Experimental constraints on the differentiation process and pre-eruptive conditions in the magmatic system of Phlegraean Fields (Naples, Italy)

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Journal of Volcanology and Geothermal Research 171 (2008) 88 – 102 www.elsevier.com/locate/jvolgeores

Experimental constraints on the differentiation process and pre-eruptive conditions in the magmatic system of Phlegraean Fields (Naples, Italy) Alessandro Fabbrizio a,⁎, Michael R. Carroll b a

Institute for Mineralogy and Petrology, Clausiusstrasse 25, NW D77.2, ETH Zurich, CH 8092, Switzerland Dipartimento di Scienze della Terra, Via Gentile III da Varano, Università di Camerino, 62032 MC, Italy

b

Received 4 May 2007; accepted 4 November 2007 Available online 17 November 2007

Abstract Phase relations of two samples of the Breccia Museo Eruption (BME), BME is an explosive event that took place (about 20 ky ago) during the caldera-forming phase of the Ignimbrite Campana eruption, have been determined experimentally as a function of temperature (700 to 885 °C), pressure (50 to 200 MPa) and water content of the melt. The crystallization experiments were carried out at fO2 = NNO + 1 log unit. Melt water content ranged from 3.4 to 8 wt.% (H2O saturation). The synthetic products are compared to the natural phases to constrain the pre-eruptive conditions of trachytic magma in the presence of an H2O-rich fluid. The major phases occurring in the BME have been reproduced. The stability of biotite is favoured at pressures higher than 135 MPa. Phase equilibria at 200 MPa reproduce the phase assemblage of the magma only at temperatures below 775 °C. Phase abundances and melt fractions indicate that the eruption tapped a magma body that was at a temperature of 780 °C and a pressure in the range 200–140 MPa. The observed major element variations are fully consistent with a fractional crystallization of a sanidine-dominated assemblage starting from the least differentiated trachytes. The compositions of the experimental products are compatible with the progressive tapping of a shallow magma chamber that was chemically zoned. These results suggest that after an early eruptive phase during which the upper, most differentiated level of the magma chamber was tapped, the sudden collapse of the roof of the reservoir triggered drainage of the less evolved remaining magma. © 2007 Elsevier B.V. All rights reserved. Keywords: fractional crystallization; phase relations; Phlegraean Fields; magma chamber; trachyte

1. Introduction 1.1. State of the art While there exist various studies of phase relations for rhyolitic and granitic compositions (e.g. Tuttle and Bowen, 1958; Holtz and Johannes, 1991; Scaillet et al., 1995) and for common calcalkaline magma compositions (andesite and dacite) (e.g. Rutherford et al., 1985; Cottrell et al., 1999; Scaillet and Evans, 1999; Hammer et al., 2002), there is little information on the phase relations and activities of volatiles in phonolitic–trachytic magma and the effect of these volatiles on mineral stabilities and phase ⁎ Corresponding author. Tel.: +41 44 63 26 394; fax: +41 44 63 21 636. E-mail addresses: [email protected] (A. Fabbrizio), [email protected] (M.R. Carroll). 0377-0273/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2007.11.002

assemblages and compositions (Berndt et al., 2001; Signorelli and Carroll, 2002; Freise et al., 2003). Despite the potentially explosive nature of phonolitic eruptions (e.g. Vesuvius, 79 AD; Campanian Ignimbrite, 39 ky ago; Neapolitan Yellow Tuff, 12 ky ago; Tenerife recent eruptions: 1563 AD, 1640 AD; Tambora, 1815) there is little experimental work to quantitatively constrain pre-eruptive pressures, temperatures and volatile activities (Harms et al., 2004). Research presented in this article provides new experimental data on the possible pre-eruptive conditions for phono-trachytic magmas in the presence of H2O-rich fluid, which may represent an important end-member of the Phlegraean Fields magmatic system. The experimental results are also of interest for others alkaline systems (e.g. Vesuvius, Tenerife). Special attention is given to the pre-eruptive volatile activities and to the influence of volatiles (mainly H2O) on phase relations (Signorelli and Carroll, 2002). Crystallization conditions of the Phlegraean Fields

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magma body have previously been estimated at ∼100 MPa and 800–980 °C, based on comparing whole-rock compositions with the Nepheline (Ne)-Kalsilite (Ks)-Quartz (Qz) system (Armienti et al., 1983; Melluso et al., 1995), from two-feldspar geothermometry (Melluso et al., 1995; Civetta et al., 1997; Fulignati et al., 2004), and from homogenization temperatures of melt and fluid inclusions in clinopyroxene and K-feldspar (Fulignati et al., 2004). However, many Phlegraean Fields eruptions involve trachytic magmas of variable composition, some showing evidence of magma mixing (Civetta et al., 1997). As a result, P–T estimates based on Ne–Ks–Qz phase relations and two-feldspar geothermometry do not focus on the stability and compositions of mineral phases formed in different parts of the thermally and chemically zoned magma bodies and are therefore sometimes incompatible. For example the temperatures obtained from whole-rock data suggest temperatures of 860–950 °C at 100 MPa of water pressure, by analogy with Ne–Ks–SiO2 diagram, (Armienti et al., 1983; Melluso et al., 1995), whereas coexisting feldspars yield lower temperatures of 800–840 °C (Melluso et al., 1995), and temperatures estimated from melt and fluid inclusions are in the range 820 °C–980 °C (Fulignati et al., 2004). Geochronological, geochemical, and Srisotopic data on volcanics erupted in Phlegraean Fields (Orsi et al., 1995; Civetta et al., 1997; De Vita et al., 1999; Pappalardo et al., 1999) show that the most prominent feature of the Phlegraean Fields magmatic system was the existence of a large trachytic magma chamber, episodically recharged by new batches of magma that mixed with the resident magma and was periodically tapped (Marianelli et al., 2006). The products of each eruption suggest that the magma was chemically and isotopically zoned. The D.I. (normative Or + Ab + Ne) for a given event vary by as much as 75 to 90 (Melluso et al., 1995; Civetta et al., 1997), the normative nepheline range from 0 to 10 wt.% (Melluso et al., 1995), Sr-isotope disequilibria detected between groundmass (0.70723 ± 6) and mineral phases (diopside = 0.70729 ± 5, salite = 0.70718 ± 3, biotite = 0.70729 ± 7) (Vollmer et al., 1981) and Sr-isotope variations in whole-rock samples (0.70733 ± 3–0.70755 ± 3) (Cortini and Hermes, 1981) were interpreted as due to mixing and/or incorporation of megacrysts derived from different magmas (Civetta et al., 1997), bimodality in mineral and glass compositions suggests the presence of a compositional gap inside the same event (Civetta et al., 1997). The observed major- and trace element variations in the eruptive products are fully consistent with a strong fractional crystallization (Armienti et al., 1983; Villemant, 1988) of a sanidinedominated assemblage, based on mass–balance calculations (Stormer and Nicholls, 1978), starting from the least differentiated trachyte (Melluso et al., 1995) or, alternatively, as due to an addition of a trachybasalt or a shoshonitic parental magma to this magma system, it is equally likely that none of the trachytic magmas were the products of fractionation alone, but rather were the product of many cycles of injections, differentiation, assimilation, and mixing (Rutherford, personal communication). Here we present an experimental study that focuses on phase equilibria in a differentiated trachytic magma of the Breccia Museo Eruption that occurred at the end of the Campanian Ignimbrite eruption, about 20 ky ago. Phase equilibrium experi-

89

ments and analyses of mineral and melt compositions provide new experimental and analytical data to explain the observed compositional variations in natural samples. Using a trachytic obsidian named as ZAC (more evolved) and a trachytic pumice indicated as PR38 (less evolved) we were able to determine a range of conditions (T, P) at which these magmas were stored prior to eruption. 1.2. Volcanic history and volcanic risk of the Phlegraean Fields The eruptive history and compositional evolution of the Phlegraean Fields magma have been well documented, and are summarized briefly here (Armienti et al., 1983; Melluso et al., 1995; Orsi et al., 1995; Orsi et al., 1996; Civetta et al., 1997; De Vita et al., 1999; Pappalardo et al., 1999). The compositional spectrum of Phlegraean rocks ranges from trachybasalts to latites, trachytes, alkali trachytes, and peralkaline phonolitic trachytes. Evolved compositions (trachytes) are much more voluminous than primitive compositions (trachybasalt and latites). Chemical and mineralogical data are sufficiently coherent to suggest that fractional crystallization within a shallow magma chamber was the dominant process for the generation of Phlegraean rock series. The volume of the magma chamber was estimated to have been at least 240 km3 at the moment of the eruption of the Campanian Ignimbrite (CI), nearly 39 ky ago (De Vivo et al., 2001). The volume of erupted material, or Dense-Rock Equivalent (DRE) was estimated to be about 150 km3. This eruption was followed by a large caldera collapse (nearly 12 km in diameter). The top of the chamber is probably at a depth of 4–5 km, as suggested by contact metamorphic rocks obtained from deep geothermal wells within the caldera (Armienti et al., 1983). During the caldera-forming phase of the CI eruption, about 20 ky ago, explosive events took place in the SW sector of Phlegraean Fields and produced the Breccia Museo Member (Civetta et al., 1997; Fulignati et al., 2004). The collapse of the CI eruption produced the Phlegraean Fields Caldera (PFC) in which are located the vents for the eruption of the Neapolitan Yellow Tuff (NYT; 12 ky), this event was a trachytic phreatoplinian eruption with a volume N 40 km3, the tuff is the second largest pyroclastic deposit of the Campanian Volcanic Area after the CI. During the NYT eruption, a caldera of about 10 km in diameter collapsed inside the PFC and then was the site of the later volcanism. The volcanic activity in the last 12 ky was concentrated inside the NYT caldera. About 60 eruptions occurred after the NYT eruption, during three epochs of volcanic activity (De Vita et al., 1999). During the first (12–9.5 ky) and the second (8.6– 8.2 ky) epochs, vents were located along the structural boundary of the NYT caldera. During the third epoch (4.8–3.8 ky) the eruptive vents were mostly located in the northern and eastern parts of the NYT caldera. The composition of the erupted magmas ranges from trachytic to alkalitrachytic and most of the eruptions were explosive with phreatomagmatic phases that reflect water–magma interaction. The largest eruptions generated the Astroni tuff ring and the complex Agnano-Monte Spina tephra. This period of volcanism ended with the Senga, Averno, and Astroni eruptions at about 3.8 ky. The most recent eruption occurred in September 1538 A.D., after a quiescence of nearly

