Geochemical zoning, mingling, eruptive dynamics and depositional processes — the Campanian...
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Jouma l of volcanology andgeothermaircseatch ELSEVIER Journal of Volcanology and Geothermal Research 75 (1997) 183-219 Geochemical zoning, mingling, eruptive dynamics depositional processes - the Campanian Ignimbrite, Flegrei caldera, Italy and Campi L. Civetta a,b, *, G. Orsi a, L. Pappalardo a, R.V. Fisher ‘, G. Heiken d, M. Ort e a Dipurtimento di Geojisica e Vulcanologia, Uniuersitb Federico II, Large San Marcellino 10, 80138 Naples, Ital) b Ossewatorio Vesuuiano, Ercolano, Naples, Italy ’ Department of Geological Sciences, University of California, Santa Barbara, CA 93106, USA ’ Las Alamos National Laboratory, Earth and Enuironmental Sciences Division, L.os Alamos, NM 87545, USA e Department of Geology and Environmental Sciences, Northern Arizona University, PO Box 4099, Flagstaff; AZ 8601 I-4099, USA Received 22 August 1995; accepted 4 April 1996 Abstract The Campanian Ignimbrite (CI) is a large-volume trachytic tuff erupted at 37 ka from the Campi Flegrei and composed of a fallout deposit overlain by ignimbrite. The ignimbrite was spread over an area of about 30,000 km* including the Campanian Plain and the Apennine Mountains, with ridges over 1000 m a.s.1. The pumice fragments of the CI range in composition from trachyte to phonolitic-trachyte (DI = 75-90). They do not show any systematic compositional variation with stratigraphic height, but the analyzed sections can be divided into three groups on the basis of chemical composition of pumices. Least-evolved pumices (DI = 75-83) occur in the ignimbrite in the central sector of the Campanian Plain up to 30 km from the vent, while the most-differentiated pumices (DI = 88-90) characterize the cogenetic fallout deposit and the ignimbrite in the western sector of the Campanian Plain, on the Tyrrhenian scarp of the Apennines between Caserta and Mt. Maggiore, on Roccamonfina volcano, and on the Sorrento Peninsula, up to 50 km from the source. Pumice fragments of intermediate composition (DI = 84-87) occur in the ignimbtite on the Apennine Mountains and Roccamonfina volcano, up to 65 km from the vent. In one exposure at Mondragone, at the base of a calcareous ridge, an ignimbrite with pumices of most-evolved composition is overlain by an ignimbrite with pumices of intermediate composition. The observed compositional variation between most- and least-evolved ignimbrite was generated in part by crystal-liquid fractionation, although other magmatic processes such as syn-eruptive mingling between most- and least-evolved magmas accounts for the mineralogical disequilibria and for the bimodality of the glass compositions in the intermediate-composition rocks. Pumice Sr-isotope ratios are positively correlated with degree of differentiation. Feldspar crystals separated from pumices of different compositions have a homogeneous Sr-isotope composition similar to that of the least-evolved pumices. Interaction between fluids and strongly fractionated Sr-poor less-dense magma can account for these isotopic features. Geochemical, mineralogic, stratigraphic and volcanologic data, together with the stratigraphic relations between most-, intermediate- and least-evolved ignimbrite as detected at Mondragone and from bore-hole drillings suggest that: (1) the CI magmatic system was composed of two distinct magma layers - the upper layer was more differentiated and homogeneous in composition, while the deeper was less evolved and slightly zoned; and (2) the CI was mostly emplaced in three main * Corresponding author. 0377-0273/97/$17.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved. PII SO377-0273(96)00027-3
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Transcript of Geochemical zoning, mingling, eruptive dynamics and depositional processes — the Campanian...
Joumalof volcanology andgeothermaircseatch
ELSEVIER Journal of Volcanology and Geothermal Research 75 (1997) 183-219
Geochemical zoning, mingling, eruptive dynamics depositional processes - the Campanian Ignimbrite,
Flegrei caldera, Italy
and Campi
L. Civetta a,b, * , G. Orsi a, L. Pappalardo a, R.V. Fisher ‘, G. Heiken d, M. Ort e
a Dipurtimento di Geojisica e Vulcanologia, Uniuersitb Federico II, Large San Marcellino 10, 80138 Naples, Ital) b Ossewatorio Vesuuiano, Ercolano, Naples, Italy
’ Department of Geological Sciences, University of California, Santa Barbara, CA 93106, USA ’ Las Alamos National Laboratory, Earth and Enuironmental Sciences Division, L.os Alamos, NM 87545, USA
e Department of Geology and Environmental Sciences, Northern Arizona University, PO Box 4099, Flagstaff; AZ 8601 I-4099, USA
Received 22 August 1995; accepted 4 April 1996
Abstract
The Campanian Ignimbrite (CI) is a large-volume trachytic tuff erupted at 37 ka from the Campi Flegrei and composed of a fallout deposit overlain by ignimbrite. The ignimbrite was spread over an area of about 30,000 km* including the Campanian Plain and the Apennine Mountains, with ridges over 1000 m a.s.1. The pumice fragments of the CI range in composition from trachyte to phonolitic-trachyte (DI = 75-90). They do not show any systematic compositional variation with stratigraphic height, but the analyzed sections can be divided into three groups on the basis of chemical composition of pumices. Least-evolved pumices (DI = 75-83) occur in the ignimbrite in the central sector of the Campanian Plain up to 30 km from the vent, while the most-differentiated pumices (DI = 88-90) characterize the cogenetic fallout deposit and the ignimbrite in the western sector of the Campanian Plain, on the Tyrrhenian scarp of the Apennines between Caserta and Mt. Maggiore, on Roccamonfina volcano, and on the Sorrento Peninsula, up to 50 km from the source. Pumice fragments of intermediate composition (DI = 84-87) occur in the ignimbtite on the Apennine Mountains and Roccamonfina volcano, up to 65 km from the vent. In one exposure at Mondragone, at the base of a calcareous ridge, an ignimbrite with pumices of most-evolved composition is overlain by an ignimbrite with pumices of intermediate composition.
The observed compositional variation between most- and least-evolved ignimbrite was generated in part by crystal-liquid fractionation, although other magmatic processes such as syn-eruptive mingling between most- and least-evolved magmas accounts for the mineralogical disequilibria and for the bimodality of the glass compositions in the intermediate-composition rocks. Pumice Sr-isotope ratios are positively correlated with degree of differentiation. Feldspar crystals separated from pumices of different compositions have a homogeneous Sr-isotope composition similar to that of the least-evolved pumices. Interaction between fluids and strongly fractionated Sr-poor less-dense magma can account for these isotopic features.
Geochemical, mineralogic, stratigraphic and volcanologic data, together with the stratigraphic relations between most-, intermediate- and least-evolved ignimbrite as detected at Mondragone and from bore-hole drillings suggest that: (1) the CI magmatic system was composed of two distinct magma layers - the upper layer was more differentiated and homogeneous in composition, while the deeper was less evolved and slightly zoned; and (2) the CI was mostly emplaced in three main
* Corresponding author.
0377-0273/97/$17.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved. PII SO377-0273(96)00027-3
184 L. Civetta et al. /Journal of Volcanology and Geothermal Research 75 (1997) 183-219
pulses of pyroclastic flows that tapped the chamber at variable levels and with distinct withdrawal dynamics. The eruption began with emission of the most differentiated magma, which gave rise to the fallout deposit. It continued with generation of expanded, turbulent pyroclastic flows that reached the Sorrento Peninsula in the southeast and Roccamonfina volcano in the northwest. These flows, whose thickness was greater than the overtopped relief, were able to travel over the water of the bay of Naples. Subsequently an intermediate-composition magma resulting from mingling of different portions of the magma chamber generated similar flows that spread radially and traveled not less than 65 km from the vent. During the last pulse the least-evolved magma was tapped and generated flows that spread within the Campanian Plain.
Variation in eruptive dynamics and composition of magma during the course of the eruption likely reflected variations of both geometry of vent and plumbing system, and efficiency of water/magma interaction, which in turns affected the dynamics of the magma chamber and the withdrawal mechanism, and resulted from the dynamics of the caldera collapse.
Keywords: calderas; campanian Ignimbrite; Campi Plegrei caldera; chemostratigraphy; large-volume magma chamber; magma mingling; eruptive dynamics
1. Introduction
Many ignimbrites are compositionally zoned, re- flecting pre-eruptive gradients in magma chambers (i.e., Smith and Bailey, 1966; Lipman, 1967; Hil- dreth, 1979, 1983). Compositional studies of both small- and large-volume ignimbrites have provided some understanding of processes that operate during transport, storage and evacuation of magma from high-level magma chambers (e.g., Giannetti and Luhr, 1983; Wolff, 1985; Wiimer et al., 1985; Hil- dreth, 1987; Mahood and Halliday, 1988; Palacz and Wolff, 1989; Vogel et al., 1989; Brian et al., 1990; Johnson and Fridrich, 1990; Freundt and Schmincke, 1992; Orsi et al., 1992, 1995; Briggs et al., 1993; Wohletz et al., 1995). Many studies have concen- trated on rhyolitic magmatic systems, while less information is available for more alkaline magmas (Wolff and Storey, 1984; Wolff et al., 1990; Civetta et al., 1991a,b; Orsi et al., 1992, 1995).
The Campanian Ignimbrite (CI), a large-volume trachytic tuff emplaced during the most powerful eruption of the last 200,000 years in the Mediter- ranean region (Barberi et al., 1978), gives a good opportunity to address the above mentioned pro- cesses.
We present the results of a mineralogic, geochem- ical and isotopic study of the CI carried out on pumice clasts from 25 stratigraphic sections located at various distances from the vent. The study was aimed at detecting possible compositional zonation of the tuff, and at understanding both the processes operating in a large trachytic magma chamber and the relationships between magma composition, erup-
tive dynamics, depositional mechanisms and caldera collapse.
2. The Campanian Ignimhrite
The CI is a trachytic nonwelded to partially welded tuff, underlain by a cogenetic southeasterly dispersed fallout deposit. It was emplaced at 37 ka (Deino et al., 1992, 1994) in the Campanian area (Fig. 1). Location of the source for the CI is controversial. Rosi et al. (1983, 1991), Armienti et al. (1983), Rosi and Sbrana (1987) and Barberi et al. (1991) sug- gested that the eruption took place in the Campi Flegrei, and was accompanied by the collapse of the Campi Flegrei caldera (CFc). Di Girolamo (19701, Di Girolamo et al. (1984) and Lirer et al. (1987) proposed that the CI was erupted north of Naples through a fracture system. Scandone et al. (1991) proposed a source in the Voltumo Plain, in the northern part of the Campanian Plain (Fig. 1). Re- cently published data on flow directions defined by means of anisotropy of magnetic susceptibility (AMS; Fisher et al., 1993), support a source within the Campi Flegrei area. Orsi et al. (1992, 1996) using data from field studies, drill core analyses, and gravi- metric and magnetic surveys, defined the CFc as a nested caldera generated during two major eruptive episodes: the Campanian Ignimbrite (37 ka) and the Neapolitan Yellow Tuff (12 ka).
The CI covered a wide part of the Campanian region, which includes the Campanian Plain and the Southern Apennine Mountains. These two morpho- logic and structural sectors are connected by the
L. Ciuetta et al. /Journal of Volcanology and Geothermal Research 75 (1997) 183-219 185
Tyrrhenian scarp of the Apennines. Formation of the Campanian Plain is related to intense extensional tectonism that affected the Tyrrhenian margin of the Southern Apennines in Plio-Quatemary times. At the time of emplacement of the CL the main morpholog- ical features were similar to the present ones. The Gulf of Naples, delimited southward by the Sorrento
Peninsula, was already formed, while the Somma- Vesuvio edifice did not yet exist. The CI attains its maximum thickness in the Campanian Plain, where it is buried under recent alluvial and volcanic sedi- ments, and therefore is exposed almost exclusively in quarries. In the Apennines, the CI is ponded within valleys that drain limestone blocks over 1000 m in
GULF OF NAPOLI
“LF OF SALERNO
Fig. 1. Geological map of the Campanian Ignimbrite outcrop area. (I) Recent and active sediments; (2) volcanic rocks of Somma-Vesuvio, Campi Plegrei, Procida and Ischia; (3) volcanic rocks of Roccamonfina: (4) Campanian Ignimbrite; (5) sedimentary rocks of the Apennine; Campanian Ignimbrite caldera rim (from Orsi et al., 1996). Numerals on the map indicate locations of the Campanian Ignimbrite studied sections: I = Santa Maria di Mortola; 2 = Marzano Appio; 3 = Tuoro di Teano; 4 = Calvi; 5 = Massa; 6 = Tocco Caudio; 7 = Mondragone; 8 = Ttiflisco; 9 = Melizzano; 10 = Moiano; I1 = Maddaloni; 12 = Villa di Briano; 13 = San Marco Evangelism; 14 = San Nicola La Strati, I5 = Scarafea; I6 = Lago Patria; I7 = Acquafidia; I8 = Capriglia; 29 = Ponti Rossi; 20 = Sant’Anna di Lavorate; 22 = Gioia- Pignolelle; 22 = Pucara; 23 = Pacognano; 24 = Marina di Cassano; 25 = Sant’Agata dei due Golfi. Symbols marking sections locations refer to different pumice compositions discussed later in the paper. Circles = sections with most-evolved pumice composition; triangles = sections with intermediate pumice composition; squares = sections with least-evolved pumice composition. Solid symbols = sections analyzed in this work; open symbols = sections analyzed by Di Girolamo (1970) and Barberi et al. (1978).
186 L. Ciuetta et al. /Journal of Volcanology and Geothermal Research 75 119971 183-219
elevation and in the caldera of the Roccamonfina volcano. The CI also occurs on both sides of the Sorrento Peninsula. An ash layer correlated to this tuff occurs in cores drilled in the Mediterranean sea (Keller et al., 1978; Thunnel et al., 1978; Cornell et al., 1983; Pateme et al., 1988; Pateme and Guichard, 1993; McCoy and Cornell, 1990).
Volcanologic and paleomagnetic studies on the distal facies of the CI were recently presented by Fisher et al. (1993). A generalized section of the CI comprises a fallout deposit overlain by an ignimbrite. The fallout deposit contains several depositional units of contrasting grain size composed of pumice frag- ments, lithic clasts, shards and mineral grains (clinopyroxene, plagioclase, sanidine and biotite) in variable percentages. The ignimbrite is composed of a thin, fines-poor massive to laminated layer 1, overlain by layer 2, a massive, gray to yellow ign- imbrite. At some localities, the base of layer 2 is composed of 10 to 20 cm of fine-grained, inversely graded, and generally pale gray to yellowish tuff (layer 2a) followed transitionally upward by a coarser grained, massive and darker gray tuff (layer 2b). The Campanian pyroclastic flow spread over an area of roughly 30,000 km2 and deposited about 150 km3 (DRE) of tuff. AMS flow directions indicate that the CI flowed radially outward from the vent area, but where it encounted topography it flowed in the direc- tions of slope as a gravity flow. To have ascended 1000 m ridges, the CI originally must have been very energetic and/or greatly expanded. Dense accumula- tions of debris at the base of the flow moved as gravity-driven flows that followed topography and ponded in valleys. The distribution of the CI together with AMS measurements indicate that the flow must have surmounted ridges higher than 1000 m, yet AMS clearly shows that the Campanian flows fol- lowed stream valleys. This indicates that the trans- port and depositional mechanisms were different (Fisher et al., 1993).
Di Girolamo (1970) detected a lateral chemical zoning in the CI with the more differentiated prod- ucts at greater distances from the vent. He proposed the existence of a magma chamber compositionally zoned by pre-eruptive fractionation processes. Bar- beri et al. (1978) recognized vertical chemical varia- tions in the deposit, which they interpreted as due to post-depositional alteration. Sr-isotope disequilibria
detected between groundmass (0.70723 &- 6) and mineral phases (diopside = 0.70707 k 5, salite = 0.70718 _t 3, biotite = 0.70729 + 7) (Vollmer et al., 1981) and Sr-isotope variations in four whole-rock samples (0.70733 1_ 3-0.70755 + 3) (Cortini and Hermes, 1981) were interpreted as due to mixing and/or incorporation of megacrysts derived from different magmas. Recently Cornell et al. (1993) recognized three main superimposed depositional units in a CI sequence drilled in the Campanian Plain, about 17 km from the CFc rim. The results of petrographic and modal analyses showed a vertical compositional zonation in the tuff with less evolved compositions upsection. Three superimposed deposi- tional units have been recognized (L. Civetta; un- publ. data) in a CI sequence drilled at Ponti Rossi (Fig. l), also with less evolved compositions upsec- tion.
