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

We have examined the stabilities of different polycyclic aromatic hydrocarbons (PAHs) at 7 GPa and 773–973 K and at 16 GPa and 300 K. Experiments were performed using a large-volume multi-anvil apparatus. Quenched products were analyzed by matrix-assisted laser desorption/ionization (MALDI) and Raman spectroscopy. The MALDI measurements revealed the considerable oligomerization of PAHs at 7 GPa and 773–873 K and insignificant PAH oligomerization at 16 GPa and 300 K. At 7 GPa and 773 K, oligomers with molecular weight up to 3400 Da were found, while only a small number of dimers of the starting PAHs were detected at 16 GPa and 300 K. PAH decomposition at 7 GPa occurred from 873 to 973 K, and the decomposition products consisted of nanocrystalline graphite. The determined decomposition temperatures of the PAHs (873–973 K) are much lower than Earth’s geotherms and the subduction slab P–T profiles at 7 GPa; therefore, PAH inclusions in mantle-derived minerals, which can be crystallized at 6–7 GPa and 1600–1700 K, should be secondary phases and could be formed by the successive polycondensation of simple hydrocarbon molecules under natural catalysts during eruption processes at sub-ambient pressures and temperatures.