Lithium isotope fractionation between Li-bearing staurolite, Li-mica and aqueous fluids: An experimental study

Abstract

We determined the lithium isotope fractionation between synthetic Li-staurolite and aqueous fluids containing excess LiCl or LiOH at 3.5 GPa between 670 and 880 °C and between Li-mica and similar fluids at 2.0 GPa and 300 to 500 °C. In all experiments for the system Li-staurolite-fluid, 7Li was weakly partitioned into Li-staurolite, whereas for Li-mica-fluid, 7Li preferentially fractionated into the fluid. The difference in the Li-isotope fractionation behaviour results from different Li-coordination in staurolite (Li [4]) and mica (Li [6]). For the system Li-mica-fluid, fractionation is about half as large along the same T-range as for spodumene-fluid [Wunder, B., Meixner, A., Romer, R.L., Heinrich, W., 2006. T-dependent isotopic fractionation of lithium between clinopyroxene and high-pressure hydrous fluids. Contrib. Mineral. Petrol. 151, 112-120]. This behaviour is consistent with differences in the bonding strengths of the Li-octahedra in Li-mica and spodumene. Subduction and concomitant dehydration of metabasic oceanic crust, containing chlorite and clinopyroxene with Li in six-fold coordination, releases fluids enriched in Li and 7Li into the fore-arc and arc mantle and thus introduces a light Li-component into the deeper mantle. Ongoing dehydration and Li-loss along the subduction path produces fluids with increasingly lower Li-concentrations and lighter lithium isotope compositions. However, as staurolite is the most important carrier of Li in metapelitic assemblages, the inverse fractionation of Li-isotopes relative to fluids enables staurolite to transport high Li-concentrations in combination with an isotopic composition enriched in 7Li into the mantle. Thus, depending on the Li-contents and Li-isotope signature of the protolite, breakdown of Li-bearing staurolite at high P and T can release fluids with high Li-contents and isotopically heavy Li into the deep mantle wedge.