G N Fabbro
email: g.fabbro@opgc.univ-bpclermont.fr

Timothy H Druitt

Stéphane Scaillet

Evolution of the crustal magma plumbing system during the build-up to the 22 ka caldera-forming eruption of Santorini (Greece)

Keywords: Santorini, magma reservoirs, melt accumulation, residence timescales, calderas

Evolution of the crustal magma plumbing system during

the build-up to the 22-ka caldera-forming eruption of Santorini (Greece)

to constrain the crustal residence time of the Cape Riva magma reservoir. The lavas were erupted between 39 and 25 ka, forming a ~2 km 3 complex of dacitic flows, coulees and domes up to 200 m thick (Therasia dome complex). The Therasia dacites show little chemical variation with time, suggesting derivation from one or more thermally buffered reservoirs. Minor pyroclastic layers occur intercalated within the lava succession, particularly near the top. A prominent pumice fall deposit probably correlates with the 26 ka Y-4 ash layer found in deep-sea sediments SE of Santorini. One of the last Therasia lavas to be discharged was a hybrid andesite formed by the mixing of dacite and basalt. The Cape Riva eruption occurred no more than 2,800 ± 1,400 y after the final Therasia activity. The Cape Riva dacite is similar in major element composition to the Therasia dacites, but is poorer in K and most incompatible trace elements (e.g., Rb, Zr, LREE). The same chemical differences are observed between the Cape Riva and Therasia hybrid andesites, and between the calculated basaltic mixing endmembers of each series. The Therasia and Cape Riva dacites are distinct silicic magma batches and are not related by shallow processes of crystal fractionation or assimilation. The Therasia lavas were therefore not simply precursory leaks from the growing Cape Riva magma reservoir. The change 21.8 ky ago from a magma series richer in incompatible elements to one poorer in those elements is one step in the well documented decrease

Introduction

Caldera-forming ignimbrite eruptions discharge large volumes (1-10 3 km 3 ) of silicic magma from shallow reservoirs [START_REF] Smith | Ash-flow magmatism[END_REF][START_REF] Mason | The size and frequency of the largest explosive eruptions on Earth[END_REF][START_REF] Miller | Supervolcanoes and their explosive supereruptions[END_REF]. The processes that generate such reservoirs, and the timescales on which those processes operate, are not completely understood (e.g., [START_REF] Costa | Residence times of silicic magmas associated with calderas[END_REF][START_REF] Bachmann | The magma reservoirs that feed supereruptions[END_REF][START_REF] Gelman | On the longevity of large upper crustal silicic magma reservoirs[END_REF]. This is particularly true of caldera systems such as Santorini that erupt crystal-poor silicic magmas [START_REF] Bachmann | On the origin of crystal-poor rhyolites: extracted from batholithic crystal mushes[END_REF][START_REF] Hildreth | Compositional zoning of the Bishop Tuff[END_REF][START_REF] Wilson | Rapid Rates of Magma Generation at Contemporaneous Magma Systems, Taupo Volcano, New Zealand: Insights from U-Th Model-age Spectra in Zircons[END_REF][START_REF] Druitt | Decadal to monthly timescales of magma transfer and reservoir growth at a caldera volcano[END_REF][START_REF] Gualda | Timescales of Quartz Crystallization and the Longevity of the Bishop Giant Magma Body[END_REF][START_REF] Allan | From mush to eruption in centuries: assembly of the super-sized Oruanui magma body[END_REF] The formation of caldera-sized reservoirs of crystal-poor silicic magma requires the generation of large volumes of silicic melt, followed by the segregation of that melt and its accumulation in the upper crust [START_REF] Lindsay | Magmatic evolution of the La Pacana caldera system, central Andes, Chile: compositional variation of two cogenetic, large-volume felsic ignimbrites[END_REF]Bachmann andBergantz 2004, 2008). Large volumes of silicic melt are generated in crustal hot zones by fractional crystallization of mantlederived basalt, partial melting of crustal rocks, defrosting of incompletely crystallized plutons [START_REF] Hildreth | Gradients in silicic magma chambers: Implications for lithospheric magmatism[END_REF][START_REF] Hildreth | Crustal contributions to arc magmatism in the Andes of Central Chile[END_REF][START_REF] Annen | The genesis of intermediate and silicic magmas in deep crustal hot zones[END_REF][START_REF] Solano | Melt segregation in deep crustal hot zones: a mechanism for chemical differentiation, crustal assimilation and the formation of evolved magmas[END_REF]. Fractional crystallization is driven by outgassing and cooling of magma during ascent [START_REF] Blundy | Petrologic reconstruction of magmatic system variables and processes[END_REF]. Partial melting of crustal rocks and pre-existing plutons is driven by heat advected by mantle-derived magma and magmatic volatiles [START_REF] Hildreth | Gradients in silicic magma chambers: Implications for lithospheric magmatism[END_REF][START_REF] Bachmann | Gas percolation in upper-crustal silicic crystal mushes as a mechanism for upward heat advection and rejuvenation of near-solidus magma bodies[END_REF][START_REF] Huber | Thermo-mechanical reactivation of locked crystal mushes: Melting-induced internal fracturing and assimilation processes in magmas[END_REF]. Silicic melt generation can occur over timescales of 10 5 to 10 1 years [START_REF] Brown | SHRIMP U-Pb dating of the preeruption growth history of zircons from the 340 ka Whakamaru Ignimbrite, New Zealand: Evidence for >250 k.y. magma residence times[END_REF][START_REF] Charlier | Magma generation at a large, hyperactive silicic volcano (Taupo, New Zealand) revealed by U-Th and U-Pb systematics in zircons[END_REF][START_REF] Bachmann | Gas percolation in upper-crustal silicic crystal mushes as a mechanism for upward heat advection and rejuvenation of near-solidus magma bodies[END_REF][START_REF] Burgisser | A rapid mechanism to remobilize and homogenize highly crystalline magma bodies[END_REF][START_REF] Klemetti | Magmatic perturbations in the Okataina Volcanic Complex, New Zealand at thousand-year timescales recorded in single zircon crystals[END_REF][START_REF] Huber | Thermo-mechanical reactivation of locked crystal mushes: Melting-induced internal fracturing and assimilation processes in magmas[END_REF]. Magmatic crystal mushes generated by these processes may remain at depth, or may erupt en masse to form crystal-rich ignimbrites (the monotonous intermediates of [START_REF] Hildreth | Gradients in silicic magma chambers: Implications for lithospheric magmatism[END_REF].

The formation of reservoirs of crystal-poor silicic magma requires the separation of melt from residual mush crystals, and the accumulation of that melt at a particular crustal level-either in situ within the mush (e.g. [START_REF] Bachmann | On the origin of crystal-poor rhyolites: extracted from batholithic crystal mushes[END_REF], or at higher levels in the plumbing system (e.g. [START_REF] Allan | From mush to eruption in centuries: assembly of the super-sized Oruanui magma body[END_REF]. The rate at which silicic melt can separate from crystals is limited by its high viscosity [START_REF] Mckenzie | The extraction of magma from the crust and mantle[END_REF][START_REF] Bachmann | The magma reservoirs that feed supereruptions[END_REF]. Possible mechanisms include gravitational separation through hindered settling of individual crystals, or compaction of a porous crystal network [START_REF] Bachmann | On the origin of crystal-poor rhyolites: extracted from batholithic crystal mushes[END_REF]. Melt migration driven by shear or gas filter pressing may generate small, local segregations [START_REF] Stevenson | Spontaneous small-scale melt segregation in partial melts undergoing deformation[END_REF][START_REF] Brown | Shear-zone systems and melts: feedback relations and self-organization in orogenic belts[END_REF][START_REF] Sisson | Gas-driven filter pressing in magmas[END_REF] that are subsequently concentrated into large melt lenses. Runaway reservoir growth requires rates of heat (and hence magma) input high enough to limit crystallisation and avoid plutonic death [START_REF] Annen | From plutons to magma chambers: Thermal constraints on the accumulation of eruptible silicic magma in the upper crust[END_REF][START_REF] Schöpa | The effects of magma flux variations on the formation and lifetime of large silicic magma chambers[END_REF][START_REF] Gelman | On the longevity of large upper crustal silicic magma reservoirs[END_REF].

Estimates of melt accumulation timescales for crystal-poor silicic magmas vary widely.

Assembly of the rhyolitic magma reservoir that discharged the 600-650 km 3 Bishop Tuff has been estimated as ~10 5 y, based on zircon ages and on the chemical similarity between the Bishop magma and earlier rhyolites [START_REF] Hildreth | Compositional zoning of the Bishop Tuff[END_REF]. However, element diffusion profiles and melt inclusion faceting in Bishop quartz crystals yield much shorter timescales (10 3 -10 2 y; [START_REF] Wark | Pre-eruption recharge of the Bishop magma system[END_REF][START_REF] Gualda | Timescales of Quartz Crystallization and the Longevity of the Bishop Giant Magma Body[END_REF]. The reservoir that fed the 530 km 3 Oruanui Ignimbrite at Taupo was assembled over 1600 years [START_REF] Wilson | Rapid Rates of Magma Generation at Contemporaneous Magma Systems, Taupo Volcano, New Zealand: Insights from U-Th Model-age Spectra in Zircons[END_REF][START_REF] Allan | From mush to eruption in centuries: assembly of the super-sized Oruanui magma body[END_REF], and that of the ~35 km 3 post-Oruanui 'Y' eruption was assembled over <1000 years [START_REF] Sutton | Late Quaternary evolution of a hyperactive rhyolite magmatic system: Taupo volcanic centre, New Zealand[END_REF]. The magma reservoir of the 30-60 km 3 Minoan eruption at Santorini underwent a spurt of rapid growth in the centuries preceding venting [START_REF] Druitt | Decadal to monthly timescales of magma transfer and reservoir growth at a caldera volcano[END_REF]. Ascent of silicic melt into shallow reservoirs can occur in pulses with durations that are short compared to the repose period between eruptions [START_REF] Druitt | Decadal to monthly timescales of magma transfer and reservoir growth at a caldera volcano[END_REF][START_REF] Parks | Evolution of Santorini Volcano dominated by episodic and rapid fluxes of melt from depth[END_REF].

In this paper we study a sequence of silicic and intermediate lavas at Santorini that were discharged prior to a ≳10 km 3 caldera-forming eruption of crystal-poor silicic magma (the 22 ka Cape Riva eruption). The lavas record the development of the crustal plumbing system over many thousands of years preceding the Cape Riva eruption. We present field, petrological and chemical data for these lavas, along with new, high-precision 40 Ar/ 39 Ar ages. The results enable us to place constraints on the residence timescale of the Cape Riva magma reservoir in the upper crust.

Geological Setting

Santorini Volcano lies on the Hellenic volcanic arc, which stretches between Greece and Turkey through the Aegean Sea, and owes its origin to the subduction of the African plate beneath the Eurasian plate (Le [START_REF] Pichon | The Hellenic arc and trench system: A key to the neotectonic evolution of the eastern Mediterranean area[END_REF][START_REF] Papazachos | The geometry of the Wadati-Benioff zone and lithospheric kinematics in the Hellenic arc[END_REF][START_REF] Nocquet | Present-day kinematics of the Mediterranean: A comprehensive overview of GPS results[END_REF]). Santorini lies on continental crust about 23 km thick [START_REF] Tirel | Aegean crustal thickness inferred from gravity inversion. Geodynamical implications[END_REF][START_REF] Karagianni | Shear velocity structure in the Aegean area obtained by inversion of Rayleigh waves[END_REF]). The boundary between the upper crust and lower crust lies at about 15 km depth [START_REF] Konstantinou | Crustal rheology of the Santorini-Amorgos zone: Implications for the nucleation depth and rupture extent of the 9 July 1956 Amorgos earthquake, southern Aegean[END_REF].

The volcanic history of Santorini has been described by [START_REF] Druitt | Santorini Volcano[END_REF]. Volcanism commenced about 650 ka (Fig. 1). From about 360 ka onwards, activity consisted of multiple large (>1 km 3 ) explosive eruptions alternating with periods of lava extrusion and minor explosive events.

