Experimental Constraints on Lithium Exchange between Clinopyroxene, Olivine and Aqueous Fluid at High Pressures and Temperatures

Clinopyroxene, olivine, plagioclase and hydrous fluid lithium partition coefficients have been measured between 800-1100oC at 1 GPa. Clinopyroxene-fluid partitioning is a function of temperature (ln DLicpx/fluid = -7.3 (+0.5) + 7.0 (+0.7) * 1000/T) and appears to increase with increasing pyroxene Al2O3 content. Olivine-fluid partitioning of lithium is a function of temperature (ln DLiol/fluid = -6.0 (+2.0) + 6.5 (+2.0) * 1000/T) and appears to be sensitive to olivine Mg/Fe content. Anorthite-fluid lithium partitioning is a function of feldspar composition, similar to the partitioning of other cations in the feldspar-fluid system. Isotopic fractionation between clinopyroxene and fluid, Licpx-fluid, has been measured between 900-1100oC and ranges from -0.3 to -3.4 ‰ (±1.4 ‰). Lithium diffusion has been measured in clinopyroxene at 800-1000oC and in olivine at 1000oC. The lithium diffusion coefficient is independent of the diffusion gradient as values are the same if the flux of lithium is into or out of the crystal and ranges from -15.19 ± 2.86 m2/s at 800oC to -11.97 ± 0.86 m2/s at 1000oC. Lithium diffusion in olivine was found to be two orders of magnitude slower than for clinopyroxene at similar conditions. Closure temperatures calculated for lithium diffusion in clinopyroxene range from ~400 to ~600oC. These results demonstrate that lithium equilibration between fluids and minerals is instantaneous, on a geological timescales. The confirmation of instantaneous equilibration, combined with min-fluid partition coefficients and values for Licpx-fluid, permits quantitative modeling of the evolution of lithium concentration and isotopic composition in slab-derived fluids during transport to the arc melt source. Our results indicate that fluids migrating by porous flow will rapidly exchange lithium with the mantle, effectively buffering the fluid composition close to ambient mantle values, and rapidly attenuating the slab lithium signature. Fluid transport mechanisms involving fracture flow are required to maintain a slab-like lithium signature (both elemental and isotopic) from the slab to the melt source of island arc basalts. This study demonstrates that mineral-fluid equilibration is rapid, and as a result the lithium content of minerals can only reliably represent chemical exchange in the very latest stages of the sample’s history.Ph