Hydrogen, carbon, nitrogen and helium in gases of igneous rocks

Abstract

The gases of closed pores of igneous rocks were investigated. A research of such kind offers a new geochemical method that can be used for the elucidation of complicated petrogenetic problems of gaseous regime of the earth's crust and for practical geological problems. In the igneous rocks of earth-alkaline series the content of gases varies from 0.4 to 9.5 cm 3/kg of rock. Granitoids contain a minimum quantity of gases. In the composition of gaseous phase hydrogen evidently prevails (up to 60–88% of gaseous mixture); hydrocarbon gases, nitrogen, helium, argon and sometimes carbon dioxide are the rest of the species. Agpaitic alkaline rocks (Khibiny, Lovosero, Illimaussak, Middle-Tatarien and other massifs) in their gaseous phase amounts and in composition differ sharply. The predominating component of the composition of these gases is not hydrogen, as in the previous case, but methane which amounts to 92% of the whole volume of gases, maximal quantity of gases in rocks reaches up to 150 cm 3/kg of rock. Agpaitic alkaline rocks by gaseous phase composition resemble the rocks of sedimentary formation, but methane of alkaline rocks differ from methane of sedimentary rocks in the carbon isotopic composition. The average isotopic composition of methane carbon, expressed in per cent of δC 13, is 0.85 for the Khibini massif, 0.53 for the Lovosero massif and 0.77 for Illimaussak (PDB etalon). The carbon isotope distribution between different fractions of hydrocarbon gases (methane-ethane-propane) is the reverse of that by sedimentary rock gases. The former allows the confirmation with a sufficient degree of certainty the abiogenetic origin of hydrocarbons in agpaitic alkaline rocks. The prevailing part of non-carbonic carbon in the igneous rocks is concentrated not in hydrocarbon gases, but in so-called dispersed carbon (C disp), which represents, presumably, carbon-graphite matter, uniformly disseminated in the rocks. The content of C disp varies from 0.008 to 0.09 wt.% on the rock. In the fracture zones the filtration of helium is revealed. The content of helium in water-dissolved gases of bore-holes in these zones is up to 4%. Such differences in helium content permit us to reveal and trace the fracture zones by superficial gaseous and water-helium mapping.