Contrib Mineral Petrol (1996) 124: 19—28 ( Springer-Verlag 1996

D.B. Dingwell · C. Romano · K.-U. Hess

The effect of water on the viscosity of a haplogranitic melt underP-T-X conditions relevant to silicic volcanism

Received: 17 August 1995/Accepted: 8 January 1996

Abstract The viscosities of hydrous haplograniticmelts synthesized by hydrothermal fusion at 2 kbarpressure and 800 to 1040°C have been measured attemperatures just above the glass transition and ata pressure of 1 bar using micropenetration techniques.The micropenetration viscometry has been performedin the viscosity range of 109 Pa s to 1012 Pa s. Thesamples ranged in water content from 0.4 to 3.5 wt%.For samples with up to 2.5 wt% H

2O, the water con-

tents have been determined using infrared spectroscopyobtained before and after each viscometry experimentto be constant over the duration of the measurements.Above this water content a measurable loss of wateroccurs during the viscometry.

The viscosity data illustrate an extremely nonlineardecrease in viscosity with added water. The viscositydrops drastically with the addition of 0.5 wt% of waterand then shallows out at water contents of 2 wt%. Anadditional viscosity datum point obtained from theanalysis of fluid inclusions in a water-saturated HPG8confirms a near invariance of the viscosity with theaddition of water between 2 and 6 wt%. Thesemeasurements may be compared directly with the dataof Hess et al. (1995, in press) for the effects of excessalkali and alkaline earth oxides on the viscosity ofHPG8 (also obtained at 1 bar). The viscosity of themelts, compared on an equivalent molar basis, in-creases in the order H

2O((Li

2O(Na

2O(

K2O(Rb

2O,Cs

2O(BaO(SrO(CaO(MgO(

BeO). The extraordinary decrease in melt viscosity withadded water is poorly reproduced by the calculationscheme of Shaw (1972) for the range of water contentsinvestigated here. The speciation of water in the quen-

D.B. Dingwell ( ) · C. Romano, K.-U. HessBayerisches Geoinstitut, Universitat Bayreuth, D-95440 Bayreuth,Germany

Editorial responsibility: J. Hoefs

ched glasses can be used to quantify the dependence ofthe viscosity on hydroxyl content. Considering only thehydroxyl groups as active fluidizers in the hydrousmelts the nonlinearity of the viscosity decrease and thedifference with the effects of the alkali oxides becomeslarger. Consequences for degassing calcalkalinerhyolite are discussed.

Introduction

A serious gap in present knowledge of the propertiesof degassing subvolcanic silicic magmas is thedescription of melt viscosity at the P-¹-X conditionsimmediately prior to and during volcanic eruptions.Modeling of the dynamics of such systems is flourish-ing at present and a further optimization of modelsfor the kinetics of melt degassing, vesiculation andfragmentation would be aided greatly by reliable vis-cosity data obtained under the appropriate P-¹-Xconditions.

Equally important is the realization that fully gene-ralizable models for the P-¹-X dependence of meltviscosity will only be achievable when structure pro-perty relationships for the melts of interest, based onsystematic studies of the variation of both structure andproperties with P, ¹ and X, are available.

Both of these considerations, together with the ob-servations from glass transition studies (Dingwell andWebb 1992) and recent viscometry (Lejeune et al. 1994;Schulze et al. in press) that present methods of estima-ting melt viscosity are not very accurate at low watercontents, have led to the present investigation. Wehave chosen to study of the influence of minor amountsof water on the viscosity of a haplogranitic meltcomposition serving as a model for calcalkalinerhyolite. The results provide the most precise compari-son of the effects of differing network-modifyingcomponents on the melt viscosity hitherto possible andsystematics emerge in the temperature and composition

dependence of the viscosity of granitic melts in general,one of the outstanding problems remaining in meltphysics.

The present measurements are focused on the vol-canologically vital but previously underinvestigated re-gion of water contents of 0.4 to 3.5 wt%. By investigat-ing such melts at 1 bar pressure and very low temper-atures of 500 to 750°C we access a range of viscositiesof 109 to 1012 Pa s which corresponds to those of cal-calkaline rhyolites during the final stages of degassing,vesiculation and/or fragmentation. The results pro-vided below are unfortunately not well predicted byavailable calculation schemes based on data obtainedat much higher water contents. We conclude that thediscrepancies between experiment and calculation arelarge enough that the predictions of much modelingwork regarding the degassing of rhyolitic melts, i.e.,those melts involved in highly explosive, rhyolitic anddacitic volcanic systems, are in need of revision.

Method

Synthesis

For our investigations we have chosen as a base composition a hap-logranitic melt composition (designated HPG8 by Holtz et al. 1992)which lies near the 1-kbar (pH

2O) ternary minimum in the system

SiO2-NaAlSi

3O

8-KAlSi

3O

8. This melt composition was chosen to

enable a direct comparison of the present results for the influence ofwater on melt viscosity with the data provided by Hess et al. (1995,in press) for the effects of alkali and alkaline earth oxides and toserve as a model for a calcalkaline rhyolites.

