Carbon Isotope and Abundance Systematics Reveal an Insignificant Crustal Sink of Mantle‐Derived CO ₂ in On‐Land Rift Zone Geothermal Systems

Abstract Rift zones, both on land and submarine, are key pathways for mantle‐derived CO 2 degassing to the Earth's surface. Fixation of CO 2 as carbonates plays a critical role in regulating these emissions. This study examines carbon abundance and isotope compositions in hydrothermal fluids, altered rocks, and carbonates from geothermal systems in both Iceland rift and off‐rift. Hydrothermal fluids exhibit wide CO 2 concentrations (6.1–70.4 mmol kg −1 ) but narrow δ 13 C values (−4.8 to −2.5‰) assuming limited to no fractionation between vapor and liquid phases, in contrast to the broader δ 13 C range of carbonates (−14.5 to +0.5‰). Altered rocks contain up to 4.33 wt.% carbon, with enrichment in the upper 1,000 m of geothermal systems. Isotopic and geochemical modeling indicate that carbon is sourced from the mantle. Decompression boiling and water‐rock interaction drive carbonate formation, but only 1%–10% of mantle‐derived CO 2 is sequestered in on‐land rift carbonates, with the majority emitted via hydrothermal fluids. In contrast, Icelandic off‐rift low‐temperature systems represent significant CO 2 sinks due to limited boiling processes. This study suggests that submarine rift zones, lacking prominent boiling processes, may sequester mantle‐derived CO 2 more effectively into oceanic crust, facilitating long‐term recycling into the mantle. These findings highlight the limited capacity of on‐land rift systems to retain mantle‐derived CO 2 , underscoring the contrasting roles of terrestrial and submarine environments in global carbon cycling.