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Hydrocarbon gases released from the crystalline rocks of the Ilimaussaq intrusion either by crushing or acid digestion show an abiogenic isotopic signature, δ13C of methane and ethane ranging from = -1.6 to -5.0 ‰ and -9.2 to -12.5 ‰ respectively. Propane of one of the samples gave an isotopic value of -16.5‰. The bitumen extracted from various rock samples, on the other hand showed hydrocarbon characteristics pointing to a biogenic origin.
GASES Numerous gas inclusions, mostly less than 10 m, are present in the various rocks of the Ilimaussaq intrusion. The gas released by crushing consisted of 90 percent hydrocarbons (C1-C5) and 10 percent inorganic gases, mostly hydrogen [1; 2]. By reaction with warm hydrochloric acid larger yields of hydrocarbons were obtained, up to 65 mg of methane per kg of rock, compared to the crushing method [3]. The composition of the gas released by the two different methods was similar with respect to both chemical and isotopic composition [4], and do not fit any known biogenic origin. The gas was therefore considered to be of abiogenic origin [1; 4].
BITUMEN Dispersed bitumen present in the various rock types, up to 300 mg HC per kg of rock, was extracted and analyzed either by IR spectroscopy [1] or GC and GC-MS [3]. The results of the latter point to hydrocarbons generated by thermal maturation of a marine type of source rock, and not to an abiogenic origin as had previously been suggested [1]. Stable carbon isotopic analyses of different extracts of bitumen gave δ13C values in the range -17 to -24 ‰ [4], thereby confirming its organic origin, and the authors therefore suggested that methane and bitumen were of different origin.
Bitumen having δ13C = -19.5 ‰ were also found in the sandstone formation into which the magma intruded c. 1100 Ma ago [3]. No potential source rock exists presently in the vicinity of the Ilimaussaq intrusion, however, 1800-2000 Ma old organic rich rocks, containing thermally mature bitumen are found less than 100 km north of the area [5; 6]. Therefore organic rich rocks expelling petroleum into the sandstones intruded by the magma may have been present. Results of new GC and GC-MS analyses of both Ketilidian and Ilimaussaq rock extracts will be presented and the most likely origin of the bitumen will be discussed. [1] Petersilie IA & Sørensen H (1970) Lithos, 3, 59-76. [2] Konnerup-Madsen J et. al. (1979) Bull. Soc. Fr. Minéral., 102, 642-653. [3] Laier T & Nytoft P (1995) 17th Meeting Organic Geochem., San Sebastian, 4-8 Sept. 1109-1111. [4] Konnerup-Madsen J et al. (1988). Bull. Soc. Fr. Minéral., 111, 567-576. [5] Bondesen, E et. al.(1967) Bull Grønlands Geologiske Undersøgelse, 67 [6] Pedersen, K R & Lam, J (1970) Bull Grønlands Geologiske Undersøgelse, 82.
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