Raman spectroscopic study of omphacite at variable pressures

Omphacitic clinopyroxenes ((Ca,Na)(Mg,Al)Si2O6) made up of a solid solution of jadeite, augite and aegirine are among the principal constituents of eclogites and can be found in blueschist facies or ultra-high-pressure rocks. They can be also recovered in the Earth’s upper mantle and in the subducted oceanic crust. Natural omphacites have two distinct polymorphs, a low-temperature phase with a space group P2/n, which shows splitting of the octahedral sites and the relative cation ordering, and a high-temperature chemically disordered phase with space group C2/c. Raman scattering of a mineral is very sensitive to structural deformations developed while heating or under external stresses, a phenomenon that is the basis of Raman elastic geobarometry. This novel method is based on the contrast in elastic properties between the host and the inclusion, and allows estimating pressure and temperature of entrapment of an inclusion, to the condition of knowing the elastic properties of both the host and the inclusion and the residual strain of the latter, allowing estimating the conditions attained during metamorphic processes (Angel et al., 2018). Therefore, the application of this methodology to omphacites requires an accurate knowledge of their elastic behaviour, for which several data are available (for example Pandolfo et al., 2012) and its response to stresses accounted through Raman spectroscopy, which has never been systematically applied for studying omphacites under variable pressure conditions. This technique has already been used to investigate the order-disorder phenomenon in the octahedral sites of omphacites (Katerinopoulou et al., 2008) and it has been shown how the position and width of some Raman peaks depends on the ordering state. Therefore, this study would permit to calculate the entrapment pressure of omphacite inclusions and would potentially allow calibrating which Raman peaks are insensitive to changes in cation ordering. Here we report the results from in situ high-pressure Raman spectroscopy on omphacites crystals from Münchberg Massif (Bavaria, Germany) formed at metamorphic-peak conditions of 2.5 GPa/600-700°C (O’Brien, 1993).