The effect of CO2 on the viscosity of silicate liquids at high pressure

0016-7037/89/163.00 t .oO

Geochsmica n Cosmockimica Ana Vol. 53, pp. 2609-2616 Copyight @ 1989 Perwnon Ress plc. Printed in U.S.A.

The effect of CO2 on the viscosity of silicate liquids at high pressure* MARK BREARLEY’.’and ART MONTANA’.’ ‘Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90024-I 567. U.S.A. *Department of Earth and Space Sciences, University of California, Los Angeles, CA 90024-1567. U.S.A. (Received July 6, 1988; accepted in

revisedform July 7, 1989)

Abstract-The effect of carbon dioxide on the viscosity of silicate liquids has been measured at high pressure and temperature using the filing-sphere method. Our m~surem~nts for the viscosity of volatilefree NaAlSisOs liquid at 1400°C at 1.5-25 kbar are significantly lower than those determined previously by KUSHIRO (1978). CO* reduces the viscosity of NaAlSisOs liquid at high pressure by a factor of two, probably as a result of the depolymerization of the liquid on solution of COZ. Based on previous data for the solubility mechanisms of CO2 in NaAlSisOs liquid, it may be inferred that CO2 dissolves in the liquid, at least in part, as the CO:- ion complexing with Na+, producing some non-tetrahedral A13’. CO1 probably decreases the viscosity of all fully polymerized silicate liquids provided that at least some of the CO2 dissolves as the CO:- ion. The magnitude of the viscosity reduction of NaAISisOs liquid by COZ is comparable to that of both Hz0 and F on a molar basis. In contrast, CO2 appears to have little effect on the viscosity of sodium melilite liquid, possibly because CO2 plays two contrasting structural roles in this relatively depolymerized composition: as a depolymerizing agent analogous to that in albite by complexing with alkali or alkaline earth cations and expelling A13+ from tetrahedral coordination, and as a polymerizer by forming new bridging oxygens and linking aluminosilicate anionic units in the liquid. Because of the low solubihty of CO,, and because CO, is probably a minor volatile constituent in silicic magmas, it will have a minor influence, compared to HZ0 and F, on the modification of the physical properties of these magmas. Despite the relatively high solubility of CO* in basic magmas, it may be predicted that the viscosity of these magmas will not be greatly affected by the addition of CO*. INTRODUCPION

cosity by adding Hz0 and F is interpreted as resulting from de~lyme~~tion of the NaAlSisOs liquid (this is possibly true for a11fulfy poIymerized compositions). consistent with phase equilibria (e.g., MANNING et al., 1980) and spectroscopic observations (e.g., MYSENet al.. 1980). This is achieved by breaking bonds linking the three-dimensional aluminosilicate network, presumably resulting in smaIIer anionic units that are able to flow more readily. The general effect of volatiles on the viscosity of hapiogranitic liquids may be estimated from these important results, but the influence of CO* on the physical properties of silicate liquids has not been investigated, although it is an important volatile constituent, especiaIIy in basic magmas. There are several lines of evidence that indicate that COZ may affect the structure and, consequently, the viscosity of silicate liquids. BOETTCHERet al. (1987) determined that, under CO*-saturated conditions, tectosilicate minerals (albite, anorthite, and sanidine), which melt to fully polymerized liquids, exhibit a depression of the freezing-point, resulting from the solution of COz in and depolymerization of the liquid. Fu~hermore, CO2 dissolves in fully polymerized liquids (except, perhaps. for liquid SiOz) both in its molecular form and as CO:“’ (MYSEN and VIRGO, 1980a; FINEand STOLPER,1985; STOLPER et al., 1987). In order to stabilize the carbonate ion in liquid albite, Nat relinquishes its chargebalancing role for tetrahedrally coordinated Al’+ and forms a Na2COs complex (MYSEN and VIRGO, 1980a), resulting in some non-tetrahedral A13+,with a concomitant depolymerization of the liquid and, presumably, a decrease in the viscosity. The inference for fully polymerized liquids is that COt decreases the viscosity.