The last major eruption (Minoan eruption) occurred about 1639-1616 BCE [START_REF] Manning | Chronology for the Aegean Late Bronze Age 1700-1400 B[END_REF] and discharged 30-60 km 3 of magma [START_REF] Pyle | New estimates for the volume of the Minoan eruption[END_REF][START_REF] Sigurdsson | 34 Marine investigations of Greece's Santorini volcanic field[END_REF]. Since the Minoan eruption there have been at least ten dacitic effusive eruptions, building up the Kameni intra-caldera volcano [START_REF] Pyle | Quantitative morphology, recent evolution, and future activity of the Kameni Islands volcano, Santorini, Greece[END_REF].

Santorini magmas are calc-alkaline to mildly tholeiitic, and range from basalt to rhyodacite in composition [START_REF] Druitt | Santorini Volcano[END_REF]. The silicic magmas are generally poor in phenocrysts (<5-20 vol% on a vesicle-free basis). Phase-equilibria experiments [START_REF] Andújar | Differentiation conditions of a basaltic magma from Santorini and its bearing on andesitic/dacitic magma production[END_REF]) have shown that mantle-derived basalt ascending beneath Santorini stagnates and fractionates to basaltic andesite at about 4 kb (~15 km, assuming a mean upper crustal density of 2640 kg.m -3 ; Konstantinou 2010), near the boundary between the upper and lower crust. The silicic magmas discharged during large eruptions are stored in the upper crust at 2 ± 0.5 kb (~8 km) immediately prior to eruption [START_REF] Cadoux | Magma storage conditions of large Plinian eruptions of Santorini Volcano[END_REF]).

Our study focuses on the products of the volcano from the period between 70 and 22 ka (Fig. 2). About 70 ky ago a caldera formed in the northern half of the volcanic field, probably as a result of one or more explosive eruptions [START_REF] Druitt | Santorini Volcano[END_REF]. Eruption of basaltic to andesitic lavas then built up a lava shield within this caldera (Skaros lava shield; 70-54 ka). The shield grew to over 350 m above present-day sea level, filling and partly overspilling the caldera [START_REF] Huijsmans | Calc-alkaline lavas from the volcanic complex of Santorini, Aegean Sea, Greece. A petrological and stratigraphical study[END_REF][START_REF] Huijsmans | Polybaric geochemical evolution of two shield volcanoes from Santorini, Aegean Sea, Greece: evidence for zoned magma chambers from cyclic compositional variations[END_REF][START_REF] Druitt | Santorini Volcano[END_REF]. The volume of the shield has been estimated at about 12 km 3 from reconstructions based on lavas exposed in the caldera cliffs [START_REF] Druitt | Santorini Volcano[END_REF].

Construction of the shield culminated 54 ky ago with an explosive eruption called Upper Scoria 2, which generated a rhyodacitc Plinian fall deposit overlain by voluminous andesitic scoria flows [START_REF] Mellors | Spatter-rich pyroclastic flow deposits on Santorini, Greece[END_REF]. Following Upper Scoria 2, about 2 km 3 of silicic lavas were extruded across the summit and western flank of Skaros to form the Therasia dome complex.

At ~22 ka, a large silicic explosive eruption (the Cape Riva eruption) collapsed the Skaros-Therasia edifice [START_REF] Druitt | Caldera formation on Santorini and the physiography of the islands in the late Bronze Age[END_REF]. The eruption had an initial Plinian phase from a vent located somewhere in the northern part of the volcanic field. This was followed by the discharge of pyroclastic flows that laid down welded ignimbrite, non-welded ignimbrite and lithicrich lag deposits up to 25 m thick all over the islands [START_REF] Druitt | A proximal ignimbrite breccia facies on Santorini, Greece[END_REF]. The volume of magma discharged during the Cape Riva eruption is poorly constrained, since most of the ignimbrite lies under the sea. However distal tephra from the eruption, recognised as the Y-2 marine ash bed, is found over a very wide area of the eastern Mediterranean and as far north as the Island of Lesvos and the Sea of Marmara [START_REF] Keller | Explosive volcanic activity in the Mediterranean over the past 200,000 yr as recorded in deep-sea sediments[END_REF][START_REF] Thunell | The age, origin, and volcanological significance of the Y-5 ash layer in the Mediterranean[END_REF][START_REF] Federman | Electron microprobe correlation of tephra layers from Eastern Mediterranean abyssal sediments and the Island of Santorini[END_REF][START_REF] Wulf | Marine tephra from the Cape Riva eruption (22 ka) of Santorini in the Sea of Marmara[END_REF][START_REF] Margari | Mediterranean tephra stratigraphy revisited: Results from a long terrestrial sequence on Lesvos Island, Greece[END_REF][START_REF] Asku | Occurrence, stratigraphy and geochemistry of late Quaternary tephra layers in the Aegean Sea and the Marmara Sea[END_REF]. The dispersal area and thickness of the Y-2 ash are similar to those of the Z-2 ash from the Minoan eruption, suggesting that the Cape Riva and Minoan eruptions were of comparable magnitude [START_REF] Narcisi | Quaternary stratigraphy of distal tephra layers in the Mediterranean-an overview[END_REF][START_REF] Asku | Occurrence, stratigraphy and geochemistry of late Quaternary tephra layers in the Aegean Sea and the Marmara Sea[END_REF]. Graphical integration of Y-2 ash thicknesses recorded by the above authors yields a minimum volume of ~10 km 3 . Since distal ashes commonly have comparable volumes to the parent ignimbrite (e.g. Pyle 1990), we very approximately infer a volume of > 20 km 3 for the Cape Riva products, equivalent to >10 km 3 of magma.

Remnants of the Skaros and Therasia lavas are widely exposed in the northern half of the present day complex. The Skaros lavas are thickest (up to 300 m) on Thera, and the Therasia lavas are thickest (up to 200 m) on Therasia. The products of the Therasia and Cape Riva eruptions are the focus of the present paper.

Methodology

Lavas of the Therasia dome complex cropping out in the caldera cliffs were photographed from a boat, and the photos were merged using computer software and interpreted to produce synthetic sections detailing the relationships and lateral extents of individual lavas. Correlations were checked by onland observations, and stratigraphic relationships were mapped out. Pumice layers and palaeosols between the lavas were also mapped.

Lavas from four key stratigraphic levels were dated using the 40 Ar/ 39 Ar technique. The groundmass of each sample was separated, hand-picked, and cleaned in an ultrasonic bath of dilute nitric acid prior to irradiation in the Cd-lined fast neutron slot β1 of the Osiris reactor (CEA, Saclay) with sanidine ACR (1.206 ± 0.002 Ma, Renne et al. 2010, 2011). Upon receipt from the nuclear reactor, the samples were analysed by multiple laser fusion using a high-sensitivity MM5400 mass spectrometer operated in pulse-counting mode following the experimental and correction procedures of [START_REF] Scaillet | Oldest human footprints dated by Ar/Ar[END_REF][START_REF] Scaillet | High-resolution 40 Ar/ 39 Ar chronostratigraphy of the post-caldera (<20 ka) volcanic activity at Pantelleria, Sicily Strait[END_REF]. More than 25 individual ages were extracted from each sample via a two-step fusion of ~10 mg of groundmass replicates (see procedural details in [START_REF] Scaillet | High-resolution 40 Ar/ 39 Ar chronostratigraphy of the post-caldera (<20 ka) volcanic activity at Pantelleria, Sicily Strait[END_REF].

Representative samples of lava and pumice from the Therasia complex, and pumice and scoria from the Cape Riva deposits, were collected for chemical analysis. All samples were chosen to be as fresh and glassy as possible. Groundmass separates of selected lavas were obtained in order to analyse the compositions of the melt phases of the magmas. This was done using a magnetic separator, and interstitial glasses of pumice samples were concentrated by flotation in water. Remaining crystals were then removed by hand picking under an optical microscope. Major elements were analysed using inductively coupled plasma atomic emission spectroscopy (ICP AES) at the Laboratoire Magmas et Volcans, Université Blaise Pascal, Clermont-Ferrand. Measurements were calibrated using three standards: a blank (LiBO 2 ), basalt (BR) and granite (GH). The DR-N and BHVO-2 standards were then passed as unknowns. Trace elements were analysed using inductively coupled plasma mass spectroscopy (ICP MS) at the Institut des Sciences de la Terre, Université de Grenoble. The ICP MS analyses were calibrated using the BR standard, and the BVHO-2 and AGV-1 standards were passed as unknowns. Some previous Cape Riva samples [START_REF] Druitt | Santorini Volcano[END_REF] were re-analysed for comparison with the data of those authors. These comparisons showed good agreement between the two datasets for the elements used in the present paper.

Phenocryst contents were calculated by mass balance from Zr concentrations in whole rock and groundmass analyses of individual samples, by assuming that the crystals contain no Zr. Zirconium behaves incompatibly in all magmas younger than 530 ka at Santorini [START_REF] Huijsmans | Calc-alkaline lavas from the volcanic complex of Santorini, Aegean Sea, Greece. A petrological and stratigraphical study[END_REF][START_REF] Druitt | Santorini Volcano[END_REF], and zircon has never been observed as a mineral phase in those magmas.

Proportions of the different phenocryst phases were estimated by image analysis of thin sections using the ImageJ software package (Rasband 1997). Mineral compositions were analysed using the Cameca SX 100 electron microprobe at the Laboratoire Magmas et Volcans, Université Blaise Pascal, Clermont-Ferrand, using a beam current of 15 nA. Glasses were analysed with a beam current of 4 nA and a defocussed beam (10-15 µm) in order to limit Na loss. Fe-Ti oxide compositions were analysed either in touching pairs or in pairs (that would have been in contact with the same melt) adhering to the outside of the same pyroxene crystal. Magmatic temperatures and oxygen fugacities were calculated with the ILMAT software package [START_REF] Lepage | ILMAT: an Excel worksheet for ilmenite-magnetite geothermometry and geobarometry[END_REF] using the formulation of [START_REF] Andersen | New (and final!) models for the Ti-magnetite/ilmenite geothermometer and oxygen barometer[END_REF] and [START_REF] Stormer | The effects of recalculation on estimates of temperature and oxygen fugacity from analyses of multicomponent iron-titanium oxides[END_REF]. This formulation has been found to give good agreement with data from phase-equilibria experiments within the 850-950 °C temperature range [START_REF] Cottrell | Petrologic and experimental evidence for the movement and heating of the pre-eruptive Minoan rhyodacite (Santorini, Greece)[END_REF][START_REF] Blundy | Petrologic reconstruction of magmatic system variables and processes[END_REF]. Values we obtained using the formulation of Ghioso and Evans (2008) are10-20 °C and 0.2-0.4 fO 2 log units higher than those obtained using Andersen and Lindsley (1988), for the temperature range in question.

Results

Field and stratigraphic relationships

Photographs and sketches of the Therasia cliffs are shown in Fig. 3; schematic diagrams summarising the architecture of the lavas are shown in Fig. 4 (with individual lavas numbered for reference). Lavas of the Therasia dome complex overlie Upper Scoria 2, separated by a palaeosol.

They make up much of the present-day cliffs of Therasia (flows 1-24), and one lava crops out at the top of the caldera wall north of Fira town (flow 25). Thin lava flows occupying the same stratigraphic position (between Upper Scoria and Cape Riva) occur beneath the town of Oia (flow 26; Andesite of Oia of [START_REF] Druitt | Santorini Volcano[END_REF]).

The lava succession on Therasia consists of many individual lava flows, coulees and domes (termed flows for short). Individual flows range in thickness up to 60 m (Fig. 5a); thin flows tend to be dark grey and glassy, whereas thicker ones are pale grey and de-vitrified. Many exhibit flow banding that is most evident in the thicker, de-vitrified flows. The greatest accumulated thicknesses occur near Cape Tripiti (~150 m) and Mount Viglos (~200 m), where, at each location, nine flows are stacked. Correlations of individual flows between the Tripiti and Viglos sections is difficult, as only two flows (3 and 4) are continuous between them. Flows 1 and 24 are compositionally very similar, and are probably the same flow. This is also true of flows 11 and 22.