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3000 years and formed the Monte Nuovo tuff cone. It is one of the smallest eruptions of the CF and lasted one week. These compositions are the most evolved produced in CF. The erupted products are mostly pyroclastic-surge and -flow beds distributed within 1 km around the vent. The recent bradyseismic episodes in 1969–1972 and 1982–1984 that have generated a net uplift of 3.5 m around the town of Pozzuoli, the widespread fumarolic and thermal springs activity (De Vita et al., 1999) testify that the magmatic system is still active. Considering that the Phlegraean Fields Caldera is inhabited by about 1.5 million people the volcanic risk might be very high. In an area with such characteristics, the definition of pre-eruptive conditions is crucial for understanding the behaviour of the system. Knowledge of past activity, magma storage, variations and data on current state help to understand how the system functions and the prediction of possible future behaviour. 2. Experimental and analytical techniques 2.1. Starting products, preparation of charges The experiments were performed on two natural trachyte samples kindly supplied by Dr. V. Di Matteo of the Department of Geophysic and Volcanology, Naples University “Federico II” Italy. The starting materials comprise an obsidian (ZAC) from the basal unit of the Breccia Museo deposit, Phlegraean Fields Italy (Ricci, 2000), and a pumice (PR38) from the Upper Flow Unit (UPFU) unit of the Breccia Museo (Melluso et al., 1995). They contain up to 30% by volume of crystals and in some case more representing samples of progressively crystallized magma at the chamber walls (Fulignati et al., 2004). Phenocrysts are dominantly alkali-feldspar, with subordinate plagioclase, clinopyroxene, biotite and magnetite in order of decreasing abundance; apatite and titanite occur as accessory phases (Fulignati et al., 2004). Compositions of the experimental starting materials are reported in Table 1. The analyses have been performed by X-ray Table 1 Starting compositions (wt.%) Trachyte a

SiO2 Al2O3 FeO b MgO MnO CaO Na2O K2O TiO2 P2O5 Total c AI d Na/K

PR38

ZAC

60.36 19.09 3.29 1.12 0.15 2.45 3.81 9.09 0.41 0.22 99.23 0.84 0.42

62.18 18.7 3.19 0.23 0.27 1.65 6.16 7.14 0.45 0.02 99.25 0.96 0.86

a Bulk rock analyses from Di Matteo et al. (2004). All analyses are normalized to 100% anhydrous. b Total iron as FeO. c Original totals. d AI = molar (Na2O + K2O)/molar Al2O3.

fluorescence (XRF) at the Department of Earth Sciences of the University of Naples “Federico II” with a Philips PW 1400 following the method described by Melluso et al. (1997). These two compositions have been chosen because they represent different degrees of magma evolution (ZAC more evolved than PR38), thus allowing us to investigate a range of equilibrium conditions and their influence on magma differentiation. These rocks were ground under acetone to 20 μm and used as starting material. Rock powder was loaded into Ag70Pd30 capsules of 15 mm length, 2.2 mm internal diameter and 0.125 mm wall thickness. Distilled and deionized water was first loaded with a microsyringe (1–5 μl, 3–10 wt.%), followed by silicate powder. The amount of distilled water added was sufficient to ensure that each experiment was run at water-saturated conditions (Pwater =Ptotal). The water solubility data of Carroll and Blank (1997) were used to estimate the amount of water needed to achieve H2O saturation. Fluid/silicate ratio [mass of fluid components / (mass of fluid components + mass of silicate mixture)] was maintained low (15–20 wt.%) to reduce the incongruent dissolution of melt components into the fluid phase at high temperatures and pressures (Scaillet et al., 1995). Capsules were weighed after each addition of material and then were crimped and welded shut, using a small oxy-acetylene torch. After welding, the capsules were checked for leaks by heating them in a furnace at 110 °C for 24 h and weighed. 2.2. Experimental equipment Water-saturated experiments were performed in three waterpressurized vertical cold seal pressure vessels (CSPV, Nimonic 105) at the Dipartimento di Scienze della Terra at University of Camerino (Italy). The oxidation condition of this apparatus is ∼0.8 log fO2 units above the Ni–NiO buffer (Di Matteo et al., 2004; Fabbrizio et al., 2006). Experimental temperatures were from 700 to 885 °C with pressures from 50 to 200 MPa; run durations varied between 142 and 449 h depending on temperature, with longer durations at lower T and lower P conditions. Samples were quenched by removing the bomb from the furnace and immersing it in a high-pressure stream of compressed air. The samples cooled to below 500 °C within 3–4 min and to room temperature within 15 min. The quench was isobaric because pressure was maintained during quenching by use of a large volume pressure reservoir and hand operated pressure generator. Temperature was measured in the sample position with a sheathed type K thermocouple placed in a borehole at the base of the autoclave. The temperature was checked against type B Pt–Rh thermocouple calibrated using the melting temperature of Au. Differences between the sample temperature and the external thermocouple were checked using a second thermocouple in the sample position at atmospheric pressure. No significant temperature differences between this internal thermocouple and the external thermocouple (b±5 °C) were observed over the length OD a typical sample capsule (∼2 cm). Pressure was recorded with a pressure transducer or Bourdontube pressure gauges, considered accurate to ±2 MPa. The sample was first pressurized to approximately half of the desired final

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pressure, then it was heated and the increasing pressure was controlled by manually operating on the pressure line. After the quench, capsules were weighed to verify that they had remained sealed during the experiment and then were Table 2 Experimental conditions Charge

Temperature Pressure H2O a (°C)

Time Products b

(MPa)

(wt.%) (h)

ZAC experiments 2A 810 Alfa 840 Beta 870 1F 775 1R 790 1D 800 1S 825 1Q 825 1B 850 6 810 5 815 1L 750 1O 775 1P 800 1T 825 9 840 Gamma 860 2B 700 1G 700 1H 725 1N 745 1W 760 1V 775 1U 800 1 835 2 860

50 50 50 100 100 100 100 100 100 120 120 150 150 150 150 150 150 200 200 200 200 200 200 200 200 200

4.8 3.4 9.5 8.9 9.9 8.8 9.9 8.9 9.1 13.6 5.0 8.3 9.1 9.1 9.4 8.7 9.7 10.0 8.4 9.2 8.8 10.0 10.0 10.0 7.0 10.0

336 192 192 142 288 146 167 189 146 192 432 190 188 188 192 449 192 313 198 198 192 256 262 238 192 192

San, Gl, Cpx, Mt San, Gl, Cpx, Mt Gl, Mt, Cpx San, Gl, Cpx, Mt, (Ap) San, Gl, Cpx, Mt, (Ap) San, Gl, Cpx, Mt, (Ap) Gl, Mt, Cpx Gl, Mt, Cpx Gl, Mt, Cpx Gl, Mt, Cpx, (Ap) Reversal, Bt reabsorbed San, Gl, Cpx, Bt, Mt, (Ap) Gl, San, Cpx, Bt, Mt, (Ap) Gl, Cpx, Bt, Mt, (Ap) Gl, Bt, Cpx, Mt Reversal, Bt reabsorbed Gl, Mt San, Cpx, Bt, Mt, (Ap) San, Cpx, Bt, Mt, (Ap) San, Gl, Cpx, Bt, Mt, (Ap) San, Gl, Cpx, Bt, Mt, (Ap) Gl, San, Cpx, Bt, Mt, (Ap) Gl, Bt, Cpx, Mt, (Ap) Gl, Bt, Cpx, Mt Gl, Mt Gl, Mt

PR38 experiments 38A1 810 38alfa 840 38beta 870 8 885 38M 775 38V 790 38P 800 38D 825 38W 850 38T 870 38N 750 38O 775 38H 800 38B 825 38Z 725 38X 745 38F 760 38E 775 38I 800 1 835 2 860 3 880

50 50 50 50 100 100 100 100 100 100 150 150 150 150 200 200 200 200 200 200 200 200

