Mineralogy of metamorphosed carbonatite of the Vesely occurrence, Northern Transbaikal region, Russia

Metamorphism of carbonatite is exemplified in the Vesely occurrence. According to available data, the age of the carbonatite is 596 ± 3.5Ma, whereas metamorphism is dated at 550 ± 14 Ma. The rocks at the Vesely occurrence were metamorphosed under conditions of greenschist facies (epidote-muscovite-chlorite subfacies) under elevated pressure. Microthermometry of fluid inclusions in minerals indicates that the temperature of metamorphism is 377−450°C and the pressure estimated from phengite geobarometer is 6−8 kbar. The low-grade metamorphism led to the partial recrystallization of carbonates and apatite with removal of trace elements. This process resulted in a change of the oxygen isotopic composition of the studied minerals. Metamorphism was accompanied by formation of talc, phengite, chlorite, quartz, tremolite-actinolite, and anthophyllite, which are not typical of carbonatite. The data obtained show that the metamorphism exerted an effect on the mineralogical, isotopic, geochemical, and technological properties of the carbonatite. The effect of metamorphism should be taken into account in determination of the nature of ore mineralization and estimation of ore quality and perspective of the occurrence.     

A rare earth element-rich carbonatite dyke at Bayan Obo, Inner Mongolia, North China

Summary ?A carbonatite dyke, extremely enriched in rare earth elements (REE), is reported from Bayan Obo, Inner Mongolia, North China. The REE content in the dyke varies from 1 wt% to up to 20 wt%. The light REEs are enriched and highly fractionated relative to the heavy REEs, and there is no Eu anomaly. Although carbon isotope δ¹³C (PDB) values of the carbonatites (−7.3 to −4.7‰) are within the range of normal mantle (−5±2‰), oxygen isotope δ¹⁸O (SMOW) (11.9 to 17.7‰) ratios apparently are higher than those of the mantle (5.7±1.0‰), indicating varying degrees of exchange with hydrothermal fluids during or after magmatic crystallization. The carbonatite is the result of partial melting followed by fractional crystallization. Primary carbonatite melt was formed by less than 1% partial melting of enriched mantle, leaving a garnet-bearing residue. The melt then rose to a crustal magma chamber and underwent fractional crystallization, producing further REE enrichment. The REE and trace element distribution patterns of the carbonatites are similar to those of fine-grained dolomite marble, the ore-host rock of the Bayan Obo REE–Nb–Fe giant mineral deposit. This fact may indicate a petrogenetic link between the dykes described here and the Bayan Obo mineral deposit. Received November 1, 2001; revised version accepted June 16, 2002     

Partitioning of rare earth elements between phosphate-rich carbonatite melts and mantle peridotites

Summary Near solidus equilibria in the system mantle peridotite-carbonate-phosphate doped with Ce and Yb have been studied at 20 kbar and 950°C. Carbonatitic melts in this system may be quenched into homogeneous glasses. Such melts intensely extract REE from rock-forming mantle minerals, and their migration may cause processes of mantle metasomatism.

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Verteilung von Seltenen Erden zwischen phosphatreichen karbonatitischen Schmelzen und Mantel-Peridotiten

Zusammenfassung Gleichgewichte nahe dem Solidus im System Mantel-Peridotit-Karbonat-Phosphat, das mit Ce und Yb dotiert wurde, wurden bei 20 kbar und 950°C untersucht. Karbonatitische Schmelzen in diesem System können zu homogenen Gläsern abgeschreckt werden. Solche Schmelzen extrahieren SEE aus gesteinsbildenden Mantelmineralen und ihre Migration könnte für Vorgänge der Mantel-Metasomatose verantwortlich sein.

