Olaya Dorado
email: odorado@geo3bcn.csic.es

Joan Andújar
email: juan.andujar@cnrs-orleans.fr

Joan Martí
email: jmarti@geo3bcn.csic.es

Adelina Geyer
email: ageyer@geo3bcn.csic.es

Olaya Dorado García

Pre-eruptive conditions at satellite vent eruptions at Teide-Pico Viejo complex (Tenerife, Canary Islands)

Keywords: Teide-Pico Viejo, Tenerife, dome, phonolite, explosive-effusive Highlights

The Teide-Pico Viejo (T-PV) stratovolcanoes constitute one of the most potentially active volcanic complexes in Europe. T-PV was traditionally considered as a non-explosive system, but recent studies have pointed out the explosive character of their phonolitic magmas, including plinian and subplinian eruptions and the generation of pyroclastic density currents, as is evidenced in the volcanological record of the last 30 kyr. This explosive activity is mostly associated with satellite dome-like vents, which are characterised by presenting progressive transitions between explosive and effusive activity. In order to

improve our understanding of these types of eruptions and their controlling mechanisms, we conducted a petrological and mineral characterisation of the products from the different eruptive phases of Pico Cabras dome. This permitted us to constrain the pre-eruptive conditions of the magma and to infer the potential factors that control the changes in the volcanic activity of dome-forming eruptions at T-PV and, in particular, the transition between explosive and effusive phases.

Feldspar (anorthoclase and sanidine), clinopyroxene (diopside and augite), biotite, amphibole (kaersutite), magnetite, ilmenite, sodalite, and glass samples were analysed using EPMA and Micro-XRF. Our results suggest the presence of a compositionally, thermally, and volatile stratified magma chamber at 100 ± 50 MPa prior to the Pico Cabras eruption (between 9210 and 5911 years BP), which was characterised by a progressive change in the style of activity, from an efficiently fragmented magma dispersed by a sustained plume, to a poorly-fragmented, low-height, non-convective fountain (forming the fountain-fed lava dome) to a glass-bearing lava flow activity. The transition in the eruptive style was controlled by changes in temperature and amount of volatiles dissolved in the phonolitic melt (i.e., differences in magma rheology). The initial explosive phase was related to the emission of highly evolved phonolitic magma ponding at 725-800 ºC while containing 3.5-5 wt.% H 2 O and located in the upper part of the magma reservoir. The magma that fed the phonolitic lava flow formed the main body of the reservoir and was stored below the preceding melt at 880 ± 30 ºC and with 2.5-3 wt.% H 2 O dissolved. Feldspar zonation suggests that the eruption was triggered by the intrusion of hotter mafic magma at the base of the phonolitic chamber, which induced the overturn of the interior of the reservoir. However, the interaction between the internal magmas was short enough to prevent the hybridization phenomena among them. The compositions of some feldspars from the most explosive phase are equivalent to those found in the El Abrigo eruption, the last caldera-forming episode (ca. 190 ka), demonstrating that T-J o u r n a l P r e -p r o o f

Introduction

Eruption dynamics are assumed to be controlled mainly by the composition and physical properties of magma [START_REF] Cassidy | Controls on explosive-effusive volcanic eruption styles[END_REF][START_REF] Sparks | The dynamics of bubble formation and growth in magmas: A review and analysis[END_REF][START_REF] Wilson | Explosive volcanic eruptions --IV. The control of magma properties and conduit geometry on eruption column behaviour[END_REF]. They determine the amount of gases that magma may dissolve, the rate at which these gases will separate from magma when approaching to the surface and, consequently, whether the eruption will be mainly effusive or explosive. Totally effusive or explosive volcanic eruptions are not rare, but eruptions alternating between the two types of activity, or showing a complete gradation from highly explosive to highly effusive, are even more common and may indeed occur for all magma compositions. The causes that determine these changes in eruption dynamics have already been the subject of many studies. Different factors have been proposed to explain such evolution, including changes in magma degassing that occur both prior to and during the eruptions, variations in magma permeability, and differences in conduit properties (e.g., [START_REF] Cashman | Volatile controls on magma ascent and eruption[END_REF][START_REF] Cassidy | Controls on explosive-effusive volcanic eruption styles[END_REF][START_REF] Degruyter | The effects of outgassing on the transition between effusive and explosive silicic eruptions[END_REF][START_REF] Eichelberger | Non-explosive silicic volcanism[END_REF][START_REF] Gonnermann | Explosive volcanism may not be an inevitable consequence of magma fragmentation[END_REF][START_REF] Lavallée | Thermal vesiculation during volcanic eruptions[END_REF][START_REF] Woods | Transitions between explosive and effusive eruptions of silicic magmas[END_REF].

Teide and Pico Viejo (T-PV) stratovolcanoes, on the island of Tenerife (Canary Islands), have alternating effusive and explosive activities that show both abrupt and gradual transitions among them during their phonolitic period, which covers the last 30 ka [START_REF] Ablay | Stratigraphy, structure, and volcanic evolution of the Pico Teide-Pico Viejo formation, Tenerife, Canary Islands[END_REF][START_REF] Martí | Assessing the potential for future explosive activity from Teide-Pico Viejo stratovolcanoes (Tenerife, Canary Islands)[END_REF]. This phonolitic activity has been mostly central, but also includes numerous satellite flank eruptions with both effusive and explosive phases, which J o u r n a l P r e -p r o o f are characterised by the construction of large dome-like structures. In calc-alkaline magmas, such an occurrence of effusive and explosive phases in dome-forming eruptions is common, having been described for many volcanoes (e.g., [START_REF] Boudon | What factors control superficial lava dome explosivity?[END_REF][START_REF] Lavallée | Thermal vesiculation during volcanic eruptions[END_REF][START_REF] Martel | Textures, water content and degassing of silicic andesites from recent plinian and dome-forming eruptions at Mount Pelee volcano (Martinique, Lesser Antilles arc)[END_REF][START_REF] Nakada | Manner of magma ascent at Unzen Volcano (Japan)[END_REF][START_REF] Sparks | Causes and consequences of pressurisation in lava dome eruptions[END_REF]. However, in the case of T-PV, dome formation shows an atypical pattern compared to classical calc-alkaline counterparts, as the lava domes correspond to the product of syn-depositional agglutination from spattering activity (e.g., [START_REF] Ablay | The-2 ka subplinian eruption of Montaña Blanca, Tenerife[END_REF]Andújar and Scaillet, 2012a and b), rather than a protrusion resulting from the slow extrusion of viscous lava.

Despite the fact that most historical volcanism on Tenerife has been related to basaltic magmas and has occurred outside T-PV stratovolcanoes, these should be considered active volcanoes, being able to produce new eruptions in the near future [START_REF] Martí | Assessing the potential for future explosive activity from Teide-Pico Viejo stratovolcanoes (Tenerife, Canary Islands)[END_REF]. Hence, understanding the causes and characteristics of satellite dome eruptions is vital to correctly interpret monitoring data and conduct a precise hazard assessment.

In this study we contribute to the understanding of the mechanisms that control a characteristic dome-forming eruption at Teide volcano in which a progressive change in the style of activity has occurred from an efficiently fragmented magma dispersed by a sustained plume to a poorly-fragmented, low-height, non-convective fountain (forming a fountain-fed lava dome) to a glass-bearing lava flow. We describe the petrology and geochemistry of the Pico Cabras dome-forming eruptive products to determine their pre-eruptive parameters and to thereby propose a conceptual model for the magmatic reservoir prior to eruption.

Geological background 2.

T-PV stratovolcanoes started to grow simultaneously at about 180 ka ago within the Las Cañadas caldera (Fig. 1), a volcanic depression formed by several vertical collapses of the former Tenerife central volcanic edifice (Las Cañadas edifice), following explosive J o u r n a l P r e -p r o o f emptying of a shallow magma chamber. Several large scale, lateral collapses of the volcano flanks also occurred, modifying some sectors of the resulting caldera depression [START_REF] Martí | Las Cañadas caldera, Tenerife, Canary Islands: A review, or the end of a long volcanological controversy[END_REF][START_REF] Martí | The Las Cañadas caldera (Tenerife, Canary Islands): an overlapping collapse caldera generated by magma-chamber migration[END_REF].

Eruptions at T-PV have evolved compositionally from basanitic to phonolitic and have occurred from both their central vents and from a multitude of satellite vents distributed on their flanks. Felsic products, however, dominate the recent output of the T-PV system and represent 83% of the total volume emitted during the last < 30 ka [START_REF] Ablay | Stratigraphy, structure, and volcanic evolution of the Pico Teide-Pico Viejo formation, Tenerife, Canary Islands[END_REF][START_REF] Martí | Assessing the potential for future explosive activity from Teide-Pico Viejo stratovolcanoes (Tenerife, Canary Islands)[END_REF].

