The viscosity of trachytes, and comparison with basalts, phonolites, and rhyolites

Chemical Geology 213 (2004) 49 – 61 www.elsevier.com/locate/chemgeo

The viscosity of trachytes, and comparison with basalts, phonolites, and rhyolites D. Giordanoa,b,1,*, C. Romanoa, P. Papaleb, D.B. Dingwellc a

Dipartimento di Scienze Geologiche, Terza Universita` degli Studi di Roma, Largo Leonardo Murialdo, 00154-Roma, Italy b Istituto Nazionale di Geofisica e Vulcanologia, sede di Pisa, via della Faggiola, 32, 56126 Pisa, Italy c Department of Earth and Environmental Sciences, Munich University, Theresienstr. 41/III, 80333, Germany Received 2 December 2003; received in revised form 4 June 2004; accepted 31 August 2004

Abstract The viscosity of natural liquids representative of the glassy portion of pumice collected from the deposits of the Campanian Ignimbrite (IGC) and Monte Nuovo (MNV) eruption of Phlegrean Fields has been measured in the temperature range from 1770 K down to the glass transition, and for a dissolved water content range from dry to nearly 4 wt.%. Measurements were performed by a combination of techniques involving concentric cylinder and micropenetration apparatuses, depending on the specific viscosity range. These measurements, together with those made on samples from the Agnano Monte Spina (AMS) eruption of Phlegrean Fields presented in a companion paper, represent the first viscosity determinations for natural trachytic liquids. Liquid viscosities have been parameterized by means of a modified VFT equation that allows the calculation of viscosity as a function of temperature and water content. Calculated viscosities are compared with those pertaining to natural liquids of phonolitic, rhyolitic, and trachybasaltic composition, showing that trachytes are intermediate between rhyolites and phonolites, consistent with the dominant eruptive style associated with the different magma compositions (mainly explosive for rhyolites and trachytes, either explosive or effusive for phonolites, mainly effusive for basalts). Compositional diversity among the analyzed trachytes corresponds to liquid viscosity differences of one to two orders of magnitude, with higher viscosities approaching that of rhyolite at the same temperature–water content conditions. All hydrous natural trachytes and phonolites become indistinguishable when isokom temperatures (i.e., temperatures corresponding to the same viscosity) are plotted versus a compositional parameter given by the molar ratio on an element basis (Si+Al)/(Na+K+H). In contrast, rhyolitic and basaltic liquids display distinct trends, with the more fragile basaltic liquids crossing the curves pertaining to all other compositions. D 2004 Elsevier B.V. All rights reserved. Keywords: Natural silicate melt; Viscosity; Natural liquid

* Corresponding author. Dipartimento di Scienze della Terra, via S. Maria 53, I-56126 Pisa, Italy. Tel.: +39 050 847273; fax: +39 050 2214333. E-mail addresses: [email protected], [email protected] (D. Giordano)8 [email protected] (C. Romano)8 [email protected] (P. Papale)8 [email protected] (D.B. Dingwell). 1 Tel.: +39 050 533010, +39 339 4721920. 0009-2541/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2004.08.032

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1. Introduction

2. Sample selection and characterisation

Viscosity is known to be a critical quantity governing transport mechanics in magmatic and volcanic processes (e.g. Webb, 1997; Dingwell, 1998; Papale, 1999). Recent investigation focuses on the complex roles of compositional and structural quantities determining the viscosity of silicate melts (Dingwell et al., 1998a,b; Whittington et al., 2000, 2001; Romano et al., 2001), as well as on the determination of viscosity of natural liquids (Giordano et al., 2000; Giordano and Dingwell, 2003; Romano et al., 2003) relevant for the modeling of magmatic processes and assessment of volcanic hazards. Composition, temperature, solid and gas bubble content, dissolved volatiles, and pressure are all parameters which influence viscosity to various extents. Pressure up to about 20 kbar and solid content within 30 vol.% do not influence viscosity as much as temperature, composition, or dissolved water content (Marsh, 1981, Pinkerton and Stevenson, 1992, Dingwell et al., 1993, Lejeune and Richet, 1995). In this work, we investigate the viscosity of silicate melts representative of the composition of liquids erupted during the Campanian Ignimbrite (IGC, 36,000 BP—Rosi et al., 1999) and Monte Nuovo (MNV, AD 1538—Civetta et al., 1991) eruptions of Phlegrean Fields. The liquids are trachytic in composition, and the present measurements, together with the viscosity data on the Agnano Monte Spina eruption (AMS, 4100 BP—de Vita et al., 1999) of Phlegrean Fields that we have presented elsewhere (Romano et al., 2003), constitute the only viscosity measurements available to date on natural trachytic liquids. We present here viscosity measurements made at dry as well as hydrous conditions (H2O up to about 3.9 wt.%), in a temperature range from 1770 K for dry liquids, and from 880 K for hydrous liquids, down to near the glass transition temperature. The new viscosity measurements allow a parameterization of the Newtonian viscosity of different trachytic liquids from Phlegrean Fields as a function of temperature and water content. Comparison with natural silicate liquids of phonolitic, basaltic, and rhyolitic composition from the literature shows that trachytic liquids occupy a well-defined viscosity field intermediate between that of rhyolitic and phonolitic liquids, with hydrous viscosities that can be close to those characterising rhyolitic melts.

