Isotopic composition of carbon and oxygen in carbonatites from the Kosyu massif (Middle Timan)
Kovalchuk N.S.
Institute of Geology, Komi Science Centre of RAS Urals Division, Syktyvkar, Russia

In the Middle Timan, in the south-eastern part of the Chetlass Stone, carbonatites with rare-earth mineralization have been known for a long time. The genesis of these carbonatites is insufficiently studied. Their age is 660 ± 23 Ma by K–Ar dating on phlogopite [1]. Alkali-ultramafic magmatite dykes, alkaline metasomatites and hydrothermal goethite-feldspar and quartz-goethite-hematite formations are closely related spatially, temporally and structurally. Their detailed description are given in the works by Yu.P. Ivensen [2], V.I. Stepanenko [3] and others. According to V.I. Stepanenko, this association of rocks formed in subplatform tectonic mode; it tends to fault zones of north-eastern strike with long-time tectonic activity and deep deposition. Regarding the origin of carbonatites of the Middle Timan, there are two points of view. According to V.I. Stepanenko bodies and zones of carbonatites evolved from previously formed enclosing sedimentary-metasomatic rocks (fenites and phlogopite mica) and have a mantle source of substance [1]. O.S. Kochetkov previously assumed [4] that these structures arose in the process of regional metamorphism by a metamorphic hydrothermal way due to carbon dioxide with source from sedimentary rocks.

To test these assumptions we have determined the isotopic composition of carbon and oxygen in carbonatites from the Kosyu massif.

Isotope studies of carbonatites were performed at an analytical complex, which includes the system of preparation and input of samples Gas Bench II, connected to mass spectrometer DELTA V Advantage in the laboratory of isotope geochemistry of the Institute of Geology of Komi Science Centre (analyst I.V. Smoleva). δ¹³C values are in ppm to PDB standard, δ¹⁸О – SMOW standard. Decomposition of carbonates and recalculation of δ¹⁸О values was performed by a standard technique [5].

In general, carbonatites from the Kosyu massif are characterized by significant variations in the isotopic composition of carbon and oxygen: δ¹³С = -4.2 ÷ -9.8 ‰; δ¹⁸О = +8.6 ÷ +20.8 ‰. The average δ¹³C value of Kosyu carbonatites is -5.4 ‰. According to J. Hefs [6] the majority of carbonatites have δ¹³С from -5 to -7 ‰, indicating the deep source of carbon, and formations, which show the values beyond these limits, were subject to hydrothermal alteration. In this case, higher values of δ¹³C are explained by thermodynamic effects of fractionation of carbon isotopes, due to decrease of carbonatite formation temperature [1].

In general Kosyu carbonatites are characterized by mantle or close to them δ¹³С values, but vary by δ¹⁸О (average δ¹⁸О is +14.3 ‰). It is possible that the observed separation reflects fractionation during the formation of carbonatites or the rocks were exposed to the hydrothermal-metasomatic alterations [7]. These differences seem to be connected to various degrees of ability to concentrate oxygen in rock-forming minerals, partly to post-magmatic transformations.

According to V.I. Stepanenko [1], carbonatites, characterized by such isotope composition of carbon and oxygen, testify to juvenile genesis of carbon dioxide. If sedimentary-metamorphic strata were source material for Kosyu carbonatites, then one would expect almost complete identity in the carbon isotope composition of carbonatites and Precambrian carbonate sedimentary rocks of the Middle Timan (δ¹³C value for the latter varies from +0.7 to -0.7 ‰) [1]. Since, according to E.M. Galimova [8], the isotopic composition of carbon does not undergo significant changes during metamorphism.

Thus, judging from the isotopic composition of carbon, the carbonatites from the Kosyu massif had a deep source of substance. Compared to typical carbonatites of alkaline-ultramafic composition [1] the Kosyu carbonatites are greatly enriched by ¹⁸O, which can be interpreted as a sign of participation of solutions contacted with the enclosing rocks in their formation. This participation might include both the post-magmatic transformation of pre-existing carbonatites body, and the formation of carbonatite stockwork by hydrothermal-metasomatic way.

This work was supported by the Uralian grant for young scientists.

References:

1. Stepanenko V.I., Sukhanov N.V. The isotopic composition of carbon and oxygen carbonatites in the Middle Timan // Reports of USSR Academy of Sciences. V. 251. № 3. 1980. P. 699–702 (in Russian).

2. Ivensen Yu.I. Magmatism of Timan and Kanin Peninsula. Moscow: Nauka, 1964 (in Russian).

3. Kostyukhin M.N., Stepanenko V.I. Baikalian magmatism in the Kanin-Timan region. Leningrad: Nauka, 1987. 232 pp (in Russian).

4. Kochetkov O.S. In collection: Rare elements in rocks of different metamorphic facies. Moscow: Nauka, 1967 (in Russian).

5. Friedman I., O'Neil J.R. Compilation of stable isotope fractionation factors of geochemical interest // Data of Geochemistry, 6th. Wash. D.C. U.S. Gov. Print. Office. 1977. 116 pp.

6. Hefs J. Geochemistry of stable isotopes. Moscow: Mir, 1983. 198 pp (in Russian).

7. Lokhov K.I., Lobach-Zhuchenko S.B., Prasolov E.M., Arestova N.A. The isotopic composition of sulfur in sulfides, carbon and oxygen of carbonates from Archaean subalkaline high-MG intrusions (sanukitoids) of Karelia: on the characterization of mantle source // Proceedings of 18th symposium on isotope geochemistry named by Academician A.P. Vinogradov. Moscow, 2007. P. 161–162 (in Russian).

8. Galimov E.M. Geochemistry of stable isotopes of carbon. Moscow: Nedra, 1968. 167 pp (in Russian).