THE PHYSICALPROPERTIESof silicate liquids are of fundamental importance to our understanding of the behavior of magmas; for example, the viscosity and density of a magma control whether a magma will separate from its source, rise, and eventually erupt and flow (e.g., SHAW, 1963; BOTTINGA and WEILL, 1972; STOLPER et al., 1981). Our knowledge of the viscosity, density, and other the~~ynamic properties of silicate liquids at atmospheric pressure is adequate to permit estimation or calculation of the physical properties of magmas at the Earth’s surface (e.g., BOTTINGA and WEILL, 1972; SHAW, 1972; BOTTINGA et al., 1982; LANGE and CARMICHAEL,1987). We need information on the properties of magmas at depth to model magmatic processes during the initial stages of magma evolution and transport to the surface. Measurements of the physical properties of sihcate liquids at elevated pressure are scarce (see SCARFE,1986, for a review), and there are even fewer data concerning the effect of volatile components on physical properties. Recently, DINGWELL (1987) demonstrated that Hz0 and F reduce the viscosity of NaAISisOs liquid at high pressure while preserving the negative pressure dependence of viscosity exhibited by volatile-free albite liquid (KUSHIRO, 1978). Because NaAISisOs liquid is fully polymerized, the reduction in vis-

* Institute of Geophysics and Planetary Physics Contribution Number 3 I 14. ’ Present address: Bayerisches Geoinstitut, Universitat Bayreuth, Postfach 10 12 5 1, 8580 Bayreuth, F.R.G. 2609

M. Brearley and A. Montana

2610

In contrast, the data for less polymerized compositions indicate that CO* may polymerize these liquids. High-pressure phase equilibria in the system diopside-forsterite-silica show that CO2 stabilizes orthopyroxene, a metasilicate, relative to olivine, a less polymerized orthosilicate (KUSHIRO, 1975: EGGLER, 1978), suggesting that CO2 acts as a polymerizer. at least in this part of the system. The spectroscopic observations of MYSEN (1976) support this contention. As a result of the apparently contrasting role of CO2 in silicate liquids, we have determined the effect of CO2 on the viscosity of silicate liquids at high pressure. Because the highpressure viscosity and CO2 solubility of albite and sodium melilite have been studied previously (KUSHIRO, 1978; SHARMA, 1979; MYSEN and VIRGO, 1980b; SCARFE, 198 I : STOLPERet al., 1987), we used these compositions to test the effect of CO2 on the structure and properties of silicate liquids. Because NaAlSi30B liquid is fully polymerized, it may be used as a model for silicic magmas. NaCaAlSizO,, on the other hand, is relatively depolymerized and, similar to many basalts, has a calculated NBO/T of 0.67, by assuming that Al is entirely in tetrahedral coordination (MYSEN et al., 1982). EXPERIMENTAL PROCEDURE

Two starting materials were used for albite; a natural low-albite from Langwitz, Switzerland, and albite glass made from oxides. Both starting materials were ground to -200 + 325 mesh and stored in a vacuum desiccator. The albite glass was prepared by melting at I bar and 1600°C in a covered Pt crucible for two hours. The compositions of both materials are shown in Table I. We employed both crystalline and glassy starting materials to assess whether crystalline starting materials produce higher viscosities because they require a longer time to become molten (KUSHIRO, 1978). The sodium melilite starting material was a bubble-free glass prepared by melting oxides at I bar and l6OO’C in a covered F’tcrucible for 12 hours. The composition is also shown in Table 1. It was also ground to -200 + 325 mesh and stored in a vacuum desiccator. Spheres of Pt were fabricated bq a method similar to that of HAZEN and SHARPE (1983). and their radii were measured to a precision of ~0.0002 cm using a petrographic microscope equipped with a graduated eyepiece. Ag&O, was used as the source of CO*. The Ag&O, was stored in an opaque container in a vacuum desiccator and periodically checked for purity and HZ0 adsorption. A high oxygen fugacity was maintained in all runs with a double-capsule technique using FezOX that had been fired at 1000°C for 48 hours. The reduction of hematite to the hematite-magnetite assemblage during the run ensures a IOU .fco andjHI(FSxTW~~~ et al., 1973: LUTH and BOETTCHER, 1986).

