Ivan Vlastélic
email: i.vlastelic@opgc.univ-bpclermont.fr

Gabrielle Menard

Abdelmouhcine Gannoun

Jean-Luc Piro

Thomas Staudacher

Vincent Famin

J.-L Piro

collapse of Piton de la Fournaise: The record of semi-volatile trace elements (Li, B, Cu, In, Sn, Cd, Re, Tl, Bi)

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Réunion Island (60 × 40 × 3 km) is located in the Indian Ocean at 21°10′S, 55°30′E( Fig. 1a). It is the emergent part of a basaltic cone of 240 × 200 × 7 km. The island is the present location of the hotspot that created the Deccan Traps, the Chagos-Maldive-Laccadive ridge, the Mascarene Plateau and Mauritius Island. The island is presently composed of two main volcanoes, the extinct Piton des Neiges in the NW and the active Piton de la Fournaise in the SE (Fig. 1b). Piton de la Fournaise has been regularly active since at least 530 ka [START_REF] Gillot | Eruptive history of the Piton de la Fournaise volcano, Réunion Island, Indian Ocean[END_REF] and is still presently in its shield building stage. Three major events, either collapses or landslides, marked the growth history of the volcano 250, 35 and 4.7 ka ago [START_REF] Gillot | Model for the structural evolution of the volcanoes of Réunion Island[END_REF]. The result is three concentric calderas or depressions, the most recent of which (the Enclos Fouqué) is U-shaped (8 × 13 km) and open to the sea on its eastern side (Fig. 1c). A 400 m high cone with two coalescent summit craters (Bory and Dolomieu) rises inside the Enclos Fouqué.

Although collapse calderas are relatively common features of basaltic volcanoes, very few have been observed during their formation. The collapse of the Piton de la Fournaise summit in April 2007 is one of these formations. Due to the advanced monitoring network, the timing of the events and the temporal relationship between the magma withdrawal from the shallow reservoir and the collapse of the summit crater in particular are known in detail [START_REF] Michon | April 2007 collapse of Piton de la Fournaise: a new example of caldera formation[END_REF][START_REF] Staudacher | The April 2007 eruption and the Dolomieu crater collapse, two major events at Piton de la Fournaise (La Réunion Island, Indian Ocean)[END_REF]. Since 1977, the emitted lavas have been sampled regularly during the course of the eruption, providing a unique opportunity to track compositional changes that may be linked to caldera-forming processes. Previous geochemical studies of the April 2007 samples [START_REF] Villemant | Evidence for a homogeneous primary magma at Piton de la Fournaise (La Réunion): a geochemical study of matrix glass, melt inclusions and Pélé's hairs of the 1998-2008 eruptive activity[END_REF][START_REF] Vlastélic | Pb isotope geochemistry of Piton de la Fournaise historical lavas[END_REF][START_REF] Collins | Two phases of sulphide saturation in Réunion magma: evidence from cumulates[END_REF]h a v ed i s c u s s e d general issues related to the magma source, magma differentiation and degassing trends, but none of them focused on the temporalcompositional evolution of the lavas erupted during, or just before, this major event, which is the object of this paper. This study focuses on trace elements, including fluid mobile and semi-volatile species (e.g., Li, B, Re, In, Cd, Tl, Bi). In particular, volatile trace metals are used to reconstruct the degassing history of this exceptional eruption, which is constrained from remote observations [START_REF] Bhugwant | Atmospheric sulfur dioxide measurements during the 2005 and 2007 eruptions of the Piton de la Fournaise volcano: implications for human health and environmental changes[END_REF][START_REF] Gouhier | Satellite-based evidence for a large hydrothermal system at Piton de la Fournaise volcano (Réunion Island)[END_REF][START_REF] Tulet | Large scale modeling of the transport, chemical transformation and mass budget of the sulfur emitted during the April 2007 eruption of Piton de la Fournaise[END_REF].

We first address the question of whether this exceptional, calderaforming eruption was preceded by geochemical precursors. Thus, we investigate the previous eruptions of 2006 and 2007. Then, we consider the possibility that the temporal-compositional evolution of the April 1 2007 lavas recorded the processes that affected the shallow magma reservoir shortly before, or even during, the eruption. These processes potentially include (1) pre-eruptive vapour transfer and accumulation at the top of the magma column, which could play a role in triggering major eruptions [START_REF] Berlo | Geochemical precursors to volcanic activity at Mount St[END_REF][START_REF] Kent | Vapor transfer prior to the October 2004 eruption of Mount St[END_REF][START_REF] Rowe | Using amphibole phenocrysts to track vapor transfer during magma crystallization and transport: an example from Mount St. Helens, Washington[END_REF]; (2) massive volatile exsolution in response to static decompression of the magma reservoir as magma drained to feed lateral eruptions [START_REF] Johnson | Dynamics of magma storage in the summit reservoir of Kilauea Volcano, Hawaii[END_REF][START_REF] Poland | Magma degassing triggered by static decompression at Kilauea Volcano, Hawai'i[END_REF]; (3) the rapid crystallisation of clinopyroxene and plagioclase microlites, depending on the thermal state of the shallowest magma reservoir and the dynamics of magma transfer to the eruption site [START_REF] Welsch | Microcrysts record transient convection at Piton de la Fournaise volcano (La Réunion Hotspot)[END_REF]; and (4) magma contamination during storage. Interaction with or assimilation of oceanic crust or edifice material could occur as magma accumulates before high-flux eruptions [START_REF] Vlastélic | Rapid change of lava composition from 1998 to 2002 at Piton de la Fournaise (Réunion) inferred from Pb isotopes and trace elements: evidence for variable crustal contamination[END_REF][START_REF] Vlastélic | Short-term (1998-2006) fluctuations of Pb isotopes at Piton de la Fournaise volcano (Réunion Island): origins and constraints on the size and shape of the magma reservoir[END_REF] or when volcanic activity resumes after a long period of quiescence, as in 1998 [START_REF] Salaün | Cannibalism of olivine-rich cumulate xenoliths during the 1998 eruption of Piton de la Fournaise (La Réunion hotspot): implications for the generation of magma diversity[END_REF]. The scenarios of submarine collapse [START_REF] Kent | Widespread assimilation of a seawater-derived component at Loihi Seamount, Hawaii[END_REF] and hydrothermal caldera [START_REF] Merle | Hydrothermal calderas[END_REF] also raise the possibility of the entrainment of hydrothermally altered material that may compose the interior of the volcano [START_REF] Lénat | Geoelectrical structure of the central zone of Piton de la Fournaise volcano (Réunion)[END_REF]. These different possibilities are discussed in light of the trace element data presented.

Eruptive background

The Piton de la Fournaise volcano is very frequently active, but most eruptions are of small volume and short duration. Amongst the 105 eruptions estimated to have occurred between 1920 and 2007 [START_REF] Stieltjes | A statistical and probabilistic study of the historic activity of Piton de la Fournaise, Réunion Island, Indian Ocean[END_REF]Peltier et al., 2009a), only four produced more than 50× 10 6 m 3 of lava, and only two more than 100 × 10 6 m 3 . These two most voluminous eruptions (July 1931 andApril 2007) caused major summit collapses. Unlike in 1931, the eruptive history (2) 2σ reproducibility on BHVO-2 (9 analyses). The low reproducibility of Pb (and possibly Mo and As) concentrations may reflect the heterogeneity of BHVO-2 powder (see [START_REF] Chauvel | Comprehensive chemical and isotopic analyses of basalt and sediment reference materials[END_REF].

(3) Isotope dilution data. BHVO-2 was not analysed for Re. The uncertainty of Re data is 1%, as inferred from replicate analyses of Piton de la Fournaise basalts [START_REF] Schiano | Osmium isotope systematics of historical lavas from Piton de la Fournaise (Réunion Island, Indian Ocean)[END_REF]. prior to the April 2007 eruption is known in great detail, especially since 1972. This recent history is first marked by an unusually long quiet period, between 1992 and 1998, which ended with a no-less unusual long eruption (March-September 1998). This eruption led to the suggestion that the volcano had entered a new, major eruptive sequence in 1998 [START_REF] Staudacher | Piton de la Fournaise volcano, Réunion island, starts a new cycle of high eruptive activity[END_REF]. Perhaps more importantly, the behaviour of the plumbing system changed markedly in 2000, as evidenced by the reappearance of olivine-rich lavas after more than 20 years of absence. From 2000, the volcano also showed repeated short-term eruptive cycles, each being characterised by (1) decreasing elevation of the eruptive vents, (2) increasing abundance in cumulative olivine, (3) summit inflation and (4) deepening of dyke roots [START_REF] Peltier | Cyclic magma storages and transfers at Piton de la Fournaise volcano (La Réunion hotspot) inferred from deformation and geochemical data[END_REF]. Based on these criteria, Peltier et al. (2009a)

The July 2006-April 2007 eruptive cycle

The sequence of events that led to the April 2007 paroxysmal activity started in July 2006. Following a modest eruption in July (2.8 × 10 6 m 3 ), a four-month long eruption (August 30, 2006 to January 1, 2007) filled the Dolomieu Crater with circa 20 × 10 6 m 3 of lava. The summit eruptive activity resumed brieflyi nF e b r u a r y1 8 ,2 0 0 7 (b 10 6 m 3 ). As the volcano continued to inflate, an eruption occurred on March 30 at 1900 m along the south-east flank and produced again a small volume of lava (b10 6 m 3 ). The subsequent April events are described in detail by [START_REF] Staudacher | The April 2007 eruption and the Dolomieu crater collapse, two major events at Piton de la Fournaise (La Réunion Island, Indian Ocean)[END_REF], [START_REF] Michon | April 2007 collapse of Piton de la Fournaise: a new example of caldera formation[END_REF] and Peltier et al. (2009b).Br iefly, the eruption started on April 2, at 590 m above sea level (asl), 7 km away from the summit along the south-east rift zone (Fig. 1c). The seismic tremor, the thermal anomaly, and the height of lava fountains all indicated that the rate of magma production increased drastically until the summit Dolomieu crater collapsed in the night of the 5 to 6 April, forming a caldera of 90× 10 6 m 3 . [START_REF] Staudacher | The April 2007 eruption and the Dolomieu crater collapse, two major events at Piton de la Fournaise (La Réunion Island, Indian Ocean)[END_REF] noted that the maximum tremor occurred during the day of April 6, 12 to 24 hours after the main collapse event. Following the paroxysmal phase, the eruption continued at this more or less regular, but elevated, rate until May 1. The total volume of lava produced, initially estimated to be approximately 130 × 10 6 m 3 [START_REF] Staudacher | The April 2007 eruption and the Dolomieu crater collapse, two major events at Piton de la Fournaise (La Réunion Island, Indian Ocean)[END_REF], has been re-evaluated to be 240 × 10 6 m 3 . The eruption rate was 52 m 3 /s on average but most likely exceeded 200 m 3 /s during the paroxysmal phase. Thermal monitoring from space revealed that the magma output rate increased from 55 to 75 m 3 / sb e t w e e nA p r i l3a n d4 [START_REF] Coppola | Lava discharge rate and effusive pattern at Piton de la Fournaise from MODIS data[END_REF]. These rates are typically one order of magnitude higher than those estimated for normal eruptions. Based on the satellite data, the degassing paroxysm (1800 kg/s of SO 2 ) occurred in the morning of April 6 [START_REF] Tulet | Large scale modeling of the transport, chemical transformation and mass budget of the sulfur emitted during the April 2007 eruption of Piton de la Fournaise[END_REF]. Summit emanations during collapse exceeded those expected from lava effusion, possibly as a result of opening of the hydrothermal system [START_REF] Gouhier | Satellite-based evidence for a large hydrothermal system at Piton de la Fournaise volcano (Réunion Island)[END_REF]. The summit collapse was ascribed to a sub-surface pressure drop caused by rapid, lateral withdrawal of magma from the shallow reservoir [START_REF] Staudacher | The April 2007 eruption and the Dolomieu crater collapse, two major events at Piton de la Fournaise (La Réunion Island, Indian Ocean)[END_REF].

