Limited Fluid Transport from the Gneiss Core Toward the Schist Envelope of the Dome de L’agout, France

Abstract

The Dome de l’Agout consists mainly of gneisses and granites, mantled by low to medium grade micaschists with relatively large amounts of graphite. Kreulen and Schuiling (1982) observed high nitrogen contents in fluid inclusions, with the highest gas contents increasing with metamorphic grade. They suggested that the nitrogen was mantle derived and moved from the centre of the dome outwards. Duit et al. (1986) analyzed the ammonium- and rubidium-contents in biotite and muscovite fractions and they observed a negative correlation between the highest ammonium contents in both minerals and metamorphic grade (Figure 1). Bos et al, (in press) studied the partitioning of ammonium and potassium between synthetic phlogopite and a chloride vapour. The distribution coefficient of ammonium was determined to be larger than 1 at the PT-conditions of 2000 bars at 550 and 650·C. The distribution coefficient increases, although slightly, with temperature, indicating that nitrogen storage in biotite is a valid source for N₂-gas in the lower crust (Bos et al., in press). In contrast to ammonium, rubidium contents increase in both micas with increasing metamorphic grade (Figure 2), pointing at a difference in behaviour and origin of the ions in the metamorphic rocks. Rubidium and ammonium both substitute for potassium in natural silicates.

Died July 26, 1985