Morgane Rondet

Caroline Martel

Jean-Louis Bourdier

The intermediate step in fractionation trends of mildly alkaline volcanic suites: an experimental insight from the Pavin trachyandesite (Massif Central, France)

Keywords: trachyandesite, Pavin, experimental petrology, differentiation

We examined magma storage conditions and eruptive dynamics for the trachyandesite (~58 wt% SiO2, 9-10 wt% alkalis) of the Pavin monogenetic volcano, a maar-like explosive crater belonging to a small group of youngest volcanoes in the Massif Central. By confronting the natural samples to experimental products, we constrained pre-eruptive conditions around 950-975 °C, 150-200 MPa (~5.5-7.0 km in depth), NNO+1.5, and 4.5-5.5 wt% melt H2O. There is petrological evidence of magma crystallization in the conduit up to shallow levels (~50 MPa; 2 km deep) before fragmentation into pumice clasts in the last km of ascent. The experiments highlight the role of biotite and crystallization pressure in defining separate compositional trends of residual liquids, i.e. alkaline (trachytes) versus sub-alkaline (dacite-rhyolite).

Introduction

The Tertiary to Quaternary volcanic province of the Massif Central (France) is part of an intraplate alkaline volcanism, the Cenozoic European Volcanic Province (CEVP) that spreads from Spain to Poland and is spatially associated with a large-scale rift system surrounding the Alps orogen. Rifting and volcanism globally formed due to local extension and mantle upwelling in response to the Alpine orogeny (Lustrino and Wilson, 2007, and references therein). In the Massif Central, discrete volcanic fields are recognized based on spatiotemporal criteria, ranging in age mostly from the Middle Miocene to the Holocene. Most of those fields are uniquely basaltic, while some erupted magmas of intermediate compositions, i.e. trachyandesites s.l. and/or phonolites, along with subordinate volumes of trachytes (Velay, Cantal, Chaîne des Puys) and even rhyolites (Monts Dore) (e.g. [START_REF] Brousse | Minéralogie et pétrologie des roches volcaniques du Massif du Mont Dore[END_REF][START_REF] Downes | Tertiary and Quaternary volcanism in the Massif Central, France[END_REF].

Comparable alkaline suites with differentiated terms have been documented in other segments of the CEVP, notably the Upper Rhine Valley (e.g. [START_REF] Schreiber | Mixing and chemical interdiffusion of trachytic and latitic magma in a subvolcanic complex of the Tertiary Westerwald (Germany)[END_REF][START_REF] Wörner | Trace element abundances and mineral/melt distribution coefficients in phonolites from Laacher See Volcano (Germany)[END_REF], and the Bohemian Massif [START_REF] Ulrych | Cenozoic intraplate volcanic rock series of the Bohemian Massif: a review[END_REF][START_REF] Ackerman | Geochemical characteristics and petrogenesis of phonolites and trachytic rocks from the České Středohoří Volcanic Complex, the Ohře Rift, Bohemian Massif[END_REF]. The differentiated terms of the CEVP suites have been largely interpreted as resulting from polybaric differentiation of variously silica under-saturated primary mafic magmas by assimilation-fractional crystallization (AFC) processes ending in the upper crust. Such a case has been made for the Massif Central magmatic suites: Cantal [START_REF] Downes | Sr and Nd isotope geochemistry of coexisting alkaline magma series, Cantal, France[END_REF][START_REF] Wilson | Contrasting fractionation trends in coexisting continental alkaline magmas series; Cantal, Massif Central, France[END_REF], Monts Dore [START_REF] Briot | Geochemical evolution in crustal magma reservoirs: Trace-element and Sr-Nd-O isotopic variations in two continental intraplate series at Monts Dore, Massif Central, France[END_REF], and Velay and Chaîne des Puys [START_REF] Villemant | Etude géochimique des éléments en traces dans les séries volcaniques du Massif Central[END_REF][START_REF] Maury | Cristallisation fractionnée d'un magma basaltique alcalin: la série de la Chaîne des Puys (Massif Central, France)[END_REF][START_REF] Villemant | Distribution coefficients of major and trace elements, fractional crystallization in the alkali basalts of Chaine des Puys (Massif Central, France)[END_REF]. Crystal fractionation trends in those suites have been defined mainly from the mineral assemblages of the various terms of the suites, and on mass-balance calculations using major and trace elements. However, constraints by phase-equilibrium experiments on the storage conditions and fractionation paths of such mildly alkaline, intermediate and silicic magmas remain currently scarce. A previous experimental study [START_REF] Martel | Phase relations in trachytes and implication for magma storage conditions in the Chaîne des Puys (French Massif Central)[END_REF] investigated the storage conditions of trachytic magmas from the youngest volcanic field in the Massif Central, i.e. the partly Holocene Chaîne des Puys. In the present paper, we go back to an intermediate step in the differentiation trend by examining phaseequilibrium conditions for a representative trachyandesite (~58 wt% SiO2) taken from the neighbouring Holocene Pavin volcano. Pavin is a maar-like explosive crater belonging to a small group of youngest volcanoes in the Massif Central, i.e. slightly younger than the Chaîne des Puys volcanoes, and located only about 40 km South of them in a similar tectonic setting.

The aim of this study is (1) to constrain the storage conditions of the Pavin trachyandesite, erupted from a likely small-volume reservoir ; (2) to investigate the conditions that can generate trachytic liquids (such as those of the Chaîne des Puys) from a trachyandesitic parent by crystal fractionation.

Geological setting and sampling

The Pavin volcano is a roughly circular explosive crater 900-1000 m in diameter, filled by a crater lake about 750 m in diameter and 92 m maximum depth. The crater rim crosscuts lava flows from an adjacent scoria cone, the Montchal volcano. Four kilometers South, stands another morphologically well-preserved scoria cone, the Montcineyre volcano. At least one neighbouring maar, Estivadoux, is demonstrated by typical phreatomagmatic basaltic deposits exposed in its surroundings, though it is currently buried by Montchal scoria flows and proximal Pavin deposits. Therefore, Pavin belongs to a small group of at least four recent monogenetic volcanoes (the Pavin group herein; Fig. 1), whose tephrostratigraphical and field relationships unambiguously define the relative eruption chronology as Montcineyre-Estivadoux-Montchal-Pavin, from the oldest to the youngest [START_REF] Bourdier | Contribution à l'étude volcanologique de deux secteurs d'intérêt géothermique dans le Mont Dore : le group du Pavin et le Massif du Sancy[END_REF]. The lack of paleosoil and the erosive contact between tephra layers from those volcanoes strongly suggest eruption within a short period of time. The most recent radiometric assessment gives an age of 4720 170 B.C. (current age of 6730 170) for the Pavin eruption, and a time elapse of 100 to 700 years between the oldest (Montcineyre) and youngest (Pavin) eruptions [START_REF] Juvigné | Distribution, tephrostratigraphy and chronostratigraphy of the widespread eruptive products of Pavin volcano[END_REF]. So far, no volcanic eruption in the Massif Central has been dated younger than the Pavin one, nor any unequivocal tephra has been recovered above the Pavin one, so that Pavin currently stands as the youngest volcano in continental France [START_REF] Juvigné | Distribution, tephrostratigraphy and chronostratigraphy of the widespread eruptive products of Pavin volcano[END_REF].

Pavin is located only about 40 km South of the southernmost volcano of the Chaîne des Puys, a monogenetic field of about 80 volcanoes, mostly scoria cones, forming a North-Southtrending chain about 30 km long, West of the city of Clermont-Ferrand. The Chaîne des Puys volcanoes range in age from less than 100 000 to about 6000-7000 years. The Pavin volcanic group is thus close in age to of the youngest volcanoes of the Chaîne des Puys. Both monogenetic fields occur in the same tectonic setting, in which eruptive vents location is mainly controlled by regional faults dating from the Variscan orogeny and rejuvenated during the post-Alpine rifting episode. In those respects, the Pavin group could be viewed as a southern extension of the larger Chaîne des Puys volcanic field [START_REF] Boivin | Volcanologie de la Chaîne des Puys[END_REF].

The Pavin proximal pyroclastic deposits [START_REF] Camus | Mise au point sur le dynamisme et la chronologie des volcans holocènes de la région de Besse-en-Chandesse (Massif Central français)[END_REF][START_REF] Bourdier | Contribution à l'étude volcanologique de deux secteurs d'intérêt géothermique dans le Mont Dore : le group du Pavin et le Massif du Sancy[END_REF][START_REF] Boivin | Le point sur l'éruption du lac Pavin il y a 7 000 ans[END_REF][START_REF] Leyrit | Characterization of phreatomagmatic deposits from the eruption of the Pavin maar (France)[END_REF] have juvenile components consisting of dominant macroscopically homogeneous, yellowish-weathering pumices, and subordinate greyish, denser clasts and texturally heterogeneous pumices [START_REF] Arbaret | Assessing the role of phreatomagmatism in the eruption dynamics of the trachyandesitic maar-like Pavin crater (Massif central, France): constraints from the pyroclasts microtextures[END_REF]. All textural types are distinctively amphibole-phyric and display similar mineralogy and bulk-rock composition [START_REF] Bourdier | Contribution à l'étude volcanologique de deux secteurs d'intérêt géothermique dans le Mont Dore : le group du Pavin et le Massif du Sancy[END_REF][START_REF] Villemant | Magmatic evolution of Pavin's group of volcanoes: petrology, geochemistry and modeling of differentiation processes[END_REF]. The juvenile magma is a mildly alkaline (peralkalinity index, P.I. = 0.6-0.8) trachyandesite (or benmoreite) (~58 wt% SiO2; Table 1, Fig. 2). In the Pavin group, the Montcineyre and Estivadoux magmas are nepheline-normative alkaline basalts to basanites and the Montchal one is a trachybasalt (or hawaiite). Thus, the Pavin magma is the most differentiated [START_REF] Bourdier | Contribution à l'étude volcanologique de deux secteurs d'intérêt géothermique dans le Mont Dore : le group du Pavin et le Massif du Sancy[END_REF]. Previous petrological and geochemical studies have linked the magma compositions of the Pavin group as a differentiation trend dominated by crystal fractionation [START_REF] Bourdier | Contribution à l'étude volcanologique de deux secteurs d'intérêt géothermique dans le Mont Dore : le group du Pavin et le Massif du Sancy[END_REF][START_REF] Villemant | Magmatic evolution of Pavin's group of volcanoes: petrology, geochemistry and modeling of differentiation processes[END_REF]. In this respect, the Pavin magmatic suite might be compared to the Chaîne des Puys series, this latter being compositionally more extensive since it shows silica-oversaturated trachytes. The Chaîne des Puys has been consistently modelled as resulting dominantly from crystal fractionation processes in crustal reservoirs [START_REF] Maury | Cristallisation fractionnée d'un magma basaltique alcalin: la série de la Chaîne des Puys (Massif Central, France)[END_REF][START_REF] Villemant | Distribution coefficients of major and trace elements, fractional crystallization in the alkali basalts of Chaine des Puys (Massif Central, France)[END_REF], in agreement with petrological and experimental constraints suggesting storage of the Chaîne des Puys trachytes at depths around 10 km [START_REF] Martel | Phase relations in trachytes and implication for magma storage conditions in the Chaîne des Puys (French Massif Central)[END_REF]. Recent geochemical modelling of the Pavin suite [START_REF] Villemant | Magmatic evolution of Pavin's group of volcanoes: petrology, geochemistry and modeling of differentiation processes[END_REF] has stressed the petrogenetic similarities between the Chaîne des Puys and Pavin suites, while pointing to some differences in the fractionation paths. The remoteness of the Pavin volcano from the differentiated volcanoes of the Chaîne des Puys and the small magma volumes involved (of the order of 30-40 Mm 3 DRE for the Pavin magma) imply that the Pavin upper reservoir should be discrete and likely of small volume, so that some differences with the Chaîne des Puys differentiation processes are to be anticipated.