3. Studied sequences
The basis for this compositional study was a sampling of the CI in natural sections and quarries, located at various distances from the postulated vent area (Fig. 1). In seven sections, six of which were complete CI sequences, pumice fragments were col- lected at heights spaced at about 50- to lOO-cm intervals. In eighteen other sections, in which only layer 2b is present and the base of the sequence is not exposed, samples were collected at the base and at the top of the sequence. Descriptions of the seven best exposed sections that were sampled in detail are reported below, following the generalized sequence proposed by Fisher et al. (1993). The center of Pozzuoli Bay is used as a reference of source in defining the distances of the sampled sections.
The CI in the Campanian Plain was sampled in a quarry at San Nicola La Strada, about 30 km from the postulated vent and at 50 m a.s.1. (Fig. 1, Sta 14). Although the base is not exposed, the tuff attains a maximum thickness of 12 m and varies in color from gray in the lower 4 m, to yellow in the uppermost 8 m. It is intensely lithified and contains large pumice fragments and lithic clasts. Columnar joints and de- gassing pipe structures cut through all the exposed tuff.
Along the western scat-p of the Apennines three
L. Ciuetta et al. /Journal of Volcanology and Geothermal Research 75 (1997) 183-219 187
sections, from north to south, were sampled - Mondragone, Triflisco and Sant’Anna di Lavorate (Fig. 1, Sta 7, 8 and 20). The section at Mondragone, located at 411 m a.s.1. on the southeastern slope of Monte Massico, is 38 km from the Pozzuoli Bay. There, the CI includes two superimposed flow units separated by a continuous slope debris deposit com- posed mostly of limestone clasts. The lower unit is 4 m thick, light to dark gray, and composed of layers 1, 2a and 2b. The upper unit, 3 m thick, changes color upward from light brown, to reddish, to dark gray, and is composed of layers 2a and 2b. The Triflisco section is located on the southern slope of Monte Maggiore, at 38 km from the Pozzuoli Bay. The CI, 2.9 m thick, is gray and includes layers 1, 2a and 2b. The Sant’Anna di Lavorate section is 46 km from the Pozzuoli Bay, and shows a 35 cm thick fallout deposit overlain by 4 m of gray tuff com- posed of layers 1, 2a and 2b.
On the Apennines, including Roccamonfina vol- cano, three sections were sampled - Santa Maria di Mortola, Massa and Capriglia (Fig. 1, Sta 1, 5 and 18). The section of Santa Maria di Mortola is located in a valley that flows from east to west on the western slope of Roccamonfina volcano, at 62 km from the Pozzuoli Bay. There the CI, 12 m thick, is a columnar jointed, gray tuff that includes layers 1, 2a and 2b. The Massa section is located along the Titemo river at 200 m a.s.1. and 60 km from the Pozzuoli Bay, where it is 8 m thick, strongly sintered and columnar jointed, and composed of layers 1, 2a and 2b. The section at Capriglia is located in a quarry in a northward draining valley, at 400 m a.s.1. and 58 km from the Pozzuoli Bay. The CI there includes 50 cm of fallout deposit overlain by a 13-m-thick ignimbrite. The ignimbrite, which is gray in the lower 6 m and becomes yellow upsection, is intensely lithified, columnar jointed and composed of layers 1, 2a and 2b.
The other eighteen sections are located in the Campanian Plain (Lago Patria, Scarafea, Villa di Briano, Ponti Rossi, San Marco), at the base of the Apennine (Calvi, Maddaloni), in the Apennines (Tocco Caudio, Moiano, Melizzano, Acquafidia), on Roccamonfina Volcano (Tuoro, Marzano Appio), on the Sorrento Peninsula (Marina di Cassano, Pucara, Pacognano, Sant’Agata dei due Golfi), and southeast of Salerno (Gioia Pignolelle) (Fig. 1).
4. Sample descriptions and analytical techniques
The pumice fragments from most of the studied sequences are small. Exception include those located on the Campanian Plain (San Nicola La Strada, Villa di Briano, San Marco, Scarafea, Ponti Rossi, Lago Patria) and the Moiano, Marina di Cassano, Sant’ Agata and Triflisco sections. Therefore, mostly multiple pumice samples were analyzed, especially for sequences at distal locations, which are character- ized by the smallest pumices (less than l-2 cm in diameter). Each sample was composed of a number of clasts similar in color, texture and crystal content collected at the same stratigraphic height. When possible, a single pumice from the same stratigraphic layer was analyzed separately. Pumice clasts of in- tensely welded portions of the tuff were extracted from the matrix by means of a diamond drill. All the pumice samples were washed in distilled water, crushed to lapilli-size particles, then ground and homogenized in an agate mortar.
Eighty-seven pumice and 21 separated glass sam- ples were analyzed by inductively coupled plasma emission (ICP), at the Centre de Recherches Petro- graphiques et Geochimiques (Vandouvre, France). Precision is 3% for major elements, 10% for trace elements, and 2-5% for REE and Y. Major-element concentrations in minerals and glass of pumice frag- ments were determined by a Cameca SX 50 electron microprobe at the CNR-Centro di Studio per il Qua- temario e 1’Evoluzione Ambientale (Roma). Analyti- cal uncertainties are 1%. Trace-element concentra- tions in clinopyroxene and feldspar phenocrysts were detected by a Cameca IMS 4F ion microprobe at the CNR-Centro di Studio per la Cristallochimica e Cristallografia (Pavia). Analytical uncertainties are < 10% for REE, and < 5% for SC, Ti, V, Cr, Sr and Zr.
Sr-isotope ratios of pumice, glass from pumice and feldspar phenocrysts were determined by a VG- 350 double-collector mass-spectometer at the Dipar- timento di Geofisica e Vulcanologia (Naples). The powders (multiple or single pumice clasts as dis- cussed previously) were leached with cold 2.5 N HCl for 10 minutes and with hot 2.5 N HCl for 30 minutes, rinsed thoroughly in pure water, then dis- solved with high purity acids. The quoted error is the standard deviation of the mean (2~~) and refers to
188 L. Civetta et al. /Journal of Volcanology and Geothermal Research 75 (1997) 183-219
the last digit. Repeated analyses of NBS-987 gave a mean value of 0.710264 f 8. The blank was 6 ng during analyses.
5. Petrography and geochemistry
CI pumice fragments are glassy, vesicular, and contain a small amount of phenocrysts (< 3% by volume). Only those of San Nicola La Strada, Ponti Rossi, Villa di Brian0 and San Marco are more porphyritic (= 10% of phenocrysts by volume). Phe- nocrysts are dominantly alkali feldspar, with subordi- nate plagioclase, clinopyroxene, biotite, magnetite, and apatite as trace phases. Although hydration of glass and vapor phase crystallization are common, only pumice fragments not affected, on the base of petrographic investigations, by post-depositional al- teration and vapour phase crystallization were ana- lyzed.
Chemical analyses of CI pumice and separated glass samples are reported in Table 1. DI ( = Q + Or + Ab + Ne + Lc normative) values of the CI pumice samples range from 75 to 90, assuming an FeO/Fe,O, ratio of 0.5 in CIPW norm calculations. The 0.5 value, assumed for FeO/Fe,O, ratios, is the average value reported by Barberi et al. (1978) for CI rocks. Although LO1 content ranges from 0.5 to 6%, most of the samples show values between 1 and 3%. Samples with LO1 > 4% that also have norma- tive Q, Hy and C, were not considered in this study.
CI whole pumice and glass samples plot in the trachyte field of the total alkali/silica diagram (TAS; Le Bas et al., 1986) with the most evolved reaching the phonolite field (Fig. 2A). In the Ne/DI diagram (Armienti et al., 1983) commonly used for the Cam- panian potassic rocks, they range in composition from trachyte to alkali-trachyte to phonolitic-trachyte (Fig. 2B).
None of the studied depositional units show SYS-
tematic variations in pumice composition (both sin- gle or multiple samples) with stratigraphic height. The only exception is the section of Mondragone which is composed of two superimposed ignimbrite units, with different compositions. The lower unit is highly evolved (DI = 88-90) (Table 1; Fig. 3) whereas the upper is intermediate in composition (DI = 86-87, Table 1). Single pumice clasts col-
lected at the same stratigraphic height generally have similar chemical composition; the only exception is the Moiano section (Fig. l), in which two single pumices with different composition co-exist at the same stratigraphic height (Table 1; Fig. 3). Further- more, two single pumices collected from Triflisco and San Nicola La Strada (Fig. 1) sections have different compositions with respect to the pumice composition of the whole section (Table 1; Fig. 3). The glass of pumices shows a wider range of compo- sition, expecially at Capriglia and Massa sections (Fig. 1, Sta 18, 5) as discussed later.
Pumice: Major-element variation diagrams (Fig. 4) show that CaO, FeO,,,, MgO, MnO, K,O and P,O, decrease, SiO, and Na,O increase, while TiO,, and Al,O, remain constant at increasing DI. REE (except Eu), Y, Nb, Zr and Rb behave as incompati- ble elements and are positively correlated to DI (Fig. 5). Sr, Ba and ferromagnesian trace elements (SC, V) display a marked and continuous depletion with in- creasing differentiation (Fig. 5). The REE show moderate LREE and HREE enrichment (Fig. 6). All
57 sio2 Wt% 72
12
Ne * Phonolitic trachyte
0
Trachyte trachytc 8-
.
74 78 82 86 DI 90
Fig. 2. Classification diagrams of the Campanian Ignimbrite ana- lyzed samples. (A) K,O/Na,O vs. SiO, diagram (TAS; Le Bas et al., 1986). (B) Ne vs. DI ( = Q + Or + Ab + Ne + Lc normative) diagram (Armienti et al., 1983).
L. Civetta et al./ Journal of Volcanology and Geothermal Research 75 (1997) 183-219 189
samples display a negative Eu anomaly (Eu/Eu* decreases from 0.4 to 0.1 with differentiation), im- plying that feldspar fractionation occurred.
Glass in pumices: Glass separated from pumice samples and analyzed by XRF (Table l>, generally yielded the same composition as whole pumice, al- though it is more evolved than host pumice for
samples with the highest crystal content (- 10%). Electron microprobe analyses of least-evolved (DI = 75-83) and most-evolved pumices (DI = 88-90) show that each glass type has a homogeneous major-element composition, similar to, or slightly more differentiated then its host pumice, whereas intermediate-composition pumices (DI = 84-87) are
5. Nicola L C
0
a 0
00
“riflisco A
75 79 83 DI 97 j 79 83 ~1 91 75 79 83 DI 91
Q
P onti Rossi L C
0
Moiano B
5. Anna 0
Fig. 3. Chemostratigraphy of selected CI sections. Location of sections is reported in Fig. 1. 0 = whole-pumice samples; 0 = glass samples; n = glass shards (by microprobe). (A) Section with most-evolved composition. @I Sections with intermediate composition. (C) Sections with least-evolved composition.
Tabl
e 1
Chem
ical
an
alys
es
of C
i pu
mic
e an
d se
para
ted
glas
s sp
= s
ingl
e pu
mic
e;
p =
com
posit
e pu
mic
e;
vt =
gla
ss;
(oxi
des
= w
t.%,
trace
el
emen
ts =
ppm
) Sa
mpl
es
are
orde
red
from
th
e bo
ttom
up
sect
ion
for
each
an
alyz
ed
sect
ions
.
Sec
tions
: V
. Br
ian0
V
. Br
ian0
Po
nti
Ross
i Po
nti
Ross
i S.
Mar
co
S. M
arco
S.
Mar
co
S. N
icol
a S.
Nic
ola
S. N
icol
a S.
Nic
ola
S. N
icol
a Sa
mpl
es:
OFV
bA
sp
OFV
6bB
sp
OFP
rA
sp
OFP
rB
sp
OF3
B90
OF3
Dch
i90
OF3
Dsc
u90
OF1
04a
sp
OF1
04b
sp
OF
104~
sp
OFl
04d
sp
OF1
04e
sp
58.7
2 56
.73
SiO
, Ti
O,
A’2
03
Fe20
3
56.9
6 58
.24
59.1
6 0.
38
18.9
1 3.
97
0.08
0.
98
3.47
2.
76
9.46
0.
22
1.56
10
0.95
60.6
6 0.
4 1
18.9
3 3.
73
0.15
0.
68
2.62
3.
79
8.29
0.
17
3.88
10
3.3
1
60.0
8 0.
41
18.8
9 3.
87
0.13
0.
83
2.86
3.
85
8.32
0.
17
2.69
10
2.1
58.9
2 0.
35
18.3
2 3.
75
0.08
0.
8 2.
72
3.47
8.
74
0.14
2.
44
39.7
3
58.5
9 0.
38
18.2
6 3.
72
0.14
0.
76
2.74
4.
05
8.03
0.
13
2.67
99
.47
58.6
5 0.
38
18.3
8 3.
85
0.1
0.89
2.
95
3.22
a.9
4 0.
17
I .94
99.4
7
59.6
5 0.
39
18.1
4 3.
4 0.
17
0.4 1.85
5.
09
7.46
0.
06
3.14
99
.75
57.6
0.
4 18
.29
4.37
0.
08
1.14
3.
5 2.
82
9.3
0.2 1.94
99
.64
MnO
Mg
O
Ca
O
Na
,O
K2O
p205
LO
1 To
tal
0.38
18
.37
4.05
0.
08
0.98
3.
26
3.11
9.
23
0.2
1 1.
3 99
.69
DI
76.4
8
Ba
Nb
Rb
SC
Sr
V
Zr
La
Ce
Nd
Sm
Eli
Gd DY
Er
Y
b LU
Y
638 26
24
7 5 49
8 66
187 48
90
35 6 2.3
6 4 1.9
2.1
0.3
21
0.45
0.
42
18.5
7 18
.49
4.62
4.
33
0.07
0.
08
1.3
I .04
3.92
3.
39
2.75
3.
8 9.
3 8.
9 0.
23
0.22
I .4
5 0.
73
99.6
2 99
.64
80.6
6 74
.68
948
707
23
44
230
281
5 5
686
512
87
71
161
295
42
63
82
120
32
46
6 a
2.3
2.2
5 6
4 5
1.8
2.7
1.9
2.x
0.3
0.4
20
30
0.46
18
.5
4.9
0.06
1.
38
4.23
2.
71
8.87
0.
31
0.66
98
.81
80.9
8 78
.13
80.7
4 78
.13
82.6
6 80
.48
82.4
8 87
.57
81.2
8
1111
23
23
4
777
101
170 43
83
35 6 2.4
5 4 1.8
1.9
0.3
22
745
178
425
289
482
459
44
807
21
56
48
25
45
27
100
21
276
324
322
260
284
244
322
209
5 4
5 5
5 6
4 7
610
203
392
444
433
536
60
695
63
40
51
48
42
52
29
67
135
359
295
168
281
178
558
137
42
85
70
51
72
51
111
44
77
148
129
96
133
96
207
83
31
55
48
37
80
38
76
35
6 9
9 7
9 7
12
7 2.
0 1.
7 1.
9 2.
0 1.
9 2.
1 I5
2.
3 5
a 7
5 7
5 10
5
4 6
6 4
6 4
9 4
I .9
3.2
3.0
2.0
2.9
2. I
4.7
1.9
1.5
2.9
2.7
2.0
3.0
2.0
4.9
1.8
0.3
0.5
0.5
0.3
0.5
0.3
0.7
0.3
19
34
31
21
30
21
52
19
Sect
ions
: S.
Nic
ola
S. N
icol
a S.
Nic
ola
S. N
icol
a S.
