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

The content and isotopic composition of ammonium (NH4+) were measured by us ing Kjeldahl distillation fur 63 samples of predominantly metasedimentary r ocks from two different geological settings of the European Variscides, i.e ., the Erzgebirge and the Zone of Erbendorf-Vohenstrauss in Germany. The st udied Erzgebirge schists represent an early Paleozoic passive margin sequen ce that reappears in different deep subducted metamorphic units. This allow s an efficient examination of nitrogen loss and isotope fractionation durin g prograde metamorphism. Ammonium is found to be progressively depleted, ac companied by a shift in delta N-15 from the Low-grade Units (approximate to 2 kbar/300 degreesC) with 638 +/- 124 ppm and delta N-15 = +2.2 +/- 0.6 par ts per thousand, to the Garnet-Phyllite Unit (approximate to9 kbar/470 degr eesC) 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 Raylei gh distillation and batch volatilization suggest that the nitrogen depletio n tool; place by ammonia release. Only for the mica schists, this isotope f ractionation can be explained by the loss of molecular nitrogen. In compari son with the early Paleozoic schists, the Proterozoic ortho- and paragneiss es of the Erzgebirge Gneiss Unit ( approximate to6-8 kbar/650 degreesC) con tain significantly lower amounts of ammonium (approximate to 70 ppm) and re ach 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 amp hibolite facies (7 kbar/650-700 degreesC). The metagraywackes have NH4+ con tents 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 200 0 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 L td.