In situ Raman and X-ray diffraction studies on the high pressure and temperature stability of methane hydrate up to 55 GPa

Abstract

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.

Observation of methane filled hexagonal ice stable up to 150 GPa

“…It had almost no detectable intensity above 50 to 60 GPa. The 2 D 2 O stretching modes shifted to smaller frequencies and lost intensity swiftly between 35 and 45 GPa, as previously reported in the literature for methane hydrate (15, 16)> and in strong analogy to what is commonly observed in pure ice (19) and salt-doped ices (20). In our previous work (11), we described in detail the mode mixing occurring between the stretching mode of D 2 O and the rocking mode of CH 4 in this pressure range.…”

“…This phase is stable in the probed pressure range up to 150 GPa and accounts for the additional detected Raman modes. The MH-IV structure is also consistent with previous X-ray diffraction investigations (13–16) and represents the highest-pressure gas hydrate phase known up to now. The transition from MH-III to MH-IV is shown to proceed in 3 steps as pressure increases: 1) symmetrization of the hydrogen bonds in MH-III at ∼30 GPa (giving rise to a structure we name “MH-IIIs”), 2) structural change from MH-IIIs to the MH-IV phase at ∼40 GPa, and 3) symmetrization of the hydrogen bonds of MH-IV at ∼50 GPa (giving rise to “MH-IVs”).…”

“…However, the appearance of the higher-frequency excitation, which stems neither from pure ice nor from MH-III, suggests the existence of a high-pressure phase of methane hydrate above 100 GPa. The first indication of a high-pressure phase of methane hydrate (for P>40 GPa) was provided by X-ray diffraction experiments (14–16). They showed peaks appearing in the diffraction patterns beyond 40 GPa, which are not compatible with the MH-III structure.…”

“…Recent studies also suggested that under high-pressure conditions methane and water could form stoichiometric mixed molecular crystals beyond the known MH-III phase: The existence of a new high-pressure phase in methane hydrate at pressures beyond 40 to 50 GPa was first hinted by Hirai and coworkers (13–16), who observed a slow transition from MH-III into a different, yet unsolved structure during synchrotron X-ray diffraction experiments. The high-pressure structure was observed to survive up to 86 GPa, but the diffraction patterns did not allow the authors to identify it in detail.…”

The Equation of State of MH-III: A Possible Deep CH4 Reservoir in Titan, Super-Titan Exoplanets, and Moons

“…This scatter in the data probably arises from the kinetic inhibition of the phase transition, and from the difficulty in interpreting the neutron diffraction patterns of phases that have disordered protons. Black square is a data point from Kadobayashi et al (2018). Black crosses are the sampled points of this work.…”