Chemical and Sr-isotopical evolution of the Phlegraean magmatic system before the Campanian...

Ž .Journal of Volcanology and Geothermal Research 91 1999 141–166www.elsevier.comrlocaterjvolgeores

Chemical and Sr-isotopical evolution of the Phlegraean magmaticsystem 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 OsserÕatorio VesuÕiano, Via dell’OsserÕatorio, 80056 Ercolano, Napoli, Italyb 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

ŽNew geochronological, geochemical, and Sr-isotopic data on volcanics erupted before the Campanian Ignimbrite CI, 37. Ž . Ž .ka and the Neapolitan Yellow Tuff NYT, 12 ka caldera-forming eruptions at Campi Flegrei CF have allowed us to

investigate the behavior and temporal evolution of the Phlegraean magmatic system. The most prominent feature of the CFmagmatic system was the existence of a large, trachytic magma chamber, episodically recharged, which fed eruptions fortens of thousands years before the CI and NYT eruptions. During the pre-CI caldera activity, magmas were episodicallyerupted from vents located outside the present caldera structure. These magmas ranged in composition from trachyte toalkali-trachyte, with Sr-isotope ratios increasing through time, and becoming identical to that of the CI magma, at about 44ka ago. This suggests that the Phlegraean magmatic system before the CI eruption was acting as an open system. It wasbeing progressively replenished by new batches of magma that mixed with the resident less radiogenic, fractionatingtrachytic magmas and was periodically tapped. The magma chamber evolution culminated in the catastrophic eruption of the

Ž 3 .voluminous 150 km DRE , chemically and isotopically zoned CI trachytic magmas, and in the resultant CI calderaformation. 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 magmawere erupted from vents located inside the CI caldera. The temporal trend shown by Sr-isotope ratios provides evidence for anew input of alkali-trachytic magma, at ca. 15 ka, with 87Srr86Sr ratio identical to that of the alkali-trachytic magma feedingthe first phase of the NYT eruption. These data testify to the arrival in a short time span of a new trachytic to alkali-trachytic

Žmagma in the system, isotopically distinct from the CI magma, that gave rise about 3 ka later to eruption of the NYT 403 .km DRE . q 1999 Elsevier Science B.V. All rights reserved.

Keywords: Phlegraean magmatic system; Campanian Ignimbrite; Neapolitan Yellow Tuff; geochronology; Sr isotope; geochemistry

) 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.Ž .PII: S0377-0273 99 00033-5

( )L. Pappalardo et al.rJournal of Volcanology and Geothermal Research 91 1999 141–166142

1. Introduction

Climactic caldera-forming eruptions are often pre-ceded by eruptions fed by the same magma chamberŽe.g. Halliday et al., 1989; Christensen and De Paolo,

.1993 . Highly differentiated magmas, which evolvewithin the upper parts of magma chambers, arepreferentially tapped before such cataclysmic events.Volcanism after a caldera collapse testifies a pro-gressive emptying of the pre-existing magma cham-ber, or a recharging by new magma which can mixwith the possible residue in the chamber. The studyof pre- and post-caldera volcanism may thereforeprovide important information about key topics suchas replenishment mechanisms of the system, evolu-tion processes undergone by the magmas while theyresided in the system, interaction processes amongdistinct batches of magma entering the system,time-scale of magma differentiation, and the lifespan of a large-volume magmatic system.

Many petrological and geochronological studiesare available in the literature on the products of thetwo most powerful Phlegraean eruptions, the Campa-

Ž 3nian Ignimbrite CI; 37 ka; 150 km DRE; Barberi etal., 1978; Deino et al., 1992, 1994; Civetta et al.,

. Ž1997 and the Neapolitan Yellow Tuff NYT; 12 ka;)40 km3 DRE; Alessio et al., 1971; Orsi et al.,1992, 1995; Scarpati et al., 1993; Wohletz et al.,

. Ž . Ž .1995 . Orsi et al. 1995 and Civetta et al. 1997have pointed out substantial geochemical and iso-topical differences between the magmas feeding theseeruptions. However, few data are available in theliterature on Phlegraean products erupted before andbetween these two climactic eruptions.

We present geochronological, major- and trace-elemental, and isotopic data for pre-CI and post-CIrpre-NYT magmas erupted at the Campi FlegreiŽ .CF . These data help define the chemical evolutionof the Phlegraean magmatic system, with particularreference to the time scales of magma replenishmentand differentiation processes.

2. Geological outlines

Ž .The Campi Flegrei caldera CFc; Fig. 1 is anested, resurgent caldera, partially submerged in thebays of Pozzuoli and Napoli and resulting from two

main collapses related to the CI and the NYT erup-Ž .tions, respectively Orsi et al., 1992, 1995, 1996 .

Many stratigraphical, volcanological, and petrologi-cal studies are available in the literature on theproducts of these two most powerful Phlegraean

Ž .eruptions. The CI 37 ka; Deino et al., 1992, 1994 isthe largest pyroclastic deposit in the Phlegraean areaŽDi Girolamo, 1970; Barberi et al., 1978, 1991; Rosiand Sbrana, 1987; Fisher et al., 1993; Orsi et al.,

.1996; Rosi et al., 1996; Civetta et al., 1997 . Itscatastrophic eruption was accompanied by a calderacollapse affecting an area of about 230 km2, includ-ing the CF, the city of Napoli, the bay of Pozzuoli,and the northwestern sector of the Gulf of NapoliŽ .Orsi et al., 1996 . The CI deposits covered an areaof about 30,000 km2 with an estimated volume of

3 Ž . Žerupted magma of about 150 km DRE Fisher et.al., 1993; Civetta et al., 1997 . The eruption gener-

ated at least three pyroclastic-flow pulses, whichwere fed by trachytic magmas geochemically and

Ž .isotopically distinct Civetta et al., 1997 . The flowsof each pulse moved in different directions andreached variable distances from the eruptive vent. In

Žproximal areas four units have been recognized Rosi.et al., 1996 and compositionally correlated to the

Ž .distal CI deposits Civetta et al., 1997 , althoughŽtheir attribution to the CI eruption is debated Per-

.rotta and Scarpati, 1994; Melluso et al., 1995 .Ž .The NYT 12 ka; Alessio et al., 1971 is the

second largest pyroclastic deposit of the Phlegraeanarea, and represents the largest known trachyticphreatoplinian eruption. It covered an area of about1000 km2, including the Pozzuoli and Napoli bays,with a volume of erupted magma of more than 40

3 Ž . Žkm DRE Orsi 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 charac-terized by an early, mostly phreatoplinian phase,followed by alternating phreatomagmatic and mag-

Žmatic phases Orsi and Scarpati, 1989; Orsi et al.,1992, 1995; Cole and Scarpati, 1993; Scarpati et al.,

.1993; Wohletz et al., 1995 . A caldera collapseoccurred during the course of the eruption and gener-ated a volcano-tectonic depression over an area of

2 Žabout 90 km nested inside the CI caldera Fig. 1;.Orsi et al., 1992, 1996 . At least three geochemically

Ždistinct magmas alkali-trachyte; trachyte; latite to.alkali-trachyte were involved in the eruption. The

https://www.researchgate.net/publication/223595233_Geochemical_zoning_mingling_eruptive_dynamics_and_depositional_processes-The_Campanian_Ignimbrite_Campi_Flegrei_caldera_Italy_J_Volcanol_Geotherm_Res?el=1_x_8&enrichId=rgreq-33cdb2a6-01c3-4c10-a132-ee72a60edc4e&enrichSource=Y292ZXJQYWdlOzIyMzkxMTM4ODtBUzo5OTIxNDQzNjYwMTg2MEAxNDAwNjY1OTczMzkx

https://www.researchgate.net/publication/223595233_Geochemical_zoning_mingling_eruptive_dynamics_and_depositional_processes-The_Campanian_Ignimbrite_Campi_Flegrei_caldera_Italy_J_Volcanol_Geotherm_Res?el=1_x_8&enrichId=rgreq-33cdb2a6-01c3-4c10-a132-ee72a60edc4e&enrichSource=Y292ZXJQYWdlOzIyMzkxMTM4ODtBUzo5OTIxNDQzNjYwMTg2MEAxNDAwNjY1OTczMzkx

https://www.researchgate.net/publication/238366799_The_Neapolitan_Yellow_Tuff_a_large-magnitude_trachytic_phreatoplinian_eruption_eruptive_dynamics_magma_withdrawal_and_caldera_collapse_J_Volcanol_Geotherm_Res?el=1_x_8&enrichId=rgreq-33cdb2a6-01c3-4c10-a132-ee72a60edc4e&enrichSource=Y292ZXJQYWdlOzIyMzkxMTM4ODtBUzo5OTIxNDQzNjYwMTg2MEAxNDAwNjY1OTczMzkx

https://www.researchgate.net/publication/238366799_The_Neapolitan_Yellow_Tuff_a_large-magnitude_trachytic_phreatoplinian_eruption_eruptive_dynamics_magma_withdrawal_and_caldera_collapse_J_Volcanol_Geotherm_Res?el=1_x_8&enrichId=rgreq-33cdb2a6-01c3-4c10-a132-ee72a60edc4e&enrichSource=Y292ZXJQYWdlOzIyMzkxMTM4ODtBUzo5OTIxNDQzNjYwMTg2MEAxNDAwNjY1OTczMzkx

( )L. Pappalardo et al.rJournal of Volcanology and Geothermal Research 91 1999 141–166 143

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 thanŽ .the other two magmas 0.70752 vs 0.70756 . It

probably entered the chamber just before the onset ofthe eruption, perhaps constituting its triggering factorŽ .Orsi et al., 1995 .

Few data are available in the literature on Phle-graean products erupted before and between thesetwo caldera-forming eruptions, probably becausethese deposits are rarely exposed, having been largelydestroyed during the caldera collapses or buried un-

Ž .derneath younger deposits. Orsi et al. 1996 reportthe results of a new field survey on the volcanics

older than the NYT synthesizing their observationswith those available in literature.

