M Gaborieau
email: marion.gaborieau@uca.fr

M Laubier

N Bolfan-Casanova

C A A Mccammon

D Vantelon

A I I Chumakov

F Schiavi

D R R Neuville

S Venugopal

Fe3+/ΣFe of olivine-hosted melt inclusions inferred from Mössbauer and XANES 1spectroscopy

Keywords: beam damage, oxygen fugacity, arc magma, synchrotron Mössbauer, melt inclusion, XANES

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Introduction

Many studies have demonstrated that arc basalts are more oxidized (~ QFM+1.5 ± 1; fO2 given relatively to the quartz-fayalite-magnetite buffer; Gaillard et al. 2015 and references therein) than MORBs (QFM-1.2 to ~ QFM; e.g. [START_REF] Berry | A re-assessment of the oxidation state of iron in MORB glasses[END_REF][START_REF] Bézos | The Fe3+/ΣFe ratios of MORB glasses and their implications for mantle melting[END_REF][START_REF] Birner | Peridotites and basalts reveal broad congruence between two independent records of mantle fO2 despite local redox heterogeneity[END_REF][START_REF] Christie | Oxidation states of midocean ridge basalt glasses[END_REF]Cottrell andKelley, 2011, 2013;[START_REF] Mallmann | The crystal/melt partitioning of V during mantle melting as a function of oxygen fugacity compared with some other elements (Al, P, Ca, Sc, Ti, Cr, Fe, Ga, Y, Zr and Nb)[END_REF][START_REF] O'neill | The oxidation state of iron in Mid-Ocean Ridge Basaltic (MORB) glasses: Implications for their petrogenesis and oxygen fugacities[END_REF][START_REF] Zhang | Determination of Fe3 +/ΣFe of XANES basaltic glass standards by Mössbauer spectroscopy and its application to the oxidation state of iron in MORB[END_REF]. However, the oxidation state of the subarc mantle is still debated. Kelley andCottrell (2009, 2012) and [START_REF] Brounce | Variations in Fe3+/PFe of Mariana Arc Basalts and MantleWedge fO2[END_REF][START_REF] Brounce | Temporal evolution of mantle wedge oxygen fugacity during subduction initiation[END_REF] suggest that variations in the Fe 3+ /ΣFe ratio measured in basaltic glasses and olivine-hosted melt inclusions from mid-ocean ridges and arcs can be explained by the sub-arc mantle being more oxidized than the mantle beneath ridges. On the other hand, [START_REF] Mallmann | The crystal/melt partitioning of V during mantle melting as a function of oxygen fugacity compared with some other elements (Al, P, Ca, Sc, Ti, Cr, Fe, Ga, Y, Zr and Nb)[END_REF] and [START_REF] Lee | Similar V/Sc systematics in MORB and arc basalts: Implications for the oxygen fugacities of their mantle source regions[END_REF][START_REF] Lee | The redox state of arc mantle using Zn/Fe systematics[END_REF] proposed the use of other fO2 proxies, namely V/Sc and Zn/ΣFe ratios in lavas and suggested that the oxidation state of the mantle wedge cannot be distinguished from that of the MORB mantle (see also [START_REF] Dauphas | Iron isotopes may reveal the redox conditions of mantle melting from Archean to Present[END_REF][START_REF] Li | The constancy of upper mantle fO2 through time inferred from V/Sc ratios in basalts[END_REF].

Fe is an abundant multivalent 3d-transition element in silicate glasses, and its speciation is linked to the oxygen fugacity of the system [START_REF] Kress | Mineralogy and Petrology The compressibility of silicate liquids containing Fe203 and the effect of composition, temperature, oxygen fugacity and pressure on their redox states[END_REF]. Many techniques allow measurements of the Fe oxidation state, such as wet chemistry [START_REF] Bézos | The Fe3+/ΣFe ratios of MORB glasses and their implications for mantle melting[END_REF][START_REF] Christie | Oxidation states of midocean ridge basalt glasses[END_REF], Mössbauer spectroscopy (e.g. [START_REF] Jayasuriya | A Mössbauer study of the oxidation state of Fe in silicate melts[END_REF][START_REF] Mccammon | Mössbauer spectroscopy: Applications[END_REF][START_REF] Partzsch | Mössbauer spectroscopic determination of Fe3+/Fe2+in synthetic basaltic glass: A test of empirical fO2equations under superliquidus and subliquidus conditions[END_REF]Zhang et al., 2018), the Flank method on the electron probe (e.g. [START_REF] Fialin | Fe3+/ΣFe vs. FeLα peak energy for minerals and glasses: Recent advances with the elec-tron microprobe[END_REF]Zhang et al., 2018) and X-Ray absorption near-edge structure (XANES) spectroscopy. Micro-XANES has been widely used to provide Fe 3+ /ΣFe in silicate glasses (e.g. [START_REF] Berry | XANES calibrations for the oxidation state of iron in a silicate glass[END_REF][START_REF] Cottrell | High-precision determination of iron oxidation state in silicate glasses using XANES[END_REF][START_REF] Wilke | Determination of the iron oxidation state in basaltic glasses using XANES at the K-edge[END_REF]. Its low detection limit and high spatial resolution of ~1-5 µm makes it the ideal technique to analyze melt inclusions trapped in olivine (e.g. [START_REF] Brounce | Variations in Fe3+/PFe of Mariana Arc Basalts and MantleWedge fO2[END_REF][START_REF] Hartley | Olivinehosted melt inclusions as an archive of redox heterogeneity in magmatic systems[END_REF]Kelley andCottrell, 2009, 2012;[START_REF] Moussallam | The impact of degassing on the oxidation state of basaltic magmas: A case study of Kīlauea volcano[END_REF]. Because magmas trapped as melt inclusions in primitive crystals are less affected by secondary processes such as degassing of volatiles and fractional crystallization than external magmas, they provide a unique window into the pre-eruptive conditions of magmas.

However, a recent study by [START_REF] Cottrell | A Mössbauer-based XANES calibration for hydrous basalt glasses reveals radiation-induced oxidation of Fe[END_REF] showed that hydrous glasses can be affected by beam-induced oxidation during XANES analysis. According to this study, the iron oxidation state of glasses can be modified as a function of the radiation dose (which they defined as the total photons delivered per square micrometer) used to perform analyses, their water contents and their initial iron oxidation state. They found that the higher the radiation dose and water content of glasses and the lower their initial Fe 3+ /ΣFe, the higher the over-estimation of their Fe 3+ /ΣFe ratios.

In this study, Fe 3+ /ΣFe ratios in olivine-hosted melt inclusions from various arcs, OIB and MORB localities were analyzed by µ-XANES spectroscopy after determination of the analytical conditions that allowed us to mitigate beam damage. In addition, for the first time, the Fe 3+ /ΣFe ratios of a subset of those melt inclusions were analyzed by synchrotron Mössbauer source spectroscopy. The Fe 3+ /ΣFe ratios obtained with these two methods on the same sample set allow us to constrain the oxidation state of iron in those magmas by avoiding the effect of beam-induced oxidation that may occur during XANES analysis. Then we were able to confirm that arc magmas are more oxidized than those from hot spots and mid-ocean ridges. Our results suggest that the variation in the oxidation state observed in our melt inclusion dataset may be linked to changes in the oxidation state of the mantle beneath arcs due to subduction processes.

Natural samples

Secondary processes such as magmatic differentiation and degassing of volatile species can affect magmas during their ascent to the surface and modify their chemical composition and their oxidation state (e.g. [START_REF] Cottrell | The oxidation state of Fe in MORB glasses and the oxygen fugacity of the upper mantle[END_REF][START_REF] Kelley | The influence of magmatic differentiation on the oxidation state of Fe in a basaltic arc magma[END_REF][START_REF] Moussallam | The impact of degassing on the oxidation state of basaltic magmas: A case study of Kīlauea volcano[END_REF]. To investigate the oxidation state of primitive magmas, we selected melt inclusions trapped in high-Mg olivines (i.e., with a forsterite content > 80%). We selected primary melt inclusions that were far from any crack, did not display daughter minerals or devitrification and therefore did not require re-homogenization. The 33 studied melt inclusions are larger than 30 µm and most of them contain a bubble that most likely formed after entrapment (Figure 1 of the Supplementary material).

We selected glassy melt inclusions from a picritic lapilli layer collected on the Torgil tuff ring and from the Red Cliff pyroclastic sequence (Ao15, Ao17, Ao2T-T, Ao-T; [START_REF] Sorbadere | Insights into the origin of primitive silica-undersaturated arc magmas of Aoba volcano (Vanuatu arc)[END_REF] of Aoba volcano in the Vanuatu arc. Melt inclusions from Mont Etna were selected from the FS marker bed located on its eastern flank, and formed during a subplinian picritic eruption dated at 3930 ± 60 BP (SVP291c; [START_REF] Coltelli | Explosive eruption of a picrite: The 3930 BP subplinian eruption of Etna volcano (Italy)[END_REF][START_REF] Kamenetsky | Arrival of extremely volatile-rich high-Mg magmas changes explosivity of Mount Etna[END_REF]. We also selected melt inclusions from scoria samples from the ~50 000 year old La Sommata pyroclastic cone on Vulcano Island (Som; [START_REF] Rose-Koga | Mantle source heterogeneity for South Tyrrhenian magmas revealed by Pb isotopes and halogen contents of olivine-hosted melt inclusions[END_REF], and from a tephra layer located in a trench dug at 450 m a.s.l. on the northeastern flank of Stromboli volcano (St82p; [START_REF] Métrich | Crystallization driven by decompression and water loss at Stromboli volcano (Aeolian Islands, Italy)[END_REF][START_REF] Rosi | Onset of the persistent activity at Stromboli Volcano (Italy)[END_REF] in the Aeolian arc. Melt inclusions were selected from a Pleistocene-aged tephra layer from the mosaic ridge in the Mount Meager Volcanic Complex in the Garibaldi volcanic belt in the Cascades volcanic arc Venugopal et al.,in prep). Previously studied melt inclusions from a picritic scoria sample from Piton de Caille on the Northwest rift-zone of Piton de la Fournaise, Reunion Island (CAI; Bureau et al., 1998a;Bureau et al., 1998b;Laubier, 2006) and from mid-ocean ridge basalts from the FAMOUS zone (Northern Mid-Atlantic Ridge; ARP73-10-03; [START_REF] Kamenetsky | Methodology for the study of melt inclusions in Cr-spinel, and implications for parental melts of MORB from FAMOUS area[END_REF][START_REF] Laubier | Olivine-hosted melt inclusions and melting processes beneath the FAMOUS zone (Mid-Atlantic Ridge)[END_REF][START_REF] Laubier | Melting and crustal processes at the FAMOUS segment (mid-atlantic ridge): New insights from olivine-hosted melt inclusions from multiple samples[END_REF] and the Gakkel ridge are also part of our dataset.