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With 2 figures     

Mobility of trace elements during subduction metamorphism as exemplified by the blueschists of the Kurtushibinsky Range,Western Sayan

In order to evaluate the mobility of trace elements during subduction metamorphism, the geochemistry of blueschists of the Dzhebash Group from the Kurtushibinsky Range of the Western Sayan (basins of the Koyard and Oresh rivers) was studied, and the chemical compositions of high-pressure rocks were compared with weakly altered basalts from the same region. The protoliths of the blueschists were probably metabasalts of similar age from the ophiolitic dike complex, the pillow lavas of the Verkhnekoyardsky Formation crowning them, and the pillow basalts of the Kurtushibinsky Formation. The spatial association of the blueschists with the Kurtushibinsky Formation basalts and identical trace element patterns in these rocks allow us to suppose the cogenetic character of their protoliths. Geological and geochemical data suggest their formation in an oceanic plateau setting, whereas the mafic rocks of the dike complex and the Verkhnekoyardsky Formation show island-arc affinity. A comparison of the Dzhebash Group blueschists with the chemically equivalent Kurtushibinsky basalts showed that high-pressure metamorphism caused only minor changes in their compositions. These rocks are almost indistinguishable with respect to such fluid-immobile components as Ti, P, Zr, Hf, Y, and middle and heavy rare earth elements. On the other hand, the blueschists are strongly depleted in potassium. The selective removal of Rb and Ba during blueschist metamorphism was observed only in those samples that showed the most extensive removal of potassium.     

Composition and genesis of the Konevinsky gold deposit,Eastern Sayan,Russia

The Konevinsky gold deposit in southeast Eastern Sayan is distinguished from most known deposits in this region (Zun-Kholba, etc.) by the geological setting and composition of mineralization. To elucidate the cause of the peculiar mineralization, we have studied the composition, formation conditions, and origin of this deposit, which is related to the Ordovician granitoid pluton 445–441 Ma in age cut by intermediate and basic dikes spatially associated with metavolcanic rocks of the Devonian–Carboniferous Ilei Sequence. Four mineral assemblages are recognized: (1) quartz–pyrite–molybdenite, (2) quartz–gold–pyrite, (3) gold–polysulfide, and (4) telluride. Certain indications show that the ore was formed as a result of the superposition of two distinct mineral assemblages differing in age. The first stage dated at ~440 Ma is related to intrusions generating Cu–Mo–Au porphyry mineralization and gold–polysulfide veins. The second stage is controlled by dikes pertaining to the Devonian–Carboniferous volcanic–plutonic association. The second stage is characterized by gain of Hg and Te and formation of gold–mercury–telluride paragenesis.     

新疆塔什库尔干县含稀有金属伟晶岩-碳酸岩的时代——对帕米尔构造结稀有金属成矿作用的启示

帕米尔构造结位于青藏高原北缘,记录了自新元古代晚期以来到新生代完整的特提斯构造演化过程。在长期的构造—岩浆演化过程中,发育了不同时代、不同类型的成矿事件。野外调查表明,在帕米尔北东段,发育了含绿柱石的伟晶岩及含氟碳铈矿的碳酸岩(碳酸岩—伟晶岩组合)。通过独居石及锆石U-Pb定年,识别出该区两期伟晶岩带(包括碳酸岩),分别是新生代和中生代(19～18 Ma和200 Ma),其中早期含绿柱石伟晶岩形成于古特提斯洋关闭后的伸展阶段,而新生代含氟碳铈矿碳酸岩—伟晶岩组合,与新生代陆内走滑伸展事件相关,表明帕米尔构造结伸展走滑启动的时间可能在19 Ma。结合区域地球化学异常特征,表明在帕米尔地区有望在稀有、轻稀土找矿上获得突破。     

Petrology of plagiogranites of the Yenisei Batholith, Western Sayan

The tonalite-plagiogranite (tonalite-trondhjemite) association only occasionally occurs in the form of large granitoid bodies, such as the Yenisei Batholith (>500 km² in area). The granitoids of the Yenisei Batholith belong to Na-rich tholeiitic rock series and differ from granitoids of the calc-alkaline series in having lower contents of alkalis and alumina (12–14 wt % Al₂O₃) and low contents of granitophile elements (Rb, Li, Cs, Be, Nb, Ta, and W), Cr, and Ni. The Cr/V (<0.10) and Rb/Sr (0.01–0.1) ratios of these rocks are at a minimum, and their K/Rb (600–1000) and Na/K (5–10) ratios are at a maximum compared to those of the rocks of the most widely spread granitoid batholiths. The plagiogranites typically have REE concentrations higher than those in oceanic plagiogranites and display weakly fractionated REE patterns (La/Yb = 1.4–3.4) with weak (or without) Eu anomalies. The lower initial Sr ratios of these rocks (0.704) and their relatively high concentrations of Pb, Zr, and B testify to the predominantly mantle provenance of their protolithic material. Geological and geochemical characteristics of the Yenisei pluton suggest that its genesis can be considered within the scope of the model of retrograde-type magmatic replacement and that the batholith was produces by the earliest granitization processes in the oceanic crust. The granitic melt was derived at low pressures (<5 kbar) and intermediate temperatures (～700°C), at the inflow of an aqueous transmagmatic fluid into the magma-generating area and the subsequent fluid-magmatic differentiation. Considering the volumes and compositions of rocks composing the Yenisei Batholith, the latter can be attributed, similarly to other typical granitoid batholiths, to crustal plutons, which differ from both oceanic plagiogranites in ophiolitic belts and continental trondhjemites. The rocks can be regarded as an individual geochemical type of crustal plagiogranites.     

