Acosta Et Al_H2O Diffusion

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ORIGINAL PAPER

Antonio Acosta-Vigil David LondonGeorge B. Morgan VI

Contrasting interactions of sodium and potassium with H 2 Oin haplogranitic liquids and glasses at 200 MPa from hydrationdiffusion experiments

Received: 2 August 2004 / Accepted: 8 December 2004 / Published online: 3 February 2005 Springer-Verlag 2005

Abstract This study examines hydrationdiffusion inthe metaluminous haplogranite system at 200 MPaH 2O and 800300 C. At 800 C hydration is accom-panied by melting and uphill diffusion of sodium from

anhydrous glass toward the region of hydration andmelting, whereas potassium diffuses away from thehydration front and into anhydrous glass. Silicon andaluminum are simply diluted upon hydration. There isno change in molecular Al/(Na + K) throughout theentire hydration-diffusion aureole and, therefore, (1)there is no loss of alkalis to the vapor, and (2) Kmigrates to replace Na in order to maintain localcharge balance required by IV Al. Alkali diffusionoccurs over a viscosity contrast from 10 4.1 Pa s inhydrous liquid to % 1011.8 1013.5 Pa s in anhydrousglass. From these results, we interpret that: (1) Na isstructurally or energetically favored over K as a

charge-balancing cation forIV

Al in hydrous graniticliquids, whereas the opposite behavior has beenobserved for anhydrous melts, and (2) the diffusion of alkalis through silicate melts is largely independent of viscosity. Results from 600 C are similar to those at800 C, but hydration at 300 C involves a loss of Naand concomitant increase in molar Al/(Na + K) inthe hydration zone due to hydrogen-alkali exchange

between uid and glass. Hydration behavior at 400 Cis transitional between those at 300 C and 600 C,suggesting that the change in hydration mechanismoccurs near the glass transition.

Introduction

The component H 2O exerts profound effects on phaserelationships, viscosity, density, and diffusivities in sili-cate liquids (Tuttle and Bowen 1958 ; Lange 1994 ;Watson 1994 ). Though interactions of H 2O with meltcomponents in the haplogranite system (AbOrQtz)have been thoroughly investigated, the interpretation of results remains a matter of debate (Burnham 1975;Kohn et al. 1989 ; McMillan 1994; Kohn 2000; Schmidtet al. 2000; Zeng et al. 2000; Schmidt et al. 2001). Inmost models, the alkalis (Na and K) are treated asequivalent during the solvation of H 2O in melt, thoughdifferences in water solubility between albite- andorthoclase-rich compositions have been noted (423%relative, Holtz et al. 1992a ; Behrens 1995 ; Holtz et al.1995 ; Romano et al. 1996).

Diffusion experiments in silicate melts can providevaluable information about the interactions amongchemical constituents because the concentration gradi-ents that reveal these interactions occur in melt atmagmatic temperatures (e.g. Wolf and London 1994 ;Acosta-Vigil et al. 2002). This is in contrast to spectro-scopic measurements on glasses, for which the structureand associations of some components in melt maychange upon quenching to glass, and may vary with rateof quench (e.g. the speciation of H 2O: Dingwell andWebb 1990; Nowak and Behrens 1995 ; Romano et al.1995 ; Shen and Keppler 1995; Zavelsky et al. 1998). Insome instances, the information derived from diffusionexperiments in melts has found verication in spectro-scopic studies of similar glasses. This is the case for thespeciation of phosphorus in granitic melts, for whichWolf and London ( 1994) found evidence of an Al-Passociated species that was also indicated by spectro-

Editorial Responsibility: T.L. Grove

A. Acosta-Vigil D. London ( & ) G. B. Morgan VI

School of Geology and Geophysics,University of Oklahoma,100 East Boyd Street Room 810 SEC,Norman, OK 73019, USAE-mail: [email protected].: +1-405-3253253Fax: +1-405-3253140

Present address : A. Acosta-VigilDipartimento di Mineralogia e Petrologia,Consiglio Nazionale delle Ricerche,Instituto di Geoscienze e Georisorse,Universita ` di Padova, Corso Garibaldi 37,35137, Padova, Italy

Contrib Mineral Petrol (2005) 149: 276287DOI 10.1007/s00410-004-0648-1


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scopic work (Mysen et al. 1981 , 1999 ; Gan and Hess1992 ; Toplis and Schaller 1998 ; Cody et al. 2001; Mysenand Cody 2001 ).

