Scholarly article on topic 'Mineralogical, geochemical and isotopic features of tuffs from the CFDDP drill hole: Hydrothermal activity in the eastern side of the Campi Flegrei volcano (southern Italy)'

Mineralogical, geochemical and isotopic features of tuffs from the CFDDP drill hole: Hydrothermal activity in the eastern side of the Campi Flegrei volcano (southern Italy) Academic research paper on "Earth and related environmental sciences"

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Abstract of research paper on Earth and related environmental sciences, author of scientific article — A. Mormone, C. Troise, M. Piochi, G. Balassone, M. Joachimski, et al.

Abstract A 506m drill-hole has been recently drilled in the framework of the Campi Flegrei Deep Drilling Project (CFDDP) and the International Continental Scientific Drilling Program (ICDP) with the intention of coring the subsurface in the eastern sector of the Campi Flegrei caldera. The borehole, located in the western district of the Neapolitan city (Bagnoli Plain) 3km to the east of the most active volcanic area and about 5m above sea level, is now targeted for monitoring purposes. This paper reports the results obtained from the analysis of two short cores collected at depths of −443 and −506m below the ground level. The cores sampled two pre-caldera tuffs. Observations performed by optical and scanning electron microscopy, energy dispersive spectroscopy and powder X-ray diffraction were used to achieve data on the primary lithology, both primary and secondary mineralogical assemblages, and the relationship between texture and secondary mineralization. Sr isotope ratios were determined on selected primary feldspars, whereas δ13C and δ18O analyses were performed on carbonates from veins and filled-voids in tuffs. Our results provide information on the hydrothermal system in the eastern sector of the caldera that was not among the goals in the previous drilling programs. Secondary mineralization suggests a saline hydrothermal environment characterized by fluids that progressively evolved from boiling toward more alkaline and cooler conditions. A paleo-temperature of ca. 160°C has been inferred from authigenic mineral occurrences and calculated on the basis of equilibria between cored calcites and fluids presently emitted at the surface, by using carbon and oxygen isotope data. The temperature measured at the bottom of the drilling is about 80°C.

Academic research paper on topic "Mineralogical, geochemical and isotopic features of tuffs from the CFDDP drill hole: Hydrothermal activity in the eastern side of the Campi Flegrei volcano (southern Italy)"

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Journal of Volcanology and Geothermal Research

journal homepage: www.elsevier.com/locate/jvolgeores

VOLCANOLOGY

Review

Mineralogical, geochemical and isotopic features of tuffs from the CFDDP drill hole: Hydrothermal activity in the eastern side of the Campi Flegrei volcano (southern Italy)

CrossMark

A. Mormone a* C. Troise a, M. Piochia, G. Balassone b, M. Joachimskic, G. De Natale '

a Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Vesuviano, Naples, Italy b Dipartimento di Scienze della Terra, dell'Ambiente e delle Risorse, Universita Federico II, Naples, Italy c GeoZentrum Nordbayern, University ofErlangen-Nuremberg, Erlangen, Germany

ARTICLE INFO

Article history: Received 8 September 2014 Accepted 1 December 2014 Available online 16 December 2014

Keywords: Campi Flegrei Pilot-hole Geothermal field Hydrothermal activity Core samples

ABSTRACT

A 506 m drill-hole has been recently drilled in the framework of the Campi Flegrei Deep Drilling Project (CFDDP) and the International Continental Scientific Drilling Program (ICDP) with the intention of coring the subsurface in the eastern sector of the Campi Flegrei caldera. The borehole, located in the western district of the Neapolitan city (Bagnoli Plain) 3 km to the east of the most active volcanic area and about 5 m above sea level, is now targeted for monitoring purposes.

This paper reports the results obtained from the analysis of two short cores collected at depths of — 443 and - 506 m below the ground level. The cores sampled two pre-caldera tuffs. Observations performed by optical and scanning electron microscopy, energy dispersive spectroscopy and powder X-ray diffraction were used to achieve data on the primary lithology, both primary and secondary mineralogical assemblages, and the relationship between texture and secondary mineralization. Sr isotope ratios were determined on selected primary feldspars, whereas 813C and 618O analyses were performed on carbonates from veins and filled-voids in tuffs. Our results provide information on the hydrothermal system in the eastern sector of the caldera that was not among the goals in the previous drilling programs. Secondary mineralization suggests a saline hydrothermal environment characterized by fluids that progressively evolved from boiling toward more alkaline and cooler conditions. A paleo-temperature of ca. 160 °C has been inferred from authigenic mineral occurrences and calculated on the basis of equilibria between cored calcites and fluids presently emitted at the surface, by using carbon and oxygen isotope data. The temperature measured at the bottom of the drilling is about 80 °C.

© 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/3.0/).

Contents

1. Introduction...............................................................40

2. Geological background..........................................................40

2.1. The volcanological and geothermal setting..............................................40

2.2. The CFDDP drilling and studied samples...............................................40

3. Results.................................................................41

3.1. Texturaldata...........................................................41

3.2. X-ray diffraction data........................................................42

3.3. Mineral chemistry.........................................................42

3.4. Carbon, oxygen and strontium isotope geochemistry (Table 4).....................................43

4. Discussion................................................................43

5. Conclusions...............................................................48

Acknowledgments...............................................................48

Appendix 1. Analytical methods.......................................................49

References..................................................................51

* Corresponding author. Tel.: +39 081 6108528. E-mail address: angela.mormone@ov.ingv.it (A. Mormone).

http://dx.doi.org/m1016/j.jvolgeores.2014.12.003

0377-0273/© 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).

1. Introduction

Campi Flegrei is an active volcano partially submerged in the Gulf of Pozzuoli (Fig. 1) (Rosi and Sbrana, 1987; Barberi et al., 1991; Piochi et al., 2014 for a review). It hosts a hot and saline geothermal system characterized by a geothermal gradient between 100 and 170 °C km-1 (Guglielminetti, 1986; AGIP, 1987; Rosi and Sbrana, 1987) as well as ground deformation and low-magnitude seismicity dynamics, which are continuously monitored (De Natale et al., 1991; Troise et al., 2008; D'Auria et al., 2011; Troiano et al., 2011). At the surface, fumaroles and CO2-rich gas emissions produce a large area of solfataric phenomena (Rosi and Sbrana, 1987; Chiodini et al., 2001, 2011; Vaselli et al., 2011).

The drilling carried out as part of the CFDDP project (De Natale and Troise, 2011) is the evolution of the exploratory activities of decades 1940-1980 made by the Azienda Geologica Italiana Petroli (AGIP) and the Societa Anonima Forze Endogene Napoletane (SAFEN) (e.g., AGIP, 1987; Rosi and Sbrana, 1987; Piochi et al., 2014). The new drill hole reached a depth of 506 m (or 501 m with respect to the sea level) providing insights into the eastern side of the volcano subsurface, the only area not explored by previous drilling exploration activity. The results of the analyses performed on mud cutting collected during the drilling are synthesized at the site (https://sites.google.com/site/cfddpproject/) and presented elsewhere (De Natale et al., 2014). This paper is dedicated to the results obtained from mineralogical, petrographic and geochemical analyses of the core samples. Our final aim is gaining insights into the functioning of the geothermal system through a combination of textural, chemical and isotopic data.

