Search

Comprehensive studies of peridotitic xenoliths from the Udachnaya kimberlite (Yakutian diamond province, Siberian craton) confirm that garnet shows inverse correlation of its volumetric percentage with its Cr2O3 contents in refertilizated peridotites, but no such correlation is observed in depleted peridotites. The correlation relationship plots as an isosceles hyperbola, which is consistent with the existing knowledge of origin of refertilized peridotite. The bulk content of aluminum is proportional to the garnet percentage both in depleted and refertilized peridotites, but Al2O3 in the rock correlates with Cr2O3 in garnet only in the refertilized varieties, while the two parameters are independent in depleted mantle rocks. According to the modeling of refertilization-related composition changes in the Udachnaya peridotites, garnet percentages are directly proportional to the amount of clinopyroxene (Gnt = 0.879*Cpx + 0.022, R2 = 0.78) and inversely proportional to that of olivine (Gnt = 0.026/Ol3.141, R2 = 0.79). As the shares of Gnt and Cpx increase from minimum values, orthopyroxene first increases (to 0.16) and then decreases since 0.65 Ol, 0.09 Cpx, and 0.10 Gnt. This model can constrain the place of the parent rock in the refertilization series knowing Cr2O3 contents in separate garnet grains. The average refertilization degree of lithospheric mantle in the region estimated from the compositions of more than 800 garnet xenocrysts in the Udachnaya kimberlite is expressed in the rock modal composition as: Ol = 0.72, Opx = 0.15, Gnt = 0.07, and Cpx = 0.06 (median values).

Sulfide-bearing polymineralic inclusions in mantle-derived chromium pyrope garnets of lherzolite paragenesis from lamprophyres of the Chompolo field (Aldan shield, southern Siberian craton) have been studied. The inclusions are composed of either only sulfides or sulfides in association with other minerals (carbonates, silicates, oxides, etc.). The sulfide part of the inclusions is represented by up to four minerals. Among the sulfides, minerals rich in Cu and Ni have been found, whereas Fe sulfides (pyrrhotite, troilite) are absent. This distinguishes the inclusions studied from the majority of sulfide inclusions in mantle minerals and diamonds, as well as in mantle xenoliths from kimberlites. The formation of polymineralic inclusions in chromium garnets of the Chompolo field is attributed to the effect of a carbonate–silicate metasomatic melt/fluid on mantle peridotites, as evidenced by the mineral suite associated with the sulfides. The research results indicate significant differences in the nature of metasomatic processes that occurred in the lithospheric mantle of the southern and central parts of the Siberian craton.

We have studied variations in the structure and composition of minerals from the pipes of the Yakutian kimberlite province (YKP) in different mantle terranes of the Siberian craton. The study was based on an extensive database, including the microprobe analysis datasets consolidated by IGM, IG, IEC and IGDNM SB RAS and ALROSA and geochemical analysis of minerals performed by LA‐ICP‐MS (Laser Ablation Inductively Coupled Plasma Mass Spectrometry). The reconstruction shows layering under the tubes, including 6–7 slab that were probably formed due to subduction; the slabs are separated by pyroxenitic, eclogitic and metasomatic layers and dunite lenses. Transects and mantle profiles across kimberlite fields are constructed. Within the limits of the revealed tectonic terranes, we assume a collage of microplates formed in the early – middle Archean. Extended submeridional structures of the tectonic terranes are not always confirmed at the mantle level. Beneath the Anabar and Aldan shields, the mantle sections show more coarse layers and 3–4 large horizons of dunites with garnet and pyroxene nests separated by ilmenite‐ phlogopite metasomatites and pyroxenites. Terranes representing the suture zones between the protocratons (e.g. Khapchan) are often saturated with eclogites and pyroxenites that may occur as leghthy ascending bodies of magmatic eclogites penetrating through the mantle lithosphere structure (ML). A nearly ubiquitous pyroxenite layer at the level of 3.5–4.5 GPa formed probably in the early Archean with a high heat flux during melting of eclogites and was subsequently traced by plume melts. Within the early Archean protocratons – granite‐greenstone terranes (Tungus, Markha, Berekta, and Sharyzhalgai, ~3.8–3.0 Gyr [ Gladkochub et al., 2019 ], the mantle lithosphere is less depleted and largely metasomatized. The ML structure of the Daldyn and Magan granulite‐orthogneiss terranes is layered with folding revealed in the north‐to‐south sections from the Udachnaya pipe to the Krasnopresnenskaya pipe, which is less pronounced in the latitudinal direction. From the Daldyn field to the Alakit field, there is an increase in the degree of metasomatism, and higher alkalinity of pyroxenes and larger amounts of phlogopite are noted. The most productive Aikhal and Yubileinaya pipes are confined to a dunite core, which is accompanied by a change in the specialization of high‐charge elements Ta‐Nb to Zr‐Hf. Within the limits of the Magan terrane, the thin‐layer structure of the middle and upper parts of the craton keel is replaced with a sharply depleted productive horizon at its base. The mantle of the granite‐greenstone Markha terrrein comprises eclogite (often pelitic) horizons, which suggests subduction of the continental lithosphere or sediments. In the central and northern parts of the Siberian craton, most structures in the mantle are sinking to the west at small angles. The geochemistry features of garnets and pyroxenes from various mantle terranes are considered in detail.

