as a marker of relatively low temperature-high pressure metamorphism, and it has been widely used in reference geobarometers for so-called ultra-high pressure metamorphic rocks (Hermann et al., 2016). Nonetheless, as Ca-carbonates are stable over a very wide range of temperatures (Suito et al., 2001; Li et al., 2017; Müller et al, 2017; Shatskiy et al. 2018), up to the average mantle adiabat beneath ridges and ocean islands (Fig. 1), a large range of carbonated eclogites and pyroxenites may contain aragonite at mantle conditions, as revealed by several experimental studies (Hammouda, 2003; Kiseeva et al., 2013; Grassi and Schmidt, 2011; Brey et al., 2015). Evidence of CaCO 3bearing eclogitic assemblages have been also provided by the occurrence of inclusions of carbonates in the diamonds from Juina (Brazil), possibly originated from the lower part of the transition zone (Brenker et al., 2007), and of aragonite as inclusions in olivine phenocrysts from leucitite lava flows at Calatrava, Spain (Humphreys et al., 2010). The geochemistry of carbonatites (Woolley and Kjarsgaard, 2008) and kimberlites (Becker and Le Roex, 2006) points to the importance of components CaO and CO 2 for describing their diversity and magmatic evolution. Carbonatites are thought to be among the major metasomatic agents in the sublithospheric mantle due to their low density, low viscosity and high reactivity (Green and Wallace, 1988). Whether calcite or aragonite occur on the liquidus surface it may affect the fractionation of trace elements as aragonite was experimentally found to preferentially partition Sr with respect to calcite (Carlson, 1980), and intermediate REE with respect to Fe-Mg carbonate (Brey et al., 2015). Thermodynamic properties of CaCO 3 polymorphs, and of liquid CaCO 3 , are barely known