Chemical Geology 287 (2011) 66–80

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Chemical Geology

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A CO2-rich magma source beneath the Phlegraean Volcanic District (Southern Italy):Evidence from a melt inclusion study

A. Mormone a, M. Piochi a,⁎, F. Bellatreccia b,c, G. De Astis d, R. Moretti a,e,⁎⁎, G. Della Ventura b,c,A. Cavallo d, A. Mangiacapra a

a Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Napoli, Osservatorio Vesuviano, Napoli, Italyb Dipartimento di Scienze Geologiche, Università Roma Tre, Roma, Italyc Istituto Nazionale di Fisica Nucleare, Laboratori Nazionali di Frascati, Frascati, Roma, Italyd Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Roma 1, Roma, Italye Centro Interdipartimentale di Ricerca in Ingegneria Ambientale (CIRIAM) & Dipartimento di Ingegneria Civile, Seconda Università degli Studi di Napoli, Aversa (CE), Italy

⁎ Correspondence to: M. Piochi, Istituto NazionaleSezione di Napoli Osservatorio Vesuviano, Via DioclezTel.: +39 81 6108529.⁎⁎ Correspondence to: R. Moretti, Centro Interdipartim

Ambientale (CIRIAM) & Dipartimento di Ingegneria CiStudi di Napoli, Aversa (CE), Italy. Tel.: +39 81 501041

E-mail addresses: monica.piochi@ov.ingv.it (M. Pioc(R. Moretti).

0009-2541/$ – see front matter © 2011 Elsevier B.V. Adoi:10.1016/j.chemgeo.2011.05.019

a b s t r a c t

a r t i c l e i n f o

Article history:Received 2 November 2010Received in revised form 18 May 2011Accepted 30 May 2011Available online 13 June 2011

Editor: D.B. Dingwell

Keywords:Melt inclusionsPhlegraean Volcanic DistrictCO2-rich magma sourceDegassingFluxing

We present a study of olivine-hosted glass/melt inclusions (MIs) in the most primitive rocks erupted atProcida Island, within the Phlegraean Volcanic District (PVD), Southern Italy. MIs were analyzed by combinedScanning ElectronMicroscopy coupledwith Energy Dispersive X-ray-detectors,Wavelength Dispersive X-ray-equipped Electron Microprobe and Fourier Transform Infrared (FT-IR) Spectroscopy; notably, the novel Focal-Plane-Array mode provided high-resolution FT-IR images evidencing the distribution of the C–H–O speciesacross samples. Olivines range in composition from Fo85 to Fo90, do not show chemical zoning and are totallyanhydrous. The majority of theMIs are glassy, while only few are completely crystallized. SomeMIs, however,show the occurrence of crystal nuclei, i.e., nano- to micro-sized pyroxenes and oxides, and appear as low-crystallized MIs. The glass of crystal-free and low-crystallized MIs shows K-affinity and a compositional rangealong the basalt, trachy-basalt, shoshonite, tephrite basanite and phono-tephrite array. H2O and CO2 contentsup to 2.69 wt.% and 2653 ppm, respectively, define amajor degassing trendwith small isobaric deviations. Thecollected data allow recalculating entrapment pressures from ~350 MPa to b50 MPa and suggest that themagma ascentwas dominated by degassing. Crystallizationwas aminor process, likely also consequent to localCO2-fluxing. Mingling occurred between variable degassed and crystallized magma portions duringdecompression. The geochemical and isotopic data of Procida glasses and rocks, and the compositionalrelationship between our MIs and those from slightly more evolved and radiogenic Phlegraean products,indicate that Procida basalts are an adequate parental end-member for the PVD. Our data suggest that a CO2-rich magma source was stored at depths of at least 13–14 km (i.e., 350 MPa) beneath the PVD. Fast ascent ofmagma batches directly started from this depth shortly before PVD trachy-basaltic to shoshonitic eruptions.Such results have implication on volcanic hazard assessment in the PVD area.

di Geofisica e Vulcanologia,iano 328, 80124 Napoli, Italy.

entale di Ricerca in Ingegneriavile, Seconda Università degli6.hi), roberto.moretti@unina2.it

ll rights reserved.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Melt inclusions (MIs) are μm-sized and variable-shaped parcels ofglass±vesicles±solids trapped within a growing crystal (Roedder,1984; Lowenstern, 1995a,b; Sobolev, 1996; Frezzotti, 2001 andreferences therein included). Most of them are frozen snapshots of theinitial melt droplet and record the physico-chemical conditions of thehosting system. Therefore, MIs provide a unique probe of near-liquidus

magmatic conditions, otherwise inaccessible to Earth scientists, and arewidely used to integrate conventional petrological and volcanologicalstudies. Microthermometry, electron and ion microprobe analyzers aswell as infrared spectroscopy are analytical techniques commonly usedto determine temperature, composition and the volatile budget of themagma from which MIs formed. The knowledge of these parameterstogether with melt-gas saturation models allows assessing theentrapment pressure and elucidates themechanisms ofmagma genesis,crustal storage, as well as the eruptive dynamics.

Here, we report on the major oxides and volatiles data obtained onMIs within olivines from the Solchiaro I products (De Astis et al.,2004), sampled at Procida, a volcanic island of the PhlegraeanVolcanic District (PVD) in Southern Italy (Fig. 1). Previous papers(Cecchetti et al., 2001; De Astis et al., 2004) indicate that this ≈18 kaold activity spilled a deep reservoir and erupted the least evolved –

both for general chemistry and isotopic features – PVD magma. To

Fig. 1. Procida Island (a) and its location within the PVD area (b) in Southern Italy (c). In (a), dashed lines delimit the volcanic craters and the arrow indicates the outcrop where theSolchiaro I sequence (d) has been sampled. Diagram in (e) shows the compositional range of the Solchiaro sequence and the least evolved (SiO2b55 andMgON3 wt.%) rocks from theCampi Flegrei, Procida and Ischia published in the literature (De Astis et al., 2004; Piochi et al., 2004, 2005 and references therein included).

67A. Mormone et al. / Chemical Geology 287 (2011) 66–80

understand the main features of the ascent and evolution processescharacterizing this pristine magma, likely the most representativefor investigating the PVD magmatic system (De Astis et al., 2004;Piochi et al., 2004; and references therein), we combined ScanningElectron Microscopy with X-ray-Energy Dispersive (SEM-EDS),Electron MicroProbe with X-ray-Wavelength Dispersive (EMP-WDS)and Fourier Transform Infrared (FT-IR) Spectroscopy. Moreover, weexploited the capabilities of a novel Focal-Plane-Array technologyapplied to FT-IR (FT-IR-FPA) to map the distribution of H2O and CO2

species through the MIs. Compared to conventional single-pointanalyses, the use of the FPA detectors allows the whole spectroscopicimage of a sample in a single data collection to be obtained (DellaVentura et al., 2010). The used multidisciplinary approach – inparticular, combined SEM and FPA observations – increases datareliability, yielding more constrained hypotheses on those phenom-ena that superimposed during the evolution of a plumbing system.

Our data, compared with those in the literature (Cannatelli et al.,2007; Mangiacapra et al., 2008; Arienzo et al., 2010) from the otherPVD volcanoes, shed light on the possible behavior of the PVD systemduring its recent period of activity (≤18 ka). As a whole, the datahighlight the role of volatile degassing, particularly CO2 fluxing,during magma storage and evolution in a tectonic setting showingextensional features (Piochi et al., 2005).

2. Geo-volcanological background

Procida Island is located along one of the main tectonic trend of thePVD that also includes theCampi Flegrei caldera and Ischia (Fig. 1) (Piochiet al., 2005 for a review). The island showscomposite volcanic successionswhere home-erupted, Campi Flegrei and Ischia deposits are often

interlayered. Its volcanism is related to five monogenic cones – TerraMurata, Vivara, PozzoVecchio, Fiumicello andSolchiaro–whoseactivitiesspread out fromabout 70 to 18 ka (DeAstis et al., 2004), althoughmost ofthem are not age-constrained. Among these volcanic structures, theSolchiaro tuff ring is the youngest and occurred shortly before theemplacement of the Neapolitan Yellow Tuff products (~15 ka — (Deinoet al., 2004)), just outside the Campi Flegrei caldera borders. Recently, DeAstis et al. (2004) have distinguished three different deposits along theSolchiaro sequence (i.e. Solchiaro I, II, III), separated by weak erosionalsurfaces and/or pumiceous lapilli layers of un-indentified source. Inparticular, the Solchiaro I deposits studiedhere (Fig. 1d) are constitutedoflithified tuffs with abundant blocks and bombs in proximal facies andstratified yellow to gray tuffs in distal outcrops. These products show sub-planar to cross stratification, with tractive structures, and an increasingpresence toward the top of well-sorted scoriaceous lapilli layers. Thelatter is believed to indicate a transition from dominating phreato-magmatic surge to dominatingmagmatic fall activity (Fig. 1d). FollowingDe Astis et al. (2004), the Solchiaro I rocks are the least evolved olivine-bearing products erupted at the PVD in the most recent period ofvolcanism and may be considered as the mafic end-member for theCampi Flegrei magmatism. This is illustrated in Fig. 1e, in which theircompositions are plotted together with literature data for the leastevolved whole-rock products (SiO2b55 wt.% and MgON3 wt.%) of theCampi Flegrei, Procida and Ischia (De Astis et al., 2004; Piochi et al., 2004,2005 and references therein included).

