CO2-DRIVEN LARGE MAFIC EXPLOSIVE ERUPTIONS: A CASE STUDY FROM THE COLLI ALBANI VOLCANIC DISTRICT (ITALY)

RESEARCH ARTICLE

CO2-driven large mafic explosiveeruptions: the Pozzolane Rosse case studyfrom the Colli Albani Volcanic District (Italy)

Carmela Freda & Mario Gaeta & Biagio Giaccio &

Fabrizio Marra & Danilo M. Palladino &

Piergiorgio Scarlato & Gianluca Sottili

Received: 2 November 2009 /Accepted: 14 August 2010# Springer-Verlag 2010

Abstract Generally, the intensity and magnitude of explo-sive volcanic activity increase in parallel with SiO2 content.Pyroclastic-flow-forming eruptions in the Colli Albaniultrapotassic volcanic district (Italy) represent the moststriking exception on a global scale, with volumes on theorder of tens of cubic kilometres and K-foiditic composi-tions (SiO2 even <42 wt.%). Here, we reconstruct the pre-eruptive scenario and event dynamics of the ~456 kaPozzolane Rosse (PR) eruption, the largest mafic explosiveevent of the Colli Albani district. In particular, we focus onthe driving mechanisms for the unusually explosiveeruption of a low-viscosity, mafic magma. Geologic,petrographic and geochemical data with mass balancecalculations, supported by experimental data for ColliAlbani magma compositions, provide evidence for signif-icant ingestion of carbonate wall rocks by the PozzolaneRosse K-foiditic magma. Moreover, the scattered occurrenceof cored bombs in Pozzolane Rosse pyroclastic-flowdeposits records carbonate entrainment even at the eruptivetime scale, as also tested quantitatively by thermal modelling

of magma–carbonate interaction and carbonate assimilationexperiments. We suggest that the addition of free CO2 fromdecarbonation of country rocks was the major factorcontrolling magma explosivity. High CO2 activity in thevolatile component, coupled with magma depressurisation,produced extensive leucite crystallisation at short timescales, resulting in a dramatic increase in magma viscosityand volatile pressurisation, which was manifested a changeof eruptive dynamics from early effusion to the PozzolaneRosse's highly explosive eruption climax.

Keywords Mafic explosive eruptions . Eruptionmagnitude . Pyroclastic flow . Colli Albani . Potassicvolcanism . Carbonate assimilation . CO2

Introduction

Volcanic explosivity, in terms of mass discharge rate(intensity) and erupted magma volume (magnitude), gener-ally correlates with SiO2 content (Fig. 1 and referencestherein), i.e. relatively low-viscosity mafic magmas mostlyfeed effusive or mildly explosive eruptions (e.g. hawaiianand strombolian), whereas high-viscosity silicic magmasfeed plinian and pyroclastic-flow-forming eruptions.

Although some examples of subplinian to plinian, andsmall-volume pyroclastic-flow-forming, mafic eruptions arereported in the literature (e.g. Fuego, Guatemala, Davies etal. 1978; Rose et al. 1978; Tarawera, New Zealand, Walkeret al. 1984; Houghton et al. 2004; Mount Etna, Coltelli etal. 2005; Nicaragua, Costantini et al. 2009) and evenbasaltic welded ignimbrite associated with caldera collapse(Gran Canaria, Freundt and Schmincke 1995), the productsof major explosive eruptions from the ultrapotassic Colli

Editorial responsibility: D.B. Dingwell

C. Freda :M. Gaeta : F. Marra : P. ScarlatoIstituto Nazionale di Geofisica e Vulcanologia,Rome, Italy

M. Gaeta :D. M. Palladino (*) :G. SottiliDipartimento di Scienze della Terra,Sapienza-Università di Roma,P.le Aldo Moro 5,I00185 Rome, Italye-mail: [email protected]

B. GiaccioIstituto di Geologia Ambientale e Geoingegneria–CNR,Rome, Italy

Bull VolcanolDOI 10.1007/s00445-010-0406-3

Albani Volcanic District (Roman Province, Italy) representthe most noticeable exception (Fig. 1). In fact, the ColliAlbani pyroclastic-flow deposits attain individual volumeson the order of tens of cubic kilometres (de Rita et al. 1995)and are K-foiditic in composition, with SiO2 contents aslow as 42 wt.% (Trigila et al. 1995; Palladino et al. 2001;Gaeta et al. 2006), much lower even than those of basalts.

Here, we focus on the driving mechanisms for largemafic explosive eruptions producing voluminous scoriaflow deposits and, in particular, on the largest Colli Albanieruptive event: the Pozzolane Rosse explosive eruption(Fornaseri et al. 1963; de Rita et al. 1988a, b, 1995;Giordano and Dobran 1994; Trigila et al. 1995; Karner etal. 2001).

Since the relationship between eruption intensity andSiO2 content for the Colli Albani eruptions is remarkableamong all those described in publication (Fig. 1), the styleof Colli Albani eruptions that produce large Colli Albaniscoria flows cannot be easily explained using conventionalmodels for silicic magmatic. They are also not wellexplained by models for hydromagmatic, explosive erup-tions in which external water is the dominant driving factorfor explosivity. We infer that the reason for this discrepancyis that traditional models explaining the range of explosiv-ity of silicic eruptions in terms of magma ascent rate andconduit stresses (e.g. Geschwind and Rutherford 1995;

Jaupart 1996; Couch et al. 2003; Burgisser and Gardner2004) refer to the behaviour of H2O-dominated, typicallyhighly viscous magmas; application of these models to low-viscosity, mafic, ultrapotassic magmas is questionable.

We reconstruct the pre-eruptive magma system anderuptive dynamics of the Pozzolane Rosse, based ongeologic, petrographic and geochemical data and massbalance calculations which, consistent with laboratoryexperiments using Colli Albani magmas, provide evidenceof an uncommon magma evolution process controlled bycarbonate assimilation (Freda et al. 1997, 2008; Gaeta et al.2009; Mollo et al. 2010). The scattered occurrence of coredbombs in Pozzolane Rosse pyroclastic-flow deposits tes-tifies that ingestion of carbonate country rocks occurredeven at the eruptive time scale, as also supported by thermalmodelling of magma–carbonate interaction. We thus pro-pose that CO2 influx from country rocks triggered theanomalous explosivity of very low silica, ultrapotassicmagmas at Colli Albani.

