Chemical and Sr-isotopical evolution of the Phlegraean magmatic system before the Campanian Ignimbrite and the Neapolitan Yellow Tuff eruptions

Journal of Volcanology and Geothermal Research 91 Ž1999. 141–166 www.elsevier.comrlocaterjvolgeores

Chemical and Sr-isotopical evolution of the Phlegraean magmatic system before the Campanian Ignimbrite and the Neapolitan Yellow Tuff eruptions L. Pappalardo

a,)

, L. Civetta a,b, M. D’Antonio b, A. Deino c , M. Di Vito a , G. Orsi a , A. Carandente a , S. de Vita a , R. Isaia a , M. Piochi a a

b

OsserÕatorio VesuÕiano, Via dell’OsserÕatorio, 80056 Ercolano, Napoli, Italy Dipartimento di Geofisica e Vulcanologia, UniÕersity ‘‘Federico II’’ of Napoli, L.go S. Marcellino, 10, 80138 Napoli, Italy c Berkeley Geochronological Center, 2455 Ridge Rd., Berkeley, CA 94709, USA

Abstract New geochronological, geochemical, and Sr-isotopic data on volcanics erupted before the Campanian Ignimbrite ŽCI, 37 ka. and the Neapolitan Yellow Tuff ŽNYT, 12 ka. caldera-forming eruptions at Campi Flegrei ŽCF. have allowed us to investigate the behavior and temporal evolution of the Phlegraean magmatic system. The most prominent feature of the CF magmatic system was the existence of a large, trachytic magma chamber, episodically recharged, which fed eruptions for tens of thousands years before the CI and NYT eruptions. During the pre-CI caldera activity, magmas were episodically erupted from vents located outside the present caldera structure. These magmas ranged in composition from trachyte to alkali-trachyte, with Sr-isotope ratios increasing through time, and becoming identical to that of the CI magma, at about 44 ka ago. This suggests that the Phlegraean magmatic system before the CI eruption was acting as an open system. It was being progressively replenished by new batches of magma that mixed with the resident less radiogenic, fractionating trachytic magmas and was periodically tapped. The magma chamber evolution culminated in the catastrophic eruption of the voluminous Ž150 km3 DRE., chemically and isotopically zoned CI trachytic magmas, and in the resultant CI caldera formation. Subsequent to the CI eruption, during a period of moderate subaereal volcanic activity of about 20 ka duration, magmas predominantly trachytic to alkali-trachytic in composition and isotopically similar to the last emitted CI magma were erupted from vents located inside the CI caldera. The temporal trend shown by Sr-isotope ratios provides evidence for a new input of alkali-trachytic magma, at ca. 15 ka, with 87Srr86 Sr ratio identical to that of the alkali-trachytic magma feeding the first phase of the NYT eruption. These data testify to the arrival in a short time span of a new trachytic to alkali-trachytic magma in the system, isotopically distinct from the CI magma, that gave rise about 3 ka later to eruption of the NYT Ž40 km3 DRE.. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Phlegraean magmatic system; Campanian Ignimbrite; Neapolitan Yellow Tuff; geochronology; Sr isotope; geochemistry

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Corresponding author. Tel.: q0039-81-7777149r150; fax: q0039-81-7390644; E-mail: [email protected]

0377-0273r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 7 - 0 2 7 3 Ž 9 9 . 0 0 0 3 3 - 5

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1. Introduction Climactic caldera-forming eruptions are often preceded by eruptions fed by the same magma chamber Že.g. Halliday et al., 1989; Christensen and De Paolo, 1993.. Highly differentiated magmas, which evolve within the upper parts of magma chambers, are preferentially tapped before such cataclysmic events. Volcanism after a caldera collapse testifies a progressive emptying of the pre-existing magma chamber, or a recharging by new magma which can mix with the possible residue in the chamber. The study of pre- and post-caldera volcanism may therefore provide important information about key topics such as replenishment mechanisms of the system, evolution processes undergone by the magmas while they resided in the system, interaction processes among distinct batches of magma entering the system, time-scale of magma differentiation, and the life span of a large-volume magmatic system. Many petrological and geochronological studies are available in the literature on the products of the two most powerful Phlegraean eruptions, the Campanian Ignimbrite ŽCI; 37 ka; 150 km3 DRE; Barberi et al., 1978; Deino et al., 1992, 1994; Civetta et al., 1997. and the Neapolitan Yellow Tuff ŽNYT; 12 ka; ) 40 km3 DRE; Alessio et al., 1971; Orsi et al., 1992, 1995; Scarpati et al., 1993; Wohletz et al., 1995.. Orsi et al. Ž1995. and Civetta et al. Ž1997. have pointed out substantial geochemical and isotopical differences between the magmas feeding these eruptions. However, few data are available in the literature on Phlegraean products erupted before and between these two climactic eruptions. We present geochronological, major- and traceelemental, and isotopic data for pre-CI and postCIrpre-NYT magmas erupted at the Campi Flegrei ŽCF.. These data help define the chemical evolution of the Phlegraean magmatic system, with particular reference to the time scales of magma replenishment and differentiation processes.

2. Geological outlines The Campi Flegrei caldera ŽCFc; Fig. 1. is a nested, resurgent caldera, partially submerged in the bays of Pozzuoli and Napoli and resulting from two

main collapses related to the CI and the NYT eruptions, respectively ŽOrsi et al., 1992, 1995, 1996.. Many stratigraphical, volcanological, and petrological studies are available in the literature on the products of these two most powerful Phlegraean eruptions. The CI Ž37 ka; Deino et al., 1992, 1994. is the largest pyroclastic deposit in the Phlegraean area ŽDi Girolamo, 1970; Barberi et al., 1978, 1991; Rosi and Sbrana, 1987; Fisher et al., 1993; Orsi et al., 1996; Rosi et al., 1996; Civetta et al., 1997.. Its catastrophic eruption was accompanied by a caldera collapse affecting an area of about 230 km2 , including the CF, the city of Napoli, the bay of Pozzuoli, and the northwestern sector of the Gulf of Napoli ŽOrsi et al., 1996.. The CI deposits covered an area of about 30,000 km2 with an estimated volume of erupted magma of about 150 km3 ŽDRE. ŽFisher et al., 1993; Civetta et al., 1997.. The eruption generated at least three pyroclastic-flow pulses, which were fed by trachytic magmas geochemically and isotopically distinct ŽCivetta et al., 1997.. The flows of each pulse moved in different directions and reached variable distances from the eruptive vent. In proximal areas four units have been recognized ŽRosi et al., 1996. and compositionally correlated to the distal CI deposits ŽCivetta et al., 1997., although their attribution to the CI eruption is debated ŽPerrotta and Scarpati, 1994; Melluso et al., 1995.. The NYT Ž12 ka; Alessio et al., 1971. is the second largest pyroclastic deposit of the Phlegraean area, and represents the largest known trachytic phreatoplinian eruption. It covered an area of about 1000 km2 , including the Pozzuoli and Napoli bays, with a volume of erupted magma of more than 40 km3 ŽDRE. ŽOrsi et al., 1992, 1995; Scarpati et al., 1993; Wohletz et al., 1995.. The eruption occurred in the western part of the CI caldera and was characterized by an early, mostly phreatoplinian phase, followed by alternating phreatomagmatic and magmatic phases ŽOrsi and Scarpati, 1989; Orsi et al., 1992, 1995; Cole and Scarpati, 1993; Scarpati et al., 1993; Wohletz et al., 1995.. A caldera collapse occurred during the course of the eruption and generated a volcano-tectonic depression over an area of about 90 km2 nested inside the CI caldera ŽFig. 1; Orsi et al., 1992, 1996.. At least three geochemically distinct magmas Žalkali-trachyte; trachyte; latite to alkali-trachyte. were involved in the eruption. The

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Fig. 1. Schematic map of the Campi Flegrei region, with sampling locations. Key for locations: PM, Punta Marmolite; CU, Cuma; TL, Trefola; TG, Torregaveta; MS, Montesanto; EC, Monte Echia; VR, Verdolino; TM, Trentaremi; VT, Veterinaria; PG, Parco Grifeo; MT, Monticelli; PR, Ponti Rossi; MM, Masseria del Monte; CO, Coroglio.

last erupted magma had lower Sr-isotope ratios than the other two magmas Ž0.70752 vs 0.70756.. It probably entered the chamber just before the onset of the eruption, perhaps constituting its triggering factor ŽOrsi et al., 1995.. Few data are available in the literature on Phlegraean products erupted before and between these two caldera-forming eruptions, probably because these deposits are rarely exposed, having been largely destroyed during the caldera collapses or buried underneath younger deposits. Orsi et al. Ž1996. report the results of a new field survey on the volcanics

older than the NYT synthesizing their observations with those available in literature. Volcanics older than the CI are exposed only along the scarps bordering the CF depression and are mostly alkali-trachytic in composition. They include the lava domes of Punta Marmolite Ž47 ka; Cassignol and Gillot, 1982. and Cuma Ž37 ka; Cassignol and Gillot, 1982.; the Tufi di Torre Franco pyroclastic deposits Ž) 42 ka; Rittmann, 1950; Alessio et al., 1973.; the remnant of the Monte Grillo tuff-cone; a sequence of pyroclastic deposits separated by paleosols, composed of both proximal and distal units

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exposed at Torregaveta; and a sequence of proximal pyroclastic deposits, intercalated by paleosols and generated by at least 10 eruptions, northeast of the Quarto locality at Trefola quarry ŽFig. 1.. The vents for some of these deposits were located outside the CI caldera, as inferred from sedimentological characteristics ŽOrsi et al., 1996.. Pyroclastic deposits older than the CI have been cored east and north of the city of Napoli. They are variable in number and thickness in the different bore-holes; the maximum number was recognized at Ponti Rossi where 11 pyroclastic deposits are separated by paleosols. Although lack of outcrops makes it difficult to reconstruct the areal distribution of these deposits, their sedimentological characteristics suggest that volcanism before the CI eruption was characterized by high-energy explosive activity ŽOrsi et al., 1996.. Volcanism younger than the CI and older than the NYT eruptions was confined inside the CI caldera ŽOrsi et al., 1996.. The majority of the rocks were produced by explosive, mostly hydromagmatic eruptions. They occur in scattered outcrops across the central part of the city of Napoli, and along the northwestern and southwestern scarps of the Posillipo hill ŽMonte di Procida, Cuma, Punta Marmolite, Trefola, Masseria del Monte, Vallone del Verdolino, Moiariello, Ponti Rossi, Sant’Arpino, Monte Echia, San Martino hill, Villanova, Coroglio, and Trentaremi. ŽOrsi et al., 1996; Fig. 1.. Paleosols interbedded between Tufi Biancastri pyroclastic deposits cropping out at Verdolino and Vallone delle Fontanelle were dated respectively at 16,390 " 180 and 15,090 " 140 years by Alessio et al. Ž1973.. An age of 14,770 " 420 years was reported by Scandone et al. Ž1991. for a paleosol between the Breccia Museo and Torregaveta units.

