A new body of highly diamondiferous kimberlites in the Nakyn field of the Yakutian kimberlite province

We report the first data on the contents of main oxides and REE in rocks and the compositions of pyropes and almandines from the Maiskoe kimberlite body recently discovered in the Nakyn field of the Yakutian kimberlite province.The kimberlites are characterized by low contents of Ti, a slight domination of Mg over Ca, and high contents of K₂O in some samples. The pyropes have high contents of Cr₂O₃ (up to 14.5 wt.%); many of them (～16%) are poor in Ca. In petrochemical and mineralogical features the kimberlites of the Maiskoe body are complementary to the highly diamondiferous kimberlites of the nearby Botuobinskaya and Nyurbinskaya pipes. At the same time, they are not the final link in the evolution of kimberlite magmatism in the Nakyn field, which makes the latter still more promising for diamonds.     

Middle Paleozoic kimberlite magmatism in the northeastern Siberian craton

The mineral chemistry and crystal morphology of kimberlite pyropes from the Billyakh River placer in the northeastern Siberian craton are characterised in terms of the placer history. The pyropes bear signatures of chemical weathering (dissolution), presumably in a Middle Paleozoic laterite profile, and therefore were originally hosted by Middle Paleozoic kimberlites. The broad occurrence of placer pyropes with lateritic dissolution signatures points to the presence of Middle Paleozoic diamond-bearing kimberlites in the study area.     

Petrogeochemical characteristics of the kimberlites from the Middle Markha region with application to the problem of the geochemical heterogeneity of kimberlites

A comparative geochemical investigation of kimberlites from the Middle Markha region and traditional diamond-bearing areas of Yakutia supported the division of kimberlites into two geochemical types. One of them includes kimberlites from the traditional diamond-bearing areas of Yakutia, and the other is represented by the kimberlites of the Middle Markha region and the rocks of the Zolotitsa field and V. Grib pipe of the Arkhangelsk province. The obtained representative parameters of the two kimberlite types indicate a sharp geochemical contrast between them, and the individual features of correlation relationships between the elements in the rocks of the two types suggest that these differences are related to fundamental genetic factors, which concern primarily the group of highly charged incompatible trace and radioactive elements. The presence of geochemically contrasting rocks within a rather uniform petrochemical association and the geochemically specialized occurrences of kimberlites and related rocks are consequences of repeated metasomatic transformations of mantle rocks under the influence of deep-derived fluids or volatiles released during the recycling of subneous source and subsequent derivation of geochemically specialized types of deep magmas showing signatures of individual mantle sources.     

Inclusions of crichtonite group minerals in pyropes from the Internatsionalnaya kimberlite pipe,Yakutia

The results of study of crichtonite group minerals in pyropes from the Internatsionalnaya kimberlite pipe are reported. Most of the studied samples are characterized by high concentrations of Sr, Ca, Na, and LREEs in comparison with minerals of the LIMA series from kimberlites of South Africa, whereas the average concentrations of Ba and K are significantly lower. Crichtonite group minerals in pyropes are characterized by predomination of Na over K in most samples and by a high concentration of Al₂O₃ (up to 4.5 wt %). Findings of inclusions of crichtonite group minerals with high concentrations of incompatible elements provide evidence for the metasomatic origin of host chromium-rich pyropes.     

Coalingite from kimberlite breccia of the Manchary pipe,Central Yakutia

Coalingite, Mg₁₀Fe₂(CO₃)(OH)₂₄ · 2H₂O, rare Mg–Fe hydrous carbonate, has been found in the course of the mineralogical study of a disintegrated kimberlite breccia from the Manchary pipe of the Khompu–May field located in the Tamma Basin, Central Yakutia, 100 km south of Yakutsk. Coalingite occurs as small reddish brown platelets, up to 0.2 mm in size. It is associated with lizardite, chrysotile and brucite, which are typical kimberlitic assemblage. Coalingite is a supergene mineral, but in this case, it is produced by the interaction of brucite-bearing kimberlite and underground water circulating through a vertical or oblique fault zone.     