Nic
ola
S. N
icol
a Ca
prig
lia
Capr
iglia
Ca
prig
lia
Capr
iglia
Ca
prig
lia
Capr
iglia
Sa
mpl
es:
OFl
04f
sp
OFl
O4a
vt
O
F 10
4b v
t O
F104
c vt
O
F104
d vt
O
FlO
4e
vt
OF5
92a
p O
FS92
blp
OF5
92b2
p O
F592
b3
p O
F592
b4p
OF5
92b5
p
SiO
, Ti
O,
*I,‘,
Fe20
3 M
nO
MgO
Ca
O
Na,O
K2O
p205
LO
1 To
tal
DI
Ba
Nb
Rb
SC
Sr
V
Zr
La
Ce
Nd
Sm
Eu
Gd DY
Er
Y
b Lu
Y
57.6
5 58
.71
59.7
7 59
.12
59.8
7 57
.79
58.5
3 58
.12
57.0
0 58
.54
57.4
2 58
.75
0.39
0.
41
0.38
0.
35
0.4
0.4
0.44
0.
4 0.
4 0.
43
0.44
0.
4 18
.32
17.7
1 18
.01
18.2
1 18
.35
18.1
1 7.
92
18.1
4 17
.6
17.8
5 18
.5
17.9
8 4.
16
4.69
3.
5 3.
62
3.24
3.
95
3.75
3.
89
3.87
3.
95
4.15
3.
65
0.08
0.
11
0.14
0.
08
0.17
0.
1 0.
17
0.17
0.
15
0.17
0.
17
0.17
1.
08
0.83
0.
65
0.83
0.
39
I.31
0.59
0.
69
0.78
0.
75
0.7
I 0.
6 3.
54
2.52
2.
31
2.62
1.
67
3.65
2.
25
2.47
2.
72
2.61
2.
62
2.24
3.
04
4.07
4.
57
3.42
5.
19
3.06
4.
94
4.62
4.
65
4.69
4.
33
4.9
I 8.
92
7.5
7.39
8.
99
7.28
8.
64
7.3
7.66
7.
93
7.8
7.3
7.58
0.
17
0.17
0.
13
0.13
0.
06
0.17
0.
15
0.14
0.
15
0.16
0.
15
0.1
I 2.
27
3.03
2.
88
2.35
3.
13
2.53
3.
7 3.
4 2.
4 I
2.79
3.
9 3.
32
99.6
2 99
.75
99.7
3 99
.72
99.7
5 99
.7
99.7
4 99
.69
97.6
6 99
.74
99.6
9 99
.7 1
78.5
9 81
.69
84.8
4 83
.25
87.9
3 78
.03
85.4
4 84
.07
84.2
9 84
.39
_
702
115
171
325
32
441
198
211
203
230
242
109
26
30
59
33
93
28
86
65
57
63
75
60
229
305
315
338
445
290
362
373
3.50
33
6 32
6 34
3 7
6 5
6 4
8 4
4 4
4 4
4 64
7 27
1 22
7 46
0 50
53
2 15
2 20
4 24
.5
224
226
114
64
62
33
58
19
62
35
37
38
42
42
23
169
203
364
232
579
196
522
420
345
413
420
464
48
58
87
56
I16
51
116
98
89
97
101
86
92
I10
164
106
217
98
219
192
IS9
175
191
156
36
43
63
42
Xl
41
76
64
58
63
66
57
7 8
11
7 14
8
15
12
11
12
13
II
2.0
2.0
1.8
2.3
I.6
2.4
2.3
2.11
2.
2 2.
0 2.
2 I.7
5
6 9
6 I2
6
12
IO
9 9
10
8 4
5 7
5 9
5 9
8 7
7 8
7 2.
1 2.
5 3.
9 2.
4 4.
9 2.
4 4.
3 3.
6 3.
2 3.
5 3.
8 3.
3 2.
0 2.
4 3.
9 2.
4 5.
1 2.
2 4.
5 3.
9 3.
4 3.
7 4.
0 3.
5 0.
3 0.
4 0.
7 0.
4 0.
8 0.
3 0.
7 0.
6 0.
6 0.
6 0.
7 0.
6 21
25
36
27
52
25
51
45
40
43
47
39
Tabl
e I (
cont
inue
d)
Sect
ions
: Ca
prig
lia
Capf
iglia
Ca
prig
lia
Capr
iglia
Ca
prig
lia
Capr
iglia
Ca
prig
lia
Capr
iglia
Ca
prig
lia
Capr
iglia
Ca
prig
lia
Capr
iglia
Sa
mpl
es:
OF5
92b6
p
OF5
92b7
p
OF5
92a
vt
OF5
92bl
vt
O
F592
b2
vt
OF5
92b3
vt
O
F592
b4
vt
OF5
92b5
vt
O
F592
b6
vt
OF5
9Fa
p O
F59F
b p
OF5
9Fd
p
sio,
56
.84
55.7
5 58
.59
59.0
3 58
.79
58.7
5 57
.59
57.6
9 57
.7
57.8
2 58
.2
58.3
2 Ti
O,
0.43
0.
45
0.39
0.
38
0.39
0.
38
0.44
0.4
0.4
0.44
0.
44
0.44
A
W,
18.7
6 19
.75
17.9
5 18
.03
18.0
7 18
.23
18.1
7 18
.28
18.5
3 17
.48
17.5
3 17
.67
Fe&
%
4.00
4.
22
3.45
3.
44
3.34
3.
29
3.77
3.
58
3.52
3.
62
3.54
3.
58
MnO
0.
17
0.17
0.
17
0.17
0.
17
0.16
0.
17
0.17
0.
16
0.2
0.22
0.
2 M
gO
0.63
0.
53
0.64
0.
58
0.64
0.
64
0.7
0.78
0.
6 0.
44
0.4
0.4
CaO
2.
45
2.2
2.66
2.
47
2.8
1 2.
83
2.47
2.
7 2.
66
1.97
1.
87
1.87
N
a,O
4.44
4.
33
5.09
5.
19
5.07
4.
94
4.55
4.
54
4.66
5.
62
5.72
5.
66
K2O
7.3
6.44
7.
3 7.
38
7.55
7.
58
7.25
7.
35
7.48
7,
00
6.89
6.
69
p205
0.14
0.
14
0.13
0.
12
0.14
0.
13
0.13
0.
13
0.13
0.
12
0.11
0.
12
LO1
4.56
5.
73
3.33
2.
94
2.86
2.
8 4.
43
4.12
3.
84
4.96
4.
83
4.77
To
tal
99.7
2 99
.7 1
99
.7
99.7
3 99
.83
99.7
3 99
.67
99.7
4 99
.68
99.6
7 99
.75
99.7
2
DI
_ 83
.87
84.3
X
84.5
1
84.4
6 _
88.0
5 88
.49
87.8
6
Ba
141
164
156
104
123
139
210
180
143
59
30
43
Nb
74
81
81
74
69
73
89
71
69
110
115
116
Rb
341
341
328
352
332
379
353
314
323
373
384
432
SC
4 4
4 4
4 4
4 4
4 3
3 4
Sr
172
161
175
152
192
214
193
193
193
54
35
34
V
30
31
28
24
27
29
42
30
29
23
21
20
Zr
489
534
468
4.55
44
0 43
2 45
8 43
1 43
3 65
8 64
8 68
2 La
93
96
10
4 90
98
10
1 10
5 93
89
14
8 15
0 16
2 Ce
16
9 17
5 19
4 16
3 17
3 17
9 19
8 16
9 16
3 26
5 27
0 29
7 N
d 61
64
70
58
63
66
70
63
59
96
98
10
8 Sm
12
12
13
11
12
13
13
12
11
18
18
20
EU
1.
9 1.
9 2.
0 1.
8 1.
9 2.
0 2.
1 2.
0 1.
9 2.
3 2.
2 2.
6 G
d 9
9 10
9
9 10
10
9
9 14
14
16
D
Y
7 8
8 7
7 8
8 7
7 11
11
13
Er
3.
6 3.
8 3.
9 3.
2 3.
6 3.
7 4.
0 3.
5 3.
3 5.
4 5.
4 6.
2 Y
b 3.
9 4.
2 4.
0 3.
6 3.
8 3.
9 4.
2 3.
8 3.
6 5.
8 5.
9 6.
7 Lu
0.
6 0.
7 0.
7 0.
6 0.
7 0.
7 0.
7 0.
6 0.
6 1.
0 1.
0 1.
1 Y
44
47
47
42
43
45
48
43
41
64
66
76
Sect
ions
Ca
prig
lia
Capr
iglia
Ca
prig
lia
Capr
iglia
M
assa
M
assa
M
assa
M
assa
M
assa
M
assa
M
assa
M
assa
Sa
mpl
es:
OF5
9Fa
vt
OF5
9Fb
vt
OF5
9Fc
vt
OF5
9Fd
vt
OFl
Ola
p
OFl
Olb
p
OFl
Olc
p
OFl
Olg
p
OF1
01
1 p
OFl
Olf
p O
F101
2 p
OF1
013
p
SiO
, Ti
O
Al,&
Fe
A
MnO
M
gO
CaO
N
a,O
W
p205
LO
1 To
tal
58.3
4 58
.25
58.4
5 58
.37
56.3
2 55
.34
55.2
7 0.
4 0.
4 0.
43
0.4
0.43
0.
44
0.44
17
.53
17.4
6 17
.57
17.5
3 18
.57
19.5
1 18
.57
3.33
3.
4 3.
45
3.37
3.
99
4.04
3.
97
0.2
0.22
0.
22
0.22
0.
25
0.26
0.
24
0.44
0.
4 0.
4 0.
34
0.59
0.
53
0.55
2.
04
1.87
1.
87
1.81
2.
4 1
2.22
2.
29
5.62
5.
83
5.83
5.
87
4.3
4.25
4.
19
7,00
6.
94
6.91
6.
98
6.37
6.
44
6.23
0.
I 0.
1 0.
08
0.1
0.14
0.
14
0.14
4.
7 4.
8 4.
51
4.7
6.37
6.
54
6.82
99
.7
99.6
7 99
.72
99.6
9 99
.74
99.7
I
98.7
1
61.3
2 60
.78
0.38
0.
4 18
.48
18.2
8 3.
5 3.
5 0.
17
0.17
0.
39
0.44
1.
82
1.92
5.
04
4.94
7.
4 7.
45
0.12
0.
13
1.14
1.
69
99.7
6 99
.7
60.3
2 61
.46
60.8
2 0.
4 0.
39
0.4
18.2
3 18
.39
18.2
5 3.
57
3.39
3.
72
0.19
0.
16
0.17
0.
4 0.
4 0.
56
1.81
1.
86
2.16
4.
83
4.94
4.
82
7.16
7.
45
7.23
0.
14
0.13
0.
16
2.7
1.13
I .4
5 99
.75
99.7
99
.74
DI
89.1
89
.46
89.5
9 89
.59
_ _
87.2
86
.9 1
85
.96
87.2
85
.16
Ba
s4
24
36
26
15s
97
90
119
92
121
81
120
Nb
91
95
95
110
88
89
90
91
90
90
91
84
Rb
392
391
391
459
295
312
285
343
403
346
374
364
SC
3 3
3 3
4 3
4 3
3 3
3 4
Sr
56
29
33
26
156
164
190
99
97
118
86
136
V
18
17
18
18
27
27
28
26
28
27
2s
34
Zr
589
624
637
640
494
505
510
504
502
506
496
473
La
131
139
137
140
96
113
120
104
I10
110
95
101
Ce
239
249
253
258
206
269
267
186
203
219
179
165
Nd
86
89
89
91
66
77
80
66
73
72
60
67
Sm
16
17
17
17
13
14
15
12
14
14
12
13
Eu
2.0
2.0
2.0
2.0
2.0
2.2
2.2
1.9
1.9
2.0
1.8
2.0
Gd
12
13
13
13
10
12
12
9 I1
II
9
10
DY
10
II
II
II
8
9 10
7
8 8
7 8
Er
4.8
5.1
5.1
5.2
3.9
4.4
4.6
3.5
4.0
4.1
3.3
3.6
Yb
5.2
5.5
5.6
5.7
4.1
4.6
4.8
3.9
4.3
4.4
3.7
3.9
Lu
0.9
0.9
0.9
1.0
0.7
0.8
0.8
0.7
0.8
0.8
0.6
0.7
Y
58
61
61
66
47
55
58
43
49
49
40
44
Tab
le
I (co
nti
nu
ed)
Sec
tion
s:
Mas
sa
Mas
sa
T.C
audi
o T
.Cau
dio
Pig
nol
elle
P
ign
olel
le
Moi
ano
Moi
ano
Moi
ano
Acq
uaF
idia
M
ondr
agon
e M
ondr
agon
e
Sam
ples
: O
FlO
14p
OF
1015
p
OF
TcA
p
OF
TcB
p
OF
GP
mid
p
OF
GP
top
p O
F5A
I sp
2 O
F5A
l sp
l O
F5B
sp
F
16A
90
p O
F15
2al
p O
F15
2a2
p
SiO
, 60
.59
60.3
4 58
.2
56.8
4 60
.08
59.9
7 58
.34
57.8
59
.91
60.8
6 58
.12
57.1
5 T
iO,
0.4
0.4
0.4
0.43
0.
41
0.43
0.
39
0.41
0.
41
0.41
0.
39
0.39
Ah
O,
18.3
2 18
.17
17.8
5 17
.64
18.1
7 18
.26
17.8
8 18
.22
18.8
7 19
.15
17.6
7 17
.56
F%
O,
3.15
3.
71
3.84
4.
25
4.14
4.
11
3.5
4.36
3.
81
3.4
3.41
3.
4 M
nO
0.
17
0.17
0.
15
0.14
0.
16
0.15
0.
2 0.
09
0.16
0.
21
0.2
0.2
MgO
0.
44
0.44
0.
66
0.93
0.
61
0.58
0.
51
1.16
0.
7 0.
45
0.34
0.
34
CaO
1.
87
1.63
2.
53
3.06
2.
43
1.94
2.
55
3.61
2.
59
1.82
2.
25
2.79
N
a,O
4.
34
4.4
5.13
4.
75
4.46
4.
5 1
5.48
2.
96
4.69
5.
87
5.87
5.
19
“20
7.75
8.
05
7.85
7.
66
7.83
7.
6 6.
82
8.46
8.
04
7.21
6.
64
6.65
P,O
s 0.
15
0.13
0.
14
0.16
0.
11
0.12
0.
09
0.2
1 0.
17
0. I
0.08
0.
08
LO
I 1.
91
2.22
3.
01
3.89
1.
31
2.05
3.
99
2.38
3.
1 I
3.72
4.
73
4.76
T
otal
99
.69
99.7
2 99
.76
99.7
5 99
.71
99.7
2 99
.75
99.6
6 10
2.52
10
3.2
99.7
99
.7 1
DI
84.8
7 87
.13
85.9
84
.19
85.1
5 85
.5
77.3
6 83
.12
86.9
9 _
Ba
178
121
Nb
97
93
Rb
400
386
SC
4
3 S
r 15
1 I1
4 V
34
29
Z
r 49
1 48
8 L
a 99
10
4 C
e 16
4 17
2 N
d 68
71
S
m
13
14
Eu
I .
94
2s)
Gd
10
II
DY
8
8 E
r 3.
7 3.
9 Y
b 4.
0 4.
0 L
u
0.7
0.7
Y
46
47
175
181
70
61
443
568
149
201
108
580
304
100
29
71
78
79
25
64
89
105
326
317
282
189
318
426
453
4 4
3 5
4 3
2
149
142
86
484
276
89
23
34
3s
23
70
44
22
15
460
515
515
175
379
550
605
87
95
107
45
87
12.5
13
3
160
168
195
84
157
211
235
64
69
71
35
57
77
82
II
13
12
6 10
I3
16
1.6
1.7
1.6
2.1
1.8
1.5
1.9
9 9
10
5 8
II
12
7 8
8 4
7 9
10
3.9
4.2
4.4
I .9
3.5
4.5
4.6
4.0
4.4
4.4
I .9
3.3
4.2
5.2
0.7
0.7
0.7
0.3
0.6
0.7
0.9
41
45
48
22
38
50
60
41
103
169
213
39
60
449
403
97
91
179
166
65
62
11
11
1.8
1.9
9 9
8 7
4.2
3.8
4.2
3.7
0.6
0.6
42
40
_ 3 21
1s
608
122
215 78
15
1.8
II 9 4.3
4.9
0.8
55
Sect
ions
: M
ondr
agon
e M
ondr
agon
e M
ondr
agon
e M
ondr
agon
e M
ondr
agon
e M
ondr
agon
e M
ondr
agon
e M
ondr
agon
e M
ondr
agon
e M
ondr
agon
e M
ondr
agon
e M
ondr
agon
e Sa
mpl
es:
OF1
52bl
p
OF1
52b2
p
OFl
5263
chp
OFl
52b3
p
OFl
52b4
p O
Fl52
b5
p O
Fl52
b6
p O
Fl52
b7
p O
F15U
l vt
O
Fl5U
3 vt
O
F15U
l P -
OF1
5U3
p
57.6
4 0.
