PII S0016-7037(99)00209-4 Chemical diffusivities of 18 trace elements in granitoid melts J. E. MUNGALL, 1,2,3, * D. B. DINGWELL, 1 and M. CHAUSSIDON 4 1 Bayerisches Geoinstitut, Universita ¨t Bayreuth, D-95440 Bayreuth, Germany 2 Ursa Minor, 4824 Avenue de l’Esplanade, Montre ´al, QC, Canada 3 Department of Geology, University of Toronto, 22 Russell Street, Toronto, ON M5S 3B1, Canada 4 CRPG-CNRS, BP 20, F-54501, Vandoeuvre, France (Received December 22, 1998; accepted in revised form June 14, 1999) Abstract—We have measured effective binary chemical diffusion coefficients for Be, B, Mg, Ca, Ti, Ge, Sr, Y, Zr, Nb, Cs, Ba, Nd, Tb, Lu, Hf, Ta, and W at concentrations below 1000 ppm in synthetic granitoid melts by performing infinite couple diffusion anneals. By imposing systematic variations in the viscosity of a granitic melt through changes in temperature and composition, we have created an internally consistent set of diffusivities that can be used to assess quantitatively the relationship between melt viscosity, melt composi- tion, and tracer diffusivities. One set of experiments were performed at 1 atm in an anhydrous haplogranitic melt near to the eutectic composition under air at temperatures of 1600, 1400, and 1137°C. A second set of experiments investigated the independent effects of the addition of approximately 3.7 wt.% H 2 O at 1600 and 1300C at 1.0 GPa pressure, and of 20 wt.% Na 2 O at 1 atm at 1200, 960, and 810°C. Within experimental error the measured diffusivities are Arrhenian, and show clear dependencies on ionic charge and radius, as well as on the viscosity of the melt. Monovalent, divalent, and trivalent cations show increasing activation energies with increasing field strength, whereas cations with higher charges have activation energies subequal to that of viscous flow. The Eyring equation is moderately successful in relating diffusivities of high field strength cations to melt viscosity, but underestimates the diffusivities of other cations by up to 4 orders of magnitude. The database of diffusivities reported in this contribution can serve to calibrate empirical models for the prediction of tracer diffusivities over a large range in melt composition, temperature, and viscosity. Copyright © 1999 Elsevier Science Ltd 1. INTRODUCTION The rates and mechanisms of diffusion of minor constituents of natural magmas are relevant to a number of problems in igne- ous petrology, and to the study of silicate melts in general. Chemical diffusion controls, for example, the partitioning of trace elements between melt and rapidly growing crystals (Ghi- orso, 1987); the efficiency of contamination of magmas by xenoliths and wall-rocks (Watson, 1982; Watson and Jurewicz, 1984); the rate of chemical and isotopic homogenization of comingled magmas (Lesher, 1994); and the partitioning of volatile elements between melt and vapor during rapid vapor exsolution (Sparks et al., 1994). In all of these processes differences in diffusivity among diverse elements, which may span several orders of magnitude, can result in a dispersion of the behavior of trace elements which might otherwise be ex- pected to act in similar ways. Comparisons of the rates of diffusion of different elements with ionic parameters such as radius and charge can lead to insights into the details of their solvation and transport mechanisms (Chakraborty, 1995; Mun- gall and Dingwell, 1997). It has long been recognized that chemical diffusivities are broadly dependent on melt viscosity (Glasstone et al., 1941), however in detail the relations between these two rate processes (i.e., viscous and diffusive relaxation) are complex (Hofmann, 1980; Dingwell, 1990). In a previous paper (Mungall and Dingwell, 1997) we re- ported that the diffusivities of U and Th at trace concentrations could be predicted within analytical error in a haplogranitic melt near the eutectic composition over wide ranges of tem- perature, pressure, and water content. In this paper we extend our coverage to experimental determinations of the diffusivities of 18 trace elements in the same dry haplogranitic melt near the eutectic composition. To further investigate the relation be- tween viscosity and diffusivity we adjusted the viscosity of the melt by adding H 2 O or Na 2 O, both of which drastically reduce the viscosity of granitic liquids. Diffusivities measured in sim- ilar melts with different viscosities can be fruitfully compared to assess the relationships between viscous and diffusive pro- cesses. Our results can be compared with previously deter- mined viscosity and diffusivity data determined in the same melt compositions (Hess et al., 1995; Hess and Dingwell, 1996; Mungall and Dingwell, 1997; Mungall et al., 1998). Additions of 3.7 wt.% H 2 O and 20 wt.% Na 2 O and variation of temper- ature between 1600 and 810°C allowed us to access a range of viscosities spanning 7 orders of magnitude. 2. THEORETICAL BACKGROUND Chemical diffusion in multicomponent melts can be de- scribed quantitatively in the context of irreversible thermody- namics. The basis of the irreversible thermodynamic approach can be found elsewhere (e.g., Kuiken, 1994), and we take the diffusion equation for a liquid having n components as our point of departure. If we assume that all fluxes and thermody- namic forces are oriented along a single direction in space, and *Address reprint requests to J. E. Mungall, Department of Geology, University of Toronto, 22 Russell Street, Toronto, Canada ON M5S 3B1. E mail address: mungall@zircon.geology.utoronto.ca Pergamon Geochimica et Cosmochimica Acta, Vol. 63, No. 17, pp. 2599 –2610, 1999 Copyright © 1999 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/99 $20.00 + .00 2599