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
hydrocarbon (HC) generation through Fischer-Tropsch (FT) type reactions
suggests the hydration of Fe-bearing phases and release of H-2 which
reacts with magmatically derived CO2 to form CH4 and higher HCs.
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 in the complex. 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 conditions for
fluid inclusions are 450-550 degrees C at 2.8-4.5 kbar. These
temperatures are too high for hydrocarbon 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 the abundance
of fluid inclusion planes (FIPs) as reported elsewhere. delta C-13
values for methane range from -22.4% to -5.4%, confirming a largely
abiogenic origin for the gas. The presence of primary CH4-dominated
fluid inclusions and melt inclusions, which contain a methane-rich gas
phase, indicates a magmatic origin of the HCs. An increase in methane
content, together with a decrease in delta C-13 isotope values towards
the intrusion margin suggests that magmatically derived abiogenic
hydrocarbons may have mixed with biogenic hydrocarbons derived from the
surrounding country rocks. (C) 2006 Elsevier BV. All rights reserved.