Most of the lava flows on Therasia are dacitic to rhyodacitic, with two exceptions. First, the basal flows 1 and 24 are andesitic, and we refer to them jointly as the lower Therasia andesite.

Second, the topmost flow on Mount Viglos (flow 22) is also andesitic, and we refer to this (and the compositionally similar flow 11) as the upper Therasia andesite. Enclaves of quenched basaltic magma with crenulated margins occur in some of the lowest lavas (flows 1 and 3) and towards the top of the succession (flows 22 and 25); rare gabbroic enclaves also occur. The widespread distribution of the Therasia lavas show that they were fed from vents extending from the summit to the western flank of the Skaros shield (Fig. 2). The feeder dyke of flow 3 is preserved at Cape Tripiti (Fig. 3 b,c). The dyke is oriented NE-SW, parallel to the main dyke trend in northern Thera and the alignment of the recent vents on the Kameni Islands [START_REF] Heiken | Caldera development during the Minoan eruption, Thira, Cyclades, Greece[END_REF][START_REF] Druitt | Santorini Volcano[END_REF].

At least four dacitic pumice fall units and a phreatomagmatic tuff occur intercalated within the Therasia succession (Fig. 5c). The pyroclastic units are concentrated towards the top of the succession (younger than flow 7 at Tripti, and younger than flow 20 at Viglos), showing an increasing tendency for explosive activity with time during eruption of the Therasia dome complex. A single pumice fall deposit crops out between flows 24 and 27 below Manolas. The most prominent fall deposit is up to a metre thick and widespread in the cliffs of southern Therasia (Fig. 5c). It occurs stratigraphically between lava flows 7 and 8, and is the product of a Plinian not recognized in previous studies. We name this unit the Cape Tripiti pumice fall deposit.

Palaeosols occur at several levels in the Therasia succession. A thick one separates the entire Therasia succession from Upper Scoria 2 (Fig. 5d). Another separates the lower Therasia andesite from overlying dacitic flows, showing that eruption of the lower Therasia andesite was both preceded, and followed, by significant time breaks. Local palaeosols also occur between some of the pyroclastic layers.

The andesite of Oia is both underlain and overlain by thick palaeosols. Long periods therefore separated its eruption from both the preceding Upper Scoria 2 eruption and the subsequent Cape Riva eruption. Four thin pumice fall layers (5-25 cm thick) occur within the palaeosol overlying the lava (sequence M11 of [START_REF] Vespa | Interplinian explosive activity of Santorini volcano (Greece) during the past 150,000 years[END_REF]; they may correlate with the pumice layers intercalated within the lavas on Therasia, but this has not been checked chemically.

Products of the Cape Riva eruption are observed to overlie all lavas of the Therasia dome complex. They have been described in detail by [START_REF] Druitt | A proximal ignimbrite breccia facies on Santorini, Greece[END_REF], [START_REF] Druitt | Vent evolution and lag breccia formation during the Cape Riva eruption of Santorini, Greece[END_REF], [START_REF] Druitt | Santorini Volcano[END_REF]. The products of the eruption are predominantly dacitic, but minor amounts of andesitic scoria were erupted during the initial Plinian phase.

Age constraints

Radiometric dating of the Therasia lavas

The ages of flows 1, 3, 21 and 25 are reported as probability density plots in Fig. 6, along with the corresponding Gauss plots that reflect the statistical distribution of individual ages for each sample. Complete 40 Ar/ 39 Ar analytical data are reported in Supplementary Table S1 andare summarized in Table 1, along with 2σ errors. All four samples exhibit relatively well behaved 40 Ar/ 39 Ar systematics in the form of unimodal density plots, with no (or only slightly) pronounced tails on either side of the mode. The homogeneity of the samples is reflected by the linear arrays formed by individual ages on the Gauss plots, indicating that they follow the distribution expected from the propagated Gaussian experimental errors. One exception is flow 3, which exhibits an age spread in excess of the variance expected from the analytical errors (i.e., excess-error scatter). This sample is, along with flow 1, the least glassy of the four, and both are characterized by slightly higher errors and some excess-error scatter. This suggests that flows 1 and 3 may have been affected by postcooling alteration close to sea level near the base of the sequence, resulting in higher apparent ages (presumably due to K loss). In contrast, flows 21 and 25 are very glassy and pristine, with unusually tight error bars; especially flow 21.

To account for secondary alteration effects, the data from flows 1 and 3 were statistically screened by computing a weighted mean age using a MSWD cut-off value. This includes only the youngest sub-population conforming to a Gaussian distribution within each sample (see procedure in [START_REF] Gansecki | 40 Ar/ 39 Ar geochronology of rhyolites erupted following collapse of the Yellowstone caldera, Yellowstone Plateau volcanic field: implications for crustal contamination[END_REF][START_REF] Scaillet | High-resolution 40 Ar/ 39 Ar chronostratigraphy of the post-caldera (<20 ka) volcanic activity at Pantelleria, Sicily Strait[END_REF]. In every instance, the weighted mean age agrees with the age derived from the best-fit line through the corresponding Gauss-plot array (Fig. 6). In what follows we cite the weighted mean ages.

The ages all are consistent with observed field stratigraphic constraints, as summarized in Fig. 7. The age of the basal flow (flow 1; 48.2 ± 2.4 ka) is consistent with the presence of a palaeosol separating it from the underlying Upper Scoria 2 (previously dated by 40 Ar/ 39 Ar at 54 ± 3 ka by [START_REF] Druitt | Santorini Volcano[END_REF], and with another palaeosol separating it from the overlying flow 3 (39.4 ± 2.2 ka). Flow 21 yields an age of 33.2 ± 1.1 ka, and flow 25 (at Fira) gives an age of 24.6 ± 1.3 ka.

Taken as a whole, our 40 Ar/ 39 Ar data between the base (48.2 ka) and the top (24.6 ka) of the lava sequence define a ~24 ky duration for the constuction of the Therasia dome complex

Correlation of the Cape Tripiti Pumice with the Y-4 deep-sea ash layer

The Cape Tripiti pumice is the most prominent pyroclastic layer in the Therasia sequence, and we have explored the possibility that, like most Plinian eruptions of Santorini, [START_REF] Keller | Explosive volcanic activity in the Mediterranean over the past 200,000 yr as recorded in deep-sea sediments[END_REF][START_REF] Federman | Electron microprobe correlation of tephra layers from Eastern Mediterranean abyssal sediments and the Island of Santorini[END_REF][START_REF] Vinci | Distribution and chemical composition of tephra layers from Eastern Mediterranean abyssal sediments[END_REF][START_REF] Schwarz | Tephrakorrelation im östlichen Mittelmeer (Meteor M40/4 Kerne)[END_REF][START_REF] Wulf | Marine tephra from the Cape Riva eruption (22 ka) of Santorini in the Sea of Marmara[END_REF][START_REF] Asku | Occurrence, stratigraphy and geochemistry of late Quaternary tephra layers in the Aegean Sea and the Marmara Sea[END_REF], the Cape Tripiti eruption left a recognisable ash layer in deep-sea sediments of the Aegean area. Previous studies have recognized a 2-7 cm-thick ash layer (Y-4 ash) preserved to the SE of Santorini; this ash lies stratigraphically beneath the Cape Riva Y-2 ash layer, and has an age of 25.8 ka estimated by interpolation in the sedimentary sequence of one core [START_REF] Schwarz | Tephrakorrelation im östlichen Mittelmeer (Meteor M40/4 Kerne)[END_REF]. The uncertainty on this age could be ± 2 ka (J Keller, pers. comm). The mineralogy of the Y-4 ash (plag, opx, cpx) pinpoints its source to Santorini [START_REF] Vinci | Distribution and chemical composition of tephra layers from Eastern Mediterranean abyssal sediments[END_REF]. [START_REF] Schwarz | Tephrakorrelation im östlichen Mittelmeer (Meteor M40/4 Kerne)[END_REF] explored the possibility that the Y-4 correlates with the rhyodacitic Plinian phase of Upper Scoria 2; however, the 40 Ar/ 39 Ar age data described above rule out this correlation, and show that the Y-4 lies chronologically in the period of the Therasia dome complex. We have analysed the interstitial glass of three pumice lumps from the Cape Tripiti deposit, and find excellent agreement with glass composition of the Y-4 (Fig. 8, Supplementary Table S2). A 26 ka age for the Cape Tripiti is consistent with all other age constraints (Fig. 7).

Synthesis of published dates for the Cape Riva eruption

The Cape Riva eruption has been dated previously by radiocarbon on charcoal from beneath the ignimbrite and via δ 18 O wiggle matching in deep-sea sequences hosting the distal equivalent Y-2 tephra layer (data and sources in Table 2). Calibration of the raw radiocarbon data against the curve of Fairbanks et al. ( 2005) returns a mean age of 21.8 ± 0.4 ka for Cape Riva (Table 2).

This yields an interval of 2,800 ± 1,400 (2σ) y between the youngest dated Therasia lava (flow 25) and the Cape Riva eruption. This is a maximum estimate for the interval separating the Cape Riva eruption from the Therasia lavas, since some of the undated flows on Therasia may be younger than flow 25, or a younger flow could have been erupted and not preserved.

Mineral chemistry and assemblages

In this section we present the petrology and chemistry of Therasia lavas and pumices younger than ~39 ka (i.e., flow 3), as well as the products of the Cape Riva eruption (Fig. 9). We exclude the lower Therasia andesite, which is significantly older, as well as the andesite of Oia. In so doing we focus on the effusive leaks of dacite during the build-up to the Cape Riva eruption.

Therasia products

Therasia dacites. The Therasia dacites have whole rock SiO 2 contents of 64.6-68.7 wt% and MgO contents of 0.7-1.5 wt%, with groundmass (i.e., melt) SiO 2 contents of 64.7-69.1 wt %. No systematic evolution of either whole rock or groundmass composition is observed with height in the lava succession (Fig. 7). The dacites contain 1-17 wt % of phenocrysts (with a smallest dimension >0.5 mm) of plagioclase (75-85 vol%), 2 pyroxenes (10-20 %, with opx > cpx), Fe-Ti oxides

(5-8 %) and trace amounts of olivine. Apatite occurs as inclusions in orthopyroxene crystals. The glassy groundmass contains microlites of feldspar and Fe-Ti oxides. Plagioclase phenocrysts are normally zoned, with cores of An 38-89 and rims of An 39-51 , and commonly contain multiple internal dissolution surfaces and sawtooth zoning. The rims are euhedral, and are compositionally similar to plagioclase microlites in the groundmass (An 30-51 ). Orthopyroxene phenocrysts are euhedral and unzoned, and have compositions of Wo 3-4 En 54-60 Fs 36-42 . Clinopyroxenes are also euhedral and unzoned, and their compositions are Wo 39-42 En 36-43 Fs 15-22 . Some phenocrysts occur as aggregates of multiple euhedral crystals with brown interstitial glass. Xenocrysts of calcic plagioclase (An 60- 89 ) occur frequently in the dacites, along with more calcic clinopyroxenes (Wo 47-50 En 30-44 Fs 8-20 ).

There are also fragments of sub-solidus olivine-bearing gabbro, with some olivines that have broken down to form pyroxene-magnetite symplectites. Fe-Ti oxides from three different dacitic pumice fall deposits within the Therasia sequence record magmatic temperatures of 875 ± 5 °C at an f O2 of -13.4 ± 0.1, 896 ± 12 °C at an f O2 of -12.7 ± 0.3, and 926 ± 9 °C at an f O2 of -11.4 ± 0.2 (2σ, where σ is the standard error of the mean, Supplementary Table S3).

