Nitrogen-Isotope Compositions of Metasedimentary Rocks of Different Tectonometamorphic Units

Birgit Mingram GeoForschungsZentrum Potsdam, Telegrafenberg A 50, 14473 Potsdam, Germany

Karin Bräuer Institut für Geophysik und Geologie, Universität Leipzig, Permoserstr.15, 04303 Leipzig, Germany

Kerstin Rötzler GeoForschungsZentrum Potsdam, Telegrafenberg A 50, 14473 Potsdam, Germany

Fluids play an essential role during mineralogical, geochemical and structural processes in metamorphic rocks. The N-isotope system represents a useful way to study volatile transport processes in open- and closed-metamorphic systems (Bebout & Fogel, 1992). NH4+ and d15N in metasedimentary rocks of the Erzgebirge and the KTB-pilot hole were used to prove large scale volatile movements. NH4+ was extracted for quantitative and isotopic measurement by Kjeldahl distillation techniques. The amount of fixed nitrogen was determined by titration, isotopic composition was determined on a mass spectrometer (DELTA-E) In metasediments, nitrogen is commonly found as N2 in fluid inclusions and as NH4+. substituted for K+ in silicate minerals (mostly micas and feldspars). Studies of N concentration and isotopic composition in regional and contact-metamorphic rocks indicate a depletion of NH4+ and an increase in d15N during prograde metamorphism. Nitrogen isotope fractionation occurs because of a preferentially release of 14N and a relative enrichment of 15N in the residual nitrogen of the rock with increasing metamorphism.

Petrological, lithological and geochemical studies of the metamorphic rocks in the Erzgebirge have shown the presence of four different metamorphic units of early Paleozoic age tectonically overlying amphibolite facies Precambrian basement. The main rock types of the metamorphic units are highly mature metapelites with similar lithologies and similar major and trace element compositions. Schists of different metamorphic units in the Erzgebirge exhibit a significant depletion of H2O and NH4+ and an increase in d15N from the low grade phyllite unit to the high grade mica schist-eclogite unit. Metapelites with the highest mica content show a loss of about 1,5 % H2O and 700 ppm NH4+ and an increase in d15N values from 1 ” to 5 ” from the low grade garnet-phyllite unit to the high grade gneiss-eclogit unit. These results indicate a loss of fluids on a large scale. The metapelites of the mica schists-eclogite unit exhibit an average NH4+ content (400 ppm) and d15N (4 ”). These values are significantly higher than the average contents of NH4+ (100 ppm) and d15N (2 ”) within gneisses (metagreywackes) of the same metamorphic unit. The gneisses occur in a shear zone with blastomylonitic texture and retrograde mineral growth. Mica schists and gneisses which were affected by contact metamorphism around Variscan granites show shifts to higher d15N (8-10 ”) accompanied with a decrease in NH4+ concentration. In contrast to the Erzgebirge, the rocks of the KTB- pilot drill hole represent a separate tectonometamorphic unit composed of metabasites and paragneisses. They underwent an amphibolite facies metamorphism and younger ductile and brittle deformations. The KTB paragneisses indicate contents of about < 100 ppm NH4+ and d15N values of around 6 ”. Gneisses in the vicinity of a late graphite-bearing cataclastic shear zone at 2000 m depth are characterised by an increase in NH4+ (> 250 ppm) and d15N (>11 ”).


1. The investigation of metapelites of the Erzgebirge crystalline complex with a similar major and trace element chemistry show a continues decrease of H2O and NH4+ -contents accompanied by an increase in d15N with increasing metamorphic grade.

2. Contact metamorphism caused by intrusion of Variscan granites leads to a higher rate of devolatilization reactions in these metasediments. This is reflected by low H2O- and NH4+ -contents and high d15N values.

3. The metagreywackes of the KTB pilot hole exhibit higher d15N (6 ”) than the metagreywackes of the Erzgebirge with d15N of around 2 ”. The KTB-gneisses reflect prograde amphibolite facies metamorphism (7 kbar, 650 °C) with temperature increase and reduction of water activity, whereas the gneisses of the Erzgebirge (12 kbar, 550°C) are characterised by temperature increase during decompression after a high pressure stage. The pressure decrease during decompression is connected with an increase in water activity and leads to reequilibration of most of the micas.

4. Fluid rock interaction in late cataclastic zones caused an increase in nitrogen content of the KTB-gneisses and a shift of nitrogen-isotope ratios.


Bebout, G. E. & Fogel, M. L. Geochim. Cosmochim. Acta 56, 2139-2149 (1992)