Run procedure

We placed one to four Ft spheres on a thin layer of powdered starting material at the base of a triple-welded, 3.5-mm diameter. i ij mm long, Pt capsule that had been previously formed to provide ;I flat base. The capsule was then tightly packed with more powder and dried overnight at 4OO”C, which minimtzes any loss of Na from albite and sodium melilite glass (see FINEand STOLPER,1985). The capsule was then sealed and triple-welded. For runs involving CQ sufficient A&C20, was placed at or near the top of the capsule tn produce 0.5 wtb CO2 in albite and 3.0 wt% CO2 in sodium melihre. These concentrations of Cot ensure that the liquid is undersaturated with respect to CO2 at run conditions (see MYSENand VIRGO,1980b: STOLPER et al., 1987). The capsule was inverted and placed II-Itriple welded, 5-mm diameter pt capsule along with ,250 mg of Fe@,. which acts as a trap for H2 and is partially reduced to Fe+& during the experiment. All experiments were conducted at high pressure in piston-cylirrdrr apparatus by using the falling-sphere method (e.g., SHAW, 1963; Kr SHIRO, 1976). We used furnace assemblies, 2.54 cm in diameter composed of NaCl, pyrex, graphite, BN, and MgO (RXXTCHER C: al., 198 1). The graphite furnace had a 5’ taper to reduce the vertical temperature gradient within tbe furnace (KUSHIRO,1976). Our tapered graphite furnace results in a symmetrical temperature profile along the length ofa IO-mm Pt capsule, with the center of the capsule being 15°C hotter than the top and bottom at run conditions ( I!) 25 kbar, 1400 and 1450°C). By using a hot piston-in technique, WC brought the runs cold to approximately 3 kbar below run pressurt’ and then heated them as rapidly as possible (~5 minutes) to r:~ temperature, resulting in an increase in the pressure because of thermal expansion of the pressure cell. Temperature was monitored with 1%. PtlO%Rh thermocouples encased in pure A1201 ceramic and w~ti? the tip of the thermocouple in contact with the top of the capsule. Experiments were controlled with a precision of +3”C and ‘0. f khar Run times varied from I minute for NaCaAISilO, up to 90 minute> for NaAlS&O(. Several experiments of different duration were per” formed at each pressure for NaAIS&Os composition to assess Ihc effect of melting-time and the initial position of the Pt spheres on the velocity of sinking of a given size of sphere (see Discussion belowi Runs were quenched isobarically at a rate of I SO-XKPC/s by rurninp off the power to the furnace. Run products

After the run, the Fe203-Fe30, mixture was exammed opncailv those runs with ~50% Fe203 remaining were discarded, because UUI experience has been that a greater amount of reduction of Fe&I, results in anomalously low viscosities, suggesting that a significant amount of Hz0 has been produced inside the sample capsule. Ail runs containing A&C20, resulted in glasses that were slightly amber at the base of the capsule, presumably because of disseminated Ag (SEWARD, 1980). The sample capsule was set in epoxy, sectioned longitudinally on both sides to expose glass, and ground until the spheres were exposed at the surface. The sinking distance of the spheres was measured to a precision of +O.OI cm with a graduated. mechanical stage attached to a petrographic microscope

Table 1.

Compositions of starting materials (wt%.) 2

3

A1203 Fag3

68.50 19.29 0.08 0.03

68.23 19.29 0.00 0.06

46.49 19.93 22.13 0.04

Na20

11.85

11.99 0.07

II.80

0.04

99.79

99.64

100.39

1 SiO2

K20

Total

1. Swiss Albite 2. Albite glass 3. Sodium melilite Analyses by electron of at least six points material

0.00

glass microprobe on each

RESULTS The results of the viscosity experiments are in Tables .I! and 3 and in Fig. 1 (for NaAlS&Os only). The viscosities wert: calculated using the duration of the experiment, the sinking distance and radius of the sphere according to Stokes’ law. incorporating the Faxen correction to account for edge effects (e.g., SHAW, 1963: KUSHIRO. 1976):

rl=

2Apr’g -------[I 9c

-

2.104(r/r,t

2611

Effect of CO2 on the viscosity of silicate liquids Table

2.

Results

of viscosity

r km)

d (cm)

experiments

for

N’AISi~Os

and NaAlSiJOg

+ CO2 at

I4OtW

1. NaAlSijOa

P W) 15

Run I

II.

15

10766k633

4.03k.o.03

0.0063

(2)

-0.02 (1)

0.774 (2)

10990+490

4.04lto.02

“0%

0.155 0.40 0.165 0.53

242

0.027

0.23

0.0115

0.00

0.683

10749+257

85 86 87

0.009 0.009 0.009

0.19 0.12 0.21

0.0027 (4)

0.04 (2)

0.892 (2)

674321113

0.013 0.013 0.013

0.33 0.14 0.34

0.0065

(S)

-0.06

(3)

0.844 (2)

S464+448

3.74+0.04

0.24 0.17s 0.10 0.165

0.0044

(5)

-0.04 (2)

0.904 (2)

3281+408

3.52f0.06

;:

0.008 0.008 0.008 0.008

tg” 91 92

0.011 0.011 0.011 0.011

0.35 0.28 0.15 0.26

0.0065

(5)

-0.04 (3)

0.868

(2)

4050+374

3.61+0.04

(2)

71092441

3.855zO.03

0.015 0.015 0.015 0.015

0.10

0.019 0.019

t;

Run #

r (cm)

97 96 98

d fcm)

t(min)

(2)

0.04

0.0067

(3)

0.01 (2)

0.809 (2)

7739*392

3.X9+0.02

0.0060

(4)

0.01 (2)

0.868 (2)

4357+358

3.6440.04

0.0097

(7)

(3)

0.845 (2)

3674+278

3.56kO.03

0.0048

(3)

0.02 (2)

0.904 (2)

29?7+224

3.4750.03

0.0078

(3)

-0.01 (2)

0.868 (2)

3336i182

3.SZf0.02

9s 41 41

0.011 0.011 0.011

0.38 0.20 0.21

:i

0.26 0.17

45 :ft

0.57 11.11::

0.41 0.28

it 30

93

0.008

0.31

60

PZ

~*~~~ .