The similarity between phenocryst rims and groundmass microlite compositions suggests an equilibrium phenocryst rim assemblage in these lavas. Sawtooth zoning of plagioclase is interpreted as recording repeated recharge of the magmas by hotter magma during phenocryst growth. Glassbearing phenocryst clusters were probably derived from crystal mush of the reservoir margins. Plagioclase populations 1 and 2 texturally and compositionally resemble plagioclase populations 1 and 2 (respectively) in the basaltic enclaves; population 3 resembles plagioclase phenocrysts in the dacites. The olivine rims have a molar Mg/Fe ratio of 1.99-2.22, which is in, or close to, equilibrium with the groundmass (Mg/Fe = 0.76), assuming a partition coefficient of between 0.26 and 0.36 [START_REF] Roeder | Olivine-liquid equilibrium[END_REF]; the cores have an Mg/Fe ratio of 4.11-4.81 and grew in equilibrium with a basaltic melt. The two pyroxenes are indistinguishable from the same phases in the dacites. The upper Therasia andesite is interpreted as a hybrid magma formed by the mixing of the basalt (containing plagioclase of populations 1 and 2 + olivine) represented by the enclaves, with typical Therasia dacite (containing population-3 plagioclase + opx + cpx). Mixing occurred long enough prior to eruption to permit physical homogenization of the resulting hybrid melt, but not long enough for crystals to grow rims in equilibrium with that melt, or for those from the dacite to be resorbed.

Cape Riva products

Cape Riva Dacite. Dacitic pumices of the Cape Riva eruption have whole rock compositions of 64-67 wt% SiO 2 and 1.0-1.9 wt% MgO, and interstitial glasses with 70-72 wt% SiO 2 . Phenocryst The olivines and population-1 plagioclases in the andesite resemble phenocrysts present in the basaltic enclaves, whereas population-2 plagioclase resembles phenocrysts in the dacites.

Genesis of the Cape Riva andesite is inferred to have involved the mixing of basaltic and dacitic magmas. Eruption occurred long after mixing for the hybrid glass to become homogeneous at the scale of the electron beam (~10 µm).

Whole rock chemistry and mixing systematics

Representative samples of each unit are shown in Table 3, and the complete dataset is presented in Supplementary Table S4. We have used a series of variation diagrams showing the whole rock compositions of the Therasia and Cape Riva magmas, plus those of the lavas of the Skaros shield (from [START_REF] Huijsmans | Calc-alkaline lavas from the volcanic complex of Santorini, Aegean Sea, Greece. A petrological and stratigraphical study[END_REF] to gain insight into the petrogenesis of the different magmas (Figs 11 and 12). Typical fractionation trends for Santorini magmas are also shown [START_REF] Mann | Trace element geochemistry of high alumina basalt-andesite-dacite-rhyodacite lavas of the Main Volcanic Series of Santorini Volcano, Greece[END_REF][START_REF] Huijsmans | Calc-alkaline lavas from the volcanic complex of Santorini, Aegean Sea, Greece. A petrological and stratigraphical study[END_REF][START_REF] Druitt | Santorini Volcano[END_REF]. Fig. 11 shows the variations of five key major oxides (CaO, MgO, FeO, TiO 2 , P 2 O 5 ) and two strongly compatible trace elements (Cr and Ni). On the plots of CaO and MgO (also Na 2 O, Al 2 O 3 , V and Sc) on which typical fractionation trends are weakly curved, all the Therasia and Cape Riva magmas fall on, or close to, the fractionation trend. However, on the plots of FeO, TiO 2 , P 2 O 5 , Cr and Ni, on which the fractionation trends are strongly curved, the Cape Riva hybrid andesite falls systematically off the fractionation trend. This is what we would expect to see if it was generated by the mixing of mafic and silicic end-members. The upper Therasia hybrid andesite also falls off the fractionation trend on plots of FeO and TiO 2 , (it does not on the other plots because mixing occurred along the fractionation trend, not across it).

Mixing models using the 'PetroGraph' software [START_REF] Petrelli | PetroGraph: A new software to visualize, model, and present geochemical data in igneous petrology[END_REF]) successfully reproduce the compositions of the upper Therasia and Cape Riva hybrid andesites (Table 4). In the case of the upper Therasia andesite, low Cr and Ni require the mafic end-member to also have low Cr and Ni. The relatively high P 2 O 5 , close to the fractionation trend, requires the silicic end-member to have a high P 2 O 5 content, limiting it to a silica content of 64-67 wt%. The upper Therasia andesite can be successfully reproduced by mixing ~60 wt% of a typical Therasia dacite with ~40 wt% of mafic magma with the same composition as a basaltic enclave (GS10-43) collected from the same flow (sum of the squares of the residuals of 0.16). The Cape Riva andesite has higher Cr and Ni than the upper Therasia andesite, requiring that the mafic end-member also has higher contents of these elements. The composition of the Cape Riva andesite can be modelled by mixing ~60 wt% of Cape Riva dacite with ~40 wt% of an average Skaros basalt (sum of the squares of the residuals of 0.14). However, the calculated Ni content is higher than that measured in the Cape Riva andesite, suggesting that the mafic endmember had lower Ni than the average Skaros basalt.

Despite their broadly similar compositions in terms of silica content and many other major and trace elements, most of the Therasia dacites are enriched in incompatible elements such as K, Rb and Zr (also Nb, Ta, Th, Hf and LREE) compared to the Cape Riva dacite (Fig. 12). LREE are also more enriched in the Therasia dacites relative to the HREE. For example, the Therasia dacites have an average La/Yb ratio of 5.91 ± 0.16 (2σ), while the Cape Riva dacite has a ratio of 4.85 ± 0.04. Amongst the HFSE, Nb and Ta are more enriched than Zr and Hf. The 23 analysed samples of Cape Riva pumice form a tight linear cluster on Fig. 12, showing that the magma was well mixed. All of the 11 analysed Therasia lavas younger than ~39 ka, and most of the intercalated pumice horizons, similarly form a tight linear cluster (at higher incompatible contents than the Cape Riva, for a given SiO 2 content). However, some of the Therasia pumices overlap with the Cape Riva field for some elements. Most prominent of these is the Cape Tripiti pumice (Fig. 5c), which lies in, or close to, the Cape Riva field for most incompatible elements except K, suggesting that the magma that fed the Cape Tripiti eruption had some chemical characteristics intermediate between the two groups of dacite.

The differences between the Therasia and Cape Riva dacites are also seen between the corresponding hybrid andesites of these two series. Despite having a similar SiO 2 content, the upper Therasia hybrid andesite is enriched in incompatible elements compared to the Cape Riva hybrid andesite (e.g. 2.1 wt% K 2 O compared to 1.7 wt%, respectively). Like the Therasia dacites, the Therasia andesite is also enriched in LREE relative to HREE, and enriched in Nb and Ta relative to Zr and Hf compared top the Cape Riva andesite. This is also reflected in the calculated mafic mixing end-members of the two hybrid andesites shown on Fig. 12, although the difference is subtle.

Discussion

Origin of the magma series

The Therasia and Cape Riva dacites have similar whole rock major element compositions (for all major elements except K), phenocryst assemblages, phenocryst rim compositions and Fe-Ti oxide temperatures, although the Cape Riva dacite has slightly more evolved interstitial melt. The main difference between the two magmas is that the Cape Riva dacite is depleted in incompatible 20 elements compared to the Therasia dacite. The Cape Riva hybrid andesite is also incompatibledepleted relative to the upper Therasia hybrid andesite, and there are indications that the basaltic mixing end-members were similarly different.

Since the Therasia and Cape Riva magma series form parallel trends on incompatible element variation diagrams (Fig. 12), and have different LREE/HREE ratios, they cannot be related to each other simply by closed-system crystal fractionation schemes like those explored in previously published papers [START_REF] Nicholls | Petrology of Santorini Volcano, Cyclades, Greece[END_REF][START_REF] Mann | Trace element geochemistry of high alumina basalt-andesite-dacite-rhyodacite lavas of the Main Volcanic Series of Santorini Volcano, Greece[END_REF][START_REF] Druitt | Santorini Volcano[END_REF]. Neither can the Cape Riva dacite be generated by simple backmixing of Therasia dacite with an incompatible-depleted basalt, since mixing would displace the silicic compositions almost parallel to the compositional trends rather than perpendicular to them. Crustal contamination of Therasia dacite to produce Cape Riva dacite is also unlikely, as this would be expected to increase incompatible element concentrations, not decrease them [START_REF] Barton | Sr isotope and trace element evidence for the role of continental crust in calc-alkaline volcanism on Santorini and Milos, Aegean Sea, Greece[END_REF][START_REF] Druitt | Santorini Volcano[END_REF]. Moreover, the two dacite types have very similar whole-rock 87 Sr/ 86 Sr ratios (0.7050 and 0.7049 respectively; [START_REF] Briqueu | Isotope geochemistry of recent magmatism in the Aegean arc: Sr, Nd, Hf, and O isotopic ratios in the lavas of Milos and Santorinigeodynamic implications[END_REF][START_REF] Druitt | Santorini Volcano[END_REF][START_REF] Zellmer | Timescales of destructive plate margin magmatism: new insights from Santorini, Aegean volcanic arc[END_REF], ruling out a significant difference in the extent of upper crustal contamination. The Therasia and Cape Riva magma series (basalt, dacite and hybrid andesite in each case) represent two fundamentally distinct magma batches that cannot be related to each other in any simple way by shallow-level processes. This conclusion is supported by comparison of the two magma series with longerterm geochemical trends at Santorini [START_REF] Huijsmans | Calc-alkaline lavas from the volcanic complex of Santorini, Aegean Sea, Greece. A petrological and stratigraphical study[END_REF][START_REF] Huijsmans | Geochemistry and evolution of the calc-alkaline volcanic complex of Santorini, Aegean Sea, Greece[END_REF][START_REF] Druitt | Santorini Volcano[END_REF].

The same incompatible elements have decreased progressively in Santorini magmas over the last 530 ka, such that the lavas of the historical Kameni Volcano are the most incompatible-depleted (Fig. 12). The difference between the (older) Therasia and (younger) Cape Riva series represents one step in this longer-term evolution. The magnitude of the decrease is similar for most incompatible elements, so that the ratios between them (e.g. K/Zr, Rb/Zr, Rb/La) have remained approximately constant with time. However, the LREE have become depleted relative to HREE, a change that is also apparent between Cape Riva and Therasia (Fig. 12d). Similar changes also occur between different high field strength elements: Nb and Ta concentrations drop faster than those of Zr and Hf. Progressive depletion in K and other incompatible elements with time has also occurred at other centres in the Aegean region [START_REF] Francalanci | A West-East traverse along the magmatism of the south Aegean volcanic arc in the light of volcanological, chemical and isotope data[END_REF]. A decrease in the extent of crustal contamination with time, as might be expected from the progressive sealing-off of ascending magmas from the crust, is not tenable; there is no evidence for a decrease in the 87 Sr/ 86 Sr ratio with time at Santorini. Isotopic signatures fluctuate with time [START_REF] Barton | Sr isotope and trace element evidence for the role of continental crust in calc-alkaline volcanism on Santorini and Milos, Aegean Sea, Greece[END_REF][START_REF] Briqueu | Isotope geochemistry of recent magmatism in the Aegean arc: Sr, Nd, Hf, and O isotopic ratios in the lavas of Milos and Santorinigeodynamic implications[END_REF][START_REF] Druitt | Santorini Volcano[END_REF][START_REF] Martin | Using the Sr isotope compositions of feldspars and glass to distinguish magma system components and dynamics[END_REF], and some young melts are amongst the most radiogenic in the history of the volcano [START_REF] Vaggelli | Highly Sr radiogenic tholeiitic magmas in the latest inter-Plinian activity of Santorini volcano, Greece[END_REF][START_REF] Martin | Using the Sr isotope compositions of feldspars and glass to distinguish magma system components and dynamics[END_REF]. Incompatible trace element contents and isotopic signatures at Santorini are decoupled, ruling out a simple common origin. A more likely explanation for the observed trends lies in the nature of the mantle sources of the parental basalts feeding the volcanic system. Possibilities include an increase with time in the degree of source depletion, an increase of source melt fraction, or a decrease in degree of source metasomatism by slab-derived fluids or melts. All of these mechanisms could potentially account for basaltic parents with decreasing incompatible element contents with time [START_REF] Huijsmans | Geochemistry and evolution of the calc-alkaline volcanic complex of Santorini, Aegean Sea, Greece[END_REF][START_REF] Clift | The trace-element characteristics of Aegean and Aeolian volcanic arc marine tephra[END_REF][START_REF] Zellmer | Timescales of destructive plate margin magmatism: new insights from Santorini, Aegean volcanic arc[END_REF][START_REF] Francalanci | A West-East traverse along the magmatism of the south Aegean volcanic arc in the light of volcanological, chemical and isotope data[END_REF][START_REF] Bailey | Formation of heterogeneous magmatic series beneath North Santorini, South Aegean island arc[END_REF]. A mantle origin is supported by the recent discovery at Santorini of multiple co-existing basalt types with different trace element and isotopic signatures [START_REF] Bailey | Formation of heterogeneous magmatic series beneath North Santorini, South Aegean island arc[END_REF][START_REF] Vaggelli | Highly Sr radiogenic tholeiitic magmas in the latest inter-Plinian activity of Santorini volcano, Greece[END_REF].

