Transition from melting to carbonization of naphthalene, anthracene, pyrene and coronene at high pressure

- Decomposition temperatures of PAH at 1.3-4.0 GPa are 773-973 K. Decomposition products consist of nanocrystalline graphite and graphite.
- PAH inclusions in deep-seated garnet and diamonds must have a secondary origin. Defined PAH decomposition parameters allow their stability at the PT-conditions of subduction zones.

We have examined the decomposition of naphthalene, anthracene, pyrene and coronene at high pressures and temperatures. Experiments were performed using in situ X-ray diffraction in multianvil apparatus at the SPring-8 synchrotron radiation facility. In the pressure range of 1.5-3.7 GPa decomposition of studied polycyclic aromatic hydrocarbons (PAHs) was detected at 773-973 K. Melting was identified only for naphthalene at 727-730 K and 1.5 GPa. Quenched products analyzed by Raman spectroscopy consist of nano- and microcrystalline graphite. The triple points between solid, liquid and carbonized (decomposed) PAHs were placed at 1-2 GPa and 800-850 K. Analyses of P-V-T data indicate that anthracene and coronene possess very low thermal expansion at 1.3-4.2 GPa. The obtained melting and decomposition parameters for PAH restrict PT-conditions of their formation by local impacts during early planetary history, as well as provide evidences for secondary origin of PAH inclusions in natural mantle minerals from kimberlites.

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