In situ X-ray diffraction study of decomposition of polycyclic aromatic hydrocarbons at pressures of 7–15 GPa: Implication to fluids under the Earth's and planetary environments

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The behavior of polycyclic aromatic hydrocarbons (PAHs) at high pressures and temperatures has been investigated as a part of study of the deep-seated C–O–H fluids in the Earth and planetary interiors. The theoretical calculations of fluid compositions at the Earth's mantle conditions in the simple C–O–H system indicate that dominant fluid species are CH4 and H2O, with subordinate H2, and heavier hydrocarbons. However, calculations of equations of state for a broader range of hydrocarbons predict an appearance of heavy alkanes and PAHs under high pressures (>6–7GPa). Here, we determined stability of the major PAHs (from naphthalene to coronene) using in situ X-ray diffraction in multianvil apparatus at the SPring-8 synchrotron radiation facility (Japan). It was found, that at 7–9GPa, PAHs become unstable at temperatures above 873–1073K. We suggest that stability of PAHs is limited by benzene ring decomposition. The data for coronene only at 15GPa confirms limited temperature stability of PAHs. The PAH decomposition products consist of amorphous hydrogenated carbon at 7–9GPa at 973–1073K. Coronene decomposition products consist of diamond and trans-polyacetylene at 15.5GPa and 973K. Determined PAH decomposition temperatures (873–1073K) are lower than known Earth's geotherms and subduction slab P–T profiles at pressures of 7–9GPa (220–280km depths). According to these data, PAH inclusions in mantle garnet and diamond should be of secondary origin, precipitating from mantle-derived fluids after or prior to kimberlite magma eruption. Limited temperature stability of PAHs restricts the parameters of their formation in meteorite by high-temperature impact events during early Solar System history.