The dry and hydrous viscosities of alkaline melts from Vesuvius and Phlegrean Fields

www.elsevier.com/locate/chemgeo

Chemical Geology 202 (2003) 23–38

The dry and hydrous viscosities of alkaline melts from

Vesuvius and Phlegrean Fields

Claudia Romanoa,*, Daniele Giordanob,1, Paolo Papalec,2, Valeria Mincioned,3,Donald B. Dingwellb,4, Mauro Rosie,5

aDipartimento di Scienze Geologiche, Universita Roma Tre, L.go S. L. Murialdo, 1, 00154 Rome, ItalybDepartment of Earth and Environmental Sciences, University of Munich, Theresienstr. 41/III, D-80333 Munich, Germany

c Istituto Nazionale di Geofisica e Vulcanologia, sede di Pisa, via della Faggiola 32, I-56126 Pisa, ItalydDipartimento di Scienze della Terra, Universita La Sapienza, P.le Aldo Moro, 5 00185 Rome, Italy

eDipartimento di Scienze della Terra, via S. Maria 53, 56126 Pisa, Italy

Received 30 May 2002; accepted 4 June 2003

Abstract

Sophisticated models of volcanic scenarios are increasingly sensitive to the accuracy of their input parameters and

constitutive equations for magma properties. Viscosity is certainly one of the most important magma properties, but only

recently systematic investigations on silicate liquids with natural compositions have started. We investigated the Newtonian

viscosity of dry and hydrous phonolitic and trachytic melts from Vesuvius and Phlegrean Fields volcanic complexes,

respectively. The analysed samples come from the deposits of the AD 1631 (Vesuvius) and ca. 4400 BP Agnano Monte Spina

(AMS) (Phlegrean Fields) eruptions that are commonly taken as reference events for the most hazardous scenarios in case of

reactivation of the two volcanoes. Samples were hydrated via piston cylinder synthesis at P= 10 kbar and T= 1600 jC. The dryhigh temperature and dry or hydrous low temperature viscosities were measured by a combination of micropenetration and

concentric cylinder techniques, covering a total temperature range from about 400 to 1500 jC, water content range from

virtually dry to 3.8 wt.%, and viscosity range from 102 to 1012 Pa s. The viscosity data for each composition were fitted by a

modified Tamman–Vogel–Fulcher equation, allowing viscosity calculations at eruption temperatures and for dissolved water

contents in the range of those examined. The viscosity data and model calculations were used for a comparison with other

natural or synthetic phonolitic and trachytic melts, as well as with rhyolitic melts, for which viscosities had been measured. At

water contents less than 1 wt.%, a trend of increasing viscosity from phonolitic to trachytic to rhyolitic melts is found. At water

contents larger than 1 wt.%, the viscosity of trachytic melts is close to that of rhyolitic melts, while the viscosity of phonolitic

melts is one to two orders of magnitude lower. A compositional parameter given by the (Na +K+H)/(Si +Al) molar ratio is

found to be linearly related to the low-T hydrous viscosity of the trachytic and phonolitic melts considered, either analysed in

0009-2541/$ - see front matter D 2003 Elsevier B.V. All rights reserved.

doi:10.1016/S0009-2541(03)00208-0

* Corresponding author. Tel.: +39-6-54888018; fax: +39-6-54888201.

E-mail addresses: [email protected] (C. Romano), [email protected] (D. Giordano), [email protected] (P. Papale),

[email protected] (V. Mincione), [email protected] (D.B. Dingwell), [email protected] (M. Rosi).1 Fax: +48-89-2180-4176.2 Fax: +39-50-500675.3 Fax: +39-6-4454729.4 Fax: +49-89-2180-4176.5 Fax: +39-50-500675.

C. Romano et al. / Chemical Geology 202 (2003) 23–3824

this work or taken from literature. Differently, the rhyolitic melt shows significant variations from the trend found for phonolitic

and trachytic melts.

D 2003 Elsevier B.V. All rights reserved.

Keywords: Viscosity; Hydrous silicate melts; Trachytes; Phonolites

1. Introduction

During the last 10 years, significant efforts have

been made in modeling and simulating volcanic

processes (see the recent reviews in Gilbert and

Sparks, 1998; Freundt and Rosi, 1998). Recent fluid

dynamic models of volcanic eruptions, although still

incomplete, can reproduce several processes occurring

in magma chambers, along volcanic conduits, and in

the subaerial environment (Folch et al., 1998; Jaupart,

1998; Neri et al., 1998; Herzog et al., 1998; Papale,

2001). These studies account for a large number of

processes that contribute to determine the dynamics of

volcanic eruptions and the volcanic scenarios, and are

currently used for the evaluation of volcanic hazard at

many potentially dangerous volcanoes.

Fluid dynamic models need the definition of mag-

ma properties in the range of conditions encountered

from the deep regions of magma storage to the

surface. Among such properties, those related to

magma rheology play a critical role, with viscosity

variations well within the range of those occurring

during a single eruption resulting in significant

changes of the large-scale eruption dynamics (Neri

et al., 1998; Herzog et al., 1998; Papale et al., 1998).

Natural magmas are known to display a variety of

rheological behaviors depending on temperature,

composition, distribution of phases, and stress–strain

conditions (Webb and Dingwell, 1990; Stein and

Spera, 1992; Bagdassarov and Dingwell, 1993; Richet

and Bottinga, 1986, 1995; Smith, 1997; Webb, 1997;

Dingwell, 1998a,b). Unfortunately, even Newtonian

viscosities are still poorly known on a wide variety of

natural compositions. The well-known Shaw’s (1972)

model has provided a way to calculate such viscosity

as a function of composition, and has been extensively

used in many numerical applications (Webb and

Dingwell, 1990; Bagdassarov and Dingwell, 1993).

This model is now known to poorly estimate New-

tonian viscosities and bad trends of viscosity versus

dissolved water content over a large range of con-

ditions (Hess and Dingwell, 1996; Richet et al., 1996;

Giordano and Dingwell, 2003).

In this paper, we present the results of measure-

ments of the Newtonian viscosity of dry and hydrous

trachytic and phonolitic samples representative of the

glassy portion of pumice discharged during two

relevant eruptions of Vesuvius and Phlegrean Fields.

These are the AD 1631 (Vesuvius) and ca. 4400 BP

Agnano Monte Spina (Phlegrean Fields) eruptions.

The Vesuvius and Phlegrean Fields volcanoes are

among the most potentially dangerous in the world,

threatening the city of Naples and its densely

inhabited suburbs and represent a continuous menace

to more than one million people. We chose to focus

our attention on these two eruptions because both are

currently used as a reference for the most dangerous

possible eruptive scenarios at each volcano. Accord-

ingly, the reconstructed dynamics of such eruptions

and the associated pyroclast dispersal patterns are

used for the preparation of hazard maps and civil

defense plans (Rosi and Santacroce, 1984; Scandone

et al., 1991; Rosi et al., 1993).

Vesuvius and Phlegrean Fields belong to the po-

tassic alkaline province of Central Italy. The compo-

sition of magmas discharged during the two above

eruptions corresponds to phonolite (Vesuvius) and

trachyte (Phlegrean Fields). Current knowledge of

the viscosity of alkaline and peralkaline magmas is

at an early stage, and models that have been proposed

are limited to a very narrow compositional variety of

rocks (Dingwell et al., 1998; Giordano et al., 2000;

Whittington et al., 2000, 2001). Additionally, the

model accounting for the important non-Arrhenian

variation of viscosity of calc-alkaline magmas (Hess

and Dingwell, 1996) is proven to greatly fail for

alkaline magmas (Dingwell et al., 1998). Therefore,

there is a great need for viscosity measurements and

modeling pertaining to such compositions. With the

present work, we contribute to fill the existing gap.

The new viscosity data are used for a parameterization

of viscosity–temperature–water content relationships

C. Romano et al. / Chemical Geology 202 (2003) 23–38 25

by means of a modification of the well-known Tam-

man–Volgel–Fulcher (TVF) equation, that allows

calculation of the Newtonian viscosity in the range

of considered temperatures and water contents. Final-

ly, we make a comparison with viscosity determina-

tions from the literature pertaining to silicate liquids

with phonolitic, trachytic, and rhyolitic composition,

and discuss the complex role of compositional diver-

sities in determining Newtonian viscosities. Subse-

quent work will use the new viscosity equations in

conjunction with fluid dynamic modeling of magma

ascent and fragmentation (Papale, 2001), in order to

investigate the peculiarities of alkaline versus calc-

alkaline eruption dynamics and describe possible

future eruptive scenarios at Vesuvius and Phlegrean

Fields.

