ORIGINAL PAPER Antonio Acosta-Vigil David London George B. Morgan VI Contrasting interactions of sodium and potassium with H 2 O in haplogranitic liquids and glasses at 200 MPa from hydration–diffusion experiments Received: 2 August 2004 / Accepted: 8 December 2004 / Published online: 3 February 2005 Ó Springer-Verlag 2005 Abstract This study examines hydration–diffusion in the metaluminous haplogranite system at 200 MPa H 2 O and 800–300°C. At 800°C hydration is accom- panied by melting and uphill diffusion of sodium from anhydrous glass toward the region of hydration and melting, whereas potassium diffuses away from the hydration front and into anhydrous glass. Silicon and aluminum are simply diluted upon hydration. There is no change in molecular Al/(Na + K) throughout the entire hydration-diffusion aureole and, therefore, (1) there is no loss of alkalis to the vapor, and (2) K migrates to replace Na in order to maintain local charge balance required by IV Al. Alkali diffusion occurs over a viscosity contrast from 10 4.1 Pa s in hydrous liquid to 10 11.8 –10 13.5 Pa s in anhydrous glass. From these results, we interpret that: (1) Na is structurally or energetically favored over K as a charge-balancing cation for IV Al in hydrous granitic liquids, whereas the opposite behavior has been observed 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 at 800°C, but hydration at 300°C involves a loss of Na and concomitant increase in molar Al/(Na + K) in the hydration zone due to hydrogen-alkali exchange between fluid and glass. Hydration behavior at 400°C is transitional between those at 300°C and 600°C, suggesting that the change in hydration mechanism occurs near the glass transition. Introduction The component H 2 O exerts profound effects on phase relationships, viscosity, density, and diffusivities in sili- cate liquids (Tuttle and Bowen 1958; Lange 1994; Watson 1994). Though interactions of H 2 O with melt components in the haplogranite system (Ab–Or–Qtz) have been thoroughly investigated, the interpretation of results remains a matter of debate (Burnham 1975; Kohn et al. 1989; McMillan 1994; Kohn 2000; Schmidt et al. 2000; Zeng et al. 2000; Schmidt et al. 2001). In most models, the alkalis (Na and K) are treated as equivalent during the solvation of H 2 O in melt, though differences in water solubility between albite- and orthoclase-rich compositions have been noted (4–23% relative, Holtz et al. 1992a; Behrens 1995; Holtz et al. 1995; Romano et al. 1996). Diffusion experiments in silicate melts can provide valuable information about the interactions among chemical constituents because the concentration gradi- ents that reveal these interactions occur in melt at magmatic 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 structure and associations of some components in melt may change upon quenching to glass, and may vary with rate of quench (e.g. the speciation of H 2 O: Dingwell and Webb 1990; Nowak and Behrens 1995; Romano et al. 1995; Shen and Keppler 1995; Zavel’sky et al. 1998). In some instances, the information derived from diffusion experiments in melts has found verification in spectro- scopic studies of similar glasses. This is the case for the speciation of phosphorus in granitic melts, for which Wolf and London (1994) found evidence of an Al-P associated 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, USA E-mail: dlondon@ou.edu Tel.: +1-405-3253253 Fax: +1-405-3253140 Present address: A. Acosta-Vigil Dipartimento 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: 276–287 DOI 10.1007/s00410-004-0648-1