Nitrogen inventory of iron meteorite parent bodies constrained by nitrogen partitioning between Fe-rich solid and liquid alloys

Abstract

Delivery of nitrogen (N), one of the most important elements for life, to Earth thought to have occurred via both undifferentiated and differentiated bodies, lasting at least 50–100 Ma from the birth of the Solar System. Therefore, to understand how Earth got its N, it is imperative to know the N budget of the earliest formed bodies in our Solar System. The best astromaterials available for providing constraints on N budget of the earliest formed planetesimals are the iron meteorites. However, iron meteorites are crystallized products of a liquid alloy and do not represent the N budget of the bulk cores of various iron meteorite parent bodies (IMPBs). Therefore, to determine how N partitioned between solid alloy (sa) and liquid alloy (la) (DNsa/la) during crystallization of molten metal alloy core, we present a series of equilibrium partitioning experiments at 1–2 GPa and 1150–1550 ℃ for various initial starting compositions having different sulfur (S), nickel (Ni), iron (Fe) and fixed nitrogen (N) concentrations. We observe that N changes from mildly incompatible to mildly compatible with increasing S concentration in the liquid alloy. Furthermore, we observed that N concentration in solid alloy decreases with increasing temperature, while pressure and Ni content showed almost no effect on the partitioning behavior of N. We used a regression model based on the results of our study and a previous study to establish a parameterization for DNsa/la. Using our parameterized DNsa/la, we determine potential siderophile element proxies of N in metallic systems and model the initial N budget of various IMPBs groups pertaining to the inner (Non-Carbonaceous (NC) reservoir) and outer Solar System (Carbonaceous (CC) reservoir). Between two possible end-member styles of IMPB differentiation (IMO – Internal Magma Ocean; EMO – External Magma Ocean), EMOs result in a higher initial N budget with a major fraction getting lost via atmospheric loss. Importantly, our calculations suggest a gradation in the N budget of CC and NC IMPBs with CC IMPBs hosting lesser N than NC IMPBs. Therefore, the early Solar protoplanetary disk likely showed a gradation in N both in its elemental and isotopic composition.