3. Sampling and analytical procedures This study concerns with rocks younger than NYT exposed in scattered outcrops at Torregaveta, Cuma, Trefola, Punta Marmolite, Monte Echia, Masseria del Monte, Ponti Rossi, along the scarps bordering the Camaldoli ŽVerdolino sections., S. Martino ŽVeterinaria, Parco Grifeo sections. and Posillipo hills ŽTrentaremi, Coroglio sections. and Monticelli.

The location of sampled deposits is shown in Fig. 1. All the stratigraphic sections, except for that of Torregaveta and Monticelli, are described in detail by Orsi et al. Ž1996., that measured the stratigraphic sequences, and reconstructed the geometrical relationships, and made when possible correlation among units. The description of sequences is briefly summarized below. Following Orsi et al. Ž1996. the units have been designated with two capital letters, which refer to the locality, followed by one lower-case letter in alphabetic and stratigraphic order. At TorregaÕeta a lava flow Žunit TGa. is the lowermost deposit of the exposed stratigraphic sequence. It is overlain by a sequence of 12 pyroclastic deposits separated by paleosols underneath the CI unit Žunits from TGb to TGm.. Units TGb and TGc are composed of cross-laminated surge beds. Units TGd–TGe–TGf include distal pumice fallout deposits intercalated by surge beds. Unit TGf 1 , which includes surge and fallout deposits, is known in the literature as the Fiumicello deposit, erupted from a vent located on Procida Island ŽPescatore and Rolandi, 1991.. Units TGg–TGj–TGk–TGl are fallout deposits, whereas units TGh–TGi–TGm are pyroclastic-flow deposits. At Cuma the lowermost unit is composed of a lava dome, overlain by a sequence that includes from the base upward, a fallout deposit ŽCUa unit., the CI and the NYT. At Trefola a thick sequence of pyroclastic units is exposed in a quarry. It includes 12 units below the CI Žunits from TLa to TLm., five units between CI and NYT Žunits from TLo to TLs., and five units above the NYT. At Punta Marmolite a sequence of deposits from older than CI to very young is exposed. The lowermost unit is a lava dome. Interposed between the lava dome and CI is a succession of a coarse fallout deposits with minor ashy layers intercalated by paleosols. This sequence is sedimentologically correlatable with the upper part of the succession underlying the CI at Trefola section. At Ponti Rossi a sequence of pyroclastic deposits from CI up to very young is exposed; nine units Žunits from PRa to PRi. intercalated by paleosols have been recognized between CI and NYT. The part of the sequence not exposed has been drilled, and it is composed by CI and 11 underlying pyroclastic deposits separated by paleosols Žunits from PRl to PRv.. At Monte Echia the exposed deposit is a single depositional unit consti-

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tuted by surge beds with minor fallout deposits. The sequence of the Tufi Biancastri pyroclastic deposits, composed of surge beds and fallout deposits emplaced between the CI and NYT eruptions, is exposed at Masseria del Monte and Verdolino sections Žunits from VRa to VRe.. A lava dome overlain by a sequence of pyroclastic-surge deposits is exposed at Montesanto along a railway tunnel. These pyroclastic rocks are in the same stratigraphic position as the similar sequence of Tufi Antichi exposed along the eastern and southern slopes of the San Martino hill Ž Veterinaria and Parco Grifeo sections.. The remnant of a tuff cone is exposed along the southwestern scarps, at Trentaremi and Coroglio sections. Furthermore, at Trentaremi the cone is underlain by a pyroclastic sequence and in turn by the NYT. At Monticelli the products of a tuff cone are exposed overlain by NYT products. Volcanic rock samples Ž112. from selected units exposed along the described sequences and volcanic rock samples Ž34. from drilled CF deposits older than the NYT have been collected for analysis. Sampled units have been selected in order to: Ž1. be representative of volcanism occurred before CI and NYT events, Ž2. allow geochemical and isotopical correlation among units when these are uncertain or impossible on the basis of stratigraphic data. Thicker and most representative units have been sampled at different stratigraphic heights. The collected samples consist mainly of pumice and scoria fragments from pyroclastic deposits, and subordinately of lavas from lava domes. The pumice fragments for most of the studied sequences are small, therefore, mostly multiple pumice samples were analyzed. Each sample was composed of a number of clasts collected from the same stratigraphic layer, similar in terms of structure of vesicles, glass color and phenocrysts content. When more than one pumice type is present, they were analyzed separately. Generally, the analyzed pumice and lava samples are aphyric. Phenocrysts account for less than 1% by volume and include feldspar, clinopyroxene, black mica, opaques and apatite. All the pumice and lava samples were washed in distilled water, crushed to lapilli-size particles, then ground and homogenized in an agate mortar. Powders were analyzed for major elements and Sc by inductively coupled plasma-atomic emission spec-

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trometry ŽICP-AES., and for the remainder of trace elements by inductively coupled plasma-mass spectrometry ŽICP-MS. at the Centre de Recherches Petrographiques et Geochimiques ŽCRPG, Vandouvre Cedex, France.. Precision is 0.5% for major elements, and variable from 2%–5%, for trace element contents in the range 50–150 ppm, to 2%–10%, for trace element contents in the range 10–50 ppm, to 5%–25%, for trace element contents in the range 0–10 ppm ŽJ. Morel, pers. commun., 1997.. Sr-isotopic compositions of whole-rock samples and separated feldspar phenocrysts were determined at the Dipartimento di Geofisica e Vulcanologia ŽUniversity ‘‘Federico II’’ of Napoli.. The powders were leached with cold 2.5 N HCl for 10 min and with hot 2.5 N HCl for 30 min, then rinsed thoroughly in pure sub-boiling distilled water, and finally dissolved with high purity HF–HNO 3 –HCl mixtures. Sr was extracted by conventional ion exchange chromatographic techniques. Measurements were made using a VG 354 double-collector thermal ionization mass spectrometer running in jumping mode, by normalizing to 86 Srr88 Sr s 0.1194 for mass fractionation effects. The quoted error is the standard deviation of the mean Ž2 sm . and refers to the last digit. Repeated analyses of NBS-987 International Reference Standard yielded a mean value of 0.71024 " 1 Ž N s 50.. The total blank was on the order of 6 ng during the period of measurements. Because of the young ages Žthe oldest 40Arr39Ar dated rock is 60 ka old. and the low RbrSr ratios, all initial Sr isotope ratios are equal to the measured ratios within the analytical uncertainty. 40 Arr39Ar dating has been determined on phenocryst concentrates from pumice fragments of 11 pyroclastic units. The sampled units ranged stratigraphically from just below the NYT to units of unknown age older than the CI. Most phenocryst separates were of pure sanidine; however, two separates were mainly plagioclase with minor sanidine Žsamples 9601 F10 and 9601 C2.. All separates were dated at least once, and five in replicate, by the laser IH method, using a broad Ž6 = 6 mm2 ., uniform-energy-profile CO 2 beam ŽSharp and Deino, 1996; see Deino and Potts, 1990 and Deino et al., 1990, 1998 for additional details of the analytical procedure.. Approximately 40–80 mg sample sizes were used, on material 0.4 mm and larger. In addition, eight of

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Table 1 40 Arr39Ar data on feldspars separated from CF rocks

Notes: ‘Phases’ refers to mineral phases analyzed, where ‘P’ is plagioclase and ‘S’ is sanidine. All uncertainties are "1 s . The integrated age is the combined age of all gas fractions weighted on the basis of the 39Ar abundance, with an uncertainty calculated as the square root of the sum of the squares of the 39Ar-abundance-weighted individual errors. The plateau age is calculated as the inverse-variance weighted mean of the plateau steps. The weighted-mean age of the single-crystal analyses is calculated in an analogous manner using weights equal to the inverse variance of the analytical uncertainties. The stated uncertainties for the plateau age and weighted-mean single-crystal age is one standard error of the weighted mean, calculated as the maximum formulations of Taylor Ž1982. and Samson and Alexander Ž1987.. ‘Ž40Arr39Ar. tr ’ is the ‘trapped’ 40Arr39Ar component from the isochron analysis. ‘MSWD’ is Mean Sum of Weighted Deviates. All stated errors in age include uncertainty in the neutron fluence parameter, J. J s 1.669 = 10y5 " 1 = 10y7 . Isotopic interference corrections: Ž36Arr37Ar. Ca s 2.64 = 10y4 " 1.7 = 10y6 , Ž39Arr37Ar. Ca s 6.73 = 10y4 " 3.7 = 10y6 , Ž40Arr39Ar. K s 7 = 10y4 " 3 = 10y4 , l s 5.543 = 10y1 0 yry1 .

the samples were dated by the single-crystal, totalfusion ŽSCTF. method. This latter heating method also employed a CO 2 laser, but focused to a sub-millimeter diameter beam. Samples were irradiated in the Cd-lined CLICIT facility of the University of

Oregon TRIGA reactor for 3 min in two batches. Sanidine from the rhyolite of Alder Creek at Cobb Mountain, California ŽTurrin et al., 1994., with a reference age of 1.194 Ma ŽRenne et al., 1998. was used as the monitor mineral to calibrate the neutron

Fig. 2. Incremental-heating apparent age spectra. Apparent-age uncertainties of the individual steps are shown at 2 s , whereas uncertainties in the plateau age and integrated age are shown at 1 s . Due to space limitations, a replicate experiment on sample 9601 F10 is not shown Žsee Table 1 for summary results..

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Fig. 2 Žcontinued..

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flux. Table 1 contains a summary of analytical data for the 40Arr39Ar analyses.

4. Results 4.1. Geochronology Fig. 2 shows the results of the incremental heating ŽIH. experiments as cumulative % 39Ar release spectra. All sanidine experiments exhibited a plateau of stable apparent age across most of the 39Ar release Žfor a definition of ‘plateau’ as used here see Fleck et al., 1977., and with one exception Ž9601 M1, Lab IDa 20873., plateau ages are indistinguishable from integrated ages Žan integrated age is the age generated when all steps are mathematically recombined to simulate a total-fusion experiment.. In addition, all sanidine experiments gave stable CarK ratios across almost the entire spectra, apart from anomalies in the first and last 5% or so of gas release. Radiogenic 40Ar contents Ž% 40ArU . are in the 50%– 90% range in the plateau regions. Inverse isochron analyses Ž36Arr40Ar vs. 39Arr40Ar, corrected for decay, mass spectrometer discrimination, and isotopic interferences. of the plateau regions Žcombined plateaus in the case of replicate IH analyses. yield ages that are all in agreement with the plateau ages. The ‘trapped’ 40Arr36Ar component obtained from the inverse isochron analysis yielded values that were all within error of the expected atmospheric composition of 295.5 ŽSteiger and Jager, 1977., sug¨ gesting that excess Ar is not present in these samples in detectable amounts. The generally excellent 40 Arr39Ar release characteristics and analytical parameters of these samples suggest that they are unaltered, homogeneous phases that should yield accurate ages. The best age for these samples amongst the three ages obtained for the IH experiments Žintegrated age, plateau age, and isochron age. is taken to be the isochron age, since this computational procedure inherently accounts for deviations from ideal atmospheric composition in correcting for ‘trapped’ 40Arr36Ar components. In contrast to the sanidine IH experiments, the plagioclase-dominant separate 9601 F10 failed to form a plateau in two experiments, and exhibited an MSWD for the inverse-isochron analysis that indi-

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cated excess scatter far beyond that expected from analytical measurements alone Ži.e., the excess scatter was due to geological effects.. The integrated age for this sample is also 20–30 ka, too old relative to a high-quality sanidine sample analyzed from the base of the section Ž9601 A1.. Sample 9601 C2, a plagioclase-dominant separate from a tuff lying stratigraphically between 9601 F10 and 9601 A1 in the same section, also yielded an age that is about 30 ka, too old relative to 9601 A1. Although this sample yielded a plateau and in all respects apart from CarK content appears to be a high-quality sample, the accuracy of the result must be questioned. Although the sample forms an isochron with an acceptable MSWD, the uncertainty of the ‘trapped’ component is so high ŽŽ40Arr36Ar. trapped s 358 " 56. that it masks whether a significant excess Ar component is present or not. We postulate that the plagioclase in these two samples may bear a significant quantity of excess Ar that is responsible for the too-old ages relative to the sanidine sample at the base of the section, and that the results from 9601 F10 and 9601 C2 should be ignored. Fig. 3 shows age–probability density spectra for the single-crystal total fusion ŽSCTF. analyses, with auxiliary plots showing related analytical information. A principal motivation for pursuing the singlecrystal approach is to examine the grain-to-grain reproducibility of the samples. Homogeneity is verified by the generally near-Gaussian shape of the age–probability density spectra in which only a single well-defined mode is present. Weighted mean ages of the SCTF experiments agree within analytical error with the IH ages. Uncertainties in the weighted mean SCTF ages are typically much greater than the corresponding uncertainties for the IH experiments, reflecting the inherent advantages of expelling adsorbed atmospheric Ar in the early phases of an IH experiment Žthus requiring a smaller correction for atmospheric Ar., and increased measurement precision derived from greater gas yields during IH of multi-grain samples. Ages derived from inverse isochron analyses of the SCTF data are generally in agreement with the conventionally calculated SCTF ages, although two samples yielded statistically significant differences Ž9601 M1 and 9602 I1.. As with the IH results, the best ages for the SCTF samples is taken to be those derived from the isochron analysis.

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A comparison of the isochron ages derived from the IH and SCTF experiments shows no discernable pattern of age bias attributable to dating technique;

four are younger by the SCTF method and four are older. For this sample suite, indications are that the bulk IH samples are uncontaminated by outliers Žsuch

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as xenocrysts. that detectably influence the age. Accepting that geological outliers are not present, the IH results are preferred to the SCTF results. This is because in IH dating controlled, progressive outgassing of the sample permits anomalous components, typically in the early and final steps of the experiment, to be identified and excluded, an advantage not attainable with the SCTF results. Comparison to previously dated units and relationship to marker horizons: The CI serves as a stratigraphic marker in all four sections studied. We have previously dated this widespread tuff using the 40 Arr39Ar technique by the SCTF ŽDeino et al., 1992. and resistance furnace IH methods ŽDeino et al., 1994., with an overall mean age of 37.1 " 0.4 ka. Units dated in this study immediately overlying the CI in the Trefola quarry, Ponti Rossi, and Verdolino Valley of 17.9 " 0.5 ka Ž9601 O1., 16.1 " 0.2 ka Ž9602 A1., and 30.3 " 0.2 ka Ž9603 A1., respectively, are completely resolved chronologically from that of the CI, and document a hiatus of variable duration Ž7–21 ka. in this interval. Units directly underlying or very nearly directly underlying the CI in the Trefola quarry and Ponti Rossi give remarkably similar ages of 45.6 " 0.7 ka Ž9601 M1. and 44.3 " 0.7 ka ŽME 28., indicating a hiatus of about 7–9 ka at both localities. 4.2. Geochemistry Chemical and isotopical analyses, CIPW normative nepheline and D.I. values Žnormative Or q Ab q Ne. of selected samples representative of the base and top of each recognized unit are listed in Tables 2–4. No significant chemical variations have been recognized between samples collected from the same unit. The complete set of analyses is available on request. Loss on ignition ŽL.O.I.. contents are variable from 1 to 7 wt.%; however, most samples have

151

L.O.I. contents ranging from 1 to 3 wt.%. Those with higher contents, from 3 to 7%, do not show any particular evidence for alteration in normative or Sr-isotopic compositions and are considered usable in this study. D.I. values range from 67 to 92, assuming an Fe 2 O 3rFeO ratio of 0.5 in the CIPW norm calculations, which is the mean value of Phlegraean volcanics for which ferrous Fe was determined by KMnO4 titration ŽRosi and Sbrana, 1987.. The analyzed pumice and lava samples older than NYT have Sr-isotope ratios ranging from 0.70649 to 0.70756 ŽTables 2–4.; a similar range Ž0.70650– 0.70756. was reported by Cortini and Hermes Ž1981. for ejecta in the NYT formation; a higher value Ž0.70769. was reported by these authors for a sample from the Cuma lava dome. This can be explained by the lack of a leaching procedure in their work, necessary considering the low Sr content Žlower than 10 ppm. of this lava sample. The chemical data are plotted in the diagrams of Figs. 4–11. Chemostratigraphy of the four main representative studied sections is reported in Fig. 5. 4.3. Pre-CI pyroclastic deposits and laÕa domes Most of the analyzed volcanic rocks plot in the trachyte and phonolite fields of the total alkalirsilica diagram ŽTAS, Fig. 4A; Le Bas et al., 1986.; only one plot in the tephri-phonolite field. All samples have a potassic alkaline affinity, with Na 2 O y 2 F K 2 O. In the D.I.–Ne diagram ŽFig. 4B; Armienti et al., 1983., commonly used for the Campanian potassic rocks, the studied samples range in composition from latite to phono-trachyte, through trachyte and alkali-trachyte. Alkali-trachytic and phono-trachytic samples, although having similar D.I. values, show different degree of evolution, with the phonotrachytes more enriched in incompatible trace elements. This characteristic is generally observed in CF rocks Žsee also D’Antonio et al., 1999..

Fig. 3. Age–probability density spectrum for the single-crystal analyses, with compositional parameters Žmoles 39Ar, % 40ArU , CarK, and values of individual analyses with 1 s analytical uncertainties in rank order.. Open symbols indicate analyses that fell more than two standard deviations from the overall weighted-mean age, and on this basis were culled from the data set. The age–probability density spectrum that includes all samples is shown by the dashed line; the spectrum that excludes the culled samples is indicated by the solid line. The modal value Žin ka. of the age–probability density curve is also indicated at the top of the peak; the weighted-mean age exclusive of culled analyses, with 1 S.E.M., is given near the bottom axis.

10.91 87.54

Ne D.I.

14.53 87.96

59.23 0.41 18.66 3.67 0.24 0.26 1.69 7.42 6.60 0.06 1.51 99.75 6.28 91.27

60.46 0.49 18.38 2.90 0.16 0.43 1.23 6.37 6.74 0.08 2.43 99.67 16.12 87.16

55.58 0.40 18.98 4.15 0.26 0.35 2.01 6.54 6.82 0.07 4.01 99.17 18.19 88.07

57.02 0.41 19.53 4.22 0.26 0.35 2.01 7.05 7.00 0.07 1.72 99.64

19 3 17 1 137 25 346 24 52 582 106 11 126 226 25.2 87 15.7 1.6 11.2 1.7 10.5 2.1 5.3 0.8 5.0 0.9 51 –

9.48 90.02

58.76 0.40 17.97 3.39 0.22 0.34 1.72 6.33 6.84 0.11 3.65 99.73

9536C b TL CI

11.18 89.02

58.70 0.39 17.93 3.37 0.22 0.35 1.75 6.27 7.35 0.09 3.35 99.77

9535C b TL CI

0.14 83.07

60.09 0.43 18.95 3.83 0.15 0.65 2.37 4.36 7.59 0.10 1.12 99.64

9559 Ac TG a

5.19 67.41

53.26 0.66 18.29 6.80 0.15 1.84 5.11 3.72 6.35 0.37 2.94 99.49

9559 B1c TG b

8.17 91.93

58.68 0.54 17.84 3.06 0.22 0.37 1.08 6.65 6.30 0.05 4.97 99.76

8.00 91.87

58.64 0.55 17.82 3.06 0.22 0.37 1.08 6.61 6.31 0.05 4.99 99.70

15 3 25 1 100 25 348 8 62 559 89 8 131 249 28.1 97 17.4 1.1 13.8 2.0 11.3 2.3 5.8 0.9 6.1 1.0 43 –

7.85 92.22

58.83 0.56 17.82 3.00 0.23 0.34 1.02 6.60 6.38 0.04 4.81 99.63

14 3 25 1 97 24 341 9 62 559 88 13 130 248 27.7 98 17.1 1.1 13.6 2.0 11.2 2.3 5.8 0.9 6.0 0.9 42 –

8.47 92.06

58.71 0.56 17.83 3.02 0.23 0.33 1.04 6.72 6.31 0.05 5.03 99.83

9559 E2 c 9559 E4 c 9559 F5 c 9559 F6 c TG TG TG TG e base e top f base f top

16 16 19 10 8 13 14 3 3 3 4 7 3 3 15 16 17 47 125 26 29 1 2 1 1 10 1 3 121 125 126 97 85 89 98 24 25 24 19 18 22 24 372 371 369 165 193 319 340 19 20 25 383 862 18 19 52 53 54 34 31 55 61 589 583 583 356 273 504 538 99 101 99 56 44 78 85 12 18 24 420 1199 23 24 123 120 125 88 72 118 124 230 225 232 159 137 224 239 25.0 25.0 24.9 16.9 14.8 24.6 27.2 86 88 90 59 54 85 93 15.5 15.0 15.5 10.6 9.9 15.1 16.7 1.5 1.5 1.5 2.2 2.3 1.1 1.2 11.4 11.4 12.6 8.3 7.9 12.1 12.8 1.8 1.8 1.8 1.2 1.1 1.8 1.9 9.9 9.9 9.6 6.4 5.9 10.1 10.4 2.1 2.0 2.2 1.3 1.2 2.2 2.2 5.4 5.0 5.5 3.2 2.9 5.5 5.5 0.8 0.8 0.8 0.5 0.4 0.8 0.9 5.4 5.5 5.9 3.3 2.8 5.7 5.6 0.9 0.8 1.0 0.5 0.4 0.9 0.9 53 53 57 32 23 40 40 0.70746 0.70745 0.70746 0.70701 0.70700 0.70680 –

12.85 10.81 89.74 89.45

57.59 58.24 0.41 0.39 18.30 17.79 3.64 3.33 0.27 0.21 0.32 0.34 1.72 1.72 6.80 6.31 6.51 7.06 0.10 0.09 4.07 4.16 99.73 99.64

9545a 9537 b TL TL m top fall CI

21 23 3 3 12 13 1 1 147 142 27 27 397 389 16 17 62 63 694 684 125 128 12 13 144 154 263 279 30.0 28.6 105 103 18.6 18.0 1.4 1.4 13.4 14.5 2.1 2.1 12.4 11.3 2.4 2.5 6.4 6.5 1.0 1.0 6.2 6.5 1.1 1.1 64 67 0.70735 –

11.21 12.34 84.48 89.86

56.12 57.58 0.43 0.41 17.90 18.10 3.91 3.60 0.23 0.25 0.83 0.31 2.65 1.69 5.79 6.73 6.73 6.52 0.13 0.09 5.01 4.43 99.73 99.71

18 19 5 4 31 30 3 3 128 131 24 25 336 344 65 73 51 53 572 592 100 106 79 88 121 124 220 234 24.5 25.9 84 86 15.0 15.4 1.6 1.5 11.4 11.7 1.7 1.8 10.0 10.6 1.9 2.0 5.3 5.3 0.8 0.9 4.9 5.2 0.9 0.9 50 53 U 0.70735 –

14.01 11.58 10.29 88.37 88.29 83.26

57.70 56.92 56.05 0.39 0.40 0.42 18.64 18.22 17.73 3.80 3.66 3.97 0.21 0.23 0.23 0.39 0.36 0.98 2.12 1.93 2.86 6.09 6.08 5.58 7.72 7.01 6.70 0.12 0.11 0.13 2.56 4.77 5.11 99.74 99.69 99.76

16 16 3 3 19 22 1 1 101 120 22 24 310 334 29 26 45 46 511 518 87 96 12 10 108 115 198 212 21.0 22.6 72 78 12.7 14.2 1.6 1.9 9.2 10.3 1.5 1.5 7.7 9.3 1.7 1.8 4.2 4.8 0.6 0.7 4.7 4.5 0.6 0.8 43 45 0.70704 –

11.05 86.86

56.69 0.39 18.83 3.80 0.20 0.39 1.94 5.69 7.33 0.06 4.37 99.69

9547 a 9546 a TL TL i m base

b

Pre-CI. CI. L.O.I.s Loss on ignition. Normative Ne and D.I. Žs Ab q Or q Ne . are calculated assuming Fe 2 O 3 rFeO s 0.5. Sr isotopic ratio with U is determined on separated feldspar, the remainder are whole-rock analyses. The internal error on all Sr isotopic ratios is "0.00001. Key for locations: PM, Punta Marmolite; CU, Cuma; TL, Trefola; TG, Torregaveta.

a

Be 16 25 8 24 22 Sc 3 3 3 3 3 V 18 8 33 15 13 Co 1 1 2 2 2 Zn 118 146 73 129 123 Ga 24 28 20 26 25 Rb 365 476 289 356 349 Sr 26 5 23 22 30 Y 49 72 38 62 60 Zr 560 840 314 681 693 Nb 93 139 49 123 120 Ba 7 2 22 22 28 La 115 161 82 141 138 Ce 220 301 157 256 253 Pr 22.5 31.8 17.0 27.5 27.1 Nd 80 105 63 94 93 Sm 13.6 16.0 11.4 16.1 16.1 Eu 1.8 1.3 1.3 1.5 1.6 Gd 10.7 15.2 9.2 12.7 12.1 Tb 1.6 2.3 1.3 2.0 1.9 Dy 8.6 12.7 7.1 11.0 9.8 Ho 1.8 2.6 1.4 2.2 2.2 Er 4.7 7.0 3.3 5.4 5.7 Tm 0.7 1.1 0.5 1.0 0.9 Yb 4.8 7.7 3.4 6.1 6.1 Lu 0.8 1.2 0.5 0.9 0.9 Th 46 72 21 65 60 87 86 Srr Sr 0.70705 0.70730 0.70685 0.70715 –

58.63 0.40 18.89 3.84 0.19 0.39 2.08 5.69 7.87 0.09 1.70 99.77

SiO 2 TiO 2 Al 2 O 3 Fe 2 O 3 tot MnO MgO CaO Na 2 O K2O P2 O 5 L.O.I. Sum

Sample: 947 a 948 a 9601 A1a 9601 C1a 9601 C24 a 9601 F1a 9550 a 9549 a 9548 a Location: PM CU TL TL TL TL TL TL TL Unit: lava dome lava dome a c base c top f base f top g h

Table 2 Major oxide Žwt.% . and trace element Žppm . contents, and Sr-isotopic composition of selected Campi Flegrei volcanics older than NYT

152 L. Pappalardo et al.r Journal of Volcanology and Geothermal Research 91 (1999) 141–166

3.88 41.44

Ne D.I.

8.84 35.68

45.98 1.12 17.17 8.87 0.12 4.53 11.94 2.97 2.72 0.61 3.53 99.56

9559 F11a TG Fiumicello

c

a

Pre-CI. Post-CIrPre-NYT.

9559 H1a TG h base

9559 Ia TG i

9559 K1a TG k

9559 L1a TG l

18 3 23 1 105 23 300 48 47 508 88 16 108 203 21.8 75 13.0 1.6 9.3 1.5 8.2 1.7 4.4 0.7 4.4 0.7 47 –

12.96 89.45

57.18 0.41 18.31 3.58 0.25 0.45 1.66 6.66 6.59 0.05 4.62 99.76

9559 Ma TG m

18.53 87.64

57.00 0.39 19.09 4.23 0.25 0.33 2.17 6.99 7.16 0.09 2.04 99.74 0.00 86.35

58.74 0.36 17.38 3.61 0.11 0.62 1.09 3.80 7.59 0.15 6.28 99.73

MS tuff

MS lava dome

5.44 80.31

57.29 0.40 17.73 4.03 0.11 1.13 3.48 4.06 8.00 0.17 3.21 99.61

EC

MT29 c 9419 c

ML6 c

3.25 80.85

56.09 0.37 16.86 3.11 0.10 0.53 3.96 4.14 7.11 0.10 7.40 99.77

VR b base

946 c

2.72 85.80

58.09 0.37 17.25 2.97 0.10 0.37 2.64 4.53 7.48 0.08 5.90 99.78

VR b top

945 c

1.31 85.27

58.08 0.40 17.41 3.25 0.11 0.45 2.40 4.48 7.29 0.10 5.80 99.77

VR d base

943 c

1.89 85.22

57.88 0.41 17.48 3.37 0.10 0.50 2.35 4.34 7.58 0.10 5.64 99.75

VR d top

944 c

2.50 76.79

57.92 0.49 18.41 4.63 0.08 1.19 3.96 3.23 8.51 0.25 0.92 99.59

TM base

9411c

3.12 82.49

57.35 0.40 17.65 3.75 0.10 0.65 2.96 3.82 8.18 0.14 4.62 99.62

TM top

9413 c

5.21 87.95

58.90 0.38 17.59 3.13 0.10 0.43 2.17 4.41 8.63 0.09 3.95 99.78

CR base

9416 c

4.08 87.62

59.15 0.39 17.65 3.14 0.10 0.41 2.17 4.49 8.32 0.09 3.87 99.78

CR top

9418 c

20 25 22 8 7 8 9 10 10 7 9 10 9 3 3 3 5 5 4 3 3 3 6 4 3 3 18 15 12 46 65 52 43 50 56 95 66 45 49 1 1 1 3 4 2 1 1 2 6 3 1 1 110 123 143 76 91 67 70 74 76 58 67 70 75 24 26 27 18 19 18 19 19 19 19 18 18 19 322 376 424 277 251 291 311 322 316 232 288 300 316 22 20 15 267 683 299 166 228 272 700 538 205 185 53 63 66 28 32 30 33 34 33 24 29 31 34 571 678 782 300 313 327 358 361 346 249 330 343 378 100 120 133 40 45 44 51 52 48 32 44 48 52 14 13 2 163 1034 190 47 143 255 1277 1040 118 72 121 142 153 64 82 70 81 84 80 55 68 77 84 222 255 284 120 151 136 159 162 154 107 131 151 157 24.2 27.2 30.2 11.8 15.7 14.5 16.6 16.7 16.2 11.6 13.6 16.1 16.3 84 94 104 45 53 52 59 61 61 43 48 54 57 14.9 16.1 18.0 8.5 9.4 9.1 10.3 11.0 10.5 8.6 8.9 9.7 10.3 1.4 1.3 1.7 1.9 2.5 2.0 2.0 2.1 2.1 2.3 2.1 2.1 2.1 10.6 13.0 13.9 6.6 8.0 7.3 8.0 8.5 8.1 6.4 7.0 7.7 8.0 1.7 2.0 2.1 1.0 1.1 1.0 1.2 1.2 1.2 0.9 1.0 1.1 1.1 9.1 11.1 11.8 4.8 5.6 5.4 6.4 6.4 6.1 4.9 5.4 5.9 5.8 1.9 2.2 2.4 1.0 1.2 1.1 1.3 1.3 1.3 0.9 1.0 1.2 1.3 5.0 5.7 6.2 2.6 2.9 3.0 3.1 3.3 3.2 2.2 2.7 2.9 3.2 0.8 0.9 1.0 0.4 0.4 0.4 0.5 0.5 0.5 0.3 0.4 0.5 0.5 5.0 6.3 7.0 2.8 2.8 3.0 3.2 3.3 3.2 2.2 3.0 3.3 3.1 0.8 0.9 1.1 0.4 0.4 0.4 0.5 0.5 0.5 0.4 0.4 0.5 0.5 54 62 66 26 25 31 34 35 35 23 31 35 35 0.70735 0.70730 0.70730 – 0.70678 0.70735 0.70730 0.70748 0.70754 0.70738 0.70728 0.70734 0.70736

