Patrick Allard
email: pallard@ipgp.fr

Mike Burton

Georgina Sawyer

Philipson Bani

Degassing dynamics of basaltic lava lake at a top-ranking volatile emitter: Ambrym volcano, Vanuatu arc

Keywords: Ambrym, lava lake, infrared spectroscopy, volcanic gases, magma degassing dynamics, volatile fluxes Highlights •

Persistent lava lakes are rare on Earth and provide volcanologists with a remarkable opportunity to directly investigate magma dynamics and degassing at the open air. Ambrym volcano, in Vanuatu, is one of the very few basaltic arc volcanoes displaying such an activity and voluminous gas emission, but whose study has long remained hampered by challenging accessibility. Here we report the first high temporal resolution (every 5 sec) measurements of vigorous lava lake degassing inside its 300 m deep Benbow crater using OP-FTIR spectroscopy. Our results reveal a highly dynamic degassing pattern involving (i) recurrent (100-200 sec) short-period oscillations of the volcanic gas composition and temperature, correlating with pulsated gas emission and sourced in the upper part of the lava lake, (ii) a continuous long period (∼8 min) modulation probably due to the influx of fresh magma at the bottom of the lake, and (iii) discrete CO 2 spike events occurring in coincidence with the sequential bursting of meter-sized bubbles, which indicates the separate ascent of large gas bubbles or slugs in a feeder conduit with estimated diameter of 6±1 m. This complex degassing pattern, measured with unprecedented detail and involving both coupled and decoupled magma-gas ascent over short time scales, markedly differs from that of quieter lava lakes at Erebus and Kilauea. It can be accounted for by a modest size of Benbow lava lake and its very high basalt supply rate (~20 m 3 s -1 ), favouring its rapid overturn and renewal. We verify a typical basaltic arc signature for Ambrym volcanic gas and, based on contemporaneous SO 2 flux measurements, we evaluate huge emission rates of 160 Gg d -1 of H 2 O, ~10 Gg d -1 of CO 2 and ~8 Gg d -1 of total acid gas (SO 2 , HCl and HF) during medium activity of the volcano in 2008. Such rates make Ambrym one of the three most powerful volcanic gas emitters at global scale, whose atmospheric impact at local and regional scale may be considerable.

Introduction

Persistent lava lake activity, occurring at rare volcanoes worldwide, is a remarkable phenomenon where molten magma steadily supplied to an open reservoir degasses directly into the atmosphere from across its surface, while loosing little to no solid mass (e.g. Le [START_REF] Le Guern | Erta 'Ale lava lake: heat and gas transfer to the atmosphere[END_REF][START_REF] Tazieff | Permanent lava lakes: observed facts and induced mechanisms[END_REF][START_REF] Oppenheimer | Sulfur, heat, and magma budget of Erta 'Ale lava lake, Ethiopia[END_REF][START_REF] Oppenheimer | Pulsatory magma supply to a phonolite lava lake[END_REF]. Such an activity, which can persist for decades (e.g. at Kilauea and Erebus volcanoes) to more than a century (Nyiragongo and Erta'Ale volcanoes), is of great value in volcanological research as it exposes what is normally hidden within persistently active volcanoes: the innermost magma dynamics capable to sustain continuous gas and heat flow. Its study therefore permits direct comparison between measured geophysical and geochemical parameters and volcanic activity, allowing features such as magma convection, passive degassing, lava fountains or explosive gas bubbling to be understood in a quantitative framework.

Because degassing contributes most of the total mass output from a lava lake, key insights into the processes controlling this rare volcanic activity can be obtained by measuring the chemistry and flux of magmatic gas emissions. Gas fluxes provide quantitative constraints on the rates of magma supply and convection, while gas compositions provide constraints upon the degassing mechanisms and their source depth if the abundance and behaviour of volatiles in the magma are known (e.g. [START_REF] Allard | Spectroscopic evidence for a lava fountain driven by previously accumulated magmatic gas[END_REF][START_REF] Oppenheimer | Pulsatory magma supply to a phonolite lava lake[END_REF][START_REF] Beckett | Conduit convection driving persistent degassing at basaltic volcanoes[END_REF]. Efficient convective magma overturn in lava lake-conduit systems, with gravitational sinking of the unerupted degassed magma, is strongly required from heat and mass balances (e.g. [START_REF] Tazieff | Permanent lava lakes: observed facts and induced mechanisms[END_REF][START_REF] Kazahaya | Excessive degassing of Izu-Oshima volcano: magma convection in a conduit[END_REF][START_REF] Stevenson | Modelling the dynamics and thermodynamics of volcanic degassing[END_REF][START_REF] Harris | Mass flux measurements at active lava lakes: Implications for magma recycling[END_REF][START_REF] Oppenheimer | Sulfur, heat, and magma budget of Erta 'Ale lava lake, Ethiopia[END_REF][START_REF] Oppenheimer | Pulsatory magma supply to a phonolite lava lake[END_REF][START_REF] Beckett | Conduit convection driving persistent degassing at basaltic volcanoes[END_REF]. It is also supported by theoretical modelling [START_REF] Witham | Stability of lava lakes[END_REF] and cyclic crystal zonation in erupted solid products [START_REF] Moussallam | Megacrystals track magma convection between reservoir and surface[END_REF]. However, separate bubble flow is another mechanism able to carry heat and gas to a lava lake (Bouche et al., 2013) and to control its level oscillations through bubble accumulation at its top [START_REF] Patrick | Shallow driven fluctuations in lava lake outgassing (gas pistoning), Kilauea volcano[END_REF]. Variations in magma-gas supply rates, degassing modes and the style of convection actually determine a wide range in lava lake behaviour, from vigorous bubbling and overturn to quieter degassing and resurfacing, interrupted or not by recurrent gas outbursts (e.g. Le [START_REF] Le Guern | Erta 'Ale lava lake: heat and gas transfer to the atmosphere[END_REF][START_REF] Tazieff | Permanent lava lakes: observed facts and induced mechanisms[END_REF][START_REF] Oppenheimer | Sulfur, heat, and magma budget of Erta 'Ale lava lake, Ethiopia[END_REF][START_REF] Oppenheimer | Pulsatory magma supply to a phonolite lava lake[END_REF], and to cyclic upheaval and drain back due to gas-piston effects [START_REF] Edmonds | Vapor segregation and loss in basaltic melts[END_REF][START_REF] Patrick | Shallow driven fluctuations in lava lake outgassing (gas pistoning), Kilauea volcano[END_REF].

The composition and mass output of magmatic gases from a lava lake can be measured using up-to-date spectroscopic methods. In the past fifteen years, open-path Fourier transform infrared (OP-FTIR) spectroscopy has been demonstrated to be a remarkable tool for remotely quantifying magmatic gas compositions during effusive and explosive eruptions at high frequency and from a safe position (e.g. [START_REF] Allard | Spectroscopic evidence for a lava fountain driven by previously accumulated magmatic gas[END_REF]Burton et al., 2007a;[START_REF] Spina | Open-path FTIR spectroscopy of magma degassing processes during eight lava fountains on Mount Etna[END_REF]. Using infrared radiation emitted by molten lava, OP-FTIR absorption spectroscopy allows simultaneous determination of the principal components of hot volcanic gases: H 2 O, CO 2 , SO 2 , HCl, HF, CO, apart from H 2 and H 2 S. Therefore, this tool is particularly wellsuited to study the degassing of lava lakes. It has already been applied to study gas emissions from the exceptional long-lived lava lakes hosted by Nyiragongo on the East-African Rift (Sawyer et al., 2008a), Erta'Ale in Ethiopia (Sawyer et al., 2008b), Erebus in Antarctica [START_REF] Oppenheimer | Probing the magma plumbing of Erebus volcano, Antarctica, by open-path FTIR spectroscopy of gas emissions[END_REF][START_REF] Oppenheimer | Pulsatory magma supply to a phonolite lava lake[END_REF]Ilanka et al., 2015) and Kilauea in Hawaii [START_REF] Edmonds | Vapor segregation and loss in basaltic melts[END_REF][START_REF] Edmonds | Magma storage, transport and degassing during the 2008-10 summit eruption at Kilauea volcano, Hawaii[END_REF][START_REF] Patrick | Shallow driven fluctuations in lava lake outgassing (gas pistoning), Kilauea volcano[END_REF]. These four volcanoes, located on either hot spots or rift zones, are fueled by magmas ranging in composition from basalt (Kilauea, Erta'Ale) to nephelinite (Nyiragongo) and phonolite (Erebus), which is reflected in widely differing gas compositions. OP-FTIR spectroscopy has also been applied on two of the rare basaltic to andesitic volcanoes in subduction zones where lava lakes have been observed to recurrently form and persist: Masaya in Nicaragua [START_REF] Burton | Field measurement of CO 2 and H 2 O emissions from Masaya Volcano, Nicaragua, by Fourier transform spectrometry[END_REF] and Villarrica in Chile [START_REF] Sawyer | Gas and aerosol emissions from Villarrica volcano, Chile[END_REF]. However, until now comprehensive OP-FTIR investigation of the degassing dynamics of a lava lake has been achieved only on Erebus [START_REF] Oppenheimer | Pulsatory magma supply to a phonolite lava lake[END_REF]Ilanka et al., 2015) and Kilauea [START_REF] Edmonds | Vapor segregation and loss in basaltic melts[END_REF][START_REF] Edmonds | Magma storage, transport and degassing during the 2008-10 summit eruption at Kilauea volcano, Hawaii[END_REF][START_REF] Patrick | Shallow driven fluctuations in lava lake outgassing (gas pistoning), Kilauea volcano[END_REF], where long or detailed enough series of measurements could be realized.

Here we report on the first OP-FTIR study of the dynamics of lava lake degassing at Ambrym, in Vanuatu, a remarkable basaltic arc volcano displaying such an activity and intense gas emissions in the southwest Pacific region. Ambrym island, located in the central part of Vanuatu arc (Fig. 1a), is the subaerial exposure of a massive (~500 km 3 ) basaltic shield volcano rising 1800 m above the sea floor. At its summit, a 12-km wide caldera which formed about 2 ka ago [START_REF] Maccall | The geology and geophysics of the Ambrym caldera, New Hebrides[END_REF] hosts two large active cones, Benbow and Marum (Fig. 1b), where the degassing of recurrent lava lakes sustains voluminous gas release.

Because of the remote location of the volcano, challenging access to its vents and adverse tropical weather conditions, this activity has long remained unstudied. The very first airborne and ground-based measurements, performed in 2005-2007, revealed prodigious gas emission rates from Ambrym [START_REF] Bani | Surge in sulphur and halogen degassing from Ambrym volcano, Vanuatu[END_REF][START_REF] Bani | First estimate of volcanic SO 2 budget for Vanuatu island arc[END_REF][START_REF] Allard | Prodigious emission rates and magma degassing budget of major, trace and radioactive volatile species from Ambrym basaltic volcano, Vanuatu island Arc[END_REF], ranking it amongst the strongest persistent emitters of volcanic volatiles on Earth [START_REF] Allard | Prodigious emission rates and magma degassing budget of major, trace and radioactive volatile species from Ambrym basaltic volcano, Vanuatu island Arc[END_REF]. However, no high temporal resolution data were collected in these studies.

In October 2008 we therefore conducted OP-FTIR spectroscopic measurements on Ambrym in order to measure the dynamics of its lava lake activity at high temporal resolution. Our measurements were made from the shortest possible distance to the lava lake that was actively degassing for ~1.5 years at the bottom of its ~300 m deep Benbow vent, and volcanic gas composition was retrieved every 5 sec. While providing new constraints on Ambrym gas chemistry, our results reveal short-term oscillations in volcanic gas composition, with different periodicities, that track a highly dynamic degassing pattern of the lava lake.

Morlet wavelet analysis of these compositional variations is combined with our field observations and melt inclusion data for dissolved volatiles [START_REF] Allard | Prodigious emission rates and magma degassing budget of major, trace and radioactive volatile species from Ambrym basaltic volcano, Vanuatu island Arc[END_REF] to infer the magmatic processes controlling this degassing pattern. Moreover, by combining our OP-FTIR data set with the SO 2 plume flux determined from contemporaneous airborne UV sensing we quantify the gas emission rates. The results, discussed in comparison with data for other lava lakes in different geodynamic contexts, confirm Ambrym volcano's status as one of the three most powerful persistent emitters of magmatic volatiles on Earth. Hence, our study provides new information on the high frequency dynamics of lava lake degassing at a top-ranking arc emitter of basaltic gases, and further insights into magma dynamics at basaltic volcanoes in general.

