Nitrogen isotope fractionation during terrestrial core-mantle separation

Abstract

doi: 10.7185/geochemlet.1614The origin and evolution of the terrestrial nitrogen remains largely unresolved. In order to understand the potential influence of core-mantle separation on terrestrial nitrogen evolution, experiments were performed at 1.5 to 7.0 GPa and 1600 to 1800 °C to study nitrogen isotope fractionation between coexisting liquid Fe-rich metal and silicate melt. The results show that the metal/silicate partition coefficient of nitrogen D N metal/silicate ranges from 1 to 150 and the nitrogen isotope fractionation d15 N metal-silicate is −3.5 ± 1.7 ‰. Calculations show that the bulk Earth is more depleted in d 15 N than the present-day mantle, and that the presentday mantle d 15 N of −5 ‰ could be derived from an enstatite chondrite composition via terrestrial core-mantle separation, with or without the addition of carbonaceous chondrites. These results strongly support the notion that enstatite chondrites may be a main component from which the Earth formed and a main source of the terrestrial nitrogen. Moreover, in the deep reduced mantle, the Fe-rich metal phase may store most of the nitrogen, and partial melting of the coexisting silicates may generate oceanic island basalts (OIBs) with slightly positive d15 N values.

Nitrogen and carbon fractionation during core–mantle differentiation at shallow depth

“…In the last decade, the deep carbon cycle and inventories have received considerable attention (c.f., Dasgupta, 2013), and in recent years there has been growing focus on nitrogen (e.g., Johnson and Goldblatt, and references therein). When examining the origin of global inventories and deep cycling, it is natural to compare nitrogen and carbon together because both are volatile during mantle degassing, incompatible during mantle melting and siderophile during core formation (e.g., Miyazaki et al, 2004;Dasgupta, 2013;Roskosz et al, 2013;Li et al, 2016).…”

Core formation and magma ocean outgassing set planetary N-S-C ratios

“…to literature data to validate its predictive power (Supplementary Figure 3). We predict literature data 20,22,25,26 Figure 4).…”

“…Core formation fractionates elements on the basis of their preference for the metallic phase. For the relatively low pressure and temperature (P-T) conditions associated with core formation in planetesimals, sulfur and carbon both display a greater preference for the metal phase (more siderophile behavior) than nitrogen e.g., [18][19][20][21][22][23][24][25][26] , provided planetesimal S and C concentrations remain at levels associated with planets 27 . Strongly siderophile S and C would lead to low S/N and C/N observable ratios for planetesimals, the opposite of what is observed for Earth 2,5,6 .…”

Nitrogen Content in the Earth's Outer Core

“…Such late addition of nitrogen is not expected to influence the N budget of the core. Moreover, a fraction of terrestrial core growth might have happened through near-disequilibrium merger of a differentiated planetary embryo (e.g., Li, Marty, et al, 2016;Rudge et al, 2010), in which case merger of a nitride-rich metallic core of more exotic composition formed from a different bulk composition could take place. Despite the geochemical arguments against significant concentration of nitrogen in the core, nitrogen cannot be ruled out as an important light element in the Earth's core on the basis of cosmochemical and early accretion process alone.…”

“…For example, nitrogen may be delivered during core formation in the form of relatively refractory nitrides rather than less stable organics (Rubin & Choi, 2009), minimizing the volatility-induced loss. Moreover, a fraction of terrestrial core growth might have happened through near-disequilibrium merger of a differentiated planetary embryo (e.g., Li, Marty, et al, 2016;Rudge et al, 2010), in which case merger of a nitride-rich metallic core of more exotic composition formed from a different bulk composition could take place. The presence of nitrogen during core-mantle differentiation of Earth is also potentially supported by recent findings of nitride and carbonitride inclusions in lower mantle diamond (Kaminsky & Wirth, 2017) if lower mantle domains preserve memory of inefficient core formation.…”