This study concentrates on hydration and H 2O dif-fusion in cylinders of glass with a composition corre-sponding to the metaluminous haplogranite minimum at200 MPa H 2O. Most experiments were conducted at800 C with the goal of investigating the interaction be-tween H 2O and other components in granitic melt thathelp to delineate the systematics of diffusion by whichthe chemical potential gradients in H 2O are erased.Hydration-diffusion experiments at 300, 400, and 600 Cwere conducted with the aim of examining the hydrationreaction as a function of temperature to elucidate dif-ferences in hydration and diffusion mechanisms betweenglass and melt.

Materials and methods

Starting materials and experimental methods

The starting material (Table 1) is a synthetic anhydroushaplogranite glass (Corning Lab Services, New York,USA) with the eutectic composition (Ab 38.20 Or 27.94Qtz 33.60 Crn 0.25 ) (mineral symbols after Kretz 1983) of the haplogranite system at 200 MPa H 2O (Tuttle andBowen 1958). Glass cylinders measuring 3.5 mm indiameter and 46 mm in length were loaded into goldcapsules (I.D. of $ 3.6 mm) with enough deionized-ul-tra ltered water ( $ 10 wt%) to permit saturation of

the entire silicate mass. Capsules were sealed by DCargon plasma arc welding while keeping the capsulefrozen to prevent volatilization of added water, placedovernight in an oven at $ 130 C and then re-weighed toensure no leakage. Experiments were conducted insubhorizontal NIMONIC 105 cold-seal pressure vesselswith water as the pressure medium. Two sets of experiments were performed (Table 2): one in whichglasses were taken to a target temperature and pressureof 800 C and 200 MPa for difference lengths of time, and a second set in which glasses were taken totemperatures of 300, 400, and 600 C at the samepressure (200 MPa) for a single experimental time.Variations in the target temperature and pressure dur-ing the experiments were % 3 C and 1 MPa,respectively. Temperature was monitored with aninternal Chromel-Alumel thermocouple, and pressurewas monitored with a factory-calibrated Heise bourdontube gauge; uncertainties in temperature and pressureare


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and London 1996 ). H 2O concentrations in glass werecalculated by difference of the electron microprobeanalysis totals from 100%. With the current analyticalmethods, accuracy for H 2O by difference is generallybetter than 10% relative for perfectly at and well-polished samples with H 2O concentrations in the range210 wt% (Morgan and London 1996 ; Acosta-Vigilet al. 2003 ). The starting anhydrous glass often con-tains tiny (0.55 l m) air-lled vesicles that tend todisappear as hydration and melting proceed. Hence,small surface imperfections (pits) arising from thesevesicles (that may be hidden by the carbon coat)account for most of statistical variation in nearly tocompletely anhydrous glasses (H 2O 1.0 wt%) inthe present experiments. The maximum uncertainty forthe reported ASI values is 0.035 (ASI = molarAl2O3/[CaO + Na 2O + K 2O]), calculated by thepropagation of errors. To ensure (1) statistical accu-racy at critical points in the diffusion prole, and (2)that diffusion perpendicular to the cylinder axis didnot interfere with diffusion parallel to the cylinderaxis, we conducted several analytical transversesthroughout the experimental glasses: two transversesnear the center and parallel to the cylinder axis, andone transverse at the middle and perpendicular to thecylinder axis.

Results

800 C and variable experimental time

Melting of the glass cores accompanied the inward dif-fusion of H 2O. Concomitantly, Na diffused uphill fromanhydrous glass into and toward the hydrated liquid,whereas K diffused uphill away from the H 2O diffusionfront and into the anhydrous portion of the charges(Figs. 1, 2, 3). These results demonstrate that Na and Kwere coupled with H 2O during diffusion in the sense thattheir migration occurred in response to the inux of H 2O. Note that in the 3-h experiment, for instance, thediffusive exchange of Na and K occurred over 1.5 mm inthe silicate cylinder, through the hydration front

(% 1.0 mm) and % 0.5 mm into the anhydrous glass(Figs. 1, 2).