1.11 8.75 7.46 87.21 85.41 87.20

58.34 55.80 56.53 0.45 0.40 0.42 18.38 18.59 18.17 3.03 3.85 3.65 0.14 0.22 0.23 0.50 0.44 0.43 1.48 1.99 1.68 5.06 5.31 5.56 6.78 7.08 6.80 0.07 0.07 0.06 5.48 5.82 5.63 99.71 99.57 99.16

9559 J2 a TG j

24 8 3 3 17 42 2 2 124 60 25 19 335 241 34 59 60 34 658 315 117 46 31 56 135 72 252 143 27.4 15.4 94 56 16.0 10.0 1.5 1.4 11.8 6.9 1.8 1.1 10.2 6.0 2.2 1.3 5.6 3.2 0.9 0.5 6.0 3.1 1.0 0.5 62 22 0.70682 –

7.54 13.93 91.90 86.92

59.55 55.45 0.54 0.40 17.98 18.76 2.92 4.06 0.19 0.25 0.41 0.35 1.16 1.92 6.80 6.26 6.16 6.73 0.07 0.05 3.43 5.41 99.21 99.64

9559 H2sp a TG h top

16 11 3 3 27 32 1 1 102 79 24 22 349 293 26 17 61 51 576 415 91 65 28 15 132 100 249 200 27.5 22.8 94 85 16.2 15.6 1.1 1.3 13.1 10.7 1.9 1.7 10.7 9.0 2.3 1.9 5.8 4.8 0.9 0.7 6.2 4.6 1.0 0.7 45 31 0.70671 –

9.03 9.00 91.94 91.52

58.69 58.54 0.55 0.54 17.91 17.83 3.02 3.14 0.25 0.24 0.31 0.41 0.98 1.10 6.71 6.67 6.48 6.41 0.04 0.05 4.98 4.84 99.92 99.77

9559 Ga TG g

Be 4 4 15 Sc 17 17 3 V 227 232 24 Co 28 28 1 Zn 69 67 100 Ga 17 17 25 Rb 131 98 352 Sr 880 867 14 Y 25 25 63 Zr 130 129 586 Nb 17 16 92 Ba 1167 1093 9 La 37 36 134 Ce 75 74 251 Pr 9.1 9.1 27.9 Nd 38 38 97 Sm 8.2 7.8 16.7 Eu 2.3 2.3 1.0 Gd 7.1 7.0 13.8 Tb 1.0 0.9 2.0 Dy 4.8 4.8 11.4 Ho 1.0 0.9 2.3 Er 2.1 2.2 5.8 Tm 0.3 0.3 0.9 Yb 1.9 1.9 6.4 Lu 0.3 0.3 1.0 Th 8 8 46 87 Srr 86 Sr 0.70649 0.70654 –

48.01 1.15 17.95 9.10 0.12 4.66 9.68 2.80 3.31 0.53 2.24 99.55

SiO 2 TiO 2 Al 2 O 3 Fe 2 O 3 tot MnO MgO CaO Na 2 O K2O P2 O 5 L.O.I. Sum

Sample: 9559 F8 a Location:TG Unit: Fiumicello

Table 3 Major oxide Žwt.% . and trace element Žppm . contents, and Sr-isotopic composition of selected Campi Flegrei volcanics older than NYT L.O.I.s Loss on ignition. Normative Ne and D.I. Žs Ab q Or q Ne . are calculated assuming Fe 2 O 3 rFeO s 0.5. Sr isotopic ratio is determined on whole-rock samples. The internal error on all Sr isotopic ratios is 0.00001. Key for locations: TG, Torregaveta; MS, Montesanto; EC, Monte Echia; VR, Verdolino; TM, Trentaremi; CR, Coroglio.

L. Pappalardo et al.r Journal of Volcanology and Geothermal Research 91 (1999) 141–166 153

c

18.69 87.01

55.91 0.39 18.65 4.09 0.26 0.38 2.18 6.67 7.37 0.12 3.70 99.72

9530 c PG

0.30 78.73

57.24 0.43 17.35 4.53 0.10 1.00 3.48 3.31 8.01 0.24 3.87 99.56

9533 c MT base

0.01 78.64

58.47 0.44 17.74 4.60 0.10 1.04 3.54 3.26 8.30 0.24 1.87 99.60

9531 sp1c MT top

2.22 91.66

60.58 0.39 16.98 2.71 0.17 0.22 1.51 5.78 6.76 0.07 4.58 99.75

9544 c TL o base

Post-CIrPre-NYT.

3.80 89.34

59.61 0.40 17.47 2.89 0.14 0.30 1.90 5.06 7.69 0.09 4.23 99.78

9540 c TL s base

3.78 89.28

59.48 0.40 17.43 2.90 0.14 0.31 1.90 4.98 7.77 0.09 4.37 99.77

9539 c TL s top

1.05 86.12

59.32 0.38 18.22 3.18 0.13 0.40 2.06 4.26 8.01 0.08 3.82 99.86 0.81 86.67

59.19 0.37 18.22 3.17 0.12 0.35 1.86 4.38 7.82 0.05 4.18 99.71 1.27 87.08

59.36 0.38 18.18 3.11 0.13 0.30 1.88 4.50 7.80 0.05 3.97 99.66 1.20 86.29

59.05 0.38 18.11 3.23 0.12 0.39 2.00 4.32 7.90 0.06 4.11 99.67

8 6 99 5 70 18 259 904 26 265 34 1874 62 119 12.8 48 8.5 2.3 6.2 0.9 4.8 1.0 2.3 0.3 2.4 0.3 23 –

1.90 78.35

57.08 0.47 18.41 4.74 0.12 0.98 3.25 3.29 8.36 0.20 2.99 99.89 2.03 88.71

59.82 0.40 18.03 2.89 0.14 0.23 1.71 5.02 7.43 0.04 3.99 99.70

11 14 3 3 54 35 2 1 74 77 19 21 295 317 266 59 34 42 372 473 53 67 169 8 82 99 154 189 16.5 20.0 60 72 10.1 12.4 2.1 1.9 7.9 9.0 1.1 1.3 5.9 7.5 1.2 1.5 3.1 3.9 0.5 0.6 3.3 4.1 0.5 0.6 36 47 0.70748 –

1.72 85.77

58.73 0.40 18.25 3.41 0.13 0.42 2.02 4.32 7.90 0.07 4.07 99.72

14 3 36 1 77 20 316 63 42 465 67 8 99 188 19.5 70 11.9 1.8 9.1 1.4 7.4 1.5 3.8 0.6 3.9 0.6 47 –

2.14 88.67

59.70 0.40 17.97 2.89 0.14 0.25 1.72 5.02 7.41 0.03 4.18 99.71

2.73 85.97

58.58 0.40 18.18 3.33 0.12 0.41 2.11 4.32 8.06 0.07 4.10 99.68

11 11 3 3 53 51 2 1 74 71 19 19 289 286 281 261 33 33 356 356 50 49 146 101 81 80 150 150 15.9 16.0 56 57 9.8 9.8 2.1 2.1 7.5 7.5 1.1 1.1 5.7 5.7 1.2 1.2 3.0 3.1 0.5 0.5 3.1 3.0 0.5 0.5 33 34 0.70755 0.70756

2.06 85.64

58.77 0.40 18.18 3.37 0.13 0.43 2.14 4.30 7.97 0.07 3.93 99.69

9602 A1c 9602 C1c 9602 D1c 9602 D2c c 9602 D2 c 9602 E1c 9602 F1c 9602 F3 c 9602 H1c 9602 I1c PR PR PR PR PR PR PR PR PR PR a c d d white d black e f base f top h i

10 13 13 9 10 11 10 3 3 3 3 3 3 3 55 39 39 48 46 44 52 2 1 1 2 1 1 1 80 86 87 64 67 66 74 19 21 20 17 18 18 19 305 344 335 251 270 274 293 252 61 58 263 202 183 246 32 42 42 29 32 33 34 361 471 475 306 344 360 360 53 71 71 41 48 51 52 170 9 11 167 82 69 118 83 103 100 69 77 83 80 156 195 189 128 144 153 153 16.6 20.4 21.7 13.9 15.3 16.4 15.7 57 72 75 52 55 59 57 10.4 13.1 13.4 8.9 9.6 10.1 10.1 2.4 2.0 1.9 1.9 1.8 1.9 2.0 7.9 10.1 9.8 6.6 7.1 7.6 7.4 1.1 1.4 1.4 1.0 1.0 1.1 1.1 6.2 7.6 8.0 5.3 5.7 6.1 5.6 1.2 1.6 1.6 1.1 1.2 1.2 1.2 3.4 4.2 4.0 2.6 2.8 2.9 3.1 0.5 0.7 0.6 0.4 0.4 0.4 0.4 3.0 4.3 4.2 2.9 3.1 3.2 3.1 0.5 0.6 0.6 0.4 0.5 0.5 0.5 35 49 50 31 34 36 34 0.70749 0.70745 0.70750 0.70736 0.70734 0.70738 –

2.10 3.59 91.37 86.74

60.59 58.71 0.40 0.40 17.04 17.60 2.69 3.39 0.17 0.12 0.24 0.45 1.55 2.22 5.72 4.38 6.80 8.22 0.08 0.12 4.48 4.11 99.76 99.72

9542 c 9541c TL TL o top r

20 22 7 7 21 21 3 3 5 5 2 2 12 15 93 89 23 24 1 2 5 6 1 1 1126 180 74 74 91 93 25 27 19 18 23 23 385 358 258 262 411 401 14 18 628 621 18 23 61 61 24 25 51 52 661 692 257 260 660 654 114 125 34 33 99 98 9 25 987 1036 7 8 137 140 53 53 135 134 271 260 101 98 244 244 27.7 29.2 10.6 11.0 24.7 25.2 94 97 42 40 83 84 16.6 17.4 7.9 7.7 14.5 14.5 1.5 1.6 1.9 2.0 1.4 1.5 13.8 13.1 5.9 5.6 10.5 11.4 2.1 2.1 0.9 0.9 1.6 1.7 11.0 11.3 4.6 4.8 9.0 9.1 2.1 2.3 1.0 1.0 1.9 1.9 5.9 6.2 2.4 2.3 5.4 5.4 0.9 0.9 0.3 0.3 0.8 0.9 6.4 6.6 2.4 2.4 5.6 5.6 1.0 1.1 0.4 0.4 1.0 0.9 63 62 22 21 77 77 0.70755 0.70720 0.70747 0.70747 0.70738 –

11.65 89.49

Ne D.I.

Be Sc V Co Zn Ga Rb Sr Y Zr Nb Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Th 87 Srr 86 Sr

57.77 0.40 17.90 3.43 0.24 0.31 1.80 6.59 6.63 0.11 4.54 99.72

SiO 2 TiO 2 Al 2 O 3 Fe 2 O 3 tot MnO MgO CaO Na 2 O K2O P2 O 5 L.O.I. Sum

Sample: 9529 bis c Location: VT Unit:

Table 4 Major oxide Žwt.% . and trace element Žppm . contents, and Sr-isotopic composition of selected Campi Flegrei volcanics older than NYT L.O.I.s Loss on ignition. Normative Ne and D.I. Žs Ab q Or q Ne . are calculated assuming Fe 2 O 3 rFeO s 0.5. Sr isotopic ratio is determined on whole-rock samples. The internal error on all Sr isotopic ratios is 0.00001. Key for locations: TL, Trefola; VT, Veterinaria; PG, Parco Grifeo; MT, Monticelli; PR, Ponti Rossi.

154 L. Pappalardo et al.r Journal of Volcanology and Geothermal Research 91 (1999) 141–166

L. Pappalardo et al.r Journal of Volcanology and Geothermal Research 91 (1999) 141–166

155

Fig. 4. Classification diagrams. ŽA. Total alkalirsilica classification grid ŽTAS; Le Bas et al., 1986.; ŽB. normative nepheline versus Differentiation Index Žs normative Or q Ab q Ne. classification grid ŽArmienti et al., 1983.. Legend: open triangles, CF pre-CI rocks; solid triangles, samples from volcanic deposits attributed to Ischia Island erupted before 37 ka, see text for further explanations; squares, CF post-CIrpre-NYT rocks.

Two samples collected at the Torregaveta section from deposits emplaced during the Fiumicello eruption ŽProcida Island. are trachybasaltic in composition and have been plotted only in the chemostratigraphic diagrams ŽFig. 5.. The latitic sample ŽD.I.s 67, Zr s 273 ppm. was collected from the pyroclastic deposit Žunit TRb. at the base of the stratigraphic sequence of the Torregaveta section ŽFig. 5.. The lowermost pyroclastic deposits of the stratigraphic sequence drilled at Ponti Rossi section Žunits from PRa to PRg. and the pyroclastic deposit at the base of Trefola section Žunit TLa. are trachytes ŽD.I.s 76–80; Zr s 300–350 ppm.. Alkali-trachytes ŽD.I.s 84–92; Zr s 300–700 ppm. characterize the pyroclastic deposits of the upper part of the stratigraphic sequence drilled at Ponti Rossi Žunits from PRi to PRm., and five pyroclastic deposits exposed in the

middle part of the stratigraphic sequence at Torregaveta section Žunits TGe, f 1 – 6 , g, h, j, k, l.. In particular, those exposed at Torregaveta, except for TRj–l, have the highest D.I. values ŽD.I.s 90–92, Zr s 400–580 ppm.. Samples from Punta Marmolite Žsample PM. and Cuma Žsample CU. lava domes, and the upper pyroclastic deposits exposed at the Trefola Žunits from TLe to TLm. and Torregaveta Žunits from TRi to TRm. sections are phono-trachytes ŽD.I.s 87–89; Zr s 500–700 ppm; Figs. 4 and 5; Tables 2–4.. By comparing the results of chemical analyses of all the studied sections, a general correlation between composition and stratigraphic height is observed, i.e. the Zr content is almost constant at 300 ppm before 60 ka, then, between 60 to 44 ka, before increases until 700 ppm, and after decreases to ca. 500 ppm, to

156

L. Pappalardo et al.r Journal of Volcanology and Geothermal Research 91 (1999) 141–166

L. Pappalardo et al.r Journal of Volcanology and Geothermal Research 91 (1999) 141–166

157

Fig. 7. Harker diagrams of trace elements Žppm. versus D.I. for pre-CI volcanics. Trend A: samples from volcanic deposits erupted from CF. Trend B: samples from volcanic deposits attributed to Ischia Island, see text for further explanations. Symbols and full field as in Fig. 6. White field: samples from volcanics produced from vents located on Ischia Island Ž ) 35 ka; data from Civetta et al., 1991c.. Fig. 6. Harker diagrams of major elements Žwt.%. versus D.I. for pre-CI volcanics. Major element data were normalized to 100% on volatile-free basis. Legend: open triangles, CF pre-CI rocks; solid triangles, samples from volcanic deposits attributed to Ischia Island erupted before 37 ka, see text for further explanations; Field: CI rocks Ždata from Civetta et al., 1997..

increase again to ca. 700 ppm before the CI eruption Ž37 ka.. Major element variation diagrams of the volcanics older than 37 ka Žpre-CI. show similar pattern with

Fig. 5. Chemostratigraphy of selected investigated sections. Vertical axes is not in scale. Legend: open triangles, CF pre-CI rocks; solid triangles, samples from volcanic deposits attributed to Ischia Island erupted before 37 ka, see text for further explanations; squares, CF post-CIrpre-NYT rocks.

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between incompatible element contents and the degree of differentiation. Sr, Ba, Eu, and ferromagnesian elements display single depletion trends with increasing differentiation. The trend A overlaps the

Fig. 8. Chondrite-normalized REE abundance patterns for analyzed samples. Normalization values from Henderson Ž1984.. Legend: open triangles, CF pre-CI rocks; solid triangles, samples from volcanic deposits attributed to Ischia Island erupted before 37 ka, see text for further explanations; squares, post-CIrpre-NYT rocks.

respect to that of CI products ŽFig. 6.. SiO 2 , MnO, and Na 2 O contents increase, whereas Fe 2 O 3 tot, MgO, CaO and P2 O5 contents decrease at increasing D.I. ŽFig. 6.. K 2 O and TiO 2 contents increase for D.I. values from 67 to 81 and then decrease. Al 2 O 3 content is roughly constant, although with a large scatter, at increasing D.I. ŽFig. 6.. REE Žexcept Eu., Y, Nb, Zr, Rb, and Th plotted versus D.I. describe two trends named A and B in Fig. 7, both characterized by a positive correlation

Fig. 9. Harker diagrams of major elements Žwt.%. versus D.I. for volcanics erupted between CI and NYT. Major element data were normalized to 100% on volatile-free basis. Legend: squares, postCIrpre-NYT rocks. Field: NYT rocks Ždata from Orsi et al., 1995..

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Fig. 10. Variation diagrams of trace elements Žppm. versus D.I. for volcanics erupted between CI and NYT. Legend: squares, post-CIrpre-NYT rocks. Field: NYT rocks Ždata from Orsi et al., 1995..

compositional range of CI products. The trend B, characterized by the highest values of D.I., is described by the samples collected from the pyroclastic deposits of the middle part of the Torregaveta stratigraphic sequence. The geochemical characteristics of this group of volcanics correspond to those of deposits erupted from vents located on Ischia Island during the same period of time Ž) 35 ka; Civetta et al., 1991c.. Therefore, they do not belong to the CF and will not be taken into account in the following discussion.

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All the volcanic rocks older than 37 ka show enrichments in both LREE and HREE at increasing degrees of differentiation ŽTables 2–4 and Fig. 8., these are only partially comparable to CI that show a wide distribution. Chondrite-normalized REE distributions display enrichment in LREE relative to HREE, with the latter showing increasing but almost flat patterns. All samples display negative Eu anomalies. EurEuU decreases from 0.98 to 0.21 with differentiation ŽFig. 8A.. 87 Srr86 Sr values were measured on whole-rock pumice Ž64 samples. and lava fragments Žsix samples., and on feldspar phenocrysts separated from CI pumice Žsix samples.. 87 Srr86 Sr ratios of leached pumice and lava samples range from 0.70681 " 1 to 0.70735 " 1. CI feldspars have Sr-isotope ratios ranging from 0.70730 to 0.70732, similar to that of the least-evolved CI pumice, as also reported previously by Civetta et al. Ž1997.. The least-evolved rocks ŽD.I.s 67–79. have the least radiogenic 87 Srr86 Sr values Ž0.70681 " 1 to 0.70694 " 1.. These samples represent the two lowermost units of the Torregaveta and the basal unit of the Trefola stratigraphic sequences ŽTGa-b and TLa, respectively., and the six lowermost units cored in the borehole at Ponti Rossi ŽPRb to PRg.. 40Arr39Ar age determinations made on the trachytic samples exposed at the Trefola section show that unit TLa, at the base of the sequence, has an age of 58 " 3 ka. The 87 Srr86 Sr variations among these trachytic units are related neither to stratigraphic height nor to degree of chemical evolution. The most-evolved ŽD.I.s 82–91. pumice samples in the upper part of the studied sequences ŽPonti Rossi, Trefola, Torregaveta, Punta Marmolite, and Cuma. have the most radiogenic values Ž0.70705 " 1 to 0.70735 " 1. ŽFig. 5.. The Sr-isotope ratios of these samples increase with D.I., from 0.70705 " 1 to 0.70730 " 1, and are also time-related, with the samples higher in the section also being more radiogenic ŽFig. 5, Tables 2–4.. The uppermost units in the studied sequences ŽPRl, TLh, TLm, TGl, TGm. have 87 Srr86 Sr ratios similar to those of the feldspars and the least-evolved pumice fragments of the CI ŽFig. 5.. 40Arr39Ar age determinations made on the uppermost of these phono-trachytes immediately underlying the CI at the Trefola and Ponti Rossi sections are 45.6 " 0.7 and 44.3 " 0.8 ka, respectively.