Volcanics older than the CI are exposed onlyalong the scarps bordering the CF depression and aremostly alkali-trachytic in composition. They include

Žthe lava domes of Punta Marmolite 47 ka; Cassignol. Žand Gillot, 1982 and Cuma 37 ka; Cassignol and

.Gillot, 1982 ; the Tufi di Torre Franco pyroclasticŽdeposits )42 ka; Rittmann, 1950; Alessio et al.,

.1973 ; the remnant of the Monte Grillo tuff-cone; asequence of pyroclastic deposits separated by pale-osols, composed of both proximal and distal units

https://www.researchgate.net/publication/215755668_Step-filling_and_development_of_a_three-layers_magma_chamber_the_Neapolitan_Yellow_Tuff_case_history_J_Volcanol_Geotherm_Res?el=1_x_8&enrichId=rgreq-33cdb2a6-01c3-4c10-a132-ee72a60edc4e&enrichSource=Y292ZXJQYWdlOzIyMzkxMTM4ODtBUzo5OTIxNDQzNjYwMTg2MEAxNDAwNjY1OTczMzkx


exposed at Torregaveta; and a sequence of proximalpyroclastic deposits, intercalated by paleosols andgenerated by at least 10 eruptions, northeast of the

Ž .Quarto locality at Trefola quarry Fig. 1 . The ventsfor some of these deposits were located outside theCI caldera, as inferred from sedimentological charac-

Ž .teristics Orsi et al., 1996 . Pyroclastic deposits olderthan the CI have been cored east and north of thecity of Napoli. They are variable in number andthickness in the different bore-holes; the maximumnumber was recognized at Ponti Rossi where 11pyroclastic deposits are separated by paleosols. Al-though lack of outcrops makes it difficult to recon-struct the areal distribution of these deposits, theirsedimentological characteristics suggest that volcan-ism before the CI eruption was characterized by

Ž .high-energy explosive activity Orsi et al., 1996 .Volcanism younger than the CI and older than the

NYT eruptions was confined inside the CI calderaŽ .Orsi et al., 1996 . The majority of the rocks wereproduced by explosive, mostly hydromagmatic erup-tions. They occur in scattered outcrops across thecentral part of the city of Napoli, and along thenorthwestern and southwestern scarps of the Posil-

Žlipo hill Monte 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

. Ž .Trentaremi Orsi et al., 1996; Fig. 1 . Paleosolsinterbedded between Tufi Biancastri pyroclastic de-posits cropping out at Verdolino and Vallone delleFontanelle were dated respectively at 16,390"180

Ž .and 15,090"140 years by Alessio et al. 1973 . Anage of 14,770"420 years was reported by Scandone

Ž .et al. 1991 for a paleosol between the BrecciaMuseo and Torregaveta units.

3. Sampling and analytical procedures

This study concerns with rocks younger than NYTexposed in scattered outcrops at Torregaveta, Cuma,Trefola, Punta Marmolite, Monte Echia, Masseriadel Monte, Ponti Rossi, along the scarps bordering

Ž .the Camaldoli Verdolino sections , S. MartinoŽ .Veterinaria, Parco Grifeo sections and Posillipo

Ž .hills Trentaremi, Coroglio sections and Monticelli.

The location of sampled deposits is shown in Fig. 1.All the stratigraphic sections, except for that ofTorregaveta and Monticelli, are described in detail

Ž .by Orsi et al. 1996 , that measured the stratigraphicsequences, and reconstructed the geometrical rela-tionships, and made when possible correlation amongunits. The description of sequences is briefly summa-

Ž .rized below. Following Orsi et al. 1996 the unitshave been designated with two capital letters, whichrefer to the locality, followed by one lower-caseletter in alphabetic and stratigraphic order.

Ž .At TorregaÕeta a lava flow unit TGa is thelowermost deposit of the exposed stratigraphic se-quence. It is overlain by a sequence of 12 pyroclasticdeposits separated by paleosols underneath the CI

Ž .unit units from TGb to TGm . Units TGb and TGcare composed of cross-laminated surge beds. UnitsTGd–TGe–TGf include distal pumice fallout de-posits intercalated by surge beds. Unit TGf , which1

includes surge and fallout deposits, is known in theliterature as the Fiumicello deposit, erupted from a

Žvent located on Procida Island Pescatore and.Rolandi, 1991 . Units TGg–TGj–TGk–TGl are fall-

out deposits, whereas units TGh–TGi–TGm are py-roclastic-flow deposits. At Cuma the lowermost unitis composed of a lava dome, overlain by a sequencethat includes from the base upward, a fallout depositŽ .CUa unit , the CI and the NYT. At Trefola a thicksequence of pyroclastic units is exposed in a quarry.

ŽIt includes 12 units below the CI units from TLa to. ŽTLm , five units between CI and NYT units from

.TLo to TLs , and five units above the NYT. AtPunta Marmolite a sequence of deposits from olderthan CI to very young is exposed. The lowermostunit is a lava dome. Interposed between the lavadome and CI is a succession of a coarse falloutdeposits with minor ashy layers intercalated by pale-osols. This sequence is sedimentologically correlat-able with the upper part of the succession underlyingthe CI at Trefola section. At Ponti Rossi a sequenceof pyroclastic deposits from CI up to very young is

Ž .exposed; nine units units from PRa to PRi interca-lated by paleosols have been recognized between CIand NYT. The part of the sequence not exposed hasbeen drilled, and it is composed by CI and 11underlying pyroclastic deposits separated by pale-

Ž .osols units from PRl to PRv . At Monte Echia theexposed deposit is a single depositional unit consti-


tuted by surge beds with minor fallout deposits. Thesequence of the Tufi Biancastri pyroclastic deposits,composed of surge beds and fallout deposits em-placed between the CI and NYT eruptions, is ex-posed at Masseria del Monte and Verdolino sectionsŽ .units from VRa to VRe . A lava dome overlain by asequence of pyroclastic-surge deposits is exposed atMontesanto along a railway tunnel. These pyroclas-tic rocks are in the same stratigraphic position as thesimilar sequence of Tufi Antichi exposed along theeastern and southern slopes of the San Martino hillŽ .Veterinaria and Parco Grifeo sections . The rem-nant of a tuff cone is exposed along the southwesternscarps, at Trentaremi and Coroglio sections. Fur-thermore, at Trentaremi the cone is underlain by apyroclastic sequence and in turn by the NYT. AtMonticelli the products of a tuff cone are exposedoverlain by NYT products.

Ž .Volcanic rock samples 112 from selected unitsexposed along the described sequences and volcanic

Ž .rock samples 34 from drilled CF deposits olderthan the NYT have been collected for analysis.

Ž .Sampled units have been selected in order to: 1 berepresentative of volcanism occurred before CI and

Ž .NYT events, 2 allow geochemical and isotopicalcorrelation among units when these are uncertain orimpossible on the basis of stratigraphic data. Thickerand most representative units have been sampled atdifferent stratigraphic heights.

The collected samples consist mainly of pumiceand scoria fragments from pyroclastic deposits, andsubordinately of lavas from lava domes. The pumicefragments for most of the studied sequences aresmall, therefore, mostly multiple pumice sampleswere analyzed. Each sample was composed of anumber of clasts collected from the same strati-graphic layer, similar in terms of structure of vesi-cles, glass color and phenocrysts content. Whenmore than one pumice type is present, they wereanalyzed separately. Generally, the analyzed pumiceand lava samples are aphyric. Phenocrysts accountfor less than 1% by volume and include feldspar,clinopyroxene, black mica, opaques and apatite.

All the pumice and lava samples were washed indistilled water, crushed to lapilli-size particles, thenground and homogenized in an agate mortar. Pow-ders were analyzed for major elements and Sc byinductively coupled plasma-atomic emission spec-

Ž .trometry ICP-AES , and for the remainder of traceelements by inductively coupled plasma-mass spec-

Ž .trometry ICP-MS at the Centre de Recherches Pet-Žrographiques et Geochimiques CRPG, Vandouvre

.Cedex, France . Precision is 0.5% for major ele-ments, and variable from 2%–5%, for trace elementcontents in the range 50–150 ppm, to 2%–10%, fortrace element contents in the range 10–50 ppm, to5%–25%, for trace element contents in the range

Ž .0–10 ppm J. Morel, pers. commun., 1997 .Sr-isotopic compositions of whole-rock samples

and separated feldspar phenocrysts were determinedat the Dipartimento di Geofisica e VulcanologiaŽ .University ‘‘Federico II’’ of Napoli . The powderswere leached with cold 2.5 N HCl for 10 min andwith hot 2.5 N HCl for 30 min, then rinsed thor-oughly in pure sub-boiling distilled water, and finallydissolved with high purity HF–HNO –HCl mix-3

tures. Sr was extracted by conventional ion exchangechromatographic techniques. Measurements weremade using a VG 354 double-collector thermal ion-ization mass spectrometer running in jumping mode,by normalizing to 86Srr88Srs0.1194 for mass frac-tionation effects. The quoted error is the standard

Ž .deviation of the mean 2s and refers to the lastm

digit. Repeated analyses of NBS-987 InternationalReference Standard yielded a mean value of 0.71024

Ž ."1 Ns50 . The total blank was on the order of 6ng during the period of measurements. Because of

Ž 40 39the young ages the oldest Arr Ar dated rock is 60.ka old and the low RbrSr ratios, all initial Sr

isotope ratios are equal to the measured ratios withinthe analytical uncertainty.

40Arr39Ar dating has been determined on phe-nocryst concentrates from pumice fragments of 11pyroclastic units. The sampled units ranged strati-graphically from just below the NYT to units ofunknown age older than the CI. Most phenocrystseparates were of pure sanidine; however, two sepa-rates were mainly plagioclase with minor sanidineŽ .samples 9601 F10 and 9601 C2 . All separates weredated at least once, and five in replicate, by the laser

Ž 2 .IH method, using a broad 6=6 mm , uniform-en-Žergy-profile CO beam Sharp and Deino, 1996; see2

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


Table 140Arr39Ar 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 "1s . The integratedage is the combined age of all gas fractions weighted on the basis of the 39Ar abundance, with an uncertainty calculated as the square root ofthe sum of the squares of the 39Ar-abundance-weighted individual errors. The plateau age is calculated as the inverse-variance weightedmean of the plateau steps. The weighted-mean age of the single-crystal analyses is calculated in an analogous manner using weights equal tothe inverse variance of the analytical uncertainties. The stated uncertainties for the plateau age and weighted-mean single-crystal age is one

Ž . Ž .standard error of the weighted mean, calculated as the maximum formulations of Taylor 1982 and Samson and Alexander 1987 .Ž40 39 . 40 39‘ Arr Ar ’ is the ‘trapped’ Arr Ar component from the isochron analysis. ‘MSWD’ is Mean Sum of Weighted Deviates. All statedtr

errors in age include uncertainty in the neutron fluence parameter, J. Js1.669=10y5 "1=10y7. Isotopic interference corrections:Ž36 37 . y4 y6 Ž39 37 . y4 y6 Ž40 39 . y4 y4Arr Ar s2.64=10 "1.7=10 , Arr Ar s6.73=10 "3.7=10 , Arr Ar s7=10 "3=10 , ls5.543Ca Ca K

=10y10 yry1.

the samples were dated by the single-crystal, total-Ž .fusion SCTF method. This latter heating method

also employed a CO laser, but focused to a sub-mil-2

limeter diameter beam. Samples were irradiated inthe 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

Ž .Mountain, California Turrin et al., 1994 , with aŽ .reference age of 1.194 Ma Renne 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 2s , whereas uncertaintiesin the plateau age and integrated age are shown at 1s . Due to space limitations, a replicate experiment on sample 9601 F10 is not shownŽ .see Table 1 for summary results .