3. Sample description and preparation

We sampled the basal products (sample PRO 7/8) of the Solchiaro Isequence (Fig. 1). Our sample corresponds to the PRO7/11whole-rock

68 A. Mormone et al. / Chemical Geology 287 (2011) 66–80

of De Astis et al. (2004). It shows low radiogenic Sr, Nd and Pbisotopes (

87Sr/

86Sr=0.705238,

143Nd/

144Nd=0.512685,

206Pb/

204Pb=

19.088,207Pb/

204Pb=15.678,

208Pb/

204Pb=39.153; De Astis et al.

(2004)) and is close to isotopic equilibrium between phases (i.e.,δ18O 5.63 vs. 5.88; De Astis, unpublished data), testifying its near-primary nature and closed-system (i.e., without newmagma rechargeand/or crustal contamination) magma evolution processes. Sampledjuvenile clasts range from dense to vesicular and, on average, arelapilli-sized. The rocks show porphyritic to glomeroporphyritictextures. Phenocryst assemblage consists of dominant euhedralolivine, with subordinate pale-green clinopyroxene, feldspar andFe–Ti oxide. The groundmass may contain feldspar microlites, whichare from tabular to acicular and dendritic in shape; they frequentlyform radial clots and, rarely, spherulites.

After washing in distilled water, the scoriaceous lapilli werecrushed in an agate mortar and then sieved. The procedure wasrepeated for those larger than 4 mm in diameter, in order to obtain agreater number of 2, 1 and 0.5 mm-sized fragments from whicholivine crystals were separated by handpicking under a binocularmicroscope. More than 70mineral grains were selected on the basis ofthe MIs abundance, glued on a glass slide with Crystalbond™ anddoubly-polished using abrasive foils. Crystals were removed from theglass slides and carefully washed in acetone to remove any residualfrom the binding agent. As a result, thin freestanding doubly-polishedolivine wafers were obtained. The target inclusion was exposed onboth sides for transmission FT-IR analyses. The MI thickness wasfirstly measured by using an electronic Mitutoyo micrometer(accuracy is ±1 μm) and checked under the optical microscopeusing a standard slice with known thickness (accuracy ±2 μm).Therefore, the latter method was preferred to avoid crystal break that,unfortunately, often occurred during relocation on support specific forthe diverse investigations. Final sample thickness was typically in therange 25–35 μm.

For SEM–EDS and EMP–WDSanalyses, the doubly polished olivinesand MI were simply glued on a graphite disk. Glassy groundmasseswere analyzed on conventional thin section of scoriaceous lapilli.

4. Analytical methods

FT-IR spectra were collected at Laboratori Nazionali di Fisicadell'Istituto Nazionale di Fisica Nucleare (LNF-INFN; Frascati, Roma,Italy) by using a Bruker Hyperion 3000 microscope. Sample-chamberwas constantly evacuated with an argon flux. Single-spots wereacquired through a liquid nitrogen-cooled MCT detector and a beamsize of 20 μm. The nominal resolutionwas 4 cm−1 and 128 scans wereaveraged for both background and sample spectral acquisition.Microspectroscopic images were acquired through a liquid N2-cooledFPA detectors by using a conventional (globar) IR source; theresolution was set at 8 to 16 cm−1 and 64 spectra were co-added toobtain the final result, such as to avoid data over-flow. Preliminarysingle-spot spectra collection on few glasses showed the presence ofweak but resolvable absorptions in the in the 4000–5000 cm−1 range,assigned to combination modes of OH− groups (e.g., Ihinger et al.,1994). These absorptions clearly emerge in glassy MIs, whereas low-crystallized MIs characterized by presence of widespread crystalnuclei (b50% by area coverage; see results and discussion later on) didnot show hydroxyl-related modes. We, thus, examined systematicallythe spectral domain between 4000 and 900 cm−1, and quantified theconcentrations of H2O, CO2 and CO3

2− species within the MIs by usingthe bands at ~3535, 2350, and 1430 and 1515 cm−1, respectively(e.g., Stolper, 1982; Stolper and Ahrens, 1987; Dixon and Pan, 1995).The absolute concentrations (C) were calculated according to theBeer–Lambert's relationship:

C = 100AM= t ˙ ρ ˙ εh i

where A is the measured absorbance, M is the molar mass (g mol−1)of the volatile specie, ε is the molar absorptivity, t is the samplethickness (cm), and ρ is the glass density (g cm−3). Following Langeand Carmichael (1987), the glass density (2.78 g cm−3) has beencalculated on the basis of the average content of major oxides(Table 1). Linear absorbance data were determined by using theOMNIC software (version 1; Thermo Electron Corporation 1992–2004) above a baseline as defined later in Section 5.2. For CO3

2−

absorbance, we averaged the height of the peaks at 1430 and1515 cm−1 (Dixon and Pan, 1995); the same was done for the twoCO2 component peaks (at 2342 and 2361 cm−1). The value of ε is themost critical issue for a correct application of the Beer–Lambertrelationship; ideally, it should be calibrated for the glass compositionof interest (see for example Ihinger et al., 1994; King, et al., 2002).Unfortunately, the absorption coefficients for H2O, CO2, and carbonatespecies are presently unknown for trachybasaltic compositions.Therefore, we used literature data on compositions as close aspossible to those of our MIs: i) ε3535=65.7 L mol−1 cm−1 for H2O,according to Bertagnini et al. (2003); ii) ε2350=1140 L mol−1 cm−1

by averaging the values of 1214 L mol−1 cm−1 from Behrens et al.(2004) [and references therein] and 1066 L mol−1 cm−1 from Blanket al. (1993), formolecular CO2; and iii) ε1515–1430=349 L mol−1 cm−1

for the CO32− group, as derived from the relationship ε1515–1430=451–

342 [Na/(Ca+Na)] L mol−1 cm−1 of Dixon and Pan (1995) and con-sidering average Na (0.21 mol) and Ca (0.096 mol) molar contents inthe our glass (from Table 1).

FT-IR spectra are highly reproducible, as demonstrated bymultipleanalyses on the same MI, which point to errors on the absorbance≤3%. Moreover, due to uncertainties on ε and, to a minor extent,density and thickness, we estimated maximum errors at 10% for H2Oand 20% for both CO2 and CO3

2− contents, as also suggested byMangiacapra et al. (2008) and in line with the discussion inBotcharnikov et al. (2006).

Detailed qualitative and quantitative imaging (Wysoczanski andTani, 2006; Della Ventura et al., 2010) of H–C–O groups in selectedsamples was done by using a 64×64-pixel FPA detector coupled witha 15× objective; the nominal resolution was set at 8 cm−1, and 128scans were averaged for each image. The covered area in a single datacollection was 170×170 μm2. As discussed by Della Ventura et al.(2010), the spatial resolution is a function of several factors, includingthe investigated wavenumber range, the FPA type and the microscopeobjective. The above set-up provided a favorable contrast for thetarget molecule between MI and their host-crystal, allowing us toachieve a resolution close to the diffraction limit (~5 μm in the H2Oregion, and ~10 μm in the CO3

2− region) (Della Ventura et al., 2010).Electron microprobe (EMP–WDS) analyses and backscattered

images (SEM–EDS) were carried out by using the JEOL JXA 8200WDS-EDS combined instrument at Istituto Nazionale di Geofisica eVulcanologia (INGV) of Roma, Italy. During EMP–WDS analyses, theoperating conditions were 15 kV accelerating voltage and 7 nA probecurrent, providing signal stability. A probe diameter of 5 μm, with afinal spot size of about 7 μm, was used to reduce alkalis migrationduring glass analysis. In particular Na, F, Si, K, and Ca were countedfirstly and simultaneously. Counting time was 10 s on both peakand background; data reduction was carried out using a so-called ZAF(Z — atomic number, Absorption and Fluorescence) correctionmethod, provided within the JEOL software package. The followingmaterials were used as standards for the various elements (betweenparenthesis): 1) in the case of glass analyses: Fluorite (F), Albite (Na),Island Basalt BIR-1 (Mg, Al, Si, Ca, Fe), Orthoclase (K), Sodalite (Cl),Barite (Ba), Nickel metal (Ni), Titanium Oxide (Ti), Apatite (P),Chromite (Cr) and Spessartine (Mn); 2) in the case of olivine analyses:Forsterite (Mg, Si), Pargasite, (Al, Ca,) Almandine (Fe), Albite (Na). Forglass analyses, maximum uncertainties were: 3.5% for Si, Al, Mg, Fe,Ca, K and Na, and 17% for Cl and S, and 24% for P. F, Cr and Nicontents were generally near or below the detection limit, implying

Table 1EMPa and FT-IR data of MIs in olivine phenocrysts from the Solchiaro I eruption.