Geological setting

The Colli Albani is located along the Tyrrhenian Seamargin of Central Italy (Fig. 2), a region where Quaternarygeodynamic evolution has been strongly influenced by the

Fig. 1 Relationship of erupted magma volumes (DRE) vs. SiO2

contents for well-known major explosive eruptions. The ~456-kaPozzolane Rosse (PR) ultrapotassic eruption in the Colli AlbaniVolcanic District (Roman Province) plots strikingly out of the globaltrend of increasing magnitude with SiO2 content: The PR event isremarkably SiO2 poor not only relative to the main trend defined byintermediate- to large-scale calc-alkaline explosive eruptions but alsorelative to major events of the potassic active volcanoes of theNeapolitan area (Somma-Vesuvius and Campi Flegrei). References forvolume and compositional data and interpretation of eruptivedynamics: Fish Canyon Tuff, Scaillet et al. (1998); Toba (71 kyrBP),Scaillet et al. (1998); Bishop Tuff (0.7 Ma), Anderson et al. (1989);Campanian Ignimbrite (39 kyrBP), Palais and Sigurdsson (1989),

Civetta et al. (1997); Crater Lake (7.6 kyrBP), Bacon and Druitt(1988), Klug et al. (2002); Tambora 1815, Devine et al. (1984);Neapolitan Yellow Tuff (12 kyrBP), Orsi et al. (1992, 1995); Taupo181AD, Palais and Sigurdsson (1989); Gran Canaria P1 (14.1 Ma),Freundt and Schmincke (1995); Minoan (3,650 yearsBP), Devine et al.(1984); Krakatau 1885, Mandeville et al. (1996); Katmai 1912,Westrich et al. (1991); Laacher See (12.9 kyrBP), Harms andSchmincke (2000); Pinatubo 1991, Westrich and Gerlach (1992);Somma-Vesuvius 79AD, Cioni et al. (1999); Bezymianny 1955–1956,Palais and Sigurdsson (1989); Agung 1963, Self and King (1996);Tarawera 1886, Palais and Sigurdsson (1989); Mount St. Helens 1980,Devine et al. (1984), Gerlach and McGee (1994); Laki 1783 (tephravolume only), Thordarson et al. (1996)

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superposition of two major tectonic features. A NE–SWextensional regime, due to the back-arc extension related tothe opening of the Tyrrhenian Basin at the rear of theApennine Chain, is considered responsible for the devel-opment of the volcanic districts of the Roman Province(Serri et al. 1993, Peccerillo 2005, and references therein).At the same time, strike–slip faulting associated with a N–Sshear zone known as the Sabina Fault Zone (Alfonsi et al.1991) affected the area of Rome (Faccenna et al. 1994),generating a transpressive regime characterised by a NE-striking, horizontal σ1 (Marra 2001). The presence of acrustal discontinuity along the Sabina Fault has beensuggested as the causal mechanism for the superpositionof the two competing stress fields, generating alternatingtranspressive and extensional tectonics (Marra 1999). TheSabina Fault developed at the rear of the Olevano-Antrodoco Line: the outermost front along which thesilicic–carbonatic terrains of the Sabina succession over-thrust the Latium-Abruzzi carbonate platform (Parotto andPraturlon 1975). The Colli Albani, located on the southernprolongation of this tectonic lineament, is rooted in anoverthickened carbonate succession (Funiciello and Parotto1978; Chiarabba et al. 1997) extending from 0.5 to 1 km of

depth down to the transition to the crystalline basement,which tomographic studies tentatively place between 5 and7 km (Bianchi et al. 2008).

Colli Albani magmatism is known for its peculiardifferentiation trend within the Quaternary ultrapotassicRoman Province, leading to constant or even decreasingSiO2 contents with increasing alkali contents (Trigila et al.1995; Gaeta et al. 2006, and references therein). Despite therather uniform magma composition, volcanic activity(~608–36 ka, Marra et al. 2003, 2009, and referencestherein) encompassed a wide spectrum of eruption styles,intensities and magnitudes, ranging from effusive to mildlyexplosive (strombolian and hydromagmatic) to largepyroclastic-flow- and caldera-forming events. Colli Albanivolcanism was traditionally subdivided into three mainperiods of activity (e.g. de Rita et al. 1988a, 1995): (1)Tuscolano-Artemisio (0.56–0.35 Ma), dominated by largeexplosive eruptions; (2) Faete (0.31–0.25 Ma), mainlycharacterised by strombolian and effusive activities, and;(3) Hydromagmatic (0.20–0.04 Ma), including monogenet-ic and polygenetic maar-forming eruptions (Fig. 2).Recently, Giordano et al. (2006) proposed a re-organization of the Colli Albani products into lithosome

Fig. 2 Location and geological sketch map of the Colli AlbaniVolcanic District (CA), which comprises the main Tuscolano-Artemisio(or Vulcano Laziale) central volcanic edifice, truncated by ahorseshoe-shaped caldera hosting the Faete stratocone, and a network

of intra- and circum-caldera scoria cones and eccentric tuff rings (e.g.Albano, Ariccia and Nemi). The location of the Sulmona and L’Aquilainter-mountain basins, where distal tephra correlated to the PReruption are identified, is also shown

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units, broadly related to the above three activity periods,corresponding to the main, old edifice (renamed VulcanoLaziale), the peri- and intra-caldera composite edifices(Tuscolano-Artemisio and Faete) and the most recent Viadei Laghi composite lithosome.

