Tendencies in the distribution and hypotheses of the genesis of condensed naphthides in magmatic rocks from various geodynamic environments

Abstract

Literature data suggest that concentrated naphthides (CN)—bitumen and oil—can occur in intrusive and volcanic rocks of various silicity and alkalinity whose age ranges from the Proterozoic to Recent. The qualitative composition of heavy (high-molecular) hydrocarbons (HHC) in CN is the same in various rocks: heavy alkanes, polycyclic aromatic hydrocarbons (PAH), oxygen-bearing derivatives of hydrocarbons, etc. The presence of CN in rock-forming minerals in magmatic rocks and pegmatites confirms that CN condensed during the epimagmatic stage, and the molecular mass of the CN, for example, in the Khibina alkaline massif decreased with decreasing temperature simultaneously with the evolution of its mineral assemblages. The synthesis of CN continues during the hydrothermal stage, but high-temperature associations of HHC gave way during this stage to low-temperature ones. During all evolutionary stages of magmatic and related processes, the contents of CN are correlated with those of trace elements, which likely occur as organoelement compounds (OEC). Hypotheses of endogenic and exogenic genesis of CN in magmatic rocks are discussed. The endogenic hypotheses include HHC synthesis from inorganic gases and light hydrocarbons (HC) on catalytic minerals. The fact that HHC and CN occur in mantle xenoliths and high-pressure minerals and the results of experimental and thermodynamic modeling are discussed as supporting the hypothesis of the mantle genesis of CN in magmatic rocks. The metastable ascent of gaseous HHC, the low oxygen fugacity, and the high alkalinity of the melts could be favorable for the partial preservation of HHC in these rocks at crustal depth levels. Endogenic CN can serve as a nutrient medium for microorganisms in the upper lithosphere. Exogenic hypotheses for the genesis of HHC in magmatic rocks assume that the melts should have assimilated sedimentary rocks rich in organic matter and that biogenic oil could be transported from sedimentary reservoirs to magmatic rocks. An important role in determining the genesis of CN in magmatic rocks is played by endogenic and biogenic markers.