The eruptive style of T-PV phonolitic eruptions depends on vent location (central or lateral). In central eruptions (with origins in T-PV main craters) the eruptive style has been dominated by effusive to transient explosive eruptions, with ash columns of up to a few kilometres and the production of scoria and spatter deposits, together with lava flows [START_REF] Ablay | Stratigraphy, structure, and volcanic evolution of the Pico Teide-Pico Viejo formation, Tenerife, Canary Islands[END_REF][START_REF] Martí | Assessing the potential for future explosive activity from Teide-Pico Viejo stratovolcanoes (Tenerife, Canary Islands)[END_REF]. Meanwhile, satellite vents are characterised by the highest explosivity and the generation of major ash columns, the collapse of which produced important fallout and PDCs deposits. Some of these satellite eruptions (e.g., Montaña Blanca and El Boquerón) have been classified as sub-plinian to plinian eruptions [START_REF] Ablay | The-2 ka subplinian eruption of Montaña Blanca, Tenerife[END_REF]Andújar & Scaillet, 2012b;[START_REF] García | The 5,660 yBP Boquerón explosive eruption, Teide-Pico Viejo complex, Tenerife[END_REF]. Furthermore, these lateral eruptions are characterised by the formation of a typical dome-like structure constructed on the vent by agglutination of large spatter clasts, at the same time or preceding the emplacement of clastogenic, glassy lava flows (e.g., [START_REF] Ablay | The-2 ka subplinian eruption of Montaña Blanca, Tenerife[END_REF].

Pico Cabras dome is approximately 600 m long, 450 m wide and has a height of 130 m (Fig. 2). Based on its stratigraphic position, its age has been placed between 9210 and 5911 years BP [START_REF] Ablay | Stratigraphy, structure, and volcanic evolution of the Pico Teide-Pico Viejo formation, Tenerife, Canary Islands[END_REF][START_REF] Carracedo | Eruptive and structural history of Teide Volcano and rift zones of Tenerife, Canary Islands[END_REF]. The deposits of this eruption are well preserved in the geological record and show a progressive gradation from highly explosive to poorly or non-explosive activity.

J o u r n a l P r e -p r o o f

Field characteristics of Pico Cabras deposits

The Pico Cabras succession includes at its base, a pumice fall deposit, which is partially reworked and eroded out, from a few centimetres to a maximum of 2 m thick, formed of angular to sub-rounded lapilli pumices. It occurs in a few well-preserved outcrops composed of well-sorted, greenish to grey, poorly vesiculated pumices, with a few obsidian lithic fragments (3-4% vol.), showing well-defined stratification. However, there are no stratigraphic indicators that could be used to correlate individual layers among the different outcrops, so the Pico Cabras pumice deposit has been therefore characterised as a single unit [START_REF] García | Stratigraphic correlation of Holocene phonolitic explosive episodes of the Teide-Pico Viejo Volcanic Complex, Tenerife[END_REF]. The basal contact of the pumice deposit with previous lavas is not well exposed; upwards it grades into agglutinated spatter deposits that form the dome structure. This transition is only visible at certain points around the dome while in most places the pumice fallout is covered by the rheomorphic welded spatter (Fig. 2).

The dome shows an internal clastogenic appearance, similar to most of the T-PV domes (e.g., [START_REF] Ablay | The-2 ka subplinian eruption of Montaña Blanca, Tenerife[END_REF], also showing the piling up of different units of strongly welded spatter clasts, ranging in size from a few centimetres to several decimetres. They behaved rheomorphically forming short lava-like lobes in all directions from the dome centre.

Pico Cabras also emitted several lava flows, with a total volume of 0.28 km3 DRE (Dense-Rock-Equivalent). They all flowed down through a narrow channel at the centre of the dome towards the north of the island. All these lavas are glassy and autobrecciated forming large irregular blocs in a chaotic distribution. Compared to the spatter clasts of the dome, the obsidianic lavas look very fresh.

Methodology

J o u r n a l P r e -p r o o f

Representative samples from Pico Cabras deposits, from the sub-plinian fallout (including spatter fragments) and the lava flows, were collected for further mineralogical and geochemical characterisation. The crystal content of the samples was obtained by the point counting method on respective thin sections and, in the case of the pumice samples, from separate minerals. Owing to the low amounts of phenocrysts present in the pumice samples (less than 0.5%), different fall-out pieces were crushed and sieved until obtaining a coarsegrained powder, from which several minerals and glass fragments were hand-picked in order to proceed with their observation under different microscope techniques.

The samples were analysed with a Scanning Electron Microscope (SEM) Merlin Compact ZEISS with an EDS Bruker detector (QUANTAX-XFlash6) at the Institut des Science de la Terre d'Orleans (ISTO, Centre National de la Recherche Scientifique, Orléans, France). Additionally, we performed a micro-X-Ray Fluorescence (Micro-XRF) analysis using the XTRACE detector (Tube Rhfilters Al, Ti, and Ni) connected to the SEM, which was calibrated against inner Br glassy standards containing different amounts of Br [START_REF] Faranda | An X-Ray Micro-Fluorescence spectrometry analysis for determination of bromine content in alkaline rocks[END_REF] in order to analyse Br in sodalite and glass in the pumice samples. Mineral phases and glass compositions were obtained with an electron microprobe SX-FIVE, also installed at ISTO. An acceleration voltage of 15 kV, a sample current of 6 nA, and a counting time of 10 s were used. Na and K were analysed first and a ZAF correction procedure was applied if necessary. Major elements calibration was performed using standards of: albite (Na, Si), orthoclase (K), andradite (Ca), topaz (F), apatite (P), vanadinite (Cl), baryte (S), corundum (Al), magnesium oxide (Mg), nickel oxide (Ni), hematite (Fe) and pyrophanite (Mn, Ti). For the oxides, the relative analytical errors are 1% (SiO 2 , Al 2 O 3 , CaO), 3% (FeO, MgO, TiO 2 ) and 5% (MnO, Na 2 O, K 2 O, P 2 O 5 ). For the glasses, a large defocused beam of 10x10 or 20x20 μm was used to minimise alkali-migration during their analysis, whereas for minerals a focused beam was employed. In this work, total iron content is noted as FeO*.

J o u r n a l P r e -p r o o f

Whole-rock data was obtained by analysing two glass samples resulting from the two-step melting (with grinding in between) of lava and pumice samples at 1400 ºC for 4 hours in total time, respectively.

As a way to make the results of this work accessible to the scientific community, all In order to constrain the pre-eruptive parameters of the magmas related to the different phases, we used available phonolitic-melt thermometry [START_REF] Martí | Controls of magma chamber zonation on eruption dynamics and deposits stratigraphy: The case of El Palomar fallout succession (Tenerife, Canary Islands)[END_REF], the clinopyroxene-liquid barometric and thermometric equations of [START_REF] Masotta | Clinopyroxene-liquid thermometers and barometers specific to alkaline differentiated magmas[END_REF] and the K-feldespar-liquid hygrometer of [START_REF] Mollo | A K-feldspar-liquid hygrometer specific to alkaline differentiated magmas[END_REF], since these were previously calibrated on magmas from Tenerife that were compositionally similar to those emitted from Pico

Cabras. These models back-calculate the above parameters with a precision of ± 16-18 ºC for T; ± 0.75 wt.% for H 2 O-melt contents, and ± 120 MPa for P, respectively.

Results

.1. Petrography 5

Pico Cabras pumices

Representative pumices examined in this work are sub-rounded, greenish to gray lapilli fragments, with an average size of ~1-2 cm. They are microvesicular, glassy, phonolitic in composition, and crystal poor (with a maximum of 0.5% crystal content). The J o u r n a l P r e -p r o o f mineral assemblage found in the pumices includes feldspar (fsp), clinopyroxene (cpx), biotite (bt), magnetite (mt), ilmenite (ilm), apatite (ap), and traces of sodalite (sdl) (Fig. 3). A few titanite crystals, associated with clinopyroxene and magnetite, were also identified.

Moreover, spatter clasts were also sampled but they were not useful to this analysis as all them were formed of totally aphyric, partially devitrified obsidian, occasionally showing surface microliths formed during cooling.

Feldspar and biotite are the most abundant and largest phenocrysts (between 0.5 and 1.5 mm). They are mostly euhedral, but some slightly rounded biotite crystals were also found (Fig. 3g). Clinopyroxene, magnetite and ilmenite are usually found as aggregates, however with smaller sizes (maximum of 0.5 mm) when compared to previous ones. Apatites of pumice and lava flow products currently appear as inclusions within other minerals (i.e., clinopyroxene, biotite, or magnetite (Fig. 3).

In addition, it is also remarkable to observe the presence of sparse, small (1 mm) crystals of sodalite (Cl-rich feldspathoid), which present rounded edges, numerous fluid inclusions and, to a lesser extent, glass inclusions (Fig. 3h). They also have small iron sulphide (pyrite or pyrrhotite) crystals inside, whose exact identification was not possible due to their small size. Along with [START_REF] Beard | Clinopyroxene/Melt Trace Element Partitioning in Sodic Alkaline Magmas[END_REF], this is the second time that sodalite is reported in the most recent phonolites from Teide. This low-abundant mineral, however, also appears frequently in previous deposits from the Las Cañadas edifice (< 180 ka; [START_REF] Araña | Variation trends in the alkaline salic rocks of Tenerife[END_REF][START_REF] Bryan | Petrology and geochemistry of the Bandas des Sur Formation, Las Cañadas Edifice, Tenerife (Canary Islands)[END_REF][START_REF] Martí | Controls of magma chamber zonation on eruption dynamics and deposits stratigraphy: The case of El Palomar fallout succession (Tenerife, Canary Islands)[END_REF][START_REF] Wolff | Zoning in highly alkaline magma bodies[END_REF].

