composition

Rb-Sr isotopic dating of phlogopite megacryst samples separated from Sturgeon Lake kimberlite, Saskatchewan, yields a crystallization age of 98±1 Ma (2 , MSWD=1.2; ⁸⁷Sr/⁸⁶Sr(t)=0.7059). The ⁴⁰Ar/³⁹Ar analyses of a phlogopite megacryst sample indicate the presence of large amounts of excess ⁴⁰Ar and yield an excessively old age of 410 Ma. Assessment of the Ar data using isotope correlation plots indicates clustering of the data points about a mixing line between the radiogenic ⁴⁰Ar component at 98 Ma and a trapped component with uniform ³⁶Ar/⁴⁰Ar and Cl/⁴⁰Ar. Values of {ie212-1} as high as +20%. (VSMOW) for calcite from the groundmass and a whole-rock sample indicate pervasive lowtemperature alteration. The {ie212-2} of matrix carbonate is-11.3%. (PDB), slightly lighter than typical values from the literature. The {ie212-3} values of about +5%. (VSMOW) for brown phlogopite megacrysts may be primary, green phlogopites are interpreted to be an alteration product of the brown variety and are 2%. heavier. Initial Nd-Sr-Pb isotopic ratios for a whole-rock sample {ie212-4}; ⁸⁷Sr/⁸⁶Sr=0.7063, ²⁰⁶Pb/²⁰⁴Pb=18.67, ²⁰⁷Pb/²⁰⁴Pb=15.54, ²⁰⁸Pb/²⁰⁴Pb=38.97) suggest an affinity with group I kimberlites. Initial {ie212-5} values of +1.7 and +0.5 (⁸⁷Sr/ ⁸⁶Sr(t)=0.7053 and 0.7050) for eclogitic and lherzolitic garnet megacryst samples, and values of 0.0 for two phlogopite megacryst samples reflect an origin from an isotopically evolving melt due to assimilation of heterogeneous mantle. Lilac high-Cr lherzolitic garnet megacrysts give an unusually high {ie212-6} of +28.6 (⁸⁷Sr/⁸⁶Sr=0.7046) indicating a xenocrystic origin probably from the lithospheric mantle. The very radiogenic ⁸⁷Sr/⁸⁶Sr and ²⁰⁶Pb/²⁰⁴Pb ratios of the kimberlite are consistent with melting of EM II (enriched) mantle components.