Copper isotope fractionation during partial melting and melt percolation in the upper mantle: Evidence from massif peridotites in Ivrea-Verbano Zone, Italian Alps

To investigate the behavior of Cu isotopes during partial melting and melt percolation in the mantle, we have analyzed Cu isotopic compositions of a suite of well-characterized Paleozoic peridotites from the Balmuccia and Baldissero massifs in the Ivrea-Verbano Zone (IVZ, Northern Italy). Our results show that fresh lherzolites and harzburgites have a large variation of delta Cu-65 ranging from -0.133 to 0.379%, which are negatively correlated with Al2O3 contents as well as incompatible platinumgroup (e. g., Pd) and chalcophile element (e. g., Cu, S, Se, and Te) contents. The high delta 65Cu can be explained by Cu isotope fractionation during partial melting of a sulfide-bearing peridotite source, with the light isotope (Cu-63) preferentially entering the melts. The low delta Cu-65 can be attributed to precipitation of sulfides enriched in Cu-63 during sulfur-saturated melt percolation. Replacive dunites from the Balmuccia massif display high delta Cu-65 from 0.544 to 0.610% with lower Re, Pd, S, Se, and Te contents and lower Pd/Ir ratios relative to lherzolites, which may result from dissolution of sulfides during interactions between S-undersaturated melts and lherzolites at high melt/rock ratios. Thus, our results suggest that partial melting and melt percolation largely account for the Cu isotopic heterogeneity of the upper mantle. The correlation between delta Cu-65 and Cu contents of the lherzolites and harzburgites was used to model Cu isotope fractionation during partial melting of a sulfide-bearing peridotite, because Cu is predominantly hosted in sulfide. The modelling results indicate an isotope fractionation factor of alpha(melt-peridotite) = 0.99980-0.99965 (i. e., 10 3 lnamelt-peridotite = -0.20 to -0.35%). In order to explain the Cu isotopic systematics of komatiites and mid-ocean ridge basalts reported previously, the estimated alpha(melt-peridotite) was used to simulate Cu isotopic variations in melts generated by variable degrees of mantle melting. The results suggest that high degrees (>25%) of partial melting extracts nearly all source Cu and it cannot produce Cu isotope fractionation in komatiites relative to their mantle source, and that sulfide segregation during magma evolution have modified Cu isotopic compositions of mid-ocean ridge basalts. (C) 2017 Elsevier Ltd. All rights reserved.