Phenocrysts are set in a microvesiculated phonolitic glassy matrix containing less than 0.1% Fe-Ti oxide, biotite, clinopyroxene, and alkaline feldspar.

Pico Cabras lavas

The studied lava products have porphyric texture and a phenocryst content of ~5%.

Feldspars from 0.5 to 5 mm in length dominate the mineral assemblage, together with lesser J o u r n a l P r e -p r o o f amounts of diopside (sizes from 0.1 to 1.5 mm), biotite (from 0.1 to 0.5 mm), magnetite and ilmenite (from 0.1 to 0.4 mm), as well as small apatite crystals. The groundmass is also phonolitic with glass feldspar microlites and, to a lesser extent, tiny crystals of clinopyroxene, biotite, magnetite, and ilmenite (Figs. 3a-f).

Lava feldspars are subhedral to anhedral, prismatic phenocrysts with (Fig. 3a,d) some displaying partially rounded and reabsorbed rims. They can also contain inclusions of smaller mineral phases such as biotite, clinopyroxene, Fe-Ti oxide, and apatite (Fig. 3a).

The rest of the mineral assemblage (cpx-bt-mt-ilm-ap) appear as both isolated phenocrysts or within agglomerates (Fig. 3b). Biotite and clinopyroxene are often euhedral to subhedral, however, rare crystals with completely rounded edges are also present in the samples (Fig. 3e). Frequently, magnetite and ilmenite appear together, with subhedral to euhedral shapes, indicating cosaturation. One feature common to all magnetites is the presence of ilmenite exsolution lamellae within the phenocrysts forming grids smaller than 1 μm in thickness, which suggest sub-solidus re-equilibration of these phases during ascent, or after emplacement [START_REF] Lattard | Experimental evidence for the exsolution of ilmenite from titaniferous spinel[END_REF]Fig. 3c). It is also remarkable to observe the presence of just one amphibole (kaersutite) crystal in Pico Cabras phonolite lavas (Fig. 3f), whose external origin will be discussed later based on chemical composition and calculated mineralglass K D .

Mineral chemistry

Feldspars

Analysed feldspars from the studied samples when plotted in the Orthoclase (Or)-Albite (Ab)-Anorthite (An) classification diagram (Fig. 4; [START_REF] Deer | Rock-forming Minerals[END_REF] show a continuous evolutionary line from anorthoclase-oligoclase type to sanidine feldspars (Ab 74 Or 12 An 14 to Ab 54 Or 44 An 2 ), encompassing the lineages defined for the actual T-PV system [START_REF] J O U R N A L P R E -P R O O F Ablay | Basanite-Phonolite Lineages of the Teide-Pico Viejo Volcanic Complex, Tenerife, Canary Islands[END_REF][START_REF] Triebold | Anorthite-calibrated backscattered electron profiles, trace elements, and growth textures in feldspars from the Teide-Pico Viejo volcanic complex (Tenerife)[END_REF]. Several distinct feldspar populations have J o u r n a l P r e -p r o o f been found within the different eruptive phases, as well as different zoning trends (Figs. 5 and6; Table 1).

All feldspar cores found in the lava flow are anorthoclase with compositions being in the range An 4-7 (Figs. 4 and5). Despite the subtle differences, population histograms and EMPA compositional profiles reveal the presence of a dominant groups at Ab 68-70 Or 23-25 An 6 -7

and a less frequent but also important one at Ab 67 Or 29 An 4-5 (Figs. 5,6a; Table 1).

Compositional profiles performed on feldspar crystals (Fig. 6a, 6b) also allowed us to identify the presence of rims (~50 µm in size) having two distinct compositions. The first rim type is characterised by an enrichment in An and FeO* content (Ab 65-69 Or 22-26 An 9-10; 8-10 wt.% FeO*) compared to the cores (Fig. 6a) and the second shows a depletion in these two elements (Ab 64 Or 35 An 2 ) in comparison to crystal centres (Figs. 6b,c; Table 1).

Meanwhile, feldspar cores in the pumices show a much larger variation and can be divided into two distinct populations (Fig. 5; Table 1). The compositional range of the first group coincides with the cores found in the lavas (Ab 67-70 Or 25-28 An 4-7 ) and are also classified as anorthoclases (Fig. 4). The second group, in contrast, have relatively low An contents (Ab 57-64 Or 35-42 An 0-1.5 ) compared to the former ones and can be classified as much more evolved anorthoclases and sanidines as shown by [START_REF] J O U R N A L P R E -P R O O F Ablay | Basanite-Phonolite Lineages of the Teide-Pico Viejo Volcanic Complex, Tenerife, Canary Islands[END_REF] and [START_REF] Andújar | Experimental constraints on the pre-eruptive conditions of the phonolitic magma from the caldera-forming the Abrigo eruption, Tenerife (Canary Islands)[END_REF]. Some of them also present relatively wide variations in Ab and Or content within the same crystal (up to 5 mol.% in several crystals). Only one zoning trend has been found in this second group, with a rim composition of Ab 61 Or 38 An 1 , which is common in cores from the two populations (Figs. 5 and6c).

The genetic relationship between the different feldspar phenocryst populations from Pico Cabras products is also supported by the trend described in Figure 7, which plots FeO* content in feldspars versus log (Or/An). This parameter works as a fractionation index and a proxy for identifying the degree of differentiation of the magma from which the minerals J o u r n a l P r e -p r o o f have crystallised [START_REF] Triebold | Anorthite-calibrated backscattered electron profiles, trace elements, and growth textures in feldspars from the Teide-Pico Viejo volcanic complex (Tenerife)[END_REF]. Feldspars from the pumices are the ones that present a higher degree of differentiation (log Or/An between 1.4 and 2.5), except the group that coincides in composition with the phenocrysts found in the lava flow (values between 0.5 and 0.8). Rims depart slightly from these values, both towards more evolved or more mafic compositions.

Microlites analysed from both lava flow and pumices show a large chemical variation that overprint the evolutionary line described by the cores, from anorthoclase-oligoclase to sanidine (from Ab 74 Or 12 An 14 to Ab 58 Or 38 An 4 ) (Fig 4 and5; Table 1). This large variation occurs even within the same sample for nearby microlites. Some of the microlites enriched in An are also enriched in iron (up to 1.4 wt.% FeO*) compared to most of the feldspar analysed (average of 0.28 wt.% FeO* in the lava flow and 0.23 wt.% FeO* in the pumices). The huge variation in microlite composition is also represented in Figure 7, with some of them reaching values typical from intermediate magmas (log(Or/An) values close to zero or even negative, with high FeO* contents).

Clinopyroxenes

Clinopyroxenes from Pico Cabras can be classified mainly as diopside according to the classification of [START_REF] Morimoto | Nomenclature of Pyroxenes[END_REF], with only one core from a pumice clast presenting augite composition (Fig. 8a, Table 2).

Clinopyroxene phenocrysts found in the lava flow present a fairly homogeneous compared to the cores.

Amphibole

One subhedral kaersutite amphibole crystal was found in the Pico Cabras lava flow close to a biotite and separated by a reaction rim. As amphibole is not a common phenocryst that crystallises from evolved phonolitic magmas at T-PV [START_REF] Andújar | Experimental constraints on the pre-eruptive conditions of the phonolitic magma from the caldera-forming the Abrigo eruption, Tenerife (Canary Islands)[END_REF]2013), we used the Putirka (2016) crystal-liquid distribution coefficients (K D ) to find out if both magma and amphibole were in equilibrium. The calculated kaersutite-melt for this crystal (K D ) falls out of the equilibrium range values (equilibrium K D Fe-Mg Amp-liq between 0.077 and 0.117) determined by [START_REF] Putirka | Special collection: Rates and depths of magma ascent on earth: Amphibole thermometers and barometers for igneous systems and some implications for eruption J o u r n a l P r e -p r o o f mechanisms of felsic magmas at arc volcanoes[END_REF], which testifies for the xenocrystic origin of this mineral.

Iron-titanium oxide

The iron and titanium oxides fall within the compositional series of T-PV magnetite and ilmenite [START_REF] J O U R N A L P R E -P R O O F Ablay | Basanite-Phonolite Lineages of the Teide-Pico Viejo Volcanic Complex, Tenerife, Canary Islands[END_REF][START_REF] Andújar | Experimental constraints on the pre-eruptive conditions of the phonolitic magma from the caldera-forming the Abrigo eruption, Tenerife (Canary Islands)[END_REF]2013). Magnetite phenocrysts always present tiny ilmenite exsolution lamellae (Fig. 3c). This feature made it difficult to obtain precise compositional results, with values ranging from Ilm 87 to Ilm 6 (the latter from a J o u r n a l P r e -p r o o f magnetite rim present in an ilmenite phenocryst without exsolutions). In contrast, ilmenite phenocrysts display more homogeneous values (between Ilm 70 and Ilm 100 ) due to the lack of exsolutions (Table 3). Based on textural observations, magnetite and ilmenite cosaturate from the melt.