During the oldest activity (Tuscolano-Artemisio orVulcano Laziale), individual pyroclastic-flow eventsemplaced >10 km3 of products over an area of about1,500 km2, including present-day Rome. Two eruptiveperiods, characterised by different eruptive behaviours,have been recognised (Marra et al. 2009), with twopyroclastic current deposit end-members associated; theseare volumetrically dominant over co-eruptive scoria falloutfrom sustained column phases (de Rita et al. 1995; Trigilaet al. 1995; Freda et al. 1997; Palladino et al. 2001; Watkinset al. 2002): (1) well-stratified, accretionary-lapilli-bearingash deposits, driven by explosive magma-water interaction(early Tuscolano-Artemisio eruptive phase; 561–527 ka;Marra et al. 2009); (2) massive, coarse deposits bearingscoria lapilli and blocks (instead of true pumices), whichshow no evidence of hydromagmatic fragmentation (lateTuscolano-Artemisio eruptive phase; 456–366 ka; Marra etal. 2009), including those known locally as “pozzolane”, e.g. Pozzolane Nere, Villa Senni upper flow unit and thePozzolane Rosse, which we deal with in this paper.

The shift from the early to the late eruptive period wasaccompanied by longer inter-eruptive breaks among high-magnitude explosive events, in the order of 104 year (Marraet al. 2004). The local transpressive tectonic regime isinferred to regulate the length of inter-eruptive periods(Marra 1999, 2001). On the other hand, the localdecompression along NE-striking fractures associated withthis stress field, as well as that induced by crustal block-rotation along the N-S strike–slip faults, capable of tappingmagma chambers at depth, may have triggered voluminouslava effusions from peripheral vent systems (e.g. theVallerano lava flow, Marra et al. 2009, at the onset of thePR eruption, see below).

Tomography studies (Amato and Chiarabba 1995)revealed the presence of a high-velocity body at 1–6 kmbeneath CA, which was interpreted as the crystallisedmagma chamber related to the late Tuscolano-Artemisiophase, thus providing supporting evidence to the hypothesisthat the main CA magma chamber (also feeding the PReruption) was located within the Meso-Cenozoic carbonatesubstrate. The peculiar geological–structural location of thedistrict, characterised by overthickened carbonate countryrocks as extensive source of CO2, is thought to have playeda key role on both magma evolution and eruptive regime(Freda et al. 1997). Present-day intense CO2 release throughthe CA (~1.7×105 tonsyear−1, Chiodini and Frondini 2001)also supports the relevant presence of this volatile phase andits role on the magmatic processes occurred in the past.

Analytical and experimental methods

Lava and scoria samples from the whole PR eruptivesuccession were investigated in order to characterisepetrographic and geochemical features of the juvenilecomponent. Microprobe analyses of glasses and mineralphases were performed at the CNR-Istituto di GeologiaAmbientale e Geoingegneria (Rome, Italy) by a CamecaSX-50 EMP, equipped with five wavelength-dispersivespectrometers, using 15-kV accelerating voltage, 15-nAbeam current, 10-μm beam diameter and 20-s countingtime. Major elements of bulk samples were determined onglass beads at the XRF Laboratory of the Dipartimento diScienze della Terra (Sapienza-Università di Roma). Matrixeffects for major elements were corrected followingFranzini et al. (1972).

Carbonate assimilation experiments were conducted witha piston cylinder apparatus (see Freda et al. 2008 andDeegan et al. 2010 for details of equipment and procedures)at the HP-HT Laboratory of Experimental Volcanology andGeophysics (Istituto Nazionale di Geofisica e Vulcanologia,Rome, Italy) at P=0.5 GPa, T=1,200°C, ~2.5 wt.% H2O,using as starting material a phono-tephritic rock powder,doped with a limestone lithic fragment (~1 mm3; 4/1powder/lithic mass ratio).

Characteristics of the Pozzolane Rosse eruptionproducts

The PR eruptive succession and areal distribution areillustrated by Fig. 3. According to the most recent40Ar/39Ar ages (Marra et al. 2009), the PR eruptive event(II Tuscolano-Artemisio pyroclastic-flow unit, de Rita et al.1988a, 1995; Giordano and Dobran 1994; Corcolle Erup-tion Unit, Trigila et al. 1995) occurred at 456±3 ka. Theemplacement of the PR pyroclastic units was preceded bythe effusion of a lava plateau (2–3 km3 in volume) from aperipheral vent system (Vallerano lava flow; 457±5 ka,Marra et al. 2009), which, based on the lack of interveningstratigraphic discontinuities in the deposits and comparableradiometric ages, is regarded as part of the PR eruptivecycle.

The lava is slightly porphyritic (<10 vol.% from thinsection observations) and contains submillimetric leucite(prevailing), and clinopyroxene phenocrysts and rareolivine xenocrysts. The groundmass is formed by leucite+clinopyroxene+Ti-magnetite+melilite+phlogopite+nephe-line and variable amounts of calcite. The occurrence ofnepheline and calcite intergrowths and of clinopyroxene,leucite and oxide inclusions in interstitial calcite (Fig. 4)provides evidence of calcite crystallisation above solidusconditions. The lava body is characterised by an increase in

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Fig. 3 CA reference map showing the areal distribution of the PRpyroclastic-flow deposits and the measured thicknesses of thePR basal scoria fall bed (a); representative stratigraphic log of thePR eruptive succession in intermediate setting (b); field aspect ofthe main PR pyroclastic-flow deposit at Corcolle locality (c); detail

of the typical “pozzolane” texture, i.e. a poorly lithified, massive,coarse-grained deposit of scoria lapilli and blocks (d); representa-tive poorly vesicular, juvenile scoria block, featuring scatteredmillimetre-sized fresh leucite phenocrysts (e)

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vesicularity from bottom to top (up to 3 vol.%), parallel to adecrease in calcite content (from 3 to 0 vol.%).