6.9 9.2 10.0 9.0 9.6 9.9 9.2 10.1 9.9 9.7 9.8 8.7 4.3 10.0 10.1 10.0 9.9 10.8 10.0 12.7 8.9 7.6

335 192 192 192 189 190 190 189 191 185 190 191 215 185 211 196 256 262 215 192 192 192

San, Plg, Bt, Cpx, Mt San, Plg, Bt, Cpx, Mt San, Plg, Bt, Cpx, Mt Gl, Cpx, Mt San, Plg, Bt, Cpx, Mt San, Plg, Bt, Cpx, Mt San, Plg, Bt, Cpx, Mt San, Plg, Bt, Cpx, Mt Gl, San, Bt, Cpx, Mt Gl, San, Bt, Cpx, Mt San, Plg, Bt, Cpx, Mt San, Plg, Bt, Cpx, Mt San, Gl, Plg, Bt, Cpx, Mt San, Gl, Bt, Cpx, Mt San, Plg, Bt, Cpx, Mt San, Plg, Bt, Cpx, Mt San, Plg, Gl, Bt, Cpx, Mt San, Gl, Plg, Bt, Cpx, Mt San, Gl, Bt, Cpx, Mt Gl, Bt, Cpx, Mt Gl, Bt, Cpx, Mt Gl, Bt, Cpx, Mt

a

H2O loaded in the capsule. Phases listed in decreasing abundance, see Table 3 for estimated modal abundances in selected experiments. Phases in parentheses are trace. Gl, Glass; San, Sanidine; Plg, Plagioclase; Bt, Biotite; Cpx, Clinopyroxene; Mt, Magnetite; Ap, Apatite. b

Fig. 1. Back-scattered images of selected experimental runs. (a) Run ZAC 1O (775 °C, 150 MPa, 9.1 wt.% H2O) contains ∼56 wt.% glass (Gl) and crystals of sanidine (San), clinopyroxene (Cpx), probably very small biotite crystals (Bt?), and titanomagnetite (Mt). (b) Run PR38H (800 °C, 150 MPa, 4.3 wt.% H2O) contains ∼ 9 wt.% glass, plus sanidine, plagioclase (Plg), clinopyroxene, biotite, and titanomagnetite. It should be noted that crystals are not zoned and are euhedral.

opened. Charges indicating H2O loss during the experiment (weight change) were discarded. Experimental conditions for each sample are reported in Table 2. 2.3. Analytical techniques Run products were characterized by reflected light microscopy, scanning electron microscopy (SEM) and electron microprobe methods. Major element analyses of glasses and minerals were carried out with a Cameca SX 50 microprobe at CNR-Istituto di Geologia Ambientale e Geoingegneria-Roma (Italy). For analysis of oxides, the standards used were: albite for Na, forsterite for Mg, Al2O3 for Al, CaSiO3 for Si and Ca, orthoclase for K, SrTiO3 for Ti, MnO for Mn, fayalite for Fe. For analysis of biotite, alkali-feldspar, plagioclase, clinopyroxene,

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crystalline phases present, checking both rim and core compositions to look for any zoning where crystal size permitted. Analyses were obtained for multiple grains from different locations within the charge. Within error no significant compositional variations were observed within single experiments. 3. Experimental results 3.1. General observations

Fig. 2. Phase diagram of ZAC sample shown as a function of experimental temperature and pressure. ■, Crystallization experiments. ▴, Reversal experiments. Mt: magnetite, cpx: clinopyroxene, bt: biotite, san: sanidine. The outlined area represents the estimated conditions in which the magma was stored prior to eruption.

and glass the standards used were: albite for Na, diopside for Ca, Mg and Si, spinel (MgAl2O4) for Al, orthoclase for K, SrTiO3 for Ti, MnO for Mn, fayalite for Fe. Analytical conditions for the electron microprobe were: accelerating voltage 15 kv, beam current 10 nA, counting time of 15 s on peak for Mg, Al, Ca, Ti, Mn, Fe, of 30 s for F and Cl, and of 8 s for Na, K, and Si (the last three always analyzed first). Analyses used a focused beam (diameter 1–2 μm) for minerals and defocused beam for glasses (10 μm), in order to minimize the Na migration. For each experiment we attempted to obtain multiple analyses of the

Fig. 3. Phase diagram of PR38 sample shown as a function of experimental temperature. ■, Crystallization experiments. Mt: magnetite, cpx: clinopyroxene, bt: biotite, san: sanidine, plag: plagioclase. The magnetite, clinopyroxene, and biotite field boundaries are drawn dashed because their first appearance is outside the temperature range investigated. The outlined area represents the estimated conditions in which the magma was stored before the differentiation towards more evolved compositions.

The hydrothermal experiments, run at P(H2O)'s of 50 to 200 MPa, and temperatures of 700 to 880 °C, were successful in that they produced a phenocryst assemblage similar to that which occurs in the natural Phlegraean Field trachytes. These phenocrysts are in apparent textural equilibrium with melt (glass). The phenocryst abundance ranged from a few volume% to essentially 100% in a few experiments that were interpreted to be subsolidus. The presence of fluid inclusions (bubbles) trapped in glass was observed and taken as evidence of saturation of the melt with respect to a fluid phase under the experimental conditions. In addition to the presence of bubbles, the presence of a fluid phase was also confirmed by small drops of water released from the capsules on opening, and water loss on opening and heating at 110 °C shows presence of a fluid phase (weight change). Backscattered images of the charges show that minerals are unzoned and euhedral (Fig. 1). Table 3 Natural and selected experimental modes Sample

°C/MPa a

Σr2

Phase proportions by mass balance b Gl

San

Plg

Bt

Cpx

Mt

Obsidian ZAC c 2A Alfa Beta 1R 1S 1L 1O 1P 1T 1H 1N 1W 1V 2

810/50 840/50 870/50 790/100 825/100 750/150 775/150 800/150 825/150 725/200 745/200 760/200 775/200 860/200

63.8 18 25 98 29 96 22 56 95 94 25 22 60 94 98

26.8 76 69 – 64 – 66 38 – – 65 69 32 – –

– – – – – – – – – – – – – – –

2 – – – – – 2 2 2 4 2 2 2 2 –

4.7 3 2 0.5 4 3 3 2 2 1 5 3 3 1 –

1.1 3 4 1.5 3 1 2 2 1 1 3 4 3 3 2

0.19 0.23 0.68 0.82 1.64 0.61 0.93 0.93 0.86 0.19 0.85 0.07 0.07 0.54

Pumice PR38 c 38W 38H 38B 38F 38E 38I 3

850/100 800/150 825/150 760/200 775/200 800/200 880/200

36.6 58 9 36 9 12 34 97

50 30 67 47 66 70 49 –

5.7 – 8 – 10 7 – –

4.1 6 7 8 7 4 8 1

2.1 3 5 7 5 5 7 1

1.5 3 3 1 3 2 1 1

0.07 0.09 0.18 0.31 0.57 0.59 0.27

a

Run conditions. Phase proportions calculated using a constrained least-square mass balance with propagation of analytical errors of the experimental phases (Albarede, 1995). c Modal abundances in natural rocks calculated from data of Civetta et al. (1997). b

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Table 4 Selected experimental glass compositions normalized to 100% anhydrous Sample

ZAC 2A (4) a

Alfa (3)

Beta (4)

1R (4)

1S (5)

1L (2)

1O (5)

1P (7)

1T (5)

61.44 (63) 0.34 (8) 18.84 (54) 0.70 1.12 (48) 0.22 (7) 0.22 (13) 1.57 (39) 7.73 (42) 6.68 (35) 0.66 (10) 0.49 (6) 94.62 (82)

61.01 (42) 0.38 (2) 18.44 (16) 0.96 1.46 (6) 0.22 (1) 0.19 (0) 1.38 (15) 8.40 (22) 6.47 (20) 0.41 (10) 0.67 (1) 95.96 (90)

63.14 (40) 0.20 (3) 20.17 (12) 0.79 1.61 (9) 0.33 (7) 0.10 (3) 1.18 (6) 5.18 (11) 5.49 (12) 1.00 (17) 0.78 (4) 92.99 (78)

62.21 (53) 0.29 (6) 19.02 (16) 0.67 1.02 (22) 0.19 (6) 0.13 (1) 1.28 (3) 7.28 (42) 6.88 (18) 0.39 (13) 0.45 (5) 94.38 (64)

62.28 (22) 0.09 (0) 19.11 (3) 0.28 0.35 (8) n.a. n.a. 0.95 (1) 9.91 (83) 6.76 (138) 0.14 (0) 0.01 (0) 98.68 (23)

63.04 (58) 0.16 (2) 19.79 (11) 0.70 1.24 (11) 0.18 (6) 0.07 (1) 1.14 (15) 6.81 (13) 5.75 (12) 0.61 (15) 0.47 (3) 90.85 (82)

61.69 (26) 0.22 (4) 18.88 (13) 0.77 1.15 (29) 0.23 (9) 0.11 (7) 1.47 (31) 7.89 (37) 6.66 (18) 0.39 (16) 0.47 (3) 94.55 (75)

