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     

Nature of diamonds in Yakutian eclogites: views from eclogite tomography and mineral inclusions in diamonds

We have performed dissections of two diamondiferous eclogites (UX-1 and U33/1) from the Udachnaya kimberlite, Yakutia in order to understand the nature of diamond formation and the relationship between the diamonds, their mineral inclusions, and host eclogite minerals. Diamonds were carefully recovered from each xenolith, based upon high-resolution X-ray tomography images and three-dimensional models. The nature and physical properties of minerals, in direct contact with diamonds, were investigated at the time of diamond extraction. Polished sections of the eclogites were made, containing the mould areas of the diamonds, to further investigate the chemical compositions of the host minerals and the phases that were in contact with diamonds. Major- and minor-element compositions of silicate and sulfide mineral inclusions in diamonds show variations among each other, and from those in the host eclogites. Oxygen isotope compositions of one garnet and five clinopyroxene inclusions in diamonds from another Udachnaya eclogite (U51) span the entire range recorded for eclogite xenoliths from Udachnaya. In addition, the reported compositions of almost all clinopyroxene inclusions in U51 diamonds exhibit positive Eu anomaly. This feature, together with the oxygen isotopic characteristics, is consistent with the well-established hypothesis of subduction origin for Udachnaya eclogite xenoliths. It is intuitive to expect that all eclogite xenoliths in a particular kimberlite should have common heritage, at least with respect to their included diamonds. However, the variation in the composition of multiple inclusions within diamonds, and among diamonds, from the same eclogite indicates the involvement of complex processes in diamond genesis, at least in the eclogite xenoliths from Yakutia that we have studied.     

A study of garnets from eclogite and peridotite xenoliths found in kimberlite

The chemical compositions of garnets from 58 eclogite, 72 peridotite and 4 pyroxenite xenoliths in kimberlites have been estimated from their unit cell edge length and refractive indices. The samples studied were obtained from 17 kimberlite occurrences and include all those of known source which remain in the famous Williams (1932) collection which is stored at the University of Cape Town. Every suitable sample available to the authors has been examined.A gap in the range of garnet volume percentages occurs in the samples studied between approximately 15 and 30%. Garnet peridotites characteristically have <15% garnet and eclogites >30% garnet. Very rare exceptions occur. Our collection contains no eclogites with olivine and only one with orthopyroxene. All but two of the peridotite-pyroxenite group contain orthopyroxene. The garnets from the peridotites and pyroxenites plot on a pyrope-almandine-uvarovite triangle in a narrow band with a remarkably constant almandine/uvarovite ratio. Garnets from the eclogites are plotted on a pyrope-almandine-grossularite triangle and have a wide spread of compositions. These fall into 4 groups viz. eclogite I, eclogite II, kyanite eclogite and corundum eclogite.The reasons for the differences in garnet chemistry are considered and a tentative evolutionary scheme suggested by partial melting of the garnet peridotite which is assumed to occur in the upper mantle. Recent models of upper mantle composition and the genesis of garnet-bearing xenoliths in kimberlite are briefly and critically examined.S.A. UMP Publication No. 9.     

Eclogite xenoliths from stockdale kimberlite,Kansas

Six kimberlite pipes of late Cretaceous or Tertiary age occur in Riley Co., east-central Kansas. Within the pipes xenoliths of local sedimentary and exotic igneous rocks are common, especially in the Stockdale pipe. Igneous rocks which occur as xenoliths include granite, gabbro, metagabbro, pyroxenite and eclogite. In the eclogites omphacitic clinopyroxene (approx. Di₅₂Jd₂₄mol%) and pyropic garnet (approx. Py₄₇Al₃₅Gr₁₂mol%) are the predominant minerals with subordinate amounts of rutile and sulphides (pyrrotite-pentlandite (?)-chalcopyrite). Interstitial kaersutitic amphibole is a minor constituent. The eclogites are chemically equivalent to olivine-basalt. The texture, composition and mineralogy of the eclogites from Kansas are similar to those of eclogites from kimberlite pipes in South Africa and Siberia. Whereas the rocks from these latter localities display a range in composition, those examined to date from Kansas are of fairly restricted composition. Furthermore it seems probable that the eclogites from Stockdale formed under limited P-T conditions within the mantle. This is the first record of such eclogites on the North American continent.     

