Article Volume00,Number00 0MONTH2013 doi:10.1002/ggge.20221 ISSN:1525-2027 Magma storage and ascent conditions beneath Pico and Faial islands (Azores archipelago): A study on fluid inclusions VittorioZanon Centro de Vulcanologia e Avaliac¸~ao de Riscos Geol(cid:2)ogicos, Universidade dos Ac¸ores, Rua M~ae de Deus, Ponta Delgada,PT-9501-801,Portugal([email protected]) MariaLuceFrezzotti DipartimentodiScienzeGeologicheeGeotecnologie,UniversityofMilan‘‘Bicocca’’,Milan,Italy [ ] In the islands of Faial and Pico (the Azores), fluid inclusions are hosted in megacrysts of olivine 1 (Mg#80–88) and clinopyroxene (Mg#79–90) in highly porphyritic lavas and in mineral assemblages of ultramafic xenoliths. Rare inclusions are contained in olivine phenocrysts (Mg#<80) and plagioclases in poorly porphyritic lavas. Trails of late-stage inclusions are predominant over isolated early-stage inclusions. Almost all inclusions are re-equilibrated and the trapped fluid consists of pure CO (Tm from (cid:2)56.5 to (cid:2)57.2). Rare early-stage inclusions may contain dypingite or Mg-calcite, 2 which indicates that in earlier times some water was present along with CO . Barometric data indicate 2 that CO inclusions in xenoliths from the two islands equilibrated at maximum pressures of 570–586 2 MPa (19.7–21.2 km), while in poorly porphyritic lavas from all the fissure zones at 465–508 MPa (16.4–18.1 km). Maximum pressure values of 463 MPa (16.8 km) and 492 MPa (17 km) were recorded for the central volcanoes of Pico and Faial, respectively. Further trapping/re-equilibration was recorded at 156 MPa in Faial (5.6 km), in plagioclase phenocrysts in mugearites. All these pressures correspond to magma ponding sites and to its crystallization and can be useful for tracing the progressive thickening of a dense transition zone, below the geophysical Moho. The ability to extract rapidly the stored magmas from these volcanic systems strictly depends on the different tectonic styles, acting in this transition zone. Magmatic evolution in small and short-lived intracrustal reservoirs, not necessarily coaxial with main conduit system, was enhanced at the intersection of differentlyorientedlineaments. Components:11,912words,14figures. Keywords:uppermantle;cumuliticxenoliths;fissurezones;basalts;storagesystem;underplating. Index Terms: 1043 Fluid and melt inclusion geochemistry: Geochemistry; 1033 Intra-plate processes: Geochemistry; 1036Magmachamberprocesses:Geochemistry;8415Intra-plateprocesses:Volcanology;3651Thermobarometry:Miner- alogyandPetrology;3615Intra-plateprocesses:MineralogyandPetrology;3618Magmachamberprocesses:Mineralogy andPetrology. Received5March2013;Revised2July2013;Accepted2July2013;Published 00Month2013. Zanon, V., and M. L. Frezzotti (2013), Magma storage and ascent conditions beneath Pico and Faial islands (Azores Islands):1.Astudyonfluidinclusions,Geochem.Geophys.Geosyst.,14,doi:10.1002/ggge.20221. ©2013.AmericanGeophysicalUnion.AllRightsReserved. 1 ZANONANDFREZZOTTI:MAGMASTORAGEBENEATHPICOANDFAIAL 10.1002/ggge.20221 1. Introduction Moreiraetal.,1999; Schaeferetal.,2002; Turner et al., 1997; Widom et al., 1997; Widom and Far- [ ] Volcanic eruptions in areas characterized by quhar, 2003]. Little is known about the shallow 2 extensional tectonics typically originate from evolution of magmas in central eruptive systems fissuresystems,whichgenerallyfollowlocalorre- and associated rift zones [Self and Gunn, 1976; gional weakness patterns, or from composite vol- M(cid:2)etrich et al., 1981; Beier et al., 2006; Snyder et canoes. Magmas erupted from these systems are al.,2004; Snyderetal.,2007; Widometal.,1992; characterized by different patterns of evolution, Widom et al., 1993]. The few existing barometric linked to different pathways to the surface [Kl€ugel data are from the islands of Pico [Franc¸a et al., et al., 2009; Eason and Sinton, 2009; Sigmunds- 2006]andCorvo[Larreaetal.,2012],andthevol- son et al., 2010]. Thin mafic lava flows issued canoes of Sete Cidades and A(cid:2)gua de Pau, in the from fissure zones constitute the bulk of the island of S~ao Miguel [Mattioli et al., 1997; eruptedmaterialfromthesevolcanoes. Renzulli and Santi, 2000; Beier et al., 2006]. A first attempt todefine theevolutionhistory andthe [ ] Magmabuoyancyplays amajorroleinthedy- 3 different ascent paths of magmas from the fissure namics of magma flow and storage [Wilson and zones and a central