Ammonium concentration and nitrogen isotope composition in metasedimentary rocks from different tectonometamorphic units of the European Variscan Belt

摘要

The content and isotopic composition of ammonium (NH4+) were measured by using Kjeldahl distillation fur 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 delta N-15 from the Low-grade Units (approximate to2 kbar/300 degreesC) with 638 +/- 124 ppm and delta N-15 = +2.2 +/- 0.6 parts per thousand, to the Garnet-Phyllite Unit (approximate to9 kbar/470 degreesC) with 621 +/- 190 ppm and delta N-15 = +3.5 +/- 0.9 parts per thousand, the Mica Schist/Eclogite Unit (approximate to 12 kbar/550 degreesC) with 394 +/- 113 ppm and delta N-15 = +3.9 +/- 0.8 parts per thousand and to the Gneiss/Eclogite Unit (>12 kbar/730 degreesC) with 99 +/- 32 ppm and delta N-15 = +7.7 +/- 2.0 parts per thousand. Using equilibrium models for Rayleigh distillation and batch volatilization suggest that the nitrogen depletion tool; 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 ( approximate to6-8 kbar/650 degreesC) contain significantly lower amounts of ammonium (approximate to 70 ppm) and reach relatively low delta N-15 values (+2.5 parts per thousand to +3.6 parts per thousand). 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 degreesC). The metagraywackes have NH4+ contents of approximate to 80 ppm and delta N-15 of approximately +6 parts per thousand. Gneisses in the vicinity of a late cataclastic shear zone at 2000 m depth are characterized by a shift in NH4+ (>250 ppm) and delta N-15 up to +12 parts per thousand, which suggest late fluid-rock interactions with a nitrogen- and N-15-enriched fluid. Copyright (C) 2001 Elsevier Science Ltd. [References: 59]