Equilibrium thallium isotope fractionation and its constraint on Earth’s late veneer

Abstract

Equilibrium isotope fractionation of thallium (Tl) includes the traditional mass-dependent isotope fractionation effect and the nuclear volume effect (NVE). The NVE dominates the overall isotope fractionation, especially at high temperatures. Heavy Tl isotopes tend to be enriched in oxidized Tl3+-bearing species. Our NVE fractionation results of oxidizing Tl+ to Tl3+ can explain the positive enrichments observed in ferromanganese sediments. Experimental results indicate that there could be 0.2–0.3 e-unit fractionation between sulfides and silicates at 1650 °C. It is consistent with our calculation results, which are in the range of 0.17–0.38 e-unit. Importantly, Tl’s concentration in the bulk silicate Earth (BSE) can be used to constrain the amount of materials delivered to Earth during the late veneer accretion stage. Because the Tl concentration in BSE is very low and its Tl isotope composition is similar with that of chondrites, suggesting either no Tl isotope fractionation occurred during numerous evaporation events, or the Tl in current BSE was totally delivered by late veneer. If it is the latter, the Tl-content-based estimation could challenge the magnitude of late veneer which had been constrained by the amount of highly siderophile elements in BSE. Our results show that the late-accreted mass is at least five-times larger than the previously suggested magnitude, i.e., 0.5 wt% of current Earth’s mass. The slightly lighter 205Tl composition of BSE relative to chondrites is probable a sign of occurrence of Tl-bearing sulfides, which probably were removed from the mantle in the last accretion stage of the Earth.