The lavas emitted until April 4, 2007 are poorly phyric transitional basalts with little compositional variability, often referred to as steady-state basalts. The amount of olivine crystals increased drastically between April 4 and 5 (one day before the summit collapse) and remained elevated (≥ 50 wt.%) until the end of the eruption. Because no geochemical study focused on this period, chemical data are sparse. Naturally quenched samples (Pélé's hairs) from the early stage of the April 2007 eruption show melt compositions (MgO up to 8.6 wt.‰) amongst the most primitive ever measured at Piton de la Fournaise [START_REF] Villemant | Evidence for a homogeneous primary magma at Piton de la Fournaise (La Réunion): a geochemical study of matrix glass, melt inclusions and Pélé's hairs of the 1998-2008 eruptive activity[END_REF]. The Pb isotopic compositions of April 2007 lavas are indistinguishable from previous eruptions (2005)(2006)(2007), with the exception of an unradiogenic anomaly (April 5) coinciding with the paroxysmal phase of the eruption [START_REF] Vlastélic | Pb isotope geochemistry of Piton de la Fournaise historical lavas[END_REF].

Methods

Lava samples were collected during the course of the eruptions. Some were quenched in deionised water (see Table 1). The major element concentrations were measured on fused beads by X-ray fluorescence (XRF) spectrometry at Laboratoire GéoSciences Réunion. A subset of samples was analysed for the total S, Cl, F and H 2 O (Service d'Analyse des Roches et Minéraux, CRPG, Nancy). Sulphur converted to SO 2 was measured by IR spectrometry. The water content was determined by volumetric Karl Fisher titration. The chlorine content was determined by a spectrophotometric method based on the formation of ferrithiocyanate, and the F content was determined by potentiometry using an ion-selective electrode [START_REF] Vernet | Dosage du Fluor et du Chlore dans les matériaux géologiques y compris les échantillons hyperalumineux[END_REF]. Trace element analyses were performed on powders made by crushing rock chips in an agate mortar. Handpicked chips, including intersticial glass from an olivine-rich sample (070420-2) and Pélé's hairs and tears (070405-2), were also analysed. The boron concentration was determined using the new direct isotope dilution method of Menard et al. (submitted), where the B isotopic compositions of HF-digested rocks are measured by ICPMS without any evaporation step. The rhenium concentration was measured by isotope dilution and ICPMS following the method described in [START_REF] Schiano | Osmium isotope systematics of historical lavas from Piton de la Fournaise (Réunion Island, Indian Ocean)[END_REF]. Other trace elements were measured more classically using external calibration. Approximately 100 mg of the sample were dissolved in 2 ml of 28 M HF and 1 ml of 14 M HNO 3 in a savillex beaker for 36 hours at 70 °C. The solutions were evaporated to near dryness at 65 °C (to prevent the loss of the volatile fluoride species, such as AsF 3 ). After the addition of a few ml of 7 M HNO 3 , solutions were evaporated to near dryness, and clear solutions were obtained by adding 20 g of 7 M HNO 3 . These solutions were diluted with deionised water by a factor of 20 to 10 to reach rock dilution factors ranging from 4000 to 2000. The trace element abundances were determined by quadrupole ICPMS (Agilent 7500, Laboratoire Magmas et Volcans). Given the instrumental detection limit of typically 1 ppt, it is possible to analyse elements that are present at the ppb level in rocks (e.g., Bi, Tl, Cd). The analyses were performed in plasma robust mode (1550 W). The reaction cell (He mode) was used to reduce interferences on masses ranging from 45 (Sc) to 75 (As). The signal was calibrated externally (every 4 samples) with a reference basaltic standard (USGS BHVO-2, batch 759) dissolved as samples and using the GeoRem recommended values (http://georem.mpch-mainz.gwdg.de/). The BHVO-2 standard was used because the trace element contents are very similar to those of Réunion basalts. For elements that are not well characterised (As, Mo, Sb, Sn, Cd, In, Bi, Tl), or which obviously show heterogeneity (e.g., Pb) in BHVO-2 powder [START_REF] Chauvel | Comprehensive chemical and isotopic analyses of basalt and sediment reference materials[END_REF], the signal was calibrated using the certified concentrations of a synthetic standard, which was also repeatedly measured. The concentrations of non-conventional elements obtained for the BHVO-2 standard are listed in Table 1 instead of the GeoRem information values. The values generally agree well with previously published values except for Cd (0.12 against 0.06 ppm). Samples were measured in a random order, and most solutions were measured twice. The external reproducibility (2σ error) of the method, as estimated by repeatedly dissolving and analysing BHVO-2 as an unknown, is b 9% for most elements, 11% for Be, B and Bi, 15% for Pb, 26% for As and 38% for Mo (Table 1). The low reproducibility of Pb, and possibly As and Mo, could reflect the heterogeneity of the BHVO-2 powder. Because of error correlation during the ICP-MS measurement (instrumental drift affecting similarly different masses), the precision of the trace element ratios is often better than the precision of the concentration data, which is especially true for ratios involving elements belonging to the same family and/or with small differences of atomic masses. For instance, the precision (2σ error) of the La/Sm, Nb/Zr and Bi/Cd ratios are better than 2%, 5% and 10%, respectively. The accuracy of the method was checked by measuring different standards (BCR-2, BIR and BEN). For most elements, the differences with respect to recommended values were less than 20%. Chemistry blanks were systematically checked and were found to be negligible compared to the natural abundances of elements in lava samples.

Results

Volatile and semi-volatile element systematics in Piton de la

Fournaise lavas Whole-rock S, Cl, F and H 2 O concentrations of recent basalts of Piton de la Fournaise (Supplementary Table A) do not exceed 400 ppm, 285 ppm, 600 ppm and 0.26%, respectively. Sulphur and H 2 O are much less abundant than in melt inclusions (1000-2000 ppm and ca. 1%, respectively [START_REF] Bureau | Fluid-magma decoupling in a hot-spot volcano[END_REF]), indicating that the lavas are extensively degassed with respect to major volatile species. The lowest F and Cl concentrations are found in the olivinerich samples while the highest concentrations are found in the 1998 Hudson lava (986-115) and in the 1977 pumice (77-14) (Fig. 2). The F/Cl ratio is constant at 2. Menard et al. (submitted). Too few samples have been analysed for both major gaseous species (S, Cl, F and H 2 O) and semi-volatile elements to infer any robust relationship between the two types of data. All semivolatile elements investigated in this study (Li, B, As, Cu, Sn, Sb, In, Cd, W, Re, Pb, Bi, Tl) decrease with Ni, indicating that all are incompatible in olivine (Fig. 3). However, unlike non-volatile elements, they show large variations at a given Ni content. According to the composition of the high-temperature (400 °C) gas sublimates [START_REF] Vlastélic | Lithium isotope fractionation during magma degassing: constraints from silicic differentiates and natural gas condensates from Piton de la Fournaise volcano (Réunion Island)[END_REF], Cd, Bi, and Tl are the most volatile trace elements at Piton de la Fournaise. These elements are enriched in tephras and a few lava samples (e.g., the 1998 Hudson lava, and lavas from the early stage of the April 2007 eruption). Lithium and B as well as Cu, In and Sn have a more complex behaviour, in particular during the April 2007 eruption where they show both transient enrichments or depletions.

The relationships between Bi, Tl, In and Cd are investigated in more detail in Fig. 4. Given that Bi and Tl are respectively slightly less and slightly more volatile than Cd at Piton de la Fournaise [START_REF] Vlastélic | Lithium isotope fractionation during magma degassing: constraints from silicic differentiates and natural gas condensates from Piton de la Fournaise volcano (Réunion Island)[END_REF], the lavas have slightly higher Bi/Cd and slightly lower Tl/Cd compared to the gas sublimates. Because In is markedly less volatile than Cd, In/Cd in lavas is much higher than in the gas phase. Artificially quenched lavas do not display a distinctive Bi-Cd-In-Tl signature compared to lavas that were sampled after cooling, suggesting that latestage degassing is not significant for these elements. However, naturally quenched samples (Pélé's hairs and tears, ashes), the 1977 pumice, and the 1998 Hudson lava generally have elevated Bi/Cd and Bi/Tl ratios and low In/Cd and In/Tl ratios, while Tl/Cd is not significantly different from common lavas (with the possible exception of Tl enrichment). The excess of Bi in these samples is not consistent with the incorporation of gas sublimates.

The 2006-2007 time evolution

The lavas erupted between July 2006 and March 2007 show little compositional variation, most trace element variations being within analytical error (Table 1 and [START_REF] Vlastélic | Short-term (1998-2006) fluctuations of Pb isotopes at Piton de la Fournaise volcano (Réunion Island): origins and constraints on the size and shape of the magma reservoir[END_REF]). The exceptions are spikes in some volatile trace elements. Thallium is elevated in the first lavas of the August-December 2006 eruption (66 ppb against 40-45 ppb in subsequent lavas). Molybdenum and rhenium are both enriched (by a factor of two at least) in sample 0702-18-3 from February 2007. Such coupled enrichments suggest the occurrence of rhenian molybdenite ((Mo,Re)S 2 ), a gas sublimate mineral that has been essentially described at the Kudryavy volcano, Kurile Islands, Russia [START_REF] Tessalina | Sources of unique rhenium enrichment in fumaroles and sulphides at Kudryavy volcano[END_REF]. The following activity, in April 2007, is first marked by a transition from steady-state basalts to olivine-rich basalts between April 4 and 5. This event is recorded in whole-rock compositions by a drastic increase in the concentration of elements that are compatible in olivine (Ni, Co and Cr) (Fig. 5)andacorresponding decrease in moderately (e.g., Yb, Li) to highly (e.g., La, Th, Nb) incompatible elements. A detailed inspection of Ni (or La) temporal trend reveals that the transition was initiated as early as February 2007. The tight negative correlations between Ni and refractory incompatible elements (not shown) indicate that olivine accumulation is the cause of much of the chemical variability seen in whole-rock samples and thus that the melt composition changed little over that period. The interstitial glasses from the olivine-rich tephra (070405-2) and the oceanite sample 0704-20-2 have similar compositions as steadystate basalts, although with higher Ni (180-200 ppm against 90-110 ppm) and lower incompatible element contents. This difference may reflect either a more primitive composition or the occurrence of very small olivine crystals in the handpicked silicate glass. The features described above are rather common for Piton de la Fournaise, however. The April 2007 eruption stands out from previous eruptions in showing chemical anomalies that occurred successively during the first few days of the eruption.

(1) Semi-volatile elements display anomalous behaviours between April 2 and 6. On the one hand, Li, Cu and Tl show low concentrations that cannot be explained by olivine accumulation (Figs. 3a and5). Menard et al. (submitted) also note that these early stage lavas have abnormally low B/Nb ratios. The most pronounced depletion is found in the first lavas erupted on April 2 (by a factor of 0.70, 0.76 and 0.82 for Li, Cu, and Tl, respectively, taking for reference the average composition of 2006 eruptions).

The intersticial glass from the olivine-rich tephra produced on April 5 is also depleted in Li and Cu, even when its higher Ni content is taken into account (Fig. 3a). These observations agree with the recent identification of Cu depletion in olivine-hosted melt inclusions from the same eruption [START_REF] Collins | Two phases of sulphide saturation in Réunion magma: evidence from cumulates[END_REF].

