Oxygen is necessary for the maintenance of all life forms on Earth. Subduction of oceanic plate carries abundant redox-sensitive elements such as sulfur, iron and carbon to the deep earth, drives the oxygen cycle between Earth’s interior and exterior and regulates the long-term atmospheric O2 level, playing a critical role in building and maintaining a habitable Earth. Arc volcanics are well known to be more oxidized than mid-ocean ridge basalts, but it is highly debated whether this is a mantle (oxidized by subduction contribution) or crustal (oxidized during magmatic evolution) feature. However, volcanic rocks that can represent primary arc magma compositions are extremely rare, and thus the origin of more oxidized arc magmas remains unclear.
The redox states of primary arc magmas control the contents of the redox-sensitive S and sulfur-loving Cu in magma. On this basis, Ph.D. candidate Siyu Zhao and Dr. Alexandra Yang Yang from Guangzhou Institute of Geochemistry, Chinese Academy of Sciences and collaborators found that the Cu/Zr ratios in arc volcanics stay constant during early-stage magma differentiation (Fig. 1a), based on the synthesis of ~100,000 global arc volcanic data. They proposed for the first time that the Cu/Zr ratios of arc volcanics with MgO > 6% can represent the primary arc magma compositions, and record the sulfur contents and redox states of the mantle. This study estimated the sulfur contents in the sub-arc mantle, adopted the Cu/Zr ratios in global arc volcanics to identify the geochemical behavior of Cu and S in primary arc magma, and revealed a more oxidized sub-arc mantle than sub-ridge mantle (the oxygen fugacity up to FMQ + 1.3, Fig. 1b and c), clarifying that the oxidized arc magma is a mantle feature.
Figure 1. (a) Cu/Zr contents of arc volcanics during magma evolution. (b) and (c) Cu/Zr systematics in primary mid-ocean ridge basalts (MORB) and arc magma during mantle melting, suggesting that the sub-arc mantle is more oxidized than sub-ridge mantle.
The Cu systematics in arc volcanics not only record the redox states of their primary arc magma, but also are the key to form porphyry Cu deposits (PCD), which supply 75% of the world’s Cu, and mainly develop in subduction zones. Super-large PCD mostly occur in thick-crusted continental arcs, and lower Cu contents in volcanics from thick-crusted arcs were previously proposed to result from deep crustal sulfide fractionation which is considered as the key step to form PCD. However, this study suggests instead that low Cu contents of thick-crusted arc volcanics result from lower extents of melting, thus obviating the need for deep-crustal sulfide accumulation, calling for alternative mechanism for the anomalous Cu enrichments in PCD.
This study resolves the long-standing debate concerning the origin of the oxidized nature of arc magmas, provides evidence for deep oxygen cycle that is vital for the habitability of our planet, and has substantial implications for the origin of porphyry-Cu deposits. This study was published in Science Advances.
This study was supported by the National Natural Science Foundation of China (41773025), the Key Special Project for Introduced Talents Team of Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou) (GML2019ZD0202), the Strategic Priority Research Program (B) of Chinese Academy of Sciences (CAS) (XDB18000000), and the Youth Innovation Promotion Association of CAS (2020349) to A.Y.Y.