Experimental investigation of formation and decomposition of roaldite in ammonia atmosphere at 300–700 °C and associated nitrogen isotope fractionations

Abstract

Roaldite (Fe4N) is one of the few nitride minerals found in meteorites. Their nitrogen (N) isotopic signatures carry important information for understanding the early N cycle in the proto-solar nebula. However, the lack of knowledge on the N isotopic effects from nitride formation to its survival from frictional heating during landing impedes the interpretation and application of N isotope compositions of nitride minerals in meteorites. Here, we carried out laboratory experiments under a recently proposed roaldite forming condition, i.e., NH3 (as starting N source) reacting with metallic Fe at medium temperatures. We observed Fe4N formation over a large range of temperatures from 300 °C to 700 °C. The formation of Fe4N was associated with equilibrium N isotope fractionations with αFe4N-NH3 values of 0.9907 (±0.0004) at 300 °C and 0.9936 (±0.0004) at 500 °C, respectively. In the experimental pressure conditions (initial PNH3 = 3.9–6.4 bar, PTotal < 8.3 bar), the formed Fe4N remained stable at 300 °C, but was unstable and quickly decomposed to Fe and N2 at 500 °C and 700 °C. The decomposition of Fe4N was associated with large kinetic isotope fractionations with αN2-Fe4N values of 0.9811 (±0.0009) at 500 °C and 0.9839 (±0.0011) at 700 °C, respectively. Our experimental results suggest that roaldite formed from NH3 can carry an isotopic signature very close to that of its source, but partial decomposition (if there is any) can easily shift its N isotope composition for several tens of per mil, and in extreme cases, to >300‰. Thus, great caution is needed when using N isotope composition of roaldite (and probably other nitride minerals as well) to trace source information.