Sample 9d MI1 9d MI2 14d MI1 14d MI2 10d MI1 10d MI2 13d MI1 13d MI 12d MI 11d MI2 5d MI 4d MI2 4d MI1 3d MI 8d MI 7d MI 6d MI

Mi type Glassy Glassy Glassy Glassy Highlycrystallized

Highlycrystallized

Glassy Glassy Partiallycrystallized

Partiallycrystallized

Glassy Highlycrystallized

Highlycrystallized

Glassy Glassy Glassy Highlycrystallized

SiO2 47.9 47.0 45.4 46.0 48.7 48.2 49.7 46.2 49.9 47.3 48.3 46.6 48.1 46.4 47.0 48.2 50.3TiO2 1.25 1.46 1.34 1.00 1.23 1.29 1.04 0.94 0.86 1.21 1.48 1.20 1.40 1.16 1.13 1.41 1.31Al2O3 15.8 16.8 17.5 17.6 15.1 15.2 15.1 16.8 15.6 16.7 18.7 17.2 20.2 16.0 17.1 15.2 14.9FeO 11.2 11.4 12.6 11.5 10.3 9.5 9.7 11.1 10.1 10.5 9.3 12.0 10.2 10.8 10.5 12.1 8.7MnO 0.22 0.16 0.13 0.16 0.07 0.05 0.08 0.19 0.12 0.15 0.09 0.14 0.09 0.12 0.12 0.12 0.08MgO 5.73 6.34 5.26 5.74 6.19 5.58 6.19 6.15 5.32 6.35 3.19 5.30 3.18 6.17 6.20 5.87 6.39CaO 12.8 11.2 11.6 11.1 10.8 11.4 11.0 11.3 8.2 10.8 11.8 12.6 10.9 12.2 11.3 11.8 10.9K2O 1.74 2.34 3.26 3.79 4.05 4.77 3.92 3.92 5.36 3.35 2.96 1.77 2.15 3.66 3.34 2.10 3.47Na2O 3.05 2.96 2.50 2.46 3.03 3.25 2.56 2.59 3.75 3.12 3.78 2.86 3.57 2.78 2.86 2.81 3.27P2O5 0.32 0.32 0.40 0.58 0.58 0.75 0.62 0.79 0.75 0.54 0.42 0.29 0.24 0.63 0.47 0.35 0.68Original Total 101.0 101.1 101.2 102.1 99.9 98.4 101.0 100.6 100.5 102.2 98.8 100.0 98.7 98.9 99.0 98.6 98.9F,Cl≡O 0.02 0.07 0.04 0.07 0.06 0.21 0.09 0.05 0.19 0.06 0.05 0.05 0.04 0.09 0.06 0.03 0.07Fb b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. 0.21 b.d.l. b.d.l. b.d.l. b.d.l. 0.11 b.d.l. b.d.l. b.d.l.Clb 0.10 0.15 0.19 0.29 0.22 0.93 0.31 0.23 0.44 0.28 0.24 0.16 0.16 0.21 0.25 0.15 0.31SO3

b 0.23 0.21 0.24 0.25 0.18 0.24 0.42 0.29 0.35 0.28 0.37 0.09 0.22 0.23 0.37 0.22 0.23H2O (wt.%) 1.74 0.77 1.20 0.80 1.50 1.10 1.12 1.70 0.86 2.70 0.99 0.35 0.20 1.46 1.40 1.30 1.31CO3

2— (ppm) 2892 226 789 423 b.d.l. b.d.l. 970 1412 b.d.l. 675 277 b.d.l. b.d.l. 1590 787 1468 1238CO2molecular (ppm) b.d.l. b.d.l. b.d.l. b.d.l. 891 483 b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l.CO2 TOT (ppm) 2892 226 789 423 891 483 970 1412 b.d.l. 675 277 b.d.l. b.d.l. 1590 787 1468 1238Fo (mol%)c 86.2 86.2 86.2 85.8 87.9 87.9 87.8 87.1 88.7 86.8 87.1 87.3 86.8 87.4 86.3 86.3 89.3KD

d 0.293 0.281 0.279 0.277 0.278 0.270 0.283 0.279 0.265 0.286 0.276 0.279 0.278 0.277 0.281 0.291 0.280P(MPa)e 346.7 86.2 179.4 122.3 215.6 144.6 209.8 250.2 72.1 185.6 69.5 21.2 26.1 256.7 171.3 261.8 240.4H2Ogas (wt.%)e 4.94 4.60 4.09 3.16 5.46 4.35 3.93 5.54 5.42 16.74 8.17 5.41 2.96 4.27 6.00 3.98 4.63CO2gas (wt.%)e 95.06 95.40 95.91 96.84 94.54 95.65 96.07 94.46 94.58 83.26 91.83 94.59 97.04 95.73 94.00 96.02 95.37

(continued on next page)

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Table 1 (continued)

Sample 1d MI2 1d MI1 2d MI2 2d MI1 MI2b2 OL4MI OL1Mib Ol1Mib OL1Mib0 OL1Mia0 OL1Mia1 OL2MIa OL2MIa Ol2MIb1 OL3MI Ol5MI OL5MI MI1

Mi type Glassy Glassy Partiallycrystallized

Partiallycrystallized

Glassy Partiallycrystallized

Glassy Glassy Notdefined

Glassy Glassy Glassy Glassy Glassy Glassy Highlycrystallized

Notdefined

Glassy

SiO2 47.3 47.6 46.1 49.9 – 48.0 48.5 47.9 49.7 48.8 48.9 48.8 49.5 51.3 50.5 50.9 50.6 –

TiO2 1.18 1.46 0.97 0.99 – 1.21 1.35 1.50 1.37 1.53 1.43 1.27 1.52 1.09 1.29 1.27 1.65 –

Al2O3 15.6 15.2 17.3 18.9 – 17.0 18.1 17.5 15.9 17.0 17.0 17.3 16.1 16.7 16.7 19.2 19.2 –

FeO 10.2 10.2 11.3 8.4 – 8.2 8.0 8.8 7.6 8.3 7.9 7.6 6.9 5.9 7.3 6.4 6.0 –

MnO 0.16 0.13 0.11 0.14 – 0.16 0.15 0.17 0.13 0.18 0.21 0.14 0.18 0.10 0.14 0.11 0.17 –

MgO 6.40 6.07 5.67 3.52 – 6.07 4.88 5.85 6.20 6.25 5.01 5.39 6.70 4.81 5.78 4.59 4.21 –

CaO 12.1 12.2 10.9 9.5 – 12.4 12.8 12.5 13.7 11.6 13.4 12.9 12.1 12.2 13.1 12.3 12.7 –

K2O 3.37 3.57 4.60 4.30 – 4.22 3.01 2.73 1.68 2.87 2.60 3.11 3.14 3.80 1.67 2.28 2.28 –

Na2O 3.02 2.99 2.42 3.44 – 2.13 2.81 2.56 3.36 2.88 3.06 3.01 3.15 3.56 3.09 2.53 2.60 –

P2O5 0.65 0.61 0.63 0.87 – 0.61 0.45 0.53 0.36 0.60 0.46 0.55 0.65 0.58 0.40 0.30 0.50 –

Original Total 100.34 99.50 100.87 96.75 – 96.43 96.32 97.11 96.74 97.56 95.97 96.41 96.56 96.46 91.27 97.16 95.66 –

F,Cl≡O 0.06 0.07 0.09 0.08 – 0.07 0.06 0.03 0.02 0.05 0.06 0.06 0.04 0.05 0.04 0.05 0.05 –

Fb b.d.l. b.d.l. b.d.l. b.d.l. – – – – – – – – – – – – – –

Clb 0.25 0.29 0.33 0.34 – 0.31 0.27 0.16 0.09 0.21 0.27 0.28 0.20 0.24 0.16 0.21 0.23 –

SO3b 0.24 0.30 0.22 0.22 – 0.28 0.23 0.24 0.18 0.28 0.27 0.30 0.25 0.22 0.27 0.19 0.18 –

H2O (wt.%) 1.54 1.18 0.50 0.35 0.73 1.26 n.d. 1.30 0.74 1.58 1.50 0.51 1.01 1.14 1.39 0.74 0.74 0.55CO3

2— (ppm) 1179 1108 b.d.l. b.d.l. b.d.l. b.d.l. n.d. 528 1451 b.d.l. 528 363 975 205 1662 b.d.l. b.d.l. b.d.l.CO2molecular (ppm) b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. n.d. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l.CO2 TOT (ppm) 1179 1108 b.d.l. b.d.l. b.d.l. b.d.l. n.d. 528 1451 b.d.l. 528 363 975 205 1662 b.d.l. b.d.l. b.d.l.Fo (mol%)c 90.1 90.1 86.6 86.9 – 85.9 – 86.6 85.4 86.6 86.6 87.7 87.7 87.4 86.7 87.5 87.5 –

KDd 0.276 0.274 0.270 0.272 – 0.283 – 0.287 0.289 0.287 0.288 0.278 0.281 0.279 0.296 0.293 0.292 –

P(MPa)e 223.0 216.1 19.8 32.8 – 17.6 – 100.9 174.4 28.1 95.5 61.0 154.0 48.4 194.3 10.9 11.1 –

H2Ogas (wt.%)e 5.35 3.68 8.52 4.60 – 82.20 – 10.09 3.26 78.81 12.79 3.77 5.50 15.74 7.93 94.21 94.51 –

CO2gas (wt.%)e 94.65 96.32 91.48 95.40 – 17.80 – 89.91 96.74 21.19 87.21 96.23 94.50 84.26 92.07 5.79 5.49 –

Refer to (Papale et al., 2006) for details on calculations of entrapment pressures (P) and water and carbon dioxide in gas phases. b.d.l., below detection limit, set to 0 in the diagrams. n.d. = not determined. All FT-IR spectra are available onthe request to the corresponding authors.

a EMP analytical procedure and standards are in Section 4. Major elements reported on a volatile-free basis.b For underlined Cl and S values, errors are b20%, for Fb30% (this must be considered as an indicative value).c From Table 2.d Exchange coefficient between olivine and melt following Toplis (2005).e Recalculated by using Papale's updated model (Papale et al., 2006).

