Raman elastic geobarometry: investigating cation order in omphacitic clinopyroxenes

Omphacitic clinopyroxenes exhibit widespread occurrence in various geological settings and rock types, making them promising candidates for Raman elastic geothermobarometry applications. Raman elastic geobarometry uses the deformation recorded by mineral inclusions to determine the pressure and temperature conditions under which they were entrapped. Raman scattering, which is highly sensitive to structural deformations, provides valuable insights into the variations in crystal structure that occur due to heating or compression. While several host-inclusion systems have been investigated, clinopyroxene inclusions have not been extensively studied. Therefore, the application of Raman elastic geobarometry to omphacites in different mineral hosts necessitates an accurate calibration of Raman-peak positions against hydrostatic pressure.

Natural omphacite crystals can exhibit cationic ordering associated with crystallization temperature, which affects their elastic behaviour. To address these aspects, we conducted a study on natural ordered (P2/n) and experimentally disordered (C2/c) omphacite crystals from Münchberg Massif (Germany). In situ high-pressure Raman spectroscopy measurements were performed using a diamond anvil cell. As expected, the frequencies of the modes increased with increasing pressure. By examining omphacite crystals with varying degrees of order obtained through isothermal annealing experiments, we observed that progressive cationic disorder primarily led to peak broadening, while changes in Raman peak positions were predominantly influenced by pressure variations.

Additionally, the chemical composition of omphacites influences the Raman-peak positions and their pressure evolution. Therefore, we analysed Fe³⁺-rich omphacites from Lugros and Camarate (Bétic Cordilleras, SE Spain), and Voltri (Italy) along with synthetic iron-free omphacites. This analysis is an initial step towards the chemical calibration of omphacites using Raman spectroscopy.

To gain a deeper understanding of the elastic behaviour of modes suitable for elastic geobarometry, we simulated the Raman spectrum of a fully ordered omphacite (Jd₅₀Di₅₀ composition) at different pressures using ab initio Hartree-Fock/Density Functional Theory simulations. The simulated data exhibited excellent agreement with experimental spectra, enabling us to comprehend the pressure dependence of specific modes. Leveraging these findings, we can calculate the entrapment pressure of omphacite inclusions still confined within their host rocks by determining Raman shifts of the main peaks alongside changes in cation order.

Our results provide valuable insights into the calibration and application of Raman elastic geobarometry for omphacitic clinopyroxenes. By considering the influence of chemical composition and cationic ordering, we have enhanced our understanding of the elastic behaviour of omphacite and its potential for geobarometric calculations. These findings offer a valuable tool for determining the pressure and temperature conditions of geological processes involving omphacitic clinopyroxenes.