RESEARCH ARTICLE

Deep explosive focal depths during maar forming magmatic-hydrothermal eruption: Baccano Crater, Central Italy

M. Buttinelli & D. De Rita & C. Cremisini & C. Cimarelli

Received: 7 September 2009 /Accepted: 12 January 2011# Springer-Verlag 2011

Abstract We describe the eruptive activity of thePleistocene composite Baccano maar crater in theSabatini Volcanic Complex (Central Italy) combiningstratigraphy, grain size/componentry and rare earthelement and Yttrium (REY) composition of its eruptiveproducts with the stratigraphy and geothermal dataderived from deep wells drilled on the Baccano structuralhigh. The main lithological characteristics of the basalBaccano maar pyroclastic deposit, composed of morethan 60% wt of non-thermometamorphosed lithic clastsfrom the sedimentary basement, show that the firsteruption was magmatic-hydrothermal in nature. Thelithology of the sedimentary lithic clasts indicates thatthe fragmentation level was at a depth of −1,000 to −1,200 m,with fragment depth verified by deep well stratigraphy. The

15% wt juvenile non-vesicular glass components suggest thatmagma played a minor role in powering the eruption.Assuming that the high-salinity hot hydrothermal fluids(365<T<410°C and P∼25 MPa), hosted in the highlypermeable and confined aquifer below the Baccano maarare representative of those at the time of the eruption, wepropose that hydrofracturing would have triggered theeruption caused by overpressure at the top of the geothermalaquifer. REY analysis performed on pyroclastic fragmentsand basement rocks suggest that partial dissolution of thedeeper limestones (>−1,400 m) by the aggressive hydro-thermal fluids enriched in acid components (HF, HCl, andH2SO4) may have contributed to increased CO2 partialpressure that helped to drive the hydrofracturing. This couldhave caused rapid vapour separation and pressure drop,allowing the almost simultaneous breaking of the aquifercover and brecciation of the calcareous units down to −1,000to −1,200 m depth. The relative abundance of calcareouslithics in the basal part of the first Baccano eruptive unit,representing about the upper 200 m of stratigraphy below thetop of the Baccano structural high, reveals the descent of thepiezometric surface during the eruption. Combining deepwell information and maar product stratigraphy, using alsoREY data from maar pyroclastic fragments and the basementrocks we draw an interpretative model for the Baccano maar-forming eruption, concluding that a) magmatic-hydrothermaleruptions may originate deeper than previously thought,and b) hydrothermal fluids circulating in limestoneaquifers may play an important role in triggering sucheruptions.

Keywords Hydrothermal eruptions . Hydromagmatism .

Explosion depth . REY. Hydrothermal fluids .

Baccano maar

Editorial responsibility: B. van Wyk de Vries

M. Buttinelli (*) :D. De Rita : C. CimarelliUniversità degli Studi di Roma Tre,L.go S.Leonardo Murialdo 1,00146 Roma, Italiae-mail: mbuttinelli@uniroma3.it

D. De Ritae-mail: derita@uniroma3.it

C. CremisiniCentro Ricerche ENEA Casaccia,Via Anguillarese,Roma, Italiae-mail: cremisini@casaccia.enea.it

Present Address:C. CimarelliLudwig Maximilians Universität,Theresienstrasse 41,80333 Munich, Germanye-mail: cimarelli@min.uni-muenchen.de

Bull VolcanolDOI 10.1007/s00445-011-0466-z

Introduction

Magmatic-hydrothermal eruptions generally occur wheninjection of magma into a pre-existing convecting hydrother-mal system causes a heat pulse that triggers an eruption. In thistype of volcanic activity, although the energy for theexplosions is directly derived from the hydrothermal system,the magmatic input is considered of primary importance intriggering the eruption. However, hydrothermal eruptions donot necessarily require any direct input of either mass orenergy derived directly from magma, thus differing fromphreatomagmatic eruptions (Wohletz 1983, 1986, 2002;Lorenz 1987; Zimanowski et al. 1991; Browne and Lawless2001; Lorenz and Kurszlaukis 2006; Mastin 2007).

A hydrothermal eruption may be triggered when liquidwater or a two-phase water mixture at boiling-point conditionsbelow the surface is exposed to a sudden pressure release.Such conditions can be caused by earthquakes (Yamamoto etal. 1999), hydraulic fracturing (Heiken et al. 1988), gravita-tional slope instabilities or even human intervention. Thesudden pressure release causes the instantaneous disequilib-rium of the hydrothermal system and the formation of steamthat drives the eruption. This can also occur without thedirect energetic input of magmatic heating (Germanovichand Lowell 1995; Browne and Lawless 2001; Fontaine et al.2002). Hydrothermal eruptions usually form at shallowdepths (of up to 450 m; Mastin 1991; Browne and Lawless2001) with estimated energy in the order of 108–1012 kJ(Browne and Lawless 2001).

Hydrothermal eruptions may occur with no evidentprecursor signals (Yamamoto et al. 1999). They are suddenviolent events, during which large volumes of rock particlesmixed with liquid water, water vapour and other gases areejected. They may last from minutes to several hours andshower ejecta over areas up to several km2, in the processgenerating craters ranging from 2 to 500 m in diameter. Thedeposits of hydrothermal eruptions are generally finelyfragmented debris, un-layered and poorly sorted. Theircomponentry is generally enriched in substrate lithic fragments(Browne and Lawless 2001; Sottili et al. 2009). Individualpyroclasts typically reflect repeated brecciation, hydrothermalalteration and possibly silicification prior to eruption. Examplesof hydrothermal eruptions have been reported in the USA, ElSalvador, Japan, Greece, the Philippines, and Guatemala(Browne and Lawless 2001 and references therein).

In the Sabatini Volcanic District (SVD; Fig. 1) the basaldeposit of the Baccano maar succession, mainly outcrop-ping in the eastern sector of the district, is particularlyenriched in non-thermometamorphosed lithics of the sedi-mentary basement.

We have analyzed in detail the volcanic succession of theBaccano maar and focus particularly on the nature and relativeabundance of juvenile, crystal and lithic components in its

different eruptive units. On the basis of data from several deepgeothermal drillholes (ENEL 1991; Baldi et al. 1975, 1982a,b, 1993) that allowed the reconstruction of the structuralsetting of the basement below the volcanic rocks (de Rita etal. 1983), we correlate the relative abundance of componentsof the eruptive units to the depth of the fragmentation level.We also analyzed the REY (Rare Earth Elements—REE—and Yttrium—Y) patterns of the Baccano units andhydrothermal alteration-related deposits in order to investi-gate the high temperature hydrothermal processes thatoccurred in the Baccano area. The REY pattern may beindicative of both shallow low temperature processesinvolving mixing of hydrothermal fluids and meteoric water(Schwinn and Markl 2005) and deep high temperatureprocesses that involved mingling of magma and geothermalfluids. The REY pattern of the hydrothermal fluids is usuallypreserved in Ca-bearing minerals such as fluorite or calcite(Möller et al. 1998). Considering that the hydrothermal fluidsin the Baccano area are particularly enriched in fluorine, wehave referred to the REY complexation in newly formed F-rich minerals operated by magmatic fluids during hydrother-mal processes. According to the Kolonin and Shironosova(2002) model, at relatively high hydrothermal temperatures(100–500°C) and pressures of 1–2 kbar, the leaching of F−

and LREE (light REE, from La to Nd) of host rocks isenhanced under the form of LREE hydroxides in F−-enrichedfluids. The following re-deposition of the newly-formedfluorite usually shows a strong LREE enrichment.

