Various antiphase domains in garnet-hosted omphacite in low-temperature eclogite: A FIB-TEM study on heterogeneous ordering processes

Low-temperature omphacite has peculiar microstructures called “antiphase domains (APDs),” which can be formed via phase transition from disordered C2/c to ordered P2/n structure during cooling. Hence morphological analyses of the APDs of undeformed omphacite have a potential to unravel the temperature-time (T-t) histories of the eclogite. We investigated five omphacite inclusions in a euhedral garnet porphyroblast obtained from low-temperature eclogite in Syros. The garnet (~6 mm in size) exhibits a distinct prograde chemical zoning and contains abundant mineral inclusions. Transmission electron microscope (TEM) observations of the focused ion beam (FIB) foils confirmed a heterogeneous distribution of equiaxed APDs (10–280 nm in diameter) and columnar APDs. Size distributions of the equiaxed APDs are characterized by kurtosis values of –0.45–3.91, which are larger than those in the matrix omphacite. The columnar APDs are subdivided into two types: dislocation-related (Type I) and inclusion–host interfacial (Type II). The presence of Type I APDs may suggest the inclusions were deformed prior to the host garnet growth. In contrast, Type II APDs, which are characterized by a bundle of stripe-like APDs (~40 nm in width) aligned perpendicular to the host garnet, imply the simultaneous growth of omphacite and garnet in a non-deformation state. The presence of these two contrasting APDs of omphacite inclusions in the single prograde-zoned garnet prevents a simple application of geospeedometry based on APD sizes. Nevertheless, our observations demonstrate that APDs are keys to understanding thermodynamic equilibrium states and the mineral growth kinetics during eclogitization.