An Erratum to this paper has been published: https://doi.org/10.1134/S1028334X23060028

To gain insight into the source of ore material, this study presents new Re–Os and Pt–Os isotopic and highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re) abundance data for chromitite and native osmium from the Guli massif of ultramafic alkaline rocks and carbonatites located in the Maimecha–Kotui province, Polar Siberia. The study utilized a number of analytical techniques, including electron probe microanalysis (EPMA), negative thermal ionization mass spectrometry (N-TIMS), and high pressure asher digestion and isotope dilution inductively-coupled plasma mass-spectrometry (ID ICP-MS). The HSE concentrations in chromitite samples range from 191 to 866 ppb with a predominance of Ir-group platinum-group elements (PGE) (Os, Ir, and Ru) over Pt-group PGE (Rh, Pt, and Pd) and Re, which is consistent with the platinum-group mineral control (i.e., Os–Ir alloys and laurite, RuS2) within the chromitite. The Re–Os and Pt–Os isotope data indicate that the HSE budget of the chromitite and native osmium from the Guli massif was largely controlled by that of their mantle source, which evolved with long-term near-chondritic Re/Os and Pt/Os ratios; this source had time-integrated Re/Os and Pt/Os within the range of those for the majority of komatiites and abyssal peridotites worldwide.

The origin of the alkaline magmatism, including kimberlites and carbonatites, is believed to be related to deep-seated mantle plumes. A chondritic Earth’s mantle contains very low amounts of alkaline elements, with Na prevailing over K. Consequently, the source of the alkaline rocks cannot be the ‘chondritic’ mantle and most likely a mantle modified by subducted crustal materials. Alkaline magmas and carbonatites appear first in the Mesoarchean (~ 3 Ga) and possibly coincided with the onset of plate tectonics. Melting and degassing of subducted slabs into the deep mantle caused widespread metasomatism and formation of reservoirs enriched in the alkaline and lithophile trace elements. These served as sources of alkaline and carbonatitic magmas, and from ~ 2 Ga onwards of kimberlite magmas. Theoretical and experimental modeling predict the lower mantle and transition zone to be largely composed of bridgmanite, ferropericlase, Ca-Si-perovskite, ringwoodite, wadsleyite, majorite, NAL (a hexagonal aluminous phase of the lower mantle containing Na, Al and K), breyite and carbonates. The alkaline elements, isomorphic in CaSi-perovskite, bridgmanite and NAL, can be released during the ascent of mantle plume and transferred to the melt/fluid-enriched reservoir of carbonatites and alkaline magmas. At ~ 600 km depth where majorite is stable, an extensive fractionation of K and Na occurs, as the partitioning coefficient of Na is an order of magnitude larger than that of K. This results in K enrichment of the metasomatic melt/fluid that contribute to prospective sources of kimberlitic and other deep-mantle K magmas.

The Fernando de Noronha archipelago (southwest Atlantic, 345 km from the coast of Brazil) is considered as the result of mantle plume activity. However, data on the isotopic composition of helium and neon, which are, perhaps, the only unambiguous geochemical criterion for deep mantle plumes have not been published yet for the region. In this paper, we present the first data on the isotopic composition of helium, neon, argon, and nitrogen, obtained by stepwise crushing of mantle xenoliths from the basanites of the San José Formation. The results obtained may indicate that fluid inclusions contain the very first portions of the exsolved gases—they are ultra-depleted in helium in relation to neon and especially argon. This conclusion is also supported by He–Ar–CO2 systematics. The isotopic composition of helium (4He/3He = 31 879 ± 6796) and neon (21Ne/22Ne(mantle) = 0.0453 ± 0.0012) indicates that it was indeed a mantle plume, identical in noble gas composition to the Kerguelen plume. According to the Ar–Ne isotope systematics 40Ar/36Ar (mantle) = 7455 ± 2290. Nitrogen is characterized by a heavy isotopic composition (δ15N = +5.4 ± 0.2‰), which corresponds to the hypothesis of the subduction nature of nitrogen in deep mantle plumes.