Behavior of major and rare-earth elements during the postmagmatic alteration of kimberlites

During serpentinization and subsequent alteration in the absence of brucite, kimberlites accumulate uncompensated silica. Its amount can be calculated from the average compositions of the rock-forming minerals (olivine, calcite, phlogopite) and the chemical compositions of the rocks. The contents of rock-forming oxides and REE were determined in 12 kimberlite pipes of the Yakutian kimberlite province, in 413 samples from secondary-alteration zones and of unaltered kimberlites. Columns of successive kimberlite alterations were constructed for each pipe on the basis of secondary-quartz data; here, the behavior of rock-forming oxides and REE was assessed. All the studied rocks had experienced different degrees of postmagmatic hydrothermal metasomatism at different depths in all the pipes. The changes were reflected in the supply/loss of rock-forming oxides and REE. The supply of REE during the hydrothermal metasomatism enriched the kimberlites with TiO₂, P₂O₅, and CaO. During the removal of REE, most of the rock-forming oxides were partially lost. The maximum REE supply was 67% in the Udachnaya-Vostochnaya pipe and 59% in the Nyurbinskaya pipe as compared with the unaltered kimberlites. The maximum REE loss was 87% in the Aikhal pipe and 81% in the Internatsional’naya pipe as compared with the unaltered kimberlites. The initial REE contents of the postmagmatically altered kimberlites changed considerably in all the studied cases. This conclusion was drawn owing to the use of normative-quartz content as a criterion for secondary alteration.     

Polygenetic sources of kimberlites, magma composition, and diamond potential exemplified by the East European and Siberian cratons

The petrological and geochemical characteristics of kimberlites from two Russian provinces of the northern East European craton (EEP) and the Siberian craton (SC) (especially the Yakutian diamondiferous province, YDP), and aphanitic kimberlites from the Jericho pipe (Canada) were compared for the elucidation of some aspects of the genesis of these rocks. The comparison of the EEP and YDP showed that they comprise identical rock associations with some variations in kimberlite composition between particular fields and regions, which are clearly manifested in the TiO₂-K₂O, TiO₂-(Y, Zr, HREE), SiO₂-MgO, SiO₂-Al₂O₃, MgO-Ni, MgO-CO₂, and MgO-H₂O diagrams and in variations in light element ratios (Li/Yb, Be/Nd, and B/Nb). The compositions of YDP kimberlites are confined mainly to quadrant III; i.e., their source was mainly the depleted mantle, whereas the compositions of EEP kimberlites fall within all four quadrants in the fields of both enriched and slightly depleted mantle reservoirs. The initial (¹⁴³Nd/¹⁴⁴Nd) _i ratio of kimberlites from the Yakutian collection is 0.5121–0.5126. The lead isotopic characteristics of the EEP and YDP kimberlites are similar to mantle values: ²⁰⁶Pb/²⁰⁴Pb of 16.19–19.14, ²⁰⁷Pb/²⁰⁴Pb of 15.44–15.61, and ²⁰⁸Pb/²⁰⁴Pb of 34.99–38.55. In the ²⁰⁷Pb/²⁰⁴Pb-²⁰⁶Pb/²⁰⁴Pb diagram, part of the kimberlites, including those from the Botuobiya pipe, fall within the lower part of the field of group I kimberlites from southern Africa near the Pb isotopic composition of the depleted mantle. It was shown that the chemical compositions of the aphanitic kimberlites of the Jericho pipe (supposedly approaching the composition of primary magmas) are similar to those of some individual kimberlite samples from the YDP and EEP. It was supposed that the initial kimberlite melt arrived from the asthenosphere and was enriched in water and other volatile components (especially CO₂). During its ascent to the surface, the melt assimilated mantle components, primarily MgO; as a result, it acquired the compositional characteristics observed in kimberlites. Subsequent compositional modifications were related to diverse factors, including the type of mantle metasomatism, degree of melting, etc. We emphasized the importance of petrological and geochemical criteria (low contents of HREE and Ti in the rocks and a kimberlite source similar to BSE or EMI) for the estimation of the diamond potential of rocks.     

Zoning of Faults and Secondary Mineralization of Host Rocks of Kimberlites of the Maiskoe Diamond Deposit,Nakyn Field,Yakutia

Study of faults and secondary mineralization of host rocks of diamond-bearing kimberlites yields important data for local prediction of kimberlite bodies. Of special methodological interest are exploration data on deposits where the study of host rocks is based on a dense observation network. Factual material for this paper was collected from cores of all inclined exploration boreholes of the Maiskoe diamond deposit found in the Nakyn field in Yakutia in 2006. The paper shows a nonuniform distribution of tectonic deformations, stringer mineralization, O and C isotopes of calcite, and CO₂ content of Lower Paleozoic host carbonate rocks of kimberlites. Our data agree with different diamond potentials of two areas of the Maiskoe kimberlite body, which can be used to search for and explore deposits.     

