Viscosity of the albite melt to 7 GPa at 2000 K

Shohei Mori *, Eiji Ohtani, Akio SuzukiInstitute of Mineralogy, Petrology, and Economic Geology, Tohoku University, Sendai 980-8578, Japan

Received 2 August 1999; accepted 10 November 1999

Abstract

Viscosity of melt at high pressure provides an important clue to understanding the dynamics of magmas in theEarth's deep interior. We measured viscosity of the albite melt to 7 GPa at 2000 K by the falling sphere method using asingle crystalline diamond as a density marker. Viscosity of the melt decreases significantly from 5.28 þ 1.78 Pa s at3 GPa to 0.90 þ 0.34 Pa s at 5 GPa and 0.99 þ 0.27 Pa s at 6 GPa. We observed a further decrease in viscosity of the meltto 0.28 þ 0.06 Pa s at 7 GPa. The change in viscosity with pressure may suggest existence of some structural change inthe albite melt at high pressure. ß 2000 Elsevier Science B.V. All rights reserved.

Keywords: viscosity; density; albite; silicate melts; high pressure

1. Introduction

Physical properties of silicate melts at highpressures and temperatures are essential to under-stand behavior of magmas in the Earth's interior.Viscosity for several silicate melts has beenstudied at pressures to 3 GPa [1^6]. Physicalproperties of silicate melts depend on the degreeof polymerization of their tetrahedral framework.The ratio of non-bridging oxygens to tetrahe-drally coordinated network-forming cations(NBO/T) has been used to represent the degreeof polymerization [7]. Previous works made upto 3 GPa suggest that viscosity of depolymerizedsilicate melts (NBO/Ts 1) increases with increas-ing pressure, whereas viscosity of highly polymer-ized silicate melts (NBO/T9 1) decreases with in-

creasing pressure [3^6]. In the present study, wedetermined the pressure dependence of viscosityof the albite melt (NBO/T = 0) at 2000 K up to7 GPa using the falling sphere method.

2. Experimental procedure

The starting material used in this study waspowdered albite glass synthesized at one atmos-phere. Na2Si2O5 was ¢rst made from a mixture ofreagent grade Na2CO3 and SiO2, then, the start-ing material was prepared from a mixture of re-agent grade Al2O3, SiO2 and Na2Si2O5. The mix-ture was fused for 20 min in a platinum crucibleat 1573 K. Then, the glass was crushed andground under acetone. High-pressure experimentswere carried out using the Kawai anvil (MA8multianvil) apparatus driven by a 1000-ton uniax-ial press and the split-cylinder guide block systeminstalled at Tohoku University. Toshiba F-grade

0012-821X / 00 / $ ^ see front matter ß 2000 Elsevier Science B.V. All rights reserved.PII: S 0 0 1 2 - 8 2 1 X ( 9 9 ) 0 0 2 8 4 - 8

* Corresponding author.

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WC anvils with truncated edge lengths of 12 mmand preformed pyrophylite gaskets were used forpressure generation. Temperature was measuredby using a W97Re3^W75Re25 thermocouple with0.15 mm in diameter. No correction was appliedfor the e¡ect of pressure on the electromotiveforce. A cylindrical stepped graphite heater wasused to reduce the temperature gradient within agraphite capsule. With this graphite heater, thetemperature gradient inside the capsule was re-duced to less than 30 K up to 2073 K. A semi-sintered MgO octahedron was used as a pressuremedium. Pressure was calibrated at room temper-ature using transitions in Bi [8] and ZnTe [9]. Thehigh-temperature correction was employed at1473 K by using the ¢xed point of the garnet-perovskite phase boundary in CaGeO3 [10] andthe coesite-stishovite phase boundary in SiO2 [11].

Viscosity of the albite melt was determined withthe falling sphere method [3,4]. A single crystal ofdiamond was used as a falling density markerbecause it can meet with the following require-ments : (1) The equation of state is known pre-cisely; (2) Density of the marker is close to thatof the albite melt ; (3) The density marker must bestable and unreactive with the albite melt withinthe time-scale of the experiment. Diamond is ther-modynamically not stable less than 5 GPa and2000 K, but we did not observe any evidencefor graphitization of the recovered diamond onthe basis of the Raman spectroscopic measure-ments and the X-ray powder di¡raction experi-ments. The isothermal compression curve of dia-mond can be calculated using the third-orderBirch^Murnaghan equation of state with the fol-lowing data sources: McSkimin and Andreatch[12], Zouboulis and Grimsditch [13] and Saxenaet al. [14]. The average of the longest axis of dia-mond crystals was 162 þ 14 Wm. The diamond wasplaced on the packed surface of the albite glasspowder at the same height in the capsule and thediamond was covered by a thin layer of the albiteglass in order to prevent contact of the diamondto the lid of the graphite capsule. All the ceramiccomponents of the sample assembly were heatedat 1073 K for about 24 h, and all of the graphitecomponents of the cell were ¢red by a gas torchfor a few seconds. The assembled run charge was