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Fig. 11. Modeling of fractional crystallization and mixing processes obtained using the IGPET calculation program, based on the formulation by De Paolo Ž1981.. Curve a: combined process of fractional crystallization of pre-CI trachytic magmas and mixing with the least-evolved CI trachytic magma. Curve b: process of fractional crystallization of the magmas erupted immediately after the CI eruption. Curve c: combined process of fractional crystallization of magmas erupted immediately before the NYT eruption and mixing with the NYT magma. D Sr s 3.5, DCaO s 1.5 ŽVillemant, 1988; Pappalardo, 1994., F s fraction of residual magma. R s 0.2% fraction of mixed magma. Legend: open triangless CF pre-CI rocks; squaress post-CIrpre-NYT rocks.

The five pyroclastic deposits exposed in the middle part of the stratigraphic sequence of the Torregaveta section have Sr-isotope ratios Ž0.7067 to

0.7068. similar to those of volcanics erupted from vents located on the island of Ischia during the same time period Ž0.7068 to 0.7069; Civetta et al., 1991c;

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Fig. 5.. This evidence corroborates the hypothesis that they erupted from Ischia, as proposed on the basis of their trace elements compositions. The trachybasalts from the Fiumicello deposit ŽProcida Island. have 87 Srr86 Sr ranging from 0.70649 " 1 to 0.70654 " 1, significantly different from those of the Phlegraean deposits ŽFig. 5.. All the studied stratigraphic sequences older than 37 ka are overlain by CI deposits. The distal CI pyroclastic deposits have been recently studied by Civetta et al. Ž1997.. These authors demonstrated that during the CI eruption three magmas were erupted. The first erupted had the most evolved phono-trachytic composition, and 87 Srr86 Sr ratios of about 0.70745. The last erupted magma was trachytic and had 87 Srr86 Sr ratios of about 0.70730. During the intermediate phase of the eruption, a mingled magma was erupted, with an alkali-trachytic composition and 87 Srr86 Sr ratios intermediate between the most- and least-radiogenic magmas. Feldspar crystals separated from pumice fragments representative of the three magmas have the same 87 Srr86 Sr ratios that of the least-differentiated magma Žabout 0.70730; Civetta et al., 1997; this paper.. 4.4. Post-CI pyroclastic deposits and laÕa domes In the total alkalirsilica diagram ŽTAS, Fig. 4A; Le Bas et al., 1986. most analyzed samples from deposits emplaced between the CI and NYT plot in the fields for trachyte and phonolite, and few plot in the fields for latite and tephri-phonolite. All samples have a potassic alkaline affinity, with Na 2 O y 2 F K 2 O. In the D.I.–Ne classification grid ŽFig. 4B; Armienti et al., 1983. the samples range in composition from trachyte to alkali-trachyte to phono-trachyte ŽD.I.s 77–91; Fig. 4B.. The least-evolved trachytes occur in the pyroclastic deposit erupted from the Monticelli volcano ŽD.I.s 78–79; Zr s 270 ppm. and in the pyroclastic deposit at the base of the stratigraphic sequence of the Trentaremi volcano ŽD.I.s 77–82; Zr s 250–330 ppm.. Pumice samples collected from the pyroclastic deposits exposed at the Camaldoli hill ŽVerdolino section., at the Ponti Rossi, Coroglio, and Trefola sections, and the products of the Monte Echia tuff cone are alkali-trachytes ŽD.I.s 80–92; Zr s 265–660 ppm.. Most alkalitrachytes exposed in the studied sections have homo-

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geneous major and trace element compositions ŽD.I.s 80–90 and Zr s 265–470 ppm., only slightly more differentiated then the last erupted CI magma; only the alkali-trachyte cropping out at Trefola immediately over the CI has a more differentiated composition ŽD.I.s 91–92; Zr s 650 ppm.. The most-evolved phono-trachytic compositions ŽD.I.s 87–90; Zr s 660–780 ppm. are shown by the Montesanto lava dome and by the Tufi Antichi pyroclastic deposits ŽVeterinaria and Parco Grifeo sections.. The post-CI trachytes are characterized by major and trace element contents, and REE patterns, similar to pre-CI trachytes. Major element oxides and selected trace element contents for post-CI samples are plotted versus D.I. in Figs. 9 and 10. Generally SiO 2 , MnO, and Na 2 O contents increase with increasing D.I., whereas Fe 2 O 3 tot, MgO, CaO, K 2 O and P2 O5 contents decrease. TiO 2 contents decrease with D.I. values increasing from 76 to 83, then remain constant. Al 2 O 3 contents are scattered. REE Žexcept Eu., Y, Zr, Nb, Rb, Be, and Zn generally show positive correlations, whereas Sr, Ba, Eu, Sc, and V show negative correlations, relative to D.I. ŽFig. 10 and Tables 2–4.. A group of four phono-trachytic samples, i.e. those collected from the Montesanto lava dome and the Tufi Antichi pyroclastic deposits, are the most evolved samples and show the highest enrichments in several incompatible trace elements. The chondrite-normalized REE patterns are characterized by a high degree of enrichment of LREE, and relatively flat HREE distributions. All samples display negative Eu anomalies ŽEurEuU s 0.94– 0.29., which increase with increasing degree of chemical evolution ŽFig. 8B.. Sr isotopic compositions of selected post-CI volcanics range from 0.70720 " 1 to 0.70756 " 1 ŽTables 2–4.. The only exception is the sample collected from the Monte Echia tuff, which is less enriched in radiogenic Sr Ž0.70678 " 1.. No correlation is observed between Sr isotope ratios and degree of chemical evolution. Conversely, a good correlation exists between the Sr isotopic composition and the stratigraphic position of the analyzed samples, with samples being more radiogenic upsections. In particular, the lowermost deposits in the investigated stratigraphic sections have lower 87 Srr86 Sr ratios Ž0.70728 " 1 to 0.70738 " 1. similar to that of the

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CI feldspars and least-evolved CI pumice Ž0.70730; Civetta et al., 1997.. 40Arr39Ar determinations made on these deposits exposed at the base of the stratigraphic successions of Ponti Rossi, Trefola, and Verdolino sections gave ages of 16.1 " 0.2, 17.9 " 0.5 and 30.3 " 0.2 ka, respectively ŽFig. 5.. The upper post-CI deposits in the investigated stratigraphic sections have higher 87 Srr86 Sr ratio Ž0.70745 " 1 to 0.70756 " 1. with values that increase upsections reaching the value of the magma feeding the first phase of the NYT eruption Žca. 0.70756; Orsi et al., 1995.. 40Arr39Ar determinations made on samples from these deposits exposed at the top of the stratigraphic successions at Ponti Rossi, Trefola, and Verdolino sections give ages of 15.9 " 0.5, 14.8 " 0.3 and 14.6 " 0.6 ka respectively. All the studied sequences are overlain by NYT products. The geochemical and Sr-isotopical features of NYT deposits were studied by Orsi et al. Ž1995.. These authors demonstrated that during the NYT eruption three distinct magmas were erupted. The first-erupted magma had a homogeneous alkalitrachytic composition, with a 87 Srr86 Sr ratio about 0.70756. Successively, a less evolved trachytic magma was erupted, with a slight, continuous compositional variation, and a 87 Srr86 Sr ratio similar to that of first-erupted magma. The last-erupted magma was strongly zoned from alkali-trachyte to latite, with a 87 Srr86 Sr ratio about 0.70752. Feldspar separated from pumice representative of the three magma compositions are in isotopic equilibrium with whole-rock.

5. Discussion 5.1. The eÕolution of the Phlegraean magmatic system before the CI eruption The pre-CI trachytes show continuous major and trace element variations with respect to degree of differentiation ŽFigs. 6 and 7.. This could be accounted for by a simple fractional crystallization process in a closed system with feldspars as the dominant phase. However, the Sr-isotopic variations Ž0.7068–0.7073. suggest that other processes must have operated during the chemical evolution of these magmas. In particular, before about 60 ka, less evolved, trachytic magmas with fairly homogeneous

chemical and Sr-isotopic compositions Ž87 Srr86 Sr s 0.70681 " 1 to 0.70694 " 1; D.I.s 76–80; Zr s 300–350 ppm. were erupted, testifying that the system acted as a closed system. From about 60 to 44 ka, alkali-trachytic magmas became progressively more differentiated and their Sr-isotopic compositions became progressively more radiogenic Ž87 Srr86 Sr s 0.7070 to 0.7073; D.I.s 84–90; Zr s 300–700 ppm.. Thus, from 60 to 44 ka the Phlegraean magmatic system displayed open-system behavior, the possible causes of which will be discussed later. Then, from 44 to 37 ka, alkali-trachytic magmas similar in 87 Srr86 Sr to the least evolved trachytic magma of the CI were erupted, suggesting that the Phlegraean system was fed by the most differentiated, uppermost portion of the growing CI magma chamber, well before the CI eruption. Several processes could have produced the chemical and isotopic variations observed between 60 and 44 ka: Ž1. contamination of a magma reservoir by hydrothermal fluids enriched in radiogenic Sr; Ž2. country rock assimilation by the magma combined with fractional crystallization ŽAFC process.; Ž3. in situ 87 Sr in growth by 87 Rb decay; Ž4. mixing process; or Ž5. a complex process of fractionation, replenishment, and mixing. In the first possibility seawater could be the source of the radiogenic Sr, as suggested by Sr-isotope ratios for hydrothermal phases Ž87 Srr86 Sr s 0.709; Sr s 200 ppm; Villemant, 1988; Civetta et al., 1991a. cored in the deep geothermal wells at the CF ŽRosi and Sbrana, 1987.. Results of quantitative modeling Že.g., Faure, 1986., however, indicate that contamination of magma by up to 90% by such fluids would be needed to explain the observed increase in 87 Srr86 Sr. Such high amounts of fluids should change heavily the concentrations of mobile elements, such as alkalies and alkali-earth elements in the magmas or rocks, that on the contrary show the same behavior of immobile elements, so that this possibility can be easily ruled out. To test the second possibility, an AFC process ŽDe Paolo, 1981. was simulated using as a potential assimilant Mesozoic carbonate rocks Ž87 Srr86 Sr s 0.70769–0.70775; Sr s 300–1000 ppm; Civetta et al., 1991b., even though the absence of limestone xenoliths in the pyroclastic sequences of the CF seems to exclude that the magmatic system was located in or below a sedimentary basement. Model-

