**5. Discussion**

As previously mentioned, the actual source mechanism of the current degassing unrest and associated phenomena at La Soufrière still remains unclear. Hence it remains of key interest to decipher whether the current degassing unrest and associated phenomena may result from relatively shallow processes operating in the volcanic pile and the hydrothermal system or, instead, imply a deeper source mechanism involving the magma reservoir at about 5.6–8.5 km depth (e.g., [32,40,41]) or even a

shallow magmatic intrusion [10,36,51]. Below we examine these possibilities in light of our results for La Soufrière gas emissions in 2016–2017 and their relations with both hydrothermal fluid circulation and geophysical signals.

#### *5.1. 2016–2017 Gas Fluxes in the Context of La Soufrière Degassing Unrest*

The magmatic origin of CO2, S, HCl and He in La Soufrière fumarolic gases (e.g., [12] and references therein) and their increased emission from central vents of the summit lava dome since 1992 provides evidence of an enhanced release of magma-derived volatiles with respect to the 1984–1992 quiescent period (Section 2). As highlighted above and in Table 2, the first in situ measurements of fumarolic gas fluxes indicated a possible factor 3 increase in the emission rate of CO2, H2S and total dry gas from 2006 to 2012 [12], in broad agreemen<sup>t</sup> with an estimate based on thermal imaging [52]. If we consider gas fluxes derived from H2S ICAs, our 2016–2017 data (Table 2) reveal a smaller but continuing increase of the overall gas discharge with respect to 2006. When compared to MultiGAS-based results in 2012, our data also reveal a spatial redistribution of the fumarolic gas pathways and emission rates in the shallow part of the edifice: the total gas flux is broadly modulated by increasing degassing at G56, whose magnitude has become of same order as that from TAS and CS. Moreover, because we have no flux data for the new fumaroles (e.g., NapN and NPE2) and new steaming ground that have extensively developed in the northern sector of the dome and along the Breislack fault, it is definitely possible that the overall gas flux from La Soufrière was increased in 2016–2017 compared to 2012. This would be fully coherent with the opening of new vents and the widening of fractures in the north-east sector of the lava dome since 2004, at an extension rate greater than for the 1995–2003 period. Given that fracture opening would have increased the permeability of that part of the volcanic edifice, a higher fumarolic gas flux in 2016-2017 would imply a higher fluid pressure in the hydrothermal system. This is supported by the increase in seismic energy release in 2017 then in early 2018 [22,28] and by the continued expansion of the thermal ground anomaly and degassing at the Napoleon fumarolic field.

#### *5.2. Insight from the Fumarolic Gas Compositions*

As shown by the ternary plots of Figure 5, the chemical composition of La Soufrière fumarolic gases in 2016–2017 and in previous years evidences two main trends with respect to a Montserrat-type magmatic compositional end-member and La Soufrière gases during the 1976 phreatic crisis. As discussed below, these two trends can be interpreted in terms of two main processes.

#### *5.3. SO2 Scrubbing in the Hydrothermal System*

SO2 scrubbing in liquid water is a common process at volcanoes displaying hydrothermal activity (e.g., [53,54]). During gas-water interactions magma-derived SO2, which is much more soluble in liquid water than coexisting H2S and CO2, is e fficiently removed from the gas phase through both hydrolysis and disproportionation reactions: 4SO2 (g) + 4H2O (aq) = H2S (aq) + 3H2SO4 (aq) and 3SO2 (g) + 2H2O (aq) = S (s,l) + 2H2SO4 (aq). SO2 scrubbing in two-phase hydrothermal systems with moderate temperature (100–300 ◦C), such as occurs beneath La Soufrière (e.g., [22,36,43]), thus leads to surface gas emissions essentially composed of CO2 and H2S besides water vapor. The observed variations of both CO2/STOT and SO2/H2S ratios in La Soufrière fumaroles (Figure 5a,b) with respect to the hypothetical magmatic pole (Montserrat) typically demonstrate variable but extensive scrubbing of SO2 in the liquid water phase of the hydrothermal system.

It is worth noting that the "hot" (~ 95 ◦C) NAPN fumarole keeps relatively high SO2 proportions (Figure 4b) while being but is clearly a ffected by H2S loss (Figure 4a). Around the fumarolic outlet, encrustations with multiple colored zoning can be observed, in addition to elemental sulphur. Although not analyzed yet, these encrustations are very likely determined by the precipitation of sulfide minerals, depleting the fumarolic fluid in H2S. Reed and Palandri [55] showed that dilution and cooling of a hydrothermal fluid by cold water causes precipitation of metals into sulfide minerals, following the destabilization of chloride complexes (see run 9 by these authors). We thus sugges<sup>t</sup> that NAPN

fumarole is thermally and chemically bu ffered by near-surface interaction with very shallow cold groundwater, circulating in a very limited zone in the fumarole surroundings.

