An experimental study of Li partitioning between olivine and diopside at mantle conditions

Measured 7Li/6Li of mineral separates from mantle xenoliths from diverse localities show unexpectedly large differences between olivines and pyroxenes, often with lighter Li found in the pyroxenes (Jeffcoate et al., 2007; Rudnick and Ionov, 2007; Ionov and Seitz, 2008). Although changes in isotopic fractionation with temperature could explain the differences, a kinetic isotope effect is as likely. Because 6Li diffuses faster than 7Li, bulk lithium exchange between two phases could result in dynamic isotopic fractionation, with the receiving phase becoming lighter and the donating phase becoming heavier. Thus if Li becomes more compatible in cpx upon cooling, that is, if DLiol/cpx is temperature-dependent, the diffusive exchange of Li will generate temporary 6Li enrichment in cpx and depletion in olivine. Experiments were conducted using a piston cylinder apparatus at 1100°C and 1.4 GPa (1-5 days) to determine DLiol/cpx. San Carlos olivine and Dekalb diopside were finely ground for starting materials. A mixture of olivine (52 wt%), diopside (34 wt%), albite (7 wt%), and quartz (7 wt%) powders (0.0145 g) was loaded into a Pt capsule inside of a Ni crucible. Milli-Q water with 100 ppm Li and 500 ppm Ba (obtained through dilution of stock solutions) was added (0.1100 g) to serve as the lithium source. Lithium concentrations in olivine and diopside from experiments held for 1, 3, and 5 days were determined by laser ablation ICP-MS. Partition coefficients DLiol/cpx from runs at 3 and 5 days are, within error, the same, 1.9 (0.3). These fall in the lower portion of the range, D = 2-7, of limited previous measurements (Brenan et al., 1998b, Blundy and Dalton, 2000; Caciagli-Warman 2010). Partitioning experiments at 700 and 900°C are ongoing. References Blundy, J. and Dalton, J. (2000) Experimental comparison of trace element partitioning between clinopyroxene and melt in carbonate and silicate systems, and implications for mantle metasomatism. Contrib. Mineral. Petrol. 139, 356-371. Brenan, J.M., Ryerson, F.J., Shaw, H.F. (1998b) The role of aqueous fluids in the slab-to-mantle transfer of boron, beryllium, and lithium during subduction: Experiments and models. Geochim. Cosmochim. Acta 62, 3337-3347. Caciagli-Warman, N. (2010) Experimental constraints on lithium exchange between clinopyroxene, olivine and aqueous fluids at high pressures and temperatures. Unpublished PhD thesis, University of Toronto. Ionov, D.A. and Seitz, H.M. (2008) Lithium abundances and isotopic compositions in mantle xenoliths from subduction and intra-plate settings: Mantle sources vs. eruption histories. Earth Planet. Sci. Lett. 266, 316-331. Jeffcoate, A.B., Elliot, T., Kasemann, S.A., Ionov, D., Cooper, K., Brooker, R. (2007) Li isotope fractionation in peridotites and mafic melts. Geochim. Cosmochim. Acta 71, 202-218. Rudnick, R.L. and Ionov, D.A. (2007) Lithium elemental and isotopic disequilibrium in minerals from peridotite xenoliths from far-east Russia: Product of recent melt/fluid-rock reaction, Earth Planet. Sci. Lett. 256, 278-293.