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

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Debjeet Pathak, Rajdeep Dasgupta

Geochimica et Cosmochimica Acta, In Press, Journal Pre-proof, Available online 6 March 2024

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“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) (D N^sa/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 D N^sa/la. Using our parameterized D N^sa/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.”

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