2. Geological background

2.1. The volcanological and geothermal setting

Campi Flegrei is presently an ~ 10 km large caldera (Fig. 1)(Rosi and Sbrana, 1987; Barberi et al., 1991; Orsi et al., 1996; Perrotta et al., 2006; Vitale and Isaia et al., 2014) of debated origin (Piochi et al., 2014 and references therein). The erupted products are dated from at least 60 ka (Pappalardo et al., 1999) and discontinuously distributed over time up to 1538 AD (Rittmann, 1950; Rosi and Sbrana, 1987; Di Vito et al., 1999; Insinga et al., 2006; Di Renzo et al., 2011; Smith et al., 2011).

The volcano produced mostly low-volume tephra, subordinate lava domes and, two high-volume ignimbrites at 39 ka BP (De Vivo et al., 2001; Campanian Ignimbrite eruption) and 14.9 ka (Deino et al., 2004; Neapolitan Yellow Tuff eruption). Commonly, the caldera is attributed to the 39 ka eruption (Rosi and Sbrana, 1987; Barberi et al., 1991; Orsi et al., 1996; Perrotta et al., 2006; Piochi et al., 2014; Vitale and Isaia, 2014), besides other authors (Orsi et al., 1996; De Vivo et al., 2001; Vitale and Isaia, 2014) suggest the occurrence of a collapse during the Neapolitan Yellow Tuff. Phlegraean volcanic products range from shoshonitic to phono-trachytic compositions, with dominant alkali-trachyte and phono-trachyte (Piochi et al., 2005, 2014 for a review). The primary mineralogical assemblage of the volcanic rocks consists of alkali feldspars (typically potassic), plagioclase, Fe-oxides, apatite, diopside and biotite (e.g., Di Girolamo et al., 1984; Rosi and Sbrana, 1987; Pappalardo, 1994; Melluso et al., 1995; Pappalardo et al., 2002, and references therein; Fedele et al., 2008). Olivine occurs rarely as phenocryst, leucite and sodalite are sporadically observed, and quartz can also occur as xenocryst (Rosi and Sbrana, 1987). The radiogenic isotope values of the Phlegraean bulk pumice sample (shoshonite to trachy-phonolite) are well-known as they are measured on a large dataset: 87Sr/86Sr varies between 0.7068 and 0.7086,143Nd/144Nd between 0.51236 and 0.51252, 206Pb/204Pb between 18.85 and 19.25 and SnB between - 6.8 and —10.6%» (see Pappalardo et al., 2002; Tonarini et al., 2009; Di Renzo et al., 2011). Few 618O measurements have been published and range from 6.97 to 8.28% (Turi and Taylor, 1976; Taylor et al., 1979).

The Sr-isotope ratios are generally higher than those measured in the rocks from the nearby volcanic fields of Ischia and Procida, with the exception of products older than the Campanian Ignimbrite (De Astis et al., 2004), which overlap in terms of Sr-isotope geochemistry (e.g., De Vivo et al., 1989; Caprarelli et al., 1997).

The temperatures reach up to ~350 °C in the caldera center ~2500 m deep (Guglielminetti, 1986; AGIP, 1987; Rosi and Sbrana, 1987; Piochi et al., 2014). Fluid inclusion studies indicate paleo-temperatures higher than temperatures directly measured in the drill holes at the Mofete site (west caldera, Fig. 1), but similar to the temperatures recorded at the San Vito 1 well (central sector of the caldera, Fig. 1) (De Vivo et al., 1989).

Four major hydrothermal/metamorphic mineral assemblages have been recognized in the caldera subsurface, according to increasing temperatures. They are represented by the argillic, phyllic, propylitic and thermometamorphic facies (Agip, 1987; De Vivo et al., 1989; Caprarelli etal., 1997; Mormone etal., 2011). The mineralogical assemblages are in agreement with those found in other hydrothermal systems at the given temperature and depth (Browne, 1978; Schott et al., 2001; Gebreiwot, 2010). The argillic facies is mainly represented by clay minerals (illite, glauconite, chlorite and montmorillonite), which replace plagioclase and mafic minerals, and by zeolites. The strong increase in glauconite, chlorite and illite contents and the appearance of mixed-layer clay minerals characterize the transition to the phyllic facies, also termed illite-glauconite facies, which develops at temperatures close to 200-250 °C. Phillipsite is the typical zeolite mineral under these temperature conditions, followed by chabazite nucleation (de Gennaro et al., 2000). Quartz, calcite and pyrite can be also found in the mineralogical assemblage. The propylitic facies or zone of Ca-Al-silicates is characterized by the presence of secondary minerals, which are close to equilibrium with neutral, sodium-chloride aqueous solution and develop up to temperatures of300 °C. Here, epidote is usually accompanied by abundant pseudo-orthorhombic adularia (Smith, 1974), albite, sulfide minerals, and in lower abundance by glauconite and illite. The thermometamorphic facies is characterized by remarkable textural re-organizations of the original lithotypes and the appearance of high-temperature mineral phases, such as amphiboles (e.g. actinolite and tremolite), pyroxenes (e.g. diopside), biotite, and garnet. The temperature of the thermometamorphic facies is at least 350 °C (Rosi and Sbrana, 1987; De Vivo etal., 1989).

The aquifers range from dilute cold waters (total dissolved solids, TDS, < 1 g l-1; T = 15-18 °C) to high-temperature brines (TDS up to 33 g l-1; T up to 95 °C) resulting from mixing between meteoric water, deep brines rich in Na and Cl, hydrothermal volatile constituents, as well as infiltrating seawater (e.g., Celico et al., 1992; Aiuppa et al., 2006). Three deeper main aquifers have been detected downhole (see De Vivo et al., 1989): (1) between -550 and -896 m, with 42,860 ppm l-1 TDS, 25,304 ppm Cl and T «220 °C; (2) between -1273 and -1989 m, with 37,880-65,509 ppm l-1 TDS, 21,16937,800 ppm Cl and T «300 °C; and (3) between - 2310 and - 2699 m, with 515,902 ppm l-1 TDS, 313,850 ppm Cl and T « 360 °C (all values under atmospheric conditions). Considering the borehole closer to the CFDDP site (CF23 in Fig. 1 ), at Agnano, the water has a Na plus Cl content of up to 27 g l-1 and a temperature of approximately 300 °C, at -1840 m of depth (Carlino et al., 2012).

