Beeskow, B., Rankin, A.H., Treloar, P.J., Potter, J. and Nivin, V. (2003) Alkaline igneous rocks of the Kola Peninsula: potential source rocks for abiogenic hydrocarbons via Fischer-Tropsch type reactions. In: EuroCarb ESF Network on Mantle Carbon and Carbon Cycling Workshop: What controls the location and timing of transfer of mantle carbon through the crust?; 16 - 21 Sep 2003, Lanzarote and Fuerteventura, Canary Islands, Spain.
Abstract
The Kola alkaline province of NW Russia is the type example of the occurrence of abiogenic hydrocarbons (HCs) in crystalline rocks. Methane and higher hydrocarbons, up to C5H12, are contained primarily within abundant fluid inclusions in rock-forming minerals or as free gas contained in sealed microfractures or along grain boundaries. Such occurrences are not unique to Kola. High-temperature, hydrocarbon-rich fluid inclusions are present in alkaline rocks from Ilimaussaq, Greenland, and in recent years they have been described in other silica-undersaturated igneous rocks as well as in hydrothermal systems associated with basic igneous rocks. Two main models (Potter and Konnerup-Madsen, 2003) have been proposed for the genesis of these abiogenic HCs: (1) a late-magmatic model whereby the HCs are equilibrium products of volatiles exsolved directly from magmas, and; (2) a post-magmatic, disequilibrium model whereby HCs are generated via Fischer-Tropsch type reactions at ca. 350°C involving CO2 and H2O bearing fluids, linked to redox reactions accompanying hydrothermal alteration. This paper reports on recent and current research on the origin and distribution of HCs from the Khibina and Lovozero intrusions of the Kola Peninsular that supports an abiogenic origin through Fischer- Tropsch synthesis. An approximate estimate of the total volume of HCs contained within these rocks can be made on the assumption that CH4-rich inclusions, with a mean density of 0.1 g/cc occupy 0.1% on the total rock volume. For a body of rock 20 x 10km (less than 10% of the area of the Khibina and Lovozero intrusions) and 1km thickness this corresponds to some 40 million tonnes of methane and associated higher HCs. Due to the low permeability these complexes cannot yet be considered as potentially economic. However, large volumes of free-flowing methane have been recorded as being released when rocks are fractured as a result of drilling during underground mining (Nivin et al., 2001) suggesting the ease with which sealed microfractures and fluid inclusions planes may be opened up and connected. Hence they are potential source rocks for abiogenic HCs especially in areas where there has been major faulting and accompanying hydrothermal alteration. Where these faults extended to sufficient depth to intersect suitable conditions for Fischer-Tropsch reactions they would have acted as focal points for HC generation as well as having acted as suitable pathways for their migration to potential traps at higher levels in the crust. Modern day reactivation of these fault systems, by either natural or anthropogenic processes will cause the release of significant amounts of methane and higher HCs trapped on grain boundaries and in secondary fluid inclusions.
Actions (Repository Editors)
 |
Item Control Page |