0.16 0.45

;II

;;

0.011

0.45 0.71

94

0.011

0.24

60 90 30

d

.

regression

sinking

distance,

(see text),

(2)

3”:

0:013

r - sphere radios,

?

(3)

60

least-squares

0.845

0.0050

0.39 0.62 0.22

105

F

60

0.016 0.016 0.016

0.011

i km)

Y (cm/mio)

0.33 0.50 0.20

96 98

95 100

Numbers capsule,

0.29 0.11 0.35

+ CO:!

*“Ott

2s

log10rl

0.821 (2)

97

20

rl

.O.Ol (2)

NaAlSijOs

P WI

F

(2)

81

ti 82 83

25

i (cm)

v (cm/min)

0.0043

80

::

20

t(min)

i.

t - experiment intercept

value,

-0.01

duration,

in parentheses are oacer~inties in the last decimal velocity of sinking calculated from one experiment.

where q is the viscosity (poise), Ap is the density contrast between the sinking sphere and the liquid, r is the radius of the sphere, g is the gravitational constant, v is the velocity of the sinking sphere and r, is the radius of the container (i.e., the F’t capsule). For the calculation of the viscosities of NaAlSi@s and NaAlSi,OB + CO2 compositions, we used at least three experiments of different duration and the same sphere size to compute the velocity of sinking of the sphere. These runs are important in that they permit evaluation of the effects of melting time and initial positions of the Pt spheres in the measurement of viscosity at high pressure. Figure 2 illustrates the results for 1400°C and 2.5 kbar as an example, and shows that the data may be fit with a straight line to calculate the velocity of sinking. We used a linear

Y - velocity

F - Faxen place.

correction, * Ron

of sinking

calculated

11 - viscosity

conducted

from

(poise).

in a S-mm

graphite

least-squares regression calculation (YORK, 1966) incorporating estimated errors in experimental duration (k 1 min.) and measurement of sinking distance (+O.Ol cm) to compute the sinking velocity and its error (Table 2). Also calculated is the intercept value and its associated error. In general, the intercept value is within error of the origin, as has been determined in other studies (e.g., SCARFE et al., 1987). The density of the Pt spheres at high temperature and pressure was calculated using the thermal expansivities of EDWARDS et al. (195 1) and compr~ibiliti~ of BIRCH (1966). Because liquid densities are difficult to measure with any degree of accuracy by the falling-sphere method, we calculated the densities of the liquids at pressure and temperature using a third-order Birch-Murnaghan equation of state, the volume

M. Brearley and A. Montana

2613 Table 3. I.

Results of viscosity experiments fur NaCaAISiZO? and NaCaAISiz07 + CO2 at 145O’C.

NaCaAISizOT

P(kb) Run# 10

123

r (cm) d lcmj t

(mint 2

twon

123

i

16.0 17.2 15.1

1.5

117 121 121

0.0025 0.0025 0.0023

0.56 0.20 O.l?

5 2 2

13.4 15.1 IS.0

1.13’ 1.18’ I.18

20

110 127 127

0.0040 0.0014 0.0020

0.25 0.10 0.13

I

15.1 9.6 14.9

1.18 0.98

123

0.16 0.32 0.155

11

0.0023 0.0034 0.0022

2

1.20 I.24 I.18

0

,_-“-----__+--___.

0 II.

-.

--

.

1.17

1s

,.

:

i il

.f,

NaCaAISizO7 + CO? 15

130 240 241

0.0030 0.0039 0.0036

0.31 0.72 0.75

s

20

.?lJ

0.0020 0.0036

0.19 fl.43

239

4

13.Y 15.0 16.4

l.14 1.18 I.!?

3 2

15.3 14.3

1.18 1.16

Symbols as delinod in Table 2. ** Runs 117 and 121 were used to regress the distance-time data for this sphere size in the same manner as for NaAISijOs: the results (12.5k3.6) encompass the whole range of viscosities studied for NaCaAISiZO, and NaCaAISi~O7 + cot

and thermal expansivity data of LANGE and CARMICHAEL (1987). the bulk moduli of HERZBERG (1987), and set K’ = 5, which is an average value for silicate liquids (RIGDEN et al.. 1988). The calculated densities are similar to those measured by KUSHIRO (1978)for NaA1Si30s composition. Because of the lack of thermodynamic data on C02, the effect of CO: on the liquid density was ignored in the calculations. This appears justified considering the effect of the variation of the liquid density on the Ap term (e.g., BREARLEY et al.. 1986). The errors in the variables r, Ap, V, and the Faxen correction were finally used to calculate the error in the viscosities (Table 2). For NaCaAISizO, and NaCaAlS&O, t CO2 compositions. the uncertainty involved with the duration ofthe experiment (the approximate time it takes to reach the run temperature after melting) swamps all other errors during the least-squares regression of the distance-time relationships and leads to large uncertainties in the viscosity. For example, regressing exper-