Results and discussion

Petrology of the Pavin trachyandesite

3.1.1. Lithology and whole-rock composition. The Pavin trachyandesite exhibits various textures, from dominantly pumice to denser and darker clasts that differ by vesicularity and groundmass microtexture, while their phenocryst assemblage and whole-rock composition do not show significant differences within the limits of our investigations. Major-element wholerock compositions are given in Table 1 where the Pavin magma composition is compared to a closely analogous composition (Nugère trachyandesite) and a slightly more evolved one (Clierzou trachyte), both from the Chaîne des Puys volcanic field. All rocks show overall similarities, being metaluminous (P.I. in the 0.6-0.9 range) and close to silica saturation in the CIPW norm. The differentiation trend in the Chaîne des Puys has been thoroughly documented and is compositionally marked by a consistent decrease of FeO, MgO, CaO and increase of alkalis with increasing SiO2 [START_REF] Camus | La Chaîne des Puys (Massif Central Français): Etude structurale et volcanologique[END_REF][START_REF] Maury | Cristallisation fractionnée d'un magma basaltique alcalin: la série de la Chaîne des Puys (Massif Central, France)[END_REF][START_REF] Martel | Phase relations in trachytes and implication for magma storage conditions in the Chaîne des Puys (French Massif Central)[END_REF]. The Nugère (Chaîne des Puys) and Pavin trachyandesites have close contents for most major elements, though the Pavin composition is consistently richer in TiO2 and K2O whereas poorer in P2O5. In the Chaîne des Puys differentiation trend, the Na2O/K2O ratio does not vary from the Nugère to the Clierzou compositions (Table 1) then it tends to decrease with increasing SiO2 in the trachyte range, through an increase of K2O at almost constant Na2O (Martel et al., 2013 and references therein). The compositional differences between the Pavin and Nugère magmas, notably the K2O-richer character of the Pavin magma, are sufficient to indicate that the Pavin magma belongs to a specific differentiation trend, i.e. distinct from the Chaîne des Puys one, as also evidenced by comparing trace-element compositions [START_REF] Villemant | Magmatic evolution of Pavin's group of volcanoes: petrology, geochemistry and modeling of differentiation processes[END_REF].

3.1.2. Phenocryst assemblage and modal proportion. The phenocryst assemblage of the Pavin magma mostly consists of plagioclase, amphibole, clinopyroxene and Fe-Ti oxides (Appendix B). Phenocrysts are found both isolated and as small glomeroporphyritic agglomerates. Apatite is a common accessory phase. Scarce biotite can be found as inclusions in amphibole and isolated microphenocrysts. Modal proportions range from 10 to 23% (recalculated to 7 to 20 wt% crystallinity; Table 2). Such a variability of the phenocryst proportions could result from heterogeneous distribution at sample scale (e.g. due to the tendency of phenocrysts to agglomerate) or might be acquired through crystallisation along the course of the eruption. No attempt has been made in this study to evaluate the variability of magma crystallinity through the Pavin deposit stratigraphy.

To some extent, the phenocryst assemblage of the Pavin trachyandesite can be compared qualitatively to counterparts from the Chaîne des Puys. Although compositionally close to the Pavin sample, the Nugère trachyandesite is almost aphyric. Other trachyandesites in the Chaîne des Puys, although scarce and volumetrically trivial, typically contain as Fe-Mg phenocrysts both amphibole (more or less stable) and stable clinopyroxene, and lack biotite [START_REF] Maury | Cristallisation fractionnée d'un magma basaltique alcalin: la série de la Chaîne des Puys (Massif Central, France)[END_REF]. Further, the couple clinopyroxene-amphibole is still present in the less differentiated trachytes, Aumone and Clierzou, which also lack biotite [START_REF] Boivin | Building and ornamental use of trachyte in the center of France during antiquity: Sources and criteria of identification[END_REF]. In contrast, biotite is a systematic phase in the more differentiated trachytes, either as the single Fe-Mg phase or accompanied by either amphibole or clinopyroxene, not both [START_REF] Martel | Phase relations in trachytes and implication for magma storage conditions in the Chaîne des Puys (French Massif Central)[END_REF][START_REF] Boivin | Building and ornamental use of trachyte in the center of France during antiquity: Sources and criteria of identification[END_REF]. The Pavin phenocryst assemblage thus compares well with the Chaîne des Puys magmas at comparable levels of differentiation. The rare occurrence of biotite in the Pavin magma, at lower SiO2 than needed for biotite appearance in the Chaîne des Puys series, might reflect the higher amount of K2O in the Pavin magma compared to the Chaîne des Puys ones at similar differentiation degree.

Mineralogy.

The natural phases were analysed following the method given in Appendix A2 and the compositions are given in Appendix C.

Plagioclase, although frequently fractured, is euhedral with clear rims, and variously zoned with frequent cloudy to sieved core textures. The plagioclase composition spreads from 32 to consistently increasing as An decreases, (Fig. 3a). Plagioclase rims have a restricted compositional range of An35-38 while most of the cores have compositions of An45-62, with the exception of one profile showing a reverse zoning with core composition down to An32.

From core to rim, the plagioclases show either smooth An decrease or drastic drops (Fig. 3b).

In the Chaîne des Puys for comparison, plagioclases in the trachyandesites are reported as being mostly in the range of andesine (An30-50; [START_REF] Maury | Cristallisation fractionnée d'un magma basaltique alcalin: la série de la Chaîne des Puys (Massif Central, France)[END_REF]. The plagioclases of the less differentiated Chaîne des Puys trachytes are also andesine, in the range An30-42 with a mean composition of An38 [START_REF] Martel | Phase relations in trachytes and implication for magma storage conditions in the Chaîne des Puys (French Massif Central)[END_REF][START_REF] Boivin | Building and ornamental use of trachyte in the center of France during antiquity: Sources and criteria of identification[END_REF].

Amphibole occurs as 1-2 mm long, euhedral and unresorbed phenocrysts with common inclusions of oxides, apatite and biotite (Appendix B2). The amphibole are kaersutites and hastingsites after [START_REF] Leake | Nomenclature of amphiboles: Report of the subcommittee on amphiboles of the International Mineralogical Association, Commission on New Minerals and Mineral Names[END_REF], showing a rather scattered range in contents of SiO2 (38.5-40.7 wt%), Al2O3 (11.1-13.9 wt%), Al IV (1.9-2.2), TiO2 (4.1-5.3 wt%), and Mg# (0.61-0.66) (Appendix C2). Amphibole-rich cumulates are common in the Pavin deposit, notably in the upper part of the deposit stratigraphy. The amphibole from the cumulates is mostly kaersutite that plots separately from the amphiboles analysed in the juvenile pyroclasts (Fig. 4). This is good evidence that the latter form a truly phenocrystic population, without any contribution of xenocrysts of cumulative origin. Within the phenocrystic amphibole population, core and rim compositions do not show systematic variations and are close to each other in the limits of our data set. Thus, the compositional scattering of the amphibole phenocrysts is not related to any significant zoning and likely reflects variations in the crystallization conditions (liquid composition and/or intensive parameters) within the magma reservoir. Amphiboles from the low-silica Clierzou trachyte are also compositionally scattered, but commonly show slightly lower contents in A-site cation number, Al IV content, and Mg# (Fig. 4).

The clinopyroxenes in the juvenile pyroclasts are 1-2 mm long, have smoothed more or less resorbed rims, and contain frequent glass inclusions. They are salites with 44-48 mol% wollastonite (Wo44-48) and 34-45 mol% enstatite (En34-45) (Fig. 5a), with notable scattering in SiO2 and Al2O3 contents (Fig. 5b; Appendix C3). By comparison, clinopyroxenes from cumulates are mostly less calcic (Wo41-47) and more magnesian (En39-46), and are on average poorer in SiO2 and richer in Al2O3 and TiO2 (Fig. 5b; Appendix C3). The compositions of the clinopyroxenes from the cumulates are even more scattered than those of the trachyandesite, suggesting large variations in the crystallization conditions. Compared to the Pavin pumice, clinopyroxenes from the Clierzou trachyte are mostly less calcic (Wo41-47) and more magnesian (En39-46; Fig. 5a), and are on average poorer in SiO2 and richer in Al2O3 (Fig. 5b).

Biotite is scarce and contains about 6 wt% TiO2, 14.5 wt% MgO and 13-14 wt% Al2O3, less than 0.4 wt% MnO and F, with Mg# ~0.65 (Fig. 6; Appendix C4). Comparatively, the Clierzou trachyte does not contain biotite. Biotites are only present in the most differentiated trachytes (>65 SiO2 wt%) in which they are poorer in magnesian (e.g. Mg# ~0.55; Fig. 6) and slightly richer in alumina (14-15 wt% Al2O3 in the Vasset trachyte; [START_REF] Martel | Phase relations in trachytes and implication for magma storage conditions in the Chaîne des Puys (French Massif Central)[END_REF] than the Pavin biotites.

Fe-Ti oxides are both magnetite-ulvöspinel and ilmenite-hematite solid solutions (Appendix C5). The isolated phenocrysts often show strong textures of exsolution, whereas Fe-Ti oxides found as mineral inclusions in mostly clinopyroxene and amphibole are texturally and compositionally homogeneous. Fe-Ti oxides in amphibole and clinopyroxene do not show systematic compositional differences. Apparently homogeneous isolated phenocrysts show compositions similar to those found as inclusions, i.e. titanomagnetite with 67-78 mol% magnetite (Mt67-78; with 0.8-1.3 wt% MnO, 79-83 wt% FeO, and 7.5-11.5 wt% TiO2) and ilmenite with 63-68 mol% ilmenite (Ilm63-68; with 0.8-1 wt% MnO, 55-57 wt% FeO, and 36-38 wt% TiO2). For comparison, the magnetites and ilmenites from the Clierzou trachytes are Mt-and Ilm-richer, respectively (Mt80-90 and Ilm70-90;[START_REF] Martel | Phase relations in trachytes and implication for magma storage conditions in the Chaîne des Puys (French Massif Central)[END_REF].

Glass inclusions are frequently trapped in plagioclases and clinopyoxenes, but are mostly devitrified in the former whereas mostly glassy in the latter, so that all analysed glass inclusions were clinopyroxene-hosted, with one in a magnetite. The compositions are trachytic, showing narrow ranges of 64-66 wt% SiO2 and 11.5-12.5 wt% alkalis (Appendix C6). The microlitepoor matrix glasses of the Pavin pumices have compositions similar to those of the glass inclusions (Fig. 7). The Pavin glasses overall broadly compare with the bulk-rock compositions of the intermediate-silica trachytes of the Chaîne des Puys (Kilian, Vasset, Sarcouy, Puy de Dôme), except for lower Na2O and higher K2O contents (both by ~1 wt%; Fig. 7). The K2Oricher character of the Pavin glasses conforms to the K2O-richer content of the Pavin wholerock trachyandesite compared to Chaîne des Puys analogues.

The H2O contents of the Pavin glasses are inferred from a calibrated by-difference method (see Appendix A2), which assumes that volatiles other than H2O can be neglected. Fluorine contents are below 0.5 wt% in the Pavin glass inclusions (Appendix C6), in agreement with the fluorine, chlorine, and sulphur contents below 0.5 wt% for the glass inclusions of the Chaîne des Puys trachytes [START_REF] Martel | Phase relations in trachytes and implication for magma storage conditions in the Chaîne des Puys (French Massif Central)[END_REF]. We thus assume that H2O is by far the most abundant volatile species and that the inferred values cannot be overestimated by more than 0.5 wt%. The range of H2O content is large, with the matrix glasses showing low contents (< 1.5 wt%) and the glass inclusions ranging between 1.1 and 5.3 wt% H2O (Fig. 8; Appendix C6).

Crystallization conditions retrieved from the natural samples

3.2.1. Phase assemblage and compositions relevant to the reservoir conditions. The natural pumice pyroclasts contain phenocrysts of plagioclase, amphibole, clinopyroxene, magnetite, ilmenite, and minor apatite. The scarcity of biotite, only found as inclusions in amphibole or rare isolated microphenocrysts, likely reflects inherited crystals.