Throughout the entire charges at all experimentaldurations, the molar ratios Al/[Na + K] (ASI) and Al/Si remained constant and equal to those in the startingglass (Fig. 1). When normalized to an anhydrous basis(Fig. 2), concentration proles for silica and alumina areat and similar to those in the starting glass withinanalytical uncertainty. In most of the central anhydrousregion of the charges, the proles for sodium andpotassium remain at and equal to the starting valuesbut develop a minimum in Na and a maximum in K wellabove analytical uncertainties within 250500 micronsof the hydration front(s), and then monotonicallyincrease (Na) and decrease (K) from the hydrationfront(s) toward the hydrated edges of the glass cylinder(Fig. 2). These observations indicate that SiO 2 andAl2O3 were simply diluted during the incorporation andmigration of H 2O, that there is no loss of alkalis to thevapor, and that the molar uxes of sodium and potas-sium were equivalent but opposite in direction tomaintain the local charge balance required by IV Al.Based on the immobility of SiO 2 during the diffusion of H 2O, we calculated the molar decrease in K ( D K,equivalent to moles of Na gained) in the region of hy-drated glass, and observed that the molar ratio D H/ D Kis approximately constant and equal to $ 100 to 200.These observations suggest that H 2O diffuses along adirection in the oxide composition space in which mol.H/Na $ 100200 and mol. Na= K.

Between 12 h and 24 h the hydration fronts meet atthe center of the charges (Figs. 1, 2), and after 24 h the

direction of alkali migration reverses. Sodium ionsspatially associated with water (in whatever form) nearthe edges of the core return toward the core interior asthe H 2O concentration gradientthe driving force forthe segregation of Na and Kdecreases with the pro-gressive hydration of the melt (Figs. 1, 2). Concomitantwith the hydration of the granitic liquid to a uniformsaturation value of $ 6.6 wt% H 2O at 200 MPa (Ta-ble 1), alkali concentration gradients erase in about48 h and do not change in experiments of up to 120 hduration.

Table 2 Starting materials and durations of the conducted experiments

Run no. Starting materials Temperature( C)

H 2O a

(wt%)Duration(min)

Acasi 276 H 2O + Metaluminous haplogranite glass core 300 10.54 28,720Acasi 283 H 2O + Metaluminous haplogranite glass core 400 17.71 1,375Acasi 284 H 2O + Metaluminous haplogranite glass core 600 17.58 1,388Acasi 270 H 2O + Metaluminous haplogranite glass core 800 10.11 0Acasi 271 H 2O + Metaluminous haplogranite glass core 800 10.64 211Acasi 272 H 2O + Metaluminous haplogranite glass core 800 9.94 368Acasi 274 H 2O + Metaluminous haplogranite glass core 800 10.47 540Acasi 275 H 2O + Metaluminous haplogranite glass core 800 10.80 740Acasi 277 H 2O + Metaluminous haplogranite glass core 800 11.62 1,454Acasi 279 H 2O + Metaluminous haplogranite glass core 800 10.62 7,268

All experiments conducted at 200 MPaa Proportion of water sealed inside the capsule with respect to the haplogranite glass

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300600 C and single experimental time

As a comparison to hydration-melting at 800 C, weconducted three more hydration experiments at 300,400, and 600 C (Table 2). For a granitic melt of eutectic composition containing 6 wt% H 2O, the glasstransition temperature is calculated at $ 375 C based

on the viscosity of silicate melt at the glass transitionfrom the data of Dingwell ( 1998 ). These three experi-ments, therefore, span this transition temperature. Theresults (Figs. 4, 5) at 600 C are similar to those at800 C: Na diffuses from the anhydrous glass towardthe region of hydration, K migrates into the anhydrousglass in the opposite direction, and there is no nettransfer of alkalis to the aqueous vapor (ASI remainsconstant). The results at 400 C, close to the calculatedglass transition temperature for this composition, beginto show a change in the alkali migration patterns. The

ASI remains essentially constant through the charge,so there is no noticeable loss of alkalis to vapor. TheNa/K ratio increases in the direction of increasinghydration, as in the experiments at higher temperature,but alkali migration across the hydration front, whereH 2O content goes to zero, is not so evident, and theelement proles mostly reect dilution of the glasscomponents by H 2O. The result at 300 C reveals achange in the behavior of alkalis upon hydration: thereis a slight loss of Na (Na/K falls) across the hydrationaureole, and there is a corresponding increase in ASI.Thus, at 300 C, hydration of the glass is accompaniedby loss of Na to the vapor phase. Potassium, however,is enriched at the interface of the hydration front inthe glass, as in all other experiments at higher tem-peratures.