Volcanic activity and methodology

Our measurements, on 5 October 2008, targeted the active lava lake that was vigorously degassing at the bottom of Ambrym's Benbow cone (1160 m elevation a.s.l. and ~300 m deep). In order to determine magmatic gas compositions with OP-FTIR spectroscopy, we climbed down the eastern inner wall of the cone with our equipment (Fig. 1c) then climbed across the eastern edge of the crater terrace before reaching the northern pit crater (815 m a.s.l.) hosting the lake. It is the first time OP-FTIR spectroscopy was operated so deeply inside a volcanic vent. The lava lake was ponding at ~110 m depth in the southern part of the pit, a few meters lower than the crater floor (Figs. 1d), as measured with laser range-finding binoculars. It was approximately circular, with a diameter of ~25 m, but possibly extended more widely under the crater floor (Fig. 2). Even though concentrated gas fumes sometimes obscured its viewing, we could observe that the overall lava lake surface was repeatedly turned over by vigorous spattering, associated with a pulsated degassing (puffs), and, intermittently, by the sequential bursting of very large (meter-sized) gas bubbles. Otherwise, its level remained broadly stable during our measurements.

We positioned our spectrometer, its controlling laptop and a 12V DC powering battery on the northern rim of the pit (16°15.312S and 168°06.240E), outside the volcanic gas plume.

The spectrometer, occasionally realigned to optimize viewing around a mobile dense gas plume, targeted the center of the lava lake at an average slanting distance of 140±10 m (Fig. 2). Its 20 mrad field of view allowed us to measure infrared radiation emitted by a ~7±1m 2 lake area. Our spectrometer was a MIDAC 4401-S equipped with a ZnSe beam splitter and a Stirling engine cooled MCT detector (avoiding the logistically challenging need of liquid nitrogen). The detector has a 600 to 5000 cm -1 sensitivity and 0.5 cm -1 resolution. Singlesided interferograms of the IR radiation emitted by molten lava and absorbed by atmospheric and volcanic gases were collected at a frequency of 1 Hz over a period of 1.6 hours (except for a 20 min interruption due to rainfall). To improve the signal-to-noise ratio and hence the quality of recorded spectra, five consecutive interferograms were co-added and subsequently Fourier transformed by applying power spectrum phase correction and Norton-Beer medium apodization. We thus obtained 943 FTIR spectra of Benbow lava lake degassing at 5 sec period. Spectroscopic gas features were in absorption, indicating a lower temperature of the measured gas compared to the radiating source. They were analyzed following the retrieval procedures developed for OP-FTIR spectrum analysis [START_REF] Burton | Field measurement of CO 2 and H 2 O emissions from Masaya Volcano, Nicaragua, by Fourier transform spectrometry[END_REF], refined for higher temperature gases [START_REF] Allard | Spectroscopic evidence for a lava fountain driven by previously accumulated magmatic gas[END_REF][START_REF] Spina | Open-path FTIR spectroscopy of magma degassing processes during eight lava fountains on Mount Etna[END_REF]. For each spectrum we retrieved the column amounts (expressed in units of molecules.cm -2 ) of eight IR-active gas species in the following wave bands: H 2 O and CO 2 (2040-2165 cm -1 ), SO 2 (2490-2550 cm -1 ), HCl and CH 4 (2690-2810 cm -1 ), HF (4035-4210 cm -1 ), CO and N 2 O (2145-2245 cm -1 ). Gas column amounts were obtained from nonlinear least-squares best fitting (Rodgers 1976's optimal estimation algorithm) between the measured spectra and simulated spectra based on the radiative transfer forward model of [START_REF] Edwards | Reference Forward Model: High level algorithms definition[END_REF] and infrared absorption line parameters from the HITRAN database [START_REF] Rothman | The HITRAN 2004 molecular spectroscopic database[END_REF]. The standard deviation of fitting residuals across the respective spectral windows yields an estimate of the error on the retrieved column amount of each gas, illustrated by error bars in the plots of Figure 3.

Radiative transfer in the two layers of hot volcanic gas and ambient atmospheric gases viewed by the spectrometer was calculated using the temperature of each layer. For the atmospheric layer we used the ambient temperature (292.6°K) and pressure (919 hPa) measured on site with a hand-held meteorological sensor. The temperature of the volcanic gas layer was determined for each spectrum from the highly T-dependent rotational band structure of SO 2 at 2500 cm -1 [START_REF] Allard | Spectroscopic evidence for a lava fountain driven by previously accumulated magmatic gas[END_REF]Burton et al., 2007a). We also determined the temperature of the radiating source from the ratio of IR intensity recorded at 4400 and 4460 cm -1 and by fitting to a scaled Planck curve [START_REF] Allard | Spectroscopic evidence for a lava fountain driven by previously accumulated magmatic gas[END_REF].

Our data time series for the volcanic gas amounts and gas ratios are depicted in Figures 4 and5. In order to identify and quantify possible degassing periodicities, we performed a continuous Morlet wavelet transform (CMWT) analysis of the gas ratios time series, using the code of [START_REF] Grinsted | Application of the cross wavelet transform and wavelet coherence to geophysical time series[END_REF]. CMWT analysis is particularly suitable for studying natural processes not stationary in time (Torrence and Combo, 1998), such as volcanic degassing (e.g. [START_REF] Tamburello | Periodic volcanic degassing behavior: Mount Etna example[END_REF][START_REF] Ilanko | Cyclic degassing of Erebus volcano, Antarctica[END_REF]; it permits to examine periodicities of a time series by moving and stretching a Morlet guide-wave and analyzing step-by-step its correlation with the signal. Wavelet power spectra reveal the strength of periodicities at different times and for different periods. Our data time series for Ambrym were close to normal distribution and thus did not require prior log-transformation. They were simply resampled at 1 Hz in order to get a time series with uniform step from 0 to 2885 seconds. The code of [START_REF] Grinsted | Application of the cross wavelet transform and wavelet coherence to geophysical time series[END_REF] was used with the default parameters (e.g. automatic estimation of the background power spectrum, 1/12 octaves per scale) and result significance was assessed by assuming a red noise background, automatically modeled by a first order autoregressive (AR1) estimator. The code adds a cone of influence (COI) in which edge effects related to the length of the time series cannot be ignored. In our study this prevented from detecting periodicities lower than 6 seconds. We also checked the phase relationship between selected pairs of gas ratios by computing the coherence spectra between their wavelet transforms (Torrence and Combo, 1998). Results are displayed in Figures 6 and7 and discussed in section 4.2.

Finally, during our field campaign, on October 8, the bulk SO 2 plume flux from Ambrym and the respective flux contribution from Benbow and Marum craters were determined using airborne UV (DOAS) absorption spectroscopy. The results are reported in [START_REF] Allard | Prodigious emission rates and magma degassing budget of major, trace and radioactive volatile species from Ambrym basaltic volcano, Vanuatu island Arc[END_REF].

Combining these data with our OP-FTIR results therefore allows us to quantify both the single and total gas emission rates from Benbow crater and Ambrym in October 2008.

Results

Gas temperature and column amounts

The temperature of air-diluted Benbow volcanic gas was found to vary between 300 and 420°K, with a mean of 340±20°K. Such low temperatures, compared to ~1400°K at emission from the molten basalt (section 4.2), demonstrate a rapid cooling of the magmatic gas upon emission and dilution by entrained ambient air. A higher mean temperature of 770±120°K was determined for the radiating source, consistent with all spectra being in absorption. The mean thermal contrast of 330°K between the IR source and the volcanic gas warrants a negligible contribution of thermal emission from hot gases themselves in the spectral region of interest (>2000 cm -1 ).

As previously mentioned, eight IR-active gas species (H 2 O, CO 2 , SO 2 , HCl, HF, CO, CH 4 and N 2 O) were retrieved in our spectra. The overall uncertainty on their amounts due to measurement and retrieval ranges between 4 and 7% for SO 2 , HCl, and HF, 8-10% for H 2 O, CO 2 and N 2 O, and 10-12% for CO. CH 4 , absent in hot basaltic gases and affected by a greater uncertainty, is left out in the following. Whilst SO 2 , HCl and HF are purely volcanic in origin, H 2 O, CO 2 and CO are both volcanic and atmospheric components, so their amounts must be corrected for air dilution. N 2 O, which is purely atmospheric, is a reliable indicator for that correction. For the abundance of N 2 O (0.32 ppmv) and other gases in Vanuatu's atmosphere in October 2008 we refer to the data recorded by the CSIRO air monitoring station of Cap Ferguson (http://www.csiro.au/greenhouse-gases/), eastern Australia, which is the closest station of the World Meteorological Organization (Fig. 1a).

Figure 3 shows scatter plots of the bulk path amounts of (a) SO 2 versus HCl, (b) HCl versus HF, (c) CO 2 versus SO 2 and (d) H 2 O versus HCl in our 943 spectra. In each plot the gradient of linear regression defines the average molar ratio of the species in the pure volcanic gas. The values are 3.1±0.4 for SO 2 /HCl, 1.9±0.3 for HCl/HF, 2.0±1.4 for CO 2 /SO 2 , and 560±140 for H 2 O/HCl. Besides analytical uncertainties, the standard deviation on these ratios includes temporal fluctuations in the volcanic gas composition, as described thereafter. In Figures 3c and3d, the offset on the y-axis characterizes the respective air background amount of CO 2 (1.55x10 20 molecules.cm -2 ) and H 2 O (6.5x10 21 molecules.cm -2 ) in the pit crater in the absence of any volcanic gas. The variable proportion (from 3% to 24%) of admixed volcanic gas implies significant temporal changes in the thickness of the measured volcanic layer. This is verified by computing the optical thickness of the purely atmospheric gas layer from the amount of N 2 O in each spectrum: L a = (N N2O ).T a /(7.243x10 14 .P a .[N 2 O] a ), where [N 2 O] a = 0.32 ppmv and T a and P a are the ambient temperature and pressure. The average N 2 O amount in all spectra, (9.5±0.7)x10 16 molecules.cm -2 , constrains a mean atmospheric path length of 130±10 m, somewhat lower than our measuring distance to the center of the lake (140±10 m), suggesting a mean thickness of order 10 m for the volcanic gas layer. However, the range in single air path values, from 100 to 160 m, demonstrates important fluctuations of the volcanic layer, in agreement with the pulsated degassing of the lava lake. The longest atmospheric paths, corresponding to low volcanic gas amounts, approach the slanting distance (170 m) between our measuring site and the southern wall of the pit behind the lava lake (Fig. 2); they thus indicate a thinned volcanic layer during transient episodes of reduced lake degassing.

Instead, the shortest air paths correlate with both a higher gas temperature and a greater amount of total volcanic gas (a rough proxy for the gas flux), indicating a thicker volcanic layer and more intense lake degassing. On a few occasions, however, a thicker volcanic layer was an artefact due to backward turbulent transport of the volcanic gas cloud that, in addition to the targeted hot gases, was seen to cross the field of view of our spectrometer.

By subtracting the atmospheric background amounts discussed above, we thus obtain the air-corrected volcanic amounts of CO 2 and H 2 O. For a finer air correction on much less abundant carbon monoxide, we combined the atmospheric path value derived from N 2 O in each spectrum with Vanuatu's atmospheric CO content (0.07 ppmv) in October 2008. Figure 4 depicts the time series of purely volcanic H 2 O, CO 2 , SO 2 , HCl, HF and CO, as well as of the volcanic gas temperature and of the N 2 O-based atmospheric path length. As expected, the temperature and the column amounts of volcanic gases display significant co-variations that are anti-correlated with changes in the atmospheric path length: the shorter the latter, the higher the former, and reciprocally.