2.2. The CFDDP drilling and studied samples

The CFDDP well was drilled in the Bagnoli Plain that lies at the foot of the Posillipo cliff, on the eastern sector of the caldera (Fig. 1). The Bagnoli Plain is characterized by two cones, Nisida and Santa Teresa that were active in the past 14.9 ka (Di Vito et al., 1999; Di Renzo et al., 2011), and a complex and variable depositional marine and sub-aerial volcanic sequence recovered by few <223 m bore holes (Orsi et al., 1996; Calderoni and Russo, 1998) and down to 506 m of depth at the CFDDP drill hole (De Natale et al., 2014). The drilling operations

Fig. 1. Google image of the CampiFlegrei volcano with caldera structures as derived by Rosi and Sbrana (1987), Orsi et al. (1996),andVitaleandIsaia (2014) and well location (see Rosi and Sbrana, 1987; Piochi et al., 2014). The CFDDP borehole lies between the CF23 drill hole, in Agnano, and the Posillipo hill.

were carried out by Perazzoli Drilling Inc.; BJ Inc and Baker Huges Company Inc., which have provided the technical and logistical mud services, as well as gas analyses in the mud.

Based on our mud cutting analyses, the sequence includes (from top to bottom): (1) pyroclastic deposits composed of variably vesicular and porphyritic fragments; (2) from about 45 to 165 m, a succession of py-roclastic and volcanoclastic rocks made up of sub-rounded or rounded vesicular to dense, heterogeneous pyroclastic fragments containing a variable amount of siliceous fossils (i.e. spiculae and diatoms; Aiello etal., 2013; https://sites.google.com/site/cfddpproject/), organic carbon remains, wood fragments and peat; (3) an ~80 m-thick level dominated by brown dense to vesicular glass fragments; (4) a pumiceous rich deposit; (5) greenish tuffs with a low degree of crystallinity between — 270 and — 470 m; and (6) a basal gray pumice- and scoriae-bearing tuff. The marine paleoenvironment is recognizable by the occurrence of exclusively siliceous fossils at depth <260 m depth (Aiello et al., 2013). Our analyses show the authigenic mineralization of the mud cuttings and, in the next section, of cores as: from ca. 150 m pyrite, from ca. 290 m feldspar (also adularia), from ca. 320 m clay minerals and sulfates, and from ca. 380 m carbonate. The tuff sequence that begins at ca. 320 m has lost its primary volcanic glass feature, as evidenced by the greenish aspect. At the bottom of the borehole the gray tuff also contains albite as part of the secondary paragenesis.

During drilling, two short cores were collected at — 443 m and — 506 m depths (Fig. 2). Both cores have diameters of about 10 cm and are less than 1 m long. They are macroscopically homogeneous in texture, color and components, representing two different pre-caldera tephra (i.e., >39 ka). Here, we focus on the upper (up = top of the cores as from Fig. 2; hereafter indicated with 443up and 506up), intermediate (int = intermediate of the cores as from Fig. 2; hereafter, 443int and 506int) and lower (dw = bottom of the cores as from Fig. 2; hereafter, 443dw and 506dw) part of each core.

3. Results

3.1. Textural data

At the macroscopic scale, the 443 m drill core is representative of a low-cohesive tephra, matrix-dominated and grayish-greenish in color, with a high alteration degree. The matrix consists of a fine to coarse ash with a few dispersed mm- to few cm-sized pumice fragments with stretched and sub-rounded vesicles, various types of crystals and rare mm-sized blackish, dense lithic fragments, with a slight degree of crystallinity. At the microscopic scale, a heterogeneous texture and structure of the sample becomes evident; this heterogeneity is due to the different components (i.e. lithic, pumice, scoriae and crystals) and the alteration which intensely affects the low amount of pumices, however, still exhibiting their original glassy, vesicular and aphyric textures. Few lithics characterized by a blackish glassy matrix and feldspar micro-crystals (< 100 |jm) occur. Isolated crystals within the matrix can reach up to several mm in size (Fig. 3a). K-feldspars and subordinate clinopyroxene, biotite, and magnetite, and rare plagioclase form the primary assemblage, being generally euhedral-to-subeuhedral and often cracked or fractured. Clinopyroxene phenocrysts include green diop-side, commonly fractured and irregularly shaped, with corroded cores or, sometimes, with cores replaced by an argillic phase (i.e. glauconite). Biotite is a minor constituent as large brown laminae, only slightly altered and idiomorphic (Fig. 3b). Secondary mineralization is dominated by calcite and very fine intergrowths of clay minerals.

Calcite is widespread, filling the voids (Fig. 3c) or crystallizing in veins that cross-cut entire samples, locally coexisting with adularia (Fig. 3d). Calcite-rich veins are typical macroscopic features of the studied rock. Under crossed polarizer, this mineral shows characteristic lamellar twinning. Interstitial calcite grains are also present. SEM observations using the backscattered electron mode (BSE) display the

Fig. 2. Photos of the drill cores sampled at 443 m and 506 m of depths.

typical cuspidic shape of voids in the matrix filled by calcite (Fig. 4a and b) or in veins (Fig. 4c and d). Fig. 4e shows a typical texture of pumice partially replaced by clay mineral phases (i.e. glauconite) and deeply devitrified. Locally, the pumice texture is overprinted by thin crystals of glauconite forming radial and efflorescent-like aggregates within the tube-shaped vesicles (Fig. 4f). Accessory phases are represented by apatite, in the primary assemblage (Fig. 4g), and rare sulfides, in the secondary assemblage. Crystals of pyrite (< 3%) have grown around feldspar relics, forming clots with apatite (Fig. 4h). Locally, in the secondary assemblage, albite also occurs.

The 506 m drillcore is a texturally and structurally heterogeneous tuff made of chaotic coarse-to-fine-ashy matrix including mm-to-cm sized, gray-to-silvery and gray scoria-like aphyric juvenile fragments with (elongated or tubular) stretched and sub-rounded vesicles. Fig. 5a displays the pumice groundmass intensely replaced by clay minerals. Under polarized light microscopy, the matrix is fine-grained and strongly altered, and includes up to 20-40% of predominantly millimeter-sized alkali feldspar phenocrysts and rare pyroxenes associated with the main secondary mineralization.

The 506 m core sample includes small blackish lithic grains with variable crystal content, due to the presence of feldspar microcrystals (< 100 |jm), most of which are oxidized. Feldspar occurs as euhedral isolate alkaline crystals with Carlsbad twins or rarely as sodium plagioclase (< 100 |am); pyroxenes are subeuhedral to anhedral and generally fragmented. Microcrysts of magnetite are present in small amounts. Among the secondary phases, quartz microcrysts are very common, but rarely display well-formed crystal faces. Calcite represents a pervasive secondary crystal growing in veins, where it shows the rhombohe-dral habit (Fig. 5b and c), or in grains as a cement of the rock (Fig. 5d). Calcite is usually surrounded by fragment and/or cubic of pyrite; this feature is also observed under BSE-SEM, with several pyrite grains occurring around single (Fig. 6a, b and c) or clustered Fe-Mn-calcite crystals (Fig. 6d). Micro-textural investigation also shows the remnants of intensely devitrified tube vesicular pumices with secondary mineralization including pyrite (Fig. 6e, f and g), in higher amount if compared to the 443 m deep core samples, and glauconite infillings forming at the expense of feldspar (Fig. 6h). Apatite and REE (Rare Earth Elements) bearing secondary sulfides are dispersed in the matrix as accessory minerals.