Discussion

Mechanism of hydration: liquid versus glass

An experimental H 2O diffusion study by Zhang et al.(1991 ) reported no alkali concentration gradients, buttheir methodology entailed diffusion of H 2O out of hy-drous glasses at 1 atm pressure, which imposed sub-stantially different conditions than those employed inthis study. Perhaps the most important difference is the

Fig. 1 Composition (oxide concentrations and molar ratios) of the800C experimental glasses along microprobe transverses parallelto the cylinder axis, as a function of experimental time and distanceto the H 2O-core interface. Each prole represents the mean valuesof two transverses near the center of the cylinder axis. H 2O prolescalculated by difference of totals from 100%. In this and followinggures, dashed vertical lines mark the position of the H 2O diffusionfront, and error bars represent maximum analytical uncertainties of the analyses (analytical uncertainties are equal or smaller than thesymbols when not shown)

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H 2O concentration. Their glasses contained 1.8 wt%H 2O, most of them


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equilibrated with alkali feldspars from 450 C to 350 C(Lagache and Weisbrod 1977 ).

Interactions between alkalis and other oxidecomponents in hydrous granitic melt

The interaction of H 2O with silicate melt componentshas a long history of inquiry and contrasting interpre-tations. In a quasi-crystalline albite melt model, Burn-ham ( 1975 ) proposed a solvation mechanism in whichthe rst mole of dissolved H 2O dissociated to form twoOH groups, one of which served to break a bridgingoxygen bond between Si-O-Si, and another that dis-placed Na from its charge-balancing position on IV Al to

an interstitial site. From spectroscopic measurements,Kohn et al. ( 1989 , 1998 ; Kohn 2000 ) observed a stronginteraction between Na and OH or H 2O, but they foundno evidence for disruption of AlOSi or SiOSi bonds(i.e. no AlOH or SiOH formed from bridging oxygenatoms). Ab initio models, however, led Sykes andKubicki ( 1993; Kubicki and Sykes 2004) to concludethat TOT bond-breaking was probable. Other nuan-ces of hydration models lie between these two extremes(e.g. see review by McMillan 1994; Schmidt et al. 2000 ),but the crux of the differences concerns the extent towhich solvation of H 2O breaks bridging oxygen linkagesin the TOT structure of melt. The potential signi-cance of different charge-balancing cations, Na + or K + ,which appear to be involved in the hydration process,has not been examined in the context of these meltstructural studies, and only Schmidt et al. ( 2001 ) havepresented recently some data (see below).

The current experiments demonstrate that the smallerNa + (effective ionic radius of 1.16 A in CN=6: Shan-

Fig. 3 Composition of the 800 C experimental glasses alongmicroprobe transverses perpendicular to the cylinder axis nearthe middle of the cylinder, as a function of experimental time anddistance to the interface

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non 1976) is preferred over K + (effective ionic radius of 1.72 A in CN=9: Shannon 1976) in the environmentcreated by the hydration melting of metaluminous gra-nitic glass (NBO/T=0: Mysen 1988). In anhydrousaluminosilicate liquids, M + OAl bonds become morestable with decreasing charge density among Group IAcations (Shannon 1976 ; Roy and Navrotsky 1984 ). Thisbehavior stems from the inability of smaller cations toeffectively shield highly charged T cations like Si (e.g.Pauling 1960), and from the tendency of network-mod-ifying cations with higher eld strength to destabilizemelt or glass structures by drawing electron densityaway from T-O bonds (e.g. Shannon 1976; Roy andNavrotsky 1984 ). For instance, in a study of partitioningbetween immiscible anhydrous melts (Watson 1976 ),only Cs + , the Group IA cation of lowest eld strength,partitioned into the higher-silica melt. In our experi-ments, dilution of silica accompanies hydration-meltingof the glass. It might be construed, therefore, that the