Temporal variations in gas ratios

In contrast to gas column amounts, the volcanic gas ratios are insensitive to variable air dilution and thus track purely volcanic processes. Now, our data set reveals significant short-term fluctuations of the gas ratios (Figs. 5a-c) and hence of the bulk volcanic gas composition, which are well beyond the errors in our spectral retrievals. These variations reveal a highly dynamic degassing pattern of Benbow lava lake, as verified by the Morlet wavelet analysis of our data time series. Figure 6 shows the wavelet periodograms for temperature and five selected gas ratios, while Figure 7 depicts the wavelet coherence and phase relationship between (a) SO 2 /HF and HCl/HF and (b) CO 2 /SO 2 and HCl/HF. The following features arise from our results:

(i)
Well defined short-period co-oscillations of the volcanic gas temperature and of most chemical ratios in the 8-32 sec band, sometimes extended to about 64 sec, that recur every ~100 to 200 sec (Fig. 6);

(ii) A continuous modulation of all parameters at around 500 sec (~8 min), especially well characterized by SO 2 /HCl and SO 2 /HF ratios (Fig. 6). Less well defined modulations also appear in the ~100-250 sec period band.

(iii) Close co-variations of both SO 2 /HF and HCl/HF ratios with the volcanic gas temperature (Fig. 5a). Coherence tests (Fig. 7) verify a synchronous behaviour of these two gas ratios at all periods and at almost all times (except in the 250-750 sec window). SO 2 /HCl ratio broadly co-varies with SO 2 /HF (Fig. 5b and5c), especially at longer periodicity (Fig. 6), while displaying fewer apparent oscillations at short periods.

(iv) Both highly correlated and poorly correlated variations of CO 2 /HCl and CO 2 /SO 2 ratios with respect to both SO 2 /HCl and SO 2 /HF (Figs. 5b-c and6). Coherence tests using HCl/HF as a reference do show a rare consistent behaviour of CO 2 /SO 2 and HCl/HF ratios at short (8-64 sec) and long (500 sec) periods (Fig. 7), with a common phase shift (left-pointing arrows) prevalently due to CO 2 /SO 2 (down-pointing arrows). In particular, our time series (Figs. 5b-c) reveal discrete spike increases of both CO 2 /HCl and CO 2 /SO 2 (by a factor 2 to 4) occurring at constant S/Cl/F ratios, which demonstrate intermittent episodes of selective CO 2 enrichment of the volcanic gas. Such events are outlined between 03:07:59 and 03:10:22 during our first measuring sequence (Fig. 5b) and between 04:21:51 and 04:22:20 during our second sequence (Fig. 5c). These CO 2 spikes, lasting from ~1 to 2 min, were observed to coincide with the sequential bursting of very large (meter-sized) gas bubbles at the lake surface.

Discussion

Average composition of Benbow basaltic gas

From Figure 3 and the air-corrected gas amounts in each spectrum we first derive the average molar composition of Benbow basaltic gas during our measurements. This composition (Table 1) is highly dominated by water vapor (96.1%), followed by CO 2 (2.2%), SO 2 (1.1%), HCl (0.35%), HF (0.19%) and CO (0.01%). Except for CO, reported here for the first time, we find that it is fairly comparable to the composition of cold Benbow plume emissions first analyzed with in situ methods in October 2007 [START_REF] Allard | Prodigious emission rates and magma degassing budget of major, trace and radioactive volatile species from Ambrym basaltic volcano, Vanuatu island Arc[END_REF]. Despite completely different instrumental tools and data processing techniques, both data sets (Table 1) show closely similar H 2 O content, SO 2 /HCl (3.1±0.4 versus 4.3±1.7) and HCl/HF (1.9±0.3 versus 2.3±0.3) ratios, while possibly differing in a somewhat higher CO 2 /SO 2 ratio in 2008

(2.0±1.4) than 2007 (1.0±0.2). This compositional resemblance for the same volcanic activity but at a one-year interval thus suggests quite steady magma degassing conditions. Long-term compositional gas stability at persistently active lava lakes, indicative of steady degassing processes, is a feature previously verified from repeated OP-FTIR measurements at Masaya [START_REF] Burton | Field measurement of CO 2 and H 2 O emissions from Masaya Volcano, Nicaragua, by Fourier transform spectrometry[END_REF], Nyiragongo (Sawyer et al., 2008a) and Erebus [START_REF] Oppenheimer | Pulsatory magma supply to a phonolite lava lake[END_REF]Ilanka et al., 2015).

The CO/CO 2 ratio of Benbow volcanic gas, measured for the first time, averages 4.7x10 -3

(Table 1). This ratio can be used to estimate the unknown redox conditions of Ambrym basaltic system if the gas equilibration temperature is determined. We assess this temperature from standard thermochemical data [START_REF] Barin | Thermochemical Properties of Inorganic Substances[END_REF] comm. 2007) and ≥100 [START_REF] Allard | Prodigious emission rates and magma degassing budget of major, trace and radioactive volatile species from Ambrym basaltic volcano, Vanuatu island Arc[END_REF]. From these values we compute an apparent gas equilibrium temperature of between 1085°K and 1022°K at ambient pressure. This is much higher than the physical temperature of the volcanic gas (340±20°K) derived from our FTIR spectra (section 3), demonstrating a relatively fast chemical quenching of the cooling magmatic gas phase after emission. From the equilibrium reaction CO 2 = CO + ½ O 2 at 1022-1085°K and the CO/CO 2 ratio of Benbow gas, we then compute an oxygen fugacity of between -0.13 and +0.14 log-unit with respect to the NNO (nickel-nickel oxide) redox buffer.

Therefore, we infer that the redox state of Ambrym basaltic system may be close to NNO, which is also typical for the parental basalt of Siwi-Yasur magmatic system in southern

Vanuatu [START_REF] Métrich | Magma and volatile supply to post-collapse volcanism and block resurgence in Siwi caldera (Tanna island, Vanuatu arc)[END_REF] and, more broadly, for basaltic arc volcanism (e.g. [START_REF] Kelley | Water and the oxidation state of subduction zone magmas[END_REF].

Lava lake degassing pattern and mechanisms

Our OP-FTIR measurements performed at high temporal resolution and at short distance from Benbow lava lake provide unique insight into its degassing dynamics. The temporal variations in volcanic gas ratios reveal a highly dynamic degassing pattern (Figs. 5 and6), involving both recurrent short-period (8-64 sec) compositional oscillations, a longer-period (~8 min) continuous degassing modulation, and CO 2 -rich discrete events coinciding with the bursting of large gas bubbles. Moreover, they reveal intervals of both coherence and incoherence between the gas ratios (Figs. 5 and7), indicating the bearing of quite different physical processes at different times.

Because there is no permanent monitoring network on Ambrym, we cannot compare these high frequency geochemical variations with other (e.g. geophysical) parameters recorded synchronously. We therefore attempt to interpret them on basis of our field observations and melt inclusion data recently obtained for Ambrym arc basalt [START_REF] Allard | Prodigious emission rates and magma degassing budget of major, trace and radioactive volatile species from Ambrym basaltic volcano, Vanuatu island Arc[END_REF]. The basalt was found to be moderately rich in volatiles (e.g. ~1.2 wt% H 2 O and 0.1 wt% CO 2 ) and to prevalently degas in closed system upon ascent from a crustal reservoir emplaced at about 4 km beneath Benbow and Marum vents [START_REF] Allard | Prodigious emission rates and magma degassing budget of major, trace and radioactive volatile species from Ambrym basaltic volcano, Vanuatu island Arc[END_REF]. Whereas CO 2 and sulphur start to exsolve at high pressures (~200 MPa and <150 MPa, respectively), H 2 O, Cl and plausibly F (not analysed) mainly exsolve at low to very low pressure during this process [START_REF] Allard | Prodigious emission rates and magma degassing budget of major, trace and radioactive volatile species from Ambrym basaltic volcano, Vanuatu island Arc[END_REF]. These contrasted solubility behaviours in the melt are reflected in the P-related compositional evolution of the co-existing magmatic gas phase (Fig. 8), computed using

VolatileCalc [START_REF] Newman | VolatileCalc: a silicate melt-H 2 O-CO 2 solution model written in Visual Basic for excel[END_REF] and the K 2 O-normalized volatile content of the melt inclusions (see details in Figure 8's caption). While a bulk magma degassing process well reproduces the average composition of Benbow volcanic gas, the results reveal sharp variations in the total gas volume fraction, H 2 O/CO 2 and S/Cl ratios associated with the prominent degassing of H 2 O and Cl at low pressures of between 25 and <10 MPa (Fig. 8).

Below we use these observations to interpret the degassing pattern of Benbow lava lake.

We first notice that the recurrent (~100-200 sec) short-period oscillations of Benbow gas temperature and composition (Fig. 6) match well the observed puffing activity of the lava lake. Their frequency, together with the relatively modest amplitude of variation of each parameter around its time-averaged value (Figs. 5a and5c), point to a shallow degassing process sourced within the upper portion of the lava lake. We do interpret these recurrent short-term variations as reflecting the periodic bursting of S-rich (high S/Cl and S/F ratios) and S-depleted (low S/Cl and S/F ratios) gas bubbles associated with the alternation, at the lake surface, of fresh lava blobs and partly degassed (S-depleted but still Cl-F-bearing), less hot lava. This interpretation is consistent with the modest amplitude of S/Cl fluctuations (from 2.5 to 4) around a mean value of 3.1±0.4 (Fig. 3a), which imply a very shallow degassing process (Fig. 8). It also well accounts for the temporal changes in degassing intensity inferred from the anti-correlated variations in atmospheric path length and total volcanic gas amount (Fig. 4 and Section 3.1). In this framework, the ~100-200 sec recurrence of the short-lived Tgas oscillations represents a time scale of convective resurfacing of the lava lake during our measurements.

Instead, the continuous modulation of almost all parameters with ~8 min period, especially well defined by SO 2 /HCl and SO 2 /HF ratios (Fig. 6), evidences a constant degassing periodicity of the volcanic system. Because it concerns all gas ratios this persistent dynamic signature likely typifies a somewhat deeper process involving both the lava lake and its feeder conduit. A plausible mechanism is the continuous influx of bubble-rich magma to the bottom of the lake, whose rate will determine the renewal time of the latter. The magma supply rate to Benbow crater was estimated to be as high as ~20 m 3 s -1 from the SO 2 flux emitted by this crater in October 2008 (see section 4.3) and from the magma sulfur content [START_REF] Allard | Prodigious emission rates and magma degassing budget of major, trace and radioactive volatile species from Ambrym basaltic volcano, Vanuatu island Arc[END_REF]. If the degassing periodicity of ~8 min is taken to approximate the renewal time of the lava lake controlled by this supply rate, we infer a lake volume of ~10 4 m 3 , about twice that of Erebus lava lake [START_REF] Oppenheimer | Pulsatory magma supply to a phonolite lava lake[END_REF].

Finally, we invoke a deeper, conduit source for the discrete CO 2 degassing events characterized by sharp spike increases of CO 2 /HCl and CO 2 /SO 2 at constant S/Cl/F ratios (Figs. 5b and5c). The chemical amplitude of these intermittent events, their duration (~1-2 min) and their coincidence with the bursting of series of meter-sized bubbles at the lake surface strongly suggest the separate ascent, in Benbow conduit, of large gas bubbles or slugs preferentially enriched in early exsolved CO 2 (Fig. 8). This means that, despite prevalent closed-system degassing of the basalt upon ascent, bubble accumulation and modest coalescence would also occur at some depth in the feeding system. A quantitative estimate is however impossible at present owing to the natural lack of CO 2 -bearing melt inclusions trapped at less than ~75 MPa [START_REF] Allard | Prodigious emission rates and magma degassing budget of major, trace and radioactive volatile species from Ambrym basaltic volcano, Vanuatu island Arc[END_REF].