Geological and geothermal background

The Sabatini Volcanic District (Fig. 1) is part of the K-alkalicRoman magmatic province (Washington 1906; Peccerillo2005 and references therein) bordering the eastern Tyrrhe-nian Sea and aligned along a NW-SE tectonic trend.Volcanism, active between 0.5 and 0.08 Ma (Cioni et al.1993; Villa 1993; Karner et al. 2001), was mainlycharacterised by explosive activity, producing large volumesof pyroclastic deposits (about 158 km3) with a subordinatevolume of lavas. Numerous eruptive centres (∼20) give thearea a “crater-field” morphology. Explosive eruptions in thedistrict showed a progressive transition from mainly mag-matic to mainly phreatomagmatic through time (Fig. 1). Theyoungest dated product is the 85±8.5 ka Baccano pyroclasticflow deposit (Villa 1993). However stratigraphic relation-ships indicate that products younger than this were eruptedfrom the Martignano, Le Cese, and Stracciacappe craters (DiFilippo 1993). The composition of products erupted withinthe SVD is considered to be transitional between maficleucitites and the trachytic differentiated suites and is relatedto a crystal–liquid differentiation in a subvolcanic (low-pressure) regime (Belkin et al. 1988; Conticelli et al. 1997).

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The stratigraphy of the area, from the younger units tothe older ones, can be summarised as follows: (i) thevolcanic succession of the Sabatini volcanic centres, (ii)the allochtonous successions of Plio-Pleistocene clay andsandy-clay sediments and a siliciclastic complex (LateCretaceous to Oligocene-Miocene) that varies fromalternating calcarenites, marly limestones, and clay toalternating calcarenites and quartz-feldspathic sandstones(200 to 1,000 m) (ii) a lower carbonate basal complex (oldestrocks dated as Late Triassic) of marly limestones overlyingmarly limestones alternating with dolomitic and anhydritelayers (thickness of the basal complex >2,000 m; Fig. 2).

The sedimentary basement is dislocated and organized intohorsts and grabens. The structural highs and lows are usuallyelongated on NW- and NE-trends (Cipollari and Cosentino1995; de Rita et al. 1996). The structural highs lack the Plio-Pleistocene cover.

The Baccano coalesced craters developed between 0.36and 0.08Ma (de Rita et al. 1983; de Rita and Zanetti 1986) inthe eastern sector of the district, along the Baccano-Cesanostructural high. The hydromagmatic activity of the Baccanomaar was conditioned by the geometry and the lithology ofthe sedimentary basement hosting the local and regionalaquifers (de Rita and Sposato 1986).

The Baccano area shows the highest geothermal gradient ofthe SVD (100–150°C/km; Calamai et al. 1976). Four of the14 deep wells drilled in the area for geothermal research(Fig. 3) produced a hot supersaturated chloride-sulfate brine(200°C, reservoir conditions) characterized by unusuallyhigh values of total dissolved salts (up to 400 g/L, post-flash; flashing steam estimated to be 20–25% of total flow;Calamai et al. 1976). Hydrothermal minerals have beenfound in all the rocks penetrated during the drillingoperations: the most frequent are sulphates and calcite;

N

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*

*

*

**

**

**

**

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**

*

* *

*

Trevignano

Sacrofano

Bracciano LakeBaccano

Manziana DomesComplex

Bracciano

Ceriti DomesComplex

Anguillara

Sasso Mt.

Bagnarello

Stracciacappa

Martignano

Polline

Agusciello

Monterosi

Le Cese

SH2 well

Cesano

Sacrofano/Baccano units

Pyroclastic units of northern Sabatini centres

Scoria cones*

Lava flows

Vican “Red Tuffwith black scorias”

“Bracciano Tuff”

Vigna di Valleignimbrite

“Yellow Sacrofano Tuff”

Early Sacrofano activity deposits“Pizzo Prato”ignimbriteCerite-Manziate Volcanic District

Allochthonous sedimentary covers

Sabatini “Red Tuffwith black scorias”

Plio-PleistoceneClaystones/Sandstones

Deep geothermalexploration wells

0 5km

alignment (Scoria conesand/or presumed faults in the basement)

Crater/Caldera Rims Fault traces

Baccano-Cesano

Structural high

N

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*

*

*

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Trevignano

Sacrofano

Bracciano LakeBaccano

Manziana DomesComplex

Bracciano

Ceriti DomesComplex

Anguillara

Sasso Mt.

Bagnarello

Stracciacappa

Martignano

Polline

Agusciello

Monterosi

Le Cese

SH2 well

Cesano

Sacrofano/Baccano units

Pyroclastic units of northern Sabatini centres

Scoria cones*

Lava flows

Vican “Red Tuffwith black scorias”

“Bracciano Tuff”

Vigna di Valleignimbrite

“Yellow Sacrofano Tuff”

Early Sacrofano activity deposits“Pizzo Prato”ignimbriteCerite-Manziate Volcanic District

Allochthonous sedimentary covers

Sabatini “Red Tuffwith black scorias”

Plio-PleistoceneClaystones/Sandstones

Deep geothermalexploration wells

0 5km

alignment (Scoria conesand/or presumed faults in the basement)

Crater/Caldera Rims Fault traces

Baccano-Cesano

Structural high

Fig. 1 Schematic geological map of the Sabatini Volcanic District (modified from de Rita et al. 1996)

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occasionally ankerite, dolomite, K-feldspar, magnesiancalcite, sulphides, and zeolites have been also detected.Fluid and melt inclusions in xenoliths indicate that fluidsenriched in sulphates were trapped during crystallization attemperatures between 365°C and 410°C (Tecce et al. 2000).

The SH2 deep well, drilled to the north of the BraccianoLake (Fig. 1), intercepted a F-rich hot brine at a depth ofabout 1,100 m (Cavarretta and Tecce 1987). Data from thisdrillhole show the presence of acidic solutions enriched inH2S, HF, and typical hydrothermal minerals, whose genesiscan be attributed to the Sabatini thermomethamorphic andhydrothermal deep systems (Cavarretta and Tecce 1987;Belkin et al. 1988; Cavarretta et al. 1981).

The presence of hydrocarbons in the phlogopite fluidinclusions suggests some interaction with the local sedi-mentary environment (Belkin et al. 1988).

Cavarretta and Tecce (1987) suggested that in theBracciano area the ascending hydrothermal solutionswere rich in SO2 and CO2, plus other volatiles (B, HF)and possibly a shallow Ca-rich fluid. They also suggestedthat the brine formed as the result of remobilization ofsedimentary (Triassic) sulphates in a low-pressure envi-ronment. They concluded that the hydrothermal solutionswere generated by a volatile-rich intermediate to trachyticmagma, which was intruded below the northern Brac-ciano Lake area, and which was the principal heat andfluid source. The large-scale zoning of phlogopitecrystals in the SH2 well and their element variationsmay suggest that during the evolution of the SH2geothermal system an episodic introduction of magmaticfluid into the geothermal regime occurred (Belkin et al.1988).

vertical exageration 3X

C1C8

C3

Top of the Basal calcareous successions (3)

C5

C6

C2

C4

RC-1

Top of Allochthonous successions

Base of volcanic units

Allochthonous successions (2)

Volcanic units (1)

Lower carbonate basal complex (3)

Alluvial units (0)

N S

N-S section along the Baccano Valley

(1)

(2)

(3)

(0)sea level

C8

S

C2 C1 C3

U1U2

C5

N

400

0

-500

-1000

U3

Fig. 2 3D structural model ofthe Baccano-Cesano high andthe Baccano maar. Structuralsurfaces have been drawn bycorrelating geothermal wellstratigraphies. Structuralsurfaces highlight the location ofthe conduits of U1 and U2coalescent craters (interceptedby drillholes C1 and C8respectively). A N-Sinterpretative geological sectionof the Baccano maar through thedeep wells, is also shown. Forthe position of geothermal wellsand the trace of the geologicalsection see Fig. 3

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Methods

Detailed field work was used to map the Baccano area at1:10,000 scale (Fig. 3a) and to correlate stratigraphicsections. A digital Terrain Model (DTM) has been createdby digitazing 1:10,000 scale georeferenced topographicmaps. A geographic database has been created starting withfield information and integrated with the stratigraphy of8 deep geothermal wells drilled in the Baccano area (deRita et al. 1983; ENEL 1991). Interpolation of informationfrom surface geology and deep wells allowed the recon-struction of several surfaces: a) the base of the lacustrinedeposits filling the composite crater of Baccano; b) the baseof the underlying volcanic successions, which include thebreccia deposits intercepted by the deep wells; c) the topsurfaces of the lower carbonate basal complex; d) thesiliciclastic complex (late Cretaceous-Oligocene-Mioceneunits); and e) the base of the Neogenic (Plio-Pleistocene)sedimentary units (Fig. 2).

