Hydrogenation of carbon at 5.5–7.8 GPa and 1100–1400 °C: Implications to formation of hydrocarbons in reduced mantles of terrestrial planets

Abstract

Formation of hydrocarbons (HCs) by reactions of hydrogen-bearing fluids with carbon (13C soot-like carbon, graphite, or diamond) has been simulated in experiments at pressures of 5.5–7.8 GPa and temperatures of 1100–1400 °C. Hydrogen fugacity (fH2) in tightly sealed Pt and Au capsules was controlled using the double capsule technique at the Mo + MoO2 + H2O (MMO) or Fe + FeO + H2O (IW) equilibria. Synthesis of light alkanes (C2 > C1 > C3 > C4), with smaller amounts of unsaturated hydrocarbons and O-bearing species, occurred all over the experimental P-T-fH2 ranges. For the first time, formation of hydrocarbons from inorganic compounds was directly proved by the reaction of 13C carbon with hydrogen, which yielded isotopically pure 13C light alkanes. In 6.3 GPa runs, the fluid-graphite reaction rate progressively grew, and the process became avalanche-like as the run duration exceeded 1 h. The greatest amounts of HCs (CH4/C2H6 < 1, CH4/C3H8, and CH4/C4H10 ≤ 10) formed at 1400 °C in the 10-hr run. The amount of HC fluid synthesized at 1200 °C was twice smaller. An increase in the experiment duration to 40 h had no effect on amounts of HCs and the composition of species, which indicated that the system achieved equilibrium. The rate of HCs formation was slowest in runs with diamond. The fluid composition varied with pressure and temperature: it contained less methane and slightly more unsaturated hydrocarbons and O-bearing species as pressure and temperature were increased from 5.5 to 7.8 GPa and from 1150 to 1350 °C. An increase in the CO2 concentration in the fluid led to a drastic decrease in the yield of hydrocarbons. The absence of C17O2 in products of the reaction between graphite and fluid containing 17O-labeled water indicated that water was not directly involved in HCs synthesis.The experiments have provided the first unambiguous evidence that volatile-rich and reduced mantles of terrestrial planets (at fO2 near or below IW) provided favorable conditions for formation of a wide range of HCs, mainly light alkanes. Carbon for the HC synthesis may come from graphite, diamond, and soot-like carbon. It should be expected that the efficiency of HCs formation by carbon hydrogenation should be maximal for an increased heat flux (>40 mW/m2) at mantle depths corresponding to the graphite stability region.