Sodalite

Sparse sodalite crystals were found in the pumices from the explosive phase, with a homogeneous composition (Table 3). Small iron sulphide crystals were found within the sodalite as inclusion bands (reaching a FeO*+SO 3 content of up to 1.6 wt.% in those zones), textures which are similar to those identified in haüynes from pre-Teide evolved phonolites [START_REF] Cooper | Volatile budget of Tenerife phonolites inferred from textural zonation of S-rich haüyne[END_REF] and some fluid and melt inclusions which were not analysed in this work. A mean of 130 ± 13 ppm of bromine was measured inside the crystal using micro-XRF analysis.

Whole-rock and residual glass

EMPA data from pumices and lava flow representative glasses (resulting from the complete melting of the samples -see above) indicate that all the products are phonolitic in composition (Le [START_REF] Bas | A Chemical Classification of Volcanic Rocks Based on the Total Alkali-Silica Diagram[END_REF] with some products straddling the trachyte-phonolite border (Fig. 10; Table 4). The only observable difference between the two phases is found in the TiO 2 content, where the lava sample is slightly enriched (up to 1.2 wt.%) compared to the pumice sample (up to 0.8 wt.%).

Residual glasses are also phonolites (Fig. 10; Table 4) (Le [START_REF] Bas | A Chemical Classification of Volcanic Rocks Based on the Total Alkali-Silica Diagram[END_REF], the whole set of data displays a continuous liquid line of descent, with SiO 2 depletion along with alkali enrichment [START_REF] J O U R N A L P R E -P R O O F Ablay | Basanite-Phonolite Lineages of the Teide-Pico Viejo Volcanic Complex, Tenerife, Canary Islands[END_REF][START_REF] Wolff | Crystallisation of nepheline syenite in a subvolcanic magma system: Tenerife, canary islands[END_REF][START_REF] Wolff | Trapped liquid from a nepheline syenite: a re-evaluation of Na-, Zr-, F-rich interstitial glass in a xenolith from Tenerife, Canary Islands[END_REF]. The less evolved liquid corresponds to the bulk lava composition and the most evolved corresponds to the residual matrix glass found in crystal-rich lava (Table 4).

J o u r n a l P r e -p r o o f 4). We should consider that applying this geothermometer to phonolites with a significant number of crystals (i.e., residual melts with compositions significantly different from those corresponding to the whole rock) may provide inaccurate results [START_REF] Martí | Controls of magma chamber zonation on eruption dynamics and deposits stratigraphy: The case of El Palomar fallout succession (Tenerife, Canary Islands)[END_REF]. Thus, for retrieving temperatures with this model we considered the bulk melted and residual glass compositions analysed for the fallout products and the composition of the full melted glass for the lava, since for this last one the glassy groundmass experienced profuse syn-eruptive crystallisation and therefore do not match the utilisation criteria provided by the authors. This empirical equation back-calculates

T with an uncertainty of ± 16 ºC.

The calculations yield average temperatures of 867 ± 23 ºC and 842 ± 19 ºC for the bulk and residual glass of the fall-out and 902 ± 24 ºC for the whole rock lava. Errors on calculated temperatures consider both the standard deviation of the average calculated T and the model uncertainty. Comparatively, the probability distribution analyses of the calculated temperatures point towards somewhat lower values (7-12 ºC) compared to the average, J o u r n a l P r e -p r o o f however such differences are smaller than the uncertainty associated with the calculations (Fig. 11a). These results are in good agreement with those determined for similar Teide phonolites [START_REF] Ablay | The-2 ka subplinian eruption of Montaña Blanca, Tenerife[END_REF][START_REF] J O U R N A L P R E -P R O O F Ablay | Basanite-Phonolite Lineages of the Teide-Pico Viejo Volcanic Complex, Tenerife, Canary Islands[END_REF]Andújar and Scaillet, 2012b;[START_REF] Andújar | Magma storage conditions of the last eruption of Teide volcano (Canary Islands, Spain)[END_REF][START_REF] Andújar | Storage conditions and eruptive dynamics of central versus flank eruptions in volcanic islands: the case of Tenerife (Canary Islands, Spain)[END_REF][START_REF] Wolff | Zonation, mixing and eruption of silica-undersaturated alkaline magma: A case study from Tenerife, Canary Islands[END_REF].

Water contents dissolved in magma were calculated using the alkali feldspar-melt hygrometer of [START_REF] Mollo | A K-feldspar-liquid hygrometer specific to alkaline differentiated magmas[END_REF]. The inputs for the model were the alkali feldspar-melt compositions of the Pico Cabras products and the temperatures obtained from Martí et al.

(2020). To make sure that the considered alkali-feldspar-melt pair was in equilibrium, we used the test based on feldspar-melt exchange reactions [K D (Or-An) kfs-liq ] proposed in [START_REF] Mollo | A K-feldspar-liquid hygrometer specific to alkaline differentiated magmas[END_REF]. The compositions plotted within 10% of the one-to-one line between the predicted K D and the measured K D are considered to be in equilibrium. According to the authors [START_REF] Mollo | A K-feldspar-liquid hygrometer specific to alkaline differentiated magmas[END_REF], a margin of error of ± 0.75 wt.% is normally associated with the magma water content estimated using this method. However, when the errors of the above calculated temperatures are also considered, the accuracy on water-determination increases to ± 0.9 wt.%.

Unfortunately, only 43 of 217 pairs tested in the model were in equilibrium. In general, all feldspars with anorthite contents exceeding 5 mol.% (phenocrysts from lava, pumices and microlites) were out of the accepted range (Fig. 12; Appendix A). In contrast, the population with An ≤ 5 mol.% from both fall-out and lava products, passed the equilibrium test. Regardless, we decided to present the results here. The applicability of the results and the use of this method with this type of magma will be discussed later in this article.

For the lava flow, water contents retrieved for feldspars accomplishing the equilibrium requirements (An 2-4 ) yield average water contents of 4.5 ± 0.9 wt.% (standard deviation, σ = 0.42, number of calculations, n = 5), which match the maximum probability J o u r n a l P r e -p r o o f peak. In contrast, the An-rich crystals and rims (An 7-10 ; not in equilibrium) yield lower H 2 O contents from 3.5 down to 2.9 ± 0.9 wt.% (Figs. 11b,12).

For the pumices, the distribution of probabilities shows the existence of two well distinct populations depending on their An content (Figs. 11b,12). Those with An 4-5 yield relatively similar water contents of ~ 4.5 ± 0.9 wt.%, values that are in close agreement with the ones retrieved for the same K-feldspar population present in the lava flow. In contrast, Ca-poor crystals (An ≤ 1.5 mol.%) back-calculated a large range of H 2 O, from 4.5 to up to 7 wt.%; most of the population being at ~ 6 ± 0.9 wt.%, contents that are somewhat higher than the calculated average (5.2 ± 0.9 wt.% H 2 O, σ = 0.86, n = 29).

Quantitative pressure and temperature estimates were also obtained using the clinopyroxene-liquid P-independent geothermometer (Eq. Talk2012) and T-independent geobarometer (Eq. Palk2012) of [START_REF] Masotta | Clinopyroxene-liquid thermometers and barometers specific to alkaline differentiated magmas[END_REF]. According to the authors, these equations have a standard error of the estimate (SEE) of ± 18 ºC and ± 120 MPa, respectively. In this case, when the uncertainty of above calculated H 2 O contents and the standard deviation of the different bulk compositions are considered, the retrieved P and T are impacted by ± 10 MPa and 5 ºC respectively. This yields total uncertainties of ± 130 MPa and ± 23ºC for the retrieved P-T.

As a test for equilibrium, this geothermobarometer uses the T-sensitive model for Fe-Mg exchange [K D (Fe-Mg) cpx-liq ] of [START_REF] Putirka | Thermometers and barometers for volcanic systems[END_REF]. In our calculations we only considered clinopyroxene-liquid pairs with K D (Fe-Mg) cpx-liq values between 0.09 to 0.37, which according to the authors, are in equilibrium [START_REF] Masotta | Clinopyroxene-liquid thermometers and barometers specific to alkaline differentiated magmas[END_REF]. Additionally, we assumed the previous calculated average melt-water contents of 4.5 wt.% in the effusive phase and 5.2 wt.% in the explosive phase.

The application of the [START_REF] Masotta | Clinopyroxene-liquid thermometers and barometers specific to alkaline differentiated magmas[END_REF] parameterisation on lava flow clinopyroxene phenocrysts yield temperatures between 884 and 911 ºC, with an average J o u r n a l P r e -p r o o f value of 898 ± 23 ºC (σ = 7.8, n = 15; Fig. 11c). These in turn returned crystallization pressures between 60 and 140 MPa, with only an extremely high value at 290 MPa (Fig. 11d). The average P of the effusive cpx dataset is 110 ± 130 MPa (σ = 0.55, n = 15), a somewhat higher value compared to the position of the dominant probability distribution peak at 80 ± 130 MPa. For pumices, the obtained temperatures are somewhat lower (846-880 ºC) compared to the lavas, with an average value of 863 ± 23 ºC (σ = 9.6, n = 17). For pressures, these show a continuous distribution from 90 to 340 MPa, with an average P at 180 ± 130 MPa (σ = 0.64, n = 17). This last one matches the top of the probability distribution poorly defined region fairly well at 170 ± 130 MPa (Fig. 11d).