The dry materials investigated here were obtained by fusion of the glassy matrix of pumice samples collected within stratigraphic units corresponding to the peak mass discharge rate of the Plinian phase of the Campanian Ignimbrite eruption (IGC), and Monte Nuovo (MNV) eruption. Level V3 (Voscone outcrop, Rosi et al., 1999) and basal fallout for IGC and MNV, respectively, were sampled. These events, together with the 4100 BP AMS eruption (de Vita et al., 1999), cover a large part of the magnitude, intensity, and compositional spectrum characterising Phlegrean Fields eruptions. The Campanian Ignimbrite is the largest event so far recorded at Phlegrean Fields (Rosi et al., 1999). The Agnano Monte Spina eruption is the most powerful event of the last period of activity (de Vita et al., 1999), whereas the Monte Nuovo eruption is the last eruptive event that occurred at the Phlegrean Fields, following a quiescent period of about 3000 years (Civetta et al., 1991). Compositions of the matrix glasses fall well within the trachytic field in the TAS diagram of Fig. 1 (see also Table 1). The composition of the IGC falls well within the field of matrix glass compositions found for the same eruption by Signorelli et al. (1999), confirming that our sample

Fig. 1. TAS diagram showing the composition of samples analyzed in this work (IGC and MNV, trachytes) or used for comparison of their viscosity. Compositions from Giordano et al. (2000) (Td_Ph, phonolite), Romano et al. (2003) (AMS_B1 and _D1, trachyte; Ves1631_W and _G, tephriphonolite), and Giordano and Dingwell (2003) (ETN, trachybasalt). The composition for rhyolite corresponds to the range of compositions employed in the calibration of the Hess and Dingwell (1996) viscosity equation for typical rhyolites.

D. Giordano et al. / Chemical Geology 213 (2004) 49–61 Table 1 Composition (wt.%) of the analyzed liquids from Campanian Ignimbrite (IGC) and Monte Nuovo (MNV) eruptions of Phlegrean Fields SiO2 Al2O3 FeOa TiO2 MnO MgO CaO Na2O K2O P2O5 a

IGC

MNV

60.74 19.22 3.37 0.27 0.18 0.28 2.11 5.28 6.32 0.06

63.88 17.10 2.90 0.31 0.13 0.24 1.82 5.67 6.82 0.05

Total iron as FeO.

did not suffer significant post-emplacement alteration. A similar conclusion comes from the comparison of our MNV composition in Table 1 and the compositional range recently found for the Monte Nuovo eruption (M. Rosi, personal communication). In the same diagram, the compositions of phonolites, rhyolite, and trachybasalt, which are used here for comparison, are also reported. The different trachytic liquids from Phlegrean Fields cover a silica range from 61 to 64 wt.%, and a total alkali range from 11.6 to 12.6 wt.%. The total compositional range considered spans a silica content of 48–79 wt.%, and a total alkali range of 6–15 wt.% (Fig. 1, Table 1).

3. Analytical and experimental methods The starting glasses used for the viscosity determinations were prepared by fusion of the pumice sample matrices after separation from crystals and lithics. Optical, densimetric and electromagnetic techniques were employed for the separation. Homogenisation by stirring of the molten materials was performed at 1 atm and 1400–1600 8C until the melts were free of bubbles. Once homogenised at high temperature, viscosities in the interval from 102.37 to 104.80 Pa s were determined, using a Brookfield DVIII+ concentric cylinder viscometer (full-scale torque=5.75
101 Nm) calibrated against a soda silica lime standard glass (DGG-1). The material (about 100 g) was contained in a cylindrical Pt80Rh20 crucible. The viscometer head drives a

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spindle at a range of constant angular velocities (0.5 to 100 rpm) and digitally records the torque exerted on the spindle by the sample. The spindles are made from the same material as the crucible and vary in length and diameter. The furnace used was a Deltech furnace with six MoSi2 heating elements. The crucible was loaded into the furnace from the base. The temperature range of the measurements varied between 1150 and 1500 8C. Viscosity was determined in steps of decreasing temperature until the minimum temperature value was reached. Possible instrumental drift was checked by subsequently returning to the highest temperature data point (further procedure details are found in Dingwell and Virgo, 1988). Melt samples were then allowed to cool to room temperature. The anhydrous glass compositions were analyzed by a Cameca SX 50 electron microprobe, using a spot size of about 20 Am and 10 nA beam current (Table 1). Doubly polished 3-mm disks of the cooled glass were prepared for the dry micropenetration viscometry as described in Hess et al. (1995) and Dingwell et al. (1996). Part of the glass from the concentric cylinder experiments was retrieved by drilling and used for hydration experiments conducted at the Bayerisches Geoinstitut (BGI), Bayreuth. Hydrothermal syntheses were performed in piston cylinder apparatus at 10 kbar and temperature between 1450 and 1600 8C followed by rapid quench. Loss of iron from the sample within the Pt capsule was limited by reducing the synthesis times. Experimental times were always shorter than 1 h. This reduces loss of iron while still ensuring total dissolution and homogenisation of water into the melt. The quenched hydrous glasses (typical initial quench rates are 200 K/s to about 100 8C in about 3 min) were then recovered and prepared for micropenetration viscometry. FTIR spectroscopy before and after the viscometry, and Karl-Fisher Titration (KFT), confirmed the homogeneity, stability and absolute water content values of the samples. KFT analyses were performed at the Institute of Mineralogy, Hannover University, following the method described by Behrens et al. (1996), prior to viscosity measurements. The rest of the samples were used for micropenetration measurements. The measured water contents are reported in Tables 2 and 3.

