Redox control on nitrogen isotope fractionation during planetary core formation

Area of Science:

• Geochemistry
• Planetary Science
• Isotope Geochemistry

Background:

• Earth's nitrogen isotopic composition shows distinct surficial (15N-enriched) and deep (15N-depleted) reservoirs.
• Current models of mantle degassing and subduction do not fully explain this observed nitrogen isotope distribution.
• The impact of planetary differentiation on nitrogen (N) isotopes remains poorly understood.

Purpose of the Study:

• To experimentally determine nitrogen isotopic fractionations during metal-silicate partitioning, simulating planetary core formation.
• To investigate the influence of oxygen fugacity on nitrogen isotope behavior under high pressure and temperature.

Main Methods:

• Experiments were conducted at 1 GPa and 1,400 °C across a range of oxygen fugacities (ΔIW -3.1 to -0.5).
• An in situ analytical method was developed to measure nitrogen elemental and isotopic compositions in metal alloys and basaltic melts.
• Nitrogen-isotopic fractionations between metal alloys and silicate glasses were quantified.

Main Results:

• Substantial nitrogen isotopic fractionations were observed, ranging from -257 ± 22‰ to -49 ± 1‰ across the studied oxygen fugacity range.
• Large fractionations under reduced conditions are attributed to differences in nitrogen bonding within metal alloys (Fe-N) versus silicate glasses (N2, NH complexes).
• Nitrogen segregation into the core during formation could have increased the silicate mantle's δ15N value by approximately 20‰.

Conclusions:

• Planetary core formation is a critical process that significantly fractionated nitrogen isotopes.
• Nitrogen partitioning into the metallic core during differentiation provides a viable mechanism for establishing the observed mantle and deep Earth nitrogen isotopic reservoirs.
• These findings offer a new perspective on the evolution of Earth's volatile elements and isotopic composition.