The rheological evolution of alkaline Vesuvius magmas and comparison with alkaline series from the Phlegrean Fields, Etna, Stromboli and Teide

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Geochimica et Cosmochimica Acta 73 (2009) 6613–6630 www.elsevier.com/locate/gca

The rheological evolution of alkaline Vesuvius magmas and comparison with alkaline series from the Phlegrean Fields, Etna, Stromboli and Teide D. Giordano a,*, P. Ardia b, C. Romano a, D.B. Dingwell c, A. Di Muro d, M.W. Schmidt b,e, A. Mangiacapra f, K.-U. Hess c a

Department of Geological Sciences, Third University of Rome, Largo S. Leonardo Murialdo 1, 00154 Rome, Italy b Department of Earth Sciences, ETH, 8092 Zurich, Switzerland c Department of Earth and Environmental Sciences, Ludwig Maximilians University, Theresienstr. 41/III, 80333 Munich, Germany d Laboratoire de Ge´ologie des Syste`mes Volcaniques, IPGP, 4 Place Jussieu, 75005 Paris, France e Faculty of Sciences, University of Zurich, Switzerland f Osservatorio Vesuviano, Istituto Nazionale di Geofisica e Vulcanologia, Via Diocleziano 328, I-80124 Napoli, Italy Received 21 March 2009; accepted in revised form 22 July 2009; available online 3 August 2009

Abstract Somma-Vesuvius is considered one of the highest-risk volcanic systems in the world due to its high population density and record of highly destructive explosive activity. Eruptive style at Vesuvius varies greatly, alternating between effusive and explosive activities, and is likely strongly controlled by the evolution of the physical and chemical properties of the magma. Nevertheless, with the exception of the 1631 eruption, the rheological properties of Vesuvius magmas remain largely unconstrained. Here, we investigate the Newtonian shear viscosity (g) of dry and hydrous melts from the Mercato (plinian) and 1906 (violent strombolian) eruptions. These eruptions differ in size, eruptive style and magma chemistry (from phonolite to phonotephrite). To evaluate the dry liquid viscosity variation covered by the eruptive products of the recent activity at Vesuvius, we measured the melt viscosities of bulk rock compositions and, for highly crystalline samples, of the separated groundmasses of tephras from the Pollena and 1906 eruptions. Hydrated samples with up to 4.24 wt% dissolved water were synthesised in a piston cylinder apparatus at confining pressure up to 10 kbar. The dry high temperature and the dry and hydrous low-temperature viscosities were obtained by combining the concentric cylinder and micropenetration techniques. The measured viscosities were parameterized by a modified Vogel–Fulcher–Tammann equation, accounting for the effect of water content, and were compared with previous measurements and models. At magmatic temperatures, the viscosities of Mercato samples are about four orders of magnitude higher than that of the least viscous investigated products from the 1906 eruption. Complex numerical models to forecast eruptive scenarios and their environmental impact are extremely sensitive to the accuracy of the input parameters and constitutive equations of magma properties. As a consequence, the numerical expressions obtained here are of particular relevance in the context of hazard assessment related to the different possible eruptive scenarios at Vesuvius through numerical simulation tools. The effect of composition on the liquid viscosities is compared to other high-Na (e.g., samples from Teide and Etna) and high-K (e.g., samples from Stromboli and Phlegrean Fields) alkaline magmas. Ó 2009 Elsevier Ltd. All rights reserved.

1. INTRODUCTION * Corresponding author. Tel.: +39 06 57338018; fax: +39 06 57338201. E-mail address: [email protected] (D. Giordano).

0016-7037/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2009.07.033

Predicting future eruptive scenarios of volcanic activities at Vesuvius is a central challenge for volcanologists. Vesuvius is one of the highest-risk volcanic systems on earth by

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virtue of its proximity to numerous highly densely populated cities and villages. Its reawakening could endanger the lives of more than 700,000 people. Critical for the evaluation of the hazard associated with possible future eruptions is the characterization of the evolution of physical properties consequent to the chemical evolution of the Vesuvius magmatic system which occurred during its past volcanic activity. Among these properties, viscosity is the most important one governing the production, transport and eruption dynamics of magmas (e.g., Dingwell, 1996; Papale, 1999; Sparks and Aspinall, 2004; Giordano et al., 2008a). Viscosity strongly influences eruptive style and yet it is this property above all that might span more than 15 orders of magnitude due to variations in temperature (T), composition (X) and volatile content (Giordano et al., 2008a). In this study, we present measurements of the Newtonian shear viscosity of dry and hydrous phono-tephritic to phonolitic samples representative of the groundmass portion and of bulk rock pumices and scoriae of the Mercato Pumices (also known as Pomici Gemelle or Pomici di Ottaviano, 7900 BP) and of the 1906 eruptions, respectively. Dry viscosities of the 79 AD Pompei and 472 AD Pollena eruptive products were also investigated. About 150 viscosity data are presented and compared with previous data for bulk rock and groundmass products (Giordano and Dingwell, 2003a; Romano et al., 2003) of the Vesuvius 1631 eruption (Rosi et al., 1993). 2. SAMPLE SELECTION AND CHARACTERIZATION The sampling philosophy employed here was to obtain the widest possible chemical interval of explosive products erupted at Vesuvius, to encompass their complete range of rheological behaviour. Similarly, for the selected Pollena samples, we measured the anhydrous viscosity of both the separated groundmass (GM) and the whole rock (TR) compositions. This in principle allows constraining the variations in viscosity for individual eruptive events, where large differences between the highly crystalline bulk rock and groundmass are observed. The Mercato Pumice and 1906 eruption products were chosen as they represent two end-members of explosive potential at Vesuvius, in terms of magnitude, eruptive style, composition and timing in the volcano’s activity. The Mercato Pumice eruption is the oldest Plinian eruption of the second period of activity of Vesuvius (Aulinas et al., 2008). It occurred after a period of quiescence of about 7 kyr and was followed by a period of unrest of about 4 kyr, which produced K-phonolites to K-tephri-phonolites. Despite being a low-energy event, the 1906 eruption is probably the most intense one in the recent period of activity after the 1631 eruption (Scandone et al., 2008; Cioni et al., 2008). The 1906 products represent the chemically most undersaturated (SiO2-poor) compositions from this last period of activity (Scandone et al., 2008; Cioni et al., 2008). The eruptive dynamics and magma chemistry addressed here significantly extend the range explored in our previous studies (GM–Romano et al., 2003; TR–Giordano and Dingwell, 2003a) of the subplinian 1631 Vesuvius eruption (Rosi et al., 1993).

The starting materials used for glass synthesis and viscosity measurements are the groundmass (GM) and total rock (TR) samples representative of the pyroclasts erupted during the peak of mass discharge rate of the plinian Mercato and Pompei Pumice; of the subplinian Pollena Pumice and of the 1906 violent strombolian eruptions. The pumiceous samples from the Pollena and the Pompei eruptions were collected at the Scudieri locality, whereas the white pumice specimens of the Mercato eruption and the scoriae of the 1906 eruption were collected at the Vallone San Severino and Ottaviano (Amarischia) localities, respectively. Before rheological investigation samples were cleaned and lithics removed. Viscosities of both dry and hydrous melts were measured from the Mercato Pumice and 1906 eruption and on the anhydrous Pompei and Pollena products. The Mercato and Pompei pumices are nearly aphiric with less than 3 and 10 vol% of phenocrysts, respectively; the composition of the residual phonolitic groundmass being fairly homogeneous. The Pollena pumices have ca. 13 vol% phenocrysts (Scaillet et al., 2008) and very similar compositions of the bulk rock and residual glass (Table 1; Fig. 1). The crystal content of the 1906 scoria sample was >50 vol%. 3. EXPERIMENTAL RATIONALE The dry materials investigated were obtained by direct fusion of total rock (TR) samples for the Mercato, Pollena, Pompei and 1906 eruptions and a groundmass (GM) sample for the Pollena eruption. A concentric cylinder (CC) technique was used for sample homogenization and determination of anhydrous liquid viscosities of 101–104.7 Pa s at 1053–1600 °C, whereas micropenetration viscometry was used to measure viscosities of between 109.1 and 1012.7 Pa s at temperatures from 380 to 781 °C. Major element compositions determined using a JEOL JXA 8200 electron microprobe are reported in Fig. 1 and Table 1 together with compositions of previously investigated samples. No significant compositional differences were measured between the remelted TR and GM products of the Pollen eruption investigated here. The analytical conditions of microprobe analysis are reported in Table 1. Glasses were retrieved after the concentric cylinder experiments by drilling and used for the hydration experiments. Water-bearing samples, containing up to 4.24 wt%, have been synthesised at 1200 °C and pressures between 5 and 10 kbar in a piston cylinder apparatus for several hours (Table 2). To ensure dissolution and homogenization of water in the melt, Au80–Pd20 capsules were employed. Ten and four syntheses were performed for the Mercato and 1906 eruption products, respectively. Finally, the water-bearing glasses were cut into 1–2 mm thick disks, doubly polished, and stored in a desiccator until further use. Distribution, homogeneity and absolute water content of the hydrated samples were determined by FTIR spectroscopy and Karl-Fischer titration (KFT), following the methods described in detail by Behrens et al. (1996) and Nowak and Behrens (1997). Measured water contents (Table 2) were corrected for the unextracted water content