Time

(min I

FIG. 2. Plot of sinking distance of Pt spheres versus expenmentai duration for spheres of 0.008 and 0.01 I cm radius. Results shown for NaAISi30s liquid at 1400°C and 25 kbar. Similar results are ob tained at different pressures and for other compositions. The siop: of the line and its error, calculated by least-squares regression, is used as input for the velocity of sinking in Eqn. (I). Note that both lines pass very close to the origin, indicating that the spheres began to sink at the start of the experiment and that they were close to the top :.>I the capsule in each case (see Table 2).

tments at 145O“C and I5 kbar m this way leads to a vtscosrt\ of 12.5 + 3.6 (poise) or a greater than 25%, error (Table 1)

The results for the viscosity experiments ot NaAlSilQ+ and NaAISiJOs + CO2 are shown in Table 2 and Fig. I. All ex periments were conducted isothermally at 140W’f. Our measured viscosities for volatile-free albite liquid are a factor of 2.4 lower at 15 kbar and 2.95 lower at 20 kbar than those of KUSHIRO ( 1978). However. the decrease in viscosity with increasing pressure is similar to that of KUSHIK~) ( 1978) (that is. the ‘*activation volume” is similar). By adding 0.5 ~~7 COz. the viscosity of liquid albite decreases at i 5 and 20 kbat by a factor of 1.5 and 25 kbar by a factor of I 2. This regular decrease in the viscosity of NaA1Si30s liquid with solution of CO2 is similar to that observed for other volatiles (DEW. WELL. 1987). .Sodium meliiite -?rCO2

c

c

-

4.0

Albilc

-t

.. 4 Iy-----4.__.

I_

-+

z

--4JJ----+

a t

Albite + 0.5% CO2

/

3.0 L~-i--_.-.---f____”

15

25

20 Pressure

(kbar)

FIG. 1. Results of viscosity experiments for Na41Si,0s liquid (open circles) and NaAISilOs liquid + 0.5 wt% CO;! (filled circles). Also shown are the results of KlJSHIRO (1978) for NaA1Si30s liquid at 15 and 20 kbar (squares).

The results for the viscosity experiments wtth sodmm meiihte and sodium melilite + CO2 are given in Table 3. Al) experiments were conducted at 1450°C. Our viscosity focil sodium melilite at 15 kbar is comparable to that measured by SCARFE (1981). Adding 2 wt% CO2 to sodium melilitc results in no detectable change in viscosity at 15 or 20 kbar outside the uncertainty of measurement of these low-viscosity liquids. DISCUSSION

Viscosity cfNaAISi308 liquid UI high pressurtOur values for the viscosity of NaAlS& liquid are srgnificantly lower than those of KIJSHIRO (1978) (see Fig. 1i. Our experiments were conducted in the presence of hematitemagnetite to maintain low /;(? within the sample capsule

Effect of CO2 on the viscosity of silicate liquids whereas those of KUSHIRO (1978) were unbuffered and conducted, generally, in graphite capsules. It is now well known that diffusion of Hz into nominally anhydrous experiments reduces the melting temperatures on solution of Hz in silicate liquids (LUTH and BOETTCHER, 1986; LUTH et al., 1987). This is likely to depolymerize the NaAlSi30B liquid by forming HZ0 and will reduce the viscosity (DINGWELL, 1987). However, our values for the viscosity of NaA1Si30s liquid are lower than those of KUSHIRO (1978). Our experimental configuration is also different from that employed by KUSHIRO (1978). We used 0.35-cm diameter Rt tubing as the sample capsule and Et spheres -0.0 15 cm in radius. The ratio (r/r=), which determines the magnitude of the Faxen correction (see Eq. (1) and Table 2) in our configuration, is approximately half of that of KUSHIRO (1978) and uncertainties in this value may contribute to this discrepancy. To reproduce Kushiro’s experimental configuration, we performed an experiment at 15 kbar and 14OO“C using a graphite capsule with a S-mm inside diameter. The result, shown in Table 2, is indistinguishable from our other results obtained using the double-capsule technique. Finally, it is conceivable that the temperature in our experiments is anomalously high. Based on the viscosity data for NaA1SiJ08 liquid of BREARLEYet al. (1986) at 1600°C and those of KUSHIRO (1978) at 14OO”C, our temperature would have to be approximately 1450°C rather than 14OO”C, to explain our lower viscosity measurements. To this end, we have calibrated the temperature profile along the graphite furnace and documented a vertical temperature gradient of 15°C at 1400°C within our experimental configuration. This cannot explain that we have unusually high temperatures in our experiments, and we conclude that our results for the viscosity of NaAISijOB liquid are reliable. Effect of CO, on the viscosity