Experimental samples

A series of 14 basaltic glasses were synthetized experimentally and used as reference material for XANES measurements. The starting composition was a MORB glass collected in the axial rift of the Pacific mid-ocean ridge (Searise 1 DR 04; Latitude 6.71, and depth = 2800 m). The sample was ground under ethanol in an agate mortar to a homogeneous powder mixture.

Experiments under anhydrous conditions at 0.1 MPa (ML1 to 9) were performed at Laboratoire Magmas et Volcans (Clermont-Ferrand, France) in a vertical quenching furnace, with a CO2-H2 gas atmosphere. The oxygen fugacity was monitored using a ZrO2-CaO oxygen cell calibrated against the Fe-FeO, Ni-NiO and Cu-Cu2O buffers. In order to cover a range of redox conditions, experiments were performed at different fO2 conditions ranging from -3,5 log units below the quartz-fayalite-magnetite buffer (QFM-3.5) to QFM+4. The sample material for these experiments consisted of ~ 50-70 mg of the mixture that had been mixed with polyvinyl alcohol and pressed into a pellet. This pellet was then sintered onto an iron-platinum alloy loop, which had been previously annealed with the starting composition in order to prevent Fe loss to the alloy during the experiment. The run temperature was continuously monitored using a Pt-Pt90Rh10 thermocouple. The thermocouple was placed in the hotspot of the furnace, where the thermal gradient is <1°C over the length of the sample. The sample was suspended alongside the thermocouple in order to minimize the difference between the temperature of the sample and the reading of the thermocouple. The reproducibility and measurement of this arrangement is better than 5°C. The experiments were run isothermally at 1350 °C at the run conditions and typically for 24h.

The BH hydrous glasses (BH1, BH3, BH6 BH7 and BH10) were synthesized at Laboratoire Magmas et Volcans in a piston-cylinder apparatus at 1 GPa and 1300 °C using a 1/2" assembly. The starting materials consisted of anhydrous glasses that were first synthesized by re-melting the starting material in a gas-mixing furnace at one bar, 1350 °C and three different fO2 conditions of ~QFM, QFM+2 and QFM+4. The glasses were then ground under ethanol in an agate mortar to a homogeneous powder mixture. For the piston-cylinder experiments, we used a Pyrex-MgO assembly, a graphite heater, an outer NaCl sleeve, and Au80-Pd20 capsules (with a 4 mm outer diameter for 1/2″ assemblies, 0.2 mm wall thickness, 4-5 mm length). To prepare the capsules, we first added deionized water at the bottom of the capsule using a micro-syringe. After loading and compacting ~50 mg of glass powder, the capsule was welded shut. We checked for leaks during capsule preparation by weighing the welded capsules before and after ≥ 1h heating at 120 °C in an oven. Before starting the experiments, the capsules were placed in an oven at 120 °C overnight to ensure homogenous distribution of the volatiles. During the experiments, the samples were held at 1300 °C and 1 GPa for only 15 min in order to minimize fO2 re-equilibration and H2O loss. The temperature was measured very close to the capsule by a W95Re5-W74Re26 thermocouple. Quenching of the hydration experiments was done by switching off the heating power, which led to a cooling rate > 80 °C/s.

The glasses recovered from the experiments were polished on both sides for major element characterization by electron microprobe, H2O concentration measurements by Raman spectroscopy (for hydrous glasses only) and XANES measurements. Fragments of the anhydrous glasses were also analyzed by conventional Mössbauer spectroscopy using a radioactive point source at the Bayerisches Geoinstitüt. Details of the experimental conditions for each run, as well as their chemical compositions can be found in Table 1-2 of the Supplementary material.

Three basaltic and three basanitic experimental glasses from [START_REF] Magnien | Kinetics of iron oxidation in silicate melts: A preliminary XANES study[END_REF] and [START_REF] Schiavi | Water quantification in silicate glasses by Raman spectroscopy: Correcting for the effects of confocality, density and ferric iron[END_REF], respectively, were added to our dataset to complete the range of Fe 3+ /ΣFe ratios and water contents covered by our experimental glasses. Their chemical compositions can be found in Tables 12of the Supplementary material.

Analytical methods

Electron microprobe analyzes

Melt inclusions and their host olivines were analyzed for major elements by electron microprobe at Laboratoire Magmas et Volcans with a CAMECA SX 100. All analyzes were performed with a 15 kV accelerating voltage. Olivines were measured with a 1 µm diameter focused beam and a 100 nA beam current. Counting times for each element were: Al (160-220 s), Si (20 s), Mg (30 s), Ca (160 s), Fe (40 s), Mn (60 s) and Ni (120 s). Analyzes of Si, Mg and Fe of olivines presenting a slightly tilted surface were performed in EDS mode using the electron microprobe to avoid bias on their stoichiometry. Counting time for these three elements using this analytical condition was 30 s. Melt inclusions were analyzed with a 10 µm diameter beam and two beam current conditions. The elements measured with a 8 nA beam and their respective counting times were: Na (10 s), Mg (30 s), Si (10 s), Al (20 s), Ca (10 s), Ti (30 s) and Fe (30 s). The elements measured at 50 nA and the respective counting times were: K (40 s), P (40 s) and Mn (80 s). Volatile elements were analyzed with a 10 µm diameter beam and a beam current of 40 nA. Respective counting times were: S (50 s), Cl (50 s) and F (300 s).

Analytical uncertainties on glass data for major elements were obtained from replicate measurements on the natural glass Ve32 (n = 23). The total analytical 2σ error, which corresponds to the maximum value between the precision and accuracy of measurements, for the Ve32 basaltic glass was 1.4% rel. for SiO2, 4.1% for TiO2, 2.8% for Al2O3, 4.8% for FeO*, 11% for MnO, 3.4% for MgO, 3.2% for CaO, 9.2% for Na2O, 9.9% for K2O and 4.6% for P2O5.

The major and volatile element compositions of melt inclusions and the major element compositions of their host crystals are given in Table 3 of the Supplementary material. The values on the Ve32 standard are also reported in Table 3 of the Supplementary material.

Raman spectroscopy

Water content of 14 melt inclusions was determined by Raman spectroscopy. Spectra were collected at Laboratoire Magmas et Volcans using an InVia (Renishaw) confocal Raman micro-spectrometer. Analytical details can be found in [START_REF] Schiavi | Water quantification in silicate glasses by Raman spectroscopy: Correcting for the effects of confocality, density and ferric iron[END_REF]. The diode laser output power was 150 mW. Laser power on the sample was reduced by filters in order to operate at powers of ~ 1 mW. Laser power on the sample was periodically checked. The analytical conditions used result in lateral and axial spatial resolutions of ~1 and 3 μm, respectively, near the sample surface. Daily calibration of the spectrometer was performed based on a Si 520.5 cm -1 peak. Analyzes were carried out at 2 µm depth on polished melt inclusions. The spectra were recorded in the wavenumber ranges from ~100 to 1350 cm -1 (alumino-silicate network domain) for 4 cycles of 30 s and from ~2900 to 3800 cm -1 (water domain) for 7 cycles of 30 s, using Wire 4.2 software. Basaltic, andesitic and rhyolitic glasses, with water content from 0 to 6.70 ± 0.35 wt% were used as external reference standards [START_REF] Médard | The effect of H2O on the olivine liquidus of basaltic melts: Experiments and thermodynamic models[END_REF][START_REF] Schiavi | Water quantification in silicate glasses by Raman spectroscopy: Correcting for the effects of confocality, density and ferric iron[END_REF] and analyzed several times during each analytical session at the same conditions as the samples, in order to correct for the dependence of band intensities on delivered energy.

Analytical precision calculated based on repeated daily measurements of the reference glass 82-72f#9 was better than 16 % rel. Analyzes of host olivines were performed with the same conditions in order to correct melt inclusion spectra that suffered from signal contamination from the host crystal.

Absolute intensity areas of the water band and silicate band were determined after subtraction of a cubic and non-parametric baseline, respectively, using PeakFit© software. For determination of water content in glasses, we used both the external calibration procedure, which is based on the absolute intensities of the water band area, and an internal calibration procedure which normalizes the intensity of the water band area to the area of the silicate bands [START_REF] Schiavi | Water quantification in silicate glasses by Raman spectroscopy: Correcting for the effects of confocality, density and ferric iron[END_REF]. The two methods often gave different results (up to ~30% rel.). As demonstrated by [START_REF] Schiavi | Water quantification in silicate glasses by Raman spectroscopy: Correcting for the effects of confocality, density and ferric iron[END_REF], the internal calibration is sensitive to matrix effects that are related to changes in glass density, the oxidation state or the presence of dissolved carbonate.

For those reasons, we used an external calibration to determine the water content of our melt inclusions. The analytical uncertainty was calculated as the 2σ relative standard deviation (RSD) on replicate measurements. When melt inclusions could only be analyzed once, we report an analytical uncertainty that corresponds to the maximal 2σ RSD determined on replicate analyzes of other melt inclusions in the same session. Water contents of melt inclusions are given in Table 3 of the Supplementary material.