The diameter of Benbow conduit can tentatively be estimated from the magma supply rate of ~20 m 3 s -1 mentioned above. Extensive convection in Benbow feeding system, with downward recycling of the denser degassed magma, is required to accommodate for such a rate, little of which exits as lava clots and ash [START_REF] Allard | Prodigious emission rates and magma degassing budget of major, trace and radioactive volatile species from Ambrym basaltic volcano, Vanuatu island Arc[END_REF]. Magma convection is also supported by the textural features of Benbow eruptive products [START_REF] Polacci | Degassing behaviour of vesiculated basaltic magmas: an example from Ambrym volcano, Vanuatu Arc[END_REF]). Here we focus on the uppermost conduit, assumed to be vertical and cylindrical for the sake of simplicity. As shown by [START_REF] Beckett | Conduit convection driving persistent degassing at basaltic volcanoes[END_REF], the net volumetric flux (Q) of magma upraising in a vertical conduit with bi-directional exchange flow does not depends on the flow regime and can be approximated by the following relationship:

𝑄 = 0.059𝛽 *+.,-𝑔𝛥𝜌𝑅 - 2𝜇 4
where β is the viscosity ratio between the degassed and the volatile-rich buoyant magma components, Δρ is their difference in density, µ b is the dynamic viscosity of the buoyant component, g is gravitational acceleration and R is the effective conduit radius. We use the rhyolite-MELTS+H 2 O-CO 2 software (version 1.2.x; [START_REF] Ghiorso | An H 2 O-CO 2 mixed fluid saturation model compatible with rhyolite-MELTS[END_REF] to compute magma densities and viscosities at a temperature of 1400°K which, under NNO redox conditions (such as inferred in section 4.1), well accounts for the melt composition, the low crystal content (<5 to 10%) and the mineral chemistry of Benbow products [START_REF] Allard | Prodigious emission rates and magma degassing budget of major, trace and radioactive volatile species from Ambrym basaltic volcano, Vanuatu island Arc[END_REF]N. Métrich, unpub. data, 2012;[START_REF] Picard | Concomitant evolution of tectonic environment and magma geochemistry, Ambrym volcano (Vanuatu, New Hebrides arc)[END_REF]. As shown in Figure 8, during its closed-system ascent Ambrym basalt will reach 50% vesicularity -a threshold for permeability-controlled gas percolation and hence open degassing (Burton et al., 2007b) while decompressing to only about 10 MPa. Under these conditions the buoyant magma has a density of ~1500 kg m -3 and an effective viscosity of 595.5 Pa s if one assumes sphericalshaped bubbles and a low capillary number [START_REF] Pal | Rheological behaviour of bubble-bearing magmas[END_REF]. Once degassed at or near the surface of the lake, the magma becomes much denser (2700 kg m -3 ) and more viscous (1445 Pa s)

and, therefore, should rapidly sink down the lake and the feeder conduit. From these numbers and the above equation we compute a diameter of 6±1 m for Benbow upper conduit, taking account of propagating uncertainties on the different parameters. Before entering the lava lake, the buoyant basalt would flow at 1.4 m s -1 across the inner half section of the conduit, which is a favourable condition to maintain bubble-melt coupling up to very shallow depth.

Finally, we emphasize that the degassing pattern of a vigorously active basaltic lake on Ambrym markedly differs from that recorded during quieter lava lake activity at either Erebus or Kilauea, the only two volcanoes where detailed OP-FTIR investigations have been achieved until now. At Erebus, passive degassing with relatively constant composition was found to prevail between the explosive bursting of deeper-derived large gas bubbles or slugs happening every ~10 mn [START_REF] Oppenheimer | Pulsatory magma supply to a phonolite lava lake[END_REF]Ilanka et al., 2015), while at Kilauea ~30 mn long cycles of low passive degassing and lava lake upheaval, controlled by shallow bubble accumulation (gas pistoning), were observed to alternate with intervals of spattering and increased H 2 O-SO 2 -rich gas release after a sudden drop in lake level [START_REF] Patrick | Shallow driven fluctuations in lava lake outgassing (gas pistoning), Kilauea volcano[END_REF].

With an estimated volume of 10 4 m 3 , an upper (surface) diameter of ~25 m and a lower (conduit) diameter of ~6 m, Benbow lava lake (as an inverted truncated cone) would have had a modest height of ~12 m in October 2008. Such small dimensions, coupled with the high basalt supply rate of ~20 m 3 s -1 , well explain its vigorous degassing and rapid renewal rate.

Instead, the much quieter degassing patterns of lava lakes at Kilauea and Erebus is fully consistent with their longer renewal rate (about 4 days and 4 hours, respectively; [START_REF] Patrick | Shallow driven fluctuations in lava lake outgassing (gas pistoning), Kilauea volcano[END_REF][START_REF] Oppenheimer | Pulsatory magma supply to a phonolite lava lake[END_REF], due to a lower magma supply rate but also a much larger size for Kilauea basaltic lake (2.5x10 6 m 3 and 100 m deep in 2010; [START_REF] Carbone | Continuous gravity measurements reveal a low-density lava lake at Kilauea Volcano, Hawaii[END_REF] and a much higher viscosity in case of Erebus phonolite lake [START_REF] Oppenheimer | Pulsatory magma supply to a phonolite lava lake[END_REF].

Ambrym volcanic gas composition and emission rates compared with other lava lakes worldwide

Ambrym basaltic gas from Benbow lava lake in October 2008 is compared in Table 1 with magmatic gases from other active lava lakes measured with OP-FTIR spectroscopy and/or classical (in situ) methods. Ambrym basaltic gas is markedly richer in water and halogens than hot spot or rift basaltic gases from Kilauea and Erta'Ale lava lakes, respectively, whilst being poorer in CO 2 than magmatic gases from Nyiragongo and Erebus alkaline lakes. These features are coherent with differences in magma compositions and geodynamic contexts.

Ambrym volcanic gas more closely resembles arc volcanic gases from Masaya [START_REF] Burton | Field measurement of CO 2 and H 2 O emissions from Masaya Volcano, Nicaragua, by Fourier transform spectrometry[END_REF] and Villarrica [START_REF] Sawyer | Gas and aerosol emissions from Villarrica volcano, Chile[END_REF] lava lakes (Table 1) and typically plots within the compositional field for arc volcanism for both major components and trace halogen species [START_REF] Allard | Prodigious emission rates and magma degassing budget of major, trace and radioactive volatile species from Ambrym basaltic volcano, Vanuatu island Arc[END_REF].

Airborne DOAS measurements on 8 October 2008 revealed a total SO 2 plume flux of ~100±30 kg s -1 from the volcano, 75% of which arose from Benbow and 25% from Marum craters [START_REF] Allard | Prodigious emission rates and magma degassing budget of major, trace and radioactive volatile species from Ambrym basaltic volcano, Vanuatu island Arc[END_REF]. Combining the specific SO 2 flux from Benbow (~75 kg s -1 , or 6500 Mg d -1 ) with the average gas composition measured with OP-FTIR spectroscopy (Table 1), we compute the simultaneous emission of 1.6x10 2). In terms of H 2 O, CO 2 and SO 2 fluxes, whose sum approximates the total gas flux, Benbow crater (and Ambrym as a whole) is of comparable strength as Nyiragongo (Sawyer et al., 2008a), despite widely differing magma compositions. The CO 2 discharge of ~10 Gg per day from Benbow in October 2008 actually ranks amongst the highest known emissions of igneous carbon from persistently degassing volcanoes [START_REF] Burton | Deep carbon emissions from volcanoes[END_REF]. As a matter of fact, our study provides further evidence that Ambrym volcano ranks among the top-three persistent emitters of magma-derived volatiles at a global scale, together with Nyiragongo and Etna (Table 2). In addition to their impact on local ecosystems [START_REF] Bani | Surge in sulphur and halogen degassing from Ambrym volcano, Vanuatu[END_REF][START_REF] Allibone | Dental fluorosis linked to degassing of Ambrym volcano, Vanuatu: A novel exposure pathway[END_REF], volcanic gas emissions from Ambrym may thus have a considerable atmospheric impact at regional scale in the southwest Pacific [START_REF] Lefèvre | Distribution of sulfur aerosol precursors in the SPCZ released by continuous volcanic degassing at Ambrym, Vanuatu[END_REF].

Conclusions

We performed the first OP-FTIR measurements of gas emissions from Benbow crater on Ambrym volcano, in Vanuatu. Ambrym is one of the rare basaltic arc volcanoes displaying persistent lava lake activity, but has long remained undocumented for its degassing compared to other, more accessible lava lakes worldwide. We measured gas absorption spectra at short range (~140 m) using the vigorously active Benbow lava lake itself as a radiation source.

Analysis of these spectra allowed retrieval of volcanic H 2 O, CO 2 , SO 2 , HCl, HF and CO path amounts at high temporal resolution (5 sec period). Our gas composition data were combined with simultaneously measured SO 2 fluxes to quantify fluxes for each measured magmatic gas species from Ambrym.

We outline the following main observations:

(i) Lava lake degassing at Ambrym clearly differs in its water-halogen-rich average composition from basaltic or alkaline volcanic gases from long-lived lava lakes on rifting plate boundaries (Erta'Ale, Nyiragongo, Erebus) or hot spots (Kilauea). It more closely resembles volcanic gases from other basaltic arc volcanoes, in agreement with Ambrym' subduction zone setting.

(ii) Temporal variations of Benbow volcanic gas composition during our measuring interval reveal a highly dynamic degassing pattern of the lava lake. Short-period oscillations of most ratios, recurring every ~100-200 sec, match the observed pulsated degassing (puffing) of the lake. These oscillations, especially clear and coherent for SO 2 /HF, HCl/HF and SO 2 /HCl ratios and correlated with temperature variations, indicate an inside-lake shallow degassing process marked by the periodic bursting of S-rich (high S/Cl ratio) and S-depleted (low S/Cl) gas bubbles associated with the alternation of fresh lava and degassed lava at the lake surface. A longer-period (~8 min) persistent modulation of the gas ratios, particularly well characterized for SO 2 /HF and SO 2 /HCl, probably tracks the influx of bubble-rich magma at the bottom of the lake and thus the rate of convective renewal of this latter. Occasional sharp increases of CO 2 /HCl and CO 2 /SO 2 at constant S/Cl/F ratios, lasting 1-2 min, evidence discrete degassing events during which carbon dioxide was selectively enriched in the emitted gas. These events, coinciding with the bursting of meter-sized bubbles at the lake surface, are attributed to the separate ascent of deeper derived (CO 2 -rich) large gas bubbles or slugs in Benbow feeder conduit, the diameter of which is estimated as 6±1 m.

(iii) The highly dynamic degassing pattern of Benbow basaltic lake markedly differs from that of quieter lava lakes at either Kilauea or Erebus. In first order, this can be related to a smaller size and much faster renewal rate of Benbow lava lake, due to a very high basalt supply rate, compared to Kilauea basaltic lake, and to a much lower magma viscosity when compared to the phonolite lake of comparable size occurring at Erebus.

(iv) Gas emission rates from Benbow crater in October 2008 include 160 Gg d -1 of H 2 O, ~10 Gg d -1 of CO 2 and ~8 Gd d -1 of total acid gases (SO 2, HCl and HF). Such emission rates are far larger than those from other lava lakes currently active worldwide, with the exception of Nyiragongo lava lake on the East African rift. We further verify that Ambrym volcano actually ranks amongst the three most powerful persistent emitters of volcanic gases at global scale and, hence, may have a considerable environmental impact in the southwest Pacific region.

Our study of lava lake degassing on Ambrym arc volcano further illustrates the great potential of OP-FTIR spectroscopy to investigate magma degassing processes at high temporal resolution. It reveals an unprecedented dynamism in gas emissions from a basaltic lake, reflecting the superimposed processes of coupled and decoupled ascent of magma and gas which feed this remarkable volcanic system. Such new insights into shallow magma dynamics at a highly active basaltic volcano provide a valuable basis for quantitative interpretation and modeling of gas composition variations and magma dynamics at other lava lakes and open-vents basaltic volcanoes. The gradient of linear regression in each plot defines the average molar ratio between the gas species in the pure volcanic gas. In Figures 3c and3d Figure 5a-c -Temporal variations of volcanic gas ratios during Benbow lava lake degassing:

Figures

(a) SO 2 /HF, HCl/HF, SO 2 /HCl and gas temperature; SO 2 /HCl, SO 2 /HF, CO 2 /HCl and CO 2 /SO 2 ratios during (b) our first measuring sequence and during (c) an enlarged window of our second sequence.

In Figure 5a ratios are displayed on logarithmic scale to render HCl/HF oscillations more readable. In Figure 5b data time series are fitted with a moving average of 15 sec period (3 consecutive spectra).

Framed time windows in Figures 5b and5c highlight the discrete CO 2 degassing events (high CO 2 /HCl and CO 2 /SO 2 at otherwise steady S/Cl/F ratios) corresponding to the bursting of meter-sized gas bubbles at the surface of the lava lake, which we attribute to the intermittent ascent and bursting of deeper-derived CO 2 -rich large bubble or slugs in Benbow volcanic conduit (Section 4.2). See text for discussion.

