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

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 degrees C, and oxygen fugacity (fO(2)) 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 (P3(1)12) 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 similar to 1500 ppm in phengitic muscovite at 6.3 GPa and 1000 degrees C and to 610-990 ppm in the intermediate between the dioctahedral and trioctahedral mica phase at 7.8 GPa and 10701090 degrees C. Ammonium in the analyzed metapelite shows incompatible behavior (D-NH4(Mica-Fluid)=0.05-0.15) at >= 5.5 GPa and temperatures and fO(2) 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 Nbearing micas, and changes of fluid composition near the second critical point.

Automated summary

The partitioning of nitrogen between different hosts was studied using partially devolatilized natural quartz-muscovite-chlorite schist (mica schist) containing NH4+-rich muscovite, under conditions corresponding to hot subduction. The results showed that the contents of NH4+ in the analyzed metapelite decrease with increasing pressure and temperature, suggesting the efficiency of ammonium outgassing from subducting metasediments. The nitrogen partitioning under hot subduction conditions is controlled by devolatilization of metasediments, phase transitions in Nbearing micas, and changes in fluid composition.