Komsomolskaya diamondiferous eclogites: evidence for oceanic crustal protoliths

The Komsomolskaya kimberlite is one of numerous (>1,000) kimberlite pipes that host eclogite xenoliths on the Siberian craton. Eclogite xenoliths from the adjacent Udachnaya kimberlite pipe have previously been geochemically well characterized; however, data from surrounding diamond-bearing kimberlite pipes from the center of the craton are relatively sparse. Here, we report major- and trace-element data, as well as oxygen isotope systematics, for mineral separates of diamondiferous eclogite xenoliths from the Komsomolskaya kimberlite, suggesting two distinct subgroups of a metamorphosed, subducted oceanic crustal protolith. Using almandine contents, this suite can be divided into two subgroups: group B1, with a high almandine component (>20 mol%) and group B2, with a low almandine component (<20 mol%). Reconstructed REE profiles for B1 eclogites overlap with typical oceanic basalts and lack distinct Eu anomalies. In addition, elevated oxygen isotope values, which are interpreted to reflect isotopic exchange with seawater at low temperatures (<350 °C), are consistent with an upper-oceanic crustal protolith. Reconstructed REE profiles for B2 eclogites are consistent with oceanic gabbros and display distinct Eu anomalies, suggesting a plagioclase-rich cumulate protolith. In contrast to B1, B2 eclogites do not display elevated oxygen isotope values, suggesting an origin deep within the crustal pile, where little-to-no interaction with hydrothermal fluids has occurred. Major-element systematics were reconstructed based on mineral modes; group B1 eclogites have higher MgO wt% and lower SiO₂ wt%, with respect to typical oceanic basalts, reflecting a partial melting event during slab subduction. Calculated residues from batch partial melt modeling of a range of Precambrian basalts overlap with group B1 trace-element chemistry. When taken together with the respective partial melt trajectories, these melting events are clearly linked to the formation of Tonalite–Trondhjemite–Granodiorite (TTG) complexes. As a result, we propose that many, if not all, diamondiferous eclogite xenoliths from Komsomolskaya represent mantle ‘restites’ that preserve chemical signatures of Precambrian oceanic crust.     

Phlogopite in mantle xenoliths and kimberlite from the Grib pipe,Arkhangelsk province,Russia:Evidence for multi-stage mantle metasomatism and origin of phlogopite in kimberlite

We present petrography and mineral chemistry for both phlogopite,from mantle-derived xenoliths(garnet peridotite,eclogite and clinopyroxene-phlogopite rocks)and for megacryst,macrocryst and groundmass flakes from the Grib kimberlite in the Arkhangelsk diamond province of Russia to provide new insights into multi-stage metasomatism in the subcratonic lithospheric mantle(SCLM)and the origin of phlogopite in kimberlite.Based on the analysed xenoliths,phlogopite is characterized by several generations.The first generation(Phil)occurs as coarse,discrete grains within garnet peridotite and eclogite xenoliths and as a rock-forming mineral within clinopyroxene-phlogopite xenoliths.The second phlogopite generation(Phl2)occurs as rims and outer zones that surround the Phil grains and as fine flakes within kimberlite-related veinlets filled with carbonate,serpentine,chlorite and spinel.In garnet peridotite xenoliths,phlogopite occurs as overgrowths surrounding garnet porphyroblasts,within which phlogopite is associated with Cr-spinel and minor carbonate.In eclogite xenoliths,phlogopite occasionally associates with carbonate bearing veinlet networks.Phlogopite,from the kimberlite,occurs as megacrysts,macrocrysts,microcrysts and fine flakes in the groundmass and matrix of kimberlitic pyroclasts.Most phlogopite grains within the kimberlite are characterised by signs of deformation and form partly fragmented grains,which indicates that they are the disintegrated fragments of previously larger grains.Phil,within the garnet peridotite and clinopyroxene-phlogopite xenoliths,is characterised by low Ti and Cr contents(TiO_21 wt.%,Cr_2 O_31 wt.% and Mg# = 100 × Mg/(Mg+ Fe)92)typical of primary peridotite phlogopite in mantle peridotite xenoliths from global kimberlite occurrences.They formed during SCLM metasomatism that led to a transformation from garnet peridotite to clinopyroxene-phlogopite rocks and the crystallisation of phlogopite and high-Cr clinopyroxene megacrysts before the generation of host-kimberlite magmas.One of the possible processes to generate low-Ti-Cr phlogopite is via the replacement of garnet during its interaction with a metasomatic agent enriched in K and H_2O.Rb-Sr isotopic data indicates that the metasomatic agent had a contribution of more radiogenic source than the host-kimberlite magma.Compared with peridotite xenoliths,eclogite xenoliths feature low-Ti phlogopites that are depleted in Cr_2O_3 despite a wider range of TiO_2 concentrations.The presence of phlogopite in eclogite xenoliths indicates that metasomatic processes affected peridotite as well as eclogite within the SCLM beneath the Grib kimberlite.Phl2 has high Ti and Cr concentrations(TiO_22 wt.%,Cr_2O_31 wt.% and Mg# = 100× Mg/(Mg + Fe)92)and compositionally overlaps with phlogopite from polymict brecc:ia xenoliths that occur in global kimberlite formations.These phlogopites are the product of kimberlitic magma and mantle rock interaction at mantle depths where Phl2 overgrew Phil grains or crystallized directly from stalled batches of kimberlitic magmas.Megacrysts,most macrocrysts and microcrysts are disintegrated phlogopite fragments from metasomatised peridotite and eclogite xenoliths.Fine phlogopite flakes within kimberlite groundmass represent mixing of high-Ti-Cr phlogopite antecrysts and high-Ti and low-Cr kimberlitic phlogopite with high Al and Ba contents that may have formed individual grains or overgrown antecrysts.Based on the results of this study,we propose a schematic model of SCLM metasomatism involving phlogopite crystallization,megacryst formation,and genesis of kimberlite magmas as recorded by the Grib pipe.     

