Abstract
The content and isotopic composition of ammonium ( NH 4 +) were measured by using Kjeldahl distillation for 63 samples of predominantly metasedimentary rocks from two different geological settings of the European Variscides, i.e., the Erzgebirge and the Zone of Erbendorf-Vohenstrauss in Germany. The studied Erzgebirge schists represent an early Paleozoic passive margin sequence that reappears in different deep subducted metamorphic units. This allows an efficient examination of nitrogen loss and isotope fractionation during prograde metamorphism. Ammonium is found to be progressively depleted, accompanied by a shift in δ 15N from the Low-grade Units (≈2 kbar/300°C) with 638 ± 124 ppm and δ 15N = +2.2 ± 0.6‰, to the Garnet-Phyllite Unit (≈9 kbar/470°C) with 621 ± 190 ppm and δ 15N = +3.5 ± 0.9‰, the Mica Schist/Eclogite Unit (≈12 kbar/550°C) with 394 ± 113 ppm and δ 15N = +3.9 ± 0.8‰ and to the Gneiss/Eclogite Unit (>12 kbar/730°C) with 99 ± 32 ppm and δ 15N = +7.7 ± 2.0‰. Using equilibrium models for Rayleigh distillation and batch volatilization suggest that the nitrogen depletion took place by ammonia release. Only for the mica schists, this isotope fractionation can be explained by the loss of molecular nitrogen. In comparison with the early Paleozoic schists, the Proterozoic ortho- and paragneisses of the Erzgebirge Gneiss Unit (≈6–8 kbar/650°C) contain significantly lower amounts of ammonium (≈70 ppm) and reach relatively low δ 15N values (+2.5‰ to +3.6‰). The Paragneisses from the Zone of Erbendorf-Vohenstrauss (ZEV) sampled from the KTB pilot hole represents a separate tectonometamorphic unit that has undergone polyphase metamorphic conditions with the major imprint being amphibolite facies (7 kbar/650–700°C). The metagraywackes have NH 4 + contents of ≈80 ppm and δ 15N of approximately +6‰. Gneisses in the vicinity of a late cataclastic shear zone at 2000 m depth are characterized by a shift in NH 4 + (>250 ppm) and δ 15N up to +12‰, which suggest late fluid-rock interactions with a nitrogen- and 15N-enriched fluid.
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