Fe-rich antigorite: A rock-forming mineral from low-temperature/high-pressure meta-ophicarbonates

This study provides the first characterization of Fe-rich antigorite (FeOtot up to 12 wt%), a rock-forming mineral occurring in ophicarbonate rocks from different low-temperature/high-pressure meta-ophiolitic suites: Acceglio (Traversiera Valley, external Piemonte Zone, NW Italy), Macedonia and Verias (Thessaloniki and Vurinos-Kozani ophiolitic complexes, NE Greece), Tinos (Tinos Island, Cyclades Archipelago, Greece). Fe-rich antigorite has been characterized through optical and transmission electron microscopy (TEM), and its mineral chemistry has been investigated by means of wavelength-dispersive and TEM-based energy-dispersive spectrometry. In thin section, Fe-rich antigorite is characterized by a strong, peculiar pleochroism (α = green–dark green; γ = bright-yellow–orange). It occurs in both mesh and bastite microstructures, and it is locally associated with relics of lizardite and/ or chrysotile. The modulated lattice parameters of disordered Fe-rich antigorites have been determined by electron diffraction in the transmission electron microscope. The values are highly variable, even within each ophicarbonate sample. Verias dominantly has a superlattice parameter a clustering around 43.5 Å (corresponding to the m = 17 polysome); Tinos and Macedonia have around 35.4 Å (m = 14); Acceglio may even go down to 29 Å (m = 12). Globally, the disorder features (i.e., reduced size of crystals, polysomatic faults, wobbling, misalignment among sublattice and superlattice reflections, etc.) increase from Macedonia to Verias, Tinos and Acceglio, respectively. The analyzed Fe-rich antigorites accommodate up to 12 wt% FeOtot, with XFe values (XFe =Fetot/[Mg + Fetot]) in the range 0.10–0.16 for Macedonia, 0.05–0.17 for Acceglio, 0.10–0.12 for Tinos and 0.05–0.10 for Verias. The intensity of the pleochroism seems to be directly correlated with the Fe content, with the Fe-richer samples showing the deeper absorption colours. Mineral relationships and TEM observations suggest that Fe-rich antigorite replaces former mesh and bastite microstructures consisting of lizardite ± chrysotile, only locally preserved as relict phases. The thermodynamic modelling approach (i.e., P/T–X(CO2) pseudosection) qualitatively shows that the stability of Fe-rich antigorite is compatible with low-temperature, high-pressure conditions (i.e., blueschist-facies metamorphic conditions), and is enhanced by the occurrence of CO2 in the fluid, consistent with the systematic occurrence of this mineral in meta-ophicarbonate rocks.