Diffusive fractionation of trace elements and their isotopes in silicate melts

Abstract

PART I and II: To assess the influence of diffusion on the trace element composition of magmatic systems, we determined the mobilities of 21+ trace elements (including transition metals, high field strength elements, low field strength elements and rare earth elements (REE)) in basaltic (part I) and hydrous rhyolitic (part II) melts. High temperature, high pressure experiments were executed in a piston-cylinder apparatus using simple diffusion couple assemblies. The resulting trace element concentration gradients developed in all experimental materials were simultaneously characterized using laser ablation ICP-MS. Diffusivities of the lanthanides in basaltic melt decrease monotonically from lanthanum to lutetium at a given temperature. Trace element diffusivities in hydrous rhyolitic melt cover nearly two orders of magnitude at a single temperature. The considerable differences among the measured trace element diffusion coefficients indicates that there is the potential for significant diffusive fractionation effects to develop in natural silicate magmas. The new data for trace element diffusion in melts is used in kinetic models to explore how diffusive fractionation occurs in “interrupted” processes in systems that have not yet reached equilibrium. Results indicate that diffusion in silicate melt is an effective mechanism to significantly fractionate trace elements from one another and produce anomalies in REE concentrations as diffusive boundary layers develop across melt-melt and crystal-melt interfaces.

Ion microprobe analyses were run on two diffusion couple experiments to determine the mass dependence of lithium diffusion in wet rhyolite. The lithium isotope mass effect is parameterized by the empirical equation D⁶⁄D⁶ =(7⁄6)^β. Over the investigated temperature range, D6Li is roughly 3.5% faster than D7Li, corresponding to β= 0.228 for diffusive fractionation of Li isotopes in hydrous rhyolite liquids. This value is slightly higher than β= 0.215 determined by Richter et al. (2003) for Li in a dry basalt-rhyolite diffusion couple and may reflect an increased potential for Li fractionation in low temperature, highly silicic melts. Diffusion in silicate melt may significantly fractionate ⁶Li from ⁶Li and create large gradients in δ⁷Li in rhyolite magma bodies.

PART III: The discovery of large lithium isotopic gradients in geologic materials has motivated recent work examining the diffusion and kinetic fractionation of Li isotopes. The rapid transport properties of Li and its large isotope mass effect make δ⁷Li uniquely suited to trace magmatic events. Lithium diffusion measurements in a variety of silicate media are needed to accurately model kinetic fractionation effects in natural samples. Here, diffusion couple experiments were used to determine Li mobility in rhyolite melts containing ~6 wt% H₂O over the temperature range 790-875 ºC. Laser ablation ICP-MS was used to characterize bulk lithium concentration gradients in all samples to calculate ⁷Li diffusion coefficients. Lithium transport in rhyolite melt is very fast and Li diffusivities are significantly increased by the addition of dissolved H2O to the melt network. Li diffusion coefficients conform to the expected Arrhenius relation D=D0exp(-Ea/RT), where the constants log(D0, m²/s)=-7.35±0.14 and Ea =39.31±2.91 kJ/mol.

Trace elements and their isotopes are often considered “blind passengers” in igneous and metamorphic reactions (i.e., they do not significantly alter phase assemblages or melt properties in most cases), making them uniquely suited to record geologic processes and the rates and conditions at which these processes occur. However, correct interpretation of the trace element contents of silicate materials may be complicated by kinetic processes that disrupt the establishment of chemical equilibrium in crystal-melt systems. When disequilibrium trace element profiles are recognized in natural samples, they can be used to resolve time-temperature paths for kinetically controlled phenomena if the trace element diffusion coefficients are known.