Volcanic Carbon: Global Variations in Gas Emissions

Abstract:

Magmas degas volatiles during ascent from the mantle and mafic melts with 7 wt% H₂O attain volatile saturation at ~15km depth. Magmatic gases are dominated by H₂O, CO₂ and S species, independent of their tectonic setting. At rift volcanoes, C is sourced from the mantle whereas arc volcanoes sample both mantle and subducted C.

Volcanic gases provide detailed information on volatile sources and degassing processes. Comparison of fumarole gases with melt inclusions and volcanic plumes shows that most fumaroles sample degassed magma. Water, CO₂ and S vary significantly between tectonic settings. The Kuriles, Japan, and Kamchatka have H₂O/CO₂ of 40 to 800 while other arcs such as the Cascades, Central America, S. America, Java, and Aeolian have ratios of 1 to 70. Gases from rift volcanoes have H₂O/CO₂ between 3 and 9. Some of these variations are due to addition of meteoric and subducted water, as evidenced by O and H isotopes. Speciation of H and C in volcanic gases are typically controlled by redox buffer reactions imposed by the Fe³⁺-Fe²⁺ (i.e. QFM) rock buffer or the SO₂-H₂S gas buffer. In more exotic systems such as Poás, hydrothermal S phases such as liquid native S can play a role in high T gas C and H speciation.

Arcs dominate the global subaerial volcanic CO₂ emission budget and arc total fluxes vary significantly i.e. only about 2 t/yr/km from the Aleutians and about 65 t/yr/km from Central America. Reasons for this are poorly constrained and may include variability in subducted material or slab/mantle conditions at depth. A large uncertainty results from use of generalized arc-wide C/S ratios, used in calculating C fluxes, and the paucity of data for remote arcs. Resolving C fluxes from subducted versus mantle or crustal (assimilated) C relies on C isotope ratios, which can vary spatially and temporally as a function of source or degassing processes. Therefore, when considering the deep C cycle and Cexchange between the interior and surface of the Earth, integrated studies with complete gas compositions, plume C/S and flux measurements, C isotopes and melt inclusions are needed.