DIFFUSIVE FRACTIONATION OF LI IN WET, HIGHLY SILICIC MELTS

The discovery of large lithium isotopic gradients in geologic materials has motivated recent work examining diffusion and diffusive fractionation of Li isotopes in silicate crystals and melts. The rapid transport properties of Li and its large isotope mass effect make δ⁷Li an excellent tracer of short-lived 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 mobilities 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 profiles in all samples, from which ⁷Li diffusion coefficients were extracted. Lithium transport in rhyolite melt is very fast even in dry melts, and diffusivities are significantly increased by the presence of dissolved H₂O in the melt network. Li diffusion coefficients conform to the expected Arrhenius relation D=D₀exp(-E_a/RT), where the constants log(D₀, m²/s)=-7.35±0.14 and E_a=39.31±2.91 kJ/mol.

Ion microprobe analyses were run on a Cameca 1280 at the CRPG in Nancy, France to determine the mass dependence of lithium diffusion in wet rhyolite. The lithium isotope mass effect is parameterized by the empirical equation (D₆/D₇)=(7/6)^β. Over the investigated temperature range, D_6Li is roughly 3.5% faster than D_7Li, 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. Diffusion in silicate melt may significantly fractionate ⁶Li from ⁷Li and create large gradients in δ⁷Li in rhyolite magma bodies.

Richter, F.M., Davis, A.M., DePaolo, D.J., and E.B. Watson (2003) Isotope fractionation by chemical diffusion between molten basalt and rhyolite. Geochimica et Cosmochimica Acta, v. 67, p. 3905-3923, doi:10.1016/S0016-7037(03)00174-1.