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Table 2 Viscosity data for the dry (dissolved water content of 0.02 wt.%) and hydrous trachytic liquids from the Campanian Ignimbrite (IGC) and the Monte Nuovo (MNV) eruptions Sample

H2O (wt.%)

T meas (8C)

log10g (Pa s)

IGC IGC IGC IGC IGC IGC IGC IGC IGC IGC IGC IGC IGC IGC IGC IGC IGC IGC IGC IGC IGC IGC IGC IGC IGC IGC IGC IGC IGC IGC IGC IGC IGC IGC IGC MNV MNV MNV MNV MNV MNV MNV MNV MNV MNV MNV MNV MNV MNV MNV

dry dry dry dry dry dry dry dry dry dry dry dry dry dry dry dry dry dry 0.81 0.81 0.81 0.81 0.81 0.81 1.54 1.54 1.54 2.01 2.01 2.01 2.96 2.96 3.41 3.41 3.41 dry dry dry dry dry dry dry dry dry dry dry dry dry dry dry

1496 1471 1446 1422 1397 1373 1348 1323 1299 1274 1249 1225 1200 1176 861 836 803 783 562 569 579 595 596 604 529 546 553 508 506 538 459 467 418 442 457 1496 1471 1446 1422 1397 1373 1348 1323 1299 1274 1249 1225 1200 1176 817

2.37 2.49 2.63 2.77 2.92 3.08 3.24 3.40 3.58 3.76 3.94 4.14 4.34 4.54 9.32 9.84 10.44 10.83 11.12 10.94 10.75 10.44 10.30 10.09 10.86 10.36 10.14 10.52 10.44 9.94 10.76 10.27 11.28 10.45 9.88 2.50 2.62 2.75 2.89 3.03 3.18 3.33 3.49 3.65 3.82 3.97 4.17 4.36 4.55 8.76

Table 2 (continued) Sample

H2 O (wt.%)

MNV MNV MNV MNV MNV MNV MNV MNV MNV MNV MNV MNV MNV MNV MNV MNV

dry dry dry 1.00 1.00 1.00 1.00 1.39 1.39 1.39 2.41 2.41 3.86 3.86 3.86 3.86

T meas (8C)

log10g (Pa s)

769 744 706 566 575 589 597 515 545 570 472 515 401 400 398 385

9.56 10.03 10.71 10.64 10.47 10.22 10.08 10.97 10.31 9.90 10.59 9.67 10.85 10.80 10.79 11.60

Water content values are those obtained by average of those measured by KFT technique (Table 3). Temperature accuracy is F0.5 8C for the low temperature data, and F1 8C for the high temperature data. Low T viscosities are accurate to F0.06 log units based on DGG Standard Glass determinations (Hess et al., 1995). High T viscosities have a maximum error of 5% (Dingwell and Virgo, 1988).

The low temperature hydrated viscosities were measured using the micropenetration technique. This involves determining the rate at which an Ir indenter under a fixed load moves into the melt surface. These measurements were performed in a BAHR DIL 802 vertical push dilatometer. Details about the experimental procedure are reported in Dingwell et al. (1996) and Hess et al. Table 3 Water content values (wt.%) and average values determined by the Karl-Fischer Titration (KFT) technique Sample IGC

MNV

H2O content KFT_1

KFT_2

2.87 1.98 1.50 3.39 3.66 2.35 1.37 0.98

3.05 2.03 1.56 3.43 4.06 2.47 1.40 1.02

Average H2O KFT_3

1.56

2.96 2.01 1.54 3.41 3.86 2.41 1.39 1.00

KFT_1,2,3 refer to the measurement number of done on single chips of glasses belonging to the same hydrous synthesis. Accuracy of KFT technique is, as reported by Behrens et al. (1996), of the order of F0.10 wt.%.

D. Giordano et al. / Chemical Geology 213 (2004) 49–61

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(1995). The one advantage of using a micropenetration technique is the small amount of material needed (10 to 30 mg) and the short time required to obtain reproducible measurements. After the heating stage, a relaxation time before the measurement on the order of 15–20 min was typically adopted. For hydrous samples, measurement times from 1 to 10 min were sufficient to obtain the viscosity data. Viscosity determinations of water-bearing trachytic samples were obtained using 1-mm-thick samples. Experimental accuracy was confirmed by frequent measurements performed on the DGG-1 standard glass. Tests to evaluate the influence of the heating phase on the viscosity were made by performing repeated measurements at approximately the same temperature on selected samples. No viscosity variation due to nucleation of gas or solid phases was detected.

4. Viscosity results and data modeling Fig. 2 and Table 2 show the measured dry and hydrous viscosities for the analyzed samples from the Campanian Ignimbrite and Monte Nuovo eruptions of Phlegrean Fields. In general, the viscosity interval explored becomes more and more restricted at higher water contents. While viscosity appears to show relatively Arrhenian behavior over the restricted range of each individual technique, a variable degree of nonArrhenian behavior emerges over the entire temperature range explored. The viscosities investigated here, as well as those for the Agnano Monte Spina eruption of Phlegrean Fields and for the other magma types are reported in Fig. 1, are well represented in the range of investigated temperature and water content, by a modified form of the VFT equation, proposed by Giordano et al. (2000): logg ¼ A þ

B T C

ð1Þ

with A ¼ a1 þ a2 logwH2 O B ¼ b1 þ b2 wH2 O C ¼ c1 þ c2 logwH2 O

ð2Þ

Fig. 2. Viscosity measurements (symbols) and calculations (lines) for the IGC (a) and MNV (b) samples. Numbers on lines and for the different symbols indicate the dissolved water content (wt.%).

where g is viscosity, a 1, a 2, b 1, b 2, c 1 and c 2 are fit parameters, and w H2O is water concentration in wt.%. Particular care must be taken in interpreting the fitting to the viscosity data. Russell et al. (2002) demonstrated that fitting viscosity–temperature data to non-Arrhenian rheological models can result in strongly correlated or even nonunique, sometimes unphysical, model parameters (A, B, C) of a VFT equation (Eq. (1)). Russell et al. (2002) quantify and discuss the possible error sources for typical magmatic or magmatic-equivalent fragile to strong silicate melts. In particular, those authors demonstrate that data distribution must not be limited to a single technique and more than one datum must be provided from both high and low temperature techniques. Particular care must be taken when working with strong liquids (liquids following a close to Arrhenian rheological behavior, e.g. Angell, 1995). In fact, the range of acceptable values for parameters A, B and C for strong liquids is demonstrated to be 5–10 times greater than the range of values estimated for fragile