2. Analytical methods

Pumice samples were collected by hand-picking

from two separate fall-out layers within the deposits of

each investigated eruption. The layers correspond to

level B1 and D1 (De Vita et al., 1999) of the Agnano

Monte Spina (AMS) eruption of Phlegrean Fields, and

white and gray levels (Rosi et al., 1993) of the AD

1631 Vesuvius (V1631) eruption. Level B1 underlies

level D1, and white level underlies gray level in the

stratigraphy of the AMS and V1631 eruptions, respec-

Table 1

Dry composition (wt.%) of glassesa

Analysed in this work

AMS_B1 AMS_D1 V_1631_W V_1

SiO2 61.26 60.86 53.52 53.1

TiO2 0.38 0.39 0.60 0.

Al2O3 18.38 18.27 19.84 19.8

FeOb 3.50 3.88 4.80 4.

MnO 0.14 0.12 0.14 0.

MgO 0.74 0.90 1.76 1.

CaO 2.97 2.96 6.76 6.

Na2O 4.58 4.12 4.66 4.

K2O 8.04 8.50 7.91 8.

a AMS_B1 and AMS_D1: glasses of trachytic composition from the A

respectively. V1631_W and V1631_G: glasses of phonolitic compositio

respectively. W_Tr and W_Ph: synthetic glasses of trachytic and phonolit

glass of phonolitic composition from Teide volcano, from Giordano et al. (2

et al. (1996). Small amounts of P2O2 are neglected.b Total Fe as FeO.

tively. Samples corresponding to each one of the above

layers were processed and analysed separately.

Glasses were separated from the coarsely crushed

samples of the selected rocks with the aid of a

binocular microscope. The melts used in this study

were then generated by fusion and homogenisation by

stirring of the separated phonolitic and trachytic glass

matrices at room pressure and 1400–1650 jC. Melt

samples were then allowed to cool to room tempera-

ture in the crucible.

All glasses were checked by optical microscopy for

the possible presence of crystalline phases and were

found to be crystal-free. We also failed to find any

evidence of phase separation in the glasses. Cylinders

8 mm in diameter were drilled out of the cooled

glasses, and sawed into disks 3 mm long. The disks

were then polished on both sides and stored in a

desiccator until used in the micropenetration experi-

ments. The remaining glass from the same crucible

was broken and reloaded into a crucible which was

then reheated to the starting temperature for concen-

tric cylinder viscosity measurements. The high tem-

perature anhydrous viscosities were determined in a

temperature range between 1050 and 1500 jC and for

a log10 viscosity range (Pa s) from 1.9 to 4.8 using the

method described in Dingwell and Virgo (1988).

The anhydrous glass compositions were chemical-

ly analysed by electron microprobe (Table 1). The

nominally anhydrous glasses were assumed to have

Not analysed in this work

631_G W_Tr W_Ph T_Ph HPG8

4 64.44 58.82 60.72 78.69

59 0.50 0.79 0.56

4 16.71 19.42 18.89 12.51

72 3.32

13 0.20

77 2.92 1.87 0.36

75 5.36 2.35 0.68

77 6.70 9.31 9.80 4.60

28 3.37 7.44 5.47 4.20

gnano Monte Spina (Phlegrean Fields) pumice, levels B1 and D1,

n from the AD 1631 Vesuvius eruption, levels White and Gray,

ic composition, respectively, from Whittington et al. (2001). T_Ph:

000). HPG8: synthetic glass of rhyolitic composition, from Dingwell

Table 2

Infrared data and water contents of analysed samples

Sample Density

(kg/m3)

Thickness

(Am)

Absorbance H2O

(wt.%)

G 637 2595 50 0.64 1.26

G 638 2582 21 0.43 2.04

G 639 2546 27 0.82 3.07

G dry 2586

W 640 2558 50 0.58 1.17

W 642 2496 27 0.87 3.32

W 643 2541 24 0.52 2.21

W dry 2549

D1 643 2500 98 1.09 1.15

D1 639 2413 31 1.09 3.75

D1 641 2468 42 0.82 2.04

D1 641 2485 42 0.82 2.02

D1 640 2448 49 1.11 2.38

D1 dry 2475

B1 642 2503 82 0.63 0.79

B1 638 2560 49 0.58 1.19

B1 637 2396 31 1.09 3.78

B1 636 2588 97 1.23 1.26

B1 dry 2481

Uncertainties in density determinations are estimated at F 0.05 kg/

m3. Calibrated density equations:

Agnano Monte Spina (B1 and D1): q (kg/m3) = 2480.3 +

20.557wH2O� 11.024(wH2O

)2.

Vesuvius 1631 White: q (kg/m3) = 2548.8 + 20.945wH2O� 11.104

(wH2O)2.

Vesuvius 1631 Gray: q (kg/m3) = 2585.6 + 20.948wH2O� 11.107w

(wH2O)2. wH2O

is wt.% dissolved water.


0.02 wt.% dissolved H2O based on previous IR

measurements on nominally dry glasses of similar

composition (Ohlhorst et al., 2001). After the concen-

tric cylinder experiments, the dry glass was retrieved

by drilling and used for the hydrous experiments. For

each composition, four to five glasses with different

water contents in the range 0.8–3.8 wt.% were

synthesised at T= 1600 jC and P= 10 kbar in a piston

cylinder apparatus for several hours, in order to ensure

dissolution and homogenisation of water into the melt.

The quenched hydrous glasses (isobaric quench rate

from dwell T to Tg on the order of 100 jC/s) were thenrecovered and prepared for micropenetration viscom-

etry and infrared spectroscopy.

The low temperature viscosities were measured

using a micropenetration technique. This involves de-

termining the rate at which an Ir indenter under a fixed

load moves into the melt surface. Details about the

experimental procedure are reported in Dingwell et al.

(1996) and Hess et al. (1995). The equilibration time at

dwell temperature before the viscosity measurements

was minimized to about 15 min, to prevent water

exsolution and crystallization. The measurement time

was also kept to a minimum (approximately 5 min).

Different sample plates from the same hydrous glasses

were used and only one viscosity measurement was

performed on each plate. The viscosity measurements

were performed in the viscosity range 108–1012 Pa s

and in the temperature range 404–814 jC. Reproduc-ibility of viscosity measurements is ensured within an

error of + 0.06 log units, based on prior calibrations

(Hess et al., 1995; Giordano and Dingwell, 2003).

The homogeneity and stability of water content

were checked by FTIR spectroscopy. Before and after

each viscosity measurement, the hydrous samples

were analysed to measure the total water content

and checked for water loss during viscometry. A

Bruker IFS 120 HR Fourier Transform Spectropho-

tometer was used to obtain transmission infrared

spectra in the NIR region (2500–8000 cm� 1), using

a W source, CaF2 beam splitter and MCT detector.

The H2O content of the samples was determined by

measuring the heights of the peaks at approximately

3570 cm� 1 attributed to the fundamental OH-stretch-

ing vibration (Nakamoto, 1997). Details about the

background subtraction procedure can be found in

Romano et al. (1995) and Behrens et al. (1996).

Uncertainty of the results is about 2% based on the

reproducibility of measurements and on the error

associated with the background subtraction procedure.

In order to calculate the water content from the

measured absorbances, thickness and density of each

sample were also measured (Table 2). The thickness

of each glass plate was measured with a digital

Mitutoyo micrometer (precision F 3 Am). Densities

of the dry glasses and of some of the hydrous glasses

were determined by weighing in air and in ethanol

using a Mettler Toledo AG 204 balance. Uncertainties

are estimated to be F 10 kg/m3. Polynomial expres-

sions for densities (reported in Table 2) have been

obtained by least-square regression.