FTIR spectroscopy

The water contents of 16 melt inclusions were determined by Fourier Transform infrared spectroscopy. Analyzes were performed at LMV using a Bruker Vertex 70 spectrometer coupled with a Hyperion microscope. The spectrometer is equipped with a Globar light source, a KBr beamsplitter and a Mercury-Cadmium-Tellurium alloy (MCT) detector. Melt inclusions polished on both sides were placed on a CaF2 window for the acquisition and the CaF2 window was used for background correction. The beam size varied as a function of the melt inclusion size. Absorbance and background spectra were obtained by acquisition of 600 and 300 scans, respectively. The spectral resolution was 4 cm -1 . Spectra were collected in the wavenumber range from 7500 to 700 cm -1 . Spectra were corrected for atmospheric contamination using OPUS software. The water contents of melt inclusions were calculated using the Beer-Lambert law. Absorption was derived from the area of the 3550 cm -1 band after baseline subtraction using OPUS software. Sample thicknesses were measured as the vertical displacement of the optical microscope stage between the top of the sample and the sample holder using a focused Raman laser beam. The uncertainty on the thickness measurement was estimated to be ~2 µm.

The density was calculated following [START_REF] Bouhifd | Densities and volumes of hydrous silicate melts: New measurements and predictions[END_REF]. For basaltic glasses, we used a molar absorption coefficient of 62.8 ± 0.8 Lmol -1 cm -1 as proposed by [START_REF] Mercier | Spectroscopic analysis (FTIR, Raman) of water in mafic and intermediate glasses and glass inclusions[END_REF]. For silicate under-saturated samples, we chose to use a molar absorption coefficient of 43.96 ± 0.6 Lmol -1 cm -1 [START_REF] Mercier | Spectroscopic analysis (FTIR, Raman) of water in mafic and intermediate glasses and glass inclusions[END_REF]. The analytical uncertainty was calculated as the 2σ relative standard deviation (RSD) on replicate measurements. When melt inclusions could only be analyzed once, we report an analytical uncertainty that corresponds to the maximal 2σ RSD determined on replicate analyzes of other melt inclusions in the same session. Water contents of melt inclusions are given in Table 3 of the Supplementary material.

A few melt inclusions were both analyzed by Raman and FTIR spectroscopy. From the comparison between the two methods, we concluded that both methods gave consistent results, but that the uncertainties on the water contents derived from FTIR spectroscopy were larger due to the error on the thickness of melt inclusions and the uncertainty resulting from the range in the published values for the molar absorption coefficient. Therefore, when both types of data were available, the water contents derived from Raman spectroscopy were preferentially used.

Mössbauer analyzes

The iron speciation of the 12 anhydrous experimental glasses (Table 1 of the Supplementary material) was determined by conventional Mössbauer spectroscopy. Analyzes were carried out at the Bayerisches Geoinstitut, Bayreuth. Glasses were mounted between two pieces of plastic film and centered in a 1000 µm diameter hole drilled in a 25 µm thick Pb foil.

Data were collected in transmission mode on a constant acceleration Mössbauer spectrometer with a nominal 370 MBq 57 Co point source in a 12 µm Rh matrix with active dimension of 500 × 500 µm 2 . The velocity scale was calibrated with a 25 µm thick natural α-Fe foil. A line width of 0.36 mm/s for the outer lines was obtained for α-Fe at room temperature. Analyzes were performed at room temperature with measuring times from 7 hours to 5 days. Fe 3+ /ΣFe in 14 olivine-hosted melt inclusions were determined using a synchrotron Mössbauer source (SMS; [START_REF] Potapkin | The 57Fe Synchrotron Mössbauer Source at the ESRF[END_REF][START_REF] Smirnov | Synchrotron Mössbauer source[END_REF]. Analyzes were carried out at the Nuclear Resonance beamline ID18 [START_REF] Rüffer | Nuclear-resonance beamline at ESRF[END_REF] of the European Synchrotron Facility, Grenoble. Melt inclusions were polished on both sides to avoid signal contamination from their host crystal and were centered under a 20-30 µm hole drilled in a 25 µm-thick Re foil. Data were collected using a multi-bunch operating mode and a 10 × 15 µm 2 focalized beam for 4.5 to 9 hours at room temperature. The SMS provided 57 Fe resonant radiation with the energy of 14.4125 keV and the bandwidth of ~13 neV tunable in the energy range of about ± 0.3 µeV. The velocity scale was calibrated with a 25 µm-thick natural α-Fe foil and the source line width was checked after each analysis. Spectra were fitted using MossA software using a full transmission integral [START_REF] Prescher | MossA: A program for analyzing energy-domain Mössbauer spectra from conventional and synchrotron sources[END_REF].

The fitting methods applied to spectra collected using conventional Mössbauer and SMS spectroscopy are further described in the Results section.

XANES analyzes

Fe K-edge X-ray absorption near edge structure (XANES) were collected at the LUCIA [START_REF] Vantelon | The LUCIA beamline at SOLEIL[END_REF] beamline of SOLEIL synchrotron, France, during five different sessions in 2014, 2015, 2017, 2018 and 2019. Data were collected in fluorescence mode between 7050 and 7300 eV, with 0.1 eV steps in the pre-edge region (7106-7120 eV). The excitation energy was selected using a Si(311) or a Si(111) double crystal monochromator depending on the analytical session, leading to a spectral energy resolution of 0.2 and 1 eV, respectively. The monochromator was calibrated by setting the first inflexion point of a Fe metallic foil to 7112 eV. The beam was focused using KB mirrors down to ∼ 3 × 3 μm 2 (Table 4 of the Supplementary material). The fluorescence detector was a Bruker silicon drift diode monoelement with a total active area of 60 mm 2 . The distance between the sample and the detector was adjusted to ensure optimal counting rate for each sample. Samples were tilted at an angle of 5° with respect to the incident beam. X-ray fluorescence (XRF) chemical maps were used to select spots for XANES measurements. Spectra collected in anhydrous reference glasses during each session were used to refine the alignment of the spectra collected during the different cessions and to calibrate Fe 3+ /ΣFe ratios obtained from the pre-edge region of XANES spectra collected on our unknown samples. For unknown samples, two consecutive XANES spectra were collected at the same location. More details on the analytical conditions are given in Tables 45of the Supplementary material. [START_REF] Cottrell | A Mössbauer-based XANES calibration for hydrous basalt glasses reveals radiation-induced oxidation of Fe[END_REF] demonstrated that hydrous glasses can be affected by beam-induced oxidation depending on their H2O content, their initial oxidation state and the radiation dose used to collect spectra. Therefore, during our 2019 session, we acquired XANES spectra on hydrous glasses using different radiation doses ranging from 7.9 × 10 5 to 2.4 × 10 8 photons/s/µm². This was done by changing the beam size from 3.7 × 3 to 35 × 15 µm 2 and/or attenuating the beam flux by placing a 60 µm thick Al foil along the beam path (Tables 45of the Supplementary material). The radiation dose corresponds to the number of photons delivered to the sample per square micrometer. The sample area exposed to radiation was determined assuming that the beam had an elliptic shape. Melt inclusions were then analyzed during that 2019 session with a 4 × 3 µm 2 focused beam size using radiation doses of 3.7 × 10 7 photons/s/µm².

To better constrain conditions at which beam-induced oxidation occurs, we tested for a time dependent variation in the intensity of the pre-edge multiplets. Following [START_REF] Debret | Redox state of iron during high-pressure serpentinite dehydration[END_REF] and [START_REF] Cottrell | A Mössbauer-based XANES calibration for hydrous basalt glasses reveals radiation-induced oxidation of Fe[END_REF], we measured the peak amplitude at two fixed energies that correspond to the energies of the Fe 2+ and Fe 3+ pre-edge peaks, respectively (7112.6 and 7114 eV), during 300 s. The duration of a time series acquisition corresponds to the time needed to collect the pre-edge part of our XANES spectra. The time series were acquired at the same conditions as the XANES spectra (Tables 4-5 of the Supplementary material).

XANES spectra were normalized and corrected for self-absorption using Athena software (IFFEFIT suite; [START_REF] Ravel | ATHENA, ARTEMIS, HEPHAESTUS: data analysis for X-ray absorption spectroscopy using IFEFFIT[END_REF]. Melt inclusion spectra were tested for potential contamination from the host olivine crystal by carrying out a comparison with spectra collected on synthetic glasses and using the principal component analysis procedure described in [START_REF] Hartley | Olivinehosted melt inclusions as an archive of redox heterogeneity in magmatic systems[END_REF] and [START_REF] Shorttle | Fe-XANES analyses of Reykjanes Ridge basalts: Implications for oceanic crust's role in the solid Earth oxygen cycle[END_REF]. The Fe 3+ /ΣFe ratios of samples were derived by fitting the pre-edge region using PeakFit© software. The pre-edge region was fitted between 7108 and ∼7118 eV with two Gaussian peaks, constrained to have the same width and line shape, simultaneously with the background, which was modeled as the tail of two other Gaussian functions. The pre-edge centroid energies, integrated areas and intensity ratios calculated as 𝐼 𝐹𝑒 3+ /(𝐼 𝐹𝑒 3+ + 𝐼 𝐹𝑒 2+ ) are reported in Table 6 of the Supplementary material.

Intensities collected as a function of time at 7112.6 and 7114 eV in the same sample were normalized as I[(Fe 3+ )/[I(Fe 3+ )+I(Fe 2+ )]] = I(7114)/[I(7114)+I(7112.6)] in order to track the evolution of the Fe 3+ /ΣFe ratios through time.