Figure 6 -Morlet wavelet periodograms of the volcanic gas temperature and five volcanic gas ratios (HCl/HF, SO 2 /HF, SO 2 /HCl, CO 2 /HCl and CO 2 /SO 2 ) over a 2885 sec sampling time interval.

Colour scale shows wavelet power normalised by variance. Black contours mark 95 % confidence from the significance tests [START_REF] Grinsted | Application of the cross wavelet transform and wavelet coherence to geophysical time series[END_REF]. Areas outside the cone of influence correspond to the edge effects from the wavelet (see section 2 for details). Two main degassing modulations appear :

(i) recurrent short-period oscillations of T and most gas ratios in the band 8-32 sec, sometimes extending up to 64 sec, especially well defined for HCl/HF and SO 2 /HF ratios; and (ii) a continuous long period modulation of all parameters at around 500-sec (~8 min), especially well defined by SO 2 /HF and SO 2 /HCl ratios. Results are discussed in section 4.2. Figure 8 -Pressure-related compositional evolution of the magmatic gas phase during closedsystem decompression of Ambrym arc basalt.

The magmatic gas phase evolution was computed from the K 2 O-normalised volatile content of olivinehosted melt inclusions in Ambrym's basalt [START_REF] Allard | Prodigious emission rates and magma degassing budget of major, trace and radioactive volatile species from Ambrym basaltic volcano, Vanuatu island Arc[END_REF], representative of the bulk basalt, and using VolatileCalc [START_REF] Newman | VolatileCalc: a silicate melt-H 2 O-CO 2 solution model written in Visual Basic for excel[END_REF] for the P-related degassing of H 2 O and CO 2 at 1400°K from 220 MPa to the surface. K 2 O varies from 1.85 wt% in the deepest, most primitive and volatile-richest inclusions (~1.2 wt% H 2 O and 0.1 wt% CO 2 ) to 2.10-2.14 wt% in the matrix glass of Benbow erupted products, implying a slight magma differentiation upon ascent [START_REF] Allard | Prodigious emission rates and magma degassing budget of major, trace and radioactive volatile species from Ambrym basaltic volcano, Vanuatu island Arc[END_REF].

Illustrated are evolutions of the total gas volume fraction (α), of the molar percentages of H 2 O and CO 2 and of the molar S/Cl ratio in the magmatic gas phase. The cumulated exsolved amount of each volatile at every pressure step was computed by difference with respect to its initial dissolved content in the most primitive inclusions. A bulk magma degassing process well reproduces the average composition of Benbow volcanic gas. Note however the prominent degassing of both H 2 O (H 2 O/CO 2 increase) and Cl (sharp drop of S/Cl ratio) in the low-pressure range from 25 to <10 MPa, resulting in a marked increase of the total gas volume fraction and, hence, basalt vesicularity. See discussion in section 4.2.

Figure 1 -

 1 Figure1-Ambrym volcano and its degassing lava lake in Benbow crater.

Figure 2 -

 2 Figure 2 -Schematic drawing of OP-FTIR spectroscopic measuring conditions at the bottom

Figure 3 a

 3 Figure 3 a-d -Scatter plots of measured gas path amounts: (a) SO 2 versus HCl, (b) HCl

  the y-intercept of regression line indicates the average background amount of CO 2 and H 2 O in ambient air of the pit crater. Error bars indicates the overall uncertainty (1σ) for each species. See section 3.2.

Figure 4 -

 4 Figure 4 -Time series of the pure volcanic gas amounts, the gas temperature and the

  Figure 1

Figure 4 -

 4 Figure4-Time series of the volcanic gas path amounts, the volcanic gas temperature and the atmospheric optical path length for each spectrum during the 1.6-hour interval of OP-FTIR measurement of Benbow lava lake degassing in October 2008.

  Figure 5c

  for the reaction: 3 CO 2 + H 2 S = 3 CO + SO 2 + H 2 O, assuming ideal gas behaviour and combining the CO/CO 2 ratio and H 2 O molar proportion (0.96) measured in 2008 with the SO 2 /H 2 S molar ratio of Benbow plume measured in 2007 with MultiGas sensors. The latter ranged between 10 (A. Aiuppa, pers.

  Benbow fluxes of H 2 O, HCl and HF exceed by factors as high as 190-670, 50-460 and 17-1000 the ranges in corresponding fluxes from Erebus, Kilauea and Erta'Ale lava lakes (Table

	5 Mg d -1 of H 2 O, 9x10 3 Mg d -1 of CO 2 ,
	1.2x10

3

Mg d -1 of HCl, 350 Mg d -1 of HF and 8 Mg d -1 of CO (with overall ±30% uncertainty; Table

2

). Such gas emission rates are far larger than those from most other lava lakes. For instance,

  Figure 7 -Power spectra of wavelet coherence for two pairs of gas ratios: (a) SO 2 /HF-HCl/HF and (b) CO 2 /SO 2 -HCl/HF.Colour scale defines the degree of coherence (1 for full correlation) and black lines delineate areas within 95 % confidence level from Monte Carlo simulations. Arrows indicate the phase behaviour of the gas ratios: in phase (pointing right), out of phase (pointing left); first ratio leading (down), second ratio leading (up). A highly coherent behaviour is verified for SO 2 /HF and HCl/HF at all times (except in the 250-750 sec interval) and at all periods (though more pronounced in the bands 32-64 and 256-512 sec). Instead, poor coherence exists between CO 2 /SO 2 and HCl/HF, except at around 500 sec periodicity and by intermittence in the band 8-64 sec, and the two ratios are clearly shifted in phase owing to a prevalent distinct behaviour of CO 2 /SO 2 . See text for discussion.

Table 1 -

 1 Molar composition of Ambrym's basaltic gas from Benbow lava lake in October 2008 and October 2007, compared with volcanic gases from other basaltic arc volcanoes and from lava lakes active in different geodynamic contexts. Mean composition of bulk Ambrym gas emissions from both Benbow and Marum craters in October 2007. In situ = standard gas sampling/analysis methods. nd: not determined; bd: below detection.

	Volcano	Measuring Geodynamic	Magma	H2O	CO2	SO2	HCl	HF	CO H2O/CO2 CO2/S Cl/S	F/S	Method	Ref.
		time	setting	type	mol?% mol?% mol?% mol?% mol?% mol?%
	Benbow,?Vanuatu	2008	Arc	basalt	96.2	2.2	1.1	0.35	0.19	0.003	44	2.0	0.32 0.17	FTIR	1
	Ambrym*,?Vanuatu	2007	Arc	basalt	95.0	2.1	2.1	0.49	0.21	nd	45	1.0	0.23 0.10	in?situ	2
	Masaya,?Nicaragua 1998S1999	Arc	basalt	94.2	3.3	1.4	0.86	0.19	bd	29	2.4	0.61 0.14	FTIR	3
	Villarica,?Chile	2009	Arc	bas.?andesite 90.5	5.7	2.6	0.90	0.30	nd	16	2.2	0.35 0.12	FTIR	4
	Villarica,?Chile	2004	Arc	bas.?andesite 95.0	2.0	2.1	0.63	0.23	nd	48	1.0	0.30 0.11	in?situ	5
	Miyakejima,?Japan	2000?	Arc	basalt	94.9	2.0	2.7	0.24	nd	nd	47	0.7	0.09		in?situ	6
	Stromboli,?Italy	2006	Arc	KSrichSbasalt	82.9	13.6	1.7	1.7	bd	0.03	6.1	8.0	1.00		FTIR	7
	Etna,?Italy	2000	Cont.?Margin trachybasalt	92.0	7.3	1.0	0.10	0.07	0.004	12.6	7.3	0.10 0.07	FTIR	8
	Erta?Ale,?Afar	1974	Rift	basalt	76.5	11.0	8.1	0.41	nd	0.54	7.0	1.4	0.05		in?situ	9
	Erta?Ale,?Afar	2005	Rift	basalt	93.6	3.7	2.5	0.19	0.04	0.06	26	1.5	0.08 0.02	FTIR	10
	Kilauea,?Hawaii	2004S05	Hot?spot	basalt	81.8	6.4	10.7	1.1	0.45	0.05	13	0.6	0.10 0.04	FTIR	11
	Kilauea,?Hawaii	1940	Hot?spot	basalt	59.9	32.2	7.8	0.05	nd	0.73	1.9	4.1	0.01		in?situ	12
	Erebus,?Antarctica	2004	Cont.?Rift	phonolite	57.9	36.4	1.4	0.69	1.27	2.3	1.6	26	0.49 0.91	FTIR	13
	Nyiragongo,?EAR 2005S2007 Cont.?Rift	nephelinite	70.0	24.0	4.6	0.26	0.11	0.87	2.9	5.2	0.06 0.02	FTIR	14
	Data sources: (1) This work; (2) Allard et al., 2015; (3) Burton et al., 2000; (4) Sawyer et al., 2011; (5) Shinohara and Witter, 2005; (6) Shinohara et al., 2003;
	(7) Burton et al., 2007a; (8) Allard et al., 2005; (9) Giggenbach and Le Guern, 1974; (10) Sawyer et al., 2008b; (11) Edmonds and Gerlach, 2007; (12) Gerlach
	and Graeber, 1985; (13) Oppenheimer and Kyle, 2008; (14) Sawyer et al., 2008a. EAR: East African Rift.

*

Table 2 -

 2 Gas emission rates from Benbow crater in 2008 and Ambrym in 2007, compared to gas emissions rates from other active lava lakes worldwide. Data sources: (1) This work; (2) Allard et al., 2015, bulk Ambrym emissions; (3) Burton et al., 2000;(4)[START_REF] Sawyer | Gas and aerosol emissions from Villarrica volcano, Chile[END_REF] (5)Sawyer et al., 2008b; (6) [START_REF] Edmonds | Magma storage, transport and degassing during the 2008-10 summit eruption at Kilauea volcano, Hawaii[END_REF] (7) [START_REF] Oppenheimer | Probing the magma plumbing of Erebus volcano, Antarctica, by open-path FTIR spectroscopy of gas emissions[END_REF] (8) Sawyer et al., 2008a.

	Volcano	Measuring	H 2 O	CO 2	SO 2	HCl	HF	CO	Total6Gas	Ref.
		time	Mg6d =1	Mg6d =1	Mg6d =1	Mg6d =1	Mg6d =1	Mg6d =1	Mg6d =1
	Benbow,6Vanuatu	2008	1.6x10 5	9x10 3	6500	1200	350	8.0	2.1x10 5	1
	Ambrym,6Vanuatu	2007	1.1x10 5	1.4x10 4	7800	1100	240	nd	1.3x10 5	2
	Masaya,6Nicaragua	1998=1999	3.5x10 4	2900	1800	640	78	nd	4.0x10 4	3
	Villarrica,6Chile	2009	3100	480	315	60	11	nd	4.0x10 3	4
	Erta6'Ale,6Afar	2005	630	60	60	2.6	0.4	0.7	7.5x10 2	5
	Kilauea,6Hawaii	2009	240	80	900	23	10	nd	1.3x10 3	6
	Erebus,6Antarctica	2004	860	1330	75	21	21	54	2.4x10 3	7
	Nyiragongo,6EAR	2005	1.5x10 5	1.2x10 4	3300	105	26	285	2.7x10 5	8

Acknowledgements

Our study was supported by the French National Agency for Research, through 2007Research, through -2010 contracts "VOLGASPEC" (06-CATT-012-01) and "Arc Vanuatu" (06-CATT-012-002), and benefited from funding from the European Research Council (FP/2007-2013)/ERC Grant Agreement n. 279802). We acknowledge B. Pelletier (IRD, Noumea), E. Garaebiti (GEOHAZARD, Vanuatu), guides and porters from Ambrym's Lalinda village for local assistance, and G. Tamburello (Palermo University, Italy) for its help in Morlet wavelet analysis. Thorough reviews by K. Iacovino, M. Patrick and two anonymous referees, as well as careful editing by T. Mather, greatly improved our manuscript.