We selected pumice clasts from an explosive event during which magma was supposed to ascend rapidly from the storage region to the surface. This has the advantage to minimize syneruptive crystallization, although the samples do contain some microlites that evidence crystallization after reservoir exit. If such syn-eruptive crystallization is negligible, the preeruptive phase compositions are provided by the phenocryst rims in equilibrium with the interstitial glass (or alternatively glass inclusions if they were not compositionally modified after entrapment). The plagioclase phenocrysts show rim compositions of An35-40 (Fig. 3b) that turned out to be impossible to reproduce under realistic storage conditions, as evidenced experimentally below. Rather, the plateau range of An46-52 present in most of the plagioclase phenocrysts seems to represent the pre-eruptive compositions, whereas compositions <An40 may reflect crystallization stages at shallower levels. Contrary to plagioclase, the other phenocrysts do not show significant compositional zonings, possibly reflecting their slower crystallization kinetics or lower compositional sensitivity to (lower) pressure and melt H2O content. The crystallization of plagioclase rims and microlites after reservoir exit lead to residual liquids more differentiated (silica-richer) than those in equilibrium in the reservoir. In contrast, melt trapped as inclusions by phenocrysts growing in the reservoir may be more representative of the pre-eruptive melt composition. Yet, microlite-bearing residual glasses and glass inclusions globally cover the same compositional range (Fig. 7), suggesting a similar compositional evolution. In particular, the Fe-Mg partition coefficients between glass inclusion and host clinopyroxene give a mean value of 0.11+0.03, whereas reaching 0.25+0.04 for the experimental melt-clinopyroxene pairs (in agreement with the 0.28+0.09 given by [START_REF] Putirka | Thermometers and barometers for volcanic systems[END_REF], which likely suggests some compositional modification of the melt inclusions after entrapment.

Therefore, the experiments should reproduce 7-20 wt% phenocrysts consisting of An49+3 plagioclase, amphibole with 12+1 wt% Al2O3 (Am12+1), Wo46+2 clinopyroxene, Mt72+5 magnetite, Ilm65+2 ilmenite, and apatite in a glass with less than 65+1 wt% SiO2 and up to 5.5 wt% H2O as shown by the glass inclusions (keeping in mind though that they were mostly modified after entrapment).

Intensive parameters inferred from the natural assemblage. Pairs of coexisting Mt and

Ilm, either trapped in phenocrysts (clinopyroxene or amphibole) or as individual phenocrysts, can be used to constrain the temperature (T) and oxygen fugacity (fO2) prevailing at the time of their crystallization, provided they co-crystallized from the same parental melt. Pairs that were positively tested for chemical equilibrium (after [START_REF] Bacon | Mg/Mn partitioning as a test for equilibrium between coexisting Fe-Ti oxides[END_REF] were used to calculate T-fO2 values following the formulation of Sauerzapft et al. (2008). Since this formulation is not calibrated originally for trachyandesitic to trachytic compositions, we tested it against the experimental oxides that crystallized under controlled T and fO2. The results give T estimates up to 40 °C and fO2 up to 1.3 log units, but mostly agree within 0.3 log units with the experimental conditions (Appendix D1), so that we validated the method for the Pavin compositions. The calculations on the natural Mt-Ilm pairs give T between 850 and 970 °C and fO2 in the range of NNO+1.3 to +1.9.

No thermodynamic model is adequately calibrated to estimate total pressure (P) for trachyandesite compositions and phase assemblages such as the Pavin ones. Nevertheless, the formulations of [START_REF] Ridolfi | Calcic amphiboles in calc-alkaline and alkaline magmas: thermobarometric and chemometric empirical equations valid up to 1,130° C and 2.2 GPa[END_REF], based on amphibole composition, were tested.

Compared to the run P, the calculations give significant and systematic P overestimations by over 100 MPa (e.g. 105 to 304+50 MPa; Appendix D2). Such calculations also overestimate T by 41 to 146 °C, whereas underestimating fO2 by about 1 log unit (Appendix D2). Therefore, we cannot consider the formulations of [START_REF] Ridolfi | Calcic amphiboles in calc-alkaline and alkaline magmas: thermobarometric and chemometric empirical equations valid up to 1,130° C and 2.2 GPa[END_REF] relevant for amphiboles crystallizing from the Pavin trachyandesite.

The pre-eruptive H2O content may be inferred from H2O dissolved in glass inclusions, assuming volatile tightness after reservoir exit. Although the large variability in H2O contents (1.1-5.3 wt%; Fig. 8) likely suggests that not all glass inclusions remained sealed after reservoir exit, the H2O-richer ones suggest entrapment pressures of ~120 MPa [START_REF] Di Matteo | Water solubility in trachytic melts[END_REF] minimum value if not H2O saturated).

Experimentally-determined storage conditions of the Pavin trachyandesite

In order to better constrain the storage conditions, phase-equilibrium experiments starting from the fused Pavin trachyandesite were performed following the method given in Appendix A1. The experimental conditions were chosen to cover the range of intensive parameters inferred from the natural assemblage. Therefore, crystallization experiments were performed from 850 to 950 °C, 150 to 400 MPa, NNO to NNO+2, and XH2Oin ranging from 0.7 to 1 (i.e. melt H2O contents from 4.5 to 9.5 wt%; Table 3). Below, we compare the natural phase assemblage, proportion, and compositions to those obtained experimentally (Fig. 9, with phase relationships and compositions described in Appendix E).

At ~400 MPa and NNO+1.5, stable plagioclase, amphibole, and clinopyroxene require H2O contents around 8 wt% and T close to 900 °C (Fig. 9c). Yet, the crystallinities of the run products at these conditions are much higher (~70 wt%) than those of the Pavin samples (11-20 wt%), biotite is ubiquitous although not considered as stable in the reservoir, and the experimental glass compositions are systematically off the trend defined by the Pavin wholerock and natural glasses in CaO, Al2O3, and alkali contents (Fig. 7). Varying T from 850 to 1000 °C would not help reconciling these discrepancies, since biotite would still be present, amphibole and clinopyroxene would not coexist, and/or the modal proportions would be too low (Fig. 9). Therefore, 400 MPa is not a realistic P for storing the Pavin magma.

Similarly, at ~300 MPa and NNO+1, plagioclase, amphibole, and clinopyroxene are stable at T-H2O conditions for which crystallinities are > 40 wt%, biotite is present (Fig. 9b), and the residual glasses hardly agree with the trend defined by the Pavin whole-rock and natural glasses (Fig. 7). Therefore, it rules out 300 MPa as a possible storage P for the Pavin reservoir.

At 150-200 MPa, ~NNO+1, and 950 to 975 °C, H2O saturation conditions (7.0 wt% H2O) do not allow clinopyroxene crystallization. Even if slightly more oxidized conditions could compensate for this, the plagioclase An content would be significantly higher than An52 (Fig. 9a). Similarly, for lower H2O contents of ~6 wt%, the plagioclase An content is between 51 and 57 wt% (Fig. 9b). For H2O contents between 4.5 and 5.5 wt%, however, ~An50 crystallizes, together with Wo44-47 and Am11-12 (Fig. 9c). the Fe-Ti oxide compositions, Mt60-61 and Ilm84-87, likely suggest too low fO2 conditions compared to the natural Fe-Ti oxides (Mt72+5 and Ilm65+2). In view of the Mt84 that crystallized together with hematite at NNO+2, it is reasonable to predict that a fO2 intermediate between NNO+1 and +2 would allow Fe-Ti oxide compositions close to the natural ones. This is in agreement with the natural Mt-Ilm pairs suggesting NNO+1.3 to +1.9 for T between 850 and 970 °C. The 15-22 wt% crystallinity at 975 °C for H2O contents of 4.5-5.5 wt% is compatible with the natural crystallinities (7-20 wt%), but the residual liquid contains ~62 wt% SiO2, i.e. significantly less than for the natural glasses (64-66 wt% SiO2). Vice versa, experimental liquids with 64 wt% SiO2 result from crystallinities much higher than expected from the natural samples (e.g. 76 wt% crystals, including 36 wt% plagioclase, at 950 °C, ~5.5 wt% H2O, and NNO+2). Yet, the natural residual liquids crystallized and differentiated during ascent in the conduit, thus not excluding liquids with 62 wt% SiO2 as possible pre-eruption compositions.

In summary, the storage conditions for the Pavin trachyandesite could be close to 150-200 MPa (~5.5-7.0 km in depth), 950-975 °C, NNO+1.5, and melt H2O content about 4.5-5.5 wt% (Fig. 10). Noteworthy, these P-T conditions are outside, but close to the stability field of biotite, and inside, but close to clinopyroxene destabilization, which may in both cases accounts for their presence but low abundance in the natural samples.

Reservoir and eruption dynamics

The Pavin juvenile products provide petrological information on (1) the reservoir dynamics prior to eruption, and (2) the eruptive conditions upon ascent of the magma towards surface.

Reservoir dynamics.

The juvenile pumice shows a great variability in many petrological features as shown above, notably phenocryst proportions, amphibole and clinopyroxene compositional scatter, and plagioclase zonings. Such heterogeneities likely points to variability in the differentiation process, in terms of liquid-crystal ratio and intensive parameters. This is further demonstrated by the inability of any of our phase-equilibrium experiments to fulfill the observed natural assemblage and phase compositions. Actually, the inferred intensive parameters can only be bracketed, the somewhat limited range of parameters fixed above representing the conditions for the mean assemblage and compositions. Indeed, the delimited P-T field for the Pavin storage conditions is tightly bordered by the clinopyroxene, ilmenite, and biotite appearance curves, so that slight P-T-fO2 variations may drastically modify the phase assemblage and the liquid-crystal ratio (e.g. decreasing T to 950 °C leads to biotite crystallization and total crystallinities of ~70 wt%; Fig. 9c).

The Pavin volcano is an isolated monogenetic crater of trachyandesitic composition, suggesting it is fed by a short-lived reservoir of likely small volume. Small polycrystalline cumulates and free cumulative crystals are abundant in the uppermost part ash fraction of the pyroclastic sequence, suggesting that the eruption tapped lower cumulative parts of the reservoir. This highlights that the reservoir was likely emptied and thus of small volume. We argue that the observed variability in phase compositions is well explainable if considering a small volume reservoir where magma evolves under strong gradients of physical parameters (notably temperature that in turn controls phase assemblage and magma crystallinity). In this respect, the Pavin case contrasts with many documented large volume silicic eruptions, notably ignimbritic ones, where phase assemblage and compositions are more homogeneous, likely resulting from the buffering effect of large magma volume on the controlling physico-chemical parameters.

Conduit dynamics.

The incompatibility between the plagioclase rim compositions of An32-40 (Fig. 3b) and the experimental compositions in the 150-400 pressure range (Fig. 9) argues for plagioclase crystallization upper in the conduit. Some of the plagioclase rims show a gradual An decrease, whereas others show a drastic drop (Fig. 3b), suggesting that the magma may have reached this intermediate storage level following different ascent rates. Similarly, some clinopyroxene-hosted melt inclusions preserved the pre-eruptive melt H2O contents of ~4.5-5.5 wt%, whereas most of them are H2O poorer (Fig. 8), likely reflecting imperfect tightness and connection with the residual liquid during magma ascent [START_REF] Humphreys | Magma ascent rates in explosive eruptions: constraints from H2O diffusion in melt inclusions[END_REF]. Most of the glass inclusions contain ~3 wt% H2O, suggesting quench pressures around 50 MPa [START_REF] Di Matteo | Water solubility in trachytic melts[END_REF]. Such a melt inclusion re-equilibration pressure is compatible with crystallization of An32-40 plagioclase. Therefore, we assume that parts of the magma crystallized at an intermediate level of ~50 MPa (i.e. 2.0 km depth), whereas other batches ascended fast enough to preserve the pre-eruptive phase compositions and H2O contents. The 0.3 to 1.2 wt% H2O of the residual glasses mark a later degassing that is not associated with crystallization, probably due to the slow kinetics and large crystal nucleation delays prevailing at low pressures [START_REF] Mollard | Decompression-induced crystallization in hydrated silica-rich melts: Empirical models of experimental plagioclase nucleation and growth kinetics[END_REF]. The residual glass H2O contents below 1.2 wt% reflect pumice clast quenching at fragmentation depth below 1 km (P < 20 MPa; Fig. 10).