Changing proportions of different parental basalts that ascend into the crust, where they mix and differentiate at between 4 and 2 kb to intermediate and silicic compositions [START_REF] Andújar | Differentiation conditions of a basaltic magma from Santorini and its bearing on andesitic/dacitic magma production[END_REF][START_REF] Cadoux | Magma storage conditions of large Plinian eruptions of Santorini Volcano[END_REF], may account for the observed temporal variations of trace element chemistry.

Irrespective of the exact explanation, our results demonstrate the availability of chemically distinct batches of silicic magma within the crustal plumbing system beneath Santorini, as has been demonstrated previously for basalts [START_REF] Bailey | Formation of heterogeneous magmatic series beneath North Santorini, South Aegean island arc[END_REF].

Reconstruction of events leading up to the Cape Riva eruption

Construction of the ~12 km 3 basaltic-to-andesitic Skaros shield between 67 and 54 ka represented a period where the eruption rate was close to the average for Santorini (~1 km 3 .ky -1 ; [START_REF] Druitt | Santorini Volcano[END_REF]). The Skaros period culminated at 54 ka in the Upper Scoria 2 explosive eruption.

Following Upper Scoria 2, the system stagnated and entered a ~15 ky-long period of near-repose until effusive activity resumed at about 39 ka. Only two lava flows are preserved in the cliffs of Therasia from this period: the ~48 ka lower Therasia andesite (flows 1 and 24) and an inaccessible flow (flow 2) immediately above it. The andesite of Oia could also belong to this period; it is chemically very similar to Upper Scoria 2 and could be residual magma from that eruption. This period of reduced activity, during which the apparent eruption rate based on preserved products was very low (<0.1 km 3 .ky -1 ), is marked by the development of thick palaeosols.

Any mantle-derived basalt injected into the crust during this period must have been trapped at depth, perhaps due to the stress imposed by the high Skaros edifice [START_REF] Pinel | The effect of edifice load on magma ascent beneath a volcano[END_REF]. Accumulation of heat from prolonged, deep intrusion probably generated silicic melt by a combination of fractional crystallization, partial melting of crustal rocks and defrosting of extant mushy intrusions [START_REF] Mann | Trace element geochemistry of high alumina basalt-andesite-dacite-rhyodacite lavas of the Main Volcanic Series of Santorini Volcano, Greece[END_REF][START_REF] Barton | Sr isotope and trace element evidence for the role of continental crust in calc-alkaline volcanism on Santorini and Milos, Aegean Sea, Greece[END_REF][START_REF] Huijsmans | Calc-alkaline lavas from the volcanic complex of Santorini, Aegean Sea, Greece. A petrological and stratigraphical study[END_REF][START_REF] Huijsmans | Polybaric geochemical evolution of two shield volcanoes from Santorini, Aegean Sea, Greece: evidence for zoned magma chambers from cyclic compositional variations[END_REF][START_REF] Druitt | Santorini Volcano[END_REF]. Then, between 39 and 25 ka, a chemically and thermally (895-925°C) monotonous series of dacites (the Therasia dome complex) was extruded from the summit of Skaros and from dykes on its western flank. The lack of any systematic variation of whole rock or groundmass (i.e., melt) composition with time during this period (Fig. 7) suggests thermal buffering of the crustal storage region by an approximate balance of heat input, heat output and latent heat of crystallisation. The mean eruption rate during construction of the Therasia dome complex was very approximately 0.1-0.2 km 3 .ky -1 : lower than the long-term average on Santorini (~1 km 3 .ky -1 ), but higher than that during the preceding repose period. Towards the end of the Therasia activity, lava extrusion became increasingly punctuated by explosive activity. Throughout most of this 15 ky period, the eruptions tapped typical Therasia-type dacite. However, the 26 ka Cape Tripiti eruption tapped dacite with some incompatible trace element contents intermediate between those of Therasia and Cape Riva. Any basaltic magma intruded beneath the summit region over the 15 ky was unable to reach the surface, except as rare quenched enclaves of dacite-contaminated olivine basalt. Towards the end of the period, basalt mixed with dacite in approximately equal proportions, forming the upper Therasia hybrid andesite.

Following extrusion of the last Therasia lava, no more than 2,800 ± 1,400 years elapsed before the 21.8 ± 0.4 ka Cape Riva eruption took place. At least 10 km 3 of 880°C Cape Riva dacite, poorer in incompatible elements, was then erupted as Plinian fallout and pyroclastic flows. The eruption also discharged a small quantity (<< 1 km 3 ) of hybrid andesite formed by the mixing of olivine basalt and incompatible-depleted dacite in sub-equal proportions, and the Skaros-Therasia edifice collapsed [START_REF] Druitt | Santorini Volcano[END_REF].

Constraints on magma reservoir development

The Therasia dacites have been previously interpreted as 'precursory leaks' from the growing Cape Riva magma body [START_REF] Druitt | Vent evolution and lag breccia formation during the Cape Riva eruption of Santorini, Greece[END_REF][START_REF] Bacon | Implications of silicic vent patterns for the presence of large crustal magma chambers[END_REF]. Precursory leaks of silicic magma from diffuse vents prior to caldera-forming eruptions are believed to record the prolonged, incremental growth of large crustal magma reservoirs [START_REF] Bacon | Implications of silicic vent patterns for the presence of large crustal magma chambers[END_REF]. For example, leaks of rhyodacite at Mount Mazama (Crater Lake, Oregon) bear witness to the progressive growth, over about 20,000 years, of the magma body that was discharged 6,850 years ago [START_REF] Bacon | Implications of silicic vent patterns for the presence of large crustal magma chambers[END_REF][START_REF] Bacon | Eruptive history and geochronology of Mount Mazama and the Crater Lake region, Oregon[END_REF]. However, the Therasia lavas cannot simply be leaks from a single, growing Cape Riva reservoir, because they represent a chemically different magma batch.

One possibility is that the two dacite types were stored in separate, long-lived reservoirs that coexisted in the upper crust, perhaps for much of the 30 ky since Upper Scoria 2. The Therasia eruptions would have been fed from one or more small reservoir(s), and the Cape Riva eruption from another, larger reservoir. The Cape Tripiti pumice, with its intermediate chemical character, might have resulted from a temporary connection between the two reservoirs. However, field and phase equilibria data provide quite stringent constraints on the possible locations of the two reservoirs in such a model. First, the vents that fed the Therasia lavas lay within the area that subsequently collapsed to form the Cape Riva caldera [START_REF] Druitt | Vent evolution and lag breccia formation during the Cape Riva eruption of Santorini, Greece[END_REF][START_REF] Druitt | New insights into the initiation and venting of the Bronze-Age eruption of Santorini, from component analysis[END_REF][START_REF] Druitt | Caldera formation on Santorini and the physiography of the islands in the late Bronze Age[END_REF], showing that the reservoirs were geographically coincident. Second, the similarities in whole rock major element composition, phenocryst assemblage, phenocryst rim compositions and Fe-Ti oxide temperatures, show that the Therasia dacites must have been stored immediately prior to eruption at a pressure (2 ± 0.5 kb, equivalent to 8 ± 2 km) similar to that determined experimentally for the Cape Riva dacite by [START_REF] Cadoux | Magma storage conditions of large Plinian eruptions of Santorini Volcano[END_REF]. The two reservoirs must therefore have been situated within the ~4 km vertical distance represented by the ± 0.5 kb uncertainty on the pressure.

Moreover, this uncertainty is on the absolute pressure, not on the relative pressure difference between the two dacite types. It is difficult to see how two magma reservoirs within this confined space could remain largely isolated over many thousands of years. We cannot rule out this model, but it raises problems that are hard to surmount.

Our preferred interpretation is that the Therasia and Cape Riva magma batches were emplaced sequentially into the upper crust beneath the summit of the volcano, the first then being partially, or wholly, flushed out by the arrival of the second. In this model the 15 ky of Therasia activity was fed either from a single long-lived, melt-dominated reservoir or by the ascent, partial eruption and rapid freezing of multiple melt packets (e.g., [START_REF] Zellmer | Magma emplacement and remobilization timescales beneath Montserrat: insights from Sr and Ba zonation in plagioclase phenocrysts[END_REF]. At 26 ka the system may have been replenished by incompatible-depleted dacite, which mixed with Therasia dacite and was discharged as the Cape Tripiti Pumice. The subsequent return to eruption of 'pure' Therasia dacite suggests that if a discrete Cape Riva reservoir already existed below the summit at this time, it had probably not yet reached its full size. Following extrusion of the last Therasia lava flow, the input of incompatible-depleted dacite continued, and perhaps accelerated, during the <2,800 ± 1,400 y preceding the Cape Riva eruption. The new input magma mixed with any remaining Therasia magma to generate the dacite that subsequently discharged during the Cape Riva eruption. This may have had the composition of pure Cape Riva dacite, or may have been a dacite even poorer in incompatibles (e.g. Kameni-like dacite; Fig. 12). Amalgamation and homogenization of the different magma batches resulted in the formation of a single, contiguous reservoir.

Assuming injection of most of the >10 km 3 of Cape Riva magma in <2800 y yields a timeaveraged supply rate of >0.004 ± 0.002 km 3 .y -1 , although the peak value may have been much higher. For comparison, the mean accumulation rate estimated over the 1600 y to prior the Oruanui eruption has been estimated as >0.33 km 3 .y -1 , culminating in values of ~1 km 3 .y -1 [START_REF] Wilson | Rapid Rates of Magma Generation at Contemporaneous Magma Systems, Taupo Volcano, New Zealand: Insights from U-Th Model-age Spectra in Zircons[END_REF][START_REF] Allan | From mush to eruption in centuries: assembly of the super-sized Oruanui magma body[END_REF]. A late-stage growth spurt of the Minoan magma reservoir has been estimated as >0.05 km 3 .y -1 [START_REF] Druitt | Decadal to monthly timescales of magma transfer and reservoir growth at a caldera volcano[END_REF]. Intrusion rates comparable to, or higher than, that estimated for the Cape Riva are implied by measured deformation rates at silicic volcanoes such as Uturuncu (~0.01 km 3 .y -1 ; [START_REF] Pritchard | An InSAR-based survey of volcanic deformation in the central Andes[END_REF][START_REF] Sparks | Uturuncu volcano, Bolivia: Volcanic unrest due to mid-crustal magma intrusion[END_REF], Kameni (~0.01 km 3 .y -1 ; [START_REF] Parks | Evolution of Santorini Volcano dominated by episodic and rapid fluxes of melt from depth[END_REF], Yellowstone (0.1 km 3 .y -1 ; [START_REF] Chang | An extraordinary episode of Yellowstone caldera uplift, 2004-2010, from GPS and InSAR observations[END_REF], and Lazufre (~0.01 km 3 .y -1 ; Froger et al. 2007; Ruch et al. 2009).

