Contrib Mineral Petrol (2001) 140: 543±553 DOI 10.1007/s004100000213
S. Signorelli á G. Vaggelli á C. Romano á M. R. Carroll
Volatile element zonation in Campanian Ignimbrite magmas (Phlegrean Fields, Italy): evidence from the study of glass inclusions and matrix glasses
Received: 2 May 2000 / Accepted: 4 October 2000 / Published online: 29 November 2000 Ó Springer-Verlag 2000
Abstract The distribution of H2O, F, Cl and S in the Campanian Ignimbrite (CI) magma chamber was investigated through study of primary glass inclusions and matrix glasses from pumices of the Plinian fall deposit. The eruption, fed by trachytic to phono-trachytic magmas, mainly produced a trachytic non-welded to partially welded tu, underlain by a minor cogenetic fallout deposit. The entire chemical variability of the eruptive products is well represented in the pumices of the Plinian fall deposit, which we divide into a basal Lower Fall Unit (LFU) and an overlying Upper Fall Unit (UFU). Primary glass inclusions were only found in clinopyroxenes associated with the LFU pumice and contain a mean of 1.60 0.32 wt% H2O (analysed by FTIR), 0.11 0.08 wt% F, 0.37 0.03 wt% Cl and 0.08 0.04 wt% SO3 (EMP analysis); CO2 concentrations were below the FTIR detection limit (10±20 ppm). The coexisting matrix glasses contain similar amounts of halogens and sulfur but less water (0.60 wt%). Partially degassed matrix glasses from UFU pumices contain a mean of 0.30 0.02 H2O, 0.28 0.10 F, 0.04 0.02 SO3 and 0.80 0.04 wt% Cl. To reconstruct the total amount of volatiles dissolved in the most S. Signorelli (&) Consejo Superior de Investigaciones Cientõ ®cas (C.S.I.C.) Institut de CieÁnces de la Terra ``Jaume Almera'' Lluõ s Sole i Sabarõ s s/n, 08028 Barcelona, Spain e-mail:
[email protected] Tel.: +34-93-409 54 10; Fax: +34-93-411 00 12 G. Vaggelli C.N.R. ± Centro di Studi per la Minerogenesi e la Geochimica Applicata, Via La Pira 4, 50121 Firenze, Italy C. Romano Universita' degli Studi di Roma Tre, Dipartimento di Scienze della Terra, L.go Murialdo 1, 00146 Roma, Italy M. R. Carroll Dipartimento di Scienze della Terra, Universita' di Camerino, Via Gentile III da Varano, 62032 Camerino, Italy Editorial responsibility: J. Hoefs
evolved trachytes we have used experimental solubility data and mass balance calculations concerning the amount of crystal fractionation required to produce the most evolved trachyte from the least evolved trachyte; these yield an estimated pre-eruptive magma volatile content (H2O + Cl + F) of 5.5 wt% for the most evolved magmas. On the basis of new determinations of Cl solubility limits in hydrous trachytic melts coexisting with an aqueous ¯uid phase + hydrosaline melt (brine), we suggest that the upper part of the magma chamber which fed the CI eruption was ¯uid(s) saturated and at a minimum depth of 2 km. Variations in eruptive style (Plinian fallout, pyroclastic ¯ows) do not appear to be related to signi®cant variations in pre-eruptive volatile contents.