Differentiation trend and comparison with the volcanic suite of the Chaîne des Puys

Plotted in Harker diagrams, the experimental residual glasses show major compositional differences with pressure, i.e. the 150-200 MPa glasses are trachyandesitic to trachytic (alkaline trend) whereas the 300-400 MPa glasses are dacitic to rhyolitic in terms of silica and alkali contents (sub-alkaline trend; Fig. 7). In details, the compositional differences in the residual glasses from both pressure series come from major differences in the crystallized mineral assemblage and proportion, as demonstrated for basalts in the pioneer work by [START_REF] Grove | Phase equilibrium controls on the tholeiitic versus calcalkaline differentiation trends[END_REF]. This is highlighted by the total silica content of the crystallized minerals, which is ~50 wt% SiO2 in the 150-200 MPa charges, whereas ~40 wt% in the 300-400 MPa samples (Table 3). Early fractionation of plagioclase and amphibole has already been recognized as a key process in generating alkaline liquids [START_REF] Villemant | Magmatic evolution of Pavin's group of volcanoes: petrology, geochemistry and modeling of differentiation processes[END_REF], but the quasi ubiquity of both phases in the experimental products does not allow deciphering the residual glass composition.

Yet, biotite (low-silica phase) seems to be a major component driving the residual liquids towards high-silica low-alkali contents (sub-alkaline trend), whereas biotite-absent products show residual liquids following the alkaline trend. The lack of biotite in the Pavin magma may result from crystallization at low pressure (150-200 MPa), but also from relatively oxidized conditions (NNO+1.5) that favour clinopyroxene at the expense of biotite (Fig. 9a). Therefore, the pressure-driven crystallization sequence of a trachyandesitic melt deciphers the liquid line of descent, with major implications for alkaline versus sub-alkaline volcanic suites. Typically, the low-pressure biotite-lacking Pavin magma has a trachytic residual liquid, but it could have been different in case of crystallization at deeper levels in the stability field of biotite.

The trachytic residual liquid of the Pavin magma overlaps in major oxides with the trachytic whole-rocks of the neighbouring Chaîne des Puys (Fig. 7; except that the Pavin glasses are enriched by ~1 wt% K2O and depleted by ~1 wt% Na2O compared to those of the Chaîne des Puys, which has been attributed to preferential fractionation of amphibole at the expense of clinopyroxene; [START_REF] Villemant | Magmatic evolution of Pavin's group of volcanoes: petrology, geochemistry and modeling of differentiation processes[END_REF]. However, the accordance of the other oxides (and total alkali) makes it possible the generation of trachytic liquids by main fractionation of amphibole, plagioclase, and clinopyroxene from a trachyandesitic parent at upper crustal conditions (< 10 km), in agreement with previous mineralogical and geochemical studies on the Massif Central magmatic suites (e.g. [START_REF] Maury | Cristallisation fractionnée d'un magma basaltique alcalin: la série de la Chaîne des Puys (Massif Central, France)[END_REF][START_REF] Villemant | Distribution coefficients of major and trace elements, fractional crystallization in the alkali basalts of Chaine des Puys (Massif Central, France)[END_REF]. The trachyandesitic melts (such as the ones of the Pavin or Nugère for the Chaîne des Puys) may have fractionated early amphibole and plagioclase at upper crust level, but our experimental results bring the additional constraint that they did not fractionate significant biotite to produce trachytic residual liquids.

Conclusion

The Pavin pyroclasts are relatively heterogeneous in composition (crystal zonings, glomerocrysts, cumulates, etc…), which may result from two main processes: (i) a small volume of stored magma that makes it significantly sensitive to any physico-chemical changes SiO 2 (wt%) 1980) and ( 2) [START_REF] Leyrit | Characterization of phreatomagmatic deposits from the eruption of the Pavin maar (France)[END_REF]. b pumice for pumice clasts and all denotes analyses on 17 thin sections encompassing pumice to dense clasts (mean mineral content). c % gives the void-free modal proportion determined from point counting using an optical microscope and wt% is mass proportion recalculated with the respective mineral densities as defined in text. d Plus apatite (<0.5) and very rare biotite. 53,58 0,67 0,15 0,07 27,17 0,12 0,65 0,02 0,03 0,00 0,03 0,01 11,04 0,18 4,73 0,67 0,57 0,18 0,10 0,11 98,16 3,32 42,24 54,44 900Pav3X7 59,25 0,20 25,93 0,41 0,07 0,07 8,23 5,88 1,06 0,00 0,00 101,1 6,25 52,84 40,91 2 950Pav3X7 56,58 1,08 0,09 0,05 26,23 0,75 0,51 0,04 0,01 0,01 0,04 0,02 9,69 0,80 5,39 0,40 0,81 0,22 0,17 0,12 99,62 4,73 47,78 47,49 3 850Pav3X1 52,81 0,64 0,02 0,01 27,56 0,61 0,47 0,08 0,01 0,01 0,04 0,03 11,04 0,36 4,69 0,29 0,38 0,04 0,00 0,00 97,01 2,26 42,49 55,25 850Pav3X8 57,23 0,10 25,27 0,64 0,06 0,03 8,21 6,18 0,77 0,05 0,00 98,54 4,54 55,08 40,39 850Pav3X7 58,11 0,54 0,09 0,12 23,84 0,34 0,77 0,01 0 0,00 0,09 0,03 7,12 0,41 6,30 0,14 1,02 0,07 0,06 0,00 97,40 6,16 57,75 36,09 4 950Pav2X1ox 53,78 0,71 0,02 0,02 26,99 0,58 0,87 0,03 0,02 0,03 0,05 0,01 10,55 0,42 5,20 0,11 0,48 0,04 0,08 0,24 98,28 2,79 45,85 51,36 950Pav2X7ox 57,02 0,74 0,07 0,07 24,30 0,71 0,81 0,12 0,06 0,02 0,09 0,04 7,85 0,21 6,06 0,52 1,06 0,14 0,22 0,20 97,55 6,34 54,53 40,62 5 900Pav4X8 53,17 0,50 0,00 0,00 28,17 0,37 0,47 0,11 0,04 0,06 0,05 0,01 10,94 0,03 4,75 0,12 0,45 0,03 0,00 0,00 98,04 2,66 42,82 54,52 900Pav4X7 58,78 1,95 0,06 0,06 24,46 1,44 0,57 0,10 0,00 0,00 0,01 0,01 7,13 1,37 5,90 0,14 1,61 0,66 0,00 0,00 67 0,48 0,13 0,13 26,94 0,16 0,65 0,10 0,03 0,05 0,08 0,02 10,46 0,08 5,34 0,02 0,55 0,10 nd nd 97,85 3,16 46,51 50,34 8 975Pav1.5X9 51,74 0,36 0,09 0,10 27,38 0,39 0,75 0,16 0,03 0,05 0,09 0,03 11,45 0,54 4,76 0,30 0,46 0,07 nd nd 96,75 2,65 41,81 55,54 Appendix B 975Pav1.5X8 3 51,74 0,33 0,14 0,01 27,48 0,39 0,74 0,13 0,09 0,05 0,08 0,04 11,46 0,34 4,87 0,07 0,53 0,10 nd nd 97,12 3,02 42,16 54,82 975Pav1.5X7 3 53,27 0,92 0,05 0,05 26,71 0,87 0,72 0,07 0,01 0,01 0,09 0,01 10,45 1,11 5,31 0,47 0,68 0,19 nd nd 97,30 3,87 46,08 50,04 a For the natural plagioclases, core or rim analyses; * for data from [START_REF] Bourdier | Contribution à l'étude volcanologique de deux secteurs d'intérêt géothermique dans le Mont Dore : le group du Pavin et le Massif du Sancy[END_REF] and ** for cumulate analyses; For the experimental plagioclases, run number as in Table 3. All oxides in wt%, with number of analyses (n) and statistical errors (italic numbers) for the experimental compositions. Or, Ab, An give the orthoclase, albite, and anorthite molar %, respectively; nd for not determined. 3 39,51 0,93 3,54 0,21 13,05 0,26 12,81 0,60 0,22 0,02 12,01 0,35 11,77 0,14 2,40 0,06 1,13 0,05 0,03 0,14 96,60 0,82 2,05 0,63 900Pav3X8 3 38,73 0,16 3,72 0,49 13,31 0,66 13,40 0,72 0,22 0,22 11,90 0,19 11,80 0,20 2,40 0,03 1,18 0,03 0,10 0,23 97,00 0,83 2,17 0,62 2 950Pav3X8 3 39,62 0,41 4,33 0,34 12,98 0,21 11,73 0,61 0,18 0,06 12,78 0,21 11,73 0,14 2,47 0,11 1,22 0,06 0,10 0,01 97,13 0,82 2,09 0,66 950Pav3X7 40,33 0,28 4,31 0,13 12,03 0,36 13,32 0,14 0,27 0,03 11,95 0,31 11,46 0,10 2,51 0,06 1,12 0,06 0,04 0,21 97,55 0,78 1,97 0,62 3 850Pav3X1 38,88 0,68 3,26 0,10 12,57 0,51 13,28 0,47 0,20 0,06 11,80 0,20 11,50 0,36 2,39 0,01 1,25 0,03 0,28 0,19 95,61 0,84 2,05 0,62 850Pav3X8 36,38 3,52 10,94 14,48 0,33 10,67 13,00 2,31 1,25 1,39 0,11 94,37 0,98 2,08 0,57 4 950Pav2X1ox 40,36 0,85 3,68 0,22 11,97 0,22 10,91 0,53 0,35 0,17 14,42 0,41 11,66 0,16 2,54 0,15 1,12 0,06 0,13 0,19 97,34 0,78 2,04 0,65 950Pav2X7ox 41,07 0,67 3,99 0,13 11,44 0,29 10,38 0,42 0,33 0,22 14,34 0,17 11,38 0,04 2,62 0,09 1,21 0,11 0,07 0,24 96,76 0,78 1,93 0,67 5 900Pav4X1 37,79 0,39 3,05 0,08 13,82 0,17 12,74 0,38 0,25 0,10 11,72 0,29 12,28 0,41 2,22 0,02 1,27 0,11 0,50 0,00 95,70 0,89 2,20 0,62 900Pav4X8 38,90 0,48 3,70 0,27 13,09 0,11 14,09 0,22 0,33 0,09 11,40 0,06 11,41 0,14 2,35 0,05 1,31 0,11 0,10 0,00 96,67 0,78 2,13 0,60 6 950Pav2X1 39,06 0, 55 4,49 0,27 12,85 0,04 11,61 0,15 0,30 0,03 12,81 0,17 11,63 0,31 2,32 0,09 1,19 0,11 0,12 0,00 96,40 0,77 2,14 0,65 950Pav2X8 39,61 0,43 4,75 0,27 12,12 0,44 11,37 0,70 0,19 0,15 13,22 0,39 11,36 0,13 2,36 0,06 1,14 0,09 1,10 0,10 96,20 0,73 2,06 0 nd nd 95,44 0,88 2,11 0,68 975Pav1.5X7 39,40 0,50 4,99 0,20 11,24 0,34 11,14 0,69 0,20 0,08 13,52 0,33 11,49 0,02 2,52 0,14 1,13 0,08 nd nd 95,63 0,87 2,07 0,69 a For the natural amphiboles, core or rim analyses; * for data from [START_REF] Bourdier | Contribution à l'étude volcanologique de deux secteurs d'intérêt géothermique dans le Mont Dore : le group du Pavin et le Massif du Sancy[END_REF] and ** for cumulate analyses; For the experimental amphiboles, run number as in Table 3. All oxides in wt%, with number of analyses (n) and statistical errors (italic numbers) for the experimental compositions. A_Cat and Al IV give the cation number in the alkali site and the tetra-coordinated aluminium in p.f.u., respectively, calculated after [START_REF] Leake | Nomenclature of amphiboles: Report of the subcommittee on amphiboles of the International Mineralogical Association, Commission on New Minerals and Mineral Names[END_REF] on a H 2 O-free 23 oxygen basis with iron as FeO. Mg# gives the magnesium number as molar Mg/(Mg+Fe tot ); nd for not determined. [START_REF] Bourdier | Contribution à l'étude volcanologique de deux secteurs d'intérêt géothermique dans le Mont Dore : le group du Pavin et le Massif du Sancy[END_REF] and ** for cumulate analyses; For the experimental clinopyroxenes, run number as in Table 3.