Eclogite xenoliths from Wajrakarur kimberlites, southern India

Summary Mineralogical characteristics of eclogite xenoliths from three kimberlite pipes (KL2, P2 and P10) of the Proterozoic Wajrakarur kimberlite field of southern India have been studied. In a rare sample of enstatite eclogite from the KL2 pipe garnet contains microscopic triangular arrays of needles or blebs of omphacite, enstatite and rutile consistent with an origin by exsolution parallel to the isometric form {111}. Discrete omphacite grains in the sample contain exsolved needles or blebs of enstatite and garnet. Kyanite eclogites are abundant in the KL2 pipe which occasionally show a secondary ring of pure celsian around kyanite grains. Omphacite Na₂O contents in the eclogites of the KL2 and P2 pipes are typically between 3 and 6 wt%, and garnet has widely variable composition with end member ranges of Prp22-81Grs0-47Alm10-30Sps0-1Adr0-5Uv0-3. Eclogites of the P10 pipe comprise chromian omphacite and garnet. Phase relations in the ACF projection exhibit systematic increase of the Ca-Tschermak’s component in omphacite from enstatite eclogite through biminerallic eclogite to kyanite eclogite. Garnet-clinopyroxene Fe–Mg geothermometry yields temperatures mostly in the range of 900–1100 °C. A formerly supersilicic nature of garnet in enstatite eclogite as inferred from exsolution mineralogy indicates minimum peak pressure of 5 GPa.     

A model for the origin of ilmenite in kimberlite and diamond: implications for the genesis of the discrete nodule (megacryst) suite

Mineralogical and chemical relationships indicate that the majority of ilmenites recovered from Group I kimberlites crystallized directly from the kimberlite magma in two contrasting P-T regimes: Ilmenites of the discrete nodule association formed in pegmatitic veins and apophyses surrounding the kimberlite magma at depth. Compositional ranges of the discrete nodule assemblage reflect essentially isobaric crystallization across the thermal aureole about the magma reservoir. Early crystallization of high pressure Cr-rich phases (garnet, clinopyroxene and possibly spinel) could result in later forming megacryst ilmenites being Cr-poor. During ascent of the kimberlite magma (essentially identical to the liquid injected into the pegmatitic veins), crystallization of garnet and clinopyroxene would be inhibited as a result of the expansion of the olivine phase field. The magma would not undergo Crdepletion, with the result that later crystallizing (ground-mass) ilmenites would be Cr-rich relative to associated ilmenite megacrysts.Rare ilmenite inclusions in diamonds show chemical affinities with those of the discrete nodule suite. It is proposed that large Type IIa diamonds may be late-crystallizing members of the discrete nodule assemblage. They are in other words related to the kimberlite event itself, and would represent a third diamond paragenesis, distinctly younger than those related to peridotites and eclogites.The mode of formation of rare MARID suite and metasomatized mantle xenoliths is not clearly understood, although mineralogical and chemical evidence point to a direct or indirect link to the host kimberlite.     

