| Summary: | Although carbonates are the primary form of carbon subducted into the mantle, their fate during recycling is debated. Here we report the first coupled high-precision Zn and Mo isotope data for Cenozoic intraplate basalts from western China. The exceptionally high delta 66Zn values (+0.39 to +0.50 parts per thousand) of these lavas require involvement of recycled carbonates in the mantle source. Variable delta 98Mo compositions (-0.39 to +0.27 parts per thousand) are positively correlated with Mo/Ce, best interpreted as mixing between isotopically light Mo from dehydrated oceanic crust and heavy Mo from recycled carbonates, which is also supported by positive coupling between delta 66Zn and delta 98Mo. Modeling reveals that involvement of <= 5% carbonate-bearing oceanic crust fully resolves the observed delta 66Zn-delta 98Mo mantle heterogeneity probed by intracontinental basalts. Our study demonstrates that combined delta 66Zn-delta 98Mo data sets for mantle-derived magmas can track recycled surficial carbonates in Earth's interior, providing a powerful geochemical tool for deep carbon science. Carbon is an element of life and studying its geological cycle is crucial for understanding Earth's evolution including formation of a life-supporting atmosphere. Here we report the first combined high-precision Zn and Mo isotope data for Cenozoic intraplate lavas from western China, showing that the basalts record <= 5% carbonate-bearing oceanic crust components in their mantle source. Our results provide new evidence for surficial carbonates being delivered into the deep upper mantle, which adds to the debate about the deepest extent of the terrestrial carbon cycle. First combined zinc (Zn) and molybdenum (Mo) isotope data for mantle-derived magmas to track the fate of subducted carbonates Zn-Mo isotopic compositions of Cenozoic Tarim basalts suggest surficial carbonates being delivered into the deep upper mantle We highlight the utility of combined Zn-Mo isotope data as a powerful tool in deep carbon science
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