Coupled Li-Cu anomalies, including both enrichments and depletions, were previously described in phenocrysts (plagioclase and amphibole) from the early stages of paroxysmal eruptions of Mount St. Helens [START_REF] Berlo | Geochemical precursors to volcanic activity at Mount St[END_REF][START_REF] Kent | Vapor transfer prior to the October 2004 eruption of Mount St[END_REF][START_REF] Rowe | Using amphibole phenocrysts to track vapor transfer during magma crystallization and transport: an example from Mount St. Helens, Washington[END_REF]. It is noteworthy that the April 2007 samples depleted in Li show Li isotopic compositions (4.0b δ 7 Lib 4.5‰)thatare slightly heavier than the average signature of steady-state basalts (δ 7 Li = 3.6 ± 0.4 (1σ), n = 27 [START_REF] Vlastélic | Lithium isotope fractionation during magma degassing: constraints from silicic differentiates and natural gas condensates from Piton de la Fournaise volcano (Réunion Island)[END_REF]), although the difference is barely significant. On the other hand, Bi, Cd, In and Sn are enriched (by a factor of 1.85, 1.23, 1.28 and 1.10, respectively) during the same period (Figs. 3ba n d6), although there is a short (1-2 days) time offset between the Li-Cu depletion (mostly on April 2) and the Bi-Cd-In-Sn enrichment (April 3 and 4). Such enrichment does not seem to occur for Tl, Pb, As, Re and B. In detail, Bi, Cd, In and Sn show contrasting responses over these two days, the Bi concentration increasing first (dominant anomaly on April 3), followed by Cd (similar anomalies on April 3 and 4) and In-Sn (anomaly on April 4). The samples from April 3 and 4 have thus anomalously high Bi/Cd and In/Cd ratios, respectively (Fig. 4). A chlorine excess occurred slightly later, during the eruption paroxysm on April 6 (Fig. 2). The chlorine most likely originates from the huge steam plume that formed when the lava flow entered the sea [START_REF] Staudacher | The April 2007 eruption and the Dolomieu crater collapse, two major events at Piton de la Fournaise (La Réunion Island, Indian Ocean)[END_REF]. ( 2 Arrows point toward the composition of high temperature (400 °C) gas sublimates from Piton de la Fournaise [START_REF] Vlastélic | Lithium isotope fractionation during magma degassing: constraints from silicic differentiates and natural gas condensates from Piton de la Fournaise volcano (Réunion Island)[END_REF].

(Fig. 7). On April 6, La/Sm decreases down to 2.77, which is the lowest value measured in recent lavas of Piton de la Fournaise [START_REF] Vlastélic | Rapid change of lava composition from 1998 to 2002 at Piton de la Fournaise (Réunion) inferred from Pb isotopes and trace elements: evidence for variable crustal contamination[END_REF][START_REF] Vlastélic | Short-term (1998-2006) fluctuations of Pb isotopes at Piton de la Fournaise volcano (Réunion Island): origins and constraints on the size and shape of the magma reservoir[END_REF] and Supplementary Table B).

Discussion

The geochemical evolution of lavas in 2006 and early 2007 does not show strong precursors to the April 2007 events. The most significant signal is the trend of increasing compatible element concentration (e.g., Ni) and decreasing ratios of elements that are more versus less incompatible (e.g., La/Sm or Nb/Zr) (Figs. 5 and7). This trend, initiated in February 2007, may reflect the increasing contribution of a less differentiated melt, which has been previously identified in April 2007 glassy samples on the basis of major elements [START_REF] Villemant | Evidence for a homogeneous primary magma at Piton de la Fournaise (La Réunion): a geochemical study of matrix glass, melt inclusions and Pélé's hairs of the 1998-2008 eruptive activity[END_REF]. The coincidence of a depleted trace element signature with the eruption paroxysm raises the possibility that fast rising magma undergoes limited crystallisation during its transfer to the eruption site. The following discussion will focus on the degassing history of the April 2007 magmas as recorded by semi-volatile elements.

6.1. Origin of Li, Cu, and Tl depletions (April 2 to 4)

The depletion of Li, Cu and Tl in the first lavas of the April 2007 eruption is difficult to explain by crystal fractionation because no mineral, or simple mineral assemblage, contains all three elements. The entrainment or the assimilation of hydrothermally altered material is also unlikely because the samples are fresh and do not show a systematic fractionation of fluid-mobile elements (e.g., Rb, U, Pb). Instead, Li, Cu and Tl could be lost during magma degassing [START_REF] Berlo | Geochemical precursors to volcanic activity at Mount St[END_REF][START_REF] Kent | Vapor transfer prior to the October 2004 eruption of Mount St[END_REF][START_REF] Rowe | Using amphibole phenocrysts to track vapor transfer during magma crystallization and transport: an example from Mount St. Helens, Washington[END_REF]. However, these elements are only moderately volatile, and only a very small fraction of Li, Cu and Tl (0.02, 1 and 0.22%, respectively) is expected to ultimately leave magmas [START_REF] Rubin | Degassing of metals and metalloids from erupting seamount and midocean ridge volcanoes: observations and predictions[END_REF]. The observed loss, in the range of 15-30%, thus requires abnormal degassing conditions. The transition from steady-state basalts to olivine-rich lavas (also referred to as oceanites), the summit collapse (main event) and the eruption paroxysm (maximum tremor, highest lava fountains and degassing peak) are indicated. The lithium and Cu contents of the olivine-rich samples were corrected for olivine accumulation with 100 ppm Ni as reference: [X] Ni=100 = [X] sample -S X-Ni . ([Ni] sample -100), where S X-Ni is the slope of the linear regression line through data points in X versus Ni plot. S X-Ni was inferred using the data given in Table 1, removing the anomalous concentrations from the early stage of the April 2007 eruption (April 2 to 4). Slopes of -0.0023 and -0.0593 were calculated for Li and Cu, respectively (see also Fig. 3). Distinctive symbols are used for concentrations corrected for olivine accumulation (circle with a dark dot), for the handpicked interstitial glass from sample 0704-20-2 (circle with a cross), and for the tephra collected on April 5 (circle with a star).

The unique properties of Li, including a high affinity for aqueous fluids and a high diffusivity in silicate melts, raise the possibility of Li transport in a H 2 O-rich phase rapidly exsolving at shallow depth. Vapour transfer may result in Li depletion at the bottom of the magma chamber and the accumulation of Li at the top [START_REF] Berlo | Geochemical precursors to volcanic activity at Mount St[END_REF]. The complementary enriched Li reservoir was not found in April 2007 products and could have been lost either as vapour, brines or phreatic ash during the major degassing event of the summit, between April 6 and April 14 [START_REF] Gouhier | Satellite-based evidence for a large hydrothermal system at Piton de la Fournaise volcano (Réunion Island)[END_REF]. Nevertheless, the hypothesis of a pre-eruptive exsolution of a Li-bearing aqueous phase agrees with (1) the evidence for shallow magma storage prior to the April 2007 eruption. The lack of deep pre-eruptive seismic activity, as well as the large volume of magma produced over a relatively short time-period (240× 10 6 m 3 in 3 weeks), suggest that the April 2007 magmas were stored approximately at the level of the main vent (590 m above sea level), where a magma reservoir was previously identified [START_REF] Peltier | Constraints on magma transfers and structures involved in the 2003 activity at Piton de la Fournaise from displacement data[END_REF]. On this basis, the April 2007 eruption stands out from the previous voluminous eruption of March 1998, which sampled progressively (60 × 10 6 m 3 in 6 months) a magma reservoir located at least 5 km below sea level [START_REF] Battaglia | Pre-eruptive migration of earthquakes at the Piton de la Fournaise volcano (Réunion Island)[END_REF]. The first lavas produced in March 1998 showed no chemical anomaly, neither in Li nor in any other semi-volatile elements (Supplementary Table B). ( 2) It is generally recognised that, unlike CO 2 , water solubility in basaltic magmas barely decreases with decreasing pressure until 200-100 MPa and decreases subsequently. Recent decompression experiments simulating magma degassing in a closed-system suggest that H 2 O solubility drops at even lower pressure, near 50 MPa [START_REF] Lesne | Experimental simulation of closed-system degassing in the system basalt-H2O-CO2-S-Cl[END_REF]. This pressure corresponds to a basaltic column of 1800 m (with a density of 2700 kg/m 3 ), which is approximately the depth of the shallow magma reservoir below the summit crater (2500 m above sea level). At such a shallow depth, the fluid phase is typically composed of 85% H 2 O, 10% CO 2 ,a n d 5% S (mol%). (3) A large amount of lithium could be lost in equilibrium degassing conditions. For instance, the 2007 eruption also emitted a small volume of differentiated lava that most likely underwent closedsystem degassing (Vlastelic et al., 2011). The extreme Li depletion (>90%) in these silicic differentiates suggests a vapour-melt partition coefficient of Li (D V-M ) greater than 60. [START_REF] Koga | Experimental demonstration of lithium-boron depletion during magma degassing[END_REF] measured a Li depletion of 43% near bubbles of an experimentally degassed sample and estimated D V-M up to 30.

Copper and Tl volatilise as sulphide and chloride complexes [START_REF] Churakov | Evolution of composition of high-temperature fumarolic gases from Kudryavy Volcano, Iturup, Kuril Islands: the thermodynamic modeling[END_REF]. Like water solubility, sulphur solubility in melts drops at low pressure while chlorine solubility remains unchanged [START_REF] Lesne | Experimental simulation of closed-system degassing in the system basalt-H2O-CO2-S-Cl[END_REF]. The partitioning of Cu and Tl in a sulphur-rich phase could thus explain why they are lost as Li. Yet, unlike H 2 O, S makes only a small fraction (b 7 mol%) of the shallow fluid phase. Only elements with the highest affinity for gaseous sulphur are thus expected to significantly leave the magma. Considering a shallow gas phase with a H 2 O/S ratio of 17:the partition coefficient of Cu between sulphur vapour and magma must be 17 times that of Li between H 2 O vapour and magma to yield losses of similar magnitude. [START_REF] Collins | Two phases of sulphide saturation in Réunion magma: evidence from cumulates[END_REF] noted that Cu has less affinity for vapour than for sulphide melt, which is the dominant form of dissolved S at Réunion [START_REF] Métrich | The oxidation state of sulfur in synthetic and natural glasses determined by X-ray absorption spectroscopy[END_REF]. They suggested that Cu could be trapped in sinking sulphides prior to being lost to the gas phase. However, because melt inclusions from the April 2007 eruption are undersaturated in S, they acknowledge that S loss by degassing must also be important. Another possibility is that Cu hydrosulphide complexes that may form in reducing conditions partition to high temperature aqueous supercritical fluids [START_REF] Sun | Release of gold-bearing fluids in convergent margin magmas prompted by magnetite crystallization[END_REF]. Bi,Cd,In and Sn enrichments (April 3 and 4) Overlapping with the Li-Cu depletion (April 2 to 5), elevated concentrations of Bi, Cd, In and Sn occurred on April 3 and 4. Such excesses in elements that are dominantly chalcophile can either reflect the entrainment of dense sulphide melts, the incorporation of microscopic gas sublimates as lavas interact with the gas plume, the eruption of less-degassed magmas, incomplete degassing during eruption, or combinations of theses different causes.

Origin of

The entrainment of sulphide melts is expected to yield systematic enrichments of chalcophile elements, which is not observed. The presence of gas sublimates (minerals formed directly from the gas phase) is possible in the case of small magma products (ashes, Pele's hairs) [START_REF] Moune | Pele's hairs and tears: natural probe of volcanic plume[END_REF] but is not expected in the case of massive lava samples such as those studied here. In addition, the incorporation of gas sublimates would decrease Bi/Cd while the opposite trend is observed (Fig. 4a). Our current understanding of the behaviour of semi-volatile elements does not allow us to distinguish degassing at depth from sub-aerial degassing. The lack of semi-volatile element enrichment in artificially quenched samples only tells us that semi-volatile elements are lost prior to the final stages of lava cooling. The variable but generally high Bi/Cd ratio that characterises tephras appears as a fingerprint of less degassed melts (Fig. 4a). The explanation most likely lies in the difficulty of the slow diffusing Bi [START_REF] Johnson | The degassing behavior of Au, Tl, As, Pb, Re, Cd and Bi from silicate liquids: experiments and applications[END_REF]inreaching the melt-gas interface in the case of rapidly cooling samples undergoing incomplete degassing. However, the fact that some massive lava samples also display elevated Bi/Cd suggests that this signature is not only due to the quenching rate but must also reflect to some extent the eruption of less degassed magma.

