Synthesis of NH4-Substituted Muscovite at 6.3 GPa and 1000°C: Implications for Nitrogen Transport to the Earth’s Mantle

Abstract

The synthesis of NH₄-bearing muscovite at P = 6.3 GPa and T = 1000°C in equilibrium with NH₃–H₂O fluid is performed. It is determined that the newly formed muscovite is enriched in celadonite minal and contains ~370 ppm of NH₄. The obtained data make it possible to conclude that ammonium-bearing micas have sufficient thermal stability and can transport crustal nitrogen to the mantle in the presence of a reduced water–ammonia fluid at fO₂ less than the values of IW + 2 log units even in the regime of “hot” subduction. The key parameter that determines the efficiency of this mechanism for the deep nitrogen cycle is redox stability of NH₄-bearing muscovite at the mantle PT–parameters.