61.48 (22) 0.36 (4) 18.61 (11) 0.80 1.20 (7) 0.30 (8) 0.15 (1) 1.75 (6) 7.75 (16) 6.64 (11) 0.44 (10) 0.44 (5) 94.30 (23)

1H (5)

1N (5)

1W (5)

1V (5)

2 (4)

38W (5)

38H (4)

38B (4)

3 (4)

SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O F Cl Total

61.07 (26) 0.43 (49) 20.28 (14) 0.66 1.07 (6) 0.18 (5) 0.04 (1) 0.78 (11) 8.79 (20) 4.93 (15) 1.35 (15) 0.28 (2) 91.71 (51)

62.34 (65) 0.11 (2) 19.91 (14) 0.66 1.08 (12) 0.21 (5) 0.06 (1) 0.92 (5) 8.00 (18) 5.57 (8) 0.92 (8) 0.27 (3) 91.03 (67)

63.89 (7) 0.12 (1) 19.23 (14) 0.54 0.92 (7) 0.18 (6) 0.08 (2) 0.95 (14) 6.89 (20) 6.14 (37) 0.80 (11) 0.24 (3) 92.81 (15)

63.39 (47) 0.17 (3) 19.42 (9) 0.50 0.83 (6) 0.21 (5) 0.08 (1) 1.05 (4) 7.84 (42) 5.74 (26) 0.35 (12) 0.28 (4) 92.22 (52)

59.43 (50) 0.35 (4) 17.77 (18) 0.91 1.08 (7) 0.18 (4) 0.23 (2) 1.71 (3) 11.02 (25) 6.64 (8) 0.32 (4) 0.32 (1) 96.15 (103)

63.39 (47) 0.30 (2) 18.22 (17) 0.66 1.09 (5) 0.16 (4) 0.15 (1) 1.63 (6) 6.28 (33) 7.39 (16) 0.29 (2) 0.43 (3) 95.07 (100)

65.10 (38) 0.18 (2) 18.21 (8) 0.53 0.93 (8) 0.17 (4) 0.12 (2) 1.38 (4) 6.09 (8) 6.50 (6) 0.39 (9) 0.38 (3) 93.26 (42)

64.51 (35) 0.24 (3) 18.83 (14) 0.58 1.08 (12) 0.15 (5) 0.18 (5) 1.74 (8) 5.45 (8) 6.70 (19) 0.19 (7) 0.34 (3) 93.41 (78)

59.34 (45) 0.30 (2) 17.83 (21) 0.79 0.95 (8) 0.05 (3) 0.31 (1) 2.62 (3) 9.12 (5) 8.16 (7) 0.23 (2) 0.26 (3) 96.70 (82)

Sample

PR38 38F (5)

38E (5)

38I (5)

68.56 (181) 0.08 (2) 16.83 (43) 0.43 0.79 (5) 0.12 (3) 0.18 (22) 1.09 (4) 6.49 (17) 4.84 (131) 0.14 (58) 0.03 (7) 92.33 (28)

66.60 (26) 0.09 (3) 17.56 (8) 0.42 0.72 (5) 0.13 (4) 0.08 (1) 1.45 (44) 6.58 (5) 5.70 (17) 0.58 (12) 0.07 (1) 92.33 (28)

63.91 (21) 0.17 (2) 19.01 (13) 0.53 0.93 (5) 0.17 (7) 0.12 (2) 1.71 (5) 6.35 (8) 6.49 (10) 0.34 (12) 0.27 (1) 93.83 (29)

b

SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O F Cl Total

59.38 (73) 0.21 (10) 19.42 (109) 0.89 c 1.25 (84) d 0.27 (19) 0.12 (7) 1.39 (24) 9.09 (91) 6.39 (80) 1.02 (69) 0.57 (43) 94.54 (53)

Sample

ZAC

SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O F Cl Total

PR38

n.a. Not analyzed. a Numbers in parentheses in the second row give the number of analyses. b Numbers in parentheses in oxide columns are the standard deviation * 100. c Fe2O3 wt.% is calculated by the equation of Kress and Carmichael (1991). d The standard deviation value is referred to total FeO all Fe2+.

Crystalline phases identified in run products, in order of decreasing abundance, were sanidine, plagioclase, biotite, clinopyroxene, magnetite, and apatite (very rarely). To investigate the approach to equilibrium, two reversal experiments were conducted (Fig. 2). The first (ZAC 9, Table 2) was conducted by first holding the sample at 820 °C for 224 h and then

raising the temperature to 840 °C for another 224 h. The second reversal (ZAC 5, Table 2) involved holding the sample at 150 MPa and 815 °C for 216 h and then lowering the pressure to 120 MPa at constant temperature for another 216 h. The results obtained from these experiments are that biotite was reabsorbed, in agreement with phase relations indicated by crystallization experiments.

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Fig. 4. (a) Variations in SiO2 concentration in glass for ZAC experiments (recalculated to 100 wt.% anhydrous) with change in temperature. The open white circle shows the SiO2 content of the starting material. The dashed horizontal lines show the maximum and minimum SiO2 content in natural glasses of the BME eruption (data from Melluso et al., 1995; Fulignati et al., 2004). The black lines show the first appearance of sanidine at different experimental pressures. The vertical bars show the standard deviations (1σ). ●, 50 MPa; ■, 100 MPa; ♦,150 MPa; ▴, 200 MPa. In all cases the appearance of sanidine leads to a drop in melt SiO2 content. (b) Variations in Al2O3 concentration in glass for ZAC experiments (recalculated to 100 wt.% anhydrous) with change in temperature. The open white circle shows the Al2O3 content of the starting material. The dashed horizontal lines show the maximum and minimum Al2O3 content in natural glasses of the BME eruption (data from Melluso et al., 1995; Fulignati et al., 2004). The black lines show the first appearance of sanidine at different experimental pressures. The vertical bars show the standard deviations (1σ). ●, 50 MPa; ■, 100 MPa; ♦, 150 MPa; ▴, 200 MPa. (c) Variations in K2O concentration in glass for ZAC experiments (recalculated to 100 wt.% anhydrous) with change in temperature. The open white circle shows the K2O content of the starting material. The dashed horizontal lines show the maximum and minimum K2O content in natural glasses of the BME eruption (data from Melluso et al., 1995; Fulignati et al., 2004). The black lines show the first appearance of sanidine at different experimental pressures. The vertical bars show the standard deviations (1σ). ●, 50 MPa; ■, 100 MPa; ♦, 150 MPa; ▴, 200 MPa. (d) Variations in CaO concentration in glass for ZAC experiments (recalculated to 100 wt.% anhydrous) with change in temperature. The open white circle shows the CaO content of the starting material. The dashed horizontal lines show the maximum and minimum CaO content in natural glasses of the BME eruption (data from Melluso et al., 1995; Fulignati et al., 2004). The vertical bars show the standard deviations (1σ). ●, 50 MPa; ■, 100 MPa; ♦, 150 MPa; ▴, 200 MPa.

3.2. Phase relations The experimental results for each sample are reported in Table 2. The phase relations for the ZAC (Fig. 2) and PR38 (Fig. 3) compositions are represented in terms of the pressure–temperature stability fields of observed minerals. Phase boundaries (Figs. 2 and 3) are drawn based on crystallization experiments, in which fO2 was near NNO + 1. Each phase boundary marks the beginning

of crystallization of a given phase in equilibrium with melt (‘phase-in’ or saturation boundary). Each boundary is, ideally, bracketed by charges showing the presence or absence of the respective phase. It is important to evidence that run durations are long (1 or 2 weeks, see Table 2), in many cases we have relatively large melt/phenocryst ratios, and we use a natural starting material that (for the PR38 starting material at least) contains all of the phenocryst phases. All these arguments should facilitate achieving

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95

Table 5 Selected microprobe analyses of experimental K-feldspar and plagioclase Sample

ZAC 2A San (5)

Alfa a

1R

1L

1O

1H

1N

1W

San (4)

San (5)

San (5)

San (5)

San (4)

San (5)

San (5)

64.29 (83) 19.02 (33) 0.43 (17) 0.04 (5) 0.00 (0) 0.87 (18) 5.76 (36) 7.44 (58) 98.02 (62) 4.3 51.7 44.0

63.77 (12) 19.77 (51) 0.43 (27) 0.11 (9) 0.03 (3) 0.99 (26) 5.38 (18) 8.05 (15) 98.78 (42) 4.9 47.9 47.2

64.89 (10) 19.64 (44) 0.54 (34) 0.13 (0) 0.08 (8) 0.74 (27) 5.63 (92) 7.78 (16) 99.47 (13) 3.7 50.4 45.9

65.51 (40) 19.61 (14) 0.23 (7) 0.00 (0) 0.01 (0) 0.65 (8) 4.01 (43) 9.94 (76) 100.07 (40) 3.3 36.7 59.9

64.91 (75) 19.04 (22) 0.23 (8) 0.11 (0) 0.02 (0) 0.36 (13) 5.42 (41) 8.53 (69) 98.67 (99) 1.8 48.2 50.0

66.07 (65) 19.87 (43) 0.26 (4) 0.08 (0) 0.01 (0) 0.82 (29) 4.76 (33) 7.84 (15) 99.75 (41) 4.4 45.9 49.7