Samples of the Baccano pyroclastic units for grain sizeand componentry analyses were collected from the bestexposed and preserved proximal section located at km 27 ofthe Via Cassia (Fig. 3a).

Grain size analysis has followed the methodology of Casand Wright (1987), and components larger than 32 mm havebeen separately weighed. Each size class has been analyzedunder the stereomicroscope to distinguish the juvenile, lithic(sedimentary versus volcanic) and crystal components.Samples with grain size ranging between 125 and 63 μmhave been subjected to image analysis using a microscopeequipped with a digital camera. Different grain size classeshave been summed for each component and a total% wtobtained. In the juvenile class we have individual pumiceclasts with size between 500 and 125 μm (−4/−0.5 Φ) andfine ash clasts with size less than 63 μm (>4 Φ). Within thelithic component we have distinguished volcanic lithics fromsedimentary ones. These latter have been further analyzed inthin section to determine their stratigraphic position inrelation to the deep well logs.

We have performed REY (Rare Earth Elements—REE—and Yttrium—Y) analysis on the fine-ash matrix (>4 Φ)and on the sedimentary lithics. The samples were firsttreated with standard techniques to create solutions withhigh concentrations of REE and Y. Then, the analysis wasperformed using an ICP-MS (Perkin-Elmer-ELAN-6100)configured with the following operational parameters: RF

recent alluvium

First hydromagmaticBaccano unit (U1)Second phreatomagmaticBaccano unit (U2)Third phreatomagmaticBaccano unit (U3)

Le Cese unit

Martignano units

Mt. Razzano unitSacrofano activities units

Deep geothermalwells - depth Main crater shapes

Ancient crater shapes

crater rimgrain size sample site

stratigraphic sections

shallow geothermalwells - depth

1000 m

Km 27 via Cassiasample site

U1p

U2p

U1d

U1m

U2m

U2dU3p

U3d

U3m

Fig. 3 a Geological map of theBaccano maar. Location of geo-thermal deep wells are given inthe map. Sites of grain sizeanalysis and representativestratigraphic logs are also given(see Fig. 5 for details). b Craterrim correlation with the Baccanomaar deposits. Note the pro-gressive vent migration fromsouth to north and then to theeast along the axis of theBaccano-Cesano structural high.Vent migration caused coales-cence of the Baccano craters andpartial obliteration of older Mt.Razzano and Sacrofano calderarims

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power 1,100 W, nebuliser flow 0.92 L min-1, resolution0.7 amu, dwell time 40 ms, replicates 3 and peak hoppingmeasurement mode. The dissolution procedure waschecked, with reference to REEs recovery, analysing thecertified reference material GSP-2 (purchased by USGS).Rhodium (Rh) was added as internal standard to all thesolutions, in order to correct any instrumental drifts. Limitsof detection for REEs have been calculated as Xmb+3Smb,where Xmb and Smb are respectively the mean and thestandard deviation of the measurements of method blanksand are 27 μg kg-1 for La, 75 μg kg-1 for Ce and in therange 0.5–5 μg kg-1 for the other elements.

Field investigation

Detailed mapping of the Baccano deposits (Fig. 3a) revealsthat the coalescing crater morphology of the maar, as shownalso by the DTM (Fig. 3b), is due to three main explosiveevents. Each eruption caused the partial collapse “into thecrater” and the emplacement of an eruptive unit. The ventmigration was from south to north and then to the east, asindicated by angular unconformities between the eruptiveunits and by the morphology of the maar itself (Fig. 3b).The present morphology of the eruptive centre has beenmodelled as resulting from outwards erosion. A lake waspresent presence until historical times, as indicated by thelacustrine deposits constituting the present crater floor.These data are supported by the deep drilling logs that havebeen used to reconstruct the 3D model of the sedimentarybasement. The model has provided the shape and thelocation of two of the three vents, by revealing the presenceof explosion breccias encountered in the C8 and C1 deepgeothermal wells (Fig. 2).

The 3D model has also allowed the calculation of theapproximate volume of the material removed from thesubstrate by excavation during the first Baccano eruption(∼3 km3).

The main lithological characteristics of the three eruptiveunits are summarised in the following paragraphs.

First baccano unit (U1)

The first Baccano unit (U1; “lower hydromagmaticBaccano unit” of de Rita et al. 1993) is widespread inthe southern and eastern sectors of the maar (Fig. 3a). TheU1 deposit has a limited distribution (about 15 km2) and amaximum thickness of 7 m, with an approximatedminimum volume of 1.5×106 m3. Deposits are lithifiedby zeolites (e.g. chabasite and phillipsite). This unit ismatrix supported, poorly sorted, and contains cm-sizedsubrounded lithic clasts, which show a general verticaland lateral grading. The largest lithic clasts (none larger

than few cms) have been observed in exposures closest tothe inferred vent. Different laterally and vertically chang-ing facies characterize the unit (Fig. 5). Massive facies areprevalent in the proximal area and in the basal part of theunit (Fig. 5—U1p), whereas poorly developed sandwaveand planar bedded facies are found in medial, distal areasand in the middle and upper part of the unit (Fig. 5—U1m/d).In proximal areas, the basal part of the unit is constituted bya crudely stratified lithic breccia (Fig. 5—U1p; Fig. 7),laterally passing into massive ash with lithic-rich breccialenses. Lithic clasts show inverse grading. The middle part ofthe unit is dominated by thin and diffusely-stratified lapilli-tuff, with very low-angle sandwave structures (Fig. 5—U1m).In the northern sectors this facies is rich in accretionarylapilli and has impact sag structures. The upper part isdominated by parallel stratified ash tuff (Fig. 7). The unit ispervaded by fractures that do not intersect the overlying U2unit. Fractures are filled with hydrothermal precipitates suchas halloysite and calcite (see paragraph dedicated to REEanalysis).

The U1 unit is particularly enriched in sedimentarylithics (65% wt). There are 20% wt of volcanic lithics (lavaand pyroclastics from the ancient Sacrofano volcano, eastof Baccano maar) and the remaining are 15% of the depositcomprises juvenile glass and crystal components (Fig. 7).Almost half of the sedimentary lithics (about 30% wt) arefragments of non-thermometamorphosed limestones whosesedimentological and paleontological analyses indicate thatthey are derived from the Eocene-Oligocene calcareousunits of the siliciclastic complex at −1,000 to −1,200 m ofdepth (Fig. 7). The remaining lithic content (almost 30% wt)is constituted by small marly and sandy clasts derived fromthe upper part of the siliciclastic complex and by rarecalcareous clasts, probably derived from older sedimentaryunits of the basal calacareous complex.