Its remains unclear why Bi and Cd fractionate so much despite similar volatilities [START_REF] Vlastélic | Lithium isotope fractionation during magma degassing: constraints from silicic differentiates and natural gas condensates from Piton de la Fournaise volcano (Réunion Island)[END_REF] and why less volatile elements (In, Sn) are enriched while more volatile elements (e.g., Tl) are not. Regarding the first issue, it can be argued that Cd is a more robust tracer of magma degassing than Bi, which can be variably enriched in undegassed magmas [START_REF] Norman | Rhenium and chalcophile elements in basaltic glasses from Ko'olau and Moloka'I volcanoes: magmatic outgassing and composition of the Hawaiian plume[END_REF]. Indium and Sn are significantly less volatile than Bi and Cd at Piton de la Fournaise [START_REF] Vlastélic | Lithium isotope fractionation during magma degassing: constraints from silicic differentiates and natural gas condensates from Piton de la Fournaise volcano (Réunion Island)[END_REF], and, consistently, are not significantly enriched in tephras (Fig. 4c). The In-Sn excess in the April 4 sample is unique in the Piton de la Fournaise basalts analysed so far and requires abnormal conditions in the shallow magma reservoir just before summit collapse.

Kinetic fractionation of semi-volatile elements: constraints on degassing time-scales

The equilibrium partitioning of elements between melts and fluids clearly fails to explain the systematic of semi-volatile elements in April 2007 lavas. For instance, B is not depleted as Li, although B has higher emanation coefficient [START_REF] Rubin | Degassing of metals and metalloids from erupting seamount and midocean ridge volcanoes: observations and predictions[END_REF]. Similarly, Tl and Re are not enriched as Cd and Bi while less volatile In is. Given the short time-periods involved, kinetic (diffusive) fractionation might control the final concentration of semi-volatile elements in the lavas.

The lithium diffusivity in silicate melts, on the order of 10 -8.1 m 2 /s [START_REF] Richter | Isotope fractionation by chemical diffusion between molten basalt and rhyolite[END_REF], is at least two orders of magnitude higher than that of other semi-volatile elements (Cd, Tl, Re, Bi, As, Pb), in the range of 10 -10.8 -10 -13.6 m 2 /s [START_REF] Mackenzie | Volatile heavy metal mobility in silicate liquids: implications for degassing and eruption prediction[END_REF][START_REF] Johnson | The degassing behavior of Au, Tl, As, Pb, Re, Cd and Bi from silicate liquids: experiments and applications[END_REF]. Amongst the chalcophile elements, Tl diffuses much faster (10 -10.8 m 2 /s) than Bi (10 -13.6 m 2 /s) and Cd (10 -12.6 m 2 /s). Little is known about Cu diffusion, but [START_REF] Johnson | The degassing behavior of Au, Tl, As, Pb, Re, Cd and Bi from silicate liquids: experiments and applications[END_REF] observed the absence of a diffusion profile during degassing experiments.

Fig. 8 compares the diffusion length scale of elements to the half distance between gas bubbles inferred from Bubble Number Density (BND) [START_REF] Toramaru | BND (bubble number density) decompression rate meter for explosive volcanic eruptions[END_REF][START_REF] Bai | Experimental study of bubble growth in Stromboli basalt melts at 1 atm[END_REF]. It is assumed that an element can leave the melt phase significantly only if it diffuses sufficiently fast to reach bubbles before they segregate. This simple statistical approach does not need to define the geometry nor the boundary conditions of the degassing system. To explain the selective degassing of Li and Tl while preserving the elevated concentration in Cd and Bi (what happened between April 2 to 4), Li and Tl must diffuse over distances longer than the half distance between bubbles while Cd and Bi must diffuse over shorter distances. Given a typical BND of 10 12 /m 3 for basalts [START_REF] Bai | Experimental study of bubble growth in Stromboli basalt melts at 1 atm[END_REF], this condition is fulfilled for time scales ranging from a few minutes to a few hours (Fig. 8). Following the same reasoning, the elevated Bi/Cd observed on April 3 (and in many tephras from previous eruptions) can be explained by the preferential loss of Cd over Bi, which occurs when the bubble residence time in the melt ranges from a few hours to two days. These estimated degassing time-scales strikingly coincide with the time offsets of typically 1-2 days observed between the different chemical anomalies, strongly supporting the model of the diffusive fractionation of elements.

6.4. Reconciling Li-Cu depletion and Bi-Cd enrichment with the events that led to summit collapse Two degassing scenarios may explain the early depletion in Li-Cu.

(1) The very short degassing time scales estimated above raise the possibility of magmas degassing during their transport to the eruption site. According to [START_REF] Staudacher | The April 2007 eruption and the Dolomieu crater collapse, two major events at Piton de la Fournaise (La Réunion Island, Indian Ocean)[END_REF], the eruption drained a shallow magma reservoir through a 7 km long, sub-horizontal conduit. This conduit was first modelled as a 7 × 10 6 m 3 dyke (1000 m high and 1 m thick) [START_REF] Staudacher | The April 2007 eruption and the Dolomieu crater collapse, two major events at Piton de la Fournaise (La Réunion Island, Indian Ocean)[END_REF] before interferometric data allowed the identification of a sill of comparable size that emplaced between March 30 and April 2 [START_REF] Froger | Some considerations about the April 2007 eruption at Piton de la Fournaise suggested by InSAR data[END_REF]. This size of the magma conduit roughly corresponds to the volume of lava produced during the first 1-2 days of the eruption (4.5× 10 6 m 3 of lava being produced in 24 hours at a rate of 52 m 3 /s) and, thus, to the volume of lava showing the most depleted Li signature (April 2 and 3), which suggests that fast diffusing elements exsolved during this early, sub-horizontal channelling of magma. (2) Alternatively, depressurisation may have occurred in the shallow magma chamber in response to magma withdrawal to feed a lateral intrusion or the forthcoming eruption. As proposed for recent eruptions of Kilauea volcano, Hawaii [START_REF] Johnson | Dynamics of magma storage in the summit reservoir of Kilauea Volcano, Hawaii[END_REF][START_REF] Poland | Magma degassing triggered by static decompression at Kilauea Volcano, Hawai'i[END_REF], such a static decompression event can trigger the massive exsolution of H 2 Oan dSO 2 . Lithium and Cu could have been lost at this stage. In this case, depressurisation must have occurred between March 30 and April 2.

The following Bi-Cd enrichment requires conditions that prevent magma outgassing. Transient pressurisation of the shallow magma reservoir as the overlying rock column started to collapse is one possibility. However, there is no evidence for pressurisation of the system or the onset of collapse before April 5 [START_REF] Michon | Basaltic calderas: collapse dynamics, edifice deformation, and variations of magma withdrawal[END_REF]. Alternatively, the increasing rate of magma transfer on April 4 [START_REF] Coppola | Lava discharge rate and effusive pattern at Piton de la Fournaise from MODIS data[END_REF] could have prevented the loss of semi-volatile elements such as Bi and Cd. Whatever the exact scenario of early degassing, the short time-scales required to fractionate Li from Cd (minutes to hours) and Cd from Bi diffusively (a few hours to two days) support the idea that the shallow magma reservoir underwent rapid pressure variations a few days before summit collapse.

Conclusion

Lavas erupted during the first few days of the April 2007 eruption show a transient depletion in Li, Cu and Tl (mostly on April 2) and an enrichment in Bi, Cd, In and Sn (on April 3 and 4). Both anomalies are ascribed to unusual degassing conditions that prevailed within the shallow magma plumbing system a few days before the summit collapse. The first anomaly could reflect the degassing of a magma intrusion that emplaced a shallow depth between March 30 and April 2 [START_REF] Froger | Some considerations about the April 2007 eruption at Piton de la Fournaise suggested by InSAR data[END_REF] and subsequently sampled on April 2. Alternatively, it could record the static decompression of the magma reservoir as magmas drained to feed the lateral intrusion or the forthcoming eruption. The second anomaly must result more or less directly from a transient pressurisation of the system, even though such an event was not Comparison between the diffusion length scale of elements and the half distance between gas bubbles. The diffusivity (D) of Li, Tl, Cd, and Bi in silicate melts are 10 -8.1 m 2 /s, 10 -10.8 m 2 /s, 10 -12.6 m 2 /s and 10 -13.6 m 2 /s, respectively [START_REF] Richter | Isotope fractionation by chemical diffusion between molten basalt and rhyolite[END_REF][START_REF] Johnson | The degassing behavior of Au, Tl, As, Pb, Re, Cd and Bi from silicate liquids: experiments and applications[END_REF]. The diffusion length-scale (in metre) is given by (D.t) 0.5 , where t is time (in second). The half distance between gas bubbles is inferred from the Bubble Number Density (BND) [START_REF] Toramaru | BND (bubble number density) decompression rate meter for explosive volcanic eruptions[END_REF]. The BND is typically 10 12 bubbles per cubic metre in basaltic melt [START_REF] Bai | Experimental study of bubble growth in Stromboli basalt melts at 1 atm[END_REF]. With such a BND value, the selective degassing of Li and Tl, while Cd and Bi remain in magmas, occurs for time-scales ranging from minutes to hours (grey field). The selective degassing of Cd while Bi remains in magmas occurs for degassing durations ranging from a few hours to two days (grey field).

recorded by geophysical data [START_REF] Michon | Basaltic calderas: collapse dynamics, edifice deformation, and variations of magma withdrawal[END_REF]. The increasing rate of magma outflow just prior to collapse [START_REF] Coppola | Lava discharge rate and effusive pattern at Piton de la Fournaise from MODIS data[END_REF] could also have prevented the loss of semi-volatile elements such as Cd and Bi. The short, transient anomalies in the semi-volatile elements are consistent with kinetic rather than equilibrium fractionation. By comparing the diffusion length-scale of the semi-volatile elements to the distance between gas bubbles, it is estimated that the selective loss of fast diffusing elements (Li and Tl), while not mobilising slow diffusing elements (Bi and Cd), occurs for time-scales ranging from minutes to hours. Similarly, the selective enrichment of a very slowly diffusing element such as Bi, which occurred on April 3, 2007, as well as in many tephras from recent eruptions, requires degassing time-scales ranging from a few hours to two days. These very short durations are consistent with rapid changes in the shallow magma plumbing system a few days before summit collapse.

Supplementary data to this article can be found online at http:// dx.doi.org/10.1016/j.jvolgeores.2012.12.027.

Fig. 1 .

 1 Fig. 1. Location maps. (a) Map of the Western Indian Ocean showing the location of Réunion Island. The 3000-m depth contour is indicated. (b) Map of Réunion Island showing the location of Piton des Neiges and Piton de la Fournaise. (c) Map of the Piton de la Fournaise volcano showing the lava flows produced during the eruptions of August-December 2006, February 2007, March 2007, and April 2007.

  identified five cycles between the end of2000 and April 2007 (end of 2000-January 2002, November 2002-January 2004, May 2004-February 2005, October 2005-December 2005 and July 2006-April 2007). To explain the change of eruptive regime in 2000, the authors also proposed that magma recharge and the drainage of the shallow reservoir were balanced before 2000. After 2000, the increasing magma flux from the depth possibly led to magma accumulation at shallow depths. Since 2007, the volcanic activity progressively vanished (very small eruptions occurred between 2008 and 2010, and no eruption occurred in 2011 and 2012).

Fig. 3 .

 3 Fig. 3. Li and Cd versus Ni. Li and Cd are both incompatible in olivine, and both decrease with Ni (a proxy for olivine accumulation). These plots show the Li depletion and the Cd enrichment of the early stage lavas of the April 2007 eruption. The slopes of the Li-Ni and Cd-Ni arrays are used to correct whole-rock Li and Cd concentrations for the influence of olivine accumulation (see Figs. 5 and 6).

Fig. 4 .

 4 Fig. 4. Bi-Tl-In-Cd covariations in recent lavas (1977-2010) of Piton de la Fournaise. Data are from Table 1 (August 2006 to April 2007 eruptions) and Supplementary Table B (other eruptions). "H" designates the Hudson lava erupted in March 1998. "P77" designates a pumice (77-14) erupted in 1977.Arrows point toward the composition of high temperature (400 °C) gas sublimates from Piton de la Fournaise[START_REF] Vlastélic | Lithium isotope fractionation during magma degassing: constraints from silicic differentiates and natural gas condensates from Piton de la Fournaise volcano (Réunion Island)[END_REF].

