Effects of Melt Percolation on Zn Isotope Heterogeneity in the Mantle: Constraints From Peridotite Massifs in Ivrea‐Verbano Zone, Italian Alps
2018
Huang, Jian | Chen, Sha | Zhang, Xing‐Chao | Huang, Fang
We determined Zn isotopic compositions of 21 orogenic peridotites from the Baldissero and Balmuccia peridotite massifs in Ivrea‐Verbano Zone, Italian Alps, to investigate Zn isotope behaviors during partial melting and melt percolation in the mantle. The samples include lherzolites, harzburgites, and dunites. Lherzolites are strongly depleted in light rare earth element relative to middle and heavy rare earth element with (La/Sm)PM from 0.009 to 0.265 and (La/Yb)PM from 0.003 to 0.125, which can be explained by 5–15% fractional melting of a primitive mantle source. Harzburgites and dunites with nearly identical Mg# (molar 100 * Mg/(Mg + Fe) = 90.2–91.0) have (La/Sm)PM and (La/Yb)PM higher than but Zn contents similar to or lower than those of the parental lherzolites, suggesting that they were influenced by Zn‐depleted silicate melt percolation. Lherzolites have δ⁶⁶Zn from 0.13 to 0.27‰ showing no correlations with indicators of melt extraction (e.g., Al₂O₃, Mg#, and La/Yb) and Zn contents. Three sulfide melt‐affected lherzolites show similar δ⁶⁶Zn to the other normal ones. These observations indicate that 5–15% partial melting and sulfide melt percolation cause limited Zn isotope variations in the mantle. The metasomatic harzburgites and dunites display high δ⁶⁶Zn (up to 0.46‰) negatively correlated with Zn contents. Such correlations are attributed to kinetic effect during silicate melt percolation, whereby ⁶⁴Zn preferentially diffuses out from mantle minerals (e.g., olivine) to the percolating silicate melts. A diffusion model suggests that the negative correlation between δ⁶⁶Zn and Zn contents in dunites can be explained by an empirical βZₙ (i.e., βZₙ‐exponent in D₆₆Zₙ/D₆₄Zₙ = (m₆₄Zₙ/m₆₆Zₙ)ᵝZⁿ) of 0.05–0.06 in olivine.
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