For compositions varying from basalt to rhyolite

the molar absorptivity of the peak at 3500 cm� 1

ranges from 63 to 88� 10� 4 m2/mol (Carroll and

Holloway, 1994 and references therein). For compo-

sitions closer to those investigated here, the range

narrows down to values from 68 (Yamashita et al.,

1997, dacitic compositions) to 70� 10� 4 m2/mol


(Silver and Stolper, 1989, albitic compositions). We

have adopted here a molar absorptivity of 70� 10� 4

m2/mol. The total error expected on the calculated

water contents from the dependence of molar absorp-

tivities on composition is F 4%.

3. Results of viscosity measurements and data

modeling

Viscosity determinations for the dry (dissolved

water content of 0.02 wt.%) and hydrous melts are

presented in Tables 3 and 4 for the Phlegrean Fields

Table 3

Viscosity data for dry and hydrous trachytic samples from the Agnano M

Sample H2O

(wt.%)

T (jC) log g(Pa s)

AMS_D1 dry 0.02 1496 2.49

AMS_D1 dry 0.02 1446 2.74

AMS_D1 dry 0.02 1397 3.01

AMS_D1 dry 0.02 1348 3.30

AMS_D1 dry 0.02 1299 3.62

AMS_D1 dry 0.02 1249 3.96

AMS_D1 dry 0.02 1200 4.33

AMS_D1 dry 0.02 1151 4.73

AMS_D1 dry 0.02 814.1 8.45

AMS_D1 dry 0.02 765.3 9.32

AMS_D1 dry 0.02 736.5 9.77

AMS_D1 dry 0.02 712.0 10.56

AMS_D1 dry 0.02 700.2 10.75

AMS_D1 dry 0.02 683.8 11.29

AMS_D1 643 1.15 644.3 9.04

AMS_D1 643 1.15 612.3 9.72

AMS_D1 643 1.15 591.9 10.08

AMS_D1 643 1.15 576.7 10.42

AMS_D1 643 1.15 546.5 11.24

AMS_D1 641 2.04 548.5 9.55

AMS_D1 641 2.04 522.3 10.20

AMS_D1 641 2.04 502.6 10.80

AMS_D1 641 2.04 490.7 11.04

AMS_D1 640 2.38 521.4 9.70

AMS_D1 640 2.38 503.5 10.05

AMS_D1 640 2.38 488.1 10.65

AMS_D1 640 2.38 470.7 10.97

AMS_D1 639 3.75 450.5 9.90

AMS_D1 639 3.75 436.2 10.31

AMS_D1 639 3.75 415.7 11.05

AMS_B1 dry 0.02 1446 2.79

AMS_B1 dry 0.02 1397 3.06

Temperature accuracy is F 0.5 jC for the low temperature data, and F 1

log units based on DGG Standard Glass determinations. Sample names ide

and Vesuvius samples, respectively. Fig. 1 shows such

viscosities as a function of reciprocal temperature.

The addition of water to the melts results in a large

shift of the viscosity– temperature relationship to

lower temperatures for all the compositions investi-

gated, in good agreement with the trend observed for a

wide range of natural and synthetic melts (Richet et

al., 1996; Dingwell et al., 1996; Schulze et al., 1996;

Holtz et al., 1999; Romano et al., 2001; Giordano et

al., 2000; Whittington et al., 2000, 2001). The vis-

cosity drops dramatically when the first 1 wt.% H2O

is added to the melt, then tends to level off at higher

water contents. The drop of viscosity as water is

onte Spina (Phlegrean Fields) eruption

Sample H2O

(wt.%)

T (jC) log g(Pa s)

AMS_B1 dry 0.02 1348 3.35

AMS_B1 dry 0.02 1299 3.67

AMS_B1 dry 0.02 1249 4.02

AMS_B1 dry 0.02 1200 4.39

AMS_B1 dry 0.02 1151 4.80

AMS_B1 dry 0.02 784.8 9.06

AMS_B1 dry 0.02 768.3 9.41

AMS_B1 dry 0.02 732.5 10.39

AMS_B1 dry 0.02 693.9 11.18

AMS_B1 642 0.79 686.3 8.83

AMS_B1 642 0.79 628.4 9.88

AMS_B1 642 0.79 585.9 10.70

AMS_B1 642 0.79 567.6 11.35

AMS_B1 638 1.19 589.6 9.54

AMS_B1 638 1.19 571.9 10.01

AMS_B1 638 1.19 554.4 10.24

AMS_B1 638 1.19 529.7 10.74

AMS_B1 638 1.19 525.1 11.02

AMS_B1 638 1.19 513.3 11.56

AMS_B1 636 1.26 599.7 9.33

AMS_B1 636 1.26 580.6 9.74

AMS_B1 636 1.26 557.7 10.09

AMS_B1 636 1.26 541.0 10.56

AMS_B1 636 1.26 538.1 10.62

AMS_B1 636 1.26 526.0 10.85

AMS_B1 636 1.26 521.5 11.19

AMS_B1 637 3.78 451.1 9.85

AMS_B1 637 3.78 444.9 10.05

AMS_B1 637 3.78 425.6 10.52

AMS_B1 637 3.78 420.2 10.95

AMS_B1 637 3.78 406.3 11.33

AMS_B1 637 3.78 404.2 11.43

jC for the high temperature data. Viscosities are accurate to F 0.06

ntify the origin (B1 =B1 level, D1 =D1 level) and synthesis number.

Table 4

Viscosity data for dry and hydrous phonolitic samples from the AD 1631 Vesuvius eruption

Sample H2O

(wt.%)

T (jC) log g(Pa s)

Sample H2O

(wt.%)

T (jC) log g(Pa s)

V_1631_W dry 0.02 1397 1.96 V_1631_W 642 3.32 431.2 10.82

V_1631_W dry 0.02 1348 2.25 V_1631_G dry 0.02 1397 2.28

V_1631_W dry 0.02 1299 2.56 V_1631_G dry 0.02 1348 2.54

V_1631_W dry 0.02 1249 2.91 V_1631_G dry 0.02 1299 2.83

V_1631_W dry 0.02 1200 3.29 V_1631_G dry 0.02 1249 3.15

V_1631_W dry 0.02 1151 3.72 V_1631_G dry 0.02 1200 3.48

V_1631_W dry 0.02 1102 4.22 V_1631_G dry 0.02 1151 3.87

V_1631_W dry 0.02 1053 4.77 V_1631_G dry 0.02 1102 4.29

V_1631_W dry 0.02 770.0 8.98 V_1631_G dry 0.02 1053 4.75

V_1631_W dry 0.02 755.1 9.01 V_1631_G dry 0.02 805.1 8.81

V_1631_W dry 0.02 752.2 9.44 V_1631_G dry 0.02 771.1 9.58

V_1631_W dry 0.02 723.0 9.97 V_1631_G dry 0.02 756.3 9.78

V_1631_W dry 0.02 708.5 10.26 V_1631_G dry 0.02 726.7 10.20

V_1631_W dry 0.02 689.2 10.68 V_1631_G dry 0.02 707.3 10.66

V_1631_W 640 1.17 594.2 9.17 V_1631_G dry 0.02 689.0 11.05

V_1631_W 640 1.17 586.9 9.20 V_1631_G 637 1.26 575.1 9.40

V_1631_W 640 1.17 567.5 9.84 V_1631_G 637 1.26 563.6 9.71

V_1631_W 640 1.17 545.7 10.32 V_1631_G 637 1.26 542.2 10.35

V_1631_W 640 1.17 532.4 10.64 V_1631_G 637 1.26 510.0 11.29

V_1631_W 640 1.17 516.4 11.05 V_1631_G 638 2.04 522.0 9.16

V_1631_W 643 2.21 526.8 8.90 V_1631_G 638 2.04 505.9 9.62

V_1631_W 643 2.21 505.4 9.40 V_1631_G 638 2.04 486.3 10.16

V_1631_W 643 2.21 473.6 10.29 V_1631_G 638 2.04 461.3 11.00

V_1631_W 642 3.32 480.6 9.32 V_1631_G 639 3.07 462.9 9.84

V_1631_W 642 3.32 475.6 9.46 V_1631_G 639 3.07 444.5 10.33

V_1631_W 642 3.32 454.6 9.82 V_1631_G 639 3.07 435.7 10.61

V_1631_W 642 3.32 444.5 10.25

Temperature accuracy is F 0.5 jC for the low temperature data, and F 1 jC for the high temperature data. Viscosities are accurate to F 0.06

log units based on DGG Standard Glass determinations. Sample names identify the origin (W=white pumice, G = gray pumice) and synthesis

number.


introduced in the melt is slightly higher for the

Vesuvius phonolites than for the AMS trachytes.