Tetrad effect in rare earth element distribution patterns: Evidence from the Paleozoic granitoids of the Oka zone,Eastern Sayan

The tetrad effect is a systematic change in REE properties, which is expressed as a split of a normalized REE pattern into four rounded segments (tetrads) and caused by the formation of complex compounds. It was identified in the granitoids of the Oka zone of the Eastern Sayan by ICP determination of REE contents. The granitoids have a high F content, a prominent negative Eu anomaly, and a low K/Rb ratio. An increase in the tetrad effect in the granitoids correlates with increasing Nb/Zr, Y/Ho, and Rb/Sr ratios and SiO₂ contents and decreasing Zr/Hf, Ce/Pb, La/Sm, and La/Ta ratios and the contents of Fe₂O₃ and Na₂O. In terms of correlation of the tetrad effect with the Y/Ho, Zr/Hf, and K/Rb ratios, the Li-F and alkali rare metal granitoids of the Oka zone are correlated with, respectively, the granitoids of the northern and northwestern part of the Bohemian Massif, eastern Germany, and the Baerzhe Massif, northeastern China.     

新疆塔什库尔干县含稀有金属伟晶岩-碳酸岩的时代——对帕米尔构造结稀有金属成矿作用的启示

帕米尔构造结位于青藏高原北缘,记录了自新元古代晚期以来到新生代完整的特提斯构造演化过程。在长期的构造-岩浆演化过程中,发育了不同时代、不同类型的成矿事件。野外调查表明,在帕米尔北东段,发育了含绿柱石的伟晶岩及含氟碳铈矿的碳酸岩（碳酸岩-伟晶岩组合）。通过独居石及锆石U-Pb定年,识别出该区两期伟晶岩带（包括碳酸岩）,分别是新生代和中生代（19~18 Ma和200 Ma）,其中早期含绿柱石伟晶岩形成于古特提斯洋关闭后的伸展阶段,而新生代含氟碳铈矿碳酸岩-伟晶岩组合,与新生代陆内走滑伸展事件相关,表明帕米尔构造结伸展走滑启动的时间可能在19 Ma。结合区域地球化学异常特征,表明在帕米尔地区有望在稀有、轻稀土找矿上获得突破。     

昆阳磷矿中稀土元素赋存状态研究

对昆阳磷矿中稀土元素的赋存状态进行了系统研究,结果表明矿体中稀土元素总量在50×10-6～250×10-6之间,以轻稀土元素富集、重稀土元素亏损为特征,Y含量在30×10-6～155×10-6之间,稀土元素总量与胶磷矿含量呈正相关关系。重砂矿物查找结果显示,昆阳磷矿中缺乏独立的稀土矿物。物相分析结果表明,稀土元素主要赋存在胶磷矿中,碳酸盐相和硅酸盐相中稀土元素含量极低。LA-ICPMS测试结果表明,胶磷矿单矿物中稀土元素总量在280×10-6～420×10-6之间,而白云石单矿物中的稀土元素总量均低于10×10-6。综合分析结果表明,昆阳磷矿中的稀土元素应主要以类质同像的形式存在于胶磷矿中。     

Magmatic graphite in dolomite carbonatite at Pogranichnoe,North Transbaikalia,Russia