Ž .Fig. 2 continued .


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

4. Results

4.1. Geochronology

Fig. 2 shows the results of the incremental heatingŽ . 39IH experiments as cumulative % Ar release spec-tra. All sanidine experiments exhibited a plateau ofstable apparent age across most of the 39Ar releaseŽfor a definition of ‘plateau’ as used here see Fleck

. Žet al., 1977 , and with one exception 9601 M1, Lab

.IDa 20873 , plateau ages are indistinguishable fromŽintegrated ages an integrated age is the age gener-

ated when all steps are mathematically recombined.to simulate a total-fusion experiment . In addition,

all sanidine experiments gave stable CarK ratiosacross almost the entire spectra, apart from anoma-lies in the first and last 5% or so of gas release.

40 Ž 40 U .Radiogenic Ar contents % Ar are in the 50%–90% range in the plateau regions. Inverse isochron

Ž36 40 39 40analyses Arr Ar vs. Arr Ar, corrected for de-cay, mass spectrometer discrimination, and isotopic

. Žinterferences of the plateau regions combined.plateaus in the case of replicate IH analyses yield

ages that are all in agreement with the plateau ages.The ‘trapped’ 40Arr36Ar component obtained fromthe inverse isochron analysis yielded values thatwere all within error of the expected atmospheric

Ž .composition of 295.5 Steiger and Jager, 1977 , sug-¨gesting that excess Ar is not present in these samplesin detectable amounts. The generally excellent40Arr39Ar release characteristics and analytical pa-rameters of these samples suggest that they are unal-tered, homogeneous phases that should yield accu-rate ages. The best age for these samples amongst

Žthe three ages obtained for the IH experiments in-.tegrated age, plateau age, and isochron age is taken

to be the isochron age, since this computationalprocedure inherently accounts for deviations fromideal atmospheric composition in correcting for‘trapped’ 40Arr36Ar components.

In contrast to the sanidine IH experiments, theplagioclase-dominant separate 9601 F10 failed toform a plateau in two experiments, and exhibited anMSWD for the inverse-isochron analysis that indi-

cated excess scatter far beyond that expected fromŽanalytical measurements alone i.e., the excess scat-.ter was due to geological effects . The integrated age

for this sample is also 20–30 ka, too old relative to ahigh-quality sanidine sample analyzed from the base

Ž .of the section 9601 A1 . Sample 9601 C2, a plagio-clase-dominant separate from a tuff lying stratigraph-ically between 9601 F10 and 9601 A1 in the samesection, also yielded an age that is about 30 ka, tooold relative to 9601 A1. Although this sample yieldeda plateau and in all respects apart from CarK con-tent appears to be a high-quality sample, the accu-racy of the result must be questioned. Although thesample forms an isochron with an acceptable MSWD,the uncertainty of the ‘trapped’ component is so highŽŽ40 36 . .Arr Ar trapped s 358 " 56 that it maskswhether a significant excess Ar component is presentor not. We postulate that the plagioclase in these twosamples may bear a significant quantity of excess Arthat is responsible for the too-old ages relative to thesanidine sample at the base of the section, and thatthe results from 9601 F10 and 9601 C2 should beignored.

Fig. 3 shows age–probability density spectra forŽ .the single-crystal total fusion SCTF analyses, with

auxiliary plots showing related analytical informa-tion. A principal motivation for pursuing the single-crystal approach is to examine the grain-to-grainreproducibility of the samples. Homogeneity is veri-fied by the generally near-Gaussian shape of theage–probability density spectra in which only a sin-gle well-defined mode is present. Weighted meanages of the SCTF experiments agree within analyti-cal error with the IH ages. Uncertainties in theweighted mean SCTF ages are typically much greaterthan the corresponding uncertainties for the IH ex-periments, reflecting the inherent advantages of ex-pelling adsorbed atmospheric Ar in the early phases

Žof an IH experiment thus requiring a smaller correc-.tion for atmospheric Ar , and increased measurement

precision derived from greater gas yields during IHof multi-grain samples. Ages derived from inverseisochron analyses of the SCTF data are generally inagreement with the conventionally calculated SCTFages, although two samples yielded statistically sig-

Ž .nificant differences 9601 M1 and 9602 I1 . As withthe IH results, the best ages for the SCTF samples istaken to be those derived from the isochron analysis.


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

four are younger by the SCTF method and four areolder. For this sample suite, indications are that the

Žbulk IH samples are uncontaminated by outliers such


.as xenocrysts that detectably influence the age. Ac-cepting that geological outliers are not present, theIH results are preferred to the SCTF results. This isbecause in IH dating controlled, progressive out-gassing of the sample permits anomalous compo-nents, typically in the early and final steps of theexperiment, to be identified and excluded, an advan-tage not attainable with the SCTF results.

Comparison to previously dated units and rela-tionship to marker horizons: The CI serves as astratigraphic marker in all four sections studied. Wehave previously dated this widespread tuff using the40 39 ŽArr Ar technique by the SCTF Deino et al.,

. Ž1992 and resistance furnace IH methods Deino et.al., 1994 , with an overall mean age of 37.1"0.4

ka. Units dated in this study immediately overlyingthe CI in the Trefola quarry, Ponti Rossi, and Ver-

Ž .dolino Valley of 17.9"0.5 ka 9601 O1 , 16.1"0.2Ž . Ž .ka 9602 A1 , and 30.3"0.2 ka 9603 A1 , respec-

tively, are completely resolved chronologically fromthat of the CI, and document a hiatus of variable

Ž .duration 7–21 ka in this interval. Units directlyunderlying or very nearly directly underlying the CIin the Trefola quarry and Ponti Rossi give remark-

Ž .ably similar ages of 45.6"0.7 ka 9601 M1 andŽ .44.3"0.7 ka ME 28 , indicating a hiatus of about

7–9 ka at both localities.

4.2. Geochemistry

Chemical and isotopical analyses, CIPW norma-Žtive nepheline and D.I. values normative OrqAb

.qNe of selected samples representative of the baseand top of each recognized unit are listed in Tables2–4. No significant chemical variations have beenrecognized between samples collected from the sameunit. The complete set of analyses is available onrequest.

Ž .Loss on ignition L.O.I. contents are variablefrom 1 to 7 wt.%; however, most samples have

L.O.I. contents ranging from 1 to 3 wt.%. Thosewith higher contents, from 3 to 7%, do not show anyparticular evidence for alteration in normative orSr-isotopic compositions and are considered usablein this study. D.I. values range from 67 to 92,assuming an Fe O rFeO ratio of 0.5 in the CIPW2 3

norm calculations, which is the mean value of Phle-graean volcanics for which ferrous Fe was deter-

Ž .mined by KMnO titration Rosi and Sbrana, 1987 .4

The analyzed pumice and lava samples older thanNYT have Sr-isotope ratios ranging from 0.70649 to

Ž . Ž0.70756 Tables 2–4 ; a similar range 0.70650–. Ž .0.70756 was reported by Cortini and Hermes 1981

for ejecta in the NYT formation; a higher valueŽ .0.70769 was reported by these authors for a samplefrom the Cuma lava dome. This can be explained bythe lack of a leaching procedure in their work,

Žnecessary considering the low Sr content lower than.10 ppm of this lava sample.

The chemical data are plotted in the diagrams ofFigs. 4–11. Chemostratigraphy of the four mainrepresentative studied sections is reported in Fig. 5.

4.3. Pre-CI pyroclastic deposits and laÕa domes

Most of the analyzed volcanic rocks plot in thetrachyte and phonolite fields of the total alkalirsilica

Ž .diagram TAS, Fig. 4A; Le Bas et al., 1986 ; onlyone plot in the tephri-phonolite field. All sampleshave a potassic alkaline affinity, with Na Oy2F2

ŽK O. In the D.I.–Ne diagram Fig. 4B; Armienti et2.al., 1983 , commonly used for the Campanian potas-

sic rocks, the studied samples range in compositionfrom latite to phono-trachyte, through trachyte andalkali-trachyte. Alkali-trachytic and phono-trachyticsamples, although having similar D.I. values, showdifferent degree of evolution, with the phono-trachytes more enriched in incompatible trace ele-ments. This characteristic is generally observed in

Ž .CF rocks see also D’Antonio et al., 1999 .

Ž 39 40 UFig. 3. Age–probability density spectrum for the single-crystal analyses, with compositional parameters moles Ar, % Ar , CarK, and.values of individual analyses with 1s 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 densityspectrum that includes all samples is shown by the dashed line; the spectrum that excludes the culled samples is indicated by the solid line.

Ž .The modal value in ka of the age–probability density curve is also indicated at the top of the peak; the weighted-mean age exclusive ofculled analyses, with 1 S.E.M., is given near the bottom axis.


Tab

le2

Ž.

Ž.

Maj

orox

ide

wt.

%an

dtr

ace

elem

ent

ppm

cont

ents

,an

dS

r-is

otop

icco

mpo

siti

onof

sele

cted

Cam

piF

legr

eivo

lcan

ics

olde

rth

anN

YT

aa

aa

aa

aa

aa

aa

bb

bc

cc

cc

cS

ampl

e:94

794

896

01A

196

01C

196

01C

2496

01F

195

5095

4995

4895

4795

4695

4595

3795

36C

9535

C95

59A

9559

B1

9559

E2

9559

E4

9559

F5

9559

F6

Loc

atio

n:P

MC

UT

LT

LT

LT

LT

LT

LT

LT

LT

LT

LT

LT

LT

LT

GT

GT

GT

GT

GT

GU

nit:

lava

dom

ela

vado

me

ac

base

cto

pf

base

fto

pg

hi

mba

sem

top

fall

CI

CI

CI

ab

eba

see

top

fba

sef

top

SiO

58.6

359

.23

60.4

655

.58

57.0

256

.69

57.7

056

.92

56.0

556

.12

57.5

857

.59

58.2

458

.76

58.7

060

.09

53.2

658

.68

58.6

458

.83

58.7

12

TiO

0.40

0.41

0.49

0.40

0.41

0.39

0.39

0.40

0.42

0.43

0.41

0.41

0.39

0.40

0.39

0.43

0.66

0.54

0.55

0.56

0.56

2

Al

O18

.89

18.6

618

.38

18.9

819

.53

18.8

318

.64

18.2

217

.73

17.9

018

.10

18.3

017

.79

17.9

717

.93

18.9

518

.29

17.8

417

.82

17.8

217

.83

23

Fe

Oto

t3.

