Pressure effect on Fe3+/FeT in silicate melts and applications to magma redox, particularly in magma oceans

Abstract:

The proportions of Fe³⁺ and Fe²⁺ in magmas reflect the redox conditions of their origin and influence the chemical and physical properties of natural silicate liquids, but the relationship between Fe³⁺/Fe^T and oxygen fugacity depends on pressure owing to different molar volumes and compressibilities of Fe³⁺ and Fe²⁺ in silicates. An important case where the effect of pressure effect may be important is in magma oceans, where well mixed (and therefore potentially uniform Fe³⁺/Fe^T) experiencses a wide range of pressures, and therefore can impart different ƒO₂ at different depths, influencing magma ocean degassing and early atmospheres, as well as chemical gradients within magma oceans.

To investigate the effect of pressure on magmatic Fe³⁺/Fe^T we conducted high pressure expeirments on ƒO₂-buffered andestic liquids. Quenched glasses were analyzed by Mössbauer spectroscopy. To verify the accuracy of Mössbauer determinations of Fe³⁺/Fe^T in glasses, we also conducted low temperature Mössbauer studies to determine differences in the recoilless fraction (ƒ) of Fe²⁺ and Fe³. These indicate that room temperature Mössbauer determinations of on Fe³⁺/Fe^T glasses are systematically high by 4% compared to recoilless-fraction corrected ratios.

Up to 7 GPa, pressure decreases Fe³⁺/Fe^T, at fixed ƒO₂ relative to metal-oxide buffers, meaning that an isochemical magma will become more reduced with decreasing pressure. Consequently, for small planetary bodies such as the Moon or Mercury, atmospheres overlying their MO will be highly reducing, consisting chiefly of H₂ and CO. The same may also be true for Mars. The trend may reverse at higher pressure, as is the case for solid peridotite, and so for Earth, Venus, and possibly Mars, more oxidized atmospheres above MO are possible. Diamond anvil experiments are underway to examine this hypothesis.