Residual water in hydrous minerals as a kinetic factor for omphacite destabilization into symplectite in the eclogites of VÃ¥rdalsneset (WGR, Norway)

Symplectitic intergrowths of sodic plagioclase + diopside ± amphibole as replacement of omphacite are commonly found in eclogites. These symplectites are interpreted as the exhumation-related decompression of eclogite into the amphibolite facies. The role of aqueous fluid in symplectite development, which would act as a catalyst and favor open-system reaction, has been suggested but not yet clearly established. In the Vårdalsneset outcrop of the Western Gneiss Region (Norway), eclogites that were not amphibolitized either display a primary eclogitic dry paragenesis (garnet + omphacite + rutile ± quartz) or a paragenesis including phengite. In the last case, omphacite is partly transformed into symplectite. The two groups have been further distinguished from a combined petrological, geochemical, and thermochemical study. Group I samples have a fine-grained unaltered microstructure, with medium Al2O3 (14-16 wt.%), high Fe2O3 (13-16 wt.%) and TiO2 (1.4-2.4 wt.%). Group II samples have a coarse-grained microstructure and are characterized by the presence of symplectites and phengite. They display higher Al2O3 (17.5-23 wt.%) and lower Fe2O3 (5.5-8 wt.%) and TIO2 (0.2-0.5 wt.%) contents than Group I. The P-T estimates for samples from both Groups I and II lead to similar conditions for peak eclogite metamorphism: temperatures range from 590 to 720 °C and pressures from 15 to 25 kbar. Perple_X modeling indicates that for Group I eclogites, temperature range is similar to the temperature range of the water saturation curve, whereas for Group II eclogites, due to slightly different chemical composition, the water saturation curve is located at much higher temperatures (770-900 °C), so that OH− remains in residual phengite at peak eclogite temperature. With no residual hydrous phase in the eclogite assemblage, and although some structural water may persist in nominally anhydrous minerals, Group I eclogites were preserved without change during exhumation. In contrast, Group II eclogites retain hydrous minerals, and this hydrous component is available as a kinetic factor for symplectite formation during exhumation. This suggests that symplectite development can be promoted by an internally derived hydrous component, signaled by the presence of a hydrous mineral, and, so, does not necessarily require an influx of external fluid.