843.

672.

904.

154.

223.

803.

803.

663.

973.

913.

603.

643.

333.

393.

373.

836.

803.

063.

063.

003.

022

3

MnO

0.19

0.24

0.16

0.26

0.26

0.20

0.21

0.23

0.23

0.23

0.25

0.27

0.21

0.22

0.22

0.15

0.15

0.22

0.22

0.23

0.23

MgO

0.39

0.26

0.43

0.35

0.35

0.39

0.39

0.36

0.98

0.83

0.31

0.32

0.34

0.34

0.35

0.65

1.84

0.37

0.37

0.34

0.33

CaO

2.08

1.69

1.23

2.01

2.01

1.94

2.12

1.93

2.86

2.65

1.69

1.72

1.72

1.72

1.75

2.37

5.11

1.08

1.08

1.02

1.04

Na

O5.

697.

426.

376.

547.

055.

696.

096.

085.

585.

796.

736.

806.

316.

336.

274.

363.

726.

656.

616.

606.

722

KO

7.87

6.60

6.74

6.82

7.00

7.33

7.72

7.01

6.70

6.73

6.52

6.51

7.06

6.84

7.35

7.59

6.35

6.30

6.31

6.38

6.31

2

PO

0.09

0.06

0.08

0.07

0.07

0.06

0.12

0.11

0.13

0.13

0.09

0.10

0.09

0.11

0.09

0.10

0.37

0.05

0.05

0.04

0.05

25

L.O

.I.

1.70

1.51

2.43

4.01

1.72

4.37

2.56

4.77

5.11

5.01

4.43

4.07

4.16

3.65

3.35

1.12

2.94

4.97

4.99

4.81

5.03

Sum

99.7

799

.75

99.6

799

.17

99.6

499

.69

99.7

499

.69

99.7

699

.73

99.7

199

.73

99.6

499

.73

99.7

799

.64

99.4

999

.76

99.7

099

.63

99.8

3

Ne

10.9

114

.53

6.28

16.1

218

.19

11.0

514

.01

11.5

810

.29

11.2

112

.34

12.8

510

.81

9.48

11.1

80.

145.

198.

178.

007.

858.

47D

.I.

87.5

487

.96

91.2

787

.16

88.0

786

.86

88.3

788

.29

83.2

684

.48

89.8

689

.74

89.4

590

.02

89.0

283

.07

67.4

191

.93

91.8

792

.22

92.0

6

Be

1625

824

2216

1619

1819

2123

1616

1910

813

1415

14S

c3

33

33

33

35

43

33

33

47

33

33

V18

833

1513

1922

1731

3012

1315

1617

4712

526

2925

25C

o1

12

22

11

13

31

11

21

110

13

11

Zn

118

146

7312

912

310

112

013

712

813

114

714

212

112

512

697

8589

9810

097

Ga

2428

2026

2522

2425

2425

2727

2425

2419

1822

2425

24R

b36

547

628

935

634

931

033

434

633

634

439

738

937

237

136

916

519

331

934

034

834

1S

r26

523

2230

2926

2465

7316

1719

2025

383

862

1819

89

Y49

7238

6260

4546

5251

5362

6352

5354

3431

5561

6262

Zr

560

840

314

681

693

511

518

582

572

592

694

684

589

583

583

356

273

504

538

559

559

Nb

9313

949

123

120

8796

106

100

106

125

128

9910

199

5644

7885

8988

Ba

72

2222

2812

1011

7988

1213

1218

2442

011

9923

248

13L

a11

516

182

141

138

108

115

126

121

124

144

154

123

120

125

8872

118

124

131

130

Ce

220

301

157

256

253

198

212

226

220

234

263

279

230

225

232

159

137

224

239

249

248

Pr

22.5

31.8

17.0

27.5

27.1

21.0

22.6

25.2

24.5

25.9

30.0

28.6

25.0

25.0

24.9

16.9

14.8

24.6

27.2

28.1

27.7

Nd

8010

563

9493

7278

8784

8610

510

386

8890

5954

8593

9798

Sm

13.6

16.0

11.4

16.1

16.1

12.7

14.2

15.7

15.0

15.4

18.6

18.0

15.5

15.0

15.5

10.6

9.9

15.1

16.7

17.4

17.1

Eu

1.8

1.3

1.3

1.5

1.6

1.6

1.9

1.6

1.6

1.5

1.4

1.4

1.5

1.5

1.5

2.2

2.3

1.1

1.2

1.1

1.1

Gd

10.7

15.2

9.2

12.7

12.1

9.2

10.3

11.2

11.4

11.7

13.4

14.5

11.4

11.4

12.6

8.3

7.9

12.1

12.8

13.8

13.6

Tb

1.6

2.3

1.3

2.0

1.9

1.5

1.5

1.7

1.7

1.8

2.1

2.1

1.8

1.8

1.8

1.2

1.1

1.8

1.9

2.0

2.0

Dy

8.6

12.7

7.1

11.0

9.8

7.7

9.3

10.5

10.0

10.6

12.4

11.3

9.9

9.9

9.6

6.4

5.9

10.1

10.4

11.3

11.2

Ho

1.8

2.6

1.4

2.2

2.2

1.7

1.8

2.1

1.9

2.0

2.4

2.5

2.1

2.0

2.2

1.3

1.2

2.2

2.2

2.3

2.3

Er

4.7

7.0

3.3

5.4

5.7

4.2

4.8

5.3

5.3

5.3

6.4

6.5

5.4

5.0

5.5

3.2

2.9

5.5

5.5

5.8

5.8

Tm

0.7

1.1

0.5

1.0

0.9

0.6

0.7

0.8

0.8

0.9

1.0

1.0

0.8

0.8

0.8

0.5

0.4

0.8

0.9

0.9

0.9

Yb

4.8

7.7

3.4

6.1

6.1

4.7

4.5

5.0

4.9

5.2

6.2

6.5

5.4

5.5

5.9

3.3

2.8

5.7

5.6

6.1

6.0

Lu

0.8

1.2

0.5

0.9

0.9

0.6

0.8

0.9

0.9

0.9

1.1

1.1

0.9

0.8

1.0

0.5

0.4

0.9

0.9

1.0

0.9

Th

4672

2165

6043

4551

5053

6467

5353

5732

2340

4043

4287

86U

Srr

Sr

0.70

705

0.70

730

0.70

685

0.70

715

–0.

7070

4–

–0.

7073

5–

0.70

735

–0.

7074

60.

7074

50.

7074

60.

7070

10.

7070

00.

7068

0–

––

aP

re-C

I.bC

I.Ž

.U

L.O

.I.s

Los

son

igni

tion

.N

orm

ativ

eN

ean

dD

.I.

sA

bq

Orq

Ne

are

calc

ulat

edas

sum

ing

Fe

Or

FeO

s0.

5.S

ris

otop

icra

tio

wit

his

dete

rmin

edon

sepa

rate

dfe

ldsp

ar,

the

rem

aind

erar

ew

hole

-roc

kan

alys

es.

The

23

inte

rnal

erro

ron

all

Sr

isot

opic

rati

osis

"0.

0000

1.K

eyfo

rlo

cati

ons:

PM

,P

unta

Mar

mol

ite;

CU

,C

uma;

TL

,T

refo

la;

TG

,T

orre

gave

ta.


Tab

le3

Ž.

Ž.

Maj

orox

ide

wt.

%an

dtr

ace

elem

ent

ppm

cont

ents

,an

dS

r-is

otop

icco

mpo

siti

onof

sele

cted

Cam

piF

legr

eivo

lcan

ics

olde

rth

anN

YT

Ž.

L.O

.I.s

Los

son

igni

tion

.N

orm

ativ

eN

ean

dD

.I.

sA

bq

Orq

Ne

are

calc

ulat

edas

sum

ing

Fe

Or

FeO

s0.

5.S

ris

otop

icra

tio

isde

term

ined

onw

hole

-roc

ksa

mpl

es.

The

inte

rnal

erro

ron

all

Sr

isot

opic

rati

osis

23

0.00

001.

Key

for

loca

tion

s:T

G,

Tor

rega

veta

;M

S,

Mon

tesa

nto;

EC

,M

onte

Ech

ia;

VR

,V

erdo

lino

;T

M,

Tre

ntar

emi;

CR

,C

orog

lio.

cc

cc

cc

cc

cc

cS

ampl

e:95

5995

5995

5995

5995

5995

5995

5995

5995

5995

59M

L6

MT

2994

1994

694

594

394

494

1194

1394

1694

18a

aa

aa

aa

aa

aF

8F

11G

H1

H2s

pI

J2K

1L

1M

Loc

atio

n:T

GT

GT

GT

GT

GT

GT

GT

GT

GT

GM

SM

SE

CV

RV

RV

RV

RT

MT

MC

RC

RU

nit:

Fiu

mi-

Fiu

mi-

gh

base

hto

pi

jk

lm

lava

tuff

bba

seb

top

dba

sed

top

base

top

base

top

cell

oce

llo

dom

e

SiO

48.0

145

.98

58.6

958

.54

59.5

555

.45

58.3

455

.80

56.5

357

.18

57.0

058

.74

57.2

956

.09

58.0

958

.08

57.8

857

.92

57.3

558

.90

59.1

52

TiO

1.15

1.12

0.55

0.54

0.54

0.40

0.45

0.40

0.42

0.41

0.39

0.36

0.40

0.37

0.37

0.40

0.41

0.49

0.40

0.38

0.39

2

Al

O17

.95

17.1

717

.91

17.8

317

.98

18.7

618

.38

18.5

918

.17

18.3

119

.09

17.3

817

.73

16.8

617

.25

17.4

117

.48

18.4

117

.65

17.5

917

.65

23

Fe

Oto

t9.

