Pressure effect on the oxidation state of a terrestrial magma ocean, from experimental perspective

Magma oceans (MO) are a pivotal stage during early terrestrial planetary evolution. The oxygen fugacity (fO₂) of a MO controls the speciation and solubility of volatile elements at depth, and their degassing to the primitive atmosphere. The volatiles in turn affect MO crystallization and its subsequent geodynamic evolution. The fO₂ of the MO also affects the initial state of the solid mantle, which has a key influence on processes such as magma generation and differentiation, element partitioning, rheology, and volcanic degassing. However, fO₂ variations with depth within a terrestrial MO are poorly constrained.

Magmatic Fe³⁺/ΣFe ratios are directly linked to MO fO_2, as iron is the most abundant multi-valent element in silicate melts. In a vigorously convecting, well-mixed MO, magma-metal equilibration at depth will impose the Fe³⁺/∑Fe ratio and the fO₂ will be mainly affected by pressure (P) and temperature (T). Experiments up to 7GPa and 1750 °C show the Fe³⁺/∑Fe ratio decreases with increasing P and T. This suggests that a shallow MO with fixed Fe³⁺/ΣFe becomes more reduced relative to the IW toward the surface, at least for planetary bodies where the pressure at the base of the MO is less than 7 GPa like the Moon.

The cores of large planets form, on average, at much higher pressures. To better constrain the fO₂ gradient in a MO for larger planets, we performed LHDAC experiments up to 78 GPa and 4000 K. The centroids resolved from u-XANES spectra indicate the Fe³⁺/ΣFe ratios decrease as pressure increases, assuming that calibration curves derived from 100 kPa are applicable. However, theoretical calculations show that compression in silicate liquids leads to a continuous increase in average ion coordination state, mimicking the discontinuous transitions that occurs in solids (Stixrude and Karki, 2005). Consequently, the centroids from 100 kPa glasses may not resolve Fe³⁺/ΣFe ratios in high pressure glasses owing to changes in Fe ion coordination. Further increases in pressure may favor Fe³⁺stability relative to Fe²⁺, such that a deep MO may become more oxidized relative to IW toward the surface.

To clarify the relationship among Fe³⁺/ΣFe ratios, ion coordination, pressure and fO₂, we have collected spectra using SMS at APS. Results will be discussed here and a revised thermodynamic parameterization is also in progress.