Diego F Narvaez
email: dnarvaez@igepn.edu.ec

Estelle F Rose-Koga

Pablo Samaniego

Kenneth T Koga

Silvana Hidalgo

Constraining magma sources using primitive olivine-hosted melt inclusions from Puñalica and Sangay volcanoes (Ecuador)

Keywords: Melt inclusions, olivine, primary magmas, volatile elements, subduction zone, Ecuador

Introduction

Theoretical and experimental studies on dehydration and melting reactions which occur during subduction have identified two different slab components (e.g. [START_REF] Elliott | Tracers of the slab[END_REF][START_REF] Kessel | Trace element signature of subduction-zone fluids, melts and supercritical liquids at 120-180 km depth[END_REF][START_REF] Spandler | Element recycling from subducting slabs to arc crust: A review[END_REF]. On the one hand, the dehydration of the subducted slab produces a low-density aqueous phase (hereafter called "fluid"), which is characterized by enrichment in large-ion lithophile elements (LILE, e.g. B, Rb, Ba, Pb) coupled with low concentrations of high-field strength elements (HFSE, e.g. Nb, Ta, Zr) and heavy rare earth elements (HREE, e.g. Yb and Y). On the other hand, the partial melting of the subducted slab (i.e. the altered oceanic crust and/or the sedimentary cover) produces a high-density hydrous siliceous phase (hereafter called "melt"), which is enriched in the so-called fluidimmobile elements, such as the light rare earth elements (LREE, e.g. La, Ce), HFSE (e.g. Nb), as well as some LILE (e.g. Be, Th). These fluids and/or melts percolate into the hotter and shallower mantle wedge, decreasing its solidus temperature and triggering its partial melting via a porous flow process [START_REF] Grove | The Role of H2O in Subduction Zone Magmatism[END_REF]. Hence, the trace element signature of primitive arc magmas is typically ascribed to the nature of the subduction component (fluid or melt; e.g. [START_REF] Elliott | Element transport from slab to volcanic front at the Mariana arc[END_REF][START_REF] Class | Distinguishing melt and fluid subduction components in Umnak Volcanics, Aleutian Arc[END_REF][START_REF] Labanieh | Martinique: a Clear Case for Sediment Melting and Slab Dehydration as a Function of Distance to the Trench[END_REF], whereas its major element composition is mainly controlled by the mineralogy of the mantle wedge (e.g. [START_REF] Grove | The role of an H2O-rich fluid component in the generation of primitive basaltic andesites and andesites from the Mt. Shasta region, N California[END_REF].

The dichotomy between fluids and melts issued from the subducted slab was corroborated by numerical models of the thermal regime of subduction zones around the world (e.g. [START_REF] Syracuse | The global range of subduction zone thermal models[END_REF]. These models take into account the geodynamic parameters of subduction systems (age and geometry of the slab, convergence rate, etc.) as well as a temperature-and stress-dependent mantle rheology, yielding large temperature variations at the slab-mantle wedge interface. In some cases, such as Cascadia, Mexico, or the Andean Colombia-Ecuador subduction systems, the estimated temperatures are high enough to confirm the partial melting hypothesis of the subducted sediments and the altered oceanic crust [START_REF] Syracuse | The global range of subduction zone thermal models[END_REF]Gomez-Tuena et al. 2011;[START_REF] Wallowski | Slab melting beneath the Cascade Arc driven by dehydration of altered oceanic peridotite[END_REF]. The Ecuadorian segment of the Andean Northern Volcanic Zone (NVZ) represents a unique place to test the influence of subduction parameters on the thermal regime and thus the slab component signature of primitive arc magmas. In fact, the Ecuadorian volcanic arc is characterized by an along-arc change in both the subduction parameters (age and geometry, [START_REF] Yepes | A new view for the geodynamics of Ecuador: implication in seismogenic source definition and seismic hazard assessment[END_REF], and the geochemical signature of the eruptive products [START_REF] Hidalgo | Role of crustal and slab components in the Northern Volcanic Zone of the Andes (Ecuador) constrained by Sr-Nd-O isotopes[END_REF][START_REF] Ancellin | Across-arc versus along-arc Sr-Nd-Pb isotope variations in the Ecuadorian volcanic arc[END_REF], suggesting that the thermal regime is probably not the same along the length of of the arc from north to south.

In this work, we identify the nature of the mantle source that melts to produce the primitive magmas of the Ecuadorian arc and the characteristics of the slab components added to this source. In order to fulfill these objectives, we focused on the southern part of the Ecuadorian arc. This choice was based on two main reasons: first, this part of the Andean NVZ is developed above a young oceanic crust to the north, which is separated from an older oceanic crust located at the southern termination of the NVZ. Secondly, given that the differentiation processes (e.g. fractional crystallization, crustal assimilation, magma mixing, etc.) obscure the composition of the primitive magmas, the study of magma sources and slab components should be restricted to the most primitive lavas displaying a high Mg# (e.g. [START_REF] Kelemen | One View of the Geochemistry of Subduction-Related Magmatic Arcs, with an Emphasis on Primitive Andesite and Lower Crust[END_REF]. These kind of primitive magmas are common in the southern part of the Ecuadorian arc, namely at Puñalica and Sangay volcanoes. We sampled the most primitive rock of the whole Andean NVZ in this part of the arc, which is also one of the most primitive samples of the entire Andean arc (e.g. the SAN20B basalt from Sangay volcano, [START_REF] Monzier | Sangay volcano, Ecuador: structural development, present activity and petrology[END_REF].

Several regionally oriented whole-rock geochemical studies have investigated the Ecuadorian arc magmatism [START_REF] Barragan | Subduction controls on the compositions of lavas from the Ecuadorian Andes[END_REF][START_REF] Bourdon | Magmatic response to early aseismic ridge subduction: the Ecuadorian margin case (South America)[END_REF][START_REF] Garrison | Dubious case for slab melting in the Northern volcanic zone of the Andes[END_REF][START_REF] Bryant | Geochemical Constraints on the Origin of Volcanic Rocks from the Andean Northern Volcanic Zone, Ecuador[END_REF][START_REF] Chiaradia | Adakite-like volcanism of Ecuador: lower crust magmatic evolution and recycling[END_REF][START_REF] Samaniego | Evolving metasomatic agent in the Northern Andean subduction zone, deduced from magma composition of the long-lived Pichincha volcanic complex (Ecuador)[END_REF][START_REF] Hidalgo | Role of crustal and slab components in the Northern Volcanic Zone of the Andes (Ecuador) constrained by Sr-Nd-O isotopes[END_REF][START_REF] Ancellin | Across-arc versus along-arc Sr-Nd-Pb isotope variations in the Ecuadorian volcanic arc[END_REF]. In this contribution, and following the melt inclusion (MI) work of Le [START_REF] Voyer | Petrogenesis of arc lavas from the Rucu Pichincha and Pan de Azucar volcanoes (Ecuadorian arc): Major, trace element, and boron isotope evidences from olivine-hosted melt inclusions[END_REF] on volcanoes from the northern part of the Ecuadorian arc (Pichincha and Pan de Azúcar), we performed a similar approach by analyzing the major, trace element and volatile contents of a group of selected MIs trapped in the Mg-rich olivines hosted in primitive lavas from the southern part of the Ecuadorian arc (Puñalica and Sangay volcanoes). This work enabled us to constrain the mantle and slab components of the most primitive magmas studied in the NVZ and to relate them to the particular geodynamical setting of this part of the Andean arc. In addition, the new Sangay melt inclusion data represent a new example of primitive, Ca-rich, Nenormative melt inclusions in a context of a volcanic arc constructed on a thick continental crust.

Geodynamical and geological context

Geodynamical setting of the Ecuadorian Andes

The Northern Volcanic Zone (NVZ) of the Andes stretches from ~6°N to ~2°S along the cordilleras of Colombia and Ecuador, and is situated at the convergent zone where the oceanic Nazca plate plunges under the continental South American plate (Fig. 1a). In Ecuador (i.e. from 1°N to 2°S), the relatively young Nazca plate (12-16 Ma;[START_REF] Gutscher | Tectonic segmentation of the North Andean margin: impact of the Carnegie Ridge collision[END_REF]Sallarès and Charvis 2003) converges at an azimuth of N85°E, a rate of 5-6 cm/year [START_REF] Kellogg | Tectonic development of Panama, Costa Rica, and the Colombian Andes: constraints from global positioning system geodetic studies and gravity[END_REF]) and a subduction angle of 25° [START_REF] Guillier | Seismological evidence on the geometry of the Orogenic System in central-northern Ecuador (South America)[END_REF]). In addition, at these latitudes the subducting Nazca plate hosts the Carnegie ridge, a 200-250 km-wide, 2 km-high submarine mountain range issued from the activity of the Galápagos hotspot. Based on geodynamic reconstruction, Sallarès and Charvis (2003) suggest that the portion of the Carnegie ridge involved in the subduction system is younger than 10 Ma. Due to its morphology, the sedimentary blanket of the subducting Carnegie ridge is relatively thin (maximum 200-400 m, Michaud et al. 2005;[START_REF] Proust | Subduction of seafloor asperities revealed by a detailed stratigraphic analysis of the active margin shelf sediments of Central Ecuador[END_REF]). To the south (i.e. roughly to the south of 2-3°S), the subducting slab is older (25-30 Ma;[START_REF] Gutscher | Tectonic segmentation of the North Andean margin: impact of the Carnegie Ridge collision[END_REF]. The boundary between these two contrasting domains is the Grijalva Fracture Zone (GFZ), which consists of a 500 m-high escarpment separating a "young" crust which originated in the Galápagos spreading center to the north, and a relatively "old" crust formed at the East Pacific rise to the south. The inland prolongation of this structure was recently identified by [START_REF] Yepes | A new view for the geodynamics of Ecuador: implication in seismogenic source definition and seismic hazard assessment[END_REF] in the southern part of the Ecuadorian volcanic arc (Fig. 1b). These authors also show that the slab displays a bend around the GFZ that abruptly increases the angle of subduction from 20-25° to the north of the GFZ to 30-35° towards the south [START_REF] Yepes | A new view for the geodynamics of Ecuador: implication in seismogenic source definition and seismic hazard assessment[END_REF].