Introduction The abundance of volatile components can signi®cantly aect the density, viscosity and crystallisation of magmas, and can in¯uence magma dierentiation, ascent and eruption. For explosive eruptions, it is the exsolution and expansion of CO2 and H2O in particular that provides the eruptive driving force. Volatile sulfur compounds (e.g. H2S and SO2) and halogens (F, Cl) are minor components in most magmas, but it has been observed that the volcanic release of these species to the atmosphere may aect global climate (e.g. Johnston 1980; Devine et al. 1984; Symonds et al. 1988; Albritton 1989; Carroll and Holloway 1994 and references therein). The amount of degassing of dierent volatile species will vary with pressure, solubility laws and magma ascent rate. Direct evidence that silicate melts become saturated with a volatile phase at crustal pressures comes from ¯uid inclusions and remote sensing studies (e.g. Roedder 1984; Gerlach and McGee 1994; Gerlach et al. 1994, 1996; Lowenstern 1995). Knowledge of solubility limits and pre-eruptive volatile content of magmas is of fundamental importance for understanding volatile
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element behaviour in magmatic reservoirs and for predicting volatile outputs during eruptions. Traditionally, it has been assumed that the pre-eruptive volatile content of magmas controls the explosivity and evolution of eruptions. However, recent studies have pointed out that sometimes volatile content is not the most important factor controlling eruptive styles (e.g. Dunbar and Kyle 1993; Sparks et al. 1994; Barclay et al. 1996). The aim of this paper is to determine pre-eruptive volatile element abundance in the 145 km3 of trachytic magma produced during the Campanian Ignimbrite (CI) eruption, the major explosive event in the last 200,000 years in the Mediterranean region (Barberi et al. 1978). The CI eruption progressed through a number of different eruptive stages, ranging from Plinian fallout to pyroclastic ¯ows and surge activity in¯uenced by caldera collapse(s) (Rosi et al. 1996). Several lines of evidence (major, trace element and isotopic geochemistry, mineralogy) suggest that the CI eruption tapped a compositionally zoned magmatic reservoir (Di Girolamo 1970; Barberi et al. 1978; Di Girolamo and Morra 1987; Cornell et al. 1993; Civetta et al. 1997; Signorelli et al. 1999a). The causes of pre-eruptive zoning are still a matter of debate: crystal-liquid fractionation in a closed (Di Girolamo 1970; Di Girolamo and Morra 1987) or open system (Civetta et al. 1997), post-depositional alteration of a homogeneous magma (Barberi et al. 1978) and preeruptive magma mixing, due to input of ma®c magma into a more silicic magma chamber (Signorelli et al. 1999a) have all been proposed. Information on pre-eruptive volatile abundance in the CI magmas comes from the study of melt (now glass) inclusions trapped during phenocryst growth prior to eruption. Such inclusions can provide samples of undegassed magma and the dierence in volatile contents between glass inclusions and matrix glasses (``petrologic'' method) from the Plinian deposit associated with the CI provide an estimate of syn-eruptive volatile degassing. We have concentrated on the cogenetic fallout deposit for three reasons: (1) in general, it represents an inverted continuous sampling of the magmatic reservoir; (2) it has been shown that the Plinian deposit compositional range is comparable to that found in the ignimbrite, indicating that the two eruptive phases drained the same magmatic reservoir (Signorelli et al. 1999a); (3) the quickly cooled tephra from the fallout deposit are often only partially degassed (e.g. Stix and Layne 1996; Signorelli and Capaccioni 1999; Signorelli et al. 1999b). Previous studies of volatiles associated with the CI eruption are few. Devine et al. (1984) and Palais and Sigurdsson (1989), in studies of glass inclusions and partially degassed matrix glasses from the Plinian deposit (our approach), found higher concentrations of halogens and sulfur in the matrix glasses compared to those in the glass inclusions; they suggested that this could re¯ect post-depositional hydrothermal alteration or metasomatism associated with cooling of the overlying ignimbrite deposit. In contrast, in the following
discussion we present an alternative interpretation which proposes that CI magmas were pre-eruptively zoned in volatiles and that the high volatile abundances in some pumice samples are in fact primary characteristics.
