Effect of Water on Thermal Reactions of <i>n</i> -C ₂₅ under High-Pressure and High-Temperature Conditions of Subducting Slab

Hydrocarbons in organic sediments subducted into the interior of the Earth play a crucial role in the deep carbon cycle. In this study, the thermal reaction of pentacosane (n-C25), a long-chain n-alkane, was experimentally investigated under high-pressure and high-temperature conditions relevant to a subducting slab. Gas chromatography–mass spectrometry (GC/MS) analyses of the reaction products revealed the formation of lighter n-alkanes, indicating that thermal cracking of n-C25 occurred between 360 and 400 °C at 0.5 GPa in the presence of SiO2. A comparison of the residual n-C25 under dry and wet conditions suggested that the presence of water enhanced thermal cracking, even at 0.5 GPa. Heavier alkanes and amorphous carbon preferentially formed under dry conditions from addition and dehydrogenation reactions, respectively. By contrast, the IR spectra indicated the formation of oxygenated organic compounds containing –OH and –CO groups under wet conditions, implying that polymerization with oxidation preferentially progresses under wet conditions. These results suggest that the presence of water promotes the formation of diverse organic compounds in subducting slabs, including oxygen-bearing compounds, rather than amorphous carbon or graphite.