Bridgmanite's ferric iron content determined Earth's oxidation state

Area of Science:

• Geochemistry
• Mineral Physics
• Planetary Science

Background:

• Bridgmanite, a magnesium-rich silicate perovskite, is the most prevalent mineral in Earth's lower mantle.
• The lower mantle contains substantial ferric iron, influencing Earth's overall oxidation state.
• The origin of the lower mantle's oxygen content remains a key question in understanding early Earth processes.

Purpose of the Study:

• To investigate the factors controlling ferric iron content in bridgmanite.
• To model the crystallization of bridgmanite from a magma ocean to determine lower mantle oxygenation.
• To assess if lower mantle oxygen could explain the current upper mantle oxidation state.

Main Methods:

• High-pressure and temperature multi-anvil experiments were conducted at controlled oxygen fugacities.
• Thermodynamic modeling was employed to simulate bridgmanite crystallization from a reduced magma ocean.
• Experimental data on bridgmanite ferric iron content as a function of temperature and pressure were utilized.

Main Results:

• Bridgmanite's ferric iron content is independent of pressure but decreases with increasing temperature.
• The thermodynamic model indicates that crystallizing bridgmanite from a reduced magma ocean yields sufficient ferric iron.
• This process explains the observed ferric iron levels in the present-day upper mantle.

Conclusions:

• Early Earth's lower mantle oxygenation was established through bridgmanite crystallization from a magma ocean.
• The mixing of this oxygen-rich lower mantle material is a plausible mechanism for the upper mantle's current oxidation state.
• Bridgmanite's thermodynamic properties are crucial for understanding planetary evolution and differentiation.