offilly

polymerized liquids

Regardless of the difference between our results and those of KUSHIRO (1978) our data show conclusively that CO2 reduces the viscosity of NaAISiJOB liquid at high pressure. The decrease in viscosity may be rationalized by recently proposed solution mechanisms and structural arguments (e.g., FINEand STOLPER, 1985; BOETTCHERet al., 1987: STOLPER et al., 1987). COZ dissolves in NaAlSirO* liquid as molecular CO* and as the CO:- ion (MYSEN and VIRGO, 1980a; STOLPER et al., 1987).CO:- presumably requires charge-balancing by Na+, in the case of albite, to stabilize the CO:- as a dissolved species (MYSENand VIRGO, 1980a). These Na+ cations relinquish their network-stabilizing role for tetrahedrally coordinated A13+in order to form a carbonate complex (e.g., MYSEN and VIRGO, 1980a), resulting in a depolymerization of the liquid (albeit slight because of the small amount of dissolved CO:-) (STOLPER et al., 1987) and a concomitant reduction in viscosity. As pressure increases, our data suggest that (aq/aP), decreases, which, on the basis of the previous arguments, would favor a decrease in CO:- relative to molecular CO2. In other words, the liquid structure may be less disrupted by the solution of total CO* at higher pressures. This proposal is consistent with that of BOEIXHER et al. (1984) who maintain that the activity of SiOZ increases with increasing pressure in NaAlSi30s liquid, increasing the ratio

2613

of molecular COz to CO:- (FINE and STOLPER, 1985). Alternatively, it can be calculated from the data of FINE and STOLPER(1985) and STOLPERet al. (1987) that the ratio of molecular COz to CO:- is approximately independent of pressure at 1450°C for both COz-undersaturated and COzsaturated glasses close to NaAlSi308 in composition. This would suggest a regular decrease in the viscosity of NaA1Si30s liquid on addition of CO2 . Although this may be inconsistent with our data, in the absence of spectroscopic data documenting the speciation of CO1 in our vapor-undersaturated glasses and the errors associated with both the viscosity and COZ speciation data, we are reluctant to assign any significance to this discrepancy. By measuring the solubility and speciation of CO2 in fully polymerized silicate liquids, we can estimate the effect of CO* on viscosities at high pressure. Because CO2 dissolves in SiOZ liquid mostly as molecular CO2 (BOETTCHER,1984; STOLPERet al., 1987) reflecting the lack of available networkstabilizing cations with which CO1 can form carbonate complexes, we predict that it will have little effect on the viscosity of Si02 liquid at high pressure, assuming that molecular CO2 does not disrupt the silicate network to any degree. In the case of KAlSi30g liquid, BOETTCHERet al. (1987) proposed that a significant proportion of CO> is dissolved as CO:-, based on the large freezing-point depression when sanidine is saturated with COr. This is consistent with the results of WHITE and MONTANA(1988) which show that COZ reduces the viscosity of KAlSi308 liquid by a larger amount than for NaAlS130s liqutd. MYSEN and VIRGO (1980a) proposed that COz dissolves in CaAlzSilOs liquid entirely as CO:- by forming CaC03 complexes and expelling tetrahedrally coordinated A13+similar to that for NaA1Si30B liquid. MONTANAet al. ( 1988) also detected only CO:-, with no molecular CO*, in the infrared spectra of CaAl$i208 glasses quenched from liquids at 15 and 25 kbar. For CaAlzSiZOs liquid, only one Ca’+ is required to charge-balance two A13+cations in tetrahedral coordination, and complexing of one Ca*+ by a CO:- ion expels these A13+ cations from tetrahedral coordination. In this case, CaA12Si20s liquid must also be depolymerized, and adding CO2 probably decreases the viscosity. However, the viscosity of CaA12SizOe liquid is so low (KUSHIRO, 198 1) that any change when adding CO2 may be undetectable, as in the case of NaCaA1Si20, (discussed below). Comparison with other volatiles Recently, DINGWELLand MYSEN (1985) and DINGWELL (1987) examined the effects of Hz0 and F on the viscosity of NaA1Si30s liquid at high pressure. At first glance, it appears that Hz0 and F are more effective than CO2 in reducing the viscosity of NaAlSi308 liquid at high pressure. However, when considered on a mole-percent basis, COZ is almost as effective as HZ0 and more effective than F in reducing the viscosity of NaA1Si30s liquid, where one mole of NaA1Si30s is defined on the basis of eight oxygens (Fig. 3). At 15 kbar, CO* reduces the viscosity by a factor of 0.62/mol% CO*, compared to 0.94/mol% H20, and 0.30/mol% F. However, the solubility of COz in NaAlS&Og liquid at these P-T conditions is only 4.4 mol% (0.77 wt%; STOLPERet al., 1987) limiting the ca-

