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India-Asia continental collision–induced volcanic gas emissions are thought to have played an important role in driving Cenozoic atmospheric CO2 variations, yet the details of how the deep carbon cycle may influence volcanic CO2 degassing are not understood. We present a novel study employing Mg-Zn-Mo isotopic compositions of Cenozoic ultrapotassic lavas from NW Tibet. The negative Mg-Zn isotope correlation (δ26Mg = −0.39‰ to −0.19‰; δ66Zn = +0.27‰ to +0.36‰), bolstered by petrographic analysis of mantle-derived xenoliths from these lavas, demonstrates that the ultrapotassic magmas originated from a lithospheric mantle source that had been enriched by recycled carbonate-bearing sediments rich in calcite and dolomite. Very low δ98Mo values (−0.78‰ to 0‰) relative to the average continental crust (δ98Mo = +0.10‰ to +0.35‰) further indicate that the sedimentary components were derived from the subducted Indian continental crust after its dehydration. Monte Carlo modeling estimates that the input flux of carbon (elemental C) from such sediments into the lithospheric mantle is ∼5.6 Mt/yr, with a predicted CO2 emission rate of ∼15.5 Mt/yr. We suggest that the still ongoing subduction of the Indian tectonic plate has played a crucial role in introducing substantial quantities of carbonate-rich sediments into the Tibetan lithospheric mantle, leading to the sequestration of large amounts of CO2 via carbonatite metasomatism. Hence, partial melting of such a carbon-rich mantle reservoir in an orogenic setting provides the positive feedback mechanism that can explain the high flux of volcanic CO2 during IndiaAsia collision. These findings not only highlight the importance of continental subduction, sediment recycling, and mantle metasomatism by carbon-rich melts/fluids in the generation of Tibetan ultrapotassic volcanism, but they also show how the deep carbon cycle influences volcanic CO2 degassing. [ABSTRACT FROM AUTHOR]

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 δ66Zn values (+0.39 to +0.50‰) of these lavas require involvement of recycled carbonates in the mantle source. Variable δ98Mo compositions (−0.39 to +0.27‰) 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 δ66Zn and δ98Mo. Modeling reveals that involvement of ≤5% carbonate‐bearing oceanic crust fully resolves the observed δ66Zn–δ98Mo mantle heterogeneity probed by intracontinental basalts. Our study demonstrates that combined δ66Zn–δ98Mo data sets for mantle‐derived magmas can track recycled surficial carbonates in Earth's interior, providing a powerful geochemical tool for deep carbon science. Plain Language Summary: 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. Key Points: First combined zinc (Zn) and molybdenum (Mo) isotope data for mantle‐derived magmas to track the fate of subducted carbonatesZn−Mo isotopic compositions of Cenozoic Tarim basalts suggest surficial carbonates being delivered into the deep upper mantleWe highlight the utility of combined Zn−Mo isotope data as a powerful tool in deep carbon science [ABSTRACT FROM AUTHOR]

Mass-independent tungsten isotope variations provide critical insights into the timing and nature of processes that occurred in the early Solar System and during planetary differentiation. However, W isotope analyses are often compromised by an analytical artifact manifesting itself as an apparent deficit in 183W, whose origin and nature have remained enigmatic. Here, by evaluating previously published high-precision W isotope data for a large and diverse set of terrestrial samples, we demonstrate that this artifact occurs independent of the type of mass spectrometer and confirm that it can be attributed to mass-independent fractionation of 183W. Contrary to previous proposals, we find that this 'analytical183W effect' cannot be explained by a nuclear field shift, but may instead reflect a magnetic isotope effect. Regardless of its exact origin, our investigation reveals that this artifact is induced during the chemical separation of W, and that the specific combination of chromatographic purification and dry-down procedure determines its overall magnitude. Within a given analytical protocol, however, its size is strongly controlled by the amount of W that is processed, where the 183W effect increases with decreasing amount of W. Therefore, this work resolves apparent inconsistencies between previous studies regarding the occurrence and magnitude of the 183W effect, and provides directions for its mitigation and reliable correction. This in turn is crucial for the accurate interpretation of W isotope data with respect to radiogenic and nucleosynthetic anomalies for both terrestrial and meteoritic materials. [ABSTRACT FROM AUTHOR]