Plume composition and volatile flux of Nyamulagira volcano, Democratic Republic of Congo, during birth and evolution of the lava lake, 2014–2015

Very little is known about the volatile element makeup of the gaseous emissions of Nyamulagira volcano. This paper
tries to fill this gap by reporting the first gas composition measurements of Nyamulagira’s volcanic plume since the onset of its
lava lake activity at the end of 2014. Two field surveys were carried out on 1 November 2014, and 13–15 October 2015. We
applied a broad toolbox of volcanic gas composition measurement techniques in order to geochemically characterize
Nyamulagira’s plume. Nyamulagira is a significant emitter of SO2, and our measurements confirm this, as we recorded SO2
emissions of up to ~ 14 kt/d during the studied period. In contrast to neighbouring Nyiragongo volcano, however, Nyamulagira
exhibits relatively low CO2/SO2 molar ratios (< 4) and a highH2O content (> 92%of total gas emissions). Strong variations in the
volatile composition, in particular for the CO2/SO2 ratio, were measured between 2014 and 2015, which appear to reflect the
simultaneous variations in volcanic activity.We also determined the molar ratios for Cl/S, F/S and Br/S in the plume gas, finding
values of 0.13 and 0.17, 0.06 and 0.11, and 2.3·10−4 and 1·10−4, in 2014 and 2015, respectively. A total gas emission flux of 48 kt/
d was estimated for 2014. The I/S ratio in 2015 was found to be 3.6·10−6. In addition, we were able to distinguish between
hydrogen halides and non-hydrogen halides in the volcanic plume. Considerable amounts of bromine (18–35% of total bromine)
and iodine (8–18%of total iodine) were found in compounds other than hydrogen halides. However, only a negligible fraction of
chlorine was found as compounds other than hydrogen chloride.