B2. Amphibole

B3. Clinopyroxene

All oxides in wt%, with number of analyses (n) and statistical errors (italic numbers) for the experimental compositions. Wo, En, and Fs give the wollastonite, enstatite, and ferrosilite mole %, respectively; Mg# gives the magnesium number as molar Mg/(Mg+Fe tot ); nd for not determined. 84 0,41 4,19 0,08 15,09 0,22 13,46 0,39 0,23 0,09 14,68 0,56 0,11 0,04 1,01 0,09 8,25 0,03 0,06 0,08 92,99 0,67 850Pav3X8 35,66 0,80 4,84 0,19 14,52 0,56 15,08 0,28 0,13 0,12 13,66 0,66 0,19 0,13 0,75 0,05 8,40 0,16 0,05 0,22 93,50 0,60 850Pav3X7 37,74 2,18 4,94 0,29 14,17 0,30 14,68 1,08 0,30 0,13 12,07 1,18 0,58 0,61 1,03 0,41 8,12 0,35 0,19 0,20 94,03 0,60 4 950Pav2X7ox 35,95 0,20 5,85 0,19 14,27 0,32 10,75 0,27 0,21 0,08 16,81 0,43 0,13 0,06 0,94 0,01 8,26 0,20 0,02 0,22 93,41 0,74 5 900Pav4X1 35,74 0,49 4,18 0,09 15,99 0,14 13,27 0,43 0,15 0,06 15,20 0,28 0,11 0,06 0,87 0,08 8,53 0,24 nd nd 94,05 0,68 900Pav4X8 36,77 1,40 4,93 0,19 16,08 0,49 14,89 0,09 0,17 0,24 13,23 1,15 0,06 0,02 0,78 0,00 8,34 0,40 nd nd 95,25 0,62 900Pav4X7 35,42 0,49 5,64 0,17 14,69 0,33 15,75 0,52 0,19 0,06 13,33 0,10 0,45 0,50 0,82 0,02 8,64 0,08 nd nd 94,94 0,61 6 950Pav2X1 35,47 0,22 5,98 0,30 15,39 0,22 11,33 0,68 0,17 0,06 16,16 0,68 0,07 0,05 0,06 0,06 8,50 0,14 nd nd 94,10 0,73 950Pav2X8 36,81 1,17 5,93 0,53 15,13 0,14 11,66 0,67 0,16 0,08 15,01 0,54 0,23 0,20 1,07 0,15 8,36 0,56 nd nd 94,36 0,71 950Pav2X7 36,42 1,37 6,11 0,10 14,97 0,44 12,89 0,92 0,12 0,10 14,58 0,77 0,26 0,19 0,99 0,06 8,27 0,30 nd nd 94,62 0,68 a For the natural biotites, inclusions in amphibole or microlite in the matrix; * for data from [START_REF] Bourdier | Contribution à l'étude volcanologique de deux secteurs d'intérêt géothermique dans le Mont Dore : le group du Pavin et le Massif du Sancy[END_REF]; For the experimental biotites, run number as in Table 3. All oxides in wt%, with number of analyses (n) and statistical errors (italic numbers) for the experimental compositions. Mg# gives the magnesium number as molar Mg/(Mg+Fe tot ); nd for not determined. 94,50 Mt65 0,14 0,03 42,55 0,74 0,38 0,04 47,42 0,86 0,79 0,00 3,63 0,19 0,21 0,05 0,02 0,03 0,09 0,03 95,23 Ilm79 900Pav3X8 0,13 0,14 11,76 0,03 4,19 0,30 73,65 0,05 0,65 0,10 2,40 0,08 0,06 0,01 0,02 0,00 0,04 0,05 92,90 Mt63 900Pav3X7 0,21 0,03 12,67 0,33 3,00 0,15 74,15 1,19 0,98 0,05 1,88 0,06 0,13 0,04 0,00 0,00 0,07 0,05 93,09 Mt61 2 950Pav3X1 0,24 0,00 12,85 0,30 4,66 0,02 68,30 0,91 0,53 0,00 3,59 0,24 0,12 0,02 0,04 0,06 0,07 0,03 90,40 Mt60 950Pav3X8 0,05 0,06 46,84 0,74 0,39 0,03 43,06 0,86 0,66 0,00 5,00 0,08 0,11 0,01 0,03 0,03 0,04 Mt52 0,11 0,07 46,58 0,06 0,39 0,03 44,01 0,04 0,73 0,08 4,04 0,02 0,15 0,01 0,02 0,02 0,09 0,02 96,13 Ilm87 3 850Pav3X1 0,37 0,18 7,93 0,46 3,44 0,20 75,34 0,44 0,75 0,13 2,15 0,10 0,18 0,11 0,05 0,01 0,1 0,02 90,31 Mt75 0,39 0,22 37,72 0,96 0,57 0,12 50,74 0,58 0,92 0,12 2,83 0,08 0,17 0,05 0,02 0,02 0,1 0,04 93,46 Ilm71 850Pav3X8 0,30 0,17 8,98 0,17 2,73 0,14 75,84 0,63 1,03 0,14 1,81 0,04 0,23 0,09 0,01 0,01 0,11 0,07 91,03 Mt72 850Pav3X7 0,35 0,05 9,36 0,34 2,36 0,11 77,12 0,32 1,02 0,24 1,76 0,07 0,22 0,12 0,01 0,02 0,05 0,00 92,25 Mt 4 950Pav2X1ox 0,21 0,16 25,30 1,09 0,91 0,12 61,98 1,88 0,38 0,16 2,69 0,52 0,15 0,06 0,01 0,01 0,08 0,02 91,71 Hem 0,19 0,03 5,48 0,15 3,91 0,03 77,86 0,83 0,78 0,08 4,17 0,20 0,16 0,07 0,00 0,00 0,08 0,08 92,64 Mt83 950Pav2X7ox 0,48 0,37 26,93 0,38 0,81 0,05 59,98 0,70 0,52 0,05 2,94 0,05 0,21 0,04 0,03 0,03 0,17 0,02 92,08 Hem 0,15 6,80 3,43 78,99 0,94 3,92 0,08 0,03 0,05 94,38 Mt84 5 900Pav4X1 0,24 0,10 8,51 0,12 4,07 0,15 77,09 0,80 0,71 0,14 2,33 0,08 0,15 0,05 0,02 0,02 0,08 0,04 93,20 Mt74 900Pav4X8 0,21 0,01 9,25 0,14 3,69 0,08 76,41 0,66 0,85 0,09 2,08 0,00 0,23 0,14 0,00 0,00 0,08 ,51 0,58 0,43 0,06 43,78 0,59 0,69 0,17 4,94 0,13 0,14 0,04 0,02 0,01 0,10 0,03 97,10 Ilm85 950Pav2X8 0,20 0,04 13,67 1,52 4,89 0,50 68,44 0,54 0,73 0,17 3,90 0,03 0,11 0,08 0,02 0,01 0,08 0,03 92,04 Mt57 0,20 46,69 0,42 44,02 0,78 5,30 0,20 0,00 0,10 97,71 Ilm84 7 975Pav2X1 0,07 0,07 8,99 0,21 4,35 0,16 74,02 0,60 0,50 0,09 3,74 0,10 0,10 0,01 0,02 0,01 0,08 0,05 91,87 Mt72 975Pav2X9 0,09 0,01 9,91 0,19 4,83 0,03 72,70 0,77 0,51 0,27 3,95 0,07 0,09 0,06 0,01 0,01 0,06 0,08 92,13 Mt69 975Pav2X8 0,13 0,01 11,40 0,32 4,87 0,09 70,34 0,51 0,62 0,15 4,09 0,17 0,14 0,04 0,02 0,03 0,05 0,07 91,65 Mt64 975Pav2X7 0,10 0,05 12,64 0,32 3,99 0,08 71,37 0,92 0,68 0,05 3,98 0,06 0,07 0,03 0,02 0,02 0,02 0,01 92,88 Mt61 8 975Pav1.5X1 0,07 0,02 9,42 0,22 4,62 0,01 75,09 0,93 0,52 0,05 3,97 0,09 0,08 0,05 0,05 0,01 0,04 0,05 93,86 Mt71 975Pav1.5X9 0,13 0,11 10,84 0,15 4,47 0,18 71,99 2,41 0,44 0,04 4,44 0,19 0,09 0,04 0,00 0,00 0,02 0,02 91,41 Mt66 975Pav1.5X8 0,39 0,15 12,53 0,37 4,29 0,30 69,41 0,56 0,71 0,12 4,40 0,14 0,08 0,05 0,01 0,02 0,10 0,03 91,94 Mt61 975Pav1.5X7 0,18 0,12 12,61 0,51 4,07 0,44 69,37 0,77 0,70 0,05 4,29 0,08 0,09 0,07 0,00 0,00 0,05 0,06 91,38 Mt60 a For the natural Fe-Ti oxide, occurrence as phenocryst (pheno) or as inclusion in clinopyroxene (in Cpx) or amphibole (in Am); * for data from [START_REF] Bourdier | Contribution à l'étude volcanologique de deux secteurs d'intérêt géothermique dans le Mont Dore : le group du Pavin et le Massif du Sancy[END_REF]; For the experimental Fe-Ti oxides, run number as in Table 3. All oxides in wt%, with number of analyses (n) and statistical errors (italic numbers) for the experimental compositions. Mt and Ilm give the magnetite and ilmenite molar %, respectively, calculated after [START_REF] Sauerzapf | The titanomagnetite-ilmenite equilibrium: new experimental data and thermo-oxybarometric application to the crystallization of basic to intermediate rocks[END_REF]; Hem for hematite. 64,84 1,07 0,69 0,19 20,28 0,14 1,51 0,31 0,09 0,02 0,04 0,02 5,18 0,21 3,59 0,12 3,26 0,16 0,21 0,30 88,47 900Pav3X8 65,56 0,69 20,34 1,18 0,06 0,00 4,44 3,60 3,99 0,12 0,00 88,87 2 950Pav3X1 63,64 1,37 0,69 0,20 19,63 0,17 1,92 0,90 0,10 0,04 0,05 0,01 5,89 0,11 4,57 0,27 2,73 0,03 0,61 0,17 89,46 950Pav3X8 64,30 1,96 0,62 0,42 19,70 0,14 1,71 0,90 0,16 0,04 0,08 0,09 5,77 0,23 4,44 1,21 2,68 0,25 0,13 0,41 90,01 3 850Pav3X1 68,73 0,22 0,41 0,17 19,60 0,27 0,99 0,39 0,07 0,06 0,04 0,01 3,69 0,10 2,62 0,31 3,51 0,15 0,09 0,25 85,83 850Pav3X8 67,68 0,18 0,57 0,09 18,12 0,28 2,28 0,21 0,11 0,07 0,31 0,12 2,45 0,03 4,07 0,06 4,25 0,17 0,06 0,10 88,77 850Pav3X7 71,63 0,26 0,43 0,01 17,63 0,27 1,51 0,05 0,10 0,01 0,08 0,06 1,37 0,38 2,91 0,00 4,24 0,45 0,03 0,09 87,16 4 950Pav2X1ox 63,36 0,60 0,85 0,08 18,55 0,40 2,94 0,06 0,13 0,06 1,09 0,05 3,23 0,11 4,92 0,34 4,58 0,10 0,29 0,05 950Pav2X7ox 64,34 0,52 0,88 0,10 17,55 0,42 2,88 0,34 0,12 0,11 1,06 0,26 2,61 0,42 5,47 0,34 4,96 0,19 0,04 0,09 5 900Pav4X1 65,30 0,20 0,50 0,08 20,78 0,11 1,13 0,16 0,18 0,07 0,09 0,04 4,46 0,10 4,01 0,15 3,48 0,09 0,07 nd 900Pav4X8 69,94 3,92 0,33 0,10 18,92 1,37 0,84 0,41 0,12 0,09 0,01 0,07 2,06 1,78 3,87 0,91 3,83 0,45 0,07 nd 7 975Pav2X1 59,69 0,29 1,38 0,06 19,02 0,25 4,74 0,26 0,16 0,12 1,69 0,06 4,92 0,07 4,50 0,27 3,91 0,14 nd nd 90,07 975Pav2X9 59,41 0,12 1,42 0,04 19,03 0,26 4,75 0,32 0,20 0,13 1,70 0,05 4,97 0,06 4,78 0,12 3,75 0,06 nd nd 91,60 975Pav2X8 59,52 0,24 1,30 0,03 18,84 0,29 4,66 0,27 0,05 0,05 1,71 0,03 4,90 0,11 5,03 0,07 3,98 0,14 nd nd 92,22 975Pav2X7 60,71 0,10 1,14 0,02 18,25 0,07 4,54 0,19 0,21 0,07 1,50 0,01 3,91 0,10 5,24 nd nd 93,18 975Pav1.5X7 61,77 0,25 0,97 0,07 17,97 0,27 4,30 0,17 0,13 0,02 1,43 0,04 3,23 0,04 5,14 0,06 5,07 0,15 nd nd 92,42 a For the natural glasses, MG for matrix glass and GI for glass inclusion; For the experimental glasses, run number as in Table 3. All oxides in wt% recalculated to 100 wt%, with Na 2 O corrected by a factor 1.2. The number of analyses (n) and the statistical errors (italic numbers) are given for the experimental compositions. Total gives the raw sum of the analysed oxides without Na correction. H 2 O gives the water contents of the natural glasses determined by the 'by-difference' method, as defined in text ('Analytical method' section); nd for not determined. 3 and Mt-Ilm are taken as the average compositions given in Appendix B5. b Chemical equilibrium test after [START_REF] Bacon | Mg/Mn partitioning as a test for equilibrium between coexisting Fe-Ti oxides[END_REF], i.e. log(Mg/Mn) in Mt and Ilm compares within an error of +0.20.