The base melt used in the present study was generated by directfusion of powder mixes of oxides and carbonates at 1 atm and1400—1650°C. The fusions were performed in thin-walled platinumcrucibles with a MoSi

2box furnace. The partially fused products of

this initial fusion were transferred in the crucibles to the viscometerfurnace and a stirring spindle was entered into the samples fromabove. The samples were fused for hours to days until inspection ofthe stirring spindle, removed periodically from the melt, indicatedthat the melts were crystal- and bubble-free. The melts were removedfrom the furnace and allowed to cool in air in the Pt crucibles. Theglass composition was analyzed by solution-based ICP-AES (induc-tively coupled plasma-atomic emission spectrometry) methods. Theresults of a bulk anhydrous analysis are presented by Holtz et al.(1992).

Glass cylinders (diameter: 3 mm; length: 3—4 mm) were drilled bywater-cooled diamond coring tools, cleaned ultrasonically inacetone and then dried at 110°C to remove any residue of acetone.Glass samples (&100 mg) were then loaded together with knownamounts of doubly distilled H

2O in capsules formed from platinum

tubing (outer diameter : 3.2 mm; length :10 mm; wall thickness0.1 mm) and sealed with an arc-welder.

The capsules were checked for possible leakage by testing forweight loss after drying in an oven at 110°C for at least one hour.After an hour at 110°C, the added water was considered to bedistributed randomly in the platinum capsule, and thus around theglass sample. Synthesis of hydrous glasses was performed with twodifferent apparatus.

The experiments performed at 800°C were carried out in ex-ternally heated rapid quench cold seal bombs operating vertically(water as a pressure medium). In this apparatus the temperature iscontrolled by Ni—NiCr thermocouple (accuracy $10°C). The pres-

sure has an accuracy of $20 bar. The experiments at 1040°C and2 kbar were placed in TZM bombs (Ar as a pressure medium) Thetemperature was recorded by a Ni—NiCr thermocouple (accuracy$15°C) and the pressure was monitored with a strain-gaugemanometer (accuracy $50 bar). The samples were held for a timesufficient to allow complete homogenization of H

2O dissolved in the

melt by diffusion through the sample (run durations ranged from7 to 17 days).

After the high pressure - high temperature dwells, the sampleswere quenched rapidly and isobarically by dropping the sample intothe cold part of the bomb (estimated quench rate 200°C/s). Specialcare was given during the quench to maintain isobaric conditions byopening the vessel to the pressure line (2000`50

~0bar).

The quenched run products consisted of crystal-free limpid glass-es. Doubly polished sections of 1 mm thickness were prepared formicropenetration viscometry by grinding with alumina abrasive andthen polishing with diamond paste using water as a lubricant.Polished samples were cleaned with acetone to remove epoxy. Thehomogeneity of the glasses was tested by analysis with an infraredspectrometer (see below).

Micropenetration viscometry

The viscosities were measured using a micropenetration technique.This involves determining the rate at which an Ir-indenter undera fixed load moves into the melt surface. These measurements wereperformed in a BA® HR DIL 802 V vertical pushrod dilatometer. Thesample is placed in a silica rod sample holder under an Argon gasflow. The indenter is attached to one end of an alumina rod, which isattached at the other end to a weight pan. The metal connectionbetween the alumina rod and the weight pan acts as the core ofa calibrated linear voltage displacement transducer (LVDT). Themovement of this metal core as the indenter is pushed into the meltyields the displacement. The present system uses hemispherical Ir-indenters with diameters of 2 mm, and a force of 1.2 N. The absoluteshear viscosity is determined from

g"

0.1875 Pt

r0.5 a1.5(1)

(Pocklington 1940; Tobolsky and Taylor 1963) for the radius of thehalf-sphere r, the applied force P, indent distance a, and timet (t"0, a"0 upon application of the force). The measurements areperformed over indentation distances of less than 100 lm. Thetechnique yields an absolute determination of viscosity up to1100°C in the range of 108 to 1011.5 Pa s. Viscosities determined onthe DGG-1 standard glass using 3 mm samples have been repro-duced within an error of $0.06 log units (see Fig. 1).

All samples were heated up to the measuring temperature, heldat this temperature for 1 hour to allow thermal equilibration andcomplete structural relaxation of the sample and then the viscositymeasurement was performed. After the measurement, each samplewas cooled at approximately 20 K/min.

The advantages of the micropenetration technique are the abilityit provides of using relatively small amounts of sample and simplesample geometry constraints compared to other high-viscositymethods such as parallel plate or fiber elongation (cf. Dingwell et al.1993; Dingwell 1995a). Using the formulation of Pocklington (1940)the penetration distance should be small compared to the sampleheight. As this criterion cannot always be honored in our presentexperiments, we have evaluated the effect of sample size on a DGG-1standard glass with a thickness of 1 mm (Fig. 1). Between 108 to1010.5 Pa s appears a slight, but significant deviation to highervalues of the determined viscosities from the calculated values re-ported in the Deutsche Glastechnische Gesellschaft data sheet. Thedata listed in Table 2, all performed on samples of 1 mm thickness,are corrected for this sample size effect.