Multiply exsolved clinopyroxene megacrysts from the Frank Smith Mine,Cape Province,South Africa

Two clinopyroxene megacrysts from the Frank Smith kimberlite show multiple exsolution of garnet, orthopyroxene and oxide phases. In one of the megacrysts the lamellar texture has partly recrystallized to a granular one. The chemical compositions of the exsolved phases are similar to those of discrete nodules and of granular pyroxenites found in the same kimberlite. Conditions of formation of the original homogeneous clinopyroxene solid solution are estimated to be ～1500°C and 37 kb, whereas the re-equilibration conditions are closer to 900°C and 35 kb. The megacrysts may have belonged to the Karroo age magnetism which predated kimberlite activity and have been brought to the surface as accidental xenocrysts by the Frank Smith kimberlite.     

Petrology and geochemistry of eclogite xenoliths from the Rietfontein kimberlite,Northern Cape,South Africa

Petrographic and geochemical features of a suite of eclogite xenoliths from the Rietfontein kimberlite that erupted through probable Proterozoic crust west of the Kaapvaal Craton in the far Northern Cape region of South Africa, are described. Group II eclogites dominate the suite both texturally and chemically, but can be subdivided into bimineralic, opx-bearing and kyanite-bearing groups. Temperature estimates from different geothermometers range from 700 to 1,000°C, indicating derivation from relatively shallow mantle depths. Orthopyroxene-bearing eclogites are inferred to originate from depths of 85 to 115 km and lie close to the average cratonic thermal profile for southern Africa. These uppermost mantle temperatures during the late Cretaceous provide evidence for equilibration of the off-craton lithosphere to craton-like thermal conditions following Namaqua-Natal orogenesis. The kyanite eclogites are distinct from the remaining eclogites in terms of both major and trace element compositions and their lesser degree of alteration. Garnets are richer in Ca, and are Cr-depleted relative to garnets from the bimineralic and opx-bearing eclogites, which tend to be more magnesian. Clinopyroxenes from the kyanite eclogites are more sodic, with higher Al₂O₃ and lower MgO contents than the bimineralic and opx-bearing eclogites. LREE-depletion, positive Sr and Eu anomalies, and the Al-rich, Si-poor bulk composition suggest a plagioclase-rich, probably troctolitic protolith for the kyanite eclogites. In contrast, the major and trace element bulk compositions of the high-MgO bimineralic and orthopyroxene-bearing eclogites are consistent with gabbroic or pyroxenitic precursors, or high-pressure cumulates, rather than mafic to ultramafic lavas. δ¹⁸O values for garnets do not deviate significantly from typical mantle values. The observations reported do not discriminate unambiguously between continental and oceanic origins for the various eclogite components in the mantle lithosphere of this region.     

Monomineral universal clinopyroxene and garnet barometers for peridotitic,eclogitic and basaltic systems

New versions of the universal Jd-Di exchange clinopyroxene barometer for peridotites,pyroxenites and eclogites,and also garnet barometer for eclogites and peridotites were developed.They were checked using large experimental data sets for eciogitic(~530) and peridotitic systems(650).The precision of the universal Cpx barometer for peridotites based on Jd-Di exchange is close to Cr-Tschermalite method produced by Nimis and Taylor(2000).Cpx barometer was transformed by the substitution of major multiplier for K_D by the equations dependent from Al-Na-Fe.Obtained equation in combination with the thermometer of Nimis and Taylor(2000) allow to reconstruct position of the magma feeder systems of the alkali basaltic magma within the mantle diapirs in modern platforms like in Vitim plateau and other Southern Siberia localities and several localities worldwide showing good agreement of pressure ranges for black and green suites.These equations allow construct PTX diagrams for the kimberlite localities in Siberia and worldwide calculating simultaneously the PT parameters for different groups of mantle rocks.They give very good results for the concentrates from kimberlite lamproites and placers with mantle minerals.They are useful for PT estimates for diamond inclusions.The positions of eclogite groups in mantle sections are similar to those determined with new Gar—Cpx barometer produced by C.Beyer et al.(2015).The Fe rich eclogites commonly trace the boundary between the lower upper parts of subcontinental lithospheric mantle(SCLM) at 3-4 CPa marking pyroxenite eclogites layer.Ca-rich eclogites and especially grospydires in SCLM beneath Precambrian kimberlites occurs near pyroxenite layer but in younger mantle sections they became common in the lower parts.The diamondiferous Mg Cr-less group eclogites referring to the ancient island arc complexes are also common in the middle part of mantle sections and near 5-6 GPa.Commonly eclogites in lower apart of mantle sections are remelted and trace the high temperature convective branch.The Mg-and Fe-rich pyroxenites also show the extending in pressure trends which suggest the anatexic melting under the influence of volatiles or under the interaction with plums.     