To gain further insight into the origin of Ru–Os sulfides, the first in-situ sulfur isotopic data for Ru–Os sulfides from different polyphase platinum-group mineral (PGM) assemblages derived from the Quaternary deposits of the Guli massif located in the Maimecha-Kotui province are presented. A number of analytical techniques, including electron microprobe analysis and laser ablation attached to multiple-collector inductively coupled-plasma mass-spectrometry, were utilized. Polyphase platinum-group mineral (PGM) assemblages are represented by two types: (1) osmium and iridium alloys associated with Ru–Os sulfides of the laurite (Ru,Os)S 2 –erlichmanite (Os,Ru)S 2 solid solution series, cuproiridsite (CuIr 2 S 4 ), and unnamed Os–Ir sulfide (Os,Ir)S 2 ; (2) ferroan platinum containing inclusions of laurite, Ru–Os–Ir alloys, and other PGMs. These detrital PGM assemblages were sourced from different bedrocks. It is concluded that PGM assemblages of types 1 and 2 were derived from dunite/chromitite and clinopyroxenite, respectively. The sulfur isotope signatures of Ru–Os sulfides of type 1 (δ 34 S = 0.9 ± 0.4‰, n = 8) imply that sulfur derived from a subchondritic source. The slightly lighter δ 34 S values in type 2 laurite (δ 34 S = –1.7 ± 0.2‰, n = 10) are likely due to the evolved composition of the ore-forming fluid. Despite these differences, the S-isotope data are consistent with the origin of sulfur from a common near-chondritic source.

Experimental studies of melting relations in the MgSiO3-Na2CO3 (+/- Al2O3, Fe2O3) multicomponent alkaline carbonate-silicate system were carried out at 23-24 GPa and 1100-1700 degrees C to shed light on the conditions and mechanisms of formation of Na-bearing bridgmanite. At 1100-1300 degrees C, an assemblage of low-Na bridgmanite (<0.6 wt% Na2O), ringwoodite, periclase (ferropericlase), alkaline carbonate with composition close to Na2Mg (CO3)(2), and sodic carbonate-silicate melts is formed. With increase in temperature up to 1700 degrees C, carbonate disappears and the content of Na2O increases in all mantle phases (up to 1.6, 4.4, and 2.7 wt% in bridgmanite, ringwoodite, and ferropericlase, respectively), which indicates that Na-bearing bridgmanite may crystallize at moderate degrees of partial melting. The clear positive correlation between the contents of Na and Al, and negative correlation between Na and Fe in bridgmanite, as well as the preferable structural incorporation of Al and Na with temperature, provide evidence for the predominant crystallization of Na-rich bridgmanite via the vacancy mechanism Si-B(4+) + Mg-A(2+) + O-O(2-) = Al3+ (B) + Na-A(+) + V-O from Al-bearing alkaline carbonate-silicate melts, with composition similar to those detected as inclusions in lower-mantle diamonds.

The paper presents the first ever data obtained on minerals concentrating Mn in the lower zone (which is not exposed at the surface) of the rare-metal deposit of the Lovozero alkaline massif in the Kola Peninsula. Detailed data are presented on the chemical composition of the pyrophanite, ilmenite, and magnetite. EPMA data indicate that the ilmenite and pyrophanite make up a continuous solid-solution series with Mn concentrations varying from 3.9 to 42.1 wt

Sodium plays an important role in the crystal structures of eudialyte-group minerals given that it can occupy different crystallographic sites. Predominantly, it distributes between the N(1–5) sites situated in the large cavities of the heteropolyhedral framework. Rarely, Na occupies split sites of the M(2) microregion where it can predominate over other elements (predominantly Mn, Fe2+, and Fe3+). The crystal structure of the Mn-deficient manganoeudialyte from the Lovozero alkaline complex (Kola Peninsula, Russia) has been refined. The trigonal unit–cell parameters are: a = 14.1848(2) Å, c = 30.4726(3) Å, V = 5309.90(11) Å3. The sample is a rare example of a high-sodium and high-calcium representative of the eudialyte group with Fe + Mn < 2 apfu. The idealized formula is Na14Ca6[(Mn,Fe)2Na]Zr3Si2[Si24O72]O(OH)·2H2O with bivalent components, Mn2+ and Fe2+, dominating at the M(2) site. The regularities of isomorphism involving M(2)Na in EGMs and the problem of the existence of the M(2)Na-dominant analogue of eudialyte are discussed. The new data obtained in this work confirm the previous conclusion that the complete isomorphism between Ca-deficient and Ca-rich members of the eudialyte group cannot be realized in frames of a single-space group (R3m, R-3m or R3). Thus, the existence of the M(2)Na analogue of eudialyte remains questionable.