The Ecuadorian segment of the NVZ is characterized by at least 84 volcanic centers of Pleistocene and Holocene age, which are distributed along four principal alignments constructed over different terranes of contrasting ages and lithologies [START_REF] Hall | Ecuadorian Andes volcanism: A review of Late Pliocene to present activity[END_REF][START_REF] Hidalgo | Role of crustal and slab components in the Northern Volcanic Zone of the Andes (Ecuador) constrained by Sr-Nd-O isotopes[END_REF]. The crustal thickness is inferred to be >50 km beneath the active volcanic arc (Feinenger and Seguin 1983;[START_REF] Prevot | Mapping of the P-wave velocity structure beneath the Ecuadorian Andes: evidence for continuity of the Central Andes[END_REF][START_REF] Guillier | Seismological evidence on the geometry of the Orogenic System in central-northern Ecuador (South America)[END_REF]. Besides the primitive nature of the Puñalica and Sangay lavas, we chose these volcanoes for this study because they are a priori constructed on slab segments with different ages and seismic characteristics [START_REF] Yepes | A new view for the geodynamics of Ecuador: implication in seismogenic source definition and seismic hazard assessment[END_REF]. Puñalica is roughly above (or immediately to the north) of the GFZ projection, while Sangay lies to the south of this boundary. We expect that the presence of the GFZ in the Ecuadorian subduction system (i) might act as a preferred fluid transport region, and (ii) should allow us to identify different slab inputs from "young" and "old" slabs into the mantle wedge.

Geological context of Sangay and Puñalica volcanoes

Sangay volcano (5230 m above sea level -a.s.l.) is the southernmost active edifice of the NVZ. It was constructed over the last 500 ka by three successive edifices [START_REF] Monzier | Sangay volcano, Ecuador: structural development, present activity and petrology[END_REF]) separated by two major sector collapses whose deposits outcrop to the east, on the upper Amazonian plain. Sangay is thought to be located ~120-130 km (see red dotted line in Fig. 1b) above an old, seismogenic slab to the south of the GFZ [START_REF] Monzier | Sangay volcano, Ecuador: structural development, present activity and petrology[END_REF][START_REF] Yepes | A new view for the geodynamics of Ecuador: implication in seismogenic source definition and seismic hazard assessment[END_REF] and is constructed on continental crust that consists of felsic and mafic igneous and metamorphic rocks [START_REF] Aspden | The geology and Mesozoic collisional history of the Cordillera Real, Ecuador[END_REF][START_REF] Pratt | An autochthonous geological model for the eastern Andes of Ecuador[END_REF]. In this work, we focus on the primitive basalt (SAN20B, Mg# 69, [START_REF] Monzier | Sangay volcano, Ecuador: structural development, present activity and petrology[END_REF], from the Sangay II edifice, roughly dated at between 100 and 50 ka. Puñalica volcano (3988 m a.s.l.) is a 300 m-high, cone-shaped edifice slightly elongated in a north-south direction. It is constructed on the eastern flank of the Late Pleistocene Carihuairazo volcano [START_REF] Ordóñez | Depósitos volcánicos del Pleistoceno Tardío en la cuenca de Ambato: caracterización, distribución y origen. Proyecto de Titulación para obtener el título de Ingeniero Geólogo[END_REF]. [START_REF] Clapperton | Glacial and volcanic geomorphology of the Chimborazo-Carihuairazo massif, Ecuadorian Andes[END_REF] and [START_REF] Ordóñez | Depósitos volcánicos del Pleistoceno Tardío en la cuenca de Ambato: caracterización, distribución y origen. Proyecto de Titulación para obtener el título de Ingeniero Geólogo[END_REF] described a series of lava flows from Puñalica that overly a sequence of tephra fallout deposits from the neighboring Chimborazo volcano [START_REF] Hidalgo | Role of crustal and slab components in the Northern Volcanic Zone of the Andes (Ecuador) constrained by Sr-Nd-O isotopes[END_REF]. Based on this stratigraphic information, we expect an age younger than 18-25 ka for this edifice. The depth of the subducted slab at this location is ~120 km with the inland projection of the GFZ roughly below Puñalica volcano [START_REF] Yepes | A new view for the geodynamics of Ecuador: implication in seismogenic source definition and seismic hazard assessment[END_REF]. As for Sangay, we selected the most primitive lavas and tephras from Puñalica, which correspond to scoriaceous bombs sampled on the Puñalica cone (CAR83A, CAR96A and CAR113B), and a lava flow (RIO17A) that outcrops 4 km away from Puñalica's crater.

Methodology

Sample selection, olivine preparation and heating experiments

The chosen rocks (CAR83A, CAR96A, CAR113B, RIO17A from Puñalica, and SAN20B from Sangay volcano) were crushed and the 0.4-1.5 mm fraction was separated. We picked about 500 olivine crystals under a binocular microscope. We used crystal-bound to attach crystals onto glass slides in order to polish and select olivines in which melt inclusions were observed (about 100 crystals). We then selected about 50 crystals with melt inclusions bigger than 30 µm. All melt inclusions were crystallized and presented bubbles. Thus, we performed homogenization using a Vernadsky-type microscope heating stage following the method described by [START_REF] Voyer | Two contrasting H2O-rich components in primary melt inclusions from Mount Shasta[END_REF]. A detailed description of melt inclusion homogenization procedure and analytical methods is provided in Online Supplementary Material 1. The data obtained for major elements by electron microprobe (EMP), volatile element concentrations by SIMS and trace element analysis by LA-ICPMS are provided in Table 1. We have taken specific care over the volatile element analysis in the MI (e.g. fluorine, chlorine) and post-analysis verifications of their quality such as the identification of misplaced beam-craters.

Post-entrapment crystallization correction

The composition of a melt inclusion can be modified by crystallization of olivine from the wall of the melt inclusion after its formation, by crystallization of minerals inside the melt inclusions during slow cooling or by overdissolution of the olivine wall during heating stages. In order to account for these effects, after homogenization of the melt inclusions, the major element compositions of the melt inclusions were corrected numerically by dissolving/adding equilibrium olivine through iterative steps, recalculating the melt composition at every step until the melt reached Fe-Mg exchange equilibrium with the host olivine (Le [START_REF] Voyer | Petrogenesis of arc lavas from the Rucu Pichincha and Pan de Azucar volcanoes (Ecuadorian arc): Major, trace element, and boron isotope evidences from olivine-hosted melt inclusions[END_REF]. We used a Kd Fe-Mg model to calculate the olivine composition added to the MI [START_REF] Toplis | The thermodynamics of iron and magnesium partitioning between olivine and liquid: criteria for assessing and predicting equilibrium in natural and experimental systems[END_REF]. For the model calculation, we adopted the average homogenization temperature (1218 ± 15°C, min-max temperature range of 1143-1309ºC) and an oxidation state calculated following [START_REF] Wallace | Sulfur in basaltic magmas[END_REF]. The speciation of sulfur was measured using the S 2-and S 6+ peak positions in the melt inclusions obtained by the electron microprobe [START_REF] Jugo | Experimental data on the speciation of sulfur as a function of oxygen fugacity in basaltic melts[END_REF]. The Fe +3 /∑Fe ratios for Puñalica melt inclusions vary from 0.16 to 0.23, while for Sangay the ratio is 0.28 for all melt inclusions (Table A1, Online Supplementary Material 1). The resulting post-entrapment olivine crystallization correction for most melt inclusions was less than 10%, except for "BC" and "K" melt inclusions that reach 17 and 22 % respectively. In spite of the large corrections for these two MIs, we kept them in our database because their compositions are coherent with the others and thus they do not change our conclusions.