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Table 2Whole-rock composition (PRO 7/11) and EMPa data on the glassy groundmass of the scoriae from the Solchiaro I eruption.

Sample PRO 7/11 PRO 7/8

Spot Whole-rock Vt1 ol4.1

Vt2 ol4.1

Vt3cpx2

Vt4cpx4.5

Vt5cpx4.5

Vt7 invt1

Vt8 invt1

Vt9 invt1

Vt10 invt1

Vt11 invt1

Vt12 invt1

Vt14 invt2

Vt15 invt2

Vt16 invt2

Vt1.17 Vt1.18 Vt1.19

SiO2 48.3 49.8 49.8 49.3 49.2 49.0 47.5 49.1 49.1 49.1 48.6 48.7 49.2 49.4 49.7 48.0 49.5 48.9TiO2 1.27 1.28 1.31 1.32 1.25 1.39 1.37 1.31 1.39 1.34 1.30 1.38 1.25 1.36 1.38 1.39 1.34 1.34Al2O3 16.2 17.3 17.1 17.0 18.4 16.7 16.3 16.5 16.6 16.6 16.8 17.2 17.0 16.9 16.9 16.4 17.3 16.7FeO 8.55 8.77 8.62 7.32 6.75 8.01 7.28 8.14 7.97 8.28 7.57 8.74 7.88 7.26 8.99 8.25 7.94 8.09MnO 0.14 0.28 0.20 0.12 0.12 0.14 0.14 0.17 0.13 0.10 0.15 0.12 0.14 0.16 0.12 0.19 0.06 0.04MgO 8.58 5.20 5.45 6.30 5.10 6.30 7.27 6.25 5.96 6.09 5.80 5.60 5.89 6.24 5.83 5.89 5.36 6.15CaO 12.0 11.0 11.5 13.5 14.1 13.2 14.4 11.6 11.6 11.9 11.9 11.4 12.6 13.4 10.9 11.9 12.5 12.0Na2O 2.84 3.53 3.43 2.87 2.67 2.89 2.38 3.23 3.29 3.17 2.77 3.07 3.03 2.88 2.73 3.16 3.07 3.24K2O 1.78 1.72 2.02 1.15 0.88 1.09 1.37 1.76 1.76 1.82 1.65 1.62 1.92 1.57 2.22 1.82 1.58 1.70P2O5 0.34 0.42 0.44 0.37 0.32 0.37 0.49 0.33 0.35 0.33 0.39 0.43 0.37 0.35 0.35 0.36 0.33 0.31Cl – 0.20 0.19 0.10 0.10 0.12 0.13 0.17 0.18 0.15 0.17 0.20 0.16 0.14 0.19 0.18 0.16 0.20Cr2O3 – 0.03 0.05 0.02 0.04 0.00 0.18 0.02 0.00 0.04 0.02 0.00 0.00 0.04 0.00 0.05 0.01 0.03Total 100.0 99.6 100.2 99.3 98.8 99.2 98.9 98.6 98.4 99.0 97.1 98.5 99.4 99.6 99.2 97.6 99.0 98.7

Anhydrous whole-rock from De Astis et al. (2004).a Details on the analytical procedure and standards are in Section 4.

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uncertainties up to 30%. For the olivine analyses, maximum un-certainties were: 2% for Si, Mg and Fe, 10% for Ca, near or belowdetection limit for Ni, Mn and Cr. SEM images were collected at 15 kV(accelerating voltage), 5 nA (probe current) and a working distance of11 mm.

The possible effect of the post-entrapment chemical exchangebetween MIs and host-crystal due to olivine crystallization (PEC) hasbeen calculated according to Danyushevsky et al. (2000) by using theFeOEQ2 software package (downloadable at the web site http://www.geol.utas.edu.au/userpages/leonid/Inclusions.html), the Roedder andEmslie (1970) olivine-melt equilibrium and the average FeO contentof the glassy matrix (Table 2).

5. Analytical results

5.1. Olivines: optical features, mineral chemistry and FT-IR spectra

Studied olivines are pale green in color and have at least one, andoften multiple, trapped melt inclusions (Fig. 2); these typically occurclose to the crystal core. Olivine crystals range in composition(Table 3) from Fo85 to Fo90 with a relatively high CaO content (0.20to 0.41 wt.%). Based on Roedder and Emslie (1970), Fo87 is a

Fig. 2. Typical melt inclusions from the Solchiaro I olivines under the optical microscope (a,bNote i) the multiple inclusions within chemically homogeneous host-olivine (ol); ii) the ccrystallized MIs in olivine of (a) (d); iv) the fresh glass of a low-crystallized MI containingeuhedral pyroxene and magnetite in the encircled MI in (b) (f); vi) the dendrithic chromit

composition characteristic of an olivine in equilibrium with the bulksystem (at first approximation represented by the whole-rockchemistry from De Astis et al. (2004); in Table 2). Toplis' (2005)algorithm suggests this olivine-melt equilibrium for temperature of~1250 °C. Each crystal is chemically homogeneous, without signifi-cant differences between rim and core. Also, detailed EMP–WDSanalyses show no significant variation of the mineral chemistry in thesurroundings of the inclusions (Table 3). FT-IR spectra on the studiedsamples did not show any peak in the 4000−3000 cm−1 region,indicating that the studied Solchiaro I olivines are anhydrous. FPAimages (see later Section 5.3) show that this is also true in the closevicinity of inclusions.

5.2. Melt inclusions: optical features and FT-IR spectra

Under optical microscope, MIs are brown colored, spherical toelliptical in shape; their measured sizes vary from 40 to 200 μm(Fig. 2). MIs may contain one or more bubbles of variable size; theplanar view generally points to a low vapor/melt volume ratio (Fig. 2).Sometimes, several minute bubbles mark the MI portions adjacent tothe host crystal, evenwhen the surfaces of their central part are highlyuniform and bubble-free. Polarized-light observations showed no sign

,c) and at the SEM (d,e,f,g). Optical images refer to crystallized (a,b) and glassy (c) MIs.rystallized nearly black color (a,b) vs. the glassy dark brown (c) MIs; iii) the partiallya large chromite microlite (white element) and a bubble in (e); v) the mixture of thines (g). See text for details.

Table 3EMPA data of selected olivines in the scoriae from the Solchiaro I eruption.

Sample Ol 12d Ol 12d Ol 12d Ol 9d Ol 9d Ol 9d Ol 14d Ol 14d Ol 14d Ol 15d Ol 15d Ol 15d Ol 10d Ol 10d Ol 13d Ol 13d Ol 13d Ol 11d Ol 11d Ol 11d Ol 2d Ol 2d

Spot Rim Core Near MI Rim Core Core nearMIs

Core Rim Near MI2 Core Rim Near MI Rim Core nearMIs

Near MI Rim Core Core nearMI1

Core Rim Rim nearMI1

Rim nearMI2

SiO2 41.1 40.2 41.0 42.0 41.1 40.3 41.0 41.4 41.3 41.9 41.9 39.9 41.8 41.6 40.1 39.9 39.9 41.0 42.2 41.2 40.8 41.2FeO 10.9 10.7 10.7 13.2 13.6 12.7 13.7 12.3 13.2 13.1 12.6 12.3 11.5 11.4 12.6 12.5 12.5 12.3 12.3 12.1 12.3 12.7MnO 0.16 0.19 0.07 0.19 0.38 0.25 0.25 0.20 0.20 0.31 0.20 0.19 0.20 0.22 0.17 0.19 0.16 0.24 0.17 0.25 0.19 0.14MgO 47.7 47.1 47.1 44.2 44.2 44.7 44.8 47.4 45.0 46.7 46.4 47.2 47.2 46.5 47.5 47.0 46.8 46.3 46.3 45.2 45.6 45.8CaO 0.28 0.32 0.35 0.38 0.25 0.33 0.30 0.35 0.24 0.35 0.27 0.40 0.33 0.39 0.35 0.33 0.32 0.30 0.30 0.32 0.36 0.33NiO 0.21 0.19 0.24 0.26 0.13 0.07 0.24 0.13 0.13 0.17 0.13 0.17 0.10 0.25 0.25 0.13 0.23 0.15 0.24 0.29 0.17 0.19Cr2O3 0.07 0.02 0.03 0.01 0.03 0.01 0.00 0.01 0.05 0.00 0.00 0.00 0.04 0.01 0.01 0.00 0.03 0.01 0.00 0.00 0.00 0.00Total 100.4 98.7 99.4 100.2 99.6 98.3 100.3 101.8 100.2 102.6 101.5 100.1 101.1 100.4 101.0 99.9 99.9 100.3 101.5 99.3 99.4 100.3Fo mol% 88.6 88.7 88.7 85.7 85.3 86.2 85.3 87.3 85.8 86.4 86.8 87.2 88.0 87.9 87.1 87.0 87.0 87.1 87.0 87.0 86.9 86.6

