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Routine Measurements

We perform routine measurements on passive and active degassing as well as on eruptive products using various techniques:

SO2 emission rates determined by ground-measurements (UV Spectrometers : DOAS / FLYSPEC) :

SO2 emission rates, emitted during eruptive crisis or passive degassing, are often correlated with other parameters such as: (i) the rising conditions and the rheological properties of magma, (ii) the chemical and physical magmatic conditions (iii) tremor and deformation, and (iv) the presence and intensity of any hydrothermal activity (e.g. Rose et al., 1980; Andres et al., 1993; Symonds et al., 1994; Edmonds et al., 2003 ). Hence, a better understanding of SO2 fluxes improves not only our knowledge on the various physic-chemical parameters that control the dynamics of degassing, but also our prediction of volcanic hazards. Since the 1990s, SO2 emission rates have been measured by UV correlation absorption spectroscopy (COSPEC; e.g. Stoiber et al., 1983; Allard et al.,1994 ; Williams-Jones et al., 2008 ) and, more recently, by differential absorption spectroscopy (DOAS/FLYSPEC; Galle et al., 2002 ; Horton et al., 2006 ). DOAS/FLYSPEC spectrometers, which have a small size (20 x 20 cm), low weight (0.4 kg), minimal power consumption and low price, represent a huge step forward for volcanic monitoring. The column abundances of SO2 in a plume cross-section are measured along transects (slant or orthogonal) of the plume via traverses or scans from a fixed position. SO2 flux is obtained by integrating these quantities and then scaling with the wind speed along the plume axis.

At OPGC we have two USB4000 spectrometers characterised by a 50μm entrance slit with wavelengths between 280 and 422 nm. Two portable stations with USB2000 spectrometers are used at OVPF. In the frame of NOVAC EU project (http://www.novac-project.eu/), three permanent stations have been installed at Piton de la Fournaise since 2007. The stations are located on the edge of the caldera of Enclos Fouque and perform regular scans of the atmosphere above the summit of Piton de la Fournaise every day (one full scan every 13 minutes on average). The network geometry allows a reliable estimate of the height of the gas plume and its dispersion direction. Test measurements at La Soufrière de Guadeloupe were performed in 2015 and, from 2017 onwards, regular SO2 flux measurements will be performed.

In-Situ measurements of fumaroles and volcanic plumes by MultiGAS :

Since volcanic plumes are necessarily a mixture of volcanic gases and air, remote sensing of H2O and CO2 is limited by their high concentration in the atmosphere. This can be overcome by the in-situ measurement of plumes near the crater or active fumaroles, being careful to minimize chemical reactions or condensation effects. Analysis of the volcanic contribution to the H2O and CO2 concentrations is obtained after subtracting the atmospheric signal. The MultiGAS system is a robust, custom-made sensor allowing high-frequency (0.1 Hz) real-time detection of multiple species like H2O and CO2 with NDIR spectroscopy and of SO2, H2S, CO, CH4 and H2 using electrochemical sensors, along with key parameters (pressure, temperature, humidity) of the gaseous mix ( Shinohara, 2005 ; Aiuppa et al., 2005 ). Since about 10 years, MultiGAS has become widely used for gas studies (plumes and fumaroles) and monitoring on volcanoes worldwide (see Aiuppa et al., 2011 , Roberts et al., 2012 and references therein). The precision on concentrations in typically +/- 5-10%. The most common method to provide flux data for various species is to combine MultiGAS measurements (elemental ratios X/SO2, where X is any element in the gas) by SO2 flux data.

The OPGC has two portable MultiGas (ca. 5 kg of weight) with a variety of sensors and concentration ranges in order to be adapted to different volcanic environments. At Piton de la Fournaise, a permanent station has been installed at the volcano summit in 2011 in collaboration with Istituto Nazionale di Geofisica e Vulcanologia (INGV) - Palermo (A. Aiuppa; G. Giudice, G. Tamburello). Portable MultiGAS is used to monitor the emissions from the summit of Piton de la Fournaise and to analyse in situ the syn-eruptive emissions. At Soufrière de Guadeloupe, regular MultiGAS measurements are done since 2015. Moreover, since May 2016, a MultiGAS type-INGV (A. Aiuppa; G. Tamburello) is available at the OVSG.

Direct gas measurements by Giggenbach bottles :

Gas direct sampling by Giggenbach bottles (Giggenbach et Gogue, 1989) is dangerous and often impossible, analyses can be long to perform and gases can sometimes be contaminated by air. However, when possible, this technique provide the most complete gas composition (concentrations and isotopes) .Volcanic gas streaming in a tube is collected in pre-evacuated glass flasks partially filled with concentrated aqueous sodium hydroxide (NaOH) or P2O5 dessiccant. When the tube is inserted into the fumarole or vent, the gases bubble through the solution and acid gases like CO2, H2S, SO2, HCl and HF will dissolve into the liquid. Inert gases that remain undissolved, such as N2, O2, H2, CO and He, are collected in the head-space of the bottle and then analysed by gas chromatography. Dissolved species are analysed by ion chromatography or traditional wet-chemical techniques. Bulk analysis allows assessing the origin and nature of gas, its physical-chemical conditions, and the different processes that modify the original composition during ascent to the surface (e.g. Aguilera et al., 2011).

