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Abstract 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/STratio measured in high‐T volcanic gases in the run‐up to the eruption.
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Carbon Fluxes and Primary Magma CO 2 Contents Along the Global Mid‐Ocean Ridge System
Abstract The concentration of carbon in primary mid‐ocean ridge basalts (MORBs), and the associated fluxes of CO2outgassed at ocean ridges, is examined through new data obtained by secondary ion mass spectrometry (SIMS) on 753 globally distributed MORB glasses. MORB glasses are typically 80–90% degassed of CO2. We thus use the limited range in CO2/Ba (81.3 ± 23) and CO2/Rb (991 ± 129), derived from undegassed MORB and MORB melt inclusions, to estimate primary CO2concentrations for ridges that have Ba and/or Rb data. When combined with quality‐controlled volatile‐element data from the literature (n = 2,446), these data constrain a range of primary CO2abundances that vary from 104 ppm to 1.90 wt%. Segment‐scale data reveal a range in MORB magma flux varying by a factor of 440 (from 6.8 × 105to 3.0 × 108m3/year) and an integrated global MORB magma flux of 16.5 ± 1.6 km3/year. When combined with CO2/Ba and CO2/Rb‐derived primary magma CO2abundances, the calculated segment‐scale CO2fluxes vary by more than 3 orders of magnitude (3.3 × 107to 4.0 × 1010mol/year) and sum to an integrated global MORB CO2flux of × 1012mol/year. Variations in ridge CO2fluxes have a muted effect on global climate; however, because the vast majority of CO2degassed at ridges is dissolved into seawater and enters the marine bicarbonate cycle. MORB degassing would thus only contribute to long‐term variations in climate via degassing directly into the atmosphere in shallow‐water areas or where the ridge system is exposed above sea level.
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- Award ID(s):
- 1634421
- PAR ID:
- 10457135
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geochemistry, Geophysics, Geosystems
- Volume:
- 20
- Issue:
- 3
- ISSN:
- 1525-2027
- Format(s):
- Medium: X Size: p. 1387-1424
- Size(s):
- p. 1387-1424
- Sponsoring Org:
- National Science Foundation
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