volcano has been recently pro- Head, 1981], as it controls the depth at which posedfortheIslandofFaial[Zanonetal.,2013]. magma reservoirs formed either in the crust or along large lithospheric discontinuities [Ryan, [6] In this study, barometric data were obtained 1993; Ida, 1995; McLeod, 1999]. Magma buoy- through the microthermometric study of the fluid ancy is related to the density difference between inclusions hosted in the lavas and xenoliths from host rocks and magma and depends on the pres- PicoandFaial,followingtheapproachofZanonet sure, temperature, and composition of both host al. [2003]. These two islands are the emerged tops rocks and magma [Ryan, 1994] and also on the of a larger volcanic edifice, resulting from the tectonicstress[Watanabeetal.,1999]. interplay of fissural volcanism and eruptions from central volcanoes. The style and typology of mag- [ ] Thestudyofthegeophysicalanomaliescaused 4 matism is apparently linked to the interaction by intruding magma (i.e., seismicity, deforma- between the activity of normal fault systems with tions, and gravity variations), is traditionally used themainregionaltransextensionaltrend.Thepres- to model the location of the storage system ent data provide the first evolutionary model of beneath a volcano, the dynamics of magma ascent magma flow and storage beneath these two andthemassinvolved.However,intheabsenceof islands. This model outlines how the interplay a reliable model for the composition and density between tectonics and magma supply rate influ- of the crust, the application of geophysical meth- enced the ascent and ponding of mafic magmas at ods cannot provide precise information about the depth, and the geometry of the plumbing system location of magma ponding sites, as is the case of beneaththedifferentvolcanicsystems. the Azores. An alternative way of obtaining the pressure of magma accumulation at various levels consists of a combination of petrological studies 2. Geological Background onmagmaevolution,withfluidinclusionbaromet- ric studies [e.g., Roedder, 1983; Belkin and De [ ] The Azores archipelago consists of nine main 7 Vivo, 1993; Andersen and Neumann, 2001; Hans- islands and several islets located in the Atlantic teen et al., 1998; Kl€ugel et al., 1997; Schwarz et Ocean and extends from northeast to southwest, al., 2004]. Previous studies have shown that baro- betweenlatitudes37(cid:3)Nand40(cid:3)N(Figure1).Mag- metric conditions of magma storage and ascent matisminthisareapresumablystartedinthelower can be estimated from densities of fluid inclusions Miocene [Luis and Miranda, 2008], with the for- trapped in phenocrysts and xenoliths [e.g., Zanon mation of a submarine plateau. The latter is et al., 2003; Zanon and Nikogosian, 2004; crossed by important tectonic structures: the Mid- Frezzotti and Peccerillo, 2004; Peccerillo et al., Atlantic Ridge, theEast Azores Fracture Zone and 2006; Kl€ugeletal.,2005]. the Terceira Rift. Georgen and Sankar [2010] reviewed the geodynamic importance of these [ ] Studies on the magmatism of the Azores 5 structures and on their relationship with the mag- islands focused on the geochemical characteriza- matismintheAzoresarea. tion of the mantle source [e.g., Beier et al., 2008, 2010; Elliott et al., 2007; Claude-Ivanaj et al., [ ] The islands are the emerged tops of larger vol- 8 2001; Franc¸a et al., 2006; Millet et al., 2009; canic elongated ridges that crop up from the 2 ZANONANDFREZZOTTI:MAGMASTORAGEBENEATHPICOANDFAIAL 10.1002/ggge.20221 Figure 1. Digital ElevationModel of the islandsofFaial and Pico, showingthe principal geographicfea- tures,historiceruptions,mainfaults(inwhite),andfissuresystems(yellowdashedlines).Tectonicdataare fromMadeira[1998]andMadeiraandBrumdaSilveira[2003].Fissurezonesarelabeledasfollows:CFZ— Capelo Fissure Zone; HFZ—Horta Fissure Zone; PAFZ—Planalto da Achada Fissure Zone. Stars indicate samplingsites.Coordinatesreportedinthegridmustbemultipliedby10,000.Theinsetshowsageneralloca- tion map of the Azores archipelago and the main geostructural lineaments. Acronyms are as follows: MAR—Mid-AtlanticRidge;TR—TerceiraRift;EAFZ—EastAzoresFractureZone;GF—GloriaFault. submarine plateau; their subaerial eruptions most developed system isthe Capelo Fissure Zone occurred since at least 7.1 Ma ago in the island of in the western part of the island, where it climbs Santa Maria [Abdel-Monem et al., 1975]. A tem- along the western