Alkaline magmas viscosity/The H2O dissolution mechanism in silicates

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Table 1 Average composition of major oxides (wt%) of the investigated natural glasses. The column in italics represent the uncertainty associated with the technique. Experimental conditions used during microprobe analysis were: 10 kV, 6 nA, spot 5 lm. Mercato 1400 °C SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 F Cl Tot. #d a b c d

58.80 0.12 20.68 1.94 0.18 0.07 1.68 8.05 6.72 0.00 0.08 0.27 98.61

a

1500 °C 0.26 0.03 0.21 0.06 0.03 0.02 0.06 0.08 0.08 0.01 0.04 0.02 0.48 15

58.90 0.12 20.58 2.00 0.18 0.07 1.69 7.91 6.66 0.00 0.05 0.25 98.40

a

1600 °C 0.22 0.02 0.17 0.06 0.03 0.02 0.05 0.10 0.08 0.01 0.06 0.01 0.36 15

58.84 0.10 20.75 2.05 0.18 0.09 1.68 7.98 6.63 0.00 0.01 0.20 98.52

Pollena TRb

Pompei TRb

Pollena GMc

1906 GMc

48.05 0.76 17.69 6.08 0.14 3.32 9.31 3.45 7.55 0.46 0.15 0.22 97.21

53.90 0.51 18.95 4.20 0.14 1.86 5.77 4.59 8.42 0.21 0.00 0.23 98.78

48.74 0.85 17.64 6.84 0.15 3.39 9.82 3.48 7.35 0.45 0.00 0.22 98.94

47.84 1.06 16.02 8.54 0.16 4.88 10.99 2.36 6.21 0.80 0.00 0.25 99.11

a

0.34 0.02 0.18 0.06 0.04 0.02 0.05 0.07 0.07 0.01 0.01 0.02 0.40 15

0.34 0.05 0.21 0.11 0.03 0.09 0.11 0.10 0.09 0.04 0.16 0.04 0.39 14

0.33 0.04 0.16 0.13 0.02 0.04 0.07 0.10 0.09 0.02 0.00 0.04 0.43 14

0.20 0.04 0.14 0.15 0.03 0.07 0.07 0.08 0.09 0.03 0.00 0.03 0.32 14

0.25 0.05 0.14 0.11 0.03 0.10 0.10 0.08 0.06 0.03 0.00 0.01 0.42 15

Mercato re-melted at three different temperatures. A slight halogens depletion is observed for samples remelted at higher temperatures. Total rock (TR) or buk rock composition, after remelting. Glass matrix (GM) analysis after remelting. Number of EMPA analysis collected for each samples.

16 Mercato Pompei Pollena GM Pollena TR 1631W_GM 1631W_TR 1631G_GM 1631G_TR 1906 GM IGC Etna MNV AMS B1 Td ph STR

Phonolite

Tot. alkali (wt%)

14 12 Tephriphonolite

10

Trachyte

Phono-tephrite

8 6

Trachybasalt Basalt

4 46

48

50

52

54 56 58 SiO 2 wt%

60

62

64

Fig. 1. TAS (total alkali vs. SiO2 contents in wt%) diagram showing melt compositions for which viscosity was measured in this study (Mercato, 1906 GM)and in the literature. Following Romano et al. (2003), the 1631 AD Vesuvius samples, equivalent to the groundmasses (GM) of erupted products are reported as 1631W_GM (White) and 1631G_GM (Grey), whereas the sample from the Agnano Monte Spina eruption (4.1 Ky, Phlegrean Fields) is reported as AMSB1. Samples 1631W_TR and 1631 G_TR correspond to the total rock samples of the 1631 Vesuvius eruption from Giordano and Dingwell (2003a). IGC is the Campanian Ignimbrite (35 Ky) and MNV the Monte Nuovo 1538 AD (Giordano et al., 2004). ETN is the Etna trachybasalt from Giordano and Dingwell (2003c) and the phonolite from the Montana Blanca eruption (Teide, Tenerife) is reported as Td ph (Giordano et al., 2000).

(Behrens and Stuke, 2003; Leschik et al., 2004). MicroRaman spectroscopy was performed following Di Muro et al. (2006) and Mercier et al. (2009) as an additional test for water homogeneity and distribution and to check for micro-crystallization of the samples. The doubly polished disks were then used for low-T micropenetration measurements under Ar atmosphere. Further details are summarized in the Appendix A and can be found in Dingwell et al. (1996) and Giordano et al. (2004, 2005).

4. RESULTS 4.1. Viscosities of anhydrous melts The dry viscosity of re-melted bulk rocks (TR) and groundmasses (GM) of products from the different Vesuvian eruptions are presented as a function of reciprocal temperature in Fig. 2. The anhydrous viscosity data, for which both high and low-temperature data were obtained

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Table 2 Synthesis condition of the hydrous samples and measured water contents.

mer_0.5 mer_0.5_II mer_0.5_i mer_0.5_ii mer_1.0 mer_2.0 mer_2.5 mer_3.0 mer_3.5 mer_4.0

1 1 0.7 0.8 1 1 0.7 1 0.8 1

1200 1200 1200 1200 1200 1200 1200 1200 1200 1200

24 19 24 41 19.5 19 14.7 12 18.3 18.5

1906_0.5 1906_1.5 1906_2.5 1906_3.0

1 0.5 1 1

1200 1200 1200 1200

3 2 2 2

a b c d e

H2ORaman H2O (wt%)d (mol%)e

1.19 0.59 0.60 0.51 0.81 2.68 2.66 3.03 3.50 4.00

0.95 2.84

0.64 0.60 0.92 2.56

2.92

3.03

4.24

4.21

0.55 1.26 2.50 3.08

4.36 2.20 2.23 1.90 3.50 9.96 9.37 10.22 12.07 14.35

6 Mercato_TR 1906 eruption_GM Pollena GM 1631 W_GM 1631 G_GM Pollena TR Pompei TR

4 2

5

6

7

8

9

10

11

10000/T (K) 6

(B)

5

are fitted by using the following Vogel–Fulcher–Tammann (VFT) (Vogel, 1921; Fulcher, 1925; Tammann and Hesse, 1926) expression: BVFT T ðKÞ  C VFT

8

0

1.97 4.43 8.53 10.36

Synthesis time. Water content (wt%) measured by Infrared spectroscopy. Water content (wt%) measured by Karl-Fischer titration (KFT). Water content (wt%) measured by microRaman spectroscopy. Water content (in mol%) considered for the parameterization.

log g ¼ AVFT þ

log [viscosity Pa s)]

P T t H2OFTIR H2OKFT (wt%)c (GPa) (°C) (h)a (wt%)b

log [viscosity Pa s)]

Syntehsis

(A)

10

4 3 Mercato_TR 1906 eruption_GM Pollena GM 1631 W_GM 1631 G_GM Pollena TR Pompei TR

2 1 0 5

6

13

8

9

(C)

12

log [viscosity Pa s)]

where g is viscosity in Pa s, T (K) is the absolute temperature, and AVFT, BVFT and CVFT are the pre-exponential factor, the pseudo-activation energy and the VFT temperature, respectively. For those samples for which low-T data are not available, such as the Pollena TR and Pompei TR, no regression curves are included in the figures. Fig. 2A shows a significant variation of fragility (deviation from Arrhenian behaviour) for the Vesuvius liquids investigated here and elsewhere (1631W_GM and 1631G_GM from Romano et al., 2003). Fig. 2B shows that the high temperature dry viscosities of the analysed liquids differ by about 2.5 orders of magnitude in this temperature range. In particular, the Mercato sample is more viscous than the 1631 Vesuvius samples (Romano et al., 2003) and the Pompei TR sample. In turn, the Pompei TR sample is more viscous than both the Pollena GM and TR and the 1906 GM compositions. A more complex scenario is observed at the lowest temperatures, where the viscosities of different samples cross over at different temperatures. At about 660 °C (10,000/T (K)  10.70), the calculated viscosity curves of 1906 GM and Pollena GM are higher than those of the other compositions (Fig. 2A and C). The crossover temperatures are lowest between the least and most fragile samples, whereas they are at higher temperature for intermediate compositions. On the other hand the viscosity curve of the re-melted Mercato sample shows the lowest fragility but crosses all other viscosity curves.

7

10000/T (K)

ð1Þ

11 10 9 Mercato_TR 1906 eruption_GM Pollena GM 1631 W_GM 1631 G_GM

8 7 9.0

9.5

10.0

10.5

11.0

10000/T (K) Fig. 2. Anhydrous viscosities of Vesuvius products. (A) Anhydrous viscosity as measured for bulk rock (TR) and groundmass (GM) re-melted materials. (B) and (C) show details for the low (high temperature) and high viscosity (low-temperature) intervals, respectively. The low-temperature viscosities for Pompei_TR and Pollena_TR were not measured.

