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

Abstract

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 δ 15N values during progressive metamorphism. Nitrogen concentrations decrease and δ 15N increases with increasing metamorphic grade (sub-chlorite zone: 120 ppm N, δ 15N = 2.3‰; chlorite zone: 110 ppm N, δ 15N = 3.0‰; biotite and andalusite zone: 85 ppm N, δ 15N = 3.8 ‰; sillimanite and migmatite zones: 40 ppm N, δ 15N = 10.7‰). Covariation of K and N contents is consistent with N substituting for K as NH 4 + in micas. Observed trends of increasing δ 15N values with decreasing nitrogen concentrations can be explained by a continuous release of nitrogen depleted in 15N with progressive metamorphism, which causes an enrichment of 15N 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–NH 4 + exchange at temperatures of 300–600 °C, whereas observed large fractionations for the upper amphibolite-facies and melt products in the migmatite-grade samples may be interpreted as NH 3–NH 4 + exchanges at temperature of 650–730 °C. Lower values in the highest grade zones may also stem in part from input of 15N-depleted fluids from the granodiorite.The magnitude of isotope fractionation of nitrogen is about 1–2‰ 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 δ 15N values in Archean greenschist-facies metasedimentary rocks of −6‰ to 30‰ can be accounted for by variable mixtures of mantle plume-dominated volatiles with a δ 15N of −5‰, and a 15N-enriched marine sedimentary kerogen component inherited from a CI chondrite veneer having δ 15N of 30‰ to 42‰.