20

Fig. 1 Viscosities determined on the Deutsche GlastechnischeGesellschaft soda-lime glass-melt viscosity standard (DGG-1) usingthe micropenetration method. Micropenetration viscometry has theadvantage of applicability to very small sample dimensions. Themeasurable influence of reducing size to 1 mm thickness is incorpor-ated as a small correction factor based on the data presented here

An important aspect of the present study is the demonstration ofthe stability of water-bearing samples during measurement at 1 atm.On each of the samples 526—3, 527—1, 527—2 and 144—1 three to fourviscosity measurements were performed with no drift in viscosityvalues as a function of time. This observation, combined with thelack of evidence for concentration gradients of water in spatiallyresolved IR measurements (described below), argues against signifi-cant water loss. Nevertheless the shapes of the indentation curvesfrom the present experiments show minor irregularities which mayindicate a very thin ((10 lm) boundary layer. Although theseirregularities add noise to the viscosity determinations, the influenceof such effects on the derived viscosities remains within the quoteduncertainties previously obtained by this method (Hess et al. 1995 inpress). This is consistent with the evidence from the sample size effectnoted above that the stress and resultant strain are distributed overthe lengthscale of the entire sample ("1 mm).

Control of water content

Water contents of the samples, both as total water dissolved in themelt and as the fractions of water dissolved as molecular water andas hydroxyl groups, were determined by IR spectroscopy. Becausethe present samples are investigated under water-oversaturated con-ditions at 1 bar, samples were analyzed before and after each viscos-ity measurement to check for water loss during viscometry. Sampleswere then positioned over an aperture in a brass disc in order to aimthe beam at areas of interest in the glasses. A Bruker IFS 120 HRfourier transform spectrophotometer was used to obtain transmis-sion infrared spectra in the NIR region (2500—8000 cm~1 ), usinga W source, CaF

2beamsplitter and a narrow band MCT detector.

The spectrophotometer operated at a resolution of 4 cm~1 witha scanning speed of 20.0 KHz and the objective used was a Casseg-ranian 15X. Typically, 200—1000 scans were collected for each spec-trum. Background was recorded and subtracted from every spec-trum. The two bands of interest of this regions are at 4500 cm~1 and5200 cm~1.

The H2O content of the samples has been determined by measur-

ing the heights at the maximum of the absorption bands near 4500

and 5200 cm~1 attributed respectively to the combination stretch-ing#bending mode of X-OH groups and a combination of stretch-ing#bending modes of molecular water groups (Stolper 1982a;Newman et al. 1986).

For evaluation of the spectra we have applied a baseline correc-tion different to that used by Stolper and coworkers (Stolper1982a,b; Newman et al. 1986; Silver et al. 1990). For the band due tothe stretching and bending of molecular water at 5200 cm~1, thebackground is essentially flat, so a linear background tangent to theminima on either side of the peak was chosen. The procedureadopted is different for the OH band at 4500 cm~1, where thebackground is complicated because of the contribution from thehigh-energy tail of the fundamental OH-stretch band at 3570 cm~1and the band at 4000 cm~1. We have adopted linear backgroundtangent to the minima in the 5200 cm~1 peaks for the determinationof the heights of the 4500 cm~1 band. This procedure assumes thatboth bands do not overlap and that the low frequency bands at4000 cm~1 do not contribute to the band at 4500 cm~1 at itsmaximum (Behrens et al. in press).

This procedure allows extremely precise and reproduciblemeasurements of the height of the peaks and we prefer it to themeasurements of the integrated absorptivities because of the higherrelative uncertainty of the latter arising from problems in accuratefitting of the background.

The precision of the measurements presented here is based on thereproducibility of the measurements of glass fragments repeatedover a long period of time as well as on the uncertainty assigned tothe background subtraction procedure. We estimate that the typicaluncertainty in our measurements for the 4500 cm~1 and the5200 cm~1 band is approximately 0.003 absorbance units.

The concentration of dissolved ‘‘water’’ in a glass contributing toa given band can be determined as follows:

c"(18.02) (absorbance)/(d) (q) (e) (2)

where c is the H2O concentration in weight fraction, 18.02 is the

molecular weight of water, absorbance is the height of the absorp-tion peak, d is the thickness of the specimen in cm, e is the molarabsorptivity (or extinction coefficient) in liter/mol-cm and q is thedensity of the sample in g/liter.

Density measurements have been performed using a BermanBalance with toluene as a reference liquid (Table 1). Precision, forsamples whose masses range from 0.02 to 0.06 g, is estimated to be$3 g/liter based on reproducibility of measurements on differentsamples. The thickness of each glass plate was measured witha digital Mitutoyo micrometer (precision $ 3.10~4 cm) (Table 1).

In order to determine quantitatively the wt% of the water speciesin the samples investigated, the molar absorptivities are required.We considered for our study molar absorptivities values presentedby Nowak and Behrens (1995) for an haplogranitic composition verysimilar to that here investigated (e

OH"1.56; e

H2O"1.79). The pro-

cedure to derive molar absorptivities is extensively described inNewman et al. (1986) and it is based on a weighted linear leastsquare method. An extended discussion of the procedures adoptedfor the derivation of the absorptivities and the errors involved isprovided by Behrens et al. (in press). Several measurements, per-formed along sections perpendicular and parallel to the length,showed no difference in the background subtracted peak area, at4500 cm~1 (hydroxyl groups) and at 5200 cm~1 (molecular H

2O)

and therefore indicate that H2O is homogeneously distributed

throughout the sample and that the run durations were long enoughto produce complete hydration via diffusion of water through thesample.