List and captions of Tables and Figures

Tables

Table 1 - Giggenbach and Le Guern, 1974;(10) Sawyer et al., 2008b;(11) [START_REF] Edmonds | Vapor segregation and loss in basaltic melts[END_REF](12) [START_REF] Gerlach | Volatile budget of Kilauea volcano[END_REF](13) [START_REF] Oppenheimer | Probing the magma plumbing of Erebus volcano, Antarctica, by open-path FTIR spectroscopy of gas emissions[END_REF](14) Sawyer et al., 2008a. Table 2 -Gas emission rates from Ambrym in 2007 and 2008, compared to gas emissions rates from other lava lakes worldwide.

Data sources: (1) This work; (2) Allard et al., 2015, bulk Ambrym emissions; (3) [START_REF] Burton | Field measurement of CO 2 and H 2 O emissions from Masaya Volcano, Nicaragua, by Fourier transform spectrometry[END_REF] (4) [START_REF] Sawyer | Gas and aerosol emissions from Villarrica volcano, Chile[END_REF](5) Sawyer et al., 2008b;(6) [START_REF] Edmonds | Magma storage, transport and degassing during the 2008-10 summit eruption at Kilauea volcano, Hawaii[END_REF](7) [START_REF] Oppenheimer | Probing the magma plumbing of Erebus volcano, Antarctica, by open-path FTIR spectroscopy of gas emissions[END_REF](8) Sawyer et al., 2008a.


domain: phys chim phys.astr

text:
Shubhadip Chakraborty
email: shubhadip.chakraborty@univ-rennes1.fr

Karine Demyk

Ludovic Biennier

Robert Georges
email: robert.georges@univ-rennes1.fr

Absorption Spectroscopy of Solid-Phase

Keywords: Fullerene, Infrared spectroscopy, Astrochemistry, Quantum Chemistry, Diffuse interstellar bands

HAL is

Introduction

Fullerenes were discovered in laboratory experiments using laser-induced vaporization of graphite, aimed at understanding how long carbon chains could form in the circumstellar envelope of dying stars. [START_REF] Kroto | C 60 : Buckminsterfullrene[END_REF] Over the last decades, fullerenes and their derivatives have been the subject of intense research, as much for their chemical and physical properties as for their technological applications. Following bulk-production of fullerenes by a method developed by Krätschmer et al. [START_REF] Krätschmer | Solid C 60 : A New Form of Carbon[END_REF] , major efforts have been made to investigate the spectroscopic fingerprints of C 60 , especially in the mid-infrared. In the solid-phase, absorption studies have been conducted by embedding C 60 crystals in KBr 3 or in films [START_REF] Wang | Observation of Higher-Order Infrared Modes in Solid C 60 Films[END_REF][START_REF] Martin | Vibrational Study of 13 enriched C 60 Crystals[END_REF] maintained at low temperature. Some experiments in which C 60 was trapped in a rare-gas matrix have also been performed. Fulara et al. [START_REF] Fulara | Electronic and Infrared Spectra of C + 60 and C -60 in Neon and Argon Matrices[END_REF] reported the IR spectrum of C 60 isolated in a Ne matrix. Sogoshi et al. [START_REF] Sogoshi | High-Resolution Infrared Absorption Spectroscopy of C 60 Molecules and Clusters in Parahydrogen Solids[END_REF] performed some experiments in a para-hydrogen matrix while more recently Wakabayashi et al. 8 reported the IR spectra of isotopically substituted C 60 in a para-hydrogen matrix.

In the gas-phase, absorption measurements of neutral C 60 were hindered by the huge vibrational partition function of C 60 , even at room temperature, 9 that limits the signal from the ground state. It is only very recently that Changala et al. 10 managed to successfully record the IR absorption spectrum at 8.5 µm of cold C 60 in the gas-phase using a collisional cooling cell coupled with frequency comb spectroscopy. The gas-phase IR spectra of C 60 was also recorded in emission by Nemes et al. 11 over the 879-1212 K temperature range and to date, this is the only available broadband gas-phase data for neutral C 60 . In parallel to the experiments, a few theoretical harmonic frequency calculations on C 60 were performed, primarily focusing on the mid-infrared region. 12,13 However, no attempt has been made for a complete anharmonic frequency calculation of C 60 including various resonances even in the mid-infrared region.

In 2010, twenty-five years after their accidental discovery during laboratory astrophysics experiments, their presence in space was finally observed. The fullerenes C 60 and C 70 were identified in the Tc1 planetary nebula 14,15 lae 16 through their mid-infrared emission features at 7.0, 8.5, 17.4 and 18.9 µm. Since then, fullerenes have been detected in various astronomical objects ranging from diffuse clouds, 17 evolved stars, 15,18-21 star forming regions [START_REF] Roberts | Detection of C 60 in Embedded Young Stellar Objects, a Herbig Ae/Be Star and an Unusual Post-Asymptotic Giant Branch Star[END_REF][START_REF] Peeters | The 15-20 µm Emission in the Reflection Nebula NGC[END_REF] to photodissociation regions. [START_REF] Castellanos | C 60 in Photodissociation Regions[END_REF] In spite of these series of positive detections, their presence remains rare. It is estimated that only a small fraction of gas-phase carbon is locked in the form of C 60 fullerene in the interstellar medium (ISM). 17 Notably, the physical and chemical routes leading to the formation of fullerenes, which include bottom-up and top-down pathways, remain open to debate. [START_REF] Scott | Polycyclic Aromatic Hydrocarbons and Fullerenes as Decomposition Products of Hydrogenated Amorphous Carbon[END_REF][START_REF] Chuvilin | Direct Transformation of Graphene to Fullerene[END_REF][START_REF] Zhen | Laboratory Formation of Fullerenes from PAHS: Top-Down Interstellar Chemistry[END_REF] Photo-chemistry, [START_REF] Lebeault | Decay of C 60 by Delayed Ionization and C 2 Emission: Experiment and Statistical Modeling of Kinetic Energy Release[END_REF] photo-dynamics 29 but also shock processing [START_REF] Biennier | Shock-wave Processing of C 60 in Hydrogen[END_REF][START_REF] Sommer | Thermal Stability of Fullerenes: A Shock Tube Study on the Pyrolysis of C 60 and C 70[END_REF] studies of C 60 aim to shed light on this issue.

Recently, the presence of C + 60 was firmly established in diffuse clouds through an observed match between its electronic spectrum and a few DIBs. [START_REF] Campbell | Laboratory Confirmation of C + 60 as the Career of Two Diffuse Interstellar Bands[END_REF][START_REF] Cordiner | Confirming Interstellar C + 60 Using the Hubble Space Telescope[END_REF] The approach was based on the photo-fragmentation of C + 60 -He clusters confined in a cryogenic radiofrequency multipole ion trap. This work confirmed the hypothesis proposed by Foing and Ehrenfreund [START_REF] Foing | Detection of Two Interstellar Absorption Bands Coincident with Spectral Feature of C + 60[END_REF] based on low-temperature Ne matrix-isolation spectra obtained by Fulara et al. [START_REF] Fulara | Electronic and Infrared Spectra of C + 60 and C -60 in Neon and Argon Matrices[END_REF] . DIBs are broad absorption features (with bandwidths varying between 2 and 80 cm -1 ) predominantly covering the 0.4 to 0.9 µm range in the spectra of reddened stars. [START_REF] Jenniskens | A Survey of Diffuse Interstellar bands (3800-8680 Å)[END_REF][START_REF] Hobbs | Studies of the Diffuse Interstellar Bands. III. HD 183143[END_REF][START_REF] Sarre | The Diffuse Interstellar Bands: A Major Problem in Astronomical Spectroscopy[END_REF][START_REF] Herbig | The Diffuse Interstellar Bands[END_REF] The original DIB spectral window has been progressively extended to the NIR range through several observational studies. [START_REF] Joblin | Detection of Diffuse Interstellar Bands in the Infrared[END_REF][START_REF] Groh | Spectral Atlas of Massive Stars around He I 10 830 Å[END_REF][START_REF] Geballe | Infrared Diffuse Interstellar Bands in the Galactic Centre Region[END_REF][START_REF] Cox | VLT/X-Shooter Survey of Near-Infrared Diffuse Interstellar Bands[END_REF][START_REF] Rawlings | A High-Resolution Study of Near-Infrared Diffuse Interstellar Bands[END_REF][START_REF] Hamano | Near Infrared Diffuse Interstellar Bands Toward the Cygnus OB2 Association[END_REF][START_REF] Elyajouri | Near-Infrared Diffuse Interstellar bands in APOGEE Telluric Standard Star Spectra . Weak bands and Comparisons with Optical Counterparts[END_REF][START_REF] Rawlings | Near-Infrared Diffuse Interstellar Bands towards Her 36[END_REF][START_REF] Galazutdinov | Infrared Diffuse Interstellar Bands[END_REF] Primarily driven by the PAH-DIB hypothesis, [START_REF] Salama | Assessment of the Polycyclic Aromatic Hydrocarbon-Diffuse Interstellar Band Proposal[END_REF][START_REF] Cox | The PAH-DIB Hypothesis[END_REF] the matrix isolated NIR spectra of a few ionic PAHs were recorded and several electronic transitions between 0.7 and 2.5 µm spectral range were measured. [START_REF] Mattioda | Experimental Near-Infrared Spectroscopy of Polycyclic Aromatic Hydrocarbons between 0.7 and 2.5 µm[END_REF] Spectral shifts in the position of the rovibronic bands, induced by the interaction of the isolated PAHs with the matrix, however prevented decisive identification of the NIR DIBs observed by Cox et al. [START_REF] Cox | VLT/X-Shooter Survey of Near-Infrared Diffuse Interstellar Bands[END_REF] . Barring electronic transitions, the NIR spectral region is also associated with higher order vibrational transitions [START_REF] Geballe | Detection of the Overtone of the 3.3 µm Emission Feature in IRAS 21282+5050[END_REF] which have been less scrutinized in the context of laboratory astrophysics. the band positions measured in the laboratory in rare gas matrices at low temperatures. [START_REF] Sassara | Absorption Wavelengths and Bandwidths for Interstellar Searches of C 60 in the 2400-4100 Å Region[END_REF] Surprisingly, none of the PNe showed the transitions in the expected wavelength range.

According to Cami [START_REF] Cami | Can Fullerene Analogues be the Carriers of the Diffuse Interstellar Bands?[END_REF] , neutral C 60 is unlikely a carrier of visible strong DIBs as this would require a too large fraction of cosmic carbon to be locked up in this species. The nonobservation of C 60 did not however, question the fact that a significant fraction of this stable molecule is expected to be neutral. Bakes and Tielens 55 had inferred that in the diffuse ISM ∼ 60% of C 60 is in the neutral form, ∼ 10% is in the cationic and ∼ 30% is in the anionic form. In a more recent work, Berné et al. 17 reported the observation of neutral C 60 in the diffuse ISM and in a star forming region using both emission and absorption measurements.

They derived a neutral to ion ratio in between 0.15 and 2.

The major goal of this paper is to provide the first spectroscopic data of neutral C 60 from 1.65 to 2.78 µm at various temperatures and to give a tentative assignment to the observed bands to the calculated transitions using quantum chemical calculations. This work paves the way for the future gas-phase study of C 60 using an ultra-sensitive absorption spectrometer coupled with a hypersonic source developed recently. [START_REF] Louviot | Strong Thermal Nonequilibrium in Hypersonic CO and CH 4 Probed by CRDS[END_REF][START_REF] Dudás | High-Temperature Hypersonic Laval Nozzle for Non-LTE Cavity Ringdown Spectroscopy[END_REF] The paper is structured as follows: in Section 2 we provide the experimental and theoretical methodology employed in this work. In Section 3 we present our results and discuss them. In Section 4, we provide some insights for gas-phase spectroscopy of C 60 and discuss astrophysical implications. Finally, in Section 5 we wrap-up with conclusions.

Methodology

Experimental Details

Fullerene C 60 used in this experiment was purchased from SES Research (99.5% purity) and was used without further purification. About 52.09 mg of C 60 was mixed with ∼ 200 mg of KBr (FTIR grade 98%) in an agate mortar and was ground thoroughly for 15 minutes. The mixture was put in a hydraulic press and was pressed under 10 tonnes of pressure for 5 minutes to form a pellet. The diameter and thickness of the pellet was 12.76 mm and 0.85 mm, respectively. The concentration of C 60 in the pellet was 6.66 × 10 -4 mol/cm 3 . A blank pellet containing ∼ 200 mg of KBr was prepared in the same way without C 60 .