65.62 (25) 19.58 (36) 0.43 (7) 0.08 (0) 0.01 (0) 0.66 (17) 4.04 (40) 9.65 (75) 100.11 (42) 3.4 37.5 59.0

38W

38H

38B

38F

38E

38I

38O

38Z

San (4)

San (5)

San (4)

San (6)

San (5)

San (4)

San (7)

San (4)

SiO2 Al2O3 FeO MnO MgO CaO Na2O K2O Total An Ab Or

65.44 (26) 19.24 (19) 0.32 (7) 0.01 (1) 0.00 (0) 0.77 (15) 2.51 (16) 12.18 (43) 100.56 (22) 3.9 22.9 73.2

63.58 (26) 19.49 (20) 0.19 (6) 0.00 (0) 0.02 (0) 1.01 (34) 3.06 (42) 11.75 (74) 99.28 (35) 4.9 26.9 68.1

63.82 (81) 18.83 (9) 0.26 (5) 0.03 (3) 0.01 (1) 0.91 (21) 1.94 (32) 12.78 (24) 98.68 (59) 4.7 17.9 77.5

62.78 (51) 19.08 (15) 0.43 (22) 0.06 (1) 0.06 (7) 0.66 (17) 2.69 (94) 12.00 (88) 98.26 (13) 3.3 24.5 72.2

62.53 (70) 18.96 (45) 0.42 (23) 0.07 (2) 0.16 (11) 0.79 (14) 3.01 (34) 11.63 (36) 97.79 (12) 3.9 27.1 68.9

64.30 (16) 19.12 (21) 0.49 (41) 0.00 (0) 0.10 (21) 0.56 (27) 2.08 (27) 12.92 (51) 99.75 (47) 2.9 19.1 78.0

63.96 (31) 19.70 (60) 0.60 (71) 0.02 (4) 0.08 (17) 1.16 (65) 4.10 (88) 9.18 (46) 98.95 (21) 38.0 5.9 56.0

63.65 (55) 18.88 (20) 0.22 (4) 0.01 (2) 0.00 (0) 0.51 (8) 1.75 (36) 13.16 (68) 98.27 (93) 16.3 2.6 81.0

Sample

PR38 38H

38F

38alfa

38D

38E

Plg (5)

Plg (5)

Plg (4)

Plg (3)

Plg (4)

47.18 (34) 32.59 (51) 0.65 (8) 0.09 (3) 0.04 (1) 15.62 (89) 2.20 (10) 0.42 (20) 98.89 (39) 77.2 20.0 2.8

47.34 (30) 32.30 (29) 0.67 (5) 0.12 (6) 0.04 (1) 15.55 (63) 2.23 (89) 0.47 (20) 98.80 (47) 77.7 19.8 2.5

48.68 (61) 30.37 (19) 0.63 (6) 0.02 (3) 0.04 (2) 13.97 (39) 3.11 (79) 0.54 (11) 97.48 (14) 69.0 27.8 3.2

45.77 (78) 33.01 (55) 0.76 (1) 0.00 (0) 0.05 (1) 17.54 (70) 1.31 (32) 0.24 (5) 98.76 (24) 86.9 11.7 1.4

46.33 (84) 32.48 (51) 0.72 (9) 0.01 (2) 0.04 (2) 16.43 (68) 1.93 (82) 0.36 (27) 98–30 (76) 81.0 17.0 2.0

SiO2 Al2O3 FeO c MnO MgO CaO Na2O K2O Total An Ab Or

64.48 (60) b 19.72 (40) 0.51 (20) 0.06 (9) 0.01 (2) 1.13 (25) 5.60 (57) 7.67 (97) 99.45 (41) 5.6 49.7 44.8

Sample

PR38

SiO2 Al2O3 FeO MnO MgO CaO Na2O K2O Total An Ab Or a b c

Numbers in parentheses in the second row give the number of analyses. Numbers in parentheses in oxide columns are the standard deviation * 100. Total iron as Fe2+.

phase equilibrium. The solidus for the runs carried out with the PR38 composition (Fig. 3) is bracketed using crystallization experiments yielding a small amount of liquid. 3.2.1. Phase relations, proportions, and composition of ZAC composition Essential features of the phase relations for the ZAC composition are as follows (Fig. 2): magnetite is the liquidus phase,

crystallizing above 870 °C and 860 °C for pressures of 50 and 200 MPa. It is followed by clinopyroxene near 870 °C and 810 °C for pressures of 50 MPa and 200 MPa. Biotite is stable only at pressures above 140 MPa, and it appears at 820 °C to 825 °C for pressures of 150 to 200 MPa. The first appearance of alkalifeldspar is around 840 °C at 50 MPa and 760 °C at 200 MPa. Given the scarce occurrence of apatite no attempt was made to draw its stability field; it was identified occasionally during the

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microprobe analyses in some experimental runs performed below 820 °C. In all of these runs, with the exception of the experiment carried out at 700 °C and 200 MPa, residual liquid (glass) was observed. 3.2.2. Crystallinity and phase proportions At a given pressure the amount of crystals increases with decreasing temperature (Table 3). At 870 °C, 50 MPa; and at 860 °C, 200 MPa the obsidian is close to its liquidus with ∼ 2 wt.% of crystals; the highest crystal content (∼ 82 wt.%) was obtained at 810 °C and 50 MPa. At 760 °C and 200 MPa the crystallinity of the charge is ∼ 40 wt.%, which is similar to the crystallinity of the trachytic lava (∼ 35 wt.%). Mass–balance calculations, based on a constrained least-squares mass balance (Albarede, 1995), show that sanidine is the most abundant mineral. Its proportion increases with decreasing temperature. For a given pressure the appearance of sanidine increases fastly the crystallinity. It should be noted that except at 760 °C and 200 Mpa, and at 775 °C and 150 MPa the experimental crystallinity is significantly higher than those of the lava, which suggests that the pre-eruptive temperature was probably not much lower than 780 °C. 3.2.3. Phase compositions Experimental glass compositions vary mainly according to temperature (Table 4; Fig. 4a; b; c; d), and range from phonolitic (SiO2 59.43 wt.%; Na2O + K2O 17.66 wt.%) at 860 °C and 200 MPa, to trachytic (SiO2 63.89 wt.%; Na2O + K2O 13.03) at 760 °C and 200 MPa. The charges with the glass composition that best matches that of the natural glass are those at temperature above 800 °C (Fig. 4a; b; c; d), unfortunately in this condition we are not able to reproduce the natural mineralogical assemblage. Variations in melt composition are already linked with the types and abundances of the various minerals crystallized. Regarding melt SiO2 content, the crystallization of oxides and Fe–Mg minerals (Bt, Cpx) produces an increase of silica in glass as shown in Fig. 4a, and the crystallization of alkali-feldspars causes a depletion of silica in the residual liquid (e.g. compare Figs. 2 and 4a). The effect of alkali-feldspar crystallization is also shown by variations in melt alumina content; Al2O3 content increases until the first appearance of alkali-feldspar (Fig. 4b) and then decreases. Melt K2O content is nearly constant until the appearance of alkali-feldspar, after which it decreases, or increases slightly, depending on experimental pressure (Fig. 4c). These K2O values are low compared with natural compositions suggesting that K2O was lost to the excess fluid during the experiments. In fact (Table 2) H2O content is ∼ 10 wt.% and Di Matteo et al. (2004) demonstrated that the maximum H2O content in this trachyte is ∼ 7.5 wt.% at 200 MPa. The effect caused by the crystallization of oxides, clinopyroxene, and biotite is shown in Fig. 4d for calcium and in Table 4 for iron, with a continuous depletion of these components in the residual liquid. Sanidine compositions (Table 5; Fig. 5) range from Or60 at 775 °C and 150 MPa to Or44 at 840 °C and 50 MPa. This variation reproduces only the more evolved natural sanidine (natural range: Or61–88). Clinopyroxene compositions (Table 6; Fig. 6) range from

Fig. 5. Compositions of experimental and natural feldspars in mol%. Analyses of natural samples were taken from Melluso et al. (1995) and from Fulignati et al. (2004). ○, natural feldspars; □, ZAC experimental feldspars; +, PR38 experimental feldspars.