38
17.8
9 3.
4 0.
17
0.48
2.
41
4.83
7.
58
0.08
3.
9 98
.76
SiO
, Ti
O,
A’2
03
WA
59.1
7 58
.59
MnO
M
gO
CaO
N
a,O
K2O
p20,
LO
1 To
tal
0.4
0.4
17.9
6 17
.85
3.45
3.
44
0.22
0.
2 0.
34
0.3
1.81
1.
77
5.99
5.
84
6.91
7.
83
0.08
0.
07
3.55
3.
4 99
.88
99.6
9
58.5
9 58
.2
58.8
4 58
.46
59.7
4 60
.7 1
0.
4 0.
4 0.
4 0.
4 0.
4 0.
4 17
.82
17.6
7 17
.x1
17.9
2 18
.41
18.3
1 3.
47
3.4
3.47
3.
54
3.54
3.
4 0.
22
0.22
0.
2 0.
22
0.22
0.
2 0.
34
0.32
0.
32
0.35
0.
4 0.
34
1.92
2.
11
1.87
2.
16
2.16
1.
81
5.64
5.
55
5.75
6.
04
6.05
6.
16
7.94
7.
88
7.23
7.
13
7.28
7.
54
0.07
0.
07
0.08
0.
08
0.08
0.
1 3.
32
3.79
3.
71
3.36
1.
45
0.76
99
.73
99.7
1 99
.68
99.6
6 99
.73
99.7
3
59.0
7 58
.89
0.4
0.4
18.2
1 18
.16
3.4
3.45
0.
19
0.19
0.
53
0.45
2.
11
2.11
5.
29
5.6
7.36
6.
8 0.
06
0.06
3.
29
3.64
99
.9 1
99
.75
57.8
7 0.
39
17.8
9 3.
4 0.
17
0.46
2.
12
5.00
7.
56
0.08
3.
5 98
.44
DI
89.0
6 89
.3 1
89
.45
88.7
8 89
.02
88.5
7 88
.21
90.2
9 87
.13
87.1
2 87
.17
85.9
5
Ba
Nb
Rb
SC
Sr
V
Zr
La
Ce
Nd
Sm
El!
Cd
DY
Er
Y
b LU
Y
34
104 2 17
14
63
1 12
3 22
2 79
15 1.8
12 9 4.4
4.9
0.8
56
25
41
18
20
24
47
41
89
_ 76
10
8 11
0 10
9 10
4 95
93
91
10
3 80
84
78
69
45
7 44
5 46
4 48
5 56
9 28
5 30
8 34
8 35
9 34
2 32
3 1
1 1
3 3
3 3
4 4
4 5
30
34
37
28
41
59
37
119
75
III
149
8 8
8 16
19
21
19
12
12
12
11
60
6 60
7 59
8 59
3 60
2 59
6 60
7 48
0 50
8 46
5 41
2 12
2 13
4 12
7 12
8 12
9 13
1 12
5 10
5 10
6 10
1 94
21
9 24
0 21
3 21
8 22
2 21
8 22
4 19
5 19
4 18
9 17
2 77
84
79
80
80
80
80
70
69
70
62
15
16
15
15
15
15
15
12
12
12
I1
1.
8 1.
9 1.
8 1.
8 1.
8 1.
9 1.
8 1.
5 1.
3 1.
7 1.
5 II
13
11
11
12
12
12
I 0
9
9 8
9 10
9
9 9
9 9
8 8
8 7
4.3
4.8
4.5
4.4
45
4.6
4.9
4.0
4.1
4.2
3.6
4.9
5.3
5.0
5.0
5.0
5.0
5.0
4.3
4.3
4.3
3.7
0.8
0.9
0.9
0.8
0.X
0.
9 0.
8 0.
6 0.
7 0.
7 0.
5 55
61
56
55
56
56
55
46
50
45
41
Tabl
e 1
(con
tinue
d)
Sect
ions
: Tr
iflisc
o Tr
iflisc
o Tr
iflisc
o Tr
iflisc
o S.
Ann
a S.
Ann
a S.
Ann
a S.
Ann
a S.
Ann
a S.
Ann
a S.
Mar
ia
S.M
aria
Sa
mpl
es:
OFl
7b
sp
OFl
7cl
sp
OFl
7d
sp
OFl
7e
sp
OFs
aF3
p O
FsaB
vt
O
F sa
D v
t O
FsaC
vt
O
FsaD
p
OFs
aG
p O
F92b
l p
OF9
2b3
p
SiO
, Ti
O,
A’,%
Fe
z%
MnO
M
@
CaO
N
a,O
W’
p205
LO
I To
tal
DI
Ba
Nb
Rb
SC
Sr
V
ZC
La
Ce
Nd
Sm
Eu
Gd DY
Er
Y
b Lu
Y
60.8
6 56
.64
0.41
0.
44
18.8
1 18
.2
3.42
4.
69
0.23
0.
08
0.35
1.
43
1.24
4.
00
6.23
2.
83
7.8.
5 8.
69
0.08
0.
25
3.74
2.
32
103.
22
99.5
7
89.2
1 75
.57
22
1066
96
24
44
6 27
9 3
7 28
74
2 16
87
59
5 14
7 12
6 44
21
6 81
79
33
14
7
1.5
2.2
12
6 9
4 4.
8 1.
7 4.
7 1.
7 0.
8 0.
3 54
22
61.3
2 59
.59
58.0
2 60
.84
61.7
5 61
.00
61.6
4 60
.65
60.6
7 60
.24
0.4
0.4
0.4
0.4
0.4
0.4
0.41
0.
43
0.41
0.
41
18.1
7 17
.91
17.9
5 18
.67
18.7
3 18
.7
18.8
6 18
.6
18.7
9 18
.67
3.5
3.54
3.
47
3.29
3.
08
3.15
3.
24
3.34
3.
52
3.5
0.2
0.22
0.
22
0.14
0.
14
0.15
0.
13
0.15
0.
17
0.17
0.
35
0.34
0.
36
0.4
0.39
0.
38
0.41
0.
5 0.
4s
0.45
1.
82
1.58
1.
66
1.82
1.
85
1.75
1.
85
2.00
1.
6 I .6
5.
3 5.
37
5.87
5.
52
5.65
5.
7 5.
67
5.58
5.
05
5.00
7.
3 7.
25
7.2
6.94
6.
7 6.
69
6.8
6.73
7.
23
7.11
0.
11
0.1
0.06
0.
07
0.05
0.
06
0.06
0.
08
0.07
0.
08
1.2
3.38
4.
01
0.54
0.
59
0.73
0.
69
0.97
I .7
5 1.
57
99.6
7 99
.68
99.2
2 98
.63
99.3
3 98
.75
99.7
6 99
.03
99.7
1
98.8
88.0
3 88
.79
89.8
2 87
.85
87.9
1 88
.07
87.7
9 87
.03
87.2
7 87
.09
39
39
32
57
43
39
44
58
109
104
101
107
100
98
98
100
100
91
115
117
505
565
378
317
305
330
322
326
423
445
3 3
4 4
4 4
4 4
4 4
42
40
34
65
56
57
60
86
119
109
20
18
1.5
13
13
13
12
12
14
1s
588
609
586
57s
537
562
558
527
589
601
122
127
126
116
115
114
111
104
121
122
220
228
234
213
213
216
207
194
236
251
81
82
85
77
78
76
76
71
81
83
15
15
15
13
13
13
12
12
14
15
1.8
1.8
1.5
1.6
1.6
1.6
1.5
1.6
1.8
1.8
12
12
12
10
10
11
10
10
11
12
9 IO
IO
9
9 9
9 8
9 9
4.5
4.7
5.2
4.6
4.6
4.7
4.7
4.4
4.6
4.9
4.8
5.2
5.7
4.8
4.7
4.7
4.9
4.5
4.8
4.9
0.8
0.9
0.8
0.7
0.7
0.8
07
0.7
0.7
0.7
55
58
55
SO
48
51
52
50
48
51
Sect
ions
: S.
Mar
ia
Mar
zano
M
arza
no
Tuor
o Ca
lvi
Calv
i Sc
araf
ea
Scar
afea
Sc
araf
ea
Scar
afea
La
goPa
tria
Lago
Patri
a Sa
mpl
es:
OF9
2b5
p O
FlO
a p
OFl
Ob
p O
F13b
p
OF1
2a
p O
F12b
p
OFS
cA
p O
FScB
sp
O
FScA
d
OFS
cB
d O
FLpA
sp
O
FLpB
sp
SiO
, Ti
O,
AI,%
Fe
@,
MnO
M
gO
CaO
N
a,O
IW
P&J
LO1
Tota
l
60.4
61
.4
0.43
0.
42
18.8
5 18
.67
3.5
3.35
0.
17
0.1
0.48
0.
47
1.58
2.
03
4.9
5.49
7.
00
7.28
0.
07
0. I
I .57
0.46
98
.95
99.7
7
DI
85.6
5
Ba
106
Nb
117
Rb
382
SC
4 Sr
10
4 V
13
zr
57
2 La
11
3 Ce
31
5 N
d 73
Sm
13
EU
1.
8 Cd
10
D
Y
8 Er
4.
4 Y
b 4.
2 Lu
0.
72
Y
50
87.4
3
47
81
350 63
27
54
1 _ _ _ _ _ _ _ _ 23
61.2
6 0.1
1
18.8
5 3.
4 0.
13
0.41
1.
84
5.61
7.
01
0.11
0.
7 1
99.4
4
87.5
3
32
97
403 35
25
s9
1 _ _ _ _ _ _ 36
61.2
1 60
.86
60.6
8 59
.23
60.9
60
.28
60.7
3 61
.25
61.4
6 0.
42
0.42
0.
4 I
0.4
0.42
0.
41
0.41
0.
41
0.4
18.6
5 18
.55
18.4
8 18
.12
18.5
2 18
.44
18.5
18
.68
18.8
4 3.
52
3.49
3.
43
3.42
3.
44
3.46
3.
48
3.41
3.
48
0.13
0.
18
0.15
0.
18
0.17
0.
19
0.17
0.
14
0.14
0.
36
0.34
0.
43
0.33
0.
35
0.34
0.
35
0.32
0.
41
1.6
I .76
1.
81
I .74
I .82
1.77
I .7
9 1.
67
1.35
5.
18
5.9
5.36
5.
91
6.13
5.
99
5.98
5.
55
5.57
7.
66
7.05
7.
34
7.26
7.
08
7.37
7.
33
7.07
7.
36
0.1
I 0.
04
0.09
0.
07
0.08
0.
09
0.08
0.
14
0.11
0.
92
1.11
1.
59
3.11
0.
86
1.41
0.
99
1.19
0.
66
99.7
6 99
.75
99.7
7 99
.77
99.7
7 99
.75
99.8
1 99
.83
99.7
8
88.3
5 88
.7
87.8
2 89
.49
89.2
1 89
.44
89.2
9 88
.29
89.1
8
26
98
390 2.5
21
594
:21
217 80
!4
1.5
II 9 4.7
5.3
0.8
49
22
41
20
19
18
28
38
79
105
100
108
108
105
104
108
97
395
375
391
366
414
393
393
381
3 3
2 3
2 2
3 2
24
38
23
26
28
31
46
69
18
19
18
18
18
18
27
25
636
620
662
653
644
641
658
590
131
121
136
136
135
131
130
116
241
236
254
2.51
25
1 25
1 25
0 22
9 88
84
89
92
89
89
88
80
15
15
15
16
15
15
15
14
1.
51
1.6
1.5
1.5
1.5
1.53
1.
6 1.
8 13
12
12
13
13
13
12
11
10
10
10
10
11
10
10
9
5.3
5.0
5.7
6.1
5.7
5.8
5.3
4.4
6.2
5.7
6.1
6.3
5.4
5.7
6.1
4.9
0.9
0.9
0.9
0.9
0.8
0.9
0.9
0.7
56
53
59
59
S8
58
58
44
Tab
le
1 (c
onti
nu
ed)
Sec
tion
s:
Pac
ogn
ano
Pu
cara
P
uca
ra
M.
Cas
san
o M
Cas
san
o M
.Cas
san
o M
.Cas
san
o S
. A
gata
S
. A
gata
S
. A
gata
M
eliz
zan
o M
eliz
zan
o M
adda
lon
i S
ampl
es:
OF
Pa
p O
FP
uA
p
OF
Pu
B
p O
FM
clit
p
OF
Mcb
otsp
O
FM
cmid
sp
OF
Mct
op
sp
OFP
S sp
2 O
FPS
spl
OF
PS
sp
3 O
FM
Em
id
p O
FM
Eto
p p
OF
490
p
SiO
, 60
.65
59.X
59
.3
60.5
2 60
.69
61.3
2 T
iO 2
0.
41
0.41
0.
41
0.41
0.
42
0.42
A’
203
1x.5
5 1X
.32
18.0
4 18
.52
1x.4
7 18
.73
Fez%
3.
42
3.58
3.
56
3.42
3.
46
3.49
M
nO
0.
16
0.19
0.
18
0.14
0.
18
3.12
M
gO
0.34
0.
41
0.71
0.
43
0.35
0.
35
CaO
1.
87
2.08
2.
22
1.63
1.
71
1.56
N
a,O
6.
22
5.67
5.
04
6.21
5.
79
5.93
Ih
O
7.00
7.
19
7.36
6.
86
7.24
6.
91
p205
0.
11
0.09
0.
1 0.
11
0.08
0.
12
LO
1 1.
12
2.01
2.
82
1.49
1.
39
0.8
Tot
al
99.8
5 99
.75
99.7
4 99
.74
99.7
8 99
.75
DI
89.1
9 X
7.5
X5.
75
X9.
43
XX
.92
XX
.97
Ba
39
39
54
27
21
21
Nb
103
97
97
109
113
103
Rb
366
518
402
379
402
341
SC
3
3 3
3 2
3 S
r 37
51
55
29
27
23
V
17
21
23
19
1X
24
Z
r 61
6 60
1 59
7 66
9 69
0 62
4 L
a 12
7 12
6 13
1 13
3 14
0 12
9 C
e 23
8 23
4 23
4 25
0 26
1 24
0 N
d 87
83
86
89
94
86
S
m
15
14
15
16
16
15
Eu
1.
4 1.
5 1.
6 1.
6 1.
6 1.
5 C
d 12
12
12
12
13
12
D
Y
10
9 9
10
:o
I1
Er
5.3
5.0
5.3
5.4
5.7
5.4
Yb
5.7
5.4
5.7
6.2
6.5
5.9
LU
0.
8 0.
8 0.
9 0.
9 0.
9 a.8
Y
55
54
56
5X
61
56
61.2
3 61
.16
61.7
1 61
.33
61.4
3 61
.15
62.0
1 0.
42
0.42
0.
43
0.42
0.
4 I
0.42
0.
41
1x.7
18
.66
18.7
4 1X
.72
1 X.6
2 1X
.58
18.8
7 3.
5 3.
52
3 57
3.
52
3.53
3.
3 0.
43
0.14
0.
22
0.2
0.2
0.17
0.
17
0.2
0.37
0.
38
0.38
0.
4 0.
42
0.44
0.
37
1.64
1.
63
1.67
1.
72
1.78
2.
01
1 .x
5.77
5.
77
5.7
5.76
5.
5 5.
75
5.53
7.
03
6.56
6.
67
6.65
6.
X
6.53
7.
48
0.1
0.09
0.
09
0.08
0.
09
0.08
0.
07
0.86
1.
04
3.67
0.
96
1 1.
32
1.03
99
.76
99.4
5 10
2.83
99
.76
99.7
5 99
.75
9X.2
XX
.53
X7.
72
X7.
57
X7.
54
X7.
17
X6.
94
X7.
97
41
43
19
37
58
105
103
102
101
98
325
372
312
327
297
2 3
3 3
3 22
22
22
25
54
19
17
18
18
20
63
6 65
2 63
9 62
2 58
6 13
0 13
4 13
3 12
8 12
0 24
3 25
0 24
7 23
7 21
5 91
90
86
85
X
l 16
15
14
14
13
1.
