Trace element diffusion in andesitic melts: An application of synchrotron X-ray fluorescence analysis

We have investigated the diffusivity of trace elements in hydrous iron-free andesitic melts containing 4.5 to 5.2 wt.% water at a pressure of 500 MPa and at temperatures between 1100 and 1400 degreesC using the diffusion coup le technique. The studied elements can be combined in several groups of par ticular geochemical interest: low field strength elements (LFSE: Rb, Sr, Ba ), transition elements (Cr. Fe, Ni, Zn), rare earth elements (REE: La, Nd, Sm, Eu, Gd, Er, Yb, Y), and high field strength elements (HFSE: Zr, Nb, Hf) . The diffusion profiles of the trace elements were measured using the sync hrotron X-ray fluorescence (SYXRF) microprobe. H2O distribution in the samp les was analyzed by IR microspectroscopy. Diffusion profiles are excellently reproduced, assuming concentration-indep endent diffusion coefficients. For all trace elements, the temperature depe ndence of diffusion in the hydrous melt can be described by a simple Arrhen ius law. In general, the diffusivity decreases from the LFSE, to transition elements, to REE, and to the HFSE, a trend that can be correlated to the i ncrease of charge in the same order. The activation energy shows a similar trend, increasing from 129 kJ/mol for Rb to 189 kJ/mol for Zr. For the tran sition elements Cr and Fe, the activation energy is relatively high (228 an d 193 kJ/mol, respectively), which can be explained by increasing contribut ions of divalent cations to the diffusion flux with increasing temperature. Higher diffusivity of Eu compared to its neighbor elements also is attribu ted to contributions of divalent cations. Modeling Eu-diffusivity using dat a of Sr as representative fur Eu2+, and of Sm and Gd as representative for Eu3+ shows that at all temperatures Eu3+ is clearly dominating in the hydro us melt. To quantify the effect of water, an additional experiment was perf ormed at 1400 degreesC using a nominally anhydrous melt. The obtained diffu sion coefficients are (for most of the elements) by one and a half orders o f magnitude lower than for a melt containing 4.5 wt.% H2O. Chemical diffusion coefficients D-eta, which were calculated from the visco sity data of Richet et al. (1996) using the Eyring equation, and which assu med a jump distance of 3 Angstrom, are in excellent agreement with the diff usivity of the HFSE for both dry and hydrous melt. Most of the investigated elements show a linear relation between log diffusivity and log viscosity. enabling the prediction of diffusivities in hydrous andesite systems at va rious conditions. Provided viscosity data are available, we suggest that th is relation can be a useful tool to estimate trace element diffusivities fo r silicate melts with different compositions. The new diffusion data show that water strongly enhances diffusivity of tra ct: elements in andesitic melts. After 10,000 yr at 1300 degreesC, diffusio n produces in the dry melt relatively short profiles with lengths (defined as x = (Dt)(1/2)) between 0.8 and 0.07 m (for Sr and Zr, respectively), whe reas in hydrous melts (5 wt.% water), profiles are much longer with lengths between 3.9 and 0.92 m (for Sr and Zr, respectively). Copyright (C) 2001 E lsevier Science Ltd.