migration of K into the anhydrous portions of the high-temperature glasses reported here reects these tenden-cies in alkali ion stabilization, and that the entire patternof diffusion behavior observed here is driven by themovement of K rather than of Na. However, two linesof evidence indicate otherwise. (1) A parallelism can beestablish between these and previous diffusion experi-ments in which chemical gradients in Al in hydroushaplogranite melts induced the molar Na/Al ratio toadjust quickly to a constant value everywhere in the meltthrough the diffusion of Na, whereas K diffused con-comitantly in such a way as to maintain or bring the ASIof melt to an equilibrium value (Acosta-Vigil et al 2002 ,2004 , and unpublished work in review). This constancyof the Na/Al ratio demonstrates a special structuralassociation of Na and Al in hydrous alkali aluminosili-cate melts. (2) As in the experiments reported here, Naalso migrates toward higher concentrations of B, P, andF where gradients in these ligands exist in hydrous

Fig. 4 Oxide concentrationsand molar ratios in the 300,400, and 600 C experimentalglasses along microprobetransverses parallel to thecylinder axis, as a function of distance to the H 2O-coreinterface. Each prolerepresents the mean values of two transverses near the centerof the cylinder axis

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haplogranite melts, and liquid lines of descent producedby crystallization culminate in melts highly enriched inNa (over K) and all the above ligands (London 1999 ,2004a , b; London et al. 2004 ). The liquid line of descentin uxed granitic melts clearly demonstrates that theseliquids are stabilized by Na, whereas K becomes insol-uble and precipitates as alkali feldspar and mica (Lon-

don et al. 1989 ; 2004 ). Furthermore, the fractionation of Na and K observed here is consistent with the rela-tionships found in bulk equilibrium studies on graniticcompositions, in which the solubility of H 2O is greater inNa-rich (albitic) than in K-rich (orthoclase-rich) liquids(Holtz et al. 1992a ; Behrens 1995 ; Holtz et al. 1995 ;Romano et al. 1996). Together with other diffusion andbulk equilibrium studies (Dingwell et al. 1984 ; Holtzet al. 1992b ; Holtz et al. 1995 ; Linnen et al. 1996 ;Romano et al. 1996 ; Mungall et al. 1998 ; Acosta-Vigilet al. 2002 , 2003 , and unpublished work in review), these

results demonstrate a greater compatibility and inter-action of H 2O with Na 2O and Al 2O3 than with K 2Oand SiO 2 in granitic liquids, although the underlyingcause of these interactions remains uncertain. Regard-ing interaction between H 2O and alkalis, NMR spec-troscopy on albitic, orthoclase-rich, and haplograniticglasses shows a progressive variation in the 23 Na

spectra with increasing water concentration (Kohnet al. 1989, 1998 ; Zeng et al. 2000 ; Schmidt et al. 2000 ,2001). Kohn et al. ( 1989 , 1998 ) interpreted the changesin the 23 Na spectra as being due to the presence of more hydrated Na complexes, whereas Schmidt et al.(2001 ) suggested that the changes are consistent withthe formation of a hydration shell around Na. Zenget al. ( 2000) argued that the variations do not consti-tute clear evidence for the presence of hydrated Nacomplexes. None of these studies report any NMRspectra for potassium, although Schmidt et al. ( 2001 )

Fig. 5 Composition of the 300,400, and 600 C experimentalglasses along microprobetransverses parallel to thecylinder axis, normalized to ananhydrous basis. H 2O proleadded for reference

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found that spectra for 1H have no measurable depen-dence on composition (from albitic to orthoclase-richto haplogranitic glasses).

We suggest that the preference of IV Al for Na overK in hydrous granitic liquids expresses a steric effectthat maximizes distances of separation, and henceminimizes repulsion, between neighboring M + (H, Na,or K) cations by the incorporation of the smaller of the alkali ions when protons are added to the melt.This should apply whether or not H + displaces Na +or K + from Al, though we cannot evaluate this sub-stitution with our current experiments. Acosta-Vigilet al. ( 2002 ), however, proposed that H + contributesto the charge balance on excess Al in hydrous pera-luminous granitic melts; this would allow Al to remainfour-coordinate (T) without the need to invoke tetra-hedral clustering (Lacy 1963) or a coordination changeto VI Al, for which there is little supporting evidencefrom spectroscopic measurements or numerical simu-lations (e.g. Stebbins et al. 2001; Kubicki and Toplis2002 ). This is based on results from diffusion and bulkequilibrium studies (Dingwell et al. 1984 , 1997 ; Holtzet al. 1992b ; Linnen et al. 1996 ; Behrens and Jantos2001 ; Acosta-Vigil et al. 2002 , 2003 ) which stronglysuggest an interaction between excess aluminum andH 2O in melt.

In any case, the constancy of ASI across the entireglass prole in our diffusion experiments above 400 Creveals that if H displaces Na or K from coordinationwith Al upon glass/melt hydration, then the displacedalkali ion is conserved locally, i.e. in the immediatevicinity of the hydration reaction. If the dissolution of H 2O involves the creation of a hydration shell of molecular water around alkali cations, as proposed bySchmidt et al. ( 2001), then the volumetric constraints onthat hydration sphere may again favor the smaller alkalication.

Effective binary diffusion coefficient for total H 2O

The calculated concentration proles for H 2O in ha-plogranitic glass are controlled by diffusion in theglass/liquid rather than by convection in the liquid.This is indicated by three lines of evidence. (1) Theadvance of compositional proles into glass is pro-portional to the square root of time (Fig. 6); i.e. theconcentration proles overlap when plotted againstdistance normalized to the square root of time (seeCrank 1975 ). (2) The concentration proles obtainedare regular and similar inward from both the ends andsides of the glass cylinders (compare Figs. 1, 2, 3 withShaw 2000 ). (3) The high viscosity of the glass/liquid(see below) precludes convection on the time scale of these experiments. We have used the data from the 3-hto 12-h experiments to calculate an effective binarydiffusion coefficient (EBDC, Cooper 1968 ) by theBoltzmann-Matano method (e.g. Crank 1975 ) for totalH 2O without differentiating between OH groups and

molecular H 2O (e.g. Stolper 1982). Corrections for theeffects of the run-up step on the diffusion proles(isobaric heating from 25 C to 800 C at 200 MPa in$ 27 min, 0-h run, Fig. 1) followed procedure (2) of Zhang and Behrens ( 2000). These methods yield EBDC$ 1.7 10

11 to 2.6 1011 m 2 s

1 for melts with 4 wt%H 2O, and EBDC $ 3.0 10

11 to 4.8 1011 m2 s

1 formelts with 5 wt% H 2O. These EBDC compare wellwith previously determined H 2O diffusivities for granitemelts under similar P T X conditions: 1 10

10.4 to1 10

10.7 m2 s1 (Nowak and Behrens 1997) and

1.2 1011 to 1.9 10

11 m2 s1 (Zhang and Behrens

2000).

Fig. 6 Al 2O3 , K 2O and H 2O concentrations versus distance to theH 2O-core interface normalized to the square root of experimentaltime, in glasses of the 3-, 6-, 12-, and 24-h hydration experiments at800C

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Alkalis and diffusion in the H 2O-saturatedhaplogranite system

Together with recent diffusion studies, this paper showsthat alkalis are important for the diffusion of other meltcomponents in the H 2O-saturated haplogranite systemat 800 C and 200 MPa. With the exception of silica,alkali migration is clearly related to the movement of other oxide components, and sodium and potassiumdiffuse at least as rapidly as the fastest-moving compo-nent in the system. In experiments involving the disso-lution of corundum (Acosta-Vigil et al. 2002 ), thecreation and diffusion of a peraluminous component(diffusivity of $ 1 10

14 m2 s1) controlled the rates of

alkali migration through the melt. In the present study,H 2O is the rate-limiting component with a diffusivity$ 2 10

11 m2 s1, and hence the diffusion of the alkalis

occurs at this same rate. This value is essentially iden-tical to the diffusivities of the alkalis when their migra-tion is driven by their own chemical potential gradientsin otherwise homogeneous hydrous melts($ 1 10

11 m 2 s1 : Freda and Baker 1998).