Cr-rich megacrysts of clinopyroxene and garnet from Lac de Gras kimberlites,Slave Craton,Canada – implications for the origin of clinopyroxene and garnet in cratonic lherzolites

Kimberlites from the Diavik and Ekati diamond mines in the Lac de Gras kimberlite field contain abundant large (>1 cm) clinopyroxene (Cr-diopside) and garnet (Cr-pyrope) crystals. We present the first extensive mineral chemical dataset for these megacrysts from Diavik and Ekati and compare their compositions to cratonic peridotites and megacrysts from the Slave and other cratons. The Diavik and Ekati Cr-diopside and Cr-pyrope megacrysts are interpreted to belong to the Cr-rich megacryst suite. Evidence for textural, compositional, and isotopic disequilibrium suggests that they constitute xenocrysts in their host kimberlites. Nevertheless, their formation may be linked to extensive kimberlite magmatism and accompanying mantle metasomatism preceding the eruption of their host kimberlites. It is proposed that the formation of megacrysts may be linked to failed kimberlites. In this scheme, the Cr-rich megacrysts are formed by progressive interaction of percolating melts with the surrounding depleted mantle (originally harzburgite). As these melts percolate outwards, they may contribute to the introduction of clinopyroxene and garnet into the depleted mantle, thereby forming lherzolite. This model hinges on the observation that lherzolitic clinopyroxenes and garnets at Lac de Gras have compositions that are strikingly similar to those of the Cr-rich megacrysts, in terms of major and trace elements, as well as Sr isotopes. As such, the Cr-rich megacrysts may have implications for the origin of clinopyroxene and garnet in cratonic lherzolites worldwide.

     

Megacrysts from the Grib kimberlite pipe (Arkhangelsk Province, Russia)

The megacryst suite of the Grib kimberlite pipe (Arkhangelsk province, Russia) comprises garnet, clinopyroxene, magnesian ilmenite, phlogopite and garnet-clinopyroxene intergrowths. Crystalline inclusions, mainly of clinopyroxene and picroilmenite, occur in garnet megacrysts. Ilmenite is characterized by a wide range in the contents of MgO (10.6–15.5 wt.%) and Cr₂O₃ (0.7–8.3 wt.%). Megacryst garnets show wide variations in Cr₂O₃ (1.3–9.6 wt.%) and CaO (3.6–11.0 wt.%) but relatively constant MgO (15.4–22.3 wt.%) and FeO (5.2–9.9 wt.%). The pyroxenes also show wide variations in such oxides as Cr₂O₃, Al₂O₃ and Na₂O (0.56–2.95; 0.86–3.25; 1.3–3.0 wt.%, respectively). The high magnesium and chromium content of all these minerals puts them together in one paragenetic group. This conclusion was confirmed by studies of the crystalline inclusions in megacrysts, which demonstrate similar variations in composition. Low concentration of hematite in ilmenite suggests reducing conditions during crystallization. P–T estimates based on the clinopyroxene geothermobarometer (Contrib. Mineral. Petrol. 139 (2000) 541) show wide variations (624–1208 °C and 28.8–68.0 kbars), corresponding to a 40–45 mW/m² conductive geotherm. The majority of Gar-Cpx intergrowths differ from the corresponding monomineralic megacrysts in having higher Mg contents and relatively low TiO₂. The minerals from the megacryst association, as a rule, differ from the minerals of mantle xenoliths, but garnets in ilmenite-bearing peridotite xenoliths are compositionally similar to garnet megacrysts. The common features of trace element composition of megacryst minerals and kimberlite (they are poor in Zr group elements) suggest a genetic relationship. The origin of the megacrysts is proposed to be genetically connected with kimberlite magma-chamber evolution on the one hand and with associated mantle metasomatism on the other. We suggest that, depending on the primary melt composition, different paragenetic associations of macro/megacrysts can be crystallized in kimberlites. They include: (1) Fe–Ti (Mir, Udachnaya pipes); (2) high-Mg, Cr (Zagadochna, Kusova pipes); (3) high-Mg, Cr, Ti (Grib pipe).     

