Mineral–fluid partitioning of lithium and implications for slab–mantle interaction

Measurements of the partitioning of lithium between clinopyroxene, olivine, plagioclase and hydrous fluid at 800–1100 °C and 1 GPa indicate this element is mildly incompatible in the solid relative to the fluid phase, similar to mineral–melt systems. Both clinopyroxene- and olivine-fluid partitioning decrease with increasing temperature (T, K) by the relations:lnDLicpx/fluid=−7.3±0.5+7.0±0.7∗1000/TandlnDLiol/fluid=−6±2+6±2∗1000/T.The lithium partition coefficients increase with pyroxene Al2O3 content and olivine FeO content, and decrease with plagioclase An content. Isotopic fractionation between clinopyroxene and fluid, ΔLicpx–fluid, between 900 and 1100 °C ranges from − 0.3 to − 3.5‰ (± 1.4‰).Quantitative modeling of the evolution of lithium concentration and isotopic composition in slab-derived fluids during transport to the arc melt source indicate that fluids migrating by porous flow 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 would be required to propagate a slab-like lithium signature (both elemental and isotopic) from the slab to the melt source of island arc basalts.

Research Highlights
► Li partitioning was measured between hydrous fluid and cpx, ol, plag.
► Li isotopic fractionation was measured between hydrous fluid and cpx.
► Li is strongly retained in mantle minerals with 6Li preferred over 7Li in cpx.
► Fluids from subducted slab will lose Li signature of slab to overlying mantle wedge.
► Li from slab may reach melt source if fluids have minimal interaction with mantle.