Carbon precipitation from heavy hydrocarbon fluid in deep planetary interiors

Abstract

The phase diagram of the carbon-hydrogen system is of great importance to planetary sciences, as hydrocarbons comprise a significant part of icy giant planets and are involved in reduced carbon-oxygen-hydrogen fluid in the deep Earth. Here we use resistively-and laser-heated diamond anvil cells to measure methane melting and chemical reactivity up to 80 GPa and 2,000 K. We show that methane melts congruently below 40 GPa. Hydrogen and elementary carbon appear at temperatures of 41,200 K, whereas heavier alkanes and unsaturated hydrocarbons (424 GPa) form in melts of 41,500 K. The phase composition of carbon-hydrogen fluid evolves towards heavy hydrocarbons at pressures and temperatures representative of Earth's lower mantle. We argue that reduced mantle fluids precipitate diamond upon re-equilibration to lighter species in the upwelling mantle. Likewise, our findings suggest that geophysical models of Uranus and Neptune require reassessment because chemical reactivity of planetary ices is underestimated.

Melting temperatures of H2O up to 72 GPa measured in a diamond anvil cell using CO2 laser heating technique

“…1 Some experimental studies reported that the CH 4 molecule at high pressures polymerizes into C 2 H 6 or higher hydrocarbons at 2000-3000 K and dissociates into C and H 2 at higher temperatures. [32][33][34] Recently, the NH 3 molecule was confirmed to dissociate into N 2 and H 2 at high pressure and temperature along with CH 4 . 35 The heavier components such as hydrocarbons, C, and N 2 may sink into the deeper interior and form the stably stratified layer.…”

Oligomerization and carbonization of polycyclic aromatic hydrocarbons at high pressure and temperature

“…According to estimations of Chekalyuk [18] and Zubkov [19], some heavy alkanes, alkenes, and polycyclic aromatic hydrocarbons (PAH) as well as some C-H-O, C-N, and C-S compounds can be stable in mantle fluids at depths of 300-600 km and deeper. Recent experimental studies in laser heated diamond anvil cell indicated the enhanced stabilities of heavy hydrocarbons at pressures up to 80 GPa and temperature of $2000 K with methane as the starting material [20,21].…”

Immiscible hydrocarbon fluids in the deep carbon cycle

“…Methane has been found in fluid inclusions in metasomatized ophicarbonates6 and diamonds7. Experiments have produced methane from carbonate89 or from gas-water-rock reactions10, ethane from methane11 and heavy hydrocarbons from methane1213, but experimental evidence of the formation of abundant hydrocarbons heavier than methane in the presence of water and silicate rocks in the deep Earth is not known.…”