Search for: All records

Subduction transports volatiles between Earth’s mantle, crust, and atmosphere, ultimately creating a habitable Earth. We use isotopes to track carbon from subduction to outgassing along the Aleutian-Alaska Arc. We find substantial along-strike variations in the isotopic composition of volcanic gases, explained by different recycling efficiencies of subducting carbon to the atmosphere via arc volcanism and modulated by subduction character. Fast and cool subduction facilitates recycling of ~43 to 61% sediment-derived organic carbon to the atmosphere through degassing of central Aleutian volcanoes, while slow and warm subduction favors forearc sediment removal, leading to recycling of ~6 to 9% altered oceanic crust carbon to the atmosphere through degassing of western Aleutian volcanoes. These results indicate that less carbon is returned to the deep mantle than previously thought and that subducting organic carbon is not a reliable atmospheric carbon sink over subduction time scales.

The degassing of CO₂and S from arc volcanoes is fundamentally important to global climate, eruption forecasting, ore deposits, and the cycling of volatiles through subduction zones. However, all existing thermodynamic/empirical models have difficulties reproducing CO₂‐H₂O‐S trends observed in melt inclusions and provide widely conflicting results regarding the relationships between pressure and CO₂/SO₂in the vapor. In this study, we develop an open‐source degassing model, Sulfur_X, to track the evolution of S, CO₂, H₂O, and redox states in melt and vapor in ascending mafic‐intermediate magma. Sulfur_X describes sulfur degassing by parameterizing experimentally derived sulfur partition coefficients for two equilibria: RxnI. FeS (m) + H₂O (v)  H₂S (v) + FeO (m), and RxnII. CaSO₄(m)  SO₂(v) + O₂(v) + CaO (m), based on the sulfur speciation in the melt (m) and co‐existing vapor (v). Sulfur_X is also the first to track the evolution offO₂and sulfur and iron redox states accurately in the system using electron balance and equilibrium calculations. Our results show that a typical H₂O‐rich (4.5 wt.%) arc magma with high initial S⁶⁺/ΣS ratio (>0.5) will degas much more (∼2/3) of its initial sulfur at high pressures (>200 MPa) than H₂O‐poor ocean island basalts with low initial S⁶⁺/ΣS ratio (<0.1), which will degas very little sulfur until shallow pressures (<50 MPa). The pressure‐S relationship in the melt predicted by Sulfur_X provides new insights into interpreting the CO₂/S_Tratio measured in high‐T volcanic gases in the run‐up to the eruption.