B4. Biotite

B5. Fe-Ti oxides

B6. Glasses

Appendix C (Thermobarometry).

C1. Two-oxides thermo-oxymetry after Sauerzapft et al. (2008).

Sample

C2. Amphibole thermo-baro-oxy-hygrometry after Ridolfi and Renzulli (2012).

Sample

  induced by deep magma recharge and (ii) an intermediate magma storage within the conduit (up to ~50 MPa; ~2.0 km deep) that promotes crystallization of microlites, microphenocrysts, and phenocryst rims. Evaluating the phase assemblage and compositions that prevail in the reservoir, we infer magma storage conditions around 950-975 °C, 150-200 MPa (~5.5-7.0 km in depth), NNO+1.5, and 4.5-5.5 wt% melt H2O. The residual liquids in equilibrium at these conditions are trachytic and compare to those of the neighbouring Chaîne des Puys trachytes (whole-rock), making it possible a differentiation trend by fractionation of at least 10-20 wt% of amphibole, plagioclase, and clinopyroxene. The experiments highlight the importance of crystallizing this assemblage at 150-200 MPa and out of the stability field of biotite to produce the trachytic liquids. In contrast, biotite plays a major role at 300-400 MPa in driving liquids towards sub-alkaline trends.

Figure captions Figure 1 .

 captions1 Figure captions

Figure 2 .

 2 Figure 2. Total alkalis versus SiO2 bulk-rock compositions of the magmas from the Pavingroup (in green) and the Chaîne des Puys (CDP; in red), as given in Table1(recalculated without P2O5 and total iron as FeO) and in[START_REF] Martel | Phase relations in trachytes and implication for magma storage conditions in the Chaîne des Puys (French Massif Central)[END_REF] for the CDP trachytes; Volcanic rock classification after Le[START_REF] Bas | A chemical classification of volcanic rocks based on the total alkali-silica diagram[END_REF].

Figure 3 .

 3 Figure 3. a. Natural and experimental plagioclase compositions recalculated as anorthite (An), albite (Ab), and orthoclase (Or) mol%, as given in Appendix C1 and b. Analytical profiles from rim to core, with the yellow box showing the An45-50 plateau value present in nearly all plagioclases and interpreted as the pre-eruptive compositions in the reservoir (see text). Higher An content may be inherited cores from the Montchal trachybasalt (as suggested by the

Figure 4 .

 4 Figure 4. Natural and experimental amphibole compositions recalculated as Al IV content versus a. cations in A-site (pfu) and b. Mg#, as given in Appendix C2.

Figure 5 .

 5 Figure 5. Natural and experimental clinopyroxene compositions recalculated as a. enstatite (En), wollastonite (Wo), ferrosilite (Fs) mol% and b. Al2O3 versus SiO2 wt%, as given in Appendix C3.

Figure 6 .

 6 Figure 6. Natural and experimental biotite compositions recalculated as Mg# versus SiO2 wt%, as given in Appendix C4.

Figure 7 .

 7 Figure 7. Natural and experimental glass compositions plotted in Harker diagrams, as given in Appendix C6 (glasses are corrected for Na2O and normalized to 100 wt% on a volatile-free basis).

Figure 8 .

 8 Figure 8. Melt H2O content of glass inclusions (circles; clinopyroxene-or magnetite-hosted) and matrix glasses (squares) as given in Appendix C6.

Figure 9 .

 9 Figure 9. Pressure-temperature phase stability of the Pavin trachyandesite as a function of H2O content: a. XH2Oin=1 (H2O saturation), b. XH2Oin~0.8, and c. XH2Oin~0.7 (black and green circles for NNO+1 and NNO+2, respectively). The right Y-axes give melt H2O content calculated after equation 1 in Appendix A2. The point labels give the crystallinity in wt% (in bracket) followed by the phase compositions : plagioclase as anorthite mol% (An), amphibole (Am) as Al2O3 wt%, clinopyroxene as wollastonite mol% (Wo), biotite as Mg#, titanomagnetite as magnetite mol% (Mt), and glass as liquid (Liq) SiO2 wt% (all compositions given in Appendix C). The crystallinity (void-free basis) and phase compositions of the natural pumice samples are given in figure title. The dashed red circle shows the experimental conditions that best reproduce the natural phase assemblage and compositions.

Figure 10 .

 10 Figure 10. Schematic view of the plumbing system beneath the Pavin volcano showing the magma storage conditions and the eruptive scenario based on petrological information. Phase abbreviations and composition as in Fig. 9. See text for explanations.

  Figure 4

  Figure 9

Table 1

 1 Major-element composition and CIPW norm of the Pavin group and comparison with samples from the Chaîne des Puys.

		Pavin group				Chaîne des Puys
		Pavin			Montchal Montcineyre Nugère Clierzou
	Sample type	fused	dense dense	lava	lava	lava	lava	fused
		pumice	clast clast	sample	sample	sample sample rock
	Reference	this work 1	2	1	1	3	3	4
	Major elements (wt %)
	SiO 2	58,57	58,45 57,61	49,23	43,80	57,69	61,60	62,67
	TiO 2	1,38	1,30 1,37	2,35	2,80	1,13	0,85	0,75
	Al 2 O 3	18,93	18,05 17,98	16,40	14,10	18,08	18,32	17,19
	Fe 2 O 3	nd	nd	nd	nd	nd	6,90	4,80	nd
	FeO	5,49	6,11 5,98	9,39	11,16	nd	nd	4,04
	MnO	0,12	0,15 0,15	0,17	0,17	0,19	0,23	0,19
	MgO	1,71	2,11 1,78	6,88	12,70	1,96	1,09	0,96
	CaO	4,77	4,71 4,79	8,97	10,00	4,58	3,17	3,22
	Na 2 O	4,79	5,01 5,49	3,67	3,20	5,48	5,71	6,59
	K 2 O	3,82	4,11 4,49	1,97	1,45	3,44	3,91	4,09
	P 2 O 5	0,40	nd	0,37	nd	nd	0,56	0,32	0,29
	Diff. to 100	-1,29	-0,09 0,18	0,97	0,62	0,21	1,98	0,18
	Na 2 O/Na 2 O+K 2 O 0,56	0,55 0,55	0,65	0,85	0,61	0,59	0,62
	P.I.	0,64	0,70 0,77	0,34	0,33	0,70	0,74	0,89
	CIPW norm
	quartz	2,14	-	-	-	-	-	3,63	0,78
	orthoclase	22,57	24,29 26,54	11,64	8,62	20,30	23,10	24,15
	albite	40,54	42,39 40,96	22,40	3,51	46,34	48,31	55,77
	anorthite	18,87	14,62 11,14	22,45	19,95	14,59	12,80	5,24
	nepheline	-	-	2,98	4,69	12,86	-	-	-
	apatite	0,99	-	0,92	-	-	1,39	0,80	0,73
	ilmenite	2,62	2,47 2,60	4,46	5,35	2,15	1,61	1,43
	magnetite	1,15	1,28 1,26	1,97	2,36	1,31	0,91	0,85
	diopside	1,86	7,31 8,58	17,94	24,1	3,77	0,73	7,48
	hypersthene	9,39	4,78 -	-	1,10	6,80	7,75	3,68
	olivine	-	2,94 5,18	13,61	23,42	2,87	-	-
	Fused sample of the Pavin trachyandesite was analyzed by EMP (corrected for Na loss; Analytical
	methods given in Appendix A2), all other analyses from the literature, with references as follows: (1)
	Bourdier, 1980 ; (2) Villemant et al., 2016 ; (3) Maury et al., 1980 ; (4) Martel et al., 2013 (corrected
	for Na loss). Sum of 10 oxides recalculated on 100% for all analyses, with difference to 100 of the
	original analyses given (negative when original sum > 100). P.I. is the peralkalinity index given as
	molar (Na 2 O+K 2 O)/Al 2 O 3 . nd for not determined.

Table 1

 1

Table 2

 2 Modal proportions of the Pavin pyroclasts.

	Reference a	This study		(1)		(2)
	Lithology b	pumice	pumice	pumice	all
	Proportion c	% wt% % wt% %	wt% % wt%
	Plagioclase	14 13	5 4	6.5 6	6 6
	Amphibole	4 3	3 2	2	1	7 6
	Clinopyroxene 1 2	1 1	1.5 1	7 5
	Fe-Ti oxides	4 2	1 <1	0.5 <1	3 1
	Crystallinity c 23 20	10 7	10.5 8	23 18
	a From (1) Bourdier (

Table 2 Table 3

 23 Experimental conditions, phase assemblages and proportions. SiO 2 content in wt% of the cumulate (Am, Bt, Pl, Cpx) calculated as the sum of the mass fractions of each mineral (from the 'Major phase' column, divided by the total fraction) by each mineral SiO 2 contents (as given in Appendix B1-4).

	Sample #	X H2Oin H 2 O melt (wt%) a	Major phases (proportion in wt%)	 (wt%) b	 SiO2 (wt%) c
	Run 1, 300 MPa, 900°C, NNO+0.8 (sensors), 4 days
	900Pav3X1	1.00	8.5	Gl (84) + Am (3) + Bt (9) + Mt (4) + Ilm (0.2) + Ap	17	36.7
	900Pav3X8	~0.9	8.0	Gl (78) + Am (8) + Bt (5) + Pl (5) + Mt (4 ) + Ilm (0.1)	22	41.7
	900Pav3X7	0.75	7.4	Gl + Am + Bt + Pl + Mt + Ilm	~70
	Run 2,

a Melt H 2 O content calculated after equation (1). b Crystallinity. c

294 MPa, 950°C, ~NNO+0.5 (from Fe-Ti oxide thermo-oxymetry), 5 days

	950Pav3X1	1.00	8.5	Gl (83) + Am (<1) + Bt (15) + Mt (2)	16	38.8
	950Pav3X8	0.84	7.7	Gl (77) + Am (3) + Bt (14) + Mt (5) + Ilm (1)	23	36.6
	950Pav3X7	0.69	7.0	Gl + Am + Bt + Pl + Mt + Ilm	~70
	Run 3, 284 MPa, 850°C, NNO+0.8 (sensors), 5 days
	850Pav3X1	1.00	8.5	Gl (65) + Am (5) + Bt (9) + Pl (17) + Mt (3) + Ilm (1)	35	45.6
	850Pav3X8	0.83	7.5	Gl (60) + Am (14) + Bt (3) + Pl (24) + Cpx (7) + Mt (2) + Ilm (0.3) 40	48.3
	850Pav3X7	0.67	6.7	Gl (60) + Bt (11) + Pl (45) + Cpx (4) + Mt (3) + Ilm (0.6)	63	53.8
	Run 4, 192 MPa, 950°C, NNO+2.1 (sensors), 8 days
	950Pav2X1ox 1.00	6.9	Gl (76) + Am (5) + Pl (15) + Cpx (0.5) + Mt (2) + Hem (2)	24	50.2
	950Pav2X7ox ~0.75	5.7	Gl (24) + Am (15) + Bt (10) + Pl (36) + Cpx (11) + Mt (3) + Hem (1) 76	49.9
	Run 5,

