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The fate of carbon subducted to mantle depths remains uncertain, yet strongly influences the distribution of terrestrial carbon on geologic timescales. Carbon fluxes into subduction zones are exceptionally high where downgoing plates contain thick sedimentary fans. This study uses volcano geochemistry to assess sedimentary carbon recycling in the high-flux Makran subduction zone, where the Arabian plate subducts northward beneath Eurasia. Based on strontium isotope geochemistry and 40Ar-39Ar geochronology, I show that a portion of the submarine Indus Fan entered the Makran trench, melted, and ascended as magmas that erupted in southern Afghanistan. The resulting volcano—composed primarily of carbonate minerals—formed at approximately 3.8 million years ago. The 87Sr/86Sr of the lavas indicates that their magmatic precursors derived from marine sediments deposited at 28.9 ± 1.4 Ma. This implies that sedimentary carbon subducted to and returned from mantle depths in less than 27 million years, indicating that magmas can efficiently recycle sedimentary carbon from subducting slabs to the overlying plate.