We present the petrography, bulk major and trace elements, and isotopic compositions for the recently identified Mohnatye Roga agpaitic syenite dyke, located 30 km from the Kovdor carbonatite complex, Kola Peninsula, Russia. The dyke was identified below Quaternary deposits as the result of a gravimetric survey and subsequent drilling. A K-Ar age on lamprophyllite of 368 +/- 9 Ma, coincides with the age of the Devonian Kola Alkaline Carbonatite Province (KACP). In the total alkalis vs. silica diagram, the Mohnatye Roga dyke exhibits a compositional trend from lamprophyre to peralkaline phonolite as well as mineralogical and geochemical affinities with the nearby Niva intrusion. The dyke is composed of orthoclase, Ti-rich aegirine-augite, aenigmatite, lamprophyllite-group minerals, K-amphibole, and astrophyllite with accessory noonkanbahite, calcite, ilmenite, rutile, barite, apatite, and sulfides. Late stage minerals include natrolite and ferripyrophyllite. A notable feature of this d?ke is its extremely high agpaicity and enrichment in HFSEs (high-field strength elements). The con-centrations of a number of HFSEs are higher in this dyke than in the Lovozero rare metal deposit. Mohnatye Roga dyke and Niva intrusion are examples of a gradual transition from melt to hydrothermal fluid. Isotopic data indicate a mantle source for the dyke, and seemingly an absence of contamination by continental crust during its formation. The geochemical and isotopic data suggest that the source of the KACP rocks, including the Mohnatye Roga dyke, is FOZO mantle which is believed to be related to plume magmatism.

A giant deposit of loparite ores, a source of Nb, Ta, and rare earth metals, is related to the world’s largest layered Lovozero pluton in the Kola Peninsula. The Sr and U contents and Nb/Ta ratio increase and the REE and Ba contents decrease in loparite in the vertical section of the Lovozero deposit to a depth of 2450 m. Detailed studies have shown that the change in the morphology of loparite and in the time of its crystallization is a new geochemical indicator of the ore potential of alkaline magmas for rare metals (loparite ores). The ore-bearing zones with rare metals of the Lovozero intrusion are only those that contain euhedral (cumulative) loparite. The lowermost zone of the intrusion (70 m from the lower contact), which is characterized by anhedral loparite, is not promising for rare metals. Thus, early cotectic saturation of the melt in the ore mineral is a necessary condition for the formation of magmatic rare metal deposits. In this case, the ore minerals have a euhedral morphology. If the content of the ore component is significantly below cotectic, the crystallization of the ore mineral would occur at late stages of the formation of rocks in a small volume of the interstitial melt, when the events of differentiation of convective gravitation and segregation of the mineral phases are difficult, leading to the dispersion of ore components as anhedral grains of accessory minerals.

Here we present summarizing of isotopic compositions and element ratios of noble gases, nitrogen, carbon and hydrogen in carbonatites of different generations of the Guli massif (West Siberia, Russia) obtained by stepwise crushing. The data point to the subcontinental lithospheric mantle (SCLM) as a primary source of the fluid phase in Guli carbonatites. However, the estimated 40Ar/36Ar ratio in the Guli mantle source of about 5400 is similar to the Kola plume value of 5000 ± 1000 (Marty et al., 1998). One explanation of such a low estimated 40Ar/36Ar ratio in the mantle end-member with SCLM type helium (4Не/3Не ~ 120000) and neon (21Nе/22Nеmantle ~ 0.7) is an admixture of atmospheric argon to the local mantle source. This assumption is supported by the Ar-Ne systematics as well as by the data for hydrogen isotopic composition. Early carbonatite differs significantly from the later ones by the concentration of highly volatile components, as well as by the isotopic compositions of carbon (CO2), argon, and hydrogen (H2O). The mantle component dominated in fluids at the early formation stages of the Guli massif rocks, whereas the late stages of carbonatite formation were characterized by an additional fluid source, which introduced atmospheric argon and neon, and most likely a high portion of CO2 with isotopically heavy carbon. The argon-neon-hydrogen isotope systematics suggest that the most plausible source of these late stage fluids are high temperature paleometeoric waters. The absence of a plume signature could be explained in terms that Guli carbonatites have been formed at the waning stage of plume magmatic activity with an essential input of SCLM components.