4.2. Viscosity of hydrous melts Hydrous and dry viscosities of the Mercato phonolite and the 1906 phono-tephrite are shown in Fig. 3. The viscosity interval accessible with the micropenetration technique becomes increasingly restricted with increasing water content and decreasing melt polymerisation. This is because above a certain temperature the diffusion process

Alkaline magmas viscosity/The H2O dissolution mechanism in silicates

log [viscosity (Pa s)] calculated

log [viscosity (Pa s)]

14

Mercato TR

13 11 9

dry 0.51 0.59 0.60 0.95 1.19 2.66 2.84 2.92 3.50 4.24

7 5 3

(A)

1

6617

(A)

12 10 8 6 4 2

Mercato 1906 eruption

0 14

log [viscosity (Pa s)]

log [viscosity (Pa s)] calculated

1906 GM

13 11 9 7 5

dry 0.55 1.26 2.50 3.08

3 1

(B) 4

6

8

10

12

14

12

(B)

10 8 6 4 2 0

16

0

10000/T (K)

4

6

8

10

12

14

log [viscosity (Pa s)] measured

allowing water to exsolve from the melt, is faster than the timescale of the measurement. Note that whereas viscosity appears to show a nearly Arrhenian behaviour over the restricted range of each individual technique (for each H2O content), a variable degree of non-Arrhenianity emerges over the complete temperature range explored. In particular, fragility decreases as water is increasingly dissolved in the melts (Giordano et al., 2008a) (Fig. 2) as well as for more polymerised melts such as Mercato. The viscosity data (Tables 3 and 4) have been fitted by assuming that AVFT is a constant independent of composition (Russell et al., 2003; Giordano et al., 2008a) the effect of water being incorporated solely into the BVFT and CVFT parameters, so that: ð2Þ

where, b1, b2, c1 and c2 are fit parameters, and H2O is the water concentration in mol%. The value of the pre-exponential parameter AVFT is taken as 4.55 (g = 104.55 Pa s) (Giordano et al., 2008a). Fitted values and standard deviations of the bi and ci parameters are provided in Table 5 for each of the datasets and Fig. 4 compares the calculated and measured viscosity values. Fig. 5 shows, for the Mercato and 1906 eruptive magmas, calculated viscosities as a function of H2O content at different temperatures (700–1600 °C). The viscosity of

Fig. 4. Comparison of viscosities as calculated from Eqs. (1) and (2) and the fit parameters of Table 4 with measured viscosities. (A) Mercato and 1906 samples; (B) previously investigated liquids reported in Table 4.

12 Mercato TR 10

log [viscosity (Pa s)]

Fig. 3. Measured viscosities as a function of water content for the re-melted (A) Mercato and (B) 1906 eruption materials. Numbers in the legends are dissolved water contents (wt%).

BVFT ¼ b1 þ b2  H2 O C VFT ¼ c1 þ c2 logð1 þ H2 OÞ

2

1906 GM

8 6 4 2 0 -2 0

1

2

3

4

5

H2O wt% Fig. 5. Isothermal viscosity vs. water content of the Mercato Pumice (grey) and 1906 GM (dashed black) calculated using the parameterizations in Table 5 at temperatures of 1600–700 °C (from top to bottom, steps of 100 °C).

the Mercato liquid is, with the exception of the lowest temperatures and for >4 wt% H2O higher than that of the 1906 liquid. For any given temperature the viscosity decreases with increasing water content is more marked for the 1906 than for the Mercato liquid.

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Table 3 Viscosity measurements (109 data) of the nominally anhydrous samples. Measurements were performed by concentric cylinder and micropenetration technique. Mercato 1600 T(°C)

Mercato 1500

Mercato 1400

Pollena TR

Pompei TR

1.15 1.26 1.38 1.49 1.61 1.74 1.88 2.02 2.17 2.32 2.49 2.65 2.84 3.04

1.68 1.80 1.92 2.05 2.18 2.33 2.47 2.63 2.79 2.96 3.13 3.33 3.52 3.73 3.95 4.18 4.43

Pollena GM

1906 GM

log g (Pa s)

1594 1569 1545 1520 1496 1471 1446 1422 1397 1372 1348 1323 1299 1274 1249 1225 1200 1176 1151 1126 1102 1077 1053

1.86 1.97 2.09 2.21 2.34 2.47 2.60 2.74 2.88 3.03 3.18 3.35 3.51 3.68 3.85 4.03 4.22 4.41 4.60

781 737 726 702 687 670 668

9.10 9.87

2.34 2.46 2.59 2.72 2.86 3.01 3.16 3.32 3.48 3.65 3.82 4.00 4.18 4.38 4.57

2.84 2.98 3.13 3.28 3.44 3.61 3.78 3.95 4.13 4.32 4.51

1.19 1.31 1.44 1.56 1.70 1.84 1.98 2.13 2.30 2.48 2.67 2.87

1.01 1.13 1.25 1.38 1.51 1.64 1.79 1.94 2.10 2.26 2.45 2.63

9.77 10.00 10.52

10.54 11.20 11.71

11.59

4.3. Comparison with previous results To compare our results with previous studies on Vesuvius and similar compositions, we have refitted the literature data according to Eqs. (1) and (2). Using different kinds of expressions for each dataset could in fact introduce unwanted forcing effects of the resulting fit curve shapes which could be also significantly different for each dataset. Table 5 reports the resulting values of the bi and ci fit parameters and their standard deviations. The excellent accuracy of the predictions are shown in Fig. 4B and Table 5. Fig. 6 shows calculated isothermal viscosity variations at 1050 K as a function of water content for previously investigated rhyolites, trachytes, phonolites, tephri-phonolites, phono-tephrites and basalts. Most of these are from Italian volcanic systems such as Vesuvius, Phlegrean Fields, Stromboli and Etna. The viscosity of a phonolitic melt from Teide (Giordano et al., 2000) and rhyolitic melts from Ardia et al. (2008) are also reported. As to be expected, water decreases strongly the viscosity of all liquids examined. The decrease is larger for more polymerised compositions, such as the rhyolite from Ardia et al. (2008), than for depolymerised liquids, such as the trachybasalt from Etna. For all compositions, a proportionally larger viscosity de-

crease is observed when small amounts of water are incorporated into the melt, with further addition of water a continuous but less pronounced decrease occurs (e.g., Shaw, 1963; Hess and Dingwell, 1996; Giordano et al., 2004). As expected, rhyolitic melts are on average more viscous than trachytes; and in turn trachytes are more viscous than phonolites, tephri-phonolites and basalts. This trend is particularly evident at low H2O contents, whereas at 3–4 wt% water the viscosity of trachytic and phonolitic melts overlap. The basaltic liquid from Mount Etna has a viscosity similar to the phono-tephritic liquid of the 1906 eruption and only slightly lower than the tephi-phonolitic 1631W_GM Vesuvius liquid (Fig. 6). Finally, to 2 wt% water, the viscosities of the high potassium (HK) basalt from the paroxymal phase of April 2003 at Stromboli (Giordano et al., 2006; Misiti et al., 2009) are similar to those of the Etna trachybasalt, but become up to 1 order of magnitude lower at 4 wt% H2O. The viscosity decrease of the Stromboli basalt as a function of H2O content is, in contrast to that of the Etna trachybasalt, very sharp. Such a rapid viscosity decrease is similar and almost parallel to the viscosity decrease of the more viscous HGG rhyolitic melt and is not observed for the other compositions, whose viscosities tend to level off more rapidly as water is added to the melt.

