The Magmatic Gas Signature of Pacaya Volcano, With Implications for the Volcanic CO2 Flux From Guatemala

Pacaya volcano in Guatemala is one of the most active volcanoes of the Central American Volcanic Arc (CAVA). However, its magmatic gas signature and volatile output have received little attention to date. Here, we present novel volcanic gas information from in-situ (Multi-GAS) and remote (UV camera) plume observations in January 2016. We find in-plume H2O/SO2 and CO2/SO2 ratios of 2-20 and 0.6-10.5, and an end-member magmatic gas signature of 80.5 mol. % H2O, 10.4 mol. % CO2, and 9.0 mol. % SO2. The SO2 flux is evaluated at 885 +/- 550 tons/d. This, combined with co-acquired volcanic plume composition, leads to H2O and CO2 fluxes of 2,230 +/- 1,390 and 700 +/- 440, and a total volatile flux of similar to 3,800 tons/d. We use these results in tandem with previous SO2 flux budgets for Fuego and Santiaguito to estimate the total volcanic CO2 flux from Guatemala at similar to 1,160 +/- 600 tons/day. This calculation is based upon CO2/total S (S-t) ratios for Fuego (1.5 +/- 0.75) and Santiaguito (1.4 +/- 0.75) inferred from a gas (CO2/S-t ratio) versus traceelement (Ba/La ratio) CAVA relationship. The H2O-poor and low CO2/S-t ratio (similar to 1.0-1.5) signature of Pacaya gas suggests dominant mantle-wedge derivation of the emitted volatiles. This is consistent with He-3/He-4 ratios in olivine hosted fluid inclusions (FIs), which range between 8.4 and 9.0 Ra (being Ra the atmospheric He-3/He-4 ratio) at the upper limit of MORB range (8 +/- 1 Ra). These values are the highest ever measured in CAVA and among the highest ever recorded in arc volcanoes worldwide, indicating negligible He-4 contributions from the crust/slab.