32. X-ray diffraction data

Table 1 lists the X-ray diffraction data on the whole-core powders. The secondary mineralogical assemblage of the drill hole, both from cores and mud-cuttings (see section The CFDDP drilling and studied samples), is consistent with a depth-dependent hydrothermal alteration zoning due to temperatures rising through the system, downward

(Browne, 1978; Rosi and Sbrana, 1987; Reyes, 1990; Mormone et al., 2011).

Feldpar, calcite, pyroxene, quartz, clay minerals, magnetite and apatite are commonly identified in the XRD spectra. The clay minerals are mainly represented by glauconite and subordinately interlayer smectite and/or montmorillonite. Interlayered clay minerals occur in greater amounts in the shallower core (443 m). X-ray pattern of the 443 m core samples indicates biotite and calcite (~20-30%), together with alkali feldspar (>40%).

The comparison of the XRD patterns of the two cores shows important differences. First, a diffraction peak at 7.05 A (Fig. 7) can be related to the zeolite mineral phillipsite in the 506 m core. A pervasive, but very fine-grained zeolitization (not visible under the microscope), may be responsible for the compactness of the core at a macroscopic scale. Besides, the XRD spectra indicate a higher content of calcite, gypsum (<5%), barite (<5%) and pyrite (<5%) in the 506 m bottom core.

Magnetite occurs in very low amounts (below 2 wt.% on average) and sodic plagioclase can be detected in all investigated cored samples.

3.3. Mineral chemistry

Chemical data of the most representative observed crystals are reported in Tables 2 and 3.

Feldspar phenocrysts and microlites in the primary assemblage are mainly represented by sanidine (Fig. 8a). The 443 m feldspars are Abi2_25-Or92_75, thus showing a higher compositional variability than the 506 m feldspars with a smaller range of orthoclase molecula (Ory^-^yg.s) and enrichment in the albitic component. The albitic plagioclase, in the secondary assemblage, from the 506 m core has An0.2-3.5-Abgg-g6 composition. Only two 443 m feldspar crystals are andesine/lab-radorite having a slightly high Or% molecule (Or-^.^-^.o). Similar plagioclase compositions were already detected in volcanic products from the Campi Flegrei (Fig. 8a), i.e. the Campanian Ignimbrite where Or is up to 14%(Pappalardo, 1994; Mellusoetal., 1995; Fedele etal.,2008) and,toa lesser extent, the Neapolitan Yellow Tuff (Orsi et al., 1995). This similarity provides evidence for a primary composition for these plagioclases. Furthermore, it suggests that cored tephra belong to an oldest period of volcanism; in fact, they are buried under 15 ka deposits, as derived by integration of our mud cutting analyses and previous shallower borehole data (Orsi et al., 1996; Calderoni and Russo, 1998).

EDS analyses support for both core samples, the presence of adularia.

Biotites are Al-, Ti- and Fe-rich-type.

Calcite contains highly variable amounts of Fe, sometimes occurring as distinct ferroan calcite (Tab. 3). Fe contents range from 0.15 to 13.90 wt.% in the 443 m level samples, whereas in the 506 m level samples Fe is in the range 8.15 to 15.96 wt.%. Mg and Mn concentrations for 443 m drill core are always between 0.05 and 8.04 and 0.29 and 1.01 wt.%, respectively. Occasionally, the shallowest 443 m sample

Fig. 3. Thin section photomicrographs of 443 m core samples: a) alkali-feldspar crystals within the matrix, crossed polars; b) biotite plat in rineritic matrix, plane polarize light; c) widespread Fe-Mn calcite crystals in the matrix and d) within veins, crossed polars. Afs, alkali feldspar; bt, biotite; Cal, calcite.

shows pure calcites. For the 506 m core, the contents range from 6.80 to 11.17 wt.% and 0.88 to 1.29 wt.%, respectively (Fig. 8b).

Pyrite is present in both samples, with molar proportions of Fe and S close to the ideal ratio of 1 mol Fe to 2 mol S. Minor amounts of As up to about 2 wt.% were measured in all the pyrites.

The tiny size of clay minerals does not allow quantitative chemical analyses to be performed, although some EDS spectra confirm a glauco-nite chemistry (Fig. 6h), as derived by XRD spectra.

3.4. Carbon, oxygen and strontium isotope geochemistry (Table 4)

Calcites from the CFDDP 443 m core have S13C (VDPB) and S18O (SMOW) values between -0.2 and - 0.3%» and between 13.1 and 13.3%, respectively. The CFDDP 506 m calcites show ô13C values of —1.6 and — 2.02% and ô18O values of 11.7 and 11.6%. Fig. 9 allows the comparison between the values measured in this study and literature data. With respect to the leachate carbonates from the AGIP cores (Caprarelli et al., 1997; Piochi et al., 2014), measured ô13C values are higher than published values ( — 2.4 to — 3.1%), while ô18O values are comparable to the isotope ratios from the Mofete cores (12 to 16.5%). Gas emissions and calcites show comparable carbon isotope ratios as derived by Fig. 9 showing the ô18O and ô13C frequency distribution of the present gas emissions.

Feldspars in the 506 m core are characterized by 87Sr/86Sr value of 0.707523 ± 8, which is in the range of isotopic compositions of both outcropping and cored rocks (Piochi et al., 2014 for a review). In particular, the obtained value is recurrent at the Campi Flegrei in the rocks younger than 14.9 ka and in the products of the Neapolitan Yellow Tuff, but, more consistently with mud cutting analyses, it was measured in pre-caldera domes drilled in the Mofete area (Piochi et al., 2014 for a review).

4. Discussion

The two tephra layers/intervals cored at 443 and 506 m depth in the CFDDP drill hole present the typical primary mineral association of the Phlegraean rocks with high abundance of sanidine, apatite, magnetite and limited abundances of pyroxene and biotite; plagioclase is rare.

The tephra show a marked alteration in terms of authigenic mineralization and different degrees of argillic alteration related to the active hydrothermal system. The secondary mineralization is reflected in the formation of calcite and clay minerals, quartz and pyrite. At a depth of 506 m, albite appears in the clay minerals to illite-to-chlorite hydrothermal facies. This low-grade secondary assemblage has already been described in the Campi Flegrei caldera for the Mofete 1 well, at a depth comparable with that of the CFDDP coring (Rosi and Sbrana, 1987; De Vivo et al., 1989; Mormone et al., 2011). Here, albite was similarly detected from a depth of around 500 m.

The formation of glauconite is responsible for the greenish color of the tephra, while calcite and the precipitation of phillipsite (zeolitization) likely contribute to variation in the cohesion of the rock at 506 m depth.

The occurrence of glauconite in the deeper core suggests slightly reducing conditions important mostly in the early stages related to a proto-glauconite formation, then becoming slightly oxidizing; (McConchie et al., 1979) and a pH « 7-8.