5 1.
6 1.
6 1.
6 1.
6 12
12
11
II
I1
I1
9
9 9
9 5.
3 4.
9 4.
5 4.
X
4.5
6.0
5.2
4.x
4.9
4.7
0.9
0.8
0.8
0.8
0.7
56
49
43
47
51
34
40
103
x9
292
418
3 3
43
58
19
22
594
570
118
114
216
197
82
77
14
15
1.5
1.9
I1
12
9 9
4.9
4.x
4.X
4.
9 0.
8 0.
9 52
55
63
61
59
57
0.2
0.1
4
2
5
i
s
k
(
1
,-
r_
L. Cirretta et al./Joumal of Volcanology and Geothermal Research 75 (1997) 183-219
iO2 “4 FeO’
\I a20
4
3
1.5
1
0.5
6
4
0.3
0.1
72 76 80 84 D, 72 76 80 84 D,
Fig. 4. Variation diagrams of major elements (wt.%) vs. DI for CI pumice. Major-element data were normalized to 100% on a volatile free base. 0 = ignimbrite; + = fallout deposit.
characterized by the coexistence, also in the same pumice, of glasses with different compositions. In two cases, i.e., at the bottom of the Triflisco section and at the upper level of Capriglia section, glass and pumice compositions are quite different (Fig. 3), the most likely explanation is the coexistence at the same stratigraphic heights of pumices with different compositions. Fig. 7 shows the K,O/Na,O ratio of glass in pumices varies from 0.5 to 3.3, with two main peaks at 1 and 2.7, corresponding to the most- and least-evolved host-pumice compositions. In con- trast, glass from pumice fragments with intermediate compositions show a bimodal distribution with K,O/Na,O ratios representative of the most-evolved
1200
1 cb
Ba 0
0 0
8 Sr
0 8
0 6
199
800
600
400
2ou
0
600
4Oc
200
60
40
20
0 72 76 80 $4 ,,I 72 76 80 84 Dl
Fig. 5. Variation diagrams of trace elements (ppm) vs. DI for CI pumice. Symbols as in Fig. 4.
and intermediate whole-rock compositions. Such variations in the glass compositions match the miner- alogical data presented below.
As previously described, some sections located at different distances and directions from the vent have
1000 ] 1
11liII lI(I II(1 If 11 La Ce Nd SmEuGd Dy Er YbLu
Fig. 6. Chondrite-normalized REE abundance patterns for CI pumice. Symbols as in Fig. 4. Normalization constants from Nakamura (1974).
200 L. Ciuena et al. /Journal of Volcanology and Geothenal Research 75 (1997) 183-219
been analyzed in detail. On the basis of chemical composition of the pumices, these sections can be divided into three groups: (1) sections (Mondragone lower unit, Triflisco and Sant’Anna) characterized by pumices of fairly homogeneous evolved composition (DI = 88-90). Significant compositional variations of single or multiple pumices or glass in pumices are not observed with stratigraphic height or at the same stratigraphic level (Table 1; Fig. 3A). These sections are defined hereafter as “most evolved” in composi- tion; (2) sections (San Nicola La Strada, Ponti Rossi, San Marco) characterized by pumices of least- evolved composition (DI = 75-83) generally show a slight decrease of differentiation upsection (Table 1; Fig. 3B). Glass in pumices has homogeneous compo- sition always more evolved than the whole pumice. These sections are defined hereafter as “least- evolved”; (3) sections (Moiano, Massa, Capriglia) characterized by pumice compositions ranging from DI 84 to 87, are intermediate to the previously described groups. Glasses of different compositions coexist in the same pumice at various stratigraphic heights, as well as pumices of different compositions (Table 1; Fig. 3C). These sections hereafter are referred to as “intermediate” in composition.
Major-element contents and trace-element ratios versus DI of pumices also show the three distin- guishable compositional groups: the most-evolved
40 \
Least evolved host-pumice
2 K20/Na20
Fig. 7. Frequency histogram of K,O/Na,O ratio of glass from CI pumice. Note the bimodality of glass composition in intermediate-composition host pumices.
40 I
Zr /Sr
i 72 76 80 84
DI 92
Fig. 8. DI vs. Zr/Sr for CI pumice. 0 = sections with most- evolved composition (Mondragone lower unit, Triflisco, Sant’Anna, Calvi Melizzano, Maddaloni, Lago Patria, Scarafea, Pacognano, Pucara, Marina di Cassano, Sant’Agata, Marzano Appio, Tuoro di Teano); A = sections with intermediate-composi- tion (Mondragone upper unit, Massa, Capriglia, Tocco Caudio, Pignolelle, Moiano, Acquafidia, Santa Maria di Mortola); 0 = sections with least-evolved composition (San Nicola La Strada, San Marco, Villa di Briano, Ponti Rossi); l = fallout deposit; A
(half filled) = CI samples from Barberi et al. (1978).
pumices have Zr/Sr = 9-35, the least-evolved have Zr/Sr < 1, whereas the intermediate composition pumices lie between the two extreme compositions (Fig. 8). These geochemical data have been used to recognize the three groups in scattered samples col- lected from other sections, as well as in the CI analyses reported by Di Girolamo (1970) and Bar- beri et al. (1978) (Fig. 8).
6. Mineral chemistry
Feldspars: Sunidine is the dominant phase, ac- counting for 90% of the total phenocryst content. Composition ranges from Or,, to Or,, (Table 2; Fig. 9) with the more potassic, and Sr-, Ba- and Eu-rich variants occurring in the least-evolved and interme- diate-composition host-pumices. Sanidine phe- nocrysts are always homogeneous in composition. Major- and trace-element variation diagrams do not display any compositional gap (Figs. 10 and 11A).
L. Civetta et al. /Journal of Volcanology and Geothermal Research 75 (1997) 183-219 201
Table 2 Selected microprobe analyses of K-feldspars of CI pumice
Sections: S. Nicola S. Nicola S. Nicola S. Nicola S. Nicola Samples: OF104e OF104e OF104f OF104f OF104b Label: kf2core kf5core kf 1 -core kf3-core kf4
sio, 63.82 64.19 64.37 64.65 64.43 A1203 19.23 19.08 19.32 19.05 19.17 Fe&h 0.27 0.23 0.27 _ _ CaO 0.34 0.5 0.43 _ 0.57 Na,O 1.42 1.45 1.55 1.56 2.19 K2O 14.62 14.36 14.37 14.37 13.14 Total 99.70 99.81 100.04 99.85 99.5
S. Nicola S. Nicola Capriglia Capriglia OF104b OF104b OF592a OF592bl kf2 kfl kt3core kf5core
64.37 64.39 64.02 64.03 18.93 19.43 19.10 19.22 0.26 0.26 0.18 0.28 0.58 0.57 0.66 0.59 2.24 2.21 2.47 3.22
13.08 13.34 14.09 12.85 99.46 100.2 100.56 100.19
An 1.66 2.49 Ab 12.65 12.94 Or 85.69 84.57 Sr 362 Zr 0.2 0.2 Ba _ 148 La 6.7 6.5 Ce 4 4.3 Nd 0.4 0.5 ELI 1.7 1.9
2.11 2.18 2.82 13.77 13.87 19.64 84.12 83.95 77.53 _ _ _ 0.1 0.3 0.1 _ _ _ 6.7 6.9 _ 4.1 4.7 _ 0.4 0.5 0.3 1.8 1.6
2.87 2.78 3.02 2.73 20.05 19.58 20.38 26.83 77.08 17.65 76.6 70.45
169 183 265 284 0.1 0.2 0.6 0.1
279 276 248 435 7.7 8 7.4 5.1 4.9 5.1 5.8 3.6 0.4 0.5 0.6 0.3 1.6 1.7 1.7 1.5
Sections: Capriglia Capriglia Capriglia Mondragone Mondragone Mondragone Mondragone Mondragone Mondragone Samples: OF592bl OF592bl OF592a OF152b5 OF152b5 OF152a2 OF152a2 OF152bl OF152bl Label:
SiO,
A1203
Fe203
CaO Na,O
K2O
Total
An 3.18 3.22 3.64 2.79 3.57 4.15 2.96 4.42 4.57 Ab 32.91 32.21 33.25 30.56 33.53 32.9 31.49 37.71 31.56 Or 63.91 64.57 63.11 66.64 62.91 62.95 65.55 57.87 57.87 Sr 225 180 207 175 152 184 161 186 191 Zr 0.2 0.2 0.4 0.3 0.8 0.2 0.2 0.7 0.4 Ba 247 274 241 259 133 196 210 246 214 La 6.2 6.6 10.6 6.3 7.2 8 6.2 7 9.3 Ce 4.5 4.5 8.1 4.9 5.3 5.8 4.2 5.4 7.3 Nd 0.3 0.5 0.8 0.6 0.6 0.5 0.4 0.5 0.6 ELI 1.3 1.3 1.8 1.4 1.6 1.6 1.3 1.4 1.5
kf4 core kf4rim kf4core kflrim kflcore kfhore
64.49 64.69 65.15 64.81 64.95 63.55 19.69 19.82 19.37 19.28 19.32 19.26 0.17 0.12 0.27 0.29 0.29 0.19 0.70 0.71 0.76 0.6 0.78 0.87 4.02 3.95 3.85 3.63 4.07 3.82
11.88 12.02 11.09 12.03 11.6 11.1 100.05 101.32 100.49 100.64 100.02 98.78
kf2rim kf3core kfl
63.58 64.53 63.67 18.88 19.48 19.89 0.10 0.26 0.22 0.62 0.93 0.95 3.66 4.37 4.3
11.59 10.2 10.08 98.44 99.77 98.09
Major elements by electron microprobe: trace elements by ion microprobe. Oxides = wt.%; trace elements = ppm
202 L. Civetta et al./Joumal of Volcanology and Geothermal Research 75 (1997) 183-219
An
Ab IO 50 90 Or
Fig. 9. Ab-An-Or plots for feldspars from CI pumice. 0 = core; 0 = rim. (A) Most-evolved samples (Mondragone lower unit and Triflisco sections). (B) Intermediate-composition samples (Capriglia and Massa sections). (C) Least-evolved samples (San Nicola La Strada section)
BEE, with the exception of Eu, decrease with in- creasing K,O content, whereas Sr, Ba and Eu in- crease (Fig. 10). Least- and most-evolved pumice samples generally contain least- and most-evolved sanidine, respectively. However, pumices of interme- diate composition contain sanidine of variable com- position (Table 2; Fig. 9).
.
Plugiocluse varies in composition from calcic bytownite (An,_,,) to sodic oligoclase (An,,_,,) (Table 3; Fig. 9). Individual phenocrysts are gener- ally homogeneous in composition; they rarely show normal or reverse zoning. Major- and trace-element contents (Fig. 11B and Fig. 12) show a composi- tional gap separating the least-evolved bytownitic plagioclase from the most-evolved oligoclase. At decreasing CaO content, incompatible-element con- centrations increase whereas compatible-element contents decrease. The least-evolved pumice samples contain labradorite, whereas andesine and oligoclase occur in intermediate- and most-evolved pumice, respectively. Bytownite is present in pumice samples covering the whole range of composition.
300
150
1.8
1.5
8
4
. . . .
l l . . . 8
lu .
‘
E
L
8
. . . . .
l . .
. ’ . . l
.
. .a .
. 0’
’ > ‘, .
.
I
12 w Temperature estimates from feldspar pairs by the
method of Fuhrman and Lindsley (1988) yield values Fig. 10. K,O (wt.%) vs. trace elements (ppm) for sanidine from CI pumice.
L. Civetta et al. /Journal of Volcanology and Geothermal Research 75 (1997) 183-219 203
ranging from 878 + 40 to 836 & 40°C (PHzO = 1.5 kbar) (Table 4), using labradorite to oligoclase and sanidine (Or,, _a,>. The method does not give acceptable results using bytownite. These data, to- gether with the occurrence of corroded rims, suggest that bytownite crystals likely are xenocrysts. Tem- perature values for the least-, intermediate- and most-evolved pumice compositions are similar within the errors.
Clinopyroxene: a greenish Fe-rich diopside is the dominant clinopyroxene and is rarely associated with a colorless Mg-rich diopside (Table 5; Fig. 13). Two clinopyroxenes coexist in magmas erupted from sev- eral Italian volcanoes. Debate persists on their origin from magma mixing (Vollmer et al., 1981; Civetta et al., 1981, 1991a; Barton and van Bergen, 1982; Giannetti and Luhr, 1983; Villemant, 1988; Becca- luva et al., 1990) or from complex crystallization from a single magma (Ghiara et al., 1979; Barberi et al., 1981; Dolfi and Trigila, 1983; Metrich, 1983; Cundari and Salviulo, 1987). Mg-rich diopside crys- tals of the CI pumices likely are xenocrysts because they are rare and their rims are always corroded. This interpretation is supported by Sr isotopic data (Vollmer et al., 1981) giving evidence that diopside is clearly isotopically different from the other phe- nocrysts and glass. Major-element concentrations of Fe-rich diopside show continuous variations of Mg#(= Mg/Mg + Fe”) from 0.7 to 0.84 (Table 51, while a bimodality is apparent in trace-element con- tents (Fig. 11C). Major- and trace-element concentra- tions show good correlations with Mg# and Zr content (Fig. 14). Least-evolved pumice fragments contain predominantly phenocrysts of Fe-rich diop- side (Fs i4_i6), which, in a few cases, are slightly reversely zoned. Pumice samples with intermediate compositions include phenocrysts of Fe-rich diopside with variable compositions (Fs,,_ ,s) and are slightly zoned, both reversely and normally. The most- evolved pumice clasts have unzoned Fe-rich diopside (Fs,,_,,). Xenocrysts of Mg-rich diopside are rarely present in poorly evolved and intermediate pumice fragments.
Biotite: Biotite crystals are un-zoned (Table 6) and commonly contain apatite inclusions.
Ti-magnetite: Ti-magnetite commonly occurs in glomeroporphyritic clots with clinopyroxene (Table 7). Despite the small number of analyses due to
sanidine 0.001 h
Ba La Ce Sr Nd Sm Eu Zr Y
Ba La Ce Sr Nd Sm ELI Zr Y
1000 j I
La Ce Sr Nd Sm Zr Ti Y Yb Eu Cd Dy
Fig. 11. Spidergrams for feldspars and clinopyroxenes from CI pumice. Solid symbols = core: open symbols = rim. (A) Sanidine. (B) Plagioclase, circles = An(,,_,,,, triangles = An(,,_,,,. (C) Fe-rich diopside, diamonds = Mg# = 0.72-0.76, circles = Mg# = 0.78-0.80.
scarcity of crystals, compositions of Ti-magnetite and host pumices are positively correlated.
In conclusion, although compositional variations of minerals and host pumices roughly correlate (Fig.
Tabl
e 3
Sele
cted
m
icro
prob
e an
alys
es
of p
lagi
ocla
se
of C
I pu
mic
e
Sect
ions
: S.
Nic
ola
S. N
icol
a S.
Nic
ola
Sam
ples
: O
F104
e O
F104
e O
F104
e La
bel:
pl3a
-cor
e pl
3-co
re
pl3r
im
S. N
icol
a S.
Nic
ola
S. N
icol
a S.
Nic
ola
S. N
icol
a M
ondr
agon
e M
ondr
agon
e O
F104
b O
FlO
4b
OFl
Wf
OF1
04f
OF1
04f
OF1
52b3
O
F152
b3
pl4b
-cor
e pl
4b-ri
m
plZc
ore
pl2-
rim
p12a
pl
2cor
e pl
2rim
SiO
, AW
3
Fe20
3
CaO
N
a,O
K2O
Tot.
All
Ab
Or Sr
Ba
La
Ce
Nd
Sm
Eu
46.4
7 46
.23
33.5
9 34
.27
0.58
0.
73
16.9
8 17
.09
1.61
1.
44
0.3
0.29
99
.53
100.
04
83.8
4 85
.29
14.4
12
.98
1.75
1.
73
2679
22
23
216
156
10.7
9.
6 13
.0
11.9
4.
6 3.
7 0.
5 0.
4 1.
5 1.
4
46.0
6 33
.29
0.59
17
.1 I
1.51
0.
27
98.8
3 84
.84
13.5
9 1.
58
_ 159 _ 14.7
0.
