Researchers have introduced the “potential oxygen fugacity” parameter to track the evolution of Earth’s mantle and atmosphere, showing that mantle cooling, rather than intrinsic fO2 changes, influenced atmospheric oxygen levels.
The oxygen fugacity (fO2) of the mantle determines how volatiles behave and move within it, affecting the type and amount of volatiles emitted during magmatic activities sourced from the mantle. This, in turn, regulates the composition of the atmosphere.
Researchers from the Institute of Oceanology of the Chinese Academy of Sciences (IOCAS), together with their collaborators, have recently proposed a new parameter, “potential oxygen fugacity,” to directly compare the fO2 characteristics of melts formed at different depths.
Current research on the fO2 of the mantle primarily focuses on studying the fO2 of mantle-derived melts. However, due to the increasing stability of Fe3+ in garnet with pressure, mantle fO2 decreases with depth if mantle composition remains unchanged. Therefore, the fO2 differences in melts originating from different depths might reflect variations in the depth of magma origin, which is strongly dependent on the mantle temperature, rather than inherent differences in mantle fO2 (Fe3+/ΣFe ratio).
Defining Potential Oxygen Fugacity
The parameter researchers proposed is analogous to the classical definition of “potential temperature” and represents the fO2 of the mantle at 1 GPa with an assumption of no melting during decompression.
Using the “potential oxygen fugacity” parameter allows direct comparison of the redox states of mantle sources from different depths, thereby constraining the evolution of the mantle’s redox state.
“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. Fangyi Zhang, first author of the study and also a researcher from IOCAS.
The study was published in Nature Communications on Aug. 10.
Results of the Study: Ancient and Modern Mantle fO2
Using the “potential oxygen fugacity” parameter they had developed, the researchers collected data on normal ambient mantle-derived basalts and mantle plume-derived komatiites and picrites globally since 3.8 Ga to constrain the evolution of the mantle’s redox state and thermal history.
The results showed that the fO2 of Archean magmas was significantly lower than that of post-Archean magmas. Meanwhile, 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,” said Dr. Fangyi Zhang.
After normalizing the fO2 of all mantle-derived magmas to the “potential oxygen fugacity,” Zhang and his colleagues 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 and a researcher from Sapienza University of Rome.
Changes in the fO2 of mantle-derived magmas affected the composition of released volatiles and thus influenced the composition of the atmosphere. 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 in fact driven by a long-term cooling of the mantle, which resulted in decreased melting depth and thereby impacted atmospheric composition.
This study uniquely integrates the thermal state and redox state of the mantle as well as the evolution of the atmosphere’s composition, thus providing “a new perspective for understanding the co-evolution history of Earth’s multi-sphere system,” said Prof. SUN Weidong, corresponding author of the study.
Reference: “The constant oxidation state of Earth’s mantle since the Hadean” by Fangyi Zhang, Vincenzo Stagno, Lipeng Zhang, Chen Chen, Haiyang Liu, Congying Li and Weidong Sun, 10 August 2024, Nature Communications.
DOI: 10.1038/s41467-024-50778-z
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1 Comment
This fake hypothesis is based upon the validity of ancient age theory, uniformitarian thinking, and not based upon scientific method. This is bad science. It may even be a lie?