Sr-Nd-C-O isotope composition of carbonatites of the Petyayan-Vara REE deposit (Vuoriyarvi, Kola Region, NW Russia): Insight to the origin

The Petyayan-Vara area of the alkaline-ultramafic carbonatite complex Vuoriyarvi, located in Kola region (NW Russia; N 66°47’, E 30°05’), hosts abundant REE-Sr-Ba-rich magnesiocarbonatite veins. Magnesiocarbonatites containing burbankite are primary magmatic. These rocks underwent alterations during several magmatic-metasomatic events, which resulted in the formation of other varieties of carbonatites, including ancylite-dominant and bastnäsite-dominant magnesiocarbonatites (ores). We studied the Sr-Nd-C-O isotopic characteristics of both the most common varieties of carbonatites of the Petyayan-Vara area and calciocarbonatites (søvites) of its nearest surroundings. The isotopic composition of the least altered magnesiocarbonatites (ε_Sr³⁷⁰=-13.9, ε_Nd³⁷⁰=5.2, δ¹³C_PDB=-3.8‰, δ¹⁸O_SMOW=9.9‰) is close to that of søvites (ε_Sr³⁷⁰=-13.5±0.1, ε_Nd³⁷⁰=4.95±0.05, δ¹³C_PDB=-3.85±0.25‰, δ¹⁸O_SMOW=7.9±0.7‰). Analysis of other Petyayan-Vara carbonatites (including ancylite and bastnäsite ores) showed wide variations in signatures of all studied isotopic systematics. All altered carbonatites are enriched with crustal strontium (ε_Sr³⁷⁰ of -12.8 to -2.0), and an increase in ε_Sr³⁷⁰ is accompanied by an increase in the content of heavy isotopes of carbon (up to -1.0‰) and oxygen (up to 23.8‰). Most Petyayan-Vara carbonatites (including ancylite ores) have close values of ε_Nd³⁷⁰=5.1±0.2. Isochron dating based on the figurative points of these rocks yielded an age of 365 Ma, indicating that the Sm-Nd radiogenic isotope system in the studied samples was unperturbed after carbonatites were crystallized. The similarity of the obtained ε_Nd³⁷⁰ value with estimates of this parameter for different (both carbonate and silicate) rocks of the Vuoriyarvi complex indicates the isotopic homogeneity of the mantle source and its small contamination with the crustal material. Samples with a disturbed Sm-Nd system (ε_Nd³⁷⁰ of -1.1 to 4.7) have petrographic signs of alterations during later processes (e.g., superimposed silicification, crystallization of the late strontianite, etc.). Bastnäsite ores also exhibit severely disturbed Sm-Nd system (ε_Nd³⁷⁰=2.9). The change in ε_Nd³⁷⁰ can be explained by either (1) an addition of crustal Nd or (2) chemical fractionation of Sm and Nd during events that occurred much later than the crystallization of Petyayan-Vara carbonatites. The obtained isotope data refine the sequence of the magmatic-metasomatic events that led to the formation of the Petyayan-Vara REE deposit, which we proposed earlier. They also clarify the contribution of the sources of elements and the scale of their redistribution at different formation stages of Petyayan-Vara carbonatites.

This research was funded by the Russian Science Foundation, grant number 19-77-10039. Field work was supported by the Geological Institute KSC RAS, state order number 0226-2019-0053.