Table 3 (continued)

Sample Ol 2d Ol 2d Ol 1d Ol 1d Ol 1d Ol 1d Ol 8d Ol 8d Ol 5d Ol 5d Ol 3d Ol 4d Ol 4d Ol 4d Ol 4 Ol 1 Ol 1 Ol 2 Ol 2 Ol 3 Ol 5 Ol 5

Spot Rim Core Rim Rim1 Core nearMis

Core Core Core nearMI

Rim Core Rim Rim Rim nearMI2

Core nearMI1

2 Rim Core Core Rim

SiO2 42.0 41.6 41.0 41.8 42.4 41.7 41.7 41.8 40.8 42.0 41.1 41.0 42.0 40.5 36.8 39.1 38.3 39.5 38.2 39.2 37.8 38.9FeO 12.5 12.6 10.4 10.6 9.5 9.3 12.3 13.0 12.7 12.5 11.6 11.5 12.0 12.2 13.6 13.5 14.3 11.9 12.1 13.0 11.8 11.9MnO 0.22 0.21 0.15 0.20 0.20 0.16 0.16 0.25 0.23 0.22 0.21 0.08 0.16 0.18 0.19 0.21 0.23 0.21 0.23 0.21 0.15 0.15MgO 46.1 45.8 46.7 48.8 48.6 48.6 45.4 46.0 47.7 47.1 45.5 45.2 46.1 45.3 45.9 48.6 46.8 47.4 46.9 47.3 46.2 49.7CaO 0.35 0.29 0.31 0.30 0.32 0.30 0.32 0.36 0.28 0.34 0.29 0.33 0.28 0.31 0.41 0.29 0.36 0.30 0.30 0.28 0.20 0.26NiO 0.12 0.23 0.19 0.21 0.16 0.19 0.25 0.09 0.19 0.13 0.31 0.29 0.19 0.29 0.24 0.20 0.24 0.12 0.24 0.21 0.35 0.40Cr2O3 0.01 0.04 0.05 0.00 0.04 0.07 0.08 0.00 0.00 0.00 0.02 0.00 0.17 0.08 0.04 0.03 0.03 0.00 0.07 0.05 0.03 0.06Total 101.4 100.8 98.8 101.9 101.2 100.4 100.2 101.4 101.9 102.3 99.0 98.4 100.9 98.9 97.2 101.9 100.2 99.3 98.0 100.2 96.4 101.4Fo mol% 86.8 86.6 88.9 89.1 90.1 90.3 86.8 86.3 87.0 87.1 87.4 87.5 87.3 86.8 85.7 86.6 85.4 87.7 87.4 86.7 87.5 88.1

*Details on the analytical procedure and standards are in Section 4. Fo is the forsteritic component whose mole content, %, is given by the subscript in the main text.

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Fig. 3. Selected FT-IR spectra showing H2O (bands between 3731 and 2506 cm−1), CO32− (between 1610 and 1345 cm−1 only in a) and CO2 (molecular form, around 2350 cm−1 in b

only) peaks detected for glassy (bold black curve in a) and partially crystallized (gray curve in a, all curves in b) Solchiaro I MIs. Note the doublet for the CO32− and also molecular CO2

species. Spectra in (b) refer to the MI within olivine in Fig. 2a. See text for details.

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of crystallization in the inclusions. On the contrary, SEM investigationsrevealedmore complex textures, allowing twomainMI populations tobe distinguished: a) glassy MIs, which constitute the most abundantpopulation, and b) subordinately, partially to totally crystallized MIs.By comparing SEM images with the optical ones, we note that thecrystallized MIs are typically darker, with tonalities toward brown/black, than the glassy ones. SEM–EDS investigations on the partially tototally crystallized MIs often evidence minute pyroxene and oxidesmicrolites, usually present as tiny euhedral (Fig. 2d,e,f) or, in few cases,dendritic crystals (Fig. 2g). These oxides cannot be analyzed due totheir small size; however, EDS data on larger grains (Fig. 2e) suggestcompositions close to chromite, (Fe2+, Mg)Cr2O4. Commonly, pyrox-ene micro-crystals are accompanied by small bubbles, which fre-quently dip into glassy portions. Extensive micro-crystallization(N70%, by area coverage) characterizes only few MIs (Fig. 2f,g),which were not analyzed; most of the MIs belonging to the partiallycrystallized population are affected by crystal nucleation, which isgenerally estimated at b10% (hereafter low-crystallized MIs), andsometimes reach 40–50% (hereafter highly-crystallized MIs), by areacoverage. Noteworthy, all the studied MIs have sharp crystal/glassinterface and no cracks prior to sample preparation (Fig. 2). Moreover,the glass portion is lucid and homogeneous (Fig. 2d,e), particularly inthe glassy MIs.

The collected FT-IR spectra show a broad asymmetric absorptionextending from ~3600 to 2600 cm−1 (Fig. 3), which results from thedifferent O–H stretching modes of hydroxyls and molecular water.This absorption band has been successfully used since (Stolper, 1982),to quantify the totalwater content of the glass. Several spectra (Fig. 3)show the typical doublet of CO3

2− groups dissolved in the glass (Dixon

Fig. 4. Reflected light image (a) and related FPA maps showing H2O (b; bands between 3731melt inclusions in Solchiaro I olivines. The color scale indicates the relative peak intensity, fcurve).

and Pan, 1995). Then, few glasses of the highly-crystallized MIs areinterestingly characterized by the presence of molecular CO2 (notethe doublet peaks at 2342 and 2361 cm−1 in Fig. 3b) (e.g., King et al.,2002). All these measurements were done in a sample-chamberconstantly evacuated with a flux of argon and checked on diversepoints of a same inclusion. Therefore, the above feature cannot berelated to pollution from atmospheric carbon dioxide and is related toenclatrated or even structurally bound molecules of CO2. A single andsharp IR antisymmetric stretching band at ~2350 cm−1 can beascribed to CO2 dissolved in glasses (e.g., Stolper and Ahrens, 1987;King et al., 2002; Mandeville et al., 2002) or present in minerals (e.g.,Della Ventura et al., 2007, 2009; Bellatreccia et al., 2009), and thedoublet peak to gaseous CO2 (e.g., Herzbeg, 1956; Stolper and Ahrens,1987). Therefore, these features in Fig. 3 agree with the presence ofCO2-rich gaseous nano-inclusions within the MIs.

In the range 3600 to 2600 cm−1, backgrounds were generally flat(Fig. 3a,b), allowing the use of a linear baseline to accurately measurepeak intensity. Calculated H2O contents range from 0.80 to 1.73 wt.%in the glassyMIs, and reach 2.69 wt.% in the low-to-highly crystallizedones, which also show CO2 (molecular) up to 890 ppm (Table 1). Thechoice of a baseline in the 1200–2000 cm−1 region is not sostraightforward (Fig. 3a,b). In this range, some authors use curvedpolynomials (French curves) or subtract the spectrum of a carbonate-free counterpart as a reference (see King et al., 2002).We prefer not touse this procedure in the absence of reference spectra for potassium-rich carbonate-free trachy-basaltic glasses or for compositionallysimilar MIs in rocks from other Italian volcanoes. Therefore, we used alinear baseline definition as depicted in Fig. 3. Corresponding CO3

2−

contents vary up to 2653 ppm in glassyMIs and up to 1293 ppm in the

and 2506 cm−1) and CO32− (c; bands between 1610 and 1345 cm−1) content of glassy

rom low (blue) to high (violet). A related FT-IR spectrum is given in Fig. 3a (bold black

Fig. 5. Reflected light image (a) and FPA maps showing H2O (b; bands between 3731 and 2506 cm−1) and CO32− (c; bands between 1610 and 1345 cm−1) content of partially

crystallized melt inclusions in Solchiaro I olivines. The color scale indicates the relative peak intensity from low (blue) to high (violet). A related FT-IR spectrum is given in Fig. 3a(gray curve).

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low-to-highly crystallized ones (Table 1). Notably, the carbonabsolute concentrations obtained using the above method are inexcellent agreement with measurements done on selected inclusionsby ion microprobe (Piochi et al., 2011).