We emphasize that almost all gases were measured just at the vent exit, which excludes the possibility that higher SO2/H2S ratios in the peripheral emissions could result from an enhanced air oxidation of H2S into SO2 [56]. Such a possibility might only apply to G56 and TAS emissions that arise from a quite deep open vent and lake, respectively. Otherwise, the observed spatial contrast in SO2/H2S ratios between central and peripheral summit vents on La Soufrière dome strongly suggests greater gas interaction with liquid water and, thus, enhanced SO2 scrubbing beneath the central summit vents, compared to the more peripheral vents. This chemical contrast was systematic in 2016–2017. However, we note that the extent of SO2 scrubbing can vary over time: this is shown, for instance, by the evolution of G56 emission during 2016 or by the variations at both G56 and CSN from 2012 to 2016. On a longer-term basis, Figure 5b also reveals a potential trend of increasing SO2 scrubbing over time at CSN when we compare the domains of variation (dotted areas) of fumarolic gases directly sampled at that vent in 2004–2017 with respect to 1997–2003.

#### *5.4. Sulfur Precipitation in the Volcanic Ground*

Gas emissions from the low-temperature and/or low-flux vents (TY, BLK1, and NPE2, NF1 and NA1) display a specific trend of H2S depletion with respect to the hottest fluids, reflected in their much higher CO2/H2S ratios (Table 1 and Figure 5b). The degree of H2S depletion depends on the exit gas temperature but can vary over time at a given site, as illustrated for instance by temporal CO2/H2S changes at NF2. We can discard that this pattern may result from partial H2S oxidation in air-filled fractures, forming SO2, since most of these low-T emissions are also depleted in total sulfur (higher CO2/STOT ratios; Figure 5a). Metastable precipitation of elemental sulfur within the volcanic ground through the gaseous reactions SO2 (g) + 2H2S (g) = 3S (native) + 2H2O (g) and 2H2S (g) + O2 (g) = 2S (native)<sup>+</sup> 2H2O(g) can deplete low-T volcanic gas in both H2S and SO2, but is likely to be of secondary importance at La Soufrière given the initial large predominance of H2S over SO2 in the hottest and SO2-richest fumaroles (the least a ffected by SO2 scrubbing). Field observations of abundant pyrite (FeS2) in the shallow ground around the low-T vents rather sugges<sup>t</sup> that H2S depletion in these low-temperature and/or low-flux emissions mainly results from superficial gas-water-rock interactions involving ferrous iron in the wet volcanic ground. Reaction 2H2S (g) + FeO (rock) + 1 2 O2 = FeS2 (rock) + 2H2O (g,l) (e.g., [22,53,57,58]) well illustrates the bu ffering e ffect played by pyrite and the pyroclastic rock in the hydrothermal system. The presence of sulfur deposits at CS and other hot vents (e.g., NAPN) sugges<sup>t</sup> a limited but additional e ffect of direct sulfur precipitation through rapid H2S oxidation in air.

#### *5.5. Compositional Gas Variations and Geophysical Signals*

Since its marked reactivation at the onset of degassing unrest in 1992, volcanic seismicity at La Soufrière has remained essentially confined within a shallow depth interval (between 1 and 4 km below the summit; Figure 2b), with no peculiar temporal trend in the hypocenter distribution of seismic events. Together with a surface deformation field limited to the lava dome, as revealed by extensometric survey since 1995 (Figure 6d–f; see also ground deformation velocities determined by the Global Navigation Satellite System, GNSS, in [22]), this strongly suggests that relatively shallow processes, operating in a rather constant volume, have been responsible for both renewed seismicity and ground deformation over the past decades. Hydrothermal pressurization in this seismic volume could well account for the observed phenomena. In particular, the localized and minor deformations of the dome, with a fault opening rate of 1–2 mm·y<sup>−</sup>1, are compatible with both hydrothermal pressure increase and local gravitational basal spreading of the southern sector of the lava dome ([30] and GNSS data in [22]). Preferential extension along the Breislack fault system in the past decade does coincide with shallow seismicity concentrating beneath this eastern sector of the lava dome (Figure 2c). Note, however, that these limited phenomena do not discard the possibility of a deeper triggering mechanism of the current unrest. In particular, it is much possible that increased degassing and geophysical signals recorded at La Soufrière since 1992 have resulted from an increased gas transfer from the crustal magma reservoir at 6–7 km depth, or even a new replenishment of that reservoir. Allard et al. [12] assessed that the fumarolic gas fluxes measured in 2012 could be accounted for by the bulk degassing of about 10<sup>3</sup> <sup>m</sup>3·d−<sup>1</sup> of parental basaltic magma feeding La Soufrière andesitic magma reservoir. Taken as representative for the degassing unrest phase from 1998 until present, such a rate would imply the cumulative degassing of ~6.5 × 10<sup>6</sup> m<sup>3</sup> of dense magma. Though an upper limit, this first-order estimation is intended to highlight that a magma intrusion of a few millions of cubic meters, sustaining the recent and current fumarolic gas fluxes, would have hardly escaped detection by the monitoring network of the OVSG-IPGP in terms of associated seismic and geodetic signals. Unless we imagine a series of small volume, sill-like intrusions emplaced at depth. Based on more recent gas data, Moretti et al. [22] estimated that a magmatic intrusion of 2.7 × 10<sup>6</sup> m<sup>3</sup> might have been responsible for the intense seismic crisis between February and late April 2018.