Fig. 5 .

 5 Fig.5. Concentration of Ni, Li and Cu plotted versus time. Different scales of time are used along the "x" axis to emphasise the rapid chemical variations occurring during the very first days of the April 2007 eruption. The transition from steady-state basalts to olivine-rich lavas (also referred to as oceanites), the summit collapse (main event) and the eruption paroxysm (maximum tremor, highest lava fountains and degassing peak) are indicated. The lithium and Cu contents of the olivine-rich samples were corrected for olivine accumulation with 100 ppm Ni as reference: [X] Ni=100 = [X] sample -S X-Ni .([Ni] sample -100), where S X-Ni is the slope of the linear regression line through data points in X versus Ni plot. S X-Ni was inferred using the data given in Table1, removing the anomalous concentrations from the early stage of the April 2007 eruption (April 2 to 4). Slopes of -0.0023 and -0.0593 were calculated for Li and Cu, respectively (see also Fig.3). Distinctive symbols are used for concentrations corrected for olivine accumulation (circle with a dark dot), for the handpicked interstitial glass from sample 0704-20-2 (circle with a cross), and for the tephra collected on April 5 (circle with a star).

Fig. 6 .

 6 Fig.6. Concentration of semi-volatile elements (Bi, Cd, In, Sn) plotted versus time. Bi, Cd, In and Sn contents of olivine-rich samples were corrected for olivine accumulation as described in the caption of Fig.5, with S Bi-Ni = -1.00 × 10 -5 ,S Cd-Ni = -6.67 × 10 -5 ,S In-Ni = -4.32 × 10 -5 and S Sn-Ni = -9.35 × 10 -4 . Distinctive symbols are used for concentrations corrected for olivine accumulation (circle with a dark dot), for the handpicked interstitial glass from sample 0704-20-2 (circle with a cross), and for the tephra collected on April 5 (circle with a star).

Fig. 7 .

 7 Fig. 7. Trace element ratios (La/Sm and Nb/Zr) plotted versus time. Distinctive symbols are used for the handpicked interstitial glass from sample 0704-20-2 (circle with a cross) and for the tephra collected on April 5 (circle with a star).

  Fig. 8. Comparison between the diffusion length scale of elements and the half distance between gas bubbles. The diffusivity (D) of Li, Tl, Cd, and Bi in silicate melts are 10 -8.1 m 2 /s, 10 -10.8 m 2 /s, 10 -12.6 m 2 /s and 10 -13.6 m 2 /s, respectively[START_REF] Richter | Isotope fractionation by chemical diffusion between molten basalt and rhyolite[END_REF][START_REF] Johnson | The degassing behavior of Au, Tl, As, Pb, Re, Cd and Bi from silicate liquids: experiments and applications[END_REF]. The diffusion length-scale (in metre) is given by (D.t) 0.5 , where t is time (in second). The half distance between gas bubbles is inferred from the Bubble Number Density (BND)[START_REF] Toramaru | BND (bubble number density) decompression rate meter for explosive volcanic eruptions[END_REF]. The BND is typically 10 12 bubbles per cubic metre in basaltic melt[START_REF] Bai | Experimental study of bubble growth in Stromboli basalt melts at 1 atm[END_REF]. With such a BND value, the selective degassing of Li and Tl, while Cd and Bi remain in magmas, occurs for time-scales ranging from minutes to hours (grey field). The selective degassing of Cd while Bi remains in magmas occurs for degassing durations ranging from a few hours to two days (grey field).

Table 1

 1 Major-trace element concentrations of Piton de la Fournaise lavas erupted between August 2006 and April 2007.

		Reference standard	August 2006
	Sample name	BHVO-2 (1)	2σ (%) (2)	0608-302	0608-302	0608-311	0609-131	0609-281	0610-111	0610-261	0611-142	0612-111
					dup
	Eruption date			30/08/06		31/08/06	13/09/06	28/09/06	11/10/06	26/10/06	14/11/06	12/12/06
	(d/m/y)
	Sample type			SSB		SSB	SSB	SSB	SSB	SSB	SSB	SSB
						qqqqqqq
	SiO 2			49.97		49.50
	Al 2 O 3			14.23		14.20
	Fe 2 O 3			12.73		11.97
	MgO			7.22		6.84
	CaO			11.89		11.21
	Na 2 O			2.65		2.80
	K 2 O			0.71		0.75
	TiO 2			2.72		2.59
	MnO			0.17		0.16
	P 2 O 5			0.32		0.31
	Li	4.8	5.6	5.86	5.83	5.88	5.83	5.90	5.80	5.83	5.81	5.87
	Be	1	11.3	1.06	1.04	1.10	1.03	1.09	1.07	1.08	1.06	1.08
	B (3)	2.76*	11.5	2.67
	Sc	32	2.7	33.4	33.1	34.2	33.2	33.5	33.3	33.9	33.4	33.2
	Ti	16300	3.7	16026	15888	16256	15930	16076	16068	16269	16028	16033
	V	317	2.2	306	305	314	306	309	306	311	306	308
	Cr	280	6.8	244	245	250	240	242	237	239	239	246
	Co	45	2.3	45.6	44.9	46.0	44.9	45.3	45.2	46.0	45.3	45.8
	Ni	119	6.0	92.2	90.7	90.8	87.9	88.8	85.8	88.2	87.1	88.9
	Cu	127	2.8	109	107	110	108	109	109	111	109	109
	As	0.92*	26.4	0.84	0.79	0.80	0.79	0.75	0.80	0.82	0.84	0.79
	Rb	9.11	4.0	17.4	17.4	17.4	17.4	17.4	17.5	17.4	17.4	17.3
	Sr	396	6.3	354	356	352	354	353	355	354	352	352
	Y	26	7.1	29.3	28.5	28.9	28.8	28.3	28.5	28.6	28.9	29.1
	Zr	172	8.3	193	193	193	193	191	192	193	192	192
	Nb	18.1	4.0	21.9	21.9	22.1	22.0	22.0	22.1	22.1	22.0	21.9
	Mo	3.4*	38.3	0.9	1.2	1.0	1.5	1.2	1.8	1.1	1.0	1.0
	Cd	0.12*	8.6	0.130	0.132	0.138	0.132	0.122	0.133	0.129	0.114	0.121
	In	0.087*	3.9	0.090	0.087	0.085	0.090	0.086	0.085	0.085	0.085	0.086
	Sn	1.69*	4.1	1.70	1.70	1.74	1.74	1.66	1.73	1.71	1.70	1.71
	Sb	0.095*	8.9	0.063	0.059	0.070	0.065	0.061	0.068	0.070	0.071	0.067
	Cs	0.1	6.2	0.267	0.259	0.268	0.262	0.270	0.261	0.265	0.263	0.268
	Ba	131	4.2	135	136	136	136	135	136	135	135	136
	La	15.2	5.6	19.4	18.6	19.4	19.5	19.3	19.5	19.5	19.4	19.4
	Ce	37.5	6.0	44.3	44.2	44.2	44.8	44.2	44.2	44.2	44.3	43.9
	Pr	5.35	5.9	5.91	5.92	5.92	5.94	5.86	5.93	5.91	5.91	5.91
	Nd	24.5	4.8	25.9	25.9	25.9	26.1	26.0	26.0	25.9	25.9	25.9
	Sm	6.07	5.2	6.22	6.22	6.33	6.29	6.20	6.26	6.23	6.27	6.30
	Eu	2.07	5.2	2.09	2.11	2.10	2.09	2.07	2.09	2.10	2.07	2.10
	Tb	0.92	6.1	0.960	0.963	0.973	0.971	0.970	0.970	0.964	0.969	0.978
	Gd	6.24	5.3	6.47	6.50	6.47	6.53	6.40	6.53	6.46	6.47	6.50
	Dy	5.31	5.4	5.63	5.65	5.63	5.67	5.58	5.68	5.63	5.63	5.71
	Ho	0.98	7.3	1.04	1.06	1.06	1.06	1.04	1.06	1.06	1.06	1.06
	Er	2.54	4.8	2.75	2.76	2.77	2.78	2.71	2.77	2.75	2.75	2.77
	Tm	0.33	6.4	0.361	0.358	0.363	0.361	0.360	0.359	0.362	0.359	0.364
	Yb	2	5.5	2.20	2.21	2.23	2.21	2.18	2.23	2.22	2.20	2.22
	Lu	0.274	6.0	0.299	0.306	0.306	0.303	0.308	0.305	0.300	0.302	0.309
	Hf	4.36	4.9	4.69	4.67	4.64	4.72	4.67	4.69	4.67	4.69	4.69
	Ta	1.14	5.1	1.37	1.38	1.38	1.38	1.37	1.37	1.37	1.37	1.38
	W	0.21	3.9	0.288	0.286	0.293	0.296	0.295	0.296	0.291	0.291	0.301
	Re (ppt) (3)	-	-	805
	Tl	0.026*	3.3	0.065	0.066	0.045	0.044	0.040	0.043	0.044	0.042	0.041
	Pb	1.56*	14.8	1.70	1.60	1.71	1.73	1.73	1.70	1.70	1.67	1.70
	Bi	0.018*	10.9	0.020	0.020	0.021	0.022	0.012	0.020	0.019	0.012	0.016
	Th	1.22	5.7	2.21	2.22	2.22	2.24	2.24	2.24	2.23	2.22	2.23
	U	0.403	6.0	0.555	0.551	0.552	0.560	0.565	0.569	0.554	0.559	0.553
	Major element concentrations are in wt.% (XRF data acquired at Laboratoire GéoSciences Réunion).
	Trace element concentrations are in ppm, with the exception of Re (in ppt).

SSB: Steady State Basalt; Oceanite: basalt with cumulative olivine (MgO >20 wt.%). q: quenched.

(1) BHVO-2 concentration data used as reference (from GEOREM data base, at http://georem.mpch-mainz.gwdg.de/). Lead and non-conventional elements (*) were determined in this study. See text for more details.

  1, except for one sample erupted on April 6, 2007, which has excess Cl. In contrast, H 2 O/Cl varies widely between 2.6 and 16. The April 2007 eruption displays amongst the lowest H 2 O/Cl ratios. Trace element data are reported in Table 1 (2006 and 2007 eruptions) and Supplementary Table B (1977 to 2010 eruptions). The boron data for the 1977 to 2010 eruptions are reported in

  Variations of Cl, F and H 2 O concentrations in recent lavas (1977-2007) of Piton de la Fournaise. Data are from Supplementary Table A. "P77" designates a pumice (77-14) erupted in 1977. The lava from April 6, 2007, has excess Cl.

		0.06	April 2007 lavas 1977 -2006 lavas			P77	H
	F (%)	0.04				April 6, 2007
		0.02	F /C l= 2 .1		Cl excess
		0.00
		0.30						H
	2 O (%) H	0.10 0.20	H 2 O/Cl				P77
			1 4
			9
			3
		0.00	05 0	100	150	200	250	300
					Cl (ppm)
	Fig. 2.
								Hudson lava with a clear

Table 1 (

 1 August 2006 to April 2007 eruptions) and Supplementary Table B (other eruptions). "H" designates the Hudson lava erupted in March 1998. "P77" designates a pumice (77-14) erupted in 1977.

Acknowledgements

This paper benefited from the constructive comments of A. Kent and an anonymous reviewer. Thanks to M.J. Rutherford for handling this manuscript. The SARM (CRPG Nancy) is thanked for carrying out S, Cl, F and H 2 O analyses. This work benefited from the financial support from the CNRS (Institut National des Sciences de l'Univers) and the Agence Nationale de la Recherche (DEGAZMAG project, contract no. ANR 2011 Blanc SIMI 5-6 003). This study is a Laboratory of excellence ClerVolc contribution no 50.


