Mantle Zn Isotopic Heterogeneity Caused by Melt-Rock Reaction: Evidence From Fe-Rich Peridotites and Pyroxenites From the Bohemian Massif, Central Europe

摘要

To investigate the effect of melt-rock reaction on Zn isotope fractionation and mantle Zn isotopic heterogeneity, we analyzed Zn isotopic compositions of peridotites, pyroxenites, and mineral separates from the Bohemian Massif, Central Europe. The Mg-lherzolites (Mg#=90.9 to 89.1, FeOT=7.9 to 9.0wt%) are melting residues with only moderate metasomatism and have Zn-66 from 0.11 to 0.20. In contrast, the Fe-rich peridotites (Mg#=88.2 to 80.3, FeOT=10.0 to 14.5wt%) and pyroxenites have larger ranges of Zn-66 from 0.11 to 0.31 and -0.33 to 0.42, respectively. Large disequilibrium intermineral Zn isotope fractionation occurs in the Fe-rich peridotites and pyroxenites with Zn-66(Opx-Ol)=-0.50, Zn-66(Grt-Ol)=-0.55 to -0.39 parts per thousand, Zn-66(Grt-Opx)=-0.28 to -0.05 parts per thousand, and Zn-66(Grt-Cpx)=-0.50 to 0.12 parts per thousand. Combined with their low SiO2 contents and radiogenic Sr-Nd-Os isotopic compositions, the high Zn-66 of the Fe-rich peridotites is attributed to reaction between Mg-lherzolites and percolating SiO2-undersaturated basaltic melts that incorporated isotopically heavy crustal components. Crystallization of the isotopically heavy percolating melts migrating through the lithospheric mantle yield the high-Zn-66 pyroxenites. The low Zn-66 of the pyroxenites and large intermineral Zn isotopic disequilibrium may result from kinetic Zn isotope fractionation during melt-rock reaction. Collectively, these observations indicate that melt-rock reaction can cause intermineral Zn isotopic disequilibrium and significant Zn isotopic heterogeneity in the mantle. This study thus highlights the potential use of Zn isotopes to trace melt-rock reaction events in the mantle.