Compared to xenoliths, kimberlite xenocrysts provide, although less accurate, more complete information about the deep structure and processes in the subcratonic lithospheric mantle (SCLM). This work is devoted to the study the composition of xenogenic olivine from kimberlites as the main mineral constituting SCLM. Olivine in kimberlites has a different origin, including those not related to depleted rocks of the lithosphere. It can crystallize directly from kimberlite or belong to the so-called Cr-poor megacryst association. In this regard, for the correct interpretation of data on its composition, it is necessary to have sufficiently clear criteria for the separation of olivine xenocrysts from kimberlites into various genetic types. In order to remove olivines crystallizing directly from kimberlite from consideration, in our study we used only central homogeneous parts of crystals larger than 0.5 mm in size [Giuliani, 2018].Based on unique and literature data on the composition of olivines from 230 xenoliths of peridotites from 12 kimberlites of the North American, South African and Siberian cratons we proposed a general division into 4 genetic types: olivines of ultrahigh-temperature (HTP-1), high-temperature (HTP-2), low-temperature (LTP) peridotites, olivines of low-chromium megacrystal association (MCA). The separation scheme uses the CaO content as an indicator of the temperature of formation and the ratio Mg/Mg+Fe as an indicator of the degree of enrichment.A study of more than 1,500 olivines from a number of kimberlite bodies of the Siberian platform according to this scheme revealed three characteristic distributions of olivine types in kimberlite bodies: 1) without high-temperature differences (Obnazhennaya pipe), 2) with significant development of HTP-2 (Olivinovaya and Vtorogodnitsa pipes) and 3) with significant development of HTP-1 (Dianga pipe). Only the latter type is characterized by the presence of a noticeable amount of olivines of the megaryst association.In general, variations in the composition of LTP olivines correspond to granular ones, while HTP-1 and HTP-2 correspond to deformed (shared) peridotites. Interestingly, the enrichment of olivines with incompatible components in these three types does not correlate directly with the formation temperature. Olivines of ultrahigh-temperature peridotites (HTP-1) have unexpectedly small compositional variations and occupy an intermediate position between low-temperature and high-temperature in content of incompatible elements.A study of the content of impurity elements (TiO2, NiO) in olivines made it possible to propose the way of formation of two different types of high-temperature olivines. It belongs to the model [Harte et al., 1993; Burgess and Harte, 1999; Burgess and Harte, 2004] where megacrystal melt of various stages of fractionation [Moore et al., 1992] effects depleted rocks of lithospheric mantle. According to it, HTP-2 olivines arose upon exposure to a fractionated melt characteristic of late crystallization stages, and HTP-1 olivines due to unfractionated (less enriched with incompatible components) megacrystal melt at higher temperatures characteristic of the initial crystallization stage.Funded by RFBR grant 18-05-01143, T.V.A. was supported by RSF grant 16-17-10067.

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.