Results

Mössbauer spectroscopy

Independent characterization of the Fe 3+ /ΣFe ratios in reference glasses

Several methods were tested to fit the Mössbauer spectra (e.g. [START_REF] Berry | A re-assessment of the oxidation state of iron in MORB glasses[END_REF][START_REF] Cottrell | High-precision determination of iron oxidation state in silicate glasses using XANES[END_REF] in order to define the best fitting method to apply to our data. The best fit as judged by statistics and consistency of parameters across all spectra for reference glasses was obtained with three pseudo-Voigt doublets (Figure 1). Figure 1: Results of the fitting procedure of four Mössbauer spectra collected for anhydrous basaltic reference glasses (Table 1 of the Supplementary material). The Fe 3+ /ΣFe ratios of the glasses are given in brackets. The red dashed line corresponds to the linear baseline. The black line represents the modeled spectra. Blue, green and pink lines correspond to the three pseudo-Voigt doublets used to characterize iron environments. For the most reduced glasses (a) the three doublets are attributed to the Fe 2+ environment. For reduced and oxidized glasses (b-c), the green pseudo-Voigt doublet was attributed to the Fe 3+ environment whereas the two others were attributed to the Fe 2+ environment. For the more oxidized sample (d), the pink pseudo-Voigt doublet was attributed to the Fe 2+ environment whereas the two others were attributed to the Fe 3+ environment. Residuals obtained from the fitting procedure are given at the top of the spectra.

The values for the center shift (δ), quadrupole splitting (∆) and the full width at half maximum (FWHM) for each doublet were set as free to vary at first. Then, the FWHMs of the three doublets were constrained to be equal when unrealistic values were obtained for hyperfine parameters. Reduced χ² values range from 0.95 to 1.93. Values obtained for hyperfine parameters are summarized in Table 7 of the Supplementary material. The values obtained for the hyperfine parameters indicate that two doublets are attributed to the Fe 3+ environment, whereas one doublet is attributed to the Fe 2+ environment in our most oxidized glass. In contrast, two doublets are attributed to the Fe 2+ environment and one doublet is attributed to the Fe 3+ environment in the less oxidized glasses. Three doublets are attributed to the Fe 2+ environment in our most reduced glasses (e.g. [START_REF] Alberto | Analysis of Mössbauer spectra of silicate glasses using a two-dimensional Gaussian distribution of hyperfine parameters[END_REF][START_REF] Berry | A re-assessment of the oxidation state of iron in MORB glasses[END_REF][START_REF] Cottrell | High-precision determination of iron oxidation state in silicate glasses using XANES[END_REF]Zhang et al., 2018); Table 7 of the Supplementary material). Fe 3+ /ΣFe ratios were obtained from the relative area of each doublet. Determination of the 2σ uncertainty is based on both the statistical uncertainty derived from the fit and the reproducibility given by results obtained from the several tested methods. The fit results are reported in Table 7 and Figure 1.

The values obtained for the hyperfine parameters in our most reduced glass standards suggest that no Fe 3+ component is required. However, the doublets are characterized by broad lines and the hyperfine parameters show a systematic overlap of the Fe 2+ components that suggests that a small amount of Fe 3+ could be present in these reference glasses (Figure 1a). Therefore, we conclude that we are not able to determine the iron speciation accurately in glasses with < 10 % Fe 3+ /ΣFe, and for that reason we decided not to use those in our XANES calibration (Table 1 of the Supplementary material).

Fe 3+ /ΣFe in melt inclusions

Due to a limited beamtime, synchrotron Mössbauer source spectra collected on melt inclusions have a lower signal to noise ratio than conventional Mössbauer spectra in the reference glasses (Figure1-2). The consequence of this lower signal quality is that we were not able to apply the same fitting method for melt inclusions and reference glasses (Figure 2). However, in order to be as consistent as possible with the fitting procedure used for reference glasses, spectra collected on melt inclusions were fitted using two pseudo-Voigt doublets, where one doublet was attributed to the Fe 2+ environment and the other one was attributed to the Fe 3+ environment. As for reference glasses, hyperfine parameters (δ and ∆) and the FWHM of the doublets were first set as free to vary. Then, the FWHM of the doublets attributable to the Fe 3+ environment was fixed to 0.4 mm/s when unrealistic values were obtained for hyperfine parameters. An asymmetric Lorentzian doublet with fixed hyperfine parameters was added to the fitting procedure (Figure 2) to correct the spectra for 57 Fe contamination from the Be lenses.

Reduced χ² values range from 0.86 to 1.15. Hyperfine parameters are summarized in Table 8 of the Supplementary material.

Since the 57 Fe contamination due to the Be lenses is larger for spectra collected on ironpoor or thin melt inclusions, we tested for a correlation between the relative area of the Lorentzian doublet from the Be lenses and the Mössbauer thickness of the different samples (Table 9 and Figure 2 of the Supplementary material). The Mössbauer thickness is calculated from the density (g/cm 3 ), porosity (%), thickness (µm) and total iron content (in wt% of element) of the sample [START_REF] Long | The ideal Mössbauer effect absorber thickness[END_REF]. The broad negative trend follows the expected behavior of the two parameters, i.e. the area of the Lorentzian doublet increases as either the iron content or the sample thickness decreases, confirming our approach to correct for 57 Fe contamination by the Be lenses.

XANES

Calibration of the XANES spectra

To derive Fe 3+ /ΣFe ratios from the pre-edge region of XANES spectra collected on unknown samples, these spectra have to be calibrated using XANES spectra collected on reference glasses with independently known Fe 3+ /ΣFe ratios (e.g. derived from Mössbauer spectroscopy). In previous studies, the XANES calibration uses either the pre-edge centroid energy (i.e., intensity-weighted energy) or the pre-edge intensity ratio (i.e., the ratio of the total area of the Fe 3+ peak [I(Fe 3+ )] to the sum of the areas of the Fe 3+ peak [I(Fe 3+ )] and the Fe 2+ peak [I(Fe 2+ )], or I(Fe 3+ )/[ I(Fe 3+ )+I(Fe 2+ )]). Pre-edge centroid energy-based calibrations are less sensitive to changes in the coordination environment [START_REF] Berry | XANES calibrations for the oxidation state of iron in a silicate glass[END_REF][START_REF] Wilke | Determination of the iron oxidation state in basaltic glasses using XANES at the K-edge[END_REF], however they are more sensitive to energy shifts that can occur during and between analytical sessions [START_REF] Cottrell | High-precision determination of iron oxidation state in silicate glasses using XANES[END_REF]. As spectra were collected on the same reference glasses during every session, we were able to correct our spectra for any shift in energy between sessions (no drift occurred within single sessions). Therefore, we applied the pre-edge centroid energy based calibration to estimate the Fe 3+ /ΣFe ratios in the unknown samples. [START_REF] Cottrell | High-precision determination of iron oxidation state in silicate glasses using XANES[END_REF] also used a pre-edge centroid energy based calibration and found that the use of a second order polynomial fit to their most oxidized reference glasses was appropriate for their data. The use of this calibration leads to the determination of an average Fe 3+ /ΣFe ratio of 0.16 ± 0.01 for MORB glasses [START_REF] Cottrell | The oxidation state of Fe in MORB glasses and the oxygen fugacity of the upper mantle[END_REF]. More recently, [START_REF] Berry | A re-assessment of the oxidation state of iron in MORB glasses[END_REF] proposed that a linear calibration can be used to determine Fe 3+ /ΣFe of MORB glasses when only considering the appropriate Fe 3+ /ΣFe range for MORBs (Fe 3+ /ΣFe < 0.3). They derived an average Fe 3+ /ΣFe ratio of 0.11 ± 0.02 for MORB glasses when they use the linear calibration and 0.10 ± 0.02 when they use the polynomial calibration. Results derived from these two calibrations are similar within error, which indicates that both calibrations can be applied to determine Fe 3+ /ΣFe ratios in the unknown samples. These authors have also proposed that the difference in the average Fe 3+ /ΣFe ratio in MORB glasses arises from the method applied to fit the Mössbauer spectra of reference glasses. On the one hand, Cottrell et al. (2009) fitted their Mössbauer spectra using two 2D-gaussian doublets, with one doublet being attributed to the Fe 2+ environment and the other one to the Fe 3+ environment. On the other hand, [START_REF] Berry | A re-assessment of the oxidation state of iron in MORB glasses[END_REF] have proposed that a third doublet has to be attributed to the Fe 2+ environment, in particular in the case of reduced samples, otherwise their Fe 3+ /ΣFe would be over-estimated. In the current study, we used three doublets to fit the Mössbauer spectra collected on our reference glasses and the third doublet was attributed to the Fe 2+ environment in our most reduced samples, therefore we expect that our derived Fe 3+ /ΣFe ratios are not overestimated.

Fe 3+ /ΣFe ratios derived from XANES spectra were determined for our natural glass samples from the centroid energy of the fitted pre-edge region using a polynomial calibration derived from seven basaltic reference glasses (Figure 3; Table 1 of the Supplementary material).

As the Fe 3+ /ΣFe ratios of our more reduced standards are below the detection limit of Mössbauer spectroscopy (<10 %), we did not use them in our calibration (Table 1 of the Supplementary material).

The XANES calibration is given by the equation:

𝐸 𝑐𝑒𝑛𝑡𝑟𝑜𝑖𝑑 = -1.0540 (𝐹𝑒 3+ Σ𝐹𝑒) ⁄ 𝑀ö𝑠𝑠𝑏𝑎𝑢𝑒𝑟 2 + 2.6146 (𝐹𝑒 3+ Σ𝐹𝑒) ⁄ 𝑀ö𝑠𝑠𝑏𝑎𝑢𝑒𝑟 + 7 112.8004 (1)
in which 𝐸 𝑐𝑒𝑛𝑡𝑟𝑜𝑖𝑑 corresponds to the centroid energy (in eV) and (𝐹𝑒 3+ Σ𝐹𝑒) ⁄ 𝑀ö𝑠𝑠𝑏𝑎𝑢𝑒𝑟 is the Fe 3+ /ΣFe value obtained by Mössbauer spectroscopy. This calibration is applicable to samples with Fe 3+ /ΣFe ratios ranging between 0.10 and 0.90. The accuracy of the calibration was tested by using one of our anhydrous experimental glasses (ML4; Table 1 of the Supplementary material) as an external standard. The mean Fe 3+ /ΣFe ratio of 0.22 ± 0.02 (n=6) calculated for the ML4 glass using this calibration is in good agreement with the Fe 3+ /ΣFe of 0.21 ± 0.03 obtained by Mössbauer spectroscopy. This result demonstrates that our calibration can be used to determine Fe 3+ /ΣFe ratios of our unknown samples. The uncertainty on the ML4 glass is reported as the 2σ standard deviation of the replicate analyzes. This uncertainty will then be used as the 2σ error on the Fe 3+ /ΣFe ratios of our unknown samples.