Where the calcite grows in veins, it is coexisting with neoformed K-feldspar (i.e., adularia) microcrysts. By analogy with active geothermal systems (Browne and Ellis, 1970; Browne, 1978; Keith and Muffler, 1978; Henley et al., 1986; Simmons and Christenson, 1994) and thermodynamic studies (Drummond and Ohmoto, 1985; Reed and Spycher, 1985; Simmons and Browne, 2000), this circumstance, together with the evidence of a clay alteration process, is a direct evidence of boiling conditions. During boiling the release of CO2 to the vapor phase leads to a pH increase in the solution, causing the calcite

.'n0..n 1 1 3Ô0_m'

Fig. 4. Back-scattered electron images (core at 443 m): a) widespread hydrothermal clay alteration; b) Fe-Mn calcite grains grown within voids and c) and d) in rock fractures; e) and f) pumice intensely alterated to clay minerals; g) alkali-feldspar relicts; and h) alkali-feldspar with apatite, diopside, as primary crystals and authigenic pyrite. Afs, alkali feldspar; Cal, cal-cite; Di, diopside; Py, pyrite; Ap, apatite.

precipitation (see Eq. (1)) first and K-feldspar precipitation through the interaction of the vapor phase (i.e. a fluid bearing CO2 from a deep source) with the previously formed clay minerals (see Eq. (2)) (Andre-Mayer et al., 2002).

Ca + 2HCO- = CaCO3| + CO2t + H2O

3KAlSi3O8 + 2CO2(g) + 2H2O + KAl2(Al, Si)4O10(OH)2 + 6SiO2 (2) +2HCO- + 2K

This is in agreement with our results showing the formation of clay minerals at the expense of alkali-feldspars, as well as the glass matrix, in the primary assemblage (Fig. 6h).

The glauconitization process is generally explained by inter-layering with degraded illites through addition of Fe, and progressive addition of

Table 2

Representative chemical composition of feldspar from the investigated drill cores of the CFDDP drill hole.

Sample 443 m 506 m M - - - o

Location Intermediate Down Up Down

SiO2 60.55 63.86 63.74 64.79 63.37 56.09 62.42 61.92 62.68 63.78 63.64 54.93 63.35 63.95 63.68 64.53 67.32 68.29 66.89 63.35 62.61 64.03 63.71 63.18 68.80 67.17 63.68

Al2Û3 18.66 19.45 18.70 19.16 18.57 26.33 18.74 18.75 18.87 19.17 18.37 27.39 18.92 18.83 19.17 18.97 19.54 19.60 19.02 18.52 18.71 18.40 18.82 18.71 19.85 19.45 18.21

FeO 0.25 0.35 0.12 0.59 0.63 0.56 0.17 0.34 0.10 0.17 0.44 0.60 0.41 0.16 0.35 0.19 0.04 0.40 0.16 0.00 0.00 0.43 0.00 0.45 0.10 0.00 0.44

MnO 0.00 0.00 0.46 0.21 0.00 0.13 0.15 0.00 0.00 0.06 0.24 0.00 0.09 0.00 0.28 0.03 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

MgO 0.00 0.02 0.00 0.00 0.00 0.00 0.00 0.10 0.12 0.00 0.00 0.05 0.00 0.06 0.00 0.00 0.00 0.00 0.15 0.00 0.00 0.18 0.00 0.00 0.07 0.05 0.00

CaO 0.48 0.40 0.34 0.57 0.61 9.59 0.43 0.57 0.26 0.94 0.42 9.97 0.65 0.35 0.60 0.53 0.00 0.11 0.36 0.88 0.70 0.70 0.75 0.54 0.04 0.22 0.60

Na2O 1.36 2.48 2.41 2.39 1.92 5.44 1.14 1.30 1.45 2.67 1.67 4.62 1.50 2.28 2.20 2.36 10.81 12.00 11.83 2.62 2.31 2.33 2.12 2.22 11.65 11.39 2.28

K2O 13.38 13.07 13.13 12.97 13.65 1.94 14.15 14.00 14.70 12.23 14.02 2.31 15.06 13.55 13.54 13.40 1.53 0.16 0.06 13.44 13.39 12.98 12.92 13.41 0.32 0.30 13.47

Total 98.30 100.76 100.21 101.24 99.56 100.30 99.32 100.10 99.80 101.12 100.02 100.88 101.26 99.82 100.73 100.01 99.73 100.97 99.60 100.30 99.07 100.29 100.44 100.07 101.43 99.81 99.31

No. of ions on the basis of 8 O No. of ions on the basis of 8 O

Si 2.91 2.93 2.95 2.95 2.95 2.55 2.94 2.92 2.94 2.93 2.96 2.50 2.93 2.96 2.93 2.96 2.98 2.98 2.97 2.94 2.94 2.96 2.95 2.94 2.98 2.97 2.97

Al 1.06 1.05 1.02 1.03 1.02 1.41 1.04 1.04 1.04 1.04 1.01 1.47 1.03 1.03 1.04 1.03 1.02 1.01 0.99 1.01 1.03 1.00 1.03 1.03 1.01 1.01 1.00

Fe 0.01 0.01 0.00 0.02 0.02 0.02 0.01 0.01 0.00 0.01 0.02 0.02 0.02 0.01 0.01 0.01 0.00 0.01 0.01 0.00 0.00 0.02 0.00 0.02 0.00 0.00 0.02

Mn 0.00 0.00 0.02 0.01 0.00 0.01 0.01 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Mg 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.00

Ca 0.02 0.02 0.02 0.03 0.03 0.47 0.02 0.03 0.01 0.05 0.02 0.49 0.03 0.02 0.03 0.03 0.00 0.01 0.02 0.04 0.04 0.03 0.04 0.03 0.00 0.01 0.03

Na 0.13 0.22 0.22 0.21 0.17 0.48 0.10 0.12 0.13 0.24 0.15 0.41 0.13 0.20 0.20 0.21 0.93 1.01 1.02 0.24 0.21 0.21 0.19 0.20 0.98 0.98 0.21

K 0.82 0.77 0.78 0.75 0.81 0.11 0.85 0.84 0.88 0.72 0.83 0.13 0.89 0.80 0.80 0.79 0.09 0.01 0.00 0.80 0.80 0.76 0.76 0.80 0.02 0.02 0.80

Total 4.99 5.00 5.01 5.00 5.01 5.02 5.00 5.00 5.01 5.00 5.00 5.02 5.02 5.02 5.02 5.02 5.01 5.03 5.01 5.03 5.04 5.00 4.99 5.01 5.00 5.00 5.01

%An 2.54 1.96 1.67 2.80 3.00 44.10 2.22 2.91 1.28 4.62 2.09 47.29 3.05 1.70 2.90 2.55 0.00 0.50 1.65 4.07 3.36 3.44 3.76 2.63 0.19 1.04 2.89

%Ab 13.04 21.95 21.45 21.27 17.08 45.27 10.67 12.01 12.87 23.76 15.01 39.66 12.75 20.02 19.23 20.58 91.48 98.64 98.02 21.93 20.07 20.70 19.21 19.57 98.04 97.28 19.87

%Or 84.42 76.10 76.88 75.93 79.92 10.62 87.11 85.08 85.85 71.61 82.91 13.05 84.20 78.28 77.87 76.87 8.52 0.87 0.33 74.00 76.56 75.87 77.03 77.80 1.77 1.69 77.24

Fig. 5. Thin section photomicrographs of 506 m core: a) pervasive alteration, plane polarize light; b) Fe-Mn calcite veins, crossed polars; c) Fe-Mn calcite and apatite crystals, crossed polars; and d) vuggy Fe-Mn calcite, crossed polars. Cal, calcite; Apa, apatite.