Wo35En5Fs60 (825 °C, 100 MPa) to Wo48En38Fs14 (825 °C, 150 MPa) with ∼2–7 wt.% Al2O3. These compositions are similar only at the more salitic clinopyroxene (Wo47–46En44–35Fs9–19, 2–5 wt.% Al2O3) found in the trachyte. Analysis of the experimental biotites was difficult because of their small size and good analyses could only be obtained in runs with relatively big biotite crystals. In most runs, biotite could only be identified by back-scattered electron microscopy. Only analyses that are not affected by glass contamination are given in Table 7. It was found only in the experiments at the higher pressures (150 and 200 MPa). It has a mg-number of 47–54 and 3–3.5 wt.% TiO2, whereas natural biotite has higher mg-number ∼67 and up to ∼ 5 wt.% TiO2. Fluorine was measured during microprobe analysis, and is found to fill b 20% of the OH− site. Cl content of the experimental biotites is negligible, and the hydroxil site not occupied by F are assumed to be filled with OH − . In all experiments Fe–Ti oxides crystallized like titanomagnetite. Quantitative analyses of the Fe–Ti oxides were difficult to obtain because of the small crystal size of ≤5 μm in most of the experimental runs. The analyses given in Table 8 show high amounts of Si which are the result of contamination of the surrounding glass. Titanomagnetites have ulvöspinel contents that range from 1 to 27. Content of Al2O3 is lower with those reported in the Fe–Ti oxide mineral of the natural trachyte, whereas MnO and MgO are comparable. 3.2.4. Phase relations, proportions, and composition of PR38 composition For the PR38 composition (Fig. 3); magnetite, clinopyroxene, and biotite are all near-liquidus phases, being observed in small amounts (2–5%) in all of the runs at the highest temperatures investigated (870 °C, 50 MPa; 880 °C, 200 MPa). Compared with the ZAC composition, the stability field of alkali-feldspar extends to temperatures ∼ 50 °C higher; it appears at a pressure of 50 MPa near 870 °C and at a pressure of 200 MPa above 800 °C. The last phase that crystallizes close the solidus is the plagioclase, and it is stable below 880 °C and 770 °C for pressures

A. Fabbrizio, M.R. Carroll / Journal of Volcanology and Geothermal Research 171 (2008) 88–102

of 50 and 200 MPa, respectively. Because of the high content of crystals, residual glass suitable for analysis was not found in many experimental charges, and the solidus is estimated at: 870 °C, 50 MPa and 755 °C, 200 MPa. The main differences between the two compositions are that in composition PR38 the stability fields of all minerals are increased by ∼ 50 °C, biotite is stable also at lower pressures (below 150 MPa), there is the crystallization of plagioclase,

97

and the quantity of residual liquid at a given pressure and temperature is much lower in PR38, reflecting his less evolved bulk composition. 3.2.5. Crystallinity and phase proportions The crystallinity and phase proportions of PR38-bearing experiments (Table 3) are appreciably different from those for ZAC experiments. At 760–775 °C and 200 MPa, and at

Table 6 Selected microprobe analyses of experimental clinopyroxene Sample

ZAC 2A (2) a

Alfa (3)

Beta (3)

1R (3)

1S (3)

1L (2)

1O (5)

1P (4)

1T (3)

49.12 (98) 0.68 (7) 2.71 (20) 9.52 (53) 0.81 (10) 11.72 (72) 22.12 (31) 0.80 (11) 0.08 (6) 97.64 (10) 47.7 35.1 17.2

45.78 (70) 1.86 (48) 4.99 (47) 12.42 (30) 1.13 (4) 8.74 (28) 21.30 (17) 1.00 (7) 0.10 (5) 97.42 (11) 48.5 27.7 23.8

46.48 (46) 0.90 (7) 3.26 (8) 16.73 (34) 1.98 (7) 5.85 (23) 20.72 (77) 1.04 (5) 0.18 (4) 97.61 (21) 47.7 18.7 33.6

43.71 (82) 2.14 (33) 7.10 (51) 7.86 (18) 21.07 (11) 1.28 (5) 13.18 (99) 1.28 (21) 0.25 (25) 97.97 (74) 34.9 4.7 60.4

48.70 (47) 0.66 (31) 2.26 (10) 15.26 (67) 2.20 (18) 7.47 (76) 21.70 (77) 0.85 (37) 0.05 (3) 99.36 (6) 39.8 19.0 41.2

45.20 (82) 1.71 (55) 4.74 (66) 19.99 (18) 1.86 (15) 3.54 (51) 21.02 (8) 0.88 (6) 0.11 (5) 99.12 (14) 48.9 11.5 39.7

46.53 (53) 1.22 (40) 4.52 (56) 15.86 (12) 1.62 (9) 6.15 (25) 20.73 (65) 0.97 (8) 0.30 (29) 97.99 (93) 48.3 19.9 31.8

50.34 (34) 0.35 (4) 2.67 (9) 8.20 (22) 0.42 (0) 12.93 (10) 22.84 (61) 0.49 (9) 0.02 (1) 98.36 (12) 48.0 37.8 14.2

1H (5)

1N (4)

1W (5)

1V (4)

38W (4)

38H (5)

38B (4)

38F (4)

38E (5)

SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O Total Wo En Fs

47.29 (61) 0.72 (29) 4.36 (22) 16.71 (26) 1.74 (22) 5.12 (27) 19.42 (29) 1.20 (71) 0.71 (15) 97.37 (51) 47.4 17.4 35.2

50.18 (21) 0.60 (9) 2.78 (77) 9.45 (81) 0.78 (38) 12.11 (72) 22.47 (17) 0.54 (17) 0.03 (2) 98.98 (18) 47.5 35.6 16.9

46.81 (24) 1.29 (77) 5.09 (23) 12.73 (39) 1.56 (22) 9.15 (11) 21.54 (10) 1.10 (32) 0.18 (17) 99.51 (92) 47.4 28.0 24.6

47.59 (27) 0.90 (45) 4.07 (40) 11.89 (20) 1.62 (23) 9.57 (10) 22.08 (44) 1.06 (17) 0.07 (2) 98.98 (82) 48.0 29.0 23.0

48.78 (79) 0.63 (10) 6.42 (63) 12.07 (42) 0.63 (29) 8.42 (77) 20.43 (11) 0.43 (11) 0.66 (26) 98.51 (80) 48.6 27.8 23.6

49.26 (21) 0.62 (7) 3.89 (27) 10.58 (41) 0.58 (15) 11.48 (97) 22.61 (9) 0.41 (5) 0.02 (1) 99.56 (62) 47.8 33.8 18.4

44.79 (23) 0.98 (16) 6.48 (96) 16.15 (63) 0.60 (13) 6.46 (15) 22.56 (19) 0.40 (3) 0.05 (2) 98.51 (29) 50.6 20.1 29.3

49.52 (97) 0.56 (31) 3.80 (51) 8.26 (79) 0.43 (37) 12.01 (47) 22.80 (49) 0.36 (24) 0.07 (4) 98.01 (58) 49.2 36.1 14.7

45.33 (45) 1.31 (81) 6.94 (6) 11.93 (15) 0.53 (12) 9.81 (10) 22.72 (26) 0.61 (26) 0.02 (2) 99.41 (54) 49.3 29.6 21.1

Sample

PR38

SiO2 TiO2 Al2O3 FeO c MnO MgO CaO Na2O K2O Total Wo En Fs

49.61 (23) 0.76 (14) 2.80 (40) 11.64 (31) 1.09 (24) 10.52 (87) 22.41 (45) 0.75 (3) 0.10 (3) 99.72 (39) 47.8 31.2 21.0

Sample

ZAC

SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O Total Wo En Fs a b c

b

PR38

38I (4)

3(2)

46.39 (50) 0.87 (28) 5.15 (50) 15.48 (45) 0.86 (20) 7.15 (37) 22.41 (34) 0.56 (15) 0.03 (2) 98.94 (42) 49.7 22.1 28.3

46.91 (4) 0.80 (40) 5.54 (91) 10.28 (55) 0.36 (8) 11.66 (66) 23.30 (5) 0.49 (18) 0.03 (4) 99.49 (21) 49 34 17

Numbers in parentheses in the second row give the number of analyses. Numbers in parentheses in oxide columns are the standard deviation * 100. Total iron as Fe2+.

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Fig. 6. Compositions of experimental and natural clinopyroxenes in the ternary Wo–En–Fs diagram. Analyses of natural samples were taken from Melluso et al. (1995) and from Fulignati et al. (2004). ○, natural clinopyroxenes; □, ZAC experimental clinopyroxenes; +, PR38 experimental clinopyroxenes.

800–825 °C and 150 MPa the runs performed with the PR38 composition have much less glass than the ZAC experiments, which is mainly due to higher sanidine proportions and to plagioclase crystallization in PR38-bearing experiments. We do not reproduce exactly the natural modal abundances of phases because when plagioclase crystallize the charges have ∼ 25 wt. % less glass than the natural situation. 3.2.6. Phase compositions Experimental glass compositions change with temperature (Fig. 7a; b; c; d) and range from phonolitic at 880 °C and 200 MPa (SiO2 59.34 wt.%; Na2O + K2O 17.28 wt.%) to trachytic at 760 °C and 200 MPa (SiO2 68.56 wt.%; Na2O + K2O 11.33). There is a good correspondence between the experimental and natural composition above 820 °C, but at this temperature we do not crystallize the natural mineralogical assemblage. Compositional changes with temperature are due to the early crystallization of oxides, clinopyroxene, and biotite (Fig. 3) that produced an increase in SiO2 (Fig. 7a), and decrease of CaO and

FeO (Fig. 7b, Table 4); later plagioclase crystallization enhances the decrease in CaO and Al2O3 as shown by the change of the trends in Fig. 7a. The behaviour of K2O is driven by the crystallization of alkali-feldspar, with a strong depletion of K2O in residual liquid following the alkali-feldspar crystallization (Fig. 7d). Experimental sanidine is Or56–81 (Table 5; Fig. 5) reproducing well most of the natural variation. Plagioclase compositions (Table 5; Fig. 5) range from An69 (840 °C, 50 MPa) to An87 (825 °C, 100 MPa), reproducing only the most (An89) natural An-rich plagioclase crystals. Experimental clinopyroxene (Table 6; Fig. 6) has a composition varying from Wo49En36Fs15 to Wo47En17Fs35 with Al2O3 4–7 wt.%, this variation covers most of the natural clinopyroxene composition. Biotite composition (Table 7) in PR38-bearing experiments is strongly different from that of ZAC-bearing experiments. Mgnumber is 63–80 and TiO2 content is in the range 2–4 wt.% overlapping very well the observed natural compositions. Ulvöspinel (Table 8) contents in titanomagnetite range from 2 to 30, covering the natural compositional variation. Al2O3, MnO, and MgO contents are comparables with those reported in the oxide of the natural trachyte. 4. Discussion Crystallization experiments carried out during this study revealed an overall good agreement with natural samples, in terms of the nature and abundance of phases present. In the following sections we will demonstrate that the PR38 (less evolved trachyte) and ZAC (more evolved trachyte) magmas erupted during the Breccia Museo Eruption are linked between them by fractional crystallization processes. We will use the experimental results to infer the pre-eruptive, pressure–temperature conditions of the magmas. The experimental data are for water saturation, and so the compositions used for starting materials are initially assumed to have been water saturated.

