Résumé
Nitrogen and hydrogen isotopic compositions together with N, K, Rb, Cs and H2O contents were measured on several high-pressure (HP) to ultrahigh-pressure (UHP) metasediments from the Schistes Lustrés nappe (western Alps) and on unmetamorphosed sedimentary protoliths from the Apennines (Italy). These samples represent a sequence of pelagic sediments subducted to different depths down to 90 km along a ‘cold’ geothermal gradient (∼8°C/km). Nitrogen isotopic composition (δ15N between +2.6 and +4.8‰) does not show any specific evolution with increasing metamorphic conditions and can be considered constant during subduction. Large variations of the N content (between 169 and 1721 ppm N) together with K, Rb and Cs content are observed but the constancy of K/N (14), K/Rb (385) and K/Cs (10 190) molar ratios in protoliths and metamorphic rocks indicates that none of these fluid-mobile elements was lost through devolatilization processes. This suggests that fluid circulation was limited to sample scale and that the rocks behaved as closed systems during subduction. This interpretation is also supported by the small range of δD values (from −54.1 to −78.0‰). The present results indicate that N, K, Rb and Cs were retained in the subducted sedimentary veneer at least down to the depth locus of island arc magmatism. Based on the correlation between N and K contents, the flux of sedimentary N recycled in subduction zones is estimated at 7.6×1011 g/yr. Mass balance calculations strongly support the fact that nitrogen is efficiently recycled to the deeper mantle.