Volcanological background The Campanian Ignimbrite (CI) eruption was the most intense volcanic episode of the Campanian Volcanic Area during the Quaternary (36 ka b.p.; Deino et al. 1992) and it ®nished with production of a caldera of 12 km diameter. The eruption produced a large-volume alkali-trachytic unwelded to partially welded tu, emplaced by dierent pyroclastic ¯ows. Recent estimates (Civetta et al. 1997) of the covered area and volume of the CI are 30,000 km2 and 145 km3 DRE (Dense Rock Equivalent) respectively; the volume of erupted material is approximately equal to estimated caldera volume (Orsi et al. 1996). The pumice fragments of the CI range from trachytic to phono-trachytic in compositions (Civetta et al. 1997; Signorelli et al. 1999a). Civetta et al. (1997) estimated DRE volumes of about 25 km3 for the most evolved magma, 20 km3 for the least evolved magma and 100 km3 for the magma of intermediate composition. Mineralogical, isotopic, and major and trace element data have been used to suggest that the intermediate magmas were produced by the syn-eruptive mingling/mixing of the least dierentiated and the most evolved magmas (Civetta et al. 1997). Near the postulated vent of the CI eruption, ESE of Pozzuoli town, the ignimbrite deposit is anticipated by a pumiceous fallout level (Fig. 1), alkali-trachytic in composition. The deposit covers an area of 1,500 km2. Based on stratigraphic, lithologic and compositional criteria, it is possible to distinguish a Lower Fall Unit (LFU) and an Upper Fall Unit (UFU) in the Plinian deposit (Castelmenzano et al. 1995; Rosi et al. 1999). The maximum thickness (130 cm) of the Plinian deposit was found at the Voscone locality (Fig. 1), in a disused quarry. Here the deposit is 42 km from the postulated vent and shows an 84-cm-thick LFU (Vos1-Vos3 samples) and a 46-cm-thick UFU (Vos4-Vos7 samples) overlain by a 15-m-thick ignimbrite. The pumice is aphyric to slightly porphyritic or glomeroporphyritic (around 5 vol%) with sanidine, clinopyroxene, plagioclase, biotite, scarce apatite and Fe-Ti oxides set in variably vesiculated colourless fresh matrix glass (Rosi et al. 1999; Signorelli et al. 1999a). The phenocryst content decreases from LFU to UFU pumice. Pyrrhotite is particularly abundant as inclusions in clinopyroxenes from LFU, whereas it is scarce in those from UFU. Details of the deposit, as well as volumes of the deposit and estimated heights of the eruptive columns are reported in Rosi et al. (1999). Juvenile clasts (pumice and obsidian) were also collected for comparison from two co-ignimbrite breccia units associated with CI (Rosi et al. 1996), at P.ta Marmolite (sample PM8) from unit B and at Scarafea (sample Sc11) from unit C (units B and C as described in Rosi et al. 1996).
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Fig. 1 Sketch map showing the areal distribution of the Campanian Ignimbrite (CI) and of the associated Plinian fall deposit, including the stratigraphic type section at Voscone. 1 Campanian Ignimbrite deposit; 2 Plinian fallout deposit related to the Campanian Ignimbrite eruption; 3 caldera rim; 4 inferred caldera rim; g.l. ground layer. (Redrawn after Rosi et al. 1999; Signorelli et al. 1999a)
10 and 7.5% respectively. Measurements of loss on ignition (LOI) provide a rough estimate of pumice water contents. In order to measure LOI, samples were dried at about 900 °C for 30 min and, after subtracting the other volatile elements, the remainder driven o is assumed to be H2O. Electron microprobe, Raman and infrared analysis of glasses
Methodology Sample preparation Selected pumice clasts from each stratigraphic horizon were cleaned in double distilled water to remove sur®cial impurities and salts, were lightly crushed to lapilli-size particles, then were ground and homogenised in an agate mortar. In order to check for the presence of further leachable constituents (soluble salts), 1 g of homogenised sample was shaken with 25 mL of deionised water for 30 min and then the solution was analysed by titration. The chlorine abundance in the solution was always less than analytical whole-rock uncertainty. Hand-picked separates of clinopyroxene phenocrysts were mounted in epoxy and polished on both sides for optical examination and micro-analysis. Matrix glasses were studied both in pumice fragments mounted in epoxy and as glass adhering to crystal rims. Whole-rock analysis Cl and S analyses of whole pumice were carried out by X-ray ¯uorescence (XRF), using the method of De Vries and Jenkins (1971). Fluorine was analysed by potentiometric methods. The estimated relative uncertainties (1r) in the Cl, S and F determinations are 7,
