Nitrogen isotope fractionations during progressive metamorphism: A case study from the Paleozoic Cooma metasedimentary complex, southeastern Australia

The well-studied Paleozoic Cooma metamorphic complex in southeastern Australia is characterized by a uniform siliciclastic protolith, of uniform age, with a continuous range of metamorphic grade from subgreenschist- to upper amphibolite-facies, and migmatite-grade in an annular pattern around the Cooma granodiorite. Those conditions are optimal for investigating variations of N concentrations and delta N-15 values during progressive metamorphism. Nitrogen concentrations decrease and delta N-15 increases with increasing metamorphic grade (sub-chlorite zone: 120 ppm N, delta N-15 = 2.3 parts per thousand; chlorite zone: 110 ppm N, delta N-15 = 3.0 parts per thousand; biotite and andalusite zone: 85 ppm N, delta N-15 = 3.8 parts per thousand; sillimanite and migmatite zones: 40 ppm N, delta N-15 = 10.7 parts per thousand). Covariation of K and N contents is consistent with N substituting for K as NH4+ in micas. Observed trends of increasing delta N-15 values with decreasing nitrogen concentrations can be explained by a continuous release of nitrogen depleted in N-15 with progressive metamorphism, which causes an enrichment of N-15 in the residual nitrogen of the rock. Equilibrium models for Rayleigh distillation and batch volatilisation for data of the greenschist and amphibolite facies metasedimentary rocks can be explained by N-2-NH4+ exchange at temperatures of 300-600 degrees C, whereas observed large fractionations for the upper amphibolite-facies and melt products in the migmatite-grade samples may be interpreted as NH3-NH4+ exchanges at temperature of 650-730 degrees C. Lower values in the highest grade zones may also stem in part from input of N-15-depleted fluids from the granodiorite. The magnitude of isotope fractionation of nitrogen is about 1-2 parts per thousand during progressive metamorphism of metasedimentary rocks from sub-chlorite zone to biotite-andalusite zone, which is consistent with previous studies. Consequently, the large spread of delta N-15 values in Archean greenschist-facies metasedimentary rocks of -6 parts per thousand to 30 parts per thousand can be accounted for by variable mixtures of mantle plume-dominated volatiles with a delta N-15 of -5 parts per thousand, and a N-15-enriched marine sedimentary kerogen component inherited from a CI chondrite veneer having delta N-15 of 30 parts per thousand. to 42 parts per thousand. (c) 2006 Elsevier Inc. All rights reserved.