Detailed studies have shown that a change in the eudialyte occurrence forms (and the moment of its crystallization) is a new geochemical criterion for rare metal ore content in alkaline magmas (eudialyte ores). A new principle of the presence of ores in alkaline magmas has been formulated: a prerequisite for the formation of an ore deposit is early saturation of alkaline magmas with an ore mineral. If the ore component concentration is significantly lower than the cotectic (saturation) one, then melt saturation and crystallization of an ore mineral will take place at later stages of rock formation in a small volume of the interstitial melt, when the phenomena of convective–gravity differentiation and segregation of mineral phases in the form of ore deposits are hampered. This leads to dispersion of the ore components in the form of xenomorphic grains of accessory minerals. Rocks of the differentiated complex (lower zone of the Lovozero deposit) and rocks of the Khibiny massif contain xenomorphic eudialyte and are not promising for eudialyte ores. Eudialyte deposits are associated with the upper zone of the Lovozero intrusion where euhedral early eudialyte occurs. The initial magma is saturated with eudialyte after crystallization of about 85% of the intrusion. The proposed criterion is applicable to the largest alkaline massifs in the world. The Ilimaussaq massif (Greenland), the rocks of which contain early crystallized euhedral eudialyte, hosts a superlarge eudialyte ore deposit. Unlike the Khibiny massif and the Pilanesberg alkaline complex, the rocks of which contain late xenomorphic eudialyte, this massif has no deposits of this type.

The distribution patterns of rare earth metals (REM) in the rocks of the Kugda massif (Polar Siberia) are assessed. The REM content decreases from early olivinite rocks, containing, on average, 1938 ppm, to the end products of syenite differentiation and increases again in carbonatites. The difference in the distribution coefficients of light and heavy rare earth metals is the reason for the noticeable fractionation of these elements during the evolution of the magmatic system of the Kugda massif. The ratio of light REM to heavy Ce/Yb drops by almost an order of magnitude in later differentiation products. The main process of the Kugda massif formation was continuous crystallization differentiation, characterized by a wide crystallization field of perovskite. An interesting feature of the process is the very early crystallization of perovskite, associated with the high potential of carbon dioxide.

The Lovozero complex, Kola peninsula, Russia and the Ilímaussaq complex in Southwest Greenland are the largest known layered peralkaline intrusive complexes. Both host world-class deposits rich in REE and other high-tech elements. Both complexes expose spectacular layering with horizons rich in eudialyte group minerals (EGM). We present a detailed study of the composition and cryptic variations in cumulus EGM from Lovozero and a comparison with EGM from Ilímaussaq to further our understanding of peralkaline magma chambers processes. The geochemical signatures of Lovozero and Ilímaussaq EGM are distinct. In Lovozero EGMs are clearly enriched in Na + K, Mn, Ti, Sr and poorer Fe compared to EGM from Ilímaussaq, whereas the contents of ΣREE + Y and Cl are comparable. Ilímaussaq EGMs are depleted in Sr and Eu, which points to plagioclase fractionation and an olivine basaltic parent. The absence of negative Sr and Eu anomalies suggest a melanephelinitic parent for Lovozero. In Lovozero the cumulus EGMs shows decrease in Fe/Mn, Ti, Nb, Sr, Ba and all HREE up the magmatic layering, while REE + Y and Cl contents increase. In Lovozero EGM spectra show only a weak enrichment in LREE relative to HREE. The data demonstrates a systematic stratigraphic variation in major and trace elements compositions of liquidus EGM in the Eudialyte Complex, the latest and uppermost part of Lovozero. The distribution of elements follows a broadly linear trend. Despite intersample variations, the absence of abrupt changes in the trends suggests continuous crystallization and accumulation in the magma chamber. The crystallization was controlled by elemental distribution between EGM and coexisting melt during gravitational accumulation of crystals and/or mushes in a closed system. A different pattern is noted in the Ilimaussaq Complex. The elemental trends have variable steepness up the magmatic succession especially in the uppermost zones of the Complex. The differences between the two complexes are suggested to be related dynamics of the crystallization and accumulation processes in the magma chambers, such as arrival of new liquidus phases and redistributions by mush melts.

