## 2 The Vanaatu Island Arc  
The New Hebrides Island Arc is located in the southwest Pacific (Figure 1). It is composed of a narrow chain of Neogene to recent volcanic islands originating from the eastward subduction of the Australian plate beneath the Pacific plate, with associated back-arc extension of the North Fiji basin on the Pacific side [38; 39]. The recent tectonic evolution of the arc is dominated by its collision with the D'Entrecasteaux Zone (DEZ) [40], which began 1.5-3 Ma ago near Epi island, and has been migratingnorthward as a result of the obliquity of the DEZ relative to the trench [40; 41]. The New Hebrides Island Arc is subdivided in four different segments, the northernmost being essentially submarine and poorly studied. The other three include the central and south segments (CVS and SVS, respectively) and the high-Mg andesite volcanic segment (HMAVS) [42]. Vanuatu extends over the CVS and SVC, for about 700 km. It consists of a partly emergent ridge with an average width of 200 km, that overlies a steeply eastward-dipping subducted slab [42]. The DEZ collision triggered the detachment of the Australian plate lithosphere beneath the central part of Vanuatu. However, this detachment is less pronounced in the Northern part of the Vanuatu arc (due to a westward and upward invasion of the Indian-type MORB mantle) [42] where the island of Gaua is located. Gaua, also named Santa Maria, represents the emerged part of a 3000 m-high and 40 km-wide composite volcano [43]. It consists of a little-dissected symmetrical cone, about 25 km-across, truncated at an elevation between 500 m and 690 m (a.s.l.) by an \(8.5\times 6\) km central caldera, filled by a large crater lake (Lake Letas; Figure 1). The active cone of Mt. Garet rises in the middle of Lake Letas. Mt. Garet (797 m, a.s.l.) has two distinct craters. Since 1991, the activity has clustered on the southeast crater, which was also the site of the 2009-2010 explosions [44] and the survey location for the in-situ volcanic gas measurements reported here. A previous study [45] on the geology of the volcano identified a thick pile of basaltic and andesitic lava flows with associated rubble horizons and beds of red cinders or agglomerates. [PERSON] and [PERSON] [45] also recognized an extensive cap of ash surrounding the caldera. On the flanks, small parasitic cones and fissure-fed lava flows are distributed along fractures concentric to the caldera rim. The morphology of vents and lava surfaces mantling the lower slopes suggest that these peripheral extrusions are very young. Analysis of 2009-2010 lavas, products of the last major eruption, revealed identical compositions to those previously reported in literature [43; 45; 46]. The occurrence of basaltic andesite and basaltic trachyandesite volcanites with highly calcite plagioclase suggest that the 2009-2010 eruption of Gaua was fed by an alkaline (K-rich) magmatic source, typical of the northern part of the Vanuatu arc [44]. Figure 1: On the left: illustration of the overall tectonic context of the Vanuatu Island Arc, with currently active volcanoes and their location marked in red triangles. In the center: digital elevation model (DEM) of the island of Gaua; land features above 700 m (a.s.l. ), including Mount Garet’s summit crater, are highlighted in red. On the right: photos of Mount Garet and its currently degassing southeast crater (December 2019); photos of National Geographic property. The 2009-2010 Eruption of Mount Garet (Gaua)  
According to local inhabitants, the 2009-2010 eruption of Mount Garet was the strongest witnessed since 1963. [PERSON] et al. [44] assessed magma composition, lithology of volcanites, ash mineralogy, SO\({}_{2}\) mass discharge, halogen release and lake water chemistry, and provided a comprehensive understanding of the 2009-2010 sequence of eruptive events described below. A series of mild explosions on 29 September 2009, marked the beginning of a year-long eruption at Mount Garet.