A recently discovered dolomite carbonatite at Pogranichnoe, North Transbaikalia, Russia, dated at 624 ± 3 Ma, contains xenoliths of calcite-bearing dolomite carbonatite with graphite spherulites. Apatite and aegirine are the other rock-forming minerals. Chemically the carbonatites are ferrocarbonatite and ferruginous calciocarbonatite. The graphite forms <1 mm up to 1.5 mm diameter spherulites, with Raman spectra similar to published spectra of microcrystalline, amorphous carbon and disordered graphite, with G and D bands at 1,580−1,600 cm⁻¹ and at around 1,350 cm⁻¹. Alteration has formed Fe-bearing calcite to Ca-bearing siderite compositions not previously reported in nature around the graphite along cracks and fractures. Mineral and stable isotope geothermometers and melt inclusion measurements for the carbonatite all give temperatures of 700°–900°. It is concluded that the graphite precipitated from the ferrocarbonatite magma. There are three candidates to control the precipitation of graphite (a) a redox reaction with Fe^II in the magma, (b) potential presence of organics in the magma (c) seeding of, or dissolution in, the magma of graphite/diamond from the mantle, and further work is required to identify the most important mechanism(s). Graphite in carbonatite is rare, with no substantial published accounts since the 1960s but graphite at other localities seems also to have precipitated from carbonatite magma. The precipitation of reduced carbon from carbonatite provides further evidence that diamond formation in carbonate melts at high mantle pressures is feasible.     

Geochemistry and petrology of superpure quartzites from East Sayan Mountains,Russia

Acta Geochimica - Quartzites are widespread within Earth’s lithosphere, but their highly pure varieties occur quite infrequently. With the development of alternative energy sources, including...     

云南安宁磷矿中稀土元素分布规律和赋存状态研究

磷矿伴生稀土元素是获取稀土资源的重要途径。我国磷块岩型稀土矿分布广,稀土含量高,具有综合回收价值,是仅次于独立稀土矿床的伴生稀土资源。本文主要研究云南安宁磷矿中稀土元素分布规律和赋存状态,并比较了磷矿石中稀土元素总量与磷含量的关系,结果表明磷矿石中稀土氧化物总量为72×10^-6～1 050×10^-6,与磷含量呈一定的正相关关系。另外通过光学显微镜及电子显微镜观察发现,安宁磷矿中缺乏独立的稀土矿物,只在部分海绿石中找到了独立的稀土矿物(可能为独居石和褐帘石)。LA-ICP-MS分析结果表明,胶磷矿单矿物稀土元素含量在770×10^-6～920×10^-6之间,而白云石单矿物稀土元素含量均低于34×10^-6,石英单矿物的稀土元素平均含量为180×10^-6。由于部分独立的稀土矿物的存在,海绿石矿物中稀土元素总量可高达2 947.27×10^-6～3 159.87×10^-6。综合分析结果表明,安宁磷矿中稀土元素主要以类质同像的...     

The rare earth element geochemistry of granite,gneiss, and migmatite from the Western Metamorphic Belt of South-eastern Australia

Four muscovite-biotite granites from the Western Metamorphic Belt of South-eastern Australia have rare earth element patterns characterized by: (i) light rare earth element enrichment; (ii) slight Eu depletion; (iii) varying degrees of heavy rare earth element depletion. The rare earth element and major element chemistry of three of these muscovite-biotite granites (the Koetong, Lockharts and Yabba Granites) can be approximated very closely by a model involving 20% partial melting of an ultrametamorphosed pelitic sediment and contamination of this minimum melt by the residual material left after melting, in the ratio 60% melt: 40% residue. Granitoids can be very largely solid material at the time of emplacement.The other muscovite-biotite granite studied (the Hawksview Granite) has major and trace element characteristics which distinguish if from the other three granitoids and these differences are attributed to variations in source material at the site of melt generation.The rare earth element and major element chemistry of a garnet-cordierite gneiss from the Western Metamorphic Belt can be modelled assuming 5% partial melting of a pelitic metamorphic rock and contamination of the minimum melt by the residue in the ratio 30% melt: 70% residue.Separated granitic and biotitic portions of a migmatite from the Western Metamorphic Belt have rare earth element characteristics which are inconsistent with a simple partial-melting model, but it is suggested that re-equilibration following, or during, separation of the vein material could obscure the process by which the vein of the migmatite developed. It is however certain that the vein developed in situ from a pelitic meta-sediment leaving the biotite rich selvage, without the introduction of material from an external source.Leucogranites which crop out to the east of the Western Metamorphic Belt are high level intrusions of highly fractionated granitic melt. Their Sr isotopic characteristics and features of their major and trace element chemistry suggest that they derive from an igneous source and are not directly related to the granites and gneisses to the west.     