Jonathan De Clermont-Gallerande

Shunsuke Saito

Maggy Colas

Philippe Thomas

Tomokatsu Hayakawa

New understanding of TeO 2 -ZnO-Na 2 O ternary glass system

Thermal, mechanical structural and optical properties of the 80TeO 2 -xZnO-(20-x)Na 2 O (TZN) system were investigated. The structural study of the glasses showed that in the TZN system, the zinc had a coordination and hence a behavior change in the tellurite matrix as a function of the concentration. Those results were correlated to the elastic properties of the glasses, like the Young modulus, the shear modulus and the Poisson ratio, thermal properties with the index of thermal stability and the crystallization behavior and the optical properties of refractive index which presented nonlinear evolutions as a function of the ZnO concentration. An in-depth study of the Boson peak is proposed with a study on the correlation length, here named as the blob size, performed from its evolution and the evolution of the mechanical properties.

Introduction

Tellurite glasses have interested not only material researchers but also device developers in the field of photonics for many years. Indeed, they are good candidates for photonic applications thanks to their high linear refractive index, high dielectric constant, wide infrared transmittance, and their high third order non-linear optical susceptibilities [START_REF] El-Mallawany | Tellurite glasses handbook: physical properties and data[END_REF][START_REF] Mirgorodsky | Ab initio study of the nonlinear optical susceptibility of TeO 2 -based glasses[END_REF][START_REF] Wang | Tellurite glass: a new candidate for fiber devices[END_REF][START_REF] Kim | Linear and Nonlinear Optical Properties of TeO 2 Glass[END_REF].

Ab-initio studies showed that the pure TeO 2 glass has the highest non-linear optical properties among tellurite glasses [START_REF] Soulis | Local molecular orbitals and hyper-susceptibility of TeO 2 glass[END_REF] (fifty times more than fused silica glasses [START_REF] Kim | Linear and Nonlinear Optical Properties of TeO 2 Glass[END_REF][START_REF] Duclère | Third order nonlinear optical properties of a paratellurite single crystal[END_REF]), but extreme quenching conditions are needed to obtain the glass [START_REF] Sarjeant | New Glassy and Polymorphic Oxide Phases Using Rapid Quenching Techniques[END_REF]. Consequently, modifier oxides are required to be added. A variety of multicomponent tellurite glass systems have been studied in their abilities to form glasses (glassy domains) and their structural, linear and non-linear optical properties for many years [START_REF] Udovic | Thermal characteristics, Raman spectra and structural properties of new tellurite glasses within the Bi 2 O 3 -TiO 2 -TeO 2 system[END_REF][START_REF] Udovic | Formation domain and characterization of new glasses within the Tl 2 O-TiO 2 -TeO 2 system[END_REF][START_REF] Linda | New glasses within the Tl 2 O-Ag 2 O-TeO 2 system: Thermal characteristics, Raman spectra and structural properties[END_REF][START_REF] Noguera | Dynamics and crystal chemistry of tellurites. II. Composition-and temperaturedependence of the Raman spectra of x(Tl 2 O)+(1-x)Te 2 O glasses: evidence for a phase separation?[END_REF][START_REF] Dutreilh-Colas | The TeO 2 -rich part of the TeO 2 -Ga 2 O 3 system: equilibrium and non-equilibrium phase diagram[END_REF][START_REF] Blanchandin | New heavy metal oxide glasses: investigations within the TeO 2 -Nb 2 O 5 -Bi2O3 system[END_REF][START_REF] Jeansannetas | Glass Structure and Optical Nonlinearities in Thallium(I) Tellurium(IV) Oxide Glasses[END_REF][START_REF] Soulis | The role of modifier's cation valence in structural properties of TeO 2 -based glasses[END_REF][START_REF] Zaki | Synthesis, thermal, structural and linear optical properties of new glasses within the TeO 2 -TiO 2 -WO 3 system[END_REF]. Among them, adding Na 2 O is well known to increase the glass forming ability resulting in homogeneous glasses with only slight modifications of TeO 2 glass structure [START_REF] Udovic | Thermal characteristics, Raman spectra and structural properties of new tellurite glasses within the Bi 2 O 3 -TiO 2 -TeO 2 system[END_REF][START_REF] Soulis | The role of modifier's cation valence in structural properties of TeO 2 -based glasses[END_REF][START_REF] Zaki | Synthesis, thermal, structural and linear optical properties of new glasses within the TeO 2 -TiO 2 -WO 3 system[END_REF].

Adding ZnO in tellurite glasses significantly increases the glass forming domain (up to 40%mol ZnO) [START_REF] Ahmmad S Kareem | Physical and optical properties of zinc arsenic tellurite glasses[END_REF]. Binary TeO 2 -ZnO glasses are useful media for ultra-low loss optical fibers (1 dB/km) in the wavelength range of 3.4-4 µm [START_REF] Van Uitert | ZnCl 2 glass: A potential ultralow-loss optical fiber material[END_REF]. Different studies have been done on TeO 2 -ZnO glasses based on Raman spectroscopic data [START_REF] Sekiya | Raman spectra of MO-TeO 2 (M = Mg, Sr, Ba and Zn) glasses[END_REF][START_REF] Bürger | Glass formation, properties and structure of glasses in the TeO 2 -ZnO system[END_REF][START_REF] Duverger | Raman spectroscopic investigations of the effect of the doping metal on the structure of binary tellurium-oxide glasses[END_REF][START_REF] Hoppe | Structure of zinc and niobium tellurite glasses by neutron and x-ray diffraction[END_REF][START_REF] Kozhukharov | Atomic arrangement of a zinc-tellurite glass[END_REF][START_REF] De Clermont-Gallerande | Correlation between mechanical and structural properties as a function of temperature within the TeO 2 -TiO 2 -ZnO ternary system[END_REF], and X-ray absorption fine structure (XAFS) [START_REF] Thorbahn | Compositional dependence of the stress-optic response in zinc tellurite glasses[END_REF]. The studies showed that adding ZnO to the original TeO 2 glass matrix results in (i) a structural transformation of trigonal bipyramids TeO 4 into trigonal pyramid TeO 3 units and so a linear decrease of the optical properties; (ii) a band gap decrease as the glasses turn more transparent upon adding ZnO [START_REF] Ghribi | Thermal, optical and structural properties of glasses within the TeO 2 TiO 2 ZnO system[END_REF]; (iii) a change in the Zn-O coordination (6-coordinated for low concentration of ZnO and 4-coordinated for high concentrations of ZnO) and structural behaviors (between the tellurite chains when 6-coordinated and substituting the TeO 2 inside the chains when 4-coordinated) [START_REF] De Clermont-Gallerande | Correlation between mechanical and structural properties as a function of temperature within the TeO 2 -TiO 2 -ZnO ternary system[END_REF].

The goal of the present study is to highlight new discoveries in the 80TeO 2 -xZnO-(20x)Na 2 O (x = 0, 5, 10, 15, 20%mol) (TZN) ternary system in accordance to thermal, mechanical, structural and optical properties. In this paper, the short and medium-range structure of TZN glasses is reported using Raman spectroscopy, mechanical properties by ultrasound echography [START_REF]Standard Test Method for Dynamic Young's Modulus, Shear Modulus and Poisson's Ratio by Impulse Excitation of Vibration[END_REF], thermal properties by differential thermal analysis (DTA) experiment and optical properties by UV(ultraviolet)-visible-infrared spectroscopy and refractometer. The TZN glasses were also doped with 0.5%wt Nd 3+ ions to study photoluminescent properties of the glasses, which will be reported elsewhere. As the doping was small, the structural data given in this paper for the TZN glasses with different ZnO concentration are not affected by the Nd 3+ ions [START_REF] De Clermont-Gallerande | Optical properties of Nd 3+ -doped TeO 2 -TiO 2 -ZnO glasses with lower hydroxyl content[END_REF]. Low-frequency Raman spectroscopy is applied to gain structural information of medium range of orders for the TZN glasses with constant TeO 2 content 80%mol and varying ZnO content from 0 to 20%mol, which shed light on how the network evolution of tellurite glasses is influenced by ZnO (covalent) and Na 2 O (ionic) entities.

Experimental

Sample preparation

The relevant amount (2g) of the different initial powders (α-TeO 2 obtained by the decomposition of telluric acid (Aldrich): anhydrous Na 2 CO 3 (Asahi Glass Co.): ZnO (Kishida Chem. Co.)) within the ternary system 80TeO 2 -xZnO-(20-x)Na 2 O, with x= 0; 5; 10; 15 and 20%mol (respectively named TZ0N20, TZ05N15, TZ10N10, TZ15N05 and TZ20N0) were ground in a mortar for 30 minutes and then melted in a platinum crucible at 850°C for one hour before being quenched on a 8mm diameter brass mold preheated at ~100°C. The obtained glasses were optically polished with a 2mm thickness after they were annealed at 30°C below the glass transition temperature (Tg) for 12 hours in order to release stresses resulting from the rapid quenching of melt. The final concentrations of the samples have been controlled by energy dispersive X-ray spectroscopic (EDS; JEOL, JSL-6010LA) experiments which evidence a deviation less than 2% from the theoretical composition. The obtained glasses were ground only for thermal analysis and Raman spectroscopy measurements.

Thermal properties were measured by DTA experiments with a RIGAKU TGD9600 in a Pt pan at a 10°C/min rate in a 100~700°C temperature range. Density measurements were done by the Archimedes method. Raman spectra were recorded in the 1.7-950cm -1 spectral range, using a JASCO NRS-2000 spectrometer at 532 nm excitation at ~30 mW. The mechanical properties have been obtained by ultrasound echography (Olympus, 5072PR). UV-Vis-IR measurements were recorded on JASCO V-570 spectrometer. The refractive index was taken by a refractometer (Five-lab, Mary-102).

Results and discussion

Thermal properties and density measurements

The DTA curves are presented in Figure 1 and the relevant information (Glass transition temperature Tg, on-set crystallization temperature Tx, first crystallization peak Tc and the index of thermal stability ΔT (=Tx-Tg)) are reported on Table 1. The density data were also added to the Table 1 for indication. Figure 1 shows that with an increase of the zinc oxide concentration from 0 to 5%mol, there is a sharp decrease of the crystallization peak intensity. For further increase of the zinc oxide concentration, two different crystallization peaks appear with temperature increases. Those peaks are more defined with an increase of zinc oxide concentration. With an increase of zinc oxide content, it can also be seen in Table 1 that there is an increase of the glass transition temperature and a sharp decrease of the thermal stability for concentrations higher than 5%mol of zinc oxide.

The density increases linearly with an increase of the zinc oxide concentration except 10%mol of zinc oxide. This evolution is almost in line with the Tg evolution as a function of the concentration. The molar volume is given from the experimental value of density and the formal composition, which is tabulated in Table 1.

Raman spectroscopy

All Raman spectra were normalized (total area normalization) and baseline corrected (linear correction).

The normalized spectra were then decomposed using the Focus program [START_REF] De Sousa Menses | FOCUS version 1.0, software utility for the creation of optical function[END_REF] based on previous studies [START_REF] Linda | New glasses within the Tl 2 O-Ag 2 O-TeO 2 system: Thermal characteristics, Raman spectra and structural properties[END_REF][START_REF] Ghribi | Thermal, optical and structural properties of glasses within the TeO 2 TiO 2 ZnO system[END_REF][START_REF] Ghribi | Structural, mechanical and optical investigations in the TeO 2 -rich part of the TeO 2 -GeO 2 -ZnO ternary glass system[END_REF], with 10 oscillators: 9 Gaussian functions and 1 log-normal function. Due to the width and overlapping of the bands of the normalized Raman spectra (Figure 2) all Gaussian functions had full width at half maximum (FWHM) and frequency fixed, and only the intensity was left free. As the log-normal function is well defined, all fitting parameters were left totally free (frequency, FWHM and intensity).