Glass inclusions and matrix glasses were analysed for major elements, Cl, S and F by a JEOL-JXA 8600 electron microprobe at the C.N.R. Centro di Studi per la Minerogenesi e la Geochimica Applicata (Firenze), using Bence and Albee (1968) matrix correction procedures. Analyses were performed using a 15 kV accelerating voltage, 10 nA beam current and 10 to 40 s counting times as detailed in Signorelli et al. (1999b) and Vaggelli et al. (1999). In the present work, the estimated analytical precision (1r), based on replicate analyses (n=30) of single standards, is 200 ppm for Cl at 0.33 wt% and 300 ppm for Cl at 0.50 wt%, 600 ppm for F at 0.25 wt% and 200 ppm for S at 0.1 wt%. The detection limits for Cl, S and F were estimated to be 400, 200 and 2000 ppm respectively. Chlorine and major element compositions of glasses from hydrothermal experiments were determined using a Cameca Camebax electron microprobe at the Department of Earth Sciences, University of Bristol. Analyses were performed with an accelerating voltage of 15 kV, a beam current of 7±10 nA and a 15-lm-diameter rastered beam to minimise alkali loss. Chlorine was counted for 60 s on peak and background. The analyses were calibrated against well-documented mineral and glass standards, as described in Signorelli and Carroll (2000). Raman analyses of volatile species (CO2, H2S and SO2) in the bubbles of the bubble-bearing glass inclusions were performed at the C.N.R., Firenze, using a Jobin-Yvon (S-3000) microspectrograph. Doubly polished thin sections of pyroxenes and pumice clasts were prepared for infrared (IR) analysis performed at the
546 Bayerisches Geoinstitut, Bayreuth (Germany) and at Dipartimento di Scienze della Terra di Napoli (Italy) respectively. The apparatus and the analytical procedure are described in detail in Signorelli et al. (1999b). Each glass inclusion was preserved within its host crystal without exposing it to the surface. The thickness of the glass inclusion was estimated by approximating the inclusions to a sphere, or in the case of a more elongated shape, a two-axis ellipsoid. The thickness of the inclusion in the latter case was taken to be the smaller of the two diameters. Given the uncertainty related to the assignment of the thickness, only inclusions with a more regular shape were analysed. In this study, the glass density was assumed to be 2,410 g/L (Rosi et al. 1999) and then corrected for the eect of water according to the equation for albite reported by Behrens et al. (1996). We chose a molar absorptivity value of 70 for the 3,570 cm±1 band used in this study (Silver and Stolper 1989). Hydrothermal experiments on Cl solubility Experiments were conducted in rapid-quench, water-pressurised cold seal pressure vessels at pressures from 25 to 200 MPa and temperatures from 860 to 900 °C using Au or Ag75Pd25 capsules at the Department of Earth Sciences, University of Bristol. The apparatus and the experimental procedure are described in detail in Carroll and Blank (1997). Chlorine was added as water-(Na,K)Cl solutions whose composition under run conditions was calculated by mass balance following the procedure of Signorelli and Carroll (2000). The (Na/K)m ratio in the ¯uid phase was chosen to be equivalent to the powder starting material. The initial Cl molality (m) in the ¯uid phase was approximately 5m, a suitable Cl concentration to have sub-critical ¯uids during the run (i.e. silicate melt coexists with both H2O-rich aqueous ¯uid and brine). The observed ¯uid inclusions (i.e. liquid + vapour and liquid + vapour + salts) in the glass run products support this interpretation. The sample holder consisted of an Inconel ®ller rod (Ni-rich), resulting in an oxygen fugacity around the capsule of NNO+1(0.5). Experiments were run for 7±14 days depending on the P and T conditions of the experiment; approach to equilibrium and other