The characterisation and origin of hydrocarbons in alkaline rocks of the Kola Alkaline Province
The origin of hydrocarbon-bearing fluid inclusions in igneous rocks remains controversial. The hydrocarbons may be of biogenic or thermogenic origin but most occurrences indicate an abiogenic origin. A variety of mechanisms have been proposed for the generation of abiogenic hydrocarbons which include a direct mantle origin, the respeciation of a magmatic fluid at low pressures and temperatures or the generation of hydrocarbons by a post-magmatic, low P-T process. The large Devonian alkaline intrusions, Khibina and Lovozero, in the Kola Alkaline Province, northwest Russia contain abundant CH[sub]4-dominant inclusions, which consist predominantly of a low-density methane vapour. These inclusions also contain significant concentrations of higher hydrocarbons up to C[sub]6 and hydrogen. They are secondary in nature, occurring as large regular inclusions or small trails of inclusions, which follow the cleavage planes of the host minerals. They are closely associated with the late minerals magnetite, biotite, aegirine and hydrated Na/K-silicates along with occasional H[sub]2 O-dominant fluid inclusions. A comparative study of the fluid inclusions in the nearby Devonian carbonatite intrusions, Kovdor and Sokli, shows that these intrusions are dominated by complex primary aqueous fluid inclusions. The main gas phase present is carbon dioxide. Hydrocarbons are not found at Kovdor but may be present in Sokli as small, secondary, monophase gas inclusions associated with phlogopitization and serpentinization of the mineral assemblage. The CH[sub]4-dominant fluid inclusions in Khibina and Lovozero were trapped as secondary inclusions at low pressures and temperatures at or near the CH[sub]4-H[sub]2 O solvus around 350 °C and 0.5 to 2 kbar. Isotopic results indicate the methane is abiogenic in origin ([delta][sup]13 C [sub]CH4 < -30 %). Therefore, a post-magmatic, abiogenic process generated the CH[sub]4-dominant fluid. The methane and higher hydrocarbons were generated via the Fischer-Tropsch reaction; nCO[sub]2 + (3n+1)H[sub]2 -> C[sub]n H[sub]2n+2 + 2nH[sub]2 O. A magmatic CO[sub]2 -rich fluid, that may be represented by the CO[sub]2 dominant fluid inclusions found in the Khibina carbonatite, reacted with hydrogen produced from hydrothermal reactions that replaced arfvedsonite, nepheline, augite and titanomagnetite with aegirine, biotite, magnetite and hydrous Na/K-silicates. The water produced by the Fischer-Tropsch reaction was recycled initiating further hydrothermal reactions setting up a cycle of alteration and hydrocarbon generation. Fischer-Tropsch synthesis of hydrocarbons in igneous rocks linked to post-magmatic hydrothermal alteration may be a lot more widespread than previously thought. This may have implications in the future for finding, or producing further natural gas resources.