RELATIONSHIPS BETWEEN FLUID COMPOSITION AND RHEOLOGY IN ECLOGITES FROM THE EASTERN ALPS

Finely banded, dolomite-rich mafic eclogites from the Tauern Window equilibrated at >2 GPa/600-650°C. Previous work (Selverstone et al. 1992 CMP; Getty & Selverstone, 1994 JMG) showed that (1) a(H₂O) varied from ~0.2 to 0.9 across layers at peak pressure, (2) δ¹⁸O of dolomite varied by up to 1.0 per mil between layers, (3) δ¹⁸O data reflect modal abundance of dolomite, which also controlled a(H₂O), and (4) there is little to no change in a(H₂O) or δ¹⁸O 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±czo±zoi±dol. 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±qtz 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° to the foliation are abundant in layers with a(H₂O)≥0.6 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 JSG; Ji & Zhao, 1993 Tectonophys). 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.