Mineralogical studies of pyropes in kimberlites from China

Based on the measurements of refractive index,specific gravity,unit cell parameter,and mineral chemistry and infrared absorption spectrum analyses of pyropes in kimberlites from China,systematic studies of the Physical properties and compositional variations of pyropes of different colors and diverse paragenetic types,within and between kimberlite provinces have been undertaken,The origin of pyropes in the Kimberlites and the depth of their formation have been discussed.Pyropes of the purple series are different from those of the orange series in physical and chemical properties,for exaple,pyropes of the puple series are higher in α0,RI,SG,Cr2O3,MgO,Cr/(Cr Al),Mg/(Mg Fe),and Mg/(Mg Ca),and lower in Al2O3,Fe2O3 FeO than those of the orange series.The classification of garnets in kimberlites from china by the Dawson and Stephens‘ method(1975) has been undertaken and clearly demonstrates that pyropes of diamond-rich kimberlites contain much more groups than those of diamond-poor,especially diamond-free kimberlites.The higher in α0,RI,SG,Cr2O(3.Cr/(Cr Al),knorringite and Cr-component the pyropes are ,the richer in diamond the kimberlites will be.The infrared absorption spectrum patterns of pyropes change with their chemical composition regularly,as reflected in the shape and position of infrared absorption peaks.Two absortpion bands at 862-901 cm^-1 will grade into degeneration from splitting and the absorption band positions of pyropes shift toward lower frequency with increasing Cr2O3 content and Cr/(Cr Al) ratio of pyropes,LREE contents of orange pyrope megacrysts are similar to those of porple pyrope macrocrysts,but the former is higher in HREE than the latter,showing their different chondrite-normalized patterns.The formation pressures of pyropes calculated by Cr-component,Ca-component,knorringite molecules of pyropes show that some pyropes of the purple series in diamondiferous kimberlites fall into the diamond stability field.but all pyropes of diamond-free kimberlites lie outside the diamond stability field.The megacrysts were formed through early crystallization of kimberlites magma at high pressure condition,the majority of the purple pyrope macrocrysts have been derived from disaggregated xenoliths but the minoirty of them appear to be fragments of the discrete megacryst pyropes,or phenocrysts.     

Kimberlites and lamproites of the East European Platform: Petrology and geochemistry

Several episodes of kimberlite magmatism occurred in the East European Province (EEP) during a long (about 1.5 Gyr) time period, from the Late Paleoproterozoic (ca. 1.8 Ga) in the Archean Ukrainian and Baltic shields to the Middle Paleozoic (ca. 0.36 Ga) mainly in the Arkhangelsk, Timan, and adjacent regions. Based on the analysis of data on 16 kimberlite occurrences and four lamproite occurrences within the EEP, five time stages can be distinguished; one of them, the Middle Paleozoic stage (Middle Ordovician and Devonian), is the most productive epoch for diamond in the northern hemisphere (EEP, Siberian Craton, and part of the China Craton). The analysis of petrological and geochemical characteristics of kimberlites (lamproites were studied less thoroughly) revealed variations in rock composition and their correlation with a number of factors, including the spatial confinement to the northern or southern Archean blocks of the craton, time of formation of the source of kimberlite melts, contents of volatiles and autoliths, etc. Three petrogeochemical types of kimberlites were distinguished: high-, medium-, and low-Ti (TiO₂ > 3 wt %, 1–3 wt %, and <1 wt %, respectively). There are two time intervals of the formation of kimberlite and lamproite sources in the EEP, corresponding to T_Nd(DM) values of about 2 Ga (up to 2.9 Ga in the Por’ya Guba occurrence) and 1 Ga. The latter interval includes two groups of occurrences with model source ages of about 1 Ga (low-and medium-Ti kimberlites of the Zolotitsa and Verkhotina occurrences) and about 0.8 Ga (high-Ti kimberlites of the Kepino and a number of other occurrences); i.e., there seems to be an evolutionary trend in the composition of kimberlites. Concentric zoning patterns were recognized. The role of the crust in kimberlite sources is discussed; it is assumed that buried remnants of the oceanic lithosphere (megaliths) may underlie whole continents. A unique feature of the composition of low-Ti kimberlites, for instance, kimberlites of the Zolotitsa occurrence (to a smaller extent, medium-Ti kimberlites of the V. Grib pipe) is the distinct depletion of highly charged elements and pronounced negative anomalies of Ti, Zr, Th, U, Nb, and Ta in trace-element distribution patterns, which indicates a contribution of crustal material to the source of these kimberlites. It was shown that autoliths exert a significant influence on the differentiation of kimberlite material, resulting in the enrichment of rocks in the whole spectrum of incompatible elements. It was argued that geochemical criteria can be used together with traditional criteria (including those based on indicator minerals) for the assessment of diamond potential in EEP occurrences. We hope that such a combined approach will yield important outcomes in the future.     