Infrared determination of the water content (molecular waterand water dissolved as hydroxyl groups) were performed after eachviscosity measurement in order to test the homogeneity of thesample and to rule out significant water loss.

Two samples were cut in slices perpendicular to the indentationpoint of micropenetration viscometry and traverses were made inorder to investigate possible water gradient across the samples.

21

Table 1 Synthesis conditionsand spectroscopic data Sample Time! P ¹ Density" Thickness# AOH$ AH

2O4

(h) (kbar) (°C) (g/l) (cm)

528—1-1 168 2 1100 2356 0.160 0.133 0.005528—1-2 168 2 1100 2356 0.161 0.129 0.009528—1-3 168 2 1100 2356 0.164 0.130 0.011526—2-0 168 2 1100 2346 0.149 0.260 0.062526—2-2 168 2 1100 2346 0.150 0.235 0.077526—2-3 168 2 1100 2346 0.150 0.229 0.073526—3-0 168 2 1100 2346 0.147 0.256 0.058526—3-2 168 2 1100 2346 0.148 0.232 0.075526—3-3 168 2 1100 2346 0.149 0.227 0.071526—3-4 168 2 1100 2346 0.149 0.228 0.069527—1-1 168 2 1100 2340 0.151 0.286 0.148527—1-2 168 2 1100 2340 0.152 0.285 0.144527—1-3 168 2 1100 2340 0.153 0.283 0.140527—2-1 168 2 1100 2340 0.154 0.297 0.148527—2-2 168 2 1100 2340 0.155 0.291 0.142527—2-3 168 2 1100 2340 0.157 0.290 0.142527—2-4 168 2 1100 2340 0.158 0.290 0.141144—1-1 408 2 800 2331 0.101 0.231 0.176144—1-2 408 2 800 2331 0.101 0.226 0.174144—1-3 408 2 800 2331 0.102 0.222 0.171280—2-1 288 2 800 2327 0.099 0.237 0.244280—1-1 288 2 800 2327 0.099 0.243 0.246300—1-1 168 2 800 2318 0.079 0.274 0.295300—1-3 168 2 800 2318 0.107 0.283 0.321

! Duration of the experiments" Density measurements; error$3 g/l (relative error"1%)# Thickness; error"$3.10~4 cm (relative error"1%)$ Absorbance; error"$0.003 units; values reported refer to multiple spectra

Single spots of 100 lm were used to evaluate the presence of a gradi-ent. The samples all appeared homogeneous and no evidence ofwater concentration gradients was observable within the uncertaintyof the infrared determinations. The infrared measurements cannotrule out the possibility of some water loss in the immediate vicinityof the indentation point ((10 lm from the surface). Such smallgradients are however unlikely to influence the present viscositydeterminations in a significant way, as discussed above.

The water contents of all samples with the exception of no. 300remain unchanged during the high temperature dwells employed toassure fully relaxed viscosity determinations in the viscometrymeasurements. The viscosity data obtained thus pertain to struc-turally equilibrated, fully relaxed, water-bearing melts at 1 bar pres-sure and the temperatures of measurement. The results for sampleno. 300 must be treated more cautiously. The high temperatureexcursions employed here involved (as noted above) a 1 hour dwellat the measurement temperature followed by loading the indenter onthe melt surface and measuring viscosity over approximately 5 min.Given that the viscosity is determined in the last 13% of the hightemperature dwell time, the postviscometry water content is likelya very good approximation to the water content during the visco-metry determination. With sample no. 300 we have identified thelimit of water content for which the present methodology can beapplied.

Results

The results of the viscosity determinations aretabulated in Table 2 together with the total water

contents and species concentrations as determined byIR spectroscopy. The viscosity data are presented ver-sus the reciprocal absolute temperature in Fig. 2. Themicropenetration viscosity measurements show a de-creasing viscosity and an approximately constant tem-perature dependence of viscosity with increasing watercontent. The decrease is extremely nonlinear, beinghigher at low water content. These data have been fit tothe Arrhenius equation (1) to describe the temperaturedependence of viscosity.

log10

g"a#b/¹ (3)

These fits are used as the basis for interpolation ofviscosity data to construct the isothermal and isovis-cous comparisons used below. Figure 3a illustrates thedata extrapolated to isotherms of 700, 800 and 900°C.Due to the likelihood of significantly non-Arrhenianbehavior of these melts (see D.B. Dingwell, in press,Dingwell et al., in press) the Arrhenian extrapolation ofthe present database to viscosities as low as 105 Pa s inFig. 3a is somewhat uncertain. An alternative repres-entation is to plot the temperature at which a constantviscosity is obtained (an isokom) as a function of watercontent. This involves no extrapolation of the presentdata set and is presented in Fig. 3b. All of the essentialfeatures of Fig. 3a are retained.