State of rare earth elements in the rare earth deposits of Northwest Guizhou,China

We studied the states of rare earth elements in ore of the Xianglushan rare earth deposit. Rare earth ore samples were tested and examined by scanning electron microscope, electron probe, and chemical leaching. No independent rare earth minerals were detected by scanning electron microscope. Elements detected by the electronic probe for the in situ micro-zone of the sample included: O, Al, Si, Ca, Mg, Fe, Ti, K, Na, S, Cl, C, Cu, Cr, V, and Pt. Rare earth elements were not detected by electron probe. (NH4)₂SO₄, (NH₄)Cl, NaCl, and H₂SO₄ were used as reagents in chemical leaching experiments that easily leached out rare earth elements under the action of 10% reagent, indicating that the rare earth elements in ore are mainly in the ionic state rather than present as rare earth minerals.     

Pogranichnoe Au-Bi occurrence, Eastern Sayan: Composition and link to magmatism

The Pogranichnoe ore occurrence of gold-bismuth type is closely related to Ordovician granitic rocks pertaining to the Sarkhoi granodiorite-granite complex of calc-alkaline series. This type of mineralization in the Eastern Sayan is described for the first time. The orebodies are represented by quartz veins and veinlets with greisenized granite as selvages. Three mineral assemblages consecutively follow one another: (1) pyrite-arsenopyrite, (2) base-metal with fahlore and Sb sulfosalts, and (3) gold-bismuth. The geological position of orebodies and character of microinclusions in accessory minerals of granites suggest that goldbismuth mineralization is related to granitoids.     

Genesis of the Khaluta alkaline-basic Ba-Sr carbonatite complex (West Transbaikala, Russia)

The Khaluta carbonatite complex comprizes fenites, alkaline syenites and shonkinites, and calcite and dolomite carbonatites. Textural and compositional criteria, melt inclusions, geochemical and isotopic data, and comparisons with relevant experimental systems show that the complex formed by liquid immiscibility of a carbonate-saturated parental silicate melt. Mineral and stable isotope geothermometers and melt inclusion measurements for the silicate rocks and carbonatite all give temperatures of crystallization of 915–1,000°C and 890–470°C, respectively. Melt inclusions containing sulphate minerals, and sulphate-rich minerals, most notably apatite and monazite, occur in all of the lithologies in the Khaluta complex. All lithologies, from fenites through shonkinites and syenites to calcite and dolomite carbonatites, and to hydrothermal mineralisation are further characterized by high Ba and Sr activity, as well as that of SO3 with formation of the sulphate minerals baryte, celestine and baryte-celestine. Thus, the characteristic features of the Khaluta parental melt were elevated concentrations of SO₃, Ba and Sr. In addition to the presence of SO₃, calculated fO₂ for magnetites indicate a high oxygen fugacity and that Fe⁺³>Fe⁺² in the Khaluta parental melt. Our findings suggest that the mantle source for Khaluta carbonatite and associated rocks, as well as for other carbonatites of the West Transbaikalia carbonatite province, were SO₃-rich and characterized by high oxygen fugacity.     

Petrostructural evolution of ultramafic rocks of the Kalninsky chromite-bearing massif, Western Sayan