According to these estimates two different magmas would have been present in Pico

Cabras eruption. The lava flows of this eruption would have tapped a magma ponded at ~900 ºC, containing 4.5 wt.% of H 2 O and located at a depth of 0.5-1.5 km b.s.l (below sea level) (~100 MPa). Whereas in turn, the pumices would have been related to a somewhat cooler (~865 ºC), water-rich (~5-7 wt.% H 2 O) but deeper body ponding at 170 MPa (~2.5 km b.s.l).

These conditions are consistent (considering errors) with the melt thermometry estimates and those experimentally determined for similar Teide-Pico Viejo phonolitic products (Andújar and Scaillet, 2012b;[START_REF] Andújar | Magma storage conditions of the last eruption of Teide volcano (Canary Islands, Spain)[END_REF][START_REF] Andújar | Storage conditions and eruptive dynamics of central versus flank eruptions in volcanic islands: the case of Tenerife (Canary Islands, Spain)[END_REF]. However, when the uncertainties associated with each calculated parameter are considered, which are especially high for P (± 130 MPa), the storage conditions overlap to a high extent (Fig. 13). Thus, in the next section we integrate the stratigraphic information, eruptive sequence, mineral compositions and zoning along with existing phase equilibria studies to discriminate between the following scenarios: a single reservoir being thermally, compositionally and volatile-zoned versus two different magma bodies that interacted prior to the eruption.

Discussion

J o u r n a l P r e -p r o o f 6.1. Refining pre-eruptive conditions by comparison with phase equilibrium results.

In the case of the Pico Cabras eruption, pre-eruptive parameters estimated by different geothermobarometers and the geohygrometer show significant differences between the distinctive products of the eruption (Figs. 11 and 13), especially with regard to temperature and, to a lesser degree, H 2 O and pressure. To provide tighter constraints, here we refine the above estimates by comparing the petrology of the natural phonolites to the information gained in experimental works that were performed on similar phonolitic compositions.

Compared to other methods (i.e., statistical studies), phase equilibrium experiments can provide first-order constraints under magma pre-eruptive conditions since these are powerful tools to simulate realistic magmatic conditions and explore the sensitivity of mineral stability and composition to changes in the intensive parameters [START_REF] Martel | Experimental simulations of magma storage and ascent[END_REF].

Magma corresponding to the lava flows shows higher temperatures: average of 902 ºC The lava samples present a mineral assemblage consisting of ~5% alkali feldspar, magnetite, ilmenite, clinopyroxene, and biotite. This low crystallinity and paragenesis is well reproduced in experiments between 50 and 100 MPa, 850-900 ºC and H 2 O contents between 3-4.5 wt.% (Andújar and Scaillet, 2012b;[START_REF] Andújar | Storage conditions and eruptive dynamics of central versus flank eruptions in volcanic islands: the case of Tenerife (Canary Islands, Spain)[END_REF]. The absence of titanite also suggests T > 850 ºC, since this phase is always present at lower temperatures in these magmas (Andújar and Scaillet, 2012b). These conditions correspond to the ones obtained here using the available geothermobarometer and geohygrometer, with the only exception being one analysis that yielded a value of 280 MPa. However, this is a non-realistic value for this magma, since at such a pressure and considering the previous T-H 2 O contents, the crystal-cargo would exceed 15% (Andújar and Scaillet, 2012b;[START_REF] Andújar | Storage conditions and eruptive dynamics of central versus flank eruptions in volcanic islands: the case of Tenerife (Canary Islands, Spain)[END_REF]).

J o u r n a l P r e -p r o o f

Mineral chemistry also confirms this first estimate: the two feldspar core populations (Ab 68-70 Or 23-25 An 7 and Ab 67 Or 29 An 4 ) were closely reproduced experimentally at 900 ºC and 850 ºC respectively [START_REF] Andújar | Experimental constraints on the pre-eruptive conditions of the phonolitic magma from the caldera-forming the Abrigo eruption, Tenerife (Canary Islands)[END_REF][START_REF] Andújar | Storage conditions and eruptive dynamics of central versus flank eruptions in volcanic islands: the case of Tenerife (Canary Islands, Spain)[END_REF]. So, we consider the average temperature of the magma for the Pico Cabras lava to be 880 ± 30 ºC, with slight temperature variations within the chamber (see below Natural biotites are also well reproduced under the conditions stated above. In terms of clinopyroxenes, the analysed compositions slightly differ from the experiments: they have a little higher enstatite content (from 38 to 41 mol.%) and Mg# (from 72.5 to 76.5). This can be explained by higher temperatures (900-950 ºC), in line with the most An-rich feldspars.

Concerning pumices, storage conditions retrieved by empirical equations span a large range of pressures (90-340 MPa), water contents (4.5-7 wt.%) and temperatures (842-880 ºC). Compared to the lavas, the crystallinity of the phonolitic pumices is significantly lower and the paragenesis contains sodalite, titanite, and some mineral compositions that were not identified in the former (feldspars with An ≤ 1.5 mol.%).

In terms of bulk-rock composition and, specially, mineralogy, pumice products share some similarities with those of the El Abrigo eruption [START_REF] Andújar | Experimental constraints on the pre-eruptive conditions of the phonolitic magma from the caldera-forming the Abrigo eruption, Tenerife (Canary Islands)[END_REF], which was the last caldera-forming eruption from Las Cañadas Edifice, occurring about 180 kyr. ago [START_REF] Martí | Assessing the potential for future explosive activity from Teide-Pico Viejo stratovolcanoes (Tenerife, Canary Islands)[END_REF]. The most notable difference between these two magmas is that El Abrigo carries haüyne instead of sodalite, otherwise the paragenesis is almost the same. Haüyne and sodalite are present in trace amounts in phonolites and share a similar P-stability dependence on phonolitic melts [START_REF] Andújar | Experimental constraints on the pre-eruptive conditions of the phonolitic magma from the caldera-forming the Abrigo eruption, Tenerife (Canary Islands)[END_REF][START_REF] Berndt | Experimental constraints on storage conditions in the chemically zoned phonolitic magma chamber of the Laacher See volcano[END_REF]Wellmann, 1970). According to J o u r n a l P r e -p r o o f these works, the presence of these phases in the magma sets a maximun pressure limit at 125-170 MPa, the variation on the H2O-content havinga limited impact on it (±25 MPa)

In this case, the coexistence of all the phases of the mineral assemblage can be only possible at T < 850 ºC; P < 150 MPa, and H 2 O < 5 wt.%., as at higher P-T-H 2 O, the magma will never crystallize feldspar and the paragenesis would be hence dominated by mt+bt or mt+bt+cpx+tit [START_REF] Andújar | Experimental constraints on the pre-eruptive conditions of the phonolitic magma from the caldera-forming the Abrigo eruption, Tenerife (Canary Islands)[END_REF]. This observation indicates that the extreme water content values we obtained using the [START_REF] Mollo | A K-feldspar-liquid hygrometer specific to alkaline differentiated magmas[END_REF] geohygrometer (H 2 O > 5 wt.%) are not realistic for this magma.

Thus, to refine our previous estimates we use the composition of the mineral phases present in the pumices. Feldspars found in the Pico Cabras pumices, as mentioned in previous sections, can be divided into two distinguishable populations. The first is similar to the ones found in the lava flow (Ab 67-70 Or 25-28 An 4-7 ), while the second is much poorer in anorthite (Ab 57-64 Or 35-42 An 0.5-1.5 ). The latter is closely reproduced at temperatures between 725 and 825 ºC, 100 MPa and 3-4.9 wt.% H 2 O [START_REF] Andújar | Experimental constraints on the pre-eruptive conditions of the phonolitic magma from the caldera-forming the Abrigo eruption, Tenerife (Canary Islands)[END_REF][START_REF] Andújar | Storage conditions and eruptive dynamics of central versus flank eruptions in volcanic islands: the case of Tenerife (Canary Islands, Spain)[END_REF]. However, when considering the sample crystallinity (< 1%), only the experiments at 725 ºC successfully reproduce feldspars with that composition (An < 1.5 mol.%). The An-rich population, on the other hand, would have crystallised under conditions similar to those described for the lava flow (880 ± 30 ºC; see above

J o u r n a l P r e -p r o o f

One point that requires special attention is the presence of highly evolved sanidines (An ≤ 1.5) and iron-rich clinopyroxenes in the pumices, which, according to our previous estimates, could both not directly crystallise from the magma compositions sampled in this work, nor at the P-T-H 2 O values previously determined for both of the magmas. However, the highly evolved character of these phases compared to those predominating in the lavas suggests that the former crystallised from a more evolved phonolitic melt. The comparison with available experimental data confirms that the potential parental magma producing such pumices phenocrysts has a composition similar or equivalent to that of the El Abrigo magma [START_REF] Andújar | Experimental constraints on the pre-eruptive conditions of the phonolitic magma from the caldera-forming the Abrigo eruption, Tenerife (Canary Islands)[END_REF]. If this magma was stored at 725 ºC and contained a higher water content (3.5-5 wt.%), the crystallisation of these highly evolved mineral phases from an El Abrigo equivalent magma could be possible. Further evidence is provided by the subtle differentiation trend described by residual glasses from the pumices. The most evolved compositions of this series have total alkali and SiO 2 contents that are identical to those of El Abrigo (Fig. 10).

