**4. Results**

#### *4.1. Fumarolic Gas Compositions*

Table 1 reports the H2S-normalized molar ratios as well as the overall molar compositions of fumarolic gases from the di fferent active vents of La Soufrière measured in 2016 and 2017. The bulk molar percentages for H2O, CO2, H2S, SO2 and H2 are reported only when water was successfully determined using an external Rh sensor. We observe that gas compositions vary significantly as a function of the gas exit temperature but also the spatial location of the vents. Water vapour greatly prevails (∼86–97%) in all gas mixtures emitted at above or near the water boiling temperature (96 ◦C) at ambient elevation (CS, NAPN), but is typically depleted in the 'colder' (40–60 ◦C) emissions (NAP1, NPE1, NPE2 and the peripheral TY and BLK1) due to shallow steam condensation in the ground. Carbon dioxide is the second most abundant component, followed by H2S. Figure 3 illustrates clear co-variations of CO2, H2S, SO2 and H2 in cross-sections of the volcanic plumes, as recorded with MultiGAS. CO2/H2S ratios display a relatively restricted range (2.9–6.5) in the hottest fluids but also in cooled emissions from Gou ffre-56, except for NaPN (Figure 5b), while more variable and higher values (up to 190) in the 'coldest' gas emissions (NA1, NF2, BLK1 and TY). SO2/H2S ratios vary by three orders of magnitude among the di fferent fumaroles and are generally higher in emissions from the east-southeast sector of the dome (G56, NAP1, NAPN and NPE2). Finally, both H2/H2S and H2/H2O ratios tend to be about 10 times higher in cooler than in hotter gas emissions (Table 1), supporting the idea of simultaneous fractionations due to partial water condensation and sulfur loss prior to gas exit. Figure 5 provides further insight into the compositional di fferences and temporal evolution of La Soufrière fumaroles in 2016 and 2017 compared to previous periods.

Figure 5a shows the fumarole compositions in an H2O-CO2-Stot ternary diagram. Displayed here are only our May 2016 gas samples in which the H2O molar proportion was accurately determined with an external Rh sensor. The data are compared with the 2012 MultiGAS dataset [12], unpublished data for Col de l'Echelle fumarolic emissions during the 1976 eruptive crisis (P. Allard, in prep.) and high-temperature (720 ◦C) SO2-rich gas collected in 1996 from extruding andesite in nearby Montserrat island [9]; the latter is taken as a reliable proxy for the andesitic magmatic end-member at La Soufrière. The diagram reveals a relatively restricted compositional domain for gas emissions from the major fumarolic vents (TAS, SCS and G56) in 2012–2017, at least in terms of H2O/Stot ratios. Instead, 'colder' gas emissions from the TY, BLK1 and NA1, as well as NPE1 and NPE2 vents, display widely di fferent and variable H2O/Stot and CO2/Stot ratios.

**Table 1.** Molar gas ratios in fumarolic emissions from La Soufrière volcano measured with MultiGAS in 2016-2017. Coordinates and gas temperature ranges are given for each fumarole. Overall molar compositions are reported only when H2O was determined.


**Table 1.** *Cont.*


**Figure 5.** Gas compositions and trends in ternary diagrams (**a**) H2O-CO2-Stot and (**b**) CO2-SO2-H2S. Coloured circles represent MultiGAS-derived molar ratios of La Soufrière fumaroles in the period 2012–2017. Grey triangle: Montserrat-type magmatic end-member. Grey crosses: La Soufrère fumarolic gases during the 1976 eruptive crisis (see text). Dashed blue and red lines delineate the compositional ranges of CS fumarole obtained from direct sampling during the period 1997–2003 and 2004–2017, respectively.

Figure 5b provides further insight into the spatial and temporal variations of sulfur species in a ternary diagram CO2-SO2-H2S. From 2012 to 2016, one observes that fumarolic gases from SC and to a lesser extent NAPN and G56 display large temporal variations in SO2/H2S ratio at relatively steady CO2/H2S ratio, along a trend that extends from the SO2-rich magmatic end-member (left corner) towards strongly SO2-depleted samples with very low SO2/H2S ratio. Such a trend is best explained by variable SO2 scrubbing in the hydrothermal liquid water. Gas emissions from Tarissan are systematically impoverished in SO2 through this process. Colder gas emissions from TY, BLK1, NA1 and generally NF1 and NF2 vents are not only strongly depleted in SO2 but also variably impoverished in H2S, as shown by their plot on a second trend of increasing CO2/H2S ratios. Such a trend strongly suggests a variable but extensive loss of H2S in the volcanic ground prior to gas exit. Only fumarole NPE1 in May 2016 deviates from this trend, but we cannot exclude an influence of the measuring conditions.

#### *4.2. Fumarolic Gas Fluxes*

CO2 and H2S concentrations typically exceed tens of ppmv in the core of the volcanic plumes and progressively decrease toward the plume margins (Figure 4b–c). Single fluxes of H2S and CO2 at each vent were determined from the respective ICAs values and the wind speed. Total gas fluxes were then computed by scaling the overall gas composition to either the H2S flux or the CO2 flux. We found that total gas fluxes derived from the H2S flux tend to be lower (by up to ~70%) than those derived from the CO2 flux. In addition to a more conservative behavior of CO2, compared to more reactive H2S, such a discrepancy most likely results from the slower response of the electrochemical H2S sensor compared to the infrared CO2 sensor [50]; while the latter is able to detect rapid concentration changes during a plume transect, the H2S sensor needs comparatively more time to reach a full read at each position and, therefore, tends to provide smoothed concentration profiles. Therefore, our flux calculations were safely based on single determinations of the CO2 flux at each vent. The H2S flux was inferred by multiplying the CO2 flux by the H2S/CO2 weight ratio.