slope of the central volcano of poralgapofabout5Maseparatestheformationof Caldeira. Its activity, which started presumably these rocks from those cropping out in Flores about6kaago,includesalsotheeruptionsof1672 island, while all the other islands were apparently and 1957–1958. The Horta Fissure Zone, in the formed at least 1.3 Ma ago [Caniaux, 2005]. Sev- southeastern part of the island is less extended eral eruptions have taken place since the coloniza- (only(cid:4)4km long).The activity ofthis systemhas tion of the archipelago (i.e., around A.D. 1427— not been dated, however the presence of 16 ka old Zbyszewski, 1963) in the islands of Faial, Pico, pyroclastic fallout generated by the Caldeira Vol- S~aoJorge,TerceiraandS~aoMiguel.Thelasterup- cano,coveringtheHortalavas,excludesmagmatic tion was submarine and occurred in 1998–2001, activityduringtheHolocene.Poorlyevolvedmag- (cid:4)20kmoffshoreTerceira. mas, from basalt to hawaiites, poured out from these two fissure zones [M(cid:2)etrich et al., 1981; [ ] The islands of Faial and Pico developed along 9 Zanonetal.,2013]. a WNW-ESE alignment and formed in a period of 850 ka [Hildenbrand et al., 2012] and about 240 [ ] ThecentralvolcanoofCaldeiraisabroadedi- 10 ka [Madeira and Brum da Silveira, 2003], respec- fice topped by a deep circular caldera. Its main tively. These islands originated from the interplay morphological feature is the presence of the Pedro between fissure volcanism and eruptions from Miguel graben, which dissects the whole edifice composite volcanoes. Fissure volcanism occurred from WNW to ESE. This edifice rises where along a few-kilometer-long subparallel clusters of N150(cid:3) striking tectonic systems interacted with fractures, whose alignment is generally similar to fissure systems and it is possible that this interac- that of the other nearby islands (Figure 1). Lava tion favored the formation of short-lived and accumulation along these fractures caused the for- shallow-level magma reservoirs, leading to the mation of volcanic ridges that consist of several establishment of a centralized feeding conduit or a rows of cinder cones. In the island of Faial, the seriesofcloselyspacedfeedingdykes[Mirandaet 3 ZANONANDFREZZOTTI:MAGMASTORAGEBENEATHPICOANDFAIAL 10.1002/ggge.20221 al., 1998]. Basalt-to-trachyte magmas [M(cid:2)etrich et cathodoluminescencedetectors,(UniversityofMi- al., 1981; Zanonet al., 2013] were erupted by this lan). A spot size of 1 (cid:2)m with a beam current of volcano starting from (cid:4)120 ka ago [Hildenbrand 15 nA was considered for all the mineral phases, et al., 2012], and its last eruptive event took place whereas a spot size of 5–7 (cid:2)m, according to the (cid:4)1.2kaago[Madeira,1998]. available surface to be analyzed, and a beam cur- rent of 2 nA were applied to glasses. Counting [ ] Pico is an elongated island characterized by 11 timeswere30sonthepeakand10soneachback- the presence, in its western part, of the steep Pico ground. Natural and synthetic minerals and Mountain (2300 m asl). This 240 ka old central glasses, used as standards, were calibrated within volcano [Chovelon, 1982] has an almost perfectly 2% at 2(cid:3) standard deviation. Raw data were cor- circularbase andnumerouslateral vents whichare rected applying a Phi-Rho-Z quantitative analysis either isolated or arranged along small radial fis- program. The typical detection limit for each ele- suresthatpunctuateitsflanks.Twoareasofpartial ment is0.01%. Allthegeochemicaldata are inthe sector collapses are located north and south of the supportinginformation(TableS3). summit area. On its summit, there is a small and young caldera, partially filled with the lavas emit- [ ] Fluid inclusions were studied in 31 lava and 16 ted by a steep and tall nested hornito. A well- six xenolith samples. About 0.5–1 kg of each lava evident NW-SE trending fault intersects the whole sample was coarsely crushed up to obtain the final edifice, passing through the summit caldera. This grain size of the largest crystal that could be visi- fault drove magma toward the surface during the ble at a macro scale. About 150/200 olivines and A.D. 1718 eruption. Another eruption from this 100/150 clinopyroxenes were embedded in epoxy volcano occurred along a more southeasterly and doubly polished up to attain a final thickness locatedradialfracture,inA.D.1720. of about 120–80 (cid:2)m. For xenoliths, 100–130 (cid:2)m