2013

The first data on carbon-hydrogen isotope composition of fluid inclusions from the Seblyavr massif carbonatites (Kola Peninsula). 

Buikin A.I., Sorokhtina N.V., Nevinny Yu. A., Kuznetsova O.V., Sevastyanov V.S., Smirnova E.P., Kogarko L.N.

Vernadskiy Institute of Geochemistry and Analytical Chemistry RAS, Moscow, Russia

The Seblyavr ultramafic–alkaline–carbonatite complex is one of the largest (4x5 km) massifs intruded Archean gneisses of the Kola Shield. The Seblyavr massif is multiphase central intrusion with concentrically zoned structures, and located in the northwestern part of the Kola Peninsula. The massif consists of five magmatic rock series, which formed in sequence from olivinites to clinopyroxenites and ijolites-melteigites and carbonatite series (magmatic stage – apatite-phlogopite-diopside (or forsterite) phoscorites and coexistent calcite carbonatites, apatite-phlogopite phoscorites and calcite carbonatites, apatite-amphibole-calcite phoscorites and amphibole-calcite carbonatites, hydrothermal tipe – dolomite-calcite and dolomite carbonatites). Often the early ultramafic, alkaline rocks, phoscorites and carbonatites of magmatic stage are intensively carbonatized and hydrothermally altered.

According to the last Th-Pb isochrones data based on dolomite carbonatites the crystallization age of the Seblyavr massif is 382.3±5.3 Ma (Rukhlov &  Bell, 2010). The U-Pb and Pb-Pb isotopic data of the late carbonatites (by Rukhlov & Bell) have the large scatter and show mantle source heterogeneity and open system behavior possibly due to a late  hydrothermal and metasomatic event during cooling of the intrusion.

We have studied isotope characteristics of fluid phase trapped as fluid inclusions in carbonatites of the Seblyavr massif. Fractions of carbonate, manly calcites were separated from drill core samples of altered carbonatized pyroxenite (Sbl97-23), fresh amphibole-phlogopite-calcite carbonatite (Sbl97-11) and the late calcite-dolomite carbonatite (Sbl99-22), and analyzed for gases from fluid inclusions by stepwise crushing method (Buikin et al., 2013). δ13C values in CO2 vary from -6.6 to -11.3 ‰ PDB, which is lighter than usual mantle value of around -3 ÷-5 ‰ in MORB chilled glasses (Buikin et al., 2013 and references there in), and could be the result of low temperature isotope exchange between CO2 and carbon from mineral matrix (i.e. calcite, Bottinga 1968). The same carbon behavior was observed in fluid inclusions from carbonate minerals of Guly carbonatites (Buikin et al. 2011). Hydrogen isotope composition was analyzed for fluid inclusions in calcites from Sbl97-11, Sbl99-22 and Sbl97-23 samples. The obtained δD values are -53 ‰, -110 ‰ and -135 ‰ SMOW respectively, which suggests magmatic (mantle?) water in amphibole-phlogopite-calcite carbonatite, and high temperature meteoric or hydrothermal water in the last one. This data indicate the occurrence of two magmatic and late hydrothermal or metasomatic events for Seblyavr carbonatites formation.

This study was financially supported by RFBR grants 13-05-12021 and 13-05-01009.