Mantle xenocrysts from early Triassic kimberlite pipes from Kharamai, Ary-Mastakh and Kuranakh fields in the Anabar shield of Siberia revealing similar compositional trends were studied to estimate the superplume influence on the subcratonic lithosphere mantle (SCLM). Pressure-temperature (PT) reconstructions using monomineral thermobarometry for 5 phases show division of the SCLM beneath the Kharamai field into 6 units: pyroxenitic Fe-rich (1–2 GPa) and Mg-rich (2–3 GPa) layers; middle with two levels of Gar-Sp pyroxenites at ∼3 and 4–5 GPa; Gar-dunite–harzburgites ∼4.5–6.5 GPa subjected to Ilm-Px vein metasomatism; and a Mg-rich dunite lower part. In the Anabar shield (Ary-Mastakh, Dyuken and Kuranakh fields) mantle lithosphere is composed of three large units divided into two parts: upper part with amphiboles and phlogopite; two levels of pyroxenites and eclogites at 3 and 4 GPa, and a lower part composed of refertilized dunites. Diagrams showing P-Fe # Gar clusters for garnets and omphacites illustrate the differences between SCLM of these localities. Differences of Triassic SCLM from Devonian SCLM are in simple layering; abundance of Na-Cr-amphiboles and metasomatism in the upper SCLM part, thick pyroxenite-eclogite layer and lower part depletion, heated from SCLM base to 5.0 GPa. Kharamai mantle clinopyroxenes represent three geochemical types: (1) harzburgitic with inclined linear REE, HFSE troughs and elevated Th, U; (2) lherzolitic or pyroxenitic with round TRE patterns and decreasing incompatible elements; (3) eclogitic with Eu troughs, Pb peak and high LILE content. Calculated parental melts for garnets with humped REE patterns suggest dissolution of former Cpx and depression means Cpx and garnets extraction. Clinopyroxenes from Ary-Mastakh fields show less inclined REE patterns with HMREE troughs and an increase of incompatible elements. Clinopyroxenes from Kuranakh field show flatter spoon-like REE patterns and peaks in Ba, U, Pb and Sr, similar to those in ophiolitic harzburgites. The PT diagrams for the mantle sections show high temperature gradients in the uppermost SCLM accompanied by an increase of P-Fe # Ol upward and slightly reduced thickness of the mantle keel of the Siberian craton, resulting from the influence of the Permian–Triassic superplume, but with no signs of delamination.

The results of the first study of the PGE distribution in deformed lherzolites of the Udachnaya kimberlite pipe (Yakutia) are presented here. The complex character of evolution of the PGE composition in the Deformed lherzolites is assumed to be the result of silicate metasomatism. At the first stage, growth in the amount of clinopyroxene and garnet in the rock is accompanied by a decrease in the concentration of the compatible PGE (Os, Ir). During the final stage, the rock is enriched with incompatible PGE (Pt, Pd) and Re possible due to precipitation of submicron-sized particles of sulfides in the interstitial space of these mantle rocks.

Paragenetic analysis of pyropes from alluvial deposits of the Muna—Markha interfluve (Sakha–Yakutia Republic) made it possible to distinguish relatively uniform areas that are promising for the discovery of kimberlite bodies.

The alkaline igneous rocks of the Chompolo field (Aldan shield, Siberian craton), previously defined as kimberlites or lamproites, are more correctly classified as low-Ti lamprophyres. The emplacement age of the Ogonek pipe (137.8 &plusmn, 1.2 Ma) and the Aldanskaya dike (157.0 &plusmn, 1.6 Ma) was obtained using 40Ar/39Ar K-richterite dating. The Chompolo rocks contain abundant xenocrysts of mantle minerals (chromium-rich pyropic garnets, Cr-diopsides, spinels, etc.). The composition of the mantle xenocrysts indicates the predominance of spinel and garnet&ndash, spinel lherzolites, while the presence of garnet lherzolites, dunites, harzburgites, and eclogites is minor. The Chompolo rocks are characterized by large-ion lithophile element (LILE) and Light Rare Earth Element (LREE) enrichments, and high field strength element (HFSE) depletions. The rocks of the Ogonek pipe have radiogenic Sr (87Sr/86Sr (t) = 0.70775 and 0.70954), and highly unradiogenic &epsilon, Nd(t) (&minus, 20.03 and &minus, 20.44) isotopic composition. The trace element and isotopic characteristics of the Chompolo rocks are indicative of the involvement of subducted materials in their ancient enriched lithospheric mantle source. The Chompolo rocks were formed at the stage when the Mesozoic igneous activity was triggered by global tectonic events. The Chompolo field of alkaline magmatism is one of the few available geological objects, which provides the opportunity to investigate the subcontinental lithospheric mantle beneath the south part of the Siberian craton.

