First In-Situ Measurements of Plume Chemistry at Mount Garet Volcano, Island of Gaua (Vanuatu)

Recent volcanic gas compilations have urged the need to expand in-situ plume measurements
to poorly studied, remote volcanic regions. Despite being recognized as one of the main volcanic
epicenters on the planet, the Vanuatu arc remains poorly characterized for its subaerial emissions and
their chemical imprints. Here, we report on the first plume chemistry data for Mount Garet, on the
island of Gaua, one of the few persistent volatile emitters along the Vanuatu arc. Data were collected
with a multi-component gas analyzer system (multi-GAS) during a field campaign in December 2018.
The average volcanic gas chemistry is characterized by mean molar CO2/SO2, H2O/SO2, H2S/SO2 and
H2/SO2 ratios of 0.87, 47.2, 0.13 and 0.01, respectively. Molar proportions in the gas plume are
estimated at 95.9 11.6, 1.8 0.5, 2.0 0.01, 0.26 0.02 and 0.06 0.01, for H2O, CO2, SO2, H2S and
H2. Using the satellite-based 10-year (2005–2015) averaged SO2 flux of ~434 t d􀀀1 for Mt. Garet,
we estimate a total volatile output of about 6482 t d􀀀1 (CO2 ~259 t d􀀀1; H2O ~5758 t d􀀀1; H2S ~30 t d􀀀1;
H2 ~0.5 t d􀀀1). This may be representative of a quiescent, yet persistent degassing period at Mt.
Garet; whilst, as indicated by SO2 flux reports for the 2009–2010 unrest, emissions can be much
higher during eruptive episodes. Our estimated emission rates and gas composition for Mount Garet
provide insightful information on volcanic gas signatures in the northernmost part of the Vanuatu Arc
Segment. The apparent CO2-poor signature of high-temperature plume degassing at Mount Garet
raises questions on the nature of sediments being subducted in this region of the arc and the possible
role of the slab as the source of subaerial CO2. In order to better address the dynamics of along-arc
volatile recycling, more volcanic gas surveys are needed focusing on northern Vanuatu volcanoes.

This research was conducted as part of the Trail by Fire II—Closing the Ring Project (PI: Y. Moussallam)
funded by the National Geographic Society (grant number CP-122R-17), the Rolex Awards for Enterprise and the
French national Research Institute for Development (IRD). J.L. also acknowledges travel funding support from
Ministero dell’istruzione, dell’università e della ricerca (MIUR;) under grant n. PRIN2017-2017LMNLAW).

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