HR: 1340h
AN: V23A-1222    [Abstracts]
TI: Experimental Constraints on the Transformation of Granulite-Facies Metapelites to Eclogites and the Role of Brines in Subduction-Zone Processes
AU: * Tropper, P
EM: c71437@mail1.uibk.ac.at
AF: Institute of Mineralogy & Petrography, University of Innsbruck, Innrain 52, Innsbruck, A-6020, Austria
AU: Manning, C E
EM: manning@ess.ucla.edu
AF: Dept. of Earth and Space Sciences, University of California, Los Angeles, CA 90095, United States
AB: A common feature of HP and UHP terranes is the subduction of lower crustal rocks to great depths. Investigations in the Bergen Area in Norway have shown that this process is triggered by fluids present during eclogite-facies metamorphism (e.g. Austrheim, 1987, EPSL, 81, 221-232). Fluid inclusions in eclogite-facies minerals range from dilute solutions to chloride-rich brines and previous studies have shown that highly saline fluids may occur in subduction zones where continental material is involved. A key example of these processes is exposed at Val Savenca of the Sesia-Lanzo Zone, Italy, where Eo-Alpine eclogite-facies metamorphism and fluid flow led to partial transformation of Hercynian amphibolite-eclogite facies metapelites (garnet + biotite + sillimanite + K-feldspar + plagioclase + quartz) to zoisite + jadeite + kyanite + phengite + quartz with associated jadeite veins. Application of the K-feldspar-jadeite-quartz barometer to the plagioclase domains yields P-T conditions of 1.7-2.1 GPa at 600°C and low a(H₂O) of 0.3-0.6 (Tropper et al., 1999, JMG, 17, 195-209). In order to understand the role of brines in the transformation exposed at Val Savenca, we carried out piston- cylinder experiments with a fresh, natural granulite from the Moldanubic Unit in upper Austria with the assemblage garnet + biotite + K-feldspar + plagioclase + quartz. The experiments were conducted in the presence of H₂O-NaCl fluids at 600°C and 2 GPa for 2-4 days. The fluids had the compositions X(H₂O) = 1.0, 0.90, 0.80 and 0.70, and fluid/rock ratio varied from 1:1 to 1:10. Oxygen fugacity was buffered at NNO and HM in most experiments. Preliminary data from unbuffered experiments show increasing reaction progress with increasing salinity in the fluid. At X(H₂O) = 1.0, no visible reaction occurred, whereas at X(H₂O) = 0.7, the protolith assemblage is almost completely replaced by large (up to 100 mm long), euhedral crystals of jadeite + phengite + quartz. Biotite is the only relict phase. The experiments also show a strong indication of Na-Ca exchange: the anorthite component of plagioclase goes readily into solution, producing zoisite needles only upon quench. Future experiments will be conducted on fluids in the system H₂O-NaCl-CaCl₂ to quantify this behavior. These experiments show that brines are highly effective in promoting reaction progress in subduction zone processes.
DE: 3612 Reactions and phase equilibria (1012, 8412)
DE: 3630 Experimental mineralogy and petrology
DE: 3653 Fluid flow
DE: 3654 Ultra-high pressure metamorphism
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting