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Diffusive fractionation of Li isotopes in wet, highly silicic melts

Holycross, M.E. ; Watson, E.B. ; Richter, F.M. ; Villeneuve, J., Geochem. Persp. Lett.

Diffusive fractionation of Li isotopes in wet, highly silicic melts

Holycross, M.E. ; Watson, E.B. ; Richter, F.M. ; Villeneuve, J.

Geochemical Perspectives Letters, 2018, 6, 39-42

The discovery of large lithium isotopic gradients in geologic media has motivated recent work examining the kinetic fractionation of Li isotopes in silicate materials. Here, piston-cylinder experiments were used to determine Li diffusivities in rhyolitic melts containing 6 wt. % H2O at 1 GPa pressure and 790-875 ºC. Lithium transport in wet rhyolitic melt is almost an order of magnitude faster than diffusion in dry obsidian glass over the investigated temperature range. Li isotope profiles collected by ion microprobe show that the kinetic exponent β = 0.228 for diffusive fractionation of Li isotopes in wet rhyolite. This value is very close to β = 0.215 determined by Richter et al. (2003) for Li isotope diffusion in a dry basalt-rhyolite couple at 1350 °C. The similarity of the two values indicates little or no dependence of βLi in silicate melts on either temperature or melt composition. The new data confirm a very high potential for diffusive fractionation of 6Li from 7Li and can be confidently used to model deviations in δ7Li to determine the time-temperature histories of natural rhyolite samples.

Voir en ligne : http://dx.doi.org/doi:10.7185/geoch...




publié jeudi 10 janvier 2019