The viscosity data summarized in Fig. 1 have been

first fitted using the approximation of Arrhenian

viscosity–temperature relationship:

logg ¼ aA þEa

RTð1Þ

where aA is a pre-exponential term and Ea is the

activation energy of viscous flow. The Arrhenian

approximation for silicate melts has been demonstrat-

ed to be invalid for large temperature ranges (e.g.

Richet and Bottinga, 1995; Richet et al., 1996; Hess

and Dingwell, 1996; Whittington et al., 2000, 2001;

Giordano et al., 2000; Giordano and Dingwell, 2003).

However, the Arrhenian approximation is still useful

to compare data over a limited temperature or viscos-

ity range. The results pertaining to the Arrhenian fit

are presented in Table 5 in terms of aA and Ea

parameters, and have been obtained by considering

separately the different compositions including the

dissolved water content, and referring to only the

low-T data in Tables 3 and 4 and Fig. 1. In Table 5,

the activation energies calculated for synthetic pho-

nolites and trachytes (Whittington et al., 2001), natu-

ral Teide phonolite (Giordano et al., 2000), and

synthetic rhyolite (Dingwell et al., 1996), listed in

Table 1, are also reported for comparison. As water is

introduced into the melt the activation energies de-

crease, in agreement with previous results on both

natural and synthetic melts (Dingwell, 1987; Persikov,

1991; Schulze et al., 1996; Holtz et al., 1999; Romano

et al., 2001). At water contents higher than about 2

Table 5

Pre-exponential coefficient A and activation energy Ea for Arrhenian

viscosity– temperature relationship

Composition H2O (wt.%) Ea (KJ/mol) A

AMS 0.02 189 � 13.9

0.79 138 � 8.5

1.15 138 � 9.1

1.19 136 � 9.5

1.26 128 � 8.4

2.04 137 � 10.6

2.38 129 � 9.8

3.78 137 � 13.0

V_1631 0.02 160 � 9.2

1.17 142 � 10.5

1.26 161 � 13.4

2.04 146 � 13.0

2.21 130 � 10.7

3.07 122 � 10.1


wt.%, the activation energies tend to converge to a

value from 120 to 140 kJ/mol, with exception only to

the rhyolitic melt (HPG8) having Ea value as low as

114 kJ/mol at a dissolved water content of only 1.33

wt.%. The slight increase of Ea with increasing water

content shown by the trachytic liquids from the AMS

eruption of Phlegrean Fields at the largest dissolved

water contents used in the present viscosity measure-

ments (3.75–3.78 wt.%) might possibly reflect unre-

laxed conditions of some of the experimental liquids

prior to measurements.

In order to fit the whole dry and hydrous viscosity

data in Fig. 1, a non-Arrhenian model must be

employed. The Adam–Gibbs theory, also known as

configurational entropy theory (e.g. Richet and Bot-

tinga, 1995; Angell, 1995), provides a theoretical

Fig. 1. Measured viscosities of trachytic samples from the AMS

eruption of Phlegrean Fields, and of phonolitic samples from the

V1631 eruption of Vesuvius. Also shown in the figure is a

comparison between measured viscosities (symbols), and viscosities

calculated by the use of Eq. (6) (lines). The numbers in the lines

refer to water content.

3.32 128 � 11.2

W_Tr 0.02 234 � 17.1

0.57 201 � 15.4

0.83 180 � 13.8

1.19 164 � 12.7

2.19 143 � 11.5

2.90 142 � 12.4

4.92 138 � 14.7

W_Ph 0.02 213 � 15.9

0.88 180 � 14.3

1.46 153 � 13.3

1.52 156 � 13.7

2.15 134 � 11.5

2.83 137 � 13.0

4.72 123 � 12.9

Td_Ph 0.02 160 � 10.0

0.85 145 � 11.3

0.95 134 � 10.3

2.10 144 � 13.3

3.75 136 � 13.7

HPG8 0.02 199 � 9.6

0.99 141 � 8.7

1.33 114 � 5.7

background for the approximation of viscosity data.

The model equation is:

logg ¼ aþ b

TSCð2Þ

where a and b are weak functions of temperature and

are commonly taken as T-independent, and SC is the

configurational entropy of the liquid given by:

SC ¼ SCTg þZ T

Tg

CCP

TdT ð3Þ

Table 6

Calibrated parameters of Eq. (6)

Run a1 a2 b1 b2 c1 c2

Td_Ph � 5.8996 � 0.2857 10775 � 394.83 148.71 � 21.650

W_Ph � 3.0850 0.05194 7127.2 � 419.51 305.42 � 37.869

V1631 � 6.7898 � 0.02653 12143.2 � 541.20 145.14 � 33.342

AMS � 3.5405 0.14467 9618.9 � 498.79 191.78 � 35.518

W_Tr � 2.2091 0.48789 7873 � 552.28 304.91 � 47.851

HPG8 � 6.6955 � 0.10556 15864 � 623.50 3.93 � 63.339

Values correspond to use of wt.% H2O and absolute temperature in

the equation, and restitute viscosity in Pa s.


with Tg being the glass transition temperature, and CPC

the configurational specific heat at constant pressure,

given by the difference between the specific heat at

system and glass transition temperatures. The Adam–

Gibbs theory represents a reliable way to incorporate

the viscosity data into a model, since the theoretical

basis outlined in Eqs. (2) and (3) allows confident

extrapolation beyond the range of conditions of the

viscosity measurements. Unfortunately, the effects of

dissolved water on the parameters a and b, on the

configurational entropy at glass transition temperature

STg

C, and partly on the configurational specific heat CPC,

are still poorly known. This implies that the use of Eq.

(2) as a viscosity model covering dry and hydrous data

requires arbitrary functions for the dissolved water

dependence of each of the above parameters, resulting

in a semi-empirical form of the viscosity equation and

in the loss of sound theoretical basis. Therefore, there

is no strong reason to prefer the configurational

entropy theory at Eqs. (2) and (3) to simpler empirical

relationships like the TVF equation (Angell, 1995;

Romano et al., 2001):

logg ¼ aþ b

T � cð4Þ

with a, b and c being fit parameters with values

depending on the dissolved water content. The capa-

bility of Eq. (4) to reproduce dry and hydrous viscosity

data is tested in this as well as in other papers (e.g.

Angell, 1995).

In order to better reproduce the viscosity data in

Fig. 1, as well as many others from literature, we have

found that the best functional forms of parameters a, b

and c in Eq. (4) are the following:

a ¼ a1 þ a2lnwH2O

b ¼ b1 þ b2wH2O

c ¼ c1 þ c2lnwH2O ð5Þ

where wH2Ois the dissolved water content in wt.%.

The TVF equation used to interpolate the viscosity

data is therefore the following:

logg ¼ a1 þ a2lnwH2O þ b1 þ b2wH2O

T � ðc1 þ c2lnwH2OÞð6Þ

The results of the fits are listed in Table 6 for the

compositions analysed in this work as well as for others

that are used for comparison. Only one set of param-

eters has been derived for the trachytic liquids from

Phlegrean Fields and the phonolitic liquids from Vesu-

vius, due to small compositional differences between

the groundmass of analysed pumice samples from the

different stratigraphic layers (Table 1), reflecting rela-

tively small viscosity differences (Tables 3 and 4, and

Fig. 1).

The results of fitting based on Eq. (6) are shown in

Fig. 1 together with the experimental data, for the

trachytic and phonolitic compositions analysed in this

work. As it emerges from the figures, the TVF Eq. (6)

satisfactorily reproduces the low-T dry and hydrous,

and the high-T dry data. However, an uncertainty

remains as to the extent to which the curves in Fig. 1

can be used to predict viscosities at conditions relevant

for the magmatic and volcanic processes, that is, for

hydrous liquids in a region in Fig. 1 corresponding to

values on the horizontal axis between 8 and 10

(� 10000) K� 1. The acquisition of viscosity data in

such conditions is hampered by the too rapid water

exsolution and crystallization kinetics that occur on a

time scale similar to that of measurements. The meas-

urements of the viscosity of liquids at high pressure via

the falling sphere method (Dobson et al., 1996; Kush-

iro, 1978; Scaillet et al., 1996; Schulze et al., 1996;

Dorfman et al., 1996) gives the possibility of reducing

or eliminating the water exsolution-related problems

(but possibly requiring the use of P-dependent terms in

the viscosity modeling). The liquid viscosities at erup-

tive temperatures calculated with Eq. (6) therefore need

to be confirmed by future high-pressure measurements.

