N2 AND CO2 IN DEEP-CRUSTAL FLUIDS - EVIDENCE FROM THE CALEDONIDES OF NORWAY

High-pressure metamorphic rocks (eclogites and high-pressure granulite s) occur along the entire length of the Norwegian Caledonides, and hav e formed from a variety of protoliths. In some cases, the relationship between protoliths, high-pressure rocks and their later retrogression products have been preserved in-situ. Fluid-inclusion data suggest a simple correlation between metamorphic grade and metamorphic fluid com position: (1) Eclogites and high-pressure granulites contain N2-bearin g fluids (pure N2, or mixtures with CO2 or H2O, with X(N), > 5%). In s ome eclogite-facies rocks, CO2-N2 inclusions are associated with aqueo us brine inclusions (ca. 30 wt% NaCl), the two compositions representi ng immiscible fluids at peak metamorphic conditions. (2) Granulite-fac ies protoliths and eclogites reworked in the granulite-facies contain pure CO2 or CO2-dominated fluids with less than 2.5% N2. (3) Rocks ret rograded in the amphibolite facies contain H2O-NaCl fluids. Immiscibil ity between brine and anhydrous N2-CO2 fluid, and between anhydrous fl uid and water-bearing aluminosilicate-melt have taken place in some ec logites. During high-grade metamorphism, nitrogen may be incorporated in minerals, as NH4 substituting for K, or it may occur as N2 in the f ree fluid phase. The partitioning of nitrogen between minerals and flu ids depends upon the water activity and oxygen fugacity during metamor phism, low a(H2O) and/or high f(O2) partitioning nitrogen to the fluid phase. A rock interacting with a carbonic fluid at granulite-facies P T conditions will be depleted in mineralogically bound nitrogen. In ca ses where the protoliths of high-pressure rocks have been through a pr evious, granulite-facies event, a local source for the nitrogen contai ned in high-pressure fluid is therefore unlikely.