THE INTERPLAY BETWEEN CATION ORDERING AND ELASTICITY OF OMPHACITES TO UNRAVEL PTT PATHS OF METAMORPHIC ROCKS

Omphacite, a clinopyroxene stable across a diverse range of pressure and temperature conditions, is one of the main rock-forming minerals of eclogites and is also found in blueschist facies and ultra-high-pressure rocks. Its widespread occurrence makes it a valuable candidate for Raman elastic geothermobarometry, a method that 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, offers significant insights into the variations in crystal structure resulting from thermal or compressive influences. While various host-inclusion systems typical of high-pressure and ultra-high-pressure rocks have been extensively studied, clinopyroxene inclusions remain less examined. Consequently, precise calibration of Raman peak positions against hydrostatic pressure is crucial for employing Raman elastic geobarometry on omphacites found in different mineral hosts. Natural omphacite crystals exhibit cationic ordering associated with crystallization temperature, which affects their elastic properties; the ordered configuration, characterised by space group P2/n, is indicative of lower temperature compared to the disordered configuration, with space group C2/c. The order–disorder process affects the six- and eight coordinated polyhedra in the pyroxenes structure, M1 and M2. The aim of this study is to provide constraints to the history of crustal rocks subducted to high-pressure or ultra-high-pressure conditions by developing the necessary tools to exploit the chemical, structural and elastic properties of omphacite. To achieve this goal, we focus on analysing omphacite samples using Raman spectroscopy, a widely recognized, non-destructive analytical technique. Raman spectroscopic analysis at high-pressure conditions were performed on omphacite single crystals from Münchberg Massif (Germany) with two different compositions, Jd43Di57 and Jd52Di48 (Jd = jadeite, Di = diopside), including variable cationic order degree. By examining omphacite crystals with different degrees of cationic-site order achieved through isothermal annealing experiments, it was observed that increasing cationic disorder leads to peak broadening, whereas pressure variations influence Raman peak positions, and the magnitude of these changes was quantified. However, the peak position is also affected by the chemical composition. Therefore, Fe3+-rich crystals from Lugros and Camarate (SE Spain), and Voltri massif eclogites (Italy) (Cámara, 1995; Cámara et al., 1998), along with synthetic Fe-free omphacites (Pandolfo et al., 2015), were analysed, as an initial step towards the chemical calibration of omphacites using Raman spectroscopy. The study was accomplished by modelling the omphacite’s elastic properties and atomic dynamics by ab initio calculations. This study can enhance our perception of the closure temperature of ordering at M polyhedra and the residual pressure conditions of omphacite inclusions within garnet hosts, and consequently the story of the processes that led to the formation of the rock in which they are present. By considering the influence of chemical composition and cationic ordering, we have expanded our understanding of the elastic behaviour of omphacite obtained through Raman spectroscopy and its potential use for geobarometric calculations, important for determining the pressure and temperature conditions involved in geological processes where omphacite growths.