Grain size analyses of samples from the massive faciesin proximal sections show polymodal size distributions andpoor sorting (Fig. 7). The non-vesicular glass shards mainlyshow blocky equant morphologies. Crystal-coating glassshowing incipient vesicules has also been observed.

Second baccano unit (U2)

The U2 unit is matrix-supported and shows concentrationsof sub-rounded pumice in layers or lenses (Fig. 5—U2m)and its basal part constitutes a pumice fall deposit (Figs. 4and 7). The unit displays is generally massive (Figs. 4 and5—U2p), but dune-bedded structures have been alsoobserved in its central and medial portions. The U2 unit isoverlain by a deposit consisting almost entirely of crystalsand juvenile ash (Figs. 4 and 7). The massive facies ischaracterized by abundant tephri-phonolitic to latitic pumice(about 16% wt), up to 10 cm in diameter, homogeneously

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dispersed in an ash matrix. Scanning electron microscopeobservations indicate that two populations of pumice arepresent: one type, common in the basal part of U2 deposit, ischaracterized by very small (20 μm) sub-spherical vesicles(Fig. 5—U2m), whereas the second type, more common inthe upper part of U2, is characterized by large (500 μm)elongate vesicles (Fig. 5—U2d). No adhering particleshave been observed on either pumice type. Grain sizeanalyses performed on the basal fall deposit showunimodal distributions of the pumice components whereas

crystals are more concentrated in the smaller grain sizes(Fig. 7).

At the best preserved proximal section outcropping atkm 27 of the Cassia road, along the southern rim of thecrater, the top of this pumice level is well exposed. Herethis portion of the deposit is altered and reveals a highconcentration of halloysite giving to the deposit a darkbrown colour, which makes this level resemble a soil. Thislevel is overlain by a concordant travertine deposit enrichedin fluorite minerals (Fig. 6). The fluorite-rich travertine has

U1U1

U2

U3U2 ash-cloud surge deposit

halloysite altered ash layer (HL)

halloysite altered ash layer (HL)

travertine layer (T)travertine layer (T)

pumice fall-out

Fig. 4 Summary of theBaccano stratigraphy relative toKm 27 Cassia section. Note thepresence of a pumice fall outlayer at the base of U2 unit andan association of adiscontinuous alteration leveland a travertine layer in thebasal part of its massive facies.The only recovered paleosoil isevident at the passage betweenU2 and U3

U3 coarse ash stratified facies

U1 massive facies

PROXIMAL

U3

U2

U1

DISTALMEDIAL

U1

U2 pumicefall-out

U2 ash-cloud(altered in soil)

U3

U2 massive facies

U3 vent proximal facies

U2 - coarse ash matrix with pumicesand small lithics U1

U2 coarse ash matrix

U2 pumices

U1p

U1 dune cross-bedding

U1m U1d

U2dU2mU2p

U3p U3m U3d

Fig. 5 Summary of facies characteristics of the three Baccano units.Images are organized showing the proximal, medial and distal faciescharacteristics of each unit. No evident paleosoil can be recognizedbetween U1 and U2. Passing from proximal to medial facies the unitstend to loose the lithic component, acquiring traces of a more

turbulent behaviour and developing organised flow structures suchas dunes, cross stratified and parallel bedding. Distal facies are usuallycharacterized by only matrix supported (coarse or fine ash) deposits(without lithic or other accessory component). For the location of eachfacies outcrop image see Fig. 3a

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been dated at between 70 and 80 ka (Taddeucci andVoltaggio 1988) whereas the U2 unit has been dated at 85±8.5 ka (Villa 1993).

The abundance of finer particles in the pumice falldeposit relative to both juvenile and crystal componentsmay indicate higher eruptive fragmentation of the matrixmaterials, although a contribution from the mechanicalabrasion of pumice during the sieving procedure cannot beexcluded a priori (Fig. 7).

The lithic component of U2 forms 50% wt of the totaldeposit. About 20% wt of the lithic component is representedby sedimentary lithics. Marble and holocrystalline lithic clastswith augite and andradite from the thermometamorphiccontact aureole (−2,500 m from drilling data) are also present(Fig. 7). This unit extends several kilometres to the south-east of the crater, covering an area of about 35 km2 and withan average thickness of 10–15 m (see Fig. 4 for therelationship between erupted units), usually filling thepalaeodepressions of the Sacrofano caldera. The estimatedvolume of the U2 deposit is ∼5×108 m3.

Third baccano unit (U3)

The third Baccano unit (U3) is characterized by a coarse tofine ash matrix. We observed both regressive and progres-sive dune structures (Fig. 5—U3m/d). Accretionary lapillihave also been observed in proximal facies. The unitgenerally does not contain a significant amount of substratexenoliths, except for the very proximal vent facies (Fig. 5—U3p), being essentially made of crystals and ash. SEManalyses show glass shards with morphologies usuallymasked by diffuse aggregation of globular dust: when

visible they show blocky or platy shapes. Due to stronglithification of the different components, grain size analysescould not be performed. Chabasite and phillipsite zeolitesare both present in U3, and halloysite is common.

The C8 breccia deposit

In order to help characterize deep structures of the vent, wehave also analyzed the breccia deposits recovered from theC8 geothermal deep well, interpreted as the infilling depositof the U2 crater.

Drill cuttings from the C8 geothermal well have beenanalyzed (0–1,300 m depth). Cuttings comprise hydrother-mally altered breccia fragments. More than 60% wt of thesefragments are angular to very well rounded limestones fromthe sedimentary basement, altered exotic volcanic rocks,and sanidine, pyroxene and mica crystals. Abundantmarble-like clasts derived from metamorphosed limestonesof the sedimentary complex are also present. There is noinformation on the nature of the matrix, because no corewas collected from the well.

X-ray diffraction analyses (XRD) of several breccia clastsindicate the presence of calcite, gypsum, clinopyroxene(augite), dolomite, ankerite, phlogopite, sanidine andmeionite.No pumice or glass shards were observed in the cuttings.

Thin sections of the cuttings reveal that lava andlimestone fragments are frequently altered at the marginswith crystallization of calcite. Leucite crystals are com-pletely replaced by sanidine. Calcite has replaced clinopyr-oxene, sanidine and leucite crystals. Meionite also appearsto have replaced pre-existing crystals.

REY analyses

The analytical results of the ICP-MS analyses are reportedin Table 1 (see also Appendix for further information aboutanalytical procedures). Samples U1am, U2am, U2 p,U2pm,U2pc, U2B and U3B are from Baccano eruptiveunit samples. We sampled also deposits where XRD spectraindicated the presence of halloysite and/or other alteration-related minerals, such as the fluorite-rich travertine at the topof U1 (sample FrT), the C8 geothermal drillhole cuttingsbelonging to deep crater breccias, clearly affected byhydrothermal fluid circulation (samples p C8-650, C8-770,C8-950), and the fracture-filling materials (halloysite andcalcite, HF, CF) in the U1 unit. We have also analyzed severalEocene-Oligocene calcareous lithic clasts in U1 (samples:U1SL, U1S4, U1S6) and one andradite-augite clot collectedfrom the U2 unit (sample AAC). As a comparison for thefluorite-rich travertine, we have also analyzed a travertinedeposit from Bagnarello thermal springs (western Sabatiniarea) that is not enriched in fluorite (sample BT; location in

Fluorite-richTravertine

HL pumice fall-out

U2 massive facies50

cm

Fig. 6 Discontinuous halloysite alteration levels, overlain by aconcordant fluorite-rich travertine at the top of U2 pumice fall-out.The dark brown alteration due to hydrothermal fluid circulation makesthe layer resemble a soil. Outcrop of km 27 Cassia southern maarsection

Bull Volcanol

Fig. 1), and a pure (70% CaF2) fluorite deposit of Mt. Sasso(South-western Sabatini area, sample SF; location in Fig. 1).As a comparison for the calcareous lithics we have alsoanalyzed a sample collected in the Apennines from theEocene limestones of the Scaglia Formation (sample SC).

