Li-isotope fractionation between silicates and fluids: Pressure dependence and influence of the bonding environment

Abstract

Isotope fractionation experiments and molecular simulations were performed to determine the relation of Li-isotope fractionation between silicates and fluids and the corresponding cation coordination environments. The effect of pressure-induced changes of Li hydration in aqueous fluids on solid-fluid Li-isotope fractionation was studied by performing experiments in the system spodumene-fluid at three different pressures of 1, 4 and 8 GPa at temperatures ranging from 500 to 625 °C. 7 Li preferentially partitioned into the fluid in all three experiments. The Li-isotope fractionation of experiments at 1 and 4 GPa does not show a significant P dependence in comparison to previously published data at 2 GPa. At 8 GPa the Li-isotope fractionation is slightly decreased compared to the low-pressure data. In addition, the fractionation of lithium isotopes between Li-bearing amphibole and fluid was determined experimentally at 700 °C and 2 GPa, which resulted in a Δ 7 Li (Li-amph-fluid) of −1.7 ‰. Our experiments are complemented by ab initio molecular dynamics simulations of Li-bearing aqueous fluids aimed to determine structural properties at high P and T . Despite the increase in Li coordination from 4.0 to 5.4 with pressure at isothermal conditions, the mean Li-O distance of the fluid is almost unchanged between 1 and 8 GPa at 727 °C. This might explain the insignificant effect of pressure over a large P range observed experimentally. The new experimental results indicate a partial inapplicability of the coordination-principle on isotope fractionation. Therefore, we additionally analyzed the relation of isotope fractionation and Li-O bond length and applied the bond valence model. Using the available structural data of solids and fluids, in a first approximation, the bond valence model seems to be more appropriate to relate the local atomic structure to isotope fractionation than the simple coordination-dependent principle.