Petrography,Sr-isotope geochemistry and geochronology of the Nxau Nxau kimberlites,north-west Botswana

The Nxau Nxau kimberlites in northwest Botswana belong to the Xaudum kimberlite province that also includes the Sikereti, Kaudom and Gura kimberlite clusters in north-east Namibia. The Nxau Nxau kimberlites lie on the southernmost extension of the Congo Craton, which incorporates part of the Damara Orogenic Belt on its margin. The Xaudum kimberlite province is geographically isolated from other known clusters but occurs within the limits of the NW-SE oriented, Karoo-aged Okavango Dyke Swarm and near NE-SW faults interpreted as the early stages of the East African Rift System. Petrographic, geochronological and isotopic studies were undertaken to characterise the nature of these kimberlites and the timing of their emplacement. The Nxau Nxau kimberlites exhibit groundmass textures, mineral phases and Sr-isotope compositions (⁸⁷Sr/⁸⁶Sr_i of 0.7036 ± 0.0002; 2σ) that are characteristic of archetypal (Group I) kimberlites. U-Pb perovskite, ⁴⁰Ar/³⁹Ar phlogopite and Rb-Sr phlogopite ages indicate that the kimberlites were emplaced in the Cretaceous, with perovskite from four samples yielding a preferred weighted average U-Pb age of 84 ± 4 Ma (2σ). This age is typical of many kimberlites in southern Africa, indicating that the Xaudum occurrences form part of this widespread Late Cretaceous kimberlite magmatic province. This time marks a significant period of tectonic stress reorganisation that could have provided the trigger for kimberlite magmatism. In this regard, the Nxau Nxau kimberlites may form part of a NE-SW oriented trend such as the Lucapa corridor, with implications for further undiscovered kimberlites along this corridor.

     

The age of predictable primary diamond sources in the northeastern Siberian Platform

The U–Pb (SHRIMP) age was determined for zircons collected from 26 observation and sampling sites of diamonds and index minerals in the northeastern Siberian Platform. This part of the region hosts 15 low-diamondiferous Paleozoic and Mesozoic kimberlite fields, excluding the near economic Triassic Malokuonapskaya pipe in the Kuranakh field. Four epochs of kimberlite formation (Silurian, Late Devonian to Early Carboniferous, Middle to Late Triassic, and Middle to Late Jurassic) of the Siberian Platform, including its northeastern part, are confirmed as a result of our studies. Most observation points, including economic Quaternary diamond placers, contain Middle to Late Triassic zircons, which confirms the abundant Late Triassic volcanism in this region. The positive correlation of diamonds and major index minerals of kimberlites (mostly, garnets) at some observation sites indicates the possible Triassic age of the predictable diamondiferous kimberlites.     

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 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.

     

Olivine in the Udachnaya-East Kimberlite (Yakutia, Russia): Types, Compositions and Origins