Content of the samples have been normalized to theupper crust concentration of REY (UCC), Chondrite C1,and also to the REY content of a tephritic Sabatini lava,which is considered representative of the original REYcontent of the Sabatini magma.

The Baccano deposits show a general enrichment inLREE by a factor 5–10 with respect to the UCC LREE

concentration (Fig. 8a), while the LREE trend of theBaccano units and of the C8 well cuttings are comparablewith that of the Sabatini lava. Nevertheless, the C8 wellcuttings from shallow depths show a lower REE concen-tration, suggesting a deep geothermal fluid circulation athigh-temperature and high pressure conditions. Further-more, the Baccano deposits are generally enriched in REYwith respect to the lava. The andradite-augite clot sample(AAC) shows depletion of LREE, whereas the fluorite richtravertine (FrT) is strongly enriched in LREE, with respectto the C8 and Sabatini lava samples (Fig. 8b). The REYpattern of halloysite veins (HF) pervading the U1 deposits

0

20

40

60

80

100

120

-6 -5 -4 -3 -2 -1 0 1 2 3 4 5

U1 massive facies

sedimentary

lithics (65%)Eocene

Oligocenecalc. fragm.

(35%)Siliciclasticcomplexfragm.(30%)

volcaniclithics(20%)xll+juv

(15%)

0

10

20

30

40

50

60

70

-6 -5 -4 -3 -2 -1 0 1 2 3 4 5

pumice fall-out-6 -5 -4 -3 -2 -1 0 1 2 3 4 5

we

igh

t (g

)

U2 massive facies

0

10

20

30

40

50

60

70

80

U2 ash-cloud

U3

U2

U1

pumice fall-out

not to scale

HL

THL

volcanic lithics

coarse ash

fine ash

sedimentarylithics

pumicesdunes structures

paleosoil

altered ashlayer

travertinesampleposition

GLASSXLL

PUMLIT

GLASSXLLLIT

GLASSXLL

PUMLIT

PHI units

we

igh

t (g

)

PHI units

we

igh

t (g

)

PHI units

S1

S2

S3

S3

S2

S1

Fig. 7 Summary of the stratigraphical relations of Baccano unit. Coloured units boundaries can be related to the analogous ones in Fig. 4. Grain-size, component analyses and the stratigraphical position of samples is also shown. Location of samples is given in Fig. 3a

Bull Volcanol

Tab

le1

ICP-M

SREYanalysisvalues.UCCREYvalues

from

Taylor

andMcL

ennan(198

5).Cho

ndrite

C1REYvalues

from

And

ersandGrevesse(198

9)

Sam

ple

La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

YHo

Er

Tm

Yb

Lu

mg/kg

mg/kg

mg/kg

mg/kg

mg/kg

mg/kg

mg/kg

mg/kg

mg/kg

mg/kg

mg/kg

mg/kg

mg/kg

mg/kg

mg/kg

SF

0.35

0.6

0.12

0.5

0.14

0.04

0.14

0.02

0.08

1.4

0.02

0.04

0.01

0.02

0.01

CT

4221

7.3

255.2

1.3

7.2

1.1

6.0

691.5

4.3

0.7

3.7

0.7

HF

7517

014

417.3

1.4

8.8

0.9

3.8

190.7

2.1

0.32

2.0

0.32

U2p

c57

120

1442

7.8

1.7

8.3

1.0

4.3

250.8

2.4

0.34

2.2

0.34

U2p

m86

170

1649

8.2

1.9

8.7

0.9

3.9

190.8

2.1

0.30

1.8

0.30

U2p

120

230

2471

112.5

131.4

5.7

291.1

3.3

0.47

2.9

0.47

BT

1.6

1.2

0.43

0.6

0.13

0.03

0.13

0.02

0.11

0.9

0.03

0.07

0.01

0.04

0.01

FrT

420

490

4095

8.8

1.8

141.0

3.1

140.6

1.8

0.21

1.2

0.22

C8-65

061

110

1237

7.1

1.6

7.5

0.8

3.2

170.6

1.6

0.23

1.2

0.23

C8-77

072

130

1441

8.1

1.9

8.3

0.9

3.5

190.7

1.7

0.25

1.4

0.25

C8-95

077

150

1754

112.4

9.9

1.3

4.9

230.9

2.4

0.37

1.8

0.32

U1am

190

390

38110

194.0

212.1

8.3

411.5

4.2

0.6

3.3

0.6

U2am

8919

016

437.2

1.6

8.8

0.9

3.9

240.8

2.2

0.34

2.1

0.34

U2B

210

340

3284

122.7

8.7

1.2

4.5

230.9

2.6

0.35

2.0

0.35

U3B

120

240

2363

123.1

131.3

5.2

260.9

2.6

0.35

2.3

0.40

AAC

41110

1890

224.8

192.4

1150

1.7

4.8

0.6

3.6

0.5

U1S

L29

455.8

193.5

0.9

3.1

0.45

2.1

110.41

1.2

0.17

1.1

0.17

SC

2424

4.5

162.9

0.6

3.0

0.43

2.2

130.43

1.3

0.17

1.1

0.17

U1S

48.0

8.1

1.5

4.0

0.7

0.16

0.7

0.10

0.46

3.0

0.10

0.29

0.04

0.24

0.04

U1S

638

537.4

285.0

1.1

5.3

0.8

4.1

230.8

2.5

0.35

2.3

0.35

GSP-2

(CRM)

170±6

420±11

50±3

160±23

23±2

2.3±0.2

14±3

1.9±0.2

5.7±0.4

24±4

0.84

±0.14

2.6±0.3

0.25

±0.6

1.3±0.2

0.20

±0.04

Normalization/ref.values—CRM

Upper

continentalcrust(U

CC)

3067

7.1

264.5

0.9

3.5

0.9

3.5

291.2

2.3

0.48

3.0

0.51

Cho

ndrite

C1A&G

×1.36

0.32

0.82

0.12

0.62

0.20

0.08

0.27

0.05

0.33

2.12

0.08

0.22

0.03

0.22

0.03

Sabatinilava

56120

1446

8.9

2.0

9.1

1.1

4.4

220.8

2.2

0.3

1.8

0.3

GSP-2

certifiedvalues

(*=indicativ

e)18

0±12

410±30

51±5

200±12

27±1

2.3±0.1

12±2

nd6.1*

28±2

1.0±0.1

2.2*

0.23*

1.6±0.2

0.23

±0.03

SFMon

teSasso

Fluorite,BTBagnarello

travertin

e,U1a

mashy

matrixof

U1Baccano

unit;

U2a

mashy

matrixof

U2un

it;U2p

pumiceclastfrom

thefallou

tlayerat

thebase

ofU2un

it,U2p

mashy

matrixof

thepu

micefallou

tlayer.U2p

cfree

crystalsfrom

thefallo

utlayer;U2B

bulk

compo

nentry

ofU2un

it;U3B

bulk

compo

nentry

ofU3un

it;C8-65

0/95

0cutting

sfrom

theC8deep

wellat

theirrelativ

edepths

bulkly

analyzed;CTcalcite

fracture

filling

inU1un

it;FrT

fluo

rite

rich

travertin

e;HFhallo

ysite

fracture

filling

;AAC

Aug

ite-A

ndradite

clot

from

U2un

it.GSP-2

standard;U1S

Lsedimentary

lithics

from

U1un

it;SC

marly

limestone

from

theScaglia

Formationof

theApenn

ine;

U1S

4Eocene-Olig

ocenecalcareous

sedimentary

lithicin

U1un

it;U1S

6Eocene-Olig

ocenecalcareous

sedimentary

lithicfrom

U1un

it

Bull Volcanol

(Fig. 8d), is very similar to that of the ash matrix of the U1and U2 Baccano deposits (U1am–U2am). The REY patternof the calcite sample (CT), sampled in pervasive fracturesof unit U1, shows a general depletion in LREE and a lowenrichment in MREE and HREE (Fig. 8d) with negative Ceanomaly, in association with a positive Y anomaly. The C1-

chondrite normalized REE pattern of the calcareoussedimentary xenoliths shows a light LREE enrichment,and a small HREE impoverishment, with smooth Ce andEu negative anomalies (Fig. 8e). Finally, the REY pattern ofU1 sedimentary xenoliths (U1SL, which do not show thermalor hydrothermal alteration) shows a negative Ce anomaly.