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melts (melts whose rheological behavior significantly deviate from Arrhenian, e.g. Angell, 1995). Such a problem is partially solved if the interval of measurement and the number of experimental data is large. Empirically obtained best fit parameters must also have physically consistent values. In fact, B and C cannot be negative (as they represent, respectively activation energies and absolute temperature). The value of the high temperature parameter (A) can be constrained, according to the Maxwell theory, by the lower limit to melt viscosity (g 0), given by sG l (where s is the relaxation time scale and G l is the bulk shear modulus at infinite frequency). If we take into account that an average value of G l is ~1010 Pa (Toplis, 1998; Dingwell and Webb, 1989) and the relaxation time scale (s) of the melt is dictated by the quasi-lattice vibration period (~1014 s) (representing the time between successive attempts to cross the energy barriers to melt rearrangement, see Angell, 1991; Toplis, 1998; Angell et al., 2000), thus, the lower limiting value to viscosity (g 0) should approximate 104 Pa s. The mathematical regression-based values of A can be different from the theoretical value of 104 Pa s, but, in order to have a physical meaning, they should always be negative. The results of our empirical fitting for the A, B and C parameters are in agreement with such theoretical premises. Finally, the validity of the calibrated equation must be verified case to case in order to prevent unphysical results as, for example, a viscosity increase with addition of water or with temperature increase (Russell et al., 2002). Extrapolation of data outside the experimental range should be avoided or limited. When fitting the data via Eqs. (1) and (2), w H2O is assumed to be always z0.02 wt.%. This constraint is supported by several infrared spectroscopic analyses as, for example, those from Ohlhorst et al. (2001) and Hess et al. (2001). These authors, on the basis of their IR analysis performed on polymerised as well as depolymerised melts, nominally anhydrous (after KFT measurements) conclude that a water content on the order of 200 ppm is always present even in the most degassed glasses. Calculated viscosities are reported in Fig. 2 (lines) together with measured values (symbols). Fig. 3 shows the calculated versus measured viscosities using the above parameterization. Values of the a 1, a 2, b 1, b 2, c 1 and c 2 fit parameters for the compositions investigated in the present work,

Fig. 3. Comparison between experimental and calculated viscosities for IGC and MNV liquids analyzed in this work.

together with their standard errors, are listed in Table 4. The fitted parameters corresponding to the other compositions used for comparison are reported in Romano et al. (2003) and Giordano and Dingwell (2003). Fig. 4 shows the calculated viscosity curves at 1100 K for liquids of rhyolitic, trachytic, phonolitic, and basaltic composition, including those from Campanian Ignimbrite and Monte Nuovo analyzed in this work, as a function of water content. The curves are clearly distinct for each different compositional group. As also pointed out by Romano et al. (2003), the viscosities of rhyolites and trachytes at dissolved water contents larger than about 1–2 wt.% are very similar, while at lower water contents the viscosity of rhyolites is up to four orders of magnitude larger. The new viscosity data presented in this work confirm this trend, with the exception that the dry viscosity of the Campanian Ignimbrite liquid is about two orders of magnitude larger than that of the other analyzed trachytic liquids from Phlegrean Fields, and that the hydrous viscosities of IGC and MNV samples are appreciably lower (by less than one order of magnitude) than that of the AMS sample. Phonolitic liquids have substantially lower viscosities than trachytic liquids, except at dry conditions, where viscosities are comparable to those of trachytic

D. Giordano et al. / Chemical Geology 213 (2004) 49–61

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Table 4 Calibrated parameters for Eq. (2) Sample

a1

a2

b1

b2

c1

c2

IGC 4.415 0.226 9243 428.1 255.3 127.0 1r deviation 0.898 0.253 1027 36.0 37.3 10.89 MNV 5.863 0.117 12747 673.5 103.4 59.85 1r deviation 1.445 0.401 2084 90.2 69.1 21. 53 Listed values restitute the log10 of viscosity expressed in Pad s when temperature is in K and water content in wt.%. The standard error of estimations of the global fits for the IGC and the MNV samples are 0.097 and 0.119 log units (R 2=0.999), respectively. The 1r deviation of fit parameters is reported under the same parameter values.

liquids. Finally, basaltic liquids from Mount Etna are significantly less viscous than trachytes both at dry and hydrous conditions (Fig. 4). Fig. 5 shows the calculated viscosity curves at representative eruptive temperatures for each composition. The general relationships between the different compositional groups remain the same. At equal dissolved water contents larger than 1–2 wt.%, the trachytes have viscosities from close to, to about two orders of magnitude less than that of rhyolite, and from less than one to about three orders of magnitude

Fig. 4. Viscosity as a function of water content for rhyolitic, trachytic, phonolitic, and basaltic liquids with natural composition, all at T=1100 K. In this figure, and in Figs. 5–8, the different compositional groups are indicated with different lines: solid thick line for rhyolite, dashed lines for trachytes, solid thin lines for phonolites, long-dashed gray line for basalt. The suffixes Tr, Ph, Rh, Bas refer to trachytic, phonolitic, rhyolitic and basaltic compositions. These abbreviation are also used Figs. 5, 6 and 8.

Fig. 5. Viscosity as a function of water content for rhyolitic, trachytic, phonolitic, and basaltic liquids with natural composition, each at the corresponding estimated eruptive temperature. Eruptive temperatures from Ablay et al. (1995) (Td_Ph phonolite, T=870 8C), Roach and Rutherford (2001) (AMS trachyte, T=830 8C), Rosi et al. (1993) (Ves1631 tephriphonolite, T=1000 8C). A typical eruptive temperature for rhyolite is assumed equal to 830 8C, whereas for the MNV and IGC trachytes a temperature of 900 8C is considered. An eruptive temperature of 1100 8C is taken for the Etna trachybasalt.

