Nitrogen storage capacity of phengitic muscovite and K-cymrite under the conditions of hot subduction and ultra high pressure metamorphism

Abstract

Nitrogen storage capacity and partitioning between main N hosts have been investigated in N-bearing Al-rich natural pelite at 3.0–7.8 GPa, 825–1070 °C and oxygen fugacity (fO2) from NNO (Ni-NiO buffer)+0.3 to NNO-4.1 log units. Under the conditions of hot subduction, pelite converts into a phase assemblage typical of ultrahigh-pressure (UHP) eclogites. Phengitic muscovite in this assemblage is in equilibrium with a volatile-rich granite-like melt at 3.0 GPa and with supercritical fluid at 5.5–7.8 GPa, and contains, respectively, 115–135 and 759–962 wt ppm NH4+ at fO2 values close to NNO. Both melt and supercritical fluid have H2O and CO2 as predominant volatiles and the NH3/(NH3 + N2) ratio from 0.04 to 0.12. The pattern of nitrogen partitioning between its main hosts in pelite (DNH4Ms-Melt = 0.41–0.58 and DNH4Ms-Fluid = 0.63–2.4) proves that the ammonium exhibits moderately incompatible to compatible behavior within a hot oxidized slab in the pressure range from 3.0 GPa to 7.8 GPa. Therefore, even relatively oxidized sediments containing phengitic muscovite can efficiently transport nitrogen to the mantle at sub-arc depths under the hot subduction conditions. At the same time, the changeover of the N host from biotite to muscovite at the arc depths in combination with subsequent pelite melting and an abrupt decrease in phengitic muscovite abundance in pelite may lead to an avalanche-like N outgassing of the slab. The incorporation of an additional nitrogen source of (NH3 + N2) into pelite reduces fO2 to NNO-3.1 – NNO-4.1 log units and increases both the NH3/(NH3 + N2) ratio in the fluid (up to 0.44–0.86) and the NH4+ concentration in phengitic muscovite (up to 3750–3820 wt ppm). K-cymrite was produced in pelite at P ≥ 6.3 GPa and the bulk nitrogen content 3.2–5.9 wt%. K-cymrite possesses exceptional nitrogen storage capacity: it hosts 1.4–1.6 wt% NH4+, up to 0.5 wt% NH3, and 4–6 wt% N2. The DN-NH4Cym-Ms coefficient is as high as 20. Being stable in sediments subducted to mantle depths, K-cymrite, with its extremely high storage capacity, can act as a huge hidden redox-insensitive nitrogen reservoir in the mantle and thus can be involved in the deep nitrogen cycle.