Table 7 Selected microprobe analyses of experimental biotite Sample

ZAC 1P (3)

SiO2 TiO2 Al2O3 FeO c MnO MgO CaO Na2O K2O F Cl Total Mg-no.

PR38 a

36.73 (68) b 3.51 (26) 13.08 (36) 17.62 (15) 0.63 (6) 12.78 (86) 0.04 (2) 0.56 (3) 8.74 (17) 0.92 (25) 0.09 (2) 95.99 (23) 0.54

1T (3)

38W (4)

38H (9)

38B (4)

38F (4)

38E (2)

38I (3)

3 (2)

35.83 (22) 3.04 (1) 13.12 (24) 20.95 (22) 0.59 (52) 10.63 (11) 0.12 (3) 0.54 (9) 8.62 (4) 1.09 (21) 0.09 (2) 94.64 (46) 0.47

35.89 (99) 4.32 (12) 14.18 (53) 14.23 (102) 0.33 (20) 14.58 (111) 0.10 (4) 0.34 (2) 9.02 (32) 0.95 (11) 0.11 (1) 94.08 (19) 0.65

35.65 (41) 4.69 (19) 14.44 (24) 13.51 (45) 0.26 (12) 14.66 (38) 0.21 (8) 0.36 (4) 9.12 (17) 1.51 (29) 0.10 (3) 94.62 (34) 0.66

35.66 (27) 4.22 (9) 14.43 (32) 13.69 (48) 0.20 (11) 14.25 (32) 0.07 (4) 0.36 (5) 8.81 (11) 0.72 (7) 0.12 (3) 92.55 (63) 0.65

35.06 (12) 4.88 (23) 14.53 (53) 13.27 (21) 0.26 (12) 13.94 (99) 0.18 (12) 0.32 (1) 9.27 (20) 0.98 (25) 0.11 (1) 92.86 (77) 0.65

34.25 (37) 4.49 (1) 13.94 (60) 11.47 (114) 0.42 (20) 13.95 (39) 0.41 (38) 0.42 (2) 9.08 (43) 1.32 (4) 0.11 (4) 92.05 (32) 0.68

35.53 (89) 4.36 (7) 14.51 (38) 13.27 (46) 0.14 (4) 14.39 (65) 0.19 (16) 0.29 (2) 8.86 (35) 0.64 (5) 0.08 (1) 92.28 (61) 0.66

36.23 (33) 4.36 (12) 14.66 (57) 12.86 (7) 0.21 (6) 15.19 (36) 0.12 (2) 0.41 (2) 9.00 (10) 0.49 (4) 0.17 (9) 93.73 (99) 0.76

mg-number = Mg/(Mg + Fe c). a Numbers in parentheses in the second row give the number of analyses. b Numbers in parentheses in oxide columns are the standard deviation * 100. c Total iron as Fe2+.

A. Fabbrizio, M.R. Carroll / Journal of Volcanology and Geothermal Research 171 (2008) 88–102

99

Table 8 Selected microprobe analyses of experimental Ti-magnetite Sample

ZAC 2A (2) a b

Alfa (3)

Beta (2)

1R (2)

1S (3)

1L (3)

1O (5)

1P (5)

1T (2)

0.24 (11) 6.95 (62) 1.27 (39) 77.66 (61) 3.78 (33) 0.93 (7) 0.11 (2) 0.20 (10) 0.11 (2) 91.28 (69)

0.14 (1) 6.68 (17) 1.45 (2) 79.43 (10) 2.24 (17) 1.05 (1) 0.02 (3) 0.05 (1) 0.10 (0) 91.22 (76) 19 81

0.12 (3) 5.19 (7) 1.45 (5) 76.15 (73) 2.15 (14) 0.61 (3) 0.00 (0) 0.02 (1) 0.08 (2) 85.83 (170)

1.29 (84) 1.87 (11) 1.50 (29) 78.28 (81) 3.97 (12) 1.69 (5) 0.40 (43) 0.15 (12) 0.21 (11) 89.41 (59) 19 81

1.29 (84) 1.87 (11) 1.50 (29) 78.28 (81) 3.97 (12) 1.69 (5) 0.40 (43) 0.15 (12) 0.21 (11) 89.41 (59)

0.44 (12) 4.91 (45) 1.08 (7) 83.01 (59) 2.45 (19) 0.26 (4) 0.08 (2) 0.02 (1) 0.07 (2) 92.37 (49) 16 84

2.17 (81) 7.70 (60) 1.77 (67) 76.53 (85) 2.38 (15) 0.42 (3) 0.09 (5) 0.39 (37) 0.32 (29) 91.79 (60)

0.13 (4) 4.89 (4) 1.25 (9) 82.58 (62) 2.46 (1) 0.79 (6) 0.10 (13) 0.01 (0) 0.03 (1) 92.26 (61) 5 95

SiO2 TiO2 Al2O3 FeO c MnO MgO CaO Na2O K2O Total Ulv Mt

0.14 (9) 9.24 (22) 1.00 (16) 78.72 (69) 4.18 (14) 0.34 (1) 0.03 (1) 0.07 (2) 0.09 (7) 93.83 (103) 27 73

Sample

ZAC 1H (5)

1N (6)

1W (2)

1V (2)

2 (2)

38W (4)

38H (3)

38B (4)

38F (3)

SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O Total Ulv Mt

1.01 (69) 5.65 (40) 0.97 (35) 79.93 (85) 2.73 (20) 0.14 (5) 0.40 (36) 0.09 (6) 0.11 (6) 90.92 (50) 17 83

1.01 (76) 4.99 (50) 1.04 (21) 80.74 (80) 2.96 (20) 0.21 (2) 0.13 (8) 0.12 (10) 0.18 (11) 91.53 (65) 15 85

1.30 (11) 0.46 (7) 1.01 (38) 80.04 (74) 6.50 (13) 1.22 (2) 0.19 (15) 0.10 (0) 0.23 (15) 91.14 (22) 1 99

2.48 (66) 1.38 (72) 1.66 (73) 78.33 (11) 4.76 (64) 1.22 (34) 0.14 (5) 0.36 (10) 0.31 (11) 90.72 (63) 3 97

0.12 (7) 1.63 (3) 1.58 (23) 81.72 (32) 2.70 (11) 2.05 (5) 0.01 (2) 0.02 (3) 0.07 (2) 89.94 (51) 4 96

0.10 (2) 5.65 (42) 0.97 (35) 79.93 (85) 2.73 (20) 0.14 (5) 0.40 (36) 0.09 (6) 0.11 (6) 90.92 (1.05) 17 83

1.01 (69) 4.45 (31) 2.54 (70) 82.71 (44) 1.38 (6) 0.90 (3) 0.18 (11) 0.07 (4) 0.14 (10) 93.30 (75) 13 87

0.85 (60) 8.72 (125) 0.28 (4) 78.18 (72) 0.50 (5) 0.29 (2) 0.09 (4) 0.03 (1) 0.04 (3) 88.42 (50) 30 70

0.26 (4) 5.95 (22) 0.65 (56) 78.69 (74) 1.44 (85) 0.53 (46) 0.15 (7) 0.02 (1) 0.05 (2) 87.92 (92) 19 81

Sample

38I (5)

3(2)

38E (3)

SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O Total Ulv Mt

2.52 (41) 3.45 (42) 2.20 (24) 76.67 (32) 1.49 (8) 0.47 (18) 0.24 (11) 0.20 (19) 0.42 (0) 88.78 (56) 11 89

0.11 (7) 0.74 (3) 2.30 (3) 82.63 (38) 1.58 (0) 3.17 (12) 0.06 (0) 0.02 (1) 0.04 (0) 90.65 (27) 2 98

0.34 (5) 4.71 (34) 2.13 (21) 80.67 (61) 1.86 (11) 1.00 (5) 0.04 (2) 0.02 (1) 0.05 (2) 90.69 (65) 13 87

a b c

PR38

Numbers in parentheses in the second row give the number of analyses. Numbers in parentheses in oxide columns are the standard deviation * 100. Total iron as Fe2+.

