The oxygenation of the Earth’s inaccessible mantle controls the speciation and mobility of light elements such as hydrogen, carbon, and sulfur within it, influencing large-scale geologic processes i.e. the composition of those gases released during volcanic activity, the plate tectonics and thereby the composition of the atmosphere.

Current research on mantle oxygenation (i.e. oxygen fugacity, fo2) primarily focuses on studying the fO2 of mantle-derived melts without considering the effect of temperature and depth at which magmas have formed through time. 

To directly compare the fO2 characteristics of melts formed at different times and depths, researchers from the Institute of Oceanology of the Chinese Academy of Sciences (IOCAS), and Sapienza University of Rome have proposed a new parameter, "potential oxygen fugacity". This parameter is analogous to the classical "potential temperature" used by geologists and represents the fO2 of the Earth’s interior (i.e., mantle) at about 30 km in depth, assuming no melting or decompression. Using this "potential oxygen fugacity" parameter allows for direct comparison of the redox states of mantle sources from different depths, thereby constraining the evolution of the mantle redox state.The study was published in Nature Communications on Aug. 10.

By establishing the "potential oxygen fugacity" parameter, the researchers collected data on normal ambient mantle-derived magmas like basalts and deep mantle (plume-) derived komatiites and picrites globally since 3.8 Ga to constrain the evolution of the mantle redox state and thermal history. 

The results show that the fO2 of Archean magmas was significantly lower than that of post-Archean magmas. Concurrently, the fO2 of magmas displayed a strong negative correlation with mantle potential temperature and melting pressure. “This indicates that the high potential temperature of the Archean mantle, causing deep and extensive partial melting, might have resulted in the lower fO2 of Archean magmas,” explained Dr ZHANG Fangyi, first author of the study.

After normalizing all mantle-derived magmas' fO2 to "potential oxygen fugacity", it was found that the fO2 of both ambient mantle and mantle plume sources (lower mantle) has remained constant since the Hadean. “The variations in the fO2 of mantle-derived magmas were due to changes in melting depth and extent,” said Associate Prof. Vincenzo Stagno, co-author of the study.

Changes in the fO2 of mantle-derived magmas affect the composition of released volatiles, influencing atmospheric composition. Previous studies suggested that the increase in mantle fO2 since the Archean promoted a rise in atmospheric O2 levels. However, this study reveals that the increase in fO2 of mantle-derived magmas was driven by a long-term cooling of the mantle, resulting in decreased melting depth, rather than changes in mantle fO2 itself, thereby impacting atmospheric composition.

“Deciphering the evolution of the mantle's redox state since the Hadean is crucial for understanding important scientific questions such as deep carbon cycling, atmospheric composition evolution, and the origins of life,” said Dr. ZHANG.

“This study uniquely integrates the thermal state, redox state, and atmospheric composition evolution of the mantle, providing a new perspective for understanding the co-evolution history of Earth's multi-sphere system,” added Prof. SUN Weidong, the corresponding author. 

“This study made possible through a collaboration with Dr. Zhang and Prof. Sun represents an important step towards the investigation of the oxygenation of Earth through time, its possible formation from chemically different accreting building blocks and, ultimately, the analogies and diversities among planets of the same solar system”.

References:

The constant oxidation state of Earth’s mantle since the Hadean - Fangyi Zhang, Vincenzo Stagno, Lipeng Zhang, Chen Chen, Haiyang Liu, Congying Li, Weidong Sun - Nature Communications. doi: 10.1038/s41467-024-50778-z

Further Information

Vincenzo Stagno
Department of Earth Sciences
vincenzo.stagno@uniroma1.it