Olivine is the principal mineral of kimberlite magmas, and isthe main contributor to the ultramafic composition of kimberliterocks. Olivine is partly or completely altered in common kimberlites,and thus unavailable for studies of the origin and evolutionof kimberlite magmas. The masking effects of alteration, commonin kimberlites worldwide, are overcome in this study of theexceptionally fresh diamondiferous kimberlites of the Udachnaya-Eastpipe from the Daldyn–Alakit province, Yakutia, northernSiberia. These serpentine-free kimberlites contain large amountsof olivine (50 vol.%) in a chloride–carbonate groundmass.Olivine is represented by two populations (olivine-I and groundmassolivine-II) differing in morphology, colour and grain size,and trapped mineral and melt inclusions. The large fragmentalolivine-I is compositionally variable in terms of major (Fo_85–94)and trace element concentrations, including H₂O content (10–136ppm). Multiple sources of olivine-I, such as convecting andlithospheric mantle, are suggested. The groundmass olivine-IIis recognized by smaller grain sizes and perfect crystallographicshapes that indicate crystallization during magma ascent andemplacement. However, a simple crystallization history for olivine-IIis complicated by complex zoning in terms of Fo values and traceelement contents. The cores of olivine-II are compositionallysimilar to olivine-I, which suggests a genetic link betweenthese two types of olivine. Olivine-I and olivine-II have oxygenisotope values (+ 5·6 ± 0·1 VSMOW, 1 SD)that are indistinguishable from one another, but higher thanvalues (+ 5·18 ± 0·28) in ‘typical’mantle olivine. These elevated values probably reflect equilibriumwith the Udachnaya carbonate melt at low temperatures and ¹⁸O-enrichedmantle source. The volumetrically significant rims of olivine-IIhave constant Fo values (89·0 ± 0·2 mol%),but variable trace element compositions. The uniform Fo compositionsof the rims imply an absence of fractionation of the melt'sFe²⁺/Mg, which is possible in the carbonatite melt–olivinesystem. The kimberlite melt is argued to have originated inthe mantle as a chloride–carbonate liquid, devoid of ‘ultramafic’or ‘basaltic’ aluminosilicate components, but becameolivine-laden and olivine-saturated by scavenging olivine crystalsfrom the pathway rocks and dissolving them en route to the surface.During emplacement the kimberlite magma changed progressivelytowards an original alkali-rich chloride–carbonate meltby extensively crystallizing groundmass olivine and gravitationalseparation of solids in the pipe. KEY WORDS: kimberlite; olivine; partial melting; carbonatitic melt; oxygen isotopes; H₂O     

Middle Paleozoic basaltic and kimberlitic magmatism in the northwestern shoulder of the Vilyui Rift,Siberia: relations in space and time

A Middle Paleozoic tectonothermal event in the eastern Siberian craton was especially active in the area of the Vilyui rift, where it produced a system of rift basins filled with Devonian–Early Carboniferous volcanics and sediments, as well as long swarms of mafic dikes on the rift shoulders. Basalts occur mostly among Middle Devonian sediments and are much less spread in Early Carboniferous formations. The dolerite dikes of the Vilyui–Markha swarm in the northwestern rift border coexist with the Mirnyi and Nakyn fields of diamond-bearing kimberlites. The voluminous dikes and sills intruded before the emplacement of kimberlites. The Mir kimberlite crosscuts a dolerite sill and a dike in the Mirnyi field, while a complex dolerite dike (monzonite porphyry) cuts through the Nyurba kimberlite in the Nakyn field. Thus, the kimberlites correspond to a longer span of Middle Paleozoic basaltic magmatism. The basalts in Middle Paleozoic sediments have faunal age constraints, but the age of dolerite dikes remains uncertain. The monzonite porphyry dike in the Nyurba kimberlite has been dated by the 40Ar/39Ar method, and the obtained age must be the upper bound of the dike emplacement. The space and time relations between basaltic and kimberlitic magmatism were controlled by Devonian plume–lithosphere interaction.     