5.3. H and C distributions in melt inclusions: FPA images

FPA images (Figs. 4, 5 and 6) show the distribution of the C–H–Ospecies in the MIs and surrounding olivines; the images wereobtained by integrating, across the scanned area, the absorbancerelated to the specific molecular arrangements. Hydrous and carbon-bearing glassy and low-crystallized MIs show similar features, i.e.,they consist of a major, inner H2O- and CO3

2−-richer portionsurrounded by a thin, inhomogeneous, anhydrous and decarbonatedrim (Figs. 4 and 5). In some cases, this lack of homogeneity may be anartifact of the MI bowed geometry at the edge. This is evident, for

Fig. 6. Partially crystallized melt inclusions in Solchiaro I olivine (a). The reflected light ima(d; bands at around 2350 cm−1) and H2O (e; bands between 3731 and 2506 cm−1) content.images evidence the micro (f) and nano (g) crystallization, together with the vesiculated g

example, in Fig. 4, which displays an homogeneous distribution ofwater and carbon dioxide (1.54 wt.% and 1179 ppm, Table 1) acrossthe entire glassy inclusion, while the decreasing contents of bothspecies (Fig. 4a,b) toward the rim coincides with the fuzzy zone underreflected light (Fig. 4a). It is worth to note that bubbles as small as5–10 μm, such as the one in the upper side of the MI imaged inFig. 4, are visible thanks to a distortion of the FPA images. The watercontent in this bubble is significantly lower than that of thesurrounding MI matrix.

FPA images of a sample containing only molecular CO2 (Fig. 6)reveal that the MI core is water- and CO2-free. Although the samplebroke-up during the transfer from the FT-IR to the SEM sample holder,detailed SEM data collected across the core-rim boundary of asurvived fragment allow understanding the intricate zoning shownin Fig. 6d and e. The inclusion is partially crystallized due to μm-sizedpyroxene and Cr-rich oxide crystallites, as already inferred on the

ge of the encircled (in a) MI (b), related FT-IR spectra (c) and FPA maps showing CO2

The color scale indicates the relative peak intensity from low (blue) to high (violet). SEMlass. See text for details.

Fig. 7. TAS classification diagram (Le Bas et al., 1986) (a) for analyzed MIs in Solchiaro Iolivines, whole-rock (data from de Astis et al., 2004) and glassy matrix; b) for feldspar-,pyroxene- and olivine-hostedMIs of Fondo Riccio andMinopoli 1 samples (Campi Flegrei;data from Cannatelli et al., 2007, Mangiacapra et al., 2008, Arienzo et al., 2010) and least-evolved PVD whole-rocks (data from De Astis et al., 2004; Piochi et al., 2004). In(c) compositions ofMIs recalculated for the PEC effect; see Sections 4.2 and 5.4 for details.

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basis of its color under the optical microscope (Fig. 6a). The FPAimages suggest that the core is highly to totally crystallized and, thus,volatile-free. However, high magnification SEM pictures (Fig. 6g)show that the apparently homogeneous glass also contains severalnanometer-sized crystals (possibly clino-pyroxene) and bubbles. Theresolution of the FPA image (~3–5 μm in the H2O stretching region)cannot resolve the glass interspersed with the crystals. Therefore,although an averaged distribution of volatiles across the sample isobserved, some red pixels in Fig. 6d document the presence ofmolecular CO2 local enrichments, corresponding to proto-bubbles

Fig. 8. Selected oxides in MIs vs. Fo (mol%) of the hosting olivine. The thin line connects MIs wCannatelli et al., 2007; Mangiacapra et al., 2008; Arienzo et al., 2010). The bold dashed bar inand maximum shift due to the PEC effect (see Sections 4.2 and 5.4 for details); where not i

(of size nearly comparable to the FPA image resolution) where thecarbon dioxide gas is concentrated (see also Section 5.2).

Finally, all the FPA maps (Figs. 4, 5, 6) corroborate the sharptransition between the host-crystal and the MI, as observed at theSEM.

5.4. MIs chemistry by EMPA

The data (Table 1) show SiO2 in the range 45.7–51.32 wt.% and aK-alkaline affinity (Fig. 7a,c) and highlight the coexistence of chem-ically different MIs within the same, homogenous, olivine (Fig. 8).GlassyMIs show a chemical range (basalt to trachy-basalt, with someshoshonitic and tephrite basanitic terms)more restricted than that ofthe low-to-highly crystallized MIs, which show scattered composi-tions extending toward more evolved phono-tephrite terms. Thewhole MIs compositional range partially overlaps the one defined bythe entire sequence of Solchiaro whole-rocks (Fig. 7a vs. 1e). Inparticular, the trachy-basaltic MIs are equivalent to the Solchiaro Iwhole-rock and glass-matrix (Table 2) compositions. Besides, themost evolved compositions of our MIs are comparable to thoseanalyzed in the olivines (less forsteritic than the Procida ones) fromFondo Riccio and Minopoli I volcanics (Fig. 7b) of the Campi Flegreicaldera (Cannatelli et al., 2007; Mangiacapra et al., 2008). The MgOcontent ranges between 4.20 and 6.45 wt.%, with three data pointsbetween 0.52 and 1. 7 wt.% (Fig. 8); these latter values suggest nochemical-equilibrium of MIs with olivines having Fo contents largerthan 87 mol%. Table 1 reports the partition coefficient (KD) of eacholivine-melt pair calculated on the basis of the Toplis (2005)algorithm for a temperature of 1160 °C (same temperature as forvolatile saturation modeling, in line with Cannatelli et al., 2007, seelater). The resulting KD is equal to 0.281±0.015, falling in the rangeof igneous rocks reported in literature (Ford et al., 1983; Toplis,2005); KD moderately decreases with pressure (not shown), asobserved by Toplis (2005). The PEC shift tends obviously to overridethe melt-crystal equilibrium, but its effect is to magnify thedifferences in Mg, Na, and Si abundances between MIs and glass-

ithin the same crystal. CF MIs are in FoN85 mol% olivines from Campi Flegrei (data fromeach diagram corresponds to the maximum error, and the arrow indicates the directionndicated the error is within the symbol width.

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matrix. On the other hand, PEC shift discounts the result of crystalnucleation and microlite formation, and, more importantly, does notexplain the variability within a single olivine grain. Furthermore,apart for the recalculated MgO and FeO shift, the PEC effect producessmall changes of the olivine Fo content and does not modify the MIclassification on the TAS (Fig. 7c). Finally, the PEC effect is not reallyrelevant also because encompassed by EMPA error bars. Therefore,later on we shall use raw analytical data. Fig. 8 reports the measuredvariations of some major elements in MIs and major oxide ratios vs.the Fo content (mol%) of the host-olivines; the arrow is reported forthe sake of completeness and indicates the chemical shift induced byPEC effect. SiO2 (ranging from 45.37 to 51.32 wt.%), CaO/Al2O3

(ranging from 0.64 to 0.83, plus three points at ~1.7 also having thelowest MgO content) and, at least extent, K2O (ranging from 1.65 to5.36 wt.%) show a rough decrease with decreasing Fo% values, thatcannot be explained by data scattering and PEC shift; on the contrary,Al2O3 (14.89–20.21 wt.%, not shown) and FeO (5.8–12.61 wt.%, notshown) contents increase with decreasing Fo% values. FeO isnegatively correlated to SiO2 (Fig. 9a). Some MIs are characterizedby correlations betweenMgO and Na2O vs. CaO/Al2O3 (Fig. 9b,c). TheSolchiaro I whole-rock scoriae show only lower FeO and higher MgOcontent than MIs. Olivine-hosted MIs from the nearby Campi Flegrei(Cannatelli et al., 2007;Mangiacapra et al., 2008; Arienzo et al., 2010)partially overlap the chemical trend defined by our MIs, but theyshow significantly scattered values (Fig. 9). The chlorine contentsrange between 0.1 and 0.45 wt.% ca., except for one partiallycrystallized MI, which records a value of Cl ~0.9 wt.% (Fig. 9d).These are among the lowermost values actually measured for thePVD products (see data in Cannatelli et al., 2007; Piochi et al., 2008;Arienzo et al., 2010). Solchiaro I MIs and glass-matrix define apositive Cl vs. K2O trend (Fig. 9d), similarly to what described in theliterature also for the Campanian volcanoes (Marianelli et al., 1995,2005;Webster et al., 2001). SO3 is constant, being its variationmostlywithin the analytical error.

Fig. 9. FeO (a) and K2O (b) vs. CaO/Al2O3, CaO/Al2O3 vs. MgO (c) Cl vs. K2O in MIs from thearrowed lines show the effect of olivine and pyroxene crystallization. In (c), black dots ocomposition. The bold dashed bar corresponds to the maximum error, and the arrow indicdetails); where not indicated the errors are within the symbol width. Literature data for CF M(2008) and Arienzo et al. (2010).

