Carbon Isotope and Abundance Systematics Reveal an Insignificant Crustal Sink of Mantle-Derived CO2 in On-Land Rift Zone Geothermal Systems

Abstract

Rift zones, both on land and submarine, are key pathways for mantle-derived CO₂ degassing to the Earth's surface. Fixation of CO₂ 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 CO₂ concentrations (6.1–70.4 mmol kg⁻¹) but narrow δ¹³C values (−4.8 to −2.5‰) assuming limited to no fractionation between vapor and liquid phases, in contrast to the broader δ¹³C 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 CO₂ is sequestered in on-land rift carbonates, with the majority emitted via hydrothermal fluids. In contrast, Icelandic off-rift low-temperature systems represent significant CO₂ sinks due to limited boiling processes. This study suggests that submarine rift zones, lacking prominent boiling processes, may sequester mantle-derived CO₂ 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 CO₂, underscoring the contrasting roles of terrestrial and submarine environments in global carbon cycling.