Results

Whole-rock petrological background of Sangay and Puñalica

Sangay samples display a high-K calc-alkaline magmatic series spanning from basaltic andesites to dacites (53.6-68.1 wt.% SiO2, Fig. 2) and include a primitive basalt (SAN20B) that plots at lower silica content than the whole Sangay series (Fig. 2,3). This latter is the most primitive basalt that has so far been sampled in the NVZ [START_REF] Monzier | Sangay volcano, Ecuador: structural development, present activity and petrology[END_REF]) with 50.3 wt.% SiO2, 11.1 wt.% MgO, and a Mg# of 69 (Fig. 2,3; Table 2). The high Ni, and Cr contents (273 ppm Ni, 410 ppm Cr) and the relatively low Al2O3 concentration (14.8 wt.%) corroborate its primitive character. The trace element abundances in SAN20B basalt show a typical arc pattern (e.g. enriched LREE and LILE, negative HFSE anomaly) and consistently plot at the primitive end of the Sangay series. It is worth noting that the Nb content in SAN20B basalt is slightly higher than that of the other NVZ basalts and basaltic andesites. [START_REF] Monzier | Sangay volcano, Ecuador: structural development, present activity and petrology[END_REF] interpreted this feature as the result of partial melting of a relatively enriched mantle source. SAN20B mineralogy consists of phenocrysts of Mg-rich olivine (7 vol.%; up to 1 mm), and euhedral clinopyroxene (3 vol.%, 0.4-0.5 mm) surrounded by an intersertal groundmass consisting of clinopyroxene and plagioclase microlites. Olivine phenocrysts frequently include Cr-rich spinel inclusions. The Mg-rich olivines (Fo86-90) from this sample were selected for this study.

Puñalica lavas are medium-K calc-alkaline basaltic andesites and andesites (54.6-60.1 wt.% SiO2, Fig. 2, 3; Table 2), including some primitive andesites characterized by Mg# >60. On the basis of the MgO content, two groups are identified. A first group includes samples with low silica content (up to 57 wt.% SiO2) and variable MgO concentrations contents (4.8-6.2 wt.%). It is worth noting that the first group display large incompatible elements variations, namely for LREE (e.g. 15-35 ppm La) and LILE (4-9 ppm Th). The selected Puñalica samples correspond to both groups (Fig. 2, 3; Table 2). They show variable phenocryst abundances of olivine (Fo80-86), orthopyroxene, clinopyroxene and plagioclase surrounded by an intersertal groundmass that includes plagioclase and clinopyroxene microlites. Olivine (up to 7 vol.%, up to 1 mm) forms euhedral and subhedral phenocrysts with homogeneous compositions.

Melt inclusion compositions

Major elements

Olivine-hosted (Fo86-89, Fig. 4a) melt inclusions from Sangay volcano are nepheline-normative (from 14.9 to 21.1, Ne, Table 1) and are characterized by restricted concentrations in SiO2 (from 41.9 to 44.5 wt.%, expressed in a volatilefree basis, Fig. 3), MgO (6.4-9.1 wt.%) and FeOt (7.7-10.6 wt.%), as well as homogeneous Mg# values ranging from 59 to 64 (Fig. 3,4b). Sangay MIs mostly plot in the basaltic field belonging to a shoshonitic series (Fig. 2). In most Harker diagrams, Sangay MI compositions do not form an extension of the whole-rock trend (Fig. 3), showing higher contents in Al2O3, CaO and TiO2 than their host lava. Sangay MI represent, together with Pan de Azúcar melt inclusions studied by [START_REF] Voyer | Petrogenesis of arc lavas from the Rucu Pichincha and Pan de Azucar volcanoes (Ecuadorian arc): Major, trace element, and boron isotope evidences from olivine-hosted melt inclusions[END_REF], the most SiO2-poor magmas found in the Ecuadorian volcanic arc. Olivine-hosted (Fo82-86, Fig. 4a) melt inclusions from Puñalica display high-K calc-alkaline basaltic to basaltic andesitic compositions (Fig. 2), which are characterized by a variable content of SiO2 (from 46.3 to 56.2 wt.%), MgO (3.9-10.2 wt.%) and FeOt (5.4-15.2 wt.%) (Fig. 3). The Mg# values vary from 53 to 58 (Fig. 4). As at Sangay, Puñalica MI have lower SiO2 contents than Puñalica whole-rocks and do not form a continuous trend with them. Puñalica MI show higher contents of K2O, Na2O, FeOt and lower contents of TiO2 and CaO than the values expected from the whole-rock trend at the same SiO2 content (Fig. 2 and3).

Trace and volatile elements

Sangay MI show trace element patterns typical of arc lavas and similar to those of Puñalica MI, except for the relatively high values of Ti and HFSE (e.g. Ta, Nb), expressed as a less marked negative anomaly in Nb (Fig. 5). The trace element patterns are also similar to those displayed by Sangay whole-rocks. Sangay MI have (La/Yb)N values ranging from 7 to 15 (normalized to primitive mantle, [START_REF] Sun | Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes[END_REF]. Puñalica MI are also enriched in LILE (e.g. Ba, K, Rb, Sr) and depleted in HREE (e.g. Yb) relative to MORB. The enrichment in LREE (e.g. La) relative to HREE is represented as (La/Yb)N and ranges from 5 to 45. These MI also display a marked negative anomaly in HFSE (e.g. Nb, Fig. 5). Three different MI groups are distinguished at Puñalica volcano based on trace and volatile elements concentrations (see below).

Sangay MI are volatile-rich, with H2O between 2.6 and 3.2 wt.% and CO2 between 2776 and 6088 ppm. Puñalica melt inclusions vary from 0.05 wt.% to 3.1 wt.% H2O and from 29 to 4307 ppm CO2 (Table 1). H2O concentrations measured in melt inclusions are considered as minimum values because melt inclusions are leaky containers for H2O [START_REF] Chen | The rate of water loss from olivine-hosted melt inclusions[END_REF][START_REF] Gaetani | Rapid re-equilibration of H2O and oxygen fugacity in olivine-hosted melt inclusions[END_REF]). CO2 concentrations are also considered minimum because melt inclusions could suffer from pre-entrapment CO2 degassing due to degassing of CO2 from the magma at depth. Sangay MI are characterized by relatively high S concentrations (3836-5473 ppm), which overlap with the S concentrations of some Puñalica MI (most of them from CAR113B, Table 1). Melt inclusions from RIO17A have less S, between 2310 and 4335 ppm, with two inclusions more degassed in S at 906 and 1457 ppm. Samples from CAR83A have the least S (between 609 and 1571ppm). The total range of S contents in melt inclusions from Sangay and Puñalica (609-5479 ppm) is similar to that of melt inclusions from other continental subduction zones such as Colima, Mexico (400-4900 ppm S; [START_REF] Vigouroux | Volatiles in High-K Magmas from the Western Trans-Mexican Volcanic Belt: Evidence for Fluid Fluxing and Extreme Enrichment of the Mantle Wedge by Subduction Processes[END_REF]) and Mt. Shasta, California (in melt inclusions from High Alumina Olivine Tholeite, HAOT: 842-4800 ppm S; Le [START_REF] Voyer | Two contrasting H2O-rich components in primary melt inclusions from Mount Shasta[END_REF]. Sangay MI are richer in F (627-1183 ppm) than melt inclusions from Puñalica (samples CAR96A, RIO17A and CAR83A, 439-1008 ppm, Table 1). Cl is the most contrasted volatile element. Sangay and Puñalica CAR83A melt inclusions have low and similar Cl concentrations (1636-2257 ppm and 1352-1948 ppm, respectively), while all other Puñalica MI are Cl-rich, with concentrations between 4021-6112 ppm (Table 1). The Cl compositions of Sangay and most Puñalica MI are extremely high (maximum 6112 ppm Cl), only exceeded by rare melt inclusions (<9000 ppm) from Izu [START_REF] Straub | The systematics of chlorine, fluorine, and water in Izu arc front volcanic rocks: Implications for volatile recycling in subduction zones[END_REF], Aeolian [START_REF] Métrich | The 1989-1990 activity of Etna magma mingling and ascent of H2O-Cl-S-rich basaltic magma. Evidence from melt inclusions[END_REF][START_REF] Métrich | 2001 flank eruption of the alkali-and volatile-rich primitive basalt responsible for Mount Etna's evolution in the last three decades[END_REF][START_REF] Vaggelli | Silicate-melt inclusions in recent Vesuvius lavas (1631-1944): II. Analytical chemistry[END_REF][START_REF] Clocchiatti | Possible deep origin of sulfur output at Vulcano (Southern Italy) in the light of melt inclusion studies[END_REF][START_REF] Schiano | The relationship between potassic, calc-alkaline and Na-alkaline magmatism in South Italy volcanoes: A melt inclusion approach[END_REF][START_REF] Spilliaert | S-Cl-F degassing pattern of water-rich alkali basalt: modelling and relationship with eruption styles on Mount Etna volcano[END_REF][START_REF] Collins | Melt inclusions track pre-eruption storage and dehydration of magmas at Etna[END_REF][START_REF] Koga | Mantle source heterogeneity for South Tyrrhenian magmas revealed by Pb isotopes and halogen contents of olivine-hosted melt inclusions[END_REF]Sorbadère et al. 2013) andVanuatu arcs (Sorbadère et al. 2011;2013). F concentrations are the highest found in arc melt inclusions to date. Overall, the halogen compositions (F and Cl) of the Sangay and Puñalica melt inclusions are high for continental arc settings, in the same range as that of the melt inclusions from the HAOT of Mt. Shasta, while Puñalica melt inclusions have the highest Cl concentrations so far reported for this type of geodynamic setting.

Classification of Puñalica melt inclusion groups

Based on the description of the chemical compositions of the Puñalica melt inclusions, we have defined three different groups. All these melt inclusions are trapped in Fo>82 olivines and most of them have a lower SiO2 content than the host lavas.