Following the early effusive phase, the onset of the PRexplosive phase is recorded by a well-sorted, centimetre- todecimetre-thick, fallout bed made up of poorly vesicular,brownish-grey, highly porphiritic (40–60 vol.% of submil-limetric leucite phenocrysts) fine scoria lapilli, loose leucitecrystals and scarce lithic fragments. On microscopicobservation, leucite crystals occur in a fresh glassy

groundmass and typically show a skeletal, star-like habit(Fig. 5). Thickness data for the scoria fall bed (Fig. 3)suggest a source vent approximately located in the CAcentral area and a ENE-trending dispersal axis. Althoughreliable calculations of eruptive parameters are preventedby poorly constrained isopach and isopleth lines, asubplinian to moderate plinian intensity and a volume oferupted magma in the order of 0.1–1 km3 can be inferredfor the early PR fallout phase by comparison with thickness

Fig. 4 Photomicrographs of thevesicle-bearing middle–top por-tion (a, b; parallel light) andvesicle-free, calcite-bearing,bottom–middle portion(c, crossed polars; d, parallellight) of the PR co-eruptive lavaplateau. Also note the occur-rence of nepheline and calciteintergrowths (c) and of leucite,clinopyroxene and oxide inclu-sions in calcite (c), consistentlyindicating crystallisation ofmagmatic calcite. The rightdiagram shows upsection varia-tions of calcite, vesicle and CaOcontents (h/htot=normalisedthickness). Lc leucite, Cc calcite,Ox Fe–Ti oxides, Ne nepheline,Vs vesicle

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and clast size data reported for other fallout deposits of theRoman Province (e.g. Palladino and Agosta 1997; Sottili etal. 2004).

The main PR unit consists of a single massive, poorlysorted pyroclastic-flow body (Fig. 3) made up of reddish-purple to dark grey, poorly to moderately vesicular, scorialapilli and blocks (up to several decimetre-sized), abundantlava and thermally metamorphosed sedimentary lithic lapilliand blocks, and scarce granular (leucite+clinopyroxene)inclusions, embedded in a coarse ash matrix. The latter isusually loose to poorly consolidated, or locally incipientlylithified due to vapour-phase zeolite crystallisation. Fieldexposures, integrated by drilling data (de Rita et al. 1988b),show how deposit thickness largely varies, in response tothe pre-existing topography, from a few metres up to>30 m. The pyroclastic-flow deposit is axisymmetricallydistributed around the present Tuscolano-Artemisio caldera,which approximately corresponds to the source vent area,as far as 25 km from the caldera centre (Fig. 3). The fairlymonotonous appearance of the deposit both vertically anddowncurrent, as far as the most distal reaches, contrastswith the notable flow mobility and size. The conservativevolume estimate of about 30–34 km3 (de Rita et al. 1988b)makes this as the largest CA pyroclastic-flow deposit.

Scoria clasts in the PR flow deposit are scarcelyporphyritic and contain millimetre-sized fresh leucite, subor-dinate clinopyroxene and scarce dark mica phenocrysts; thegroundmass is largely formed by leucite microcrysts showingstar-like habit and scarce interstitial glass turned to zeolitesand/or halloysite. Scoria bombs may host millimetre- tocentimetre-sized, variably preserved carbonate lithic cores,which show evidence of high thermo-chemical alteration and/or incipient to advanced hollowing (Turi 1970) and may evenappear as “ghost” lithic remnants in scoria holes. Noteworthy,diffuse, up to decimetre-sized, cored bombs occur as far asthe peripheral CA areas (e.g. Corcolle, Fig. 3). We will focuson this peculiar feature in the next section.

The PR eruption succession ends with a pedogenized co-ignimbrite ash fall horizon, which is overlain by decimetre-

thick layered scoria lapilli fallout deposits (442±2 ka,Marra et al. 2009) dispersed to the east (Fig. 3).

Overall, the PR deposits lack the general features ofother CA products indicative of the dominant presence ofeither magmatic or external H2O in the eruptive andemplacement regimes, typically resulting in extensivedeposit lithification by zeolitization (e.g. Villa Senni lowerflow unit, Freda et al. 1997) and/or the diffuse occurrenceof accretionary lapilli and other hydromagmatic textures(e.g. Trigoria-Tor de’ Cenci, Palladino et al. 2001; Albanoand Prata Porci maars, Freda et al. 2006, Sottili et al. 2009).

Ongoing tephrostratigraphic studies (e.g. Giaccio et al.2009; Galli et al. 2010) recognised a couplet of K-foiditictephra layers in the Middle Pleistocene fluvio-lacustrinedeposits of the Sulmona and L’Aquila inter-mountainbasins of the Central Apennine chain (>100 km ENE andNE of CA; Fig. 2). The ~5 mm-thick lower layer is madeup of ash-sized, leucite-bearing, brown-black scoria andloose leucite and clinopyroxene crystals; the upper layer, upto 3 cm-thick, is normally graded, fine lapilli- to coarse ash-grained, and contains moderately porphyritic dark greyscoria, and lava, holocrystalline and thermally metamor-phosed carbonate lithic fragments. The mutually consistentmorpho-stratigraphic context and hardly diagnostic compo-sitional features among the other co-existing tephra fromthe Roman Province volcanoes provide a robust correlationto the PR eruption products. In particular, based on thestratigraphic position, and quite distinctive componentry,microtextural and chemical features (Table 1, Fig. 6), thetwo tephra layers are interpreted as the distal equivalents ofthe basal fallout and the main pyroclastic-flow (co-ignimbrite ash cloud) deposits, respectively.

According to new chemical data, the PR-feeder magmais K-foiditic in composition (Table 1, Fig. 6). In particular,the glass in the scoria clasts from the basal fall unit showsK2O/Na2O<2 and relatively low amounts of leucite-compatible elements (i.e. K, Al, Si, Rb and Cs), as aconsequence of extensive leucite crystallisation. We con-sider the glass particles from the distal ash-cloud deposit as

Fig. 5 Photomicrograph (a) andbackscattered FE-SEM image(b) of juvenile scoria lapilli fromthe PR basal fall deposit. Notethe high amount of leucite crys-tals showing skeletal star-likehabit

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the best representative samples of the melt compositionerupted during the pyroclastic-flow phase, as they virtuallyescaped syn-depositional vapour-phase transformation,which commonly affected the juvenile fraction in theproximal-intermediate deposits (cf. bulk scoria analyses inTrigila et al. 1995). This glass composition shows K2O/Na2O>2 and matches the lava plateau bulk chemistry, asalso concerns the 87Sr/86Srcpx ratios (Gaeta et al. 2006),consistently indicating a common magma body feedingboth the effusive and the climactic, pyroclastic-flow-forming, phases.