experimental details are described in Signorelli and Carroll (2000). Description of glass inclusions and matrix glasses In the CI fallout tephra, primary glass inclusions are abundant in clinopyroxene phenocrysts (Fig. 2A) from LFU, rare in feldspars and absent in the other mineral phases. Unfortunately, no glass inclusions suitable for study were found in the UFU phenocrysts. In feldspar phenocrysts, glass inclusions are characterised by the presence of fractures, daughter minerals and large bubbles, leading us to consider them unreliable for recording unaltered pre-eruptive melt compositions. Clinopyroxene-hosted glass inclusions consist of translucent brown glass, sometimes with a small bubble (Fig. 2B). Generally, bubble-bearing inclusions display constant glass/bubble volume ratios of 100 or more. Glass inclusions have
mostly sub-spherical or sub-cylindrical shape with a maximum size of 100 lm, but diameters commonly range from 20 to 30 lm. Backscattered electron images and X-ray traverses indicate that the inclusions are homogeneous and the crystal/glass interface is sharp, without apparent and/or signi®cant signs of host mineral re-crystallisation. Major element compositions of most inclusions do not show any signi®cant depletion of CaO or relative enrichment of other components that would re¯ect post-entrapment crystallisation of clinopyroxene (Signorelli et al. 1999a). Host clinopyroxenes range in composition from diopside to salite and glass inclusions occur both in zoned crystals at diopside-salite interfaces and in homogeneous salitic clinopyroxenes from LFU (Vos1 and Vos3 samples). All matrix glass samples analysed were colourless and isotropic under crossed polars, with no evidence of chemical zoning visible in back-scattered electron images. Figure 3 shows the variation diagrams for SiO2, MgO and Na2O in the glass inclusions and matrix glasses, including the stratigraphic position of the samples. Analytical results for all major elements are published and discussed in detail in Signorelli et al. (1999a). The matrix glasses and the glass inclusions of sample Vos1 display the largest compositional variability, whereas Vos3 and the UFU samples are more homogeneous. The analysed matrix glasses of ignimbrite pumices and obsidians (Sc11 and PM8) are individually homogeneous, but show distinct compositions, with PM8 matrix glass most similar to matrix glasses in UFU and Vos3, and Sc11 matrix-glass analyses most similar to the Vos1 glass inclusions. The least evolved glass compositions are found in LFU samples, whereas the matrix glasses of the UFU comprise the most evolved compositions. The chemical heterogeneity observed in the Vos1 samples is mainly due to processes of magma mixing (Civetta et al. 1997; Signorelli et al. 1999a).
Results Table 1 presents the volatile contents of the glasses and whole pumices; the glass analyses include mean and median compositions based on the analysis of 66 glass inclusions and 174 matrix glasses. Analytical results are plotted in Figs. 4 and 5. In general, glass inclusions are richer in S and H2O but poorer in Cl and F than matrix glasses. In particular, glass inclusions show signi®cantly lower Cl contents than matrix glasses with the exception of the least evolved (CaO ³ 2 wt%) Vos1 matrix glasses, which display compositions similar to the associated glass inclusions (Fig. 4A). The variation diagrams vs. Fig. 2 A Transmitted light photomicrograph of a typical clinopyroxene containing primary glass inclusions. B Back-scattered SEM image of a typical bubble-bearing glass inclusion in clinopyroxene
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Fig. 3 Variation diagrams of major elements (wt%) for the matrix glasses and the glass inclusions. Data reported on water-free basis (redrawn after Signorelli et al. 1999a)
CaO for glass inclusions show that with decreasing CaO, S decreases and Cl remains constant (Fig. 4A). In matrix glasses, Cl exhibits a hyperbolic trend with decreasing CaO similar to that displayed by Na2O, whereas S is positively correlated with CaO. Given that the vast majority of analyses of S for Vos3 and UFU are below the detection limit (200 ppm by weight), it is only possible to say that the S contents of these samples appear to be uniformly low (