The TZN glass Raman spectra's frequencies range can be divided in two different parts: the low wavenumber region with the Boson peak (BP) and the high wavenumber region (300-800 cm -1 ) with the deformation of bridges (asymmetric and symmetric Te-O-Te) and the elongation vibrations of Te-O bonds for TeO 3 and TeO 4 polyhedra (Figure 2). 3 shows the structural evolution of the high wavenumber bands as a function of the concentration. When looking at the bands associated with the Te-O-Te bonds elongation (Fig. 3a) there is an overall increase of the intensity of the band associated to the symmetric elongation of Te-O-Te bridges (band E) and a decrease of the intensity of the band associated to the asymmetric elongation Te-O-Te bridges (band F). When looking at the bands associated with the TeO 3 and TeO 4 elongation (Fig. 3b), it can be seen that for concentrations lower than 10%mol ZnO there is a decrease of the intensity non-bridging elongation of Te-O (NBO) bonds in TeO 3 and TeO 4 polyhedra (band I and J) and for higher concentration an increase of the intensity of the same bands. There is also a linear decrease of the nonsymmetric elongation of Te-O bonds in TeO 4 polyhedra (band G) and between 0 and 5%mol ZnO a slight increase of the intensity of the band associated to the symmetric elongation of Te-O bonds in TeO 4 and TeO 3 polyhedra (band H) and a decrease of its intensity for higher concentrations.

1,0E-3 1,1E-3 1,2E-3 1,3E-3 1,4E-3 1,5E-3
G band (595 cm -1 ) H band (654 cm -1 ) I band (723 cm -1 ) J band (784 cm -1 )

Intensity

ZnO concentration (% mol) b)

Figure 3 : Intensity evolution of a) E and F bands and b) G, H, I and J bands as a function of the ZnO concentration

The complete analysis of the evolution of the high wavenumber region become more relevant by looking at the TeO 3 /TeO 4 ratio, which should give an idea on the depolymerization of the tellurite network. This ratio is calculated from the following equation:

(I I +I H )/(I J +I H +I G ), I being the intensity of the corresponding bands. Its evolution with an increase amount of ZnO is shown in Figure 4.

The TeO 3 /TeO 4 ratio is overall increasing with an increase of the ZnO content, but between 5 and 15%mol ZnO the ratio is almost constant. This means that between these concentrations, there is a change in structural behavior in this range of concentrations. By looking at the evolution of the different bands inside the high wavenumber region and comparing it to the literatures, it is possible to understand the present evolution of intensity.

 For concentrations lower than 5%mol ZnO there is an increase of TeO 3 because of the introduction of ZnO (band F more intense than band E, slight increase of band H, strong increase of the TeO 3 /TeO 4 ratio). This means that the ZnO is behaving like a modifier. It can be explained by the low content of ZnO.  For concentration between 5 and 10%mol ZnO, a slight decrease of the TeO 3 /TeO 4 ratio and a decrease of the I and J band intensity is noted. By comparing these results to the literatures [START_REF] Sekiya | Raman spectra of MO-TeO 2 (M = Mg, Sr, Ba and Zn) glasses[END_REF][START_REF] Bürger | Glass formation, properties and structure of glasses in the TeO 2 -ZnO system[END_REF][START_REF] Duverger | Raman spectroscopic investigations of the effect of the doping metal on the structure of binary tellurium-oxide glasses[END_REF][START_REF] Kozhukharov | Atomic arrangement of a zinc-tellurite glass[END_REF][START_REF] De Clermont-Gallerande | Correlation between mechanical and structural properties as a function of temperature within the TeO 2 -TiO 2 -ZnO ternary system[END_REF][START_REF] Thorbahn | Compositional dependence of the stress-optic response in zinc tellurite glasses[END_REF][START_REF] Ghribi | Thermal, optical and structural properties of glasses within the TeO 2 TiO 2 ZnO system[END_REF], it is possible to hypothesize that for concentrations lower than 10%mol ZnO the zinc oxide will be octahedra formed and between the chains, linking to the network, decreasing the number of NBO, and slightly increasing the network polymerization.  For concentration higher than 10%mol ZnO, there is an increase of the TeO 3 /TeO 4 ratio and an increase of the intensity of the I and J bands. This shows a change in the structural behavior, with a change of the zinc coordination, going toward tetrahedra formed, and substituting the tellurium oxide inside the chains [START_REF] De Clermont-Gallerande | Correlation between mechanical and structural properties as a function of temperature within the TeO 2 -TiO 2 -ZnO ternary system[END_REF]. With higher the concentration of ZnO, there will be more tetrahedra formed ZnO, increasing the TeO 3 /TeO 4 ratio and the number of NBO. For better understanding of the structural changes, it is interesting to look at the low wavenumber region and more specifically at the Boson peak (BP).

Study of the low wavenumber region

Non-crystalline materials (glasses and amorphous solids) have notable differences compared to crystalline solids in vibratory dynamics around 1THz (33,3 cm -1 ) [START_REF] Phillips | Amorphous Solids: Low-Temperature Properties[END_REF]. Those low energy excitation are used in different cases and among them it is used for explaining the presence of dispersed intensities, which are not present inside parent crystals on Raman spectra and incoherent neutron diffusion (INS) [START_REF] Phillips | Amorphous Solids: Low-Temperature Properties[END_REF][START_REF] Nakayama | Boson peak and terahertz frequency dynamics of vitreous silica[END_REF][START_REF] Pohl | Low-temperature thermal conductivity and acoustic attenuation in amorphous solids[END_REF].

This point has been researched for a number of years theoretically or experimentally [START_REF] Schirmacher | Harmonic Vibrational Excitations in Disordered Solids and the "Boson Peak[END_REF][START_REF] Taraskin | Origin of the Boson Peak in Systems with Lattice Disorder[END_REF][START_REF] Grigera | Phonon interpretation of the 'boson peak' in supercooled liquids[END_REF][START_REF] Parisi | On the origin of the boson peak[END_REF][START_REF] Lubchenko | The origin of the boson peak and thermal conductivity plateau in low-temperature glasses[END_REF][START_REF] Gurevich | Anharmonicity, vibrational instability, and the Boson peak in glasses[END_REF][START_REF] Gurevich | Pressure dependence of the boson peak in glasses[END_REF][START_REF] Pilla | The low energy excess of vibrational states in v-SiO 2 : the role of transverse dynamics[END_REF][START_REF] Gurarie | Phonons in Random Elastic Media and the Boson Peak[END_REF][START_REF] Johari | Librational heat capacity of fullerenes in the Einstein model[END_REF][START_REF] Johari | Molecular inertial effects in liquids: Poley absorption, collision-induced absorption, low-frequency Raman spectrum and Boson peaks[END_REF][START_REF] Power | Relaxation strength of localized motions in D-sorbitol and mimicry of glass-softening thermodynamics[END_REF][START_REF] Chumakov | Collective Nature of the Boson Peak and Universal Transboson Dynamics of Glasses[END_REF][START_REF] Yannopoulos | Low-energy excitations in noncrystalline arsenic trioxide[END_REF][START_REF] Greaves | Identifying Vibrations That Destabilize Crystals and Characterize the Glassy State[END_REF]. Unfortunately, after more than four decades of research, the origin of this peak, named 'Boson peak', because the vibrational population of the energetic levels of this peak was dependent on the temperature following the Bose-Einstein statistical prediction, is still not satisfactorily explained.

Because it is difficult to establish one unique theory valid for all glasses, empiric correlations were established from their properties. Indeed, one spectral characteristic of the Boson peak (intensity, position …) is correlated to a specific property of the glass. But even these empiric correlations are dispersed. The Boson peak is associated with different models: (i) The dispersion of acoustic phonons in a disordered medium by polarizability ( 49) ; (ii) The fluctuation of the intrinsic densities ( 50) ; (iii) The vibrations of small structured clusters inside the glass ( 51) ; (iv) The vibrations of localized clusters inside the glass showing elastic fluctuations [START_REF] Pang | Local vibrational states of glasses[END_REF].

Some authors [START_REF] Elliott | A Unified Model for the Low-Energy Vibrational Behaviour of Amorphous Solids[END_REF][START_REF] Malinovsky | The nature of boson peak in Raman scattering in glasses[END_REF][START_REF] Takahashi | Anomalous Boson Behavior and Nanometric Heterogeneity in Glassy Fresnoite[END_REF] have connected the Boson peak to the medium distance order inside the glass. The following expression has been proposed [START_REF] Elliott | A Unified Model for the Low-Energy Vibrational Behaviour of Amorphous Solids[END_REF][START_REF] Duval | Vibrational dynamics and the structure of glasses[END_REF], linking the frequency of the Boson peak ω BP to the correlation size ξ of the structural clusters (thereafter named blobs) :

(1) where V av is the average sound velocity inside the glass and is calculated by the following equation with the longitudinal velocity (V L ) and transversal velocity (V T ).

(2)

The evolution of the Boson peak intensity is also linked to the heterogeneity of the blobs inside the glass [START_REF] Duval | Inelastic light, neutron, and X-ray scattering related to the heterogeneous elasticity of glasses[END_REF][START_REF] Champagnon | Boson peak, structural inhomogeneity, light scattering and transparency of silicate glasses[END_REF].

This makes the study of the Boson peak interesting to understand the evolution of the structure at intermediate distance. At the same time, a comparison of the evolution of the Boson peak and the mechanical properties makes this study even more important. Figure 5a represents the evolution of the Boson peak (BP) intensity and Figure 5b the evolution of the BP frequency as a function of the ZnO concentration.

In Figure 5a, a decrease of the intensity with an increase of the ZnO concentration is observed until 5%mol ZnO. Between 5%mol and 15%mol ZnO, an increase of this intensity is noted and finally, a sharp decrease of the intensity is noted for further increase of the ZnO concentration. In Figure 5b, the evolution of the frequency shown as a function of the zinc oxide concentration is stranger. There is a first decrease of the frequency between 0 and 5%mol ZnO and then an increase of the frequency until 15%mol ZnO and a decrease for further concentrations.

The evolution of the intensity of the BP (Figure 5a) could be explained by the change in structural behavior, which will lead to a change in ZnO coordination.

As for the evolution of the Boson peak frequency, in our previous study [START_REF] De Clermont-Gallerande | Correlation between mechanical and structural properties as a function of temperature within the TeO 2 -TiO 2 -ZnO ternary system[END_REF] it was shown that it is more correct to look at the evolution of the blob size than the evolution of the BP frequency alone, which will be done in the discussion part.

Mechanical approach

The mechanical properties were studied by ultrasound echography. The relevant information (Young modulus (E), shear modulus (G), Poisson ratio ()) as well as the average sound velocities (Eq.( 2)) are calculated from the experimentally obtained longitudinal and transversal velocities (respectively (V L and V T )) using (3) (4) [START_REF] Soulis | Local molecular orbitals and hyper-susceptibility of TeO 2 glass[END_REF] and are reported in Table 3 and Figure 6 Sample

E (GPa) G (GPa) Poisson ratio  V L (m.s -1 ) V T (m.s -1 ) V av (m.s -1 ) TZ0N20

Table 3 : Young modulus (E), shear modulus (G), Poisson ratio (), longitudinal and transversal velocities (respectively (V L and V T ) values for each sample

The evolution of E and G (Figure 6a and 6b) shows an overall increase with an increase of the zinc oxide content. There is, though, a slope break after 10%mol ZnO. Looking at the Poisson ratio (Figure 6c), which is inversely proportional to the connectivity of the network [START_REF] Greaves | Poisson's ratio and modern materials[END_REF], an addition of ZnO will overall increase the connectivity of the network. But it is also evident that there is a slope break at 10%mol ZnO, showing once more a different structural behaviour. These results are well in order with the evolution of the structural properties showed previously, showing a change in behaviour at the same concentration in zinc oxide content.

Optical properties

Transmission spectra were taken on 2mm thick optically polished pellets. Figure 7 shows the evolution of the transmission of the samples as a function of the wavelength for each sample. TZ0N20 glass showed lower transmission, which is due to macroscopic inhomogeneity of the glass and 2mm thickness. The in-line transmission could be lowered by small refraction of incident light inside the glass.

This evolution is actually easier to understand by knowing the fact that the refractive index of TZ0N20 was measured at different point of the sample and the obtained results showed a larger standard deviation (± 0.04) compared to the other glasses (Fig. 8).

