Zinc isotope fractionation in mantle rocks and minerals, and a revised? 66Zn value for the Bulk Silicate Earth

The isotope composition of zinc (Zn) can be an important tracer of planetary formation and differentia-tion, 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 mu g/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 abun-dance ratios of [Zn]Sp/[Zn]Ol = 15-47, [Zn]Ol/[Zn]Opx = [[Zn]Ol/[Zn]Cpx =-6. Spinel has higher .566Zn values than coexisting silicates (-0.12 %. higher than in Ol). Minerals in orogenic peridotites show dise-quilibrium Zn element and isotope partitioning due to slow inter-grain Zn diffusion at retrograde meta-morphism.Fertile peridotites (& GE;3 wt% Al2O3) contain 52-60 mu g/g Zn, usually higher than melt-depleted (< 3 wt% Al2O3) rocks (40-53 mu g/g). The .566Zn values in the majority of spinel and garnet peridotites range from 0.11 to 0.26 %. and show no systematic co-variations with melt extraction indices (e.g., Al2O3, Mg#), sug-gesting, unlike some previous studies, that partial melting does not appreciably affect Zn isotope compo-sition of residual mantle. Garnet lherzolites yield an average .566Zn of 0.22 & PLUSMN; 0.09 %. (2SD), similar to that (0.18 & PLUSMN; 0.07 %., 2SD) for shallower spinel peridotites, indicating uniform Zn isotope composition with depth. Four fertile lherzolites from earlier work, duplicated in this study to assess inter-laboratory bias, fall in the same .566Zn range, suggesting that previously reported higher .566Zn for these samples could be due to analytical problems. We thus estimate the .566Zn of the BSE at 0.19 & PLUSMN; 0.08 %. (2SD) based on suit-able fertile peridotites from several localities in this study and the literature.The .566Zn values (-0.10 to 0.04 %.) are much lower in Fe-rich (9.3-13.1 wt% FeOT) and veined spinel peridotites as well as in spinel pyroxenite veins from Mongolia (-0.20 to 0.11 %.). The Fe-rich peridotites and pyroxenites are rich in Zn (up to 113 mu g/g) and were previously found to have anomalously low .556Fe (-0.33 to -0.27 %.) and .553Cr (-1.36 to -0.05 %.). We attribute these features to kinetic isotope frac-tionation during reaction with mafic melts migrating in conduits in the lithospheric mantle or intrusion of Fe-Zn-rich melts with light Fe-Zn isotope compositions. Kinetic Zn isotope fractionation, rather than equilibrium partial melting, may be a major cause of Zn isotope heterogeneity in the mantle.(c) 2022 Elsevier Ltd. All rights reserved.

Automated summary

The isotope composition of zinc is an important tracer for planetary formation and differentiation. However, there is still debate about the estimates of the Bulk Silicate Earth and the extent and origin of zinc isotope variations in the mantle. Through analysis of samples from different locations, it was found that there are differences in the zinc content and isotope composition in different minerals, and partial melting has little effect on the zinc isotope composition of the residual mantle.