Table 2 reports the computed CO2 and H2S gas fluxes from La Soufrière in 2016–2017 and compares them to previous data. Note that flux data in 2006 and 2012 [12] were based on the H2S flux measured with either MultiGAS (2012) or a specific H2S sensor (2006) and, therefore, represent minimum figures. Fumarolic steam, calculated for all the three vents only on May 2016 (and thus not displayed in Table 2), contributes a predominant fraction (75–203 <sup>t</sup>·d−1) of the total gas flux emitted by La Soufrière volcano over the past decade, followed by CO2 (2–18 <sup>t</sup>·d−1) and H2S (1–4 <sup>t</sup>·d−1). The greater variability of H2O fluxes, compared to other gas fluxes, simply reflects the larger uncertainty in H2O determination due to both high ambient humidity on top of La Soufrière (Rh close to 100% [12]) and occasional partial steam condensation on cold surfaces at the inlet of analytical instruments. The lack of an external water sensor in our MultiGAS setting after the May 2016 campaign prevented reliable calculation of H2O/H2S ratio in the fumarolic emissions (Table 2). SO2 and H2 gas fluxes are in the order of 10−<sup>2</sup> and 10−<sup>4</sup> t·d−<sup>1</sup> and contribute negligibly to the total gas output. At the TAR crater, fumarolic degassing appears relatively steady over time. Instead, since 2012, the total gas flux from SC, the historically most active vent, tended to decrease, whereas in the same time G56 displayed a noticeable flux increase accompanying its progressive reactivation since 2007 [10]. From March 2012 to October 2017, gas fluxes from G56 have varied from below detection limit to values that are comparable to those at SC and TAR. As a whole, we find that the total gas discharge from La Soufrière measured after 2016 was approximately equivalent to that determined in March 2012 (see Section 5.3).

#### *4.3. Patterns of Fracturing and Shallow Ground Deformation*

Among the numerous fractures dissecting La Soufrière lava dome (Figure 1b–c), 12 have been monitored since 1995 by OSVG using extensometers [30]. The dataset collected until 2017 reveals that some fractures displayed systematic trends in extension over the last 20 years, whereas others displayed no extension or even a contraction. Figure 6f shows the displacement vectors computed from the extensometric data set since 2007, with particular focus on deformation that a ffected the fractures hosting most of the fumarolic activity. The main fractures of Napoléon (NAP1, in Figure 6), Faille du 30 Août (F303, in Figure 6), Breislack (BLK1, in Figure 6), 8 July 1976 (F8J1, in Figure 6), all show an extension pattern since 1995. For these fractures, three main temporal phases of extension can be identified (Figure 6d,e): (i) a period of moderate extension from 1995 to 1999, coinciding with the initial seismogenic and degassing phase of the ongoing unrest; (ii) a period of no extension or minor contraction between 1999 and 2004; then (iii) a new period of extension, at a much higher rate, from 2004 to 2017. Phases of increased extension were previously attributed to a pressure increase upon the solid dome rocks that host the upper hydrothermal reservoir of La Soufrière [30].

Here we find that, since 2004, the largest displacement change has a ffected the WNW-ESE oriented Breislack fracture (Figure 1b). The maximum opening rate averages 2.49 ± 0.18 mm·yr<sup>−</sup><sup>1</sup> in its upper part at the Napoleon Crater (blue arrow in Figure 6f), with a total opening of 40.54 mm over the past 10 years, and 67.25 mm since June 1995. This extensional trend along the Breislack fracture coincides with the renewal (since 2007) of fumarolic degassing along this structure cutting the lava dome, especially marked at the G56 vent. Increased extension along this fault zone propagated more recently to other summit structures such as Peyssonnel Crater and Dupuy Crater, where thermal anomalies and di ffuse degassing started to become observed since 2016 (OVSG-IPGP Report). Jacob et al. [30] modelled the displacements of four of the most important fractures and suggested that the main source of displacement can be accounted by a hydrothermal reservoir of ellipsoidal shape centered within the lava dome, at ~100 m depth, undertaking pressure changes.


#### *Geosciences* **2019**, *9*, 480

**Table 2.** CO2 and H2S gas fluxes at La Soufrière in 2016–2017 computed from both CO2 and H2S ICAs and CO2/H2S average molar ratios (see Section 4.1). Dry gas

**Figure 6.** Compared temporal variations of fumarolic gas fluxes, seismicity and dome's fault extensometry at La Soufrière between 1992 and 2017. (**a**) Total dry gas flux and (**b**) and H2S and CO2 fluxes calculated from both H2S and CO2 ICAs (for 2006 and 2012 we used only H2S ICA-derived values). (**c**) Seismicity and (**d**,**<sup>e</sup>**) extent of opening or closing of fractures cross-cutting La soufrière summit dome revealed by extrensometric survey. (**f**) Vectors and amplitudes of fracture width variations and location of active fumaroles (yellow circles) shown in Figure 1. BLK1: Gouffre Breislack, DOL1: Fracture Dolomieu Est, DOL2: Fracture Dolomieu Ouest, DUP1: Gouffre Dupuy Ouest, DUP2: Gouffre Dupuy Est, F302: Faille du Nord-Ouest, F303: Faille 30 Août Bas, F8J1: Faille 8 Juillet 1976, FNO1: Fente du Nord, FNW1: Faille du Nord-Ouest, LCX1: Fracture Lacroix, NAP1: Cratère Napoléon, PEY1: Gouffre Peyssonnel ([28,30], and this work).
