Zinc and iron isotopic compositions of Cenozoic basalts in Inner Mongolia: New insights into deep carbon recycling related to Paleo-Asian slab subduction

Abstract

The origin of Cenozoic basalts in Inner Mongolia has been debated in recent years. Although the heavier-than-mantle zinc isotopic compositions indicate recycled carbonate in their sources, the recycled carbonate has been attributed to two different subducting oceanic slabs (Paleo-Asian vs. Paleo-Pacific). To further address this issue, Cenozoic basalts located in the west of the North-South Gravity Lineament (NSGL) in Inner Mongolia are investigated, and the sampling profile is approximately parallel to the NSGL (i.e., from southwest to northeast; SW-NE). These basalts have notably heavier zinc (δ⁶⁶Zn = 0.47–0.52 ‰) and iron (δ⁵⁶Fe = 0.18–0.24 ‰) isotopic compositions than those of the mantle, which cannot be interpreted by magmatic processes (differentiation and partial melting). Instead, the involvement of recycled carbonates in sources is a viable explanation for the high δ⁶⁶Zn characteristics, supported by high P₂O₅ contents and low Hf/Hf*. In addition, samples with high δ⁵⁶Fe also display high Fe/Mn (69–81) related to Cenozoic basalts from the east of the NSGL, indicating the existence of a pyroxenite component in sources in addition to carbonated peridotites. Combining with literature data, we found that basalts distributed in the SW-NE direction in Inner Mongolia have similarly heavy Zn and Fe isotopic compositions. This provides further constraints that mantle carbonate metasomatism in the west of the NSGL was most likely to have been caused by southeastward subduction of the Paleo-Asian oceanic slab, instead of northward subduction of the Paleo-Asian oceanic slab or subduction of the Paleo-Pacific oceanic slab.