365 MPa, 900°C, ~NNO+1.5 ( from Fe-Ti oxide thermo-oxymetry), 4.5 days

	900Pav4X1	1.00	9.5	Gl (84) + Am (4) + Bt (7) + Mt (4) + Ilm (<1) + Ap (1)	16	36.5
	900Pav4X8	0.80	8.3	Gl (60) + Am (11) + Bt (6) + Pl (20) + Mt (3) + Ilm (1)	40	45.9
	900Pav4X7	~0.70	7.8	Gl + Am/Cpx + Bt + Pl + Mt + Ilm	~70
	Run 6, 214 MPa, 950°C, NNO+0.7 (sensors), ~2 days (failed)
	950Pav2X1	1.00	6.9	Gl + Am + Bt + Pl + Mt + Ilm	-
	950Pav2X8	0.76	6.3	Gl + Am + Bt + Pl + Mt + Ilm + Ap	-
	950Pav2X7	0.67	5.8	Gl + Am + Bt + Pl + Cpx + Mt + Ilm	-
	Run 7, 192 MPa, 975°C, ~NNO+1 (no sensors), 7.5 days
	975Pav2X1	1.00	6.9	Gl (99) + Mt (1)	1
	975Pav2X9	0.90	6.6	Gl (99) + Mt (1)	1
	975Pav2X8	0.77	6.3	Gl (96) + Am (1) + Pl (2) + Mt (1)	4	47.9
	975Pav2X7	0.68	5.8	Gl (85) + Am (2) + Pl (11) + Mt (2)	15	51.5
	Run

8, 148 MPa, 975°C, ~NNO+1 (no sensors), 4 days

	975Pav1.5X1 1.00	5.8	Gl (99) + Mt (1)	1
	975Pav1.5X9 0.89	5.4	Gl (93) + Pl (5) + Mt (2)	7	51.7
	975Pav1.5X8 0.77	5.0	Gl (87) + Am (1) + Pl (9) + Cpx (2) + Mt (1)	13	50.0
	975Pav1.5X7 0.69	4.5	Gl (78) + Am (1) + Pl (17) + Cpx (2) + Mt (2)	22	52.1

Table 3 Appendix B (Natural and experimental compositions). B1. Plagioclase

 3

	Type a Sample#	n SiO 2	TiO2	Al 2 O 3	FeO	MnO	MgO	CaO	Na 2 O	K 2 O	P 2 O 5 F	Total Or Ab	An
	Natural (selected)
	core	PAV-P1-01-3	54,44	0,00	27,03	0,34	0,05	0,01	10,52	5,53	0,45	0,00 0,19 98,56 2,53 47,51 49,96
	core	PAV-P1-04-4	55,32	0,08	26,03	0,35	0,00	0,06	9,34	6,12	0,54	0,00 0,21 98,05 3,07 52,60 44,33
	core	PAV-P1-05-2	58,40	0,02	23,93	0,35	0,00	0,03	6,78	7,36	0,97	0,00 0,09 97,93 5,42 62,68 31,90
	core	PAV-P1-15-1	57,54	0,00	25,00	0,35	0,07	0,03	7,99	6,70	0,81	0,00 0,00 98,49 4,56 57,50 37,93
	core	PAV-P1-15-3	55,40	0,01	26,45	0,28	0,00	0,01	9,30	6,08	0,54	0,00 0,48 98,55 3,04 52,54 44,41
	core	P20-P1-25	55,29	0,04	26,01	0,45	0,11	0,05	9,18	6,08	0,56	0,00 0,35 98,12 3,19 52,78 44,03
	core	P20-P1-26	57,65	0,10	25,18	0,43	0,00	0,00	8,16	6,52	0,81	0,15 0,00 99,00 4,63 56,37 38,99
	rim	PavP115plgbo	58,32	0,00	24,95	0,35	0,00	0,03	8,02	6,54	0,73	0,00 0,00 98,94 4,20 57,09 38,71
	core* 9	52,75	0,10	28,88	0,40	0,00	0,06	11,74	4,54	0,37	0,00 nd 98,84 2,16 40,28 57,56
	core* 10	57,78	0,00	25,79	0,41	0,08	0,00	7,94	6,77	0,83	0,06 nd 99,66 4,67 57,84 37,49
	core* 11	55,24	0,05	26,83	0,33	0,00	0,02	8,91	6,04	0,61	0,02 nd 98,05 3,53 53,15 43,32
	core* 12	58,25	0,00	25,54	0,38	0,02	0,03	7,22	6,93	0,89	0,00 nd 99,26 5,09 60,23 34,68
	core** Cum1_11	51,92	0,14	28,17	0,71	0,07	0,06	11,47	4,35	0,37	0,00 0,00 97,26 2,16 40,28 57,56
	core** Cum1_13	53,09	0,17	27,85	0,78	0,01	0,08	11,21	4,55	0,34	0,07 0,22 98,37 4,67 57,84 37,49
	core** Cum3_4	56,80	0,11	24,74	0,78	0,00	0,04	7,87	6,44	1,00	0,03 0,00 97,81 3,53 53,15 43,32
	Experimental (average)
	1	900Pav3X8

  For the natural clinopyroxenes, core or rim analyses; * for data from

	core** Cum1_14	45,20	2,20	7,65	8,29	0,16	12,91	20,06	0,56	0,00 0,00 0,00 97,02 45,21 40,49 14,29 0,74
	core** Cum1_15	45,25	2,51	8,15	7,77	0,07	12,40	20,98	0,63	0,00 0,00 0,10 97,87 47,56 39,12 13,32 0,75
	core** Cum1_16	46,56	2,11	6,88	8,21	0,24	13,34	20,23	0,63	0,01 0,00 0,00 98,21 44,83 41,12 14,05 0,75
	core** Cum3_5	49,23	1,23	3,92	7,73	0,26	15,59	19,36	0,43	0,00 0,00 0,09 97,82 41,15 46,11 12,74 0,79
	core** Cum3_7	45,70	2,13	7,31	7,96	0,00	13,13	21,29	0,46	0,00 0,00 0,19 98,18 46,77 40,14 13,09 0,75
	core** Cum3_8	47,52	1,98	5,09	6,68	0,00	14,18	20,70	0,59	0,06 0,03 0,00 96,83 45,56 43,42 11,01 0,80
	core** Cum3_10	48,46	1,73	5,47	7,62	0,05	14,51	20,58	0,54	0,00 0,00 0,19 99,15 44,24 43,39 12,37 0,78
	core** Cum3_11	49,76	1,13	4,38	7,13	0,20	15,37	20,67	0,38	0,00 0,00 0,00 99,02 43,46 44,99 11,56 0,80
	Experimental (average)
	3	850Pav3X8	1 46,74	1,33	5,15	12,94	0,81	9,36	20,70	0,92	0,25 0,05 0,01 96,33 46,97 29,56 23,47 0,57
		850Pav3X7	3 49,66 1,58 0,86 0,07 4,64 0,54 9,84 0,86 0,65 0,23 10,49 0,57 20,01 1,05 0,87 0,04 0,41 0,06 0,08 97,59 47,09 34,35 18,57 0,66
	4	950Pav2X1ox 3 45,96 0,93 1,73 0,39 6,64 1,10 8,83 0,60 0,38 0,12 13,57 0,16 19,58 1,64 1,12 0,30 0,37 0,08 0,22 98,48 43,11 41,64 15,25 0,74
		950Pav2X7ox 4 49,64 0,35 1,24 0,17 5,57 0,64 9,79 1,77 0,63 0,14 11,50 0,92 19,66 0,55 0,91 0,07 0,48 0,10 0,01 99,62 45,20 36,79 18,02 0,69
	6	950Pav2X7	2 47,25 0,96 1,28 0,26 5,42 0,69 9,28 1,27 0,73 0,03 12,02 0,07 21,84 0,50 0,71 0,04 0,20 0,40 0,00 99,09 47,39 36,31 16,30 0,71
	8	975Pav1.5X8 3 47,91 0,40 1,49 0,06 3,60 0,22 9,24 0,51 0,41 0,05 13,55 0,37 20,76 0,08 0,49 0,04 0,17 nd nd 97,62 44,03 40,22 15,75 0,73
		975Pav1.5X7 4 48,24 0,40 1,44 0,10 4,21 0,60 9,68 1,09 0,49 0,21 12,56 0,51 20,02 0,53 0,60 0,09 0,26 nd nd 97,51 44,34 38,74 16,92 0,71
	Type a Sample#	n SiO 2	TiO 2	Al 2 O 3	FeO	MnO	MgO	CaO	Na 2 O	K 2 O P 2 O 5 F	Total Wo En	Fs	Mg#
	Natural (selected)
	core	PAV-1-02-1	50,43	0,27	1,81	10,14	0,85	12,60	22,60	0,83	0,01 0,00 0,00 99,53 46,69 36,22 17,09 0,70
	rim	PAV-1-02-6	50,09	0,70	2,55	8,55	0,54	12,72	22,79	0,81	0,04 0,00 0,00 98,78 48,16 37,40 14,44 0,73
	core	PAV-P1-14-3	50,66	0,37	1,80	9,42	1,10	12,13	22,39	0,96	0,00 0,00 0,00 98,82 47,43 35,77 16,80 0,71
	rim	PAV-P1-15-2	49,04	1,12	3,84	9,38	0,35	13,05	22,51	0,67	0,00 0,00 0,22 100,18 46,92 37,85 15,22 0,72
	core	P20-P1-04	50,45	0,34	2,30	10,76	0,62	12,19	21,92	0,96	0,01 0,00 0,00 99,54 46,21 35,76 18,03 0,68
	core	P20-P1-19	49,08	1,18	3,74	8,41	0,27	13,54	22,11	0,68	0,00 0,02 0,00 99,02 46,58 39,70 13,72 0,75
	core	P20-P1-27	49,01	1,06	3,68	8,73	0,50	13,10	22,09	0,75	0,00 0,00 0,00 98,92 46,75 38,57 14,68 0,74
	core* 6	49,70	0,55	2,92	10,93	0,52	11,22	21,72	1,17	0,00 nd nd 98,73 47,29 33,99 18,72 0,66
	core* 7	48,19	1,38	5,99	5,09	0,14	14,59	21,61	0,50	0,00 nd nd 97,49 47,15 44,29 8,56 0,84
	rim*	8	50,19	1,15	4,22	6,11	0,07	15,36	20,96	0,44	0,00 nd nd 98,50 44,63 45,51 9,86 0,82
	rim*	9	49,32	1,23	4,48	7,01	0,17	14,38	21,59	0,47	0,00 nd nd 98,65 45,95 42,59 11,46 0,79
	core* 11	46,94	2,09	7,13	7,57	0,07	12,87	21,94	0,66	0,00 nd nd 99,27 48,14 39,30 12,57 0,76
	core* 12	51,02	0,63	2,71	8,58	0,26	13,22	22,68	0,72	0,00 nd nd 99,82 47,54 38,56 13,90 0,74
	rim*	13	49,43	0,81	2,91	9,53	0,61	12,25	22,38	1,02	0,00 nd nd 98,94 47,57 36,23 16,20 0,70

a

  0,21 4,50 0,27 nd nd 93,42975Pav1.5X1 59,59 0,81 1,27 0,18 19,19 0,19 4,77 0,55 0,03 0,04 1,76 0,14 4,84 0,00 4,80 0,01 3,74 0,19 nd nd 90,82 975Pav1.5X9 60,03 0,80 1,15 0,16 18,62 0,43 4,64 0,15 0,13 0,03 1,73 0,05 4,63 0,28 4,98 0,12 4,09 0,10 nd nd 91,43

	975Pav1.5X8 60,40	1,23	18,47	4,69	0,16	1,65	3,82	4,95	4,63

  For the experimental samples, run conditions are given in Table

	a	log(Mg/Mn) b	Sauerzapft et al. (2008)
		Mt	Ilm	T °C	log fO2	NNO
	Selected natural (Mt/Ilm)
	PAV-P1-01-4/ PAV-P1-04-6	0.68 0.77	969	-9.42	+1.3
	PAV-P1-02-4/ PAV-P1-04-6	0.45 0.77	851	-10.94	+1.8
	Experimental
	900PAV3X1 (NNO +0.8)	0.95 0.91	901	-10.95	+0.9
	950PAV3X7 (~NNO +1)	0.95 0.99	924	-11.26	+0.2
	850PAV3X1 (NNO +0.8)	0.70 0.73	870	-10.85	+1.5
	900PAV4X8 (~NNO +1)	0.63 0.63	860	-11.25	+1.3
	950PAV2X1 (NNO +0.7)	0.95 1.10	924	-11.13	+0.4
	950PAV2X8 (NNO +0.5)	0.97 1.08	926	-10.99	+0.4

a

Acknowledgements

We thank Ida Di Carlo for assistance with SEM and EMP analyses, Rémi Champallier for assistance with the IHPV, and Michel Pichavant for fruitful discussions. This work is part of MR's undergraduate research project which was partly funded by TLS Geothermics (France) and the french Investissements d'Avenir program Equipex_Planex (ANR-11-EQPX-0036; B. Scaillet). We greatly thank the reviewers for their useful suggestions that improved the manuscript.