FrTC8 - 650C8 - 950

C8 - 770

U3BU2BU2amU1am

U2pU2pc U2pm

AAC

Sabatini Lava

C8 - 650

C8 - 770

C8 - 950

FrT

CTU2pcU2pFrT

U2amU1am

U3bU2bAAC

BT

SFHF

C8 - 650C8 - 950

C8 - 770C8 - 650C8 - 950C8 - 770

CT HFU2pmU2pc

U1am U2am

U2p

U2B U3B

U2pm

a b

c d

U1S4

U1S6

U1SL

SC

e

Fig. 8 Graphs showing: a the pattern and REY concentrations ofBaccano volcanic units compared to halloysite and fluorite-richtravertine alteration layers; b LREE enrichment in fluorite-richtravertine compared with C8 samples and Augite-Andradite clot; cREY pattern of Baccano activity-related deposits compared with a

non-rich travertine and sedimentary fluorite REY pattern sampledfrom Sabatini area; d REY pattern of Baccano activity depositscompared to fracture filling materials in U1; e REY pattern ofsedimentary xenoliths in U1

Bull Volcanol

Data interpretation

The poor sorting, massive appearance and polymodal sizedistribution of the massive to stratified lithic breccia faciesof the U1 basal part, its geometry and extension and thelack in the upper part of a fine-grained associated deposit,indicate the deposition from a relatively high-concentratedflow. Of specific interest is the low amount of juvenilecomponents relative to lithic fragments, which are assumedto be largely derived by erosion or collapse of the eruptiveconduit and/or vent walls.

Limited extent and small volume of U1, its poorsorting and matrix-supported character, the weak verticaland lateral grading of the clasts and the presence ofsubrounded clasts (Browne and Lawless 2001). More-over, the presence of accretionary lapilli and plasticdeformation of the deposit in impact sags, suggest thatthe unit was deposited in a wet (vapour rich) environment.The 30% wt of calcareous lithics contained in U1 arederived from a part of the siliciclastic complex thatpresently hosts a regional aquifer. Assuming that theseconditions are the same as at the time of the first eruption,following Wohletz’s (2002) model we can estimate theratio of saturated sediments to magma erupted (Rs), whichis about 1.3, considering the abundance of geothermalaquifer fragments in the unit (19.5 wt.% counted as the30% wt of 65% wt).

The U2 unit is a pyroclastic flow deposit with a pumicefall deposit at the base, well preserved on topographic lows(Fig. 4). The pyroclastic flow surmounted the topographicbarrier of the U1 crater rim and emplaced the massive,coarse-to-fine-grained deposit and the dune-bedded ash-cloud-surge on the top. The pumice fall deposit in the basalpart of the unit strongly suggest that the eruption startedwith a sustained column, which shortly after collapsed toform a dilute pyroclastic flow.

Shard morphologies and vesicles types in U2 suggestthat after the emplacement of U1 unit, the decompression ofthe underlying volume of magma caused more energeticexolution processes, which produced the juvenile compo-nent of the U2 deposit. The presence in the lithiccomponent of marble and augite and andradite from thethermometamorphic contact aureole hosted at −2,500 m(drilling data) suggests that the composition of thesedimentary lithics in U2 correlates to stratigraphic depthsgreater than those indicated by lithics in U1. Thus, wesuggest that the vesiculation-driven or water–magmainteraction/fragmentation occurred at a greater depth(−2,500 to −3,000 m) than that responsible for the U1eruption. (ENEL 1991). The presence of thermometamor-phic contact aureole lithics in the pumice-rich diluitepyroclastic flow deposit suggests that the U2 eruptioncould have been generated shortly after the U1 crater-

opening phase, when the conduit was opened from thechamber to the surface allowing vesiculation and explosiveentrainment of possibly water-saturated thermometamorphicrocks (Self and Wright 1983; Valentine et al. 1989;Druitt 1992).

The third Baccano unit is a dilute pyroclastic flow depositcharacterized by an ash-dominated sandwave facies andwidespread occurrence of accretionary lapilli. The presenceof abundant accretionary lapilli suggests that the Baccanotephra was emplaced in wet conditions. The presence of bothregressive and progressive dune structures indicates thepulsatory nature of the eruption and the resulting changes inthe flow regime of the pyroclastic currents.

All the Baccano deposits show evidence of pervasivehydrothermal fluid circulation. Halloysite is often present inall the deposits and in particular it is characteristic of thefractures pervading U1, sutured by the overlying U2 unit,and of two levels: one at the top of U1 and the other at thetop of the pumice fall deposits at the basal part of U2 unit(Figs. 4, 6, 7). The fluorite-rich travertine (composed of83% CaCO3, 12% CaF2 and 5% silicate compound anddated at 70–80 ka by Taddeucci and Voltaggio 1988) at thetop of the pumice fall layer in the basal part of U2 indicatesthat the deposition of CaCO3 was from a hydrothermalsolution enriched in F− and CO2, during the U2 eruption.The lack of a soil or of an unconformity between U1 andU2 deposits strongly suggests that the two eruptionsfollowed each other closely in time. The analyses of deepcuttings of C8 geothermal drillhole, interpreted as the deepcrater filling breccias of the U2 crater, also show a diffusealteration due to hot fluid circulation.

We suggest that the REY enrichment of the juvenilecomponent of Baccano deposits occurred directly in themagma, during crystallization. The hypothesis is supportedby the comparison with the REY content in the andradite-augite clot sample and in the fluorite-rich travertine. The clotand travertine REY patterns are comparable, suggesting thatthe LREE depletion and enrichment processes are related toeach other. According to the experiments of Kolonin andShironosova (2002), conducted at temperatures between100°C and 500°C, the LREE enrichment may be inter-preted as a leaching process from an acid fluid withsubsequent deposition at lower temperature. If we considerthe present geothermal gradient of Baccano area and thetemperature of the hot brine produced by the SH2 deep well(Calamai et al. 1976) as representative of those at the time ofthe eruption, we may hypothesize that the aquifer hosted inthe Baccano structural high had a temperature of >200°C,thus falling in the experimental temperature range ofKolonin and Shironosova (2002).

The presence of fluorite (Y-fluorite) indicates that fluorinewas leached and transported from depth by the hydrothermalfluids and consequently was not present in the C8 deep

Bull Volcanol

cuttings; it was subsequently deposited as fluorite associatedto LREE and Y in the travertine, when pH and T of thegeothermal fluids allowed the deposition. This interpretationis supported by the comparison of the REY concentration inthe Mt. Sasso Fluorite deposits and in the Terme di Bagnarellotravertine (Fig. 8c). In these cases, in fact, the REY trend(lacking negative Eu anomaly) is typical of CaF2 and CaCO3

deposited by geothermal fluids at low temperatures (below200°C), from solutions eventually mixed with meteoricwaters (Schwinn and Markl 2005).