108.

873.

023.

142.

924.

063.

033.

853.

653.

584.

233.

614.

033.

112.

973.

253.

374.

633.

753.

133.

142

3

MnO

0.12

0.12

0.25

0.24

0.19

0.25

0.14

0.22

0.23

0.25

0.25

0.11

0.11

0.10

0.10

0.11

0.10

0.08

0.10

0.10

0.10

MgO

4.66

4.53

0.31

0.41

0.41

0.35

0.50

0.44

0.43

0.45

0.33

0.62

1.13

0.53

0.37

0.45

0.50

1.19

0.65

0.43

0.41

CaO

9.68

11.9

40.

981.

101.

161.

921.

481.

991.

681.

662.

171.

093.

483.

962.

642.

402.

353.

962.

962.

172.

17N

aO

2.80

2.97

6.71

6.67

6.80

6.26

5.06

5.31

5.56

6.66

6.99

3.80

4.06

4.14

4.53

4.48

4.34

3.23

3.82

4.41

4.49

2

KO

3.31

2.72

6.48

6.41

6.16

6.73

6.78

7.08

6.80

6.59

7.16

7.59

8.00

7.11

7.48

7.29

7.58

8.51

8.18

8.63

8.32

2

PO

0.53

0.61

0.04

0.05

0.07

0.05

0.07

0.07

0.06

0.05

0.09

0.15

0.17

0.10

0.08

0.10

0.10

0.25

0.14

0.09

0.09

25

L.O

.I.

2.24

3.53

4.98

4.84

3.43

5.41

5.48

5.82

5.63

4.62

2.04

6.28

3.21

7.40

5.90

5.80

5.64

0.92

4.62

3.95

3.87

Sum

99.5

599

.56

99.9

299

.77

99.2

199

.64

99.7

199

.57

99.1

699

.76

99.7

499

.73

99.6

199

.77

99.7

899

.77

99.7

599

.59

99.6

299

.78

99.7

8

Ne

3.88

8.84

9.03

9.00

7.54

13.9

31.

118.

757.

4612

.96

18.5

30.

005.

443.

252.

721.

311.

892.

503.

125.

214.

08D

.I.

41.4

435

.68

91.9

491

.52

91.9

086

.92

87.2

185

.41

87.2

089

.45

87.6

486

.35

80.3

180

.85

85.8

085

.27

85.2

276

.79

82.4

987

.95

87.6

2

Be

44

1516

1124

818

2025

228

78

910

107

910

9S

c17

173

33

33

33

33

55

43

33

64

33

V22

723

224

2732

1742

2318

1512

4665

5243

5056

9566

4549

Co

2828

11

12

21

11

13

42

11

26

31

1Z

n69

6710

010

279

124

6010

511

012

314

376

9167

7074

7658

6770

75G

a17

1725

2422

2519

2324

2627

1819

1819

1919

1918

1819

Rb

131

9835

234

929

333

524

130

032

237

642

427

725

129

131

132

231

623

228

830

031

6S

r88

086

714

2617

3459

4822

2015

267

683

299

166

228

272

700

538

205

185

Y25

2563

6151

6034

4753

6366

2832

3033

3433

2429

3134

Zr

130

129

586

576

415

658

315

508

571

678

782

300

313

327

358

361

346

249

330

343

378

Nb

1716

9291

6511

746

8810

012

013

340

4544

5152

4832

4448

52B

a11

6710

939

2815

3156

1614

132

163

1034

190

4714

325

512

7710

4011

872

La

3736

134

132

100

135

7210

812

114

215

364

8270

8184

8055

6877

84C

e75

7425

124

920

025

214

320

322

225

528

412

015

113

615

916

215

410

713

115

115

7P

r9.

19.

127

.927

.522

.827

.415

.421

.824

.227

.230

.211

.815

.714

.516

.616

.716

.211

.613

.616

.116

.3N

d38

3897

9485

9456

7584

9410

445

5352

5961

6143

4854

57S

m8.

27.

816

.716

.215

.616

.010

.013

.014

.916

.118

.08.

59.

49.

110

.311

.010

.58.

68.

99.

710

.3E

u2.

32.

31.

01.

11.

31.

51.

41.

61.

41.

31.

71.

92.

52.

02.

02.

12.

12.

32.

12.

12.

1G

d7.

17.

013

.813

.110

.711

.86.

99.

310

.613

.013

.96.

68.

07.

38.

08.

58.

16.

47.

07.

78.

0T

b1.

00.

92.

01.

91.

71.

81.

11.

51.

72.

02.

11.

01.

11.

01.

21.

21.

20.

91.

01.

11.

1D

y4.

84.

811

.410

.79.

010

.26.

08.

29.

111

.111

.84.

85.

65.

46.

46.

46.

14.

95.

45.

95.

8H

o1.

00.

92.

32.

31.

92.

21.

31.

71.

92.

22.

41.

01.

21.

11.

31.

31.

30.

91.

01.

21.

3E

r2.

12.

25.

85.

84.

85.

63.

24.

45.

05.

76.

22.

62.

93.

03.

13.

33.

22.

22.

72.

93.

2T

m0.

30.

30.

90.

90.

70.

90.

50.

70.

80.

91.

00.

40.

40.

40.

50.

50.

50.

30.

40.

50.

5Y

b1.

91.

96.

46.

24.

66.

03.

14.

45.

06.

37.

02.

82.

83.

03.

23.

33.

22.

23.

03.

33.

1L

u0.

30.

31.

01.

00.

71.

00.

50.

70.

80.

91.

10.

40.

40.

40.

50.

50.

50.

40.

40.

50.

5T

h8

846

4531

6222

4754

6266

2625

3134

3535

2331

3535

8786

Srr

Sr

0.70

649

0.70

654

–0.

7067

1–

0.70

682

––

0.70

735

0.70

730

0.70

730

–0.

7067

80.

7073

50.

7073

00.

7074

80.

7075

40.

7073

80.

7072

80.

7073

40.

7073

6

aP

re-C

I.cP

ost-

CIr

Pre

-NY

T.


Tab

le4

Ž.

Ž.

Maj

orox

ide

wt.

%an

dtr

ace

elem

ent

ppm

cont

ents

,an

dS

r-is

otop

icco

mpo

siti

onof

sele

cted

Cam

piF

legr

eivo

lcan

ics

olde

rth

anN

YT

Ž.

L.O

.I.s

Los

son

igni

tion

.N

orm

ativ

eN

ean

dD

.I.

sA

bq

Orq

Ne

are

calc

ulat

edas

sum

ing

Fe

Or

FeO

s0.

5.S

ris

otop

icra

tio

isde

term

ined

onw

hole

-roc

ksa

mpl

es.

The

inte

rnal

erro

ron

all

Sr

isot

opic

rati

osis

23

0.00

001.

Key

for

loca

tion

s:T

L,

Tre

fola

;V

T,

Vet

erin

aria

;P

G,

Par

coG

rife

o;M

T,

Mon

tice

lli;

PR

,P

onti

Ros

si.

cc

cc

cc

cc

cc

cc

cc

cc

cc

cS

ampl

e:95

29bi

s95

3095

3395

31sp

195

4495

4295

4195

4095

3996

02A

196

02C

196

02D

196

02D

2c96

02D

296

02E

196

02F

196

02F

396

02H

196

02I1

Loc

atio

n:V

TP

GM

TM

TT

LT

LT

LT

LT

LP

RP

RP

RP

RP

RP

RP

RP

RP

RP

RU

nit:

base

top

oba

seo

top

rs

base

sto

pa

cd

dw

hite

dbl

ack

ef

base

fto

ph

i

SiO

57.7

755

.91

57.2

458

.47

60.5

860

.59

58.7

159

.61

59.4

859

.32

59.1

959

.36

59.0

557

.08

58.7

359

.82

59.7

058

.77

58.5

82

TiO

0.40

0.39

0.43

0.44

0.39

0.40

0.40

0.40

0.40

0.38

0.37

0.38

0.38

0.47

0.40

0.40

0.40

0.40

0.40

2

Al

O17

.90

18.6

517

.35

17.7

416

.98

17.0

417

.60

17.4

717

.43

18.2

218

.22

18.1

818

.11

18.4

118

.25

18.0

317

.97

18.1

818

.18

23

Fe

Oto

t3.

434.

094.

534.

602.

712.

693.

392.

892.

903.

183.

173.

113.

234.

743.

412.

892.

893.

373.

332

3

MnO

0.24

0.26

0.10

0.10

0.17

0.17

0.12

0.14

0.14

0.13

0.12

0.13

0.12

0.12

0.13

0.14

0.14

0.13

0.12

MgO

0.31

0.38

1.00

1.04

0.22

0.24

0.45

0.30

0.31

0.40

0.35

0.30

0.39

0.98

0.42

0.23

0.25

0.43

0.41

CaO

1.80

2.18

3.48

3.54

1.51

1.55

2.22

1.90

1.90

2.06

1.86

1.88

2.00

3.25

2.02

1.71

1.72

2.14

2.11

Na

O6.

596.

673.

313.

265.

785.

724.

385.

064.

984.

264.

384.

504.

323.

294.

325.

025.

024.

304.

322

KO

6.63

7.37

8.01

8.30

6.76

6.80

8.22

7.69

7.77

8.01

7.82

7.80

7.90

8.36

7.90

7.43

7.41

7.97

8.06

2

PO

0.11

0.12

0.24

0.24

0.07

0.08

0.12

0.09

0.09

0.08

0.05

0.05

0.06

0.20

0.07

0.04

0.03

0.07

0.07

25

L.O

.I.

