A reassessment of models for hydrocarbon generation in the Khibiny nepheline syenite complex, Kola Peninsula, Russia

Beeskow, B., Treloar, P.J., Rankin, A.H., Vennemann, T.W. and Spangenberg, J. (2006) A reassessment of models for hydrocarbon generation in the Khibiny nepheline syenite complex, Kola Peninsula, Russia. Lithos, 91(1-4), pp. 1-18. ISSN (print) 0024-4937


Although hydrocarbon-bearing fluids have been known from the alkaline igneous rocks of the Khibiny intrusion for many years, their origin remains enigmatic. A recently proposed model of post-magmatic HC generation through Fischer-Tropsch type reactions suggests a primary CO2 fluid. This hypothesis is based upon textural data, gas bulk and isotopic composition and low entrapment temperature of fluid inclusions that indicate an abiogenic origin for the HC gases. However, new petrographic, microthermometric, laser Raman, bulk gas and isotope data are presented and discussed in the context of previously published work in order to reassess models of HC generation. The gas phase is dominated by CH4 with only minor proportions of higher hydrocarbons. No remnants of the proposed primary CO2-rich fluid are found anywhere in the complex. Primary and secondary fluid inclusions show similar compositions and entrapment conditions. The majority of the fluid inclusions are of secondary nature and trapped in healed microfractures. This indicates a high fluid flux after magma crystallisation. Entrapment condtions for primary fluid inclusions are 450-550°C at 2.8 - 4.5 kbar. These temperatures are too high for HC gas generation through the FT reaction. Chemical analyses of rims of Fe-rich phases suggest that they are not the result of alteration but instead represent changes in magma composition during crystallisation. Furthermore, there is no clear relationship between the presence of Fe-rich minerals and fluid inclusion abundance as reported elsewhere. Fluid inclusion planes generally start and terminate at mineral boundaries . δ13C values for methane range from –22.4 to -5.4‰, confirming also an abiogenic origin for the gas. These data support an abiogenic HC origin, but are not consistent with a post-magmatic FT-type reaction. The presence of primary CH4-dominated fluid and melt inclusions, which contain a methane-rich gas phase, indicate that a magmatic origin is more likely. An increase in methane content, together with a decrease in δ13C isotope values towards the intrusion margin suggest that magmatically derived abiogenic hydrocarbons may have mixed with biogenic hydrocarbons derived from the surrounding country rocks.

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