Prodigious emission rates and magma degassing budget of major, trace and radioactive volatile species from Ambrym basaltic volcano, Vanuatu island Arc

Abstract

Ambrym volcano, in the Vanuatu arc, is one of the most active volcanoes of the Southwest Pacific region, where persistent lava lake and/or Strombolian activity sustains voluminous gas plume emissions. Here we report on the first comprehensive budget for the discharge ofmajor,minor, trace and radioactive volatile species fromAmbrymvolcano, as well as the first data for volatiles dissolved in its basaltic magma (olivine-hosted melt inclusions). In situ MultiGAS analysis of H2O, CO2, SO2 and H2S in crater rim emissions, coupled with filter-pack determination of SO2, halogens, stable and radioactive metals demonstrates a common magmatic source for volcanic gases emitted by its two main active craters, Benbow and Marum. These share a high water content (~93 mol%), similar S/Cl, Cl/ F, Br/Cl molar ratios, similar (210Po/210Pb) and (210Bi/210Pb) activity ratios, as well as comparable proportions in most trace metals. Their difference in CO2/SO2 ratio (1.0 and 5.6–3.0, respectively) is attributed to deeper gasmelt separation at Marum (Strombolian explosions) than Benbow (lava lake degassing) during our measurements in 2007. Airborne UV sensing of the SO2 plume flux (90 kg s−1 or 7800 tons d−1) demonstrates a prevalent degassing contribution (~65%) of Benbow crater in that period and allows us to quantify the total volatile fluxes duringmedium-level eruptive activity of the volcano. Results reveal that Ambrym ranks among the most powerful volcanic gas emitters on Earth, producing between 5% and 9% of current estimates for global subaerial volcanic emissions of H2O, CO2, HCl, Cu, Cr, Cd, Au, Cs and Tl, between 10% and 17% of SO2, HF, HBr, Hg, 210Po and 210Pb, and over 30% of Ag, Se and Sn. Global flux estimates thus need to integrate its contribution and be revised accordingly. Prodigious gas emission from Ambrym does not result from an anomalous volatile enrichment nor a differential excess degassing of its feedingbasalt: this latter contains relativelymodest dissolved amounts ofH2O(≤1.3wt%), CO2 (~0.10 wt%), S (0.075 wt%) and Cl (0.05 wt%), and its degassing under prevalent closed-systemconditions well reproduces the composition of emitted volcanic gases. Instead,we show that the gas discharge is sustained by a very high basalt supply rate of 25m3 s−1, from a large (~ 0.5 km3)magma reservoir probably emplaced at ~3.8 km depth below the summit caldera according to both the H2O-CO2 content of bubble-free melt inclusions and preliminary seismic data. Radioactive disequilibria in the volcanic gases constrain that this reservoir may be entirely renewed in about 240 days. The comparatively low magma extrusion rate requires extensive convective overturn of the basaltic magma column and recycling of the unerupted (denser) degassed magma in the plumbing system, in agreement with textural features of erupted products. Finally, our results suggest that the Indian MORB-type mantle source of Ambrym basalts is modestly enriched in slab-derived water and other volatiles, in agreement with the prevalent volcanoclastic nature of subducted sediments and their lower subduction rate under the central Vanuatu arc due to its collision with the D'Entrecasteaux Ridge.