Figure 3: XANES calibration curve for the determination of the Fe 3+ /ΣFe ratios of melt inclusions and glasses. The solid curve is the second order polynomial fit result. Averaged centroid energies and Fe 3+ /ΣFe ratios from Mössbauer spectroscopy of anhydrous basaltic reference glasses are shown in Table 1 of the supplementary material. Error bars are reported as 2σ uncertainties (Table 1 of the supplementary material).

Time series in hydrous synthetic glasses

The measurement of the evolution of the pre-edge peak intensities with time at fixed energies gives us access to the evolution of the iron speciation of glasses during the time required to acquire the XANES pre-edge region, which, in our set-up, is about 5 minutes. The time series allows us to spot the occurrence of beam damage during acquisition. Time series collected on hydrous experimental glasses with water contents ranging from 0.02 ± 0.002 to 2.86 ± 0.34 wt% and equilibrated at various fO2 show no clear evidence of beam damage when using radiation doses lower than 1.2 × 10 8 photons/s/µm². When using higher doses, the time series collected on water-rich glasses (≥ 2.48 ± 0.20 wt% H2O) show a systematic increase in the intensity ratio I(7114)/[I(7114)+I(7112.6)] (Figure 4). As time series are collected at fixed energies, the effect of beam-induced oxidation cannot be monitored using centroid positions.

However, when considering a XANES spectrum, the use of the centroid position allows us to determine more accurate Fe 3+ /ΣFe ratios.

At 2.4 × 10 8 photons/s/µm², time series collected on the BH1 glass (0.74 ± 0.02 wt% H2O) also show an increase in its intensity. This suggests that beam damage can even occur for very low water contents when using such high radiation doses. 2 of the Supplementary material) using radiation doses from 7.9 × 10 5 to 2.4 × 10 8 photons/s/µm². No evidence of beam damage is visible when using radiation doses lower than 1.2 × 10 8 photons/s/µm². When using higher radiation doses (≥ 1.2 × 10 8 photons/s/µm²) beam damage effects are recorded as an increase in the intensity through time. The overestimation of the Fe 3+ /ΣFe calculated from these time series is 10% rel. and 15% rel. using 1.2 × 10 8 and 2.4 × 10 8 photons/s/µm² respectively.

We converted the intensity of absorption recorded in the time series into Fe 3+ /ΣFe ratios using a calibration based on non-normalized pre-edge peak intensity ratios in the reference glasses. Then we applied a linear regression through the variation of Fe 3+ /ΣFe with time to calculate the over-estimation of Fe 3+ /ΣFe induced by beam damage. We find that Fe 3+ /ΣFe ratios are over-estimated by 6 to 10 % rel. and 6 to 15 % rel. when using radiation doses of 1.2 × 10 8 and 2.4 × 10 8 photons/s/µm², respectively. The percentage of over-estimation of the Fe 3+ /ΣFe ratios shows a poor positive correlation with the water content of glasses (Figure 5a)

and no correlation with their original Fe 3+ /ΣFe ratios (i.e., as determined from the pre-edge centroid energy of a spectrum collected in non-damaging conditions (5.1 × 10 6 photons/s/µm²; Figure 5b). [START_REF] Cottrell | A Mössbauer-based XANES calibration for hydrous basalt glasses reveals radiation-induced oxidation of Fe[END_REF] found that the beam damage susceptibility φ, which they define as the product of the water concentration and the ferrous/ferric oxide ratio on a molar basis, is positively correlated with the percentage of over-estimation of Fe 3+ /ΣFe ratios recorded for hydrous glasses. Our data do not show such correlation (Figure 5c). The lack of correlation between these parameters implies that we were not able to quantify the over-estimation of Fe 3+ /ΣFe ratios for glasses analyzed using standard radiation doses (~10 8 photons/s/µm²) and for which time series have not been collected. 2 of the Supplementary material).

XANES spectra in hydrous synthetic glasses

We first compared the pre-edge features of XANES spectra successively collected at the same location in our synthetic hydrous glasses to look for evidence for beam-induced oxidation between the two consecutive spectra (Figure 6). Changes in the pre-edge feature indicating that beam damage occurred during the acquisition were visible for water-rich glasses (≥ 2.48 ± 0.20 wt% H2O) that were analyzed with radiation doses of 1.2 × 10 8 and 2.4 × 10 8 photons/s/µm². When we compared the Fe 3+ /ΣFe ratios obtained for these spectra, the overestimation of the Fe 3+ /ΣFe ratios derived from the second spectrum is systematically contained in the 2σ uncertainty on these ratios. Similarly, we compared the Fe 3+ /ΣFe ratios obtained for these spectra with their original Fe 3+ /ΣFe ratios (i.e., as determined from the pre-edge centroid energy of a spectrum collected in non-damaging conditions of 5.1 × 10 6 photons/s/µm²). The over-estimation of Fe 3+ /ΣFe derived from XANES spectra that were affected by beam damage (as evidenced by time series and modification of the pre-edge region of XANES spectra) lies within the error except for BH7 at the highest radiation dose. We can also note that all the glasses that show changes in the pre-edge feature between two consecutive spectra also show evidence of beam induced oxidation in the time series. However, the inverse is not systematically observed (and this might indicate that the photo-oxidation has reached a plateau and/or that it is irreversible).

When no beam damage was detected either from the time series and/or the comparison of the pre-edge regions of the two consecutive spectra, we calculated a mean Fe 3+ /ΣFe from the two spectra collected at the same spot. 2σ uncertainties were calculated as the propagation of the 2σ uncertainty of the Fe 3+ /ΣFe ratios of both spectra. On the other hand, when there was evidence for oxidation from the difference in the two spectra, we discarded the value from the second spectrum and only kept the Fe 3+ /ΣFe ratio derived from the first spectrum. 2 of the Supplementary material) with radiation doses of 1.2 × 10 8 and 2.4 × 10 8 photons/s/µm², respectively. The first and second spectra are shown in red and black, respectively. The two spectra collected in the BH10 glass are identical. In contrast, the second spectrum collected in the BSN081117 -2 glass shows a small decrease in its Fe 2+ component and an increase in its Fe 3+ component compared to the first spectrum, indicating beam damage.

When using a radiation dose of about 10 7 photons/s/µm², we record no evidence of beam-induced oxidation from the time series and the two consecutive spectra. This indicates that decreasing the radiation dose of about one order of magnitude provides more accurate Fe 3+ /ΣFe ratios while still keeping the small beam size that is required for melt inclusion analysis.

Determination of Fe 3+ /ΣFe in melt inclusions

Time series were collected using a radiation dose of 3.7 × 10 7 photons/s/µm² in melt inclusions that were large enough to accommodate several XANES spots. As we saw no evidence for beam-induced oxidation in the hydrous experimental glasses at the same beam conditions, we expected no beam damage in the melt inclusions. As expected, we found no evidence for beam damage in the time series of most melt inclusions except for three arc melt inclusions: the relative increase of the Fe 3+ /ΣFe ratio with time ranges from 6 to 63% rel. for water contents ranging from 2.31 to 4.14 wt% (Table 3 of the Supplementary material). The percentage of over-estimation of the Fe 3+ /ΣFe ratios does not, however, correlate with the water content in the melt inclusions. One should note that the largest amount of over-estimation of the Fe 3+ /ΣFe ratio (63% rel.) is comparable or sometimes lower than the 2σ uncertainties attributed to the Fe 3+ /ΣFe ratios calculated from Mössbauer spectra. However, we have demonstrated that the absolute Fe 3+ /ΣFe value calculated from the XANES spectrum is biased.

In addition to the time series, we also compared the pre-edge features of two spectra that were successively acquired at the same location. Beam damage affecting the second spectrum is clearly visible in eleven water-rich arc melt inclusions with water contents ranging from 2.31 to 4.59 wt%. Still, this effect is minimal and only one of them (inclusion SVP291c n4) shows an over-estimation of its Fe 3+ /ΣFe ratio that is larger than the 2σ uncertainty on the Fe 3+ /ΣFe ratio. Surprisingly this melt inclusion shows no clear evidence for beam damage in the time series, which suggests that the first spectrum has not been affected by beam damage and that the Fe 3+ /ΣFe ratio derived from it is reliable. In the end, more than half of the water-rich (> 1 wt% H2O) melt inclusions did not show modification of the pre-edge feature between the two consecutive spectra.

When no sign of beam damage was recorded in the time series and/or in the comparison of the pre-edge regions of two spectra collected on the same spot, we calculated a mean Fe 3+ /ΣFe ratio from the two spectra. 2σ uncertainties were calculated as the propagation of the 2σ uncertainties of the Fe 3+ /ΣFe ratios of both spectra. When time series could not be collected for lack of space in the inclusion, we calculated a mean Fe 3+ /ΣFe ratio from the two spectra when their Fe 3+ /ΣFe ratios lie within the 2σ errors. Melt inclusions for which time series exhibit evidence of beam damage were removed from our dataset. When melt inclusions have been analyzed in different spots, mean Fe 3+ /ΣFe ratios were calculated using only the first spectra collected at each spot when the time series didn't record any beam damage. Results are given in Table 3 of the Supplementary material.