5 1.6
45.6
2 45
.21
56.0
8 33
.35
33.7
2 26
.5 1
0.
72
0.71
0.
53
16.9
9 17
.26
9.21
1.
42
1.37
5.
01
0.16
0.
15
1.7
98.2
6 98
.49
99.0
4 86
.00
86.6
2 45
.36
13.0
1 12
.48
44.6
8 0.
99
0.9
9.95
17
65
2127
52
3 10
1 12
6 27
13
.5
10.9
20
.8
17.4
12
.9
22.7
6.
0 4.
2 5.
0 0.
8 0.
4 0.
5 1.
4 1.
4 1.
8
55.2
8 27
.28
0.56
9.
45
5.29
1.
15
99.0
0 46
.39
46.9
2 6.
69
415
26 _ 1.8
55.2
7 45
.69
45.3
4 27
.56
33.6
2 34
.18
0.48
0.
57
0.84
9.
75
16.7
7 17
.24
5.41
1.
25
1.2
1.12
0.
29
0.23
99
.59
98.1
9 99
.18
46.7
1 86
.54
87.5
6 46
.89
11.6
7 11
.05
6.4
1.78
1.
39
510
2665
24
38
24
138
145
20.3
8.
2 10
.2
22.3
11
.1
13.8
5.
3 3.
3 3.
7 0.
6 0.
4 0.
5 1.
8 1.
1 1.
3
Sec
tion
s:
Sam
ples
: M
ondr
agon
e M
ondr
agon
e C
apri
glia
C
apri
glia
C
apri
glia
C
apri
glia
C
apri
glia
C
apri
glia
C
apri
glia
C
apri
glia
O
Fl5
2bl
OF
152b
5 O
F59
2bl
OF
592b
l O
F59
2b7
OF
592b
7 O
F59
2a
OF
592a
O
F59
2bl
OF
592b
l L
abel
: pl
2cor
e pl
2cor
e P
lSri
m
pllc
ore
plco
re
plri
m
pllc
ore
pllr
im
plfc
ore
plfr
im
SiO
, 60
.76
61.1
4 46
.87
46.9
9 47
.91
45.9
5 47
.26
49.8
60
.84
59.9
9
Al,%
24
.5
23.6
5 34
.76
33.4
3 32
.78
23.2
6 33
.32
31.4
6 24
.13
25.4
6
Fe&
0.
48
0.43
0.
81
0.73
0.
61
0.79
0.
66
0.67
0.
49
0.48
C
aO
5.73
5.
21
17.1
3 16
.41
15.4
16
.86
16.1
4 14
.13
5.6
6.77
Na,
O
7.39
6.
72
1.43
1.
65
1.78
1.
33
1.91
2.
78
6.51
6.
44
K2O
1.
91
2.41
0.
19
0.23
0.
5 1
0.26
0.
24
0.5
2.99
2.
09
Tot
. 10
0.82
99
.52
101.
2 99
.51
98.9
7 99
.55
99.5
3 99
.39
100.
51
101.
24
An
26
.81
25.7
2 85
.9
83.3
6 80
.12
73.8
1 81
.18
71.5
1 26
.73
32.3
7 A
b 62
.57
60.0
8 12
.97
15.1
2 16
.73
26.1
9 17
.38
25.4
9 56
.28
55.7
5 O
r 10
.62
14.2
1.
13
1.52
3.
15
0 1.
44
3 16
.78
11.8
8 S
r 18
4 19
1 17
93
1779
18
12
2054
14
53
1570
18
1 25
0 B
a 23
22
71
52
75
74
63
59
21
18
L
a 25
.2
25.5
10
.1
11.8
12
.1
10.7
13
.4
13.9
24
.4
23.3
C
e 28
.3
29.1
14
.8
16.4
16
.4
15.4
18
.5
19.2
28
.1
27.6
N
d 4.
3 4.
8 3.
7 4.
6 4.
4 3.
9 4.
9 5.
1 4.
7 4.
8 S
m
0.3
0.3
0.4
0.5
0.5
0.5
0.5
0.5
0.3
0.4
Eu
1.
5 1.
5 1.
2 1.
3 1.
3 1.
4 1.
4 1.
5 1.
7 1.
6
Maj
or e
lem
ents
by
elec
tron
mic
ropr
obe;
tra
ce e
lem
ents
by
ion
mic
ropr
obe.
Oxi
des
= w
t.%
; tr
ace
elem
ents
= p
pm.
206 L. Ciuetta et al. /Journal of Volcanology and Geothermal Research 75 (19971 183-219
2000
ImO-
O-
20.
10-
Fig. 12. CaO (wt.%) vs. selected trace-element contents (ppm) for plagioclase from CI pumice.
151, mineralogical disequilibrium is a common fea- ture, particularly in the rocks of intermediate compo- sition. Least-evolved pumice samples (DI = 75-83) contain phenocrysts of Fe-rich diopside (Fs,,_ ,& in a few cases slightly reversely zoned, labradorite, and K-rich sanidine. Pumice samples of intermediate compositions (DI = 84-87) have phenocrysts of Fe- rich diopside with variable compositions (Fs r4 _ rs) and both slight reverse and normal zoning, andesine, and K-sanidine associated with Na-sanidine. The most-evolved pumices (DI = 88-90) have unzoned Fe-rich diopside (Fs ,6 _ rs), oligoclase, and sodic sanidine. Xenocrysts of bytownite are present in pumices of all compositions, whereas rare xenocrysts of Mg-rich diopside occur in the least-evolved and intermediate-composition pumice. These crystals may have been detached either from partly liquid mush along the chamber wall or from accidental xenoliths.
7. Fractional crystallization
The compositional variations displayed by the pumice samples indicate that the magma in the chamber was not homogeneous. To test a fractional- crystallization model from the least-differentiated trachyte to the most-evolved phonolitic-trachyte, least-squares mass-balance calculations were per- formed for major elements (Table 81, using mineral compositions measured by electron microprobe. Ac- cording to the model, variations in major-element compositions from the least- to the most-evolved trachyte are well accommodated by removal of 37.5% sanidine, 8.4% plagioclase, 7.4% clinopyroxene, 2.7% magnetite and 0.6% biotite (xr2 = 0.291). Trace-element models (Table 8) based on the calcu- lated fractionating assemblage and on measured par- tition coefficients (Pappalardo, 1994) were used to test the results of major-element modelling. A good match was achieved between the results of trace-ele- ment Rayleigh fractionation modeling and the com- position of the evolved trachyte.
As an alternative to the use of measured partition coefficients, the trace elements were plotted on log- log diagrams against Zr (Villemant et al., 1981; Villemant, 1988). The bulk distribution coefficients (D) for fractional crystallization processes can be measured by the slope (p) of log-log plots of trace-element concentrations against strongly incom- patible-element concentrations. Slopes lower than 1 indicate that distribution coefficients are compatible with fractional crystallization processes. The CI plots (Fig. 16) give slopes lower than 1. Calculated distri- bution coefficients (0, in Table 9) agree well with those calculated from modal abundances and mea- sured trace-element values (D, in Table 9). Further-
Table 4 Estimated temperatures from feldspar pairs
IC P Plagioclase Sanidine Fuhrman and Lindsley
(kbar) Ab Or An Ab Or An TAb To, TAn
OFl52b3 1.5 61.2 10.3 28.4 35.2 61.0 3.1 879 874 881 OF152b5 1.5 61.5 10.4 28.5 33.5 62.9 3.6 868 864 863 OFlOlG 1.5 61.0 9.9 29.0 30.3 65.7 3.9 849 848 852 OFlO4A 1.5 25.0 3.4 70.6 15.3 81.7 3.0 838 838 830 OF104B 1.5 38.1 4.9 57.0 19.7 77.1 3.2 839 837 840
T”C(media)
878 + 40 865 + 40 850 f 43 836 + 40 839 + 40
Tab
le
5
Sel
ecte
d m
icro
prob
e an
alys
es
of
clin
opyr
oxen
e of
C
I pu
mic
e
Sec
tion
s:
S.
Nic
ola
S.
Nic
ola
S.
Nic
ola
S.
Nic
ola
Cap
rigl
ia
Cap
rigl
ia
Cap
rigl
ia
Cap
rigl
ia
Cap
rigl
ia
Cap
rigl
ia
Cap
rigi
ia
Cap
rigl
ia
Cap
rigi
ia
Cap
rigl
ia
Mon
drag
one
Mon
dmgo
ne
Sam
ples
: O
F
104e
O
F
104e
O
F
I@&
O
F
104e
O
FS
92b7
O
F59
2b7
OF
592b
7 O
F59
2b3
OF
592b
3 O
F59
2bl
OF
592b
l O
F59
2bl
OF
592b
l O
F59
2bl
OF
152b
3 O
F15
2bl
Lab
el:
cpx2
core
cp
x2ri
m
cpx3
core
cp
x3li
m
cpx2
core
cp
x2
rim
C
PX
l co
re
rim
co
x3ri
m
cox3
rim
co
x3co
re
coxl
core
co
xln
m
CD
XI
CD
X
- S
iO,
51.9
4 49
.04
52 7
2
TiO
, 0.
67
1.09
0.
45
AL
203
3.69
5.
89
2.65
Fe%
, 5.
09
6.84
4.
58
Mn
O
0.16
0.
21
0.05
MgO
16
.06
14.1
7 16
.5
cao
23.3
6 22
.98
23.1
3
Na,
0
0.25
0.
29
0.16
Tot
. 10
1.2
1005
1 10
0.23
51.1
1 50
.78
50.8
7 50
.44
49.8
6
0.51
0.
53
0.71
3.06
3.
6 3.
56
8.16
8.
33
8.52
0.36
0.
46
0.59
13.8
7 13
.58
13.2
2
22.1
9 22
.27
22.2
0.57
0.
31
0.38
99.6
5 99
.58
99.0
4
77
2945
154
<
LO
O
118
49
80
15.6
57.6
50.4
14.6
4.6
13.5
10.0
4.2
2.6
50.7
6 50
.76
so.7
5 49
.53
50.9
2 50
.18
0.59
0.
59
0.76
1.
08
0.78
0.
86
3.13
3.
13
2.67
3.
58
2.8
2.97
8.67
8.
67
9.92
10
.63
10.1
4 9.
78
0.59
0.
59
0.7
0.94
0.
96
0.78
12.8
9 12
.89
12.2
2 II
56
12
.3
II.9
5
22.2
7 22
.27
21.8
8 21
.6
21.8
7 22
.26
0.52
0.
52
1.02
0.
83
0.88
0.
69
99.5
1 99
.51
100.
06
99.7
3 10
0.5
99.5
6
50.2
6
0.84
P
2.
96
10.4
2 Q
0.87
E
3
Num
ber
of cario
ns
on rh
e ba
sis
of 6
oxy
gen
s
Si
1.88
Ti
0.02
Al
IV
0.12
Al
VI
0.04
Fe3
+
0.07
Fe2
+
0.09
Mn
0
Mg
0.87
Ca
0.9
Nkl
0.
02
z 4
WO
47
.02
En
44
.99
FS
7.
99
Mg#
0.
91
SC
Ti
V
Cr
Sr
Y
ZI
La
Ce
Nd
Sm
EU
Gd
DY
E
I
Yb
1.8
1.92
1.
88
0.03
0.
01
0.01
0.2
0.08
0.
12
0.05
0.
03
0.02
0.07
0.
02
0.11
0.14
0.
12
0.17
0.01
0
0.01
0.77
0.
9 0.
76
0.9
0.9
0.9
0.02
0.
01
0.03
4 4
4
47.8
4 46
.57
46.3
2
41.0
6 46
.24
39.4
Il.1
1 7.
2 14
.27
0.85
0.
88
0 82
0.42
0.
47
3.28
2.
87
8.99
8.
21
0.32
0.
5 I
13.9
3 13
.26
22.7
8 22
.47
0.36
0.
65
101.
18
99.2
1
I.9
I .89
1.88
I .
87
0.01
0.
01
0.01
0.
02
0.1
0.11
0.
12
0.13
0.03
0.
03
0.04
0.
03
0.06
0.
06
0.05
0.
06
0.19
0.
19
0.21
0.
21
0.02
0.
01
0.01
0.
02
0.74
0.
77
0.76
0.
74
0.9
0.88
0.
89
0.89
0.05
0.
04
0.02
0.
03
4 4
4 4
47.4
7 46
.37
46.7
2 46
.99
38.9
8 40
.32
39.6
4 38
.93
13.5
4 13
.3
13.6
4 14
.08
0.79
0.
8 0.
78
0.78
89
84
77
86
4225
32
35
3285
36
05
246
175
185
169
<
100
<
LO
O
i IO
0 35
0
142
140
I55
119
38
49
s2
59
82
88
89
122
Il.7
14
.9
17.1
18
.6
41.4
55
.9
65.3
69
.2
39.9
51
.4
60.4
63
.7
11.6
15
.0
17.6
18
.3
3.4
5.0
5.4
5.5
10.6
13
.7
15.9
16
.5
7.3
9.3
12.0
12
.3
3.2
4.0
5.0
5.0
2.2
3.0
3.7
3.7
I9
1.9
1.9
0.02
0.
02
0.02
0.1
0.1
0.1
0.04
0.
04
0.01
0.05
0.
05
0.08
0.22
0.
22
0.23
0.02
0.
02
0.02
0.72
0.
72
0.68
0.89
0
89
0.88
0 04
00
4 0.
07
4 4
4
47 4
47
.4
46.9
3
38.2
38
.2
36.4
6
14.4
14
.4
16.6
1
0.76
0.
76
0.75
81
74
71
4010
34
10
4060
126
102
107
<
100
<
100
250
19
19
I2
83
81
91
220
223
292
33.1
31
.6
41.6
127
120
155
101
93.2
11
7
26.8
25
.2
30.4
4.8
3.9
3.1
22.1
20
.0
24.1
17.5
15
.7
IX.5
7.6
7.2
8.2
6.0
5.6
7.0
1.87
0.03
0.13
0.02
0.11
0.22
0.03
0.65
0.87
0.06
4
46.9
7
34.9
9
18.0
4
0.74
76
4161
<
101
7.5
106
326
48.2
183
134
33.9
2.x
27.2
21.0
9.2
8.3
1.9
I .89
0.02
0.
02
0.1
0.11
0.02
0.
02
0.07
0.
06
0.25
0.
25
0.03
0.
02
0.67
0.
67
0.87
0.
9
0.06
0.
05
4 4
47.0
1 47
.86
35.9
9 35
.73
17.0
1 16
.41
0.73
0.
73
76
77
4225
37
70
97
82
<
100
i 10
0
7.5
7.2
109
102
357
308
52.6
47
.3
193
177
149
134
37.6
34
.2
3.3
3.2
29.1
27
.0
23.2
20
.2
10.0
8.
0
8.8
7.5
12
21.9
7 n
0.69
e \
1caO
l s 5
1.89
E
L
0.02
9
0.11
8
0.02
6
0.07
s
0.26
0 r
0.03
B
0.67
0.
88
s_
0.05
Q
4 8
46.9
5 !e
35.6
7 17
.38
2
0.72
69
8
3330
B
76
;: s
<
100
2 6.
5 2
85
s 23
0 Y
38
.4
138
5
108
26.5
2.5
20.4
16.0
6.8
5.9
Oxi
des
=
wt.
%;
trac
e el
emen
ts
= p
pm.
208 L. Civetta et al./Joumal of Volcanology and Geothermal Research 75 (1997) 183-219
wo f J / l
‘En 10 20 Fs
I
En 10 l
20 FS
Fig. 13. En- Wo-Fs plots for clinopyroxenes from CI pumice. 0 = core; 0 = rim. (A) Most-evolved host pumices (Mondragone lower unit and Triflisco sections). (B) Intermediate-composition host pumices (Capriglia and Massa). (C) Least-evolved host pumices (San Nicola La Strada section).
more, log-log plots of compatible versus incompati- ble trace-element concentrations show steep negative slopes, consistent with derivation by fractional-crys- 150- Sr : V
. - 250
tallization of a single parental magma. . . loo-
8. . -150
8. Isotope geochemistry 50- . .*
. l a* Sr-isotope ratios were measured on whole-rock 0 . so*
. 50
pumice fragments (25 samples), and glass (7 sam- La .