heated at 523 K in an oven for 1 h before eachexperiment. After the pressure was applied, thetemperature was ¢rst kept 200V300 K belowthe target value for 2V5 min to homogenize thetemperature in the cell assembly and then raisedto the target value within 10 s. After heating for aperiod of time, the sample was quenched by shat-ting o¡ the electric power supply. The recoveredsamples were polished in both sides. The fallingdistance was then measured under the micro-scope.

Measurements of viscosity of the albite meltwere made at 3.0, 4.0, 4.5, 5.0, 6.0, 6.5 and 7.0GPa at 2000 K. The run durations were rangedfrom 20 to 550 s. A microscopic view of the re-covered sample is shown in Fig. 1. The viscositywas calculated based on the Stokes' law, incorpo-rating the Faxen correction to account for thewall e¡ect [3,4] :

R � 2r2vbg9v

132:104rrc

� ��2:09

rrc

� �3�

30:95rrc

� �5�A

where R is the viscosity, r is the sphere radius, vbis the density di¡erence between the melt and thesphere, g is the acceleration due to gravity, v is thefalling velocity, rc is the capsule radius and A is aparticle shape correction factor. When the fallingsphere method is employed to determine the vis-cosity, a falling material must be a perfect sphere.However, a single crystal of diamond used in thisstudy was not sphere. Therefore, the particleshape correction of diamond was employed. Atone atmosphere, the falling velocity of diamondwas measured using a standard viscosity solution(Showa Shell, JS2000) of which density and vis-cosity are known. The viscosity values were calcu-lated from the falling velocities and the grain sizeof diamond using the Stokes' law. The particleshape correction factor, A, was obtained as theratio of the true viscosity of the solution to theviscosity measured by assuming sphericity of thefalling marker in this experiment. The particleshape correction factor was found to be A = 0.79( þ 0.08). Parentheses indicate a standard devia-

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tion (1c). We used at least three experiments ofdi¡erent run duration to calculate the fallingvelocity. Fig. 2 illustrates the result for 5 GPa,6 GPa and 7 GPa as examples, and the datawere ¢tted with a straight line to calculate thefalling velocity.

Calculation of viscosity using Stokes' law re-quires the densities of the melt and falling materi-al. We calculated the density of diamond at high

pressure and temperature using the third-orderBirch^Murnaghan equation of state with the fol-lowing data source: McSkimin and Andreatch[12], Zouboulis and Grimsditch [13] and Saxenaet al. [14]. The density of the albite melt at highpressure was estimated based on the moleculardynamics simulation. Potential parameters forthe albite melt were optimized by ¢tting the den-sity and thermal expansion data of the albite melt

Fig. 1. A photomicrograph and a sketch of a polished charge showing sinking of diamond. The run condition is 5 GPa, 2000 Kand 213 s (run no. avis13).

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at one atmosphere [15] and the nearest-neighborbond distance data of the albite melt at one at-mosphere [16]. The detailed procedure of the den-sity calculation is given somewhere [17]. The den-sity of the albite melt at 2000 K was determinedto be 2.86, 2.98, 3.05, 3.10, 3.17, 3.20 and 3.25g/cm3 at 3.0, 4.0, 4.5, 5.0, 6.0, 6.5 and 7.0 GPa,respectively. The errors in the variables r, vb, v, Aand the Faxen correction were ¢nally used to cal-culate the error in viscosity.