Group 1 have the highest SiO2 and the lowest MgO and FeOt contents and plot close to the trend drawn by the wholerocks in Harker and trace element diagrams (Fig. 3 and5). Group 1 consists of four MI found in sample CAR83A, which have similar trace element patterns to those of the whole-rocks, and the lowest LREE (e.g. La, Ce) values normalized to primitive mantle (Fig. 5). They are trapped in Fo83-84 olivines. Their (La/Yb)N values vary from 5 to 14.

Group 1 is also characterized by the lowest contents of Cl (1350-1950 ppm), F (439-560 ppm) and S (610-1570 ppm) compared to other groups of MI from Puñalica.

Group 2 has similar compositions to Group 3 for FeOt, MgO and SiO2. Group 2 consists of twenty-six MI from CAR113B and RIO17A samples. Group 2 MI are enclosed in Fo84 olivines. This group of MI show intermediate Cl (4037-6112 ppm) and F (483-1000 ppm) contents and (La/Yb)N values that vary from 9 to 25. A marked difference between CAR113B and RIO17A MI is the elevated S content, as CAR113B MI contains 1635 to 6738 ppm S and RIO17A from 906 to 4335 ppm S.

Group 3 is depleted in Al2O3 (Fig. 3) and enriched in trace elements compared to Groups 1 and 2. Group 3 consists of twelve MI from CAR96A. The olivine crystals are Fo83. Group 3 MI are characterized by high (La/Yb)N values that vary from 13 to 45, high Sc (up to 46 ppm, Groups 1 and 2 up to 23 ppm), high F (from 596 to 2187 ppm) and similar Cl (3460-5985 ppm) contents to those of Group 2. Moreover, Group 3 MI have LREE primitive mantle-normalized values (e.g. La, Ce), higher than the Puñalica whole-rocks (Fig. 5). Group 3 has the highest F concentrations (even compared to Sangay melt inclusions), all between 922 and 2187 ppm, except for one MI at 596 ppm (Table 1).

Discussion

Magmatic processes prior to entrapment

Melt inclusions may record dissolution-reaction-mixing (DRM) processes within the magmatic plumbing system [START_REF] Danyushevsky | Melt Inclusions in Primitive Olivine Phenocrysts: the Role of Localized Reaction Processes in the Origin of Anomalous Compositions[END_REF]). These types of inclusions are commonly trapped in Fo>85 olivines, and are numerous, often clustered in single crystals and are due to rapid crystallization. Such characteristics differ significantly from those of our samples. Puñalica and Sangay melt inclusions are trapped in primitive olivines, and the melt compositions are independent of the Fo content of the host crystal. Melt inclusions in our samples were scarce, and we never found groups of melt inclusions in the same crystal as reported by [START_REF] Danyushevsky | Melt Inclusions in Primitive Olivine Phenocrysts: the Role of Localized Reaction Processes in the Origin of Anomalous Compositions[END_REF]. Based on these textural and compositional characteristics, the melt inclusions represent a primary inclusion character that formed at the time of the first growth of olivine. Thus, the compositions of the melt inclusions reported here have captured primitive melts that are different from host lavas. Our homogenization procedure and post-entrapment crystallization correction closely reconstitutes such primitive melt compositions at the time of capture, except for the abundance of H2O and CO2 (which must be considered as minimum values).

Primitive magmas determined from melt inclusions

Assessment of post-entrapment modifications

Puñalica and Sangay melt inclusions are trapped in Fo82-86 and Fo86-89 olivines respectively. Compositional corrections for post-entrapment crystallization and degassing of melt inclusions are viable only for inclusions efficiently isolated from the influence of the external magma surrounding the host olivine. In fact, the composition of melt inclusions in readily exchanged major elements is susceptible to irreversible, post-entrapment modifications.

Specifically, the host olivine can continuously exchange Fe and Mg with the surrounding magma, potentially leading to "Fe-loss" when comparing the melt inclusion composition to that of the whole-rock. Therefore, the exact reconstruction of the FeO and MgO contents of the parental magma cannot be achieved [START_REF] Danyushevsky | Re-equilibration of melt inclusions trapped by magnesian olivine phenocrysts from subduction-related magmas: petrological implications[END_REF][START_REF] Gaetani | Open system behavior of olivine-hosted melt inclusions[END_REF][START_REF] Danyushevsky | Melt Inclusions in Olivine Phenocrysts: Using Diffusive Re-equilibration to Determine the Cooling History of a Crystal, with Implications for the Origin of Olivine-phyric Volcanic Rocks[END_REF]). However, most MI extend the whole-rock trend towards low silica compositions (excepting those from Puñalica Group 1 and Z, T and U from Group 2, Table 1), thus it seems reasonable to postulate that Fe-Mg exchange is limited. Other species, such as H + in melt inclusions, can also re-equilibrate with an external melt through the host olivine crystal and therefore the H2O contents of melt inclusions represent minimum H2O values and do not systematically represent either the initial H2O concentrations at the time of entrapment or the primitive H2O abundance [START_REF] Chen | The rate of water loss from olivine-hosted melt inclusions[END_REF][START_REF] Gaetani | Rapid re-equilibration of H2O and oxygen fugacity in olivine-hosted melt inclusions[END_REF][START_REF] Gaetani | Hydration of mantle olivine under variable water and oxygen fugacity conditions[END_REF]. Melt inclusions from this study are therefore considered as representative of minimum H2O concentrations of Puñalica and Sangay magmas and we will not use them in the rest of the discussion. Because trace elements (e.g. the rare earth elements) diffuse significantly more slowly than H + , Fe, and Mg [START_REF] Cherniak | REE diffusion in olivine[END_REF], abundances of such elements should closely resemble those at the time of entrapment following the crystallization correction (see section on post-entrapment crystallization correction in the supplementary material).

Thus, the ratios of incompatible trace elements remain unmodified, as they are not partitioned into olivine, and are therefore representative of the primitive parental magma.

Lavas and magmatic inclusions, are they related?

A petrogenetic decoupling between olivine-hosted melt inclusions and the host eruptive products (lavas and tephra) is clearly seen in Harker diagrams (Fig. 3). If the parental magma of the erupted products crystallized the olivines (with their inclusions), as it crossed the liquidus, the melt inclusion and lava/tephra compositions should plot along the olivine-controlled liquid line of descent (Fig. 3). We show the liquid line of descent calculated with R-MELTS algorithm (Gualda et al. 2015) from the more primitive MI compositions for both Puñalica (solid line) and Sangay (dashed line) magmatic suites, with a fixed pressure of 400 MPa, a fO2 at NNO and 4 wt.% H2O. The liquid-line-ofdescent curves, for higher pressure values (i.e. 800 MPa, not shown), would display similar results. In fact, given that the first crystallization stages of Puñalica and Sangay MI are mainly controlled by olivine fractionation, there are almost no changes with pressure. Therefore, the Puñalica and Sangay host eruptive products cannot be derived from their respective melt inclusions by either simple olivine or olivine + clinopyroxene + plagioclase fractionation.

The projection from olivine into a Di-Ne-Q ternary plot (Fig. 6) yields additional constraints on the genesis of these MI. This projection from the olivine apex allows us to avoid any correction effect or chemical exchange between the melt inclusion and the host olivine (e.g. Sorbadère et al. 2013a, b). When plotted on the Di-Ne-Q plane, the studied Puñalica whole-rock samples are hypersthene-normative, whereas most of the 42 olivine-hosted (Fo82-86, Fig. 4a) melt inclusions from Puñalica volcano are nepheline normative (up to 10.1 Ne, Table 1); only six (AA, AC, AG1, K, S1 and BB) are hypersthene normative. Sangay whole-rock (SAN20B) and melt inclusions are all nepheline normative (Fig. 6).

Magma compositions of low pressure crystallization sequences (especially at anhydrous conditions) cannot plot across the boundary separating nepheline and hypersthene normative magmas, which is defined by the Di-An join (Fig. 6).

Typical mineral phases crystallizing from a basaltic magma (ol+cpx+plg) define a Fo-Di-An plane, which is projected on the Di-Ne-Q plane as a line joining Di-An, and magmas plotting to the left of the line can only evolve further to the left with crystallization, and vice-versa for the right. Occurrence of these Ca-rich, Ne-normative primitive melts in continental arcs, like at Sangay, are scarce and so far have only been reported at Mt. Shasta, Cascades (Le Voyer et al. [START_REF] Métrich | Transfer of sulfur in subduction settings: An example from Batan island (Luzon volcanic arc, Philippines)[END_REF], Lombok, Indonesia [START_REF] Elburg | The origin of medium-K ankaramitic arc magmas from Lombok (Sunda arc, Indonesia): Mineral and melt inclusion evidence[END_REF]) and Pan de Azucar and Pichincha, Ecuador (Le [START_REF] Voyer | Petrogenesis of arc lavas from the Rucu Pichincha and Pan de Azucar volcanoes (Ecuadorian arc): Major, trace element, and boron isotope evidences from olivine-hosted melt inclusions[END_REF]).

An alternative process to evolve magmas from the nepheline to hypersthene normative fields is fractional crystallization of silica-undersaturated minerals. For instance, amphibole crystallization can significantly increase the silica content in the residual liquid, eventually causing it to cross the nepheline-hypersthene divide (e.g. [START_REF] Frezzotti | Silicate-melt inclusions in magmatic rocks: applications to petrology[END_REF].

