Abstract

Cenozoic intraplate basalts are widespread above the Big Mantle Wedge (BMW) and its front in East Asia. While the mantle source lithology and redox-hydration state have been demonstrated to be crucial in generation of the basalts above the BMW, their nature and role on the basalts above the front of the BMW is poorly constrained. To address this, 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 signatures (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 lie −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. In the light of our findings, we propose a model in which mantle upwelling carried the recycled oceanic crust and EMI component from the MTZ to shallow mantle, due to the Pacific slab stagnating in the MTZ, to form pyroxenite, which subsequently melted to generate the Datong basalts.

Supplementary material at https://doi.org/10.6084/m9.figshare.c.5227668