3. Results and discussion

Experimental results are summarized in Table 1and Fig. 3. Viscosity of the albite melt up to2 GPa at 2000 K was calculated from the lowertemperature data [12,13] by Arrhenius extrapola-tion using the activation energy Ea = 363 kJ/mol[12]. This result is consistent with ours. The vis-cosity at 2000 K greatly decreases from5.28 þ 1.78 Pa s at 3 GPa to 0.90 þ 0.34 Pa s at5 GPa, whereas it was 0.99 þ 0.27 Pa s at 6 GPa.It decreases again from 0.99 þ 0.27 Pa s at 6 GPato 0.28 þ 0.06 Pa s at 7 GPa. We observed anapparent minimum of the viscosity at around5 GPa, however, we need caution about the exis-tence of the viscosity minimum since uncertaintyof the viscosity is large in this pressure range. Thechange in viscosity of the melt should be associ-

ated with some structural changes of the melt.Thus, the observation that a possible existenceof a viscosity minimum or in£ection at around5V6 GPa may suggest that two types of thestructural change occur in the pressure rangesfrom 3 to 5 GPa, and above 6 GPa. This obser-vation might support the previous estimation ofthe viscosity based on the oxygen self-di¡usivitydata by Poe et al. [18] who suggested that theoxygen self-di¡usivity maximum (viscosity mini-mum) around 5 GPa is due to the structurechange of four-coordinated Al species. However,in this pressure range, there has been no evidencefor ¢ve- or six-coordinated Al species in thequenched albite melt [19^24]. Mossbauer, Ramanand EXAFS study of Fe and Ga analogues of thealbite glass quenched from high pressures up to3 GPa has also shown no evidence for cationcoordination changes with pressure [25].

We have also measured the Raman spectra ofthe albite glass quenched from high pressures and2000 K. We observed a shift of the Raman bandin 485 cm31 into higher wave number, which issimilar to those reported in the pressure range

Fig. 2. Examples of falling distance of diamond with time athigh pressure and temperature.

Fig. 3. The pressure dependence of viscosity of the albitemelt. A circle, triangles and solid squares represent Scarfeand Cronin [29], Kushiro [30] and this study, respectively.The results by Scarfe and Cronin and Kushiro were extrapo-lated to 2000 K using the activation energy Ea = 363 kJ/mol[12].

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below 3 GPa previously [19^22]. The previousworks implied a decrease of T^O bond lengthand T^O^T bond angle (T = Si, Al) with increas-ing pressure. Thus, weakening of bridging T^O^Tbonds resulting from the decrease of the T^O^Tangle might explain the continuous decrease inviscosity of the albite melt up to 5 GPa [19^25].However, we cannot rule out the possible coordi-nation change in this pressure range, since sub-stantial relaxation of the melt structure may occurduring pressure release as was discussed by Wil-liams and Jeanloz [26].

It has been suggested that higher coordinatedAl and Si species exist at pressures above 8 GPain the polymerized melt, and the increase in thecoordination number of Al was observed at lowerpressure than that of Si [24,27,28]. Therefore, de-crease in viscosity of the albite melt above 6 GPamight be accounted for by formation of higher-coordinated Al species in the melt, although wecould not observe clear evidence for such a coor-dination change in the quenched albite melt formthe Raman spectra.

Acknowledgements

We appreciate T. Taniguchi for valuable sug-gestions and supports on this work. This workwas partly supported by the Grants-in-Aid ofthe Scienti¢c Research (A) of the Ministry of Ed-ucation, Science, Sport and Culture of the Japa-nese Government, No. 09304051 to E. Ohtani andNo. 10304041 to H. Taniguchi.[FA]

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Table 1Experimental conditions and results at 2000 K

Run no. Pressure(GPa)

Duration(s)

Falling distance(mm)

avis27 3.0 252 0.37avis31 3.0 320 0.71avis30 3.0 427 0.80avis29 3.0 67 0.20avis21 4.0 212 0.14avis23 4.0 365 0.43avis26 4.0 269 0.57avis24 4.0 541 1.09avis44 4.5 72 0.37avis39 4.5 180 0.79avis43 4.5 220 1.26avis25 5.0 123 0.38avis11 5.0 190 0.54avis13 5.0 213 0.97avis12 5.0 242 1.14avis09 6.0 121 0.46avis08 6.0 182 0.83avis10 6.0 237 1.00avis49 6.5 29 0.20avis48 6.5 49 0.49avis45 6.5 121 0.77avis46 6.5 184 1.20avis17 7.0 60 0.43avis19 7.0 89 0.86avis16 7.0 118 1.14

Pressure(GPa)

Density di¡erence(g/cm3)

Velocity(mm/sU1033)

Viscosity(Pa s)

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