K to the growing glauconitic smectites (Burst, 1958a, 1958b; Hower, 1961 ; Odin and Matter, 1981 ; Meunier and El Albani, 2007). This process is favored in K-enriched rocks, and generally occurs in marine or hypersaline environments and at the interface between oxidizing and reducing environments (Ireland et al., 2006; Meunier and El Albani, 2007). The alteration of the studied rocks by clay, represented mainly by glauconite is in agreement with: (1) the drop of the tephra below the sea level that was at least 120-80 m lower in the 15-200 ka time (Orsi et al., 1996; Antonioli et al., 2004); (2) the geochemistry of the secondary calcites; (3) the formation of As-rich pyrites; (4) the fluids circulating in the borehole; and (5) previous data on the geothermal fluids circulating in the caldera (Caprarelli et al., 1997; Valentino et al., 1999; Aiuppa et al., 2006).

The geochemistry of the studied calcites shows that they are enriched in Mg, Mn and Fe; where, the enrichment in Mg content can be attributed to the Mg-rich nature of involved marine waters (Coleman, 1985). Moreover, in particular at 506 m depth, the rather high MgO content can also be due to Mg vs. Ca exchange reactions between calcite and Mg-rich fluids under hydrothermal conditions (Jonas et al., 2014). On the other hand, the growth of pyrite around the calcite crystals - describing an inverse correlation between FeO and MgO/CaO - suggests the progressive enrichment in Fe2+ (and S2+) in the environment. This circumstance suggests both higher permeability around the calcite crystals, as demonstrated by experimental studies (Roberts et al., 1969), and the circulation of H2S enriched fluids (Curtis and Coleman, 1986). It is worth noting that H2S exhalations were detected from the 300 to 313 m depth range in the drilling mud during perforation.

The principal stage of pyrite formation takes place with the reaction Fe2+ + S2~ ^ FeS2, which requires the H2S action in aqueous solution under weakly oxidizing conditions to form polysulfide and Fe2+ ions, as demonstrated by some experiments (Curtis and Coleman, 1986). This is also supported by pyrite chemistry (Table 2). In fact, pyrite shows As concentrations between 1.3 and 2.1 wt.%, which are in the range of sulfide deposits and hydrothermal systems (from about 6 ppm to 10 wt.%; Abraitis et al., 2004). As shown by previous studies

(Heinrich and Eadington, 1986; Ballantyne and Moore, 1988; Spycher and Reed, 1989), arsenic derives from hydrothermal solutions where this element is easily dissolved (so transported). Therefore, the As-rich pyrites in our samples support boiling conditions and a pH increase, which occurs when H2S gas is driven off (Ballantyne and Moore, 1988). Based on previous investigations at Campi Flegrei wells (Rosi and Sbrana, 1987) and particularly fluid inclusion studies on secondary mineral phases (De Vivo et al., 1989), pyrite is indicative of temperatures of 100 to 350 °C.

Similarly, following Rosi and Sbrana (1987), the authigenic albite occurs within the secondary paragenesis and suggests temperatures of 160 to 300 °C at the time of its growth.

Isotope compositions help to constrain the hydrothermal conditions, at the moment of calcite crystallization at the CFDDP site. In order to calculate reservoir temperature, White et al. (1990) used the 613C and 618O values of calcites and hydrothermal fluids. We calculated the 13C and 18O partitioning between calcite and present fluids as well as the temperature using the relationships given by Bottinga (1968). Table 5 reports the calculated fractionation factors and equilibrium temperatures determined on the basis of the carbon and oxygen isotope compositions of the fluids as shown in Fig. 9. The best agreement (within 10%) between calculated and measured temperatures is obtained assuming fluids dominated by H2O and CO2 with average isotope compositions similar to the fluids from the present fumarolic field (see values in Fig. 10 and Table 5). This interpretation is supported by the comparison of the fractionation factors from the calculation in Table 5 and the theoretical curves obtained by applying the Bottinga's formula (Fig. 10). Therefore, we are highly confident to suggest that equilibrium paleo-temperatures of the hydrothermal system were 125 °C at 443 m and 160 °C at 506 m. These conditions are consistent with the thermal gradient measured at the Campi Flegrei (Rosi and Sbrana, 1987; see also the Introduction section), although each specific site could show chemico-physical conditions related to the variable intra-caldera tectonics and dynamics. For example, the Mofete 1 borehole, sited on the opposite caldera rim (Fig. 1), in the first 500 m intercepts lithologies

Fig. 6. Back-scattered electron images (core at 506 m): a), b) andc) Fe-Mn calcite in isolated cuspate grains and d) within rock fractures; e) glauconite plates grown in pumice pipes; f) and g) widespread hydrothermal alteration in argillitic phases; h) glauconite lamellar crystals forming at the expense of feldspar. Afs, alkali feldspar; Cal, calcite; Py, pyrite; Glt, glauconite.

similar to those encountered in the CFDDP borehole, with fossiliferous volcanoclastic sediments inter-bedded among pyroclastic products. Indeed, the calculation we made on calcites from Mofete 1 cores collected at depth of 1299-1503 m restituted temperature (180-222 °C, Table 5) slightly lower than those of homogenization (250 °C) in the related fluid inclusion studies (De Vivo et al., 1989), possibly due to the action of fluids compositionally different with respect to those in Bagnoli (and in the active fumarolic field of Solfatara).

The temperatures re-calculated from the calcite (T = 125-160 °C; Table 5) and fluid equilibrium, as well as the ones inferred on the basis of albite presence (T = 160-300 °C by Rosi and Sbrana, 1987) and by occurrence of pyrites (T = 100-350 °C by Rosi and Sbrana, 1987 and De Vivo et al., 1989), are higher than the temperature measured downhole (T = 60 °C). These cooling conditions were indicated also for the Mofete area (De Vivo et al., 1989). However, here the downhole temperature measured during AGIP drilling activities is

Table 1

Mineralogical association of the cored samples at the CFDDP as derived by optical and electron microscopy and XRD measurements.