The authors wish to make the following erratum to this paper [...]

This work is devoted to the systematization of the composition of xenogenic olivine from kimberlites as the main mineral composing the lithospheric mantle. Based on data on the composition of olivines from xenoliths and megacrysts from kimberlites, a general division into four types is proposed: olivines of ultrahigh-temperature (HTP-1), high-temperature (HTP-2) and low-temperature (LTP) peridotites, as well as olivines of low-chromium megacryst association (MCA). The separation scheme uses the CaO content as an indicator of the formation temperature and the Mg/(Mg + Fe) ratio as an indicator of the degree of enrichment in olivines. In contrast to Al, the Ca content in olivines from cratonic peridotites is high enough to use only EPMA when applying the proposed scheme. According to this scheme the study of more than 1500 individual olivine xenocrysts from a number of kimberlite bodies of the Siberian platform was made. It revealed three characteristic distributions of olivine types: without high-temperature differences (Obnazhennaya pipe), with significant development of HTP-2 (Olivinovaya and Vtorogodnitsa pipes), and with a significant development of HTP-1 (Dianga pipe). Only the latter type of distribution is characterized by the presence of a noticeable amount of megacryst association olivines. The study of other minor elements (TiO2 and NiO) in olivines allowed us to propose a model for the formation of high-temperature olivines of two different types due to the interaction of megacryst melt of various fractionation stages on depleted rocks of the lithospheric mantle. HTP-2 olivines arose upon exposure to a fractionated melt of the late stages of crystallization, and HTP-1 olivines appeared upon exposure to unfractionated (less enriched with incompatible components) megacryst melt at higher temperatures of the initial stage of crystallization.

Single-grain thermobarometric studies of xenocrysts were used to compile local SCLM transects through the major regions of kimberlite magmatism in Siberia and longer transects through the subcontinental mantle lithosphere (SCLM) beneath the Siberian craton. The mantle structure was obtained using P-Fe#, Ca in garnets, oxygen fugacity values fO2 and calculated temperatures T°C. The most detail transect obtained for the Daldyn field on the Udachnaya-Zarnitsa reveals layering showing an inclination of > 35° to Udachnaya. Mantle layering beneath the Alakit field determined from the Krasnopresnenskaya-Sytykanskaya transect shows a moderate inclination from N to S. The inflection near Yubileinaya-Aykhal is also supported by the extreme depletion in peridotites with low-Fe sub-Ca garnets. Beneath the Malo-Botuobinsky field the sharply layered mantle section starts from 5.5 GPa and reveals step-like P-Fe#Ol trends for garnets and ilmenites. The deeper part of SCLM in this field was originally highly depleted but has been regenerated by percolation of protokimberlites and hybrid melts especially beneath Internationalnaya pipe. The three global transects reveal flat layering in granite-greenstone terranes and fluctuations in the granulite-orthogneiss Daldyn collision terranes. The mantle layering beneath the Daldyn - Alakite region may have been created by marginal accretion. Most of southern fields including the Malo-Botuobinsky field reveal flat layering. The primary subduction layering is smoothed beneath the Alakit field. Lower Jurassic kimberlites from the Kharamai-Anabar kimberlite fields reveal a small decrease of the thickness of the SCLM and heating of its base. The Jurassic Kuoyka field shows an uneven base of the SCLM inclined from west to east. SCLM sequences sampled at this time started mainly from depths of 130 km, but some pipes still showed mantle roots to 250 km. The garnet series demonstrates an inclined straight line pyroxenite P-Fe# trend due to interaction with superplume melts.