Evidence of magma–carbonate rock interaction

The available geochemical data, mass balance calculationsand experimental data (Dallai et al. 2004; Gaeta et al. 2006;Freda et al. 2008) show that CA K-foiditic magmas mayderive from a phono-tephritic parental magma, aftercrystallisation of clinopyroxene+leucite (leucite/clinopyr-oxene <1; Fig. 6) plus assimilation of ~7 wt.% of calcitefrom carbonate wall rocks. Moreover, Freda et al. (1997,

2008) experimentally demonstrated that CA magmasdifferentiate under H2O-poor, CO2-rich conditions, at P=200–300 MPa (Gaeta et al. 2009). Although phono-tephriticmelts can host almost double the amount of CO2 thanbasaltic melts (Blank and Brooker 1994; Behrens et al.2009), the assimilation of ~7 wt.% of carbonate rockswould actually result into a significant amount of free CO2

in the system.A key aspect is the balance of CO2 input vs. output from

the magma reservoir during long inter-eruptive periods (inthe order of 104 year; Marra et al. 2004). In the PR casestudy, evidence of high fCO2 in the magma at the eruptiononset is provided by the occurrence of magmatic calcite inthe groundmass of the lava flow (up to 3 vol.% in thebottom and middle lava portions; Fig. 4). In fact, theformation of calcite crystals follows the reaction:CO2 gasð Þ þ CaO meltð Þ ! CaCO3 solidð Þ, where extensive leu-cite crystallisation at ambient pressure enhances excess ofCaO(melt) (Freda et al. 2008). The observed decrease in thecalcite content, parallel to increasing vesicularity, towardthe lava top (Fig. 4) suggests CO2 exsolution and degassingduring lava emplacement.

Table 1 Representative chemical compositions (major oxides, wt.%; selected trace elements, ppm) of the main PR rock types, including the earlylava plateau (XRF analyses), the basal scoria fall deposit (EMP and LA-ICP-MS analyses) and the distal pyroclastic-flow equivalent (EMP analyses)

Lava plateau Basal scoria fallout Distal pyroclastic flow

LVb LVt LV2–72.8

LV1–78.9

LV1–81.9

AH20-PRa

sd(6)

SULb sd(11)

PGa sd(3)

SULm sd(5)

PGb sd(3)

SiO2 42.46 42.57 45.07 44.32 44.23 40.41 0.16 41.14 0.31 39.70 0.33 44.52 0.35 44.30 0.42

TiO2 0.90 0.88 0.96 0.89 0.89 1.33 0.04 1.22 0.11 1.17 0.04 0.95 0.06 0.92 0.06

Al2O3 15.72 15.71 15.55 15.62 15.60 12.14 0.05 13.93 0.10 13.77 0.10 16.17 0.15 15.95 0.10

MgO 4.45 3.97 4.30 4.91 4.44 6.19 0.17 5.10 0.11 5.24 0.26 3.93 0.08 4.00 0.03

CaO 10.98 11.84 10.33 12.09 10.19 14.77 0.30 14.07 0.29 14.29 0.72 10.78 0.09 10.82 0.20

MnO 0.19 0.19 0.16 0.18 0.17 0.26 0.03 0.31 0.03 0.31 0.05 0.23 0.04 0.25 0.06

FeOtot 9.14 9.06 8.06 8.51 8.44 11.21 0.13 11.92 0.28 11.71 0.35 9.50 0.14 9.34 0.27

SrO 0.32 0.41 nd nd nd 0.27 0.04 nd nd nd nd

BaO 0.30 0.30 nd nd nd 0.39 0.06 nd nd nd nd

Na2O 2.86 2.33 1.91 1.91 1.93 3.38 0.08 3.91 0.10 3.85 0.37 3.02 0.10 3.19 0.15

K2O 8.60 7.62 8.76 7.63 8.57 4.83 0.36 5.12 0.27 5.02 0.12 8.63 0.13 7.81 0.81

P2O5 0.78 0.77 0.87 0.96 0.95 1.08 0.12 1.05 0.06 0.88 0.03 0.79 0.03 0.76 0.05

F 0.30 0.30 nd nd nd 0.59 0.11 0.48 0.09 0.48 0.08 0.33 0.12 0.27 0.05

SO3 0.10 0.07 nd nd nd 0.08 0.02 0.11 0.02 0.23 0.16 0.24 0.07 0.36 0.05

Total 97.10 96.02 95.97 97.02 95.41 96.94 98.34 96.65 99.19 97.97

Leucite-compatible trace elements

Rb 250 242 nd nd nd 45 7.45 nd nd nd nd

Cs 24.3 24.8 nd nd nd 0.9 0.68 nd nd nd nd

LV and AH samples from the Colli Albani central area; SUL and PG samples from distal tephra layers in the Sulmona and L’Aquila basins,respectively

sd standard deviation of EMPA (numbers of point analyses in brackets); nd not determined

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The high δ18O values of calcite from the lava at theVallerano locality (Fornaseri and Turi 1969) indicateisotopic disequilibrium between magma and CO2(gas), thussuggesting the shallow origin of CO2 from decarbonationduring magma rise. In addition, the finding of carbonatelithic cores showing variable degrees of decarbonation inPR scoria bombs (Fig. 7) allows us to hypothesise thatwall-rock entrainment and magma–carbonate interactiontook place even at the eruptive time scale. In the followingsection, we discuss the kinetics of decarbonation of wallrocks.

Constraints on magma–carbonate rock interaction

Theoretical modelling

In order to estimate the initial temperature difference at themagma–wall-rock contact, we applied the industrially

derived coating model of Jiao and Themelis (1993) to thePR cored bomb textures. This model was previouslyapplied to the kinetics of re-melting of rhyolitic wall-rockfragments in basaltic magmas (Rosseel et al. 2006) and tothe ingestion of substrate rocks during CA maar-formingeruptions (Sottili et al. 2009, 2010).