Metasomatic Eclogitic Diamond Growth: Evidence from Multiple Diamond Inclusions

Diamond formation from metasomatic fluids, rather than from igneous melts, remains controversial but is paramount to our understanding of diamonds' mantle origin(s). Physical and chemical properties of diamonds, their inclusions, and host eclogites from the Mir kimberlite, Yakutia, Russia form the basis for our evaluation of diamond origin. Mir eclogitic diamonds and their multiple inclusions show a definite break in time and temperature between the formation of the core zones and the rims of the diamonds. Extreme changes in chemistry for multiple diamond inclusions (DIs) between the cores and the rims cannot be accounted for by magmatic fractional crystallization. Evidence also exists for large temperature decreases (40° to 140°C) from the cores to the rims of some diamonds. The distinct changes in nitrogen contents and aggregation states from cores to rims of diamonds would appear to reflect different residence times for these portions of the diamonds in the mantle- i.e., formation of cores and rims at vastly different times (e.g., 2 Gy). Many of the mineral-chemical characteristics, including C and N isotopes and N aggregation states of the diamond, can best be explained by crystallization of the diamonds after formation of the eclogite host. This suggests that the formation of the eclogite and the nucleation and growth of some diamonds are not coeval and possibly not cogenetic.

Most diamondiferous eclogite xenoliths probably have never experienced a major magmatic episode (i.e., complete melt stage) after subduction of their crustal protoliths into the mantle. Carbon isotopes in diamond, sulfur isotopes from sulfide DIs, and oxygen isotopes from eclogite minerals all point to crustal protoliths for many eclogites.

All of the factors above, taken as a whole, indicate that many eclogitic diamonds are the result of petrogenesis by metasomatism over a prolonged period of time. Introduction of metasomatic fluids facilitates the precipitation of the diamonds, either in tolo or as rims on previously formed diamonds. Inasmuch as some eclogites are considered to be igneous in origine.g., Group-A eclogites of Taylor and Neal (1989)-it is entirely possible that these eclogites may contain truly igneous diamonds. However, even some of these diamonds may have later metasomatic overgrowths.     

The significance of groundmass ilmenite and megacryst ilmenite in kimberlites

Criteria are suggested for distinguishing xenocrystic ilmenites from those indigenous to the host kimberlite. For instance, in contrast to groundmass grains, ilmenite xenocrysts usually are larger, have reaction rims of leucoxene and perovskite, exhibit strong magnesium enrichment outward, and sometimes have exsolution lamellae and deformation features. Most of the abundant ilmenite macrocrysts found in kimberlite appear to have been phenocrysts in a crystal mush unrelated to kimberlite. On the other hand, kimberlitic groundmass ilmenite is rare, but consistently more magnesian than the cores of macrocrysts. Strong Mg-enrichment patterns evident in the ilmenite macrocrysts probably developed during their attempt to equilibrate with the more magnesian, fractionating kimberlitic liquid. The hypothesis of extensive reaction of ilmenite with kimberlite melt/ fluid has implications with regard to the following: (1) the degree of differentiation of kimberlite melts; (2) the genesis of mantle megacrysts; (3) the reactivity of kimberlite; and (4) the usefulness of groundmass ilmenite as a petrogenetic indicator.     

青藏高原羌塘中部不同时代榴辉岩的识别及其意义——来自榴辉岩及其围岩~（40）Ar-~（39）Ar年代学的证据

羌塘中部高压变质带由榴辉岩、石榴子石白云母片岩、蓝片岩等组成,与蛇绿混杂岩、晚古生代浅变质地层岩片等共同构成了龙木错-双湖板块缝合带这一构造混杂岩带。目前已先后在片石山地区、果干加年山地区和冈玛错地区发现典型的榴辉岩,以片石山和果干加年山地区的榴辉岩为研究对象。片石山地区的榴辉岩为低温型榴辉岩,围岩为石榴子石白云母片岩,变质作用峰期温压条件为T=500℃,p=2.3GPa。已获得230～244Ma锆石SHRIMP U-Pb年龄和石榴子石Lu-Hf等时线年龄,代表榴辉岩相变质作用的时代。榴辉岩及其围岩在误差范围内具有相同的Ar-Ar年龄,为210～220Ma,代表了榴辉岩及其围岩冷却抬升至近地表的时代。果干加年山地区的榴辉岩具有和片山地区榴辉岩相似的野外产状、矿物组合、温压条件和围岩。Ar-Ar年代学研究显示,果干加年山地区的榴辉岩在240Ma左右即已折返并抬升至近地表,其变质作用峰期时代明显要早于片石山地区。     