higher than the viscosity of phonolites. The Etnean trachybasalt shows viscosities at eruptive temperature which are nearly two orders of magnitude less than those of Vesuvius phonolites, three orders of magnitude less than those of Teide phonolite, and up to five orders of magnitude less than those of trachytes and rhyolite. Fig. 6 shows the isokom temperature (i.e., the temperature at fixed viscosity) corresponding to 1012 Pa s, for the same compositions shown in Fig. 5. Such a high viscosity corresponds approximately to the glass transition (Richet and Bottinga, 1986), and it is close to the 1010 to 1011 Pa s measured viscosities at all water contents employed in the experiments (Table 2 and Fig. 2). This ensures that the errors introduced by extrapolating the viscosity parameterization of Eqs. (1) and (2) are small, giving an accurate picture of the viscosity relationships for the considered compositions. The most striking feature in the figure is represented by the crossover between the isokom temperatures of trachybasalt and rhyolite, and trachybasalt and trachyte from IGC eruption, at a water content less than 1 wt.%. Such a

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Fig. 6. Isokom temperature corresponding to 1012 Pad s as a function of water content for rhyolitic, trachytic, phonolitic, and basaltic liquids with natural composition.

crossover was also found to occur for synthetic tephritic and basanitic liquids when compared to synthetic trachytes, and interpreted as due to the larger depolymerising effect of water in liquids which are more polymerised at dry conditions (Whittington et al., 2000). Our data and parameterization show also that, even at dry conditions, the isokom temperature and hence viscosity of Etnean trachybasalt remains larger than that of phonolites and of AMS and MNV trachytes, pointing to differences in the fragility of the anhydrous liquids. The crossover effect disappears at viscosities less than about 1010 Pa s (not shown in the figure). Apart from trachybasalt, the other liquids considered in Fig. 6 show relationships similar to those in Fig. 4, with phonolites occupying the lower part of the diagram, followed by trachytes, then by the rhyolite. Other changes with respect to the lower viscosity fields in Fig. 4 are represented by the position of the IGC curve, which is above those of other trachytes for most of the water content range considered, and by the position of the Ves1631 phonolite, which is still below but very close to the curves pertaining to trachytes. In a previous work (Romano et al., 2003), it has been shown that the viscosity of trachytic and phonolitic liquids in the high viscosity range follows a well-defined trend with a compositional parameter given by a modification of the total alkali/silica ratio, namely, the ratio on an

elemental molar basis TAS*=(Na+K+H)/(Si+Al). Such a trend is best evidenced in an isokom temperature versus 1/TAS* diagram, where the isokom temperature is the temperature corresponding to a constant viscosity value chosen to be equal to 1010.5 Pa s. Such a high viscosity falls within the range of measured viscosities for all conditions from dry to hydrous (Fig. 2); therefore, the error introduced by the viscosity parameterization at Eqs. (1) and (2) is minimal. Fig. 7 shows the same diagram, with the addition of the new curves for the Campanian Ignimbrite and Monte Nuovo trachytes from Phlegrean Fields, and the curve corresponding to the Etnean trachybasalt. As can be seen, the existence of a unique trend for hydrous trachytes and phonolites is confirmed by the new determinations and parameterizations performed in this work. In spite of the large viscosity differences between trachytes and phonolites, which are shown in Fig. 4, all these liquids become the same as long as water contents (w H2ON0.3 wt.%, or N0.6 wt.% for the Teide phonolite) are considered together with the compositional parameter TAS* defined above. Conversely, the Etnean trachybasalt and the HPG8 rhyolite display very different curves in Fig. 7, this having been interpreted as due to the very large structural differences characterising highly polymerised (HPG8) or highly depolymerised (ETN) liquids as compared with the moderately polymerised liquids with trachytic and phonolitic composition (Romano et al., 2003).

Fig. 7. Isokom temperature corresponding to 1010.5 Pad s plotted against the inverse of TAS* parameter defined in the text. The HPG8 rhyolite (Dingwell et al., 1996) has been used to obtain appropriate TAS* values for rhyolites.

D. Giordano et al. / Chemical Geology 213 (2004) 49–61

5. Discussion In this paper, we present measurements of the viscosities of dry and hydrous trachytes from Phlegrean Fields, representative of the liquid fraction flowing along the volcanic conduit during phases of the Campanian Ignimbrite and Monte Nuovo eruptions. Together with similar measurements reported in a companion paper (Romano et al., 2003) on samples from the Agnano Monte Spina (AMS) eruption, the above represent the first viscosity data for Phlegrean Fields trachytes, and for natural trachytes in general. Viscosity measurements on a synthetic trachytic composition were presented by Whittington et al. (2001), and are discussed below together with the AMS data. The results shown in the present paper clearly show the existence of separate viscosity fields for the different considered compositions, with trachytes being in general more viscous than phonolites and less viscous than rhyolites. This trend is demonstrated for near-experimental conditions in Fig. 6, and is also clear in the extrapolations of Figs. 4 and 5, corresponding to temperatures and water contents similar to those characterising the magmatic liquids at natural conditions. In such cases, the viscosity curve of the AMS liquid tends to merge with that of the rhyolitic liquid for water contents larger than a few wt.%, deviating from the trend shown by IGC and MNV trachytes. This deviation is well evidenced in Fig. 4, which refers to the 1100 K isotherm, and corresponds to a lower slope of the viscosity versus water content curve of the AMS with respect to the IGC and MNV liquids. Since the only points in Fig. 4 which are well constrained by the viscosity data are those corresponding to dry conditions (see Fig. 2), in which case the accuracy of the calculated viscosity at the relatively high-temperature conditions in Figs. 4 and 5 decreases with increasing water content, the possibility exists that the diverging trend of AMS with respect to IGC and MNV in Fig. 4 is due to the approximations introduced by extrapolating the viscosity parameterization of Eqs. (1) and (2) beyond the range of experimental conditions. It is however worth noting that the synthetic trachytic liquid analyzed by Whittington et al. (2001) produces viscosities at 1100 K which are closer to that of AMS trachyte, or even slightly more