This paper reports the results of the first study of pyroxenes from the deepest zones of the Lovozero deposit. The geochemical and mineralogical study of these rocks is of great scientific interest, as they are the least differentiated rocks and provide insight into the composition of a parental magma. According to microprobe analysis, clinopyroxenes evolve from early diopside–hedenbergite–augite to later alkaline aegirine–augite species. Upsection, the contents of Na, Fe3+ and Ti increase, while Mg, Ca, Fe2+, and Zr decrease. Thus, isomorphic substitution in pyroxenes of the lower zone follows the scheme (Ca, Mg, Fe2+, Zr) → (Na, Fe3+, Ti).

Geochemical data on three superlarge rare-metal deposits of Eastern Fennoscandian are generalized for the first time. Using loparite, eudialyte, and apatite deposits of the Kola Peninsula as an example, it is shown that the early saturation of an alkaline magma in an ore phase is a necessary condition for the formation of a magmatic deposit. An important factor is an active convection in the magma chamber, which causes the mineral phases to be sorted by size, thus leading to the formation of magmatic deposits. The absence of Eu anomaly in the REE distribution patterns for apatite, loparite and eudialyte ores indicates that primary magma did not experience fractionation of plagioclase. The primary magma of these deposits was olivine-melanephelinite in composition.

Alkaline magmatism has occurred since 2.7–3Ga and its abundance has continuously increased throughout the Earth’s history. Alkaline rocks appeared on the Earth with changes in the geodynamic regime of our planet, i.e., when plume tectonics was supplemented by plate tectonics. Global scale development of plate tectonics at the Archean–Proterozoic boundary initiated subduction of already significantly oxidized oceanic crust enriched in volatiles and largescale mantle metasomatism caused the formation of enriched reservoirs as sources of alkaline and carbonatite magmatism. Study of metasomatized mantle material showed the occurrence of traces of primary carbonatite melts, which are strongly enriched in rare elements, according to ionmicroprobe analyses. The results obtained allowed us to propose a new two-stage genetic model for Ca-rich carbonatites including (1) metasomatic wehrlitization and carbonatization of mantle material and (2) partial melting of wehrlitized mantle with formation of carbonaterich melts or three immiscible liquids (at high alkali contents), i.e., silicate, carbonatitic, and sulfide (at high sulfur activity).

Lovozero complex, the world’s largest layered peralkaline intrusive complex hosts gigantic deposits of Zr-, Hf-, Nb-, LREE-, and HREE-rich Eudialyte Group of Mineral (EGM). The petrographic relations of EGM change with time and advancing crystallization up from Phase II (differentiated complex) to Phase III (eudialyte complex). EGM is anhedral interstitial in all of Phase II which indicates that EGM nucleated late relative to the main rock-forming and liquidus minerals of Phase II. Saturation in remaining bulk melt with components needed for nucleation of EGM was reached after the crystallization about 85 vol. % of the intrusion. Early euhedral and idiomorphic EGM of Phase III crystalized in a large convective volume of melt together with other liquidus minerals and was affected by layering processes and formation of EGM ore. Consequently, a prerequisite for the formation of the ore deposit is saturation of the alkaline bulk magma with EGM. It follows that the potential for EGM ores in Lovozero is restricted to the parts of the complex that hosts cumulus EGM. Phase II with only anhedral and interstitial EGM is not promising for this type of ore. Nor is the neighboring Khibiny complex despite a bulk content of 531 ppm of Zr. Khibiny only has interstitial and anhedral EGM. The evolution of the Lovozero magma is recorded in the compositions EGM up through a stratigraphy of 2400 m in Phase II and III of the complex, and distinct in elements like rare earth elements (REE), Sr, Ba, Th, U, Rb, Mn, Fe. The compositional evolution reflects primarily fractional crystallization processes within the magma chamber itself in combination with convective magma flow and layering by precipitation of minerals with different settling velocities. The suggested mechanism for the formation of the EGM deposits is flotation of very small, suspended EGM crystals in the convective magma and concentration below the roof of the magma chamber. Phase III EGM is enriched in total REE (1.3%) and in HREE (Ce/Yt = 8.8) and constitutes a world class deposit of REE in the million tons of Phase III eudialyte lujavrites.