Above, we showed that two distinct periods can be recognized in La Soufrière degassing sequence over the past decades. We highlight here that the transition between these two periods coincides with the gradual but sustained widening of the dome fractures where new fumarolic activity has developed (NAP1, F8J1, BLK1, Figure 6d). As shown in Figures 5 and 6b, our MultiGAS measurements in 2016–2017 further verify a concomitant evolution of the fumarolic gas fluxes since 2004. However, they also reveal rapid (short-term) compositional gas changes (between March 2012 and May 2016; Figure 5), as well as at newly active vents (G56, NAP1, NAPN). We argue that such rapid gas changes do not result from deeply sourced mechanism but, instead, from fractionation processes a ffecting the fumarolic gases during their shallow paths. In particular, as outlined above, the quite short-lived variations of CO2/H2S and SO2/H2S ratios in various fumaroles between 2012 and 2017 but also during 2016–2017 (e.g., at G56; Figure 5) strongly support the idea of a key control of gas compositions at the di fferent vents by the degree of SO2 scrubbing in hydrothermal water and late-stage sulfur precipitation in the volcanic ground (Figure 5). Below we show how the variability in fumarolic gas compositions relates to the shallow circulation of hydrothermal fluids inside the lava dome.

#### *5.6. Spatial Relationships of 2016—2017 Gas Compositions with Hydrothermal Fluid Circulation*

The main fumarolic activity on La Soufrière currently extends along the upper sections of both the Breislack fault and the 30 August 1976 eruption fault (Figure 1b–c). The TAR pit crater, at the center of the dome, marks the intersection point of these two discontinuities. We present here two slices (Figure 7b,c) of the 3D electrical conductivity model obtained by Rosas-Carbajal et al. [29] for the period 2003–2011, in order to examine the relation between the pathways connecting the surface fumarolic vents to the hydrothermal reservoir and their influence upon the fumarolic gas compositions and fluxes. We have additionally superimposed the topographical survey of the cavities and pits on the summit plateau [59]; these reach depths of a few tens of meters before becoming narrow fissures or being obstructed by fallen blocks (Figure 7a). In particular, TAR crater is a 20–30 m wide and ≥100 m deep vertical pit that, since 2001, has been hosting a boiling acid pond whose level has varied between 60 and 100 m depth below the rim (average depth: 82 ± 9 m, n = 173 measurements from 26 November 2002 to 26 June 2017 [28]). In March 1993 a team of speleologists had also explored the CS pit crater [59] and found it to host an acid lake with bubbling gas at ~140 m depth (pH ~1.82, 74,060 ppm of SO4 2− and 79.52 ppm of Cl<sup>−</sup>; OVSG, G. Hammouya, personal communication). As previously mentioned, an acid boiling pond (pH <sup>~</sup>−0.5, see Section 1), associated with geyser-like pulsating jets of boiling water, temporarily surfaced in CS vent as well between April 1997 and December 2004 (see Figure S4 in [29]).

One first important observation from the 3D electrical tomography of La Soufrière lava dome is that the upper boundary of the conductive hydrothermal region closely corresponds to the measured depth of the boiling acid water lake persisting in TAR pit crater and temporarily observed at the bottom of CS (Figure 7b,c). This means that the upper conductive boundary tracks the upper surface of the main body of thermal liquid water, saturated in dissolved sulfate and chlorine scrubbed from the magma-derived gas upflow. The TAR acid pond has an electrical conductivity of ~25 S/m at ambient temperature. Since its actual temperature is ~90 ◦C, it can be expected that the liquid saturating the extremely conductive region has an electrical conductivity of at least 50 S/m (e.g., [60]). This acidic fluid has infiltrated the pore space of the hydrothermally altered and fractured host-rock of the lava dome and likely accumulated in the resulting dome's cavities. Obviously, the potential water storage capacity of such cavities is larger at the centre of the dome than in its peripheral sectors. This inference is supported by the observations of recurrent exurgence (see Figure 1b) of hot acidic hydrothermal water, forming mud flows, during La Soufrière phreatic eruptions in 1797–1798, 1836–1837, 1956, and 1976–1977 [6,29,34].