As a whole, major oxide data and related trends, together withSEM images of the microlite abundance, indicate that a post-entrapment crystallization ≥50% (by area coverage) is necessary toinduce significant compositional change of trapped-melt. Widespreadcrystal nucleation during cooling generated variable abundances ofmicrolites within the MIs. Therefore, even the most micro-crystallized(up to 40–50%) MIs can be safely used for data interpretation togetherwith glassy ones. The variation of major oxides suggests thatcrystallization occurred, but it was not the primary differentiationmechanism of the Solchiaro I magma. The isotopic equilibrium amongphases (see Section 3) excludes mingling between chemical andisotopically different magma batches. Additional phenomena canexplain the features detected in the Solchiaro I MI as discussed below.

6. Discussion

6.1. Significance of MIs chemical variability

On the basis of the presented data, PEC diffusion between MI andthe host mineral can be discarded, in line with Danyushevsky et al.(2002). SEM and FPA images also corroborate the lack of anappreciable PEC shift, because of the sharp contacts between the MIand the surrounding olivine. As a matter of fact, the compositionalvariations of major and volatile elements of the Solchiaro I MIs arequite independent of the host crystal chemistry, as evidenced byappreciable chemical differences recorded by MIs within a singlehomogeneous crystal grain (symbols connected by thin lines in Figs. 8and 9). Furthermore, MI re-equilibration during magma ascent(Danyushevsky et al., 2000) is inadequate to explain the FeOenrichment and the MgO constancy of MIs with respect to the olivineFo content. Moreover, if we consider the whole rock and the leastevolved glassy MI composition (sample 14 d in Table 1), a variation ofthe Fo proportion (up to 10 mol%) in olivine can be obtained forrelatively small temperature changes (≤50 °C), as suggested by tests

Solchiaro I olivine phenocrysts. In b) and c), in line with Marianelli et al. (1995, 2005),n the thin lines indicate the fractionation of 10% starting from the least evolved MIates the direction and maximum shift due the PEC effect (see Sections 4.2 and 5.4 forIs and glass matrix from Cannatelli et al. (2007); Mangiacapra et al. (2008); Piochi et al.

Fig. 10. Variation of CO2 and H2O dissolved in the MIs glass trapped in the olivines fromSolchiaro I scoriae. Isobars and the isopleths showing the pressure-related CO2–H2Oevolution are re-calculated for the average composition of our analyzed MIs andassuming FeO/Fe2O3=2 (see text for details) and temperature of 1160 °C (as derivedby homogenization data from Cannatelli et al., 2007), using Papale et al. (2006) model.Paths 1, 2, 3, and 4 represent plausibly closed-system degassing conditions for the leastevolved magmas erupted at the Campi Flegrei caldera. In particular, Paths 1, 2 and 3 aregas-buffered systems, coincident with gas isopleths for CO2

gas/H2Ogas ratios of about 4,8 and 20 (molar), respectively. Path 4 is a closed-system degassing trend characterizedby total H2O and CO2 contents of 2 and 5 wt.%, respectively; Path 4bis is thecorresponding open-system degassing pattern (for Pb200 MPa). Arrows indicateisobaric processes of crystallization (white arrow) or CO2 fluxing (gray-filled arrows).

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of equilibrium conditions via MELTS software (Ghiorso and Sack,1995; Asimow and Ghiorso, 1998).

Therefore, the measured chemical variations (Figs. 7a, 9), togetherwith isotopic data (see Section 3), suggest that the Solchiaro I magmamostly evolved under closed-system conditions, i.e. characterized by alimited crystallization (~10% in Fig. 9c). The magma evolution yieldedcompositions that include some of the most evolved ones of PVD(Figs. 1, 7b). However, olivine crystallization explains the abovevariations only to some extent. In agreement with mineralogicalobservation, it is necessary to invoke the crystallization of clinopyr-oxene and opaque minerals to account for i) the general FeO decreasegoing toward the SiO2-richer glass-matrix compositions (Fig. 9a) and,ii) the correlations between CaO/Al2O3, MgO, K2O and Cl (Fig. 9b,c,d).These features imply some analogy with the nearby Vesuvius case(see also Marianelli et al., 1995, 2005). However, the possible pre-cipitation of biotite (recognized in some PVD rocks; De Astis et al.,2004), together with fluid transfer and self-mingling phenomena (asreported in the later Section 6.3), may explain why variations of CaO/Al2O3 (0.6–0.8) and K2O (2–5 wt.%) measured in our samples aresmaller and larger, respectively, than those measured at Vesuvius(0.4–1.1. for CaO/Al2O3, and 4.5–6.5 wt.% for K2O (Marianelli et al.,1995, 2005)). In particular, the K variability displayed by Procida MIsand the observed K2O–Cl correlation (Fig. 9d) can be considered anindicator of alkali transfer through a fluid phase, as suggested at theEtna volcano (Ferlito and Lanzafame, 2010). Besides, the drop of CaO/Al2O3 in the MIs of Fo-poor (b87 mol%) olivines and, particularly, onlyin some of the crystallized ones (Fig. 8c, with particular regard to theoutliers), is consistent with additional post-entrapment diffusenucleation and nano-to micro scale crystallization of the melt as aclosed-system within the armoring olivine. MELTS-based runs(Ghiorso and Sack, 1995; Asimow and Ghiorso, 1998) at fO2 fixedalong the Ni–NiO buffer, substantiate the hypothesis that equilibriumof clinopyroxene and oxide with a liquid compositionally similar toour MIs occurs for T≥1100 °C. Notably, this latter value is consistentwith homogenization temperatures reported by Cannatelli et al.(2007) for Campi Flegrei shoshonites, which are compositionallysimilar to our products. Temperatures larger than 1160 °C increase theKD values (Table 1), further suggesting the absence of the PEC effect.

6.2. Polybaric processes in the Solchiaro I magma

Magma entrapment pressures (Table 1) have been calculated forany specific MI composition according to the model of Papale et al.(2006) (Fig. 10). Because whole-rock data show that Mg-rich (N6 wt.%) Procida magmas (including Solchiaro) are more reduced (FeO/Fe2O3~3) (D'Antonio et al., 1999) than Mg-depleted ones (FeO/Fe2O3~1) (De Astis et al., 2004), amean FeO/Fe2O3 value of 2was usedin drawing the saturation surface for the trachy-basaltic compositionthat was obtained by averaging our MI analyses in Table 1. Thetemperature was set at 1160 °C, i.e., the homogenization temperaturemeasured by Cannatelli et al. (2007), lacking ad hocmeasurements onSolchiaro I olivines. Based on these parameters, possible degassingtrends are plotted for the trachy-basaltic composition, together withrepresentative isopleths (Fig. 10).

Entrapment pressures range from b50 MPa to ~350 MPa. We notethat: a) the glassy MIs contain ~1 wt.% of dissolved H2O in the 350 to100 MPa range; b) the crystallized MIs mostly lie between ~50 and1 MPa (CO2 was below the detection limit); and c) four of MIs recordpressures between ~150 and ~225 MPa (Fig. 10). Dissolved volatilesin MIs define closed-system degassing trends featured by a CO2

dominated gas phase (CO2gas/H2Ogas ranging from 4 to ≥20 M) (see

also Table 1), except one H2O-rich MI at ~180 MPa. Going into details,H2O–CO2 pairs are embraced by the gas-buffered degassing trendswith CO2

gas/H2Ogas between 4 (Path 1) and 20 (Path 3). The gasisopleth for CO2

gas/H2Ogas=8 (Path 2) goes through most of thepoints, including the one at ~350 MPa. These gas-buffered patterns

are closed-system degassing trends inwhich the gas fraction is so highthat the whole system becomes insensitive to CO2 and H2O chemicalexchange between gas and melt. The right-side bound of all theH2O–CO2 but one pairs can be modeled as a closed system-degassingsystem featured by a total (TOT: exsolved+dissolved) watercontent, H2OTOT, around 2 wt.%, which is close to the H2O concen-tration measured in the ~350 MPamelt inclusion. Along this pattern,the total carbon dioxide content, CO2

TOT, was set at 5 wt.%. This highvalue allows the degassing system to avoid a rapid depression of CO2

contents in the melt for pressure b150 MPa, then approaching thegas-buffered conditions of Path 1. Data points plotting along theX-axis (CO2 below the detection limit) require the occurrence ofdegassing under open-system conditions, i.e. the separation of thegas phase starting from one, or more, of the above patterns (e.g.Path 4bis represents an open-system degassing trend originatingfrom the closed-system degassing trend 4 at P~200 MPa). Fewdata points spread out the analytical error and may indicateisobaric enrichment of CO2 and consequent melt dehydration(gray arrows in Fig. 10). This possibly requires an additional styleof open-system degassing, related to the upstream of CO2-rich gas,formed at larger depth, hence making CO2-fluxing responsible ofgas-buffered patterns and strong CO2

gas/H2Ogas enrichments (e.g.,Spilliaert et al., 2006; Mangiacapra et al., 2008; Vigoroux et al.,2008; Barsanti et al., 2009; Collins et al., 2009; Arienzo et al.,2010). However, we have to hypothesize that some magma por-tions were isolated by the arrival of the deep CO2 gas and not affectedby fluxing. In fact, one of the crystallized MI is particularly H2O-enriched (2.7 wt.%, at ~170 MPa, Fig. 10), as a result of the joint effectof gas escaping in open-system and closed-system crystallization(~50 wt.%), leading to enrichment in the H2O-component, muchmore soluble than CO2.