4.1. Comparison with previous works There exist very few experimental studies of phase relations for phonolitic–trachytic magma compositions. Hay and Wendlandt (1995) studied the melting phase relations of a phonolite from the Kenya Rift at pressures of 0.5–1.2 Gpa and temperature of 800–1150 °C. Phlogopite was the phase with the highest melting temperature, followed by calcic amphibole, augite, and andesine/oligoclase at lower temperatures, liquid compositions varied with experimental pressures, reflecting differences in mineral phase stabilities, abundances, and relative proportions. Harms et al. (2004) determined the phase relations of a natural highly evolved phonolite of the Laacher See volcano (East Eifel,

Germany). Their experiments were performed at water pressures of 75–175 MPa, temperatures of 725–800 °C, and fO2 of 1 log unit above Ni–NiO buffer. The liquidus temperature is near 800 °C at 125 MPa. Magnetite is the first phase that crystallizes from the liquidus. Amphibole is the hydrous phase observed by Harms et al. (2004), and the presence of this phase instead of biotite may be related to the low value (0.5) of the molar ratio K2O/Na2O in the starting material. Sanidine is the last phase that crystallizes. The crystal content increases with decreasing temperature and pressure. Their sanidines vary in composition from Or50Ab47 to Or37Ab61 becoming more potassic with lower temperature; they found that experimental sanidines were chemically homogeneous. In our study we determined the phase equilibrium

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Fig. 7. (a) Variations in SiO2 concentration in glass for PR38 experiments (recalculated to 100 wt.% anhydrous) with change in temperature. The open white circle shows the SiO2 content of the starting material. The dashed horizontal lines show the maximum and minimum SiO2 content in natural glasses of the BME eruption (data from Melluso et al., 1995; Fulignati et al., 2004). The black lines show the first appearance of sanidine and plagioclase at different experimental pressures. The vertical bars show the standard deviations (1σ). ■, 100 MPa; ♦, 150 MPa; ▴, 200 MPa. (b) Variations in CaO concentration in glass for PR38 experiments (recalculated to 100 wt.% anhydrous) with change in temperature. The open white circle shows the CaO content of the starting material. The dashed horizontal lines show the maximum and minimum CaO content in natural glasses of the BME eruption (data from Melluso et al., 1995; Fulignati et al., 2004). The black line shows the first appearance of plagioclase at different experimental pressures. The vertical bars show the standard deviations (1σ). ■, 100 MPa; ♦, 150 MPa; ▴, 200 MPa. (c) Variations in Al2O3 concentration in glass for PR38 experiments (recalculated to 100 wt.% anhydrous) with change in temperature. The open white circle shows the Al2O3 content of the starting material. The dashed horizontal lines show the maximum and minimum Al2O3 content in natural glasses of the BME eruption (data from Melluso et al., 1995; Fulignati et al., 2004). The black lines show the first appearance of sanidine and plagioclase at different experimental pressures. The vertical bars show the standard deviations (1σ). ■, 100 MPa; ♦, 150 MPa; ▴, 200 MPa. (d) Variations in K2O concentration in glass for PR38 experiments (recalculated to 100 wt. % anhydrous) with change in temperature. The open white circle shows the K2O content of the starting material. The dashed horizontal lines show the maximum and minimum K2O content in natural glasses of the BME eruption (data from Melluso et al., 1995; Fulignati et al., 2004). The black line shows the first appearance of sanidine at different experimental pressures. The vertical bars show the standard deviations (1σ). ■, 100 MPa; ♦, 150 MPa; ▴, 200 MPa.

of two natural trachytes of the Phlegraean Fields (Naples, Italy). We performed experiments at water pressures of 50–200 MPa, temperatures of 700–885 °C, and fO2 equal to NNO + 1. Comparison of the phase relations observed by Harms et al. (2004) with the results of this study shows that the liquidus temperature is approximately 100 °C lower for the Na-rich Lacheer See phonolite. Like in the Harms et al. (2004) experiments, magnetite is the first phase that crystallizes from the

liquidus. In both systems sanidine is the last phase that crystallizes and there is no plagioclase observed for the Laacher See phonolite. The absence of plagioclase is likely related to the low CaO content (0.62 wt.%) in the starting material. Instead of amphibole we observe biotite, likely reflecting the higher K2O/ Na2O values (2.4 for PR38, and 1.2 for ZAC) in the trachytic compositions from Phlegraean Fields. Probably the biotite crystallization is favoured by higher K2O/Na2O ratio. In conclusion

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the presence of hydrous minerals, biotite vs amphibole, is primarily affected by differences in alkali abundances, with more Na2O favouring amphibole and more K2O favouring biotite; also, Cawthorn (1974) verified that the thermal stability of amphibole increases with increasing melt Na2O content. 4.2. Comparison of Breccia Museo products and experimental results Comparing the experimental results with the natural products of the Breccia Museo Eruption. We have the following observations: 1. The natural mineralogical assemblage is well reproduced only with PR38-bearing experiments. 2. Sanidine is the most abundant phase, it has no zonation, and observed compositions range from Or44 to Or82 with the more potassic sanidines associated with the less evolved glasses. 3. The plagioclase crystallize only on the less evolved magma (PR38) and the An content is comparable to the most An-rich natural plagioclase crystals. 4. The experimental clinopyroxene reproduces well only a portion of the salitic natural clinopyroxene. 5. Composition of experimental biotite reproduces well the natural biotite. 6. Oxides are well reproduced in the experimental runs. 7. The experimental glasses are in general more evolved than the corresponding natural samples. The listing point above suggests the following interpretation of data. We have assumed an H2O-rich fluid with no CO2, if this assumption is not correct the estimation of the temperature indicated by the phase assemblage is too low. Three of the observational points suggest that the fluid was likely CO2-bearing. (1) All of the more primitive magmas in the Phlegraean Fields are not particularly crystal-rich, but are plagioclase-bearing, and except for significant assimilation it is unlikely that the more evolved trachyte and phonolite melts are not plagioclase saturated. In the PR38 magma it appears very late in the crystallization sequence, and its high An content suggests that the experimental P (H2O) is too high compared to what it was in the natural magma. (2) This higher P(H2O) would explain why the experimental glasses are more evolved than the natural ones. (3) Some studies (Rutherford, personal communication) evidence that clinopyroxene and plagioclase in trachytic magmas can contain a large amount of dissolved CO2, suggesting these magmas are generally quite CO2-rich. In conclusion the data presented here can be seen as the phase equilibria for trachytic magmas in the presence of a water-rich fluid, an important end-member of the real system which often contains an intermediate H2O–CO2 fluid.

101

800 °C, plagioclase coexists with melt at 780–800 °C in PR38 magma at pressures between 200 and 150 MPa (Fig. 3). The liquid PR38 had a molar ratio (Na2O + K2O)/Al2O3 lower than ZAC liquids (0.84 vs 0.96, Table 1) consequently the plagioclase crystallization was favoured in the less evolved liquid (PR38). The differentiation of this less evolved trachyte produced more evolved ZAC-like magma that rose to shallower reservoirs in the crust. These batches (ZAC) were more evolved than the trachyte (PR38) because they were enriched in silica and depleted in calcium. The most abundant phenocryst in the magma is sanidine (Table 3), and at 780 °C it coexists with melt at pressures below 170 MPa for the ZAC composition (Fig. 1). The trachyte magma (ZAC) was in a reservoir at a pressure of about 140–150 MPa just before the eruption, because our experimental results show that biotite is not stable below 135 MPa (Fig. 1). This liquid could have spent no more than a couple of days at pressures below 135 MPa, or biotite would have broken down in a reaction with the groundmass melt. This suggests that the travel of the magma from the reservoir at 150 MPa to the surface was very fast (hours to a maximum of 1–2 days). Together, these arguments suggest a final pre-eruption equilibration of the more evolved trachytes (ZAC magma) at a pressure near 150 Mpa at a temperature of ∼780 °C, assuming H2O saturation. Assuming that the ZAC magma came from the roof of the chamber, our estimated pressures place the top at a depth of about 5–6 km, which suggests that the entire magma body was stored between 5 and 8 km below the surface (assuming H2O-saturated conditions). However considering that the microthermometric experiments carried out on glass inclusions (Fulignati et al., 2004), that the two-feldspar geothermometer (Melluso et al., 1995; Fulignati et al., 2004) have given temperature in the range 860–980 °C (Fulignati et al., 2004), and that our experimental evidences indicates the probably presence of CO2 dissolved in this trachytic magma it seems unlike that the more evolved trachytic magma equilibrated at a temperature lower than 800–820 °C. Additional experimental work, using a mixed-fluid H2O–CO2, is needed to check the presence of CO2 in this trachytic magma and its influence on phase equilibria. Acknowledgements This manuscript is a part of the PhD project of the first author, we wish to thank Dr. B. Scaillet for his comments on the PhD thesis and Dr. V. Di Matteo who provided the natural samples used for the experiments in this study. We thank Fidel Costa and Malcolm Rutherford for their helpful reviews that improved the quality of a first version of the manuscript. Assistance with electron microprobes was provided by Marcello Serracino (Roma). This work has been financially supported by the COFIN 04 INGV, FIRB and Dip. Prot. Civ. Italy.

4.3. Constraints on pre-eruptive conditions

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