The infrared spectra of blank and C 60 sample pellets were recorded from 6052 to 3596 cm -1 (1.65 to 2.78 µm) with the ESPOIRS apparatus at the IRAP laboratory in Toulouse [START_REF] Chakraborty | Experimental Approach to the Study of Anharmonicity in the Infrared Spectrum of Pyrene from 14 to 723K[END_REF][START_REF] Demyk | Low Temperature MIR to Submillimeter Mass Absorption Coefficient of Interstellar Dust Analogues. I. Mg-rich Glassy Silicates[END_REF] from 11 to 300 K with an increasing temperature step of 50 K. A smaller temperature step size of 10 K was used near the phase transition temperature (∼ 260 K) of C 60 . The transmittance spectrum at each specific temperature was calculated by dividing the sample spectrum with the background spectrum at that temperature. The spectral resolution was kept at 4 cm -1 and the acquired interferograms were averaged over 256 scans to increase the signal-tonoise ratio. All spectra were processed using the OPUS suite of programs provided by Bruker optics. Local linear continua were subtracted from the observed spectra at each temperature using the same anchor points for regions I and II (see Figure 1).

The ESPOIRS setup consists of a Bruker Vertex 70 vacuum FTIR spectrometer equipped with multiple sample and detector compartments. For this experiment, a CaF 2 beamsplitter and a room temperature, deuterated triglycine sulfate (DLaTGS) detector was used. This setup is coupled with a helium cryo-cooling device. The optical bench of the spectrometer is pumped via a primary pump while the cryostat in which the sample was placed was pumped using a turbomolecular pump. The pressure inside the sample compartment was brought down to ∼ 10 -5 mbar before turning on the cryo-cooler. By lowering the temperature, the pressure dropped to 10 -7 mbar. The experimental temperature was set using a Lakeshore temperature controller.

Theory

The icosahedral fullerene the remaining 32 are silent modes. However, in the crystal, the intermolecular interactions reduce the local symmetry of the fullerene cage, leading to very efficient activation of all the silent modes, that can then be seen both through infrared and Raman spectroscopy. [START_REF] Procacci | Computer Simulation of Solid C 60 using Multiple Time-Step Algorithms[END_REF] In addition, one has to consider the so-called isotopically activated modes. In natural fullerene, the abundance of the isotopologue 13 C 12 C 59 is about 37.5%, which is far from being negligible.

It has been demonstrated that the silent modes are also activated by the presence of the 13 C isotope in the molecule which reduces its symmetry from I h to C s . [START_REF] Love | Near-infrared Raman spectroscopy of Solid C 60 . Raman Activation of Silent Modes by 13 C and Sample Disorder[END_REF] Considering these two effects, we therefore decided to compute all the possible combination and overtone bands falling in the observed region as all of them can contribute to the infrared spectrum.

The Hessian matrix at the optimized geometry of C 60 was calculated using the B3LYP level of theory in conjunction with the 6-311G* basis set in the Cartesian framework. Diagonalization of the mass-weighted Cartesian Hessian matrix gives the harmonic frequencies (ν unscaled ) as listed in Table S1 (cf. supporting information). Then, instead of considering the anharmonic effects by scaling the harmonic frequencies with some ad hoc scale factor, as done by Schettino et al. 12 , an inhouse python program [START_REF]AnharmonicIPR20[END_REF] was employed to establish a linear regression between the calculated and the reported experimental frequencies [START_REF] Martin | Vibrational Study of 13 enriched C 60 Crystals[END_REF]12 (cf. Table S1). For that, we used the assignment of the modes provided by Schettino et al. 12 . Considering only the Raman (A g , H g ) and infrared (T 1u ) active modes, our approach leads to a Root-Mean-Square (RMS) of 4.8 cm -1 , very similar that of 4.6 cm -1 obtained by Schettino et al. 12 However, our approach leads to a better RMS of 5.5 cm -1 (versus 6.6 cm -1 for Schettino et al. 12 ) by considering the complete set of 46 fundamental modes (Raman, infrared and silent modes).

As the NIR spectrum of neutral C 60 from 1.65 to 2.78 µm is dominated by the vibrational transitions due to ∆v = 3 and 4 quanta, we then computed all possible 3-and 4-quanta combination and overtone transitions in the ranges of 6050 to 5565 and 4765 to 3596 cm -1 (cf.

Table S2 andS3). We did not include in our calculation, the various anharmonic resonances bands by several cm -1 . We also did not calculate the different hot bands, negligible at very low temperatures though not at room temperature, for which the population of the lowest vibrational level located at 264 cm -1 is about 1.4 times greater than in the ground state.

In the gas-phase, the modes of C 60 which are activated by the intermolecular interactions specific to the crystal will disappear. The isotopically activated modes will remain but their overall infrared intensity was estimated by Schettino et al. 12 to be only 3.4% that of the T 1u derived modes. This means that, in the framework of a future infrared spectroscopic study or detection of the C 60 molecule in the gas-phase, it is important to distinguish the IR active modes which are dominating the infrared absorption spectrum, that is to say those which are characterized by at least one T 1u symmetry element. We performed a group theoretical analysis of the calculated combination modes of T 1u symmetry using a standard quantum chemical approach, [START_REF] Atkins | Molecular Quantum Mechanics: Fourth Edition[END_REF] as listed in Tables S2 andS3. The theoretical methodologies have been described in detail in the supporting information.

Results and Discussion

NIR spectrum of C 60

In Figure 1, we present the absorption spectra of C 60 at 11 and 300 K ranging from 6052 to 3596 cm -1 . Strong and marked spectral features are noticed between 4765 and 3596 cm -1 (Region I) while several weak absorption features are distributed over the 6052 to 5565 cm -1 region (Region II). A quick look shows no significant change in the 11 and 300 K spectra. A closer observation however, reveals slightly sharper and more intense peaks at 11 K. A combination band of water is observed at 5275 cm -1 (1.89 µm) (see Figure 1 In the left and right panels of Figure 2, we present the absorption spectra of C 60 from 4765 to 3596 and from 6052 to 5565 cm -1 ranging from 11 to 300 K, respectively. The observed spectra contain many peaks and shoulders. Bands of Region I are more structured than those in Region II, being comparatively less resolved with several humps and shoulders.

The positions of the peak maxima of the bands of both regions and their relative absorbance values at 11 K are listed in Table 1. There are 39 distinct absorption bands in Region I.

On the other hand, in Region II, three bands dominate at 5937.4, 5889.9 and 5846.6 cm -1 .

Apart from these three, six less isolated absorption features are observed below 5840 cm -1 . 9

Spectral interpretation of the bands in Regions I and II

Wang et al. [START_REF] Wang | Observation of Higher-Order Infrared Modes in Solid C 60 Films[END_REF] previously recorded the IR spectrum of a C 60 film deposited on a KBr substrate at 300 K in the range from 4000 to 400 cm -1 . To date, this is the only study where the In order to comment on the intensity, it is important to include the higher order dipole derivatives of the Taylor series expansion of the dipole moment surface. This is a huge task which is beyond the scope of the present work. In

Figures 3 and4 we present however, the calculated cold band positions for Regions I and II, together with the experimental spectrum of C 60 recorded at 11 K for which the contribution of the hot bands is totally absent. These calculations do not reveal any particular grouping of energy levels in packets which could explain the presence of the well identified absorption peaks. Indeed, accidental vibrational degeneracies are responsible for the vibrational polyad structure of highly symmetric molecules, such as methane for example.

Our calculations show that the bands appearing between 1.65 and 2.78 µm must have at least 3-or 4-quanta states. Due to its lower energy, Region I is populated with both 3 and 4 quanta states. Since the oscillator strengths of transitions starting from 3-quanta states are higher than that starting from 4-quanta states, we presume that the observed bands in Region I are primarily due to 3-quanta transitions. For Region II, our calculations predict only 4-quanta transitions and nullify the possibility of 3-quanta states due to energy mismatch. However, there could be 5-quanta states in Region II but the associated oscillator strengths will be too weak to show up in this region. We simulated a total of and 1797 4-quanta transitions in Region I and II, respectively and these are marked with green vertical lines in Figures 3 and4. In total, our calculation predicts 3895 transitions in the spectral range from 1.65 to 2.78 µm Surprisingly, the overall shape of the spectrum hardly changes from 11 to 300 K while the contribution of a large number of hot bands is expected at room temperature. Amongst the 46 normal modes of C 60 , 7 of them have vibration frequencies below 500 cm -1 and should contribute significantly to the room temperature spectrum. A closer look reveals that the positions of the peaks actually evolve with temperature, either towards low or high wavenumbers depending on the bands. In the supporting information we report the evolution of a few peaks with temperature. At room temperature, the main peaks are broader while their amplitude is smaller. The peak broadening results in the overlapping contribution of hot bands shifted by vibrational anharmonicities, while the smaller amplitude relates to the population of the ground state which is smaller at room temperature.

Interestingly, we observe a more pronounced shift for a number of peaks at 260 K which nicely confirms the observation of Narasimhan et al. [START_REF] Narasimhan | Infrared Spectroscopy Through the Orientational Phase Transition in Fullerene Films[END_REF] to 1083 K. They also observed 14 additional weak features between 2170 and 951 cm -1 but could not assign them. The weak feature they observed at 1538 cm -1 has only been recently assigned to a 2-quanta combination transition involving an H g fundamental mode at 772 cm -1 and a G u fundamental mode at 764 cm -1 . 66,67 None of these two modes are IR active, however their 2-quanta combination possesses a T 1u symmetry element and is therefore active in the infrared. In particular, this band was also observed in the astronomical spectrum of C 60 by Cami et al. 14 . Of very valuable help, the data produced by Nemes et al. 11 are nevertheless affected by a very significant contribution of hot bands responsible for a shift of several cm -1 of the peak maxima, as can be seen by comparing the values of their reported T 1u active modes with those deduced from combined ab initio and solid phase studies.

We believe that the solid phase spectra produced at very low temperature in this work can give some interesting insights for a detection of C 60 in the gas-phase. What changes are going to take place from the solid to the gas-phase? Firstly, the weak transitions due to the silent modes activated by intermolecular interactions will disappear. Secondly, the peak splitting due to the removal of degeneracy of the IR active modes induced by the symmetry lowering associated with the sc crystal phase 65 will no longer exist, and a number of low intensity satellite peaks will also disappear. On the other hand, the isotopically activated modes will be present and will continue to produce IR transitions of weaker intensity. In addition, we expect that the absorption bands in the NIR will be broadened by strong intramolecular resonances coupling multiple high-order vibrational states, preventing access to their rotational structure. In conclusion, the NIR gas-phase spectrum is expected to be slightly more decongested with sharper peaks.

Therefore, in support of our future low temperature, gas-phase absorption spectroscopy of C 60 we have computed the IR active combination and overtone transitions in the region be- We anticipate these 1862 transitions will appear in the absorption spectrum of gas-phase C 60 , however those involving at least one or more quanta of the four IR active fundamentals (c.f. Table S1) will have the strongest intensity. Figure 5 presents a statistical analysis of the calculated bands by considering all posssible combination and overtone bands of C 60 in the solid-phase together with those expected to be observed in the gas-phase (having T 1u symmetry). From Figs. 5 (a) and (b) it is evident that approximately 50% of the calculated transitions in the solid-phase under each bin, have T 1u symmetry and therefore will be observed in the gas-phase. It can be seen that the density of 3-quanta states decreases from 3596 to 4765 cm -1 and the density of the 4-quanta states peaks between 5600 and 5700 cm -1 . A decisive identification of higher quanta vibrational transitions of C 60 and the NIR DIBs in the astronomical absorption spectra can be made upon comparison with the band positions obtained from the gas-phase absorption spectroscopy. In Table 2 we have listed a few observed DIBs in the near infrared range together with our experimental band positions.

We note that three DIBs observed at 16588, 17803 and 21887 Å by Cox et al. [START_REF] Cox | VLT/X-Shooter Survey of Near-Infrared Diffuse Interstellar Bands[END_REF] are only 4, -66 and 18 Å away from our solid-phase measurement. Two more DIBs observed by Galazutdinov et al. [START_REF] Galazutdinov | Infrared Diffuse Interstellar Bands[END_REF] at 21274 and 23919 Å are 32 and -37 Å away from the data reported in this work.