The Origins of Yakutian Eclogite Xenoliths

Owing to the association with diamonds, eclogite xenoliths havereceived disproportionate attention given their low abundancein kimberlites. Several hypotheses have been advanced for theorigin of eclogite xenoliths, from the subduction and high-pressuremelting of oceanic crust, to cumulates and liquids derived fromthe upper mantle. We have amassed a comprehensive data set,including major- and trace-element mineral chemistry, carbonisotopes in diamonds, and Rb–Sr, Sm–Nd, Re–Os,and oxygen isotopes in ultrapure mineral and whole-rock splitsfrom eclogites of the Udachnaya kimberlite pipe, Yakutia, Russia.Furthermore, eclogites from two other Yakutian kimberlite pipes,Mir and Obnazhennaya, have been studied in detail and offercontrasting images of eclogite protoliths. Relative to eclogitesfrom southern Africa and other Yakutian localities, Udachnayaeclogites are notable in the absence of chemical zoning in mineralgrains, as well as the degree of light rare earth element (LREE)depletion and unradiogenic Sr; lack of significant oxygen, sulfur,and carbon isotopic variation relative to the mantle; and intermineralradiogenic isotopic equilibration. Several of these eclogitescould be derived from ancient, recycled, oceanic crust, butmany others exhibit no evidence for an oceanic crustal protolith.The apparent lack of stable-isotope variation in the Udachnayaeclogites could be due to the antiquity of the samples and consequentlack of deep oceanic and biogenically diverse environments atthat time. Those eclogites that are interpreted to be non-recycledhave compositions characteristic of Group A eclogites from otherlocalities that also have been interpreted as being directlyfrom the mantle. At least two separate and diverse isotopicreservoirs are suggested by Nd isotopic whole-rock reconstructions.Most samples were derived from typical depleted mantle. However,two groups of three samples each indicate both enriched mantleand possible ultra-depleted mantle present beneath Yakutia duringthe late Archean and early Proterozoic. The vast majority ofeclogites studied from the Obnazhennaya pipe also exhibit characteristicsof Group A eclogites and are probably derived directly fromthe mantle. However, the eclogites from the Mir kimberlite aremore typical of other eclogites world-wide and show convincingevidence of a recycled, oceanic crustal affinity. We concurwith the late Ted Ringwood that eclogites can be formed in avariety of ways, both within the mantle and from oceanic crustalresidues. KEY WORDS: diamonds; eclogite xenoliths; isotopic composition; REE; Yakutia     

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.

     

Major element composition of concentrate garnets in Proterozoic kimberlites from the Eastern Dharwar Craton,India: Implications on sub-continental lithospheric mantle

This study examines the major element composition of mantle-derived garnets recovered from heavy mineral concentrates of several Proterozoic kimberlites of the diamondiferous Wajrakarur Kimberlite Field (WKF) and the almost barren Narayanpet Kimberlite Field (NKF) in the Eastern Dharwar Craton of southern India. Concentrate garnets are abundant in the WKF kimberlites, and notably rare in the NKF kimberlites. Chemical characteristics of the pyropes indicate that the lithology of the sub-continental lithospheric mantle (SCLM) beneath both the kimberlite fields was mainly lherzolitic at the time of kimberlite eruption. A subset of green pyropes from the WKF is marked by high CaO and Cr₂O₃ contents, which imply contribution from a wehrlitic source. The lithological information on SCLM, when studied alongside geobarometry of lherzolite and harzburgite xenoliths, indicates that there are thin layers of harzburgite within a dominantly lherzolitic mantle in the depth interval of 115–190 km beneath the WKF. In addition, wehrlite and olivine clinopyroxenite occur locally in the depth range of 120–130 km. Mantle geotherm derived from xenoliths constrains the depth of graphite–diamond transition to 155 km beneath the kimberlite fields. Diamond in the WKF thus could have been derived from both lherzolitic and harzburgitic lithologies below this depth. The rarity of diamond and garnet xenocrysts in the NKF strongly suggest sampling of shallower (<155 km depth) mantle, and possibly a shallower source of kimberlite magma than at the WKF.     

Mineralogy and petrography of a kimberlite-picrite dike in the northwestern Urik-Iya Graben,the eastern Sayan region

Mica kimberlite and alkali picrite were identified in the northwestern Urik-Iya Graben of the eastern Sayan region. Typomorphism of Cr-diopside and high-Cr (up to 55.22 wt % Cr₂O₃) spinel from kimberlite of the Bushkanai dike indicate that the melt was generated in the mantle, composed of spinel peridotite. The high content of Cr-spinel (45–55 wt % Cr₂O₃) microlites in the groundmass of kimberlite and small amounts of ulvospinel and titanomagnetite in the absence of perovskite testifies to the diamond potential of this kimberlite. Picroilmenite, manganoilmenite with an anomalously high MnO content (11.37–17.78 wt %), and barium titanate with (wt %) 62.21 TiO₂, 0.61 Cr₂O₃, 15.89 FeO, 4.05 MnO, 1.71 CaO, and 11.13 BaO close in composition to a new mineral species from the Murun pluton were identified in the groundmass for the first time. Kimberlite from the Bushkanai dike belongs to the Zolotitsa low-Ti geochemical type of kimberlites derived from the slightly enriched lithospheric mantle EM1. The distribution of trace elements, including REE, in picrite from the same dike corresponds to the slightly depleted asthenospheric mantle. Different mantle sources of kimberlite and picrite from the same dike indicate that these rocks are related to independent melts rather than to products of fractionation of a common parental alkaline ultramafic magma.