Alkaline magmas viscosity/The H2O dissolution mechanism in silicates Table 4 Micropenetration viscosity data for hydrous samples (46 data). H2O (wt%)

H2O (mol%)

Mercato eruption mer_0.5 3639 mer_0.5 4108 mer_0.5 4109 mer_0.5 3629 mer_0.5_II 3676 mer_0.5_II 3677 mer_0.5_II 4119 mer_0.5_i 4107 mer_0.5_i 4112 mer_0.5_i 4111 mer_0.5_i 4120 mer_0.5_ii 4130 mer_0.5_ii 4131 mer_0.5_ii 4132 mer_1.0 3625 mer_1.0 3632 mer_1.0 3653 mer_1.0 3633 mer_2.0 3626 mer_2.0 3627 mer_2.5 4113 mer_2.5 4114 mer_2.5 4115 mer_3.0 3628 mer_3.0 3631 mer_3.0 3634 mer_3.5 4116 mer_3.5 4117 mer_3.5 4118 mer_4.0 3654

500 500 550 570 590 630 540 550 570 530 590 600 570 620 560 530 500 590 475 440 430 460 410 400 430 450 400 380 420 400

11.98 11.98 10.52 9.91 10.34 9.60 12.04 11.06 10.70 12.00 10.03 10.66 11.35 10.13 10.20 10.98 12.15 9.57 9.62 10.84 11.23 10.12 11.80 11.82 10.55 9.94 10.96 11.28 10.31 10.74

1.19 1.19 1.19 1.19 0.59 0.59 0.59 0.60 0.60 0.60 0.60 0.51 0.51 0.51 0.95 0.95 0.95 0.95 2.84 2.84 2.66 2.66 2.66 2.92 2.92 2.92 3.50 3.50 3.50 4.24

4.36 4.36 4.36 4.36 2.20 2.20 2.20 2.23 2.23 2.23 2.23 2.05 2.05 2.05 3.50 3.50 3.50 3.50 9.96 9.96 9.37 9.37 9.37 10.22 10.22 10.22 12.07 12.07 12.07 14.35

1906 eruption 1906_0.5 1906_0.5 1906_0.5 1906_1.5 1906_1.5 1906_1.5 1906_1.5 1906_2.5 1906_2.5 1906_2.5 1906_3.0 1906_3.0 1906_3.0 1906_4.5 1906_4.5 1906_4.5

620 580 600 530 540 560 510 460 490 520 490 450 520 430 410 440

10.14 11.65 10.75 11.35 11.03 10.33 12.50 12.70 11.43 10.20 10.72 12.00 10.21 10.93 11.68 11.22

0.55 0.55 0.55 1.26 1.26 1.26 1.26 2.50 2.50 2.50 3.08 3.08 3.08 4.45 4.45 4.45

1.97 1.97 1.97 4.43 4.43 4.43 4.43 8.53 8.53 8.53 10.36 10.36 10.36 14.48 14.48 14.48

3647 3645 4124 3630 3641 3646 4122 3642 3643 3644 3649 4125 3651 3652 4121 3650

A different picture is obtained when comparing viscosities as a function of water content at the eruptive temperatures (Terupt) as detailed in Fig. 7. At the eruptive temperatures the viscosities of rhyolitic melts (Terupt = 830 °C) are the highest up to 3.5 wt% dissolved H2O. Due to their low eruptive temperatures of 775–785 °C, the phonolites, have eruptive viscosities higher than the trachytes (Terupt = 890–945 °C). The viscosity of the trachytes is always lower than that of the rhyolites and phonolites, with the phonolites having a viscosity much closer to that of

log [viscosity (Pa s)] @ 1050 K

log g (Pa s)

Exp. n

Rhyolite (HGG) Trachyte (AMSB1) Trachyte (MNV) Trachyte (IGC) Phonolite (Mercato_TR) Phonolite (Td_ph) Tephri-phonolite (1631W_GM) Trachybasalt (Etna) Phono-tephrite (1906_GM) Basalt (STR)

11 9 7 5 3 0

1

2 H2O wt%

3

4

Fig. 6. Isothermal viscosity variation at 1050 K as a function of water content as calculated from Eqs. (1) and (2). Liquid viscosities for rhyolites (black), trachytes (blue), phonolites (red), tephriphonolites (green), phono-tephrites (grey dashed) and basalts (grey) are given. For references see Fig. 1.

log [viscosity (Pa s)] @ eruptive temperature

T (°C)

Sample

13

6619

Rhyolite (HGG) Trachyte (AMSB1) Trachyte (MNV) Trachyte (IGC) Phonolite (Mercato_TR) Phonolite (Td_ph) Tephri-phonolite (1631W_GM) Trachybasalt (Etna) Phono-tephrite (1906_GM) Basalt (STR)

14 12 10 8

INCREASING EXPLOSIVITY

6 4 2 0 0

1

2 H2O wt%

3

4

Fig. 7. Calculated viscosity vs H2O variation at eruptive temperatures. The eruptive temperatures are as follows: 830 °C for rhyolites (Ardia et al., 2008); 785 °C for the Mercato phonolite (Scaillet et al., 2008); 1100 °C for the phono-tephrite of the 1906 eruption (Scaillet et al., 2008); 977 °C for the tephri-phonolite of the 1631_W_GM Vesuvius eruption (Scaillet et al., 2008); 890, 900 and 945 °C for the trachytic melts of Monte Nuovo (MNV), Ignimbrite Campana (IGC) and Agnano Monte Spina (AMSB1), respectively (Piochi et al., 2008); 775 °C for the Teide phonolite (Td_ph, Ablay et al., 1995); 1125 °C for the Etna trachybasalt (Etna, Giordano and Dingwell (2003c)) and 1150 °C for the Stromboli (HK) basalt (Misiti et al., 2009). A clear relation between viscosity values and associated eruptive styles emerges: higher viscosity products are those commonly producing explosive activity (plinian and subplinian), lower viscosity products produce effusive, strombolian or fire fountaining activity and, intermediate viscosity products produce a range of activities varying from violent strombolian (e.g., 1906 eruption) to subplinian (Vesuvius 1631 and Pollena 472 AD) and plinian (e.g., Mercato, Pompei) eruptive events. Colours are as in Fig. 6.

the rhyolites. Finally, Etna trachybasalt shows viscosities at eruptive temperatures of 1125 °C which are two orders of magnitude less than those of the Vesuvius tephri-phonolites

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Table 5 Fit parameters for Eq. (2). The pre-exponential factor A is fixed to 4.55 (g = 104.55 Pa s) as from Giordano et al. (2008a). The fit parameter values, uncertainties and standard deviations for Stromboli sample (STR in the table) were obtained by fitting the experimental viscosity from Giordano et al. (2006) and Misiti et al. (2009). Also considering only the data provided by Misiti et al. (2009) our five parameters equation slightly improves over the predictions provided by the six parameters expression of these previous authors. Parameter of Eq. (2) This study Mercato +/

b1

b2

c1

c2

r2

a

Std. Dev.

b

Std. Dev. Prev.

10085 145

64.93 16.72

312.2 13.4

212.7 14.5

0.9972

0.21

6054 81

25.47 12.80

572.8 6.6

209.4 8.6

0.9989

0.16

New fittings of previously measured viscosity databases Etna 5676 52.70 623.7 +/ 130 20.84 9.7

245.3 11.9

0.20

0.16

Teide +/

10261 95

26.21 12.59

263.8 7.9

257.8 9.2

0.13

0.13

1631 +/

8206 131

35.06 13.73

437.3 10.3

227.8 9.9

0.16

0.18

AMSB1 +/

10382

44.69 12.31

313.2 3.9

227.7 10.3

0.17

0.15

MNV +/

10449 82

101.63 9.22

303.7 6.9

193.8 7.1

0.10

0.12

IGC +/

8889

82.10 17.64

492.9 3.6

385.5 12.2

0.19

0.10

STR +/

6101 74

63.66 10.02

567.0 6.6

160.3 8.0

0.21

1906 +/

a b

Standard deviation in logarithmic scale obtained by using Eqs. (1) and (2). Standard deviation in logarithmic scale obtained by using equations in the literature (see text).

(Terupt = 977 °C), three to four orders of magnitude less than the trachytic liquids, up to five orders magnitude less than the phonolitic liquids and up to six orders of magnitude less than the rhyolitic liquids. The viscosities of the Etna trachybasaltic and 1906 GM phono-tephritic melts (Terupt = 1100 °C) are quite similar and slightly higher than those from the Stromboli (HK) basalt (Terupt = 1150 °C). The viscosity variations at eruptive temperatures may help to explain the wide spectrum of eruptive styles shown by Vesuvius magmas during its past activity. Effusive to violent strombolian eruptions are typically associated with silica poor magmas, such as the 1906 phono-tephrite and the Etna trachybasalt; highly explosive plinian eruptions are associated with melts richer in silica, such as the phonolitic products of Mercato, Pompei. Finally, intermediate subplinian eruptions (Vesuvius 1631 and Pollena 472 AD) are associated with intermediate compositions. As far as the activity at Stromboli is concerned, we recognize that its most common activity is a low intensity strombolian which is interrupted from time to time by low volume paroxysmal events and lava flows. We calculated the isokom temperatures as a function of water at fixed viscosities of 1012 and 108 Pa s (Fig. 8). These values, represent at first approximation the glass transition temperature Tg (Dingwell and Webb, 1989, 1990) and the viscosity at which a magmatic mixture ascending along volcanic conduits likely disrupts at the fragmentation level, passing

from a continuum liquid with dispersed bubbles to a continuum gas with dispersed liquid droplets (Papale, 1999; Polacci et al., 2004). Such high viscosities are close to the measured viscosities at the water contents employed in the experiments, which ensures that the errors introduced by extrapolating the viscosity parameterisation of Eqs. (1) and (2) are small. Isokom temperatures strongly decrease with water content (Fig. 8A and B), the phonolites having the lowest glass transition temperatures (Giordano et al., 2005). Tg decreases in the following order: 1631 W tephri-phonolite, trachytes, trachybasaltic and basaltic compositions from Etna and Stromboli and the 1906 phono-tephrite, and finally the rhyolite. Less dramatic rates of decrease are observed for the basaltic and rhyolitic compositions followed by tephri-phonolite, trachyte and phonolites, with the exception of IGC trachyte, which isokom temperatures cross the Etna trachybasalts. Several crossovers are observed for the isokom at 108 Pa s, all lying within <50 K. Rhyolites have isokom temperatures which are 100–250 K higher than those calculated for the other suites. Though strongly dependent on the model equation, another parameter reflecting the temperature-dependence of the viscosity and thus flow property changes as the glass transition temperature is approached, is the fragility. This derivative property provides information about the sensitivity of the melt structure and of rheological properties to temperature changes and increases with deviation from

