Stagnant subducted Pacific slab-derived CO2 emissions: Insights into magma degassing at Changbaishan volcano, NE China

Changbaishan volcano, ∼1400 km away from Pacific subduction zone, has long been regarded as significant source of carbon released by episodic eruptions since Quaternary and magma-related geothermal activities at present (e.g. hot springs and soil microseepage). Soil CO2 flux measurements following accumulation chamber method indicate that average soil CO2 flux of western flank of the Changbaishan volcano is ∼19.4 g m−2 d−1, which suggests relatively quiescent status of diffuse soil microseepage and might be informative for volcanic monitoring. Bubble gases from hot springs close to crater are characterized by arc-like 3He/4He ratios (2.7-6.33 RA) and δ13CCO2 values (−3.5‰ to −7.5‰), whereas the sample far away from crater has lower 3He/4He ratio (0.85 RA) and δ13CCO2 value (−12.3‰). Three-component C-He isotope coupling model reveals that, upper mantle (MORB), slab carbonate (CAR) and organic metasediments (ORG) from both subducted Pacific slab and overriding continental crust are involved in origin and evolution of the Changbaishan volcanic gases. Combined with evidences from geochemical and geophysical studies, we propose that CO2-rich volcanic gases released by Changbaishan volcano might originate from stagnant subducted Pacific slab in the mantle transition zone (MTZ). The MTZ-derived volatiles mixed by components from upper mantle, slab carbonate and subducted organic metasediments underwent variable degrees of contamination by organic metasediments in continental crust during transportation from magma chamber to the surface. Subducted Pacific slab-derived materials contribute over half of the total carbon inventory at Changbaishan volcano, which could shed light on the significance of carbonatitic and carbonated silicate melts on liberating carbon from Earth's interior to the exosphere.