The mechanism of carbon recycling into orogenic lithosphere: A Li isotope perspective

Abstract

Subducting slabs transport carbon to deep mantle depths and release it into the overlying mantle wedge and lithospheric mantle through multiple mechanisms, including mechanical removal via diapirism, metamorphic decarbonization, carbonate dissolution and parting melting. Identifying the dominant carbon recycling mechanism responsible for carbonation of subcontinental lithospheric mantle (SCLM) remains challenging, yet it is critical for understanding the genesis of post-collisional carbonatites and associated rare earth element deposits. To address this issue, we investigate the Li isotopic systematics of typical post-collisional carbonatite-alkalic complexes from Mianning-Dechang (MD), Southeast Tibet. Our results show that the less-evolved magmas (lamprophyres) have mantle-like or slightly lower δ⁷Li values (0.3‰–3.6‰) with limited variability, contrasting sharply with the wider δ⁷Li range observed in associated carbonatites and syenites. We interpret this dichotomy as reflecting distinct processes: while the variable and anomalous δ⁷Li values in differentiated rocks (carbonatites and syenites) were caused by late-stage magmatic-hydrothermal processes (including biotite fractionation, fluid exsolution and hydrothermal alteration), the lamprophyres retain the primary Li isotopic signature of their mantle source. Together with their arc-like trace element and EM1-EM2-type Sr-Nd-Pb isotopic signatures, such mantle-like or slightly lower δ⁷Li values of the lamprophyres preclude carbon derivation from high-δ⁷Li reservoirs (altered oceanic crust, serpentinites) and recycling of sedimentary carbon through metamorphic decarbonization or dissolution. Instead, these features indicate that the carbon was predominantly transported into the mantle source via partial melting of subducted carbonate-bearing sediments. This study demonstrates that Li isotopes can serve as a tracer for identifying the mechanism of carbon recycling in collision zones.