Carbon cycling during the India-Asia collision revealed by δ26Mg–δ66Zn–δ98Mo evidence from ultrapotassic volcanoes in NW Tibet

Abstract

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 India-Asia 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.