Fig. 2 shows the isokoms (lines at constant viscos-

ity) corresponding to 1010.5 Pa s as a function of

dissolved water content, for the compositions investi-

gated here and for others from Table 1 using TVF

parameters (Eq. 6) given in Table 6. This type of

Fig. 2. Isokom temperatures corresponding to a viscosity of 1010.5

Pa s for the AMS trachytes and V1631 phonolites analysed in this

work, and for other phonolitic, trachytic, and rhyolitic liquids from

literature, plotted against the dissolved water content. Compositions

in Table 1. Calculations based on the modified TVF Eq. (6).


representation allows a comparison between viscosity

calculations without requiring an extrapolation out-

side the range of experimental measurements; there-

fore, minimizing the errors derived from the fitting

procedure. In Fig. 2, the isokom calculated from a

TVF equation for rhyolites calibrated by Hess and

Dingwell (1996) is also reported for comparison.

It should be noted that the use of Eq. (6) as a

viscosity model prevents taking into account dry

viscosities, since in such a condition the calculated

viscosities do not take a finite value. On the other

hand, true anhydrous compositions are virtually never

reached during laboratory measurements, where even

the most degassed glasses still retain water contents of

the order of 0.02 wt.% (Ohlhorst et al., 2001).

Therefore, throughout this paper we refer to dry

conditions by actually considering a water content

of 0.02 wt.%. It is worth noting that the semi-empir-

ical nature of Eq. (6) implies that our aim is not to

produce a theoretically sound model working down to

true dry conditions. On the contrary, it provides a

useful method for calculating liquid viscosities over a

wide range of practical purposes. Regressions done by

assuming that the dissolved water content at nominal-

ly dry conditions corresponds to 0.04 or 0.06 produce

small differences in model predictions for the trachyt-

ic and phonolitic liquids that we investigate. Maxi-

mum differences of 0.5 log units are obtained at very

low water contents of the order of a few hundreds

ppm. At larger water contents, differences in model

predictions are less than 0.3 log units in the temper-

ature range from 900 to 1300 jC.Fig. 2 allows further insights into the viscosities of

the analysed samples, by relating such viscosities to

those of alkaline as well as calc-alkaline liquids

analysed in previous papers. The figure shows the

decrease of isokom temperature with increasing water

content (conceptually analogous to decrease of vis-

cosity with increasing water content). Below about 1

wt.% dissolved water the curves show a clear trend of

increasing isokom temperature from phonolites to

trachytes to rhyolites. While the isokom temperature

of phonolites remains lower than that of trachytes

along the entire water content range of measurements,

the isokom temperature of trachytes becomes equal to

or even larger than that of rhyolites at water contents

exceeding 1 wt.%. The synthetic trachytes of Whit-

tington et al. (2001) display higher isokom temper-

atures compared with the natural trachytic liquids

from Phlegrean Fields. Differently, the phonolitic

liquid of Whittington et al. (2001) exhibits isokom

temperatures which are close to those of phonolites

from Vesuvius. The phonolitic liquid from Teide dis-

plays the lowest isokom temperatures up to about 2

wt.% dissolved water, then it merges with the other

phonolitic liquids in the figure.

Fig. 3 shows the calculated viscosities as a function

of water content for the liquid compositions analysed

in this work, and for others in Tables 1 and 6 that are

used for comparison. The curves refer to a constant

temperature of 830 jC, at which the calculations for

hydrated conditions represent an extrapolation based

on Eq. (6). Consequently, the associated uncertainties

are larger than those of the curves in Fig. 2, which, in

contrast, represents an interpolation of data points. In

spite of this, we can note that the anhydrous viscosity

calculations on the vertical axis of the figure are well

constrained by measurements at high and low tem-

perature (Table 1 and Fig. 1), and that the viscosity

trends in Fig. 3 are very similar to those in Fig. 2. In

fact, in both figures the different compositional groups

show very distinct viscosity ranges at low water

contents, with viscosity increasing from phonolites

to trachytes to rhyolites. While the difference in

calculated viscosity between trachytes and rhyolite

increases when the water content approaches zero,

the same difference decreases markedly when com-

Fig. 3. Viscosity as a function of dissolved water content, at

constant T= 830 jC, for the AMS trachytes and V1631 phonolites

analysed in this work, and for other phonolitic, trachytic, and

rhyolitic liquids from literature, calculated on the basis of Eq. (6)

with calibrated parameters in Table 6.


paring dry trachytes with dry phonolites. At water

contents greater than about 1 wt.% the viscosities of

rhyolite and trachytes tend to merge, whereas those of

phonolites maintain lower and distinct values. There-

fore, we can conclude that an extrapolation based on

Eq. (6) does not modify the viscosity relationships

shown by the data and isokoms in Fig. 2. Extrapola-

tion up to water contents of 4 wt.% (Fig. 3) shows that

the viscosity of phonolites at such conditions can be

as low as about 103 Pa s, similar to that of basaltic

Table 7

Compositional/structural parameters for samples in Table 1

A/AEa M1/M2b AIc TA/Sd

Td_Ph 20.52 9.2 1.17 0.25

W_Ph 5.19 5.19 1.2 0.28

V1631_W 1.94 1.59 0.82 0.23

V1631_G 2.01 1.66 0.85 0.25

AMS_B1 4.46 3.26 0.88 0.21

AMS_D1 4.17 3.02 0.87 0.21

W_Tr 1.71 1.71 0.88 0.16

HPG8 l l 0.97 0.11

Where present, the three values separated by hyphens refer to water conta A/AE is the ratio of alkaline to alkaline earths (moles of elements).b M1/M2 is the ratio of single charge to double charge cations (molesc AI is the agpaicity index given by the molar ratio (Na2O +K2O)/Al2d TA/S is the total alkali to silica ratio (mass of oxides).e TA/S* is the total alkali to silica ratio (moles of elements).f Al/Si is the ratio aluminum–silica in moles of elements.g NBO/T is the ratio of non-bridging oxygens to number of structuralh m is the fragility of liquid calculated as in Romano et al. (2000).

liquids with a few wt.% dissolved water (Giordano

and Dingwell, 2003). Differently, for trachytes and

water content of 4 wt.% the calculated viscosity is as

high as about 105 Pa s, close to or even larger than

that of rhyolitic liquids at the same conditions.

4. Discussion

The results in Figs. 2 and 3 clearly show that

phonolites, trachytes, and rhyolites are characterized

by distinct viscosity trends, and that the compositional

variations within each compositional type play a

second-order role in determining viscosity. The main

results from Figs. 2 and 3 can be summarized as

follows: (i) the dry viscosity of trachytes is close to

one order of magnitude higher than that of phonolites,

while the dry viscosity of rhyolites is about three

orders of magnitude higher than that of trachytes; (ii)

the initial viscosity drop due to addition of small

amounts of water appears to be largest for rhyolites,

and smallest for trachytes; (iii) the hydrous (wt.%

H2O>1) viscosity of trachytes is close to that of

rhyolites, while the hydrous viscosity of phonolites

is one to two orders of magnitude lower.

The above distinct viscosity trends must reflect

some substantial difference among the structural/com-

positional characteristics of phonolites, trachytes, and

rhyolites. In Table 7, we report a summary of relevant

TA/S*e Al/Sif NBO/Tg mh

0.43 0.37 0.10-0.12-0.34 22.9-22.6-22.5

0.47 0.39 0.19-0.21-0.44 30.2-26.7-25.2

0.36 0.44 0.23-0.25-0.49 27.6-25.5-24.4

0.37 0.44 0.24-0.26-0.50 27.6-25.5-24.4

0.31 0.35 0.10-0.12-0.34 26.0-22.6-20.6

0.31 0.35 0.10-0.12-0.34 26.0-22.6-20.6

0.27 0.31 0.21-0.23-0.45 32.3-25.1-21.3

0.18 0.19 0.00-0.02-0.22 23.6-20.3-17.2

ents of 0.02, 0.3, and 3 wt.%, respectively.

of elements).