The molecular CO2 occurs in two of the four MIs crystallized atpressures between ~150 and ~225 MPa. Based on FPA images (Fig. 6)and inferred isobaric processes (i.e., fluxing; Fig. 10), this presence

78 A. Mormone et al. / Chemical Geology 287 (2011) 66–80

may results either i) by micro-vesiculation induced by internal MIcrystallization or ii) by abundance – and possibly fluxing – of carbondioxide in the melt that provokes the reduction of water solubility, and,consequently, dehydration-induced crystallization (e.g., Holloway,1976; De Natale et al., 2004; Quareni et al., 2007; Blundy and Cashman,2008; Blundy et al., 2010). Although our data are not really conclusive,we consider molecular CO2 in the melts as an intermediate speciesforming during dissolution of the gas phase; full equilibrium conversionto CO3

2− was not achieved because prevented by the fast crystallizationof the trapped melt.

6.3. Implications on Solchiaro I feeding eruption system and PVD magmapetrology

The variability detected in the Solchiaro I olivine-MI pairs is theresult of self-physicalmingling (mixing sensu latu) process that affecteda chemically (and isotopically (De Astis et al., 2004)) homogeneous,poorly evolved magma batch that upraised through a narrow conduitand stalled at shallow levels shortly before eruption. The following dataand considerations support self-mingling processes: 1) studied samplesare from the same single eruptive layer; 2) O-isotopic equilibrium wasestablished between olivines and the glassy matrix (De Astis personalcommunication); 3) no relationship occurs between olivine chemistryandMI-entrapment pressure (Fig. 11); 4) MIs of different compositionsare observed in the homogenous olivines crystallized at same depth

Fig. 11. Variation of olivine and MI composition as function of crystallization depth in the SoArienzo et al., 2010) products. Thin lines connect MIs hosted within the same crystal; dashedcouple values. The bold dashed bar corresponds to the maximum error. See text for details

(Fig. 11); and 5) little compositional differences exist between themostevolved MI and glass-matrix (Fig. 9). Magma degassing (Fig. 10) wasthen themain process featuring the ascent of the Solchiaro Imagma. Onthe other hand, crystallization was a minor process, also likelyconsequent to local CO2-fluxing and melt dehydration (e.g., Blundyand Cashman, 2008). The evidence that an upstreaming (CO2-dominated and halogen-rich) gas phase partially contributed tomagma differentiation is supported by the detected K-variability ofMIs and its correlation with chlorine (Fig. 9). Furthermore, i) thepresence of MIs trapped at b50 MPa (Fig. 10), ii) the late-formation offeldspars in scoriae and iii) the occurrence of spherulitic microlitesindicate that a major step in chemical differentiation occurred after theSolchiaro I magma reached shallow depth. Therefore, the chemicalsimilarity among MIs, glass-matrix and whole-rock compositions, aswell the relatively limited temperature changes (≤50 °C, modeled byMELTS software, see Section 6.1) suggest a relatively fast magmaascent for P≤350 MPa. By adopting Stocke's law, the settling velocity of1–1.5 mm-sized (at first approximation spherical) olivine grains(density=3.4 g/cm3) within a melt column compositionally similarto our MIs (density=2.8 g/cm3; temperature=1160 °C; viscosity~99 Pa·s, from the equation of Giordano et al., (2008)) can be used toestimate a minimum ascent velocity of 1–2 m/day.

Finally, the scattered (0.094–0.42 wt.%; Table 1) SO3 distribution(Fig. 11d), together with the sulfide absence (at themicroscopic view)and the variation of the FeO/Fe2O3 ratio in whole rocks (D'Antonioet al., 1999; DeAstis et al., 2004) suggest that variable redox conditions

lchiaro I and Campi Flegrei (data from Cannatelli et al., 2007; Mangiacapra et al., 2008;lines envelop Campi Flegrei MIs for which it was not possible to recover the exact data

.

79A. Mormone et al. / Chemical Geology 287 (2011) 66–80

were associated to degassing and differentiation, in terms of anincreasing oxidation state duringmagma ascent. Notably, variations ofredox conditions were recognized to affect the Fo of a crystallizingolivine (e.g., Matzen et al., 2011).

Solchiaro I olivine-hosted MIs show compositional relationshipwith MIs in olivines from younger (≤10 ka) (Mangiacapra et al.,2008) Campi Flegrei caldera products (Figs. 2 and 7), supporting theidea that Solchiaro I magma is an adequate parental end-member forthe PVD. Themost evolved latitic and trachyticMIs compositionswererecovered in late- and shallow- (b200 MPa) crystallized pyroxenesfrom the Agnano Monte Spina and Fondo Riccio eruptions within theCampi Flegrei caldera (Cannatelli et al., 2007; Mangiacapra et al.,2008; Arienzo et al., 2010). Interestingly, at ~200 MPa (≈8 Km),pyroxene-hosted trachyticmelts from Campi Flegrei caldera are richerin both Cl and H2O (Fig. 11) than those from Procida. Admitting thesame deep volatile source for PVD magmatism, magma ponding anddifferentiation at depth ≤8 km beneath PVD may be invoked for theproduction of the more hydrous (white arrow in Fig. 10) andexplosive magmas erupted at the Campi Flegrei caldera. The observedCl increment is consistent with this idea, and documents thecoexistence of magmas with exsolved aqueous and Cl-rich fluids inthe shallow reservoir(s), in line with the findings of Carroll (2005).Alternatively, the high CO2/H2O ratio characterizing Procida magmascould reflect source effects due to 1) differential H2O–CO2 supply,from the devolatilizing slab upward, or 2) the contribution of anupwelling, carbonate-contaminated asthenospheric mantle to thesource of Italian magmas (e.g., Frezzotti et al., 2009). Noteworthy, theinvolvement of an asthenospheric MORB-type mantle was suggestedto be much larger at Procida than at the Campi Flegrei caldera (Piochiet al., 2004, 2005), where, instead, the signature inherited by slab-derived fluids and/or crustal contamination is stronger (e.g. Tonariniet al., 2004; Piochi et al., 2005; D'Antonio et al., 2007).

7. Conclusions

H2O and CO2 contents from the Solchiaro I MIs indicate that meltentrapment (and olivine crystallization) occurs over a wide range ofpressures, during magma ascent. The highest detected pressures of~350 MPa (~13–14 km of depth, for crustal rocks with density of2700 kg/m3) in our samples reveal that a volatile-saturated magmasource provided the parental melts to the PVD volcanism. Thesepressures, and related depths are among the highest so far discoveredwithin PVD and are comparable to those underneath the Vesuvius(Marianelli et al., 1999) and Stromboli (Metrich et al., 2001; Bertagniniet al., 2003) volcanoes. Starting from the depth of 13–14 km upward,H2O and CO2 variations record the trapping of variably degassedmeltsduring magma ascent.

Moreover, the CO2gas/H2Ogas ratios up to 20 M revealed by data

modeling, are the highest found in melt inclusions from Campanianvolcanic rocks. The unexpected occurrence of CO2 dissolved in meltsin its molecular form is argued to be indicative of CO2 upstreamthrough or from magmas. Therefore, the whole data set from thispaper point to the existence of a deep, CO2 rich, continuouslydegassing magma source beneath the PVD.

According to the most recent literature (e.g., De Astis et al., 2004),Solchiaro I scoriae are the most primitive and least contaminatedmagma compositions erupted in the PVD. Crystallization within thecrust occurred only to a limited extent, and was not the main processdriving magma differentiation. The potassium variability is likely toreflect alkali transport in the CO2-rich fluid phase up-streamingthrough the magmatic system. Therefore, the low differentiationdegree of the Solchiaro I magma, its gas-rich conditions during ascent(or even gas-dominated because of CO2-fluxing), and the highlysimilar compositions of MIs and glassy groundmass, possibly suggestthat the Solchiaro I-type trachy-basaltic magma batches rapidlyascended from a large reservoir sited beneath the PVD at a depth

larger than that detected by seismic tomography (i.e., ~8 km) (Zolloet al., 2008). This magma reservoir could have been the supply of CO2

for the magma batches progressively formed during time at shallowdepths in the PVD. We also suggest that such a deep reservoir is theultimate feeder of the present hydrothermal discharges (e.g., Chiodiniet al., 2001; Caliro et al., 2007) at the PVD surface.

Acknowledgments

This work was supported by INGV-DPC 2005–2006 and by PRIN2007M4K94A projects. We are grateful to P. Scarlato (INGV) andL. Freda (INGV) for allowing access to the analytical facilities in Roma,Italy. The paper received greatly benefited in both its form andcontents from the constructive criticism of N. Metrich (IPGP) and ananonymous referee.

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