It is possible to examine whether or not such processes are responsible for deriving the erupted lavas from the melt inclusion compositions by investigating trace element abundances and their variations. Because amphibole preferentially incorporates Dy over Yb and La, the role of amphibole during differentiation is illustrated by a La/Yb increase and a decrease in Dy/Yb during differentiation [START_REF] Davidson | Amphibole "sponge" in arc crust?[END_REF]). The Dy/Yb (not shown) is highly variable in all MI and does not form a clear negative correlation with SiO2. In addition to REE systematics, other observations argue against a role of amphibole fractionation in these inclusions. Significant amphibole fractionation is not expected in a basalt or primitive andesite at high temperature (e.g. [START_REF] Krawczynski | Amphibole stability in primitive arc magmas: effects of temperature, H2O content, and oxygen fugacity[END_REF]) such as that recorded in the inclusions (Thomogenization=1218 °C).

In conclusion, these observations indicate that the melts trapped in olivine crystals are not the parental magmas of the erupted lavas. Puñalica and Sangay melt inclusions represents primitive melts that are not related to host eruptive products, extending the magmatic series towards low silica contents.

Mantle sources for Puñalica and Sangay primitive magmas

Experimental studies have striven to characterize the different lithologies melting under arcs (see Fig. 6 caption for references). The experimentally-determined fields for magma compositions issued from melting of different mantle lithologies (peridotite, pyroxenite) are reported in Fig. 6 for pressures between 0.5 and 3.5 GPa (see figure caption for references). Puñalica melt inclusions plot in the field of peridotite melts (dotted field in Fig. 6) for different pressures.

Group 1 and Group 2 MI have compositions similar to peridotite-melts coming from lower pressures (up to 2 GPa; Fig. 6) than Group 3 (up to 3.5 GPa). Such a discrepancy is mainly due to lower Al2O3 contents of Group 3 MI (Fig. 3c, Fig. 7), which is in agreement with peridotite melting experiments (e.g. [START_REF] Walter | Melting of garnet peridotite and the origin of komatiite and depleted lithosphere[END_REF]). These experiments have shown that garnet stability increases with pressure, reducing the Al2O3 content in the partial melt. The stability of garnet in Group 3 MI is also shown by an increase in La/Yb when Al2O3 decreases, creating a negative correlation (Fig. 7). Therefore, the chemical differences recorded by the Ne-normative Puñalica MI could be explained by variable depths of partial melting and thus the mineralogy of the mantle, with garnet probably present in Group 3 MI and spinel/plagioclase for Group 1 and 2 sources, implying a deeper origin for Group 3 compared to Groups 1 and 2.

The Sangay MI plot in the field of amphibole-bearing clinopyroxenite-derived melts (Fig. 6) occurring at pressures lower than 1.5 GPa and temperatures between 1200-1300°C [START_REF] Médard | Melting of Amphibole-bearing Wehrlites: an Experimental Study on the Origin of Ultra-calcic Nepheline-normative Melts[END_REF]. The homogenization temperature of Sangay MI varies from 1189 to 1254°C and is therefore in good agreement with the temperatures stipulated for amphibole-bearing clinopyroxenite-derived melts. The high CaO, relatively high FeOt, low silica contents and high Sc contents (from 26 to 43 ppm, Table 1) of Sangay MI, are also in agreement with compositions coming from the partial melting of an amphibole-bearing clinopyroxenite lithology. In fact Sorbadère et al. (2013) proposed that compositions similar to those of Sangay MI could be produced by mixing between partial melts of amphibole-bearing clinopyroxenite and peridotite. The compositions of the Sangay MI correspond to an end-member of the amphibole-bearing clinopyroxenite-derived melt. Similar Ca-rich, Ne-normative melt inclusions from Pan de Azúcar volcano, located in the back-arc of the Ecuadorian Arc, were also interpreted as the result of melting of pyroxenite-rich lithologies (Le [START_REF] Voyer | Petrogenesis of arc lavas from the Rucu Pichincha and Pan de Azucar volcanoes (Ecuadorian arc): Major, trace element, and boron isotope evidences from olivine-hosted melt inclusions[END_REF].

The presence of clinopyroxenitic lithologies in arc sources has been widely discussed in the literature (e.g. [START_REF] Schiano | Primitive CaO-rich, silica-undersaturated melts in island arc: Evidence for the involvement of clinopyroxene-rich lithologies in the petrogenesis of arc magmas[END_REF][START_REF] Médard | Melting of Amphibole-bearing Wehrlites: an Experimental Study on the Origin of Ultra-calcic Nepheline-normative Melts[END_REF]Sorbadère et al. 2013a, b). Clinopyroxenite is probably found in the lower crust since the differentiation process of primitive basalt produces ultramafic cumulates (i.e. clinopyroxenites), allowing the magma to evolve toward andesitic compositions (e.g. [START_REF] Greene | A Detailed Geochemical Study of Island Arc Crust: the Talkeetna Arc Section, South-Central Alaska[END_REF][START_REF] Blatter | Crystallization of oxidized, moderately hy-drous arc basalt at mid-to lower-crustal pressures: implications for andesite genesis[END_REF]Nandeckar et al. 2014). These ultramafic cumulates react with new ascending hydrous magmas triggering amphibole crystallization at the expense of clinopyroxene [START_REF] Smith | Clinopyroxene precursors to amphibole sponge in arc crust[END_REF], producing the amphibole-bearing clinopyroxenites. Then, the amphibole-bearing clinopyroxenite is melted when a new hot primitive magma (T > 1100°C) ascends through the clinopyroxenite.

Alternatively, the amphibole-bearing clinopyroxenite delaminates and sinks into the hotter mantle wedge [START_REF] Smith | Clinopyroxene precursors to amphibole sponge in arc crust[END_REF][START_REF] Jagoutz | Role of Arc Processes in the Formation of Continental Crust[END_REF][START_REF] Schmidt | The global systematics of primitive arc melts[END_REF]. The lower solidus temperature and higher melt productivity of pyroxenites [START_REF] Hirschmann | A possible role for garnet pyroxenite in the origin of the "garnet signature[END_REF] would promote olivine growth and enable entrapment of amphibole-bearing clinopyroxenite-derived melts.

Constraining the slab components by trace element systematics

Previous experimental studies have demonstrated the variable mobility of LILE, HFSE, REE and other trace elements (e.g. Li, Be) during the dehydration (low temperature) or melting (high temperature) process of the basaltic oceanic crust and/or its sedimentary cap [START_REF] Kessel | Trace element signature of subduction-zone fluids, melts and supercritical liquids at 120-180 km depth[END_REF][START_REF] Klimm | Trace Element Partitioning and Accessory Phase Saturation during H2O-Saturated Melting of Basalt with Implications for Subduction Zone Chemical Fluxes[END_REF][START_REF] Hermann | Accessory phase control on the trace element signature of sediment melts in subduction zones[END_REF]. These studies show that aqueous fluids resulting from dehydration processes are enriched in fluid-mobile elements (e.g. Ba, Rb, Cs, Pb) and strongly depleted in fluid-immobile elements (e.g. Be, Nb, La, Th). In contrast, hydrous siliceous melts, resulting from a melting process, are enriched in both fluid-mobile and fluid-immobile elements. Based on this dichotomy, fluid-mobile/fluid-immobile element ratios have been used to identify the nature of the slab components (e.g. [START_REF] Elliott | Element transport from slab to volcanic front at the Mariana arc[END_REF][START_REF] Elliott | Tracers of the slab[END_REF]. Fig. 8a, b shows plots of Sr/Th and Ba/Th, respectively vs. (La/Yb)N of the melt inclusions. Because the mobile/immobile element ratio (e.g. Ba/Th, Sr/Th) of the pre-metasomatized mantle wedge (i.e. MORB composition) is low, the addition of aqueous fluids from the slab increases this ratio. In contrast, mantle metasomatism by hydrous silicate melts does not change the ratio due to the fact that both groups of elements (fluid mobile and immobile) are transported by siliceous melts. The melt inclusions from Puñalica plot at relatively low Ba/Th values (60 to 190, average 90 ± 34) compared to the range of word-wide primitive arc magmas (Fig. 8b, dotted area).

There is no significant contrast in Sr/Th between Groups 1, 2 and 3 of Puñalica MI (Fig. 8a). However, the three Puñalica groups have distinct and relatively constant Ba/Th ratios within each group (Fig. 8b). The combination of the low Ba/Th and (La/Yb)N (values ranging from 10 to 40) for Puñalica melt inclusions is characteristic of magma most likely derived from the partial melting of a source metasomatized by hydrous siliceous melts. The mean values of Group 3 MI compositions for Ba/La~15, Ba/Th~66 and Th/La~0.22 contrast with the ratios obtained for Sangay MI (Ba/La~29, Ba/Th~261 and Th/La~0.11), and are in agreement with those reported by Brandt et al. (2017a) for the northern segment of Payenia volcanic field located in the Andean Southern Volcanic Zone (Ba/La=16, Ba/Th=63, Th/La=0.25) whose source is also interpreted as having been enriched by siliceous melts.