The OPGC has various Giggenbach bottles. This technique cannot be applied to the Piton de la Fournaise, which does not have a permanent accessible fumarole field. On the contrary, monthly sampling and analysis of gases from the main fumaroles of La Soufrière have been performed for over 40 years by the OVSG ( Bernard et al., 2006 , Villemant et al. , 2014)

Plume filtration by Filter Packs :

Volcanic gases and aerosols are considerably enriched in major (H2O, CO2, SO2, H2S, HCl, HF, etc.) and minor (N2, rare gases, CO, CH4, H2, etc.) components but also in many trace elements including alkali, alkali-earth, transition, and heavy metals (e.g. Zoller et al., 1974). Volcanic gases and particles emitted into the atmosphere are well known to participate significantly to pollution on a local and regional scale, and represent the most important source of natural emissions for many significant pollutants, such as sulphur and heavy metals for instance (e.g. Nriagu, 1989). The diluted plume is pumped through 3 filters in series: 2 impregnated filters with a caustic solution to trap acid gases (SO2, HCl, HF) and 1 filter to trap volcanic aerosols rich in trace elements. Ionic chromatography and ICP-MS are used to measure the gas and trace element concentrations, respectively ( Moune et al., 2010 ; Menard et al., 2014; Gauthier et al., 2016).

The OPGC has several pumping systems with various filters. This technique can not be used in an extremely humid environment like La Soufrière, Guadeloupe and will soon be tested at Piton de la Fournaise as part of a project financed by the ANR (STRAP project) .

CO2 diffuse emissions from the soil

Although the majority of volcanic gases are emitted around the main crater, Allard et al. (1991) showed that a significant fraction of the degassing can percolate through the sides of volcanic structures and be emitted by the ground. While reactive gases, such as SO2 and HCl, are released at the crater, degassing at ground level releases less reactive gases such as CO2. The study of soil degassing is important to complete the mass balance of gaseous emissions from a volcano, as well as to detect significant structures (e.g., fault, fractures; Giammanco et al., 2006 ; Lee et al, 2016 ) and delineate the hydrothermal areas (e.g., Werner et al., 2008 ). Indeed, correlation of soil gases and tectonic structures is an increasingly used approach to monitor volcano activity. The temporal evolution of the gas fluxes near fault zones, which represent highly permeable zones, is expected considering that CO2 is a poorly soluble species in the silicate liquid, released to great depths during magma ascent and decompression.

Since 2012, surface mapping of CO2 fluxes, using portable systems for the quantification of diffuse CO2 flux and permanent stations for the continuous (1 measurement /hour) soil CO2 concentration measurement in small boreholes are used at Piton de la Fournaise to track the time and space evolution of CO2 diffuse emissions over the whole volcano massif (Liuzzo et al., 2015 ). OVPF portable systems are a portable storage chamber ( Chiodini et al., 1998 ) manufactured by WestSystem. Continuous real-time measurement of diffuse degassing and regular measurement of its isotopic composition are performed on three stations at the main rift zone cutting the western flank of PdF, in collaboration with INGV-Palermo (M. Liuzzo; G. Giudice; S. Gurrieri). Environmental parameters (rain, wind speed and direction, pressure, temperature, humidity) are simultaneously recorded and their influence on the gas fluxes is evaluated in a temporal perspective.

Volatile budget: Combining the study of fluid and melt inclusions :

Measuring the amount and behaviour of dissolved volatile components in magmas constitutes the first step to constrain modelling and interpretation of the degassing processes and their link with volcanic activity. The pre-eruptive abundances of dissolved volatiles can be retrieved, over a large pressure range (from mantle depth to the surface), by probing the tiny droplets that become trapped in crystal phases during magma ascent and are preserved as quenched melt (glassy) and fluid inclusions during cooling (Anderson, 1974; Lowenstern, 2003 ). The residual volatile content of the degassed magma is determined by analysing the groundmass (glass + microlites). Microanalysis is performed by integrating several approaches (electron microprobe, LA-ICP-MS, Raman, SIMS, FTIR - see below for the description) to get a full chemical characterization of the samples. These analyses are performed at LMV and IPGP where we have extensive experience in performing such analyses (e.g. Métrich et Wallace, 2008 ; Di Muro et al., 2006 ; Schiano, 2003 ; Moune et al., 2007 ; Moune et al., 2012 ; Sigmarsson et al., 2013 ; Venugopal et al., 2016). Expected gas fluxes can be estimated by integrating degassing modelling with the volume of erupted magma. The comparative study of expected (modelled) and measured gas compositions permit to identify and quantify sink and sources of the volatile budget for a given volcanic system. Quantification of source terms is of paramount importance for accurate modelling of eruptive plume dynamics (Di Muro et al., 2015).

In the framework of GAZVOLC, we perform sampling and analysis of melt and fluid inclusions in past and present eruptive products from Piton de la Fournaise and La Soufrière de Guadeloupe.

Chemistry :

GAZVOLC use also facilities at LMV and IPGP :



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