Nitrogen behavior during fluid-rock interaction in a continental subduction zone: Results from the UHP Dora-Maira Massif whiteschists and Eastern Alps leucophyllites

Abstract

Deep-Earth cycling of nitrogen (N) along the subduction pathway, remains relatively poorly understood, particularly that related to deep subduction of continental crust. The ultrahigh-pressure (UHP) Dora-Maira Massif whiteschists in the Western Alps and their lower-P-T equivalent leucophyllites in the Eastern Alps show a decrease in N concentration and an increase in δ15N relative to their presumed country rock protoliths. On average, the whiteschists contain 20.7 ± 10.5 ppm N (mean ± 1σ) with δ15Nair = +2.7 ± 2.5‰ (mean ± 1σ) and the leucophyllites contain 59.8 ± 12.3 ppm N with δ15Nair = +4.3 ± 2.0‰, whereas the metagranitic country rocks from Dora-Maira contain 41.3 ± 12.5 ppm N with δ15Nair = −1.9 ± 3.2‰ and the country rocks from the Eastern Alps contain 91.1 ± 42.5 ppm N with δ15Nair = +1.7 ± 2.1‰. The loss of N and isotope shift in the whiteschists and leucophyllites from these two localities was accompanied by loss of LILE (Rb, Ba), Sr, CaO, Na2O, and FeOtotal. The other stable isotope systems applied to study both suites (Mg, Fe, O, Li and Ba) show no obvious correlation with variations in δ15N. Infiltration by and interaction with a serpentinite-derived fluid depleted in N and LILE could have led to leaching of N from the country rock protoliths and production of the +4.6‰ (Dora-Maira Massif) and +2.6‰ (Eastern Alps) isotope shifts adequately explained using a Rayleigh distillation model. The N exchange and leaching process is best explained as involving N2-NH4 or NH3-NH4 at about 550–600 °C, the latter the approximate temperatures at which the fluid infiltration is thought to have occurred. These results provide new insights regarding N behavior and isotopic fractionation during fluid-rock interactions occurring in UHP rocks and point to the importance of H2O-rich fluids from serpentinite dehydration as agents of metasomatism along deep subduction interfaces. Furthermore, the data for the metagranites indicate the potential of this lithology to retain significant amounts of N to at least 100 km and beyond sub-arc depths.