thickdoublypolishedwaferswereprepared. [ ] East of thePico Volcano, there isthe Planalto 12 da Achada Fissure Zone. This structure is similar [ ] Microthermometry was carried out on a 17 to the Capelo Fissure Zone and follows the same Linkam MDSG600 heating-cooling stage, geographic direction, but it is considerably longer calibrated according to synthetic fluid inclusion ((cid:4)30km).About170cindercones,someofwhich standards of pure CO and H O. Melting and 2 2 have a considerable size, constitute this ridge. homogenization temperatures are reproducible The oldest lava flow unit dated here is 230 ka old to 60.1(cid:3)C with heating rates in the range of [Chovelon, 1982], but magmatism continued until 0.2–0.5(cid:3)C/min. thelasteruptionin1562. [ ] Fluid inclusions were further analyzed with a 18 [ ] Both the Pico Mountain and the Planalto da confocal Labram multichannel spectrometer of 13 AchadaFissureZoneeruptedpoorlyevolvedmag- Jobin-Yvon Ltd. at the University of Siena (Italy). mas, whose composition ranges from basalt to An Arþ laser produced the excitation line at 514.5 hawaiites. The only exception regards a mugearite nm. The Raman intensity was measured with a eruptedin1718[Franc¸aetal.,2006]. peltier-cooled carbonate compensation depth (CCD) detector. The beam was focused to a spot size of about 1–2 (cid:2)m using an Olympus 100(cid:6) 3. Methods lens. The scattered light was analyzed using a Notch holographic filter with a spectral resolution [ ] Activation Laboratories (Canada) performed of 1.5 cm(cid:2)1 and grating of 1800 grooves/mm. 14 24 whole-rock analyses of major elements using Mineral identification was based on our spectral inductively coupled plasma (ICP). Reproducibility database [Frezzotti et al., 2012a]. Raman identifi- was generally better than 1%. Densities of bulk cation of optically ‘‘hidden’’ water (i.e., <10 mole xenoliths andlava samples weremeasuredmaking %)influidinclusionswasperformedfollowingthe use of a MD 200s electronic densimeter and cor- approach described in Frezzotti and Peccerillo rected for porosity. The density resolution was is [2007]andFrezzottietal.[2012b]. 0.001g(cid:5)cm(cid:2)3. [ ] ThedensitiesoftheCO fluidwerecalculated 19 2 [ ] Electron microprobe analyses of mineral on the basis of the equations (3.14) and (3.15) of 15 phases in lava and xenolith samples were obtained Span and Wagner [1996]. Isochores for a pure with a JEOL JXA 8200 Superprobe, equipped CO fluid were calculated through the application 2 with five wavelength-dispersive spectrometers, of the equation of state of CO of Sterner and Pit- 2 Energy Dispersive X-ray spectroscopy (EDS), and zer [1994], valid up to at least 2000(cid:3)K and 10 4 ZANONANDFREZZOTTI:MAGMASTORAGEBENEATHPICOANDFAIAL 10.1002/ggge.20221 GPa. For CO -H O inclusions (H O:CO vine and clinopyroxene and few phenocrysts. 2 2 2 2 ratio¼1:9), densities were calculated on the basis Lavas issued from the northwestern vents showed of the equation provided in Sterner and Bodnar no megacrysts, but, on the contrary, there were [1991], after the application of the fluid density numerous phenocrysts of plagioclase, olivine, cli- correction suggested by Hansteen and Kl€ugel nopyroxene and many gabbroic and cumulitic [2008]. Isochores were calculated using the soft- xenoliths. The lava emitted in 1720 was poorly ware ‘‘Fluids’’ [Bakker, 2003]. The microthermo- porphyritic with few plagioclases and mafic metric data are shown in the supporting phases. information(TableS4). [ ] In Faial, the whole surface of the central vol- 24 cano of Caldeira exhibited a thick pyroclastic cover produced during its Holocene activity. Two 4. Description of the Samples lavaswerethereforecollectedfromrareaccessible outcrops along the coast and from some gullies on [ ] Fluidinclusionswerestudiedinthelavaflows 20 the southeastern flank of the volcano. These sam- emitted from both the fissural systems and from ples were poorly porphyritic lavas issued during a the central volcanoes of the islands of Pico and period starting from 120 ka ago. The first one Faial. Most of the samples date back to the Holo- showed similar characteristics to the southern his- ceneageandwerecollectedfromquitewellrecog- toric lavas from Pico, while the second rock had nizablevolcanicunits. onlyfewmacroscopicplagioclases. [ ] All the lavas ranged in composition from ba- 21 salt to hawaiite and presented different degrees of 4.2. Fissure Systems porphyricity. Three mugearites from the central volcanoes were collected in order to check if the [25] All thelavas and pyroclasts collected from all evolution of magma was associated to changes in the fissure zones had in general very similar char- the feeding systems. We also carefully examined acteristics, i.e., they were poorly porphyritic with theultramaficxenolithsfoundinsomelavaflows. a phenocrysts content (olivine, clinopyroxene6 plagioclase) amounting to less than 20% in vol- umeandweredevoidofmegacrysts. 