So far, all DIBs are observed using ground based telescopes and their detection relies on a good subtraction of telluric features. Hence, more NIR DIBs between 10,000 to 25,000

Å could be hidden in the noise and not retrieved by the telluric line removal process. [START_REF] Cox | VLT/X-Shooter Survey of Near-Infrared Diffuse Interstellar Bands[END_REF] The upcoming launch of the James Webb Space Telescope with its onboard NIRSpec instrument, will greatly increase the sensitivity of the observations in the NIR region without contribution from telluric atmospheric lines. It should allow new NIR DIBs to be detected in various astronomical environments. 17 We finally thank the referees who greatly contributed improving of the manuscript.

Supporting Information Available

• Theoretical methodology

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 333 and the NGC 7023 and 2023 reflection nebu-Paragon Plus Environment ACS Earth and Space Chemistry A c c e p t e d M a n u s c r i p t

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 433 The identification of C + 60 as the carrier of five electronic NIR DIBs inevitably raised the possibility of other fullerene species as potential DIB carriers. In an earlier study, García-Hernández and Díaz-Luis[START_REF] García-Hernández | Diffuse Interstellar Bands in Fullerene Planetary Nebulae:The Fullerenes -Diffuse Interstellar Bands Connection[END_REF] searched for the three strong electronic transitions of C 60 at ∼ Paragon Plus Environment ACS Earth and Space Chemistry A c c e p t e d M a n u s c r i p t 3760, 3980 and 4024 Å in two planetary nebulae (PNe) Tc1 and M 1-20 in proximity to

5 Page

 5

6 Page

 6 C 60 molecule belongs to the I h symmetry group and possesses 46 normal modes of which only 4 are infrared active (T 1u symmetry), 10 are Raman active and

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 733 such as Fermi and Darling Dennison resonances which can shift the interacting vibrational Paragon Plus Environment ACS Earth and Space Chemistry A c c e p t e d M a n u s c r i p t

Figure 1 :

 1 Figure 1: Variable temperature NIR spectra of C 60 from 6052 to 3596 cm -1 (from 1.65 to 2.78 µm). Region I and II correspond to the spectral range from 4765 to 3596 and from 6052 to 5565 cm -1 , respectively.

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 33 higher order vibrational transitions (up to 2 quanta) of C 60 have been assigned, though these assignments were questioned by the more recent study by Schettino et al.12 . Along with the 4 IR active fundamental modes, they identified 98 two-quanta combination transitions in the 4000-400 cm -1 range. They simulated two bands at 4287.6 and 3548.7 cm -1 that were 10 Paragon Plus EnvironmentACS Earth and Space ChemistryA c c e p t e d M a n u s c r i p t assigned to the third overtone of the two highest frequency IR active fundamental modes of C 60 belonging to T 1u symmetry. However, they did not observe any bands above 3500 cm -1 . In contrast, we could observe 15 peaks between 3500 and 4000 cm -1 , probably due to a better signal-to-noise ratio.Each of the clearly identifiable peaks observed in our laboratory spectra is in fact made up of dozens of transitions connecting the vibrational ground state and the thermally populated excited vibrational states to many upper combination and overtone vibrational modes, eventually interacting via Fermi or Darling-Dennison resonances. These transitions overlap to form a "global" peak which cannot be interpreted simply on the sole basis of a calculation of the position of the vibrational energy levels and without taking into account the intensities of the different transitions involved.

Figure 3 :

 3 Figure 3: Experimental NIR spectrum of C 60 at 11 K together with the position of the combination and overtone bands computed at 0 K and limited to 3 quanta. The green vertical lines show a total of 2098 combination and overtone transitions computed in this region. Peak positions at the band maximum are marked in the figure.

Figure 4 :

 4 Figure 4: Experimental NIR spectrum of C 60 at 11 K together with the position of the combination and overtone bands computed at 0 K and limited to 4 quanta. The green vertical lines show a total of 1797 combination and overtone transitions computed in this region. Peak positions at the band maximum are marked in the figure.

4

  Insights for gas-phase spectroscopy of C 60 and astrophysical implicationsA key issue for the detection of molecular C 60 in the cold ISM in the NIR is how different the gas-phase spectra would be from the solid phase? So far, the infrared data recorded in the gas-phase is very scarce. Apart from the IR active mode at 1180 cm -1 recorded recently by Changala et al. 10 at ∼ 135 K by absorption spectroscopy the only study dealing with the gas-phase spectroscopy of C 60 is that of Nemes et al. 11 . They observed the 4 IR active T 1u 14 Page 14 of 33 ACS Paragon Plus Environment ACS Earth and Space Chemistry A c c e p t e d M a n u s c r i p t modes at 1407.2, 1165.8, 570.0 and 527.5 cm -1 by emission spectroscopy of C 60 vapor heated

tween 1 .

 1 65 and 2.78 µm arising from all the 46 fundamental modes. Regions I and II contain 986 and 876 combination and overtone transitions of T 1u symmetry element, respectively. 15 Page 15 of 33 ACS Paragon Plus Environment ACS Earth and Space Chemistry A c c e p t e d M a n u s c r i p t

Figure 5 :

 5 Figure 5: Density of 3-and 4-quanta transitions of C 60 in Region I and II. Black and red histograms shows the number of calculated transitions in the solid and gas-phase, respectively.

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 142127 Cami, J.; Bernard-Salas, J.; Peeters, E.; Malek, S. E. Detection of C 60 and C 70 in a Young Planetary Nebula. Science 2010, 329, 1180-1182. (15) García-Hernández, D. A.; Manchado, A.; García-Lario, P.; Stanghellini, L.; Villaver, E.; Shaw, R. A.; Szczerba, R.; Perea-Calderón, J. V. Formation of Fullerenes in H-Containing Planetary Nebulae. Astrophys. J. 2010, 724, L39-L43. (16) Sellgren, K.; Werner, M. W.; Ingalls, J. G.; Smith, J. D. T.; Carleton, T. M.; Joblin, C. C 60 in Reflection Nebulae. Astrophys. J. 2010, 722, L54-L57. (17) Berné, O.; Cox, N. L. J.; Mulas, G.; Joblin, C. Detection of Buckminsterfullerene Emission in the Diffuse Interstellar Medium. Astron. Astrophys. 2017, 605, L1. s c r i p t (65) van Loosdrecht, P. H. M.; van Bentum, P. J. M.; Meijer, G. Rotational Ordering Transition in Single-Crystal C 60 Studied by Raman spectroscopy. Phys. Rev. Lett. 1992, 68, 1176-1179. (66) Menéndez, J.; Page, J. B. In Light Scattering in Solids VIII: Fullerenes, Semiconductor Surfaces, Coherent Phonons; Cardona, M., Güntherodt, G., Eds.; Springer Berlin Heidelberg: Berlin, Heidelberg, 2000; pp 27-95. (67) Brieva, A.; Gredel, R.; Jäger, C.; Huisken, F.; Henning, T. C 60 As a Probe for Astrophysical Environments. Astrophys. J. 2016, 826, 122.

  Environment ACS Earth and Space ChemistryA c c e p t e d M a n u s c r i p t

Table 1 :

 1 Figure 2: Variable temperature NIR spectra of C 60 in Region I (from 4765 to 3596 cm -1 ) and Region II (from 6052 to 5565 cm -1 ) from 11 to 300 K. A vertical offset has been added to each spectrum for clarity. Enlarged spectra at 11 K for both region I and II are shown in Figs 3 and 4 Experimental band positions and their relative absorbances of C 60 between 6052 and 3596 cm -1 at 11 K. Abs. No Position Rel. Abs. No Position Rel. Abs. No Position Rel. Abs. No Position Rel. Abs. No Position Rel. Abs.

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Table 2 :

 2 Experimental band positions of neutral C 60 at 11 K (cf. Fig.3and Fig.4) and the observed NIR DIBs in various astronomical objects. The experimental band positions reported in this work were obtained from the solid phase study.This study aims to provide guidance for future gas-phase absorption spectroscopy of neutral C 60 . Our work is a first building block towards the systematic exploration of overtone and combination bands of neutral C 60 , in connection with the C 60 -DIBs hypothesis. In this framework we have measured the NIR spectrum of C 60 from 11 to 300 K between 1.65 and 2.78 µm and the observed bands are tentatively assigned to 3-and 4-quanta combination and overtone transitions. We computed a total of 3895 combination and overtone transitions amongst which 1862 transitions have IR active T 1u symmetry. We expect them to be observed in the spectrum of gas-phase C 60 . Due to the presence of a large number of normal modes, a complete anharmonic calculation including various resonances of C60 has not yet been attempted, even in the frequency range from 4000 to 400 cm -1 . The theoretical methodology presented in this work does not compute the intensities of the high order vibrational transitions and the associated anharmonic effects but can be considered as the best effort to interpret the higher order vibrational transitions of C 60 . Although five frequencies observed in this work are close to the observed DIBs at 16588, 17803, 21887, 21274 and 23919 Å, conclusive evidence can only be established with the aid of gas-phase absorption spectra. This work was funded by the Indo-French Center for the Promotion of Advanced Research (CEFIPRA/IFCPAR) under project 6005-N and the Programme National "Physique et Chimie du Milieu Interstellaire" (PCMI) of CNRS/INSU with INC/INP co-funded by CEA and CNES. We thank Prof. Manuel Goubet from the Université de Lille and Dr. Rosine Lallement from GEPI, Observatoire de Paris for reading the manuscript and providing some insightful comments. SC thanks Dr. Sumanta Mukherjee from IBM India Research, for helping to develop the matrix minimization algorithm. SC is grateful to Dr. Subrata Banik from the School of Chemistry and Biotechnology, SASTRA University, India for many helpful discussions. SC thanks Dr. Christine Joblin, from the Institut de Recherche en Astrophysique et Planétologie, Toulouse for many fruitful discussions on C 60 and its astrophysical relevance.

	λ expt (Å) λ obs (Å) λ expt (Å) λ obs (Å)
		16567 a	21724
		16578 a	21842 a
	16592	16588 a	21887 a
	16842		21905
	16978		22107
	17104		22201
	17221		22268
	17333		22144 b
	17420		22585
	17535		22816
	17643		22971
	17737		23047
		17803 a	23205
		19940 b	23359
		20427 a	23505
		20877 b	23577
		21274 b	23795
		21387 a	23919 b
	21467		23951
	5 Conclusions

  • Table S1 Theoretical and experimental frequencies of C 60 Effect of temperature on the observed band position • Table S3 1797 combination and/or overtone bands of C 60 between 6005-5580 cm -1 • Table S4 2098 combination and/or overtone bands of C 60 between 4765-3596 cm -1 Wakabayashi, T.; Momose, T.; Fajardo, M. E. Matrix Isolation Spectroscopy and Spectral Simulations of Isotopically Substituted C 60 Molecules. J. Chem. Phys. 2019, 151, 60 and C 6- 60 : Scaled Quantum Mechanical Results Performed in Redundant Internal Coordinates and Compared to Experiments. J. Phys. Chem. A 2000, 104, 102-112.

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	(8) 234301.
	M a n u s c r i p t M (9) Stewart, J. T.; Brumfield, B. E.; Gibson, B. M.; McCall, B. J. Inefficient Vibrational Cooling of C 60 in a Supersonic Expansion. ISRN Phys. Chem. 2013, 2013, 1-10. (10) Changala, P. B.; Weichman, M. L.; Lee, K. F.; Fermann, M. E.; Ye, J. Rovibrational a n u s c r i p t Quantum State Resolution of the C 60 Fullerene. Science 2019, 363, 49-54.
	A c c e p t e d A c c e p t e d
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• (11) Nemes, L.; Ram, R. S.; Bernath, P. F.; Tinker, F. A.; Zumwalt, M. C.; Lamb, L. D.; Huffman, D. R. Gas-phase Infrared Emission spectra of C 60 and C 70 . Temperature-Dependent Studies. Chem. Phys. Lett. 1994, 218, 295 -303. (12) Schettino, V.; Pagliai, M.; Ciabini, L.; Cardini, G. The Vibrational Spectrum of Fullerene C 60 . J. Phys. Chem. A 2001, 105, 11192-11196. (13) Choi, C. H.; Kertesz, M.; Mihaly, L. Vibrational Assignment of All 46 Fundamentals of C