Mantle Redox State Evolution in Eastern China and Its Implications

Abstract

Using the secondary spinel standard, the authors have precisely measured the Fe3+/Σ Fe values of spinels in mantle xenoliths from Cenozoic basalts in eastern China, and estimated the oxygen fugacities recorded by 63 mantle xenoliths through olivine‐orthopyroxene‐spinel oxygen barometry. The results indicate that the oxygen fugacities of the lithospheric mantle in eastern China are higher in the south than in the north. Among them, the oxygen fugacity of the North China craton lithospheric mantle is the lowest, similar to that of the oceanic mantle, while that of Northeast and South China are the same as that of the global continental mantle. The variations of mantle redox state in eastern China are mainly controlled by the C‐O‐H fluids derived from the asthenospheric mantle. According to the mantle oxidation state, it can be concluded that the C‐O‐H fluids in the lithospheric mantle of eastern China consist mainly of CO2 and minor H2O, but CH4‐rich fluids should come from the asthenosphere where the oxidation state is lower.

Hot, volatile‐poor, and oxidized magmatism above the stagnant Pacific plate in Eastern China in the Cenozoic

“…We infer olivine-melt and spinel-olivine equilibration for phenocrysts cores and thus fO 2 of the magmas for partial crystallization during intermittent storage at ≥~400 MPa (Table 1), in which case degassing-related fO 2 variation should have had no large effect on our estimates. The effect of melt-rock reaction on the redox state of the magmas remains unconstrained; it would have reduced the melts if they were H 2 O-poor, as the normal upper mantle beneath eastern China is inferred to be buffered at ≤FMQ−1.5 to FMQ (Li & Wang, 2002), but could have oxidized them if they were H 2 O-rich (e.g., Tollan & Hermann, 2019). We therefore propose that the Menluhe, Xunke, and Wuchagou magmas were either derived from mantle sources at~FMQ−1±1 to~FMQ±1, and thus from comparatively oxidized mantle domains compared to typical asthenosphere at <FMQ−1 to~FMQ−3 (Foley, 2011), or that the melts acquired a relatively oxidized signature during ascent in the mantle as a result of H 2 loss.…”