Alkaline magmas viscosity/The H2O dissolution mechanism in silicates Rhyolite (HGG) Trachyte (AMSB1) Trachyte (MNV) Trachyte (IGC) Phonolite (Mercato_TR) Phonolite (Td_ph) Tephri-phonolite (1631W_GM) Trachybasalt (Etna) Phono-tephrite (1906_GM) Basalt (STR)

(A)

1200

1000

800

1400

Isokom Temperature (K)

Isokom Temperature (K)

1400

T(K) @ η = 10 Pa s 0

1

(B)

1200

1000

800

12

600

6621

8

600

2 H2O wt%

3

4

T(K) @ η = 10 Pa s 0

1

2 H2O wt%

3

4

Fig. 8. Isokom temperature variation as a function of H2O contents calculated at 1012 Pa s (a proxy for the glass transition temperature Tg). and 108 Pa s (a proxy for magma fragmentation). Colours as in Fig. 6.

Arrhenian behaviour. Energy landscape (e.g., Stillinger, 1995) and configurational entropy theories (Adam and Gibbs, 1965) correlate the fragility with the number of accessible basins of local potential energy minima or configurations available in the melt structure. For these reasons, we believe the fragility to be intimately correlated with the ease of a material to adapt to thermal (e.g., upon magma degassing or cooling during magma–water interaction) or mechanical (e.g., upon differential stress–strain regimes during the ascent of the magma along a volcanic conduit) perturbations therefore providing a viscous or a brittle rheological response. Fragility pathways are therefore particularly relevant for the magma flow characterisation (Giordano and Dingwell, 2003b; Giordano et al., 2008a). Near-Arrhenian behaviour can be described as “strong” melt, whereas “fragile” melts are those exhibiting significant deviation from Arrhenian behaviour (Angell, 1991; Giordano et al., 2008a). For melt fragility (Fig. 9) we use the definition provided by Plazek and Ngai (1991) and Bohmer and Angell (1992) via the steepness index (m), recently confirmed by Nascimento and Aparicio (2007):

dðlog10 gÞ BVFT m¼ ¼ dðT g =T Þ T ¼T g T g ð1  C VFT=T g Þ2

ð3Þ

where BVFT, CVFT and Tg are specific for each individual melt. The addition of water decreases the steepness index m for all compositions (Giordano et al., 2008a). As expected, the most depolymerised melts (the Etna trachybasalt and the 1906 phono-tephrite) have the highest m values, the most polymerised melts (the Teide phonolite from and the rhyolite) have the lowest m values, and the 1631 W Vesuvius melt has intermediate m values. The fragilities for the most depolymerised melts are a factor of two higher than those of the most polymerized melts. In addition, the rate of fragility changes decreases as water is added to the melt and is higher for the most “fragile” than for the most “strong” melts. The sole exception is the IGC trachyte which has both high m values (especially at low H2O content) and a significant decrease of m upon addition of water. 5. DISCUSSION

60

Rhyolite (HGG)

Trachyte (AMSB1) Trachyte (MNV) Trachyte (IGC) Phonolite (Mercato_TR) Phonolite (Td_ph) Tephri-phonolite (1631W_GM) Trachybasalt (Etna) Phono-tephrite (1906_GM) Basalt (STR)

steepness intex (m)

50

40

30

20 0

1

2 H2O wt%

3

4

Fig. 9. Steepness index (m) vs. water content for the discussed compositions (Fig. 7) calculated employing Eqs. (1)–(3) and the parameterizations of Table 5. More fragile compositions show the highest decrease in the steepness index with first water addition.

5.1. Effect of composition on melt viscosity: Structural implications 5.1.1. Anhydrous melts The variation of viscosity as a function of reciprocal temperature can be discussed in the framework of the configurational entropy theory of viscous flow proposed by Adam and Gibbs (1965), according to which: log g = AAG + BAG/(T*Sconf (T)), where AAG is the pre-exponential factor, BAG the activation energy of viscous flow and Sconf (T), is the temperature-dependent configurational entropy. This theory has been shown to account for deviations from Arrhenian behaviour observed in silicate melts. It also relates variations in physical properties to the structure of silicate melt networks (Richet, 1984; Richet and Neuville, 1992; Stebbins, 2008). Sconf can be considered as the sum of a “topological” and a “chemical” contribution (Richet and Neuville, 1992; Toplis et al., 1997; Romano

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16

16

(A)

(C)

14

12

log [viscosity (Pa s)]

log [viscosity (Pa s)]

14

10 8 6 4 2 0 -2

660 °C

12 10 8

800 °C

6 4

1000 °C

2

1200 °C

0

1400 °C 1600 °C

-2 15

20

25

30

35

5

15

SM 16

35

16

(B)

(D)

14

12

log [viscosity (Pa s)]

log [viscosity (Pa s)]

14

25

SM

10 8 6 4 2

660 °C

12 10 8

800 °C

6 4

1000 °C

2

1200 °C 1400 °C 1600 °C

0

0

-2

-2 0.0

0.2

0.4

0.6

0.0

NBO/T*

0.2

0.4

0.6

NBO/T*

Fig. 10. Anhydrous isothermal viscosity curves as a function of the SM and NBO/T* parameters approximating the degree of polymerization. Numbers in the figures are isotherm temperatures and are the same for all panels. Vesuvius samples are reported in (A) and (B), whereas (C) and (D) report alkaline samples from the Phlegrean Fields (IGC, AMSB1, MNV), Teide (Td_ph) Etna (ETN) and Stromboli (STR). For the Vesuvius samples viscosity always follows linear trends. This observation does not hold for wider compositional range.

et al., 2001). The mainly temperature-dependent topological contribution derives from variations in the topology of the oxygen lattice, mainly due to variations in the distribution of T*–O distances and T*OT* angles (where T* represent cations in tetrahedral coordination), and represents the short- and intermediate-range order of the silicate network (Henderson, 2005; Stebbins, 2008). The topological contribution is believed to be dominant at high temperature (viscosities in the interval 100–105 Pa s), whereas the chemical contribution becomes more important, as T decreases (at viscosities between 108 and 1012 Pa s; Richet, 1984; Richet and Neuville, 1992). The ability of O to explore new configurations is expected to depend on the strength of its bonding with cations in the lattice. In the low-viscosity regime, Sconf should be proportional to the average bond strength of the melt, which is proportional to the degree of polymerisation and to melt fragility (Toplis et al., 1997; Bottinga and Richet, 1995; Giordano et al., 2008b). The degree of polymerisation may be expressed by the SM parameter, i.e., the sum of all the structure modifier oxides (in mol%) as calculated by Giordano and Dingwell (2003a) or by the NBO/T* parameter (NBO: non-bridging oxygens) divided by tetrahedrally coordinated cations, as defined by Mysen (1988)

(Table 6). On a molar basis, NBO/T* is calculated as (2O–4T*)/T* and for both the SM and NBO/T* parameters, half of the total iron (FeOtot (wt%) in Table 1) is taken as Fe2O3 (Fe3+ being a network former while Fe2+ is a network modifier, Giordano and Dingwell, 2003a). Such an oxidation state of iron is realistic and fits with that of most of the anhydrous glasses synthesised at atmospheric condition (Mercier et al., 2009). The simplified parameters SM or NBO/T* can be considered as useful proxies to correlate to a first order complexity physical properties of silicate melts with their structural configuration (Giordano and Dingwell, 2003a; Giordano et al., 2008a; Mercier et al., 2009) (Table 6). An example of the correspondence of the rheological properties with polymerization parameters is shown in Fig. 10. The viscosity of the Vesuvius liquids at high T may be described as a linear function of SM or NBO/T* (Fig. 10A and B). The Mercato phonolite has the highest viscosities (SM = 16.58, NBO/T* = 0.05), followed by the 1631 W Vesuvius magmas (SM = 24.09, NBO/T* = 0.26), the trachyte from Pompei (SM = 23.49, NBO/T* = 0.23), the tephri-phonolite from Pollena GM (SM = 29.77, NBO/T* = 0.44) and the phono-tephrite from 1906 (SM = 33.13, NBO/T* = 0.59).

log [viscosity (Pa s)]

Alkaline magmas viscosity/The H2O dissolution mechanism in silicates

16 14 12 10 8 6 4 2 0 H2O = 0 -2 5 10

(A)

(B)

H2O = 2

15

20

25

30

6623

15

20

(C)

H2O = 4

25

SM + H2O

30

35

SM + H2O

40

45

30

35

40

45

50

55

60

SM + H2O

Fig. 11. Dry (A) and hydrous (B and C) isothermal viscosities as a function of the molar sum of SM + H2O (a proxy for the network weakening components). Similar trends are observed if the NBO/T* parameter is used. Added water contents in wt% are indicated in each panel. Non linear low-T trends are observed both for dry and hydrous melts. Curves and associate symbols are as in Fig. 10.

