Heavy boron isotopes reveal deep carbon recycling in hot subduction zones

The efficiency of deep carbon recycling in hot subduction systems remains debated, with contrasting models predicting either extensive shallow decarbonation or substantial carbonate retention within the subducted slab. Here we present boron-isotope and geochemical data from olivine-hosted melt inclusions in volcanic arc adakites from western Panama and in forearc high-Mg andesites from SW Japan, representing modern and Archean-like hot subduction systems. Olivine-hosted melt inclusions exhibit elevated δ 11 B values (up to +23.2‰) coupled with high Ce/Pb, La/Yb, Eu/Ti, and Th/Zr ratios, indicating melt contributions from subducted carbonate-bearing lithologies. Integration with regional CO 2 flux estimates from western Panama suggests that more than ~60% of slab carbonate escapes shallow decarbonation, likely owing to extensive forearc and arc dehydration causing substantial fluid loss, thereby suppressing fluid-mediated decarbonation beneath both the deeper forearc and arc front. These results demonstrate that substantial carbon can bypass shallow decarbonation in both modern and Archean hot subduction systems.