Polyamorphic Transformations in Fe‐Ni‐C Liquids: Implications for Chemical Evolution of Terrestrial Planets

Abstract

AbstractDuring the formation of the Earth's core, the segregation of metallic liquids from silicate mantle should have left behind evident geochemical imprints on both the mantle and the core. Some distinctive geochemical signatures of the mantle‐derived rocks likely own their origin to the metal‐silicate differentiation of the primitive Earth, setting our planet apart from undifferentiated meteorites as well as terrestrial planets or moons isotopically and compositionally. Understanding the chemical evolution of terrestrial planetary bodies requires knowledge on properties of both liquid iron alloys and silicates equilibrating under physicochemical conditions pertinent to the deep magma ocean. Here we report experimental and computational results on the pressure‐induced structural evolution of iron‐nickel liquids alloyed with carbon. Our X‐ray diffraction experiments up to 7.3 gigapascals (GPa) demonstrate that Fe‐Ni (Fe90Ni10) liquids alloyed with 3 and 5 wt % carbon undergo a polyamorphic liquid structure transition at approximately 5 GPa. Corroborating the experimental observations, our first‐principles molecular dynamic calculations reveal that the structural transitions result from the marked prevalence of three‐atom face‐sharing polyhedral connections in the liquids at >5 GPa. The structure and polyamorphic transitions of liquid iron‐nickel‐carbon alloys govern their physical and chemical properties and may thus cast fresh light on the chemical evolution of terrestrial planets and moons.