Olivine chemistry of the Quaternary Datong basalts in the Trans-North China Orogen: Insights into mantle source lithology and redox–hydration state

Cenozoic basalts are widespread above the front edge of the Big Mantle Wedge (BMW) in East Asia, but their mantle source lithology and redox–hydration state are poorly constrained. To address this issue, we report olivine compositions of the Quaternary Datong basalts. The Datong basalts exhibit OIB-like trace-element compositions and depleted Sr–Nd isotopes with slightly enriched (EMI) for tholeiitic basalts. Olivines of the Datong basalts show high Ni and Fe/Mn, and low Ca, Mn, and Mn/Zn values, pointing to a pyroxenite source. Applying V and Ca partition coefficients between olivine and whole-rock, respectively, the Datong basalts have −0.44 to 0.64 log units above the fayalite–magnetite–quartz buffer for fO2 , and contain 2.1–3.4 wt.% H2O but highly variable H2O/Ce values (265–1498). Both fO2 and H2O/Ce in the basalts vary with whole-rock and olivine compositions, indicating the source was the main control, thus, a heterogeneous redox–hydration state in the source: the EMI component being relatively reduced but extremely wet, and recycled oceanic crust being relatively oxidized but dry. The extremely wet EMI component was likely derived from the mantle transition zone. A model of mantle upwelling from the mantle transition zone to generate the Datong basalts is proposed