4.54

3.70

3.87

1.87

4.58

4.48

4.11

4.23

4.37

3.82

4.18

3.97

4.11

2.99

4.07

3.99

4.18

3.93

4.10

Sum

99.7

299

.72

99.5

699

.60

99.7

599

.76

99.7

299

.78

99.7

799

.86

99.7

199

.66

99.6

799

.89

99.7

299

.70

99.7

199

.69

99.6

8

Ne

11.6

518

.69

0.30

0.01

2.22

2.10

3.59

3.80

3.78

1.05

0.81

1.27

1.20

1.90

1.72

2.03

2.14

2.06

2.73

D.I

.89

.49

87.0

178

.73

78.6

491

.66

91.3

786

.74

89.3

489

.28

86.1

286

.67

87.0

886

.29

78.3

585

.77

88.7

188

.67

85.6

485

.97

Be

2022

77

2121

1013

139

1011

108

1114

1411

11S

c3

35

52

23

33

33

33

63

33

33

V12

1593

8923

2455

3939

4846

4452

9954

3536

5351

Co

12

56

11

21

12

11

15

21

12

1Z

n11

2618

074

7491

9380

8687

6467

6674

7074

7777

7471

Ga

2527

1918

2323

1921

2017

1818

1918

1921

2019

19R

b38

535

825

826

241

140

130

534

433

525

127

027

429

325

929

531

731

628

928

6S

r14

1862

862

118

2325

261

5826

320

218

324

690

426

659

6328

126

1Y

6161

2425

5152

3242

4229

3233

3426

3442

4233

33Z

r66

169

225

726

066

065

436

147

147

530

634

436

036

026

537

247

346

535

635

6N

b11

412

534

3399

9853

7171

4148

5152

3453

6767

5049

Ba

925

987

1036

78

170

911

167

8269

118

1874

169

88

146

101

La

137

140

5353

135

134

8310

310

069

7783

8062

8299

9981

80C

e27

126

010

198

244

244

156

195

189

128

144

153

153

119

154

189

188

150

150

Pr

27.7

29.2

10.6

11.0

24.7

25.2

16.6

20.4

21.7

13.9

15.3

16.4

15.7

12.8

16.5

20.0

19.5

15.9

16.0

Nd

9497

4240

8384

5772

7552

5559

5748

6072

7056

57S

m16

.617

.47.

97.

714

.514

.510

.413

.113

.48.

99.

610

.110

.18.

510

.112

.411

.99.

89.

8E

u1.

51.

61.

92.

01.

41.

52.

42.

01.

91.

91.

81.

92.

02.

32.

11.

91.

82.

12.

1G

d13

.813

.15.

95.

610

.511

.47.

910

.19.

86.

67.

17.

67.

46.

27.

99.

09.

17.

57.

5T

b2.

12.

10.

90.

91.

61.

71.

11.

41.

41.

01.

01.

11.

10.

91.

11.

31.

41.

11.

1D

y11

.011

.34.

64.

89.

09.

16.

27.

68.

05.

35.

76.

15.

64.

85.

97.

57.

45.

75.

7H

o2.

12.

31.

01.

01.

91.

91.

21.

61.

61.

11.

21.

21.

21.

01.

21.

51.

51.

21.

2E

r5.

96.

22.

42.

35.

45.

43.

44.

24.

02.

62.

82.

93.

12.

33.

13.

93.

83.

03.

1T

m0.

90.

90.

30.

30.

80.

90.

50.

70.

60.

40.

40.

40.

40.

30.

50.

60.

60.

50.

5Y

b6.

46.

62.

42.

45.

65.

63.

04.

34.

22.

93.

13.

23.

12.

43.

34.

13.

93.

13.

0L

u1.

01.

10.

40.

41.

00.

90.

50.

60.

60.

40.

50.

50.

50.

30.

50.

60.

60.

50.

5T

h63

6222

2177

7735

4950

3134

3634

2336

4747

3334

8786

Srr

Sr

0.70

755

0.70

720

0.70

747

0.70

747

0.70

738

–0.

7074

90.

7074

50.

7075

00.

7073

60.

7073

40.

7073

8–

–0.

7074

8–

–0.

7075

50.

7075

6

cP

ost-

CIr

Pre

-NY

T.


Ž . Ž . Ž .Fig. 4. Classification diagrams. A Total alkalirsilica classification grid TAS; Le Bas et al., 1986 ; B normative nepheline versusŽ . Ž .Differentiation Index snormative OrqAbqNe classification grid Armienti 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, CFpost-CIrpre-NYT rocks.

Two samples collected at the Torregaveta sectionfrom deposits emplaced during the Fiumicello erup-

Ž .tion Procida Island are trachybasaltic in composi-tion and have been plotted only in the chemostrati-

Ž . Žgraphic diagrams Fig. 5 . The latitic sample D.I.s.67, Zrs273 ppm was collected from the pyroclas-

Ž .tic deposit unit TRb at the base of the stratigraphicŽ .sequence of the Torregaveta section Fig. 5 . The

lowermost pyroclastic deposits of the stratigraphicŽsequence drilled at Ponti Rossi section units from

.PRa to PRg and the pyroclastic deposit at the baseŽ . Žof Trefola section unit TLa are trachytes D.I.s

. Ž76–80; Zrs300–350 ppm . Alkali-trachytes D.I.s.84–92; Zrs300–700 ppm characterize the pyro-

clastic deposits of the upper part of the stratigraphicŽsequence drilled at Ponti Rossi units from PRi to

.PRm , and five pyroclastic deposits exposed in the

middle part of the stratigraphic sequence at Torre-Ž .gaveta section units TGe, f , g, h, j, k, l . In1 – 6

particular, those exposed at Torregaveta, except forŽTRj–l, have the highest D.I. values D.I.s90–92,

.Zrs400–580 ppm . Samples from Punta MarmoliteŽ . Ž .sample PM and Cuma sample CU lava domes,and the upper pyroclastic deposits exposed at the

Ž .Trefola units from TLe to TLm and TorregavetaŽ .units from TRi to TRm sections are phono-trachytesŽD.I.s87–89; Zrs500–700 ppm; Figs. 4 and 5;

.Tables 2–4 .By comparing the results of chemical analyses of

all the studied sections, a general correlation betweencomposition and stratigraphic height is observed, i.e.the Zr content is almost constant at 300 ppm before60 ka, then, between 60 to 44 ka, before increasesuntil 700 ppm, and after decreases to ca. 500 ppm, to


Ž .Fig. 6. Harker diagrams of major elements wt.% versus D.I. forpre-CI volcanics. Major element data were normalized to 100% onvolatile-free basis. Legend: open triangles, CF pre-CI rocks; solidtriangles, samples from volcanic deposits attributed to IschiaIsland erupted before 37 ka, see text for further explanations;

Ž .Field: CI rocks data from Civetta et al., 1997 .

Ž .Fig. 7. Harker diagrams of trace elements ppm versus D.I. forpre-CI volcanics. Trend A: samples from volcanic deposits eruptedfrom CF. Trend B: samples from volcanic deposits attributed toIschia Island, see text for further explanations. Symbols and fullfield as in Fig. 6. White field: samples from volcanics produced

Žfrom vents located on Ischia Island )35 ka; data from Civetta et.al., 1991c .

increase again to ca. 700 ppm before the CI eruptionŽ .37 ka .

Major element variation diagrams of the volcanicsŽ .older than 37 ka pre-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; solidtriangles, samples from volcanic deposits attributed to Ischia Island erupted before 37 ka, see text for further explanations; squares, CFpost-CIrpre-NYT rocks.


Fig. 8. Chondrite-normalized REE abundance patterns for ana-Ž .lyzed samples. Normalization values from Henderson 1984 .

Legend: open triangles, CF pre-CI rocks; solid triangles, samplesfrom volcanic deposits attributed to Ischia Island erupted before37 ka, see text for further explanations; squares, post-CIrpre-NYTrocks.

Ž .respect to that of CI products Fig. 6 . SiO , MnO,2

and Na O contents increase, whereas Fe O tot,2 2 3

MgO, CaO and P O contents decrease at increasing2 5Ž .D.I. Fig. 6 . K O and TiO contents increase for2 2

D.I. values from 67 to 81 and then decrease. Al O2 3

content is roughly constant, although with a largeŽ .scatter, at increasing D.I. Fig. 6 .

Ž .REE except Eu , Y, Nb, Zr, Rb, and Th plottedversus D.I. describe two trends named A and B inFig. 7, both characterized by a positive correlation

between incompatible element contents and the de-gree of differentiation. Sr, Ba, Eu, and ferromagne-sian elements display single depletion trends withincreasing differentiation. The trend A overlaps the

Ž .Fig. 9. Harker diagrams of major elements wt.% versus D.I. forvolcanics erupted between CI and NYT. Major element data werenormalized to 100% on volatile-free basis. Legend: squares, post-

ŽCIrpre-NYT rocks. Field: NYT rocks data from Orsi et al.,.1995 .


Ž .Fig. 10. Variation diagrams of trace elements ppm versus D.I.for volcanics erupted between CI and NYT. Legend: squares,

Žpost-CIrpre-NYT rocks. Field: NYT rocks data from Orsi et al.,.1995 .

compositional range of CI products. The trend B,characterized by the highest values of D.I., is de-scribed by the samples collected from the pyroclasticdeposits of the middle part of the Torregaveta strati-graphic sequence. The geochemical characteristics ofthis group of volcanics correspond to those of de-posits erupted from vents located on Ischia Island

Žduring the same period of time )35 ka; Civetta et.al., 1991c . Therefore, they do not belong to the CF

and will not be taken into account in the followingdiscussion.

All the volcanic rocks older than 37 ka showenrichments in both LREE and HREE at increasing

Ž .degrees of differentiation Tables 2–4 and Fig. 8 ,these are only partially comparable to CI that show awide distribution. Chondrite-normalized REE distri-butions display enrichment in LREE relative toHREE, with the latter showing increasing but almostflat patterns. All samples display negative Eu anoma-lies. EurEuU decreases from 0.98 to 0.21 with dif-

Ž .ferentiation Fig. 8A .87Srr86Sr values were measured on whole-rock

Ž . Žpumice 64 samples and lava fragments six sam-.ples , and on feldspar phenocrysts separated from CI

Ž . 87 86pumice six samples . Srr Sr ratios of leachedpumice and lava samples range from 0.70681"1 to0.70735"1. CI feldspars have Sr-isotope ratiosranging from 0.70730 to 0.70732, similar to that ofthe least-evolved CI pumice, as also reported previ-

Ž .ously by Civetta et al. 1997 .Ž .The least-evolved rocks D.I.s67–79 have the

87 86 Žleast radiogenic Srr Sr values 0.70681"1 to.0.70694"1 . These samples represent the two low-

ermost units of the Torregaveta and the basal unit ofŽthe Trefola stratigraphic sequences TGa-b and TLa,

.respectively , and the six lowermost units cored inŽ . 40 39the borehole at Ponti Rossi PRb to PRg . Arr Ar

age determinations made on the trachytic samplesexposed at the Trefola section show that unit TLa, atthe base of the sequence, has an age of 58"3 ka.The 87Srr86Sr variations among these trachytic unitsare related neither to stratigraphic height nor todegree of chemical evolution.