O3.

tetrahedra in the liquid calculated as in Mysen (1988).


quantities that describe structural and compositional

features of the liquids in Table 1. We must anticipate

that the use of natural instead of synthetic composi-

tions largely complicates a compositionally based

interpretation of the viscosity data, since it is not

possible to isolate the influence of each compositional

factor that varies from one liquid to another. Such an

interpretation is more easily done when comparing

synthetic liquids, appropriately designed to investigate

the role of any single compositional parameter (Whit-

tington et al., 2000, 2001; Romano et al., 2001). On

the other hand, data on natural compositions are

required in order to use the laboratory determinations

for applications to real volcanic systems. In the

present case, the very distinct trends of viscosity with

composition, either natural or synthetic, that are

summarized above, suggest that some basic difference

should appear from Table 7. Since such viscosity–

composition relationships largely depend on the

amount of dissolved water, structural/compositional

parameters that depend on water are reported in Table

7 for three different water contents of 0 (actually 0.02

wt.% as explained above), 0.3 wt.% (at which the

three compositional types considered show most dis-

tinct viscosity values), and 3 wt.% (at which trachytes

and rhyolites have similar viscosity while phonolites

have significant lower viscosity) (Figs. 2 and 3).

Two parameters have been successfully used in

previous works to explain viscosity–composition

relationships. These parameters are the ratio of non-

bridging oxygens to tetrahedrally coordinated, net-

work-forming cations (NBO/T), and the type and

abundance of network-modifying cations present in

the liquid (expressed in Table 6 as the ratio of alkaline

to alkaline earth cations, A/AE) (Hess et al., 1995;

Hess and Dingwell, 1996; Romano et al., 2001;

Whittington et al., 2001). The NBO/T value is repre-

sentative of the degree of polymerization of the liquid.

In a fully polymerized liquid all of the oxygens are

bridging, and therefore, NBO/T is zero. Progressively

larger values indicate progressively larger numbers of

oxygens which are not used to link network-forming

cations, therefore, progressively lower degree of po-

lymerization (Mysen, 1988).

At constant NBO/T, the value of A/AE has been

found to be inversely proportional to viscosity in the

high viscosity range (109 to 1012 Pa s) (Whittington et

al., 2001). Moreover, for fully polymerized liquids in

the high viscosity range (109 to 1012 Pa s) the viscosity

of alkali-bearing liquids is lower than the cor-

responding viscosity of alkaline-earth-bearing liquids,

whereas the opposite occurs in the low viscosity range

(100 to 105 Pa s) (Romano et al., 2001). This was

interpreted as due to the different relative importance of

cooperative motion of single flow units and average

bond strength of the melt at high and low viscosity (or

low and high temperature), with the former factor

dominating at high viscosity, and the latter at low

viscosity. Since alkaline cations interact with the equiv-

alent of just one NBO, whereas alkaline earth cations

require the equivalent of two, the average size of

rearranging flow units is greatly reduced when the

value of A/AE is increased, resulting in increased

configurational entropy and favoring cooperative mo-

tion, thus reducing viscosity at low temperature. On the

contrary, the higher field strength of the divalent

alkaline earth cations results in stronger bonds with

oxygen, leaving weaker, more destabilized Al–O and

Si–O bonds, resulting in a decrease of viscosity at high

temperature with decreasing A/AE. In the case of

natural samples, the calculation of NBO/T must be

considered as a first approximation of the melt’s degree

of polymerization, given the uncertainty in the calcu-

lation of the Fe3 +/Fe2 + ratio and the complexity of the

structural role of Fe and water in the melt. Although we

have not determined Fe3 +/Fe2 + quantitatively, we

estimate it to be 0.5 based on the bulk composition of

our samples (Middlemost, 1989). Deviation from this

value (e.g. due to treatment at high temperature) would

have only a slight effect on the calculated NBO/T

because of the relatively small amounts of total Fe in

the samples. In our calculation, we have considered the

water dissolved in the liquid as total water content (in

mol fraction), not taking into consideration its specia-

tion (OH� andmolecular H2O) (see discussion further).

From the parameters in Table 7, it emerges that

neither the NBO/T nor the A/AE values show mono-

tonic trends with viscosity. As an example, the dry

phonolite from Teide shows the highest degree of

polymerization (lowest NBO/T) among trachytes and

phonolites, but lowest viscosity (Figs. 2 and 3).

Another example is provided by the dry synthetic

trachyte from Whittington et al. (2001), which shows

lower polymerization but higher viscosity with respect

to the dry AMS trachytes. Similar relationships

emerge from the hydrous data.

Fig. 4. Calculated dry viscosities as a function of the TA/S* (based

on moles of elements, solid symbols) parameters, at constant

T= 1100 K.


However, a trend seems to appear when consider-

ing together the variations of NBO/T and A/AE. For

instance, the phonolitic anhydrous melt from Teide

displays the lowest viscosities among the liquids here

considered. Both the Teide phonolitic and the AMS

trachytic liquids have the same abundance of NBOs,

but the A/AE value is much higher for the Teide

phonolites than for the AMS trachytes (Table 7).

Another example is provided by the W_Tr trachyte

in Table 7, which displays higher anhydrous viscosity

compared to AMS trachytes, despite the higher NBO/

T, together with lower A/AE ratio. When comparing,

at dry as well as hydrous conditions, cases with

similar NBO/T value, the viscosity at a given temper-

ature turns out to be lower when the A/AE parameter

is higher, in agreement with the results of Whittington

et al. (2001). This is observed both at high and low

viscosity (see Tables 3 and 4), differently from the

case of fully polymerized liquids investigated by

Romano et al. (2001). However, our efforts to quan-

tify these relationships by performing a multifunction

fitting of the viscosity data as a function of both NBO/

T and A/AE parameters unfortunately resulted in v2

values too high to be considered relevant.

An interesting observation is that if each composi-

tional group is associated with a distinct viscosity

trend, then we should expect a relationship between

viscosity and the total alkali/silica ratio (TA/S in Table

7), since such a ratio is used to distinguish among

rhyolites, trachytes, and phonolites. As a matter of fact,

the TA/S parameter is, together with the Al/Si ratio, the

only quantity in Table 7 to show well-distinct ranges

for phonolites (0.23–0.28), trachytes (0.16–0.21), and

rhyolite (0.11). We parameterize the viscosity as a

function of a modified TA/S parameter, considering

the elemental molar ratio (Na +K)/Si, denoted TA/S*

(Table 7). Fig. 4 shows the dry viscosity of rhyolite,

trachytes, and phonolites at 1100 K versus TA/S*. The

trend shows phonolites and trachytes merging with the

same nearly linear behavior, and rhyolite showing a

large positive deviation.

The TA/S* parameter, however, does not account

for the dissolved water content, nor for the Al content

in the melt. Hydrous viscosities can be accounted for

by considering a modified form of TA/S* parameter

obtained by considering water together with the alkali

components. After having noted that the effect of a

higher alkali content is qualitatively similar to that of

a larger water content, both cases result in a viscosity

decrease (Figs. 3 and 4). There has been ample debate

on the role and effect of speciation of water on the

physical properties of silicate liquids and recent inves-

tigations (Richet et al., 1996; Romano et al., 2001;

Whittington et al., 2001) suggest that the effect of

water is independent from the concentration of any

single dissolved H-bearing species. In our discussion,

we have therefore considered (as for the calculation of

NBO/T) water dissolved in the liquid as total water

content, not taking into consideration its speciation

(OH� and molecular H2O).

Fig. 5 shows a plot of viscosity versus the new

compositional parameter (Na +K +H)/(Al + Si) that

takes into account total water and total aluminum

content in the liquid. As is evident from the figure,

a general trend of increasing viscosity with decreasing

(Na +K+H)/(Al + Si) ratio emerges (Fig. 5). A simi-

lar relationship between the molar proportion of

(Si +Al)/(H +Na +K) and the viscosity of the melt

has been envisaged by Holtz et al. (1999) for synthetic

compositions along the Ab-Qz join.

In order to minimize the effects of approximations

due to modeling, the plot in Fig. 5 is repeated for a low

temperature similar to that of the viscosity determina-

tions. This procedure rules out the possibility to

consider rhyolite together with phonolites and tra-

chytes, since the low-T range of viscosity determina-

tions for phonolites and trachytes is far below the glass

transition temperature of rhyolite (Tables 3 and 4). Fig.

