Zinc isotope fractionation in mantle rocks and minerals, and a revised δ66Zn value for the Bulk Silicate Earth

The isotope composition of zinc (Zn) can be an important tracer of planetary formation and differentiation, but estimates of the Bulk Silicate Earth (BSE) and the extent and origin of Zn isotope variations in the mantle continue to be debated. We report 153 high-precision Zn isotope analyses of whole-rock samples and minerals of well-characterized, mantle-derived spinel and garnet peridotites from five sites in Siberia, Mongolia and China: 38 xenoliths in basaltic rocks (fertile to strongly melt-depleted as well as variably metasomatized or veined peridotites) and 3 orogenic peridotites. Data quality was rigorously tested by twenty-two duplicated analyses using several dissolution techniques. Spinel (Sp) contains 760–1460 μg/g Zn and hosts up to 58 % of whole-rock Zn budget in the peridotites followed by olivine (Ol, 30–66 % of whole-rock Zn budget), orthopyroxene (Opx), clinopyroxene (Cpx) and garnet, with abundance ratios of [Zn]Sp/[Zn]Ol = 15–47, [Zn]Ol/[Zn]Opx = ∼1.5, [Zn]Ol/[Zn]Cpx = ∼6. Spinel has higher δ66Zn values than coexisting silicates (∼0.12 ‰ higher than in Ol). Minerals in orogenic peridotites show disequilibrium Zn element and isotope partitioning due to slow inter-grain Zn diffusion at retrograde metamorphism.