Structure of the Pico Cabras reservoir prior to the eruption

The comparison of the petrological information with experimental data allowed us to refine the estimates provided by the empirical methods. In summary, both magmas were stored at ~100 MPa (± 50 MPa) and contained slightly different water contents (3 wt.% for the lavas and 5 wt.% for the pumices) but showed very different temperatures, these being in the range 725-825 ºC and 850-900 ºC for the pumices and the lavas, respectively. This points towards the existence of a unique reservoir where magmas co-existed prior to the eruption.

Field evidence along with the petrology of the natural samples further support such a scenario. The lack of An-poor feldspar crystals (An <1.5 ) or other evolved mineral phases like the ones found in the pumices within the lavas deposits, rules out the unlikely situation where J o u r n a l P r e -p r o o f the most evolved magma rising from deeper levels interacts and passes through the most mafic and shallower one, as suggested by cpx-geobarometry (Fig. 13). Additionally, in the case studied here, the emplacement of pumices precedes that of the lavas, the former not showing evidence of long-term chemical interaction (e.g., reverse zoning in sanidines An 1.5 to An 4 ) with its more mafic counterpart.

By integrating this information, we are able to propose the following conceptual model to explain the Pico Cabras dome-forming eruption. The phonolitic reservoir that fed Pico Cabras eruption was located at a pressure of about 100 ± 50 MPa (around 1 km b.s.l).

This was chemically, thermally, and volatile-zoned prior to the eruption (Fig. 14). The main body of the reservoir was formed by a phonolitic magma being at temperatures of 880 ± 30 ºC and containing 2.5-3 wt.% dissolved H 2 O. Least squares mass-balance calculations relate the lava and pumice WR compositions by the crystallization of 0.8 wt.% of Fe-Ti oxides, 0.1 wt. % Bt, 0.5 wt. % Cpx and 3.9 wt. % of An 4 K-feldspar (ΣR 2 = 0.08). Since, these mineral proportions are similar to those determined for the phonolite lavas, we can confirm that the uppermost part of the reservoir hosted the residual melt derived from the crystallization of the former one, it being cooler (725-825 ºC) and containing a higher water content (3-5 wt.%) than the main magma. At these pressures, the estimated water contents are slightly far from saturation (5-6 wt.% H 2 O at 100-150 MPa, respectively; [START_REF] Schmidt | Water solubility in phonolite melts: Influence of melt composition and temperature[END_REF], an observation that supports the general water-undersaturated character of Tenerife phonolites (Andújar and Scaillet, 2012a). Other volatiles (S, Cl, Br) were also present in this upper part.

However, our results cannot assess the co-existence of an independent fluid-phase containing an excess of these minor gas-phases (i.e., brine-rich fluid phase, [START_REF] Bureau | Volcanic degassing of bromine and iodine: Experimental fluid/melt partitioning data and applications to stratospheric chemistry[END_REF][START_REF] Signorelli | Solubility and fluid-melt partitioning of Cl in hydrous phonolitic melts[END_REF] with the Pico Cabras magmas.

Eruption started with the explosive emission of the uppermost magma, forming a subplinian eruption column, which rapidly waned in intensity given rise to a low fire fountaining In addition, there is evidence for the existence of different pre-eruptive processes which affected the interior of this magma chamber as recorded by the zonation patterns of single feldspar crystals. The step zoning found in alkali feldspar crystals from the lavas (with An 6-7 and An 4 ), indicates that this phase registered slight temperature variations of between 850 and 900 ºC (Fig. 6). As the zoning does not extend to other chemical elements, we can rule out the idea of contrasting compositional layers inside the main magmatic body.

Moreover, An-rich feldspar rims and microlites (Figs. 56) are indicators of higher temperatures (up to 950 ºC). This could have been caused by an external heat supply prior to eruption, possibly by a magma intrusion, which in turn could have triggered the event. This less evolved and hotter magma (1000-1050 ºC according to [START_REF] J O U R N A L P R E -P R O O F Ablay | Basanite-Phonolite Lineages of the Teide-Pico Viejo Volcanic Complex, Tenerife, Canary Islands[END_REF][START_REF] Martí | Controls of magma chamber zonation on eruption dynamics and deposits stratigraphy: The case of El Palomar fallout succession (Tenerife, Canary Islands)[END_REF]) could have caused a temperature rise at the bottom of the resident magma, responsible J o u r n a l P r e -p r o o f

for the slight increase in the amount of FeO* (up to 1.4 wt.%) and CaO (up to An 9-14 ) observed in the alkali feldspar rims and microlites, and the increase of enstatite and Mg# in the rims of some clinopyroxenes. However, mingling/mixing between the two magma was rather restricted, as suggested by: 1) the presence of feldspar phenocrysts of evolved compositions (< 1.5 mol.% An) without calcium-rich rims in the explosive phase; 2) the low thickness (~ 5 μm) of the more mafic rims; and 3) the absence of banded pumices or the emission of hybrid magmas. Most likely, both magmas would only have coexisted for a short time (hours to few weeks) prior to the eruption. The consequent volatile exsolution from the more mafic magma disturbed the equilibrium in the magma chamber and produced a syneruptive "self-mixing" process [START_REF] Couch | Mineral disequilibrium in lavas explained by convective self-mixing in open magma chambers[END_REF][START_REF] Triebold | Anorthite-calibrated backscattered electron profiles, trace elements, and growth textures in feldspars from the Teide-Pico Viejo volcanic complex (Tenerife)[END_REF], which caused the extrusion of the main magma into the upper part of the chamber. This explains the presence of phenocrysts of very different compositions in the pumice samples and the rather similar bulk rock composition of both products.

According to the results obtained, the Pico Cabras eruption constitutes another documented example, together with those from Roques Blancos [START_REF] Andújar | Storage conditions and eruptive dynamics of central versus flank eruptions in volcanic islands: the case of Tenerife (Canary Islands, Spain)[END_REF], Montaña Blanca [START_REF] Ablay | The-2 ka subplinian eruption of Montaña Blanca, Tenerife[END_REF]Andújar and Scaillet, 2012b), and El Palomar (Martí et al., 2020), of how rapidly Tenerife phonolitic magmas may enter into eruption when they receive an energy input from deeper magmas. Data presented here confirms that T-PV explosive flank dome eruptions are associated with very shallow magma batches and that they could be well anticipated with precise monitoring. Moreover, it is worth highlighting here the presence of sodalite in the explosive products from Pico Cabras, an indicator of the presence of a significant amount of halogen gases within the fluid phase. These gases (especially chlorine and, above all, bromine) play a large role in ozone layer depletion once they reach the stratosphere [START_REF] Bureau | Volcanic degassing of bromine and iodine: Experimental fluid/melt partitioning data and applications to stratospheric chemistry[END_REF][START_REF] Cadoux | Stratospheric Ozone destruction by the Bronze-Age Minoan eruption (Santorini Volcano, Greece)[END_REF][START_REF] Cadoux | The role of melt composition on aqueous fluid vs. silicate melt partitioning of bromine in magmas[END_REF][START_REF] Daniel | Stratospheric ozone destruction: The importance of bromine relative to chlorine[END_REF].

J o u r n a l P r e -p r o o f

Conclusions.

In this work we have undertaken a study of the pre-eruptive conditions of the Pico Cabras dome eruption (Teide-Pico Viejo magmatic system, Tenerife) and their relation to the changes in eruptive dynamics during the eruption. The magma of this eruption was stored in a reservoir at 100 ± 50 MPa, which was chemically and thermally zoned. The main body would have had a temperature of 880 ± 30 ºC and 2.5-3 wt.% H 2 O and it is the magma that gave rise to the lava flow phase at the end of the eruption. However, the initial eruptive phase that formed the pumice fall deposits and the dome structure derived from the upper part of the chamber, where magma was stored at 725-800 ºC and 3.5-5 wt.% H 2 O. This data allows us to connect the evolution of the eruption with changes in temperature and the amount of volatiles within the reservoir.

The magmatic evolution of Teide-Pico Viejo volcanic system is likely to continue producing these kinds of explosive eruptions. Furthermore, the presence of feldspar relicts with a composition equivalent to those found in the Las Cañadas caldera episode in Pico

Cabras most evolved products indicates that the current T-PV is already producing phonolites with a high explosive potential. An eruption with these characteristics would have a dramatic impact on Tenerife, particularly towards the north of the island. Therefore, there is a need to continue increasing our knowledge of the geological record and the processes occurring inside the active central volcanic system in order to be able to undertake an accurate hazard assessment of the island. In particular, studies regarding the volatile components of these magmas should be carried out in order to evaluate the effects they could have on atmospheric Scaillet, and M. Pichavant during sampling was highly appreciated. English text was corrected by Grant George Buffett of Terranova Scientific (www.terranova.barcelona). [START_REF] Deer | Rock-forming Minerals[END_REF]. The blue arrow shows the compositional evolution in a phonolitic magma of feldspars. Cabras dome (the horizontal scale is equal to vertical scale). The scheme shows the location of the reservoirs for central Teide magmas [START_REF] Andújar | Magma storage conditions of the last eruption of Teide volcano (Canary Islands, Spain)[END_REF] and Pico Cabras magmas.

J o u r n a l P r e -p r o o f

Pre-eruptive conditions estimated here in the different levels of the magma chamber are also shown. Magma reservoir volumes are not to scale.