On the other hand, Ba/Th values from Sangay melt inclusions are around three times higher than those of Puñalica (from 230 to 280, average 261 ± 17) and Sr/Th ratio is also high with values between 444 to 594 (Fig. 8a; Puñalica between 103 and 209). Although not exceptionally high, the higher values for both Sr/Th and Ba/Th in Sangay melt inclusions suggests that aqueous fluids might have metasomatized the source of Sangay. This aqueous fluid would have been enriched in fluid mobile elements such as Ba, Sr, Pb. In a previous whole-rock study on Pichincha Volcanic Complex (north of the Ecuadorian arc) a threshold value of Ba/Th ~ 200 was used to distinguish magmas metasomatized by aqueous fluids (Ba/Th > 200) from hydrous siliceous melts [START_REF] Samaniego | Evolving metasomatic agent in the Northern Andean subduction zone, deduced from magma composition of the long-lived Pichincha volcanic complex (Ecuador)[END_REF]. Based on this threshold value, the Ba/Th ratios of Sangay melt inclusions can be taken as resulting from aqueous fluid metasomatism.

We would stress that in the whole-rock studies at the southern termination of the Ecuadorian arc, this discrimination is not evident (e.g. [START_REF] Ancellin | Across-arc versus along-arc Sr-Nd-Pb isotope variations in the Ecuadorian volcanic arc[END_REF], possibly because of the overprint by crustal contamination that has blurred the geochemical signal, whereas olivine-hosted melt inclusions are shielded from this contamination.

Modelling the metasomatized mantle source

In order to test the hypothesis that two different subduction components are injected from the slab into the mantle wedge, we performed a three-step trace element modeling procedure. Firstly, we considered the melting or dehydration of the oceanic crust for Puñalica and Sangay, respectively. Secondly, given that the sources for Puñalica and Sangay MI are different, we mixed the slab component with a depleted peridotite for Puñalica and with an amphibole-bearing clinopyroxenite composition for Sangay. Finally, we melted these enriched source lithologies using a batch melting equation (Shaw 1970).

1. For the basalt partial melting step, we used the high-temperature experimental Kd (1000°C) from [START_REF] Kessel | Trace element signature of subduction-zone fluids, melts and supercritical liquids at 120-180 km depth[END_REF] and the average composition of the Carnegie ridge basalts (Harpp et al. 2005). This step, with a melting degree set at 5% leaves an eclogitic residue. The degree of melting is imposed by the strong negative-Nb anomaly in the Puñalica MI. Such an anomaly is a characteristic only attained with >1 vol.% of rutile in the residue and accordingly to Gaetani et al. (2008), the rutile vol.% in the residue peaks at roughly 4% of melting. Between 4 and 5% melting does not change our conclusion. For sediment melting, we used the bulk distribution coefficients given by Plank (2005), and an average composition of sediments from the Nazca plate (Plank and Langmuir 1998). We set the melting degree at a fairly low value (i.e. 8 %). The dehydration process was modeled with the same parameters as for the melting except for Kd, which was that given by [START_REF] Kessel | Trace element signature of subduction-zone fluids, melts and supercritical liquids at 120-180 km depth[END_REF] at low temperature (800°C).

2. The trace element composition of the depleted peridotite is from Workman and Hart (2005), and that of the clinopyroxenite from [START_REF] Smith | Clinopyroxene precursors to amphibole sponge in arc crust[END_REF]. The mixing proportion between slab fluid or melt component and the depleted source varies from 10:90 to 15:85. When the slab component is a siliceous melt, the amount of sedimentary melt relative to basaltic melt varies in a proportion of 0 to 20%.

3. For Puñalica MI, the metasomatized mantle melt is 10%, leaving a residue of ol:opx:cpx in a proportion of 68:20:12 and a variable amount of grt (1-8 vol.%). We used the Kds given by Halliday et al. (2005) for basaltic melts. For Sangay, the minerals in the residue after 5% of melting are ol:cpx:hbl:sp in a proportion of 24:70:2:4. This mineral assemblage was computed by R-Melts software [START_REF] Gualda | Rhyolite-MELTS: A modified calibration of MELTS optimized for silica-rich, fluid-bearing magmatic systems[END_REF]) at 1 GPa and 1 wt.% H2O for an amphibolebearing clinopyroxenite composition (from [START_REF] Médard | Melting of Amphibole-bearing Wehrlites: an Experimental Study on the Origin of Ultra-calcic Nepheline-normative Melts[END_REF].

Results are plotted in Fig. 9 as multi-element diagrams normalized to primitive mantle values [START_REF] Sun | Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes[END_REF]. The Puñalica modelled composition fits very well with natural MI compositions, supporting the hypothesis that the metasomatic agent beneath Puñalica is a silicate melt (Fig. 9a). For Sangay, the modeled composition shows a reasonable agreement, although some differences (e.g. Rb, Nb) are highly source-dependent and can be explained by the scarce trace element data for amp-bearing clinopyroxenite compositions in the literature (Fig. 9b). Lastly, given the primitive signature of SAN20B whole-rock, we also modelled its origin as a result of a hydrous fluid-metasomatized peridotite (Fig. 9c). The parameters for dehydration were similar to those of Sangay MI modelling and the source was that of a depleted mantle (Workman and Hart 2005). The mixing proportion between the slab component and the mantle was fixed at 15:85 and the partial melting of the mantle at 7%. The relatively good fit indicates an aqueous fluid as the metasomatic agent responsible for the composition of the SAN20B olivine-hosted melt inclusions.

Constraints provided by the volatile element compositions on the nature of the slab flux in a continental subduction zone Sangay and Puñalica MI have high concentrations of fluorine and chlorine (Fig. 10), which is typical of volcanic arcs from continental subduction (e.g. Izu, [START_REF] Straub | The systematics of chlorine, fluorine, and water in Izu arc front volcanic rocks: Implications for volatile recycling in subduction zones[END_REF]Mt. Shasta, Cascades, Le Voyer et al. 2010; Vulcano, Italy and Iwate, Japan, Rose- [START_REF] Koga | Mantle source heterogeneity for South Tyrrhenian magmas revealed by Pb isotopes and halogen contents of olivine-hosted melt inclusions[END_REF][START_REF] Koga | Volatile (F and Cl) concentrations in Iwate olivine-hosted melt inclusions indicating low-temperature subduction[END_REF]. Puñalica Group 3 melt inclusions display amongst the highest F contents in continental arc-related MI (up to 2187 ppm F). This high F content is accompanied by high Cl concentrations (Cl commonly between 5000 and 6000 ppm in Puñalica MI). For comparison, an olivine-hosted melt inclusion from Reventador volcano has 3149 ppm chlorine and 549 fluorine [START_REF] Samaniego | Pre-eruptive physical conditions of El Reventador volcano (Ecuador) inferred from the petrology of the 2002 and 2004-05 eruptions[END_REF] and Tungurahua melt inclusions have less than 1081 ppm chlorine [START_REF] Myers | Replenishment of volatile-rich mafic magma into a degassed chamber drives mixing and eruption of Tungurahua volcano[END_REF].

High halogen contents in primitive arc magmas (the case of Puñalica) can be achieved by two processes (van den Bleeken and Koga 2015): (i) the subducted crust melts (rather than simply dehydrating), or (ii) the subducted crust is enriched in halogens and eventually melts/dehydrates. Thus, one explanation is that Puñalica volcano is roughly located above the Grijalva fracture zone and the higher permeability in the fracture zone enhances interactions with seawater (compared to a normal oceanic crust), thus increasing its F and Cl contents. Furthermore and because the Puñalica slab component seems to be a siliceous melt, this altered oceanic crust eventually melts driving F and Cl contents even higher (van den Bleeken and [START_REF] Van Den Bleeken | Experimentally determined distribution of fluorine and chlorine upon hydrous slab melting, and implications for F-Cl cycling through subduction zones[END_REF]. Both fluorine and chlorine are incompatible in the typical mineral phases of a garnet-bearing peridotite (van den Bleeken and Koga 2015), therefore, those elements would leave the mantle during partial melting.For an amphibole-bearing source lithology, as in the case of Sangay MI, fluorine compatibility must change because this element is compatible in amphibole, whereas chlorine is not. This preference of amphibole for fluorine would affect F/Cl ratios. Thus, Sangay F/Cl ratios (0.35-0.53) that are higher than those from Puñalica (0.11 to 0.39) could be explained by the interaction of ascending mantle-derived magma with an amphibole-bearing clinopyroxenite, likely enriched in F compared to Cl.