The transmission of the other glasses was almost constant, not dependent on the zinc oxide concentration. In the visible to infrared (IR) region, several sharp absorption lines owing to Nd 3+ ions doped were detected, which were assigned to the optical transitions from 4 I 9/2 level to 1) 4 I 13/2 , 2) 4 I 15/2 , 3) 4 F 3/2 , 4) 4 F 5/2 + 2 H 9/2 , 5) 4 F 7/2 + 4 S 3/2 , 6) 4 F 9/2 , 7) 4 G 5/2 + 2 G 7/2 and 8) 2 K 13/2 + 4 G 7/2 + 4 G 9/2 levels. (28) A large IR absorption band was attributed to OH vibrations. Refractive indexes of the samples were taken on the same optical flat pellets, with an incident light at 800nm. Table 1 and Figure 8 show the evolution of their values as a function of the sample. The evolution shows that there are two different average values of the refractive index depending on the ZnO concentration. Between 0 and 5%mol ZnO there is a 1.97 average value and for 10%mol ZnO or more there is a 2.02 average value of the refractive index. The latter is preferable to enhancement of third-order nonlinear optical susceptibilities. [START_REF] Kim | Linear and Nonlinear Optical Properties of TeO 2 Glass[END_REF][START_REF] Duclère | Third order nonlinear optical properties of a paratellurite single crystal[END_REF] These observations resultantly show that the change in the structural behavior seems to also have an impact on the refractive index value. Now, looking at the refractive index value of the TZ15N05 glass, it is lower than that of the TZ10N10 and TZ20N0 glass, even if the density of the TZ15N05 glass is the same as the TZ10N10 glass (Table 1), which is not a normal behavior. This evolution could then be interpreted by the increased number of microscopic heterogeneities due to different Zn coordination numbers highlighted in the BP intensity evolution.

Discussion

As explained before, Duval et al. [START_REF] Duval | Vibrational dynamics and the structure of glasses[END_REF] and Elliott et al. [START_REF] Elliott | A Unified Model for the Low-Energy Vibrational Behaviour of Amorphous Solids[END_REF] independently found a relationship that links the size of density or elastic fluctuations domains ξ (here, called blob size) with the position of the Boson peak frequency ω BP and V av . As a function of the concentration (Fig. 9), an increase in blob size can be noted between 0 and 5%mol ZnO. The blob size then decreases until 15%mol ZnO and increase for further concentrations. An increase in blob size can be associated with an increase of a number of the network linkages due to the different bonding natures of the elements (Te, Zn, and Na) with oxygens and/or the length of the average covalent bonds (Te-O, Zn-O), and vice versa.

It can be seen here that an addition of 5%mol or lower content in ZnO will increase the average number of the networking bonds. This can be explained by the difference between the natures of Na-O and Zn-O bonds. Indeed, Na 2 O have ionic bonds in nature, and will not link to the tellurite network. On the contrary, ZnO have covalent bonding characters and will link to the tellurite network, which will increase the average number of the covalent bonds inside the glass.

An increase of 5 to 15%mol ZnO, however, decrease blob size. This can be explained by the fact that ZnO components in six-fold coordination with oxygen, ZnO 6 like ones found in Zn 2 Te 3 O 8 crystal (ZnO 6 (Zn-O: 2.0~2.5 Å) and TeO 4 (Te-O:~1.9 Å with equatorial positioned oxygens and ~2.1 Å with axial bonding oxygens)), are between the tellurite chains and linking them together, which possibly induces a compaction of the glassy system by decreasing contents of the cations by substituting Na 2 O with ZnO; the total cation concentration C(x) is varied as 120-x with ZnO concentration x in 80TeO 2 -xZnO-(20-x)Na 2 O. A theoretical density linearly increases with ZnO concentration, which was experimentally confirmed as shown in Table 1, since the atomic number of Zn (Z=30) is higher than that of Na (Z=11), and thus molar volume should decrease because of the decreasing cation concentration in the glass formula, meaning that the correlation length of the network structures could be shortened with ZnO concentration. It can be noted in Table 1 that the estimated molar volume had an overall tendency of the decrease with ZnO concentration. As seen in Table 3, the ZnO concentration dependence of the mechanical properties also showed the monotonic increase. However, looking in detail, the molar volume exhibits an anomaly at 10-15%mol (See Table 1 again) and the mechanical properties appear on the different lines in the border of 10%mol ZnO (Fig. 6), as mentioned in the previous session. This is a clue to understand the structural evolution of the TZN glasses.

Further increase in the ZnO content once again increases the size of the blob, which will be associated with an increase in the length of the average bonds of the tellurite glass composed by Te-O and Zn-O as a main framework. By comparing the evolution to the literatures, it might mean that when the ZnO is 4-coordinated and substituting the TeO 2 inside the glass, the difference between the length of the Zn-O and Te-O bond will explain this increase of blob size (Zn-O bond length being longer than Te-O bond length). For example, in ZnTeO 3 , Zn-O is the bonding length of 1.9~2.3 Å in ZnO 4+1 unit, while Te-O has ~1.9 Å of trigonal pyramids TeO 3 .

By comparing the evolution of the blob size to the structural properties, there should be an increase of blob size between 10 and 15%mol ZnO, which is not the case here. This can be explained by the strong increase of the inhomogeneities between the blobs highlighted with the evolution of the BP intensity and the refractive index values. The thermal properties' evolution showed how different the TZ05N15 sample is compared to the other samples, with the lowest intensity of crystallization peak. This goes well with the idea of the fact that really low amount of ZnO content will have a modifier influence. It was also shown that the thermal stability was decreased with further introduction of ZnO content, due to the apparition of a new crystallization peak. This evolution seems to go in favor of a new structural behavior.

The structural evolution as a function of the concentration indicates also a slight influence on the optical properties of the glasses. It has been shown that an addition of more than 10%mol ZnO will change the refractive index value of the glasses.

Conclusion

Correlation between structural properties of glasses to different kind of macroscopic physic one has been one of the most important field of research on glasses for years. In this study, some interesting facts have been pointed out.

By comparing mechanical experiments and Raman spectroscopy, it has been possible to understand the structural properties of a glass having a complex structure with two entities that have a coordination number which change as a function of the Na 2 O and ZnO content. (i) As usual Raman spectroscopy gave information on the change of the Te-O polyedra inside the glass, (ii) mechanical experiment gave an understanding on the strength of the different bonds as a function of the concentration, and (iii) from both Raman and mechanical experiments by calculating the blobs size, the structural evolution of the glass at intermediate-distance has been evidenced. This paper showed that for low concentration of ZnO (≤5%mol in this paper), there was not enough zinc oxide content to see the influence of the ZnO as a network former inside the glass, but with an increase of the blob size and TeO 3 /TeO 4 ratio, and a decrease of Boson peak intensity. This paper also showed that by increasing the zinc oxide content to 10%mol ZnO, there would be an increase the tellurium polymerization, increase the mechanical properties, decrease of the NBO content and increase the connectivity of the network, resulting in higher refractive index. This indicated that the zinc oxide, which had a 6 coordination, would behave like inside the Zn 2 Te 3 O 8 crystalline phase, linking the tellurite chains together and moreover compacting the glassy system, which were evidenced by the decreasing molar volume. A change in the structural behavior was noted for higher ZnO concentration, associated with an increase of the number of NBO in the tellurium network, an increase of the TeO 3 /TeO 4 ratio and a slower increase of the mechanical properties with an increase of ZnO content. This change indicated that the zinc oxide would change its behavior, going toward a 4 coordination, and acting like in the ZnTeO 3 crystalline phase by substituting the tellurium inside the chains.

Acknowledgement

This work was performed in JSPS KAKENHI Grant Number JP19F19798. The authors of JdeCG and TH gratefully acknowledge the financial support of the Japan Society for the Promotion of Science (JSPS). The absorption lines of Nd 3+ ions in the visible to infrared region were assigned to the optical transitions from 4 I 9/2 level to 1) 4 I 13/2 , 2) 4 I 15/2 , 3) 4 F 3/2 , 4) 4 F 5/2 + 2 H 9/2 , 5) 4 F 7/2 + 4 S 3/2 , 6) 4 F 9/2 , 7) 4 G 5/2 + 2 G 7/2 and 8) 2 K 13/2 + 4 G 7/2 + 4 G 9/2 levels.(28) A large IR absorption band was attributed to OH vibrations.

Figure 1 :

 1 Figure 1 : DTA evolution for every TZN samples. The arrow shows the respective glass transition temperature.

Figure 2 :

 2 Figure 2 : Raman bands attribution of a typical TZN sample (Example of the TZ15N05 sample)

Figure 4 :

 4 Figure 4 : Evolution of the TeO 3 /TeO 4 ratio as a function of the ZnO concentration

Figure 5 :

 5 Figure 5: a) BP intensity evolution; b) ω BP evolution as a function of ZnO content.

Figure 6 :

 6 Figure 6 : Evolution of a) the Young modulus (E); b) the shear modulus (G); c) the Poisson ratio () as a function of the ZnO concentration

Figure 8 :

 8 Figure 8 : Evolution of the refractive index as a function of ZnO concentration.

Figure 9 :

 9 Figure 9 : Evolution of the size of the blobs as a function of the concentration in ZnO

Figure 1 :

 1 Figure 1 : DTA evolution for every TZN samples. The arrow shows the respective glass transition temperature.

Figure 2 :Figure 3 :Figure 4 :

 234 Figure 2 : Raman bands attribution of a typical TZN sample (Example of the TZ15N05 sample)

Figure 5 :

 5 Figure 5 : a) BP intensity evolution; b) ω BP evolution as a function of ZnO content.

Figure 6 :

 6 Figure 6 : Evolution of a) the Young modulus (E); b) the shear modulus (G); c) the Poisson ratio () as a function of the ZnO concentration

Figure 7 :

 7 Figure 7 : Transmission spectra of each sample in the wavelength range of 200 to 3000nm.The absorption lines of Nd 3+ ions in the visible to infrared region were assigned to the optical transitions from 4 I 9/2 level to 1) 4 I 13/2 , 2) 4 I 15/2 , 3) 4 F 3/2 , 4) 4 F 5/2 + 2 H 9/2 , 5) 4 F 7/2 + 4 S 3/2 , 6) 4 F 9/2 , 7) 4 G 5/2 + 2 G 7/2 and 8) 2 K 13/2 + 4 G 7/2 + 4 G 9/2 levels.(28) A large IR absorption band was attributed to OH vibrations.

Figure 8 :

 8 Figure 8 : Evolution of the refractive index as a function of ZnO concentration.

Figure 9 :

 9 Figure 9 : Evolution of the size of the blobs as a function of the concentration in ZnO

Table 1 : Tg, Tx, Tc and ΔT ( 1 °C) as well as density, molar volume and refractive index of TZN glasses

 1

	Sample	Tg (°C)	Tx (°C) Tc (°C)	ΔT (°C)	Density, d (g/cm 3 )	Molar Volume / cm 3 mol -1	Refractive index, n
	TZ0N20	250	363	390	113	4.89 0.01	2867 4	1.976 0.037
	TZ5N15	264	345	374	81	5.03 0.01	2801 8	1.952 0.007
	TZ10N10	299	359	381	67	5.35 0.01	2664 3	2.030 0.006
	TZ15N5	298	361	390	63	5.35 0.01	2671 4	2.013 0.002
	TZ20N0	312	371	394	59	5.53 0.05	2602 23	2.033 0.002

Table 2 :

 2 Frequencies and mode attribution of the used oscillators for the decomposition of the TZN Raman spectra.

NBO: Non-bridging oxygen 2.2.1. Study of the high wavenumber region Figure

  Figure 7 : Transmission spectra of each sample in the wavelength range of 200 to 3000nm. The absorption lines of Nd3+ ions in the visible to infrared region were assigned to the optical transitions from4 I 9/2 level to 1)4 I 13/2 , 2) 4 I 15/2 , 3) 4 F 3/2 , 4) 4 F 5/2 + 2 H 9/2 , 5) 4 F 7/2 + 4 S 3/2 , 6) 4 F 9/2 , 7) 4 G 5/2 + 2 G 7/2 and 8) 2 K 13/2 + 4 G 7/2 + 4 G 9/2 levels.(28) A large IR absorption band was attributed to OH vibrations.