4.1. Central Volcanoes [ ] Thebulkrockcompositionrangedfrombasalt [ ] On the slopes of the central volcano of Pico 26 22 to hawaiite. Some lavas with a higher crystal con- we collected 16 samples from recent lava flow tent were collected only in the Capelo Fissure units, issued from isolated vents far from the sum- Zone area, where numerous phenocrysts of mit cone, or from vents located along radial fis- olivine were scattered over the flow and did not sures connected to the main feeding system. One form any evident accumulation texture. In any lava sample issued from the summit cone was col- case, only a lava sample that contained some lected on the southwestern rim of the caldera. All xenoliths had a crystal content comparable to that samples except one were younger than 40 ka (sup- of the highly porphyritic lavas from Pico central portinginformation,TableS1). volcano. [ ] Most of these samples had a very high crystal 23 [ ] In the island of Pico, seven samples were col- content and were macroscopically characterized 27 lected along the axial ridge of the Planalto da bythepresenceofalargeproportionofmaficphe- AchadaFissureZoneandfromtheverticalcliffon nocrysts(Ø1.5–6.0mm)andmegacrysts(upto25 the northern coast, where a succession of thin mm in length), totaling between 25 and 70% in lavas is capped with the A.D. 1562 flow. With the volume. Plagioclase was sometimes present only exception of a single sample, which is older than as phenocrysts of 8–12 mm in size. Megacrysts 50 ka, all the other rocks are of the Holocene or were commonly either widespread inside lava historic. flows or concentrated in discontinuous layers or lenses. In a single case, they appeared as patches [ ] In the island of Faial eight rocks collected 28 of a dense network of euhedral and equidimen- along the fissure zones come from scoria cones or sional crystals, physically linked to one another. associated lava flows and were representative of The historical lavas had a variable degree of por- the whole activity which developed there. The phyricity. Those collected from the southern vents samples from Capelo Fissure Zone are younger ofthe1718eruptionweremacroscopicallycharac- than6 ka,those from HortaFissureZone are older terized by the presence of some megacrysts of oli- than11ka. 5 ZANONANDFREZZOTTI:MAGMASTORAGEBENEATHPICOANDFAIAL 10.1002/ggge.20221 4.3. UltramaficXenoliths [ ] Ultramafic xenoliths were only found in lavas 29 from Planalto da Achada Fissure Zone, in Pico and Capelo Fissure Zone, in Faial. In both cases, these lava flow units cropped out at the morpho- logical boundary between the central volcano and the fissure zone. The flow from Planalto da Achada Fissure Zone (Pico Isl.) is older than 50 ka. It crops out at the base of the northern cliff, covered by the A.D. 1562 lava, at a distance of (cid:4)15 km from the crater of the central volcano. However, its source vent can be reasonably located close to that of 1562, at a short distance from the eastern slopes of the central volcano, i.e., at the boundary between a central volcano and a Figure2. Chemicalclassificationofcollectedlavasamples. fissure zone. The host lava was poorly porphyritic Openversusfullsymbolsdistinguishsamplesbetweencentral and contained olivines and clinopyroxenes. The volcanoesandthefissurezones.Light-coloredarearepresents sizeofxenolithsdidnotexceed3–4cm. the entire compositional field from existing literature [Zanon etal.,2013;Franc¸aetal.,2006]. [ ] The flow from Capelo Fissure Zone (Faial 30 Isl.) was a crystal-rich lava which cropped out on the northern coastal cliff. This lava is not dated; cryptocrystalline textures werealso observed (Fig- however, due to the existing dating of the Capelo ure3a). rocks, we can assume that this flow is younger [ ] Olivine phenocrysts (rarely exceeding 2 mm 33 than 6 thousand years. Xenoliths were composi- across) were