Oxygen levels play a key role in early Earth's magma ocean formation

Oxygen levels play a key role in early Earth's magma ocean formation
by Riko Seibo
Tokyo, Japan (SPX) Aug 27, 2024

The early Earth is believed to have been largely covered by a molten magma ocean, formed due to the extreme heat generated by the collision of smaller celestial bodies with the planet. Understanding this early phase is essential for grasping Earth's formation process. However, current models of magma ocean formation are challenged by disagreements over the melting temperatures of deep mantle rocks. Traditional models rely on specific experimental data, but recent findings suggest these temperatures may be off by as much as 200 - 250 C.

Some research indicates that oxygen fugacity, or the available oxygen in the mantle, could significantly affect the melting temperatures of deep mantle rocks. This, in turn, may have impacted the formation of the magma ocean. While the mantle's oxygen fugacity likely increased during accretion, core formation, and subsequent evolution, the precise effect of this change on deep mantle melting temperatures has remained uncertain.

To address this, a research team led by Associate Professor Takayuki Ishii of the Institute for Planetary Materials at Okayama University, Japan, and Dr. Yanhao Lin from the Center for High Pressure Science and Technology Advanced Research, China, explored how oxygen fugacity influenced the formation of the magma ocean during Earth's early evolution. "The evolution of early Earth has been greatly influenced by oxygen fugacity, which may necessitate the reconsideration of current models. To this end, we assessed the effect of oxygen fugacity on the melting temperatures of deep mantle materials to constrain the conditions at the floor of a deep terrestrial magma ocean," said Prof. Ishii.

Collaborating with Professor Wim van Westrenen from the Department of Earth Sciences at Vrije Universiteit Amsterdam, Netherlands, Professor Tomoo Katsura from Bayerisches Geoinstitut at the University of Bayreuth, Germany, and Dr. Ho-Kwang Mao from the Center for High Pressure Science and Technology Advanced Research, China, the team published their findings in 'Nature Geoscience' on July 16, 2024.

The researchers conducted experiments under high pressure (16 - 26 GPa, corresponding to mantle depths of 470 - 720 km) and high oxygen fugacity on mantle pyrolite, which simulates the Earth's mantle composition. They found that increased oxygen fugacity led to lower melting temperatures, by 230 - 450 C, compared to those under low oxygen fugacity. If the magma ocean maintained a constant temperature, this suggests the magma ocean floor would deepen by approximately 60 km for each logarithmic increase in mantle oxygen fugacity. This strong influence implies that existing models of early Earth's thermal evolution and core formation may need significant revision.

Additionally, these results could explain the discrepancy between the low oxygen fugacities predicted for the Earth's deep mantle post-core formation and the high oxygen fugacities observed in ancient magmatic rocks, over 3 billion years old, formed from deep mantle materials.

"Beyond Earth's formation, our findings on the dependence of melting temperatures on oxygen fugacity can also be applied to understand the formation of other rocky planets that can support human life," commented Dr. Lin, emphasizing the broader implications of the study. "For example, these results can improve our understanding of Mars, which is a recent hot topic regarding human habitability."

This study is expected to significantly enhance models of Earth's formation and provide insights into the formation of other rocky planets.

Research Report:Melting at the base of a terrestrial magma ocean controlled by oxygen fugacity