Discussion

The effect of beam-induced oxidation on the determination of Fe 3+ /ΣFe ratios in silicate glasses

The time series collected on hydrous experimental glasses by XANES indicate that beam damage systematically takes place at high water contents (≥ 2.5 wt% H2O) for durations of 5 minutes and standard radiation doses (~10 8 photons/s/µm²; Figure 4). At lower water contents (< 1 wt% H2O), beam damage is not systematically observed. Beam induced oxidation recorded in the time series is usually, but not systematically, supported by changes in the preedge feature between the two XANES spectra collected successively at the same location (one spectrum acquisition lasts for 7 to 20 min; Figure 6), which suggests that the oxidation process continues to occur during acquisition of the second spectrum. To explain the cases where time series showed oxidation through time, but no change was noticeable between the pre-edge features of the two spectra successively collected, we can make two hypotheses. The first hypothesis is that beam damage is "completed" after full acquisition of the pre-edge [START_REF] Cottrell | A Mössbauer-based XANES calibration for hydrous basalt glasses reveals radiation-induced oxidation of Fe[END_REF]. The second hypothesis is that the modification due to beam damage is not systematically large enough to be visible given our spectral resolution. The Fe 3+ /ΣFe ratios calculated for hydrous experimental glasses from their pre-edge centroid energies suggest that the effect of this process may be within the error associated with the calculation of these ratios.

However, based on time series, an over-estimation of Fe 3+ /ΣFe of about 15 % rel. may occur when using a radiation dose of 2.4 × 10 8 photons/s/µm². Thus, the Fe 3+ /ΣFe ratios of melt inclusions, and especially of arc melt inclusions that are usually water-rich, will likely be overestimated when analyzing them with such doses.

A large number of melt inclusions have been analyzed with standard radiation doses (~10 8 photons/s/µm²) during our first synchrotron sessions. At the time, as we did not realize how large the effects of beam damage could be, we did not systematically collect time series.

The lack of or the poor correlation between the chemical composition of glasses (in particular their H2O contents and/or initial Fe 3+ /ΣFe) and the over-estimation of the Fe 3+ /ΣFe ratios recorded by time series collected on hydrous synthetic glasses (Figure 5) means that we are not able to quantify the over-estimation that potentially affected the Fe 3+ /ΣFe ratios derived from XANES spectra collected on these melt inclusions using standard beam conditions. Therefore, we were not able to correct them and we decided not to use them.

Time series collected on hydrous experimental glasses using radiation doses lower than ~10 8 photons/s/µm² do not show any evidence for oxidation. However, time series acquired in three hydrous melt inclusions using a radiation dose of 3.7 × 10 7 photon/s/µm² show evidence of beam damage. Moreover, melt inclusions that exhibit an over-estimation of the Fe 3+ /ΣFe ratio derived from the second spectrum compared to the first one do not systematically display an evolution of the speciation through time in the time series. These observations corroborate the ones carried out by [START_REF] Cottrell | A Mössbauer-based XANES calibration for hydrous basalt glasses reveals radiation-induced oxidation of Fe[END_REF] showing that lowering the radiation dose to about ~10 7 photons/s/µm² allows us to mitigate but not definitively eliminate beam-induced oxidation during XANES analyzes. Using these more appropriate beam conditions, only three melt inclusions out of thirty show clear signs of beam damage during the acquisition of the first preedge spectrum.

Fe 3+ /ΣFe inferred from Mössbauer vs. XANES spectroscopy

For the first time, the Fe 3+ /ΣFe ratios of olivine-hosted melt inclusions were determined by Mössbauer spectroscopy. Despite counting times of 4.5 to 9 h, the limited thickness of our melt inclusions resulted in much larger uncertainties in Fe 3+ /ΣFe than the ones determined for reference glasses using conventional Mössbauer spectroscopy (Tables 1 and3 of the Supplementary material). The difference can be explained by the lower signal to noise ratio of spectra collected on melt inclusions (Figure 1-2). Furthermore, these uncertainties are even larger for the most reduced melt inclusions (Fe 3+ /ΣFe < 0.15; Figure 3 of the Supplementary material). The increased uncertainties with decreasing Fe 3+ /ΣFe ratio are explained by decreasing signal to noise ratio with decreasing Fe 3+ content in the melt inclusions.

Figure 7 shows the comparison between Fe 3+ /ΣFe ratios obtained by synchrotron Mössbauer source and XANES spectroscopy. 3 of the Supplementary material).

The figure shows that Fe 3+ /ΣFe ratios derived from these two methods mostly overlap within the 2σ uncertainties, with a distribution close to the 1:1 line. Nevertheless, we note that the absolute Fe 3+ /ΣFe values calculated from Mössbauer spectra are generally lower than the ones obtained from XANES. Only two arc melt inclusions from Aoba (inclusions Ao17-D6 and Ao-T8) show significantly lower Mössbauer-derived Fe 3+ /ΣFe. The first hypothesis we can make is that these two melt inclusions were oxidized during XANES analysis. However, we see no evidence for beam-induced oxidation in the time series nor in the comparison of two consecutive spectra. A second hypothesis is that a small proportion of host olivine was sampled by the beam during Mössbauer analyses. The spectra collected in the Ao-T8 melt inclusion do not show evidences of signal contamination from the host olivine (Figure 2). Thus, it is more likely that the Fe 3+ /ΣFe ratio obtained from Mössbauer spectroscopy in Ao-T8 is underestimated because of the low signal to noise ratio and increased error. The poor signal to noise ratio of the spectra collected in the Ao17-D6 melt inclusion does not allow us to be as definitive as for the spectra collected in the Ao-T8 melt inclusion (Figure 4 in the supplementary material).

Generally, the comparison of Fe 3+ /ΣFe ratios inferred from these two methods seems to indicate that we managed to detect beam damage when it occurred during XANES analysis.

However, the fact that the Fe 3+ /ΣFe ratios obtained by Mössbauer spectroscopy exhibit larger uncertainties because of the low spectral resolution indicates that we are not able to determine precise Fe 3+ /ΣFe ratios using synchrotron Mössbauer yet, unless we increase counting times significantly.

The oxidation state of melt inclusions

In order to discuss further the oxygen fugacity at which melt inclusions equilibrate, their compositions have to be corrected for post-entrapment modification. Melt inclusions were corrected for host crystallization and/or 'Fe loss' using Petrolog3 software [START_REF] Danyushevsky | Petrolog3: Integrated software for modeling crystallization processes[END_REF]. Details on the correction method used in this study are available in the Supplementary material. Corrected compositions and 'original' Fe 3+ /ΣFe ratios are reported in Table 3 of the Supplementary material.

Our results show that melt inclusions from MORB display a mean Fe 3+ /ΣFe ratio of 0.10 ± 0.05 (n=5). Melt inclusions from OIB and arcs display mean Fe 3+ /ΣFe ratios of 0.13 ± 0.05 (n=2) and 0.25 ± 0.15 (n=19) respectively. Mount Etna is not classified as an arc volcano and has a more complex geodynamic setting than the other volcanoes presented in this study. Many authors suggested that Mount Etna has an OIB signature but for which the mantle is metasomatized by subduction-derived fluids. Nevertheless, the composition of the mantle beneath Mount Etna is still debated (e.g. [START_REF] Correale | A two-component mantle source feeding Mt. Etna magmatism: Insights from the geochemistry of primitive magmas[END_REF][START_REF] Schiano | Transition of Mount Etna lavas from a mantle-plume to an island-arc magmatic source[END_REF][START_REF] Schiavi | Geochemical heterogeneities in magma beneath Mount Etna recorded by 2001-2006 melt inclusions[END_REF][START_REF] Tonarini | Subduction-like fuids in the genesis of Mt. Etna magmas: evidence from boron isotopes and fuid mobile elements[END_REF][START_REF] Viccaro | Mantle ingredients for making the fingerprint of Etna alkaline magmas: implications for shallow partial melting within the complex geodynamic framework of Eastern Sicily[END_REF]. Our results show that melt inclusions collected in our Mount Etna sample display a mean Fe 3+ /ΣFe ratio of 0.26 ± 0.05 (n=7) that is closer to the mean value obtained for arc melt inclusions than the one obtained for OIB melt inclusions.