euhedral, subhedral, and/or skeletal; tionally heterogeneous, with the typical size(cid:7)20 their core Mg number [Mg#¼100(cid:5)Mg/(MgþFe)] cmandhadeitheraroundedoranangularshape. ranged,onaverage,from77to82(Figure4).Only the last tens of microns at their rim were Fe enriched. Euhedral and zoned augites (Wo , 31(cid:2)42 En , Fs ) were always present in all fissure 5. Petrographic Characteristics 31(cid:2)47 3(cid:2)14 zones, with the exception of magmas from the Horta Fissure Zone (Faial Isl.), where their pres- 5.1. LavaandScoriaSamples ence was rarer. Their Mg# values ranged from 72 [ ] Allthesamplesshowedthesamepetrographic to 86. The negative correlation between Ti and Si 31 and geochemical characteristics. The only differ- evidenced in the diagram of Figure 5a, indicated ence was represented by the total crystal content the occurrence of a common crystallization path that in rocks from fissure zones did not exceed for all the samples collected. The AlIV/AlVI ratio 19%involume,whileinthelavasfromthecentral decreased irregularly with titanium, indicating dif- volcano of Pico, it ranged between 25% and 70% ferent and variable pressures of crystallization of (supporting information, Table S2). These latter clinopyroxenes from microphenocrysts to mega- lavas, known as ankaramites (i.e., dark porphyritic crysts [Wass, 1979] (Figure 5b). Plagioclase was mafic lavas with a high content of pyroxene and ubiquitous in all fissure zones while scarce or olivine crystals and minor amounts of plagio- absent in the central volcano of Pico. Crystals clase), contained also numerous mafic megacrysts. were typically euhedral, tabular and with oscilla- Lavas of this kind are common on many other tory or reverse zoning and with composition from islands of the archipelago and have already been bytownite to labradorite [Franc¸a et al., 2006; described for the islands of Corvo [Larrea et al., Zanonetal.,2013]. 2012]andFaial[Zanonetal.,2013]. [ ] Olivines, clinopyroxenes and plagioclases, 34 [ ] Collected samples ranged in composition plus ilmenites or titanomagnetites or both, com- 32 frombasalttomugearites(Figure2; supportingin- monly represented microphenocrysts. Amphibole formation, Table S3) with mineral assemblage (Ø(cid:8)2.5mm)waspresentinthelavasemitteddur- constituted by olivineþclinopyroxene6plagio- ing the A.D. 1718 eruption from the northern seg- clase6oxides. The typical textures were intergra- ment of the fissure. Apatite was frequently found nular and intersertal. Rarer hyaloophitic and intimately associated with clinopyroxenes and/or 6 ZANONANDFREZZOTTI:MAGMASTORAGEBENEATHPICOANDFAIAL 10.1002/ggge.20221 amphiboles. Nepheline was occasionally found in somebasaltsfromPicoVolcano. [ ] The megacrysts of olivines and clinopyrox- 35 enes (Figure 3b) were commonly euhedral or sub- hedral or anhedral, and showed disequilibrium features, such as reaction rims and embayments. It was sometimes very difficult to discriminate between phenocrysts and megacrysts especially for olivines, due to overlapping compositions and crystalssize. [ ] A comparison was made between the forster- 36 ite content (Fo) in phenocrysts and in megacrysts and the composition of olivines in equilibrium Figure4. Comparisonbetweenthetheoreticalconditionsof olivinecrystallizationandthemeasuredforsteritecontent.(a) Thevariationofforsteriteofmegacryst(ingray)andofphe- nocrysts (in black), hosted in various samples of the two islands.(b)Theblackcurverepresentsthevalueofforsterite in equilibrium with coexisting silicate melt at various pres- sures.Thiscurvewassimulatedinabsenceofwaterandunder the QFM buffer, with MELTS software [Ghiorso and Sack, Figure 3. Microphotographs of representative samples. (a) 1995], starting from a primary melt composition of Faial Poorly porphyritic lava of the Horta Fissure Zone, showing Island, recalculated from the sample Fys 301, a poorly por- two small and partly iddingsitized olivine dispersed among phyritic basalt from Horta Fissure Zone (Faial Isl.), already plagioclase microphenocrysts. (b) Highly porphyritic lava describedinZanonetal.[2013].Theprimarymeltwasrecal- samplefromthePicosummitcone.Itisevidentthepresence culatedafterequilibratingthesamplecompositionwithFo89, oflargeeuhedralmegacrystsofolivineandclinopyroxene.(c) usingthegeothermometersofLeeetal.