Mineralsa Sample # 443 506

Albite NaAlSi3O8 xx xx

Alkali feldspar KAlSi3O8 xxxx xxxx

Barite BaSO4 tr tr

Biotite K(Mg,Fe2 + )3(AlSi3O10)(OH,F)2 x x

Calcite CaCO3 xx xx

Diopside CaMgSi2O6 x x

Glauconite (K,Na)(Fe3+,Al,Mg)2(Si,Al)4O10(OH)2 x x

Gypsum CaSO4-2H2O x x

Illite(K,H3O)(Al,Mg,Fe)2(Si,Al)4Oi0[(OH)2(H2O)] x x

Hematite Fe2O3 x x

Magnetite Fe2 + Fe3 + 2O4 x x

Montmorillonite (Na,Ca)0.3(Al,Mg)2Si4O10(OH)2-n(H2O) tr tr

Phillipsite (K,Na,Ca)i _2(Si,Al)8Oi6-6(H2O) - x

Pyrite FeS2 x x

Quartz SiO2 x x

a Ideal formulas from webmineral.com. xxxx > 40%; xxx = 40-20%; xx = 20-5%; x < 5%; tr = trace amounts.

nearly 200 °C. Therefore, the two opposite rims of the caldera presents a different temperature (60 vs. 200 °C) setting possibly in relation to several reasons, among which (1) the variable cooling rates of the hydrothermal system; (2) the earlier disappearing of volcanism in Bagnoli with respect to Mofete (Rosi and Sbrana, 1987; Smith et al., 2011); and/or (3) the peculiar geometry of the magmatic-geothermal system.

In summary, the results of this study indicate cooling of the hydrothermal system through time later than the sequence tuff formation. In such a context, the formation of phillipsite should represent the final step of the evolution of the system, more than the syn-depositional welding of the cooling tuff. As a matter of fact, alkaline or almost neutral conditions in the low-temperature saline solution interacting with the glassy precursor is a precondition for the zeolitization processes to the detriment of feldspatization, as demonstrated by de Gennaro et al. (2000).

5. Conclusions

Detailed investigations of two cored samples collected during the CFDDP drilling program highlight pervasive rock alteration

characterized by a well-developed and depth-dependent mineral alteration zoning related to the increasing temperature of the hydrothermal system downhole. The study of the mineralogy and geochemistry of the neogenic mineralogical assemblages helped to better understand the mechanism involved during the hydrothermal alteration in the eastern sector of the caldera and lead to the following conclusions:

(1) the coexistence of calcite and neo formed feldspar, together with the As-rich pyrite grains records a past hydrothermal state and supports boiling conditions.

(2) glauconite alteration is the result of K-feldspar disequilibrium and seawater ingression, both favoring the uptake of K, Fe and Mg that, in turn, determines Mg- and Sr-rich calcites.

(3) pyrite and glauconite following calcite formation are an indirect evidence of alkaline and possibly slightly oxidizing or oxidiz-ing-reducing interface conditions.

(4) The equilibrium temperature, calculated using the isotope ratios of calcite and estimated from the mineralogical assemblage, was at least 160 °C, i.e. several tens of degrees higher than the 60 and 80 °C measured at depth of443 m and at the bottom of the borehole, respectively, suggesting cooling of the hydrothermal system.

Acknowledgments

This work has been supported by the ICDP-CFDDP and PON-MON.I.-C.A. (PON 01152 M.O.N.I.C.A.) projects. The VULCAMED project provided funds to install the XRD laboratory at the Istituto Nazionale di Geofisica e Vulcanologia in Naples. Thanks are due to key partner Bagnolifutura S.p.A. and Baker Huges. Dott. Roberto Isaia and Mauro Di Vito are kindly thanked for their discussions and encouragement. We acknowledge the technician dott. Roberto de Gennaro for skillful assistance in SEM observations and EDS analyses at "Centro Interpartimentale per Analisi Geomineralogiche (CISAG)" — Federico II-University, Napoli, Italy. We thank the staff of the Laboratorio of Mass Spectrometry at the Istituto Nazionale di Geofisica e Vulcanologia — Osservatorio Vesuviano and in particular I. Arienzo for the kind laboratory assistance. MP and AM kindly thank Gianfilippo De Astis for a last reading of the manuscript. The authors wish to express their gratitude to the Editor Prof. M.J. Rutherford, to Prof. F. Pirajno and an anonymous reviewer for their revision of the manuscript and providing constructive comments and suggestions.

Fig. 7. XRD patterns of two powder cored samples with interpretation of main peaks. Refer to the text for further details.

Table 3

Calcite chemical composition for the two drill cores of this study.

Sample Core 443 m Core 506 m

MgO 0.55 0.70 7.57 2.88 8.04 0.76 7.64 7.53 0.16 0.05 7.30 0.47 11.32 11.17 8.91 8.80 6.80 10.58

CaO 50.42 51.22 32.80 44.12 32.21 50.59 32.44 31.72 52.17 52.62 32.40 52.34 32.35 32.30 31.68 31.64 30.67 31.71

MnO 0.30 0.63 0.57 1.01 0.29 0.75 0.48 0.74 0.78 0.77 0.69 0.78 1.71 1.03 0.88 1.29 1.04 1.18

FeO 0.86 0.15 12.13 5.37 13.43 1.00 14.44 13.90 0.35 0.00 13.69 1.13 6.91 8.58 12.88 12.10 15.96 8.15

CO2a 40.88 41.54 41.85 41.69 42.46 41.67 42.94 42.09 41.88 41.83 42.26 42.89 43.21 43.49 43.21 42.73 42.00 42.16

Total 93.01 94.49 95.05 95.07 96.43 94.97 97.94 95.98 95.58 95.27 96.49 98.05 95.98 96.68 98.14 96.75 96.66 93.78

No. of ions on the basis of 6 O

Mg 0.03 0.04 0.40 0.15 0.41 0.04 0.39 0.39 0.01 0.00 0.38 0.02 0.57 0.56 0.45 0.45 0.35 0.55

Fe2+ 0.03 0.00 0.36 0.16 0.39 0.03 0.41 0.40 0.01 0.00 0.40 0.03 0.20 0.24 0.37 0.35 0.47 0.24

Mn 0.01 0.02 0.02 0.03 0.01 0.02 0.01 0.02 0.02 0.02 0.02 0.02 0.05 0.03 0.03 0.04 0.03 0.03

Ca 1.94 1.94 1.23 1.66 1.19 1.91 1.19 1.18 1.96 1.97 1.20 1.92 1.18 1.17 1.15 1.16 1.15 1.18

C 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00

a Calculated from stoichiometry.

Appendix 1. Analytical methods

Cores were retrieved through bit in a 6 inch-large borehole, subsequently drilling crust with a circulating water mud stabilized with organic additives. For the thin section preparation of cored rocks, the samples were impregnated with Araldite D and Rajy Hardner EH2950, and prepared at ca. 30 mm thickness (OMT Laboratory), for both petro-graphic studies under a reflected-light-equipped optical microscope, and Scanning Electron Microscope (SEM) and energy dispersed scanning microscopy (EDS).

SEM observations and semi-quantitative and quantitative EDS analysis were carried out at CISAG Laboratory (Universita di Napoli Federico II), by using a JEOL-JSM 531 instrument, equipped with a Link EDS and a Inca 4.08 software and using the following reference standards: ortho-clase (Si,Al), wollastonite (Ca), almandine (Fe), diopside (Mg), albite (Na), orthoclase (K), celestine (Sr), bentonite (Ba), pyrite (S), sylvite (Cl) and fluorite (F). Operating conditions were 15 kV accelerating voltage, 50-100 mA filament current, 5-10 |jm spot size and 50 s net acquisition time.