In Figure 8, we report the number of melt inclusions as a function of their geodynamic setting (melt inclusions from Mount Etna are reported here as arc melt inclusions) and their calculated Fe 3+ /ΣFe ratios (light grey). Also reported are previously published data for melt inclusions in high-Mg olivines (> Fo80; dark grey) and glasses (black) from MORBs (Berry et al., 2018;[START_REF] Birner | Peridotites and basalts reveal broad congruence between two independent records of mantle fO2 despite local redox heterogeneity[END_REF]Cottrell andKelley, 2011, 2013;[START_REF] Kelley | Water and the Oxydation State of Subduction Zone Magmas[END_REF]Zhang et al., 2018), OIBs [START_REF] Brounce | Redox variations in Mauna Kea lavas, the oxygen fugacity of the Hawaiian plume, and the role of volcanic gases in Earth's oxygenation[END_REF][START_REF] De Moor | Sulfur degassing at Erta Ale (Ethiopia) and Masaya (Nicaragua) volcanoes: Implications for degassing processes and oxygen fugacities of basaltic systems[END_REF][START_REF] Hartley | Olivinehosted melt inclusions as an archive of redox heterogeneity in magmatic systems[END_REF][START_REF] Moussallam | The impact of degassing on the oxidation state of basaltic magmas: A case study of Kīlauea volcano[END_REF] and arcs [START_REF] Brounce | Variations in Fe3+/PFe of Mariana Arc Basalts and MantleWedge fO2[END_REF]Kelley andCottrell, 2009, 2012). This figure shows that the Fe 3+ /ΣFe ratios in melt inclusions from MORBs are in good agreement with previous data and comparable to the ones obtained by [START_REF] Berry | A re-assessment of the oxidation state of iron in MORB glasses[END_REF]0.10 ± 0.02). Fe 3+ /ΣFe ratios calculated for melt inclusions from OIBs show similar or lower values than the previously published XANES data. This result may indicate beam-induced oxidation or that the range of Fe 3+ /ΣFe ratios recorded for OIBs is larger than previously reported [START_REF] Brounce | Redox variations in Mauna Kea lavas, the oxygen fugacity of the Hawaiian plume, and the role of volcanic gases in Earth's oxygenation[END_REF][START_REF] De Moor | Sulfur degassing at Erta Ale (Ethiopia) and Masaya (Nicaragua) volcanoes: Implications for degassing processes and oxygen fugacities of basaltic systems[END_REF][START_REF] Hartley | Olivinehosted melt inclusions as an archive of redox heterogeneity in magmatic systems[END_REF][START_REF] Moussallam | The impact of degassing on the oxidation state of basaltic magmas: A case study of Kīlauea volcano[END_REF]. Melt inclusions from arcs from this study display a range in Fe 3+ /ΣFe ratios that is in good agreement with previous data [START_REF] Brounce | Variations in Fe3+/PFe of Mariana Arc Basalts and MantleWedge fO2[END_REF]Kelley andCottrell, 2009, 2012). A few melt inclusions exhibit Fe 3+ /ΣFe ratios that are higher than literature data. This suggests that the range of oxidation state recorded in arc magmas may be larger than previously observed. Therefore, our results confirm that arc magmas are more oxidized than those from hot spots and mid-ocean ridges. 3 of the Supplementary material). When XANES spectra showed evidence of beam damage, we report Fe 3+ /ΣFe ratios, we report Fe 3+ /ΣFe ratios derived from Mössbauer spectroscopy instead (6 melt inclusions). Also reported are literature data for glasses (black) and melt inclusions from high-Mg olivines (> Fo80) from arc basalts, OIBs and MORB for comparison (dark grey; Berry et al., 2018;[START_REF] Birner | Peridotites and basalts reveal broad congruence between two independent records of mantle fO2 despite local redox heterogeneity[END_REF][START_REF] Brounce | Variations in Fe3+/PFe of Mariana Arc Basalts and MantleWedge fO2[END_REF][START_REF] Brounce | Redox variations in Mauna Kea lavas, the oxygen fugacity of the Hawaiian plume, and the role of volcanic gases in Earth's oxygenation[END_REF]Cottrell andKelley, 2011, 2013;[START_REF] De Moor | Sulfur degassing at Erta Ale (Ethiopia) and Masaya (Nicaragua) volcanoes: Implications for degassing processes and oxygen fugacities of basaltic systems[END_REF][START_REF] Hartley | Olivinehosted melt inclusions as an archive of redox heterogeneity in magmatic systems[END_REF]Kelley andCottrell, 2009, 2012;[START_REF] Moussallam | The impact of degassing on the oxidation state of basaltic magmas: A case study of Kīlauea volcano[END_REF]Zhang et al., 2018).

Similarly observations by [START_REF] Kelley | Water and the Oxydation State of Subduction Zone Magmas[END_REF], Fe 3+ /ΣFe correlates positively with the H2O content in melt inclusions (Figure 9). These authors suggested that the positive correlation between these two parameters most likely reflects changes in the oxidation state of the mantle beneath arcs because of subduction processes.

Figure 9: Fe 3+ /ΣFe ratios of melt inclusions as a function of their water content. Fe 3+ /ΣFe ratios reported in this graph are mainly derived from XANES measurements at a radiation dose of 3.7 × 10 7 photons/s/µm² (light grey; Table 3 of the Supplementary material). When XANES spectra showed evidence of beam damage, we report Fe 3+ /ΣFe ratios, we report Fe 3+ /ΣFe ratios derived from Mössbauer spectroscopy instead (6 melt inclusions). Data from this study are represented by black symbols. Error bars are 2σ (Table 3 of the Supplementary material).

Previous data (grey symbols) from [START_REF] Brounce | Variations in Fe3+/PFe of Mariana Arc Basalts and MantleWedge fO2[END_REF][START_REF] Brounce | Redox variations in Mauna Kea lavas, the oxygen fugacity of the Hawaiian plume, and the role of volcanic gases in Earth's oxygenation[END_REF], [START_REF] Cottrell | The oxidation state of Fe in MORB glasses and the oxygen fugacity of the upper mantle[END_REF], Kelley andCottrell (2009, 2012), [START_REF] Moussallam | The impact of degassing on the oxidation state of basaltic magmas: A case study of Kīlauea volcano[END_REF] and Zhang et al. (2018) collected in glasses and melt inclusions from high-Mg olivines (> Fo80) are also reported.

fO2 values of our melt inclusions were calculated relative to the QFM buffer using the algorithm of [START_REF] Kress | Mineralogy and Petrology The compressibility of silicate liquids containing Fe203 and the effect of composition, temperature, oxygen fugacity and pressure on their redox states[END_REF] and are reported in Table 3 of the Supplementary material and in Figure 9. The QFM values reported in Figure 10 show that melt inclusions from arc basalts are generally more oxidized (QFM +0.1 to +2.8) than those from MORBs (QFM -1.9 to -0.1) and OIBs (QFM -0.8 and +0.3).

Conclusions

We used XANES and Mössbauer spectroscopy to determine the Fe 3+ /ΣFe ratios of primitive olivine-hosted melt inclusions from basaltic samples from various volcanic arcs, midocean ridges and hot spots.

First, a careful evaluation of beam damage effects during XANES analysis was carried out by monitoring changes in the pre-edge feature of XANES spectra and intensities variations of time series. Beam induced oxidation recorded in the time series collected in hydrous experimental glasses using standard radiation doses (~10 8 photons/s/µm²) is usually, but not systematically, supported by changes in the pre-edge feature between the two XANES spectra collected successively at the same location. This suggests that the oxidation process continues to occur during acquisition of the second spectrum and may be "completed" after full acquisition of the pre-edge [START_REF] Cottrell | A Mössbauer-based XANES calibration for hydrous basalt glasses reveals radiation-induced oxidation of Fe[END_REF]. Our results show that Fe 3+ /ΣFe ratios in waterrich glasses (≥ 2.5 w% H2O) are likely to be over-estimated when derived from XANES spectra collected using standard radiation doses (~10 8 photons/s/µm²). The amount of over-estimation of the Fe 3+ /ΣFe ratios recorded by the time series does not or poorly correlates with the water content, the 'initial' Fe 3+ /ΣFe ratio or the beam damage susceptibility φ of glasses as defined by [START_REF] Cottrell | A Mössbauer-based XANES calibration for hydrous basalt glasses reveals radiation-induced oxidation of Fe[END_REF]. Therefore, it is difficult to assess beam damage effects in melt inclusions that were analyzed using standard radiation doses when time series have not been collected. When a lower radiation dose is used (≤ 3.7 × 10 7 photons/s/µm²), we record no evidence of beam damage in hydrous synthetic glasses. However, time series acquired in three hydrous melt inclusions using a radiation dose of 3.7 × 10 7 photon/s/µm² show evidence of beam damage. Moreover, melt inclusions that exhibit an over-estimation of the Fe 3+ /ΣFe ratio derived from the second spectrum compared to the first one do not systematically display an evolution of the speciation through time in the time series. Therefore, decreasing the radiation dose allowed us to mitigate but not definitively avoid beam damage in all melt inclusions. For that reason, systematic acquisition of time series is recommended even at low radiation doses in order to assess beam damage in glasses.

The iron speciation in a subset of melt inclusions was also analyzed using a Mössbauer synchrotron source. The comparison of Fe 3+ /ΣFe ratios inferred from these two methods suggests that we successfully identified the melt inclusions that suffered from beam damage.

However, Fe 3+ /ΣFe ratios obtained by Mössbauer spectroscopy exhibit large uncertainties because of low signal to noise ratio. A significant increase of counting times (in the order of a day) would lead to a better resolution, but it would decrease the number of samples that can be analyzed during a synchrotron session.

The Fe 3+ /ΣFe ratios of melt inclusions derived from XANES spectroscopy using a low radiation dose are in good agreement with previously published data. Our results confirm that primitive arc magmas are more oxidized than those from hot spots and mid-ocean ridges. A positive correlation between the Fe 3+ /ΣFe ratios calculated for melt inclusions and their water contents suggests that the oxidation recorded in primitive arc magmas is likely linked to changes in the oxidation state of the mantle beneath arcs because of subduction processes [START_REF] Kelley | Water and the Oxydation State of Subduction Zone Magmas[END_REF]. The calculation of the fO2 relative to the QFM buffer confirms that arc melt inclusions and Mount Etna melt inclusions are more oxidized than MORB and OIB melt inclusions. The results also indicate that the range of oxidation state recorded in arc melt inclusions varies from one subduction zone to another.

Fe

  3+ /ΣFe ratios were derived from the relative areas of each doublet and 2σ uncertainties were calculated from the statistical fitting uncertainties reported by the software. Results are shown in Table3of the Supplementary material.

Figure 2 :

 2 Figure 2: Results of the fitting procedure applied to two room temperature Mössbauer spectra collected on melt inclusions using the synchrotron Mössbauer source. The Fe 3+ /ΣFe ratios of

Figure 4 :

 4 Figure 4: Time series of the intensity ratio I(7114)/[I(7114)+I(7112.6)] collected over 300 s in

Figure 5 :

 5 Figure 5: Overestimation of the Fe 3+ /ΣFe ratios recorded by time series collected in hydrous glasses as a function of (a) their water content, (b) their 'initial' Fe 3+ /ΣFe ratios determined in non-photo-oxidizing conditions (5.1 × 10 6 photons/s/µm²) and (c) the beam damage

Figure 6 :

 6 Figure 6: Pre-edge regions after baseline subtraction of two successive XANES spectra

Figure 7 :

 7 Figure 7: Fe 3+ /ΣFe ratios from Mössbauer spectroscopy compared to those from XANES

Figure 8 :

 8 Figure 8: Number of melt inclusions and glasses as a function of their Fe 3+ /ΣFe ratios and geological settings. Fe 3+ /ΣFe ratios plotted in this graph are mainly derived from XANES measurements at a radiation dose of 3.7 × 10 7 photons/s/µm² (light grey; Table3 of the

Figure 10 :

 10 Figure 10: Number of melt inclusions as a function of their fO2 calculated relative to the QFM

  Table3of the Supplementary material). Fe 3+ /ΣFe ratios plotted in this graph are mainly derived from XANES measurements at a radiation dose of 3.7 × 10 7 photons/s/µm² (light grey; Table3of the Supplementary material). When XANES spectra showed evidence of beam damage, we report Fe 3+ /ΣFe ratios derived from Mössbauer spectroscopy instead (6 melt inclusions).Within our population of melt inclusions from arc basalts, we see a range in fO2 values. Melt inclusions from Stromboli (Eolian arc) and Mount Meager (Cascades) are the least oxidized with mean fO2 values of QFM +0.1 and QFM +0.6 (n=6), respectively. Melt inclusions from La Sommata (Vulcano island, Eolian arc) are the most oxidized with a mean fO2 of QFM +2.1 (n=4). Melt inclusions from Aoba volcano (Vanuatu arc) display a mean fO2 value that is intermediate(QFM +1.3, n=8). Furthermore, we note that melt inclusions from Aoba display the largest range in fO2, overlapping nearly the entire range in fO2 in all the arc melt inclusions from this study. Melt inclusions from Mount Etna display a mean fO2 value of QFM +1.2 (n=7). This fO2 value is closer to arc values than the OIB values determined in this study.