6 shows the low-T plot, for water contents from 0.8 to

Fig. 5. Calculated hydrous viscosities as a function of the mole ratio

(Na +K+H)/(Si +Al), at constant T= 1100 K.


3 wt.% (lower water contents push the conditions

beyond the glass transition). As can be seen, a well-

defined linear trend emerges between log g and the

molar ratio (Na +K+H)/(Si +Al), for all the compo-

sitions including Phlegrean Fields trachytes, Vesuvius

and Teide phonolites, and the synthetic trachytic and

phonolitic liquids from Whittington et al. (2001).

The diagram in Fig. 6 shows that the different

viscosities of hydrous phonolites and trachytes at the

temperature of 800 K can be explained in terms of

Fig. 6. Calculated hydrous viscosities as a function of the mole ratio

(Na +K+H)/(Si +Al), at constant T= 800 K, for the AMS trachytes

and V1631 phonolites analysed in this work, and for other

phonolitic and trachytic liquids from literature. The line is the

linear least square fit to the points. The parameters of the regression

curve are also reported.

different abundance of Na, K, and H, relative to Si and

Al. As an example, a trachytic liquid with 3 wt.%

dissolved water at 800 K has a viscosity close to 109

Pa s. At the same temperature, phonolitic liquids have

the same viscosity with only 2 wt.% dissolved water.

In both cases, the (Na +K+H)/(Si +Al) molar ratio is

close to 0.5.

Fig. 5 shows a similar general trend, but a much

larger dispersion of points that might be due to a

larger role on viscosity of components other than

those included in the (Na +K+H)/(Si +Al) ratio at

higher temperature, or to the approximations intro-

duced by the viscosity modeling with Eq. (6). A

definite answer requires the production of dry as well

as hydrous viscosity data in the intermediate to high

temperature range.

The inclusion of rhyolites in a plot similar to that in

Fig. 6 can be done by considering isokom temper-

atures, instead of viscosities, as a function of the

(Na +K+H)/(Si +Al) ratio. This is shown in Fig. 7,

for conditions from dry to hydrous. As is evident from

the figure, the trachytic and phonolitic compositions

still display well-defined, very close (with the partial

exception of the synthetic phonolite from Whittington

et al., 2001), and essentially linear trends down to

water contents of at least 0.8 wt.%, below which a

steep increase of the isokom temperature is observed.

In contrast, the hydrous rhyolitic composition is

Fig. 7. Calculated isokom temperatures corresponding to a viscosity

of 1010.5 Pa s for the AMS trachytes and V1631 phonolites analysed

in this work, and for other phonolitic, trachytic, and rhyolitic liquids

from literature, plotted against the mole ratio (Na +K+H)/(Si +Al),

for dissolved water contents from 0 (0.02 wt.% as explained in the

text) to 4 wt.%.


shifted toward lower isokom temperature at equal

(Na +K+H)/(Si +Al) ratio (or lower magma viscosity

at equal magma temperature and (Na + K + H)/

(Si +Al) ratio). As a conclusion, Fig. 7 shows that

at very high viscosity conditions the (Na +K+H)/

(Si + Al) ratio explains the different viscosities of

hydrous trachytes and phonolites, whereas the hy-

drous rhyolite follows a different trend. On the con-

trary, the hydrous rhyolites in Fig. 5, pertaining to

intermediate viscosity conditions (less constrained by

the viscosity data), show the same trend of trachytic

and phonolitic compositions.

The reason why rhyolitic liquids display different

trends at high viscosity compared with the trachytes

and phonolites can be complex. From Table 7, we can

observe that the HPG8 displays compositional fea-

tures quite different from those of the trachytic and

phonolitic liquids (NBO/T = 0, low Al/Si ratio, low

TA/S ratio, A/AE and M1/M2 =l). In such a case,

the mechanism of activation of viscous flow can be

different, for instance by breaking Al–O or Si–O

bonds (a nominally fully polymerized liquid does not

contain non-bridging oxygens). The lower Al/Si ratio

could also play a role in increasing viscosity, due to

higher strength of the liquid (Al destabilizes the

silicate framework by lengthening the average TO

distance and narrowing the average TOT angle, Nav-

rotsky et al., 1985), as well as due to lower Si/Al order

disorder and therefore lower configurational entropy

of the liquid (see Eq. (2)). Moreover, comparison of

synthetic (HPG8) versus natural (trachytes and pho-

nolites) compositions certainly introduces further

approximations and uncertainties in the calculation

of the above compositional features hampering possi-

ble correlations among them.

It seems therefore likely that in the case of liquids

with large differences in their structural/compositional

characteristics, several parameters should be taken

into account in order to understand the viscosity–

composition relationships and additional structural

information should be available in order to predict

the compositional variation of melt viscosity.

5. Conclusions

In this paper, we have presented the results of

measurements of the Newtonian viscosity of dry and

hydrous trachytic and phonolitic samples representa-

tive of two relevant eruptions of Vesuvius and Phle-

grean Fields. Trends of increasing viscosity from

phonolitic to trachytic to rhyolitic melts were found

at water contents of less than 1 wt.%. Interestingly, at

water contents greater than 1 wt.%, the viscosities of

trachytic melts were found to be close to those of

rhyolitic melts, greatly simplifying the tasks of pre-

dicting the behaviour of natural magmas of such

compositions, while the viscosity of phonolitic melts

was found to be one to two orders of magnitude lower.

A compositional parameter given by the (Na +K+H)/

(Si +Al) molar ratio was found to be linearly related

to the low-T hydrous viscosities of the trachytic and

phonolitic melts considered. This parameter was suc-

cessfully applied to trachytic and phonolitic melts

either analysed in this work or taken from literature.

Prediction of possible future eruptive scenarios and

the evaluation of the volcanic hazard at Vesuvius and

Phlegrean Fields areas is greatly aided by the acquired

knowledge of the viscosity behaviour of magmas

discharged from these volcanoes.

Acknowledgements

We wish to thank Hubert Schulze for technical

support during the preparation of experimental

materials. We are grateful for the very helpful

comments provided by A. Whittington and an

anonymous reviewer. [RR]

References

Angell, C.A., 1995. Formation of glasses from liquids and biopol-

ymers. Science 67, 1924–1935.

Bagdassarov, N.S., Dingwell, D.B., 1993. Frequency dependent

rheology of vesicular rhyolite. J. Geophys. Res. 98, 6477–6487.

Behrens, H., Romano, C., Nowak, M., Holtz, F., Dingwell, D.B.,

1996. Near infrared spectroscopic determination of water spe-

cies in glasses of the system Mal3SiO8 (M=Li, Na, K): an

interlaboratory study. Chem. Geol. 128, 41–63.

Carroll, M., Holloway, J.R., 1994. Volatiles in magmas. Mineral.

Soc. of Am., Rev. Mineral., Washington, DC, p. 30.

De Vita, S., Orsi, G., Civetta, L., Carandente, A., D’Antonio, M.,

Deino Di Cesare, A.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. Volca-

nol. Geotherm. Res. 91, 269–301.


Dingwell, D.B., 1987. Melt viscosities in the system NaAlSi3O8–

H2O–F2O-1. In: Mysen, B.O. (Ed.), Magmatic Processes:

Physicochemical Principles, Spec. Publ., vol. 1. The Geochem-

ical Society, Pennsylvania State University, University Park,

PA, pp. 423–433.

Dingwell, D.B., 1998a. Magma degassing and fragmentation. Re-

cent 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., 1998b. Recent experimental progresses in the

physical description of silicic magma relevant to explosive vol-

canism. 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 polymerized silicate melts. Geochim. Cos-

mochim. Acta 52, 395–403.

Dingwell, D.B., Romano, C., Hess, K.-U., 1996. The effect of water

on the viscosity of a haplogranitic melt under P–T–X condi-

tions relevant to silicic volcanism. Contrib. Mineral. Petrol. 124,

19–28.

Dingwell, D.B., Hess, K.-U., Romano, C., 1998. Extremely fluid

behaviour of hydrous peralkaline rhyolites. Earth Planet. Sci.

Lett. 158, 31–38.