Conventional diamond exploration seldom involves searching for diamonds in rock and soil samples; rather, it focuses on the search for “indicator minerals.” Kimberlite indicator minerals include garnet, olivine, chromite, pyroxene, and ilmenite, and these can be used to infer the presence of kimberlites and lamproites in the vicinity of where the samples were collected. Ilmenite has served as an effective indicator mineral for more than 40 years due to its resistance to chemical and physical weathering. As a result of its relatively high density compared to other indicator minerals, ilmenite grains often accumulate in placer deposits downstream from a kimberlite source. Although the ideal formula for ilmenite is FeTiO3, the crystal structure is also favorable to cation substitution owing to similarities in ionic radii and charge between Ti and Fe and other trace elements associated with its formation. We have investigated ilmenite trace element chemical signatures that can be related to the presence of diamond-bearing or diamond-free kimberlites. Our results suggest that the diamond potential of kimberlites is best reflected in the Zr/Nb ratio of ilmenite— these elements substitute for Ti in the ilmenite structure. An extensive compilation of compositions of ilmenite collected from heavy-mineral placers and from 14 kimberlites in northern Siberia (Yakutia) indicates that diamond pipes that have economically favorable diamond grades and abundances are associated with ilmenites having a Zr/Nb ratio of >0.37. Because of this, we suggest that ilmenite trace-element chemistry can be a useful tool to identify high-priority targets for diamond potential on the Siberian craton, and reconnaissance studies of other areas suggest that this relationship may be universally applicable.

1062 Subcalcium chromium pyropes with a CaO concen� tration of ~3 wt % or lower and widely variable Cr2O3 concentration of ~5 wt % and higher are observed as inclusions in diamonds of peridotitic assemblage, xeno� liths of harzburgite (dunite) and lherzolite, and as indi� vidual grains in the heavy fraction of kimberlite [1–3]. Of special interest are chromium pyropes with a CaO concentration of

More than 1000 pyropes from the Muza (J3) and Ivushka (D–C) (northeastern Siberian Platform, and Khorkich (Mz) (southwestern part of the platform) kimberlite pipes, alluvial deposits of the Muna-Markha area, and granular peridotites of the Udachnaya pipe have been analyzed for major and some minor elements. As a result, a group of pyropes was distinguished whose composition is not typical of the lherzolite paragenesis (LAC pyropes). They are predominant in the Muza pipe and are widespread over the world. This group is described as a separate paragenetic type. In all known cases, LAC pyropes belong to granular clinopyroxene-bearing harzburgites, and in situ conditions for this suite are typically below 50 kbar and 1000 °C. Our own and literature data suggest that LAC pyropes may appear when the magmas corresponding to the high-degree melting of the primary magma affect the depleted peridotites of the lithosphere mantle. The character of paleogeotherm and distribution of LAC pyropes in kimberlites and secondary collectors of varying age that occur on the Siberian Platform indicate that the lithosphere mantle was considerably thinner in the northeast and the rocks characterized by LAC pyropes played an increasingly important role in this region in the period from Paleozoic to Mesozoic and that these rocks were abundant in the lithosphere mantle of the platform’s interior. These facts as well as a considerable change in the rock composition in the lithospheric mantle and in the southwestern part of the platform in the same range of time suggest that the effect of the Permian-Triassic Siberian plume on the lithospheric mantle of the platform considerably changed its composition and structure in its separate parts.

The relative abrasive stability of kimberlite indicator minerals such as pyrope, picroilmenite, olivine, and apatite as well as diamonds and kimberlite fragments was studied experimentally, and the following sequence of mineral abrasive stability was established: pyrope–olivine–picroilmenite–apatite–kimberlite fragments. Diamond did not virtually change during the experiment. Kimberlite fragments appeared to be rather stable. Their relics were preserved until the end of the experiment, whereas the other minerals acquired wearing-resistant shapes. Pyrope, olivine, and apatite were shaped into an oval. Owing to anisotropy of microhardness, picroilmenite forms hexagonal tablets, which are typical of ancient haloes of indicator minerals in all diamondiferous regions. The parity analysis of the abrasive stabilities of pyrope and picroilmenite has shown that in the haloes of “mature” littoral zone represented by pyrope alone, with a possible admixture of diamond, picroilmenite is completely destroyed by abrasion.

The dissolution stability of garnets from kimberlite has been studied in new experiments in which etching in HF simulated natural chemical weathering. The experiments lasted 42 days and included weight loss monitoring and analysis of chemistry and parageneses of the output grains. The etched garnets of Cr2O3-rich parageneses became corroded less strongly. The greater chemical resistance of Cr-rich pyropes is consistent with the behavior of bulk Cr2O3 observed in a natural garnet assemblage from a weathered placer derived from the Mir kimberlite. Chemical weathering being the principal control of the assemblage composition, the placer assemblages with uncorroded pyropes may be compositionally proximal to their counterparts in the kimberlite.