Nitrogen fractionation in mica metapelite under hot subduction conditions: Implications for nitrogen ingassing to the mantle

Abstract

Nitrogen partitioning between its main hosts is investigated using partly devolatilized natural quartz–muscovite–chlorite schist (mica schist), with NH4+-rich muscovite, under conditions corresponding to hot subduction: 3.0–7.8 GPa, 750–1090 °C, and oxygen fugacity (fO2) about the NNO (Ni-NiO) buffer. At these conditions, mica schist transforms into an eclogitic assemblage, while muscovite (C2/c) recrystallizes successively into two high-pressure white mica phases. The resulting phases are phengite (P3112) and a phase intermediate between the dioctahedral and trioctahedral mica series (C2/m). During dehydration and decarbonation reactions at run P-T conditions mica schist releases an H2O-CO2 fluid containing 0.3–14 rel.% N2 and < 0.1–3.7 rel.% NH3. The contents of NH4+ determined by Fourier transform infrared spectroscopy (FTIR) decrease from 1600 to 2000 ppm in primary muscovite to ∼1500 ppm in phengitic muscovite at 6.3 GPa and 1000 °С and to 610–990 ppm in the intermediate between the dioctahedral and trioctahedral mica phase at 7.8 GPa and 1070-1090 °C. Ammonium in the analyzed metapelite shows incompatible behavior (DNH4Mica−Fluid=0.05–0.15) at ≥ 5.5 GPa and temperatures and fO2 common to hot oxidizing slabs, which is expected to cause its efficient outgassing from subducting metasediments depleted in volatiles. Nitrogen partitioning under hot subduction conditions at sub-arc depths is generally controlled by devolatilization of measediments, phase transitions in N-bearing micas, and changes of fluid composition near the second critical point.