22

Table 2 Viscosity and water contents

Sample OH! H2O" H

2O total# T Viscosity

(wt%) (wt%) (wt%) (°C) (Pa s)

528—1-1 0.41 0.01 0.42 648.7 11.80528—1-2 0.39 0.02 0.41 703.6 10.76528—1-3 0.39 0.03 0.42 687.3 11.40526—2-0*$ 0.86 0.18 1.04 — —

526—2-2 0.77 0.22 0.99 649.6 9.68526—2-3 0.75 0.21 0.96 611.4 10.45526—3-0*$ 0.86 0.17 1.03 — —

526—3-2 0.77 0.22 0.99 611.1 10.48526—3-3 0.75 0.21 0.96 593.7 10.98526—3-4 0.75 0.20 0.95 666.6 9.42527—1-1 0.93 0.42 1.35 528.4 11.39527—1-2 0.93 0.41 1.33 549.4 10.96527—1-3 0.92 0.39 1.31 602.8 9.88527—2-1 0.95 0.41 1.37 567.4 10.58527—2-2 0.92 0.39 1.32 582.9 10.38527—2-3 0.91 0.39 1.30 621.4 9.60527—2-4 0.90 0.38 1.28 612.5 9.83144—1-1 1.13 0.75 1.88 546.6 10.09144—1-2 1.11 0.74 1.85 507.0 11.30144—1-3 1.08 0.72 1.80 576.6 9.69280—2-1 1.19 1.07 2.25 545.3 9.62280—1-1 1.22 1.07 2.29 501.9 10.49300—1-1 1.73 1.62 3.35 515.1 9.93300—1-3 1.31 1.30 2.61 548.7 9.12

! Water content as derived from IR determination of absorbances;average relative error is 2%" Average relative error is 7%# Total water content is derived as a sum of molecular and waterdissolved as hydroxyl groups$ Samples indicated by !0 are referred to IR measurements per-formed before the first viscosity determination

Fig. 2 The viscosities of HPG8 melt containing up to 3 wt% waterdetermined using micropenetration viscometry on samples annealedat 1 bar at various temperatures above the glass transition for1 hour. The viscosities decrease strongly whereas the temperature-dependence of viscosity is little affected within the range of watercontents presented in this study

Fig. 3a Isotherms at 700, 800 and 900°C for the influence of wateron the viscosity of HPG8 melt at 1 bar pressure. The estimatedviscosity for this range of conditions using the method of Shaw(1972) at 700°C is included for comparison (line). The viscositydecrease is much more nonlinear than the method predicts.b ‘‘Isokoms’’ or lines of constant viscosity at 109, 1010, 1011 Pa sversus the prediction of the 1010 Pa s isokom from the method ofShaw (1972) (solid line)

Discussion

Comparison with the scheme of Shaw

For over two decades estimates of the viscosity ofhydrous silicate melts have been possible using thecalculational scheme of Shaw (1972). The input of hy-drous melt viscosities upon which the Shaw (1972)scheme was based are the data obtained by the sameauthor on a calcalkaline rhyolite sample with approxi-mately 4 and 6 wt% of added H

2O (Shaw 1963). Those

23

experiments were performed in the temperature rangeof 800 to 1000°C and at 2 kbar pressure using thefalling sphere method in hydrothermal bombs. Theresults of the present viscosity determinations can nowbe compared with the viscosities of the present meltspredicted from the method of Shaw (1972). This com-parison is made in Fig. 3a where the solid line repres-ents the results of the Shaw (1972) calculation for700°C and in Fig. 3b where the solid line represents theresults of the Shaw (1972) calculation for the 1010 Pa sisokom. Figure 3 illustrates that the present experi-mental data deviate seriously from the predictivescheme of Shaw (1972). The decrease in viscosity withwater content is more extreme at low water contentsthan the Shaw (1972) method predicts. Thus the pres-ence of minor amounts of water in silicic magmas willhave a much greater fluxing effect on the melt than hasbeen previously appreciated.

Comparison with the effects of alkali and alkaline earthoxides

Shaw (1963) noted the similarity in the effects of theakali oxides on the viscosity of SiO

2melt and water on

the viscosity of a rhyolite melt, and for some time nowthis has more or less intuitively implied that the struc-tural roles of alkali oxides in excess of aluminum oxide,and that of water, were similar. With the obtainment ofthe present data for the effect of water on the viscosityof HPG8 melt, determined on melts at 1 bar pressure,together with the data of Hess et al. (1995), we are ablefor the first time to provide a direct and precise com-parison of the effects of water and other depolymerisingcomponents on the viscosity of a single polymerizedmelt composition at identical temperatures and pres-sures. Figure 4 illustrates that the influence of de-polymerizers on the viscosity of the haplo-granitic melt is stronger for water than for any of theother components. Hess et al. (1995) have alreadypointed out that a trend of decreasing viscosity withdecreasing cationic size or field strength is discernibleamongst the data for the effects of K

2O, Na

2O and

Li2O (see Fig. 4). The location of the hydrous melt

viscosity data would seem consistent with the extra-polation of such a trend to H

2O but its quantification

appears difficult. To analyze this trend further weshould consider the speciation of water (see below).

Comparison with the results of SFG experiments.

The behavior of sample no. 300 illustrates that with thehighest water contents reported here we appear to havereached the limit of the applicability of our presentmethods for the investigation of hydrous melt viscosi-ties. In order to obtain the extension of the trend tohigher water contents in this range of viscosities we are

Fig. 4 Comparison of the effects of the addition of alkali oxides andwater to the viscosity of HPG8 melt at 1 bar pressure and 700°C.This is the first direct comparison of these various depolymerizingcomponents on the viscosity of a single silicic melt compositionunder identical temperature and pressure conditions

forced to employ complementary information for theeffects of water on the viscosity of HPG8 melt.

The cooling of hydrous melts using drop quenchmethods yields glasses with a viscosity of 109 Pa s at theglass transition temperature (Dingwell and Webb1989). Thus glass transition temperatures obtained forrapidly quenched hydrous melts yield viscosity-tempe-rature data points for those melts. The determination ofthe glass transition temperature of very water richmelts has been performed recently for compositions onthe albite-orthoclase anhydrous base system (Romanoet al. 1994). Equivalent data for the granitic melt com-position investigated here, HPG8, has been obtainedby Romano (1994).

The methods for the determination of the glasstransition temperature are based on the behavior ofvesicles in water-saturated melts which behave as fluidinclusions during the quench (Dingwell 1993). Themethod has been developed in full by Romano et al.(1994) and is not discussed further here. The fluid inclu-sion-based glass transition temperature for the haplo-granitic HPG8 composition, water saturated at 2 kbar(6.07 wt% water; 2.19 wt% OH) is 544$10° C. Theviscosity corresponding to this temperature is 109 Pa s.(Dingwell and Webb 1990). Estimation of the activa-tion energy of viscous flow for these samples (Shaw1963) permits the selection of a viscosity value at theglass transition temperature corresponding to anyother cooling rate due to the equivalence of the activa-tion energies of viscous flow and of the shift of glasstransition temperature with cooling rate (Stevenson etal. 1995). In Fig. 5 the 1010.2 Pa s viscosity isokom forthe results of the present study is presented togetherwith the glass transition temperature corresponding to

24

Fig. 5 Comparison of the micropenetration viscometry data at the1010.2 Pa s isokom including the glass transition temperature at thatisokom from the rapid quench synthetic fluid inclusion method ofRomano et al. (1994). Increase of water content from 3 to 6 wt%causes only a very minor decrease in viscosity or, as presented here,isokom temperature. The hydrous melt isokoms are presented fortotal water and for hydroxyl content (as determined from quenchedglasses whose fictive temperatures correspond to this isokom). Thedata labelled SFG are the glass transition temperature results fromthe study of synthetic fluid inclusions in glasses corrected to thepresent isokom with the activation energy calculated from Shaw(1972). Also included are isokom data for the dry peralkaline meltsof Hess et al. (1995) on the same melt composition (HPG8). (See textfor further explanation)

this viscosity for the water-saturated HPG8 melt con-taining 6.07 wt% water. The water-saturated melt vis-cosity appears consistent with a smooth continuationof the decrease in viscosity versus water content trend.Although this comparison involves data obtained at1 bar as well as at 2 kbar the pressure dependence ofthe viscosity of water-rich silicic melts is expected to beslight in this pressure range (Dingwell 1987).

Water speciation

The relationship between melt structure and viscosity isa central theme of current research into structure-pro-perty relationships of silicate melts. The solution ofwater in silicate melts plays a special role in that dis-cussion because: (1) the effect of water on melt viscosityis extremely large (Shaw 1963; Dingwell 1987; Romanoet al. 1994); (2) water is known to be dissolved in meltsas structurally bound OH groups and as molecularwater (Stolper 1982a,b). The speciation of water in suchmelts is dependent on temperature, water content andpossibly pressure (Zhang et al. 1991, 1995; Romano1994; Romano et al. 1995; Nowak and Behrens 1995).In order to estimate the speciation of water at the runtemperatures of the viscometry in the present study we

need only be concerned with the effect of temperature,as pressure and composition do not change betweenviscometry and spectroscopic investigation.

The speciation of water obtained in quenched sam-ples of silicate glasses does not correspond directly tothat present in the melts at very high temperatures(Dingwell and Webb 1989; Stolper 1989). Nevertheless,to a very close approximation (cf. Romano et al. 1995;Nowak and Behrens 1995), the temperature-depend-ence of the speciation of water can be estimated fromdata obtained on quenched glasses. This estimation isbased on the premise that the speciation of water islargely frozen in during the quenching of hydrous silicicmelts at the glass transition temperature. The tests ofthis assumption using viscosity estimates of the glasstransition are in very good agreement with spectro-scopic anneal-and-quench estimates of the relaxationtimes for water speciation in such melts (see discussionsin McMillan, 1994; Dingwell, 1995b).

An outstanding question regarding the effect ofwater on melt viscosity concerns the presence of mole-cular water in the melts. The strongly nonlinear influ-ence of water on the melt viscosity has been interpretedto indicate the structural consequences of the decreas-ing fraction of water dissolved as hydroxyl groupswithin the melt with increasing total water content(Stolper 1982a,b). If molecular water has a negligibleeffect on melt viscosity then the melt viscosity is deter-mined solely by the amount of hydroxyl groups presentin the melt structure. Given that there is no evidence forsignificant amounts of associated alkali oxides (i.e., freeoxygens in the sense of Toop and Sammis, 1962) per-sisting in the melt structure (see Mysen, 1988) it followsthat comparisons between the effects on viscosity ofwater, alkali oxides and alkaline earth oxides should bemade on the basis of hydroxyl content present in themelts at the temperature of viscosity measurementrather than total water content.

