Lithium and Li-isotopes in young altered upper oceanic crust from the East Pacific Rise

Lithium contents and Li-isotope ratios have been measured in a section of young oceanic upper crust formed at the East Pacific Rise and exposed in the Hess Deep rift. The Li contents of both lavas and sheeted dikes are lower than in the fresh protolith (by 43% and 74%, respectively) suggesting that axial hydrothermal systems leach Li from the entire upper crust. The Li-isotopic composition of the lavas (4.1 + 1.7 parts per thousand) is similar to the protolith (3.4 + 1.4 parts per thousand) whereas the dikes extend to both higher and lower isotopic compositions (delta Li-7 from -11 to 14 parts per thousand). Loss of Li from all lava samples was unexpected due to the petrographically fresh appearance of these rocks and the relatively cool fluids generally thought to exist within the lava pile. This Li loss is interpreted as indicating that warm (50-100 degrees C) fluids pervade the lava pile, at least episodically, leaching Li from the lavas without recrystallization of the primary igneous minerals. This hypothesis has been tested experimentally. Hydrothermal experiments at 125 degrees C, using synthetic basalts cooled at different rates to produce different starting material texture (glass to crystalline rock), demonstrate that Li is leached rapidly at this temperature and substantially more rapidly from crystalline rock than from glass. The natural and experimental data are consistent with Li diffusion out of plagioclase into warm hydrothermal fluids acting to leach Li from the lavas without any concomitant mineralogical changes. The wide range of delta Li-7 in the sheeted dike complex, and the lack of a systematic correlation of delta Li-7 with the concentration of Li, suggest that more than one process leads to Li depletion. A combination of mineral-fluid reactions that break-down igneous minerals and produce secondary minerals (principally amphibole, chlorite and secondary plagioclase), along with diffusive loss of Li from plagioclase into the fluid, can explain the Li-systematics of the sheeted dike complex at Hess Deep. (C) 2012 Elsevier Ltd. All rights reserved.