The Kalininsky ultramafic massif is a fragment of lower structural zone of the Kurtushiba ophiolitic belt in the extreme northeastern part of the Western Sayan. The massif is composed largely of rocks making up the dunite-garzburgite banded complex. The northeastern part of the massif is composed mainly of dunite with linear NW-trending chromite-bearing zones, the localization of which is controlled by banding of the dunite-harzburgite complex. Harzburgite and dunite are characterized by inhomogeneous structures and textures caused by nonuniform ductile deformation, which is expressed as heterogeneous extinction, kink bands, and syntectonic and annealing recrystallization. The petrostructural patterns of olivine in harzburgite and dunite provide evidence for three stages of ductile deformation. At the first stage under deep mantle-crustal conditions, the ductile flow of ultramafic rocks developed mainly in a regime of axial compression, high temperature (>1000°C), and low strain rate (? < 10^?6 s^?1), which resulted in translational gliding along the (010)[100] and (100)[001] systems in olivine and enstatite, respectively, in combination with a subordinate role of syntectonic recrystallization. Consequently, the rocks acquired a medium-grained (mesogranular) microstructure. At the second stage, related to the thermal effect on ultramafics, the ductile flow developed under the settings of low strain rate (? < 10^?6 s^?1) and rising temperature (>1000°C). The translational gliding in olivine proceeded largely along (010)[100] and was accompanied by diffusion creep. As the temperature rose, ductile deformation gave way to secondary recrystallization of annealing, which facilitated the growth of olivine grains free of dislocations owing to absorption of individual grains oriented adversely relative to the compression axis and deformed grains saturated with dislocations. As a result, dunite and harzburgite with a coarse-grained porphyroblastic microstructure have been formed. The third stage of ductile flow was apparently related to their transport along deep-seated thrust faults under settings of intense shear deformations at a high temperature (～1000°C) and strain rate (? >10^?4 s^?1). The ductile flow in olivine resulted in heterogeneous translational gliding along (010)[100] and accompanied by intense syntectonic recrystallization with the formation of a porphyroblastic microstructure. Chromite mineralization in dunite is controlled by internal banding. Intense ductile flow facilitated the metamorphic separation of linearbanded Cr-spinel segregations. Thus, the results of a petrostructural study show that ultramafic rocks of the Kalninsky massif, ascending to the upper lithosphere, underwent both axial and shear ductile deformations in the mantle and lower crust, and these deformations controlled chromite mineralization.     

Compositions of magmas,formation conditions,and genesis of carbonate-bearing ijolites and carbonatites of the Belaya Zima alkaline carbonatite complex,eastern Sayan

Based on the investigation of melt inclusions using electron and ion microprobe analysis, we estimated the composition, evolution, and formation conditions of magmas responsible for the calcite-bearing ijolites and carbonatites of the Belaya Zima alkaline carbonatite complex (eastern Sayan, Russia). Primary melt and coexisting crystalline inclusions were found in the nepheline and calcite of these rocks. Diopside, amphibole (?), perovskite, potassium feldspar, apatite, calcite, pyrrhotite, and titanomagnetite were identified among the crystalline inclusions. The melt inclusions in nepheline from the ijolites are completely crystallized. The crystalline daughter phases of these inclusions are diopside, phlogopite, apatite, calcite, magnetite, and cuspidine. During thermometric experiments with melt inclusions in nepheline, the complete homogenization of the inclusions was attained through the dissolution of a gas bubble at temperatures of 1120–1130°C. The chemical analysis of glasses from the homogenized melt inclusions in nepheline of the ijolites revealed significant variations in the content of components: from 36 to 48 wt % SiO₂, from 9 to 21 wt % Al₂O₃, from 8 to 25 wt % CaO, and from 0.6 to 7 wt % MgO. All the melts show very high contents of alkalis, especially sodium. According to the results of ion microprobe analysis, the average content of water in the melts is no higher than a few tenths of a percent. The most salient feature of the melt inclusions is the extremely high content of Nb and Zr. The glasses of melt inclusions are also enriched in Ta, Th, and light rare earth elements but depleted in Ti and Hf. Primary melt inclusions in calcite from the carbonatites contain a colorless glass and daughter phlogopite, garnet, and diopside. The silicate glass from the melt inclusions in calcite of the carbonatite is chemically similar to the glasses of homogenized melt inclusions in nepheline from the ijolites. An important feature of melt inclusions in calcite of the carbonatites is the presence in the glass of carbonate globules corresponding to calcite in composition. The investigation of melt inclusions in minerals of the ijolites and carbonatites and the analysis of the alkaline and ore-bearing rocks of the Belaya Zima Massif provided evidence for the contribution of crystallization differentiation and silicate-carbonate liquid immiscibility to the formation of these rocks. Using the obtained trace-element compositions of glasses of homogenized melt inclusions and various alkaline rocks and carbonatites, we determined to a first approximation the compositions of mantle sources responsible for the formation of the rock association of the Belaya Zima alkaline-carbonatite complex. The alkaline rocks and carbonatites were derived from the depleted mantle affected by extensive metasomatism. It is supposed that carbonate melts enriched in sodium and calcium were the main agents of mantle metasomatism.