Appendix A (Methods).

A1. Experimental methods

Starting material and experimental charges. Crystallization experiments were performed starting from trachyandesitic pumices sampled in the Clidères section of [START_REF] Leyrit | Characterization of phreatomagmatic deposits from the eruption of the Pavin maar (France)[END_REF], 1 km south of the Pavin crater rim (Fig. 1). A tenth of macroscopically homogeneous pumiceous clasts was ground into fine powder (~20 µm) and fused in a Pt-crucible at 1400 °C and 1 atm for 2 h, air-quenched and powdered in a mortar. The cycle of fusion and grinding was repeated twice to ensure chemical homogeneity of the starting glass powder (PAV). The EMP analyses show that the PAV composition is close to the respective whole-rock ones (Table 1). Charges consisted of ~30 mg of PAV glass powder, added to deionized H 2 O, and silver oxalate as source of CO 2 , which were introduced in Au containers (2.5 mm inner diameter, 0.2 mm wall thickness, and 20 mm length) sealed by arc-welding and checked for leakage by immersion in an oil bath at 120 °C. All experiments were vapour-saturated, either with pure H 2 O (X H2Oin = X H2O /(X H2O +X CO2 ) = 1, in moles) or with a H 2 O-CO 2 fluid mixture (X H2Oin from ~0.7 to 0.9). The fluid/silicate ratio was kept constant around 10 wt% to avoid significant changes in the silicate composition.

Equipments. All experiments were performed in vertically-working internally-heated pressure vessels pressurized with Ar-H 2 gas mixtures. Up to four capsules (containing PAV with three different X H2Oin and one redox sensor; see below) were placed at the hotspot of a Mo-or Kwindings furnace where the thermal gradient over ~3 cm is less than 5 °C. The furnace temperature was regulated by a Eurotherm controller and temperature (T) was read continuously by S-type thermocouples with typical uncertainty of ±5 °C. Total pressure (P) was recorded continuously by a transducer calibrated against a Heise Bourdon gauge (+0.4 MPa). Run durations varied between 3 and 5 days, depending on T, in agreement with run durations in [START_REF] Martel | Phase relations in trachytes and implication for magma storage conditions in the Chaîne des Puys (French Massif Central)[END_REF]. The experiments were stopped either by isobaric quench (by switching off the furnace while maintaining P constant) or by rapid quench (using a device allowing capsule drop in the cold part of the furnace with a cooling rate of ~100 °C/s; modified after [START_REF] Roux | A fast quench device for IHPV[END_REF]. After experiments, each capsule was weighed to check for leakage and opened. Clasts of the samples were embedded in epoxy resins for analysis.

Appendix A

Control of redox conditions. The redox state of the experimental charges was controlled by the H 2 fugacity of the Ar-H 2 gas pressure medium. Half of the runs contained Ni-Pd-O solid sensors that were used to measure the experimental H 2 fugacity following the method of Pownceby and O'Neill (1994). The oxygen fugacity (f O2 ) was finally calculated from the H 2 O dissociation reaction that requires the H 2 O fugacity determined from the H 2 O content of the experimental glasses following the model of [START_REF] Burnham | The importance of volatiles constituents[END_REF]. The resulting uncertainty on log f O2 is +0.3 log unit (Martel et al., 1999).

A2. Analytical methods

Textural assessment and major-element analysis. The textural characteristics of the natural and experimental samples were examined using a scanning electron microscope (SEM; MIRA3 TESCAN at ISTO). SEM imaging allowed identification of minerals or glass areas for chemical analysis using an electron microprobe (EMP; SX Five at ISTO). The analytical conditions for the natural and experimental crystals were set to an acceleration voltage of 15 kV, a beam current of 10 nA, and a focused beam (spot size < 2 µm). Both peak and background counting times were set at 10 s for all elements. The redox sensors were analysed for S, Fe, Co, Ni, Pd, Pt and Au (using metallic elements and pyrite standards) with a beam current of 20 nA (15 kV and 10 s counting time). Glasses were analysed with a beam current of 6 nA, 15 k, 10 s counting time, and a defocused 10 µm² beam size to minimize alkali migration under the electron beam. However, alkali-and H 2 O-rich compositions such as those of the Pavin trachyandesite commonly need to be corrected for Na migration [START_REF] Pichavant | The effects of boron and water on liquidus phase relations in the haplogranite at 1 kbar[END_REF]. Therefore, in agreement with the analyses of the trachytic glasses from the Chaîne des Puys [START_REF] Martel | Phase relations in trachytes and implication for magma storage conditions in the Chaîne des Puys (French Massif Central)[END_REF], we applied a similar correction factor of 1.2 for Na 2 O on the analyses of the Pavin glasses on the ground that they are chemically close and were run at comparable P-T-H 2 O conditions. The analytical errors on major elements were calculated at ~1% (relative) for SiO 2 , Al 2 O 3 and CaO, 3% for FeO, MgO and TiO 2 , and 5% for MnO, Na 2 O and K 2 O.

Melt H 2 O contents. The H 2 O contents of the glass inclusions and residual glasses of the pumice samples were measured by EMP using a modified by-difference method [START_REF] Devine | Comparison of microanalytical methods for estimating H 2 O contents of silicic volcanic glasses[END_REF], which requires calibration of the differences to 100 wt% of the glass EMP analyses against chemically-close glass standards whose H 2 O contents are known and bracket the range of those anticipated for the samples. As glass standards, we used two trachytic glasses (800CHO2 and 800CHO3) containing 6.9 and 8.5 wt% H 2 O (determined by Karl-Fisher titration) as defined in [START_REF] Martel | Phase relations in trachytes and implication for magma storage conditions in the Chaîne des Puys (French Massif Central)[END_REF]. The resulting uncertainty on H 2 O content measurements is ±0.5 wt% absolute.

H 2 O contents could not be measured in all experimental glasses because of glass areas often too small in sizes for EMP analyses. The H 2 O-pressure (P) empirical equation of Di [START_REF] Di Matteo | Water solubility in trachytic melts[END_REF] calculates H 2 O solubility for trachytic liquids at 850-895 °C up to P of 200 MPa. To extrapolate this formulation to higher P, we used the near-liquidus trachytic glasses obtained by [START_REF] Martel | Phase relations in trachytes and implication for magma storage conditions in the Chaîne des Puys (French Massif Central)[END_REF] Phase proportions. In the natural samples, modal proportions of phenocrysts and void-free groundmass were obtained by point counting on two thin sections (~1200 points) using an optical microscope. The error related to this method is estimated to ±10% relative. Because the experimental minerals are commonly too small (<20 µm) to be clearly identified using an optical microscope, the phase proportions were determined by mass balance using the formulation of [START_REF] Stormer | XLFRAC: a program for the interactive testing of magmatic differentiation models[END_REF] and the EMP chemical compositions of the crystals, residual glasses, and bulk rock. To compare both methods, the modal proportions of the natural samples were converted into wt% using the following mineral densities: 2.7 for feldspars (andesine), 5.2 for magnetite and ilmenite, 3.0 for biotite, 3.3 for clinopyroxene (augite), 3.2 for amphibole (pargasite), 3.0 for biotite, and 2.4 for the glass.

Appendix D (Phase-equilibrium run).

D1. Phase relationships

In order to constrain the storage conditions of the Pavin trachyandesite, experimental conditions were chosen to cover the range of intensive parameters inferred from the natural assemblage. Therefore, phase equilibrium experiments were performed from 850 to 950 °C, 150 to 400 MPa, NNO+1 and +2, and X H2Oin ranging between 0.7 and 1 (melt H 2 O contents from 4.5 to 9.5 wt%) (Table 3).

At 365 MPa, 900 °C, and ~NNO+1.5, biotite, magnetite, ilmenite, and rare apatite are present at H 2 O-saturation conditions (~9.5 wt% H 2 O) in proportion of ~16 wt% (Fig. 9a).

Amphibole and plagioclase appear for H 2 O contents around 8-9 wt% (Fig. 9b), with an increase of crystallinity to ~40 wt%, and clinopyroxene replaces amphibole for H 2 O contents < 8 wt% (~70 wt% crystallinity; Fig. 9c).

At ~300 MPa and NNO+0.5 to +1.0, biotite and magnetite are liquidus phases at 950 °C and H 2 O-saturated conditions, with possibly amphibole (~8.5 wt% H 2 O, ~16 wt% crystallinity; Fig. 9a). Ilmenite crystallize for ~8 wt% H 2 O (~23 wt% crystallinity; Fig. 9a), followed by plagioclase for ~7 wt% H 2 O (~70 wt% crystallinity; Fig. 9c). At 900 °C, the assemblage of biotite, magnetite, ilmenite, amphibole, and apatite prevailing at H 2 O saturation (~16 wt% crystallinity; Fig. 9a) is completed by plagioclase for <8 wt% H 2 O (~22 wt% crystallinity; Fig. 9b). Clinopyroxene replaces amphibole for H 2 O contents <7 wt% (~70 wt% crystallinity; Fig. 9c). At 850 °C, biotite, amphibole, magnetite, ilmenite, and plagioclase are stable at H 2 O-saturation conditions (~35 wt% crystallinity; Fig. 9a), clinopyroxene appears between 7.5 and 8.5 wt% H 2 O (~40 wt% crystallinity; Fig. 9b) whereas amphibole disappears for H 2 O contents below 7.0-7.5 wt% (~63 wt% crystallinity; Fig. 9c).

At ~200 MPa, 975 °C and ~NNO+1, magnetite is the liquidus phase at H 2 O saturation (~7 wt%; Fig. 9a), followed by amphibole and plagioclase for H 2 O contents <6 wt% (~4% and ~15 wt% crystallinity for ~6.0 and ~5.5 wt% H 2 O, respectively; Fig. 9b-c). At 950 °C and NNO+1, the experiment failed after about two days, showing evidence of incomplete maturation of the minerals (e.g. hollow plagioclases). Yet, although total crystallinities and the residual glass compositions cannot be used, the crystal assemblages and compositions may be relevant to the imposed P-T conditions. In this experiment, magnetite, ilmenite, amphibole, plagioclase, biotite, and apatite are present for H 2 O contents from saturation to ~6.5 wt% (Fig.

9a-b), followed by clinopyroxene for H 2 O contents <6 wt% (Fig. 9c). An additional

Appendix D

experiment was performed at 950 °C under f O2 conditions of NNO+2, in which magnetite, hematite, amphibole, plagioclase, and clinopyroxene are present at H 2 O saturation condition (~24 wt% crystallinity; Fig. 9a). Biotite appears for H 2 O contents of 6-7 wt% (~76 wt% crystallinity; Fig. 9c), highlighting the predominance of clinopyroxene at NNO+2 at the expense of biotite at NNO+1.

D2. Phase compositions

The