Diffusion of alkalis and viscosity

The migration of Na and K along chemical potentialgradients in these experiments is largely independent of liquid viscosity. In the 3-h experiment at 800 C, theexchange of Na and K via diffusion extended up to$ 1.5 mm into domains of the silicate core that remainedanhydrous (Figs. 1, 2). The calculated viscosity (Schulzeet al. 1995 ) of this nominally anhydrous composition(wt% H 2O varied from 0.01 to 0.001) at 800 C is 10

11.8

to 10 13.5 Pa s, which spans the structural transition of liquid to glass (Dingwell 1998). The alkali diffusionproles are continuous up to the outer edge of thehydration aureole, where the liquid is saturated in H 2Ofor this P and T , and for which the calculated viscosity(Schulze et al. 1995) is 104.1 Pa s. Mungall ( 2002) simi-larly concluded that the diffusion of alkalis was largelyindependent of melt viscosity (at a specied tempera-ture), though his interpretation arose from multiple datasets rather than single hydration-diffusion experimentsof the type employed in this study. Moreover, the samemechanisms and directions in the diffusion of alkalisoperate to comparatively low temperatures, at least totemperatures near the glass transition, and consequently

across much higher gradients in viscosity.

Concluding remarks

Diffusion studies such as the present work represent animportant tool for examining the interactions of com-ponents in silicate liquids. A basic difference betweendiffusion studies versus bulk equilibrium and spectro-scopic studies is the degree of chemical homogeneity inthe glass/melt products. Spectroscopic and bulk equi-

librium studies deal with chemically homogeneous glass/melt at any given experimental time. Diffusion studies,on the other hand, produce melts with noticeable con-centration gradients at a given time and, therefore, havethe potential to magnify the effects of the interactionsamong components. Such behavior helps to explain thelarge fractionation in the Na/K molar ratio in melt dueto varying H 2O concentrations (45% relative, this study)as compared to the moderate increase in H 2O concen-tration arising from changes in the Na/K ratio in meltsat equilibrium (423% relative, Holtz et al. 1992a , 1995 ;Behrens 1995; Romano et al. 1996). This indicates theutility of diffusion studies in deciphering the interactionsamong melt components that may be extensively erasedonce the melts achieve chemical homogeneity or arequenched to glass.

The current hydration-diffusion experiments extendour recent series of diffusion studies (Acosta-Vigil et al.2002 , and unpublished work in review) that haverevealed differences in behavior between Na and K inhydrous granitic liquids, but consistencies of the samealkali ions among differing experimental programs. BothNa and K diffuse in response to the creation of Al and Hchemical gradients in hydrous haplogranite melts.Sodium diffuses to maintain a constant Al/Na molarratio throughout the entire melt reservoir when gradi-ents in Al are present, or toward H-rich environmentswhen gradients in H are present. Potassium diffuses inthe direction needed to bring the ASI of melt to theequilibrium value and to maintain local charge balanceon IV Al (Acosta-Vigil et al. 2002 ; unpublished work inreview; and this study). The diffusion of alkalis canoperate over the dimensions of the entire melt volume(i.e. the diffusion caused by chemical potential gradientsis independent of distance, Acosta-Vigil et al. 2002 ;London et al. 2004 ; Acosta-Vigil et al. unpublished workin review) and across very large viscosity gradients (thisstudy). The diffusion of Na and K along chemical po-tential gradients induced by the addition of other com-ponents (e.g. excess Al, H 2O) or by removal of Na or Kvia the crystallization of feldspars represents anotherviable mechanism for the fractionation of Na and K ingranitic systems (London 2004c; London et al. 2004 ).

Acknowledgements Support for this research was provided by Na-tional Science Foundation grants EAR-990165 and EAR-0124179.The Electron Microprobe Laboratory at the University of Okla-homa was created with US DOE grant DE-FG22-87FE1146 andupgraded with NSF grant EAR-9404658 and support from the

University of Oklahoma Office of Research Administration.Comments on the manuscript by James Kubicki and thoughtfulreviews by Simon Kohn and Yang Liu improved the paper and arevery much appreciated.

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