This linear relationship between viscosity and SM or NBO/T* at high temperatures (although a significantly larger scatter is observed compared to the SM parameter at low NBO/T* values) holds also when a wider range of silicate liquid compositions is included (Fig. 10C and D). In the low-viscosity regime, the viscosity of the rhyolitic melts HGG as calculated from Ardia et al. (2008), has the highest anhydrous viscosity (SM = 7.34, NBO/T* = 0), whereas the lowest viscosity pertains to the phono-tephrite from the 1906 eruption (SM = 33.13, NBO/T* = 0.59), the Etna trachybasalt (SM = 31.16, NBO/T* = 0.50) and the high-K basalt from Stromboli (SM = 31.00, NBO/T* = 0.50). Between the two extremes, trachytes and phonolites are in decreasing order of viscosities: IGC (SM = 15.34, NBO/ T* = 0.04) > MNV (SM = 15.17, NBO/T* = 0.07) > AMSB1 (SM = 17.29, NBO/T* = 0.10), Mercato (SM = 16.58, NBO/T* = 0.05) > Teide phonolites (Td_ph)

Table 6 SM, NBO/T* and steepness indexes (m) of discussed compositions.

HGG MNV IGC Mercato_TR Td_ph AMSB1 Pompei_TR 1631W_GM Pollena_GM Etna 1906_GM STR

SMa

NBO/T*b

mc

7.3 15.2 15.3 16.6 17.7 19.3 23.5 24.1 29.8 31.2 33.1 31

0.00 0.07 0.04 0.05 0.10 0.10 0.23 0.26 0.44 0.50 0.56 0.50

22.3 24.5 31.7 25.0 25.9 24.8 31.1 46.5 42.5 42.0

a Structure Modifiers parameter. On a molar basis (mol%) of oxides, SM is the sum of all structure modifier oxides. As defined by Giordano and Dingwell (2003a), it is calculated by assuming that half of the total iron (FeOtot) in wt% partitioned as Fe2O3 (Fe3+ being a network former cation) and the other half is FeO (Fe2+ being a network modifier) (see text). b Non-bridging oxygen per tetrahedra on a molar basis (Mysen, 1988). c Steepness index (e.g., Plazek and Ngai, 1991) using the new parameterization (Table 5).

(SM = 17.67, NBO/T* = 0.10) > 1631 W_GM phonolites (SM = 24.09, NBO/T* = 0.26). This inverse correlation between degree of polymerisation and liquid viscosity is observed at high but not at low temperatures. At the lowest temperatures (<660 °C), the trend is inverted (Fig. 10A), with the depolymerised melts from Vesuvius having viscosities higher than those of more polymerized samples (i.e., Mercato and Pompei). Even if the limited amount of data does not allow for quantitative interpretations, a clearly different behaviour between high and low temperatures is evident (Fig 10). This behaviour can also be correlated with the fragility of the melt. The fragility of dry glasses and melts depends on the degree of polymerisation such that fragility increases as the degree of depolymerisation increases (Hess et al., 1995; Giordano and Dingwell, 2003b; Giordano et al., 2008a). Fragility changes of anhydrous liquids are given in Table 6. The Pollena tephri-phonolite and the 1906 phono-tephrite have the lowest viscosities at high T, due to their low degree of polymerisation, but both display the highest viscosities at low-T, because of their higher fragility. Above we have defined two viscosity regimes: a high temperature, low-viscosity regime where the overall strength of the melt, considered as the average strength of the T*–O bonds, is the main factor influencing flow properties, and a low-temperature high viscosity regime. As temperature decreases, the energy necessary to break T*–O bonds may not be high enough to activate viscous flow (therefore directly relating the topological contribution to viscous flow). Other mechanisms can then have a greater influence on viscous flow, such as the cooperative motion of different configuration units in the melt, (the so called cation or chemical contribution, like Q speciation, Al/Si order-disorder, triclusters, ion channelling, bond dangling, [5] and [6] coordinated Al and Si, coordination changes of network modifiers, (see Stebbins, 2008; Giordano et al., 2008b). These reactions have often dCp/dT curves with positive slopes at low-temperature, which then reach maxima and have negative slopes at higher temperature. These curves mirror different structural mechanisms which become more or less relevant at different temperature ranges (Richet, 1984; Richet and Neuville, 1992; Stebbins, 2008) making it difficult

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D. Giordano et al. / Geochimica et Cosmochimica Acta 73 (2009) 6613–6630

to establish the individual contribution of specific reactions. However, Stebbins, 2008 have observed that reactions like Q speciation, or changes in the coordination number of Al and Si or Al/Si ordering may contribute quite significantly to the overall energetics especially near Tg. 5.1.2. Hydrous melts As to be expected, water strongly decreases the viscosity of all examined liquids (Figs. 6 and 11). This decrease is larger for more polymerised compositions, such as the rhyolite from Ardia et al. (2008), than for depolymerised liquids, such as the trachybasalt from Etna. That water has a stronger effect on polymerised than depolymerised melts is a feature commonly observed in various natural and synthetic melts (Persikov et al., 1990; Whittington et al., 2000, 2001; Romano et al., 2003) and is related to the dissolution mechanism of water in the aluminosilicate melts. Several models have been proposed to account for the dissolution of water into silicate melts but a unique view on this matter has not yet been reached. Water is known to dissolve in melts as both hydroxyl groups and molecular (Stolper, 1982a,b) and the structural role of OH groups is still a matter of debate. Traditional models of water dissolution involve dissociation reactions with consequent disruption of the network (and decrease of viscosity), according to the following reactions: Si–O–Si þ H2 O () 2Si–OH Si–O–Al þ H2 O () Si–OH þ Al–OH Al–O–Al þ H2 O () 2Al–OH

ð4Þ ð5Þ ð6Þ

(Burnham, 1975; Stolper, 1982a,b; Sykes and Kubicki, 1993, 1994; McMillan, 1994). More complex mechanisms have been proposed, involving coordination changes, creation of anhydrous non-bridging oxygen and hydrous Al and/or Na complexes, all pointing to depolymerisation of the network (see Kohn, 2000 for review). According to Kohn and co-workers (Kohn et al., 1989, 1992, 1994, 1998; Kohn, 2000), on dissociation, a proton substitutes for an alkali (or alkaline earth), which charge balances an aluminium in tetrahedral coordination, and the remaining OH group complexes with the liberated alkali (or alkaline earth) cation, according to the following reaction: Naþ ðAl–O–SiÞ þ H2 O ¼ Si–OH–Al þ NaOH

ð7Þ

This protonation reaction leads to a viscosity decrease through weakening of the T*–O–T* bonds without breaking them, but producing a NaOH complex. Hence, the extraordinary effect of water in decreasing the viscosity of silicate (especially SiO2-rich) melts is likely due to an enhanced stability for the activated complex involved in viscous flow, rather than to melt depolymerisation. A NMR study on a hydrous synthetic phonolite suggests that both depolymerisation and protonation may occur and operate simultaneously (Robert et al., 2001). Other studies support both mechanisms for tectosilicate melts; the prevalence of “depolymerisation” mechanism

(reaction (4)–(6)) for more silica rich compositions, and the “protonation” mechanism (reaction (7)) for more NaAlO2-rich compositions (Kohn, 2000; Holtz et al., 1995; Mysen, 1992). Both mechanisms explain the observed decrease in viscosity through an overall weakening of the structure. The first mechanism involves breaking the strong T*–O–T* bonds that control viscous flow, the second weakens the same T*–O–T* bonds by the protonation reaction, facilitating the rate of viscous flow. Spectroscopic studies (Fraser, 1977; Xue and Kanzaki, 2004, 2008; Mysen and Cody, 2005) and models (Moretti, 2005) have suggested the presence of free OH groups in addition to SiOH and AlOH groups formed by the dissociation of water and disruption of the aluminosilicate network. According to these studies, in both Al-free and Al-bearing systems water dissolves into the silicate network according to the reaction: 2Si–O–M þ H2 O ¼ Si–O–Si þ 2MOH