Ž .The most-evolved D.I.s82–91 pumice samplesŽin the upper part of the studied sequences Ponti

Rossi, Trefola, Torregaveta, Punta Marmolite, and. ŽCuma have the most radiogenic values 0.70705"1

. Ž .to 0.70735"1 Fig. 5 . The Sr-isotope ratios ofthese samples increase with D.I., from 0.70705"1to 0.70730"1, and are also time-related, with thesamples higher in the section also being more radio-

Ž .genic Fig. 5, Tables 2–4 . The uppermost units inŽ .the studied sequences PRl, TLh, TLm, TGl, TGm

have 87Srr86Sr ratios similar to those of the feldsparsand the least-evolved pumice fragments of the CIŽ . 40 39Fig. 5 . Arr Ar age determinations made on theuppermost of these phono-trachytes immediately un-derlying the CI at the Trefola and Ponti Rossi sec-tions are 45.6"0.7 and 44.3"0.8 ka, respectively.


Fig. 11. Modeling of fractional crystallization and mixing processes obtained using the IGPET calculation program, based on theŽ .formulation by De Paolo 1981 . 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

Ž .magma. D s3.5, D s1.5 Villemant, 1988; Pappalardo, 1994 , Fs fraction of residual magma. Rs0.2% fraction of mixed magma.Sr CaO

Legend: open trianglessCF pre-CI rocks; squaresspost-CIrpre-NYT rocks.

The five pyroclastic deposits exposed in the mid-dle part of the stratigraphic sequence of the Torre-

Žgaveta section have Sr-isotope ratios 0.7067 to

.0.7068 similar to those of volcanics erupted fromvents located on the island of Ischia during the same

Žtime period 0.7068 to 0.7069; Civetta et al., 1991c;


.Fig. 5 . This evidence corroborates the hypothesisthat they erupted from Ischia, as proposed on thebasis of their trace elements compositions.

The trachybasalts from the Fiumicello depositŽ . 87 86Procida Island have Srr Sr ranging from0.70649"1 to 0.70654"1, significantly different

Ž .from those of the Phlegraean deposits Fig. 5 .All the studied stratigraphic sequences older than

37 ka are overlain by CI deposits. The distal CIpyroclastic deposits have been recently studied by

Ž .Civetta et al. 1997 . These authors demonstratedthat during the CI eruption three magmas wereerupted. The first erupted had the most evolvedphono-trachytic composition, and 87Srr86Sr ratios ofabout 0.70745. The last erupted magma was tra-chytic and had 87Srr86Sr ratios of about 0.70730.During the intermediate phase of the eruption, amingled magma was erupted, with an alkali-trachyticcomposition and 87Srr86Sr ratios intermediate be-tween the most- and least-radiogenic magmas.Feldspar crystals separated from pumice fragmentsrepresentative of the three magmas have the same87Srr86Sr ratios that of the least-differentiated magmaŽ .about 0.70730; Civetta et al., 1997; this paper .

4.4. Post-CI pyroclastic deposits and laÕa domes

ŽIn the total alkalirsilica diagram TAS, Fig. 4A;.Le Bas et al., 1986 most analyzed samples from

deposits emplaced between the CI and NYT plot inthe fields for trachyte and phonolite, and few plot inthe fields for latite and tephri-phonolite. All sampleshave a potassic alkaline affinity, with Na Oy2F2

ŽK O. In the D.I.–Ne classification grid Fig. 4B;2.Armienti et al., 1983 the samples range in composi-

tion from trachyte to alkali-trachyte to phono-trachyteŽ .D.I.s77–91; Fig. 4B . The least-evolved trachytesoccur in the pyroclastic deposit erupted from the

Ž .Monticelli volcano D.I.s78–79; Zrs270 ppmand in the pyroclastic deposit at the base of thestratigraphic sequence of the Trentaremi volcanoŽ .D.I.s77–82; Zrs250–330 ppm . Pumice samplescollected from the pyroclastic deposits exposed at

Ž .the Camaldoli hill Verdolino section , at the PontiRossi, Coroglio, and Trefola sections, and the prod-ucts of the Monte Echia tuff cone are alkali-trachytesŽ .D.I.s80–92; Zrs265–660 ppm . Most alkali-trachytes exposed in the studied sections have homo-

geneous major and trace element compositionsŽ .D.I.s80–90 and Zrs265–470 ppm , only slightlymore differentiated then the last erupted CI magma;only the alkali-trachyte cropping out at Trefola im-mediately over the CI has a more differentiated

Ž .composition D.I.s91–92; Zrs650 ppm . TheŽmost-evolved phono-trachytic compositions D.I.s

.87–90; Zrs660–780 ppm are shown by the Mon-tesanto lava dome and by the Tufi Antichi pyroclas-

Ž .tic deposits Veterinaria and Parco Grifeo sections .The post-CI trachytes are characterized by major andtrace element contents, and REE patterns, similar topre-CI trachytes.

Major element oxides and selected trace elementcontents for post-CI samples are plotted versus D.I.in Figs. 9 and 10. Generally SiO , MnO, and Na O2 2

contents increase with increasing D.I., whereasFe O tot, MgO, CaO, K O and P O contents de-2 3 2 2 5

crease. TiO contents decrease with D.I. values in-2

creasing from 76 to 83, then remain constant. Al O2 3Ž .contents are scattered. REE except Eu , Y, Zr, Nb,

Rb, Be, and Zn generally show positive correlations,whereas Sr, Ba, Eu, Sc, and V show negative corre-

Ž .lations, relative to D.I. Fig. 10 and Tables 2–4 . Agroup of four phono-trachytic samples, i.e. thosecollected from the Montesanto lava dome and theTufi Antichi pyroclastic deposits, are the mostevolved samples and show the highest enrichmentsin several incompatible trace elements.

The chondrite-normalized REE patterns are char-acterized by a high degree of enrichment of LREE,and relatively flat HREE distributions. All samples

Ž Udisplay negative Eu anomalies EurEu s0.94–.0.29 , which increase with increasing degree of

Ž .chemical evolution Fig. 8B .Sr isotopic compositions of selected post-CI vol-

Žcanics range from 0.70720"1 to 0.70756"1 Ta-.bles 2–4 . The only exception is the sample col-

lected from the Monte Echia tuff, which is lessŽ .enriched in radiogenic Sr 0.70678"1 . No correla-

tion is observed between Sr isotope ratios and degreeof chemical evolution. Conversely, a good correla-tion exists between the Sr isotopic composition andthe stratigraphic position of the analyzed samples,with samples being more radiogenic upsections. Inparticular, the lowermost deposits in the investigatedstratigraphic sections have lower 87Srr86Sr ratiosŽ .0.70728"1 to 0.70738"1 similar to that of the


ŽCI feldspars and least-evolved CI pumice 0.70730;. 40 39Civetta et al., 1997 . Arr Ar determinations made

on these deposits exposed at the base of the strati-graphic successions of Ponti Rossi, Trefola, andVerdolino sections gave ages of 16.1"0.2, 17.9"

Ž .0.5 and 30.3"0.2 ka, respectively Fig. 5 . Theupper post-CI deposits in the investigated strati-graphic sections have higher 87Srr86 Sr ratioŽ .0.70745"1 to 0.70756"1 with values that in-crease upsections reaching the value of the magma

Žfeeding the first phase of the NYT eruption ca.. 40 390.70756; Orsi et al., 1995 . Arr Ar determinations

made on samples from these deposits exposed at thetop 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 featuresŽ .of NYT deposits were studied by Orsi et al. 1995 .

These authors demonstrated that during the NYTeruption three distinct magmas were erupted. Thefirst-erupted magma had a homogeneous alkali-trachytic composition, with a 87Srr86Sr ratio about0.70756. Successively, a less evolved trachyticmagma was erupted, with a slight, continuous com-positional variation, and a 87Srr86Sr ratio similar tothat of first-erupted magma. The last-erupted magmawas strongly zoned from alkali-trachyte to latite,with a 87Srr86Sr ratio about 0.70752. Feldspar sepa-rated from pumice representative of the three magmacompositions are in isotopic equilibrium withwhole-rock.

5. Discussion

5.1. The eÕolution of the Phlegraean magmatic sys-tem before the CI eruption

The pre-CI trachytes show continuous major andtrace element variations with respect to degree of

Ž .differentiation Figs. 6 and 7 . This could be ac-counted for by a simple fractional crystallizationprocess in a closed system with feldspars as thedominant phase. However, the Sr-isotopic variationsŽ .0.7068–0.7073 suggest that other processes musthave operated during the chemical evolution of thesemagmas. In particular, before about 60 ka, lessevolved, trachytic magmas with fairly homogeneous

Ž87 86chemical and Sr-isotopic compositions Srr Srs0.70681"1 to 0.70694"1; D.I.s76–80; Zrs

.300–350 ppm were erupted, testifying that the sys-tem acted as a closed system. From about 60 to 44ka, alkali-trachytic magmas became progressivelymore differentiated and their Sr-isotopic composi-

Ž87 86tions became progressively more radiogenic SrrSrs0.7070 to 0.7073; D.I.s84–90; Zrs300–700

.ppm . Thus, from 60 to 44 ka the Phlegraean mag-matic system displayed open-system behavior, thepossible causes of which will be discussed later.Then, from 44 to 37 ka, alkali-trachytic magmassimilar in 87Srr86Sr to the least evolved trachyticmagma of the CI were erupted, suggesting that thePhlegraean system was fed by the most differenti-ated, uppermost portion of the growing CI magmachamber, well before the CI eruption.

Several processes could have produced the chemi-cal and isotopic variations observed between 60 and

Ž .44 ka: 1 contamination of a magma reservoir byŽ .hydrothermal fluids enriched in radiogenic Sr; 2

country rock assimilation by the magma combinedŽ . Ž .with fractional crystallization AFC process ; 3 in

87 87 Ž .situ Sr in growth by Rb decay; 4 mixing pro-Ž .cess; or 5 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-isotopeŽ87 86ratios for hydrothermal phases Srr Srs0.709;

.Srs200 ppm; Villemant, 1988; Civetta et al., 1991aŽcored in the deep geothermal wells at the CF Rosi

.and Sbrana, 1987 . Results of quantitative modelingŽ .e.g., Faure, 1986 , however, indicate that contami-nation of magma by up to 90% by such fluids wouldbe needed to explain the observed increase in87Srr86Sr. Such high amounts of fluids should changeheavily the concentrations of mobile elements, suchas alkalies and alkali-earth elements in the magmasor rocks, that on the contrary show the same behav-ior of immobile elements, so that this possibility canbe easily ruled out.