Wall-rock entrainment may occur on condition that themagma is well above the freezing temperature, Tf (Rosseelet al. 2006). Provided that wall-rock temperature, Tl, ismarkedly lower than magma temperature, then lithicfragment entrainment will induce a substantial magmatemperature decrease and form a microcrystalline, glass-bearing chilled rim around the lithic core (Fig. 7). On thesegrounds, from the measured thicknesses of chilled rims, Δr,we can write (Jiao and Themelis 1993):

Δr

a¼ 1þ rl

rc

Cpl Tf � Tlð ÞΔHc

� �1=3� 1 ð1Þ

Fig. 6 Compositional features of the PR rock types: a PR chemicalcompositions (including the co-eruptive lava plateau and the distaltephra layers in the Sulmona and L’Aquila basins) plotted in the TASdiagram (data from Table 1). The chemistry of an olivine-bearing lavaflow (sample AH7a in Gaeta et al. 2006) is also reported asrepresentative of the parental magma composition of CA largeexplosive eruptions; b differentiation trend of the PR magma feedingthe different eruption phases, schematised in the K2O/Na2O vs. Na2Odiagram; c phase diagram for the hydrous-carbonated, parental phono-tephritic composition, illustrating the widening of the leucite stability

field relative to clinopyroxene with decreasing pressure (diamonds:experimental data from Iacono Marziano et al. 2007 and Freda et al.2008). According to experimental phase relationships and massbalance calculations, the differentiation of the lava plateau magmaand the PR pyroclastic-flow glass occurred at relatively hightemperature (i.e. >1,100°C, above oxide-in temperature) and pressure(P>200 MPa; leucite/clinopyroxene <1 in the fractionated solid).Differently, extensive leucite crystallisation (leucite/clinopyroxene >2in the fractionated solid) occurred in the magma feeding the basalscoria fallout (Fig. 5) due to a pressure drop

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where a is clast radius, ρl and ρc are the densities of thelithic clast and the chilled rim, respectively, Cpl is the heatcapacity of the lithic clast at constant P, and ΔHc is thelatent heat of solidification of the melt.

By using appropriate parameters for the PR case study(Table 2), we calculated Δr/a values as a function of Tl, fora carbonate rock type interacting with a K-foiditic magma.

Model output, through the comparison of Δr/a valuescalculated from Eq. 1 to Δr/a values measured in 90 PRcored bombs, provides inferences on the thermal state ofcountry rocks interacting with the PR magma (see resultsbelow; Fig. 7).

Heat transfer from a K-foiditic magma to an entrainedcarbonate lithic clast can be modelled through the theory of

Fig. 7 Model for magma/wall-rock thermal interaction from coredbomb textures. a Scoria lapilli and bombs in the PR pyroclastic-flowdeposits often show variably preserved carbonate lithic cores, asrelated to different degrees of wall-rock assimilation (i.e. incipient onthe left, complete on the right); b plot of core equivalent radius vs.thickness of chilled shell (ΔR), measured for 90 cored scoria clasts.Straight lines represent Eq. 1 solutions (see text) for carbonate wallrocks at different initial lithic-magma contact temperatures (i.e. 100°C,300°C and 500°C). Data points provide thermal constraints to theinteracting wall rocks, consistent with the temperature range withinthe depth interval of the carbonate substrate (ca. 7–0.5 km belowpresent CA, Bianchi et al. 2008), considering a ca. 100°C/km gradient

beneath the high thermal flux Roman area (C.N.R. 1982; Cavarrettaand Tecce 1987); c FE-SEM image of carbonate assimilation experi-ments using a phono-tephritic rock powder as starting material. Notethe reaction zone around the remnants of the lithic fragment in thezero-time experiment; d model output of heat transfer rate from K-foiditic magma to carbonate lithic clasts (clast radii of 0.05 and 5 cm;contact temperatures of 100°C, 300°C and 500°C), based on texturalanalysis of PR cored scoria clasts (see text for model frame and inputparameters). Theoretical thermal re-equilibration times at near-magmatic temperatures, depending on the initial contact temperatureand lithic size, are in the order of fractions of seconds to fractions ofhours, as also supported by experimental data

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heat conduction in isotropic media (e.g. Bird et al. 2002),by assuming that (1) lithic clasts are spherical; (2) magmaand lithic clasts are at specific homogeneous and constanttemperatures at the initial contact; (3) heat transfer isessentially conductive; and (4) relative magma-clast motionis negligible on the interaction time scale. Relevant magmaand limestone clast physical properties are reported inTable 2. Model input parameters for heat transfer rateinclude the thermal conditions for country rocks, Tl, asderived from Eq. 1, and the temperature of the K-foiditicmagma, Tm=1,320 K (Table 2). Three-dimensional con-ductive heat transfer with internal energy conversion isgiven by:

r2T þ q0

K¼ 1

k

@T

@tð2Þ

where T is temperature, K is the thermal conductivity and kis the thermal diffusivity. By assuming that heat q′ is neithergenerated nor consumed (i.e. crystallisation is negligible),then for q′=0 and spherical limestone clasts, Eq. 2 can beexpressed in spherical coordinates:

r2T ¼ 1

a

@2

@t2a;Tð Þ þ 1

a2 siny@

@ysiny

@T

@y

� �þ 1

a2sin2y

@2T

@f2

ð3Þ

where y and f are the azimuthal and equatorial coordinates,respectively.

Since T depends only upon clast radius (a) and time (t),Eq. 3 can be expressed as:

@T

@t¼ k

@2T

@a2þ 2

a

@T

@a

� �ð4Þ

A solution to the differential equation of heat transfer(Eq. 4) from magma to a solid limestone sphere is given by(Carslaw and Jaeger 1959):

T a; tð Þ � T1 � Tm2

erf"þ 1

2ffiffiffit

p�

� erf"� 1

2ffiffiffit

p � 2

ffiffiffit

p"

ffiffiffip

p

e�"�1ð Þ24t � e�

"þ1ð Þ24t

� ��þ Tm ð5Þ

where ε is the dimensionless distance from clast centre(" � r

a), and τ is the Fourier number Fo (t ¼ kta2 � Fo,

where k is the limestone thermal diffusivity).

Model results

Here, we test quantitatively the hypothesis that decarbon-ation of wall rocks may have represented a significantsource of free CO2 on a short time scale (i.e. minutes tohours) and thus controlled the pre-eruptive conditions andthe peculiar eruptive dynamics of the large mafic PRexplosive event. Based on evidence from cored bombs, wemodelled the conductive heat transfer rate from a K-foiditicmagma (composition reported in Table 1) to the entrainedcarbonate lithic clasts (see “Theoretical modelling”).