Isotopic-geochemical evidence for crustal contamination of eclogites in the Kokchetav subduction-collision zone

This paper reports isotopic and geochemical studies of eclogites from the western ultrahigh pressure (UHP) and eastern high-pressure (HP) blocks of the Kokchetav subduction-collision zone. These HP and UHP eclogites exhumed in two stages: (1) The rocks of the western block metamorphosed within the field of diamond stability (e.g., Kumdy-Kol and Barchy); (2) In contrast, the metamorphic evolution of the eastern block reached the pressure peak within the stability field of coesite (e.g., Kulet, Chaglinka, Sulu-Tyube, Daulet, and Borovoe). The eclogites vary widely in the ratios of incompatible elements and in the isotope ratios of Nd (¹⁴³Nd/¹⁴⁴Nd = 0.51137-0.513180) and Sr (⁸⁷Sr/⁸⁶Sr = 0.703930.78447). The Sulu-Tyube eclogites display isotope-geochemical features close to N-MORB, while those from the other sites are compositionally similar to E-type MORB or island arc basalts (IAB). The model ages T_Nd(DM) of eclogites vary between 1.95 and 0.67 Ga. The Sulu-Tyube eclogite yields the youngest age; it has the values of ε_Nd(T) (7.2) and ⁸⁷Sr/⁸⁶Sr (0.70393) close to the depleted mantle values. The crustal input to the protolith of the Kokchetav eclogites is evident on the ε_Nd(T)-⁸⁶Sr/⁸⁷Sr and ε_Nd(T)-T plots. The eclogites make up a trend from DM to country rocks. Some eclogites from the Kulet, Kumdy-Kol, and Barchy localities display signs of partial melting, such as high Sm/Nd (0.65-0.51) and low (La/Sm)_N (0.34-0.58) values. The equilibrium temperatures of these eclogites are higher than 850 °C. The geochemical features of eclogites testify to the possibility of the eclogite protolith formation in the tectonic setting of passive continental rift margin subducted to depths over 120 km.     

Tale of the Kulet eclogite from the Kokchetav Massive,Kazakhstan: Initial tectonic setting and transition from amphibolite to eclogite

The Kulet eclogite in the Kokchetav Massif, northern Kazakhstan, is identified as recording a prograde transformation from the amphibolite facies through transitional coronal eclogite to fully recrystallized eclogite (normal eclogite). In addition to minor bodies of normal eclogite with an assemblage of Grt + Omp + Qz + Rt ± Ph and fine‐grained granoblastic texture (type A), most are pale greyish green bodies consisting of both coronal and normal eclogites (type B). The coronal eclogite is characterized by coarse‐grained amphibole and zoisite of amphibolite facies, and the growth of garnet corona along phase boundaries between amphibole and other minerals as well as the presence of eclogitic domains. The Kulet eclogites experienced a four‐stage metamorphic evolution: (I) pre‐eclogite stage, (II) transition from amphibolite to eclogite, (III) a peak eclogite stage with prograde transformation from coronal eclogite to UHP eclogite and (IV) retrograde metamorphism. Previous studies made no mention of the presence of amphibole or zoisite in either the pre‐eclogite stage or coronal eclogite, and so did not identify the four‐stage evolution recognized here. P–T estimates using thermobarometry and Xprp and Xgrs isopleths of eclogitic garnet yield a clockwise P–T path and peak conditions of 27–33 kbar and 610–720 °C, and 27–35 kbar and 560–720 °C, respectively. P–T pseudosection calculations indicate that the coexistence of coronal and normal eclogites in a single body is chiefly due to different bulk compositions of eclogite. All eclogites have tholeiitic composition, and show flat or slightly LREE‐enriched patterns [(La/Lu)_N = 1.1–9.6] and negative Ba, Sr and Sc and positive Th, U and Ti anomalies. However, normal eclogite has higher TiO₂ (1.35–2.65 wt%) and FeO (12.11–16.72 wt%) and REE contents than those of coronal eclogite (TiO₂ < 0.9 wt% and FeO < 12.11 wt%) with one exception. Most Kulet eclogites plot in the MORB and IAB fields in the 2Nb–Zr/4–Y and TiO₂–FeO/MgO diagrams, although displacement from the MORB–OIB array indicates some degree of crustal involvement. All available data suggest that the protoliths of the Kulet eclogites were formed at a passive continent marginal basin setting. A schematic model involving subduction to 180–200 km at 537–527 Ma, followed by slab breakoff at 526–507 Ma, exhumation and recrystallization at crustal depths is applied to explain the four‐stage evolution of the Kulet eclogite.     