[2009].Theresulting Wherlitesampledfromapoorlyporphyriticlavaofthefissure conditionsofmeltingwereusedasinputforfractionalcrystal- zone of Pico. The presence of numerous trails of fluid inclu- lization simulations (i.e., 1450(cid:3)C and 25 kbar). The segment sionsatthecentreofthemainisperfectlyvisibleatthecentre of the liquid line of descent shown is relative to the litho- oftheclinopyroxene. sphere thickness (41.9 km) simulated beneath the island of Faial[Dasguptaetal.,2010]. 7 ZANONANDFREZZOTTI:MAGMASTORAGEBENEATHPICOANDFAIAL 10.1002/ggge.20221 tion (Figure 5b) and made it possible to consider also those crystals as antecrysts. However, the pa- rental link between antecrysts and phenocrysts was not relevant for the purposes of the present work; therefore, in the following discussion, we continued to distinguish megacrysts from phenocrysts. [ ] Mugearites were subaphyric with trachytic 38 texture and phenocrysts assemblages constituted by numerous oligoclase-to-Na-sanidine feldspars (Ø>4 cm), rare olivines (Ø>0.15 cm) (Fo ) 57(cid:2)60 and kaersutite amphiboles. The same phases were present as microphenocrysts together with clino- pyroxene, apatite, and oxides (both ilmenite and titanomagnetite). 5.2. UltramaficXenoliths [ ] Ultramafic xenoliths were coarse-grained lith- 39 ologies made up of olivine, clinopyroxene, ortho- pyroxene, and spinel in various amounts (Figure 3c). Plagioclase and plagioclase-and-amphibole- bearinglithologieswerealsofrequentbuthavenot been described here, since they did not contain fluid inclusions. Mantle xenoliths consisted of three harzburgites from Pico, and one websterite Figure 5. (a) The variations of the Ti in clinopyroxenes of andonewherlitefromFaial. thetwodimensionalclassesarenegativelycorrelatedwithSi, andformasingletrend.Thisrelationshipindicatesacontinu- [ ] All peridotites showed porphyroclastic to 40 ous variation in the crystallization pressure of clinopyroxene equigranular textures, where olivine and orthopyr- [Wass, 1979]. (b) Data of AlIV/AlVI versus Ti in the mega- oxene grains were present as large porphyroclasts crysts of clinopyroxenes in lavas of the central volcano of (up to 4 mm) and polygonal grains typically<1 Pico are poorly variable, while corresponding variations in mm. Clinopyroxene was present as polygonal poorly porphyritic lavas are wider. This indicates that the grains in the wherlite and as interstitial grains in largercrystals in the highlyporphyriticlavas were generated deeper than the smaller ones in the poorly porphyritic lavas. harzburgites. Spinel was interstitial (up to 0.5 mm A.p.f.u.standsforAtomsporFormulaUnit. in diameter) or present as inclusions in olivine. Plagioclase was absent; all studied xenoliths equi- libratedinthespinelstabilityfield. along the liquid line of descent of a recalculated primitive magma composition (Figure 4). Fo was [ ] Olivine was the dominant phase in all perido- 41 generallyhigherinmegacryststhaninphenocrysts tite samples. In harzburgites, it had Mg# ranging (Figure 4a). However, many phenocrysts also from 88 to 90.5, NiO contents ranging from 0.26 showed disequilibrium conditions with the melt. to0.47wt%andlowCaO(<0.17wt%)andCr O 2 3 Forthis reason,both ofthem shouldbe considered (<0.07 wt%). In harzburgites, orthopyroxene as ‘‘antecrysts’’ [Davidson et al., 2007], i.e., crys- showed higher Mg# than olivine (91–92), high tals, which may have formed from a different and Cr O (0.74–0.48 wt%), and low Al O (1.56– 2 3 2 3 moreprimitivemagmathanthepresenthost. 2.27 wt%) and TiO (<0.13 wt%). Clinopyroxene 2 had also very high Mg# (0.91–0.93), and spinel [ ] Similar considerations were made for clino- 37 was Cr rich. In the websterite and in the wherlite pyroxenes. Both megacrysts and phenocrysts fol- Mg# of both olivine and orthopyroxene were lowed the same evolution path (Figure 5a) and between 87 and 88, while slightly higher in clino- were therefore comagmatic. However, Mg# in pyproxene(Mg#¼89–90). megacrysts was higher and less variable (i.e., 79–90) and the variation of the AlIV/AlVI ratio [ ] Based on their mineral chemistry, harzbur- 42 with Ti was limited, which evidenced higher pres- gites from Pico were interpreted as fragments of sure conditions during clinopyroxene crystalliza- upper mantle, which underwent partial melting 8 ZANONANDFREZZOTTI:MAGMASTORAGEBENEATHPICOANDFAIAL 10.1002/ggge.20221 processes. These were similar to ultrarefractory mantle xenoliths previously reported from S~ao Miguel[Simonetal.,2008].Incontrast,thepyrox- eniteandthewherlitefromFaialhadcompositions that are more fertile and appeared to represent melts crystallized or cumulated at mantle depth, butnotnecessarilyinamagmachamber. 