M. Brearlev and A. Montana

ing oxygens. They also proposed a stmtlar mechamsm ior CO2 in sodium melilite liquid. However, because Al”’ requtres charge-balancing in tetrahedral coordination f by Na * or C’a’in sodium melilite), the formation of CaCO,, which IS man stable than Na$O, (MYSEN and VIRGO, 1980b: DICKINSON. 1986; BOETTCHER et al., 1987). may result m expulsion or A13+from tetrahedral coordination. The net result of these: two competing structurally modifying processes may be that the viscosity remains unchanged. AI” is more likely to ix stabilized in tetrahedral coordination by Na’ rather than h> Ca2+ (BOTTINGA and WEILL, 1972; VIRGO et al.. i 980) how

Pressure

(kbarb

Flc. 3. Comparative plot of viscosity versus pressure at 1400°C to illustrate the effect of various dissolved volatiles on the viscosity of NaA1Si,08 liquid. Data for H20 and F are from DING~ELL ( 1987). Also shown are the data of KUSHIRO ( 1978) for volatile-free NaA1Si308. Relative effects of HrO. F, and COZ on the viscosity of NaAISi308 liquid are discussed in the text.

ever, and with the preferential formation of calcium carbonate complexes, some polymerization is favored. More work IS required to quantify the potentially dual role ofCO? in such depolymerized liquids.

.@plicatinn to the viscosity qf magmatrc iu&.\ Although it may be premature to predict the effect of vatatile components on the physical properties of magmas from

the measurement of synthetic liquid compositions, severa; conclusions can be drawn from available data. Although CO: pacity of CO,, to reduce the viscosity. In contrast, an extrapolation of experimental data (HAMILTON and OXTOBY, 1986: and see also MCMILLAN and HOLLOWAY, 1987) indicates that greater than 50 moI% (around 8 wt%) HZ0 may be dissolved in NaAlSisOs liquid at 1400°C and 15 kbar, which permits HZ0 to reduce the viscosity to a greater extent than CO* (Fig. 4). Because the viscosity-reducing capacities of these volatiles are similar on molar basis, one might expect CO2 and Hz0 (or F) to dissolve in fully polymerized silicate liquids in an analogous manner. However, it is well known that Hz0 reacts with the three-dimensional network of the liquid and shows a limited compositional dependence of solubility (e.g.. STOLPER, 1982; MYSEN, 1988; SILVER, 1988). The solubility of CO2 in silicate liquids is very dependent on composition (e.g., MYSEN, 1976; FINE and STOLPER. 1985) and especially on the activity of non-bridging oxygens, with which CO: preferentially bonds to form carbonate complexes (MYSEN. 1988). Thus, in fully polymerized silicate liquids. in which the activity of non-bridging oxygens is very low. the possibilities for CO* to disrupt the structure of the liquid are limited and its solubility and effect on the bulk viscosity are severely restricted.

may not be important in the generation of silicic magmas. except perhaps in partial melting of granulites (e.g., GRhi\;l 1986) it does decrease the viscosity of such highly polymerized liquids. In contrast, as discussed by DINGWELI and Mi SEN ( 1985) and DINGWELL (1987), Hz0 and Fare importam

in the evolution of granitic liquids. Both are highly soluble and extremely effective in reducing the viscosity of these 1~ uids, and together they have a profound influence on crystal,’ liquid equilibria, rheological properties, and compositiona@ modifying processes such as crystal fractionation, CO: is G principal volatile constituent of basic alkaline magmas (e.g.. EGGLER, 1978: WYLLIE, 1980. 1987), and the genesis of thcsc

The effect qf c’OI on the viscosit,v (!f‘depoi~.rnrrr=~,[l liquid.7 The absence of a detectable change in viscosity by adding CO2 to sodium melilite liquid may reflect that COZ is in molecular form and does not disrupt the silicate network. Raman spectra indicate that this is unlikely because there are bands that are characteristic of the CO:- ion (MYSEN and VIRGO. 1980b). Furthermore, it has been demonstrated that CO? dissolves predominantly as the CO:- ion in depolymerized liquid compositions (MYSEN, 1976; MYSEN and VIRGO. 1980b; FINE and STOLPER, 1986). A more interesting speculation concerns the solubility mechanisms of CO* in these depolymerized liquids. In diopside liquid, for example, MYSEN and VIRGO (1980b) postulated that CO2 polymerizes the liquid, as a result of formation ofalkali-metal-carbonate complexes and concomitant bridg-