Petrology of two diamondiferous eclogite xenoliths from the Lahtojoki kimberlite pipe, eastern Finland

Diamondiferous Group A eclogites constitute a minor portion of the mantle-derived xenoliths in the eastern Finland kimberlites. They have been derived from the depth interval 150–230 km where they are inferred to occur as thin layers or small pods within coarse-grained garnet peridotites. The chemical and isotopic composition of minerals suggest that they represent (Proterozoic?) mantle-derived melts or cumulates rather than subducted oceanic lithosphere. During magma ascent and emplacement of the kimberlites, the eclogite xenoliths were mechanically and chemically rounded judging from the types of surface markings. In addition, those octahedral crystal faces of diamonds that were partially exposed from the rounded eclogite xenolith became covered by trigons and overlain by microlamination due to their reaction with the kimberlite magma. The diamonds bear evidence of pervasive plastic deformation which is not, however, evident in the eclogite host. This suggests that annealing at ambient lithospheric temperatures has effectively recrystallised the silicates while the diamond has retained its lattice imperfections and thus still has the potential to yield information about ancient mantle deformation. One of our samples is estimated to contain approximately 90,000 ct/ton diamond implying that some diamonds occur within very high-grade pods or thin seams in the lithospheric mantle. To our knowledge, this is one of the most diamondiferous samples described.     

Zircon megacrysts from kimberlite: oxygen isotope variability among mantle melts

The oxygen isotope ratios of Phanerozoic zircons from kimberlite pipes in the Kaapvaal Craton of southern Africa and the Siberian Platform vary from 4.7 to 5.9‰ VSMOW. High precision, accurate analyses by laser reveal subtle pipe-to-pipe differences not previously suspected. These zircons have distinctive chemical and physical characteristics identifying them as mantle-derived megacrysts similar to zircons found associated with diamond, coesite, MARID xenoliths, Cr-diopside, K-richterite, or Mg-rich ilmenite. Several lines of evidence indicate that these ¹⁸O values are unaltered by kimberlite magmas during eruption and represent compositions preserved since crystallization in the mantle, including: U/Pb age, large crystal size, and the slow rate of oxygen exchange in non-metamict zircon. The average ¹⁸O of mantle zircons is 5.3‰, ∼0.1 higher and in equilibrium with values for olivine in peridotite xenoliths and oceanic basalts. Zircon megacrysts from within 250 km of Kimberley, South Africa have average ¹⁸O=5.32±0.17 (n=28). Small, but significant, differences among other kimberlite pipes or groups of pipes may indicate isotopically distinct reservoirs in the sub-continental lithosphere or asthenosphere, some of which are anomalous with respect to normal mantle values of 5.3±0.3. Precambrian zircons (2.1–2.7 Ga) from Jwaneng, Botswana have the lowest values yet measured in a mantle zircon, ¹⁸O=3.4 to 4.7‰. These zircon megacrysts originally crystallized in mafic or ultramafic rocks either through melting and metasomatism associated with kimberlite magmatism or during metamorphism. The low ¹⁸O zircons are best explained by subduction of late Archean ocean crust that exchanged with heated seawater prior to underplating as eclogite and to associated metasomatism of the mantle wedge. Smaller differences among other pipes and districts may result from variable temperatures of equilibration, mafic versus ultramafic hosts, or variable underplating. The narrow range in zircon compositions found in most pipes suggests magmatic homogenization. If this is correct, these zircons document the existence of significant quantities of magma in the sub-continental mantle that was regionally variable in ¹⁸O and this information restricts theories about the nature of ancient subduction. Received: 8 August 1997 / Accepted: 6 May 1998