To test the second possibility, an AFC processŽ .De Paolo, 1981 was simulated using as a potential

Ž87 86assimilant Mesozoic carbonate rocks Srr Srs0.70769–0.70775; Srs300–1000 ppm; Civetta et

.al., 1991b , even though the absence of limestonexenoliths in the pyroclastic sequences of the CFseems to exclude that the magmatic system waslocated in or below a sedimentary basement. Model-


ing was made using the lowest and the highestsolidrliquid partition coefficients measured for CF

Ž .trachytic rocks by Villemant 1988 and PappalardoŽ .1994 . Results show that the Ca content of the

Ž .carbonate assimilant Cas30 wt.%; Faure, 1991 istoo high to be consistent with the observed negativecorrelation between CaO content and 87Srr86Sr ratio.Thus, the AFC process cannot be responsible for thechemical and isotopic variations shown by the prod-ucts erupted between 60 and 44 ka.

The third possibility involves in situ 87Rb decayŽ .in a high RbrSr magma chamber. Cavazzini 1994

proposed equations to calculate the increase in87Srr86Sr ratio during a time-protracted fractionalcrystallization process of magmas with high RbrSr.

Ž .We used this procedure to calculate the time tŽ .required for crystallization of a magma assuming: 1

Žthe solidrliquid distribution coefficients for Rb DRb. Ž .s0.6 and Sr D s3 reported in the literature forSr

ŽCF trachytic rocks Villemant, 1988; Pappalardo,. Ž .1994 , 2 a residual liquid fractionation of 0.3,

which is obtained by simulating a fractionation pro-cess from the least-evolved trachytic to the most-evolved phono-trachytic magmas using mineral com-

Žposition measured by electron microprobe our un-. Ž . 87 86published data , and 3 the Srr Sr ratios detectedŽ . Žin the least- s0.70681 and most-evolved s

.0.70730 rocks. The resulting age is 600 ka, muchhigher than the age measured on the stratigraphicallylowest products, isotopically similar to the CI. Thegeochronological and isotopic data which are earlierpresented indicate that before 60 ka the system hadgeochemical and isotopic characteristics distinct withrespect to the CI magmas, and that the latter proba-bly recharged the system at this age. As a conse-quence, the value of 600 ka is unrealistic on the basisof geochronological and isotopic data.

A simple mixing processes between two end-members can be ruled out. In fact in 87Srr86Sr ratios

Ž .vs. 1rSr diagrams not reported samples do notdescribe a straight line indicative of mixing between

Ž .two components Faure, 1986 .Finally, the last possibility is a process of com-

Ž . Ž .bined fractionation F , replenishment R , and mix-Ž .ing M mechanisms. In this hypothesis we assume

the existence of a magma chamber in which trachyticmagma evolves by crystal fractionation, is replen-ished by a fresh batch of trachytic and more radio-

genic CI-type magma, and undergoes contemporane-ous mixing with the newly arrived magma. To testthis model we made calculations using the IGPET

Ž .program version 1994 and the values ofsolidrliquid partition coefficients comprised in therange reported for CF trachytic rocks by VillemantŽ . Ž .1988 and Pappalardo 1994 . The obtained resultsshow that the Sr-isotope variation observed in sam-ples older than the CI products can be reproduced

Žassuming that the trachytic magma magma 1 in Fig..11 underwent 70% fractional crystallization with

alkali-feldspar as the dominant phase, and mixedwith about 20% of a magma with the composition ofthe least-evolved trachytic CI magma, to reach thecomposition of magma 2 in Fig. 11, where thetheoretical curve ‘‘a’’ fits the experimental data well,supporting this hypothesis. This model suggests ahigh amount of crystal fractionation, although mostanalyzed rocks are aphyric. This apparent contradic-tion can be reconciled hyphothesizing ‘‘in situ’’

Žgrowth of crystals e.g. McBirney et al., 1985; Nil-.son et al., 1985; Turner and Campbell, 1986 . Ac-

cording to this model crystals are thought to nucleateand grow in situ on the floor and walls of thechamber, while the evolved liquid, originated at thecontact with the growing crystals, migrates towardsthe upper part of the chamber due to its lowerdensity. This mechanism can generate poorly-porphyritic strongly evolved liquids. This hypothesis

Žis corroborated by thermal modeling Wohletz et al.,.1999 , indicating that a great volume of magma is

present beneath the CF caldera, as well as magneticŽ .modeling Orsi et al., 1999 , indicating that this

magma should be hot, presumably molten, since itgives no magnetic anomalies.

Ž .Civetta et al. 1997 demonstrated that the CItrachytic magma evolved by fractional crystallizationin a closed system before the CI eruption. Thevariations in 87Srr86Sr ratio observed in the CI rockswere explained by interaction of the uppermost dif-ferentiated layer with fluids enriched in radiogenic

Ž .Sr just before eruption Civetta et al., 1997 .

5.2. The eÕolution of the Phlegraean magmatic sys-tem after the CI eruption and before the NYT erup-tion

ŽGeochronological data Alessio et al., 1973; Scan-.done et al., 1991; this paper and stratigraphic data


Ž .Orsi et al., 1996 suggest that a period of moderateeruptive activity occurred between 37 and 16 ka, inagreement with tephrostratigraphic data from the

Žcentral Mediterranean Sea Paterne and Guichard,.1993 . In this period, a series of explosive eruptions

ejected alkali-trachytic to trachytic products isotopi-cally very similar to the last-erupted CI magma. Asin the case of the products older than 37 ka, majorand trace element contents of these products can bequalitatively modeled by fractionation processes in-volving the observed mineral phases. However, thevariations in Sr-isotope ratios suggest that processesother than simple fractional crystallization must haveoperated. The observed isotopic variations cannot bedue to processes involving bulk or selective assimila-tion, since no correlation is observed between87Srr86Sr ratio and degree of chemical evolution. Itcannot be the result of in situ growth of radiogenicSr, since the increase of radiogenic Sr is not corre-lated to the 87Rbr86Sr ratios. We suggest that thePhlegraean magmatic system, after the CI eruption,acted first as a closed system: the 16 ka old, lower-most exposed deposits have constant 87Srr86Sr ratiossimilar to that of the least-evolved CI volcanics.

Ž .Quantitative modeling curve b in Fig. 11 madeusing the IGPET program shows that the most-

Ž .evolved of these volcanics magma 4a in Fig. 11could be derived by 70% of fractionation of the

Žleast-evolved CI trachytic magma magma 3 in Fig..11 . This modeling implies that, between 37 and 16

ka, the least-evolved trachytic magma left in thechamber after the CI eruption evolved in a closedsystem to produce an uppermost-differentiated, al-kali-trachytic layer. Afterwards, magmas were

87 86 Žerupted with higher Srr Sr ratios 0.70745–.0.70756 , increasing with time up to 14.6 ka, and

reaching that of the magma that fed the first phase ofŽ .the NYT eruption 0.70756; Orsi et al., 1995 . A

process of mixing between the newly arrived, moreradiogenic NYT magma with a resident fractionatingmagma similar to the least-evolved CI trachyte is

Ž .suggested. Quantitative modeling IGPET shows thatŽ .the Sr isotopic variations curve c are well ac-

counted for by 70% of fractional crystallization of aparental magma with the composition of the least-evolved CI trachyte and mixing with 50% of amagma similar in composition to the NYT magmaŽ .curve c in Fig. 11 , reaching the composition ofmagma 4b in Fig. 11.

6. Conclusions

Geochronological data and petrological investiga-tions on products older than the CI eruption andproducts erupted between the CI and NYT eruptionsshow that the Phlegraean magmatic system was peri-odically recharged by Sr-isotopically distinct mag-mas tapped at different times. In particular, in theearliest stages of the Phlegraean magmatic activityup to 60 ka, magmas with trachytic composition and

87 86 Ž .low Srr Sr ratios about 0.7068 were feeding thesystem and periodically erupted. The products of thisactivity crop out at the base of the Trefola andTorregaveta sections and occur in the lowermostportion of the bore-hole drilled in the city of Napoliat Ponti Rossi. Subsequently, more evolved magmas,alkali-trachytic to phono-trachytic in composition,characterized by more radiogenic Sr-isotopic compo-sitions, intermediate between those of the earlier

Ž87 86 .trachytic magmas Srr Sr s 0.7068 and theŽ87 86 .least-evolved CI magma Srr Srs0.7073 were

feeding the system and periodically erupted from atleast 60 to 44 ka. These have been interpreted as‘‘hybrid’’ magmas generated by mixing of the resi-dent fractionating magmas with newly arrived, more

Ž .radiogenic CI trachytic magma s . These data indi-cate that before 60 ka the system had geochemicaland isotopic characteristics distinct with respect tothe CI magmas that probably recharged the system atthis age; this hypothesis excludes the model pro-

Ž .posed by Cavazzini personal communication thatestimated an evolution time of 350–520 ka for theCI magma chamber.

Then during a period of moderate subaerial vol-canic activity between 37 and 16 ka, trachytic toalkali-trachytic magmas isotopically similar to the

Ž .last-erupted CI magma s and presumably derived by70% fractionation of the least-evolved CI trachyticmagma left in the system, were erupted. Subse-quently, magmas isotopically similar to that eruptedduring the first phase of the NYT eruption reachedthe system and mixed with the resident magmasproducing hybrid magmas with isotope ratios inter-mediate between that of the least-evolved CI tra-

Ž .chytic magma 0.70730 and that of the NYT magmaŽ .0.70756 . The age of the youngest of these productsis 14.6 ka. At 12 ka, during the NYT eruption,magmas geochemically and Sr-isotopically well dis-tinct from the CI magmatic system were emitted,


testifying that in a short time interval, i.e. few thou-sands years, a large trachytic magma body wasemplaced above the uppermost part of the residualCI magma reservoir.

Thus, geochronological, geochemical and Sr-iso-topic data on the CF volcanics older than the NYTeruption, combined with stratigraphical and vol-canological data, strongly suggest that the Phle-graean magmatic system before 12 ka behaved as acomplex system, undergoing periods of simple frac-tional crystallization of the resident magmas, andperiods of replenishments by distinct, fractionatingbatches of magmas that mixed with the residentmagmas. These hybrid magmas were periodicallyextracted to feed moderate- to large-volume erup-tions.

Acknowledgements

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

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Chemical and Sr-isotopical evolution of the Phlegraean magmatic system before the Campanian...

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