The formation of a chilled rim in the PR cored bombs(Fig. 7) indicates that a significant temperature differenceexisted between the entrained lithic clast and the magma atthe initial contact, and that the time elapsed between lithicentrainment and eruption was shorter than the time requiredfor re-melting the chilled rim. Therefore, core textures mayput constraints to the thermal state of the interactingsubstrate rocks before entrainment. On these grounds, atemperature range of 100–500°C is obtained for carbonatecountry rocks at the initial contact with magma, consistentwith the likely temperature range within the depth intervalof the carbonate substrate (from 7 km up to 0.5–1.5 kmbelow present CA; Bianchi et al. 2008), by considering atypical temperature gradient of ca. 100°C/km beneath thehigh thermal flux Roman area (C.N.R. 1982, Cavarretta andTecce 1987).

From the above-obtained temperature range for thecarbonate wall rocks, the heat transfer rate from the PR K-foiditic magma to the entrained carbonate clasts can beestimated. In Fig. 7, temperature changes with time in themagma–carbonate system are reported for two carbonatecore sizes (i.e. clast radii of 0.05 and 5.0 cm, respectively,and initial contact temperatures of 100°C, 300°C and 500°C). It results that the times required for thermal re-equilibration at magmatic temperatures of carbonate clasts,depending on the initial contact temperature and lithicsize, are in the order of fractions of seconds to fractions ofhours.

Table 2 Model input parameters for the K-foiditic magma andcarbonate country rocks

Parameter (units) K-foiditic magma Limestone

Thermal conductivity,K (Jm−1s−1K−1)

1.7 1.5

Thermal diffusivity,k (m2s−1)

6.3×10−7 5.7×10−7

Heat capacity Cp(Jkg−1K−1)

1,050 840

Temperature T (K) 1,320 373–573–773

Density ρ (kgm−3) 2,600 2,500

Latent heat of solidificationΔH (Jkg−1)

3.5×105

Latent heat of melt solidification from Rosseel et al. (2006). Otherthermodynamic properties of the K-foiditic melt calculated followingthe “MELTS” code (Ghiorso and Sack 1995). Thermodynamicproperties of limestone from Vosteen and Schellschmidt (2003)

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Theoretical analysis thus confirms our hypothesis thatdecarbonation may occur significantly even on syn-eruptivetime scales. This is also supported by carbonate assimila-tion experiments performed on a phono-tephritic startingmaterial, doped with a ~1-mm3 limestone fragment, at P=0.5 GPa, T=1,200°C, ~2.5 wt.% H2O. For instance, in the t=0 s experimental run, where the sample was heated to1,200°C, with a heating rate of 200°C/min, and thenquenched, the carbonate fragment was partially resorbed,as evidenced by a clear reaction zone (Fig. 7). In the t=60 s(i.e. the sample was maintained at 1,200°C for 60 s) thecarbonate fragment was almost completely digested.

Effects of CO2 on the magma systemand eruptive dynamics

Scoria-bearing pyroclastic-flow deposits similar to PR infield appearance are widely described elsewhere in thecontext of both magmatic and hydromagmatic eruptions,including the Roman Province (e.g. Santorini, Greece,Mellors and Sparks 1991; Vulsini, Italy, Palladino andSimei 2005). However, these examples strongly differ fromPR in terms of the relationship of erupted volume vs. SiO2

content, as they plot in the “normal” global trend. Thus, thePR magma fragmentation and eruption style cannot beeasily interpreted within the spectrum of “usual” magmaticor hydromagmatic processes. In fact, the PR eruptionproducts lack the general features typical of dominantlyH2O-driven eruptive regimes. PR fresh glasses (Table 1)provide further evidence of low H2O content in the magmasystem, with respect to the H2O-saturation amount in CO2-poor potassic magmas (Behrens et al. 2009) at the pressureof interest. Instead, a strong CO2 control on the PR eruptivescenario is inferred, as previously proposed for the“pozzolane”-type Villa Senni upper flow unit (Freda et al.1997).

Here, we discuss the PR eruption trigger and dynamics,as possibly related to a CO2 influx from the carbonatecountry rocks during the 104 year-long quiescence intervalpreceding the PR event. Long-lasting dormancies amongCA high-magnitude explosive events have been explainedby the peculiar geological-structural setting of the interact-ing substrate and tectonic regime (Marra 1999, 2001). Thedevelopment of the huge PR magma reservoir was possiblyfavoured by the NE–SW-oriented, horizontal σ1 associatedwith the local transpressive regime, which may have causedthe sealing of the major NW–SE oriented extensional faultsand ultimately prevented large magma batches to reach thesurface before the PR event.

It is likely that the long-lasting CO2 flux from thecarbonate wall rocks led to diffuse free CO2 in the PRmagma chamber. In this regard, the abundance of skarn

inclusions in CA volcanics, as well as of cumulate rocksformed in Ca+CO2-rich environments (Gaeta et al. 2009),records the presence of long-living, large magma reservoirsin carbonate country rocks at P=200–300 MPa. Indeed, theemplacement of several tens of cubic kilometres-sizedmagma reservoirs in carbonate country rocks at CA impliesnecessarily carbonate digestion of comparable volume.

Recent experimental results on ultrapotassic magmacompositions (Mollo et al. 2010) show that carbonateassimilation is a three-phase-process (i.e. melt, solid andgaseous fluid) that involves a strongly SiO2-undersaturated,CaO-rich melt, a crystalline assemblage made up ofdiopside–hedenbergite–Ca–Tschermak clinopyroxene solidsolution±leucite and a C–O–H fluid phase.