**Figure 7.** (**a**) Northwest-southeast cross-section of the La Soufrière lava dome with geometries of the main vents and cavities (from [59]). (**b**) West-East and (**c**) South-North slices of measured electrical conductivity with main isoconductivity contours shown for 1, 0.1, 0.001, and 0.001 S·m<sup>−</sup><sup>1</sup> from deep red to blue, respectively, as well as superimposed vents geometries of TAR, G56, BLK1 and CSS. Arrows with dashed lines represent hypothetical rising gas paths crossing the >0.1 S·m<sup>−</sup><sup>1</sup> hydrothermal water body and reaching TAR and CSS fumarolic vents. Instead, gases emitted from G56 and rising along the Breislack fault structure apparently bypass the main hydrothermal body (modified from [29]).

The highly conductive body detected below the southern part of the summit extends southwards and downwards to a few hundred meters depth below the base of the dome, but also rises vertically along a structural contact to reach the surface at the level of the Galion thermal springs (Figure 1b) [29]. This is in agreemen<sup>t</sup> with geochemical data that sugges<sup>t</sup> a direct connection between the fluids reaching the dome summit and the fluids that ultimately feed these thermal springs [10,27]. It is noteworthy that the electrical conductivity is much smaller (<0.05 <sup>S</sup>·m<sup>−</sup>1) in the upper region extending between the bottom of the central pits and the surface of the lava dome, where no stagnant liquid water occurs and where volcanic gas pathways are instead located (Figure 7). This feature, together with the high electrical conductivity values of the saturating liquid, suggests that the electrical conductivity of the host rock medium is mainly dependent on its degree of liquid saturation (e.g., [61] and references therein). This strong dependence seems to be particularly important in volcanic rocks, as shown by Ghorbani et al. (2018). The mid-range electrical conductivity values (>0.1 <sup>S</sup>·m<sup>−</sup>1) found in this region most likely correspond to partially fluid-saturated rocks that have been altered by the intense hydrothermal activity and thus contain abundant clay-rich minerals which contribute to an increased bulk electrical conductivity. On the other hand, the region of the dome where G56 is located is

characterized by a rather resistive massive dome rock (<0.1 <sup>S</sup>·m<sup>−</sup>1). This indicates that host rocks in that sector are much less altered and not liquid-saturated (Figure 7b,c), as actually observed by speleologists who explored G56 [59].

Fumarolic gas from the CSC, the single vent regularly sampled and analyzed over the past 28 years, has long been considered to be the most representative of the mixed magmatic-hydrothermal end-member fluid at La Soufrière [10,12]. The boundaries of the liquid hydrothermal system, imaged through the 3D electrical tomography ([29] and our Figure 7b,c), as well as the depth of the explored pit craters in La Soufrière lava dome [59], offer new insight into the interplay between the shallow hydrothermal system and fumarolic degassing at the surface. Based on this imaging and our 2016–2017 gas results (Figure 5), here we demonstrate that fumarolic gases from both CS and TAR central pit craters strongly interact with the acid liquid water of the hydrothermal system, directly positioned at 80–100 m under these vents, and thereby undergo intense water/gas interactions. Gas scrubbing in fact accounts for their low SO2 content (dissolved as SO4<sup>2</sup>− in the hydrothermal water) with respect to H2S (much less soluble in acidic boiling water). Instead, higher SO2/H2S ratios in fumarolic gas emitted from the more peripheral G56 pit (Figure 1c), located on the NE border of the liquid hydrothermal system (Figure 7b,c), point to weaker (albeit variable) SO2 scrubbing and thus more limited fractionation of sulfur species at that vent. As a matter of fact, the gas pathway beneath G56 is characterized by a weaker electrical conductivity anomaly, indicative of a small proportion of thermal liquid water in the rock column underneath. It is notable that the distinct chemical signature of fumarolic gas at that vent has persisted from 2012 through 2017, even though with temporal oscillations (Figure 5). Furthermore, our analyses of Napoleon vents (NAPN, NAP) and new peripheral fumaroles in 2016 (NPE1 and NPE2) reveal that other gas emissions markedly richer in CO2 and SO2 than CS fumarolic fluid are simultaneously active in the N-NE sector of the lava dome. Therefore, in contrast to previous exp1ectations, we argue that current gas emissions from peripheral vents on La Soufrière lava dome (G56 and other new smaller vents of the Napoleon fracture system) may be more closely representative of the unfractionated magmatic-hydrothermal gas end-member than emissions from the longer-lived, more central vents of the dome (SC and TAR). Based on that conclusion, we thus recommend that gas compositions and fluxes from peripheral vents of La Soufrière lava dome become carefully monitored in the future.
