SALT MATTERS: COMPARING NATURAL AND EXPERIMENTAL OBSERVATIONS OF THE TRANSFORMATION OF HIGH-GRADE METAPELITES AND GRANITES TO ECLOGITES

A common feature of HP and UHP terranes is the subduction of lower crustal rocks to great depths. Fluid inclusions in eclogite-facies minerals have shown that highly saline fluids occur in subduction zones, where continental material is involved. Key examples of these processes is exposed in the metapelites at Val Savenca and the metagranites of Monte Mucrone, both in the Sesia Lanzo Zone, Italy, where Alpine eclogite-facies metamorphism and fluid flow led to partial transformation of Variscan amphibolite-eclogite facies metapelites (garnet + biotite + sillimanite + K-feldspar + plagioclase + quartz) and Permian metagranites (garnet + biotite + K-feldspar + plagioclase + quartz) to zoisite ± jadeite + kyanite + phengite + quartz with associated jadeite veins under P-T conditions of 1.7 – 2.1 GPa at 600°C and low a(H₂O) of 0.3-0.6. In order to understand the role of brines in the mineralogical and textural transformation exposed at Val Savenca and Monte Mucrone, we carried out piston-cylinder experiments with a fresh, natural orthogneiss granulite from the Moldanubic Unit in upper Austria with the assemblage garnet + biotite + K-feldspar + plagioclase + sillimanite + 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 the fluid/rock ratio varied from 1:1 to 1:10. Oxygen fugacity was buffered at NNO in the experiments. The experiments show increasing reaction progress with increasing fluid salinity. At X(H₂O) = 0.7, the protolith assemblage is completely replaced by the assemblage jadeite + phengite + quartz indicating the reaction: 3Anorthite + 2K-feldspar + Phlogopite + H₂O = 3Diopside + 3Muscovite. Clinopyroxene composition also changes as a function of NaCl content in the fluid ranging from omphacite at X(H₂O) = 1 to jadeite at X(H₂O) <1. The experiments also show a strong indication of Na-Ca exchange since the anorthite component of plagioclase goes readily into solution, producing zoisite needles upon quench. These experiments so far show that the biotite breakdown reaction is similar to the one observed in the natural samples, that clinopyroxene composition can be a monitor of fluid composition and that brines highly effective promote reaction progress in subduction zone processes.