Finely banded, dolomite-rich mafic eclogites from the Tauern Window equilibrated at >2 GPa/600-650\xb0C. Previous work (Selverstone et al. 1992; Getty & Selverstone, 1994) showed that: a(H2O) varied from ~0.2 to 0.9 across layers at peak pressure; \u03b418O of dolomite varied by up to 1.0 per mil between layers; \u03b418O data reflect modal abundance of dolomite, which also controlled a(H2O); and there is little to no change in a(H2O) or \u03b418O along layers, despite large changes across layers. These data indicate that the rocks acted as a closed system during high-P metamorphism, with little to no fluid communication across layers only mm to cm in thickness. This study evaluates the mechanical behavior of the eclogites as a function of fluid composition within individual layers. All layers show a single, well-developed foliation defined by elongate omp\xb1czo\xb1zoi\xb1dol. Average matrix grain size varies from 40 to >200 microns, but shows no correlation with fluid composition. Garnet is abundant throughout and typically displays rounded, inclusion-rich cores with sub- to euhedral inclusion-free overgrowths. Garnets are subequant in layers with low a(H2O) and elongate (aspect ratios up to 1.6) in layers with high a(H2O). Garnets in layers with low a(H2O) were shattered into jagged, puzzle-like pieces at different stages during growth. Foliation-parallel omp\xb1qtz fills spaces between gar fragments. It is likely that decarbonation reactions in these layers produced a nonwetting CO2-rich fluid that facilitated high-P cataclasis. Mode I cracks oriented at 70\xb0 to the foliation are abundant in layers with a(H2O)\u22650.6 but are absent in layers with lower a(H2O). Gar and czo zoning in these layers indicates strain accommodation via dissolution/reprecipitation creep prior to cracking. High a(H2O) facilitated solution creep and growth of grains with high aspect ratio. These elongate inclusions were subsequently favored for brittle failure during ongoing ductile deformation of surrounding layers (Mandal et al., 2001; Ji & Zhao, 1993). The mode of ductile strain accommodation thus preconditioned certain layers to localize later brittle deformation. This study highlights the role that fluid composition plays in controlling rheology during high-P metamorphism.
National Science Foundation EAR-0509937
rheology, fracturing, dissolution-precipitation creep
Selverstone, Jane. "Relationships between fluid composition and rheology in ecologites from the eastern Alps." (2007). https://digitalrepository.unm.edu/eps_fsp/2