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viscous, when the data are fitted by Eqs. (1) and (2) (Romano et al., 2003). The distinct viscosity trends presented in Figs. 4–6 reflect some structural differences among the various magmas investigated. A detailed analysis of the structural parameters, which can affect the rheological behavior of melts, has been presented in Romano et al. (2003) and will not be repeated here. It is worth noting, however, that when considering the variations of A/AE (alkali versus alkaline earth ratio, Whittington et al., 2001; Romano et al., 2003), a relationship can be envisaged among AMS; MNV and IGC. However, as demonstrated by Romano et al. (2003), fitting of the viscosity data as a function of the A/AE parameter result in values of v 2 too high to be considered reliable. On the other hand, a good relationship between the viscosity of trachytes and phonolites considered in this work and the inverse of TAS* parameter (Romano et al., 2003) seems to hold true at anhydrous as well as hydrous conditions. The equivalence of all hydrous trachytes and phonolites considered in this work when the isokom temperature is plotted against the inverse of TAS* parameter in the high viscosity range is presented in Fig. 7, despite relatively large compositional differences, with total FeO ranging from 2.90 (MNV) to 4.80 wt.% (Ves1631), CaO from 0.7 (Td_ph) to 6.8 wt.% (Ves1631), MgO from 0.2 (MNV) to 1.8 (Ves1631) (Romano et al., 2003, and Table 1). Conversely, dry viscosities (w H2Ob0.3 wt.%, or 0.6 wt.% for Td_ph) lie outside of the hydrous trend, with a general tendency to increase with 1/ TAS* but with significant deviations shown by AMS and MNV liquids (Fig. 7). The curves in Fig. 7 shown by rhyolite and trachybasalt are very different from those of trachytes and phonolites, pointing to some substantial difference in their structures maybe related to the very different values of NBO/T. It is also worth noting that rhyolite, trachytes, and phonolites show similar slopes in Fig. 7, while the Etnean trachybasalt shows a much lower slope with its curve crossing the curves pertaining to all other compositions. Such a crossover is related to that shown by ETN in Fig. 6. As a conclusion, while it is now clear that hydrous trachytes have viscosities intermediate between those of hydrous rhyolites and phonolites, the actual range of possible viscosities for trachytic

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liquids from Phlegrean Fields at close-to-eruptive temperature conditions can only be approximately constrained at present. These viscosities vary, at equal water content, from that of hydrous rhyolite as an upper limit, to about one order of magnitude less (Fig. 4), or two orders of magnitude less when taking into account possible different eruptive temperatures of rhyolitic and trachytic magmas (Fig. 5). In order to improve our capability of calculating the viscosity of magmatic liquids at temperatures and water contents approaching those in magma chambers or volcanic conduits, it is necessary to perform viscosity measurements at such conditions. This, in turn, requires the development and standardization of experimental techniques capable to maintain the dissolved water content in the high temperature liquids for a sufficiently long time, larger than the time required for the measurement. Some steps have been made in this direction, by employing the falling sphere method, especially in conjunction with a centrifuge apparatus (Dorfman et al., 1996). This system increases the apparent gravity acceleration, thus reducing significantly the time required for each measurement. It is highly desirable that similar techniques will be routinely employed in the future to measure hydrous viscosities of silicate liquids from intermediate to high temperature conditions. The viscosity relationships between different compositional groups of liquids in Figs. 4 and 5 are also consistent with the dominant eruptive styles associated with each different composition. A relationship between magma viscosity and eruptive style is described in Papale (1999) on the basis of numerical simulations of magma ascent and fragmentation along volcanic conduits. Other conditions being equal, a higher viscosity favours a more efficient feedback between pressure decrease, ascent velocity increase, and further multiphase magma viscosity increase, that culminate in magma fragmentation and in the onset of an explosive (fragmented) eruption. Conversely, low viscosity magma does not easily reach the conditions for magma fragmentation, even when the volume occupied by the gas phase becomes larger than 90% of the total volume of magma, giving rise to effusive (nonfragmented) eruptions. The present results show that at eruptive conditions, and to a large extent irrespective of the dissolved water content, the

trachybasaltic liquid from Mount Etna has the lowest viscosity, consistent with the dominant effusive style of its eruptions. Phonolites from Vesuvius are characterised by viscosities higher than those of Mount Etna trachybasalt, but lower than those of Phlegrean Fields trachytes. This means that, along the phonolitic evolutionary line, magmas with slightly lower silica content may give low viscosities comparable to those of basalts. Accordingly, while lava flows are virtually absent in the long volcanic history of Phlegrean Fields, the activity of Vesuvius is characterised by periods with dominant effusive activity alternated with periods with dominant explosive activity. Finally, rhyolites are the most viscous liquids among those here considered, and accordingly rhyolitic volcanoes typically produce highly explosive eruptions. Different from hydrous conditions, the calculated dry viscosities are well constrained from the measured data at all temperatures from very high to close to the glass transition (Fig. 2). Therefore, the calculations for dry conditions made via Eqs. (1) and (2) can be regarded as an accurate description of the actual (measured) viscosities. Figs. 4–6 show that at temperatures comparable with those of eruptions the general trends outlined above for hydrous conditions are maintained, with increasing viscosity from trachybasalt to phonolites to trachytes to rhyolite. In contrast, at low temperature close to the glass transition (Fig. 6) the dry viscosity (or the isokom temperature) of phonolites from the 1631 Vesuvius eruption becomes similar to that of AMS and MNV trachytes, and even more striking, the dry viscosity of trachybasalt from Mount Etna becomes higher than that of trachytes, except for IGC which shows the highest dry viscosity among trachytes. The crossover between trachybasalt and rhyolite isokom temperatures corresponding to a viscosity of 1012 Pa s, which is evident from Fig. 6, does not occur only as a consequence of less steep slope of viscosity versus water content curve of trachybasalt with respect to rhyolite, as pointed out by Whittington et al. (2000), but it is also the consequence of a much more rapid increase of the dry viscosity of the trachybasalt with decreasing temperature approaching the glass transition temperature (Fig. 8), related to the more fragile nature of the basaltic liquid with respect to liquids of different composition (Giordano and Dingwell, 2003). Trachy-

D. Giordano et al. / Chemical Geology 213 (2004) 49–61

Fig. 8. Viscosity versus temperature for ddryT rhyolitic, trachytic, phonolitic, and basaltic liquids with natural composition considered in the present work.

basalt and rhyolite isokoms corresponding to lower viscosity values (1010 Pa s, not shown in the paper) do not cross. From the analysis of Fig. 6, and considering the range of temperatures, water contents and viscosities of natural basalts and trachybasalts, we do not believe that the observed crossover would ever occur in real geological environments, unless rhyolitic compositions remarkably different from those used for the calibration in Hess and Dingwell (1996) are considered.