FIG. 4. Solubility fraction (detined as ‘?ovolatile added divided hj total volatile solubility) versus reduction in viscosity illustrating thr greater effectiveness of Hz0 relative to CO2 in reducing the viscositv of NaA1Si30s liquid at 1400°C. Viscosity data for CO&earing NaAISiJOp liquid from this study. Two points for 15 and 25 kbar have been plotted. Solubility of CO2 from STOLPER et al. (19873 Viscosity of HIO-bearing NaAISipOs liquid from DINGWELLI1987! and solubility of HZ0 from the data compilation of MCMILLANand HOLLOWAY(1987).Solubility of Hz0 at I5 kbar is extrapolated from the data of HAMIITON and OXTORY I 1986)

2615

Effect of CO2 on the viscosity of silicate liquids

magmas will be influenced by the effect of CO2 on physical properties (SPERA and BERGMAN, 1980). Alkali basaltic magmas at depth contain dissolved COZ (BREYand GREEN, 1976), but we predict little effect of CO1 on the viscosity of these compositions. More depolymerized compositions that are charged with C02, such as kimberlites, may experience an increase in viscosity similar to that predicted for diopside liquid. However, carbonatite magmas are extremely fluid at atmospheric pressure (D. B. DINGWELL,unpubl. data), and it will be interesting to measure their viscosity at high pressure. One further application and test of the structural role of COZ in silicate liquids would be to measure the combined effect of CO2 and Hz0 on viscosities at high pressure. In addition, we need experimental data on the influence of COZ and HZ0 on the density of silicate liquids. Once this information is available, we can make progress toward modelling magmatic processes at depth within the Earth.

DINGWELLD. B. (1987) Melt viscosities in the system NaAISiJOsH20-F20-, . In Magmatic Processes: Physicochemical Principles (ed. B. 0. MYSEN), Geochem. Sot. Spec. Publ. 1, pp. 423-432. DINGWELLD. B. and MYSEN B. 0. (1985) Effects of fluorine and water on the viscosity of albite melt at high pressure: a preliminary investigation. Earrh Planet. Sci. Lett. 74, 266-274. EDWARDSJ. W., SPEISERR., and JOHNSONL. (195 1) High temperature structure and thermal expansion of some metals as determined by X-ray diffraction. 1. Platinum, tantalum, niobium and molybdenum. J. Appl. Phys. 22,424-428. EGGLI%RD. H. (1978) The effect of COr upon partial melting of neridotite in the svstem Na,O-CaO-Al*O?-Mao-SiO,-CO* to 35 kb with an analysis of melting in a pehdotit&HrO-CO, system.

Acknowledgments-This

HAMILTOND. L. and OXTOBYS. (1986) Solubility of water in albite melt determined by the weight-loss method. J. Geol. 94,626-630. HAZEN R. M. and SHARPEM. R. (1983) Radiographic determination of the position of platinum spheres in density-viscosity studies of silicate melts. Carnegie Inst. Wash. Yearb. 82,428-430. HERZBERGC. T. (1987) Magma density at high pressure Part 1: the effect of composition on the elastic properties of silicate liquids. In Magmatic Processes: Physicochemical Principles (ed. B. 0. MYSE~), Geochem. Sot. Spec. Publ. 1, pp. 25-46. KUSHIRO1. ( 1975) On the nature of silicate melt and its sianificance in magma’genesis: Regularities in the shift of the liquid& boundaries involving olivine, pyroxene. and silica minerals. Amer. J. Sri. 275,41 I-430. KUSHIRO I. (1976) Changes in viscosity and structure of melt of NaA1Si206 composition at high pressures. J. Geophys. Res. 81, 6347-6350. KUSHIRO I. (1978) Viscosity and structural change of albite (NaAISirOa) melt at high pressure. Earth Planet. Sci. Lett. 41,87-

research was supported by National Science Foundation Grants EAR87-05870 and EAR87-20289 to Montana. We thank Jim Dickinson of Coming Glass for the albite and sodium melilite glasses and David Moecher and Donald Peacor of the University of Michigan for the Swiss albite. Discussions with Brad White of UCLA were very helpful, and we thank him for comments, criticisms, and review ofthe manuscript. Bjom Mysen and an anonymous reviewer provided valuable input. In particular, Bjom’s penetrating analysis is appreciated.

Editorial handling: P. C. Hess

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