We consider the occurrence of a CO2-rich fluid phase asa major factor controlling the differentiation path, therheological properties and the eruptive behaviour of thePR ultrapotassic magma. Concerning the state of CO2 in thesystem, we remark that CO2 solubility in a typical CA K-foiditic magma is as low as 0.8–0.3 wt.% at 5–2 MPa(Behrens et al. 2009) and that trachybasaltic melt inclu-sions, representative of the CA parental magma, contain0.4 wt.% CO2 (Gaeta et al. 2009). Thus, we can assume thatthe PR magma interacting with carbonate wall rocks wasalready CO2-saturated at the depth of interest (7–0.5 km),thus resulting in a mechanical coupling of excess CO2 asfree gas in the melt.

A significant interacting volume of magma throughprolonged time periods demands a high surface/volumeratio of the reservoir and/or high magma mobility (e.g.piecemeal stoping of country rocks), compatible with thelow viscosity of potassic magmas. Moreover, the magmadensity decrease due to free CO2 addition would haveenhanced magma buoyancy and allowed sinking ofentrained carbonate clasts.

In light of the above geological, theoretical andexperimental evidence, we suggest that variably preservedlithic cores in PR scoria clasts consistently indicate thatcarbonate ingestion and addition of free CO2 to theascending magma took place even in the syn-eruptiveregime at shallow levels, i.e. up to the top of the carbonatesubstrate (0.5–1.5 km depth at present, Bianchi et al. 2008),at 100–500°C (Fig. 7). Concerning the PR eruptiondynamics, we thus recognise the dominant role of freeCO2 from carbonate ingestion relatively to that dissolved inthe melt ab initio.

If we assume an “excess” source of gas, interacting withmagma even during the eruption, then the change in the PReruptive dynamics from effusive to highly explosive can beexplained by a change in the magma rheology. Scoria clastsfrom subplinian fallout following lava flow effusion areindeed characterised by a high amount of skeletal, star-likeleucite, suggesting sudden nucleation and growth (Fig. 5),

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in turn indicative of a significant widening of the leucitestability field. According to phase relations, the widening ofthe leucite stability field in potassic magmas is related to apressure decrease (Thompson 1977). In particular, phaserelations for a hydrous-carbonated, CA parental phono-tephritic composition (Iacono Marziano et al. 2007; Fredaet al. 2008) point out the widening of the leucite stabilityfield relative to clinopyroxene with decreasing pressure(Fig. 6). Moreover, experimental evidence of diffusecrystallisation of skeletal leucite in potassic magmas bysudden decompression is also provided (Shea et al. 2009).

We hypothesise that the significant magma withdrawalrelated to the lava flow effusion (~2–3 km3, presumably inthe time frame of some 102 days) depressurised asignificant portion of the PR magma chamber, leading tofast, extensive leucite crystallisation (Fig. 5). Consequently,magma viscosity rapidly increased (by at least two orders ofmagnitude, as estimated via Einstein–Roscoe equation formultiphase flows; Roscoe 1952), thus interfering with theescape of pervasive free CO2 from the system. Magmadecompression may have thus caused CO2-bearing bubbleexpansion and increasing tensile stress in the melt,ultimately leading to fragmentation and triggering the earlyPR explosive activity represented by subplinian scoriafallout.

We speculate that the cumulative magma dischargerelated to the effusive+subplinian phases resulted in anincipient caldera collapse and the development of a fissure-or multi-vent system, the direct contact of the CO2-rich PRmagma reservoir with the surface eventually driving a largepyroclastic-flow eruption. The abundance of coarse coredbombs in the PR pyroclastic-flow deposits of the CAperipheral areas (e.g. Corcolle, Fig. 3) would be consistentwith magma stoping and eruption through a widened ventsystem during ongoing caldera collapse concomitant to theclimactic phase of the PR eruption.

Conclusions

To our knowledge, the PR event represents the moststriking scenario of anomalously high explosive behaviourof SiO2-poor magmas on planetary scale. We propose thatthe PR eruption was the end result of a positive feedback ofcumulative causations including (1) the local transpressivetectonic regime controlling long-lasting dormancy periods(some 104 year) among high-magnitude explosive events;(2) the development of a large mafic magma body in theupper crust (a few km depth) during a long inter-eruptiveperiod; (3) the occurrence of thick carbonate country rocksat the depth of interest that may release unusually highamounts of CO2 interacting with magma; (4) the peculiarphase relationships of leucite-bearing ultrapotassic magmas

as related to CO2- vs. H2O-controlled conditions; (5) theinherent rheological behaviour of low-viscosity ultrapotas-sic magmas favouring pervasive thermo-chemical interac-tion with the surroundings.

Concerning these controlling factors, it is known that theoccurrence of ultrapotassic magma reservoirs in carbonatesuccessions does not apply uniquely to Colli Albani, astestified in other districts of the Roman and CampanianProvinces. However, we remark the most significantdifferences between CA and another typical volcanicsystem with a carbonate substrate, such as the activeSomma-Vesuvius, concerning individual eruption size (tensof cubic kilometres of deposits for essentially non-Plinianscoria flows vs. a few cubic kilometres for Plinian events,respectively), recurrence periods of major explosive erup-tions (104 vs. 102–103 years) and composition of explo-sively erupted magmas (K-foidite vs. relatively SiO2-richphonolite).

We stress that, in spite of the common regional geologic-tectonic frame, a considerable thickening of the carbonatesuccessions occurs beneath CA, compared with Somma-Vesuvius and other potassic districts of central Italy,coupled with a different local tectonic regime. In ourinterpretation, these concomitant conditions, which controlthe duration of quiescence periods, and thus magmachamber size, acted as the most important factors thatpromoted prolonged magma–carbonate interaction, leadingto the peculiar CO2-dominated magma differentiation trendtoward very-low-silica composition and the diffuse pres-ence of a free volatile phase through a large portion of themagma chamber up to considerable depth, ultimatelyincreasing the explosive potential of the system to asurprising extent.

The PR event may thus be regarded as the CO2-dominated end-member of a wide spectrum of volatileconditions controlling magma chamber processes, as wellas eruptive and emplacement dynamics. The present casestudy provides evidence that the addition of free CO2 fromentrained country rocks through a wide depth interval (upto 200–300 MPa) may drive mafic H2O-undersaturatedmagmas toward anomalously high-intensity explosivebehaviour.

Acknowledgements We are grateful to Piero Dellino and twoanonymous referees for meaningful comments and suggestions.

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