Rapid intrusion of the Cape Riva dacite into the upper crust would have favoured runaway growth of a melt-dominated magma reservoir [START_REF] Annen | From plutons to magma chambers: Thermal constraints on the accumulation of eruptible silicic magma in the upper crust[END_REF][START_REF] Schöpa | The effects of magma flux variations on the formation and lifetime of large silicic magma chambers[END_REF][START_REF] Gelman | On the longevity of large upper crustal silicic magma reservoirs[END_REF]. Driving mechanisms for magma ascent may have included increased basaltic flux from the mantle, tectonic forces, or gravitational instability of crustal magma storage regions. Possible evidence of increased basaltic flux is provided by the production of hybrid andesites (formed by mixing of basalt and dacite in approximately equal proportions) in the few thousand years prior to the Cape Riva eruption, as well as in the Cape Riva reservoir itself. Pressurization of the upper crustal plumbing system by sustained, high-flux injection of dacite and basalt may have triggered the transition from prolonged, largely effusive activity to catastrophic explosive eruption and caldera collapse.

Conclusions

We have combined field, high-resolution radiometric, petrological and geochemical approaches in a study of the evolution of the crustal magma plumbing system of Santorini prior to a silicic caldera-forming eruption. The main conclusions are listed below.

1. Between 39 ka and 25 ka, a sequence of compositionally (65-68 wt% SiO 2 ) and thermally (895-925 °C) monotonous dacitic lavas leaked out to form the ~2 km 3 Therasia dome complex across the summit and western flank of the high, extant Skaros lava shield.

Pyroclastic units interbedded with the lavas towards the top of the sequence record increasing explosivity of the system towards the end of this period. One prominent pumice fall deposit within the Therasia complex probably correlates with the 26 ka Y-4 ash layer observed in deep-sea sediments SE of Santorini. Following a pause in activity no longer than 2,800 ± 1,400 ky, the 21.8 ± 0.4 ka Cape Riva explosive eruption discharged >10 km 3 of 880 °C dacite, and collapsed the Skaros shield.

2. The Therasia and Cape Riva dacites are similar in most major elements, but the Cape Riva dacite has lower contents of K and incompatible trace elements (e.g., Rb, Zr, Th, LREE) than the Therasia dacites at a given silica content. This decrease in incompatibles that took place at 21.8 ka is one step in the well-documented longer-term decrease in incompatible elements with time observed at Santorini over the last 530 ka. The Therasia and Cape Riva dacites represent distinct magma batches that are unrelated by shallow-level processes.

3. Given that the source vents for the Therasia and Cape Riva dacites were geographically coincident, and that the reservoir depths estimated from phase equilibria were similar, accumulation of most of the Cape Riva magma in the upper crust probably took place within the 2,800 ± 1,400 y period between the last Therasia eruption and the Cape Riva. This would have required a time-averaged magma accumulation rate in excess of 0.004 ± 0.002 km 3 y -1 .

4. Discharge of basaltic magma during this time period is limited to <<1 % quenched enclaves of olivine basalt in some Therasia lavas and in Cape Riva pumice. However, hybrid andesite magmas formed by the mixing of olivine basalt and dacite in approximately equal proportions were erupted as lava towards the end of the Therasia, and as scoria in the Cape Riva eruption. These hybrids may record an increased influx of basalt into the upper crust over the several thousands of years leading up to the Cape Riva eruption. Increased basaltic flux may have played a role in the rapid accumulation of incompatible-depleted Cape Riva magma beneath the summit of Skaros Volcano prior to its 21.8 ka eruption. Tables Table 1 New Mixing models for the upper Therasia andesite and the Cape Riva andesite are shown as blue and green lines, respectively. The mixing calculations are presented in Table 4. The typical fractionation trend for Santorini magmas discussed by [START_REF] Nicholls | Petrology of Santorini Volcano, Cyclades, Greece[END_REF], [START_REF] Mann | Trace element geochemistry of high alumina basalt-andesite-dacite-rhyodacite lavas of the Main Volcanic Series of Santorini Volcano, Greece[END_REF] and [START_REF] Druitt | Santorini Volcano[END_REF] is shown on the diagrams as a grey arrow. Composition of the Skaros lavas are also plotted for reference [START_REF] Huijsmans | Calc-alkaline lavas from the volcanic complex of Santorini, Aegean Sea, Greece. A petrological and stratigraphical study[END_REF], as are scoria from the Upper Scoria 2 eruption [START_REF] Druitt | Santorini Volcano[END_REF]

  Therasia mafic enclaves. Quenched basaltic (49.6-51.8 wt % SiO 2 ; 5.4-8.6 wt% MgO) enclaves 1-10 cm in diameter are found in flows 3 and 11, where they make up <1% of the erupted volume. They contain phenocrysts of plagioclase (~55 vol%), pyroxenes (~35 vol%, with cpx >> opx) and olivine (~10 vol%) set in a glassy, diktytaxitic groundmass. Two populations of plagioclase phenocrysts with different core compositions, but similar rim compositions, are observed: (1) normally zoned crystals with cores of An 83-91 and rims of An 64-71 ; (2) reversely zoned crystals with cores of An 51-61 , separated by a sieve-textured zone from rims normally zoned from An 82-86 to An 64-71 . Plagioclase in the groundmass is An 32-42 . Olivines are normally zoned from cores of Fo 77-82 to rims of Fo 53-60 . Two clinopyroxene populations are found, although their textural relationships with the plagioclase populations are ambiguous. Both cpx populations are euhedral and unzoned, with compositions of (1) Wo 43-46 En 42-46 Fs 10-12 and (2) Wo 41-44 En 39-43 Fs 15-20 . Rare orthopyroxenes with compositions of Wo 3 En 67 Fs 30 also occur.The occurrence of two plagioclase populations with different core compositions, but similar, intermediate rim compositions is indicative of magma mixing. Plagioclases of population 1 are interpreted as derived from a basaltic melt, and those of population 2 from a more evolved melt. The cores of olivine crystals (molar Mg/Fe = 3.37-4.64) are in equilibrium with the whole rock (i.e., basaltic) composition (Mg/Fe = 1.20), assuming a crystal-melt partition coefficient of between 0.26 and 0.36[START_REF] Roeder | Olivine-liquid equilibrium[END_REF]. The composition of population-2 clinopyroxenes is similar to that of the clinopyroxene phenocrysts in the dacite. The enclaves are interpreted as having formed by the inmixing of a small proportion of more evolved magma (possibly dacitic, containing population-2 plagioclase cores + population-2 cpx) into a basalt (containing population-1 plagioclase cores + olivine + population-1 cpx; Fig.10). Mixing occurred long enough prior to eruption for plagioclase from the evolved component to partially melt (generating sieve texture), followed by overgrowth of equilibrium rim compositions on plagioclases from both populations.Upper Therasia andesite. The upper Therasia andesite (60.5 wt % SiO 2 , 2.6 wt% MgO) contains 26 wt% of macroscopic crystals: plagioclase (75 vol%), pyroxenes (20 %, with opx ≈ cpx), Fe-Ti oxides (5 %) and trace amounts of olivine set in a glassy, 64 wt % SiO 2 groundmass containing microlites of plagioclase and magnetite. Three distinct populations of plagioclase are observed: (1) normally zoned crystals with cores of An 83-89 , and euhedral rims of An 57-61 ; (2) crystals with cores of An 53-60 separated by a sieve-textured zone from rims normally zoned from An 75-87 to An 55-70 ; (3) normally zoned crystals with cores as calcic as An 76 and rims of An 36-55 . Rare olivines have cores of Fo 80-83 , and rims of Fo 67-69 . Orthopyroxenes have compositions of Wo 3 En 57- 59 Fs 38-39 , and clinopyroxenes have compositions of Wo 39-42 En 40-43 Fs 15-20 .

  phases and proportions are the same as in the Therasia dacites, with total contents ranging from 15 to 20 wt%. Plagioclase phenocrysts are euhedral, with rims of An 31-65 , and cores as calcic as An 70 . As in the Therasia dacites, plagioclase phenocrysts in the Cape Riva dacite contain complex dissolution surfaces and saw tooth zoning. Rare xenocrysts of An 70-96 also occur. Orthopyroxene phenocrysts have compositions of Wo 3 En 52-68 Fs 45-29 , and clinopyroxenes from Wo 44 En 41 Fs 15 to Wo 40 En 36 Fs 24 . Touching Fe-Ti oxide pairs give temperatures of 879 ± 15 °C and fO 2 of -12.9 ± 0.4 (Cadoux et al. 2013). Cape Riva mafic enclaves. Millimetre-sized quenched blebs of basaltic magma occur dispersed (<<1%) through the Cape Riva dacite, and in banded pumices containing the dacite and andesite mingled together. They contain An 90-96 plagioclase, Fo 72-84 olivine, Wo 41-44 En 36- 41 Fs 15-23 cpx and rare Wo 3 En 68 Fs 29 opx. The enclaves have micro-crenulated surface textures, and many have a single crystal or xenocrystic fragment at their centres. They are interpreted as small fragments of chilled basaltic magma. Cape Riva andesite. The Cape Riva andesitic scoria has 60-62 wt% SiO 2 and 3.2-2.5 wt% MgO; it contains ~12 wt% macroscopic crystals of plagioclase, olivine, clinopyroxene and magnetite set in brown dacitic glass with 63.5 wt% SiO 2 . The pure andesitic component (free of any in-mingled streaks of dacite) contains two populations of plagioclase: (1) a calcic population of An 70-96 , with a discrete population of euhedral, unzoned grains of An 90-96 , and (2) a less abundant population with cores up to An 52 and rims of An 30-40 . Olivines are compositionally uniform (Fo 84 ). Augites occur sparsely as microphenocrysts of Wo 40-41 En 41-43 Fs 19 . No orthopyroxene has been observed.

5.

  Pressurization of the upper crustal plumbing system by sustained, high-flux injection of dacite and basalt may have triggered the transition from prolonged, largely effusive activity (Therasia) to catastrophic explosive eruption and caldera collapse (Cape Riva). Agence National de Recherche [ANR-08-BLAN-0249-01]. GN Fabbro was funded by the Région d'Auvergne. This is Laboratory of Excellence ClerVolc contribution number 76.

FiguresFig. 1 Fig. 2 Fig. 3 Fig. 4 Fig. 5 Fig. 6

 123456 Figures

Fig. 7 Fig. 8 Fig. 10

 7810 Fig. 7 Summary of magma compositions and ages between 70 and 20 ka. Major explosive

Table 2

 2 40 Ar/ 39 Ar-ages for the Therasia dacites Published ages for the Cape Riva eruption. The average of these ages is

	Sample	Unit	Gauss-plot age (ka) Weighted mean age (ka)
	SAN 09-43 Flow 25		25.3 ± 1.4	24.6 ± 1.3
	SAN 10-13 Flow 21		33.1 ± 1.1	33.2 ± 1.1
	SAN 10-12 Flow 3		40.1 ± 2.2	39.4 ± 2.2
	SAN 10-11 Flow 1		49.4 ± 2.5	48.2 ± 2.4
	21.8 ± 0.4 ka (2σ)
	Uncalibrated	Calibrated age	Method	Sample dated	Reference
	14 C age (ka) a	(ka)
	18.05 ± 0.34 18.17 ± 0.21 18.88 ± 0.23	21.46 ± 0.49 b 21.63 ± 0.33 b 22.47 ± 0.24 b	14 C	Charred trees from ignimbrite	Pichler and Friedrich (1976)
	18.15 ± 0.20 21.60 ± 0.32 b 14 C	Charcoal from ignimbrite	Eriksen et al. (1990)
	17.38 ± 0.23 18.53 ± 0.15 18.24 ± 0.14	20.53 ± 0.26 b 21.75 ± 0.24 b 22.14 ± 0.17 b	14 C	Peat below Y-2 tephra, Philippi basin, Greece	St Seymour et al. (2004)
	-	21.62		Interpolation between sapropels dated using 14 C		Asku et al. (2008)
				Interpolation between
		19 c		istopic stages dated by		Thunell et al. (1979)
				231 Pa/ 230 Th

a Published uncalibrated 14 C age b Calibrated using the curve of

[START_REF] Fairbanks | Radiocarbon calibration curve spanning 0 to 50,000 years BP based on paired 230 Th/ 234 U/ 238 U and 14 C dates on pristine corals[END_REF]

c not included in the average

Table 3

 3 Representative whole rock and groundmass analyses of the Therasia and Cape Riva products FeO T is the total FeO and Fe 2 O 3 content calculated as FeO