The Halloysite REY pattern suggests that the source ofelements and the HT conditions of leaching, migration andprecipitation were the same during the Baccano eruptions.The calcite REY pattern is different from any otherobserved trend and is typical of deposits precipitated frommeteoric and karst water (Möller 2002). In this case,oxygen-rich surface water acquires a strong negative Ceanomaly and positive Y anomaly (Bau and Dulski 1996;Möller 2002; Leybourne et al. 2000). The negative Ceanomaly was produced by the rapid oxidation of Ce3+ toCe4+ and preferential removal of Ce4+ from solutions uponleaving the shallow groundwater environment (Leybourneet al. 2000). This pattern is also typical of depositsassociated with geothermal CO2-rich waters (Möller et al.2004a, b) suggesting that it can occur not only due tometeoric infiltration but also by magmatic fluid circulation,which is the principal source of the REY.

The strong negative Eu anomaly observed in sedimen-tary limestones has generally been interpreted as due to lateor post-magmatic alteration (Meloni et al. 1987), resultingfrom strong hydrothermal activity with F− and/or Cl−-richfluids that produce a strong LREE and Eu depletion(Meloni et al. 1987; Möller 1998). In our samples, however,the Ce and Eu negative anomalies are not particularlymarked, and are unlikely to be associated with hydrothermalalteration processes. Unfortunately, REY abundance anddistribution in pelagic sediments compared to those resultingfrom hydrothermal alteration, are not well documented;several studies (Meloni et al. 1987; Parekh et al. 1977)report the distribution of REY in marine limestone, inrelationship to the genesis and the mobilization of theseelements during the depositional and weathering processes.

The REY pattern of U1 sedimentary xenoliths is typicalof marine seawater limestones, where negative Ce anomalyis generally interpreted as confirming oxidizing conditionsin the sedimentary environment. Even stronger negative Euand Ce anomalies have been observed in the limestones ofthe basal sedimentary complex and they have beeninterpreted as due to weak weathering (Meloni et al. 1987).

Our analytical data may suggest that the enrichment inLREE of the Baccano deposits and the impoverishment inthe lithic clot could be due to leaching processes connectedto F−-rich magmatic fluids. Leaching of F− and LREE

probably occurred at high temperatures, whereas re-deposition of LREE-enriched fluorite in the travertine, andthe halloysite in the fractures occurred between 100°C and500°C (Kolonin and Shironosova 2002), and more specif-ically between 200°C and 450°C.

The leaching processes did not involve the sedimentaryEocene-Oligocene limestone hosted at −1,000 to −1,200 ma.s.l, suggesting that the hydrothermal system was locatedat higher depth.

A possible scenario for the baccano eruption

The structural, textural and lithological characteristics ofthe basal part of the U1 Baccano deposit—in particular thepredominance of the Eocene-Oligocene sedimentary lithicclasts, with respect to the limited abundance of juvenilecomponents, are comparable with those usually shown bymagmatic-hydrothermal eruption deposits. Deep drillingdata indicate that Eocene-Oligocene calcareous units are atdepths of −1,000 to −1,200 m and are part of the Baccano-Cesano structural high, which hosts the regional aquifer inthe Sabatini area. Under the assumption that most lithicclasts were eroded from the conduit at the fragmentationlevel, the abundance of the Eocene-Oligocene lithic clastsindicates that the fragmentation level of U1 eruption mostlyoccurred at these depths, which is an unusual depth forhydrothermal and hydrovolcanic eruptions. Browne andLawless (2001) discussed the possible mechanisms andcauses for hydrothermal and magmatic-hydrothermal erup-tions. In their model, the initial steam burst occurs a fewmeters below the ground surface inducing decompression;the flashing front and brecciation surface move progres-sively downward within the reservoir, followed by theeruption front (“top-down model”). However, the sameauthors point out that the model is applicable to eruptionsoccurring at depths of 300–500 m. We think that the “top-down model” could also potentially be applied to the caseof the U1 eruption, assuming that the flashing front andbrecciation surface could have rapidly descended until theyreached the depth of −1,000 to −1,200 m (Fig. 9c and d).

This hypothesis is reasonable if we take into accountthe boundary conditions of the geothermal system at thetime of the U1 eruption. In the Baccano case we canestimate an overburden of more than 25 MPa oflithostatic pressure at −1,000 to −1,200 m. Based ongeothermal exploration data we could expect temper-atures much greater than 200°C and probably temper-atures between 365°C and 410°C (Tecce et al. 2000) andhigh salinities for the brines in the geothermal aquiferhosted by the Baccano-Cesano structural high. Because ofthe lithologies, we can reasonably assume that the aquiferwas confined and highly permeable. This is mainly based

Bull Volcanol

on the geometry and mechanical characteristics of thesedimentary structural high that is confined by major NE-trending regional faults and consists mostly of tectonicallyfractured limestones. Thus the piezometric surface wasmost likely at −1,000 to −1,200 m, i.e. at the top of theEocene-Oligocene calcareous units (Fig. 9a). If weconsider that pure water at 22 MPa and 374°C is closeto the critical conditions (Keenan et al. 1978; Henley andEllis 1983), then we can speculate that the fluid in theBaccano aquifer was also close to critical conditions. Thischange could be caused by seismic activity, changing offluid pressure in the geothermal aquifer and changes ofpermeability and/or porosity of the reservoir rocks. This

scenario is also in accordance with the model proposed byYamamoto et al. (1999), in which they correlated thetriggering of a phreatic explosion with seismic activityimmediately before the eruption. Similarly to the U1 atBaccano (Fig. 9c), the eruption reported by Yamamoto etal. (1999) could not be justified by a disequilibrium of themagmatic system, due to the fact that the ejected materialswere almost completely (78%) made of pre-existing rockfragments, with little or no presence of juvenile glass shards.

Our hypothesis, supported by the REY analyses, is thatanother possible contributing factor could be associatedwith the corrosive action of acid hydrothermal fluids on thecountry-rocks. The action of these fluids could have

heat and fluid source

surface

enhancedfracturing, leaching

and overpressuringin the aquifer

Loweringof the piezometric

surface1000

- 1

500

m30

00 -

500

0 m

B

risinghydrothermal

acid fluids

Initialbrecciation

heat and fluid source

surface

calcarerousstructural high

hosting aregional aquifer

Top of theconfined

hot acquifer(piezometric

surface)

marly - clayeylow-permeability

cover

1000

- 1

500

m30

00 -

500

0 m

A

risinghydrothermal

acid fluids

contact thermo-metamorphic

aureola

heat and fluid source

surface

crisis and “opening” of the system due tooverpressuring

rising dyke/s ?

