The effect of cation ordering in omphacite on the phonon compressibility: A step towards Raman elastic geothermobarometry

Omphacite-in-garnet is a common host-inclusion system that has the potential to be used in Raman elastic geobarometry. However, the application of omphacite for this purpose requires the study of the effect of cation order on the phonon compressibility βω=1/ω0dω/dP, i.e. on the pressure dependency of the relative changes of phonon wavenumbers. Here, we report high-pressure Raman spectra of six omphacite crystals with the same composition (∼Jd52Di48) but having a different state of cation occupancy at the six- and eight-coordinated (M) cation sites and compare results with those derived for ordered and disordered omphacite with composition ∼Jd43Di57. We demonstrate that the position of the well-resolved Raman peak near 688 cm−1, which originates from Si-O-Si bond bending, is strongly influenced by pressure but its phonon compressibility remains unaffected by both the chemical composition and state of M-site cationic order. Thus, the Si-O-Si bond-bending wavenumber can be used as a marker of the residual pressure in omphacite-garnet systems. However, the omphacite grain has to be subsequently disclosed to determine the peak position at atmospheric pressure because the phonon wavenumber also varies with the omphacite chemical composition. On the other hand, the width of the peak near 688 cm−1 varies with chemical order, but not with pressure, making it a valuable indicator for determining the temperature of inclusion entrapment or the closure temperature of the system, as the cationic order in omphacite provides insights into the closure temperature of the cation-exchange reaction.