Arc magmas are more oxidized than the magmas erupted from the mid-ocean ridges, which is probably attributed to the input of oxidizing subduction components into the sub-arc mantle. However, it remains unclear what subduction components control the oxygen fugacity of arc magmas. 

Some researchers suggested that serpentinite dehydration can produce highly oxidizing fluids, which may be responsible for the high oxygen fugacity of arc magmas, but it is still under debate.

Dr. ZHANG Yuxiang from the research team led by Prof. ZENG Zhigang at the Institute of Oceanology of the Chinese Academy of Sciences (IOCAS), in collaboration with researchers from Cornel University and Woods Hole Oceanographic Institution, investigated the relationship between subducted serpentinites and oxygen fugacity of arc magma. 

The study was published in Science Advances. 

The copper (Cu) behavior in arc magmas is significantly influenced by oxygen fugacity. Thus, the researchers used Cu enrichment or depletion during magma evolution (△Cu) as a proxy of magma oxygen fugacity. 

In addition, they adopted another experimentally determined oxygen fugacity indicator. They used boron (B) isotope (δ¹¹B) and B/niobium (Nb) ratio as proxies for recycled serpentinites and investigated their relationships with oxygen fugacity proxies (△Cu and V/Yb).

They found that these two groups of proxies showed robust correlations on a global scale, suggesting that subducted serpentinites had an important influence on the oxygen fugacity of arc magmas. 

Another important finding of this study is that the oxygen fugacity of primitive arc magmas are not homogeneous globally: continental arc magmas are probably more oxidized than intra-oceanic arc magmas.

The subducting plate beneath the continental arcs are generally young, hot, and flat, and has longer arc-trench distance. Thus, the serpentinite within the plate dehydrates more efficiently and might have released oxidizing fluids at early-stage subduction, leaving a subducting plate that is depleted in oxidizing components when reaching the sub-arc depth. The intra-oceanic subduction zone is just the opposite. 

The study was supported by the National Natural Science Foundation of China, the Strategic Priority Research Program (B) of Chinese Academy of Sciences, and the Shandong Provincial Natural Science Foundation, China.