6. Conclusions With the present work, we have determined the dry and hydrous viscosity of natural trachytic liquids representative of the glassy portion of pumice samples from eruptions of Phlegrean Fields, and have calibrated the parameters of a modified VFT equation that allows the calculation of viscosity as a function of temperature and water content, for each different composition. The viscosity of natural trachytic liquids falls between that of natural phonolitic and rhyolitic liquids, consistent with the dominantly explosive eruptive style of Phlegrean Fields volcano as compared with the similar style of rhyolitic volcanoes, the mixed explosive-effusive style of phonolitic volca-

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noes like Vesuvius, and the dominantly effusive style of basaltic volcanoes which are associated with the lowest viscosities among those considered in this work. Compositional variations between the different considered trachytes translate into liquid viscosity differences of nearly two orders of magnitude at dry conditions, and less than one order of magnitude at hydrous conditions. Such differences can increase significantly when taking into account estimated eruptive temperatures of different eruptions at Phlegrean Fields. An interesting result is that in the high viscosity range all hydrous trachytes and phonolites become indistinguishable when the isokom temperature is plotted against a compositional parameter which expresses the molar ratio on an element basis given by (Si+Al)/(Na+K+H)=1/TAS*. However, rhyolitic and basaltic liquids show distinct behaviors, the interpretation of which is not possible with the presently available data. A combination of macroscopic determinations of the rheology of magmas and the investigation of their structure through spectroscopic means is therefore required to explore the microscopic origin of the viscous behavior of melts and magmas. For hydrous liquids in the low viscosity range, or for temperatures close to those of natural magmas, the uncertainty of calculations is large even if not quantifiable, due to lack of measurements at such conditions. Although special care has been taken in the regression procedure in order to obtain physically consistent parameters (such as avoiding negative values of B and C, and positive values of A in Eq. (1), as discussed above), such an uncertainty represents a limitation to the use of the present results for the modeling and interpretation of volcanic processes. Future improvements require therefore the development and standardization of experimental techniques for the determination of hydrous viscosities in the intermediate to high temperature range.

Acknowledgement This work has been supported by the Italian Civil Protection through GNV (National Group for Volcanology) Project 2001-03/17 and the Volcano

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Dynamics RTN of the EU. We thank the Bayerisches Geoinstitut for the use of the piston cylinder apparatus and the electron microprobe, Hubert Schulze for the technical support during the preparation of the experimental material, and M. Rosi for having kindly provided the IGC and MNV samples. We gratefully acknowledge the assistance of Harald Behrens for the Karl-Fisher-Titration determinations. Reviews by A. Whittington and Y. Bottinga provided constructive criticisms and helpful suggestions which helped improve the original manuscript. [RR]

References Ablay, G.J., Ernst, G.G.J., Marti, J., Sparks, R.S.J., 1995. The ~2 ka subplinian eruption of Montana Blanca, Tenerife. Bull. Volcanol. 57, 337 – 355. Angell, C.A., 1991. Relaxation in liquids, polymers and plastic crystals—strong/fragile patterns and problems. J. Non-Cryst. Solids 131–133, 13 – 31. Angell, C.A., 1995. Formation of glasses from liquids and biopolymers. Science 265, 1924 – 1935. Angell, C.A., Ngai, K.L., McKenna, G.B., McMillan, P.F., Martin, S.W., 2000. Relaxation in glass-forming liquids and amorphous solids. J. Appl. Phys. 88, 3113 – 3149. Behrens, H., Romano, C., Nowak, M., Holtz, F., Dingwell, D.B., 1996. Near-infrared spectroscopic determination of water species in glasses of the system MAlSi3O8 (M=Li, Na, K): an interlaboratory study. Chem. Geol. 128, 41 – 63. Civetta, L., Carluccio, E., Innocenti, F., Sbrana, A., Taddeucci, G., 1991. Magma chamber evolution under Phlegrean Fields during the last 10 ka: trace element and isotope data. Eur. J. Mineral. 3, 415 – 428. de Vita, S., Orsi, G., Civetta, A., Carandente, A., D’Antonio, M., Deino, A., di Cesare, T., Di Vito, M.A., Fisher, R.V., Isaia, R., Marotta, E., Necco, A., Ort, M., Pappalardo, L., Piochi, M., Southon, J., 1999. The Agnano Monte Spina eruption (4100 years BP) in the restless Campi Flegrei caldera (Italy). J. Volcanol. Geotherm. Res. 91, 269 – 301. Dingwell, D.B., 1998. Magma degassing and fragmentation. Recent experimental advances. In: Gilbert, J.S., Sparks, R.S.J. (Eds.), The Physics of Explosive Volcanic Eruptions, Special Publications, vol. 145. Geological Society, London, pp. 9 – 26. Dingwell, D.B., Virgo, D., 1988. Melt viscosities in the Na2O–FeO– Fe2O3–SiO2 system and factors controlling the relative viscosities of fully polymerised silicate melts. Geochim. Cosmochim. Acta 52, 395 – 403. Dingwell, D.B., Webb, S.L., 1989. Structural relaxation in silicate melts and non-Newtonian melt rheology in geologic processes. Phys. Chem. Miner. 16, 508 – 516. Dingwell, D.B., Bagdassarov, N.S., Bussod, G.Y., Webb, S.L., 1993. Magma rheology. In: Luth, R.H. (Ed.), Short Course