. Yb 50-
.
ples) and feldspar phenocrysts (19 samples) sepa- 6 :
rated from pumices (Table 10). To test for alteration, . . ? -6
leaching was carried out on samples assuming that 30- B
surticial Sr deposited during alteration should be .*
fully leachable. Both leachate and residual sample were analyzed and yielded similar 87Sr/ 86Sr ratios.
4. i 10 l , I 1
In a few cases, the leachate had a slightly more 100 300 Zr 100 &I Zr ’ radiogenic isotopic composition, probably as the re- Fig. 14. Zr vs. selected trace-element contents (ppm) for clinopy- sult of surficial alteration processes. roxenes from CI pumice.
Tab
le 6
S
elec
ted
mic
ropr
obe
anal
yses
of
biot
ite
of C
I pu
mic
e
Sec
tion
s:
Tri
flis
co
Tri
flis
co
Tri
flis
co
Tri
flis
co
Cap
rigl
ia
Cap
rigl
ia
Cap
rigl
ia
Cap
rigl
ia
Cap
rigl
ia
Cap
rigl
ia
Cap
rigl
ia
Cap
rigl
ia
Sam
ples
: O
Fl7
cl
OF
17cl
O
Fl7
cl
OF
l7cl
O
F59
2bl
OF
592b
l O
F59
2bl
OF
592b
3 O
F59
2b3
OF
592b
5 O
F59
2b5
OF
592b
5 L
abel
: bt
lcor
e bt
lrim
bt
2cor
e bt
2rim
bt
1 co
re
btlr
im
bt2c
ore
btlc
ore
btlr
im
btlc
ore
bt2c
ore
bt2r
im
SiO
, 36
.75
36.5
1 36
.35
36.8
37
.41
36.4
6 36
.54
37.3
1 37
.2
35.9
2 36
.33
36.0
8 T
iO,
4.86
4.
71
4.74
4.
89
4.81
4.
69
4.7
4.99
4.
92
4.5
1 4.
68
5.00
N2o
3 14
.43
14.3
7 14
.19
14.6
8 14
.43
14.4
4 14
.4
13.8
14
.35
15.1
7 15
.32
14.7
4 F
e0
13.6
6 13
.71
13.1
2 13
.84
13.0
9 13
.41
13.0
1 12
.75
13.1
9 13
.24
13.1
1 13
.38
Mn
O
0.25
0.
15
0.25
0.
12
_ 0.
18
0.15
0.
23
0.18
0.
16
0.08
0.
07
MgO
15
.56
15.3
5 15
.55
15.5
5 15
.56
15.4
8 15
.58
15.9
6 15
.79
15.3
4 16
.07
15.4
2 N
a,O
0.
4 0.
45
0.43
0.
4 0.
46
0.33
0.
35
0.36
0.
35
0.35
0.
31
0.32
K2O
9.
39
9.61
9.
51
9.56
9.
57
9.49
9.
39
9.47
9.
74
9.42
9.
57
9.47
F
1.
05
1.26
0.
87
0.81
0.
96
2.58
0.
99
0.87
1.
13
1.01
1.
01
1.12
C
l 0.
06
0.06
0.
07
0.13
0.
09
0.1
0.09
0.
08
0.04
0.
12
0.08
0.
12
Mg
* 2.
84
2.85
2.
79
2.85
2.
75
2.78
2.
76
2.72
2.
76
2.8
2.76
2.
81
Sec
tion
s:
Cap
rigl
ia
Cap
rigl
ia
Cap
rigl
ia
Cap
rigl
ia
Cap
rigl
ia
Cap
rigl
ia
Cap
rigl
ia
Mas
sa
Mas
sa
Mas
sa
Mas
sa
Mon
drag
one
Sam
ples
: O
F59
2b5
OF
592b
5 O
F59
2b5
OF
592b
S
OF
592b
5 O
F59
2b5
OF
592b
5 O
FlO
lb
OF
lOlb
O
FlO
lb
OF
lOlb
O
Fl5
2b4
Lab
el:
bt3c
ore
bt3r
im
b&or
e bt
llri
m
b&or
e bt
6cor
e bt
6rim
bt
lcor
e bt
lrim
bt
2cor
e bt
2rim
bt
lcor
e
SiO
36
.5 1
36
.7
36.1
1 36
.19
36.5
9 36
.11
35.8
7 35
.1
35.2
9 37
.39
36.6
1 36
.87
TiO
: 5.
06
4.98
4.
83
5.00
4.
82
4.71
4.
99
5.08
4.
69
4.78
4.
84
3.88
A’20
3 15
.13
15.1
3 15
.02
14.9
1 15
.13
14.8
7 14
.87
14.4
6 14
.53
14.0
8 14
.85
14.0
0
Fe0
13
.55
13.0
5 13
.3
13.4
9 13
.45
13.3
4 13
.2
13.2
2 12
.82
12.9
6 13
.44
15.9
3
Mn
O
_ 0.
07
0.16
0.
17
0.19
0.
12
0.06
0.
1 0.
21
0.34
0.
26
MgO
15
.73
15.2
2 15
.63
15.7
1 15
.84
15.6
5 15
.5
15.3
2 14
.86
15.9
6 15
.84
14.5
N
a,O
0.
33
0.34
0.
33
0.33
0.
33
0.31
0.
28
0.27
0.
35
0.37
0.
36
0.57
K2O
9.
58
9.41
9.
55
9.65
9.
64
9.44
9.
39
9.47
9.
23
9.59
9.
68
9.17
F
1.04
0.
94
0.95
0.
82
0.66
0.
79
0.71
0.
92
1.19
1.
00
0.99
1.
94
Cl
0.06
0.
05
0.09
0.
07
0.08
0.
07
0.06
0.
07
0.09
0.
05
0.08
0.
14
Mg
* 2.
81
2.75
2.
8 2.
82
2.8
2.81
2.
8 2.
81
2.78
2.
14
2.19
3.
15
Oxi
des
in w
t.%
Tab
le 7
S
elec
ted
mic
ropr
obe
anal
yses
of
Ti-
mag
net
ite
from
CI
pum
ice
Sec
tion
s:
Tri
flis
co
Tri
flis
co
Tri
flis
co
Tri
flis
co
Sam
ples
: O
Fl7
cl
OF
l7cl
O
Fl7
cl
OF
l7cl
Lab
el:
mtl
core
m
tlri
m
mt2
core
m
t2ri
m
Tri
flis
co
Tri
flis
co
Tri
flis
co
Tri
flis
co
Tri
flis
co
Tri
flis
co
Mas
sa
OF
l7cl
O
F17
cl
OF
17cl
O
F17
cl
OF
l7cl
O
Fl7
cl
OF
lOlb
m
t3co
re
mt3
rim
m
t4co
re
mt4
rim
m
&or
e m
t5ri
m
mt 1
core
SiO
, 0.
3 0.
27
0.24
T
i0,5
.91
5.95
6.
01
6.08
A
hO
, 3.
47
3.26
3.
38
Fe0
79
.58
79.8
9 79
.6
Mn
O
0.91
0.
82
0.81
M
gO
2.23
2.
21
2.07
C
rA
0.11
0.
09
- Z
nO
_
0.48
P
92
.51
92.4
9 92
.59
0.39
0.
28
6.1
5.97
3.
35
3.41
79
.8
79.5
1 0.
82
0.92
2.
11
2.22
_ 0.
22
92.7
7
_ 92.4
4
0.28
6.
06
3.4
79.7
1
0.75
2.
17
0.13
0.
28
92.6
9
0.26
6.
01
3.39
79
.78
0.7
2.03
_ 92
.22
0.34
0.
4 6.
01
5.85
3.
48
3.57
79
.05
79.5
1
0.84
0.
75
2.18
2.
12
0.07
0.
11
0.3
0.17
92
.27
92.6
4
0.32
5.
57
3.32
79
.08
0.84
1.
97
0.08
_ 91
.46
0.29
5.
57
4.56
c*
79.4
4 2 P
0.
53
3 2.
7 3
0.17
a
0.16
\
” 6 93
.42
As
Sto
rmer
(198
3)
a F
e,O
, r.
52
.32
52.5
6 52
.11
51.9
7 52
.1
52.2
51
.98
51.6
2 51
.65
51.6
9 52
.43
3 F
e0 r
. 32
.49
32.5
8 32
.7
33.0
3 32
.62
32.7
3 32
.99
32.6
33
.02
32.5
6 32
.25
R
Tot
. r.
97
.73
97.7
5 97
.31
97.7
4 97
.64
97.6
4 97
.41
97.1
3 97
.62
96.6
2 98
.49
g
Usp
%
17.9
4 17
.83
18.2
3 18
.44
18.3
4 18
.12
18.5
18
.35
18.4
9 18
.02
17.1
8 $ B 3 :
Sec
tion
s:
Mas
sa
Mas
sa
Mas
sa
Mas
sa
Tri
flsc
o T
rifl
ico
Tri
flis
co
Mas
sa
Mon
drag
one
Mon
drag
one
8 S
ampl
es:
OF
lOlb
O
FlO
lb
OF
lOlb
O
FlO
lb
OF
l7d
OF
l7d
OF
l7d
OF
lOla
O
Fl5
2b4
OF
l52b
4 x
Lab
el:
mtl
rim
m
t2co
re
mt2
rim
m
t3co
re
mt5
rim
m
tlri
m
mt 1
core
m
t lco
re
mt l
core
m
tlri
m
8 S
iO
TiO
:
0.29
0.
34
0.33
0.
25
0.32
0.
23
0.3
0.4
0.3
1 0.
34
z
5.5
5.61
5.
36
5.97
5.
85
9.43
9.
65
9.5
9.35
9.
15
c 3 W
’, 4.
48
4.36
4.
34
4.06
3.
32
1.2
1.96
2.
12
1.98
1.
92
;: a-
Fe0
79
.79
79.2
9 78
.59
79.4
2 79
.08
78.2
76
.91
78.6
2 79
.04
78.3
6 2
Mn
O
0.6
0.61
0.
49
0.68
0.
84
1.93
1.
79
1.9
1.98
1.
87
2 M
gO
2.83
2.
59
2.6
2.69
1.
97
1.41
1.
33
1.48
1.
33
1.25
s
Cr2
03
_ 0.
11
0.12
0.
07
0.08
0.
07
0.08
0.
11
2 _
Zn
O
0.16
_
0.2
0.25
0
0.43
_
0.2
0.26
2
c 93
.65
92.9
1 92
.13
93.3
9 91
.46
92.8
3 91
.85
94.0
9 94
.27
93.2
6 Y
Y
\
o
As S
torm
r (1
983)
F
e,O
, r.
53
.12
52.2
3 51
.85
52.3
5 47
.63
47.1
2 51
.69
45.7
8 47
.73
47.2
5 F
e0 r
. 31
.99
32.9
9 31
.92
32.3
1
35.3
4 36
.22
32.5
6 35
.7
36.0
9 35
.83
Tot
. r.
98
.8
98.1
3 97
.1
98.3
8 97
.16
98.8
96
.62
96.4
4 98
.83
97.7
2 U
sp%
16
.6
17.3
1 17
.04
17.9
5 26
.75
27.7
4 18
.02
28.5
27
.13
27.0
1
Oxi
des
in w
t.%
L. Civetta et al. / Joumaf of Votcanofogy and Geothemal Research 75 (1997) 183-219 211
;e-rich diopside I-- C
.
8 10 Fe0
Fig. 15. Frequency histograms of composition for sanidine (A), plagioclase (B) and Fe-rich diopside (Cl from CI pumice. Note the bimodality of sanidine and Fe-rich diopside in intermediate host- pumice compositions.
87Sr/ 86Sr initial ratios of leached pumice sam- ples range from 0.70731 k 1 to 0.70746 t_ 1 (Table 10). The Sr-isotope ratios of four unleached pumice samples reported by Cortini and Hermes (198 1) are compatible with the detected range.
Pumice 87Sr/ 86Sr ratios are related to chemical compositions; the most differentiated samples have
Table 8 Fractional-crystallization model
Initial Final Obs-talc % of removed magma magma mineral phases OF17Cl OF152bl
SiOL 58.24 61.42 -0.18 TiO Al,;,
0.45 0.42 - 0.03 sanidine -37.51 18.71 18.64 - 0.09 plagioclase - 8.38
Fe0 tot 4.82 3.58 -0.1 clinopyroxene - 7.38 MnO 0.08 0.23 0.06 biotite - 0.65 MgO 1.47 0.35 -0.1 magnetite - 2.13 CaO 4.11 1.88 0.1 Na,O 2.9 1 6.22 0.35 Hr* = 0.291 W 8.94 7.17 0.21 P*O, 0.26 0.08 - 0.23
Partition coefficients observed
La 43.6 123.2 -16 0.3 Eu 2.2 1.8 0 0.9 Ba 1066 34 -3 3.1 Sr 742 17 -0.3 3.3 SC 7 2 -0.20 1.7
l ) p=o.‘:
La
2 &
p=o.73
2 Zr 32 Zr 3
Nb
p=l
Fig. 16. Zr vs. selected trace-element contents (ppm) log-log diagrams from CI pumice: p = slope.
212 L. Civetta et al./ Journal of Volcanology and Geothermal Research 75 (1997) 183-219
Table 9 Total distribution coefficients for CI pumice
Dl D2 Dl D2
Ba 3 3.1 Y 0.3 _
Nb 0 _ Zr _ Rb 0.6 - La 0.3 0.3 SC 1.7 1.7 Ce 0.3 _ Sr 3 3.3 Nd 0.4 _ V 1.9 - Sm 0.5 _
Dl and D2 see text.
the highest amount of radiogenic Sr (Fig. 17). Feldspars separated from pumice fragments covering the whole range of composition have homogeneous Sr-isotope ratios (0.70730 + l-0.70732 f 11, similar to those of the least-differentiated host pumice frag- ments, with the exception of two samples from the
Table 10 Isotope ratio of CI samples
Mondragone lower unit (0.70741 f 1) and the fallout deposit (0.70748 + 1).
