High-temperature and high-pressure experiments were performed under 2-55 GPa and 298-653 K using in situ Raman spectroscopy and X-ray diffraction combined with externally heated diamond anvil cells to investigate the stability of methane hydrate. Prior to in situ experiments, the typical C-H vibration modes of methane hydrate and their pressure dependence were measured at room temperature using Raman spectroscopy to make a clear discrimination between methane hydrate and solid methane which forms through the decomposition of methane hydrate at high temperature. The sequential in situ Raman spectroscopy and X-ray diffraction revealed that methane hydrate survives up to 633 K and 40.3 GPa and then decomposes into solid methane and ice VII above the conditions. The decomposition curve of methane hydrate estimated by the present experiments is >200 K lower than the melting curves of solid methane and ice VII, and moderately increases with increasing pressure. Our result suggests that although methane hydrate may be an important candidate for major constituents of cool exoplanets and other icy bodies, it is unlikely to be present in the ice mantle of Neptune and Uranus, where the temperature is expected to be far beyond the decomposition temperatures.

No abstract

In situ Raman spectroscopy was performed to investigate the stability and dissociation behavior of filled-ice Ih of methane hydrate at high-pressure (up to 5.1 GPa) and high-temperature (up to 408 K) using an externally heated diamond anvil cell. The results revealed that filled-ice Ih was stable up to 383 K at 4.5 GPa, and the dissociation conditions intersected with the melting curve of ice VII at approximately 2.5 GPa and with that of solid methane at approximately 2.2 GPa. Consequently, the dissociation into water and methane components occurred via two different mechanisms depending on pressure: (1) melting into liquid water and fluid methane below approximately 2.5 GPa and (2) solid-solid decomposition into ice VII and solid methane above approximately 2.5 GPa. Visual observations with an optical microscope synchronized with in situ Raman spectroscopy detected considerable changes in the sample in the case of melting, whereas they could not detect any changes in the decomposition case because both decomposition products were solid phases with similar refractive indices. This result demonstrated that the synchronized and sequential Raman spectroscopy with optical observations is indispensable for evaluating the dissociation behavior; otherwise, the solid-solid decomposition is overlooked when the evaluation is performed using a microscope only. The stability field of filled-ice Ih obtained in the experiment can help infer the states of methane hydrates in the interior of icy bodies such as Saturn's largest moon, Titan.

Hydrocarbon chemistry in the C–O–H system at high pressure and high temperature is important for modelling the internal structure and evolution of giant icy planets, such as Uranus and Neptune, as their interiors are thought to be mainly composed of water and methane. In particular, the formation of diamond from the simplest hydrocarbon, i.e., methane, under the internal conditions of these planets has been discussed for nearly 40 years. Here, we demonstrate the formation of diamond from methane hydrate up to 3800 K and 45 GPa using a CO2 laser-heated diamond anvil cell combined with synchrotron X-ray diffraction, Raman spectroscopy, and scanning electron microscopy observations. The results show that the process of dissociation and polymerisation of methane molecules to produce heavier hydrocarbons while releasing hydrogen to ultimately form diamond proceeds at milder temperatures (~ 1600 K) and pressures (13–45 GPa) in the C–O–H system than in the C–H system due to the influence of water. Our findings suggest that diamond formation can also occur in the upper parts of the icy mantles of giant icy planets.

High-pressure experiments were performed to understand the structural evolution of methane hydrate (MH) up to 134 GPa using x-ray powder diffraction (XRD) and Raman spectroscopy with diamond anvil cells. XRD revealed the distinct changes in the diffraction lines of MH owing to phase transition from a guest-ordered state phase [MH-III(GOS)] to a new high-pressure phase (MH-IV) at 33.8–57.7 GPa. MH-IV was found to be stable up to at least 134 GPa without decomposition into solid methane and high-pressure ices. Raman spectroscopy showed the splits in the C–H vibration modes ν3 and ν1 of guest methane molecules in filled-ice Ih (MH-III) at 12.7 GPa and 28.6 GPa, respectively. These splits are caused by orientational ordering of guest methane molecules contained in the hydrate structure, as observed in a previous study. These results suggest that the structural evolution of the filled-ice structure of MH is caused by successive orientational ordering of guest methane molecules, thereby inducing changes in the host framework formed by water molecules.