6. Study on the Fluid Inclusions [ ] In all the samples the fluid contained in the 43 inclusions was rich in CO . The distribution of 2 fluid inclusions varied considerably among the samples. In general, they could be frequently found in ultramafic xenoliths, but they were found in (cid:4)10–20% of megacrysts of olivine, whereas they were scarce in (cid:4)1% of phenocrysts. They were also found in (cid:4)5% of clinopyroxene, both megacrysts and phenocrysts, and in some plagio- clasephenocrysts. 6.1. Petrographic Analysis [ ] On the basis of their textural characteristics, 44 two populations of fluid inclusions were observed. Early carbonic inclusions (Type I) were either iso- lated inclusions or in small and spatially defined clusters, located both inside the grains and at its boundaries(Figure6a). This kind of inclusions was rarely found in few olivine and clinopyroxene grains in highly porphyritic lavas of Pico Volcano and in a single olivine from a poorly porphyritic lava of the Horta Fissure Zone (Faial Isl.). The inclusions were generally rounded or rarely nega- tive crystal shaped, with a size (cid:8)20–30 (cid:2)m. At room temperature, Type I inclusions were single phase (L) or might contain a vapor bubble (LþV). Their texture often showed evidences of partial density re-equilibration, such ashaloes of tiny fluid inclusions(Ø<1(cid:2)m)surroundingamaininclusion cavity, and/or short cracks radiating from the microcavity[VitiandFrezzotti,2000,2001].Afew inclusions, contained in a single olivine megacryst from a single sample of the volcano of Pico hosted a tiny (Ø(cid:8)5 (cid:2)m) Mg-calcite [(Ca,Mg)CO3—Fig- Figure 6. Photomicrographs of the different types of fluid ure 7a], identified by Raman microspectroscopy by inclusions hosted in olivines, and their textures. (a) Olivine its diagnostic vibrations at 714 and 1087 cm(cid:2)1 from the 1718 eruption in Pico Island, showing a cluster of [Frezzotti et al., 2012a]. In another group of inclu- TypeIfluidinclusionwithdecrepitationfeatures(halooftiny sions of the same samples, the presence of dypin- bubbles), marked by red arrows. (b) Olivine from a highly gite (Mg [(OH)(CO3) ] (cid:5)5H O [Frost et al., 2008] porphyritic lava from the central volcano of Pico, showing 5 2 2 2 numerous Type II diffuse fluid inclusions, coexisting with wasidentifiedforitsmainRamanvibrationat1122 cm(cid:2)1—Figure7b. several silicate melt inclusions of different size. (c) Olivine from a dunite fragment from Faial Island, showing various [ ] Late carbonic fluid inclusions (Type II) were trailsoftiny(2–15mm)TypeIIfluidinclusions. 45 dominant in all the studied samples, where they 9 ZANONANDFREZZOTTI:MAGMASTORAGEBENEATHPICOANDFAIAL 10.1002/ggge.20221 Figure7. RamanspectraofcrystallinephasesfoundinsideTypeIfluidinclusions(threeacquisitionsfor30 sbandareaintegration,withalaserpowerof300mW).Themainvibrationsat1285and1388cm(cid:2)1arechar- acteristic of CO . Raman bands of host olivine are marked with asterisks. (a) Peaks of Mg-calcite— 2 (Ca,Mg)CO .(b)Thevibrationofthehydratedcarbonatedypingite—Mg [(OH)(CO ) ] 5H O. 3 5 32 2 2 formed trails of variable length and thickness, 6.2. Composition which lined completely healed fractures, limited or crosscutting grain boundaries (Figures 6b and [46] On cooling, CO2-rich inclusions froze 6c). The inclusions were typically (cid:8)30 (cid:2)m, usu- between (cid:2)68 and (cid:2)92(cid:3)C. A further decrease of ally rounded (Figure 6c) or with a negative- temperature did not produce any visible phase crystal shape. At room temperature, Type II change. Most of the inclusions melted instantane- inclusions were single phase (L) or two phases ously within a temperature interval between 57.1 (LþV), commonly liquid rich. Many inclusions and56.4(cid:3)Cwithmostofthedataat56.6(cid:3)C. Clath- showed evidence of re-equilibration and coex- rates were not observed and final homogenization isted with numerous silicate melt inclusions with occurred to either the liquid (Th ) or the vapor L a variable size and crystallization degree. These phase (Th ) at <31.1(cid:3)C. Liquid water was not V silicate melt inclusions may contain a large detected, neither optically nor by Raman spectros- CO -richbubble. copy,inanyofthemeasuredinclusions. 2 10