Lithium partitioning between olivine and diopside at upper mantle conditions: An experimental study

Experiments were conducted at 1.5GPa and temperatures between 700°C and 1100°C in order to assess the equilibrium distribution of lithium between olivine and diopside in the upper mantle. Lithium in olivine and diopside from natural mantle xenoliths displays a broad array of apparent partition coefficients ranging from ~0.2 to 10. In addition, a strikingly large range of lithium isotope ratios is observed in olivine and diopside from mantle xenoliths, with Δ ⁷Li _ol-di (=δ ⁷Li _ol-δ ⁷Li _di) ranging from nearly zero to greater than 20%. Both of these observations might be explained if the distribution of Li between olivine and diopside is strongly temperature dependent at mantle conditions such that a change in temperature, i.e. cooling upon exhumation, initiates diffusive re-equilibration of Li between phases in the xenolith. Accompanying dynamic fractionation of ⁶Li from ⁷Li due to differing diffusion rates of the two isotopes could then be permanently recorded in the xenolith if its temperature drops below the closure temperature before a new equilibrium is reached.The results of this study indicate a partition coefficient for Li between olivine and diopside (D _Li ^ol/di) of 2.0±0.2 that is independent of temperature (within the error of our analyses) over the range 700°C to 1100°C. This lack of temperature dependence holds true when data from previous experiments at temperatures as high as 1375°C are considered. Thus it appears that closed-system diffusion of Li between olivine and diopside in response to changing temperature is not an appropriate explanation for the observed range of elemental and isotopic distributions in natural xenoliths. Other possible explanations include Li redistribution in response to changing oxygen fugacity in the system, or diffusive addition or subtraction of Li during open-system interaction with an infiltrating melt or fluid.