Appendix A. Measured and predicted Or-Ab partition coefficient in K-feldspar-liquid pairs from Pico Cabras products used as an equilibrium test by [START_REF] Mollo | A K-feldspar-liquid hygrometer specific to alkaline differentiated magmas[END_REF] geohygrometer. The model assumes as reliable the compositions plotting within 10% of the 1:1 line (± 0.1). analyses from the Pico Cabras lavas and pumices. Water content calculated using the method by difference [START_REF] Devine | Comparison of microanalytical methods for estimating H20 contents of silicic volcanic glasses[END_REF] and temperatures calculated by [START_REF] Martí | Controls of magma chamber zonation on eruption dynamics and deposits stratigraphy: The case of El Palomar fallout succession (Tenerife, Canary Islands)[END_REF] geothermometer are also shown. [START_REF] Devine | Comparison of microanalytical methods for estimating H20 contents of silicic volcanic glasses[END_REF]. Temperatures from [START_REF] Martí | Controls of magma chamber zonation on eruption dynamics and deposits stratigraphy: The case of El Palomar fallout succession (Tenerife, Canary Islands)[END_REF]  Pico Cabras stratified chamber was trigger by an intermediate intrusion.

 T-PV system is already producing phonolites with a high explosive potential.

J o u r n a l P r e -p r o o f

Journal Pre-proof

  and 898 ºC with the calculations of[START_REF] Martí | Controls of magma chamber zonation on eruption dynamics and deposits stratigraphy: The case of El Palomar fallout succession (Tenerife, Canary Islands)[END_REF] and[START_REF] Masotta | Clinopyroxene-liquid thermometers and barometers specific to alkaline differentiated magmas[END_REF] (SEE of ± 16 ºC and ± 23 ºC, respectively). On the other hand, it presents lower pre-eruptive water content (average of 4.5 wt.%)

J

  o u r n a l P r e -p r o o f eruptive phase of hotter and less volatile-rich magma. Hot, large scoria fragments accumulated and agglutinated on the vent, experiencing rheomorphism and forming the dome-like structure. The last part of the eruption corresponded to the emission of the main part of the chamber in the form voluminous obsidianic lava flows that branched at the southern side of the dome. The profuse surface crystallization of feldspar microlites, clinopyroxene and Fe-Ti oxides in the lava during its downward flow (see above), generated the highly Na-rich residual melts measured in this work. In essence, this eruption sequence reflects the eruption of progressively hotter and less volatile-rich phonolitic magmas, without any other clear interference of internal or external factors (e.g., final mingling, variation in conduit size). The progressive change in the style of activity, from an efficiently fragmented magma to a poorly-fragmented, low-height, non-convective fountain, to a glass-bearing lava flow activity reflect the sequential withdrawal of a zoned magma chamber with slight differences in pre-eruptive parameters (T and H 2 O melt content) of the resident magma prior the eruption.

  chemistry and, as a result, on human health. This study has been partially funded by EC Grant EVE (ref: DG ECHO H2020 826292), the Ministry of Science, Innovation, and Universities of Spain (FPU18/02572) and the CSIC grant JAEINT_18_00808. The authors kindly acknowledge the analytical SEM, EMPA and Microfluorescence support provided by I. Di Carlo and the financial support of Equipex-Planex ANR-11-EQPX-0036 and Labex-Voltaire ANR-10-LABX-100-01 from Orléans.Raffaello Cioni and an anonymous reviewer are kindly thanked for the extensive reviews of the original manuscript. We thank M. Roden for his help with the editorial handling of this work. Authors kindly thank S. Meletlidis, M.J. Blanco, and the IGN from the Canary Islands for their support provided during the field campaign. The help from M. Jiménez-Mejías, B.

Fig. 1 .

 1 Fig. 1. Simplified geological and volcanostratigraphy of Tenerife. DRZ: Dorsal Rift

Fig. 2 .Fig. 3 .

 23 Fig. 2. A and B) View of Pico Cabras dome, located on the north flank of Teide

Fig. 4 .

 4 Fig. 4. Feldspar compositions of all analysed feldspars from Pico Cabras eruption

Fig. 5 .

 5 Fig. 5. Histograms of feldspar compositions expressed as anorthite (An), albite (Ab),

Fig. 6 .

 6 Fig. 6. Zoning profiles showing compositional variations within feldspar phenocrysts

Fig. 13 .

 13 Fig. 13. P-T data obtained using the geothermobarometer of Masotta et al. (2013),

Fig. 14 .

 14 Fig. 14. S-N schematic cross section of the Teide-Pico Viejo complex and the Pico

Figure 1 Figure 2

 12 Figure 1

Figure 3

 3 Figure 3

Figure 4

 4 Figure4

Figure 5

 5 Figure5

Figure 6

 6 Figure6

Figure 7

 7 Figure 7

Figure 8

 8 Figure8

Figure 9 Figure 10

 910 Figure9

Figure 11

 11 Figure11

Figure 12

 12 Figure12

Figure 13

 13 Figure13

Figure 14

 14 Figure14

Table 3 )

 3 composition at Wo 46-48 En 38-41 Fs 12-16 with Mg# between 71 and 77 (where Mg# =X MgO /(X MgO +X FeO* )x100, and X i is the mole fraction of i in the liquid) without core-rim Only one analysed crystal from the explosive products shows core-to-rim variation, with a Fe-rich core (Wo 43 En 29 Fs 28 ) and a Mg-rich rim (Wo 46 En 33 Fs 21 ). This more evolved core is also enriched in Na (up to 3.2 wt.%) and Mn (up to 1.5 wt.%) (Table2). . Biotites from the lava flow have higher Mg# (average of 65) values than those in the pumices (average of 63). This last group is also the only one showing moderate chemical variation within the crystals, with rims having lower Mg# (between 56 and 61), Na 2 O, K 2 O (~0.3 wt.%) and higher SiO 2 , Al 2 O 3 and MnO contents (up to 2 and 0.3 wt.% respectively)

	zoning. In comparison, the phenocrysts found in the pumices are slightly more evolved
	(Wo 43-47 En 29-38 Fs 15-28 ; Mg# from 51 to 71) and contain somewhat lower Al 2 O 3 (0.4 wt.%) and
	TiO

2 (0.2 wt.%) compared to the former ones.

J o u r n a l P r e -p r o o f

  Residual glass from pumice samples plots on the differentiation lineage defined by previous products being this way, however, slightly depleted in SiO 2 and CaO and enriched in Na 2 O compared to the whole-rock (Figs. 10; Table4). Additionally, we analysed the

	amount of halogen volatiles (Cl, Br, and F) present in the glasses. Cl varied between 0.2 and
	0.6 wt.% and F from ~0 up to 0.3 wt.% in the phonolitic glasses. The XRF analysis of Br
	yielded concentrations of 10 to 15 ppm.
	5.4. Pre-eruptive parameters: P, T and H 2 O

The first estimates concerning pre-eruptive temperature were obtained by applying the new empirical and experimentally-derived geothermometer for phonolitic melts from

Martí et al. (2020)

to the different melts analysed in this work (whole rock (WR) lava, WR pumice, residual matrix pumice and lava; Table

  ). On the other hand, the same experiments reproduced feldspar rim populations and microlites (Ab 65-69 Or 22-26 An 9-19 and Ab 64 Or 35 An 1.5 ) at 950 ºC

	and 825 ºC, respectively. At this temperature, the lowest crystallinity (5-10%) is found in
	experiments at 100 MPa and 2.5-3 wt.% H 2 O (1-2 wt.% below the ranges estimated by the
	geohygrometer).

  ).

	Under these conditions, natural biotites are also well reproduced: cores (Mg# between
	61-65) at 100 MPa, 800 ºC, and 4 wt.% H 2 O; and rims (Mg# between 56 and 61) with
	temperatures below 800 ºC. Clinopyroxenes crystallised at 100 MPa and ~825 ºC closely
	reproduce the Fs-rich phenocrysts in pumices (Wo 43 En 29 Fs 28 ). En-rich cores and rims
	(Wo 46 En 33 Fs 21 ) are also well reproduced with the P-T-H 2 O conditions we constrained for the
	lavas.

Table 1 .

 1 Major element composition and end-members of representative feldspars from Pico Cabras effusive (lava) and explosive (pumices) phases. Calculated H 2 O using the[START_REF] Mollo | A K-feldspar-liquid hygrometer specific to alkaline differentiated magmas[END_REF] geohygrometer are also shown.