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ing was made using the lowest and the highest solidrliquid partition coefficients measured for CF trachytic rocks by Villemant Ž1988. and Pappalardo Ž1994.. Results show that the Ca content of the carbonate assimilant ŽCa s 30 wt.%; Faure, 1991. is too high to be consistent with the observed negative correlation between CaO content and 87 Srr86 Sr ratio. Thus, the AFC process cannot be responsible for the chemical and isotopic variations shown by the products erupted between 60 and 44 ka. The third possibility involves in situ 87 Rb decay in a high RbrSr magma chamber. Cavazzini Ž1994. proposed equations to calculate the increase in 87 Srr86 Sr ratio during a time-protracted fractional crystallization process of magmas with high RbrSr. We used this procedure to calculate the time Ž t . required for crystallization of a magma assuming: Ž1. the solidrliquid distribution coefficients for Rb Ž D Rb s 0.6. and Sr Ž D Sr s 3. reported in the literature for CF trachytic rocks ŽVillemant, 1988; Pappalardo, 1994., Ž2. a residual liquid fractionation of 0.3, which is obtained by simulating a fractionation process from the least-evolved trachytic to the mostevolved phono-trachytic magmas using mineral composition measured by electron microprobe Žour unpublished data., and Ž3. the 87 Srr86 Sr ratios detected in the least- Žs 0.70681. and most-evolved Žs 0.70730. rocks. The resulting age is 600 ka, much higher than the age measured on the stratigraphically lowest products, isotopically similar to the CI. The geochronological and isotopic data which are earlier presented indicate that before 60 ka the system had geochemical and isotopic characteristics distinct with respect to the CI magmas, and that the latter probably recharged the system at this age. As a consequence, the value of 600 ka is unrealistic on the basis of geochronological and isotopic data. A simple mixing processes between two endmembers can be ruled out. In fact in 87 Srr86 Sr ratios vs. 1rSr diagrams Žnot reported. samples do not describe a straight line indicative of mixing between two components ŽFaure, 1986.. Finally, the last possibility is a process of combined fractionation ŽF., replenishment ŽR., and mixing ŽM. mechanisms. In this hypothesis we assume the existence of a magma chamber in which trachytic magma evolves by crystal fractionation, is replenished by a fresh batch of trachytic and more radio-

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genic CI-type magma, and undergoes contemporaneous mixing with the newly arrived magma. To test this model we made calculations using the IGPET program Žversion 1994. and the values of solidrliquid partition coefficients comprised in the range reported for CF trachytic rocks by Villemant Ž1988. and Pappalardo Ž1994.. The obtained results show that the Sr-isotope variation observed in samples older than the CI products can be reproduced assuming that the trachytic magma Žmagma 1 in Fig. 11. underwent 70% fractional crystallization with alkali-feldspar as the dominant phase, and mixed with about 20% of a magma with the composition of the least-evolved trachytic CI magma, to reach the composition of magma 2 in Fig. 11, where the theoretical curve ‘‘a’’ fits the experimental data well, supporting this hypothesis. This model suggests a high amount of crystal fractionation, although most analyzed rocks are aphyric. This apparent contradiction can be reconciled hyphothesizing ‘‘in situ’’ growth of crystals Že.g. McBirney et al., 1985; Nilson et al., 1985; Turner and Campbell, 1986.. According to this model crystals are thought to nucleate and grow in situ on the floor and walls of the chamber, while the evolved liquid, originated at the contact with the growing crystals, migrates towards the upper part of the chamber due to its lower density. This mechanism can generate poorlyporphyritic strongly evolved liquids. This hypothesis is corroborated by thermal modeling ŽWohletz et al., 1999., indicating that a great volume of magma is present beneath the CF caldera, as well as magnetic modeling ŽOrsi et al., 1999., indicating that this magma should be hot, presumably molten, since it gives no magnetic anomalies. Civetta et al. Ž1997. demonstrated that the CI trachytic magma evolved by fractional crystallization in a closed system before the CI eruption. The variations in 87 Srr86 Sr ratio observed in the CI rocks were explained by interaction of the uppermost differentiated layer with fluids enriched in radiogenic Sr just before eruption ŽCivetta et al., 1997.. 5.2. The eÕolution of the Phlegraean magmatic system after the CI eruption and before the NYT eruption Geochronological data ŽAlessio et al., 1973; Scandone et al., 1991; this paper. and stratigraphic data

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ŽOrsi et al., 1996. suggest that a period of moderate eruptive activity occurred between 37 and 16 ka, in agreement with tephrostratigraphic data from the central Mediterranean Sea ŽPaterne and Guichard, 1993.. In this period, a series of explosive eruptions ejected alkali-trachytic to trachytic products isotopically very similar to the last-erupted CI magma. As in the case of the products older than 37 ka, major and trace element contents of these products can be qualitatively modeled by fractionation processes involving the observed mineral phases. However, the variations in Sr-isotope ratios suggest that processes other than simple fractional crystallization must have operated. The observed isotopic variations cannot be due to processes involving bulk or selective assimilation, since no correlation is observed between 87 Srr86 Sr ratio and degree of chemical evolution. It cannot be the result of in situ growth of radiogenic Sr, since the increase of radiogenic Sr is not correlated to the 87 Rbr86 Sr ratios. We suggest that the Phlegraean magmatic system, after the CI eruption, acted first as a closed system: the 16 ka old, lowermost exposed deposits have constant 87 Srr86 Sr ratios similar to that of the least-evolved CI volcanics. Quantitative modeling Žcurve b in Fig. 11. made using the IGPET program shows that the mostevolved of these volcanics Žmagma 4a in Fig. 11. could be derived by 70% of fractionation of the least-evolved CI trachytic magma Žmagma 3 in Fig. 11.. This modeling implies that, between 37 and 16 ka, the least-evolved trachytic magma left in the chamber after the CI eruption evolved in a closed system to produce an uppermost-differentiated, alkali-trachytic layer. Afterwards, magmas were erupted with higher 87 Srr86 Sr ratios Ž0.70745– 0.70756., increasing with time up to 14.6 ka, and reaching that of the magma that fed the first phase of the NYT eruption Ž0.70756; Orsi et al., 1995.. A process of mixing between the newly arrived, more radiogenic NYT magma with a resident fractionating magma similar to the least-evolved CI trachyte is suggested. Quantitative modeling ŽIGPET. shows that the Sr isotopic variations Žcurve c. are well accounted for by 70% of fractional crystallization of a parental magma with the composition of the leastevolved CI trachyte and mixing with 50% of a magma similar in composition to the NYT magma Žcurve c in Fig. 11., reaching the composition of magma 4b in Fig. 11.

6. Conclusions Geochronological data and petrological investigations on products older than the CI eruption and products erupted between the CI and NYT eruptions show that the Phlegraean magmatic system was periodically recharged by Sr-isotopically distinct magmas tapped at different times. In particular, in the earliest stages of the Phlegraean magmatic activity up to 60 ka, magmas with trachytic composition and low 87 Srr86 Sr ratios Žabout 0.7068. were feeding the system and periodically erupted. The products of this activity crop out at the base of the Trefola and Torregaveta sections and occur in the lowermost portion of the bore-hole drilled in the city of Napoli at Ponti Rossi. Subsequently, more evolved magmas, alkali-trachytic to phono-trachytic in composition, characterized by more radiogenic Sr-isotopic compositions, intermediate between those of the earlier trachytic magmas Ž87 Srr86 Sr s 0.7068. and the least-evolved CI magma Ž87 Srr86 Sr s 0.7073. were feeding the system and periodically erupted from at least 60 to 44 ka. These have been interpreted as ‘‘hybrid’’ magmas generated by mixing of the resident fractionating magmas with newly arrived, more radiogenic CI trachytic magmaŽs.. These data indicate that before 60 ka the system had geochemical and isotopic characteristics distinct with respect to the CI magmas that probably recharged the system at this age; this hypothesis excludes the model proposed by Cavazzini Žpersonal communication. that estimated an evolution time of 350–520 ka for the CI magma chamber. Then during a period of moderate subaerial volcanic activity between 37 and 16 ka, trachytic to alkali-trachytic magmas isotopically similar to the last-erupted CI magmaŽs. and presumably derived by 70% fractionation of the least-evolved CI trachytic magma left in the system, were erupted. Subsequently, magmas isotopically similar to that erupted during the first phase of the NYT eruption reached the system and mixed with the resident magmas producing hybrid magmas with isotope ratios intermediate between that of the least-evolved CI trachytic magma Ž0.70730. and that of the NYT magma Ž0.70756.. The age of the youngest of these products is 14.6 ka. At 12 ka, during the NYT eruption, magmas geochemically and Sr-isotopically well distinct from the CI magmatic system were emitted,

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testifying that in a short time interval, i.e. few thousands years, a large trachytic magma body was emplaced above the uppermost part of the residual CI magma reservoir. Thus, geochronological, geochemical and Sr-isotopic data on the CF volcanics older than the NYT eruption, combined with stratigraphical and volcanological data, strongly suggest that the Phlegraean magmatic system before 12 ka behaved as a complex system, undergoing periods of simple fractional crystallization of the resident magmas, and periods of replenishments by distinct, fractionating batches of magmas that mixed with the resident magmas. These hybrid magmas were periodically extracted to feed moderate- to large-volume eruptions.

Acknowledgements The authors wish to thank R. Romanelli and F. Castorina for their help during sample collection and preparation. They are grateful to J. Luhr and W. Duffield for critical reviews improving the quality of the paper. This work has been financially supported by the Gruppo Nazionale per la Vulcanologia of C.N.R., Italy.

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