Trace Element Diffusion in Basaltic Melt

Abstract:

We conducted high pressure, high temperature experiments to determine simultaneously the diffusivities of 24 trace elements (Sc, V, Rb, Y, Zr, Nb, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu, Hf, Ta, Th, U) in liquids of basaltic composition. Pre-synthesis runs were conducted in graphite capsules in a piston-cylinder apparatus to create two glasses having relatively high and low trace element contents. These glasses were then powdered and paired in diffusion couples by repacking in graphite capsules. All diffusion experiments were executed in a piston cylinder apparatus at 1 GPa pressure and temperatures ranging from 1250-1500º C.

Concentration gradients that developed in the glasses were characterized using a laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS). Diffusion coefficients were determined from concentration profiles and show Arrhenian behavior within experimental error. Errors were assigned based on the linear fit of five time series experiments conducted over 500-9000 s to accurately represent the total experimental reproducibility of our results. Data show the highest activation energies are obtained for high field strength elements. Values for the pre-exponential factor, D₀, also peak for the high field strength elements. We suggest that trace element diffusion in basaltic melts follows the compensation law (Winchell, 1969), with log D₀ exhibiting linear dependence on activation energy. Calculated diffusivities indicate that transport through basaltic melt could be an effective mechanism for fractionating high field strength elements over geologically relevant time scales.

Winchell (1969) High Temp. Sci. 1: 200-215