COMPOSITION AND SPECIATION OF FLUID IN THE SYSTEM C-H-N-O-S AT P-T CONDITIONS OF THE UPPER MANTLE

Abstract

During transportation by magma to the surface, the mantle fluid (MF) included in deep rock xenolitlis and high-pressure minerals evolves in composition and splits into gaseous, liquid, and solid phases. A part of the fluid is released during this transportation. The estimation of the gas phase composition in the MF is of tentative character due to drawbacks pertaining to mechanical and thermal methods of gas extraction. Nevertheless, it was established that the reduced gases (H2, CH4, CO) and oxidized gases (H2O, CO2) are major components of the gaseous phase. In addition to H2O and CO2, the liquid phase consists of heavy hydrocarbons, alkanes, and polycyclic aromatic hydrocarbons. Solid phases are represented by diamond, graphite, sulfides, carbonates, and other minerals that should be taken into account in the MF modeling. The thermodynamic modeling of evolution in the system C-H-N-O along the gcotherm supports the existence of two MF branches: inorganic and hydrocarbon. The formation of diamond from MF is controlled by the chemical composition of the latter. Stepwise variations in the fluid composition in the C-H-N-O-S system under equilibrium conditions are noted; these variations correspond to changes in the mineralogic composition of mantle rocks. In the transitional zone between the lower and upper mantle, N-bearing inorganic and organic compounds, as well as arenes, give way to nitrogen and heavy alkanes. Nitrogen becomes the most chemically active volatile component in the lower mantle. The transformation of heavy alkanes into CH4, H2O, H2S, and CO2 coincides with the zone of graphite-diamond transition in the asthenosphere. The important role of organoelement compounds and inorganic complexes in the MF composition is stressed. To model the MF composition in terms of thermodynamics, it is important to determine the relationship between independent components in this fluid. The presence of heavy hydrocarbons in mantle rocks and minerals testifies to metastable fluid ascent through the upper mantle and the Earth's crust. The further progress in the study of MF composition and speciation may be provided by a complex (analytical, experimental, and thermodynamic) approach.