Slab components in the geodynamic context of the continental Ecuadorian arc and beyond

The origin of the metasomatic agent in the Ecuadorian arc has long been debated in the scientific literature. Some researchers consider that the metasomatic agent of the subarc mantle wedge is an aqueous fluid issued from dehydration reactions of the slab [START_REF] Barragan | Subduction controls on the compositions of lavas from the Ecuadorian Andes[END_REF][START_REF] Garrison | Dubious case for slab melting in the Northern volcanic zone of the Andes[END_REF][START_REF] Bryant | Geochemical Constraints on the Origin of Volcanic Rocks from the Andean Northern Volcanic Zone, Ecuador[END_REF][START_REF] Chiaradia | Adakite-like volcanism of Ecuador: lower crust magmatic evolution and recycling[END_REF]. Based on the overall adakitic signature of the Ecuadorian arc, other researchers consider that the metasomatic agent is a siliceous-melt associated with slab partial melting [START_REF] Bourdon | Magmatic response to early aseismic ridge subduction: the Ecuadorian margin case (South America)[END_REF][START_REF] Hidalgo | Role of crustal and slab components in the Northern Volcanic Zone of the Andes (Ecuador) constrained by Sr-Nd-O isotopes[END_REF]). In addition, some long-lived volcanic complexes of the Ecuadorian arc are characterized by a progressive variation of the trace element signature that has been interpreted as a change of the metasomatic agent throughout its evolution. This variation in whole-rock trace elements, described at Cayambe [START_REF] Samaniego | Transition from calc-alkalic to adakitic magmatism at Cayambe volcano, Ecuador: Insights into slab melts and mantle wedge interactions[END_REF][START_REF] Samaniego | Temporal Evolution of Magmatism in the Northern Volcanic Zone of the Andes: The Geology and Petrology of Cayambe Volcanic Complex (Ecuador)[END_REF]; Illinizas [START_REF] Hidalgo | Adakitic magmas in the Ecuadorian Volcanic Front: Petrogenesis of the Iliniza Volcanic Complex (Ecuador)[END_REF]), Mojanda-Fuya Fuya [START_REF] Robin | Evolution of the late Pleistocene Mojanda-Fuya Fuya volcanic complex (Ecuador), by progressive adakitic involvement in mantle magma sources[END_REF]) and Pichincha [START_REF] Samaniego | Evolving metasomatic agent in the Northern Andean subduction zone, deduced from magma composition of the long-lived Pichincha volcanic complex (Ecuador)[END_REF], implies that the older magmas of these volcanic complexes came from a hydrous-fluid metasomatized mantle wedge, whereas the younger magmas are related to partial melting of a siliceous-melt metasomatized mantle wedge. Such a temporal evolution suggests a change from dehydration to partial melting of the slab that could be associated with an increase in the thermal regime along the slab due to the presence of the subducted Carnegie ridge (e.g. [START_REF] Yepes | A new view for the geodynamics of Ecuador: implication in seismogenic source definition and seismic hazard assessment[END_REF]. In this context, the younger age and structure of the Carnegie ridge would induce an enhanced thermal gradient along the slab, which may trigger slab partial melting. This hypothesis is supported by thermal modeling studies (e.g. [START_REF] Syracuse | The global range of subduction zone thermal models[END_REF]) that estimate elevated slab-surface temperatures below the Ecuadorian arc.

In this work, we demonstrated that the subarc mantle below two of the southern Ecuadorian volcanoes is metasomatized by different slab components. The cause of such variation might be related to the geodynamical setting, where the Grijalva Fracture Zone (GFZ) sharply separates a young slab to the north from an older slab to the south. As recently pointed out by [START_REF] Yepes | A new view for the geodynamics of Ecuador: implication in seismogenic source definition and seismic hazard assessment[END_REF], this difference in age is coupled to a contrasted seismic behavior and an increase in the subduction angle: to the north of the GFZ, the slab displays low intermediary seismicity (50-150 km depth) and subducts at an angle of 20-25º, whereas to the south, the slab is highly seismogenic and plunges at a higher angle (30-35º). This contrasted seismic behavior could be associated with a different thermal regime on both sides of the GFZ [START_REF] Yepes | A new view for the geodynamics of Ecuador: implication in seismogenic source definition and seismic hazard assessment[END_REF], which in turn could control the dehydration, and the melting process happening at depth.

Based on whole-rock major, trace element, and Sr-Nd-Pb isotopic signature of the Ecuadorian arc lavas, [START_REF] Ancellin | Across-arc versus along-arc Sr-Nd-Pb isotope variations in the Ecuadorian volcanic arc[END_REF] describe a N-S variation of the metasomatic agent along the entire Ecuadorian volcanic arc. They interpreted these along-arc variations as an increase of the slab input northward and southward on both sides of a divide that they placed at 0.5ºS (i.e. at the latitude of the Carnegie ridge subduction). They concluded that the most likely process able to explain these variations is a variable influx of slab-derived fluids or melts. In the present olivine-hosted melt inclusion study, we refine the previous findings and argue that Puñalica MI come from a mantle wedge metasomatised by a hydrous siliceous melt, whereas Sangay MI are most likely produced by partial melting of a hydrous fluid metasomatized amphibole-bearing clinopyroxenite source.

In a more general context, the singularity of Sangay melt inclusions is striking in Fig. 6 and9. High-Ca, Nenormative primitive arc melts exist in other arc systems around the world (cf. the section on Primitive magmas determined from melt inclusions), but are not that common in continental subduction zone settings. However, whereas the crustal thickness in the previously mentioned arc systems (Cascades, Italy, Vanuatu) is around 20-30 km thick, the Ecuadorian continental arc is characterized by a very thick arc crust (> 50 km). The influence of crustal thickness on melt compositions is not new and has already been proposed for arc lavas (e.g. [START_REF] Wallace | Sulfur in basaltic magmas[END_REF]. In the case of Sangay MI, we therefore have to reconcile the amphibole-bearing clinopyroxenite source (Ca-rich, Nenormative melts), the high volatile (F and Cl) contents of MI and the high Fo of the host olivines, with the presence of a 50 km-thick crust. One hypothesis is that melting of the amphibole-bearing clinopyroxenite happens at depth, probably at the interface between the lower crust and the upper mantle. In order to accommodate the deep origin of the MI and the required fast migration to the surface, the occurrence of trans-crustal major faults seems an important requirement.

Conclusions

Melts inclusions from Puñalica and Sangay volcanoes are Ca-rich and have mostly Ne-normative compositions.

The magmas sampled as melt inclusions are not the parental magmas of the whole-rocks but they constitute exceptional compositions that show the complexity of the processes occurring at subduction zones and the heterogeneity of the sources of primitive arc magmas.

All melt inclusions from Puñalica have major element compositions similar to experimental melts derived from peridotite. Sangay MI have homogeneous compositions and their origin is attributed to mixing processes between melts derived from peridotite (represented by the whole-rock SAN20B composition) and amphibole clinopyroxene-rich sources.

The slab component that metasomatizes the mantle source of Puñalica volcano is a siliceous melt (low mobile/immobile ratios), which displays a variable enrichment in LREE. Moreover, the high content of volatiles (e.g. F, Cl) is attributed to the presence of the Grijalva Fracture Zone that separates a younger and hotter oceanic crust to the north from a colder and older oceanic crust to the south. Under Sangay volcano, the melt inclusions are produced by partial melting of metasomatized (most likely by aqueous fluids) amphibole-bearing clinopyroxenite source and the relatively higher F compared to Cl is likely due to fluorine enrichment in the source linked to fluorine compatibility with amphibole.

Ca-rich, Ne-normative primitive melts are scarce in subduction zone settings, specially those associated with thick continental crust, and could possibly be related to the presence of major trans-crustal faults. [START_REF] Jaillard | Sedimentary record of terminal Cretaceous accretions in Ecuador: The Yunguilla Group in the Cuenca area[END_REF]]. Analyses of melt inclusions were performed in rock samples from Sangay and Puñalica volcanoes (in red). Puñalica volcano is a small, young edifice associated with the older Carihuairazo volcano .

Figure captions

Pichincha Volcanic Complex (Pichincha) and Pan de Azúcar volcanoes, which are mentioned in the text, are in cyan.

White dashed line is the surface projection of the Grijalva fracture zone (GFZ) that affects subduction and separates old oceanic lithosphere to the south from young oceanic lithosphere to the north. This projection is based on the recent seismological work of [START_REF] Yepes | A new view for the geodynamics of Ecuador: implication in seismogenic source definition and seismic hazard assessment[END_REF] studying the flexure of the subducted slab along the GFZ The black arrows correspond to olivine fractionation lines calculated using R-MELTS program (Gualda et al. 2015), starting from one of the most primitive melt inclusions of each volcano, using a pressure of 4 kbar and an estimated water content of 2 wt. %. For description of symbols see Fig. 2 Fig. 4 Frequency histogram of the forsterite content of the olivine phenocrysts (a) and of melt inclusions (b) from Puñalica and Sangay volcanoes. Diagram (a) shows data from this study and that of [START_REF] Monzier | Sangay volcano, Ecuador: structural development, present activity and petrology[END_REF]. Forsterite contents for melt inclusions are plotted using homogenization-corrected values and Fe 2+ /∑Fe obtained using sulfur speciation. We did not differentiate the Puñalica groups [START_REF] Hidalgo | Role of crustal and slab components in the Northern Volcanic Zone of the Andes (Ecuador) constrained by Sr-Nd-O isotopes[END_REF] and [START_REF] Ancellin | Across-arc versus along-arc Sr-Nd-Pb isotope variations in the Ecuadorian volcanic arc[END_REF]] normalized to Primitive Mantle composition [START_REF] Sun | Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes[END_REF]. d REE spider diagram for Puñalica MI. N-MORB (dashed gray line) from [START_REF] Sun | Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes[END_REF] [START_REF] Hirose | Partial melting of dry peridotite at high pressures: determination of compositions of melts segregated from peridotite using aggregates of diamond[END_REF][START_REF] Baker | Determining the composition of high-pressure mantle melts using diamond aggregates[END_REF]Kushiro 1996;[START_REF] Falloon | Peridotite melting at 1 GPa: reversal experiments on partial melt compos-itions produced by peridotite-basalt sandwich experiments[END_REF][START_REF] Wasylenki | Near-solidus Melting of the Shallow Upper Mantle: Partial Melting Experiments on Depleted Peridotite[END_REF][START_REF] Laporte | A new experimental technique for extracting liquids from peridotite at very low degrees of melting: application to partial melting of depleted peridotite[END_REF]) and hydrous peridotites between 0.5 and 3.5 GPa (continuous lines are isobars with pressure given in GPa; [START_REF] Hirose | Hydrous partial melting of lherzolite at 1GPa: The effect of H2O on the genesis of basaltic magmas[END_REF][START_REF] Gaetani | The influence of water on melt-ing of mantle peridotite[END_REF], amphibole-free [START_REF] Kogiso | Experimental study of clino-pyroxenite partial melting and the origin of ultra-calcic melt inclu-sions[END_REF] and amphibole-bearing clinopyroxenites between 0.5 and 1.5 GPa [START_REF] Médard | Melting of Amphibole-bearing Wehrlites: an Experimental Study on the Origin of Ultra-calcic Nepheline-normative Melts[END_REF][START_REF] Pilet | Metasomatized Lithosphere and the Origin of Alkaline Lavas[END_REF] Fig. 7 La/Yb vs. wt.% Al2O3 of melt inclusions and whole-rocks. La/Yb ratio is explained by 1 to 8% of garnet in the mantle residue following the model proposed for Puñalica MIs (see text) Whole-rock data variation for Pichincha Volcanic Complex field is taken from [START_REF] Samaniego | Evolving metasomatic agent in the Northern Andean subduction zone, deduced from magma composition of the long-lived Pichincha volcanic complex (Ecuador)[END_REF]. Purple field corresponds to the older Rucu Pichincha magmatic suite whereas yellow field corresponds to the field of the younger Guagua Pichincha suite. MORB composition represented by the small black rectangle is from [START_REF] Leroux | The effects of variable sources, processes and contaminants on the composition of northern EPR MORB (8-10°N and 12-14°N): Evidence from volatiles (H2O, CO2, S) and halogens (F, Cl)[END_REF]. The primitive arc magma composition (dotted field) is taken from [START_REF] Elliott | Tracers of the slab[END_REF]. Symbols are those of Fig.

