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

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 degrees C to 700 degrees C. The formation of Fe4N was associated with equilibrium N isotope fractionations with alpha Fe4N-NH3 values of 0.9907 (perpendicular to 0.0004) at 300 degrees C and 0.9936 (+/- 0.0004) at 500 degrees C, respectively. In the experimental pressure conditions (initial P-NH3 = 3.9-6.4 bar, P-Total < 8.3 bar), the formed Fe4N remained stable at 300 degrees C, but was unstable and quickly decomposed to Fe and N2 at 500 degrees C and 700 degrees C. The decomposition of Fe4N was associated with large kinetic isotope fractionations with alpha N-2-Fe4N values of 0.9811 (+/- 0.0009) at 500 degrees C and 0.9839 (+/- 0.0011) at 700 degrees 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. (C) 2021 Elsevier Ltd. All rights reserved.

Automated summary

Experimental results show that roaldite can be formed under different temperature conditions, with equilibrium nitrogen isotope fractionations observed at specific temperatures, but decomposition occurs at high temperatures. Caution is needed when using nitride minerals like roaldite to trace source information.