ð8Þ

where M are network modifier cations. The net effect of water dissolution, according to these spectroscopic results, depends on the relative importance of reactions (4)–(6) vs. (8) and on the abundance of free OH species relative to SiOH and AlOH species. The depolymerisation reactions (4)–(6) will predominate in polymerised liquids, whereas reaction (8) will be dominant in depolymerised liquids, such that at NBO/T > 0.5 (SM  30) the governing mechanism is the polymerisation mechanism of reaction (8), as shown by the spectroscopic analysis of the same samples as measured here (Mercier et al., 2009; Di Muro et al., 2009). The stronger effect of water on the viscosity of polymerised melts can therefore be due to the higher abundance of AlOH and SiOH species, the absence of free OH and the overall stronger depolymerising effect of water. Our viscosity data would also be consistent with a model where all three mechanisms operate simultaneously. In this case, the effect of water in reducing viscosity would then be due to the disruption of the network and creation of SiOH and AlOH groups (reactions (4)–(6)), to the weakening of the structure by protonation and to formation of Si–OH– Al species (reaction (7)). The latter two mechanisms would have a reduced effect on viscosity in more depolymerised melts through the formation of free OH groups (reaction (8)), which could lead to an opposite effect of increasing polymerisation and viscosity. The decrease in viscosity due to the addition of water is not linear (Figs. 6 and 11). Such a large decrease in viscosity with small amounts of added water and a continuous but less pronounced decrease in viscosity with further added water is commonly observed (Jewell et al., 1993; Hess and Dingwell, 1996; Giordano et al., 2004; Vetere et al., 2006). This behaviour has been explained by an increasing concentration of molecular H2O over OH groups with total water dissolved (Stolper, 1982a,b; Silver and Stolper, 1989). These studies investigated water speciation on quenched glasses. Dingwell and Webb (1990), however, demonstrated, on the basis of fictive temperature analysis, that the speciation reaction for water is not quenchable from eruptive temperatures. Subsequent studies, both in situ and fictive temperature based, have confirmed the tempera-

Alkaline magmas viscosity/The H2O dissolution mechanism in silicates

ture-dependent trends (Nowak and Behrens, 1995; Shen and Keppler, 1995; Romano et al., 1995; Sowerby and Keppler, 1999) and have shown that the fraction of H2O present as hydroxyl groups increases with heating above Tg. The disproportionation equilibrium moves toward OH groups both with temperature and H2O, and therefore cannot totally account for the non linear variation of viscosity as a function of H2O. Instead, this effect could be due to the competition between reactions (4)–(6) and (7) (depolymerisation and protonation) and (8) (polymerisation), such that as water content increases, reaction (8) becomes relatively more important than reactions (4)–(6) and (7) and the overall depolymerisation effect of water diminishes. A non linear dependence of viscosity on the concentration of the added depolymerising component has also been observed for the addition of alkali, or alkaline earth cations, or for the addition of other volatiles such as fluorine and chlorine. These components are thought to be completely dissociated in the melt and produce non linear variations of viscosity with excess component. The effect of water in reducing the viscosity of the melts decreases noticeably as the temperature increases and at 1000–1200 °C, the reduction of viscosity is almost negligible (Figs. 5 for the Mercato and 1906 melts). In the framework of the Adam and Gibbs theory, this behaviour can be explained as due to the contribution of Smix to the overall Scont (T, x) (i.e., Sconf (T) + Smix) at varying temperatures. It has been suggested that in binary systems, at low-temperature and intermediate composition, strong viscosity decrease is produced by an Smix term. Considering the hydrous silicate melt as a pseudobinary mixture of an anhydrous silicate endmember and pure water endmember, we observe the same trend with our liquids (Fig. 5). A similar non linear variation of viscosity is observed at the lowest temperatures for anhydrous intermediate compositions (Fig. 10C and D). This indicates an important contribution of Smix to Sconf (T, x) due to mixing of network modifiers or configurational species in the liquid. Water therefore behaves as any other network modifier or any other configurational species as its presence in the melt introduces a mixing term which is strongest for intermediate compositions and low-T. From the combination of Figs. 5 and 10 we obtain Fig. 11, where isothermal viscosity is reported as a function of SM + H2O. For this chemical variable, which includes all depolymerising species, we observe the same strong non linear variation of viscosity as with SM + H2O, with possible minima for intermediate compositions and low-T, again indicating an important chemical contribution to Sconf (T, x) also in presence of water. As temperature increases, the total Sconf (T, x) of the melt increases considerably and the relative contribution of Smix becomes less significant, reducing the viscosity decrease for intermediate compositions (Richet, 1984; Neuville and Richet, 1991; Whittington et al., 2001). From about 1000 °C the reduction of viscosity with H2O becomes then almost negligible. The same trend, although less marked, can be observed for other natural and synthetic compositions, both at 1 atm and at high pressure (Romano et al., 2001; Misiti et al., 2006; Poe et al., 2006). This effect

6625

occurs despite the disproportionation equilibria (4)–(6) shifting in favour of OH which led Richet et al. (1996) to argue against a simple direct dependence of viscosity on OH concentration. The complexity of the correlation between viscosity and water content also emerges from the analyses of Fig. 9 where water decreases the fragility of the liquids. The fragility of dry glasses and melts increases with melt depolymerisation (Hess et al., 1995, 1996a,b; Del Gaudio et al., 2007). Depolymerised liquids are composed by a large number of configurational species which can easily accommodate changes in temperature by changes in their arrangement or in the type of cooperative motion that activate viscous flow. For this capability, they are fragile, in the sense of Angell. Excess of alkali or alkaliearth cations depolymerise the liquid, decreases the viscosity and increases the fragility of the melt (Hess et al., 1995,1996b). Silica-rich, polymerised liquids have stiffer structures with a lower degree of freedom to accommodate thermal perturbations, and therefore are less fragile. Depolymerisation of the liquid through formation of Si–OH and Al–OH groups should increase liquid fragility. Literature data are however quite scarce and controversial in this respect. A slight decrease in fragility, expressed as the gradient of the viscosity curve at Tg on a reduced temperature scale (Toplis et al., 1997), has been reported as a function of water content for albite and andesite liquids (Richet et al., 1996), while a stronger decrease of fragility with water has been observed for basanite liquids (Whittington et al., 2000). Del Gaudio et al. (2007) report hydrous viscosity data for a tephritic liquid but the scatter is too large to derive any trend. Deubener et al. (2008) show a clear trend of decreasing fragility with increasing water content (to 1.5 wt% H2O) for a series of commercial glasses. On the other hand, Romano et al. (2001) observe for a series of liquids of feldspatic composition that fragility increases with water content. The same conclusions are reached by Misiti et al. (2006) investigating the effect of water on the viscosity of K-trachytic melts from the Phlegrean Fields. Whittington et al. (2004), indicate a compositional dependence of fragility, the latter increasing with water for albite and decreasing with water for leucogranites. For rhyolitic liquids, Ardia et al. (2008) calculated a decrease of fragility with increasing water content, indicating a stronger melt at higher water contents. Using the model of Giordano et al. (2008a), we observe similar trends for the magmas from Vesuvius. It is difficult to reconcile the traditional view of water disrupting the silicate network and depolymerising the melts with the decrease in fragility. Apparently, the mechanism of water dissociation, as discussed above, is more complex than traditionally thought and various equilibrium reactions above illustrated have to be taken into account. It is possible, as we have suggested earlier, that all these three equilibrium reactions operate simultaneously but to a different extent as a function of increasing water content and melt polymerization. It has already been suggested, that as the liquids become increasingly water rich, reaction (8) moves to the

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right hand side. However, it is difficult to reconcile the resulting hypothetical overall polymerisation of the liquid (which would results from reaction (8)) with the observed continuously decreasing viscosity. However, if both reaction (7) and (8) shift to the right hand side with increasing water content a liquid could be envisaged whose NBO/T* is not strongly changing with water content but whose weakened structure could still facilitate, though to a lesser extent, viscous flow.

thoughtful comments and suggestions. We are grateful to two anonymous reviewers and associated editor J.K. Russell for their insightful suggestions which allowed us to further improve the clarity of the manuscript. Finally, we thank Dominique Massare for the density measurements of glass samples.