  2010), Vulcano, Italy[START_REF] Gioncada | A study of melt inclusions at Vulcano (Aeolian Islands, Italy): insights on the primitive magmas and on the volcanic feeding system[END_REF][START_REF] Koga | Mantle source heterogeneity for South Tyrrhenian magmas revealed by Pb isotopes and halogen contents of olivine-hosted melt inclusions[END_REF], Ambrym and Gaua, Vanuatu (Sorbadère et al. 2013a, b), Grenada and St Vincent, Lesser Antilles (Bouvier et al. 2010), Batan, Philippines

Fig. 1 a

 1 Fig. 1 a Geodynamical setting of Ecuadorian arc. b Schematic map of the main geological zones in Ecuador [modified from Jaillard et al. (2008)]. Analyses of melt inclusions were performed in rock samples from Sangay and Puñalica

Fig. 2 aFig. 3

 23 Fig. 2 a K2O vs. SiO2 classification diagram (Peccerillo and Taylor 1976) of melt inclusions (homogenizationcorrected compositions; colored diamonds and triangles) and whole-rocks (shaded areas and star symbols colored to the same as their corresponding MI) of Sangay and Puñalica volcanoes. 50 melt inclusion compositions are plotted; 7 belonging to SAN20B sample from Sangay volcano and the others belonging to Puñalica samples. Groups 1, 2 and 3 in the legend box correspond to the groups defined for Puñalica MI. See text for more details. b Molar 100 * (MgO / (FeOt + MgO) vs. SiO2 wt.% for melt inclusions and whole-rock lavas from Sangay and Puñalica volcanoes

Fig. 5

 5 Fig. 5 Trace element compositions of Puñalica (a-c) and Sangay (e) melt inclusions and whole-rock lavas [shaded areas,

Fig. 6

 6 Fig. 6 Projection from olivine (M2S) onto the Ne-Di-Qz (CMS2-S-CA) plane for whole-rock lavas and melt inclusions from Puñalica and Sangay. Schema using O'Hara (1976) CMAS component calculation. C = CaO -10/3P2O5 + 2Na2O + 2K2O, M = MgO + FeO + MnO -TiO2, A = Al2O3 + Fe2O3+ TiO2 + Na2O + K2O, S = SiO2 -2Na2O -2K2O. Also shown in the diagram are the fields compiled by Sorbadere et al. (2013a) of experimentally determined melts of anhydrous[START_REF] Hirose | Partial melting of dry peridotite at high pressures: determination of compositions of melts segregated from peridotite using aggregates of diamond[END_REF][START_REF] Baker | Determining the composition of high-pressure mantle melts using diamond aggregates[END_REF] Kushiro 1996;[START_REF] Falloon | Peridotite melting at 1 GPa: reversal experiments on partial melt compos-itions produced by peridotite-basalt sandwich experiments[END_REF][START_REF] Wasylenki | Near-solidus Melting of the Shallow Upper Mantle: Partial Melting Experiments on Depleted Peridotite[END_REF]

Fig. 8

 8 Fig. 8 Plot of (La/Yb)N normalized to primitive mantle (Sun and McDonough 1989) versus a Sr/Th and b Ba/Th.

  2

Fig. 9

 9 Fig. 9 Modeled composition (colored dashed lines) compared to natural melt inclusion (colored solid lines) from (a) Puñalica and (b) Sangay volcanoes. (c) Modeled composition for SAN20B whole-rock sample from Sangay volcano. Gray dot-dash lines are compositions of the non-metasomatized source lithologies: a depleted mantle (Workman and Hart, 2005) for Puñalica and SAN20B, and a clinopyroxenite (Smith 2014) for Sangay. The proportion of slab component compared to peridotite/clinopyroxenite in the mixing process is 10:90 for Puñalica MI, 12:88 for Sangay MI and 15:85 for SAN20B. The amount of sedimentary-origin melt in the slab component is 20% (only for Puñalica).

Fig. 10 F

 10 Fig.10F vs. Cl for Sangay and Puñalica melt inclusions. Our data is compared to other melt inclusion compositions compiled by Rose-[START_REF] Koga | Volatile (F and Cl) concentrations in Iwate olivine-hosted melt inclusions indicating low-temperature subduction[END_REF]. Melt inclusion compositions from Payenia backarc are fromBrandt et al. (2017b).

  ).

		A	B	C	D	E	E1	F	G	AA	AB	AC	AE	AF	AG
	SiO2	42.43	42.76	44.45	42.83	41.95	42.95	43.9	42.3	56.16	48.01	53.82	48.22	49.95	51.99
	TiO2	1.53	1.7	1.48	1.36	1.8	1.65	1.45	1.41	0.41	0.75	0.53	0.67	0.68	0.88
	Al2O3	19.11	18.44	16.59	17.09	18.53	18.82	18.02	18.75	15.32	14.41	15.88	14.67	13.25	15.93
	FeOt	8.92	9.02	10.56	10.16	8.75	7.66	8.82	8.51	8.39	15.16	9.37	14.26	12.1	8.48
	MnO	0.17	0.14	0.1	0.06	0.22	0.12	0.22	0.15	0.08	0.03	0.22	0.13	0.16	0.11
	MgO	8.6	8.17	8.72	9.11	7.69	6.38	7.53	8.3	5.44	9.52	6.07	9.19	8.01	5.77
	CaO	13.25	14.49	13.32	14.21	15.25	16.05	14.33	14.5	7.4	6.77	7.66	7.48	10	11.64
	Na2O	4.24	3.73	3.35	3.55	4.25	4.62	4.17	4.28	4.48	3.48	4.17	3.5	3.83	3.6
	K2O	1.32	1.12	0.99	1.02	1.19	1.31	1.13	1.22	1.97	1.53	1.91	1.54	1.72	1.36
	P2O5	0.44	0.45	0.44	0.62	0.37	0.44	0.44	0.57	0.35	0.34	0.36	0.33	0.3	0.25
	Total	94.56	94.75	93.82	95.3	96.39	98.36	96.41	92.92	99.27	99.15	99.68	98.35	97.6	99.3
	H2O			3.17	2.6					0.09	0.1	0.09	0.1	0.07	0.07
				2776	6088					252	625	201	658	826	123
	F (ppm)	913

Table 1

 1 Major, trace elements and volatile concentrations of olivine-hosted melt inclusions from Puñalica and Sangay lavas. Total given are the measured values. Major elements are the corrected values after wall olivine crystallization.

	3140	2829	3109	4805	2152

Acknowledgments

This research was conducted as part of Diego Narvaez's PhD, which is financed by the Secretaría Nacional de Educación Superior, Ciencia, Tecnología e Innovación of Ecuador (SENESCYT, Ecuador) and the ARTS program of the French Institut de Recherche pour le Developpement (IRD). It is part of a cooperation program carried out between the Instituto Geofısico, Escuela Politécnica Nacional (IGEPN), Quito, Ecuador and the IRD, through the Laboratoire Mixte International "Séismes et Volcans dans les Andes du Nord". ER-K acknowledges funding from the French INSU scientific program SYSTER. Thank you F. Van Wyk de Vries for This work also benefited from financial support from the Laboratory of Excellence ClerVolc. This is Laboratory of Excellence ClerVolc contribution n° XXXX.