		Therasia Mafic	Upper Therasia	Therasia Dacite	Cape Riva Andesite Cape Riva Dacite	Upper Therasia	Therasia Dacite	Cape Riva Andesite Cape Riva Dacite
		Enclave	Andesite				Andesite
		Whole Rock	Whole Rock	Whole Rock	Scoria	Pumice	Groundmass	Groundmass	Groundmass	Groundmass
	Sample	GS10-43	GS10-22	GS10-17	S09-41	S09-40	GS10-22	GS10-17	S09-41	S09-40
	Unit	Flow 22	Flow 22	Flow 5	Cape Riva A	Cape Riva A	Flow 22	Flow 5	Cape Riva A	Cape Riva A
	Major elements (ICP-AES, wt% dry)
	SiO 2	51.87	60.26	66.00	60.19	65.84	63.67	67.99	61.53	69.88
	Al 2 O 3	19.07	17.16	15.38	16.43	15.40	15.44	14.70	16.98	15.05
	TiO 2	0.89	0.86	0.75	0.88	0.76	0.79	0.63	0.74	0.48
	FeO T a	8.08	6.05	4.66	6.58	4.88	5.60	4.03	5.96	3.31
	MgO	5.44	2.66	1.08	3.17	1.16	2.39	0.85	2.62	0.53
	CaO	10.16	5.98	3.15	6.20	3.28	4.62	2.34	5.86	2.14
	Na 2 O	3.30	4.52	5.48	4.63	5.84	4.55	5.91	4.92	5.86
	K 2 O	0.90	2.18	3.19	1.56	2.45	2.60	3.25	1.82	2.95
	MnO	0.16	0.14	0.13	0.16	0.16	0.14	0.12	0.15	0.12
	P 2 O 5	0.14	0.19	0.18	0.19	0.22	0.20	0.19	0.17	0.16
	Trace elements (ICP-MS, ppm)
	Li	16.20	18.10	25.50	16.4	17.4	20.3	26.9	17.7	20.4
	Sc	25.90	17.40	13.90	23.4	13.8	18.5	14.2	24.6	14.8
	V	193.0	109.0	30.1	139.0	30.2	80.8	14.5	126	8.3
	Cr	31.30	9.52	1.17	27.10	0.60	8.41	0.76	21.2	4.33
	Ni	15.80	5.65	0.86	14.20	2.57	4.62	0.75	14.6	5.48
	Rb	25.8	68.1	104.0	50.0	71.7	89	105	60.2	101
	Sr	323	231	133	185	127	176	108	197	105
	Y	23.8	37.9	50.8	38.5	45.9	44.9	53.6	44.2	61.1
	Zr	108	200	321	175	250	271	334	214	341
	Nb	4.71	9.47	12.90	7.12	9.44	11.4	13.3	7.58	11.6
	Ba	251	391	513	297	375	455	542	323	460
	La	11.9	23.5	32.2	19.0	23.7	27.9	33.9	20.5	28.9
	Ce	26.9	49.5	66.5	40.7	50.5	63.2	70.2	43.6	61.6
	Pr	3.37	5.93	7.70	4.98	6.09	7.07	8.22	5.41	7.4
	Nd	14.2	23.4	30.1	20.4	24.6	27.9	31.9	21.6	29.5

Table 4

 4 Calculation for the mixing of (1) Therasia dacite plus Therasia basalt (mafic enclave) to produce the upper Therasia hybrid andesite, and (2) Cape Riva dacite plus Skaros basalt to produce the Cape Riva hybrid andesite. For the upper Therasia andesite, the mixing proportions are 38:62 mafic:silicic. For the Cape Riva andesite, the proportions are 36:64. Calculated using the PetroGraph software of[START_REF] Petrelli | PetroGraph: A new software to visualize, model, and present geochemical data in igneous petrology[END_REF]

				Upper Therasia Hybrid Andesite				Cape Riva Hybrid Andesite
		Mean Therasia Dacite	Mafic Enclave (GS10-43)	Calculated	Measured (GS10-22)	Residual	Mean Cape Riva Dacite	Mean Skaros Basalt	Calculated	Mean Measured	Residual
	Major Elements (wt%)
	SiO 2	65.50	51.86	60.28	60.26	0.02	66.41	52.16	61.22	61.14	0.08
	TiO 2	0.83	0.89	0.85	0.86	-0.01	0.74	0.87	0.79	0.83	-0.04
	Al 2 O 3	15.51	19.07	16.87	17.16	-0.29	15.46	18.38	16.52	16.39	0.13
	FeO tot	4.93	8.08	6.13	6.05	0.08	4.75	8.20	6.01	6.27	-0.26
	MnO	0.14	0.16	0.15	0.14	0.01	0.15	0.17	0.16	0.15	0.01
	MgO	1.21	5.44	2.83	2.66	0.17	1.23	5.96	2.95	2.93	0.02
	CaO	3.41	10.16	5.99	5.98	0.01	3.35	10.78	6.06	5.92	0.14
	Na 2 O	5.50	3.30	4.66	4.52	0.14	5.33	2.72	4.38	4.53	-0.15
	K 2 O	2.74	0.90	2.04	2.18	-0.14	2.40	0.64	1.76	1.68	0.08
	P 2 O 5	0.23	0.14	0.20	0.19	0.01	0.18	0.12	0.16	0.15	0.01
	Total	100.00	100.00	100.00	100.00	0.00	100.00	100.00	100.00	100.00	0.00
					Sum of the Squares: 0.16				Sum of the Squares: 0.14
	Trace Elements (ppm)
	Cr	2	31	13	10	3	8	82	35	33	2
	Ni	1	16	7	6	1	6	31	15	10	5
	Nb	12	5	9	9	0	10	4	8	8	0
	Rb	92	26	66	68	-2	74	21	55	51	4
	Zr	291	108	221	200	21	266	94	203	193	10
	Sr	155	323	219	231	-12	143	226	173	179	-6

Acknowledgments

We thank M Benbakkar and C Chauvel for providing major element and trace element analyses, respectively, and J-L Devidal for help with the electron microprobe. J Keller kindly drew our attention to the Y-4 deep-sea ash layer. E Klemetti, D Morgan and G Zellmer provided helpful reviews, and F Costa commented on an earlier version of the manuscript. The work was carried out in the framework of the project 'Storage and Mixing at Santorini' financed by the French

Supplementary Table

 S1Full 40 Ar/ 39 Ar data Supplementary Table S2 Major element composition of glass from pumices in the Cape Tripiti deposit Supplementary Table S3 Major element composition, temperature and fO 2 determinations for pairs of magnetite and ilmenite from Therasia pumices. S4 Whole rock and groundmass chemical analyses of the Therasia and Cape Riva deposits [START_REF] Huijsmans | Calc-alkaline lavas from the volcanic complex of Santorini, Aegean Sea, Greece. A petrological and stratigraphical study[END_REF] and Upper Scoria 2 data of [START_REF] Druitt | Santorini Volcano[END_REF]. Mixing models for the upper Therasia andesite and the Cape Riva andesite are plotted, as in Fig. 11. Fields of Santorini magma during the volcanic history since 530 ka are plotted, showing the long-term decrease in incompatible elements at Satonrini [START_REF] Huijsmans | Calc-alkaline lavas from the volcanic complex of Santorini, Aegean Sea, Greece. A petrological and stratigraphical study[END_REF][START_REF] Druitt | Santorini Volcano[END_REF][START_REF] Martin | Geochemical and textural analysis of mafic enclaves from Nea Kameni[END_REF]. (d) REE diagram showing the Therasia and Cape Riva magmas. The Therasia dacite is an average of 14 flows with 65-68 wt% SiO 2 , and the Cape Riva dacite is an average of four analyses with SiO 2 contents of 66-67 wt% (average: 66.5 wt%). The upper Therasia andesite is an average of flows 11 and 22 (average SiO 2 : 60.5 wt%), and the Cape Riva andesite is an average of three analyses with SiO 2 contents between 60.2-60.4 wt% (average: 60.4 wt%).

Supplementary Table


Sébastien Campocasso
email: sebastien.campocasso@ensam.eu.

Jean-Philippe Costes

Guillaume Fromentin

Stéphanie Bissey-Breton

Gérard Poulachon

Improvement of cutting forces modeling based on oriented cutting tests

Keywords: Cutting forces modeling, Edge discretisation, Oriented cutting tests, Pure copper

In order to predict the characteristics of the machined part, such as geometry, surface roughness and fatigue or corrosion resistance, the cutting forces values should be known as precisely as possible. The edge discretisation methodology can be used to model the three components of the cutting forces. The results are generally considered as suitable, even if the considered cutting operation is complex, because the geometry is well described. Usually, the local cutting forces model is identified from orthogonal or oblique cutting tests and the local contributions are assumed to be independent of the orientation of the elementary edge in the reference plane r P . However, when turning in the tool nose or with round inserts, the tool cutting edge angle r  (or Side Cutting Edge Angle) evolves along the active cutting edge and the values of this angle are very small compared to that of 90° used in orthogonal/oblique cutting. For this study, a new elementary cutting operation, called "oriented cutting", has been tested. In this configuration, the active cutting edge is rectilinear, without inclination, but oriented by an angle r  different from 90°. In addition, cylindrical turning tests have been done. The measurements, performed in pure copper, show an influence of the tool cutting edge angle on the cutting forces. An interaction between r  and the workpiece radius is also highlighted.

Introduction

The calculation of cutting forces is of interest to predict the characteristics of the machined part as geometry, surface roughness or material integrity.

One possible way to model the forces is to use the edge discretisation as presented on Figure 1.

This methodology allows considering the local uncut chip thickness [START_REF] Colwell | Predicting the Angle of Chip Flow for Single-Point Cutting Tools[END_REF], which is particularly interesting in finish turning ( p ar   ). Indeed, when cylindrical turning in the nose or with round insert, the uncut chip thickness evolves along the active cutting edge.

The maximum uncut chip thickness max h and the local uncut chip thickness () h  can be analytically calculated respectively according to Huang and Liang [START_REF] Huang | Force Modelling in Shallow Cuts with Large Negative Rake Angle and Large Nose Radius Tools -Application to Hard Turning[END_REF] and Armarego and Samaranayake [START_REF] Armarego | Performance Prediction Models for Turning with Rounded Corner Plane Faced Lathe Tools -I[END_REF]. The angular sector corresponding to the active cutting edge can be determined by Equations ( 1) and (2).

The local cutting model is often identified by doing orthogonal cutting tests on disks [START_REF] Bissey-Breton | Identification of Cutting Relations in High Speed Milling[END_REF][START_REF] Germain | Développement d'un modèle d'efforts de coupe intégrant le contact en dépouille : Application au tournage de superfinition du cuivre Cu-c2[END_REF] or tubes [START_REF] Bissey-Breton | Identification of Cutting Relations in High Speed Milling[END_REF][START_REF] Budak | Prediction of Milling Force Coefficients From Orthogonal Cutting Data[END_REF].

The most often used forces model is expressed in a semi-global basis ( , , ) o v h oriented by both the machine ( v // c V ) and the tool ( h normal and o tangent to the cutting edge) [START_REF] Bissey-Breton | Identification of Cutting Relations in High Speed Milling[END_REF].

In a first approximation, the local forces can be expressed only in function of the local uncut chip thickness h as follows (Equation ( 3)) [7].

(

) with or

i ci ei f K h k i v h    (3)
As the tool is finely described by the discretisation, the local cutting geometry can also be taken into account in the model [START_REF] Armarego | Drilling with Flat Rake Face and Conventional Twist Drills -I. Theoretical Investigation[END_REF].