In principle, the hydroxyl contents of HPG8 withadded water may be derived on the basis of spectro-scopic data from quenched glasses together with a re-liable estimate of the temperature-dependence of thespeciation using the methods of fluid inclusion deter-mination of the glass transition (Romano et al. 1994,1995). In practice the present study provides a simplermethod. If the viscosity comparison is performed at1010.2 Pa s then the speciation of water quenched intopostviscometry glasses corresponds to the speciation ofwater at the temperatures of the viscosity determina-tions (for g"1010.2 Pa s). The estimated hydroxyl con-tents of the HPG8 melt for the 1010.2 Pa s isokom havebeen used to plot the 1010.2 Pa s isokom for the hy-drous melts of this study in Fig. 5. This test of thehypothesis that the hydroxyl content is controlling theviscosity of the hydrous melts generates in Fig. 5 a ‘‘hy-droxyl’’ viscosity isokom which is much lower in tem-perature that those of the lighter alkali oxide networkmodifiers. A comparison at constant added network

25

modifier yields a trend of viscosities which exhibitsa strong convergence as the network-modifying cationis enlarged. The results of both of the isokom compari-sons (total water and hydroxyl) yield families of sub-parallel curves. Further quantification of the cationicspecificity of the viscosity reducing trend is difficult.

Before generalizing the viscosity-temperature rela-tionships of water-bearing melts within the context ofthe alkali- and alkaline earth-depolymerized graniticmelts, the critical issue of the degree of non-Arrhenianbehavior (the ‘‘fragility’’ of Angell, 1985) of the hydrousmelts must be dealt with. The fragility of the dry meltsof Hess et al. (1995) appears to be generally related tothe nature of the depolymerizing cation. The fragilitycan be expressed as a smooth function of the fieldstrength or electronegativity of the network-modifyingcation and correlates inversely with the partial molarvolumes of the modifying oxides (Hess et al. in press).The parameterization of Hess et al. (in press) predictshighly fragile behavior for hydrous melts. The presentdata set on the temperature dependence of the viscosi-ties of hydrous granitic melts is wholly inadequate totest these predictions and thus studies aimed at thefragility of hydrous melts are required for progress.Newly developed extensions of the falling spheremethod may provide this opportunity (Dorfman et al.in press)

Petrological significance

There a number of aspects concerning the petrologicalsignificance of the present results which are worth not-ing here. Firstly, the severe disagreement of the Shaw(1972) method with the present determinations meansthat we are not yet in a position to predict the viscosi-ties of water-bearing granitic melts with the accuracynormally ascribed to that method. This disappointingconclusion leads to especially relevant problems atcontents below 4 wt% and the discrepancy at lowerwater contents is extreme (several orders of magnitude).The present observation of a much more extreme de-crease of melt viscosity with water content than pre-dicted by the Shaw (1972) method implies that thefluxing role of water in petrogenetic processes shouldbe much more efficient at lower water contents thancurrently thought. Modeling of the variation of visco-sity during the petrogenesis of ascending crystallizingplutons has been conducted by Holtz and Johannes(1992). They pointed out that the viscosities ofhigh temperature relatively dry intermediate toacidic magmas may be lower than the viscosities ofthe water-saturated derivatives of these magmatic sys-tems. The present data clearly indicate that that state-ment is correct down to 1.5 wt% water. In fact thetrends indicated by Holtz and Johannes (1992) areprobably more pronounced than previously thoughtpossible.

The present observations should have, in particular,consequences for the fate of subvolcanic calcalkalinerhyolitic magmas undergoing degassing. Many aspectsof the physics of degassing, including vesicle nucleationand growth, are influenced by the nonlinear response ofmelt properties, amongst them viscosity, to decreasingwater content during the degassing process (Sparks etal. 1994; Wilding et al. 1995). The strong increase inviscosity during degassing of the melt can, through therate-limiting control of viscosity on certain degassingprocesses, effectively act as a brake on the efficiency ofdegassing by diffusive means. It is for this reason thatthe functional form of the effect of water content on theviscosity is a critical similarity parameter in experi-ments involving analog materials (e.g., Sparks et al.1994). The general effect of a correction of results gene-rated from the Shaw (1972) method in the direction ofthe presently defined trend, combined with the likeli-hood of a strongly negative temperature dependence ofthe water solubility (Holtz et al. 1995), will mean thatdegassing processes linked kinetically to the viscosityshould go much more effectively to completion thanhas been previously anticipated. The more effectivedegassing of rhyolites to be expected based on thecurves of Fig. 3 may be a factor in the relatively efficientdegassing inferred from the low water contents of manyrhyolitic obsidians in comparison to the relatively highwater contents inferred from other sources such as meltinclusions or phase equilibria. In particular, the possi-bility of a viscous resistance influencing the rate ofbubble growth has been discussed extensively in therecent volcanological literature (see review by Sparks etal., 1994). It is of utmost importance to constrain theviscosity and water content at which viscous forcesbegin to impede the expansion of bubbles duringmagma ascent leading to eruption because such effectscan potentially stop bubble growth leading to a frag-mentation event. Such a scenario, termed a ‘‘viscosityquench’’ by Thomas et al. (1994) would generate brittlefailure of rhyolitic foam at a certain critical viscosity.Thomas et al. (1994) have suggested this viscosityquench should occur at 109 Pa s for decompressionrates and timescales typical of explosive eruptions(Sparks et al., 1994). The water contents correspondingto this viscosity value are drastically influenced by thediscrepancy between the results of calculations usingthe Shaw (1972) method and the present data (Fig. 3.).The Shaw (1972) calculation generates this critical vis-cosity value at water contents of 1.1 wt% for 800°Cwhereas the present data indicate 0.55 wt%. Thus if weappeal to the scenario of the viscosity quench, (ignoringother possible causes of fragmentation) we observe thatthe magma is capable of degassing far more extensivelythan previously estimated. About the only consolationfrom the comparison of Fig. 3 is that at dissolved watercontents of about 0.1 to 0.2 wt% (typical levels forunaltered calcalkaline obsidians), the calculationand the data agree. Thus the ‘‘target’’ viscosities of

