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

Abstract

—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°C using the diffusion couple technique. The studied elements can be combined in several groups of particular 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 synchrotron X-ray fluorescence (SYXRF) microprobe. H 2O distribution in the samples was analyzed by IR microspectroscopy. Diffusion profiles are excellently reproduced, assuming concentration-independent diffusion coefficients. For all trace elements, the temperature dependence of diffusion in the hydrous melt can be described by a simple Arrhenius 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 increase 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 transition elements Cr and Fe, the activation energy is relatively high (228 and 193 kJ/mol, respectively), which can be explained by increasing contributions of divalent cations to the diffusion flux with increasing temperature. Higher diffusivity of Eu compared to its neighbor elements also is attributed to contributions of divalent cations. Modeling Eu-diffusivity using data of Sr as representative for Eu 2+, and of Sm and Gd as representative for Eu 3+, shows that at all temperatures Eu 3+ is clearly dominating in the hydrous melt. To quantify the effect of water, an additional experiment was performed at 1400°C using a nominally anhydrous melt. The obtained diffusion coefficients are (for most of the elements) by one and a half orders of magnitude lower than for a melt containing 4.5 wt.% H 2O. Chemical diffusion coefficients D η, which were calculated from the viscosity data of Richet et al. (1996) using the Eyring equation, and which assumed a jump distance of 3 Å, are in excellent agreement with the diffusivity 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 various conditions. Provided viscosity data are available, we suggest that this relation can be a useful tool to estimate trace element diffusivities for silicate melts with different compositions. The new diffusion data show that water strongly enhances diffusivity of trace elements in andesitic melts. After 10,000 yr at 1200°C, diffusion 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), whereas in hydrous melts (5 wt.% water), profiles are much longer with lengths between 3.9 and 0.92 m (for Sr and Zr, respectively).