X-ray diffraction (XRD) intensity data were collected on selected grains, minerals or whole-rocks previously powdered in an agate mill. We have used a X'Pert Powder diffractometer by PANalytical, at the Istituto Nazionale di Geofisica e Vulcanologia — Osservatorio Vesuviano, equipped with a high speed PIXcel detector, a Ni-filter, diffracted-beam monochromator, and CuKa radiation, at 40 kV and 40 mA in a 3-70126 range, with 0.02° steps at 8 s/step. Diffraction patterns were elaborated and interpreted using the X'Pert HIGH Score Plus computer program.

87Sr/86Sr measurements on very pure feldspar crystals hand-picked from cores were performed at the Istituto Nazionale di Geofisica e Vulcanologia, Sezione Osservatorio Vesuviano di Napoli. Feldspars were leached with cold and warm 2.5 N HCl for 10 min, then rinsed several times in pure sub-boiling distilled water, and finally dissolved with high-purity HF-HNO3-HCl mixtures. Sr was separated by standard cation-exchange methods. Isotope ratios were measured statically by Thermal Ionization Mass-Spectrometer (ThermoFinnigan™ Triton TI) and were corrected for mass fractionation using 86Sr/88Sr = 0.1194. Replicate analysis of NIST NBS 987 Reference Standard gave average values of 0.710192 ± 0.000017 (2ct, n = 25); Sr blank was of the order of 0.1 ng during the period of chemistry processing.

Stable carbon and oxygen isotope analyses were carried out at the University of Erlangen-Nurnberg (Germany). Carbonate powders and picked minerals were reacted with phosphoric acid at 70 °C using a GasBench II connected to a Thermo Finnigan Five Plus mass spectrometer. All values are reported in per mil relative to V-PDB by assigning a S13C value of +1.95%» and a S18O value of - 2.20% to NBS19. Reproduc-ibility was checked by replicate analysis of laboratory standards and was better than ±0.07% (1ct) for both carbon and oxygen isotope analyses.

Table 4

Isotope compositions of analyzed carbonate minerals.

Fig. 8. Classification diagrams for a) feldspar and b) calcite crystals of the studied core samples. The field shows the plagioclase compositions from the Campanian Ignimbrite (IC, Pappalardo, 1994; Melluso et al., 1995; Fedele et al., 2008) and the Neapolitan Yellow Tuff (NYT, Orsi et al., 1995); see text for details.

Sample # 87Sr/86Sr S13C VDPB ö1sO VDPB S18O SMOW

CFDDP 443 cx dw - 0.20 -17.05 13.33

CFDDP 443 cxi - 0.27 - 17.27 13.10

CFDDP 443 pw i - 0.26 - 17.07 13.31

CFDDP 506core i 0.707523a - 2.02 - 18.67 11.67

CFDDP 506core up - 1.64 - 18.78 11.55

Measured in feldspars.

Fig. 9.813C and 818O of CFDDP calcites (numbers refer to the cored depth) and Mofete 1 leachated carbonates (cores at 1299-1306 and 1495-1503 m of depth) in the frequency distribution of measurements in present thermal waters and gaseous emissions. Literature data from Baldietal. (1975), Corteccietal. (1978), Ghiaraetal. (1988), Allardetal. (1991), Caprarelli et al. (1997), Valentino et al. (1999), Chiodini et al. (2000,2008), Minissale (2004), and Caliro et al. (2007).

Comparison of temperature estimated by isotope compositions of analyzed carbonate minerals and measured downhole as function of average fluid compositions from histograms of Fig. 9.

CFDDP 443 cxdw CFDDP 443 cxi CFDDP 443 pw i CFDDP 506core i CFDDP 506core up MF1 1299- 1306a MF1 1495-1503Aa

Measured data

813C calcite - 0.20 - 0.27 - 0.26 - 2.02 - 1.64 - 2.40 - 3.10

818O calcite 13.33 13.10 13.31 11.67 11.55 7.50 7.60

Downhole temperature 65 65 65 80 80 295 320

Calculated temperatures and fractionation values (a) following White et al. (1990) on the basis of 13C fractionation between calcites and CO2 and18 O fractionation between calcite

and H2O

Thermal fluids

813C thermal watera -5 -5 -5 -5 -5 -5 -5

818O thermal watera -5 -5 -5 -5 -5 -5 -5

Calculated a13C 1.00482 1.00476 1.00476 1.00300 1.00338 1.00261 1.00191

Calculated a18O 1.01843 1.01820 1.01840 1.01840 1.01675 1.01256 1.01266

Tcalcite-CO2 79 80 80 107 100 86 98

Tcalcite-H2O 90 92 90 105 106 152 151

Gas emissions

813C gas averagea - 2.5 - 2.5 - 2.5 - 2.5 - 2.5 - 2.5 - 2.5

818O gas averagea -1 -1 -1 -1 -1 -1 -1

Calculated a13C 1.00230 1.00224 1.00224 1.00048 1.00086 1.00010 0.99940

Calculated a18O 1.01435 1.01412 1.01433 1.01268 1.01256 1.00851 1.00861

Tcalcite-CO2 120 121 121 167 155 180 210

Tcalcite-H2O 130 132 130 151 152 222 219

813C gas minimuma -5 -5 -5 -5 -5 -5 -5

818O gas minimuma -2 -2 -2 -2 -2 -2 -2

Calculated a13C 1.00482 1.00476 1.00476 1.00300 1.00338 1.00261 1.00191

Calculated a18O 1.01537 1.01514 1.01534 1.01369 1.01358 1.00952 1.00962

Tcalcite-CO2 79 80 80 107 100 114 128

Tcalcite-H2O 118 121 119 138 138 201 199

813C gas maximuma 0 0 0 0 0 0 0

818O gas maximuma 0 0 0 0 0 0 0

Calculated a13C 0.99980 0.99973 0.99974 0.99798 0.99836 0.99760 0.99690

Calculated a18O 1.01333 1.01310 1.01331 1.01167 1.01155 1.00750 1.00760

Tcalcite-CO2 192 195 194 328 281 433 imp

Tcalcite-H2O 142 145 142 165 167 245 243

813C and 818O in per mil, T = temperature in °C. imp: impossible calculation, data for MF1 1299-1306a and MF1 1495-1503Aa cores from Caprarelli et al. (1997). a Fluid and gas compositions from literature as data in Fig. 9; the 813C and 818O are the CO2 and H2O values measured at the present fields of fluid emissions.

Fig. 10. Fractionation values (a) for18O (left) and 13C (right) vs. temperature (°C) calculated for our cores compared with Mofete 1 data (Caprarelli etal., 1997) and the theoretical curves based on Bottinga's (1968) formula. In the legend: (e) points for recalculated values from Table 1; 65° and 80 °C are the measured downhole temperatures; 125 °Cand 160 °C are the best fit temperatures in Table 1.

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