26

degassing calculations down to these levels are approx-imately in order.

Quite separate from the issue of calculation repro-ducing the magnitudes of viscosity of Fig. 3 is theobservation that the rate of decrease of the viscosityversus water content is in reality far more nonlinearthan calculations predict. This too has important con-sequences for modeling magmatic processes accom-panying volcanic eruptions such as degassing and frag-mentation. The existence of a very sharp pressuregradient below the choking point of the magma ina volcanic conduit where fragmentation occurs is basedon the variation of melt viscosity with water loss be-tween 4—5 wt% prior to vesiculation and 1.5—1.8 wt%after vesiculation and fragmentation as recorded inpumice (Thomas et al. 1994). Instead of the estimatedfour log units of viscosity variation from the higher tothe lower water contents, the data of Fig. 3 indicate, ina way consistent with what we know of the speciationof water and the behavior of other melt depolymerizers(Fig. 3) very little viscosity variation between thesewater contents.

The abruptness of the viscosity increase itself, atlower water contents, could indeed be a critical factorcontributing to the relative constancy of observedmean vesicularities in the Plinian and pyroclasticphases tabulated by Thomas et al. (1994). The viscosi-ties estimated in their Table 2 might be substantiallyless certain than their estimated uncertainties and likelyless precise than the total variation presented in theiranalysis.

The dehydration films which can be expected in thepresence of any diffusion-controlled bubble growth orother process involving the chemical diffusion of waterwill generate little or no viscosity gradients down toabout 1.5 wt% water. Below this water content, theviscosity gradient and with it the gradient in brittle-ductile transition at the glass transition, will rise moredrastically in temperature than has previously beenwidely appreciated (cf. Dingwell and Webb 1992).Water loss from 6 to 2 wt% water results in a negligibleviscosity and glass transition increase whereas the lossfrom 2 to 0.2 wt% water generates a viscosity increaseof some five orders of magnitude. In terms of the glasstransition temperature (calculated for a cooling rate of60°C/s) the increase from 1.5 wt% water to 0.2 wt%water is a full 200 K from 850 to 1050°C.

It remains open how well the present data set can beapplied to the influence of minor amounts of water onthe viscosity of more complex natural compositions.All indications from studies of the effect of traces ofincluded water and other depolymerizing agents on theglass transition of natural and synthetic melts indicates,however, that the effect is very extreme at water con-tents even lower than those considered here (e.g., Jewellet al. 1993). For volcanological considerations we needdata for water contents down to those preserved in thecommonly erupted rhyolitic obsidians of calcalkaline

and peralkaline volcanic fields. This means data ina range down to one tenth of a weight percent of water.Considering the relative importance of melt kinetics atrelatively low water contents during volcanic degassingwe wish to emphasize the need for more and better meltviscosity determinations under the P-¹-X conditionsrelevant to subvolcanic processes. The present datamust be built into new calculations of these phenomenain order to approach more closely realistic models ofviscous magma degassing.

Concluding marks

The viscosity of hydrous silicic melts is much morestrongly affected by water content than that which isestimated from previous calculation schemes based ondata for water-rich melts. The discrepancy leads tomuch lower viscosities in the range of water content upto 3 wt% H

2O.

The influence of water on melt viscosity falls withina trend of decreasing viscosity with increasing fieldstrength of the cationic depolymerizers. Nevertheless,before the systematic trends observed for water, alkalioxides and alkaline earth oxides can be parameterizedwith the aim of developing a general predictive schemefor the viscosity of granitic/rhyolitic melts the issue ofthe fragility of hydrous melts must be dealt with experi-mentally.

This extreme influence of water on viscosity meansthat degassing rhyolitic magmas will experience rela-tively low viscosities to a much later stage in the de-gassing process than has previously been anticipated.Thus the efficiency of degassing, even in the limit ofviscosity controlled degassing kinetics, will be relativelyhigh. This efficient degassing is consistent with therelatively low water contents of commonly eruptedrhyolitic obsidians.

Acknowledgments We are grateful to Hubert Schulze for samplepreparation and to Markus Nowak and Frank Schulze for access tomanuscripts. This work was supported by the Deutsche Forschun-gsgemeinschaft (DFG) and the Deutsche Akademischer Austaus-chdienst (DAAD). Helpful reviews were provided by Pascal Richetand Bjorn Mysen.

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