	Sa mp le IG SN Zo ne An aly sis Si O 2	Lav a IEO DG 000 1 Cor e PC1 -01-34 64.1 8	Lav a IEO DG 000 1 Cor e PC1 -02-115 65.3 5	Lav a IEO DG 000 2 Cor e PC2 -05-59 64.9 4 J o u r n a l P r e -p r o o f Lav a Lav a Lav a Lav a Pum ice Pum ice Pum ice IEO DG 000 1 IEO DG 000 2 IEO DG 000 1 IEO DG 000 2 IEO DG 000 3 IEO DG 000 3 IEO DG 000 3 Rim Rim Mic rolit e Mic rolit e Cor e Cor e Cor e PC1 -01-51 PC2 -10-124 PC1 -01-72 PC2 -04-21 PB-03-109 PB-02-86 PB-01-63 63.8 4 67.1 6 63.2 0 65.3 2 66.3 7 67.0 1 66.8 9	Pum ice IEO DG 000 3 Rim PB-02-92 66.4 5	Pum ice IEO DG 000 3 Mic rolit e PA-15-19 65.6 4	Pum ice IEO DG 000 3 Mic rolit e PC-05-7 64.4 2
	Ti O 2	0.03 0.09 0.03 0.49 0.26 0.37 0.16 0.13 0.00 0.01 0.07 0.51 0.54
	Al 2 O	20.5 9	19.5 2	20.0 7	20.8 5	18.7 7	20.1 9	19.1 8	19.6 5	18.8 2	19.1 6	18.7 6	19.9 4	20.5 5
	3
	M gO	0.02 0.00 0.03 0.01 0.00 0.02 0.01 0.02 0.00 0.04 0.00 0.05 0.04
	Ca O	1.49 0.90 1.39 2.13 0.46 2.39 0.69 0.99 0.18 0.10 0.20 0.84 2.76
	Fe O	0.39 0.22 0.34 0.53 0.26 0.80 0.43 0.34 0.32 0.14 0.13 0.65 1.35
	Na 2 O	8.10 7.96 7.97 7.79 7.42 7.91 7.28 7.78 6.63 7.43 6.90 6.11 8.50

Table 2 .

 2 Major element composition, end-members, and Mg# of representative clinopyroxenes from the Pico Cabras lavas and pumices. .83 99.66 98.85 99.17 99.39 99.05 En 38.51 40.07 39.53 39.02 38.97 38.28 28.95 32.89 31.96 33.93 Fs 15.48 13.15 13.50 13.31 14.10 15.37 25.51 20.96 21.80 19.93 J o u r n a l P r e -p r o o f Wo 46.01 46.77 46.97 47.67 46.93 46.36 45.54 46.15 46.24 46.14 #M g 71.32 75.29 74.54 74.56 73.44 71.35 53.17 61.08 59.44 62.99

	Sam ple	Lava	Lava	Lava	Lava	Lava	Pumic es	Pumic es	Pumic es	Pumic es	Pumic es
	IGS N	IEOD G000 1	IEOD G000 1	IEOD G000 1	IEOD G000 1	IEOD G000 1	IEOD G000 3	IEOD G000 3	IEOD G000 3	IEOD G000 3	IEOD G000 3
	Zon e	Core	Core	Core	Rim	Rim	Core	Core	Core	Rim	Rim
	Ana lysi s	PC1-10-126	PC1-10-139	PC1-06-63	PC1-06-76	PC1-10-123	PC-07-17	PC-13-31	PC-13-34	PC-13-28	PC-13-36
	SiO	49.93 52.87 53.12 51.39 51.46 52.03 50.31 51.59 51.97 51.42
	2
	TiO	1.54	0.72	0.94	0.83	0.83	0.65	0.67	0.51	0.77	0.51
	2
	Al 2 O 3	2.24	1.21	1.48	1.64	1.64	0.95	0.85	1.18	1.08	1.11
	Mg O	12.75 13.64 13.28 12.98 13.28 12.90	9.05	10.49	9.95	11.00
	CaO 21.19 22.14 21.95 22.06 22.24 21.73 19.80 20.47 20.03 20.81
	Mn O	0.85	0.58	0.69	0.62	0.71	1.06	1.39	1.00	1.07	1.01
	FeO 9.14	7.98	8.09	7.89	8.56	9.23	14.21 11.91 12.10 11.52
	Na 2 O	1.23	0.97	1.08	1.15	1.06	1.10	2.56	2.02	2.41	1.67
	K 2 O 0.05	0.00	0.00	0.00	0.05	0.02	0.023	0.02	0.00	0.01
	Su m	98.91	100.1 0	100.6 2	98.54 99

Table 3 .

 3 Major element composition of representative biotites, magnetites, ilmenites, and sodalites from the Pico Cabras lavas and pumices. Mg# for biotites is also shown. : no data. *Magnetite analysis do not represent pure magnetite composition, as they are contaminated by ilmenite exsolutions (see text).

	Mi
	ner			Biotite			Magnetite*	Ilmenite	Sodalite
	al
	Sa mpl e IGS N Zon e An alys is SiO 2 TiO 2 Al 2 O 3 Mg O Mn O Fe O	Lava Lava IEO DG0 001 IEO DG0 001 Core Core Core Core Rim Pumi ces Pumi ces Pumi ces IEO DG0 003 IEO DG0 003 IEO DG0 003 PC1-11-185 PC2-01-53 PA-02-07 PA-28-33 PA-49-46 37.33 37.04 36.79 36.96 38.92 0.00 Lava IEO DG0 001 Core with Pumi ces IEO DG0 003 exsolutions PC1-02-91 PC-13-25 0.08 7.52 7.55 7.62 7.46 7.92 9.75 14.14 48.81 50.77 Lava Pumi ces IEO DG0 001 IEO DG0 003 Core Core Core Rim Pumi ces Pumi ces IEO DG0 003 IEO DG0 003 PC1-06-53 PC-04-12 PC-PS-98 PC-PS-130 0.04 0.00 37.29 37.34 s.d s.d 13.05 12.25 12.90 12.76 13.58 1.53 0.70 0.11 0.06 29.76 29.97 14.31 14.14 13.83 13.97 11.29 1.68 0.94 3.67 2.80 s.d s.d 0.34 0.43 0.23 0.39 0.63 2.30 2.79 3.06 3.39 s.d s.d 13.40 12.64 14.72 14.23 15.64 83.69 77.43 44.10 43.57 0.81 0.15 J o u r n a l P r e -p r o o f
	Ca O	0.00	0.00	0.00	0.00	0.11	0.00	0.02	0.03	0.03	0.26	0.29
	Na 2 O	0.93	1.03	1.11	1.09	0.81	0.03	0.05	0.04	0.04 22.76 23.11
	K 2 O	8.66	8.61	8.37	8.49	8.23	0.06	0.03	0.00	0.00	1.91	1.84
	Cl	n.d	n.d	0.04	0.05	0.07	n.d	0.00	n.d	n.d	6.67	6.86
	SO 3 n.d	n.d	n.d	n.d	n.d	n.d	n.d	n.d	n.d	0.59	0.33
	Su m	95.58 93.67 95.60 95.49 97.32 98.98 96.10 99.83	100.6 0	100.0 4	99.89
	#M g	65.56 66.61 62.61 63.65 56.27
	Mt						72.78 59.24 18.28 12.21

d

Table 4 .

 4 Major element composition of representative whole rock and residual melt

  . = no data. W.R. = Whole rock. R.M. = Residual melt. Water content calculated using the method by difference

	Sampl	W.R.	W.R.	R.M.	R.M.	R.M.	R.M.	R.M.	R.M.	Inclusi
	e	Lava	Pumice	Lava	Lava	Lava	Pumice	Pumice	Pumice	on
	IGSN	IEOD G0001	IEOD G0003	IEOD G0001	IEOD G0001	IEOD G0001	IEOD G0003	IEOD G0003	IEOD G0003	IEOD G0001
	Analys	PC1-	P-SM-	PC1-	PC1-	PC1-	PA-04-	PB-07-	PB-07-	PC1-
	is	SM-22	12	10-283	10-302	01-69	27	20	13	06-01
	SiO 2	60.78	61.22	57.81	56.01	55.50	59.72	59.93	59.43	56.82
	TiO 2	0.91	0.64	0.92	1.11	1.07	0.69	0.64	0.63	0.86
	Al 2 O 3	19.02	19.34	19.11	18.46	18.37	18.98	18.78	19.05	18.26
	MgO	0.46	0.45	0.59	0.79	0.97	0.38	0.35	0.34	2.18
	CaO	0.97	0.80	0.39	0.35	0.29	0.55	0.70	0.64	0.24
	MnO	0.14	0.12	0.39	0.32	0.65	0.32	0.17	0.31	0.26
	FeO	3.71	3.27	4.74	4.97	5.20	3.61	3.53	3.33	8.42
	Na 2 O	8.58	8.61	10.93	11.71	11.59	9.66	9.60	9.66	5.67
	K 2 O	5.39	5.11	5.00	6.21	6.27	5.62	5.89	5.94	7.06
	Cl	n.d	0.13	n.d	n.d	n.d	0.39	0.24	0.29	n.d
	Sum	100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00
	Origin al sum	99.23	99.08	99.88	98.77	97.80	99.02	98.97	97.14	99.61
	Na 2 O+ K 2 O	13.97	13.72	15.93	17.92	17.86	15.28	15.49	15.60	12.72
	H 2 O wt.%	0.77	0.92	0.12	1.23	2.20	0.98	1.03	2.86	0.39
	T (ºC)	919	856	812	839	816	823	834	832	665
	n.d

Acknowledgements:

J o u r n a l P r e -p r o o f

Data Availability

Datasets related to this article can be found at http://dx.doi.org/10.20350/digitalCSIC/12715, hosted at the open-access repository DIGITAL.CSIC [START_REF] Dorado | Major elements geochemistry of Pico Cabras dome's explosive and effusive products (Tenerife)[END_REF].

Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.