N 2 and CO 2 in deep crustal fluids: evidence from the Caledonides of Norway

Abstract

High-pressure metamorphic rocks (eclogites and high-pressure granulites) occur along the entire length of the Norwegian Caledonides, and have 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 composition: (1) Eclogites and high-pressure granulites contain N 2-bearing fluids (pure N 2, or mixtures with CO 2 or H 2O, with X N 2 > 5%). In some eclogite-facies rocks, CO 2/1bN 2 inclusions are associated with aqueous brine inclusions (ca. 30 wt% NaCl), the two compositions representing immiscible fluids at peak metamorphic conditions. (2) Granulite-facies protoliths and eclogites reworked in the granulite-facies contain pure CO 2 or CO 2-dominated fluids with less than 2.5% N 2. (3) Rocks retrograded in the amphibolite facies contain H 2O/1bNaCl fluids. Immiscibility between brine and anhydrous N 2/1bCO 2 fluid, and between anhydrous fluid and waterbearing aluminosilicate-melt have taken place in some eclogites. During high-grade metamorphism, nitrogen may be incorporated in minerals, as NH 4 substituting for K, or it may occur as N 2 in the free fluid phase. The partitioning of nitrogen between minerals and fluids depends upon the water activity and oxygen fugacity during metamorphism, low a H 2O and/or high f O 2 partitioning nitrogen to the fluid phase. A rock interacting with a carbonic fluid at granulite-facies PT conditions will be depleted in mineralogically bound nitrogen. In cases where the protoliths of high-pressure rocks have been through a previous, granulite-facies event, a local source for the nitrogen contained in high-pressure fluid is therefore unlikely.