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

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.