APPENDIX A A.1. High-T viscosity determination

6. CONCLUSIONS In this study, we have determined the Newtonian shear viscosity of anhydrous and hydrous natural melts from the past and recent activity of the Vesuvius volcanic system. The results have been parameterized by using the same VFT empirical equation, which allows calculating viscosities in the T–H2O space for each of the investigated products. The adopted VFT expression was also used to recalibrate the empirical parameters necessary to describe the rheological variation from other Italian alkaline volcanic systems (Phlegrean Fields, Etna, Stromboli HK) and from Teide (Tenerife) and provides a first order evaluation of the relation between the viscosity path and the eruptive style for high-Na and high-K alkaline volcanic systems. The results from the present analysis indicate that at Vesuvius the most polymerised phonolitic melts, which produced the most violent eruptions, are the most viscous products at their eruptive temperatures. Largely, the viscosity relationships between the different compositional groups analysed here are also consistent with the dominant eruptive styles that vary from catastrophic plinian (Mercato Pumices; Pompei Pumices) to subplinian (1631 and Pollena) to violent strombolian and effusive (1906 eruption) with decreasing viscosity (Fig. 7). Also fragility (m) relates strongly to the style of eruption, with the most fragile melts associated with lower intensity eruptions and the strongest melts being associated with the most violent Plinian eruptions. The pre-eruptive temperature, considered as a proxy for the eruptive temperatures itself, has shown to be a crucial variable to define proper viscosity values necessary to constrain eruption dynamics. Further improvements to the rheological characterization of natural magmas will have to account for the multiphase nature of the investigated magmas. ACKNOWLEDGMENTS This work was supported by the 2005–2006 INGV-DPC project V3-4/UR05. D. Girordano and C. Romano also acknowledge support from the FIRB AIRPLANE project 2007–2010 and the Experimental Volcanology and Petrology Laboratory (Laboratorio di Vulcanologia e Petrologia Sperimentale) of the Geological Sciences Department of the Third University of Rome. P. Ardia and M.W. Schmidt acknowledge grant TH-27/05-3, the grant No. 2-77182-02 of the Swiss National Science Foundation, and E. Reusser for support. D.B. Dingwell acknowledges support of a Research Professorship (LMU excellent) of the German Bundesexzellenzinitiativ. We would like to acknowledge Prof. R. Cioni for assistance in fieldwork, helpful insights and discussions and Prof. F. Barberi for

Measurements of viscosities between 102 and 105 Pa s were obtained using the concentric cylinder system and methods described by Giordano et al. (2004,2005) These measurements were performed on anhydrous samples, directly after homogenisation, in a Deltech furnace using a Brookfield DVIII (full-scale torque = 5.75  101 N m) viscometer head and a Pt80Rh20 low-viscosity spindle (e.g., Dingwell and Virgo, 1988). Batches of about 100 g were loaded into the Deltech furnace and melted at 1300– 1600 °C (depending on composition) and stirred for several hours. After inspection of the stirring spindle revealed that the melts were physically homogeneous, the melt viscosities were measured Viscosity was determined in steps of decreasing temperature with a final measurement again at the highest temperature to check for drift. The viscosity standard DGG-1 was used to calibrate the system and the viscosities are accurate to within 5%. After quenching, cylinder of a few mm in diameter were cored from the glasses and cut into disks 2–3 mm thick. The disks were then doubly polished, dried and stored in a desiccator until further use. A.2. Low-T viscosity determinations Micropenetration measurements for both dry and waterbearing samples in a viscosity interval 109.1–1012.7 Pa s were performed using a vertical push-rod Ba¨hr 802 V dilatometer (e.g., Hess et al., 1995; Giordano et al., 2005, 2008b). This method is based upon the determination of the rate at which a hemispherical Ir-indenter penetrates under a fixed load into a melt surface. The absolute shear viscosity is determined via the following equation: g¼

0:1875  P  t r0:5 d 1:5

ðA:1Þ

(Pocklington, 1940; Tobolsky and Taylor, 1963) where P is the applied force, r is the radius of the hemisphere, t is the penetration time and d is the indentation distance. This provides an accurate viscosity value for indentation distance < 150–200 microns. The applied force was about 1.2 N. One of the main advantages of this technique is the small amount of material (<10 mg) required to perform a measurement making this technique suitable for piston cylinder synthesis products. Samples were heated in the dilatometer at 25 K/min to a temperature 100 K below the temperature of measurement. The samples were then further heated at 5 K/min to the target temperature, where they were allowed to relax during an isothermal dwell of 10–15 min. Subsequently the indenter was lowered to pene-

Alkaline magmas viscosity/The H2O dissolution mechanism in silicates

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Table A1 Measured absorbances, thicknesses and densities for Mercato and 1906 samples measured by FTIR spectroscopy and calculated content of dissolved hydroxyl and molecular water species. The adopted absorptivity coefficients are those estimated by Carroll and Blank (1997) on Narich phonolites. Sample

A4520

A5230

d (mm)

q (g/cm3)

OH

H2Om

FTIR H2Ot

Raman H2Ot

MER MER MER MER MER MER

0.084 0.082 0.110 0.224 0.228 0.243

0.014 0.013 0.033 0.244 0.286 0.411

1.14 1.01 1.06 1.11 1.09 1.07

2.474 2.474 2.480 2.429 2.419 2.396

0.43 0.47 0.60 1.20 1.25 1.37

0.08 0.08 0.21 1.48 1.78 2.63

0.51 0.55 0.81 2.68 3.03 4.00

0.60 0.64 0.92 2.56 3.03 4.21

0.5_ii 0.5 1.0 2.0 3.0 4.0

trate the sample and the viscosity measurement was stopped, once a stable indentation rate was reached. Each measurement was performed at isothermal conditions using a new sample. The recorder indentation – time allows evaluation of whether exsolution or other kinetics processes affecting the measurement occurred during the run. Measurements with evidence for such processes were discarded (Giordano et al., 2008b). A.3. Determination of dissolved water content Homogeneity and stability of the water contents of our samples were checked by FTIR and microRaman spectroscopy before and after the micropenetration viscometry using the methods described by Behrens et al. (1996), Nowak and Behrens (1997) and Di Muro et al. (2006). Transmission FTIR spectroscopy (Nicolet Magna-IR 550, CEA Saclay) was performed on 1 mm thick double polished samples of hydrous Mercato pumice glasses prepared for micropenetration experiments. We used the absorption coefficients of Carroll and Blank (1997) for phonolitic Mercato glasses and Dixon et al. (1995) for the 1906 glasses. In the NIR domain of these samples, two main bands at 4500 and 5200 cm1 can be identified. The 4500 cm1 band is due to a combination between O–H stretch at 3600 cm1 and the protonated T*–O, T*–O–T* linkage at 900 cm1. The 5200 cm1 band is a combination mode of molecular H2O. A weak overtone band at 7000 cm1 due to both OH and molecular water was observed in the spectra of the most water-rich glasses. Concentrations of each species were calculated according to Beer–Lambert’s law: C = 100AM/(eqd), where A is the absorbance, M the molar mass (g mol1) of the species, e the molar absorptivity (Carroll and Blank, 1997), d the sample thickness (cm) and q is the glass density (Table A1). Total water content was determined as the sum of the 4500 and 5200 contributions. Absorbance was measured after subtraction of a cubic baseline. Density measurements were performed in a pyrex pycnometer by immersion of small glass chips in a solution of heavy liquid (Na-polytungstate). This method requires the dilution with distilled water of the heavy liquid till equilibrium with the immersed glass chip is attained. MicroRaman analyses were performed on glass chips of Mercato to detect possible quench crystals and to quantify the total amount of dissolved water. Raman spectroscopy was carried out at the Centre for Nuclear studies (CEA) at Saclay using a Renishaw inVia Raman system fitted to

a Leica DMi microscope with a 100 objective. Raman analyses were carried out at the surface of undisturbed glass chips and of glasses that experienced one heating cycle during micropenetration. A green diode laser (532 nm) was used to minimize the effects of heating and oxidation during measurement. An external calibration procedure for the intensity of the H2OT peak was carried out using a range of matrix matched phonolitic synthetic glass standards from Pollena (H2Ot: 0–6.8 wt%). A cubic baseline correction was applied before determining peak intensities. Counting times were 30  3 s on both the H2O band and the bands related to vibrations of the aluminosilicate network. Calibrations were tested on a set of six Mercato glasses previously analyzed by FTIR. Precision, estimated as the average deviation from FTIR data, is 0.06 wt%. Finally, total water contents were also measured by KarlFischer titration (KFT) (Department of Earth Sciences, ETH, Zurich) of glass samples of Mercato and 1906. We followed the method described by (Behrens and Stuke, 2003) with a correction for unextractable water. KFT, FTIR and microRaman analysis performed on undisturbed glasses provided consistent water contents. Water contents determined by KFT and FTIR technique on samples before and after the micropenetration measurements, provided similar water contents also consistent, within the experimental error, with the values found before the micropenetration measurement. On the other hand, microRaman analysis of samples which underwent thermal treatment provided important differences in the water contents compared to concentrations obtained by KFT and FTIR analysis. This indicates that a certain water loss may occur at the sample surface most likely towards the end of the micropenetration measurements. Nevertheless, we firmly believe that whether volatile loss occurred during viscosity measurements that is restricted to few microns at the sample surface, and therefore is not detected by FTIR and KFT analyses. We sustain, that this loss will not significantly affect measured viscosities for the following reasons: (1) there is no indication of any significant indentation time relation drift during measurement; (2) log (viscosity) vs. reciprocal temperature trends maintain a nearly Arrhenian behaviour; (3) the indentation distances are significantly larger than the depth where degassing would be efficient; (4) degassing would be more efficient at the end of the measuring stage and still being activated during the cooling stage and (5) the consistency of our results with those obtained by experimental techniques evaluating viscosity

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through stress vs. strain-rate deformation on larger samples, which have a smaller surface vs. volume ratio and therefore a more limited degassing efficiency.

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