Interface Processes and Anomalous Oxygen Transport in Rapid Metal Oxidation and Magnetite Formation at Protoplanetary Conditions

Abstract

It has long become evident that the stability and abundance of water across the solar protoplanetary disk was affected by extensive gradients of temperature and oxygen fugacity, but it is unclear to which extent water was directly involved in gas−solid interactions with minerals. In order to better understand the chemical and mineralogical record of water in the early stages of the solar system, I have studied the reaction of metallic Fe 94 Ni 5 Co 1 Cr 1 an analog to common minerals in the protoplanetary diskwith gaseous H 2 /H 2 O mixtures at total pressures of 1 and 10 mbar and temperatures of 350, 450, and 600 °C. The experiments used a specifically designed gas-mixing furnace equipped with a controlled evaporator-mixer for precise H 2 O dosing. The interpretation of the reaction products is aided by nanoscale chemical and microstructural analysis using focused ion beam methods and transmission electron microscopy, providing quantitative measures of the reaction rates and insights into the reaction mechanisms. The kinetics of magnetite formation in these reactions, determined as parabolic rate constants, are orders of magnitude faster than previously reported and suggest a strong contribution of anomalous oxygen transport via "short-circuit diffusion". The total pressure of the H 2 −H 2 O gas relates nonlinearly to the growth rates of magnetite, suggesting that adsorption of H 2 O at the magnetite−gas interface is another controlling mechanism besides diffusion. The growth of magnetite whiskers and the selective retention of slowly diffusing Cr and Ni provide further evidence for growth control by the gas phase and bidirectional transport of iron and oxygen across the magnetite scale, respectively. These phenomena could provide diagnostic tools for identifying magnetite formed under protoplanetary conditions in meteorites and returned samples.

Iron whiskers on asteroid Itokawa indicate sulfide destruction by space weathering

“…EDX analysis was performed in scanning TEM (STEM) mode aided by high-angle annular dark-field (HAADF) imaging. Quantitative elemental abundances within the whisker metal were calculated using the Cliff-Lorimer thin film approximation with k-factors determined from an artificial Fe-Ni-Co-Cr alloy of known composition 67 . High-resolution elemental profiles (Fig.…”

Re-distribution of volatiles on the airless surface of the C-type carbonaceous asteroid Ryugu.

“…Interestingly, morphologies of iron metals are different between iron sulfides and magnetite. The self-diffusion coefficient of iron in magnetite 30 is several orders of magnitudes lower than in iron sulfides 31 through a wide range of temperature and oxygen fugacity. Iron nano-particles on magnetite may nucleate from excess iron atoms in the vicinity, while iron whiskers may be derived from excess iron atoms that diffuse long distance to the limited nucleation sites on iron sulfides, perhaps via accelerated surface and/or grain boundary diffusion.…”

Influx of nitrogen-rich material from the outer Solar System indicated by iron nitride in Ryugu samples

“…The self-diffusion coefficient of iron in magnetite 25 is several orders of magnitudes lower than in iron sulfides 26 through a wide range of temperatures and oxygen fugacities. Iron nanoparticles on magnetite may have nucleated from excess iron atoms in the vicinity, whereas iron whiskers may be derived from excess iron atoms that diffused long distances to the limited nucleation sites on iron sulfides, perhaps through accelerated surface or grain boundary diffusion.…”