Mantle Oxidation Induced by Recycled Carbonate: Insights From Mg‐Zn‐Fe‐Cu Isotopic Systematics of Intraplate Basalts

Abstract

Mantle oxidation plays an essential role in volatile cycling between Earth’s interior and exterior. However, the conceivable oxidizer remains enigmatic and needs to be explored by geochemical tools. Here, we examine the potential of recycled carbonates in inducing redox changes of the deep mantle through combined isotopic studies of Mg (δ26Mg), Zn (δ66Zn), Fe (δ56Fe), and Cu (δ65Cu) on a suite of intraplate alkaline basalts proposed to have originated from the mantle transition zone (∼440–660 km). Compared with mid‐ocean ridge basalts (MORB), these basalts have variably lower δ26Mg and higher δ66Zn, the typical features of marine carbonates, indicating the presence of various amounts of deeply recycled carbonates in their sources. As the estimated amounts of recycled carbonates in sources increase (i.e., lower δ26Mg and higher δ66Zn), the basalts have higher δ56Fe and lower δ65Cu compared with MORB. These correlations cannot be explained alone by binary mixing between recycled carbonates and the pristine mantle because of the much lower Fe and Cu contents of marine carbonates than those of the mantle and instead require redox‐driven isotope fractionation induced by mantle carbonate metasomatism. During partial melting of an oxidized mantle source, isotopically heavy ferric iron and isotopically light sulfide were preferentially incorporated/dissolved into the melts that are enriched in heavy Fe and light Cu isotopes. These observations highlight that recycled carbonates serve as an important oxidizer for the deep mantle and facilitate the extraction of mantle sulfides, which can be effectively tracked by Fe and Cu isotopic systematics of intraplate basalts.