The observed variations in Sr-isotope composition of pumice samples cannot be related to post-eruptive alteration because all samples were strongly leached and they are not correlated to LO1 values. Mixing among magmas of distinct series can also be ruled out because of the constant “Sr/ @Sr ratio of feldspars separated from pumices of various degrees of evolution and variable 87Sr/ 86 Sr ratios. The de- tected Sr-isotope variations could result from two distinct processes: (1) In situ aging of magma with high Rb/Sr ratios, after crystallization of feldspar (Christensen and De Paolo, 1987, 1993). In this interpretation, the slope calculated from the 87Sr/ 86Sr vs. Rb/Sr plot gives an age of 250 k 100 ka for the CI magmatic system. Long-lived mag-
Sections: S. Nicola La Strada Capriglia
Samples: OFlO4a OFl04b OFlO4c OFlO4d OFlO4e OFlO4f OF592a OF592b2
(*‘Sr/ s6Sr)pumice 0.70728 + 1 0.70735 + 1 0.70732 + 1 0.70744 f 1 0.70732 + 1 0.70735 f 1 0.70731 + 1 - (*‘Sr/ 86Sr)glass - _ _ _ _ 0.70732 + 1 - c8’Sr/ s6Sr)feldspars 0.70730 + 1 - 0.70730 * 1 - 0.70730 f 1 - 0.70730 * 1 0.70731 * 1 Sr (ppm) 444 433 536 72 695 647 152 245
Sections: Capriglia Massa S. Anna
Samples: OF592b3 OF592b4 OF592b6 OF1011 OF1013 OF1015 F sa D C8’Sr/ 86Sr)pumice 0.70734 + 1 0.70738 f 1 0.70738 f I 0.70745 * 1 0.70740 + 1 0.70736 + 1 0.70739 * 1 (*‘Sr/ 86Sr)glass 0.70736 f 1 - _ _ 0.70736 + 1 - C8’Sr,/ 86Sr)feldspa_rs 0.70731 + 1 - 0.70732 f 1 - _ 0.70733 + 1 Sr (ppm) 224 226 172 97 136 114 60
Sections: Mondragone lower unit Mondragone upper unit Triflisco
Samples: OFl52a2 OFl52bl OFl52b2 OFl52b6 OFl5Ul OFl5U3 OFl7b OFl7cl
C8’Sr/ 86Sr)pumice 0.70745 + 1 - 0.70744 * 1 0.70745 f 1 0.70733 f 1 0.70731 + 1 0.70745 + 1 0.70731 + 1 (“Sr/ s6Sr)glass 0.70745 f 1 0.70747 + 1 - 0.70747 * 1 - - _ (*‘Sr/ 86Sr)feldspars 0.70731 + 1 0.70732 +_ 1 - 0.70741 + 1 0.70731 i 1 0.70731 + 1 - 0.70732 +_ 1 Sr bd 27 17 30 59 111 149 28 742
Sections: Triflisco Fallout deposit
Samules: OFl7d OFl7e OF59 Fa OF59 Fb OF59 Fc OF sa Fl OF sa F3
(s’Sr/ 86Sr)pumice 0.70744 f 1 - 0.70744 + 1 0.70745 i 1 - 0.70740 + 1 PSr/ a6Sr)glass 0.70742 + 1 _ - _ _ _ (“Sr/ 86Sr)feldspars - 0.70731 f 1 0.70731 + 1 0.70748 0.70734 + 1 0.70731+ 1 - Sr (ppm) 42 40 54 35 34 34 65
L. Civetta et al./ Journal of Volcanology and Geothermal Research 75 (1997) 183-219 213
0.70750 0
22 q A0A A 0 0 p A - A
-O 0 $A #5I$ A A 0 0.70730 q o 3 AA m
0 q q
f 1 , / 9 I * 8 1 I I / 60 SiO2 64 0 300 Sr 600 10 87&/86Sr 40
Fig. 17. *‘Sr/ 86Sr vs. SiO, (wt.%), Sr (ppm) and 87Rb/ *6Sr for CI pumice. Crosses = feldspars; other symbols as in Fig. 8.
matic systems were postulated by many authors (Hildreth, 1987; Mahood, 1981; Halliday et al., 1989). Halliday et al. (1989) calculated a residence time of magma in the Long Valley system of 0.7 ma before the Bishop Tuff eruption. However, there are not enough geochemical and geochronological data on the Campi Flegrei products older than CI to support this hypothesis, (2) Contamination of the most differentiated part of the magma chamber by radiogenic Sr, after feldspar crystallization (Wiimer et al., 1985; Palacz and Wolff, 1989). This contami- nation could result from interaction of magma with hydrothermal fluids prior to eruption. Seawater (*‘Sr/ 86Sr = 0.709) could be the source for the radiogenic Sr as suggested by the 87Sr/ “Sr ratios of hydrothermal minerals found in deep geothermal wells at the Campi Flegrei (Rosi and Sbrana, 1987). Selective assimilation of 20% of a fluid with an isotopic ratio and Sr concentration similar to that of modem seawater can account for the isotopic varia- tions detected in the CI pumice samples. A similar hypothesis has been proposed by Villemant (1988) and Civetta et al. (1991a) for the deeper part of the Campi Flegrei magma chamber in the last 10 ka.
9. Chemostratigraphy and compositional tions
varia-
Plots of various geochemical parameters versus normalized stratigraphic height of the analyzed sam- ples collected from sections including complete se- quences of the deposit, grouped according to the three composition previously defined, are shown in Fig. 18. Two units found in the Mondragone section have been plotted according to their composition.
The mineralogical, chemical and isotopical data presented here lead to the following observations:
- Pumice samples from outcrops located in the central sector of the Campanian Plain up to 30 km from the Pozzuoli Bay (San Nicola La Strada, San Marco, Villa di Briano, Ponti Rossi) are the least evolved (DI = 75-83) and are more porphyritic (= 10% of phenocrysts by volume). The pumice compo- sitions are slightly more mafic upsection. Pumices contain phenocrysts of Fe-rich diopside (Fs i4 _ i6), labradorite, and K-rich sanidine, and xenocrysts of bytownite and rare Mg-rich diopside. Pumices and separated feldspar are isotopically in equilibrium and pumices display the lowest *‘Sr/ “Sr ratios (0.70730-0.70735) measured for the CL Glass com- positions are fairly homogeneous and more evolved than the host pumice.
- Pumice samples from sections located in the western sector of the Campanian Plain (Lag0 Patria, Scarafea) and along the western scarp of the Apen- nines (Mondragone lower unit, Calvi, Triflisco, Sant’Anna di Lavorate, Maddaloni, Melizzano), on the Sorrento Peninsula (Marina di Cassano, Pacog- nano, Sant’Agata, Pucara), on Roccamonfina (Marzano, Tuoro di Teano), up to 50 km from the vent and from the fallout deposit, are the most evolved (DI = 88-90) and are fairly homogeneous in composition throughout the section. Pumices contain phenocrysts of unzoned Fe-rich diopside (Fs ,6 _ ,s), oligoclase and sodic sanidine, and only rarely xenocrysts of bytownite. Pumices have the highest 87Sr/ 86Sr ratios (0.70740-0.70745) and are in iso- topic disequilibrium with separated feldspars, which have less-radiogenic Sr. The glass in pumices have a homogeneous chemical composition.
Pumice samples from sections located in the
214 L. Ciuetta et al./ Journal of Volcanology and Geothermal Research 75 (1997) 183-219
h
7:
5c
25
C
h’
75
50
25
0
h
75
50
25
0
%
i-
)-
l--
%
%
C
0 0 Cl 0
0 0
0 ?
AA %
A A A A
A AA A A
A % !w AA A
0
0 0 0
0
0 0 &al
I I
A
6
74 78 82 86 90 0 10 20 0.70730 0.70750
DI Zr/Sr g7Sr/%r
Fig. 18. DI, Zr/Sr (ppm) and *‘Sr/ 86Sr vs. stratigraphic height of the analyzed samples. Crosses = feldspar& other symbols as in Fig. 8.
Apennines and on Roccamonfina volcano, up to 65 km from the vent (Santa Maria di Mortola, Ac- quafidia, Massa, Tocco Caudio, Capriglia, Moiano, Pignolelle) and from the Mondragone upper unit, have an intermediate composition (DI = 84-87). Pumices are characterized by coexistence of both reverse- and normal-zoned Fe-rich diopside with
variable compositions (Fe,,_,,), by andesine and K-sanidine associated with Na-sanidine, and by xenocrysts of bytownite and rare Mg-rich diopside. Pumices have s7Sr/ 86Sr ratios between 0.70730 and 0.70745, in most cases in disequilibrium with sepa- rated feldspar. Glass from pumices of different com- position have been measured in the same pumice through all the sequence, suggesting syn-eruptive magma mingling. Generally, as previously described, the glass from pumices of intermediate composition have a bimodal distribution with the two peaks cor- responding to the most- and least-evolved pumice compositions.
The occurrence of two superimposed flow units with variable composition at Mondragone, corrob- orated by the sequence of three flow units also with variable composition drilled in the Campanian Plain at Afragola (Cornell et al., 1993) and at Ponti Rossi (Civetta, unpubl. data), is strong evidence that the compositional variations detected in the CI rocks are time-related. Therefore the chemostratigraphic se- quence of the CI deposit includes from the base upward: (1) the most-evolved magma (fallout and ignimbrite up to 50 km from the vent); (2) the intermediate composition magma (ignimbrite up to 65 km from the vent); and (3) the least-evolved magma (ignimbrite in the Campanian Plain up to 30 km from the vent).
We estimate volumes of about 25 km3 (DRE) for the most-evolved magma, about 100 km3 (DRE) for the magma of intermediate composition and about 20 km3 (DRE) for the least-evolved magma. This esti- mation was crudely made by circumscribing circles with a radii similar to the maximum distance reached from the vent; the thickness was the supposed maxi- mum at the center and zero at the perimeter of the circle. An important point is that the estimated vol- ume of magma emitted during the CI eruption nicely matches the volume of the caldera collapse estimated by Orsi et al. (1996).
The Y5 ash layer cored in the Ionian sea was related to CI by Thunnel et al. (19781, Cornell et al. (1983) and McCoy and Cornell (19901, and was dated by the oxygen isotope record at approximately 38 ka (Thunnel et al., 1978). It has the same compo- sition of the most-evolved and intermediate CI ign- imbrites. The five ash layers cored in the central Mediterranean Sea, which were dated at 38.7, 36.0,
L. Civetta et al./ Journal of Volcanology and Geothermal Research 75 (1997) 183-219 215
33.5, 26.9 and 24.1 ka and interpreted as the deposits of a sequence of phlegraean eruptions referred to as “CI series” by Pateme et al. (1988) and Pateme and Guichard (1993) are not compositionally correlatable to the CI distal products. These data are interpreted as an evidence that during the course of the CI eruption only shards with the most-evolved and in- termediate composition reached the Ionian Sea, whereas those with less-differentiated composition were deposited only close to the source area.
Recently Rosi et al. (1996) have studied the prox- imal deposits of the CI eruption and have identified four units which, from base to top, are: unit A (densely welded ignimbrite and lithic-rich breccia), characterized by a relatively uniform composition of pumice (SiO, = 62-62.5%; Na,O = 6.6-6.8%); unit B (sintered ignimbrite, low-grade ignimbrite and lithic-rich breccias), also with relatively homoge- neous composition of pumice (SiO, = 61-63%; Na,O = 4.5-6.5%); unit C (lithic-rich breccia and spatter agglutinate), with pumice and spatter clasts of composition variable over the whole chemical spec- trum detected for the CI (SiO, = 58-63%; Na,O = 2.5-6.5%); and unit D (low-grade ignimbrite), in- cluding pumices of variable composition, although those with less-differentiated composition are the most abundant. Other authors (Lirer et al., 1991; Perrotta and Scarpati, 1994; Melluso et al., 1995) suggested that these breccias postdate the CI erup- tion and those occurring in the western sector of the Campi Flegrei were erupted from a vent presently located in the sea between Campi Flegrei and the island of Procida. The compositional characteristics have been presented by Melluso et al. (1995). Com- parison of these compositional data with those pre- sented in these paper (Fig. 19) permits to correlate the lower proximal units [LPFU and SU of Melluso et al. (1995) and units A and B of Rosi et al. (1996)] with the most evolved CI distal products (this study), the medial proximal unit [BU of Melluso et al. (1995) and unit C of Rosi et al. (1996)] with the intermediate composition CI distal products (this study), and the upper proximal unit [UPFU of Mel- luso et al. (1995) and unit D of Rosi et al. (1996)] with the least-differentiated CI distal deposits (this study). Furthermore, Sr-isotopic ratios on pumice (0.70736 f l-0.70742 f 1) and separated feldspars (0.70730) collected at Marina di Vitafumo at the
so
60
40
30
400
ml
40
30
20
10
a
i
I-
!r/Sr ::
3; ;’ 0
;;.L$ ! ,,... ’
! I’_ r.c I A-’ I I 1
71 76 60 6-L DI 92
Fig. 19. DI vs. Zr/Sr, Rb, Y. Data fields are from CI pumice, 0 = UPFU of Melluso et al. (1995); A = BU of Melluso et al. (1995); 0 = LPFU-SU of Melluso et al. (1995).
base [LPFU of Melluso et al. (1995) and unit B of Rosi et al. (1996)] and top [BU of Melluso et al. (1995) and unit C of Rosi et al. (1996)] of this proximal sequence (L. Civetta, unpubl. data) are similar to those measured in CI distal exposures (this study). Chemical data from both distal and proximal CI deposits support the view that the eruption began with the emission of the most differentiated magma, followed with the simultaneous tapping of variable portions of the magma chamber, and ended with the extrusion of the least-evolved magma.
216 L. Civetta et al./ Journal of Volcanology and Geothermal Research 75 (1997) 183-219
10. Evolution of the CI magmatic system
Several lines of evidence suggest that the detected compositional variations from the least-differentiated to the most-evolved samples could result from crys- tal fractionation processes. They include: (1) the systematic trends of major and trace elements versus DI (Figs. 3 and 4); (2) the results of major- and trace-element modelling; and (3) the homogeneous Sr-isotope composition of feldspar separated from pumices of different composition.
Pumice samples of intermediate composition show strong evidence of mineralogic disequilibria, such as coexistence of sanidine and diopside of variable composition (Fig. 151, bimodality of trace-element distribution of Fe-rich diopside (Fig. 11) and bi- modal distribution of glass compositions. These data suggest that the magma of intermediate composition resulted from mingling of the most-differentiated, first-erupted magma with a less-differentiated magma, probably representing the most-differenti- ated part of the least-evolved, last-erupted magma.
The CI magma chamber must have been charac- terized by an uppermost differentiated-layer tapped during the initial phases of the eruption, whose products are represented by the fallout deposit and the first flows that reached the Sorrento Peninsula in the southeast and Roccamonfina Volcano in the northwest, and by a least-differentiated slightly zoned deeper-layer tapped during the final phases of the eruption. This deeper layer was separated by a com- positional gap from the most-differentiated upper-part as suggested by the bimodality in mineral composi- tion and in glasses of intermediate-composition pumices. The compositional zonation was obliterated during mingling prior to or during eruption of the intermediate composition magma. It is then interest- ing that the mingling was so efficient as to affect about 70% of the whole erupted volume.
11. Conclusions
Before eruption the CI magma chamber contained two magma layers characterized by different degrees of evolution, and separated by a compositional gap, as suggested by bimodahty in mineral and glass compositions. The upper magma had a more-evolved
homogeneous composition (DI = W-90), while the deeper magma was more basic and zoned (DI = 75% 83). The process of crystal-liquid fractionation was probably responsible for the detected chemical varia- tions. Both magmas had similar Sr isotope composi- tions before feldspar crystallization, as suggested by similarity in 87Sr/ 86Sr ratios of separated feldspars. This support a cogenetic relationship between them. They probably originated from a geochemically simi- lar deeper source. Interaction between fluids and strongly fractionated Sr-poor less-dense magma, af- ter crystallization of feldspars, could account for the isotopic variations displayed by the glasses of the most differentiated pumices.
Most of the CI deposit was emplaced by pyroclas- tic flows erupted in three major pulses that reached various distances and directions from the eruptive vent, and were fed by magmas of three distinct compositions. In Fig. 1 is reported the location of studied sections, using different symbols for the most-, intermediate- and least-evolved products char- actering the sections; location and composition of the products analyzed by Di Girolamo (1970) and Bar- beri et al. (1978) are also reported. All the presented and published data on the CI allow us to speculate on the eruption history, caldera collapse and magma withdrawal dynamics. The eruption began with a central-vent plinian-phase that drained only part of the upper most-evolved magma in the chamber. At this stage there was a change to fissure eruption related to the beginning of a caldera collapse, through activation of discrete fractures probably located to the north and to the south of the present Campi Flegrei, as suggested by the distribution of the most- evolved products (Fig. 1). During caldera collapse the foundering roof acted as a piston and forced the magma layer to move laterally toward the conduit, producing flattened shape (isochrons are defined as the locus of points in two-dimensional space such that magma parcels along a given isochron arrive at the entrance of the volcanic conduit concurrently (Spera, 1984; Spera et al., 1986). Therefore the tapped magma was still the most-evolved upper layer in the chamber. Extrusion of this magma allowed further collapse of the caldera with activation of almost all the ring fractures that intersected the side of the chamber where less-evolved magma resided. Blake and Ivey (1986) and Blake and Campbell
L. Civetta et al. /Journal of Volcanology and Geothermal Research 75 (1997) 183-219 217
(1986) showed that eruptions from the borders of a subsiding caldera block can produce a complex evac- uation leading syneruptive mixing of different com- position magmas. Therefore this intense deforma- tional event could produce such a decompression in the chamber that the least-evolved, deeper magma layer buoyantly rose into the chamber and mingled with the remnant most-evolved, upper layer. The mingling process was so efficient as to affect about 70% of the whole erupted volume. At this stage pyroclastic flows, fed by the mingled magma, poured out from the caldera’s ring fracture and moved across the landscape in all directions. This eruptive phase completely drained the most-evolved magma layer, and only part of the least-evolved layer was left in the chamber. Late collapses along part of the ring fractures generated the last flows, which were fed by the least-evolved magma and spread only over part of the surrounding plain. High expansion and turbu- lence of the flows could likely be due to efficient interaction of the magma with a large amount of seawater flowing into the plumbing system during caldera collapse.
Acknowledgements
The authors are grateful to J. Luhr and J. Wolff for reviewing the manuscript.
M. Serracino and Piero Bottazzi are acknowl- edged for kind assistance during electron and ion microprobe analyses, respectively. The research was carried out with the support of the Italian National Group for Volcanology.
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