Dobson, D.P., Jones, A.P., Robe, R., Sekine, T., Kurita, K., Tanigu-

chi, T., Kondo, T., Kato, T., Shinomura, O., Urakawa, S., 1996.

In situ measurement of viscosity and density of carbonatitic

melts at high pressure. Earth Planet. Sci. Lett. 143, 207–215.

Dorfman, A., Hess, K.-U., Dingwell, D.B., 1996. Centrifuge-assis-

ted falling-sphere viscometry. Eur. J. Mineral. 8, 507–514.

Folch, A., Marti, J., Codina, R., Vazquez, M., 1998. Numerical

model for temporal variations during explosive central vent

eruptions. J. Geophys. Res. 103, 20883–20899.

Freundt, A., Rosi, M., 1998. From Magma to Tephra—Modeling

Physical Processes of Explosive Volcanic Eruptions. Springer,

Berlin.

Gilbert, J.S., Sparks, R.S.J., 1998. The Physics of Explosive Vol-

canic Eruptions, Special Publications. Geological Society, Lon-

don, p. 145.

Giordano, D., Dingwell, D.B., 2003. Viscosity of hydrous Etna

basalt: implications to the modelling of Plinian basaltic erup-

tions. Bull. Volcanol. 65, 8–14.

Giordano, D., Dingwell, D.B., Romano, C., 2000. Viscosities of

Teide phonolites in the welding interval. J. Volcanol. Geotherm.

Res. 103, 239–245.

Herzog, M., Graf, H.F., Textor, C., Oberhuber, J.M., 1998. The

effect of phase changes of water on the development of volcanic

plumes. J. Volcanol. Geotherm. Res. 87, 55–74.

Hess, K.-U., Dingwell, D.B., 1996. Viscosities of hydrous leu-

cogranitic 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.

Holtz, F., Roux, J., Behrens, H., Pichanvant, M., 1999. Water sol-

ubility in SiO2–NaAlSi3O8 melts. Am. Mineral. 85, 682–686.

Jaupart, C., 1998. Gas loss from magmas through conduit walls

during eruption. In: Gilbert, J.S., Sparks, R.S.J. (Eds.), The

Physics of Explosive Volcanic Eruptions, Special Publications,

vol. 145. Geological Society, London, pp. 73–90.

Kushiro, I., 1978. Viscosity and structural changes of albite

(NaAlSi308) melt at high pressures. Earth Planet. Sci. Lett.

41, 87–90.

Middlemost, E.A.K., 1989. Iron oxidation ratios, norms and the

classification of volcanic rocks. Chem. Geol. 77, 19–26.

Mysen, B.O., 1988. Structure and Properties of Silicate Melts

Elsevier, Amsterdam.

Nakamoto, K., 1997. Infrared and Raman Spectra of Inorganic and

Coordination Compounds: Theory and Applications in Inor-

ganic Chemistry, 5th ed. Wiley, New York.

Navrotsky, A., Geisinger, K.L., McMillan, P., Gibbs, G.V., 1985.

The tetrahedral framework in glasses and melts: influence from

molecular orbital calculations and implications for structure,

thermodynamics, and physical properties. Phys. Chem. Miner.

11, 284–298.

Neri, A., Papale, P., Macedonio, G., 1998. The role of magma

composition and water content in explosive eruption: II. Py-

roclast dispersion dynamics. J. Volcanol. Geotherm. Res. 87,

95–115.

Ohlhorst, S., Behrens, H., Holtz, F., 2001. Compositional depend-

ence of molar absorptivities of near-infrared OH� and H2O

bands in rhyolitic to basaltic glasses. Chem. Geol. 174, 5–20.

Papale, P., 2001. Dynamics of magma flow in volcanic conduits

with variable fragmentation efficiency and nonequilibrium pum-

ice degassing. J. Geophys. Res. 106, 11043–11065.

Papale, P., Neri, A., Macedonio, G., 1998. The role of magma

composition and water content in explosive eruption: I. Conduit

ascent dynamics. J. Volcanol. Geotherm. Res. 87, 75–93.

Persikov, E.S., 1991. The viscosity of magmatic liquids: experi-

ment, generalized patterns. A model for calculation and predic-

tion. Applications. Adv. Phys. Geochem. 9, 1–40.

Richet, P., Bottinga, Y., 1986. Thermochemical properties of silicate

glasses and liquids: a review. Rev. Geophys. 24, 1–25.

Richet, P., Bottinga, Y., 1995. Rheology and configurational entro-

py of silicate melts. In: Stebbins, J.F., McMillan, P.F., Dingwell,

D.B. (Eds.), Structure, Dynamics and Properties of Silicate

Melts, vol. 32. Mineral. Soc. Am. Rev. Mineral., Washington,

DC, pp. 67–94.

Richet, P., Lejeune, A.M., Holtz, F., Roux, J., 1996. Water and the

viscosity of andesite melts. Chem. Geol. 128, 185–197.

Romano, C., Dingwell, D.B., Behrens, H., 1995. The temperature

dependence of the speciation of water in NaAlSi3O8–KAlSi3O8

melts: an application of fictive temperatures derived from syn-

thetic fluid inclusions. Contrib. Mineral. Petrol. 122, 1–10.

Romano, C., Hess, K.-U., Mincione, V., Poe, B.P., Dingwell, B.T.,

2001. The viscosities of dry and hydrous XAlSi3O8 (X =Li, Na,

K, Ca0.5, Mg0.5) melts. Chem. Geol. 174, 115–132.

Rosi, M., Santacroce, R., 1984. Volcanic hazard assessment in the

Phlegrean Fields: a contribution based on stratigraphic and his-

torical data. Bull. Volcanol. 47, 359–370.

Rosi, M., Principe, C., Vecci, R., 1993. The Vesuvius 1631 erup-

tion. A reconstruction based on historical and stratigraphical

data. J. Volcanol. Geotherm. Res. 58, 151–182.


Scaillet, B., Holtz, F., Pichavant, M., Schmidt, M.O., 1996. The

viscosity of Himalayan leucogranites: implications for mecha-

nisms of granitic magma ascent. J. Geophys. Res. 101, 27691–

27699.

Scandone, R., Bellocci, F., Lirer, L., Rolandi, G., 1991. The struc-

ture of the Campanian Plain and the activity of Neapolitan

volcanoes (Italy). J. Volcanol. Geotherm. Res. 48, 1–31.

Schulze, F., Behrens, H., Holtz, F., Roux, J., Johannes, W., 1996.

The influence of water on the viscosity of a haplogranitic melt.

Am. Mineral. 81, 1155–1165.

Shaw, H.R., 1972. Viscosities of magmatic silicate liquids: an em-

pirical method of prediction. Am. J. Sci. 272, 870–893.

Silver, L.A., Stolper, E.M., 1989. Water in albitic glasses. J. Petrol.

30, 667–709.

Smith, J.V., 1997. Shear thickening dilatancy in crystal-rich flows.

J. Volcanol. Geotherm. Res. 79, 1–8.

Stein, D.J., Spera, F.J., 1992. Rheology and microstructure of mag-

matic emulsions: theory and experiments. J. Volcanol. Geo-

therm. Res. 49, 157–174.

Webb, S.L., 1997. Silicate melts: relaxation, rheology, and the glass

transition. Rev. Geophys. 35, 191–218.

Webb, S.L., Dingwell, D.B., 1990. Non-Newtonian rheology of

igneous melts at high stresses and strain rates: experimental

results for rhyolite, andesite, basalt, and nephelinite. J. Geophys.

Res. 95, 15695–15701.

Whittington, A., Richet, P., Holtz, F., 2000. Water and the viscosity

of hydrous depolymerized aluminosilicate melts. Geochim. Cos-

mochim. Acta 64, 3725–3736.

Whittington, A., Richet, P., Linard, Y., Holtz, F., 2001. The vis-

cosity of hydrous phonolites and trachytes. Chem. Geol. 174,

209–223.

Yamashita, S., Kitamura, T., Kusakabe, M., 1997. Infrared spectro-

scopy of hydrous glasses of arc magmas compositions. Geo-

chem. J. 31, 169–174.

The dry and hydrous viscosities of alkaline melts from Vesuvius and Phlegrean Fields

Documents

Transcript of The dry and hydrous viscosities of alkaline melts from Vesuvius and Phlegrean Fields