Thus, a lot of studies add the effect of the local cutting geometry in the model, as the rake angle [START_REF] Bissey-Breton | Identification of Cutting Relations in High Speed Milling[END_REF][START_REF] Germain | Développement d'un modèle d'efforts de coupe intégrant le contact en dépouille : Application au tournage de superfinition du cuivre Cu-c2[END_REF][START_REF] Budak | Prediction of Milling Force Coefficients From Orthogonal Cutting Data[END_REF][START_REF] Armarego | Drilling with Flat Rake Face and Conventional Twist Drills -I. Theoretical Investigation[END_REF], the inclination angle [START_REF] Bissey-Breton | Identification of Cutting Relations in High Speed Milling[END_REF][START_REF] Budak | Prediction of Milling Force Coefficients From Orthogonal Cutting Data[END_REF] or the clearance angle [START_REF] Germain | Développement d'un modèle d'efforts de coupe intégrant le contact en dépouille : Application au tournage de superfinition du cuivre Cu-c2[END_REF].

However, in industrial context, the first step needed is to evaluate the forces for a given tool along a tool path in order to select the suitable cutting parameters.

That is why, this study focuses on the effects of the workpiece radius and the tool cutting edge angle and their possible interactions, as these parameters can evolve along a tool path.

The effect of the side cutting edge angle (SCEA, equal to 90 r    ) has been yet experimentally studied [START_REF] Koenigsberger | An Investigation into the Cutting Force Pulsations during Milling Operations[END_REF][START_REF] Venkatesh | An Analysis of Cutting Tools with Negative Side Cutting Edge Angles[END_REF][START_REF] Kattan | Developing New Trends of Cutting Tool Geometry[END_REF][START_REF] Noordin | Application of Response Surface Methodology in Describing the Performance of Coated Carbide Tools when Turning AISI 1045 Steel[END_REF]. Unfortunately, the ranges of variations of r  are close to 90  (except in [9]). Furthermore, the tests and the observations have been done only at a macroscopic scale. Indeed, the nose of the tool was engaged during the tests and the measured forces are studied in the measurement basis. Some changes have been observed in the chip morphology when changing r  [START_REF] Venkatesh | An Analysis of Cutting Tools with Negative Side Cutting Edge Angles[END_REF][START_REF] Kattan | Developing New Trends of Cutting Tool Geometry[END_REF][START_REF] Khettabi | Effect of Tool Lead Angle and Chip Formation Mode on Dust Emission in Dry Cutting[END_REF]]. However, the experimental work presented in this paper suggests an explanation, detailed in the following sections, based on the contact between the clearance face and the machined surface.

In the next section, the elements of the experimental device common to all tests are briefly described.

Then, the operation of cylindrical turning is analysed to point the need of taking into account both r  and the workpiece radius in the local cutting forces model.

Finally, new elementary cutting tests are presented in order to highlight the effect of r  at a mesoscopic scale.

Experimental set-up

The machined material for the whole tests is a pure copper Cu-OFE. The homogeneity has been verified by micro-hardness tests from the heart of the workpiece to the surface (average measured hardness: 100 HV 0.5 ). ) and the edge radii r  are between 11 and 17 µm.

Effect of the workpiece radius on the cutting forces

Orthogonal cutting on disks

It has been shown by Germain [START_REF] Germain | Développement d'un modèle d'efforts de coupe intégrant le contact en dépouille : Application au tournage de superfinition du cuivre Cu-c2[END_REF] that, in the case of orthogonal cutting on disks, when the radius increases the cutting forces could increase for some cutting geometries. This has been supposed to be due to the enlargement of the clearance contact.

Cylindrical turning -Theoretical investigation

When finish cylindrical turning, in addition to the uncut chip thickness, the local cutting edge angle r  (equal to  in the present case) and the radius of curvature of the workpiece in the plane o P (where is done the elastic recovery and the friction on the clearance face due to the first motion) evolve along the cutting edge too.

This radius, called local effective workpiece radius, denoted W o R and shown on Figure 3, can be expressed by the Equation ( 4) according to Germain [START_REF] Germain | Force Modelling for Superfinish Turning of Pure Copper with Scale Effects[END_REF].

R p a f V  m Z R R r  W o max R ( ) W o R 
   , W o
R is equal to the workpiece radius, but it does not correspond to orthogonal cutting on disk because r  is equal to zero.

Cylindrical turning tests

Five tests of cylindrical turning, with different feeds and depths of cut, have been done at different diameters. Some results are given in Table 1 and on Figure 4.

The depth of cut p a is small enough compared to r  to have W o max R less than 1.3 times R . So, each group of five tests reflects the effect of a small range of radii.

It should be noted that, due to this reason, max  is lower than 34  . These tests have been conducted with a Sandvik RCGX1204M0-AL H10 (6 r   mm) insert mounted on a SRDCL2025M12 holder. The high nose radius r  has been chosen because:

.5 R R  mm 12.5 R R  mm N° f (mm/rev) p a (mm) max  (°) c F (N) f F (N) p F (N) c F (N) f F (N) p F (N) 1 0.
• The curvature of the active edge is small. • It is possible to have low ratios / p ar  with high cutting forces (noise reduction).

As the wear increases throughout the tests, a second insertused only for two radiihas been used to verify that there is no change due to wear (Figure 4). As it can be seen on Figure 4 (a), the increase of the forces can reach 30 %.

F c -Insert 1 F f -Insert 1 F p -Insert 1 F c -Insert 2 F f -Insert 2 F p -Insert 2
F c -Insert 1 F f -Insert 1 F p -Insert 1 F c -Insert 2 F f -Insert 2 F p -Insert 2 (b)
The comparison between the five tests shows that the effect of the workpiece radius is even more important as the feed and the depth of cut are high.

When p a increases, the growth of forces with the workpiece radius may be attributed to the evolution of W o R along the active cutting edge, which can affect the chip formation and the clearance contact.

Concerning the interaction between the radius and the feed, it could be due to the relative motion of the cutting edge and the machined surface. Indeed, at 0    the edge "slides" in parallel to the surface while at 90    the edge comes closer to the surface due to the feed rate. And so, the force acting on the edge and the clearance face due to the elastic deformation of the workpiece can increase, all the more since W o R is high.

It should be noted that the variation of forces due to the workpiece radius can be more important for higher p a , particularly around r  ( 90 max    ); since, as previously mentioned, max  does not exceed 34  in this study.

In order to verify these assumptions concerning the effect of the orientation of the edge, elementary cutting tests have been conducted.

The goal of these tests, detailed in the next paragraph, is to reproduce what occurs on each segment of the discretised tool.

Elementary oriented cutting tests

Oriented cutting tests have been conducted using two types of configurations: either by cutting a tube or a corner as shown on Figure 5.

The second configuration allows testing very low values of r

 without risk of excessive bending, even vibrations, of the tube. The rectilinear cutting edges of Sandvik VCGX160404 AL H10 inserts have been used. The inserts are assembled with a SVJCL2020K16 body (modified to get r  equal to 90°) or a SVHCR2020K16 (modified to obtain 40 r   on a rear turret lathe).

In order to change the cutting edge angle r  , the tool is oriented by a wedge (in red on the Figure 5).

Experimental procedure of tests on the corner

These tests consist in machining the corner of a cylindrical workpiece with a rectilinear cutting edge and an axial feed rate.

During the tests, three steps can be identified: • The uncut chip thickness h and the width of cut b increase during the first revolution of cut.

• Then, b increases linearly over time and h is constant on the main part of the edge.

• Finally, h decreases due to the deceleration of the machine.

Because the global cutting forces evolve along the time, they should be divided by the width of cut b to be analysed. To be as accurate as possible, the gauge tool is made by electrical contact between the cutting edge and the workpiece edge, and the tool position is measured by a laser sensor Keyence LK-G82.

As the workpiece radius R is high (rough diameter of 183 mm) and the machine acceleration too (4 m/s²), the third step does not mask the second one in the present case. However, this point could be troublesome under different conditions, particularly when r  is small because the feed becomes substantial.

Results obtained by the corner tests

Typical signal and chip obtained by these tests are shown on Figures 6 and7 o v h linked to the tool as shown on Figure 8.

p F f F h F () bt f V ( ) r h f sin   r  Fig. 8

. Global and local forces

A typical evolution of the linear force as a function of the width of cut b is presented on Figure 9. The first revolution corresponds to the part AC of the curve, with a scaling effect due to the small uncut chip thicknesses between the beginning and the point B. The increase between C and D could be attributed to side effects like burrs, which are not negligible for small width of cut. Then, there is a plateau (DE) which should correspond to the value obtained by the tube tests. The part between E and F corresponds to the deceleration, while the end of the evolution to the retract of the tool.



The graphics show that the new corner tests give forces levels in accordance with the tube tests. So, this method makes possible the realisation of elementary cutting tests without preparing tubes or disks specimens, which is a laborious work.

The trends observed on Figures 10 and11 seem to consolidate the assumption that if the feed is high enough, the cutting edge angle affects the cutting forces.

However, the experimental results are very scattered, in spite of the attention given to the experimental procedure. It should be noted that, whilst the tube tests have been done with different inserts, all twelve corner tests have been conducted with a unique insert.

Observation of the tools

The four tube tests at 0.2 h  mm have been realised with four different inserts. As the feed is important, the contact between the clearance face and the machined surface was supposed to increase with r  .

In order to confirm this statement, the four inserts have been observed by SEM. The backscattered electrons mode allows distinguishing the copper from the carbide of the insert as shown on Figure 12 (and confirmed by XDS analyses). The contact lengths are between 3 and 8 µm and do not allow to draw conclusion about the effect of r  on the tool-workpiece contact on the clearance face. Due to the high clearance angle, it seems that the contact is located in the rounded cutting edge.

µm

Cu layer

Conclusion

The new experimental methodology proposed through the corner tests is helpful to conduct elementary cutting tests without making thin specimens beforehand and avoiding vibrations during the cut.

Even if the tests have to be confirmed and completed, it seems that the introduction of the local cutting edge angle r  and the effective workpiece radius W o R in the local model may improve the accuracy of the cutting forces model. Thereby, the modeling of the forces along a tool path with varying radii would be enhanced. Future work on this topic will focus on the decoupling of the parameters r  and W o R during the elementary cutting tests.

In-situ observations and post-mortem analysis of the clearance contact and the chip formation will also be developed in order to clarify the observed phenomena.

Fig. 1 .

 1 Fig. 1. Edge discretisation principle, illustrated in the case of a round turning insert

  Cutting tests have been done on a 2-axis lathe Somab Transmab 400 at a cutting speed c V of 250 m/min with micro-lubrication (Total Valona MQL 3046). The cutting forces have been measured with a piezoelectric dynamometer Kistler type 9121 and an amplifier Kistler 5019A. The signals have been digitised by a data acquisition card NI PCI-6221 (at a frequency of 2 kHz) and treated with DasyLab software.

Fig. 2 .

 2 Fig. 2. Experimental set-up The local cutting geometries of the cutting inserts have been measured by stereovision from SEM photographs (Alicona Mex software). The normal wedge angles n  are approximately

Fig. 3 .

 3 Fig. 3. Local effective workpiece radius when cylindrical turning

Fig. 4 .

 4 Fig. 4. Measured forces vs. workpiece radius: (a) 0.25 f  mm/rev, 1 p a  mm (Test N°1); (b) 0.05 f  mm/rev, 0.5 p a  mm (Test N°5)

Fig. 5 .

 5 Fig. 5. Oriented cutting configurations: (a) Tube ( 80 ); r 

  .

Fig. 6 .

 6 Fig. 6. Measured global cutting force c F in function of time for 10 r 

  Fig. 9. Linear cutting force v f (or c f ) vs. the width of cut b for 10 r 

  on the plateau - for corner tests are compared to the values obtained by tube tests. The component o f is always near zero.

  Fig. 10. Linear cutting force v f (or c f ) vs.r 

  r

Furthermore, the results

  of the elementary tests do not show the only effect of r  , since the change of this angle induces a modification of the effective radius W o R . However, this coupled effect is representative of the cylindrical turning, where both r  and W o R vary along the cutting edge.

Fig. 12 .

 12 Fig. 12. SEM image of the clearance face after cutting at 50 r 

Table 1 .

 1 Measured forces when cylindrical turning in function of the workpiece radius

	Test	Parameters	Measured forces for radius 87