?1000

- 1

500

m30

00 -

500

0 m

C

risinghydrothermal

acid fluids

breaking ofthe cover

and cratering

scarce juvenile andsedimentary lithics with

no thermo-metamorphism

First Baccano event

heat and fluid source

surface

deepeningof the focalexplosion

depth

1000

- 1

500

m30

00 -

500

0 m

Dpumice and

thermo-methamorphed marbles

pumices

Second Baccano event

deep magmaticlithics

Fig. 9 Schematic representationof the evolution of Baccanomaar activity: a structural settingof the area in the pre-eruptivestage; b initial stage ofinteraction between the risinghydrothermal acid fluids and thecalcareous structural highhosting the regional aquifer;c First explosive activity ofBaccano maar, characterized byan hydrothermal-phreaticeruption occurred involving theupper part of the confinedaquifer on top of the structuralhigh, with a no water–magmainteraction. d Second explosiveactivity of Baccano maar,characterized by a more evidentphreato-magmatic behaviour ofthe eruption, showing efficientwater–magma interaction, whichis accompanied by thedeepening of the explosive focaldepth (and/or a potentialnorthward shifting of the crater)involving thethermo-metamorphic contactaureole of the magma chamber

Bull Volcanol

produced partial high-temperature dissolution of the CaCO3

of the basement limestones, causing release of CO2, leadingto the overpressuring of the entire system and consequenthydrofracturing. This in turn may have caused rapid vapourseparation and a pressure drop, allowing the almostsimultaneous breaking of the cover and the brecciation ofthe calcareous units (Fig. 9b and c). The calcareousbrecciated lithic clasts in the basal part of U1, representingmost of the first 200 m below the top of the Baccano-Cesano structural high, may represent the descent of thepiezometric surface during the eruption (Fig. 9b). Thishypothesis is supported by the relative abundance ofsedimentary and volcanic lithic clasts in the U1 deposit.

Following this scenario, we may expect evidence offluid/rock interaction such as precipitation and self-sealingprocesses in the limestones at the fragmentation level.However, the Eocene-Oligocene calcareous lithics do notshow any evidence of these processes. One possibility isthat fluid/rock interaction occurred at a deeper level(>1,400 m) and/or that the rapid propagation of thebrecciation front did not allowed alteration of the upperpart of the calcareous units hosting the aquifer. Afterdecompression, magma rose up in the conduit and cameinto direct contact with water in the regional aquifer hostedin the Baccano-Cesano structural high and experienced anefficient fuel coolant interaction. This occurrence couldjustify the characteristics of the U2 and U3 Baccano units,i.e. a lower percentage of sedimentary lithics, a moreabundant and vesicular juvenile component and a higherdegree of clast fragmentation with respect to U1 (Fig. 9d).The rise of the hydrothermal fluids closed the first eruptionand deposited the fluorite-rich travertine that is character-ized by a LREE-enriched REY pattern. The deposition ofhalloysite in the fracture system pervading unit U1 occurredin the same period (before the U2 euption, which followedthat of U1 by an unknown time period) but at lowertemperatures. The lithology of the sedimentary lithic clastspresent in the U2 unit suggests that the fragmentation levelof the explosions progressively deepened (with a potentialnorthward shifting of the crater, Fig. 9d), as is generallyexpected in hydrothermal and hydromagmatic eruptions(Browne and Lawless 2001).

Conclusions

In this paper we report the occurrence at Baccano maar of avent-opening explosion breccia generated by a deep-seatedmagmatic-hydrothermal eruption. The lithology of theBaccano breccia clasts, compared with the stratigraphy ofdeep wells for geothermal exploration in the Sabatini,suggests that hydrothermal eruptions here occurred atdepths greater than 1,000–1,200 m. We suggest that a

mechanism similar to that proposed by Browne andLawless (2001) for hydrothermal eruptions may be valideven at depths exceeding 300–500 m. This is possiblebecause a rapid overpressure and decompression of ahydrothermal system may be caused by high-temperaturedissolution processes driven by acid hydrothermal fluids.At Baccano, CaCO3 dissolution of the sedimentary base-ment, mainly caused by H2S, HF and H2SO4 enrichedfluids, enhanced CO2 overpressure and subsequent fractur-ing of the limestones, causing the decompression of thehydrothermal system and triggering the first explosion.Subsequently, magma rose up in the conduit and came intocontact with the water of the regional aquifer hosted in theBaccano-Cesano structural high. This promoted moreefficient fuel-coolant interactions between magma andwater, which allowed the progressive deepening of thefragmentation level driving the waxing phase of thephreatomagmatic Baccano activity.

Acknowledgements This work benefited from the help of severalpeople. We want to thank Fabio Spaziani for helping us in the REEanalysis, Francesca Tecce who provided us the Cesano-1 and Cesano-8 well cuttings and supported our EPMA analysis. Daniela Dolfi, andMonia Procesi for constructive criticism and debate during laboratoryand field work. Alfredo Mancini and Marcello Sarracino are thankedfor assistance at SEM and EPMA analyses respectively. Weparticularly acknowledge Richard Brown and Kenneth Wohletz forall the useful suggestions that strongly improved the draft version ofthe manuscript. We would also thank an anonymous reviewer for histhorough revision of the manuscript. We would finally thankBenjamin van Wyk de Vries for the review and all the precioussuggestions.

Appendix: Analytical procedure used for REY analysis

After the preliminary grinding and homogenization,samples were dissolved by means of microwave-assistedacid dissolution (with a FKV—Milestone—MLS 1200Mega). Each dissolution was executed on approximately400 mg of sample, accurately weighed (directly) inTeflon vessels, using the following acid mixture: 4 mlHNO3 + 1 ml H2O2 + 2 ml HF + 1 ml HClO4. All thesereagents were of high purity grade (BDH-Aristar®). Themicrowave cycle was prearranged in four steps: 5 min at250 W (heating power), 10 min at 400 W, 10 min at600 W and 5 min at 250 W. After cooling at roomtemperature, the solutions were transferred into Teflonopen recipients and then heated near to dryness on ahotplate, in order to remove HF and the excess of the otheracids. This heating process was repeated three times and atthe beginning of each time a small amount of acid wasadded (0.5 ml of HClO4 the first time, then 1 ml ofHNO3). The final nearly dry residue was transferred to a50 mL volumetric flask adding 0.5 ml of HNO3 and ultra-

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pure (U.P.) deionized water (∼18.4 MΩ cm) up to the finalvolume. Twenty blanks were also prepared, using thesame analytical scheme but omitting the sample, in orderto establish the practical limits of detection of the method(whole analytical procedure). The dissolution procedurewas checked, with reference to REEs recovery, analysingthe GSP-2 (CRM purchased by USGS). The determinationof the REEs in the solutions coming from the dissolutionof the samples was performed by means of an ICP-MS(Perkin-Elmer-ELAN-6100) configured with the followingoperational parameters: RF power 1,100 W, nebiliser flow0.92 L min-1, resolution 0.7 amu, dwell time 40 ms,replicates 3, measurement mode peak hopping. REEs areeasily determined by ICP-MS since they occur at themiddle to the high portion of the mass range and have fewinterferences from polyatomic species. Rhodium (Rh) wasadded as internal standard to all the solutions, in order tocorrect any instrumental drifts. Blanks, calibration stand-ards and sample solutions were prepared and, if necessary,diluted in a 1% HNO3 matrix (obtained using U.P.deionized water and high purity grade HNO3). Standardsolutions, containing the REEs in the range 0.01–50 μg/L,were prepared daily by dilution of a stock multi-elementsolution (CAL1-1 supplied by AccuStandard) at 10 mg/L.The solutions obtained at the end of the dissolutionprocedure were generally diluted with a factor of 1 to 10before the final determination by ICP-MS.

Limits of detection (LOD) for REEs have beencalculated as Xmb+3Smb, where Xmb and Smb arerespectively the mean and the standard deviation of themeasurements of method blanks and are 27 μg kg-1 forLa, 75 μg kg-1 for Ce and in the range 0.5–5 μg kg-1for the other elements.

The geochemical reference material GSP-2 was used toverify the accuracy and precision of the analytical method.Five distinct tests were carried out on this CRM, thereforefive separate solutions were analyzed with the ICP-MS.Table 1 shows the excellent agreement between the resultsobtained in our laboratory and the certified values.

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