Handbook on Experiments at High Pressure and Applications to Earth’s Mantle, vol. 21. Dingwell, D.B., Romano, C., Hess, K.U., 1996. The effect of water on the viscosity of a haplogranitic melt under P–T–X conditions relevant to silicic volcanism. Contrib. Mineral. Petrol. 124, 19 – 28. Dingwell, D.B., Hess, K.U., Romano, C., 1998a. Viscosity data for hydrous peraluminous granitic melts: comparison with a metaluminous model. Am. Mineral. 83, 236 – 239. Dingwell, D.B., Hess, K.U., Romano, C., 1998b. Extremely fluid behaviour of hydrous peralkaline rhyolites. Earth Planet. Sci. Lett. 158, 31 – 38. Dorfman, A., Hess, K.U., Dingwell, D.B., 1996. Centrifuge-assisted falling-sphere viscometry. Eur. J. Mineral. 8, 507 – 514. Giordano, D., Dingwell, D.B., 2003. Viscosity of hydrous Etna basalt: implications to the modeling of Plinian basaltic eruptions. Bull. Volcanol. 65, 8 – 14. Giordano, D., Dingwell, D.B., Romano, C., 2000. Viscosity of a Teide phonolite in the welding interval. J. Volcanol. Geotherm. Res. 103, 239 – 245. Hess, K.U., Dingwell, D.B., 1996. Viscosities of hydrous leucogranitic melts: a non-Arrhenian model. Am. Mineral. 81, 1297 – 1300. Hess, K.U., Dingwell, D.B., Webb, S.L., 1995. The influence of excess alkalis on the viscosity of a haplogranitic melt. Am. Mineral. 80, 297 – 304. Hess, K.U., Dingwell, D.B., Gennaro, C., Mincione, V., 2001. Viscosity–temperature behaviour of dry melts in the Qz–Ab–Or system. Chem. Geol. 174, 133 – 142. Lejeune, A.M., Richet, P., 1995. Rheology of crystal-bearing melts. J. Geophys. Res. 100 (B3), 4215 – 4229. Marsh, B.D., 1981. On the crystallinity, probability of occurrence, and rheology of lava and magma. Contrib. Mineral. Petrol. 78, 85 – 98. Ohlhorst, S., Behrens, H., Holtz, F., 2001. Compositional dependence of molar absorptivities of near-infrared OH- and H2O bands in rhyolitic to basaltic glasses. Chem. Geol. 174, 5 – 20. Papale, P., 1999. Strain-induced magma fragmentation in explosive eruptions. Nature 397, 425 – 428. Pinkerton, H., Stevenson, R.J., 1992. Methods of determining the rheological properties of magmas at sub-liquidus temperatures. J. Volcanol. Geotherm. Res. 53, 47 – 66. Richet, P., Bottinga, Y., 1986. Thermochemical properties of silicate glasses and liquids: a review. Rev. Geophys. 24 (1), 1 – 25. Roach, A., Rutherford, M.J., 2001. Experimental petrology of the trachytes of Agnano Monte Spina, Campi Flegrei. Proceedings of the meeting bFrame Programme for the Monitoring and Research Activity on Italian Volcanoes 2000– 2002Q, Gruppo Nazionale per la Vulcanologia, Roma, 9–11 October. , pp. 196 – 197. Romano, C., Poe, B., Mincione, V., Hess, K.U., Dingwell, D.B., 2001. The viscosities of hydrous XAlSi3O8 (X=Li, Na, K, Ca0.5, Mg0.5) melts. Chem. Geol. 174, 115 – 132. Romano, C., Giordano, D., Papale, P., Mincione, V., Dingwell, D., Rosi, M., 2003. The dry and hydrous viscosities of alkaline melts from Vesuvius and Phlegrean Fields. Chem. Geol. 202, 23 – 38.

D. Giordano et al. / Chemical Geology 213 (2004) 49–61 Rosi, M., Principe, C., Vecci, R., 1993. The Vesuvius 1631 eruption. A reconstruction based on historical and stratigraphical data. J. Volcanol. Geotherm. Res. 58, 151 – 182. Rosi, M., Vezzoli, L., Castelmenzano, A., Greco, G., 1999. Plinian pumice fall deposit of the Campanian Ignimbrite eruption (Phlegren Fields, Italy). J. Volcanol. Geotherm. Res. 91, 179 – 198. Russell, J.K., Giordano, D., Dingwell, D.B., Hess, K.U., 2002. Modelling the non-Arrhenian rheology of silicate melts: numerical considerations. Eur. J. Mineral. 14, 417 – 427. Signorelli, S., Vaggelli, G., Francalanci, L., Rosi, M., 1999. Origin of magmas feeling the Plinian phase of the Campanian Ignimbrite eruption, Phlegrean Fields (Italy): constraints based

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on matrix-glass and glass-inclusion compositions. J. Volcanol. Geotherm. Res. 91, 199 – 220. Toplis, M.J., 1998. Energy barriers to viscous flow and the prediction of glass transition temperatures of molten silicates. Am. Mineral. 83, 480 – 490. Webb, S.L., 1997. Silicate melts: relaxation, rheology, and the glass transition. Rev. Geophys. 35, 191 – 218. Whittington, A., Richet, P., Linard, Y., Holtz, F., 2000. Water and the viscosity of depolymerised aluminosilicate melts. Geochim. Cosmochim. Acta 64, 3725 – 3736. Whittington, A., Richet, P., Linard, Y., Holtz, F., 2001. The viscosity of hydrous phonolites and trachytes. Chem. Geol. 174, 209 – 223.