Acknowledgments

The staff of the LUCIA beamline, Nicolas Trcera and Pierre Lagarde, are thanked for help with data collection, and we acknowledge the European Synchrotron Radiation Facility

for provision of synchrotron radiation facilities at the beamline ID18. Nicolas Cluzel, Jean-Luc Devidal, Franck Pointud, Didier Laporte are warmly thanked for lab assistance. Oliver Shorttle is thanked for sharing his R codes. Massimo Pompilio is thanked for sharing his SVP291c sample. MG was supported by the Région Auvergne (Nouveau chercheur fellowship to ML). This research was founded by the French Government Laboratory of Excellence initiative (ClerVolc LabEx) and the Région Auvergne (Nouveau chercheur fellowship to ML). This is ClerVolc contribution number XXX.


Jonathan Chibois

Jonathan Chibois How Far

How Far is The Parliamentary Work a Product of Collective Action?

Ethnographies of Legislatures Workshop -Edinburgh, UK

How Far is The Parliamentary Work a Product of Collective Action?

12 October 2017, by Jonathan Chibois, IIAC (EHESS-CNRS)

To start, I would thank all the organizers, and especially Marc who has done a lot, to give me the opportunity to present my PhD work to you, today.

To begin with, I should say a few words about my research project, and explain my own approach to Parliament. My work is within the French National Assembly, which takes place in a building called the "Palais Bourbon" in central Paris.

In short, I am questioning the digital revolution within the French Parliament. More precisely, I am trying to understand whether parliamentary work has changed, or is in the process of changing, because of the computerization of activities, procedures and interactions. That is to say that, whilst I am not overlooking or ignoring the new digital tools (use of social media, websites, web tools, etc.) and the processes used within the legislative process, my focus is on how such processes are made possible, and the network of activities which support it.

Why have I chosen such an approach? Because social-technical system are themselves strong powerful instruments. They are both produced by the social groups in which they are integrated, and additionally, they participate to produce such social groups. Further, using a new tool impacts the manner in which we deliver the tasks that we have been assigned.

Additional consequences include the manner in which the work is divided between us and our colleagues, and also how the group itself is structured.

The key focus to this approach is "infrastructure". I am studying the evolution of the French Parliamentary infrastructure and, within this, an "ethnography of the infrastructure". I have interviewed several application developers who the Parliament have hired. I have personally used many of the computer applications developed for MPs as well as those for the Civil Service. I have even read several current and previous user manuals! All this in addition to the fundamental things we do as ethnographers! For fieldwork -and this was a key focus -I aimed to describe the networks of the working relations between the actors, which allowed me in a second step to try to describe their variance over the past decades. I've based my article on an ethnographic issue, that is to say a situation I observed during my fieldwork, which for a longtime, I have been trying to explain. I wanted to talk about it today, because it has several important methodological implications to my work. Indeed, to resolve such an issue, and to understand why it was hardly understandable, I felt the need to step back from several common concepts, and particularly the term "MP" which I needed to slightly redefinate! How did I come to such a conclusion?

The digital footprint does not really match what we know about MPs. The fact is that MPs' parliamentary work is more or less absent from digital networks. MPs are always saying what they do, how they do it, why it is important to do it, but there is often no digital footprint of their activities. To be sure, we see their names, and also their faces; we see these everywhere throughout the channels of the legislative process, and also in media channels which they use to project their activities. However, it is very easy for someone else to take the (digital) place of the MP! What may we observe within the networks; especially the actors we commonly call 'MPs'?

On the one hand, there are the digital footprints of the individuals, in the form of metadata. It is about the access point of their connections when they sign-in to a parliamentary web service or to a social network. It is also the timestamps recorded when, say, they have opened a door with their badge. But, such individual data is generally limited. Firstly, because they are mostly unavailable for an ethnographer; but that does not preclude our awareness that such data is indeed collected. Secondly, because it is difficult to verify that a personal account or badge is being used by the actual owner! Thirdly, because MPs usually make limited use of such digital tools.

On the other hand, there are traces of work tagged by the MPs' name, but which could have been prepared by someone else. For instance, assistants can sign-in to the parliamentary information system using the MP's credentials, rather than their own. Things like, updating the daily schedule, making changes, or sending an email in the name of the MP. For instance, they can also tweet or send blog posts in the name of the MP.

Yet, when we ask MPs and their assistants how they work, they readily acknowledge they share the accounts between themselves. That is to say not only that we cannot trust such traces to know who exactly does what, but also the work done is largely collective, despite the names appended to the different outputs, and despite also whatever such arrangements suggest. So, everything happens as if MPs are not individuals, but always as a collective: the parliamentary team as the inner circle, the civil servants as the second circle, and the all acquaintances (for example the partner, or the children) as the outer circle. It is clear that, the work done by a MP has always done by a many different individuals.

Actually, such division of work is no secret. We know that MPs cannot fulfill their objectives in isolation. There is too much for an individual, they need help. That's why they have assistants, and in France, many civil servants work in the background both for them, but also working to support the legislative process. If we spend time within parliament or within the constituencies; when we live with the parliamentary actors for a few weeks and even months -as we do as ethnographers -we don't have any difficulties in understanding how such assistants are vital to the MPs' work.

And if do I not see the MP's personal digital footprints within the networks, I now realize that it is only because they are busy somewhere else, which does not necessarily require their use of a digital device but rather, just an ordinary telephone! To not have digital traces of them within the network does not mean that they are inactive! But can we really consider that MPs are only a collective entity, simply because we cannot find digital footprints which show that they act as individuals? Can we really consider that MPs are just a collective entity, whereas the electorate know full well whom they have elected!

Therefore what should we call a MP?

Here, we face a situation which has a lot to do with opacity and media visibility issues. The challenge was to understand situations where MPs are mainly collective in the day to day outward presentation, but individuals when caught 'off camera'. Whatever the reasons for such discrepancy between a MP's outward show and the factual reality of their parliamentary work it begs the question whether this divergence is acceptable in a democratic system. This is a point for discussion.

If I wanted to be precise; I would not use the term 'MP' both to talk about the actions done by the individual elected, and the actions done by the team (including civil servants) working for the said individual. But how do we name or describe an MP who sometimes represents a collective entity, sometimes represents an individual entity, and sometimes both at the same time!? Such considerations led me finally to redefine the term "MP". Firstly, in order to be able to clearly identify the elected individual and how they are perceived. Secondly, in order to be able to describe the links between the two terms.

• "The Collective". I've chosen to keep the term of "Member of Parliament" or "MP" to retain the perception that this is the actual work of the mandate holder, or its media figure. It never specifies individuals nor specific objectives, but always a representation of the democratic ideals. It is important to note that a MP could always be a product of a collective work, even if it has only one official face.

• "The Elected Individual" -the "Mandate Holder". I have opted for the term "mandate holder" to name specifically the individuals who have been elected. They always work as a team, of which the elected individual is the leader. That doesn't mean they personally produce the work, but that they provide the direction and objectives and have the final say in every decision. They are also the ones who have taken the greatest risks during the election, who now elected, rewards them with privileges over others actors, such as the right to attend and vote in Parliament. That does not mean that they are alone in their team to work on the Parliamentary bills, but that they are also able to represent their whole team. And, finally, they have the very specific task to lend their own face to the figures of the MPs, even if such figure is always collectively built by its team.

Of course, very often these two dimensions match together so much that it is not easy to distinguish them. For example, when we meet in the constituency office, or when you watch it on TV. In such cases, we see both the individual and its representation. The mandate holder is using standard media in order to bring alive the figure of the MP.

But that does not prevent us from considering situations where the two can be uncoupled. For example, when an assistant of the mandate-holder replies to an email and sign as if he is the mandate holder, they are bringing the MP figure to life. As another example, when an assistant is tweeting for the mandate-holder's account, the MP figure comes alive without any intervention from the mandate holder! For a final example, when a civil servant writes an entire parliamentary report in the name of a mandate holder, the mandate holder may have made no direct contribution, but this work is nevertheless attributed to the MP.

To conclude

The step back from the notion of the "MP" has also led me to see this social world a lot more flat and decentralized than I did. In doing so, I indeed removed what separated those elected from the others, in order to present all actors' activities on an equal footing. It allows thinking how MP's work can be equally well achieved by elected and non-elected individuals. From this perspective, an MP is a parliamentary worker among others -and not above the others.

But, I would like to make it clear that if the greater part of their work is similar to that of other parliamentary actors, it does not mean that the mandate holders have the same place in the parliamentary teams or in the parliament itself. It doesn't remove the responsibilities, the privileges, the conflicts, the tensions, and power-struggle in all such relationships. On the contrary, such perspective leads us to consider the fact that the division of work in a parliamentary team is not based on a natural difference of roles, but are only arrangements between individuals in their desire to achieve specific outcomes.