Composition and sources of volatiles and noble gases in fluid inclusions in pyroxenites and carbonatites of the Seblyavr Massif, Kola Peninsula

Abstract

Carbonatites and related pyroxenites from the Seblyavr alkaline-ultramafic massif were analyzed for isotopic composition and concentrations of carbon (in carbon dioxide), nitrogen, and noble gases using the stepwise crushing technique. The C isotopic composition in crushing steps of calcite from the carbonatite varies from −6.6 to −15.0‰ (PDB) with average values from −8.5 to −10.5‰, which is lower than the mantle range for $\delta ^{13} C_{(CO_2 )} $ (from −3 to −5‰) and can likely be explained by long-term isotopic exchange between the carbon of CO2 in inclusions and their host Ca carbonate. The 40Ar/36Ar ratios in the crushing extractions of the calcites vary from the atmospheric value of 296 to 3200. Diopside from the pyroxenite has these ratios as high as 26000–33000 (such high values for pyroxenite in the Kola alkaline-ultramafic province have been obtained for the first time), which corresponds to the values obtained for MORB chilled glasses. Nitrogen in the samples is isotopically heavy, δ15N from +1 to +2 on average, which is consistent with earlier data on carbonatite massifs in the Kola alkaline province (Dauphas and Marty, 1999) and carbonatites of the Guli Massif (Buikin et al., 2011). The N2 content in the crushing extractions is correlated with the 36Ar concentration, which is an indicator of atmospheric contamination and suggests the dominance of the crustal N component in the samples, likely as a result of subduction or penetration of the ancient meteoric water into the magma chamber or a metasomatic source. The variations in the isotopic and elemental composition of the gas components between crushing steps suggest that the investigated samples contain inclusions of at least two populations.